Eduardo E. Wolf

Methane utilization by oxidative coupling. I: A study of reactions in the gas phase during the cofeeding of methane and oxygen

Abstract The oxidative coupling of methane has been investigated in the absence of catalysts by cofeeding methane and oxygen. A survey on the effects of varying several operating conditions shows that under certain conditions significant gas-phase oxidative coupling can occur in the absence of catalysts. The reaction products were C 2 H 6 , C 2 H 4 , C 3 H 8 , C 3 H 6 , CO (the main oxidation product), CO 2 , H 2 , H 2 O, and traces of HCHO and C 4 and C 5 hydrocarbons when conversions were high. The general trend dictated by the gas-phase kinetics is that the hydrocarbon product selectivity falls as the conversion increases. Specifically, at methane conversions of 2%, the hydrocarbon product selectivity was around 65%, but when methane conversions were increased to 32%, the hydrocarbon selectivity decreased to 29%. The apparent activation energy for methane conversion in the gas phase was found to be around 55 kcal/mole which is similar to that reported for the gas-phase reaction of methane and molecular oxygen forming methyl and hydroperoxy radicals. Several gas-phase results have been presented for comparative purposes with catalytic oxidative coupling studies. A reaction pathway for the gas-phase oxidative coupling network has been considered and compared to proposed catalytic reaction pathways. The implications that the observed results might have on catalytic oxidative coupling of methane have also been considered.

Crystallite size effects during the catalytic oxidation of propylene on Ptγ-Al2O3

Abstract This work reports results on the kinetics and crystallite size effects during the catalytic oxidation of propylene on Pt γ- Al 2 O 3 . The experiments were carried out in a reactor-chemisorption apparatus that can be used to study the reaction kinetics and in situ characterization of the catalyst by measuring the chemisorption of H 2 . It was found that reaction rate decreased during reaction and that oxygen pretreatment restored the catalyst activity. Kinetic studies were carried out using catalysts with 1% Pt and average crystallite sizes of 11, 60, and 144 A. The kinetic results revealed a complex dependence between reaction rate and C 3 H 6 concentration. At low propylene concentration, the reaction rate increased with concentration until it reached a maximum. A further increase in reactant concentration produced a decrease in reaction rate, and at high reactant concentration the reaction rate became zero order. The reaction rates per unit mass of catalyst were quantitatively similar for catalysts with different crystallite sizes; consequently, the reaction rates per unit area of catalyst (specific reaction rates) were different since there was a tenfold difference in average crystallite size. The results indicated that the reaction exhibits demanding behavior and that the specific reaction rate increases significantly with crystallite size. This finding indicates that larger crystallites catalyze the oxidation of propylene more effectively which has also been observed in the case of CO oxidation on Pt α- Al 2 O 3 .

Fourier transform infrared spectroscopy studies of surface reaction dynamics. II: Surface coverage and inhomogeneous temperature patterns of self-sustained oscillations during CO oxidation on Pt/SiO2

Abstract Transient methods of Temperature-Programmed Reaction (TPR) and Concentration-Programmed Reaction (CPR) have been used to uncover regions of self-sustained oscillations of CO oxidation on a 5 wt% Pt SiO 2 catalyst. The surface dynamics of the oscillations were studied using Fourier Transform Infrared (FTIR) spectroscopy and sensitive surface-temperature measurements. Surface-temperature excursions up to 150 °C are recorded while adsorbed CO oscillates by as much as 60% of total coverage. The oscillations are shown to occur only within the high-to-low steady-state transition region which shifts from the stoichiometric point toward the lean CO region as the temperature is lowered. Multiple surface-temperature measurements reveal that extreme spatial nonuniformity and propagating waves of high reaction fronts are common in the region of CO inhibition and during oscillations. It is shown that the localized hot spots occur in regions which sustain higher reaction rates while CO inhibition prevails in other regions of the catalyst. Oscillations in CO2 production appear to be the result of fluctuating zones of high and low reaction rates, and therefore represent averages over spatially inhomogeneous surface states, rather than over spatially uniform states as has been customarily assumed.

