Pulse-response TAP studies of the reverse water–gas shift reaction over a Pt/CeO2 catalyst

The temporal analysis of products (TAP) technique was successfully applied for the first time to investigate the reverse water–gas shift (RWGS) reaction over a 2% Pt/CeO₂ catalyst. The adsorption/desorption rate constants for CO₂ and H₂ were determined in separate TAP pulse-response experiments, and the number of H-containing exchangeable species was determined using D₂ multipulse TAP experiments. This number is similar to the amount of active sites observed in previous SSITKA experiments. The CO production in the RWGS reaction was studied in a TAP experiment using separate (sequential) and simultaneous pulsing of CO₂ and H₂. A small yield of CO was observed when CO₂ was pulsed alone over the reduced catalyst, whereas a much higher CO yield was observed when CO₂ and H₂ were pulsed consecutively. The maximum CO yield was observed when the CO₂ pulse was followed by a H₂ pulse with only a short (1 s) delay. Based on these findings, we conclude that an associative reaction mechanism dominates the RWGS reaction under these experimental conditions. The rate constants for several elementary steps can be determined from the TAP data. In addition, using a difference in the time scale of the separate reaction steps identified in the TAP experiments, it is possible to distinguish a number of possible reaction pathways.     

NO decomposition and reduction on Pt/Al2O3 powder and monolith catalysts using the TAP reactor

A systematic study over Pt/Al₂O₃ powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H₂. NO pulsing and NO–H₂ pump-probe experiments demonstrate the effect of catalyst temperature, NO–H₂ pulse delay time and H₂/NO ratio on N₂, N₂O and NH₃ selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H₂, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H₂ at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N₂O or recombine to form N₂. In excess H₂, higher temperatures and longer delay times favor the production of N₂. The longer delay enables NO decomposition on reduced Pt with the role of H₂ being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N₂ formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.     

Fischer–Tropsch synthesis over Co/TiO2 catalyst: Effect of catalyst activation by CO compared to H2

Co/TiO₂ catalyst activation for Fischer–Tropsch (FT) reaction by CO in comparison to H₂ has been performed. The catalyst, prepared by incipient wetness impregnation, has been characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses after separate reduction using CO and H₂ respectively. Temperature programmed reduction (TPR) analyses were also conducted to study the reduction behaviour of the catalyst in presence of H₂ and CO respectively. CO improved catalyst reduction and produced a more stable and active catalyst with higher selectivity and yield for C_5 + hydrocarbons at extended time-on-stream.     

Mechanistic aspects of the oxidative dehydrogenation of propane over an alumina-supported VCrMnWOx mixed oxide catalyst

Mechanistic and kinetic aspects of the catalytic oxidative dehydrogenation of propane (ODP) were studied within a wide range of temperatures (673–773 K), partial pressures of oxygen (0–20 kPa), propane (0–40 kPa) and propene (0–4 kPa) under both steady-state ambient-pressure and transient, vacuum conditions in the temporal analysis of products (TAP) reactor. A Mn_0.18V_0.3Cr_0.23W_0.26O_x–Al₂O₃ catalyst was identified as a selective catalyst for ODP by high-throughput experiments. For comprehensive catalyst characterization, XRD, BET, and in situ UV–visible techniques were applied. The results from transient experiments in combination with UV–visible tests reveal that selective and non-selective propane oxidation occurs on the same active surface sites, i.e., lattice oxygen. CO_x formation takes place almost exclusively via consecutive propene oxidation, which involves both lattice and adsorbed oxygen species, with the latter being active in CO formation. However, the adsorbed species play a minor role. CO₂ formation was found to increase in the presence of propene in the reaction feed. Optimized operating conditions for selective propane oxidation were derived and discussed based on the experimental observations with respect to the influence of temperature and partial pressures of O₂, C₃H₆ and C₃H₈ on the reaction. In co-feed mode with a propane to oxygen ratio of 2, optimal catalytic performance is achieved at low partial pressures of oxygen and high temperature. Propene selectivity can be also improved by carrying out the ODP reaction in a periodic mode; that is an alternate feed of propane and air.     

