1310
Ind. Eng. Chem. Res. 1998, 37, 1310-1316
Effect of Dispersion and Distribution on Activity of Alumina-Supported Cadmium Sulfide Photocatalysts for Hydrogen Production from Water Manjit K. Arora, A. S. K. Sinha,* and S. N. Upadhyay Department of Chemical Engineering and Technology, Banaras Hindu University, Varanasi 221 005, India
Activity of a cadmium sulfide photocatalyst, for hydrogen production by decomposition of water using visible light, depends on the protocol used for its preparation. In the present work, unsupported and alumina-supported CdS photocatalysts were prepared by various techniques and characterized by N2 adsorption, X-ray diffraction, electron diffraction, and X-ray photoelectron spectroscopy. The results indicate that the activity is strongly affected by the dispersion and distribution of CdS on alumina. High dispersion and preferential distribution of CdS on the surface of alumina yield superior activity. The catalyst activity has been explained with the help of its microstructure. Introduction Hydrogen, because of its physical and chemical properties, has been accepted as an ideal energy source for the future. On combustion, it releases energy and does not produce harmful CO and CO2. Furthermore, since the autoignition temperature of hydrogen is only 585 K, the formation of NOx, thermodynamically favored at temperatures above 1000 K, is suppressed. The production of hydrogen from water by photocatalysis using solar radiation is a promising technique of harnessing solar energy. Amongst many semiconductors, cadmium sulfide is one of the suitable photocatalysts for this reaction (Darwent and Porter, 1981; Matsumura et al., 1983; Borell et al., 1992; De et al., 1996). Owing to its narrow band gap (2.4 ev), it can be activated by visible light and thus has a potential to harvest solar energy. It is a well-known fact that, apart from the chemical composition, activity of a catalyst is largely dependent upon the protocol used for its preparation. Supported catalysts in general exhibit better activity because the active ingredients can be highly dispersed on the surface of the support. Supports can also provide heterojunction for electrons and holes, thus, restricting the charge recombination which is an essential requirement for photocatalysis (Borgarello et al., 1985, 1986; Kobayashi et al., 1987). However, since light cannot penetrate deep inside the porous matrix of a support, a highly active photocatalyst should have the active ingredients deposited only on or near the external surface of the support. Thus, dispersion as well as distribution of active ingredients on the support, which are strongly affected during the preparation stage of a catalyst, are of critical importance. A search of literature reveals that such studies, on the effect of the preparation technique on CdS microstructure and consequently activity, have not been reported though difference in activity with the preparation technique has been noticed. Barbeni et al. (1985) have observed that the activity of CdS depends on the precursor salt used and also CdS prepared in the presence of excess Cd2+ is more active. Borell et al. * Author to whom correspondence should be addressed. Phone: 091-0542-317192. Fax: 091-0542-316427.
(1992) have reported that crystalline rather than amorphous CdS is more active. The amorphous CdS is considered to have structural defects which could trap the charge carriers before they reach the surface and react. Kobayashi et al. (1985) prepared CdS and CdSbased catalysts supported over alumina. CdS was precipitated by reacting Na2S with CdSO4 solution. It was observed that CdS-ZnS co-precipitated on alumina gave a better performance than the physical mixture of the catalysts prepared by separately precipitating CdS and ZnS and then mixing them. The authors propose that, on activation, the intimate contact obtained by precipitation leads to the transfer of electrons from the conduction band of CdS to one of the impurity levels in the band of the ZnS semiconductor, hence better activity. In the present work, alumina-supported cadmium sulfide photocatalysts were prepared by various techniques. Their activities for hydrogen production by photodissociation of water utilizing visible light were measured. Catalysts were characterized by N2-adsorption, X-ray diffraction (XRD), electron diffraction, X-ray