Supported metals reduction 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. 

We have demonstrated that supported Pd and Cu catalysts are effective in catalyzing the oxidative carbonylation at low pressure reaction condition and the supported metal catalysts can be easily separated from the product mixture in both fixed bed and slurry phase reactors (12,17). The objective of this study is to investigate the feasibility of using Al203-supported Pd catalysts for catalyzing the reductive carbonylation of nitrobenzene with ethanol. 

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). 

Polymers are attracting much attention as functional materials to construct photochemical solar energy conversion systems. Polymers and molecular assemblies are of great value for a conversion system to realize the necessary one-directional electron flow. Colloids of polymer supported metal and polynuclear metal complex are especially effective as catalysts for water photolysis. Fixation and reduction of N2 or C02 are also attractive in solar energy utilization, although they were not described in this article. If the reduction products such as alcohols, hydrocarbons, and ammonia are to be used as fuels, water should be the electron source for the economical reduction. This is why water photolysis has to be studied first. 

Similar effects of the cation of the supporting electrolyte occur, to a greater or lesser extent, in the reductions of alkali and alkaline earth metal ions in basic aprotic solvents [26a]. In dimethylacetamide (DMA), the reductions of alkaline earth metal ions are electrochemically masked by Et4N+. In DMF and DMSO, the reversibility of the reductions of alkali and alkaline earth metal ions decreases with the decrease in the cationic size of the supporting electrolyte. This effect is apparent from the kinetic data in Table 8.3, which were obtained by Baranski and Fawcett [23 b] for the reductions of alkali metal ions in DMF. 

Copper-catalysts promoted with i) other group VIA or VIIIA metals and ii) alcaline or alcaline earth elements (IA or IIA) are used for selective hydrogenation of various organic compounds (1). Moreover Cu(Co) Zn-Al catalysts were extensively studied for the synthesis of methanol and of light alcohols (2,3). More recently, due to the development of fine chemical processes, detailed studies of copper catalysts were carried out in order to show, like for noble metals, the effect of supports (SMSI), of promoters and of activation-on metal dispersion or reduction, on alloy formation... For example modified copper catalysts are known for their utilization in the dehydrogenation of esters (4-6), in the hydrolysis of nitriles (7), in the selective hydrogenation of nitriles (8), in the amination of alcohols (9)... 

The preparation of real supported catalysts will involve the deposition of a precursor salt followed by decomposition and/or reduction to the final metallic state. We shall consider the influence of the precursors and the effect of oxidative pretreatments later. First, we consider how the shapes of supported metal particles will vary with time under reducing conditions, since this represents the working condition for most metal catalysts. A comprehensive review of sintering and redispersion in supported metals has been presented by Ruckenstein and Dadyburjor.232... 

Redispersion through an oxidation-reduction cycle as described previously is, indeed, an effective way to regenerate supported metal catalysts that have been deactivated because of sintering, and the underlying principle is spontaneous monolayer dispersion. 

No support can be regarded as inert with respect to the active centres. By its universally positive effect on the activity of centres, MgCl2 is superior to any other support. In spite of the great technical importance of Mg in active centres, generally not much is known of their structure in third-generation catalysts (or perhaps because of its positive effects all the important producers have published hundreds of patents, but the crucial factors may still be kept secret). It is suspected that the separation (dilution) of transition metal atoms by a barrier of Mg atoms enables the majority of transition metals to become part of the active centres on these centres, the polymer grows more rapidly than on centres without Mg. Mutual contact of the centres is hindered, bimolecular termination of centres (transition metal reduction to a less active oxidation state) is limited, and the centres live longer. 

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. 

For catalysts containing reducible oxide supports, as is the case of systems, the chemisorption studies may also be used for detecting the metal deactivation effects due to the occurrence of a SMSI effect (300,301). On MT1O2 catalysts, the classic SMSI systems, it is now well established that reduction at about 773 K strongly inhibits the metal chemisorptive capability (171,302,318-320). The chemisorption data reported for M/Cc(M)02. catalysts have also suggested the occurrence of such an effect. It is certainly an interesting question which deserves some further discussion in this chapter. 

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