Adsorption Of Hydrogen Species Research Articles

Ru surface density effect on ammonia synthesis activity and hydrogen poisoning of ceria-supported Ru catalysts

Evaluating the effect of metal surface density on catalytic performance is critical for designing high-activity metal-based catalysts. In this study, a series of ceria (CeO2)-supported Ru catalysts (Ru/CeO2) were prepared to analyze the effect of Ru surface density on the catalytic performance of Ru/CeO2 for ammonia synthesis. For the Ru/CeO2 catalysts with Ru surface densities lower than 0.68 Ru nm−2, the Ru layers were in close contact with CeO2, and electrons were transferred directly from the CeO2 defect sites to the Ru species. In such cases, the adsorption of hydrogen species on the Ru sites in the vicinity of O atoms was high, leading to a high ammonia synthesis activity and strong hydrogen poisoning. In contrast, the preferential aggregation of Ru species into large particles on top of the Ru overlayer resulted in the coexistence of Ru clusters and particles, for catalysts with a Ru surface density higher than 1.4 Ru nm−2, for which Ru particles were isolated from the direct electronic influence of CeO2. Consequently, the Ru−CeO2 interactions were weak, and hydrogen poisoning can be significantly alleviated. Overall, electron transfer and hydrogen adsorption synergistically affected the synthesis of ammonia over Ru/CeO2 catalysts, and catalyst samples with a Ru surface density lower than 0.31 Ru nm−2 or exactly 2.1 Ru nm−2 exhibited high catalytic activity for ammonia synthesis.

Hydrogen adsorbed at N-polar InN: Significant changes in the surface electronic properties

The interaction of atomic hydrogen and ammonia with as-grown N-polar InN surfaces is investigated using in situ photoelectron spectroscopy. Changes in the surface electronic properties, including the band alignment and work function, as well as the chemical bonding states of the substrate and adsorbates are characterized. Ammonia molecules are dissociating at the InN surface, resulting in adsorption of hydrogen species. Consequently, the considerable changes of the chemical and electronic properties of the InN surface during ammonia interaction are almost identical to those found for adsorption of atomic hydrogen. In both cases, hydrogen atoms preferentially bond to surface nitrogen atoms, resulting in the disappearance of the nitrogen dangling-bond-related occupied surface state close to the valence band edge at $\ensuremath{\sim}1.6$ eV binding energy and the formation of new occupied electron states at the conduction band edge. Furthermore, a decrease in work function during adsorption from 4.7 to 3.7--3.8 eV, as well as an increase in the surface downward band bending by 0.3 eV, confirm that hydrogen is acting as electron donor at InN surfaces and therefore has to be considered as one main reason for the surface electron accumulation observed at N-polar InN samples exposed to ambient conditions, for example as the dissociation product of molecules. The measured formation and occupation of electronic states above the conduction band minimum occur in conjunction with the observed increase in surface electron concentration and underline the relationship between the energy position of occupied electron states and surface band alignment for InN as a small-band-gap semiconductor.

