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High surface area mixed oxide 8.7% NiO-CeO2nanoparticles (122 m2/g; 6–7 nm) were prepared using a reversed microemulsion method, and were tested for dry methane reforming (DRM). The catalytic activity of these nanoparticles remains stable under the severe conditions of DRM (700 °C), and they show better carbon resistance than conventional NiO-CeO2catalysts prepared without control of the size. The activity and selectivity of nanoparticles and reference catalyst are similar, but nanoparticles reduce the accumulation of carbon by 63% during the DRM tests, which is a key feature for this reaction. XPS and H2-TPR suggest that the improved carbon resistance of the nanoparticles is due to the better interaction and cooperation between NiO and CeO2mixed phases. In nanoparticles, the participation of cerium cations in the redox processes taking place during DRM stabilizes cationic species of nickel. On the contrary, the catalyst prepared without control of the size suffers segregation of Ni during DRM reaction, and segregated Ni explains the higher catalytic formation of carbon.

The effect of the three-dimensionally ordered macroporous (3DOM) structure and the Ni doping of CeO2on 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, CeO2supports were synthesized with uncontrolled (Ref) and 3DOM-structured morphology, and their respective Ni/CeO2catalysts 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 CeO2with 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 CeO2crystal 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 CeO2catalyst.

3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO2methanation active phases (NiO-CeO2, 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO2, showing great stability and cyclability. Two monolith designs were used: a conventional parallel-channeled structure (honeycomb) and a complex 3D network of non-linear channels built upon interconnected circular sections (circles), where flow turbulences along the reactant gas path are spurred. The effect of the active phases particle size on the catalyst distribution and the overall performance has been assessed by comparing NiO-CeO2nanoparticles of 7 nm average (Np), with a reference counterpart of uncontrolled structure (Ref). The improved radial gases diffusion in the circles monolith design is confirmed, and nanoparticles show enhanced CO2methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH4production at higher temperatures, where the reaction kinetics is controlled by mass transfer limitations (T > 300 ºC). SEM and Hg porosimetry evidence that nanoparticles are deposited at deeper penetration through the narrow micropores of the carbon matrix of the monolithic supports, which tend to accumulate on the channels surface remaining more accessible to the reactant molecules. Altogether, this study examines the impact of the channel tortuosity and the active phase sizing on the CO2methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications.

A series of α-alumina-supported cobalt spinel catalysts of various expositions of the (100), (111), and (110) planes was synthesized and tested in the preferential oxidation of CO (CO-PROX) reaction. Successful control of the polyhedral shape of the nanospinel active phase was achieved via glycerol and/or Zn addition to the impregnation solution. This allowed for the application of such catalysts for resolving the shape–reactivity relationships in a more controlled fashion in comparison to previous studies, where various catalysts of an entirely unlike origin were used. The catalysts were thoroughly characterized by means of X-ray fluorescence, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning transmission electron microscopy, and temperature-programmed reduction techniques. The CO-PROX catalytic performance of the obtained catalysts was examined in the temperature-programmed surface reaction mode. A strong impact of the spinel nanograin morphology on the activity was observed, with the (100) termination found as the most active among all of the exposed low-index (111) and (110) planes. Furthermore, a linear correlation between the abundance of the surface Co3+cations and the yield of carbon monoxide oxidation reveals their vital relevance as the catalytically CO-PROX active sites. The results are discussed in terms of a thermodynamic diagram of the surface oxygen, water, and oxygen vacancy stabilities on the Co3O4most abundant planes, providing the rational background for the superior behavior of the (100) termination in the CO-PROX reaction. The obtained results speak in favor of the involvement of the suprafacial oxygen species in the CO oxidation process.

A new nanocomposite catalyst consisting of high-loading cobalt oxide (CoO) on nitrogen-doped reduced graphene oxide (rGO) for oxygen reduction reaction (ORR) was prepared in this work. Its high activity for the ORR in alkaline electrolyte was determined using the rotating disk electrode technique, and further confirmed in real alkaline membrane fuel cells. A combination of physicochemical characterization (e.g., X-ray absorption and X-ray photoelectron spectra) and density functional theory (DFT) calculation suggests that cobalt(II) cations in the composite catalyst may coordinate with the pyridinic nitrogen atoms doped into graphene planes, most likely the active species for the ORR. Especially, the DFT calculations indicate that a stable rGO(N)–Co(II)–O–Co(II)–rGO(N) structure can be formed in the nitrogen-doped graphene catalyst. Kinetic parameter analysis shows a high selectivity of four-electron reduction on the composite catalyst during the ORR with an average electron transfer number of 3.75. A synergistic effect between the rGO(N) and CoO may exist, yielding a much higher catalytic activity on the CoO/rGO(N) catalyst, compared to either rGO(N) or CoO controls. The novel synthesis procedure utilizing rGO(N) to further couple Co(II) yields a high loading of Co species (24.7 wt %). Thus, a relatively thinner cathode in fuel cell can accommodate more active Co species and facilitate O2transfer. Due to the high intrinsic activity and efficient mass transport, the CoO–rGO(N) ORR catalyst achieved approaching performance to state-of-the-art Pt/C cathodes in anion-exchange-membrane alkaline fuel cells.

PrOxand CuO/PrOxcatalysts have been prepared with conventional (Ref) and three dimensionally ordered macroporous (3DOM) structures, and the effect of the structure on soot combustion has been studied. It has been demonstrated that the 3DOM structure significantly improves the catalytic combustion of soot with O2. The activity follows the trend PrOx-3DOM > CuO/PrOx-3DOM ∼ CuO/PrOx-Ref PrOx-Ref, which is explained considering two aspects: the production of active oxygen and its transfer from catalyst to soot. FESEM microscopy, N2adsorption, Hg porosimetry and He density show that the 3DOM catalysts have ordered macroporosity with pores of 80 nm in diameter, which favors the carbon–catalyst contact. In addition, the 3DOM catalysts present higher surface density of active oxygen (Oads), which follows the trend CuO/PrOx-3DOM > PrOx-3DOM ∼ CuO/PrOx-Ref > PrOx-Ref. Consequently, the PrOx-3DOM catalyst combines a good production of active oxygen and an efficient transfer to soot, making it the most active catalyst to accelerate soot combustion. In contrast, PrOx-Ref is the least active since it is the least efficient in producing and transferring active oxygen. The impregnation of copper with the conventional support (CuO/PrOx-Ref) enhances the production and transfer of active oxygen, improving the activity with respect to PrOx-Ref. However, CuO blocks the porosity of the 3DOM support, hindering the contact with soot. Soot combustion is accelerated in the presence of NOxdue to the production of NO2. This NO2, once produced, is mostly readsorbed on the surface of the catalysts producing active oxygen that must be transferred to soot. For this reason, the porosity of the catalysts also plays a relevant role during combustion with NOx/O2because it affects the transfer of active oxygen produced by NO2to soot.

The reaction kinetics of CO2methanation over a highly active 8.5% Ni/CeO2catalyst 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–1was determined, as well as the effect of reactants (positive) and water partial pressures (negative) on CO2methanation rate. Three mechanistic models reported in the literature, in which CO2is 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 Ni0sites as well as oxygen vacant near to Ni-CeO2interface participate in CO2methanation, is able to predict the kinetics of Ni/CeO2catalyst accurately for a wide range of operational conditions.