Determining Structure‐Activity Relationships in Oxide Derived CuSn Catalysts During CO₂ Electroreduction Using X‐Ray Spectroscopy

Abstract

The development of earth‐abundant catalysts for selective electrochemical CO2 conversion is a central challenge. CuSn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide‐derived CuSn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods—X‐ray absorption spectroscopy for in situ study, and quasi in situ X‐ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0 under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types: the CO‐selective catalysts exhibit a surface Sn content of 13 at. % predominantly present as oxidized Sn, while the formate‐selective catalysts display an Sn content of ≈70 at. % consisting of both metallic Sn0 and Sn oxide species. Density functional theory simulations suggest that Snδ+ sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalysts.

A comparative spectroscopic study revealing why the CO2 electroreduction selectivity switches from CO to HCOO- at Cu-Sn and Cu-In based catalysts

“…This charge transfer from Sn/In into Cu led to partial positive charge on Sn/In atoms, indicated by the oxidized Sn and In species, and hence stabilizing the *COOH intermediate (CO-pathway). 17,24 Within the resolution of our XPS analysis we do not observe clear evidence of charge transfer, but rather both metals are f ully reduced on the surf ace. Thus, Cu-poor catalysts (HCOO-selective) is thought to enhance the HCOO-pathway by inhibiting the H-adsorption and stabilizing the *OCHO intermediate (HCOO-pathway), see in-situ SERS below.…”

“…Many other studies revealed a similar behavior f or Cu-Sn systems, where researchers reported Cu-Sn catalysts with high FE towards either CO or HCOO -. 9,17,22 For instance, Li et al 23 investigated CO2ER on Cu/SnO2 core-shell nanoparticles with various Sn shell thickness, finding the selectivity to strongly depend on said thickness, with thicker Sn shells showing Sn-like activity with high HCOOselectivity (FE ~90%), while thinner shell exhibited a high CO selectivity (FE ~93%). Moreover, our recently published work 17 reported a similar behavior f or Cu nanowires coated with ultrathin SnOx layers grown using atomic layer deposition, and detailed X-ray spectroscopic investigation revealed differing Sn speciation (i.e.…”

“…A variety of mechanistic hypotheses, such as alloying and electronic effects, have been proposed to explain these bimetallic synergetic effects, mostly centered around the crucial role of Cu-In and Cu-Sn interfaces in tuning the binding strength of the key intermediates (e.g., *COOH, *OCHO and *H) and consequently, CO and HCOOselectivity. 17,19,[23][24][25] For instance, DFT calculations attribute the selectivity shifts (f rom CO to f ormate) with increasing of In or Sn contents to the gradual weakening of the adsorbed *COOH intermediate (which leads to CO), concurrently with enhancing the adsorption of *OCHO intermediate (resulting in HCOO -). A plausible explanation of this observed shif t f rom carbophilic (*COOH, C-bound) to oxophilic (*OCHO, O-bound) adsorption modes could be the charge transfer from Sn or In to Cu sites resulting in localized positive charge regions.…”

The role of crystal facets and disorder on photo-electrosynthesis

“…[89][90][91] This includes exciting opportunities in microscopies, such as transmission electron and atomic force microscopies (TEM and AFM, respectively), as well as spectroscopies, such as IR, Raman and X-ray absorption, photoelectron and emission spectroscopies (XAS, XPS and XES, respectively). [92][93][94][95][96] Experimentally, one challenge that lies ahead of performing these advanced operando studies, will be to establish whether the observed surface changes result from the catalytic reaction or the measurement itself. For example, it has recently been shown that a chemical segregation of Bi in BiVO 4 crystals takes place during TEM measurements forming Bi nanoparticles on the photoanode surface.…”

CO2 electroreduction activity and dynamic structural evolution of in situ reduced nickel-indium mixed oxides

“…3,9,11 Bimetallic catalysts offer possible synergetic effects among different metals to yield optimal binding of CO 2 and the key intermediate CO and have been demonstrated as a successful strategy for tuning selectivity in Cu-based catalysts. [18][19][20][21][22] Such an approach can be adapted for the modulation of CO binding on other early transition metals; for instance, the strong CO binding on Ni can be modulated by combination with a weak CO 2 /CO binding surface like In. 9,11 Nickel-indium composites were previously reported by He and coworkers 23 as a potential bimetallic composition with decreased CO binding energy capable of producing CO with appreciable selectivity.…”