Microkinetic Modeling for the Water-Gas Shift Reaction over Cobalt Catalysts Supported on Multi-Walled Carbon Nanotubes

The development of microkinetic models allows gaining an understanding of fundamental catalyst surface phenomena in terms of elementary reaction steps without defining a priori a rate-determining step, yielding more meaningful reaction rates. This work aimed at developing such a microkinetic model that accurately describes the Water-Gas Shift (WGS) reaction, i.e., one of the major routes for hydrogen production, over cobalt (Co) catalysts supported on multi-walled carbon nanotubes (MWCNTs). Co is a sulfur-tolerant active phase, and the functionalized MWCNT support has exceptional conductivity properties and defects that facilitate electron transfer on its surface. The model was formulated based on a well-known mechanism for the WGS reaction involving the highly reactive carboxyl (COOH*) intermediate. The kinetic parameters were either estimated or computed from theoretical prediction models (such as the Collision and Transition-State theory). The derived system of differential-algebraic equations was solved using the DDAPLUS package available in AthenaVISUAL Studio. The developed model was capable of simulating the experimental data (R² = 0.96), presenting statistically significant kinetic parameters. Furthermore, some of the catalysts descriptors introduced in the model were experimentally determined by characterization techniques, such as the specific surface area (S_P = 22000 m²/kg_cat) and the density of active sites (σ = 0.012 mol_{Act.Surf.­­­­}/kg_cat). The characterization results along with the model confirm that the COOH* formation reaction (CO* + OH* → COOH* + *) is the rate-determining step and explain the optimal catalyst performance at elevated temperatures (350-450^oC) and space times (70-80 kg.s/mol), as indicated by the experimental results.