Accurate Measurements of NH₃ Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites

Water Radiocatalysis for Selective Aqueous-Phase Methane Carboxylation with Carbon Dioxide into Acetic Acid at Room Temperature

Methane (CH4) carboxylation with carbon dioxide (CO2) into acetic acid (CH3COOH) is an ideal chemical reaction to utilize both greenhouse gases with 100% atom efficiency but remains a great challenge under mild conditions. Herein, we introduce a concept of water (H2O) radiocatalysis for efficient and selective aqueous-phase CH4 carboxylation with CO2 into CH3COOH at room temperature. H2O radiolysis occurs under γ-ray radiation to produce ·OH radicals and hydrated electrons that efficiently react with CH4 and CO2, respectively, to produce ·CH3 radicals and ·CO2– species facilely coupling to produce CH3COOH. CH3COOH selectivity as high as 96.9 and 96.6% calculated respectively from CH4 and CO2 and a CH3COOH production rate of as high as 121.9 μmol·h–1 are acquired. The water radiocatalysis driven by γ-rays is also applicable to selectively produce organic acids from other hydrocarbons and CO2

Accurate Measurements of NH₃ Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites

Compositions, Structures, and Catalytic Activities of CeO₂@Cu₂O Nanocomposites Prepared by the Template-Assisted Method

CeO₂@Cu₂O nanocomposites were prepared from Cu₂O cubes and octahedra by the template-assisted method involving the liquid (Ce­(IV))–solid (Cu₂O) interfacial reaction. Their compositions, structures, and catalytic activities in CO oxidation were studied in detail. Under the same reaction conditions, CeO₂@Cu₂O nanocomposites prepared from cubic and octahedral Cu₂O templates exhibit different compositions and structures. With an increasing amount of Ce­(IV) reactant, a smooth CeO₂–CuO_<i>x</i> shell develops on the surface of Cu₂O cubes and eventually void cubic core/multishell Cu₂O/CeO₂–CuO_<i>x</i> nanocomposites form; however, a rough CeO₂–CuO_<i>x</i> shell develops on the surface of Cu₂O octahedra, and eventually hollow octahedral CeO₂–CuO_<i>x</i> nanocages form. The formation of different compositions and structures of CeO₂@Cu₂O nanocomposites was correlated with the different exposed crystal planes and surface reactivities of Cu₂O cubes and octahedra. The catalytic activity of CeO₂@Cu₂O nanocomposites in CO oxidation depends on their compositions and structures. The most active CeO₂@Cu₂O nanocomposites become active at 70 °C and achieve a 100% CO conversion at 170 °C. These results broaden the versatility of Cu₂O nanocrystals as the sacrificial template for the fabrication of novel nanocomposites with core/shell and hollow nanostructures and exemplify the morphology effect of Cu₂O nanocrystals in liquid–solid interfacial reactions with respect to the composition, structure, and properties of nanocomposites prepared by the template-assisted method

Size-Dependent Reaction Pathways of Low-Temperature CO Oxidation on Au/CeO₂ Catalysts

Via a comprehensive time-resolved operando-DRIFTS study of the evolutions of various surface species on Au/CeO₂ catalysts with Au particle sizes ranging from 1.7 ± 0.6 to 3.7 ± 0.9 nm during CO oxidation at room temperature, we have successfully demonstrated size-dependent reaction pathways and their contributions to the catalytic activity. The types and concentrations of chemisorbed CO­(a), carbonate, bicarbonate, and formate species formed upon CO adsorption, their intrinsic oxidation/decomposition reactivity, and roles in CO oxidation vary with the size of the supported Au particles. The intrinsic oxidation reactivity of CO­(a) does not depend much on the Au particle size, whereas the intrinsic decomposition reactivity of carbonate, bicarbonate, and formate species strongly depend on the Au particle size and are facilitated over Au/CeO₂ catalysts with large Au particles. These results greatly advance the fundamental understanding of the size effect of Au/CeO₂ catalysts for low-temperature CO oxidation

CeO₂ Thickness-Dependent SERS and Catalytic Properties of CeO₂‑on-Ag Particles Synthesized by O₂‑Assisted Hydrothermal Method

Oxide-on-metal particles with exposed interfaces exhibit wealthy structures and functions, but their facile synthesis remains as a challenge. Herein we report a facile O₂-assisted hydrothermal method to synthesize CeO₂-on-Ag particles with different CeO₂ thicknesses. In this novel approach Ag particles catalyze the O₂ + H₂O reaction to form surface hydroxyls that induce the preferential nucleation of Ce­(OH)₃ on the surfaces of Ag particles, eventually forming CeO₂ adlayers on Ag particles. CeO₂-on-Ag particles exhibit the CeO₂ thickness-dependent SERS effect in which their best SERS effect is 2 orders stronger than that of Ag particles. They also exhibit CeO₂ thickness-dependent catalytic performance in CO oxidation in which the best one is as active as traditional CeO₂-supported Ag catalyst. These results open up new opportunities to synthesize oxide-on-metal particles and explore their functions by tuning the oxide adlayer thickness

Size-Dependent Interaction of the Poly(<i>N</i>-vinyl-2-pyrrolidone) Capping Ligand with Pd Nanocrystals