FTIR studies of potassium catalyst-treated gasified coal chars and carbons

Abstract The catalytic effect of K 2 CO 3 , KHCO 3 , KOH, K 2 SO 4 and KCl during steam gasification of coal and carbon chars has been evaluated by gravimetric measurements at 750 °C. The order of catalytic activity found was KOH ~ K 2 CO 3 > KHCO 3 > KNO 3 > K 2 SO 4 with almost no activity for KCl. Transmission i.r. spectra were obtained on thin wafers of KBr mixed with carbon and coal char samples after impregnation, devolatilization and partial gasification. All catalytically activated salts presented two characteristic bands at 1050 and 1400 cm −1 while no such bands appeared in the case of catalytically inactive KCl. Furthermore, such bands were also present in a different carbon substrate impregnated with K 2 CO 3 and in an oxygen-exposed intercalate (C 8 K). The bands appeared at the same frequencies as those of KHCO 3 in pure, carbon impregnated, devolatilized, and partially gasified form.

FTIR studies of surface reaction dynamics I. Temperature and concentration programming during CO oxidation on Pt/SiO2

Abstract A transient technique integrating FTIR spectroscopy with Temperature-Programmed Reaction (TPR) and Concentration-Programmed Reaction (CPR) using a small-volume infrared reactor, has been developed. Studies of CO oxidation on a Pt SiO 2 catalyst using these methods, have shown the occurrence of remarkable surface temperature behavior in regions of ignition and quenching. Extensive documentation of linear-bonded CO behavior is presented and the relevance of a Langmuir-Hinshelwood and other reaction models is discussed. Two different energy transport mechanisms are described which might lead to the observed catalyst temperature excursions.

Spurious limit cycles and related phenomena during CO oxidation on supported platinum

A study of CO oxidation on Ptα-Al2O3 catalyst was carried out in the temperature range 370 to 420 °K, in oxygen. Attention was devoted specifically to isothermal limit-cycle phenomena in this reaction. Spurious limit-cycle phenomena resulting from trace impurities in the oxygen stream were observed. Some characteristics of these, and the conditions under which they occur were elucidated. Further, changes in catalytic behavior proceeding from modifying treatments in oxidizing and reducing environments were noted and related to similar phenomena reported by other authors.

An integrated study of Pt/WO3/SiO2 catalysts for the NOCO reaction: I. Catalyst characterization by XRD, chemisorption, and XPS

Abstract Pt/WO 3 /SiO 2 catalysts were characterized by X-ray diffraction (XRD), selective CO chemisorption, and X-ray photoelectron spectroscopy (XPS). It was found that the presence of WO 3 decreased final Pt crystallite size, in part by reducing the formation of bulk PtCl 2 precursor. However, CO chemisorption was suppressed; this indicated strong metal-promoter interactions. Changes in Pt electron binding energy suggest that charge transfer may also take place. A model of the surface consisting of Pt crystallites decorated by partially reduced WO x is proposed. This morphological model was supported by XRD and XPS results which revealed a surface-localized phase of partially reduced WO x (likely WO 2 ). Decoration was also indicated in the trend of XPS Pt Si ratios.

Selected area fourier transform infrared studies of surface reaction dynamics III. Spatial coverage and temperature patterns during self-sustained oscillations of CO oxidation on Pd/SiO2

Abstract Surface dynamics of self-sustained oscillations during CO oxidation on a 2 wt% Pd SiO 2 catalyst have been studied using Selected Area Fourier Transform Infrared (FTIR) spectroscopy along with temperature measurements from an array of four sensitive surface thermocouples. These experiments demonstrate that self-sustained oscillations involve spatially propagating regions of nonuniform coverage and surface temperature. The results elucidate how the multipeak structure of the oscillations arises from inhomogeneous patterns in surface reaction rate controlled by localized CO inhibition.

Infrared thermography and ftir studies of catalyst preparation effects on surface reaction dynamics during CO and ethylene oxidation on Rh/SiO2 catalysts

Abstract The use of infrared imaging has shown the spatial nature of non-uniform behavior on Rh/SiO 2 catalysts that could previously only be inferred through isolated thermocouple measurements at various points on the catalyst wafer. Catalysts prepared by different methods show different dynamic behavior.

A scanning tunneling microscopy study of the platinum catalysts particles supported on graphite

Scanning tunneling microscopy has been used to image platinum catalyst precursors and reduced platinum catalyst crystallites supported on graphite substrate at ambient condition. The catalysts were prepared by impregnation and ion exchange of precursor salt solutions unto the graphite substrate. Both nonfunctionalized and nitric acid functionalized nuclear grade high purity graphites were used as substrate. The crystallites’ size and distribution, microstructure, morphology, and surface structures are dependent on the substrate functionalization, the choice of precursors and the preparation method. Surface atomic structures of the crystallites deviate from that of single crystal studies but closely approximate the structure of O2 induced surface reconstruction.