Growth Dynamics and Processes Governing the Stability of Electrodeposited Size-Controlled Cubic Cu Catalysts

The renewable energy-powered conversion of industrially generated CO2 into useful chemicals and fuels is considered a promising technology for the sustainable development of our modern society. The electrochemical reduction of CO2 (CO2RR) is one of the possible conversion processes that can be employed to close the artificial carbon cycle, mimicking nature’s photosynthesis. Nevertheless, to enable green catalytic processes, selectivity for the desired products must be achieved. In the case of CO2RR, the selectivity is strongly dependent on the electrocatalyst structure. Here, we explore the phase space of synthesis parameters required for the electrodeposition of Cu cubes with {100} facets on glassy carbon substrates and elucidate their influence on the size, shape, coverage, and uniformity of the cubes. We found that the concentration of Cl– ions in solution controls the cube size, shape, and coverage, whereas the ratio of the reduction versus oxidation time and number of cycles in the alternating potential electrodeposition protocol can be used to further tune the cube size. Cyclic voltammetry experiments were complemented with in situ electrochemical scanning electron microscopy to follow the growth dynamics and ex situ transmission electron microscopy and electron diffraction. Our results indicate that the cube growth starts from nuclei formed during the first cycle, followed by a layered deposition and partial dissolution of new material in subsequent cycles

Revealing the CO Coverage Driven C-C Coupling Mechanism for Electrochemical CO₂ Reduction on Cu₂O Nanocubes via Operando Raman Spectroscopy

Electrochemical reduction of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chemicals and fuels. However, the highly selective generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent reduction of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu–CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with Density Functional Theory, we proved that this ratio is determined by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C–C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction

Size effects and active state formation of cobalt oxide nanoparticles during the oxygen evolution reaction

Water electrolysis is a key technology to establish CO2-neutral hydrogen production. Nonetheless, the near-surface structure of electrocatalysts during the anodic oxygen evolution reaction (OER) is still largely unknown, which hampers knowledge-driven optimization. Here using operando X-ray absorption spectroscopy and density functional theory calculations, we provide quantitative near-surface structural insights into oxygen-evolving CoOx(OH)y nanoparticles by tracking their size-dependent catalytic activity down to 1 nm and their structural adaptation to OER conditions. We uncover a superior intrinsic OER activity of sub-5 nm nanoparticles and a size-dependent oxidation leading to a near-surface Co–O bond contraction during OER. We find that accumulation of oxidative charge within the surface Co3+O6 units triggers an electron redistribution and an oxyl radical as predominant surface-terminating motif. This contrasts the long-standing view of high-valent metal ions driving the OER, and thus, our advanced operando spectroscopy study provides much needed fundamental understanding of the oxygen-evolving near-surface chemistry

Operando Investigation of Ag‐Decorated Cu₂O Nanocube Catalysts with Enhanced CO₂ Electroreduction toward Liquid Products

Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO2 electrochemical reduction reaction (CO2RR) can contribute to the decarbonization of the global economy. Here, well‐defined Cu2O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2O NCs, the catalyst with 5 at% Ag on Cu2O NCs displayed a two‐fold increase in the Faradaic efficiency for C2+ liquid products (30% at ‐1.0 VRHE), including ethanol, 1‐propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X‐ray absorption spectroscopy revealed the partial reduction of Cu2O during CO2RR, accompanied by a reaction‐driven redispersion of Ag on the CuOx NCs. Operando surface‐enhanced Raman spectroscopy data further uncovered significant variations in the CO binding to Cu, which were assigned to Ag‐Cu sites formed during CO2RR that appear crucial for the C‐C coupling and the enhanced yield of liquid products

Role of the Oxide Support on the Structural and Chemical Evolution of Fe Catalysts during the Hydrogenation of CO₂

Iron-based catalysts are considered active for the hydrogenation of CO2 toward high-order hydrocarbons. Here, we address the structural and chemical evolution of oxide-supported iron nanoparticles (NPs) during the activation stages and during the CO2 hydrogenation reaction. Fe NPs were deposited onto planar SiO2 and Al2O3 substrates by dip coating with a colloidal NP precursor and by physical vapor deposition of Fe. These model catalysts were studied in situ by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) in pure O2, then in H2, and finally in the CO2 + H2 (1:3) reaction mixture in the mbar pressure range and at elevated temperatures. The NAP-XPS results revealed the preferential formation of Fe(III)- and Fe(II)-containing surface oxides under reaction conditions, independently of the initial degree of iron reduction prior to the reaction, suggesting that CO2 behaves as an oxidizing agent even in excess of hydrogen. The formation of the iron carbide phase, often reported for unsupported Fe catalysts in this reaction, was never observed in our systems, even on the samples exposed to industrially relevant pressure and temperature (e.g., 10 bar of CO2 + H2, 300 °C). Moreover, the same behavior is observed for Fe NPs deposited on nanocrystalline silica and alumina powder supports, which were monitored in situ by X-ray absorption spectroscopy (XAS). Our findings are assigned to the nanometer size of the Fe particles, which undergo strong interaction with the oxide support. The combined XPS and XAS results suggest that a core (metal-rich)–shell (oxide-rich) structure is formed within the Fe NPs during the CO2 hydrogenation reaction. The results highlight the important role played by the oxide support in the final structure and composition of nanosized catalysts

