Dissertations / Theses on the topic 'Platinum adsorption'

The Pt-W(lOO) system has been studied using Auger spectroscopy. X-ray photoelectron spectroscopy and X-ray photoelectron diffraction. Several different regimes were discerned and characterized; a pseudomorphic first Pt layer, distorted hexagonal Pt overlayers, 3D Pt microcrystallites and alloy films. XPS indicated electron transfer from Pt to W at the W/Pt interface and in the alloy layers. Surface Pt inhibits the dissociative chemisorption of CO via an ensemble effect resulting in enhanced molecular adsorption. Distortion in the pseu-dohexagona! layer results in CO bonding to Pt not typical of Pt(l 11). On alloy films CO appears to bind only at W sites with the bonding modified by neighbouring Pt atoms (ligand effect). Surface Pt inhibits adsorption and dissociation of hydrogen on W. At submonolayer coverages there is evidence for occupation of mixed W-Pt sites. Pseudomorphic Pt shows H2 desorption behaviour not typical of pure Pt surfaces while thicker layers resemble Pt(lll). In the presence of large quantities of Pt, H2 desorption from W sites occurs in a new feature at 270K. In the presence of adsorbed CO, the H2 desorption features are shifted to lower temperature, while CO seems little affected by hydrogen. In some cases new H2 desorption features indicate some local mixing of adsorbed CO and hydrogen. Low coverages of methanol or formaldehyde on W(100) lead to CO and H2 as the only decomposition products while at higher coverages, complexes giving rise to desorption of H2, Co, CH4, H2CO and CH3OH are formed. Small quantities of surface Pt eliminate the formation of these complexes. Methane is also formed via a different intermediate which is observed for Pt coverages < 0.5ML. At higher Pt loadings, adsorption of methanol or formaldehyde on W is inhibited. Thick Pt layers behave like pure Pt surfaces towards decomposition of methanol or formaldehyde giving CO and H2 only as decomposition products. On alloy surfaces, there is evidence for the formation of an H/CO complex with methanol decomposition but not with formaldehyde.

The unique Cambridge single crystal adsorption microcalorimeter has been applied to a range of systems of catalytic interest to extract fundamental thermodynamic data. Coverage-dependent adsorption heats and sticking probabilities have been obtained with CO, NO, oxygen and ethylene adsorbates and Pt{110}, Ni{100}, Ni{110} and K/Ni{110} substrates. The initial adsorption heats for CO, NO, and O2 on Pt{110} are 183, 160 and 335 kJ mol⁻¹ respectively. Catalytic heat of reaction data for CO + O2 on Pt{110}, the first obtained on a single crystal surface, suggest that "hot" adatoms are involved in producing thermally excited CO2 product molecules. The first measurement of the heat of adsorption of a hydrocarbon on a single crystal surface has been made for ethylene on Pt{110}. The initial value of 200 kJ mol⁻¹, together with coverage dependent values, yield an average Pt–C binding energy of 231 kJ mol⁻¹. A detailed comparison of new data for oxygen adsorption on Ni{100} and Ni{110} with existing data for Ni{111} is presented, with initial adsorption heats of 550, 475 and 440 kJ (mol O2) -1 respectively, while the corresponding sticking data suggest that the oxide film created is four atomic layers thick in each case. Novel temperature-dependent data have been successfully collected using a pyroelectric detector for oxygen on Ni{100} at 100, 300 and 410 K with a Monte Carlo simulation of the highest-temperature data yielding a strong second-nearest neighbour interaction energy of +30 ± 5 kJ mol⁻¹. The adsorption of CO on K-predosed Ni{110} was found to differ markedly from previous data collected on Ni{100} for potassium coverages up to 0.35 ML, with the adsorption heat promoted by only ~ 30 kJ mol⁻¹ compared to ~ 180 kJ mol⁻¹ for the {100} surface, a difference ascribed to the missing row reconstruction of Ni{110}.

The adsorption and separation of platinum(IV) and palladium(II) chlorido species (PtCl₆²⁻ and PdCl₄²⁻) on polystyrene-based beads and nanofibers as well as silica microparticles functionalized with polyamine centres derived from ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetriamine (TETA) and tris-(2-aminoethyl)amine (TAEA) is described. The functionalized sorbent materials were characterized by using microanalysis, SEM, XPS, BET and FTIR. The nanofiber sorbent material functionalized with ethylenediamine (F-EDA) had the highest loading capacity which was attributed to its high nitrogen content (10.83%) and larger surface area (241.3m²/g). The adsorption and loading capacities of the sorption materials were investigated using both the batch and column studies in 1 M HCI. The adsorption studies for both PtCl₆²⁻ and PdCl₄²⁻ on the polystyrene-based sorbent materials fit the Langmuir isotherm while the silica-based sorbents fitted the Freundlich isotherm with R² values > 0.99. In the column experiment the highest loading capacity of Pt and Pd were 7.4 mg/g and 4.3 mg/g respectively on the nanofiber sorbent material based on ethylenediamine (EDA). The polystyrene and silica-based resins with triethylenetetramine (TETA) functionality (M-TETA and S-TETA) showed selectivity for platinum and palladium, respectively. Metal chlorido complexes loaded on the sorbent materials were recovered by using 3% m/v thiourea solution as teh eluting agent with quantitative desorption efficiency under the selected experimental conditions. The separation of platinum from palladium was partially achieved by selective stripping of PtCl₆²⁻ with 0.5 M of NaClO₄ in 1.0 M HCI with PdCl₄²⁻ was eluted with 0.5 M thiourea in 1.0 M HCI. The selectivity of the M-TETA and S-TETA sorbent materials was proved by column separation of platinum(IV) and palladium(II), respectively, from synthetic solutions containing iridium(IV) and rhodium(III). The loading capacity for platinum on M-TETA was 0.09 mg/g while it was 0.27 mg/g for palladium on S-TETA.
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This thesis deals with the influence of the structure andchemical composition of platinum surfaces on the adsorption ofsome molecules. Three main lines can be distinguished : 1) thecharacterisation of clean/modified surfaces, 2) the adsorptionof some simple atoms and molecules on these surfaces andfinally 3) the adsorption of 2-butenal, a bi-functionalmolecule containing a C=C group conjugated with a C=O group.The main tools used in this work are scanning tunnellingmicroscopy, photoelectron spectroscopy and high-resolutionelectron energy loss spectroscopy, in combination with quantumchemical calculations.

The platinum (111)(1× 1) and (110)(1× 2) surfacesare chosen as substrates. Pt(111) is a non-reconstructedclose-packed surface, while the Pt(110) surface is open, due toits missing row (MR) reconstruction, which results in thealternation of ridges, {111} microfacets and valleys. Titaniumgrows on Pt(111) in a Volmer-Weber mode. Pronounced reactionsbetween Pt and Ti are detected already at room temperature asthe Ti2p and Pt4f_7/2core-level shifts are characteristic of the Pt₃Ti alloy. Carbon segregated on the (110) surfaceappears as extended graphitic regions, which smoothen thesubstrate. Sn deposition at room temperature on Pt(110) resultsin the appearance of small islands, randomly spread over thesurface. The presence of mobile Sn ad-atoms and Pt-Sn-Ptalloyed chains in the valley of the MR reconstruction is alsoevidenced. Annealing the surface results in the rearrangementof the tin in the surface layer, together with a globalshortening of the terraces in the [110]direction and anincreased density of (1× n) (n>2) defects.

The adsorption of atomic hydrogen and oxygen has beenperformed on the Pt(111)( √ 3x √3)R30º surfacealloy and on Pt(110)(1× 2) respectively. The adsorptionsite of these atoms is changed as compared to the onedetermined on the clean Pt(111) surface (i.e. fcc hollow site): H is adsorbed on-top site on the (√3× √3)surface alloy, and O sits on the ridge of the missing rowreconstruction in bridge site. Carbon monoxide adsorption wasperformed on Pt(111) and Pt(110)(1× 2) surfaces, modifiedor not by tin. On the unmodified (111) surface, CO adsorbsfirst in top site, thenin bridge. Changing the geometry of thesurface to the (110) results in the vanishing of the bridgesite population. Modifying these surfaces by Sn does not changethe CO adsorption site.

