Journal of Chemistry
Volume 2015 (2015), Article ID 912905, 8 pages
http://dx.doi.org/10.1155/2015/912905

Research Article

Co₃O₄ Catalysts on CeO₂-ZrO₂ Supports and Co₃O₄-CeO₂ Catalysts on Al₂O₃/SiO₂ Supports for the Oxidation of Propylene

Tien The Nguyen,¹ Phuong Thi Mai Pham,¹ Thang Minh Le,² and Isabel Van Driessche³

¹Laboratory of Environmental Friendly Materials and Technology, Advanced Institute of Science and Technology, Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi 10000, Vietnam
²Department of Organic and Petrochemical, School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi 10000, Vietnam
³Department of Inorganic and Physical Chemistry, SCRiPTS, Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium

Received 10 November 2014; Revised 27 February 2015; Accepted 1 March 2015

Academic Editor: Albert Demonceau

Copyright © 2015 Tien The Nguyen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Different compositions of Co₃O₄ catalysts on CeO₂-ZrO₂ solid solution (Ce0.9Zr0.1O₂ and Ce0.8Zr0.2O₂) have been studied for the oxidation of propylene. The optional amount of Co₃O₄ active phase on CeO₂-ZrO₂ support of 30 wt% was found. The mixed Co₃O₄-CeO₂-ZrO₂ with the same composition of the optimal supported ones showed approximately the same activity, which was not higher than the activity of the mixed Co₃O₄-CeO₂ catalyst. Catalytic activities of mixed Co₃O₄-CeO₂ with different loading contents supported on high surface area supports (Al₂O₃, SiO₂) were then measured. The optimal composition of active phase was still 30 wt% but the minimum temperature of the highest activity increased to above 300°C due to the inert nature influence of the support.

1. Introduction

Cobalt-based oxides are one of the most active catalysts in catalytic oxidation of hydrocarbons. Co₃O₄ is only slightly weaker than the noble metal catalysts for low temperature catalytic combustion [1, 2]. Furthermore, cobalt spinel in both pure and bimetallic forms seems promising for its low cost and good activity in volatility organic compound combustion [3–5], diesel soot oxidation [6], and particularly total oxidation of light hydrocarbons [7, 8]. The high catalytic activity in oxygen involving reactions of the Co₃O₄-based catalysts is most likely related to the high bulk oxygen mobility [9] and facile formation of highly active electrophilic oxygen (O⁻ or O⁻²) species for hydrocarbon oxidation [10].

Ceria has been the most frequently used catalyst due to its beneficial properties like the ability to enhance the water-gas shift reaction, to improve catalytic activity at the interfacial metal-support sites, to thermally stabilize the alumina support, to promote noble metal dispersion, and to exhibit its own catalytic activity especially when it is used in nanoparticles [11–15]. However ceria’s most prominent effect is its ability to undergo rapid redox cycles, 2CeO₂ → Ce₂O₃ + (1/2)O₂, thus acting as an oxygen buffer by storing/releasing O₂ due to the Ce⁴⁺/Ce³⁺ redox couple. Addition of zirconium is considered the most effective way for ceria stabilization against its sintering at high temperature reactions. Ceria-zirconia solid solution also yields an improvement in ceria’s oxygen storage capacity (OSC), redox properties, thermal resistance, and catalytic activity at low temperatures [16–18]. Therefore, system based on CeO₂ is widely used as support or active phase for oxidation reaction of hydrocarbon.

