CeO2 Cubes Research Articles

Promoted oxygen adsorption on porous CeO2 cubes with abundant oxygen vacancies for efficient gaseous formaldehyde removal

Photocatalytic degradation stands as a promising method for eliminating gas-phase pollutants, with the efficiency largely hinging on the capture of photogenerated electrons by oxygen. In this work, we synthesized a porous CeO2 single crystal cube with abundant oxygen vacancies as photocatalyst, employing urea as a pore-forming agent and for gas-phase formaldehyde degradation. Compared with the CeO2 cubes without pores, the porous ones were superior in specific surface area, akin to conventional CeO2 nanoparticles. The photocatalytic degradation for gas-phase formaldehyde on porous CeO2 cubes was significantly accelerated, of which degradation rate is 3.3 times and 2.1 times that of CeO2 cubes without pores and CeO2 nanoparticles, respectively. Photoelectric tests and DFT calculations revealed that this enhancement stemmed from facilitated oxygen adsorption due to pronounced oxygen vacancies. Consequently, the capture of photoelectrons by oxygen was promoted and its recombination with holes was suppressed, along with an accelerated generation of curial free radicals such as ·OH. This work reveals the pivotal role of surface oxygen vacancies in promoting adsorbed oxygen, proposing a viable strategy to enhance the photocatalytic degradation efficiency for gas-phase pollutants.

Facet-dependent haloperoxidase-like activities of CeO2 nanoparticles contribute to their excellent biofilm formation suppression abilities

Biofilms adhering to different surfaces have significant negative impacts in various fields. Cerium oxide nanoparticles can serve as mimics of haloperoxidase for biological biofilm inhibition applications. The regulation of the exposed facet of CeO2 nanoparticles influences their efficiency in various catalytic processes. However, there is still a lack of systematic studies on the facet-dependent haloperoxidase-like activity of CeO2. In the present study, the facet-dependent haloperoxidase activities and antibiofilm performance of CeO2 nanoparticles were elucidated through experiment analysis and density function theory calculation. The as-prepared CeO2 nanoparticles inhibited bacterial survival and catalyzed the oxidative bromination of quorum sensing signaling molecules, achieving biofilm inhibition performance. The antibacterial and biofilm formation suppression abilities were consistent with their haloperoxidase activities. The {111}- and {110}-facet CeO2 nanopolyhedra, as well as the {110}- and {100}-facet CeO2 nanorods, which had higher haloperoxidase activity showed better antibiofilm performance than the {100}-facet CeO2 cubes. The present findings provide a comprehensive understanding of the facet-dependent haloperoxidase-like activity of CeO2. Furthermore, engineering CeO2 morphologies with different crystal facets may represent a novel method for significantly adjusting their haloperoxidase-like activity.

A Detailed Insight into the Effects of Morphologies of Cerium Oxide on Fenton-like Reactions for Different Applications.

As an exceptional Fenton-like reagent, cerium oxide (CeO2 ) finds applications in biomedical science and organic pollutants treatment. The Fenton-like reaction catalyzed by CeO2 typically encompasses two distinct processes: one resembling the classical Fenton reaction, wherein cerium (Ce3+ ) triggers the decomposition of hydrogen peroxide (H2 O2 ) to yield reactive oxygen species (ROS), and the other involves the complexation of H2 O2 on the Ce3+ surface, leading to the formation of peroxides. However, the influence of diverse CeO2 morphologies on these two reaction pathways has not been comprehensively explored. In this study, CeO2 exhibiting three typical morphologies, rods, cubes, and spheres, were prepared. The generation of ROS and peroxides was evaluated using the 3,3,5,5-tetramethylbenzidine (TMB) oxidation reaction and the reduction current of H2 O2 , respectively. Moreover, the impacts of pH variations and CeO2 /H2 O2 concentrations on the production and conversion of these two reaction products were investigated. To corroborate the distinctions between the resultant products and their applicability, apoptosis assays and acid orange 7 (AO7) degradation analyses were performed. Notably, CeO2 rods exhibited the highest proportion of Ce3+ , predominantly engaging in complexation with H2 O2 to foster peroxide formation, thereby facilitating the robust degradation of AO7. However, the generated peroxides appeared to occupy Ce3+ sites, thereby impeding the H2 O2 decomposition process. Conversely, Ce3+ species on the surface of CeO2 cubes were primarily involved in H2 O2 decomposition, leading to heightened ROS production, and thus showcasing substantial potential for damaging A549 tumor cells. It is worth noting that the ability of these Ce3+ species to form peroxides through complexation with H2 O2 was comparatively reduced. In summation, this study sheds light on the intricate interplay between distinct CeO2 morphologies and their divergent impacts on Fenton-like reactions. These findings expand our comprehension of the influences on its reactivity of CeO2 morphologies and open new insights for applications in diverse domains, from organic dye degradation to tumor therapy.

