Kinetics of the alkali carbonate catalysed gasification of carbon: 1. CO2 gasification

The kinetics of CO₂ gasification of carbon, catalysed by Na, K, Rb and Cs is adequately described by a two step model in which the first step represents a reversible oxidation of the carbon and the second, a rate determining step, the release of CO from the carbon matrix. The catalyst only increases the steady state concentration of oxygen at the carbon surface. The activation energy of the second step is 20–40 kJ mol⁻¹ lower than for the uncatalysed reaction. The reaction rate is independent of the CO₂ pressure over a range of 4–30 bar, increases with catalyst loading and in the period order Na < K < Rb < Cs. In case of Na the number of active sites probably increases with temperature due to carbonate decomposition.     

Effects of preparation methods on the properties of cobalt/carbon catalyst for methane reforming with carbon dioxide to syngas

The effect of different preparation methods on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. The catalysts were prepared by the impregnation and characterized by the XRD and scanning electron microscopy (SEM). The result indicated that the active components of cobalt/carbon catalyst prepared by using ultrasonic wave distributed evenly, activity was high and the loading time was short. The Co/Carbon catalyst prepared by incipient-wetness impregnation, 10 wt% loading and 300 °C calcination, achieved the best activity. Furthermore, the effect of reaction temperature, air speed and CH₄/CO₂ ratio on the catalyst activity and CO/H₂ ratio in products was investigated. It was found that the conversion of CO₂ and CH₄ increased with the increasing of reaction temperature. However, the conversion of CO₂ and CH₄ increased first and then decreased with the increasing of air speed. With the increasing of CH₄/CO₂ in feed gas, both the catalyst activity and the CO/H₂ ratio in products decreased.     

生物柴油副产物粗甘油催化氧化脱水制备丙烯酸

用生物柴油副产物粗甘油催化氧化脱水制丙烯酸,该过程耦合了甘油脱水制丙烯醛和丙烯醛选择性氧化制备丙烯酸两步反应。结果表明,在甘油脱水反应中,使用Cs3PW12O40, P-ZSM-5和Co0.5H2PO4/SiO2等固体酸催化剂,可得到较高的丙烯醛收率(最高86.9%)。利用上述催化剂和MoVW基氧化催化剂,在脱水/氧化双催化剂床层构型反应器中,以甘油为原料合成丙烯酸的收率达50%~80%,直接加入粗甘油可获得相似的丙烯酸收率。     

Hydrogenation of carbon dioxide on cobalt catalysts – activation,deactivation, influence of carbon monoxide

A batch reactor directly combined with an ultrahigh vacuum apparatus, which is equipped with facilities for catalyst preparation and Auger electron spectroscopy, was used to answer some questions which had arisen in recent studies concerning carbon dioxide hydrogenation on pure metallic and supported Co catalysts. Both oxygen incorporated during oxidation/reduction cycles and carbon deposited when CO₂ is hydrogenated penetrate deep into the bulk. This kind of carbon can easily be hydrogenated. CO strongly hinders the reduction of the oxidized Co surface in the H₂/CO₂ reaction mixture (4 : 1). CO hydrogenation is favoured over CO₂ hydrogenation and leads to a higher percentage of C₂ to C₄ hydrocarbons as compared with CH₄ formation.     

Kinetics of the reaction of oxygen with carbon monoxide adsorbed on palladium

A study of the kinetics of the reaction of adsorbed carbon monoxide with oxygen on polycrystalline palladium is reported in which a pressure jump method was used to induce transients in the carbon dioxide production. Through an analysis of these transients under a variety of conditions of temperature and oxygen pressure, some details of the kinetics have been delineated. At relatively low temperatures and under a significant O₂ pressure, CO(a) is desorbed more readily as CO₂, via the reaction CO(a) + O(a) → CO₂, than as CO. The reaction is first order in oxygen and the rate is limited by the rate of adsorption of oxygen onto sites which are in close proximity to CO(a). Oxygen adsorption at sites which are further than a critical distance from CO(a) are unreactive. The critical distance increases with temperature reflecting increased mobility. Under conditions where both CO(a) and O(a) are significant and both CO(g) and O₂(g) are small the rate is limited by the mobility of CO(a) and/or O(a). The amount of CO(a) during the course of the steady-state oxidation reaction can be determined by analyzing the transient CO₂ production which occurs following a pressure jump in carbon monoxide.     

