Ceria Nanocubes Research Articles

Ceria Nanocubes: Dependence of the Electronic Structure on Synthetic and Experimental Conditions

The properties of nanostructured ceria are both size- and shape-dependent. Here, ∼3–10 nm ceria nanocubes having exposed (002) surfaces were hydrothermally synthesized. For the first time, the electronic properties of such nanocubes were systematically studied using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Comparisons of the collected Ce 3d XPS spectra after a single sweep revealed that the concentration of Ce3+ present within the nanocubes is independent of the particle size and the Ce precursor used. Ce 3d spectra collected after averaging 20 sweeps demonstrated the reduction of Ce4+, and that the degree of reduction was also independent of the synthetic conditions. This observed surface reduction/damage is mainly the result of exposure to high intensity X-ray radiation. These conclusions were further confirmed through examination of Ce M4,5- and N4,5-edge XANES spectra. On the basis of these results, it is clear that the concentration of Ce3+ on the surface of ceria nanocubes...

Controlled synthesis of ceria nanoparticles for the design of nanohybrids

Ceria nanoparticles were synthesized from reaction mixture of cerium nitrate/hexamethylenediamine/water–ethylene glycol. Lamellar, particle-aggregated array, platelet, rice, cube, quasi-sphere shapes of the ceria nanoparticles can be controlled by tuning reaction parameters (reagent concentration, reagent components, pH, and reaction conditions). Studies on shape-dependent catalysis of the bare ceria samples toward CO oxidation indicated that the cube-shaped ceria nanoparticles show better catalytic activity than the nanospheres and the commercial micropowders. As capped by hexamethylenediamine (HEA) molecules, amine-functionalized ceria nanoparticles act as platforms for depositing copper particles to produce efficient Cu/CeO2 hybrid nanocatalysts for CO conversion. Coupling of the copper clusters with the HEA-capped ceria nanocubes was achieved with the Cu contents up to 15wt.%. The Cu/CeO2 nanohybrids show an enhanced catalytic efficiency of low temperature CO conversion. This could be due to high exposure of the reactive {100} facets in the ceria nanocubes and interfacial copper–ceria interactions.

Preferential oxidation of CO in excess H2 over CuO/CeO2 catalysts: Characterization and performance as a function of the exposed face present in the CeO2 support

A series of oxidised copper-cerium nanostructured catalysts prepared by impregnation of copper over ceria supports synthesized by different methods (hydrothermal with varying preparation parameters, microemulsion/precipitation), in order to achieve different specific morphologies (nanocubes, nanorods and nanospheres), have been examined with respect to their catalytic properties for preferential oxidation of CO in excess H2 (CO-PROX). The catalysts have been characterized in detail by XRD, Raman, SBET measurement, HREM, XPS, TPR and EPR, which allows establishing a model of structural characteristics of the catalysts. The characterization results have been correlated with analysis of CO-PROX catalytic properties by means of catalytic activity measurements complemented by operando-DRIFTS. Structural dependence of the CO oxidation reaction on the dispersed copper oxide entities as a function of the exposed face present at the surface of the different ceria supports is revealed. An important overall enhancement of the CO-PROX performance is detected for the sample supported on ceria nanocubes which is proposed to be a consequence of the interaction between copper oxide and (100) faces of the ceria support.

Catalysis oxidation of 1,2-dichloroethane and ethyl acetate over ceria nanocrystals with well-defined crystal planes

The ceria nanorods, nanocubes and nano-octahedrons exposed crystal planes of (110)/(100), (100) and (111), respectively, are synthesized via simple hydrothermal method, and characterized by XRD, N2 adsorption, TEM/HRTEM, Raman, OSC and H2-TPR. Comparing with nanocubes and nano-octahedrons, the ceria nanorods showed higher surface area, smaller crystallite size, more oxygen vacancies, higher OSC and mobility of oxygen. The catalysis oxidation of DCE and EA was investigated over these ceria exposed well-defined crystal planes for the first time. The results indicated that ceria nanorods showed the highest catalysis activities, followed by nanocubes and nano-octahedrons. Moreover, a possible reaction mechanism for the catalytic oxidation of DCE over the ceria catalyst was proposed.

Tuning Growth Modes of Ceria-Based Nanocubes by a Hydrothermal Method

We control two growth modes, Ostwald ripening (OR) and oriented attachment (OA), to yield ceria nanocubes by a simple hydrothermal method, using an oleate modified precursor. Using a Ce(III) salt as an initial material, the OA mode is highly activated to form ceria nanocubes with well-developed (002) facets (ca. 8 nm), while OA growth is not allowed so that smaller nanocubes (ca. 5 nm) were formed if Ce(IV) salt is used for the synthesis. A combination analysis by X-ray photoelectron spectroscopy and Fourier transform IR indicates that the surface of ceria nanocubes synthesized with Ce(III) precursor have surface valence (Ce3+) sites coordinating with oleate through bidentate bonding. We propose that the difference in Ce3+ from the grain interior (Ce4+) provides the high surface energy needed to activate the OA growth mode. The luminescence spectra of ceria nanocubes grown by OA rather resemble those of a microcrystalline sample, suggesting the presence of a nonstoichiometric ceria phase, such as Ce2O3, a...