Electroreduction of CO2 at Cu and Sn Foams

Previous studies on the electrochemical reduction of CO2 at metallic electrodes suggest that the adsorption of hydrogen species is structure-sensitive.[1] Surface roughening is likely to introduce defects favorable for the reaction of adsorbed hydrogen atoms, an important intermediate step in the electro-reduction of CO2 in protic media.[1, 2] Furthermore, roughened surfaces have been shown to be more selective toward hydrocarbon products, which was attributed to a higher number of uncoordinated metal sites.[2] These finding were confirmed by DFT calculations, which suggest that CO2 activation and reduction occurs at these sites. [3] To further gauge the effect of morphology on the faradaic efficiency and distribution of products obtained during the electroreduction of CO2, Cu and Sn foams were electrosynthesized on Cu substrates using a recently reported process.[4] The metal foams were found to be mechanically stable during their preparation, handling, and use in the electrocatalytic reduction of CO2. Both Cu and Sn foams are attractive metals for the electrocatalytic reduction of CO2because of their low cost and non-toxic nature.Electroreduction of CO2 was performed in a typical H-cell under potentiostatic conditions. The faradaic efficiency of producing formate from CO2 at the metal foams and the equivalent planar metal electrode are compared in Figure 1a and 1b. The faradaic efficiencies obtained with the Cu foam electrode were found to be higher at all potentials with a maximum efficiency of 37% at -1.5V for HCOOH. Previously reported data is included for comparison.[5] Likewise, the faradaic efficiencies for HCOOH generation at the Sn foam electrode were found to be higher at all potentials with a maximum efficiency of 89.5% at -1.7V. Previously reported data with slightly different conditions (Sn gas diffusion cell in 0.5 M NaHCO3) is included for comparison.[6] XRD analysis of the Cu and Sn foams did not reveal major differences in the relative ratio of dominant crystal facets when compared to the equivalent planar samples of high purity metal. The hierarchical nature of the pore architecture along with high surface area of the metal foams could affect the reaction kinetics and contribute to the observed increase in faradaic efficiency. For example, the porous nature of the metal foams may promote interactions between incoming CO2, adsorbed surface species, electrolyte, and reducing equivalents that are novel relative to the same reaction at a planar electrode. The effect of varying the electrodeposition time and resulting foam architecture on the electroreduction of CO2will be the focus of this presentation. References Hori, Y., A. Murata, and R. Takahashi, Formation of hydrocarbons in the electrochemical reduction of carbon dioxide at a copper electrode in aqueous solution. Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases, 1989. 85(8): p. 2309-2326.Tang, W., A.A. Peterson, A.S. Varela, Z.P. Jovanov, L. Bech, W.J. Durand, S. Dahl, J.K. Norskov, and I. Chorkendorff, The importance of surface morphology in controlling the selectivity of polycrystalline copper for CO2 electroreduction. Physical Chemistry Chemical Physics, 2012. 14(1): p. 76-81.Durand, W.J., A.A. Peterson, F. Studt, F. Abild-Pedersen, and J.K. Nørskov, Structure effects on the energetics of the electrochemical reduction of CO2 by copper surfaces. Surface Science, 2011. 605(15–16): p. 1354-1359.Shin, H.C., J. Dong, and M. Liu, Nanoporous Structures Prepared by an Electrochemical Deposition Process. Advanced Materials, 2003. 15(19): p. 1610-1614.Kuhl, K.P., E.R. Cave, D.N. Abram, and T.F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy & Environmental Science, 2012. 5(5): p. 7050-7059.G.K.S. Prakash, F.A. Viva, G.A. Olah, Electrochemical reduction of CO2 over Sn-Nafion® coated electrode for a fuel-cell-like device, Journal of Power Sources, 223 (2013) 68-73.

Hydrogen Detection with Noble Metal-TiO2 Schottky Diodes

Fast detection of hydrogen at a wide concentration range is desired for many applications. We report detecting hydrogen using Au-TiO2-Ti and Ag-TiO2-Ti diodes. While hydrogen-Au and hydrogen-Ag interactions are very different, at a constant biasing voltage, the measured current in both diodes is highly sensitive to the partial pressure of hydrogen contamination in the surrounding atmosphere. Work function variations were investigated by connecting the I-V specifications to the energy barrier height established at the Au-TiO2 and Ag-TiO2 junctions. Electronic features of the devices were described based on the assumption of two different hydrogen-noble metal interactions: Hydrogen reduces Ag work function by reducing the adsorbed oxygen species from the silver surface, while Au work function is reduced by the same mechanism as well as the direct adsorption of hydrogen species to the gold surface. Both of these mechanisms result in hydrogen detection by Schottky barrier height reduction and current increase.

Gas-phase Hydrodechlorination of Chlorobenzene Over Silica-supported Ni2P Catalysts Prepared Under Different Reduction Conditions

The influence of reduction conditions on the properties and reactivity of silica-supported nickel phosphide (Ni2P/SiO2) catalysts for gas-phase hydrodechlorination (HDC) of chlorobenzene was investigated. The catalysts prepared under different reduction conditions had the similar specific surface area (∼370 m2 g-1), and pore diameter (∼5.2 nm) and Ni2P crystallites size (10–13 nm). However, comparing with Ni2P/SiO2 catalyst prepared at 923 K with the H2 space velocity of 15,000 mL g−1 h−1 for 2 h, with increasing the H2 space velocity from 15,000 to 19,200 mL g−1 h−1, or the reduction temperature from 923 to 1023 K or the reduction time from 2 to 6 h, the Ni/P ratio in the prepared catalyst was increased from 1/0.56 to about 1/0.50, and the HDC reaction induction period over the catalyst was also shortened from above 30 to 2–4 h. It is suggested that the induction period might be due to the blocking of active sites by excess phosphorus, which results in hindering the activation of chlorobenzene and the adsorption of hydrogen species on Ni2P. Under the conditions of 573 K and WNi/FCl = 186.6 gNi\({\text{mol}}{}_{{\text{Cl}}}^{ - 1} \) min, the chlorobenzene conversion over the Ni2P/SiO2 catalyst reached 99%, and it did not change during 36 h. The good activity and stability of the Ni2P/SiO2 catalyst was ascribed to the weak interaction between chlorine and Ni2P and a great of spilt-over hydrogen species on the catalyst surface.