Enhancing catalytic performance and hot electron generation through engineering metal-oxide and oxide-oxide interfaces

Interfaces are of utmost importance in catalytic reactions, influencing reaction kinetics and electron transfer processes. However, investigations in combined interfaces of metal-oxide and oxide-oxide at heterogeneous catalysts still have challenges due to their complex structure. Herein, we synthesized well-defined Co3O4 and CeO2 cubes with distinct facets and investigated their catalytic performance when deposited on a Pt-thin film, focusing on the influence of metal-oxide and oxide-oxide interfaces. Catalytic measurements demonstrated that the CeO2/Pt interface significantly enhanced turnover frequency (TOF) and selectivity for partial methanol oxidation compared to Co3O4/Pt and bare Pt. Notably, the CeO2/Co3O4/Pt nanodevice exhibited improved partial oxidation selectivity, highlighting the role of the CeO2/Co3O4 interface in methyl formate production. Chemicurrent measurements demonstrate enhanced hot electron generation due to increased overall TOF and partial oxidation production. We also conducted near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) analysis, revealing a higher concentration of Ce3+ ions and increased oxygen vacancies in the CeO2/Co3O4/Pt catalyst, suggesting oxygen migration from CeO2 to Co3O4, leading to methoxy species stabilization and promoting methyl formate formation.

Investigation on morphologies and supporter of cerium dioxide nanostructure on oxime based electrochemical sensors for organophosphorus detection

Pralidoxime (PAM) chloride has been taken as be an effective electrochemical probe to detect organophosphorus pesticides (OPs). In our previous work, CeO2 nanoparticles were proved to be able to significantly improve the sensitivity of the as-fabricated electrochemical sensor. In this paper, PAM-based sensors with different morphologies and supporters of nano-structured CeO2 were investigated. Firstly, CeO2 particles, CeO2 cubes and CeO2 rods were prepared and characterized. The results show that the CeO2 rods exhibits the best electrochemical performance because of its larger specific surface and better electrocatalytic activity. Secondly, multi-walled carbon nanotubes (MWCNTs), carbon powder (CP) and Carbon aerogel (CA) were adopted as supporter to disperse CeO2 rods. The results show that CA with large specific surface area and excellent adsorption property makes the fabricated sensor exhibit the best performance. Under the optimized conditions, the sensor has a chlorpyrifos detecting limit of 3.4 × 10−12 M in the range from 5 × 10−11 M to 1 × 10−7 M. Besides, the sensor also shows good repeatability, stability and reliability in the detection of chlorpyrifos.

Nanoshaped Cerium Oxide with Nickel as a Non-Noble Metal Catalyst for CO2 Thermochemical Reactions

Four different nanoshapes of cerium dioxide have been prepared (polycrystals, rods, cubes, and octahedra) and have been decorated with different metals (Ru, Pd, Au, Pt, Cu, and Ni) by incipient wetness impregnation (IWI) and ball milling (BM) methods. After an initial analysis based on oxygen consumption from CO2 pulse chemisorption, Ni-like metal, and two forms of CeO2 cubes and rods were selected for further research. Catalysts were characterized using the Brunauer-Emmett-Teller formula (BET), X-ray spectroscopy (XRD), Raman spectroscopy, scanning electron microscopy (SEM), UV-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (H2-TPR) and CO2 pulse chemisorption, and used to reduce of CO2 into CO (CO2 splitting). Adding metals to cerium dioxide enhanced the ability of CeO2 to release oxygen and concomitant reactivity toward the reduction of CO2. The effect of the metal precursor and concentration were evaluated. The highest CO2 splitting value was achieved for 2% Ni/CeO2-rods prepared by ball milling using Ni nitrate (412 µmol/gcat) and the H2 consumption (453.2 µmol/gcat) confirms the good redox ability of this catalyst.

