Au CeO2 Research Articles

A comparative study of the deactivation mechanisms of the Au/CeO2 catalyst for water–gas shift under steady-state and shutdown/start-up conditions in realistic reformate

The deactivation mechanisms of the Au/CeO2 catalyst in steady-state and shutdown/start-up WGS reactions were investigated in realistic reformate at 250°C. Catalyst deactivation due to sintering was excluded. After steady-state operation, the original activity was not recovered by removing the deposited carbonate species. The influences of the component gases of realistic reformate on the activity suggest that loss of the Au–CeO2 interaction caused by the reducing H2 and CO is the main reason for catalyst deactivation. Under the shutdown/start-up condition, the catalyst suffered more drastic deactivation, although it underwent a lesser degree of reduction. A good correlation between the extent of deactivation and the amount of the carbonate species indicates that catalyst deactivation is mainly caused by enhanced formation of the carbonate species, especially through a combined effect of CO2 and H2O, during the low-temperature shutdown and start-up steps. The implications of these findings for improved applications of the Au/CeO2 catalyst in WGS are indicated.

Mesoporous-shelled CeO2 hollow nanospheres synthesized by a one-pot hydrothermal route and their catalytic performance

Uniform-sized and monodisperse CeO2 mesoporous hollow nanospheres composed of ceria nanocrystals were synthesized via a template-free and surfactant-assisted benign hydrothermal route. On the basis of a time-dependent experiment, the precipitation–dissolution–renucleation–assembly and Ostwald ripening processes mechanism is proposed for the formation of the CeO2 mesoporous hollow spheres. Poly(vinyl-pyrrolidone) (PVP) was applied as a surfactant to facilitate the reassembly of the CeO2 nanoparticles and the formation of the mesoporous shell of the hollow nanospheres. By increasing the amount of PVP, the mesoporous shell of the hollow spheres could be fabricated and became more apparent. Meanwhile, altering the amounts of H2O2 could control the size of the primary nanocrystals via affecting the nucleation/growth process of them, which is another key factor in the formation of the hollow structure and the uniform, monodispersed nanoparticles. The catalytic test results show that the as-obtained CeO2 mesoporous hollow spheres exhibited desirable CO catalytic properties. The synthesized Au–CeO2 nanospheres demonstrate a higher catalytic activity in CO oxidation than pure ceria nanospheres.

Effect of ceria structural properties on the catalytic activity of Au–CeO2 catalysts for WGS reaction

Two gold based catalysts supported on ceria prepared by different methods (urea gelation coprecipitation, UGC, and coprecipitation, CP) have been synthesized and tested in the WGS reaction, showing quite different catalytic behaviors. Interestingly, the two catalysts have the same gold loading (3 wt% Au was inserted by deposition-precipitation) and the FTIR spectroscopy of the adsorbed CO revealed the same amount of gold exposed sites. With the aim to elucidate how the preparation method affects the properties of the support, a morphological, structural and textural characterization has been performed by HRTEM, XRD, BET and Raman analyses, as well as FTIR spectroscopy to probe both the Au and the support exposed sites. It was found that the UGC method gave rise to an enhancement of the defectivity of ceria and to an increase of the reactivity under reductive treatment. Further FTIR measurements of adsorbed acetone demonstrated the presence of two kinds of Ce(4+) sites with different coordination, (CUS) Ce(4+) A and (CUS) Ce(4+) B, on the UGC sample. Such sites can influence the catalytic activity, possibly favoring the water dissociation, making ceria prepared by UGC a better support for Au catalysts than the CP-prepared one.

In situ growth of Au@CeO2core–shell nanoparticles and CeO2nanotubes from Ce(OH)CO3nanorods

Core–shell (CS) nanoparticles (NPs) have many applications in areas such as catalysis and sensing. The utilization of hollow nanostructured materials as the supports, such as nanotubes (NT), is a growing interest to anchor NPs. Generally, several steps are necessary to prepare CS NP–NT nanocomposites, including: (i) the synthesis of CS NPs; (ii) the preparation of NTs; and (iii) the combination of CS NPs and NTs. Moreover, surface modifications with organic ligands are often involved during the synthesis of CS NPs and/or the step combining CS NPs and supports. Here we report a facile method for in situ growth of Au@CeO2 CS NPs and CeO2 NTs by mixing HAuCl4 and Ce(OH)CO3 nanorods under mild conditions. The formation of Au–CeO2 nanocomposite is due to the interfacial oxidation–reduction reaction between HAuCl4 and Ce(OH)CO3, where Au(III) in HAuCl4 is reduced to Au(0) by Ce(III) in Ce(OH)CO3, while Ce(III) is oxidized into Ce(IV), followed by hydrolysis to generate CeO2. The slow hydrolysis rate of Ce(IV) leads to the coverage of CeO2 on the Au NPs, and on the residual Ce(OH)CO3 surface, developing into Au@CeO2 and Ce(OH)CO3@CeO2 CS structures. Further depletion/dissolution of Ce(OH)CO3 results in Au@CeO2 CS NP–CeO2 NT nanocomposite eventually. The advantages of our synthetic strategy are independent of foreign reducing agents and additional surface modification. And, such CS NP–NT nanocomposite can be obtained in one step, simplifying the synthesis procedures greatly. This method based on interfacial oxidation–reduction may be employed as a unique entry to other nanocomposites.