Although both cobalt oxide and ceria/ceria-zirconia catalysts have been widely applied for the oxidation of hydrocarbons, their mixtures are still rare investigated. A series of Co₃O₄ on mesoporous (, 0.85, 0.95, 1) catalysts synthesized by surfactant-assisted coprecipitation with subsequent incipient wetness impregnation method were studied. Results showed that the appropriate Ce/(Ce + Zr) ratio in ceria-zirconia support and cobalt oxide loading is required, and the 20 wt.% Co₃O₄/meso-Ce0.85Zr0.15O₂ catalyst exhibits outstanding catalytic performance [19]. Another investigation by Xu et al. has shown that -Co₃O₄ catalysts exhibit a better catalytic activity when compared with pure Co₃O₄. The catalyst with the Ce/Co atomic ratio 1 : 16 exhibits the best activity [20]. Nanometric CeO₂-supported cobalt oxide catalysts (molar ratio of 100 Co/Ce = 0.1, 1, 4, 10, 20, 50, 100) were prepared by the method of ultrasonic-assisted incipient wetness impregnation. The best catalytic activity was obtained over the catalyst containing 100 Co/Ce molar ratio of 20 [21]. Studies by Iwaneka et al. have shown that -CeO₂ catalysts are in general more active than the undoped cobalt catalyst. It has been found that catalysts with a Co/(Co + Ce) ratio not higher than 0.64 sinter less than those with a higher cobalt content. The catalyst containing 86% cobalt exhibits the highest activity among the studied systems [22].

For the treatment of unburned hydrocarbons, which are toxic components in automobile exhaust gases, mixtures of Co₃O₄ (30 wt %) with CeO₂ and CeO₂-ZrO₂ (Ce/Zr = 6/4) prepared by precipitation method have also been tested for the combustion of CH₄ under exceeds oxygen conditions. The results showed that the methane oxidation activity of Co₃O₄ (30 wt.%)-CeO₂/(CeO₂-ZrO₂) oxides is comparable with pure Co₃O₄ for fresh samples, while the composite systems exhibit higher activity and thermal stability than single Co₃O₄ after ageing at 750°C. The catalysts were able to convert 100% CH₄ at rather high temperature (500°C). The role of ceria and ceria-zirconia is assumed to maintain a good combustion activity of the cobalt composite oxides by dispersing the active phase Co₃O₄ [23].

Meanwhile, the study on the mixtures of Co₃O₄ with CeO₂ or CeO₂-ZrO₂ for the oxidation of hydrocarbons is still limited with only one ratio of Ce/Zr (Ce/Zr = 6/4) while we found from our previous study that the optimal Ce/Zr ratios for the highest conversion of hydrocarbons are 8/2 and 9/1 [24]. Moreover, the catalytic ability of the reported mixed Co₃O₄ with CeO₂ or CeO₂-ZrO₂ catalysts was only high at high temperatures (500°C) [24]. Therefore, it is still worth studying more logically the different mixtures of Co₃O₄ with CeO₂ or CeO₂-ZrO₂ or Co₃O₄ catalysts supported on CeO₂ or CeO₂-ZrO₂ under different operation oxidation conditions of hydrocarbons (rich and stoichiometric conditions). It may also be interesting to compare the use of active CeO₂-ZrO₂ support with other popular nonactive but high surface area and cheap supports as Al₂O₃, SiO₂. That will be the focus of this present paper.

2. Experimental

2.1. Catalyst Synthesis

The work uses several commercial supports such as Al₂O₃ (purity 100%, Merck) and SiO₂ (purity 100%, Merck). To prepare active phase and mixtures of different components, a sol-gel method was used as described in more detail. It was previously shown that sol-gel method leads to the formation of very pure and homogeneous catalyst powders exhibiting high surface area [25].

13.5697 g Ce(NO₃)₃·6H₂O (98.5%, Merck) was dissolved in 250 mL distilled water under stirring condition (solution A).

10.0703 g ZrOCl₂·8H₂O (99.0%, Merck) was dissolved in 250 mL distilled water under stirring condition (solution B).

27.7778 g citric acid monohydrate, C₆H₈O₇·H₂O (99.5%, Merck), was dissolved in 250 mL distilled water under stirring condition (solution C).

9.0937 g Co(NO₃)₂·6H₂O (99.0%, Merck) was dissolved in 250 mL distilled water under stirring condition (solution D).

To prepare mixed oxide CeO₂-ZrO₂ supports (Ce0.9Zr0.1O₂ and Ce0.8Zr0.2O₂), solution B was dropped into a suitable volume of solution A corresponding to different CeO₂/ZrO₂ molar percentages. If precipitation occurred, a suitable amount of concentrated HNO₃ solution was added. A suitable amount of solution C was dropped into the obtained solution in order to obtain molar ratio of citric acid to metals of 2.6.