CO2 Activation over Nanoshaped CeO2 Decorated with Nickel for Low-Temperature Methane Dry Reforming.

Dry reforming of methane (DRM) is a promising way to convert methane and carbon dioxide into H2 and CO (syngas). CeO2 nanorods, nanocubes, and nanospheres were decorated with 1–4 wt % Ni. The materials were structurally characterized using TEM and in situ XANES/EXAFS. The CO2 activation was analyzed by DFT and temperature-programmed techniques combined with MS-DRIFTS. Synthesized CeO2 morphologies expose {111} and {100} terminating facets, varying the strength of the CO2 interaction and redox properties, which influence the CO2 activation. Temperature-programmed CO2 DRIFTS analysis revealed that under hydrogen-lean conditions mono- and bidentate carbonates are hydrogenated to formate intermediates, which decompose to H2O and CO. In excess hydrogen, methane is the preferred reaction product. The CeO2 cubes favor the formation of a polydentate carbonate species, which is an inert spectator during DRM at 500 °C. Polydentate covers a considerable fraction of ceria’s surface, resulting in less-abundant surface sites for CO2 dissociation.

Facet-Dependent selectivity of CeO2 nanoparticles in 2-Propanol conversion

CeO2 nanoshapes, cubes with dominant (100) facets and octahedra with dominant (111) facets, were synthesized to investigate the influence of surface structure on acid-base properties. An optimization of calcination temperatures, coupled with Raman and TEM, was employed to minimize the intrinsic (Frenkel-type) defect sites and their potential complications on the facet studies involved in this work. The acid-base properties of these CeO2 nanoshapes were characterized with in situ pyridine and CO2 adsorption using infrared spectroscopy and quantified using pyridine, ammonia, and CO2 temperature-programmed desorption (TPD). The (100) facet displayed weaker acid sites with lower site density than the (111) facet which had a similar density of base sites but those on the (100) facet were stronger. A strong correlation was observed between 2-propanol conversion and each facet’s acid-base properties. CeO2 cubes exhibited greater base-site catalyzed dehydrogenation to acetone while the more acidic CeO2 octahedra were more active in dehydration to propene.

In-situ construction of ceria-metal/titanate heterostructure with controllable architectures for efficient fuel electrochemical conversion

Construction of ceria-metal/titanate heterostructure via exsolution is a promising strategy to improve the catalytic activity of titanate perovskites and broaden their applications in various energy conversion scenarios. However, the species exsolved after reduction are limited to reducible metal cations, such as Ni, Co, Fe, and precious metals. Herein, we report a modified exsolution approach for co-exsolving active oxides and metal nanoparticles from a titanate perovskite, La0.8Ce0.1Ni0.4Ti0.6O3-δ (LCeNT). We highlight strained facet-specific CeO2 cubes can be grown on the support after an air-annealing process with their morphology tunable by varying annealing temperature, whilst exsolution of Ni nanoparticles form subsequently following chemical/electrical reduction. An electrolyte-supported SOFC utilizing CeO2-Ni@LCeNT anode achieves maximum power density of 642 mW cm−2 at 900 ℃ in H2 (∼3% H2O). Exceptional robustness of the heterostructure is illustrated after running the cell in CH4 (∼3% H2O) for 20 h. Overall, this work demonstrates an intriguing pathway to constructing stable and active ceria-metal/titanate heterostructure for energy applications.

Catalytic performance and mechanistic evaluation of sulfated CeO2 cubes for selective catalytic reduction of NOx with ammonia

Sulfated CeO2 cubes were prepared by the impregnation of CeO2 cubes by ammonium sulfates, and further evaluated in selective catalytic reduction of NOx with ammonia (NH3-SCR). Catalytic activity tests indicated that NOx reduction conversions and N2 selectivity of sulfated CeO2 cubes could be significantly improved compared to pure CeO2 cubes. The synthesized sulfated CeO2 cubes were further characterized by atom-resolved high angle annular dark-field (HAADF) imaging, Fourier-transform infrared spectroscopy (FTIR) by pyridine adsorption, and temperature-programmed reduction by H2 (H2-TPR). The characterization results showed that sulfates were primarily dispersed through the corners, edges, and surfaces of CeO2 cubes, and did not significantly affect the crystal structures of CeO2 cubes. Sulfation treatment could create and strengthen Brønsted acid sites originated from the protons on surface sulfates, further facilitating ammonia adsorption and activation. The kinetic data indicated that the apparent reaction order of NO, O2, and NH3 was 0.95 to 1.01, −0.01 to 0.00, and −0.18 to −0.15, respectively. It could speculate that gaseous phase NO involving in NO catalytic oxidation was the rate-determining step over sulfated CeO2 cubes for NH3-SCR reaction. The presence of NH3 slightly inhibited the SCR reaction rate due to the competitive adsorption blocking NO oxidation sites.

