The effect of the three-dimensionally ordered macroporous (3DOM) structure and the Ni doping of CeO2 on the physicochemical properties and catalytic activity for soot combustion was studied. ...Moreover, the way in which Ni is introduced to the ceria support was also investigated. For this, CeO2 supports were synthesized with uncontrolled (Ref) and 3DOM-structured morphology, and their respective Ni/CeO2 catalysts were prepared by impregnation of the previously synthesized supports or by successive impregnation of both precursors (Ni and Ce) on the 3DOM template. Conclusions reached in this study are: (1) the 3DOM structure increases the surface area of the catalysts and improves the catalyst–soot contact. (2) The doping of CeO2 with Ni improves the catalytic activity because the NiO participates in the catalytic oxidation of NO to NO2, and also favors the production of active oxygen and the catalyst oxygen storage capacity. (3) Ni incorporation method affects its physicochemical and catalytic properties. By introducing Ni by successive infiltration in the solid template, the CeO2 crystal size is reduced, Ni dispersion is improved, and the catalyst reducibility is increased. All of these characteristics make the catalyst synthesized by successive infiltration to have higher catalytic activity for soot combustion than the Ni-impregnated CeO2 catalyst.
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Effect of Pr in CO2 Methanation Ru/CeO2 Catalysts Rodríguez, Sergio López; Davó-Quiñonero, Arantxa; Juan-Juan, Jerónimo ...
Journal of physical chemistry. C,
06/2021, Volume:
125, Issue:
22
Journal Article
Peer reviewed
Open access
CO2 methanation has been studied with Pr-doped Ru/CeO2 catalysts, and a dual effect of Pr has been observed. For low Pr content (i.e., 3 wt %) a positive effect in oxygen mobility prevails, while for ...high Pr doping (i.e., 25 wt %) a negative effect in the Ru–CeO2 interaction is more relevant. Isotopic experiments evidenced that Pr hinders the dissociation of CO2, which takes place at the Ru–CeO2 interface. However, once the temperature is high enough (200 °C), Pr improves the oxygen mobility in the CeO2 support, and this enhances CO2 dissociation because the oxygen atoms left are delivered faster to the support sink and the dissociation sites at the interface are cleaned up faster. In situ Raman spectroscopy experiments confirmed that Pr improves the creation of oxygen vacancies on the ceria lattice but hinders their reoxidation by CO2, and both opposite effects reach an optimum balance for 3 wt % Pr doping. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments showed that Pr doping, regardless of the amount, decreases the population of surface carbon species created on the catalysts surface upon CO2 chemisorption under methanation reaction conditions, affecting both productive reaction intermediates (formates and carbonyls) and unproductive carbonates.
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Vanadium‐coated carbon‐xerogel microspheres are successfully prepared by a specific designed sol–gel method, and their supercapacitor behavior is tested in a two‐electrode system. Nitrogen adsorption ...shows that these composite materials present a well‐developed micro‐ and mesoporous texture, which depends on the vanadium content in the final composite. A high dispersion of vanadium oxide on the carbon microsphere surface is reached, being the vanadium particle size around 4.5 nm. Moreover, low vanadium oxidation states are stabilized by the carbon matrix in the composites. The complete electrochemical characterization of the composites is carried out using cyclic voltammetry, chronopotentiometry, cycling charge–discharge, and impedance spectroscopy. The results show that these composites present high capacitance as 224 F g−1, with a high capacitance retention which is explained on the basis of the presence of vanadium oxide, texture, and chemistry surface.
Vanadium‐coated carbon microspheres are, for the first time, prepared in a onepot polymerization synthesis. V2+ state is stabilized and highly dispersed on the surface of carbon microspheres. The electrochemical behavior of these materials is highly promising, since composites has high capacitance (224 F g−1) and capacitance retention which is explained on the basis of VOx presence, texture, and chemistry surface.
