Kinetic and in situ FTIR study of CO methanation on a Rh/Al₂O₃ catalyst

Abstract

Carbon monoxide hydrogenation was studied over a γ-alumina-supported 1 wt.% Rh catalyst by means of kinetic and in situ-Infrared measurements. The study was carried out at 200-300ºC, 0 -22.5 kPa H 2 and 1 -7.5 kPa CO. The in-situ FTIR scrutiny of catalyst surface shows that adsorbed CO* species and vacancies dominate the Rh surface, while no effect of H 2 and H 2 O pressures on surface coverage was observed at the conditions studied. Kinetic data are consistent with a mechanism in which the C-O bond dissociation is assisted by a double H-addition while H 2 dissociative adsorption, CO molecular adsorption and the HCO* formation are quasi-equilibrated steps. A two-parameter Langmuir-Hinshelwood rate expression is deduced for CH 4 formation, in agreement with the proposed sequence of elementary steps and kinetic data. The effect of temperature on parameters α and K CO leads to an apparent activation energy of 82.3 kJ·mol -1 , an average CO adsorption enthalpy of -14.1 kJ·mol -1 and an entropy change of -17.9 J·mol -1 ·K -1 . Insitu FTIR experiments show a full coverage of Rh surface with adsorbed CO below 200ºC and this CO* coverage decreases as temperature increases in the range 200-300ºC; it is also observed that the heat of CO adsorption on Rh surface decreases with CO* coverage.

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

“…H 2 (1−60 kPa) and H 2 O (0−32 kPa) pressures did not influence the intensity or the frequency of the CO infrared bands (SI, Figure S5), indicating that the coverages of adsorbed species derived from H 2 or H 2 O (e.g., H*, O*, OH*, H 2 O*) are much smaller than those of CO*. The invariance of the CO* adlayer with H 2 or H 2 O pressure is consistent with previous studies on Ru,35,39,38,115 Rh,61 and Co48,116,117 that…”

“…The K CO equilibrium constants regressed from eq 1 are clearly not "constant" with CO pressure or CO* coverage at the conditions used to obtain the rate data reported to obey eq 1 in previous studies. 38,48,49,61 Eq 1, derived from Langmuirian treatments of surfaces, cannot capture the strong effects of coadsorbate interactions that prevail as CO* adlayers densify with increasing CO pressure. The enhancements in rates over those predicted by eq 1 with K CO values at low coverages (η):…”

“…This thermodynamic formalism accounts for co-adsorbate interactions in the context of nonideal thermodynamic treatments of reactivity over the entire range of surface coverages that prevail in the practice of CO hydrogenation. [35][36][37][38][39][40]48,49,61 Such descriptions also provide fundamental insights into the dynamics of the many other reactions that occur on nearly saturated surfaces, such as CO oxidation, 1−8 NO reduction, 66−69 g −1 ) was treated in flowing dry air (Praxair, 99.999%, 0.33 cm 3 s −1 g −1 ) by heating to 1073 at 0.083 K s −1 and holding for 5 h. Ru was then dispersed on this SiO 2 using incipient wetness impregnation methods (5% wt. Ru).…”

“…Defect sites attributed to direct CO* dissociation events when examined at low coverages − are inactive at high coverages , because of the inability to form the required vacancies in the presence of strongly bound CO*. H 2 linearly increases the rate of CO hydrogenation and does so by H* assisting the kinetically relevant CO* activation step, which weakens CO bonds via the formation of *HCOH* intermediates. ,,,,− These species dissociate to form *OH and *CH species, which then form H 2 O, CH 4 , as well as the adsorbed monomers and chain initiators required to form C–C bonds and to grow hydrocarbon chains.…”

A short review on Ni-based catalyst supporter for carbon monoxide (CO) methanation process

“…There has been a substantial amount of effort put into the development of CO methanation catalysts, such as Ni [1][2][3][4][5][6][7][8][9][10][11][12], Co [13][14][15], Ru [16][17][18], Rh [19,20], and Pd [21] based catalysts. Ni-based catalysts were shown to be superior to other types of catalysts due to their comparatively strong methanation capabilities and inexpensive cost.…”