Density Functional Kinetic Monte Carlo Simulation of Water–Gas Shift Reaction on Cu/ZnO

We describe a density functional theory based kinetic Monte Carlo study of the water–gas shift (WGS) reaction catalyzed by Cu nanoparticles supported on a ZnO surface. DFT calculations were performed to obtain the energetics of the relevant atomistic processes. Subsequently, the DFT results were employed as an intrinsic database in kinetic Monte Carlo simulations that account for the spatial distribution, fluctuations, and evolution of chemical species under steady-state conditions. Our simulations show that, in agreement with experiments, the H₂ and CO₂ production rates strongly depend on the size and structure of the Cu nanoparticles, which are modeled by single-layer nano islands in the present work. The WGS activity varies linearly with the total number of edge sites of Cu nano islands. In addition, examination of different elementary processes has suggested competition between the carboxyl and the redox mechanisms, both of which contribute significantly to the WGS reactivity. Our results have also indicated that both edge sites and terrace sites are active and contribute to the observed H₂ and CO₂ productivity

Mechanistic Insight through Factors Controlling Effective Hydrogenation of CO₂ Catalyzed by Bioinspired Proton-Responsive Iridium(III) Complexes

Reversible H₂ storage near room temperature and pressure with pH as the “switch” for controlling the direction of the reaction has been demonstrated (<i>Nat. Chem.</i>, <b>2012</b>, <i>4</i>, 383–388). Several bioinspired “proton-responsive” mononuclear Ir­(III) catalysts for CO₂ hydrogenation were prepared to gain mechanistic insight through investigation of the factors that control the effective generation of formate. These factors include (1) kinetic isotope effects by water, hydrogen, and bicarbonate; (2) position and number of hydroxyl groups on bpy-type ligands; and (3) mono- vs dinuclear iridium complexes. We have, for the first time, obtained clear evidence from kinetic isotope effects and computational studies of the involvement of a water molecule in the rate-determining heterolysis of H₂ and accelerated proton transfer by formation of a water bridge in CO₂ hydrogenation catalyzed by bioinspired complexes bearing a pendent base. Furthermore, contrary to expectations, a more significant enhancement of the catalytic activity was observed from electron donation by the ligand than on the number of the active metal centers

Mechanistic Insight through Factors Controlling Effective Hydrogenation of CO₂ Catalyzed by Bioinspired Proton-Responsive Iridium(III) Complexes

Reversible H₂ storage near room temperature and pressure with pH as the “switch” for controlling the direction of the reaction has been demonstrated (<i>Nat. Chem.</i>, <b>2012</b>, <i>4</i>, 383–388). Several bioinspired “proton-responsive” mononuclear Ir­(III) catalysts for CO₂ hydrogenation were prepared to gain mechanistic insight through investigation of the factors that control the effective generation of formate. These factors include (1) kinetic isotope effects by water, hydrogen, and bicarbonate; (2) position and number of hydroxyl groups on bpy-type ligands; and (3) mono- vs dinuclear iridium complexes. We have, for the first time, obtained clear evidence from kinetic isotope effects and computational studies of the involvement of a water molecule in the rate-determining heterolysis of H₂ and accelerated proton transfer by formation of a water bridge in CO₂ hydrogenation catalyzed by bioinspired complexes bearing a pendent base. Furthermore, contrary to expectations, a more significant enhancement of the catalytic activity was observed from electron donation by the ligand than on the number of the active metal centers

Mechanisms for CO Production from CO₂ Using Reduced Rhenium Tricarbonyl Catalysts

