Regular Article
Arming wood carbon with carbon-coated mesoporous nickel-silica nanolayer as monolithic composite catalyst for steam reforming of toluene

https://doi.org/10.1016/j.jcis.2021.04.112Get rights and content

Highlights

  • A thin Ni-silica layer was in-situ grown on the wall of wood carbon channels.

  • Small Ni nanoparticles are embedded in mesoporous silica with carbon protection.

  • The hardness and hydrothermal stability of monolithic wood carbon was improved.

  • The multilevel pore structure enhances mass transfer and avoids coke deposition.

  • The monolithic catalyst exhibits excellent tar steam reforming performance.

Abstract

Steam reforming is an effective measure for biomass tar elimination as well as H2-rich syngas (H2 + CO) production. However, the granular or powdery Ni-based catalysts are prone to deactivation, which is caused by inappropriate mass transfer and clogging of catalyst bed. Herein, monolithic wood carbon (WC) with low-tortuosity microchannels is armed with a carbon-coated mesoporous nickel-silica nanocomposite (Ni-SiO2@C) layer via an evaporation-induced self-assembly and calcination procedure for toluene (tar model compound) steam reforming. The quality of the Ni-SiO2@C layer growing on the surface of WC microchannel is affected by the molar ratios of Si/Ni feed. A uniform thin-layer coverage is obtained on the Ni-15SiO2@C/WC (Si/Ni = 15) catalyst, where highly dispersed Ni nanoparticles (average size of 6.6 nm) with appropriate metal-support interaction and remarkable mechanical strength are achieved. The mass transfer, coke resistance, and hydrothermal stability of the Ni-15SiO2@C/WC catalyst were significantly improved by the multilevel structure assembled from the WC microchannels and the secondary ordered SiO2 mesopores. A stable toluene conversion over 97% with an H2 yield of 135 μmol/min was obtained at 600 °C on the Ni-15SiO2@C/WC catalyst. This work opens a new window for facilely constructing high-performance wood carbon-based monolithic tar reforming catalyst.

Graphical abstract

Monolithic wood carbon armed with mesoporous nickel-silica nanocomposite layer is a stiff and highly efficient catalyst for toluene steam reforming.

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Introduction

The development of clean and renewable energy is an imperative path to alleviate the energy crisis and environmental issues caused by the combustion of fossil fuels [1]. The utilization of hydrogen (H2) is considered as one of the attractive measures for realizing energy supply security and greenhouse gas reduction due to its high energy density and zero discharge of air pollutants [2], [3], [4]. Nevertheless, up to 95% of H2 yield is associated with fossil fuel-based techniques and the hydrogen energy derived from renewable resources is still in a small proportion [5], [6]. Biomass, as a renewable and carbon–neutral fuel, has great potential for economical H2 production [7], [8]. Among different technologies, biomass gasification is a cost-effective approach to convert biomass into H2-rich syngas (H2 + CO) [9], [10], which can be further utilized in fuel cell systems and C1 synthetic processes, such as Fischer-Tropsch synthesis (FTS) and CO2 hydrogenation [11], [12]. However, tar formation (500–700 °C, anoxic condition) during biomass gasification is a major obstacle that restricts large-scale syngas production. Tar is a condensable mixture of aromatic compounds including toluene (24%), benzene (22%), and naphthalene (15%) [13], [14], [15], which would cause fouling and plugging in the pipes, engines, and turbines of the gasification system [13]. Thus, additional equipment and technologies are required to remove tar from the gasification apparatus for safe operation and promoted application of biomass gasification [16]. Conventional methods, such as wet scrubbing and thermal cracking (>1000 °C) have been widely adopted for biomass tar arresting [17]. However, wet scrubbing would reduce the thermal energy of syngas and cause water pollution, while the high-temperature thermal cracking needs additional energy supply. In contrast, catalytic steam reforming can be carried out at moderate temperature (600–900 °C), and therefore it can be coupled into a gasification system without additional energy supplement. In other words, tar can be removed on-line during biomass gasification with the assistance of appropriate catalysts. Moreover, tar steam reforming can raise the H2/CO ratio of syngas from biomass gasification (normally < 1.0), which is beneficial for its subsequent use, such as FTS (the need H2/CO ratio of 1.0–2.0) [18].

