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Abstract: The CO2 capture properties of a new nanocrystalline CaO-based mixed oxide (CaCeZrO x ) were studied in both TGA and a fixed bed reactor. The kinetics of CO2 capture at dry conditions was compared between these two reactors. The application of the new CO2 acceptor in sorption enhanced steam methane reforming was studied by a fixed-bed reactor in the presence of 40wt% nickel hydrotalcite catalysts. Experimental results show that the new CO2 acceptor, CaCeZrO x has a significantly improved CO2 capture capacity and cyclic reaction stability compared to natural dolomites, because of the large pores and stable backbone structure. It has also been observed that the sorption enhanced steam methane reforming in the fixed bed reactor produces more than 95% H2 in a single-step process. Increase in production of hydrogen and higher CO2 capture capacity are also observed with an increase in the residence time of methane. Steam has a significant effect on the stability of the acceptors, but not very much on the kinetics and capacity of CO2 capture. The capacity of CO2 capture on the acceptors in the SESMR process cannot be predicted by the independent CO2 capture kinetic study on acceptor alone. A synergy effect between the sorption enhanced reactions and CO2 capture on the CO2 capture capacity was observed on the all solid CaO based CO2 acceptors. [Copyright &y& Elsevier]

Abstract: Pre-reforming of natural gas has been studied on a nickel catalyst at 480–550°C and 20bar using a TEOM (Tapered Element Oscillating Microbalance) reactor. The focus has been on carbon formation and the main objective was to study the influence of C2–C3 hydrocarbons in the methane feed on carbon deposition on the catalyst. Coking thresholds for different mixtures of hydrocarbons were determined by varying the steam to carbon (S/C) ratio at various temperatures. The steady-state coking rate decreases with increasing S/C ratio, and increases with increasing carbon number of the hydrocarbon. Unsaturated hydrocarbons show a strong effect on coking rates and on carbon thresholds. For mixtures of methane and propane/propene, the steady-state coking rate as well as the coking threshold decreased with a decrease in the mole fraction of propane/propene and with an increase in the hydrogen mole fraction. The coking rate revealed a complicated temperature dependency, and a minimum in the coking rate and in the coking threshold was detected at 500°C. A relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed based on a suggested reaction mechanism, which can properly predict the role of hydrocarbon, hydrogen and water in carbon formation. [Copyright &y& Elsevier]

Hydrogen is highly demanded in biorefinery, and hydrogen production from renewable sources is essential to produce truly green transportation fuels from biomass. The present work demonstrates experimentally the production of high pressure H 2 with high purity by pressure swing sorption enhanced steam reforming (PS-SESR) of the byproducts from biorefinery. Pure hydrogen was produced by one-pot high pressure sorption enhanced reforming of a mixture of acetic acid (AA), glycolaldehyde (Gl) and hydroxyacetone (Hy), as model of the byproducts obtained from biomass fast-hydropyrolysis, which is integrated in the H 2 Bioil process, aimed at producing liquid transportations fuels. SESR was performed using Pd/Ni-Co derived from a hydrotalcite-like material as catalyst and dolomite as CO 2 acceptor. Both thermodynamic analysis and experimental study revealed enhanced hydrogen purity but lower hydrogen yield at high pressure compared to atmospheric pressure. Moreover, a compromise between pressure and temperature is needed to get high purity and yield of hydrogen. A H 2 purity as high as 99.6 vol% can be obtained at atmospheric pressure and 550 °C, while a H 2 purity of 99.0 vol% can be reached at 5 bar and 600 °C. Under these conditions, the H 2 yield is of 92.3% at 1 bar and 92.6% at 5 bar. These results show that the SESR of the biomass-derived compounds is an efficient method for the production of highly pure, hot and high pressure hydrogen, which is required for the overall process studied. According to the thermodynamic analysis, and given that the process is performed at high pressure, a pressure swing decarbonation process is suggested for the sorbent regeneration. [ABSTRACT FROM AUTHOR]

