Surface chemistry of Cu in the presence of CO2 and H2O

Role of Cu Oxide and Cu Adatoms in the Reactivity of CO2 on Cu(110)

We investigate the interaction of CO2 with metallic and oxidized Cu(110) surfaces using a combination of near-ambient pressure scanning tunneling microscopy (NAP-STM) and theoretical calculations. While the Cu(110) and full CuO films are inert, the interface between bare Cu(110) and the CuO film is observed to react instantly with CO2 at a 10 mbar pressure. The reaction is observed to proceed from the interfacial sites of CuO/Cu(110). During reaction with CO2, the CuO/Cu(110) interface releases Cu adatoms which combine with CO3 to produce a variety of added Cu-CO3 structures, whose stability depends on the gas pressure of CO2. A main implication for the reactivity of Cu(110) is that Cu adatoms and highly undercoordinated CuO segments are created on the Cu(110) surface through the interaction with CO2, which may act as reaction-induced active sites. In the case of CO2 hydrogenation to methanol, our theoretical assessment of such sites indicates that their presence may significantly promote CH3OH formation. Our study thus implies that the CuO/Cu(110) interfacial system is highly dynamic in the presence of CO2, and it suggests a possible strong importance of reaction-induced Cu and CuO sites for the surface chemistry of Cu(110) in CO2-related catalysis.

Modulating the Moderate d-Band Center of Indium in InVO4 Nanobelts by Synergizing MnOx and Oxygen Vacancies for High-Efficiency CO2 Photoreduction

Modulating the electronic properties of transition metal sites in photocatalysts at the atomic level is essential for achieving high-activity carbon dioxide photoreduction (CO2PR). An electronic strategy is herein proposed to engineer In-d-band center of InVO4 by incorporating MnOx nanoparticles and oxygen vacancies (VO) into holey InVO4 nanobelts (MnOx/VO-InVO4), which synergistically modulates the In-d-band center to a moderate level and consequently leads to high-efficiency CO2PR. The MnOx/VO-InVO4 catalyst with optimized electronic property exhibits a single carbon evolution rate of up to 145.3 µmol g-1 h-1 and a carbon monoxide (CO) product selectivity of 92.6%, coming out in front of reported InVO4-based materials. It is discovered that the modulated electronic property favors the interaction between the In sites and their intermediates, which thereby improves the thermodynamics and kinetics of the CO2PR-to-CO reaction. This work not only demonstrates the effective engineering of the d orbital of the low-coordination In atoms to promote CO2PR, but also paves the way for the application of tuning d-band center to develop high-efficiency catalysts.

Observed kinetics for the production of diethyl carbonate from CO2 and ethanol catalyzed by CuNi nanoparticles supported on activated carbon

Monometallic and bimetallic Cu:Ni catalysts with different Cu:Ni molar ratios (3:1, 2:1, 1:1, 1:2, 1:3) were synthesized by wetness impregnation on activated carbon and characterized by TPR (temperature programmed reduction), XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). The synthesized catalysts were evaluated in the gas phase production of diethyl carbonate from ethanol and carbon dioxide. The largest catalytic activity was obtained over the bimetallic catalyst with a Cu:Ni molar ratio of 3:1. Its improved activity was attributed to the formation of a Cu-Ni alloy on the surface of the catalyst, evidenced by XPS and in agreement with a previous assignment based on Vegard law and TPR analysis. During the reaction rate experiments, it observed the presence of a maximum of the reaction rate as a function of temperature, a tendency also reported for other carbon dioxide-alcohol reactions. It showed that the reaction rate-temperature data can be adjusted with a reversible rate equation. The initial rate as a function of reactant partial pressure data was satisfactorily adjusted using the forward power law rate equation and it was found that the reaction rate is first order in CO2 and second order in ethanol.

