Suchergebnisse

The authors acknowledge the financial support from the Spanish Ministry of Science and Innovation (PID2019-105960RB-C22), the University of Alicante (Project GRE18-01A), the Generalitat Valenciana (Projects PROMETEO/2018/076 and GV2020-075, PhD grant GRISOLIAP/2017/177 and contract APOSTD/2019/030), the Junta de Andalucia (Project P18-RTJ-2974) and the UE (FEDER funding). ; A new method to synthesize integral carbon monoliths with a controlled channel morphology has been developed in this work by combining 3D-printing technology and sol–gel polymerization. By this method, robust and consistent carbon monoliths were obtained with a perfect replica of the channel architecture at a microscale range. As a proof of concept, a carbon monolith with tortuous channels that split and join successively along the monolith length has been designed, fabricated and tested as a CuO/CeO2 support for the preferential oxidation of CO in the presence of H2 (CO-PrOx), which is a topic of ongoing research for H2 purification in fuel cells. The behavior of this novel carbon monolith catalyst has been compared with that of a counterpart catalyst prepared with a conventional honeycomb design. Results shown that the wide macroporosity of the carbon network favors the anchoring and dispersion of the active phase both in the channel surface and the carbon network. The channel architecture affects the gas diffusion both through the channel and the carbon network and consequently, affects the active phase accessibility and activity. T50 (the temperature to achieve 50% CO conversion) decreases by almost 13 °C at 240 mL min−1 in the carbon monolith with tortuous channels (T50 = 79.7 °C) compared to the honeycomb monolith (T50 = 93.1 °C). The turbulent path created by the tortuous channels favours the active phase–gas contact and even the gas diffusion inside the macropores of the carbon skeleton improving the catalytic performance of the active phase compared to that by the conventional honeycomb design. Thus, this work demonstrates the potential of 3D printing to improve the catalytic supports currently available. ; Spanish Government PID2019-105960RB-C22 ; University of Alicante GRE18-01A ; Generalitat Valenciana ; European Commission ; General Electric PROMETEO/2018/076 GV2020-075 GRISOLIAP/2017/177 APOSTD/2019/030 ; Junta de Andalucia P18-RTJ-2974 ; UE (FEDER funding)

The conversion of CO2 towards added valuable products is considered as a potential alternative to achieve the increase of the petrochemical industry while reducing the CO2 emissions. In the present work, the effect of Ru loading on CeO2 supports has been studied for the CO2 methanation reaction, and catalysts with different Ru loading in the 1–5 wt. % range have been prepared, characterized, and tested. The optimum Ru loading has been found to be 2.5 wt. %. Ruthenium cations are reduced at the lowest temperature for this optimum loading according to H2-TPR experiments (even at room temperature), and the highest proportion of ruthenium cations with strong interaction with ceria is achieved, as deduced from XPS. XRD characterization suggests partial insertion of ruthenium cations into the ceria lattice. In situ DRIFTS experiments evidenced that the balance between formation upon CO2 chemisorption and further hydrogenation of surface carbon intermediates is optimum for 2.5 wt. % Ru/CeO2. For low metal contents, the CO2 chemisorption is limited and no relevant, while as the metal content is increased, the hydrogenation of carbon species is less favourable. The 2.5 wt.% Ru/CeO2 catalyst comprises a balance between surface-carbon groups formation and further hydrogenation. ; Generalitat Valenciana (PROMETEO/2018/0765); MICINN (PID2019-105960RB-C22); Junta de Andalucía (Project P18-RTJ-2974); European Union's Horizon 2020 research and innovation program (Marie Skłodowska-Curie grant agreement No 713567); Science Foundation Ireland Research Centre (award 12/RC/2278_P2).

