The principles and practical use of steady-state isotopic transient kinetic analysis (SSITKA) for studying heterogeneous catalytic reactions are presented. SSITKA combines the advantages of steady-state and transient techniques. A basic requirement for SSITKA is that steady-state conditions are maintained during the isotope switch, and for 12C/13C, 14N/15N, and 16O/18O, the isotope effect is small and these isotopes are all used. The use of H2/D2 is not straightforward since a H2/D2 switch may induce kinetic isotopic effects. Hydrogen spillover to the support and exchange with surface hydroxyl groups may also take place during a H2/D2 switch. The SSITKA principle is outlined based on a simple reaction with adsorbed intermediates. The presentation of the technique is then extended to include complex multi-pool SSITKA and to reactivity distributions. A standard experimental set-up is described and several examples of reactions studied with SSITKA are included. The technique makes it possible to identify the abundance of intermediates and their kinetic parameters. It is not possible to determine the composition of the adsorbed intermediates, but recent developments combining SSITKA with spectroscopic techniques are promising developments. Examples of the combination of SSITKA and spectroscopic techniques are included.

The fundamentals of promoter effects in the Fischer - Tropsch synthesis over Co-based catalysts were systematically investigated by employing the Rh, Ir, Sb and Ga as promoters of Co/Al2O3. Various characterizations showed the higher dispersion and reducibility over Rh- and Ir- promoted catalysts would cause a promoted adsorption property, while the Sb- and Ga- promoted catalysts showed a contrary result because the formation of alloy suppressed the accessibility of Co species. Based on the experimental results and kinetic calculations, a linear relationship for the activation energies of these catalysts as a function of activation entropies was proposed. Besides, steady state isotopic transient kinetic analysis (SSITKA) revealed that promoters mainly affected the surface CHx concentration to alter the CO reaction rate, and the chain growth rate constants would affect the product distribution. Since CoRhA and CoIrA greatly enhanced the CHx concentration and inhibited the chain growth ability, they owned a higher CO reaction rate and CH4 selectivity. While the CoSbA and CoGaA suppressed the CHx concentration and intrinsic activity, but enhanced the chain growth ability, thereby revealing a lower CO reaction rate and CH4 selectivity. Furthermore, the reactivity and selectivity difference were attributed to the modification of adsorption strength dictated by the enthalpy of activation on cobalt catalysts with different promoters. These findings provide kinetic insights into the promoter effects and pave the way for optimizing catalysts.

Fischer-Tropsch synthesis over CoMn oxides is a surface-catalyzed structure-sensitive reaction. Active site changes greatly alter product distribution. The dynamics and essence of the changes in Co species during the reaction have seldom been explored. Herein, multiple characterizations were applied to confirm that two types of active sites, i.e., Co and Co2C, coexist on CoMnOx under the reaction conditions. Apart from the direct partici-pation of carbon species in the reaction, the steady-state isotopic transient kinetic analysis also suggested the mutual transformation of carbon species in Co2C. The ratio of the two paths altered with the temperature, thus leading to Co2C-rich and Co-rich surfaces at intermediate and high temperatures, respectively. Kinetic calcula-tions and microkinetic modeling showed that Co2C-rich surfaces facilitated CO adsorption and suppressed hydrogen adsorption. This gave high carbon-chain growth activity. The Co-rich surfaces showed high vacancy and hydrogen coverages, lowering the CO activation and CH4 formation energies.

Transition metals supported on carbons play an important role in catalysis and energy storage. By pyrolysis of metal alginate, highly active catalysts for the Fischer-Tropsch synthesis (FTS) can be produced. However, the evolution of the carbon (alginate) and transition metal (Fe3+) during pyrolysis remains largely unknown and was herein corroborated with several advanced in situ techniques. Initially, Fe3+ was reduced to Fe2+, while bound to alginate. FeO nucleated above 300 C, destabilizing the alginate functional groups. Increasing temperatures improved carbonization of the carbon support, which facilitated reduction of FeO to alpha-Fe at 630 ?. Catalysts were produced by pyrolysis between 400 and 700 ?, where the highest FTS activity (612 mu molCO gFe(-1) s(-1)) was achieved for the sample pyrolyzed at low temperature. Lower metal loading, due to less decomposition of alginate, moderated sintering and yielded larger catalytic surface areas. The results provide valuable knowledge for rational design of metal-alginate-based materials.

