Dissociative Adsorption Of Molecular Hydrogen Research Articles

An apparatus for investigating the kinetics of plasmonic catalysis

Plasmonic catalysis, which is driven by the localized surface plasmon resonance of metal nanoparticles, has become an emerging field in heterogeneous catalysis. The microscopic mechanism of this kind of reaction, however, remains controversial partly because of the inaccuracy of temperature measurement and the ambiguity of reagent adsorption state. In order to investigate the kinetics of plasmonic catalysis, an online mass spectrometer-based apparatus has been built in our laboratory, with emphases on dealing with temperature measurement and adsorption state identification issues. Given the temperature inhomogeneity in the catalyst bed, three thermocouples are installed compared with the conventional design with only one. Such a multiple-point temperature measuring technique enables the quantitative calculation of equivalent temperature and thermal reaction contribution of the catalysts. Temperature-programmed desorption is incorporated into the apparatus, which helps to identify the adsorption state of reagents. The capabilities of the improved apparatus have been demonstrated by studying the kinetics of a model plasmon-induced catalytic reaction, i.e., H2+D2→HD over Au/TiO2. Dissociative adsorption of molecular hydrogen at Au/TiO2 interface and non-thermal contribution to HD production have been confirmed.

Thermodynamic Aspects in Non-Ideal Metal Membranes for Hydrogen Purification.

In this paper, an overview on thermodynamic aspects related to hydrogen-metal systems in non-ideal conditions is provided, aiming at systematically merging and analyzing information achieved from several different studies present in the open literature. In particular, the relationships among inner morphology, dissolved hydrogen and internal stresses are discussed in detail, putting in evidence the conformation complexity and the various types of dislocations induced by the presence of H-atoms in the lattice. Specifically, it is highlighted that the octahedral sites are preferentially occupied in the FCC metals (such as palladium), whereas tetrahedral sites are more energetically favored in BCC-structured ones (such as vanadium). These characteristics are shown to lead to a different macroscopic behavior of the two classes of metals, especially in terms of solubility and mechanical failure due to the consequent induced stresses. Furthermore, starting from the expression of the chemical potential generally presented in the literature, a new convenient expression of the activity of the H-atoms dissolved into the metal lattice as a function of the H-concentration is achieved. Such an activity expression is then used in the dissolution equilibrium relationship, which is shown to be the overall result of two different phenomena: (i) dissociative adsorption of molecular hydrogen onto the surface; and (ii) atomic hydrogen dissolution from the surface to the metal bulk. In this way, the obtained expression for equilibrium allows a method to calculate the equilibrium composition in non-ideal conditions (high pressure), which are of interest for real industrial applications.

Anomalous Dependence of the Reactivity on the Presence of Steps: Dissociation of D2 on Cu(211).

Stepped metal surfaces are usually assumed to exhibit an increased catalytic activity for bond cleavage of small molecules over their flat single-crystal counterparts. We present experimental and theoretical data on the dissociative adsorption of molecular hydrogen on copper that contradicts this notion. We observe hydrogen molecules to be more reactive on the flat Cu(111) than on the stepped Cu(211) surface. We suggest that this exceptional behavior is due to a geometric effect, that is, that bond cleavage on the flat surface does not occur preferentially over a top site.

Dissociative Adsorption of Molecular Hydrogen on BN-Doped Graphene-Supported Aluminum Clusters

The present work, demonstrates dissociative adsorption of molecular hydrogen on supported and unsupported aluminium clusters (Aln, n=4 - 8, 13) using Density Functional Theory based calculations. The studies reveal that the presence of a BN doped graphene surface support reduces the dissociative adsorption barrier of the bond in molecular hydrogen on even atom clusters. In particular, supported Al6 demonstrates a barrier-less dissociative adsorption towards the H2 molecule. These results demonstrate the excellent potential of supported Al nanoparticles for hydrogen storage and also the potential of doped graphene systems are catalyzing supports.

