3-fold Hollow Sites Research Articles

The dissociation of H2O on the γ-U (110) and (100) surfaces: an ab initio study

The dissociation of H2O on γ-U (110) and γ-U (100) surfaces has been studied by using ab initio molecular dynamics simulations at an elevated temperature of 800 K. The simulation results show the dissociation of H2O into the OH group and H atom, which are finally adsorbed on the uranium surface. The dissociation results from electronic interactions between surface uranium 6d/5 f states and the s orbitals of H and the 2p orbitals of O. Additionally, the hybridization between the 6d orbital of surface uranium and the 2p orbital of oxygen plays a dominant role in dissociative adsorption. A significant charge transfer from the uranium surface to the O and H atoms is observed, indicating the formation of U–O and U–H chemical bonds. Specifically, for γ-U (110) surface, the most preferred site for OH is the 3-fold hollow site and H occupies the bridge site or the 3-fold hollow site. On the other hand, for γ-U (100) surface, OH prefers to adsorb on the bridge site and H occupies the 3-fold hollow site or the bridge site. Furthermore, when H2O is placed on the TOP site, its initial dissociation on the γ-U (110) surface is easier compared to the γ-U (100) surface.

Selective Cleavage of α-Olefins to Produce Acetylene and Hydrogen.

Acetylene production from mixed α-olefins emerges as a potentially green and energy-efficient approach with significant scientific value in the selective cleavage of C-C bonds. On the Pd(100) surface, it is experimentally revealed that C2 to C4 α-olefins undergo selective thermal cleavage to form surface acetylene and hydrogen. The high selectivity toward acetylene is attributed to the 4-fold hollow sites which are adept at severing the terminal double bonds in α-olefins to produce acetylene. A challenge arises, however, because acetylene tends to stay at the Pd(100) surface. By using the surface alloying methodology with alien Au, the surface Pd d-band center has been successfully shifted away from the Fermi level to release surface-generated acetylene from α-olefins as a gaseous product. Our study actually provides a technological strategy to economically produce acetylene and hydrogen from α-olefins.

Regulation of cerium species in Keggin structure of phosphotungstic acid for efficient nitrogen electroreduction to ammonia

As a sustainable and economical strategy, electrochemical nitrogen reduction reaction (eNRR) is a promising alternative of energy-intensive Haber-Bosch process to produce ammonia. Fabricating efficient catalysts for eNRR is still challenging due to the bottleneck of insufficient activity and low electron selectivity. Herein, phosphotungstic acid (PTA) as substrate to anchor cerium species on the 4-fold hollow site was prepared, which reaches high ammonia yield rates of 42 μg h−1 mgcat−1 and Faradaic efficiencies of 9.2 % at −0.6 V versus reversible hydrogen electrode. Experiments and theoretical calculations reveal that the introduction of Ce species could significantly improve the electronic density of adjacent exposed W atoms, further boosting the effective adsorption and activation of N2 to form active *N2. Finally, the designed Ce-PTA catalysts show high electrocatalytic activity of N2 reduction into ammonia. This work highlights the important role of using PTA as substrate to prepare stable supported metal species for heterogeneous electrocatalysis.

Enriching Pt3 ensemble with isolated 3-fold hollow site by crystal-phase engineering of Pt3Fe single-nanoparticle for acetylene hydrogenation

Tuning the crystal-phase of bimetallic particles enables a precise control of catalytically active sites but is experimentally challenged by the averaging ensemble effects that are caused by the particle communications occurred during the crystal-phase transformation. Here, H2-treatment at 773–973 K of Pt3Fe single-nanoparticle, confined by a permeable silica shell, achieved crystal-phase transformation from the disordered A1 phase to the chemically ordered L12 phase. Detailed microscopic and spectroscopic characterizations have identified that the disordered A1 particle possessed contiguously connected large Pt domains while the ordered L12 particle afforded regularly populated Pt3 ensembles surrounded by Fe single-atoms. The isolated 3-fold hollow Pt site in the three-atom ensemble efficiently boosted the adsorption of acetylene, yielding a much higher hydrogenation activity. The reaction rate of the L12 particle was twice of that for the A1 particle. Moreover, the charge transfer from the neighboring Fe single-atoms to the Pt atoms on the L12 particle expedited the desorption of ethylene, promoting the selectivity as well. Overall, both geometric and electronic interactions in the ordered crystal-phase synergistically facilitated the selective hydrogenation of acetylene to ethylene.

