M. A Vannice - Academia.edu (original) (raw)
Papers by M. A Vannice
Preprints-American Chemical Society Division of Petroleum Chemistry, 1997
The strong thermodynamic potential for carbon formation during CH 4 -CO 2 reforming at low temper... more The strong thermodynamic potential for carbon formation during CH 4 -CO 2 reforming at low temperatures and equimolar feed ratios necessitates use of a catalyst which inhibits carbon deposition.
Journal of Catalysis, 1976
The catalytic behavior of unsupported nickel and nickel dispersed on a variety of supports has be... more The catalytic behavior of unsupported nickel and nickel dispersed on a variety of supports has been studied in the CO/H/sub 2/ synthesis reaction. The catalysts weree characterized by chemisorption and X-ray diffraction measurements. Specific activities and product distributions were determined and kinetic parameters for the methanation reaction were obtained. The specific activity is sensitive to the support material but the variation in activity is less than an order of magnitude. There is also evidence that maximum activity may occur over a certain nickel particle size range. The kinetic parameters are relatively insensitive to the support; however, dispersing nickel on any of the supports studied enhanced the formation of higher molecular weight hydrocarbons compared to unsupported nickel. These differences in catalytic behavior are attributed to changes in the adsorbed state of CO on nickel surfaces which are a result of metal-support interactions or variations in nickel crysta...
Preprints-American Chemical Society Division of Petroleum Chemistry, 1995
A microwave absorption technique based on cavity perturbation theory is applicable for electrical... more A microwave absorption technique based on cavity perturbation theory is applicable for electrical conductivity measurements of both small, single-crystal particles and finely divided powder samples when {sigma} values fall in either the low ({sigma}<0.1{Omega}{sup -1}cm{sup -1}) or the intermediate (0.1 <{sigma}<100{Omega}{sup -1}cm{sup -l}) conductivity region. If the skin depth of the material becomes significantly smaller than the sample dimension parallel to the E-field, an appreciable error can be introduced into the calculated conductivity values; however, this discrepancy is eliminated by correcting for the field attenuation associated with the penetration depth of the microwaves and accurate absolute values can be obtained. When combined with microwave Hall effect measurements of mobility, {mu}, carrier densities can be calculated, for electrons N{sub o}={sigma}/{rho}e{mu} where e is the electron charge and {sigma} is the density of the solid. This approach eliminat...
Etude des proprietes d'adsorption et catalytiques de melanges physiques de 7,0% Ni/TiO 2 et 0... more Etude des proprietes d'adsorption et catalytiques de melanges physiques de 7,0% Ni/TiO 2 et 0,6Z Pt/TiO 2 dans l'hydrogenation de l'oxyde de carbone
Catalyst Characterization Science, 1985
Catalysis, 1982
Interest in carbon monoxide has increased dramatically during the past decade. This is due primar... more Interest in carbon monoxide has increased dramatically during the past decade. This is due primarily to a new interest in energy resources other than natural gas and petroleum resources which include coal, oil shale, and heavy residua. In any process which involves gasification to convert these hydrogen-deficient materials to hydrocarbons or other organic compounds, CO is one of the principal products of the gasification step, and its subsequent hydrogenation to form the required final products is of extreme importance. It is thermodynamically possible to produce methane as SNG, hydrocarbon liquids as fuels, and alcohols and olefins as chemical intermediates — the major problem is the selectiveproduction of the desired product. During the overall conversion process, the reaction between CO and water is important, viathe water gas shift reaction, not only because it is used to adjust the H2/CO ratio in the gas stream between the gasifier and the reactor, but also because H2O is a by-product of the CO hydrogenation reaction and can react with unconsumed CO in the syngas reactor to form CO2.
