Dissociative Adsorption Of H2 Research Articles

Unusual dissociative adsorption of H2 over stoichiometric MgO thin film supported on molybdenum

The dissociation of a hydrogen molecule on perfect MgO(001) films deposited on Mo(001) surface is investigated systematically using periodic density-functional theory (DFT) method. The unusual adsorption behavior of heterolytic dissociative hydrogen molecule at neighboring surface oxygen and surface magnesium, is clarified here. To our knowledge, this heterolytic dissociative state has never been found before on bulk MgO(001) or metal supported perfect MgO(001) surfaces (without low coordination sites). The results confirm that, in all cases, the heterolytic dissociation is much more favorable that homolytic dissociation both energetically and kinetically. The energy differences between two dissociative states are very large, in the range of 1.1eV–1.5eV for Mo supported 1ML–3ML oxide films, which inhibits, to a great extent, the homolytic dissociation in the respect of reaction thermodynamics. The energy barriers of heterolytic dissociation are about 0.5eV, much lower that the barrier of homolytic dissociation. The transformation reaction on thick films will be more endothermic. Passing through heterolytic dissociation state has significantly lowered the reaction heat and the energy barrier for obtaining homolytic dissociative structure, which makes the homolytic splitting of H2 easier on 2ML oxide films. The results provide a useful strategy for enhancing the reactivity of the nonreducible metal oxide.

Defections induced hydrogenation of silicene: a density functional theory calculation study

Vacancy defects significantly depress the energy barrier for dissociative adsorption of H2 on silicene, which can open the band gap of silicene.

Variation in the c-axis conductivity of multi-layer graphene due to H2 exposure.

The variation of the c-axis conductivity of a multilayer graphene (MLG) as a function of H2 pressure from vacuum to 20 bar has been investigated. MLG was connected to the electrodes vertically using a wet transfer process. After exposure to H2 gas pressure up to 20 bar, the chemisorption of dissociated atomic hydrogen on MLG affects its electrical and structural properties. The formation of C-H bonds causes a decoupling of graphene layers, and then interferes with charge transfer through the out of plane. As a result, the c-axis conductivity decreases. Furthermore, the electron doping effect and the decoupling of the layers were confirmed using Raman spectroscopy. Hydrogenated carbons induce a defect structure of MLG which results in the expansion of layers. We observed a 43.54% increase in the thickness of the MLG after H2 exposure using atomic force microscopy.

Effect of PdIn bimetallic particle formation on CO2 reduction over the Pd–In/SiO2 catalyst

A Pd–In/SiO2 catalyst was prepared by the wet impregnation method and used for CO2 reduction at atmospheric pressure. The catalyst was shown to have 100% selectivity toward CO with no detectable CH4 formation. In order to understand the high CO selectivity of Pd–In/SiO2, a detailed catalyst characterization in combination with the DFT study was conducted and compared with that over the Pd/SiO2 catalyst. The XRD and TEM analyses indicate the formation of the PdIn bimetallic nanoparticles on Pd–In/SiO2 after H2 reduction. The unique catalytic property of Pd–In/SiO2 was attributed to the formation of the bimetallic particles. Characterizations including H2 chemisorption and DRIFT and DFT modeling show that the formation of PdIn bimetallic nanoparticles reduces the activity toward H2 dissociative adsorption, weakens CO binding on the surface and increases the barrier of CO hydrogenation, suppressing the further hydrogenation beyond CO.