A density functional theory study of thiophene and pyridine adsorption on Pt/Rh-doped Cu (100) surface

The adsorption of pyridine and thiophene molecules on clean, doped Rh-Cu (100) and Pt-Cu (100) surfaces was investigated using density functional theory calculations including van der Waals interactions (optB86b-vdW). Various Rh-Cu and Pt-Cu slabs, in which the Cu surface atoms are substituted by a monomer, dimer, and tetramer, were tested by adsorbing pyridine and thiophene molecules. The adsorption properties of these systems were examined regarding their adsorption energy, adsorption heights, electron density difference (EDD), and density of states (DOS). Our results indicate that, for the thiophene molecule, the adsorption energy was increased from 0.98 to 3.55 eV after alloying the Cu surface with an Rh tetramer. Concerning the adsorption of pyridine, the adsorption energy is increased from 0.96 to 3.06 eV after substituting the Cu atoms with an Rh tetramer. Therefore, the Rh-Cu (100) doped surface was found to be more suitable for the adsorption of thiophene and pyridine in comparison with the clean surface. Bader charge analysis shows that the charge transfer was from surface atoms to the thiophene molecule for all systems except the case of the adsorption on the Pt4-Cu (100) surface. However, concerning the adsorption of pyridine on doped surfaces, all charge transfers are moved from the surface to the molecule and attained 0.34 electron charge when the Cu atoms are substituted by Rh dimer. Furthermore, the electron density difference (EDD) calculations for doped surfaces display a charge accumulation on the thiophene molecule. The EDD confirmed the results of the charge transfer obtained for the pure and doped Cu (100) surface with Pt tetramer and Rh tetramers. The results of PDOS of four metal atoms in the top layer making a 4-fold hollow site indicate an upshift of the d orbitals, which can explain the redistribution of charge between the surface and both molecules.

Hydrogen-Induced Restructuring of a Cu(100) Electrode in Electroreduction Conditions

The rearrangement of Cu surfaces under electrochemical conditions is known to play a key role in the surface activation for major electrocatalytic reactions. Despite the extensive experimental insights into such rearrangements, from surface-sensitive spectroscopy and microscopy, the spatial and temporal resolution of these methods is insufficient to provide an atomistic picture of the electrochemical interface. Theoretical characterization has also been challenged by the diversity of restructuring configurations, surface stoichiometry, adsorbate configurations, and the effect of the electrode potential. Here, atomistic insight into the restructuring of the electrochemical interface is gained from first principles. Cu(100) restructuring under varying applied potentials and adsorbate coverages is studied by grand canonical density functional theory and global optimization techniques, as well as ab initio molecular dynamics and mechanistic calculations. We show that electroreduction conditions cause the formation of a shifted-row reconstruction on Cu(100), induced by hydrogen adsorption. The reconstruction is initiated at 1/6 ML H coverage, when the Cu-H bonding sufficiently weakens the Cu-Cu bonds between the top- and sublayer, and further stabilized at 1/3 ML when H adsorbates fill all the created 3-fold hollow sites. The simulated scanning tunneling microscopy (STM) images of the calculated reconstructed interfaces agree with experimental in situ STM. However, compared to the thermodynamic prediction, the onsets of reconstruction events in the experiment occur at more negative applied voltages. This is attributed to kinetic effects in restructuring, which we describe via different statistical models, to produce the potential- and pH-dependent surface stability diagram. This manuscript provides rich atomistic insight into surface restructuring in electroreduction conditions, which is required for the understanding and design of Cu-based materials for electrocatalytic processes. It also offers the methodology to study the problem of in situ electrode reconstruction.

