Opportunities for ceria-based mixed oxides versus commercial platinum-based catalysts in the soot combustion reaction. Mechanistic implications

The aim of this paper is to study the activities of ceria–zirconia and copper/ceria–zirconia catalysts, comparing with a commercial platinum/alumina catalyst, for soot combustion reaction under different gas atmospheres and loose contact mode (simulating diesel exhaust conditions), in order to analyse the kinetics and to deduce mechanistic implications. Activity tests were performed under isothermal and TPR conditions. The NO oxidation to NO2 was studied as well. It was checked that mass transfer limitations were not influencing the rate measurements. Global activation energies for the catalysed and non-catalysed soot combustion were calculated and properly discussed. The results reveal that ceria-based catalysts greatly enhance their activities under NOx/O2 between 425 °C and 450 °C, due to the “active oxygen”-assisted soot combustion. Remarkably, copper/ceria–zirconia shows a slightly higher soot combustion rate than the Pt-based catalyst (under NOx/O2, at 450 °C).The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEOII/2014/010 project) and the Spanish Ministry of Economy and Competitiveness (CTQ2012-30703 project, UE-FEDER funding)

Catalytic performance of CuO/Ce0.8Zr0.2O2 loaded onto SiC-DPF in NOx-assisted combustion of diesel soot

This work presents a comparative study between the catalytic performance of the 2% CuO/ceria-zirconia powder catalyst and the same catalyst supported on silicon carbide DPF (Diesel Particulate Filter) towards NO oxidation reaction and soot combustion reaction. The ceria-zirconia catalyst was prepared by the co-precipitation method and 2 wt% copper was incorporated by the incipient wetness impregnation method. The catalyst was incorporated onto the ceramic support using a simple and organic solvent-free procedure by a simply dipping the DPF into an aqueous solution of the catalyst. The powder catalyst has been characterized using N2 adsorption at −196 °C, XRD and Raman Spectroscopy; whereas the catalytic coating morphology has been evaluated by SEM and the mechanical stability by an adherence test. Both catalyst configurations were tested for NO oxidation to NO2 and for soot combustion under NOx/O2. The results revealed that incorporation of the very active copper/ceria-zirconia catalyst onto SiC-DPF has been successfully achieved by a simple coating procedure. Furthermore, the catalytic coating has shown suitable mechanical, chemical and thermal stability. A satisfactory catalytic performance of the catalytic-coated filter was reached towards the NO oxidation reaction. Moreover, it was proved that the catalytic coating is stable and the corresponding coated DPF can be reused for several cycles of NO oxidation without a significant decrease in its activity. Finally, it was verified that the loose-contact mode is a good choice to simulate the catalytic performance of this active phase in a real diesel particulate filter.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEOII/2014/010 project), the Spanish Ministry of Economy and Competitiveness (CTQ2012-30703 project, UE-FEDER funding). S.Q.D. wishes to thank VIDI-University of Alicante her Master Thesis Grant

Ceria-based catalysts for NOx removal in NSR processes: A fundamental study of the catalyst modifications explored by in situ techniques

In this work, a fundamental and systematic study was conducted, leading to a better understanding of the phenomena occurring on the catalyst’s surface during the NOx reduction process in NSR systems. For this purpose, ceria-based catalysts, with Cu in substitution of noble metal, have been synthesized and deeply characterized by means of XRF, XPS, in situ (XRD, Raman spectroscopy and DRIFTS), temperature-programmed reduction under H2 (H2-TPR) and under NO reaction (NO isothermal reaction + NO-TPR). The whole results show the key role of copper to promote the reducibility and the creation of oxygen vacancies, allowing a high NO consumption and fast kinetics of N2O and N2 formation, until the oxygen vacancies consumption takes place. The study of the surface reactions taking place in the formation of adsorbed NOx species and the oxygen vacancies consumption with NO uptake is complex; however, a hydroxyl consumption route is found to be involved. The reduction of NO provided higher levels of N2 at higher temperatures; also, a very high efficiency of the previously created oxygen vacancies was found for this process.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Science and Innovation (PID2019-105542RB-I00 project) and the UE-FEDER funding. Martínez-Munuera also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603)

Ceria-Praseodymia Mixed Oxides: Relationships Between Redox Properties and Catalytic Activities Towards NO Oxidation to NO2 and CO-PROX Reactions

