Coadsorption Of Hydrogen Research Articles

Metal–organic frameworks and related materials for hydrogen purification: Interplay of pore size and pore wall polarity

The separation of hydrogen from other weakly adsorbing gases is a topic of high industrial relevance. Microporous materials, such as zeolites, metal–organic frameworks (MOFs), and nanoporous molecular crystals, hold much promise as adsorbent materials for adsorption-based hydrogen separation units. However, most experimental and theoretical studies that have been reported so far have focused on relatively few gas mixtures (mainly CO2/H2 and CH4/H2). In this work, the suitability of five materials (zeolite: silicalite; MOFs: Mg-formate, Zn(dtp), Cu3(btc)2; porous molecular crystal: cucurbit[6]uril) for the adsorption-based separation of carbon monoxide/hydrogen and oxygen/hydrogen mixtures is assessed using force-field based grand-canonical Monte Carlo simulations. The simulations are employed to predict single-component and mixture isotherms, as well as adsorption selectivities. Moreover, a detailed analysis of the solid-fluid interactions is carried out on an atomistic level. The choice of materials is motivated by their structural properties: four systems contain relatively narrow channels (diameters < 6.5 A), but differ in pore wall composition and polarity. The fifth system possesses coordinatively unsaturated metal sites, which can act as preferential adsorption sites for some guest molecules. The role of electrostatic interactions is fundamentally different for the two mixtures considered: for CO/H2 separation, the employment of polar adsorbents is beneficial due to the enhanced electrostatic interaction with carbon monoxide. On the contrary, an increased polarity of the pore wall tends to reduce the O2/H2 selectivity, because electrostatic interactions favour hydrogen over oxygen due to its larger quadrupole moment. In general, materials with narrow channels perform best in the separation of hydrogen from weakly adsorbing species, because the dispersive interactions are maximized in the channels. Moreover, they provide little space for the co-adsorption of hydrogen.

Ab-initio study of the coadsorption of Li and H on Pt(001), Pt(110) and Pt(111) surfaces

The coadsorption of Li and H atoms on Pt(001), Pt(110) and Pt(111) surfaces is studied using density functional theory with generalised gradient approximation. In all calculations Li, H and the two topmost layers of the metal were allowed to relax. At coverage of 0.25 mono-layer in a p(2×2) unit cell, lithium adsorption at the hollow site for the three surfaces is favoured over top and bridge sites. The most favoured adsorption sites for H atom on the Pt(001) and Pt(110) surfaces are the top and bridge sites, while on Pt(111) surface the fcc site appears to be slightly favoured over the hcp site. The coadsorption of Li and atomic hydrogen shows that the interaction between the two adsorbates is stabilising when they are far from each other. The analysis of Li, H and Pt local density of states shows that Li strongly interacts with the Pt surfaces.

Ab Initio Molecular Dynamics Investigation of the Coadsorption of Acetaldehyde and Hydrogen on a Platinum Nanocluster

By means of ab initio molecular dynamics, we have investigated the molecular adsorption of acetaldehyde on Pt 13 nanoparticles in the presence of coadsorbed hydrogen on the surface of the metal particle. The acetaldehyde molecules exclusively interact with low-coordinated metal atoms of the nanoparticle while they remain inert toward direct interaction with adsorbed hydrogen, thus confirming the key role of the metal as intermediate binding site for hydrogenation. At room temperature within a time scale of picoseconds aldehyde–metal bonds are formed. Coadsorbed hydrogen decreases the reactivity of the aldehyde molecules toward the metal. Kinetically, the first adsorption modes to occur are of the η1 type, either via the oxygen or via the carbon atom. A tendency for double adsorption on the same metal site is observed. Upon addition of ammonia molecules to the simulation box, the interesting phenomenon of the conversion of η1 to η2 carbonyl bonding appears, mediated by the adsorption of the ammonia nitrogen to a platinum atom. This investigation highlights the richness of the interaction modes of a carbonyl group with a platinum nanoparticle, reached in the very brief time scale of a few picoseconds. In particular the adsorption modes of the aldehyde are modified by the presence of a second electron-donor molecule, such as another aldehyde molecule or an ammonia molecule, in the latter case even changing the adsorption mode of the carbonyl moiety from η1 to η2.

ChemInform Abstract: Electrochemical and Infrared Spectroscopic Comparison of Pt, ZrPt3, and HfPt3 Catalytic Properties: Hydrogen Evolution and CO Adsorption.

