Dissociative Chemisorption Of H2 Research Articles

In Situ DRIFTS Analysis during Hydrogenation of 1-Pentyne and Olefin Purification with Ag Nanoparticles.

The catalytic performance of nanoparticles (NPs) of Ag anchored on different supports was evaluated during the selective hydrogenation of 1-pentyne and the purification of a mixture of 1-pentene/1-pentyne (70/30 vol %). The catalysts were identified: Ag/Al (Ag supported on ɣ-Al2 O3 ), Ag/Al-Mg (Ag supported on ɣ-Al2 O3 modified with Mg), Ag/Ca (Ag supported on CaCO3 ) and Ag/RX3 (Ag supported on activated carbon-type: RX3). In addition, in situ DRIFTS analysis of 1-pentyne adsorption on each support, catalyst, and 1-pentyne hydrogenation were investigated. The results showed that the synthesized catalysts were active and very selective (≥85 %) for obtaining the desired product (1-pentene). Different adsorbed species (-C≡C- and -C=C-) were observed on the supports and catalysts surface using in situ DRIFT analysis, which can be correlated to the activity and high selectivity reached. The role of the supports and electronic properties over Ag improve the H2 dissociative chemisorption during the hydrogenation reactions; promoting the selectivity and the high catalytic performance. Ag/Al and Ag/Al-Mg were the most active catalysts. This was due to the synergism between the active Ag/Ag+ species and the supports (electronic effects). The results show that Ag/Al and Ag/Al-Mg catalysts have favorable properties and are promising for the alkyne hydrogenation and olefin purification reactions.

Ring Polymer Molecular Dynamics Approach to Quantum Dissociative Chemisorption Rates.

A ring polymer molecular dynamics (RPMD) method is proposed for the calculation of the dissociative chemisorption rate coefficient on surfaces. The RPMD rate theory is capable of handling quantum effects such as the zero-point energy and tunneling in dissociative chemisorption, while it relies on classical trajectories for the simulation. Applications to H2 dissociative chemisorption are demonstrated. For the highly activated process on Ag(111), strong deviations from Arrhenius behavior are found at low temperatures and attributed to tunneling. On Pt(111), where the dissociation has a barrierless pathway, the RPMD rate coefficient is found to agree with the experimentally derived thermal sticking coefficient within a factor of 2 over a large temperature range. Significant quantum effects are also found.

Accurate description of the quantum dynamical surface temperature effects on the dissociative chemisorption of H2 from Cu(111).

Accurately describing surface temperature effects for the dissociative scattering of H2 on a metal surface on a quantum dynamical (QD) level is currently one of the open challenges for theoretical surface scientists. We present the first QD simulations of hydrogen dissociating on a Cu(111) surface, which accurately describe all relevant surface temperature effects, using the static corrugation model. The reaction probabilities we obtain show very good agreement with those found using quasi-classical dynamics (QCD), both for individual surface slabs and for an averaged, thus Monte Carlo sampled, set of thermally distorted surface configurations. Rovibrationally elastic scattering probabilities show a much clearer difference between the QCD and QD results, which appears to be traceable back toward thermally distorted surface configurations with very low dissociation probabilities and underlines the importance of investigating more observables than just dissociation. By reducing the number of distorted surface atoms included in the dynamical model, we also show that only including one surface atom, or even three surface atoms, is generally not enough to accurately describe the effects of the surface temperature on dissociation and elastic scattering. These results are a major step forward in accurately describing hydrogen scattering from a thermally excited Cu(111) surface and open up a pathway to better describe reaction and scattering from other relevant crystal facets, such as stepped surfaces, at moderately elevated surface temperatures where quantum effects are expected to play a more important role in the dissociation of H2 on Cu.

Dynamics of Initial Hydrogen Spillover from a Single Atom Platinum Active Site to the Cu(111) Host Surface: The Impact of Substrate Electron-Hole Pairs.

The initial impulsive diffusion of hot hydrogen atoms resulted from the dissociative chemisorption of H2 at atomically dispersed Pt atoms embedded in Cu(111) is investigated using ab initio molecular dynamics. Upon dissociation, one of the two hydrogen atoms tends to roam away from the dissociation site while the other remains trapped. It is shown that the fraction of diffusion and the average diffusion length increase with the incident energy and H2 vibrational excitation, due apparently to the increased initial kinetic energy of the hot atoms. Most importantly, the strong interaction with surface electron-hole pairs, modeled using an electronic friction model, is shown to play an important role in rapid energy dissipation and significant retardation of the impulsive diffusion.

