Base-Catalyzed Bifunctional Addition to Amides and Imides at Low Temperature. A New Pathway for Carbonyl Hydrogenation

In this study, we have compared the results of using chemical probes on model surfaces to investigate reactivity of monolayer Ni films on Pt(111), W(110), and Ru(0001) single crystal surfaces. Using H2, cyclohexene, and ethylene as probe molecules, we have studied the metal–H bond strength, the hydrogenation of cyclohexene, and the bonding and decomposition of ethylene. The Ni/Pt(111) and Ni/W(110) surfaces exhibit a low-temperature desorption of hydrogen at monolayer Ni coverages [also at 0.4 ML for the Ni/W(110) surfaces]. In contrast, the presence of monolayer Ni on Ru(0001) does not induce the low-temperature desorption state of hydrogen on the Ni/Ru(0001) surface. This desorption at low temperature is an indication of a weaker metal–H bond, which will affect the hydrogenation activity of the surfaces. For example, on the 1 ML Ni/Pt(111) and the 0.4 ML Ni/W(110) surfaces, a low-temperature hydrogenation pathway is detected. Previous high-resolution electron energy loss spectroscopy results indicate that cyclohexene is weakly π bonded to the Ni/Pt(111) surface, but it is strongly bonded to the Ni/W(110) surface. These results further confirm the importance of weakly bonded hydrogen for the enhancement of the hydrogenation pathway. Finally, the 1 ML Ni/Pt(111) surface is relatively inactive toward ethylene decomposition, while the 1 ML Ni/Ru(0001) surface remains active toward the dissociation of ethylene.

Sum frequency generation (SFG) surface vibrational spectroscopy and kinetic measurements using gas chromatography have identified at least two reaction pathways for benzene hydrogenation on the Pt(100) and Pt(111) single-crystal surfaces at Torr pressures. Kinetic studies at low temperatures (310-370 K) show that benzene hydrogenation does not proceed through cyclohexene. A Langmuir-Hinshelwood-type rate law for the low-temperature reaction pathway is identified. The rate-determining step for this pathway is the addition of the first hydrogen atom to adsorbed benzene for both single-crystal surfaces, which is verified by the spectroscopic observation of adsorbed benzene at low temperatures on both the Pt(100) and Pt(111) crystal faces. Low-temperature SFG studies reveal chemisorbed and physisorbed benzene on both surfaces. At higher temperatures (370-440 K), hydrogenation of benzene to pi-allyl c-C(6)H(9) is observed only on the Pt(100) surface. Previous single-crystal studies have identified pi-allyl c-C(6)H(9) as the rate-determining step for cyclohexene hydrogenation to cyclohexane.

Theoretical studies on dynamics and thermochemistry of the reactions CHClFCHO, CHF 2CHO and CClF 2CHO with the Cl atom

Ammonia and its derived products are vital to modern societies. Artificial nitrogen fixation to ammonia via the Haber–Bosch process has been employed industrially for over 100 years. However, the Haber–Bosch process is energy intensive and not sustainable in its current form as it uses hydrogen sourced from steam methane reforming to reduce N2. The roadmap to sustainable NH3 production demands the discovery of novel approaches for nitrogen fixation under near ambient conditions that preferably use water as the reducing agent. Over the last decade, great efforts have been made to develop catalysts capable of N2 fixation under mild reaction conditions, using strategies such as low temperature thermal catalysis, nonthermal plasma catalysis, enzymatic catalysis, photocatalysis, and electrocatalysis to generate ammonia and other valuable nitrogen‐containing chemicals. In parallel with catalytic performance studies, researchers have also placed emphasis on the mechanistic understanding of natural and artificial nitrogen fixation catalysts. In this work, the various routes now being explored for nitrogen fixation are summarized. The different dinitrogen activation and hydrogenation pathways are described, whilst describing key advances made to date on the journey toward near ambient ammonia synthesis. Key obstacles that need to be overcome to attract industry interest are also discussed.

