H2 Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H2 Activation at the Metal–Support Interface

Upside-Down Adsorption: The Counterintuitive Influences of Surface Entropy and Surface Hydroxyl Density on Hydrogen Spillover

Although hydrogen spillover is often invoked to explain anomalies in catalysis, spillover remains a poorly understood phenomenon. Hydrogen spillover (H*) is best described as highly mobile H atom equivalents that arise when H2 migrates from a metal nanoparticle to an oxide or carbon support. In the 60 years since its discovery, few methods have become available to quantify or characterize H*-support interactions. We recently showed in situ infrared spectroscopy and volumetric chemisorption can quantify reversible H2 adsorption on Au/TiO2 catalysts, where adsorbed hydrogen exists as H* and interacts with titania surface hydroxyl (TiOH) groups. Here, we report parallel thermogravimetric analysis and Fourier transform infrared spectroscopy methods for systematically manipulating the surface TiOH density. We examine the role of surface hydroxylation on spillover thermodynamics using van't Hoff studies to determine apparent adsorption enthalpies and entropies at constant H* coverage, which is necessary to maintain constant H* translational entropy. Although surface TiOH groups are the likely adsorption sites, the data show removing hydroxyl groups increases spillover. This surprising finding─that adsorption increases as the adsorption site density decreases─is associated with improved thermodynamics on dehydroxylated surfaces. A strong adsorption enthalpy-entropy correlation implicates the changing surface entropy of the titania support itself (i.e., an initial state effect) is deeply intertwined with the H* configurational entropy. These effects are surprising and should apply to all low-coverage adsorbates where entropy terms dominate more traditional enthalpic considerations. Moreover, this study points toward a kinetic test for invoking spillover in a reaction mechanism: namely, in situ dehydroxylation should enhance spillover processes.

Real-Time Simulation of the Reaction Kinetics of Supported Metal Nanoparticles

A common issue with supported metal catalysts is the sintering of metal nanoparticles, resulting in catalyst deactivation. In this study, we propose a theoretical framework for realizing a real-time simulation of the reactivity of supported metal nanoparticles during the sintering process, combining density functional theory calculations, microkinetic modeling, Wulff-Kaichew construction, and sintering kinetic simulations. To validate our approach, we demonstrate its feasibility on α-Al2O3(0001)-supported Ag nanoparticles, where the simulated sintering behavior and ethylene epoxidation reaction rate as a function of time show qualitative agreement with experimental observation. Our proposed theoretical approach can be employed to screen out the promising microstructure feature of α-Al2O3 for stable supported Ag NPs, including the surface orientation and promoter species modified on it. The outlined approach of this work may be applied to a range of different thermocatalytic reactions other than ethylene epoxidation and provide guidance for the development of supported metal catalysts with long-term stability.

Towards bridging thermo/electrocatalytic CO oxidation: from nanoparticles to single atoms

Proton exchange membrane fuel cells (PEMFCs), as a feasible alternative to replace the traditional fossil fuel-based energy converter, contribute significantly to the global sustainability agenda. At the PEMFC anode, given the high exchange current density, Pt/C is deemed the catalyst-of-choice to ensure that the hydrogen oxidation reaction (HOR) occurs at a sufficiently fast pace. The high performance of Pt/C, however, can only be achieved under the premise that high purity hydrogen is used. For instance, in the presence of trace level carbon monoxide, a typical contaminant during H2 production, Pt is severely deactivated by CO surface blockage. Addressing the poisoning issue necessitates for either developing anti-poisoning electrocatalysts or using pre-purified H2 obtained via a thermo-catalysis route. In other words, the CO poisoning issue can be addressed by either thermal-catalysis from the H2 supply side or electrocatalysis at the user side, respectively. In spite of the distinction between thermo-catalysis and electro-catalysis, there are high similarities between the two routes. Essentially, a reduction in the kinetic barrier for the combination of CO to oxygen containing intermediates is required in both techniques. Therefore, bridging electrocatalysis and thermocatalysis might offer new insight into the development of cutting edge catalysts to solve the poisoning issue, which, however, stands as an underexplored frontier in catalysis science. This review provides a critical appraisal of the recent advancements in preferential CO oxidation (CO-PROX) thermocatalysts and anti-poisoning HOR electrocatalysts, aiming to bridge the gap in cognition between the two routes. First, we discuss the differences in thermal/electrocatalysis, CO oxidation mechanisms, and anti-CO poisoning strategies. Second, we comprehensively summarize the progress of supported and unsupported CO-tolerant catalysts based on the timeline of development (nanoparticles to clusters to single atoms), focusing on metal-support interactions and interface reactivity. Third, we elucidate the stability issue and theoretical understanding of CO-tolerant electrocatalysts, which are critical factors for the rational design of high-performance catalysts. Finally, we underscore the imminent challenges in bridging thermal/electrocatalytic CO oxidation, with theory, materials, and the mechanism as the three main weapons to gain a more in-depth understanding. We anticipate that this review will contribute to the cognition of both thermocatalysis and electrocatalysis.