ChemInform Abstract: Electrochemical and Infrared Spectroscopic Comparison of Pt, ZrPt3, and HfPt3 Catalytic Properties: Hydrogen Evolution and CO Adsorption.

The Stereoselectivity of the Dehydrogenation of Alkyl Groups on Pt(111) Single-Crystal Surfaces

The thermal chemistry of erythro- and threo-2-butyl-3-d1 species adsorbed on Pt(111) single-crystal surfaces was studied under ultrahigh vacuum (UHV) by temperature-programmed desorption (TPD). The alkyl intermediates were prepared via thermal activation of the appropriate erythro- and threo-2-iodobutane-3-d1 precursors, which were made in house via a three-step synthesis starting with cis- and trans-2,3-epoxybutanes, respectively. Several products were detected in the TPD experiments, including butenes and butanes with different degrees of isotope substitutions and hydrogen from deep dehydrogenation reactions. At least two distinct temperature regimes were identified at higher coverages for the β-hydride elimination steps that produce the olefins, the focus of this work, but one single high-temperature regime could be isolated by working with low coverages of the iodoalkanes. On clean Pt(111), an inverse kinetic isotope effect was observed for the conversion of 2-butyls to 2-butenes, and a slight preference for trans- (rather than cis-) 2-butene production. In addition, a reversal in relative rates for 2-butene-d0 versus 2-butene-d1 was detected in the high-temperature side of the TPD peaks obtained with the two alkyl epimers, an observation that we explain by a high degree of reversibility of the alkyl-to-alkene conversion. Coadsorption of hydrogen or deuterium on the surface reverses the kinetic isotope effect, but not the selectivity for trans olefin production. However, these effects, in particular, the preference for trans olefin formation, are small, indicating an early transition state for the β-hydride elimination step and a thermodynamic (rather than kinetic) control of selectivity in olefin isomerization reactions.

Coadsorption of Hydrogen and CO on Hydrogen Pre‐Covered PtRu/Ru(0001) Surface Alloys

The influence of pre-adsorbed hydrogen on the adsorption of CO on well-defined PtRu/Ru(0001) surface alloys and the modification of the adsorption properties of both adsorbate species due to coadsorption was studied by a combination of temperature-programmed desorption and infrared spectroscopic measurements. The hydrogen binding strength is weakened both by surface alloy formation and by CO coadsorption. In the densely packed coadsorbate layers, which can be formed after CO post-adsorption, the adsorption properties of the adsorbed CO molecules are also affected significantly. Depending on the Pt surface concentration, the CO adsorption process on hydrogen pre-covered surfaces is blocked to a different extent with a decreasing blocking tendency for Pt-rich alloys.

Formation of an Oxametallacycle Surface Intermediate via Thermal Activation of 1-Chloro-2-methyl-2-propanol on Ni(100)

The thermal chemistry of 1-chloro-2-methyl-2-propanol (CH2Cl(CH3)2COH) on a Ni(100) single-crystal surface, clean and after hydrogen preadsorption, was studied by temperature-programmed desorption and X-ray photoelectron spectroscopy. The 1-chloro-2-methyl-2-propanol adsorbs molecularly on the metal surface at 100 K. An initial chemical reaction is seen at around 180 K involving the sequential scission of the C−Cl and O−H bonds to produce hydroxyalkyl −CH2(CH3)2COH and oxametallacycle −CH2(CH3)2CO− intermediates. Hydrogenation of the first surface species produces tert-butyl alcohol, whereas further conversion of the oxametallacycle follows at least three reaction pathways, a cyclization to isobutene oxide, a hydrogenation to tert-butyl alcohol, and a dehydration reaction to produce isobutene, water, H2, and CO. It was also shown that hydrogen coadsorption on the surface enhances the production of tert-butyl alcohol and partially inhibits the decomposition to isobutene.

Interaction of atomic H and CO with Cu(1 1 0) and bimetallic Ni/Cu(1 1 0)

Co-adsorption of hydrogen and CO on Cu(1 1 0) and on a bimetallic Ni/Cu(1 1 0) surface was studied by thermal desorption spectroscopy. Hydrogen was exposed in atomic form as generated in a hot tungsten tube. The Ni/Cu surface alloy was prepared by physical vapor deposition of nickel. It turned out that extended exposure of atomic hydrogen leads not only to adsorption at surface and sub-surface sites, but also to a roughening of the Cu(1 1 0) surface, which results in a decrease of the desorption temperature for surface hydrogen. Exposure of a CO saturated Cu(1 1 0) surface to atomic H leads to a removal of the more strongly bonded on-top CO ( α 1 peak) only, whereas the more weakly adsorbed CO molecules in the pseudo threefold hollow sites ( α 2 peak) are hardly influenced. No reaction between CO and H could be observed. The modification of the Cu(1 1 0) surface with Ni has a strong influence on CO adsorption, leading to three new, distinct desorption peaks, but has little influence on hydrogen desorption. Co-adsorption of H and CO on the Ni/Cu(1 1 0) bimetallic surface leads to desorption of CO and H 2 in the same temperature regime, but again no reaction between the two species is observed.