Adsorption of benzene on defective Pt surfaces: A DFT study

Defects occur naturally on surfaces during formation. The interaction of a benzene molecule with a defective surface of platinum is investigated using plane wave DFT. Adsorption energy, adsorbate geometry and electronic structure of benzene on defective surfaces of platinum is obtained and compared with those on a perfect platinum (111) facet. We observe that the presence of defects in the form of steps and vacancy leads to an increase in the strength of adsorption. It is found that the atop is the most stable site for adsorption on a Pt (111) surface having a vacancy defect, which agrees with the available experimental results. The terrace region, which has a symmetry similar to a perfect Pt(111) surface, gives the 3-fold symmetric fcc hollow site as energetically preferred for adsorption. The step edge has the hcp hollow as the preferred site for adsorption. Analysis of electronic structure and charge transfer at the preferred sites is performed. The presence of defects brings out major changes in adsorption process, as compared to that on perfect surfaces. Reduction in coordination of surface sites due to defects is identified as the main reason for more reactive nature of defective surfaces.

Lithium/bismuth co-functionalized phosphotungstic acid catalyst for promoting dinitrogen electroreduction with high Faradaic efficiency

Ammonia occupies a significant standing in the global economy. As a promising alternative to the energy-intensive Haber-Bosch process, electrochemical nitrogen reduction could achieve sustainable ammonia synthesis. However, the attractive alternative suffers from the bottleneck of insufficient activity and low electron selectivity. Here, we report an efficient electrocatalyst using phosphotungstic acid as platforms to anchor bismuth species on the 4-fold hollow site and anchor lithium species on the 3-fold hollow site, which reaches high ammonia yield rates of 61 ± 1 μg h −1 mg cat. −1 and Faradaic efficiencies of 85% ± 2% at −0.1 V versus reversible hydrogen electrode (RHE). Experiments and theoretical calculations reveal that the Bi δ+ species can promote the activation and hydrogenation of nitrogen at adjacent unsaturated tungsten active sites, and the incorporated lithium species reduce the hydrogen reduction rate in undesired water splitting. This synergistic functionalization of bismuth and lithium species contributes to the high-efficient catalyst. Li/Bi species are anchored on phosphotungstic acid by impregnation Li/Bi species synergistically contribute to the activation of nitrogen Li species are favorable to inhibit the competitive hydrogen evolution reaction Li/Bi species teamwork achieves a high Faradic efficiency for aqueous ammonia synthesis Electrochemical nitrogen reduction could achieve sustainable ammonia synthesis as a promising alternative to the energy-intensive Haber-Bosch process. Liao et al. design Li/Bi co-functionalized phosphotungstic acid by precisely anchoring Bi and Li species, where Bi δ+ species facilitate nitrogen reduction and Li species suppress competitive hydrogen evolution reaction. The cooperation promotes electrocatalytic nitrogen reduction to ammonia with, to the best of our knowledge, record Faradic efficiencies for aqueous systems.

Beyond the Reverse Horiuti–Polanyi Mechanism in Propane Dehydrogenation over Pt Catalysts

The catalytic dehydrogenation of light alkanes over Pt catalysts is generally accepted to follow a reverse Horiuti–Polanyi mechanism. Using a microkinetic analysis in combination with results from density functional theory calculations, we show that although propane dehydrogenation (PDH) occurs by two successive dehydrogenation steps on terraces, an unexpected non-reverse Horiuti–Polanyi mechanism accounts for more than half the propylene production at the under-coordinated active sites that dominate the kinetics of PDH. The main reaction is composed of three dehydrogenation steps that have two β-H atoms and one α-H atom removed from propane, followed by the hydrogenation of CH3CCH2; starting from this species, the formation of propylene and byproducts proceed by way of two parallel competing reactions. The proposed mechanism has been verified by exploring several key and general aspects of the kinetic behavior observed in the dehydrogenation of light alkanes, and it is found that only when adsorbate–adsorbate interactions are taken into consideration can the experimentally determined kinetics be properly reproduced. Increasing the H2 partial pressure from low values favors an increase in the coverage of free sites due to the gasification of adsorbed coke precursors, which in turn gives rise to lowered energy barriers for C–H bond breaking, thereby achieving an increased propane consumption rate. As the H2/C3H8 ratio increases, the rate of propylene production first goes up and then declines, and a maximum is observed at a H2/C3H8 ratio of 1.33, which occurs when the negative effect of the increased free 4-fold hollow sites that bring about deep dehydrogenation begins to dominate the positive effect of the increased free step sites that are responsible for activating propane. The mechanism formulated here proves to be valid even if the temperature, pressure, or the H2/C3H8 ratio is varied and hence provides a foundation for the rational design of metal and alloy catalysts for light alkane dehydrogenation.

Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

Summary Propane dehydrogenation (PDH) is a commercial propylene production technology that has received much attention, but high reaction temperatures result in a decrease of propylene selectivity and catalyst stability. This paper describes a single-site [PtZn4] catalyst by assembling atomically ordered intermetallic alloy (IMA) as a selective and ultrastable PDH catalyst. The catalyst enables more than 95% of propylene selectivity from 520°C to 620°C. No obvious deactivation is observed within the 160-h test, superior to PtSn/Al2O3 and state-of-the-art Pt-based catalysts. Based on in situ X-ray absorption fine structure, X-ray photoelectron spectroscopy measurements, and density functional theory calculations, we reveal that the surface [PtZn4] ensembles in PtZn IMAs serve as the key active site structures, wherein the geometry-isolated and electron-rich Pt1 site in [PtZn4] ensembles readily promotes the first and second C–H cleavage of propane but inhibits further dehydrogenation of surface-bounded propylene. This significantly improves the selectivity and stability by prohibiting coke side reactions.

A Comprehensive Study of the Bridge Site and Substrate Relaxation Asymmetry for Methanethiol Adsorption on Au(111) at Low Coverage.

We use dispersion-corrected density functional theory to explore the bridge-site asymmetry for methanethiol adsorbed on Au(111) with two different S–C bond orientations. We attribute the asymmetry to the intrinsic character of the Au(111) surface rather than the adsorbate. The preference for bridge-fcc versus bridge-hcp SCH3 adsorption sites is controlled by the S–C bond orientation. The system energy difference favors the bridge-fcc site by 8.1 meV on the unrelaxed Au(111) surface. Relaxing the Au substrate increased this energy difference to 26.1 meV. This asymmetry is also reflected in the atomic displacement of the relaxed Au surface. Although in both cases, the bridge-site Au atoms shift away from the fcc 3-fold hollow site, the motion is greater for the bridge-fcc allowing a more favorable geometry for the sulfur atom to bond to the bridging atoms. We confirm that the adsorption energy is strongly dependent on the S–C bond orientation and position, which can be understood in terms of a simple coordination geometry model. This work has important implications for alkanethiol surface diffusion and the structure of their self-assembled monolayers.

Cyanate and Cyanurate Adsorption at Silver Electrodes in Neutral Solutions: In Situ ATR-SEIRAS and DFT Studies

The adsorption and reactivity of cyanate at silver electrodes is studied spectroelectrochemically in sodium perchlorate solutions and compared to those of cyanuric acid (1,3,5-triazine-2,4,6-trione, C3H3N3O3). The surface enhanced infrared reflection absorption spectra obtained under attenuated total reflection conditions (ATR-SEIRAS) for low cyanate concentrations show that some amount of adsorbed CO is formed at the silver electrode surface at low electrode potentials. Adsorbed cyanate species are formed at higher potentials and predominate at the electrode surface. However, for cyanate concentrations above 1 mM, the ATR-SEIRA spectra show potential-dependent absorption features between 1800 and 1300 cm-1, which are similar to those obtained after dosing cyanuric acid to the sodium perchlorate solution. This observation suggests that adsorbed cyanuric acid related species are formed from cyanate anions at the silver surface sites as the result of an electroless trimerization reaction. The infrared spectra collected in the cyanuric acid containing solutions also show a higher amount of adsorbed CO at low potential when compared to that observed in cyanate-containing solutions, thus indicating certain instability of cyanuric acid at the silver surfaces. Density functional theory (DFT) calculations have allowed the study of the effect of crystallographic orientation and bonding geometry on the adsorption energy and vibrational frequencies of adsorbates. Also, the effect of adsorbate coverage on the vibrational frequencies has been investigated. In the case of cyanate, DFT calculations show that the most favorable adspecies is N-bonded cyanate adsorbed in 4-fold hollow (100) sites. The experimental spectra in cyanuric acid solutions agree with the formation of adlayers of specifically adsorbed triketo-monocyanurate species that adsorb, in a tridentate configuration, perpendicular to the electrode surface. The polyoriented nature of the silver surface used in the spectroelectrochemical experiments, together with the eventual existence of cyanate in domains with different local coverages, can explain the broadness of the band for the asymmetric OCN stretching. For adsorbed triketo-monocyanurate, high local coverages are at the origin of the band observed experimentally at ca. 1790 cm-1 that can be associated with the in-phase collective CO stretching.

