Direct Detection of Reactive Gallium-Hydride Species on the Ga2O3 Surface via Solid-State NMR Spectroscopy

Ultrafine PtGa Clusters Confined in Porous ZrOx/SiO2 Nanofibers for Enhanced Propane Dehydrogenation

Ultrafine Pt clusters exhibit superior activity for propane dehydrogenation compared to larger Pt nanoparticles; however, they are prone to sintering at high operating temperatures, leading to a decline in both activity and selectivity. In this work, porous ZrOx/SiO2 nanofibers featuring highly dispersed ZrOx nanodomains within a SiO2 matrix were successfully fabricated via a high-throughput blow-spinning process. The abundant and thermal-stable 1.6 nm micropores significantly stabilize 1.5 nm PtGa clusters against sintering at temperatures over 800 °C, due to the pore confinement. Moreover, the electron transfer from Ga to Pt is significantly enhanced in close proximity to ZrOx, contributing to metallic Pt with exceptional activity toward C-H bond activation. Thereby, the sinter-resistant PtGa/ZrOx/SiO2 nanofibers maintained 98.8% propylene selectivity and 43.2% propane conversion rate over 100 h of reaction, with a deactivation rate constant down to 0.0045 h-1. This work explores a sinter-resistant catalytic system based on oxide nanofibers and elaborates a new logic for the design of high-performance propane dehydrogenation catalysts with long-term stability.

2H Quadrupolar Coupling Constant: A Spectroscopic Ruler for Transition Metal-Hydride Bond Distances in Molecular and Surface Sites

Transition metal hydrides (TMHs) find numerous applications across fields from catalysis to H2 storage. Yet, determining the structure of TMHs can remain a challenge, as hydrogen is difficult to detect by X-ray based or classical spectroscopic techniques. Considering that the deuterium isotope (D) is a quadrupolar nucleus (I = 1) and that a quadrupolar coupling constant (CQ) depends on the distance between D and its bonding partner E (dED), we evaluate this trend across molecularly defined transition metal deuterides (TMDs) through a systematic investigation across TM block elements using both computations and experiments. We show that the M-D bond distance (dMD) in [Å] correlates with the CQ values in [kHz] as dMD = 7.83(CQ + 28.7)-1/3─independently from the nature of the TM─with an accuracy >0.04-0.08 Å. Based on experimental CQ values measured by 2H solid-state NMR, this simple correlation is then used to obtain the M-D bond distances in two silica-supported TMDs (M = Zr and Ir), notable heterogeneous catalysts representing early and late TMDs, where evaluating M-D bond distances by other means is very challenging. Considering the ease of measurement, this method is readily applicable to a large range of diamagnetic terminal M-Ds, from molecular to surface sites, making 2H NMR a method of choice to measure TMD bond distances.

Towards Stable Metal-I2 Battery: Design of Iodine-Containing Functional Groups for Enhanced Halogen Bond

The redox chemistries of iodine have attracted tremendous attention for charge storage owing to their high theoretical specific capacity and natural abundance. However, the practical capacity and cycle life are greatly limited by the active mass loss originating from the dissolved iodine species in either non-aqueous or aqueous batteries. Despite intensive progress in physical and physicochemical confinements of iodine species (I2/I3 -/I-), less attention has been paid to confining iodine species beyond the host-iodine interface, inhibiting further development of iodine cathodes with high I2 contents. Here a halogen bond (XB)- enhanced design concept is proposed between I2 molecules to achieve stable cycling performances, as exemplified by the Na-I2 battery. The enhanced XB is derived from the incorporation of -B(OH)I3 groups in highly integrated porous carbon/I2 cathode (HOCF-BIn), which can generate extended interactions between -B(OH)I3 and following I2 molecules. Due to the strong intermolecular force between I2 molecules, the HOCF-BIn cathodes exhibit substantially strengthened I2/I3 -/I- confinement, enabling outstanding cycling stability at I2 loading ranging from 1.8 to 6.2 mg cm-2. This findings demonstrate a functional group to manipulate XB chemistry within I2 molecules and polyiodides for stable and low-cost metal-iodine batteries.