Graphene-based frustrated Lewis pairs as bifunctional catalysts for CO2 reduction via the dissociative chemisorption of molecular H2: a periodic density functional perspective

Nanocarbon-based frustrated Lewis pair (FLP) bifunctional catalysts, on account of their unquenched electron transfer property, are becoming increasingly attractive as catalysts for the CO2 reduction reaction via the dissociative chemisorption of H2.

Quantum Monte Carlo calculations on dissociative chemisorption of H2 + Al(110): Minimum barrier heights and their comparison to DFT values.

Reactions of molecules on metal surfaces are notoriously difficult to simulate accurately. Density functional theory can be utilized to generate a potential energy surface, but with presently available functionals, the results are not yet accurate enough. To provide benchmark barrier heights with a high-quality method, diffusion Monte Carlo (DMC) is applied to H2 + Al(110). Barrier heights have been computed for six geometries. Our present goal is twofold: first, to provide accurate barrier heights for the two lowest lying transition states of the system, and second, to assess whether density functionals are capable of describing the variation of barrier height with molecular orientation and impact site through a comparison with DMC barriers. To this end, barrier heights computed with selected functionals at the generalized gradient approximation (GGA) and meta-GGA levels are compared to the DMC results. The comparison shows that all selected functionals yield a rather accurate description of the variation of barrier heights with impact site and orientation, although their absolute values may not be accurate. RPBE-vdW-DF and BEEF-vdW were found to perform quite well even in terms of absolute numbers. Both functionals provided barrier heights for the energetically lowest lying transition state that are within 1 kcal/mol of the DMC value.

Tuning the Catalytic Activity of Pd x Ni y (x + y = 6) Bimetallic Clusters for Hydrogen Dissociative Chemisorption and Desorption.

Density functional theory was used to study dissociative chemisorption and desorption on PdxNiy (x + y = 6) bimetallic clusters. The H2 dissociative chemisorption energies and the H desorption energies at full H saturation were computed. It was found that bimetallic clusters tend to have higher chemisorption energy than pure clusters, and the capacity of Pd3Ni3 and Pd2Ni4 clusters to adsorb H atoms is substantially higher than that of other clusters. The H desorption energies of Pd3Ni3 and Pd2Ni4 are also lower than that of the Pd6 cluster and comparable to that of the Ni6 cluster, indicating that it is easier to pull the H atom out of these bimetallic catalysts. This suggests that the catalytic efficiency for specific PdxNiy bimetallic clusters may be superior to bare Ni or Pd clusters and that it may be possible to tune bimetallic nanoparticles to obtain better catalytic performance.

On the mechanism of alkyne hydrogenation catalyzed by Ga-doped ceria

The hydrogenation of acetylene by Ga-doped ceria is investigated using density functional theory (DFT). Different from a previously proposed mechanism that contains an unattainable barrier, our models are based on two Ga-doped ceria surfaces with oxygen vacancies, representing respectively low and high Ga dopant concentrations. The doping of Ga is shown to promote the formation of surface oxygen vacancies, which create a template containing frustrated Lewis pairs to facilitate heterolytic H2 dissociative chemisorption. This mechanism is similar in spirit to our recently proposed mechanism for bare and Ni-doped CeO2 catalysts (J. Am. Chem. Soc., 2018, 140, 12964–12973) and calls for the formation of GaH hydride species along with OH ones on the ceria surface. The former has recently been experimentally observed, thus providing strong support for this mechanism. The rate-limiting step in doped CeO2(1 1 1) with low Ga concentrations has a barrier that is 0.18 eV for the Ga/O FLP and 0.05 eV for the Ce/O FLP higher than that of undoped CeO2(1 1 1), while that for high Ga concentrations is 0.21 eV lower, consistent with the experimental observed concentration dependence of catalytic activity. However, different from the Ni dopant, which was found to be a promoter, the Ga dopant is not only a promoter but also an active participant in the catalysis.

Hydrogen Chemisorption on Pd-Doped Copper Clusters

The structural evolution, electronic, and magnetic properties of Pd-doped Cun (n = 1–12) clusters and the dissociative chemisorption of H2 on the lowest energy structures are investigated on the ba...

