Effect of aging on the behavior of copper-chromium compounds supported on activated carbon

CarbonVol. 29. No. 3. pp. 297-303, 1991 Printed inGreatBritain.

OOOS-6223191 $3.00+ .OO Copyright 0 1991Pergamon Pressplc

EFFECT OF AGING ON THE BEHAVIOR OF COPPER-CHROMIUM COMPOUNDS SUPPORTED ON ACTIVATED CARBON P.

EHRBURGER

and J.

LAHAYE

Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24 Avenue du Prt%ident Kennedy, 68200 Mulhouse. France

and P.

DZIEDZINL

and R.

FANGEAT

Centre d’Etudes du Bouchet. 91710 Vert-le-Petit. (Received 15 February

France

1990; accepted in revised form 2 May 1990)

Abstract-Copper-chromium compounds deposited on activated carbon by the impregnation technique are used for the removal of low boiling point gases from air. Their efficiency decreased markedly after exposure to humid air. Hexavalent chromium and divalent copper salts are the precursor of the supported species. Metallic silver is also added to the impregnation solution in the preparation of whetlerite-type materials. Since Cr( +6), Cu( +2) compounds can be exothermally reduced in an inert atmosphere by the carbon substrate, the thermal behavior of whetlerite-type adsorbents was investigated by means of differential scanning calorimetry (DSC). For unaged impregnated samples, all copper species are in the oxidation state +2, whereas chromium is found to have oxidation states +6 and +3. This fact is attributed to a partial reduction of the hexavalent chromium precursor during preparation of the supported phase. Exposure of the impregnated carbon to weathering conditions (5o”C, 90% relative humidity) leads to a segregation of copper from the copper-chromium complexes and to a progressive consumption of the Cr( + 6) fraction left on the carbon. The efficiency of the impregnated samples for the removal of cyanogen chloride from an air stream has been measured using a dynamic adsorption system. The breakthrough time of CNCI for samples weathered at different periods of time is directly linked to the amount of hexavalent chromium associated with copper species. After consumption of the total amount of Cr( +6) compounds, the efficiency of the impregnated carbon is equivalent to that of nonimpregnated carbon. A carbon only impregnated with copper behaves similarly to a nonimpregnated one for the removal of CNCI. Hence it is the association of Cr( +6) with Cu( +2) species which leads to an efficient destruction of CNCI in dynamic adsorption conditions. Key Words--Copper-chromium

compounds supported on carbons, whetlerite, aging.

1. INTRODUCTION

Copper and chromium compounds supported on activated carbons are used for the removal from air of gases of low boiling point. Basically, the overall destruction process of the gaseous contaminants includes two steps: (i) physical adsorption of the vapor in the carbon micropores; and (ii) chemical reaction with the supported metallic compounds, leading to non-lethal products. Typical adsorbents are whetlerite charcoals, which are prepared by impregnation of a highly porous carbon with ammoniacal carbonate solutions of copper ( +2), chromium ( + 6), and silver salts. It is well known, however, that the efficiency of impregnated activated carbon decreases markedly after heat treatment or exposure to high humidity (weathering). Since the carbon support is in contact with hexavalent chromium and divalent copper compounds, a redox-type reaction may occur at the interface leading to Cr( + 3) and Cu( + 1) species, which are known to be inefficient in the removal of the toxic gases[l]. Several attempts to elucidate the nature of the active phase and to determine the aging mechanism have been made during the last

two decades[l,2]. According to Deitz et al.[ 11, carbon is not an inert support, and commercial whetlerites contain Cr( +6), Cr( + 3), Cu( + 2), and Cu( + 1) species. More recently, the conversion of cupric species into cuprous oxide during the thermal decomposition of whetlerite above 325°C has also been reported[2]. In an attempt to follow more closely the physicochemical changes occurring during thermal treatment of an impregnated charcoal, Ehrburger et a1.[3] investigated the thermal decomposition of supported Cu( +2) by differential scanning calorimetry (DSC) and x-ray diffraction. Two decomposition steps were evidenced: reduction of Cu( +2) into cuprous oxide, and subsequent formation of metallic copper. The thermal behavior of chromium trioxide deposited on carbon was also investigated using DSC[4]. It was found that Cr( +6) was partially reduced by the carbon support even during the impregnation and/or the drying steps of preparation. Hence the supported chromium species are generally present in the oxidation states + 6 and + 3, as also found by x-ray photoelectron spectroscoPYN In a recent paper Hammarstrom 291

