Mechanism of methanol decomposition on the Pd/WC(0001) surface unveiled by first-principles calculations

In this study, the decomposition of methanol into the CO and H species on the Pd/tungsten carbide (WC)(0001) surface is systematically investigated using periodic density functional theory (DFT) calculations. The possible reaction pathways and intermediates are determined. The results reveal that saturated molecules, i.e., methanol and formaldehyde, adsorb weakly on the Pd/ WC(0001) surface. Both CO and H prefer three-fold sites, with adsorption energies of −1.51 and −2.67 eV, respectively. On the other hand, CH₃O stably binds at three-fold and bridge sites, with an adsorption energy of −2.58 eV. However, most of the other intermediates tend to adsorb to the surface with the carbon and oxygen atoms in their sp³ and hydroxyl-like configurations, respectively. Hence, the C atom of CH₂OH preferentially attaches to the top sites, CHOH and CH₂O adsorb at the bridge sites, while COH and CHO occupy the three-fold sites. The DFT calculations indicate that the rupture of the initial C–H bond promotes the decomposition of CH₃OH and CH₂OH, whereas in the case of CHOH, O–H bond scission is favored over the C–H bond rupture. Thus, the most probable methanol decomposition pathway on the Pd/WC(0001) surface is CH₃OH → CH₂OH → trans-CHOH → CHO → CO. The present study demonstrates that the synergistic effect of WC (as carrier) and Pd (as catalyst) alters the CH₃OH decomposition pathway and reduces the noble metal utilization.     

Mechanistic Aspects of Hydrogen Production by Partial Oxidation of Methanol Over Cu/ZnO Catalysts

In this work, mechanistic aspects of the partial oxidation of methanol (POM) to hydrogen and carbon dioxide over Cu/ZnO catalysts have been investigated. The data obtained with different catalyst compositions and different Cu^o metal surface areas showed that the reaction depends on the presence of both the phases ZnO and Cu^o. On the other hand, for catalysts with Cu concentrations in the range 40-60 wt%, the copper metal surface area seems to be the main factor determining the reaction rate. Kinetic isotope effects using CH₃OH and CH₃OD showed that both C–H and O–H bonds are at least partially involved in the rate-limiting step. TPD experiments with pure Cu^o, pure ZnO and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface methanol can form intermediates which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H₂, HD, D₂, H₂O, HDO and D₂O in the temperature-programmed reaction of CH₃OD revealed a slight enrichment of the products with H, suggesting that during methanol activation on the ZnO some of the D atoms might be retained by the support. The effect of oxygen partial pressure suggests that oxygen atoms on the copper surface strongly promote methanol activation and H₂ and CO₂ formation. It is proposed that oxygen atoms participate in methanol activation by the abstraction of the hydroxyl H atom to form methoxide and OH_surf. This OH_surf species rapidly loses H to the surface regenerating the O_surf.     

Reaction kinetics of phenol synthesis through one-step oxidation of benzene with N<Subscript>2</Subscript>O over Fe-ZSM-5 zeolite

Based on the information from GC-MS on-line measurement and thermodynamic analysis, the reaction network of gas-phase hydroxylation of benzene with nitrous oxide over Fe-ZSM-5 zeolite was systematically investigated. The main reactions and side reactions were identified, and a kinetic reaction network was proposed as follows: benzene+N₂O→phenol→CO/CO₂. According to the mechanism, the experimental results were interpreted reasonably. The hydroxylation kinetic experiments were carried out in an isothermal integral microreactor under the conditions of n(benzene)/n(N₂O)=8–12, T=663–763 K and atmospheric pressure. Based on the reaction network proposed, the parameters in the rate model of power-law were estimated by means of Gauss-Newton optimal method with the Levenberg-Marquardt modifications, and the results were in good agreement with the experimental data.     

