Abstract
A series of catalysts of Ag2O-Co3O4/γ-Al2O3 was prepared by equivalent volume impregnation method. The effects of the metal loading, calcination time, and calcination temperatures of Ag and Co, respectively, on the catalytic activity were investigated. The optimum preparing condition of Ag2O-Co3O4/γ-Al2O3 was decided, and then the influence of different preparation conditions on catalytic activity of Ag2O-Co3O4/γ-Al2O3 was analyzed. The results showed the following: (1) at the same preparation condition, when silver loading was 8%, the Ag2O-Co3O4/γ-Al2O3 showed higher catalyst activity, (2) the catalyst activity had obviously improved when the cobalt loading was 8%, while it was weaker at loadings 5% and 10%, (3) the catalyst activity was influenced by different calcination temperatures of silver, but the influences were not marked, (4) the catalyst activity can be influenced by calcination time of silver, (5) different calcination times of cobalt can also influence the catalyst activity of Ag2O-Co3O4/γ-Al2O3, and (6) the best preparation conditions of the Ag2O-Co3O4/γ-Al2O3 were silver loading of 8%, calcination temperature of silver of 450°C, and calcinations time of silver of 4 h, while at the same time the cobalt loading was 8%, the calcination temperature of cobalt was 450°C, and calcination time of cobalt was 4 h.
1. Introductions
Humans suffered from serious environmental pollution owing to the burning of fossil fuels. Moreover, as a kind of nonrenewable energy, fossil fuels are decreasing gradually even drying up in some cases [1]. In order to maintain the sustainable development of society, we have to develop and use reusable energy and new energy. New energies like hydrogen are becoming important as a kind of replaced energy and playing a more and more important role in primary energy [2–4]. Hydrogen can be prepared with fossil fuels or nonrenewable energy [5–7]. But according to the characteristics of China’s coal resource [1], the preparation for hydrogenation of coal pyrolysis may be more appropriate in reality. Because the pyrolysis temperature is low and the technical process is fixable, therefore, it can be both final disposal to prepare some hydrogen-rich gas, tar, and semicoke, and so forth, for the future of the people’s demand and middle disposal to realize the polygeneration with gases, tar, and semicoke which produced in pyrolysis process [8–10]. But the output in simple hydrogenation of coal pyrolysis is low presently, and adding appropriate catalysts will improve the efficiency of traditional coal pyrolysis process because the study of catalyst plays a vital role. This paper discussed the influences of six different preparation conditions of catalyst (Ag2O-Co3O4/γ-Al2O3) activity in loading, calcination time, and calcination temperatures of silver and cobalt, analyzed the influences of different preparation conditions on catalytic activity of Ag2O-Co3O4/γ-Al2O3 for hydrogenation of coal pyrolysis, and then found the optimum conditions to prepare Ag2O-Co3O4/γ-Al2O3.
2. Experiment Device and Method
2.1. Catalyst Preparation
Catalysts were prepared with equivalent volume impregnation method. Optimum dose of AgNO3 and Co(NO3)26H2O was taken to prepare a certain concentration liquor according to the 5%, 8%, and 10% loadings of silver and cobalt. Primarily, move corresponding Co(NO3)2 soak into γ-Al2O3 and bake or dry it with a slow fire after placing 24 h. Then, put it into an oven and let it dry overnight. Thirdly, place it into a muffle furnace to roast. Finally, soak γ-Al2O3 in AgNO3 and repeat above steps; then Ag2O-Co3O4/γ-Al2O3 bimetallic catalyst can be prepared.
2.2. Evaluation of Catalytic Activity
In this experiment, samples were placed in a tubular reactor housed in a furnace. In each test, the inside temperature of the furnace was raised from room temperature to 1100°C at a rate of 17.8°C min−1 at the system pressure of 101.3 kPa with 1 h of reaction time. The gas product of pyrolysis was collected at an increase of every 100°C, dedusted, dried, and analyzed by gas chromatography. The hydrogen concentration was analyzed using nitrogen as the carrier while helium was used for analysis of carbon dioxide and carbon monoxide. The gas product can be calculated using , where is the yield of interested gas (mL), is the gas concentration, and is gas the total volume (mL). The schematic diagram of experimental device is shown in Figure 1.