Catalytic removal of formaldehyde at room temperature over supported gold catalysts

Two kinds of Au/CeO2, prepared by deposition–precipitation (DP) using urea (U) or NaOH (N) as precipitants were investigated as catalysts for HCHO oxidation. H2-TPR and XPS techniques were used to characterize the Au/CeO2 samples. Due to the generation of increased amounts of active surface oxygen species resulting from the strong Au–CeO2 interaction, the Au/CeO2 (DPU) catalyst showed higher activity than the DPN catalyst, achieving 100% conversion of HCHO into CO2 and H2O at room temperature, even in the presence of water and at high GHSV (143,000h−1); moreover, the conversion was stable for at least 60h. The reaction mechanism and the rate limiting steps for HCHO oxidation over the Au/CeO2 catalysts were identified by means of in situ DRIFTS studies. The influence of oxygen and water on the formation and consumption of the formate reaction intermediates was also investigated. Results suggest that Au/CeO2 (DPU) is a promising catalyst for HCHO removal under real world conditions.

Facile synthesis of core–shell Au@CeO2 nanocomposites with remarkably enhanced catalytic activity for CO oxidation

Uniform Au@CeO2 core–shell submicrospheres, in which a Au nanoparticle core is coated with a shell composed of CeO2 nanoparticles, are easily synthesized by using hydrothermal and calcinating processes. When the Au@CeO2 core–shell submicrospheres are used for catalytic oxidation of CO to CO2, the full conversion temperature is decreased from over 300 °C to 155 °C with respect to the conventional supported Au–CeO2 catalysts. Furthermore, the Au@CeO2 core–shell submicrospheres show superior catalytic stability, and no deactivation occurs after 72 h reaction. The mechanisms of the growth and the high catalytic performance of Au@CeO2 core–shell submicrospheres are discussed in detail.

Water-soluble Au–CeO2 hybrid nanosheets with high catalytic activity and recyclability

A facile one-pot aqueous method has been fabricated for synthesis of Au-CeO(2) hybrid nanosheets. L-Lysine molecules are employed as the junctor to connect the two components of Au and CeO(2) nanoparticles. The obtained catalysts exhibited high catalytic activity, stability, and recyclability for the reduction reaction of p-nitrophenol into p-aminophenol by NaBH(4).

Aerobic oxidation of 5-hydroxylmethylfurfural with homogeneous and nanoparticulate catalysts

The aerobic oxidation of 5-hydroxymethylfurfural (HMF) with homogeneous Co(OAc)2/Zn(OAc)2/Br−, and heterogeneous Au–TiO2 and Au–CeO2 nanoparticulate catalysts was carried out in acetic acid with and without a trifluoroacetic acid (HTFA) additive. The selectivity of the oxidation products, 2,5-furandicarboxylic acid (FDCA) and 5-formyl-2-furancarboxylic acid (FFCA), improved in the presence of the HTFA (1 wt%) additive. The homogeneous oxidation catalyst gave higher selectivity for FDCA (60% yield) under 1 atm of O2 at 90 °C while Au–TiO2 afforded 80% FFCA under the same conditions.

Novel CeO2 yolk–shell structures loaded with tiny Au nanoparticles for superior catalytic reduction of p-nitrophenol

Direct fabrication of core-shell or yolk-shell functional nanomaterials via a facile template-free method remains a challenge. In this work, we present a novel approach that involves straightforward chemical transformation and thermal treatment of the infinite coordination polymer particles to obtain composition-tunable CeO(2) yolk-shell structures. Uniform CeO(2) yolk-shell hollow spheres with a high surface area are promising support materials for tiny gold nanoparticles (ca. 4 nm), forming Au-CeO(2) nanocomposites which exhibit a remarkable catalytic activity and high stability for the reduction of p-nitrophenol. A possible mechanism for the formation of CeO(2) yolk-shell microspheres is also proposed.

‘Shape effects’ in metal oxide supported nanoscale gold catalysts

We report the activity of shape-controlled metal oxide (CeO(2), ZnO and Fe(3)O(4)) supported gold catalysts for the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions. Metal oxide nanoshapes, prepared by controlled hydrolysis and thermolysis methods, expose different crystal surfaces, and consequently disperse and stabilize gold differently. We observe that similar to gold supported on CeO(2) shapes exposing the {110} and {111} surfaces, gold supported on the oxygen-rich ZnO {0001} and Fe(3)O(4) {111} surfaces shows higher activity for the SRM and WGS reactions. While the reaction rates vary among the Au-CeO(2), Au-ZnO and Au-Fe(3)O(4) shapes, the apparent activation energies are similar, indicating a common active site. TPR data further indicate that the reaction lightoff coincides with the activation of Au-O-M species on the surface of all three oxide supports evaluated here. Different shapes contain a different number of binding sites for the gold, imparting different overall activity.

