Surface electronic states mediate concerted electron and proton transfer at metal nanoscale interfaces for catalytic hydride reduction of -NO2 to -NH2

Influence of Nitrogen-Doped Carbon Dots on H• Radical-Mediated Au-H Formation in the Hydrogenation of 4-Nitrophenol Using NCDs-Au Nanohybrids

The hydrogenation of 4-nitrophenol using carbon dot-stabilized gold (Au) nanoparticles is well-studied, with Au-H species known to catalyze the reaction. However, the impact of specific nitrogen moieties in nitrogen-doped carbon dots on Au-H formation and catalytic activity remains unexplored. These nitrogen species, acting as surface ligands, may influence the catalytic properties through the generation of Au-H species via H• radicals. In this regard, modulation of the catalytic properties of Au nanoparticles has been explored by conjugating their surface with nitrogen-doped carbon dots (NCDs). Three distinct nanohybrid formulations comprising NCDs and Au nanoparticles (i.e., NCDs-Au) have been prepared, where the NCDs were derived from different carbon sources (e.g., citric acid and l-malic acid) and varying mole ratios of the nitrogen source (i.e., urea). The impact of NCDs on Au nanoparticle-mediated catalysis has been investigated using the model reaction of hydrogenation of 4-nitrophenol (4-NP) in the presence of NaBH4. The fractions of different nitrogen species (such as pyrrolic, pyridinic, and amidic) in the different NCDs-Au nanohybrids were quantified through XPS analysis, and their roles in catalytic performance have been studied. Further, the size, shape, crystallinity, defects, and exposed facets of the NCDs-Au nanohybrids have also been assessed (through XRD, HRTEM, and Raman studies), and their structure-activity relationships have been corroborated. The hydrogenation of 4-NP is proposed to happen through the formation of gold-hydride (Au-H) species facilitated by H• radicals, as confirmed by EPR analysis. The NCDs-Au nanohybrid, synthesized from NCDs derived from a 1:3 molar ratio of l-malic acid and urea (MU13-Au), exhibits superior catalytic efficiency with a rate constant of 1.013 min-1, attributed to its abundant defects and a notably high relative content of catalytically favorable pyridinic nitrogen species compared to other tested nanohybrids.

The nature of crystal facet effect of TiO2-supported Pd/Pt catalysts on selective hydrogenation of cinnamaldehyde: electron transfer process promoted by interfacial oxygen species

Supported noble metal nanocatalysts typically exhibit strong crystal plane dependent catalytic behavior, but their working mechanism is still unclear. Herein, using anatase TiO2 with well-exposed crystal facets of {101}, {100} and {001} as a prototype support, Pd- and Pt-based supported TiO2 nanocatalysts (TiO2-Pd and TiO2-Pt) were prepared by chemical reduction with NaBH4 as reducer, and they showed a distinct metal-dependent crystal facet effect in the selective hydrogenation of cinamaldehyde (CAL). For Pd-based nanocatalysts, most Pd species on the {100} plane of TiO2 are present in the oxidized form with positive charges and unexpectedly show higher reactivity than the Pd species in the zero-valence state on the {101} and {001} planes. On the contrary, Pt species on all three crystal planes of TiO2 show zero-valence state, with relatively low conversion, but much better selectivity for hydrogenation of a CO bond than Pd-based catalysts. Well-designed experiments manipulating the stability and type of surface oxygen species confirmed that the essence of the crystal facet effect of the catalyst support actually creates a unique nanoconfined interface at the molecular level to construct a surface p-band intermediate state (PBIS), which provides a new alternative channel for surface electron transfer and consequently accelerates the reaction kinetics.

P-Band Intermediate States Mediate Electron Transfer at Confined Nanoscale

In this Perspective, mainly based on the model of structural water molecules (SWs) as bright color emitters, we briefly summarize the development and theoretical elaboration of P-band intermediate state (PBIS) theory as well as its application in several typical catalytic redox reactions. In addition, with a simple equation (2∫ψ2σ1' + ∫ψ2σ2 + ∫ψ2π = 1), we clearly define how the interface states correlate with the three basic parameters of heterogeneous catalysis (conversion, selectivity, and stability), and what is the dynamic nature of catalytic active sites. Overall, the proposal of SW-dominated PBIS theory establishes an internal physical connection between the decay kinetics of excited electrons and the catalytic reaction kinetics and provides new insights into the physical origin of photoluminescence emission of low-dimensional quantum nanodots and the physical nature of nanoconfinement and nanoconfined catalysis.