Exploring Ligand Effects on Structure, Bonding, and Photolytic Hydride Transfer of Cationic Gold(I) Bridging Hydride Complexes of Molybdocene and Tungstenocene

A diverse family of heterobimetallic bridging hydride adducts of the type [LAu(μ-H)2MCp2][X] (L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, IPr; 1,3-bis(1-adamantyl)imidazole-2-ylidene, IAd; 1,3-bis(2,6-di-iso-propylphenyl)-5,5-dimethyl-4,6-diketopyrimidinyl-2-ylidene, DippDAC; triphenylphosphine, PPh3; 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl, tBuXPhos; X = SbF6-, BF4- or TfO-) was synthesized by reacting group VI metallocene dihydrides Cp2MH2 (Cp = cyclopentadienyl anion; M = Mo, W) with cationic gold(I) complexes [LAu(NCMe)][X]. Trimetallic [L'Au2(μ-H)2WCp2][X]2 and tetrametallic [L'Au2{(μ-H)2WCp2}2] [X]2 complexes (L' = rac-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene or bis(diphenylphosphinomethane)) were obtained by reacting digold [L'{Au(NCMe)}2][X]2 with Cp2WH2 in a 1:1 and a 1:2 stoichiometry. Accessing such a broad structural diversity allowed us to pinpoint roles played by the ancillary ligands and group VI metals on the bonding properties of this family of bridging hydrides. In particular, a clear effect of the ligand on the interaction energy and electronic structure was observed, with important implications on photolytic reactivity. UV or visible light irradiation, indeed, leads to the selective cleavage of the heterobimetallic Au(μ-H)2M arrangement and formation of molecular gold hydrides. The photolysis was found to be chromoselective (wavelength-dependent), which can be ascribed to different charge redistributions upon excitation to the first (Kasha's reactivity) and higher (anti-Kasha's reactivity) excited states.

Structural regulation of amorphous molybdenum sulfide by atomic palladium doping for hydrogen evolution

Molybdenum sulfide materials have long been considered as attractive non-precious-metal electrocatalysts for the hydrogen evolution reaction (HER). However, comparing with the crystalline counterpart, amorphous MoSx has been less investigated previously. We here propose to increase the catalytical activity of a-MoSx by raising the reactant concentration at the catalytic interface via a chemical doping approach. The reconstruction of coordination structure of a-MoSx via Pd doping induces the formation of abundant unsaturated S atoms. Moreover, the reactant friendly catalytic interface is constructed through introducing hydrophilic groups to a-MoSx. The doped a-MoSx catalyst exhibits significantly enhanced HER activity in both acid and alkaline media.

Theoretical search for characteristic atoms in supported gold nanoparticles: a large-scale DFT study

The size and site dependences of atomic and electronic structures in isolated and supported gold nanoparticles have been investigated using large-scale density functional theory (DFT) calculations using multi-site support functions. The effects of the substrate on nanoparticles with diameters of 2 nm and several different shapes have been examined. First, isolated gold nanoparticles with diameters of 0.6 nm (13 atoms) to 4.5 nm (2057 atoms), which have comparable sizes to nanoparticles used in experiments, were considered. To analyse huge amounts of data obtained from large-scale DFT calculations, we performed principal component analysis (PCA), which helps systematically and efficiently clarify the electronic structures of large nanoparticles. The PCA results reveal the site dependence of the electronic structures. Notably, the atoms in the surface and subsurface have different electronic structures to those located in the inner layers, especially at the vertexes of the particles. The convergence of local electronic structures with respect to the particle size has also been demonstrated. For supported nanoparticles, PCA helps indicate which atoms are affected, and how much, by the substrate. The correlation between the PCA results and site dependence of reaction activity is also discussed herein.

