Active site components challenges

These two functions require different catalysts, so the development of a successful catalyst system is a difficult challenge. The catalysts must be effective over a wide range of operating temperatures. They must continue to be active despite the fact that various components of the exhaust can block the active sites of the catalyst. And the catalysts must be sufficiently rugged to withstand exhaust gas turbulence and the mechanical shocks of driving under various conditions for thousands of miles. 

This adds yet another aspect of complexity to a task which is challenging in itself the search for the active sites. Catalysis occurs on metastable structures rather than on idealized stable surfaces, which usually offer low activities (=reaction rates related to surface area under specified conditions as temperature and reactant composition). We have to look for defect sites, for atomic arrangements to be found only on small particles, on interaction structures with a second component, e.g., a support or a promoter. Usually, the arrangement of atoms requested by the catalytic reaction is not the only one exposed by the catalytic element. The coexistence of the active sites with indifferent structures or even with sites catalyzing the same reaction with different activation energy and, therefore... 

In addition, the reactors used for upgrading and refining heavy petroleum are complicated to model and design. The composition and properties of the heavy petroleum that is converted in reactors are such that the reaction system can involve various phases, types of catalysts, reactor configuration, reaction conditions, catalyst deactivation, and so on, making the development of a model a challenging task. Moreover, hundreds of components are present in heavy petroleum that undergo different reaction pathways and compete for the active sites of catalysts, which increases the complexity for the formulation of the kinetics and reactor models. 

As stated in Section 1, the Bagnoli brownfield site contains metallic elements whose source overlaps a natural component (ascribable to the CF hydrothermal activity connected to the quiescent volcanism) and an anthropogenic one (due to the industrial activity). The challenge was demonstrating and separating the contribution of these two components. 

---