Bottom-up precise synthesis of stable platinum dimers on graphene

Abstract

Supported metal clusters containing only a few atoms are of great interest. Progress has been made in synthesis of metal single-atom catalysts. However, precise synthesis of metal dimers on high-surface area support remains a grand challenge. Here, we show that Pt₂ dimers can be fabricated with a bottom–up approach on graphene using atomic layer deposition, through proper nucleation sites creation, Pt₁ single-atom deposition and attaching a secondary Pt atom selectively on the preliminary one. Scanning transmission electron microscopy, x-ray absorption spectroscopy, and theoretical calculations suggest that the Pt₂ dimers are likely in the oxidized form of Pt₂O_x. In hydrolytic dehydrogenation of ammonia borane, Pt₂ dimers exhibit a high specific rate of 2800 mol_H2 mol_Pt ⁻¹ min⁻¹ at room temperature, ~17- and 45-fold higher than graphene supported Pt single atoms and nanoparticles, respectively. These findings open an avenue to bottom–up fabrication of supported atomically precise ultrafine metal clusters for practical applications.

Controlled fabrication of few-atoms supported catalysts is a major challenge in the synthesis of nanomaterials. Here, the authors show a bottom-up approach to precisely synthesize platinum dimers supported on graphene, which display higher catalytic activity and stability than single atoms and nanoparticles.

Introduction

Supported metal catalysts are among the most important categories of heterogeneous catalysts in many reactions including chemical upgrading, automobile exhaust treatment, Fischer-Tropsch synthesis, biomass conversions, and many other processes^1–7. Decreasing metal particle size is desirable for improving metal utilization, since catalytic reactions take place on the surface of metal nanoparticles (NPs). When a metal cluster contains only a few metal atoms, it could have a discrete energy band structure, tightly correlated with the number of metal atoms. Changing one atom in the ultrafine cluster might largely alter the electronic structure and drastically change its catalytic properties. Such atom-dependent catalytic behaviors have been successfully demonstrated by the model catalysts of mass-selected metal clusters, which were fabricated by soft landing of mass-selected ions from their physical vapor under ultrahigh vacuum conditions^8–14. However, such complicated approach is only limited to model catalyst studies and is not applicable to high-surface area supports for practical applications.

Recently, synthesis of supported metal single-atom catalysts (SACs) has been extensively explored and a number of successful examples have been demonstrated^15–23. Nonetheless, synthesis of atomically precise ultrafine metal clusters, such as dimers, on high-surface area supports, remains a grand challenge. The decisive limitation is the lack of precise control over the aggregation process, which often causes metal NPs formation with a broad size distribution. Protecting metal clusters with a strong ligand can certainly inhibit metal aggregation to a large extent, such as in the case of thiolate-protected Au magic clusters²⁴. However, these strong protective ligands typically poison the metal clusters, and decrease their catalytic activities considerably^25–30. Alternatively, Gates et al. demonstrated that precisely defined iridium and rhodium clusters were achieved by grafting the corresponding carbonyl complexes with a specific number of metal atoms onto oxide supports^31–33. But the success is limited. As a consequence, a general bottom–up approach to synthesize atomically precise metal clusters on high-surface area supports is still missing. Atomic layer deposition (ALD) relies on two sequential self-limiting surface reactions at the molecular level, which are separated by inert gas purging^34–36. This unique character makes ALD possible to bottom–up construct catalytic materials on a high-surface area substrate uniformly and precisely^37–39.

Here, we show that Pt₂ dimers can be bottom–up fabricated on a graphene support by depositing Pt on phenol-related oxygen anchor sites atom-by-atom in a sequential manner using Pt ALD. The dominant presence of isolated Pt₁ single atoms and Pt₂ dimers in the corresponding samples were confirmed by both aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS). Their structures were determined through a combination of density function theory (DFT) calculations and XAFS spectra simulations. In hydrolysis of ammonia borane (AB) for hydrogen generation, graphene supported Pt₂ dimers (Pt₂/graphene) exhibited a striking activity, which is ~17- and 45-fold higher than that of graphene supported Pt₁ single atoms and Pt NPs, respectively. Compared to Pt₁ single atoms and Pt NPs, the decreased adsorption energies of both AB and H₂ molecules on Pt₂ dimers are likely the major reason for the high activity. More importantly, the Pt₂ dimers were stable under the current reaction condition and in the inert environment at below 300 °C.

Results

Synthesis and morphology of Pt₁/graphene and Pt₂/graphene

Based on our recent strategy²², the nucleation sites of isolated phenols or phenol–carbonyl pairs suggested by Shenoyl et al⁴⁰. for Pt ALD were first created on pristine graphene nanosheets through acid oxidation followed by high-temperature thermal reduction, as illustrated in the schematic model in Fig. 1. The reduced graphene oxide support was defect-rich multilayered graphene films with a thickness of about a few nanometers (Supplementary Fig. 1). It had a surface area of 570 m²/g. Next, Pt ALD was performed on the graphene support by alternately exposing trimethyl(methylcyclopentadienyl)-platinum(IV) (MeCpPtMe₃) and molecular O₂ at 250 °C. The self-limiting surface reactions between MeCpPtMe₃ and the support ensure nucleation of one MeCpPtMe₃ molecule on one phenol-related nucleation site during the saturated MeCpPtMe₃ exposure (Supplementary Fig. 2). It should be noted that Pt nucleation on graphene defect sites, such as edges and line defects, was inhibited at 250 °C, although it is possible at 300 °C (Supplementary Table 1)⁴¹. A similar temperature effect on inhibiting metal ALD on oxides was also observed by Elam et al⁴². Next, the ligands were removed through a combustion reaction during the O₂ exposure step^43–45 and Pt₁ single atoms are formed (denoted as Pt₁/graphene).

