How do defects affect hydrogen spillover on graphene-supported Pt? A DFT study

The hydrogen dissociation and spillover mechanism on oxide-supported Cu catalysts play a pivotal role in the hydrogenation of carbon dioxide to methanol. This study investigates the hydrogen spillover mechanism on Cu/CeO2 catalysts using in situ spectral characterization under high-pressure reaction conditions and density functional theory (DFT) simulations. The research confirms that the Cu sites serve as the initial dissociation points for the hydrogen molecules. The chemically adsorbed hydrogen (H*) then spills over onto the CeO2 support and interacts with the lattice oxygen to form special hydroxyl groups, while simultaneously reducing the surrounding Ce4+ to form Ce3+. Interestingly, the temperature-programmed desorption (TPD) results found that heating the hydroxyl-containing surface mainly reverses H2 dissociation by desorbing H2 instead of forming H2O, while no significant vacancy formation was detected. The DFT calculation identified a subsurface pathway favoring hydrogen migration, which explained the dominating H2 in the TPD products. A chemical loop study after CO2/H2 cofeeding on the catalyst reveals that hydrogen spillover facilitates the highly reduced surface serving as the active centers, enabling a secondary methanol synthesis in a vacuum. This study provides a model of the formation and desorption pathways of hydrogen species on Cu/CeO2 catalysts and illustrates the key role of the hydrogen spillover mechanism in promoting the CO2 hydrogenation to methanol reaction through important experimental analysis.

Adsorption and dissociation of molecular hydrogen on Pt/CeO 2 catalyst in the hydrogen spillover process: A quantum chemical molecular dynamics study

Unveil the potential in hydrogen activation and spillover towards NiMoS by Ni3S2 - A theoretical study

Steric and chemical effects on the hydrogen adsorption and dissociation on free and graphene–supported palladium clusters

Defect-Driven hydrogen Evolution: Enhanced hydrogen spillover on Pt-MoS2 interface via sulfur vacancies.

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO2 substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 A. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO2 surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H2 dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.

ConspectusHydrogen spillover, as a well-known phenomenon for thermal hydrogenation, generally involves the migration of active hydrogen on the surface of metal-supported catalysts. For thermocatalytic hydrogenation, hydrogen spillover generally takes place from metals with superiority for dissociating hydrogen molecules to supports with strong hydrogen adsorption under a H2 environment with high pressures. The former can bring high hydrogen chemical potential to largely reduce the kinetic barrier of the migration of active hydrogen species from metals to supports. At the same time, the latter can make H* migration thermodynamically spontaneous. For these reasons, hydrogen spillover is a common interfacial phenomenon occurring on metal-supported catalysts during thermocatalysis. Recently, this phenomenon has been observed for the exceptionally enhanced electrocatalytic performance for hydrogen evolution and other electrocatalytic organic synthesis. Different from hydrogen spillover for thermocatalysis under high H2 pressure, hydrogen spillover for electrocatalysis involves the migration of active hydrogen species (H*) from metals with strong hydrogen adsorption to supports with weak hydrogen adsorption, thereby suffering from a thermodynamically unfavorable process accompanied by a high kinetic barrier. Thus, the occurrence of hydrogen spillover at the electrocatalytic interface is not easy, and successful cases are rare. Understanding the underlying nature of hydrogen spillover at the electrocatalytic interface of metal-supported catalysts is critical to the rational design of advanced electrocatalysts.In this Account, we provide in-depth insights into recent advances in hydrogen spillover at the electrocatalytic interface for a significantly enhanced hydrogen evolution performance. Electron accumulation at the metal-support interface induces severe interfacial H* trapping and is recognized as the main factor in the failed hydrogen spillover. Given this, we developed two novel strategies to promote the occurrence of hydrogen spillover at the electrocatalytic interface. These strategies include (i) the introduction of ligand environments to enrich the local hydrogen coverage on metals and lower the barrier for interfacial hydrogen spillover and (ii) the minimization of work function difference between metals and supports (ΔΦ) to relieve electron accumulation and lower the kinetic barrier for hydrogen spillover. Also, we summarize the previously reported strategy of shortening the metal-support interface distance to lower the kinetic barrier for interfacial hydrogen spillover. Afterward, some criteria and methodologies are proposed to identify the hydrogen spillover phenomenon at the electrocatalytic interface. Finally, the remaining challenges and future perspectives are also discussed. Based on this Account, we aim to provide new insights into electrocatalysis, particularly the targeted control of hydrogen spillover at the electrocatalytic interface, and then to offer guidelines for the rational design of advanced electrocatalysts.

