The aim of this work is an investigation on a series of Pd-doped cobalt spinels catalysts for the lean CH4 combustion reaction. All the catalysts were synthesized and fully characterized from the structural and surface point of view (XRD, XRF, RS, BET, XPS, and FESEM) and then tested towards the oxidation of CH4 in lean conditions. The work was divided into two parts. In the first part, different catalysts at powder level were screened to optimize the design of Pd-doped cobalt spinel catalysts. In the second part, the best performing Pd-based catalysts previously selected were coated on structures of various nature (ceramic monoliths and foams) and tested towards the lean methane combustion to simulate a possible real applications (abatement of unburned CH4 residues from compressed natural gas vehicles or ventilation air methane emissions in coal mines). This thesis is organized as a collection of papers, either published during the Ph.D. or submitted for publications. In the first part (catalysts at powder level), the influence of different synthesis methods on the preparation of Pd/Co3O4 catalysts was evaluated (Paper I). Next, the role of the Pd doping on Co3O4 was investigated to determine the optimal Pd loading (Paper II). Then the Pd/Co3O4 catalyst formulation was optimized: the important catalysts’ features determining the reactivity were exploited in a technologically relevant methane concentration range (Paper III). In the end, a series of cobalt iron spinels was investigated to synthesize active catalysts, with a lower content of Co, with the aim of reducing the production costs (Fe cheaper than Co) (Paper IV). As main results, the synthesis method influences the catalytic activity. Indeed, the undoped spinels synthesized by solution combustion synthesis exhibit a better activity respect to the undoped spinel synthesized via precipitation. Thus, evaluating all the informations coming from the various characterizations, the influence of the synthesis method on the catalytic activity of cobalt oxide seems to be related with its redox state. Palladium doping improves the catalytic activity independently of the synthesis method, palladium load, and CH4 inlet concentration. Indeed a complete CH4 oxidation can be reached at a temperature lower than 430 °C for undoped spinels. The addition of palladium led to the formation of a reduced cobalt oxide phase which could contribute to a generation of active oxygen species under reaction conditions. The optimal Pd load is 3wt.%, calculated as PdO. The benefit was due to the existence of well-dispersed Pd nanocrystals. At lower Pd concentrations (0.5% Pd) the amount of Pd was insufficient to catalyze CH4 combustion effectively in the applied conditions, while the specific activity was lost for higher Pd concentrations (5% Pd) because of the agglomeration of Pd nanoparticles. Finally, the addition of Fe to Co3O4 did not affect the catalytic activity of undoped catalysts, supposedly because the rate-determining step of the reaction is the activation of the C–H bond in the CH4 molecule, and apparently, Fe is not influencing the lattice oxygen stability. In the second part of the work, undoped and 3 wt.% Pd-doped cobalt spinel catalysts were deposited on monoliths and open cell foams via solution combustion synthesis using glycine as precursors. The catalyzed structures were impregnated with Pd via wetness impregnation. The catalytic activity were tested toward the methane oxidation in lean conditions, in a gas mixture containing 0.5 vol.% or 1 vol.% CH4 at three different weight hourly space velocity (30, 60, and 90 NL h–1 gcat–1). The addition of Pd improved the catalytic activity of all structures independently on the test conditions. The pressure drop and heat transfer properties were evaluated for monoliths and foams as well. The results show that the foams exhibit an higher catalytic activity than the monolith. Moreover, all the catalysts show better activity at lower weight hourly space velocity. The open cell foam based on zirconia, with the biggest average pore diameter, exhibits the best catalytic activity. In general, zirconia-based foams show a better activity than alumina and silicon carbide ones for all test conditions. In order to well understand the different behavior of the foams, pressure drop measurements and thermal conductivity tests were carried out. From these measurements, the zirconia-based foams have lower pressure drop and lower overall heat exchange coefficients than the monolith and alumina and silicon carbide foams. This aspect can be explained by the higher thermal conductivity of alumina and silicon carbide materials. In conclusion, the obtained results represent a promising scientific advance because they demonstrate the good and stable performance of a 3% Pd/Co3O4 catalyst on a zirconia-based structured support for methane combustion in adiabatic or quasi-adiabatic applications (Papers V and VI). Finally, the basic Co3O4 spinel, synthesized with different methods, was tested as an alternative anodic catalyst for the electrochemical oxygen evolution reaction, the typical reaction of an electrolyzer (Paper VII).

Catalytic combustion of methane in lean conditions on Pd/Co​3O4 ​: from powdered to open-cell foam supported catalysts / Ercolino, Giuliana. - (2017).

