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3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO 2 methanation active phases (NiO-CeO 2 , 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO 2 , 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-CeO 2 nanoparticles 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 CO 2 methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH 4 production 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 CO 2 methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications. [Display omitted] • NiO-CeO 2 /3D-printed carbon monoliths were efficient CO 2 methanation catalysts. • Unconventional circular design improves gas mixing along the monolith. • Nanoparticles show higher activity when intrinsic kinetic control prevails. • Above 300 °C, the reference outperforms in mass transfer rate control. [ABSTRACT FROM AUTHOR]

Ni/CeO 2 catalysts are very active and selective for total hydrogenation of CO 2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO 2 catalyst has been monitored under CO 2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that t he species involved in the redox processes taking place during the CO 2 methanation mechanism are the Ni2+-CeO 2 /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 CO 2 methanation conditions the H 2 reduction rate of the Ni2+-CeO 2 /Ni0 and Ce4+/Ce3+ couples is much faster than their CO 2 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/CeO 2 catalysts for CO 2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO 2 and Ni0 active sites where CO 2 and H 2 are expected to be efficiently dissociated, respectively. [Display omitted] • The redox processes during CO 2 methanation involve the Ni2+-CeO 2 /Ni0 and Ce4+/Ce3+ pairs. • H 2 reduction rate of the Ni2+-CeO 2 /Ni0 and Ce4+/Ce3+ couples is faster than their CO 2 reoxidation rate. • Ni/CeO 2 catalysts keep simultaneously Ni2+-CeO 2 and Ni0 active sites under reaction conditions. • NiO and Ni2+-carbonates/hydroxyls do not participate in redox processes [ABSTRACT FROM AUTHOR]

[Display omitted] • CO 2 methanation kinetics on low-loaded Ni/Al 2 O 3 catalyst is described based on DRIFTS and kinetic data. • LHHW rate equations are deduced including the role of water, carbonyls and formates. • Formates decomposition as RDS rate equation represents most accurately the kinetic performance. • The participation of both carbonyls and formates in the CO 2 hydrogenation is confirmed. The mechanism and kinetic of CO 2 methanation reaction of 9.5 % Ni/Al 2 O 3 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/Al 2 O 3 catalyst shows an apparent activation energy of 80.1 kJ mol−1 in CO 2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H 2 O inhibition, with a water adsorption constant of 3.1 atm−1 and apparent orders of 0.24 and 0.27 for H 2 and CO 2 , 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 CO 2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al 2 O 3 catalyst with high accuracy under relevant process conditions (315−430 °C, 1−6 bar, H 2 to CO 2 molar ratios between 1–16 and, different reagents and products partial pressures). [ABSTRACT FROM AUTHOR]

• The optimum Ru loading for Ru/CeO 2 CO 2 methanation catalysts is 2.5 wt. %. • 2.5 wt. Ru/CeO 2 is reduced at the lowest temperature. • 2.5 wt. Ru/CeO 2 achieves the highest proportion of ruthenium cations with strong interaction with ceria. • 2.5 wt. Ru/CeO 2 combines good CO 2 chemisorption capacity with efficient further hydrogenation. • The balance between formation and hydrogenation of surface carbon intermediates is optimum for 2.5 wt. Ru/CeO 2. The conversion of CO 2 towards added valuable products is considered as a potential alternative to achieve the increase of the petrochemical industry while reducing the CO 2 emissions. In the present work, the effect of Ru loading on CeO 2 supports has been studied for the CO 2 methanation reaction, and catalysts with different Ru loading in the 1–5 wt. % range have been prepared, characterized, and tested. The optimum Ru loading has been found to be 2.5 wt. %. Ruthenium cations are reduced at the lowest temperature for this optimum loading according to H 2 -TPR experiments (even at room temperature), and the highest proportion of ruthenium cations with strong interaction with ceria is achieved, as deduced from XPS. XRD characterization suggests partial insertion of ruthenium cations into the ceria lattice. In situ DRIFTS experiments evidenced that the balance between formation upon CO 2 chemisorption and further hydrogenation of surface carbon intermediates is optimum for 2.5 wt. % Ru/CeO 2. For low metal contents, the CO 2 chemisorption is limited and no relevant, while as the metal content is increased, the hydrogenation of carbon species is less favourable. The 2.5 wt.% Ru/CeO 2 catalyst comprises a balance between surface-carbon groups formation and further hydrogenation. [Display omitted] [ABSTRACT FROM AUTHOR]

