Influence of CeO2 addition on the physicochemical and catalytic properties of Pd/TiO2 catalysts in CO oxidation

The influence of CeO₂ addition on the formation of the microstructure, electronic state, and catalytic properties of Pd/TiO₂ supported catalysts in CO oxidation were investigated. It was shown that, when Pd is supported on titanium dioxide modified with cerium dioxide, annealing at 500°C results in the formation of Pd/(CeO₂-TiO₂) catalysts with a nanocrystalline structure composed of incoherently intergrown fine anatase crystals and interblock boundaries in which palladium and cerium are stabilized. The higher catalytic activity of Pd/(CeO₂-TiO₂) catalysts compared to Pd/TiO₂ catalysts is explained by the smaller size of Pd particles and the higher proportion of palladium in the Pd^δ+ state.     

Influence of NH<Subscript>3</Subscript>-treating temperature on visible light photocatalytic activity of N-doped P<Subscript>25</Subscript>-TiO<Subscript>2</Subscript>

The influence of NH₃-treating temperature on the visible light photocatalytic activity of N-doped P₂₅-TiO₂ as well as the relationship between the surface composition structure of TiO₂ and its visible light photocatalytic activity were investigated. The results showed that N-doped P₂₅-TiO₂ treated at 600°C had the highest activity. The structure of P₂₅-TiO₂ was converted from anatase to rutile at 700°C. Moreover, no N-doping was detected at the surface of P₂₅-TiO₂. There was no simply linear relationship between the visible light photocatalytic activity and the concentration of doped nitrogen, and visible light absorption. The visible light photocatalytic activity of N-doped P₂₅-TiO₂ was mainly influenced by the synergistic action of the following factors: (i) the formation of the single-electron-trapped oxygen vacancies (denoted as V_o^·); (ii) the doped nitrogen on the surface of TiO₂; (iii) the anatase TiO₂ structure.     

Co oxidation with oxygen in the presence of hydrogen on CoO/CeO<Subscript>2</Subscript> and CuO/CoO/CeO<Subscript>2</Subscript> catalysts

The catalytic activity of the CoO/CeO₂ and CuO/CoO/CeO₂ systems in selective CO oxidation in the presence of hydrogen at 20–450°C ([CuO] = 1.0–2.5%, [CoO] = 1.0–7.0%) is reported. The maximum CO conversion (X) decreases in the following order: CuO/CoO/CeO₂ (X = 98–99%, T = 140–170°C) > CoO/CeO₂ (X = 67–84%, T = 230–240°C) > CeO₂ (X = 34%, T = 350°C). TPD, TPR, and EPR experiments have demonstrated that the high activity of CuO/CoO/CeO₂ is due to the strong interaction of the supported copper and cobalt oxides with cerium dioxide, which yields Cu-Co-Ce-O clusters on the surface. The carbonyl group in the complexes Co^δ+-CO and Cu⁺-CO is oxidized by oxygen of the Cu-Co-Ce-O clusters at 140–160°C and by oxygen of the Co-Ce-O clusters at 240°C. The decrease in the activity of the catalysts at high temperatures is due to the fact that hydrogen reduces the clusters on which CO oxidation takes place, yielding Co⁰ and Cu⁰ particles, which are inactive in CO oxidation. The hydrogenation of CO into methane at high temperatures is due to the appearance of Co⁰ particles in the catalysts.     