photoelectron spectroscopy (XPS) and inter-relations among preparation variables, microstructure, and activities of the catalysts have been developed. Further, it should be emphasized that the present work does not aim at obtaining the best catalyst formulation; hence, a comparison of activities of the photocatalysts reported in the present study with earlier reported works has not been included. Also, such a comparison could have been meaningful only if the light sources and their intensities were comparable in the reported works. Experimental Section Catalyst Preparation. Predried alumina is widely used as a support for catalyst preparation. In the present work also, alumina was used as a support. However, a departure was made and hydrogel of alumina, instead of predried alumina, was mixed with the impregnating solutions. Sinha and Shankar (1993) and also Rao and Shankar (1988) have reported that hydrogel leads to better dispersion of active ingredients. A high surface area hydrogel was prepared in the labora-
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Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1311
tory using the method described by Lippens and Steggerda (1970). A concentrated solution of aluminum nitrate was neutralized with ammonia at pH 9 and temperature 303 K. The hydrogel was aged for 48 h and then washed with distilled water until it was free of nitrate ions. Alumina-supported cadmium sulfide photocatalysts were prepared by the impregnation technique. The details of catalyst preparation are given below. Unsupported CdS and CdS supported on predried alumina were also prepared. Catalyst 1. Unsupported cadmium sulfide was prepared by bubbling H2S gas in an aqueous solution of cadmium sulfate. The precipitate was washed with distilled water and dried in an air oven at 383 K for 12 h. Catalyst 2. H2S gas was passed over granules of cadmium sulfate at 473 K in a fixed-bed reactor. Catalyst 3. Predried alumina (i.e., the hydrogel prepared in the laboratory and dried at 473 K for 12 h was impregnated with a requisite amount of ammoniacal solution of cadmium sulfate to yield the final composition by weight as CdS:Al2O3 ) 1:2. The mixture was stirred for 20 h. It was then dried over a water bath with continuous stirring and finally in an air oven at 383 K for 12 h. The dried granules were then reacted with H2S at 473 K for 5 h in a packed-bed reactor. Catalyst 4. This catalyst was prepared by the same procedure as catalyst 3. The only difference was that prior to the reaction with H2S, the dried granules were flushed with nitrogen for 2 h at 473 K to remove any residual ammonia adsorbed on the surface. Catalyst 5. The only difference in the preparation of this catalyst and catalyst 3 was that, instead of predried alumina, hydrogel of alumina prepared in the laboratory was impregnated. Catalyst 6. It was prepared in the same manner as catalyst 4 except that the hydrogel rather than the predried alumina was impregnated. Catalyst 7. The preparation procedure was the same as catalyst 4 except that the hydrogel of alumina was taken and the reaction with H2S was carried out at a higher temperature of 673 K. Catalyst 8. The preparation technique of this catalyst was similar to that of catalyst 3 except that alumina hydrogel was used and it was impregnated with an aqueous rather than ammoniacal solution of cadmium sulfate. Activity Measurement. The photocatalytic activity of various catalysts for hydrogen production from water was studied in a batch reactor. The photoreactor was a 250-mL flat-bottomed flask; one of its sides was also made flat to permit the entry of light through a plane wall. The reactor had a provision for measurement and control of pH and temperature. A 150-W Philips tungsten-halogen lamp was used as the light source. The choice of the light source was based on the availability as well as on the spectral characteristics of the emitted light. No UV or IR filter was used because the spectrum of the light used showed a very negligible emission in the UV range and the use of a Pyrex glass reactor further prevented this radiation from reaching the catalyst. IR radiation can only be absorbed in the form of heat and could increase the temperature of the solution. During experimentation, the temperature could be maintained at the desired value; therefore, the IR filter was also not used.
