Carbon Chain Growth by Formyl Insertion on Rhodium and Cobalt Catalysts in Syngas Conversion

Abstract

Syngas (CO/H 2 ) produced from coal, natural gas, or biomass has attracted much attention as alternative to petroleumderived fuels and chemicals. Syngas can be selectively converted to oxygenates, such as alcohols, aldehydes, and carboxylic acids, or hydrocarbons by Fischer-Tropsch synthesis (FTS). [1] Industrially, rhodium- [2] and cobalt-based [3] catalysts are often used for production of C 2 oxygenates and hydrocarbons. Despite numerous studies, the exact mechanism remains in debate, and represents a major challenge in catalysis. [4] Formyl, formed by CO hydrogenation, has been implicated as of the key reactive intermediates in syngas conversion, [4e, 5] and it was proposed that hydrogenation of HCO followed by C = O bond scission leads to the formation of a CH x monomer. Then chain growth proceeds by CO insertion into CH x , by carbene coupling, or by condensation of C 1 oxygenates with elimination of water, with formation of C n (n ! 2) oxygenates or hydrocarbons. However, the short lifetime of HCO prevents its characterization, which typically requires elevated pressures, and identification of its role in syngas conversion. [6] Recently, direct evidence for HCO as the key intermediate for CO methanation was obtained by in situ spectroscopic experiments on supported Ru catalysts. [7] Herein we report on the use of DFT calculations (for computational details, see Methods) to explore the role of HCO in syngas conversion and its dependence on the catalyst. Insertion of HCO was revealed to be an efficient alternative for chain growth on Rh(111) and Co(0001) surfaces in syngas conversion for the first time. Since HCO was proposed to be a prerequisite for the formation of a CH x monomer (the key intermediate involved in chain growth), there should be a sufficiently high concentration of HCO for the formation of C 2 oxygenates and hydrocarbons. The results were compared to reaction pathways of CO insertion and carbene coupling. This work offers a mechanistic understanding of syngas chemistry, by achieving fundamental insight that could be used to design and develop improved catalysts for these and other important reactions of technological interest.We first investigated competitive CO versus HCO insertion into CH x (x = 1-3) on Rh(111), as shown in Figure 1 a. The calculated activation energy barriers for CO insertion into CH, CH 2 , and CH 3 of 1.34, 1.25, and 1.55 eV, respectively, are significantly higher than the corresponding barriers for HCO insertion (0.89, 0.75, and 1.02 eV). Compared to the most commonly studied CO insertion pathway, the kinetic preference for the HCO insertion pathway is immediately apparent. Moreover, HCO insertion into CH x is slightly endothermic or exothermic, with reaction energies of 0.27, À0.10, and À0.04 eV, whereas CO insertion is endothermic by 1.11, 0.69, and 0.35 eV, respectively. Therefore, the HCO insertion pathway is preferred on thermochemical grounds. Regardless of the pathway, insertion into CH 2 (CH 3 ) is the most kinetically favorable (unfavorable) step among al...

Ru-Cluster-Modified Ni Surface Defects toward Selective Bond Breaking between C–O and C–C

“…Previous theoretical studies reported that C−O bond cleavage shows strong structural effects, which intensively requires a double stepedge site with a high-lying d-state; 18,20 the barrier for C−O bond cleavage is obviously higher than that for C−C cleavage. 47,48 This indicates that C−O bond cleavage requires a higher d-band-energy-state of active metal than C−C bond breaking. As a result, based on the desirable modulation of the metal d-band center position to a moderate energy state, the Ru−Ni interfacial sites as active sites substitute the original Ni surface defects, resulting in the inhibition of C−O breaking and the promotion of C−C bond cleavage.…”

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

“…The first layer is , the second layer , the third layer and the forth layer of chain propagation and termination is crucial for the production of desired products. Although it is difficult to simulate the elementary steps involving large molecules on the catalyst surface due to the limitation of current computational capacity, some theoretical efforts [36,37,116,119,120,143] have been devoted to this field concerning the formation of C 2 and C 3 . Hu and coworker have investigated all the possible reaction pathways for C 1 ?…”

Insight into the mechanism and possibility of ethanol formation from syngas on Cu(100) surface

“…It is well known that syngas mainly contains CO and H 2 along with a small amount of CO 2 [52], meanwhile, the extensive studies about C 2 H 5 OH formation form syngas on Rh-based catalysts have been reported [7,9,11,15], in which only CO hydrogenation to form C 2 H 5 OH are considered. On the other hand, when CO 2 hydrogenation is considered to form C 2 H 5 OH, on the basis of the number of O atom in C 2 H 5 OH, we think CO 2 firstly needs to be converted to CO, then, CO hydrogenates to product C 2 H 5 OH.…”

“…For C 2 oxygenates formation, the studies on Rh and Co catalysts by Zhao et al [15] found that CHO insertion into CH x (x = 1-3) is superior and/or competitive to CO insertion and carbene coupling for chain growth; moreover, Choi and Liu [7] have found that CH 3 is the most favored monomer on Rh(1 1 1) surface among all CH x (x = 1-3) species, meanwhile, the productivity and selectivity of C 2 H 5 OH is controlled by CH 3 hydrogenation to CH 4 formation and CO insertion into CH 3 to C 2 oxygenates. In this study, CH 3 are the most favored monomer formed by syngas on Cu(1 0 0) surface, in order to illustrate C 2 oxygenates formation, we further investigate CO insertion into CH 3 , CHO insertion into CH 3 , as well as CH 3 hydrogenation, dissociation and coupling on Cu(1 0 0) surface.…”

“…Nowadays, Rh-based catalysts are the most studied systems and the only group of materials for syngas conversion into ethanol, rather than via methanol [12,13], in which two key steps is involved in ethanol formation, one is surface hydrocarbon CH x (x = 1-3) formed by the direct CO dissociation or CO dissociation with hydrogen-assisted, the other is the C C chain formation [7,11,14,15]. However, the expensive cost and limited supply of Rh-based catalysts have restricted their applications to be used as industrial catalysts [2,4].…”