Operando X-ray Absorption Spectroscopy Studies Research Articles

Novel high-pressure/high-temperature reactor cell for in situ and operando x-ray absorption spectroscopy studies of heterogeneous catalysts at synchrotron facilities.

This paper presents the development of a novel high-pressure/high-temperature reactor cell dedicated to the characterization of catalysts using synchrotron x-ray absorption spectroscopy under operando conditions. The design of the vitreous carbon reactor allows its use as a plug-flow reactor, monitoring catalyst samples in a powder form with a continuous gas flow at high-temperature (up to 1000 °C) and under high pressure (up to 1000bar) conditions, depending on the gas environment. The high-pressure/high-temperature reactor cell incorporates an automated gas distribution system and offers the capability to operate in both transmission and fluorescence detection modes. The operando x-ray absorption spectroscopy results obtained on a bimetallic InCo catalyst during CO2 hydrogenation reaction at 300 °C and 50bar are presented, replicating the conditions of a conventional microreactor. The complete setup is available for users and permanently installed on the Collaborating Research Groups French Absorption spectroscopy beamline in Material and Environmental (CRG-FAME) sciences and French Absorption spectroscopy beamline in Material and Environmental sciences at ultra-high dilution (FAME-UHD) beamlines (BM30 and BM16) at the European Synchrotron Radiation Facility in Grenoble, France.

An Electrochemical Cell for Operando Grazing-Incidence X-Ray Absorption Spectroscopic Studies of Low-Loaded Electrodes

X-ray absorption spectroscopy (XAS) is a powerful technique that can provide element specific information on the local electronic and structural properties of newly developed electrocatalysts, especially when performed under operating conditions (i.e., operando). However, the large amounts of catalyst typically needed to achieve sufficiently high spectral quality and temporal resolution can result in working electrodes of several micrometers in thickness. This can in turn lead to an inhomogeneous potential distribution across the electrode, delamination and/or incomplete utilization of the catalyst layer, severe bubble formation and accumulation due to poor mass transport properties.[1, 2] In addition, the activity and selectivity of a catalyst are often measured in a different cell geometry (e.g., using rotating disk electrodes (RDEs)), leading to uncertainties in comparison with the results inferred from spectroscopic data.[3]To tackle these problems we have developed a new spectroelectrochemical XAS flow cell that enables spectral acquisition in fluorescence mode using an X-ray beam incidence angle of ≤ 0.1° with regards to the working electrode’s substrate plane, i.e., in a so called grazing incidence (GI) configuration (see Figure 1). In this acquisition configuration we successfully tracked the formation of palladium hydride with a time resolution of 10 seconds per spectrum while using a Pd-loading as low as 50 µgPd∙cm-2. Moreover, the careful design of the working electrode flow field allows the study of faradaic processes (e.g., O2-reduction) under mass-transport controlled conditions entailing currents comparable to those attained in RDE measurements (≤ 10 mA∙cm−2). The combination of these features with an ion-conductive membrane to separate the working- and counter-electrode compartments additionally enables the detection of gaseous products (e.g., from CO2-electroreduction) by degassing them out of the electrolyte and analyzing them in time-resolved fashion by means of mass spectrometry.[4] References Diklić, N., et al., Potential Pitfalls in the Operando XAS Study of Oxygen Evolution Electrocatalysts. ACS Energy Letters, 2022. 7(5): p. 1735-1740. Diercks, J.S., et al., Spectroscopy vs. Electrochemistry: Catalyst Layer Thickness Effects on Operando/In Situ Measurements. Angew Chem Int Ed Engl, 2023: p. e202216633. Diercks, J.S., et al., Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions. ACS Catalysis, 2022: p. 10727-10741. Khanipour, P., et al., Electrochemical Real-Time Mass Spectrometry (EC-RTMS): Monitoring Electrochemical Reaction Products in Real Time. Angew Chem Int Ed Engl, 2019. 58(22): p. 7273-7277 Figure 1

Exploration of the lithiation/de-lithiation mechanisms of MoSe2@rGO composite anode by operando X-ray absorption spectroscopy