Abstract–Sustained hydrogen photoevolution fromChlamy domonas reinhardtiiandC. Moewusiiwas measured under an anoxic, CO2‐containing atmosphere. It has been discovered that light intensity and temperature influence the partitioning of reductant between the hydrogen photoevolution pathway and the Calvin cycle. Under low incident light intensity (1‐3 W m‐2) or low temperature (approx. 0°C), the flow of photosynthetic reductant to the Calvin cycle was reduced, and reductant was partitioned to the hydrogen pathway as evidenced by sustained H2photoevolution. Under saturating light (25 W m‐2) and moderate temperature (20±5°C), the Calvin cycle became the absolute sink for reductant with the exception of a burst of H2occurring at light on. This burst of H2corresponded to the expression of about 450 electrons for each photosynthetic electron transport chain. These results suggest that the hydrogen pathway and the Calvin cycle compete for reductant under anoxic conditions and that partioning between the two pathways can, to a certain extent, be controlled by the appropriate choice of experimental conditions.

Low temperature electrolysis offers a viable option towards $2/Kg hydrogen at source with an efficiency greater than 43KWh/Kg. Towards this goal it is important to consider the criticality of the materials used. Typically a membrane based electrolyzer provides the convenience of being able to achieve high current and voltage efficiency via affording higher current density operation at or below 2V. Current state of the art membrane technology resides on the use of proton exchange membranes which necessitates the use of noble metals such as Pt and even rarer Ir. Achieving similar performance levels using anion exchange membranes with non-noble metals provides a clear pathway for mass deployment of this technology. This presentation will provide detailed materials strategy along with mechanistic insight into electrocatalytic pathways for both hydrogen and oxygen evolution (HER/OER) reactions as well as transport mechanisms involved in high current density (>1 A/cm2) operation while maintaining the potentials well below 2 V. Materials advantage will not only be discussed in the context of catalysts, membranes and electrodes but also in the context of current collectors and stack engineering.Advanced Ni based catalyst design principles for HER will be discussed in the context of fundamental studies of electrocatalytic reactions in alkaline pH using Pt based catalysts. Its direct translation to Ni based catalysts will be demonstrated from both activity and durability perspectives. Both a metal-metal oxide and a functionalized monometallic designs will be discussed. Performance in terms of RDE studies as well as full and half cell measurements will be presented. Similarly OER electrocatalysis will be presented from the point of view of layered double hydroxides. Doping of β-Ni(OH)2 with Fe and Co will be described in terms of their effects on crystal structure band gaps and possible mechanism changes. Full cell performance will be presented in terms of activity, use of electrolytes at the anode (OER) side, operational safety and durability. In addition use of this low temperature electrolyzer will also be described in terms of its use with salt water. Voltage window for OER vs hypochlorite formation will be discussed.

The selective hydrogenation of CO2 to methanol by renewable hydrogen source represents an attractive route for CO2 recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8mmol gcat-1 h-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513K, making Cu@FAU a potential methanol synthesis catalyst from CO2 hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO2 hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Chapter 3 Adsorption and Hydrogenation of Carbonyl and Related Compounds on Transition Metal Catalysts