Green carbon science: fundamental aspects

Carbon energy has contributed to the creation of human civilization, and it can be considered that the configuration of the carbon energy system is one of the important laws that govern the operation of everything in the universe. The core of the carbon energy system is the opposition and unity of two aspects: oxidation and reduction. The operation of oxidation and reduction is based on the ternary elemental system composed of the three elements of carbon, hydrogen and oxygen. Its operation produces numerous reactions and reaction products. Ancient Chinese philosophy helps us to understand in depth the essence of green carbon science, to explore its scientific basis, and to identify the related platforms for technology development.

Structural water molecules dominated p band intermediate states as a unified model for the origin on the photoluminescence emission of noble metal nanoclusters: from monolayer protected clusters to cage confined nanoclusters

In the past several decades, noble metal nanoclusters (NMNCs) have been developed as an emerging class of luminescent materials due to their superior photo-stability and biocompatibility, but their luminous quantum yield is relatively low and the physical origin of the bright photoluminescence (PL) of NMNCs remain elusive, which limited their practical application. As the well-defined structure and composition of NMNCs have been determined, in this mini-review, the effect of each component (metal core, ligand shell and interfacial water) on their PL properties and corresponded working mechanism were comprehensively introduced, and a model that structural water molecules dominated p band intermediate state was proposed to give a unified understanding on the PL mechanism of NMNCs and a further perspective to the future developments of NMNCs by revisiting the development of our studies on the PL mechanism of NMNCs in the past decade.

Activation of H2O Tailored by Interfacial Electronic States at a Nanoscale Interface for Enhanced Electrocatalytic Hydrogen Evolution

Despite the fundamental and practical significance of the hydrogen evolution reaction (HER), the reaction kinetics at the molecular level are not well-understood, especially in basic media. Here, with ZIF-67-derived Co-based carbon frameworks (Co/NCs) as model catalysts, we systematically investigated the effects of different reaction parameters on the HER kinetics and discovered that the HER activity was directly dependent not on the type of nitrogen in the carbon framework but on the relative content of surface hydroxyl and water (OH-/H2O) adsorbed on Co active sites embedded in carbon frameworks. When the ratio of the OH-/H2O was close to 1:1, the Co/NC nanocatalyst showed the best reaction performance under the condition of high-pH electrolytes, e.g., an overpotential of only 232 mV at a current density of 10 mA cm-2 in the 1 M KOH electrolyte. We unambiguously identified that the structural water molecules (SWs) in the form of hydrous hydroxyl complexes absorbed on metal centers {OHad·H2O@M+} were catalytic active sites for the enhanced HER, where M+ could be transition or alkaline metal cations. Different from the traditional hydrogen bonding of water, the hydroxyl (hydroxide) groups and water molecules in the SWs were mainly bonded together via the spatial interaction between the p orbitals of O atoms, exhibiting features of a delocalized π-bond with a metastable state. These newly formed surface bonds or transitory states could be new weak interactions that synergistically promote both interfacial electron transfer and the activation of water (dissociation of O-H bonds) at the electrode surface, i.e., the formation of activated H adducts (H*). The capture of new surface states not only explains pH-, cation-, and transition-metal-dependent hydrogen evolution kinetics but also provides completely new insights into the understanding of other electrocatalytic reductions involving other small molecules, including CO2, CO, and N2.

Dynamic Pt-OH-·H2O-Ag species mediate coupled electron and proton transfer for catalytic hydride reduction of 4-nitrophenol at the confined nanoscale interface