A series of CexPr1−xO2−δ catalysts was prepared by co-precipitation method in alkali media. These catalysts were characterized by N2 adsorption–desorption isotherms at −196 °C, X-ray diffraction, thermogravimetry combined with mass spectrometry (TG-MS), and temperature-programmed reduction with H2 and CO (H2-TPR and CO-TPR, respectively). Catalytic tests were performed for temperature programmed NO oxidation to NO2 (from 25 to 750 °C) and for the preferential oxidation of CO in H2 rich stream (CO-PROX reaction) in the range of 150–500 °C. The trends in the order of catalytic activities towards NO oxidation and CO-PROX are correlated with the redox properties of the catalysts and their composition. CexPr1−xO2−δ mixed oxides present very different catalytic behaviours towards NO oxidation and CO-PROX reactions. These experimental trends might be explained by the balance of several factors: the acid character of the NO and CO molecules, the different lattice oxygen mobility of the catalysts, the presence of surface carbonates species in the samples, and the catalysts’ reducibility under H2 and CO. The understanding of the features that govern the activity towards these environmentally relevant oxidation reactions is important in the designing of effective catalysts.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEOII/2014/010), MINECO (CTQ2015-64801-R, MAT2013-40823-R, CSD2009-00013) and the UE (FEDER funding). X. Chen thanks the program of “Ramón y Cajal” from Ministry of Science and Innovation of Spain

Study of Ce/Pr ratio in ceria-praseodymia catalysts for soot combustion under different atmospheres

The effect of Ce/Pr ratio on ceria-praseodymia on structural and surface properties is studied. The catalytic activity towards soot combustion under NOx/O2 and O2/N2 atmospheres is also evaluated, together with the NO oxidation activity to NO2. CexPr1-xO2-δ compositions (x = 0.8, 0.5 and 0.2) have been prepared along with ceria and praseodymia. Catalysts were prepared by co-precipitation. The Ce0.5Pr0.5O2-δ composition, was also prepared by nitrate calcination. Ceria-praseodymia mixed oxides yield reduction profiles shifted to lower temperatures, higher NO oxidation activities to NO2 and improved catalytic activities for soot combustion with respect to pure ceria. Ce0.5Pr0.5O2-δ was the most active one. Under NOx/O2, it has greater soot combustion activity if it is prepared by nitrate calcination. However, under O2/N2, the co-precipitation method is more favorable because of the better dopant insertion achieved in the ceria lattice, which seems to lead better oxygen mobility on the surface and in the bulk oxide.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076 project) and the Spanish Ministry of Economy and Competitiveness (CTQ2015-64801-R project) and the UE-FEDER funding. J.C.M.M. also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603)

Unraveling the nature of active sites onto copper/ceria-zirconia catalysts for low temperature CO oxidation

The aim of this research is an attempt to shed some light on the understanding of the nature of the active sites and the generated synergies in the copper/ceria-zirconia formulations for low temperature CO oxidation by means of the creation of copper entities with different physico-chemical nature. For this reason, several CuOx/ceria-zirconia catalysts, with different Cu contents and different methods to incorporate copper species, were synthesized. Focus was specially put in this case trying to link the results of CO oxidation catalytic tests with the CO-temperature programmed reduction profiles/approximate estimations and selected characterization parameters in order to find out correlations among catalysts' properties/reducibility and catalytic behaviors, especially those corresponding to the nature and roles of the different CuOx species in contact with ceria-based support on catalytic activity. Results reveal a significant improvement in CO conversion compared to the ceria-zirconia support by adding a small amount of copper loading (as low as 0.5 %), emphasizing the paramount role of copper incorporated by the method of IWI. From 0.5 up to 2% of copper loading, an interesting increase gradual trend in activity and reducibility can be noted. It should be mentioned that all the catalysts obtained by this procedure are more catalytically active towards CO oxidation than 1%Pt/Al2O3 at low temperatures (T < 130 degrees C). CO-TPR results show that the reducibility of these catalysts is in line with their CO oxidation activity. The method of preparation has been revealed as a critical variable in the catalytic performance, and quite similar catalytic activities can be reached from different synthesis methods and different copper contents, due to the similar nature and type of CuOx species generated over the catalysts' surface, identified by the CO-TPR profiles and the rest of characterization data. Finally, IWI method seems to be the best one among those tested, thus combining superior areas of both alpha and beta contributions assigned on CO-TPR profiles, which seem to be critical in the interpretation of the catalytic behaviors

Potential of Cu–saponite catalysts for soot combustion

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

Lattice oxygen activity in ceria-praseodymia mixed oxides for soot oxidation in catalysed Gasoline Particle Filters

