Heterolytic Dissociation Of H2 Research Articles

Preparation and Catalytic Properties of Gold Single-Atom and Cluster Catalysts Utilizing Nanoparticulate Mg-Al Layered Double Hydroxides.

Au single atoms and clusters were stabilized on Mg-Al layered double hydroxide nanoparticles (LDH NPs), and the obtained Au@LDH NPs were supported on SiO2 and CeO2. After hydrogen reduction, Au single atoms were found together with Au clusters on LDH/SiO2. In contrast to Au single-atom catalysts which are deposited in metal vacancies of oxide supports, the LDH NPs stabilize very small Au species despite the absence of metal vacancies. The obtained Au(0)@LDH/SiO2 catalyzed aerobic oxidation of alcohols, and Au single atoms maintained after the reaction. Given that only Au NPs were observed on bulk LDH, the abundant surface OH group of LDH NPs would contribute to stabilize Au, resulting in higher activity than Au/LDH-bulk. After calcination to transform LDH to mixed metal oxide (MMO), the obtained Au(0)@MMO/SiO2 also exhibited high catalytic activity. Moreover, Au(0)@LDH/CeO2 exhibited higher activity and excellent selectivity for hydrogenation of 4-nitrostyrene to 4-aminostyrene than conventional Au catalysts such as Au/CeO2 and Au/TiO2. We demonstrated that Au size can be minimized using LDH NPs, exhibiting high catalytic performance. The basic surface OH groups of LDH would be also beneficial for deprotonation of alcohols and heterolytic dissociation of H2 in the catalytic reactions.

First-principles study of CO2 hydrogenation on Cd-doped ZrO2: Insights into the heterolytic dissociation of H2.

Cd-doped ZrO2 catalyst has been found to have high selectivity and activity for CO2 hydrogenation to methanol. In this work, density functional theory calculations were carried out to investigate the microscopic mechanism of the reaction. The results show that Cd doping effectively promotes the generation of oxygen vacancies, which significantly activate the CO2 with stable adsorption configurations. Compared with CO2, gaseous H2 adsorption is more difficult, and it is mainly dissociated and adsorbed on the surface as [HCd-HO]* or [HZr-HO]* compact ion pairs, with [HCd-HO]* having the lower energy barrier. The reaction pathways of CO2 to methanol has been investigated, revealing the formate path as the dominated pathway via HCOO* to H2COO* and to H3CO*. The hydrogen anions, HCd* and HZr*, significantly reduce the energy barriers of the reaction.

Heterolytic Dissociation of H2 in Heterogeneous Catalysis

H2 dissociation plays crucial roles in catalytic hydrogenation reactions and hydrogen storage. Metal-nanoparticle-based heterogeneous catalysts often dissociate H2 via a homolytic pathway. The heterolytic H2 dissociation pathway was identified in several heterogeneous catalytic systems, including single-atom catalysts, metal–support interfaces, and bulk metal oxides/sulfides/nitrides/phosphides. The active site structures of these heterogeneous catalysts resemble homogeneous catalysts where the metal centers (Lewis acids) are coordinated with O/S/N/P atoms (Lewis bases). These Lewis acid–base pairs dissociate H2 molecules heterolytically into proton–hydride pairs, which favor the hydrogenation of polar functional groups in unsaturated hydrocarbons. In this review, we summarize the common structural features of heterogeneous, homogeneous, and enzyme catalysts in the heterolytic dissociation of H2. The active sites, Lewis acid–base pairs, are discussed throughout this review. The energy barriers and kinetic contributions of heterolytic and homolytic H2 dissociation pathways in heterogeneous catalytic systems are discussed. The spectroscopic evidence of the heterolytic H2 dissociation pathways is critically reviewed.

Rational Design of Main Group Metal-Embedded Nitrogen-Doped Carbon Materials as Frustrated Lewis Pair Catalysts for CO2 Hydrogenation to Formic Acid.

