Spectroscopic Investigation of Surface-Dependent Acid–Base Property of Ceria Nanoshapes

In addition to their well-known redox character, the acid–base property is another interesting aspect of ceria-based catalysts. Herein, the effect of surface structure on the acid–base property of ceria was studied in detail by utilizing ceria nanocrystals with different morphologies (cubes, octahedra, and rods) that exhibit crystallographically well-defined surface facets. The nature, type, strength, and amount of acid and base sites on these ceria nanoshapes were investigated via in situ IR spectroscopy combined with various probe molecules. Pyridine adsorption shows the presence of Lewis acid sites (Ce cations) on the ceria nanoshapes. These Lewis acid sites are relatively weak and similar in strength among the three nanoshapes according to the probing by both pyridine and acetonitrile. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the ceria nanoshapes, as probed by CO₂ adsorption. CO₂ and chloroform adsorption indicate that the strength and amount of the Lewis base sites are shape dependent: rods > cubes > octahedra. The weak and strong surface dependence of the acid and base sites, respectively, are a result of interplay between the surface structure dependent coordination unsaturation status of the Ce cations and O anions and the amount of defect sites on the three ceria nanoshapes. Furthermore, it was found that the nature of the acid–base sites of ceria can be impacted by impurities, such as Na and P residues that result from their use as structure-directing reagent in the hydrothermal synthesis of the ceria nanocrystals. This observation calls for precaution in interpreting the catalytic behavior of nanoshaped ceria where trace impurities may be present

Support Shape Effect in Metal Oxide Catalysis: Ceria-Nanoshape-Supported Vanadia Catalysts for Oxidative Dehydrogenation of Isobutane

The support effect has long been an intriguing topic in catalysis research. With the advancement of nanomaterial synthesis, the availability of faceted oxide nanocrystals provides the opportunity to gain unprecedented insights into the support effect by employing these well-structured nanocrystals. In this Letter, we show by utilizing ceria nanoshapes as supports for vanadium oxide that the shape of the support poses a profound effect on the catalytic performance of metal oxide catalysts. Specifically, the activation energy of VO_<i>x</i>/CeO₂ catalysts in oxidative dehydrogenation of isobutane was found to be dependent on the shape of ceria support, rods < octahedra, closely related to the surface oxygen vacancy formation energy and the numbe of defects of the two ceria supports with different crystallographic surface planes

Gold Nanoparticles Supported on Carbon Nitride: Influence of Surface Hydroxyls on Low Temperature Carbon Monoxide Oxidation

This paper reports the synthesis of 2.5 nm gold clusters on the oxygen free and chemically labile support carbon nitride (C₃N₄). Despite having small particle sizes and high enough water partial pressure these Au/C₃N₄ catalysts are inactive for the gas phase and liquid phase oxidation of carbon monoxide. The reason for the lack of activity is attributed to the lack of surface −OH groups on the C₃N₄. These OH groups are argued to be responsible for the activation of CO in the oxidation of CO. The importance of basic −OH groups explains the well documented dependence of support isoelectric point versus catalytic activity

Surface Structure Dependence of SO₂ Interaction with Ceria Nanocrystals with Well-Defined Surface Facets

The effects of the surface structure of ceria (CeO₂) on the nature, strength, and amount of species resulting from SO₂ adsorption were studied using in situ IR and Raman spectroscopies coupled with mass spectrometry, along with first-principles calculations based on density functional theory (DFT). CeO₂ nanocrystals with different morphologies, namely, rods (representing a defective structure), cubes (100 facet), and octahedra (111 facet), were used to represent different CeO₂ surface structures. IR and Raman spectroscopic studies showed that the structure and binding strength of adsorbed species from SO₂ depend on the shape of the CeO₂ nanocrystals. SO₂ adsorbs mainly as surface sulfites and sulfates at room temperature on CeO₂ rods, cubes, and octahedra that were either oxidatively or reductively pretreated. The formation of sulfites is more evident on CeO₂ octahedra, whereas surface sulfates are more prominent on CeO₂ rods and cubes. This is explained by the increasing reducibility of the surface oxygen in the order octahedra < cubes < rods. Bulk sulfites are also formed during SO₂ adsorption on reduced CeO₂ rods. The formation of surface sulfites and sulfates on CeO₂ cubes is in good agreement with our DFT results of SO₂ interactions with the CeO₂(100) surface. CeO₂ rods desorb SO₂ at higher temperatures than cubes and octahedra nanocrystals, but bulk sulfates are formed on CeO₂ rods and cubes after high-temperature desorption whereas only some surface sulfates/sulfites are left on octahedra. This difference is rationalized by the fact that CeO₂ rods have the highest surface basicity and largest amount of defects among the three nanocrystals, so they bind and react with SO₂ strongly and are the most degraded after SO₂ adsorption cycles. The fundamental understanding obtained in this work on the effects of the surface structure and defects on the interaction of SO₂ with CeO₂ provides insights for the design of more sulfur-resistant CeO₂-based catalysts

Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes

The kinetics and mechanism of methanol dehydration, redox, and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO₂ nanoplates and truncated-bipyramidal nanocrystals in order to understand the surface structure effect of TiO₂. The two TiO₂ nanoshapes displayed both (001) and (101) facets, with a higher fraction of the (001) facet exposed on the nanoplates, while truncated-bipyramidal nanocrystals were dominated by the (101) facet. A kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and nonequivalent in comparison to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions, while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO₂ surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C–H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the 5-fold-coordinated Ti cations on the (101) surface, similar to the mechanism reported on the (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD₃OH and CD₃OD), confirming that the cleavage of the C–H bond is the rate-limiting step for the formation of these products. A comparison between estimated kinetic parameters for methanol dehydration over various TiO₂ nanocrystals suggests that (001) has a higher dehydration reactivity in comparison to (101), but the surface density of active sites could be limited by the presence of residual fluorine atoms originating from the synthesis. The (001) surface of TiO₂ is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) in comparison to the (101) surface