Co-synthesis and Electrochemical Investigation of the Nitrogen-Doped Carbon Layer with Metallic Nano Beads on the SiOx Anode for Lithium Secondary Batteries

The high theoretical capacity (∼2000 mAh g-1) of silicon suboxide (SiOx, with 1 < x < 2) can solve the energy density issue of the graphite anode in Li-ion batteries. In addition, it has an advantage in terms of volume expansion or side reactions compared to pure Si or Li metals, which are considered as next-generation anode materials. However, the loading content of SiOx is limited in commercial anodes because of its low cycle stability and initial coulombic efficiency. In this study, a nitrogen-doped carbon layer with Cu beads (N-C/Cu) derived from copper phthalocyanine (CuPc) is applied to a SiOx electrode to improve its electrochemical performance. The SiOx electrode is simultaneously coated with a Cu- and N-doped carbon layer using CuPc. N-C/Cu synergistically enhances the electric conductivity of the electrode, thus improving its electrochemical performance. The SiOx/N-C/Cu composite has better cyclability and higher capacity (1095.5 mAh g-1) than the uncoated electrode, even after 200 cycles in the 0.5 C condition. In full-cell cycling with NCM811 cathodes, the SiOx (60 wt % of SiOx, with a n/p ratio of 1.1) and graphite-mixed (7.8 wt % of SiOx, with a n/p ratio of 1.1) anodes also show improved electrochemical performances in the same conditions.

In Situ Probing of CO2 Reduction on Cu-Phthalocyanine-Derived Cux O Complex

An in situ measurement of a CO2 reduction reaction (CO2 RR) in Cu-phthalocyanine (CuPC) molecules adsorbed on an Au(111) surface is performed using electrochemical scanning tunneling microscopy. One intriguing phenomenon monitored in situ during CO2 RR is that a well-ordered CuPC adlayer is formed into an unsuspected nanocluster via molecular restructuring. At an electrode potential of -0.7 V versus Ag/AgCl, the Au surface is covered mainly with the clusters, showing restructuring-induced CO2 RR catalytic activity. Using a measurement of X-ray photoelectron spectroscopy, it is revealed that the nanocluster represents a Cu complex with its formation mechanism. This work provides an in situ observation of the restructuring of the electrocatalyst to understand the surface-reactive correlations and suggests the CO2 RR catalyst works at a relatively low potential using the CuPC-derived Cu nanoclusters as active species.

Edge-rich molybdenum disulfide tailors carbon-chain growth for selective hydrogenation of carbon monoxide to higher alcohols

Selective hydrogenation of carbon monoxide (CO) to higher alcohols (C2+OH) is a promising non-petroleum route for producing high-value chemicals, in which precise regulations of both C-O cleavage and C-C coupling are highly essential but remain great challenges. Herein, we report that highly selective CO hydrogenation to C2-4OH is achieved over a potassium-modified edge-rich molybdenum disulfide (MoS2) catalyst, which delivers a high CO conversion of 17% with a superior C2-4OH selectivity of 45.2% in hydrogenated products at 240 °C and 50 bar, outperforming previously reported non-noble metal-based catalysts under similar conditions. By regulating the relative abundance of edge to basal plane, C2-4OH to methanol selectivity ratio can be overturned from 0.4 to 2.2. Mechanistic studies reveal that sulfur vacancies at MoS2 edges boost carbon-chain growth by facilitating not only C-O cleavage but also C-C coupling, while potassium promotes the desorption of alcohols via electrostatic interaction with hydroxyls, thereby enabling preferential formation of C2-4OH.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.

High-performance electrode of ZIF-67 metal-organic framework (MOF) loaded laser-induced graphene (LIG) composite for all-solid-state supercapacitor

This work demonstrates a facile and efficient methodology to synthesize a composite material of zeolitic imidazolate frameworks (ZIFs) and laser-induced graphene (LIG). This ZIF-67 loaded LIG composite (ZIF-67/LIG) has been adequately characterized for its morphology and structure, and its electrochemical performance has been specifically examined. As supercapacitors (SCs) electrode material, the ZIF-67/LIG composite exhibits superb electrochemical performance, owing to the inherent high porosity, abundant active sites, large specific surface area of ZIF-67, and the excellent conductive three-dimensional hierarchical porous network structure provided by LIG. In three-electrode system, ZIF-67/LIG composite electrode displays outstanding areal specific capacitance (C_A) of 135.6 mF cm^-2at a current density of 1 mA cm^-2with 1 M Na₂SO₄aqueous electrolyte, which is far greater than that of pristine LIG (7.7 mF cm^-2). Furthermore, the ZIF-67/LIG composite has been fabricated into an all-solid-state planar micro-supercapacitor (MSC). This ZIF-67/LIG MSC exhibits an impressiveC_Aof 38.1 mF cm^-2at a current density of 0.20 mA cm^-2, a good cycling stability of 80.3% capacitance retention after 3000 cycles, and a high energy density of 5.29μWh cm^-2at a power density of 0.1 mW cm^-2. All electrochemical results clearly manifest that as-prepared ZIF-67/LIG composite can be a candidate in energy storage field with exciting possibilities.