Finally, 2-butenal (CH₃-CH=CH-CH=O) has been adsorbed at 100 K on thePt(111) surface and the Sn/Pt(111) surface alloys. On theunmodified Pt surface, comparison with results obtained forpropene (CH₂=CH-CH₃) adsorption evidences the involvement of the C=Cgroup of the 2-butenal molecule in the bonding to the Ptsurface. The carbonyl group is also suggested to take part inthe bonding, through a σ_ccη₁(O) configuration. This carbonyl group bondingdisappears when Sn is alloyed to the surface, and the formationof a new physisorbed phase is detected.

This thesis reports results on the investigation of the thermodynamics and kinetics of adsorption of bovine serum albumin (BSA) on a platinum electrode at pH 7.4 in a wide temperature range using the electrochemical techniques of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and differential capacitance (DC).
The adsorption of BSA at anodic potentials has been found to be a competitive process occurring in parallel with platinum oxide formation. It has been found that the adsorption charge is directly proportional to the amount of protein adsorbed. With the increase in temperature, the maximum amount of adsorbed BSA (saturated surface concentration) also increases. This dependence has been found to be linear. It has also been shown that the adsorption process is dependent on the electrode surface charge.

The heterogeneous gas/solid enantioselective hydrogenation of methyl pyruvate over cinchonidine modified supported platinum catalysts (the Orito reaction) in a differential flow system was studied using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and capillary gas chromatography. The reaction was studied over a temperature range of room temperature to 160°C. The adsorption of methyl pyruvate on a Pt(111) single crystal was also studied using Reflection Adsorption Infrared Spectroscopy (RAIRS). DRIFTS spectra are presented showing that methyl pyruvate absorbs on Pt/alumina catalysts principally via the keto-carbonyl (T1 adsorption mode). At room temperature, methyl pyruvate adsorbs on the unmodified catalyst in a T1 mode with the keto-carbonyl forming a carbon-oxygen-metal bond (COM), with a slight preference for the trans conformer. Over the modified catalyst at room temperature, methyl pyruvate adsorbs with a strong preference for the T1-cis_COM mode. Other modes of adsorption are observed: a cis bidentate mode utilising both carbonyls; a trans mode using the keto-carbonyl and the ester oxygen; and T1 adsorption via the keto-carbonyl lone-pairs. As the temperature is increased, these additional modes increase in population. The unmodified reaction was found to be temperature independent over the range studied while the modified reaction showed changes in conversion and enantioselectivity. The deactivation of the modified reaction is discussed in terms of an enantiodirecting function of the modifier and a ligand accelerating function. The enantiodirection decreases above room temperature, while the rate-enhancing ligand acceleration is stable up to 100°C. Above 100°C, it is proposed that the modifier destructively hydrogenates producing non-volatile side-products that block the active catalyst sites. Possible side-reactions and products are also discussed. The implications of a successful gas/solid Orito reaction are discussed with reference to current literature.

