[show abstract][hide abstract] ABSTRACT: In the present work, two different methods of catalyst promotion, the electrochemical promotion (EP) and the conventional promotion (CP), were comparatively applied on a catalytic system of significant environmental and practical importance: the N2O reduction by hydrocarbons (alkanes and alkenes), in the presence or absence of O2, over Pd catalysts. A galvanic cell of the type Pd/K+-conducting β″-Al2O3/Au was constructed for the application of the EP concept whereas the CP concept was investigated via a series of highly dispersed Pd/γ-Al2O3 catalysts, conventionally promoted (by impregnation) with K modifier. Given that EP is a straightforward, efficient and in situ way for investigating the effect of a promoter on a catalytic system, the present study is dealing with its prior use as a rapid “research tool” for exploring the effect of K promoter on the catalytic system under consideration. Subsequently, the insight obtained from EP studies is applied to the design of conventional catalysts' composites, i.e. Pd/γ-Al2O3 catalysts conventionally promoted by K at loadings indicated from EP studies. For the system investigated, the optimal promoter loading was in the range of ∼ 0.45–0.55, in terms of K-coverage. In this range of K-loadings significant enhancement on de-N2O activity was obtained under reducing conditions using both methods of K-promotion. However, in the presence of excess oxygen in the reaction mixture the effect of K-promotion was less pronounced, independently of the reducing agent used.
Solid State Ionics 06/2011; 192(1):653-658. · 2.05 Impact Factor
[show abstract][hide abstract] ABSTRACT: The present work aims to explore the activity of Cu/CeO2 composites as anodic electrodes in direct iso-octane SOFCs. When the cell was operated as a membrane reactor, the effect of temperature, Pi-C8H18 and applied anodic overpotentials on the electrocatalytic activity and products' distribution, at both open and closed circuit conditions, was examined. Additionally, in situ DRIFT spectroscopy was carried out in order to correlate the performance of Cu/CeO2 with its surface chemistry during iso-octane decomposition. Under the “fuel cell” mode of operation, the electrochemical performance and stability of Cu/CeO2 were investigated by voltage–current density–power density and AC impedance measurements. The results reveal that at high anodic polarization conditions, carbon formation can be noticeably restricted (verified also by EDAX analysis), while H2 production was enhanced due to partial oxidation, steam reforming, dehydrogenation and water gas shift reactions. Achieved power densities were found to substantially increase both with temperature and Pi-C8H18, while minor performance degradation was indicated in the step-change tests, where the overall activity of Cu–CeO2 electrodes remained essentially unaffected.
Solid State Ionics 01/2011; 192(1):435-443. · 2.05 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the present work the basic transport processes occurring in a planar solid oxide fuel cell (SOFC) were simulated. The Navier–Stokes and energy equations, including convective and diffusive terms, were numerically solved by the commercial CFD-ACE+ program along with the mass and charge transport equations. To achieve this, a three-dimensional geometry for the planar fuel cell has been built. It was also assumed that the feedstream was a mixture of methane and steam in a ratio avoiding carbon formation. In accordance with the literature, the steam reforming reaction, the water–gas shift reaction as well as electrochemical reactions were introduced to the model. The spatial variation of the mixture's velocity, the temperature profiles and the species concentrations (mass fractions) were obtained. Furthermore, the effect of temperature on the produced current density was investigated and compared to the outcomes from isothermal imposed conditions.
Chemical Engineering Research & Design - CHEM ENG RES DES. 01/2011; 89(2):224-229.
[show abstract][hide abstract] ABSTRACT: Mathematical modeling of transport and electrochemical phenomena within SOFCs can lead to improved understanding of the involved physical, electrical, and chemical processes and represents a powerful tool for their development. In this context, the present work illustrates a three-dimensional CFD simulation of a planar SOFC unit cell fuelled by modeled biogas/steam mixtures. The simulations estimate the distribution of gas species, the current densities and the potentials, as well as the temperature gradients and confirm that equimolar CH4/CO2 biogas leads to improved performance, while minimal steam addition can prevent carbon deposition.
[show abstract][hide abstract] ABSTRACT: A series of metal catalysts (Pd, Rh, Ru, Cu, Fe, In and Ni) supported on γ-Al2O3 carrier, were evaluated during N2O catalytic conversion to N2 in the absence and presence of excess oxygen and reducing agents (CH4 or C3H8). Among all catalysts tested, Pd-, Ru- and Rh-based samples exhibited the best catalytic performance, in all reaction conditions
examined. The reaction was inhibited by O2, in particular at lower temperatures, while its effect was essentially negligible at higher ones. In the presence of reducing
agents and under lean reaction conditions, N2O conversion was comparably enhanced, with C3H8 being more efficient than CH4; however even in the presence of hydrocarbons N2O decomposition is the major pathway for N2O abatement, since reducing agents mainly act as oxygen scavengers reducing and concurrently activating the metal sites. The
influence of different co-existing gases (CO, H2O and SO2) on the performance of Pd supported catalysts was also investigated, whereas thermal stability tests in the presence of SO2 indicate a gradual irreversible decrease in activity until a new steady state was established.
