Mechanistic Study of the Selective Methanation of CO over Ru/TiO₂ Catalysts: Effect of Metal Crystallite Size on the Nature of Active Surface Species and Reaction Pathways

Abstract

The effect of metal crystallite size on the reaction mechanism of competitive methanation of CO and CO2 is investigated over Ru/TiO2 catalysts of variable metal content (0.5 and 5.0 wt %) and crystallite size (2.1 and 4.5 nm) employing in situ FTIR and transient mass spectrometry techniques. Results show that although both catalysts follow the same mechanistic pathways, the relative population of reaction intermediates differs. Specifically, the nature of Ru-bonded carbonyl species detected on the catalyst surface upon interaction with the CO/CO2-containing gas mixtures is the same for both catalysts. However, their relative population depends significantly on ruthenium crystallite size and determines catalytic activity. The population of multicarbonyl species adsorbed on partially oxidized Ru sites (Ru n+-(CO) x ) decreases significantly with increasing Ru crystallite size, whereas the opposite is observed for CO species linearly bonded on reduced Ru crystallites (Ru x -CO). This is correlated with the higher catalytic activity found for the 5%Ru/TiO2 catalyst, since evidence has been provided that surface species adsorbed on reduced ruthenium sites are direct precursors of methane.

Highly efficient Ru/TiO2 -NiAl mixed oxide catalysts for CO selective methanation in hydrogen-rich gas

“…Notably, the temperature width associated with the Ru/15TiO 2 ‐MMO catalyst (175‐260°C) equals the one obtained with the Ru/25TiO 2 ‐MMO (200‐285°C), but the former is located at a lower temperature range. Thus, Ru/15TiO 2 ‐MMO has the highest performance for CO‐SMET among the catalysts prepared herein and is much better than the commercial Ru/TiO 2 and Ru/Al 2 O 3 catalysts. Compared with the previously reported RuNi‐doped TiO 2 ‐Al 2 O 3 and Ru‐doped mesoporous Ni‐Al‐oxide catalysts, the Ru/15TiO 2 ‐MMO catalyst in this work presents a comparable reactivity but has a much wider reaction temperature window (175‐260°C).…”

“…To improve the catalytic performance of MMO, various composite materials with enhanced reactivity have been developed by maximizing their intrinsic properties (eg, increased SSA, metal dispersion, and structural stability) . In this regard, TiO 2 attracted our attention as it has been reported to be effective in CO‐SMET . Until now, TiO 2 ‐MMO coupled composites obtained from TiO 2 ‐LDH precursors have shown promising applications in photodegradation and solar cells .…”

Noble-metal-free and Pt nanoparticles-loaded, mesoporous oxides as efficient catalysts for CO2 hydrogenation and dry reforming with methane

“…Among these metals, Ni and Co are promising candidates due to its lower cost than Pd, Ru, and Rh. A variety of oxide supports have been used for the CO 2 methanation that includes Al 2 O 3 [57][58][59], SiO 2 [47,60,61], TiO 2 [52,53,62,63], CeO 2 [50,61,64], NiO [65], Co 3 O 4 [66].…”

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

“…S13), we can conclude that a direct CO2 hydrogenation path to CH4 takes place on the Ru@C-EDTA-20 sample, while contribution of a RWGS reaction mechanism occurs on the Ru/C sample in the presence of Ru 0 , in agreement to previous studies. 35,[37][38][39][40][41][42] Moreover, the fact that 13 CH4 is not observed in the isotopic studies of the Ru@C-EDTA-20 sample allows to discard the co-existence of Ru 0 species on the catalyst surface or, if present, it should be in a very low amount, being the activity ascribed predominately to the presence of RuC species. Based on it, a core shell structure containing a metallic core and an upper shell of ruthenium carbide and carbon species can be proposed in our catalysts.…”