Table 1. Hydrogen Evolved in 2 h of Irradiation catalyst
hydrogen evolved (mL at NTP‚g of CdS-1)
1 2 3 4 5 6 7 8
0.8 2.0 1.6 2.7 7.0 8.1 6.1 5.9
Two grams of catalyst of size -200 mesh was suspended with a magnetic stirrer in 250 mL of aqueous solution of concentration 0.01 and 0.004 M with respect to Na2S and Na2SO3, respectively. The pH was maintained at 8.6 during the experimentation by adding requisite quantities of NaOH and acetic acid. The temperature was maintained at 333 K and the solution was deaerated by sparging nitrogen for 2 h prior to irradiation. The gas evolved was collected by a water displacement technique and analyzed by a gas chromatograph using a 5-Å molecular sieve column and thermal conductivity detector. The results showed that the evolved gas was pure hydrogen. Intrinsic activity of a photocatalyst is largely independent of the pH and temperature, since negligible changes in the electronic band structure of a semiconductor are expected. Therefore, the activity of various catalysts was compared at constant pH and temperature. The optimum value of pH depends on the electrolytes present in the solution. In the present study, with a mixture of sodium sulfide and sulfite as electrolytes, the optimum pH was observed to be 8.6 and the activities of all the photocatalysts have been compared at this pH. Further, the intensity of light was also not varied in the present study. However, experiments were conducted by varying the amount of catalysts in the reactor, and it was observed that the intensity of light was not a limiting factor for dispersion up to 3.0 g of the most active catalyst (catalyst 6). Therefore, only 2.0 g of catalysts was taken in the subsequent runs for the comparison of activity to avoid the results being affected by the intensity of the light used. Catalyst Characterization. The BET surface area and pore size distribution of the catalysts were measured by N2 adsorption at 77 K in a volumetric gas adsorption apparatus. X-ray diffraction studies were carried out using a Philips 1710 X-ray diffractrometer with Cu KR target of radiation wavelength 1.542 Å. Electron diffraction studies were carried out on a Philips (CM-12) transmission electron diffractometer equipped with a twin lens system. X-ray photoelectron spectroscopy studies were carried out on a Perkin-Elmer (φ 1800, United States) spectrometer using a Mg KR target. Carbon was used as an internal source for calibration. Results and Discussion Activity. The activities of various catalysts, measured as milliliters of hydrogen produced (at NTP) per gram of CdS in 2 h of irradiation, are reported in Table 1. The reproducibility of data in repeated runs for each catalyst was tested. Further, the reproducibility for catalysts 6-8 was also tested for two different preparations of these catalysts. In either case, consistent data were obtained for each catalyst without any significant deviation. The data reported in the table are the
1312 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 2. BET Surface Area of Catalysts
Table 3. Cumulative Pore Volume of Catalysts
sample
BET surface area (m2‚g-1)
Al2O3* Al2O3** catalyst 6 catalyst 7 catalyst 8
100 125 21 25 12
average of the repeated runs. It is observed from the table that among the unsupported catalysts (catalysts 1 and 2), catalyst 2 which was prepared by a gas-solid reaction exhibited a superior activity as compared to catalyst 1 which was prepared by a gas-liquid reaction. The stoichiometric analysis of catalysts 1 and 2 revealed that catalyst 2 had a sulfur deficiency, thus exhibiting the characteristics of an n-type semiconductor. The superior activity of this catalyst has been attributed to its n-type behavior as discussed elsewhere (Arora et al., 1998). Furthermore, in general, the supported CdS catalysts exhibited better activity compared to the unsupported CdS. It is in order to mention that the support was expected to prevent charge recombination and to promote better dispersion of CdS. It is likely that these two factors may be responsible for the better activity of the supported catalysts. It is also observed that, in spite of having the same composition, the activities of supported catalysts depend upon the preparation technique. Catalysts 5-8, which were prepared by impregnating alumina hydrogel, showed a higher activity than catalysts 3 and 4, where predried alumina was impregnated. It is also observed that ammonia in the impregnating solution (catalysts 5-7) resulted in a better activity as compared to catalyst 8 where aqueous solution was used for the impregnation. Furthermore, catalysts 4 and 6 showed much higher activity in comparison to catalysts 3 and 5, respectively. It is relevant to emphasize here that the only difference between catalysts 3 and 4 and 5 and 6 was that, prior to the reaction with H2S, the latter catalysts were flushed with nitrogen gas at 473 K to remove the residual ammonia from the surface. The temperature maintained during the reaction of cadmium sulfate with H2S is also seen to affect the activity. Catalyst 7 which showed a reduced activity was prepared by the same technique as catalyst 6 and had the same composition, but during the reaction of CdSO4 with H2S, a higher temperature (i.e., 673 K) was used. The durability of catalyst 6, which had the best activity, was also tested for repeated runs. It was observed that the activity remained constant if the concentration of the donor species was maintained and the evolved hydrogen gas was constantly removed from the reactor. The detailed results have been communicated elsewhere (Arora et al., 1998). Gas Adsorption Studies. The physical texture of the catalysts and support was studied by low-temperature nitrogen adsorption. Alumina was subjected to different treatments. Samples of alumina (Al2O3** and Al2O3*) were given the same treatments as those adopted for the preparation of catalysts 6 and 8, respectively. Al2O3* was prepared by drying alumina hydrogel at 383 K and then heating for 5 h at 473 K whereas Al2O3** was prepared by aging the gel in ammonia for 24 h followed by drying and heating at the same temperature as Al2O3*. The BET surface area of catalysts are given in Table 2. It is observed that the pretreatment of Al2O3 hydro-
pore volume × 103 (mL‚g-1) sample
30 Å g rpa
30 Å < rp e 80 Å
80 Å < rp e 150 Å
rp e 150 Å
Al2O3* Al2O3** catalyst 6 catalyst 7 catalyst 8
58.0 31.0 23.0 32.0 6.0
27.0 29.0 12.0 28.0 11.5
13.5 26.0 3.0 9.0 8.0
98.5 86.0 38.0 69.0 25.5
a
rp indicates pore radius.