The individual and simultaneous removal of NOx by Storage and Reduction with H2 (H2‐NSR) and combustion of soot have been studied for La and CuLa catalysts. The NOx storage capacity of pure lanthanum oxide is improved either by copper addition or by using a colloidal crystal hard template for the synthesis of a macroporous structure. Although the NOx storage capacity of the CuLa macroporous material is around ten times lower regarding conventional PtBa catalysts, the kinetics of the H2‐NSR cycles are fast enough to allow high NOx removal (until 90 % removal). Lanthanum oxide catalyst prepared with uncontrolled structure has no activity for soot combustion, but the activity increases for a macroporous counterpart prepared using the colloidal crystal template because the macroporous structure improves the soot‐catalyst contact and allows the nitrogen‐oxygen surface groups yielded upon NOx chemisorption to oxidize soot. The soot combustion activity of lanthanum catalysts is also enhanced upon copper loading, because copper improves the NO2‐assisted soot combustion mechanism and favors the chemisorption of NOx, enhancing soot oxidation by nitrogen‐oxygen surface species. Finally, it is demonstrated that the simultaneous removal of NOx by H2‐NSR and soot combustion can be carried out with a CuLa catalyst at 450 °C. The simultaneous removal does not affect the combustion of soot and only has a small effect in NOx removal. ; Spanish Ministry of Economy and Competitiveness (Project CTQ2015-67597-C2-2-R), Spanish Ministry of Science and Innovation (PID2019-105960RB-C22), Generalitat Valenciana (Project PROMETEO/2018/076 and APOSTD/2019/030) and European Union (FEDER funding).

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. ; This work was financially supported by Generalitat Valenciana (PROMETEO/2018/0765), MINECO (CTQ2015-67597-C2-2-R); MICINN (PID2019-105960RB-C22); Junta de Andalucía (Project P18-RTJ-2974); European Union's Horizon 2020 research and innovation program (Marie Skłodowska-Curie Grant agreement no. 713567); and Science Foundation Ireland Research Centre (award 12/RC/2278_P2).

The preparation and use of Guefoams as heterogeneous catalyst is reported. The Guefoam catalyst consists of an open-pore Al-Si foam that accommodates a freely mobile guest phase (Ni/CeO2/Al2O3 particles) in its cavities, with neither a physical nor a chemical matrix-guest bond. A eutectic Al-12Si alloy was used as a low-melting matrix precursor to prevent thermal sintering of the active phase during liquid metal infiltration. CO2 methanation was chosen as the reaction test. The activity and CH4 selectivity (close to 100%) achieved with the Guefoam catalyst were similar to those obtained with a packed bed of the same active phase particles, but with the advantages of a structured reactor such as robustness and ease of handling. The thermal conductivity of the Guefoam catalyst is significantly improved with regard to the packed bed of active phase particles, which reduces the temperature gradients in the catalytic reactor, as demonstrated by computational fluid dynamic modelling. Since the permeability of the Guefoam catalyst is 2.7 times that of the packed bed, the pressure drop caused by the passage of a fluid through the novel material is reduced, resulting in a significantly higher catalytic performance index than the packed bed. ; Financial support from the Spanish Agencia Estatal de Investigación (AEI), the Spanish Ministry of Science and Innovation and the European Union (FEDER and NextGenerationEU funds) [projects MAT2016-77742-C2-2-P, PDC2021-121617-C21 and CTQ2015-67597-C2-2-R] and the Conselleria d'Innovació, Universitats, Ciència, i Societat Digital of the Generalitat Valenciana [projects GVA-COVID19/2021/097 and PROMETEO/2018/076, PhD grant of C.Y. Chaparro GRISOLIAP/2017/177 and contract of E. Bailón APOSTD/2019/030]. L.P. Maiorano also acknowledges the financial support from the University of Alicante through grant "Programa Propio para el fomento de la I+D+I del Vicerrectorado de Investigación y Transferencia de Conocimiento" (UAFPU2019-33).

