CORRELATIONS BETWEEN SUBSTITUENT EFFECTS FOR SURFACE-REACTIONS AND CATALYTIC REACTIONS

Minor changes or substitutions in the structure or composition of reac tants can have profound effects on reaction kinetics. The effects of s ubstituents on surface and catalytic reaction kinetics can be understo od through the use of semi-quantitative empirical correlations such as the Hammett relation. These correlations provide a basis for comparis on of reaction kinetics in varying environments. For example, comparis ons can be made between reaction kinetics measured on single crystals under ultra-high vacuum (UHV) conditions and reaction kinetics measure d on catalysts at high pressures. Doing so offers a means of translati ng insights obtained from studies of surface chemistry into the field of catalysis. One such insight is into the nature of the transition st ates for heterogeneously catalyzed reactions. An example discussed in this article compares beta-hydride elimination of alkoxides on Cu(111) with alcohol dehydrogenation over ZnO-Cr2O3. In both systems inductiv e substituents (R) decrease the reaction rate suggesting that the tran sition state for the rate limiting step in both reactions has an elect ron deficient carbon atom(RCdelta+... H). A second example, phenyl cou pling on the AE(111) surface exhibits an increase in rate due to induc tive substituents. This observation is consistent with observations of homogeneously catalyzed phenyl coupling described in the literature. These comparisons demonstrate the potential for using Hammett correlat ions measured in surface science studies to understand important issue s in catalysis. (C) 1998 Elsevier Science B.V.