gel with ammonia prior to drying results in an increase in surface area from 100 to 124 m2‚g-1. Also, the surface areas of the catalysts are observed to be very low in comparison to that of the support. However, catalysts 6 and 7, prepared using an ammoniacal solution, have higher surface areas than catalyst 8, which was prepared by taking an aqueous solution. It is also observed that the BET surface areas of catalysts 6 and 7 are nearly equal. Thus, the higher temperature (673 K) used during the preparation stage did not have any effect on the BET surface area of the catalyst, although its activity was observed to be less. Pore size distribution and cumulative pore volume of radii 80 Å) has reduced. Furthermore, since Al2O3** was subjected to heat treatment at 473 K only, there is no basis of comparison between Al2O3** and catalyst 7 which was subjected to a temperature of 673 K during the preparation stage. The pore size distribution curves of various catalysts and their supports subjected to similar treatments are shown in Figures 1 and 2. These figures reveal that all the catalysts exhibit nearly similar pore size distribution patterns. The maximum is centered around 2030 Å with a fairly long tail. Further, as concluded earlier, it is also seen that, in comparison to its support (Al2O3*), catalyst 8 has a very low density of smaller size pores whereas, in catalyst 6, there is an appreciable decrease in the density of larger size pores. Hence, it appears that in catalyst 8, prepared using an aqueous solution of CdSO4, the CdS is preferentially deposited on pores of smaller radii, whereas in catalyst 6, which was prepared using an ammoniacal solution, the CdS is deposited in pores of larger radii. XRD Studies. XRD analyses of the catalysts were carried out for phase identification and crystallite size estimation. The d-value (interplaner spacing) for each peak was calculated and the peaks were identified as those of CdS, γ-Al2O3, and different phases of aluminium hydroxide. Furthermore, an analysis of peaks of CdS revealed that the hexagonal phase of CdS was largely present in all the catalysts. However, peaks corresponding to cubic CdS were also observed in all the catalysts where ammoniacal solution was used for impregnation. A comparison of catalyst 3 with 4 and
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1313 Table 4. Mean Crystallite Size of Catalysts
Figure 1. Pore size distribution of catalysts. Al2O3**: Alumina gel aged in ammonia for 24 h, dried at 373 K, and heated at 473 K.
Figure 2. Pore size distribution of catalysts. Al2O3*: Alumina gel dried at 373 K and heated at 473 K in H2S.
