R.D. Kelley

Kinetics of the hydrogenation of CO over a single crystal nickel catalyst

A specially designed ultrahigh vacuum system has been used to examine the effect of surface chemical composition on the kinetics of the catalytic methanation reaction over a single crystal Ni(100) catalyst. The surface is characterized using Auger Electron Spectroscopy (AES) in an ultrahigh vacuum chamber, and reaction kinetics are determined following an in vacuo transfer of the sample to a catalytic reactor contiguous to the AES chamber. The kinetics of CO hydrogenation on a clean Ni(100) surface at 450–800 K are compared with kinetic data reported for high-area supported nickel catalysts. Excellent agreement is observed between specific rates, activation energy, and product distribution measured for the supported catalysts and the single crystal Ni(100). The dependence of the specific rate on total pressure (1–1500 Torr) and on H2 and CO partial pressures as well as the product distribution are also reported. These data are consistent with a mechanism in which an active surface carbon species is the dominant route to product.

Neutron vibrational spectroscopy of hydrogen and deuterium on Raney nickel

Incoherent neutron inelastic scattering has been applied to the study of the chemisorption of hydrogen and deuterium on Raney nickel (a high surface area nickel powder). The binding sites have been identified by comparing the vibrational frequencies and intensities provided by the inelastic scattering with calculations for model structures of different geometries, force constants, and symmetries. We conclude that, on Raney nickel above 150 K, hydrogen chemisorbs predominantly in sites of threefold symmetry. The ’’best fit’’ corresponds to a Ni–H bond distance of 1.88±0.03 A and a force constant of 0.58±0.03 mdyn/A. In addition, we have observed lateral adsorbate interactions, deuterium isotope shifts, and temperature dependent binding sites.Incoherent neutron inelastic scattering has been applied to the study of the chemisorption of hydrogen and deuterium on Raney nickel (a high surface area nickel powder). The binding sites have been identified by comparing the vibrational frequencies and intensities provided by the inelastic scattering with calculations for model structures of different geometries, force constants, and symmetries. We conclude that, on Raney nickel above 150 K, hydrogen chemisorbs predominantly in sites of threefold symmetry. The ’’best fit’’ corresponds to a Ni–H bond distance of 1.88±0.03 A and a force constant of 0.58±0.03 mdyn/A. In addition, we have observed lateral adsorbate interactions, deuterium isotope shifts, and temperature dependent binding sites.

Activity of tungsten as a methanation catalyst

The catalytic activity of tungsten has been investigated for the methane synthesis from H/sub 2/ and CO. Starting with atomically clean tungsten, the activity for catalytic methanation rivals that of Ni, with tungsten being active at lower temperatures than Ni. These observations of high catalytic activity for tungsten suggest that the low activity seen in previous studies on tungsten catalysts may have been due to inadequate cleaning procedures.

Vibrational spectroscopy of adsorbed species on nickel by neutron inelastic scattering

Abstract In a test of the utility of neutron inelastic spectroscopy (NIS) for studying molecular vibrations in surface reactions, we have examined the adsorption, co-adsorption, and reaction of hydrogen and carbon monoxide on Raney nickel catalysts.

Interaction of vibrating H atoms on the surface of platinum particles by isotope dilution neutron spectroscopy

The first results of a study of the vibrational spectrum of hydrogen, chemisorbed on the surface of platinum by isotope dilution neutron spectroscopy are presented. (AIP)

Surface and bulk analysis of a deactivated Raney nickel methanation catalyst

In a joint PETC-NBS experiment, a Raney nickel methanation catalyst in a hot gas recycle (HGR) bench-scale reactor (used for catalyst lifetime testing) has been examined with a wide range of modern analytical techniques sensitive to both surface and bulk chemical properties of the catalyst. The reactor was designed to use a catalyst essentially identical to that used in previous lifetime testing and to allow both the sampling at various positions along the catalyst bed and the introduction of the samples into the various analytical instruments under inert atmospheric conditions (i.e., without exposure to oxygen, water, etc.). The purpose of this work was to explore the reasons for the premature failure of the catalyst in pilot plant lifetime tests. The results indicate that in spite of significant catalyst deactivation only low levels of carbon (a small fraction of a surface monolayer) are formed on the catalyst surface. The primary cause of catalyst deactivation in this lifetime test was determined to be the growth of the nickel crystallites and subsequent decrease in active catalyst surface area.

