The catalytic reduction of CO2 is an effective way to control industrial CO2 emissions and receives special attention nowadays due to the urgent environmental need to mitigate global warming and climate change. During CO2 reduction by H2, CO2 methanation (CO2 + 4H2 ↔ CH4 + 2H2O) is thermodynamically favored at low temperatures (200–500 o C), while higher temperatures favor CO production through the reverse water gas shift reaction (RWGS), (CO2 + H2 ↔ CO + H2O), turning the catalytic process to a syngas production rather than a methanation one. Herein the CO2 hydrogenation reaction is investigated over Ni catalyst supported on mesoporous MCM-41 silica. Two different synthetic methods, specifically one-pot synthesis and impregnation were followed for the preparation of the Ni-based catalysts of this study. Both support and catalysts were thoroughly characterized by several techniques and the catalysts were evaluated in the CO2 methanation reaction at the temperature interval 200-650 oC. The CO2 methanation efficiency of Ni catalysts was found to be strongly dependent on the catalyst preparation method used: One-pot synthesis provided catalysts almost completely ineffective for methanation, but very effective for the reverse-water–gas-shift (RWGS) reaction producing syngas at low temperatures. In striking contrast, compositionally identical Ni catalysts prepared by impregnation were efficient in CO2 methanation. This was found to be mainly related to the size of Ni crystallites formed during catalyst synthesis. Specifically, large Ni particles (formed when using one-pot synthesis method) favor CO production, while small Ni nanoparticles (formed when using impregnation method for catalyst synthesis) favor methanation. According to the present findings, the Ni-MCM-41 system can be used as a simple and effectively controllable system for the preparation of versatile catalysts that can be directed to produce either syngas or methane.
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