(669c) Cobalt-Based Catalysts for the Conversion of Syngas to Ethanol and Higher Alcohols: Formation and Role of Cobalt Carbide

Introduction

Ethanol is an alternative fuel that can be formed from natural gas and biomass-derived syngas. Cobalt-based catalysts have been proposed for ethanol and higher alcohols synthesis as a substitute of high-cost noble metal catalyst^1,2. In order to form ethanol and higher alcohols, two types of sites with close proximity are needed: One to dissociate CO and a second to insert CO to the intermediates to yield oxygenates ³. Metallic cobalt is responsible for CO dissociation, while the nature of the site for CO insertion is still under study. Our recent studies show that cobalt carbide is formed on the catalyst surface during the reaction, and cobalt carbide is able to associatively adsorb CO and insert CO into C_n species that lead to ethanol and higher alcohols⁴. 

Materials and Methods

Catalysts were synthesized using coprecipitation method. Samples are denoted as (1)40Cu40Co/La, which has 40 mol% Co, 40 mol% Cu and balance lanthanum, and (2) 80Co/La, which has 80mol% Co and balance lanthanum. In addition, another cobalt carbide catalyst named as (3) Co₂C/Co is made by carburizing cobalt (II) oxide powder directly with carbon monoxide. XRD and XPS characterizes the transformation of cobalt in the bulk or on the surface. X-ray absorption determines local atomic structure of the catalyst. DRIFTS under reaction conditions examine CO adsorption on the surface species and reaction mechanism. In addition, CO hydrogenation at differential conversion shows catalyst activity and selectivity.

Results and Discussion

XPS results show that the amount of surface carbon increases significantly after CO hydrogenation reaction. However, In situ XRD profiles do not identify any crystallinity changes of the catalyst in the presence of syngas at 250°C. For 40Cu40Co/La, freshly reduced sample and post-reaction sample showed distinct XANES profiles. EXAFS result on used 40Cu40Co/La shows a peak at lower distance for the first Co-C coordination shells. DRIFTS tests on 40Cu40Co/La show a CO stretch peak at 1995 cm^-1 was detected, which corresponds to CO linearly adsorbed on cobalt carbide species⁶. During the reaction, Co₂C is slowly formed. Although methane selectivity slightly increases, the selectivity to ethanol and other oxygenates has increased with time. The formation of Co₂C increases CO insertion probability, leading to greater selectivity to ethanol and higher alcohols. 

Reference

[1]        Subramani, V.; Gangwal, S. K. Energy & Fuels 2008, 22, 814.

[2]        Gupta, M.; Smith, M. L.; Spivey, J. J. ACS Catalysis 2011, 1, 641.

[3]        Spivey, J. J.; Egbebi, A. Chemical Society Reviews 2007, 36, 1514.

[4]       Volkova, G. G.; Yurieva, T. M.; Plyasova, L. M.; Naumova, M. I.; Zaikovskii, V. I. J. Mol. Catal. A-Chem. 2000, 158, 389.

[5]        Lebarbier, V. M.; Mei, D.; Kim, D. H.; Andersen, A.; Male, J. L.; Holladay, J. E.; Rousseau, R.; Wang, Y. Journal of Physical Chemistry C 2011, 115, 17440.

[6]        G. Bian, T. Nanba, N. Koizumi, M. Yamada, Journal of Molecular Catalysis A: Chemical 178 (2002) 219-228.