Highly efficient and robust Pt ensembles on mesoporous alumina for reversible H2 charge and release of commercial benzyltoluene molecules
Graphical Abstract
Introduction
Hydrogen economy will be virtually realized when technological limitations are overcome in the three sectors of H2 production (e.g., grey H2 via steam methane reforming, blue H2 combined with CO2 capture and sequestration, and green H2 via water electrolysis), storage and transportation, and utilization using fuel cells on board and standalone [1], [2], [3]. Nowadays, the biggest challenge is to find an economically feasible way of H2 storage and transportation positioned in the middle of the value chain because of low volumetric H2 density [4], [5]. Though many options including compressed H2, liquefied hydrogen, physical adsorption in nanoporous materials, metal hydrides, etc. are being studied, a great deal of attention has been paid to chemical hydrogen storage based on liquid organic hydrogen carriers (LOHC) because it appears more fit to massive storage and overseas transportation [6], [7], [8], [9]. The LOHC concept is, simply speaking, to reversibly interconvert H2-lean and H2-rich molecules by catalytic hydrogenation (H2 charge) and dehydrogenation (H2 release) [10], [11], [12]. The reported LOHC molecules can be categorized into two classes according to the presence of heteroatoms. The typical aromatic examples are toluene and heat transfer oils [13], such as monobenzyltoluenes (MBT) and dibenzyltoluenes (DBT) [14], [15], while the heteroaromatic ones are N-ethylcarbazole (NEC) and 2-(n-methylbenzyl)pyridine (MBP) [16], [17]. Each class has pros and cons, yet the universal requirement for their successful implementation is a dehydrogenation catalyst of high efficiency under mild conditions and robustness in successive uses, which will be embedded in traditional or state-of-the-art catalytic reactors [18]. Moreover, it is ideal if the same catalyst achieves full conversion at a higher rate in the hydrogenation reaction.
Among a variety of dehydrogenation catalysts, platinum (Pt) and palladium (Pd) are recognized as the most powerful metal in H2 release from cyclic and heterocyclic LOHC molecules, respectively. First, Pd is examined to be more active than Pt in most of N-containing LOHC molecules such as NEC, MBP, and alkylindoles, because of strong ability of metallic Pd to adsorb N atom of the tested substrate [19]. Generally, the important properties of Pd catalysts affecting the dehydrogenation activity are Pd dispersion, Pd–support interaction, and surface fraction of active Pd (111) plane in supported Pd nanoparticles [20]. These were all accomplished by the mesoporous Pd–Al2O3 (MPdA) generated by solvent deficient precipitation (SDP) [21], [22]. This method offers high surface area and mesoporosity without the use of structure directing agents. The MPdA of 1 wt% Pd afforded the improved dehydrogenation capability and negligible activity loss compared to the impregnated catalyst Pd/Al2O3 for N-heterocyclic molecules. This superior performance was attributed to highly dispersed Pd particles with a pronounced population of (111) plane, provided by the unique SDP environment to prevent Pd particles adjacent to alumina from being sintered.
In the case of cyclic LOHC molecules, Pt is capable of excellently rupturing C–H bonds [23]. However, the catalytic functions necessary for Pt-based catalysts should be well controlled because higher temperatures are demanded due to the higher dehydrogenation enthalpy than heterocyclic molecules [13], [24]. Under this circumstance Pt works actively in the cleavage of C–C bonds, thereby giving rise to cracking products including small alkanes and aromatics [25], [26]. Thus, various approaches have been taken, such as tuning the size and surface plane of Pt particles [27], [28], modifying surface functionality of supports [15], and adding metal promoters [29], all of which are ultimately directed toward achieving the maximum efficiency of Pt atom, reducing defect-induced side reactions, and maintaining the fresh catalyst state. In this regard, the most straightforward solution is to discover such a new synthesis method that can acquire these at the same time. Therefore, the simple, cost-effective SDP method is worthwhile being explored for supported Pt catalysts, as exemplified in the MPdA catalyst.
Here we report a great potential of the mesoporous Pt–Al2O3 (MPtA) synthesized by the SDP method in reversible H2 release and charge of commercial benzyltoluene-based LOHC molecules such as MBT and DBT. The MPtA exhibits extremely high performance in both the dehydrogenation and hydrogenation reactions, and is amazingly robust in successive H2 storage–release cycles, compared to the conventional impregnated Pt counterpart. Furthermore, the SDP method predominantly yields Pt single atoms and clusters even at higher Pt loadings, unlike the general methods producing single atoms at the Pt content of less than 1 wt% [30], [31], [32]. As a result of finding an optimal Pt loading, the ensemble comprised of Pt single atoms and clusters is verified to be accountable in both the superior H2 storage and release capabilities of the MPtA at 3 wt% Pt. This is further confirmed by the activities of the control samples prepared under different calcination conditions. Through spectroscopic characterizations and H2 desorption experiments, the strong interaction between Pt and Al2O3 governs the characteristics, (de)hydrogenation performance, and durability of the MPtA catalyst.
Section snippets
Preparation of supported Pt catalysts
The MPtA catalysts were intended to synthesize about 3 g (based on the weight of reduced catalyst) at a single batch. Typically, Pt(NH3)4(NO3)2 (0.06–0.30 g), Al(NO3)3.9 H2O (21.4–22.3 g), and NH4HCO3 (13.65–14.10 g) were placed in a mortar without solvent. Note that the mole ratio of total nitrate to bicarbonate was set at 1.0. Then, the solid mixture was ground with a pestle for twenty min at room temperature. The prepared wet solid was calcined at 600 °C for 5 h (ramping: 2 °C min−1 to
Dehydrogenation performance of mesoporous Pt–Al2O3
For the MPtA and Pt/gA catalysts with the nominal Pt loading of 3 wt% (M3PtA and 3Pt/gA, respectively), the results of ICP-OES analysis showed that the actual Pt loading was similar each other at 2.9 ± 0.02 (Table 1). N2 physisorption measurement examined that the specific surface area (347 m2 g−1) and pore volume (0.57 cm3 g−1) of mesoporous alumina (m-Al2O3) were larger than those of γ-Al2O3. This difference was similarly observed in the fresh M3PtA and 3Pt/gA (Table 1). A higher level of
Conclusions
The mesoporous Pt–Al2O3 catalyst showed extraordinary durability, and greater H2 storage and release efficiencies for commercial LOHC molecules such as MBT and DBT. Thus, the MPtA achieved reversible interconversion between H2-rich and H2-lean MBT without catalyst deactivation. The employed SDP method is intriguingly unique to offer a broad range of Pt loading in producing single atoms as well as clusters to significantly affect the catalytic performance and product selectivity. Particularly,
CRediT authorship contribution statement
Jinho Oh: Investigation, Formal analysis, Data curation, Writing – original draft. Yeongin Jo: Investigation, Formal analysis, Data curation. Tae Wan Kim: Investigation, Formal analysis, Data curation. Hari Babu Bathula: Investigation, Formal analysis, Data curation. Sungeun Yang: Formal analysis, Data curation. Joon Hyun Baik: Formal analysis, Data curation. Young-Woong Suh: Conceptualization, Supervision, Funding acquisition, Writing – review & editing.
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 work was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT, Republic of Korea (NRF-2019M3E6A1064908). The authors J.O. and H.B.B. gratefully acknowledge the financial support of the Basic Science Research Program through the National Research Foundation of Korea under the Ministry of Education, Republic of Korea (NRF-2016R1A6A1A03013422).
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Current address: Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, United Kingdom.