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Precipitation and impregnation are two common methods to synthesize heterogeneous catalysts. In this paper we compare the stability and activity of silica-supported catalysts prepared by precipitation or by impregnation, having comparable support morphology and copper weight loading. To obtain catalysts with comparable support morphology, plate-like silica was prepared from the precipitated catalyst. This porous, high surface area plate-like silica was subsequently impregnated with an aqueous copper solution and reduced in hydrogen. Catalysts characterization revealed that catalysts prepared by impregnation contained heterogeneities, present as particles significantly larger than the vast majority, whereas catalysts prepared by precipitation displayed a narrow particle size distribution. In particular the catalyst prepared by impregnation with a copper weight loading of 34 wt% displayed heterogeneities in the form of large particles, which were detrimental for both activity and stability in methanol synthesis from synthesis gas. [ABSTRACT FROM AUTHOR]

• ZnO x promotes the activity of Cu catalysts also in CO 2 -free syngas conversion. • Alternative products were obtained in CO 2 -free CO hydrogenation. • CO 2 boosts the syngas conversion and methanol selectivity for promoted Cu catalysts. • The methanol formation rate over unpromoted Cu catalysts is not influenced by CO 2. • Carbon-supported metal catalysts are valuable model systems. Methanol synthesis over Cu/ZnO/Al 2 O 3 is a key industrial reaction. Typically, a ZnO x promoter and CO 2 enrichment of the feed are applied to maximize the syngas conversion. However, understanding the effects of these additives on the performance of the Cu catalysts is obscured by the strong interaction between the ZnO x promoter and oxidic supports. Here, we use Cu nanoparticles on graphitic support to study the interplay between CO 2 concentration and ZnO x promotion. CO 2 enrichment enhanced the activity with an optimum at 3 vol% in the feed, but only if the ZnO x promoter was present, demonstrating the intricate interaction between the two. Interestingly, not only the activity but also the methanol selectivity was enhanced by CO 2 enrichment, and even further by the addition of ZnO x. Understanding not only the role of the individual components, but also the interaction between them, is important to design catalysts for processes with more flexible feed compositions. [ABSTRACT FROM AUTHOR]