The chemistry between NOx species adsorbed on La2O3 and CH4 was probed by temperature‐programmed ... more The chemistry between NOx species adsorbed on La2O3 and CH4 was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH4 in the presence of O2, NO3- does not appear to activate CH4, thus either an adsorbed O species or an NO2- species is more likely to activate CH4. In the absence of O2, a different reaction pathway occurs and NO- or (N2O2)2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH4 unidentate NO3-, which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO2 or H2O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N2 and N2O from NO was observed at much lower temperature during the TPR experiments.
Surface Science, 1994
Abstract The adsorption, desorption and decomposition of acetone on a Pt foil have been studied b... more Abstract The adsorption, desorption and decomposition of acetone on a Pt foil have been studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Two surface adsorption geometries occur for acetone on the foil: η 1 (O)-(CH 3 ) 2 CO (bonding end-on via the lone pair electrons on the oxygen) and η 2 (O,C)-(CH 3 ) 2 CO (bonding side-on via either π-bonding or di-σ bonding after rehybridization). For the η 1 species, reversible adsorption and desorption are dominant and are similar in behavior on a Pt(111) surface, while conversion of η 1 species to η 2 species may also occur. For the η 2 species, the decomposition channel is competitive with desorption, and the desorbed decomposition products are hydrogen and carbon monoxide. This process is associated with surface defects such as step sites, and preadsorbed hydrogen inhibits the decomposition channel thus providing additional support for this conclusion.
Journal of Catalysis, 1998
A combination of CO adsorption at 300 K, which provides irreversible adsorption on Cu +1 sites, a... more A combination of CO adsorption at 300 K, which provides irreversible adsorption on Cu +1 sites, and oxygen adsorption via N 2 O dissociation at 363 K, which counts surface Cu 0 atoms, was used to obtain values for the dispersion of Cu on SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , diamond powder, and graphitized carbon fibers. The crystallite sizes obtained from these estimates were compared to those obtained from TEM and XRD measurements and were found to be in very good agreement. DRIFT spectra of CO adsorbed on these catalysts were obtained at subambient temperatures as well as at 300 K, and the various amounts of Cu 0 , Cu +1 , and Cu +2 at the surface were detected by respective peaks in the regions of 2110 cm −1 and lower, 2110-2140 cm −1 , and 2145 cm −1 and above. Spectra obtained at 173 K were especially useful because irreversible CO adsorption occurred on all three sites, thus allowing the removal of gas-phase CO and clarifying the spectra of adsorbed CO. Heats of adsorption for CO were determined from the variation in the intensity of the Kubelka-Munk function versus temperature, and the following average values were obtained for Cu dispersed on SiO 2 , Al 2 O 3 , and diamond: Cu 0-4.7 kcal/mole; Cu +1-11.7 kcal/mole; and Cu +2-5.3 kcal/mole.
Journal of Catalysis, 1997
A pretreatment involving a 4-h reduction under H 2 at 573 K, compared to either 473 or 673 K, gav... more A pretreatment involving a 4-h reduction under H 2 at 573 K, compared to either 473 or 673 K, gave the highest specific activity for furfural and crotonaldehyde hydrogenation over an unpromoted copper chromite catalyst. Both H 2 and CO chemisorption declined monotonically with increasing reduction temperature, but turnover frequencies also showed a maximum after a pretreatment at 573 K. DRIFTS identified a band, associated with CO adsorbed on Cu +1 sites, which reached a maximum intensity after a 573 K pretreatment, while XRD patterns also provided evidence for a CuCrO 2 phase and showed a continuous increase in Cu 0 as temperature increased. Selectivity to furfuryl alcohol ranged from 35-80% and reaction orders on furfural and H 2 were near unity. In contrast, crotonaldehyde hydrogenation gave primarily butyraldehyde with only around 3% crotyl alcohol and a near zero-order dependence on crotonaldehyde, although the H 2 dependence was still near first order. IR spectra under reaction conditions revealed adsorbed crotonaldehyde, provided evidence for an unsaturated alkoxide intermediate and detected no butyraldehyde on the surface. A simple Langmuir-Hinshelwood sequence nicely explained both reactions, it was consistent with the DRIFTS results obtained during crotonaldehyde hydrogenation, and the resultant rate expression gave meaningful entropies and enthalpies of adsorption for H 2 and crotonaldehyde.
