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김대철·임선기

한국과학기술원 화학공학과

Deactivation of Cr-containing Catalysts for Oxidative Decomposition of Trichloroethylene

Dae-Chul Kim and Son-Ki Ihm

Dept. of Chem. Eng., KAIST

Introduction

Catalyst deactivation has been identified as one of the most serious problems associated with the commercial application of catalysts for the decomposition of chlorinated organic compounds [1]. Until recently, the use of catalytic oxidizers was restricted to non-halogenated VOC (volatile organic compound) streams, because halogens would poison the conventional noble-metal catalysts used.

Transition metal oxides, including ion-exchanged zeolites and solid acids have been known to experience catalyst deactivation [2]. Even though the higher resistance to poisoning of transition metal oxides in comparison with the noble metal catalysts are believed mainly due to their higher active surface area, the nature of deactivation in metal oxides has yet to be elucidated.

Several factors have been considered as the origins of this deactivation with transition metal oxide catalysts. It was suggested that the elemental Cl2 produced by the Deacon reaction during chlorinated VOC oxidation can cause chlorination and hence deactivation of the transition metal oxide catalysts like the case of noble metals. Therefore, the catalysts that are less efficient for the Deacon reaction would be more suitable for chlorocarbon oxidation. Another mechanism of the deactivation is the coke deposition [4-6]. For the oxidation of trichloroethylene over acidic zeolite catalysts, it was found that there were a large amount of coke deposition [5].

We have reported the high activity of supported chromium catalysts and spinel-type chromites for the decomposition of trichloroethylene (TCE) in the previous work. But these catalysts showed significant drop of their activity in the first few hours. This deactivation was more pronounced for the reactions at low temperatures. Causes of this deactivation were discussed in terms of reaction intermediates.

Experimental

Mixed oxides of Cr and Co were prepared by coprecipitation at pH 8. The precipitation was accomplished by adding dropwise the co-solution of metal nitrates with desired proportions of ammonium hydroxide at room temperature under continuous stirring. The filtered precipitate was dried and calcined at 600oC for 5 hrs. CrOx/□-Al2O3 samples were prepared by incipient wetness impregnation of □-Al2O3 using the corresponding nitrate salts as precursors. JRC-ALO-2 was chosen for □-Al2O3 support whose surface area was 350m2/g. Metal loading was varied from 3 to 16 wt% of Cr.

The activity of the catalysts in oxidative decomposition of TCE was measured in a fixed bed apparatus at the temperatures between 280oC and 330oC. The reaction feed mixture contained 1500ppm of TCE and 15000ppm of H2O with balance air. The weight hourly space velocity (WHSV) was 48 l/g-cat□hr.

UV-vis diffuse reflectance spectroscopy (DRS) was applied to identify surface chromate structures and electronic interactions, using a Hitachi U-3501 spectrophotometer. Thermogravimetric (TG) analysis was performed in a stream of air (50cc/min with 200cc/min He) by means of Cahn 2000 microbalance.

Results and discussion

Both of CoCr2O4 and alumina supported catalysts showed obvious deactivation during the first few hours of the reaction and this trend was more significant at the lower temperature as shown in Fig. 1. A distinct color change from brown to blue after the reaction at 300oC for supported catalysts suggested that there should be some change in the catalyst structure which in turn would affect the catalyst activity. The possibility of the structural collapse of supports causing successive loss in surface area and active reaction sites was excluded since there was a negligible change of pore size distributions as well as surface area.

Fig. 1. Catalytic activity of CoCr2O4 and CrOx/□-Al2O3 for the decomposition of TCE with time-on-stream.

Coke formation could be another reason for the deactivation. Coke formation did not seem to be the main cause of deactivation in the case of CoCr2O4 which showed a negligible weight loss in TGA experiment under the oxygen stream. Some weight changes, however, were observed in TGA for the used CrOx/Al2O3 after the reaction at 300oC. But it can hardly be related to the coke formation due to the fact that the fresh CrOx(12%)/Al2O3 catalyst also showed the weight change in the similar degree during the temperature programmed oxidation. This weight change of fresh CrOx/Al2O3 catalyst must be due to the removal of condensed water from the surface hydroxyl groups.

