Active sites ensembles

Of particular interest are oxidations of unsaturated alcohols, for example, oxidation of cinnamyl alcohol to cinnamaldehyde,74,75 and special promoters have been added to increase selectivity (Fig. 6.13).75 Although the functions of these promoters are still not fully undestood, some authors attribute their increased selectivity to physical blocking of reaction sites. This blocking reduces the size of the active site ensemble and suppresses the tendency for alcohols to strongly adsorb and dissociate on Pt.75... 

Bi does not adsorb hydrogen, thus a Bi/Pt coverage can be calculated from the hydrogen chemisorption data. It is seen in Figure 2 that there is an excellent correlation between the Bi-coverage of Pt and the rate of 1-phenylethanol oxidation. It seems that the hydrogen chemisorption ability of Pt or the size of active sites ensembles has to be minimized to avoid deactivation. There are indications in the literature that the suppression of hydrogen sorption on a Pt electrode can eliminate the poison formation (20). 

This contribution was aimed to review the role of metal ion - metal nanocluster active site ensembles (MIMNES) in different heterogeneous catalytic reactions. MIMNES are stabilized at a given boundary layer. These species have significantly higher catalytic activity than supported metal nanoclusters. In this review, the formation, the characterization and the catalytic activity of different MIMNES type active sites has been described and discussed. Results obtained in our laboratory were compared with those published earlier or recently in different model reactions. 

In cases in which the active site structure is known, an alternative approach would be geometry optimization and calculation of descriptors for the effector molecule-active site ensemble. Docking of the effector molecule in the active site can be done manually using computer-generated structures (Murcia et al. 2006 Huey et al. 2007 Weber et al. 2006 Tucinardi et al. 2007). 

In order to calculate 3D descriptors, the analyzed molecule is placed into a regular, virtual, three-dimensional network of points. The 3D descriptors are basically characteristics of some physical quantities, calculated in the points of the network. The value of the 3D descriptors depends on the position in space of the considered point and on the characteristics of the atoms in certain areas of the molecule. When a group of molecules is analyzed, the network of points includes the package of overlapped molecules. The methods used to overlap molecules are highly diverse. For example, molecules can be overlapped on the common skeleton if present. Other methods overlap the vectors of the dipole moments of the molecules in the package. The value of the 3D descriptors is influenced by the method used to overlap the molecules and by the distance between the network points. A different approach is to use the molecules in the package in their position within the effector-active site ensemble if such position can be determined. 

In our opinion, the increase in selectivity caused by adsorbed Cu may be explained by geometric effects. By coverage of the active Pd surface with inactive Cu, the number and size of active site "ensembles" decrease, which -beyond a certain limit - will lead to significant change in the selectivity (ref.26). 

The Holy Grail of catalysis has been to identify what Taylor described as the active site that is, that ensemble of atoms which is responsible for the surface reactions involved in catalytic turnover. With the advent of atomically resolving techniques such as scanning tunnelling microscopy it is now possible to identify reaction centres on planar surfaces. This gives a greater insight also into reaction kinetics and mechanisms in catalysis. In this paper two examples of such work are described, namely CO oxidation on a Rh(llO) crystal and methanol selective oxidation to formaldehyde on Cu(llO). 

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 concept of site isolation is important in catalysis. On metal particles one usually assumes that ensembles of metal atoms are necessary to activate bonds and to accommodate the fragments of molecules that tend to dissociate or to recombine. We present here three examples of such effects the dehydrogenation of decane into 1-decene, the dehydrogenation of isobutane into isobutene and the hydrogenolysis of acids or esters into aldehydes and alcohols. In most cases the effect of tin, present as a surface alloy, wiU be to dilute the active sites, reducing thereby the yield of competitive reactions. 

---