Elsevier

Catalysis Today

Volume 309, 1 July 2018, Pages 140-146
Catalysis Today

Rapid evaluation of coke resistance in catalysts for methane reforming using low steam-to-carbon ratio

https://doi.org/10.1016/j.cattod.2017.08.051Get rights and content

Highlights

  • Effect of steam-to-carbon ratio on coke formation during methane reforming studied.

  • Coke formation measured within 5 h using low steam-to-carbon ratio and high WHSV.

  • Quick development of Ru-Mg catalysts with high coke resistance.

  • Long-term stability for both steam and dry methane reforming demonstrated.

Abstract

The formation and subsequent accumulation of coke is one of the major reasons for the catalyst deactivation in methane reforming reaction. Although the investigation of coke-resistant catalysts is closely related to their long-term stability of given catalysts, it takes a long time to quantitatively measure the amount of carbon deposition on catalysts under normal reaction operational conditions. To overcome this problem, we used the steam deficient reaction condition, i.e. a low steam-to-carbon ratio (S/C) of 0.5 to accelerate the carbon deposition on catalysts. In this condition, the base catalyst of 10 wt.% Ni/alumina rapidly lost its catalytic activity, indicating fast coke deposition. However, adding proper additives, such as Ru among various precious metals (Ru, Rh, Pt, and Pd) and alkaline earth metals (Mg, Ca, Sr, and Ba) with the appropriate loading (5 wt.%) effectively suppressed coke formation. The optimized catalyst composition is 0.5 wt.% Ru/5 wt.% Mg/10 wt.% Ni/alumina, which displayed coke resistance in the long-term stability test of steam methane reforming and 40 h test of dry reforming of methane. These experimental results indicate that the method developed in this study is useful for the rapid evaluation of given catalysts for their coke resistance.

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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