Growth of CeO2 nanocubes showing size-dependent optical and oxygen evolution reaction behaviors.

CeO2 nanocubes with average sizes of 9, 13, and 18 nm have been synthesized by preparing a slightly basic aqueous mixture of Ce(NO3)3, Na2SO4, and NH4OH and heating the solution to 100 to 150 °C in 4 or 9 h. The nanocubes possess high crystalline quality. Their band gaps decrease gradually beyond the quantum confinement regime from 3.57 eV to 3.45 eV with increasing particle sizes. The 9 nm CeO2 nanocubes have the most positive valence band energy and correspondingly they exhibit the best electrochemical oxygen evolution reaction activity. Since band gaps of semiconductor nanocrystals can be tuned substantially through particle size control to yield different band energies, this fact can be utilized to enhance the electrochemical and photocatalytic properties.

Catalytic Conversion of Benzyl Alcohol to Benzaldehyde over a Cu-CeO2 Catalyst: Utilization of CO2 for Enhancement of Selectivity and Activity

CO2-assisted dehydrogenation of benzyl alcohol to benzaldehyde over Cu nanoparticles dispersed on CeO2 was reported. Cu nanoparticles with an average size of ∼11.4 nm dispersed over CeO2 cubes were...

Morphology-dependent CeO2 catalysis in acetylene semihydrogenation reaction

We herein investigated the catalytic performances and reaction mechanisms of CeO2 cubes (denoted as c-CeO2), CeO2 rods calcined at 500 °C (denoted as r-CeO2-500), and CeO2 rods calcined at 700 °C (denoted as r-CeO2-700) in the acetylene semihydrogenation reaction. r-CeO2-500 is most catalytically active while r-CeO2-700 is least. The acetylene semihydrogenation reaction catalyzed by various CeO2 nanocrystals was observed to follow the same reaction mechanism. C2H2 adsorbs at both O and Ce sites on CeO2 surfaces to form C2H2(a) species that facilely hydrogenates stepwise to C2H3(a) and C2H4(a). The surface hydrogenation reactions proceed more facilely at the O site than at the Ce site. The desorption of C2H4(a) proceeds slowly and is the rate-limiting step. c-CeO2, r-CeO2-500 and r-CeO2-700 do not exhibit obvious differences among these elementary surface reactions, but show much different abilities to dissociate H2 and chemisorb C2H2. The C2H2(a) coverage follows an order of r-CeO2-500 ≫ c-CeO2 ≈ r-CeO2-700. The ability to dissociate H2 follows an order of c-CeO2 > r-CeO2-500 ≫ r-CeO2-700. The observed morphology-dependent behaviors of various CeO2 nanocrystals were correlated to their defects and exposed facets. These results successfully demonstrate morphology-dependent CeO2 catalysis in acetylene semihydrogenation reaction and elucidate the underlying mechanisms.

Shape dependence and sulfate promotion of CeO2 for selective catalytic reduction of NOx with NH3

