Structural Evolution of Ga–Cu Model Catalysts for CO₂ Hydrogenation Reactions

Abstract

We studied the initial stages of Ga interaction with the Cu(001) surface and environment-induced surface transformations in an attempt to elucidate the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional (“monolayer”) Ga oxide islands uniformly covering the Cu surface. The surface morphology and the oxidized state of Ga remain in H2 as well as in a CO2 + H2 reaction mixture at elevated pressures and temperatures (0.2 mbar, 700 K). The results indicate that the Ga-doped Cu surface under reaction conditions exposes a variety of structures including GaO x /Cu interfacial sites, which must be taken into account for elucidating the reaction mechanism.

Dual Experimental and Computational Approach to Elucidate the Effect of Ga on Cu/Ceo2–Zro2 Catalyst for Co2 Hydrogenation

“…In addition to the finely dispersed Cu and Ga intermetallics (Figures2 and S5), Ga x O y layers appeared around the copper metal particles, consistent with previous studies [9]. This morphology (Figures2 and S5) was more prominent in the Ga 5 Cu 10 catalyst than in the Ga 7.5 Cu 7.5 catalyst, indicating a more uniform mixing of the Cu and Ga species in the latter.…”

“…Oxygen species bind more strongly to Ga 5 Cu 10 than to Cu, whereas C-bound species bind more strongly to Cu (111) than to Ga 5 Cu 10 (see Supplementary Information), which is possibly because oxygen-gallium bonds are significantly stronger than oxygen-copper bonds. [9] In contrast, the carbon-gallium and carbon-copper bond strengths were very similar.…”

“…[7,8] Gallium is an interesting promoter as it combines with copper to form an intermetallic material that remains stable in reaction conditions. [9] In another study, a Zn-promoted Cu catalyst produced the highest amount of active intermediates for CO formation at all operating temperatures (220 °C-280 °C), followed by Cu-Ga and finally by the unpromoted catalyst. [10] Promoters modify the active sites and drive the formation of CO and methanol by creating new sites for the adsorbed active intermediates.…”

The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis

“…Interestingly, this result resembles the residual gallium oxide found in CuGa catalysts under the highly reducing conditions of the thermal CO 2 hydrogenation reaction as long as a few ppm of oxygen were present. 59 Reports by Popova et al dating back to the 1970s claim that even after Ga activation (i.e., at voltages more negative than the reduction peak) a "residual film" of different chemical nature persists on the surface and increases the HER overpotential compared to Ga free of the surface oxide. 60,61 Having acquired this new knowledge, questions regarding the structure and the role of the oxide layer on the Ga NPs during the electrocatalysis arise.…”

Observation and Characterization of Dicarbonyls on a RhCu Single-Atom Alloy

“…As shown by the TPD results in Figure 1A, we observe the desorption of CO from the c(2 × 2) phase on Cu(100) at <180 K. 32 These results are consistent with previous reports as well as our controls on Rh-free Cu(100) presented in Figure 1A (black trace). 32,33 At higher temperatures, two features, which are found only in the presence of Rh, are observed at ∼265 and ∼440 K. The high-temperature feature at ∼440 K corresponds to CO bound in a monodentate configuration to the isolated Rh atoms. 17 However, the feature at ∼265 K has never been observed before.…”