Thesis (PhD)--Stellenbosch University, 2015.
ENGLISH ABSTRACT: In order to exploit lower grade and complex platinum group metal resources, cheaper and more efficient alternatives to the conventional mill-float-smelt-refine route are being sought. Leaching of platinum and palladium with cyanide has been proposed a number of times as a promising precious group metals (PGM) process option, and although platinum extractions are problematic, progress into the understanding of cyanide leaching of PGM containing ore and concentrate has been made. The platinum and palladium leaching will typically take place at elevated temperatures, which can range from 55°C on heaps to 180°C in autoclaves, with a better degree of leaching occurring with higher temperatures. Although this process for Pt and Pd extraction is a promising process option, research regarding the feasibility of the subsequent upgrading and recovery of the pregnant PGM leach solution, however, has been lacking. Since the carrier-phase extraction of gold using activated carbon offers significant advantages over other processes in terms of simplicity, the high pre-concentration factor, rapid phase separation, and relatively low capital and operating costs, activated carbon was deemed the most suitable sorbent for a Pt and Pd adsorption and stripping process. Very little is published on the adsorption of PGM cyanides onto activated carbon and when the effect of impurities such as base metals and thiocyanate together with a suitable elution method, are considered, no information could be found in the open literature. This study was launched and in general it was found that the activated carbon process does seem to be a viable process consideration for the upgrading of PGMs in a cyanide leach stream. Adsorption rates for dilute PGM solutions (0.15mg/L Pt, 0.38 mg/L Pd, 0.1 mg/L Au) in a stirred vessel indicated a high rate of adsorption within the first 60 minutes (giving more than 98% recovery of precious metals). A comparison of the Pt isotherm (25°C) to Au isotherms from literature indicated a similar loading capacity, while that of Pd was found to be significantly lower. In common with most diffusion controlled processes, an increase in the adsorption rate of platinum, palladium and gold cyanide with an increase in temperature was observed, while experiments with consecutive contacts of the PGM cyanide solution onto the activated carbon revealed that with an increase in temperature, the amount of PGMs that were adsorbed, decreased with each loading. In the absence of free cyanide and base metals, it was found that after 4 consecutive contacts, 99% of the total amount of platinum and palladium adsorbed at 25°C, compared to 85% of the platinum and 83% of the palladium at 50°C. No difference could be seen between the adsorption of gold cyanide at 25 and 50°C after 4 contacts. It has also been established that the detrimental effect of free cyanide on the adsorption of PGMs will increase as the temperature increases. The detrimental effect of the presence of Cu and Ni was found to depend on the amount of these base metals adsorbed, which in turn will depend on the cyanide concentration and the solution temperature. Adsorption of Pt and Pd has been found to be significantly more affected by temperature, cyanide and base metals than the adsorption of gold and needs to be carefully taken into consideration with the design of a PGM adsorption circuit to ensure sufficient Pt and Pd recovery. It is therefore highly likely that an activated carbon recovery process for Pt and Pd cyanide will not be as robust as the gold CIS (carbon-in-solution) process, which is considered to be one of its main advantages. The feasibility of eluting platinum and palladium cyanide complexes from activated carbon was investigated. It was found that platinum and palladium elute from activated carbon almost to completion in 4 to 5 bed volumes (BV) at 80°C, while the elution of gold at this temperature is slow, with a significant amount of gold (≈ 55 %) still to be eluted after 16 bed volumes. An increase in Pt and Pd elution kinetics was demonstrated with an increase in temperature with 99% recovery achieved at 4 BVs with an elution temperature of 95°C. Cyanide pre-treatment has been found to have a large influence on PGM elution. The effect of the NaCN concentration shows an increase in the recovery of Pt, Pd and Au as the cyanide increases from 0 to 2 %, after which the recovery starts decreasing again as the NaCN concentration increases from 2 to 4%. The NaOH concentration was also found to affect the PGM recovery and at 0% NaCN, an increase in the recovery is seen, while at a higher cyanide concentration (2 and 3% NaCN) a decrease in the PGM recovery occurs when the NaOH concentration is increased from 0.22% to 1.65%. A general decrease in Pt, Pd and Au recovery was seen as the ionic strength of the elution water increased and is consistent with literature on Au elution. The effect of a hydrochloric acid pre-treatment, which forms part of the process to remove calcium build-up from the activated carbon, was investigated, and for all the cases the Pt and Pd recovery increased when an acid pre-treatment was performed, compared to no acid pre-treatment. In none of the cases did any of the Pt, Pd or Au elute with the acid or the following rinsing water. The acid pre-treatment performed at 70°C removed a significant 64% to 75% of the Ni present and an additional 9.1% to 10.5% in the following rinsing water step. In the presence of copper cyanide, the elution order has been found to be copper, palladium, platinum and gold, which is the opposite order of adsorption preference. The cyanide pre-treatment has also been found to have a major influence on the elution of Cu and can be explained by the difference in the absorbance strength between the different copper cyanide complexes. The presence of Cu did not have a negative effect on the elution of the PGMs at strong pre-treatment (2% NaCN and 0.55% NaOH) conditions, but at weak pre-treatment conditions (0% NaCN) the recovery of Pt and Pd was reduced by between 10 and 18% after 5 BVs when Cu was present. The presence of 100 mg/L KSCN salt added to the leach solution during the adsorption stage, reduces the elution recovery of the PGMs at 4 BVs from 90% for Pt and Pd, when not present, to approximately 70% when present. The addition of the additional K+ ions reduced the recovery by less that 4% at 4 BV, which indicates that the possible formation of a PGM bonding with thiocyanate ([Pt(SCN)4]2- and [Pd(SCN)4]2-), which adsorb onto carbon, but doesn’t adsorb with water, cannot be ruled out completely. A maximum amount of 0.15% for Pt, 0.28% Pd and 0.6% Au was found to report to the pre-treatment solution at 25°C. For higher pre-treatment temperatures, the amount of Pt and Pd reporting to the pre-treatment solution increased significantly to approximately 8% at 80°C, while the increase in gold was marginal to 0.8% at 80°C. For the higher loading on the activated carbon (7000 mg/kg Pt and Pd), which is expected to be a better representation of plant conditions, 0.07% Pt, 0.11% Pd and 0.12% Au reported to the pre-treatment solution. The PGMs reporting to the pre-treatment solution is attributed to the distribution of the PGMs on the carbon particle, and even though very small, a certain amount of these PGMs would readily desorb or wash off the carbon. It has thus been established here that the stripping of adsorbed platinum cyanide complexes from activated carbon consists of a two step batch process, which involves the pre-treatment of the metal-loaded activated carbon with a relatively strong sodium cyanide and sodium hydroxide solution, prior to the elution step with de-ionized water at 80°C. Through the development of a mathematical model to describe this process, it was found that the rate of release of the platinum ions is governed by the amount of platinum and sodium on the activated carbon and the concentrations of these ions in the bulk of the liquid. This is mathematically described by a modified Freundlich isotherm equation and the mass transfer diffusion equation. The dependency of the platinum elution rate on the sodium concentration on the activated carbon, as suggested by gold related literature (Van Der Merwe and Van Deventer, 1990, Stange, 1990), is confirmed. Furthermore it has been found that the rate of platinum elution interchangeably depends on the equilibrium of the Pt ions at the carbon-liquid interface and the mass transfer of these Pt ions from the interface to the bulk liquid. As both of these rate-limiting factors were found to depend on the sodium concentration, the dominant platinum elution rate limiting factor shifts as the sodium concentrations change as the elution progresses. Four main time periods are used to simplify and to describe this process. The benefits of fundamentally understanding this process can ultimately lead to improved elution, better process control, shorter elution times, smaller elution columns or assist in the development of a continuous elution process.
AFRIKAANSE OPSOMMING: Nuwe prosesse om komplekse laer graad waardevolle metaal ertse te onwikkel in plaas van die konventionele maal-flotasie-smelt-raffineer proses word tans ondersoek. Loging van platinum en palladium met sianied is verskeie kere al voorgestel as n belowende proses opsie en alhoewel die ekstraksie van platinum nog 'n probleem is, is vordering al gemaak om die logings proses beter te verstaan. Die loging van platinum en palladium met sianied sal tipies plaasvind by hoër temperature wat kan wissel van 55°C in n hoop logins proses tot 180°C onder druk. Beter loging vind plaas soos wat die temperatuur styg. Alhoewel hierdie proses belowend is, is daar nog geen navorsing beskikbaar t.o.v die volgende stap in die proses wat die opgradering van die logings oplossing behels. Aangesien die gebruik van geaktiveerde koolstof in die goud industrie baie voordele inhou soos die eenvoudigheid van die proses, hoë opgraderings faktor, relatiewe vinnige adsorpsie en lae kapitaal en operationele kostes, mag hierdie tegnologie ook geskik wees vir Pt en Pd. Baie min is gepubliseer oor die adsorpsie van waardevolle metale (PGMs) met geaktiveerde koolstof en wanneer die effek van onsuiwerhede soos basis metale en thiosianied saam met n afstropings metode in ag geneem word, kon geen informasie in the literatuur gvind word nie. Hierdie studie is dus geloots en in die algemeen is daar bevind dat die geaktiveerde koolstof metode wel n geskikte oplossing bied vir die opgradering van PGMs in 'n sianied logings stroom. Adsorpsie snelheid vir verdunde PGM oplossings (0.15mg/L Pt, 0.38 mg/L Pd, 0.1 mg/L Au) in bekers wat geroer is, was vinnig vir die eerste 60 minutte (98% herwinning van die PGMs) en 'n vergelyking tussen die Pt adsorpsie isoterm en gepubliseerde Au isoterms wys op n relatiewe soorgelyke ladings kapasiteit. Die isotherm van Pd was egter laer. Soortgelyk aan meeste diffusie beheerde prosesse, het 'n verhoging van temperatuur gelei tot vinniger adsorpsie snelhede vir Pt, Pd en Au. Indien oplossings egter herhaaldelik met dieselfde koolstof in kontak gebring word, is bevind dat met 'n verhoging in temperatuur, die totale hoeveelheid PGMs wat absorbeer, met elke kontak verminder. In die afwesigheid van sianied en enige basis metale, het die totale hoeveelheid Pt en Pd wat geabsobeer is na 4 opeenvolgende kontake verminder van 99% by 25°C tot 85% adsorpsie van Pt en 83% Pd by 50°C. Daar was geen veskil tussen die adsorpsie van Au by 25°C en 50°C na 4 kontakte nie. Dit is ook vasgestel dat die negatiewe effek van vrye sianied in die oplossing, groter word soos wat die temperatuur verhoog. Die negatiewe effek van die teenwoordigheid van Cu en Ni, hang af van die hoeveelheid van hierdie metale wat absorbeer word wat weer bepaal word deur die sianied konsentrasie asook die temperatuur van die oplossing. Die mate waar in Pt en Pd ge-absorbeer word deur koolstof, word aansienlik meer beïnvloed deur temperatuur, sianied konsentrasie en die teenwoordigheid van basis metale as wat die adsorpsie van Au beïnvloed word. Met die ontwerp van n PGM adsorpsie proses, sal dit sterk in aanmerking geneem moet word, om seker te maak dat so min as moontlik Pt en Pd verlore gaan. Dit wys ook dat 'n geaktiveerde koolstof proses vir Pt en Pd, nie so onvatbaar vir proses veranderings sal wees soos wat die Au proses is nie, wat as een van die groot voordele van die Au proses beskou word. Die afstropery van die Pt en Pd sianied komplekse is ondersoek en gevind dat by 80°C, amper al die Pt en Pd binne 4 to 5 bed volumes (BV) van die geaktiveerde koolstof gestroop kan word. Die afstropery van Au by hierdie temperatuur is stadig en sowat 55% van die Au bly nog oor op die koolstof na 16 BVs. Die snelheid waarteen die Pt en Pd afgestroop word verhoog indien die temperatuur verhoog word en by 95°C is 99% van die Pt en Pd herwin na 4 BVs. Voorafbehandeling van die gelaaide koolstof met sianied, het 'n groot uitwerking op die PGM afstroping. Soos wat die NaCN konsentrasie verhoog word vanaf 0 tot by 2%, verhoog die PGM herwinning maar daal ook weer indien die NaCN konsentrasie verder verhoog word vanaf 2% tot by 4%. Die NaOH konsentrasie het ook n invloed gehad. By 0% NaCN, het die PGM herwinning verhoog soos wat die NaOH konsentrasie verhoog is maar by hoër sianied konsentrasies van 2 en 3%, is n daling in die herwinning waargeneem soos wat die NaOH verhoog is vanaf 0.22% tot by 1.65%. N algemene daling in die Pt, Pd and Au herwinning is waargeneem soos wat die ioniese sterkte van die stropings water toegeneem het. Dit stem ooreen met gepubliseerde navorsing oor die afstroping van Au. Om kalsium van die geaktiveerde koolstof te verwyder, word die koolstof vooraf met soutsuur gewas. Hierdie stap is vir Pt en Pd ondersoek en daar is bevind dat vir al die gevalle, die herwinning hoër was wanneer die gelaaide koolstof met soutsuur behandel is teenoor geen behandeling nie. In geen van die gevalle, is Pt, Pd of Au saam met die soutsuur of die daaropvolgende was water afgestroop nie. Voorafbehandeling met soutsuur teen 70°C lei wel daartoe dat tussen 64 en 75% Ni afgestroop word saam met die soutsuur en n verdere 9.1 to 10.5% saam met die daaropvolgende was water. In die teenwoordigheid van koper sianied, word Cu eerste afgestroop met Pd, Pt en dan Au wat daarop volg. Dit is presies die teenoorgestelde orde waarin die metale geabsorbeer word. Die voorafbehandeling met sianied, het ook 'n beduidende effek op die stroping van Cu. Dit kan verduidelik kan word aan die hand van die verskillende koper sianied komplekse wat vorm wat elkeen 'n verskillende affiniteit het vir adsorpsie. Wanneer 2% NaCN en 0.6% NaOH in die vooraf behandeling stap gebruik word, het die teenwoordigheid van koper geen negatiewe invloed op die afstroping van die PGMs gehad nie maar die herwinning is wel met 10% en 18% verlaag by 5 BVs wanneer geen sianied in die voorafbehandeling stap gebruik is nie. Wanneer 100 mg/L KSCN sout by die adsorpsie stap gevoeg word, daal die herwinning van die PGMs in die stropings stap van 90% tot 70% by 4 BVs. Die addisionel K+ katione verminder die herwinning met slegs 4% by 4 BVs wat beteken dat die vorming van adisionele komplekse soos [Pt(SCN)4]2- en [Pd(SCN)4]2-, wat nie op die normale metode afgestroop kan word, 'n moontlikheid mag wees. N maksimum van 0.15% vir Pt, 0.28% Pd en 0.6% Au word in die vooraf behandelings stap af gestroop. Indien die temperatuur van hierdie stap verhoog word na 80°C, verhoog die hoeveelheid Pt en Pd wat na hierdie stroom raporteer na 'n beduidende 8% tewyl Au basies onveranderd bly by 0.8%. Vir geaktiveerde koolstof wat hoër gelaai is (7000 mg/Kg Pt en Pd) en dus 'n beter verteenwoordiging van aanleg kondisies is, het 0.07% Pt, 0.11% Pd en 0.12% Au raporteer na die vooraf behandelings stap. Dit word toegeskryf aan die verspreiding van die PGMs op die koolstof wat hoofsaaklik op die oppervlakte voorkom en alhoewel die hoeveelheid klein is, word 'n sekere hoeveelheid slegs afgewas. Dit is dus vasgestel hier, dat die afstroping van Pt 'n twee stap proses is. Die eerste stap is die voorabehandeling van die koolstof met 'n sianied oplossing en daarna volg die afstroping van die Pt met suiwer water teen ongeveer 80°C. Met die ontwikkeling van n wiskundige model, is bevind dat die snelheid waarteen die Pt afgestroop word, beïnvloed word deur die hoeveelheid Pt en Na wat op die koolstof oppervlakte is, asook die konsentrasies van die metale in die vloeistof. Dit word wiskundig beskryf deur n gemodifiseerde Freundlich isotherm vergelyking asook deur die massa diffusie vergelyking. Die Pt afstropings snelheid wat afhanklik is van die Na konsentrasie kan vergelyk word met die afstroping snelheid van Au wat ook afhanklik is van die Na konsentrasie (Van Der Merwe and Van Deventer, 1990, Stange, 1990). Verder is bevind dat die snelheid van Pt stroping afhanklik is van beide die ewewig van Pt ione by die koolstof-vloeistof grens asook die massa beweging van Pt ione van die koolstof-vloeistof grens na die vloeistof. Beide hierdie snelheids bepalende faktore word bepaal deur die Na konsentrasie en skuif soos wat die Na konsentrasie verander soos wat die afstroping plaasvind. Vier hoof tyd periodes word gebruik om hierdie verskynsel te verduidelik. Deur die proses fundamenteel te verstaan, kan uiteindelik lei tot 'n beter proses, beter beheer, korter afstropings tye, kleiner toerusting of die ontwikkeling van 'n kontinue proses.

