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Impregnation of activated carbon with chromium and copper salts: Effect of porosity and metal content
Carbon. Vol. 32, No. 7, pp. 1259-1265,1994 Copyright 0 1994Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008.622304 $6.00 + .OO
Pergamon 0008-6223(94)00084-O
IMPREGNATION OF ACTIVATED CARBON WITH CHROMIUM AND COPPER SALTS: EFFECT OF POROSITY AND METAL CONTENT M. MOLINA-SABIO, V. PEREZ, and F. RODR~XJEZ-REINOSO Departamento de Quimica Inorglnica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain (Received 15 November 1993; accepted in revised form 3 June 1994) Abstract-Activated
carbons with different pore size distributions were impregnated with ammoniacal solutions of Cr(VI) and Q(H) salts to give impregnated carbons with different metal loadings. Original and impregnated carbons were analyzed by means of physical adsorption, simultaneous DTA/TG, and temperature programmed desorption (TPD). Impregnation produces a partial blocking of the porosity of the carbon, which is a function of the original pore size distribution; thus, if the carbon has a relatively important mesoporosity contribution, the loss of microporosity is more pronounced than in essentially microporous carbons. When the impregnated carbons are heat treated in inert atmosphere, there is simultaneous reduction of the metals and oxidation of the carbon. The reduction products are Cr(II1) and Cu(O), the reductions taking place in two stages. The simultaneous oxidation of carbon produces oxygen surface groups, which are different from those in the original carbon. Key Words-Activated
carbon, Cu/Cr
impregnation,
I. INTRODUCTION Activated carbon is commonly used in the purification of gases and liquids, but in the case of toxic gases such as HCN and CNCl, with low molecular weight, low boiling point, and appreciable polarity, activated carbon is not useful since the absorption capacity is low and the adsorption process is reversible[ 11. However, when salts of copper and chromium are incorporated into the activated carbon, the efficiency of the removal process of toxic gases from air is greatly enhanced because a chemical reaction of the gases with the supported metals takes place after they are physically adsorbed on the micropores of the carbon. Information on the preparation of impregnated carbons and on the effect of impregnation on the porosity of the original carbon is scarce. Kloubek[2] studied N, adsorption and mercury porosimetry on the porosity of an activated carbon and the corresponding sample impregnated with Cu and Cr; they deduced that there seems to be a minimum of pore dimension (300 nm) in which the impregnant can be deposited in a layer, the walls of smaller pores being free of impregnant layer although the pores may be plugged. Bradley et al.[3], when comparing the porosity of a commercial ASC Whetlerite carbon (activated carbon impregnated with Cu, Cr, and Ag) and a similar nonimpregnated carbon, BPL, concluded that the loss in porosity of the carbon caused by impregnation is due to the fact that the most significant effects of impregnation occur in wide micropores and mesopores, leaving a more homogeneous pore structure in the impregnated carbon. The effectiveness of an impregnated carbon for the removal of toxic gases is significantly reduced by aging. Deactivation, which takes place by either hu-
Thermal
treatment.
midity or temperature, causes a reduction of the copper and chromium originally introduced in a high-oxidation state (Cr(V1) and Cu(II)) to lower-oxidation states, not effective for the removal of toxic gases[4]. Ehrburger et al. have analyzed the physicochemical changes produced upon heat treatment of activated carbons impregnated with copper[5], chromium[6], or both metals[7], by means of Differential Scanning Calorimetry (DSC), and concluded that an increase in temperature produces a redox reaction between the metal salts and the carbon. The initial species Cr(V1) and Cu(l1) are reduced to Cr(II1) and Cu(I) (the latter will reduce to metallic copper at a later stage) at the same time that the carbon support becomes oxidized. Similar results have been described by Bat et al.[S] when using thermogravimetry (TG) to examine the kinetics of decomposition of ASC Whetlerite; these authors suggested that at low temperatures (<217”(Z) the decomposition reaction involves the reduction of CuCrO, to CuCr,O, (Cr(V1) or Cr(II1)) with the subsequent oxidation of carbon to COz. At higher temperatures (325-405°C) the CuO species react with the carbon to yield Cu,O and CO,. If the activated carbon participates in the reduction process it can not be considered an inert species. In fact, some authors[9,10] have suggested that the stronger the interaction between the carbon and the metals species, the more favored will be the reduction process. One may then deduce that both the texture of the activated carbon and the metal loading will affect the redox process. One objective of this work is to analyze the modification in porosity of several activated carbons when impregnated with different amounts of Cr(VI) and Cu(I1) salts. A second objective is to study-by Differential Thermal Analysis (DTA/TG) and Temperature Programmed Desorption (TPD)-
1259
1260
M.
MOLINA-SABIO et al.
the behavior of these impregnated carbons when subjected to heat treatment.
2. EXPERIMENTAL
2.1 Adsorben ts Four activated carbons (N, P, Q, and R), with a 0.7-l .O mm particle size, have been prepared by activation with steam (at SOO-9OO’C)of carbonized olive stones; the experimental conditions of activation have been selected to produce activated carbons with different porous structure. Although impregnation is usually carried out either from excess solution or by incipient wetness, the latter has been selected in this work, as in previous similar laboratory studies by other authors[5,7,9]. Deposition of Cr(V1) and Cu(I1) has been carried out by impregnation (0.5 ml/g of carbon) with an ammoniacal solution (1:l) containing 0.08 g of (NH&CO3 and a variable amount of CrO, and basic copper carbonate, CuC03 .CU(OH)~. The volume of the solution has been selected as a function of the pore volume of the carbons (deduced from densities measured with water and mercury) in such a way as to ensure that the solution is retained in the porosity of the carbon in approximately 10 min (in fact, after this time the carbon had a dry appearance). The impregnated carbon was immediately dried in a fluidized bed of air at 110°C for 45 min and packed under vacuum until use. The metal loading of the impregnated carbons is expressed as weight of copper (6.2,7.2, and 8.1 grams) and chromium (1.6, 2.2, and 3.2 grams) per 100 g of carbon. The nomenclature used included the letter for the original carbon, followed by two numbers giving the approximate percentages of copper and chromium, respectively. Thus, sample N6-2 is carbon N containing 6.2 grams of copper and 2.2 grams of chromium per 100 grams of original carbon.