Fig. 1.

Schematic illustration of bottom–up synthesis of dimeric Pt₂/graphene catalysts. Controlled creation of isolated anchor sites on pristine graphene; one cycle of Pt ALD on the anchor sites for Pt single atoms formation by alternately exposing MeCpPtMe₃ and molecular O₂ at 250 °C; second cycle of Pt ALD on the Pt₁/Graphene to selectively deposit the secondary Pt atoms on the preliminary ones for Pt₂ dimers formation at 150 °C. The balls in cyan, white, red, and blue represent carbon, hydrogen, oxygen, and platinum while the ball in gray represents carbon atoms in the graphene support

Next, the formed Pt₁ single atoms was further utilized as nucleation sites for anchoring the secondary MeCpPtMe₃ molecule in the following cycle. Again, the steric hindrance between MeCpPtMe₃ molecules restricts chemisorbing one MeCpPtMe₃ molecule only on one isolated Pt₁ atom. However, we noticed that a considerable amount of Pt NPs were formed after two successive cycles of Pt ALD at 250 °C (denoted as 2cPt/graphene, Supplementary Fig. 3). Therefore, the deposition temperature was decreased to 150 °C for the second ALD cycle to avoid any metal aggregation. Meanwhile, ozone (O₃), a stronger oxidizing regent was utilized to remove the ligand efficiently to form Pt₂ dimers (Pt₂/graphene) (Fig. 1)⁴⁶. The formed Pt₁ single atoms and Pt₂ dimers are expected to be in the oxidized forms, since they were exposed to O₂ and O₃ during synthesis, respectively.

Aberration-corrected HAADF-STEM measurements were carried out to investigate the morphologies of the single-atom Pt₁/graphene and dimeric Pt₂/graphene catalysts. Compared to the naked graphene (Supplementary Fig. 1), HAADF-STEM images illustrated that one cycle of Pt ALD on the graphene support at 250 °C resulted in a formation of atomically dispersed Pt₁ atoms without presence of any visible clusters or NPs (Fig. 2a–c, and Supplementary Fig. 4). These Pt₁ single atoms were well isolated from each other with a distance >2 nm in average, which is significantly larger than the effective diameter of the MeCpPtMe₃ molecule of ~0.96 nm⁴⁷, confirming the steric effect during synthesis. Similar to our recent study of Pd₁ single-atom growth on graphene²², we found that complete removal of other oxygen-contained functional groups, such as carboxyl groups, from graphene by carefully tuning the reduction temperature and time, is the key to eliminate any Pt clusters or NPs formation (Supplementary Fig. 5). These findings suggest that metal atoms anchored on carboxyl groups have a general weak interaction with the graphene support, thus aggregate aggressively to larger metal NPs under the ALD conditions, in line with literature⁴⁸.

Fig. 2.

Morphology of the single-atom Pt₁/graphene and dimeric Pt₂/graphene catalysts. Aberration-corrected HAADF-STEM images of Pt₁/graphene (a–c) and dimeric Pt₂/graphene (d, e). Scale bars, 20 nm (a, d), 2 nm (b, e), and 1 nm (c, f). Pt single atoms in b and c and dimers in e and f are highlighted by white and yellow circles, respectively. g Statistical Pt–Pt distance in the observed Pt₂ dimers. h The ratio of Pt loading of in Pt-MeCpPtMe/graphene to that in MeCpPtMe/graphene, and i the ratio of Pt loading in Pt₂/graphene to that in Pt₁/graphene in nine independent trials determined by ICP-AES

After performing another cycle of Pt ALD on Pt₁/graphene at 150 °C (Pt₂/graphene), Pt₂ dimers were dominantly formed along with a certain number of Pt₁ single atoms (Fig. 2d–f, and Supplementary Fig. 6), where neither Pt clusters nor NPs were observed. Very interestingly, we noticed that Pt₂ dimers frequently rotated by specified angles of 30, 60, and 90° under the electron beam during STEM measurements and then split into two isolated Pt₁ atoms (see more details in Supplementary Figs. 7–9). Such characteristic rotations might be related with the geometry of the graphene support and the size of carbon defect by considering the aforementioned Pt₂ dimer structure (Supplementary Fig. 10). This observation provides strong evidence of the presence of Pt₂ dimers rather than the projection coincidence of two isolated Pt₁ atoms at different Z positions. Statistical analysis of more than 80 pairs of Pt₂ dimers showed a Pt–Pt distance of 0.30 ± 0.02 nm for Pt₂ dimers (Fig. 2g), which is significantly longer than the Pt–Pt bond in Pt bulk. This indicates that the Pt₂ dimers are in the oxidized form as expected.

