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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. · 2.65 Impact Factor
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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. · 2.62 Impact Factor
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ABSTRACT: The present study aims at exploring the surface and catalytic behavior of Rh/γ-Al2O3 catalysts during the selective reduction of NO by C3H8 in the presence of excess oxygen, H2O, and SO2 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 H2O and SO2, 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/γ-Al2O3 catalyst with its corresponding surface chemistry. The results reveal that in the absence of H2O and SO2, the reaction follows a typical “reduction” type mechanism, where the active intermediates (NOX, carboxylates, isocyanates) are interacting to yield the final products. In this reaction sequence the formation of carboxylate (CxHyOz) species is considered as the rate determining step. Water affects in a different way the NO and C3H8 conversion performance of Rh/γ-Al2O3 catalyst; its effect is totally reversible in the case of C3H8 oxidation, while the NO reduction was permanently affected mainly due to the oxidation of Rh active sites. In contrast, SO2 poisons both reactions irreversibly via the formation of strongly adsorbed sulfate compounds, which hinder the adsorption and consequently the activation of reactants.
10/2009;
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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. · 2.95 Impact Factor
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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.
Solid State Ionics.
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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 192(1):435-443. · 2.65 Impact Factor
