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

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).