Cooperative coadsorption effects

Cooperative Coadsorption Effects on Small Gold Clusters. Two examples of cooperative adsorption effects on small gold cluster anions identified in temperature dependent rf-ion trap experiments (see Chemical and Catal3dic Properties of Gas-Phase Clusters for experimental details) will be presented in the following. Au3 does not react with O2 in the ion trap experiment at any reaction temperature [34]. It, however, adsorbs a maximum of two CO molecules at reaction temperatures below 250 K [185]. If the gold trimer is exposed simultaneously to CO and O2 inside the octopole ion trap, still no reaction products are observed at reaction temperatures above 250 K as can be seen... 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

We conclude this introductory with a short outline of the sections that follow. In Section 2.3.2, we offer a brief introduction to pressrue and material gap problems that arise when model catalytic systems and conditions are used to emulate working catalytic systems. In Section 2.3.3, we describe the local aspects of the catalytically reactive sites in the section titled Ensemble effects and defect sites . This is followed by four sections on environmental influences on the reactivily entitled Cluster size and metal supports , Cooperativity , Surface moderation by coadsorption of organic molecules and Stereochemistry of homogeneous catalysts . The chapter is concluded with a section on surface kinetics, dealing with surface reconstruction and transient reaction intermediates. 

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