Kinetic Study of La0.75Sr0.25Cr0.5Mn0.5O3-Δ Nanoelectrodes for Symmetrical-SOFC

One of the main technological challenges of the solid oxide fuel cells (SOFC) is to reduce its operating temperature. Nanostructured materials open this possibility since the microstructure plays a fundamental role on the electrocatalytic activity of the electrodes. In this context, the understanding of the limiting mechanism steps for both O2 reduction and H2 oxidation reactions is a main objective in order to improve the design of the nanostructured electrodes. La0.75Sr0.25Cr0.5Mn0.5O3- δ (LSCM) perovskite has demonstrated a promising combination of properties for its employ as electrode (anode and cathode, simultaneously) in symmetrical-SOFC, among which it can mention: phase and dimensional stability, high electrochemical activity in both reducing and oxidizing atmospheres, and chemical compatibility with various solid electrolytes. In recent works, we present the synthesis by combustion of pure-phase LSCM nanocrystallites [1], and the bottom-up building process of nanostructured LSCM electrodes employing La0.8Sr0.2Ga0.8Mg0.2O3-δ(LSGM) as electrolyte [2]. In particular, the influence of the electrode crystallite size on the electrochemistry behavior as both anode and cathode was studied by electrochemical impedance spectroscopy (EIS). In this work, we present an electrode kinetics study of the nanostructured LSCM material by employing EIS measurements in different oxidizing and reducing atmospheres. Figures (a) and (b) present the EIS spectra of LSCM/LSGM/LSCM cell varying the oxygen and hydrogen partial pressures, i.e. pO2 and pH2 respectively. As it can be observed, when both pO2 and pH2 increase the electrode polarization resistance decrease, which indicates that the LSCM nanocrystallites are electrocatalytically active both for the oxygen reduction as well as hydrogen oxidation. Figures (c) and (d) show the evolution of the polarization resistance with the pO2 and pH2 obtained by fitting of the EIS spectra by using Electrical Equivalent Circuits. In both cases, two EIS arcs at high and low frequency (HF and LF, respectively) can be resolved showing complex mechanisms. In the H2 oxidation, the HF contribution with n ≈ 0 is independent of H2 concentration, probably due to a charge transfer process, while the LF contribution with n ≈ 0.5 could involve atomic species, e.g. H2-dissociative adsorption or H-surface diffusion [3]. On the other hand, in the O2 reduction, the HF contribution with n ≈ 0.25 is typically assigned to O-surface transport process whereas the LF arc with n ≈ 1 is generally attributed to O2 mass transfer in the gas phase [4].

Correlation of Electronic Structure with Catalytic Activity: H2–D2Exchange across CuxPd1–xComposition Space

The relationship between alloy catalyst activity and valence band electronic structure has been investigated experimentally across a broad, continuous span of CuxPd1–x composition space. CuxPd1–x composition spread alloy films (CSAFs) were used as catalyst libraries with a 100 channel microreactor to measure the H2–D2 exchange kinetics over a temperature range of 333–593 K at 100 discrete CuxPd1–x compositions spanning the range x = 0.30–0.97. The H2–D2 exchange activity exhibits a monotonic decrease over the composition range x = 0.30–0.97. A steady state, microkinetic model was used to estimate the energy barriers to dissociative H2 adsorption, ΔEads‡, and recombinative H2 desorption, ΔEdes‡, as functions of alloy composition, x. Their values fall in the ranges ΔEads‡(x) = 0.15 to 0.45 eV and ΔEdes‡(x) = 0.55–0.65 eV. Spatially resolved UV photoemission spectra were obtained from the CuxPd1–x CSAF and used to estimate the average energy of the filled states of the valence band as a function of alloy com...

Effect of Hydrogen on the Orientation of Cinchonidine Adsorbed on Platinum: An ATR-SEIRAS Study

Abstract The orientation of cinchonidine (CD) adsorbed on a Pt surface, a model system of enantioselective catalysis for hydrogenation reactions, is studied in 1,2-dichloroethane by in situ surface-enhanced infrared absorption spectroscopy in attenuated total reflection configuration. The quinoline (QN) moiety of CD is shown to be reoriented from a π-bonded nearly parallel orientation to an N-bonded upright one by bubbling the solution with H2. The origin of the reorientation is ascribed to the repulsive interaction of a QN π-orbital and the negative surface charge induced by dissociative adsorption of H2 and hydrogenation of the vinyl moiety.