Co₃O₄ supported on Ce0.9Zr0.1O₂ or Ce0.8Zr0.2O₂ and CeO₂-Co₃O₄ supported on Al₂O₃, SiO₂ were synthesized by impregnation method by mixing solution D and solution C, or solution A, solution D, and solution C, with different ratios and suitable amount of active phases. Concentrated HNO₃ was then added to obtain solution in order to control pH of solution below 1. The obtained solutions were stabilized for 30 minutes. Then, a suitable amount of Ce0.9Zr0.1O₂ (or Ce0.8Zr0.2O₂, Al₂O₃, SiO₂) support was added to solution to obtain suspensions. After stirring vigorously for 1 hour, the suspension was vaporized at 60–80°C until the slurry was dried. The obtained solid was dried at 120°C for 2 hours. Then, these samples were calcined at 550°C for 3 hours.

Co₃O₄ supported on CeO₂-ZrO₂ was labeled as A Co/Ce0.9Zr0.1O₂ or Ce0.8Zr0.2O₂, in which A is the weight percentage of the active phase.

CeO₂-Co₃O₄ supported on Al₂O₃ (SiO₂) was identified as E CeCo P-Q/Al (Si), in which E is the weight percentage of the active phase, P-Q is the ratio of CeO₂/Co₃O₄, Ce is the symbol of CeO₂, Co is the symbol of Co₃O₄, Al is the symbol of Al₂O₃, and Si is the symbol of SiO₂.

2.2. Characterization

The catalysts were characterized by using X-ray, SEM, and BET techniques. The X-ray powder diffraction (XRD) patterns were recorded with a D8 Bruker Advanced diffractometer (Germany).

The specific surface areas of the samples were measured at 77 K by the BET method using N₂ adsorption on an ASAP 2010 and a Gemini VII Micromeritics apparatus.

The morphology of these catalysts was examined on a Hitachi X4800 (Japan) scanning electron microscope.

2.3. Measurement of Catalytic Activities

Catalytic activities (the propylene conversion, which was determined as the ratio of consumed propylene to the amount of propylene introduced into the reactor, and the CO₂ selectivity, which was determined as the amount of the obtained CO₂ per the total amount of all products) were measured in a micro (or differential) reactor set up with an internal diameter of 0.4 cm and length of 60 cm. 0.1 g of catalyst (particle sizes are within the range 250–300 μm, which were prepared by pressing catalyst powders in a hydraulic presser, grinding the obtained pellets, and sieving into desired particle sizes) was used with a total gas flow of 80 mL/min at a pressure of 1 atm. The catalyst bed length is 5 mm; thus the gas hourly space velocities (GHSV), which were determined from the standard flow rate of the gas divided by the catalyst bed volume, were 76,000 h⁻¹ in the catalyst reactor. The volume composition of gas flow was C₃H₆/O₂/N₂ = 2.5/2.5/95 (%) and the reaction temperatures range from 200°C to 500°C. Under the stoichiometric condition (oxygen sufficient), the composition of the gas flow was C₃H₆/O₂/N₂ = 2.5/11.25/86.25 (%). The reaction temperature was measured using a thermocouple attached at the position of the catalyst bed inside an electric furnace. The temperature of the catalytic bed was directly controlled by the proportional integral derivative (PID) controller of the furnace. Analysis of propylene, oxygen, CO₂, CO, and oxygenated products was performed using an online Focus-Thermo scientific gas chromatograph with a thermal conductivity detector (TCD). C₃H₆ and oxygenated products were detected with a column of 80/100 Chromosorb and a column of Carbowax 20 M in series while CO₂, CO, and O₂ were detected with a column of 60/80 Carboxen and a column of 80/100 Porapak in series.

