Oxidation of formaldehyde over Pd/Beta catalyst

The role of catalyst support in the complete oxidation of HCHO has been investigated over the metal oxide- and zeolite-supported Pd catalysts. The 0.25Pd/Beta catalyst exhibited the highest intrinsic activity for the complete oxidation of HCHO among other monometallic catalysts, mainly due to the high HCHO adsorption capacity of Beta zeolite support and the fast surface reaction for HCHO oxidation on Pd as determined by TPD and TPSR studies, respectively. Formate and dioxymethylene have been identified as major reaction intermediates by an in situ FTIR study. These reaction intermediates are the most abundant surface species on the Pd/Beta catalyst during HCHO oxidation, so their oxidation is the rate determining step of the complete oxidation of HCHO. When Mn was incorporated into the 0.25Pd/Beta catalyst, the HCHO oxidation activity was further improved, especially in the low temperature region; HCHO was completely oxidized to CO2 over the 0.25Pd/20Mn/Beta catalyst at 40 °C and 50,000 h−1. The enhanced oxidation activity of the 0.25Pd/20Mn/Beta catalyst could be understood by a kinetic synergism between MnOx and Pd for the oxidation of HCHO. Oxygen adsorbs on and diffuses through MnOx along an oxygen concentration gradient to Pd, while Pd consumes oxygen via HCHO oxidation to create the oxygen concentration gradient in MnOx on the surface of the Beta zeolite. A possible reaction pathway has been proposed that could elucidate the enhanced HCHO oxidation activity over the bimetallic 0.25Pd/20Mn/Beta catalyst by the extended Mars–van Krevelen mechanism involving direct and indirect surface reactions over the bimetallic Pd–MnOx catalyst system.

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► 0.25Pd/Beta catalyst exhibits the highest HCHO removal activity among monometallic catalysts examined.
► HCHO is completely oxidized to CO2 at 40 °C and 50,000 h−1 when 20 wt.% of Mn is impregnated onto the 0.25Pd/Beta catalyst.
► The synergistic interaction between Pd and MnOx is the key parameter in the enhancement of the HCHO oxidation activity.