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Highly Active and Stable CeO<sub>2</sub>‑Promoted CuCl<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> Oxychlorination Catalysts Developed by Rational Design Using a Rate Diagram of the Catalytic Cycle
In this study, we
have developed a method to predict the steady-state
rate and Cu oxidation state during ethylene oxychlorination from a
reaction rate diagram of the individual steps involved in the catalytic
oxychlorination cycle. The steady state of the redox cycle is represented
by a cross point of the reaction rates of the reduction and oxidation
steps as a function of the Cu<sup>2+</sup> in the rate diagram. Transient
kinetics of elementary reactions and steady-state kinetics of the
overall catalytic cycle were investigated in an operando study using
combined mass and UV–vis-NIR spectrophotometry. The catalytic
consequence of the promoters was then evaluated in terms of reduction
and oxidation activity as well as number of active sites, site activity,
and the catalyst oxidation state at steady state. Results revealed
that the neat CuCl<sub>2</sub> catalysts operated at low Cu<sup>2+</sup> at the steady-state conditions with stoichiometric feed composition,
as a result of relatively low oxidation rate of Cu<sup>1+</sup>. As
a consequence of a high content of Cu<sup>1+</sup>, ethylene conversion
and selectivity are low, and the catalyst deactivates rapidly. By
the promotion of the CuCl<sub>2</sub> catalyst by K, the reactor operates
at a high Cu<sup>2+</sup> concentration with much improved stability
as a result of enhanced oxidation rate, but the catalyst has low activity
due to significantly reduced reduction rate. Therefore, the rate diagram
has been applied as a tool for a rational design of the CuCl<sub>2</sub>-based oxychlorination catalysts, and Ce was proposed as the promoter
due to its high promotion to the oxidation and low reactivity with
Cu ions. It was found that the activity of the Ce-promoted catalysts
increased 8 times compared to the neat CuCl<sub>2</sub> catalyst and
moreover significantly improved the stability for the oxychlorination
catalyst at steady state, due to the enhancement of both the rates
of the reduction and oxidation. It is anticipated that the methodology
developed here paves the way for a general method for catalyst design
of heterogeneous catalysts where the catalyst undergoes oxidation
state changes, in particular in redox reactions