2.2 Porosity The adsorption isotherms of N2 at 77 K have been determined for all sample before and after impregnation in a conventional gravimetric system. Samples were outgassed under high vacuum for 12 h at 110°C. The adsorption isotherms have been used to calculate the volume of micropores by means of the Dubinin-Radushkevich (DR) equation[ll], Ve. and the CY~ methodH21, V, , and the volume of mesopores by subtracting the volume of micropores from the amount adsorbed at P/P,, = 0.95. The non-microporous surface area, S, , has been determined by application of the Q, method. The nonporous carbon used as reference for the CX, method was a carbon prepared from olive stones and heat treated in He at 2000°C to close the porosity [131. 2.3
Thermal treatment
Previous works[4,7,8] have shown that when an impregnated activated carbon is heated in inert atmo-
sphere there is a reduction of the Cr(VI) and Cu(I1) salts and an oxidation of the carbon. To study the changes produced during the heat treatment, two different techniques have been used: DTA/TG and TPD. 2.3.1 DTAITG. A simultaneous DTA/TG from Stanton Redcroft (STA780) has been used to obtain the plots of energy evolved as a function of temperature, from which the enthalpy of reduction of the metals salts is calculated. The reference bucket was loaded with 10 mg of calcined alumina, and the measurement bucket with 10 mg of finely grounded impregnated activated carbon. The selection of alumina instead of the unimpregnated carbon is based on the fact the former is more inert to the heat treatment than carbon in the calibration with different metals. All the experiments (heating rate of 20YYmin and nitrogen atmosphere) were carried out in three steps. The first step is preheating for 10 min. at 150°C to eliminate the water of the sample. The second step, after cooling to room temperature, is heating up to 6OO”C, and the third step, after cooling to room temperature, is heating again to 6OO”C,always under a nitrogen flow. The difference between the two curves of steps 2 and 3 gives a curve that allows the calculation of the enthalpy of the process, using the previous calibration data obtained with standard metals[l4]. Since there is not any change in weight during the third step for any of the samples studied, one may assume that the reactions of carbon with the metal salts were already completed. Blank runs with the three original activated carbons were also carried out, for reference purposes. 2.3.2 TPD. The desorption profiles of CO and COz were obtained by heating (S”C/min) 200 mg of dry, finely grounded carbon, under a flow (60 ml/min) of helium, from room temperature to 1100°C. The amounts evolved of CO and COz were measured with a gas chromatograph equipped with a thermal conductivity detector. Standard mixtures with different CO/He and COJHe ratios were used for calibration.
3. RESULTS
AND DISCUSSION
3.1 Changes in porosity Figure 1 includes the adsorption isotherms of Nz at 77 K on the four original activated carbons and the corresponding impregnated samples containing about 6% Cu and 2% Cr. Although all carbons are essentially microporous, the pore volume and the pore size distribution change from one carbon to another. Carbons N and P have narrower microporosity, with lower contribution from larger porosity, the isotherm approaching to a larger extent the type I isotherm typical of microporous carbons. Carbons Q and R exhibit isotherms in which not only the microporosity is wider and more heterogeneous (as denoted by the opening of the knee), but also the contribution from mesoporosity is important. A more quantitative comparison is possible through the data of Table 1. Thus, the two values of micropore volume (V, and V,) are very
1261
Impregnation of activated carbon with chromium and copper
n(mmol/g)
n(mmol/g)
,,v
25
5-
0 0.0
0.2
0.4
0.6
0.8
0’ 0.0
1.0
I
I
0.2
0.4
0.6
0.8
I
1.0
(b)
Fig. 1. N, (77 K) adsorption
isotherms
similar in carbon N and P, but are different in carbon Q and even more in carbon R. As shown in previous works[l5-171, this difference indicates a more heterogeneous micropore size distribution in carbons Q and R than in carbons N or P. This is so because for highly activated carbons (as is the case for Q and R) the DR plots are not linear in the whole range of relative pressure, with an upward deviation at relative pressures above 0.05. Consequently, the extrapolation of the linear portion underestimates the micropore volume, because the larger micropores are not included. This is not the case for the a, plots, and the extrapolated mi-
1. Porosity
I
P/P,
p/p, (a)
Table
I
parameters
of activated % (m2/g)
carbons v 0.x- v 0
Carbon
V0 (cm3/g)
V, (cm3/g)
N N6-1.5 N6-2 N6-3 N7-2 N8-2
0.36 0.26 0.26 0.27 0.26 0.25
0.37 0.28 0.28 0.28 0.27 0.28
47 32 36 36 39 36
0.07 0.05 0.06 0.06 0.06 0.06
P P6-2
0.44 0.35
0.46 0.36
45 32
0.07 0.05
&2
0.39 0.52
0.38 0.57
185 90
0.15 0.25
R R6-2
0.51 0.36
0.63 0.42
106 38
0.25 0.11
(cm3/g)
for original
and some impregnated
carbons
cropore volume V, is larger than V, for these activated carbons. When the adsorption isotherm is more similar to type I (more uniform microporosity and lower contribution from mesoporosity), the extrapolated micropore volumes are practically identical. Impregnation produces not only a decrease in the amount of N, adsorbed, but also a change in the shape of the adsorption isotherm (Fig. l), the change being a function of the porosity of the original carbon. Thus, the change in adsorption isotherm for carbons derived from carbons N and P is almost simply the decrease in the amount adsorbed, with only slight modification of the knee, corresponding to the microporosity. However, the change in carbons derived from Q and R is larger for both the amount adsorbed and the pore size distribution (related to the shape of the isotherm). Impregnation of carbons N and P does not much affect the mesoporosity (V0,9,-V,,), but produces a decrease in microporosity (see Table 1). However, for carbons Q and R, there is an important decrease in micropore volume, mesopore volume, and external surface area. On the other hand, the values of V, and V, become more similar for samples impregnated from carbons Q and R, whereas they were very different for the original carbons (this means that impregnation of carbons Q and R leads to samples with more uniform microporosity). All this means that impregnation of activated carbon produces a decrease in the volume of micropores and its mean pore size, rendering a more uniform and
