Activation Energy For Hydrogenation Research Articles

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi5 (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi5 nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi5 transformed into Mg–Mg2Ni–LaH3 nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg2Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H2 in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H2 in less than 10 min at 623 K and 3.0 wt.% H2 in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol−1. The high sorption kinetics was explained by the nanostructure, catalysis of Mg2Ni and LaH3 nanoparticles, and the size reduction effect of Mg2Ni formation.

Impact of solvent for individual steps of phenol hydrodeoxygenation with Pd/C and HZSM-5 as catalysts

Impacts of water, methanol, and hexadecane solvents on the individual steps of phenol hydrodeoxygenation are investigated over Pd/C and HZSM-5 catalyst components at 473K in presence of H2. Hydrodeoxygenation of phenol to cyclohexane includes four individual steps of phenol hydrogenation to cyclohexanone on Pd/C, cyclohexanone hydrogenation to cyclohexanol on Pd/C, cyclohexanol dehydration to cyclohexene on HZSM-5, and cyclohexene hydrogenation to cyclohexane on Pd/C. Individual phenol and cyclohexanone hydrogenation rates are much lower in methanol and hexadecane than in water, while rates of cyclohexanol dehydration and cyclohexene hydrogenation are similar in three solvents. The slow rate in methanol is due to the strong solvation of reactants and the adsorption of methanol on Pd, as well as to the reaction between methanol and the cyclohexanone intermediate. The low solubility of phenol and strong interaction of hexadecane with Pd lead to the slow rate in hexadecane. The apparent activation energies for hydrogenation follow the order Ea phenol>Ea cyclohexanone>Ea cyclohexene, and the sequences of individual reaction rates are reverse in three solvents. The dehydration rates 1.1–1.8×103molmolBAS-1h-1 and apparent activation energies (115–124kJmol−1) are comparable in three solvents. In situ liquid-phase IR spectroscopy shows the rates consistent with kinetics derived from chromatographic evidence in the aqueous phase and verifies that hydrogenation of phenol and cyclohexanone follows reaction orders of 1.0 and 0.55 over Pd/C, respectively. Conversion of cyclohexanol with HZSM-5 shows first-order dependence in approaching the dehydration–hydration equilibrium in the aqueous phase.

Experimental investigation of ethylene hydroformylation to propanal on Rh and Co based catalysts

Intrinsic hydroformylation kinetics have been measured in a high-throughput kinetic test setup at temperatures varying from 448 to 498K, with the total pressure ranging from 1 to 3MPa. A gaseous feed containing CO, C2H4 and H2 was used with space times varying from 2.7 kgcat s/molC2H4,in to 149 kgcat s/molC2H4,in. Three catalysts have been investigated, i.e., 5%Rh on Al2O3, 1%Co on Al2O3 and 0.5%Co–0.5%Rh on Al2O3. The main products observed were ethane, propanal and propanol. The Rh catalyst showed the highest hydroformylation and hydrogenation site time conversions in the investigated range of operating conditions. Moreover it was found on all investigated catalysts that the hydrogenation activation energy was about 15–20kJmol−1 higher than that for hydroformylation. On the Rh catalyst, higher ethylene feed concentrations have a more pronounced effect on CO conversion and production of propanal and propanol compared with an increase in the inlet concentration of the other reactants.

Human aldosterone synthase: Recombinant expression in E. coli and purification enables a detailed biochemical analysis of the protein on the molecular level

Aldosterone, the most important human mineralocorticoid, is involved in the regulation of the blood pressure and has been reported to play a key role in the formation of arterial hypertension, heart failure and myocardial fibrosis. Aldosterone synthase (CYP11B2) catalyzes the biosynthesis of aldosterone by successive 11β- and 18-hydroxylation followed by an 18-oxidation of 11-deoxycorticosterone and thus comprises an important drug target. For more than 20 years, all attempts to purify recombinant human CYP11B2 in significant amounts for detailed analysis failed due to its hydrophobic nature as a membrane protein. Here, we present the successful expression of the protein in E. coli yielding approx. 90nmol/l culture, its purification and detailed enzymatic characterization. Biochemical analyses have been performed using in vitro conversion assays which revelead a Vmax of 238±8nmolproducts/nmolhCYP11B2/min and a Km of 103±8μM 11-deoxycorticosterone. Furthermore, binding analyses indicated a very loose binding of the first intermediate of the reaction, corticosterone with a Kd value of 115±6μM whereas for 11-deoxycorticosterone a Kd of 1.34±0.13μM was estimated. Upon substrate conversion of 11-deoxycorticosterone, new intermediates have been identified as 19- and 18-hydroxylated products not described before for the human enzyme. To understand the differences in substrate conversion, we constructed a new homology model based on the 3D structure of CYP11A1, performed docking studies and calculated the activation energy for hydrogen abstraction of the different ligands. The data demonstrated that the 11β-hydroxylation requires much less abstraction energy than hydroxylation at C18 and C19. However, the C18 and C19 hydroxylated products might be of clinical importance. Finally, purified CYP11B2 represents a suitable tool for the investigation of potential inhibitors of this protein for the development of novel drugs against hypertension and heart failure as was shown using ketoconazole.

