Interfacial Chemistry in the Electrocatalytic Hydrogenation of CO_{2} over C-Supported Cu-Based Systems

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)-O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe-Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)-O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu-Zn system represents the optimal active ensembles with stabilized Cu(I)-O; DFT simulations rationalize this observation by indicating that Cu-Zn-O neighboring atoms are able to activate CO2, whereas Cu-Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies

Interfacial chemistry in the electrocatalytic hydrogenation of CO2 over C-Supported Cu-Based systems

Operando soft and hard X-ray spectroscopic techniques were used in combination with plane-wave density functional theory (DFT) simulations to rationalize the enhanced activities of Zn-containing Cu nanostructured electrocatalysts in the electrocatalytic CO2 hydrogenation reaction. We show that at a potential for CO2 hydrogenation, Zn is alloyed with Cu in the bulk of the nanoparticles with no metallic Zn segregated; at the interface, low reducible Cu(I)–O species are consumed. Additional spectroscopic features are observed, which are identified as various surface Cu(I) ligated species; these respond to the potential, revealing characteristic interfacial dynamics. Similar behavior was observed for the Fe–Cu system in its active state, confirming the general validity of this mechanism; however, the performance of this system deteriorates after successive applied cathodic potentials, as the hydrogen evolution reaction then becomes the main reaction pathway. In contrast to an active system, Cu(I)–O is now consumed at cathodic potentials and not reversibly reformed when the voltage is allowed to equilibrate at the open-circuit voltage; rather, only the oxidation to Cu(II) is observed. We show that the Cu–Zn system represents the optimal active ensembles with stabilized Cu(I)–O; DFT simulations rationalize this observation by indicating that Cu–Zn–O neighboring atoms are able to activate CO2, whereas Cu–Cu sites provide the supply of H atoms for the hydrogenation reaction. Our results demonstrate an electronic effect exerted by the heterometal, which depends on its intimate distribution within the Cu phase and confirms the general validity of these mechanistic insights for future electrocatalyst design strategies

Ternary and quaternary Cr or Ga containing ex LDH catalysts Influence of the additional oxides onto the microstructure and activity of Cu ZnAl2O4 catalysts

The stepwise substitution of Al by Cr and Ga leads to quaternary LDH precursors for Cu/ZnM₂O₄ (M = Al, Ga, Cr) catalysts. With the substitution of Al by Cr the interaction of the Cu phase with the oxide matrix is gradually weakened, which is caused by the participation of the chromium oxide phase in the redox processes during catalyst preparation. Such reactive Cr oxide matrix is less efficient than the inert Al oxide matrix in stabilizing the special microstructure of Cu/ZnM₂O₄ catalysts. These weakened interactions led to a lowering of the Cu particle embedment, coinciding with a pronounced Cu crystallite growth during reduction. Both effects partially compensate each other and a maximum in Cu surface area is observed for intermediate Cr contents. In the Ga-substituted catalysts, two distinct Cu species were found for high Ga contents. This is attributed to the presence of partially crystalline spinel and the resulting different strength of interface interaction of the CuO phase with the crystalline and the amorphous oxide. After reduction Cu catalysts with similar average Cu particle sizes as well as Cu surface areas were obtained. In both sample series, the catalytic activity in methanol synthesis does not scale with the Cu surface area and the experiments show that a strong interaction to the oxide is necessary to gain stability and activity of the Cu phase. Al substitution thus confirms that interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface

Work Function, Band Bending, and Microwave Conductivity Studies on the Selective Alkane Oxidation Catalyst MoVTeNb Oxide Orthorhombic M1 Phase under Operation Conditions

A systematic gas phase-dependence of the electron work function, electron affinity, band bending and the high frequency electrical conductivity of the prospective oxidation catalyst MoVTeNbOx with orthorhombic M1 structure was identified under selective alkane oxidation conditions. The conductivity measured in a fixed bed flow reactor at 1 bar with a noncontact microwave technique and the surface electronic properties studied by in situ X-ray photoelectron spectroscopy at 0.25 mbar were determined at 400°C in 2:1 mixtures of oxygen and the alkanes ethane, propane, and n-butane, respectively. The observed modulation of the surface electron affinity is explained by a gas phase dependent modification of the dipolar structure of the active surface, while the band alignment is interpreted in terms of the formation and modification of the space charge region due to pinning of the Fermi energy to the surface state energy as defined by the vanadium(IV,V) oxide surface layer. The thus changed charge carrier density in the space charge region gives rise to the observed conductivity response. Consequently, the catalytic system and its working mode can be described as a semiconductor heterostructure comprising the semiconducting bulk phase, a vanadium oxide termination layer, and the reactive gas phase modulating the Fermi energy of the whole system