Selective oxidation of CO in the presence of hydrogen on CuO/CeO<Subscript>2</Subscript> catalysts modified with Fe and Ni oxides

The selective oxidation of CO in the presence of hydrogen on CuO/CeO₂ systems containing Fe and Ni oxides as promoters was studied. The catalysts containing 1–5 wt % CuO and 1–2.5 wt % Fe₂O₃ supported on CeO₂ and the CuO/CeO₂ systems containing 1–2.5 wt % NiO were synthesized, and their catalytic activity as a function of temperature was determined. It was found that the additives of Fe and Ni oxides increased the activity of the CuO/CeO₂ catalysts with a low concentration of CuO. In this case, the conversion of CO at 150°C approached 100%. At the same time, these additives had no effect on the activity of the CuO/CeO₂ systems at a CuO concentration of 5 wt % or higher, which exhibited an initially high activity in the above temperature region. The forms of CO adsorption and the amounts of active sites for CO adsorption and oxidation were studied using temperature-programmed desorption. It was found that the introduction of Fe and Ni additives in a certain preparation procedure facilitated the formation of an additional amount of active centers associated with CuO. Data on the temperature-programmed reduction of samples (the amount of absorbed hydrogen and the maximum temperature of hydrogen absorption) suggested the interaction of all catalyst components, and the magnitude of this interaction depended on the sample preparation procedure. With the use of Mössbauer spectroscopy, it was found that the procedure of iron oxide introduction into the CuO/CeO₂ system was responsible for the electron-ion interactions of catalyst components and the reaction mixture.     

Fabrication of Cu-Doped CeO<Subscript>2</Subscript> Catalysts with Different Dimension Pore Structures for CO Catalytic Oxidation

Transition metal oxides (TMOs) applied as catalysts whose catalytic activities are directly affected by their pores size and pores distributions. Herein, two-dimensional Cu-doped CeO₂ (2D@Cu–CeO₂) and three-dimensional Cu-doped CeO₂ (3D@Cu–CeO₂) were prepared by adopting the mesoporous silica SBA-15 and KIT-6 as templates, respectively. Nanometer Cu-doped CeO₂ (nano@Cu–CeO₂) was synthesized by the method of precipitation. All catalysts were evaluated for the catalytic oxidation of CO, and the 3D@Cu–CeO₂ catalyst exhibited the highest catalytic activity (complete conversion temperature T₁₀₀?=?50?°C), which can be ascribed to the three-dimensional porous channel structure, larger specific surface area and abundant active surface oxygen species. In addition, complete conversion of CO had remained the same after 3D@Cu–CeO₂ was observed for 12 h, indicating it has the best catalytic stability for CO.     

Synthesis of highly ordered mesoporous CeO<Subscript>2</Subscript> and low temperature CO oxidation over Pd/mesoporous CeO<Subscript>2</Subscript>

Highly ordered mesoporous cerium dioxide (meso-CeO₂) was successfully synthesized using a facile solvent-free infiltration method from a mesoporous silica template, KIT-6. The meso-CeO₂ material, thus obtained, exhibited well-defined mesostructure and high surface area (153 m² g⁻¹). The physicochemical properties of meso-CeO₂ material and Pd-supported on meso-CeO₂ (Pd/meso-CeO₂) were characterized by electron microscopy, X-ray diffraction, N₂ adsorption–desorption, and temperature-programmed experiments. The Pd/meso-CeO₂ catalyst exhibited excellent catalytic activity for CO oxidation compared with those of other Pd/CeO₂ catalysts which were prepared using nanocrystalline CeO₂ and bulk-CeO₂ as the supports. Moreover, a hydrogen pretreatment of the Pd/meso-CeO₂ catalyst resulted in a remarkable increase of catalytic activity (T ₁₀₀ = 52 °C).     

Effect of the nanoparticle size of supported copper-containing catalysts on their activity in the selective oxidation of CO in an excess of H<Subscript>2</Subscript>

The catalytic properties of systems prepared by the supporting of CuO onto CeO₂, ZrO₂, and Zr_0.5Ce_0.5O₂ with particle sizes of 15–25 nm (nitrate pyrolysis (p)) and 5–6 nm (microemulsion method (me)) in the reaction of CO oxidation in an excess of H₂ were studied. In the latter case, the supports had an almost homogeneous surface and a small number of defects. The catalytic activity of (me) and (p) supports was low and almost the same, whereas the catalytic activity of CuO/(CeO₂, ZrO₂, and Zr_0.5Ce_0.5O₂)(me) samples was lower than that of CuO/(CeO₂ and ZrO₂)(p). The maximum CO conversion (∼100% at 125°C) was observed on 5% CuO/CeO₂ (p). The CO and CO₂ adsorption species on (p) and (me) catalysts were studied by TPD. Differences in the compositions of copper-containing centers on the surfaces of (p) and (me) systems were found using TPR. The nature of the active centers of CO oxidation and the effect of support crystallite size on the catalytic activity were considered.     