Dissociative Adsorption of H2 on PtRu Bimetallic Surfaces

We present a theoretical study of the dissociative adsorption of molecular hydrogen on PtRu bimetallic surfaces based on density functional theory (DFT) calculations. We focus on the reactivity of a pseudomorphic Pt monolayer deposited on Ru(0001), Pt1ML/Ru(0001), for which we have obtained a minimum activation energy barrier for H2 dissociation, Eb = 0.32 eV, i.e., ∼0.3 and 0.26 eV higher than on Ru(0001) and Pt(111), respectively. Accordingly, the initial sticking probability for low energy impinging molecules (Ei ≲ 0.1 eV) derived from classical trajectory calculations is various orders of magnitude smaller than on Ru(0001) in apparent contradiction with available experimental data. However, undercoordinated Pt atoms in the borders of Pt pseudomorphic islands and isolated Ru atoms or small Ru aggregates in a Pt-rich two-dimensional PtxRu1–x surface alloy provide active sites allowing nonactivated H2 dissociative adsorption. These two possible defects in a full pseudomorphic Pt monolayer deposited on Ru...

Environment-driven reactivity of H2 on PdRu surface alloys

The dissociative adsorption of molecular hydrogen on Pd(x)Ru(1-x)/Ru(0001) (0 ≤ x ≤ 1) has been investigated by means of He atom scattering, Density Functional Theory and quasi-classical trajectory calculations. Regardless of their surroundings, Pd atoms in the alloy are always less reactive than Ru ones. However, the reactivity of Ru atoms is enhanced by the presence of nearest neighbor Pd atoms. This environment-dependent reactivity of the Ru atoms in the alloy provides a sound explanation for the striking step-like dependence of the initial reactive sticking probability as a function of the Pd concentration observed in experiments. Moreover, we show that these environment-dependent effects on the reactivity of H2 on single atoms allow one to get around the usual constraint imposed by the Brønsted-Evans-Polanyi relationship between the reaction barrier and chemisorption energy.

Dissociative adsorption of molecular hydrogen onto Pt6 and Pt19 platinum clusters located on the tin dioxide surface: Quantum-chemical modeling

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO2 substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 A. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO2 surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H2 dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.

Quantum-chemical modeling of the dissociative adsorption of molecular hydrogen onto the tin dioxide surface

The hydrogen adsorption, dissociation, and migration on the tin dioxide surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that dissociative chemisorption onto the tin dioxide surface depends on the adsorption site and the surface structure and that there are places on the surface where dissociation can occur with a low barrier of 0.1–0.2 eV to yield a primary isomer in which one of the hydrogen atoms is bound to the tin atom and the other, to an oxygen atom; the second dissociation even at the same place is possible only if the primary isomer overcomes a barrier of ∼1 eV and transforms to the secondary isomer with two O-H bonds.

H/D exchange of molecular hydrogen with Brønsted acid sites of Zn- and Ga-modified zeolite BEA

Kinetics of hydrogen H/D exchange between Brønsted acid sites of pure acid-form and Zn- or Ga-modified zeolites beta (BEA) and deuterated hydrogen (D(2)) has been studied by (1)H MAS NMR spectroscopy in situ within the temperature range of 383-548 K. A remarkable increase of the rate of the H/D exchange has been found for Zn- and Ga-modified zeolites compared to the pure acid-form zeolite. The rate of exchange for Zn-modified zeolite is one order of magnitude higher compared to the rate for Ga-modified zeolite and two orders of magnitude larger compared to the pure acid-form zeolite. This promoting effect of metal on the rate of H/D exchange was rationalized by a preliminary dissociative adsorption of molecular hydrogen on metal oxide species or metal cations. The adsorbed hydrogen is further involved in the exchange with the acid OH groups located in vicinity of metal species. The role of different metal species in the possible mechanisms of the exchange with involvement of zeolite Brønsted acid sites and metal species is discussed.

Hydrogen adsorption configurations on Ge(001) probed with STM

The adsorption of hydrogen on Ge(001) has been studied with scanning tunneling microscopy at 77 K. For low doses (100 L) a variety of adsorption structures has been found. We have found two different atomic configurations for the Ge-Ge-H hemihydride and a third configuration that is most likely induced by the dissociative adsorption of molecular hydrogen. The Ge-Ge-H hemihydride is either buckled antiparallel or parallel to the neighboring Ge-Ge dimers. The latter configuration has recently been predicted by M. W. Radny et al. [J. Chem. Phys. 128, 244707 (2008)], but not observed experimentally yet. Due to the presence of phasons some dimer rows appear highly dynamic.