Insights into Electrochemical CO2 Reduction on Metallic and Oxidized Tin Using Grand-Canonical DFT and In Situ ATR-SEIRA Spectroscopy

Electrochemical CO2 reduction (CO2R) to formate is an attractive carbon emissions mitigation strategy due to the existing market and attractive price for formic acid. Tin is an effective electrocatalyst for CO2R to formate, but the underlying reaction mechanism and whether the active phase of tin is metallic or oxidized during reduction is openly debated. In this report, we used grand-canonical density functional theory and attenuated total reflection surface-enhanced infrared absorption spectroscopy to identify differences in the vibrational signatures of surface species during CO2R on fully metallic and oxidized tin surfaces. Our results show that CO2R is feasible on both metallic and oxidized tin. We propose that the key difference between each surface termination is that CO2R catalyzed by metallic tin surfaces is limited by the electrochemical activation of CO2, whereas CO2R catalyzed by oxidized tin surfaces is limited by the slow reductive desorption of formate. While the exact degree of oxidation of tin surfaces during CO2R is unlikely to be either fully metallic or fully oxidized, this study highlights the limiting behavior of these two surfaces and lays out the key features of each that our results predict will promote rapid CO2R catalysis. Additionally, we highlight the power of integrating high-fidelity quantum mechanical modeling and spectroscopic measurements to elucidate intricate electrocatalytic reaction pathways.

Specific Hydrogen Spillover Pathways Generated on Graphene Oxide Enabling the Formation of Non-Equilibrium Alloy Nanoparticles

The phenomenon of hydrogen spillover is investigated as a means of realizing a hydrogen-based society for over half a century. Herein, a graphene oxide having a precisely tuned architecture via calcination in air to introduce ether groups onto basal planes along with carbon defects is reported. This material provides specific pathways for the spillover of atomic hydrogen and has practical applications with regard to the synthesis of non-equilibrium solid-solution alloy nanoparticles. A combination of experimental work and simulations confirmed that the presence of ether groups associated with carbon defects facilitated hydrogen spillover within the basal planes of this graphene oxide. This enhanced hydrogen spillover ability, in turn, enables the simultaneous reduction of Ru3+ and Ni2+ ions to form RuNi alloy nanoparticles under hydrogen reduction conditions. Energy dispersive X-ray and X-ray absorption near edge structure simulations establish that this strategy forms unique alloy nanoparticles each comprising a Ru core with a RuNi solid-solution shell having a hexagonal close-packed structure. These non-equilibrium RuNi alloy nanoparticles exhibit greater catalytic activity than monometallic Ru nanoparticles during the hydrolysis of ammonia borane.

Strategies to improve hydrogen activation on gold catalysts

Catalytic reactions involving molecular hydrogen are at the heart of many transformations in the chemical industry. Classically, hydrogenations are carried out on Pd, Pt, Ru or Ni catalysts. However, the use of supported Au catalysts has garnered attention in recent years owing to their exceptional selectivity in hydrogenation reactions. This is despite the limited understanding of the physicochemical aspects of hydrogen activation and reaction on Au surfaces. A rational design of new improved catalysts relies on making better use of the hydrogenating properties of Au. This Review analyses the strategies utilized to improve hydrogen-Au interactions, from addressing the importance of the Au particle size to exploring alternative mechanisms for H2 dissociation on Au cations and Au-ligand interfaces. These insights hold the potential to drive future applications of Au catalysis.

Distinct Site Motifs Activate O2 and H2 on Supported Au Nanoparticles in Liquid Water

Au nanoparticles catalyze the activation and conversion of small molecules with rates and kinetic barriers that depend on the dimensions of the nanoparticle, composition of the support, and presence of catalytically culpable water molecules that solvate these interfaces. Here, molecular interpretations of steady-state rate measurements, kinetic isotope effects, and structural characterizations reveal how the interface of Au nanoparticles, liquid water, and metal oxide supports mediate the kinetically relevant activation of H2 and sequential reduction of O2-derived intermediates during the formation of H2O2 and H2O. Rates of H2 consumption are 10-100 fold greater on Au nanoparticles supported on metal oxides (e.g., titania) compared to more inert and hydrophobic materials (carbon, boron nitride). Similarly, Au nanoparticles on reducible and Lewis acidic supports (e.g., lanthana) bind dioxygen intermediates more strongly and present lower barriers (<22 kJ mol-1) for O-O bond dissociation than inert interfaces formed with silica (>70 kJ mol-1). Selectivities for H2O2 formation increase significantly as the diameters of the Au nanoparticles increase because differences in nanoparticle size change the relative fractions of exposed sites that exist at Au-support interfaces. In contrast, site-normalized rates and barriers for H2 activation depend weakly on the size of Au nanoparticles and the associated differences in active site motifs. These findings suggest that H2O aids the activation of H2 at sites present across all surface Au atoms when nanoparticles are solvated by water. However, molecular O2 preferentially binds and dissociates at Au-support interfaces, leading to greater structure sensitivity for barriers of O-O dissociation across different support identities and sizes of Au nanoparticles. These insights differ from prior knowledge from studies of gas-phase reactions of H2 and O2 upon Au nanoparticle catalysts within dilute vapor pressures of water (10-4 to 0.1 kPa H2O), in which catalysis occurs at the perimeter of the Au-support interface. In contrast, contacting Au catalysts with liquid water (55.5 M H2O) expands catalysis to all surface Au atoms and enables appreciable H2O2 formation.

