Catalytic ensemble structures

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]. 

The obtained optimally superimposed complexes are automatically annotated in terms of the complex composition homo- and hetero-multimeric receptors, catalytic metal ions as well as cofactors and their analogs are automatically identified based on the consistency of each of these features throughout the ensemble. Compositional and conformational differences between the individual ensemble structures are recorded. Where applicable, symmetry neighbors are generated and taken into account. 

Figure 13 Structures of PTPs include two important motifs, the P-loop that bears the cysteine nucleophile within the general signature motif (H/V)Cp<)5R(S/T), and the WPD-loop, which includes an important aspartic acid, a general acid-base catalyst. Substrate binding by the P-loop promotes a change of the WPD-loop conformation from an open, inactive to a closed, active conformation in which the aspartic acid completes the catalytic ensemble used for catalysis. The representation in this figure was created using PyMol from the PTP1B structures in apo-bound (PDB 2CM2) and inhibitor-bound (PDB 1BZJ) forms.

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. 

Since early in this century the concept of the active site in catalysis [1] has been a focus of attention in this area of chemistry. This was proposed to be that ensemble of surface atoms/reactants which is responsible for the crucial surface reaction step involved in a catalytic conversion. Since those days much work has been done in the area, which cites the concept of the active site. However, no such ensemble has been positively identified due to the lack of availability of techniques which could image such a structure, which is of atomic dimensions. 

The basic goal behind this approach is to find systems that perform the desired reaction without particular interest in the absolute structure of the active species. In an ensemble that possesses activity, there are likely many catalysts that are not active. The analogy to catalytic antibodies is made. Just as in the polyallylamine system reported, the identity and structure of catalytic antibodies is not determined. At this time, the authors are not interested in sorting out which species are active and which are not. Their stated goal is to find a system that catalyzes the desired reaction. 

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. 

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