D2 Exchange Research Articles

In situ real-time neutron imaging of gaseous H2 adsorption and D2 exchange on carbon-supported Pd catalysts.

We use in situ neutron imaging to observe the adsorption/absorption of hydrogen within a packed catalyst bed of a Pd/C catalyst at a spatial and temporal resolution of ∼430 μm and a ∼9 s respectively. Additionally, the H2/D2 exchange process across the catalyst bed is followed in real time.

On the Utility of Ce3+ Spin−Orbit Transitions in the Interpretation of Rate Data in Ceria Catalysis: Theory, Validation, and Application

Cerium oxide is a ubiquitous component in catalytic systems of industrial and academic interest, and the precise mechanistic function of oxygen vacancies/Ce3+ ions a persistent open question in this regard. Existing methods for quantifying vacancy densities are for the most part indirect in nature, and they either have to necessarily be applied ex situ or, if applicable in situ, are oftentimes limited in applicability or quantitative rigor. We present herein a theoretical and experimental analysis of the utility of 2F5/2–2F7/2 electronic transitions in the quantitative estimation of oxygen vacancy or Ce3+ densities. Application of crystal field theory reveals the nonforbidden nature of 2F5/2–2F7/2 transitions for Ce3+ ions present in a C3v site symmetry environment, unlike free Ce3+ ions in the gas phase that exhibit La Porte forbidden transitions between states of equivalent parity. The molar extinction coefficient for the infrared feature of interest is estimated using three reductants─H2, CO, and ethanol─and applied toward interpretation of ceria-catalyzed H2–D2 exchange and tert-butanol dehydration catalysis rate data. Linear correlations between H2–D2 exchange rates and infrared spectroscopy-derived vacancy densities point to the sole involvement of reduced domains in H2 activation steps prevalent in hydrogenation and hydrodeoxygenation catalysis. Contrastingly, areal tert-butanol dehydration rates that trend inversely with vacancy densities both in the presence and absence of α-hydrogen-containing oxygenates carrying a propensity toward stoichiometric oxidative routes suggest the participation of oxidized, not reduced domains in dehydrative turnovers producing isobutene. The nonforbidden nature of the Ce3+2F5/2–2F7/2 transition─evidenced here using symmetry considerations─likely extends beyond the C3v symmetry environment under consideration here, implying that its fortuitous appearance within the infrared region of the electromagnetic spectrum may render it amenable to quantitative application in relating local structure to catalytic function in ceria catalysis more broadly.

Direct utilization of crude and waste H2 via CO-tolerant hydrogenation

Direct utilization of crude and waste H2 via CO-tolerant hydrogenation

Catalytic Interplay of Ga, Pt, and Ce on the Alumina Surface Enabling High Activity, Selectivity, and Stability in Propane Dehydrogenation

Pt-based bimetallic catalysts have been widely investigated in propane dehydrogenation (PDH) owing to their high activity in C–H cleavage and propylene selectivity. However, upon repeated coke oxidation for catalyst regeneration, they suffer from significant metal sintering and dealloying. Recently, γ-Al2O3 doped with Ga, Pt, and Ce was reported to exhibit superior catalytic activity, selectivity, and stability in PDH, but the catalytic role of each element has not been clearly understood because of the complexity of this system. In this study, we rigorously investigated the reaction mechanism and catalytic interplay of each component (Ga, Pt, and Ce). Selective poisoning, in situ diffuse reflectance infrared Fourier transform spectroscopy, and H2–D2 exchange revealed that Ga3+ is responsible for the heterolytic dissociation of the C–H bond of propane, while Pt0 facilitates the sluggish H recombination into H2 via reverse spillover. Catalyst deactivation during repeated reaction–regeneration cycles is mainly due to the irreversible sintering of Pt0. Notably, optimal Ce doping (∼2 wt %) selectively generated atomically dispersed Ce3+ sites on the γ-Al2O3 surface, which greatly suppressed the sintering of Pt0 particles by increasing the metal–support interactions. In contrast, excessive Ce loading generated discrete CeO2 domains, which stabilized the Pt species in the form of Pt2+ inactive for H recombination. Thus, excessive Ce loading led to an even more severe loss of catalytic activity and selectivity. The present results demonstrate that the selective generation of atomically dispersed Ce3+ on the γ-Al2O3 surface is important for stabilizing Pt0 species, which is essential for simultaneously achieving high catalytic activity, selectivity, and longevity in PDH.

