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8 | Clean Energy, 2017, Vol. XX, No. XX If an equilibrium has been achieved, an increase in oxygen content should reduce hydrogen production due to increasing oxygen content favoring oxidation reaction. However, it was also noted that hydrogen yield improved with an increase in OBMR from 2.5M to 2.7M over Rh/Al2O3 and Ni/Al2O3 catalyst [5]. This may be due to improved activity of catalyst, and there was no significant reduc- tion in metal reducibility of used catalyst. There are also chances that with the prevailing set of reaction condi- tions equilibrium was not achieved. A further increase in OBMR beyond 2.7M resulted in a decrease of the hydrogen yields. This behavior was due to the decreased reduci- bility of active metals [5]. Harju et al. [41] also documented similar results. After comparing experimental results with thermodynamic equilibrium conditions, it was con- cluded that the equilibrium was not achieved otherwise it would have decreased the hydrogen yield. Few studies have reported hydrogen production by OSR of bio-butanol [51–54]. Experimental results are summa- rized and compared in Table 5. For all experiments, the SCMR, bio-butanol to oxygen molar ratio and pressure were maintained at 3, 1/1.5 and 101.3 kPa, respectively. Bimetallic catalysts, with one metal Co and other noble metals such as Rh, Ru, Pd and Ir, have been explored to de- termine their hydrogen production capability [51]. TPO and Raman spectroscopy tests confirmed the best activity and least coke deposition on catalyst for CoRh/ZnO. Addition of noble metal markedly improved activity when compared with monometallic Co/ZnO. Group 9 metals (Rh, Ru) were found to exhibit better activity. The addition of noble metal favors the stabilization of Co0 phase and reduces the crys- tallization of Co, which in turn increased reducibility and decreased sintering and carbon deposition of the catalyst [51]. Apart from the type of metal, loading of a particular metal also plays an important role in the activity of cata- lyst. Cai et al. [54] prepared four different catalysts and investigated the role of Ir in OSR of bio-butanol. An im- provement in catalytic performance in terms of hydrogen production, conversion and stability of the catalyst was found when adding a small amount of noble metal. However, the optimum amount of noble metal should be decided as a higher percentage of metal may lead to lower activity. Addition of iridium (~0.3% by wt.) to Co/ ZnO improved its catalytic performance because of the improved reduction characteristics of Co and support and improved coke resistance. However, further addition of Ir (~1% by wt.) showed no subsequent improvement in catalytic activity. This behavior was justified by separate Ir0 cluster interacting with ZnO support, as confirmed by high-resolution transmission electron microscopy. The nature of support and support structure also gov- erns the activity of catalyst [52, 53]. Cai et al. [53] studied the role of different supports such as ZnO, TiO2 and CeO2- ZrO2 (18% CeO2 by wt.) over Co-Ir bimetallic catalysts in OSR of bio-butanol. CeO2-ZrO2 was found to exhibit better catalytic performance due to better reducibility of metal, lower Co sintering and smaller coke deposition. Cai et al. [52] determined the role of support structure in OSR of bio- butanol. Calcination temperature of support was found to have an impact on crystallite size, degree of reduction, oxygen storage capacity and total number of basic sites of support. Support calcined at 500 °C indicated the best cata- lytic activity when compared with catalysts calcined at 700 and 900 °C. Support calcined at 500 °C showed higher oxygen storage capacity and basic characteristics, which favored oxygen diffusion from bulk and oxidized carbon deposited on catalyst surface. 3 Dry reforming of butanol In DR, reforming is carried out with CO2 as a reactant. DR can be an attractive option as CO2 can be converted into syngas, hydrogen or high-value-added inert carbon, and it can be removed from the carbon biosphere cycle [55]. We found few studies that indicated sequestration of CO2 and the marketable product ‘carbon nanofibers’ [55–58]. DR of methane [59–61] and ethanol [56–58] have been reported extensively. In contrast, we found only one study for butanol based on thermodynamic analysis of butanol DR [55]. One of the reasons for limited study on butanol DR may be the highly endothermic nature of reaction and high amount of coke deposition. The renewable nature of butanol, high hydrogen content and sequestration of CO2 in the form of carbon nanofibers does open the possibility of development of an active catalyst. In a study by Wang [55], coke-free regions and coke-formed regions were identified. This study encourages further development of a suitable catalyst for DR of butanol if reaction conditions are selected in coke-free regions. 4 Partial oxidation 4.1 General aspects POX of butanol is carried out in the absence of steam in feed. POX reaction is exothermic in nature unlike SR and DR [62]. In fuel-rich (deficit of oxygen) environment, POX of butanol leads to formation of hydrogen. Butanol can be completely oxidized to carbon dioxide and water in a fuel-lean (excess of oxygen) condition [63]. In the POX technique, all the reactions mentioned in Table 2 take place except stream reforming of butanol and me- thane. In addition to these reactions, complete oxidation of butanol ( C4H10O + 6O2 ↔ 4CO2 + 5H2O ), POX of butanol CH O+3O ↔4CO+5H, combustion of hydrogen 410 22 2 H +1O →HO and oxidation of carbon monoxide 222 2 CO + 1 O → CO needs to be considered in POX [64]. 22 2 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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