Developing efficient and inexpensive main group catalysts for CO2 conversion and utilization has attracted increasing attention, as the conversion process would be both economical and environmentally benign. Here, based on the main group element Al, we designed several heterogeneous frustrated Lewis acid/base pair (FLP) catalysts and performed extensive first-principles calculations for the hydrogenation of CO2. These catalysts, including Al@N-Gr-1, Al@N-Gr-2, and Al@C2N, are composed of a single Al atom and two-dimensional (2D) N-doped carbon-based materials to form frustrated Al/C or Al/N Lewis acid/base pairs, which are all predicted to have high reactivity to absorb and activate hydrogen (H2). Compared with Al@N-Gr-1, both Al@N-Gr-2 and Al@C2N, especially Al@N-Gr-2, containing Al/N Lewis pairs exhibit better catalytic activity for CO2 hydrogenation with lower activation energies. CO2 hydrogenation on the three catalysts prefers to go through a three-step mechanism, i.e., the heterolytic dissociation of H2, followed by the transfer of the hydride near Al to CO2, and finally the activation of a second H2 molecule. Other IIIA group element (B and Ga)-embedded N-Gr-2 materials (B@N-Gr-2 and Ga@N-Gr-2) were also explored and compared. Both Al@N-Gr-2 and Ga@N-Gr-2 show higher catalytic activity for CO2 hydrogenation to HCOOH than B@N-Gr-2. However, the CO2 hydrogenation path on Ga@N-Gr-2 tends to follow a two-step mechanism, including H2 dissociation and subsequent hydrogen transfer. The present study provides a potential solution for CO2 hydrogenation by designing novel and effective FLP catalysts based on main group elements.

Comparative study on the activities of different MgO surfaces in CO2 activation and hydrogenation

The first-principles calculations based on the density functional theory (DFT) have been applied to explore the CO2 adsorption as well as the activity and selectivity of CO2 dissociation and hydrogenation on six MgO surfaces. CO2 activation can be occurred by forming carbonate species on (110), (210), (211), (111)O and (111)Mg surfaces but the direct dissociation of CO2 looks thermodynamically unfavorable due to the high reaction energies (≥3.54 eV). The occurrence of CO2 protonation to carboxyl (HOCO) by hydroxyl on MgO surfaces seems to be thermodynamically and kinetically unfavorable. The upsurge noted in the hydroxyl coverage, due to the homolytic dissociation of H2, supports the CO2 conversion to HOCO with a lower barrier (1.98 eV). However, the dissociated H2O can reversibly lead to the consumption of HOCO (HOCO*+ OH*→ H*+ OH*+ CO2*). Hydride species might be produced by the heterolytic dissociation of H2 on (111)O, (111)Mg, and (211) surfaces. Two pathways have been considered for the CO2 hydrogenation to HCOO: (i) hydride H reaction with adsorbed CO2 by Langmuir-Hinshelwood mechanism and (ii) molecular CO2 electrophilic attacking the hydride H by Eley-Rideal mechanism. CO2 hydrogenation to HCOO by hydride H is more feasible on MgO surfaces in comparison with CO2 protonation.

Importance of Size and Contact Structure of Gold Nanoparticles for the Genesis of Unique Catalytic Processes.

Since the discovery of catalysis by Au nanoparticles (NPs), unique catalytic features of Au have appeared that are greatly different from those of Pd and Pt. In this Review, we aimed to disclose how the unique catalytic abilities of Au are generated with respect to (a) the contact structures between Au and its supports and (b) the size of the Au particles. For CO oxidation, the catalytic activity of Au on reducible metal oxides (MOx) is strongly correlated with the amount of oxygen vacancies of the MOx surface, which play a key role in O2 activation. Single atoms, bilayers of Au, sub-nm clusters, clusters (1-2 nm), and NPs (2-5 nm) have been proposed as the active sizes of the Au species, which may depend on the type of support. For propylene epoxidation, the presence of isolated TiO4 units in SiO2 supports is important for the production of propylene oxide (PO). Au NPs facilitate the formation of Ti-OOH species, which leads to PO in the presence of H2 and O2, whereas Au clusters facilitate propylene hydrogenation. However, Au clusters can produce PO by using only O2 and water, whereas Au NPs cannot. For alcohol oxidation, the reducibility of the MOx supports greatly influences the catalytic activity of Au, and single Au atoms more effectively activate the lattice oxygen of CeO2. The basic and acidic sites of the MOx surface also play an important role in the deprotonation of alcohols and the activation of aldehydes, respectively. For selective hydrogenation, heterolytic dissociation of H2 takes place at the interface between Au and MOx, and the basic sites of MOx contribute to H2 activation. Recent research into the reaction mechanisms and the development of well-designed Au catalysts has provided new insights into the preparation of high-performance Au catalysts.

