Interaction of Pd with Oxygen and its Effects on Activity

Burch and Urbano have attempted to correlate the combustion activity in pulse experiments with the rate and extent of oxygen uptake at 300 °C for 4% Pd/ AI2O3 catalysts exposed to various pretreatments. The shape of the oxygen uptake curves as a function of time was dependent on the type of pretreatment performed on the catalysts. However, a common characteristic in all cases was that the initial oxygen uptake was much faster than after a monolayer had been completed. As a 

A quantitative comparison between these two sets of experiments cannot be made since they were performed under very different conditions, one under static 10 torr O2, the other after the injection of a pulse containing 1% CH4 and 10% O2. However, a qualitative comparison of the shapes of the two curves implies important differences. The most important one is that while the oxygen uptake curve presented a clear break when a monolayer was completed, no break was present in the conversion curve From this behavior the authors have concluded that chemisorbed oxygen was not the active species. They proposed that oxidized Pd is much more active and that the activity continuously increases with the extent of oxidation until a plateau is reached after formation of several oxygen monolayers. 

In a more recent study, Carstens et al. have examined the relationship between the methane conversion and the fraction of oxidized Pd by simultaneously measuring the extent of oxidation and the activity of a pre-reduced Pd/ Zr02 catalyst. The extent of Pd oxidation was determined by TPR (by CH4) at various points during the activation period after rapidly quenching from the reaction temperature of 500 °C. The instantaneous methane conversion was zero until the extent of Pd oxidation was 0.05, then it increased almost linearly with the fraction of oxidized Pd until a saturation point was reached at an oxidation extent of 0.35, which corresponds to about 6-7 ML. After this point, the extent of Pd oxidation continued to increase while, in agreement with Burch and Urbano, the catalytic activity remained unchanged. 

A few surface science studies have investigated the structure of adsorbed oxygen on Pd at temperatures relevant to the methane oxidation reaction. For example, it has been found that oxygen dissociatively adsorbs on Pd (100) and occupies the four-fold hollow sites below 0.25 monolayers, forming an ordered p 

Although the explanations of the role of oxygen in the methane combustion reaction are diverse, a common concept that emerges in most studies is the realization that different forms of oxygen can be present in the catalyst and that they have different reactivities. The following sections analyse the methodology that has been used to characterize the various Pd-O species and their role in activity. 

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