Owing to their inherent advantages of large inter-planar spacing and high-rate capability transition metal di-chalcogenides have attracted much attention as active materials for lithium-ion batteries (LIBs). Herein, MoSe2 and hybrid MoSe2@rGO nanocomposites have been successfully synthesized via hydrothermal processes and have been employed as anodes in LIBs. LIBs with bare MoSe2 and MoSe2@rGO anodes yield initial discharge capacities of 825 mAhg−1 and 979 mAhg−1 respectively at the current density of 100 mAg−1 and 619 mAhg−1 and 850 mAhg−1 respectively at the high current density of 500 mAg−1 in a voltage range of 0.1–3 V respectively. Thus, LIBs with MoSe2@rGO anode showed enhanced electrochemical performance compared to that with bare MoSe2 anode. Further, LIB with MoSe2@rGO anode during the 1st discharge/charge cycle has been investigated by operando X-ray Absorption Spectroscopy (XAS) measurements using synchrotron radiation which provides a detail insight into the intercalation/conversion processes that take place during lithiation/de-lithiation. The operando measurements reveal formation of LixMoSe2 in the initial phase of Li intercalation (discharge process) followed by conversion of LixMoSe2 into Mo, Li2Se and Se. The Li de-intercalation (charging process) shows oxidation of Mo to MoSe2, though the process is found to be not fully reversible and residual unreacted Mo and Se may lead to capacity fading of the LIBs during long cycling. Thus in the present work, the distinctive lithium reaction pathway in LIBs with MoSe2@rGO anodes has been deciphered by synchrotron radiation based operando XAS study.

Spatially-resolved investigation of CO2 methanation over Ni/γ-Al2O3 and Ni3.2Fe/γ-Al2O3 catalysts in a packed-bed reactor

CO2 methanation via the Sabatier reaction with (green) H2 is promising due to its role in achieving a carbon–neutral energy balance in the context of Power-to-Gas technologies. Since Ni-based catalysts are relatively inexpensive compared to other metals and exhibit high catalytic activity, they are most commonly used. Due to the exothermic nature of the reaction, strong temperature and concentration gradients occur, which influence the catalyst structure. Thus, revealing the effects of structural changes of the catalyst along the reactor bed on local activity and selectivity is essential. A 1D packed-bed reactor model was used for numerical simulations, coupled with detailed microkinetics and mass transport limitations. The simulation results are compared with axially-resolved concentration and temperature profiles over 17 wt% Ni/γ-Al2O3 and 17 wt% Ni3.2Fe/γ-Al2O3 catalysts at oven temperatures of 623 K and 723 K. Using additional information from structural spatially-resolved synchrotron-based operando X-ray absorption spectroscopy studies, the oxidation state of Ni was considered in modeling the reactor by changing the catalytically active surface area along the reactor. Predicted surface coverages are compared with surface species experimentally determined by diffuse reflectance infrared Fourier transform spectroscopy. Overall, this study demonstrates the importance of combining modeling with spatially-resolved and temperature-dependent experiments to improve multiscale models and make predictions more accurate.

Intermediates during the Nucleation of Platinum Nanoparticles by a Reaction with Ethylene Glycol: Operando X-ray Absorption Spectroscopy Studies with a Microfluidic Cell.

Using operando X-ray absorption spectroscopy in a continuous-flow microfluidic cell, we have investigated the nucleation of platinum nanoparticles from aqueous hexachloroplatinate solution in the presence of the reducing agent ethylene glycol. By adjusting flow rates in the microfluidic channel, we resolved the temporal evolution of the reaction system in the first few seconds, generating the time profiles for speciation, ligand exchange, and reduction of Pt. Detailed analysis of the X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra with multivariate data analysis shows that at least two reaction intermediates are involved in the transformation of the precursor H2PtCl6 to metallic platinum nanoparticles, including the formation of clusters with Pt-Pt bonding before complete reduction to Pt nanoparticles.

Operando X-ray Absorption Spectroscopy Study of SnO2 Nanoparticles for Electrochemical Reduction of CO2 to Formate

Tin-based electrocatalysts exhibit a remarkable ability to catalyze CO2 to formate selectively. Understanding the size-property relationships and exploring the evolution of the active size still lack complete understanding. Herein, we prepared SnO2 nanoparticles (NPs) with a controllable size supported on commercial carbon spheres (SnO2/C-n, n = 1, 2, and 3) by a simple low-temperature annealing method. The transmission electron microscopy/scanning transmission electron microscopy images and fitting results of the small-angle X-ray scattering profile confirm the increased size of SnO2 NPs due to the increase of SnO2 loading. The catalytic performance of SnO2 has proved the size-dependent effect during the CO2 reduction reaction process. The as-prepared SnO2/C-1 displayed the maximum Faradic efficiency of formate (FEHCOO-) of 82.7% at -1.0 V versus reversible hydrogen electrode (RHE). In contrast, SnO2/C-2 and SnO2/C-3 with larger particle sizes achieved lower maximum FEHCOO- and larger overpotential. Moreover, we employed operando X-ray absorption spectroscopy to study the evolution of the oxidation state and local coordination environment of SnO2 under working conditions. In addition to the observed shifts of the rising edge of Sn K-edge X-ray absorption near-edge structure spectra to a lower energy side as the applied voltage decreases, the decreased coordination number of Sn in the Sn-O scattering path and the presence of Sn metal contribution in the extended X-ray absorption fine structure spectra verify the reduction of SnO2 to SnOx and metallic Sn.