Ultrasonically Activated Liquid Metal Catalysts in Water for Enhanced Hydrogenation Efficiency

Hydride (H-) species on oxides have been extensively studied over the past few decades because of their critical role in various catalytic processes. Their syntheses require high temperatures and the presence of hydrogen, which involves complex equipment, high energy costs, and strict safety protocols. Hydride species tend to decompose in the presence of atmospheric oxygen and water, which reduces their catalytic activities. These challenges highlight the need for further research to improve the stability and efficiency of catalytic processes and develop safer and cost-effective synthesis methods. This paper introduces an ultrasonic fabrication method for gallium hydride species on liquid metal (LM) nanoparticles (Ga-H@LM NPs) in water and describes the evaluation of their catalytic properties. The Ga-H@LM NPs were synthesized by dispersing liquid metals of eutectic gallium-indium in water using a two-step ultrasonication process in an ice bath. The presence of Ga-H species was confirmed by Fourier-transform infrared spectroscopy. The Ga-H@LM NPs demonstrated the rapid catalytic hydrogenation of 4-nitrophenol and reductive degradation of azo dyes within minutes without the need for external reducing agents like NaBH4. The proposed mechanism involves high-energy ultrasonic cavitation at the interface between LM NPs and water, which promotes the formation of H2 from water and its activation to form Ga-H on particles surface during ultrasonication. This study has significant implications for advancing the field of catalysis because it provides a novel and efficient catalytic method for the synthesis of stable hydride species on gallium oxides.

Solid-State Nuclear Magnetic Resonance Spectroscopy for Surface Characterization of Metal Oxide Nanoparticles: State of the Art and Perspectives

Metal oxide materials have found wide applications across diverse fields; in most cases, their functionalities are dictated by their surface structures and properties. A comprehensive understanding of the intricate surface features is critical for their further design, optimization, and applications, necessitating multi-faceted characterizations. Recent advances in solid-state nuclear magnetic resonance (ssNMR) spectroscopy have significantly extended its applications in the detailed analysis of multiple metal oxide nanoparticles, offering unparalleled atomic-level information on the surface structures, properties, and chemistries. Herein, we present an overview of the current state of the art from an NMR perspective. We begin with a brief introduction to contemporary ssNMR methodologies. Subsequently, we introduce and provide critical reviews on the applications of different ssNMR techniques in the detailed characterizations of the surface local structures, disorders, defects, active sites, and acidity on metal oxide nanoparticles, as well as the revelation of mechanisms behind some intriguing chemistries that occur on the surfaces, referencing representative recent studies. Finally, we address the challenges beyond the current status and provide perspectives on the future development and application of advanced ssNMR methodologies in this emerging field.

Surface Structure Dependent Activation of Hydrogen over Metal Oxides during Syngas Conversion

Despite the extensive studies on the adsorption and activation of hydrogen over metal oxides, it remains a challenge to investigate the structure-dependent activation of hydrogen and its selectivity mechanism in hydrogenation reactions. Herein we take spinel and solid solution MnGaOx with a similar bulk chemical composition and study the hydrogen activation mechanism and reactivity in syngas conversion. The results show that MnGaOx-Solid Solution (MnGaOx-SS) is a typical Mn-doped hexagonal close-packed (HCP) Ga2O3 with a Ga-rich surface. Upon exposure to hydrogen, Ga-H and O-H species are simultaneously generated. Ga-H species are highly active but unselective in CO activation, forming CHxO, and ethylene hydrogenation, forming ethane. In contrast, MnGaOx-Spinel is a face-centered-cubic (FCC) spinel phase featuring a Mn-rich surface, thus effectively suppressing the formation of Ga-H species. Interestingly, only part of the O-H species are active for CO activation while the O-H species are inert for olefin hydrogenation over MnGaOx-Spinel. Therefore, MnGaOx-Spinel exhibits a higher activity and higher light-olefin selectivity than MnGaOx-SS in combination with SAPO-18 during syngas conversion. These fundamental understandings are essential to guide the design and further optimization of metal oxide catalysts for selectivity control in hydrogenations.

