(138d) 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.