Honeycomb monoliths are the preferred supports in many industrial heterogeneous catalysis reactions, but current extrusion synthesis only allows obtaining parallel channels. Here, we demonstrate that 3D printing opens new design possibilities that outperform conventional catalysts. High performance carbon integral monoliths have been prepared with a complex network of interconnected channels and have been tested for carbon dioxide hydrogenation to methane after loading a Ni/CeO2 active phase. CO2 methanation rate is enhanced by 25% at 300 °C because the novel design forces turbulent flow into the channels network. The methodology and monoliths developed can be applied to other heterogeneous catalysis reactions, and open new synthesis options based on 3D printing to manufacture tailored heterogeneous catalysts. [ABSTRACT FROM AUTHOR]

• Increasing loading of Ni and Ru increases the surface basicity and forms new CO 2 adsorption sites. • High calcination temperature leads to an increase of RuO 2 particle size and formation of inert Ni species. • Ni/Al 2 O 3 catalysts present high metal-support interaction, so that only a relative amount of metal is active for CO 2 methanation. • Ru/Al 2 O 3 catalysts are more efficient than Ni/Al 2 O 3 in hydrogen dissociation; TOF of the former is about ten times than TOF of Ni catalysts. • Optimal behavior was found for 12% Ni and 4% Ru, which provide metal surfaces of 5.1 and 0.6 m2 g−1, respectively. The hydrogenation of CO 2 into CH 4 from H 2 produced by renewable energy is considered an interesting alternative in order to promote the development of such green energies. In the present work, the effect of Ni and Ru loadings on the catalytic performance of alumina-supported catalysts is studied for CO 2 methanation reaction. All catalysts were prepared by wetness incipient impregnation, characterized by several techniques (N 2 -physisorption, CO 2 -TPD, XRD, H 2 -chemisorption, XPS and H 2 -TPR) and evaluated for CO 2 methanation in a fixed bed reactor at GHSV = 10,000 h−1 and W / F C O 2 0 = 4.7 (g cat.) h mol−1. Characterization results showed that addition of increasing loadings of Ni and Ru lead to the formation of both CO 2 adsorption and H 2 dissociation active sites, which are necessary to carry out CO 2 hydrogenation into methane. Easily reducible ruthenium was dispersed on γ-Al 2 O 3 in form of large agglomerates, whereas Ni was better dispersed presenting a great interaction with the support. 12% Ni and 4% Ru resulted to be the optimal contents providing metal surfaces of 5.1 and 0.6 m2 g−1, T 50 values of 340 and 310 °C and activity being quite stable for 24 h-on-stream. In terms of turnover frequency (TOF), 4%Ru/Al 2 O 3 catalyst was quite more efficient than 12%Ni/Al 2 O 3 , probably due to a greater ability of ruthenium to dissociate hydrogen. The apparent activation energies for alumina supported Ni and Ru were 129 and 84 kJ mol−1, respectively. [ABSTRACT FROM AUTHOR]

CO 2 hydrogenation to CH 4 (or methanation) has been proposed to diminish CO 2 emissions producing a valuable fuel. A catalyst consisting of NiO-CeO 2 mixed oxide 6–7 nm nanoparticles with enhanced properties has been prepared, and compared with other NiO-CeO 2 reference catalysts including a mixed oxide with the same composition but without control of the size, a counterpart NiO-CeO 2 mixed oxide with three dimensionally ordered macroporous (3DOM) structure and an inverse catalyst consisting of bulk NiO-supported CeO 2 nanoparticles among others. At 275 °C the CO 2 methanation rate is near 3 times higher to that achieved with the counterpart reference catalyst prepared without control of the size, being more active than all reference catalysts. The selectivity towards CH 4 formation is ~100% in the whole range of temperature studied (until 500 °C), and kept the same activity and selectivity during a 25 h long-term test. The high activity of this catalyst is related with its high specific surface area (122 m2/g) and with the presence of highly-reducible Ni-O-Ce species on the nanoparticles surface • 6-7 nm NiO-CeO2 with enhanced activity for CO2 methanation has been prepared. • Reaction rate is 3 times higher to that of a catalyst prepared without size control. • Nanoparticles selectivity towards CH4 is near 100 % until 500 °C. • Nanoparticles kept the same activity and selectivity for 25 hours. [ABSTRACT FROM AUTHOR]