Generally, the catalytic transformation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) at heterogeneous metal surfaces follows a Langmuir-Hinshelwood (L-H) mechanism when sodium borohydride (NaBH₄) is used as the sacrificial reductant. Herein, with Pt-Ag bimetallic nanoparticles confined in dendritic mesoporous silica nanospheres (DMSNs) as a model catalyst, we demonstrated that the conversion of 4-NP did not pass through the direct hydrogen transfer route with the hydride equivalents being supplied by borohydride via the bimolecular L-H mechanism, since Fourier transform infrared (FTIR) spectroscopy with the use of isotopically labeled reactants (NaBD₄ and D₂O) showed that the final product of 4-AP was composed of protons (or deuterons) that originated from the solvent water (or heavy water). Combined characterization by X-ray photoelectron spectroscopy (XPS), ¹H nuclear magnetic resonance (NMR) and the optical excitation and photoluminescence spectrum evidenced that the surface hydrous hydroxide complex bound to the metal surface (also called structural water molecules, SWs), due to the space overlap of p orbitals of two O atoms in SWs, could form an ensemble of dynamic interface transient states, which provided the alternative electron and proton transfer channels for selective transformation of 4-NP. The cationic Pt species in the Ag-Pt bimetallic catalyst mainly acts as a dynamic adsorption center to temporally anchor SWs and related reactants, and not as the active site for hydrogen activation.

Structural Water Molecules Confined in Soft and Hard Nanocavities as Bright Color Emitters

Molecules confined in the nanocavity and nanointerface exhibit rich, unique physicochemical properties, e.g., the chromophore in the β-barrel can of green fluorescent protein (GFP) exhibits tunable bright colors. However, the physical origin of their photoluminescence (PL) emission remains elusive. To mimic the microenvironment of the GFP protein scaffold at the molecule level, two groups of nanocavities were created by molecule self-assembly using organic chromophores and by organic functionalization of mesoporous silica, respectively. We provide strong evidence that structural water molecules confined in these nanocavities are color emitters with a universal formula of {X⁺·(OH^-·H₂O)·(H₂O) _n-1}, in which X is hydrated protons (H₃O⁺) or protonated amino (NH₃ ⁺) groups as an anchoring point, and that the efficiency of PL is strongly dependent on the stability of the main emitter centers of the structural hydrated hydroxide complex (OH^-·H₂O), which is a key intermediate to mediate electron transfer dominated by proton transfer at confined nanospace. Further controlled experiments and combined characterizations by time-resolved steady-state and ultrafast transient optical spectroscopy unveil an unusual multichannel radiative and/or nonradiative mechanism dominated by quantum transient states with a distinctive character of topological excitation. The finding of this work underscores the pivotal role of structurally bound H₂O in regulating the PL efficiency of aggregation-induced emission luminogens and GFP.

Physical Origin of Dual-Emission of Au-Ag Bimetallic Nanoclusters

On the origin of photoluminescence of noble metal NCs, there are always hot debates: metal-centered quantum-size confinement effect VS ligand-centered surface state mechanism. Herein, we provided solid evidence that structural water molecules (SWs) confined in the nanocavity formed by surface-protective-ligand packing on the metal NCs are the real luminescent emitters of Au-Ag bimetal NCs. The Ag cation mediated Au-Ag bimetal NCs exhibit the unique pH-dependent dual-emission characteristic with larger Stokes shift up to 200 nm, which could be used as potential ratiometric nanosensors for pH detection. Our results provide a completely new insight on the understanding of the origin of photoluminescence of metal NCs, which elucidates the abnormal PL emission phenomena, including solvent effect, pH-dependent behavior, surface ligand effect, multiple emitter centers, and large-Stoke's shift.

Caged structural water molecules emit tunable brighter colors by topological excitation

Intrinsically, free water molecules are a colourless liquid. If it is colourful, why and how does it emit the bright colours? We provided direct evidence that when water was trapped into the sub-nanospace of zeolites, the structural water molecules (SWs) exhibited strong tunable photoluminescence (PL) emissions from blue to red colours with unprecedented ultra-long lifetimes up to the second scale at liquid nitrogen temperature. Further controlled experiments and combined characterizations by time-resolved steady-state and ultra-fast femtosecond (fs) transient optical spectroscopy showed that the singly adsorbed hydrated hydroxide complex {OH^-·H₂O} as SWs in the confined nanocavity is the true emitter centre, whose PL efficiency strongly depends on the type and stability of the SWs, which is dominated by H-bond interactions, such as the solvent effect, pH value and operating temperature. The emission of SWs exhibits the characteristic of topological excitations (TAs) due to the many-body quantum electron correlations in confined nanocavities, which differs from the local excitation of organic chromophores. Our model not only elucidates the origin of the PL of metal nanoclusters (NCs), but also provides a completely new insight to understand the nature of heterogeneous catalysis and interface bonding (or state) at the molecule level, beyond the metal-centred d band theory.