and Sacco[6] in-

P. EHRBURGER

298

vestigated the deactivation mechanism of ASC whetlerite charcoal using mainly x-ray diffraction and ESCA techniques. They showed that, upon increasing the thermal treatment temperature, the supported basic copper ammonium chromate, CuOHNH,CrO,, decomposes to copper oxide-copper chromite complex, CuO-CuCr20,, followed by the reduction of Cu( + 2) to Cu( + 1) giving CulCrlO,. The amount of copper is generally higher than the one which would lead to the formation of the aforementioned stoichiometric compounds. ESR studies on whetlerite suggest that the excess copper may be present as a polymeric-type copper species[7]. At temperatures higher than 25o”C, cupric oxide sinters away and is subsequently reduced to Cu( + 1) species[6]. According to the same authors, the degradation of the whetlerite in humid environments is due to the conversion of the supported basic copper ammonium chromate into brochantite chromate, Cu,(OH),CrO,, which thereafter decomposes to CuO, CuzCrlO,, and Cr,0,[6]. It is clear that the surface chemistry of the impregnated carbons is quite complex due to the large number of compounds which can be found on the carbon surface. Their identification is sometimes rather difficult, especially if they are amorphous. Nevertheless, it has been reported recently that the active compounds for the destruction of cyanogen chloride contain Cr( + 6) associated with copper species[8]. The purpose of the present study is to investigate further the physicochemical state of the supported species in whetlerite-type materials and its changes during aging in wet air. Since Cr( +6), Cu( +2), and Cu( +‘l) compounds can be exothermally reduced by the carbon substrate, it was thought of interest to investigate the thermal behavior of whetlerite-type adsorbents by means of DSC. A second objective is to establish a correlation between the thermal behavior of copper-chromium compounds on carbon subjected to aging and their efficiency in destroying cyanogen chloride from an air stream. 2. EXPERIMENTAL

2.1 Materials Two types of materials, a commercial one (C) and a sample impregnated in the laboratory (L), have been studied. In both cases an activated carbon (PICA) prepared from coconut shells was used. The carbon had a BET surface area measured by nitrogen adsorption at 77 K, equal to 1,190 m?/g. Its ash content was 2% by weight. The commercially available whetlerite-type material (PICA) contained copper, chromium, and silver compounds, whereas no silver was deposited on the sample prepared in the laboratory. Sample L was obtained by conventional impregnation of the carbon support, using an ammoniacal carbonate solution of copper( +2) and of chromic acid in a manner described elsewhere[3]. A carbon containing only copper was also prepared using the same procedure (sample E). Prior to the

et al.

