Density Functional Theory Study of the CO Insertion Mechanism for Fischer−Tropsch Synthesis over Co Catalysts

Abstract

Different mechanisms have been proposed for Fischer-Tropsch synthesis, the conversion of CO and H 2 to long-chain alkanes. Density functional theory calculations indicate that CO activation has a barrier of 220 kJ/mol on Co(0001), and hence the concentration of surface C or CH 2 species is likely too low to explain the high chain growth probability. Hydrogenation lowers the C-O dissociation barrier to 90 kJ/mol for HCO and to 68 kJ/mol for H 2 CO; however, CO hydrogenation has a high energy barrier of 146 kJ/mol and is +117 kJ/mol endothermic. We propose an alternative propagation cycle starting with CO insertion into surface RCH groups. The barrier for this step is 80 kJ/mol. RCHCO is subsequently hydrogenated to RCH 2 CHO, which undergoes C-O dissociation with a barrier of 50 kJ/mol. The hydrogenation barriers are 120 and 48 kJ/mol along the dominant reaction path. The calculated CO turnover frequency for the proposed CO insertion mechanism is 1 to 2 orders of magnitude faster the hydrogen-assisted CO activation mechanism and 4 orders of magnitude faster than direct CO activation on a model Co(0001) surface.

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…Inderwildi et al found the same pathway via HCO and CH 2 O molecules, which undergo a lower barrier of 0.85 eV for C-O bond cleavage of CH 2 O [131]. Besides, the similar results of C-O bond cleavage (0.93 eV via HCO and 0.70 eV via CH 2 O) were obtained by Saeys and coworkers [114]. Moreover, Iglesia and coworkers Fig.…”

“…PBE functional [124] or PW functional [123]), and other parameters were employed in these calculations. Despite this fact, similar trends can be observed Understanding of Co-Based F-T Catalysis 147 PBE functional, PAW pseudopotential in VASP package were employed [114,117], and the data in parentheses are calculated on Co (0001) with 1/3 ML CO coverage [117]. Perdew-Burke-Ernzerhor form (PBE) of generalized gradient corrections and Troullier-Martins norm-conserving scalar relativistic pesudopotential in SIESTA package were employed [116,120,121].…”

“…As an alternative to the carbide mechanism, the CO insertion mechanism on cobalt-based catalysts has been investigated by Saeys and coworkers [114]. As shown in Table 5, it is reported that the barriers of CO insertion into CH 2 and CH are 1.11 and 0.83 eV respectively, which is comparable to those of the C 1 ?…”

“…The authors found that the chain growth probability is not proportional to the surface C coverage but to the partial pressure of CO, a result that further supports their argument that CO is the chain lengthening monomer. This example shows that transient experiments could provide [114] b Ref [121] c Ref [117], where the data in the parentheses were calculated on Co (0001) with 1/3 ML CO coverage Understanding of Co-Based F-T Catalysis 155 useful information regarding reaction mechanisms, although the results need a carful interpretation. The issue regarding the chain growth mechanism, whether the carbide mechanism or CO insertion mechanism is the more favorable, has not yet been solved since both mechanisms have theoretical and experimental support.…”

“…Furthermore, Liu et al [115] elucidated that the barriers of hydrogen-assisted CO activation are 0.73, 1.04 and 0.72 eV on Co (0001), Co (10-12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) surfaces, respectively indicating that the hydrogen-assisted CO dissociation is also a structure sensitive reaction which depends on the local structure of the active sites. [40], where the data in the parentheses were calculated on the surface with 0.5 ML pre-covered CO b Ref [28] c Ref [114] d Ref [130 e Ref [131] f Ref [120] g Ref [119], where the data in the parentheses were calculated on O pre-covered surface h Ref [118] i Ref [29] j Ref [117], where the data in the parentheses were calculated on Co (0001) with 1/3 ML CO coverage k Ref [115] l Ref [113] m Ref [32], where the surface with 1/4 ML pre-covered CO Understanding of Co-Based F-T Catalysis 151…”