Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO₂(111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O₂, CO, and CO + O₂ at 300 K. The particles are found to be stable in O₂ ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above ∼1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO₂(111) support, is observed in CO + O₂ (1:1) mixture at much lower pressure (∼10⁻³ mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.     

SOx storage and release kinetics for ceria-supported platinum

The SO_x storage and release kinetics on CeO₂ have been studied by lean SO_x adsorption and temperature programmed desorption for different pairwise configurations of individual monolith samples, i.e., Pt/CeO₂ + SiO₂, Pt/SiO₂ + CeO₂, CeO₂ + Pt/SiO₂ and CeO₂ + SiO₂. In the case of sole ceria, SO_x adsorption proceeds both via SO₂ and SO₃ adsorption although the latter channel is kinetically favored. Hence, the rate of SO₂ oxidation is crucial for the overall SO_x storage kinetics. It is also found that physical contact between Pt and ceria is important for the storage process. This is attributed to efficient transport routes for SO_x (surface diffusion and spill-over processes) and/or specific adsorption sites at the platinum–ceria interface. The main route for SO_x release is found to be thermal decomposition where the effect of platinum is minor, although an indirect effect cannot be ruled out. Different mechanistic scenarios for SO_x adsorption are discussed, which may serve as a guide for future experiments.     

Activity of oxygen reduction reaction on small amount of amorphous CeOx promoted Pt cathode for fuel cell application

Pt on ceria (CeO_x) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeO_x, which consisted of the interface region between Pt and CeO_x, remained on Pt particles. Other free CeO_x particles were dissolved into H₂SO₄ aqueous solution. To develop the Pt-CeO_x/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H₂SO₄ aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO₂ with high specific surface area provides the continuous amorphous cerium oxide (Ce³⁺, Ce⁴⁺) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeO_x/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.     

Ceria-based Catalysts for the Production of H2 Through the Water-gas-shift Reaction: Time-resolved XRD and XAFS Studies

Hydrogen is a potential alternate energy source for satisfying many of our energy needs. In this work, we studied H₂ production from the water-gas-shift (WGS) reaction over Ce_1?x Cu _x O₂ catalysts, prepared with a novel microemulsion method, using two synchrotron-based techniques: time-resolved X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). The results are compared with those reported for conventional CuO _x /CeO₂ and AuO _x /CeO₂ catalysts obtained through impregnation of ceria. For the fresh Ce_1?x Cu _x O₂ catalysts, the results of XAFS measurements at the Cu K-edge indicate that Cu is in an oxidation state higher than in CuO. Nevertheless, under WGS reaction conditions the Ce_1?x Cu _x O₂ catalysts undergo reduction and the active phase contains very small particles of metallic Cu and CeO_2?x . Time-resolved XRD and XAFS results also indicate that Cu^δ+ and Au^δ+ species present in fresh CuO _x /CeO₂ and AuO _x /CeO₂ catalysts do not survive above 200 °C under the WGS conditions. In all these systems, the ceria lattice displayed a significant increase after exposure to CO and a decrease in H₂O, indicating that CO reduced ceria while H₂O oxidized it. Our data suggest that H₂O dissociation occurred on the O_vacancy sites or the Cu–O_vacancy and Au–O_vacancy interfaces. The rate of H₂ generation by a Ce_0.95Cu_0.05O₂ catalyst was comparable to that of a 5 wt% CuO _x /CeO₂ catalyst and much bigger than those of pure ceria or CuO.     

Interaction of SO2 with CeO2 and Cu/CeO2 catalysts: photoemission, XANES and TPD studies

CeO₂ and Cu/CeO₂ are effective catalysts/sorbents for the removal or destruction of SO₂. Synchrotron‐based high‐resolution photoemission, X‐ray absorption near‐edge spectroscopy (XANES), and temperature‐programmed desorption (TPD) have been employed to study the reaction of SO₂ with pure and reduced CeO₂ powders, ceria films (CeO₂, CeO_2−x, Ce₂O_3+x) and model Cu/CeO₂ catalysts. The results of XANES and photoemission provide evidence that SO₄ was formed upon the adsorption of SO₂ on pure powders or films of CeO₂ at 300 K. The sulfate decomposed in the 390–670 K temperature range with mainly SO₂ and some SO₃ evolving into gas phase. At 670 K, there was still a significant amount of SO₄ present on the CeO₂ substrates. The introduction of O vacancies in the CeO₂ powders or films favored the formation of SO₃ instead of SO₄. Ceria was able to fully dissociate SO₂ to atomic S only if Ce atoms with a low oxidation state were available in the system. When Cu atoms were added to CeO₂ new active sites for the destruction of SO₂ were created improving the catalytic activity of the system. The surface chemistry of SO₂ on the Cu‐promoted CeO₂ was much richer than on pure CeO₂. The behavior of ceria in several catalytic processes (oxidation of SO₂ by O₂, reduction of SO₂ by CO, automobile exhaust converters) is discussed in light of these results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst

The catalytic reduction of nitrogen monoxide by propene in the presence of excess oxygen over gold based ceria catalyst was studied. Adsorption and temperature programmed desorption of NO/O₂ on Au/CeO₂ reveal that the catalyst adsorbs and desorbs NO over a large range of temperature. A maximum of 26% conversion of NO _x was obtained around 210 °C, with a selectivity of 50% to N₂.     

Water�CGas Shift and CO Methanation Reactions over Ni�CCeO2(111) Catalysts

X-ray and ultraviolet photoelectron spectroscopies were used to study the interaction of Ni atoms with CeO₂(111) surfaces. Upon adsorption on CeO₂(111) at 300 K, nickel remains in a metallic state. Heating to elevated temperatures (500?C800 K) leads to partial reduction of the ceria substrate with the formation of Ni²⁺ species that exists as NiO and/or Ce_1?xNi_xO_2?y. Interactions of nickel with the oxide substrate significantly reduce the density of occupied Ni 3d states near the Fermi level. The results of core-level photoemission and near-edge X-ray absorption fine structure point to weakly bound CO species on CeO₂(111) which are clearly distinguishable from the formation of chemisorbed carbonates. In the presence of Ni, a stronger interaction is observed with chemisorption of CO on the admetal. When the Ni is in contact with Ce⁺³ cations, CO dissociates on the surface at 300 K forming NiC_x compounds that may be involved in the formation of CH₄ at higher temperatures. At medium and large Ni coverages (>0.3 ML), the Ni/CeO₂(111) surfaces are able to catalyze the production of methane from CO and H₂, with an activity slightly higher than that of Ni(100) or Ni(111). On the other hand, at small coverages of Ni (<0.3 ML), the Ni/CeO₂(111) surfaces exhibit a very low activity for CO methanation but are very good catalysts for the water?Cgas shift reaction.     

The role of cationic Au and nonionic Au species in the low-temperature water–gas shift reaction on Au/CeO2 catalysts

The roles of cationic and nonionic Au species in the water–gas shift (WGS) reaction on Au/CeO₂ catalysts were studied by comparing the reaction behavior of a cyanide leached catalyst, after removal of the Au nanoparticles by cyanide leaching, with that of non-leached catalysts, following the technique introduced by Q. Fu et al. [Science 301 (2003) 935]. Using rate measurements as well as in situ spectroscopic and structure-sensitive techniques, we found that based on the Au mass balance, cyanide leaching removed all Au except for ionic Au³⁺ species, and that leaching resulted in a pronounced decay of the catalyst mass normalized activity to 1–25% of that of a non-leached catalyst. The extent of the activity loss strongly depended on the post-treatment of the leached catalyst. Both the catalyst treatment after leaching and, in particular, the WGS reaction resulted in considerable reformation of Au⁰ species by thermal decomposition of Au oxides (Au³⁺) and subsequent nucleation and growth of very small Au⁰ aggregates and metallic Au⁰ nanoparticles, as indicated by Au(4f) signals at 85.9 eV (Au³⁺), 84.0–84.6 eV (up-shifted signal of small Au⁰ aggregates), and 84.0 eV (metallic Au⁰). In this work, correlations between ionic and nonionic Au species and between total WGS activity and activity for the formation/decomposition of bidentate formate species are evaluated, and the role of the respective Au species in the WGS reaction on Au/CeO₂ catalysts is discussed.     

Promotional role of CeO2 in reduction of NO with activated carbon under oxygen-rich atmosphere

In this study, reduction of NO with activated carbon (AC) under oxygen-rich atmosphere was investigated over Cu/AC, Cu/CeO₂ − AC and Cu/CeO₂ + AC catalysts. Among the three catalysts, Cu/CeO₂ − AC exhibited the highest catalytic activity meanwhile the burning-off of AC was restrained to the largest extent. In the Cu/CeO₂ − AC catalyst, Cu acted as catalytically active sites and CeO₂ played an important role in enhancing the selectivities of AC towards NO reduction and improving the tolerance to poison by water vapour and SO₂.     