Near-infrared-featured broadband CO2 reduction with water to hydrocarbons by surface plasmon

Imitating the natural photosynthesis to synthesize hydrocarbon fuels represents a viable strategy for solar-to-chemical energy conversion, where utilizing low-energy photons, especially near-infrared photons, has been the ultimate yet challenging aim to further improving conversion efficiency. Plasmonic metals have proven their ability in absorbing low-energy photons, however, it remains an obstacle in effectively coupling this energy into reactant molecules. Here we report the broadband plasmon-induced CO₂ reduction reaction with water, which achieves a CH₄ production rate of 0.55 mmol g^-1 h^-1 with 100% selectivity to hydrocarbon products under 400 mW cm^-2 full-spectrum light illumination and an apparent quantum efficiency of 0.38% at 800 nm illumination. We find that the enhanced local electric field plays an irreplaceable role in efficient multiphoton absorption and selective energy transfer for such an excellent light-driven catalytic performance. This work paves the way to the technique for low-energy photon utilization.

Chemisorption of CO2 on Nitrogen-Doped Graphitic Carbons

The adsorption of CO₂ on nitrogen-doped graphitic carbon materials, such as graphene nanosheet (GNS) powder and highly oriented pyrolytic graphite (HOPG), was comparatively studied using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Desorption of CO₂ was observed at approximately 380 K for both pyridinic-nitrogen (pyri-N)-doped GNS and pyri-N-doped HOPG samples in the TPD experiments, whereas no CO₂ desorption was observed for graphitic nitrogen-doped HOPG. This indicated that only pyri-N species create identical CO₂ adsorption sites on any graphitic carbon surface. The adsorption energies of CO₂ on pyri-N-doped carbons were estimated between 101 and 108 kJ mol^-1, indicating that chemisorption, rather than physisorption, took place. The CO₂ adsorption/desorption process was reproducible in repeated measurements, and no CO₂ dissociation occurred during the process, suggesting that it is a promising CO₂ capturing material. The O 1s peak of the adsorbed CO₂ clearly appeared at 531.5-532 eV in the XPS measurements. The N 1s peak of pyri-N did not change with CO₂ adsorption, indicating that CO₂ is not directly bound to pyri-N but is adsorbed on a carbon atom near the pyridinic nitrogen via the nonbonding p_z orbital of the carbon atom.

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO₂/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO₂/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO₂ gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO₂ methanol synthesis.

Oxygen vacancies in Ru/TiO2 - drivers of low-temperature CO2 methanation assessed by multimodal operando spectroscopy

Hydrogenation of CO2 is very attractive for transforming this greenhouse gas into valuable high energy density compounds. In this work, we developed a highly active and stable Ru/TiO2 catalyst for CO2 methanation prepared by a solgel method that revealed much higher activity in methanation of CO2 (ca. 4-14 times higher turnover frequencies at 140-210°C) than state-of-the-art Ru/TiO2 catalysts and a control sample prepared by wetness impregnation. This is attributed to a high concentration of O-vacancies, inherent to the solgel methodology, which play a dual role for 1) activation of CO2 and 2) transfer of electrons to interfacial Ru sites as evident from operando DRIFTS and in situ EPR investigations. These results suggest that charge transfer from O-vacancies to interfacial Ru sites and subsequent electron donation from filled metal d-orbitals to antibonding orbitals of adsorbed CO are decisive factors in boosting the CO2 methanation activity.

Dynamics over a Cu-graphite electrode during the gas-phase CO2 reduction investigated by APXPS

The electrocatalytic conversion of CO2 to fuels and chemicals using renewable energy is a key decarbonization technology. From a technological viewpoint, the realization of such process in the gas phase...