Coadsorption of hydrogen and CO on well-defined Pt35Ru65/Ru(0001) surface alloys—site specificity vs. adsorbate–adsorbate interactions

The coadsorption of CO and hydrogen on a structurally well-defined Pt(35)Ru(65)/Ru(0001) monolayer surface alloy and, for comparison, on Ru(0001) were investigated by temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). The data reveal distinct modifications in the hydrogen adsorption behavior and also in the CO adsorption properties compared to adsorption of the individual components both on the mono-metallic and on the bimetallic surface. These modifications are discussed on an atomic scale, in a picture that involves adsorbate-adsorbate interactions and site-specific variations in the (local) adsorption properties of the bimetallic surface, due to electronic ligand and strain effects and geometric ensemble effects.

A sublattice-model isotherm for the competitive coadsorption of hydrogen and bromide on a Pt(100)electrode

Previous work demonstrated that the Frumkin isotherm is inadequate to model the competitive coadsorption of species with different saturation coverages, such as hydrogen and bromide coadsorption on Pt(100) [N. Garcia-Araez et al., J. Electroanal. Chem., 2006, 588, 1]. Therefore, Monte Carlo simulations were necessary to determine meaningful values of the microscopic parameters (namely, energies of adsorption and interaction). In the present work, an alternative analytical isotherm is developed, by taking into account the occupation of two sublattices, which together compose the whole lattice of adsorption sites. Despite its relatively simple mathematical form, this isotherm presents, under certain conditions, a significant improvement over the classical Frumkin isotherm for the modeling of competitive adsorption processes, thus providing a closer agreement with results from Monte Carlo simulations. Finally, it is demonstrated that the sublattice-model isotherm will be generally applicable to systems in which the formation of segregated adlayers, whose structure is not explicitly taken into account in the model, is energetically unfavorable.

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H 2C CHCH CH 2, C 4H 6) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage ( θ H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ H investigated ( θ H = 0.6–0.9 ML). This is in strong contrast to the copious evolution of ethane (CH 3CH 3, C 2H 6) from coadsorbed hydrogen and ethylene (CH 2 CH 2, C 2H 4) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H 2 and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene–butene mixtures.

Multiadsorption and Coadsorption of Hydrogen on Model Conjugated Systems

Hydrogen interaction with conjugated aromatic molecular systems is analyzed with high level ab initio electronic structure methodology. The adsorption of hydrogen molecules on aromatic systems ranging from a single ring to double ring systems including fused and bridged molecules is investigated. We propose several functionalization schemes of the conjugated system to increase hydrogen intake. These functionalizations consist of using an asymmetrical conjugation skeleton as inherently present for the azulene molecule and the introduction of B−N heteroatom pairs as doping sites in the conjugation ring. We have also tested the possibility of involving more than a single interaction site within the same molecule by using a molecule involving several rings as the triptycene and several derived functionalized molecules. These computational studies provide insight on the interaction of hydrogen with conjugated molecular species. This insight can be utilized for designing materials such as metal organic framewor...

Coadsorption of CO and H on Fe(100)

A DFT model of the Fe(100) surface was used to study the coadsorption of CO and hydrogen. Various coverages of CO and hydrogen were considered with resulting H:CO ratios ranging from 0.25 to 4. Electrostatic repulsive interactions exist between the adsorbed CO and H as well as an electronic interaction which can have a stabilizing effect on the coadsorbed geometry. A number of geometries at both 0.25 and 0.5 ML of CO coverage are stable in a mixed coadsorbed state as opposed to island formation. This is due to the relative instability of the corresponding compressed CO and H islands. For these mixed states the optimal surface H:CO ratios for ⊖CO = 0.5 ML and ⊖CO = 0.25 ML are 1 and 3 respectively.