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•O-vacancies and Ag(s) over La0.7Ag0.3MnO3 are the most active sites for NO and oxygen adsorption.•Chelating and bridging nitrates along with Ag-nitrites are the primary reaction ...intermediates for the NO oxidation.•Monodentate nitrates and Ag-nitrates were considered NOx storage species over 400 °C.•A reaction pathway for soot and NO removal over La0.7Ag0.3MnO3 was proposed based on DRIFT results.•The silver-free catalyst was less active than its counterpart due to changes in nitrogenous species distribution.
The microwave-synthesized-LaAgMnO3-catalyst can eliminate soot and NOx simultaneously below 400 °C. To get some insight about the chemical species formed on catalyst and soot surfaces, in situ diffuse reflectanced infrared Fourier transform (DRIFT) spectroscopy under NO, O2, and NO/O2 atmospheres was performed. The DRIFTS results indicated that over 200 °C, at least four types of nitrate-species, mono- and bi-dentate nitrates (bridging and chelating) on the perovskite, as well as Ag-nitrite/nitrate, with different thermos-stabilities were formed. The decomposition of less stable surface nitrates/nitrites accounts for NO2 formation which assisted soot oxidation. The transformation-decomposition of nitrite/nitrate compounds coincided with the appearance of CO2 and carbonate-species coming from re-adsorption of soot combustion products. Monodentate nitrates, which are more stable nitrate-species, were considered NOx storage-species over 400 °C. Chelating- and bi-dentate nitrates formed on perovskite oxygen vacancies appear to be the primary reaction intermediates for the NO oxidation reaction over the Ag-doped perovskite catalyst.
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This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP–XPS) and diffuse reflectance infrared ...Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP–XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2 –δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.
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Three-dimensional (3D)-printed catalysts are being increasingly studied; however, most of these studies focus on the obtention of catalytically active monoliths, and thus traditional channeled ...monolithic catalysts are usually obtained and tested, losing sight of the advantages that 3D-printing could entail. This work goes one step further, and an advanced monolith with specifically designed geometry has been obtained, taking advantage of the versatility provided by 3D-printing. As a proof of concept, nonchanneled advanced monolithic (NCM) support, composed of several transversal discs containing deposits for active phase deposition and slits through which the gas circulates, was obtained and tested in the CO-PrOx reaction. The results evidenced that the NCM support showed superior catalytic performance compared to conventional channeled monoliths (CMs). The region of temperature in which the active phase can work under chemical control, and thus in a more efficient way, is increased by 31% in NCM compared to the powdered or the CM sample. Turbulence occurs inside the fluid path through the NCM, which enhances the mass transfer of reagents and products toward and from the active sites to the fluid bulk favoring the chemical reaction rate. The nonchanneled monolith also improved heat dispersion by the tortuous paths, reducing the local temperature at the active site. Thus, the way in which reactants and products are transported inside the monoliths plays a crucial role, and this is affected by the inner geometry of the monoliths.
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Honeycomb-shaped cordierite monoliths are widely used as supports for a large number of industrial applications. However, the high manufacturing cost of cordierite monoliths only justifies its use ...for high temperatures and aggressive chemical environments, demanding applications where the economic benefit obtained exceeds the manufacturing costs. For low demanding applications, such as the preferential oxidation of CO (CO-PrOx), alternative materials can be proposed to reduce manufacturing costs. Polymeric monoliths would be an interesting low-cost alternative; however, the limitations of the active phase incorporation to the polymeric support must be overcome. In this work, the implementation and use of polymeric monolithic structures obtained by three-dimensional printing to support CuO/CeO2 catalysts for CO-PrOx have been studied. Several approaches were used to anchor the active phase into the polymeric monoliths, such as adding inorganic materials (carbon or silica) to the polymer previous to the printing process, chemical attack with solvents of the printed resin before or during the active phase incorporation, and consecutive impregnation and modification of the channel wall design. Among those approaches, best results were obtained by the addition of silica and by channel modification. Independent of the strategy followed, a subsequent thermal treatment in N2 was required to soften the resin and favor the active phase anchoring. However, catalyst particles become embedded on the polymeric resin being not active, and thus, a final cleaning thermal treatment under air was needed to recover the active phase activity, after which the supported active phase demonstrated good catalytic activity, stability, and reusability.
