Energy & Fuels 1994,8, 1233-1237
1233
A TPD Study of Chromium Catalysts Supported on an Oxidized and Nonoxidized Activated Carbon C. Moreno-Castilla,* M. A. Ferro-Garcia, J. Rivera-Utrilla, and J. P. Jolyt Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain, and Laboratoire $Application de la Chimie a l%nvironnement, Universitk Claude Bernard, Lyon, France Received March 31, 1994. Revised Manuscript Received July 21, 1994@
A commercial activated carbon was oxidized with nitric acid and both the untreated and the oxidized activated carbon were used as supports for Cr catalysts, which was deposited on them by adsorption from aqueous solution using either chromium nitrate or ammonium chromate as precursor salts. The samples so obtained were heat treated in He flow up to 1200 K following the evolution of CO and COZ by mass spectrometry. Results found show that during this treatment the CO disproportionation reaction takes place only in those supported catalysts obtained from the oxidized activated carbon. This reaction was catalyzed by Cr203 which is formed during the heat treatment. After He treatment up to 1200 K, the samples were treated in a He-02 flow (10 vol % of oxygen) a t 573 K for 5 min. Results found show that Crz03 supported on the untreated carbon presented a higher dispersion than that supported on the oxidized one.
Introduction Adsorption of metallic salts from aqueous solutions on activated carbons is one of the most important methods of preparing supported metal catalysts, because one can control the dispersion and, thereby, the metal particle size of the catalysts by controlling the factors that affect the adsorption process.1-6 Among these factors should be cited the surface area, pore texture and surface chemical nature of the support, and the type of salt, because the metal can be in a cationic or anionic form, and the pH of the solution. In these adsorption systems, the surface chemical nature of the activated carbons determined by the amount and nature of the surface complexes (essentially oxygen complexes), has been in general, to exert a greater influence than the surface area and porosity of the adsorbent because these surface complexes can act as specific sites to adsorb the metal ions or even change their oxidation state. After preparation of the supported cataysts, they are generally heat treated in different atmospheres to obtain the active phase. In this process the oxygen surface complexes are, to a greater or lesser extent, destroyed and essentially evolve as CO, COS, and HzO from the supported catalysts. Moreover, these gases can undergo secondary gas-phase reactions, as CO dispro-
* Author to whom the correspondence should be addressed.
Universite Claude Bernard. Abstract published in Advance ACS Abstracts, September 1,1994. (1)Rodriguez-Reinoso, F.; Rodriguez-Ramos,I.; Moreno-Castilla, C.; Guerrero-Ruiz,A.; L6pez-Gonz&lez, J. D. J. Catal. 1986,99,171. (2) Prado-Burguete, C.; Linares-Solano, A.; Rodriguez-Reinoso,F.; Salinas-Martinez de Lecea, C. J. Catal. 1989,115, 98. (3) Burch, R. Catal. Today 1990, 7. (4) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990,28,369. (5) Solar, J. M.; Derbyshire, F. J.; de Beer, V. H. J.; Radovic, L. R. J . Catal. 1991,129,330. (6)Kim, K. T.; Chung, J. S.; Lee, K. H.; Kim, Y. G.; Sung, J. Y. Carbon 1992,30, 467. (7) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A.; Moreno-Castilla, C.Carbon, 1994,32,93. +
@
portionation and water-gas shift, catalyzed by the metal compound which constitutes the active phase of the catalyst. Therefore, during the heat treatment of the supported catalysts, and due to these two facts, viz., gas evolution and its possible catalyzed reaction, some changes in the metallic phase can take place. The effect of the oxygen complexes of the activated carbons on Cr adsorption from aqueous solutions was and the aim one of the objectives of a previous of this study is to investigate the effect of heat treatment in He up to 1200 K on Cr catalysts supported on an oxidized and nonoxidized activated carbon.
