Application of Spinel-Type Cobalt Chromite as a

stream of H2 and He mixture. Reaction Experiments. The activity of the catalysts in oxidative decomposition of TCE was measured in a fixed bed apparat...
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Environ. Sci. Technol. 2001, 35, 222-226

Application of Spinel-Type Cobalt Chromite as a Novel Catalyst for Combustion of Chlorinated Organic Pollutants DAE-CHUL KIM AND SON-KI IHM* National Research Laboratory for Environmental Catalysis, Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon 305-701, Korea

Various chromium-containing catalysts were tested for the total oxidation of trichloroethylene (TCE) as a model reaction for the catalytic combustion of chlorinated organic pollutants. A spinel-type cobalt chromite (CoCr2O4) among others was proven to be a very promising catalyst showing higher activity and higher CO2 selectivity than traditional alumina supported chromia. Even if both Cr3+ and Cr6+ species were observed on the surface of CoCr2O4, the Cr6+ species was stable under reducing environment. The presence of Cr3+-Cr6+ pair sites and the effect of redox treatments on the activity were investigated to explain the nature of possible active sites for TCE decomposition. Higher selectivity to CO2 of CoCr2O4 was ascribed to the abundance of its Cr3+ species, together with its activity for water gas shift reaction.

Introduction Catalytic combustion of chlorinated VOCs (CVOC) is a challenging technology (1-5). While many gas streams containing pollutants can be incinerated with conventional supported noble metal catalysts, the combustion of chlorinated hydrocarbons presents several complications. Supported noble metal catalysts are very expensive and vulnerable to the poisoning by Cl2 or HCl produced in the reaction (5-8). Development of a new type of catalysts for this reaction does not appear to be an easy task due to the poisoning and the existence of water vapor (3, 9, 10). Even though a lot of works have been devoted to supported or unsupported metal oxides such as transition metal oxides (7-20), ion exchanged zeolites (21-25), TiO2 based catalysts (26-28), and perovskite type oxides (29-31), little had been reported on the direct comparison of catalytic activity among different catalysts under the same reaction conditions. Among the transition metal oxides, chromium oxide has been known to be the most active component. The reason for the unique activity of chromium in the catalytic oxidation of chlorinated organics (10, 18, 22) is not yet clear, and the nature of active sites of chromium species is still under debate (32-51). Most studies of supported chromium oxides for catalytic oxidation suggested that the active phase of supported chromia catalysts might be highly oxidized Cr6+ (8, 19, 22). Cr6+ was also observed in the case of ion exchanged zeolites * Corresponding author phone: +82-42-869-3915; fax: +82-42869-5955; e-mail: [email protected]. 222

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and considered as an active phase for the decomposition of TCE. The nature of surface chromium species of supported chromia can be summarized as follows (42): (1) for low Cr loadings, most of the Cr is present as a highly dispersed Cr6+ surface phase, (2) with Cr content the fraction of Cr3+ species increases and the Cr dispersion decreases, and (3) for higher loadings, large Cr2O3 particles appears. It is rather difficult to determine the nature of the active sites due to the different oxidation state of chromium on the surface. Even if spinel-type chromites (33, 36, 40, 51-57) were reported as active materials in hydrogenation, dehydrogenation, water gas shift reaction, and combustion catalysis, there has been no report dealing with this material for the total oxidation of halogenated hydrocarbons. Although the existence of Cr6+ on the surface of unsupported Cr-containing oxides was reported for Cr2O3 (34, 41, 44-46), MgCr2O4 (36), and CoCr2O4 (51), it should not be concluded that the activities of unsupported Cr-containing oxides is solely depending on the presence of redox couples between Cr3+ and Cr6+ (or Cr3+-Cr6+ pair sites). Zaki et al. (44) reported from UV-DRS observation that the surface chromate species (Cr6+) is stable under reductive condition. Pradier et al. (49) pointed out the important role of crystalline Cr2O3 which favors the formation of CO2 in the total oxidation of ethyl acetate suggesting unsupported chromia (Cr3+) with high surface area would be a viable tool for the total oxidation of hydrocarbons as an alternative for the supported chromia. In the present work, various chromium-containing catalysts were tested for total oxidation of chlorinated organic pollutants, and the potential of cobalt chromite (CoCr2O4) among others was reported as a new type of catalysts for the catalytic combustion process. Its promising behavior was demonstrated through its high activity in the presence of water and also higher selectivity to CO2. The oxidation state of chromium and the effect of redox treatments on the catalytic activity were investigated to explain the nature of possible active sites of CoCr2O4 catalyst. Trichloroethylene (TCE) was chosen to be a model compound of chlorinated volatile organic compounds since it is one of the most hazardous volatile organic compounds.

Experimental Section Catalyst Preparation. Chromium oxide supported on MCM41 was prepared through the incipient wetness method with aqueous solution of Cr(NO3)3‚9H2O. The samples were freezedried for 3 days followed by calcination in air at 600 °C for 2 h. A pure silica MCM-41 sample was synthesized following the procedure reported by Ryoo and Jun (58). The Cr loading on supports was chosen at 3, 6, and 12%. CrOx/γ-Al2O3 samples were also prepared by incipient wetness impregnation of γ-Al2O3 using the corresponding nitrate salts as precursors. Two types of γ-Al2O3 supports were obtained from Strem and Catalysis Society of Japan (JRC-ALO-2), whose surface area was 200 m2/g and 350 m2/ g, respectively. Metal loading was varied from 3 to 16 wt % of Cr. A spinel-type oxide of Cr and Co was prepared by coprecipitation at pH 8. The precipitation was accomplished by adding dropwise the cosolution of metal nitrates with desired proportions of ammonium hydroxide at room temperature under continuous stirring. The filtered precipitate was dried and calcined at 600 °C for 5 h. Ceria addition was also carried out using cerium nitrate in the same manner. Metal ion-exchanged Y zeolites (Cr-Y and Co-Y) were prepared with Y zeolite from Strem and 0.05M solution of 10.1021/es001098k CCC: $20.00

 2001 American Chemical Society Published on Web 11/28/2000

FIGURE 2. Catalytic activity for the decomposition of TCE at 330 °C over alumina (JRC-ALO-2) supported chromium oxides and their XRD patterns (1500 ppm TCE; 15000 ppm water; WHSV: 48 L/g-cat‚h; after 4 h reaction). The numbers in parentheses are surface areas (m2/g). FIGURE 1. Catalytic activity for the decomposition of TCE at 350 °C over various catalysts (1500 ppm TCE; 15000 ppm water; WHSV: 48 L/g-cat‚h; after 4 h reaction). The numbers in parentheses are surface areas (m2/g). Cr(NO3)3‚9H2O and Co(NO3)2‚6H2O. As reference catalysts 0.5% Pd/γ-Al2O3 and Pt/γ-Al2O3 from Strem were used as representatives of conventional noble metal catalysts. Characterization of Catalysts. The crystal structures of prepared catalysts were analyzed with a Rigaku D/MAX-III diffractometer using monochromic CuKR (λ ) 0.1506 nm) radiation. The surface area of prepared catalysts was determined by ASAP2000 (Micromeritics) before and after reaction. Before the measurement, each sample was degassed at 250 °C for 3-5 h in a vacuum condition. XPS data were obtained using a ESCALAB MKII photoelectron spectrometer of VG Scientific equipped with a magnesium anode (1253.6 eV). During data processing of the XPS spectra, binding energy (BE) values were referenced to the C1s peak (284.6 eV). Temperature-programmed reduction (TPR) measurements were carried out with PulseChemiSorb 2705 (with TPD option from Micromeritics). Catalyst samples were pretreated with a mixture of 5 vol % oxygen in He at 500 °C for 2 h. Hydrogen consumption was measured with a flow of a mixture of 5 vol % H2 in argon at 20 cm3/min and a linear heating rate of 10 °C/min in the 50-500 °C range. The reducibility of (Ce added) spinel, which is a bulk property, was observed independently through a thermogravimetric method(TGA) using a Cahn-2000 microbalance under a stream of H2 and He mixture. Reaction Experiments. The activity of the catalysts in oxidative decomposition of TCE was measured in a fixed bed apparatus at the temperatures between 280 °C and 330 °C. The reaction feed was a mixture of 1500 ppm of TCE and 15000 ppm of H2O with balance air. The weight hourly space velocity (WHSV) was 48 L/g-cat‚h. Reaction products were analyzed by an online gas chromatograph equipped with TCD and FID. An electrochemical gas sensor from Sierra Monitor Corporation was used to detect chlorine. Carbon balance fell into the range between 90 and 110% with TCE, CO2, and CO which was checked by using separate CO detector (MONOXOR II from BACHARACH). Neither chlorinated methanes nor chlorinated ethylenes except TCE were detected.