Catalytic combustion of methane in lean conditions on Pd/Co​3O4 ​: from powdered to open-cell foam supported catalysts

ERCOLINO, GIULIANA
2017

Abstract

The aim of this work is an investigation on a series of Pd-doped cobalt spinels catalysts for the lean CH4 combustion reaction. All the catalysts were synthesized and fully characterized from the structural and surface point of view (XRD, XRF, RS, BET, XPS, and FESEM) and then tested towards the oxidation of CH4 in lean conditions. The work was divided into two parts. In the first part, different catalysts at powder level were screened to optimize the design of Pd-doped cobalt spinel catalysts. In the second part, the best performing Pd-based catalysts previously selected were coated on structures of various nature (ceramic monoliths and foams) and tested towards the lean methane combustion to simulate a possible real applications (abatement of unburned CH4 residues from compressed natural gas vehicles or ventilation air methane emissions in coal mines). This thesis is organized as a collection of papers, either published during the Ph.D. or submitted for publications. In the first part (catalysts at powder level), the influence of different synthesis methods on the preparation of Pd/Co3O4 catalysts was evaluated (Paper I). Next, the role of the Pd doping on Co3O4 was investigated to determine the optimal Pd loading (Paper II). Then the Pd/Co3O4 catalyst formulation was optimized: the important catalysts’ features determining the reactivity were exploited in a technologically relevant methane concentration range (Paper III). In the end, a series of cobalt iron spinels was investigated to synthesize active catalysts, with a lower content of Co, with the aim of reducing the production costs (Fe cheaper than Co) (Paper IV). As main results, the synthesis method influences the catalytic activity. Indeed, the undoped spinels synthesized by solution combustion synthesis exhibit a better activity respect to the undoped spinel synthesized via precipitation. Thus, evaluating all the informations coming from the various characterizations, the influence of the synthesis method on the catalytic activity of cobalt oxide seems to be related with its redox state. Palladium doping improves the catalytic activity independently of the synthesis method, palladium load, and CH4 inlet concentration. Indeed a complete CH4 oxidation can be reached at a temperature lower than 430 °C for undoped spinels. The addition of palladium led to the formation of a reduced cobalt oxide phase which could contribute to a generation of active oxygen species under reaction conditions. The optimal Pd load is 3wt.%, calculated as PdO. The benefit was due to the existence of well-dispersed Pd nanocrystals. At lower Pd concentrations (0.5% Pd) the amount of Pd was insufficient to catalyze CH4 combustion effectively in the applied conditions, while the specific activity was lost for higher Pd concentrations (5% Pd) because of the agglomeration of Pd nanoparticles. Finally, the addition of Fe to Co3O4 did not affect the catalytic activity of undoped catalysts, supposedly because the rate-determining step of the reaction is the activation of the C–H bond in the CH4 molecule, and apparently, Fe is not influencing the lattice oxygen stability. In the second part of the work, undoped and 3 wt.% Pd-doped cobalt spinel catalysts were deposited on monoliths and open cell foams via solution combustion synthesis using glycine as precursors. The catalyzed structures were impregnated with Pd via wetness impregnation. The catalytic activity were tested toward the methane oxidation in lean conditions, in a gas mixture containing 0.5 vol.% or 1 vol.% CH4 at three different weight hourly space velocity (30, 60, and 90 NL h–1 gcat–1). The addition of Pd improved the catalytic activity of all structures independently on the test conditions. The pressure drop and heat transfer properties were evaluated for monoliths and foams as well. The results show that the foams exhibit an higher catalytic activity than the monolith. Moreover, all the catalysts show better activity at lower weight hourly space velocity. The open cell foam based on zirconia, with the biggest average pore diameter, exhibits the best catalytic activity. In general, zirconia-based foams show a better activity than alumina and silicon carbide ones for all test conditions. In order to well understand the different behavior of the foams, pressure drop measurements and thermal conductivity tests were carried out. From these measurements, the zirconia-based foams have lower pressure drop and lower overall heat exchange coefficients than the monolith and alumina and silicon carbide foams. This aspect can be explained by the higher thermal conductivity of alumina and silicon carbide materials. In conclusion, the obtained results represent a promising scientific advance because they demonstrate the good and stable performance of a 3% Pd/Co3O4 catalyst on a zirconia-based structured support for methane combustion in adiabatic or quasi-adiabatic applications (Papers V and VI). Finally, the basic Co3O4 spinel, synthesized with different methods, was tested as an alternative anodic catalyst for the electrochemical oxygen evolution reaction, the typical reaction of an electrolyzer (Paper VII).
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11583/2675699
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