The Binding Strength of Reactive H*: A Neglected Key Factor in Rh-Catalyzed Environmental Hydrodefluorination Reaction

The catalyzed reduction process plays an important role in the safe handling of environmental contaminants. It is generally believed that the adsorption of pollutants and cleavage of chemical bonds with high stability is the key and rate-determining step. Herein, using the H2–D2 exchange experiment, we observed a rapid decrease in the yield of HD on the Rh nanocatalyst, a state-of-the-art choice for the reduction of fluorinated organics, which indicates that the overadsorption of H/D atoms poisoned the Rh surface. The high binding energy of H/D on Rh decreases its reactivity as a reductant and is unfavorable for the adsorption/activation of the C–F bond. This dilemma is easily overcome by depositing Rh atoms onto Pd nanowires or commercial Pd/Al2O3. In addition to reducing the size of the Rh ensemble and therefore decreasing the H binding energy, Pd also directly spills over reactive H to the nearby Rh center; Pd@Rh shows 50 times higher mass activity and can remove persistent pollutants such as hexafluorobenzene and perfluorooctane acid under environmentally relevant conditions. Interestingly, the Pd@Rh catalyst hydrogenates halogenated phenols into value-added cyclohexanone with high selectivity. This work provides new insight into the design of advantageous nanocatalysts for environmental improvement and the recovery of resources from wastewater.

Kinetic Fingerprints of Catalysis by Subsurface Hydrogen

Mechanisms for catalytic H2–D2 exchange involving subsurface hydrogen, H′ (D′), have been analyzed to predict the ranges of reaction conditions over which the reaction orders, nH2 and nD2, can be u...

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.

Non-noble metal catalysts for hydrogenation: A facile method for preparing Co nanoparticles covered with thin layered carbon

Metallic cobalt nanoparticles with surface CoOx patches covered by thin layered carbon (named Co@C) have been directly synthesized by thermal decomposition of Co-EDTA complex. Raman spectra and HRTEM images suggest that discontinuities can be found in the disordered layered carbon. XPS shows that the CoOx patches in the Co@C nanoparticles can reduced to metallic Co by H2 under reaction conditions (7bar at 120°C), and H2–D2 exchange experiments show that the reduced metallic Co nanoparticles covered by carbon layers can dissociate H2. The Co@C nanoparticles show excellent activity and selectivity during chemoselective hydrogenation of nitroarenes for a wide scope of substrates under mild reaction conditions. Based on the results from DRIFTS adsorption experiments, we propose that metallic Co in the Co@C nanoparticles is the active phase. The role of the carbon layers is to protect the Co from overoxidation by air, leading to the chemoselective hydrogenation of nitroarenes.

Catalytic Properties of Gold Nanoparticles Prepared in Reverse Micelles

Adsorption and catalytic properties of gold nanoparticles of various sizes prepared by two methods of reduction in reverse micellar solutions were studied. Gold nanoparticles are capable of adsorbing molecular hydrogen and exhibiting catalytic activity in reactions of ortho—para conversion of protium and deuterium—hydrogen exchange. The size dependence of the catalytic activity of gold nanoparticles in reaction of H2—D2 exchange regardless of the method of preparation was determined.

Catalytic properties of gold nanoparticles in H2—D2 exchange and ortho—para hydrogen conversion

Gold nanoparticles of 3.8, 4.6, and 19.4 nm size, unlike the bulk metal, can adsorb hydrogen and catalyze the deuterium—protium exchange in the 77—523 K range. It has been shown that as the particle size decreases, the specific catalytic activity increases. The exceptionally high catalytic activity of gold nanoparticles in the ortho—para hydrogen conversion with respect to the deuterium—hydrogen exchange may indicate that at low temperatures the nanoparticles exhibit magnetic properties.