TEM and STEM Study of the Au Nano-Particles Supported on Cerium Oxides

The structures of Au particles on CeO2 surfaces were observed by an analytical transmission electron microscopy (TEM) equipped with annular dark field scanning transmission electron microscopy (HAADF-STEM) systems. The Au/CeO2 model catalysts were prepared by using the poly-crystalline CeO2 substrates. The Au particles of 2-5 nm in diameter were deposited on the substrates. The preferential orientation relationship of (111)[1-10]Au//(111)[1-10]CeO2 was frequently observed in profile-view HRTEM images on CeO2 (111) surface. High resolution HAADF-STEM images were also obtained for Au-CeO2 interfaces. The position of atomic columns of Au and Ce at Au-CeO2 interface is directly investigated from HAADF-STEM images. The structure of the interfaces between Au particles and CeO2 (111), (100), (110) surfaces were discussed.

Conductivity and dielectric properties of thin amorphous cerium dioxide films

We present experimental studies on the dielectric properties of amorphous cerium dioxide in Au–CeO2–Au systems on strontium titanate substrates. From measurements of the capacitance versus contact area we calculated the dielectric constant of cerium dioxide at room temperature to be ϵr = 26.9 ± 0.5. These measurements also reveal an additional offset capacitance, which can be attributed to stray fields in the substrate material. I–V characteristics show that the current transport through the insulator changes from electrode-limited to bulk-limited conduction.

Catalytic wet air oxidation of p-coumaric acid on CeO2, platinum and gold supported on CeO2 catalysts

p-Coumaric acid is representative of the polyphenolic fraction typically found in olive milling waste waters (OMWW). The catalytic wet air oxidation of p-coumaric acid has been investigated at 353K at P=2MPa, using CeO2, Pt and Au supported on CeO2 catalysts. The influence of the metal and of the preparation method of the catalysts on the catalytic activity has been investigated.Upon addition of Pt to CeO2, the rate of oxidation of p-coumaric acid increases whereas addition of gold do not lead to a significant difference of the activity of CeO2. On all the catalysts investigated, the abatement of total organic carbon (TOC) was ≥80% after 300min of reaction. Catalysts containing metallic platinum are the most effective towards the mineralization of the organic carbon to CO2 and the degree of mineralization (DM%) was higher than 50%. On CeO2 and Au–CeO2 catalysts a great contribute to the abatement of TOC is given from a significant adsorption of the organic substrates on the solid.

In situ time-resolved characterization of Au–CeO2 and AuOx–CeO2 catalysts during the water-gas shift reaction: Presence of Au and O vacancies in the active phase

Synchrotron-based in situ time-resolved x-ray diffraction and x-ray absorption spectroscopies were used to study the behavior of nanostructured {Au+AuO(x)}-CeO(2) catalysts under the water-gas shift (WGS) reaction. At temperatures above 250 degrees C, a complete AuO(x)-->Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO(2)(111) single crystal corroborate that cationic Au(delta+) species cannot be the key sites responsible for the WGS activity at high temperatures. The rate determining steps for the WGS seem to occur at the gold-ceria interface, with the active sites involving small gold clusters (<2 nm) and O vacancies.

TEM observations of Au and Ir particles supported on CeO2

Au/CeO2 and Ir/CeO2 catalysts were observed by a transmission electron microscope in order to investigate the nano-structures of Au and Ir particles and CeO2 grains and the interface structure between metallic particles and CeO2. An annular dark field scanning transmission electron microscope (ADF-STEM) and an energy dispersive X-ray spectroscopy (EDS) revealed that the metallic particles smaller than 2 nm in diameter are highly dispersed on CeO2 supports in both catalysts. For model samples of larger CeO2 particles with flat facets of low-index surfaces, high resolution transmission electron microscopy observations of Au/CeO2 and Ir/CeO2 interfaces were made and the epitaxial relationship between Au and CeO2, (111)[110]Au//(111)[110]CeO2 has been found for the first time.

TEM observation of gold nanoparticles deposited on cerium oxide

The structure of Au nanoparticles supported on CeO2 using the deposition precipitation method was observed using a transmission electron microscope (TEM). The shape of Au nanoparticles and the fine structure of contact interface between the Au nanoparticles and CeO2 supports were carefully examined. It was found that there was a preferential orientation relationship between Au nanoparticles and the CeO2 support. It was also observed that small and thin Au nanoparticles disappeared during TEM observation, shrinking layer by layer down to a mono-atomic layer.

Nanostructured Au–CeO2 Catalysts for Low-Temperature Water–Gas Shift

The composite system of nanostructured gold and cerium oxide, with a gold loading 5–8 wt%, is reported in this work as a very good catalyst for low-temperature water–gas shift. Activity depends largely on the presence of nanosized ceria particles. Various techniques of preparation of an active catalyst are disscussed. The presence of gold is crucial for activity below 300°C. A dramatic effect of gold on the reducibility of the surface oxygen of ceria is found by H2-TPR, from 310–480°C to 25–110°C. All of the available surface oxygen was reduced, while there was no effect on the bulk oxygen of ceria. This correlates well with the shift activity of the Au–ceria system.