The chemical conversion of CO₂ has been studied by numerous experimental groups. Particularly the use of rhenium tricarbonyl-based molecular catalysts has attracted interest owing to their ability to absorb light, store redox equivalents, and convert CO₂ into higher-energy products. The mechanism by which these catalysts mediate reduction, particularly to CO and HCOO^–, is poorly understood, and studies aimed at elucidating the reaction pathway have likely been hindered by the large number of species present in solution. Herein the mechanism for carbon monoxide production using rhenium tricarbonyl catalysts has been investigated using density functional theory. The investigation presented proceeds from the experimental work of Meyer’s group (<i>J. Chem. Soc., Chem. Commun.</i> <b>1985</b>, 1414–1416) in DMSO and Fujita’s group (<i>J. Am. Chem. Soc.</i> <b>2003</b>, 125, 11976–11987) in dry DMF. The latter work with a simplified reaction mixture, one that removes the photo-induced reduction step with a sacrificial donor, is used for validation of the proposed mechanism, which involves formation of a rhenium carboxylate dimer, [Re­(dmb)­(CO)₃]₂(OCO), where dmb = 4,4′-dimethyl-2,2′-bipyridine. CO₂ insertion into this species, and subsequent rearrangement, is proposed to yield CO and the carbonate-bridged [Re­(dmb)­(CO)₃]₂(OCO₂). Structures and energies for the proposed reaction path are presented and compared to previously published experimental observations

Thermodynamic and Kinetic Hydricity of Ruthenium(II) Hydride Complexes

Despite the fundamental importance of the hydricity of a transition metal hydride (Δ<i>G</i>_H^–^°(MH) for the reaction M–H → M⁺ + H^–) in a range of reactions important in catalysis and solar energy storage, ours (<i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 2794) are the only values reported for water solvent, and there has been no basis for comparison of these with the wider range already determined for acetonitrile solvent, in particular. Accordingly, we have used a variety of approaches to determine hydricity values in acetonitrile of Ru­(II) hydride complexes previously studied in water. For [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ (bpy = 2,2′-bipyridine), we used a thermodynamic cycle based on evaluation of the acidity of [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ p<i>K</i>_a = 22.5 ± 0.1 and the [Ru­(η⁶-C₆Me₆)­(bpy)­(NCCH₃)_1/0]^2+/0 electrochemical potential (−1.22 V vs Fc⁺/Fc). For [Ru­(tpy)­(bpy)­H]⁺ (tpy = 2,2′:6′,2″-terpyridine) we utilized organic hydride ion acceptors (A⁺) of characterized hydricity derived from imidazolium cations and pyridinium cations, and determined <i>K</i> for the hydride transfer reaction, S + MH⁺ + A⁺ → M­(S)²⁺ + AH (S = CD₃CN, MH⁺ = [Ru­(tpy)­(bpy)­H]⁺), by ¹H NMR measurements. Equilibration of initially 7 mM solutions was slowon the time scale of a day or more. When <i>E</i>°(H⁺/H^–) is taken as 79.6 kcal/mol vs Fc⁺/Fc as a reference, the hydricities of [Ru­(η⁶-C₆Me₆)­(bpy)­H]⁺ and [Ru­(tpy)­(bpy)­H]⁺ were estimated as 54 ± 2 and 39 ± 3 kcal/mol, respectively, in acetonitrile to be compared with the values 31 and 22 kcal/mol, respectively, found for aqueous media. The p<i>K</i>_a estimated for [Ru­(tpy)­(bpy)­H]⁺ in acetonitrile is 32 ± 3. UV–vis spectroscopic studies of [Ru­(η⁶-C₆Me₆)­(bpy)]⁰ and [Ru­(tpy)­(bpy)]⁰ indicate that they contain reduced bpy and tpy ligands, respectively. These conclusions are supported by DFT electronic structure results. Comparison of the hydricity values for acetonitrile and water reveals a flattening or compression of the hydricity range upon transferring the hydride complexes to water

Highly Efficient and Selective Methanol Production from Paraformaldehyde and Water at Room Temperature

An efficient catalytic system using a water-soluble iridium complex, Cp*IrL­(OH₂)²⁺ (Cp* = pentamethylcyclopentadienyl, L = 2,2′,6,6′-tetrahydroxy-4,4′-bipyrimidine), was developed for highly selective methanol production at room temperature (initial turnover frequency of 4120 h^–1) with a very high yield (93%). This catalytic system features paraformaldehyde as the sole carbon and hydride source, leading to a record turnover number of 18200 at 25 °C. A step-by-step mechanism has been proposed for the catalytic conversion of paraformaldehyde to methanol on the basis of density functional theory (DFT) calculations. The proposed pathway holds the potential capacity to extend the scope of indirect routes for methanol production from CO₂