Aqueous phase reforming (APR) of ethylene glycol was performed at 225 °C and 30 bar in batch and continuous reaction conditions. The effect on the APR performance by functionalizing carbon supports with nitrogen and adding Mn to Pt-based catalysts was investigated. The presence of nitrogen species on the carbon surface and Mn-addition (PtMn) improved the catalytic activity and promoted H 2 production. XPS results suggest that the enhancement of the catalytic activity may be attributed to charge transfer from platinum to the nitrogen groups and Mn. Pt-based catalysts were stable under the studied reaction conditions, while up to 97 % of the manganese leached into the liquid solution during APR. However, the catalytic activity was maintained even with such significant decrease in Mn content, indicating that only a small amount of Mn is necessary to maintain the promotional effect on Pt during APR. [Display omitted] • Electronic interactions between Pt-N and Pt-Mn led to enhanced catalytic activity. • N-doped carbon nanofibers effectively improved H 2 yield in aqueous phase reforming. • Pt-based catalyst promoted with Mn presented higher H 2 yield and selectivity. • Metal-support interactions are key factors in promoting catalytic activity. [ABSTRACT FROM AUTHOR]

• Small Ni particles exhibit higher steam methane reforming activity than large ones. • The size-dependent activity of Ni catalysts is related to the surface-dependent activity. • Ni(2 1 1) is the most active surface for steam methane reforming. • Decreased Ni(2 1 1) surface fraction results in reduced Ni activity as size increases. Fundamental understanding of the size-dependent activity is essential to harness powers of the nanocatalysts. Here we report an experimental and theoretical study of the Ni particle size effect on activity of steam methane reforming (SMR) to achieve a better understanding of the size dependence of kinetic behavior at an atomic level. A kinetic study illustrated the higher forward methane turnover frequency on the smaller sized Ni particles. The size-dependent activity was well reproduced by microkinetic modeling on a truncated octahedron model with the kinetic parameters estimated by the improved unity bond index-quadratic exponential potential (UBI-QEP) and the Brønsted–Evans–Polanyi (BEP) relationship. Microkinetic modeling suggested that the size-dependent activity of Ni catalysts is associated with the surface-dependent activity. Much higher activity of Ni(2 1 1) than Ni(1 1 1) and Ni(1 0 0) accompanied by decreased Ni(2 1 1) surface fraction results in reduced Ni activity as particle size increases. The activity of Ni(1 1 1) is limited by high free energy barriers, while that of Ni(1 0 0) is limited by site blockage by C* and CH*. This work offers a feasible approach to gain insight into size-dependent activity and to aid rational catalyst design for SMR in which preparing extremely small Ni particles (≤6 nm) might be a good strategy. [ABSTRACT FROM AUTHOR]

Fuel-cell grade H 2 has been produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of the bio-oil obtained from the fast pyrolysis of biomass. A Pd/Ni–Co catalyst derived from a hydrotalcite-like material (HT) with dolomite as CO 2 sorbent was used in the process. A fixed-bed reactor with three temperature zones was employed to favor the catalytic steam reforming reaction in the high-temperature segment, the SESR reaction in the intermediate-temperature part, as well as the water-gas shift (WGS) and CO 2 capture reactions in the low-temperature segment. Different conditions of pressure, temperature, steam/C molar ratio and weight hourly space velocity (WHSV) in the feed were evaluated. Higher steam/C molar ratios and lower WHSV values facilitated the production of H 2 and reduced the concentrations of CH 4 , CO and CO 2 in the produced gas. A fuel-cell grade H 2 stream with a H 2 purity of 99.8 vol.% and H 2 yield of 86.7% was produced at atmospheric pressure, with a steam/C ratio of 3, a WHSV of 0.893 h −1 and a temperature of 575 °C in the intermediate part of the reactor (675 °C in the upper segment and 425 °C in the bottom part). At high pressure conditions (15 atm) a maximum H 2 concentration of 98.31 vol.% with a H 2 yield of 79.81% was obtained at 725 °C in the intermediate segment of the reactor (825 °C in the upper segment and 575 °C in the bottom part). Under these conditions an effluent stream with a CO concentration below 10 ppm (detection limit) was obtained at both low and high pressure, making it suitable for direct use in fuel cell applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • A scaling relation-based L-H kinetic model is proposed to assess reaction rate. • Principles for making a kinetically consistent free energy profile are suggested. • Surface site coverages is estimated from free energy diagram of intermediates. • Reaction rate is straightforwardly obtained from the free energy diagram of reaction. • The model predicted resutls fit microkinetic modeling well for three model reactions. We report a scaling relation-based L-H kinetic model to assess reaction rate, where general principles for making a kinetically consistent free energy profile are suggested. Water-gas shift, Fischer-Tropsch synthesis, and steam methane reforming are employed as three model reactions to illustrate this approach. Herein, potential energies and free energies are calculated by improved scaling relations. The different methods to construct energy profiles are compared, and free energy diagrams rather than potential energy diagrams are more favorable to be employed in the field of reaction mechanism discriminations and kinetics investigations due to consisting of thermodynamic information. The sequence of various elementary steps is important to generate appropriate rate-determining step and rate expressions. It is suggested that the sequence of the elementary step follows a principle that the reaction step is set just before it is required, and desorption occurs just after the formation of the intermediates. Based on the procedure elucidated here, it is easy to get turnover frequency straightforwardly from the free energy diagram of reaction and the surface site coverages from the free energy diagram of intermediates. The scaling relation-based L-H kinetic model exhibits a similar accuracy for predicting the surface coverages and reaction rates with full microkinetic modeling toward all the three model reactions. It demonstrates that the approach proposed herein greatly simplifies the calculation but maintains high accuracy as compared to full microkinetic modeling. This approach can be widely applied for kinetic modeling to elucidate the reaction mechanism and kinetics for chemical processes of interest. [ABSTRACT FROM AUTHOR]

