Detailed Modelling of Heterogeneous Chemistry on Palladium and Platinum

Abstract

The investigation presents the development, analysis and iterative validation of a heterogeneous chemical kinetic model for the simulation of reactive hydrocarbon flows over noble metal catalysts. An axisymmetric, two-dimensional boundary layer system is used to model a virtual pore, physically and chemically representative of the average pore through a catalytic bed in short contact time reactors. A mechanism for the modeling of heterogeneous chemistry over palladium was developed by adopting the Unity Bond Index Quadratic Exponential Potential (UBI-QEP) for the calculation of the energetics. This was integrated, where appropriate, with results from ab initio and experimental investigations. The surface mechanism was coupled with a thoroughly validated homogeneous gas mechanism to model the oxidative dehydrogenation of ethane over an alumina supported palladium catalyst (1%Pd/Al2O3). A bimetallic palladium-platinum mechanism was constructed by coupling the palladium chemistry with a platinum mechanism previously developed with the same methodology. The refinement of the mechanism was carried out and validated with experimental measurements of ethane and ethane-acetylene co-feeds over two catalysts (0.2%Pd 3%Pt/Al2O3 and 1%Pd 3%Pt/Al2O3). Extensive investigation of the sensitivity of the model to physical and chemical parameters was undertaken and evaluated in concert with the analysis of modifications to the energetics and chemical kinetic mechanisms. Of particular significance, for the palladium mechanism, was the reduction of the atomic heat of adsorption of carbon on palladium (to 363 kJ/mol). On platinum, evidence of poisoning by acetylene confirmed the indication that an increase in the adsorption energy of acetylene (to 219 kJ/mol) would correct the balance in relative heats of adsorption of key species. The study provides a framework for the construction and development of bimetallic heterogeneous chemical kinetic mechanisms and produces a validated working model for the simulation of reacting flows over pure palladium and bimetallic palladium-platinum catalysts in a range of conditions.