Highly Active and Sintering-Resistant Pt Clusters Supported on FeOx-Hydroxyapatite Achieved by Tailoring Strong Metal-Support Interactions.

The catalytic performance of supported metal catalysts is closely related to their structure. While Pt-based catalysts are widely used in many catalytic reactions because of their exceptional intrinsic activity, they tend to deactivate in high-temperature reactions, requiring a tedious and expensive regeneration process. The strong metal-support interaction (SMSI) is a promising strategy to improve the stability of supported metal nanoparticles, but often at the price of the activity due to either the coverage of the active sites by support overlay and/or the too-strong metal-support bonding. Herein, we newly constructed a supported Pt cluster catalyst by introducing FeOx into hydroxyapatite (HAP) support to fine-tune the SMSIs. The catalyst exhibited not only high catalytic activity but also sintering resistance, without deactivation in a 100 h test for catalytic CO oxidation. Detailed characterizations reveal that FeOx introduced into HAP weaken the strong covalent metal-support interaction (CMSI) between Pt and FeOx while simultaneously inhibiting the oxidative strong metal-support interaction (OMSI) between Pt and HAP, giving rise to both high activity and thermal stability of the supported Pt clusters.

The Development of the Regenerable Catalytic System in Selective Catalytic Oxidation of Ammonia with High N2 Selectivity.

Supported particulate noble-metal catalysts are widely used in industrial catalytic reactions. However, these metal species, whether in the form of nanoparticles or highly dispersed entities, tend to aggregate during reactions, leading to a reduced activity or selectivity. Addressing the frequent necessity for the replacement of industrial catalysts remains a significant challenge. Herein, we demonstrate the feasibility of the 'regenerable catalytic system' exemplified by selective catalytic oxidation of ammonia (NH3-SCO) employing Ag/Al2O3 catalysts. Results demonstrate that our highly dispersed Ag catalyst (Ag HD) maintains >90% N2 selectivity at 80% NH3 conversion and >80% N2 selectivity at 100% NH3 conversion after enduring 5 cycles of reducible aggregation and oxidative dispersion. Moreover, it consistently upholds over 98% N2 selectivity at 100% NH3 conversion after 10 cycles of Ar treatment. During the aggregation-dispersion process, the Ag HD catalyst intentionally aggregated into Ag nanoparticles (Ag NP) after H2 reduction and exhibited remarkable regenerable capabilities, returning to the Ag HD state after calcination in the air. This structural evolution was characterized through in situ transmission electron microscopy, atomically resolved high-angle annular dark-field scanning transmission electron microscopy, and X-ray absorption spectroscopy, revealing the on-site oxidative dispersion of Ag NP. Additionally, in situ diffuse reflectance infrared Fourier transform spectroscopy provided insights into the exceptional N2 selectivity on Ag HD catalysts, elucidating the critical role of NO+ intermediates. Our findings suggest a sustainable and cost-effective solution for various industry applications.

Decoration of Gold and Platinum Nanoparticle Catalysts by 1 nm Thick Metal Oxide Overlayer and Its Effect on the CO Oxidation Activity.

Exfoliated M-Al layered double hydroxide (M-Al LDH; M = Mg, Co, Ni, and Zn) nanosheets were adsorbed on Au/SiO2 and calcined to transform LDH into mixed metal oxides (MMOs) and yield Au/SiO2 coated with a thin MMO overlayer. These catalysts showed a higher catalytic activity than pristine Au/SiO2. In particular, the 50% CO conversion temperature decreased by more than 250 °C for Co-Al MMO-coated Au/SiO2. In contrast, the deposition of CoAlOx on Au/SiO2 by impregnation or the deposition of Au on Co-Al MMO-coated SiO2 resulted in a worse catalytic activity. Moreover, the presence of a thick MMO overlayer decreased the catalytic activity, suggesting that the control of the overlayer thickness to less than 1 nm is a requisite for obtaining a high catalytic activity. Moreover, the thin Co-Al MMO overlayer on Au/SiO2 possessed abundant oxygen vacancies, which would play an important role in O2 activation, resulting in a highly active interface between Au and the defect-rich MMO on the Au NP surface. Finally, this can be applied to Pt/SiO2, and the obtained Co-Al MMO-coated Pt/SiO2 also exhibited a much improved catalytic activity for CO oxidation.