Includes bibliographical references.
The aim of this work was to investigate changes in the electronic structure of platinum as a result of alloying with vanadium, and the effects of these changes on O2 adsorption. This is important for the further development of hydrogen fuel cells, because the oxygen reduction reaction (ORR) presently requires O2 adsorption to occur on pure platinum, which is a prohibitively expensive material. A computational study has therefore been undertaken on alloying platinum (which reduces cost) with vanadium (for which there is plentiful experimental data) and the consequences for O2 adsorption. The first moment of the d-band of platinum alloy DOS was used to represent the d-band centre. The d-band centre of Pt-PDOS became lower as a result of hybridisation between platinum and vanadium. The d-band centre of a pure platinum surface with respect to the Fermi level is -1.99eV, but it is shifted to -3.23eV when vanadium atoms are added to the subsurface layer. The adsorption energies of O2 are sensitive to a combination of calculation parameters used. In this work, the calculations were executed using the CASTEP code. This is a plane wave pseudo potential code. The most stabilised geometry of an adsorbed molecule on pure Pt (111) was at the fcc site and had an adsorption energy of -1,91eV. The adsorption energy at the bridge site of Pt (111) is -1.81eV. When subsurface vanadium atoms were introduced, the equilibrium surface-molecule bond lengths increased. The adsorption energy at the fcc site shifted to -1.37eV, -1.43 for the bridge site and -1.45eV for the hcp site. It was concluded that the presence of vanadium atoms in the surface region destabilises an adsorbed oxygen molecule but a more detailed study is needed to show the effect of the solute atoms on the thermodynamics and kinetics of the whole oxygen reduction reaction chain.

Este trabalho apresenta um estudo aplicando a Teoria do Funcional da Densidade (DFT), utilizando o funcional B3LYP, para estudar a estrutura eletrônica de nanoclusters de platina dopados com Al, Cr e V e suas interações com oxigênio molecular. As análises das populações de Mulliken e de NBO para a interação entre O2 e Pt2, Pt-Cr e Pt-V revelaram que ocorre transferência de carga dos orbitais s e d dos metais para os orbitais p do oxigênio, resultando no preenchimento dos orbitais antiligantes da molécula de oxigênio, provocando a quebra da ligação O--O e formação de ligações hibridizadas Metal - O, com energia de dissociação da ligação O - O em 1,0 eV sobre Pt2. Sobre Pt-Cr e Pt-V, esse valor decresce para 0,56 eV e 0,20 eV, respectivamente. Os estudos da interação entre oxigênio molecular e os clusters Pt3, Pt2Al, Pt2Cr e Pt2V mostram que ocorre adsorção não dissociativa de oxigênio molecular sobre o cluster Pt3 onde observamos uma adsorção segundo o modelo de ponte enquanto que sobre os clusters Pt2Al e Pt2V ocorre adsorção dissociativa de oxigênio molecular. Por outro lado, com o cluster Pt2Cr a adsorção segue o modelo de Pauling, com o oxigênio molecular adsorvido em apenas um sítio do cluster, que foi o átomo Pt, sem a quebra da ligação O--O. Curvas de superfície de energia potencial para a dissociação de oxigênio molecular sobre Pt2Al e Pt2V mostraram um valor de aproximadamente 0,21 eV para a barreira de dissociação da ligação O--O adsorvido sobre o cluster Pt2Al e aproximadamente 0,30 eV para a barreira de dissociação da ligação O--O adsorvido sobre o cluster Pt2V. Os estudos da interação entre oxigênio molecular e os clusters Pt4, Pt3Al, Pt3Cr e Pt3V mostram que ocorre adsorção não dissociativa de oxigênio molecular sobre os clusters Pt4 e Pt3Cr onde foi observado que a adsorção segue o modelo de ponte. Por outro lado, sobre os clusters Pt3Al e Pt3V a adsorção de oxigênio molecular também seguiu o modelo ponte, com dissociação da ligação O - O. Encontramos um valor de 0,46 eV para a barreira de dissociação da ligação O--O adsorvido sobre o cluster Pt3Al e aproximadamente 0,28 eV para a barreira energética de dissociação da ligação O--O adsorvido sobre o cluster Pt3V. Os estudos para interação entre oxigênio molecular e os clusters Pt5, Pt4Al, Pt4Cr e Pt4V revelaram que ocorre adsorção dissociativa de oxigênio molecular sobre os clusters Pt4Al e Pt4V, onde observamos uma adsorção que segue o modelo de ponte enquanto que sobre o cluster Pt5 a adsorção segue o modelo de ponte sem dissociação da ligação O--O. Por outro lado, a adsorção sobre Pt4Cr segue o modelo de Pauling, com o oxigênio molecular adsorvido em apenas um sítio do cluster, sem a quebra da ligação O--O. Curvas de superfície de energia potencial mostraram um valor de aproximadamente 0,62 eV para a barreira de dissociação da ligação O--O adsorvido sobre o cluster Pt4Al e aproximadamente 0,20 eV para a barreira energética de dissociação da ligação O--O adsorvido sobre o cluster Pt4V.
This work presents a study applying the Density Functional Theory (DFT) using the B3LYP functional to study the electronic structure of platinum nanoclusters doped with Al, Cr and V and their interactions with molecular oxygen. The analysis of Mulliken populations and NBO for the interaction between O2 and Pt2, Pt-Cr and Pt-V showed that charge transfer occurs orbital sed metals for the p orbitals of oxygen, resulting in the filling of the antibonding orbitals of the molecule oxygen, causing the cleavage of O-O and bond forming hybridized Metal - O, bond dissociation energy of O - O on Pt2 about 1.0 eV. On Pt-Cr and Pt-V, this value decreases to 0.56 eV and 0.20 eV, respectively. Studies of the interaction between molecular oxygen and the Pt3 clusters, Pt2Al, Pt2Cr and Pt2V show that no dissociative adsorption occurs for molecular oxygen on the cluster Pt3 where we observe a second adsorption model bridge while on clusters Pt2Al and Pt2V dissociative adsorption occurs of molecular oxygen. Moreover, with the cluster Pt2Cr adsorption follows the model of Pauling, with molecular oxygen adsorbed on only one site in the cluster, which was the Pt atom, without the cleavage of the O-O. Curves of potential energy surface for dissociation of molecular oxygen on Pt2Al and Pt2V showed a value of about 0.21 eV for the bond dissociation barrier of the O-O adsorbed on the cluster Pt2Al approximately 0.30 eV and for the barrier The bond dissociation O-O adsorbed on the cluster Pt2V. Studies of the interaction between molecular oxygen and clusters Pt4, Pt3Al, Pt3Cr and Pt3V show that no dissociative adsorption occurs for molecular oxygen on clusters Pt4 and Pt3Cr where it was observed that the adsorption follows the model of the bridge. On the other hand, on clusters Pt3Al and Pt3V adsorption of molecular oxygen also followed the model bridge, with bond dissociation O - O. We found a value of 0.46 eV for the barrier to bond dissociation to O-O adsorbed on the cluster Pt3Al and approximately 0.28 eV for bond dissociation of the O-O adsorbed on the cluster Pt3V. Studies on the interaction between molecular oxygen and clusters Pt5, Pt4Al, Pt4Cr and Pt4V revealed that occurs dissociative adsorption of molecular oxygen on clusters Pt4Al and Pt4V, where we observe an adsorption model that follows the bridge while on the adsorption cluster Pt5 follows the bridge model without the bond dissociation. Moreover, adsorption is modeled on Pt4Cr Pauling with oxygen adsorbed on only one site of the cluster, without the cleavage of the O-O. Curves of potential energy surface showed a value of about 0.62 eV for the bond dissociation barrier of the O-O adsorbed on the cluster Pt4Al and approximately 0.20 eV energy barrier for bond dissociation of the O-O adsorbed on the Pt­4V cluster.