Uranium Adsorbent Fibers Prepared by Atom-Transfer Radical Polymerization from Chlorinated Polypropylene and Polyethylene Trunk Fibers

Seawater contains a large amount of uranium (∼4.5 billion tons) which can serve as a nearly limitless supply for an energy source. However, to make the recovery of uranium from seawater economically feasible, lower manufacturing and deployment costs are desirable, and good solid adsorbents must have high uranium uptake, reusability, and high selectivity toward uranium. In this study, atom-transfer radical polymerization (ATRP), without the high-cost radiation-induced graft polymerization, was used for grafting acrylonitrile and <i>tert</i>-butyl acrylate from a new class of trunk fibers, forming adsorbents in a readily deployable form. The new class of trunk fibers was prepared by the chlorination of polypropylene (PP) round fiber, hollow-gear PP fiber, and hollow-gear polyethylene fiber. During ATRP, degrees of grafting (d.g.) varied according to the structure of active chlorine sites on trunk fibers and ATRP conditions, and the d.g. as high as 2570% was obtained. Resulting adsorbent fibers were evaluated in U-spiked simulated seawater, and the maximum adsorption capacity of 146.6 g U/kg, much higher than that of a standard adsorbent Japan Atomic Energy Agency fiber (75.1 g/kg), was obtained. This new type of trunk fiber can be used for grafting a variety of uranium-interacting ligands, including designed ligands that are highly selective toward uranium

Uranium Adsorbent Fibers Prepared by Atom-Transfer Radical Polymerization (ATRP) from Poly(vinyl chloride)-<i>co</i>-chlorinated Poly(vinyl chloride) (PVC-<i>co</i>-CPVC) Fiber

The need to secure future supplies of energy attracts researchers in several countries to a vast resource of nuclear energy fuel: uranium in seawater (estimated at 4.5 billion tons in seawater). In this study, we developed effective adsorbent fibers for the recovery of uranium from seawater via atom-transfer radical polymerization (ATRP) from a poly­(vinyl chloride)-<i>co</i>-chlorinated poly­(vinyl chloride) (PVC-<i>co</i>-CPVC) fiber. ATRP was employed in the surface graft polymerization of acrylonitrile (AN) and <i>tert</i>-butyl acrylate (<i>t</i>BA), precursors for uranium-interacting functional groups, from PVC-<i>co</i>-CPVC fiber. The [<i>t</i>BA]/[AN] was systematically varied to identify the optimal ratio between hydrophilic groups (from <i>t</i>BA) and uranyl-binding ligands (from AN). The best performing adsorbent fiber, the one with the optimal [<i>t</i>BA]/[AN] ratio and a high degree of grafting (1390%), demonstrated uranium adsorption capacities that are significantly greater than those of the Japan Atomic Energy Agency (JAEA) reference fiber in natural seawater tests (2.42–3.24 g/kg in 42 days of seawater exposure and 5.22 g/kg in 49 days of seawater exposure, versus 1.66 g/kg in 42 days of seawater exposure and 1.71 g/kg in 49 days of seawater exposure for JAEA). Adsorption of other metal ions from seawater and their corresponding kinetics were also studied. The grafting of alternative monomers for the recovery of uranium from seawater is now under development by this versatile technique of ATRP

Facile Synthesis of Highly Porous Metal Oxides by Mechanochemical Nanocasting

Metal oxides with high porosity usually exhibit better performance in many applications, as compared with the corresponding bulk materials. Template-assisted method is generally employed to prepare porous metal oxides. However, the template-assisted method is commonly operated in wet conditions, which requires solvents, soluble metal oxide precursors, and a long time for drying. To overwhelm those drawbacks of the wet procedure, a mechanochemical nanocasting method is developed in the current work. Inspired by solid-state synthesis, this strategy proceeds without solvents, and the ball milling process can enable pores replicated in a shorter time (60 min). By this method, a series of highly porous metal oxides were obtained, with several cases approaching the corresponding surface area records (e.g., ZrO₂, 293 m² g^–1; Fe₂O₃, 163 m² g^–1; CeO₂, 211 m² g^–1; CuO_<i>x</i>-CeO_<i>y</i> catalyst, 237 m² g^–1; CuO_<i>x</i>-CoO_<i>y</i>-CeO_<i>z</i> catalyst, 203 m² g^–1). Abundant nanopores with clear lattice fringes in metal oxide products were witnessed by scanning transmission electron microscopy (STEM) in high angle annular dark field (HAADF). By combination of mechanochemical synthesis and nanocasting, current technology provides a general and simple pathway to porous metal oxides

Toward the Design of a Hierarchical Perovskite Support: Ultra-Sintering-Resistant Gold Nanocatalysts for CO Oxidation

An ultrastable Au nanocatalyst based on a heterostructured perovskite support with high surface area and uniform LaFeO₃ nanocoatings was successfully synthesized and tested for CO oxidation. Strikingly, small Au nanoparticles (4–6 nm) are obtained after calcination in air at 700 °C and under reaction conditions. The designed Au catalyst not only possessed extreme sintering resistance but also showed high catalytic activity and stability because of the strong interfacial interaction between Au and the heterostructured perovskite support