3. Results and Discussions
3.1. The Influence of Different Silver Loadings on Catalytic Activity
The relationship graph between catalytic activity of Ag2O-Co3O4/γ-Al2O3 and silver loading is referred to in Figure 2. From Figure 2 we can see that the catalytic activity of different silver loadings was not distinct in 700°C950°C. When the temperature was within 900°C950°C, catalytic activity with silver loading at 10% showed better performance. From the above analysis, it can be seen that catalytic activity of Ag2O-Co3O4/γ-Al2O3 was not increased with the silver loading increase.
Figure 3 showed the XRD of different silver loadings; it can be seen from Figure 3 that all the samples had the complete Co3O4 crystal structure, which showed the characteristic peak of Co3O4 at 2θ = 31.249°, 36.840°, 45.005°, 55.916°, 59.508°, and 65.458°; however when the silver loading was 8%, characteristic peak of the Ag2O was shown at 2θ = 32.839° and 38.109° in the XRD, but the characteristics’ peak of Ag2O was not obvious when the silver loading was 5% and 10%. The reason was that Ag2O and Co3O4 presented a strong interaction with the increasing of the silver loading, which caused the Ag2O dispersed as the microcrystalline on the surface of catalyst.
Finally several good conclusions were drawn from the former analyzing conclusion. As a result of the above analysis Co3O4 is a kind of p-type semiconductor and conducts by cavitations. When Ag2O loaded on Co3O4, as the valence state of Ag+1 is lower than cobalt, it plays a role as acceptor impurity, which can increase the cavitations and the conductivity of Co3O4 semiconductor [11–13]. Take produced hydrogen occurring on the metal cobalt oxide catalysts for propane secondary cracking as an example. Propane became positive ions adsorbed on the catalyst, and the function of propane can be a donor impurity; the propane gave electronics to Co3O4; then the cavitations were decreased; thus decrease of the cavitations became unhelpful to accept propane electronic. If Co3O4 was added to the acceptor impurity Ag+1, the cavitations’ number will be raised, which improved the electric conductivity remarkably, availed surface adsorption step, and reduced the activation energy of hydrogen production from propane secondary cracking correspondingly. As a result, it has the highest catalytic activity when silver loading was 8% [14, 15].
3.2. The Influences of Different Cobalt Loadings on Catalytic Activity
Figure 4 was catalytic activity curves for hydrogenation of coal pyrolysis when cobalt loading was 5%, 8%, and 10%. From those curves it can be seen that the catalyst activity has obviously shown improvement when loading of the Co was 8%, but it was weaker when loading of Co was 5% and 10%.
Figure 5 showed that all the catalysts have shown the characteristics of diffraction peak of Co3O4, but the intensity of the diffraction peak differs with the change of cobalt loading. The intensity of diffraction peak was weakest at 5% cobalt loading; the diffraction peak of 8% cobalt loading showed both the characteristics of Ag2O diffraction peak and the characteristics of diffraction peak of Co3O4, while the intensity of diffraction peak was strongest at 10% cobalt loading. Then, by comparing Figure 4 with Figure 5, crystalline phase of Co3O4 was not the main active center for hydrogenation of the coal pyrolysis. The reason was that the catalytic activity of 8% the cobalt loading was optimal, but the intensity of characteristics diffraction peak at cobalt 8% loading was lower than 10% loading. This means that intense interactions exist in Ag2O and Co3O4. The catalyst activity can be influenced by this interaction mainly.
3.3. The Influence of Different Silver Calcination Temperatures on Catalytic Activity
Figure 6 showed the influence of different silver calcination temperatures on Ag2O-Co3O4/γ-Al2O3 catalytic activity. The results showed that catalytic activity was the best when calcination temperature was at 450°C, while the change was unobvious at 400°C and 500°C. It can be inferred that the different calcination temperatures had influenced the catalyst activity, but the influences were not marked.
It can be seen from Figure 7 that the characteristics’ diffraction peak of Co3O4 was sharp; the intensity was the strongest and formatted the Ag2O crystal phase simultaneously when the calcination temperature was 450°C. While the Ag2O crystalline degree was bad when the calcination temperature was 400°C, so presumably the Ag2O crystal phase has not formed not yet; therefore this made the catalyst activity low relatively. When the calcination temperature was 500°C, with the increasing of calcination temperature, silver can disperse the surface of γ-Al2O3 preferably because of the interaction of Co-Ag.