M. MOLINA-SABIO et al.
1262
narrow microporosity, and a decrease in the volume of mesopores. In order to evaluate the loss in adsorption capacity of the carbons produced by impregnations, one has to consider the fact that impregnated carbons contain salts that do not contribute to the adsorption capacity, but do contribute to the weight in the adsorption isotherms. Assuming that the impregnant salts are mainly in the form of oxides of copper and chromium after the final drying step[4,8], the results have been corrected and the differences in the parameters defining the porous texture have been calculated (Table 2). There is in all cases a decrease in porosity produced by impregnation, although the values found are larger than those calculated if the salts fill exclusively the pores of the carbon. Thus, it is estimated that the volume of the impregnated salts occupy as an average about 0.03 cm3/g of the carbon (0.023 and 0.035 cm3/g for N6-1.5 and N6-3, respectively, the two extreme cases). This value is somewhat lower than the decrease produced in microporosity (measured by V, or V,), and even lower if we consider the decrease in VO,ss(addition of columns 2 and 5 of Table 2). Consequently, the salts must be deposited on the internal surface of the carbon, thus blocking part of it. This blocking is surprisingly similar for all carbons of series N, independent of the amount of metal salt introduced (in the range studied, the volume corresponding to the metal salt is also similar). This is not the case for the other carbons with different porosity. Thus, the decrease produced in the micropore volume is lower in carbons with narrower microporosity and lower contribution from mesoporosity. The decrease in V0 is around 12% in all cases of series N, 7% for P, 12% for Q, and 16% for R. The decrease in V, is 11% for N, 9% for P, 26% for Q, and 25% for R. In addition, the different decrease in the volume of micropores calculated for carbons Q and R (with more heterogeneous micropore size distribution) when using the o(, or DR method seems to indicate that a fraction of the wide microporosity has been occupied by the salts, blocking part of the narrow microporosity, with the result that impregnated carbons have a lower micropore volume and a more uniform micropore size (more similar values of Vc and V,). The deposition of salts also takes place in the large size pores (see val-
Table
2. Decrease
in porosity parameters impregnation AS, (m’/g)
produced
ues of V,.,,-V, and S, in Table 2), especially when the original carbon has both larger mesopore volume and nonmicroporous surface area (carbons Q and R), since both decrease about 40% and 55%, proportions much larger than in the other two carbons. 3.2 DTA/TG The pretreatment of the samples for 10 min at 150°C produces in all cases a small endothermal peak and a weight loss proportional to the peak area, changes due to the loss of adsorbed water. The differential plots of energy evolved as a function of temperature (in the 150-600°C range) for impregnated carbons with the same copper but different chromium content can be found in Fig. 2, and they show the presence of an exothermal process, similar to the one described in the literature[b, 181, caused by the reduction of Cr(VI) and Cu(I1) and the subsequent oxidation of carbon. Although the separation of peaks is not good, the shape of the plots, with a minimum at about 4OO”C, seems to differentiate two stages of the process. On the other hand, the shape of the plots is the same for all samples, but the energy values increase with increasing chromium content in the whole range of temperatures. Integration of the curves of Fig. 2 allows the determination of the energy involved in the process of thermal decomposition, and the energy involved in each stage of the thermal process has been determined by deconvolution of the peaks. The values obtained for the carbons (Table 3) show a proportionality between the energy deduced for the first peak AHi and the amount of chromium introduced (samples N6-1.5, N6-2 and N6-3), whereas AH2 keeps approximately constant. On the other hand, an increase in the copper loading (samples N6-2, N7-2, and N8-2) increasesalthough slightly- the energy evolved in the second peak, due to the small reduction enthalpy of CuO, AH, remaining constant. It seems that the reduction of Cr(VI) takes place at low temperatures as a consequence of its high oxidation capacity, and the reduction of Cu(I1) takes place at higher temperatures. Similar results were described by Ehrburger et al. [5-71 when studying the effect of thermal treatment of activated carbons impregnated with copper and/or chromium;
by
Carbon
Avo (cm3/g)
AV, (cm3/g)
N6-1.5 N6-2 N6-3 N7-2 N8-2
0.04 0.04 0.04 0.05 0.05
0.04 0.04 0.04 0.05 0.04
5 5 4
0.02 0.01 0.01
1
0.01
4
0.01
P6-2 Q6-2 R6-2
0.03 0.06 0.08
0.04 0.15 0.16
7 71 58
0.02 0.10 0.13
AVo.9,-Vo
(cm3/g)
Fig. 2. Differential
energy
plots (DTA).
Impregnation Table 3. Enthalpy
of reduction
of activated
carbon
with chromium
1263
and copper
for the carbon-metal salts
Carbon N6-1.5 N6-2 N6-3 N7-2 N8-2
103 149 194 149 150
40 43 45 48 59
P6-2 Q6-2
196 125
43 58
34 t
92’
0
’
’
100
200
I 300
’
’
400
500
600
T (“Cl
they concluded that the reduction process takes place in two stages. If one assumes that the two following reactions take place during the heat treatment (enthalpy values correspond to the temperatures of the maxima of the plots of Fig. 2): 4Cr03
+ 3C = 2Cr,O,
+ 3C02
AH 325 = 316 kJ/mol 2cuo
CrO,
+ c = 2cu + co2 AH 50,, = 45.2 kJ/mol
CuO,
one would obtain values of 97, 134, and 195 J/g of carbon for 1.5, 2, and 3% Cr, respectively, and 44, 51, and 58 J/g of carbon for samples with 6, 7, and 8% Cu. These values are similar to the experimental values of Table 3 (AH1 of samples N6-1.5, N6-2, and N6-3, and AH2 of samples N6-2, N7-2, and N8-2), and it is clearly deduced that there is reduction of Cr(V1) to Cr(II1) followed by the reduction of copper to the zerovalent state, the reduction being complete at 600°C. Table 3 lists the enthalpy values deduced for carbons having different porosity but a common impregnation (N6-2, P6-2, and 46-2); there are differences even though the same amount of chromium and copper are incorporated to the carbons. In general terms, the narrower the porosity, the higher the energy for the first peak AH, and the lower for the second AH,. These differences are probably due to kinetics factors, since we are dealing with solid state reactions, meaning that the interface between the carbon and the metal oxides plays a very important role, larger crystals being expected in carbons with wider porosity.
Fig. 3. Weight
changes measured experiments.
DTA/TG
sition of oxygen surface groups of the carbon. The plot for the impregnated carbon N6-2 is initially similar to carbon N, but the weight loss is larger at higher temperatures, indicating that in addition to the decomposition of the surface groups of the original carbon there is a weight loss due to the reduction of the metal salts and the subsequent oxidation of the carbon (the latter will produce mainly CO2 in the temperature range used). Thus, the total weight loss for carbon N is about 3.5%, whereas for carbon N6-2 it is 5.7%. In order to evaluate the weight changes due to only the redox process, the curve for carbon N has been subtracted from that of carbon N6-2, and the corresponding curve can be found in Fig. 3. This curve shows that the weight loss is not continuous in the whole range of temperatures, since the weight remains constant, in general terms, at 350-400°C and 500600°C. On the other hand, the largest/lowest weight changes are coincident with the largest/lowest changes in energy plotted in Fig. 2, and this confirms that at around 400°C there is a change in the predominance of the reduction for Cr(VI), at the same time that the reduction of Cu(I1) starts. Similar subtracted curves have been obtained for the rest of the carbons, from which the corresponding weight changes have been calculated (Table 4). The overall weight losses for carbons with different Cr content (N6-1.5, N6-2, and N6-3) are practically coincident and very low-around 2% -the weight loss being almost equally distributed in the two stages. However, the weight loss for carbons with different Cu content
Table 4. Weight loss (070) produced 3.3
during
by the redox process
TG
The use of simultaneous DTA/TG allows parallel monitoring of the weight changes, and Figure 3 includes the weight changes corresponding to carbon N (non-impregnated) and carbon N6-2. There is for carbon N a continuous weight loss with increasing temperature, more marked at 200 and 550°C. Since the carbon was originally prepared at high temperature and previously subjected to a drying step at 15O”C, the weight loss must correspond to the thermal decompo-
PI
p2
P,o,al
Carbon
200-400°C
400-600°C
200-600°C
N6-1.5
1.1 0.8 1.0 1.7 1.0 0.7 0.9
1.1 1.1 I .2 1.3 2.9 1.6 0
2.2 1.9 2.2 3.0 3.9 2.3 0.9
N6-2 N6-3 N7-2 N8-2 P6-2 Q6-2
M MOLINA-SABIO et al.
