Comment on the “In situ IR studies on the mechanism of methanol synthesis from CO/H2 and CO2/H2 over Cu-ZnO-Al2O3 catalyst” by Wang et al

Abstract

Wang et al. [1] have recently reported an in situ IR investigation of the mechanism of methanol synthesis from CO/H2 and CO2/H2 mixtures over a Cu-based catalyst. Some IR bands were assigned by these authors to surface adsorbates such as formates and carbonates (Fig. 1(a)). These particular IR bands, as well as many others observed on this figure, appeared to be unusually narrow to be realistically those of species adsorbed on the catalyst. The full width at half-height of most of the narrow bands reported in Fig. 1(a) was lower than 6 cm. Such a narrow width contrasts with that of IR bands of typical carbonates and formates [2-4] observed at the surface of catalysts that can be wider than 100 cm. In fact, it is usually difficult to resolve the bands of carbonate and formate species. A closer inspection of the set of narrow bands observed by Wand et al. [1] reveals that those exactly match the rotovibrational bands associated with gas-phase water, which is given in Fig. 1(b), collected at the same resolution (i.e. 4 cm). This observation strongly suggests that all the narrow bands observed by Wang et al. [1] are due to the absorption of water molecules, from ambient air and/or present in the cell due to the release/formation of water during the temperature-programmed reaction. Similar narrow bands were also observed and assigned to formates and carbonates in the Fig. 5 reported by Wang et al. in their paper [1] and it appears again that those are all assignable to gasphase water. Truly carbonate species can actually be observed on the spectra reported by the authors [1], i.e. the very broad bands at ca. 1,550 and 1,370 cm (Fig. 1(a)), which were not indexed by the authors. In conclusion, we believe that the interpretation of the in situ IR data reported by Wang et al. [1] is partly flawed and the paper should be revised in part to ensure that the appropriate conclusions were reached.