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Methane oxidation using O2 over transition metal oxides often requires severe conditions ( >500 °C) to achieve detectable conversion. In this study, NO was used to transfer oxygen atoms from O2, through the facile gas-phase formation of NO2 at moderate conditions (0.1 MPa and 300–400 °C), to oxidize methane over silica-supported transition metal oxides (VOx, CrOx, MnOx, NbOx, MoOx, and WOx). In situ infrared spectroscopy measurements indicated that the reaction likely proceeded by the formation of surface monodentate nitrate intermediates. These nitrate species were formed by the interaction between adsorbed NO2 and the supported metal oxides. During the reaction, the oxides of vanadium, molybdenum, and tungsten formed formaldehyde and CO2, whereas the oxides of chromium, manganese, and niobium produced only CO2. These results are consistent with the known hydrocarbon oxidation chemistry of the metal oxides. Contact time measurements on VOx/SiO2 indicated that formaldehyde was a primary product and CO2 was the final product; conversely, analogous measurements on MnOx/SiO2 showed that CO2 was the sole product. The formaldehyde production rate on VOx/SiO2, MoOx/SiO2, and WOx/SiO2, based on surface sites measured by high temperature oxygen chemisorption, compared favorably to oxygenate production rates for stronger oxidants (N2O and H2O2) reported in the literature.

The Delplot method provides a means of analyzing conversion and selectivity data to derive a reaction sequence and has been applied so far to gas-phase or liquid-phase species. This study analyses the applicability of the method for catalytic reactions and considers for the first time adsorbed intermediates. The method is applied to a contact time study of the hydrodeoxygenation (HDO) of benzofuran at 0.5 MPa and 350 °C over a catalyst consisting of bimetallic CoPd phosphide supported on a potassium ion-exchanged ultra-stable Y (KUSY) zeolite. Both simple and detailed reaction networks were derived. The simple networks considered only gas-phase species and were modeled by a first-order sequence of steps, whereas the detailed network took into account adsorbed intermediates and was simulated using a rake mechanism. The networks were compared using F-statistics, which accounted for the differences in the number of fitting parameters. The detailed network gave a better fit to the experimental data because it represented a more realistic description of the transformation. A suggested sequence from the Delplot method was consistent with the simple network but not with the detailed network. The lack of applicability of the Delplot analysis to the network with adsorbates was linked to overly large equilibrium constants, a determination supported by Delplot fitting for a model sequence. This study indicated the inapplicability of the Delplot method in determining reaction sequences that involve adsorbed species with large equilibrium constants for formation.

The synthesis of bimetallic CoPdP phosphides supported on potassium ion exchanged ultra-stable Y (KUSY) zeolites was studied and catalytic activity tests on the hydrodeoxygenation of a model pyrolysis oil compound, benzofuran, were conducted. Temperature programmed reduction (TPR) profiles revealed that Pd aided the reduction of the Co, lowering both the onset and peak temperatures for reduction. The catalyst structure was probed by X-ray diffraction (XRD) and extended X-ray adsorption fine structure (EXAFS), which showed the formation of CoPdP alloys. Fourier-transform infrared spectroscopy was used to probe the Co and Pd sites on the surface. Several Co:Pd ratios were tested in the range of 225–350 °C at a medium pressure of 0.5 MPa. Synergistic effects were observed between Co and Pd, with the alloys showing higher activity than the single metal Pd or Co phosphide catalysts. High selectivity to unsaturated high carbon number products were obtained for the bimetallic phosphide catalysts.

The understanding of catalytic mechanisms is enhanced by the observation of surface intermediates at reaction conditions using spectroscopic techniques, but this is insufficient, as the observed species may not be involved in the reaction. This work describes a general method of analysis of hydrogenation or oxidation reactions which uses transient spectroscopic data to determine whether an adsorbed species is a reactive intermediate or a spectator on the surface. The assumptions and limitations of the method are summarized. Although the technique is approximate, it is easy to implement, and provides order-of-magnitude estimates of the rate of reaction of an intermediate. The method consists of measuring the change of coverage of the species with time, dθ/dt, during adsorption in inert gas or at reaction conditions. An example is given with the hydrodeoxygenation of the model compound γ-valerolactone (GVL) using a Ni2 P/MCM-41 catalyst, one of the most effective catalysts reported for the transformation. The reaction is relevant to the upgrading of bio-oil derived from pyrolysis of lignocellulosic feedstocks. The kinetics of the reaction and observation by in situ infrared spectroscopy of adsorbed GVL and its transformation to pentanoic acid are consistent with a Langmuir-Hinshelwood mechanism. Analysis by in situ transient X-ray absorption fine structure shows that the adsorbed GVL is a true reaction intermediate.