Photoinduced Water Oxidation at the Aqueous GaN (101̅0) Interface: Deprotonation Kinetics of the First Proton-Coupled Electron-Transfer Step

Photoelectrochemical water splitting plays a key role in a promising path to the carbon-neutral generation of solar fuels. Wurzite GaN and its alloys (e.g., GaN/ZnO and InGaN) are demonstrated photocatalysts for water oxidation, and they can drive the overall water splitting reaction when coupled with co-catalysts for proton reduction. The present work investigates the water oxidation mechanism on the prototypical GaN (101̅0) surface using a combined ab initio molecular dynamics and molecular cluster model approach taking into account the role of water dissociation and hydrogen bonding within the first solvation shell of the hydroxylated surface. The investigation of free-energy changes for the four proton-coupled electron-transfer (PCET) steps of the water oxidation mechanism shows that the first PCET step for the conversion of −Ga–OH to −Ga–O^•– requires the highest energy input. The study further examines the sequential PCETs, with the proton transfer (PT) following the electron transfer (ET), and finds that photogenerated holes localize on surface −NH sites, and the calculated free-energy changes indicate that PCET through −NH sites is thermodynamically more favorable than −OH sites. However, proton transfer from −OH sites with subsequent localization of holes on oxygen atoms is kinetically favored owing to hydrogen bonding interactions at the GaN (101̅0)–water interface. The deprotonation of surface −OH sites is found to be the limiting factor for the generation of reactive oxyl radical ion intermediates and consequently for water oxidation

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO₂ Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH₃)]²⁺ (6DHBP = 6,6′-(OH)₂bpy)

Ruthenium complexes with proton-responsive ligands [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]­(CF₃SO₃)₂ (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as catalysts for CO₂ reduction. Electrochemical reduction of [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]²⁺ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru­(tpy)­(<i>n</i>DHBP–2H⁺)]⁰) triggers catalysis of CO₂ reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ (bpy = 2,2′-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO₂ at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H⁺ with CO₂

CO₂ Hydrogenation Catalysts with Deprotonated Picolinamide Ligands

In an effort to design concepts for highly active catalysts for the hydrogenation of CO₂ to formate in basic water, we have prepared several catalysts with picolinic acid, picolinamide, and its derivatives, and we investigated their catalytic activity. The CO₂ hydrogenation catalyst having a 4-hydroxy-<i>N</i>-methylpicolinamidate ligand exhibited excellent activity even under ambient conditions (0.1 MPa, 25 °C) in basic water, exhibiting a TON of 14700, a TOF of 167 h^–1, and producing a 0.64 M formate concentration. Its high catalytic activity originates from strong electron donation by the anionic amide moiety in addition to the phenolic O^– functionality

Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO₂ Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH₃)]²⁺ (6DHBP = 6,6′-(OH)₂bpy)

Ruthenium complexes with proton-responsive ligands [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]­(CF₃SO₃)₂ (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as catalysts for CO₂ reduction. Electrochemical reduction of [Ru­(tpy)­(<i>n</i>DHBP)­(NCCH₃)]²⁺ generates deprotonated species through interligand electron transfer in which the initially formed tpy radical anion reacts with a proton source to produce singly and doubly deprotonated complexes that are identical to those obtained by base titration. A third reduction (i.e., reduction of [Ru­(tpy)­(<i>n</i>DHBP–2H⁺)]⁰) triggers catalysis of CO₂ reduction; however, the catalytic efficiency is strikingly lower than that of unsubstituted [Ru­(tpy)­(bpy)­(NCCH₃)]²⁺ (bpy = 2,2′-bipyridine). Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical infrared experiments suggest the reactivity of CO₂ at both the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl formed by the intermediacy of a metallocarboxylic acid is stable against reduction, and mass spectrometry analysis of this product indicates the presence of two carbonates formed by the reaction of DHBP–2H⁺ with CO₂