3. Results

3.1. Characterization and Catalytic Activity of Co₃O₄ Catalysts on CeO₂-ZrO₂ Supports

From our previous experiment, it was found that mixtures of Co₃O₄ and CeO₂ exhibited high activity for the oxidation of propylene but were unstable at high temperatures. The situation may be improved if Co₃O₄ is impregnated on a stable phase containing CeO₂. CeO₂-ZrO₂ mixtures have been determined to be active for the oxidation of propylene as in our previous publication [24]. Amongst CeO₂-ZrO₂ mixtures, ones with Ce/Zr ratios = 8/2 and 9/1 showed highest activity and possessed high ability of CO₂ formation. Therefore, CeO₂-ZrO₂ mixtures with Ce/Zr ratios = 8/2 and 9/1 were chosen as supports for Co₃O₄ catalysts. The Co₃O₄ supported on CeO₂-ZrO₂ samples was characterized and tested for the oxidation of propylene under rich condition.

BET surface area of Co₃O₄ impregnated on different CeO₂-ZrO₂ supports (Ce0.9Zr0.1O₂ and Ce0.8Zr0.2O₂) is shown in Table 1. The results shows that Ce0.8Zr0.2O₂ support possesses a little higher surface area than Ce0.9Zr0.1O₂ support. When Co₃O₄ was impregnated on the supports, surface area tends to decrease maybe due to lower surface area of Co₃O₄ catalyst (11.44 m²/g) and the influence of the calcination after the impregnation. However, samples with 5% of Co₃O₄ on CeO₂-ZrO₂ supports showed increased surface area, which is not clearly understood since both SEM and XRD of these samples showed no evidence of strange phases, particle sizes, or morphology.

Table 1: Specific surface area (m²/g) of Co₃O₄ catalyst on CeO₂-ZrO₂ supports.

Support
Content of Co₃O₄ on supports (%)

0
5
30
50

Ce0.9Zr0.1O₂
38.75
67.42
23.02
29.17

Ce0.8Zr0.2O₂
46.97
44.44
23.49
29.87

All samples show structures similar to those of CeO₂ and CeO₂-ZrO₂ mixed oxides as seen from XRD patterns (Figure 1). The evidence of Co₃O₄ was only seen clearly when the content of Co₃O₄ is over 30%. For the samples with lower Co₃O₄ contents, the presence of Co₃O₄ only made the base line rougher, which indicates the more amorphous nature of Co₃O₄ (which was seen from XRD pattern of its pure sample).

Figure 1: XRD patterns of Co₃O₄/Ce0.9Zr0.1O₂ samples.

SEM images of Co₃O₄ catalyst on Ce0.8Zr0.2O₂ support in Figure 2 show a fine dispersion of Co₃O₄ on the honeycomb-like structure of the support. The particle size of Co₃O₄ is only few nm (circle marked in Figures 4(b) and 4(c)) and became larger when Co₃O₄ content increased (sample 30% Co₃O₄). On the sample 30% Co₃O₄/Ce0.8Zr0.2O₂, there are aggregations of Co₃O₄ particles at some places (Figure 4(d)) but the support surface was not impregnated completely yet at some other places (Figure 4(c)).

(a)

(b)

(c)

(d)

Figure 2: SEM images of pure Ce0.8Zr0.2O₂ (a), 5% Co₃O₄/Ce0.8Zr0.2O₂ (b), and 30% Co₃O₄/Ce0.8Zr0.2O₂ (c and d).

The comparison of the activity of catalysts with different Co₃O₄ loading contents (5–50%) on CeO₂-ZrO₂ supports is shown in Figures 3-4. The catalysts were tested in rich condition (oxygen deficient) since it has been found that this condition is close to the real high speed operating conditions of motorcycles, the field of application of these catalysts [24]. It was also expected that if a catalyst exhibits good activity for complete oxidation under rich condition, it might exhibit even better activity under stoichiometric or lean conditions.

(a)

(b)

Figure 3: Propylene conversions (a) and CO₂ selectivity (b) under rich condition of Co₃O₄ catalyst with different loading contents on Ce0.9Zr0.1O₂ support.

(a)

(b)

Figure 4: Propylene conversions (a) and CO₂ selectivity (b) under rich condition of Co₃O₄ catalyst with different loading contents on Ce0.8Zr0.2O₂ support.