The direction of the research described in this thesis originates from earlier work in this laboratory on the reactive chemisorption of CH3 CN at Pt single-crystal surfaces, studied by means of cyclic voltammetry and integrated adsorption current transients, coupled with in situ IR reflectance spectroscopy. It was found that resonant guanidonium cations, +NH 2 = C(NH2)2 (G+), have major surface-specific influences on H underpotential deposition (UPD) voltammeric profiles at Pt(hkl) surfaces. Cyclic voltammetry, adsorbate displacement by CO, adsorption charge-transient, in situ FTIR spectroscopy, and a.c. impedance spectroscopy techniques were employed complimentarily to study the adsorption behaviour of G+ cations at Pt single-crystal surfaces. Based on the results of adsorption charge-transients obtained by introduction of 1% solution of G+ in 0.1 mol dm-3 NaOH at the Pt(111) surface, it is concluded that G+ becomes chemisorbed on the Pt surface in a 2-electron transfer process, with resulting electrooxidation of the H's dissociated from --NH2 groups upon their chemisorption. In situ FTIR experiments were conducted in aq. 0.05 mol dm-3 H2SO4 for the purpose of identifying the molecular basis of effects of co-adsorption of G + on the states of adsorbed HSO4- anions through changes of vibrational spectra. The spectroscopic results revealed a broad band in the spectra over the 1500--1600 cm-1 range which was assigned to the >C = NH2+ stretch of G+. Supported by the voltammetric behaviour, they also provided evidence about the co-operative chemisorption of G+ cations with the HSO4- ions on the Pt single-crystal surfaces, where both species are in the chemisorbed state at the Pt surface. This is an attractive interaction which leads to the observed substantial, voltammetric shift of potential for HSO4- desorption towards the RHE potential. A mechanism of this co-operative chemisorption (or ion-pairing) between the G+ cations and the HSO4- (also ClO4- and OH- in aq. HClO4 and NaOH solutions, respectively) ions, co-adsorbed at the Pt single-crystal surfaces, is proposed. Explanations are also given for the reported, in some cases appreciable (for Pt(100) and (511) surfaces), increases that arise in the total voltammetric charges, measured within the potential range for UPD of H. No significant diffusion control for the process of adsorption of G+ at the Pt surface was observed. Thus, the recorded response currents in the voltammograms are directly associated with surface electrochemical processes. (Abstract shortened by UMI.)

Relata-se um estudo sobre o processo de eletrossorção do n-propanol sobre eletrodos de platina eletrodispersa em soluções de H2SO4 1 N, a diferentes temperaturas (12 a 51 ºC) e potencíais (0,30 a 0,60 V). São abordados os aspectos relacionados com a cinética de adsorção dos possíveis intermediários formados na desidrogenação do n-propanol, bem como a determinação das constantes de velocidade em cada etapa. São apresentadas as energias de ativação do processo de adsorção para graus de cobertura, θ = 0 e θ ≠ 0. A adsorção do álcool estudado a potencial controlado na região da dupla camada elétrica, ocorre através da desidrogenação da molécula, seguida pela ionização do hidrogênio adsorvido. A isoterma cinética de adsorção foi obtida a partir dos cronoamperogramas de desidrogenação do álcool e mostra uma variação linear de θ com o logaritmo do tempo de adsorção (t), para 0,25 < θ < 0,80. Por outro lado, a relação entre E = f (log li), onde li é a máxima corrente não estacionária obtida a t = 0, é linear com coeficiente angular igual a 2,3 (2 RT/F). Esse valor indica que no início da adsorção do n-propanol, apenas um elétron está envolvido no processo. Da mesma maneira que no metanol, supõe-se que a desidrogenação do n-propanol não ocorre através da eliminação simultânea dos dois átomos de hidrogênio ligados no carbono-α, mas por duas etapas consecutivas de desidrogenação: R - CH2 - OH j1→ R - .CH - OH + H+ + e- ( 1 ) R - .CH - OH j2→ R - ..C - OH + H+ + e- ( 2 ) Desse modo, a corrente anódica não estacionária (j), proveniente da ionização do hidrogênio formado na desidrogenação do n-propanol é resultante da soma das correntes j>SUB>1 e j2 produzidas nas reações descritas nas equações (1) e (2). Quando o tempo de adsorção é muito curto, isto é , j2 = 0, a corrente não estacionária é determinada apenas pela adsorção da espécie R-.CH-OH. Considerando esta hipótese e as isotermas de Temkin e Elovich foi obtida uma equação que descreve o grau de cobertura pela espécie R-.CH-OH, (θ1) em função do tempo. θ1 = - Qmáx B/k2t (1-A-B ln t) + k1/k2 onde Qmáx é a carga máxima de cobertura, k1 e k2 são as constantes de velocidade de adsorção das espécies R-.CH-OH e R-..C-OH, respectivamente, A e B são as constantes da equação de Elovich. A equação precedente permitiu determinar as constantes de velocidade de adsorção k1 e k2. A partir desses valores em diferentes temperaturas foram obtidas as energias de ativação para as reações (1) e (2). Verificou-se que os tempos de máxima cobertura por R.CHOH e de inflexão, obtidos respectivamente de θ1= f (log t) e j-1 = f(t), são comparáveis para dada temperatura e potencial.
The kinetics and mechanism of n-propanol adsorption on a platinized platinum electrode was studied in 1 N H2SO4 at several temperatures, by means of the potential pulse method. Between 0.30 V and 0.60 V (RHE), the adsorption occurs via a dehydrogenation of the α-carbon, followed by a rapid ionization of the adsorbed hydrogen atoms. The kinetic isotherms obtained by integration of the chronoamperograms show a linear variation of the surface coverage, θ, with logarithm of the adsorption time, tads, in the range 0.25 ≤ θ ≤ 0.80. This indicates that the adsorption rate can be expressed in tems of an Elovich equation. It is shown that the relation Eads vs log Ii, where Eads is the adsorption potential and Ii is the maximum non-stationary current at t = 0, is a straight line with a slope equal to 2.3[2RT/F], independently of the temperature. These data show that the initial adsorption step envolves a monoelectronic charge transfer, and can be represented by the following equation: R-CH2-OH j1→ R-.CH-OH + H+ + e- ( 1 ) Therefore, it is assumed that the adsorption occurs via a two step consecutive reaction, given by equations (1) and (2): R-.CH-OH j2→ R-..C-OH + H+ + e- ( 2 ) with the two adsorbed species R-.CH-OH and R-..C-OH characterized by their degree of coverage θ1 and θ2, respect ively. The non-stationary anodic current, j, is then the sum of currents j1 and j2 resulting from reactions described by equations (1) and (2). When the adsorption time is very short, it can be assumed that j = j1 + j2 ≈ j1, and that θ = θ 1 + θ2 ≈ θ1. From those assumptions, the following equation relating θ1 with t was obtained: θ1 = -Qmáx . B/[k2.t (1-A-B). ln t ] + k1/k2 (3) where Qmáx is the charge related with the maximum surface coverage, k1 and k2 the apparent rate constants of reactions (1) and (2), respectively, and A and B are constants from the Elovich equation. Equation (3) permitted the evaluation of the rate constants k1 and k2 for distinct Eads values. From the data at different temperatures, the apparent activation energies of both reactions were calculated.

