Mark L. Unland

Isocyanate intermediates in the reaction of NO and CO over noble metal catalysts

A comparative study of the infrared spectra of reaction intermediates and chemisorbed species during the reaction of NO + CO over Pt, Pd, Rh, Ir, and Ru-on-alumina catalysts at elevated temperatures has been made. A strong band which is assigned to a surface isocyanate species has been found in the 2260–2270 cm−1 region on all the noble metals except Ru where the band is either quite weak or entirely absent. The weakness of the band on RuAl2O3 seems to correlate with its unique low ammonia-forming properties during NOx removal from automobile exhaust. Hydrolysis of the surface isocyanate species provides a pathway to NH3 which does not involve reduction of NO directly with hydrogen already present in the exhaust stream or generated via a water-gas shift mechanism. No assessment of the relative importance of this mechanism with respect to other mechanisms for ammonia formation in actual automobile exhaust can be made from the present results. Sulfur poisoning of the Rh and Ru catalysts is observed to inhibit the formation of the isocyanate species. However, this poisoning is temporary and the -NCO sites can be regenerated by oxidation treatment.

Isocyanate Intermediates in Ammonia Formation over Noble Metal Catalysts for Automobile Exhaust Reactions

Isocyanate species have been detected on the surface of noble metal catalysts during the reactions of carbon monoxide with nitric oxide. The intensity of the surface isocyanate infrared band correlates with the known ammonia-forming tendencies among the noble metals. The discovery of this isocyanate species suggests a new mechanistic pathway to the ammonia formed during catalytic reduction of nitrogen oxides in automobile exhaust.

Laser Raman study of benzene adsorption on alkali metal X and Y zeolites

Abstract Shifts in the laser Raman spectrum of the ring breathing mode of adsorbed benzene are used to probe the electrostatic fields within the supercage of alkali metal-exchanged X and Y zeolites. It is concluded that the excess cations in X versus Y zeolites result in higher fields at the aromatic nucleus of initially adsorbed benzene. Further increases in field are caused by crowding in the supercage as the size of the cation increases. Increased benzene loading causes the frequency of the Raman band to approach that of the liquid state. These trends are related to selectivity sequences observed when these materials are used as catalysts for the alkylation of toluene with methanol to give styrene and ethylbenzene.

Ketene in a Gaussian Basis: LCAO–MO–SCF Wavefunction, One‐Electron Properties, and Electron‐Density Maps

An SCF calculation of the wavefunction for ketene is presented. The basis functions are a (73/73/3) uncontracted set of Gaussian 1s and 2p orbitals. Energy components, population analysis, one‐electron properties, and density maps are presented and discussed. A main conclusion is that, opposite to formaldehyde, the total contribution of π orbitals to the radial extent of the charge density in the out‐of‐plane direction in ketene is greater than the total contribution of the σ orbitals. Both second moments of charge and density maps support this conclusion.

Characterization of Ground‐State Wavefunctions by Measured Electronic Properties. III. A Gaussian Basis Self‐Consistent‐Field Calculation for Nitrogen Trifluoride

An LCAO MO SCF calculation has been carried out for nitrogen trifluoride using an uncontracted (73 / 73) Gaussian basis set. Energy parameters, dipole moment, field gradient, molecular quadrupole moment, second moments of charge, and population analysis are reported. The calculation is used to evaluate earlier wavefunctions obtained by more empirical methods. Combining experimental quadrupole‐coupling‐constant data with the calculated field gradient gives a nitrogen nuclear quadrupole moment of + 0.018 ± 0.003 × 10−24 cm2 which is in good agreement with earlier, less precise, estimates of this parameter.

