Clean Technology 2010
N. Kapur, C. Sampara, X. Hao, J. Hyun, B. Shan, J.B. Nicholas, B. Davis, K. Cho
Nanostellar Inc., US
Fischer-Tropsch, microkinetics, Rhodium, Cobalt, defects, DFT
Syngas reformation into fuels is an attractive opportunity to diversify away from petroleum and address energy-dependency concerns. Rh and Co-based catalysts used within Fischer-Tropsch (FT) process selectively convert syngas into oxygenates and hydrocarbons respectively but suffer from catalytic activity issues. High CO conversion has been reported for large Rh and Co nanoparticles accompanied by variations in methanol, ethanol, and methane formation rates. Increase in CO turnover frequency has been attributed to an increase in surface step density. Insights into FT reaction network, structure-reactivity and structure-selectivity correlations required for catalyst design are however, lacking. Herein, we validate FT reaction networks on Rh and Co surfaces with theoretical and experimental efforts and investigate active site requirements. Microkinetic models are used to investigate competitive routes for forming oxygenates and hydrocarbons on both flat and stepped Rh and Co surfaces. Direct CO dissociation is promoted on defect sites and an alternative hydroxymethylene based route is kinetically favored on flat surfaces for both materials. Methylene addition to R-CH2 and CO is deduced to be the precursor for hydrocarbon and C2 oxygenate formation respectively. Our results indicate that catalytic routes are preferred on step sites irrespective of the reaction considered. Thus, further modifications to undercoordinated sites can lead to improvements in both activity and desired product selectivity.