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Microkinetic Modeling and Reduced Rate Expression of the Water–Gas Shift Reaction on Nickel

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journal contribution
posted on 05.07.2018 by Thiago P. de Carvalho, Rafael C. Catapan, Amir A. M. Oliveira, Dionisios G. Vlachos
The development of a first-principles-based microkinetic modeling of the water–gas shift (WGS) reaction on nickel surfaces is presented. The surface reaction mechanism consists of 19 elementary reversible steps among 10 adsorbates. Density functional theory (DFT) was used to calculate the binding energies and transitions states of all adsorbates and reactions on Ni(111) and Ni(211) surfaces [Catapan; DFT study of the water–gas shift reaction and coke formation on Ni (111) and Ni (211) surfaces. J. Phys. Chem. C 2012, 116, 20281−20291]. Thermodynamic consistency of the DFT-predicted energetics was taken into account in the construction of the kinetic mechanism. Lateral interactions between adsorbates were calculated via DFT and included in the microkinetic modeling using a hierarchical approach. The model predictions compare well with experimental results on Ni/Al2O3 reported in the literature, reproducing CO conversion, apparent activation energy, and reaction orders for CO and H2O. A reduced microkinetic model is derived using sensitivity and principal component analysis. The main reaction pathway analysis revealed that the carboxyl pathway is favored and the elementary step CO* + OH* ⇌ COOH* + * is the rate-determining step of WGS on Ni. A global one-step rate expression for WGS on Ni was also developed. Moreover, a novel thermodynamic-consistent treatment for the evaluation species coverages in the rate expression is proposed. This came with the use of a look-up table for the most abundant adsorbed species and has improved the performance of the one-step expression over a wide range of conditions (inlet compositions, volumetric flow rates, and different temperatures).