Dynamic Evolution of Palladium Single Atoms on Anatase Titania Support Determines the Reverse Water-Gas Shift Activity

Research interest in single-atom catalysts (SACs) has been continuously increasing. However, the lack of understanding of the dynamic behaviors of SACs during applications hinders catalyst development and mechanistic understanding. Herein, we report on the evolution of active sites over Pd/TiO₂-anatase SAC (Pd₁/TiO₂) in the reverse water-gas shift (rWGS) reaction. Combining kinetics, in situ characterization, and theory, we show that at T ≥ 350 °C, the reduction of TiO₂ by H₂ alters the coordination environment of Pd, creating Pd sites with partially cleaved Pd-O interfacial bonds and a unique electronic structure that exhibit high intrinsic rWGS activity through the carboxyl pathway. The activation by H₂ is accompanied by the partial sintering of single Pd atoms (Pd₁) into disordered, flat, ∼1 nm diameter clusters (Pd_n). The highly active Pd sites in the new coordination environment under H₂ are eliminated by oxidation, which, when performed at a high temperature, also redisperses Pd_n and facilitates the reduction of TiO₂. In contrast, Pd₁ sinters into crystalline, ∼5 nm particles (Pd_NP) during CO treatment, deactivating Pd₁/TiO₂. During the rWGS reaction, the two Pd evolution pathways coexist. The activation by H₂ dominates, leading to the increasing rate with time-on-stream, and steady-state Pd active sites similar to the ones formed under H₂. This work demonstrates how the coordination environment and nuclearity of metal sites on a SAC evolve during catalysis and pretreatments and how their activity is modulated by these behaviors. These insights on SAC dynamics and the structure-function relationship are valuable to mechanistic understanding and catalyst design.

Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Enhancing Tungsten Oxide Gasochromism with Noble Metal Nanoparticles: The Importance of the Interface

Crystalline tungsten trioxide (WO₃ ) thin films covered by noble metal (gold and platinum) nanoparticles are synthesized via wet chemistry and used as optical sensors for gaseous hydrogen. Sensing performances are strongly influenced by the catalyst used, with platinum (Pt) resulting as best. Surprisingly, it is found that gold (Au) can provide remarkable sensing activity that tuned out to be strongly dependent on the nanoparticle size: devices sensitized with smaller nanoparticles display better H₂ sensing performance. Computational insight based on density functional theory calculations suggested that this can be related to processes occurring specifically at the Au nanoparticle-WO₃ interface (whose extent is in fact dependent on the nanoparticle size), where the hydrogen dissociative adsorption turns out to be possible. While both experiments and calculations single out Pt as better than Au for sensing, the present work reveals how an exquisitely nanoscopic effect can yield unexpected sensing performance for Au on WO₃ , and how these performances can be tuned by controlling the nanoscale features of the system.

Electronic modulation of metal-support interactions improves polypropylene hydrogenolysis over ruthenium catalysts

Ruthenium (Ru) is the one of the most promising catalysts for polyolefin hydrogenolysis. Its performance varies widely with the support, but the reasons remain unknown. Here, we introduce a simple synthetic strategy (using ammonia as a modulator) to tune metal-support interactions and apply it to Ru deposited on titania (TiO₂). We demonstrate that combining deuterium nuclear magnetic resonance spectroscopy with temperature variation and density functional theory can reveal the complex nature, binding strength, and H amount. H₂ activation occurs heterolytically, leading to a hydride on Ru, an H⁺ on the nearest oxygen, and a partially positively charged Ru. This leads to partial reduction of TiO₂ and high coverages of H for spillover, showcasing a threefold increase in hydrogenolysis rates. This result points to the key role of the surface hydrogen coverage in improving hydrogenolysis catalyst performance.

Catalytic pathways of plastic waste valorization to lubricants are attractive avenues to foster circular economy. Tuning of catalyst electronic properties allows to significantly improve its activity due to boosted hydrogen storage on the surface.

