Mechanistic Study of Low-Temperature CO₂ Hydrogenation over Modified Rh/Al₂O₃ Catalysts

Abstract

The hydrogenation of CO2 on Rh/Al2O3 catalysts modified with Ni and K was studied by in situ and operando DRIFTS spectroscopy comprising transient and isotopic exchange experiments to study the influence of this modification on the catalytic performance in CO and methane formation at 250–350 °C and to gain mechanistic insight. Catalytic testing and spectroscopic studies revealed that the modification with particularly K promotes the formation of CO being the highest over Rh, K, Ni/Al2O3, whereas methane formation is preferred over the unmodified catalyst. It was found that CO2 does not dissociatively adsorb but is adsorbed at the support, forming mainly hydrogen carbonate, and in the presence of K, also carbonate species. The dissociative adsorption of H2 proceeds on Rh. The activated H2 reacts mainly with the hydrogen carbonate species forming CO adsorbed on Rh and formate (F1) species stably adsorbed on the support. On the K-containing catalysts, an additional formate species (F2) was identified as more reactive than F1 formate and can act as a reaction intermediate in the CO formation pathway. Furthermore, adsorbed formyl species were detected, which are assumed to be intermediates in the methanation reaction. The modifying additives change the surroundings of the Rh particles. This influences the strength of CO adsorption and the activation ability of Rh for H2 dissociation. Thus, desorption of the formed CO from the catalyst surface is favored, and the methanation of CO is hindered. The modification with K enhances the ability for CO2 fixation by formation of additional carbonate species which cover adsorption sites for unreactive F1 formate species and favors the formation of reactive F2 formate species.

Operando Spectroscopic Evidence of the Induced Effect of Residual Species in the Reaction Intermediates during CO2 Hydrogenation over Ruthenium Nanoparticles

“…However, the relative intensity of the peaks is hardly affected by the temperature and we believe that this fact can be better explained considering that the H 2 dissociation is favoured at high temperatures, increasing the amount of adsorbed hydrogen species on the Ru surface. The chemisorbed H* species act as electron donor and favour the back-donation of electronic density from the metal to the π* antibonding orbitals of CO. [23] This leads to the weakening of CÀ O bond and the carbonyl bands appear at lower frequencies. Considering now the spectra relating to Ru/Al 2 O 3 (NaÀ B) showed in Figure 4c, the fact that carbonyl adsorbed species on ruthenium were absent is remarkable.…”

Unravelling the Promotional Effect of La2O3 in Pt/La‐TiO2 Catalysts for CO2 Hydrogenation

“…[25] Thus, the generation of the Pt-CO speciesw ill have ag reat effect on the evolution of CH 4 and CO. In situ Fouriert ransform infrared spectroscopy (FTIR) experiments were carriedo ut to explore the relationship between Pt-CO and the catalytic efficiency.A ss hown in Figure 9, the FTIR band at about 2028cm À1 is the characteristic peak of Pt-CO over Pt/TiO 2 .W ith the introduction of La 2 O 3 ,t he intensity of the Pt-CO band of Pt/La-TiO 2 is increased coupled with as mall shift to the lower wavenumber of 2025 cm À1 .T he increased band intensity and the lower wavenumbero ft he Pt-COs pecies point to the formation of more Pt-CO species over Pt/La-TiO 2 , [27] which will boost the CO 2 conversion reaction to produce CH 4 and CO. Generally,t he Pt-CO intermediate is formed via the dissociation of the chemisorbed CO 2 . [28] As ar esult, the enhanced adsorption and activation of CO 2 over Pt/La-TiO 2 will improvet he formationo fP t-CO. All of the above results demonstrate that the introduced La 2 O 3 could enhancet he adsorption and activationo fC O 2 and consequentlyc ontribute to CO 2 hydrogenation.…”

Monitoring the Reaction Mechanism in Model Biogas Reforming by In Situ Transient and Steady‐State DRIFTS Measurements

“…The latter process participates in the reaction mechanism through the RWGS reaction, and it is clear that the formation of formate species requires the close vicinity of carbonate species and metal‐adsorbed hydrogen. Therefore, the formation of formate species proceeds mainly at the metal–support interface, although the formate species can migrate to the surface support, at which they are stably adsorbed and detected by DRIFTS …”

“…Therefore, the formation of formate speciesp roceeds mainly at the metal-support interface, althought he formate species can migrate to the surface support, at which they are stably adsorbed and detected by DRIFTS. [29] The majority of the reaction mechanismsp ostulated in the literaturef or differents upported catalysts, except those developed for inert supports, are based on ab ifunctional pathway. This bifunctional pathway is widely accepted, andt he reaction takes place on catalysts with an active metal and as upport having either electrophilic or nucleophilic character.F or active metals on inert supports, the metal catalyzesa ll the surfacer eactions and is more prone to deactivation owing to carbon deposition or sintering.…”