Interactions of oxygen and hydrogen on Pd(1 1 1) surface

The coadsorption and interactions of oxygen and hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy and density functional theory calculations. In the absence of hydrogen oxygen forms a (2 × 2) ordered structure. Coadsorption of hydrogen leads to a structural transformation from (2 × 2) to a (√3 × √3) R30° structure. In addition to this transformation, hydrogen enhances the mobility of oxygen. To explain these observations, the interaction of oxygen and hydrogen on Pd(1 1 1) was studied within the density functional theory. In agreement with the experiment the calculations find a total energy minimum for the oxygen (2 × 2) structure. The interaction between H and O atoms was found to be repulsive and short ranged, leading to a compression of the O islands from (2 × 2) to (√3 × √3) R30° ordered structure at high H coverage. The computed energy barriers for the oxygen diffusion were found to be reduced due to the coadsorption of hydrogen, in agreement with the experimentally observed enhancement of oxygen mobility. The calculations also support the finding that at low temperatures the water formation reaction does not occur on Pd(1 1 1).

Transition State for Alkyl Group Hydrogenation on Pt(111)

Substituent effects have been used to probe the characteristics of the transition state to hydrogenation of alkyl groups on the Pt(111) surface. Eight different alkyl and fluoroalkyl groups have been formed on the Pt(111) surface by dissociative adsorption of their respective alkyl and fluoroalkyl iodides. Coadsorption of hydrogen and alkyl groups, followed by heating of the surface, results in hydrogenation of the alkyl groups to form alkanes, which then desorb into the gas phase. Temperature-programmed reaction spectroscopy was used to measure the barriers to hydrogenation, DeltaE(H)(double dagger), which are dependent on the size of the alkyl group (polarizability) and the degree of fluorination (field effect). This example is one of only two surface reactions for which the influence of the substituents on DeltaE(H)(double dagger) has been correlated with both the field and the polarizability substituent constants of the alkyl groups in the form of a linear free energy relationship. Increasing both the field and the polarizability constants of the alkyl groups increases the value of DeltaE(H)(double dagger). The substituent effects are quantified by a field reaction constant of rho(F) = 27 +/- 4 kJ/mol and a polarizability reaction constant of rho(alpha) = 19 +/- 3 kJ/mol. These suggest that the transition state for hydrogenation is slightly cationic with respect to the alkyl group on the Pt(111) surface, RC + H <--> {RC(delta+)...H}(double dagger).

Challenges and opportunities for glycerol as a renewable feedstock

We have investigated the impact and role of the Pt surface modification on the coadsorption of hydrogen and CO on structurally well defined bimetallic Pt monolayer island/film modified Ru(0001) surfaces with Pt contents up to a complete Pt layer, employing temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS). Kinetic limitations in the surface diffusion are shown to play an important role for adsorption at 90 K, and lead to profound effects of the dosing sequence on the adsorption and desorption characteristics. Furthermore, they are responsible for spill-over effects during the TPD measurements, where COad becomes mobile and can spill-over from weakly bonding Pt monolayer areas to strongly bonding Pt-free Ru(0001) areas, which displaces Dad from these surface areas. The present findings are discussed in comparison with previous results on related metallic and bimetallic adsorption and coadsorption systems.

Comparative creation of surface Schottky defects on SnO2(110) and TiO2(110)

Periodic DFT calculations have been used to study the creation of Schottky defects on MO2(110) rutile surface where M=Ti or M=Sn. These defects are oxygen vacancies: a bridging oxygen atom is removed from the surface creating an F°S center and readsorbed on a vicinal site. The readsorption permits to compensate a part of the energy cost required for the removal. Along this process, the stoichiometry and the atomic oxidation states remain those of the perfect surface. The energy cost for such defect is found almost 5 times weaker for TiO2(110) than for SnO2(110). Next, we analyze the effect of hydrogen coadsorption. We start considering the hydrogenated surface, one hydrogen atom binding to a bridging oxygen atom and reducing the metal-oxide surface. The Schottky process then becomes the displacement of a hydroxyl ion which desorbs and readsorbs on surface titanium just as basic species do. This coadsorption does not affect the energy cost in the case of TiO2, while, in the case of SnO2(110), it makes it weaker. This decrease in mainly due to the magnitude of the interaction between a hydroxyl group and the Sn4+ surface atom, that is large compared with that for O.

