Angelos M. Efstathiou

Reforming of methane with carbon dioxide to synthesis gas over supported Rh catalysts

Reforming of methane with carbon dioxide to synthesis gas (CO/H2) has been investigated over rhodium supported on SiO2, TiO2, γ-Al2O3, MgO, CeO2, and YSZ (ZrO2 (8 mol% Y2O3)) catalysts in the temperature range of 650–750°C at 1 bar total pressure. A strong carrier effect on the initial specific activity, deactivation rate, and carbon accumulation was found to exist. A strong dependence of the specific activity of the methane reforming reaction on rhodium particle size was observed over certain catalysts. Tracing experiments (using 13CH4) coupled with temperature-programmed oxidation (TPO) revealed that the carbon species accumulated on the surface of the Rh/Al2O3 catalyst during reforming reaction at 750°C are primarily derived from the CO2 molecular route. The amount of carbon present on the working catalyst surface which is derived from the CH4 molecular route is found to be very small.

Transient methods in heterogeneous catalysis: Experimental features and application to study mechanistic aspects of the CH4/O2 (OCM), NH3/O2 and NO/He reactions

Experimental features of the transient method using concentration forcing functions to study the surface dynamics of a heterogeneous catalytic reaction at 1 atm and with on line mass spectroscopy are discussed. Emphasis is given in the presentation and discussion of two important transient techniques, namely the ‘steady state tracing’ or ‘steady state isotopic transient kinetic analysis (SSITKA)’ and the ‘transient isothermal hydrogenation’ of adsorbed surface reaction intermediate species. The SSITKA technique allows to measure the concentration of the active reaction intermediate species, the intrinsic reactivity of the site associated with a given adsorbed reaction intermediate species and elementary step, and also velocities of elementary steps in the reaction network. The ‘transient isothermal hydrogenation’ technique, when applied in an appropriate reaction system and coupled with the SSITKA technique, allows for the measurement of concentration of inactive (spectator) reaction intermediate species. In addition, important intrinsic kinetic parameters and other mechanistic information related to the hydrogenation process can be obtained. The bulk of this paper concerns the application of transient methods (various kinds of transient experiments) in order to elucidate mechanistic aspects of: (a) the oxidative coupling of methane (OCM) reaction to C2-hydrocarbons over mixed metal oxide catalysts, where a synergy phenomenon between the two components of the mixed metal oxide catalyst occurs, (b) the ammonia oxidation reaction over V2O5/TiO2 catalysts, and (c) the NO decomposition reaction over Rh supported on TiO2 and W6+-doped TiO2 carriers.

Mathematical modeling of the oxygen storage capacity phenomenon studied by CO pulse transient experiments over Pd/CeO2 catalyst

A mathematical model has been developed for the first time to study the oxygen storage capacity (OSC) phenomenon by the CO pulse injection technique over a 1 wt% Pd/CeO2 model catalyst in the 500–700 ◦ C range. A two-step reaction path that involves the reaction of gaseous CO with the oxygen species of PdO (pre-oxidized supported palladium particles in the 500–700 ◦ C range) and of the backspillover of the oxygen process from ceria to the oxygen vacant sites of surface PdO has been proven to better describe the outlet CO pulse transient response and the experimentally measured quantity of OSC (µatoms of O/g) obtained in a CSTR microreactor. With the proposed mathematical model, the transient rates of the CO oxidation reaction and of the back-spillover of the oxygen process can be calculated. In the 500–700 ◦ C range, the transient rate of CO oxidation was always greater than that of the back-spillover of oxygen. The ratio, ρ ,o f the maximum CO oxidation rate to the maximum back-spillover of the oxygen rate was found to decrease with increasing reaction temperature in the 500–700 ◦ C range. In particular, at 500 and 700 ◦ Ct he value ofρ was found to be 1.6 and 1.2, respectively. The present mathematical model allows also the calculation of the intrinsic rate constant k1 (s −1 ) of the Eley–Rideal step for the reaction of gaseous CO with surface oxygen species of PdO to form CO2. An activation energy of 9.2 kJ/mol was estimated for this reaction step. In addition, an apparent rate constant k app (s −1 ) was estimated for the process of back-spillover of oxygen. The ratio of the two rate constants (k1/k app ) was found to be greater than 100 in the 500–700 ◦ C range. A Langmuir–Hinshelwood surface elementary reaction step of adsorbed CO with atomic oxygen of PdO failed to describe the experimental transient kinetics of CO oxidation in the 500–700 ◦ C range. The results of the present work provide the means for a better understanding of the effects of various additives and contaminants present in a three-way commercial catalytic converter and other related model catalysts on their OSC kinetic behavior. In addition, intrinsic effects of a given regeneration method for a commercial three-way catalyst on the OSC phenomenon could better be studied by making use of the results of the present mathematical model.

Hydrogen Production Technologies: Current State and Future Developments

Hydrogen (H2) is currently used mainly in the chemical industry for the production of ammonia and methanol. Nevertheless, in the near future, hydrogen is expected to become a significant fuel that will largely contribute to the quality of atmospheric air. Hydrogen as a chemical element (H) is the most widespread one on the earth and as molecular dihydrogen (H2) can be obtained from a number of sources both renewable and nonrenewable by various processes. Hydrogen global production has so far been dominated by fossil fuels, with the most significant contemporary technologies being the steam reforming of hydrocarbons (e.g., natural gas). Pure hydrogen is also produced by electrolysis of water, an energy demanding process. This work reviews the current technologies used for hydrogen (H2) production from both fossil and renewable biomass resources, including reforming (steam, partial oxidation, autothermal, plasma, and aqueous phase) and pyrolysis. In addition, other methods for generating hydrogen (e.g., electrolysis of water) and purification methods, such as desulfurization and water-gas shift reactions are discussed.