It can be seen that CeO₂-ZrO₂ supports could only reach to high propylene conversions (refer propylene conversions higher than 22.22%, the maximum conversion of propylene for the complete oxidation at the C₃H₆/O₂ ratio of 1) at high temperatures (above 300°C). Meanwhile, pure Co₃O₄ and Co₃O₄ catalysts on CeO₂-ZrO₂ supports exhibited high conversion from 250°C. After reaching to those high conversions at this temperature, the propylene conversion was rather stable. The sample 30% Co₃O₄/Ce0.9Zr0.1O₂ exhibited a sudden increased conversion at 500°C but together with decreased CO₂ selectivity, which is the same as pure Co₃O₄ catalyst. However, pure Co₃O₄ even exhibited much lower CO₂ selectivity at high temperatures. Thus, the deposition of Co₃O₄ catalyst on CeO₂-ZrO₂ supports decreased the temperature to obtain high activity of the catalysts compared with that of the support (250°C for catalysts on support instead of 350°C for the support) even at low Co₃O₄ loading contents (3–5%). The deposition of Co₃O₄ catalyst on CeO₂-ZrO₂ supports also simultaneously increased CO₂ selectivity compared with that of the pure Co₃O₄ catalyst at high temperatures (400–500°C).

To investigate the role of active site and support, samples prepared by chemical mixing using sol-gel synthesis with the same compositions as 30% Co₃O₄/Ce0.9Zr0.1O₂ and 30% Co₃O₄/Ce0.9Zr0.1O₂ samples were also tested. The samples prepared by chemical mixing showed similar activities as supported sample prepared by impregnation method. These samples also possess high surface area as those of CeO₂-ZrO₂ support. This observation shows that CeO₂-ZrO₂ components in 30% Co₃O₄/Ce0.9Zr0.1O₂ and 30% Co₃O₄/Ce0.9Zr0.1O₂ catalysts play roles as active sites too. Therefore, it is possible to simply prepare Co₃O₄-CeO₂-ZrO₂ mixed catalyst by sol-gel method instead of more complex impregnation method while still maintaining surface area and catalytic activity of the catalysts.

3.2. Characterization and Catalytic Activity of Co₃O₄-CeO₂ Supported on Al₂O₃ and SiO₂

In our previous study, it was found that the sample containing 50 and 20 mol% of CeO₂ (CeCo 50-50 and CeCo 20-80, resp.) exhibited excellent catalytic activities, which were even higher than those of Co₃O₄ catalysts on CeO₂-ZrO₂ supports described in Section 3.1. Co₃O₄-CeO₂ catalysts were able to convert a large amount of propylene from 200°C already [24]. The catalysts also possessed the same high surface area as those of CeO₂-ZrO₂ supports (around 45 m²/g). However, these catalysts (the same as 30–50% Co₃O₄/CeO₂-ZrO₂ catalysts) were unstable at high temperatures; the catalyst particles were observed broken at high temperatures (400–500°C), resulting in blocking of the reactor. Therefore, it may be helpful to impregnate Co₃O₄-CeO₂ catalysts on other high surface area and thermal stable supports (γ-Al₂O₃ and SiO₂) to increase surface area of the catalysts as well as make them more stable at high temperatures. Thus, we decided to study Co₃O₄-CeO₂ catalysts on support. CeO₂-ZrO₂ would not be chosen as a support for Co₃O₄-CeO₂ catalysts since the composition of the catalyst already contains CeO₂; moreover, CeO₂-ZrO₂ supports possess much less surface areas. Although high surface area is not the most important factor in the reaction where a Mars Van Krevelen mechanism is usually invocated as in hydrocarbons oxidation [26], high surface area still helps in the adsorption of reactants.