Tuning product selectivity in CO2 hydrogenation over metal-based catalysts

Conversion of CO2 into chemicals is a promising strategy for CO2 utilization, but its intricate transformation pathways and insufficient product selectivity still pose challenges. Exploiting new catalysts for tuning product selectivity in CO2 hydrogenation is important to improve the viability of this technology, where reverse water-gas shift (RWGS) and methanation as competitive reactions play key roles in controlling product selectivity in CO2 hydrogenation. So far, a series of metal-based catalysts with adjustable strong metal-support interactions, metal surface structure, and local environment of active sites have been developed, significantly tuning the product selectivity in CO2 hydrogenation. Herein, we describe the recent advances in the fundamental understanding of the two reactions in CO2 hydrogenation, in terms of emerging new catalysts which regulate the catalytic structure and switch reaction pathways, where the strong metal-support interactions, metal surface structure, and local environment of the active sites are particularly discussed. They are expected to enable efficient catalyst design for minimizing the deep hydrogenation and controlling the reaction towards the RWGS reaction. Finally, the potential utilization of these strategies for improving the performance of industrial catalysts is examined.

Dynamic Restructuring of Coordinatively Unsaturated Copper Paddle Wheel Clusters to Boost Electrochemical CO2 Reduction to Hydrocarbons*

The electrochemical reduction of CO₂ to hydrocarbons involves a multistep proton-coupled electron transfer (PCET) reaction. Second coordination sphere engineering is reported to be effective in the PCET process; however, little is known about the actual catalytic active sites under realistic operating conditions. We have designed a defect-containing metal-organic framework, HKUST-1, through a facile "atomized trimesic acid" strategy, in which Cu atoms are modified by unsaturated carboxylate ligands, producing coordinatively unsaturated Cu paddle wheel (CU-CPW) clusters. We investigate the dynamic behavior of the CU-CPW during electrochemical reconstruction through the comprehensive analysis of in situ characterization results. It is demonstrated that Cu₂ (HCOO)₃ is maintained after electrochemical reconstruction and that is behaves as an active site. Mechanistic studies reveal that CU-CPW accelerates the proton-coupled multi-electron transfer (PCMET) reaction, resulting in a deep CO₂ reduction reaction.

Green carbonaceous material‒fibrous silica-titania composite photocatalysts for enhanced degradation of toxic 2-chlorophenol

In this work, fibrous silica-titania (FST) was successfully prepared by the microemulsion method prior to the addition of three types of carbonaceous materials: graphitic-carbon nitride, g-C3N4 (CN), graphene nanoplatelets (GN), and multi-wall carbon nanotubes, MWCNT (CNT), via a solid-state microwave irradiation technique. The catalysts were characterized using XRD, FESEM, TEM, FTIR, UV-Vis DRS, N2 adsorption-desorption, XPS and ESR, while their photoactivity was examined on the degradation of toxic 2-chlorophenol (2-CP). The result demonstrated that the initial reaction rate was in the following order: CNFST (5.1 × 10^-3 mM min^-1) > GNFST (2.5 × 10^-3 mM min^-1) > CNTFST (2.3 × 10^-3 mM min^-1). The best performance was due to the polymeric structure of g-C3N4 with a good dispersion of C and N on the surface FST. This dispersion contributed towards an appropriate quantity of defect sites, as a consequence of the greater interaction between g-C3N4 and the FST support, that led to narrowed of band gap energy (2.98 eV to 2.10 eV). The effect of scavenger and ESR studies confirmed that the photodegradation over CNFST occurred via a Z-scheme mechanism. It is noteworthy that the addition of green carbonaceous materials on the FST markedly enhanced the photodegradation of toxic 2-CP.

CO2 doping of organic interlayers for perovskite solar cells

In perovskite solar cells, doped organic semiconductors are often used as charge-extraction interlayers situated between the photoactive layer and the electrodes. The π-conjugated small molecule 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is the most frequently used semiconductor in the hole-conducting layer^1-6, and its electrical properties considerably affect the charge collection efficiencies of the solar cell⁷. To enhance the electrical conductivity of spiro-OMeTAD, lithium bis(trifluoromethane)sulfonimide (LiTFSI) is typically used in a doping process, which is conventionally initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light for several hours. This process, in which oxygen acts as the p-type dopant^8-11, is time-intensive and largely depends on ambient conditions, and thus hinders the commercialization of perovskite solar cells. Here we report a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with CO₂ under ultraviolet light. CO₂ obtains electrons from photoexcited spiro-OMeTAD, rapidly promoting its p-type doping and resulting in the precipitation of carbonates. The CO₂-treated interlayer exhibits approximately 100 times higher conductivity than a pristine film while realizing stable, high-efficiency perovskite solar cells without any post-treatments. We also show that this method can be used to dope π-conjugated polymers.

