Model catalyst studies of the strong metal-support interaction: Surface structure identified by STM on Pd nanoparticles on TiO2(110)

Model catalysts of Pd nanoparticles and films on TiO2 (I 10) were fabricated by metal vapour deposition (MVD). Molecular beam measurements show that the particles are active for CO adsorption, with a global sticking probability of 0.25, but that they are deactivated by annealing above 600 K, an effect indicative of SMSI. The Pd nanoparticles are single crystals oriented with their (I 11) plane parallel to the surface plane of the titania. Analysis of the surface by atomic resolution STM shows that new structures have formed at the surface of the Pd nanoparticles and films after annealing above 800 K. There are only two structures, a zigzag arrangement and a much more complex "pinwheel" structure. The former has a unit cell containing 7 atoms, and the latter is a bigger unit cell containing 25 atoms. These new structures are due to an overlayer of titania that has appeared on the surface of the Pd nanoparticles after annealing, and it is proposed that the surface layer that causes the SMSI effect is a mixed alloy of Pd and Ti, with only two discrete ratios of atoms: Pd/Ti of 1: 1 (pinwheel) and 1:2 (zigzag). We propose that it is these structures that cause the SMSI effect. (c) 2005 Elsevier Inc. All rights reserved.

Engineering Pt in ceria for a maximum metal-support interaction in catalysis

Conventional supported metal catalysts are metal nanoparticles deposited on high surface area oxide supports with a poorly defined metal−support interface. Typically, the traditionally prepared Pt/ceria catalyzes both methanation (H2/CO to CH4) and water−gas shift (CO/H2O to CO2/H2) reactions. By using simple nanochemistry techniques, we show for the first time that Pt or PtAu metal can be created inside each CeO2 particle with tailored dimensions. The encapsulated metal is shown to interact with the thin CeO2 overlayer in each single particle in an optimum geometry to create a unique interface, giving high activity and excellent selectivity for the water−gas shift reaction, but is totally inert for methanation. Thus, this work clearly demonstrates the significance of nanoengineering of a single catalyst particle by a bottom-up construction approach in modern catalyst design which could enable exploitation of catalyst site differentiation, leading to new catalytic properties.

Effects of oxide promoters on metal dispersion and metal-support interactions in Ni catalysts supported on activated carbon

Various oxide-promoted Ni catalysts supported on activated carbon were prepared, and the effect of promoters on the surface structure and properties of Ni catalysts was studied. Physical adsorption (Na adsorption), thermogravimetric analysis (TGA), temperature-programmed desorption (TPD), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used to characterize the catalysts. It is found that nickel is fairly uniformly distributed in the pores of the carbon support. Addition of promoters produces a more homogeneous distribution of nickel ion in carbon. However, distributions of promoters in the pores are varying. Addition of promoters increases the dispersion of nickel in carbon. Promoters also change the interaction between the carbon and Ni, resulting in significantly different behaviors of catalysts under various environments. CaO and MgO promoters improve the reactivity of nickel catalysts with O-2 but retard the interaction between nickel oxide and carbon. La2O3 shows some inhibiting effect on the interactions between nickel oxide and oxygen as well as carbon.

CO2 reforming of methane on Ni catalysts: Effects of the support phase and preparation technique

The effects of the support phase and catalyst preparation methods on catalytic activity and carbon deposition were systematically investigated over nickel catalysts supported on Al2O3, SiO2 and MgO for the reforming reaction of methane with carbon dioxide. It is found that the pore structure of the support and metal-support interaction significantly affected the catalytic activity and coking resistance. Catalyst with well-developed porosity exhibited higher catalytic activity. Strong interaction between metal and the support made the catalyst more resistant to sintering and coking, thus resulting in a longer time of catalyst stability. (C) 1998 Elsevier Science B.V.

A FTIR study of the metal-support interactions and hydrogen spillover on Pd/TiO2 and Ni/TiO2

Nickel and palladium dispersed on titania support were submitted to reductive treatment, under hydrogen, at 200 and 500 ºC. After the reductive thermal treatment the materials were exposed to carbon monoxide (10 Torr) and analyzed in the infrared region. The increasing of the electronic density in the metallic d subshell, produced by the reductive thermal treatment, was monitored by the infrared stretching band shift of carbon monoxide adsorbed and it was interpreted as a consequence of the metal-support interactions. The highest effect was observed for Pd/TiO2 system. From the FTIR spectra was also observed that the hydrogen spillover was stronger on Pd/TiO2 than Ni/TiO2 system.

