Pd catalysts for total oxidation of methane: Support effects

• Oxidation activity of Pd catalysts increases with increasing density of surface Pd*–PdO site pairs.
• The rate constant's pre-exponent provides a relative measure of site pair density.
• Site pair density—and activity—is low when the support stabilizes PdO.

We prepared a series of Pd catalysts for total oxidation of methane on different supports: Al2O3, ZrO2–CeO2 and CeO2. We characterized the catalysts’ structures by XRD and their surface chemistries by temperature programmed reduction (TPR) and oxidation (TPO); and measured their activities for methane oxidation in temperature programmed reaction experiments (TPRx). By fitting a kinetic model to the TPRx results, we extracted apparent activation barriers (Eapp) and pre-exponents (Aapp) for methane oxidation over each of the catalysts. Activity correlates strongly with Aapp, which we interpret as a relative measure of the density of PdO–Pd* (* is an O-vacancy) site pairs. TPR results support this view: PdO in low activity (low Aapp) catalysts, such as Pd/CeO2, is relatively resistant to reduction, meaning that formation of the Pd* partner of the site pair is difficult—and the number of site pairs that can form is low.

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