The process of precious metal refining can be up to 99.99% efficient at best, and although it may seem small, the amount of valuable metal lost to waste streams is appreciable enough to warrant recovery. The method currently used to remove entrained metal ions from refinery wastewaters, chemical precipitation, is not an effective means for selective recovery of precious metals from a wastewater. Biosorption, the ability of certain types of biomass to bind and concentrate metals from even very dilute aqueous solutions, may be an effective point-source metal recovery strategy. The yeast, Saccharomyces cerevisiae, has been found capable of sorbing numerous precious and base metals, and is a cheap and abundant source of biomass. As such, it represents a possible precious metal sorbent for application to refining wastewaters. In this investigation, S. cerevisiae biomass was immobilized, using polyethyleneimine and glutaraldehyde, to produce a suitable sorbent, which was found to be capable of high platinum uptake (150 to 170 mg/g) at low pH (< 2). The sorption mechanism was elucidated and found to be a chemical reaction, which made effective desorption impossible. The sorption process was investigated in a packed bed column conformation, the results of which showed that the diameter and height of the column require further optimization in order to attain the metal uptake values achieved in the batch studies. When applied to a refinery wastewater, two key wastewater characteristics limited the success of the sorption process; the high inorganic ion content and the complex speciation of the platinum ions. The results proved the concept principle of platinum recovery by immobilized yeast biosorption and indicated that a more detailed understanding of the platinum speciation within the wastewater is required before the biosorption process can be applied. Overall, the sorption of platinum by the S. cerevisiae sorbent was demonstrated to be highly effective in principle, but the complexity of the wastewater requires that pretreatment steps be taken before the successful application of this process to an industrial wastewater.

Les (sub)nanoclusters sont des agrégats d’atomes ou de molécules composés de quelques unités à quelques centaines d’unités. En raison de leur petite taille, ils peuvent avoir des propriétés électroniques, optiques, magnétiques et catalytiques très différentes par rapport au solide correspondant . D'un point de vue expérimental, il est encore très difficile de synthétiser des agrégats de taille calibrée. D'un point de vue théorique, le développement des puissances de calcul, des méthodes de calcul de structure électronique et des algorithmes de recherches globales de structures stables, permettent un calcul toujours plus précis de leurs propriétés physico-chimiques. L’étude théorique permet alors de déterminer de façon fiable les structures stables de ces systèmes qui président aux calculs de leurs propriétés . L’exemple qui illustre ce travail s’inspire du processus observé au sein des piles à combustible dans lequel le Platine (Pt) est couramment utilisé pour produire de l’énergie par oxydation du dihydrogène en favorisant notamment sa dissociation . L’objet de ce travail consiste à comparer la capacité des clusters de Platine de différentes tailles à adsorber la molécule de dihydrogène sous leur forme libre et adsorbée sur substrat. Le graphène , matériaux bidimensionnel cristallin formé de carbone est choisi dans ce travail en tant que substrat en raison de sa grande résistance mécanique et chimique. La première partie de ce travail est consacrée à la recherche d’éléments dopants qui vont permettent à la fois d’améliorer la capacité d’adsorption des clusters de Platine sur la surface et éviter leur migration. L’objectif est ici de proposer un substrat sur lequel peuvent être empêchés les phénomènes d’agglomération, de dissolution et de détachement du cluster qui ainsi limiteraient son efficacité catalytique . Des dopages de la surface, tel qu’ils sont réalisables expérimentalement , par l’Azote, le Bore et le Nitrure de Bore, par substitution atomique et avec ou sans considération préalable de lacunes, ont été étudiés. La seconde partie correspond à l’implémentation dans le code GSAM (Global Search Algorithm of Minima - algorithme de recherche globale de minima) développé au laboratoire , , des éléments qui permettent la recherche de structures de plus basse énergie de clusters moléculaires adsorbés sur substrat, tels que les systèmes [H2-Ptn-Graphène dopé] de cet exemple. La troisième partie concerne l’illustration de la fiabilité de la méthode de recherche globale employée et de la qualité de quelques méthodes de calcul de l’énergie moléculaire (DFT et GUPTA) vis-à-vis de résultats mentionnés dans la littérature sur les clusters de Platine. La dernière partie comporte l’investigation structurale des systèmes [H2-Ptn] et [H2-Ptn-Graphène dopé] pour différentes tailles de clusters allant de n=6 à n=20. La variation de l’énergie d’adsorption de H2 sur les clusters libres et supportés ainsi que celle du cluster moléculaire sur le substrat en fonction de la taille est reportée
A sub-nanometer sized metal cluster consists of only several to tens of atoms. Due to its small size and quantum effects, it can have very specific electronic, optical, magnetic and catalytic properties as compared with their bulk behaviors . From an experimental point of view, it is still a big challenge to realize size-controlled synthesis for (sub) nanoclusters. From a theoretical point of view, benefiting from the development of faster high-performance computational sources, more efficient electronic structure modelling software and more reliable global search methods for the determination of the most stable structures, the chemical and physical properties of clusters can be determinate more accurately. As it is experimentally a big challenge to realize size-controlled synthesis for (sub) nanoclusters, theoretical studies can provide detailed information on their geometric structure, electronic structure, as well as adsorption and reaction properties . The example chosen to be treated in this study is inspired by the fuel cell, in which the Platinum (Pt) is a typical and most commonly used precious metal catalyst for the production of energy by the oxidation of dihydrogene . Graphene is a recently discovered 2D carbon net structure, has several special properties, such as: low weight, high strength, high surface area, high electrical conductivity, etc. With these properties and their novel combinations, graphene might prove a promising candidate to be used as catalyst supports. The first part of this study is devoted to the search of the doping elements which permit both enhance the adsorption capacity of Pt clusters on the surface and prevent their migration. The aim here is propose one substrate which can avoid the problems of cluster agglomeration, dissolution and detachment, which reduce the performance of the catalysts . The ways of doping of the surface, which have already been experimentally realized , such as Nitrogen, Boron, and N-B patches substitution of Carbon atoms with or without introducing the vacancy on the pristine graphene, are studied. The second part corresponds to the implementation of some new features into the code GSAM (Global Search Algorithm of Minima) developed in our laboratory , , , which permit the search of the most stable structures of the molecular clusters adsorbed on substrate, such as the complex systems of [H2-Ptn-doped Graphene]. The third part is to evaluate the reliabilities of the global search method used, as well as the DFT and the empirical (GUPTA) potential energy surface. Thus, the main discussion appears as a comparison with the results of the literature concerning the Pt clusters. The fourth part consists of the structural investigation of [H2-Ptn] and [H2-Ptn-doped Graphene] systems for different sizes of Pt clusters with n=6 to n=20. The variation of the adsorption energy of H2 on the free and supported Ptn clusters, and the adsorption energy of (H2+Ptn) system on the surface with respect to the size of the cluster is discussed

Etude par voltammetrie cyclique du taux de recouvrement en soufre, resultats confirmes par spectrometrie uv-visible. Interaction entre le soufre et l'hydrogene, modifications electroniques de la surface du platine. Effet des thiols sur l'hydrogenation de l'acide maleique