Why are trace amounts of chloride so highly surface-active?

On many metals, small quantities of chloride are known to be adsorbed at potentials well below the potential of zero charge and to influence other electrochemical processes. In order to understand this behavior, we have investigated the adsorption of a single Cl− ion from aqueous solution by a combination of density functional theory and classical molecular dynamics, taking Au(111) as a model surface. While in the vacuum, Cl is adsorbed directly on the surface in the 3-fold hollow sites, whereas in aqueous solution, the optimal position lies a fraction of an Ångstrom towards the solution, where the ion is almost fully solvated and mobile in the direction parallel to the surface. As a result, the adsorption of a single ion is exothermic by about −0.6 eV, even at the potential of zero charge. This adsorption behavior is limited to low coverages because the adsorbates repel each other and because full solvation shells can only form at low coverage.

Atomically Dispersed Pt1-Polyoxometalate Catalysts: How Does Metal-Support Interaction Affect Stability and Hydrogenation Activity?

Unlike nanostructured metal catalysts, supported single-atom catalysts (SACs) contain only atomically dispersed metal atoms, hinting at much more pronounced metal-support effects. Herein, we take a series of polyoxometalate-supported Pt catalysts as examples to quantitatively investigate the stability of Pt atoms on oxide supports and how the Pt-support interaction influences the catalytic performance. For this entire series, we show that the Pt atoms prefer to stay at a 4-fold hollow site of one polyoxometalate molecule and that the least adsorption energy to obtain sintering-resistant Pt SACs is 5.50 eV, which exactly matches the cohesive energy of bulk Pt metal. Further, we compared their catalytic performance in several hydrogenation reactions and simulated the reaction pathways of propene hydrogenation by density functional theory (DFT) calculations. Both experimental and theoretical approaches suggest that despite the Pt1-support interactions being different, the reaction pathways of various Pt1-polyoxometalate catalysts are very similar and their effective reaction barriers are close to each other and as low as 24 kJ/mol, indicating the possibility of obtaining SACs with improved stability without compromising activity. DFT calculations show that all reaction elementary steps take place only on the Pt atom without involving neighboring O atoms and that hydrogenation proceeds from the molecularly adsorbed H2 species. Pt SACs give a weaker H2 adsorption energy than Pt clusters or surfaces, resulting in small adsorption equilibrium constants and small apparent activation barriers, which agree between experiment and theory.

Comparative computational study of CO2 dissociation and hydrogenation over Fe-M (M = Pd, Ni, Co) bimetallic catalysts: The effect of surface metal content

Density functional theory (DFT) calculations were performed to study CO2 adsorption, dissociation and hydrogenation over Fe-M (M = Pd, Ni, Co) bimetallic catalysts, with a focus on probing the effect of surface content of the added transition metals to Fe. Various Fe-M bimetallic surfaces were constructed with varied surface atomic ratios of Fe/(Fe + M), based on which CO2 and atomic H* adsorptions were systematically examined. The H* was found to be energetically favorable adsorbed at the 4-fold hollow site of Fe-M catalysts and the adsorption stability was slightly impacted by the surface content of the introduced transition metal. For CO2 adsorption, stable bent structures adsorbed on the 4-fold hollow sites were identified on Fe-Ni and Fe-Co surfaces, no matter at which Fe-M formulations. However, on Fe-Pd surfaces, CO2 adsorption configurations were found to be sensitive to surface Pd content, resulting in large distinctions in adsorption stabilities of CO2 as compared to Fe-Co and Fe-Ni surfaces. CO2 dissociation and initial hydrogenation were comparatively investigated on Fe-M bimetallic surfaces, and the calculation results demonstrated that CO2 conversion properties are similar over Fe-Ni and Fe-Co catalysts, with CO* and HCOO* as the preferred intermediates but the barriers are still above 0.8 eV. While on Fe-Pd bimetallic surfaces, CO2 reactions exhibit significant distinctions with varying the surface Pd content, showing a dramatic preference (Eact around 0.3∼0.4 eV) towards HCOO* and CO* formation at surface Pd/(Pd + Fe) atomic ratios of 4/9 and 5/9. The superior catalytic activities of Fe-Pd catalysts are attributed to the particular surface structures and electronic features at specific bimetallic formulations which result in unique adsorption configurations of CO2 and facilitate the stabilization of transition states in CO* and HCOO* formation pathways in CO2 conversion.