Effect of Alkali Metal Hydroxides on the Morphological Development and Optical Properties of Ceria Nanocubes Under Hydrothermal Conditions

Nanocrystalline cerium(LV) oxide (CeO2, ceria) particles were produced via the hydrothermal treatment of cerium nitrate hexahydrate with various alkali metal hydroxides (MOH: M = Li, Na, K). Experimental conditions such as [MOH], reaction temperature, and reaction time were studied. Particle morphology as well as size of crystallites was precisely controlled by choice of experimental conditions. While rod-shaped particles were obtained at 120 degrees C, well-defined nanocubes were formed at higher temperatures regardless of the choice of MOH. Examination of particle growth kinetics, in the final stages of crystallization, showed that particle growth rate is controlled by two different mechanisms. Grain boundary diffusion controls the particle growth in the presence of NaOH with an activation energy of 113.8 kj/mol and surface diffusion for LiOH ad KOH with the activation energy of 43.0-150.9 kj/mol, respectively. In addition, the particles exhibit strong violet and blue emissions at 400 nm and 370 nm. The former emission originates from excitation of a wide band gap of CeO2. The latter one is attributed to the trivalency of the cerium ion and appears to be sensitive to all the experimental conditions studied. Both extending reaction time and increasing temperature reduce the intensity of the 370 nm emission and increase the intensity of the 400 nm emission.

Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

The interaction of gold atoms with CeO2 nanocrystals having rod and cube shapes has been examined by cyanide leaching, TEM, TPR, CO IR and X-ray absorption spectroscopy. After deposition–precipitation and calcination of gold, these surfaces contain gold nanoparticles in the range 2–6 nm. For the ceria nanorods, a substantial amount of gold is present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis. These cations are stable against cyanide leaching in contrast to gold nanoparticles. Upon reduction the isolated Au atoms form finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced gold nanoparticles on ceria nanocubes exposing the {100} surface plane, it is inferred that the gold nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature in line with earlier findings (Y. Guan and E. J. M. Hensen, Phys Chem Chem Phys 11:9578, 2009), the presence of gold nanoparticles results in their more facile agglomeration, especially in the presence of water (e.g., WGS conditions). For liquid phase alcohol oxidation, metallic gold nanoparticles are the active sites. In the absence of a base, the O–H bond cleavage appears to be rate limiting, while this shifts to C–H bond activation after addition of NaOH. In the latter case, the gold nanoparticles on the surface of ceria nanocubes are much more active than those on the surface of nanorod ceria.

The base-free and selective oxidative transformation of 1,3-propanediol into methyl esters by different Au/CeO2 catalysts

Different Au/CeO2 catalysts, prepared by depositing gold on different facets of ceria nanocubes ({100}), nanorods ({110} and {100}) and nanopolyhedra ({111} and {100}), were separately characterized by means of XRD, N2 sorption, TPD and TPR. It was found that certain types of Au/CeO2 could selectively catalyze the oxidative transformation of 1,3-propanediol in methanol to methyl 3-hydroxypropionate, methyl 3-methoxypropionate, methyl acrylate or dimethyl malonate by molecular oxygen in the absence of any base. The selectivities of these Au/CeO2 catalysts depended on the shapes of the supporting CeO2 and the reaction temperature. The Au/CeO2 cube catalyst with less acidic and basic sites exhibited high selectivity towards methyl 3-hydroxypropionate (93.1% at 21.6% conversion). Comparatively, selectivities towards methyl acrylate (41.6% at 92.3% conversion) and methyl 3-methoxypropionate (40.2% at 92% conversion) increased using Au/CeO2 rod and polyhedron catalysts, which contained more acidic and basic sites than the cube catalyst. Moreover, we found the Au/CeO2 cube catalyst could be recycled without losing the gold nanoparticles.

Probing Defect Sites on CeO2 Nanocrystals with Well-Defined Surface Planes by Raman Spectroscopy and O2 Adsorption

Defect sites play an essential role in ceria catalysis. In this study, ceria nanocrystals with well-defined surface planes have been synthesized and utilized for studying defect sites with both Raman spectroscopy and O(2) adsorption. Ceria nanorods ({110} + {100}), nanocubes ({100}), and nano-octahedra ({111}) are employed to analyze the quantity and quality of defect sites on different ceria surfaces. On oxidized surfaces, nanorods have the most abundant intrinsic defect sites, followed by nanocubes and nano-octahedra. When reduced, the induced defect sites are more clustered on nanorods than on nanocubes, although similar amounts (based on surface area) of such defect sites are produced on the two surfaces. Very few defect sites can be generated on the nano-octahedra due to the least reducibility. These differences can be rationalized by the crystallographic surface terminations of the ceria nanocrystals. The different defect sites on these nanocrystals lead to the adsorption of different surface dioxygen species. Superoxide on one-electron defect sites and peroxide on two-electron defect sites with different clustering degree are identified on the ceria nanocrystals depending on their morphology. Furthermore, the stability and reactivity of these oxygen species are also found to be surface-dependent, which is of significance for ceria-catalyzed oxidation reactions.

