Jn. Carstens et al., FACTORS AFFECTING THE CATALYTIC ACTIVITY OF PD ZRO2 FOR THE COMBUSTION OF METHANE/, Journal of catalysis, 176(1), 1998, pp. 136-142
The catalytic combustion of CH4 has been investigated over Pd/ZrO2 wit
h the aim of identifying how the state of oxidation of the Pd affects
the catalytic activity, particularly at low temperatures. It is found
that fully reduced Pd is inactive and that the activity increases at 5
33 K until a plateau in activity is achieved when 6-7 ML of PdO have f
ormed over the surface of the Pd crystallites. The maximum activity is
identical to that for fully oxidized Pd. Examination of the PdO forme
d under reaction conditions and during oxidation in O-2 at 533 K revea
ls some differences. Temperature-programmed reduction in CH4 and Raman
spectroscopy demonstrate that the oxide formed under reaction conditi
ons is largely crystalline PdO, whereas the oxide formed in O-2 is a m
ixture of amorphous and crystalline PdO. The crystalline form of PdO i
s found to reduce more readily than the amorphous form. It is observed
that the low-temperature CH4 combustion activity of PdO can be enhanc
ed by producing a small amount of metallic Pd on the surface of PdO. T
he metallic Pd is more effective than PdO for the dissociative adsorpt
ion of CH4. It is hypothesized that the products of CH4 adsorption on
Pd, H-s and CHx,s (x = 3-1.), rapidly diffuse to the Pd/PdO interface
and then reduce the PdO to Pd. The effects of metallic Pd cannot be su
stained, though, under steady-state conditions, because the metallic P
d is oxidized back to PdO. (C) 1998 Academic Press.