High-temperature oxidation of aluminium in various gases

The oxidation of high-purity aluminium sheet in dry oxygen, moist oxygen, carbon dioxide and carbon monoxide (at total pressure 1.333 × 10³ Nm^?2) was studied in the range 673–923°K, using a vacuum microbalance to follow weight gains. ¹⁴CO₂ and ¹⁴CO were used to elucidate the mechanism of the oxidation in these gases and to estimate the extent of carbon deposition in the oxide layer. The rate of oxidation in moist oxygen was similar to that in dry oxygen, the principle reaction being 2Al + 3H₂O ← Al₂O₃ + 3H₂. It is suggested that there are three steps in the reaction in CO₂, viz. 2Al+3CO₂ ← Al₂O₃ + 3CO, followed by 2Al + 3CO ← Al₂O₃ + 3C, and about 10% of the deposited carbon reacting further by 4Al + 3C ← Al₄C₃. Only the last two reactions are operative in carbon monoxide. The Arrhenius plots show a distinct break in the region 773–823°K for both carbon monoxide and carbon dioxide, but not for dry or moist oxygen. This is tentatively explained by a change in the rate-determining process from diffusion via grain boundaries or cracks in the oxide, to lattice diffusion. It is suggested that carbon may become mobile in the oxide film between 773 and 823°K and may tend to congregate in the grain boundaries and cracks. The oxide film remained protective throughout the duration of the experiments in all the gases.     

On the determination of kinetic parameters for the regeneration of cracking catalyst

In the study of the regeneration of cracking catalyst, two approaches may be taken when determining the kinetic rate constant for the coke combustion reaction. A global coke burning rate equation may be considered, based on the observed oxygen concentration and carbon dioxide to carbon monoxide molar product ratio. This reaction may also be represented by an intrinsic coke burning equation which is a function of the oxygen concentration and the carbon dioxide to carbon monoxide molar ratio at the reaction site, combined with a carbon monoxide postcombustion equation. It is proposed in this paper that the rate constant for intrinsic coke burning, k^c, is essentially equal to the global coke burning rate constant, k_c, and that its value is independent of the rate equation chosen for the carbon monoxide post-combustion reaction.     

Kinetics of vapor-phase dehydration of glycerol into acrolein on the BAO-1 heterogeneous catalyst

The vapor-phase dehydration of glycerol on the heterogeneous catalyst 0.5B₂O₃/γ-Al₂O₃ (BAO-1) was studied. The kinetic model of the process was developed based on the data obtained in a differential reactor. To evaluate the kinetic constants of the generalized mathematical models of the kinetics of vapor-phase dehydration of glycerol, we used the differential evolution method implemented in the Mathematica 5.0 program. The calculated activation energy of the target reaction of acrolein formation was 50.18 ± 0.11 kJ/mol. The adequacy of the obtained equations was assessed using the Fisher test. The optimum conditions of the vaporphase dehydration of glycerol were determined using the proposed kinetic equations (reaction temperature 330°C, glycerol concentration in the supply stream 30%, and catalyst load 0.0338 L/(g_cat min). The obtained data may be used in calculations for large units for acrolein production by glycerol dehydration.     