First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.

Unusual localization of zinc cations in MFI zeolites modified by different ways of preparation

Comparison of DRIFT spectra of OH groups in MFI with framework Si/Al ratios equal to 25 or 40 with the spectra of the same zeolites after reaction with zinc vapour at 773 K indicated quantitative substitution of acidic hydroxyls by Zn2+ ions. Complete substitution of protons indicates that at high Si/Al ratios some of the Zn2+ cations are localized at the isolated aluminium occupied oxygen tetrahedra with single negative charges. This results in the formation of sites with only partially compensated excessive positive charges of bivalent cations. In parallel, the same amount of negatively charged aluminium occupied oxygen tetrahedra remains without compensating cations or protons. Their negative charges are neutralized indirectly by the electrostatic interaction with the surrounding positively charged cationic sites. Zinc or other bivalent ions with only partially compensated positive charges exhibit an unusually high chemical activity. This explains the specific catalytic and adsorption properties of high-silica zeolites modified with the bivalent transition metal cations. In the present study, this was demonstrated by very large red shifts of H–H stretching vibrations of H2 adsorbed by zinc cations at 77 K and by heterolytic dissociative adsorption of molecular hydrogen at moderately elevated temperatures. Application of low temperature H2 adsorption as a molecular probe indicated that similar sites of unusual localization of bivalent zinc cations also exist in zeolites modified by ion exchange or incipient wetness impregnation, but the amount of such sites is less than for the samples modified by reaction with zinc vapour.

Stereochemistry on Si(001): angular dependence of H2 dissociation.

The angular dependence of the dissociative adsorption of molecular hydrogen at terrace and step sites of vicinal single-domain Si(001) surfaces was investigated by means of molecular beam techniques and optical second-harmonic generation. A strongly anisotropic behavior was observed for terrace adsorption with polar distributions of cos3theta and cos12theta parallel and perpendicular to the dimer, respectively. The D(B)-steps show enhanced reactivity under glancing incidence in the upwards direction. The results are traced back to the directionality of the covalent surface bonds.

Real-space study of the pathway for dissociative adsorption of H2 on Si(001).

Dissociative adsorption of molecular hydrogen on clean Si(001) surfaces has been investigated by means of scanning tunneling microscopy. The dissociated hydrogen atoms are found to occupy Si atoms of adjacent dimers. In addition to this interdimer configuration associated with the adsorption of isolated hydrogen molecules, pairs of adjacent doubly occupied dimers are readily formed. They arise from the enhanced reactivity of partially occupied dimers following the initial H2 adsorption step. The results are considered in light of recent adsorption and desorption measurements.

Cluster Approach of Active Sites in an MoS 2 Catalyst

The present paper reports a Density Functional Theory (DFT) study of a Mo12S24 cluster as a model of the active phase in hydrodesulphurisation (HDS). Different types of sulphur vacancies are considered and compared. The interaction of a thiophene molecule with a double vacancy is simulated leading to the determination of a stable configuration which corresponds to a flat adsorption on the edge of the MoS2 sheets. The dissociative adsorption of molecular hydrogen on a double vacancy is also considered.

Influence of steps and defects on the dissociative adsorption of molecular hydrogen on silicon surfaces

. Neither the absolute values nor the temperature dependence of adsorption on the terraces are affected by the misorientation.

