Cerium oxide catalysts for oxidative coupling of methane reaction: Effect of lithium, samarium and lanthanum dopants

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

Highly selective and stable nickel catalysts supported on ceria promoted with Sm2O3, Pr2O3 and MgO for the CO2 methanation reaction

[Display omitted]•Microwave assisted sol gel method produces selective CO2 methanation Ni catalysts.•The incorporation of Sm3+ and Pr3+ into the CeO2 lattice generates basic positions.•Sm3+ and Pr3+ oxygen vacancies suppress the agglomeration of Ni sites.•Presence of Mg2+ increases basicity and prevents Ni sintering during reaction.•Ni on Pr-Ce highly active, selective and stable for CO2 methanation reaction.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

“Promotion by sodium in emission control catalysis: The difference between alkanes and alkenes in the Pd-catalysed reduction of NO by hydrocarbons

Δημοσίευση σε επιστημονικό περιοδικόSummarization: The activity and selectivity of Pd catalysts supported on YSZ and dosed with different amounts of Na promoter has been investigated for the reduction of NO by alkenes (C3H6) or alkanes (CH4). It is found that Na strongly promotes the reduction of NO by C3H6. Rate increases by an order of magnitude are achievable, while the N2-selectivity is improved from ~75% over the unpromoted Pd catalyst to >95% over the optimally Na-promoted catalyst. With CH4 as the reductant, a very different behaviour is observed: for all loadings, Na induces only poisoning. The experimental data indicate that Na increases the strength of NO chemisorption relative to the hydrocarbons. This is accompanied by weakening of the N-O bond, thus facilitating NO dissociation, which is proposed as the critical reaction-initiating step. According to this model the promoting or poisoning effect of Na depends on the interaction strength of the catalyst surface with the hydrocarbon. The different behaviour of propene and methane reflects the weaker interaction of alkanes with metal surfaces compared to that of alkenes. XPS and Auger data demonstrate that Na coverage increases monotonically with promoter loading and that there is no significant tendency for the promoter to agglomerate with increasing promoter loading. However, a very small but constant tendency to accumulate subsurface or dissolved Na was observed as the promoter loading is increased.Παρουσιάστηκε στο: Global-Nest: the International Journa