Catalytic decomposition/regeneration of Pt/Ba(NO3)2 catalysts: NOx storage and reduction

The introduction of lean-burn engine technology has prompted the development of NOx storage-reduction (NSR) catalysts, which are currently based on a Pt/Ba(NO3)2/Al2O3 system. In this work a series of powdered catalysts based on this system were prepared and their reactivities examined with a pulsed-flow reactor, which was used to carry out temperature programmed desorption (TPD) experiments and to simulate the NSR reaction itself. TPD experiments reveal that Ba(NO3)2 is catalytically decomposed by the presence of Pt, at a significantly lower temperature than otherwise (by >200 K), with the extent of the decomposition being dependent on the amount of Pt loaded. The products formed from the decomposition depend on the oxidation state of the Pt; firstly N2 and N2O are evolved (formed from cracking of the NO produced via the Ba(NO3)2 decomposition), but as the adsorbed oxygen resulting from their formation builds up, NO and O2 appear and N2 and N2O are lowered. NOx storage has been shown to occur even at room temperature following Ba(NO3)2 decomposition from a 0.5 wt.% Pt/Al2O3 catalyst impregnated with 10 wt.% Ba(NO3)2. These experiments can be carried out in a reversible way. Simulations of the NSR reaction, using H2 and CO as reductants, indicate a remarkable difference between the two; CO appears to facilitate Ba(NO3)2 decomposition, but not NOx reduction, whereas H2 enables both to take place, with excellent conversion to N2. The difference between the two reductants in these experiments reflects the difference in binding between the two adsorbates and consequent Pt surface poisoning by CO.