Combined with Figure 6 and Table 1, it can be inferred that although the main phase of catalyst existed was Co3O4 when the calcination temperature was 400°C and 500°C, but the specific surface area had changed. Table 1 showed that the specific surface area of 400°C had greatly improved compared to that of the 500°C; the high temperature roasting may be the main reason causing the sintering on surface, which led to the decreasing of specific surface area. Those reasons can make the catalyst activity decrease. On account of the catalyst Ag2O crystalline phase existed obviously when the calcination temperature was 450°C. Thus the catalyst activity had increased; its specific surface area was reduced, so that the catalyst activity, which existed two crystalline phases, had no corresponding relation obviously with the structure and the specific surface area of catalyst. Therefore, it was concluded that the interaction between Co3O4 and Ag2O may be more important factors affecting catalytic activity.
3.4. The Influence of Different Cobalt Calcination Temperatures on Catalytic Activity
The influence of different cobalt calcination temperatures on catalytic activity is referred to in Figures 8 and 9. The catalyst showed best activity when cobalt calcination temperature was 450°C at the range of 400°C800°C while at different temperatures catalytic activity’s changes were not distinct at the range of 800°C950°C. When cobalt calcination temperature was 400°C, the catalytic activity was better than the other two catalysts. The catalyst showed higher activity when the cobalt calcination temperature was 450°C in the whole temperature range. It was due to the fact that intensity of Co3O4 characteristic diffraction peak was lower than the other two catalysts, so the activity was awful [16–18]. The catalytic activity component particle may be sintering at 500°C; thus the number of the surface active sites was decreased, so the catalytic activity for hydrogenation of coal pyrolysis was reduced.
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3.5. The Influence of Different Silver Calcination Times on Catalytic Activity
Figure 10 showed the different silver calcination times’ influence on catalytic activity. It can be inferred from Figure 5 that different calcination times can also influence the catalytic activity. Catalyst showed the best activity while calcination time was 4 h; in contrast the catalytic activity’s change was not distinct when the calcination time was 3 h or 5 h.
From Figure 11 it can be inferred that all catalysts showed the characteristics diffraction peak of Co3O4, but the intensity of the diffraction peak varied with the silver calcination time change. The intensity of the Co3O4 diffraction peak was the strongest when the calcination time was 4 h, and also the characteristic diffraction peak of Ag2O at 2θ = 32.839° and 38.109° was observed while the catalysts whose calcination times were 3 h and 5 h had not shown the characteristic diffraction peak of Ag2O almost. Because when the calcination time was 3 h the transformation from Co(NO3)26H2O and AgNO3 to Co3O4 and Ag2O was incomplete, and some of cobalt and silver cannot be transformed to Co3O4 and Ag2O which possess catalytic activity [19, 20]. But when silver calcination time was 5 h, the time was too long to make catalyst grow up and gather on the surface and reduced the active particles, which lead to catalytic activity decrease.
3.6. The Influence of Different Cobalt Calcination Times on Catalytic Activity
Figure 12 showed the different cobalt calcination times influencing catalytic activity. Different calcination times of Co can also influence the catalyst activity of Ag2O-Co3O4/γ-Al2O3 in Figure 12. The catalytic activity of Ag2O-Co3O4/γ-Al2O3 was the best when the calcination time was 4 h; on the contrary, when calcination time was 3 h or 5 h, the change of catalytic activity was not distinct.
4. Conclusions
At the same preparation condition, Ag2O-Co3O4/γ-Al2O3 with loading 8% silver showed higher catalyst activity. When the range of calcination temperature was 700°C950°C, the change of catalytic activity was not distinct for different silver loadings, while catalytic activity only showed enhanced activity of 10% silver loading in 900°C950°C. The activity had obviously improved when loading of the Co was 8%, while it was weaker by loadings 5% and 10%. Catalytic activity was the best when calcination temperature of silver was at 450°C, while it had a little change at 400°C and 500°C. It can be inferred that the different calcination temperatures can influence the catalyst activity, but the influence was not marked. Different calcination times can also influence the catalyst activity. Catalytic activity was the best at 4 h, while the change of catalyst activity was not distinct when the calcination time was 3 h or 5 h. When loading of the silver was 8%, the calcination temperature of silver was at 450°C and the calcination time of silver was 4 h, while at the same time the Co loading was 8%, the Ag2O-Co3O4/γ-Al2O3 showed the best catalyst activity.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
The financial support of this research by the Natural Science Basic Research Plan in Shaanxi Province of China (Program no. 2011JQ2015) in China and the financial support of this research by the Scientific Research Program Funded by Shaanxi Provincial Education Department (Program no. 2013JK0869) in China are gratefully acknowledged.