Hydrodesulfurization of Thiophene over γ-Mo2N catalyst

Catalytic removal of the S-content from thiophene is a central step in efforts aiming to reduce the environmental burdens of transportation fuels. In this contribution, we investigate the hydrodesulfurization (HDS) mechanisms of thiophene (C4H4S) over γ-Mo2N catalyst by means of density functional theory (DFT) calculations. The thiophene molecule preferentially adsorbs in a flat mode over 3-fold fcc nitrogen hollow sites. The HDS mechanism may potentially proceed either unimolecularly (direct desulfurization) or via H-assisted reactions (hydrogenation). Due to a sizable activation barrier required for the first CS bond scission of 54.6 kcal/mol, we predict that the direct desulfurization to contribute rather very insignificantly in the HDS mechanism. Transfer of adsorbed hydrogen atoms on the γ-Mo2N surface to the thiophene ring substantially reduces activation barrier required in the CS bond scission to only 24.1 kcal/mol in a process that affords an adsorbed C4H6* species and an S atom. Further hydrogenation of the unsaturated C4H6* produces 2-butene. Kinetics and thermodynamics attributes dictate the occurrence of partial rather than full hydrogenation of C4H6*. Calculated rate constants for all individual steps could be utilized to construct a robust kinetic model for the overall HDS process. Estimated conversion values of thiophene predict 50–70% consumption of thiophene at 700 K and low values of gas hourly space velocities. Reaction routes and kinetic parameters provided herein are useful to design stand-alone γ-Mo2N-based catalysts for applications entailing partial hydrogenation and hydrodesulfurization of severely contaminated S-fuels.

The adsorption and dissociation of O2 on Pd and Pt modified TaC (1 0 0) surface: A first principles study

The adsorption and dissociation of O2 on the palladium and platinum modified TaC (1 0 0) surfaces were investigated based on the density functional theory calculations. It is found that the adsorption sites of O2 are the Ta-Ta bridge sites on both the partially covered TaC (1 0 0) surfaces by Pd and Pt, M4/TaC (1 0 0) (M = Pd and Pt), while the 4-fold metal hollow sites and the metal-metal bridge sites are preferred on the fully covered TaC (1 0 0) surfaces by Pd and Pt monolayer, MML/TaC (1 0 0), respectively. The deposition of Pd or Pt can enhance the oxidation resistance of TaC (1 0 0). Meanwhile, the TaC (1 0 0) decorated by monolayer Pd still exhibited outstanding catalytic activity for O2 dissociation. Our study might be useful to designing efficient catalysts for the oxygen reduction reaction.

Adsorption of thiophene on transition metal surfaces with the inclusion of van der Waals effects

We use density functional theory with the inclusion of the van der Waals interaction to study the adsorption of thiophene, C4H4S, on Pt, Rh, Pd, Au, and Ag (100) surfaces. The five van der Waals (vdW) inclusive functionals we employ are optB86b-vdW, optB88-vdW, optPBE-vdW, revPBE-vdW, and rPW86-vdW2. For comparison we also run calculations with the GGA- Perdew Burke and Ernzerhof (PBE) functional. We examine several adsorption sites with the plane of the molecule parallel or perpendicular to the surface. The most stable configuration on all metals was the site where the center of the thiophene lies over a 4-fold hollow site with the sulfur atom lying close to a top site. Furthermore, we examine several electronic and geometric properties of the adsorbate including charge transfer, modification of the d-band, adsorption energy, tilt angle, and adsorption height. For the coinage metals PBE gives the lowest adsorption energy. For reactive transition metal substrates, revPBE-vdW and rPW86-vdW2 give lower adsorption energies than PBE.

