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Thermal behavior of chromium trioxide deposited on carbon
Carbon Vol 2X. No
1.p~. 113-118, 1990
0008.6223190 $3.00+.00
Copyright 0 1990Pergamon Pressplc
Printedin Great Br~tam.
THERMAL
BEHAVIOR OF CHROMIUM DEPOSITED ON CARBON P.
EHRBURGER?,
J. DENTZER,
and J.
TRIOXIDE
LAHAYE
Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, Avenue du President Kennedy, 68200, Mulhouse, France and P. DZIEDZINL and R. FANGEAT Centre d’Etudes du Bouchet, 91710 VERT-LE-PETIT, (Received
France
1 March 1989; accepted in revised form 25 May 1989)
Abstract-The
thermal reaction of chromium trioxide with an activated carbon has been investigated by differential scanning calorimetry (DSC) and mass spectrometry analysis of the evolved gases. TWO types of deposition techniques have been used: A simple mixture of both components and the impregnation procedure. The analysis of the DSC curves in conjunction with mass spectrometry indicates that the observed exothermal effect is due to the oxidation of the carbon by chromium compounds of oxidation state higher than 3. In the case of Cr( + 6)carbon mixture an almost quantitative reaction of Cr( + 6) can be found in some cases. For CrO, impregnated carbon a significant fraction of the deposited Cr( +6) is consumed during the preparation of the sample. Aging in humid atmosphere also leads to a consumption of the deposited chromium trioxide by the carbon support. Key
Word-Activated
carbon, chromium oxides, thermal decomposition.
1. INTRODUCTION
heat treatment of cupric oxide supported on carbon have been studied by differential scanning calorimetry (DSC)[4,5]. It was found that CuO is reduced to Cu,O and thereafter to Cu when thermally treated between 220 and 550°C. The same authors also showed that during exposure to humidity at 50°C Cu( + 2) is partly reduced to Cu( + 1). The conversion of cupric oxide species into cuprous oxide during the thermal decomposition of whetlerite at 325°C has also been reported[6]. The effect of weathering of a whetlerite has also been investigated by X-ray photoelectron spectroscopy[7]. It was found that the oxidation state of the chromium depends on the extent of weathering and Cr( + 6) is specifically reduced to Cr( + 3) by the carbon supports. The purpose of the present paper is to determine more precisely the behavior of chromium trioxide in presence of carbon. The effect of the deposition technique of CrO, on the carbon support has been investigated. Furthermore, the interaction of CrO, with the activated carbon during heat treatment and after weathering has been studied in order to estimate the extent of reduction of Cr( + 6) by the support.
Chromium and copper compounds supported on activated carbons are used for the removal of toxic gases from the air. Basically, the overall destruction
process of the gaseous contaminants steps:
includes two
1. physical adsorption of the vapor in the carbon micropores and, 2. chemical reaction with the supported metallic compounds. It is also well known that the efficiency of such systems markedly decreases after heat treatment or after weathering. Since the activated carbon is usually impregnated with a solution containing hexavalent chromium and divalent copper compounds, a redox type of reaction between the deposited phase and the carbon support may explain the observed loss of efficiency. During the last two decades several attempts for the elucidation of the nature of the active species and of the mechanism of their altering during weathering have been made. Deitz et al. [l] reported that carbon cannot be considered as an inert support and found that commercial whetlerite contains Cr( + 6), Cu( + 2), Cr( + 3) and Cu( + 1) species as well. Furthermore, Berg et aL[2,3] noticed an increase of the crystallinity of the supported oxides after severe weathering conditions. More recently, the physico-chemical changes occurring during the tTo whom all correspondence
2.
EXPERIMENTAL
2.1 Materials An activated carbon (PICA) prepared from coconut shells was screened to a particle size between 1 and 1.6 mm. Its surface area measured by nitrogen adsorption at 77 K and calculated with the BET equation was equal to 1190 m2/g. The ash content