The Electrophilicity of Surface Carbon Species in the Redox Reactions of CuO-CeO2 Catalysts

Electronic metal–support interactions (EMSI) describe the electron flow between metal sites and a metal oxide support. It is generally used to follow the mechanism of redox reactions. In this study of CuO‐CeO₂ redox, an additional flow of electrons from metallic Cu to surface carbon species is observed via a combination of operando X‐ray absorption spectroscopy, synchrotron X‐ray powder diffraction, near ambient pressure near edge X‐ray absorption fine structure spectroscopy, and diffuse reflectance infrared Fourier transform spectroscopy. An electronic metal–support–carbon interaction (EMSCI) is proposed to explain the reaction pathway of CO oxidation. The EMSCI provides a complete picture of the mass and electron flow, which will help predict and improve the catalytic performance in the selective activation of CO₂, carbonate, or carbonyl species in C1 chemistry.

Optimizing Active Sites for High CO Selectivity during CO2 Hydrogenation over Supported Nickel Catalysts

Controlling the selectivity of CO₂ hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO₂ hydrogenation reaction cycle from 100 to 800 °C. The usually high CH₄ formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH₄ production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO₂ hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO₂ hydrogenation to CO processes toward high-added-value products.

Highly Selective, Defect-Induced Photocatalytic CO2 Reduction to Acetaldehyde by the Nb-Doped TiO2 Nanotube Array under Simulated Solar Illumination

The adsorption and activation of CO₂ molecules on the surface of photocatalysts are critical steps to realize efficient solar energy-induced CO₂ conversion to valuable chemicals. In this work, a defect engineering approach of a high-valence cation Nb-doping into TiO₂ was developed, which effectively enhanced the adsorption and activation of CO₂ molecules on the Nb-doped TiO₂ surface. A highly ordered Nb-doped TiO₂ nanotube array was prepared by anodization of the Ti-Nb alloy foil and subsequent annealing at 550 °C in air for 2 h for its crystallization. Our sample showed a superior photocatalytic CO₂ reduction performance under simulated solar illumination. The main CO₂ reduction product was a higher-energy compound of acetaldehyde, which could be easily transported and stored and used to produce various key chemicals as intermediates. The acetaldehyde production rate was over ∼500 μmol·g^-1·h^-1 with good stability for repeated long-time uses, and it also demonstrated a superior product selectivity to acetaldehyde of over 99%. Our work reveals that the Nb-doped TiO₂ nanotube array could be a promising candidate with high efficiency and good product selectivity for the photocatalytic CO₂ reduction with solar energy.

Cu2O nanoparticles for the degradation of methyl parathion

Methyl parathion (MP) is one of the most neurotoxic pesticides. An inexpensive and reliable one-step degradation method of MP was achieved through an aqueous suspension of copper(I) oxide nanoparticles (NPs). Three different NPs sizes (16, 29 and 45 nm), determined with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM), were synthesized using a modified Benedict's reagent. 1H nuclear magnetic resonance (NMR) results show that the hydrolytic degradation of MP leads to the formation of 4-nitrophenol (4-NPh) as the main product. While the P=S bond of MP becomes P=O, confirmed by 31P NMR. Although Cu2O is a widely known photocatalyst, the degradation of methyl parathion was associated to the surface basicity of Cu2O NPs. Indirect evidence for the basicity of Cu2O NPs was achieved through UV-vis absorption of 4-NPh. Likewise, it was shown that the surface basicity increases with decreasing nanoparticle size. The presence of CuCO3 on the surface of Cu2O, identified using X-ray photoelectron spectroscopy (XPS), passivates its surface and consequently diminishes the degradation of MP.

Identifying the catalyst chemical state and adsorbed species during methanol conversion on copper using ambient pressure X-ray spectroscopies

Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can generate a hydrogen-containing gas mixture. Understanding the chemical state of the catalyst over which these reactions occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions. Here we study polycrystalline Cu foils using ambient pressure X-ray spectroscopies to reveal the Cu oxidation state and identify the adsorbed species during partial oxidation (CH₃OH + O₂), steam reforming (CH₃OH + H₂O), and autothermal reforming (CH₃OH + O₂ + H₂O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen-rich conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu₂O. We rationalise this behaviour on the basis of the shift in equilibrium of the CH₃OH* + O* ⇌ CH₃O* + OH* reaction step caused by the addition of H₂O.