Experimental Section A commercial activated carbon supplied by Merck (sample M) was used in this study. A portion of this sample was oxidized with HN03 to introduce oxygen surface complexes, sample MO. The procedure was as follows: 50 cm3 of concentrated nitric acid was added t o 5 g of carbon and the suspension was heated at 353 K until dryness. The residue was washed with distilled water till no nitrates were present and dried overnight in an oven at 383 K. These two activated carbon samples, M and MO, were studied t o determine their pore texture, surface area, and surface chemical nature before they were used as adsorbents of CdIII) and CdVI) ions from aqueous solutions. The results obtained were the subject of a previous paper' in which they were discussed in detail. Some of the results there obtained are summarized in Table 1. The supported Cr catalysts were prepared by adsorption from aqueous solutions of either Cr(NO&9H20 or (NH&Cr04, respectively, on the activated carbons M and MO. The suspensions, 1 g of carbon and 500 cm3 of Cr solution with a concentration of 0.2 g of C r L , were shaken and thermostated at 298 K for 48 h. The Cr solution concentration was that necessary to obtain the maximum Cr uptake by the supports. Subsequently, the suspensions were filtered out and the carbon was dried in a n oven at 383 K. The Cr content of the samples was determined by burning off a portion in a thermobalance and weighing the residue as Cr203. These Cr contents are also shown in Table 1.
0887-0624/94/2508-1233$04.50/00 1994 American Chemical Society
Moreno-Castilla et al.
1234 Energy & Fuels, Vol. 8, No. 6, 1994 Table 1. Characteristics of the Supports and Their Maximum Cr Adsorption Capacity maximum adsorption capacity mg of Cr(II1). mg of Cr(VI). support % 0" m2.g-l cm3.g-l pH g of carbon-l g of carbon-l M 2.0 1089 0.654 7.0 2.7 6.9 MO 23.0 164 0.371 2.6 25.3 15.5 sNz
0
VHzO
-
\
1 " LL
From the amount of CO and COz evolved upon TPD to 1200
K.
0
The temperature-programmed desorption (TPD) experiments were carried out as follows. About 200 mg of sample was placed in a dynamic quartz reactor, about 1cm3, equipped with a short response time capillary thermocouple in its center. The samples were heated in a He flow (60 cm3/minj at a heating rate of 20 Wmin up to 1200 K. They were subsequently cooled down to 573 K, and the He flow was changed to another containing 10 vol % 02. These conditions were mainteined for 5 min. On-line gas analysis was followed with a quadrupole mass spectrometer (MS) with a capillary introduction system heated at about 373 K. A microcomputer was used for MS control, data acquisition, and processing. The apparatus and data processing have been described in detail in a previous publication.8 Calibrations were carried out by analyzing gases diluted in He (10vol %).
Results and Discussion When a carbonaceous material is subjected to a program of increasing temperature, the oxygen surface complexes desorb primarily as COZand C0,9-11 among others. Thus, COZproceeds from the decomposition of carboxylic, anhydride (acidic groups), and lactonic groups, whereas CO proceeds from the decomposition of phenolic, carbonyl, quinone, pyrone, and anhydride (acidic) groups. Therefore, TPD experiments are widely used in carbonaceous materials, in order t o determine the degree of oxidation and the nature and thermal stability of the surface oxide formed. It must borne, though, in mind that the interpretation of the TPD spectra of these porous materials can be complicated by mass-transfer effects resulting in readsorption and even further chemical reactions.1° Figure 1 shows the CO and COz evolution from the supports M and MO, and Table 2 summarizes their total amount of gases evolved up to 1200 K. The TPD pattern of sample M is typical of a nonoxidized carbonaceous material with a very low oxygen content (Table 1). The COz evolution occurs below 850 K and the CO evolution above this temperature; the CO/COz ratio (Table 2) is quite high which gives the surface of this carbon a neutral character (as shown in Table 1)and suggests that the CO-generating groups have a weakly acidic, neutral or basic character such as phenolic, carbonyl, quinone, or pyrone g r o u ~ s . l l - ~ ~ (8) Ferro-Garcia, M. A.; Utrera-Hidalgo, E.; Rivera-Utrilla, J.; Moreno-Castilla,C.; Joly, J. P. Carbon 1993,31,857. (9) Tremblay, G.; Vastola, F. J.;Walker, P. L., Jr. Carbon 1978,16, 35. (10) Calo, J. M.; Hall, P. J. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; NATO/ AS1 Series E192; Kluwer Academic: New York, 1991; p 329. (11)Roman-Martinez,M. C.; Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. Carbon 1993,31,895. (12) Linares-Solano. A; Salinas-Martinez de Lecea, C.; CazorlaAmoros, D.; Joly, J. P.; Charcosset, H. Energy Fuels 1990,4 , 467. (13)Otake, Y.; Jenkins, R. G. Prepr. Pap.-Am. Chem. SOC.,Diu.