should be sent. 113
114
P. EHRBURGER et al.
of the carbon support was equal to 2% by weight. Chromium trioxide with a purity better than 95% was used. Since the interactions between two condensed phases usually depend on the development of their interface, two deposition techniques of CrO, on the carbon support were considered. In the first one, the carbon particles were finely ground in a mortar and the desired amount of CrO, was added by weighing. This type of samples will be called “CrO,-carbon mixture.” For DSC experiments, each sample was individually prepared by taking about 15 mg of carbon and adding the desired amount of CrO, (usually between 0.5 and 1.5 mg) with the use of a balance sensitive to 0.01 mg. In the second case, CrO, was deposited on the carbon by the impregnation technique. Therefore CrO, dissolved in water was added to the carbon particle until incipient wetness was reached. The water was then removed in a dry air stream and the temperature of the carbon sample never exceeded 110°C. The amount of chromium deposited on the carbon was measured by atomic absorption after ashing of the impregnated carbon at 800°C in a crucible. 2.2 Techniques 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. In order to minimize the endothermal effect due to desorption of water from the sample, a pretreatment at 150°C for 15 min was done in the DSC equipment. After cooling to room temperature enthalpy analysis was carried out between 25 and 550°C. In some experiments the nature and the amount of gases evolved during the heat treatment of the CrO,-carbon samples have been determined using a mass spectrometer (Balzers QMG 112). A 10 mg sample was placed in a reaction vessel which was outgassed to low4 Pa. The sample was then subjected to a temperature programmed desorption at a linear heating rate of 5 Kmin-’ under a dynamic pressure range of 2.10-3-10-4 Pa. Mainly water and carbon dioxide were detected. The amount of CO2 evolved during heating was measured using a similar procedure to the one described by Causton and McEnaney[8]. The calibration of the mass spectrometer was done with an absolute manometer (Barocel 1 torr pressure range) using ultrapure CO, (L’Air Liquide N45). 2.3 Aging procedure The impregnated samples were conditioned at 18°C and 90% relative humidity (RH) for about 2 days. In these conditions, the activated carbon adsorbs about 30% by weight of water. After conditioning, the samples were aged at 50°C and 90% RH for several days. Before testing by DSC, the aged carbons were dried in an oven at 110°C overnight.
3. RESULTS
The thermal behavior of CrO, in presence of carbon will be examined respectively in the case of a mixture and of impregnated samples. 3.1 CrO,-carbon mixtures The DSC curve of a CrO,-carbon mixture (12.5% CrO, by weight) obtained at a heating rate of 10 KI min is shown on Fig. 1. An endothermal peak is seen between 196 and 206°C. It corresponds to the melting of Cr03 (m. p. 197°C). Immediately after melting a broad exothermal effect takes place up to about 460°C. In fact, two rather intense peaks respectively occurring at about 240 and 430°C can be seen. This general pattern is obtained for heating rates ranging from 10 to 40 K/min butthe peaks are slightly shifted to higher temperature with increasing heating rates. It is known that chromium trioxide is thermally unstable and decomposes upon heating into chromic oxide (Cr,03). The decomposition process leads to the intermediate formation of non-stoechiometric compounds like Cr50i3 and Crs01J8]. The DSC curve corresponding to pure Cr03 heated at 10 K/min is shown in Fig. 2. For comparison, the DSC curve of a mixture of CrO, and carbon, involving the same amount of CrO, than in the simple decomposition experiment is also shown. It is seen that the shape of the exothermal peak between 400 and 480°C for the CrO,-carbon mixture differs somewhat from the previous one (Fig. 1). In the absence of carbon the DSC curve of CrO, is markedly different. After the melting peak, a small exotherm is found around 350°C but a sharp endothermal effect can be seen at higher temperature (490°C). According to Bencovski et a/.[91 the exothermal effect corresponds to the formation of the non-stoechiometric oxide whereas the endotherm occurs when this intermediate oxide is decomposed into Cr,O,. These authors proposed therefore the two following reactions: 2 Cr03 (1) -
Cr204.68(s) + 0.66 0, (g)
(1)
Cr2Q4.68(s) -
CrZ03 (s)
(2)
10
+ 0.84 0, (g)
I
I
-700
200
I
I
300
400
TEMPERATURE
500
OC
Fig. 1. DSC curve of a CrOj-carbon mixture.
E
115
CrO, deposited on carbon
(240°C) occurs when CrO, has not yet undergone its decomposition whereas the second one would correspond to the reaction of the nonstoechiometric oxide with the carbon. From the energetic point of view the overall reaction between CrO, and carbon in the temperature range 200-500°C may be written as follows:
2 CrO, (1) + 1.5 C CrZOj (s) + 1.5 CO? (g) + AH -1
q’ 00
300
200
I
400
TEMPERATURE
500
i
600
"C
Fig. 2. DSC curves of CrOl (-) mixture (......),
and CrO,-carbon
In the presence of carbon the exothermal effect is more pronounced and certainly corresponds to the formation of carbon dioxide. In order to confirm this point, the gases released during heat treatment under vacuum of a mixture of carbon and CrO, (15.2% CrO, by weight) were analysed by mass spectrometry. No significant amount of 0, could be detected but an important evolution of CO, was measured. The rate of CO, evolution during a linear heating rate of the sample (2 K/min) is shown in Fig. 3 along with the one corresponding to carbon without CrOj. It is seen that the evolution of CO, immediately starts after CrO, has melted and shows two maxima respectively at about 240 and 440°C. Although the operating conditions of DSC and mass spectrometry experiments are not strictly the same (differences in gas pressures and in heating rates) the curves of Figs. 1 and 3 show that the occurrence of the exothermal effect compares well with the release feature of CO?. Thus it may be concluded that the exothermal effect corresponds to the oxidation of carbon by chromium compounds of oxidation state higher than +3. It is also interesting to note that the first exotherm peak
I
,
PO0
(3)