Experimental and theoretical study of the intrinsic kinetics for dimethyl ether synthesis from CO2 over Cu–Fe–Zr/HZSM‐5

An intrinsic kinetics model was established for CO2 hydrogenation to dimethyl ether (DME) with a Cu–Fe–Zr/HZSM‐5 catalyst based on H2/CO2 adsorption, simulation, and calculation of methanol synthesis from CO2 intermediates and experimental data. H2/CO2‐temperature programmed desorption results show a dissociative H2 adsorption on Cu site; CO2 was linearly adsorbed on Fe3O4 weak base sites of the catalyst; the adsorbing capacity of H2 and CO2 increased after Zr‐doping. Density functional theory analysis of methanol synthesis from CO2 and H2 revealed a formate pathway. Methanol synthesis was the rate‐limiting step (173.72 kJ·mol−1 activation energy) of the overall CO2 hydrogenation reaction, and formation of H2CO is the rate‐determining step of methanol synthesis. Relative errors between calculated and experimental data of partial pressures of all components were less than 10%. Therefore, the kinetics model may be an accurate descriptor of intrinsic kinetics of CO2 hydrogenation to DME. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1613–1627, 2015

Competition between electron doping and short-range scattering in hydrogenated bilayer graphene on hexagonal boron nitride

We studied the electron doping of bilayer graphene (BLG) on hexagonal boron nitride (h-BN) by dissociative H2 adsorption.

Interaction of H2 with gold–palladium binary clusters: Molecular and dissociative adsorption

We present density functional calculations of H2 molecular and dissociative adsorption on small bimetallic AunPdm clusters with n+m⩽5. Molecular adsorption is found to be favorable for Au2, Au3 and Au2Pd clusters, while dissociative adsorption is favored for other clusters. The H atoms draw electrons from the metal clusters in dissociative adsorption while the H2 molecule donates electron density to metal clusters in molecular adsorption. The H2 molecule prefers binding to Pd atoms when both Au and Pd sites co-exist and the metal framework is related to the ground state of bare AunPdm cluster in molecular adsorption. In contrast, the geometries of metal framework are greatly changed upon H2 dissociative adsorption. The binding energy per atom and the HOMO–LUMO gaps for molecular and dissociative adsorptions were also compared and discussed. The Au4PdH2 cluster shows the relatively higher stability among the physisorbed structures.

Quantum Dynamics Study on the Effects of Vibration, Translational Energy and Incident Angle on H2 Adsorption on a Defective Pt(111) Surface

Quantum dynamics calculations via the local reflection matrix method are performed to investigate the effects of the vibration and initial translational energy on the dissociative adsorption of H2 approaching a defective Pt(111) surface at different incident angles and adsorption sites. The sticking probability plot for H2 incident on the top site at 15° shows that as the translational energy is increased, the probability rapidly rises to unity which suggests that H2 is easily adsorbed on the Pt surface. The plot also shows that even though the adsorption process is non-activated, there is a probability that H2 will not be adsorbed on the Pt surface at low translational energies due to quantum mechanical effects. For the rest of the configurations, an S-shaped region is observed in the plots suggesting an activated adsorption process. The plots show that when the initial translational energy (Et) is less that the barrier, H2 sticks to the Pt surface by tunneling through the barrier and when Et is greater ...

The impact of porosity, pH2 and pH2O on the polarisation resistance of Ni–BaZr0.85Y0.15O3−δ cermet anodes for Protonic Ceramic Fuel Cells (PCFCs)

The effect of porosity and atmosphere on the polarisation resistance of Ni-BZY cermet anodes for protonic ceramic fuel cells (PCFCs) was investigated using electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The level of porosity was adjusted by the use of differing amounts of starch porogen. The total polarisation resistance of the cermet anode (Rp) generally showed an increase with increasing porosity. The high frequency polarisation resistance (R2) was shown to have low atmosphere dependence but presented a strong correlation to microstructural variations. In contrast, the low frequency polarisation resistance (R3) was shown to have a low dependence on humidity but demonstrated a strong negative dependence on pH2 with a value of unity. Subsequent discussion relates the lower frequency response to the dissociative adsorption of H2 on the anode surface. The present study highlights that porogens are not required for peak performance in PCFC anodes under standard operating conditions, a result contrary to that of their oxide-ion conducting cermet counterparts.