To investigate the role of steam on the structure and activities of platinum-based catalyst, a tin modified Pt/γ-Al 2 O 3 catalyst was prepared for propane dehydrogenation in the presence of steam. The propane conversion could be prominently elevated by nearly twofolds with only 1.0% loss of selectivity to propene when an optimum amount of H 2 O was added. The retarding effects of steam on the activities are also observed at higher H 2 O/C 3 H 8 molar ratios (>1). Kinetic studies results show that the apparent activation energies for steam dehydrogenations are lower than those without steam addition. The fresh, spent and steam pretreated catalysts were characterized by TG, Raman, H 2 -chem, HAADF-STEM, XPS and CO adsorption FT-IR (CO-FTIR). It is observed that the platinum particles sizes are smaller in the samples subjected to certain period of steam pretreatment. The changes of platinum dispersion in steam conditions are related to the changes of tin oxidation states and the segregation of platinum from Pt–Sn alloys, as revealed by CO-FTIR and XPS results. More accessible Pt sites that are generated in the steam due to the phase transformation of Pt–Sn catalyst as well as the removal of coke accumulated on the catalyst surface would account for the increase of catalytic activities and the decrease of activation energies in steam atmosphere. Competitive adsorption of H 2 O and other reagent over Pt sites would lead to the retarding effects of steam on the activities at high H 2 O partial pressure. [ABSTRACT FROM AUTHOR]

High-purity H 2 was produced by the sorption enhanced steam reforming (SESR) of acetic acid, a model compound of bio-oil obtained from the fast pyrolysis of biomass, in a fluidized bed reactor. A Pd/Ni–Co hydrotalcite-like material (HT) and dolomite were used as reforming catalyst and CO 2 sorbent, respectively. The hydrogen yield and purity were optimized by response surface methodology (RSM) and the combined effect of the reaction temperature ( T ), steam-to-carbon molar ratio in the feed (steam/C) and weight hourly space velocity (WHSV) upon the sorption enhanced steam reforming process was analyzed. T was studied between 475 and 675 °C, steam/C ratio between 1.5 and a 4.5 mol/mol and WHSV between 0.893 and 2.679 h −1 . H 2 yield, H 2 selectivity and H 2 purity, as well as the CH 4 , CO and CO 2 concentrations in the effluent gas, were assessed. The operating temperature proved to be the variable that had the greatest effect on the response variables studied, followed by the WHSV and the steam/C ratio. The results show that the H 2 yield, H 2 selectivity and H 2 purity increased, while the CH 4 , CO and CO 2 concentrations decreased, concurrently with the temperature up to around 575–625 °C. Higher values of the steam/C ratio and lower WHSV values favored the H 2 yield, H 2 selectivity and H 2 purity, and reduced the CH 4 concentration. It was found that the SESR of acetic acid at atmospheric pressure and 560 °C, with a steam/C ratio of 4.50 and a WHSV of 0.893 h –1 gave the highest H 2 yield of 92.00%, with H 2 purity of 99.53% and H 2 selectivity of 99.92%, while the CH 4 , CO and CO 2 concentrations remained low throughout (0.04%, 0.06% and 0.4%, respectively). The results also suggested that a slow CO 2 capture rate led to a poor level of hydrogen production when the SESR process was carried out at low temperatures, although this can be improved by increasing the sorbent/catalyst ratio in the fluidized bed. [ABSTRACT FROM AUTHOR]