Journal of Catalysis, 1981
Nickel crystallite sizes calculated from H,, CO, and OZ chemisorption are compared to those deter... more Nickel crystallite sizes calculated from H,, CO, and OZ chemisorption are compared to those determined from X-ray diffraction (XRD) and transmission electron microscopy (TEM). Good agreement is obtained among all techniques for typical Ni/SiO% and Ni/Al,OI catalysts, whereas sizes calculated from H, and CO chemisorption are far too large for Ni/TiO, catalysts. In contrast, oxygen chemisorption provides values in excellent agreement with those from XRD and TEM, and indicates that monolayer coverages of CO and HP are much lower on TiO,-supported nickel. This agreement also implies that oxygen monolayer coverage on TiO,-supported nickel does not exceed unity regardless of crystallite size, whereas coverages of two or higher are observed on SiO,-and A&O,-supported nickel, in agreement with previous studies. Isobars for CO and H, adsorption on Ni/TiO, clearly show that activated adsorption is not responsible for the low CO and H2 uptakes at 300 K. Low-contrast nickel crystallites on titania are observed in the TEM micrographs, indicating that nickel may exist with a raft-like morphology on this support. These results provide evidence that strong metal-support interaction behavior exists in the Ni/TiO* system.
Journal of Catalysis, 1995
Journal of Catalysis, 1999
A combination of techniques including controlled atmosphere electron microscopy coupled with in s... more A combination of techniques including controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the nature of a carbonaceous support can impact both the chemistry and morphological properties of small copper particles when treated in various gas environments. We have found that the ease of reduction of CuO is dependent upon the nature of the carbon material and the strength of the interaction between the two components. It is suggested that an epitaxial relationship exists between copper and a diamond surface, which facilitates the reduction of copper oxide to the metallic state. In systems where copper species are supported on active carbon or graphite, attack on the substrate surface was observed to occur at low temperatures in the presence of hydrogen. This behavior is rationalized according to the notion that dissociation of molecular hydrogen takes place on Cu 0 sites at the surface of Cu 2 O particles and that the active atomic species "spill over" onto the graphitic support media and undergo reaction with the π-electrons of the basal plane resulting in the creation of pits thus producing extensive modifications to the surface structure of these support media. Based on the data from in situ electron diffraction analysis it is tentatively concluded that at moderate temperatures under reducing conditions, Cu 2 O is the stable chemical state on amorphous and graphitic forms of carbon.
Journal of Catalysis, 1970
The net slow uptake of hydrogen by platinized carbon is decreased drastically or suppressed altog... more The net slow uptake of hydrogen by platinized carbon is decreased drastically or suppressed altogether when the carbon contaminant on the platinum surface is burned off prior to the adsorption measurements. Contamination of the metal by carbon seems to provide bridges permitting surface diffusion of hydrogen atoms from platinum to the carbon support. A convenient manostat is described for use in adsorption and kinet.ic studies at constant pressure.