According to the thermodynamic data[lO], the heat of reaction AH at 200 and 500°C is equal to - 299.7 and - 332.1 kJ/mole CrO,, respectively. Considering the small change in AH and taking in account the complexity of the reaction between CrO, and carbon, an average value -316 kJ/mole CrO> has been taken for reaction (3). The heat released during a DSC run has been measured for various content of CrO, in the mixture and for various heating rates. The results expressed per gram of mixture are listed in Table 1. The fraction a, of chromium trioxide which would react according to reaction (3) is also indicated. Furthermore the amount of heat needed for the melting of CrO,, AH,,,, is also indicated. Finally, the fraction (Y, of CrO, detected in the sample from the melting endotherm has also been determined by taking the heat of melting equal to 15.7 kJ/mole Cr03. The results listed in Table 1 show that (Y,,,is close to 1. Taking in account the experimental error which results from the weighing of small amounts of CrO,-carbon mixtures, it is seen that the heat of melting corresponds well to its amount of CrO,. In contrast, there is a greater scatter for (Y,. It seems that the heating rate does not significantly affect (Y,since nearly 90% of the expected heat can be found for rates equal to 10, 20, and 40 K/min, respectively. Thus, in some cases, an almost quantitative conversion of CrO, into Cr,03 according to reaction (3) can be obtained. Lower values probably originate from the heterogeneous character of the reduction of CrO, which leads to the formation of solid chromium oxides on the carbon surface. The amount of CO1 evolved during heating from 200 to 500°C of a mixture containing 13.7% by weight of CrO, has been measured by mass spectrometry and is equal to 1.35 mmole CO,/g. In the same conditions the amount of CO? desorbed from the carbon support alone is equal to 0.29 * 0.02 mmoleig. Thus the oxidation of the carbon by CrO, produces 1.06 mmole CO? per gram of sample. Table 1 indicates that for a mixture containing 13.6 or 13.7% of CrO, the measured exotherm values are respectively equal to -375 and -403 Jig. Taking a mean value of -389 J/g the amount of CO2 produced according to equation (3) would be equal to 0.92 mmole COz/g. This value is in reasonable agreement with the amount of CO, by mass spectrometry (1.06 mmoleig).
_-------._.:“-:, ,.4----7---ZM
--_______~____.-. u
m
400
ml
TEMPERATUREOc
Fig. 3. CO2 evolution during heat treatment of a CrO,carbon mixture () and carbon alone (......).
116
P.
EHRBURGER
et al.
Table 1. DSC data of Cr03-carbon
mixtures
CrO, content (% by weight)
Heating rate (K/min)
AH, (J/g)
Cr03 fraction detected in exothermal peak a,
13.0 13.7 13.6 6.1 8.7 9.2 15.5 16.5
10 10 10 20 20 20 40 40
- 301 -403 - 375 - 165 -216 -210 -353 - 373
0.74 0.93 0.87 0.85 0.79 0.71 0.91 0.88
3.2 CrO, impregnated carbons The DSC curve of an impregnated sample (CrO, loading 5.9% by weight) corresponding to a heating rate of 40 Kimin. is shown in Fig. 4. After a small endothermal signal around 200°C only a weak but broad exotherm can be detected. In contrast to CrO,carbon mixtures, the impregnated samples do not exhibit two maxima in the exothermal signal. Nevertheless, the exotherm is found in the same interval of temperature 200-480°C. The heat AH, released during a DSC run at 4OK/min has been measured for various loading of CrO, and the results are shown in Table 2 as well as the corresponding value of cr,. Due to the weakness of the signal, the uncertainty of AH, is equal to +l J/g. It is seen that the fraction of CrO, detected by DSC is much smaller than for the CrO,-carbon mixtures. The evolution of CO, detected by mass spectrometry during heating of the sample with a loading of 5.9% CrO, by weight is shown in Fig. 5. The formation of CO, in the temperature interval 200-550°C is only slightly higher than the one resulting from the desorption of the carbon. The amount of CO* which originates from the oxidation reaction can be estimated to about 0.2 mmole/g. Taking a mean value of AH, equal to - 5.7 J/g, the corresponding amount of CO, would be equal to 0.13 mmole/g. Considering the uncertainties involved in these measurements, one may conclude that both techniques lead to sim-
CrOz fraction detected in endothermal peak Wn 18.5 21.6 20.4 9.9 12.9 14.4 23.9 24.6
0.91 1.00 0.95 1.03 0.94 0.99 0.98 0.95
ilar figure for the oxidation of the carbon by the supported CrO,. The behavior of impregnated samples is thus strikingly different from CrO,-carbon mixtures. Since the amount of evolved CO, is much less for impregnated carbon, one may say that a significant part of the deposited chromium trioxide has already been reduced. The sample impregnated with 5.9% by weight of CrO, has been submitted to weathering at 50°C and 90% RH. The DSC curves of the samples which have been conditioned and aged for 5 days are shown in Fig. 5. No endotherm can be detected for the aged carbon and the intensity of the exothermal signal has decreased as compared to the conditioned sample. The values of the exotherm, AH,, corresponding to different aging times are listed in Table 3. It is seen that even after conditioning of the sample, i.e., contacting it with water vapor for about 48 hours, there is a decrease of AH,. Moreover, the exotherm becomes smaller with aging time and has almost vanished after 15 days of weathering. 4. DISCUSSION
The above results show a considerable difference in the thermal behavior of CrO,-carbon mixtures and CrO, impregnated carbons. In the first case, the oxidation reaction proceeds through at least two regimes which correspond to the thermal stability of CrOj. In the second case, the exothermal reaction, although it takes place in the same interval of temperature occurs in a more steady way. Furthermore, the fraction of CrO, which is detected by DSC is higher when no solubilization step of the oxide is involved in the deposition procedure. Thus, the impregnation step affects the thermal behavior of the supported chromium trioxide. In order to substanTable 2. DSC data of CrO, impregnated carbons
30
s
200
c
300
CrO, fraction detected in exothermal peak a,
CrO, loading (% by weight) 400
TEMPERATURE
500
"C
Fig. 4. DSC curve of carbon impregnated with CrOj.