Removal of gossypol from cottonseed by solvent extraction procedures

Summary The relative efficiencies of organic, polar solvents and of solvent-water pairs for use in the extraction of gossypol and related compounds from cottonseed flakes were determined in a specially devised glass laboratory extractor. Of the solvents tested a butanone-water pair containing 10% of water by volume was the most effective, and chlorine-substituted hydrocarbons were the least effective. Under equilibrium conditions maximum extraction of gossypol was obtained with a butanone solvent containing 2.5% of water by weight. The rate of extraction of gossypol from cottonseed meal with butanone-water pairs increased with increase in the amount of water in the system and with increase in temperature of the extraction system. The greater amounts of water in the extraction system resulted in swelling and packing of the flakes and in a decrease in extraction efficiency. Flakes extracted at 26°C. contained 0.08% free gossypol and those extracted at 71°, 0.054%. This decrease may be due, in part, to the reaction of gossypol with the protein to form bound gossypol. Report of a study carried on under the Research and Marketing Act of 1946. This paper is No. 9 in the series on “Processing of Cottonseed” from the Southern Regional Research Laboratory. References to other papers in this series are: J. Am. Oil Chem. Soc.24, 97–108 (1947); J. Am. Oil Chem. Soc.24, 276–283 (1947); J. Am. Oil Chem. Soc.24. 362–369 (1947); J. Am. Oil Chem. Soc.26, 28–34 (1949); Oil Mill Gaz.54 (2), 12–15 (1949); Cotton Gin and Oil Mill Press51 (9), 18–20 (1950); Official Proc. Natl. Cottonseed Products Asso,55, 32–34, 36 (1951); and J. Am. Oil Chem. Soc. (in press), “The Effect of Screw Press and Hydraulic Press Processing Conditions on Pigment Glands of Cottonseed”, by D. M. Batson. F. H. Thurber, and A. M. Altschul. One of the laboratories of the Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, U. S. Department of Agriculture.     

Partial oxidation of methanol on TiO2- supported 12-molybdophosphate catalysts

Steam reforming of ethanol, C₂H₅OH+H₂O→2CO+4H₂, was carried out over Co/Al₂O₃, Co/SiO₂, Co/MgO, Co/ZrO₂ and Co/C. The properties of the Co catalysts were greatly affected by the supports. Co/Al₂O₃ exhibited the highest selectivity for steam reforming of ethanol by suppression of methanation of CO and decomposition of ethanol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Structural change of Cu/ZnO by reduction of ZnO in Cu/ZnO with methanol

The reducibility of ZnO was investigated in the temperature range of 523–623 K in a stream of a reducing agent such as H₂, CO, and methanol. ZnO was reduced only in the presence of copper in the vicinity of ZnO with CO and methanol, but it was not reduced with H₂. Methanol was a stronger reducing agent in the reduction of ZnO than CO, while CO was stronger in the reduction of CuO than methanol. Two types of brass were observed resulting from the reduction of ZnO in the Cu/ZnO sample by XRD. Zanghengite brass started to be formed at 573 K in addition to α-brass which was observed at the temperature above 523 K in the temperature range of 523–623 K during the ZnO reduction with methanol. The carbon monoxide chemisorption showed that the copper surface areas decreased during the reduction of ZnO with methanol. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular surface science of C–H bond activation and polymerization catalysis

Surface science studies of heterogeneous catalysis use model systems ranging from single crystals to monodispersed nanoparticles in the 1–10 nm range. Molecular studies reveal that bond activation (C–H, H–H, C–C, C≡O) occurs at 300 K or below as the active metal sites simultaneously restructure. The strongly adsorbed molecules must be mobile to free up these sites for continued turnover of reaction. Oxide–metal interfaces are also active for catalytic turnover. Examples using C–H and C = O activation are described to demonstrate these properties. Polymerization catalysis demonstrates a strong dependence upon catalyst surface structure, which allows for the selectivity to be tuned by the choice of Ziegler-Natta surface preparation. Novel preparation methods of model catalyst arrays in two and three dimensions are opening the door to a complete understanding of catalytic reaction selectivity.     