Bifunctional hydroformylation on heterogeneous Rh-WOx pair site catalysts

Metal-catalysed reactions are often hypothesized to proceed on bifunctional active sites, whereby colocalized reactive species facilitate distinct elementary steps in a catalytic cycle^1-8. Bifunctional active sites have been established on homogeneous binuclear organometallic catalysts^9-11. Empirical evidence exists for bifunctional active sites on supported metal catalysts, for example, at metal-oxide support interfaces^2,6,7,12. However, elucidating bifunctional reaction mechanisms on supported metal catalysts is challenging due to the distribution of potential active-site structures, their dynamic reconstruction and required non-mean-field kinetic descriptions^7,12,13. We overcome these limitations by synthesizing supported, atomically dispersed rhodium-tungsten oxide (Rh-WO_x) pair site catalysts. The relative simplicity of the pair site structure and sufficient description by mean-field modelling enable correlation of the experimental kinetics with first principles-based microkinetic simulations. The Rh-WO_x pair sites catalyse ethylene hydroformylation through a bifunctional mechanism involving Rh-assisted WO_x reduction, transfer of ethylene from WO_x to Rh and H₂ dissociation at the Rh-WO_x interface. The pair sites exhibited >95% selectivity at a product formation rate of 0.1 g_propanal cm^-3 h^-1 in gas-phase ethylene hydroformylation. Our results demonstrate that oxide-supported pair sites can enable bifunctional reaction mechanisms with high activity and selectivity for reactions that are performed in industry using homogeneous catalysts.

Distinct Role of Surface Hydroxyls in Single-Atom Pt1/CeO2 Catalyst for Room-Temperature Formaldehyde Oxidation: Acid-Base Versus Redox

The development of highly efficient catalysts for room-temperature formaldehyde (HCHO) oxidation is of great interest for indoor air purification. In this work, it was found that the single-atom Pt₁/CeO₂ catalyst exhibits a remarkable activity with complete removal of HCHO even at 288 K. Combining density functional theory calculations and in situ DRIFTS experiments, it was revealed that the active O_latticeH site generated on CeO₂ in the vicinity of Pt²⁺ via steam treatment plays a key role in the oxidation of HCHO to formate and its further oxidation to CO₂. Such involvement of hydroxyls is fundamentally different from that of cofeeding water which dissociates on metal oxide and catalyzes the acid-base-related chemistry. This study provides an important implication for the design and synthesis of supported Pt catalysts with atom efficiency for a very important practical application-room-temperature HCHO oxidation.

Elucidating the Roles of Amorphous Alumina Overcoat in Palladium-Catalyzed Selective Hydrogenation

Amorphous alumina overcoats generated by atomic layer deposition (ALD) have been shown to improve the selectivity and durability of supported metal catalysts in many reactions. Several mechanisms have been proposed to explain the enhanced catalytic performance, but the accessibilities of reactants through the amorphous overcoats remain elusive, which is crucial for understanding reaction mechanisms. Here, we show that an AlOx ALD overcoat is able to improve the alkene product selectivity of a supported Pd catalyst in acetylene (C2H2) hydrogenation. We further demonstrate that the AlOx ALD overcoat blocks the access of C2H2 (kinetic diameter of 0.33 nm), O2 (0.35 nm), and CO (0.38 nm) but allows H2 (0.29 nm) to access Pd surfaces. A H-D exchange experiment suggests that H2 might dissociate heterolytically at the Pd-AlOx interface. These findings are in favor of a hydrogen spillover mechanism.

Observation of unsaturated platinum carbenes Pt2C2n - (n = 1-3) clusters: A photoelectron imaging spectroscopic and theoretical study

The structural and bonding properties of the Pt₂C_2n ^- (n = 1-3) complexes have been investigated by mass-selected photoelectron velocity-map imaging spectroscopy with quantum chemical calculations. The adiabatic detachment energies and vertical detachment energies of Pt₂C_2n ^- have been obtained from the measured photoelectron imaging spectra. Theoretical results indicate that the lowest-energy isomers of Pt₂C_2n ^- (n = 1-3) possess linear chain-shaped configurations. The binding motif in the most stable isomer of Pt₂C₂ ^- has a linear cumulenic structure with a Pt=C=C=Pt configuration, and the structural characteristic persists up to all the lowest-energy isomers of the Pt₂C₄ ^- and Pt₂C₆ ^- anions. The chemical bonding analyses indicate that the Pt₂C_2n ^- (n = 1-3) complexes have multicenter two-electron characteristics.