1264
(N6-2, N7-2, and NS-2) increases with copper loading, and is mainly concentrated in the second stage, corresponding to the reduction of Cu(II). There seems to be not much effect of porosity in the weight loss, as denoted by the results for N6-2 and P6-2. Only slight deviation from this general behavior is detected for carbon 46-2 (as with the enthalpy values, Table 3). Although these results are somewhat restricted by the limitations of the technique (in addition, part of the weight loss experienced up to about 250°C may be due to the decomposition of the metal complexes), the registered values are systematically lower than expected if during the redox process all the oxygen was liberated as CO*. Thus, a value of 3.2% would be expected for carbon N6-1.5 and 4.2% for carbons N6-3 and N8-2, an intermediate value being expected for the rest of the carbons. The results then seem to indicate that the product of the redox reactions may not be only COz but oxygen surface groups of different type, only a part of which is decomposed to CO2 in the temperature range used (up to 6OO”C), according to the proportion of Cr(VI) and Cu(I1) in the impregnant. The copper content seems to affect to a larger extent the proportion of COz evolved, since in the case of the sample with the largest copper content (N8-2), the registered value (3.9%) is only slightly lower than the expected one (4.2%). 3.4
TPD TPD in the temperature range 150-1100°C has been carried out on all samples of series N, and Fig. 4 includes the profiles corresponding to the evolution of CO and CO2 for some of these carbons. In general terms, the profiles are typical of activated carbons[l9]: CO2 evolves at lower temperatures up to 700-750°C and CO evolves above 600°C. Carbon N exhibits two COz peaks, corresponding to the so-called low and high temperature CO* groups[ 191, and only a peak for CO, with a maximum at 850°C. The profiles for the impregnated carbons have larger area, and this shows that the amount of oxygen groups evolved increases with impregnation.
15 -N
10
.” --
N6-1.5 N6-2
~
N6-3
On the other hand, these groups are not of the same type present in the original carbon. Thus, the CO2 groups of carbon N exhibit maxima at 350 and 65O”C, whereas there are three maxima for impregnated carbons N6-1.5, N6-2, and N6-3, only the one at 650°C being common to the original carbon; the other two peaks are at 200-300 and 410°C. The latter two temperatures are reasonably coincident with the temperature for the maximum weight loss (Fig. 3) and maximum energy evolved (Fig. 2). This confirms the existence of two relatively well differentiated stages in the transformation taking place during the thermal treatment of the carbon. Impregnation also produces an increase in the amount of groups evolving as CO, but these are not detected in the DTA/TG experiments, since they start to evolve at the maximum temperature used in DTA/TG, 600°C. The groups generated by the redox process evolve at a temperature similar to the original carbon N (850-900°C). The TPD profiles for the rest of the carbons are very similar to the plots of Fig. 4, and their integration permits the calculation of the amounts of oxygen (from CO and COz) evolved. The values given in Table 5 for the impregnated carbons have been calculated as the difference between the total amount and that corresponding to the original carbon N, and are expressed as weight of oxygen evolved (from CO or COz) per gram of carbon. It is observed that during the redox process the oxygen evolving as COz predominates. Since the reduction of chromium occurs at temperatures up to about 4OO”C, followed then by the reduction of copper, the amount of CO, evolved is caused by the reduction of both Cr(V1) and Cu(I1). On the other hand, the trend of amounts of oxygen evolved as CO* is coincident with that shown in Table 4 for the total weight loss (i.e., it does not change with increasing Cr content and it increases with Cu content). The trend for the amounts of oxygen evolved as CO is the opposite to that shown by CO*. Consequently, the oxidation of carbon with Cr(V1) and Cu(I1) seems to occur through the formation of oxygen surface groups that evolve as both CO and COz, although Cr(V1) leads to a larger contribution of thermally stable groups decomposed as CO only at high temperatures (>6OO”C). However, oxidation with Cu(I1) produces a larger contribution from groups decomposing as COz at lower temperatures (<6OO”C).
co
% E z
Table 5
a
200
400
6W
8oa
1000
T (“C) Fig. 4. TPD profiles
for samples
of series N.
1200
5. Oxygen
(evolved as CO and COz) introduced impregnation
by
Carbon
as CO1 (mg/g)
as CO (mg/g)
N6-1.5 N6-2 N6-3 N7-2 N8-2
16 17 17 20 23
11 14 17 14 17
Impregnation
of activated
carbon
with chromium
of activated
carbon
with
salts
of
Cr(VI) and Cu(II) produces a decrease not only in the volume of micro and mesopores, but also in the mean micropore size, the resulting impregnated carbons exhibiting a microporosity that is narrower and more uniform than in the original carbon. The changes in porosity are a function of the original pore size distribution of the activated carbon. In general terms, the largest decrease in porosity is seen in carbons with more heterogeneous microporosity and an important contribution of mesoporosity. If the contribution of meso and macroporosity to the total porosity of the carbon is small, the only effect of impregnation is a decrease in microporosity. The extent of the porosity decrease produced by the impregnating salts indicates that these produce a partial blocking of porosity. 2. When the impregnated carbons are heat treated in an inert atmosphere, the reduction of Cr(V1) and Cu(II) takes place with a simultaneous oxidation of the carbon. The enthalpies involved in the process indicate that the reduction products are Cr(II1) and Cu(O), the reduction taking place in stages. There is first the reduction of Cr(VI), followed by the reduction of Cu(II), the process ending at around 600°C. 3. The reduction of the metal oxides is simultaneous to the oxidation of carbon, leading to the formation of oxygen surface groups different from those present in the original carbon. It general terms, the reduction of Cu(I1) leads to a larger contribution of groups that decompose thermally to CO2 at low temperature (<6OO”C), and the reduction of Cr(V1) leads essentially to the formation of groups that decompose to CO at higher temperature (>6OO”C). Acknowledgemenr-Financial S.A. and DGICYT (Project acknowledged.