Motivated by the use of electrodes modified at the nanoscale by supported metal species, we studied computationally how the reactivity changes in such a composite system compared to the reactivity of the individual systems, metal clusters and metal surfaces. Specifically, we examined hydrogen adsorption on and hydrogen spillover from Au- and Cu-supported Pt(3) and Pd(3) clusters, using a method based on Density Functional Theory. Two distinctive types of sites were found for the adsorption of atomic hydrogen: (i) on the supported clusters and (ii) at the cluster-substrate interfaces. The adsorption energy of hydrogen on the supported clusters is ∼20 kJ mol(-1) smaller when the cluster is supported by Cu instead of Au. In contrast, the substrate has no effect on hydrogen adsorbed at the cluster-substrate interfaces. Adsorbed Pt(3) and Pd(3) clusters locally modify the reactivity of the substrates as quantified by the reduced adsorption energy of hydrogen compared to the corresponding clean substrate. Hydrogen dissociative adsorption followed by spillover is thermodynamically and kinetically favored for clusters supported on a Cu surface, but not on Au. Moreover, spillover of hydrogen is more likely from metal-supported Pd than Pt clusters as revealed by barriers that are calculated 40-50 kJ mol(-1) lower in energy.

The effects of cluster size and metal–support interaction on the catalytic activity of Au nanoparticles supported on anatase TiO2(101) and (001) surfaces for H2 adsorption, activation and dissociation were investigated by periodic density functional theory (DFT) calculations. On the stoichiometric TiO2 surfaces, it was found that the adsorptions of both Au clusters and H2 molecules are sensitive to the cluster size of gold, and the (001) facet with “soft” lattice and coordination unsaturated atoms on the surface is superior for Au adsorption stability, but H2 adsorption does not show apparent distinction on the two catalysts. The Au atoms active for H2 activation should be neutral in charge and located at the edge or corner of the Au nanoparticles in lower coordination. The metal–support interaction plays an important role for H2 activation and dissociation, and the O2––H+–H––Au structure was identified in the transition state through which H2 dissociation occurred via a heterolytic dissociation process a...

Atomic Pd dispersion in triangular Cu nanosheets with dominant (111) plane as a tandem catalyst for highly efficient and selective electrodehalogenation

