Active ensemble

The UPS indicated structure change is associated with size reduction as the discontinuous gold film is transformed into rod-shape and spherical particles with size of 5-10 nm. Accordingly, with size reduction the activity displayed in CO oxidation is also altered the rate increased from 6.7 X 10 to 2 X 10 molmin cm . Consequently, not only the gold-reducible oxide interaction is responsible for the increased activity, but also size reduction. Indeed, small clusters themselves are able to activate the reaction components shown by theoretical calculations performed for 10-15-atom clusters, which can activate easily oxygen [177,200], but in real catalyst, even at the smallest active ensemble, it consists of a few hundreds atoms. 

Active Ensemble Structures for Selective Oxidation Catalyses at Surfaces... 

Molecular-level design of catalytic ensemble structures on surfaces in a controllable manner, based on new chemical concepts and strategies regarding composition or structure, provides a promising opportunity for the development of novel and efficient catalysts active for selective oxidation. Novel strategies and concepts for the creation of active ensemble structures on flat and porous surfaces may emerge from self-assembly and in situ transformation of precursors immobilized on the surfaces, with the aid of in situ characterization by sophisticated physical techniques [1-6]. 

Kobozev s inductive theory of active ensembles (168,169,171,172a,b) postulates that the carrier of catalytic activity is a phase present in high dilution on the support. This phase, which is in the amorphous precrystalline state, consists of a number of cells separated by geometrical barriers (microfissures) which are impenetrable to molecules for movement from one group of cells to another. Thus there is no exchange of catalyst atoms, reactant molecules or catalyst poisons between these cells. The smallest group of catalytically active atoms in the cells form an ensemble which constitutes the carrier of the catalytic activity and to... 

The question as to what is the active site of Cu-based catalysts in MSR is still unclear and debated in the literature. Similar to the methanol synthesis reaction, either metallic Cu° sites, oxidized Cu+ sites dispersed on the oxide component or at the Cu-oxide interface, or a combination of both kinds of sites are thought to contribute to the active ensembles at the Cu surface. Furthermore, the oxidic surface of the refractory component may take part in the catalytic reaction and provide adsorption sites for the oxygenate-bonded species [126], whereas hydrogen is probably adsorbed at the metallic Cu surface. Similar to methanol synthesis, factors intrinsic to the Cu phase also contribute to the MSR activity in addition to SACu- There are two major views discussed in the literature relating these intrinsic factors either to the variable oxidation state of Cu, in particular to the in situ adjustment of the Cu°/Cu+ ratio at the catalyst s surface [102, 107, 127 132], or to the defect structure and varying... 

These results can be easily rationalized if ethane hydrogenolysis requires an ensemble of multiple Ni surface atoms to form an active site. The concentration of active ensembles will decline sharply with composition as the active Ni atoms are diluted with inactive Cu atoms in the surface.. Additional information on how bimetallic catalysts affect chemical reactions can be found in the excellent monograph by Sinfelt (J. H. Sinfelt, Bimetallic Catalysts Discoveries, Concepts, and Applications, Wiley, New York, 9st, . 

Attempts to determine the critical sites of the active ensembles have usually been based on expressions of the forni,... 

The reactions of butan-l-ol (Scheme 3) were explored over silica-supported Pt-Au catalysts (and also Ni-Cu powders). It was confirmed that metals are active in ether formation from higher alcohols, although sensitive to the presence of sodium ions. Alloying decreased the activity of pure Pt for ether formation, ascribed to the diminished number of active ensembles (perhaps containing 4 atoms), although it was not eliminated (unlike Ni-Cu alloys).The percentage formation of C4 hydrocarbons (butane, butene),butanal. 

In complete contrast with these developments, Kobosev approached the problem of supported metal catalysts from the viewpoint of atomic dispersion and from the viewpoint of the theory of active ensembles which bears his name. Over a period of years, Kobosev and co-workers investigated the behavior of catalysts containing very small amounts of supported metal. Many of their observations have been reviewed recently (2) and they inspired Poltorak to investigate very carefully dilute platinum catalysts on silica gel (3, 4). The studies from Poltorak s laboratory, originating from the desire to check the theoretical and experimental results of Kobosev, have led independently to many of the same conclusions reached by ourselves starting from the commercially available platinum catalysts for reforming (5). 

Early attempts to approach this problem were made by Kobosev and co-workers in the 1930s from the viewpoint of atomic dispersion and active ensembles. They studied the behavior of catalysts containing very small amounts of supported metal and were able to derive the number of atoms within the ensembles that were active for specific reactions (one atom for SO2 oxidation, two atoms for benzene hydrogenation, three atoms for ammonia synthesis, four atoms for acetylene oligomerization) (2a-c). These results as well as later ones have been reviewed by GiFdebrand (3). 

However, it is interesting that in two references the variation of TOF with d has been used to estimate a further characteristic of the active ensemble of atoms. Topsoe et al. (345) have reasoned that since N2 adsorption seems to be rate-determining for the synthesis of NH3 on Fe/MgO, it would be logical to measure the surface site concentration by N2 chemisorption. When a new TOF is defined in this way, it is found to be insensitive to particle size. Recall that we have classified this system as showing antipathetic structure sensitivity, with FE measured as usual on H2 chemisorption. 

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