Platinum-Iron(II) Oxide Sites Directly Responsible for Preferential Carbon Monoxide Oxidation at Ambient Temperature: An Operando X-ray Absorption Spectroscopy Study.

Operando X-ray absorption spectroscopy identified that the concentration of Fe2+ species in the working state-of-the-art Pt-FeOx catalysts quantitatively correlates to their preferential carbon monoxide oxidation steady-state reaction rate at ambient temperature. Deactivation of such catalysts with time on stream originates from irreversible oxidation of active Fe2+ sites. The active Fe2+ species are presumably Fe+2 O-2 clusters in contact with platinum nanoparticles; they coexist with spectator trivalent oxidic iron (Fe3+ ) and metallic iron (Fe0 ) partially alloyed with platinum. The concentration of active sites and, therefore, the catalyst activity strongly depends on the pretreatment conditions. Fe2+ is the resting state of the active sites in the preferential carbon monoxide oxidation cycle.

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., ∼−0.5 to ∼−1.0 vs ∼−0.1 to ∼−0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pd-electrocatalysts, previous studies disagree with regard to palladium’s ability to form a hydride phase (PdHx) under CO2 reduction reaction (CO2RR) conditions, as well as on the influence of this PdHx on its CO2RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO2RR with adsorbed CO, whose precise influence on the formation of PdHx and corresponding effects on the CO2RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdHx formation at CO2RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdHx (with x ≈ 0.6) at all applied potentials under CO2RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.

Nonprecious transition metal nitrides as efficient oxygen reduction electrocatalysts for alkaline fuel cells.

Hydrogen fuel cells have attracted growing attention for high-performance automotive power but are hindered by the scarcity of platinum (and other precious metals) used to catalyze the sluggish oxygen reduction reaction (ORR). We report on a family of nonprecious transition metal nitrides (TMNs) as ORR electrocatalysts in alkaline medium. The air-exposed nitrides spontaneously form a several-nanometer-thick oxide shell on the conductive nitride core, serving as a highly active catalyst architecture. The most active catalyst, carbon-supported cobalt nitride (Co3N/C), exhibited a half-wave potential of 0.862 V and achieved a record-high peak power density among reported nitride cathode catalysts of 700 mW cm−2 in alkaline membrane electrode assemblies. Operando x-ray absorption spectroscopy studies revealed that Co3N/C remains stable below 1.0 V but experiences irreversible oxidation at higher potentials. This work provides a comprehensive analysis of nonprecious TMNs as ORR electrocatalysts and will help inform future design of TMNs for alkaline fuel cells and other energy applications.

Enlightening the journey of metal-organic framework (derived) catalysts during the oxygen evolution reaction in alkaline media via operando X-ray absorption spectroscopy

The development of catalysts with enhanced activity for the oxygen evolution reaction (OER) compared to the traditionally used metal oxide catalysts is crucial for further commercialization of electrolyzers. Because of their high surface area and adjustable pore structure, metal-organic framework (MOF)-based catalysts represent a promising alternative. During the OER in alkaline media, the initial MOF structure is susceptible to transformations including the decomposition of the organic backbone and/or the formation of oxide, hydroxide and oxyhydroxide intermediates. Hence, operando characterizations of MOF catalysts during OER are essential to understand the material's progressive changes and extract the OER catalytic mechanism. This article discusses existing operando X-ray absorption spectroscopy studies of MOF(-derived) catalysts during OER and extracts important parameters for future research regarding operando X-ray absorption spectroscopy characterizations of MOFs during alkaline electrolysis.