1265
REFERENCES
4. CONCLUSIONS 1. Impregnation
and copper
support from Filtros Energia, No. PB91-0747)
is gratefully
1. R. C. Bansal, J.B. Donnet, and H. F. Stoeckli, In Active Carbon, pp. 266-269. Marcel Dekker, New York (1988). 2. J. Kloubek, Carbon 9, 303 (1981). and A. Turner. Exten. 3. R. H. Bradley, I. L. Clackson, Abstr. 20th Bienn. Conf. Carbon, Amer. Carbon Sot., p. 90 (1991). and A. Sacco, Jr., J. Cutal. 112, 267 4. J. L. Hammarstrom (1988). 5. P. Ehrburger, J. M. Henlin, and J. Lahaye, J. Cutal. 100, 429 (1986). J. Dentzer, J. Lahaye, P. Dziedzinl, and 6. P. Ehrburger, R. Fangeat, Carbon 28, 113 (1990). J. Lahaye, P. Dziedzinl and R. Fangeat, 7. P. Ehrburger, Carbon 29, 297 (1991). and A. Sacco, Jr., Carbon 8. N. Bat, J. L. Hammarstrom, 25, 545 (1987). 9. V. R. Deitz, J. N. Robinson, and E. J. Poziomek, Carbon 13, 181 (1975). 10. M. M. Ross, R. J. Colton, and V. R. Deitz, Carbon 27,
492 (1989). 11. M. M. Dubinin, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 2, pp. 84-85. Marcel Dekker, New York (1966). 12. S. J. Gregg and K. S. W. Sing, In Adsorption Surface Area and Porosity (2nd edition), pp. 94-100. Academic Press, London (1982). 13. F. Rodriguez Reinoso, J. M. Martin Martinez, C. Prado Burguete, and B. McEnaney, J. Phys. Chem. 91, 515 (1987). 14. M. I. Pope and M. D. Judd, In Differential Thermal Analysis, pp. 36-37. Heyden, London (1980). 15. F. Rodriguez Reinoso, J. Garrido, J. M. Martin Martinez, M. Molina Sabio, and R. Torregrosa, Carbon 27,
23 (1989). 16. J. M. Martin Martinez, M. Molina Sabio, F. Rodriguez Reinoso, and R. Torregrosa, Fuel 68, 204 (1989). 17. F. Rodriguez Reinoso and A. Linares Solano, In Chetnistry and Physics of Carbon (Edited by P.A. Thrower), Vol. 21, p. 120. Marcel Dekker, New York (1989). 18. P. Ehrburger, J. Lahaye, P. Dziedzini, and R. Fangeat, Exten. Abstr. 19th Bienn. Conf. Carbon, Amer. Carbon Sot. p. 36 (1989). 19. Y. Otake and R. Cl. Jenkins, Carbon 31, 109 (1993).
Pergamon 0008-6223(94)00084-O
IMPREGNATION OF ACTIVATED CARBON WITH CHROMIUM AND COPPER SALTS: EFFECT OF POROSITY AND METAL CONTENT M. MOLINA-SABIO, V. PEREZ, and F. RODR~XJEZ-REINOSO Departamento de Quimica Inorglnica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain (Received 15 November 1993; accepted in revised form 3 June 1994) Abstract-Activated
carbons with different pore size distributions were impregnated with ammoniacal solutions of Cr(VI) and Q(H) salts to give impregnated carbons with different metal loadings. Original and impregnated carbons were analyzed by means of physical adsorption, simultaneous DTA/TG, and temperature programmed desorption (TPD). Impregnation produces a partial blocking of the porosity of the carbon, which is a function of the original pore size distribution; thus, if the carbon has a relatively important mesoporosity contribution, the loss of microporosity is more pronounced than in essentially microporous carbons. When the impregnated carbons are heat treated in inert atmosphere, there is simultaneous reduction of the metals and oxidation of the carbon. The reduction products are Cr(II1) and Cu(O), the reductions taking place in two stages. The simultaneous oxidation of carbon produces oxygen surface groups, which are different from those in the original carbon. Key Words-Activated
carbon, Cu/Cr
impregnation,
I. INTRODUCTION Activated carbon is commonly used in the purification of gases and liquids, but in the case of toxic gases such as HCN and CNCl, with low molecular weight, low boiling point, and appreciable polarity, activated carbon is not useful since the absorption capacity is low and the adsorption process is reversible[ 11. However, when salts of copper and chromium are incorporated into the activated carbon, the efficiency of the removal process of toxic gases from air is greatly enhanced because a chemical reaction of the gases with the supported metals takes place after they are physically adsorbed on the micropores of the carbon. Information on the preparation of impregnated carbons and on the effect of impregnation on the porosity of the original carbon is scarce. Kloubek[2] studied N, adsorption and mercury porosimetry on the porosity of an activated carbon and the corresponding sample impregnated with Cu and Cr; they deduced that there seems to be a minimum of pore dimension (300 nm) in which the impregnant can be deposited in a layer, the walls of smaller pores being free of impregnant layer although the pores may be plugged. Bradley et al.[3], when comparing the porosity of a commercial ASC Whetlerite carbon (activated carbon impregnated with Cu, Cr, and Ag) and a similar nonimpregnated carbon, BPL, concluded that the loss in porosity of the carbon caused by impregnation is due to the fact that the most significant effects of impregnation occur in wide micropores and mesopores, leaving a more homogeneous pore structure in the impregnated carbon. The effectiveness of an impregnated carbon for the removal of toxic gases is significantly reduced by aging. Deactivation, which takes place by either hu-
Thermal
treatment.
midity or temperature, causes a reduction of the copper and chromium originally introduced in a high-oxidation state (Cr(V1) and Cu(II)) to lower-oxidation states, not effective for the removal of toxic gases[4]. Ehrburger et al. have analyzed the physicochemical changes produced upon heat treatment of activated carbons impregnated with copper[5], chromium[6], or both metals[7], by means of Differential Scanning Calorimetry (DSC), and concluded that an increase in temperature produces a redox reaction between the metal salts and the carbon. The initial species Cr(V1) and Cu(l1) are reduced to Cr(II1) and Cu(I) (the latter will reduce to metallic copper at a later stage) at the same time that the carbon support becomes oxidized. Similar results have been described by Bat et al.[S] when using thermogravimetry (TG) to examine the kinetics of decomposition of ASC Whetlerite; these authors suggested that at low temperatures (<217”(Z) the decomposition reaction involves the reduction of CuCrO, to CuCr,O, (Cr(V1) or Cr(II1)) with the subsequent oxidation of carbon to COz. At higher temperatures (325-405°C) the CuO species react with the carbon to yield Cu,O and CO,. If the activated carbon participates in the reduction process it can not be considered an inert species. In fact, some authors[9,10] have suggested that the stronger the interaction between the carbon and the metals species, the more favored will be the reduction process. One may then deduce that both the texture of the activated carbon and the metal loading will affect the redox process. One objective of this work is to analyze the modification in porosity of several activated carbons when impregnated with different amounts of Cr(VI) and Cu(I1) salts. A second objective is to study-by Differential Thermal Analysis (DTA/TG) and Temperature Programmed Desorption (TPD)-
1259
1260
M.
MOLINA-SABIO et al.
the behavior of these impregnated carbons when subjected to heat treatment.
2. EXPERIMENTAL
2.1 Adsorben ts Four activated carbons (N, P, Q, and R), with a 0.7-l .O mm particle size, have been prepared by activation with steam (at SOO-9OO’C)of carbonized olive stones; the experimental conditions of activation have been selected to produce activated carbons with different porous structure. Although impregnation is usually carried out either from excess solution or by incipient wetness, the latter has been selected in this work, as in previous similar laboratory studies by other authors[5,7,9]. Deposition of Cr(V1) and Cu(I1) has been carried out by impregnation (0.5 ml/g of carbon) with an ammoniacal solution (1:l) containing 0.08 g of (NH&CO3 and a variable amount of CrO, and basic copper carbonate, CuC03 .CU(OH)~. The volume of the solution has been selected as a function of the pore volume of the carbons (deduced from densities measured with water and mercury) in such a way as to ensure that the solution is retained in the porosity of the carbon in approximately 10 min (in fact, after this time the carbon had a dry appearance). The impregnated carbon was immediately dried in a fluidized bed of air at 110°C for 45 min and packed under vacuum until use. The metal loading of the impregnated carbons is expressed as weight of copper (6.2,7.2, and 8.1 grams) and chromium (1.6, 2.2, and 3.2 grams) per 100 g of carbon. The nomenclature used included the letter for the original carbon, followed by two numbers giving the approximate percentages of copper and chromium, respectively. Thus, sample N6-2 is carbon N containing 6.2 grams of copper and 2.2 grams of chromium per 100 grams of original carbon.
