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Diesel Particulate Filters (DPFs) have been loaded with the same amount (0.6 wt.%) of either Pt or an optimized CePr active phase, and an experimental set-up has been designated and used to investigate the catalytic combustion of soot under realistic reaction conditions. Both active phases were stable under reaction conditions, with no evidences of deactivation in consecutive combustion experiments. The presence of H2O and CO2 together with NOx and O2 in the gas mixture slightly delays soot combustion to higher temperatures, but the effect is equal for the Pt and CePr active phases. The mass of soot loaded into the filters had no effect in the catalytic regeneration of the DPFs for soot:catalyst weigh ratios below 0.4, while it was hindered above this ratio. The Pt and CePr active phases behave equal until 0.6 soot:catalyst weigh ratio and Pt performance was slightly better for higher soot loading. This difference is explaining according to the main soot combustion mechanisms occurring during Pt–DPF (NO2-assited) and CePr–DPF regeneration (active oxygen). The CO2 selectivity is near 100% for both catalysts in all the experimental reaction conditions evaluated. According to this study, the CePr active phase seems to be a promising candidate to replace Pt in real applications. ; The financial support of Generalitat Valenciana (Project PROMETEOII/2014/010)), Spanish Ministry of Economy and Competitiveness (Project CTQ2012-30703) and European Union (FEDER funding) is acknowledged.

Novel methodology, referred to as rapid-scan operando, has been demonstrated to be a highly powerful tool for studying reaction mechanisms in heterogeneous catalysis, because it combines rapid scan for IR monitoring in the milliseconds time frame and operando methodology. As proof of concept, the NOx–CO and NOx–H2 reactions were studied as model catalyzed reactions with practical interest for removal of NOx in diesel exhausts. Rapid-scan operando experiments confirmed state-of-the-art knowledge concerning the reaction mechanisms and, more importantly, allowed us to elucidate, for the first time, the different roles of the surface hydroxy groups depending on the reductant used (CO or H2). Moreover, this new tool was used to distinguish the behavior of carbonates and nitrites under reaction conditions that could not be monitored by conventional IR spectroscopy approaches owing to overlap of their absorbance bands. ; Financial support from Generalitat Valenciana (Project PROMETEOII/2014/010 and grant BEST/2014/250), Spanish Ministry of Economy and Competitiveness (Projects CTQ2012–30703 and MAT2014–61992-EXP, and grant PRX14/00249), and the European Union (FEDER funding).

Iron-containing activated carbon fibers (Fe-ACF) have been prepared by a novel method consisting of mixing an iron precursor with raw pitch before carbon fiber formation, and further CO2 activation of the Fe-CF. The NO adsorption experiment revealed that iron species are accessible to gases and that they activate NO improving adsorption into the fiber microporosity. The chemisorption of NO and the subsequent dimer (N2O2) formation is, in general, the rate-limiting step. Metallic iron and/or partially reduced iron compounds are the most active iron species. However, in the fibers with narrow micropores dimer diffusion in the microporosity is the slowest step and, consequently, an appropriate pore size is required from a kinetic point of view. The most suitable samples prepared were those with pores wider than 0.7 nm which contain partially reduced iron species. Finally, NO adsorbed can be further recovered by heating the spent adsorbents at a quite low temperature (500 K). Fresh and regenerated adsorbents show similar NO adsorption capacity. ; Spanish Government (Project CTQ2005-01358).

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).

