Boundary Conditions for Promotion versus Poisoning in Copper-Gallium-based CO2–to–Methanol Hydrogenation Catalysts

Abstract

To establish descriptors for the activity of Cu-Ga-based CO2-to-methanol hydrogenation catalysts, we prepared via co-grafting followed by a reduction in hydrogen, a series of materials containing 3-6 nm Cu1-xGax alloyed nanoparticles with variable xGa. They display a volcano-type activity behavior, where methanol formation is promoted for xGa < 0.13, while above poisoning effects are observed. In catalysts that are prepared via different methods and differ in the degree of Ga reduction, only the fraction of Ga0 in the formed Cu-Ga alloy matters for the catalytic activity. We were therefore able to establish that the concentration of Ga0 in the Cu-Ga alloyed precatalyst is the main descriptor of methanol formation activity.

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol

“…These results indicated that the Ni–Ga alloy nanoparticles retained their structure and composition under CO 2 hydrogenation conditions, with no evidence for dealloying/additional alloying, in agreement with the study of Hejral et al using unsupported Ni–Ga nanoparticles . The behavior observed for Ni–Ga/SiO 2 contrasts what have been observed for other bimetallic catalysts (i.e., Cu–Zn, Cu–Ga, , Pd–Ga) synthesized via the same SOMC approach used in this work, where dealloying, even if partial, was always observed under CO 2 hydrogenation conditions. ,,,, Note that the absence of bulk dealloying was also observed for the Au–Zn/SiO 2 system (Au:Zn ca. 33:67) .…”

“…In that context, research efforts have been undertaken to improve Cu-based catalysts, as well as explore alternative metal-based catalysts such as Ni, Pd, and Au, ,, focusing on understanding the nature of the active sites and the role of additional metals/metal oxides, which is critical to allow for a knowledge-driven catalyst design. For example, Ga-promoted transition metal catalysts, such as Cu–Ga, − Ni–Ga, − and Pd–Ga, − have shown promising activity for the CO 2 hydrogenation to methanol. In these catalysts, Ga was present in the form of an alloy that in many cases ,,, coexisted with oxidized Ga species under reaction conditions.…”

“…Indeed, dealloying under CO 2 hydrogenation conditions has been reported for related bimetallic M–M′ systems (Cu–Ga, , Cu–Zn, and Pd–Ga), generating a mixture of both metallic M (Cu/Pd) and (partially) oxidic M′ (Ga/Zn) species. Similarly, the ratio of M/M′ in bimetallic catalysts has been shown to have a pronounced influence on the catalyst activity, transitioning from promoting to poisoning effects . In the specific case of Ni–Ga, the δ-Ni 5 Ga 3 phase has been proposed to be particularly active for CO 2 hydrogenation to methanol, putting forward questions related to the optimal Ni:Ga ratio as well as the stability and the role of the alloy and/or the oxide interface on methanol selectivity and formation rate.…”