Littérature scientifique sur le sujet « Catalysis, Ageing, STM »

The global trend in restrictions on pollutant emissions requires the use of catalytic converters in the automotive industry. Noble metals belonging to the platinum group metals (PGMs, platinum, palladium, and rhodium) are currently used for autocatalysts. However, recent efforts focus on the development of new catalytic converters that combine high activity and reduced cost, attracting the interest of the automotive industry. Among them, the partial substitution of PGMs by abundant non-PGMs (transition metals such as copper) seems to be a promising alternative. The PROMETHEUS catalyst (PROM100) is a polymetallic nanosized copper-based catalyst for automotives prepared by a wet impregnation method, using as a carrier an inorganic mixed oxide (CeO2-ZrO2) exhibiting elevated oxygen storage capacity. On the other hand, catalyst deactivation or ageing is defined as the process in which the structure and state of the catalyst change, leading to the loss of the catalyst’s active sites with a subsequent decrease in the catalyst’s performance, significantly affecting the emissions of the catalyst. The main scope of this research is to investigate in detail the effect of ageing on this low-cost, effective catalyst. To that end, a detailed characterization has been performed with a train of methods, such as SEM, Raman, XRD, XRF, BET and XPS, to both ceria–zirconia mixed inorganic oxide support (CZ-fresh and -aged) and to the copper-based catalyst (PROM100-fresh and -aged), revealing the impact of ageing on catalytic efficiency. It was found that ageing affects the Ce–Zr mixed oxide structure by initiating the formation of distinct ZrO2 and CeO2 structures monitored by Raman and XRD. In addition, it crucially affects the morphology of the sample by reducing the surface area by a factor of nearly two orders of magnitude and increasing particle size as indicated by BET and SEM due to sintering. Finally, the Pd concentration was found to be considerably reduced from the material’s surface as suggested by XPS data. The above-mentioned alterations observed after ageing increased the light-off temperatures by more than 175 °C, compared to the fresh sample, without affecting the overall efficiency of the catalyst for CO and CH4 oxidation reactions. Metal particle and CeZr carrier sintering, washcoat loss as well as partial metal encapsulation by Cu and/or CeZrO4 are identified as the main causes for the deactivation after hydrothermal ageing.

Ageing of automotive catalysts is associated to a loss of their functionality and ultimately to a waste of precious resources. For this reason, understanding catalyst ageing phenomena is necessary for the design of long lasting efficient catalysts. The present work has the purpose of studying in depth all the phenomena that occur during ageing, in terms of morphological modification and deactivation of the active materials: precious metal particles and oxidic support. The topic was deeply investigated using specific methodologies (FT-IR, CO chemisorption, FE-SEM) in order to understand the behavior of metals and support, in terms of their surface properties, morphology and dispersion in the washcoat material. A series of commercial catalysts, aged in different conditions, have been analyzed, in order to find correlations between real and simulated ageing conditions. The characterization highlights a series of phenomena linked to the deactivation of the catalysts. Pd nanoparticles undergo a rapid agglomeration, exhibiting a quick loss of dispersion and of active sites with an increase of particles size. The evolution of the support allows highlighting also the contribution of chemical ageing effects. These results were also correlated with performance tests executed on synthetic gas bench, underlining a good correspondence between vehicle and laboratory aged samples and the contribution of chemical poisoning to vehicle aged ones. The collected data are crucial for the development of accelerated laboratory ageing protocols, which are instrumental for the development and testing of long lasting abatement systems.