Thermally stable CeO2 cubes and nanospheres were synthesized and modified by sulfation treatment. Non-sulfated CeO2 cubes and nanospheres did not exhibit good catalytic performance for selective catalytic reduction of NOx with NH3 (NH3-SCR), but mainly contributed towards an undesired side reaction of NH3 oxidation above 350 °C. Based on the structural, morphological and physicochemical characterization, it was observed that CeO2 nanospheres comprising small crystallites could easily release active oxygen species, which resulted in strong NH3 oxidation. However, sulfation treatment greatly improved the catalytic performance of NH3-SCR on both CeO2 cubes and nanospheres. Sulfated CeO2 catalysts did not contribute to significant NH3 oxidation due to the inhibited reducibility of Ce4+ coordinated with the surface sulfates. The adsorbed ammonia could be activated on Brønsted acid sites generated by the formation of surface Ce2(SO4)3 species, while gaseous NOx could be activated on separate surface sites of Ce4+. The presence of separate reaction sites for NH3 and NOx is believed to be important for the improved catalytic performance of SCR reaction. Sulfated CeO2 cubes outperformed sulfated CeO2 nanospheres in the entire test temperature window (200–500 °C). The improved performance of sulfated CeO2 cubes appears to be related to surfaces with abundant Brønsted acid sites and relatively weak reducibility of Ce4+. These fundamental findings contribute to a better mechanistic understanding needed for designing efficient CeO2-based NOx reduction catalysts in the future.

CO oxidation over ceria supported Au22 nanoclusters: Shape effect of the support

Gold (Au) nanoclusters have recently emerged as ideal models for understanding Au catalysis, because the nanosized Au particles have precise atomic numbers and uniform size. In this work, we studied for the first time the support shape effect on the catalysis of Au nanoclusters by using CO oxidation as a model reaction. Au22(L8)6 (L=1,8-bis(diphenylphosphino) octane) nanoclusters were supported on CeO2 rods or cubes, then pretreated at different temperatures (up to 673K), allowing the gradual removal of the organic phosphine ligands. CO oxidation test over these differently pretreated samples shows that CeO2 rods are much better supports than cubes for Au22 nanoclusters in enhancing the reaction rate. In situ IR spectroscopy coupled with CO adsorption indicates that the shape of CeO2 support can impact the nature and quantity of exposed Au sites, as well as the efficiency of organic ligand removal. Although CeO2 rods are helpful in exposing a greater percentage of total Au sites upon ligands removal, the percentage of active Au sites (denoted by Auδ+, 0<δ<1) is lower than that on CeO2 cubes. The in situ extended X-ray absorption spectroscopy (EXAFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) results show that the Au nanoclusters bound more strongly to the CeO2 rods than to the cubes where the Au nanoclusters show more sintering. Considering the typical redox mechanism for CO oxidation over supported Au nanoclusters and nanoparticles, it is concluded that the reactivity of the lattice oxygen of CeO2 is the determining factor for CO oxidation over Au22/CeO2. CeO2 rods offer more reactive lattice oxygen and abundant oxygen vacancies than the cubes and thus make the rods a superior support for Au nanoclusters in catalyzing low temperature CO oxidation.

Oxygen vacancy clusters essential for the catalytic activity of CeO2 nanocubes for o-xylene oxidation

Catalytic oxidation of o-xylene was investigated on CeO2 nanocubes calcined at 350, 450, 550, and 650 °C, among which the samples calcined at 550 °C exhibited the highest activity and long durability. Positron annihilation spectroscopy measurements revealed that the size and distribution of oxygen vacancies for CeO2 nanocubes could be tuned by carefully controlling the calcination temperature. An excellent linear correlation between a factor related to size and density of oxygen vacancy clusters and reaction rate of o-xylene oxidation was revealed on ceria nanocubes. This means that oxygen vacancy clusters with suitable size and distribution are responsible for catalytic reaction via simultaneous adsorption and activation of oxygen and o-xylene. Electron spin resonance spectra revealed that over the CeO2 cubes, water vapor significantly promoted the formation of ∙OH radicals with a sharp decrease in the signals relating to oxygen vacancies, accelerating the transformation of o-xylene to the intermediate benzoate species, resulting in an enhancement of catalytic activity. Water thus serves as a “smart” molecule; its introduction into the feed mixture further confirmed the key role of oxygen vacancies in the catalytic performance of CeO2 nanocubes. A possible mechanism of oxygen vacancy formation during the calcination process was also proposed.