5 with 6 shows that the intensity of peaks corresponding to the cubic phase of CdS is substantially less in the latter catalysts. Catalysts 4 and 6 were prepared in the same manner as catalysts 3 and 5, respectively; the only difference being that, during the preparation of the former catalysts, they were flushed with nitrogen to remove the residual ammonia before reacting with H2S gas. Therefore, the lower activities of catalysts 3 and 5 as compared to those of catalysts 4 and 6, respectively, may be attributed to the presence of the cubic phase of CdS, which has been reported to have lower activity (Matsumura et al., 1985). The cubic phase appears due to the presence of residual ammonia during the reaction of CdSO4 with H2S. Mean crystallite sizes of the catalysts were calculated by analyzing the [110] (i.e., the largest intensity) peak of CdS by the Warren-Averbatch technique (Wagner, 1966) and are shown in Table 4. Catalyst 3 could not be analyzed because of extensive overlapping of the peak
catalyst
mean crystallite size (Å)
1 2 3 4 6 7 8
61 97 140 138 68 148 76
by neighboring peaks. However, since both the catalysts, catalysts 3 and 4, were prepared using predried alumina, the crystallite size in catalyst 3 is expected to be of the same order of magnitude as that in catalyst 4. It is evident from Table 4 that the crystallite size depends upon the protocol of catalyst preparation. Further, the observed activity of the photocatalysts could be correlated to some extent with the crystallite size (or dispersion), but the better dispersion alone cannot be taken as the only criterion for the superior activity. Among the unsupported catalysts, catalyst 2, prepared by a gas-solid reaction and showing a superior activity, had a larger crystallite size than catalyst 1. It has been mentioned above that catalyst 2 exhibited characteristics on an n-type semiconductor leading to better activity. It is also observed that even though the crystallite sizes of the unsupported catalysts are low, their activities were also low as compared to those of supported catalysts. This indicates that the support alumina also imparted some catalytic activity to CdS. As discussed earlier and also reported by several previous workers (Borgarello et al., 1985, 1986; Kobayashi et al., 1987), the support prevents the recombination of photogenerated electrons and holes. Borgarello et al. (1986) have worked with CdS supported over Al2O3 and CdS supported over Al2O3-RuO2. These authors have presented evidence that the excited electrons from the conduction band of CdS are transferred to Al2O3 or Al2O3-RuO2, from where they are further transferred to the aqueous solution to generate hydrogen. Though the exact mechanism of this phenomenon is not clear, an intimate contact between the support and the semiconductor is essential for this phenomenon to occur. In the present study, this intimate contact may not be possible when the support is predried. Thus, it appears that lack of the intimate contact and larger crystallite sizes in catalysts 3 and 4, where predried alumina was used for support, are responsible for the poor activity of the catalysts. It is also seen that the crystallite size of CdS in catalyst 4 is nearly double that in catalyst 6, which was prepared by the same technique, the only difference being that, for the former, a predried rather than a hydrogel of alumina was taken as the support. Therefore, it may be concluded that the use of hydrogel leads to a better dispersion of the active ingredient. Catalyst 7 which was prepared at a higher temperature of 673 K has a larger crystallite size, indicating sintering of the CdS crystallites whereas the BET surface area was observed to remain unaffected. Thus, the lower activity of catalyst 7 may be attributed to the sintering of CdS. Furthermore, catalyst 6, where the solid mass was purged with N2 prior to the reaction with H2S in order to remove the adsorbed ammonia from the surface, has a much lower crystallite size (68 Å) as compared to catalyst 5 (138 Å). Thus, the presence of ammonia on the surface of a catalyst during the reaction of CdSO4 with H2S not only leads to the formation of the cubic CdS but also yields CdS of a larger crystallite size. Both of these factors may be responsible for the lower activity
1314 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 5. Electron Diffraction Results of Catalysts d-values (Å) for CdSa standard (fccc)
experimental
(hcpd)
zone
axis
catalyst
d1
d2
d3
d1
d2
d3
d1
d2
d3
(fcc)
(hcp)
5
2.068
3.361
1.753
2.068
1.752
2.068
1.355
3.365 (002) 2.068 (110) b
1.761 (1 h 12) 1.756 (112) b
(001)
3.361
2.068 (110) 3.365 (200) b
(112)
5
(110)
b
6
2.068
3.361
1.753
1.876
1.301
(010)
2.521
2.987
1.368
b
b
b
b
(100)
8
3.200
4.161
1.731
b
b
b
1.761 (1 h 12) 1.398 (203 h) 1.398 (02 h 3) 1.731 (201)
b
8
3.365 (002) 1.898 (103 h) 3.160 (01 h 1) 3.580 (100)
(110)
3.581
2.068 (110) 3.580 (100) 2.450 (01 h 2) 3.160 (101)
(112)
6
1.753 (31 h 1) 1.753 (3 h1 h 1) 1.337 (33 h 1) 1.753 (31 h1 h) b
(110)
3.362
3.360 (111 h) 2.058 (2 h2 h 0) 2.058 (22 h 0) 3.360 (111 h) b
(112)
5
2.058 (22 h 0) 3.360 (1 h 11) 3.360 (11 h 1) 2.058 (22 h 0) b
b
(010)
a
d-values corresponding to r1, r2, and r3. b No corresponding standard d-values. c Face-centered cubic.