A series of nickel-cobalt bimetal oxides in varying molar ratios and its single metal oxides were synthesized by reactive calcination of coprecipitated basic-carbonates. Several characterization techniques, such as: Bruneuer Emmett Teller (BET), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infra Red (FTIR), and Hydrogen Temperature Programmed Reduction (H2-TPR), were performed over the oxides. Activities of oxides were evaluated in methane total oxidation in the presence or the absence of CO. The best catalytic performance was observed over NiCo catalyst with a Ni/Co molar ratio of 1:1, and the complete conversion of CO-CH4 mixture was achieved at 390 °C. Moreover, the presence of carbon monoxide improves CH4 total oxidation over nickel-cobalt mixed oxides. Structural analysis reveals that the insertion of nickel into the spinel lattice of cobalt oxide causes the structural disorder, which probably caused the increase of the amount of octahedrally coordinated divalent nickel cations that are responsible for catalytic activity. Stability of the best-performed catalyst has been tested in the two conditions, showing remarkable long-term stability and thermal stability, however, showed deactivation after thermally ageing at 700 °C. Copyright © 2020 BCREC Group. All rights reserved

A Three-Way Catalyst (TWC) contains a cordierite ceramic monolith coated with a layer of Al2O3, CexZr1−xO2 and platinoids mixture. Under standard operation, the platinoid concentration decreases, exposing the remaining washcoat structure. After that particle release stage, the sintering process follows where the crystalline CexZr1−xO2 solution is broken and begins to separate into ZrO2 and CeO2 phases. ZrO2 is released to the environment as micro and nanoparticles, while a small amount of CeO2 generates a new AlxCe1−xO2 composite. The main effect of Ce capture is the growth in the size of the polycrystal structure from 86.13 ± 16.58 nm to 225.35 ± 69.51 nm. Moreover, a transformation of cordierite to mullite was identified by XRD analysis. Raman spectra showed that the oxygen vacancies (Vö) concentration decreased as CexZr1−xO2 phases separation occurred. The SEM-EDS revealed the incorporation of new spurious elements and microfractures favouring the detachment of the TWC support structure. The release of ultrafine particles is a consequence of catalytic devices overusing. The emission of refractory micro to nanocrystals to the atmosphere may represent an emerging public health issue underlining the importance of implementing strict worldwide regulations on regular TWCs replacement.

The obtaining and characterization of some environmental-friendly composites that are based on natural rubber and plasticized starch, as filler, are presented. These were obtained by peroxide cross-linking in the presence of a polyfunctional monomer used here as cross-linking co-agent, trimethylolpropane trimethacrylate. The influence of plasticized starch amount on the composites physical and mechanical characteristics, gel fraction and cross-link density, water uptake, structure and morphology before and after accelerated (thermal) degradation, and natural (for one year in temperate climate) ageing, was studied. Differences of two orders of magnitude between the degradation/aging methods were registered in the case of some mechanical characteristics, by increasing the plasticized starch amount. The cross-link density, water uptake and mass loss were also significant affected by the plasticized starch amount increasing and exposing for one year to natural ageing in temperate climate. Based on the results of Fourier Transform Infrared Spectroscopy (FTIR) and cross-link density measurements, reaction mechanisms attributed to degradation induced by accelerated and natural ageing were done. SEM micrographs have confirmed in addition that by incorporating a quantity of hydrophilic starch amount over 20 phr and by exposing the composites to natural ageing, and then degradability can be enhanced by comparing with thermal degradation.

