Maximizing platinum efficiency in LOHC dehydrogenation
Résumé
Introduction In the European plan of moving to carbon-neutral energy production, "A Clean Planet for All", hydrogen is considered a potential solution for decarbonizing industries and long-distance transportation. While numerous efforts have been made to make renewable hydrogen production cost-competitive, H2 transportation and storage still stand as the "Achilles heel" of the hydrogen economy. Among the available strategies for H2 transportation and storage, Liquid Organic Hydrogen Carriers (LOHCs) are promising candidates since they allow hydrogen storage in liquid form at ambient temperature and pressure. LOHCs are homo- and heterocyclic molecules, like perhydrogenated benzyl toluene (H12-BT), which can be dehydrogenated with hydrogen release. Among the dehydrogenation catalysts, Pt-based systems appear as the best-performing catalysts due to the high selectivity of Pt for breaking C-H bonds compared to C-C bonds. Despite its high efficiency, platinum is an expensive and rare material with reserves limited to a few countries. Therefore, the use of this raw material must be optimized by maximizing the metal activity (activity per Pt atom) while maintaining high H2 selectivity. Moreover, different strategies exist in the literature to modify the performance of metallic catalysts, e.g., changing the size of metal particles, using reducible metal oxide supports, or combining Pt with a second, less rare, metal. In our study, we explore multiple alternatives for maximizing the activity and stability of platinum for the dehydrogenation of H12-BT. First, platinum nanoclusters (NCs ≈ 1 nm) were prepared on zeolites, TiO2 and CeO2 (reducible supports), and Al2O3 (standard support). In addition, the platinum content in the catalyst was “diluted” or alloyed with Sn and Re, which are reported to enhance the performance of this metal. Two strategies were applied to prevent nanocluster aggregation: strong interaction between Pt and reducible metal oxides, and encapsulation in the zeolite network. The characterization of the spent catalysts was used to verify the stability of the metallic clusters. Materials and Methods In order to obtain a consistent and homogeneous size of platinum nanoclusters, the catalysts were prepared using various approaches, including incipient wetness impregnation (IWI), ion exchange (IE) or competitive ion exchange (CIE) methods, and an organometallic route. All metal precursors and supports were purchased from commercial sources and used without pretreatment. The size and dispersion of Pt were analyzed by annular dark-field scanning transmission electron microscopy (ADF-STEM) and CO chemisorption; textural properties of the catalysts were determined by N2 physisorption; Pt content was analyzed by ICP-OES; the acidity of the supports was obtained from IR spectroscopy of adsorbed pyridine. The catalytic tests were performed using a setup containing a three-neck flask connected to a chiller and to a microGC to analyze gas-phase products. Reaction conditions: H12-BT (20 g), catalyst (0.009 mol%), 260 °C. Yields were determined by GC−FID using tetradecane as an internal standard. Results and Discussion All the catalysts were prepared with a Pt loading of 0.5 wt%. The Pt-zeolite samples were synthesized by using Y-zeolites with Si/Al ratios of 2.6, 6, 15, and 30 through IE and CIE procedures. Following the IE approach, two populations of Pt particles were formed: the first one composed of small NCs of 0.9 ± 0.2 nm and the second one composed of larger nanoparticles of 16 ± 5 nm. The Pt dispersion quantified by CO chemisorption was higher than 90% for all the samples, indicating that Pt was mainly present as small nanoclusters. Notably, large particles were not observed when using the CIE method. NCs of the same size were also synthesized in Na-Y-zeolite (Si/Al=2.6) to monitor the influence of the Brönsted acidity on the properties of the catalyst. Pt NCs on TiO2 and CeO2 were prepared by the IWI method. STEM shows the formation of NCs of 1.0 nm in size. Finally, small NPs of Pt mixed with Sn or Re were synthesized on TiO2 using IWI and organometallic approaches. Significance This work presents different approaches to maximize the use of platinum as a dehydrogenation catalyst to improve the sustainability and efficiency of H 2 transportation.