Fuel Chem. 1987,32, 310. (14) Boehm, H. P. High Temp.-High Press. 1990,22,275. (15)Treptau, M. H.; Miller, D. J. Carbon 1991,29, 531.
300
500
700
900
1100
1300
900
1100
1300
T (0
6 n
" 4 0, 1. 0
E,
z * 0 300
500
700
T (0
Figure 1. CO and COz evolution profiles for the nonoxidized (M) and oxidized (MO) activated carbon heated up to 1200 K (+I co; (0)c02. Table 2. Total Amounts (mmolg-l) of CO, C02, and 0 Evolved up to 1200 K sample MO MO-Cr(II1) MO-Cr(VI) M M-Cr(II1j M-Cr(VI) a
Total amount of
0 2
co
c02
02"
4.97 3.48 4.10 0.81 1.23 0.95
4.71 5.45 5.16 0.21 0.78 0.72
7.20 7.19 7.21 0.62 1.40 1.20
2
evolved as CO and COz.
When sample M was oxidized with HN03, there was a deep change both in its surface area and porosity as well as in its surface chemical nature. Thus, Table 1 shows a dramatic lowering of S(N2) and of the pore volume accessible to water, V(HzO), together with an increase in the oxygen content. The decrease in surface area and porosity was caused by breakage of the pore walls to produce oxygenated groups and by mechanical destruction by the surface tension of the oxidizing solution.16 Comparing the TPD patterns of Figure 1and the data of Table 2 one can deduce that HNO3 treatment generated a great amount of oxygen surface complexes. Figure 1 shows that COz-generating groups are decomposed at low temperature with the maximum rate at around 600 K. The COz profile, after this temperature, presents a shoulder or tail up to 1200 K indicative of the occurrence of chemically different complexes and/ or the same oxygen complex existing on energetically different sites. Thus, it has been shownl1J7 that carboxyl anhydride groups are more stable than carboxyl acid groups and, therefore, they evolve as COz and CO above 600 K. The CO profile also presents a maximum at around 550 K and another asymmetrical peak with the maximum rate centered at around 950 K which presumably results from the decomposition of different (16) Matsumara, Y. J. Appl. Chem. Biotechnol. 1975,25,39. (17) Otake, Y.; Jenkins, R. G. Carbon 1993,31, 109.