2.2 Porosity The adsorption isotherms of N2 at 77 K have been determined for all sample before and after impregnation in a conventional gravimetric system. Samples were outgassed under high vacuum for 12 h at 110°C. The adsorption isotherms have been used to calculate the volume of micropores by means of the Dubinin-Radushkevich (DR) equation[ll], Ve. and the CY~ methodH21, V, , and the volume of mesopores by subtracting the volume of micropores from the amount adsorbed at P/P,, = 0.95. The non-microporous surface area, S, , has been determined by application of the Q, method. The nonporous carbon used as reference for the CX, method was a carbon prepared from olive stones and heat treated in He at 2000°C to close the porosity [131. 2.3
Thermal treatment
Previous works[4,7,8] have shown that when an impregnated activated carbon is heated in inert atmo-
sphere there is a reduction of the Cr(VI) and Cu(I1) salts and an oxidation of the carbon. To study the changes produced during the heat treatment, two different techniques have been used: DTA/TG and TPD. 2.3.1 DTAITG. A simultaneous DTA/TG from Stanton Redcroft (STA780) has been used to obtain the plots of energy evolved as a function of temperature, from which the enthalpy of reduction of the metals salts is calculated. The reference bucket was loaded with 10 mg of calcined alumina, and the measurement bucket with 10 mg of finely grounded impregnated activated carbon. The selection of alumina instead of the unimpregnated carbon is based on the fact the former is more inert to the heat treatment than carbon in the calibration with different metals. All the experiments (heating rate of 20YYmin and nitrogen atmosphere) were carried out in three steps. The first step is preheating for 10 min. at 150°C to eliminate the water of the sample. The second step, after cooling to room temperature, is heating up to 6OO”C, and the third step, after cooling to room temperature, is heating again to 6OO”C,always under a nitrogen flow. The difference between the two curves of steps 2 and 3 gives a curve that allows the calculation of the enthalpy of the process, using the previous calibration data obtained with standard metals[l4]. Since there is not any change in weight during the third step for any of the samples studied, one may assume that the reactions of carbon with the metal salts were already completed. Blank runs with the three original activated carbons were also carried out, for reference purposes. 2.3.2 TPD. The desorption profiles of CO and COz were obtained by heating (S”C/min) 200 mg of dry, finely grounded carbon, under a flow (60 ml/min) of helium, from room temperature to 1100°C. The amounts evolved of CO and COz were measured with a gas chromatograph equipped with a thermal conductivity detector. Standard mixtures with different CO/He and COJHe ratios were used for calibration.
3. RESULTS
AND DISCUSSION
3.1 Changes in porosity Figure 1 includes the adsorption isotherms of Nz at 77 K on the four original activated carbons and the corresponding impregnated samples containing about 6% Cu and 2% Cr. Although all carbons are essentially microporous, the pore volume and the pore size distribution change from one carbon to another. Carbons N and P have narrower microporosity, with lower contribution from larger porosity, the isotherm approaching to a larger extent the type I isotherm typical of microporous carbons. Carbons Q and R exhibit isotherms in which not only the microporosity is wider and more heterogeneous (as denoted by the opening of the knee), but also the contribution from mesoporosity is important. A more quantitative comparison is possible through the data of Table 1. Thus, the two values of micropore volume (V, and V,) are very
1261
Impregnation of activated carbon with chromium and copper
n(mmol/g)
n(mmol/g)
,,v
25
5-
0 0.0
0.2
0.4
0.6
0.8
0’ 0.0
1.0
I
I
0.2
0.4
0.6
0.8
I
1.0
(b)
Fig. 1. N, (77 K) adsorption
isotherms
similar in carbon N and P, but are different in carbon Q and even more in carbon R. As shown in previous works[l5-171, this difference indicates a more heterogeneous micropore size distribution in carbons Q and R than in carbons N or P. This is so because for highly activated carbons (as is the case for Q and R) the DR plots are not linear in the whole range of relative pressure, with an upward deviation at relative pressures above 0.05. Consequently, the extrapolation of the linear portion underestimates the micropore volume, because the larger micropores are not included. This is not the case for the a, plots, and the extrapolated mi-
1. Porosity
I
P/P,
p/p, (a)
Table
I
parameters
of activated % (m2/g)
carbons v 0.x- v 0
Carbon
V0 (cm3/g)
V, (cm3/g)
N N6-1.5 N6-2 N6-3 N7-2 N8-2
0.36 0.26 0.26 0.27 0.26 0.25
0.37 0.28 0.28 0.28 0.27 0.28
47 32 36 36 39 36
0.07 0.05 0.06 0.06 0.06 0.06
P P6-2
0.44 0.35
0.46 0.36
45 32
0.07 0.05
&2
0.39 0.52
0.38 0.57
185 90
0.15 0.25
R R6-2
0.51 0.36
0.63 0.42
106 38
0.25 0.11
(cm3/g)
for original
and some impregnated
carbons
cropore volume V, is larger than V, for these activated carbons. When the adsorption isotherm is more similar to type I (more uniform microporosity and lower contribution from mesoporosity), the extrapolated micropore volumes are practically identical. Impregnation produces not only a decrease in the amount of N, adsorbed, but also a change in the shape of the adsorption isotherm (Fig. l), the change being a function of the porosity of the original carbon. Thus, the change in adsorption isotherm for carbons derived from carbons N and P is almost simply the decrease in the amount adsorbed, with only slight modification of the knee, corresponding to the microporosity. However, the change in carbons derived from Q and R is larger for both the amount adsorbed and the pore size distribution (related to the shape of the isotherm). Impregnation of carbons N and P does not much affect the mesoporosity (V0,9,-V,,), but produces a decrease in microporosity (see Table 1). However, for carbons Q and R, there is an important decrease in micropore volume, mesopore volume, and external surface area. On the other hand, the values of V, and V, become more similar for samples impregnated from carbons Q and R, whereas they were very different for the original carbons (this means that impregnation of carbons Q and R leads to samples with more uniform microporosity). All this means that impregnation of activated carbon produces a decrease in the volume of micropores and its mean pore size, rendering a more uniform and