Platinum for hydrogen sensing: surface and grain boundary scattering antagonistic effects in Pt@Au core-shell nanoparticle assemblies prepared using a Langmuir-Blodgett method.

Hydrogen resistive sensors are fabricated through the synthesis of a series of Pt@Au core-shell nanoparticles showing various Pt shell thicknesses. Resulting colloids are assembled as hexagonal close-packed 2D monolayers of various dimension characteristics using a simple Langmuir-Blodgett method. The fabricated sensors show attractive hydrogen sensing performances with reversible responses in extended sensing ranges, a good specificity towards H2, short response and recovery times… Sensing measurements and data analyses allow the demonstration of the associated sensing mechanisms. The dissociative chemisorption of H2 and O2 on the Pt surface through a Langmuir-Hinshelwood mechanism leads to the formation of chemisorbed hydrogen and hydroxyl groups. This surface nature change induces the modification of the scattering of the conduction electrons at both the grain surface and intercontacts, tuned by the extent of hydrogen and hydroxyl group coverages. In assemblies made of particles showing thin Pt shells, the predominance of the surface scattering described by the Fuchs-Sondheimer model accounts for the observed conductive responses as the number of involved grain boundaries is limited. In contrast, in assemblies made of particles with thick Pt shells, the scattering at the grain boundaries described by the Mayadas-Shatzkes model mostly contributes to the observed resistive responses. The sensor behavior is balanced by these two antagonistic effects.

Numerical and experimental study of La–Ni hydriding kinetics based on the varying-size model

Particle size often varies during gas–solid heterogeneous reactions. For example, metal hydrides (MH) can reversibly absorb and desorb H2, accompanied by the expansion and shrinkage of the alloy particles, respectively. Although the traditional shrinking-core model (SCM) can be used to describe the H2 absorption/desorption process, the calculation results from SCM show relatively large deviations from the experimental data without considering the factor of particle expansion/shrinkage. Therefore, we proposed a shrinking-core–varying-size model (SC-VSM, henceforth VSM) for MH particles that accurately determines the kinetics equation with particle deformation. Three types of control mechanism including H2 dissociative chemisorption, H internal diffusion, and surface reaction were studied extensively, and the rate-controlling step for both VSM and SCM was determined to be the internal diffusion of H. Hydriding experiments on the systems of LaNi5 and LaNi4.5Al0.5 were performed under quasi-isothermal and variable-pressure conditions. The simulation results indicated that the new VSM was highly consistent with the experimental data for both alloys, evidently providing higher precision than the traditional SCM. The simplified forms of the VSM diffusion equations for LaNi5 and LaNi4.5Al0.5 were also developed to facilitate their practicability during the hydriding process: LaNi5:1.426-1.419(1-X)23-0.046(1-X)53-0.95X+0.038(1-X)2=tτdiff′LaNi4.5Al0.5:1.362-1.35(1-X)23-0.069(1-X)53-0.90X+0.057(1-X)2=tτdiff′

Computational Investigation on Hydrodeoxygenation (HDO) of Acetone to Propylene on α-MoO3 (010) Surface

Density functional theory (DFT) calculations were performed on the multistep hydrodeoxygenation (HDO) of acetone (CH3COCH3) to propylene (CH3CHCH2) on a molybdenum oxide (α-MoO3) catalyst following an oxygen vacancy-driven pathway. First, a perfect O-terminated α-MoO3 (010) surface based on a 4 × 2 × 4 supercell is reduced by molecular hydrogen (H2) to generate a terminal oxygen (Ot) defect site. This process occurs via a dissociative chemisorption of H2 on adjacent surface oxygen atoms, followed by an H transfer to form a water molecule (H2O). Next, adsorption of CH3COCH3 on the oxygen-deficient Mo site forms an O–Mo bond and then the chemisorbed CH3COCH3 forms CH3COCH2 by transfer of an H atom to an adjacent Ot site. The surface bound hydroxyl (OH) then transfers the H atom to the immobilized O atom to form surface-bound enol, CH3CHOCH2. The next step releases CH3CHCH2 into the gas phase, while simultaneously oxidizes the surface back to a perfect O-terminated α-MoO3 (010) surface. The adsorption of H2,...

Dynamics of H2 dissociation on the close-packed (111) surface of the noblest metal: H2 + Au(111).