Generation of Surface Ga-H Hydride and Reactivity toward CO2

Hydrogen adsorption on gallium oxide was investigated by in situ infrared (IR) spectroscopy over a temperature range of 30-450 °C. Both hydroxyl groups and Ga-H hydrides with a pair of characteristic bands at 3685 (3532) cm-1 and 2011 (1988) cm-1 were detected on the surface gallium oxide. The formation and stability of surface Ga-H hydrides were found to be highly dependent on the temperature of H2 dissociation. Through a combination of density functional theory (DFT) calculations and isotopic experiments, a mechanism involving both heterolytic and homolytic pathways for hydrogen dissociation was proposed for the generation of Ga-H hydrides. Furthermore, the reactivity of surface Ga-H hydrides was explored by their interactions with various probe molecules such as carbon dioxide, oxygen, and nitrogen. A potential reaction mechanism involving the attraction between nucleophilic hydrogen and positively charged intermediates was suggested during those hydrogenations.

Nondissociative Activated Dihydrogen Binding on CeO2 Revealed by High-Pressure Operando Solid-State NMR Spectroscopy

Dihydrogen complexes, which retain the H-H bond, have been extensively studied in molecular science and found to be prevalent in homogeneous and enzymatic catalysis. However, their counterparts in heterogeneous catalysis, specifically nondissociative chemisorbed dihydrogen binding on the catalyst surface, are rarely reported experimentally. This scarcity is due to the complexity of typical material surfaces and the lack of effective characterization techniques to prove and distinguish various dihydrogen binding modes. Herein, using high-pressure operando solid-state NMR technology, we report the first unambiguous experimental observation of activated dihydrogen binding on a reduced ceria catalyst through interactions with surface oxygen vacancies. By employing versatile NMR structural and dynamical analysis methods, we establish a proportional relationship between the degree of ceria surface reduction and dihydrogen binding, as evidenced by NMR observations of H-D through-bond coupling (JHD), T1 relaxation, and proton isotropic chemical shifts. In situ NMR analysis further reveals the participation of bound dihydrogen species in a room-temperature ethylene hydrogenation reaction. The remarkable similarities between surface-activated dihydrogen in heterogeneous catalysis and dihydrogen model molecular complexes can provide valuable insights into the hydrogenation mechanism for many other solid catalysts, potentially enhancing hydrogen utilization.

Spinel Nanostructures for the Hydrogenation of CO2 to Methanol and Hydrocarbon Chemicals

Composite oxides have been widely applied in the hydrogenation of CO/CO2 to methanol or as the component of bifunctional oxide-zeolite for the synthesis of hydrocarbon chemicals. However, it is still challenging to disentangle the stepwise formation mechanism of CH3OH at working conditions and selectively convert CO2 to hydrocarbon chemicals with narrow distribution. Here, we investigate the reaction network of the hydrogenation of CO2 to methanol over a series of spinel oxides (AB2O4), among which the Zn-based nanostructures offer superior performance in methanol synthesis. Through a series of (quasi) in situ spectroscopic characterizations, we evidence that the dissociation of H2 tends to follow a heterolytic pathway and that hydrogenation ability can be regulated by the combination of Zn with Ga or Al. The coordinatively unsaturated metal sites over ZnAl2Ox and ZnGa2Ox originating from oxygen vacancies (OVs) are evidenced to be responsible for the dissociative adsorption and activation of CO2. The evolution of the reaction intermediates, including both carbonaceous and hydrogen species at high temperatures and pressures over the spinel oxides, has been experimentally elaborated at the atomic level. With the integration of a series of zeolites or zeotypes, high selectivities of hydrocarbon chemicals with narrow distributions can be directly produced from CO2 and H2, offering a promising route for CO2 utilization.

