Platinum structure sensitivity

Platinum Nanoclusters Size and Surface Structure Sensitivity of Catalytic Reactions... 

Housmans THM, Wonders AH, Koper MTM. 2006. Structure sensitivity of methanol electrooxidation pathways on platinum An on-hne electrochemical mass spectrometry study. J Phys ChemB 110 10021 10031. 

Clavilier J, Feliu JM, Aldaz A. 1988. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J Electroanal Chem 243 419-433. 

The hydrogenation of butadiene is structure-sensitive on Pd and Rh but lacks particle-size dependence in the case of platinum. The strong complexation of the diene to atoms of low coordination number is a possible explanation for this phenomenon where it occurs37,38. 

T. H. M. Housmans, A. H. Wonders, and M. T. M. Koper. Structure sensitivity of methanol electro-oxidation pathways on platinum An online electrochemical mass spectrometry study. Journal of Physical Chemistry B 110 (2006) 10021-10031. 

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

The voltammograms are unique for each basal plane and are often used as a diagnostic tool for the identification of primary Pt-surface sites [2-6,54]. The structure-sensitivity of the voltammogram serves as a means to characterize the preferential orientation of a given sample [55]. The voltammogram for platinum nanoparticles obtained by different preparation procedures is shown in Figure 6.13. The difference in voltammetric behavior displays the influence of preparation procedures on the fraction of (100), (110) and (111) atomic sites on the surface of the nanoparticles. 

In a series of studies, the dehydrogenation and hydrogenolysis of cyclohexane was studied on both the stepped and low Miller index (111) crystal faces of platinum at a surface temperature of 300°C and a hydrogen to cyclohexane ratio of 20 1. While the rates on the stepped and low Miller index surfaces were not very different for the formation of benzene and hexane, the formation of cyclohexene was very structure sensitive, its rate being 100 times greater on the stepped surface than on the (111) crystal face. In Table III mrnnare the initial turnover numbers for the various reactions at low... 

We have been able to identify two types of structural features of platinum surfaces that influence the catalytic surface reactions (a) atomic steps and kinks, i.e., sites of low metal coordination number, and (b) carbonaceous overlayers, ordered or disordered. The surface reaction may be sensitive to both or just one of these structural features or it may be totally insensitive to the surface structure, The dehydrogenation of cyclohexane to cyclohexene appears to be a structure-insensitive reaction. It takes place even on the Pt(l 11) crystal face, which has a very low density of steps, and proceeds even in the presence of a disordered overlayer. The dehydrogenation of cyclohexene to benzene is very structure sensitive. It requires the presence of atomic steps [i.e., does not occur on the Pt(l 11) crystal face] and an ordered overlayer (it is poisoned by disorder). Others have found the dehydrogenation of cyclohexane to benzene to be structure insensitive (42, 43) on dispersed-metal catalysts. On our catalyst, surfaces that contain steps, this is also true, but on the Pt(lll) catalyst surface, benzene formation is much slower. Dispersed particles of any size will always contain many steplike atoms of low coordination, and therefore the reaction will display structure insensitivity. Based on our findings, we may write a mechanism for these reactions by identifying the sequence of reaction steps ... 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

The catalytic reduction of Cu2+ by hydrogen is faster on small platinum particles since the turn over number increases from 0.37 Cu2+ ions reduced per accessible platinum atom per hour to 3.0 when the platinum dispersion increases from 11% to 54% [8], Thus the catalytic hydrogenation of Cu2+ ions is a structure-sensitive reaction occurring preferentially on small platinum particles. 

Molecular beam scattering is also a powerfiil tool for probing the structure-sensitive nature of reactive (dissociative) scattering events. In Figure 27 the production of HD from the surface reaction of a mixed H2 D2 molecular beam is plotted versus incident angle for two crystal faces of platinum. The close-packed Pt(l 11) surface shows a low reaction probabihty... 

In this same series of reactions it was also found that C-C bond cleavage took place most readily over the 111 face catalyst (A in Fig. 3.2) and to a considerably less extent on the others. This reaction, which is stmcture sensitive, obviously teikes place over ensembles of face atoms. It was also found > 8 that the dehydrocyclization of hexane to benzene, another structure sensitive reaction, took place four times faster on the 111 face of a platinum crystal than on one cleaved to expose the 100 face. The hydrogenolysis of ethane, on the other hand, was found to be significantly faster on Ni (100) than on Ni (111).22 Thus, it is not sufficient merely to define a site as an ensemble of face atoms the orientation of the atoms must also be specified. 

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