Stable Hydrogen Storage Cycling in Magnesium Hydride, in the Range of Room Temperature to 300 °C, Achieved Using a New Bimetallic Cr-V Nanoscale Catalyst

We created a bimetallic chromium vanadium hydrogen sorption catalyst for magnesium hydride (MgH2). The catalyst allows for significant room-temperature hydrogen uptake, over 10 cycles, at absorption pressures as low as 2 bar. This is something that has never been previously achieved. The catalyst also allowed for ultrarapid and kinetically stable hydrogenation cycling (over 225 cycles) at 200 and at 300 °C. Transmission electron microscopy analysis of the postcycled samples revealed a nanoscale dispersion of Cr-V nanocrystallites within the Mg or MgH2 matrix. TEM analysis of the partially absorbed specimens revealed that even at a high absorption pressure, that is, a high driving force, relatively few hydride nuclei are formed at the surface of the pre-existing magnesium, ruling out the presence of any contracting volume (also termed contracting envelope or core shell) type growth. HRTEM of the cycled and desorbed powder sample demonstrated that the bcc Cr-V phase is crystalline and nanoscale. We experimentally demonstrated that the activation energy for hydrogen absorption is not constant but rather evolves with the driving force. This finding sheds new insight regarding the origins of the wide discrepancy in the literature – reported values of the hydrogenation activation energy in magnesium hydride and in related metal hydride systems.

Reaction Energetics for the Abstraction Process C2H3 + H2 → C2H4 + H

The fundamentally important combustion reaction of vinyl radical with hydrogen has been studied in the laboratory by at least five experimental groups. Herein, the reaction C2H3 + H2 → C2H4 + H has been examined using focal-point analysis. Molecular energies were determined from extrapolations to the complete basis-set limit using correlation-consistent basis sets (cc-pVTZ, cc-pVQZ, and cc-pV5Z) and coupled-cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples [CCSDT], and perturbative quadruples [CCSDT(Q)]. Reference geometries were optimized at the all-electron CCSD(T)/cc-pCVQZ level. Computed energies were also corrected for relativistic effects and the Born–Oppenheimer approximation. The activation energy for hydrogen abstraction is predicted to be 9.65 kcal mol–1, and the overall reaction is predicted to be exothermic by 5.65 kcal mol–1. Natural resonance theory (NRT) analysis was performed to verify the reaction pathway and describe bond-breaking and ...

Electrochemical sorption of hydrogen in single-wall carbon nanotubes encapsulated in palladium

The distribution of hydrogen in the palladium matrix and single-wall carbon nanotubes (SWCNs) in Pd-SWCN nanocomposite upon electrochemical hydrogenation is quantitatively studied in situ using voltammetry and chronoammetry. The high mobility of hydrogen in nanotubes is discovered and reflected in the low H-SWCN bond energy, which equals 0.073 ± 0.05 eV/H-atom and is close to the hydrogen activation energy (ɛH = 0.072–0.08 eV/H-atom) obtained using vacuum thermal desorption [7]. The presence of SWCN is shown to increase the hydrogen capacity of the Pd-SWCN composite compared to individual palladium by nearly 25%. The hydrogen storage capacity of nanotubes (CH) in Pd-SWCN composite increases with an increase in the Pd-to-SWCN volume ratio (V(Pd)/V(SWCN)). CH reaches the maximum limiting value of 12 wt % at V(Pd)/V(SWCN) > 12.