Pd nanocrystals were prepared by the reduction of a H₂PdCl₄ aqueous solution with C₂H₄ in the presence of different amounts of poly­(<i>N</i>-vinyl-2-pyrrolidone) (PVP). Their average size decreases monotonically as the PVP monomer/Pd molar ratio increases up to 1.0 and then does not vary much at higher PVP monomer/Pd molar ratios. Infrared spectroscopy and X-ray photoelectron spectroscopy results reveal the interesting size-dependent interaction of PVP molecules with Pd nanocrystals. For fine Pd nanocrystals capped with a large number of PVP molecules, each PVP molecule chemisorbs with its oxygen atom in the ring; for large Pd nanocrystals capped by a small number of PVP molecules, each PVP molecule chemisorbs with both the oxygen atom and nitrogen atom in the ring, which obviously affects the structure of chemisorbed PVP molecules and even results in the breaking of involved C–N bonds of some chemisorbed PVP molecules. Charge transfer always occurs from a chemisorbed PVP ligand to Pd nanocrystals. These results provide novel insights into the PVP–metal nanocrystal interaction, which are of great importance in the fundamental understanding of surface-mediated properties of PVP-capped metal nanocrystals

Reaction Sensitivity of Ceria Morphology Effect on Ni/CeO₂ Catalysis in Propane Oxidation Reactions

CeO₂ nanocubes (c-CeO₂), nanoparticles (p-CeO₂), and nanorods calcined at 500 °C (r-CeO₂-500) and 700 °C (r-CeO₂-700) were used as supports to synthesize a series of Ni/CeO₂ catalysts for the propane combustion and oxidative dehydrogenation of propane (ODHP) reactions. The Ni-CeO₂ interaction greatly promotes the reducibility of CeO₂, but CeO₂ morphology-dependent Ni-CeO₂ interaction was observed to form different speciation of Ni species (Ni²⁺ dissolved in CeO₂, highly dispersive NiO, NiO aggregate) and oxygen species (strongly activated oxygen species, medially activated oxygen species, weakly activated oxygen species) in various Ni/CeO₂ catalysts. Ni-CeO₂ interaction is stronger in Ni/c-CeO₂ catalysts than in other Ni/CeO₂ catalysts. Different morphology-dependences of Ni/CeO₂ catalysts in propane combustion and ODHP reactions were observed. The Ni/r-CeO₂-500 catalyst with the largest strongly activated oxygen species is most catalytic active in the propane combustion reaction while the Ni/c-CeO₂ catalyst with the largest amount of weakly activated oxygen species exhibits the best catalytic performance in the ODHP reaction. Thus, the CeO₂ morphology engineering strategy is effective in finely tuning the metal-CeO₂ interaction and the reactivity of oxygen species to meet the requirements of different types of catalytic oxidation reactions

Direct Observation of Reversible Transformation of CH₃NH₃PbI₃ and NH₄PbI₃ Induced by Polar Gaseous Molecules

Despite its competitive photovoltaic efficiency, the structural transformations of the prototypical hybrid perovskite, methylammonium lead iodide, are facilitated by interactions with polar molecules. Changes in optical and electronic properties upon exposure to ammonia potentially can enable the use of hybrid perovskites in gas-sensing applications. We investigated the effects of ammonia on CH₃NH₃PbI₃ by exposing perovskite films to a wide range of vapor pressures. Spectroscopic analyses indicated that ammonium cations replaced the methylammonium cations in the perovskite crystal, thereby resulting in the formation of NH₄PbI₃. The transformation of CH₃NH₃PbI₃ to NH₄PbI₃ caused distinct changes in the morphology of the film and its crystalline structure; however, the introduction of CH₃NH₂ gas reversed these changes. An in-depth understanding of the reversible chemical and structural alterations resulting from exposure to polar molecules can advance the development of hybrid perovskite sensors and provide insight into mechanisms by which perovskites convert due to interactions with polar molecules

Probing Surface Structures of CeO₂, TiO₂, and Cu₂O Nanocrystals with CO and CO₂ Chemisorption

CO and CO₂ chemisorption on uniform CeO₂, TiO₂, and Cu₂O nanocrystals with various morphologies were comprehensively studied with in-situ diffuse reflectance infrared Fourier transform spectroscopy. The formed adsorbates were observed to be morphology dependent. CO or CO₂ chemisorbed at the metal cation sites, and bidentate and bridged carbonates involving the O sites are sensitive to the surface composition and the local coordination environments of surface metal cations and O anions and can be correlated well with the surface structures of facets exposed on oxide nanocrystals. Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO₂. Carbonate species formed by CO chemisorption can probe the different facets of TiO₂. Adsorbed CO and carbonate species formed by CO chemisorption can probe the different facets of Cu₂O, and adsorbed CO₂ formed by CO₂ chemisorption can also probe the different facets of Cu₂O. These results demonstrate chemisorption of probing molecules as a convenient technique to identify surface structures of different facets of oxide nanocrystals and lay the foundations of surface structures for the fundamental understanding of catalysis and other surface-mediated functions of CeO₂, TiO₂, and Cu₂O nanocrystals