To step-wise elucidate the selective deposition of secondary Pt atom onto the preliminary ones for the formation of Pt₂ dimers in the second ALD cycle, a set of control experiments were further performed using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). First, the influence of the Pt precursor ligand on the second Pt ALD cycle was examined. In this case, half cycle of Pt ALD was executed on the graphene support by performing the MeCpPtMe₃ pulse step only (denoted as MeCpPtMe/graphene) at 250 °C. After that, the ALD reactor was cooled to near room temperature and half amount of the MeCpPtMe/graphene sample was taken out of reactor for ICP-AES analysis; while the rest of the sample was put back to the ALD reactor quickly to perform the second cycle (Pt-MeCpPtMe/graphene) at 150 °C. In nine independent trials, the ICP-AES results showed that the ratio of the Pt loadings of Pt-MeCpPtMe/graphene to those of the corresponding MeCpPtMe/graphene were all very close to one (Fig. 2h). Therefore, there was no additional Pt deposited on MeCpPtMe/Graphene, reflecting the saturated self-limiting reaction character of ALD^34–36. Second, we found that exposure of pristine graphene to O₂ at 250 °C did not cause any detectable Pt deposition either (Supplementary Table 1). This is very important to ensure that the oxygen pulse at 250 °C in the first ALD cycle did not create any additional nucleation sites. Taken together, the Pt₁ single atoms formed on graphene, confirmed by HAADF-STEM in Fig. 2a–c, are the only nucleation sites for the following ALD cycle. It is worthy to note that Pt₁ single atoms well isolated from each other could be crucial to make all the Pt₁ single atoms accessible for chemisorbing the second MeCpPtMe₃ molecule in the second ALD cycle without steric hindrance.

During the second ALD cycle, one MeCpPtMe₃ molecule anchors on one Pt₁ atom in the Pt₁/graphene sample due to the steric hindrance effect, which doubles the Pt loading. This was confirmed by the ratios of two for the Pt loadings of Pt₂/Graphene to Pt₁/graphene in nine independent trials (Fig. 2i and Supplementary Table 2), hence providing strong evidence of the formation of Pt₂ dimers. The Pt₁ single atoms observed in Fig. 2e and Supplementary Figs. 6–9 were likely formed by uncoupling of Pt₂ dimers under the high flux electron beam during STEM measurements⁴⁹. On the other hand, once Pt NPs were formed during ALD, the ratio of Pt loading of the two-ALD cycle sample to the one-cycle sample was apparently off from the stoichiometric value of two (Supplementary Table 2).

Performing an additional cycle on Pt₂/graphene to form Pt₃ trimers might be possible. However, we noticed that the O₃ in the second cycle can create additional nucleation sites on graphene. As a consequence, selective deposition was not achieved for the third cycle and resulted in a mixture of Pt₁, Pt₂ and Pt₃. Therefore, we mainly focused on the Pt₂ dimers in this work.

XAFS characterization and DFT calculations

Figure 3a shows the X-ray absorption near-edge structure (XANES) spectra of MeCpPtMe/graphene, Pt₁/graphene, and Pt₂/graphene at the Pt L ₃-edge, along with Pt foil, PtO₂, and MeCpPtMe₃ as references. Evidently, the XANES white line peaks of these three samples (11,567 eV) located at between Pt foil and PtO₂, indicating that the Pt in MeCpPtMe/graphene, Pt₁ single atoms, and Pt₂ dimers were all in a similar oxidation state between Pt⁰ and Pt⁴⁺. The MeCpPtMe₃ reference sample exhibits two well-resolved peaks at 1.62 and 1.99 Å in the Fourier transformed (FT) k ³χ(k) curve in the real-space (R-space) (Fig. 3b), assigned to the shorter Pt–C bonds (1.99−2.14 Å) in the three Pt–Me groups and longer Pt–C bonds (2.26−2.36 Å) in the Pt–MeCp group, respectively (Supplementary Fig. 11a)^47,50. Apparently, the two split peaks in the MeCpPtMe/graphene curve suggests that the MeCp group stayed on Pt in this sample. This observation is in line with both the experimental result of the formation of MeCpPtMe₂ surface species on oxides^43,51 and theoretical calculations where MeCpPtMe₃ on epoxydated and hydroxylated graphene surfaces liberates either one or two methyl groups depending on the available surface groups⁵².

Fig. 3.

XAFS structural characterization and spectra simulations. a The XANE spectra and b the K²-weighted Fourier transform spectra of MeCpPtMe/graphene, Pt₁/graphene, and Pt₂/graphene at the Pt L₃-edge. The reference samples of Pt foil, PtO₂, and MeCpPtMe₃ are also shown for comparison. Comparison of the EXAFS simulations based on the corresponding DFT calculated structural models (insets) with the experimental EXAFS spectra of MeCpPtMe/graphene (c), Pt₁/graphene (d) and Pt₂/graphene (e). The ball in gray, white, red, and dark blue represent carbon, hydrogen, oxygen, and platinum, respectively

The first shell FT peak in the Pt₁/graphene spectrum had a higher intensity and slightly shifted to 1.65 Å, while the peak at 1.99 Å disappeared. Clearly, the MeCp ligand was combusted off after the O₂ exposure step at 250 °C. The first shell peak is assigned to Pt−O and/or Pt−C coordinations. Similar to MeCpPtMe/graphene, a very weak peak at 2.4 Å was visible in the spectrum of Pt₁/graphene. However, this peak is significantly different from the Pt–Pt coordination peak (at ~2.62 Å) in Pt foil, thus assigned to the second nearest C/O neighbors of Pt. This suggests the absence of Pt NPs in Pt₁/graphene, consistent with our STEM observation (Fig. 2a–c and Supplementary Fig. 4). The dimeric Pt₂/graphene sample showed a similar FT curve with Pt₁/graphene, implying a similar local C/O coordinations in these two samples. In the Pt₂/graphene spectrum, there was no discernible peak for the Pt–Pt coordination, suggesting the Pt₂ dimers are in the oxidized form after the ozone exposure step at 150 °C.