Ni-based catalysts are frequently used for biomass tar destruction due to its abudance and comparable catalytic activity to noble metals in breaking Csingle bondC bonds [14]. Unfortunately, Ni-based catalysts are susceptible to sintering and carbon deposition at high temperatures, resulting in rapid catalyst deactivation [19], [20]. Choosing support materials is key to achieve high dispersion of Ni nanoparticles (NPs), suitable metal-support interaction (MSI), and effective mass transfer. Mesoporous silica (e.g., MCM-41 and SBA-15) with large surface area and narrow well-defined channels are one of the ideal supports for loading metal NPs [21]. However, due to the weak MSI between Ni and silica, Ni NPs would diffuse out from the mesopores of silica and aggregate at the outer surface at high temperatures [22]. Moreover, pure silica-based materials are hydrothermally unstable [23], [24], where the ordered mesoporous structure would gradually collapse under long-term steam corrosion, leading to severe pore blockage and loss of surface area [25]. To address these problems, our group developed a carbon-coated mesoporous silica-supported Ni nanocomposite catalyst (Ni-SiO2@C) with a high and stable catalytic performance in toluene steam reforming owing to the protection of carbon layers and the enhanced MSI [22]. However, the Ni-SiO2@C is in granular form, and the particle packing results in irregular gas flow channels, which would cause diffusion limitations and pressure drop in the traditional fixed-bed reactors [26], [27], [28], [29]. In such a condition, a large proportion of well-dispersed Ni NPs in the mesopores cannot be sufficiently utilized due to the inappropriate mass transfer.

Monolithic catalysts, consisting of symmetric and thin-walled parallel channels, have attracted increasing attention because of their outstanding mass transfer efficiency and tunable space–time yield [28], [30], [31]. Recently, several monolithic microreactors coated with metal oxide supported Ni-based catalysts (e.g. Cu-Ni/TiO2 and Ni/MgO-Al2O3) were developed and exhibited a higher catalytic performance than their granular counterparts in steam reforming reactions [28], [30]. However, Ni species highly dispersed on these oxide supports are difficult to be reduced during the thermal activation process owing to strong MSI or the formation of spinel structured compounds such as nickel titanate (Ni2TiO4) and aluminate (NiAl2O4) [32], [33]. Moreover, it is hard to realize the uniform dispersion and strong adhesion strength of catalyst layers onto the channel wall of microreactors (e.g. FeCrAlloy [34]) due to their limited surface areas and surface functional groups [35]. The different physicochemical properties between the two materials are also against their integration during calcination [36]. For example, the incompatible thermal expansion coefficient between oxide slurry (e.g. Fe2Al5) and substrates (e.g. Cr or 10CrMo9-10) may cause unexpected cracking or even peeling of catalyst layers [28]. In recent years, carbonaceous materials have become a research hotspot in various areas such as energy storage, flexible devices, and catalysis due to their outstanding mechanical strength, electrical and thermal conductivity [37], [38], [39], [40]. The inexpensive and biodegradable monolithic wood with abundant surface oxygen-containing groups is one of the promising carbon-based materials, which is emerged as a scaffold for the immobilization of ultrafine metal-based catalysts in the areas of electrocatalysis, water treatment, and biomass tar reforming [26], [41], [42], [43], [44], [45], [46]. In our previous work, low-tortuosity basswood was adopted to support Ni NPs and the obtained monolith catalyst displayed a good catalytic performance in toluene steam reforming at 700 °C [26]. In such a monolith catalyst, Ni NPs were formed and confined within graphitic layers on the wood carbon wall during the carbonization process of the Ni(NO3)2 loaded basswood, and no further reduction is required. However, the particle size of these Ni NPs is relatively large (20–60 nm) due to the limited confinement effect of the carbonaceous surface, and a high Ni loading amount (ca. 14.5%) is essential to compensate for the loss of active sites. Besides, the mechanical strength of the wood carbon monolithic catalyst would gradually decline as the carbon wall would be corroded under high-temperature hydrothermal conditions [47].

