Correlation Between CO Oxidation and Coke Oxidation over Cerium-Zirconium Mixed Oxides

Correlation between CO Oxidation and Coke Oxidation over Cerium-Zirconium Mixed Oxides

Kehua Yin¹, Shilpa Mahamulkar², Hirokazu Shibata³, Andre Malek⁴, Christopher W. Jones², Pradeep Agrawal², Robert J. Davis¹*

(1) University of Virginia, Charlottesville, VA 22904 (USA)

(2) Georgia Institute of Technology, Atlanta, GA 30332 (USA)

(3) Hydrocarbons R&D, The Dow Chemical Company, Dow Benelux B.V. P.O. Box 48, NL 4530 AA, Terneuzen (The Netherlands)

(4) Hydrocarbons R&D, The Dow Chemical Company, 1776 Building, Midland, MI 48674 (USA)

*rjd4f@virginia.edu

Coke formation reactions at high temperature are common in chemical processes, such as on catalysts in hydrocarbon cracking [1] and on reactor walls in steam cracking [2]. While carbon rejection via coke may be desirable in some processes (e.g. FCC), it often leads to catalyst deactivation, poor heat transfer through reactor walls and even damage to the reactor. Thus, mitigation of coke by catalytic oxidation reactions can be an important remediation step in high temperature processes. Cerium-zirconium mixed oxides have demonstrated higher oxidation activity than cerium oxide at high temperatures because of their increased thermal stability [3]. However, kinetic studies of coke oxidation catalyzed by cerium-zirconium mixed oxides are still lacking. Hence, we investigated the reaction kinetics of coke oxidation catalyzed by cerium-zirconium mixed oxides and explored the relationship between mixed oxide composition and activity for oxidation of both coke and CO.

Model coke was prepared by flowing 40 cm³ min^-1 ethylene (50%) in He through a quartz tube reactor at 1073 K and atmospheric total pressure. Soluble carbonaceous deposits in the quartz tube were removed with toluene and the remaining solid coke was collected for further study. Cerium oxide, zirconium oxide, and cerium-zirconium mixed oxides (Ce/Zr = 0.2, 0.5, 0.8) were prepared by precipitation or co-precipitation at pH=10. Coke and the oxide catalysts were characterized by XRD, BET, SEM and Raman spectroscopy. Kinetics of coke oxidation reactions were determined in TGA (TA SDT Q-600) experiments at isothermal conditions with coke and the oxide catalysts in tight contact mode [4]. Tight contact was determined by grinding the coke with the catalysts in a mortar until there was no increase in activity with further grinding. Oxidation of CO was conducted in a fixed-bed reactor system equipped with an on-line gas chromatograph.

Coke oxidation rates over four cerium-based catalysts are shown in Figure 1, with the most active catalyst composition being Ce_0.8Zr_0.2O₂. The CO oxidation rates are also correlated with coke oxidation rates in Figure 1, which likely indicates that the CO oxidation reaction can be used as a probe reaction for the screening of coke oxidation catalyst.

The apparent activation energy of both the non-catalyzed and the ceria-catalyzed coke oxidation reaction was determined by measuring the first order rate constant as a function of temperature. The presence of catalyst decreased the E_obs for coke oxidation by 20-30 kJ mol^-1. In addition to lowering the activation energy, the presence of catalyst reduced the order in dioxygen from unity for the non-catalyzed reaction to 0.26 at high loading of Ce_0.8Zr_0.2O₂.

Reaction kinetics will be interpreted in light of the results from characterization of the coke and oxide catalysts.

References

[1] Cumming, K.A., and Bohdan W. Wojciechowski. "Hydrogen transfer, coke formation, and catalyst decay and their role in the chain mechanism of catalytic cracking." Catalysis Review: Science and Engineering 38 (1996): 101-157.

[2] Cai, Haiyong, Andrzej Krzywichi, and Michael C. Oballa. "Coke formation in steam crackers for ethylene production" Chemical Engineering and Processing 41 (2002): 199-214.

[3] Atribak, Idriss, Agust¨ªn Bueno-Lopez, and Avelina Garc¨ªa-Garc¨ªa. "Thermally stable ceria¨Czirconia catalysts for soot oxidation by O₂" Catalysis Communications 9 (2008): 250-255.

[4] Neeft, John P.A., Olaf P. van Pruissen, Michiel Makkee, and Jocob A. Moulijn. "Catalysts for the oxidation of soot from diesel exhaust gases II. Contact between soot and catalyst under practical conditions" Applied Catalysis B: Environmental 12 (1997): 21-31.