Potential-Dependent Competitive Electroreduction of CO2 into CO and Formate on Cu(111) from an Improved H Coverage-Dependent Electrochemical Model with Explicit Solvent Effect

An improved density functional theory-based H coverage-dependent electrochemical model with explicit solvent effect is proposed for Cu(111), which is used to identify potential-dependent initial competitive CO₂ electroreduction pathways considering HER. We find that a chemisorbed CO₂ molecule at the present electrode/aqueous interface can be spontaneously formed and the overpotentials can affect its coordination pattern. The Eley-Rideal mechanism may be more favorable during the initial CO₂ electroreduction into CO, whereas chemisorbed CO₂ reacting with adsorbed H into HCOO^- via the Langmuir-Hinshelwood mechanism is more facile to occur. The analyses of energetics suggest that the low overpotentials have a negligible influence on CO and HCOO^- formation, and HCOO^- species with monodentate and bidentate configurations may also parallelly form with the surmountable barriers at room temperature. However, the high potentials have an interruptive effect on initial CO₂ electroreduction because of the significantly increased barriers, indicating that the chemisorbed CO₂ can be stabilized by imposing more negative potentials and thus going against initial CO₂ electroreduction. By analyzing the competing HER with initial CO₂ electroreduction into CO, we find that HER is competitive with initial CO formation because of the required lower overpotentials. Simultaneously, the present study shows that the blocked Cu surface by adsorbed H and CO can explain why the initial CO formation pathway is unfavorable at the high overpotentials. Our present conclusions can also confirm the previous experimental report on initial formation of CO and HCOO^-.

Molybdenum carbide catalyst for the reduction of CO2 to CO: surface science aspects by NAPPES and catalysis studies

Carbon dioxide is a greenhouse gas, and needs to be converted into one of the useful feedstocks, such as carbon monoxide and methanol. We demonstrate the reduction of CO₂ with H₂ as a reducing agent, via a reverse water gas shift (RWGS) reaction, by using a potential and low cost Mo₂C catalyst. Mo₂C was evaluated for CO₂ hydrogenation at ambient pressure as a function of temperature, and CO₂ : H₂ ratio at a gas hourly space velocity (GHSV) of 20 000 h^-1. It is demonstrated that the Mo₂C catalyst with 1 : 3 ratio of CO₂ : H₂ is highly active (58% CO₂ conversion) and selective (62%) towards CO at 723 K at ambient pressure. Both properties (basicity and redox properties) and high catalytic activity observed with Mo₂C around 700 K correlate well and indicate a strong synergy among them towards CO₂ activation. X-ray diffraction and Raman analysis show that the Mo₂C catalyst remains in the β-Mo₂C form before and after the reaction. The mechanistic aspects of the RWGS reaction were determined by near-ambient pressure X-ray photoelectron spectroscopy (NAPXPS) with in situ generated Mo₂C from carburization of Mo-metal foil. NAPXPS measurements were carried out at near ambient pressure (0.1 mbar) and various temperatures. Throughout the reaction, no significant changes in the Mo²⁺ oxidation state (of Mo₂C) were observed indicating that the catalyst is highly stable; C and O 1s spectral results indicate the oxycarbide species as an active intermediate for RWGS. A good correlation is observed between catalytic activity from atmospheric pressure reactors and the electronic structure details derived from NAPXPS results, which establishes the structure-activity correlation.

Dramatic differences in carbon dioxide adsorption and initial steps of reduction between silver and copper

Converting carbon dioxide (CO₂) into liquid fuels and synthesis gas is a world-wide priority. But there is no experimental information on the initial atomic level events for CO₂ electroreduction on the metal catalysts to provide the basis for developing improved catalysts. Here we combine ambient pressure X-ray photoelectron spectroscopy with quantum mechanics to examine the processes as Ag is exposed to CO₂ both alone and in the presence of H₂O at 298 K. We find that CO₂ reacts with surface O on Ag to form a chemisorbed species (O = CO₂^δ−). Adding H₂O and CO₂ then leads to up to four water attaching on O = CO₂^δ− and two water attaching on chemisorbed (b-)CO₂. On Ag we find a much more favorable mechanism involving the O = CO₂^δ− compared to that involving b-CO₂ on Cu. Each metal surface modifies the gas-catalyst interactions, providing a basis for tuning CO₂ adsorption behavior to facilitate selective product formations.

