Mechanistic insight into carbon-carbon bond formation on cobalt under simulated Fischer-Tropsch synthesis conditions.

Facile C-C bond formation is essential to the formation of long hydrocarbon chains in Fischer-Tropsch synthesis. Various chain growth mechanisms have been proposed previously, but spectroscopic identification of surface intermediates involved in C-C bond formation is scarce. We here show that the high CO coverage typical of Fischer-Tropsch synthesis affects the reaction pathways of C2Hx adsorbates on a Co(0001) model catalyst and promote C-C bond formation. In-situ high resolution x-ray photoelectron spectroscopy shows that a high CO coverage promotes transformation of C2Hx adsorbates into the ethylidyne form, which subsequently dimerizes to 2-butyne. The observed reaction sequence provides a mechanistic explanation for CO-induced ethylene dimerization on supported cobalt catalysts. For Fischer-Tropsch synthesis we propose that C-C bond formation on the close-packed terraces of a cobalt nanoparticle occurs via methylidyne (CH) insertion into long chain alkylidyne intermediates, the latter being stabilized by the high surface coverage under reaction conditions.

Correction: Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100).

Correction for 'Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100)' by B. Caglar et al., Phys. Chem. Chem. Phys., 2016, 18, 23888-23903.

The effect of C-OH functionality on the surface chemistry of biomass-derived molecules: ethanol chemistry on Rh(100).

The adsorption and decomposition of ethanol on Rh(100) was studied as a model reaction to understand the role of C-OH functionalities in the surface chemistry of biomass-derived molecules. A combination of experimental surface science and computational techniques was used: (i) temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS), work function measurements (Kelvin Probe - KP), and density functional theory (DFT). Ethanol produces ethoxy (CH3CH2O) species via O-H bond breaking upon adsorption at 100 K. Ethoxy decomposition proceeds differently depending on the surface coverage. At low coverage, the decomposition of ethoxy species occurs viaß-C-H cleavage, which leads to an oxometallacycle (OMC) intermediate. Decomposition of the OMC scissions (at 180-320 K) ultimately produces CO, H2 and surface carbon. At high coverage, along with the pathway observed in the low coverage case, a second pathway occurs around 140-200 K, which produces an acetaldehyde intermediate viaα-C-H cleavage. Further decomposition of acetaldehyde produces CH4, CO, H2 and surface carbon. However, even at high coverage this is a minor pathway, and methane selectivity is 10% at saturation coverage. The results suggests that biomass-derived oxygenates, which contain an alkyl group, react on the Rh(100) surface to produce synthesis gas (CO and H2), surface carbon and small hydrocarbons due to the high dehydrogenation and C-C bond scission activity of Rh(100).

Modeling the surface chemistry of biomass model compounds on oxygen-covered Rh(100).

Rhodium-based catalysts are potential candidates to process biomass and serve as a representation of the class of noble metal catalysts for biomass-related processes. Biomass can be processed in aqueous media (hydrolysis and aqueous phase reforming), and in this case the surface chemistry involves hydroxyl (OH) species. In our study this was modelled by the presence of pre-adsorbed oxygen. Ethylene glycol, with a hydroxyl group on every carbon atom, serves as a model compound to understand the conversion of biomass derived molecules into desirable chemicals on catalytically active metal surfaces. Ethanol (containing one OH group) serves as a reference molecule for ethylene glycol (containing two OH groups) to understand the interaction of C-OH functionalities with a Rh(100) surface. The surface chemistry of ethylene glycol and ethanol in the presence of pre-adsorbed oxygen on a Rh(100) surface has been studied via temperature programmed reaction spectroscopy (TPRS) and reflection absorption infrared spectroscopy (RAIRS) using various coverages of O(ad) and ethylene glycol and ethanol. Pre-adsorbed oxygen alters the decomposition chemistry of both compounds, thereby affecting the product distribution. Under an oxygen-lean condition, the selectivity to produce methane from ethanol is enhanced significantly (4.5-fold with respect to that obtained on the oxygen-free surface). For ethylene glycol, oxygen-lean conditions promote the formation of formaldehyde, with 10-15% selectivity. In addition, with Oad present the fraction of molecules that decompose on the surface increases 2-fold for ethanol and 1.5-fold for ethylene glycol, due to fast O-H bond activation by pre-adsorbed oxygen. Under oxygen-rich conditions, the decomposition products are mainly oxidized to carbon dioxide and water for both molecules. In this condition, the promotion effect provided by adsorbed oxygen for the dissociative adsorption of ethanol and ethylene glycol is reduced due to the site blocking effect of oxygen.