CeO2 morphology-dependent NbOx –CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts in oxidative dehydrogenation of propane

A series of NbOx/CeO2 catalysts were synthesized employing CeO2 cubes (c-CeO2), CeO2 rods calcined at 500°C (r-CeO2-500) and CeO2 rods calcined at 700°C (r-CeO2-700) as the supports, and their structures and catalytic performances in the oxidative dehydrogenation of propane reaction were studied. Strong CeO2 morphology-dependent NbOx –CeO2 interaction, structure and catalytic performance of NbOx/CeO2 catalysts were observed. The supported NbOx species evolve with the increasing Nb loading from the monomeric Nb species to the polymeric Nb species and the CeNbO4 species. The monomeric Nb5+ species interacting with oxygen vacancy/Ce3+ on CeO2 can be reduced to form the monomeric Nb4+ species. The formed monomeric Nb species follows the order of NbOx/r-CeO2–500>NbOx/c-CeO2>NbOx/r-CeO2-700 while the formed monomeric Nb4+ species is most abundant on the c-CeO2 support. The loading of NbOx suppresses the surface reduction of c-CeO2 and r-CeO2-500 but promotes the surface reduction of r-CeO2-700. R-CeO2-500 and r-CeO2-700 are more catalytic active than c-CeO2 in the oxidative dehydrogenation of propane reaction. The loading of NbOx slightly enhances the catalytic activity of c-CeO2 but decreases the catalytic activity of r-CeO2-700. A loading of 0.6 Nb/nm2 much enhances the catalytic activity of r-CeO2-500 at 200°C, but further increasing the Nb loading results in the decrease of the C3H8 conversion. These results add a solid example to vindicate the morphology engineering strategy to modify the structure and catalytic performance of CeO2-based catalysts.

Designing CuOx Nanoparticle-Decorated CeO2 Nanocubes for Catalytic Soot Oxidation: Role of the Nanointerface in the Catalytic Performance of Heterostructured Nanomaterials

This work investigates the structure-activity properties of CuOx-decorated CeO2 nanocubes with a meticulous scrutiny on the role of the CuOx/CeO2 nanointerface in the catalytic oxidation of diesel soot, a critical environmental problem all over the world. For this, a systematic characterization of the materials has been undertaken using transmission electron microscopy (TEM), transmission electron microscopy-energy-dispersive X-ray spectroscopy (TEM-EDS), high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM), scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS), X-ray diffraction (XRD), Raman, N2 adsorption-desorption, and X-ray photoelectron spectroscopy (XPS) techniques. The TEM images show the formation of nanosized CeO2 cubes (∼25 nm) and CuOx nanoparticles (∼8.5 nm). The TEM-EDS elemental mapping images reveal the uniform decoration of CuOx nanoparticles on CeO2 nanocubes. The XPS and Raman studies show that the decoration of CuOx on CeO2 nanocubes leads to improved structural defects, such as higher concentrations of Ce(3+) ions and abundant oxygen vacancies. It was found that CuOx-decorated CeO2 nanocubes efficiently catalyze soot oxidation at a much lower temperature (T50 = 646 K, temperature at which 50% soot conversion is achieved) compared to that of pristine CeO2 nanocubes (T50 = 725 K) under tight contact conditions. Similarly, a huge 91 K difference in the T50 values of CuOx/CeO2 (T50 = 744 K) and pristine CeO2 (T50 = 835 K) was found in the loose-contact soot oxidation studies. The superior catalytic performance of CuOx-decorated CeO2 nanocubes is mainly attributed to the improved redox efficiency of CeO2 at the nanointerface sites of CuOx-CeO2, as evidenced by Ce M5,4 EELS analysis, supported by XRD, Raman, and XPS studies, a clear proof for the role of nanointerfaces in the performance of heterostructured nanocatalysts.

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.

The effects of morphology of cerium oxide catalysts for dehydrogenation of ethylbenzene to styrene

Ethylbenzene (EB) dehydrogenation in presence and absence of CO2 was investigated over CeO2 catalysts of distinct morphologies: cubes, rods and particles. The presence of CO2 resulted in prolonged catalyst activity and higher initial benzene selectivity compared to the absence of CO2. However, CO2 had no effect on the catalytic activity of ceria catalysts after stabilization. CeO2 cubes exhibited about two times higher activity per m2 compared to rods and particles, independent of presence or absence of CO2. Product distribution and Raman spectroscopy characterization of the spent catalysts suggested that superior catalytic activity of CeO2 cubes originates from the enhanced amount of lattice oxygen species that can be extracted by EB on the (100) crystal planes of ceria cubes, contrary to highly intrinsically stable (111) crystal planes predominantly exposed on rods and particles. We suggest that these O species are consumed in nonselective EB conversion pathways, generating partly-reduced surface sites, which are suggested to act as active sites for selective styrene formation.