X-ray Photospectroscopy and Electronic Studies of Reactor Parameters on Photocatalytic Hydrogenation of Carbon Dioxide by Defect-Laden Indium Oxide Hydroxide Nanorods.

In the study reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) and in situ Hall charge carrier measurements provide new insights into the surface physical chemistry of gaseous H2, CO2, and H2+CO2 combined with nanostructured In2O(3−x)(OH)y nanorods, which ensue under photochemical and thermochemical operating conditions. Heterolytic dissociation of H2 in H2-only atmosphere appears to occur mainly under dark and ambient temperature conditions, while the greatest amount of OH shoulder expansion in H2+CO2 atmosphere appears to mainly occur under photoilluminated conditions. These results correlate with those of the Hall measurements, which show that the prevalence of homolytic over heterolytic dissociation at increasing temperatures leads to a steeper rate of increase in carrier concentrations; and that H2 adsorption is more prevalent than CO2 in H2+CO2 photoillumination conditions.

Understanding the Role of Rutile TiO2 Surface Orientation on Molecular Hydrogen Activation.

Titanium oxide (TiO2) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H2 with TiO2 surface plays an important role. However, the activation of hydrogen over rutile TiO2 surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H2 as a hydride–proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride–proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO2 orientations.

Room-Temperature Ethene Hydrogenation Activity of Transition-Metal-Free HY Zeolites

The conversion of C2H4 in the absence and presence of H2 was examined over dealuminated HY30, HY15, and HY2.6 zeolites with different Si/Al ratios under ambient conditions (1 atm, 25 °C). Only in the presence of H2 did these materials catalyze the conversion of C2H4 into C2H6 with measurable rates and no sign of C2H4 dimerization. The C2H4 hydrogenation activity of these materials increases with the Si/Al ratio, implying that in this case, the strength of Bronsted acid sites (i.e., Si–OH–Al) is an important factor for the hydrogenation process. The adsorption of both C2H4 and H2 over Bronsted acid sites was calculated to be exothermic without activating the H2 molecule, while the conversion of C2H4 into ethoxy species is slightly endothermic. Heterolytic dissociation of H2 is strongly unfavorable over Al/O or Si/O sites of the zeolite framework, but it is exothermic over the extraframework Al species, indicating that cooperation of Bronsted acid sites with extraframework Al species is indeed required for ...

Design of a difunctional Zn-Ti LDHs supported PdAu catalyst for selective hydrogenation of phenylacetylene

To suppress hydrogenation of alkene at complete alkyne conversion, a difunctional Zn-Ti layered double hydroxides (LDHs) supported bimetallic PdAu alloy catalyst with alkalinity was designed and prepared by a photochemical reduction method. On the basic of TEM and XPS results, the formation of Pd-Au alloy was determined. The alloy nanoparticles had incorporated into the interlayer region of LDHs, giving a strong interaction between them. As expected, the PdAu/ZnTi catalysts exhibited excellent styrene selectivity (over 90%) even when the reaction time was prolonged (6 h) after full conversion of phenylacetylene. Such excellent selectivity is attributed to the synergistic effect between bimetallic alloy nanoparticles and Zn-Ti LDHs. The selective formation of polar hydrogen species derived from the heterolytic dissociation of H2 at the interface between PdAu alloy and basic sites of Zn-Ti LDHs is more favorably reactive to alkyne compared with alkene. Moreover, the Zn-Ti LDHs supported PdAu catalyst exhibited great recyclability. The difunctional catalyst is expected to be potentially promising for industrial applications.

Screening the activity of Lewis pairs for hydrogenation of CO2

CO2 capture coupled with CO2 conversion to hydrocarbon fuels could reduce the overall anthropogenic carbon footprint but requires an efficient catalytic pathway for CO2 hydrogenation. In this work, we examine functional groups that can be integrated within nanoporous materials, such as metal organic frameworks, that could lead to the development of materials that can selectively adsorb CO2 from flue gas and convert it into a useful fuel. We focus on the use of Lewis pair (LP) moieties as catalytic functional groups because of their activity for heterolytic dissociation of H2 and subsequent hydrogenation of CO2. However, most LP functional groups also strongly bind CO2, such that the catalytic site can be poisoned if the binding energy of CO2 is much stronger than that of H2. In this work, we screen a variety of LP moieties using density functional theory to compute the adsorption energies of H2 and CO2. We consider five classes of LP functional groups, with each class designed to explore modifying the binding energies in different ways. We have developed a mathematical model for predicting the H2 adsorption energies on various Lewis pairs as a function of geometric and energetic properties of the functional moieties.

