James T. Muckerman

Hydrogen‐Evolution Catalysts Based on Non‐Noble Metal Nickel–Molybdenum Nitride Nanosheets

Hydrogen production through splitting of water has attracted great scientific interest because of its relevance to renewable energy storage and its potential for providing energy without the emission of carbon dioxide. Electrocatalytic systems for H2 generation typically incorporate noble metals such as Pt in the catalysts because of their low overpotential and fast kinetics for driving the hydrogen evolution reaction (HER). However, the high costs and limited world-wide supply of these noble metals make their application in viable commercial processes unattractive. Several non-noble metal materials, such as transition-metal chalcogenides, carbides, and complexes as well as metal alloys have been widely investigated recently, and characterized as catalysts and supports for application in the evolution of hydrogen. Nitrides of early transition-metals have been shown to have excellent catalytic activities in a variety of reactions. One of the primary interests in the applications of nitrides in these reactions was to use them in conjunction with low-cost alternative metals to replace group VIII noble metals. For example, the function of molybdenum nitride as a catalyst for hydrocarbon hydrogenolysis resembles that of platinum. The catalytic and electronic properties of transition-metal nitrides are governed by their bulk and surface structure and stoichiometry. While there is some information concerning the effect of the bulk composition on the catalytic properties of this material, there is currently little known about the effects of the surface nanostructure. Nickel and nickel–molybdenum are known electrocatalysts for hydrogen production in alkaline electrolytes, and in the bulk form they exhibited exchange current densities between 10 6 and 10 4 Acm , compared to 10 3 Acm 2 for Pt. Jaksic et al. postulated a hypo-hyper-d-electronic interactive effect between Ni and Mo that yields the synergism for the HER. Owing to their poor corrosion stability, few studies in acidic media have been reported.With the objective of exploiting the decrease in the overpotential by carrying out the HER in acidic media, we have developed a low-cost, stable, and active molybdenum-nitride-based electrocatalyst for the HER. Guided by the “volcano plot” in which the activity for the evolution of hydrogen as a function of the M H bond strength exhibits an ascending branch followed by a descending branch, peaking at Pt, we designed a material on the molecular scale combining nickel, which binds H weakly, with molybdenum, which binds H strongly. Here we report the first synthesis of NiMo nitride nanosheets on a carbon support (NiMoNx/C), and demonstrate the high HER electrocatalytic activity of the resulting NiMoNx/C catalyst with low overpotential and small Tafel slope. The NiMoNx/C catalyst was synthesized by reduction of a carbon-supported ammonium molybdate [(NH4)6Mo7O24·4H2O] and nickel nitrate (Ni(NO3)2·4H2O) mixture in a tubular oven in H2 at 400 8C, and subsequent reaction with NH3 at 700 8C. During this process, the (NH4)6Mo7O24 and Ni(NO3)2 precursors were reduced to NiMo metal particles by H2, and then they were mildly transformed to NiMoNx nanosheets by reaction with ammonia. The atomic ratio of Ni/Mo was 1/4.7 determined by energy dispersive X-ray spectroscopy (EDX) on the NiMoNx/ C sample. The transmission electron microscopy (TEM) images, as shown in Figure 1a, display NiMo particles that are mainly spherical. The high-resolution TEM image, as shown in the inset of Figure 1a, corroborated the presence of an amorphous 3 to 5 nm Ni/Mo oxide layer (see Figure S4 in the Supporting Information for resolved image), whereas NiMoNx is characterized by thin, flat, and flaky stacks composed of nanosheets with high radial-axial ratios (Figure 1b and Figure S5 in the Supporting Information for a magnified image). Figure 1c shows that some of the nanosheets lay flat on the graphite carbon (as indicated by the black arrows), and some have folded edges that show different layers of NiMoNx sheets (white arrows). The thickness of the sheets ranged from 4 to 15 nm. The average stacking number of sheets measured from Figure 1b is about [*] Dr. W.-F. Chen, Dr. K. Sasaki, Dr. J. T. Muckerman, Dr. R. R. Adzic Chemistry Department, Brookhaven National Laboratory Upton, NY 11973 (USA) E-mail: ksasaki@bnl.gov

Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures

Green plants convert CO2 to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO2 and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO2, formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies. When operating at near-ambient conditions, using water as a solvent, a high-turnover iridium catalyst enables a reversible hydrogen storage system that uses carbon dioxide, formate and formic acid. Proton-responsive ligands in the catalyst allow it to be turned on or off by controlling the pH of the solution.

Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts

The production of hydrogen by the electrolysis of water, a sustainable and greenhouse-gas-free source, requires an efficient and abundant electrocatalyst that minimizes energy consumption. Interest in transition metal carbides and nitrides has been aroused by their promising properties that make them potential substitutes for Pt-group metals as catalysts for the hydrogen evolution reaction. In this review, we discuss systematically the recent progress in the development of group IV-VI metal carbides and nitrides toward the hydrogen evolution reaction. Some strategies for designing such catalysts and improving their efficiency and reliability, including nanostructuring, optimizing hydrogen binding energy, interaction with the supporting material, and exploiting hybrid structures, are highlighted. We conclude with an outlook on the challenges in designing future HER electrocatalysts.