Results and Discussion Activity of Various Chromium Containing Catalysts. Figure 1 summarizes the results of reaction experiments with various

catalysts for the decomposition of TCE at 350 °C. Supported noble metals such as Pt or Pd were not effective for the TCE decomposition mostly due to their rapid deactivation even during the first 1-h of reaction, also as reported by Marceau et al. (6) for the alumina-supported Pt catalysts. While Cr-Y was very active for the TCE decomposition, Co-Y was inactive. Chintawar and Greene (22) also reported that only chromium showed catalytic activity for TCE decomposition among the first row transition metals. Higher activity of JRCALO-2 supported chromium oxides compared to that supported on alumina from Strem may be due to higher surface area. MCM-41 supported chromium oxides (even with its high surface area and uniform pore structure) showed a lower activity than alumina supported chromium oxide. It should be noted that alumina supported chromium oxides have been studied extensively as one of the most promising catalysts (9-11, 15, 18). Figure 2 shows the catalytic activities of alumina supported chromium oxides with various metal loadings. There was an optimum loading at 12% of Cr (or 25% of Cr2O3) as shown in Figure 2(a). Formation of large Cr2O3 particle was observed in the case of 16% loading whose XRD pattern showed a typical peak of Cr2O3 at 24° and 34° (Figure 2(b)). Decrease in the activity with loading beyond 12% seemed to be due to this agglomeration of well dispersed chromium species. Significant drop of surface area for the 16% Cr loading (Figure 2(a)) might also be connected with the formation of large Cr2O3 particles resulting in the blockage of small pores. CoCr2O4sA Promising Catalyst for the Decomposition of Chlorinated Organic Pollutants. A spinel-type cobalt chromite was prepared by coprecipitation method with the Cr/Co ratio of 2 followed by calcination at 600 °C for 5 h. It showed relatively higher surface area (70 m2/g in comparison with 18 m2/g of Cr2O3), and the spinel structure of CoCr2O4 was confirmed with the Powder Diffraction File (PDF No. 22-1084) of International Center for Diffraction Data (ICDD). Catalytic activity of CoCr2O4 measured at 4 h of timeon-stream for the decomposition of TCE was compared with that of CrOx/γ-Al2O3 (Figure 3). CoCr2O4 showed higher conversions than CrOx/γ-Al2O3 (12 wt % of Cr), whose activity was the highest among the alumina supported chromia catalysts with different loading as presented in Figure 2. Even if the catalytic activity decreased with time on stream, the relative performance among different catalysts remained almost the same. Table 1 shows the nature of the chromium species present on the surface of the catalysts as observed by X-ray photoVOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Light-off curves of Cr-containing catalysts (1500 ppm TCE, 15000 ppm H2O, WHSV: 48 l/g-cat‚h, after 4 h reaction).

FIGURE 4. XPS results of CoCr2O4 after the treatment with H2 at 500 °C for 2 h (a) and after the treatment with O2 at 500 °C for 2 h (b).

TABLE 1. XPS Spectroscopic Data of Cr2O3, CoCr2O4, and CrOx(12%)/Al2O3 BEa of Cr2p3/2 (eV)

sample

Cr6+/{Cr3++Cr6+} Cr/{total metals}b (%) (%)

Cr2O3 576.5/579.1 CoCr2O4 576.5/579.1 CrOx(12%)/Al2O3 576.8/579.3 a

Binding energy.

b

31.5 29.1 69.5

100.0 66.5 17.4

Atomic ratio of Cr/(Cr+Co+Al).