High Selectivity Cofactor NADH Regeneration Organic Iridium Complexes Used for High-Efficiency Chem-Enzyme Cascade Catalytic Hydrogen Transfer.

Our research demonstrated that novel pentamethylcyclopentadienyl (Cp*) iridium pyridine sulfonamide complex PySO2NPh-Ir (7) could highly specifically catalyze nicotinamide adenine dinucleotide (NAD+) into the corresponding reducing cofactor NADH in cell growth media containing various biomolecules. The structures and catalytic mechanism of 7 were studied by single-crystal X-ray, NMR, electrochemical, and kinetic methods, and the formation of iridium hydride species Ir-H was confirmed to be the plausible hydride-transfer intermediate of 7. Moreover, benefiting from its high hydrogen-transfer activity and selectivity for NADH regeneration, 7 was used as an optimal metal catalyst to establish a chem-enzyme cascade catalytic hydrogen-transfer system, which realized the high-efficiency preparation of l-glutamic acid by combining with l-glutamate dehydrogenase (GLDH).

Low-Temperature Hydrogenation of CO2 to Methanol in Water on ZnO-Supported CuAu Nanoalloys.

Optimizing processes and materials for the valorization of CO2 to hydrogen carriers or platform chemicals is a key step for mitigating global warming and for the sustainable use of renewables. We report here on the hydrogenation of CO2 in water on ZnO-supported CuAu nanoalloys, based on ≤7 mol % Au. Cux Auy /ZnO catalysts were characterized using 197 Au Mössbauer, in situ X-ray absorption (Au LIII - and Cu K-edges), and ambient pressure X-ray photoelectron (APXP) spectroscopic methods together with X-ray diffraction and high-resolution electron microscopy. At 200 °C, the conversion of CO2 showed a significant increase by 34 times (from 0.1 to 3.4 %) upon increasing Cu93 Au7 loading from 1 to 10 wt %, while maintaining methanol selectivity at 100 %. Limited CO selectivity (4-6 %) was observed upon increasing temperature up to 240 °C but associated with a ≈3-fold increase in CO2 conversion. Based on APXPS during CO2 hydrogenation in an H2 O-rich mixture, Cu segregates preferentially to the surface in a mainly metallic state, while slightly charged Au submerges deeper into the subsurface region. These results and detailed structural analyses are topics of the present contribution.

Metal-Support Interaction in Gold Zeolitic Octahedral Metal Oxide and the Catalytic Activity for Low-Temperature Alcohol Oxidation.

Au nanoparticles are efficient catalysts for selective oxidations. The interaction between Au nanoparticles and supports is critical for achieving high catalytic activity. Herein, Au nanoparticles are supported on a zeolitic octahedral metal oxide based on Mo and V. The charge of Au is controlled by the surface oxygen vacancies of the supports, and the redox property of the zeolitic vanadomolybdate is highly dependent on Au loading. The Au-supported zeolitic vanadomolybdate is used as a heterogeneous catalyst for alcohol oxidation under mild conditions with molecular oxygen as an oxidant. The supported Au catalyst can be recovered and reused without the loss of activity.

Gold/Substituted Hydroxyapatites for Oxidative Esterification: Control of Thin Apatite Layer on Gold Based on Strong Metal-Support Interaction (SMSI) Results in High Activity.

Gold nanoparticles (Au NPs) deposited on various cation- and anion-substituted hydroxyapatites (Au/sHAPs) show oxidative strong metal-support interaction (SMSI), wherein a thin layer of the sHAP covered the surface of the Au NPs by heat treatment in an oxidative atmosphere. Calcination of Au/sHAPs at 300 °C caused a partial SMSI and that at 500 °C gave fully encapsulated Au NPs. We investigated the influence of the substituted ions in sHAP and the degree of the oxidative SMSI on the catalytic performance of Au/sHAPs for oxidative esterification of octanal or 1-octanol with ethanol to obtain ethyl octanoate. The catalytic activity depends on the size of the Au NPs but not on the support used, owing to the similarity of the acid and base properties of sHAPs except for Au/CaFAP. The presence of a large number of acidic sites on CaFAP lowered the product selectivity, but all other sHAPs exhibited similar activity when the Au particle size was almost the same, owing to the similarity of the acid and base properties. Au/sHAPs_O2 with SMSI exhibited higher catalytic activity than Au/sHAPs_H2 without SMSI despite the fact that the number of exposed surface Au atoms was decreased by the SMSI. In addition, the oxidative esterification reaction proceeded even though the Au NPs were fully covered by the sHAP layer when the thickness of the layer was controlled to be less than 1 nm. The substrate can access the surfaces of the Au NPs covered by the thin sHAP layer (<1 nm), and the presence of the sHAP structure in close contact with the Au NPs resulted in significantly higher catalytic activity compared with that for fully exposed Au NPs deposited on the sHAPs. This result suggests that maximizing the contact area between the Au NPs and the sHAP support based on the SMSI enhances the catalytic activity of Au.