The recycling of CO₂ into storable chemicals is critical in order to mitigate climate change, although CO₂’s inert nature has limited the reduction’s mechanistic considerations. Here, authors pair in-situ spectroscopy with quantum mechanics to elucidate CO₂ adsorption on copper and silver surfaces.

Optimizing reaction paths for methanol synthesis from CO2 hydrogenation via metal-ligand cooperativity

As diversified reaction paths exist over practical catalysts towards CO₂ hydrogenation, it is highly desiderated to precisely control the reaction path for developing efficient catalysts. Herein, we report that the ensemble of Pt single atoms coordinated with oxygen atoms in MIL-101 (Pt₁@MIL) induces distinct reaction path to improve selective hydrogenation of CO₂ into methanol. Pt₁@MIL achieves the turnover frequency number of 117 h^-1 in DMF under 32 bar at 150 °C, which is 5.6 times that of Pt_n@MIL. Moreover, the selectivity for methanol is 90.3% over Pt₁@MIL, much higher than that (13.3%) over Pt_n@MIL with CO as the major product. According to mechanistic studies, CO₂ is hydrogenated into HCOO* as the intermediate for Pt₁@MIL, whereas COOH* serves as the intermediate for Pt_n@MIL. The unique reaction path over Pt₁@MIL not only lowers the activation energy for the enhanced catalytic activity, but also contributes to the high selectivity for methanol.

Interface Science Using Ambient Pressure Hard X-ray Photoelectron Spectroscopy

The development of novel in situ/operando spectroscopic tools has provided the opportunity for a molecular level understanding of solid/liquid interfaces. Ambient pressure photoelectron spectroscopy using hard X-rays is an excellent interface characterization tool, due to its ability to interrogate simultaneously the chemical composition and built-in electrical potentials, in situ. In this work, we briefly describe the “dip and pull” method, which is currently used as a way to investigate in situ solid/liquid interfaces. By simulating photoelectron intensities from a functionalized TiO2 surface buried by a nanometric-thin layer of water, we obtain the optimal photon energy range that provides the greatest sensitivity to the interface. We also study the evolution of the functionalized TiO2 surface chemical composition and correlated band-bending with a change in the electrolyte pH from 7 to 14. Our results provide general information about the optimal experimental conditions for characterizing the solid/liquid interface using the “dip and pull” method, and the unique possibilities offered by this technique.

A mini review of in situ near-ambient pressure XPS studies on non-noble, late transition metal catalysts

The rich surface chemistry of Fe, Co, Ni and Cu during heterogeneous catalytic reactions from the perspective of NAP-XPS studies.

Selective Electrochemical Reduction of CO2 to CO on Zn-Based Foams Produced by Cu2+ and Template-Assisted Electrodeposition

In this work, we aim to develop a Zn-based metal foam catalyst with very large specific area suitable for efficient CO production. Its manufacture is based on the dynamic hydrogen bubble template method that consists of the superposition of metal deposition and hydrogen evolution at the solid-liquid interface. We employed Cu ions in the Zn²⁺-rich electroplating bath as foaming agent. The concentration of Cu as foaming agent was systematically studied and an optimized Zn₉₄Cu₆ foam alloy was developed, which, to the best of our knowledge, is the most selective Zn-based CO₂ electrocatalyst toward CO in aqueous bicarbonate solution (FE_CO = 90% at -0.95 V vs reversible hydrogen electrode). This high efficiency is ascribed to the combination of high density of low-coordinated active sites and preferential Zn(101) over Zn(002) texturing. X-ray photoelectron spectroscopy investigations demonstrate that the actual catalyst material is shaped upon reduction of an oxide/hydroxide-terminating surface under CO₂ electrolysis conditions. Moreover, intentional stressing by oxidation at room conditions proved to be beneficial for further activation of the catalyst. Identical location scanning electron microscopy imaging before and after CO₂ electrolysis and long-term electrolysis experiments also showed that the developed Zn₉₄Cu₆ foam catalyst is both structurally and chemically stable at reductive conditions.