Ensemble effects

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

Liu P, Nprskov JK. 2001b. Ligand and ensemble effects in adsorption on alloy smfaces. Phys Chem Chem Phys 3 3814. 

An ensemble effect related to the variation of the number of sites of specihc configuration with particle size. Ensemble effects have been, for example, suggested to account for the decrease in Hupd coverage, and the S A in the MOR. 

A surface crystallography effect, which is, in fact, closely related to the ensemble effect. 

Describe the differences between ensemble effects and ligand effects (in relation to bimetallic surfaces), and how these may play a role in the adsorption activity of a surface. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

PtMo alloys are not as effective as PtRu for methanol, or ethanol, oxidation. As shown in Figure 29, the d band vacancy per Pt atom for the PtMo/C catalyst continues to increase until 0.6 V vs RHE, in contrast to the behavior of PtRu/C. ° The authors attribute this difference to the lack of removal of the Cl fragments from the particle surface by the oxy-hydroxides of Mo. However, the difference in the electrocatalytic activity of PtRu and PtMo catalysts may be attributed to ensemble effects as well as electronic effects. The former are not probed in the white line analysis presented by Mukerjee and co-workers. In the case of methanol oxidation, en-... 

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