Selective formation of acetate intermediate prolongs robust ethylene removal at 0 °C for 15 days.

Efficient ethylene (C2H4) removal below room temperatures, especially near 0 °C, is of great importance to suppress that the vegetables and fruits spoil during cold-chain transportation and storage. However, no catalysts have been developed to fulfill the longer-than-2-h C2H4 removal at this low temperature effectively. Here we prepare gold-platinum (Au-Pt) nanoalloy catalysts that show robust C2H4 (of 50 ppm) removal capacity at 0 °C for 15 days (360 h). We find, by virtue of operando Fourier transformed infrared spectroscopy and online temperature-programmed desorption equipped mass spectrometry, that the Au-Pt nanoalloys favor the formation of acetate from selective C2H4 oxidation. And this on-site-formed acetate intermediate would partially cover the catalyst surface at 0 °C, thus exposing active sites to prolong the continuous and effective C2H4 removal. We also demonstrate, by heat treatment, that the performance of the used catalysts will be fully recovered for at least two times.

Praseodymia-titania mixed oxide supported gold as efficient water gas shift catalyst: modulated by the mixing ratio of oxides.

Modulating the active sites for controllable tuning of the catalytic activity has been the goal of much research, however, this remains challenging. The O vacancy is well known as an active site in reducible oxides. To modify the activity of O vacancies in praseodymia, we synthesized a series of praseodymia-titania mixed oxides. Varying the Pr : Ti mole ratio (2 : 1, 1 : 2, 1 : 1, 1 : 4) allows us to control the electronic interactions between Au, Pr and Ti cations and the local chemical environment of the O vacancies. These effects have been studied study by X-ray photoelectron spectroscopy (XPS), CO diffuse reflectance Fourier transform infrared spectroscopy (CO-DRIFTS) and temperature-programmed reduction (CO-TPR, H2-TPR). The water gas shift reaction (WGSR) was used as a benchmark reaction to test the catalytic performance of different praseodymia-titania supported Au. Among them, Au/Pr1Ti2O x was identified to exhibit the highest activity, with a CO conversion of 75% at 300 °C, which is about 3.7 times that of Au/TiO2 and Au/PrO x . The Au/Pr1Ti2O x also exhibited excellent stability, with the conversion after 40 h time-on-stream at 300 °C still being 67%. An optimal ratio of Pr content (Pr : Ti 1 : 2) is necessary for improving the surface oxygen mobility and oxygen exchange capability, a higher Pr content leads to more O vacancies, however with lower activity. This study presents a new route for modulating the active defect sites in mixed oxides which could also be extended to other heterogeneous catalysis systems.

Transesterification of Ethyl-10-undecenoate Using a Cu-Deposited V2O5 Catalyst as a Model Reaction for Efficient Conversion of Plant Oils to Monomers and Fine Chemicals.

Transesterification of ethyl-10-undecenoate (derived from castor oil) with 1,4-cyclohexanedimethanol over a recyclable Cu-deposited V2O5 catalyst afforded 1,ω-diene, the corresponding cyclohexane-1,4-diylbis(methylene) bis(undec-10-enoate), a promising monomer for the synthesis of biobased polyesters, in an efficient manner. Deposition of Cu plays an important role in proceeding the reaction with high selectivity, and both the activity and the selectivity are preserved for five recycled runs by the addition of the substrates. The present catalyst was effective for transesterification with other alcohols, especially primary alcohols, demonstrating a possibility of using this catalyst for efficient conversion of plant oil to various fine chemicals.

Anti-inflammatory effect of gold nanoparticles supported on metal oxides.