Unraveling the facet-dependent and oxygen vacancy role for ethylene hydrogenation on Co3O4 (110) surface: A DFT+U study

Crystal facet engineering and defect engineering are both critical strategies to improve the catalytic hydrogenation performance of catalyst. Herein, ethylene hydrogenation on the perfect and oxygen defective Co3O4(110) surfaces has been studied by using periodic density functional theory calculations. The results are compared with that on Co3O4(111) surface to clarify the problem of which facet for Co3O4 is more reactive, and to illuminate the role of oxygen vacancy. The low oxygen vacancy formation energy suggests that Co3O4(110) surface with defective site is easily formed. The whole mechanism of H2 dissociation and stepwise hydrogenation of ethylene to ethane is examined, and the most favorable pathway is heterolytic dissociation of H2 follows two stepwise hydrogenation of ethylene process. The results show that ethyl hydrogenation to ethane on perfect Co3O4(110) surface is the rate limiting step with an activation energy of 1.19eV, and the presence of oxygen vacancy strongly reduces the activation energies of main elementary steps, and the activation energy of rate limiting step is only 0.47eV. Compared with that on Co3O4(111), ethylene hydrogenation is preferred on Co3O4(110) surface. Therefore, Co3O4 with exposed (110) facet is predicted as an excellent catalyst for ethylene hydrogenation.

Catalytic Hydrogenation of CO2 to Formates by a Lutidine-Derived Ru–CNC Pincer Complex: Theoretical Insight into the Unrealized Potential

Metal–ligand cooperative properties of a bis-N-heterocyclic carbene ruthenium CNC pincer catalyst and its activity in CO2 hydrogenation to formates were studied by DFT calculations complemented by NMR spectroscopy and kinetic measurements. The dearomatized Ru–CNC* pincer (1*) is significantly more reactive toward metal–ligand cooperative activation of H2 and CO2 than the structurally related phosphine-based Ru–PNP complex. The enhanced reactivity of Ru–CNC* stems from the combination of electronic properties of this system and the reduced geometric constraints imposed onto the Ru center by the large and flexible CNC chelate. Heterolytic dissociation of H2 by 1* results in the bis-hydrido complex 2 that is active in hydrogenation of CO2. However, under commonly applied reaction conditions, the catalyst rapidly deactivates via metal–ligand cooperative paths. The transient formation of the dearomatized complex Ru–CNC* (1*) in the course of the reaction leads to the irreversible cooperative activation of CO2,...

Illuminating CO2 reduction on frustrated Lewis pair surfaces: investigating the role of surface hydroxides and oxygen vacancies on nanocrystalline In2O(3-x)(OH)y.

Designing catalytic nanostructures that can thermochemically or photochemically convert gaseous carbon dioxide into carbon based fuels is a significant challenge which requires a keen understanding of the chemistry of reactants, intermediates and products on surfaces. In this context, it has recently been reported that the reverse water gas shift reaction (RWGS), whereby carbon dioxide is reduced to carbon monoxide and water, CO2 + H2 → CO + H2O, can be catalysed by hydroxylated indium oxide nanocrystals, denoted In2O(3-x)(OH)y, more readily in the light than in the dark. The surface hydroxide groups and oxygen vacancies on In2O(3-x)(OH)y were both shown to assist this reaction. While this advance provides a first step toward the rational design and optimization of a single-component gas-phase CO2 reduction catalyst for solar fuels generation, the precise role of the hydroxide groups and oxygen vacancies in facilitating the reaction on In2O(3-x)(OH)y nanocrystals has not been resolved. In the work reported herein, for the first time we present in situ spectroscopic and kinetic observations, complemented by density functional theory analysis, that together provide mechanistic information into the surface reaction chemistry responsible for the thermochemical and photochemical RWGS reaction. Specifically, we demonstrate photochemical CO2 reduction at a rate of 150 μmol gcat(-1) hour(-1), which is four times better than the reduction rate in the dark, and propose a reaction mechanism whereby a surface active site of In2O(3-x)(OH)y, composed of a Lewis base hydroxide adjacent to a Lewis acid indium, together with an oxygen vacancy, assists the adsorption and heterolytic dissociation of H2 that enables the adsorption and reaction of CO2 to form CO and H2O as products. This mechanism, which has its analogue in molecular frustrated Lewis pair (FLP) chemistry and catalysis of CO2 and H2, is supported by preliminary kinetic investigations. The results of this study emphasize the importance of engineering the surfaces of nanostructures to facilitate gas-phase thermochemical and photochemical carbon dioxide reduction reactions to energy rich fuels at technologically significant rates.