Detailed kinetic modelling of automotive exhaust NOx reduction over rhodium catalyst

The reduction of NO with CO and H2 as the reductants was investigated on the rhodium‐based catalyst over a temperature range of 150–400 °C. Micro‐kinetic modelling method was utilized to compare the activities of CO and H2. Quasi‐elementary mechanisms of NO‐CO and NO‐H2 consisting of 11 and 25 steps, respectively, were proposed. The reaction steps and kinetic parameters were derived from literature and some parameters were optimized to fit the experimental data. Furthermore, to verify the validity range of the developed mechanism, the reaction intermediate N2O reduction by CO and by H2 over Rh were investigated. Over a wide range of reaction conditions, the detailed mechanism predicted experimental results quite well for both NO and N2O conversions with CO and/or H2 as reductant. The phenomenon that NO‐CO and NO‐H2 reactions commenced at similar temperature can be interpreted by the hindered H2 dissociative adsorption steps by extensive NO adsorption. Reaction profiles such as fractional coverages, reaction pathway and sensitivity analysis provided valuable insight into the reaction systems. N2O plays a crucial role in N2 formation for both NO‐CO and NO‐H2 reaction systems. The routes involving N2O formation and decomposition were identified as the dominant pathway in N2 formation at low temperatures.

The different roles of Pu-oxide overlayers in the hydrogenation of Pu-metal: an ab initio molecular dynamics study based on van der Waals density functional (vdW-DF)+U.

Based on the non-local van der Waals density functional (vdW-DF)+U scheme, we carry out the ab initio molecular dynamics (AIMD) study of the interaction dynamics for H2 impingement against the stoichiometric PuO2(111), the reduced PuO2(111), and the stoichiometric α-Pu2O3(111) surfaces. The hydrogen molecular physisorption states, which cannot be captured by pure DFT+U method, are obtained by employing the vdW-DF+U scheme. We show that except for the weak physisorption, PuO 2(111) surfaces are so difficult of access that almost all of the H2 molecules will bounce back to the vacuum when their initial kinetic energies are not sufficient. Although the dissociative adsorption of H2 on PuO2(111) surfaces is found to be very exothermic, the collision-induced dissociation barriers of H2 are calculated to be as high as 3.2 eV and 2.0 eV for stoichiometric and reduced PuO2 surfaces, respectively. Unlike PuO2, our AIMD study directly reveals that the hydrogen molecules can penetrate into α-Pu2O3(111) surface and diffuse easily due to the 25% native O vacancies located along the ⟨111⟩ diagonals of α-Pu2O3 matrix. By examining the temperature effect and the internal vibrational excitations of H2, we provide a detailed insight into the interaction dynamics of H2 in α-Pu2O3. The optimum pathways for hydrogen penetration and diffusion, the corresponding energy barriers (1.0 eV and 0.53 eV, respectively) and rate constants are systematically calculated. Overall, our study fairly reveals the different interaction mechanisms between H2 and Pu-oxide surfaces, which have strong implications to the interpretation of experimental observations.

Dissociative Hydrogen Adsorption on the Hexagonal Mo2C Phase at High Coverage

Hydrogen adsorption on the primarily exposed (001), (100), (101), and (201) surfaces of the hexagonal Mo2C phase at different coverage has been investigated at the level of density functional theory and using ab initio thermodynamics. On the Mo-terminated (001) and (100) as well as mixed Mo/C-terminated (101) and (201) surfaces, dissociative H2 adsorption is favored both kinetically and thermodynamically. At high coverage, each surface can have several types of adsorption configurations coexisting, and these types are different from surface to surface. The stable coverage as a function of temperature and partial pressure provides useful information not only for surface science studies at ultrahigh vacuum condition but also for practical applications at high temperature and pressure in monitoring reactions. The differences in the adsorbed H atom numbers and energies of these surfaces indicate their different potential hydrotreating abilities. The relationship between surface stability and stable hydrogen c...

