It is observed that pyridinium is hydrogenated at Pt electrodes in electrochemical conditions consistent with those previously shown to yield selective reduction of carbon dioxide to methanol and formic acid. The hydrogenation proceeds through a heterogeneous reaction with chemisorbed hydrogen, which originates from one-electron surface proton transfer reactions. Electrochemical methods are used to show that pyridinium adsorbs on the Pt surface, consistent with the proposed heterogeneous reaction mechanism. From this first observation of the electrochemical generation of a stable hydrogenated piperidinium-like near-surface species it logically follows that dihydropyridinium, the protonated form of the previously-proposed hydride-shuttling reduction catalyst, must transiently exist under these conditions near the Pt surface in the presence of carbon dioxide. Therefore partially hydrogenated heterocycles remain strong candidates for catalytically active species that enable selective carbon dioxide reduction. More generally, the observed mild potentials required for electrocatalytic hydrogenation of stable organics implies that engineered transfer hydrogenations involving organic adsorbates can be a viable approach for achieving selective carbon dioxide reduction to fuels.

A new catalyst is presented for the oxygen evolution reaction (OER) based on cerium-modified copper oxide (CuOx) prepared using a facile electrodeposition procedure. Incorporation of Ce into CuOx leads to greatly improved OER activity, which reached an optimal value at a surface concentration of 6.9 at% Ce. Specifically, the OER current density at 400 mV overpotential for the most active Ce-modified CuOx catalyst (6.9 at% Ce) was 3.3 times greater compared to the pure CuOx. Coincident with the improved OER activity, Ce incorporation also leads to significant structural changes that manifested in increasing degrees of disorder. A further increase in the Ce concentration led to a decrease in the OER performance which can be attributed to the formation of a segregated CeO2 phase. A strong correlation was observed between the OER performance and tetravalent Ce (Ce4+) ion concentration, up to a concentration corresponding to CeO2 phase segregation. No particular trend was observed for the OER activity of these Ce-modified CuOx catalysts with respect to the surface concentration of Cu ions, surface oxygen species or catalyst structure. The stability of these CuOx catalysts at 5 mA cm−2 was also improved with Ce incorporation, and the overpotential required to sustain this current density is much lower than that of pure CuOx. Overall, this study provides new insights regarding the promoting effect of tetravalent Ce ions on the OER activity of CuOx-based OER catalysts in alkaline electrolytes.

A new electrode structure enabling low overpotentials for the oxidation of water, based on three-dimensional arrays of CoOOH nanowires, is presented. The electrocatalytic activities of pure and M-modified cobalt oxyhydroxides (M = Ni or Mn) nanowires have been investigated in detail for the oxygen evolution reaction (OER) in an alkaline environment. The pure, Ni-, and Mn-modified nanowires, with preferentially exposed low-index surfaces, were fabricated directly on stainless steel mesh current collectors using an inexpensive and scalable chemical synthesis procedure. The unique electrode structure ensures excellent substrate–catalyst electrical contact and increases the surface area accessible to the electrolyte. The OER activity of CoOOH nanowires is shown to be significantly improved through incorporation of Ni. Specifically, optimal OER activity is obtained for CoOOH nanowires with 9.7% surface Ni content, which corresponds to four-times greater current density compared to pure CoOOH. In contrast, Mn modification of the CoOOH nanowires did not improve the OER activity. Tafel analysis suggests Ni incorporation leads to change in the OER rate-determining step based on an observed decrease in the Tafel slope. Electrochemical impedance spectroscopy reveals that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. This study provides new insights regarding the influence of transition metal impurities on the OER activity of CoOOH and provides a clear strategy for the optimization of CoOOH-based OER catalysts in alkaline electrolytes.