We have performed calculations on the dissociative chemisorption of H2 on un-reconstructed and reconstructed Au(111) with density functional theory, and dynamics calculations on this process on un-reconstructed Au(111). Due to a very late barrier for dissociation, H2 + Au(111) is a candidate H2-metal system for which the dissociative chemisorption could be considerably affected by the energy transfer to electron-hole pairs. Minimum barrier geometries and potential energy surfaces were computed for six density functionals. The functionals tested yield minimum barrier heights in the range of 1.15-1.6 eV, and barriers that are even later than found for the similar H2 + Cu(111) system. The potential energy surfaces have been used in quasi-classical trajectory calculations of the initial (v,J) state resolved reaction probability for several vibrational states v and rotational states J of H2 and D2. Our calculations may serve as predictions for state-resolved associative desorption experiments, from which initial state-resolved dissociative chemisorption probabilities can be extracted by invoking detailed balance. The vibrational efficacy ηv=0→1 reported for D2 dissociating on un-reconstructed Au(111) (about 0.9) is similar to that found in earlier quantum dynamics calculations on H2 + Ag(111), but larger than found for D2 + Cu(111). With the two functionals tested most extensively, the reactivity of H2 and D2 exhibits an almost monotonic increase with increasing rotational quantum number J. Test calculations suggest that, for chemical accuracy (1 kcal/mol), the herringbone reconstruction of Au(111) should be modeled.

Mechanism of Hydrogen Spillover on WO3(001) and Formation of HxWO3 (x = 0.125, 0.25, 0.375, and 0.5)

Hydrogen spillover onto the WO3(001) surface and further into the bulk in the presence of a platinum catalyst was investigated by density functional theory calculations. The diffusion pathways are designed based on the energetic preference of hydrogen adsorption. Atomic H migrates from a platinum catalyst, where it is adsorbed upon H2 dissociative chemisorption, to terminal oxygen on WO3 (001) surface, followed by migration to in-plane oxygen and then diffusion across the surface or insertion in the lattice. We first theoretically corroborated the role of water in facilitating the hydrogen spillover process. The otherwise prohibitively high activation barriers were considerably reduced with the participation of water, which agrees well with experimental facts. The high mobility of H inside the WO3 lattice was also identified with low calculated diffusion barriers for several selected migration pathways. Finally, the effects of hydrogen concentration and electronic structures of HxWO3 were investigated. Th...

Hydrogen Interaction with the Al Surface Promoted by Subsurface Alloying with Transition Metals

Dissociative chemisorption of H2 on the Al surface is a crucial step in the regeneration of promising hydrogen-storage materials such as alane and alanates. We show from first-principles calculations that transition metals such as V and Nb can act as effective catalysts for H2 interaction with Al(100). When located at subsurface sites, V and Nb can reduce the activation barrier for H2 dissociation by significantly larger values than the well-studied catalyst Ti. In addition, the binding energy of a H atom on the surface can be enhanced by as much as 0.4 eV when V or Nb is introduced in the sublayers of Al(100). The diffusion barrier for the adsorbed hydrogen is reduced by ∼0.1 eV, showing an increased hydrogen mobility. The mechanism of promoting the metal surface reactivity by subsurface alloying with transition metals proposed in this work may serve as a new possible scheme for catalytic reactions on the metal surface.

First-Principles Study of the Hydrogenation Process of Li2NH

On the basis of ab initio simulations, we identified possible microscopic pathways for the Li2NH + H2 → LiNH2 + LiH rehydrogenation reaction of Li imide. Dissociative chemisorption of H2 at the Li2NH(001) surface is an activated process with an energy barrier of 0.5 eV. The heterolytic splitting of H2 leads to the formation of a subsurface NH2– group and a H– ion coordinated by Li+ surface ions. Surface diffusion of H– to form LiH is rate-limiting, whereas H+ (NH2–) and Li+ diffusion in the bulk to sustain the rehydrogenation reaction is easier. Heterolytic splitting of H2 is possible also in the bulk, but insertion of H2 in bulk sites is more costly than the activation energy for dissociative chemisorption at the surface, which suggests that rehydrogenation proceeds from the surface into the bulk. The large difference in lattice parameters between LiNH2/Li2NH and LiH suggests that LiH cannot grow epitaxially on Li imide/amide but flakes off, providing a free surface of the imide for hydrogen adsorption a...

