LHHW and PSSA kinetic analysis of rates and adsorbate coverages in CO/H2/C2H4 reactions on Mn-Rh/SiO2

The kinetics, adsorbate residence times and coverages, and isotopic transient responses for propionaldehyde formation from CO/H-2/C2H4 reaction have been studied over 4 wt% Mn-Rh/SiO2 (Mn:Rh = 0.1:1) catalyst at 513 K and 0.1 MPa, Reaction rates were measured under differential conditions; adsorbate residence times and coverages, and transient responses of C-13 propionaldehyde formation were measured by a steady-state isotopic transient method coupled with in situ infrared spectroscopy. Both Langmuir-Hinselwood-Hougen-Watson (LHHW) and pseudo-steady-state-approximation (PSSA) approaches mere employed to develop rate laws (i.e., kinetic models) and isotherm equations for adsorbate coverages to fit the rate and coverage data. Although the LHHW kinetic model provides a good fit of rile rate data, its adsorbed acyl (*C2H5CO) isotherm equation fails to describe the dependence of adsorbed acyl coverage on H-2 and CO partial pressures. Use of the PSSA approach without assumption of a sole rate-determining step (RDS) resulted in a rate law and an acyl isotherm equation which describe both the kinetics and adsorbate coverages behaviors with high accuracy. The PSSA analysis suggests that both CO insertion into adsorbed ethyl species and hydrogenation of adsorbed acyl species are kinetically significant steps for propionaldehyde formation. The failure of LHHW isotherm equations for filling the coverage data is due to lack of a sole RDS. The kinetics, adsorbate residence times and coverages, and transient responses of propionaldehyde formation on Mn-Rh/SiO2 ere compared with those on Rh/SiO2 to unravel the effect of Mn promotion on the reaction. This study demonstrates that measurement of transient responses for product formation and adsorbate residence times and coverages provides essential in formation for verification of kinetic models with mechanistic significance. (C) 1998 Academic Press.