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In this study, Ni catalysts supported on Pr-doped CeO2 are studied for the CO2 methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr3+ ions can substitute Ce4+ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO2 chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO2 activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution., Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public., ChemE/O&O groep, ChemE/Catalysis Engineering

Dry ethane reforming (DER) aims to utilize captured CO2 and ethane, which is found in large quantities in shale gas, towards the production of high-value synthesis gas. During the dry reforming of hydrocarbons, the interaction between the active metal and the underlying support, along with the choice of the operating temperature, are considered to be the main factors influencing a catalyst’s stability and coking resistance. In this work, the DER catalytic performance and stability of Ni-doped perovskite systems is compared with that of a typical impregnated Ni/Al2O3 catalyst. The calcined, reduced and spent catalysts are assessed using the ICP, XRD, N2 physisorption, H2-TPR, CO2-TPD, TEM, HAADF-STEM, EDS Mapping, XPS and TPO techniques. Ni-CaZrO3 (CZNO) consisting of partly exsolved Ni nanoparticles with a strong metal-support interaction is shown to be particularly stable and accumulate only a fraction of the coke that is deposited on the impregnated Ni/Al2O3 catalyst, which suffers from severe and rapid degradation under the reactant stream. By increasing the operating temperature to 750 °C, Ni-CaZrO3 can achieve almost total conversion of ethane and around 90% conversion of carbon dioxide towards synthesis gas, with no apparent loss of catalytic activity.

In this study, Ni catalysts supported on Pr-doped CeO2 are studied for the CO2 methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr3+ ions can substitute Ce4+ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO2 chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO2 activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution.

The work presented herein reports on the oxidative coupling of methane (OCM) performance of a series of Li-free and Li-doped CeO2 and CeO2 modified with Mg2+ and La3+ catalysts. The supporting materials (Ce, Mg-Ce and La-Mg-Ce metal oxides) were synthesized using the microwave assisted sol-gel method, while lithium ions were added using the wet impregnation technique, to further affect the physicochemical properties, activity and selectivity of the materials, in terms of the desirable hydrocarbon products (C2H4 and C2H6). The materials were characterized towards their textural, structural and redox properties, surface basicity, and surface morphology using N2 adsorption/desorption, X-Ray Diffraction (XRD), Raman spectroscopy, CO2-Temperature Programmed Desorption (CO2-TPD), H2-Temperature Programmed Reduction (H2-TPR), Scanning Electron Microscopy (SEM), and X-ray Photoelectron Spectroscopy (XPS). Catalytic activity was assessed between 600 and 870 °C, at atmospheric pressure and different CH4:O2 molar ratios and Weigh-basis Gas Hourly Space Velocity (WGHSV). It is concluded that low specific surface area values, the existence of surface moderate basic sites, increased concentration of oxygen vacancies and the presence of electrophilic oxygen species (O2– and O22–) on the catalyst surface had a crucial role on the improvement of the catalytic performance in terms of the desirable products, mainly ethylene. It is noted that the addition of Li changed thoroughly the reaction pathway and the products’ distribution, with the C2 selectivity values exceeding 85%. The Li/Mg-Ce catalyst, presenting the higher population of intermediate basic sites and surface superoxide species, showed an improved catalytic activity in terms of XCH4 and production of ethylene, while the incorporation of La3+ into the crystal structure of CeO2 suppressed the production of ethylene. Finally, smaller CH4:O2 molar ratios suppressed the production of hydrocarbons, while enhanced residence times

The work presented herein reports on the oxidative coupling of methane (OCM) performance of a series of Li-free and Li-doped CeO2 and CeO2 modified with Sm3+ and La3+ catalysts. The supporting materials (Ce, Sm-Ce and La-Sm-Ce metal oxides) were synthesized using the microwave assisted sol-gel method in order to achieve nanophase complex materials with increased particle surface energy and reactivity. Lithium ions were added, using the wet impregnation technique, in order to further improve the physicochemical characteristics and reinforce the activity and selectivity, in terms of C2H6 and C2H4 production. All materials were characterized using N2 adsorption-desorption, XRD, Raman spectroscopy, CO2-TPD, H2-TPR, SEM and XPS. We showed that the addition of lithium species changed the reaction pathway and drastically enhanced the production of ethylene and ethane, mainly for the promoted catalysts (Li/Sm-Ce and Li/La-Sm-Ce). In particular, the presence and the synergy between the electrophilic oxygen species (peroxide and superoxide), population of oxygen vacancy sites and the surface moderate basic sites determined the reaction pathway and the desirable product distribution.

