D2 Exchange Reaction Research Articles

Improving Catalytic Hydrogenation Performance of Pd Nanoparticles by Electronic Modulation Using Phosphine Ligands

Tuning the activity and selectivity of metal nanoparticles (NPs) is a long-term pursuit in the field of catalysis. Herein, we report successfully improving both the activity and chemoselectivity of Pd NPs (1.1 nm) with triphenylphosphine (PPh3) cross-linked in the nanopore of FDU-12. The electron-donating effect of PPh3 increases the surface electronic density of Pd NPs and weakens the Pd–H bond, as evidenced by the results of XPS, in situ FT-IR adsorption of CO, and H2–D2 exchange reactions. Consequently, Pd NPs modified with PPh3 obtain >99% selectivity to 1-phenylethanol in acetophenone hydrogenation and 94% selectivity to styrene in phenylacetylene hydrogenation. Furthermore, the activity of Pd NPs is enhanced and suppressed by PPh3, respectively, in the hydrogenation of electrophilic nitro compounds and nucleophilic carbonyl substrates. Our primary results shed some light on judiciously choosing organic ligands for modifying the catalytic performance of metal NPs toward specific chemical transformations.

A microreactor array for spatially resolved measurement of catalytic activity for high-throughput catalysis science

We describe a 100-channel microreactor array capable of spatially resolved measurement of catalytic activity across the surface of a flat substrate. When used in conjunction with a composition spread alloy film (CSAF, e.g., PdxCuyAu1−x−y) across which component concentrations vary smoothly, such measurements permit high-throughput analysis of catalytic activity and selectivity as a function of catalyst composition. In the reported implementation, the system achieves spatial resolution of 1mm2 over a 10×10mm2 area. During operation, the reactant gases are delivered at constant flow rate to 100 points of differing composition on the CSAF surface by means of a 100-channel microfluidic device. After coming into contact with the CSAF catalyst surface, the product gas mixture from each of the 100 points is withdrawn separately through a set of 100 isolated channels for analysis using a mass spectrometer. We demonstrate the operation of the device on a PdxCuyAu1−x−y CSAF catalyzing the H2–D2 exchange reaction at 333K. In essentially a single experiment, we measured the catalytic activity over a broad swathe of concentrations from the ternary composition space of the PdxCuyAu1−x−y alloy.

Influence of Au and TiO2 structures on hydrogen dissociation over TiO2/Au(100)

We performed H2–D2 exchange reactions over TiOx/Au(100) and compared the observed reaction kinetics with those reported for TiOx/Au(111) in order to clarify the influence of the Au and TiO2 structures on dissociation of H2 molecules. Low energy electron diffraction observations showed that the TiO2 produced on Au(100) was disordered, in contrast to the comparatively ordered TiO2 structure formed on Au(111). The activation energies and the turnover frequencies for HD formation over TiO2/Au(100) agreed well with those for TiO2/Au(111), clearly indicating that the hydrogen dissociation sites created over TiO2/Au(100) were the perimeter interface between stoichiometric TiO2 and Au, as was previously concluded for TiO2/Au(111). We concluded that the creation of active sites for hydrogen dissociation was independent of the Au and TiO2 structures consisting perimeter interface, and that local bonds that formed between Au and O atoms of stoichiometric TiO2 were essential for the creation of active sites.

Active Sites for Hydrogen Dissociation over TiOx/Au(111) Surfaces

We carried out H2–D2 exchange reactions over TiOx/Au(111) (x = 0–2) model surfaces to clarify the active sites for hydrogen dissociation over the Au/TiO2 catalyst. Ti oxides with controlled oxidation states were prepared on the Au(111) surface by regulation of O2 exposure and temperature. The HD formation rate depended on the concentration of TiO2 over Au(111), indicating that hydrogen dissociation sites were created between the stoichiometric TiO2 and Au. Atomic force microscopy observations indicated that the number of monolayer TiO2 islands with diameters of about 20 nm increased with increasing Ti coverage below ΘTi = 0.65 and that as ΘTi was increased further, a TiO2 multilayer began to form over Au(111). The turnover frequencies for HD formation over the TiO2/Au(111) surface, which were calculated from the length of the perimeter interface between the TiO2 islands and the Au substrate, were consistent with the frequencies reported for Au/TiO2(110) surfaces. Furthermore, the activation energies for H...