Gold (Au) can be deposited as nanoparticles (NPs) smaller than 10 nm in diameter on a variety of metal oxide (MOx) NPs. Au/MOx have high catalytic performance and selective oxidation capacity which could have implications in terms of biological activity, and more specifically in modulation of the inflammatory reaction. Therefore, the aim of this study was to examine the effect of Au/TiO2, Au/ZrO2 and Au/CeO2 on viability, phagocytic capacity and inflammatory profile (TNF-α and IL-1ß secretion) of murine macrophages. The most important result of this study is an anti-inflammatory effect of Au/MOx depending on the MOx nature with particle internalization and no alteration of cell viability and phagocytosis. The effect was dependent on the MOx NPs chemical nature (Au/TiO2 > Au/ZrO2 > Au/CeO2 if we consider the number of cytokines whose concentration was reduced by the NPs), and on the inflammatory mediator considered. The effect of Au/TiO2 NPs was not related to Au NPs size (at least in the case of Au/TiO2 NPs in the range of 3-8 nm). To the best of our knowledge, this is the first demonstration of an anti-inflammatory effect of Au/MOx.

Catalytic Activities of Various Niobium Oxides for Hydrogen Absorption/Desorption Reactions of Magnesium.

Five types of niobium(V) oxides (Nb2O5) were synthesized by hydrothermal and heat treatment processes, and their structural properties and catalytic activities for the hydrogen absorption/desorption reactions of magnesium were characterized. The synthesized Nb oxides were dispersed on magnesium hydride (MgH2), a typical hydrogen storage material, using the ball-milling method. All the synthesized Nb oxides improved the reaction kinetics of the hydrogen desorption/absorption reactions. The catalytic activities for the hydrogen desorption were comparable, while the hydrogen absorption rates were significantly different for each synthesized Nb oxide. This difference can be explained by the structural stability of Nb2O5, which is related to the formation of a catalytically active state by the reduction of Nb2O5 during the ball-milling process. Notably, the highest catalytic effect was observed for Nb2O5 with a highly crystalline pyrochlore structure and a low specific surface area, suggesting that pyrochlore Nb2O5 is a metastable phase. However, only the amorphous Nb oxide was out of order, even though there is a report on the high catalytic activity of amorphous Nb oxide. This is attributed to the initial condensed state of amorphous Nb oxide, because particle size affects the dispersion state on the MgH2 surface, which is also important for obtaining high catalytic activity. Thus, it is concluded that Nb2O5 with lower stability of the crystal structure and smaller particle size shows better catalysis for both hydrogen desorption and absorption reactions.

Synthesis of Zeolitic Mo-Doped Vanadotungstates and Their Catalytic Activity for Low-Temperature NH3-SCR.

Mo was successfully introduced into a vanadotungstate (VT-1), which is a crystalline microporous zeolitic transition-metal oxide based on cubane clusters [W4O16]8- and VO2+ linkers (MoxW4-x. x: number of Mo in VT-1 unit cell determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)). It was confirmed that W in the cubane units was substituted by Mo. The resulting materials showed higher microporosity compared with VT-1. The surface area and the micropore volume increased to 296 m2·g-1 and 0.097 cm3·g-1, respectively, for Mo0.6W3.4 compared with the those values for VT-1 (249 m2·g-1 and 0.078 cm3·g-1, respectively). The introduction of Mo changed the acid properties including the acid amount (VT-1: 1.06 mmol g-1, Mo0.6W3.4: 2.18 mmol·g-1) and its strength because of the changes of the chemical bonding in the framework structure. MoxW4-x showed substantial catalytic activity for the selective catalytic reduction of NO with NH3 (NH3-selective catalytic reduction (SCR)) at a temperature as low as 150 °C.

Bulk tungsten-substituted vanadium oxide for low-temperature NOx removal in the presence of water.