First principles calculations of the structure and V L edge X ray absorption spectra of V2O5 using local pair natural orbital coupled cluster theory and spin orbit coupled configuration interaction approaches

A detailed study of the electronic and geometric structure of V₂O₅ and its X-ray spectroscopic properties is presented. Cluster models of increasing size were constructed in order to represent the surface and the bulk environment of V₂O₅. The models were terminated with hydrogen atoms at the edges or embedded in a Madelung field. The structure and interlayer binding energies were studied with dispersion-corrected local, hybrid and double hybrid density functional theory as well as the local pair natural orbital coupled cluster method (LPNO-CCSD). Convergence of the results with respect to cluster size was achieved by extending the model to up to 20 vanadium centers. The O K-edge and the V L_2,3-edge NEXAFS spectra of V₂O₅ were calculated on the basis of the newly developed Restricted Open shell Configuration Interaction with Singles (DFT-ROCIS) method. In this study the applicability of the method is extended to the field of solid-state catalysis. For the first time excellent agreement between theoretically predicted and experimentally measured vanadium L-edge NEXAFS spectra of V₂O₅ was achieved. At the same time the agreement between experimental and theoretical oxygen K-edge spectra is also excellent. Importantly, the intensity distribution between the oxygen K-edge and vanadium L-edge spectra is correctly reproduced, thus indicating that the covalency of the metal–ligand bonds is correctly described by the calculations. The origin of the spectral features is discussed in terms of the electronic structure using both quasi-atomic jj coupling and molecular LS coupling schemes. The effects of the bulk environment driven by weak interlayer interactions were also studied, demonstrating that large clusters are important in order to correctly calculate core level absorption spectra in solids

Operando observation of nickel ceria electrode surfaces during intermediate temperature steam electrolysis

The solid oxide electrolysis cell (SOEC) technology has a huge potential for future mass production of hydrogen, mainly due to its high electrical-to-chemical energy conversion efficiency. However, the durability and the performance of SOEC devices are inferior to that of other competitive electrolysis technologies inhibiting the commercialization of SOECs. Despite the fact that Ni-based cermets are currently the most widely used cathode materials for SOEC, change of the nickel oxidation state has been accused as a major issue limiting the performance of these devices. In this work we provide operando experimental evidence of the active surface oxidation state and composition of nickel/doped-ceria cermets under water electrolysis conditions using ambient pressure X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies, combined with quantitative spectra simulation. Remarkably under specific operational conditions, nickel is maintained in a partially oxidized state which, counterintuitive to the expected behavior, can be beneficial to the cell performance. This finding may initiate new improvement strategies for SOEC electrodes based on thorough optimization of the operational conditions, in order to engineer in situ the most propitious electrode configuration

In Situ Study of the Gas Phase Electrolysis of Water on Platinum by NAP XPS

Chasing down the active state: Near-ambient-pressure X-ray photoelectron spectroscopy was used to study the surface of a Pt electrode during the oxygen evolution reaction (OER). A hydrated Pt metal phase with dissolved oxygen in the near-surface region is OER-active and considered to be the precursor of the analytically detected PtO₂, which is in fact the deactivation product of the electrode

The effect of Al doping on ZnO nanoparticles applied as catalyst support

A pure ZnO sample and a sample containing 3 mol% Al were prepared by (co)-precipitation as model materials for the oxidic support phase in Cu/ZnO/Al2O3 methanol synthesis catalysts. The samples were characterized with respect to their crystal, defect and micro-structure using various methods (XRD, TEM, XPS, UV-vis spectroscopy, EPR, NMR). It was found that a significant fraction of the Al is incorporated into the ZnO lattice and enhances the defect chemistry of the material. The defect structure, however, was not stable under reducing conditions as applied in catalytic reactions. Al ions migrated towards the surface of the ZnO nanoparticles leading to formation of an Al-rich shell and an Al-depleted core. This process proceeds during the first 10–20 hours on stream and is associated with strong modification of the optical bandgap energy and the EPR signal of donor sites present in ZnO