Density-functional study of hydrogen chemisorption on vicinal Si(001) surfaces

Relaxed atomic geometries and chemisorption energies have been calculated for the dissociative adsorption of molecular hydrogen on vicinal Si(001) surfaces. We employ density-functional theory, together with a pseudopotential for Si, and apply the generalized gradient approximation by Perdew and Wang to the exchange-correlation functional. We find the double-atomic-height rebonded D_B step, which is known to be stable on the clean surface, to remain stable on partially hydrogen-covered surfaces. The H atoms preferentially bind to the Si atoms at the rebonded step edge, with a chemisorption energy difference with respect to the terrace sites of >sim 0.1 eV. A surface with rebonded single atomic height S_A and S_B steps gives very similar results. The interaction between H-Si-Si-H mono-hydride units is shown to be unimportant for the calculation of the step-edge hydrogen-occupation. Our results confirm the interpretation and results of the recent H_2 adsorption experiments on vicinal Si surfaces by Raschke and Hoefer described in the preceding paper.

Role of zero-point effects in catalytic reactions involving hydrogen

According to the Heisenberg uncertainty principle of quantum mechanics, particles which are localized in space by a bounding potential must have a finite distribution of momenta. This leads, even in the lowest possible energy state, to vibrations, and thus, to the so-called zero-point energy. For chemically bound hydrogen the zero-point energy can be quite substantial. For example, for a free H2 molecule it is 0.26 eV, a significant value in the realm of chemistry, where often an energy of the order of 0.1 eV/atom (or 2.3 kcal/mol) decides whether or not a chemical reaction takes place with an appreciable rate. Yet, in many theoretical studies the dynamics of chemical reactions involving hydrogen has been treated classically or quasiclassically, assuming that the quantum mechanical nature of H nuclei, i.e., the zero-point effects, will not strongly affect the relevant physical or chemical properties. In this article we show that this assumption is not justified. We will demonstrate that for very basic and fundamental catalytic reaction steps, namely the dissociative adsorption of molecular hydrogen at transition metal surfaces and its time reverse process, the associative desorption, zero-point effects cannot only quantitatively but even qualitatively affect the chemical processes and rates. Our calculations (treating electrons as well as H nuclei quantum mechanically) establish the importance of additional zero-point effects generated by the H2 surface interaction and how energy of the H–H stretch vibration is transferred into those and vice versa.

Experimental and theoretical studies of fuel cell catalysts: Density functional theory calculations of H 2 dissociation and CO chemisorption on fuel cell metal dimers

Gradient-corrected density functional theory (GC-DFT) calculations have been performed for molecular hydrogen, carbon monoxide and the metal dimers PtPt, PtNi, and PtRu. The dissociative adsorption of molecular hydrogen on these metal dimers has been modelled. Derived potential energy surfaces for the dissociation of molecular hydrogen on these dimers are presented. Furthermore, the interaction with carbon monoxide has been studied. Here we find that Pt 2 binds CO significantly stronger than PtNi and PtRu. We present equilibrium geometries, calculated binding energies, Mulliken charge distributions, and orbital energies. Our results correlate well with our experimental studies of the hydrogen electro-oxidation reaction in a proton-exchange fuel cell where the fuel to the anode ( Pt Pt alloy on carbon electrode) is hydrogen, prepared by reforming of methane, which contains trace amounts of carbon monoxide.

Nonlinear optical investigations of the dynamics of hydrogen interaction with silicon surfaces

Optical second-harmonic generation (SHG) from silicon surfaces may be resonantly enhanced by dangling-bond-derived surface states. The resulting high sensitivity to hydrogen adsorption combined with unique features of SHG as an optical probe has been exploited to study various kinetical and dynamical aspects of the adsorption system H2/Si. Studies of surface diffusion of H/Si(111)7×7 and recombinative desorption of hydrogen from Si(111)7 × 7 and Si(100)2 × 1 revealed that the covalent nature of hydrogen bonding on silicon surfaces leads to high diffusion barriers and to desorption kinetics that strongly depend on the surface structure. Recently, dissociative adsorption of molecular hydrogen on Si(100)2×1 and Si(111)7×7 could be observed for the first time by heating the surfaces to temperatures between 550 K and 1050 K and monitoring the SH response during exposure to a high flux of H2 or D2. The measured initial sticking coefficients for a gas temperature of 300K range from 10−9 to 10−5 and strongly increase as a function of surface temperature. These results demonstrate that the lattice degrees of freedom may play a decisive role in the reaction dynamics on semiconductor surfaces.