deposition a particle of copper measured ples in a

step, the carbon support was screened to size between 1 and 1.5 mm. The amounts and chromium on the carbon samples were by atomic absorption after ashing the samcrucible at 800°C. The silver content in sample C was given by the supplier. Percentages by weight of the metals expressed on dry basis for the different samples are given in Table 1. From the atomic ratio Cu/Cr, it is seen that copper isin excess to chromium in both types of samples. 2.2 Differential scanning calorimetry DSC was operated using a Mettler TA 3000 thermoanalyzer. The reference was a similar but empty pan. Experiments were made under a flow of nitrogen on about 15 mg of finely crushed sample. In order to minimize the endothermal effect due to desorption of water from the sample, a pretreatment at 190°C for 10 minutes was done in the DSC equipment. After cooling to room temperature, enthalpy analysis was carried out between 25 and 550°C at 20 K/min. The data were stored on floppy disks and processed with a microcomputer. The DSC curves corresponding to sample C and to the impregnated carbon support are shown in Fig. 1. It is seen that the impregnated sample exhibits a strong exothermal signal as compared to the carbon support, which has a much smaller and slightly endothermal behavior. Since the exothermal signal of the impregnated sample is rather broad, a curved base line was taken for the determination of the heat effect. A preliminary study with nonimpregnated carbon samples had shown that the equation of the base line in the temperature range lOO-550°C is of polynomial type, a + bt + ct’ + dt’, where t is the temperature and a, b, c, and d are the coefficients determined experimentally for each type of sample using a fitting procedure. The base line obtained in this manner for sample C is also shown in Fig. 1. The reproducibility measurements of the heat effect using this procedure is equal to 21.5 J/g. The reported data are the average of at least three DSC experiments. In order to facilitate the comparison of DSC results obtained for different samples, all base lines have been drawn horizontally in the subsequent DSC runs. 2.3 Aging procedure The impregnated samples were conditioned at 18°C and 90% relative humidity (RH) for about two days (conditioned sample). During this procedure, the activated carbon adsorbs about 30% by weight

Table 1. Metal content of carbon samples cu Sample C L E

(% by !?dght) 4.27 5.60 5.20

1.84 2.70

Ag 0.2

Atomic ratio Cu/Cr 1.9 1.7

Copper-chromium ---.\

-1.

52 -1.0 -

\./.-.r

299

compounds on activated carbon

./”

E -2.0 s I+

-3.0 -

c 5 I

-4.0 -

-5

40

n

200

300

3

400

TEMPERATURE

500

’

600

I. -‘.PO0

Fu

I

200

300

1

1

TEMPERATURE

E

500

400

(OC)

Fig. 1. DSC curves for initial sample C (-) and nonimpregnated carbon (-.-); calculated base line for sample c (---).

Fig. 2. DSC curve of initial samples L (----).

of water. After conditioning, the samples were aged at 50°C and 90% RH for several days.

seen that by simply conditioning sample C to 90% RH, a slight change is found in the thermal pattern. A small endothermal effect can be detected at about 2Oo”C, which was not the case with the initial sample (Fig. 2). Changes become more pronounced as the severity of aging increases (4 and 13 days). The endothermal peak ranges from 180°C to about 240°C and its intensity increases with weathering, whereas the opposite effect is found for the first exotherm. Interestingly, the second exotherm is not significantly affected by the aging process. Therefore, the total exothermal effect AHr has been decomposed into two parts (AH, and AHr,), corresponding respectively to the peaks below and above 360°C. The values of the different thermal effects, including the endotherm, are given for untreated and aged samples C in Table 2. It is seen that peak I decreases rapidly during the first two days of aging and that thereafter the changes are less pronounced. After

2.4 Dynamic adsorption of cyanogen chloride A wet air stream with a flow rate equal to 30 limin and containing CNCl (concentration equal to 5 g/m’) was passed at 18°C through a cylindrical container filled with impregnated carbon. Taking into account the high toxicity of CNCl, the experimental set-up must comply with all safety requirements. The experimental procedure was similar to the one described in a previous paper[9]. The efficiency of CNCl removal from the air stream was estimated by measuring the breakthrough time corresponding to a concentration of lethal compound in the outlet stream equal to 30 mgim’. The experimental uncertainty is estimated to be *30 seconds.

C (-)

and

3. RESULTS

3.1 Unaged samples The DSC curves corresponding to both types of impregnated samples, C and L, are shown in Fig. 2. In the case of the commercial sample, two partially overlapping exothermal peaks can be detected. The first peak, which starts around 22O”C, is the most intense one. In the temperature interval 360-450°C. a second and less pronounced exotherm occurs. Sample L also exhibits a broad exothermal effect in the temperature range 220-49o”C, but only a shoulder can be seen around 380°C. It seems that the two exotherms are less well separated in the case of sample L. This fact may originate from a different type of impregnation procedure and/or from the absence of silver in sample L. 3.2 Aged samples The DSC curves corresponding to samples C after different stages of aging are shown in Fig. 3. It is

4.01

-‘.