NH3-SCR (selective catalytic reduction) is important process for removal of NOx. However, water vapor included in exhaust gases critically inhibits the reaction in a low temperature range. Here, we report bulk W-substituted vanadium oxide catalysts for NH3-SCR at a low temperature (100-150 °C) and in the presence of water (~20 vol%). The 3.5 mol% W-substituted vanadium oxide shows >99% (dry) and ~93% (wet, 5-20 vol% water) NO conversion at 150 °C (250 ppm NO, 250 ppm NH3, 4% O2, SV = 40000 mL h-1 gcat-1). Lewis acid sites of W-substituted vanadium oxide are converted to Brønsted acid sites under a wet condition while the distribution of Brønsted and Lewis acid sites does not change without tungsten. NH4+ species adsorbed on Brønsted acid sites react with NO accompanied by the reduction of V5+ sites at 150 °C. The high redox ability and reactivity of Brønsted acid sites are observed for bulk W-substituted vanadium oxide at a low temperature in the presence of water, and thus the catalytic cycle is less affected by water vapor.

Importance of Size and Contact Structure of Gold Nanoparticles for the Genesis of Unique Catalytic Processes.

Since the discovery of catalysis by Au nanoparticles (NPs), unique catalytic features of Au have appeared that are greatly different from those of Pd and Pt. In this Review, we aimed to disclose how the unique catalytic abilities of Au are generated with respect to (a) the contact structures between Au and its supports and (b) the size of the Au particles. For CO oxidation, the catalytic activity of Au on reducible metal oxides (MOx) is strongly correlated with the amount of oxygen vacancies of the MOx surface, which play a key role in O2 activation. Single atoms, bilayers of Au, sub-nm clusters, clusters (1-2 nm), and NPs (2-5 nm) have been proposed as the active sizes of the Au species, which may depend on the type of support. For propylene epoxidation, the presence of isolated TiO4 units in SiO2 supports is important for the production of propylene oxide (PO). Au NPs facilitate the formation of Ti-OOH species, which leads to PO in the presence of H2 and O2, whereas Au clusters facilitate propylene hydrogenation. However, Au clusters can produce PO by using only O2 and water, whereas Au NPs cannot. For alcohol oxidation, the reducibility of the MOx supports greatly influences the catalytic activity of Au, and single Au atoms more effectively activate the lattice oxygen of CeO2. The basic and acidic sites of the MOx surface also play an important role in the deprotonation of alcohols and the activation of aldehydes, respectively. For selective hydrogenation, heterolytic dissociation of H2 takes place at the interface between Au and MOx, and the basic sites of MOx contribute to H2 activation. Recent research into the reaction mechanisms and the development of well-designed Au catalysts has provided new insights into the preparation of high-performance Au catalysts.

Redox-Active Zeolitic Transition Metal Oxides Based on ε-Keggin Units for Selective Oxidation.

The design and development of zeolitic transition metal oxides for selective oxidation are interesting due to the combination of the redox properties and microporosities. Redox-active zeolitic transition metal oxides based on ε-Keggin iron molybdates were synthesized. O2 can be activated by the materials via an electron-transfer-based process, and the materials can be oxidized even at room temperature. The materials are oxidized and reduced reversibly while the crystal structures are maintained. V is uniformly incorporated in the materials without changing the basic structures, and the redox properties of the materials are tuned by V. The materials are used as robust catalysts for ethyl lactate oxidation to form ethyl pyruvate using O2 as an oxidant.

A zeolitic vanadotungstate family with structural diversity and ultrahigh porosity for catalysis.

Design of the structure and composition of crystalline microporous inorganic oxides is of great importance in catalysis. Developing new zeolites is one approach towards this design because of the tunable pore system and high thermal stability. Zeolites are limited to main group elements, which limits their applications in redox catalysis. Another promising choice is zeolitic transition metal oxides providing both porosity and redox activity, thereby further expanding the diversity of porous materials. However, the examples of zeolitic transition metal oxides are rare. Here, we report a new class of zeolitic vanadotungstates with tunable frameworks exhibiting a large porosity and redox activity. The assembly of [W4O16]8- units with VO2+ forms two isomeric porous frameworks. Owing to the complex redox properties and open porosity, the vanadotungstates efficiently catalyse the selective reduction of NO by NH3. This finding provides an opportunity for design and synthesis of inorganic multifunctional materials for future catalytic applications.

Carbon Monoxide Oxidation by Polyoxometalate-Supported Gold Nanoparticulate Catalysts: Activity, Stability, and Temperature- Dependent Activation Properties.

Nanoparticulate gold supported on a Keggin-type polyoxometalate (POM), Cs4 [α-SiW12 O40 ]⋅n H2 O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was -67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U-shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas-phase reactions.