Defective Indium/Indium Oxide Heterostructures for Highly Selective Carbon Dioxide Electrocatalysis

Recent Advances in Electrocatalytic C-N Coupling for Urea Synthesis

Urea, one of the most widely used nitrogen-containing fertilizers globally, is essential for sustainable agriculture. Improving its production is crucial for meeting the increasing demand for fertilizers. Electrocatalytic co-reduction of CO₂ and nitrogenous compounds (NO₂-/NO₃-) has emerged as a promising strategy for green and energy-efficient urea synthesis. However, challenges such as slow reaction kinetics and complex multi-step electron transfers have hindered the development of efficient urea synthesis methods. This review explores recent advances in the electrocatalytic C-N coupling process, focusing on bimetallic catalysts, metal oxide/hydroxide catalysts, and carbon-based catalysts. The review also discusses the future prospects of designing effective catalysts for electrocatalytic C-N coupling to improve urea synthesis.

C2 Product Formation over the C1 Product and HER on the 111 Plane of Specific Cu Alloy Nanoparticles Identified through Multiparameter Optimization

C2 products are more desirable than C1 products during CO2 electroreduction (CO2ER) because the former possess higher energy density and greater industrial value. For CO2ER, Cu is a well-known catalyst, but the selectivity toward C2 products is still a big challenge for researchers due to complex intermediates, different final products, and large space of the catalyst due to its morphology, plane, size, host surface etc. Using density functional theory (DFT) calculations, we find that alloying of Cu nanoparticles can help to enhance the selectivity toward C2 products during CO2ER with a low overpotential. By a systematic investigation of 111 planes (which prefer the C1 product in the case of bulk Cu), the alloys show the generation of C2 products via *CO-*CO dimerization (* indicates adsorbed state). It also suppresses the counter-pathway of hydrogenation of *CO to *CHO, which leads to C1 products. Further, we find that *CH2CHO is the bifurcating intermediate to distinguish between ethanol and ethylene as the final product. We have used simple graphical construction to identify the catalyst for CO2ER over HER, and vice versa. We have also defined the case of hydrogen poisoning and projected a parity plot to recognize the catalyst for C2 product evolution over the C1 product. Our study reveals that Cu-Ag and Cu-Zn catalysts selectively promote ethanol production on 111 planes. Moreover, an edge-doped 2SO2 graphene nanoribbon as the host layer further lowers the barrier and selectively promotes ethanol on Cu38- and Cu79-based alloys. This work provides new theoretical insights into designing Cu-based nanoalloy catalysts for C2 product formation on the 111 plane.

Enriching Metal-Oxygen Species and Phosphate Modulating of Active Sites for Robust Electrocatalytical CO2 Reduction

Direct electrochemical reduction of CO2 (CO2 RR) into value-added chemicals is a promising solution to reduce carbon emissions. The activity of CO2 RR is influenced deeply by the reaction microenvironment and electronic properties of the catalysts. Herein, the surface PO4 3- anions are tuned to modulate the local microenvironment and the electronic properties of the indium-based catalyst with abundant metal-oxygen species enabling efficient electrochemical conversion of CO2 to HCOO- . Indium nanoparticles coupled with PO4 3- anions (PO4 3- -In NPs) achieve a high selectivity of HCOO- up to 91.4% at a low potential of -0.98 V versus reversible hydrogen electrode (versus RHE) and a high HCOO- partial current density of 279.3 mA cm-2 at -1.1 V versus RHE in the electrochemical flow cell. In situ and ex situ characterizations confirm the PO4 3- anions keep stable on the surface of indium during CO2 RR, accelerating the generation of OCHO* intermediate. From density functional theory calculations, PO4 3- anions enrich the metal-oxygen species on the substrate to optimize the electronic structure of the catalysts and induce a local microenvironment with massive K+ ions on the interface, thus reducing the activation energy barrier of CO2 RR.

MOFs-derived plum-blossom-like junction In/In2O3@C as an efficient nitrogen fixation photocatalyst: Insight into the active site of the In3+ around oxygen vacancy

Nitrogen activation with low-cost, visible-light-driven photocatalysts continues to be a major challenge. Since the discovery of biological nitrogen fixation, multi-component systems have achieved higher efficiency due to the synergistic effects, thus one of the challenges has been distinguishing the active sites in multi-component catalysts. In this study, we report the photocatalysts of In/In2O3@C with plume-blossom-like junction structure obtained by one-step roasting of MIL-68-In. The "branch" is carbon for supporting and protecting the structure, and the "blossom" is In/In2O3 for the activation and reduction of N2, which form an efficient photocatalyst for nitrogen fixation reaction with the performance of 51.83 μmol h-1 g-1. Experimental studies and DFT calculations revealed the active site of the catalyst for nitrogen fixation reaction is the In3+ around the oxygen vacancy in In2O3. More importantly, the elemental In forms the Schottky barrier with In2O3 in the catalyst, which can generate a built-in electric field to form charge transfer channels during the photocatalytic activity, not only broadens the light absorption range of the material, but also exhibits excellent metal conductivity.