1H Magic Angle Spinning NMR Evidence for Dissociative Adsorption of Hydrogen on Ag+-Exchanged A- and Y-Zeolites

1H magic angle spinning NMR gave firm evidence for formation of acidic protons (4.0 ppm) and hydride species (−1.8 ppm), where the former exists as OH groups (acidic protons) and the latter has an interaction with three equivalent Ag atoms, existing as Ag3H when Ag+-exchanged A-zeolite is reduced with hydrogen. The formations of Ag3H and of acidic protons by the heterolytic dissociation of H2 over Ag3+ are reversible. The heterolytic dissociation of H2 was also observed over Ag-Y-zeolite, the silver hydride species being observed at −0.1 ppm.

Hydrogen–Deuterium Equilibration over Transition Metal Sulfide Catalysts: On the Synergetic Effect in CoMo Catalysts

H2–D2 equilibration was used to study the synergetic effect between Co and Mo in carbon-supported CoMo sulfide catalysts. The reaction was carried out in a recirculation apparatus with continuous on-line analysis of all gas-phase components (H2, D2, HD, H2S, D2S, and HDS) by a mass spectrometer. The low reaction temperature (423 K) allowed for the first time a comparison to be made of the catalytic activity between Co sulfide particles in low-temperature sulfided (373 K) carbon-supported Co and those in standard sulfided (673 K) carbon-supported CoMo, which have similar 57Co MES parameters. On the basis of the much higher activity of the bimetallic catalyst than of the monometallic catalysts (Co and Mo sulfide) it is concluded that the observed synergy is related neither to the effect of the high dispersion of the Co sulfide nor seemingly to spillover effects as put forward by the remote control model. The results indicate that the sulfur atoms that bridge between Co2+ and Mo4+ play a crucial role during the catalytic cycle. During the equilibration reaction also extensive exchange takes place between hydrogen (deuterium) atoms adsorbed on the sulfided phase and hydrogen atoms present at the carbon support surface. This spillover process is accelerated by admixture of H2S, which most probably leads to an increase of terminal SH groups by dissociative H2S adsorption. These SH groups are thought to form hydrogen bridges with the carbon support functional groups. Both H2 and H2S compete for adsorption on the same coordinatively unsaturated sites. While heterolytic dissociation of H2 is considered to lead to a hydride and a bridging SH group between Co and Mo, heterolytic dissociation of H2S would lead to the formation of two SH groups, one bridging SH group and one terminal SH group active in forming hydrogen bridges with the support surface functional groups.

Effect of incorporation of copper or nickel on hydrogen storage in ceria. Mechanism of reduction

Interaction of hydrogen with a series of mixed oxides CeMx(M = Cu, Ni; 0 < x⩽ 5) precursors of catalysts for hydrogenation reactions has been studied in the 300–1073 K temperature range. In situ XRD, XPS, EPR and thermogravimetric techniques have been used to characterize the processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing H2 have also been determined. The most interesting phenomena have been found to occur in the activation temperature range. They involve a non-negligible expansion of the ceria-based lattice which has been attributed to the high amount of hydrogen stored in the host oxide and the reduction of Ce4+ to Ce3+. The M2+ reduction is slowed down, depending on the content of M2+ in the ceria lattice: redox processes between Ce4+, Ce3+, M and M+ or M2+ have been clearly demonstrated. Finally, a reduction mechanism, partly based on heterolytic dissociation of H2, is proposed. This may be extended to bulk mixed oxides with other transition metals, and to a certain extent to metals of Group VIII (Rh, Pd, Ir, Pt) deposited on ceria.