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The reaction kinetics of CO2 methanation over a highly active 8.5% Ni/CeO2 catalyst was determined in a fixed-bed reactor, in the absence of heat- and mass-transfer limitations. Once the catalyst ...activity was stabilized, more than 120 kinetic experiments (with varying values of reaction temperature, total pressure, space velocity (GHSV), and partial pressure of products and reactants) were performed. From initial reaction rates, an apparent activation energy of 103.9 kJ mol–1 was determined, as well as the effect of reactants (positive) and water partial pressures (negative) on CO2 methanation rate. Three mechanistic models reported in the literature, in which CO2 is adsorbed dissociatively (carbon and formyl routes) or directly (formate route), were explored for modeling the entire reaction kinetics. For that, the corresponding rate equations were developed through the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach. In agreement with DRIFTS experiments, formate route, in which the hydrogenation of bicarbonate to formate is considered to be the rate-determining step, reflects the kinetic data accurately, operating from differential conversion to thermodynamic equilibrium. In fact, this mechanism results in a mean deviation (D) of 10.38%. Based on previous own mechanistic studies, the participation of two different active sites has been also considered. Formate route on two active sites maintains a high fitting quality of experimental data, providing kinetics parameters with a higher physical significance. Thus, the LHHW mechanism, in which Ni0 sites as well as oxygen vacant near to Ni-CeO2 interface participate in CO2 methanation, is able to predict the kinetics of Ni/CeO2 catalyst accurately for a wide range of operational conditions.
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Fe−N−C catalysts are an interesting option for polymer electrolyte fuel cells due to their low cost and high activity towards the oxygen reduction reaction (ORR). Since Fe−N−C active sites are ...preferentially formed in the micropores of the carbon matrix, increasing the microporosity is highly appealing. In this work, carbon xerogels (CXG) were activated by physical and chemical methods to favor the formation of micropores, used as carbon matrices for Fe−N−C catalysts, and investigated for the ORR. The catalysts were characterized by solid‐state techniques to determine chemical composition and pore structure. Physical activation increased microporosity up to 2‐fold leading to catalysts with a larger density of active sites (more than twice iron and nitrogen uptake, pyridinic N and Nx−Fe). This entailed a higher ORR intrinsic activity determined in a 3‐electrode cell (80 mV better half‐wave potential). At the cathode of a fuel cell, the catalysts based on activated carbon materials showed 26 % lower power density ascribed to a more hydrophilic surface, causing a larger extent of flooding of the electrode that counterbalances the higher intrinsic activity. Interestingly, a more stable behavior was observed for the activated catalysts, with up to 2‐fold better relative power density retention after 20‐hour operation.
Activated carbon xerogels were studied as matrix for Fe−N−C catalysts. The increased microporosity lead to catalysts with a larger density of active sites, achieving up to more than twice iron and nitrogen uptake, and consequently, a higher ORR intrinsic activity. Whereas, fuel cell power density is negatively affected by a more hydrophilic character, but stability enhances with activation.
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Abstract The present research exploits an innovative methodology for producing auto‐pressurized carbon microreactors with a precise and controlled structure analyzing the influence of their design on ...the fluid dynamics and their catalytic performance. Carbon monoliths with Tesla‐valve shape channels (Tesla, T, and modified Tesla, Tm) are synthesized through the combination of 3D printing and sol–gel process and further probed as Ni/CeO2 supports on CO2 methanation. The experimental results and mathematical modeling corroborated the improved performance obtained through the complex design compared to a conventional one. In addition to chaotic fluid flow induced by the deviation in flow direction, which improves the reagents‐active phase interaction, local pressure increases due to convergence of flows may enhance the Sabatier reaction according to Le Châtelier's principle. Conversely to straight channels, T and Tm are not affected by flow rate and presented chemical control. Tesla‐valve with curved angle (Tm) improved the mass transfer, achieving higher conversion and ≈30% reaction rate increase regarding right angle (T). Thus, this auto‐pressurized multi‐stage Tesla‐valve monolith opens the gate to design specific and advanced functional materials for multitude chemical reactions where not only the reactant‐active phase contact can be maximized but also the reaction conditions can be controlled to maximize the reaction kinetics.
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