The oxidation state of chromium species has been considered as one of the most important properties for determining their catalytic activity. The change of oxidation state of Cr-containing catalysts before and after the reaction were followed by diffuse reflectance spectroscopy of UV-vis. Fig. 2. is the UV-vis DRS spectra exhibited the presence of chromate species in the alumina supported catalysts. The surface structure of the chromate detected can be drawn from the DRS spectra on the basis of the following criteria. The band at 370 nm is due to the charge-transfer absorption of Cr(VI) and oxygen with the feature at 450 nm arising from the presence of a small proportion of Cr(III). The band at 595 nm has been ascribed to d-d electron exchange interactions Cr(III) which is intimately coupled with Cr(VI) sites (denoted gamma-phase). Hence, peaks at 370nm and 450nm, can be taken as direct evidence of an existence of surface chromate with Cr(VI).

Fig. 3. TGA results of bulk and supported chromium oxide in reactive condition without water. (Under the Flow of TCE, Oxygen and He),

Fig. 2. UV-Vis diffuse reflectance spectra of CrOx/Al2O3 (12% Cr)

The absence of a well-resolved band near 595 nm in the spectrum of fresh alumina catalysts (Fig. 2) indicates that the material contains predominantly the chromates anchored directly onto the alumina support. The disappearance of peak at 370nm after the reaction at 300oC was ascribed to the transformation of surface chromate species by the interactions with trichloroethylene.

To explain the initial deactivation (decrease of catalytic activity exhibited during the first few hours after the reaction started), it was assumed that the active sites with reduced species could not be rapidly re-oxidized or hydrolyzed to regenerate active species at low temperatures such as 300oC. This assumption might be tested by carrying out the temperature programmed reaction with TCE and oxygen with a measurement of weight change. Fig. 3. shows the TGA results of bulk and supported chromium oxide under reactive condition without water (under the flow of TCE, oxygen, He). At low temperatures such as <300oC, weights of catalysts increased with the increase of temperature. This weight increase should come from the reductive adsorption of reactants. Weight of catalyst decreased at the higher temperatures above 300oC due to the removal of the adsorbed species by rapid oxidation.

Fig. 4. Catalytic activity of CrOx/□-Al2O3 (12%, S.A.:235m2/g) and CrOx/□-Al2O3 (6%, S.A.:159m2/g) for the decomposition of TCE at different temperature.

It was expected that the initial deactivation would not be observed if we had carried out reaction experiments at higher temperatures beyond 300oC. Indeed, Fig. 4 shows that the catalytic activity carried out at 330oC for CrOx(12%)/Al2O3 or 350oC for CrOx(6%)/Al2O3 were not deactivated during the first five hours of reaction. It should be noted that the CrOx(6%)/Al2O3 catalyst was prepared with low surface area alumina support to prevent the complete conversion at that temperature (350oC). It could be concluded that the initial deactivation of supported chromium oxide catalysts were caused by the dissociative adsorption of TCE on the active sites, which could not be regenerated at the temperatures below 300oC.

참고문헌

[1] R.A. Keller and J.A. Dyer, Chem. Eng., January 100-105(1998)

[2] M. Ballikaya, S. Atalay, H.E. Alpay and F.S. Atalay, Combust. Sci. Tech., 120, 169-184(1996)

[3] S.Chatterjee and H.L. Greene, J. Catal., 130, 76-85(1991)

[3] H. Windawi and Z.C. Zhang, Catal. Today, 30, 99-105(1996)

[4] S.K. Agarwal, J.J. Spivey and J.B. Butt, Appl. Catal. A:General, 82, 259-275(1992)

[5] K. Ramanathan, J.J. Spivey, Combust. Sci. Tech., 63, 247-255(1989)

[6] Imamura, S., H. Tarumo and S. Ishida, Ind. Eng. Chem. Res. 28, 1449-1452 (1989)