Model catalysts metal-support interactions

B.L. Mojet, J.T. Miller, D.E. Ramaker, and D.C. Koningsberger, A new model describing the metal-support interaction in noble metal catalysts, J. Catal. 186, 373-386 (1999). 

Metal support interaction in Pt/SiOi model catalyst... 

Bowker M, Stone P, Morrall P, et al. Model catalyst studies of the strong metal-support interaction surface structure identified by STM on Pd nanoparticles on TiO2(110). J Catal. 2005 234 172-81. 

Fig. 56. CO adsorption on palladium nanoparticles grown at 90 K on Nb205/Cu3Au(l 00). (a) SFG spectra acquired in 10 mbar of CO at 110 K, after annealing of the model catalysts to the temperatures indicated. Values obtained for the peak position, resonant amplitude, peak width (FWHM), and phase <[) of the spectra are displayed in (b), both for on-top and bridge-bonded CO. Metal-support interactions resulting from annealing of Pd/Nb205 led to an irreversible loss of the CO adsorption capacity and formation of a mixed Pd-NbO, . phase reprinted from (523) with permission from Elsevier.

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

There is similar electrochemical promotion behavior of Rh films on YSZ and similar metal-support interaction-induced behavior of dispersed Rh on different supports for the model reaction of C2H4 oxidation on Pt (Figure 40). In particular, there are very similar p values (p ep]y[sj e 120) upon increasing the potential and work function of the Rh film or upon increasing the work function (or absolute potential) of the support of the dispersed Rh catalyst (Figure 40). 

This simple model (Figure 42) can account for the observed equivalence between electrochemical promotion and metal-support interaction-induced promotional phenomena. In both cases backspillover to the catalyst surface is the dominant promotional mechanism. It should be noted that according to the... 

The same reaction was also investigated over Rh nanoparticles dispersed over pure and doped Ti02 and other porous supports of known work functions [136]. It was established that the influence of electrochemical promotion on kinetic parameters of the model reaction is identical to the influence of metal-support interactions, under conditions at which the change of the work function of the catalyst is the same, regardless of the means by which the alteration in the work function is achieved. 

Magnetic measurements, which included magnetization-temperature behavior and particle size determination, were also made on this series of catalysts as a function of reduction treatment. These results, in conjunction with those obtained from kinetic studies, provided a physical picture of the different mechanisms for the niobia and phosphate supports. The same picture is consistent with the less interacting nature of niobia-silica, which should prove useful as a model system for the study of metal-support interactions in general. 

Recently titania appeared as a non-conventional support for noble metal catalysts, since it was found to induce perturbations in their H2 or CO adsorption capacities as well as in their catalytic activities, This phenomenon, discovered by the EXXON group, was denoted "Strong Metal-Support Interactions" (SMSI effect) (1) and later extended to other reducible oxide supports (2). Two symposia were devoted to SMSI, one in Lyon-Ecully (1982) (3) and the present one in Miami (1985) (4) and presently, two main explanations are generally proposed to account for SMSI (i) either the occurence of an electronic effect (2,5-13) or (ii) the migration of suboxide species on the metal particles (14-20). The second hypothesis was essentially illustrated on model catalysts with spectroscopic techniques.lt can be noted that both possibilities do not necessarily exclude each other and can be considered simultaneously (21). 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

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