Cobalt-Induced PdO Formation in Low-Loading Pd/BEA Catalysts for CH₄ Oxidation

Abstract

Palladium (Pd)/zeolite-based catalysts have shown great promise in low-temperature CH4 oxidation reactions. However, improving the low-temperature performance of Pd/zeolite catalysts while simultaneously decreasing Pd usage remains a challenge. Herein, we demonstrate that incorporation of cobalt (Co) into 0.5 wt % Pd/BEA (Pd (0.5)/BEA) can substantially boost the CH4 oxidation performance. In particular, increasing Co loading from 0 to 1 wt % led to a continuous improvement in the CH4 oxidation activity, with T50 (temperature at which 50% conversion is achieved) decreasing from >500 °C over Pd (0.5)/BEA to 352 °C over Pd(0.5)Co(1.0)/BEA. Moreover, the CH4 oxidation reaction rate of Pd(0.5)Co(1.0)/BEA at 250 °C was 77% greater than that of Pd(1.0)/BEA. Experimental evidence from CH4-temperature programmed reduction, CO diffuse reflectance infrared Fourier transform spectroscopy, H2-temperature programmed reduction, O2-temperature programmed desorption, high-angle annular dark field-scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and apparent activation energy studies suggested that the promotion effect of Co incorporation is attributed to the formation of highly active PdO, instead of less active ionic Pd. Stability tests over Pd(0.5)Co(1.0)/BEA showed comparable CH4 oxidation activity to Pd(1.0)/BEA and slightly improved H2O resistance. Density functional theory calculations revealed that Co is more stable than Pd at the ion-exchange sites (Al sites) of BEA zeolites. Co being more stable than Pd at the ion-exchange sites leads to fewer ion-exchange sites available for Pd and thus less ionic Pd but more PdO nanocluster formation over bimetallic PdCo/BEA catalysts.