Carbon, 1995
The direct measurement of electrical properties of a high-surface-area turbostratic carbon black ... more The direct measurement of electrical properties of a high-surface-area turbostratic carbon black by microwave techniques is reported. This approach obviates many difficulties associated with powder samples and allows absolute values of conductivity and mobility to be obtained, from which charge carrier concentrations can be calculated and compared to those obtained by ESR methods. After a treatment in Ar at 1223 K to remove sulfur, this carbon had conductivity σ = 2.4 (Ω · cm)−1, mobility μ = 5.4 cm2/V · s, and a carrier concentration of 1.5 × 1018 g−1 at 300 K. The adsorption of O2 at 300 K increased σ, decreased μ, and increased carrier concentration to a small extent. After oxidizing this carbon in HNO3, all three parameters were decreased slightly and adsorbed O2 had little effect. However, the concentration of charge carriers in both carbon blacks was 10–40 times lower than the conduction carrier concentrations determined from a typical analysis of ESR signal intensity obtained at different temperatures. These preliminary results indicate that holes are the primary charge carriers in these carbon materials, which have small crystallite size and a small extent of graphitization. Oxygen chemisorbed on the pre-cleaned carbon black surface increases the concentration of both localized spin centers and holes and creates a higher electrical conductivity. Intensive oxidation by HNO3 increases the number of localized spin centers but decreases the concentration of holes, thus also decreasing conductivity. A comparison of these microwave and ESR results implies that the traditional way of identifying and quantifying conduction charge carriers from the temperature-dependent susceptibility of ESR signals may not be satisfactory.
Applied Catalysis B: Environmental, 1998
Signi®cant NO adsorption occurred at 300 K on both La 2 O 3 and Sr-promoted La 2 O 3 , with irrev... more Signi®cant NO adsorption occurred at 300 K on both La 2 O 3 and Sr-promoted La 2 O 3 , with irreversible adsorption increasing from 3.2 to 8.1 mmol/m 2 at the maximum loading of 8% Sr. On La 2 O 3 the total NO uptake remained at 5.0AE0.5 mmol/m 2 between 300 and 773 K; however, the irreversible portion dropped sharply above 600 K. In contrast, total oxygen chemisorption was much lower at 300 K (ca. 0.1 mmol O 2 /m 2), reached a maximum between 500 and 600 K, then dropped noticeably at higher temperatures. On all catalysts, the speci®c activities (mol/s/m 2) increased in the order: NO decomposition<NO reduction by CH 4 (no O 2)<NO reduction by CH 4 (1% O 2). The addition of Sr produced 4-to 6-fold increases in the speci®c activities but it enhanced sintering, thus rates per gram typically reached a maximum at loadings below 8% Sr. For each of the three reactions on either La 2 O 3 or the Sr-promoted La 2 O 3 catalysts, turnover frequencies (TOFs) based on the number of sites for irreversible NO adsorption at 300 K agreed within a factor of 2, thus implying that Sr promotion also increased the number of active sites per unit surface area. Almost no CH 4 combustion occurred at 773 K with 0.45% CH 4 in 1% O 2 with no NO present, but the activation energy of 37 kcal/mol for this reaction on La 2 O 3 was much higher than that for the NO/CH 4 reaction and, at 973 K, 54% of the CH 4 was converted at a WHSV of 26 500 cm 3 /h g. A reaction model for NO reduction by CH 4 (no O 2) gave an excellent ®t of the data at three temperatures and produced thermodynamically consistent rate parameters which gave heats of adsorption of 28 and 20 kcal/mol for NO and CH 4 , respectively.
Journal of Catalysis, 1995
Etude experimentale en mecanisme expliquant l'augmentation de l'activite catalytique du p... more Etude experimentale en mecanisme expliquant l'augmentation de l'activite catalytique du platine par un support de TiO 2 pur ou dope par Nb ou Ga, par rapport au platine sur support d'α Al 2 O 3
Preprints-American Chemical Society Division of Petroleum Chemistry, 1985
On a utilise un calorimetre differentiel pour mesurer la somme des chaleurs d'adsorption de C... more On a utilise un calorimetre differentiel pour mesurer la somme des chaleurs d'adsorption de CO et H 2 sur du Pd disperse sur SiO 2 , Al 2 O 3 , SiO 2 -Al 2 O 3 et TiO 2
Preprints-American Chemical Society Division of Petroleum Chemistry, 1997
The strong thermodynamic potential for carbon formation during CH 4 -CO 2 reforming at low temper... more The strong thermodynamic potential for carbon formation during CH 4 -CO 2 reforming at low temperatures and equimolar feed ratios necessitates use of a catalyst which inhibits carbon deposition.
