Shapeshifting Nanocatalyst for CO2 Conversion

Abstract

The conversion of CO₂ into high-value chemicals through a photoreduction reaction in water is a promising route to reduce the dependence on fossil fuels. Enhancing selectivity toward hydrocarbons or alcohols can be achieved by Ag-Cu alloys. However, the stabilized surface state created by Ag-Cu interactions is still poorly understood. In this work, multi-modal in situ X-ray experiments reveals underlying mechanisms and the evolution of Ag-Cu nanoparticles under CO₂ reduction reaction (CO₂RR) conditions. Both morphological and chemical changes of Ag and Cu species induced by diffusion mechanics are tracked during nanocatalyst operation. The initial spheroid Ag-Cu nanoparticles are composed of a Cu-rich shell and Ag-rich core. The reduction treatment promotes Ag migration toward the surface. During photocatalytic CO₂ reduction reaction, Cu atoms migrate back to the surface, forming Ag-Cu-O species. The study observes the surface oxidation of Cu(0) to Cu⁺ and the presence of Ag at the sub-surface region. Furthermore, nanoparticles change their shape, decreasing their specific surface area, driven by Cu diffusion during the CO₂ photoreduction reaction. The results provide invaluable insights into the dynamic restructuring of the catalyst under reaction conditions and into the active species responsible for CO₂ conversion.

Keywords: CO2 reduction reaction; artificial photosynthesis; in situ measurements; morphology changes; photocatalysis.

Figure 1

Typical SEM images of AgCu nanoparticles a) before and b) after exposure to CO₂RR. c) Histogram of nanoparticle lateral width before and after exposure to CO₂RR. d) Typical AFM image obtained of the sample after exposure to CO₂RR. e) UV–vis spectroscopy measured in total reflectance mode before and after exposure to CO₂RR. The inset shows the ratio between the spectra of the sample after and before exposure.

Figure 2

AP‐XPS spectra at a) Cu 3p + Ag 4p, b) Ag 3d, c) C 1s, and d) O 1s electronic regions in the as‐prepared, after annealing at 200 °C in 20 mTorr H₂, during CO₂RR with 40 mTorr CO₂ + 40 mTorr H₂O with laser off, and during 532 nm laser on conditions. e) XAS measurements at Cu L edge measured in drain‐current mode in the same conditions. f) correlations taken out of AP‐XPS measurements between Cu/Ag, AgO_x/Ag, Cu‐Ag‐O/C, and C‐O/C ratio (atomic %) on the surface.

Figure 3

a–c) AP‐GIXS measurements of the sample as prepared, after annealing, and upon 40 mTorr CO₂ + 40 mTorr H₂O exposure. Black lines are placed as a guide to the eye centered at the high intensity lobe, while the magenta line shows where linecuts were taken for further analysis. d) Simulated scattering pattern of the as‐prepared sample using BornAgain software, and e) the nanoparticle configuration used for the simulation. f) Scattering contrast as a function of the radial distance to the surface of the average size nanoparticle, obtained after fitting a linecut with the Boucher sphere model.

Figure 4

a) Comparison between scattering patterns obtained after 532 nm laser irradiation around Cu L₃ edge, in off‐resonance and resonance conditions. b) Scattering contrast as a function of the radial distance to the surface of the average size nanoparticle obtained after fitting a linecut with the Boucher sphere model.

Figure 5

a) Atomic configurations of core‐shell 47% Ag – 53% Cu nanoparticles annealed at different temperatures in MD simulations. b) Surface stoichiometry and c) ratio of mean to maximum particle radius after thermal annealing as a function of the stoichiometry of the full nanoparticle. Shaded areas indicate points closest to experimental observation. d) Schematic representation of the overall transformations observed during CO₂RR. e) STEM measurements of the Ag‐Cu nanoparticles after CO₂RR (laser on condition), and respective EELS mapping of the region of interest marked with the red rectangle.