In the construction industry, lacquer is commonly painted onto the wood surface to improve its durability. It is expected that arming a continuous nano-coating onto the inner surface of wood carbon channels can enhance the mechanical strength and corrosion resistance of the monolith, and maintain the stream guidance function of the channels. A large number of hydroxyl groups can be retained on the low-temperature carbonized wood and these oxygen-containing groups can interact with Si-OH groups of the silica precursor through hydrogen bonds. Thus, the integration of our previously reported mesoporous Ni-SiO2@C nanocomposite catalyst into wood carbon microchannels is very likely to form a stiff and hydrothermally stable monolithic catalyst with low Ni loading and small Ni NPs.

In this work, we successfully introduced an organic–inorganic Ni-SiO2 precursor into the monolithic wood carbon microchannels by an evaporation-induced self-assembly (EISA) process. After heat treatment, the Ni-SiO2@C/WC catalyst was obtained and a thin layer of mesoporous Ni-SiO2@C nanocomposite was uniformly coated onto the surface of wood carbon microchannels. As expected, the Ni-15SiO2@C/WC (Si/Ni = 15) catalyst demonstrated high and stable activity for toluene steam reforming, where the toluene conversion efficiency over 97% with an H2 yield of 135 μmol/min was achieved at 600 °C. In the prepared monolithic catalysts, the mechanical strength and hydrothermal stability of wood carbon scaffold are promoted by the coating of Ni-SiO2 nanocomposite layer. The Ni NPs embedded in the silica mesopores can sufficiently contact with reactant gas and avoid deactivation owing to the efficient mass transfer in micron-sized low-tortuosity channels of wood carbon.

Section snippets

Materials

Pluronic triblock copolymer P123 ((EO)20(PO)70(EO)20, Mw = 5800) was purchased from Aldrich. Tetraethoxysilane (TEOS, analytical grade), nickel nitrate hexahydrate (Ni(NO3)2·6H2O, analytical grade, ≥98.0% purity), and anhydrous ethanol (analytical grade, ≥99.7% purity) were obtained from Sinopharm Chemical Reagent Co., Ltd (China). All the chemicals were used as purchased without further purification. The raw basswood was obtained from the Chenlin Wood Company (China).

Preparation of monolithic wood carbon catalysts

Before use, basswood

The formation procedure of catalyst

As illustrated in Scheme 1, the Ni-xSiO2@C/WC catalysts were prepared by in-situ growing mesoporous Ni-SiO2@C nanocomposite layer onto the 3D wood carbon microchannels for highly efficient toluene steam reforming. Natural basswood was cut perpendicularly along the growth direction of the tree and shaped into cylinders, followed by carbonization at 300 °C. Compared with the original wood, the low-temperature carbonized wood (WC-300) demonstrates a slightly intensified FT-IR (Fourier Transform

Conclusions

In conclusion, a thin layer of carbon-coated mesoporous nickel-silica nanocomposite (Ni-SiO2@C) was successfully grown onto the microchannel surface of monolithic wood carbon via evaporation-induced self-assembly and followed calcination process. In these monolithic catalysts, small Ni NPs (average size of 6.6 nm) are uniformly embedded in the ordered mesopores of silica due to suitable MSI and carbon layer protection, resulting in the improved resistance to sintering and carbon deposition.

CRediT authorship contribution statement

Haiyang Xu: Methodology, Investigation, Writing - original draft. Zhangfeng Shen: Methodology, Writing - review & editing. Siqian Zhang: Investigation, Validation. Gang Chen: Visualization. Hu Pan: Formal analysis. Zhigang Ge: Project administration. Zheng Zheng: Supervision. Yanqin Wang: Conceptualization. Yangang Wang: Conceptualization, Funding acquisition. Xi Li: Funding acquisition, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge the financial support from the National Key Research and Development Program of China (Grant No. 2018YFB1502900), Zhejiang Provincial Natural Science Foundation of China (Grant No. LY19B060006), Innovation Jiaxing Elite Leadership Plan, and Research Funding of Jiaxing University.

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