Journal of Catalysis, 1976
The catalytic behavior of unsupported nickel and nickel dispersed on a variety of supports has be... more The catalytic behavior of unsupported nickel and nickel dispersed on a variety of supports has been studied in the CO/H/sub 2/ synthesis reaction. The catalysts weree characterized by chemisorption and X-ray diffraction measurements. Specific activities and product distributions were determined and kinetic parameters for the methanation reaction were obtained. The specific activity is sensitive to the support material but the variation in activity is less than an order of magnitude. There is also evidence that maximum activity may occur over a certain nickel particle size range. The kinetic parameters are relatively insensitive to the support; however, dispersing nickel on any of the supports studied enhanced the formation of higher molecular weight hydrocarbons compared to unsupported nickel. These differences in catalytic behavior are attributed to changes in the adsorbed state of CO on nickel surfaces which are a result of metal-support interactions or variations in nickel crysta...
Preprints-American Chemical Society Division of Petroleum Chemistry, 1995
A microwave absorption technique based on cavity perturbation theory is applicable for electrical... more A microwave absorption technique based on cavity perturbation theory is applicable for electrical conductivity measurements of both small, single-crystal particles and finely divided powder samples when {sigma} values fall in either the low ({sigma}<0.1{Omega}{sup -1}cm{sup -1}) or the intermediate (0.1 <{sigma}<100{Omega}{sup -1}cm{sup -l}) conductivity region. If the skin depth of the material becomes significantly smaller than the sample dimension parallel to the E-field, an appreciable error can be introduced into the calculated conductivity values; however, this discrepancy is eliminated by correcting for the field attenuation associated with the penetration depth of the microwaves and accurate absolute values can be obtained. When combined with microwave Hall effect measurements of mobility, {mu}, carrier densities can be calculated, for electrons N{sub o}={sigma}/{rho}e{mu} where e is the electron charge and {sigma} is the density of the solid. This approach eliminat...
Etude des proprietes d'adsorption et catalytiques de melanges physiques de 7,0% Ni/TiO 2 et 0... more Etude des proprietes d'adsorption et catalytiques de melanges physiques de 7,0% Ni/TiO 2 et 0,6Z Pt/TiO 2 dans l'hydrogenation de l'oxyde de carbone
Catalyst Characterization Science, 1985
Catalysis, 1982
Interest in carbon monoxide has increased dramatically during the past decade. This is due primar... more Interest in carbon monoxide has increased dramatically during the past decade. This is due primarily to a new interest in energy resources other than natural gas and petroleum resources which include coal, oil shale, and heavy residua. In any process which involves gasification to convert these hydrogen-deficient materials to hydrocarbons or other organic compounds, CO is one of the principal products of the gasification step, and its subsequent hydrogenation to form the required final products is of extreme importance. It is thermodynamically possible to produce methane as SNG, hydrocarbon liquids as fuels, and alcohols and olefins as chemical intermediates — the major problem is the selectiveproduction of the desired product. During the overall conversion process, the reaction between CO and water is important, viathe water gas shift reaction, not only because it is used to adjust the H2/CO ratio in the gas stream between the gasifier and the reactor, but also because H2O is a by-product of the CO hydrogenation reaction and can react with unconsumed CO in the syngas reactor to form CO2.
The chemistry between NOx species adsorbed on La2O3 and CH4 was probed by temperature‐programmed ... more The chemistry between NOx species adsorbed on La2O3 and CH4 was probed by temperature‐programmed reaction (TPR) as well as in situ DRIFTS. During NO reduction by CH4 in the presence of O2, NO3- does not appear to activate CH4, thus either an adsorbed O species or an NO2- species is more likely to activate CH4. In the absence of O2, a different reaction pathway occurs and NO- or (N2O2)2- species adsorbed on oxygen vacancy sites seem to be active intermediates, and during NO reduction with CH4 unidentate NO3-, which desorbs at high temperature, behaves as a spectator species and is not directly involved in the catalytic sequence. Because reaction products such as CO2 or H2O as well as adsorbed oxygen cannot be effectively removed from the surface at lower temperatures, steady‐state catalytic reactions can only be achieved at temperatures above 800 K, even though formation of N2 and N2O from NO was observed at much lower temperature during the TPR experiments.