Finding Key Factors for Efficient Water and Methanol Activation at Metals, Oxides, MXenes, and Metal/Oxide Interfaces

Activating water and methanol is crucial in numerous catalytic, electrocatalytic, and photocatalytic reactions. Despite extensive research, the optimal active sites for water/methanol activation are yet to be unequivocally elucidated. Here, we combine transition-state searches and electronic charge analyses on various structurally different materials to identify two features of favorable O–H bond cleavage in H₂O, CH₃OH, and hydroxyl: (1) low barriers appear when the charge of H moieties remains approximately constant during the dissociation process, as observed on metal oxides, MXenes, and metal/oxide interfaces. Such favorable kinetics is closely related to adsorbate/substrate hydrogen bonding and is enhanced by nearly linear O–H–O angles and short O–H distances. (2) Fast dissociation is observed when the rotation of O–H bonds is facile, which is favored by weak adsorbate binding and effective orbital overlap. Interestingly, we find that the two features are energetically proportional. Finally, we find conspicuous differences between H₂O/CH₃OH and OH activation, which hints toward the use of carefully engineered interfaces.

Revealing hydrogen spillover pathways in reducible metal oxides

Hydrogen spillover, the migration of dissociated hydrogen atoms from noble metals to their support materials, is a ubiquitous phenomenon and is widely utilized in heterogeneous catalysis and hydrogen storage materials. However, in-depth understanding of the migration of spilled hydrogen over different types of supports is still lacking. Herein, hydrogen spillover in typical reducible metal oxides, such as TiO₂, CeO₂, and WO₃, was elucidated by combining systematic characterization methods involving various in situ techniques, kinetic analysis, and density functional theory calculations. TiO₂ and CeO₂ were proven to be promising platforms for the synthesis of non-equilibrium RuNi binary solid solution alloy nanoparticles displaying a synergistic promotional effect in the hydrolysis of ammonia borane. Such behaviour was driven by the simultaneous reduction of both metal cations under a H₂ atmosphere over TiO₂ and CeO₂, in which hydrogen spillover favorably occurred over their surfaces rather than within their bulk phases. Conversely, hydrogen atoms were found to preferentially migrate within the bulk prior to the surface over WO₃. Thus, the reductions of both metal cations occurred individually on WO₃, which resulted in the formation of segregated NPs with no activity enhancement.

The hydrogen spillover pathway in typical reducible metal oxides, such as TiO₂, CeO₂, and WO₃, was investigated by combining various in situ characterization techniques, kinetic analysis, and density functional theory calculations.

Zeolite-Encaged Isolated Platinum Ions Enable Heterolytic Dihydrogen Activation and Selective Hydrogenations

Understanding the unique behaviors of atomically dispersed catalysts and the origin thereof is a challenging topic. Herein, we demonstrate a facile strategy to encapsulate Pt^δ+ species within Y zeolite and reveal the nature of selective hydrogenation over a Pt@Y model catalyst. The unique configuration of Pt@Y, namely atomically dispersed Pt^δ+ stabilized by the surrounding oxygen atoms of six-membered rings shared by sodalite cages and supercages, enables the exclusive heterolytic activation of dihydrogen over Pt^δ+···O^2- units, resembling the well-known classical Lewis pairs. The charged hydrogen species, i.e., H⁺ and H^δ-, are active reagents for selective hydrogenations, and therefore, the Pt@Y catalyst exhibits remarkable performance in the selective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols and of nitroarenes to arylamines.

Influence of solvent structure and hydrogen bonding on catalysis at solid-liquid interfaces

Solvent molecules interact with reactive species and alter the rates and selectivities of catalytic reactions by orders of magnitude. Specifically, solvent molecules can modify the free energies of liquid phase and surface species via solvation, participating directly as a reactant or co-catalyst, or competitively binding to active sites. These effects carry consequences for reactions relevant for the conversion of renewable or recyclable feedstocks, the development of distributed chemical manufacturing, and the utilization of renewable energy to drive chemical reactions. First, we describe the quantitative impact of these effects on steady-state catalytic turnover rates through a rate expression derived for a generic catalytic reaction (A → B), which illustrates the functional dependence of rates on each category of solvent interaction. Second, we connect these concepts to recent investigations of the effects of solvents on catalysis to show how interactions between solvent and reactant molecules at solid-liquid interfaces influence catalytic reactions. This discussion demonstrates that the design of effective liquid phase catalytic processes benefits from a clear understanding of these intermolecular interactions and their implications for rates and selectivities.

