Ethanol steam reforming over heterogeneous catalysts

Depletion of non-renewable energy sources such as coal and natural gas is paving the way to generate alternative energy sources. Hydrogen is considered as one of the clean and green energy sources. Currently, 96% of hydrogen is produced from non-renewable sources. Hydrogen production from renewable sources is considered a promising route for development of sustainable energy production [1]. Ethanol steam reforming (ESR) has received considerable attention, because ethanol is mainly obtained by renewable sources, it is simple to store, handle and transport because of its low volatility and toxicity [2].Metal catalysts play an important role on ethanol conversion and hydrogen production in ESR. Both noble metals (Rh, Ru, Pt, Pd, Ir, La) and non-noble metals (Ni, Co) were used in ESR reactions. These metals are able to break the high energy C-C bond which is an important characteristic of these catalysts for ethanol conversion. Despite the active metal, the supports also interact with ethanol and effect its conversion or transformation to other products. Ni-based catalysts have been used extensively due to its high activity and selectivity in breaking C-C bonds and its low cost compared to noble metals. It also catalyzes the water-gas shift reaction, improving H2 yield and its subsequent purification from CO. However, it produces more by-products such as coke and CH4 than Co-based catalysts do. Various preparation methods, Ni precursors, and supports have been actively investigated to obtain Ni catalysts with highly dispersed small Ni particles because the Ni particle size is an important factor for controlling activity, selectivity, and carbon deposition [3]. For Ni catalysts, various supports have been investigated: Al2O3, M-Al2O3 (M: La, Zr, Ti), Sn-La2O3, K-ZrO2, LiAlO2, MgAl2O4, Ce-MgAl2O4, SiO2, Ce-SiO2, zeolite (ZSM-5), mesoporous silica (SBA-15), CeO2, MCeO2 (M = La, Sm, Zr, Ca, Mg, K), graphene, Ni containing perovskite oxides, Ni-Mg-Al hydrotalcite, and Ni-Mg-Al oxyhydride. As active metals, bi-metallic or tri-metallic catalysts such as Ni-Cu, Ni-Sn, Ni-Pt, Ni-Rh, Ni-Au, and Ni-Cu-Fe have been used [4].Among Ni-based catalysts, Al2O3 support has been applied widely to ESR because of its high surface area and stability. However, Al2O3 support has an acidic site and promotes formation of ethylene, a coke precursor that promotes coke deposition. Modification of Al2O3 support by basic oxides (La2O3 and ZrO2) neutralizes the acidic sites effectively and suppresses coke formation, although addition of excess amount of modifier decreases its specific surface area and activity [5]. The noble metals are more resistant to carbon deactivation compared to non-noble metals. Among noble metals, Rh is highly active for both C-C and C-H bond scission. It induces hydrogenation reaction and leads to very low carbon deposition. It also has the ability to evade carbonate formation that s why the oxygen vacancies are easily accessible for oxidizing carbon. Carbon formation was reported insignificantly for Rh at stoichiometric molar ratio of water and ethanol. Nevertheless, Pt catalyst over different oxide supports (CeO2, Al2O3, ZrO2) during ESR showed significant amount of carbon deposition. Comparative study of ESR on Au and Pd catalyst over SiO2 support revealed Au to be more stable compared to Pd. Overall, the carbon depositions as compared to non-noble metals are considerably less [6].The main drawback of all the steam reforming reactions is the deactivation of the catalyst. Catalyst deactivation can be mainly classified into the following types: coke formation; active metal sintering or oxidation. In order to overcome these problems, several strategies have been reported such as catalyst modification with promoter, small particle size, presence of lattice oxygen and increasing water to alcohol ratio of the reactions [1].

Keywords:

Ethanol steam reforming (ESR) , Ni catalyst , support , deactivation , coke formation , sintering