Energy & Fuels, Vol. 8, No. 6, 1994 1235
TPD Study of Chromium Catalysts types of CO complexes. Mass-transfer effects do not play an important role in the shape of the CO and C02 profiles found because (i) TPD spectum of a nitric acid oxidized activated carbon carried out under vacuum18 (10-7-10-4 Torr) presents the same shape as that of sample MO (Figure 1) which was obtained near to atmospheric pressure, and (ii)sample MO is much less porous than sample M (see their S(N2) and V(H20) values in Table 1);however, the former presents a more asymmetric COS peak than the latter. Data from Table 2 show that the HNO3 treatment of sample M t o obtain sample MO increased the C02generating groups to a greater extent than the COgenerating groups which causes both the CO/C02 ratio and the pH of the sample to decrease (Table 11, making it acidic. The presence of carboxyl groups on the surface of sample MO was also detected by FTIR as indicated el~ewhere.~ As seen from Table 1, Cr(VI) is adsorbed t o a greater extent than Cr(II1)on the surface of sample M; this has been explained in a previous paper7 to be due to the fact that the surface of the activated carbon M has a greater aEnity to specifically adsorb anions because of its pH. When activated carbon MO was used as support of either Cr(II1) or Cr(VI) ions, there was an increase in the total Cr supported compared to sample M, in spite of the fact that support MO had a lower surface area and porosity than support M (see Table 1). These results were explained elsewhere7 as follows. First, an increase in the amount of Cr(II1) adsorbed on sample MO compared to sample M (about 10 times greater) is due to an increase in the surface acidity of support MO (pH 2.6), which means that in aqueous solution the carboxyl surface groups of this carbon would be dissociated and their conjugated bases would be the specific sites to adsorb Cr(II1) ions. Second, the Cr(VI) ions have been shown7 to be reduced at acid pH to some degree t o Cr(II1) on the surface of the activated carbon MO, probably by the -OH surface groups of the carbon of phenol or hydroquinone type according to reactions 1 and 2, respectively. The asterisk represents the hydroquinone surOH C&
I
+ 3-C-
0
+
+ 3-C-
5Hf-C$+
II
+ 4H20
(1)
OH OH
Cr0;-
+
1
1.5-C-C-
I *
+ 5H+0
C$+
+
II
1.5-C-C-
0
II
+ 4H20
(2)
face group. Therefore, adsorption of chromium on sample MO from aqueous solutions of Cr(VI) would be carried out to some extent as Cr(II1). This would explain the greater adsorption of chromium by sample MO than sample M from aqueous CI-04'- solutions, in spite of the fact that the surface of the former sample would not specifically adsorb anions. Data of the amounts of CO and COZ evolved from samples MO-Cr(II1) and MO-Cr(VI) up to 1200 K are compiled in Table 2, and their TPD patterns are depicted in Figures 2 and 3, respectively. It is quite (18)Joly, J. P.; Haydar, S.; Perrard, A.; Moreno-Castilla, C.; RiveraUtrilla, J.; Ferro-Garcia, M. A. Extended Abstr. Carbon '94 1994, 346.
7
xo
dco
xu
600
7m
BCO
903
I~II
iim
12m
1303
T (K)
Figure 2. CO evolution profile for samples: MO, W; MOCr(III),O; and MO-Cr(VI), O.
301
a
xu
m
7m
803
903
1000
iim
im,
1303
T (0
Figure 3. COz evolution profile for samples: MO, W; MOCdIII), O; and MO-CdVI), O.
interesting to note that the CO desorption profile of sample MO-Cr(VI) presents a pronounced peak at around 650 K this peak appears in sample MO at around 550 K. The former sample also presents a more pronounced peak than the latter at around 1050 K. This shifting would confirm the reduction of Cr(VI) to Cr(111) according to reactions 1 and 2, because now the new ketone and quinone groups formed would be thermally more stable than the original phenol or hydroquinone groups, and therefore, they would be evolved as CO at a higher temperature. The C02 desorption profiles of samples MO, MO-Cr(1111, and MO-Cr(VI), Figure 3, show that more C02 is evolved from both samples containing chromium mainly in the temperature range between 800 and 1000 K. Table 2 shows that the total amount of oxygen evolved as C02 plus CO from the three samples was identical (around 7.2 mmo1.g-l); however, the total amount of COS evolved from both samples containing Cr was higher than that of the original MO sample, and a t the same time the CO desorbed from samples containing Cr was less than that from sample MO. Qualitatively, this behavior was also found by Otake et a l l 7 with a HN03oxidized char containing Na, and they suggested that in the presence of Na a t temperatures higher than 700 K the CO disproportionation reaction 3 is catalyzed as a secondary gas-phase reaction. According t o reaction 2co
-
CO2
+c
(3)
3, the molar quantity of C 0 2 produced is half that of the CO consumed. Table 2 shows that in the case of sample MO-Cr(III), the amount of CO consumed was 1.49 mmol-g-' (4.97-3.48) and the amount of CO2 produced 0.74 mmo1.g-l (5.45-4.71), and for sample MO-Cr(VI), the amount of CO consumed was 0.87
Moreno-Castilla et al.