Hydrogenolysis of nitrosodimethyl amine in gas phase over Au/γ-Al2O3 nanocatalyst

Nitrosodimethyl amine (NDMA), as a carcinogenic byproduct in production of unsymmetrical dimethyl hydrazine (UDMH) in space industries, should be decomposed in the vapor phase. A suitable method for this purpose is selective catalytic hydrogenolysis of NDMA over Au/γ-Al2O3 nanocatalyst. We synthesized and characterized the Au/γ-Al2O3 nanocatalyst by homogeneous deposition-precipitation (HDP)/DP-urea method. Activity of the catalyst was influenced by nanosized Au particles, Au loading and the bed temperature. The optimum parameters for the catalyst were: Au particles <5 nm, Au loading at 1.5 wt% and bed temperature of 35–45 °C. The reaction was strongly sensitive to the Au particle size. The reaction occurred over the catalyst to produce dimethyl amine (DMA) and nitroxyl in a selective manner. The kinetics of NDMA hydrogenolysis over the nanocatalyst was studied in an integral fixed bed reactor. There existed a consistency with the Langmuir-Hinshelwood mechanism involving dissociative adsorption of H2 and NDMA.

Electronic Friction Dominates Hydrogen Hot-Atom Relaxation on Pd(100)

We study the dynamics of transient hot H atoms on Pd(100) that originated from dissociative adsorption of H2. The methodology developed here, denoted AIMDEF, consists of ab initio molecular dynamics simulations that include a friction force to account for the energy transfer to the electronic system. We find that the excitation of electron-hole pairs is the main channel for energy dissipation, which happens at a rate that is five times faster than energy transfer into Pd lattice motion. Our results show that electronic excitations may constitute the dominant dissipation channel in the relaxation of hot atoms on surfaces.

Development and understanding of La0.85Sr0.15Cr1−xNixO3−δ anodes for La5.6WO11.4−δ-based Proton Conducting Solid Oxide Fuel Cells

Porous electrodes based on the system La0.85Sr0.15Cr1−xNixO3−δ (x = 0.1 and 0.2) have been investigated as anodes for proton conducting solid oxide fuel cells based on the La5.6WO11.4−δ (LWO) electrolyte material. The microstructure of the anodes was optimized by varying both the starting powder morphology and the final anode sintering temperature. Two different electrode thicknesses were studied, i.e. 15 and 30 μm. The importance of the catalytic role of Ni was also studied by using different concentrations of Ni (10% and 20%) in the chromite and by tuning the Ni particle sizes through the control of the reduction temperature. Additionally, a ceramic–ceramic (cer–cer) composite electrode comprising a physical mixture of the optimized chromite and LWO phase was also considered. Finally, a kinetics study and modeling based on Langmuir–Hinshelwood mechanism was carried out in order to quantitatively describe the rate of dissociative adsorption of H2 on the Ni particles spread on the chromite surface.

A First Principle Comparative Study on Chemisorption of H2 on C60, C80, and Sc3N@C80 in Gas Phase and Chemisorption of H2 on Solid Phase C60

The chemisorptions of H2 on fullerenes C60 and C80, endofullerene Sc3C@C80 and solid C60 were comparatively studied. A chain reaction mechanism for dissociative adsorption of H2 on solid C60 is proposed under high pressure. The breaking of H–H bond is concerted with the formation of two C–H bonds on two adjacent C60 in solid phase. The adsorption process is facilitated by the application of high pressure. The initial H2 adsorption on two adjacent C60 gives a much lower barrier 1.36 eV in comparison with the barrier of adsorption on a single C60 (about 3.0 eV). As the stereo conjugate aromaticity of C60 is destructed by the initial adsorption, some active sites are created. Hence the successive adsorption becomes easier with much low barriers (0.6 eV). In addition, further adsorption can create new active sites for the next adsorption. Thus, a chain reaction path is formed with the initial adsorption dominating the whole adsorption 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 th...