Adsorption and desorption of hydrogen in Mg nanoclusters: Combined effects of size and Ti doping

Using density functional theory formalism, we have investigated the interaction of hydrogen with pure and Ti doped Mg clusters. The objective of this study is two folds: (i) the reactivity of small Mg clusters in comparison to the extended Mg surface and (ii) the catalytic effect of Ti on the hydrogenation behavior. For Mg 55 cluster, the activation energy of hydrogenation is calculated to be 0.72 eV, which is 30% less than the bulk value of 1.04 eV. The interaction of hydrogen with Mg 55 and TiMg 54 clusters gives the binding energy of 0.217 and 0.164 eV, respectively. Moreover, the activation energy calculated by the elastic band method reveals that the dissociation barrier of hydrogen is 0.72 and 0.58 eV for Mg 55 and TiMg 54, respectively. Thus we could show a significant reduction in the activation barrier (almost 40%) of hydrogen dissociation in small clusters than the bulk. This has been attributed to the combined effects of the finite size of Mg clusters and the catalytic influence of Ti substitution. Further to underscore the hydrogen desorption mechanism, we have calculated the onset temperature of hydrogen diffusion using ab initio molecular dynamics simulation study on the hydrogenated Mg 55 cluster. The results reveal that at room temperature, the hydrogen atoms starts toggling from one Mg to another, which has been ascribed as the onset of hydrogen desorption.

Encumbering the intramolecular π donation by using a bridge: A strategy for designing metal-free compounds to hydrogen activation

On the basis of the FLP (frustrated Lewis pair) principle, a new strategy has been proposed to construct the “frustration” in designing metal-free hydrogen activation compounds, by using FMO (frontier molecular orbital) analyses and quantum mechanics calculations. Unlike the known FLPs which use bulky substituents to prevent them from forming stable Lewis acid/base complexes, the new approach encumbers the intramolecular π donation from the electron donor to the acceptor (e.g. in BH2NH2) by using a CH2 bridge (giving BH2CH2NH2). The strategy is simple and effective. Its effectiveness is demonstrated by the small hydrogen activation energy (12.0 kcal/mol) of the model molecule (BH2CH2NH2), which is significantly less than the 42.7 kcal/mol of BH2NH2 and also less than the 18.5 kcal/mol of BH2PH2 whose derivative, R2PB(C6F5)2, has been experimentally shown to be able to activate hydrogen. We also exemplified how to use the strategy to design experimentally more realizable molecules. The example shows promises as a hydrogen activation agent. The strategy can be used to design metal-free catalysts for direct hydrogenation.

Products and Mechanism of the Reaction of Chlorine Atoms with 3-Pentanone in 700−950 Torr of N2/O2 Diluent at 297−515 K

The products, kinetics, and mechanism of the reaction Cl + 3-pentanone have been measured by UV irradiation of Cl(2)/3-pentanone/N(2) (O(2)) mixtures using primarily GC analysis with selected cross checks by FTIR. In the absence of O(2), the products are 1- and 2-chloro-3-pentanone with yields of 21 and 78%, respectively. As the temperature is increased, the yield of 1-chloro-3-pentanone increases modestly relative to the 2-chloro-3-pentanone yield. On the basis of this increase, the activation energy for hydrogen abstraction at the 1 position is determined to be 500 (+/-500) cal mole(-1) relative to abstraction at the 2 position. In the presence of 500 ppm of O(2) with 900-1000 ppm of Cl(2) at 297 K, the yield of 2-chloro-3-pentanone decreases dramatically from 78 to 2.5%, while the 1-chloro-3-pentanone decreases only modestly from 21 to 17%. The observed oxygenated species are acetaldehyde (59%), 2,3-pentanedione (9%), and propionyl chloride (56%). Increasing the temperature to 420 K (O(2) = 500 ppm) suppresses these oxygenated products, and 2-chloro-3-pentanone again becomes the primary product, indicating that the O(2) addition reaction to the 2-pentanonyl radical has become reversible. At 500 K and 10 000 ppm O(2), a new product channel opens which forms a small yield ( approximately 4%) of ethylvinylketone. Computer modeling of the product yields has been performed to gain an understanding of the overall reaction mechanism in the presence and absence of O(2). The reaction of chlorine atoms with 3-pentanone proceeds with a rate constant of 8.1 (+/-0.8) x 10(-11) cm(3) molecule(-1) s(-1) independent of temperature over the range of 297-490 K (E(a) = 0 +/- 200 cal mole(-1)). Rate constant ratios of k(C(2)H(5)C(O)CHCH(3) + Cl(2))/k(C(2)H(5)C(O)CHCH(3) + O(2)) = 0.0185 +/- 0.0037 and k(C(2)H(5)C(O)CH(2)CH(2) + Cl(2))/k(C(2)H(5)CH(2)C(O)CH(2)CH(2) + O(2)) = 2.7 +/- 0.4 were determined at 297 K in 800-950 Torr of N(2)/O(2) diluent. In 800-950 Torr of N(2)/O(2) diluent, the major fate of the alkoxy radical CH(3)CH(O)C(O)C(2)H(5) is decomposition to give C(2)H(5)C(O) radicals and CH(3)CHO. These results show that the chemical mechanisms of the 3-pentanone reactions are very similar to those observed for butanone. In addition, the rate constants of the reactions of chlorine atoms with 1-chloro-3-pentanone [3 (+/-0.6) x 10(-11) over the range of 297-460 K], 2,3-pentanedione [1.4 (+/-0.3) x 10(-11) at 297 K], and ethylvinylketone [1.9 (+/-0.4) x 10(-10) over the range of 297-400 K, decreasing rapidly above 400 K] were measured at ambient pressure.