Promoting Effect of CeO2 on NO x Reduction with Propene over SnO2/Al2O3 Catalyst Studied with In situ FT-IR Spectroscopy

The mechanistic cause of the promoting effect of CeO₂ on the activity of SnO₂/Al₂O₃ catalyst for the SCR of NO _x by propene was investigated using X-ray photoelectron spectra (XPS) and in situ Fourier transform infrared (FT-IR) spectroscopy. FT-IR measurements have revealed that the role of CeO₂ on the CeO₂–SnO₂/Al₂O₃ catalyst is to contribute to the formation of formate, acetate and nitrate species, and to promote the reaction between nitrates and hydrocarbon-derived species to form isocyanate (–NCO), which is a reaction intermediate for NO _x reduction.     

Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

MnO_x–CeO₂ mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnO_x–CeO₂ catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnO_x(0.86)–CeO₂ was the most active catalyst, on which the complete combustion temperature (T_90%) of chlorobenzene was 236 °C. The stability of MnO_x–CeO₂ catalysts in the CB combustion was investigated. MnO_x–CeO₂ catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.     

Gold catalysts supported on ceria-modified mesoporous zirconia for low-temperature water–gas shift reaction

New gold catalytic system prepared on ceria-modified mesoporous zirconia used as water–gas shift (WGS) catalyst is reported. Mesoporous zirconia was synthesized using surfactant templating method through a neutral [C₁₃(EO)₆-Zr(OC₃H₇)₄] assembly pathway. Ceria modifying additive was deposited on mesoporous zirconia by deposition–precipitation method. Gold-based catalysts with different gold content (1–3 wt. %) were synthesized by deposition–precipitation of gold hydroxide on mixed metal oxide support. The supports and the catalysts were characterized by powder X-ray diffraction, high-resolution transmission electron microscopy, N₂ adsorption analysis and temperature programmed reduction. The catalytic behavior of the gold-based catalysts was evaluated in WGS reaction in a wide temperature range (140–300 °C) and at different space velocities and H₂O/CO ratios. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/ceria-modified mesoporous zirconia catalysts was compared with that of gold catalysts supported on simple oxides CeO₂ and mesoporous ZrO₂, revealing significantly higher catalytic activity of Au/ceria-modified mesoporous zirconia. A high degree of synergistic interaction between ceria and mesoporous zirconia and a positive modification of structural and catalytic properties by ceria have been achieved. It is clearly revealed that the ceria-modified mesoporous zirconia is of much interest as potential support for gold-based catalyst. The Au/ceria-modified mesoporous zirconia catalytic system is found to be effective catalyst for WGS reaction.     

Interaction of CO with planar Au/TiO2 model catalysts at elevated pressures

The interaction of CO with structurally well-defined, planar Au/TiO₂ model catalysts at elevated pressures (up to 50 mbar) was studied in-situ by polarization-modulated infrared reflection absorption spectroscopy and ex-situ by X-ray photoelectron spectroscopy performed before and after CO exposure. The results indicate a CO-induced partial reduction of the oxide surface, which is evidenced by a low frequency C–O vibration at 2060 cm⁻¹, combined with a spreading of the Au nanoparticles due to a modification of the Au-oxide interface energy. In a 2:1 CO:O₂ atmosphere, TiO₂ support reduction was not observed, and a pre-reduced surface was re-oxidized. The consequences of these results for the understanding of the CO oxidation mechanism on Au/TiO₂ (model) catalysts are discussed.     

Investigation of the Intrinsic Activity of ZrxCe1−xO2 Mixed Oxides in the CO + NO Reactions: Influence of Pd Incorporation

The intrinsic activity of various Zr _x Ce_1–x O₂ mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of Zr _x Ce_1–x O₂ depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr_0.5Ce_0.5O₂ being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO₂ and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a bifunctional mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr_0.25Ce_0.75O₂ being the most resistant to deactivation.     

A comparative study of the selective oxidation of NH3 to N2 over gold, silver and copper catalysts and the effect of addition of Li2O and CeOx

This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH₃/O₂ ratios. The effect of addition of CeO_x and Li₂O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N₂. However the multicomponent catalysts: M/Li₂O/CeO_x/Al₂O₃ (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support.     

Effect of Ceria on the Storage and Regeneration Behavior of a Model Lean NO x Trap Catalyst

In this study the effect of ceria addition on the performance of a model Ba-based lean NO _x trap (LNT) catalyst was examined. The presence of ceria improved NO _x storage capacity in the temperature range 200–400 °C under both continuous lean and lean-rich cycling conditions. Temperature-programmed experiments showed that NO _x stored in the ceria-containing catalyst was thermally less stable and more reactive to reduction with both H₂ and CO as reductants, albeit at the expense of additional reductant consumed by reduction of the ceria. These findings demonstrate that the incorporation of ceria in LNTs not only improves NO _x storage efficiency but also positively impacts LNT regeneration behavior.     