H– and Na–saponite supports have been prepared by several synthesis approaches. 5% Cu/saponite catalysts have been prepared and tested for soot combustion in a NOx + O2 + N2 gas flow and with soot and catalyst mixed in loose contact mode. XRD, FT-IR, N2 adsorption and TEM characterization results revealed that the use of either surfactant or microwaves during the synthesis led to delamination of the saponite support, yielding high surface area and small crystallite size materials. The degree of delamination affected further copper oxide dispersion and soot combustion capacity of the Cu/saponite catalysts. All Cu/saponite catalysts were active for soot combustion, and the NO2-assisted mechanism seemed to prevail. The best activity was achieved with copper oxide supported on a Na–saponite prepared at pH 13 and with surfactant. This best activity was attributed to the efficient copper oxide dispersion on the high surface area delaminated saponite (603 m2 g−1) and to the presence of Na. Copper oxide reduction in H2-TPR experiments occurred at lower temperature for the Na-containing catalysts than for the H-containing counterparts, and all Cu/Na–saponite catalysts were more active for soot combustion than the corresponding Cu/H–saponite catalysts. ; The financial support of Generalitat Valenciana (Project PROMETEOII/2014/010), Catalan Government (2014SGR1146), Spanish Ministry of Economy and Competitiveness (Projects CTQ2012-30703 and CTQ2011-24610) and UE (FEDER funding) is acknowledged.

The SCR of NOx with NH3 has been studied by using different Cu zeolite catalysts, prepared both with ZSM5 and BETA zeolite supports by ionic exchange or by impregnation. The catalysts were characterized by ICP-AES, N2 adsorption at −196 °C, XRD, TEM, XPS and H2-TPR. The catalysts characterization confirmed the presence of different Cu(II) species on all catalyst (CuO and Cu(II) exchanged on tetrahedral and octahedral positions of the zeolites framework). Clear evidences of Cu(I) or Cu(0) species were not obtained. CuO was more abundant in high copper-content catalysts and in ZSM5 catalysts, due to its lower ionic exchange capacity, while isolated Cu(II) ions are more abundant in low copper-content catalysts and in BETA catalysts. It was concluded that CuO catalyzes the oxidation of NO to NO2, and this favors the reduction of NOx at lower temperature (the NH3-NO2 reaction is faster than the NH3-NO reaction because NO2 is much more oxidizing than NO), whereas isolated Cu(II) ions maintain high NOx conversion at high temperatures. ; Financial support provided by the Spanish Ministry of Economy and Competitiveness (CTQ2012-30703), the Basque Government (IT-657-13) and the UPV/EHU (UFI11/39). One of the authors (UDLT) wants to acknowledge to the Basque Government for the PhD Research Grant (BFI-2010-330).

Alumina-supported manganese catalysts with cryptomelane and/or birnessite structure have been prepared using a simple method based on the thermal decomposition of potassium permanganate. The samples have been characterized by XRD, FTIR, TGA, DSC, N2 adsorption at −196 °C, SEM, H2-TPR and XPS, and their catalytic activity for soot combustion has been tested and compared to that of a reference Pt/alumina catalyst. The thermal decomposition of alumina-supported KMnO4 yields a mixture of supported birnessite and potassium manganate which is the most effective, among those prepared, to lower the soot combustion temperature. However, this material is not useful for soot combustion because the accelerating effect is not based on a catalytic process but on the oxidation of soot by potassium manganate. A suitable soot combustion catalyst is obtained after potassium manganate is removed by water washing, yielding only the birnessite phase on the γ-Al2O3 support. This birnessite phase can be transformed into cryptomelane by calcination at 600 °C. These two samples, γ-Al2O3-supported birnessite and cryptomelane are suitable catalysts for soot combustion in NOx/O2 mixtures, as their catalytic activity is based on the NO2-assited mechanism, that is, both catalysts accelerate the oxidation of NO to NO2 and NO2 promotes soot oxidation. The soot combustion temperatures obtained with these birnessite/cryptomelane alumina-supported catalysts are similar to that obtained with the reference Pt/alumina catalyst. ; The authors thank to Spanish government (MICINN project CTQ2009-07475), DIMA (Dirección de Investigaciones) and Laboratorio de Magnetismo y Materiales Avanzados (Universidad Nacional de Colombia, Sede Manizales) and Vicerrectoría de Investigaciones de la Universidad de Caldas for the financial support.

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).