L'objectif de ces travaux de thèse consiste à étudier et mettre en œuvre des matériaux polymères fonctionnels innovants, sous forme de poudre, pour l'extraction sélective de métaux du groupe platine (PGM) provenant d'effluents industriels. L'intérêt principal de ces matériaux polymères réside d'une part dans leur mise en forme sous forme de poudre, évitant le recours à des matériaux supports inertes, et d'autre part dans les fonctions extractantes spécifiques, à base de phosphine (MP-101) et d'oxyde de phosphine (MP-102).Ce travail a permis de montrer que la sorption d'ions Pd2+ (ion métallique cible) intervient via un phénomène de chélation du palladium par les fonctions phosphines sur le MP-101 et par les fonctions oxydes de phosphine sur le MP-102. Ce mode d'interaction induit des énergies de sorption du Pd et des sélectivités supérieures à celles généralement à l'œuvre lorsque le mode d'interaction est de l'échange ionique. Alors que les paramètres opératoires tels que le pH, la force ionique et la composition des effluents influencent la sorption du Pd sur le MP-102, la sorption sur le MP-101 est peu influencée par ces paramètres opératoires car les interactions phosphine-palladium sont très fortes. Ainsi la sorption du Pd sur le MP-102 est totalement réversible par élution en présence de thiourée ; par ailleurs la sorption sur le MP-101 permet d'atteindre des capacités de sorption très élevées, jusqu'à 8 mmol.g-1 dans des milieux représentatifs d'effluents industriels réels pour lesquels la concentration en acide sulfurique peut atteindre plusieurs moles par litre.Le MP-102 a été mis en œuvre dans un procédé couplant une étape de sorption et de microfiltration en continu. Ce système permet de confiner le polymère et d'exploiter ses propriétés de régénérabilité dans un système de cycles successifs de sorption-désorption rappelant la mise en œuvre des colonnes de résines échangeuses d'ions. Plusieurs cycles successifs ont été réalisés sans perte d'efficacité, permettant l'extraction sélective de palladium et de platine contenus dans une solution de lixiviation de pots catalytiques. Le MP-101 a été mis en œuvre au sein d'un système couplant la sorption à une étape de floculation-décantation suivi d'une incinération, pour récupérer les métaux sous forme métallique. Ce procédé a été appliqué avec succès à une solution minière permettant l'extraction sélective de Pd et Pt
The objective of this work consists in studying and implementing innovative functional polymer materials under powder form for the selective extraction of platinum group metals (PGM) from industrial effluents. The main interest of the polymeric materials studied in this work relies on their shaping powder, avoiding the use of inert support, and their binding functions: phosphine (MP-101) and phosphine oxide (MP-102).It has been shown that Pd2 + sorption (target metal ion) occurs by chelation through the phosphine functionalities onto the MP-101 and through the phosphine oxides functionalities on the MP-102. This binding mode induces higher sorption energies and selectivity toward Pd than the currently ion exchange mode. While the operating parameters such as pH, ionic strength and composition of the effluent were shown to affect the Pd sorption onto the MP-102, sorption onto MP-101 is less affected by these operating parameters since the interactions phosphine-palladium are very strong. Consequently, the Pd sorption onto MP-102 is completely reversible by elution with thiourea and sorption onto MP-101 lead to sorption capacities up to 8 mmol.g-1 in solutions representative of industrial effluents where the concentration of sulfuric acid can reach several moles per liter. The MP-102 has been implemented in a continuous process coupling a sorption stage with microfiltration. This system allowed using the polymer during successive sorption-desorption cycles without loose of sorption efficiency. This coupled process allowed the selective extraction of palladium and platinum from a leach solution with catalysts. The MP-101 was implemented in a system coupling sorption with a flocculation-decantation stage followed by the incineration of the sludge, allowing the recovery of metals in metallic form. This method has been successfully applied to mining effluents to the selective extraction of Pd and Pt

Chimie organométallique résonne avec catalyse homogène, et chimie des surfaces avec catalyse hétérogène. Mais la frontière établie entre ces deux domaines est en réalité très mince. Leur rapprochement aboutit dans les années 1990 au développement d’une nouvelle science : la chimie organométallique de surface, qui souligne leur complémentarité. L’objectif de cette science, dans laquelle s’inscrit ce travail de thèse, est de créer des catalyseurs hétérogènes à partir de composés organométalliques. Notre objectif est d’apporter une contribution à la compréhension de la formation de nanoparticules de platine - métal incontournable en catalyse hétérogène - sur des supports de silice amorphe, et ce grâce à la chimie organométallique. Au cours de ce travail, une palette de catalyseurs Pt/SiO2 a été préparée à partir de trois précurseurs de platine : le composé classique H2PtIVCl6.xH2O et deux composés organométalliques PtII(η 4-C8H12)Cl2 et Pt0(η 2-C7H10)3 , et de trois supports : une silice commerciale (Davison), une silice mésoporeuse SBA-15 synthétisée au laboratoire et unesilice naturelle, la diatomite. De plus, l’étude du catalyseur de référence au platine « EuroPt-1 » a permis de développer une méthodologie de suivi operando par Spectroscopie Infrarouge à Transformée de Fourier par Réflexion Diffuse (DRIFTS) de l’adsorption du monoxyde de carbone (CO) sur les différents sites d’une nanoparticule de platine. En conclusion, les catalyseurs préparés à partir de Pt0(η 2-C7H10)3 s’avèrent être plus actifs qu’EuroPt-1 pour la réaction d’oxydation du CO, ce qui démontre le potentiel d’utilisation des composés organométalliques dans le domaine de la catalyse hétérogène
Organometallic chemistry resonates with homogeneous catalysis, and surface chemistry with heterogeneous catalysis. But the frontier between these two fields is very thin. In the 90’s, these fields approached each other and led to the development of a new science: organometallic surface chemistry, which underlines their complementarity. The goal of this science, with which this work is associated, is to create heterogeneous catalysts from organometallic compounds. Our goal is to contribute to the understanding of platinum nanoparticle formation - platinum being an important metal in heterogeneous catalysis - onto amorphous silica supports, thanks to organometallic chemistry. During this work, various Pt/SiO2 catalysts were prepared fromthree platinum precursors: the classical one, H2PtIVCl6.xH2O and two organometallic compounds PtII(η 4-C8H12)Cl2 et Pt0(η2-C7H10)3, and involved three supports: a commercial silica (Davison), a mesoporous silica SBA-15 synthesized in the laboratory, and a natural silica, the diatomite. Moreover, the study of the standard platinum reference catalyst “EuroPt-1” lead to the development of a methodology of operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) of carbon monoxide (CO) adsorption on the different sites of a platinum nanoparticle. Catalysts prepared from Pt0(η 2-C7H10)3 are more active than EuroPt-1 for the CO oxidation reaction, and this work shows the potential of organometallic precursors in the domain of heterogeneous catalysis

Nous avons étudié l'influence de la température d'adsorption et des coadsorbats (atomes alcalins, oxygène atomique, alcalins oxydés) sur les modes d'adsorption des hydrocarbures éthyléniques (éthylène, propylène) sur Pt(111). Ces différents modes d'adsorption ont été identifiés sous ultravide par des méthodes spectroscopiques complémentaires (UPS, XPS, NEXAFS, TDS). L'étalonnage des quantités d'alcalins déposes a été obtenu par la spectrométrie des électrons Auger et les mesures de potentiel de sortie en UPS. Les oxydes de potassium ont été préparés et identifiés en utilisant la technique UPS et en comparant le nombre et les positions des orbitales moléculaires dans le régime de la sous-monocouche d'alcalin aux résultats en couches épaisses de la littérature et aux prédictions théoriques. Les résultats expérimentaux obtenus sont interprétés sur la base de modèles théoriques existants. Des conséquences probables de l'utilisation des oxydes d'alcalins comme promoteurs des catalyseurs utilises dans la synthèse catalytique des hydrocarbures ont été soulignées

L'influence de la structure cristalline superficielle du platine sur le processus d'électrosorption de l'hydrogène a été étudiée dans différents électrolytes sur les 3 orientations de base avec mise en évidence d'états d'adsorption de l'hydrogène situes à des potentiels exceptionnellement élevés, sur les surfaces à marches (511), (310), (211), (331) et sur un monocristal sphérique. Parallèlement l'effet de l'électrosorption d'oxygène sur la structure de toutes ces surfaces a été étudié en utilisant H2 comme une espèce sonde sensible à la surface cristalline superficielle.