Considering the difficulties in discriminating the C/O neighbors by EXAFS fittings, we resorted to the combination of DFT calculations with EXAFS simulations to determine the optimized structures of these three samples. Here, a graphene support containing a carbon vacancy along with either isolated phenol group or phenol–carbonyl pairs⁴⁰ was employed as the reduced graphene oxide surface. The structural models optimized by DFT calculations were further examined by EXAFS simulations.

Regarding the previous work⁵², the structures of MeCpPtMe₂ and MeCpPtMe were both considered for the MeCpPtMe/graphene sample (Supplementary Fig. 11). Compared to the MeCpPtMe₃ molecule, the five Pt–C bonds in the Pt–MeCp group in these two structures both changed significantly. According to the EXAFS simulations for these two structures, MeCpPtMe bonded to the graphene support through two interfacial O atoms might be the promising structure for the MeCpPtMe/graphene sample (Fig. 3c and Supplementary Figs. 11b and 12). Compared to the spectrum of MeCpPtMe₃, the remarkably attenuated peak at 1.99 Å in the MeCpPtMe/graphene spectrum is due to the considerable distortion of the MeCp group.

When oxygen combusts off the ligand, additional oxygen chemisorbs on the Pt₁ atom in the Pt₁/graphene sample⁵³. Indeed, Pt₁ atom with one chemisorbed O₂ molecule at the terminal position (the Pt–O bond distance: 2.00 Å) and two O atoms at the interface (the Pt–O bond distance: 2.02 Å) produces an EXAFS spectrum in good agreement with the experimental result (Fig. 3d and Supplementary Fig. 12). On the contrary, the Pt₁ atom with one O and one C atom at the interface generates split FT peaks in the first shell, in contrast with the experimental results (Supplementary Fig. 13). Nonetheless, this structure might not be completely ruled out.

During the second Pt ALD cycle, a secondary Pt atom anchors on the preliminary one and then becomes oxidized during the O₃ exposure step. Taking this information into account, a Pt₂O₆ chain structure with O atoms alternating between the terminal and bridge positions was constructed (inset in Fig. 3e). After optimization, our calculations show that the Pt–Pt bond distance in the Pt₂O₆ chain is 3.03 Å (the inset of Fig. 3e), consistent with the experimental results very well (Fig. 2g). The lengths of the Pt–O bonds in the Pt₂O₆ chain are very close to each other, ~1.93–2.03 Å. Moreover, XAFS spectrum simulation for this Pt₂O₆ chain structure also agrees very well with the experimental result (Fig. 3e and Supplementary Fig. 12). This chain structure is found to be similar to the suggested structure models for Pt_xO_y (x = 1–3) clusters by Schneider et al. previously⁵⁴. Interestingly, we also noticed that the tilted angle of the Pt₂O₆ chain could vary largely from ~8 to ~50°, depending on both the size of carbon vacancies and the configurations of two interfacial O atoms (Supplementary Fig. 14). The largely varied angles tilted from the graphene support, explain well the different Pt–Pt bond distances in the Pt₂ dimers observed by STEM (Fig. 2e–g). Again, the structure model of Pt₂ dimers with one O and one C atom at the interface might not be completely ruled out.

Catalytic activity

AB with satisfactory air stability and remarkably high hydrogen content of 19.6 wt%, has been regarded as a promising hydrogen storage media for portable applications⁵⁵. Here, hydrolysis of AB for hydrogen production was utilized as a probe reaction to investigate the catalytic properties of Pt₁ single atoms and Pt₂ dimers. According to literature, this reaction is depicted as the following Eq. (1):⁵⁶

$$\mathrm{N}{\mathrm{H}}_{\mathrm{3}}\mathrm{B}{\mathrm{H}}_{\mathrm{3}}+\mathrm{2}{\mathrm{H}}_{\mathrm{2}}\mathrm{O}\to \mathrm{N}{{\mathrm{H}}_{\mathrm{4}}}^{\mathrm{+}}+\mathrm{B}{{\mathrm{O}}_{\mathrm{2}}}^{-}+\mathrm{3}{\mathrm{H}}_{\mathrm{2}}\left(\mathrm{g}\right)$$ 1