AbstractIn this study, the effect of the cell density of monolithic catalysts was investigated and further mathematically modeled on cordierite supports used in CO2 methanation. Commercial cordierite monoliths with 200, 400, and 500 cpsi cell densities were coated by immersion into an ethanolic suspension of Ni/CeO2 active phase. SEM–EDS analysis confirmed that, owing to the low porosity of cordierite (surface area < 1 m2 g−1), the Ni/CeO2 diffusion into the walls was limited, especially in the case of low and intermediate cell density monoliths; thus, active phase was predominantly loaded onto the channels' external surface. Nevertheless, despite the larger exposed surface area in the monolith with high cell density, which would allow for better distribution and accessibility of Ni/CeO2, its higher macro-pore volume resulted in some introduction of the active phase into the walls. As a result, the catalytic evaluation showed that it was more influenced by increments in volumetric flow rates. The low cell density monolith displayed diffusional control at flow rates below 500 mL min−1. In contrast, intermediate and high cell density monoliths presented this behavior up to 300 mL min−1. These findings suggest that the interaction reactants-catalyst is considerably more affected by a forced non-uniform flow when increasing the injection rate. This condition reduced the transport of reactants and products within the catalyst channels and, in turn, increased the minimum temperature required for the reaction. Moreover, a slight diminution of selectivity to CH4 was observed and ascribed to the possible formation of hot spots that activate the reverse water–gas shift reaction. Finally, a mathematical model based on fundamental momentum and mass transfer equations coupled with the kinetics of CO2 methanation was successfully derived and solved to analyze the fluid dynamics of the monolithic support. The results showed a radial profile with maximum fluid velocity located at the center of the channel. A reactive zone close to the inlet was obtained, and maximum methane production (4.5 mol m−3) throughout the monolith was attained at 350 °C. Then, linear streamlines of the chemical species were developed along the channel.

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. ; The authors thank the financial support of the Spanish Ministry of Economy and Competitiveness (Project CTQ2015-67597-C2-2-R and grant FJCI-2015-23769), the Spanish Ministry of Science and Innovation (PID2019-105960RB-C22), Generalitat Valenciana (Project PROMETEO/2018/076), and the EU (FEDER funding). A.D.-Q. and M.G.-M. acknowledge the financial support from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 713567 and Science Foundation Ireland Research Centre award 12/RC/2278_P2. F.C.H. acknowledges the Argentinian National Research Council (CONICET) for the financial support. C.E. acknowledges funding from the MICINN/FEDER RTI2018-093996-B-32 project. The computational results of this research have been achieved using the DECI resource Salomon based in the Czech Republic at the IT4Innovations National Supercomputing Center with support from the PRACE aisbl. The DJEI/DES/SFI/HEA Irish Centre for High-End Computing (ICHEC) is also acknowledged for the provision of computational facilities.

Ni/CeO2 catalysts are very active and selective for total hydrogenation of CO2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO2 catalyst has been monitored under CO2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that the species involved in the redox processes taking place during the CO2 methanation mechanism are the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ pairs. In addition, a small fraction of nickel is present on the catalyst surface forming NiO and Ni2+-carbonates/hydroxyls (around 20% of the total surface nickel), but these species do not participate in the redox processes of the methanation mechanism. Under CO2 methanation conditions the H2 reduction rate of the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate (2 times faster, at least, at 300ºC), but a certain proportion of nickel always remains oxidized under reaction conditions. The high activity of Ni/CeO2 catalysts for CO2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO2 and Ni0 active sites where CO2 and H2 are expected to be efficiently dissociated, respectively. ; Generalitat Valenciana, Spain (PROMETEO/2018/0765) Ministry for Science and Innovation MICINN, Spain (Projects PID2019-105960RB-C21 and PID2019-105960RB-C22) Junta de Andalucía, Spain (Project P18-RTJ-2974); European Union's Horizon 2020 Research and Innovation Program (Marie Skłodowska-Curie grant agreement No 713567) Science Foundation Ireland Research Centre, Ireland (award 12/RC/2278_P2) ALBA synchrotron, Spain (Proposal number: ID 2020094556).

