COMBUSTION OF METHANE OVER PALLADIUM ZIRCONIA DERIVED FROM A GLASSY PD-ZR ALLOY - EFFECT OF PD PARTICLE-SIZE ON CATALYTIC BEHAVIOR/

A Pd25Zr75 glassy metal ahoy has been activated by controlled oxidatio n in air, resulting in highly active catalysts for the catalytic combu stion of methane. The fully oxidized ahoy was reduced in hydrogen at d ifferent temperatures prior to catalytic investigations resulting in a catalyst containing metallic palladium and zirconia. The reduced mate rials showed marked differences in BET surface area and specific surfa ce area of palladium, both decreasing with increasing reduction temper ature. The loss of surface area was accompanied by an increase of the palladium crystallite size as evidenced by XRD line broadening measure ments. Catalysts reduced at low temperatures were faster reoxidized th an catalysts reduced at high temperatures. Changes in the chemical beh avior of palladium oxide were indicated by different decomposition beh avior and reducibility with hydrogen and methane. Kinetic measurements revealed profound differences in catalytic activity for catalysts in dependence of the reduction temperature. Strong correlations between t he catalytic activity and the crystallite size as well as the reducibi lity of palladium oxide with methane were found for catalysts reduced at different temperatures. The correlation between the reduction with methane and the catalytic performance is explained by a redox mechanis m involving palladium oxide. The influence of the particle size on the catalytic activity is attributed to a strong interaction of the Pd-co ntaining phases (Pd, PdO) with zirconia. This (support) effect is sugg ested to become prominent with decreasing Pd-particle size. (C) 1997 A cademic Press.