600 J
1.2 1.9 4.5 5.9
-12 ? 1 -142 1 -41+ 1 -562 1
0.32 0.23 0.28 0.30
CrO, deposited on carbon
117
3-
, Pm
300
zoo
400
500
t
TEMPERATURE
TEMPERATURE“C Fig. 5. CO, evolution during heat treatment of carbon im) and carbon alone (......). pregnated with CrOx (-
tiate this observation,
a CrOl-carbon mixture (13.5% CrO, by weight) was wetted with a drop of water in
the DSC crucible. Thereafter the sample was dried at 110°C for 15 min and submitted to DSC. The curve recorded at 20 Kimin is shown in Fig. 6. The shape
of the exotherm is no longer comparable to those corresponding of CrO,-carbon mixture but is more similar to those obtained for impregnated carbon. In addition, the enthalpy of reaction, AH, is equal to -60.2 J/g which indicate that about 14% of the initial present CrO, have reacted with the carbon support. This experiment has been repeated three times and the fraction of Cr03 detected by DSC, o,, represents about 12 to 15% of the initial loading. According to these results, it can be said that solubilized chromium trioxide oxidizes the carbon support. Since the contact time of water at room temperature was rather small in the previous wetting experiments of small amounts of CrO,-carbon mixture (Fig. 7), the oxidation reaction probably takes place during the drying step when water evaporates from the carbon micropores. Hence, a significant amount of CrO, which is highly soluble can come in contact with the carbon surface during the preparation of the sample by the impregnation technique. A similar process takes places during aging of the impregnated samples since the carbon micropores are filled with water and during storage at 50°C the oxidation reaction can slowly proceed. After drying, the deposit of chromium is composed of CrO, and of oxides of lower oxidation state. Afterwards, during the DSC run, the release of heat originates from Table 3. AH, as a function of weathering 90% RH Weathering time Untreated Conditioned 2 days 5 days 15 days
(lS”C, 90% RH)
"C
Fig. 6. DSC curves of carbon impregnated with Cr03: conditioned sample (upper curve), aged for 5 days (lower curve).
the reaction of the carbon with the remaining part of CrO, as well as with the lower oxides. This reaction is highly heterogeneous and all particular reaction steps are smeared out, thus leading to a continuous heat release and CO, evolution. 5. CONCLUSION
The thermal decomposition of CrO? on carbon has been studied in the case of mixtures of both components and for impregnated samples. The exothermal effect found in both cases is essentially due to the oxidation of the carbon support by chromium compound of oxidation state higher than 3. For CrO,carbon mixtures, two reaction steps could be evidenced after melting of the oxide. These steps are due to the formation of an intermediate non-stoechiometric chromium oxide. When CrO, is deposited on the carbon support by the impregnation technique, a significant fraction of Cr(VI) is reduced to a lower oxidation state, probably during the drying procedure. Aging of impregnated samples in conditions where the carbon micropores are filled with water also leads to a consumption of Cr( + 6) which may be complete after sufficient weathering time.
at 50% and
AH, (Jig) -562 1 -502 1 -35 It 2 -29 + 2 -5 -r- 2
-Boo I
200
300 1
400 3
TEMPERATURE
Fig. 7. DSC curve of CrO,-carbon addition.
500
60I
='C
mixture after water
P. EHRBURGER et al.
118 REFERENCES
1. V. R. Deitz, J. N. Robinson, and E. J. Poziomek, Carbon 13, 181 (1975). 2. R. Berg, A. H. Gulbrandsen, and G. A. Neefjes, Rev. Port. Quim. 19, 378 (1977). 3.H. P. Hiermstad and R. Berg, Am. Ind. HYK. ._ Assoc. .l. 5 (38j, 311 (1977). 4. P. Ehrburger, J. M. Henlin, and J. Lahaye, 17th Bienn. Conf. Carbon, Ext. Abstr. Prog., U. Kentucky, Lexington, KY, 118 (1985). 5.P. Ehrburger, J. M. Henlin, and J. Lahaye, J. Catalysis, 100,429(1986).