介质阻挡放电等离子体处理载体对CO甲烷化Ni/SiO2催化剂性能的改进

以经介质阻挡放电等离子体处理的SiO₂为载体,用浸渍法制备了Ni/SiO₂催化剂,并进行了CO甲烷化反应评价。与载体未经处理的常规Ni/SiO₂催化剂相比,载体经处理的催化剂在400℃下的CO与H₂转化率均提高了约6%,且在经700℃烧结6 h后,活性仍高于常规催化剂。XRD、TEM和H₂-TPR结果表明,载体经处理的催化剂,Ni颗粒粒径更小、粒径分布更集中,Ni与SiO₂之间的相互作用更强,证明等离子体处理使SiO₂更有利于促进Ni的分散。     

Kinetic modeling of storage and reduction with different reducing agents (CO, H2, and C2 H4) on a Pt–Ba/-Al2O3 catalyst in the presence of CO2 and H2O

In this paper a global reaction kinetic model is used to understand and describe the NO_x storage/reduction process in the presence of CO₂ and H₂O. Experiments have been performed in a packed bed reactor with a Pt–Ba/γ-Al₂O₃ powder catalyst (1 wt% Pt and 30 wt% Ba) with different lean/rich cycle timings at different temperatures (200, 250, and ) and using different reductants (H₂, CO, and C₂H₄). Model simulations and experimental results are compared. H₂O inhibits the NO oxidation capability of the catalyst and no NO₂ formation is observed. The rate of NO storage increases with temperature. The reduction of stored NO with H₂ is complete for all investigated temperatures. At temperatures above , the water gas shift (WGS) reaction takes place and H₂ acts as reductant instead of CO. At , CO and C₂H₄ are not able to completely regenerate the catalyst. At the higher temperatures, C₂H₄ is capable of reducing all the stored NO, although C₂H₄ poisons the Pt sites by carbon decomposition at . The model adequately describes the NO breakthrough profile during 100 min lean exposure as well as the subsequent release and reduction of the stored NO. Further, the model is capable of simulating transient reactor experiments with 240 s lean and 60 s rich cycle timings.     

The effects of regeneration-phase CO and/or H2 amount on the performance of a NOX storage/reduction catalyst

The effects of regeneration-phase CO and/or H₂, and their amounts as a function of temperature on the trapping and reduction of NO_X over a model and a commercial NO_X storage/reduction catalyst have been evaluated. Overall, for both catalysts, their NO_X removal performance improved with each incremental increase in H₂ concentration. For the commercial sample, using CO at 200 °C, beyond a small amount added, was found to decrease performance. The addition of H₂ to the CO-containing mixtures resulted in improved performance at 200 °C, but the presence of the CO still resulted in decreased performance in comparison to activity when just H₂ was used. With the model sample, the presence of CO resulted in very poor performance at 200 °C, even with H₂. The data suggest that CO poisons Pt sites, including Pt-catalyzed nitrate decomposition. At 300 °C, H₂, CO, and mixtures of the two were comparable for trapping and reduction of NO_X, although with the model sample H₂ did prove consistently better. With the commercial sample, H₂ and CO were again comparable at 500 °C, but mixtures of the two led to slightly improved performance, while yet again H₂ and H₂-containing mixtures proved better than CO when testing the model sample. NH₃ formation was observed under most test conditions used. At 200 °C, NH₃ formation increased with each increase in H₂, while at 500 °C, the amount of NH₃ formed when using the mixtures was higher than that when using either H₂ or CO. This coincides with the improved performance observed with the mixtures when testing the commercial.     

Comprehensive kinetic study for Fischer–Tropsch reaction over KMoFe/CNTs nano-structured catalyst

The kinetics of the Fischer–Tropsch (FT) reaction was evaluated through detailed experimentation with a KMo bimetallic promoted Fe catalyst supported on carbon nanotubes (CNTs). The kinetic tests were conducted in a fixed-bed reactor under operating conditions of P = 6.9–41.3 bar, T = 543–563 K, H₂/CO = 1, gas hourly specific velocity (GHSV) = 2000 h⁻¹. This study aimed to investigate the mechanism prevailing in CO activation and the rate equation for CO consumption during FT reactions over a 0.5K5Mo10Fe/CNTs catalyst. To evaluate the synergistic effects of Fe, Mo, and K phases on the catalyst activity, both fresh and spent catalysts were thoroughly characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) to ascertain the different phases (active sites) present and relevant interactions. Based on the adsorption of carbon monoxide and hydrogen, 22 possible mechanisms for monomer formation were proposed for FT synthesis in accordance with the Langmuir–Hinshelwood–Hougen–Watson (LHHW) and Eley–Rideal (ER) adsorption theories. The best fit kinetic model was identified through a multi-variable non-linear regression analysis. The selected mechanistic model was based on carbide formation approach, where H₂-assisted adsorption of CO was considered for the derivation. Kinetic parameters such as activation energy, adsorption enthalpies of H₂, and CO were estimated to be 65.0, −13.0, and −54.0 kJ/mol, respectively. Considering the developed kinetic model, the effects of reaction temperature and pressure were assessed on Fischer–Tropsch synthesis (FTS) product distribution. Additionally, the kinetic model was compared with the typical Anderson–Schulz–Flory model, suggesting the effects of water-gas-shift and the existence of additional formation pathway such as secondary re-adsorption of olefins for heavier hydrocarbons.     