Open AccessCCS ChemistryRESEARCH ARTICLE1 Oct 2021Synergistic Effect of Platinum Single Atoms and Nanoclusters Boosting Electrocatalytic Hydrogen Evolution Yihui Zhu†, Pengfei Tian†, Hongliang Jiang, Jingren Mu, Lu Meng, Xiaozhi Su, Yu Wang, Yunxiang Lin, Yihua Zhu, Li Song and Chunzhong Li Yihui Zhu† Key Laboratory for Ultrafine Materials of Ministry of Education, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Pengfei Tian† Key Laboratory for Ultrafine Materials of Ministry of Education, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 , Hongliang Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 , Jingren Mu Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Lu Meng Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Xiaozhi Su Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210 , Yu Wang Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210 , Yunxiang Lin National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029 , Yihua Zhu Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Li Song National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029 and Chunzhong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 Shanghai Engineering Research Center of Hierarchical Nanomaterials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.020.202000497 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Maximizing atomic utilization of precious metal-based catalysts is of great significance in heterogeneous catalysis, also becoming a useful strategy to develop efficient electrocatalysts for hydrogen evolution reaction (HER). Although the dispersion of platinum (Pt) as single atoms (SAs) has increasingly been used in the design of HER electrocatalysts, it is still controversial if the SAs possess higher reactivity relative to the nanoparticles with identical atom loading. Here, by virtue of computational studies, we find that atomic step-rich Pt clusters and defective graphene (DG)-loaded Pt SAs are beneficial to water dissociation and hydrogen coupling, respectively, predicting that decent activity and high atomic utilization for alkaline HER electrocatalysis will be exhibited on the structure that integrates both Pt SAs and nanoclusters onto the DG matrix (PtSA/NC-DG). We experimentally synthesize the PtSA/NC-DG catalyst for alkaline HER. The optimized PtSA/NC-DG delivers an overpotential of 41 mV at a current density of 10 mA cm−2 and mass activity of 5.4 mA μgpt−1 at the overpotential of 100 mV. The mass activity is nearly 6 and 10 times higher than that of its Pt SA counterpart and commercial Pt/C catalyst. This work deepens the knowledge of the synergistic effect of single atoms and nanoclusters for alkaline HER electrocatalysis. Download figure Download PowerPoint Introduction Hydrogen production enabled by electrocatalytic methods represents an attractive approach to utilize renewable electricity from intermittent energy, such as solar, wind energy, geothermal power, and so forth.1–5 Although nonprecious materials based on earth's abundant resources have achieved substantial development,6–8 platinum (Pt) is still the benchmark catalyst for the electrocatalytic hydrogen evolution reaction (HER) in terms of low overpotentials and high mass activities.9–11 The electrocatalytic hydrogen production in alkaline electrolytes offers more benign conditions compared with that of acid media.12 More importantly, alkaline water splitting into hydrogen allows the application of active catalysts based on earth-abundant transition metals for anodic oxygen evolution reaction.1,6 However, the catalytic activity of Pt in alkaline media is nearly two to three orders of magnitude slower than that in acid media due to the high-energy barrier of water dissociation.13 In terms of cost efficiency, it is highly desirable to reduce the Pt amount to the greatest extent, while maintaining or even improving catalytic activities.14–17 In recent years, decreasing the size of catalytically active species in atomic dispersion, also referred to as single atoms (SAs), has attracted considerable attention thanks to their high atomic utilization and unique physicochemical properties.18–21 However, the catalytic nature remains highly controversial, especially since it is not clear how the SA sites achieve the splitting and recombination of chemical bonds.22,23 For instance, the hydrogen evolution process in alkaline media commonly consists of water dissociation and hydrogen coupling, a typical Volmer step (H2O + e− → H* + OH−) and Heyrovsky step (H2O + H* + e− → H2 + OH−) or Tafel step (H* + H* → H2).13 The structure with dual active components, typically a metal/oxide interfacial structure,24 has generally been adopted to optimize the water dissociation and hydrogen coupling simultaneously, boosting alkaline water electrolysis to produce hydrogen. To date, various attempts have successfully been made to obtain efficient low-Pt catalysts for acid HER electrocatalysis, but it has been determined that neither the Pt SAs nor nanoparticles deliver satisfactory performance in alkaline media.25–27 Developing efficient Pt-based catalysts with ultimate atomic utilization for alkaline HER electrocatalysis remains a severe challenge.28–31 Here, we demonstrate both theoretically and experimentally that the synergistic function from the integration of defective graphene (DG)-supported Pt SAs and nanoclusters (named as PtSA/NC-DG) boosted electrocatalytic hydrogen evolution in alkaline media. In alkaline HER, given typical elementary steps of the water dissociation and hydrogen coupling, we employed density functional theory (DFT) calculations to compute the two steps onto Pt(111), Pt(211), Pt38 cluster, and DG-supported Pt SA models. It was revealed that the Pt nanoclusters and SAs helped water dissociation and hydrogen coupling, respectively. Therefore, the coexistence of Pt SAs and nanoclusters on the DG matrix would decouple the optimization of the two steps, integrally delivering superior hydrogen evolution. We experimentally synthesized the PtSA/NC-DG and DG-supported Pt SAs (named as PtSA-DG). The electrocatalytic performance of the PtSA/NC-DG outperformed that of PtSA-DG and commercial Pt/C catalysts, verifying the synergistic effect of Pt SAs and nanoclusters for alkaline HER electrocatalysis. Experimental Methods Synthesis of DG About 20.00 g urea was dispersed uniformly in its crucible. Then the powder was calcined in a muffle furnace at 550 °C for 4 h, and the heating rate was controlled at 5 °C min−1. The temperature was reduced to 300 °C at 10 °C min−1, and then naturally cooled to obtain carbon nitride (C3N4). About 5.00 g of the prepared C3N4 and 4.50 g (C6H12O6 > 99 %) glucose were dispersed in mortar, the mixture was uniformly mixed, and then it was placed in a tube furnace at 1000 °C for 2 h under an argon atmosphere. The heating rate was controlled at 5 °C min−1. The temperature was reduced to 300 °C at 10 °C min−1, followed by natural cooling to obtain nitrogen-doped DG. Synthesis of PtSA-DG and PtSA/NC-DG In a typical preparation of Pt SAs loaded on the graphene matrix, 0.05 g of the DG was dispersed in 60.00 mL ultrapure water and sonicated for 30 min. Then 2.00 mL aqueous solution of chloroplatinic acid hexahydrate (2 mg mL−1) was added into the suspension and stirred for 12 h. The mixture was placed in an oil bath at 80 °C. It was collected by centrifuging, washing, and then drying at 75 °C for 6 h. The dark powdered sample, namely PtSA-DG, was obtained. The PtSA-DG was placed into a quartz tube and heated to 400 °C at a rate of 5 °C min−1 under a flow of gas mixture of argon and hydrogen (volume ratio = 9∶1). After 2 h of pyrolysis, the PtSA/NC-DG-400 was collected. By adjusting the annealing temperature (200 and 600 °C), the PtSA/NC-DG-X (X = 200 or 600) was also obtained. Other related experimental methods are available in the Supporting Information. Results and Discussion Structural investigation by DFT calculation The water dissociation and hydrogen coupling results on established models are shown in Figure 1 and Supporting Information Table S1. It was found that the energy barrier of water dissociation (EH2O) on a typical Pt(111) surface was estimated as 0.98 eV, and the hydrogen-binding energy (ΔGH*) was −0.20 eV, in keeping with previously published theoretical data.32,33 The results indicated the low ,activity of large size Pt crystals for alkaline HER electrocatalysis. When reducing the Pt size for the usage decrease, in addition to the thermodynamically stable Pt(111) surface, a high-index facet is increasingly exposed. Therefore, a typical high-index Pt(211) surface covered by atomic steps was here introduced.10 As shown in Figures 1a and 1b, the EH2O on the Pt(211) surface was 0.65 eV. On the contrary, the ΔGH* (−0.40 eV) on the Pt(211) surface was extremely negative (Figures 1c and 1d). The results suggested that the atomic steps can accelerate water dissociation to supply H*, but hamper the H* combination into H2. It can reasonably be anticipated that atom-assembled nanoclusters with rich atomic steps can further facilitate the water dissociation, but severely hinder the hydrogen coupling. The computational EH2O and ΔGH* on typical Pt38 clusters were 0.59 and −0.60 eV, respectively, verifying that only the presence of nanoclusters can not efficiently accomplish the water splitting into H2 (Figures 1b and 1d). In view of the ultimate in atomic utilization efficiency, Pt SAs anchored in suitable support naturally come to mind. Here, a nitrogen-doped defective carbon matrix for the anchoring of Pt SAs was considered, thanks to high structural feasibility and electronic conductivity as well as rich surface chemistry.34 Here, typical four-coordination structure is investigated in consideration of the high structure stability.10,32 It was determined that all simulated Pt-CxNy structures (x + y = 4) displayed large EH2O (Figure 1b), revealing the inferior water dissociation process. And moderate |ΔGH*| was exhibited on most of the Pt-CxNy structures, including Pt-C1N3, Pt-C2N2, and Pt-C4 (Figure 1d), thereby benefitting the hydrogen coupling process on these structures. Combined with the aim of atomic utilization maximization, these theoretical results predicted that decent activity and high atomic utilization of alkaline HER electrocatalysis would be exhibited on the structure that integrates both Pt SAs and nanoclusters onto proper nitrogen-doped defective carbon support. In the integrated structure, Pt clusters can accelerate water dissociation to supply adsorbed H atoms onto Pt clusters. The migration of H atoms from Pt clusters to the graphene support and then to the SAs can be carried out through hydrogen spillover.35–37 The DG as the substrate of Pt clusters receives excess H* from the spillover. Most of Pt-CxNy structures are beneficial to the hydrogen coupling process. Figure 1 | Structural investigation by DFT calculation. (a and c) The adsorption configurations of H2O and H on Pt(111), Pt(211), Pt38, and Pt-CxNy (x + y = 4), respectively. The dark blue, gray, blue, red, and white spheres are Pt, C, N, O, and H atoms, respectively. (b) Reaction energy diagram of water dissociation on Pt(111), Pt(211), Pt38, and Pt-CxNy (x + y = 4). (d) Free-energy barriers for HER on Pt(111), Pt(211), Pt38, and Pt-CxNy (x + y = 4). Download figure Download PowerPoint Synthesis and characterization of DG-supported Pt catalysts The nitrogen-doped DG selected for the loading of Pt species was obtained by the pyrolysis of the physical mixture of glucose and carbon nitride ( Supporting Information Figures S1 and S2). Transmission electron microscopy (TEM) images showed typical graphene nanosheets with a high degree of structural wrinkle38 ( Supporting Information Figure S3). The graphene displayed rich porosity and suitable pyridinic- or pyrrolic-nitrogen defects ( Supporting Information Figures S4 and S5), which were beneficial to the anchoring of metal species.39–41 The Pt SAs were anchored onto the DG matrix by a one-step electroless deposition to obtain the PtSA-DG with 3.89 wt % Pt loading.22,23 Aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) images showed the Pt species of the PtSA-DG were atomically dispersed onto the DG support20–22 (Figure 2a and Supporting Information Figure S6). Subjected to thermal treatment at a different temperature, the PtSA-DG was transformed into the PtSA/NC-DG-X (X = 200, 400, or 600 °C). In the thermal treatment, a part of the Pt SAs owing to its unsaturated coordination structure and low binding energy with CN matrix (Pt-CxNy, x + y < 4) were readily aggregated into Pt clusters.10,42 The Pt species in PtSA/NC-DG-X (X = 200, 400, or 600 °C ) dispersed as Pt nanoclusters with a similar average diameter ( Supporting Information Figure S7). Unless otherwise specified, the PtSA/NC-DG was the one obtained by heat treatment in 400 °C. From the HAADF-STEM images (Figure 2b and Supporting Information Figure S8), it was determined for the PtSA/NC-DG that both atom-assembled nanoclusters and SAs were dispersed onto the DG matrix. The energy-dispersive X-ray spectroscopy (EDS) mapping element further evidenced the coexistence of SAs and nanoclusters onto the nitrogen-doped carbon substrate (Figure 2c). X-ray diffraction (XRD) patterns (Figure 2d and Supporting Information S9) also revealed the presence and absence of crystalline Pt species in the PtSA/NC-DG and PtSA-DG, respectively. Figure 2 | Morphology and structure characterizations of PtSA-DG and PtSA/NC-DG. (a) High magnification HAADF-STEM image of PtSA-DG. (b) Gradually magnified HAADF-STEM images of PtSA/NC-DG. (c) Elemental mapping of PtSA/NC-DG. (d) XRD patterns of PtSA-DG and PtSA/NC-DG. Download figure Download PowerPoint X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS) measurements were carried out to identify the electronic and geometric structure of Pt species in the PtSA-DG and PtSA/NC-DG-X catalysts. The high-resolution Cl 2p XPS spectra showed the complete removal of chlorine ( Supporting Information Figure S10), suggesting the absence of Pt-Cl coordination in the obtained catalysts. The normalized X-ray absorption near-edge structure (XANES) spectra showed that white-line intensity of all the PtSA-DG and PtSA/NC-DG-X catalysts was located among PtO2 and Pt foil, demonstrating the existence of positively charged Pt species in these catalysts (Figure 3a).42 Furthermore, the decreased white-line intensity of PtSA/NC-DG-X catalysts indicated the enriching of Pt0 species with the increase of annealing temperature, which was also evidenced by high-resolution Pt 4f XPS spectra23 ( Supporting Information Figure S11). The coordination environments of Pt species were disclosed by k3-weighted extended XAFS (EXAFS). From Figure 3b and Supporting Information Figure S12, it was clearly found that only the dominant peak of Pt-C/N/O coordination was observed for the PtSA-DG catalyst, confirming the atomic dispersion of Pt species.22 For the PtSA/NC-DG-X catalysts, in addition to the peak from Pt-C/N/O contribution, those peaks close to the Pt–Pt coordination were also observed, evidencing the coexistence of Pt SAs and nanoclusters,10 in line with the above HAADF-STEM results. To more directly discriminate the coordination environment of PtSA-DG and PtSA/NC-DG catalysts, an EXAFS wavelet transform (WT) was performed (Figure 3c).43 For the PtSA-DG catalyst, only one intensity maximum region close to that of PtO2 was displayed, confirming the mononuclear centers of Pt species. Two maximum-intensity regions aligned with that of PtO2 and Pt foil were observed for the PtSA/NC-DG catalyst, strongly evidencing the presence of Pt-C/N/O and Pt–Pt coordination. Combining these spectroscopic results with the above microscopic observation, it was demonstrated that the PtSA-DG and PtSA/NC-DG were successfully constructed. Figure 3 | Spectroscopic identifications of PtSA-DG and PtSA/NC-DG-X. (a) Normalized XANES spectra and local enlargement at the Pt L3 edge. (b) k3-weighted R space Fourier-transformed spectra from the XANES. (c) WT for the EXAFS spectra. Download figure Download PowerPoint HER performance of different carbon-loaded Pt catalysts The high-performance catalytic activities of the PtSA/NC-DG for alkaline HER were verified in a typical three-electrode cell filled by 1 M KOH. First, from the polarization curves of PtSA/NC-DG-X catalysts ( Supporting Information Figure S13), the PtSA/NC-DG obtained at 400 °C was reactivity-optimized, probably due to the suitable ratio of Pt SAs and nanoclusters.32 The optimized PtSA/NC-DG achieved an overpotential (η) of 41 mV at a current density of 10 mA cm−2, significantly surpassing that of the PtSA-DG and commercial 20 wt % Pt/C catalysts (Figure 4a and Supporting Information Figure S14). The comparison of mass activities for precious metal-based catalysts is of great importance.9,44 Thus, the mass activities of PtSA-DG, PtSA/NC-DG, and commercial Pt/C were calculated based on the Pt loading at the electrode. The PtSA/NC-DG delivered significantly higher mass activity than the PtSA-DG and commercial Pt/C (Figure 4b and Supporting Information Figure S15). In particular, the mass activity of PtSA/NC-DG was as high as 5.40 mA μgpt−1 at 100 mV, which was nearly 6 and 10 times higher than that of PtSA-DG (0.83 mA μgpt−1) and commercial Pt/C (0.52 mA μgpt−1). In addition, the calculated turnover frequency (TOF) also demonstrated the remarkable activity of the PtSA/NC-DG ( Supporting Information Figure S16). From Supporting Information Figure S17, the Tafel slope of PtSA/NC-DG (40 mV dec−1) was close to that of commercial Pt/C, and was lower than that of PtSA-DG, following the Volmer–Tafel mechanism.28,45 Moreover, the exchange current density of PtSA/NC-DG was 1.25 mA cm−2 (Figure 4c), which is nearly seven and two times larger than that of PtSA-DG and commercial Pt/C, indicating rapid reaction kinetics of the PtSA/NC-DG. To further compare the intrinsic activity, the electrochemically active surface area (ECSA) was evaluated ( Supporting Information Figure S18). The ECSA-normalized mass activity of the PtSA/NC-DG at 100 mV was still five times higher than that of the PtSA-DG ( Supporting Information Figure S19), demonstrating superior intrinsic activity. To rule out the possible influence of particle size, exposed surface and the metal–support interaction, we prepared the Pt nanoclusters supported on high-surface-area carbon black support (PtNC-BP2000-400) with similar Pt loading according to previous studies.10 The PtNC-BP2000-400 without Pt SAs displayed a similar size of Pt clusters with PtSA/NC-DG ( Supporting Information Figure S20), which was also confirmed by high-resolution Pt 4f XPS spectra ( Supporting Information Figure S21). The metal–support interaction result from particle size could also be excluded. Then, we compared their electrocatalytic activity and stability. The PtSA/NC-DG catalyst displayed a lower overpotential and superior stability than PtNC-BP2000-400 ( Supporting Information Figure S22). The negligible overpotential increase after accelerated degradation test evidenced the long-term stability of PtSA-DG and PtSA/NC-DG, probably thanks to the confinement effect of the defective carbon matrix (Figure 4d and Supporting Information Figures S23 and S24).22,46 HAADF-STEM images of PtSA/NC-DG after 10,000 cycles further confirmed that PtSA/NC-DG is only slight larger than that of slight larger than that of the pristine PtSA/NC-DG ( Supporting Information Figure S25). N 1s and Pt 4f high-resolution XPS spectrum of PtSA/NC-DG further demonstrated the outstanding structural stability of PtSA/NC-DG catalyst ( Supporting Information Figures S26 and S27). The PtSA/NC-DG compared with that of PtSA-DG, PtNC-BP2000-400, commercial Pt/C as well as other Pt-based catalysts ( Supporting Information Table S2) displayed higher activity, verified the above theoretical synergistic effect of Pt SAs and nanoclusters boosting water splitting into hydrogen. Figure 4 | Electrochemical characterization. All data are IR-corrected. (a) LSV curves of PtSA-DG, PtSA/NC-DG, and commercial Pt/C in 1 M KOH. (b) Mass activities of PtSA-DG, PtSA/NC-DG, and commercial Pt/C at an overpotential of 50 and 100 mV. (c) Exchange current density of PtSA-DG, PtSA/NC-DG, and commercial Pt/C. (d) LSV curves of PtSA/NC-DG recorded initially and after 10,000 potential cycles. IR, infrared; LSV, linear sweep voltammetry. Download figure Download PowerPoint Conclusion Combining the DFT simulation and experimental verification, we have reported a highly efficient PtSA/NC-DG electrocatalyst for alkaline HER by the integration of Pt nanoclusters and SAs onto DG matrix. In the integrated structure, the Pt nanoclusters promote water dissociation into *H and *OH, and the Pt SAs facilitate the H–H coupling into gaseous hydrogen. Consequently, despite low Pt loading in the PtSA/NC-DG catalyst, the electrocatalytic performance toward alkaline HER is substantially better than that of the Pt SA counterpart and commercial Pt/C catalyst with 20 wt % Pt. The notion presented in this work will provide some guidance for the design of noble metal-based heterogeneous catalysts approaching the ultimate in atomic utilization efficiency. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (nos. 21838003, 91834301, and 21978278), the Shanghai Scientific and Technological Innovation Project (nos. 18JC1410500 and 19JC1410400), and the Fundamental Research Funds for the Central Universities (no. 222201718002). Acknowledgments The authors thank Shanghai Synchrotron Radiation Facility (BL14W1, SSRF).

Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Among various porous materials, hydrogen storage in covalent-organic frameworks (COFs) has attracted the most significant attention since they were first synthesized due to good stability, large surface area, porosity and extremely low density. Although COFs exhibit promising hydrogen storage properties at very low temperatures, their hydrogen storage capacity is not satisfactory at room temperature, which is too low to meet the uptake target set by US-DOE, thereby being unable to have practical applications. Remarkably, hydrogen spillover has been experimentally demonstrated as an effective approach to improving the hydrogen storage capacity on porous materials at ambient temperature. In some of the most promising results the metal-organic frameworks (MOFs) and COFs have been used as substrates. However, the structures of many COFs materials are quite complex and the experimental condition is difficult to control. Furthermore, the sample preparations for these hydrogen spillover experiments are also very difficult. Therefore, only COF-1 is used in experimental study of hydrogen spillover. Although some theoretical work has contributed to understanding the hydrogen spillover mechanism of COFs, many basic problems about hydrogen spillover have not been solved, which hinders its practical application to a large extent. Based on the above reasons, the hydrogen spillover mechanism of Pd doped COF-108 is studied by using density functional theory (DFT) method, which mainly includes the various deposited configurations of Pd4 cluster on COF-108, the adsorption and dissociation of H2 on Pd4 cluster of Pd4@COF-108, the migration of H atom from Pd4 cluster toward the COF-108 and the diffusion of H atom on COF-108 surface. The results show as follows. 1) The larger the number of Pd atoms interacting with HHTP or TBPM cluster, the greater the binding energy of Pd4 deposited on them is. Deposited configuration orientation has little effect on binding energy. The binding energies of all deposition configurations for TBPM cluster are larger than those for HHTP cluster, so Pd4 cluster prefers to deposit on TBPM cluster with face-contact configuration. (2) H2 molecules spontaneously dissociated into Pd4 cluster, i.e., a barrierless H2 dissociation process takes place, which meets the first condition required by hydrogen spillover. 3) Only H atom located at the bridge site may migrate to the substrate surface, and the migration process is an endothermic reaction and less stable, which indicates that H atoms will further diffuse on the substrate surface. Although H atoms located at the top site may not migrate directly to the substrate surface, it will automatically migrate to the bridge site after the H atom on the bridge site has migrated to the substrate surface, so the migration process may proceed continuously. (4) The introduction of transition metal Pd can greatly reduce the diffusion energy barrier of H atoms on substrate surface, which makes it easier for H atoms to diffuse on substrate. These results may help us understand the microscopic mechanism of hydrogen spillover influencing the properties of hydrogen storage on COFs and provide useful guidance for targeted preparing the COFs materials with excellent hydrogen storage properties experimentally.

A DFT study of CO adsorption on the pristine, defective, In-doped and Sb-doped graphene and the effect of applied electric field

Hydrogen spillover in supported metal electrocatalysts has garnered significant research attention for its potential to enhance the hydrogen evolution reaction (HER) efficiency. However, challenges remain in facilitating hydrogen spillover and reducing the associated energy barriers. Herein, PtPd alloy clusters are anchored to the CeO 2 surface, enabling short‐path hydrogen spillover and lowering the reaction energy barrier in acidic environments. During HER, hydrogen is initially adsorbed on the noble metal surface and subsequently migrates to the interface, rather than precipitating directly on the CeO 2 surface. This interface exhibits a near‐zero Gibbs free energy of hydrogen adsorption (0.023 eV). Consequently, the catalyst demonstrates an exceptionally low overpotential of only 5.7 mV at 10 mA cm −2 in acidic media, along with remarkable long‐term stability. These findings provide valuable insights into designing highly efficient HER electrocatalysts for acidic environments based on hydrogen spillover mechanisms.

Hydrogen spillover in metal-supported catalysts can largely enhance electrocatalytic hydrogenation performance and reduce energy consumption. However, its fundamental mechanism, especially at the metal-metal interface, remains further explored, impeding relevant catalyst design. Here, we theoretically profile that a large free energy difference in hydrogen adsorption on two different metals (|ΔGH-metal(i)-ΔGH-metal(ii)|) induces a high kinetic barrier to hydrogen spillover between the metals. Minimizing the difference in their d-band centers (Δϵd) should reduce |ΔGH-metal(i)-ΔGH-metal(ii)|, lowering the kinetic barrier to hydrogen spillover for improved electrocatalytic hydrogenation. We demonstrated this concept using copper-supported ruthenium-platinum alloys with the smallest Δϵd, which delivered record high electrocatalytic nitrate hydrogenation performance, with ammonia production rate of 3.45±0.12 mmol h-1 cm-2 and Faraday efficiency of 99.8±0.2 %, at low energy consumption of 21.4 kWh kgamm -1. Using these catalysts, we further achieve continuous ammonia and formic acid production with a record high-profit space.