Adsorption and decomposition of hydrocarbons on platinum black: Vibrational modes from NIS

The adsorption and decomposition of acetylene and ethylene on platinum black have been investigated by neutron inelastic scattering (NIS) studies of the vibrations of chemisorbed species in the energy range 30–200 meV. Results are compared in detail with electron energy loss spectroscopy (EELS) data and with spectra of model inorganic complexes. The NIS spectra for C2H2 and C2H4 chemisorbed at 120–150 K exhibit a number of spectral features (from both the internal modes and modes associated with vibrations against the surface metal atoms) which are in general agreement with EELS results for Pt(111). Detailed comparison of the spectral peak intensities and positions with various models for the adsorbed molecules rule out linear (C2H2) or planar (C2H4) species and are consistent with bent molecular configurations on the surface. Bond angles and force constants are derived from the model fits to the neutron data in each case. The NIS spectra of a ‘‘saturated’’C2H2 monolayer warmed to 300 K indicates a rearra...

On the mechanism of fischer-tropsch synthesis on a single crystal nickel catalyst

The ability to determine under reaction conditions the coverage of active surface carbon (from 10 to 50% of a monolayer, the highest possible coverage without causing deactivation of the catalyst surface) and to relate these measurements to products yields (spanning a range of 3 orders of magnitude) leads to the conclusion that adsorbed molecular CO is directly involved in higher hydrocarbon synthesis. Two sets of experiments were conducted, one at low carbide coverage and the other at high carbide coverage. At both of these concentrations, the rate of methane production is independent of the partial pressure of CO (P/sub CO/) while the rate of higher hydrocarbon production depends strongly on P/sub CO/. The present data show that under steady-state conditions the higher hydrocarbon production rate is dominated by a CO insertion mechanism. 9 references, 2 figures, 1 table.

Kinetics of carbon monoxide hydrogenation on a polycrystalline nickel ribbon catalyst

Abstract Ultrahigh vacuum procedures have been employed to highly purify reactant gases and to prepare a clean Ni ribbon for use as a methanation catalyst in a static reactor. The kinetics of CO-hydrogenation to produce CH4 have been studied over a Ni catalyst temperature range of 600–740 K using 4 H2 : 1 CO at a total pressure of 120 Torr. Under thse conditions, the reaction was found to be highly selective (less than 1% higher hydrocarbons produced). Small amounts of CO2 were also produced continously during methanation. The initial rate of CH4 production was used to estimate the activation energy (66 kJ mole-1 in this temperature range. Satisfactory agreement was observed between our rate measurements (turnover numbers) made on a 10 cm2 Ni ribbon and other kinetics studies made on supported Ni catalysts of much larger surface area. A comparison of data from different laboratories suggests that the methanation activation energy falls as the Ni temperature is raised, and that raising the reaction temperature also produces enhanced selectivity towards CH4 production.

Inelastic neutron scattering studies of the interaction of hydrogen with palladium black

Abstract Incoherent inelastic neutron scattering has been used to investigate the interaction of hydrogen with palladium black (a high-surface-area Pd powder). Vibrational spectra of chemisorbed hydrogen have been measured in the energy range 30–230 meV at 80 and 295 K. The dominant surface adsorption site of hydrogen has been identified by comparison of the vibrational frequencies and intensities provided by the scattering data with normal coordinate calculations for model structures of different geometries, force constants and bond lengths. The “best fit” to the data is obtained for surface adsorption sites of threefold symmetry. A vibrational feature observed at 58 meV, which is inconsistent with either surface or bulk Pdue5f8H species, is assigned to hydrogen occupying subsurface sites of bulk-like octahedral symmetry.