The dynamic nature of active sites on La-Fe-based perovskites in chemical looping methane oxidation has been studied. According to experimental observations, the reduction of the oxygen carrier is divided into three stages occurring at different active ensembles where the Fe cations are in different coordination environments. The Mars van Krevelen mechanism formulated by microkinetic analysis describes well the effect of oxygen vacancy on the catalytic performance of LaFeO3. CO2 is not produced by CO oxidation but rather is a primary product of methane oxidation, and the presence of surface oxygen vacancies would dramatically increase the overall energy barrier for total combustion, thus decreasing the selectivity toward CO2. Hence, the Fe coordination environment (and hence the oxygen vacancy concentration) is the key parameter governing the catalyst selectivity, in the sense that methane oxidation can vary from total combustion on the O-rich surface to fully selective partial oxidation on the O-deficient surface.

CO hydrogenation has been studied on cobalt foils as model catalysts for Fischer–Tropsch (FT) synthesis. The effect of pretreatment (number of calcinations and different reduction times) for cobalt foil catalysts at 220 °C, 1 bar, and H2/CO = 3 has been studied in a microreactor. The foils were examined by scanning electron microscopy (SEM). It was found that the catalytic activity of the cobalt foil increases with the number of pretreatments. The mechanism is likely an increase in the available cobalt surface area from progressively deeper oxidation of the foil, supported by surface roughness detected by SEM. The highest FT activity was obtained using a reduction time of only 5 min (compared to 1 and 30 min). Prolonged reduction caused the sintering of cobalt crystallites, while too short of a reduction time led to incomplete reduction and small crystallites susceptible to low turn-over frequency from structure sensitivity. Larger crystals from longer reduction times gave increased selectivity to heavier components. The paraffin/olefin ratio increased with the increasing number of pretreatments due to olefin hydrogenation favored by enhanced cobalt site density. From the results, it is suggested that olefin hydrogenation is not structure sensitive, and that mass transfer limitations may occur depending on the pretreatment procedure. Produced water did not influence the results for the low conversions experienced in the present study (<6%).

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 FeMnOx 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 Fe5C2 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.

A rhenium promoted Fischer-Tropsch (FT) cobalt catalyst supported on gamma-Al2O3 has been investigated by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD), before and after reduction. Electron diffraction, High Resolution TEM and Electron Energy Loss Spectroscopy were used to confirm the oxidation state. Cobalt aggregate, particle and crystallite sizes have been studied in detail and measured by TEM and XRD. A cobalt particle size of 10.0 +/- 2.4 nm obtained from bright field TEM images for the reduced material is consistent with the XRD analysis of the calcined catalyst. After reduction dark field TEM imaging gave a volume-weighted crystallite size of 7.5 +/- 2.5 nm, which is close to the value obtained by XRD. The particles had lost the parallel orientation and physical continuity within the alumina pore structure that were present before reduction. The latter was confirmed by electron tomography. Lamellae identified with the presence of Hexagonal Close Packed cobalt were observed in the predominantly Face Centred Cubic particles.