We review our recent work on dealloyed nanoparticle electrocatalysts and address their synthesis, structural characterization and surface catalytic performance in low-temperature Polymer Electrolyte Membrane fuel cells (PEMFCs). The active form of the catalyst is obtained by voltammetric dealloying of non-noble metal rich Pt alloy precursors. In the dealloying process, the less noble precursor component, here Cu, is selectively removed from the surface of the precursor alloy particles and hence a Pt enriched particle shell is formed. Single fuel cell tests showed that, when used on the cathode of PEMFCs, dealloyed Pt catalysts show reactivities for the oxygen reduction reaction (ORR) which are up to 6 times higher than those of conventional pure Pt fuel cell catalysts. Similarly, the stability of dealloyed nanoparticle catalysts is superior to that of pure Pt particles. X-ray based structural and compositional studies suggested a core—shell particle structure as the active form of the catalyst consisting of a Pt enriched particle shell surrounding a Pt alloy core. At the present time, this catalyst system constitutes one of the most active fuel cell catalyst system reported in the literature. I. THE CONCEPT OF DEALLOYED NANOPARTICLE CATALYSTS Despite much recent focus on the development of advanced Li ion batteries for use as power source for short-range inner city transportation 255 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Vol. 25. No. 4. 2009 Dealloyed Core-Shell Fuel Cell Eleclrocatalysts applications, low temperature fuel cells continue to be the solution of choice for medium and long range transportation technologies based on their gravimetric power density as well as the gravimetric energy density of commonly used fuels. Wider use of low-temperature fuel cell technology is hampered by performance, cost, and durability issues associated with materials and components of a single fuel cell. Figure 1 displays a cross section of the layered structure of a low temperature PEMFC showing the anode (left) and cathode (right) gas diffusion layers (GDLs), which sandwich the anode and cathode catalyst layers and the proton exchange membrane. Figure 1 also schematically shows the molecular as well as electrical pathways of hydrogen fuel molecules, oxygen molecules, protons as well as of electrons. The overall performance of a PEMFC in terms of its practical cell voltage is limited by kinetic, ohmic, and mass transport processes for low, medium and high current densities, respectively. Of these, the kinetic surface catalytic reactions cause the most severe fuel cell voltage losses. * Anode fl( / Cathode ·>' '..·· Electrical | O2+*H* + 4e -> 2 H2O Energy Fig. 1: Cross sectional SEM micrograph through a membrane electrode assembly sandwiched between gas diffusion layers. Reaction processes at anode and cathode, mass and charge flows are indicated (from ref) In particular, the electrocatalytic Oxygen Reduction Reaction (ORR) at the cathode according to O2 + 4H + +4e->2H 2 O E°=+1.23 V/RHE (1) represents the key challenge to improved PEMFCs. In acidic media, Pt catalysts offer the highest catalytic activities which made first unsupported, later high surface area carbon-supported Pt particles the ORR catalyst of 256 Bereitgestellt von | Technische Universitat Berlin Angemeldet Heruntergeladen am | 14.04.15 16:12 Peter Strasser Reviews in Chemical Engineering choice· . Figure 2 illustrates the associative (via O2H) as well as the dissociative (via O) pathways of reaction (1) from molecular oxygen to water. Key for the catalysis is the chemisorption energy of the adsorbed oxygenated intermediates, such as Pt-O2H, Pt-O, and Pt-OH. On pure Pt, atomic oxygen is bonded too strongly and requires considerable overpotentials to react to Pt-OH. As a result of this, Pt is covered by (hydr)oxide adsorbates near the equilibrium potential of reaction (1). There is a consensus within the fuel cell catalysis community that a moderate reduction of the Pt-O chemisorption energy would result in significant ORR activity increases. H2O2 Hydrogen peroxide 0=0 Oxygen adsorption Oxygenated OH intemediates

The intricate dynamics of hydrogen on a nickel (111) surface is investigated. The purpose is to understand the unique recombination reaction of subsurface with surface hydrogen on the nickel host. The analysis is based on the embedded diatomics in molecules many-body potential surface. This potential enables a consistent evaluation of different hydrogen pathways in the nickel host. It is found that the pathway to subsurface-surface hydrogen recombination involves crossing a potential-energy barrier. Due to the light mass of the hydrogen the primary reaction route at low temperature occurs via tunneling. A critical evaluation of tunneling dynamics in a many-body environment has been carried out, based on a fully quantum description. The activated transport of subsurface hydrogen to a surface site, the resurfacing event, has been studied in detail. It is shown that the tunneling dynamics is dominated by correlated motion of the hydrogen and the nickel hosts. A fully correlated quantum-dynamical description in a multimode environment was constructed and employed. The ``surrogate Hamiltonian method'' represents the nickel host effect on the hydrogen dynamics as that of a set of two-level systems. The spectral density, which is the input of the theory is obtained via classical molecular-dynamics simulations. The analysis then shows that the environment can both promote and hinder the tunneling rate by orders of magnitude. Specifically for hydrogen in the nickel host, the net effect is suppression of tunneling compared to a frozen lattice approximation. The added effect of nonadiabatic interactions with the electron-hole pairs on the hydrogen tunneling rate was studied by an appropriate ``surrogate Hamiltonian'' with the result of a small suppression depending on the electron density of nickel. The resurfacing rates together with surface recombination rates and relaxation rates were incorporated in a kinetic model describing thermal-desorption spectra. Conditions for a thermal-programmed-desorption peak which manifest the subsurface-surface hydrogen recombination were found.

We report the synthesis of Ru(II) and Os(II) trans hydrido-hydroxo complexes by reacting the unsaturated amido complexes MH(NHCMe2CMe2NH2)(PPh3)2 (M = Ru, Os) with stoichiometric amounts of water. Proton exchange is rapid at room temperature between the amine/water/hydroxide moieties which leads to signal averaging of the NMR properties which can be slowed at low temperature in order to see resonances of separate complexes. These compounds can also be cleanly converted back to their starting complexes by dehydration in the presence of 3 Å molecular sieves. X-ray crystal structures of these Ru(II) and Os(II) trans hydrido-hydroxo complexes reveal that the unit cell contains an additional molecule of water trapped in the crystal lattice which hydrogen bonds with a neighbouring hydroxo ligand, forming a water bridged dimer in the solid state. Although there are many cases of oxidative addition of water to transition metal complexes, relatively few cases are well characterized where water addition occurs via metal-ligand cooperation (bifunctional addition) without altering the oxidation state of the metal center.