As shown in Fig. 4a, the single-atom Pt₁/graphene catalyst generated 9.8 mL H₂ gas only in 10.8 min, which is ~42% of the theoretical volume of 23.4 mL according to the Eq. (1). In sharp contrast, the dimeric Pt₂/graphene catalyst released 23.4 mL H₂ vigorously in only 0.9 min, indicating the depletion of AB. The activity of 2cPt/graphene, synthesized by two successive cycles of Pt ALD on graphene at 250 °C, was rather close to Pt₁/graphene, by generating 7.5 mL H₂ in 10.5 min. Obviously, Pt₂/graphene and 2cPt/graphene were distinctly different in structure. As a comparison, the activities of the Pt NP catalysts of Pt/graphene-WI, Pt/carbon, and Pt/SiO₂, as well as the commercial PtO₂ were tested. The Pt/graphene-WI catalyst with a Pt particle size of 1.8 ± 0.5 nm (Supplementary Fig. 15 and Table 3), showed a very poor activity of 6.6 mL H₂ release in 15.2 min. The commercial Pt/carbon catalyst with a Pt particle size of 2.3 ± 0.7 nm (Supplementary Fig. 16 and Table 3) was considerably better, generating 23.4 mL H₂ in 9.7 min. The Pt/SiO₂ ALD catalyst with a Pt particle size of 1.9 ± 0.3 nm (Supplementary Fig. 17 and Table 3) was also very active, releasing 23.4 mL H₂ in 6.8 min. The commercial PtO₂ powder (Supplementary Fig. 18) generated ~ 21 mL H₂ in 15 min. In this case, a reduction of PtO₂ into Pt occurred during the reaction, in line with literature⁵⁶.

Fig. 4.

Catalytic activities of various Pt catalysts in AB hydrolysis. a Plots of time vs normalized volume of hydrogen gas generated from the AB hydrolysis reaction over the single-atom Pt₁/graphene and dimeric Pt₂/graphene catalysts. Pt NPs samples of 2cPt/graphene, Pt/graphene-WI, Pt/carbon, Pt/SiO₂, and commercial PtO₂ powder were also evaluated as a comparison. b The specific rates over these samples based on the mole of Pt

The specific rates of these samples were calculated based on the Pt contents. The rates were 160 and 110 mol_H2 Mol_Pt ⁻¹ min⁻¹, for Pt₁/graphene and 2cPt/graphene, respectively (Fig. 4b and Supplementary Fig. 19). For the Pt NP samples, the rates were 62, 380, and 440 mol_H2 Mol_Pt ⁻¹ min⁻¹, for Pt/graphene-WI, Pt/carbon, and Pt/SiO₂, respectively, close to the values for Pt catalysts reported in the literature (Supplementary Table 4). The rate of PtO₂ was 197 mol_H2 Mol_Pt ⁻¹ min⁻¹. Obviously, hydrolytic dehydrogenation of AB on Pt catalysts is a structure sensitive reaction, the size, and electronic properties of Pt NPs might both play important roles^57,58. In sharp contrast with the above samples, the Pt₂ dimers exhibited the highest rate of 2800 mol_H2 Mol_Pt ⁻¹ min⁻¹ ever reported in literature, which was ~17 and 45 times higher than the corresponding single-atom Pt₁/graphene and Pt/graphene-WI samples, respectively. When the mole ratio of Pt to the AB substrate was increased, Pt₂/graphene could preserve the high specific rate to a large extent (Supplementary Fig. 20). Note that all the Pt samples produced a similar product of BO₂ ⁻ in the spent reaction solution, according to the identical ¹¹B resonance at 8.9 ppm (Supplementary Fig. 21)⁵⁶.

DFT calculations were further carried out to get a deeper insight into the vast activity difference between Pt₁/graphene and Pt₂/graphene. Since AB is known as an excellent reducing agent⁵⁹, and could likely stripe off the terminal dioxygen of Pt₁/graphene and Pt₂/graphene (the insets of Fig. 3d, e) during the reaction, partially reduced structures without the terminal dioxygen were considered for both Pt₁/graphene and Pt₂/graphene (the insets of Fig. 5a). The reduced samples are denoted as Pt₁/graphene-R and Pt₂/graphene-R, respectively. First, we compared the projected density of states of the 5d orbitals of the Pt atom in Pt₁/graphene-R and the top Pt atom in Pt₂/graphene-R. It was found that the unoccupied 5d states of the top Pt atom in Pt₂/graphene-R locates at a considerably higher energy position of 0.87 eV above Fermi level (E _f) than that of the Pt atom in Pt₁/graphene-R (0.40 eV), which indicates that Pt₁/graphene-R is more prone to accept electrons than Pt₂/graphene-R (Fig. 5a). This result is in line with the recent literature where Åstrand et al. reported that Pt₁ single atom had a more strong ability to accept electrons than the top Pt atom in Pt₂ dimer, thereby showing stronger CO adsorption on Pt₁ ^60,61.

Fig. 5.

Electronic properties as well as AB and H₂ adsorptions. a The local partial density of state (LPDOS) projected on the Pt 5d orbitals of Pt₁/graphene-R and the top Pt atom in Pt₂/graphene-R. The local configurations for AB adsorption on Pt₁/graphene-R (b), Pt₂/graphene-R (c). The local configurations for H₂ adsorption on Pt₁/graphene-R (d), Pt₂/graphene-R (e). The ball in gray, white, pink, blue, red, and dark blue represent carbon, hydrogen, boron, nitrogen, oxygen, and platinum, respectively

