Microcalorimetric Studies on Co–Re/γ-Al2O3 Catalysts with Na Impurities for Fischer–Tropsch Synthesis

Microkinetic Modeling: A Tool for Rational Catalyst Design

The design of heterogeneous catalysts relies on understanding the fundamental surface kinetics that controls catalyst performance, and microkinetic modeling is a tool that can help the researcher in streamlining the process of catalyst design. Microkinetic modeling is used to identify critical reaction intermediates and rate-determining elementary reactions, thereby providing vital information for designing an improved catalyst. In this review, we summarize general procedures for developing microkinetic models using reaction kinetics parameters obtained from experimental data, theoretical correlations, and quantum chemical calculations. We examine the methods required to ensure the thermodynamic consistency of the microkinetic model. We describe procedures required for parameter adjustments to account for the heterogeneity of the catalyst and the inherent errors in parameter estimation. We discuss the analysis of microkinetic models to determine the rate-determining reactions using the degree of rate control and reversibility of each elementary reaction. We introduce incorporation of Brønsted-Evans-Polanyi relations and scaling relations in microkinetic models and the effects of these relations on catalytic performance and formation of volcano curves are discussed. We review the analysis of reaction schemes in terms of the maximum rate of elementary reactions, and we outline a procedure to identify kinetically significant transition states and adsorbed intermediates. We explore the application of generalized rate expressions for the prediction of optimal binding energies of important surface intermediates and to estimate the extent of potential rate improvement. We also explore the application of microkinetic modeling in homogeneous catalysis, electro-catalysis, and transient reaction kinetics. We conclude by highlighting the challenges and opportunities in the application of microkinetic modeling for catalyst design.

The Effect of Potassium on Cobalt-Based Fischer–Tropsch Catalysts with Different Cobalt Particle Sizes

The effect of K on 20%Co/0.5%Re/γ-Al2O3 Fischer–Tropsch catalysts with two different cobalt particle sizes (small, in the range 6–7 nm and medium size, in the range 12–13 nm) was investigated. The catalyst with the smaller cobalt particle size had a lower catalytic activity and C5+ selectivity while selectivities towards CH4 and CO2 were slightly higher than over the catalyst with larger particles. These effects are ascribed to lower hydrogen concentration on the surface as well as the lower reducibility of smaller cobalt particles. Upon potassium addition all samples showed decreased catalytic activity, reported as Site Time Yield (STY), increased C5+ and CO2 selectivities, and a decrease in CH4 selectivity. There was no difference in the effect of potassium between the sample with small cobalt particles compared to the sample with medium size particles). In both cases the specific activity (STY) fell and the C5+ selectivity increased in a similar fashion.

Potassium adsorption behavior on hcp cobalt as model systems for the Fischer-Tropsch synthesis: a density functional theory study

Potassium (K), an important impurity in syngas from biomass, can have a large influence on the activity and selectivity of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts in Biomass to Liquids (BTL) processes. In this work, the potassium adsorption behavior on hcp cobalt was systematically studied using density functional theory. The surface energy calculations and Wulff construction of the equilibrium shape of hcp cobalt showed it is dominated by 10 facets. The interaction of K with these facets has been investigated. The results showed that the stepped facet (10-12) has the highest K adsorption energy of -2.40 eV. The facets (0001), (10-10), (10-11), (10-15), and (21-30) also showed relatively high K adsorption energies in the range of -2.28 to -2.34 eV. The corrugated facets exhibited comparatively lower K adsorption energies (-2.04 to -2.18 eV), and would be less favorable for K adsorption. It was also found that the adsorption properties depend on coverage, where the K adsorption energy decreased with increasing coverage. Diffusion energy barrier calculations indicated that K was mobile on typical facets (0001) and (10-11) with very low diffusion barriers (<0.15 eV). On stepped facets, although K could move freely along the same step (diffusion barrier <0.01 eV), diffusion from one step to another had a significantly higher barrier of 0.56 eV. This suggested that K atoms would be mobile to some extent during FTS reaction conditions, and tend to occupy the most favorable sites independent of their initial position. The results obtained in this work provide valuable information on the interaction of K with cobalt surfaces, relevant for practical cobalt catalysts and their application in BTL processes.