In the study presented herein, nickel catalysts supported on CeO2 and, for the first time in the literature, on La2O3-Sm2O3-CeO2, La2O3-Pr2O3-CeO2 and La2O3-MgO-CeO2 were prepared and evaluated for the reaction of CO2 methanation. The carriers were prepared through a sol-gel microwave assisted method and the catalysts were obtained following wet impregnation. The physicochemical properties of the catalysts prior to reaction were determined through H2-TPR, H2-TPD, Raman spectroscopy, XRD, CO2-TPD, N2 physisorption-desorption, XPS and TEM. The spent catalysts, after the time-on-stream experiments were further characterised using TEM and TGA. It was shown that the simultaneous incorporation of La3+, Pr3+ and La3+, Sm3+ into the crystal structure of cerium oxide created higher population of oxygen vacant sites. Moreover, the co-presence of La3+, Mg2+ and La3+, Pr3+ into the CeO2 increased the plethos of moderate basic sites. These physicochemical properties increased the rate of CO2 methanation reaction at relatively low temperatures. Furthermore, it is argued that the addition of La3+ stabilized the Ni active sites via the probable formation of a new compound (La-O-Ni) on the catalyst surface or synergetic catalytic centers at the interfacial area improving the catalytic properties (activity and stability). Finally, the catalytic performance tests revealed that the addition of La3+ mainly improved the conversion of CO2 and yield of CH4 for the Ni/La-Mg-Ce and Ni/La-Sm-Ce samples. The rCO2 and XCO2 values at 300 °C followed the order Ni/La-Sm-Ce >> Ni/La-Mg-Ce > Ni/La-Pr-Ce > Ni/Ce.

The present work reports on the investigation of the catalytic performance for the methanation of CO2 over Ni catalysts based on CeO2, and for the first time, of Ni catalysts supported on binary CeO2-based oxides, namely, Sm2O3-CeO2, Pr2O3-CeO2 and MgO-CeO2. The supports were obtained using the microwave assisted sol-gel method under reflux, while the catalysts were prepared by the wet impregnation method. For the investigation of the morphological, textural, structural and other intrinsic properties of the catalytic materials a variety of characterization techniques were used, i.e., Raman spectroscopy, XRD, N2 physisorption-desorption, CO2-TPD, H2-TPR, H2-TPD, XPS and TEM. Carbon deposition and sintering were investigated using TEM. It was shown that the addition of Sm3+ or Pr3+, incorporated into the lattice of CeO2, generated oxygen vacancies, but the Ni/Pr-Ce catalyst was found to possess more surface oxygen vacancies (e.g. Ce4+-Ov-Pr3+ entities). Moreover, modification of CeO2 using Sm3+ or Pr3+ restricted the agglomeration of nickel active sites and led to the genesis of Lewis basic positions. These characteristics improved the hydrogenation reaction at lower temperature. On the other hand, the addition of Mg2+ resulted at strong metal support interactions reinforcing the resistance of the Ni/Mg-Ce catalyst against sintering. Furthermore, the addition of Sm3+, Pr3+ and Mg2+ cations increased the overall basicity and the moderate adsorption sites and led to the formation of smaller Ni nano particles; these physico-chemical properties enhanced the CO2 methanation reaction. Finally, the activity experiments (WGHSV = 25,000 mL g−1 h−1, H2/CO2 = 4:1, T =350 °C) showed that at lower reaction temperature the Ni/Pr-Ce had the highest catalytic performance in terms of CO2 conversion (54.5%) and CH4 yield (54.5%) and selectivity (100%). The TOF values were found to follow the order Ni/Pr-Ce >> Ni/Mg-Ce > Ni/Sm-Ce > Ni/Ce.