When AB is adsorbed on Pt₁/graphene-R, two B–H bonds were significantly elongated from 1.22 to 1.42 Å, with a strong adsorption energy of −3.20 eV (Fig. 5b). On Pt₂/graphene-R, the elongation of the two B–H bonds was slightly less, to 1.39 Å, and the AB adsorption energy was considerably weaker, about −2.81 eV (Fig. 5c). The adsorption of AB on Pt (111) was also investigated as a comparison (Supplementary Fig. 22). We found that AB could quickly dissociate to three H atoms and BNH₃ species without any barrier. The AB adsorption energy is −3.97 eV, significantly stronger than that on Pt₁ single atom and Pt₂ dimer. The strong AB adsorption on Pt (111) revealed by DFT calculations agrees well with the literature⁵⁷, where Pt NP catalyst deactivation induced by B poisoning was observed during the AB hydrolysis reaction. Bearing this in mind, we further examined the recycling stabilities of the Pt₁/graphene, Pt NP, and PtO₂ catalysts and measured the B contents in the used samples using ICP-AES. Indeed, catalyst deactivations and considerable amounts of B adsorption were observed on all the used samples (Supplementary Figs. 23 and 24), in line with the literature⁵⁷. In addition, sintering and leaching of Pt were also noticed on Pt₁/graphene, Pt/graphene-WI, and Pt/SiO₂ (Supplementary Fig. 25). Therefore, the considerable weaker adsorption of AB on Pt₂ dimer could be one key factor for its high activity as shown in Fig. 4.

H₂ adsorptions on Pt₁/graphene-R and Pt₂/graphene-R were also investigated as a descriptor of hydrogen desorption from the catalyst surface during the AB hydrolysis reaction (Fig. 5d, e). It was again found that H₂ adsorption on Pt₁/graphene-R (−2.42 eV) is remarkably stronger than that on Pt₂/graphene-R (−1.29 eV). More interestingly, we found that H₂ chemisorbs dissociatively on Pt₁/graphene-R, but molecularly on Pt₂/graphene-R, indicated by the H–H bond distance of 2.02 and 0.96 Å, respectively. Such molecular adsorption of H₂ on the Pt₂ dimer with a moderate adsorption energy favors H₂ desorption during the AB hydrolysis reaction, thereby further boosting the catalytic activity. Taken together, compared to Pt₁ single atom, the higher energy position of the unoccupied state of the Pt 5d orbital of the top Pt in the Pt₂ dimer might play an important role in weakening the adsorption of both AB and H₂ molecules, thus facilitating the activity remarkably.

Stability of Pt₂ dimers on graphene

In sharp contrast with all other Pt samples shown in Supplementary Fig. 23, the dimeric Pt₂/graphene catalyst exhibited a very high stability in the AB hydrolysis reaction during the recyclability test for five cycles (Fig. 6a). STEM measurements showed that there was no any visible Pt NPs formation and Pt₂ dimers remained as the main features in the used sample (Fig. 6d, g). The Pt₁ single atoms shown in Fig. 6g were likely produced by the electron beam during STEM measurements (Supplementary Figs. 7–9), since there was no apparent activity decrease. When Pt₂/graphene was annealed at high temperatures in helium, the activity declined considerably (Fig. 6b), but rates still remained as high as1670 and 1037 mol_H2 Mol_Pt ⁻¹ min⁻¹ for the sample annealed at 300 (Pt₂/graphene-300C) and 400 °C (Pt₂/graphene-400C), respectively (Fig. 6c). Obviously, significant amounts of Pt₂ dimers were survived after the high-temperature treatments. Indeed, HAADF-STEM revealed a mixture of Pt₂ dimers, Pt₁ single atoms, and Pt NPs in both Pt₂/graphene-300C (Fig. 6e, h) and Pt₂/graphene-400C (Fig. 6f, i). Please keep in mind that these STEM images might significantly underestimate the portion of Pt₂ dimers in the samples owing to the possible beam damage during STEM measurements (Supplementary Figs. 7–9).

Fig. 6.

Stability of the dimeric Pt₂/graphene catalyst. a Five recycles in hydrolytic dehydrogenation of AB at room temperature over the dimeric Pt₂/graphene catalyst by adding additional 0.325 mmol of pure AB into the reaction flask after each run. b Plots of time vs volume of hydrogen gas generated from AB hydrolysis and c the corresponding specific rates at room temperature over the dimeric Pt₂/graphene catalysts after different pretreatments: as-prepared, annealing in helium at 300 and 400 °C for 1 h, respectively. d, g Representative HAADF-STEM images of the used Pt₂/graphene catalyst after the recyclability test, scale bars, 20 nm (d), 1 nm (g). e, h Representative HAADF-STEM images of the Pt₂/graphene catalyst after annealing in helium at 300 °C for 1 h, scale bars, 10 nm (e), 2 nm (h). f, i Representative HAADF-STEM images of the Pt₂/graphene catalyst after annealing in helium at 400 °C for 1 h, scale bars, 10 nm (f), 2 nm (i). Pt₂ dimers in g–i are highlighted by yellow circles