Physicochemical properties of TiO<Subscript>2</Subscript> (anatase) prepared by the centrifugal thermal activation of hydrated titanium dioxide

The physicochemical properties of titania (anatase) prepared from hydrated titanium dioxide by centrifugal thermal activation (CTA) at 140–700°C were studied. It was found that the microstructure and the texture parameters of anatase prepared by the above method were considerably different from those of the samples prepared by the traditional thermal decomposition of titanium hydroxide. The conditions of centrifugal activation exerted a considerable effect on the structure and the texture parameters of the resulting anatase. The crystal structure of anatase prepared at a temperature lower than 650°C was imperfect, and it approached a regular structure only at a temperature of >650°C. At temperatures higher than 300°C, the samples of TiO₂ prepared using CTA were characterized by higher specific surface areas, fine pore structures, and comparable mesopore volumes, as compared with the samples prepared by commonly used synthetic methods.     

Light expanded clay aggregate (LECA) as a support for TiO<Subscript>2</Subscript>, Fe/TiO<Subscript>2</Subscript>, and Cu/TiO<Subscript>2</Subscript> nanocrystalline photocatalysts: a comparative study on the structure,morphology, and activity

In this work, TiO₂ and doped TiO₂ photocatalysts (Fe/TiO₂ and Cu/TiO₂) were synthesized by the sol–gel method. The main objective of this study was to investigate the influence of dopants on the structure, morphology, and activity of the catalysts in powder and immobilized states. XRF, XRD, and SEM methods were used to characterize the catalysts. The structure and phase distribution of the nanocrystalline powders were identified by XRD. Nanoparticles crystallite size and the degree of crystallinity were affected by doping. The anatase contents of catalysts were achieved as follows: TiO₂ (5.89 %) < Fe/TiO₂ (42.17 %) < Cu/TiO₂ (70.28 %). It was indicated that the activity of the catalysts strongly depends on the anatase content. Under the same circumstances, copper-modified TiO₂ exhibited a twofold higher photocatalytic activity compared with TiO₂. The nanostructured catalysts were immobilized on light expanded clay aggregate (LECA) granules in order to investigate the effect of a novel support on the activity of the catalysts. Morphological changes are recognizable in the SEM images. Activity tests indicated that the best catalytic performance was assigned to Cu/TiO₂/LECA. After 120 min of irradiation, 61 % degradation of phenol in synthetic wastewater was achieved. The high photocatalytic activity of Cu/TiO₂/LECA confirms that LECA is as an excellent support.     

The effect of PO<Subscript>2,5</Subscript> and AlO<Subscript>1,5</Subscript> additions on structural changes and crystallization behavior of SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> sol-gel derived glasses and thin films

