REDUCTION OF NITRIC-OXIDE WITH HYDROGEN ON CHROMIA ZIRCONIA CATALYSTS

The catalytic activity of CrOx/ZrO2 (Cr content, 0.26 to 1.80 atoms nm-2) for the reduction of nitric oxide with hydrogen was investigated in a flow apparatus at 473 to 773 K and in a circulation apparatus at 453 to 593 K. Before catalysis, the average oxidation state of Cr, n, was controlled by redox treatments with oxygen at 773 K (n=5.5) or hydrogen at 623 K (n=2.5). No ammonia was detected at any temperature, N2, N2O and H2O being the only reaction products. The catalytic activity of all samples was stable with time-on-stream. The selectivity to nitrogen increased with a rise in reaction temperature (90% at 773 K in flow experiments). The activity of the ZrO2 support is one to two orders of magnitude lower than that of CrOx/ZrO2 (ZC) samples. In ZC samples, the activity was proportional to the chromium content. At a given temperature, irrespective of the chromium content, turnover frequencies, calculated with respect to the total chromium (NCr/molecules s-1 atom-1) have almost the same value. This suggests that the active sites are mononuclear chromium species. The surface species formed on nitric oxide adsorption or in the presence of NO+H2 reactants were characterised by Fourier transform IR spectroscopy. The chromyl species (CrV, mononuclear on ESR evidence) and chromates were detected in the oxidised ZC samples. In the presence of NO or NO+H2, nitrosyls of CrII and CrIII, nitrites, nitrates and water were identified. On the basis of the thermal stability and reactivity with hydrogen of CrIII dinitrosyls, these species may well play an important role in the NO+H2 reaction.