Frank L. Williams

Simultaneous measurement of CO oxidation rate and surface coverage on PtAl2O3 using infrared spectroscopy: Rate hysteresis and CO island formation

Abstract The catalytic oxidation of CO on a commercial Pt Al 2 O 3 catalyst has been studied at a total pressure of one atmosphere using Fourier transform infrared spectroscopy. Simultaneous measurements of steady-state reaction rates and CO surface coverages were obtained at temperatures between 460 and 520 K. During oxidation, the frequency of the infrared absorption peak of linearbonded CO is invariant at 2081 cm −1 at 520 K over nearly a 100-fold change in CO surface coverage. This peak position is within 1 cm −1 of that found for saturation coverage in the presence of pure CO at one atmosphere. In contrast, under nonreactive desorption, large shifts in frequency are found as the CO surface coverage changes. The ratio of the intensities of the absorption bands due to both linear and bridge-bonded CO was constant during oxidation. These observations are interpreted as resulting from the formation of islands of CO during the oxidation reaction. Hysteresis in both CO reaction probability and CO surface coverage are found which are inversely related. Thus CO blocks active sites on the Pt for the reaction. The width and shape of the hysteresis loops as a function of temperature are qualitatively understood in terms of the rate of CO desorption from the Pt surface. At very high surface coverages, changes in the reaction probability are not accompanied by changes in adsorbed CO. This may be due either to the formation of surface species, unobserved in ir, which block the reaction or to the presence of a small number of very reactive Pt sites which are blocked with only a very small change in CO surface coverage.

Diffusional Effects in the Recovery of Methane From Coalbeds

This paper presents evidence that the unipore model is inadequate for describing diffusional fluxes from coal over the entire timescale of desorption. Field desorption of methane from coal pieces shows pronounced curvature, which is attributable to the bidisperse pore structure generally found for coal. A model is presented that accounts for the bidisperse pore structure of coal and predicts diffusion rates for both field and laboratory desorptions. Transient diffusivities are determined for Pittsburgh bituminous and Madrid (NM) anthracite coals by using a pulse tracer technique coupled with Fourier analysis of the elution curves. This technique allows the rapid determination of the diffusion parameters with minimal experimental effort. In addition, steady-state diffusivities are determined to identify mechanisms of diffusion. Diffusion was found to be a combination of bulk, Knudsen, and surface diffusions, depending on the coal pore structure and gas pressure.

Diffusion models for gas production from coals: Application to methane content determination

Abstract Experimental studies of methane diffusion from coal cuttings have been analysed with both a unipore diffusion model and a bi-disperse pore structure diffusion model to determine the time range of each models validity. Initial desorption, during the first 50% of volume desorbed, can be adequately described by the unipore model. For Fruitland coals of the San Juan Basin, the unipore diffusion coefficient varied by less than a factor of 2 from a value of 1 × 10−5s−1 despite large variations in sample depth and location. The unipore diffusion model has been successfully incorporated into a lost-gas-correction-factor determination as part of a new technique to delineate coal-bed methane. To describe methane diffusion over the full timescale of desorption, a more complex model which more accurately represents the observed bi-disperse pore structure of coal is required. Using that model the observed desorption rate is described over all the desorption. This model is especially applicable as a source term in describing overall well production.

Diffusion models for gas production from coal: Determination of diffusion parameters

Abstract A method for the laboratory measurement of transient diffusivity parameters for methane in coal has been developed. Previously described Chromatographic techniques for measurement of single diffusivity parameters have been modified for the bi-disperse pore diffusion model. This model, which is necessary to describe methane diffusion and desorption in coal for both short and long times, requires the determination of three parameters to characterize a particular coal sample. The experimental technique uses a pulse of methane gas which is introduced into a helium stream flowing through a long tube containing the coal sample. The three desired parameters are determined by matching the experimental and theoretical elution curves. Further refinement of the technique accounts for tracer pulse duration thus allowing operation over a greater flow rate and sample size range. This technique has been applied to anthracite, bituminous and subbituminous coals. For all three coal types, the macropore diffusivity (≈10−5 s) was about two orders of magnitude higher than the micropore diffusivity. The anthracite adsorbs more methane in the micropores than the other coals. These results showed the expected trends taking the known change of pore structure with rank into account.

