sReactivation of CeO₂‐based Catalysts in the HCl Oxidation Reaction: In situ Quantification of the Degree of Chlorination and Kinetic Modeling

Abstract

Deactivation of CeO2‐based catalysts in the HCl oxidation reaction proceeds via selective bulk chlorination of the active CeO2 component to form CeCl3×nH2O. We study the reactivation of two bulk‐chlorinated CeO2‐based Deacon catalysts by oxygen treatment at 430 °C, namely pure CeO2 and 20 mol % of CeO2 supported on preformed ZrO2 particles (20CeO2@ZrO2), with a dedicated experiment. In the flow reactor setup we determine in‐situ the degree of chlorination of the catalyst by quantifying down‐stream with in‐situ UV‐Vis spectroscopy the total amount of chlorine in the catalyst that is exchanged by reoxidation at 430 °C. The activity of deactivated 20CeO2@ZrO2 can be fully restored by oxygen exposure at 430 °C, while that of pure CeO2 declines steadily. Since the UV‐Vis analytics is fast and sensitive, we can follow the kinetics of reoxidation. To rationalize the observed kinetics, we develop a modified Johnson‐Mehl‐Avrami‐Kolmogorov (JMAK) model based on a nucleation‐and‐growth approach for the reoxidation of the catalyst starting from the chlorinated phase. The fast reoxidation kinetics of chlorinated 20CeO2@ZrO2 is traced to a fast nucleation rate.

Effect of Conversion, Temperature and Feed Ratio on In2O3/In(OH)3 Phase Transitions in Methanol Synthesis Catalysts: A Combined Experimental and Computational Study

“…[20] Kinetics can play a crucial role in determining the long-term stability of thermodynamically unstable materials in catalysis and they also determine the steady-state phase fractions if degradation is reversible. [21] In the case of In(OH) 3 /In 2 O 3 , we observe no remarkable differences between our expectations from the purely thermodynamic treatment and our experimental observations. As a consequence, considering the kinetics in addition would add little additional insight at the present stage.…”

Dynamic Response of Oxygen Vacancies in the Deacon Reaction over Reduced Single Crystalline CeO2–x(111) Surfaces

“…This process will inevitably lead to a chlorinated CeO 2– x surface, with strongly adsorbed Cl vac species in oxygen vacancy positions serving as the “active” phase. The degree of chlorination was quantified by previous prompt γ-ray neutron activation analysis (PGAA) and confirmed by dedicated in situ titration experiments . For shape-controlled particles, especially CeO 2 octahedra exposing preferentially (111) facets, Ce 3d XP spectra indicate a mean Ce 3+ concentration in the near-surface region of 30% for an oxidizing Deacon reaction feed O 2 /HCl = 2:1 (stoichiometric feed O 2 /HCl = 1:4), while the fresh CeO 2 catalyst possesses a Ce 3+ concentration of 20%.…”

“…The Deacon reaction over CeO 2 reveals a positive reaction order of O 2 and typically runs under oxidizing reaction conditions, i.e., O 2 /HCl > 1:4 . Yet, oxidizing reaction mixtures lead still to a reduction of CeO 2 so that it is able to stabilize strongly adsorbing chlorine at the surface. − The reduction process is caused by HCl that adsorbs in an acid–base reaction forming OH groups with surface lattice O and adsorbed Cl atoms coordinated to surface Ce atoms. At high enough temperature, chlorine atoms recombine to form Cl 2 , while neighboring OH groups recombine to form water and an oxygen vacancy.…”

“…The degree of chlorination was quantified by previous prompt γ-ray neutron activation analysis (PGAA) 50 and confirmed by dedicated in situ titration experiments. 52 For shape-controlled particles, 9 especially CeO 2 octahedra exposing preferentially (111) facets, Ce 3d XP spectra indicate a mean Ce 3+ concentration in the nearsurface region of 30% for an oxidizing Deacon reaction feed O 2 /HCl = 2:1 (stoichiometric feed O 2 /HCl = 1:4), while the fresh CeO 2 catalyst possesses a Ce 3+ concentration of 20%. Altogether, this renders well-ordered chlorinated CeO 2−x (111) surfaces as suitable model systems for studying the elementary steps in the Deacon process.…”

Is There a Stable Deacon Catalyst? Computational Screening Approach for the Stability of Oxide Catalysts under Harsh Conditions

“…Two interesting areas in the periodic table are identified where oxides are both stable against chlorination and non-volatile: the early transition-metal oxides and the late rare-earth oxychlorides. The early transition metals include elements such as Ti, Zr, and Hf, whose oxides are already known as stable (but by themselves catalytically inactive) catalyst supports and have been demonstrated to improve the stabilities of CeO 2 catalysts either of the supported CeO 2 catalysts , or mixed oxides . Other elements included in this area are Nb and Ta, which, to the author’s knowledge have not been considered as Deacon catalysts so far.…”