(Invited) Probing Atomic Charges at the Electrochemical Interface through Resonant Surface X-Ray Diffraction

An unsolved and crucial question in electrochemistry is the issue of charge transfer between the metal electrode and the adsorbate as this defines the nature of the bonding. Although there have been several theoretical studies of the charge transfer mechanism, initially in ideal UHV conditions but recently with extension to the electrochemical interface, there have been few experimental electrochemical studies due to the complexity of the electrochemical environment which is inaccessible to most surface probes of charge transfer. A fundamental understanding of the nature of the charge transfer, especially the influence of the applied potential and the screening by the electrolyte, is a major goal in electrochemistry. However, in-situ studies of the chemical bonding are rather difficult due to the presence of the electrolyte as standard characterisation techniques which are mostly UHV based, cannot be applied. We performed in-situ resonant scattering experiments for the halide Cu(001) and halide Cuupd-Au(001) system to investigate the suitability of this technique to probe the charge transfer and nature of the chemical bonding in-situ for such systems and to give detailed information of the atomic subsurface charge at the interface. Halide adsorption onto Cu(001) electrodes is an ideal system [1] in which to attempt this measurement, as the halides, Br and Cl, both form simple ordered c(2x2) adlayers on the Cu(001) surface. For both Cl and Br adsorption onto Cu(001) the adsorbates form c(2x2) adlayers, which are simple square structures with a coverage of 0.5 halide atoms per surface Cu atom and adsorption uniquely into the 4-fold Cu hollow site. The c(2x2) cell gives rise to additional surface scattering that is independent from the Cu crystal truncations rods (CTR’s). In the experiment it is therefore possible to probe the surface Cu atoms, by performing resonant measurements at the ‘anti-Bragg’ positions of the Cu and at the c(2x2) superstructure positions. A shift in the adsorption edge can be seen indicative of a change in the charge state of the surface Cu atoms. More detailed information can be obtained by modelling the experimental data through single atomic charges. Similar experiments were carried out for the Br adsorption on Au(001) and Cuupd halide Au(001) system [2] for which a difference in bonding, compared to the halide Cu(001) system is expected. This is the first direct experimental proof of partial charge transfer and subsurface charge distribution at the electrochemical interface. These measurements show that resonant x-ray scattering is an adequate tool to investigate the charge transfer at an electrochemical interface and will shed light on the nature of the chemical bonding at the interface and the role of the double layer.

Influence of Ga on H Reactivity with Ga-Stabilized δ-Pu Alloys

The influence of Ga on the adsorption and solubility of H on Ga-stabilized δ-Pu alloys is investigated using density functional theory and experimentally via a Sieverts’ apparatus. All-electron DFT calculations, with spin–orbit coupling and full relaxations of the surface, are implemented to calculate H adsorption on δ-Pu–Ga alloy (111) surfaces. Chemisorption energies indicate that H prefers a 3-fold hollow site that binds strongly to a Pu-rich environment regardless of Ga concentration of the surface. Sites that include a Ga–H bond yield an increase in chemisorption energies as opposed to chemisorption energies when the H is only bonded to Pu atoms. Also, solution energies for interstitial sites are less exothermic than surface adsorption energies. Experimentally, H-vacancy binding is more exothermic than interstitial binding, and since vacancy binding is the analogue of surface binding, the DFT results are in agreement. Both DFT and experimental results show that with increasing Ga content to 7 at. % Ga there is an increase in H solution energies, thus reducing H solubility. The maximal reduction of H solubility is between 4 and 5 at. % Ga stabilized δ-Pu. The bonding nature of H on the Pu–Ga surface, via analysis of the partial density of states, shows weak hybridizations between the Pu 6d and H 1s states, with a slight decrease of the Pu 5f states near the Fermi level. Overall, Ga will affect the surface adsorption and solubility of H in δ-Pu–Ga alloys.