Platinum oxide, transport model

Other groups, although accepting many aspects of the platinum oxide transport model, also concluded that it cannot explain all the observations. For... 

One caveat pertains to the platinum oxide transport model It does not appear to be able to explain the differences in metal weight loss during ammonia oxidation and hydrogen cyanide synthesis by the Andrussow process. In the Andrussow process a mixture of methane, ammonia, and air is used to maintain a high temperature (1200°C) while generating hydrogen cyanide. Alternative processes require energy input, because HCN synthesis is an endothermic reaction. Thus, in both ammonia oxidation and HCN synthesis, platinum or alloy... 

In sum, according to one postulated mechanism of catalytic etching, particularly of platinum, etching occurs via the transport of volatile metal oxide species. It has always been understood, even by the model s proponents, that it does not fully explain the observations. It has been repeatedly suggested that other volatile species may exist, or that other unaccounted-for processes are responsible for the observed behavior. Indeed, as discussed below, several recent studies suggest that other volatile species and other unaccounted-for processes may very well explain some or all catalytic etching. 

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

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