ABSTRACTPhenol is an important industrial chemical that was manufactured by the Hock process. The sulphuric acid used in the Hock process to break cumene hydroperoxide into phenol and acetone suffers from environmental problems along with low selectivity towards the desired products. There are several solid acid catalysts have been tried in cumene hydroperoxide decomposition into an ecofriendly process. In the present work, an efficient environmentally friendly and economically benefitted zeolite-based catalyst was synthesized and tested for breaking cumene hydroperoxide (CHPO) into phenol and acetone. Beta zeolite and ZSM-5 zeolite were synthesised with nano crystalline geometry by hydrothermal method and tested their efficiency to break CHPO. The morphology and structure of nanocrystalline zeolite materials was investigated by Dynamic Light Scattering (DLS), X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) methods. From DLS studies it is documented that the zeolite particle size is tuneable as evidenced from particle size growths with the ageing time under hydrothermal treatment process. XRD analysis confirms the zeolite Beta and ZSM-5. The mean crystal size of Beta and ZSM-5 obtained from SEM analysis are 45 nm and 18.3 nm respectively. The optimization of reaction conditions for CHPO decomposition such as catalyst quantity and initial concentration of CHPO has been done using nanocrystalline Beta catalyst. The optimum catalyst amount and concentration of CHPO are 0.03 g and 15 mmol respectively. The results showed that the nanocrystalline Beta zeolite catalyst exhibits exceptional activity with 100% conversion of CHPO and yielded exclusively phenol and acetone at room temperature. No other by-products are detected. Also, the catalyst is stable and exhibits high product selectivity for phenol and acetone. The nature of the enhanced activity at room temperature for the decomposition of CHPO is suggested that higher accessibility of the active sites that are proposed to be associated with the zeolite with nano configuration with higher surface area and short distances for diffusion inside the channels in the nanocrystalline zeolite particles. The possibility of commercialization of beta zeolite is proved by complete conversion of CHPO with selective formation of phenol and acetone.

Organic contaminants significantly limit the bioactivity of titanium implants, resulting in the degradation known as the ageing of titanium. To reactivate the surfaces, they can be photofunctionalized, i.e., irradiated with C-range ultraviolet (UVC) light. This descriptive in vitro study compares the effectiveness of novel light-emitting diode (LED) technology to remove contaminant hydrocarbons from three different commercially available titanium dental implants: THD, TiUnite, and SLA. The surface topography and morphology were characterized by scanning electron microscopy (SEM). The chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS), before and after the lighting treatment, by a pair of closely placed UVC (λ = 278 nm) and LED devices for 24 h. SEM analysis showed morphological differences at the macro- and micro-scopic level. XPS analysis showed a remarkable reduction in the carbon contents after the UVC treatment: from 25.6 to 19.5 C at. % (carbon atomic concentration) in the THD; from 30.2 to 20.2 C at. % in the TiUnite; from 26.1 to 19.2 C at. % in the SLA surface. Simultaneously, the concentration of oxygen and titanium increased. Therefore, LED-based UVC irradiation decontaminated titanium surfaces and improved the chemical features of them, regardless of the kind of surface.

2007/2008
This thesis concerns a surface science approach for the investigation of the ageing process of a model catalyst. It combines extreme oxidation conditions with Ultra High Vacuum (UHV) compatible characterization techniques. Our model system was the surface oxide formed on Rh(110); to grow such oxide, we used three alternative oxygen sources, optimizing for each case the preparation recipe. When dosing molecular oxygen, pressures in the ∼ 10− 4 mbar range were used, therefore bridging, to some extent, the pressure gap. For characterization of the oxides we used mainly Scanning Tunneling Microscopy (STM) and Thermal Desorption Spectroscopy (TDS), providing atomic scale and desorption mechanism information, respectively. Low Energy Electron Microscopy (LEEM) and X-ray Photoelectron Specroscopy (XPS) complemented our measurements with large scale morphology and reactivity data and with chemically resolved results. To mimic real catalytic conditions, we setup an ageing protocol consisting of cycles of oxidation and annealing in UHV (up to more than ∼ 40), with each of the three oxygen sources. In this way, we were able to observe two different kinds of ageing: a “contaminant-driven” and an “intrinsic” one, caused by the iterative oxidation procedure. The latter is connected to the presence of a new species we detected on the (1 × 1) surface obtained after the oxide desorption, that we named “units” (or ageing fingerprints). By decreasing their number we were able to show that the intrinsic ageing is, at least partially, reversible. We could not uniquely determine the structure of the “units”, but plausible models are proposed. ------------------------------------------------
Questa tesi si occupa dello studio dell’invecchiamento artificiale di un catalizzatore modello, combinando condizioni di ossidazione estreme con le tecniche di caratterizzazione, compatibili con l’ultra-alto-vuoto (UHV), proprie della scienza delle superfici. Il nostro sistema modello era l’ossido superficiale formato sul Rh(110); per crescere questo ossido, abbiamo utilizzato tre sorgenti alternative di ossigeno, ed abbiamo ottimizzato la ricetta di preparazione nei singoli casi. Utilizzando l’ossigeno molecolare abbiamo dosato a pressioni dell’ordine di ca. 10− 4 mbar, ed in questo modo abbiamo in parte colmato la “pressure gap” che solitamente divide gli studi su sistemi modello da quelli di catalisi reale. Per caratterizzare gli ossidi abbiamo usato principalmente la microscopia a scansione ad effetto tunnel STM e la spettroscopia di desorbimento termico TDS, che ci hanno fornito rispettivamente informazioni su scala atomica e sul meccanismo di desorbimento. Le tecniche LEEM e XPS hanno contribuito in modo complementare alle nostre misure con dati di morfologia e reattività su larga scala, da un lato, e con dati risolti chimicamente, dall’altro. Per simulare le condizioni della catalisi reale, abbiamo sviluppato un protocollo di invecchiamento (“ageing”) composto da cicli di ossidazione e riscaldamento in ultra-alto-vuoto (fino a ca. 40), con ognuna delle tre sorgenti di ossigeno. Seguendo questa procedura, abbiamo osservato due diversi tipi di invecchiamento dell’ossido di rodio: un primo tipo dominato dalla presenza di contaminanti, ed un secondo invece che abbiamo chiamato “intrinseco”, causato cioè dalle ripetute ossidazioni. Quest’ultimo dipende dalla presenza di una nuova specie osservata sulla superficie (1 × 1) che si ottiene a seguito del desorbimento di ciascun ossido, che abbiamo chiamato “units” (o -ageing fingerprint-). Ri- ducendo la densità di questa specie siamo stati in grado di mostrare come l’invecchiamento intrinseco sia, almeno in parte, reversibile. Non abbiamo potuto determinare univocamente la struttura delle “units”, ma alcuni modelli possibili vengono proposti.
XXI Ciclo
1981

In recent years, the pollution caused by hydrocarbon spills has increased, and this leads to research to mitigate the deterioration caused to the environment, therefore, this work has the purpose of remedying a contaminated soil due to the explosion of the Well Terra 123 of Mexican Oil (PEMEX) occurred in October 2013, which left in its wake contamination, death of animals and diseases in the population, causing both environmental and health effects on the inhabitants of the region such as vitiligo, problems vision, throat, cough and flu (Reporte Indigo, 2019). The studies carried out by Duran in 2015 on "Environmental impact on the indigenous communities of Nacajuca, Tabasco, due to the explosion of the Well Terra 123", gathered evidence of the impact that this oil accident had on the health of those close to it to the facilities and the environment. This chapter will deal with the remediation of soil contaminated by hydrocarbons due to the explosion of the Terra 123 Well, using a polymeric material (carboxymethylcellulose gel), which was synthesized at the laboratory level using carboxymethylcellulose (CMC), glutaraldehyde (GA) as agent of crosslinking and hydrochloric acid (HCl) as a catalyst for synthesis. The CMC gel was incorporated into the contaminated soil for the absorption of the hydrocarbon for a period of three months. The samples were collected from the soil contaminated with hydrocarbon from Terra 123 well, located in Oxiacaque, Nacajuca, Tabasco, and the effectiveness was studied in two stages: (1) The soil particle was decreased by sieving and (2) Pre and post soil analyzes were carried out (moisture percentage and fat content). In addition, the CMC gels were analyzed using infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques before and after being incorporated into the contaminated soil. The amount of hydrocarbon initially contained in the soil was made using the Soxhlet method, obtaining 0.99 mg of hydrocarbon / kg of soil.