Elsevier

Journal of Catalysis

Volume 234, Issue 2, 10 September 2005, Pages 476-487
Journal of Catalysis

Catalytic study and electrochemical promotion of propane oxidation on Pt/YSZ

https://doi.org/10.1016/j.jcat.2005.07.017Get rights and content

Abstract

The Pt-catalyzed oxidation of propane was studied in an oxygen ion conducting solid electrolyte cell at 623–773 K and atmospheric total pressure. Under open circuit, the solid electrolyte potentiometry (SEP) technique was used to monitor the thermodynamic activity of oxygen adsorbed on the catalyst surface during reaction. Reaction kinetics and SEP measurements were combined to elucidate the reaction mechanism. Under certain conditions, sustained oscillations in the reaction rate and the surface oxygen activity were observed. The reaction exhibited a strong non-Faradaic modification of catalytic activity (NEMCA) effect. By varying the potential of the Pt catalyst, the rate of propane oxidation could be reversibly enhanced by up to a factor of 1400. At positive potentials, the reaction exhibited a pronounced electrophobic NEMCA enhancement. At negative potentials, the reaction also exhibited a strong electrophilic enhancement, indicating that the promoting effect is of the “inverted volcano” type.

Introduction

As an environmentally benign alternative to conventional flame combustion, the catalytic oxidation of hydrocarbons has received considerable attention [1], [2], [3], [4], [5], [6]. The catalysts used for combustion are mainly supported noble metals, especially Pt and Pd [5]. Excluding Pd in the case of methane, Pt is considered the most active metal for the catalytic oxidation of hydrocarbons [3], [6].

Stable lower alkanes, such as propane, require relatively high temperatures for complete oxidization over supported Pt catalysts [6]. But if the reaction is run under the appropriate conditions, then the incomplete oxidation of propane can result in the production of industrially important compounds. Toward this end, numerous investigators have studied the Pt-catalyzed oxidative dehydrogenation of propane at high reaction temperatures (873–1373 K). These studies have focused on the production of propylene, a key chemical in the polymerization and organic synthesis industries [7], [8], [9].

Platinum is the most widely used catalyst for the combustion of propane [1], [2], [3], [4], [5], [6], [10], [11], [12], [13], [14], [15], [16], [17]. In an effort to enhance the catalytic activity of Pt, earlier work has focused on the role of the support [4], [5], [6], [10], [11], [12], the effect of particle size and catalyst dispersion [2], [3], [11], and the effects of solid additives and gas phase promoters [3], [13], [14], [15], [16], [17].

Yazawa et al. [5] studied the Pt-catalyzed combustion of propane on several supports (MgO, La2O3, Al2O3, Si2O3–Al2O3, ZrO2, and SO42–ZrO2). They concluded that support materials affect the catalytic activity of Pt by affecting the oxidation state of the catalyst. These authors also observed that the support effect in a reducing atmosphere was significantly different from that in an oxidizing atmosphere. Kiwi-Minsker et al. [4] studied the reaction on Pt supported on glass fiber materials modified by several oxides (Al2O3, ZrO2, and TiO2) and observed the highest catalytic activity on glass fibers modified by TiO2. Garetto et al. [6] studied the reaction on zeolite supports and found that their activity was much higher than that of the Pt/Al2O3 catalysts. The Pt/zeolites' superiority was attributed to their ability to maintain a higher density of propane at the metal–support interface [6].

Hubbard et al. [2] reported that the catalytic activity of highly dispersed Pt was one to two orders of magnitude higher on Pt/zirconia than on Pt/γ-alumina. With increasing metal loading, this difference gradually vanished. Yazawa et al. [11] studied the effect of catalyst dispersion on several support materials and found that the reaction turnover frequency decreased with increasing catalyst dispersion and the turnover frequency increased with increasing acid strength of the support. The variation in activity with catalyst dispersion and support acidity was attributed to the concomitant variation in the oxidation state of platinum. Large platinum particles are less oxidized than small ones, and reduced platinum is the much more active form. Similarly, platinum on acidic supports is less oxidized than platinum on basic supports.

The additive effect on supported Pt catalysts has been studied in an effort to find ways to enhance their catalytic activity [10], [15], [16]. Yazawa et al. [10], [16] investigated the variation in reaction rate with the addition of a large number of metal additives on Pt/Al2O3 and found that intrinsic catalytic activity increased with increasing electronegativity of the additive. This effect was observed in an oxidizing atmosphere only and was attributed to increased oxidation resistance of Pt [10].