Hydrogen diffusion in titanium‐hydride observed by the diffraction‐enhanced X‐ray imaging method

AbstractThe X‐ray refraction imaging technique was applied to the quantitative study of hydrogen diffusion in titanium‐hydride. Diffusivity and activation energy of hydrogen diffusion were determined by direct observation of the hydride. The hydride was formed on the surface of titanium by electrolytic‐charging at 31, 63 and 98 °C. The specimens were cut into 1 mm thick slices for cross‐sectional observations. The hydride layer was observed using the diffraction‐enhanced X‐ray imaging (DEI) method. Boundaries between titanium and the hydride were observed as thick black or white lines parallel to the specimen surface in the DEI images similar to previously reported results. Hydride distribution caused by hydrogen diffusion from the surface was calculated using an assumed hydrogen diffusivity value and from the solution of the appropriate diffusion equation. The results were converted to the intensity profiles of refraction images of the hydride using the measured rocking curve from an analyzer. The calculated intensity profiles were compared to those obtained from the experimental results and the fitted diffusivity was determined by trial and error. The activation energy of hydrogen in titanium‐hydride thus obtained, 0.55 ± 0.07 eV, showed good agreement with the widely accepted value obtained by the internal friction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Kinetics of the Hydrogenation of Sunflower Oil over Alumina Supported Palladium Catalyst

The present work studies the sunflower oil hydrogenation on supported palladium catalysts, by analyzing the surface kinetics and the mass transfer limitations of products and reactants. Initially, a simplified model was studied. This model took into account only the consecutive hydrogenation of linoleic acid (diene), to reach the production of oleic (monoene) and stearic (saturated) acids. Using the adjusted values of the kinetic constants and the activation energies of the hydrogenation obtained with this model, a new scheme was investigated considering the geometric isomerization reactions (cis-trans). The diene hydrogenation constant was larger than that of the monoene. This fact confirms the higher reaction rate of the diene hydrogenation in comparison with that of the monoene. With respect to the isomerization rates, these have an activation energy superior to that of the monoene hydrogenation, and slightly superior to the diene hydrogenation activation energy. This fact verifies the influence of temperature on the formation of trans-isomers.

Prediction of Activation Energies for Hydrogen Abstraction by Cytochrome P450

We have estimated the activation energy for hydrogen abstraction by compound I in cytochrome P450 for a diverse set of 24 small organic substrates using state-of-the-art density functional theory (B3LYP). We then show that these results can be reproduced by computationally less demanding methods, for example, by using small organic mimics of compound I with both B3LYP and the semiempirical AM1 method (mean absolute error of 3-4 kJ/mol) or by calculating the bond dissociation energy, without relaxation of the radical (B3LYP) or estimated from three-point fit to a Morse potential (AM1; errors of 4 and 5 kJ/mol, respectively). We can assign activation energies of 74, 61, 53, 47, and 30 kJ/mol to primary carbons, secondary/tertiary carbons, carbons with adjacent sp(2) or aromatic groups, ethers/thioethers, and amines, respectively, which gives a very simple and predictive model. Finally, some of the less demanding methods are applied to study the CYP3A4 metabolism of progesterone and dextromethorphan.