Kinetics of hydrogen activation on ceria–zirconia

Ceria–zirconias are popular catalysts and supports for metal particles. Even without supported metal particles, these materials are active for hydrogenation and hydrodeoxygenation reactions, where oxygen vacancies serve as the active site. To gain a detailed understanding of the ability of ceria-based catalysts to dissociatively adsorb hydrogen, H2–D2 exchange is studied as a test reaction. The density of exchangeable hydrogen per surface area approaches the values for typical metal surfaces. Below 250°C, H2–D2 exchange occurs at oxygen vacancies with an activation energy of ca. 24kJmol−1. At higher temperatures, additional sites at the edges of ceria–zirconia crystallites contribute to the reaction. The kinetics and the density of active sites for dissociative adsorption of hydrogen are correlated with physicochemical properties of the catalysts. Specifically, the crystallite size has a strong influence on the reactivity of different samples.

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

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

Investigation of the active sites of rhodium sulfide for hydrogen evolution/oxidation using carbon monoxide as a probe.

Carbon monoxide (CO) was observed to decrease the activity for hydrogen evolution, hydrogen oxidation, and H2-D2 exchange on rhodium sulfide, platinum, and rhodium metal. The temperature at which the CO was desorbed from the catalyst surface (detected by recovery in the H2-D2 exchange activity of the catalyst) was used as a descriptor for the CO binding energy to the active site. The differences in the CO desorption temperature between the different catalysts showed that the rhodium sulfide active site is not metallic rhodium. Using density functional theory, the binding energy of CO to the Rh sites in rhodium sulfide is found comparable to the binding energy on Pt. Coupled with experiment this supports the proposition that rhodium rather than sulfur atoms in the rhodium sulfide are the active site for the hydrogen reaction. This would indicate the active sites for hydrogen evolution/oxidation as well as oxygen reduction (determined by other groups using X-ray absorption spectroscopy) may be the same.

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.

Redistribution of Trialkyl Silanes Catalyzed by Iridium Silyl Complexes

[Ir(COE)2Cl]2 reacts with Et3SiH at 23 °C to form a binuclear iridium complex (Et3Si)2(H)2Ir(μ-Cl)2Ir(H)2(SiEt3)2, 1. Complex 1 reacts further with Et3SiH at 60 °C to form a second binuclear iridium complex, Et3Si(H)2Ir(μ-SiEt2)2Ir(H)2SiEt3, 2, containing bridging Et2Si groups. Activation of 2 with H2 produces trace quantities of a very highly reactive but unobservable species which rapidly and efficiently catalyzes alkyl redistribution reactions of silanes, RR′R″SiH. D2 and silane exchange experiments establish reactivity features of both 2 and the reactive intermediate. The intermediate cannot be observed, but it is likely a monomeric iridium silyl silylene complex that catalyzes alkyl scrambling via silane exchanges coupled with 1,3-alkyl migrations between silicon centers. DFT calculations support such a mechanism.

The Kinetics of H2–D2 Exchange over Pd, Cu, and PdCu Surfaces

PdCu alloy membranes are promising candidates for hydrogen separation from H2S-containing gas mixtures. Dissociative H2 adsorption and associative desorption are important steps in the process of hydrogen transport across PdCu membranes; however, the kinetics of H2 adsorption and desorption on PdCu surfaces are not well understood. In this work, the kinetics and energetics of H2 adsorption and desorption on Pd, Cu, and PdCu surfaces are investigated by microkinetic analysis of H2–D2 exchange (H2 + D2 → 2HD) over fixed beds of Pd, Cu, Pd70Cu30, and Pd47Cu53 foils at near-ambient pressure and temperatures in the range 300–900 K. The rate of H2–D2 exchange over Cu, which is the least active H2–D2 exchange catalyst used in this study, is limited by the rate of H2 adsorption due to the large activation barrier to dissociative H2 adsorption on Cu (ΔEads⧧ = 0.54 ± 0.06 eV). The Cu content of the PdCu alloys and the crystal structures of the Pd-hydride phases (α- and β-PdH) and of Pd47Cu53 (body-centered cubic an...

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...