Facile Synthesis of Heterogeneous Indium Nanoparticles for Formate Production via CO2 Electroreduction

In this study, a simple and scalable method to obtain heterogeneous indium nanoparticles and carbon-supported indium nanoparticles under mild conditions is described. Physicochemical characterization by X-ray diffraction (XRD), X-ray photoelectron microscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed heterogeneous morphologies for the In nanoparticles in all cases. Apart from In0, XPS revealed the presence of oxidized In species on the carbon-supported samples, whereas these species were not observed for the unsupported samples. The best-in-class catalyst (In50/C50) exhibited a high formate Faradaic efficiency (FE) near the unit (above 97%) at -1.6 V vs. Ag/AgCl, achieving a stable current density around -10 mA·cmgeo-2, in a common H-cell. While In0 sites are the main active sites for the reaction, the presence of oxidized In species could play a role in the improved performance of the supported samples.

Alloying strategies for tuning product selectivity during electrochemical CO2 reduction over Cu

Excessive reliance on fossil fuels has led to the release and accumulation of large quantities of CO₂ into the atmosphere which has raised serious concerns related to environmental pollution and global warming. One way to mitigate this problem is to electrochemically recycle CO₂ to value-added chemicals or fuels using electricity from renewable energy sources. Cu is the only metallic electrocatalyst that has been shown to produce a wide range of industrially important chemicals at appreciable rates. However, low product selectivity is a fundamental issue limiting commercial applications of electrochemical CO₂ reduction over Cu catalysts. Combining copper with other metals that actively contribute to the electrochemical CO₂ reduction reaction process can selectively facilitate generation of desirable products. Alloying Cu can alter surface binding strength through electronic and geometric effects, enhancing the availability of surface confined carbon species, and stabilising key reduction intermediates. As a result, significant research has been undertaken to design and fabricate copper-based alloy catalysts with structures that can enhance the selectivity of targeted products. In this article, progress with use of alloying strategies for development of Cu-alloy catalysts are reviewed. Challenges in achieving high selectivity and possible future directions for development of new copper-based alloy catalysts are considered.

Subsurface oxygen defects electronically interacting with active sites on In2O3 for enhanced photothermocatalytic CO2 reduction

Oxygen defects play an important role in many catalytic reactions. Increasing surface oxygen defects can be done through reduction treatment. However, excessive reduction blocks electron channels and deactivates the catalyst surface due to electron-trapped effects by subsurface oxygen defects. How to effectively extract electrons from subsurface oxygen defects which cannot directly interact with reactants is challenging and remains elusive. Here, we report a metallic In-embedded In2O3 nanoflake catalyst over which the turnover frequency of CO2 reduction into CO increases by a factor of 866 (7615 h-1) and 376 (2990 h-1) at the same light intensity and reaction temperature, respectively, compared to In2O3. Under electron-delocalization effect of O-In-(O)Vo-In-In structural units at the interface, the electrons in the subsurface oxygen defects are extracted and gather at surface active sites. This improves the electronic coupling with CO2 and stabilizes intermediate. The study opens up new insights for exquisite electronic manipulation of oxygen defects.

Thermodynamics of heterogeneous equilibria in the In-In2 O3 system using Knudsen effusion mass spectrometry

RATIONALE

The In-In₂ O₃ system is regarded as a potential low-temperature source of gaseous indium oxide (In₂ O) when obtaining functional materials by physical vapor deposition techniques. To date the vaporization thermodynamics of the system have been investigated in few studies, the results of which are contradictory.

METHODS

The study of the In-In₂ O₃ system was performed using Knudsen effusion mass spectrometry in the temperature range 930-1210 K, with a magnet mass spectrometer (MS-1301). Quartz effusion cells heated by a resistance furnace were employed.