Co-adsorption of water and hydrogen on Ni(111)

We have studied the surface coverage dependence of the co-adsorption of D and D(2)O on the Ni(111) surface under UHV conditions. We use detailed temperature-programmed desorption studies and high resolution electron energy loss spectroscopy to show how pre-covering the surface with various amounts of D affects adsorption and desorption of D(2)O. Our results show that the effects of co-adsorption are strongly dependent on D-coverage. In the deuterium pre-coverage range of 0-0.3 ML, adsorption of deuterium leaves a fraction of the available surface area bare for D(2)O adsorption, which shows no significant changes compared to adsorption on the bare surface. Our data indicate phase segregation of hydrogen and water into islands. At low post-coverages, D(2)O forms a two-phase system on the remaining bare surface that shows zero-order desorption kinetics. This two phase system likely consists of a 2-D solid phase of extended islands of hexamer rings and a 2-D water gas phase. Increasing the water post-dose leads at first to 'freezing' of the 2-D gas and is followed by formation of ordered, multilayered water islands in-between the deuterium islands. For deuterium pre-coverages between 0.3 and 0.5 ML, our data may be interpreted that the water hexamer ring structure, (D(2)O)(6), required for the formation of an ordered multilayer, does not form anymore. Instead, more disordered linear and branched chains of water molecules grow in-between the extended, hydrophobic deuterium islands. These deuterium islands have a D-atom density in agreement with a (2x2)-2D structure. The disordered water structures adsorbed in-between form nucleation sites for growth of 3-D water structures. Loss of regular lateral hydrogen bonding and weakened interaction with the substrate reduces the binding energy of water significantly in this regime and results in lowering of the desorption temperature. At deuterium pre-coverages greater than 0.5 ML, the saturated (2x2)-2D structure mixes with (1x1)-1D patches. The mixed structures are also hydrophobic. On such surfaces, submonolayer doses of water lead to formation of 3-D water structures well before wetting the entire hydrogen-covered surface.

Effects of Hydrogen on the Reactivity of O2 toward Gold Nanoparticles and Surfaces

Density functional theory (DFT) was used to study the coadsorption of O2 and H2 or H on Au(111) and Au(100) and on free pyramidal clusters (Au25 and Au29) observed on titania- and ceria-based catalysts. For the adsorption of isolated O2, no significant interaction was obtained between the flat metal surfaces and the oxygen molecule. Gold clusters (Au14, Au25, and Au29) showed a higher reactivity, but this reactivity depended strongly on the type of uncoordinated sites exposed, ensemble effects, and the fluxionality of the metal nanoparticles. On the Au14 and Au29 clusters, a superoxo species and a peroxo moiety were formed, respectively. In contrast, no interaction between O2 and Au25 was seen because of the high coordination number of the exposed metal sites. In general, H2 dissociates much easier on the gold clusters than O2, but again Au25 is less reactive than Au29 or Au14. The pyramidal Au25 and Au29 clusters illustrate the importance of geometry in the chemistry of gold nanoparticles: A smaller particle size does not necessarily imply a bigger chemical reactivity. With the presence of predissociated hydrogen on a gold system, the adsorption energy of oxygen is much higher in absolute value than for the O2/Au systems, accompanied by the formation of a hydroperoxo species. The origin of the synergistic effect observed for the coadsorption of hydrogen and O2 is the formation of O−H bonds that enhance the O↔Au interactions, reducing (0.15−0.30 Å) the O−Au bond lengths. The reaction H(a) + OOH(a) → H2O2(a) was found to be highly exothermic on the Au clusters. An interesting case is when the oxygen orientation allows for simultaneous interaction with two H adatoms: The formation of hydrogen peroxide is readily achieved with an energy release of more than 30 kcal/mol. This reaction is only hindered by the need of a concerted approach of O2 to the H adatoms.

AN STM OBSERVATION OF ADSORPTION OF CuPc ON THE Si(100) SURFACE WITH Bi-LINE STRUCTURES

We report the reaction dynamics of the Bi -line structure (BLS) with copper-phthalocyanine ( CuPc )molecules as well as hydrogen and Ag atoms on Si (100) surfaces observed by scanning tunneling microscopy. Co-adsorption of hydrogen and Ag on the Si (100) surface with BLSs brought about agglomeration of Ag atoms on the Si terrace. When CuPc molecules were deposited on the Si (100) surface with BLSs at room temperature, domains with c(4 × 4) periodicity appeared in the terraces near the BLS. When the surface was annealed at 200°C–400°C, the area of the c(4 × 4) domain was increased. The CuPc molecules, adsorbed on BLS, were possibly dissociated by catalytic reaction of Bi atoms.