M. MOLINA-SABIO et al.
1262
narrow microporosity, and a decrease in the volume of mesopores. In order to evaluate the loss in adsorption capacity of the carbons produced by impregnations, one has to consider the fact that impregnated carbons contain salts that do not contribute to the adsorption capacity, but do contribute to the weight in the adsorption isotherms. Assuming that the impregnant salts are mainly in the form of oxides of copper and chromium after the final drying step[4,8], the results have been corrected and the differences in the parameters defining the porous texture have been calculated (Table 2). There is in all cases a decrease in porosity produced by impregnation, although the values found are larger than those calculated if the salts fill exclusively the pores of the carbon. Thus, it is estimated that the volume of the impregnated salts occupy as an average about 0.03 cm3/g of the carbon (0.023 and 0.035 cm3/g for N6-1.5 and N6-3, respectively, the two extreme cases). This value is somewhat lower than the decrease produced in microporosity (measured by V, or V,), and even lower if we consider the decrease in VO,ss(addition of columns 2 and 5 of Table 2). Consequently, the salts must be deposited on the internal surface of the carbon, thus blocking part of it. This blocking is surprisingly similar for all carbons of series N, independent of the amount of metal salt introduced (in the range studied, the volume corresponding to the metal salt is also similar). This is not the case for the other carbons with different porosity. Thus, the decrease produced in the micropore volume is lower in carbons with narrower microporosity and lower contribution from mesoporosity. The decrease in V0 is around 12% in all cases of series N, 7% for P, 12% for Q, and 16% for R. The decrease in V, is 11% for N, 9% for P, 26% for Q, and 25% for R. In addition, the different decrease in the volume of micropores calculated for carbons Q and R (with more heterogeneous micropore size distribution) when using the o(, or DR method seems to indicate that a fraction of the wide microporosity has been occupied by the salts, blocking part of the narrow microporosity, with the result that impregnated carbons have a lower micropore volume and a more uniform micropore size (more similar values of Vc and V,). The deposition of salts also takes place in the large size pores (see val-
Table
2. Decrease
in porosity parameters impregnation AS, (m’/g)
produced
ues of V,.,,-V, and S, in Table 2), especially when the original carbon has both larger mesopore volume and nonmicroporous surface area (carbons Q and R), since both decrease about 40% and 55%, proportions much larger than in the other two carbons. 3.2 DTA/TG The pretreatment of the samples for 10 min at 150°C produces in all cases a small endothermal peak and a weight loss proportional to the peak area, changes due to the loss of adsorbed water. The differential plots of energy evolved as a function of temperature (in the 150-600°C range) for impregnated carbons with the same copper but different chromium content can be found in Fig. 2, and they show the presence of an exothermal process, similar to the one described in the literature[b, 181, caused by the reduction of Cr(VI) and Cu(I1) and the subsequent oxidation of carbon. Although the separation of peaks is not good, the shape of the plots, with a minimum at about 4OO”C, seems to differentiate two stages of the process. On the other hand, the shape of the plots is the same for all samples, but the energy values increase with increasing chromium content in the whole range of temperatures. Integration of the curves of Fig. 2 allows the determination of the energy involved in the process of thermal decomposition, and the energy involved in each stage of the thermal process has been determined by deconvolution of the peaks. The values obtained for the carbons (Table 3) show a proportionality between the energy deduced for the first peak AHi and the amount of chromium introduced (samples N6-1.5, N6-2 and N6-3), whereas AH2 keeps approximately constant. On the other hand, an increase in the copper loading (samples N6-2, N7-2, and N8-2) increasesalthough slightly- the energy evolved in the second peak, due to the small reduction enthalpy of CuO, AH, remaining constant. It seems that the reduction of Cr(VI) takes place at low temperatures as a consequence of its high oxidation capacity, and the reduction of Cu(I1) takes place at higher temperatures. Similar results were described by Ehrburger et al. [5-71 when studying the effect of thermal treatment of activated carbons impregnated with copper and/or chromium;
by
Carbon
Avo (cm3/g)
AV, (cm3/g)
N6-1.5 N6-2 N6-3 N7-2 N8-2
0.04 0.04 0.04 0.05 0.05
0.04 0.04 0.04 0.05 0.04
5 5 4
0.02 0.01 0.01
1
0.01
4
0.01
P6-2 Q6-2 R6-2
0.03 0.06 0.08
0.04 0.15 0.16
7 71 58
0.02 0.10 0.13
AVo.9,-Vo
(cm3/g)
Fig. 2. Differential
energy
plots (DTA).
Impregnation Table 3. Enthalpy
of reduction
of activated
carbon
with chromium
1263
and copper
for the carbon-metal salts
Carbon N6-1.5 N6-2 N6-3 N7-2 N8-2
103 149 194 149 150
40 43 45 48 59
P6-2 Q6-2
196 125
43 58
34 t
92’
0
’
’
100
200
I 300
’
’
400
500
600
T (“Cl
they concluded that the reduction process takes place in two stages. If one assumes that the two following reactions take place during the heat treatment (enthalpy values correspond to the temperatures of the maxima of the plots of Fig. 2): 4Cr03
+ 3C = 2Cr,O,
+ 3C02
AH 325 = 316 kJ/mol 2cuo
CrO,
+ c = 2cu + co2 AH 50,, = 45.2 kJ/mol
CuO,
one would obtain values of 97, 134, and 195 J/g of carbon for 1.5, 2, and 3% Cr, respectively, and 44, 51, and 58 J/g of carbon for samples with 6, 7, and 8% Cu. These values are similar to the experimental values of Table 3 (AH1 of samples N6-1.5, N6-2, and N6-3, and AH2 of samples N6-2, N7-2, and N8-2), and it is clearly deduced that there is reduction of Cr(V1) to Cr(II1) followed by the reduction of copper to the zerovalent state, the reduction being complete at 600°C. Table 3 lists the enthalpy values deduced for carbons having different porosity but a common impregnation (N6-2, P6-2, and 46-2); there are differences even though the same amount of chromium and copper are incorporated to the carbons. In general terms, the narrower the porosity, the higher the energy for the first peak AH, and the lower for the second AH,. These differences are probably due to kinetics factors, since we are dealing with solid state reactions, meaning that the interface between the carbon and the metal oxides plays a very important role, larger crystals being expected in carbons with wider porosity.
Fig. 3. Weight
changes measured experiments.
DTA/TG
sition of oxygen surface groups of the carbon. The plot for the impregnated carbon N6-2 is initially similar to carbon N, but the weight loss is larger at higher temperatures, indicating that in addition to the decomposition of the surface groups of the original carbon there is a weight loss due to the reduction of the metal salts and the subsequent oxidation of the carbon (the latter will produce mainly CO2 in the temperature range used). Thus, the total weight loss for carbon N is about 3.5%, whereas for carbon N6-2 it is 5.7%. In order to evaluate the weight changes due to only the redox process, the curve for carbon N has been subtracted from that of carbon N6-2, and the corresponding curve can be found in Fig. 3. This curve shows that the weight loss is not continuous in the whole range of temperatures, since the weight remains constant, in general terms, at 350-400°C and 500600°C. On the other hand, the largest/lowest weight changes are coincident with the largest/lowest changes in energy plotted in Fig. 2, and this confirms that at around 400°C there is a change in the predominance of the reduction for Cr(VI), at the same time that the reduction of Cu(I1) starts. Similar subtracted curves have been obtained for the rest of the carbons, from which the corresponding weight changes have been calculated (Table 4). The overall weight losses for carbons with different Cr content (N6-1.5, N6-2, and N6-3) are practically coincident and very low-around 2% -the weight loss being almost equally distributed in the two stages. However, the weight loss for carbons with different Cu content
Table 4. Weight loss (070) produced 3.3
during
by the redox process
TG
The use of simultaneous DTA/TG allows parallel monitoring of the weight changes, and Figure 3 includes the weight changes corresponding to carbon N (non-impregnated) and carbon N6-2. There is for carbon N a continuous weight loss with increasing temperature, more marked at 200 and 550°C. Since the carbon was originally prepared at high temperature and previously subjected to a drying step at 15O”C, the weight loss must correspond to the thermal decompo-
PI
p2
P,o,al
Carbon
200-400°C
400-600°C
200-600°C
N6-1.5
1.1 0.8 1.0 1.7 1.0 0.7 0.9
1.1 1.1 I .2 1.3 2.9 1.6 0
2.2 1.9 2.2 3.0 3.9 2.3 0.9
N6-2 N6-3 N7-2 N8-2 P6-2 Q6-2
M MOLINA-SABIO et al.
