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Abstract
Carbon species deposition is recognized as the primary cause of catalyst deactivation for hydrocarbon cracking and reforming reactions. Exploring the formation mechanism and influencing factors for carbon deposits is crucial for the design of rational catalysts. In this work, a series of NixMgyAl-800 catalysts with nickel particles of varying mean sizes between 13.2 and 25.4 nm were obtained by co-precipitation method. These catalysts showed different deactivation behaviors in the catalytic decomposition of methane (CDM) reaction and the deactivation rate of catalysts increased with the decrease in nickel particle size. Employing TG-MS and TEM characterizations, we found that carbon nanotubes which could keep catalyst activity were more prone to form on large nickel particles, while encapsulated carbon species that led to deactivation were inclined to deposit on small particles. Supported by DFT calculations, we proposed the insufficient supply of carbon atoms and rapid nucleation of carbon precursors caused by the lesser terrace/step ratio on smaller nickel particles, compared with large particles, inhibit the formation of carbon nanotube, leading to the formation of encapsulated carbon species. The findings in this work may provide guidance for the rational design of nickel-based catalysts for CDM and other methane conversion reactions.
Highlights
Nowadays, in order to solve the problems of energy shortage, increasing environmental pollution and energy demand, the development of new and renewable energy has become the primary tasks facing humanity [1]
Safe, efficient, and renewable energy source, hydrogen (H2 ) is one of the most economical and effective alternative energy sources for human beings to get rid of their dependence on fossil energy resources [3]. It has been industrialized for quite a long time, the steam methane reforming (SMR) process still cannot avoid the shortcomings of high energy consumption and high
Similar to what we found in our in our several work, several structures, for example, and encapsulated work, carboncarbon structures, for example, carbon nanotubes (CNTs), CNTs, CNFs, CNFs, carbon carbon onions onions and encapsulated carbon carbon have obtained been obtained in methane the methane conversion reactions
Summary
In order to solve the problems of energy shortage, increasing environmental pollution and energy demand, the development of new and renewable energy has become the primary tasks facing humanity [1]. Ermakova et al obtained a similar conclusion to Chen’s results They observed that the maximum yield of carbon produced by methane decomposition was at the nickel particle size of 20–60 nm [20]. In these works, the authors took the generation of carbon nanotube as the starting point, ignoring the discussion on the variation in the hydrogen formation rate during the CDM reaction, and lacked detailed analysis of other carbon deposition types. With the deep understanding of the mechanism of the CDM reaction in recent years, the descriptions of the formation of carbon deposits in these works seems to be insufficient Given such complexities, further analyses of the effect of metal particle size on the carbon formation in the CDM reaction are urgently needed to guide the design of rational catalysts. With the aid of density functional theory calculations, the mechanism of carbon formation on nickel particles of different sizes is proposed
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