The Effect of Carbon Monoxide Co-Adsorption on Ni-Catalysed Water Dissociation

The effect of carbon monoxide (CO) co-adsorption on the dissociation of water on the Ni(111) surface has been studied using density functional theory. The structures of the adsorbed water molecule and of the transition state are changed by the presence of the CO molecule. The water O–H bond that is closest to the CO is lengthened compared to the structure in the absence of the CO, and the breaking O–H bond in the transition state structure has a larger imaginary frequency in the presence of CO. In addition, the distances between the Ni surface and H₂O reactant and OH and H products decrease in the presence of the CO. The changes in structures and vibrational frequencies lead to a reaction energy that is 0.17 eV less exothermic in the presence of the CO, and an activation barrier that is 0.12 eV larger in the presence of the CO. At 463 K the water dissociation rate constant is an order of magnitude smaller in the presence of the CO. This reveals that far fewer water molecules will dissociate in the presence of CO under reaction conditions that are typical for the water-gas-shift reaction.     

Production of H2 for fuel cell applications: methanol steam reforming with sufficiently thorough cleaning of H2 from CO impurity

Methanol steam reforming (MSR) and preferential CO oxidation (PROX) were studied with the view of improving the generation of H₂-rich gases. In MSR, conventional catalysts of methanol synthesis were tested, various Cu-based catalysts were prepared and studied. A theoretic kinetic model (based on the reaction mechanism established using independent methods [1]) is developed and checked out. PROX was studied over various Ru/Al₂O₃ catalysts using a flow “quasi-adiabatic” reactor. On-line recording of gas temperature in the catalyst bed and CO residual concentration at varied reaction conditions allowed to observe ignition and extinction of the catalyst surface and the transition states of the process. It is shown that in the ignition mode a sharp decrease in CO residual concentration can be achieved. The combination of proposed catalyst and the control of the macrokinetic regime of PROX allows high degree of CO removal from gaseous mixtures produced by MSR. Residual CO content in a H₂-rich gaseous mixture can be lowered to < 15 ppm at GHSV∼100 m³/(kg cat)/h and O₂/CO ratio of 1. Obtained data show the possibility of designing a high-throughput set-up for generation of H₂-rich gases from methanol with one-step cleaning from the CO impurity.     

Intrinsic kinetics study of LPDME process from syngas over bi-functional catalyst

The intrinsic kinetics of the three-phase dimethyl ether (DME) synthesis from syngas over a bi-functional catalyst has been investigated in a agitated slurry reactor at 20–50 bar, 200–240 °C and H₂/CO feed ratio from 1 to 2. The bi-functional catalyst was prepared by physical mixing of CuO/ZnO/Al₂O₃ as methanol synthesis catalyst and H-ZSM-5 as methanol dehydration catalyst. The three reactions including methanol synthesis from CO and H₂, methanol dehydration and water gas shift reaction were chosen as the independent reactions. A kinetic model for the combined methanol and DME synthesis based on a methanol synthesis model proposed by Graaf et al. [G.H. Graaf, E.J. Stamhuis, A.A.C.M. Beenackers, Kinetics of low pressure methanol synthesis, Chem. Eng. Sci. 43 (12) (1988) 3185; G.H. Graaf, E.J. Stamhuis, A.A.C.M. Beenackers, Kinetics of the three-phase methanol synthesis, Chem. Eng. Sci. 43 (8) (1988) 2161] and a methanol dehydration model by Bercic and Levec [G. Bercic, J. Levec, Intrinsic and global reaction rate of methanol dehydration over γ-Al₂O₃ pellets, Ind. Eng. Chem. Res. 31 (1992) 399–434] has been fitted our experimental data. The obtained coefficients in equations follow the Arrhenius and the Van’t Hoff relations. The calculated apparent activation energy of methanol synthesis reaction and methanol dehydration reaction are 115 kJ/mol and 82 kJ/mol, respectively. Also, the effects of different parameters on the reactor performance have been investigated based on the presented kinetic model.     