TABLE 2. Effect of Reduction Treatment on the Catalytic Activity of CoCr2O4 for the Decomposition of TCE at 300 °Ca step

treatment

conversion of TCE (%)

selectivity to CO2 (%)

1 2 3

H2 at 500 °C for 2 h air at 500 °C for 2 h H2 again at 500 °C for 2 h

36.1 66.0 36.9

62.9 67.8 74.1

a 1500 ppm TCE; 15000 ppm water; WHSV: 48 L/g-cat‚h; after 4 h reaction at each step.

electron spectroscopy. The Cr2p3/2 peak was resolved into two peaks around 576.5 and 579.3 eV assigned to Cr3+ and Cr6+, respectively (46). The spinel CoCr2O4 possessed around 30% of Cr6+ species in the subsurface with bulk Cr3+ species. Even if it was rather well established that the active species of supported chromium oxide is Cr6+ species (37, 38, 43, 45, 47-49), the nature of active species for CoCr2O4 is yet to be further investigated. Table 2 shows that the activity for TCE oxidation over the spinel CoCr2O4 catalyst was reversible with the treatments by hydrogen and air. The activity was low with hydrogen treatment, but it was recovered by oxygen treatment. Even if the redox treatment may suggest that the activity comes from the redox couple between Cr6+ and Cr3+, the XPS observation in Figure 4 did not indicate any significant change in the oxidation state of chromium due to the hydrogen and air treatment. The Cr6+ species in the subsurface of CoCr2O4 seems to be stable under the present reduction conditions. The temperature programmed reduction by hydrogen which is of a surface-sensitive nature shows in Figure 5 that hydrogen was consumed in each of CoCr2O4 and CrOx/Al2O3. The TPR peak around 300 °C for CrOx/Al2O3 was ascribed to the reduction of Cr6+ into Cr3+ as reported by Grzybowska et al. (45). On the other hand, the TPR peak around 200 °C may not be due to the reduction of chromium species as 224

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FIGURE 5. TPR curves for CoCr2O4 and for CrOx/γ-Al2O3. observed by XPS in Figure 4 but due to a simple chemisorption of hydrogen on the coordinatively unsaturated Cr3+ species as proposed for the chromate species dispersed on Cr2O3 particles by Zaki et al. (44). The observation by XPS, TPR, and redox treatments in this work suggest that the nature of active sites for CoCr2O4 is not necessarily the redox couples of Cr6+/Cr3+. It is obvious that further investigations are desirable to understand the active species of CoCr2O4 for TCE oxidation especially the coordinatively unsaturated chromium species. Selectivity to CO2. High conversion is not the only criterion for determining good halohydrocarbon destruction catalysts. Transformation of converted carbons to the carbon monoxide, which is another toxic air pollutant, has been a common problem for most of catalytic systems for total oxidation of chlorinated hydrocarbons. Another promising behavior of CoCr2O4 was its high selectivity to CO2 as shown in Figure 3. CoCr2O4 showed 75% of carbon dioxide selectivity at 300 °C and 93% at 315 °C, while only about 50% of converted carbons of TCE was transformed into CO2 on the CrOx/γ-Al2O3 catalyst. According to the two-step combustion (10), TCE is partially oxidized into CO which is oxidized again to produce CO2 (10).

