A Review on Pd Based Catalysts for CO2 Hydrogenation to Methanol: In-Depth Activity and DRIFTS Mechanistic Study

Global warming, the environmental curse, created mainly by anthropogenic uses of fossil resources causing an excessive amount of CO2 emission in the earth's atmosphere. Scientists are focusing to utilize CO2 to produce value added chemicals, i.e. methanol, DME, formic acid, etc. to reduce the effect of this greenhouse gas (GHG) and also provide an alternative carbon source and carbon neutral pathway for valuable chemicals. Despite significant achievements so far on the conversion of CO2 to methanol via hydrogenation over Cu-ZnO-Al2O3 catalyst, palladium and palladium based bimetallic catalysts showed a superior activity (> 10% CO2 conversion) and selectivity (similar to 100%) to methanol over Cu based catalysts especially at low pressure (<= 30 bar) and low temperature (<= 250 degrees C). The alloying effect of Pd with the support ZnO, ZrO2, Ga2O3, etc. forming PdZn, PdZr2, PdGa species, which are identified as the main active phase of methanol synthesis. Also, reducible oxidic supports like CeO2, ZrO2, Ga2O3, etc. played important roles in adsorbing and activating CO2 as CO and or CO3- over the surface and hydrogenated to formate species, which has been regarded as the pivotal intermediate for methanol synthesis. Though there are challenges involving the costs of noble metal palladium, hydrogen production from renewable sources and carbon capture and storage (CSS) processes. There are several review articles on CO2 hydrogenation to methanol in the past few years but none of the existing review articles uniquely dealt with Pd-based catalysts. On this premise, this article presents a brief review comprising catalytic activity of Pd and Pd based bimetallic catalysts, effects of supports and promoters, reaction mechanism (DRIFTS studies) and perspectives on future researches necessary to achieve industrial acceptability of Pd-based catalyst for CO2 hydrogenation to methanol. [GRAPHICS]