of catalyst 5 compared to that of catalyst 6. Another significant observation is that the crystallite sizes of catalyst 6 and 8 are nearly equal but the activity of the former is much more superior. These catalysts were prepared by the same technique except that catalyst 6 was prepared using an ammoniacal solution rather than an aqueous solution used for catalyst 8. This difference in the activities of catalysts 6 and 8 cannot be explained on the basis of crystallite size and dispersion of CdS. Electron Diffraction Studies. It has been mentioned earlier that the XRD results indicated the presence of the cubic phase of CdS in a catalyst where the solid mass was not purged with N2, prior to the reaction with H2S to remove the adsorbed ammonia. Electron diffraction studies were carried out to confirm the presence of the cubic phase of CdS in catalysts 5, 6, and 8. The d-values (interplaner spacing) were calculated by the standard formula (Thomas, 1962) and were compared with the standard values of ASTM cards. The results are presented in Table 5. It is observed that, in the case of the aqueous solution based catalyst (catalyst 8), only the hexagonal phase is present whereas in the case of the ammoniacal solution based catalysts (catalysts 5 and 6), both hexagonal as well as cubic phases are present. This confirms the conclusion drawn earlier that the presence of residual ammonia, during the sulfurization reaction, causes the formation of the cubic phase of CdS. However, as discussed earlier, the intensities of the XRD peaks indicate that the concentration of the cubic phase decreases when the solid mass is purged with N2 to remove the adsorbed ammonia prior to the reaction with H2S, as in catalysts 4 and 6. XPS Studies. The results of XPS studies on catalysts 4, 6, and 8 are presented in Figure 3. The binding energies corresponding to Cd 3d5/2 and 3d3/2 peaks coincided well with those reported for CdS (Vesley and Langer, 1971) (i.e., 405 eV for Cd 3d5/2 and 411.8 eV for Cd 3d3/2). The binding energy for Al 2p was observed to be 74.6 eV which also matched well with the standard XPS data, 74.7 eV for aluminum in aluminum oxide (Wagner et al., 1978). The XRD analysis, as reported earlier, showed that cadmium was present only as CdS. Although XRD gives bulk analysis, in the absence of any contradictory result, it may be assumed that only CdS is present on the surface as well. The intensity ratios, as the ratio of the peak areas of cadmium (3d5/2) and aluminium (2p), of each catalyst
d
Hexagonal close packed.
Figure 3. X-ray photoelectron spectrograph of various catalysts. Table 6. XPS Intensity Ratios of Cd 3d5/2 and Al 2p Peaks catalyst
intensity (Cd/Al) ratio
4 6 8
1.2 ∞ 0.6
were also calculated and are reported in Table 6. The intensity ratio is a function of dispersion as well as relative concentration of the active ingredients on the surface (Best et al., 1977; Al-Ubaid, 1988). Thus, better dispersions and higher concentrations of CdS will yield higher values of the Cd/Al intensity ratio. It is observed that catalysts 4 and 6, where ammonia was used as the impregnating solution, have higher intensity ratios compared to catalyst 8, where an aqueous solution was used for impregnation. It is interesting to note that this ratio is very high for catalyst 6. It has been revealed by XRD analysis that catalysts 6 and 8 have crystallites of comparable sizes; hence the large difference in the Cd to Al ratio may be attributed to a higher surface concentration of CdS in catalyst 6 compared to that of catalyst 8. Therefore, the superior activity of catalyst 6 compared to that of catalyst 8 is due to a higher concentration of CdS on the surface of the catalyst. This
Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1315 Table 7. Summary of Resultsa
catalyst
preparation technique
A12O3*
alumina hydrogel dried at 383 K and then heated at 473 K in air alumina hydrogel aged in ammonia, dried, and heated as above unsupported CdS, H2S gas bubbled in aqueous solution of CdSO4 unsupported CdS, H2S gas reacted with CdSO4 solid at 473 K predried alumina impregnated with ammoniacal solution of CdSO4, dried, and reacted with H2S gas as in catalyst 2 same as catalyst 3 but prior to reaction with H2S solid purged with N2 at 473 K same as catalyst 3 but alumina hydrogel was taken same as catalyst 4 but alumina hydrogel was impregnated same as catalyst 6 except that reaction with H2S was carried out at 673 K same as catalyst 6 but aqueous impregnating solution was used