6.N. Bat, J. L. Hammarstrom, and A. Sacco Jr., Carbon 25 (4), 545 (1987). 7. P. Causton and B. McEnaney, Fuel 64, 1447 (1985). 8. A. Simon and T. Schmidt, Z. Anorg. Chem. 153, 191 (1926). 9. A. Bencovski, A. Caraman, D. Fatu, E. Popp, E. Segal, and Gh. Sherban, J. Thermal Analysis 5, 427 (1973).
10. Handbook of chemistry and physics (Edited by R. C. Weast), 59th ed. CRC Press, West Palm Beach, FI. (1979).
1.p~. 113-118, 1990
0008.6223190 $3.00+.00
Copyright 0 1990Pergamon Pressplc
Printedin Great Br~tam.
THERMAL
BEHAVIOR OF CHROMIUM DEPOSITED ON CARBON P.
EHRBURGER?,
J. DENTZER,
and J.
TRIOXIDE
LAHAYE
Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, Avenue du President Kennedy, 68200, Mulhouse, France and P. DZIEDZINL and R. FANGEAT Centre d’Etudes du Bouchet, 91710 VERT-LE-PETIT, (Received
France
1 March 1989; accepted in revised form 25 May 1989)
Abstract-The
thermal reaction of chromium trioxide with an activated carbon has been investigated by differential scanning calorimetry (DSC) and mass spectrometry analysis of the evolved gases. TWO types of deposition techniques have been used: A simple mixture of both components and the impregnation procedure. The analysis of the DSC curves in conjunction with mass spectrometry indicates that the observed exothermal effect is due to the oxidation of the carbon by chromium compounds of oxidation state higher than 3. In the case of Cr( + 6)carbon mixture an almost quantitative reaction of Cr( + 6) can be found in some cases. For CrO, impregnated carbon a significant fraction of the deposited Cr( +6) is consumed during the preparation of the sample. Aging in humid atmosphere also leads to a consumption of the deposited chromium trioxide by the carbon support. Key
Word-Activated
carbon, chromium oxides, thermal decomposition.
1. INTRODUCTION
heat treatment of cupric oxide supported on carbon have been studied by differential scanning calorimetry (DSC)[4,5]. It was found that CuO is reduced to Cu,O and thereafter to Cu when thermally treated between 220 and 550°C. The same authors also showed that during exposure to humidity at 50°C Cu( + 2) is partly reduced to Cu( + 1). The conversion of cupric oxide species into cuprous oxide during the thermal decomposition of whetlerite at 325°C has also been reported[6]. The effect of weathering of a whetlerite has also been investigated by X-ray photoelectron spectroscopy[7]. It was found that the oxidation state of the chromium depends on the extent of weathering and Cr( + 6) is specifically reduced to Cr( + 3) by the carbon supports. The purpose of the present paper is to determine more precisely the behavior of chromium trioxide in presence of carbon. The effect of the deposition technique of CrO, on the carbon support has been investigated. Furthermore, the interaction of CrO, with the activated carbon during heat treatment and after weathering has been studied in order to estimate the extent of reduction of Cr( + 6) by the support.
Chromium and copper compounds supported on activated carbons are used for the removal of toxic gases from the air. Basically, the overall destruction
process of the gaseous contaminants steps:
includes two
1. physical adsorption of the vapor in the carbon micropores and, 2. chemical reaction with the supported metallic compounds. It is also well known that the efficiency of such systems markedly decreases after heat treatment or after weathering. Since the activated carbon is usually impregnated with a solution containing hexavalent chromium and divalent copper compounds, a redox type of reaction between the deposited phase and the carbon support may explain the observed loss of efficiency. During the last two decades several attempts for the elucidation of the nature of the active species and of the mechanism of their altering during weathering have been made. Deitz et al. [l] reported that carbon cannot be considered as an inert support and found that commercial whetlerite contains Cr( + 6), Cu( + 2), Cr( + 3) and Cu( + 1) species as well. Furthermore, Berg et aL[2,3] noticed an increase of the crystallinity of the supported oxides after severe weathering conditions. More recently, the physico-chemical changes occurring during the tTo whom all correspondence
2.
