Supported metals reduction temperature effect

Figure 2 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3d5/2), Ni(2p3/2), or Cu(2p3/2) all measured before adsorbing CO) [66]. In general, a reduction in BE of a core level is accompanied by an enhancement in the strength of the bond between CO and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level BE. The correlation observed in Figure 2 can be explained in terms of a model based on initial-state effects . The chemisorption bond on metal is dominated by the electron density of the occupied metal orbital to the lowest unoccupied 27t -orbital of CO. A shift towards lower BE decreases the separation of E2 t-Evb thus the back donation increases and vice versa. 

High-temperature reactions with vacuum microbalance, 5 119 High-temperature reduction, 34 19 effects on titania-supported metals, 36 176-177, 180... 

Low-temperature reduction, effects on titania-supported metals... 

The term SMSl was introduced by Tauster et al. (S.J. Tauster, S.C. Funk, R.L. Garten, J. Am. Chem. Soc. 1978, 100, 170) to denote the effect responsible for the drastic decrease in CO and H2 chemisorption on titania-supported metals after increasing the reduction temperature from 200 to 500°C. More details on this effect can be found in a review paper of Hadjiivanov and Klissurski (K.I. Hadjiivanov, D.G. Klissurski, Chem. Soc. Rev., 1996, 25, 61). 

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. 

The initial reaction rate values show the effect of metal support interaction, the extent of reduction and method of preparation. For alumina supported sample the increase in reduction temperature, and an increase in the degree of reduction, results in a fourfold increase in initial... 

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. 

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. 

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