The use of high surface area Al₂O₃ and SiO₂ supports resulted in much high surface area catalysts than in the case of using CeO₂-ZrO₂ supports. The surface area of the CeO₂-Co₃O₄ catalysts on Al₂O₃ and SiO₂ supports was only slightly lower than that of pure supports even if the catalyst loading contents were high (30–50 w%) (Table 2). Morphology of CeO₂-Co₃O₄ catalysts on Al₂O₃ and SiO₂ supports is a little different. Figure 5 shows SEM images of CeO₂-Co₃O₄ catalyst with Ce/Co molar ratio of 5/5 on Al₂O₃ and SiO₂ supports. The deposition of CeO₂-Co₃O₄ catalysts on Al₂O₃ resulted in more amorphous morphology and less particle sizes. The images also show that the catalyst loading content of 30 w% made the surface of the supports covered almost completely by CeO₂-Co₃O₄ catalyst particles. This may be the reason for the fact that the samples with catalyst loading contents above 30 w% were the samples which exhibited highest propylene conversion (Figure 6).

Table 2: Specific surface area (m²/g) of CeO₂-Co₃O₄ catalyst on Al₂O₃ and SiO₂ supports.

Content of CeO₂-Co₃O₄ on supports (%)

Support
0
5
10
30
50

Ce/Co = 2/8
Ce/Co = 5/5
Ce/Co = 2/8
Ce/Co = 5/5
Ce/Co = 2/8
Ce/Co = 5/5
Ce/Co = 2/8
Ce/Co = 5/5

Al₂O₃
118
115.26
116.75
117.31
120.33
115.63
106.20
109.16
104.15

SiO₂
228

222.66
245
203.13
210.55

(a)

(b)

Figure 5: SEM images of CeO₂-Co₃O₄ catalyst with Ce/Co molar ratio of 5/5 on Al₂O₃ (a) and SiO₂ supports (b).

(a)

(b)

Figure 6: Propylene conversion (a) and CO₂ selectivity (b) of CeO₂-Co₃O₄ catalyst with Ce/Co molar ratio of 5/5 on Al₂O₃ support at different reaction temperatures.

Propylene conversions and CO₂ selectivity (under rich condition) of CeO₂-Co₃O₄ catalyst on Al₂O₃ and SiO₂ supports were presented in Figures 6-7. The catalytic loading content was investigated in a wide range (5–50 w%) with the aim of optimizing it and finding out a catalyst with as low catalytic loading content as possible but as high activity as possible. It can be seen that the pure CeO₂-Co₃O₄ active phases were able to reach to the maximum activity at lower temperatures than when they were deposited on the supports. The samples with high CeO₂-Co₃O₄ loading contents (30–50 w%) on Al₂O₃ support could convert a maximum amount of propylene at 300°C while the samples with lower CeO₂-Co₃O₄ loading contents (5–10 w%) could only obtain the maximum conversion at 400–450°C (Figure 6). Thus, when the catalyst was impregnated on the support, high loading contents of the active phase (above 30%) are required to lower the minimum temperature of the maximum conversion. High CeO₂-Co₃O₄ loading contents also help to maintain CO₂ selectivity at high temperature close to that of the pure active phase. The sample with low CeO₂-Co₃O₄ loading contents (5–10 w%) showed decreased CO selectivity at high temperatures, which is similar to that of the pure support. The results also show that catalytic activities of the samples with Ce/Co molar ratios of 5/5 and 2/8 are quite similar. The sample containing 50 w% CeO₂-Co₃O₄ (Ce/Co = 2/8) on SiO₂ support exhibited a little better activity than those of other samples since it could convert a maximum amount of propylene sooner (from 250°C). CO₂ selectivity on catalysts using SiO₂ support is also a little higher than those of the catalysts using Al₂O₃ support. This is probably due to the higher acidity of Al₂O₃ support, which made some exposed places of the support become sites for coke formation. The catalysts on Al₂O₃ support after the reaction are usually observed becoming black. Catalytic activity of the best CeO₂-Co₃O₄ catalyst on Al₂O₃ and SiO₂ supports was not higher than those of the best of Co₃O₄ catalyst on CeO₂-ZrO₂ supports although the activity of the unsupported CeO₂-Co₃O₄ catalyst was higher than that of the unsupported Co₃O₄ catalyst, showing that active CeO₂-ZrO₂ supports made certain meanings compared with inert Al₂O₃ and SiO₂ supports.