Exclusively selective oxidation of toluene to benzaldehyde on ceria nanocubes by molecular oxygen

Well-defined ceria nanocubes covered by oleic acid with exposed {100} facets have been synthesized and exhibited exclusive selectivity for the oxidation of toluene to benzaldehydes in liquid phase by O(2).

Active gold species on cerium oxide nanoshapes for methanol steam reforming and the water gas shift reactions

We report that a small amount (<1 at%) of gold on ceria single crystals prepared as nanorods (10 ± 2.8 by 50–200 nm) of {110} and {100} crystal surfaces shows excellent catalytic activity in both the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions at low temperatures (<250 °C). The ceria nanorods bind and stabilize gold as atoms and clusters (<1 nm, TEM invisible). On the other hand, gold nanoparticles (∼3 nm) are found on the {100} surfaces of ceria nanocubes. Very low rates of SRM and WGS were measured on the Au–ceria {100} cubes, while the rates on Au–ceria {110} rods were at least an order of magnitude higher. However, the apparent activation energies did not depend on the shape of ceria. Strong bonded Aun–O–Ce species are the active sites and these are present only in negligible concentrations on the {100} surfaces. Thus, both reactions are structure-insensitive on Au–ceria. SRM proceeds through the methyl formate route. The Au–ceria {110} catalyst shows both high SRM activity and high selectivity to CO2 at temperatures below 250 °C.

Steam reforming of methanol over ceria and gold-ceria nanoshapes

In this work we have found that a small amount (∼1at.%) of gold deposited on ceria nanorods exhibits excellent catalytic activity for the low-temperature steam reforming of methanol (SRM). Gold clusters (<1nm, TEM invisible) dispersed on the {110} faces of ceria nanorods catalyze the reaction in a cooperative mechanism with ceria. Gold nanoparticles (∼3nm) on the {100} surfaces of ceria nanocubes are inactive. The apparent activation energy of the SRM reaction on Au-ceria is ∼110kJmol−1. On the basis of TPSR/MS analysis, we determined that the SRM reaction on Au-ceria involves methanol dehydrogenation, methyl formate hydrolysis and formic acid decomposition steps to produce CO2 and H2. Better than 97% catalyst selectivity to CO2 was found over the temperature range from 175 to 250°C. In the presence of methanol, the water–gas shift (WGS) reaction is suppressed and is not part of the mechanism at temperatures below 250°C. The SRM stability of the Au-ceria system is good for practical application of this type catalyst.

Single-crystal CeO2 nanocubes used for the direct electron transfer and electrocatalysis of horseradish peroxidase

A new horseradish peroxidase (HRP) third-generation electrochemical biosensor based on ceria nanocubes (CeO2-NCs) and chitosan (Chit) was developed. The single-crystalline, uniform and size-controlled CeO2-NCs have been synthesized by hydrothermal method. HRP was immobilized in CeO2-NCs and Chit film on the glass carbon electrode (HRP/CeO2/Chit/GCE). Compared with HRP-chitosan modified electrode (HRP/Chit/GCE), HRP/CeO2/Chit/GCE exhibited a pair of more obvious redox peaks at −0.348V (versus Ag/AgCl). Experimental results indicate CeO2-NCs greatly promoted the electron transfer between HRP and GCE. The immobilized HRP exhibited direct electrochemical behavior toward the reduction of hydrogen peroxide (H2O2). The resulting biosensor showed a linear range of 1–150μM and a detection limit of 0.26μM estimated at a signal-to-noise ratio of 3. Stability and reproducibility of the biosensor were also studied. The biological activity of HRP immobilizing in the composite film was characterized by UV–vis and FTIR spectra.

Shape-Selective Synthesis and Oxygen Storage Behavior of Ceria Nanopolyhedra, Nanorods, and Nanocubes

Single-crystalline and uniform nanopolyhedra, nanorods, and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at temperatures in the range of 100-180 degrees C under different NaOH concentrations, using Ce(NO3)3 as the cerium source. According to high-resolution transmission electron microscopy, they have different exposed crystal planes: {111} and {100} for polyhedra, {110} and {100} for rods, and {100} for cubes. During the synthesis, the formation of hexagonal Ce(OH)3 intermediate species and their transformation into CeO2 at elevated temperature, together with the base concentration, have been demonstrated as the key factors responsible for the shape evolution. Oxygen storage capacity (OSC) measurements at 400 degrees C revealed that the oxygen storage takes place both at the surface and in the bulk for the as-obtained CeO2 nanorods and nanocubes, but is restricted at the surface for the nanopolyhedra just like the bulk one, because the {100}/{110}-dominated surface structures are more reactive for CO oxidation than the {111}-dominated one. This result suggests that high OSC materials might be designed and obtained by shape-selective synthetic strategy.