Highly active and durable nanostructured molybdenum carbide electrocatalysts for hydrogen production

In an attempt to tailor low-cost, precious-metal-free electrocatalysts for water electrolysis in acid, molybdenum carbide (β-Mo2C) nanoparticles are prepared by in situ carburization of ammonium molybdate on carbon nanotubes and XC-72R carbon black without using any gaseous carbon source. The formation of Mo2C is investigated by thermogravimetry and in situ X-ray diffraction. X-ray absorption analysis reveals that Mo2C nanoparticles are inlaid or anchored into the carbon supports, and the electronic modification makes the surface exhibit a relatively moderate Mo–H bond strength. It is found that carbon nanotube-supported Mo2C showed superior electrocatalytic activity and stability in the hydrogen evolution reaction (HER) compared to the bulk Mo2C. An overpotential of 63 mV for driving 1 mA cm−2 of current density was measured for the nanotube-supported Mo2C catalysts; this exceeds the activity of analogous Mo2C catalysts. The enhanced electrochemical activity is facilitated by unique effects of the anchored structure coupled with the electronic modification.

CO2 Hydrogenation to Formate and Methanol as an Alternative to Photo- and Electrochemical CO2 Reduction

Photoand Electrochemical CO2 Reduction Wan-Hui Wang,*,† Yuichiro Himeda,*,‡,§ James T. Muckerman, Gerald F. Manbeck, and Etsuko Fujita* †School of Petroleum and Chemical Engineering, Dalian University of Technology, Panjin 124221, China ‡National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5-1, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States

Monte Carlo Calculations of Energy Partitioning and Isotope Effects in Reactions of Fluorine Atoms with H2, HD, and D2

A total of 16 000 three‐dimensional classical trajectories on an optimized semiempirical potential energy surface were computed for collisions of F atoms with H2, HD, and D2. Analysis of the reactive trajectories indicates general agreement with results of infrared chemiluminescence and chemical laser studies with respect to the partitioning of the available energy among the translational, rotational, and vibrational degrees of freedom in the reaction products. Of particular interest is the calculated effect of reactant rotation on the reaction cross sections for all the isotropic systems, and on the intramolecular isotope effect in the reaction of F with HD. These phenomena have not as yet been seen experimentally.

Nature of hydrogen interactions with Ni(II) complexes containing cyclic phosphine ligands with pendant nitrogen bases

Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P2PhN2Ph)2(CH3CN)](BF4)2, 3a (where P2PhN2Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H2 production rate to be determined. The turnover frequency of 350 s−1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P2CyN2Bz)2](BF4)2, 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (Keq = 190 atm−1 at 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A–4C, which have been identified by a combination of one- and two-dimensional 1H, 31P, and 15N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to [HNi(P2CyN2Bz)2](BF4), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.

Some useful discrete variable representations for problems in time-dependent and time-independent quantum mechanics

Abstract We develop a simple, yet powerful approach to constructing discrete variable representations for the solution of quantum mechanical problems by focusing on properties of the underlying basis set. Two examples, one involving fixed-node boundary conditions and the other periodic boundary conditions, are explicitly derived and applied to a problem in spherical polar coordinates with azimuthal symmetry. These two DVRs have equally spaced points and equal weights.

Reactive Scattering Cross Sections III: Quasiclassical and Semiclassical Methods

Classical trajectory methods for calculating inelastic scattering cross sections are covered in earlier chapters of this book, especially Chapters 10 and 12. This chapter covers the extension of this technique to treat reactive scattering. The first question which must be answered in a classical trajectory study of a reactive system is whether one should be using this method at all. Classical trajectory studies are useful not just because they yield reaction cross sections, angular distributions, reactivity as a function of initial and final energy distribution, and other observable reaction at-tributes, but also for the insight they may offer into the actual reaction event. One may look at the atomic motions in representative trajectories, and one may calculate such nonobservables as opacity functions (probability of reaction as a function of impact parameter) and dependence on features of the potential energy surface. But one must be careful not to overinterpret the reaction by a trajectory study. Because many reaction attributes depend sensitively on quantitative and qualitative features of the potential energy surfaces which are not quantitatively understood, one must be cautious about believing that the dynamical details of a particular trajectory calculation are in general accord with reality. Trajectory calculations are discussed from this point of view in Chapter 18 of this book.

Theoretical studies of the mechanism of catalytic hydrogen production by a cobaloxime

Our theoretical studies of the standard reduction potentials of the molecular complex [Co(II)(dmgBF(2))(2)](0) (dmgBF(2) = difluoro-boryldimethylglyoximate) in acetonitrile solution shed light on its electrocatalytic mechanism for hydrogen production. Three such mechanisms have been proposed, all proceeding through the formation of Co(III)H. Our results indicate that the mechanism involving a Co(II)H intermediate is the most likely.