Precise Structural and Dynamical Details in Zeolites Revealed by Coupling-Edited 1H-17O Double Resonance NMR Spectroscopy

Despite the extensive industrial and research interests in zeolites, their intrinsic catalytic nature is not fully understood due to the complexity of the hydroxyl-aluminum moieties. 17O NMR would provide irreplaceable opportunities for much-needed fine structural determination given the ubiquitous presence of oxygen atoms in nearly all species; however, the low sensitivity and quadrupolar nature of oxygen-17 make its NMR spectroscopic elucidation challenging. Here, we show that state-of-the-art double resonance solid-state NMR techniques have been combined with spectral editing methods based on scalar (through-bond) and dipolar (through-space) couplings, which allowed us to address the subtle protonic structures in zeolites. Notably, the often-neglected and undesired second-order quadrupolar-dipolar cross-term interaction ("2nd-QD interaction") can actually be exploited and can help gain invaluable information. Eventually, a comprehensive set of 1H-17O/1H-27Al double resonance NMR with J-/D-coupling spectral editing techniques have been designed in this work and enabled us to reveal atomic-scale precise structural and dynamical details in zeolites including: 1) The jump rate of the bridging acid site (BAS) proton is relatively low, i.e., far less than 100 s-1 at room temperature. 2) The Al-OH groups with 1H chemical shift at 2.6-2.8 ppm, at least for nonseverely dealuminated H-ZSM-5 catalysts, exhibit a rigid bridging environment similar to that of BAS. 3) The Si-OH groups at 2.0 ppm are not hydrogen bonded and undergo fast cone-rotational motion. The results in this study predict the 2nd-QD interaction to be universal for any rigid -17O-H environment, such as those in metal oxide surfaces or biomaterials.

Homolytic H2 dissociation for enhanced hydrogenation catalysis on oxides

The limited surface coverage and activity of active hydrides on oxide surfaces pose challenges for efficient hydrogenation reactions. Herein, we quantitatively distinguish the long-puzzling homolytic dissociation of hydrogen from the heterolytic pathway on Ga2O3, that is useful for enhancing hydrogenation ability of oxides. By combining transient kinetic analysis with infrared and mass spectroscopies, we identify the catalytic role of coordinatively unsaturated Ga3+ in homolytic H2 dissociation, which is formed in-situ during the initial heterolytic dissociation. This site facilitates easy hydrogen dissociation at low temperatures, resulting in a high hydride coverage on Ga2O3 (H/surface Ga3+ ratio of 1.6 and H/OH ratio of 5.6). The effectiveness of homolytic dissociation is governed by the Ga-Ga distance, which is strongly influenced by the initial coordination of Ga3+. Consequently, by tuning the coordination of active Ga3+ species as well as the coverage and activity of hydrides, we achieve enhanced hydrogenation of CO2 to CO, methanol or light olefins by 4-6 times.

Insight into the Role and Evidence of Oxygen Vacancies in Porous Single-Crystalline Oxide to Enhance Catalytic Activity and Durability

Introducing and stabilizing oxygen vacancies in oxide catalysts is considered to be a promising strategy for improving catalytic activity and durability. Herein, we quantitatively create oxygen vacancies in the lattice of porous single-crystalline β-Ga2O3 monoliths by reduction treatments and stabilize them through the long-range ordering of crystal lattice to enhance catalytic activity and durability. The combination analysis of time-of-flight neutron powder diffraction and extended x-ray absorption fine structure discloses that the preferential generation of oxygen vacancy tends to occur at the site of tetrahedral coordination oxygen ions (OIII sites), which contributes to the formation of unsaturated Ga-O coordination in the monoclinic phase. The oxygen vacancies are randomly distributed in lattice even though some of them are present in the form of domain defect in the PSC Ga2O3 monoliths after the reduction treatment. The number of oxygen vacancies in the reduced monoliths gives 2.32 × 1013, 2.87 × 1013, and 3.45 × 1013 mg-1 for the Ga2O2.952, Ga2O2.895, and Ga2O2.880, respectively. We therefore demonstrate the exceptionally high C2H4 selectivity of ~100% at the C2H6 conversion of ~37% for nonoxidative dehydrogenation of C2H6 to C2H4. We further demonstrate the excellent durability even at 620 °C for 240 h of continuous operation.