Full Length ArticleSteam reforming of acetic acid over Ni/Al2O3 catalyst: Correlation of calcination temperature with the interaction of nickel and alumina
Graphical abstract
Introduction
Nickel–based catalysts have many applications in various catalytic reactions such as steam reforming, hydrogenation, methanation and etc. [1], [2], [3], [4], [5]. In some cases unsupported nickel such as Raney Ni is used as the catalyst, while more frequently nickel is supported on a support for the use as a catalyst [6], [7]. The use of a support instead of using pure nickel is more advantageous. For example, a support with a high surface area could enhance the dispersion of nickel which consequently decreases the usage of nickel and might reduce the cost of catalyst manufacturing. In addition, some supports are not inert materials, which could involve and play a role in the catalytic reactions [8], [9], [10], [11], [12]. One example is Pt/ZrO2, a bifunctional catalyst used for steam reforming of acetic acid [13]. Pt mainly activate acetic acid while ZrO2 helps to activate steam for the reforming reactions. The support interacts with nickel catalyst, which significantly affect the catalytic properties [14], [15], [16].
One typical example is nickel supported on alumina. The surface of alumina (the gamma form) has some acidity and the functionality such as hydroxyl group, which could have strong metal–support interactions [17], [18], [19] At elevated temperatures such as above 800 °C, alumina and nickel could react, forming nickel aluminum spinel (NiAl2O4) [20]. The formation of NiAl2O4 significantly affect the behaviors of nickel species as NiAl2O4 is much more difficult to be reduced than nickel oxides. The interaction of nickel with alumina is a well–known phenomenon [21]. However, how does the essential parameters for preparation of the Ni/Al2O3 catalyst affect the extent of the interaction between nickel and alumina needs further investigation. Understanding this could help to optimize the parameters for preparation of the Ni/Al2O3 catalyst for maximizing its performances for catalytic reactions, which could also provide useful information for preparation of other heterogeneous supported catalysts.
In this study, the effects of nickel loading (10 wt% and 20 wt%) and the calcination temperatures (from 500 to 1000 °C with an increment of 50 °C) on the interaction of nickel species with the alumina carrier were investigated. The catalysts prepared were characterized in detail and evaluated in steam reforming of acetic acid. The selection of steam reforming of acetic acid as a probe reaction is due to the fact that acetic acid is a main component in the bio–oil produced from pyrolysis of renewable biomass [22], [23], [24], [25], [26]. Bio–oil is a feedstock for hydrogen production [27], [28]. Nevertheless, bio–oil has a very complex composition and steam reforming of bio–oil involves a complicated reaction network [29], [30]. Since acetic acid generally has a high abundance in bio–oil, the steam reforming of acetic acid would provide useful information for understanding the reaction network in steam reforming of bio–oil. Thus, in this study, steam reforming of acetic acid was used as a model reaction to probe the effects of the interaction between nickel and alumina on the catalytic behaviors.
Section snippets
Preparation of the catalysts
The Ni/Al2O3 catalysts were prepared by an incipient wetness method. Before the impregnation, given amount of Ni(NO3)2·6H2O as a precursor was dissolved in deionized water to achieve the nickel loading of 10 wt% or 20 wt% (the loading was defined by the weight of metallic nickel divided by the weight of alumina) on Al2O3 (30–45 mesh). After impregnation, the catalysts precursor was dried at 110 °C for 2 h. After that, the resulting solid was calcined at a certain temperature for 4 h. The
Temperature programmed reduction (TPR)
TPR profiles of Ni catalysts calcined at the different temperatures were shown in Fig. 1. The reduction of nickel oxides initiated at ca. 400 °C while the main reduction peak spanned from ca. 600 to 900 °C (Fig. 1a). Unsupported nickel oxide has a sole reduction peak at 350 °C. Obviously, in Ni/Al2O3 catalyst, nickel species had different degrees of interaction with alumina support [31], [32], [33]. The reduction peak at the lower temperature represented the nickel oxides with the weak
Conclusion
To sum up, the calcination temperature and nickel loading have significant effects on the interaction of nickel species with alumina, the activity, stability, and resistivity towards coking of the catalysts as well as property of the coke formed. Higher calcination temperature led to the strong interaction between nickel species with alumina. The calcination temperature of above 700 °C led to the solid phase reaction and the formation of nickel–alumina spinel, shifting the reduction temperature
Acknowledgements
This work was supported by the Strategic International Scientific and Technological Innovation Cooperation Special Funds of National Key R&D Program of China (No. 2016YFE0204000), the Program for Taishan Scholars of Shandong Province Government, the Recruitment Program of Global Young Experts (Thousand Youth Talents Plan) and Natural Science Fund of Shandong Province (ZR2017BB002).
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