The direct partial oxidation of CH4 to dimethyl ether (DME) on a Pt/Y2O3 catalyst was studied using a mixture of NO and O2 as the oxidant. The reaction was carried out in a fixed bed reactor at 0.1 MPa and 275–400 °C using 20% CH4, 1% NO, and 1% O2 in inert gas. No methanol was detected in the effluent and a contact time study demonstrated that DME was a primary product. The DME productivities were comparable to the oxygenate (methanol, formaldehyde) productivities obtained with stronger oxidants such as N2O, H2O2, and O3. The presence of Pt and the NO + O2 gas mixture was necessary for DME formation; without NO only CO2 was produced. During the methane partial oxidation reaction NO and NO2 were not reduced to N2, indicating that they worked as a shuttle to transfer oxygen from O2 to CH4. In situ Fourier transform infrared showed the formation of a bridged nitrate species on the Pt/Y2O3 catalyst which was associated with the reaction of CH4. A comprehensive study of this bridged nitrate species indicated they were formed on yttria sites close to Pt and were likely responsible for the formation of DME. The characterization of catalysts using X-ray diffraction showed that the Pt was highly dispersed and CO uptake measurements indicated a particle size of ~3 nm. Analysis by X-ray absorption fine structure measurements showed the presence of Pt oxide with Pt-O and Pt-Pt bonds.

A series of supported Ni-Mo-P alloy catalysts was studied for the catalytic hydrodeoxygenation (HDO) of the cyclic five-membered ester γ-valerolactone (GVL-C5H8O2) as a model compound for pyrolysis oil. Alloy formation in Ni-Mo-P was indicated by X-ray diffraction analysis and X-ray absorption near-edge spectroscopy, which showed systematic shifts with composition. The number of active sites of each metal species was estimated by factor analysis combining CO-uptake measurements and infrared (IR) spectra of adsorbed CO. It was found that the catalytic activity followed the order: Ni2P/MCM-41 > NiMo(3:1)P/MCM-41 > NiMo(1:1)P/MCM-41 ≅ (Ni2P + MoP)/MCM-41 > NiMo(1:3)P/MCM-41 > MoP/MCM-41, whereas the normalized turnover frequency based on Ni sites were similar for all the catalysts, while retaining the same order. It is concluded that adjacent surface Ni atoms are the main active sites involved in the rate-determining step (rds). The Mo-containing catalysts produced more 1-pentanol and C5 hydrocarbons than Ni2P/MCM-41, indicating that while the exposed Ni sites governed catalytic activity, Mo sites controlled the selectivity to C5 hydrocarbons. Thus, steps following the rds were influenced by Mo sites, leading to preferences for different reaction pathway during the HDO of γ-valerolactone. The study reveals that the catalytic behavior of NiMoP catalysts can be tuned by the relative proportion of Ni and Mo sites.

Pyrolysis bio-oil is a promising source of liquid fuels, but requires upgrading to remove excess oxygen and produce a satisfactory fuel oil. Nickel phosphide has been shown to be an active composition for hydrodeoxygenation (HDO) of bio-oil model compounds. In this study, nickel phosphide catalysts were used for direct upgrading of an actual pyrolysis bio-oil derived from cedar chips. The activity of Ni2P deposited on an amorphous SiO2 support for HDO was first verified using the model compound, 2-methyltetrahydrofuran (2-MTHF), at the temperature of the pyrolysis oil treatment of 350 °C. The Ni2P/SiO2 catalyst showed high activity for 2-MTHF hydrodeoxygenation under atmospheric pressure hydrogen with low cracking activity. Fast pyrolysis and catalytic upgrading were conducted sequentially using a laboratory-scale, two-stage system consisting of a fluidized bed pyrolyzer and a fluidized bed catalytic reactor both operating at 0.1 MPa, with a hydrogen partial pressure of 0.06 MPa. It was found that the Ni2P/SiO2 catalyst was moderately effective in upgrading the biomass pyrolysis vapors and producing a refined bio-oil with decreased oxygen content. The moderate deoxygenation of the bio-oil was confirmed by elemental analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis. Gas chromatography–mass spectrometry (GC–MS) analysis showed that the treated bio-oil mainly consisted of phenolic compounds, and the MS spectra before and after upgrading suggested that reactions including hydrodeoxygenation, hydrogenation, decarbonylation, and hydrolysis occurred during the upgrading. Furthermore, Ni2P supported on ZSM-5 zeolite eliminated oxygen in the bio-oil with smaller reduction in the oil yield than Ni2P supported on SiO2. The deoxygenation activity of the nickel phosphide catalysts was higher than that of conventional catalysts such as Ni/SiO2, Pd/C and an FCC-catalyst.

The hydrodeoxygenation of 2-methyltetrahydrofuran (2-MTHF) at a medium pressure of 0.5 MPa is studied over a Ni 2 P/SiO 2 catalyst. Contact time studies show the formation of n -pentanal, 1-pentanol, 2-pentanone, 2-pentanol, and 2-pentene, as reaction intermediates and the production of pentane and butane as major products. The results are consistent with adsorption of 2-MTHF followed by rate-determining ring-opening to form either 1-pentoxide or 2-pentoxide alkoxide intermediates. Subsequent hydrogen-transfer steps produce the various intermediates, a decarbonylation step of the pentanal forms n -butane and CO, and further hydrodeoxygenation steps result in n -pentane. Fitting of the results using a rake mechanism that considers adsorbed intermediates gives excellent agreement with experimental data, and agrees with a simulation with a simpler first-order model. The more detailed rake analysis indicates that the surface species from the 1-pentoxide intermediate are ten-fold more plentiful than those produced from the 2-pentoxide intermediate. In situ infrared measurements support this reaction mechanism.