Surface composition of Pt/Rh alloys

Abstract Ion scattering spectroscopy has been used to determine the surface composition of Pt/Rh alloys as a function of temperature and bulk Pt/Rh atomic ratio. Three alloys, with bulk compositions of 90 at.% Pt, 50 at.% Pt and 10 at.% Pt were studied. The surfaces of the three alloys are enriched in platinum at temperatures between 800 K and 1400 K. At 1300 K the surfaces of the three alloys contain 97 at.% Pt, 77 at.% Pt, and 27 at.% Pt respectively. Segregation of Rh to these surfaces was expected from bond-breaking theories of surface segregation and no satisfactory explanation for the disparity has been found. The segregation of Pt to the surface shows a maximum near 1050 K and decreases to zero (bulk composition) at temperatures below 800 K. This behavior is attributed to a competition between non-preferential sputtering by the analysis beam and thermal diffusion. Sputter removal of quenched surface layers indicated that surface enrichment only occurs in the outer atomic layer.

Specific surface area studies of shock-modified inorganic powders

Modification of inorganic powders with high-pressure shock-wave loading is of interest for shock-activated sintering, material synthesis, shock-enhanced catalytic activity, dynamic compaction, and shock-enhanced solid-state reactivity. The specific surface area of shock-modified powders is a direct quantitative measure of powder morphology changes, yet few studies have been carried out on powders subjected to controlled shock-loading conditions. In the present work aluminium oxide, zinc oxide, aluminium nitride, titanium carbide and titanium diboride powder compacts were subjected to controlled shock-loading to peak pressures of from 4 to 27 GPa at various starting densities, and characterized with specific surface area measurements by the BET (gas adsorption) method. Low-temperature cyclical thermal pretreatment and outgassing pretreatment of the shock powders at 250° C were employed; the former improves the reliability of the BET measurements, and makes the surfaces of the shock-modified powders more chemically active than those of the starting powders. Each powder shows a somewhat different response to shock-loading, ranging from a decrease in specific surface by a factor of six for zinc oxide to a 200% increase for titanium diboride. Shock-induced changes in specific surface show four characteristic behaviours as shock pressure is increased. Well-understood and controllable shock-loading conditions are found to be essential to shock-modification studies. An update on earlier measurements on rutile, zirconia and silicon nitride is also reported.

Stability of carbon islands on 90% Rh-10% Pt (111) surfaces

Abstract Islands of carbon were created on a 90% Rh-10% Pt(111) surface by electron beam cracking of CO. The islands were one monolayer thick and either circular (approximately 60 μm) in diameter or rectangle (as a result of scanning the electron beam). The spots were stable up to a temperature of 980 K. AtT⩾990K they disappeared in a few minutes (9 min at 1000 K, 3 min at 1030 K) by dissolution of carbon into the bulk of the crystal. By analogy to other carbon-metal systems, it was concluded that 990 K is a surface phase transition temperature. Direct observation of the islands and the surrounding clean crystal surface with scanning Auger microscopy did not show carbon diffusing out across the surface from the islands.

Direct method of determining the methane content of coal ― A modification

Abstract It is shown that the usually applied method for determining the methane content of coal, while quite satisfactory when used at small surface time ratios, significantly underestimates the quantity of gas lost at large surface time ratios.

Catalytic activity of shock‐loaded TiO2 powder

Catalytic activity measurements have been carried out on a pure TiO2 powder which was explosively loaded and preserved for post‐shock examination. The oxidation of CO in a flow reactor was used to show an enhancement in catalytic activity of over two orders of magnitude at 720K. The catalytic activity does not correlate with the concentration of two paramagnetic defects as indicated by ESR measurements.

Applications of Fourier transform infrared spectroscopy to in situ catalyst surface studies

Fourier transform infrared spectroscopy (FTIR) is a powerful tool for studying surface species present on supported catalysts during normal reaction conditions of high temperature and/or pressure. We have used a special high-throughput optical bench to optimize the sensitivity of the system for high optical density catalyst samples. The infrared cell is designed to operate at high vacuum for adsorption studies or as a recycle-flow reactor. Improvements in spectral subtractions have been obtained by the precise repositioning of samples in the IR beam. The enhanced performance resulting from these modifications has been instrumental in improving the sensitivity for infrared surface studies of the reactions C6H6 + 3H2 ⇔ C6H12 and 2C0 + 02 → 2CO2 over a Pt/A1203 catalyst. The bonding and geometry of benzene chemisorbed on the Pt surface have been determined from the infrared spectra. Simultaneous reaction rate and surface coverage measurements have also been made during CO oxidation on Pt/A1203. Hysteresis effects in both reaction rate and CO surface coverage are observed. The IR spectra indicate that islands of CO form on the Pt surface during reaction.