1264
(N6-2, N7-2, and NS-2) increases with copper loading, and is mainly concentrated in the second stage, corresponding to the reduction of Cu(II). There seems to be not much effect of porosity in the weight loss, as denoted by the results for N6-2 and P6-2. Only slight deviation from this general behavior is detected for carbon 46-2 (as with the enthalpy values, Table 3). Although these results are somewhat restricted by the limitations of the technique (in addition, part of the weight loss experienced up to about 250°C may be due to the decomposition of the metal complexes), the registered values are systematically lower than expected if during the redox process all the oxygen was liberated as CO*. Thus, a value of 3.2% would be expected for carbon N6-1.5 and 4.2% for carbons N6-3 and N8-2, an intermediate value being expected for the rest of the carbons. The results then seem to indicate that the product of the redox reactions may not be only COz but oxygen surface groups of different type, only a part of which is decomposed to CO2 in the temperature range used (up to 6OO”C), according to the proportion of Cr(VI) and Cu(I1) in the impregnant. The copper content seems to affect to a larger extent the proportion of COz evolved, since in the case of the sample with the largest copper content (N8-2), the registered value (3.9%) is only slightly lower than the expected one (4.2%). 3.4
TPD TPD in the temperature range 150-1100°C has been carried out on all samples of series N, and Fig. 4 includes the profiles corresponding to the evolution of CO and CO2 for some of these carbons. In general terms, the profiles are typical of activated carbons[l9]: CO2 evolves at lower temperatures up to 700-750°C and CO evolves above 600°C. Carbon N exhibits two COz peaks, corresponding to the so-called low and high temperature CO* groups[ 191, and only a peak for CO, with a maximum at 850°C. The profiles for the impregnated carbons have larger area, and this shows that the amount of oxygen groups evolved increases with impregnation.
15 -N
10
.” --
N6-1.5 N6-2
~
N6-3
On the other hand, these groups are not of the same type present in the original carbon. Thus, the CO2 groups of carbon N exhibit maxima at 350 and 65O”C, whereas there are three maxima for impregnated carbons N6-1.5, N6-2, and N6-3, only the one at 650°C being common to the original carbon; the other two peaks are at 200-300 and 410°C. The latter two temperatures are reasonably coincident with the temperature for the maximum weight loss (Fig. 3) and maximum energy evolved (Fig. 2). This confirms the existence of two relatively well differentiated stages in the transformation taking place during the thermal treatment of the carbon. Impregnation also produces an increase in the amount of groups evolving as CO, but these are not detected in the DTA/TG experiments, since they start to evolve at the maximum temperature used in DTA/TG, 600°C. The groups generated by the redox process evolve at a temperature similar to the original carbon N (850-900°C). The TPD profiles for the rest of the carbons are very similar to the plots of Fig. 4, and their integration permits the calculation of the amounts of oxygen (from CO and COz) evolved. The values given in Table 5 for the impregnated carbons have been calculated as the difference between the total amount and that corresponding to the original carbon N, and are expressed as weight of oxygen evolved (from CO or COz) per gram of carbon. It is observed that during the redox process the oxygen evolving as COz predominates. Since the reduction of chromium occurs at temperatures up to about 4OO”C, followed then by the reduction of copper, the amount of CO, evolved is caused by the reduction of both Cr(V1) and Cu(I1). On the other hand, the trend of amounts of oxygen evolved as CO* is coincident with that shown in Table 4 for the total weight loss (i.e., it does not change with increasing Cr content and it increases with Cu content). The trend for the amounts of oxygen evolved as CO is the opposite to that shown by CO*. Consequently, the oxidation of carbon with Cr(V1) and Cu(I1) seems to occur through the formation of oxygen surface groups that evolve as both CO and COz, although Cr(V1) leads to a larger contribution of thermally stable groups decomposed as CO only at high temperatures (>6OO”C). However, oxidation with Cu(I1) produces a larger contribution from groups decomposing as COz at lower temperatures (<6OO”C).
co
% E z
Table 5
a
200
400
6W
8oa
1000
T (“C) Fig. 4. TPD profiles
for samples
of series N.
1200
5. Oxygen
(evolved as CO and COz) introduced impregnation
by
Carbon
as CO1 (mg/g)
as CO (mg/g)
N6-1.5 N6-2 N6-3 N7-2 N8-2
16 17 17 20 23
11 14 17 14 17
Impregnation
of activated
carbon
with chromium
of activated
carbon
with
salts
of
Cr(VI) and Cu(II) produces a decrease not only in the volume of micro and mesopores, but also in the mean micropore size, the resulting impregnated carbons exhibiting a microporosity that is narrower and more uniform than in the original carbon. The changes in porosity are a function of the original pore size distribution of the activated carbon. In general terms, the largest decrease in porosity is seen in carbons with more heterogeneous microporosity and an important contribution of mesoporosity. If the contribution of meso and macroporosity to the total porosity of the carbon is small, the only effect of impregnation is a decrease in microporosity. The extent of the porosity decrease produced by the impregnating salts indicates that these produce a partial blocking of porosity. 2. When the impregnated carbons are heat treated in an inert atmosphere, the reduction of Cr(V1) and Cu(II) takes place with a simultaneous oxidation of the carbon. The enthalpies involved in the process indicate that the reduction products are Cr(II1) and Cu(O), the reduction taking place in stages. There is first the reduction of Cr(VI), followed by the reduction of Cu(II), the process ending at around 600°C. 3. The reduction of the metal oxides is simultaneous to the oxidation of carbon, leading to the formation of oxygen surface groups different from those present in the original carbon. It general terms, the reduction of Cu(I1) leads to a larger contribution of groups that decompose thermally to CO2 at low temperature (<6OO”C), and the reduction of Cr(V1) leads essentially to the formation of groups that decompose to CO at higher temperature (>6OO”C). Acknowledgemenr-Financial S.A. and DGICYT (Project acknowledged.
1265
REFERENCES
4. CONCLUSIONS 1. Impregnation
and copper
support from Filtros Energia, No. PB91-0747)
is gratefully
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