The chemistry of ethylene on MoAl alloy thin films formed on dehydroxylated alumina: Hydrogenation, dehydrogenation and H–D exchange reactions

The chemistry of ethylene adsorbed on a thin MoAl layer grown in ultrahigh vacuum on a thin alumina film is studied using a combination of temperature-programmed desorption and X-ray, Auger and reflection absorption infrared spectroscopies. Both di-σ-bonded and a small proportion of π-bonded ethylene are found, where the di-σ-bonded ethylene has a σ/π parameter of ∼0.8 and a heat of adsorption of ∼70 kJ/mol. The ethylene self-hydrogenates to yield ethane and a small amount of methane is detected. The surface hydrogenation activation energy of di-σ-bonded ethylene is ∼65 kJ/mol, while the π-bonded species hydrogenates more easily. Adsorbed ethyl species grafted onto the surface by decomposing ethyl iodide predominantly undergo β-hydride elimination to yield ethylene. Ethyl species hydrogenate to ethane at a lower temperature than does di-σ-bonded ethylene implying that addition of hydrogen to adsorbed ethylene is slower than the rate of ethyl hydrogenation.

The preparation characteristics of hydrogen permselective membranes for higher performance in nuclear hydrogen production

The thermochemical water-splitting method has been proposed as a clean method for hydrogen production. The IS (iodine–sulphur) process is one of the thermochemical water-splitting processes. The silica membrane to apply in membrane reactor for the HI decomposition reaction improvement of the IS process was prepared by sol-gel and thermal CVD (chemical vapour deposition) methods. In the prepared membrane by sol-gel method, the permeation mechanism of hydrogen and nitrogen gas is dominated with the activated diffusion and Knudsen diffusion, respectively. The activation energy of hydrogen was ca. 5 kJ/mol. In the prepared membrane by thermal CVD method, the deposited silica particle size was obtained less than 11 nm and the deposited thickness layer was about 2 µm. The selectivity of H2 to N2 was 26.3 at 450°C, and the permeance of H2 was 6 × 10-8 mol Pa-1 m-2 s-1.

Activation energy of hydrogen, oxygen, and nitrogen diffusion in metals

Significant progress has recently been made in experimental studies of the diffusion of atoms of light elements in metals. In particular, for a number of metals, the diffusion coefficients of tracer atoms of hydrogen (tritium), oxygen, nitrogen, and boron were measured. Previously, such data were available only for carbon. New experimental data enabled us in this work to formulate the problem of developing a theoretical criterion for the reliability of experimental results on the activation energy Q for the interstitial diffusion mechanism. The necessity of formulating such a problem is primarily caused by the fact that the scatter of experimental data on the diffusion of interstitial impurities in metals is often very wide. For example, experimental values of the activation energy Q of hydrogen diffusion in α -Ti are within the range 24‐62 kJ/mol [1]. In this context, in this work, we analyzed experimental data on the diffusion of interstitial atoms in metals that were obtained by reliable methods. It was found that all such data on the activation energy of hydrogen, oxygen, and nitrogen diffusion are consistent with a calculation model [2, 3] based on the theory of elasticity. The calculation accuracy corresponds to the level of a modern precision experiment (on the order of 1%). The corresponding data for carbon and boron were excluded from the analysis. This is because the adequacy of the elastic model of the potential barrier for carbon was not confirmed by the results of measuring the anisotropy of the diffusion coefficients in α -Ti [4, 5]. Data on the diffusion of tracer boron atoms in metals are extremely meager and did not allow us to analyze relationships for boron. Elastic model of the potential barrier. The currently dominant concept of the nature of Q and procedures for calculating Q was formulated in the midtwentieth century [2, 3]. Within this concept, the activation energy Q for the interstitial diffusion mechanism is determined by the elastic energy of crystal strain when an impurity atom is placed at a saddle point; i.e., the larger the diffusant atom, the higher the activation energy Q . The expression for Q has the form [2]

Mechanistic aspects of hydrogen abstraction for phenolic antioxidants. Electronic structure and topological electron density analysis

