A group of researchers recently published a paper in the journal ACS Energy Letters that demonstrated the feasibility of using single-site titanium (Ti) catalysts for hydrogen (H2) spillover and storage on graphene.
Study: Hydrogen Spillover and Storage on Graphene with Single-Site Ti Catalysts. Image Credit: Sergey Dzyuba/Shutterstock.com
Chemical catalytic reactions involving H2 require the dissociation of H2 molecules into H atoms at the catalytic sites. The H2 spillover was first observed in the tungsten trioxide/platinum heterogeneous catalysis system.
Several subsequent studies on H2 spillover demonstrated that H2 molecules could be dissociated easily on a metal catalyst and the resultant H atoms migrate to adjacent support, such as reduced metal oxides, where they are chemisorbed.
Catalysts are commonly optimized to achieve the desired chemical activity by adjusting their geometry, particle size, and crystallographic structure. However, synthesizing heterogeneous catalysts with isolated active sites possessing atomic level efficiency has remained a significant challenge.
Different support materials can significantly affect catalyst optimization. In certain instances, robust interactions between the catalyst and the support material enhanced the catalytic performance of transition-metal catalysts through charge-transfer effects.
Transition-metal-based single-atom catalysts (SACs) were successfully prepared in previous studies to reduce the amount of metal catalyst needed to efficiently catalyze the hydrogen spillover. However, more improvements in the catalytic activity of SACs are required to effectively use them for hydrogen spillover.
Carbon supports, such as graphene, represent a suitable alternative to metal oxide supports to facilitate H2 spillover for storage. Carbon atoms do not directly participate as a catalyst in the hydrogen spillover reaction process due to their higher inertness compared to conventional oxide supports.
However, the migration of dissociated H atoms on graphite or graphene is kinetically unfavorable, which limits the H2 storage on graphene support to small distances around the catalyst sites. Thus, the number of H atoms that can be chemisorbed in graphene support is a crucial factor that must be determined before using graphene for H2 storage.
In this study, researchers investigated the role of single-site Ti catalysts on graphene for H2 spillover and storage using synchrotron-radiation-based methods. Previous studies have demonstrated that single Ti atoms are adsorbed at the energetically favorite hollow sites on graphene. Researchers used Ti atomic deposition to mimic SACs on graphene.
Single crystalline graphene support was grown epitaxially through thermal decomposition in a resistively heated cold-wall reactor on a hydrogen-etched 4H-silicon carbide (4H-SiC) (0001) substrate.
The pristine crystalline graphene was grown under an argon atmosphere at 780 mbar pressure and 1600 K temperature for 10 min to achieve sufficient monolayer uniformity. Subsequently, single-site catalysts were formed when 0.03−0.5 ML Ti atoms were deposited over the graphene layer.
Raman spectroscopy and atomic force microscopy (AFM) were performed to assess the homogeneity and quality of the monolayer graphene.
The graphene samples were annealed for more than seven h at 900 K under ultra-high vacuum (UHV) conditions before the ambient-pressure X-ray photoemission spectra (APXPS), X-ray absorption spectroscopy (XAS), and angle-resolved photoemission spectra (ARPES) measurements.
The electronic structure of Ti-loaded graphene, without and with the dosage of H2 molecules, and pristine graphene was investigated using ARPES, while the hydrogenation percentage of the Ti-decorated graphene with the adsorption of H2 molecules was determined using APXPS measurements.
The APXPS and ARPES data were measured after dosing the Ti-decorated graphene under continuous H2 flow for different periods. The APXPS and ARPES spectra were measured using SPECS Phoibos 150 NAP and SPECS Phoibos 150 analyzers, respectively.
All measurements were performed near 300 K temperatures. The UNIFIT 2013 software was employed to deconvolute the XPS spectra. The XAS spectra were obtained in total-electron-yield mode and recorded in on-the-fly scan mode to avoid radiation damage. The Ti coverage was determined using a quartz crystal microbalance (QCM) as it possesses a narrow resonance that leads to great accuracy and stable oscillations.
Ti-loaded graphene samples were synthesized successfully. A Ti-adatom-induced band structure renormalization of the samples was observed for 0.03 ML/low Ti coverage due to strong carbon 2pz and Ti 3d orbital hybridization of the isolated Ti atoms at the hollow sites of graphene.
The formation of C-H bonds was observed in Ti-decorated graphene samples that were exposed to H2, indicating the hydrogen spillover on the graphene support. No C-H bonds were observed in Ti-loaded graphene samples without hydrogen dosing.
A 280 meV band-gap opening in the graphene was observed when the Ti-decorated graphene sample was exposed to 4.5 L H2 near 300 K temperature, indicating the chemisorption of H atoms on graphene after the dissociation of gaseous H2 molecules into H atoms and hydrogen spillover by isolated Ti SACs.
No band structure renormalization or modification was observed without Ti loading even after the H2 exposure of graphene samples, which indicated the critical role of Ti in facilitating the dissociation of molecular H2 and graphene hydrogenation.
The H-storage capacity of the partially hydrogenated graphene with a 280 meV band gap was 10.6%, which is between the upper limit of 16.2% and a lower limit of 7.3%. The isolated Ti SACs promoted the electron transfer from Ti to graphene support at the rate of 0.45 e− per Ti atom, leading to n-type doping of graphene and a high H-storage activity, which provides a safer and more efficient method for using SACs on graphene.
The graphene hydrogenation occurred only on the first nearest-neighbor sublattices of isolated Ti, as the dissociated H atoms were restricted to small distances around the Ti catalytic center.
The H-storage capacity of 0.03 ML Ti SACs was 6.3%, which was close to the lower limit H-storage capacity value of 7.3% obtained from the partially hydrogenated graphene. The H-storage capacity increased nonlinearly to 13.5% when the Ti deposition reached 0.5 ML, indicating the aggregation of Ti atoms after 0.1 ML deposition and the formation of Ti nanoclusters with a hexagonal-close-packed cluster configuration on graphene.
The H coverage of the hydrogenated graphene of 13.5 ± 0.5% was larger than the previously reported H coverage values of 8.7−9% on the atomic hydrogenation of crystalline graphene.
To summarize, the findings of this study demonstrated that a simple H2 spillover process at Ti SACs could facilitate covalent hydrogen bonding on graphene, which provides an effective strategy for the rational design of single-site catalysts on carbon supports for chemisorbed H-storage.
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Convertino, D., Heun, S., Coletti, C. et al. Hydrogen Spillover and Storage on Graphene with Single-Site Ti Catalysts. ACS Energy Letters 2022. https://doi.org/10.1021/acsenergylett.2c00941
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