The upgrading of biosyngas to convert methane into syngas is a necessary step for hydrogen production from biomass. However, this process is highly energy-demanding. In this study, a model biosyngas containing H-2, CH4, CO, and CO2 were thermally treated between 1200 and 1500 degrees C. The gas mixture, comprised of H-2 (33 Vol %), CH4 (12 Vol %), CO (28 vol %), CO2 (25 vol %), and He (2 vol %), was diluted with argon and the effect of reaction temperature (1200-1500 degrees C), water addition (0-44.3 mol %), and residence time (23, 46, and 76 mu s in corresponding to flow rates of 1500, 2500, and 5000 inLimin at normal temperature and pressure, respectively) were studied. A possible reaction scheme for the upgrading of the model gas is proposed based on the kinetic simulation with CHEMKIN. The main reaction pathways involve dry reforming of methane and reverse water-gas shift (WGS) reactions. The kinetic simulation explained the finding that CO production was negatively influenced by the water content via the WGS reaction. The main side reaction is the methane pyrolysis reaction which causes the formation of carbon. The carbon formed in the reforming was characterized by SEM and Raman spectroscopy. SiO2 needle-like microdomains are observed at the top of the reactor heating zone, while the central part of the heating zone was covered by carbon with a disordered, amorphous, low-density soot structure. It is proposed that the SiO2 species formed by the chemical reaction between the reactor wall material and the reactants act as a support to anchor the carbon formed.

This study reports a green, inexpensive, and highly versatile procedure to synthesize well-dispersed transition-metal nanoparticles anchored on carbon supports. The resulting metal loadings are 26 ...

CO hydrogenation is one of the most important and complex chemical reactions with various product distributions in which the product formation mechanism is not well understood yet, and the design of active and selective catalysts toward the targeted products is still a major challenge. Herein, descriptor-based microkinetic modeling is employed to analyze the product distribution and favor the catalyst screening of CO hydrogenation, which predicted well industrial catalysts for different product-related processes. The microkinetic analysis demonstrates that CO activation on the close-packed surface is slow and mainly occurs via hydrogen-assisted CO dissociation, with HCO and CH3O as the most critical intermediates. The major C1/C2 chain growth in the microkinetic model used here takes place through the coupling of CH + CO/CH3C + CO pathways. Both methane and C2/C3 olefin/paraffin formation mechanisms are dependent on metal surfaces. The data analysis illustrated similar adsorption energy of the intermediates with similar structures but different carbon numbers. The chain growth in the microkinetic model was extended to higher hydrocarbon formation in CO hydrogenation by assuming the identical adsorption energy of similarly structured intermediates. Moreover, the activity and selectivity maps successfully identify four active and selective bimetallic catalysts toward CO hydrogenation to light olefin production, where Co3Rh is experimentally proven highly active.

Understanding the secondary reactions of olefins in Fischer Tropsch Synthesis (FTS) is crucial for enhancing olefin selectivity in Fischer Tropsch to Olefin (FTO) reaction. Equimolar ethene towards CO was used as a probe to be cofed into the syngas (H-2: CO = 2) to study the effect on the product distribution over cobalt-based catalysts supported on gamma-alumina, alpha-alumina and carbon nanotube (CNT), respectively. The experiments were performed at T = 210 degrees C; P-tot = 1.85 bar, P-CO = 0.11 bar; P-H2 = 0.22 bar; P-C2H4 = 0 or 0.11 bar with Ar as the balance. On all three catalysts, ethene co-feeding at this concentration inhibited CO conversion and the products were mainly from ethene. The products showed similar Anderson-Schulz-Flory (ASF) distribution as typical for FTS products. Ethene co-feeding can both enhance the chain growth probability and significantly increase the O/P ratio of C-3-C-6 hydrocarbons. An isotopic switch from (CO)-C-12/H-2 /Ar ->(CO)-C-13/H-2/C2H4/Kr proved that ethene can hydrocrack to C-1 intermediates and be incorporated into the growing chain. On the other hand, the hydrogenation activity can be significantly suppressed due to the H-2 scavenging effect and possibly competitive adsorption with H-2.