(a)

(b)

Figure 7: Propylene conversion (a) and CO₂ selectivity (b) of CeO₂-Co₃O₄ catalysts with Ce/Co molar ratios of 5/5 and 2/8 on Al₂O₃ support at different reaction temperatures.

3.3. Activity of CeO₂-Co₃O₄ Catalysts for the Treatment of Propylene under Stoichiometric Condition

To understand details about the ability of the catalysts in different reaction conditions, CeO₂-Co₃O₄ catalysts on supports were also tested for the oxidation of propylene under stoichiometric condition (enough oxygen), in which the ratio of O₂ to propylene was maintained at 4.5 according to the stoichiometric ratio of the complete oxidation reaction. The best Co₃O₄ catalyst on Ce0.9Zr0.1O₂ support (30% Co₃O₄/Ce0.9Zr0.1O₂) was also tested under stoichiometric condition for comparison. The results show that all catalysts were able to convert 100% propylene into CO₂ (Table 3). However, only 30% Co₃O₄/Ce0.9Zr0.1O₂ catalyst was able to convert almost 100% propylene from low temperature (250°C); the catalysts on Al₂O₃ and SiO₂ supports were only able to convert propylene completely at high temperatures (from 400°C). Although Al₂O₃ and SiO₂ supports possess much higher surface area than that of Ce0.9Zr0.1O₂ support, they are almost inert for the oxidation of propylene while Ce0.9Zr0.1O₂ support, itself, is a good catalyst for the oxidation of propylene as seen in Section 3.1; therefore, the catalyst on Ce0.9Zr0.1O₂ support exhibited an excellent activity. Between Al₂O₃ and SiO₂ supports, catalyst on Al₂O₃ showed worse activity than that of SiO₂, which may be due to higher acidity of Al₂O₃, making exposed Al₂O₃ particles become sites for coke formation even under stoichiometric condition.

Table 3: Propylene conversion (, %) and CO₂ selectivity (, %) of catalysts with Co₃O₄/CeO₂-Co₃O₄ catalysts on different supports at different reaction temperatures under stoichiometric condition.

Catalysts
200°C
250°C
300°C
350°C
400°C
500°C

30% Co₃O₄/Ce0.9Zr0.1O₂
6.1
100
99.7
100
100
100
100
100
100
100
100
100

30 CeCo 50-50/Si
6.1
100
12.9
100
84.8
100
98.2
100
100
100
100
100

30 CeCo 50-50/Al
6.1
100
8.5
100
16.7
100
68.0
100
100
100
100
100

Different from the rich condition, under stoichiometric condition, the only product is CO₂ at all temperatures. Thus, the catalysts catalyze well the complete oxidation of propylene but under rich condition, CO and oxygenated products were still formed since there was not enough oxygen for the reaction of propylene.

When the complete oxidation was performed under lean condition (excess oxygen), the catalytic activities of these catalysts were even better, resulting in the decrease of the minimum temperature to convert 100% propylene of all catalysts.

4. Conclusions

Two series of catalysts with suitable loading contents (above 30 w%) investigated in this work, Co₃O₄ on CeO₂-ZrO₂ supports and CeO₂-Co₃O₄ catalysts on Al₂O₃ and SiO₂ supports, exhibited good activities for the treatment of propylene under all rich, stoichiometric, and lean conditions. Under stoichiometric condition, 30% Co₃O₄/Ce0.9Zr0.1O₂ catalyst was able to oxidize 100% propylene to pure CO₂ from 250°C; 30% CeO₂-Co₃O₄ catalysts on Al₂O₃ and SiO₂ supports were able to do the same at higher temperatures (400°C). Thus, Ce0.9Zr0.1O₂ was proved to be the more active support than inert Al₂O₃ and SiO₂ supports for the catalysts for the complete oxidation of propylene since Ce0.9Zr0.1O₂, itself, was also a good active phase for the reaction. However, catalysts on Ce0.9Zr0.1O₂ exhibited much lower surface areas and much less thermal stability. Further work will be performed to explore more details on the characteristics of the catalysts and the supports.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant no. 104.03-2011.16.

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