A kinetic study of the reduction of nitrous oxide and nitric oxide by carbon monoxide on a platinum catalyst using steady-state bifurcation and forced composition cycling techniques.

摘要

The reduction reactions of N{dollar}sb2{dollar}O and NO by CO over a Pt/Al{dollar}sb2{dollar}O{dollar}sb3{dollar} catalyst have been studied using an external recycle reactor. The N{dollar}sb2{dollar}O+CO reaction was found to exhibit isothermal steady-state multiplicity at 461-520 K. The steady-state bifurcation behavior has been used to discriminate among four rival mechanisms. Only a kinetic model based on a three-step mechanism and invoking CO self-exclusion from the platinum surface could describe the observed behavior. The model was further tested using transient behavior produced by Forced square-wave cycling of CO and N{dollar}sb2{dollar}O concentrations in the feed. The phase angle between the two square-waves has a pronounced effect on the reaction-rate resonance. Time-average CO conversions as high as five times the steady-state conversion were attained during feed cycling. The model was further modified by incorporating a reaction rate enhancement effect, due to Pt surface phase transition (1 x 1 {dollar}leftrightarrow{dollar} hex), to describe both the transient behavior during feed cycling and the steady-state multiplicity.; The NO+CO reaction also exhibits isothermal steady-state multiplicity at 465-520 K. Generally, the selectivity towards N{dollar}sb2{dollar}O formation decreased with increasing NO conversion. For the high-conversion steady-states, the N{dollar}sb2{dollar}O selectivity decreased rapidly with a decrease in the feed (NO) {dollar}sb{lcub}rm o{rcub}{dollar}/ (CO) {dollar}sb{lcub}rm o{rcub}{dollar} ratio below a critical value of 1.5. It is shown that the N{dollar}sb2{dollar}O+CO reaction occurs to a significant extent (up to 50%) in the overall NO+CO reaction. The multiplicity behavior and the N{dollar}sb2{dollar}O selectivity have been described by a seven-step mechanism incorporating the CO self-exclusion effect. The NO+CO reaction also exhibits reaction-rate resonance during variable-phase feed composition cycling. Time-average CO and NO conversions of more than 20 times the steady-state conversions were obtained during feed cycling. The transient results seem to support the seven-step mechanism that was used to describe the steady-state multiplicity. The NO+CO reaction exhibits long-term transients during feed cycling. The number of cycles required to reach cycle-invariance was found to be strongly and inversely dependent on the ratio of the gas-phase capacitance to the surface capacitance.