1236 Energy & Fuels, Vol. 8, No. 6, 1994
n cn
6
91
co
2
1 .
E
21 U
0
3ocl
m
700
900
1100
1m
Table 3. Amounts of CO and COZ (mmol*g-')Evolved during the Heat Treatment at 573 K for 5 min in a He-02 Flow and the Percentage Burn-off sample co coz % BO" MO 0.30 0.27 0.68 MO-Cr(II1) 0.35 1.99 2.81 MO-Cr(VI) 0.33 1.55 2.26 M 0.23 0.18 0.49 M-Cr(II1) 0.37 0.54 1.09 M-Cr(VI) 0.35 0.82 1.40 a
Obtained from the amounts of CO and COz evolved.
T (K)
300
500
900
703
1103
1300
T (IO Figure 4. CO and COz evolution profiles for samples: M, W; M-Cr(III), U; and M-Cr(VI), 0.
mmo1.g-l (4.97-4.10) and that of CO2 produced 0.45 mmo1.g-l (5.16-4.71). Thus, in both MO samples containing Cr the amount of CO consumed was double that of the CO2 produced, and this strongly suggests that the CO disproportionation reaction takes place on both samples. This reaction would be catalyzed by Cr203, since X-ray diffraction has revealedlgthat when carbon samples containing either Cr(II1) or Cr(VI) are heated in an inert atmosphere up to 873 K that species is formed between 650 and 700 K. The CO disproportionation reaction would take place between 700 and 1000 K because Figure 2 shows that the amount of CO evolved from MO-Cr(II1) and MO-Cr(VI) decreases compared to that evolved from sample MO in that temperature range. The above results also indicate that reaction 3 occurs t o a greater extent on sample MO-Cr(II1) than on sample MO-Cr(VI), which is due t o the fact that the former contains a greater amount of Cr than the latter (Table 1). The maximum amount of both Cr(II1) and Cr(VI) adsorbed on sample M is given in Table 1. This sample adsorbs much less chromium than sample MO; however, the amount of Cr(V1) adsorbed on sample M is higher than that of Cr(III), because this sample specifically adsorbs anions due to the chemical nature of its ~urface.~ The amounts of CO and C02 evolved from these samples are given in Table 2 and the CO and C02 profiles are depicted in Figure 4. The behavior of these samples is quite different from that found before for samples MO containing chromium. Thus, the amounts of CO and C02 evolved up to 1200 K from samples M-Cr(II1) and M-Cr(VI) are higher than those found for sample M which, evidently, brings about an increase in the oxygen content of samples containing chromium compared to sample M. Since the oxygen content of this (19)Moreno-Castilla, C.; Carrasco-Marin, F.; Rivera-Utrilla, J. Fuel 1990,69,354.