Topics in Catalysis 12/2009; 52(13-20):1880-1887. · 2.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: The present study aims at exploring the surface and catalytic behavior of Rh/gamma-Al(2)O(3) catalysts during the selective reduction of NO by C(3)H(8) in the presence of excess oxygen, H(2)O, and SO(2) with particular emphasis on identifying the elementary steps that describe the reaction mechanism. To this end, detailed activity and stability tests were employed and a precise kinetic analysis was carried out at differential conditions to elucidate the effect of each reactant, including H(2)O and SO(2), on the total reaction rate. At the same time, temperature programmed desorption (TPD) studies in combination with in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy were carried out under various reaction conditions to correlate the catalytic performance of Rh/gamma-Al(2)O(3) catalyst with its corresponding surface chemistry. The results reveal that in the absence of H(2)O and SO(2), the reaction follows a typical "reduction" type mechanism, where the active intermediates (NO(X), carboxylates, isocyanates) are interacting to yield the final products. In this reaction sequence the formation of carboxylate (C(x)H(y)O(z)) species is considered as the rate determining step. Water affects in a different way the NO and C(3)H(8) conversion performance of Rh/gamma-Al(2)O(3) catalyst; its effect is totally reversible in the case of C(3)H(8) oxidation, while the NO reduction was permanently affected mainly due to the oxidation of Rh active sites. In contrast, SO(2) poisons both reactions irreversibly via the formation of strongly adsorbed sulfate compounds, which hinder the adsorption and consequently the activation of reactants.
The Journal of Physical Chemistry A 10/2009; 114(11):3969-80. · 2.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: The integration of solid oxide fuel cells (SOFCs) in biomass gasification–turbine processes was studied for the estimation of the overall electrical efficiency. Since both processes operate close to 1000 °C, heat integration is one of the benefits of the proposed scheme. Heat generated at the SOFC and the afterburner of the integrated process was found sufficient to cover the demands of gasification and reforming, in any examined case, while a significant heat excess was available to a bottoming thermal cycle for additional power generation. The electrical efficiency of the integrated process was found to overcome 60% of the low heating value of the biomass feed. SOFC's contribution to the overall electrical power output was of the order of 70%, and fuel utilization at the SOFC was recognized as the most crucial operational parameter.
[show abstract][hide abstract] ABSTRACT: The integration of biomass gasification with SOFCs offers the potential of highly efficient and renewable power generation, primarily in modular solutions. SOFC seems to be the most promising fuel cell technology of biomass gasifier producer gases. Solid oxide fuel cells, because of their high operating temperature, do not require pure hydrogen as fuel, exhibiting high fuel flexibility. Sufficient amounts of cereal, cotton, corn, olive, coffee or palm tree residues are available in Mediterranean areas, while the climatic conditions are favorable for energy crops cultivations. Their residues can be utilized for electricity production by modular biomass gasification-based solid oxide fuel cells (SOFC).
Renewable and Sustainable Energy Reviews. 01/2008;
[show abstract][hide abstract] ABSTRACT: The effect of hydrogen partial pressure (1.3–5.8 kPa) and cell temperature (600–800 °C) on the kinetics and mechanism of the charge transfer electrode reaction taking place at the three phase boundary H2–Fe–SCY (SCY = SrCe0.95Yb0.05O2.975), was thoroughly examined by performing electrode polarization measurements. A three electrode single chamber proton conducting solid electrolyte cell of the type Fe–SCY–Au, was used in order to conduct the electrode kinetic studies. The steady-state current–overpotential characteristics were analysed with the high field approximations of the Butler–Volmer equation, by taking into account the presence of limiting currents. Both, apparent exchange current density, Io, and anodic/cathodic charge transfer coefficients (αa / αc), were calculated. Limiting currents, Il, were observed in all reaction conditions. The apparent reaction order, q, was found in most cases, within experimental error, close to 0.5 suggesting a possible reaction model, where a competition exists between charge transfer and mass transport of hydrogen ad-atoms or protons along the electrode/solid electrolyte interface.