Several investigators have reported that small amounts of SO2 can promote the catalytic combustion of propane over Pt/Al2O3 [3], [13], [14], [17]. Researchers from the University of Cambridge and Ford Motor Co. jointly studied the role of SO2 in the promotion of Pt/Al2O3 catalysts [3]. Their work confirmed that PtO2 particles are essentially inactive and that an increase in metal loading increases the metallic character of Pt. These findings explain the structural sensitivity of propane combustion. Moreover, sulfation causes simultaneous reduction and partial sintering of the PtO2 particles. Finally, the effect of SO2 is not confined to the catalyst itself; it is support-specific because it facilitates the dissociative chemisorption of propane at the Pt/Al2O3 interface [3]. Burch et al. [13] studied the reaction on supported Pt catalysts in the presence and in the absence of SO2. The presence of SO2 resulted in strongly enhanced activity of the alumina-supported catalysts but produced no effect on the silica-supported catalysts. It has been suggested that SO2 results in the polarization of the perimeter sites at the Pt–support interface, sites important in the activation of the first Csingle bondH bond of propane [13].

Despite the different interpretations presented by the various research groups, it is generally understood that the intrinsic catalytic activity of Pt toward propane oxidation can be significantly enhanced by a properly chosen support and the presence of additives and gas phase promoters. Another alternative approach to catalytic promotion, non-Faradaic modification of catalytic activity (NEMCA), or electrochemical promotion, has been developed and applied in numerous catalytic reaction systems [18], [19]. In NEMCA, the catalytic reaction is run in an electrochemical cell in which one of the electrodes is placed concomitant with the catalyst under study. By appropriately controlling either the current or the cell potential, ions can be “pumped” into or away from the electrode catalyst. Ionic “pumping” changes the work function of the electrode and thus alters its catalytic properties.

Vernoux et al. [20] investigated the electrochemical promotion in the oxidation of propane on Pt, along with the oxidation of propylene. They found significant differences in the promotion behaviors of propane and propylene, and attributed these differences to the much stronger adsorption of the alkene on the catalyst surface. Kotsionopoulos and Bebelis [21] studied the NEMCA effect in the catalytic oxidation of propane on Pt and compared the results with those obtained on Rh. These authors found that the increase in the rate of oxygen consumption could exceed the rate of oxygen ion “pumping” by three orders of magnitude, resulting in enhancement of the intrinsic catalytic rate by up to a factor of 1350 [21].

This work reports results of the reaction of propane oxidation studied in a solid electrolyte cell. Experimental findings obtained under both open- and closed-circuit operation are presented. Under open circuit, the cell operated as a regular catalytic reactor, and the kinetics of the reaction were studied in conjunction with measurements of cell potential. The effect of oxygen ion “pumping” on the catalytic activity of Pt was investigated under closed circuit.

Section snippets

Experimental

The experimental apparatus (schematically illustrated in Fig. 1) consisted of the feeding unit, reactor, and analytical system. Reactant gases, C3H8, O2, and diluent He were of 99.999% purity. Analyses of the inlet (C3H8 and O2) and outlet gases (C3H8, O2, CO2, and H2O) were performed using a Shimadzu GC-14B on-line gas chromatograph with a thermal conductivity detector. A Porapak QS column was used to separate C3H8, CO2, and H2O, and a Molecular Sieve 13X was used to separate O2. CO and CO2

Catalytic (open-circuit) measurements

The catalytic reaction rate and the surface oxygen activity behavior was studied at 623–773 K and at 100 kPa total pressure. The partial pressure of inlet propane ranged from 0.2 to 1.7 kPa, and that of oxygen ranged from 0.15 to 10.0 kPa. Helium was used as the diluent, and the total volumetric flow rate was approximately 90 cm3/min. As shown previously [26], [27], the reactor behavior is very close to that of an ideal CSTR for the foregoing volumetric flow rates. At all temperatures and

Discussion

The kinetics of propane oxidation on Pt has been a stimulating problem for several decades [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [28], [29]. Otto et al. [1] reported an apparent activation energy of 22±3 kcal/mol, and apparent reaction orders of 0 for oxygen and 1 for propane. Hubbard et al. [2] also found zero- and first-order dependence on oxygen and propane, respectively, and an apparent activation energy of 17.5±3.5 kcal/mol. Garetto et

Conclusions

The catalytic oxidation of propane on Pt was studied under both open-circuit and closed-circuit conditions. With the cell operating at open circuit, the reaction was positive order in propane and, depending on the pO2/pC3H8 ratio, either first, zero, or negative order in oxygen. The apparent activation energy was about 19 kcal/mol. Under certain conditions, reaction rate and surface oxygen activity exhibited sustained oscillations.

With the cell operating at closed circuit, the reaction

Acknowledgement

The authors acknowledge financial support for this research from the Chemical Process Engineering Research Institute (CPERI) of Thessaloniki.

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