Direct Evidence of Subsurface Oxygen Formation in Oxide‐Derived Cu by X‐ray Photoelectron Spectroscopy

Abstract

Subsurface oxygen has been proposed to be crucial in oxide‐derived copper (OD‐Cu) electrocatalysts for enhancing the binding of CO intermediates during CO2 reduction reaction (CO2RR). However, the presence of such oxygen species under reductive conditions still remains debated. In this work, the existence of subsurface oxygen is validated by grazing incident hard X‐ray photoelectron spectroscopy, where OD‐Cu was prepared by reduction of Cu oxide with H2 without exposing to air. The results suggest two types of subsurface oxygen embedded between the fully reduced metallic surface and the Cu2O buried beneath: (i) oxygen staying at lattice defects and/or vacancies in the surface‐most region and (ii) interstitial oxygen intercalated in metal structure. This study adds convincing support to the presence of subsurface oxygen in OD‐Cu, which previously has been suggested to play an important role to mitigate the σ‐repulsion of Cu for CO intermediates in CO2RR.

Diffusion trapped oxygen in oxide derived Copper electrocatalyst in CO2 reduction

“…If the amount of oxygen atoms exceeds the capacity of the surface, the excess oxygen atoms aggregate in the bulk to form Cu2O grains, consistent with the experimental results. 29 The oxygen depth profile shown in Figure 2d for the low specific surface area model (S.S.A. = 0.034 Å -1 , Figure 2e), shows three peaks corresponding to Cu2O, two on the surface and one inside the bulk.…”

“…6 Moreover, the existence of oxygen species under reductive conditions were investigated by grazing incident hard X-ray photoelectron spectroscopy of OD-Cu prepared by reduction of Cu oxide with H2 without exposing to air thus being able to extract the oxygen depth distribution profiles. 29 These experiments identify separated Cu2O buried in the material, oxygen in the lattice close to the surface replacing Cu atomic positions, and O as interstitials in Cu. Computational models have attempted to disentangle such complexity, but have faced similarly severe limitations.…”

“…12,[16][17][18][19][20] Under the experimental conditions these materials show a highly dynamic behavior with profound stoichiometric and geometric rearrangements. [21][22][23][24] The unique performance of OD-Cu has been attributed to the singularities of its morphology, [18][19][20][21][22][23] however the real structure under reaction conditions remains controversial 15,16,[29][30][31][32][33][34][35][36] due to its highly dynamic behavior under experimental conditions. [31][32][33][34] According to XRD and Raman experiments, 31,41 OD-Cu shall be reduced in the bulk according to thermodynamics 42,43 , but pulse experiments 16 show that residual oxygen 44,45 can be trapped, enhancing the electrocatalytic process.…”

In situregeneration of copper catalysts for long-term electrochemical CO2reduction to multiple carbon products

“…Because oxidized Cu species and/or sub-surface oxygen are critical for CO 2 reduction to C 2+ products, the disappearance of these species makes the catalyst less selective toward C 2+ products over time. [53][54][55] Furthermore, for gas-phase ECR in ow systems using gas diffusion electrodes, the ooding of the gas diffusion layer and the precipitation of bi/ carbonate salts can block the diffusion of the CO 2 reactant, leading to the suppression of CO 2 reduction. 3 To identify the deactivation mechanism in our system, we rst checked if ooding and salt precipitation were the main reasons.…”

Unraveling Catalytic Reaction Mechanism by In Situ Near Ambient Pressure X-ray Photoelectron Spectroscopy

“…83−85 For example, POLARIS, a high-pressure XPS station at the Petra III synchrotron, can achieve in situ measurement of catalysts under industrial operando conditions by using hard Xrays. [51][52][53]86,87 To overcome the low surface sensitivity of hard X-ray XPS (usually ∼30 nm detection depth), the POLARIS setup applies the aforementioned GI approach during XPS measurements, which even allows for depth profile analysis from bulk to surface sensitivity. Goodwin et al measured the Pd/PdO ratio with GI XPS on the Pd(100) surface under a total pressure of 300 mbar (O 2 :CO = 50:4) during CO oxidation by varying incident angles.…”