Hydrothermal Synthesis of Ruthenium Nanoparticles with a Metallic Core and a Ruthenium Carbide Shell for Low-Temperature Activation of CO₂ to Methane

Abstract

Ruthenium nanoparticles with a core-shell structure formed by a core of metallic ruthenium and a shell of ruthenium carbide have been synthesized by a mild and easy hydrothermal treatment. The dual structure and composition of the nanoparticles have been determined by synchrotron XPS and NEXAFS analysis and TEM imaging. At increasing sample depth, metallic ruthenium species start to predominate, according to depth profile synchrotron XPS and XRD analysis. The herein ruthenium carbon catalyst is able to activate both CO2 and H2 showing exceptional high activity for CO2 hydrogenation at low temperatures (160-200 °C) with 100% selectivity to methane, surpassing by far the most active Ru catalysts reported up to now. Based on catalytic studies and isotopic 13 CO/ 12 CO2/H2 experiments, the active sites responsible for the unprecedented activity can be associated to those surface ruthenium carbide (RuC) species, enabling CO2 activation and transformation to methane via direct CO2 hydrogenation mechanism. The high activity and absence of CO in the gas effluent confers this catalyst interest for the Sabatier reaction, a reaction with renewed interest for storing surplus renewable energy in the form of methane.

Utility of Core–Shell Nanomaterials in the Catalytic Transformations of Renewable Substrates

“…classical Ru on Cc atalysts, the Ru/C core-shell catalysts inhibit the formation of Ru 0 ,w hich favors the reduction of CO 2 to CO. [25] Transformations of Biomass by Core-Shell…”

“…3 %H 2 ,a nd 5% N 2 (vol%). [25] Reproduced with permission;Copyright 2019,American Chemical Society. acidic sites.…”

“…Control experiments show that the catalytic performance decreases sharply following removal of the carbon shell. Compared to classical Ru on C catalysts, the Ru/C core–shell catalysts inhibit the formation of Ru 0 , which favors the reduction of CO 2 to CO [25] …”

“…(e) CO 2 conversion (left axis, black) and methane space‐time yield (STY) (right axis, blue) on the Ru/C‐20 catalyst at 160 °C, 21428 h −1 gas hourly space velocity (GHSV), 5 % CO 2 , 20 % H 2 , and 75 % N 2 (vol %). (f) Methane STY (left axis, black) at 160 °C, 21 428 h −1 GHSV, 23.8 % CO 2 , 71.3 % H 2 , and 5 % N 2 (vol %) [25] . Reproduced with permission; Copyright 2019, American Chemical Society.…”

“…[23,24] Ap owerful example of the importance of the shell may be appreciated from ac omparison of ruthenium methanation catalysts, which are generally easily deactivated by sintering, but by coating the Ru NPs with a mesoporousc arbon shell,s intering is significantly averted (Figure 2a). [25] The core-shell approach can also lead to ar eduction in the consumption of preciousm aterials, exemplified a the Cu/Au core-shell electrocatalyst. Au NPs are highly active for the eletro-reduction of CO 2 ,a nd after layeringA uo nt he surfaceo faCu core, the activity was retained with al ower Au loading ( Figure 2b).…”