Surface Science, 1994
Abstract The adsorption, desorption and decomposition of acetone on a Pt foil have been studied b... more Abstract The adsorption, desorption and decomposition of acetone on a Pt foil have been studied by high resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Two surface adsorption geometries occur for acetone on the foil: η 1 (O)-(CH 3 ) 2 CO (bonding end-on via the lone pair electrons on the oxygen) and η 2 (O,C)-(CH 3 ) 2 CO (bonding side-on via either π-bonding or di-σ bonding after rehybridization). For the η 1 species, reversible adsorption and desorption are dominant and are similar in behavior on a Pt(111) surface, while conversion of η 1 species to η 2 species may also occur. For the η 2 species, the decomposition channel is competitive with desorption, and the desorbed decomposition products are hydrogen and carbon monoxide. This process is associated with surface defects such as step sites, and preadsorbed hydrogen inhibits the decomposition channel thus providing additional support for this conclusion.
Journal of Catalysis, 1998
A combination of CO adsorption at 300 K, which provides irreversible adsorption on Cu +1 sites, a... more A combination of CO adsorption at 300 K, which provides irreversible adsorption on Cu +1 sites, and oxygen adsorption via N 2 O dissociation at 363 K, which counts surface Cu 0 atoms, was used to obtain values for the dispersion of Cu on SiO 2 , Al 2 O 3 , TiO 2 , ZrO 2 , diamond powder, and graphitized carbon fibers. The crystallite sizes obtained from these estimates were compared to those obtained from TEM and XRD measurements and were found to be in very good agreement. DRIFT spectra of CO adsorbed on these catalysts were obtained at subambient temperatures as well as at 300 K, and the various amounts of Cu 0 , Cu +1 , and Cu +2 at the surface were detected by respective peaks in the regions of 2110 cm −1 and lower, 2110-2140 cm −1 , and 2145 cm −1 and above. Spectra obtained at 173 K were especially useful because irreversible CO adsorption occurred on all three sites, thus allowing the removal of gas-phase CO and clarifying the spectra of adsorbed CO. Heats of adsorption for CO were determined from the variation in the intensity of the Kubelka-Munk function versus temperature, and the following average values were obtained for Cu dispersed on SiO 2 , Al 2 O 3 , and diamond: Cu 0-4.7 kcal/mole; Cu +1-11.7 kcal/mole; and Cu +2-5.3 kcal/mole.
Journal of Catalysis, 1997
A pretreatment involving a 4-h reduction under H 2 at 573 K, compared to either 473 or 673 K, gav... more A pretreatment involving a 4-h reduction under H 2 at 573 K, compared to either 473 or 673 K, gave the highest specific activity for furfural and crotonaldehyde hydrogenation over an unpromoted copper chromite catalyst. Both H 2 and CO chemisorption declined monotonically with increasing reduction temperature, but turnover frequencies also showed a maximum after a pretreatment at 573 K. DRIFTS identified a band, associated with CO adsorbed on Cu +1 sites, which reached a maximum intensity after a 573 K pretreatment, while XRD patterns also provided evidence for a CuCrO 2 phase and showed a continuous increase in Cu 0 as temperature increased. Selectivity to furfuryl alcohol ranged from 35-80% and reaction orders on furfural and H 2 were near unity. In contrast, crotonaldehyde hydrogenation gave primarily butyraldehyde with only around 3% crotyl alcohol and a near zero-order dependence on crotonaldehyde, although the H 2 dependence was still near first order. IR spectra under reaction conditions revealed adsorbed crotonaldehyde, provided evidence for an unsaturated alkoxide intermediate and detected no butyraldehyde on the surface. A simple Langmuir-Hinshelwood sequence nicely explained both reactions, it was consistent with the DRIFTS results obtained during crotonaldehyde hydrogenation, and the resultant rate expression gave meaningful entropies and enthalpies of adsorption for H 2 and crotonaldehyde.