Promotion and inhibition of oxidation of rich hydrogen-air mixtures by nitric oxides (NO and NO<Subscript>2</Subscript>) during adiabatic self-ignition

The tracer method was used to numerically study the effect of nitric oxides (NO and NO₂) on the oxidation of rich hydrogen-air mixtures during adiabatic self-ignition at low and high initial temperatures and a pressure of 0.1 MPa. At low temperatures, the added NO interacts with HO₂ to form NO₂, and NO₂ then interacts with H to form NO. When NO₂ is added at the same temperatures, a two-stage mechanism takes place: NO formed by the reaction NO₂ + H is not involved in the reaction until NO₂ is almost completely consumed. In the temperature range 900–1200 K, NO₂ inhibits self-ignition through participation in the reaction with H, leading to the replacement of part of the completely branched chain H → (O, OH) → 3H by the unbranched chain H → OH → H. At low initial temperatures, NO effectively promotes hydrogen oxidation due to replacement of the unbranched chain H → HO₂ → H₂O₂ → OH → H by a chain with branching.     

Changes in the hydrogenative ring-opening mechanism of cyclohexene oxide over Cu/SiO2 resulting from the addition of cyclohexene, a major product

The ring-opening mechanism influencing effect of a major product in the cyclohexene oxide–D₂ system was investigated over a Cu/SiO₂ catalyst. This product is cyclohexene, thus, the hydrogenative ring opening of a 1:1 cyclohexene oxide–cyclohexene mixture was studied in the presence of D₂ at 403 K in a closed circulation reactor. It was found that the mechanism of single C–O scission was not affected, but that of the double C–O scission was changed. Simultaneous bond cleavage was the major route of ring opening in the additive-free system and it became consecutive on cyclohexene addition. Added cyclohexene was hydrogenated with a very low rate, but it transformed the surface of the catalyst and, thus, facilitated the change in the mechanism. An explanation concerning the seemingly anomalous lack of deuterium in a product (cyclohexane) not seen in the additive-free system is also suggested. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of Pretreatment Atmosphere on CuO/TiO2 Activities in NO + CO Reaction

Using TiO₂ as carrier, CuO/TiO₂ catalysts with different CuO loading were prepared by the impregnation method. The catalytic activities in NO+CO reaction were examined with a micro-reactor gas chromatography reaction system and the methods of TPR, XPS and NO-TPD. It was found that the catalytic activities were affected by pretreatment atmosphere, i.e. H₂ atmosphere > reduction–reoxidation > 10%CO/He > reaction gas (fresh sample). NO decomposition was better by low-valence Cu species than by high-valence Cu species, i.e. Cu⁰>Cu⁺>Cu²⁺. The XPS results indicated that Cu species on CuO/TiO₂ were Cu⁰, Cu⁺, normal Cu²⁺(Cu²⁺(I)) and chain-structured Cu²⁺(Cu²⁺(II)) as –Cu–O–Ti–O–. The activities of Cu²⁺(II) were much higher than that of Cu²⁺(I), but both species were very unstable in the reaction atmosphere and easily reduced by CO, which accounted for the variable activities of fresh catalysts with increasing reaction temperature. In NO+CO reaction, the redox process was a cycle of Cu⁺–Cu²⁺(I) at low reaction temperature but was a cycle of Cu⁰–Cu⁺ at high reaction temperature. As shown by NO-TPD, high catalytic activities could be attributed to the following factors, e.g. oxygen caves on the catalyst’s surface after pretreatment with H₂ and reduction–reoxidation, formation of Cu⁰ after pretreatment with H₂, and increment of Cu species dispersion and formation of Cu²⁺(II) after pretreatment with reduction–reoxidation.     