7” 200c 00

”

300

400 -

”

500

‘I

TEMPERATURE (oC> Fig. 3. Effect of aging on the DSC curve of samples C: (-) conditioned, (-.-) aged for 4 days. (----) aged for 11 days

P. EHRBURGER et al.

300

Table 2. DSC results and breakthrough time of initial and aged samples C Exotherm (J/g) Sample

AHr

AHi

Initial Conditioned Aged 4 days Aged 6 days Aged 8 days Aged 11 days Aged 13 days

-86.2 - 79.4 - 65.6 -53.6 -51.8 -49.2 -47.2

-69.6 ‘- 64.0 -49.3 -39.0 -36.3 -33.4 - 32.8

AHi, -

16.6 15.4 16.3 14.6 15.5 15.8 14.4

11 days of weathering, the exothermal effect corresponding to peak I has decreased by a factor of 2. In an opposite way, the endothermal effect increases during aging and remains almost constant after 11 days of weathering. The breakthrough time (BT) of CNCl on conditioned and aged samples is also given in Table 2. As expected, BT is markedly affected by the aging of the impregnated carbon samples. Hence, after a weathering period of 11 days, the observed breakthrough time is almost comparable to the one corresponding to a nonimpregnated carbon, for which BT = 10 minutes. Similar DSC results are obtained during aging of sample L. The DSC curves for conditioned and aged samples are shown in Fig. 4. A decreased intensity of the exotherm associated with an increased intensity of the endotherm is observed. In particular, the second exothermal peak is more apparent after aging. However, in contrast to the case with sample C, the resolution is not sufficient to allow a separation into two peaks. The values of the thermal effects found for samples L before and after aging are listed in Table 3. It appears that the changes are occurring

essentially

Endotherm (J/g) AH,,

during the first two days of ag-

Breakthrough

1.8 7.3 4.8 8.2 11.9 12.3

time (min)

26 17 15 14 12 12

impregnated carbon[3]. Interestingly, chromium trioxide deposited on carbon does not give the endothermal peak[4]. This fact suggests that the endotherm is likely to be linked to the presence of isolated copper species. The DSC curves of sample L aged for eight days and of sample E (unaged copper impregnated carbon) are compared in Fig. 5. Although there are some differences in the shapes of both curves, they have basically the same features. In particular, the endotherms are comparable and the two exotherms occur in the same temperature interval. The thermal data corresponding to sample E are also given in Table 3. These values compare well with those of aged impregnated carbon L. Since samples L and E have nearly the same copper content, it appears that after aging copper-chromium impregnated carbons behave similarly in thermal analysis to the unaged supported Cu( +2) species. The results obtained for aged whetlerite-type samples C substantiate this observation if one takes into account their somewhat lower copper content (see Table 2). The shape of their exothermal signal differs, however, from the one observed for single Cu( + 2) impregnated carbon.

ing. The most noticeable change in the DSC curves of aged samples is the appearance of an endothermal peak. A similar signal is also found for Cu( + 2)

4. DISCUSSION

The physicochemical changes occurring to supported chromium and copper species during heat treatment and after aging will be discussed separately. 4.1 Thermal decomposition The DSC curve of copper-chromium impregnated carbon differs from that of the copper impregnated carbon by the absence of an endothermal peak. This fact indicates that for samples C and L copper species Table 3. DSC results of initial and aged samples L and E Exotherm -%0

1.

200

300

400

500

Sample

600

TEMPERATURE (OC> Fig. 4. Effect of aging on the DSC curve of samples L: (-) initial, (-.-) aged for two days, (-----) aged for eight days.

L L L L E

initial aged 2 days aged 6 days aged 8 days initial

J'g

- 122.6 -74.6 -69.4 -61.2 -58.1

Endotherm

J/g

1.5 5.9 6.2 6.1 12.0

Copper-chromium 4.01

compounds on activated carbon I

TEMPERATURE

K)

Fig. 5. Comparison of the DSC curve of initial sample E (-----) and sample L aged for eight days (-).

are probably strongly associated with chromium compounds. The origin of the exothermal effect occurring during heat treatment of samples C and L is a redox reaction between the carbon and the supported species, analogous to that of cupric oxide or chromium trioxide deposited on carbon. In the case of cupric oxide, the chemical reactions giving rise to a heat release can be written as follows[lO]: 4cuo

f c -

2cu,o

+ co,

AHIWC = -38.5 2cu,o

+ c -

kJ/mole CuO

(1)

kJ/mole Cu,O.

