Surface Alloy Composition Controlled O₂ Activation on Pd–Au Bimetallic Model Catalysts

Oxygen is an important reactant in several catalytic conversions and partial oxidation reactions on Pd–Au alloy surfaces; however, adsorption and dissociation are not fully understood, especially as a function of the surface alloy composition. In this study, we probe the influence of the atomic makeup of the surface of Pd–Au catalysts regarding control of the catalytic activity toward O₂ dissociation and the reactivity of the resulting oxygen adatoms. To experimentally investigate this, we prepared various bimetallic surfaces under ultrahigh vacuum via evaporation of Pd onto a Au(111) surface. Hydrogen molecules were used to characterize the composition of the Pd–Au surfaces, which we simplistically group into two categories: (i) Pd–Au interface sites and (ii) Pd(111)-like island sites. When the Pd coverage is 1.0 ML, which predominantly indicates Pd–Au interface sites, no dissociative adsorption of O₂ at 300 K is observed, but dissociation begins to be measurable on the surfaces with larger Pd loadings (greater than 1.5 ML), which we believe leads to Pd(111)-like islands on the surface. We also find that adsorbed oxygen atoms are very reactive at the Pd–Au interface sites via measurements of the CO oxidation reaction at relatively low temperatures (<200 K); however, CO oxidation can also take place at higher temperatures (∼400 K) and in this case is very dependent on Pd coverage, being strongly related to the number of Pd­(111)-like islands, which bind O_a relatively strongly. From our experimental results, we estimate the barrier to dissociation of O₂ and also the CO oxidation reaction barrier, which is an indirect measure of the reactivity of the adsorbed atomic oxygen. From our analysis, we find that, upon increasing Pd coverage, the dissociation barrier for O₂ steadily decreases and, further, the reaction barrier for CO oxidation continuously increases. Finally, oxygen molecularly adsorbs on the Pd–Au bimetallic surface and is a precursor to dissociative O₂ chemisorption, just as with pure Pd surfaces, and additionally, the enhanced reactivity of adsorbed atomic oxygen originates at the interfaces between Pd and Au domains

Coincorporation of N and Ta into TiO₂ Nanowires for Visible Light Driven Photoelectrochemical Water Oxidation

We report a synthesis of N- and Ta-coincorporated TiO₂ (N,Ta:TiO₂) and Ta-incorporated TiO₂ (Ta:TiO₂) nanowire (NW) arrays and their application as photoanodes for water photooxidation. Tantalum is incorporated into TiO₂ NWs with concentrations ranging from 0.11 to 3.47 atomic % by a simple solvothermal synthesis. N,Ta:TiO₂ nanowires are prepared via nitridation of Ta:TiO₂ nanowires in NH₃ flow at a relatively low temperature (500 °C). N,Ta:TiO₂ NWs with the optimum Ta concentration of 0.29 atomic % also demonstrate significant enhancement in photoelectrochemical performance with the photocurrent reaching 0.52 and 0.18 mA/cm² under AM 1.5 G and visible light (>420 nm) illumination, compared with 0.26 and 0.13 mA/cm² for that of N:TiO₂ NWs, although the active spectrum of the N,Ta:TiO₂ NW sample only extends to ∼520 nm (2.38 eV), compared to ∼540 nm (2.30 eV) for N:TiO₂ NWs. We believe that the enhancement shown by the N,Ta-coincorporated sample is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process

Hydrogen Adsorption and Absorption with Pd–Au Bimetallic Surfaces

Pd–Au bimetallic catalysts have shown promising performance in numerous reactions that involve hydrogen. Fundamental studies of hydrogen interactions with Pd–Au surfaces could provide useful insights into the reaction mechanisms over Pd–Au catalysts, which may, in turn, guide future catalyst design. In this study, the interactions of hydrogen (i.e., adsorption, absorption, diffusion, and desorption) with Pd/Au(111) model surfaces were studied using temperature-programmed desorption (TPD) under ultrahigh-vacuum conditions. Our experimental results reveal Pd–Au bimetallic surfaces readily dissociate H₂ and yet also weakly bind H adatoms, properties that could be beneficial for catalytic reactions involving hydrogen. The presence of contiguous Pd sites, characterized by reflection–absorption infrared spectroscopy using CO as a probe molecule (CO-RAIRS), was found to be vital for the dissociative adsorption of H₂ at 77 K. The H adatom binds to Pd–Au alloy sites more strongly than to Au(111) but more weakly than to Pd(111) as indicated by its desorption temperature (∼200 K). With hydrogen exposure at slightly higher temperatures (i.e., 100–150 K), extension of a low-temperature desorption feature was observed, suggesting the formation of subsurface H atoms (or H absorption). Experiments using deuterium indicate that H–D exchange over the Pd–Au bimetallic surface obeys Langmuir–Hinshelwood kinetics and that H/D adatoms are mobile on the surface at low temperatures

Oxygen-Electrode Catalysis on Oxoperovskites at 700 °C versus 20 °C

The oxygen-reduction and oxygen-evolution reactions (ORR and OER) at 700 °C on the perovskites ABO_3−δ (A = La, Sr and B = Co, Fe, Mn, Cr with undoped versus Nb- or Ta-doped SrCoO_3−δ) have been evaluated experimentally with a reversible solid oxide fuel cell (R-SOFC). The predictor for active ORR catalysis at 20 °C in alkaline solution is not applicable at 700 °C; the adsorbed water on the oxide catalyst surface is lost. In a SOFC, the ORR is split between the fuel and the oxygen electrode; in an alkaline air battery, the entire ORR occurs at the oxygen electrode. On the other hand, the OER reaction occurs by a similar process at the oxygen electrode in a R-SOFC at 700 °C and in an air battery or room-temperature fuel cell in an alkaline solution. A proposed condition for both the ORR and the OER occurring at the same oxoperovskite surface at 700 °C is a transition-metal cation of the perovskite at its equilibrium oxidation state at the operating temperature and <i>p</i>O₂