Gold Nanorods: The Most Versatile Plasmonic Nanoparticles

Gold nanorods (NRs), pseudo-one-dimensional rod-shaped nanoparticles (NPs), have become one of the burgeoning materials in the recent years due to their anisotropic shape and adjustable plasmonic properties. With the continuous improvement in synthetic methods, a variety of materials have been attached around Au NRs to achieve unexpected or improved plasmonic properties and explore state-of-the-art technologies. In this review, we comprehensively summarize the latest progress on Au NRs, the most versatile anisotropic plasmonic NPs. We present a representative overview of the advances in the synthetic strategies and outline an extensive catalogue of Au-NR-based heterostructures with tailored architectures and special functionalities. The bottom-up assembly of Au NRs into preprogrammed metastructures is then discussed, as well as the design principles. We also provide a systematic elucidation of the different plasmonic properties associated with the Au-NR-based structures, followed by a discussion of the promising applications of Au NRs in various fields. We finally discuss the future research directions and challenges of Au NRs.

Au3+ Species Boost the Catalytic Performance of Au/ZnO for the Semi-hydrogenation of Acetylene

Au nanoparticles have garnered remarkable attention in the chemoselective hydrogenation due to their extraordinary selectivity. However, the activity is far from satisfactory. Knowledge of the structure-performance relationship is a key prerequisite for rational designing of highly efficient Au-based hydrogenation catalysts. Herein, diverse Au sites were created through engineering their interactions with supports, specifically via adjusting the support morphology, that is, flower-like ZnO (ZnO-F) and disc-like ZnO (ZnO-D), and the catalyst pretreatment atmosphere, that is, 10 vol % O₂/Ar and 10 vol % H₂/Ar (denoted as -O and -H, respectively). The four samples of Au/ZnO were characterized by various techniques and evaluated in the semi-hydrogenation of acetylene. The transmission electron microscopy results indicated that the Au particle sizes are almost similar for our Au/ZnO catalysts. The charge states of Au species demonstrated by X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule, and simulation based on density functional theory, however, are greatly dependent on the ZnO shape and pretreatment atmosphere, that is, the percentage of Au³⁺ reduces following the order of Au/ZnO-F-O > Au/ZnO-F-H > Au/ZnO-D-O > Au/ZnO-D-H. The testing results showed that the Au/ZnO-F-O catalyst containing maximum of Au³⁺ possesses the optimal activity with 1.8 × 10^-2 s^-1 of specific activity at 200 °C, around 16.5-fold of that for Au/ZnO-D-H. More interestingly, the specific rate at 200 °C and the average conversion/selectivity in the entire operating temperature range are well correlated with the redox states of the Au species, indicating that Au³⁺ sites are more active for acetylene hydrogenation. A plausible explanation is that the Au³⁺ species not only facilitate acetylene adsorption via electrostatic interactions but also favor the heterolysis of H₂ via constructing frustrated Lewis pairs with O.

High Catalytic Performance of Au/Bi2O3 for Preferential Oxidation of CO in H2

Preferential oxidation (PROX) of CO in hydrogen is of great significance for proton exchange membrane fuel cells (PEMFCs) that need a CO-free hydrogen stream as fuel. The key technical problem is developing catalysts that can efficiently remove CO from the H2-rich stream within the working temperature range of PEMFCs. Herein, we design a Au/Bi2O3 interfacial catalyst for PROX with excellent catalytic performance, which can achieve 100% CO conversion in the PROX reaction over a wide temperature window (70-200 °C) and is perfectly compatible with the operating temperature window (80-180 °C) of PEMFCs. Moreover, the catalyst also demonstrates excellent high flow performance and long-term stability. Density functional theory (DFT) calculations reveal that the electrons transferring from Bi2O3 to Au and then to adsorbed perimeter CO and O2 molecules promote the activation of CO and O2, thus enhancing the catalytic performance of PROX.

Glucose-oxidase like catalytic mechanism of noble metal nanozymes

Au nanoparticles (NPs) have been found to be excellent glucose oxidase mimics, while the catalytic processes have rarely been studied. Here, we reveal that the process of glucose oxidation catalyzed by Au NPs is as the same as that of natural glucose oxidase, namely, a two-step reaction including the dehydrogenation of glucose and the subsequent reduction of O₂ to H₂O₂ by two electrons. Pt, Pd, Ru, Rh, and Ir NPs can also catalyze the dehydrogenation of glucose, except that O₂ is preferably reduced to H₂O. By the electron transfer feature of noble metal NPs, we overcame the limitation that H₂O₂ must be produced in the traditional two-step glucose assay and realize the rapid colorimetric detections of glucose. Inspired by the electron transport pathway in the catalytic process of natural enzymes, noble metal NPs have also been found to mimic various enzymatic electron transfer reactions including cytochrome c, coenzymes as well as nitrobenzene reductions.