Bio-oil obtained by the pyrolysis of woody biomass contains many oxygenated organic compounds which degrade the product quality and make necessary upgrading for its use as a liquid fuel. Hydrodeoxygenation (HDO) is a catalytic hydrotreating process for the removal of the problematic oxygen functionalities and is promising for bio-oil upgrading. In this work, 2-methyltetrahydrofuran (2-MTHF) was chosen as a model oxygenated compound, and its HDO reaction mechanism was studied on a silica-supported nickel phosphide catalyst (Ni 2 P/SiO 2 ) at a medium pressure of 0.5 MPa. The temperature dependency of the catalyst activity was determined and it was found that at 350 °C Ni 2 P/SiO 2 showed 100% conversion and 85% selectivity to n -pentane, with higher oxygen removal activity and less C C bond cracking activity than commercial noble metal Ru/C and Pd/Al 2 O 3 catalysts based on the same amount of active sites. A contact time study allowed the determination of a reaction sequence for 2-MTHF HDO on Ni 2 P/SiO 2 and it was found that C O bond cleavage of the furanic ring to generate either 2-pentanone or 1-pentanal was the rate-determining step. This was followed by hydrogen transfer steps to produce oxygen free compounds, n -pentane or n -butane. A partial pressure analysis of 2-MTHF and H 2 was consistent with a rate equation derived using a Langmuir–Hinshelwood (L–H) mechanism. This suggested that adsorption of 2-MTHF and hydrogen occurred competitively and that these species reacted on the Ni 2 P/SiO 2 surface. Although high partial pressure of H 2 was favorable for hydrogenation, too much H 2 competed with 2-MTHF adsorption, which caused lower conversion.

A series of silica-supported metal phosphides was prepared by two methods involving the reduction of phosphite (I) or phosphate (A) precursors and was studied for the hydrodeoxygenation (HDO) of 2-methyltetrahydrofuran (2-MTHF). The I method required lower temperature than the A method and resulted in catalysts with higher surface area. The activity was evaluated in a packed-bed reactor on the basis of equal CO chemisorption sites (30 μmol) loaded in the reactor with comparison made to a commercial Pd/Al 2 O 3 catalyst. At 300 °C and 1 atm, the order of activity was Ni 2 P > WP > MoP > CoP > FeP > Pd/Al 2 O 3 . The principal HDO products for the iron group phosphides (Ni 2 P and CoP) were pentane and butane, whereas for the group 6 metal phosphides (MoP and WP), the products were mostly pentenes and pentadienes. For the Pd/Al 2 O 3 and the low-activity FeP/SiO 2 catalyst the products were mostly pentenes and C4 mixtures. There were no significant differences in the turnover frequency between materials prepared by the two methods, except possibly for the case of WP. There were likewise no great changes in selectivity toward HDO products at 5% total conversion, except for WP. The differences in the case of WP were attributed to the surface P/W ratio that X-ray photoelectron spectroscopy (XPS) showed to be twice as large for the I method than the A method. Contact-time studies were used to develop reaction networks for the most active catalysts, Ni 2 P/SiO 2 and WP/SiO 2 . For Ni 2 P/SiO 2 by both methods, the selectivity profiles were similar and could be explained by a rake mechanism with pentenes as primary products, 2-pentanone as a secondary product, and pentane as a final product. In contrast, for WP/SiO 2 , the selectivity depended greatly on the preparation method, but produced a preponderance of unsaturated compounds. The results could be explained from the surface composition.

Supported Ni2P catalysts are active for hydrodesulfurization (HDS) of petroleum feedstocks and have attracted considerable attention. In order to find out the structure of the active catalyst and to obtain insight into the reaction mechanism of thiophene HDS over Ni2P/MCM-41, we conducted a simultaneous time-resolved (STR) study with in situ X-ray absorption fine structure (XAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy together with time-resolved product analysis. At 513 K, adsorbed tetrahydrothiophene (THT) was observed by FTIR, following the formation of a nickel phosphosulfide phase (NiPS). Detection of the HDS products coincided with the THT formation. All these experimental data indicated that NiPS worked as the active phase and identified THT as a reaction intermediate for the HDS reaction. The STR study proved to be a powerful tool for the precise study of reaction intermediates and the catalytic reaction path.