Catalysed carbon gasification with Ba13CO3

The interaction of barium carbonate with carbon black was studied to understand catalysed CO₂ gasification of carbon. Temperature-programmed reaction with isotopic labelling of the carbonate and the carbon showed that carbon dramatically accelerated the rate of BaCO₃ decomposition to form BaO and CO₂, which rapidly gasified carbon to form CO. Pure BaCO₃ was observed to exchange carbon dioxide with the gas-phase, and the exchange rate was increased significantly by carbon at higher temperatures, due to formation of a carbon-carbonate complex. The interaction of BaCO₃ and C to form a complex occurred well below gasification temperatures, and BaCO₃ did not decompose until after gasification began and the gas phase CO₂ concentration was low. During catalysed gasification, formation of gaseous CO from a surface oxide is shown directly to be the slow step in the reaction. The active catalyst appears to cycle between BaCO₃ and BaO (both of which interact with carbon). The rates of carbonate decomposition, catalytic gasification, and exchange with gaseous CO₂ are all slower for BaCO₃ than for K₂CO₃, indicating the large differences in carbonate-carbon interaction between alkali carbonates and alkaline earth carbonates. The two carbonates apparently follow different reaction mechanisms.     

Oxydehydrogenation of propane over Mg-V-Sb-oxide catalysts. II. Reaction kinetics and mechanism

Recently we reported that Mg₄V₂Sb₂O_x is selective for propane andn-butane Oxydehydrogenation at low hydrocarbon conversion, and that propane is oxidized in parallel reactions to propylene and CO_x. We report now on the kinetics of propane and propylene oxidations over this catalyst. The partial oxidations of propane and propylene and zero-order in oxygen, whereas deep oxidations of both hydrocarbons are half-order. This difference in reaction order indicates that different forms of reactive oxygen are involved in the partial and deep oxidation reactions. Presumably, nucleophilic lattice oxygen partakes in the partial oxidation, while electrophilic dissociatively adsorbed oxygen is involved in deep oxidation. A single activated surface adsorbed state of the hydrocarbons is thought to be involved in both the partial and deep oxidation reactions. An interpretation of the observed reaction kinetics in context of the Mg₄V₂Sb₂O_x solid state chemistry, and the partial oxidation literature in general, suggests that selective oxydehydrogenation of propane occurs on isolated (Sb-O-V-O-Sb) sites, deep oxidation on multiple vicinal vanadium sites (Sb-O-V-O-V-O-Sb), and partial oxidation of propylene to acrolein on subsurface V-promoted antimony sites (Sb-O-Sb). Therefore, unproved selectivity of desired intermediates (propylene/acrolein) should be achieved by further lowering the vanadium concentration and/or through key solid state positioning of the vanadium in the catalyst lattice. Alternatively, selective doping to electronically decrease the electrophilicity of the waste forming sites and its appended oxygen should also help depress the waste forming reaction channels in favor of the desired partial oxidation channels. Finally it is anticipated that higher useful product yields would be attained with a compositionally optimized Mg-V-Sb-oxide catalyst by opting for a more stable, isolatable intermediate, e.g., acrylonitrile, by reacting propane in the presence of ammonia and oxygen (air) over this catalyst.     

Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O₂ concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature.As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO₂ production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO₂ peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO₂ production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO₂ production rate drops abruptly as the reaction “quenches”.Catalyst layer thickness was changed over the range 0.06-1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO₂ production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.     

Rhodium-catalyzed partial oxidation of methane to CO and H2. In situ DRIFTS studies on surface intermediates

Reaction steps in the oxidation of CH₄ to CO and H₂ over a Rh(1 wt%)/-Al₂O₃ catalyst were studied using in situ DRIFTS at 973 K and 0.1 MPa. Product distribution and the resulting absorption band intensities of the respective adsorbates were strongly influenced by oxygen coverage and carbon deposits on the surface. CH₄ is dehydrogenated to carbon deposits and H₂ and is simultaneously oxidized to CO₂ and H₂O. OH surface groups in the support are involved in the CH_x conversion to CO via reforming reaction. The reaction of surface carbon with CO₂ was assumed to contribute to CO formation. Formate is a by-product of the reaction.     

