Carbon deposition–resistant Ni3Sn nanoparticles with highly stable catalytic activity for methanol decomposition
Graphical abstract
Introduction
The high hydrogen to carbon ratio and the ease of transport/storage make methanol, which is liquid at room temperature, a promising hydrogen carrier and a useful raw material for the synthesis of hydrocarbons and their derivatives [[1], [2], [3], [4]]. Methanol decomposition, which affords H2 and CO, is one of the most important reactions for methanol utilization and is catalyzed by transition metals such as Ni, Cu, Pt, Pd, and Ru [[4], [5], [6], [7], [8], [9]]. Among them, the relatively cheap and highly active Ni is viewed as a promising methanol decomposition catalyst [[10], [11], [12]]. Ni nanoparticles (NPs) are usually dispersed on large-surface-area oxide supports to obtain good catalytic performance but suffer from low selectivity for H2 and CO as well as low resistance to carbon deposition at high temperatures [10,12]. Therefore, new catalysts with high activity and selectivity for methanol decomposition in a wide temperature range are needed for decentralized H2 and/or synthesis gas production.
Intermetallic compounds (IMCs) have the potential to exhibit better catalytic performance than disordered alloy and monometallic catalysts due to their ordered crystal structure and strong chemical bonding that differs from that of their constituents. Many investigations on the use of IMCs in heterogeneous catalysis have been performed, and they were well summarized recently [[13], [14], [15], [16], [17]]. Some of IMCs were reported to exhibit excellent catalytic properties for a variety of reactions. For example, Pt3Ge shows high selectivity for butenes in the hydrogenation of 1,3-butadience, which is attributed to electron transfer from Ge to Pt and the geometrical environment change of Pt active sites [18,19]. Moreover, the selectivity of PdZn for methanol steam reforming is on par with that of Cu, possibly because of the similar valence electron density of states of these two catalysts, as revealed by energy band calculations and X-ray photoelectron spectroscopy (XPS) [20]. Co3Ta shows higher specific activity for hydrazine oxidation reaction than conventional Pt/C electrocatalysts [21]. Ni3Sn shows high selectivity for partial hydrogenation of acetylene and for dehydrogenation of cyclohexane [[22], [23], [24]]. It has also been reported that the formation of Ni3Sn phase in bimetallic Ni-Sn catalysts might enhance the selectivity for H2 during the reformation of aqueous hydrocarbon [25,26]. Our recent work on the catalytic performance of crushed single-phase Ni3Sn ingots for methanol decomposition revealed that these samples showed high selectivities for H2 and CO over a wide temperature range [27,28]. To realize an even higher catalytic activity, we synthesized Ni3Sn NPs through a thermal plasma process and examined their initial catalytic performance for methanol decomposition at 513–793 K [29], revealing that in the initial stage of the reaction, these NPs exhibited higher selectivity for methanol decomposition than Ni NPs over a wide temperature range. The above results suggest that Ni3Sn NPs are a promising catalyst for methanol decomposition. However, the catalytic performance stability of Ni3Sn NPs has not been investigated, and the mechanism of methanol decomposition over Ni3Sn remains poorly understood.
Herein, we report the catalytic performance stability and selectivity of Ni3Sn NPs for methanol decomposition at high reaction temperatures (673 and 793 K) required for decentralized H2 and/or synthesis gas production and compare the results with those obtained for monometallic Ni NPs synthesized by the same thermal plasma process. Samples before and after methanol decomposition are characterized by synchrotron radiation X-ray diffraction (SR-XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), XPS, and Brunauer–Emmett–Teller (BET) surface area measurements. Moreover, density functional theory (DFT) calculations are performed to determine the adsorption energies of methanol and possible intermediates, and the activation energies of the possible elementary steps of methanol decomposition over Ni3Sn (0001) and Ni (111) surfaces are determined to provide molecular insights into the mechanism of methanol decomposition over Ni3Sn.
Section snippets
Catalyst preparation
Ni3Sn and Ni NPs were synthesized using a radiofrequency induction thermal plasma method (Nisshin Engineering Inc., Japan) from pure Ni and Sn fine powders (Kojundo Chem. Lab. Co., Ltd., Japan; Ni: 99.9 %, 2–3 μm in diameter; Sn: 99.9 %, <38 μm in diameter) as raw materials. The composition of as-synthesized Ni3Sn NPs was determined as Ni-21.3 at% Sn by fluorescence X-ray spectroscopy.
Catalyst characterization
The BET specific surface areas of NPs before and after the catalytic reaction were measured by N2 adsorption
Catalytic performance
The Ni3Sn NP catalyst was much more stable than Ni NP catalyst under the reaction conditions. Fig. 3 shows methanol conversion over Ni3Sn and Ni NPs at 673 and 793 K as a function of time on stream, revealing that at 673 K, conversion over Ni3Sn was stable at ∼35 % without any obvious decrease even after 70 h. In contrast, Ni showed an initially higher conversion of 53 %, which, however, decreased with time on stream to a value (37 %) close to that of Ni3Sn. At 793 K, conversion over Ni3Sn
Conclusions
The catalytic properties of Ni3Sn NPs for methanol decomposition were examined by performing isothermal tests at an LHSV of 30 h−1 and a temperature of 673 or 793 K. The above NPs exhibited high selectivities for H2 and CO and featured a carbon deposition resistance significantly exceeding that of monometallic Ni NPs. The activity of Ni3Sn NPs was slightly lower but much more stable than that of monometallic Ni NPs. The Ni3Sn phase of Ni3Sn NPs was stable during methanol decomposition at
CRediT authorship contribution statement
Ya Xu: Conceptualization, Methodology, Investigation, Writing - original draft. Huixin Jin: Software, Formal analysis. Jianxin Zhang: Validation. Yoshitaka Matsushita: Investigation.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
This work was partly supported by the Japan Society for Promotion of Science (JSPS; KAKEHI Grant No. 15K06572). Synchrotron XRD experiments were performed at the SPring-8 facility on the approval of the NIMS Synchrotron X-ray Station (Proposal Nos. 2015B4901 and 2018A4907). The authors are grateful to Dr. M. Tanaka and Dr. Y. Katsuya at NIMS for their help with XRD experiments, Dr. H. Ohata at NIMS for help with XPS experiments, and Dr. J. Nara at NIMS for help with the PHASE0 software package
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