A general strategy for preparing pyrrolic-N4 type single-atom catalysts via pre-located isolated atoms

Single-atom catalysts (SACs) have been applied in many fields due to their superior catalytic performance. Because of the unique properties of the single-atom-site, using the single atoms as catalysts to synthesize SACs is promising. In this work, we have successfully achieved Co1 SAC using Pt1 atoms as catalysts. More importantly, this synthesis strategy can be extended to achieve Fe and Ni SACs as well. X-ray absorption spectroscopy (XAS) results demonstrate that the achieved Fe, Co, and Ni SACs are in a M1-pyrrolic N4 (M= Fe, Co, and Ni) structure. Density functional theory (DFT) studies show that the Co(Cp)2 dissociation is enhanced by Pt1 atoms, thus leading to the formation of Co1 atoms instead of nanoparticles. These SACs are also evaluated under hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and the nature of active sites under HER are unveiled by the operando XAS studies. These new findings extend the application fields of SACs to catalytic fabrication methodology, which is promising for the rational design of advanced SACs.

Interlayer ligand engineering of β-Ni(OH)2 for oxygen evolution reaction

Oxygen evolution reaction (OER) is a bottleneck process for many electrochemical devices due to the sluggish kinetics, for which advanced electrocatalysts should be carefully designed. Nickle-based materials have been extensively studied to catalyze OER. However, their performances are still below the expectation and the active sites are often controversial. Herein, we have successfully modulated the electronic and surface properties of layered •-Ni(OH)2 by the interlayer ligand engineering, aiming to design novel efficient electrocatalysts and unveil the catalysis mechanism. By one-step solvothermal reaction, alkoxyl substituted •-Ni(OH)2 with variable interlayer distances is obtained, and the ethoxyl substituted one (NiEt) shows great potential for efficient OER. With the assistance of powder X-ray diffraction and crystalline structure computational simulation, the formula of alkoxyl substituted •-Ni(OH)2 are determined. Operando X-ray absorption spectroscopy studies combined with ex-situ analyses revealed that the critical active species of NiEt is formed via hydroxylation and subsequent de-protonation, with high valent Niδ+ (3<δ≤3.66). The corresponding catalytic reaction pathway and mechanism are proposed.

Dynamic active-site generation of atomic iridium stabilized on nanoporous metal phosphides for water oxidation

Designing efficient single-atom catalysts (SACs) for oxygen evolution reaction (OER) is critical for water-splitting. However, the self-reconstruction of isolated active sites during OER not only influences the catalytic activity, but also limits the understanding of structure-property relationships. Here, we utilize a self-reconstruction strategy to prepare a SAC with isolated iridium anchored on oxyhydroxides, which exhibits high catalytic OER performance with low overpotential and small Tafel slope, superior to the IrO2. Operando X-ray absorption spectroscopy studies in combination with theory calculations indicate that the isolated iridium sites undergo a deprotonation process to form the multiple active sites during OER, promoting the O–O coupling. The isolated iridium sites are revealed to remain dispersed due to the support effect during OER. This work not only affords the rational design strategy of OER SACs at the atomic scale, but also provides the fundamental insights of the operando OER mechanism for highly active OER SACs.

Oscillatory Behavior in Oxidation of Propane Over Nickel Foil and Films

The oxidation of propane was studied over Ni foil, a thick Ni/Si film, and a thin Ni/Si film in a flow reactor under oxygen-deficient conditions at a total pressure near 1 mbar. The thickness of the foil was 125 µm, while the thickness of the films was 20 and 1.5 µm, respectively. It was shown that regular self-sustained reaction-rate oscillations arise only over the Ni foil and the thick Ni/Si film. The relaxation-type oscillations appear at 700 °C after an induction period of approximately 50 min, which is accompanied by the formation of a rough and porous structure over their surfaces. According to krypton adsorption measurements, the effective surface area of the treated catalysts increases significantly. CO, CO2, H2, and H2O were detected as products in the gas phase. The results of operando X-ray absorption spectroscopy study indicate that the oscillations originate due to periodical oxidation and reduction of nickel. The highly active state is Ni in the metallic state, while the transition to the poorly active state is accompanied by the growth of NiO layer on the catalyst surface. The upper layer of the catalysts with a thickness of at least 15 nm is involved in the oxidation/reduction process. In contrast, no oscillations appeared in the oxidation of propane over the thin Ni/Si film. In this case, the thickness of the Ni layer is not sufficient for the formation of a porous structure over the thin Ni film. As a result, the thin Ni film cannot provide high activity in the catalytic oxidation of propane that is sufficient for the self-sustained oscillations to occur. The reasons that lead to oscillatory behavior in the oxidation of propane over nickel are discussed.