L'adsorption des oxydes de soufre par des masses regenerables a ete etudiee. Ces travaux s'inscrivent dans le cadre de la mise au point d'un procede de desulfuration de divers effluents de raffineries petrolieres sur une masse composee de 30% d'oxyde de magnesium associe a de l'alumine et dopee par 50 ppm de platine

L'oxydation electrochimique du fructose en milieu basique et en milieu acide est realisee: une etude cinetique est effectuee, on etudie la modification des proprietes catalytiques superficielles des electrodes par adjonction d'adatomes, on fait plusieurs electrolyses a potentiel programme montrant que l'on obtient par cette reaction un melange d'acides gluconique, tartrique et citrique

Le but de ce travail etait d'etudier le role du potentiel du catalyseur sur l'adsorption et l'oxydation d'acides carboxyliques (acide maleique, acide oxalique et acide formique), en phase aqueuse. Un catalyseur modele au platine platine a ete utilise. Les trois composes etudies s'adsorbent sur le platine suivant trois modes d'adsorption differents : - l'adsorption de l'acide maleique conduit a deux especes differentes, dependant de la temperature, dont l'une est reactive en oxydation. - l'adsorption de l'acide oxalique est faible et reversible, du type anionique - l'adsorption de l'acide formique est complexe et conduit a deux especes absorbees, dont l'une est reactive et l'autre du type poison lors d'une oxydation, le potentiel du catalyseur peut etre etabli spontanement en presence d'oxygene a differentes pressions ou controle au moyen d'un potentiostat (en presence d'oxygene ou non). Ces trois modes de controle du potentiel du catalyseur ont ete employes dans ce travail. Les resultats obtenus montrent que l'acide oxalique et l'acide formique sont oxydables a 333 k, dans des domaines de potentiel differents. Les activites catalytiques dependent de la nature des anions presents dans la solution et du ph l'acide maleique peut etre oxyde dans des conditions plus severes, a 383 k et sous pression d'oxygene de un a cinq bars, dans un autoclave electrochimique. Il ressort de ce travail que chacun des trois composes etudies adopte un comportement different en fonction des divers parametres etudies. Les differents modes d'adsorption et les mecanismes reactionnels peuvent rendre compte des resultats obtenus.

L'apport essentiel des techniques nucleaires dans l'etude de l'adsorption des gaz sur les surfaces est la possibilite de mesurer des concentrations de l'adsorbat inferieures a une monocouche avec une grande precision meme en presence de la phase gazeuse dans un large domaine de pression. L'analyse de l'hydrogene par la reaction #1h(#1#5n,)#1#2c a fait l'objet d'une etude detaillee qui a permis de definir les caracteristiques analytiques de ces methodes en fonction des differents parametres experimentaux. L'analyse du deuterium par les reactions nucleaires #2d(#1#5n,p)#1#6n et #2d(#1#5n,n)#1#6o a permis l'emploi du marquage isotopique dans les etudes de coadsorption benzene-hydrogene, le dosage des atomes de carbone etant realise avec les reactions #1#2c(d,p)#1#3c et #1#3c(p,)#1#4n. Dans une premiere etude de l'adsorption de h#2 sur ni(111) a temperature ambiante, le taux de recouvrement maximal a ete mesure de 0,6 (0,05) monocouche. Cette valeur en parfait accord avec les resultats obtenus a partir d'autres techniques experimentales a montre la fiabilite des techniques nucleaires qui ont alors ete utilisees pour etudier l'adsorption de l'hydrogene sur les faces (111) et (110) de l'alliage pt#5#0ni#5#0, dans le domaine de temperature 115-400 k et le domaine de pression 10##8-10##4 mbar. Les taux de recouvrement a saturation des faces (110) et (111), de 0,92 (0,1) et 0,18 (0,04) monocouche respectivement, sont notablement plus faibles que ceux des metaux purs. A partir du trace des isobares d'adsorption, la variation de l'energie d'adsorption en fonction du recouvrement de la face (110) a pu etre determinee. Nous avons tente d'interpreter les resultats de l'analyse nucleaire en prenant en compte les differences de composition de la surface selon l'orientation cristalline et les donnees de l'adsorption de h#2 sur les monocristaux ni (110) et pt (111). L'etude de la coadsorption benzene-hydrogene sur ni(111) a montre que la reaction d'echange d'atomes d'hydrogene entre les deux especes adsorbees est la reaction de surface preponderante a temperature ambiante

Pour mieux comprendre les mecanismes de desactivation par le soufre de catalyseurs d'hydrogenation d'hydrocarbures, des systemes modeles platine-diolefine-atmosphere sulfuree ont ete etudies. L'etude de l'adsorption du butadiene et de l'isoprene sur les faces (111) et (110) du platine prerecouvertes de soufre a mis en evidence l'effet preponderant de la structure cristalline sur le mode de desactivation des surfaces. Mais une adsorption simultanee d'hydrocarbure, d'hydrogene et de sulfure d'hydrogene sur ces memes faces conduit a la formation de mercaptans, dont la nature depend du mode d'adsorption des hydrocarbures. Il existe une correlation directe entre l'adsorption et la reactivite des diolefines. En effet, la distribution des produits de premiere et deuxieme hydrogenation de l'isoprene depend des effets geometriques et electroniques induits sur les insaturations activees par le metal. La comparaison avec le butadiene a montre que la selectivite des produits depend des temps de sejour et de la concentration des especes en surface. En presence de soufre preadsorbe, la mobilite des especes adsorbees est aussi a prendre en compte pour expliquer leur reactivite. Cette etude modele a permis de proposer un mecanisme pour la regeneration par le sulfure d'hydrogene d'une surface de platine polycristallin preempoisonnee par le soufre, processus au cours duquel des mercaptans specifiques sont formes

Etude dans l'approximation des liaisons fortes et de la methode des moments de la structure electronique et des proprietes d'adsorption de plusieurs types de surfaces de metaux de transition. Application a l'etude de la reconstruction de la surface (110) du tungstene. Interpretation du comportement surprenant des dimeres de la troisieme serie des metaux de transition sur la surface w (110) en prenant en compte les effets des correlations electroniques. Influence de la presence de marches sur les proprietes d'adsorption d'une surface d'un metal de transition

L’activation du méthane (CH4) et du méthanol (CH3OH) sur la surface (100) du platine a été étudiée par des calculs de la théorie de la fonctionnelle de la densité basée sur les ondes planes et les pseudopotentiels. Nous avons optimisé le système CH4/Pt(100). Les énergies d’adsorption calculées sur les sites apical, ponté et symétrie 4 sont très faibles, dépendant peu de l’orientation moléculaire. La nature de l’interaction CH4-Pt a été examinée à travers les modifications de la structure électronique. L’adsorption du méthyle (CH3) et de l’hydrogène (H) et la coadsorption de CH3+H ont également été calculées. De ces résultats, nous avons examiné la dissociation de CH4 en CH3+H, et les énergies d’activation trouvées sont en bon accord avec les valeurs expérimentales et théoriques. L’activation de CH3OH/Pt(100) a été étudiée. Tous les sites ont presque la même énergie d’adsorption. L’adsorption de l’oxygène (O) et la coadsorption de CH4 et O ont aussi été examinées. En outre, la formation de CH3OH selon un mécanisme via la coadsorption de CH4+O a été étudiée et la barrière d’activation s’est avérée élevée
The activation of methane (CH4) and methanol (CH3OH) on Pt(100) surface has been investigated using density functional theory calculations based on plane-wave basis and pseudo-potential. We optimised CH4/Pt(100) system. The calculated adsorption energies over the top, bridge and hollow sites are small, weakly dependent on the molecular orientation. The nature of the CH4-Pt interaction was examined through the electronic structure changes. The adsorption of methyl (CH3) and hydrogen (H) and the coadsorption of CH3+H were also calculated. From these results, we examined the dissociation of CH4 to CH3+H, and the activation energies found are in good agreement with the experimental and theoretical values. The activation of CH3OH/Pt(100) has been studied. All the sites have almost the same adsorption energy. The adsorption of oxygen (O) and the coadsorption of CH4 and O were also examined. In addition, the formation of CH3OH assuming a one-step mechanism step via the coadsorption of CH4+O has been studied and the barrier height was found to be high