Abstract
AbstractMetallic Pd, under CO2 hydrogenation conditions (>175 °C, 20 bar in this work), promotes CO formation via the reverse water gas shift (RWGS) reaction. Pd‐based catalysts can show high selectivity to methanol when alloyed with Zn, and PdZn alloy catalysts are commonly reported as a stable alternative to Cu‐based catalysts for the CO2 hydrogenation to methanol. The production of CH4 is sometimes reported as a minor by‐product, but nevertheless this can be a major detriment for an industrial process, because methane builds up in the recycle loop, and hence would have to be purged periodically. Thus, it is extremely important to reduce methane production for future green methanol synthesis processes. In this work we have investigated TiO2 as a support for such catalysts, with Pd, or PdZn deposited by chemical vapour impregnation (CVI). Although titania‐supported PdZn materials show excellent performance, with high selectivity to CH3OH+CO, they suffer from methane formation (>0.01 %). However, when ZnTiO3 is used instead as a support medium for the PdZn alloy, methane production is greatly suppressed. The site for methane production appears to be the TiO2, which reduces methanol to methane at anion vacancy sites.
Highlights
Chemical vapour impregnation (CVI) was employed as the synthetic methodology because it presents several advantages over conventional wet preparation routes. It allows the preparation of highly dispersed small nanoparticles, it avoids contamination from the use of solvents or ligands, avoids chlorine contamination, which can be detrimental for CO2 hydrogenation activity and it allows the easy preparation of bimetallic catalysts.[26,27,28]
We have shown that the synthesis of PdZn alloys prepared by chemical vapour impregnation (CVI) using TiO2 as support gave improved PdZn dispersion compared to the use of ZnO and Al2O3, and resulted in improved methanol production rates.[29]
To determine the active sites responsible for CH4 formation Pd/TiO2, Pd/ZnO, Pd/ZnTiO3, PdZn/TiO2 and PdZn/ZnTiO3 catalysts were prepared by CVI as described above
Summary
CH4 is commonly reported as a minor side product on PdZn alloy catalysts.[13,15,21,22] not much attention is paid to CH4 because of its low selectivity (less than 1 %), which can lead to the misinterpretation that it is not important and that CH4 formation is inherent to PdZn catalysts. This could limit applications of PdZn based catalysts in a CH3OH synthesis plant operating with captured CO2 and green hydrogen.
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