Rapid evaluation of coke resistance in catalysts for methane reforming using low steam-to-carbon ratio
Graphical abstract
Introduction
Steam methane reforming (SMR) is important because it is the only practical solution for producing hydrogen on a large scale [1], [2], [3], [4]. A series of nickel-based commercial catalysts have been implemented for the large-scale production of hydrogen using SMR. These systems use a high steam-to-carbon ratio (S/C = 3.0) to suppress coke formation, which is closely related with the deactivation of catalysts. Recently, various kinds of oxygenates (ethanol, glycerol, and ethylene glycol) [5], [6], [7], [8], [9] and hydrocarbons (propane, benzene, and toluene) [10], [11], [12], [13], [14] have been considered in place of methane. In the dry reforming of methane (DRM), CO2 instead of H2O was used to produce syngas (CO + H2) [4], [15], [16], [17]. In these reactions, commercial Ni-based catalysts quickly lose their catalytic activity mainly due to severe coke formation. To overcome this problem, precious metals (Ru, Rh, Pt, Pd, etc.) [2], [3], [4], [18], alkaline earth metals (Mg, Ca, Ba, Sr, etc.) [19], [20], [21], [22], [23], [24], [25], and rare earth metals (La, Ce, Pr, etc.) [2], [26], [27], [28], [29], [30] have been tested as additives to enhance coke resistance. Different catalyst preparation methods [31], [32], [33] and catalyst supports [1], [2], [3], [4], [34], [35] have also been studied so far. As the formation and subsequent accumulation of coke is among the major reasons for the catalyst deactivation in methane reforming reaction, the investigation of coke-resistant catalysts is closely related to their long-term stability. However, it takes a long time to quantitatively measure the amount of carbon deposition on catalysts under normal operational conditions [36], [37], [38]. In this study, we developed a new method to rapidly evaluate the coke resistance of given catalysts, by using a low S/C of 0.5 and high space velocity of 30,000 h−1 within 5 h in SMR. Using this method, the effects of various kinds of additives were investigated. An optimized catalyst with the composition of 0.5 wt.% Ru/5 wt.% Mg/10 wt.% Ni/alumina was applied to long-term (250 h) stability test of SMR and 40 h test of DRM. These results confirmed that the method developed here is valid for the rapid evaluation of coke resistance for given catalysts.
Section snippets
Catalyst preparation
The nickel-based base catalysts were prepared by incipient wetness impregnation method. Nickel nitrate hexahydrate (1.701 g, 97%, Aldrich) was dissolved in deionized water. Then, this solution was added into alumina (3 g, γ-Al2O3, SASOL) by incipient wetness impregnation method. After impregnation, this sample was dried in an oven at 100 °C for 12 h, and further calcined in a furnace at 700 °C for 4.5 h in air. The product is denoted as “calcined 10 wt.% Ni/alumina”.
For the precursors of precious
Physical properties and nickel dispersion of catalysts
Table 1 shows the physical properties of various catalysts. All samples had the similar surface area and pore volumes. Using various additives (ruthenium and/or magnesium) on 10 wt.% Ni/alumina slightly decreased the surface areas and pore volume. Nevertheless, these additives at relatively low loading ( < 5 wt.%) did not change the surface area and pore volume significantly. The CO chemisorption results indicate that the addition of precious metals (Ru or Rh) and/or alkaline earth metal reduced
Conclusion
Severe reaction conditions favorable for the coke deposition, namely low S/C (0.5) and high space velocity (30,000 mL h−1 g−1), were applied to evaluate the coke resistance of given catalysts with the experimental duration of 5 h. This rapid evaluation method was used to assess the effects of various additives (precious metals (Ru, Rh, Pt, Pd) and alkaline earth (Mg, Ca, Sr, Ba)), resulting in an optimized composition of 0.5 wt.% Ru/5 wt.% Mg/10 wt.% Ni/alumina. This optimized catalyst was used in the
Acknowledgements
This work was conducted under the framework of research and development program of the Korea Institute of Energy Research (B7-2424-01). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (No. 2016R1A4A1012224).
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