Hydrogen production in microreactor using porous SiC ceramic with a pore-in-pore hierarchical structure as catalyst support
Introduction
Proton exchange membrane fuel cells (PEMFCs) have gained much attention by global researchers due to its high efficiency and low pollution for providing energy using hydrogen [[1], [2], [3]]. However, the safety of storage and use of hydrogen is a great challenge because of the serious danger during the direct utilization of hydrogen as energy source [4,5]. In-situ hydrogen production technology using microreactors is an economical and safe method to meet the demanding requirements of PEMFCs [6]. The channels with a size below 1 mm in the microreactors exhibit a high specific surface area, improving the efficiency of heat and mass transfer and thus easily controlling the reaction temperature and decreasing the number of hot spots [[7], [8], [9], [10]]. Therefore, plenty of previous researches have concentrated on the fabrication and application of hydrogen generation microreactor.
Methanol steam reforming (MSR) is a very promising approach for hydrogen generation on demand, especially the application in microreactor to provide hydrogen due to its low reaction temperature, low CO concentration and high H2 selectivity [[11], [12], [13]]. Hence, the microreactors with different structures and supports for MSR have been studied extensively [[14], [15], [16], [17], [18]]. For instance, Chen et al. fabricated microreactors with various tree-shaped structures and found that the reaction rate increased with the rising of temperature and pressure, while the branch angle of tree-shaped structure exhibited few effects [14]. Zhou and his coworkers revealed that the microreactor containing catalyst-loaded metal foams without clearance cascading showed the best catalytic performance [15]. Another research group compared the performances of microreactors with different cross-section shapes, spacing dimensions and layouts in MSR and found that the parallel and symmetrical distributed microchannel led to higher H2 yield and better methanol conversion [16,17]. It was reported that the maximum methanol conversion could reach 70.3% and the CO concentration was about 1% at 280 °C using a cube-post microreactor fabricated by Zeng et al. [18]. However, the minimum temperature of 100% methanol conversion rate for most microreactors is above 300 °C and the catalytic stability of microreactor is still a problem for the entire microreactor industry because of the negative impacts of catalyst composition, catalyst loading amount and loading method. Generally, the catalyst support in microreactor for MSR is composed of metal or silicon and the microchannels are fabricated via machining and etching, which could probably cause many defects in the microreactor [19]. It is widely accepted that the defects in the general microreactors such as low adhesion between the supports and catalysts, complex processing and high manufacturing costs limit the application and development of microreactors [20].
SiC ceramic is often used as a function material due to its superior resistance against oxidation, corrosion, thermal shock and high-temperature, and porous SiC ceramic with three-dimension space network structure exhibits low density, high porosity and high thermal conductivity [[21], [22], [23]]. Thus, the porous SiC ceramic is considered as more than a widely used material for gas filtration and ceramic membrane, as well as a suitable support for microreactor applied in chemical production [[24], [25], [26]]. Meanwhile, as a simple, fast and inexpensive technology for catalyst fabrication, the solution combustion synthesis (SCS) method can generate a large amount of gases during the synthetic process, resulting in some micro-pores that can facilitate the approachability of reactant to catalyst and improve the catalytic performance significantly [27,28]. Since the porous and loose structure and the expansiveness of catalyst synthesized by SCS, there exists a possibility to synthesize porous catalyst in the microchannels of catalyst support via SCS to increase the catalyst loading amount and enhance the catalytic efficiency of microreactor.
In this study, a porous SiC ceramic was fabricated as a catalyst support in microreactor and porous CuO/ZnO/CeO2/ZrO2 catalyst was loaded on the support via solution combustion method by using glycol as the fuel. The effects of fuel/nitrates molar ratio on the characteristic of prepared catalyst and the catalytic performance of microreactor for MSR were investigated in detail. Then, the optimal fuel/nitrates molar ratio of the laden ceramic support for microreactor was proposed.
Section snippets
Fabrication process of porous SiC ceramic support
The porous SiC ceramic support was fabricated by a three-step process: mixing powders, dry pressing and calcination. In the first step, the powders including SiC (D50 = 60 μm), glass frit (D50 = 25 μm) and polymethyl methacrylate (PMMA, D50 = 70 μm) were mixed adequately in a mortar. In the second step, the mixed powders were pressed into a disc with a diameter of 30 mm and a height of 10 mm. In the last step, the pressed SiC disc was calcined with air atmosphere at 700 °C for 2 h. The
The phases and particle size of synthesized catalyst
The XRD patterns of the synthesized CuO/ZnO/CeO2/ZrO2 catalysts are shown in Fig. 4. The peaks of CuO (JCPDS No.41–0254) and ZnO (JCPDS No.05–0664) were observed on all samples, while no peaks of CeO2 and ZrO2 were detected. The presence of peaks for CexZr1-xO2 implies that the solid solution between cerium and zirconium is formed. It has been proved that the CexZr1-xO2 has excellent properties as a co-catalyst such as improving the catalytic activity and suppressing the formation of CO during
Conclusions
Porous SiC ceramic support loaded with porous CuO/ZnO/CeO2/ZrO2 catalyst for MSR in a microreactor was successfully fabricated and a pore-in-pore hierarchical structure was formed on the support by using glycol as the fuel. The effects of fuel/nitrates molar ratio on the characteristic of catalyst and support and the catalytic performance of microreactor were investigated in detail. According to the results, increasing the glycol content could decrease the catalyst particle size and increase
Acknowledgments
The authors are grateful for the financial support from the National Key Research and Development Program of China (No. 2017YFB0310400) and the National Natural Science Foundation of China (Grant No. 51872082).
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