Extraordinarily effective promotion by sodium in emission control catalysis:: NO reduction by propene over Na-promoted Pt/γ-Al2O3

The catalytic activity and selectivity of Pt dispersed on gamma-Al2O3, for the reduction of NO by propene is promoted extremely strongly by sodium in the temperature range 470-770 K. Remarkable and unprecedented effects on both activity and selectivity are found. Rate increases by two orders of magnitude are achievable, while the selectivity towards to N-2 is improved from similar to 20% over the Na-free unpromoted Pt catalyst, to >95% over the optimally Na-promoted catalyst. The Na-free Pt is almost ineffective for the NO + C3H6 reaction over the gas composition and temperature range of this investigation. The promoting effect is due to a sodium-induced increase in the strength of Pt-NO chemisorption relative to propene on a catalyst surface predominantly covered by propene and its fragments. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which process is proposed as the critical reaction initiating step. (C) 1999 Elsevier Science B.V. All rights reserved.