NSRRC Activity Report 2022

018 NSRRC ACTIVITY REPORT 2022 used Ti atomic deposition to mimic a single-atom catalyst (SAC) on graphene. 5 Unlike the previous study, 6 in which dissociated hydrogen sources were used, the present study demonstrated that atomically dispersed Ti atoms on crystalline graphene can produce an efficient spillover for hydrogen, which is then chemisorbed around the anchored Ti SACs. The H storage capacity discovered in this study exceeded that of Ti nanocluster catalysts at similar Ti loading. The hydrogen spillover is illustrated in Fig. 1 ; angle-resolved photoemission spectroscopy (ARPES) spectra at TLS 08A1 reveal Ti-adatom- induced renormalization of the graphene band structure for low Ti coverage (~0.03 monolayer; ML). Compared with pristine graphene that the Ti coverage was too low to break the lattice symmetry of graphene and retain a linear and gapless feature. Moreover, the band structure renormalization could be attributed to strong Ti 3d and C 2p z orbital hybridization of the isolated Ti atoms in the graphene hollow sites. When the sample was exposed to 4.5 L of H 2 near 300 K (1 L, exposure to 1 × 10 −6 Torr for 1 s), linear dispersion of the band structure of graphene and a gap of approximately 280 meV was observed. These findings indicated that hydrogen atoms were chemisorbed on graphene, which is evidence for the dissociation of gaseous H 2 molecules and atomic hydrogen spillover by isolated Ti SACs. Without Ti, neither band structure modification nor renormalization was observed in the ARPES spectrum of graphene obtained after H 2 exposure. Thus, Ti catalyst facilitated the dissociation of molecular H 2 and hydrogenation of graphene at approximately 300 K under vacuum. The hydrogen spillover in the Ti SACs on the graphene system was characterized using ARPES and other qualitative and quantitative measurements, including X-ray photoemission (XPS) at TLS 24A1 and X-ray Fig. 1 : ARPES measurements performed on pristine epitaxial graphene, after Ti deposition, and after a subsequent dose of H 2 molecules (4.5 L of H 2 ) at approximately 300 K. Band structure of pristine and Ti-doped graphene at a low coverage of 0.03 ML. The sample was then exposed to a 4.5-L dose of H 2 . Data were measured along the direction K → Γ in the vicinity of the K point. White dashed lines indicate the energy dispersion fitted by the momentum dispersion curve. [Reproduced from Ref. 5] Fig. 2 : (a) Schematic of hydrogen spillover from single-atom to single-cluster catalysts. Low-to-high Ti deposition yielded well-isolated atoms or nanocluster catalysts on graphene. From the spillover of H atoms on an isolated Ti atom and Ti nanocluster catalyst, the 2D-diffusion length L d and nanocluster size (diameter R c ) could be used to estimate the H-storage capacity at low and high Ti coverage. (b) Calculation of H storage in the Ti SACs and Ti nanoclusters with a truncated- bipyramidal shape versus the number of Ti atoms N . (c) Schematic of full H storage (η = 100%) on graphene obtained with Ti SACs through the formation of a TiH 6 structure. [Reproduced from Ref. 5]