The oxidative dehydrogenation of methanol at the CuPd[85 : 15]{110} p(2x1)and Cu{110} surfaces: Effects of alloying on reactivity and reaction pathways

The adsorption and selective oxidation of methanol to formaldehyde over CuP d[85:15]{110}p(2 x 1) and Cu{110} has been investigated using a thermal mol ecular beam. Methanol oxidation on the alloy surface appears, at first sigh t, to be very similar to that seen on Cu{110}: the shapes of the methanol s ticking curves and product evolution curves are very similar to those seen on Cu{110}. It is found, however, that temperature dependence of the sticki ng coefficient for methanol varies in a manner opposite that seen for Cu{11 0}; at temperatures below similar to 400 K the reaction stoichiometry is co nsistently lower than seen for Cu{110}; above 400 K in the 1/4ML O/CuPd[85: 15]{110}p(2 x 1) case this stoichiometry is seen to increase with temperat ure. Preoxidation of the surface to 1/2ML oxygen coverage results in behavi our that is more Cu like with no obvious increase in reaction stoichiometry at high (similar to 400 K) temperature; in this case an increase in the ne t stoichiometry of reaction only occurs at lower surface temperatures. In b oth 1/4ML and 1/2ML cases the rate of reaction of methanol is found to be f aster on the alloy surface than on Cu{110}. The above differences in behavi our are rationalised in terms of an alloy-induced destabilisation of the su rface methoxy species of around 2.5 kJ mol-l and a thermally induced Pd seg regation during beam reactions conducted at above similar to 400 K, The app arent differences in behaviour between the 1/4ML and 1/2ML O cases on the a lloy surface are also discussed in terms of the microscopic manner in which the methanol oxidation reaction is known to proceed on Cu{110}. (C) 1999 A cademic Press.