Catalytic behavior of La-Sr-Ce-Fe-O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La 0.5 Sr x Ce y FeO z were prepared by using the nitrate method and characterized by XRD and Mossbauer techniques. The crystal phases detected were perovskites LaFeO 3 and SrFeO 3- and oxides α-Fe 2 O 3 and CeO 2 depending on x and y values, The low surface area ceramic materials have been tested for the NO+CO and NO+CH 4 +O 2 (lean-NO, ) reactions in the temperature range 250-550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La 3+ cation at A-site with divalent Sr +2 and tetravalent Ce +4 cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La 0.5 Sr x Ce y FeO z materials to reduce NO by CO or by CH 4 under lean-NO x conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s -1 ) at 300 C (based on NO chemisorption) revealed values comparable to Rh/α-Al 2 O 3 catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La 0.5 Sr x Ce y FeO z solids and their catalytic activity. O 2 TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O 2 TPD studies was observed for the La 0.5 Sr 0.2 Ce 0.3 FeO z and La 0.5 Sr 0.5 FeO z solids.

The COH2 reaction on RhAl2O3: II. Kinetic study by transient isotopic methods

Abstract Transient isotopic methods have been used to study the CO H 2 reaction over 5% Rh Al 2 O 3 in the temperature range 180–260 °C. The steady-state tracing method permits the determination of the surface coverage of CO and active carbon Cα during reaction, without the need for quenching in helium and subsequent titration by hydrogen. The results confirm that the sequence of steps for methane formation passes through a small reservoir of active carbon (θcα

The COH2 reaction on RhAl2O3: I. Steady-state and transient kinetics

Abstract Various hydrogen titrations yield the surface composition of Rh Al 2 O 3 during the CO H 2 reaction at temperatures in the range 180–260 °C. There is a very small amount of active carbon C a , 0.02–0.06 monolayer, and 0.70–0.98 monolayer of CO on the metal surface. The coverage of H is low. Also present are inactive carbon C β on the metal and formate species on the support. The surface coverages are consistent with the observed steady-state kinetics, which result in methane as the predominant product. Although the rate-limiting step may be the dissociation of surface CO, it is found that the reaction sequence passes through a small coverage of highly active surface carbon C α

Regeneration of three-way automobile catalysts using biodegradable metal chelating agent—S, S-ethylenediamine disuccinic acid (S, S-EDDS)

Regeneration of the activity of three-way catalytic converters (TWCs) was tested for the first time using a biodegradable metal chelating agent (S, S-ethylenediamine disuccinic acid (S, S-EDDS). The efficiency of this novel environmentally friendly solvent in removing various contaminants such as P, Zn, Pb, Cu and S from commercial aged three-way catalysts, and improving their catalytic performance towards CO and NO pollutants removal has been investigated. Four samples of catalysts from the front and rear inlets of two different TWCs with different mileages and aged under completely different driving conditions were investigated. The catalysts were characterized using various techniques, such as X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area measurements (N(2) adsorption at 77 K). Quantitative ICP-MS analyses and SEM-EDS studies show the removal of Zn, P and Pb. SEM-EDS images obtained at low magnification (50 μm) showed considerable differences in the surface morphology and composition after washing with S, S-EDDS. However, XRD studies indicated neither little to no removal of major contaminant compound phases nor major structural changes due to washing. Correspondingly, little or no enhancement in BET surface area was observed between the used and washed samples. Light-off curves show that the regeneration procedure employed can effectively improve the catalytic performance towards NO pollutant.

Enthalpy and entropy of H2 Adsorption on Rh/Al2O3 measured by temperature-programmed desorption

Abstract Temperature-programmed desorption spectroscopy under pseudo-equilibrium conditions has been used to obtain the coverage dependence of the heat and entropy of hydrogen adsorption for a 5-wt% Rh/Al 2 O 3 catalyst. This method is considered to be more convenient than volumetric or flow methods. The latter require more effort to obtain various isotherms and isobars, from which the heat and entropy of adsorption are obtained in a manner similar to that in this study. Extrapolation of the heat and entropy of adsorption to zero coverage yields values of 24 kcal/mol and 38 cal/mol-K, respectively. For the former value, a binding energy of 64 kcal/mol is obtained, which is comparable to reported values for a polycrystalline Rh surface.

Partial oxidation of methane to synthesis gas over Ru/TiO2 catalysts

The catalytic partial oxidation of methane to synthesis gas is investigated over Group VIII metal catalysts. It is shown that while all catalysts promote methane combustion followed by reforming with H2O and CO2, the Ru/TiO2 catalyst, to a large extent, promotes the direct formation of synthesis gas. The existence of the direct reaction route is probed by steady-state isotopic transient experiments. The extent of the direct route is found to be sensitive to modifications of the TiO2 carrier. FTIR and XANES studies indicate that the unique performance of the Ru/TiO2 catalyst is related to its high resistance to oxidation, which renders high selectivity to synthesis gas in the presence of oxygen.