The effect of low concentrations of tetrachloroethylene on H2 adsorption and activation on Pt in a fuel cell catalyst

The poisoning effect of tetrachloroethylene (TTCE) on the activity of a Pt fuel cell catalyst for the adsorption and activation of H2 was investigated at 60°C and 2atm using hydrogen surface concentration measurements. The impurity was chosen as a model compound for chlorinated cleaning and degreasing agents that may be introduced into a fuel cell as a contaminant at a fueling station and/or during vehicle maintenance. In the presence of only H2, introduction of up to 540ppm TTCE in H2 to Pt/C resulted in a reduction of available Pt surface atoms (measured by H2 uptake) by ca. 30%, which was not enough to shift the H2–D2 exchange reaction away from being equilibrium limited. Exposure of TTCE to Pt/C in a mixed redox environment (hydrogen+oxygen), similar to that at the cathode of a fuel cell, resulted in a much more significant loss of Pt surface atom availability, suggesting a role in TTCE decomposition and/or Cl poisoning. Regeneration of catalyst activity of poisoned Pt/C showed the highest level of recovery when regenerated in only H2, with much less recovery in H2+O2 or O2. The results from this study are in good agreement with those found in a fuel cell study by Martínez-Rodríguez et al. [2] and confirm that the majority of the poisoning from TTCE on fuel cell performance is most likely at the cathode, rather than the anode.

Nitrogen Doping of Graphite for Enhancement of Durability of Supported Platinum Clusters

Nitrogen doping of a graphite surface by ion bombardment has been found to enhance the durability of deposited Pt clusters for H2−D2 exchange reaction. Scanning tunneling microscopy suggests that t...

Promoted Catalytic Activity of a Platinum Monolayer Cluster on Graphite

Platinum atoms deposited on a graphite surface show distinct different catalytic activity depending on their morphology on the surface. Flat monolayer clusters of platinum on graphite show higher catalytic activity than taller Pt clusters for the H2−D2 exchange reaction. It is suggested based on scanning tunneling microscopy observations and X-ray photoelectron spectroscopy measurements that the origin of the superiority is due to the interface interaction between Pt and graphite, leading to a shift of the d-band center of platinum away from the Fermi level.

From [M≡N] and [M—N—E] Complexes to Models for Metal Oxidoreductases

AbstractThe article reviews results of research that was initially aiming at complexes containing new and unusual [M—N—E] element combinations (M = transition metal, E = main group element), but soon turned into studies on model complexes for metal enzymes such as nitrogenases, hydrogenases or CO dehydrogenases, because several of the resulting [M—N—E] complexes exhibited reactions relevant to these enzymes. It could be shown that alkylation of transition metal thiolate nitride complexes gives alkylimido complexes when bulky and mild alkylation reagents, e.g. Ph3C+, are used. Hydride addition to [Ru(NO)(pybuS4)]+ yielded [Ru(HNO)(pybuS4)], which contains a bifurcated [M—N(X, Y)] bridge. The diazene complex [μ‐N2H2{Ru(PCy3)(S4)}2] undergoes H+/D+ and H+/D2 exchange reactions that enabled to rationalize the until then inexplicable ‘N2 dependent HD formation’ catalyzed by nitrogenases. Out of a larger number of [Ni(NE)(S3)] complexes, the compound [Ni(NHPPr3)(S3)] proved capable to model structure and reactivity features of [NiFe] hydrogenases. The [Ni(L)(S3)] complexes with L = N3— and N(SiMe3)2— exhibit extremely high reactivity towards CO, CO2 and SO2. The reactions lead to NCO—, CN— and NSO— complexes and bear potential relevance for carbon monoxide dehydrogenase reactions.

Local reaction environments and their properties for ethene deuterogenation on the surfaces of SMSI catalysts

Ethene deuterogenation and H2–D2 exchange reaction over Nb2O5-supported Rh and Ir catalysts have been investigated in relation to strong metal–support interaction (SMSI) phenomena. The activation energies for these reactions were considerably changed by high-temperature reduction of the catalyst in the case of Ir/Nb2O5, but were not modified in the case of Rh/Nb2O5. The change is ascribed to a reduction in the energy barrier for deuterium dissociation. The deuterium distribution in ethane formed during ethene deuterogenation was also investigated at various reaction temperatures and as a function of the reduction time of the catalyst. By studying the catalysts in their working state instead of by static adsorption measurements two kinds of active sites in different environments are suggested to exist on the surface of these catalysts in the SMSI states. One of these sites (site I) is on the bare metal surface and the other (site II) is on the perimeter of a migrating NbOx island. The surface isotopic ratio of hydrogen during ethene deuterogenation is different at sites I and II. Site I, on which D2 dissociates, acts as a deuterium supply for site II. A model for the deuterogenation of ethene on the SMSI catalysts is proposed.