Journal of Catalysis, 1981
Nickel crystallite sizes calculated from H,, CO, and OZ chemisorption are compared to those deter... more Nickel crystallite sizes calculated from H,, CO, and OZ chemisorption are compared to those determined from X-ray diffraction (XRD) and transmission electron microscopy (TEM). Good agreement is obtained among all techniques for typical Ni/SiO% and Ni/Al,OI catalysts, whereas sizes calculated from H, and CO chemisorption are far too large for Ni/TiO, catalysts. In contrast, oxygen chemisorption provides values in excellent agreement with those from XRD and TEM, and indicates that monolayer coverages of CO and HP are much lower on TiO,-supported nickel. This agreement also implies that oxygen monolayer coverage on TiO,-supported nickel does not exceed unity regardless of crystallite size, whereas coverages of two or higher are observed on SiO,-and A&O,-supported nickel, in agreement with previous studies. Isobars for CO and H, adsorption on Ni/TiO, clearly show that activated adsorption is not responsible for the low CO and H2 uptakes at 300 K. Low-contrast nickel crystallites on titania are observed in the TEM micrographs, indicating that nickel may exist with a raft-like morphology on this support. These results provide evidence that strong metal-support interaction behavior exists in the Ni/TiO* system.
Journal of Catalysis, 1995
Journal of Catalysis, 1999
A combination of techniques including controlled atmosphere electron microscopy coupled with in s... more A combination of techniques including controlled atmosphere electron microscopy coupled with in situ electron diffraction has been used to follow the manner by which the nature of a carbonaceous support can impact both the chemistry and morphological properties of small copper particles when treated in various gas environments. We have found that the ease of reduction of CuO is dependent upon the nature of the carbon material and the strength of the interaction between the two components. It is suggested that an epitaxial relationship exists between copper and a diamond surface, which facilitates the reduction of copper oxide to the metallic state. In systems where copper species are supported on active carbon or graphite, attack on the substrate surface was observed to occur at low temperatures in the presence of hydrogen. This behavior is rationalized according to the notion that dissociation of molecular hydrogen takes place on Cu 0 sites at the surface of Cu 2 O particles and that the active atomic species "spill over" onto the graphitic support media and undergo reaction with the π-electrons of the basal plane resulting in the creation of pits thus producing extensive modifications to the surface structure of these support media. Based on the data from in situ electron diffraction analysis it is tentatively concluded that at moderate temperatures under reducing conditions, Cu 2 O is the stable chemical state on amorphous and graphitic forms of carbon.
Journal of Catalysis, 1970
The net slow uptake of hydrogen by platinized carbon is decreased drastically or suppressed altog... more The net slow uptake of hydrogen by platinized carbon is decreased drastically or suppressed altogether when the carbon contaminant on the platinum surface is burned off prior to the adsorption measurements. Contamination of the metal by carbon seems to provide bridges permitting surface diffusion of hydrogen atoms from platinum to the carbon support. A convenient manostat is described for use in adsorption and kinet.ic studies at constant pressure.