Al2O3** 1 2 3
4
5 6 7
8
a
activity of (mL of H2 in 2 h‚g of CdS-1)
cumulative pore volume × 103 (mL‚g-1)
BET surface area (m2 ‚g-1)
30 Å g rp
30 Å < rp e 80 Å
80 Å < rp e 150 Å
rp e 150 Å
100
58.0
27.0
13.5
98.5
125
31.0
29.0
26.0
86.0
mean crystallite size (Å)
crystal structure
0.8
61
hcp
2.0
91
hcp
1.6
Cd/Al XPS peak area ratio
largely hcp but fcp was also observed
2.7
140
same as catalyst 3 but fraction of fcc less
7.0
138
largely hcp but fcc was also observed same as catalyst 5 but fraction of fcc less same as catalyst 6
8.1
21
23.0
12.0
3.0
38.0
68
6.1
25
32.0
28.0
9.0
65.0
148
5.9
12
6.0
11.5
8.0
25.5
76
hcp
1.2
∞
0.6
The composition of all the supported catalysts (catalysts 3-8) was CdS:Al2O3 ) 1:2 by weight.
difference in activity could not be explained by dispersion alone (cf., XRD results). Sinha and Shankar (1993) have also reported that ammoniacal impregnating solutions lead to a higher concentration of active ingredients on the support. It may be relevant to add that Irwin and Moss (1980) and Sinha and Shankar (1993) have reported that, in catalytic combustion, the reactions being very fast, preferential distribution of active ingredients near the surface is desirable. Similarly, a higher concentration of the active ingredient on or near the surface is also desirable for the photocatalytic reactions, since light cannot penetrate deep inside the porous matrix of the support, which has been found to be true on comparing the activities of catalysts 6 and 8. Distribution of the active ingredients during the drying operation may be responsible for a higher concentration of CdS on the surface (Lee, 1981; Sinha and Shankar, 1993). When solid particles are impregnated with solutions which contain a volatile liquid such as ammonia, then on drying, in addition to the capillary induced flow, the gas evolved causes the liquid column ahead to move to the surface exposed to the drying medium. Therefore, drying largely takes place from or near the surface. This results in a higher concentration of the active ingredient on the outer surface of catalyst 6 and also in the pores of very large radii (macropores) which are formed at later stages of drying. It is to be emphasized that the pore size analysis by N2 adsorption as discussed above also shows the preferential deposition of CdS in the pores of larger radii in the case of catalyst 6.
Conclusions On the basis of the discussions presented above and a summary of results given in Table 7, the following conclusions are drawn. Activity of the alumina-supported CdS is better than that of the unsupported CdS. Dispersion as well as distribution of CdS on the support, which are affected by the preparation protocol, have critical influence on the activity. The use of alumina hydrogel as a support results in a better dispersion of the active ingredient and also promotes intimate contact between CdS and Al2O3 and thereby an enhanced activity. Use of an ammoniacal impregnating solution results in a higher surface concentration of CdS and consequently better activity. Ammonia gas evolved during the drying operation causes higher surface concentration. Any residual ammonia adsorbed on the surface during the reaction of cadmium sulfate with H2S gas at the preparation stage adversely affects the activity of the catalyst because of the formation of the cubic phase and larger crystallite size of CdS. Acknowledgment The authors thank Professor O. N. Srivastava, Department of Physics, BHU, Varanasi, India, for helping with X-ray diffraction and electron diffraction facilities. They also thank Professor M. C. Bhatnagar, Department of Physics, Indian Institute of Technology, Delhi, India, for helping with the XPS facility. The authors are also thankful to the Ministry of Non-Conventional Energy Sources, Government of India, New Delhi, India, for funding the research project. One of the authors
1316 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998
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Received for review July 9, 1997 Revised manuscript received December 12, 1997 Accepted December 12, 1997 IE970477S