In conclusion, we have successfully demonstrated that Pt₂ dimers can be bottom–up constructed on graphene with a high-surface area. We found that the type of surface nucleation sites, selective deposition, the self-limiting nature of ALD, and the high stabilities of Pt₁ single atoms and Pt₂ dimers are the keys factors for the Pt₂ dimers synthesis. The dominant presence of Pt₂ dimers on graphene in the oxidized form of Pt₂O_x were confirmed by a combination of aberration-corrected HAADF-STEM, ICP-AES, and XAFS and DFT calculations. Rotating and uncoupling of Pt₂ dimers under the electron beam during STEM measurements, provide direct evidence of the presence of Pt₂ dimers on graphene. In the AB hydrolysis reaction, the dimeric Pt₂/graphene catalyst exhibited a strikingly high activity, which was ~17- and 45-fold higher than graphene supported Pt₁ single atoms and Pt NPs, respectively. The lower adsorption energies of AB and H₂ on the Pt₂ dimers than that on Pt₁ single atoms or Pt NPs are likely the major reasons for the high activity. Importantly, the dimeric Pt₂/graphene catalyst showed a high stability under the current reaction conditions and in the inert environment at below 300 °C. Finally, our findings point out a new avenue to bottom–up synthesis of atomically precise ultrafine metal (and/or metal oxide) clusters on high-surface area supports for advanced catalysis.

Methods

Materials

Trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe₃, 98%), chloroplatinic acid (H₂PtCl₆, ≥99.9%, trace metals basis), ammonia borane (97%), the commercial PtO₂ (≥99.9%, 70 m²/g), and Pt/carbon catalysts (the Pt content, 5.0 wt%) were all purchased from Sigma Aldrich. Silica gel was purchased from Alfa Aesar (Brunauer, Emmett, and Teller (BET) surface area 300 m²/g). Pristine graphene nanosheet (99.5%) was bought from Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Sciences. All materials were used as received without further purification.

Preparation of reduced graphene oxide

Pristine graphene nanosheet was first oxidized to graphene oxide according to the procedure described previously⁶². In brief, 0.6 g graphene nanosheet and 0.3 g sodium nitrate was sequentially added into concentrated sulfuric acid (H₂SO₄, 15 mL) and stirred at room temperature for 22 h. Then the mixture was cooled down to 0 °C to add 1.8 g potassium permanganate (KMnO₄). After stirring at room temperature and 35 °C for 2 and 3 h, respectively, the mixture was heated to 98 °C and kept at this temperature for another 30 min. Next, it was cooled down to 40 °C, and 90 mL of water and 7.5 mL of hydrogen peroxide (H₂O₂, 30%) were slowly added into the mixture. After that the precipitate was filtered out by washing with HCl (5%) and ultrapure water, it was dried in a vacuum oven at 45 °C overnight. The dry material was grinded to obtain graphene oxide powder. Finally, the reduced graphene oxide was obtained by thermal deoxygenation of graphene oxide powder at 1050 °C for 2 min under helium at a flow rate of 50 mL/min.

Synthesis of Pt₁/graphene

Pt ALD was carried out on a viscous flow reactor (GEMSTAR-6 Benchtop ALD, Arradiance) by alternatively exposing to MeCpPtMe₃ precursor and O₂ (99.999%) at 250 ^oC^46,63,64. Ultrahigh purity N₂ (99.999%) was used as the carrier gas at a flow rate of 200 mL/min. The Pt precursor was heated to 65 °C to get a sufficient vapor pressure. The reactor inlets were held at 110 °C to avoid any precursor condensation. The timing sequence was 90, 120, 60, and 120 sec for the MeCpPtMe₃ exposure, N₂ purge, O₂ exposure, and N₂ purge, respectively (90-120-60-120).

Synthesis of Pt₂/graphene

The second Pt ALD cycle was performed on the Pt₁/graphene SAC at 150 °C. Here, O₃ was used as the oxidant to make sure that the precursor ligand can be fully removed⁶⁵. The timing sequence was 90-120-60-120.

Synthesis of 2cPt/graphene

Two consecutive cycles of Pt ALD was also performed on the reduced graphene oxide at 250 °C using the same timing sequence.

Synthesis of Pt/SiO₂

Pt ALD was performed on the silica gel support for one cycle at 250 °C using the same timing sequence.

Synthesis of Pt/graphene-WI

A Pt/graphene NP catalyst was synthesized by the wetness impregnation method (Pt/graphene-WI). In this case, 100 mg graphene support was slowly added into a 1.93 × 10⁻² M H₂PtCl₆ aqueous solution (0.9 mL). Then, the mixture was stirred for 30 min, and then dried in air at room temperature for 12 h. The dried material was first calcined in air at 120 °C for 12 h, and then reduced in 10% H₂ in argon at 300 °C for another 2 h to get the Pt/graphene-WI catalyst.

Catalyst characterization

The Pt loadings in these samples were determined by ICP-AES measurements; therein all samples were dissolved in hot fresh aqua regia. The BET surface area was measured on a Micromeritics ASAP 2020 system. Raman spectra were recorded on a LabRAM HR Raman spectrometer with a 514 nm Ar laser in backscattering geometry. Aberration-corrected HAADF-STEM measurements were taken on a JEM-ARM200F instrument (University of Science and Technology of China) at 200 keV. XAFS measurements at the Pt L ₃-edge (11,564 eV) were performed in the transmission mode with the Si (111) monochromator at the BL14W1 beamline of the Shanghai Synchrotron Radiation Facility (SSRF), China. The storage ring of SSRF worked at 3.5 GeV with a maximum current of 210 mA.

