Prediction of Stable Surfaces of Metal Oxides through the Unsaturated Coordination Index.

This study proposes the unsaturated coordination index, σ, as a potential descriptor of the stability of metal-oxide surfaces cleaved from bulk. The value of σ, the number of missing bonds per unit area, can be obtained very quickly using only crystallographic data, namely, the bulk geometry. The surface energies of various binary oxides, with and without atom relaxation, were calculated. Their correlations with σ had good coefficients of determination (R2) values, particularly in high-symmetry crystals. The proposed descriptor is very useful for an initial evaluation of stable metal-oxide surfaces without conducting any surface model calculations.

Designing main-group catalysts for low-temperature methane combustion by ozone.

The catalytic combustion of methane at a low temperature is becoming increasingly key to controlling unburned CH4 emissions from natural gas vehicles and power plants, although the low activity of benchmark platinum-group-metal catalysts hinders its broad application. Based on automated reaction route mapping, we explore main-group elements catalysts containing Si and Al for low-temperature CH4 combustion with ozone. Computational screening of the active site predicts that strong Brønsted acid sites are promising for methane combustion. We experimentally demonstrate that catalysts containing strong Bronsted acid sites exhibit improved CH4 conversion at 250 °C, correlating with the theoretical predictions. The main-group catalyst (proton-type beta zeolite) delivered a reaction rate that is 442 times higher than that of a benchmark catalyst (5 wt% Pd-loaded Al2O3) at 190 °C and exhibits higher tolerance to steam and SO2. Our strategy demonstrates the rational design of earth-abundant catalysts based on automated reaction route mapping.

An automated reaction route mapping for the reaction of NO and active species on Ag4 clusters in zeolites.

A computational investigation of the catalytic reaction on multinuclear sites is very challenging. Here, using an automated reaction route mapping method, the single-component artificial force induced reaction (SC-AFIR) algorithm, the catalytic reaction of NO and OH/OOH species over the Ag42+ cluster in a zeolite is investigated. The results of the reaction route mapping for H2 + O2 reveal that OH and OOH species are formed over the Ag42+ cluster via an activation barrier lower than that of OH formation from H2O dissociation. Then, reaction route mapping is performed to examine the reactivity of the OH and OOH species with NO molecules over the Ag42+ cluster, resulting in the facile reaction path of HONO formation. With the aid of the automated reaction route mapping, the promotion effect of H2 addition on the SCR reaction was computationally proposed (boosting the formation of OH and OOH species). In addition, the present study emphasizes that automated reaction route mapping is a powerful tool to elucidate the complicated reaction pathway on multi-nuclear clusters.

High-Entropy Intermetallics Serve Ultrastable Single-Atom Pt for Propane Dehydrogenation.

Propane dehydrogenation has been a promising propylene production process that can compensate for the increasing global demand for propylene. However, Pt-based catalysts with high stability at ≥600 °C have barely been reported because the catalysts typically result in short catalyst life owing to side reactions and coke formation. Herein, we report a new class of heterogeneous catalysts using high-entropy intermetallics (HEIs). Pt-Pt ensembles, which cause side reactions, are entirely diluted by the component inert metals in PtGe-type HEIs. The resultant HEI (PtCoCu) (GeGaSn)/Ca-SiO2 exhibited an outstandingly high catalytic stability, even at 600 °C (kd-1 = τ = 4146 h = 173 d), and almost no deactivation of the catalyst was observed for 2 months for the first time. Detailed experimental studies and theoretical calculations demonstrated that the combination of the site-isolation and entropy effects upon multi-metallization of PtGe drastically enhanced the desorption of propylene and the thermal stability, eventually suppressing the side reactions even at high reaction temperatures.

Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects.

Keggin-type phosphomolybdic acid (PMo12O40), treated with pyridine (Py), forms a crystalline material (PyPMo-HT) following heat treatment under an inert gas flow at ∼420 °C. Although this material is known to have attractive catalytic properties for gas-phase oxidation, the origin of this catalytic activity requires clarification. In this study, we investigated the crystal structure of PyPMo-HT. PyPMo-HT comprises a one-dimensional array of Keggin units and pyridinium cations (HPy), with an HPy/Keggin unit ratio of ∼1.0. Two oxygen atoms were removed from the Keggin unit during crystal structure transformation, which resulted in an electron being localized on the Mo atom in close contact with the adjacent Keggin unit. Upon the introduction of molecular oxygen, electron transfer from this Mo atom resulted in the formation of an electrophilic oxygen species that bridged two Keggin units. The electrophilic oxygen species acted as a catalytically active oxygen species, as confirmed by the selective oxidation of propylene. PyPMo-HT showed excellent catalytic activity for the selective oxidation of methacrolein, with the methacrylic acid yield being superior to that obtained with PMo12O40 and comparable to that obtained with an industrial Keggin-type polyoxometalate (POM) catalyst. The oxidation catalysis observed over PyPMo-HT provides a deeper understanding of POM-based industrial catalytic processes.

Local structure and NO adsorption/desorption property of Pd2+ cations at different paired Al sites in CHA zeolite.