A proton-deuterium exchange study of three types ofDesulfovibrio hydrogenases

Hydrogenases are among the main enzymes involved in bacterial anaerobic corrosion of metals. The study of their mode of action is important for a full comprehension of this phenomenon. The three types ofDesulfovibrio hydrogenases [(Fe), (NiFe), (NiFeSe)] present different patterns in the pH dependence of their activity. The periplasmic enzyme fromDesulfovibrio salexigens and the cytoplasmic enzyme fromDesulfovibrio baculatus both have pH optima at 7.5 for H2 uptake and 4.0 for H2 evolution and H+−D2 exchange reaction (measured by membrane-inlet mass-spectrometry). The H2 to HD ratio at pH above 5.0 is higher than 1.0. The periplasmic hydrogenase fromD. gigas presents the same pH optimum (8.0) for the H+−D2 exchange as for H2 consumption. In contrast, the enzyme fromD. vulgaris has the highest activity in H2 production and in the exchange at pH 5.0. Both hydrogenases have a H2-to-HD ratio below 1.0.

Inhibition by hydrogen of the heterogeneous decomposition of ammonia on platinum

Absolute reaction rates have been measured in a continuous flow microreactor for the inhibition by hydrogen of the heterogeneous decomposition of ammonia over a polycrystalline platinum wire at pressures between 10 T and 0.6 torr, with ammonia to hydrogen partial pressure ratios varying from 1:1 to 1:4 and temperatures between 400 and 1200 K. Inhibition of the decomposition is observed at relatively high total pressures (0.2-0.6 torr) for all temperatures studied. The reaction orders with respect to hydrogen and ammonia are -1.5 and 1.0, respectively, with an apparent activation energy of 38 kcal/mol. For relatively low total pressures (10 T-5 x 10 T torr), inhibition is only observed at temperatures below 650 K, where the activation energy varies from 33 to 55 kcal/mol and the reaction order with respect to hydrogen approaches -1.5 at low temperatures. The decomposition becomes uninhibited at high temperatures and the activation energy is 4.5 kcal/mol. The kinetics of this reaction as well as previous results for the NH3 + D2 exchange reaction are described quantitatively by a mechanistic model employing independently measured adsorption-desorption parameters of NH3, N2 and H2, where the rate coefficient for hydrogen desorption is a function of the fractional surface coverage ofmore » nitrogen adatoms. The hydrogenation of NH2(a) to produce molecularly adsorbed ammonia is predicted to be the dominant factor in the inhibition of the ammonia decomposition. 37 references, 5 figures, 1 table.« less

The Catalytic Properties of the Pd/Mica Model Catalyst

Abstract The apparent activation energy of an H2–D2 exchange reaction on a Pd/mica model catalyst system increased with the decrease in the particle size of palladium. The activity pattern as a function of the mean particle size was of a volcano-type, with a maximum at a particle size around 1.2 nm.

Angular and velocity distributions of HD molecules produced by the H2–D2 exchange reaction on the stepped Pt(557) surface

The H2–D2 exchange reaction on Pt(557) crystal surface was investigated using a molecular beam surface scattering technique. The angular and velocity distributions of the HD product were measured at various crystal temperatures (500–1170 K). It was found that the exchange reaction occurred via interactions of adsorbed H and D atoms (Langmuir–Hinshelwood mechanism). Although the HD that is produced desorbs with a cos2 θ angular distribution, its translational energy corresponds to a temperature slightly colder than that of the substrate with the mean energy of the desorbing molecules depending on the desorption angle. As the desorbing angle increases away from the surface normal, the mean translational energy decreases. The exchange reactivity was found to be incident azimuthal angle dependent while the translational energy of the product HD is independent of the azimuthal angle of detection.

The reaction H2+D2⇄2HD. A long history of erroneous interpretation of shock tube results

An ultraclean 2 in. i.d. single pulse shock tube coupled to an atomic resonance absorption system was constructed in order to determine hydrogen atom concentration by Lyman-α absorption. The tube was baked to 300 °C and pumped down to ∼10−7 Torr. Ultrapure argon could be shock heated to ∼2500 K with no spurious H atom absorption. The system was constructed in order to study the kinetics of chemical reactions which are strongly catalyzed by H atoms, under the conditions where no such atoms are present. Specifically, the role of H atoms in the H2+D2→2HD exchange reaction was studied. Mixtures of hydrogen and deuterium diluted in argon were shock heated to 1375–1760 K; samples were then taken from the tube and analyzed mass spectrometrically for the ratio [HD]/[D2]. 1400 K was the highest temperature at which no spurious H atom absorption was observed in a shocked mixture of 1% H2–1% D2. At 1400 K, under the conditions of no absorption, no, or ≤1%HD conversion was obtained. At higher temperatures Lyman-α absorption was detected and more HD conversion was observed. A comparison between these results and results obtained previously in conventional systems suggests that the high HD conversion observed in the past was caused by hydrogen atoms generated from impurities. The existence of a molecular mechanism in the H2–D2 exchange reaction is highly doubtful.

