The chemistry and catalysis of the water/toluene reaction 2. The role of support and kinetic analysis

The influence of support on the activity of rhodium for the water/toluene reaction and the H2/toluene reaction is investigated. It is shown that an effective catalyst for water/hydrocarbon reactions is bifunctional in the sense that both metal and support provide activation sites for reactants. In the selective steam dealkylation of toluene, as in total steam reforming reactions, the support significantly influences the catalyst's specific activity when based on the metal surface area. For example, in the steam dealkylation reaction the rate of benzene formation over a RhSiO2 catalyst is only 137th the rate over a RhAl2O3 catalyst. The absence of support leads to an even further drop in the specific activity of rhodium, as a rhodium black catalyst is only 1/200th the activity of the RhAl2O3 catalyst. In contrast the support has no influence on the specific activity of rhodium for the H2/toluene (hydrodealkylation) reaction. A Langmuir-Hinshelwood approach is used to describe the kinetics of the water/toluene reaction in terms of a surface reaction between activated water species (from support sites) and activated toluene (on metal sites for Rh and the other Group VIII metals investigated). This approach leads to a simple power-law rate expression of the form: r = kPTolnPH2O(1 − n)2. The observed kinetic parameters reported in Part I of this series of papers agree well with those predicted from this rate expression.