The rate determining steps and rate equation for the oxidative dry reforming of methane over supported Ni catalyst

Abstract

Dry reforming of methane (DRM) and its modification with O2 co-feed, the ODRM reaction, have been gaining importance. However, a general rate equation applicable for the ODRM reaction with varying O2 co-feed has not been reported. This general rate equation should be applicable for the DRM and ODRM reactions depending on the amount of O2 co-feed. Microkinetic models simulate catalytic reactions without making any a priori assumptions for the RDS and are helpful in developing rate equations. Unfortunately, microkinetic modelling for ODRM over supported monometallic Ni has not been carried out. Here, we perform microkinetic modelling using an existing reforming reaction mechanism for ODRM over an alumina-supported Ni catalyst. Validation is achieved by comparing experimental and microkinetic outlet flow rates at different feed rates of O2 and applying R2 analysis and F-tests. Once the model is validated, we identify the RDS through a hierarchical strategy using microkinetic results and sensitivity analysis for varying amounts of O2 co-feed. The results reveal that CH4 activation is the RDS. CH4 activation can occur through direct dehydrogenation on an empty site or oxidative dehydrogenation aided by adsorbed O, depending on the amount of O2 co-feed. Once the RDSs are identified, we derive a general rate equation that effectively simulates the DRM reaction with different amounts of O2 co-feed. We further examine the effect of the amounts of O2 co-feed and ascertain the presence of maximum hydrogen production for certain amounts of O2 co-feed.