A simple perturbational approach for the determination of favourable reaction pathways

A simple perturbational approach has been used to establish the symmetry conditions for energetically favourable nuclear motions on a potential energy surface. In particular, the operational rules of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) for specifying the symmetry species of the nuclear displacements, which make a symmetry forbidden pathway symmetry-allowed, have been derived. A general, symmetry-independent, procedure is then proposed for finding the energetically most favourable pathway by referring to the form of the overlap density function of non-correlating orbitals. The method is demonstrated by selecting from among the several symmetry-allowed nuclear motions on the potential energy surface for the H2 + D2 exchange reaction, that which is energetically most favourable.

ChemInform Abstract: MECHANISM OF THE CATALYTIC OXIDATION OF HYDROGEN ON COPPER

AbstractH2O‐Bildung aus H2 und O2 an Cu wird im Temp.‐Bereich 393‐423 K in einem geschlossenen Zirkulationssystem und mit Hilfe der XPS untersucht.

Mechanism of the catalytic oxidation of hydrogen on copper

The mechanism of water formation from H2 and O2 on copper at 393–423 K was studied using a closed circulating system and X-ray photoelectron spectroscopy. Under the whole pressure range of O2 studied the surface was covered with a Cu2O layer during the reaction. Where the partial pressure of O2 was higher than ca. 9 × 103 Pa, the reaction rate was expressed as follows: r=k[H2]1[O2]0 and H2–D2 exchange did not proceed during the reaction. On the other hand, where the partial pressure of O2 was lower than ca. 9 × 103 Pa, the rate of water formation showed a maximum value at 1.3 × 103 Pa and the H2–D2 exchange reaction proceeded gradually. From these results, a possible mechanism of the reaction is proposed.

Calculation of the H4 Potental Energy Surface by a Semi-empirical Method

Abstract A semi-empirical method recently proposed for the calculation of the potential energy surface of the H3 system is extended to a larger polyatomic H4 system. The energy formula is based on the valence-bond formulation and is expressed as functions of many multiple exchange integrals. These integrals can be decomposed into “diatomic” contributions by using the Mulliken approximation. The diatomic contributions are estimated from accurate values of the H2 molecule. Calculations of the H4 potential energy for the symmetric linear, linear equidistant, and square planar configurations are presented. The present method is of use for dynamical studies of the H2+D2 exchange reaction.

The Mechanism of Catalyzed Hydrogen–Deuterium Exchange on Gold Deposited over Poly(tetrafluoroethylene)

Abstract The H2–D2 exchange reaction over Au(0.3%)/PTFE(poly(tetrafluoroethylene)) was studied. The apparent activation energy was found to be 29 kj mol−1. The dependence of the reaction rate on the total pressure was measured at 518, 432, and 393 K. It was found that the pressure exponent of the exchange reaction decreases with the increase in the total pressure and that in the lower pressure range, the reaction order is as high as 2. The rate equation obtained from the pressure dependence was R=k{KP⁄(1+KP)}2. Accordingly, it was concluded that the H2–D2 exchange reaction over gold takes place between molecularly adsorbed hydrogen and deuterium through a dimeric intermediate. The heat of adsorption on the active sites in the gold surface was 5.9 kj mol−1.

A modulated molecular beam study of the mechanism of the H2–D2 exchange reaction on Pt(111) and Pt(332) crystal surfaces

The interaction of hydrogen with platinum has been studied by the exchange reaction H2+D2=2HD on two crystal surfaces, a flat (111) and a stepped (332) (in step notation, (S)−[6(111) × (111)]). The adsorption of hydrogen appears to be an activated process on the Pt(111) surface, with a barrier height of ∼0.5–1.5 kcal/mole. On the Pt(332) surface, the adsorption of hydrogen requires no activation energy. The recombination of H and D atoms to form HD follows, on both surfaces, a parallel mechanism with one of the branches operative in the entire temperature range studied, 25–800° C. This branch has an activation energy and pseudo-first-order preexponential of E1=13.0±0.4 kcal/mole and A1= (8±3) ×104sec−1 for the Pt(332) surface and E1=15.6±0.5 kcal/mole and A1= (2.7±1) ×105sec−1 for the Pt(111) surface. For temperatures above ∼300° C, the second branch is observed, but the values of the preexponential factors and activation energies could not be uniquely determined for either of the two crystals. Below ∼300° C, a third process appears in series with the first branch. The scatter in the data at low temperatures due to the small signal amplitude prevents accurate determination of the rate constants for this reaction step.