(2)

4cu + co, AH,src = -58.0

The overall reduction reaction of CuO into Cu is equal to the sum of individual steps: 2cuo

+ c -

2cu + co, AHcu = -67.5

kJ/mole CuO.

(3)

In the case of sample E containing 5.2% of copper by weight, the heat evolved during the thermal treatment according to reaction (3) would be equal to - 55.3 J/g. Considering the value measured by DSC, -58.1 J/g (Table 3), and taking in account the experimental uncertainty in the determination of the base line over such a large interval of temperature, it may be concluded that a reasonable agreement is found for the two values. Hence, for unaged copper impregnated samples, an almost quantitative measurement of the amount of deposited copper can be made by DSC. A similar result has already been obtained by considering only reaction (1)[3]. It has also been previously shown that chromium trioxide

is reduced into Cr( + 3) oxide according to the following exothermal reaction[lO]: 4Cr03 + 3C -+

2Crz03 + 3C0, AHcr = -303 kJ/mole CrO,.

(4)

301

In the case of whetlerite, copper and chromium species are probably combined, and therefore reactions (3) and (4) cannot be straightforwardly applied. In the pretreatment conditions of a DSC experiment (heating at 190°C for 10 minutes), it may be assumed that basic copper ammonium chromate is already decomposed into copper chromate CuCrO,. The standard heat of formation of CuCrO, is - 746.5 kJ/ mole[ll], which is almost equal to the sum of the heats of formation of CuO and CrO, (- 746.1 kJi mole). Hence, as a first approximation, the heat released by the reduction of CuCrO, can be estimated from eqns. (3) and (4). Furthermore, it must be kept in mind that copper is in excess of chromium, which means that all copper cannot be chemically associated with chromium. Considering the amount of copper and chromium present on the carbon, one may calculate the amount of heat which would be released according to eqns (3) and (4). The values are equal to - 152.6 J/g and - 216.9 Jig for unaged samples C and L, respectively. It is seen from Tables 2 and 3 that the corresponding experimental values are significantly lower. This fact may be attributed to a partial reduction of Cu( + 2) and/or Cr( + 6) species during sample preparation. In a similar way, as for copper impregnated carbon, the presence of Cu( + 1) species is highly improbable in unaged whetlerite. Furthermore, in the presence of Cr( +6), Cu( + 1) is highly unstable and would be reoxidized to Cu( +2) species according to thermodynamic considerations. Therefore, it appears that, as in chromium impregnated carbon, Cr( +6) species are partly reduced during the preparation of whetlerite. The fraction of Cr( +6) which can thus be detected by DSC represents 38% and 40% of the initial Cr( +6) loading of samples C and L, respectively. Although the amounts of copper and chromium salts deposited on both samples are not quite the same, it appears that the fraction of Cr( + 6) which is reduced during the impregnation and/or the drying steps is similar. Furthermore, it is interesting to note that in absence of copper salt, the amount of Cr( +6) which is reduced during sample preparation is somewhat higher, since only about 25 to 30% of the initial amount of Cr( + 6) is left[4]. From the above results, it may be concluded that Cu( + 2), Cr( +6), and Cr( +3) species are present on untreated whetlerite. During the thermal treatment these compounds are gradually reduced to lower oxidation states. The DSC curve of sample C (Fig. 3) clearly indicates the presence of two exothermal peaks. Interestingly, the second one is not significantly affected during aging, and therefore it is interesting to investigate further its origin. For single Cu( +2) species supported on carbon, it is known that the second exotherm corresponds to the reduction of cuprous oxide into metallic copper[3]. Assuming a similar reaction for sample C, the amount of heat produced according to reaction (2) is equal to 19.5 J/g. This figure is somewhat higher than the average

302

I?