•Ni films grow nearly layer-by-layer on Pd(111) at 300 K.•Interdiffusion at 600 K converts a 1-ML Ni film into a Pd–Ni–Pd(111) structure.•Oxygen induces Ni segregation for Pd–Ni–Pd(111) above 500 K.•DFT calculations predict oxygen-induced surface segregation of Ni in Pd–Ni alloys.Surface composition and structure of deposited Ni ultrathin films grown on a Pd(111) surface and their thermal stability have been studied using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) and scanning tunneling microscopy (STM). In experiments where up to 2 monolayers (ML) of Ni was deposited onto Pd(111) at 300 K, the initial film growth followed a non-ideal layer-by-layer growth mode, in which the majority of the surface was covered by a single atomic layer of Ni, but the second Ni layer started to appear before the first layer was completed. Annealing the Ni/Pd(111) surface to 600 K caused Ni interdiffusion into subsurface layers and the outermost surface was mainly Pd. This structure, designated as Pd–Ni–Pd(111), was not stable in the presence of surface oxygen. Ni segregated to the topmost surface layer to form a (2 × 2) superstructure after exposing the Pd–Ni–Pd(111) surface at 590 K to 350 L O2. The oxygen-induced segregation of Ni is consistent with predictions from density functional theory (DFT) calculations.

Surface-bound species on GaP(110) formed upon interaction with water were investigated through experiment and theory. These studies are motivated by and discussed in the context of electrocatalytic and photoelectrocatalytic schemes for solar fuel production, including especially observations of selective CO2 reduction to methanol in acidified aqueous solutions of CO2 and nitrogen-containing heteroaromatics. Experimentally, surface-bound species over 10 orders of magnitude of pressure were spectroscopically identified in situ using synchrotron-based ambient pressure photoelectron spectroscopy. Ga 3d and O 1s core-level spectra indicate that the interaction of GaP(110) with H2O induces formation of a partially dissociated adlayer, characterized by the presence of both Ga–OH and molecular H2O species. Measurements of the P 2p core level indicate formation of a negatively charged hydride that irreversibly bonds to surface P in vacuum. The surface densities of the hydroxide and hydride species increase with increasing pressure (surface coverage) of water. Periodic slab calculations using density functional theory were used to study several relevant water configurations at 298 K on this surface. Consistent with earlier theoretical predictions at 0 K, the calculations confirm that Ga–OH, molecular H2O, and P–H species are thermodynamically stable on the GaP(110) surface under experimental conditions. Isobaric measurements at elevated pressures were used to probe the thermal stabilities of adsorbed species as well as the oxidation of surface Ga and P. The observation of stable surface hydride formation induced by interaction with water is especially notable given the critical role of hydride transfer to catalysts and CO2 during chemical fuel synthesis reactions in aqueous environments. It is hypothesized that the observed high stability of the hydride on GaP may contribute to its associated remarkable near-100% faradaic efficiency for methanol generation by solar-driven CO2 reduction in acidified aqueous pyridine solutions [J. Am. Chem. Soc.2008, 130, 6342] because such stability is known to yield high overpotentials for the competing hydrogen evolution reaction.

Artificial photosynthesis by photoelectrocatalytic CO2 reduction is dependent, as is natural photosynthesis, on interfacial electron transfer to couple light excitation energy to reaction centers. For heterogeneous systems, in the context of frontier orbital theory artificial reaction centers are defined through the interactions of filled and empty orbitals within a few electronvolts of the Fermi energy of the adsorbate complex. Here we report a scanning tunneling microscopy (STM) and density functional theory investigation of the orbital-resolved adsorption state defining the dative bonding interaction between a III–V semiconductor surface [GaP(110)] and a N-containing heteroaromatic (pyridine). This system was selected for its relevance to photoelectrocatalysis utilizing heteroaromatic cocatalysts, which has been reported to yield highly selective CO2 reduction to fuels. By examining the distribution of unoccupied molecular orbitals, we show that STM images can be used to positively identify the sites on pyridine susceptible to nucleophilic attack, consistent with frontier orbital theory. This indicates that STM can be used to explore the local reaction centers of adsorbed ambidentate electrophiles and nucleophiles relevant to artificial photosynthesis, and more broadly to generate critical mechanistic information for various heterogeneous acid–base reactions.