Pathways for CO2 Formation and Conversion During Fischer–Tropsch Synthesis on Iron-Based Catalysts

CO₂ reaction and formation pathways during Fischer–Tropsch synthesis (FTS) on a co-precipitated Fe–Zn catalyst promoted with Cu and K were studied using a kinetic analysis of reversible reactions and with the addition of ¹³C-labeled and unlabeled CO₂ to synthesis gas. Primary pathways for the removal of adsorbed oxygen formed in CO dissociation steps include reactions with adsorbed hydrogen to form H₂O and with adsorbed CO to form CO₂. The H₂O selectivity for these pathways is much higher than that predicted from WGS reaction equilibrium; therefore readsorption of H₂O followed by its subsequent reaction with CO-derived intermediates leads to the net formation of CO₂ with increasing reactor residence time. The forward rate of CO₂ formation increases with increasing residence time as H₂O concentration increases, but the net CO₂ formation rate decreases because of the gradual approach to WGS reaction equilibrium. CO₂ addition to synthesis gas does not influence CO₂ forward rates, but increases the rate of their reverse steps in the manner predicted by kinetic analyses of reversible reactions using non-equilibrium thermodynamic treatments. Thus the addition of CO₂ could lead to the minimization of CO₂ formation during FTS and to the preferential removal of oxygen as H₂O. This, in turn, leads to lower average H_2/CO ratios throughout the catalyst bed and to higher olefin content and C₅₊ selectivity among reaction products. The addition of ¹³CO₂ to H₂/¹²CO reactants did not lead to significant isotopic enrichment in hydrocarbon products, indicating that CO₂ is much less reactive than CO in chain initiation and growth. We find no evidence of competitive reactions of CO₂ to form hydrocarbons during reactions of H₂/CO/CO₂ mixtures, except via gas phase and adsorbed CO intermediates, which become kinetically indistinguishable from CO₂ as the chemical interconversion of CO and CO₂ becomes rapid at WGS reaction equilibrium.     

Syngas production by autothermal reforming of methane on supported platinum catalysts

The performance of supported platinum catalysts on the autothermal reforming of methane was evaluated. The effect of the calcination temperature of the CeZrO₂ support and of the reaction conditions (reaction temperature, presence of CO₂ in the feedstock, and H₂O/CH₄ molar ratio) was studied. The catalysts were characterized by BET, XRD, and OSC analyses and the reaction mechanism was determined by TPSR experiments. The TPSR analyses indicate that autothermal reforming of methane proceeds through a two-step mechanism (indirect mechanism) over all catalysts studied. The Pt/Ce_0.75Zr_0.25O₂ catalyst presented the best stability, which depends not only on the amount of oxygen vacancies of the support but also on the metal particle size. The higher reducibility and oxygen storage/release capacity of Pt/Ce_0.75Zr_0.25O₂ catalyst promote the mechanism of continuous removal of carbonaceous deposits from the active sites, which takes place at the metal-support interfacial perimeter. The water also participates in this mechanism, favouring the carbon removal of metal particle. Furthermore, the reaction conditions influenced significantly the behaviour of Pt/Ce_0.75Zr_0.25O₂ catalysts. The increase of H₂O/CH₄ molar ratio had a beneficial effect on the methane conversion and on the H₂/CO molar ratio. However, the increase of the reaction temperature had an opposite effect. Both the methane conversion and H₂/CO molar ratio decreased with the increasing of reaction temperature. Moreover, the addition of CO₂ to feedstock increased the initial methane conversion, but decreased the stability of the catalyst.     

Hydrogen Production by Sorption‐Enhanced Steam Glycerol Reforming: Sorption Kinetics and Reactor Simulation

Sorption‐enhanced glycerol reforming, an integrated process involving glycerol catalytic steam reforming and in situ CO₂ removal, offers a promising alternative for single‐stage hydrogen production with high purity, reducing the abundant glycerol by‐product streams. This work investigates this process in a fixed‐bed reactor, via a two‐scale, nonisothermal, unsteady‐state model, highlighting the effect of key operating parameters on the process performance. CO₂ adsorption kinetics was investigated experimentally and described by a mathematical reaction‐rate model. The integrated process presents an opportunity to improve the economics of green hydrogen production via an enhanced thermal efficiency process, the exothermic CO₂ adsorption providing the heat to endothermic steam glycerol reforming, while reducing the capital cost by removing the processing steps required for subsequently CO₂ separation. The operational time of producing high‐purity hydrogen can be enhanced by increasing the adsorbent/catalyst volume ratio, by adding steam to the reaction system and by increasing the inlet reactor temperature. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2105–2118, 2013     