2C2HCl3 + O2 + 2H2O f 4CO + 6HCl Hence, CO would compete for the active sites with other reactants to be oxidized further. Relatively higher concentration of active site of CoCr2O4 (Table 1) may relax this competition leading to a higher selectivity to CO2 in comparison with the supported catalysts. Competition of carbon monoxide with the chlorocarbon for oxidation sites

TABLE 3. Effect of Water Addition on the CO2 Selectivity of CrOx/Al2O3 and CoCr2O4 for the Oxidative Decomposition of TCE at 300 °Ca selectivity to CO2 (%) step

reaction condition

CrOx(12%)/Al2O3

CoCr2O4

1 2 3

without water for 4 h with water for 4 h without water again for 4 h

39.1 45.2 38.1

39.2 58.0 34.8

a

1500 ppm TCE, WHSV: 48 L/g-cat‚h.

was also suggested by Ramanathan and Spivey (10). They reported that the low concentration of reactant would favor carbon dioxide formation over carbon monoxide. Different distribution of Cr3+/Cr6+ was considered to be another factor for CO2 selectivity. Because the control of Cr species distribution of CoCr2O4 did not seem to be possible (i.e. XPS did not show any change in oxidation state of Cr even with redox treatments at 500 °C), CO oxidation experiments were carried out independently at 340 °C with 1000 ppm CO at 48 L/g-cat‚h by using chromium-containing catalysts having different Cr3+/Cr6+ ratio, i.e., Cr2O3 (for Cr3+), Cr-Y (for Cr6+), and CrOx(12%)/Al2O3 (for Cr3+/Cr6+). The activity of CO oxidation was the highest with Cr2O3 and the lowest with Cr-Y indicating that the abundance of Cr3+, the main species of CoCr2O4, was responsible for the higher CO2 selectivity. Pradier et al. (49) reported from the total oxidation of ethyl acetate that the improvement of complete oxidation performance of supported chromia catalyst could be made by increasing the surface area of microcrystalline chromia phase rather than through the formation of surface chromium silicate or aluminates. The water vapor in the feed should also contribute to a CO2 selectivity through the water gas shift reaction (59). Mellor et al. (40) reported that CoCr2O4 is a good catalyst for water gas shift reaction. Catalytic activity of CoCr2O4 was also tested for the decomposition of TCE with or without water. Reaction experiments proceeded under dry conditions for the first 4 h and with 1.5% water were carried out for the next 4 h and proceeded without water for the last 4 h. As shown in Table 3, water addition was effective for the enhancement of CO2 selectivity. The discrepancy in CO2 selectivity between Tables 2 and 3 was ascribed to the difference in sequential treatments for the experiments to examine the reversibility of each variable, i.e., H2 treatment or water addition. Figure 3 shows that CO2 selectivity was remarkably enhanced without significant loss in activity by the addition of cerium. It should be noted that cerium oxide is known not only to have the catalytic activity for the water gas shift reaction but also to enhance the redox reactivity through its well-known oxygen storage property (60). Figure 6 shows the TGA results of CoCr2O4 and CoCr2Ce0.1Ox under the flow of H2. The decrease in weight was observed for CoCr2O4 only at above 500 °C, while that for the cerium added catalyst started even at a lower temperature (∼200 °C). Jiang et al. (61) also reported that there was a good correlation between the activity for CO oxidation and the reduction ability of catalyst.

Conclusion Total oxidation of trichloroethylene (TCE) was tested as a model reaction for the catalytic combustion of chlorinated organic pollutants over various chromium-containing catalysts. Cobalt chromite (CoCr2O4) of spinel-type was proven to be a very promising catalyst showing higher activity and higher CO2 selectivity than traditional alumina supported chromia catalysts.

FIGURE 6. TGA results of CoCr2O4 and CoCr2Ce0.1Ox under the flow of H2 (carrier: 50 cm3 H2/min and 200 cm3 He/min, heating rate: 5 °C/min). Even if both Cr3+ and Cr6+ species on the surface were observed with XPS, the Cr6+ species was very stable under reducing environment. The presence of Cr3+-Cr6+ pair sites and the effect of redox treatments on the activity were investigated to explain the nature of possible active sites for TCE decomposition. Higher selectivity to CO2 of CoCr2O4 was ascribed to the abundance of its Cr3+ species, together with its activity for water gas shift reaction.

Acknowledgments This work was supported partially by the grant (97-05021001-5) of Korea Science and Engineering Foundation (KOSEF) and also from the National Research Laboratory Project(2000-N-NL-01-C-108) of Korea.

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Received for review March 14, 2000. Revised manuscript received August 21, 2000. Accepted October 12, 2000. ES001098K