RESULTS

It was established that In(g) and In₂ O(g) are the major vapor species over heterogeneous mixtures (In(l) + In₂ O₃ (s)) and the gaseous oxide In₂ O is predominant. The partial pressures of the vapor species were determined and the quantitative vapor composition was calculated. Based on the experimental data, a p-x section of the In-In₂ O₃ system phase diagram at 1060 K was constructed. The standard enthalpies of reactions accompanying vaporization of the In and In₂ O₃ mixtures were evaluated using the second- and third-law methods. The standard enthalpy of formation of In₂ O(g) was derived from the enthalpies of reactions obtained.

CONCLUSIONS

The predominance of In₂ O in the equilibrium vapor over heterogeneous mixtures (In(l) + In₂ O₃ (s)), along with its high partial pressure at relatively low temperatures, substantiate the In-In₂ O₃ system to be suitable for physical vapor deposition methods. The obtained results can be used for physical vapor deposition parameter adjustment and optimization. The standard enthalpy of formation of In₂ O(g) obtained in an independent way in the present work is in good agreement with that from our previous In₂ O₃ (s) vaporization study.

Photoelectrochemical CO2 Reduction Products Over Sandwiched Hybrid Ga2O3:ZnO/Indium/ZnO Nanorods

Recycled valuable energy production by the electrochemical CO₂ reduction method has explosively researched using countless amounts of developed electrocatalysts. Herein, we have developed hybrid sandwiched Ga₂O₃:ZnO/indium/ZnO nanorods (GZO/In/ZnO_NR) and tested their photoelectrocatalytic CO₂ reduction performances. Gas chromatography and nuclear magnetic spectroscopy were employed to examine gas and liquid CO₂ reduction products, respectively. Major products were observed to be CO, H₂, and formate whose Faradaic efficiencies were highly dependent on the relative amounts of overlayer GZO and In spacer, as well as applied potential and light irradiation. Overall, the present study provides a new strategy of controlling CO₂ reduction products by developing a sandwiched hybrid catalyst system for energy and environment.

CO2 Conversion to Alcohols over Cu/ZnO Catalysts: Prospective Synergies between Electrocatalytic and Thermocatalytic Routes

The development of efficient catalysts is one of the main challenges in CO₂ conversion to valuable chemicals and fuels. Herein, inspired by the knowledge of the thermocatalytic (TC) processes, Cu/ZnO and bare Cu catalysts enriched with Cu⁺¹ were studied to convert CO₂ via the electrocatalytic (EC) pathway. Integrating Cu with ZnO (a CO-generation catalyst) is a strategy explored in the EC CO₂ reduction to reduce the kinetic barrier and enhance C-C coupling to obtain C₂₊ chemicals and energy carriers. Herein, ethanol was produced with the Cu/ZnO catalyst, reaching a productivity of about 5.27 mmol·g_cat^-1·h^-1 in a liquid-phase configuration at ambient conditions. In contrast, bare copper preferentially produced C₁ products like formate and methanol. During CO₂ hydrogenation, a methanol selectivity close to 100% was achieved with the Cu/ZnO catalysts at 200 °C, a value that decreased at higher temperatures (i.e., 23% at 300 °C) because of thermodynamic limitations. The methanol productivity increased to approximately 1.4 mmol·g_cat^-1·h^-1 at 300 °C. Ex situ characterizations after testing confirmed the potential of adding ZnO in Cu-based materials to stabilize the Cu¹⁺/Cu⁰ interface at the electrocatalyst surface because of Zn and O enrichment by an amorphous zinc oxide matrix; while in the TC process, Cu⁰ and crystalline ZnO prevailed under CO₂ hydrogenation conditions. It is envisioned that the lower *CO binding energy at the Cu⁰ catalyst surface in the TC process than in the Cu¹⁺ present in the EC one leads to preferential CO and methanol production in the TC system. Instead, our EC results revealed that an optimum local CO production at the ZnO surface in tandem with a high amount of superficial Cu¹⁺ + Cu⁰ species induces ethanol formation by ensuring an appropriate local amount of *CO intermediates and their further dimerization to generate C₂₊ products. Optimizing the ZnO loading on Cu is proposed to tune the catalyst surface properties and the formation of more reduced CO₂ conversion products.

Coke-resistant Ni-based bimetallic catalysts for the dry reforming of methane: effects of indium on the Ni/Al2O3 catalyst

In the quest for highly efficient coke-resistant catalysts for the dry reforming of methane (DRM) to produce syngas, a series of Ni–In/γ-Al2O3 catalysts with various Ni contents were prepared via a “two-solvent” method and used for the DRM reaction.