The hydrodesulfurization (HDS) of the refractory compound 4,6-dimethyldibenzothiophene (DMDBT) normally proceeds through a hydrogenation pathway that removes the planarity of the ring system and makes the hindered sulfur atom more accessible to the desulfurization centers. In this study, a highly active dispersed bimetallic NiFeP catalyst is found to have high selectivity for a direct desulfurization pathway, which does not require prior hydrogenation. The compound has equal numbers of Ni and Fe atoms which extended X-ray absorption fine structure analysis indicates are distributed randomly in the hexagonal Fe2P structure, with just a slight enrichment of Fe on the surface. This is supported by density functional theory calculations. The remarkable properties of the catalyst are ascribed to a ligand effect of Fe on the active Ni atoms.

Gold nanoparticles on titanium oxide and titania-silica supports are active for the formation of propylene oxide by the oxidation of propylene with hydrogen and oxygen mixtures. Cyanide treatment of gold supported on titanosilicate zeolite supports (Au/TS-1) yielded unexpected results. Catalysts treated with weak solutions of sodium cyanide resulted in preferential removal of small gold particles, while catalysts treated with strong solutions resulted in dissolution of the gold and re-precipitation as gold (+1) cyanide. X-ray absorption spectroscopy demonstrated that catalysts that produce propylene oxide in the presence of hydrogen and oxygen mixtures had supported gold (+3) oxide nanoparticles of 3 nm size after synthesis, which were reduced to gold metal at reaction conditions. Samples treated with strong solutions of sodium cyanide resulted in supported gold (+1) cyanide particles of large size, 9–11 nm. These particles did not produce propylene oxide but, surprisingly, showed high selectivity toward propylene hydrogenation. Increasing gold (+1) cyanide particle size resulted in a decrease in hydrogenation activity.

The hydrodesulfurization (HDS) of thiophene was studied over Ni 2 P/MCM-41 by in situ Fourier transform infrared (FTIR) spectroscopy and X-ray absorption near-edge structure (XANES) measurements at atmospheric pressure and 393–723 K (120–450 °C). FTIR showed the presence of adsorbed molecular thiophene principally on the MCM-41 support. In the presence of hydrogen new bands in the aliphatic C–H stretching region (2953, 2918, and 2876 cm −1 ) and the CH 2 scissoring region (1464 cm −1 ) appeared and grew with temperature up to 553 K (280 °C). These were assigned to a tetrahydrothiophene surface intermediate, whose presence is consistent with the high hydrogenation activity of nickel phosphide. XANES measurements between 353 and 573 K (180–300 °C) also showed growth of a signal (8330.9 eV) attributed to Ni–S interactions of either the adsorbed tetrahydrothiophene or a surface phosphosulfide. Transient XANES measurements showed that the sulfur species responded dynamically at the same rate as the steady-state rate, indicating that it was associated with an intermediate in the reaction.

The epoxidation of propylene with H 2 –O 2 mixtures was studied over a gold catalyst supported on a Ge-modified TS-1 catalyst at 170 °C. Shifts of X-ray diffraction lines to lower angle showed that the Ge was incorporated into the structure of the TS-1. The presence of Ge more than doubled the conversion from 1.6% to 4% while increasing selectivity from 87% to 91%. Density functional theory calculations indicated that the origin of the enhanced activity was a ligand effect of Ge on Ti, which lowered the activation energy for the critical epoxidation step.

A kinetic method is described to determine the role of adsorbed intermediates in heterogeneous reaction mechanisms. The method, denoted as analysis of coverage transients (ACT), involves comparing the time response of a spectroscopically observed species in an inert gas and a reactive gas to differentiate between adsorption–desorption processes and reaction. The method is applied in a kinetic study of pyridine hydrodenitrogenation (HDN) on a 12.2 wt% silica-supported nickel phosphide (Ni2P/SiO2) catalyst at 423 K and atmospheric pressure. In situ Fourier transform infrared spectroscopy (FTIR) measurements of pyridine adsorbed on Ni2P revealed the formation of a pyridinium surface intermediate. The concentration of the pyridinium intermediate increased with pyridine partial pressure and decreased in the presence of hydrogen, suggesting that it was a reaction intermediate. However, transient and steady-state kinetic measurements showed that the rate of reaction of the intermediate did not correspond to the overall reaction rate, and it is concluded that the pyridinium intermediate is not directly involved in the HDN reaction of pyridine over Ni2P. The studies demonstrate that mere observation of an adsorbed surface species at reaction conditions is not sufficient to prove that it is a reaction intermediate. The ACT method has potential as it can be used with any type of spectroscopy, as long as the surface coverage can be calibrated.