Estimate of methane production from rumen fermentation

A method for the assessment of CH₄ emission from dairy cows, based on in vitro volatile fatty acids (VFA) production, is described. 15 energy rich feedstuffs, 12 protein feedstuffs and 15 forages were in vitro fermented using rumen fluid as inoculum. Methane production for each feedstuff was calculated from net concentration of volatile fatty acids after 24 hours of fermentation according to the following stoechiometry: Glucose + 2 H₂O → 2 acetate + 2 CO₂ + 4 H₂; Glucose + 2 H₂ → 2 propionate + 2 H₂O ; Glucose → 1 butyrate + 2 CO₂ + 2 H₂; CO₂ + 4 H₂ → CH₄ + 2 H₂O and taking in account the `dilution rate' of feedstuffs, estimated according to the Cornell University model. These data were used to estimate the CH₄ production in cows (live weight 650 kg, annual milk production:7550 kg, fat content 3.8%). Calculated annual methane emissions based on in vitro trials were: 182.74 kg CH₄ /cow or 137.05 kg C–CH₄ /cow (diet with corn silage); 182.56 kg CH₄ /cow or 136.92 kg C–CH₄/cow (diet without corn silage). If compared with estimastes obtained from IPPC (1996) detailed methodology, the above estimates is 35% higher. A critical evaluation of reliability of in vitro data is given. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetic modeling of non-hydrocarbon/nitric oxide interactions in a flow reactor above 1,400K

The reduction of nitric oxide by reaction with non-hydrocarbon fuels under reducing conditions at comparatively higher temperature has been studied with a detailed chemical kinetic model. The reaction mechanism consists of 337 elementary reactions between 65 chemical species based on the newest rate coefficients. The experimental data were adopted from previous work. Analyses by comparing existing experimental data with the modeling predictions of this kinetic mechanism indicate that, at comparatively high temperature, apart from the reaction path NO→HNO→NH→N₂, NO+N→N₂ is also prominent. In the presence of CO, NO is partly converted to N by reaction with CO. Based on present model, the reduction of NO at high temperature, which was usually underestimated by previous work, can be improved to some extent. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

Intrinsic kinetics of one-step dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst

The reaction kinetics of the dimethyl ether synthesis from hydrogen-rich synthesis gas over bi-functional catalyst was investigated using an isothermal integral reactor at 220–260°C temperature, 3–7 MPa pressure, and 1,000–2,500 mL/g·h space velocity. The H₂/CO ratio of the synthetic gas was chosen between 3 : 1 and 6 : 1. The bi-functional catalyst was prepared by physically mixing commercial CuO/ZnO/Al₂O₃ and γ-alumina, which act as methanol synthesis catalyst and dehydration catalyst, respectively. The three reactions, including methanol synthesis from CO and CO₂ as well as methanol dehydration, were chosen as independent reactions. The Langmuir-Hinshelwood kinetic models for dimethyl ether synthesis were adopted. Kinetics parameters were obtained using the Levenberg-Marquardt mathematical method. The model was reliable according to statistical and residual error analyses. The effects of different process conditions on the reactor performance were also investigated.     

Biosynthesis of tetrahydrofuranyl fatty acids from linoleic acid by <Emphasis Type="Italic">clavibacter</Emphasis> sp. ALA2

Clavibacter sp. ALA2 converts linoleic acid into many novel oxygenated products including hydroxy FA and tetrahydrofuranyl unsaturated FA (THFA). One of them was tentatively identified by GC-MS as 12,13,16-trihydroxy-9(Z)-octadecenoic acid (12,13,16-THOA) (Hou, C.T., H.W. Gardner, and W. Brown, J Am. Oil Chem. Soc. 78∶1167–1169, 2001). We have separated and purified 12,13,16-THOA from its isomer, 12,13,17-THOA, by silica gel column chromatography and by preparative TLC. Its structure was then confirmed by proton and ¹³C NMR analyses. Purified 12,13,16-THOA was used as a substrate to study the biosynthesis of THFA. Within 24 h of incubation, cells of strain ALA2 converted 12,13,16-THOA to both 12-hydroxy-13,16-epoxy-9(Z)-octadecenoic acid (12-hydroxy-THFA) and 7,12-dihydroxy-13,16-epoxy-9(Z)-octadecenoic acid (7,12-dihydroxy-THFA). The relative abundance of 7,12-dihydroxy-THFA increased with incubation time, whereas that of 12,13,16-THOA and of 12-hydroxy-THFA decreased. Therefore, the biosynthetic pathway of THFA from linoleic acid by strain ALA2 is as follows: linoleic acid→12,13-dihydroxy-9(Z)-octadecenoic acid→12,13,16-THOA→12-hydroxy-THEA→7,12-dihydroxy-THFA.     