Effect of Pd Coordination and Isolation on the Catalytic Reduction of O2 to H2O2 over PdAu Bimetallic Nanoparticles

The direct synthesis of hydrogen peroxide (H₂ + O₂ → H₂O₂) may enable low-cost H₂O₂ production and reduce environmental impacts of chemical oxidations. Here, we synthesize a series of Pd₁Au_x nanoparticles (where 0 ≤ x ≤ 220, ∼10 nm) and show that, in pure water solvent, H₂O₂ selectivity increases with the Au to Pd ratio and approaches 100% for Pd₁Au₂₂₀. Analysis of in situ XAS and ex situ FTIR of adsorbed ¹²CO and ¹³CO show that materials with Au to Pd ratios of ∼40 and greater expose only monomeric Pd species during catalysis and that the average distance between Pd monomers increases with further dilution. Ab initio quantum chemical simulations and experimental rate measurements indicate that both H₂O₂ and H₂O form by reduction of a common OOH* intermediate by proton-electron transfer steps mediated by water molecules over Pd and Pd₁Au_x nanoparticles. Measured apparent activation enthalpies and calculated activation barriers for H₂O₂ and H₂O formation both increase as Pd is diluted by Au, even beyond the complete loss of Pd-Pd coordination. These effects impact H₂O formation more significantly, indicating preferential destabilization of transition states that cleave O-O bonds reflected by increasing H₂O₂ selectivities (19% on Pd; 95% on PdAu₂₂₀) but with only a 3-fold reduction in H₂O₂ formation rates. The data imply that the transition states for H₂O₂ and H₂O formation pathways differ in their coordination to the metal surface, and such differences in site requirements require that we consider second coordination shells during the design of bimetallic catalysts.

Kinetics of H2 Adsorption at the Metal-Support Interface of Au/TiO2 Catalysts Probed by Broad Background IR Absorbance

H₂ adsorption on Au catalysts is weak and reversible, making it difficult to quantitatively study. We demonstrate H₂ adsorption on Au/TiO₂ catalysts results in electron transfer to the support, inducing shifts in the FTIR background. This broad background absorbance (BBA) signal is used to quantify H₂ adsorption; adsorption equilibrium constants are comparable to volumetric adsorption measurements. H₂ adsorption kinetics measured with the BBA show a lower E_app value (23 kJ mol^-1 ) for H₂ adsorption than previously reported from proxy H/D exchange (33 kJ mol^-1 ). We also identify a previously unreported H-O-H bending vibration associated with proton adsorption on electronically distinct Ti-OH metal-support interface sites, providing new insight into the nature and dynamics of H₂ adsorption at the Au/TiO₂ interface.

Design of Local Atomic Environments in Single-Atom Electrocatalysts for Renewable Energy Conversions

Single-atom electrocatalysts (SAECs) have recently attracted tremendous research interest due to their often remarkable catalytic responses, unmatched by conventional catalysts. The electrocatalytic performance of SAECs is closely related to the specific metal species and their local atomic environments, including their coordination number, the determined structure of the coordination sites, and the chemical identity of nearest and second nearest neighboring atoms. The wide range of distinct chemical bonding configurations of a single-metal atom with its surrounding host atoms creates virtually limitless opportunities for the rational design and synthesis of SAECs with tunable local atomic environment for high-performance electrocatalysis. In this review, the authors first identify fundamental hurdles in electrochemical conversions and highlight the relevance of SAECs. They then critically examine the role of the local atomic structures, encompassing the first and second coordination spheres of the isolated metal atoms, on the design of high-performance SAECs. The relevance of single-atom dopants for host activation is also discussed. Insights into the correlation between local structures of SAECs and their catalytic response are analyzed and discussed. Finally, the authors summarize major challenges to be addressed in the field of SAECs and provide some perspectives in the rational construction of superior SAECs for a wide range of electrochemical conversions.

Clean protocol for deoxygenation of epoxides to alkenes via catalytic hydrogenation using gold

Au NP catalyst combined with triethylphosphite, P(OEt)₃, is remarkably more reactive than solely Au NPs for the selective deoxygenation of epoxides to alkenes.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.