Effects of catalyst aging on the activity and selectivity of commercial three-way catalysts

A Pd catalyst is particularly effective for the oxidation of CO and C₃H₆ at low temperatures, while the Pt/Rh/Ce catalyst is active for NO reduction. The TWC activity of both catalysts generally decreased as the catalyst mileage increased. However, the NO reduction activity was less affected by catalyst aging compared to the oxidation reactions. The selectivity of the catalysts in favor of the CO–O₂ reaction (vs. C₃H₆–O₂ reaction) in the O₂ partitioning experiments became less pronounced as the catalyst aged. The NO partitioning experiments reveal the superior capability of H₂ in NO reduction to the other reductants (CO and C₃H₆) examined in the present study. The reactivities of NO with both H₂ and CO were found to decrease upon catalyst aging, resulting in decreased overall NO removal activity.     

Study on the reaction mechanism of SO2 hydrogenation over Mo-based catalyst

Mo-based catalysts are widely used for the SO₂ hydrogenation process. However, the detailed reaction mechanism is still unclear and some details should be further supplemented. In this article, the SO₂ hydrogenation processes over the Mo-based catalyst were systematically studied. Several technologies including temperature-programmed experiments, isotope-tracing experiment, fourier transform infrared spectroscopy, and switching experiment were adopted to investigate the reaction steps. The results indicated that during the SO₂ hydrogenation process there were three reaction processes containing six steps: First SO₂ would adsorb on the active sites, which could start at relatively low temperature. Then the adsorbed SO₂ would react with H₂ and gradually form elemental sulfur that could be strongly anchored with S atoms of the MoS₂ surface. Afterward, the sulfur further reacted with H₂ to generate H₂S. Alongside with the consumption of S atoms, the sulfur would be reduced and the active sites was recovered, thus starting the next reaction cycle.     

Kinetic studies of char gasification by steam and CO2 in the presence of H2 and CO

Char-CO₂ gasification reactions in the presence of CO and char-steam gasification reactions in the presence of H₂ were studied at the atmospheric condition using a thermogravimetric apparatus (TGA) at various reactant partial pressures and within a temperature range of 1123 K-1223 K. The char was prepared from a lignite coal. The partial pressure of H₂ and CO varied from 0.05 to 0.3 atm. The experimental results showed that Langmuir-Hinshelwood (L-H) kinetic equation was applicable to describe the inhibition effects of CO and H₂. The kinetic parameters in L-H equations were obtained. Interactions of char gasification by steam and CO₂ in the presence of H₂ and CO were discussed. It was found that the kinetic parameters determined from pure or binary gas mixtures can be used to predict multi-component gasification rates. The results confirmed that the char-steam and char-CO₂ reactions proceed on separate active sites rather than common active sites.     

Coking kinetics of a Cr2O3/Al2O3 catalyst during 1-butene dehydrogenation: Effect of H2 partial pressure

The influence has been studied of the partial pressure of hydrogen (0–30 kPa) upon the coking rate of a Cr₂O₃/Al₂O₃ commercial catalyst during 1-butene dehydrogenation. Coke deposition has been analysed using a monolayer-multilayer reversible coke growth model (MMRC model). This model provides good fits to the experimental data, within the range of partial pressure of H₂ studied, and allows us to estimate the main kinetic parameters involved in the coking-deactivation process. The results obtained reveal a dual effect of hydrogen: competition against 1-butene for the active sites and the removal of coke precursors from the catalyst surface. Bom effects diminish the coking rate as the H₂ partial pressure is increased.     