EXPERIMENTAL
2.1 Materials An activated carbon (PICA) prepared from coconut shells was screened to a particle size between 1 and 1.6 mm. Its surface area measured by nitrogen adsorption at 77 K and calculated with the BET equation was equal to 1190 m2/g. The ash content
should be sent. 113
114
P. EHRBURGER et al.
of the carbon support was equal to 2% by weight. Chromium trioxide with a purity better than 95% was used. Since the interactions between two condensed phases usually depend on the development of their interface, two deposition techniques of CrO, on the carbon support were considered. In the first one, the carbon particles were finely ground in a mortar and the desired amount of CrO, was added by weighing. This type of samples will be called “CrO,-carbon mixture.” For DSC experiments, each sample was individually prepared by taking about 15 mg of carbon and adding the desired amount of CrO, (usually between 0.5 and 1.5 mg) with the use of a balance sensitive to 0.01 mg. In the second case, CrO, was deposited on the carbon by the impregnation technique. Therefore CrO, dissolved in water was added to the carbon particle until incipient wetness was reached. The water was then removed in a dry air stream and the temperature of the carbon sample never exceeded 110°C. The amount of chromium deposited on the carbon was measured by atomic absorption after ashing of the impregnated carbon at 800°C in a crucible. 2.2 Techniques 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. In order to minimize the endothermal effect due to desorption of water from the sample, a pretreatment at 150°C for 15 min was done in the DSC equipment. After cooling to room temperature enthalpy analysis was carried out between 25 and 550°C. In some experiments the nature and the amount of gases evolved during the heat treatment of the CrO,-carbon samples have been determined using a mass spectrometer (Balzers QMG 112). A 10 mg sample was placed in a reaction vessel which was outgassed to low4 Pa. The sample was then subjected to a temperature programmed desorption at a linear heating rate of 5 Kmin-’ under a dynamic pressure range of 2.10-3-10-4 Pa. Mainly water and carbon dioxide were detected. The amount of CO2 evolved during heating was measured using a similar procedure to the one described by Causton and McEnaney[8]. The calibration of the mass spectrometer was done with an absolute manometer (Barocel 1 torr pressure range) using ultrapure CO, (L’Air Liquide N45). 2.3 Aging procedure The impregnated samples were conditioned at 18°C and 90% relative humidity (RH) for about 2 days. In these conditions, the activated carbon adsorbs about 30% by weight of water. After conditioning, the samples were aged at 50°C and 90% RH for several days. Before testing by DSC, the aged carbons were dried in an oven at 110°C overnight.
3. RESULTS
The thermal behavior of CrO, in presence of carbon will be examined respectively in the case of a mixture and of impregnated samples. 3.1 CrO,-carbon mixtures The DSC curve of a CrO,-carbon mixture (12.5% CrO, by weight) obtained at a heating rate of 10 KI min is shown on Fig. 1. An endothermal peak is seen between 196 and 206°C. It corresponds to the melting of Cr03 (m. p. 197°C). Immediately after melting a broad exothermal effect takes place up to about 460°C. In fact, two rather intense peaks respectively occurring at about 240 and 430°C can be seen. This general pattern is obtained for heating rates ranging from 10 to 40 K/min butthe peaks are slightly shifted to higher temperature with increasing heating rates. It is known that chromium trioxide is thermally unstable and decomposes upon heating into chromic oxide (Cr,03). The decomposition process leads to the intermediate formation of non-stoechiometric compounds like Cr50i3 and Crs01J8]. The DSC curve corresponding to pure Cr03 heated at 10 K/min is shown in Fig. 2. For comparison, the DSC curve of a mixture of CrO, and carbon, involving the same amount of CrO, than in the simple decomposition experiment is also shown. It is seen that the shape of the exothermal peak between 400 and 480°C for the CrO,-carbon mixture differs somewhat from the previous one (Fig. 1). In the absence of carbon the DSC curve of CrO, is markedly different. After the melting peak, a small exotherm is found around 350°C but a sharp endothermal effect can be seen at higher temperature (490°C). According to Bencovski et a/.[91 the exothermal effect corresponds to the formation of the non-stoechiometric oxide whereas the endotherm occurs when this intermediate oxide is decomposed into Cr,O,. These authors proposed therefore the two following reactions: 2 Cr03 (1) -
Cr204.68(s) + 0.66 0, (g)
(1)
Cr2Q4.68(s) -
CrZ03 (s)
(2)
10
+ 0.84 0, (g)
I
I
-700
200
I
I
300
400
TEMPERATURE
500
OC
Fig. 1. DSC curve of a CrOj-carbon mixture.
