Catalyst-support interactions reduction temperature effect

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

Temperature programmed reduction measures the reaction of hydrogen with a sample at various temperatures. The results are interpreted in terms of the different species present in the sample and their amenability to reduction. Therefore, these results can give information on the presence of different oxidation states or the effect of a dopant in a lattice. It is useful for measuring the temperature necessary for the complete reduction of a catalyst and is commonly used to investigate the interaction of a metal catalyst with its support, or of the effect of a promoter on a metal catalyst. 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

In a recent review work (117) on the chemical and nano-structural characterization of NM/CeO catalysts, a detailed study of the H interaction with a Pt/CeO catalyst reduced at temperatures ranging fh)m 473 K to 773 K is reported. The experimental techniques used in this work were TPD-MS and Isotopic Transient Kinetics (ITK) of the H2/D2 exchange at 298 K. The catalyst sample was carefully selected in order to minimise the Pt and support sintering effects in the investigated range of reduction temperatures. Likewise, a chlorine-free metal precursor, [Pt(NH3)4](OH)2, was used in the preparation of the catalyst. 

Nanostructural Features of Metal-Support Interaction Effects in NM/Ce(M)02, Catalysts. Their Evolution with Reduction Temperature. 

Palladium-silica catalysts prepared from tetra-ammine palladous nitrate (to avoid chlorine introduction) showed a marked reduction effect , viz, the specific activity for benzene hydrogenation decreased with increased reduction temperature, i.e., 573 or 723Various explanations were considered, including a metal-support interaction. After reduction at 873 K, X-ray diffraction provided clear evidence of chemical reaction and at lower temperatures silicon insertion into palladium might still occur, which could either disrupt the palladium ensembles required for benzene adsorption or modify the properties of single palladium atoms, if these are the active sites. 

Supported Nickel. - The interaction of nickel with siUca and alumina at various stages of catalyst preparation has been extensively studied and only some recent evidence for interaction and its effect on performance are noted here. Nickel nitrate impregnated on silica decomposed to NiO during calcination, which interacted little with the support, whereas the ESCA spectrum of low nickel loadings on 7-alumina were quite different and NiO only appeared once the surface sites were filled. After reduction at 673 K and examination by ESCA without exposure to air, it was shown that percentage reduction depended on both Ni content and calcination temperature, which was considered indicative of a strong interaction between nickel and alumina. Nickel ions in octahedral sites are readily reduced and increasing the calcination temperature makes them diffuse to tetrahedral sites where they are difficult to reduce. 

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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