EHRBURGER

value taken from Table 2, which is equal to 15.3 J/g. The discrepancy may originate from the experimental determination of the second exotherm or from the occurrence of another reduction reaction. It was indeed shown that during thermal decomposition of whetlerite Cu,Cr,O, is formed[6]. The standard heat of formation of Cu,CrzO, is of the order of - 1338 kJ/mole, but its heat capacity is not accurately known[l2]. In that case, the heat of reaction corresponding to the reduction of Cu2CrzOI according to the equation 2Cu1Crz0, + C -

4Cu + 2Cr,03 + CO,

(5)

would be equal to -28 kJ/mole Cu,Cr,O,, which is lower than the one corresponding to reaction (2) (-58 kJ/mole Cu,O). Assuming that all copper is present as CuQ,O,, the heat released during the decomposition of sample C is equal to -9.4 J/g. This value is somewhat lower than the experimental one, which is not too surprising since the amount of copper exceeds that which would be present as Cu$r,O,. All these results allow a better understanding of the chemical changes occurring during the thermal decomposition of copper-chromium compounds on carbon. Hence, the first exotherm corresponds essentially to the reduction of copper species from the oxidation state +2 to + 1 and of the remaining chromium fraction from oxidation state +6 to +3. At the end of the first exotherm, all chromium and copper compounds are mainly present in the oxidation states + 3 and + 1, respectively. The second exothermal peak can be attributed to the reduction of Cu( + 1) species to metallic copper. Furthermore, the DSC results confirm that Cu( + 1) is present as Cu,O and Cu,CrZO,. It is also interesting to note that the exothermal peaks are less well separated in the case of sample L. This fact suggests that the aforementioned reduction steps are probably less well separated in the case of sample L. The reason for this situation may be either the absence of silver in sample L, which would have a catalytic effect on the decomposition of the supported species. or a difference in the impregnation procedure. 4.2 Effect of aging The two major changes in the DSC curve of aged samples as compared to an unaged one are respectively the formation of an endothermal peak and the decrease in the intensity of the first exotherm. As already mentioned, the occurrence of an endothermal peak can be attributed to isolated Cu( + 2) species. This result is in agreement with a segregation mechanism of Cu-Cr compounds into CuO and Cr,O, as proposed recently by Hammarstrom and Sacco[6]. It may be noted that even during the conditioning procedure of sample C, a slight formation of isolated Cu( +2) species takes place (see Fig. 3). The intensity of the exothermal peak decreases during aging, particularly at the beginning of weath-

et al.

ering. In the case of sample C aged for 13 days, the value of the overall exotherm (- - 47 J/g) is almost equivalent to the heat of reduction of the amount of Cu( + 2) species of the sample, which is equal to 45.4 J/g. A similar result is obtained for aged sample L (AH = -61 J/g), for which the heat of reduction of the copper species would be equal to 59.5 J/g. These results clearly indicate that during weathering the remaining fraction of Cr( + 6) compounds is progressively reduced to Cr( +3) species. As shown by Hammarstrom and Sacco[6], copper chromite (CuCr,O,) may be formed during a moderate heat treatment (220°C). The heat of formation of CuCr,O, from CuO and Cr,03 is estimated to be about - 10.4 kJ/mole at 9OO”C[13]. This value would lead to a somewhat lower heat value for the reduction of CuCr,O, as compared to CuO. However, since all copper cannot be present as chromite salts, the DSC results will not be markedly modified. Nevertheless, the shape of the DSC curve is affected by the presence of copper chromite as compared to isolated copper species (Fig. 5). The breakthrough time of CNCl on conditioned and aged samples C is plotted against the value of their total exothermal peak AHT in Fig. 6. It is seen that a linear relationship is obtained which confirms that the efficiency in removal of CNCl is actually linked to the presence of Cu( + 2) and Cr( + 6) species on the activated carbon. Since the DSC results suggest that Cu( +2) is not reduced during the investigated period of aging, it appears that it is essentially Cr( +6) which is consumed during weathering. Investigations carried out with isolated Cr( +6) also showed that hexavalent chromium species are very sensitive to aging[4]. As already mentioned, the most severely aged sample has an efficiency nearly equal to that of a nonimpregnated carbon. An experiment has also been carried out with a carbon only impregnated with copper (sample E) in order to assess the efficiency of this compound.