A theoretical study of H2 dissociation on (3×3)R30°CO/Ru(0001)

We have studied the influence of preadsorbed CO on the dissociative adsorption of H(2) on Ru(0001) with density functional theory calculations. For a coverage of 1/3 ML CO, we investigated different possible reaction paths for hydrogen dissociation using nudged elastic band and adaptive nudged elastic band calculations. One reaction path was studied in detail through an energy decomposition and molecular orbital type of analysis. The minimum barrier for H(2) dissociation is found to be 0.29 eV. At the barrier the H-H bond is hardly stretched. Behind this barrier a molecular chemisorption minimum is present. Next, the molecule overcomes a second barrier, with a second local chemisorption minimum behind it. To finally dissociate to chemisorbed atoms, the molecule has to overcome a third barrier. To move along the reaction path from reactants to products, the hydrogen molecule needs to rotate, and to significantly change its center-of-mass position. The procedure of mapping out reaction paths for H(2) reacting on low-index surfaces of bare metals (computing two-dimensional elbow plots for fixed impact high-symmetry sites and H(2) orientations parallel to the surface) does not work for H(2)+CO/Ru. The first barrier in the path is recovered, but the features of the subsequent stretch to the dissociative chemisorption minimum are not captured, because the molecule is not allowed to change its center-of-mass position or to rotate. The dissociative chemisorption of H(2) on CO/Ru(0001) is endoergic, in contrast to the case of H(2) on bare Ru(0001). The zero-point energy corrected energies of molecularly and dissociatively chemisorbed H(2) are very close, suggesting that it may be possible to detect molecularly chemisorbed H(2) on (sq.rt(3) x sq.rt(3))R30 degrees CO/Ru(0001). The presence of CO on the surface increases the barrier height to dissociation compared with bare Ru(0001). Based on an energy decomposition and molecular orbital analysis we attribute the increase in the barrier height mainly to an occupied-occupied interaction between the bonding H(2) sigma(g) orbital and the (surface-hybridized) CO 1pi orbitals, i.e., to site blocking. There is a small repulsive contribution to the barrier from the interaction between the H(2) molecule and the Ru part of the CO covered Ru surface, but it is smaller than one might expect based on the calculations of H(2) interacting with a clean Ru surface, and on calculations of H(2) interacting with the CO overlayer only. Actually, the analysis suggests that the Ru surface as a subsystem is (slightly) more reactive for the reaction path studied with CO preadsorbed on it than without it. Thus, the results indicate that the influence of CO on H(2) dissociation on Ru is not only a simple site-blocking effect, the electronic structure of the underlying Ru is changed.

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters — a review

In this review, we present our recent first principles studies on the sequential H2 dissociative chemisorption and H desorption on the Ptn and Pdn clusters (n=2–9, 13). Upon full saturation by H atoms, the calculated H2 dissociative chemisorption energy and H desorption energy on Pt13 and Pd13 clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H2 dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt13 and Pd13 from the icosahedral to fcc-like structures at certain H coverage was also investigated.

Dynamics of Hydrogen Spillover on Carbon-Based Materials

We investigate the dynamics of physisorbed atomic hydrogen on several carbon based materials (various fullerenes and a graphene sheet) using first principles molecular dynamics simulations. The physisorbed H atoms, generated upon H2 dissociative chemisorption on metal catalysts and interaction with carbonized “bridge” materials and substrates (Chen, L.; et al. J. Phys. Chem. C 2007, 111, 18995), can diffuse freely on carbon surfaces with high mobility. Our results indicate that physisorption of H atoms is a metastable state and the atoms will readily recombine to form H2 molecules, which can be recycled to generate more H atoms, or attack the substrates to form C−H bonds. The strength of the resulting C−H bonds exhibits a strong dependency on the carbon material surface curvature. The implication of C−H bond strength on the dehydrogenation of hydrogenated carbon materials to form molecular H2 is discussed.

Influence of CO Poisoning on Hydrogen Chemisorption onto a Pt6 Cluster

We present a systematic study on the reactivity of a Pt6 cluster toward H2 dissociative chemisorption in the presence of CO molecules using density functional theory (DFT). The sequential adsorption and threshold desorption energies of H at varying CO coverage were identified. It was found that the main influence of CO molecules is to block the available active surface sites for H2 dissociative chemisorption. In addition, our population and density of states analysis indicate that the poisoning effect is partially due to the loss of Pt(5d) electrons upon CO adsorption. The hydrogenation of CO is found to be endothermic.