Recently, Pd-exchanged CHA zeolites (Pd-CHA) have attracted attention as promising passive NOx adsorbers (PNAs) for reducing NOx emissions during the cold start period of a vehicle engine. In this work, the relationship between the local structures and the NO adsorption/desorption properties of the Pd cations in CHA zeolites was investigated. Pd cation formation and NO adsorption were theoretically explored by density functional theory (DFT) calculations for different paired Al sites in six-/eight-membered rings (6MR/8MR). Furthermore, we prepared a series of Pd-CHAs with different Pd loadings (0.5-5.4 wt%) and evaluated their NO adsorption/desorption properties by in situ infrared (IR) spectroscopy and temperature-programmed desorption (TPD) measurements. The increase in the Pd loading resulted in a shift in the NO desorption temperature toward a higher temperature regime. This phenomenon was ascribed to the increase in the proportion of less stable Pd cations, resulting in improved NO adsorption. Furthermore, the effect of Al distribution on the NO adsorption property of Pd-CHA was examined using CHA zeolites containing different proportions of paired Al sites in 6MR while maintaining similar Si/Al ratios (Si/Al = 12.0-16.5). The present study, based on a combination of theoretical and experimental techniques, shows that the NO adsorption/desorption properties over Pd-CHA can be tuned by controlling the Pd loading amount and the type of paired Al sites.

Transformation of Bulk Pd to Pd Cations in Small-Pore CHA Zeolites Facilitated by NO.

Atomic dispersion of metal species has attracted attention as a unique phenomenon that affects adsorption properties and catalytic activities and that can be used to design so-called single atom materials. In this work, we describe atomic dispersion of bulk Pd into small pores of CHA zeolites. Under 4% NO flow at 600 °C, bulk Pd metal on the outside of CHA zeolites effectively disperses, affording Pd2+ cations on Al sites with concomitant formation of N2O, as revealed by microscopic and spectroscopic characterizations combined with mass spectroscopy. In the present method, even commercially available submicrosized Pd black can be used as a Pd source, and importantly, 4.1 wt % of atomic Pd2+ cations, which is the highest loading amount reported so far, can be introduced into CHA zeolites. The structural evolution of bulk Pd metal is also investigated by in situ X-ray absorption spectroscopy (XAS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), as well as ab initio thermodynamic analysis using density functional theory (DFT) calculations.

Isolated Indium Hydrides in CHA Zeolites: Speciation and Catalysis for Nonoxidative Dehydrogenation of Ethane.

The study of the formation, characterization, and functionality of isolated surface hydrides on solid materials is a formidable task because of the complexity of solid surfaces and the difficulty of analyzing structures in solids. Herein, we found the formation of indium (In) hydride species supported by CHA zeolites. The In hydrides were formed by treatment of an In-exchanged CHA zeolite (In-CHA) with H2 at high temperatures (>773 K). In situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations revealed that an [InH2]+ ion on a framework anionic site is a plausible structure. In-CHA exhibited high selectivity and durable catalytic activity for the nonoxidative dehydrogenation of ethane for at least 90 h. Kinetic and in situ spectroscopic studies as well as transition state (TS) calculations suggested that [InH2]+ ions serve as catalytically active sites for selective dehydrogenation using In-CHA.

Experimental and theoretical study of multinuclear indium-oxo clusters in CHA zeolite for CH4 activation at room temperature.

We have carried out an experimental and theoretical study of CHA-zeolite supported indium (In)-oxo clusters that promote CH4 activation at room temperature. X-ray absorption fine structure (XAFS) measurements indicate the formation of multinuclear In-oxo clusters by the O2 activation of the In(i)-exchanged CHA zeolite prepared through reductive solid-state ion exchange (RSSIE). The structure of the In-oxo clusters and their locations were investigated in detail using ab initio thermodynamic analysis. The redox properties of the In species during RSSIE and the formation of the In-oxo clusters were also studied by temperature programmed reaction and in situ XAFS measurements. The reaction of CH4 on the O2-activated In-CHA zeolite was monitored using IR spectroscopy where adsorbed formic acid was generated at room temperature. The adsorption and C-H activation of CH4 on our plausible model of the In-oxo clusters were theoretically investigated using density functional theory calculations. We found that CH4 is likely to adsorb and react more easily on dinuclear In-oxo ions than on monomeric In-oxo ions and that the C-H bond cleavage reaction occurs via a heterolytic pathway rather than a homolytic pathway. This study reveals the potential of multinuclear In-oxo clusters as active sites for the transformation of CH4 to oxygenates under mild reaction conditions.

Synergistic interplay of Zn and Rh-Cr promoters on Ga2O3 based photocatalysts for water splitting.

The photocatalytic water splitting activity of a wide-bandgap material, Ga2O3, is greatly boosted with the addition of a Zn and Rh-Cr co-catalyst at optimum loadings. To date, however, the exact roles of the co-catalysts and particularly the origin of their synergistic functions have not been clarified. Herein, we present how the optimum Zn loading on Ga2O3 leads to creation of a ZnGa2O4/Ga2O3 heterojunction favorable for charge separation through information on the occupied and unoccupied electronic states of Zn and Ga elucidated by X-ray absorption and emission spectroscopic methods. The function of Rh-Cr as an electron sink and reduction site was proven by photocatalytic experiments using an electron scavenger (Ag+) and by learning where Ag deposits and its effects on photocatalytic activity. Finally, perturbation of the Zn electronic structure by photoactivation was evidenced by modulation excitation X-ray absorption spectroscopy. Importantly, Rh-Cr markedly enhanced the level of the perturbation, serving as proof of the direct communication and synergy between the electronic states of Zn, present in ZnGa2O4, and Rh-Cr deposited on Ga2O3.