Ni 2 P catalysts supported on SiO 2 and MCM-41 were prepared by temperature-programmed reduction (TPR), and the effect of the dispersion on catalyst structure and hydroprocessing performance was studied. The surface areas of the samples varied from low (Ni 2 P/SiO 2 -L, 88 m 2 g −1 ) to high (Ni 2 P/SiO 2 -H, 240 m 2 g −1 ), to very high (Ni 2 P/MCM-41, 487 m 2 g −1 ), with corresponding Ni 2 P average crystallite sizes decreasing from 10.1 to 6.5 and 3.8 nm. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) studies were used to obtain structural parameters for the supported Ni 2 P phase. The catalytic activity in hydrodesulfurization (HDS) was measured at 613 K and 3.1 MPa in a three-phase fixed bed reactor using a model liquid feed containing 4,6-dimethyldibenzothiophene (4,6-DMDBT) and quinoline in a tridecane solvent. At standard conditions using 500 ppm S as 4,6-DMDBT, 6000 ppm S as dimethyldisulfide (DMDS), and 500 ppm N as quinoline, the catalytic activity followed the sequence Ni 2 P/MCM-41 > Ni 2 P/SiO 2 -H > Ni 2 P/SiO 2 -L, based on equal sites (230 μmol) loaded in the reactor. In particular, Ni 2 P/MCM-41 gave an HDS conversion of 90%, which was much higher than that of a commercial Ni–Mo–S/Al 2 O 3 catalyst which gave an HDS conversion of 68%, based on equal number of sites (230 μmol) loaded in the reactor. The sites were counted by CO chemisorption for the phosphide and by low-temperature O 2 chemisorption for the sulfide. EXAFS analysis of the samples confirmed the presence of two types of sites, tetrahedral Ni(1) sites and square pyramidal Ni(2) sites, with the latter growing in number in the same order as the reactivity Ni 2 P/MCM-41 > Ni 2 P/SiO 2 -H > Ni 2 P/SiO 2 -L, as the crystallite size decreased. From the selectivity to the direct desulfurization (DDS) product (dimethylbiphenyl) and the hydrogenation (HYD) products (methylcyclohexyltoluenes and dimethylbicyclohexyls) it is concluded that the Ni(1) sites are responsible for DDS while the Ni(2) are highly active sites for the HYD route.

The selective oxidation of propane to acetone and 2-propanol with H 2 and O 2 was studied on Au/TS-1 by kinetic and spectroscopic analysis. A kinetic study using a factorial design at conditions where the catalyst was stable and gave propane conversions of 90%, resulted in a power-rate law expression of the form r oxyg. = k oxyg. (H 2 ) 0.74 (O 2 ) 0.36 (C 3 H 8 ) 0.29 . In situ Au L 3 -edge X-ray absorption near-edge spectroscopy (XANES) measurements showed activation of O 2 on Au, whereas in situ ultraviolet–visible (UV–vis) spectroscopy evidenced the presence of Ti-hydroperoxo species. The role of the Ti-hydroperoxo species was probed by a transient technique in which changes in Ti K -edge XANES spectra were used to determine the evolution of coverage with time ( d θ / d t ). It was shown that the rate of reaction by XANES ( 6.7 × 10 −4 s −1 ) was close to the turnover rate measured in a catalytic flow reactor ( 5.6 × 10 −4 s −1 ), indicating that the hydroperoxo species were true intermediates in the reaction. A proposed reaction sequence in which H 2 O 2 forms on Au sites and propane is partially oxidized on Ti centers accounts for the spectroscopic results and the reaction orders obtained experimentally for the power-rate law expression.

The vapor-phase epoxidation of propylene with H2 and O2 over a highly dispersed Au/TS-1 catalyst was carried out in a packed-bed catalytic membrane reactor. The membrane reactor allowed the use of high concentrations of H2 and O2 (40% each) that were well within the explosive regime, and the production rate of propylene oxide was improved by 100–200% compared with that from a conventional packed-bed reactor. At 180 °C, the C3H6 conversion was 10% and the PO selectivity was 80%, corresponding to a space–time yield of 150gPOkgcat−1h−1, whereas at 212 °C, the yield increased to 200gPOkgcat−1h−1. The kinetic dependency, rPO=krPO=k (H2)0.53 (O2)0.26 (C3H6)0.18, obtained free from mass transport and heat transfer limitations at these high concentrations was in good agreement with that reported at low concentrations.

Propane epoxidation was carried out by sequential propane dehydrogenation–propylene epoxidation steps using a two-catalyst bed and H2 and O2 as the oxidant mixture. The propane dehydrogenation step used a Au/TiO2 catalyst that was active at the low temperature (443 K) used for the propylene epoxidation step; the latter used a Au/TS-1 catalyst. In situ Au L3-edge X-ray absorption near-edge structure and ultraviolet–visible measurements on Au/TiO2 under propane dehydrogenation conditions showed activation of oxygen on gold nanoparticles and evidence for the formation of adsorbed oxygen intermediate species responsible for the production of propylene. Propane epoxidation with H2 and O2 at 443 K and 0.1 MPa with the dual Au/TiO2 and Au/TS-1 catalysts resulted in an overall propane conversion of 2%, propylene selectivity of 57%, and propylene oxide selectivity of 8%, corresponding to a propylene oxide space-time yield of 4 g kg cat −1 h −1 . The catalysts showed little deactivation and maintained their conversion and selectivity levels for the 12 h duration of the measurements.