We report the observation and molecular-scale scanning probe electronic structure (dI/dV) mapping of hydrogen-bonded cyclic water clusters nucleated on an oxide surface. The measurements are made on a new type of cyclic water cluster that is characterized by simultaneous and cooperative bonding interactions among molecules as well as with both metal and oxygen sites of an oxide surface. Density functional theory + U + D calculations confirm the stability of these clusters and are used to discuss other potential water-oxide bonding scenarios. The calculations show that the spatial distributions of electronic states in the system are similar in character to those of the lowest unoccupied molecular orbitals of hydrogen-bonded water molecules. On the partially oxidized Cu(111) investigated here, experiment and theory together suggest that Cu vacancies in the growing islands of cuprous oxide inhibit water adsorption in the centers of the islands (which have reached thermodynamic equilibrium). A stoichiometric, less stable cuprous oxide likely exists at island edges (the growth front) and selectively binds these water clusters.

A novel electrocatalytic surface consisting of a Pt monolayer (ML) on an Hf–Ir alloy substrate demonstrated significantly higher activity (six times) and higher selectivity to CO2 formation than bulk Pt in oxidizing ethylene glycol. This enhanced performance could be associated with the high reducibility of Hf oxide and altered electronic property of the Pt ML.

Photoelectrochemical solar fuel synthesis devices based on photoactive hematite (α-Fe2O3) anodes have been extensively investigated, yet a fundamental understanding regarding its associated water oxidation surface reaction mechanism is still lacking. To help elucidate detailed reaction mechanisms, we studied water chemisorption and reaction as well as structural changes induced by Ni incorporation into the α-Fe2O3(0001) surface. Investigation by scanning probe and electron diffraction techniques show that vapor deposition of Ni and subsequent annealing to 700 K leads to the interdiffusion and incorporation of Ni into the near-surface region of hematite and changes the structure of the (0001) surface by the formation of FeO-like domains on the topmost layer. These results are discussed in the context of a proposed water oxidation mechanism on this surface in which Ni doping facilitates water oxidation by increasing O hole concentrations and forms less negatively charged O anions (*O) and *O···OH species [Liao, P. L.; Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 13296−13309.]. Consistent with predictions from this theory, electrochemical measurements using cyclic voltammetry carried out on the ultrahigh vacuum-prepared surfaces demonstrated that Ni incorporation leads to higher current density and lower onset potential than the unmodified α-Fe2O3 surface. Our work utilizing a surface science approach helps to connect such theoretical predictions of reaction thermodynamics on well-defined structures and the performance of modified hematite model electrocatalysts for water oxidation.Keywords: electrocatalysis; hematite; heterogeneous catalysis; surface chemistry; water oxidation

The physical and photoelectrochemical properties of a composite oxide photoelectrode comprised of α-Fe2O3 and WO3 crystals is investigated. The composite films exhibit a water oxidation photocurrent onset potential as low as 0.43 V vs. RHE, a value considerably lower than that of pure α-Fe2O3 photoanodes prepared in comparable synthesis conditions. This result represents one of the lowest onset potentials measured for hematite-based PEC water oxidation systems. Compositional analysis by X-ray Photoelectron Spectroscopy and Energy Dispersive Spectroscopy indicates the composition of the films differs between the surfaces and bulk, with tungsten found to be concentrated in the surface region. Post-reaction Raman spectroscopy characterization demonstrates that water interacts with surface WO3 crystals, an event that is associated with the formation of a hydrated form of the oxide.