Carbon, 1995
The direct measurement of electrical properties of a high-surface-area turbostratic carbon black ... more The direct measurement of electrical properties of a high-surface-area turbostratic carbon black by microwave techniques is reported. This approach obviates many difficulties associated with powder samples and allows absolute values of conductivity and mobility to be obtained, from which charge carrier concentrations can be calculated and compared to those obtained by ESR methods. After a treatment in Ar at 1223 K to remove sulfur, this carbon had conductivity σ = 2.4 (Ω · cm)−1, mobility μ = 5.4 cm2/V · s, and a carrier concentration of 1.5 × 1018 g−1 at 300 K. The adsorption of O2 at 300 K increased σ, decreased μ, and increased carrier concentration to a small extent. After oxidizing this carbon in HNO3, all three parameters were decreased slightly and adsorbed O2 had little effect. However, the concentration of charge carriers in both carbon blacks was 10–40 times lower than the conduction carrier concentrations determined from a typical analysis of ESR signal intensity obtained at different temperatures. These preliminary results indicate that holes are the primary charge carriers in these carbon materials, which have small crystallite size and a small extent of graphitization. Oxygen chemisorbed on the pre-cleaned carbon black surface increases the concentration of both localized spin centers and holes and creates a higher electrical conductivity. Intensive oxidation by HNO3 increases the number of localized spin centers but decreases the concentration of holes, thus also decreasing conductivity. A comparison of these microwave and ESR results implies that the traditional way of identifying and quantifying conduction charge carriers from the temperature-dependent susceptibility of ESR signals may not be satisfactory.
Applied Catalysis B: Environmental, 1998
Signi®cant NO adsorption occurred at 300 K on both La 2 O 3 and Sr-promoted La 2 O 3 , with irrev... more Signi®cant NO adsorption occurred at 300 K on both La 2 O 3 and Sr-promoted La 2 O 3 , with irreversible adsorption increasing from 3.2 to 8.1 mmol/m 2 at the maximum loading of 8% Sr. On La 2 O 3 the total NO uptake remained at 5.0AE0.5 mmol/m 2 between 300 and 773 K; however, the irreversible portion dropped sharply above 600 K. In contrast, total oxygen chemisorption was much lower at 300 K (ca. 0.1 mmol O 2 /m 2), reached a maximum between 500 and 600 K, then dropped noticeably at higher temperatures. On all catalysts, the speci®c activities (mol/s/m 2) increased in the order: NO decomposition<NO reduction by CH 4 (no O 2)<NO reduction by CH 4 (1% O 2). The addition of Sr produced 4-to 6-fold increases in the speci®c activities but it enhanced sintering, thus rates per gram typically reached a maximum at loadings below 8% Sr. For each of the three reactions on either La 2 O 3 or the Sr-promoted La 2 O 3 catalysts, turnover frequencies (TOFs) based on the number of sites for irreversible NO adsorption at 300 K agreed within a factor of 2, thus implying that Sr promotion also increased the number of active sites per unit surface area. Almost no CH 4 combustion occurred at 773 K with 0.45% CH 4 in 1% O 2 with no NO present, but the activation energy of 37 kcal/mol for this reaction on La 2 O 3 was much higher than that for the NO/CH 4 reaction and, at 973 K, 54% of the CH 4 was converted at a WHSV of 26 500 cm 3 /h g. A reaction model for NO reduction by CH 4 (no O 2) gave an excellent ®t of the data at three temperatures and produced thermodynamically consistent rate parameters which gave heats of adsorption of 28 and 20 kcal/mol for NO and CH 4 , respectively.
Journal of Catalysis, 1995
Etude experimentale en mecanisme expliquant l'augmentation de l'activite catalytique du p... more Etude experimentale en mecanisme expliquant l'augmentation de l'activite catalytique du platine par un support de TiO 2 pur ou dope par Nb ou Ga, par rapport au platine sur support d'α Al 2 O 3
Preprints-American Chemical Society Division of Petroleum Chemistry, 1985
On a utilise un calorimetre differentiel pour mesurer la somme des chaleurs d'adsorption de C... more On a utilise un calorimetre differentiel pour mesurer la somme des chaleurs d'adsorption de CO et H 2 sur du Pd disperse sur SiO 2 , Al 2 O 3 , SiO 2 -Al 2 O 3 et TiO 2