A catalyst consisting of gold supported on a Ti-containing silicate mesoporous material (TUD) with 13-nm pores was used for the epoxidation of propylene with mixtures of H 2 and O 2 . The catalyst activity was enhanced by the addition of a Ba promoter. The pH of deposition was important in controlling the gold loading and particle size. A pH of 7 gave a Au loading of 2.7 wt% and an average particle size of 2.0 nm, as determined by transmission electron microscopy, whereas a pH of 9 produced a much lower Au loading of 0.11 wt% and smaller particles of size about 0.9 nm, as estimated by X-ray absorption fine structure measurements. At 423 K and 0.1 MPa total pressure, the catalyst prepared at pH 7 gave a steady-state propylene conversion of 2.1%, a propylene oxide (PO) selectivity of 79%, and a H 2 efficiency of 3.8%, whereas that prepared at pH 9 gave a conversion of 1.4%, a PO selectivity of 99%, and a H 2 efficiency of 17%. The turnover frequency for PO production based on total gold increased on going to the sample prepared at pH 9. Thus, it is concluded that very small Au particles (about 1 nm) are the most active for epoxidation, whereas larger Au particles (about 2 nm) are less active because they promote direct H 2 oxidation to H 2 O. X-ray absorption near-edge spectroscopy results indicated that at reaction conditions, the small particles had partially oxidized gold but the larger particles had metallic gold, suggesting that the smaller particles had high coverage of oxygen or oxygen-derived species.

A Ni{sub 2}P catalyst supported on a high-surface area SiO{sub 2} (350 m{sup 2} g{sub -1}) was prepared by temperature-programmed reduction, and its structural and surface properties were studied. X-ray diffraction and extended X-ray absorption fine structure measurements were used to obtain structural parameters for the supported Ni{sub 2}P phase, and Fourier transform infrared (FTIR) analysis with the probe molecules CO and pyridine was carried out to characterize the surface properties. The catalytic activity was measured at 573 K and 3.1 MPa in a three-phase fixed-bed reactor for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) using a model liquid feed. At standard conditions using 500 ppm S as 4,6-dimethyldibenzothiophene (4,6-DMDBT), 3000 ppm S as dimethyldisulfide, 200 ppm N as quinoline, and 1% tetralin in a tridecane solvent, the Ni{sub 2}P/SiO{sub 2} gave an HDS conversion of 85%, an HDN conversion of 100%, and a tetralin conversion of 37%, which were much higher than those of a commercial Ni-Mo-S/Al{sub 2}O{sub 3} catalyst, which gave an HDS conversion of 41%, an HDN conversion of 98%, and a tetralin conversion of 20% based on equal numbers of sites (240 {micro}mol) loaded in the reactor. The sites were counted by CO chemisorption for the phosphide andmore » by low-temperature O{sub 2} chemisorption for the sulfide. The Ni{sub 2}P/SiO{sub 2} catalyst favored the hydrogenation (HYD) pathway for 4,6-DMDBT HDS to generate methylcyclohexyltoluene and dimethylbicyclohexane with a relative HYD selectivity of 95%. It also favored hydrogenation for tetralin to give decalin with a relative HYD selectivity of 89%. The Ni{sub 2}P/SiO{sub 2} catalyst also showed better resistance to N-compounds than the Ni-Mo-S/Al{sub 2}O{sub 3} catalyst. The FTIR spectra of adsorbed CO showed that the Ni site in the Ni{sub 2}P phase gave rise to considerable {pi}-back bonding, which was related to the high activity of the Ni{sub 2}P/SiO{sub 2} catalyst in the hydrogenation of aromatics. The FTIR spectra of adsorbed pyridine showed that the Ni{sub 2}P phase had a P{single_bond}OH group associated with Br{o}nsted acidity that was active for the protonation of N compounds. These results thus suggest that the supported Ni{sub 2}P catalyst has bifunctional properties that are beneficial for catalytic activity in hydroprocessing.« less

A detailed mechanistic study of acetone oxidation using ozone was performed on a 10 wt% silica-supported manganese oxide catalyst using in situ Raman spectroscopy. Two adsorbed intermediates were identified at reaction conditions: an acetone species with a band at 2930 cm −1 and an adsorbed peroxide species on the manganese oxide with a band at 890 cm −1 . Quantitative temperature-programmed desorption measurements showed that the acetone species resided on the silica support, which thus acted as a noninnocent support. The rates of the acetone and ozone reactions at 318–373 K were equally well described by the power-rate law and Langmuir–Hinshelwood expressions. Transient experiments showed that the rates of formation and reaction of the peroxide surface species did not correspond to the overall reaction rate, and it was concluded that it was not directly involved in the main reaction pathway. A mechanism is proposed involving the migration of the adsorbed acetone intermediate from the silica support to manganese centers, where it reacts with atomically adsorbed oxygen species to form complete oxidation products.