E
115
CrO, deposited on carbon
(240°C) occurs when CrO, has not yet undergone its decomposition whereas the second one would correspond to the reaction of the nonstoechiometric oxide with the carbon. From the energetic point of view the overall reaction between CrO, and carbon in the temperature range 200-500°C may be written as follows:
2 CrO, (1) + 1.5 C CrZOj (s) + 1.5 CO? (g) + AH -1
q’ 00
300
200
I
400
TEMPERATURE
500
i
600
"C
Fig. 2. DSC curves of CrOl (-) mixture (......),
and CrO,-carbon
In the presence of carbon the exothermal effect is more pronounced and certainly corresponds to the formation of carbon dioxide. In order to confirm this point, the gases released during heat treatment under vacuum of a mixture of carbon and CrO, (15.2% CrO, by weight) were analysed by mass spectrometry. No significant amount of 0, could be detected but an important evolution of CO, was measured. The rate of CO, evolution during a linear heating rate of the sample (2 K/min) is shown in Fig. 3 along with the one corresponding to carbon without CrOj. It is seen that the evolution of CO, immediately starts after CrO, has melted and shows two maxima respectively at about 240 and 440°C. Although the operating conditions of DSC and mass spectrometry experiments are not strictly the same (differences in gas pressures and in heating rates) the curves of Figs. 1 and 3 show that the occurrence of the exothermal effect compares well with the release feature of CO?. Thus it may be concluded that the exothermal effect corresponds to the oxidation of carbon by chromium compounds of oxidation state higher than +3. It is also interesting to note that the first exotherm peak
I
,
PO0
(3)
According to the thermodynamic data[lO], the heat of reaction AH at 200 and 500°C is equal to - 299.7 and - 332.1 kJ/mole CrO,, respectively. Considering the small change in AH and taking in account the complexity of the reaction between CrO, and carbon, an average value -316 kJ/mole CrO> has been taken for reaction (3). The heat released during a DSC run has been measured for various content of CrO, in the mixture and for various heating rates. The results expressed per gram of mixture are listed in Table 1. The fraction a, of chromium trioxide which would react according to reaction (3) is also indicated. Furthermore the amount of heat needed for the melting of CrO,, AH,,,, is also indicated. Finally, the fraction (Y, of CrO, detected in the sample from the melting endotherm has also been determined by taking the heat of melting equal to 15.7 kJ/mole Cr03. The results listed in Table 1 show that (Y,,,is close to 1. Taking in account the experimental error which results from the weighing of small amounts of CrO,-carbon mixtures, it is seen that the heat of melting corresponds well to its amount of CrO,. In contrast, there is a greater scatter for (Y,. It seems that the heating rate does not significantly affect (Y,since nearly 90% of the expected heat can be found for rates equal to 10, 20, and 40 K/min, respectively. Thus, in some cases, an almost quantitative conversion of CrO, into Cr,03 according to reaction (3) can be obtained. Lower values probably originate from the heterogeneous character of the reduction of CrO, which leads to the formation of solid chromium oxides on the carbon surface. The amount of CO1 evolved during heating from 200 to 500°C of a mixture containing 13.7% by weight of CrO, has been measured by mass spectrometry and is equal to 1.35 mmole CO,/g. In the same conditions the amount of CO? desorbed from the carbon support alone is equal to 0.29 * 0.02 mmoleig. Thus the oxidation of the carbon by CrO, produces 1.06 mmole CO? per gram of sample. Table 1 indicates that for a mixture containing 13.6 or 13.7% of CrO, the measured exotherm values are respectively equal to -375 and -403 Jig. Taking a mean value of -389 J/g the amount of CO2 produced according to equation (3) would be equal to 0.92 mmole COz/g. This value is in reasonable agreement with the amount of CO, by mass spectrometry (1.06 mmoleig).
_-------._.:“-:, ,.4----7---ZM
--_______~____.-. u
m
400
ml
TEMPERATUREOc
Fig. 3. CO2 evolution during heat treatment of a CrO,carbon mixture () and carbon alone (......).
116
P.
EHRBURGER
et al.
Table 1. DSC data of Cr03-carbon
mixtures
CrO, content (% by weight)
Heating rate (K/min)
AH, (J/g)
Cr03 fraction detected in exothermal peak a,
13.0 13.7 13.6 6.1 8.7 9.2 15.5 16.5
10 10 10 20 20 20 40 40
- 301 -403 - 375 - 165 -216 -210 -353 - 373
0.74 0.93 0.87 0.85 0.79 0.71 0.91 0.88
3.2 CrO, impregnated carbons The DSC curve of an impregnated sample (CrO, loading 5.9% by weight) corresponding to a heating rate of 40 Kimin. is shown in Fig. 4. After a small endothermal signal around 200°C only a weak but broad exotherm can be detected. In contrast to CrO,carbon mixtures, the impregnated samples do not exhibit two maxima in the exothermal signal. Nevertheless, the exotherm is found in the same interval of temperature 200-480°C. The heat AH, released during a DSC run at 4OK/min has been measured for various loading of CrO, and the results are shown in Table 2 as well as the corresponding value of cr,. Due to the weakness of the signal, the uncertainty of AH, is equal to +l J/g. It is seen that the fraction of CrO, detected by DSC is much smaller than for the CrO,-carbon mixtures. The evolution of CO, detected by mass spectrometry during heating of the sample with a loading of 5.9% CrO, by weight is shown in Fig. 5. The formation of CO, in the temperature interval 200-550°C is only slightly higher than the one resulting from the desorption of the carbon. The amount of CO* which originates from the oxidation reaction can be estimated to about 0.2 mmole/g. Taking a mean value of AH, equal to - 5.7 J/g, the corresponding amount of CO, would be equal to 0.13 mmole/g. Considering the uncertainties involved in these measurements, one may conclude that both techniques lead to sim-