Oo 1.

20 1.

40 1.

60 1.

80 1.

100 I

HEAT OF REACTION (J/g) Fig. 6. Relationship between breakthrough time, BT, and exothermal heat values for unaged and aged samples C: n : AH,; 0: AH,,.

Copper-chromium

compcxnds on activated carbon

In that case, the breakthrough time was equal to 11 minutes. It can be deduced that copper by itself is inefficient in destroying CNCl adsorbed on the carbon. Subtracting the amount of heat corresponding to Cu( + 2) reduction from the total exothermal signal AH,, one obtains the amount of heat (AH,,) related to the presence of Cr( +6) on the carbon. Breakthrough time has also been plotted against AHcr in Fig. 6. With that kind of representation, the efficiency in scavenging CNCl by physical adsorption and by chemical reaction with Cr( + 6) can be clearly differentiated. It must be kept in mind, however, that it is the association of Cr( +6) with Cu( +2) species which is efficient in the destruction of CNCI during the dynamic adsorption test.

5.CONCLUSION

The thermal decomposition of copper and chromium compounds supported on carbon gives rise to a redox-type reaction in which two steps can be distinguished. In a first step, chromium species are reduced from the oxidation state + 6 to + 3 and copper species from +2 to + 1. Thereafter, monovalent copper compounds are reduced to metallic copper. In both cases, the reducing agent is the carbon support itself. During preparation of the supported compounds, a considerable part of the initial amount of hexavalent chromium is already reduced, as in chromium trioxide impregnated carbon. Weathering of supported copper and chromium compounds induces significant changes in the thermal decomposition reaction. During aging, segregation of copper from chromium takes place, leading to isolated Cu( +2)

303

The amount of hexavalent chromium also decreases during aging due to a redox reaction with the carbon support. The efficiency of the impregnated carbon for the removal of cyanogen chloride is directly linked to the amount of hexavalent chromium associated with copper species. Once the consumption of the Cr( + 6) compounds is complete (i.e., after sufficient weathering), the efficiency of the impregnated carbon is equivalent to that of a nonimpregnated one, indicating that cyanogen chloride is mainly removed from the gas stream by physical adsorption. compounds.

REFERENCES 1. V. R. Deitz. J. N. Robinson. and E. J. Poziomek, Carbon, 13, 181 (1975). 2. N. Bat, J. L. Hammarstrom. and A. Sacco. Jr., Curbon, 25, 545 (1987). 3. P. Ehrburger. J. M. Henlin, and J. Lahaye. J. Catal., 100.429 (1986). 3. P. Ehrburger. J. Dentzer, J. Lahaye, P. Dziedzinl. and R. Fangeat. Carbon. 28, 113 (1990). 5. M. M. Ross, R. J. Colton. and V. R. Deitz. Carbon. 27. 494 (1989). 6. J. L. Hammarstrom and A. Sacco. Jr., J. Catal.. 112. 267 (1988). 8. P. N. Krishnan. S. A. Katz, A. Birenzvige. and H. Salem. Carbon, 26, 914 (1988). 9. P. Ehrburger. J. Lahaye, and R. Fangeat. Carbon. 25. 227 (1987).

10. Handbook of’ Chemistry and Physics, 59th ed., CRC Press, West Palm Beach, FL, D51&68 (1979). 11. D. I. Chemodanov and M. Chernyak. Zh. Fiz. Khim.. 52, 2121 (1978). 12. S. C. Schaefer and N. A. Gokcen, High Temp. Sci..

24, 63 (1987). 13. F. Miiller and 0. J. Kleppa, J. Inorg. Nucl. Chem.. 35. 267 (1972).