In this study the effect of particle size on ethylene and propylene epoxidation was studied on a series of silver catalysts supported on CaCO 3 with loading levels of 0.5–56 wt%. Particle sizes determined from O 2 chemisorption uptakes at 443 K (170 °C) ranged from 50 to 660 nm, and this was confirmed by field emission scanning electron microscopy, which in addition showed crystallite agglomeration at high loadings. Reaction results show that large particles favor ethylene epoxidation by 3–5-fold at 473–493 K, whereas particle size does not have a large effect on propylene epoxidation. X-Ray diffraction measurements indicate that the bulk of the particles consist of silver in a metallic state, but in situ ultraviolet–visible (UV–vis) spectroscopy distinctly shows that in addition to a metallic component, small particles have silver in Ag + state. The small particles are probably covered by a layer of Ag 2 O, which results in lower selectivity for epoxidation for both propylene and ethylene oxidation in the small size regime. The approach to steady state is fast in propylene oxidation (3–4 h) and is accompanied by changes in the UV–vis spectra that indicate a reduction in the Ag 2 O phase. The approach to steady state is slow in ethylene oxidation (24–36 h) and is associated with changes in the UV–vis spectra that are consistent with the formation of a partially oxidized surface phase, which may involve subsurface or adsorbed oxygen.

Alumina-supported molybdenum phosphide hydroprocessing catalysts, MoP/ γ -Al 2 O 3 , with weight loadings from 3.5 to 39 wt% were prepared by temperature-programmed reduction of alumina-supported molybdenum phosphate precursors. The precursors were obtained by incipient wetness impregnation of the support with aqueous molar solutions of ammonium paramolybdate and ammonium phosphate (Mo/P=1/1). Effects of loading, reduction temperature, and heating rate on the catalysts were studied, and the samples were characterized by CO chemisorption, BET surface area, and X-ray diffraction (XRD) measurements. Compared to the bulk MoP and MoP/SiO 2 systems (which are similar to one another), the MoP/ γ -Al 2 O 3 material showed different behavior. Whereas calcined bulk and silica-supported molybdenum phosphates form Mo–P amorphous glasses which reduce directly to MoP, for the alumina-supported phosphates at high loadings (e.g., 13 wt% MoP/ γ -Al 2 O 3 ), a MoO 3 component is visible which reduces sequentially with temperature to MoO 2 and Mo metal, and then transforms to MoP. The difference in behavior between these systems is attributed to the formation of a strongly bound amorphous phosphate surface layer on the alumina support. Alumina has a strong affinity for phosphates, and appears to initially abstract a substantial amount of phosphorus from the stoichiometric molybdenum phosphate mixture. The alumina then releases the phosphorus at high temperature, allowing it to recombine with Mo metal to form MoP and generating active sites. The MoP/ γ -Al 2 O 3 catalysts performed well in hydroprocessing of a simulated distillate containing dibenzothiophene and quinoline at conditions representative of industrial hydroprocessing (643 K, 3.1 MPa). For the 13 wt% sample the hydrodesulfurization conversion was 57% and hydrodenitrogenation conversion was 62%. Catalytic activity, based on equal chemisorption sites loaded in the reactor (70 μmol), was generally independent of the amount of MoP deposited on the alumina surface, independent of the presence of X-ray visible molybdenum phases, and was associated with a relatively high temperature reduction peak found at all loading levels of MoP/Al 2 O 3 but not in bulk MoP. We conclude that the high-temperature peak gives rise to highly dispersed MoP which is responsible for the bulk of the CO titration sites and the catalytic activity, and that the large MoP particles visible by XRD make smaller contributions.

The field of catalysis is continuously invigorated by the discovery of new catalytic materials. Transition metal phosphides represent one such group of compounds that have recently been shown to have excellent activity for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN). This is an important area, as continuous decline in the quality of petroleum feedstocks and increasingly stringent environmental regulations have made the removal of sulfur a paramount problem in the refining industry. The structure of the metal-rich phosphides is based on trigonal prisms, which can well accommodate the relatively large phosphorus atoms. The prisms are similar to those in sulfides, but phosphides do not take on layered structures and are metal conductors, not insulators or semiconductors. Lack of layers leads to a more isotropic crystal morphology and potentially better exposure of surface metal atoms to fluid phase reactants. Phosphides can be prepared readily in bulk and supported forms by reduction of phosphate precursors. The catalytic activity of the phosphides for dibenzothiophene HDS and quinoline HDN at 643 K and 3.1 MPa followed the order Fe2P