Acetylene reactivity as a function of Sn concentration on Pt catalytic surfaces was studied by comparing adsorption and reactions of regular and deuterated acetylene at 90–1000 K on three surfaces, Pt(111), Pt3Sn/Pt(111), and Pt2Sn/Pt(111), using high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and density functional theory calculations. The strongly adsorbed di-σ/π-bonded acetylene species, which dominate on pure Pt, were not detected on the Pt–Sn surfaces. The presence of Sn is also shown to suppress acetylene decomposition and, as a result, to maintain adsorbed acetylene in the molecular form as weakly adsorbed π- and di-σ-bonded species. The destabilization of adsorbed acetylene makes associative reactions with the formation of dimers (C4 hydrocarbons) and trimers (benzene) progressively more energetically favorable with increasing Sn concentration. Acetylene adsorption modes and reactions on Pt catalytic surfaces can, therefore, be controlled with Sn alloying. The concentration of Sn needs to be an optimal level for catalytic activity since all hydrocarbon species bind preferentially only to Pt sites.Keywords: adsorption; cyclotrimerization; DFT; ethyne; HREELS; platinum; platinum−tin alloy; TPD

Alloy formation and chemisorption at bimetallic surfaces formed by vapor-depositing Zn on a Pt(111) single crystal were investigated primarily by using X-ray photoelectron diffraction (XPD), X-ray photoelectron spectroscopy (XPS), low-energy alkali ion scattering spectroscopy (ALISS), low electron energy diffraction (LEED), and temperature programmed desorption (TPD). A wide range of conditions were investigated to explore whether deposition and annealing of Zn films could produce well-defined, ordered alloy surfaces, similar to those encountered for Sn/Pt(111) surface alloys. These attempts were unsuccessful, although weak, diffuse (2 × 2) spots were observed under special conditions. The particular PtZn bimetallic alloy created by annealing one monolayer of Zn on Pt(111) at 600 K, which has a Zn composition in the surface layer of about 5 at. %, was investigated in detail by using XPD and ALISS. Only a diffuse (1 × 1) pattern was observed from this surface by LEED, suggesting that no long-range, ordered alloy structure was formed. Zn atoms were substitutionally incorporated into the Pt(111) crystal to form a near-surface alloy in which Zn atoms were found to reside primarily in the topmost and second layers. The alloyed Zn atoms in the topmost layer are coplanar with the Pt atoms in the surface layer, without any “buckling” of Zn, that is, displacement in the vertical direction. This result is expected because of the similar size of Pt and Zn, based on previous studies of bimetallic Pt alloys. Zn atoms desorb upon heating rather than diffusing deep into the bulk of the Pt crystal. Temperature programmed desorption (TPD) measurements show that both CO and NO have lower desorption energies on the PtZn alloy surface compared to that on the clean Pt(111) surface.

Tungsten carbide (WC) has been considered a promising replacement for precious metal-based catalysts and electrocatalysts; however, synthesis of high-quality WC that is free of surface carbon remains a major challenge. Surface carbon adversely influences the catalytic activity of WC and hinders direct interaction between metal adlayer modifiers and the WC substrate. In this letter, we report the beneficial effects of pretreatments of WC foil by atomic oxygen generated in an oxygen plasma source. We found that the graphitic carbon at the WC surface could be removed controllably by the atomic oxygen without causing oxidation of WC, and this improved performances for electrocatalytic methanol oxidation and hydrogen evolution.Keywords: hydrogen evolution reaction (HER); methanol oxidation reaction (MOR); oxygen plasma treatment; tungsten carbide;

While a high efficiency of contaminant removal by nanoscale zerovalent iron (nZVI) has often been reported for several contaminants of great concern, including aqueous arsenic species, the transformations and translocation of contaminants at and within the nanoparticles are not clearly understood. By analysis using in situ time-dependent X-ray absorption spectroscopy (XAS) of the arsenic core level for nZVI in anoxic As(III) solutions, we have observed that As(III) species underwent two stages of transformation upon adsorption at the nZVI surface. The first stage corresponds to breaking of As–O bonds at the particle surface, and the second stage involves further reduction and diffusion of arsenic across the thin oxide layer enclosing the nanoparticles, which results in arsenic forming an intermetallic phase with the Fe(0) core. Extended X-ray absorption fine-structure (EXAFS) data from experiments conducted at different iron/arsenic ratios indicate that the reduced arsenic species tend to be enriched at the surface of the Fe(0) core region and had limited mobility into the interior of the metal core within the experimental time frame (up to 22 h). Therefore, there was an accumulation of partially reduced arsenic at the Fe(0)/oxide interface when a relatively large arsenic content was present in the solid phase. These results illuminate the role of intraparticle diffusion and reduction in affecting the chemical state and spatial distribution of arsenic in nZVI materials.