The mechanism and structure requirements of selective and total oxidation of methane in a chemical looping process are both experimentally and theoretically examined on La1-xSrxFeO3-delta (x = 0, 0.2, and 0.5) and La0.5Sr0.5Fe1-xCoxO3-delta (x = 0.5 and 1) perovskites. The oxygen mobility in the perovskites described by the formation energy of oxygen vacancy is found to have a pronounced effect on the catalytic activity and selectivity. In particular, the selectivity is controlled largely by the surface oxygen concentration or the oxygen vacancy concentration on perovskites, which depends strongly on the bulk oxygen concentration and the relative rate of the lattice oxygen diffusion with respect to the surface reaction. The substitution of Sr for La at the A site and the substitution of Co for Fe at the B site of the ABO(3) perovskites dramatically increase the oxygen mobility. A higher oxygen diffusion rate, and hence enrichment of oxygen on the surface, would improve the catalyst selectivity toward total oxidation.

A mathematical model for a single slit packed microstructured reactor-heat exchanger in the synthesis of methanol from syngas was developed. The model constitutes a simplified 3D-pseudo homogeneous approach for a reaction slit with integrated pillar geometry. Literature kinetic rate expressions for methanol synthesis over commercial Cu/ZnO/support type catalysts were applied at 80 bar total pressure, temperature range of 473-558 K, and syngas composition of H-2/CO/(CO2N2)-N- :65/25/ 5/5 mol%. The model is found capable of predicting experimental CO conversion data with acceptable accuracy. Superior thermal stability of the microchannel upon variation of different parameters such as contact time, feed gas temperature and reaction temperature were shown. The simulation results also reveal that the microchannel reactor can operate free of performance loss due to concentrations field that may arise from overlaid temperature fields. Simulations have also been used to calculate the rapid temperature transients at the inlet. The agreement between simulation results and experimental data signifies the applicability of the developed model for further design and performance optimization of microstructured reactors for methanol synthesis and other exothermic processes.

Platinum supported on ceria and zirconia was prepared through different preparation methods: Coprecipitation (CP), spray drying (SD), and flame spray pyrolysis (FSP). The catalysts were characterized by XRD, TPR, N2 adsorption, and H2 chemisorption, and the water–gas shift activity in the range 190–310 °C and initial stability at 300–310 °C were tested. Although the spray-dried Pt/CeO2/ZrO2 catalyst shows the highest initial activity, it deactivates rapidly at 300 °C and levels out at similar activity as the coprecipitated Pt/CeO2 and Pt/CeO2/ZrO2 within a few hours. Flame spray pyrolysis appears to be a promising preparation method concerning the stability of catalysts, although the initial activity is rather poor. High activity is related to high Pt dispersion, low reduction temperature, and small support particles. The support particle size is also much affected by the preparation method.

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 Bronsted-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.

The ratio between propene and propane (C3 o/p) during Fischer–Tropsch synthesis (FTS) has been analyzed based on both literature reports and experiments for five catalysts. The latter comprise four cobalt catalysts on γ-alumina with variations in pore sizes, and one catalyst on α-alumina. Overall variations include H2/CO feed ratio, residence time, water addition, transients between test conditions, CO conversion, cobalt particle size, promoter (Re), and support material. It was possible to rationalize all data based on secondary hydrogenation of olefins. In fact, it was deduced that olefins are dominating termination products in FTS, estimated to ca. 90% for C3, but that some paraffins most likely are also produced directly. Increased residence time and high H2/CO feed ratio favors olefin hydrogenation, while added water presumably displaces hydrogen on cobalt giving enhanced C3 o/p. High cobalt dispersion favors hydrogenation, as also promoted by Re. Effect of intraparticle diffusion is seen in transient periods; for example, as water is added or depleted. There is frequently positive correlation between C3 o/p and selectivity to longer chains; the latter expressed as C5+ selectivity, as both are sensitive to hydrogen activity. Some modifications, however, are needed due to the accepted volcano plot for C5+ selectivity with cobalt crystallite size. Titania as support shows unexpectedly low C3 o/p; probably due to SMSI (strong-metal-support-interaction).