Density functional calculations using the B3LYP functional are used to provide insight into the hydrogen abstraction mechanism of phenolic antioxidants. The energy profiles for 13 ortho, meta, para and di-methyl substituted phenols with hydroperoxyl radical have been determined. An excellent correlation between the enthalpy (DeltaH) and activation energy (DeltaEa) was found, obeying the Evans-Polanyi rule. The effects of hydrogen bonding on DeltaEa are also discussed. Electron donating groups at the ortho and para positions are able to lower the activation energy for hydrogen abstraction. The highly electron withdrawing fluoro substituent increases the activation energies relative to phenol at the meta position but not at the para position. The electron density is studied using the atoms in molecules (AIM) approach. Atomic and bond properties are extracted to describe the hydrogen atom abstraction mechanism. It is found that on going from reactants to transition state, the hydrogen atom experiences a loss in volume, electronic population and dipole moment. These features suggest that the phenol hydroperoxyl reactions proceed according to a proton coupled electron transfer (PCET) as opposed to a hydrogen atom transfer (HAT) mechanism.

Chemistry of the t-butoxyl radical: evidence that most hydrogen abstractions from carbon are entropy-controlled.

Absolute rate constants and Arrhenius parameters for hydrogen abstractions (from carbon) by the t-butoxyl radical ((t) BuO.) are reported for several hydrocarbons and tertiary amines in solution. Combined with data already in the literature, an analysis of all the available data reveals that most hydrogen abstractions (from carbon) by (t) BuO. are entropy controlled (i.e., TdeltaS > deltaH, in solution at room temperature). For substrates with C-H bond dissociation energies (BDEs) > 92 kcal/mol, the activation energy for hydrogen abstraction decreases with decreasing BDE in accord with the Evans-Polanyi equation, with alpha approximately 0.3. For substrates with C-H BDEs in the range from 79 to 92 kcal/mol, the activation energy does not vary significantly with C-H BDE. The implications of these results in the context of the use of (t) BuO. as a chemical model for reactive oxygen-centered radicals is discussed.

CO Poisoning of Ethylene Hydrogenation over Pt Catalysts: A Comparison of Pt(111) Single Crystal and Pt Nanoparticle Activities

The ethylene hydrogenation reaction was studied on two platinum model catalyst systems in the presence of carbon monoxide to examine poisoning effects. The catalysts were a Pt(111) single crystal and lithographically fabricated platinum nanoparticles deposited on alumina. Gas chromatographic results for Pt(111) show that CO adsorption reduces the turnover rate from 101 to 10-2 molecules/Pt site/s at 413 K, and the activation energy for hydrogenation on the poisoned surface becomes 20.2 ± 0.1 kcal/mol. The activation energy for ethylene hydrogenation over Pt(111) in the absence of CO is 10.8 kcal/mol. The Pt nanoparticle system shows the same rate for the reaction as over Pt(111) in the absence of CO. When CO is adsorbed on the Pt nanoparticle array, the rate of the reaction is reduced from 102 to 100 nmol/s at 413 K. However, the activation energy remains largely unchanged. The Pt nanoparticles show an apparent activation energy for ethylene hydrogenation of 10.2 ± 0.2 kcal/mol in the absence of CO and 11.4 ± 0.6 kcal/mol on the CO-poisoned nanoparticle array. This is the first observation of a significant difference in catalytic behavior between Pt(111) and the Pt nanoparticle arrays. It is proposed that the active sites at the oxide--metal interface are responsible for the difference in activation energies for the hydrogenation reaction over the two model platinum catalysts.

Carbon dioxide reforming of methane over 5 wt.% Ni/CaO-Al 2O 3 catalyst

Methane reforming by carbon dioxide was investigated over 5 wt.% Ni/CaO-Al 2O 3 catalyst. X-ray diffraction (XRD) and temperature-programmed reduction (TPR) techniques were applied to characterise the catalyst. The catalyst exhibited high activity and very good stability at stoichiometric methane and carbon dioxide feed. The addition of steam in the reacting mixture was tested and proved beneficial for the conversion of methane and the drastic decrease in carbon deposition. The kinetic behaviour of the catalyst was investigated as a function of temperature and methane and carbon dioxide partial pressures. The apparent activation energies of the two reactants CH 4 and CO 2 were estimated 25.5±2.0 and 23.6±1.8 kcal/mol, respectively and that of CO was 24.6±1.2 kcal/mol while hydrogen activation energy was estimated at 35.2±3.2 kcal/mol. Partial pressure dependencies of the reaction rates were obtained at 630 °C. The increase of H 2 partial pressure resulted in an acceleration of the CO formation, while an increase in CO partial pressure demonstrated the inhibiting role in H 2 formation and the conversion of the reactants.