Comprehensive Operando Electrochemical XAS Study on Nanoporous Cu Oxide Foams for CO2 Reduction Reaction

To secure our global energy economy and chemical industry in the future, new and improved strategies are needed due to the strong dependence on limited fossil resources and the socio-environmental problems associated with increasing anthropogenic emissions of CO2. In this context, electrochemical CO2 reduction reaction (CO2RR) into hydrocarbons and alcohols is a very promising route to counteract this problem and simultaneously to reduce the concentration of CO2 as a well-known greenhouse gas. Different reaction mechanisms and kinetics for the CO2RR are postulated.[1-4] In particular, the role of Cu oxide during the C2 formation like ethylene and ethanol is poorly understood to date. Further critical issues need to address: (i) high overpotentials, (ii) broad product distribution, (iii) fast degradation by catalyst poisoning and (iv) competition reaction at high cathodic potentials, referred to as hydrogen evolution reaction. Our work is focusing on the fundamental understanding about the role of nano-porous copper oxide foams probed by operando electrochemical X-ray absorption spectroscopy (XAS) technique. The nano-porous Cu oxide foams prepared by H2-assisted electrodeposition and thermal annealing in air exhibit large surface area-to-volume ratios, improved catalytic performance and high C2 selectivity compared to a flat Cu surface. The catalytic properties (activity and selectivity) can be controlled by the thermal annealing process in air, where different kind of Cu oxides (Cu(I) vs. Cu(II)) are generated. By using operando XAS, we studied the stability of the Cu oxides as function of the applied potential during the CO2RR. In addition, potential jumps experiments were performed to investigate the kinetics of the reduction process of Cu oxides prepared by different annealing temperatures. Linear combination fitting and EXAFS analysis provide us information about the potential-controlled changes of oxidation state and local environment of the nanoporous Cu oxide foams. More interestingly, we compare our results with operando Raman spectroscopy and operando X-ray diffraction to highlight the enormous importance for the appropriate design of operando measurements. In addition, we identified the potential range for the catalyst aging via coarsening and suggest strategies to stabilize the structure of the Cu foams during the CO2RR. Based on our operando XAS studies, we obtained a deeper insight to the mechanism and kinetics of the CO2RR on porous Cu foams and clarified the role of Cu oxides for the C2 production.

Understanding the Lithium Storage Mechanism in Core–Shell Fe2O3@C Hollow Nanospheres Derived from Metal–Organic Frameworks: An In operando Synchrotron Radiation Diffraction and in operando X-ray Absorption Spectroscopy Study

In this work, a core–shell structure of an Fe2O3@C hollow nanosphere derived from metal–organic frameworks is used as an anode material for Li-ion batteries. This material delivers a reversible cap...

Single platinum atoms embedded in nanoporous cobalt selenide as electrocatalyst for accelerating hydrogen evolution reaction

Designing efficient electrocatalysts for hydrogen evolution reaction is significant for renewable and sustainable energy conversion. Here, we report single-atom platinum decorated nanoporous Co0.85Se (Pt/np-Co0.85Se) as efficient electrocatalysts for hydrogen evolution. The achieved Pt/np-Co0.85Se shows high catalytic performance with a near-zero onset overpotential, a low Tafel slope of 35 mV dec−1, and a high turnover frequency of 3.93 s−1 at −100 mV in neutral media, outperforming commercial Pt/C catalyst and other reported transition-metal-based compounds. Operando X-ray absorption spectroscopy studies combined with density functional theory calculations indicate that single-atom platinum in Pt/np-Co0.85Se not only can optimize surface states of Co0.85Se active centers under realistic working conditions, but also can significantly reduce energy barriers of water dissociation and improve adsorption/desorption behavior of hydrogen, which synergistically promote thermodynamics and kinetics. This work opens up further opportunities for local electronic structures tuning of electrocatalysts to effectively manipulate its catalytic properties by an atomic-level engineering strategy.