Operando‐Untersuchung von Ag‐dekorierten Cu₂O‐Nanowürfel‐Katalysatoren mit verbesserter CO₂‐Elektroreduktion zu Flüssigprodukten

Direct conversion of carbon dioxide into multicarbon liquid fuels by the CO2 electrochemical reduction reaction (CO2RR) can contribute to the decarbonization of the global economy. Here, well‐defined Cu2O nanocubes (NCs, 35 nm) uniformly covered with Ag nanoparticles (5 nm) were synthesized. When compared to bare Cu2O NCs, the catalyst with 5 at% Ag on Cu2O NCs displayed a two‐fold increase in the Faradaic efficiency for C2+ liquid products (30% at ‐1.0 VRHE), including ethanol, 1‐propanol, and acetaldehyde, while formate and hydrogen were suppressed. Operando X‐ray absorption spectroscopy revealed the partial reduction of Cu2O during CO2RR, accompanied by a reaction‐driven redispersion of Ag on the CuOx NCs. Operando surface‐enhanced Raman spectroscopy data further uncovered significant variations in the CO binding to Cu, which were assigned to Ag‐Cu sites formed during CO2RR that appear crucial for the C‐C coupling and the enhanced yield of liquid products

Enhanced Formic Acid Oxidation over SnO₂-decorated Pd Nanocubes

The formic acid oxidation reaction (FAOR) is one of the key reactions that can be used at the anode of low-temperature liquid fuel cells. To allow the knowledge-driven development of improved catalysts, it is necessary to deeply understand the fundamental aspects of the FAOR, which can be ideally achieved by investigating highly active model catalysts. Here, we studied SnO2-decorated Pd nanocubes (NCs) exhibiting excellent electrocatalytic performance for formic acid oxidation in acidic medium with a SnO2 promotion that boosts the catalytic activity by a factor of 5.8, compared to pure Pd NCs, exhibiting values of 2.46 A mg–1Pd for SnO2@Pd NCs versus 0.42 A mg–1Pd for the Pd NCs and a 100 mV lower peak potential. By using ex situ, quasi in situ, and operando spectroscopic and microscopic methods (namely, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption fine-structure spectroscopy), we identified that the initially well-defined SnO2-decorated Pd nanocubes maintain their structure and composition throughout FAOR. In situ Fourier-transformed infrared spectroscopy revealed a weaker CO adsorption site in the case of the SnO2-decorated Pd NCs, compared to the monometallic Pd NCs, enabling a bifunctional reaction mechanism. Therein, SnO2 provides oxygen species to the Pd surface at low overpotentials, promoting the oxidation of the poisoning CO intermediate and, thus, improving the catalytic performance of Pd. Our SnOx-decorated Pd nanocubes allowed deeper insight into the mechanism of FAOR and hold promise for possible applications in direct formic acid fuel cells

Steering the structure and selectivity of CO₂ electroreduction catalysts by potential pulses

Convoluted selectivity trends and a missing link between reaction product distribution and catalyst properties hinder practical applications of the electrochemical CO2 reduction reaction (CO2RR) for multicarbon product generation. Here we employ operando X-ray absorption and X-ray diffraction methods with subsecond time resolution to unveil the surprising complexity of catalysts exposed to dynamic reaction conditions. We show that by using a pulsed reaction protocol consisting of alternating working and oxidizing potential periods that dynamically perturb catalysts derived from Cu2O nanocubes, one can decouple the effect of the ensemble of coexisting copper species on the product distribution. In particular, an optimized dynamic balance between oxidized and reduced copper surface species achieved within a narrow range of cathodic and anodic pulse durations resulted in a twofold increase in ethanol production compared with static CO2RR conditions. This work thus prepares the ground for steering catalyst selectivity through dynamically controlled structural and chemical transformations