XAFS data analysis and simulation

The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages⁶¹.The EXAFS oscillation functions χ(k) were obtained by subtracting the post-edge background from the overall absorption spectra and then normalized with respect to the edge-jump step. The R _bkg value of 1.0 was used for all samples. Subsequently, k ³-weighted χ(k) fucntions in the k range of 2.2–13.5 Å⁻¹ were FT to the R space by using a Hanning window of dk = 3.0 Å⁻¹.

EXAFS simulations were performed with the FEFF8.4 code⁶² using the structural models suggested by DFT calculations. The simulated EXAFS χ(k) functions were also k ³-weighted and FT into the R-space by using the same k range of 2.2–13.5 Å⁻¹ as that in the experimental data. During simulations, the coordination numbers were set to the values of the model structures generated by DFT calculations. The amplitude reduction factor S ₀ ² was fixed at the value of 0.86 which was determined by fitting the reference metal Pt foil. The Debye–Waller factors for the nearest Pt−O/C and Pt−Pt pairs were set at the typical values of 0.0030 and 0.0065 Ų determined from the fittings of PtO₂ and Pt foil references, respectively, and they were set at 0.008 Ų for all the other distant paths which contributed barely discernible signals as seen from the experimental data in the R-space. To further improve the match between the simulation and the experimental data, for the MeCpPtMe/graphene sample, the two nearest Pt–C and Pt–O interatomic distances were optimized to 2.00 and 2.02 Å, respectively, both of which are within ~ 2% error level as compared to the optimized structure by DFT calculations (2.05 and 2.06 Å, respectively). For the other two samples, the simulated EXAFS spectra based on the DFT-generated structures match well with the experimental data, thus no further structure optimization was performed during EXAFS simulations.

DFT Calculations

All spin-polarized calculations were performed by using the DFT method. The DFT Semi-core Pseudopotential method⁶⁶ with a double numerical basis set together with polarization functions (DNP) were adopted to form the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional within the generalized gradient approximation⁶⁷, implemented in DMol³ package (DMol³ is a density functional theory quantum mechanical package available from Accelrys Software Inc.)⁶⁸. A DFT-D semi-empirical correction with Tkatchenko-Scheffler (TS) method is applied with the PBE functional to account for the dispersion interaction. Conductor-like screening model (COSMO) with a dielectric constant of 78.54 is adopted to consider the water solvent effect regarding the adsorption of molecule and fragment in AB hydrolysis. A smearing of 0.001 Ha to the orbital occupation is applied to achieve electronic convergence. The real-space global cutoff radius is set to be 4.5 Å. A hexagonal supercell containing (8 × 8) unit cells of graphene monolayer with about 17 Å vacuum layer was used as a support. The convergence tolerances of energy, force, and displacement for the geometry optimization were 1 × 10⁻⁵ Ha, 0.002 Ha/Å, and 0.005 Å, respectively. 1 × 1 × 1 k-points grid is used to describe the Brillouin zone for geometric optimization. The adsorption energy is defined by the formula: E _ads(AB) = E _{(AB/catalyst)}–(E _catalyst + E _AB) and E _ads(H2) = E _{(H2/catalyst)}–(E _catalyst + E _H2,gas), where E _{(AB/catalyst)}, E _{(H2/catalyst)}, and E _catalyst are the total energies for the optimized equilibrium configurations of catalyst with and without AB or H₂, respectively; and E _AB (E _H2,gas) is the energy of the AB (gas phase H₂) molecule in its ground state. For Pt(111), the supercell is (4×4).

Hydrolytic dehydrogenation of AB

As a probe reaction, hydrolytic dehydrogenation of AB was performed in a three-necked flask at 27 °C under atmospheric pressure. The flask was immersed in a water bath to control the reaction temperature. 10 mg of the Pt₂/graphene catalyst was used, while the weight of other catalysts was adjusted to keep the same amount of Pt with Pt₂/graphene. Typically, 5 mL aqueous AB solution (6.5 × 10⁻² M) were introduced into the glass container via a syringe. For the commercial PtO₂ catalyst, the mole ratio of Pt to AB was kept as the same with other samples, and the result was normalized to other samples based the amount of Pt. The AB solution and the catalyst were well-mixed by using a magnetic stirrer at a speed of 800 r/min to eliminate any mass-transfer issue. The generated volume of H₂ was measured by a water-filled gas burette, where the volume of water discharged was converted into the volume of hydrogen generated⁶⁹.

The specific rates (r) of these catalysts were calculated according to the Eq. (2):

$$r\mathrm{=}\frac{n_{\mathrm{H2}}}{n_{\mathrm{Pt}}\times t}$$ 2

Here $n_{\mathrm{H2}}$ is the mole of generated H₂, while $n_{\mathrm{Pt}}$ is the total mole of Pt in the sample. t is the reaction time in min.

Data availability

All the relevant data are available from the authors upon request.

Author contributions

J.L. conceived the idea and designed the experiments. H.Y. performed catalyst synthesis and catalytic performance evaluations. Y.L. performed the STEM measurements. S.W., Z.S., H.C., W.L., and T.Y. performed the XAFS measurements. W.Z., H.W., and J.Y. performed the DFT calculations. C.W., J.L., and X.H. assisted catalyst characterization and catalytic performance tests. J.L. wrote the manuscript. All the authors contributed and commented on the manuscript. H.Y. and Y.L. contributed equally to this work.

Competing interests

The authors declare no competing financial interests.