Characterization of Ground‐State Wavefunctions by Measured Electronic Properties. I. 119Sn Mössbauer Isomer Shifts

A particular type of LCAO MO wavefunction is applied to the problem of correlating experimental 119Sn Mossbauer isomer shifts with calculated electron densities at the nucleus. The results are found to be sensitive to the choice of basis functions, molecular geometry, and method of setting basis orbital exponents. Because of this sensitivity, only a rough correlation of the isomer‐shft data is obtained when conventional methods of setting orbital exponents are used. A set of wavefunction parameters which result in good correlation of the 119Sn data is found and, for each molecule, a quantitative measure of the sensitivity of the calculated electron densities to each parameter is obtained.

Measurement and Analysis of Mössbauer Data for Compounds Containing 125Te

Mossbauer isomer shifts and quadrupole splittings are reported for a number of tellurium compounds. The isomer‐shift data are analyzed by setting up localized orbitals and converting to their equivalent delocalized orbitals in order to calculate the electron density at the 125Te nucleus. Similarities between data for tellurium and tin compounds are noted. The theoretical calculations confirm the expectation that hydrogenic orbitals are more appropriate for calculations of electron density at a nucleus than the Slater‐type orbitals generally used in molecular‐orbital calculations.

Investigation of the SO2 oxidation rate limiting factors for K/V and Cs/V catalysts at low temperatures

Different potassium and cesium vanadyl sulfate catalysts were studied to determine why cesium produces a more active catalyst at low temperatures than potassium. Thermal analyses and electron paramagnetic resonance instruments were used to study catalysts in various SO2SO3O2N2 atmospheres at normal operating temperatures. Catalysts supported on controlled pore glass silica were analyzed. The results indicate that KV catalysts form large amounts of inactive V4+ crystals at temperatures under 425 °C while the CsV catalysts form much smaller amounts at temperatures below 400 °C. The reaction rates at temperatures below 425 °C drop off because a K2SO4V2O4SO3 compound crystallizes out of the active molten salt in the KV catalyst. The CsV catalyst activity is believed to drop off below 400 °C because increased V2O4SO3 concentrations reduce the availability of V2O5 for SO2 oxidation according to the six-step Boreskov reaction mechanism.

Characterization of Ground‐State Wavefunctions by Measured Electronic Properties. II. Dipole Moment and Field Gradient of Nitrogen Trifluoride

A particular localized‐orbital type of LCAO MO wavefunction is used to calculate the dipole moment and field gradient at the nitrogen atom in NF3. Because the wavefunction is not obtained by the usual self‐consistent‐field procedure, the results for the various observables depend on the choice of calculation parameters such as the polarity of the σ bonds, the amount of π bonding, and the hybridization of the lone‐pair orbitals on the fluorine atoms. This dependence is studied in detail and it is concluded that with reasonable choices of input parameters, good agreement with experiment can be obtained. Thus, the localized orbitals specified by setting the σ‐bond coefficient equal to 0.875, the π‐bond coefficient equal to 0.0, and the fluorine lone‐pair orbital angle equal to 112.7°, yield a dipole moment of 0.236 D and a total field gradient at the N atom of − 2.007 a.u. The experimental dipole moment is 0.234 ± 0.004 D and the experimental field gradient is − 1.505 to − 3.010 a.u. depending on the exact v...

The electronic structure of difluoromethane: ab initio studies in several basis sets and semiempirical calculations

LCAO–MO–SCF calculations have been carried out on the difluoromethane molecule, CH2F2, by use of the following basis sets: (31/31/1), (31/31/2), (52/52/2), (52/52/3), (52/73/3), (73/52/3), and (73/73/3), with these orbital listings being in the order of (C/F/H)[the notation (ab/cd/e) corresponds to the assignment of a 1s and b 2p atom-optimized Gaussian-type orbital-exponents to the carbon, c 1s and d 2p to each of the identical fluorine atoms, and e 1s to each of the identical hydrogens]. In addition, CNDO, INDO, and extended Huckel calculations have also been done for this molecule and the energies have been compared to a near-Hartree–Fock Gaussian calculation. Total energies, orbital energies, correlation-corrected binding energies, electron-population analyses, and various physical properties calculated from these wave-functions are compared for the various basis sets. Three-dimensional, electron-density, cross-sectional plots are presented.