Surface science studies of strong metal-oxide interactions on model catalysts

The strong metal support interaction (SMSI) was first described in 1978 by Tauster [1-4]. The effect was observed as a severely negative effect on CO and H2 uptake on the catalyst after high temperature calcination under reducing conditions (heating above ~ 700 K) [1,2]. It also had a negative effect on the reaction rate for reactions, such as alkane hydrogenolysis [5,6]. It appeared that the effect occurred for catalysts comprised of reducible supports which were treated at elevated temperature in reducing conditions [2-4]. A classic support which has manifested this behaviour in many studies is TiO2. Over the years following the first discovery of SMSI it has been recognised that the effect is not always negative – for instance for the CO-H2 reaction for which it appears to have a positive effect [5,6]. Further it was noted that hydrogen reduction was not necessary to observe the effect of CO adsorption suppression, it also occurs by vacuum treatment [7], though it should be noted that vacuum treatment at elevated temperature is, in effect, a reducing environment.

Reforming of methane with carbon dioxide over Ni/Al2O3 catalysts: Effect of nickel precursor

The catalytic activities of Ni/gamma-Al2O3 catalysts prepared using different nickel precursor compounds were studied for the reaction of methane reforming with CO2. It is found that the nickel precursor employed in the catalyst preparation plays an important role. The catalyst based on nickel nitrate exhibited higher catalytic activity and stability over a 24-h test period than the other two catalysts derived from nickel chloride and nickel acetylacetonate. A comprehensive characterisation of the catalysts showed that the weak interaction between Ni particles and gamma-Al2O3 resulted in more active sites on Ni nitrate-derived Ni/gamma-Al2O3 catalyst. Coking studies showed that carbon deposition on Ni catalysts derived from inorganic precursors (nitrate and chloride) were more severe than on the organic precursor-derived catalyst. However, the Ni nitrate-derived catalyst was found to have the highest stability (or lowest deactivation rate) mainly due to the active carbon species (-C-C-) of the resulting graphitic structure and their close contact with the metal particles. In contrast, the carbon formed on Ni-AA catalyst (from Ni acetylacetonate) is dominated by inactive -CO-C- species, thus leading to a rapid accumulation of carbon in this catalyst and more severe deactivation. (C) 1998 Elsevier Science B.V.

Role of CeO2 in Ni/CeO2-Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on gamma-Al2O3, CeO2 and CeO2-A1(2)O(3) systems were tested for catalytic CO2 reforming of methane into synthesis gas. Ni/CeO2-Al2O3 catalysts showed much better catalytic performance than either CeO2- or gamma-Al2O3-supported Ni catalysts. CeO2 as a support for Ni catalysts produced a strong metal-support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO2 had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/gamma-Al2O3 catalysts for this reaction. A weight loading of 1-5 wt% CeO2 was found to be the optimum. Ni catalysts with CeO2 promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO2-promoted catalysts are attributed to the oxidative properties of CeO2. (C) 1998 Elsevier Science B.V. All rights reserved.

Estudo do efeito do cloreto em catalisadores de paládio usando a análise no infravermelho e de fotoelétrons excitados por raios x

Chloride poisoning is known as having an inhibitor effect in the activity of metal catalysis. In this work in situ infrared spectroscopy (FTIR) of adsorbed carbon monoxide and x-ray photoelectron spectroscopy (XPS) were used to investigate the effect of chloride presence in the electronic metal density in the d subshell of palladium dispersed on alumina. The chloride poisoning effect was interpreted as an electronic effect since a weak back-bonded Pd-CO was formed due to the decrease in the electronic density of the d subshell of palladium, which could be also detected by the higher Pd 3d5/2 binding energy in the chloride presence. A similar poisoning effect was also observed for chloride free Pd/Al2O3 reduced at 500 ºC, and it was interpreted based on the interaction of metal with the alumina surface. The use of molybdena/alumina binary system as support, yield a contrary effect due to the metal-support interaction.

