Suchergebnisse

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 mechanism and kinetic of CO2 methanation reaction of 9.5 % Ni/Al2O3 catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al2O3 catalyst shows an apparent activation energy of 80.1 kJ mol−1 in CO2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H2O inhibition, with a water adsorption constant of 3.1 atm−1 and apparent orders of 0.24 and 0.27 for H2 and CO2, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al2O 3 catalyst with high accuracy under relevant process conditions (315−430 °C, 1−6 bar, H2 to CO2 molar ratios between 1–16 and, different reagents and products partial pressures). ; Support for this study was provided by the Spanish Ministry of Economy and Competiveness (Project PID2019-105960RB-C21 and PID2019-105960RB-C22) and the Basque Government (Project IT1297-19). One of the authors (JAOC) acknowledges the Post-doctoral research grant (DOCREC20/49) provided by the University of the Basque Country. Other of the ...

The reaction kinetics of CO2 methanation over a highly active 8.5% Ni/CeO2 catalyst 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–1 was determined, as well as the effect of reactants (positive) and water partial pressures (negative) on CO2 methanation rate. Three mechanistic models reported in the literature, in which CO2 is 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 Ni0 sites as well as oxygen vacant near to Ni-CeO2 interface participate in CO2 methanation, is able to predict the kinetics of Ni/CeO2 catalyst accurately for a wide range of operational conditions. ; Support for this study was provided by the Spanish Ministry of Science and Innovation (Project Nos. PID2019-105960RB-C21 and PID2019-105960RB-C22) and the Basque Government (Project No. IT1297-19). J.A.O.-C. acknowledges the Postdoctoral research grant (No. DOCREC20/49) ...