The impact of reaction conditions and material composition on the stepwise methane to methanol conversion over Cu-MOR: An operando XAS study

The direct methane to methanol (DMTM) conversion is often referred to as a ‘dream reaction’ with enormous potential to alter energy sector and chemical industry. After O2-activation, Cu-exchanged zeolites form CuxOy species that activate CH4 and release it in the form of CH3OH upon interaction with H2O. Despite extensive research efforts in the last years, several questions concerning the influence of materials composition and process parameters on the reaction mechanism remain open. Herein, we characterize Cu-MOR zeolites with different composition by operando X-ray absorption spectroscopy (XAS), monitoring their spectroscopic response under two characteristic DMTM reaction protocols varied in the duration of the key reaction steps. Linear Combination Fit (LCF) analysis of the time-resolved X-ray absorption near edge structure (XANES) spectra collected during CH4-loading and steam-assisted CH3OH extraction enabled to quantify the abundance of different Cu species during these two steps. Data analysis revealed a positive linear correlation between the methanol yield generated per incorporated copper and the Cu(I) component formed during the CH4-loading step. Cu(I) development during CH4-loading is accompanied by modifications in the extended X-ray absorption fine structure (EXAFS) spectra suggesting substantial rearrangement in the active site structure. The obtained results provide new mechanistic insights for the DMTM over Cu-MOR.

Operando X-ray absorption spectroscopy study of the Fischer–Tropsch reaction with a Co catalyst

This article describes the setting up of a facility on the energy-scanning EXAFS beamline (BL-09) at RRCAT, Indore, India, for operando studies of structure-activity correlation during a catalytic reaction. The setup was tested by operando X-ray absorption spectroscopy (XAS) studies performed on a Co-based catalyst during the Fischer-Tropsch reaction to obtain information regarding structural changes in the catalyst during the reaction. Simultaneous gas chromatography (GC) measurements during the reaction facilitate monitoring of the product gases, which in turn gives information regarding the activity of the catalyst. The combination of XAS and GC techniques was used to correlate the structural changes with the activity of the catalyst at different reaction temperatures. The oxide catalyst was reduced to the metallic phase by heating at 400°C for 5 h under H2 at ambient pressure and subsequently the catalytic reaction was studied at four different temperatures of 240, 260, 280 and 320°C. The catalyst was studied for 10 h at 320°C and an attempt has been made to understand the process of its deactivation from the XANES and EXAFS results.

Operando XAS Studies: Electro-Reduction of CO2 on Nanoporous Cu Foams

New and improved strategies to secure our global energy economy and chemical industry in the future are needed due to the strong dependence on limited fossil resources and the socio-environmental problems associated with increasing anthropogenic emissions of CO2. To counteract this problem and simultaneously to reduce the concentration of the CO2 as well-known greenhouse gas, a very promising route is the electrochemical CO2 reduction reaction (CO2RR) into hydrocarbons and alcohols. Different reaction mechanisms and kinetics for the CO2RR are postulated.[1-4] In particular, the role of Cu oxide during the C2 formation like ethylene and ethanol is poorly understood to date. Further critical issues are addressed: (i) high overpotentials, (ii) broad product distribution, (iii) fast degradation by catalyst poisoning and (iv) competition reaction at high cathodic potentials, referred to as hydrogen evolution reaction. Our work is focusing on the fundamental understanding about the mechanism and kinetics for the CO2RR on nano-porous copper (np-Cu) foams probed by XAS technique. The np-Cu foams prepared by dealloying of Cu-based alloys or by electrodeposition exhibit large surface area-to-volume ratios, improved catalytic performance and high C2 selectivity compared to a flat Cu surface. The catalytic properties (activity and selectivity) can be controlled by the pore size and curvature of the ligaments of the np-Cu. By using operando X-ray absorption spectroscopy (XAS), we studied the various structures and oxidation states of the Cu species in np-Cu foams as function of the applied potential, kind of electrolyte and pH value during the CO2RR. Linear combination fitting and multi-edge EXAFS fit analysis provide us information about the potential-controlled changes of oxidation state and local environment of Cu. In addition, we identified the potential range for the catalyst aging via coarsening and suggest strategies to stabilize the ligament structure and size of the Cu foams during the CO2RR. Based on our operando XAS studies, we obtained a deeper insight to the mechanism and kinetics of the CO2RR on porous Cu foams and clarified the role of Cu oxides for the C2 production. K. P. Kuhl , E. R. Cave, D. N. Abram, and T. F. Jaramillo, Energy Environ. Sci., 5 (2012) p. 7050.A. Dutta, M. Rahaman, N. C. Luedi, M. Mohos, and P. Broekmann, ACS Catal., 6 (2016) p. 2804.H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B. R. Cuenya, Nature Communications, 7 (2016) p. 12123.R. Reske, M. Duca, M. Oezaslan, K. J. P. Schouten, M. T. M. Koper, and P. Strasser, J. Phys. Chem. Lett., 4 (2013) p. 2410.