Solid State Ionics 04/2007; 178(s 7–10):649–656. · 2.05 Impact Factor
[show abstract][hide abstract] ABSTRACT: The (electro-)kinetics of the reverse water gas shift (RWGS) reaction was studied in a solid oxide fuel cell (SOFC) of the type Pt/YSZ/Pt. The effect of imposed potentials, cell temperature (650–800°C), H2 (1–10kPa) and CO2 (1–10kPa) partial pressures on the kinetics and mechanism of the catalytic and electrocatalytic RWGS reaction, were systematically examined. The apparent catalytic activation energy was found equal to 15.6kcal/mol, while H2 and CO2 apparent reaction orders were equal to 0.5 and 0.7, respectively. At both open and closed circuit operation, the associative formate decomposition reaction mechanism was considered to describe kinetics. Under closed circuit operation, rate enhancement factor, |Λ|, values up to 10 were achieved. Finally, current density–voltage and current density–power density characteristics of the cell were recorded at various temperatures and gas mixtures of CO2 and H2. It was found that electrical power output of the cell was optimized by increasing temperature and decreasing CO2/H2 feed ratio. Maximum power density obtained was 9mW/cm2 (at 520mV cell voltage and a current density of 17.3mA/cm2, at 800°C and PCO2/PH2=0.16).
[show abstract][hide abstract] ABSTRACT: In this paper the integrated process of biomass gasification and a solid oxide fuel cell (SOFC) was studied in terms of thermodynamics. The study is based on an ongoing project intending to develop an innovative sustainable technology with high efficiency. According to some assumptions, the energy balance revealed that the process can be auto-thermal. Furthermore, and due to the utilization of the hydrogen content of steam utilized in the reforming stage, the overall efficiencies to electrical power could reach very high levels.
International Journal of Hydrogen Energy. 01/2007;
[show abstract][hide abstract] ABSTRACT: The catalytic and electrocatalytic behavior of the La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) perovskite deposited on yttria-stabilized zirconia (YSZ) was studied during the reaction of methane oxidation. Experiments were carried out in a well-mixed (CSTR) reactor at atmospheric pressure, in the 600–900°C range. When, instead of co-feeding with methane in the gas phase, oxygen was electrochemically supplied as O2−, considerable changes in the methane conversion and product selectivity were observed. Catalytic and electrocatalytic results were compared to those obtained when the LSCF served as a dense mixed-conducting membrane supplying oxygen to the methane feed stream because of the oxygen partial-pressure gradient across the membrane.
[show abstract][hide abstract] ABSTRACT: The catalytic and electrocatalytic behavior of the La0.6Sr0.4Co0.8Fe0.2O3−a perovskite-type oxide deposited on Yttria Stabilized Zirconia, was studied during the reaction of ethylene oxidation. Experiments were carried out in a continuous-flow well-mixed reactor (CSTR) at atmospheric total pressure and at temperatures between 350 and 550 °C. Under open circuit, the technique of solid-electrolyte potentiometry (SEP) was used in order to monitor the activity of oxygen on the catalyst surface. The main products were those of complete oxidation and only small quantities of carbon were observed under certain conditions. The reaction was also studied under closed circuit, where carbon was not observed under conditions at which it was formed with using gaseous oxygen (open circuit). High rate enhancement factor (A) values were observed under fuel lean conditions. The A values increased with decreasing the ratio of ethylene to oxygen.
[show abstract][hide abstract] ABSTRACT: This work aims to investigate the efficiency of Cu–CeO2 as anodic composites in direct CH3COOH fed SOFC reactors for power generation. When the cell operated as an electrochemical membrane reactor, the effect of temperature, PCH3COOH and anodic overpotentials on the catalytic activity and selectivity of Cu/CeO2 for CH3COOH decomposition and electro-oxidation at both open and closed circuit operation was explored. In addition, in situ DRIFT spectroscopy was employed in order to correlate the Cu–CeO2 performance with its surface chemistry. In the fuel cell mode, the electrochemical performance of Cu–CeO2 was investigated by voltage–current density–power density and AC impedance measurements. The results reveal that at open circuit conditions, CH3COOH and its derived carbonaceous and oxygenate active intermediates are both thermally and catalytically decomposed to final products. At anodic polarization conditions, Cu–CeO2 exhibited high catalytic activity towards the electro-oxidation of all combustible species, while carbon deposition was noticeably limited. At fuel cell operation, ohmic losses were the prevailing source of polarization, mainly attributed to the anodic interfacial resistance, which is significantly influenced by temperature and fuel type. The deconvolution of the impedance spectra fitted into the Randles circuit, showed that at CH3COOH containing reacting mixtures, the electrode performance was mainly determined by the corresponding charge transfer processes of the existing combustible species. On the other hand, when H2/He mixtures were fed in the cell, diffusion resistance altered the performance of Cu–CeO2 electrodes.
Solid State Ionics 225:398–407. · 2.05 Impact Factor