Catalytic Mechanisms in HCl Oxidation Processes

The catalytic oxidation of hydrogen chloride to chlorine, commonly known as the Deacon process, underpins sustainable chlorine manufacture by converting corrosive waste streams into valuable product. Central to this process is the adsorption of HCl onto oxide surfaces, its dissociation into surface-bound chlorine and hydroxyl species, and the subsequent transfer of oxygen—either from molecular O₂ or from lattice oxygen—to yield Cl₂. Key mechanistic challenges arise from the strong adsorption of chloride, which can block active sites, and from the propensity of catalysts to undergo bulk chlorination, leading to deactivation. Recent advances have identified peroxo intermediates as facilitators of concerted dechlorination steps on ceria facets, substantially lowering activation barriers. Structure–activity relationships in mixed oxide systems, notably ruthenium on titania, have highlighted the role of interfacial oxygen species and support interactions in governing turnover rates. Equally, the dynamics of chlorination and reoxidation on ceria–zirconia composites have been elucidated through kinetic modelling and in situ spectroscopies. Together, these insights into surface chemistry, oxygen mobility and site-specific energetics guide the design of robust, high-surface-area catalysts with enhanced selectivity and longevity for industrial HCl oxidation.

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Technical terms

Deacon process: Industrial catalytic oxidation of hydrogen chloride to chlorine using oxygen.

Peroxo species: Adsorbed O₂²⁻ moiety that facilitates concerted oxygen transfer on catalyst surfaces.

Activation barrier: Energy threshold that must be overcome for a reaction step to occur.

Lattice oxygen: Oxygen atoms incorporated within an oxide catalyst framework that participate in redox cycles.

Bulk chlorination: Incorporation of chlorine into the catalyst lattice leading to permanent deactivation.

Catalyst support: Inert or active material that disperses the active phase, influencing surface structure and reactivity.

References

1. Critical Step in the HCl Oxidation Reaction over Single-Crystalline CeO2–x (111): Peroxo-Induced Site Change of Strongly Adsorbed Surface Chlorine. ACS Catalysis (2023).
2. Ru-Ti Oxide Based Catalysts for HCl Oxidation: The Favorable Oxygen Species and Influence of Ce Additive. Catalysts (2019).
3. sReactivation of CeO2‐based Catalysts in the HCl Oxidation Reaction: In situ Quantification of the Degree of Chlorination and Kinetic Modeling. ChemCatChem (2020).