carbon M is very low, the results found might be due to its partial oxidation after immersion in the aqueous solutions of the chromium ions,20,21the conversion of carboxyl anhydride groups to carboxyl acid groups by the water of the solution,17 and the reaction of the chromium salts with the carbon support during their decomposition in the heat treatment up to 1200 K. These phenomena were not detected with samples MO containing Cr, since in these samples the support was saturated wih oxygen surface complexes due to its previous nitric acid treatment. After heat treatment of the samples in He up to 1200 K, they were cooled down to 573 K and the He flow changed to another of He-02 following for 5 min the evolution of the gases from the samples; the results obtained are given in Table 3. All the results of these runs were corrected taking into account the new mass of sample, since during the first treatment up to 1200 K in He there was a certain weight loss (which was calculated from the amounts of CO and C02 evolved from Table 2). The % BO (burn-off)of sample MO is higher than that of sample M due to the fact that the loss of the surface oxygen complexes of sample MO (more abundant than in sample MI during the heat treatment up to 1200 K created more nascient carbon atoms than in sample M, this makes it more active to gasification by oxygen. All samples containing Cr present a higher % BO than the original samples M and MO, indicative that Cr2O3 catalyzes the gasification of the support by oxygen at to be a good 573 K, because this oxide is known19~22323 catalyst in this reaction, and there is a fairly linear relationship between the % BO and the Cr content of these samples as shown in Figure 5. However, if the amount of carbon gasified is given per gram of chromium, R in Figure 5, this value decreases with the Cr content being the best catalyst M-Cr(III), which indicates that in this sample, Cr2O3 has the highest dispersion, after the previous treatment in He at 1200 K. It is interesting to compare the variation in catalyst activity per gram of Cr to gasify the support and produce C02, rcoz, during the reaction time, for samples M-Cr(111)and MO-Cr(III), Figure 6. In contrast to sample M-Cr(II1) where there is a marked decrease in activity with reaction time after reaching its maximum rcoz value, in the case of MO-Cr(II1) the rco2value remained practically constant with time and is smaller than in the former sample. Similar results were found when comparing samples M-Cr(VI) and MO-Cr(VI). This (20) Boehm, H. P.; Voll, M. Carbon 1970,8, 227. (21) Leon y Leon, C. A,; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992,30,797. (22) McKee, D. W. Carbon 1986,24,331. (23)Moreno-Castilla, C.; Rivera-Utrilla, J.; L6pez-Peinado, A. J.; Fernandez-Morales, I.; Lbpez-Garzbn, F. J. Fuel 1985,64, 1220.
TPD Study of Chromium Catalysts
Energy & Fuels, Vol. 8, No. 6, 1994 1231
has a lower particle size which increases by sintering after reaching the maximum rCOz value. The highest CrzO3 particle size of the catalysts supported on activated carbon MO is due on the one hand to their higher Cr content and on the other to both the decomposition of the oxygen complexes of the support and the CO disproportionation reaction during their previous heat treatment in He up to 1200 K. Thus, after this heat treatment the CrzO3 mean particle size determined by XRD was 22 nm in the samples MO-Cr(II1) and MOCr(VI);however, no diffraction peaks of Cr species were detected in samples M-Cr(II1) and M-Cr(VI), probably due to their small crystallite size, or due to the fact that CrzO3 on support M was spread on its surface giving very thin oxide particles which may have lost their crystallinity, as reported in a previous work19 for chromium oxide supported on activated carbons with surface areas similar to that of support M.
3
2
0
m N
1
0
1
I
2
1
w
Cr
Figure 5. Relationship between the % BO and R with the Cr content of the activated carbons.
31
0
100
300
200
400
500
nmetr)
Figure 6. Variation in catalyst activity per gram of Cr to gasify the support and produce COz, rcoz, with the reaction time: M-Cr(III), W; MO-Cr(VI), 0.
behavior indicates that when oxygen gasification of the support catalyzed by CrzO3 begins, the particle size of this oxide is greater in supported catalysts that come from activated carbon MO and that during 5 min of reaction their size practically does not change. However, Cr2O3 supported on activated carbon M initially
Conclusions This work shows that when Cr is supported on a nitric acid oxidized activated carbon and is heat treated in He up to 1200 K, the CO evolved from the support undergoes the disproportionation reaction. This reaction is catalyzed between 700 and 1000 K by CrzO3 which is formed during the heat treatment in He. However, the above reaction is not observed in samples prepared with the as-received nonoxidized activated carbon. These supported catalysts show a higher oxygen content than the as-received activated carbon used as support, due to the fixation of oxygen after immersion in the Cr solutions, the conversion of anhydride groups to carboxyl acid groups, and the reaction of Cr salts with the surface of the support during the heat treatment. The results also show that after heat treatment up t o 1200 K in He flow, the CrzO3 supported on the nonoxidized activated carbon presents a highest dispersion than the Ci-203 supported on the oxidized one.
Acknowledgment. We thank CICYT, Project No. AMB 92-1032, and the Spanish-French Joint Research Programme, No. 246 B, for financial support.
