The direct hydrogenation of CO2 into methanol is crucial for providing a means of CO2 fixation and a way to store cleanly produced hydrogen in a more energy-dense and transportable form. Here we have prepared two series of Pd/ZnO catalysts, both by immobilisation of PVA-protected Pd colloids and by Pd impregnation of PdCl2 to investigate structure activity relationships for direct CO2 hydrogenation. Very different performances were found for the different preparation methods, and the Pd loading and pre-reduction of the catalysts were shown to be important factors for optimising methanol yield. The crucial factor for high methanol yield is the formation of a Pd–Zn alloy, either during the reaction itself, or better by high temperature pre-reduction. The formation of the alloy greatly reduces CO production by the reverse water gas shift reaction. The catalysts prepared by sol-immobilisation were relatively stable to thermal treatment. In contrast, the impregnated catalysts were much less thermally stable, due to the presence of remnant chloride on the surface of the catalyst, which was absent for the case of sol immobilisation preparation. The results illustrate the importance of controlling the PdZn particle size and its surface structure for the catalysts to achieve high methanol selectivity (60%, the rest being CO) and conversion (11%) at 250°C and 20bar. Selectivity for sol-immobilised catalysts decreases from 60% at 3nm average diameter, to 20% at 7nm

The formation of PdZn bimetallic alloys on ZnO, TiO ₂ and Al ₂ O ₃ supports was investigated, together with the effect of alloy formation on the CO ₂ hydrogenation reaction. The chemical vapour impregnation (CVI) method produced PdZn nanoparticles with diameters of 3–6 nm. X-ray photoelectron spectroscopy and X-ray diffraction revealed the changes in the structure of the PdZn alloy that help stabilise formate intermediates during methanol synthesis. PdZn supported on TiO ₂ exhibits high methanol productivity of 1730 mmol kg _cat ⁻¹ h ⁻¹ that is associated with the high dispersion of the supported PdZn alloy.

Eschewing the common trend towards use of catalysts comprising of Cu, it is reported that PdZn alloys are active for CO₂ hydrogenation to oxygenates. It is shown that enhanced CO₂ conversion is achievable through the introduction of Brønsted acid sites, which promote dehydration of methanol to dimethyl ether. We report that deposition of PdZn alloy nanoparticles onto the solid acid ZSM-5, via chemical vapour impregnation affords catalysts for the direct hydrogenation of CO₂ to DME. This catalyst shows dual functionality; catalysing both CO₂ hydrogenation to methanol and its dehydration to dimethyl in a single catalyst bed, at temperatures of > 270 ^oC. A physically mixed bed comprising 5% Pd 15% Zn/ TiO₂ and H-ZSM-5 shows a comparably high performance, affording a dimethyl ether synthesis rate of 546 mmol kgcat^-1 h^-1 at a reaction temperature of 270 ^oC.