Chemical reactivity of supported gold: IV. Reduction of NO by H2

was 150 to 300°C 10 6 pn$ 6 100 Torr, and p!+/pHa = 1. The Au preparations were characterized by wide-angle X-ray scattering and transmission electron microscopy. No characteristic effect on the rate of reaction was observed by varying the Au precursor salt or the preparative method. There was an effect of the support on the reaction selectivity to Nt, $02~supported preparations showing the lowest and AllOo-supported preparations the highest value of the selectivity. The rate of the IIs-1~2 equilibration was fastest on Au supported on SiO,. It is argued that the different tendency of the supports employed to act as electron donor-acceptor with supported Au influences t,he electron density at the surface Au site. The interpretation is consistent with the relative acid-base ranking of the supports and of the precursor Au salt. An increased electron density at the Au site increases electron back donation from metal to adsorbed NO, weakening the N-O bond and facilitating the formation of Nz over that of NHS. This interpretation of the role of the support in modifying the electronic conditions at the Au site is empIoyed to suggest that the higher HZ-D, equilibration rate on Au-SiOe results from a more ionic hydrogen chemisarption bond. Suggestions on the morphoIo~ca1 and chemical nature of the Au which interacts with the support are advanced in the context of the highly heterogeneous morphology of the supported Au, as revealed by the characterization methods employed. The meaning of these findings in the framework of present ideas for high Nz selectivity in the NO reduction as well as the impact of the conclusions from the present study on other important reactions (CO + HZ, NZ + HZ) are pointed out.

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