Two series of ceria-praseodymia catalysts with varying composition have been systematically investigated in the oxidation of soot under inert atmosphere in order to find out its potential utilization in Gasoline Particulate Filters for GDI engines. The samples have been widely characterized by XRD, Raman spectroscopy, TEM, FESEM, XPS, N2 adsorption at −196 °C and O2-TPD. The praseodymium incorporation onto the ceria enhances the oxygen mobility in the subsurface/bulk of the sample favoring higher O2 released amounts under inert atmosphere. The intermediate compositions can promote more accentuated O2 emissions at moderate temperatures (up to 500 °C). The efficiency of the own active oxygen species released from the catalyst to oxidize soot under inert atmosphere, even under loose contact mode, has been well demonstrated. The pathways of the mechanism taking place seem to be dependent on the temperature and mainly on the type of contact among soot and catalyst. Under loose contact conditions and low-medium temperatures, the O2 freshly emitted from the catalyst can oxidize soot more efficiently than a diluted O2-gas stream. Conversely, under more severe conditions (higher temperature or tight contact conditions), the soot acts as a “driving force” and the own lattice oxygen species can be transferred directly towards soot surface in an efficient way.The authors gratefully acknowledge the financial support of Generalitat Valenciana (PROMETEO/2018/076), MINECO (CTQ2015-64801-R) and the UE (FEDER funding). Also, JCMM acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603)

NO Oxidation on Lanthanum-Doped Ceria Nanoparticles with Controlled Morphology

The present work aims to assess the impact of morphology and reducibility on lanthanum-doped ceria nanocatalysts with controlled morphology on the NO oxidation reaction. Specifically, samples were prepared using a hydrothermal method incorporating lanthanum at varying molar concentrations (0, 5, 10, and 15 mol.%) into ceria with a controlled morphology (nanocubes and nanorods). The structural, compositional, and redox characterization of these catalysts has been performed via scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (X-EDS), inductively coupled plasma (ICP), hydrogen temperature-programmed reduction (H2-TPR), and oxygen storage capacity (OSC). NO oxidation catalytic tests were conducted, and the results were compared with estimated curves (obtained by considering the proportions of the corresponding components), which revealed the presence of a synergistic effect between lanthanum and ceria. The degree of enhancement was found to depend on both the morphology and the amount of lanthanum incorporated into CeO2. These findings may facilitate the optimization of features concerning ceria-based nanocatalysts for the removal of NOx emissions from exhaust gases.This research was funded by Generalitat Valenciana (CIPROM/2021/070 project), the Spanish Ministry of Science and Innovation/Research Spanish Agency (PID2019-105542RB-I00/AEI/10.13039/501100011033, PID2020-113006RB-I00/AEI/10.13039/501100011033 and PID2020-113809RB-C33 projects), and UE-FEDER funding

Improved NOx Storage/Release Properties of Ceria-Based Lean NOx Trap Compositions with MnOx Modification

Ceria/spinel-based lean NOx trap compositions with and without barium were modified with MnOx via incipient wetness impregnation. The effect of the MnOx layer on the aged materials (850 °C) as to the NOx storage and release properties was investigated via NOx adsorption (500 ppm NO/5% O2/balance N2) carried out at 300 °C in a dual-bed with a 1% Pt/Al2O3 catalyst placed upstream of the samples to generate sufficient amounts of NO2 required for efficient NOx storage. Subsequent temperature programmed desorption (TPD) experiments were carried out under N2 from 300 °C to 700 °C. The addition of MnOx to the barium free composition led to a slightly reduced NOx storage capacity but all of the ad-NOx species were released from this material at significantly lower temperatures (ΔT ≈ 100 °C). The formation of a MnOx layer between ceria/spinel and barium had a remarkable effect on ageing stability as the formation of BaAl2O4 was suppressed in favour of BaMnO3. The presence of this phase resulted in an increased NOx storage capacity and lower desorption temperatures. Furthermore, NOx adsorption experiments carried out in absence of the Pt-catalyst also revealed an unexpected high NOx storage ability at low NO2/NO ratios, which could make this composition suitable for various lean NOx trap catalysts (LNT) related applications.A.G.-G. gratefully acknowledges the general financial support of Generalitat Valenciana (PROMETEO/2018/076), MINECO (CTQ2015-64801-R) and the UE (FEDER funding). J.C.M.-M. also acknowledges Spanish Ministry of Science, Innovation and Universities for the financial support through a FPU grant (FPU17/00603)