Effect of catalyst activity in SMR-SERP for hydrogen production: Commercial vs. large-pore catalyst

In this work, we have evaluated the performance of an SMR-SERP unit (steam methane reforming sorption enhanced reaction process), using two different Ni/Al₂O₃ catalysts: commercial “Octolyst 2001” from Degussa and a large-pore catalyst (Catalyst A). The selective CO₂ sorbent was a potassium modified hydrotalcites. Several experiments were performed under different operating conditions to validate a mathematical model.Experimental results show that the Degussa catalyst is more active and more selective to CO₂ producing hydrogen with higher purity and less CO than the large-pore catalyst. Cyclic SMR-SERP experiments were also performed. The cycles comprise four different steps: reaction, depressurization, reactive regeneration and pressurization. In the cyclic experiments, conversion was 43% higher than in an SMR reactor, while H₂ purity was 75%, which is 25% higher than in normal SMR operation. Results indicate that more active catalysts also promote a better reactive regeneration optimizing the use of part of the product (H₂). The proposed mathematical model was validated in a wide range of operating conditions and in a cyclic experiment. The model was able to describe the SMR-SERP experiments without any fitting parameters.     

CO methanation over supported Mo catalysts in the presence of H2S

The effect of various Mo catalyst supports, i.e., γ-Al₂O₃, SiO₂, SiO₂–Al₂O₃, ZrO₂, yttria-stabilized zirconia (YSZ), CeO₂, and TiO₂, on CO hydrogenation in the presence of H₂S was examined. At 5 wt.% Mo loading, Mo/ZrO₂ was determined to be the most active catalyst for this reaction; its activity was dependent on the number of active sites, as determined via NO chemisorption. Raman spectroscopy revealed that MoO₃ transforms into MoS₂ during the reaction.     

A CuO-CeO2 Mixed-Oxide Catalyst for CO Clean-Up by Selective Oxidation in Hydrogen-Rich Mixtures

A CuO-CeO₂ mixed-oxide catalyst was shown experimentally to be highly active and selective for the oxidation of CO in hydrogen-rich mixtures, and an attractive alternative to the noble metal catalysts presently used for CO clean-up in hydrogen mixtures for proton-exchange membrane fuel cells (PEMFC). Although the presence of H₂O and CO₂ in the feed decreased the activity and increased the reaction temperature considerably to achieve a given CO conversion with a reactor, the selectivity profile with respect to the conversion remained virtually the same. The effect of H₂O and CO₂ on the reaction was found to increase the required energy for reduction of the active copper species in the redox cycles undergone during the reaction. The catalyst showed a slow, reversible deactivation, but the activity was restored on heating the catalyst at 300 °C, even under an inert flow. At space velocities above 42 g h m^-3, the catalyst reduced the CO content to less than 10 ppm in the temperature range 166-176 °C for a feed of 1% CO, 1% O₂, 50% H₂, 20% H₂O, 13.5% CO₂ and balance He. Hence, with this catalyst it is feasible to clean up the CO in a single-stage reactor with relatively small excess oxygen, which is in contrast to the typical multistage reactor systems using noble metal catalysts.     

Partial oxidation of methane with N2O over Fe2(MoO4)3 catalyst

The product distributions for partial oxidation of methane on Fe₂(MoO₄)₃ catalyst were changed remarkably when the oxidant was switched from oxygen to nitrous oxide. When oxygen was used as the oxidant, the main products were HCHO and CO. However, when nitrous oxide was used, the formation of HCHO was greatly suppressed and C₂ hydrocarbons (C₂H₆ and C₂H₄) were newly produced. The difference in kinetic behaviors between the two reactions using nitrous oxide and oxygen as the oxidant can be explained in terms of the competitive conversions of methyl intermediate into HCHO and C₂H₆. In the case of nitrous oxide as the oxidant, the adsorbed methyl intermediate would be transformed predominantly into C₂H₆ due to a low steady-state concentration of the active oxygen species on Fe₂(MoO₄)₃.     