Roles of Surface and Bulk Lattice Oxygen in Forming CO2 and CO During Methane Reaction over Gadolinia-Doped Ceria

Temperature-programmed reaction of methane and temperature-programmed reduction were performed over gadolinia-doped ceria (GDC). It was found that CO₂ formation can occur at very much lower temperature than CO formation. The surface lattice oxygen acts as the active site for CH₄ adsorption. This active site has a dynamic characteristic due to the mobility of the lattice oxygen. The rates of CO and CO₂ formations can be controlled by the supply rate of the lattice oxygen from the GDC bulk; this supply rate depends on the mobility and the concentration of the bulk lattice oxygen. CO₂ formation is associated with the existing surface lattice oxygen while CO formation depends on the oxygen species coming from the bulk lattice during methane reaction.     

The transient response study of CO,CO2, and O2 adsorption and CO oxidation over La0.4Sr0.6Co0.4Mn0.6O3

The transient behaviour caused by the change of the component concentration for CO oxidation on the perovskite‐type catalyst La_0.4Sr_0.6Co_0.4Mn_0.6O₃ was investigated. Results showed that CO was not adsorbed on the catalyst surface and CO oxidation was carried out between the surface oxygen species and gas phase CO. On the other hand, CO₂ can be adsorbed on the catalyst surface but its adsorption site was different from the forming site.     

Carbon-carbon dioxide reaction: Kinetics at low pressures and hydrogen inhibition

The gasification of a very high purity natural graphite was studied at temperatures between 960 and 1120°C and at CO₂ pressures below 108 millitorr. For CO₂ depletion up to at least 90%, gasification rates were first order in CO₂ pressure, that is with no inhibition by CO observed. The activation energy for the rate constant for the oxygen transfer step (103.5 ± 5.8 kcal/mole) agreed within experimental error with that found from kinetic studies at intermediate CO₂ pressures where CO does inhibit the reaction. The rate of the oxygen transfer reaction is markedly inhibited by the presence of low pressures of H₂. As H₂ pressure is increased up to 3 millitorr, the gasification rate in CO₂ at 1100°C monotonically decreases. Further increase in H₂ pressure, has a negligible effect on rate. From measurements of hydrogen uptake at reaction temperature, it is clear that inhibition is caused by dissociative chemisorption of hydrogen on to active sites. Inhibition by hydrogen is even more marked for the Graphon-CO₂ reaction and is attributed not only to its chemisorbing on carbon sites but also on to impurity catalyst sites. It is doubtful if true rate constants for the C-CO₂ reaction, uninhibited by hydrogen, have ever been reported.     

Slurry-phase CO2 hydrogenation to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst: Investigation of reaction conditions

The hydrogenation of CO₂ to hydrocarbons over a precipitated Fe-Cu-Al/K catalyst was studied in a slurry reactor for the first time. Reducibility of the catalyst and effect of reaction variables (temperature, pressure and H₂/CO₂ ratio of the feed gas) on the catalytic reaction performance were investigated. The reaction results indicated that the Fe-Cu-Al/K catalyst showed a good CO₂ hydrogenation performance at a relatively low temperature (533 K). With the increase of reaction temperature CO₂ conversion and olefin to paraffin (O/P) ratio in C₂-C₄ hydrocarbons as well as the selectivity to C₂-C₄ fraction increased, while CO and CH₄ selectivity showed a reverse trend. With the increase in reaction pressure, CO₂ conversion and the selectivity to hydrocarbons increased, while the CO selectivity and O/P ratio of C₂-C₄ hydrocarbons decreased. The investigation of H2/CO₂ ratio revealed that CO₂ conversion and CH₄ selectivity increased while CO selectivity and O/P ratio of C₂-C₄ decreased with increasing H₂/CO₂ ratio.