It is observed that pyridinium is hydrogenated at Pt electrodes in electrochemical conditions consistent with those previously shown to yield selective reduction of carbon dioxide to methanol and formic acid. The hydrogenation proceeds through a heterogeneous reaction with chemisorbed hydrogen, which originates from one-electron surface proton transfer reactions. Electrochemical methods are used to show that pyridinium adsorbs on the Pt surface, consistent with the proposed heterogeneous reaction mechanism. From this first observation of the electrochemical generation of a stable hydrogenated piperidinium-like near-surface species it logically follows that dihydropyridinium, the protonated form of the previously-proposed hydride-shuttling reduction catalyst, must transiently exist under these conditions near the Pt surface in the presence of carbon dioxide. Therefore partially hydrogenated heterocycles remain strong candidates for catalytically active species that enable selective carbon dioxide reduction. More generally, the observed mild potentials required for electrocatalytic hydrogenation of stable organics implies that engineered transfer hydrogenations involving organic adsorbates can be a viable approach for achieving selective carbon dioxide reduction to fuels.

The physical and photoelectrochemical properties of a composite oxide photoelectrode comprised of α-Fe2O3 and WO3 crystals is investigated. The composite films exhibit a water oxidation photocurrent onset potential as low as 0.43 V vs. RHE, a value considerably lower than that of pure α-Fe2O3 photoanodes prepared in comparable synthesis conditions. This result represents one of the lowest onset potentials measured for hematite-based PEC water oxidation systems. Compositional analysis by X-ray Photoelectron Spectroscopy and Energy Dispersive Spectroscopy indicates the composition of the films differs between the surfaces and bulk, with tungsten found to be concentrated in the surface region. Post-reaction Raman spectroscopy characterization demonstrates that water interacts with surface WO3 crystals, an event that is associated with the formation of a hydrated form of the oxide.

A novel electrocatalytic surface consisting of a Pt monolayer (ML) on an Hf–Ir alloy substrate demonstrated significantly higher activity (six times) and higher selectivity to CO2 formation than bulk Pt in oxidizing ethylene glycol. This enhanced performance could be associated with the high reducibility of Hf oxide and altered electronic property of the Pt ML.

A new electrode structure enabling low overpotentials for the oxidation of water, based on three-dimensional arrays of CoOOH nanowires, is presented. The electrocatalytic activities of pure and M-modified cobalt oxyhydroxides (M = Ni or Mn) nanowires have been investigated in detail for the oxygen evolution reaction (OER) in an alkaline environment. The pure, Ni-, and Mn-modified nanowires, with preferentially exposed low-index surfaces, were fabricated directly on stainless steel mesh current collectors using an inexpensive and scalable chemical synthesis procedure. The unique electrode structure ensures excellent substrate–catalyst electrical contact and increases the surface area accessible to the electrolyte. The OER activity of CoOOH nanowires is shown to be significantly improved through incorporation of Ni. Specifically, optimal OER activity is obtained for CoOOH nanowires with 9.7% surface Ni content, which corresponds to four-times greater current density compared to pure CoOOH. In contrast, Mn modification of the CoOOH nanowires did not improve the OER activity. Tafel analysis suggests Ni incorporation leads to change in the OER rate-determining step based on an observed decrease in the Tafel slope. Electrochemical impedance spectroscopy reveals that Ni incorporation improves the ability of the catalysts to stabilize surface intermediates, whereas Mn incorporation impedes intermediate stabilization. This study provides new insights regarding the influence of transition metal impurities on the OER activity of CoOOH and provides a clear strategy for the optimization of CoOOH-based OER catalysts in alkaline electrolytes.