Characterization of Au/MnO x /TiO2 for Photocatalytic Oxidation of Carbon Monoxide

The Au/MnO _x /TiO₂ catalyst was used for the photocatalytic oxidation of carbon monoxide. The catalytic activity of Au/MnO _x /TiO₂ with low concentration of manganese (3–7 mol%) was much higher than that of Au/TiO₂. The surface of Au/MnO _x /TiO₂ was characterized by XPS and Raman spectroscopy. While the main state of manganese in 13.8 mol% MnO _x /TiO₂ was Mn⁴⁺ species, Mn³⁺ was the dominant species in the samples with below 6.5 mol% manganese. Raman spectroscopy revealed that the interaction between the MnO _x and TiO₂ form Mn–O–Ti species in which the state of manganese was Mn³⁺. The Au particles also interacted with both MnO _x and TiO₂ to modify the surface of them. In the case of the Au species, low loading of manganese produced the metallic Au⁰ and perimeter interfacial Au^δ+, whereas high loading showed the coexistence of three components which were metallic Au⁰, perimeter interfacial Au^δ+, and oxidic Au³⁺. The catalytic active component was the metallic Au⁰ and perimeter interfacial Au^δ+ species, which were dispersed on TiO₂ and Mn³⁺/TiO₂.     

Preparation, electrochemical properties, and cycle mechanism of Li1? x Fe0.8Ni0.2O2-Li x MnO2 (Mn/(Fe+Ni+Mn)=0.8) material

A new type of Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ (Mn/(Fe+Ni+Mn)=0.8) material was synthesized at 350 °C in an air atmosphere by a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−x FeO₂-Li _x MnO₂ ((Fe+Ni+Mn)=0.8) with much reduced impurity peaks. It was composed of many large particles of about 500–600 nm and small particles of about 100–200 nm, which were distributed among the larger particles. The Li/Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ cell showed a high initial discharge capacity above 192 mAh/g, which was higher than that of the parent Li/Li_1−x FeO₂-Li _x MnO₂ (186 mAh/g). This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles. We suggest a unique role of doped nickel ion in the Li/Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ cell, which results in the increased initial discharge capacity from the redox reaction of Ni²⁺/Ni³⁺ between 2.0 and 1.5 V region.     

Excellent sulfur resistance of Pt/BaO/CeO2 lean NOx trap catalysts

In this work, we investigated the NO_x storage behavior of Pt/BaO/CeO₂ catalysts, especially in the presence of SO₂. High surface area CeO₂ (110 m²/g) with a rod like morphology was synthesized and used as a support. The Pt/BaO/CeO₂ sample demonstrated slightly higher NO_x uptake in the entire temperature range studied compared with Pt/BaO/γ-Al₂O₃. More importantly, this ceria-based catalyst showed higher sulfur tolerance than the alumina-based one. The time of complete NO_x uptake was maintained even after exposing the sample to 3 g/L of SO₂. The same sulfur exposure, on the other hand, eliminated the complete NO_x uptake time on the alumina-based NO_x storage catalysts. TEM images show no evidence of either Pt sintering or BaS phase formation during reductive de-sulfation up to 600 °C on the ceria-based catalyst, while the same process over the alumina-based catalyst resulted in both a significant increase in the average Pt cluster size and the agglomeration of a newly formed BaS phase into large crystallites. XPS results revealed the presence of about five times more residual sulfur after reductive de-sulfation at 600 °C on the alumina-based catalysts in comparison with the ceria-based ones. All of these results strongly support that, besides their superior intrinsic NO_x uptake properties, ceria-based catalysts have (a) much higher sulfur tolerance and (b) excellent resistance against Pt sintering when they are compared to the widely used alumina-based catalysts.     

A comparative study of the effect of addition of CeO x and Li2O on γ-Al2O3 supported copper, silver and gold catalysts in the preferential oxidation of CO

In the study described in this paper we deposited gold, silver and copper on γ-Al₂O₃ as nanoparticles (<4 nm) and investigated the behavior of these nanoparticles in the preferential oxidation of CO in presence of H₂. In addition, the effect of addition of CeO _x and/or Li₂O was investigated. All the three metals show preferential oxidation of CO at low temperatures. The oxides added to Au/γ-Al₂O₃, Ag/γ-Al₂O₃ and Cu/γ-Al₂O₃ improve the catalytic performance of the gold, silver and copper. Interesting and synergistic effects were observed when both the CeO _x and Li₂O were added. Possible mechanisms are proposed.