CrOz fraction detected in endothermal peak Wn 18.5 21.6 20.4 9.9 12.9 14.4 23.9 24.6
0.91 1.00 0.95 1.03 0.94 0.99 0.98 0.95
ilar figure for the oxidation of the carbon by the supported CrO,. The behavior of impregnated samples is thus strikingly different from CrO,-carbon mixtures. Since the amount of evolved CO, is much less for impregnated carbon, one may say that a significant part of the deposited chromium trioxide has already been reduced. The sample impregnated with 5.9% by weight of CrO, has been submitted to weathering at 50°C and 90% RH. The DSC curves of the samples which have been conditioned and aged for 5 days are shown in Fig. 5. No endotherm can be detected for the aged carbon and the intensity of the exothermal signal has decreased as compared to the conditioned sample. The values of the exotherm, AH,, corresponding to different aging times are listed in Table 3. It is seen that even after conditioning of the sample, i.e., contacting it with water vapor for about 48 hours, there is a decrease of AH,. Moreover, the exotherm becomes smaller with aging time and has almost vanished after 15 days of weathering. 4. DISCUSSION
The above results show a considerable difference in the thermal behavior of CrO,-carbon mixtures and CrO, impregnated carbons. In the first case, the oxidation reaction proceeds through at least two regimes which correspond to the thermal stability of CrOj. In the second case, the exothermal reaction, although it takes place in the same interval of temperature occurs in a more steady way. Furthermore, the fraction of CrO, which is detected by DSC is higher when no solubilization step of the oxide is involved in the deposition procedure. Thus, the impregnation step affects the thermal behavior of the supported chromium trioxide. In order to substanTable 2. DSC data of CrO, impregnated carbons
30
s
200
c
300
CrO, fraction detected in exothermal peak a,
CrO, loading (% by weight) 400
TEMPERATURE
500
"C
Fig. 4. DSC curve of carbon impregnated with CrOj.
600 J
1.2 1.9 4.5 5.9
-12 ? 1 -142 1 -41+ 1 -562 1
0.32 0.23 0.28 0.30
CrO, deposited on carbon
117
3-
, Pm
300
zoo
400
500
t
TEMPERATURE
TEMPERATURE“C Fig. 5. CO, evolution during heat treatment of carbon im) and carbon alone (......). pregnated with CrOx (-
tiate this observation,
a CrOl-carbon mixture (13.5% CrO, by weight) was wetted with a drop of water in
the DSC crucible. Thereafter the sample was dried at 110°C for 15 min and submitted to DSC. The curve recorded at 20 Kimin is shown in Fig. 6. The shape
of the exotherm is no longer comparable to those corresponding of CrO,-carbon mixture but is more similar to those obtained for impregnated carbon. In addition, the enthalpy of reaction, AH, is equal to -60.2 J/g which indicate that about 14% of the initial present CrO, have reacted with the carbon support. This experiment has been repeated three times and the fraction of Cr03 detected by DSC, o,, represents about 12 to 15% of the initial loading. According to these results, it can be said that solubilized chromium trioxide oxidizes the carbon support. Since the contact time of water at room temperature was rather small in the previous wetting experiments of small amounts of CrO,-carbon mixture (Fig. 7), the oxidation reaction probably takes place during the drying step when water evaporates from the carbon micropores. Hence, a significant amount of CrO, which is highly soluble can come in contact with the carbon surface during the preparation of the sample by the impregnation technique. A similar process takes places during aging of the impregnated samples since the carbon micropores are filled with water and during storage at 50°C the oxidation reaction can slowly proceed. After drying, the deposit of chromium is composed of CrO, and of oxides of lower oxidation state. Afterwards, during the DSC run, the release of heat originates from Table 3. AH, as a function of weathering 90% RH Weathering time Untreated Conditioned 2 days 5 days 15 days
(lS”C, 90% RH)
"C
Fig. 6. DSC curves of carbon impregnated with Cr03: conditioned sample (upper curve), aged for 5 days (lower curve).
the reaction of the carbon with the remaining part of CrO, as well as with the lower oxides. This reaction is highly heterogeneous and all particular reaction steps are smeared out, thus leading to a continuous heat release and CO, evolution. 5. CONCLUSION
The thermal decomposition of CrO? on carbon has been studied in the case of mixtures of both components and for impregnated samples. The exothermal effect found in both cases is essentially due to the oxidation of the carbon support by chromium compound of oxidation state higher than 3. For CrO,carbon mixtures, two reaction steps could be evidenced after melting of the oxide. These steps are due to the formation of an intermediate non-stoechiometric chromium oxide. When CrO, is deposited on the carbon support by the impregnation technique, a significant fraction of Cr(VI) is reduced to a lower oxidation state, probably during the drying procedure. Aging of impregnated samples in conditions where the carbon micropores are filled with water also leads to a consumption of Cr( + 6) which may be complete after sufficient weathering time.
at 50% and
AH, (Jig) -562 1 -502 1 -35 It 2 -29 + 2 -5 -r- 2
-Boo I
200
300 1
400 3
TEMPERATURE
Fig. 7. DSC curve of CrO,-carbon addition.
500
60I
='C
mixture after water
P. EHRBURGER et al.
118 REFERENCES
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