A series of iron, cobalt, and nickel metal phosphides of chemical formula Fe2P, CoP, and Ni2P with specific surface areas of around 3 m2 g−1 were synthesized by means of temperature-programmed reduction (TPR) of the corresponding phosphates. These phosphides were also successfully prepared in dispersed form on a silica support (90 m2 g−1) for use as catalysts. The phase purity of these materials was established by X-ray diffraction (XRD), and surface properties were determined by N2 BET specific surface area (Sg) measurements and CO uptake determinations. The activity of the silica-supported catalysts in hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) was evaluated in a three-phase trickle-bed reactor using a model liquid feed containing 2000 ppm nitrogen as quinoline, 3000 ppm sulfur as dibenzothiophene, 500 ppm oxygen as benzofuran, 20 wt% aromatics as tetralin, and balance aliphatics as tetradecane. The reactivity study showed that the HDS activity sequence for the three samples was Ni2P/SiO2>CoP/SiO2>Fe2P/SiO2, while the HDN activity followed the sequence CoP/SiO2>Ni2P/SiO2>Fe2P/SiO2. Compared with a commercial Ni–Mo–S/γ-Al2O3 catalyst, Ni2P/SiO2 had a higher HDS activity (90 vs 76%), but a lower HDN activity (14 vs 38%), based on equal sites loaded in the reactor. The sites were determined by CO chemisorption for the phosphide and low-temperature O2 chemisorption for the sulfide. XRD and X-ray photoelectron spectroscopy characterizations of the spent catalysts indicated that the Ni2P/SiO2 catalyst was tolerant of sulfur.

The number of surface sites on catalysts can be measured by well-established chemisorption techniques employing small molecules or by a reactant-probe method utilizing the actual reacting species in a catalytic reaction. The latter approach gives the advantage of directly titrating the actual reactive sites on the surface and has been demonstrated in a number of systems. An example is given for the case of manganese oxide supported on SiO2, Al2O3, TiO2, and ZrO2, which are good catalysts for the decomposition of ozone. Reasonable correspondence is found between oxygen chemisorption and temperature-programmed desorption of adsorbed ozone species, but the latter is better able to account for dual sites found on the ZrO2 support.

A tungsten phosphide, WP, hydrotreating catalyst was prepared by temperature-programmed reduction of an X-ray amorphous tungsten phosphate at a heating rate of 0.0167 K s−1 and a 2-h soak at the final temperature of 938 K. Characterization of intermediates quenched at different temperatures revealed a single crystalline reaction product whose specific CO uptake and BET surface areas passed through a maximum of 12 μmol g−1 and 15 m2 g−1, respectively, at 873 K. Heating rate variation measurements gave an apparent first-order activation energy of 148 kJ mol−1 for the reduction. Tungsten phosphide was catalytically active in the hydrodenitrogenation and hydrodesulfurization reactions of a simulated light oil consisting of 20 wt% tetralin, 76 wt% tetradecane, 2000 wppm N as quinoline, 3000 wppm S as dibenzothiophene, and 500 wppm O as benzofuran. Measurements were made in an upflow fixed-bed reactor operated at 3.1 MPa and 643 K and gave conversion levels at steady state for the WP catalyst of 58% HDN and 67% HDS. These conversions were superior to those found for WC, W2N, and WS2 tested under the same conditions.

A new family of bimetallic oxycarbide compounds MI–MII–O–C (MI=Mo, W; MII=V, Nb, Cr, Fe, Co, Ni) has been synthesized by carburizing bimetallic oxide precursors using a temperature-programmed method. The oxide precursors are prepared by conventional solid-state reaction between two appropriate monometallic oxides. The synthesis involves passing a 20 mol% CH4 in H2 mixture over the oxide precursors while raising the temperature at a linear rate of 8.3×10−2 K s−1 (5 K/min) to a final temperature (Tmax) which is held for a period of time (thold). The synthesis, chemisorption properties, and reactivation of the materials indicate that the compounds can be divided into two groups of different reducibility (high and low). Their surface activity and surface area are evaluated based on CO chemisorption and N2 physisorption measurements. It is found that the CO number density correlates with the reducibility of the compounds. The catalysts were evaluated for hydroprocessing in a three-phase trickle-bed reactor operated at 3.1 MPa and 643 K. The feed was a model liquid mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The bimetallic oxycarbides had moderate activity for HDN of quinoline, with Nb–Mo–O–C showing higher HDN than a commercial sulfided Ni–Mo/Al2O3 catalyst tested at the same conditions. X-ray diffraction of the spent catalysts indicated that the oxycarbides of the early transition metals were tolerant of sulfur, while those involving the late transition metals showed bulk sulfide phases.

A new family of bimetallic oxynitride compounds, MI–MII–O–N (MI, MII=Mo, W, V, Nb, Cr, Mn, and Co), has been synthesized by nitriding bimetallic oxide precursors with ammonia gas via a temperature programmed reaction. The oxide precursors are prepared by conventional solid state reaction between two appropriate monometallic oxides. The synthesis involves passing NH3gas over the oxide precursors at a flow rate of 6.80×102μmol s−1(1000 cm3/min) and raising the temperature at a heating rate of 8.3×10−2K s−1(5 K/min) to a final temperature (Tf) which is held constant for a short period of time (thold). The oxynitrides thus obtained are pyrophoric and need to be passivated before exposing them to air. All these new bimetallic oxynitrides have a face centered cubic crystal structure and high values of surface area. The surface reactivation and the thermal stability of the materials are studied by temperature programmed reduction and this indicates that the compounds can be divided into three groups of different reducibility (high, medium, and low). Their surface activity and surface area are evaluated based on CO chemisorption and N2physisorption measurements. It is found that the chemisorbed CO number density correlates with the reducibility of the compounds.