The hydrogenation of CO2 into CH4 from H2 produced by renewable energy is considered an interesting alternative in order to promote the development of such green energies. In the present work, the effect of Ni and Ru loadings on the catalytic performance of alumina-supported catalysts is studied for CO2 methanation reaction. All catalysts were prepared by wetness incipient impregnation, characterized by several techniques (N2-physisorption, CO2-TPD, XRD, H2-chemisorption, XPS and H2-TPR) and evaluated for CO2 methanation in a fixed bed reactor at GHSV=10,000 h−1 and W/FCO2 0 = 4.7 (g cat.) h mol−1. Characterization results showed that addition of increasing loadings of Ni and Ru lead to the formation of both CO2 adsorption and H2 dissociation active sites, which are necessary to carry out CO2 hydrogenation into methane. Easily reducible ruthenium was dispersed on γ-Al2O3 in form of large agglomerates, whereas Ni was better dispersed presenting a great interaction with the support. 12% Ni and 4% Ru resulted to be the optimal contents providing metal surfaces of 5.1 and 0.6m2 g−1, T50 values of 340 and 310 °C and activity being quite stable for 24 h-on-stream. In terms of turnover frequency (TOF), 4%Ru/Al2O3 catalyst was quite more efficient than 12%Ni/Al2O3, probably due to a greater ability of ruthenium to dissociate hydrogen. The apparent activation energies for alumina supported Ni and Ru were 129 and 84 kJ mol−1, respectively. ; The support from the Economy and Competitiveness Spanish Ministry (CTQ2015-67597-C2-1-R and CTQ2015-67597-C2-2-R MINECO-FEDER), the Basque Government (IT657-13 and IT1297-19) and the SGIker (Analytical Services) at the University of the Basque Country are acknowledged. One of the authors (AQ) also acknowledges University of the Basque Country by his PhD grant (PIF-15/351).

The mechanism and kinetic of CO2 methanation reaction of 9.5 % Ni/Al2O3 catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al2O3 catalyst shows an apparent activation energy of 80.1 kJ mol−1 in CO2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H2O inhibition, with a water adsorption constant of 3.1 atm−1 and apparent orders of 0.24 and 0.27 for H2 and CO2, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al2O 3 catalyst with high accuracy under relevant process conditions (315−430 °C, 1−6 bar, H2 to CO2 molar ratios between 1–16 and, different reagents and products partial pressures). ; Support for this study was provided by the Spanish Ministry of Economy and Competiveness (Project PID2019-105960RB-C21 and PID2019-105960RB-C22) and the Basque Government (Project IT1297-19). One of the authors (JAOC) acknowledges the Post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country. Other of the ...

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. ; Support for this study was provided by the Spanish Ministry of Science and Innovation (Project Nos. PID2019-105960RB-C21 and PID2019-105960RB-C22) and the Basque Government (Project No. IT1297-19). J.A.O.-C. acknowledges the Postdoctoral research grant (No. DOCREC20/49) ...

The CO2 methanation mechanism was studied for Ni/CeO2 and Ni/Al2O3 catalysts. The higher methanation activity and selectivity of Ni/CeO2 is attributed to: i) Ni/CeO2 combines two types of active sites efficient for CO2 dissociation at the NiO-Ceria interface and for H2 dissociation on Ni0 particles; ii) water desorption is the slowest mechanism step, and, due to the high oxygen mobility throughout the ceria lattice, water is not necessarily formed on the same active sites that chemisorb CO2, i.e., the CO2 chemisorption sites are not blocked by water molecules; iii) the Ni/CeO2 surface does not accumulate carbon-containing species under reaction conditions, which allows faster chemisorption and dissociation of CO2. The Ni/Al2O3 catalyst handicaps are that all the steps of the mechanism take place on the same active sites, and the slow release of water and the accumulation of surface formates on these sites delay the chemisorption of further CO2 molecules. ; Financial support of: - Economy and Competitiveness Spanish Ministry: Projects CTQ2015-67597-C2-1-R and CTQ2015-67597-C2-2-R MINECO-FEDER) and grant of EBG FJCI-2015-23769. - Generalitat Valenciana: Project PROMETEO/2018/076 and PhD grant of ACA GRISOLIAP/2017/185. - The Basque Government: Project IT657-13. - SGIker (Analytical Services) at the University of the Basque Country. - Spanish Ministry of Education, Culture and Sports grant of ADQ FPU14/01178. - University of the Basque Country PhD grant of AQ PIF-495 15/351.