Reverse Topotactic Transformation of a Cu–Zn–Al Catalyst during Wet Pd Impregnation: Relevance for the Performance in Methanol Synthesis from CO2/H2 Mixtures

The effect of palladium metal on the performance of a CuO–ZnO–Al2O3 catalyst is studied for methanol synthesis by hydrogenation of carbon dioxide. The dramatic decrease in the methanol yield (in mol CH3OH/h·gcat) seen for the Pd-containing catalysts is discussed in terms of formation, decomposition, and reconstruction of hydroxycarbonate precursors. The CuO–ZnO–Al2O3 mixed oxide, obtained by calcination (673 K) of a hydrotalcite-containing precursor, still contains interlayer CO32− anions as a result of incomplete decomposition. The presence of the remaining carbonate anion and the hydrothermal-like treatment during the Pd impregnation step allow the partial reconstruction of the original hydrotalcite-type structure. An improvement in the crystallinity of the copper oxide phase is also obtained. This improvement in crystallinity occurs during the hydrotalcite reconstruction and is a consequence of crystalline rearrangement of this oxidic phase. Such an effect gives rise to poorer copper dispersion and, consequently, to an apparent masking of the promoting effect of palladium. However, nitrous oxide chemisorption measurements allow the redefinition of the methanol yield per mole of exposed copper, a situation that clearly reveals the intrinsic promoting effect of palladium. The appearance of low-temperature reduction peaks involving Cu(II) species and the increase in the hydrogen consumption for the Pd-modified catalysts, both compared to the base CuO–ZnO–Al2O3 catalyst, indicate a hydrogen-spillover mechanism as being responsible for the intrinsic promoting effect of palladium.