In this contribution, we combine density functional theory (DFT) calculations, experimental kinetic study and DFT-assisted analysis to elucidate the impact of the interface of monolayer Pt on the Ni surface on catalytic performance of steam methane reforming including carbon formation on core-shell (Ni@Pt) catalysts and compare it with Ni and Pt catalysts. We demonstrate that core-shell structured Ni@Pt significantly lowers the carbon formation without sacrificing much the activity. The DFT results demonstrate that the metal identity, core shell structure and support have significant impacts on the reaction mechanisms. The direct methane activation is energetically favorable reaction pathway on Ni, while the OH* assisted methane activation is the favorable pathway on Pt and Ni@Pt catalysts, where methane activation is the rate-determining step on all catalysts. We unambiguously reveal that the core-shell Ni@Pt catalyst modified the surface Pt electron density and shifted d-band center away from Fermi level compared to Ni(1 1 1) and Pt(1 1 1). It results in a strong basic surface OH* which actively reacts with CHx and thus enhances carbon formation resistance. Above all, Ni-core/Pt-shell particle could decouple the activity and carbon resistance to keep the activity and reduce carbon formation simultaneously in methane steam reforming. In addition, by taking into account the activation of steam on the support, the effective activation energy estimated from DFT-assisted analysis is well consistent with the experimental value on the both Ni and Ni@Pt catalysts, which could shed some light on building a bridge between experimental work and DFT-assisted kinetic study.

The paper presents a fundamental study of the properties and functions of well-defined Ni-Ag surface alloys in methane decomposition and steam reforming, aiming at providing a better understanding of the principle for manipulating the catalytic activity of steam reforming and suppressing carbon formation. A better insight of structure-property relationship was obtained by a kinetic study of the reactions on well-defined surface Ni-Ag alloys, which were synthesized by surface redox reaction to selectively introduce Ag atoms into the surface of Ni particles supported on hydrotalcite derived support. The effects of Ni surface alloy with Ag are three-folds in general. Replacement of Ni by Ag reduces the number of active site exponentially with increasing Ag site coverage. Ag site is not only inactive, but also significantly reduces the activity of adjacent Ni sites for methane activation in both methane decomposition and steam reforming. The third effect is to block the active sites for the nucleation and growth of the filamentous carbon. The rate of methane activation at Ni step sites was found to be 16-19 times of that on Ni terrace sites. The carbon formation rate decreased linearly with Ag site coverage and the effect of Ag is divided into two regions. At low Ag site coverages (0 to 0.055), Ag atoms preferentially deposit on Ni step sites, which has a significant effect on the methane activation compared to Ag atoms on the Ni terrace sites. The results reveal that the effects of Ag site on the carbon formation in the two regions are mostly caused by the different effects of Ag on the activity of Ni step sites and terrace sites, respectively, rather than the different ensemble sizes for carbon formation proposed in the literature.

The understanding of the water effect on olefin selectivity in Fischer-Tropsch synthesis (FTS) is limited by the complexity of the reaction network. Herein, we employ propene hydrogenation as a model reaction to isolate the water effect on olefin adsorption and hydrogenation from the complex reaction of FTS. It is clearly observed that the added water inhibits the activity of propene hydrogenation on two cobalt catalysts supported on high-surface-area alumina (HAS Al2O3) and low-surface-area alumina (LSA Al2O3), respectively. The inhibiting effect is much stronger for Co/HSA Al2O3. DFT investigation demonstrates that the in situ generated OH, rather than H2O and O, impedes the adsorption of propene and thus decreases the activity of propene hydrogenation. The suppressive effect of OH on propene adsorption is attributed to the downshift of the d-band center and the Bader charge of the catalyst surface. The DFT-based kinetic analysis finds that the higher site coverage of OH results in the more pronounced negative effect on propene hydrogenation. Furthermore, the theory of OH-induced weak olefin adsorption and low olefin hydrogenation activity could rationalize the enhancement effect of water on the olefin selectivity and the particle size dependence of the water effect in FTS. The insights obtained here may inspire researchers to optimize olefin selectivity by manipulating the electronic properties of catalysts with hydroxyl species.