Effect of manganese on a potassium-promoted iron-based Fischer-Tropsch synthesis catalyst

The effects of manganese promoter on the reduction–carburization behavior, surface basicity, bulk phase structure and their correlation with Fischer-Tropsch synthesis (FTS) performances have been emphatically studied over a series of spray-dried Fe–Mn–K catalysts with a wide range of Mn incorporation amount. The catalysts were characterized by means of H₂ and CO temperature-programmed reduction (TPR), CO₂ temperature-programmed desorption (TPD), Mössbauer spectroscopy etc.. The results indicated that small amount of Mn promoter can promote the reduction of the catalyst in H₂. However, FeO phase formed during reduction is stabilized by MnO phase with the further increase of Mn content, making FeO phase difficult to be reduced in H₂. The addition of Mn promoter can stabilize the Fe²⁺ and Fe³⁺ ions, and suppresses the reduction and carburization of the catalyst in syngas and CO. Mn promoter can also enhance the amount of the basic sites and weaken the strength of the basic sites, which possibly come from the reason that the Mn–K interaction is strengthened with the addition of Mn promoter. The change of surface basicity can modify the selectivity of hydrocarbons and olefins, and the change of bulk structure phase derived from the addition of Mn promoter will affect the catalyst activity and run stability. The synergetic effects of the two main factors result in an optimized amount of Mn promoter for the highest catalyst activity and heavy hydrocarbon selectivity in slurry FTS reaction of Fe–Mn–K catalysts.     

Ni-Co bimetallic catalyst for CH4 reforming with CO2

A co-precipitation method was employed to prepare Ni/Al₂O₃-ZrO₂, Co/Al₂ O₃-ZrO₂ and Ni-Co/Al₂O₃-ZrO₂ catalysts. Their properties were characterized by N₂ adsorption (BET), thermogravimetric analysis (TGA), temperature-programmed reduction (TPR), temperature-programmed desorption (CO₂-TPD), and temperature-programmed surface reaction (CH₄-TPSR and CO₂-TPSR). Ni-Co/Al₂O₃-ZrO₂ bimetallic catalyst has good performance in the reduction of active components Ni, Co and CO₂ adsorption. Compared with mono-metallic catalyst, bimetallic catalyst could provide more active sites and CO₂ adsorption sites (C + CO₂ = 2CO) for the methane-reforming reaction, and a more appropriate force formed between active components and composite support (SMSI) for the catalytic reaction. According to the CH₄-CO₂-TPSR, there were 80.9% and 81.5% higher CH₄ and CO₂ conversion over Ni-Co/Al₂O₃-ZrO₂ catalyst, and its better resistance to carbon deposition, less than 0.5% of coke after 4 h reaction, was found by TGA. The high activity and excellent anti-coking of the Ni-Co/Al₂O₃-ZrO₂ catalyst were closely related to the synergy between Ni and Co active metal, the strong metal-support interaction and the use of composite support.     

Morphology and deactivation behaviour of Co–Ru/γ‐Al2O3 Fischer–Tropsch synthesis catalyst

Deactivation of Co–Ru/γ‐Al₂O₃ Fischer–Tropsch (FT) synthesis catalyst along the catalytic bed over 850 h of time‐on‐stream (TOS) was investigated. Catalytic bed was divided into four parts and structural changes of the spent catalysts collected from each catalytic bed after FT synthesis were studied using BET, ICP, XRD, TPR, carbon determination, H₂ chemisorption and oxygen titration techniques. Rapid deactivation was observed during first 200 h of FT synthesis. In this case, the deactivation rate was not dependent on the number of the catalyst active sites. It was zero order to CO conversion and independent of the size of active sites. Beyond the TOS of 200 h, the deactivation could be simulated with a power law expression: . The physical properties of the catalyst charged in 1st half of the reactor did not change significantly. Interaction of cobalt with alumina and formation of mixed oxides of the form xCoO·yAl₂O₃ and CoAl₂O₄ was increased along the catalytic bed. Percentage reducibility and dispersion decreased by 2.4–25.5% and 0.5–8.8% for the catalyst in the beds 1 and 4, respectively. Particle diameter increased by 0.8–6.1% for the catalyst in the beds 1 and 4 respectively suggesting higher rate of sintering at last catalytic bed. The amount of coke formation in the 4th catalytic bed was 6 times more than that of in bed 1.     

Research on unsupported nanoporous gold catalyst for CO oxidation

Nanoporous gold (NPG), a novel unsupported gold catalyst prepared by dealloying, exhibits exceptional catalytic activity for CO oxidation. Systematic studies were carried out on this new catalytic system, including the active sites of catalysts, the reaction kinetics, and activity dependence as functions of space velocity and temperature. Our results show strong evidence that metallic gold atoms on NPG are the intrinsic active sites at which the reaction of CO with O₂ occurs. The kinetic study found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O₂ concentration. We suggest that CO adsorption plays a decisive role in CO oxidation on NPG as the rate-limiting step. By completely ruling out the support influence, our findings provide considerable insight into the role of gold catalysts.