Abstract: First-principles calculations have been performed to examine the effect of doped Ag on the kinetics of Ni-catalyzed methane dissociation and coke formation. The close-packed Ag/Ni(111) and stepped Ag/Ni(211) surfaces as well as the defect facets with step sites blocked by Ag or C atoms are constructed to investigate the role of the coordinatively unsaturated sites in the catalytic performance of Ni nanoparticles. The most stable CH x (x =0–4) adsorption configurations and transition states for methane dissociation have been identified on both Ni and Ag/Ni surfaces. The calculated results indicate that the activation energy for methane dissociation is increased with the Ag coverage on Ni(111), and the C atoms deposited on the catalyst surface can be readily separated into small islands by Ag. On Ni(211) Ag atoms are predicted to bind preferentially to the middle-step sites which act as the nucleation center for the growth of filamentous carbon and therefore have the potential to prevent catalyst particles from being destroyed. Meanwhile, as the energy barrier for methane dissociation on the Ag-blocked Ni(211) surface is even higher than that on pure Ni(111), the active center is transferred from the stepped surface to the close-packed surface. These findings provide a rational interpretation of the experimental observations that Ag/Ni catalyst exhibits lower catalytic activity towards steam methane reforming but high resistance to coke deposition. [Copyright &y& Elsevier]

Fischer–Tropsch synthesis of light olefins plays a vital role in the production of major chemical building blocks from non-petroleum resources, having great academic and commercial importance. Herein, Na and S modulated FeMnO x catalysts were employed to systematically investigate the influence of additives, which not only enhanced the CO conversion but also facilitated the olefin formation and suppressed the undesired methane formation. Multiple characterizations confirmed that the existence of promoters would enhance the formation of active species of Fe 5 C 2 at FTO conditions because of the promoted carbon insertion into the iron species from the intermediates dissociation. Combined the steady-state isotopic transient kinetic analysis with dynamic calculation, it confirmed that the promoters indeed had the ability to lower CO activation energy, as well as increase the carbon chain growth activity and the energy barrier to hydrogenation. This study provides a practical strategy for exploring the highly active and stable FTO catalysts. [Display omitted] • Na and S additives over FeMnO x catalyst promoted both the CO conversion and the olefin formation. • Pomotes facilitated the Fe5C2 formation by suppressing the oxidation cycle of iron and enhancing the insertion of carbon. • The presence of promoters reduced the charge of iron and caused more compact bonds of Fe-C, lowing the CO activation energy. • Promoters indeed increased the carbon chain growth activity and the energy barrier to hydrogenation. [ABSTRACT FROM AUTHOR]

• For selectivity and stability test, CeO 2 -Ni shows the best performance. • Forward turnover rate is independent of CO 2 partial pressure. • TOF obtained from G plot is in perfect agreement with experiment value. • The increased basicity and decreased electronegativity help to activate the CO 2. Here we provide new mechanistic and kinetic insights into the functions of oxides on Ni catalysts in methane dry reforming combining kinetic studies with density functional theory (DFT) calculations. Hydrotalcite derived Ni catalysts with a small amount of oxide additive (CeO 2 , ZrO 2 , ZnO) as promoters are synthesized and characterized by different techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), N 2 physisorption, H 2 chemisorption, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and thermogravimetric analysis combined with mass spectrometry (TGA-MS). Regarding H 2 /CO ratio, the CeO 2 -Ni shows the highest the values along all the temperatures. Moreover, the CeO 2 -Ni catalyst has the best stability among the four catalysts, while ZnO-Ni experiences the most severe deactivation. Kinetic studies in terms of reaction orders and activation energies are performed and compared to the DFT investigations, to assess the functions of oxide promoters. The CeO 2 -Ni catalyst shows the lowest apparent activation energy for CO 2 activation, and it is also found that forward turnover rate is independent of CO 2 partial pressure for all the samples. In DFT calculations, CO 2 is more favorable to be activated on the support and the TOF obtained from G plot is in perfect agreement with our experiment value. In addition, it is also found that basicity of oxide additives and electronegativity of metal element can be well correlated to the activation of CO 2 and catalyst's deactivation. In general, both the increased basicity of oxide and the decreased electronegativity of metal element help to promote the CO 2 activation and enhance the catalyst's stability. We propose that the CeO 2 -Ni catalyst has best performance for CO 2 activation, thus leading to a higher surface oxygen concentration to oxidize the carbon on the catalysts, which prolongs the catalyst's life. [ABSTRACT FROM AUTHOR]