Methanol O–H Bond Dissociation on H‑Precovered Gold Originating from a Structure with a Wide Range of Surface Stability

Gold has been shown to exhibit promising catalytic activity, and understanding the fundamental interactions of reactants and hydrogen atoms on a gold surface is key to gaining insight into hydrogenation reaction mechanisms. In this paper, we report that the adsorption of methanol onto a H-precovered Au(111) surface induces an adsorbate structure, or set of structures, on the surface involving both methanol and hydrogen adatoms with a wide range of stability on the surface. Coadsorption of H/MeOD or D/MeOH indicates H/D exchange between the two surface species, providing evidence that the H-precovered gold surface can dissociate the methanol O–H bond at low temperature (<120 K). These isotopic experiments also demonstrate that hydrogen/deuterium atoms released from a methanol molecule desorb at higher temperatures than hydrogen/deuterium atoms originating from the surface, providing insight into the adsorbate structure(s) present. Additionally, the presence of MeOH on the surface is shown to inhibit the ability of adsorbed MeOD to undergo hydrogen exchange, providing additional clues regarding the exchange reaction mechanism. These phenomena are also shown to exist for ethanol on H-precovered Au(111), suggesting that this behavior may be common to alcohols or species with an O–H functional group in general. These observations give insight into the behavior of the O–H group on a gold surface, which can aid in determining reaction mechanisms and directing future catalytic research

SILAR Growth of Ag₃VO₄ and Characterization for Photoelectrochemical Water Oxidation

α-Ag₃VO₄ has shown promise as a photocatalyst for decomposition of organics and H₂O in particle dispersion studies, but no thin film studies of Ag₃VO₄ have looked at its photoelectrochemical (PEC) properties. Addressing this deficiency, we grow films via successive ionic layer adsorption and reaction (SILAR) and characterize the material using standard physical and PEC techniques. We confirm a low bandgap of 2.2 eV and report the first results on chemical and electrochemical stability, intrinsic doping behavior, flat-band potential, and potential dependence of photocurrent. While our results are not initially promising with respect to the applicability of Ag₃VO₄ to solar water splitting, they highlight the most important property changes necessary to make Ag₃VO₄ competitive with better known photocatalysts and the salience of thin-film studies for PEC material characterization

BiSI Micro-Rod Thin Films: Efficient Solar Absorber Electrodes?

The development of improved solar energy conversion materials is critical to the growth of a sustainable energy infrastructure in the coming years. We report the deposition of polycrystalline BiSI thin films exhibiting promising photoelectrochemical properties on both metal foils and fluorine-doped tin-oxide-coated glass slides using a single-source chemical spray pyrolysis technique. Their strong light absorption in the visible range and well-crystallized layered structure give rise to their excellent photoelectrochemical performance through improved electron–hole generation and separation. The structure and surface composition of the films are dependent on deposition temperature, resulting in dramatic differences in performance over the temperature range studied. These results reveal the potential of <i>n</i>-BiSI as an alternative thin film solar energy conversion material and may stimulate further investigation into V–VI–VII compounds for these applications

Simple Synthesis of Nanostructured Sn/Nitrogen-Doped Carbon Composite Using Nitrilotriacetic Acid as Lithium Ion Battery Anode

A composite of 3.5 nm Sn nanoparticles dispersed in nitrogen-doped carbon was prepared from low cost precursors, using simple equipment, by the simple process of hydrolyzing at 300 °C SnCl₄ mixed with nitrilotriacetic acid and then pyrolyzing the complexed SnO₂ at 650 °C. The affordable anode made with the composite retained at 0.2 A g^–1 specific current a specific capacity of 660 mAh·g^–1 at the 200th cycle and a 630 mAh·g^–1 capacity at 400th cycle. At 1 A g^–1 specific current the capacity was as 435 mAh·g^–1

Low-Temperature Hydrogenation of Acetaldehyde to Ethanol on H-Precovered Au(111)

Gold-based classical high surface area catalysts have been widely investigated for hydrogenation reactions, but fundamental studies on model catalysts are lacking. We present experimental measurements of the reaction of hydrogen adatoms and adsorbed acetaldehyde on the Au(111) surface employing temperature-programmed desorption. Here, we show that chemisorbed hydrogen adatoms bind weakly with desorption peaks at ∼110 K, indicating an activation energy for recombinative desorption of ∼28 kJ/mol. We further demonstrate that acetaldehyde (CH₃CHO) can be hydrogenated to ethanol (CH₃CH₂OH) on the H-atom-precovered Au(111) surface at cryogenic temperatures. Isotopic experiments employing D atoms indicate a lower hydrogenation reactivity

Effect of Si Doping and Porosity on Hematite’s (α-Fe₂O₃) Photoelectrochemical Water Oxidation Performance

The photoelectrochemical water oxidation performance under simulated solar irradiation of hematite (α-Fe₂O₃) films synthesized by coevaporation of pure Si and Fe in an oxygen ambient, a process known as reactive ballistic deposition, is studied as a function of Si doping level and film porosity, ranging from dense films to nanocolumnar films. It is found that Si segregates to the hematite surface, does not improve the bulk conductivity, and lowers the optical absorption coefficient. Nevertheless, the photoelectrochemical performance of Si-doped, porous films is significantly improved relative to undoped, porous films. However, the improvement relative to dense, undoped films is marginal. It is concluded that Si acts to passivate the hematite surface and aids charge transfer to the solution. Additionally, from incident photon conversion efficiency measurements it is found that Si doping and porosity have little effect on the normalized spectral response of 100 nm thick hematite films