مقاله انگلیسی رایگان در مورد اثر افزودن هیدروژن بر تجزیه متان به هیدروژن و کربن بر کاتالیزور کربن فعال – الزویر 2018

The effect of H2 addition on CH4 decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H2 to CH4 changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H2 flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H2, and when the H2 flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H2 to CH4 and the catalyst deactivation is restrained. Filamentous carbon is obtained when H2 is introduced into CH4 reaction gas compared with the agglomerate carbon without H2 addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Hydrogen is a clean fuel and considered as an attractive alternative for fossil fuels and renewable energy sources. Generally, the main sources of hydrogen are fossil fuels, water, and methane, in which fossil fuels are also the main source of air pollution that causes considerable damage to the environment. Conventionally, most of the industrial hydrogen production is based on the steam methane reforming (SMR) process, however, large amount of carbon oxides as by-product leads to the increasing cost of hydrogen production because of the necessary purification process [1,2]. Electrochemical splitting of water is a process to produce hydrogen without COx emission, and some efficient catalysts have been developed to improve the efficiency of process and reduce the production cost [3e7]. Catalytic methane decomposition (CMD), another simple and promising process for production of hydrogen and carbon material without by-products CO and CO2, has received more and more attention. Many metal catalysts, such as Fe [8e10], Co [11e13] and Ni [14e16], were introduced into the process and proved as effective catalysts to promote methane decomposition. However, metal catalysts easily suffer the deactivation because the carbon deposition produced from methane encapsulates the surface active site of metals [17e19]. In comparison, carbon materials are considered as the alternative catalysts for methane decomposition to hydrogen owing to several advantages compared with metal catalysts such as low cost, high-temperature resistance, tolerance to sulfur and other potentially harmful impurities in the feedstock, and so on [20,21]. Activated carbon (AC) and carbon black are considered as more catalytically active materials than the more ordered ones such as graphite, diamond, carbon fibers and the carbon nanotubes [20,22], in which AC exhibits higher initial activity but lower sustainability. The rapid deactivation of AC catalysts is due to the blocking of the mouth of AC pores by growing carbon crystallites, which are generally in irregular agglomerate form [21,23]. Regeneration of carbon catalysts by carbonaceous deposit combustion with multiple cycles of reaction/regeneration or continuous supply of CO2/H2O to methane decomposition can effectively remove the carbon deposits and recover the carbon catalysts [24e26]. However, the regeneration of carbon catalysts will lead to the generation of undesirable COx and even destroy the catalyst itself [27]. The type of carbon deposition is confirmed to be an important factor to influence the activity of catalyst during methane decomposition [19].