Estudo da reação de oxidação preferencial do co sobre o sistema CuO/CeO2-TiO2

Cu catalysts supported on CeO2, TiO2 and CeO2/TiO2 were prepared by precipitation method and used for preferential oxidation of carbon monoxide contained in a hydrogen flow generated by methane steam reforming. The samples were characterized by XRD, BET and TPR techniques. The catalytic properties were studied in the 50-330ºC range by using a quartz micro-reactor vertically positioned on an electrical furnace. The results showed that the small copper particles generated with the lower metal content are the most easily reducible and give the best catalytic performance. In respect of support effect, the strong metal-support interaction and the redox characteristics of the CuOx-CeO2 series resulted in the best catalytic results, especially with the sample with 1% copper content.

Hydrogen purification for fuel cell using CuO/CeO(2)-Al(2)O(3) catalyst

CuO/CeO(2), CuO/Al(2)O(3) and CuO/CeO(2)-Al(2)O(3) catalysts, with CuO loading varying from 1 to 5 wt.%, were prepared by the citrate method and applied to the preferential oxidation of carbon monoxide in a reaction medium containing large amounts of hydrogen (PROX-CO). The compounds were characterized ex situ by X-ray diffraction, specific surface area measurements, temperature-programmed reduction and temperature-programmed reduction of oxidized surfaces; XANES-PROX in situ experiments were also carried out to study the copper oxidation state under PROX-CO conditions. These analyses showed that in the reaction medium the Cu(0) is present as dispersed particles. On the ceria, these metallic particles are smaller and more finely dispersed, resulting in a stronger metal-support interaction than in CuO/Al(2)O(3) or CuO/CeO(2)-Al(2)O(3) catalysts, providing higher PROX-CO activity and better selectivity in the conversion of CO to CO(2) despite the greater BET area presented by samples supported on alumina. It is also shown that the lower CuO content, the higher metal dispersion and consequently the catalytic activity. The redox properties of the ceria support also contributed to catalytic performance. (C) 2010 Elsevier B.V. All rights reserved.

Preparation, structural characterization and catalytic properties of Co/CeO2 catalysts for the steam reforming of ethanol and hydrogen production

In this paper, Co/CeO2 catalysts, with different cobalt contents were prepared by the polymeric precursor method and were evaluated for the steam reforming of ethanol. The catalysts were characterized by N-2 physisorption (BET method), X-ray diffraction (XRD), UV-visible diffuse reflectance, temperature programmed reduction analysis (TPR) and field emission scanning electron microscopy (FEG-SEM). It was observed that the catalytic behavior could be influenced by the experimental conditions and the nature of the catalyst employed. Physical-chemical characterizations revealed that the cobalt content of the catalyst influences the metal-support interaction which results in distinct catalyst performances. The catalyst with the highest cobalt content showed the best performance among the catalysts tested, exhibiting complete ethanol conversion, hydrogen selectivity close to 66% and good stability at a reaction temperature of 600 degrees C. (c) 2012 Elsevier B.V. All rights reserved.

CuO-CeO2 catalysts synthesized in one-step: Characterization and PROX performance

PEM fuel cells seem to be the most affordable and commercially viable hydrogen-based cells, the biggest challenge being to obtain CO-free H-2 (<100 ppm) as the fuel. In this study, the use of CuO-CeO2 catalysts in preferential oxidation of CO to obtain CO-free H-2 (PROX reaction) was investigated. Ce1-xCuxO2 catalysts, with x (mol%) = 0, 0.01, 0.03, 0.05 and 0.10, were synthesized in one-step by the polymeric precursor method, to obtain a very fine dispersion and strong metal-support interaction, to favor active copper species and a preference for the PROX reaction. The results obtained from catalyzed reactions and characterization of the catalysts by XRD, Rietveld refinement, BET surface area, UV-Vis and TPR, suggest that this one-step synthesis method gives rise to catalysts with copper species selective for the PROX reaction, which reaches a maximum rate on Ce0.97Cu0.03O2 catalyst. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.