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6 | Clean Energy, 2017, Vol. XX, No. XX metal dispersion of 1.86% and greater active metal surface area of 12.4 m2/g of metal compared to SiO2 and ZrO2. Roy et al. [22] investigated the effect of Ni supported on Al2O3 and CeO2 in SR of butanol. CeO2 showed better activity in terms of hydrogen selectivity compared to Al2O3. The lower activity of Al2O3-supported catalyst was confirmed by hydrogen chemisorption results, which indicated that active metal grew by ~28% in size compared to the CeO2- supported catalyst. The growth in size was attributed to the higher reduction temperature, i.e. 1050 °C on Al2O3- supported catalyst. Cai et al. [30] carried out SR of n-butanol over Co supported on TiO2, CeO2 and ZnO. Out of these three catalysts, Cu/ZnO showed better activity and the highest hydrogen content in exit gas. These results were supported by Raman spectroscopy and TPO tests, which revealed the most disordered carbonaceous deposits on Cu/ZnO and the most ordered carbon deposits on Cu/TiO2. These results clearly indicated better metal support inter- action with the Cu/ZnO catalyst. The concentration and size of active metal govern the yield of hydrogen and conversion. The conversion and hydrogen yields increased from 31.2 to 100%D and 64.49 to 84.49%E, respectively, with an increase in metal loading from 10 to 25% by wt. of Ni over γ-Al2O3 support [48]. This trend was observed due to an increase in the number of active sites with an increase in metal loading. However, this trend cannot be generalized for all catalysts. Bimbela et al. [44] prepared three catalysts of different Ni loading (23, 28 and 33%) and found the catalyst with intermediate metal loading of 28% showed the best results in terms of catalytic activity. The amount of carbonaceous deposits on the catalyst, revealed after SR reaction, was less than 28% for the Ni catalyst compared to the other two catalysts. The effect of copper as an additive was investigated to find its impact on SR of butanol [46]. Ni/Al co-precipitated catalysts were prepared with 0.1 and 5.5% loading of Cu by weight. The catalyst activity of butanol SR decreased with an increase in Cu loading. This behavior could be due to in- activity of Cu toward C–C bond cracking. Further research for the optimization of additives would be useful as there are few studies reported on the effect of additive during SR of butanol. The main obstacle for steady-state operation of butanol SR is the formation of unwanted carbonaceous deposits on the catalyst surface. Coke formation is mainly due to the Boudouard reaction. The formation of coke is detri- mental to catalyst structure and it covers the active cata- lyst surface resulting in reduced activity of the catalyst. The challenge is to design a catalyst that remains stable for a prolonged period of time and also provides higher conversion and yield of hydrogen. Time on stream (TOS) data can provide evidence about the stability of a catalyst. Harju et al. [41] observed during SR of butanol that without the presence of active metal (Rh) conversion of butanol over support collapses in just 15 min. The presence of Rh over ZrO2 resulted in the complete conversion of butanol for 23 h at 700 °C. The same catalyst was found to exhibit lower stability at 500 and 600 °C. These results were sup- ported by TPO tests of the catalyst. This test confirmed for- mation of three types of carbon deposits on the catalyst: (i) carbon deposited on noble metal (Rh), (ii) carbon deposited on support (ZrO2) and (iii) less reactive and more graph- itic carbon species. The results indicated that total carbon deposits on catalysts were decreased with increases in temperature and resulted in the increased stability of the catalyst. The study also reported that the carbon depos- ited on noble metal (Rh) were very small after 25.5 h of SR at 700 °C, while the amount of carbon deposits on noble metal were relatively higher at 500 and 600 °C compared to 700 °C [41]. These results prove that noble metal and temperature are key factors in controlling coke deposition. Valant et al. [50] conducted SR of n-butanol and i-butanol over 1% Rh/MgAl2O4/Al2O3 catalyst. Water was produced and not consumed during the course of reaction, indi- cating that the dehydration reaction plays a crucial role in the alcohol conversion. Dehydration produces alkenes, which in turn polymerizes and converts to coke. The effect was more evident in the case of i-butanol compared to n-butanol because of the formation of stable carbocations. It was also documented that the acidic nature of alumina was also responsible in enhancing formations of alkenes and coke by the dehydration reaction [50]. These results indicate the type of alcohol (branched or linear) and sup- port–metal interaction decide the formation of coke. 1.2.5 Sorption-enhanced steam reforming An attractive feature of sorption-enhanced steam reform- ing (SESR) is production of high-purity hydrogen with min- imum formation of coke and carbon monoxide by shifting equilibrium of WGS reaction. This high-purity hydrogen can be used without any purification in proton exchange membrane fuel cells. In this process, carbon dioxide ad- sorbent, e.g. CaO, is used along with the catalyst, which shifts the equilibrium of WGS reaction in a forward direc- tion and thereby reduces the concentration of CO and CO2 in the product gas stream [43, 45]. There are limited studies on the thermodynamic analysis of hydrogen production from butanol via SESR [43, 45]. We found no experimental study on SESR of butanol. Thermodynamic studies revealed the possibility of generation of high-purity hydrogen from butanol, but as there is no experimentation, it is too early to comment on the feasibility of this process. 1.2.6 Steam reforming of bio-butanol SR of bio-butanol (crude mixture of butanol, acetone and ethanol in 6:3:1 mass ratio, respectively) obtained after preliminary distillation of fermentation liquid can be a lucrative option for hydrogen production as excessive amount of the energy required for the distillation pro- cess can be reduced to many fold. To the greatest of our knowledge, only one study based on SR of bio-butanol crude mixture has been conducted to explore hydrogen production capacity of this renewable feedstock. Cai et al. [30] experimented SR of bio-butanol over monometallic Co Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkx008/4743500 by guest on 15 December 2017PDF Image | Renewable hydrogen production from butanol
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