Role of atomic hydrogen diffusion in a hydrogen flame

A numerical modeling study of the propagation of a laminar flat homogeneous gas flame has shown that in a hydrogen-air flame, a rapid increase in the concentration of OH radicals begins in the range of low temperatures and the concentration profile has two maxima. The first maximum in the low-temperature region of the front is related to the diffusion of H atoms, formation of HO₂ radicals, and the quadratic branching reaction H + HO₂ → OH + OH. The second maximum in the OH concentration profile is due to the classical high-temperature branching reactions H + O₂ → OH + O and O + H₂ → OH + H. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 3–9, March–April, 2007.     

Unique adsorption and catalytic behavior of H2 and CO over hollow Silica-Rh, -Ir, and -Ru nanocomposites prepared by Reversed Micelle Technique

Hollow silica nano spheres containing Rh, Ir or Ru metal particles were synthesized by Rh(NH₃)₆Cl₃ aq, Ir(NH₃)₃Cl₃ aq or Ru(NH₃)₆Cl₃ aq/NP-6/cyclohexane reversed micelle system. Hydrolysis of TEOS surrounding metal ammine complex crystals inside the micelle caused the formation of the hollow, which contained small metal particles inside and tiny metal clusters in the silica network. The amounts of H₂ adsorption over Rh and Ir nanocomposites were two to three times more in the cases of hollow-SiO₂ catalysts compared with those of non-hollow ones, suggesting the occlusion of hydrogen inside the hollows of Rh–SiO₂ or Ir–SiO₂. CO molecules could also permeate into the silica wall and be adsorbed on the metal clusters in the silica wall after 573 K pretreatment. Especially in the case of Ru nanocomposite the amount of adsorbed CO was much more than that of H₂, suggesting some unique character of Ru metal nanoparticles. After 773 K pretreatment, however, the amount of CO(a) decreased drastically to less than 1/10 of H(a), indicating the densification of Si–O–Si bonds and the formation of ultra-micropores in the silica wall where only H₂ can selectively permeate. Selective formation of methane was observed in the CO–H₂ reaction over these nanocomposite catalysts, provably because of the higher concentration of hydrogen inside the hollow and silica network.     

The transition state for metal-catalyzed dehalogenation

Substituent effects have been used to probe the nature of the transition state to catalytic carbon–halogen bond breaking. Kinetics measurements have determined the activation energies (E _act to C–Cl bond breaking on the Pd(111) surface and C–I bond breaking on the Pd(111) and Ag(111) surfaces. These barriers have been measured using alkyl halides with varying degrees of fluorine substitution. The activation energies have been correlated with the inductive or field substituent constants (σ_F) of the fluorinated alkyl groups in order to determine reaction constants (E _act=E₀+ρσ_F) for the dehalogenation reactions. In all three cases it has been found that the barriers are insensitive to inductive substituent effects and the reaction constants are all relatively small: ρ= −0.5± 1.0 kcal/mol for C–Cl cleavage on Pd(111), ρ= −0.3±0.8 kcal/mol for C–I cleavage on Pd(111), and ρ= −2.9±0.4 kcal/mol for C–I cleavage on Ag(111). This implies that the transition state for dehalogenation is homolytic and occurs early in the reaction coordinate. The implications of this result are discussed for catalytic dehalogenation processes such as hydrodechlorination. This revised version was published online in July 2006 with corrections to the Cover Date.