The bifunctional addition of lactones/esters is unexpectedly facile at low temperatures. Catalytic hydrogenations of esters can be carried out under mild conditions, e.g. −20 °C under 4 atm of H2, but product inhibition slows these reactions over time.

The mechanism of asymmetric hydrogenation of five representative β-dehydroamino acids catalyzed by rhodium complexes of (R)-(tert-butylmethylphosphino)(di-tert-butylphosphino)methane (trichickenfootphos, TCFP) and (R,R)-1,2-bis(tert-butylmethylphosphino)benzene (BenzP*) was studied through a combination of extensive NMR experiments and state-of-the-art DFT computations in order to reveal the crucial factors governing the sense and order of enantioselectivity in this industrially important reaction. The binding mode of the substrate with a Rh(I) catalyst was found to be highly dependent on the nature of the rhodium complex and the substrate. Thus, no substrate binding was detected for [Rh((R,R)-BenzP*)S2]+SbF6– (5) and (E)-3-acetylamino-2-butenoate (2a) even at 173 K. [Rh((R)-TCFP) S2]+BF4– (3) exhibited weak reversible binding with 2a in the temperature interval 173–253 K with the formation of complex 4a, whereas at ambient temperature, slow isomerization of 2a to (Z)-3-acetylamino-2-butenoate (2b) took place. The investigations with a total of 10 combinations of the catalysts and substrates demonstrated various binding modes that did not affect significantly the enantioselectivities observed in corresponding catalytic reactions and in low temperature hydrogenations of the catalyst–substrate complexes. The monohydride intermediate 10 formed quantitatively when the equilibrium mixture of 2a, 3, and 4a was hydrogenated at 173 K. Its molecular structure including relative stereochemistry was determined by NMR experiments. These results together with the stereochemichal outcome of the low-temperature hydrogenation (99.2% ee, R) and DFT calculations led to the reasonable reaction pathway of the asymmetric hydrogenation of 2a catalyzed by 3. The conceivable catalytic pathways were computed for five combinations of the BenzP*-Rh catalyst and prochiral β-dehydroamino acids 2a,b and 21–23. In most cases, it was found that the pathways involving the hydrogenation of Rh(I) square planar chelate complexes are usually higher in free energy than the pathways with the hydrogen activation prior to the chelate formation. Computed differences in the free energies of the transition states for the double bond coordination stage of the R and S pathways reasonably well reproduce the optical yields observed experimentally in the corresponding catalytic reactions and in the low temperature hydrogenation experiments. To explain extremely high ee's (>99% ee) in some of the hydrogenations, it is necessary to analyze in more detail the participation of the solvent in the enantiodetermining step.

Structural effect of Ni/ZrO2 catalyst on CO2 methanation with enhanced activity

γ-Valerolactone (GVL) is readily accessible by catalytic hydrogenation of carbohydrate-derived levulinic acid (LA) and is an attractive biobased chemical with a wide range of applications in both the chemical (e.g., as biomass-derived solvent) and the transportation fuel sector. In this study, we used isotopic labeling experiments to provide insights into the catalytic hydrogenation pathways involved in the conversion of LA to GVL under different reaction conditions using water as an environmentally benign solvent and Ru/C as a readily available catalyst. 2H NMR experiments combined with quantum chemical calculations revealed that deuterium atoms can be incorporated at different positions as well as the involvement of the different intermediates 4-hydroxypentanoic acid and α-angelica lactone (α-AL). The insight provided by these studies revealed an as of yet unexploited sequential deuteration route to synthesize fully deuterated LA and GVL. The route starts by the conversion of LA to α-AL followed by a selective deuteration of the acidic protons of α-AL by H/D exchange with D2O. Subsequent ring-opening in D2O (d2-AL to d3-LA) and exchange of the remaining protons of d3-LA via a keto-enol tautomerization by heating in D2O under acidic conditions gives d8-LA. Finally, the d8-LA is catalytically reduced at low temperature using Ru/C with D2 in D2O to d8-GVL.