Pd/SiO2 as an active and durable CH4 oxidation catalyst for vehicle applications

Abstract

Abstract The removal of CH4 is critical to address environmental concerns for developing natural gas vehicles (NGVs), because CH4 has a 20-fold-greater contribution to the greenhouse effect than CO2. Alumina supported Pd catalysts are widely used for CH4 oxidation due to their superior catalytic activity and durability compared to other CH4 oxidation catalysts. However, the continuous deactivation of Pd-based catalysts during vehicle applications needs further development of active and durable catalysts. Here, we report that Pd/SiO2 can be active and durable catalysts for CH4 oxidation in practically relevant condition via a simple reductive regeneration. CH4 oxidation light-off curves of freshly prepared Pd/SiO2 (air, 550\u2009°C) present higher activity than those of Pd/Al2O3, but severe deactivation was observed after hydrothermal aging (HTA, air with 10% H2O) at 850\u2009°C. However, X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and volumetric CO adsorption suggest that Pd/SiO2 has similar particle sizes as Pd/Al2O3 even after HTA, indicating that Pd/PdO particle sintering is not the origin of the deactivation of Pd/SiO2. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) shows that the CO was not adsorbed on Pd/SiO2 after HTA, whereas it was adsorbed on Pd/Al2O3 after HTA. These results demonstrate that the deactivation of Pd/SiO2 originates mainly from the blockage of the Pd/PdO surface by the SiO2 overlayer formed during HTA. The Pd/PdO surfaces are re-exposed by a simple H2 treatment at 500\u2009°C, resulting in CO adsorption on the Pd/PdO surface and regeneration for CH4 oxidation activity, which suggests that the Pd/PdO surface was re-exposed by a reduction treatment. This work also demonstrates that SiO2-supported Pd catalysts can be a good candidate for active and durable CH4 oxidation catalysts by using the proper regeneration protocols.