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Catalysis and Kinetics
Hydrogen production through steam reforming of diesel over highly efficient promoted Ni/#-Al2O3 catalysts containing lanthanide series (La, Ce, Eu, Pr, Gd) promoters Muhammad Naeem Younis, Zuhair O Malaibari, Waqar Ahmad, and Shakeel Ahmed Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00890 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018
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Energy & Fuels
Hydrogen production through steam reforming of diesel over highly efficient promoted Ni/γ-Al2O3 catalysts containing lanthanide series (La, Ce, Eu, Pr, Gd) promoters Muhammad Naeem Younisa,b, Zuhair O. Malaibaria, Waqar Ahmada, Shakeel Ahmed*b Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. b Center for Refining & Petrochemicals, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia. *Email :
[email protected] a
ABSTRACT Wet incipient impregnation technique was used for the synthesis of non-promoted and lanthanide series promoted (La, Ce, Eu, Pr and Gd) Ni/γ-Al2O3 catalysts. The physicochemical properties of the catalysts were determined using XRD, BET, TEM, SEM, EDX and TPR characterization techniques. Activity and the resistance to coke formation of non-promoted and lanthanide series promoted 12%Ni/γ-Al2O3 catalysts were investigated for diesel steam reforming in a fixed-bed reaction system. BET analysis showed no remarkable difference in the surface area of 12%Ni/γ-Al2O3 and 5%Pr12%Ni/γ-Al2O3 (Pr-Ni) catalyst as compared to other promoted 12%Ni/γ-Al2O3 catalysts. Meanwhile, Pr-Ni catalyst also has the higher BET surface area among other synthesized lanthanide promoted 12%Ni/γ-Al2O3 catalysts. TEM analysis presented the uniform metal dispersion of promoted catalysts. Moreover, TPR analysis indicated an optimum interaction between the metal and the support in Pr-Ni catalyst. Therefore, experimental results presented the maximum diesel conversion, H2 selectivity, H2 yield and reforming efficiency over Pr-Ni catalyst as compared to all other studied catalysts during a 40 h run at 620 °C, 1 atm, steam to carbon ratio (S/C) = 3.0, gas hourly space velocity (GHSV) of 5800 h-1 and 6 ppm sulfur contents in the fuel. Furthermore, TGA and CHNS analysis also confirmed the low amount of coke formation over the surface of 5%Pr-12%Ni/γ-Al2O3 catalyst. Keywords: Diesel; Hydrogen; Steam Reforming; Nickel; Lanthanide Promoters 1. INTRODUCTION In the past few decades, hydrogen-based fuel cells (FC) have witnessed remarkable advances in terms of energy efficiency, alleviating the devastating effects of environmental pollution caused by the burning of fossil fuel and ultimately reducing the greenhouse gases emissions 1. Reliable generation of electricity to fulfill the needs of remote areas and mobile units (generation of electricity and refrigeration at idle mode) using FC based auxiliary power units (APUs) required a readily available hydrogen source 2–5. However, difficulties in safe storage of hydrogen and the absence of reliable methods of hydrogen transportation demanded portable, compact, and efficient hydrogen production units. Conversely, there was a sound, reliable, and well developed delivery system available for diesel throughout the world 6. Usually, hydrogen was produced by 1
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steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR) methods. Therefore, steam reforming process can be described by the following general set of reactions 7. CxHy + xH2O ←→ xCO + (x+0.5y) H2 (1) CO + H2O ←→ CO2 + H2 (2) CO + 3H2 ←→ CH4 + H2O (3) CO2 + 4H2 ←→ CH4 + 2H2O (4) Steam reforming was considered as the most economical and efficient method for hydrogen production in several industrial applications, e.g. fertilizer plants and power generation sector 8–10. Moreover, hydrogen produced from diesel steam reforming (DSR) can be used directly as a fuel or in an efficient fuel cell (in fixed or portable FC-auxiliary power units) 11,12. For DSR, there were two available methods for the mixing of fuel and steam inlet streams; (1) pre-mixing in a separate chamber (by evaporating the diesel), or (2) direct fuel injection 6,13. Since diesel was almost immiscible with water, therefore the direct injection of diesel on the catalyst surface was a challenging and risky operation 14,15 . Consequently, an ultrasonic injector system was used in current study to counter problems in diesel injection system, which not only saved the energy required to pre-heat or evaporate diesel but also increased the efficiency of the reforming process 6. The high operating temperatures of DSR was also a major problem due to sintering of the catalyst, which consequently, reduced its activity 7,16,17. Deactivation of the catalyst due to sulfur poisoning, coke formation and deposition, hydrocarbon slip, and reforming kinetics were among the other concerns in DSR 18–22. Furthermore, three types of coke usually formed during DSR can be summarized as: (1) Whisker type coke formed at the free active metal, (2) Pyrolytic carbon forms due to pyrolysis of diesel or HC at high temperatures, and (3) Encapsulating carbon forms due to polymerization of diesel or HC 9,23 . Wang et al. 2007 24 reported that the coke was formed due to acidic supports, while Li et al. 2006 25 previously suggested that the presence of the larger nickel metal (>12 µm) was the main reason for coke formation. Some reactions related to carbon deposition can be illustrated through equations 5−7. CxHy ←→ xC + 0.5yH2 (5) 2CO ←→ CO2 + C (6) CO+H2 ←→ H2O + C (7) In order to reduce the coke formation, one approach was to increase the amount of steam (higher S/C ratios), which ultimately increased the quantity of energy and water required, hence increased the cost of reaction and reduces the suitability for an APU 9. Therefore, a steam to carbon ratio of 3.0 was found optimum and more economical 7,9,20. Several reports indicated that noble metals provided the excellent efficiency and activity for the steam reforming processes 7,9,26,27. Although nickel-based catalysts have proved themselves as an economical and worthy alternative for steam reforming, still they were not as efficient as noble metals based catalysts (Pt, Pd, and Rh) in terms of catalyst stability and conversion efficiency 28. The addition of La to Ni catalyst reduced the coke 2
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Energy & Fuels
formation because the CO chemisorption capacity was decreased (metal dispersion:
