Catalyst support effects on hydrogen spillover.

In chemical catalysis, spillover is the process in which hydrogen atoms are made from hydrogen molecules at one site and then added to other atoms or molecules at another. A study reveals details of this effect. See Letter p.68 Hydrogen spillover—the surface migration of hydrogen atoms from the metal catalyst particle on which they are generated onto the catalyst support—was discovered in the early 1960s, but remains poorly understood. Waiz Karim and colleagues now use advanced nanofabrication to place multiple pairs of iron oxide and platinum nanoparticles on titanium oxide and aluminium oxide supports, with varying inter-particle distance, and observe the extent of the reduction of the iron oxide particles by hydrogen atoms generated on the platinum. The results reveal that hydrogen spillover is fast and efficient on titanium oxide, and extremely slow and short-ranged on aluminium oxide. The results should aid our understanding of hydrogen storage and catalytic hydrogenation reactions, and the approach to creating and probing model catalyst systems open up new avenues for studying fundamental processes in supported catalysts.

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

Hydrogen spillover is an extraordinary effect in heterogeneous catalysis and hydrogen storage, which refers to the surface migration of metal particle-activated hydrogen atoms over the solid supports. Historical studies on this phenomenon have mostly been limited to reducible metal oxides where the long-distance proton-electron coupled migration mechanism has been established, yet the key question remains on how to surmount short-distance and defect-dependent hydrogen migration on nonreducible supports. By demerging hydrogen migration and hydrogenation reaction, here we demonstrate that the hydrogen spillover in nonreducible metal-organic frameworks (MOFs) can be finely modulated by the ligand functional groups or embedded water molecules, enabling significant long-distance (exceed 50 nm) movement of activated hydrogen. Furthermore, using sandwich nanostructured MOFs@Pt@MOFs as catalysts, we achieve highly selective hydrogenation of N-heteroarenes via controllable hydrogen spillover from Pt to MOFs-shell. We anticipate that this work will enhance the understanding of hydrogen spillover and shed light on de novo design of MOFs supported catalysts for many important reactions involving hydrogen.

Hydrogen spillover involves the migration of hydrogen atoms from metal to another component, yet it remains challenging to achieve on non-reducible supports. This paper describes the observation of hydrogen spillover on non-reducible Al2O3-supported Pd single-atom catalysts, facilitated by phosphorus-hydroxyl (P-OH) bridge sites, for acetylene semi-hydrogenation. Through in situ spectroscopy, kinetic analysis, and computational analyses, we show that Pd single atoms promote the heterolytic activation of H2, while phosphorus doping on Al2O3 lowers energy barrier for hydrogen spillover via the formation of P-OH species. Gas-phase acetylene collides and hydrogenates with as-formed P-OH, following Eley-Rideal type mechanism, making inert Al2O3 active in hydrogenation reaction. This unconventional relay pathway reduced Pd usage by one order of magnitude and improved catalytic stability, with over 95% ethylene selectivity at 99% acetylene conversion in ethylene-rich stream at 100 °C for more than 200 hours. This work elucidates the mechanism of hydrogen spillover on non-reducible Al2O3 and relay hydrogenation on supported single-atom catalysts.

Hydrogen migration from metal particles to the support, known as hydrogen spillover, has provided insights for designing highly efficient catalysts in catalytic processes involving hydrogen. A facile and controllable strategy is highly desired to achieve an effective hydrogen spillover effect on nonreducible oxides for catalytic performance optimization and clarifying the catalytic function of hydrogen spillover. Here, we provide an organic molecular decoration (OMD) strategy obtained by the molecular layer deposition-like pulse method for facilitating hydrogen spillover over the nonreducible silica support. After decorating with the fluoroalkylsilane (FAS) molecular layer, the hydrogen spillover effect over the catalysts (xFAS-Pt/SBA-15) is greatly enhanced due to the presence of carbonaceous species compared with the original Pt/SBA-15. The amount of hydrogen spillover can also be precisely regulated by controlling the FAS pulse number. For cinnamaldehyde hydrogenation, xFAS-Pt/SBA-15 presents superior catalytic performance versus its untreated counterpart and the sample decorated via the traditional method. Also, the catalytic activity varies based on the FAS pulse number, showing a linear correlation with the amount of hydrogen spillover. The altered adsorption behavior of the reactant plays an important role in the hydrogenation selectivity. This facile OMD strategy for efficient hydrogen spillover is general and may have potential applications in many heterogeneous reactions.

Role of hydrogen spillover on the hydrogenation of N-ethylcarbazole over in-situ Encapsulation of Ru within zeolite for hydrogen storage

In some industrial processes, flowing hydrogen molecules over solid surfaces containing catalytic metals reduces organic molecules by hydrogenation. On the basis of a mechanism proposed in the 1960s, chemists think such reactions may proceed through the movement of hydrogen atoms in a process called “hydrogen spillover.” But researchers haven’t been able to confirm this mechanism because models of the reaction are difficult to create and analyze. With a new, realistic model system, researchers have shown definitively that hydrogen spillover occurs on two key types of catalytic surfaces, called reducible and nonreducible supports, but to a drastically different extent. The information could help chemists design better catalysts as well as improve hydrogenation processes and hydrogen storage for fuel cells. The approach could also help scientists determine whether hydrogen spillover occurs more generally in important research and industrial reactions. In hydrogen spillover, catalytic platinum nanoparticles ...

In this Letter, we report, for the first time, using polymer single crystal as magnetically recoverable support for nanoparticle catalysts. This catalyst system is composed of polymer single crystal, platinum nanoparticles, and iron oxide nanoparticles, which act as support, catalysts, and magnetic responsive materials, respectively. Platinum nanoparticles and iron oxide nanoparticles were bonded onto thiol groups and hydroxyl groups on a tailor-designed polymer single-crystal surface. Because of its quasi 2D nature, polymer single crystal possesses high surface area to volume ratio (2.5 × 10(8) m(-1)), which is ∼40 times higher than its nanosphere counterpart of the same volume. This high surface to volume ratio facilitates the high loading of both nanoparticles, which ensures efficient catalytic reaction and reliable nanoparticle recycling. Synergetic interactions between platinum and iron oxide nanoparticles also led to further improvement in catalytic activity.

Introduction Cobalt Fischer‐Tropsch Synthesis catalysts are generally doped with small amounts of noble metals that serve as reduction promoters to enhance the catalytic activity of the cobalt active sites [1] . This is because the promoter metals are able to dissociate hydrogen gas at a low temperature which then also lowers the reduction temperature of the cobalt oxide to cobalt. This then prevents easy deactivation of the cobalt catalysts that can be induced by high temperature activation. Hydrogen spillover has been invoked to explain the observed effect of these metals as promoters on cobalt catalysts. Two types of hydrogen spillover processes can be envisaged; (1) primary hydrogen spillover, whereby the promoter [i.e., initiator] is in contact with the cobalt oxide [i.e, acceptor] and the dissociated hydrogen atoms can move from the initiator through the direct interface to interact with the acceptor and (2) secondary hydrogen spillover, in this process the initiator and the acceptor materials are separated by some distance and hydrogen spillover can only happen by the dissociation of the hydrogen molecule on the initiator followed by a migration of the atomic hydrogen on a carrier (or catalyst support) to the acceptor material [i.e., cobalt oxide] [2]. Few model catalysts exist that can provide direct evidence of the existence of a type of hydrogen spillover that is dominant on Fischer‐Tropsch like catalysts. In this study mesoporous hollow carbon spheres (MHCS) were used as model supports to study whether both the primary and secondary hydrogen spillover were prominent during catalyst activation and Fischer‐Tropsch synthesis. Experimental MHCS were prepared as shown in Fig 1 (a). Three Co catalysts (15% loading) were prepared (1) Ru@MHCS@Co, with Ru nanoparticles and Co nanoparticles separated by the carbon shell, (2) CoRu/MHCS, Ru and Co co‐precipitated outside MHCS and (3) Co/MHCS, Co outside MHCS. Materials were thoroughly characterized using electron microscopy before being tested under Fischer‐Tropsch conditions at 220 ° C and 10 bar. Results and Discussion Scanning electron microscopy (SEM) analysis of the silica template and hollow carbon spheres gave respective average sizes of 340 nm and 290 nm, thus showing that the silica spheres shrunk as they were heated up to 900 ° C before the carbonization process (Fig 1(b,c) and Fig 2 (a,b)). The resulting hollow carbon spheres retained their spherical nature hence showing no significant breakage of the MHCS. Transmission electron microscopy (TEM) analysis of the materials showed that indeed the spheres were hollow and they had Ru nanoparticles with an average size of 4.1 nm embedded on its walls (Fig 1(e) and Fig 2 (c)). The loaded Co nanoparticles had an average particles size of approximately 5.9 nm on all three catalysts (Fig 1(f) and Fig 2 (d)). MHCS show a distinct roughness under TEM imaging suggesting high porosity of the materials which is necessary to allow reactants to access the encapsulated Ru nanoparticles. TEM tilting over a single axis proved that all the Ru nanoparticles are encapsulated inside the MHCS. Loading of Co nanoparticles outside the MHCS allowed for decoupling of the spillover effects from those that require direct Ru and Co direct contact. Electron Probe Micro‐Analysis (EPMA) large area mapping analysis proved that the metal nanoparticles are well dispersed on the MHCS and thus was ideal materials to study the spillover process (Fig 3). The Fischer‐Tropsch catalytic reaction of the three catalysts was compared and gave a Co time yield in terms of carbon monoxide and hydrogen conversion to hydrocarbons as follows; CoRu/MHCS > Ru@MHCS@Co Co/MHCS. Electron microscopy has therefore helped in following the preparation of a functional material where the promoter effects of Ru using MHCS could be evaluated. I was also observed that a close proximity of Ru and Co nanoparticles was vital for an improved catalytic performance when compared to the case where the Ru and Co nanoparticles were separated by a potential hydrogen transporting material.

Hydrogen spillover, as a common phenomenon, pervasively occurs in heterogeneous catalysis. Nevertheless, the understanding of the dynamic mechanism of hydrogen spillover in the typical Pt/CeO2 system remains limited. Herein, the pathways for hydrogen spillover on the surface of two Pt/CeO2(111) models have been systematically investigated using density functional theory (DFT) calculations. Hydrogen coverage and metal coverage are considered factors influencing hydrogen spillover in the Pt/CeO2 system. Descriptors for hydrogen migration at different metal coverages have been proposed to screen effective spillover metals within group Ⅷ: at lower metal coverages, the difference between and is considered as a descriptor, at higher metal coverages, the is used as a descriptor. Based on hydrogen spillover pathways, two dynamic mechanisms of hydrogen spillover, namely M–O–M and M–M, are introduced at different metal coverages. This study offers a deeper understanding of the hydrogen spillover phenomenon, proposes descriptors for hydrogen spillover and provides new insights into the design of heterogeneous catalysts.

Topographic nanostructuring of surfaces is a promising concept, in particular to improve the bio-integration of titanium orthopedic implants. In this context, a new method for the nanostructuring of anodised titanium surfaces was developed. Ordered topographic features in the tens of nanometre in height were created by anodising electropolished titanium in the presence of polymeric particles deposited as monolayers. To do so, an existing electropolishing method for titanium was applied and optimised, allowing the production of extremely smooth starting surfaces. The particle-substrate contact was mesured, modeled and finally modified by plasma, thermal and chemical vapour treatments and its effect on the topography of the anodic oxide layer was investigated. Different types of ordered structures were produced, and characterised by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The influence of the anodisation conditions on the topography and morphology of the surface was studied. To gain deeper understanding of the mechanisms at play, a model experiment using electron beam lithography was designed. Circular masks of increasing diameters were deposited on the surface to simulate the presence of particles of corresponding sizes, and the effect on the topography of the oxide layer was characterised. Some aspects of the structuring phenomena observed with the particles were thus cleared up, in particular the extent of oxide layer growth underneath the masks. Finally, numerical finite element modeling was applied to simulate the initial stages of anodic oxide growth around masks, leading to a better understanding of the formation of some of the topographic features observed. In parallel to this work on surface structuring, mesoporous silica particles containing different functional nanoparticles were produced. They were originally destined to serve as an alternative to the polymeric particles used to create our structurations. Their synthesis process, based on the formation of particles by sol-gel in a miniemulsion, was scaled-up and optimised to obtain narrowly distributed submicron particles with a high micropore volume and a high functional nanoparticle loading. These multifunctional particles were finally found to be unadapted for surface structuring but were assessed for various other applications, depending on the type of nanoparticles incorporated in the silica matrix. In combination with superparamagnetic iron oxide nanoparticles (SPIONS), they were evaluated as potential magnetic drug delivery vehicles. Drug loading and release of the anticancer drug paclitaxel was studied as a model system by simulation and experiment. The release kinetics was found to be very slow, restricted by the large chemical potential difference between the molecule in solution and the molecule adsorbed inside micropores. Chromium doped alumina (ruby) nanoparticles were synthesised and characterised for potential application as near-IR fluorescent markers for biomedical imaging. Their photoluminescent properties were studied and were found to be sufficiently intense for bright-field fluorescence microscopy and confocal laser scanning microscopy. They could however not be incorporated in the multifunctional particles during the gel emulsion synthesis. Manganese doped zinc sulphide quantum dots were also encapsulated in a mesoporous silica matrix and used as the basis material for the production of multicolour fluorescent surfaces. Localised laser thermal treatment of the material was successfully realised, causing a shift in its fluorescence emission wavelength. Multicolour fluorescent images could thus be drawn by programing the laser appropriately.

Hydrogen spillover is an important process in catalytic hydrogenation reactions, facilitating H2 activation and modulating surface chemistry of reducible oxide catalysts. This study focuses on the operando unveiling of platinum-induced hydrogen spillover on monoclinic tungsten trioxide (γ-WO3), employing ambient pressure X-ray photoelectron spectroscopy, density functional theory calculations and microkinetic modeling to investigate the dynamic evolution of surface states at varied temperatures. At room temperature, hydrogen spillover results in the formation of W5+ and hydrogen intermediates (hydroxyl species and adsorbed water), facilitated by Pt metal clusters. With increasing temperature, water desorption, reverse hydrogen spillover and surface-to-bulk diffusion of hydrogen atoms compete with each other, leading initially to reoxidation and then further reduction of W atoms in the near-surface. The combined experimental results and simulations provide a comprehensive understanding of the mechanisms underlying hydrogen interaction with reducible metal oxides, lending insights of relevance to the design of enhanced hydrogenation catalysts.

Hydrogen spillover is a phenomenon in which hydrogen atoms generated on metal catalysts diffuse onto catalyst supports. This phenomenon offers reaction routes for functional materials. However, due to difficulties in visualizing hydrogen, the fundamental nature of the phenomenon, such as how far hydrogen diffuses, has not been well understood. Here, in this study, we fabricated catalytic model systems based on Pd-loaded SrFeOx (x ∼ 2.8) epitaxial films and investigated hydrogen spillover. We show that hydrogen spillover on the SrFeOx support extends over long distances (∼600 μm). Furthermore, the hydrogen-spillover-induced reduction of Fe4+ in the support yields large energies (as large as 200 kJ/mol), leading to the spontaneous hydrogen transfer and driving the surprisingly ultralong hydrogen diffusion. These results show that the valence changes in the supports' surfaces are the primary factor determining the hydrogen spillover distance. Our study leads to a deeper understanding of the long-debated issue of hydrogen spillover and provides insight into designing catalyst systems with enhanced properties.

Photoredox-catalyzed aminoarylation and thioamination of unactivated alkenes have been developed, providing novel synthetic routes to access synthetically challenging quaternary carbon-centered ben...

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022Theory-Driven Design of Electrocatalysts for the Two-Electron Oxygen Reduction Reaction Based on Dispersed Metal Phthalocyanines Yang Wang†, Zisheng Zhang†, Xiao Zhang, Yubo Yuan, Zhan Jiang, Hongzhi Zheng, Yang-Gang Wang, Hua Zhou and Yongye Liang Yang Wang† Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zisheng Zhang† Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Xiao Zhang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027 , Yubo Yuan Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Zhan Jiang Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Hongzhi Zheng Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 , Yang-Gang Wang Department of Chemistry, Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055 , Hua Zhou X-Ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439 and Yongye Liang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055 Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Southern University of Science and Technology, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202000590 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The two-electron electrochemical reduction of oxygen is an appealing approach to produce hydrogen peroxide. Metal and heteroatom-doped carbon (M–X/C) materials have recently been recognized as compelling catalysts for this process, but their performance improvement is generally hindered by the ill-defined structures of active sites. Herein, we demonstrate a theory-driven design of catalysts for oxygen reduction reactions based on molecularly dispersed electrocatalysts (MDEs) with metal phthalocyanines on carbon nanotubes. Density functional theory calculations suggest that nickel phthalocyanine (NiPc) favors the formation of *H2O2 over *O, thus acting as a selective catalyst for peroxide production. NiPc MDE shows high peroxide yields of ∼83%, superior to the aggregated NiPc and pyrolyzed Ni–N/C catalysts. The performance is further enhanced by the introduction of the cyano group (CN). NiPc–CN MDE exhibits ∼92% peroxide yields and good stability. Our studies provide a new perspective for the development of heterogeneous electrocatalysts for hydrogen peroxide production from metal macrocyclic complexes. Download figure Download PowerPoint Introduction Driving economically important chemical reactions with renewable electricity offers an intriguing opportunity to replace current energy-intensive processes.1–3 For example, the electrochemical oxygen reduction reaction (ORR) through the two-electron (2e−) pathway is considered an environmentally benign alternative to the industrial anthraquinone method to produce hydrogen peroxide (H2O2), which is widely used as a green oxidizer in bleaching, waste water treatment, and the chemical industry.4–6 An ideal electrocatalyst should possess high activity toward the 2e− pathway to the peroxide product and suppress the competing 4e− process to water. Platinum (Pt) or palladium (Pd) alloys with mercury (Hg) have been demonstrated as selective electrocatalysts for the 2e− pathway in ORR.7,8 However, due to the toxicity of Hg and the limited reserve of noble metals, these elements are not preferred for practical applications. Carbon-based materials doped with earth-abundant elements, however, are compelling candidates as efficient and affordable electrocatalysts.9–14 Carbon nanotube (CNT), graphene, and activated carbon with oxygen-containing functional groups were reported to be selective in 2e− ORR.15–18 Embedding coupled boron–nitrogen (BN) domains into graphitic carbon showed enhanced selectivity and activity for reducing O2 to HO2− compared with the catalysts with individual B or N doping.19 In addition to metal-free catalysts, metal and heteroatom-doped carbon (M–X/C) catalysts with isolated heteroatom-coordinated metal moieties, one type of single-atom catalysts (SACs), have also been exploited for H2O2 production.20–23 A series of M–N/C (M = Mn, Fe, Co, Ni, and Cu) catalysts with proposed M–N4 active sites were synthesized to investigate their performance in ORR. The Co–N/C catalyst showed preference for the 2e− pathway in acidic condition.24 Moreover, SACs with transition-metal centers coordinated by different heteroatoms, such as O and S, were also reported to show high selectivities for the 2e− reduction pathway in ORR.25–28 However, the lack of well-defined structures and the copresence of various types of active sites prevent understanding the structure–performance relationships and catalyst design principles in these SAC catalysts. Metal macrocyclic complexes, such as metal phthalocyanines (MPcs) and porphyrins with well-defined M–N4 moieties, have been attractive electrocatalysts since the report of cobalt phthalocyanine as an active ORR catalyst.29–31 For instance, iron phthalocyanine (FePc) has been reported to be efficient in catalyzing ORR through the 4e− pathway to water.32,33 However, the performances of metal complexes in heterogeneous form are often limited by their low electric conductivity.34–36 Hybridizing metal macrocyclic complexes with nanocarbon materials were found to promote their catalytic performances.35,37,38 In the carbon dioxide reduction reaction, achieving molecular dispersion on conducting supports is beneficial to reveal the intrinsic performance of molecular catalysts and establish catalyst design principles.39,40 In addition, previous reports of heterogeneous molecular ORR catalysts mainly focused on optimizing the performance toward the 4e− pathway with little exploration of the 2e− pathway to peroxide production.30,41 In this work, we present a theory-driven design of electrocatalysts based on an molecularly dispersed electrocatalyst (MDE) consisting of dispersed MPcs on CNTs for electrochemical production of peroxide. From density functional theory (DFT) calculations, we identify nickel phthalocyanine (NiPc) as a selective catalyst for 2e− ORR with experimental peroxide yields of ∼83% in the form of MDE, in contrast to FePc MDE that is selective for 4e− ORR. Achieving molecular dispersion of NiPc with well-defined Ni–N4 sites is important to the high peroxide selectivity as proven by the lower peroxide yields of the physically mixed NiPc and CNT (containing aggregated NiPc) and a pyrolyzed Ni–N/C SAC. Moreover, molecular engineering of NiPc MDE with the introduction of cyano groups (CNs) to the Pc ligand (NiPc–CN MDE) further enlarges the free-energy preference to the 2e− pathway and enhances the selectivity for the electrochemical production of peroxide. NiPc–CN MDE exhibits a high peroxide yield of ∼92% in the potential range of 0.70–0.20 V versus a reversible hydrogen electrode (RHE). Experimental Methods Preparation of MPc MDEs The preparation of MPc MDEs was based on a reported procedure with the control of the ratio between MPcs and CNTs.40 NiPc and FePc were obtained from commercial sources, and NiPc–CN was synthesized according to a reported method.40 Briefly, 30 mg purified CNTs were dispersed in 25 mL of N,N-dimethylformamide (DMF) with the assistance of sonication, in which a calculated amount of MPcs in 5 mL of DMF was added to obtain a well-mixed suspension. The mixture was further sonicated for 30 min and then stirred at room temperature for 20 h. Subsequently, the precipitate was collected by centrifuge and washed with DMF (three times) and ethanol (twice). Finally, the collected precipitate was lyophilized to yield the final product. Electrochemical measurements About 4 mg of MPc MDEs and 10 μL of 5 wt % Nafion solution were dispersed in 990 μL ethanol under ultrasonication to form a homogeneous ink. About 13 μL catalyst ink was loaded onto the glassy carbon (GC) disk electrode (5.5 mm in diameter) of a rotating ring-disk electrode (RRDE) to achieve a catalyst loading of ∼0.2 mg cm−2. The ink of NiPc + CNT was prepared by dispersing 2.8 mg of NiPc, 1.2 mg of CNT, and 10 μL of 5 wt % Nafion solution in 990 μL ethanol under ultrasonication, then loaded onto the GC electrode. The RRDE experiments were conducted with a four-electrode system using a saturated calomel electrode (SCE) as the reference electrode (calibrated with a homemade RHE), a graphite rod as the counter electrode, and the catalyst-modified GC disk electrode as the working electrode. Meanwhile, the Pt ring electrode was kept at 1.5 V (vs RHE, the same for following potentials unless otherwise stated) for all experiments. The disk and ring electrodes were rotated at a speed of 1600 rpm (Pine research). Electrolytes (0.1 M KOH) were saturated with O2 by bubbling for 30 min prior to each experiment, and a flow of O2 was maintained over the electrolyte during the reaction. Linear sweep voltammetry (LSV) was conducted by scanning the disk electrode potential with a scan rate of 5 mV/s. For the stability test, the disk electrode potential was kept at 0.5 V. Experiments were also performed under an argon environment to record the background currents of the disk and ring electrodes, which were subtracted from the currents under O2. The peroxide yield and electron transfer number (n) were determined by the following equations: Peroxide yield ( HO 2 − ) = 200 × ( I r / N ) I d + ( I r / N ) % n = 4 × I d I d + ( I r / N ) where Ir is ring current, Id is disk current, and N is current collection efficiency of the Pt ring electrode (0.28, calibrated with K3[Fe(CN)6]). Computational Methods DFT calculations of gas-phase MPc molecules catalyzing ORR were conducted using the Gaussian 09 program.42 B3LYP functional43 with D3 correction (Becke–Johnson damping)44 was adopted for calculation.45 The all-electron 6-31G* basis set (for H, C, N, and O)46–48 and the Stuttgart–Dresden (SDD) basis set containing all double-ξ valence with effective core potentials (ECPs)49 (for Ni and Fe) were used. The geometric structures were all optimized at 298.15 K and under 1 atm. The harmonic vibrational frequencies were computed with no imaginary frequency found for all reaction intermediates. The Gibbs free energies of high- and low-spin forms of all intermediates were calculated with the harmonic potential approximation to determine the ground states. The electrocatalytic mechanisms were investigated with the computational hydrogen electrode (CHE) model.50 Additional details of computational methods are available in the Supporting Information. Results and Discussion Theoretical calculations of MPcs catalyzing ORR To understand how the central metals in MPc molecules affect the product selectivity in ORR, DFT calculations of the free-energy changes of ORR through the 2e− and 4e− pathways were conducted on FePc and NiPc at 1.23 V versus RHE. The calculated free-energy diagrams suggest distinctly different ORR behaviors of FePc and NiPc (Figure 1a). On FePc, O2 is first adsorbed on the Fe center, followed by a proton-coupled electron transfer (PCET) process to form *OOH with an uphill free-energy change. The divergence of the 2e− and 4e− pathways came from the preference of *OOH reduction with a protonation mechanism to *H2O2 or an O–O cleavage to *O. The downhill free-energy change to form *O and the large free-energy increase required for the generation of *H2O2 indicate a high preference for the 4e− reduction pathway on FePc, consistent with high selectivities of O2 reduction to water/hydroxide of Fe macrocyclic complexes and Fe-based SACs in previous reports.34,51 By contrast, the *OOH intermediate (generated from O2 through an *O2 intermediate with two uphill free-energy changes) on NiPc shows a slight downhill free-energy change to generate *H2O2, while the formation of *O in the 4e− reduction pathway is energetically uphill (Figure 1a). In contrast to FePc, the reversed trend in free-energy changes to form *H2O2 and *O on NiPc suggests the preference for the 2e− reduction pathway. Therefore, NiPc molecules are predicted to be selective electrocatalysts for 2e− ORR to peroxide product (Figure 1b). Figure 1 | Theoretical calculations of ORR catalyzed by MPcs. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and FePc at 1.23 V. (b) Schematic presentation of ORR selectivity on NiPc and FePc based on DFT calculations. Download figure Download PowerPoint ORR performance of MPc MDEs and aggregated MPcs Dispersed NiPc and FePc molecules were supported on the CNTs via π–π interactions to fabricate MPc MDEs according to our previous method40 to examine the calculated trends in ORR. The metal contents in MDEs were controlled to be ∼0.7 wt % ( Supporting Information Table S1), which were measured by inductively coupled plasma mass spectrometry (ICP-MS). The electrochemical behaviors of NiPc MDE and FePc MDE were first investigated in O2-saturated 0.1 M KOH electrolytes with the RRDE setup (0.2 mg cm−2 catalyst loading). The MDEs were drop-coated on the disk electrode as the working electrode to reduce O2, while the ring electrode (Pt) was maintained at 1.5 V to detect the produced peroxide. FePc MDE shows more positive onset potential (0.94 V at −0.025 mA, corresponding to a current density of ∼−0.1 mA cm−2) than that of NiPc MDE (0.79 V) (Figure 2a). The current of FePc MDE is saturated to −1.41 mA at ∼0.58 V. The saturation current for NiPc MDE is about −0.63 mA at the same potential. The peroxide yield and n of MPc MDEs calculated from the disk and ring currents are depicted in Figure 2b and Supporting Information Figure S1, respectively. NiPc MDE shows good peroxide yields of ∼83% in the potential range of 0.70–0.53 V, which decline at more negative potentials. Correspondingly, n of NiPc MDE is below 2.34 in the potential range of 0.70–0.53 V, which gradually increases to 2.83 from 0.53 to 0.20 V ( Supporting Information Figure S1). In contrast, low peroxide yields of ∼1% together with n above 3.97 in the potential range of 0.70–0.20 V are observed with FePc MDE (Figure 2b and Supporting Information Figure S1), confirming its strong preference toward the 4e− reduction pathway. These results indicate that the preferred ORR pathways of NiPc MDE and FePc MDE are the 2e− and 4e− reduction pathways, respectively, which are consistent with the DFT calculations (Figure 1a). Figure 2 | ORR performance of MPc-based electrocatalysts on RRDE. (a) Disk and ring currents of NiPc, NiPc + CNT, NiPc MDE, and FePc MDE in O2-saturated 0.1 M KOH electrolytes on RRDE test rotating at 1600 rpm. (b) Calculated peroxide yields of NiPc MDE, NiPc + CNT, and FePc MDE. Download figure Download PowerPoint The effects of aggregation state of NiPc were further investigated. The NiPc molecules directly deposited on substrates easily formed aggregates due to their strong intermolecular interactions ( Supporting Information Figure S2). Due to the poor electric conductivity and limited exposure of active sites of aggregated NiPc, the neat NiPc electrode shows minimal activity in ORR (Figure 2a). Therefore, we physically mixed NiPc with CNTs (denoted as NiPc + CNT) to enhance the conductivity. LSV shows that the physically mixed NiPc + CNT possesses higher activity than that of neat NiPc (Figure 2a). Although NiPc + CNT exhibits even more positive onset potential than NiPc MDE, the low peroxide yields of NiPc + CNT (under 60% in the potential range of 0.70–0.20 V) suggests much less preference toward the 2e− pathway compared with NiPc MDE (Figure 2b). Topological defects and N-dopants have been considered as active sites for ORR.11 However, Pc MDE (prepared by anchoring Pc molecules on CNTs) without metal centers shows inferior ORR activity and selectivity for the 2e− pathway compared with NiPc MDE ( Supporting Information Figure S3). These results suggest the critical role of dispersed Ni centers rather than the N-dopants or topological defects in the selective electrocatalysis of 2e− ORR. To investigate the origin of the different ORR behaviors of neat NiPc, NiPc + CNT, and NiPc MDE, their structures were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM ( Supporting Information Figure S4a) and SEM ( Supporting Information Figure S4b) images of NiPc MDE show the bundles of multiwalled CNTs and excluded the formation of nano- or microsized NiPc aggregates. The isolated bright spots in the high-angle annular dark-field (HAADF) image of NiPc MDE obtained with a Cs-corrected scanning TEM (STEM) indicate the existence of single-Ni sites, suggesting the molecular dispersion of NiPc on CNTs ( Supporting Information Figure S5). No appreciable signature peaks of NiPc molecules are observed in the Raman spectrum of NiPc MDE ( Supporting Information Figure S6), which could be due to the low content of NiPc in NiPc MDE. On the contrary, physically mixed NiPc + CNT contains microsized NiPc aggregates, as revealed by SEM and confirmed by energy-dispersive spectrometry (EDS) mapping of the Ni signals ( Supporting Information Figures S4c and S7). Electrochemical impedance spectroscopy (EIS) was further conducted to gain insights into the ORR kinetics of the NiPc catalysts in aggregated and dispersed states. The Nyquist plots in Supporting Information Figure S4d show that the charge transfer of NiPc MDE for ORR is more favorable than that of NiPc and NiPc + CNT. The neat NiPc exhibited the largest charge-transfer resistance, in agreement with its low activity from LSV (Figure 2a). It should be noted that DFT calculations with individual NiPc molecule-catalyzing ORR suggest high selectivity toward the 2e− transfer pathway, which is only observed in dispersed NiPc as in NiPc MDE but not in aggregated NiPc as in NiPc + CNT. These results emphasize the importance of correlating free-energy diagrams calculated with individual catalyst molecule with the electrocatalytic performance of dispersed molecular catalysts as opposed to aggregated molecules. Comparison with the pyrolyzed Ni–N/C SAC SACs have gained extensive attention recently due to their superior electrocatalytic properties. In electrocatalytic applications, SACs are generally fabricated by pyrolyzing metal salts and N-containing organic precursors at high temperatures. However, these pyrolyzed SACs often contain parasitic active sites due to the insufficient structural control during the high-temperature synthesis.52 For comparison, a nickel SAC with Ni–Nx structures (denoted as Ni–N/C) was synthesized by pyrolyzing a Ni-containing zeolitic imidazolate framework (ZIF) precursor according to the reported method with minor modifications.53 The Ni content of the Ni–N/C catalyst was also controlled to be ∼0.7 wt % (measured by ICP-MS) to compare with NiPc MDE. TEM images of Ni–N/C suggest the absence of metallic Ni particles in the catalyst ( Supporting Information Figure S8). The X-ray diffraction (XRD) pattern of Ni–N/C only shows broad features attributable to graphitic carbon ( Supporting Information Figure S9),53 further indicating the absence of metallic Ni or crystalline Ni-containing compounds. The Fourier-transformed extended X-ray adsorption fine structure (FT-EXAFS) curve of Ni–N/C exhibits a peak at ∼1.4 Å (without phase correction) corresponding to Ni–N coordination, but little signal at ∼2.1 Å corresponding to Ni–Ni coordination ( Supporting Information Figure S10), confirming the presence of single Ni sites in Ni–N/C. Although no Ni particles are observed, parasitic active sites such as N-doped carbon sites could still be present in the Ni–N/C catalyst,54,55 which are known to be active for 4e− ORR (Figure 3a). The ORR performance of the Ni–N/C catalyst was further characterized in the RRDE setup with identical conditions as NiPc MDE. Given the LSV curves (Figure 3b), Ni–N/C possesses more positive onset potential (0.83 V) than that of NiPc MDE (0.79 V). The peroxide yields of Ni–N/C are under 43% with n larger than 3.1 in the potential range of 0.70–0.20 V (Figure 3c and Supporting Information Figure S11). The much worse selectivity of the Ni–N/C catalyst toward the 2e− reduction pathway than that of NiPc MDE is attributed to the structural heterogeneity in the pyrolyzed Ni–N/C catalyst. Additionally, the stability tests were carried out at the constant potentials of 0.50 V for the disk electrode to conduct ORR and at 1.50 V for the ring electrode to detect generated peroxide. As shown in Figure 3d, Ni–N/C shows obvious decay of both the disk and ring currents in the first suggesting the of the pyrolyzed By contrast, NiPc MDE exhibits much stability without appreciable decay of the disk and ring currents during the Therefore, the molecularly dispersed and well-defined Ni–N4 sites in NiPc MDE a ORR catalyst for the 2e− reduction pathway than the pyrolyzed Ni–N/C. NiPc MDE be a catalyst system to establish the between the active structure and electrocatalytic Figure | Comparison of electrocatalytic ORR performance between NiPc MDE and Ni–N/C. (a) Schematic presentation of ORR with NiPc MDE and Ni–N/C. (b) Disk and ring currents of NiPc MDE and Ni–N/C in O2-saturated 0.1 M KOH Calculated peroxide yields of NiPc MDE and Ni–N/C. tests of NiPc MDE and Ni–N/C under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint engineering of NiPc MDEs for ORR A of the MDE system is the of the of the active sites and the electrocatalytic performance through molecular we to further the performance of NiPc MDE for 2e− ORR for peroxide production with the introduction of groups to the Pc with Figure for molecular as an ORR catalyst by DFT calculations. The free-energy diagrams suggest to NiPc, NiPc–CN shows strong preference toward the 2e− reduction pathway, and the of *H2O2 over *O is from with NiPc to with NiPc–CN (Figure indicating that NiPc–CN be a more selective catalyst for 2e− ORR. Figure 4 | ORR electrocatalysis with NiPc–CN MDE. (a) Calculated free-energy diagrams of ORR through the 2e− and 4e− reduction pathways on NiPc and NiPc–CN at 1.23 V. shows the molecular structure of (b) Disk and ring currents of NiPc and NiPc–CN MDEs in O2-saturated 0.1 M KOH Calculated peroxide yields and n of NiPc and NiPc–CN test of NiPc–CN MDE under the constant potentials of 0.50 V for the disk electrode and 1.50 V for the ring electrode. Download figure Download PowerPoint NiPc–CN MDE was synthesized ( Supporting Information Figure and characterized with the RRDE The LSV curve shows more positive onset potential of NiPc–CN MDE V) than that of NiPc MDE (0.79 V) (Figure Moreover, the saturation current for NiPc–CN MDE in the potential range of V together with constant ring The peroxide yields of NiPc–CN MDE were calculated to be ∼92% from to 0.20 V, superior to NiPc MDE with peroxide yields below V (Figure The enhanced peroxide yields of NiPc–CN MDE are consistent with the preference toward the 2e− reduction pathway from DFT calculations with the introduction of (Figure NiPc–CN MDE also shows good stability in ORR with little decay in the disk and ring currents and the peroxide yields during the (Figure The performance of NiPc–CN MDE for oxygen reduction to peroxide is the for the reported and metal catalysts, high peroxide selectivity of ∼92% at a potential in conditions ( Supporting Information Table S2). by catalyst design with DFT calculations, we identify NiPc MDE as a good ORR catalyst for the 2e− reduction pathway and further enhance its performance through molecular The enhanced NiPc–CN MDE with shows superior selectivity with peroxide yield of ∼92% in the potential range of 0.70–0.20 V. The molecularly dispersed and well-defined Ni–N4 sites on CNTs NiPc MDEs with higher 2e− selectivities than the aggregated NiPc and pyrolyzed Ni–N/C catalysts. These results also that the MPc MDE system as electrocatalysts for the of the between active structures and electrocatalytic performances of molecular catalysts and Supporting Information Supporting Information is available and Figures and and of is no of to Information was supported by Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and the from Zhejiang University, and Guangdong Provincial Key Laboratory of TEM and were measured with maintained by The were obtained with the of the Advanced Photon a Department of Energy of Science for the of Science by Argonne National Laboratory under The computational is supported from the for Computational Science and the in Energy Materials for It for to Zheng Wang Zhang Wang Pt on for O2 Reduction to H2O2 in Wang of Peroxide from and Jiang Wang Design for Electrochemical Oxygen Reduction toward H2O2 through in the Electrochemical of and by Zheng Zhou with