SiO₂-TiO₂-PO_2,5 (STP) and SiO₂-TiO₂-AlO_1,5 (STA) glasses were prepared by sol-gel processing. Their infrared absorption spectra (IR), differential thermal analysis curves (DTA) and X-ray diffraction patterns (XRD) have been recorded. In the SiO₂-TiO₂ system, the chemical homogeneity of the sol-gel glass could be evaluated by the relative concentration of Si-O-Ti heterocondensation comparing to Si-O-Si homocondensation. For the STA system, a gradual decrease of the Si-O-Ti/Si-O-Si band ratio (based on IR spectra) with the addition of Al₂O₃ is observed, with the simultaneous formation of Si-O-Al and Ti-O-Al bounds, i.e Al₃ ⁺ ions are dissolved in the SiO₂-TiO₂ glass matrix and do not promote glass-in-glass phase-separation in the composition range of 0–15 mol% AlO_1.5. In the STP system, on the other hand, P=O bond IR stretch in the ternary glasses indicates that P=O free PO₂O_2/2 ⁻ tetrahedra are formed, rather than the double bonded POO_3/2 tetrahedra that usually occur in binary SiO₂-P₂O₅ glasses. It can be concluded that SiO₂-TiO₂-P₂O₅ glass separates into a SiO₂-rich phase and a TiO₂(P₂O₅)-rich phase. During heat-treatment in STA system only anatase precipitates, even at T ~ 1,000 °C, while in for STP, anatase (TiO₂) or (TiO)₂P₂O₇ (TOP) crystals precipitate at ~600 °C, depending on the P₂O₅ concentration. The major crystal phase, cristobalite, precipitated at ~1,000 °C and at ~1,200 °C, the P-containing phase melts.     

Use of the differential charging effect in XPS to determine the nature of surface compounds resulting from the interaction of a Pt/(BaCO<Subscript>3</Subscript> + CeO<Subscript>2</Subscript>) model catalyst with SO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Changes in the chemical composition of the surface of a Pt/(BaCO₃ + CeO₂) model NO _x storage-reduction catalyst upon its interaction with SO _x (SO₂ (260 Pa) + O₂ (2600 Pa) + H₂O (525 Pa)) followed by regeneration in a mixture of CO (2100 Pa) with H₂O (525 Pa) were studied by X-ray photoelectron spectroscopy (XPS). Model catalyst samples were prepared as a thin film (about several hundreds of angstrom units in thickness) on the surface of tantalum foil coated with a layer of aluminum oxide (～100 Å). It was found that the Pt/BaCO₃ and Pt/CeO₂ catalyst constituents acquired different surface charges (differential charging) in the course of photoelectron emission; because of this, it was possible to determine the nature of surface compounds formed as a result of the interaction of the catalyst with a reaction atmosphere. It was found that barium carbonate was converted into barium sulfate as a result of reaction with SO _x on the surface of BaCO₃ at 150°C. As the treatment temperature in SO _x was increased to 300°C, the formation of sulfate on the surface of CeO₂ was observed. The sulfatization of CeO₂ was accompanied by the reduction of Ce(IV) to Ce(III). The regeneration reaction of the catalyst treated in SO _x at 300°C resulted in the consecutive decomposition of cerium(III) sulfate at ≤500°C and then barium sulfate at 600–700°C. Upon the decomposition of BaSO₄, a portion of sulfur was converted into a sulfide state, probably, because of the formation of BaS.     

Effect of calcination temperature on the microstructure of porous TiO<Subscript>2</Subscript> film

To obtain porous TiO₂ film, the precursor sol was prepared by hydrolysis of Ti isopropoxide and then complexed with trehalose dihydrate. The porous TiO₂ film was fabricated by the dip-coating technique on glass substrates using this solution. The TiO₂ film was calcined at 500 °C. The maximum thickness of the film from one-run dip-coating was ca. 740 nm. The film was composed of nanosized particle and pores. The porosity of the TiO₂ film was increased by addition of trehalose dihydrate to the sol. The porous TiO₂ films were calcined at different temperatures. The effects of calcination temperature on the microstructure of the porous TiO₂ film were investigated. The porous film prepared from sol containing trehalose still kept the porous structure after calcination at 950 °C. The phase transition temperature of the film from anatase to rutile was shifted from 650 to 700 °C by addition of trehalose to the sol.     

Methanol conversion over CuO supported on CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript>: An in situ IR spectroscopic study

The main reactions yielding hydrogen are the recombination of hydrogen atoms on copper clusters and methyl formate decomposition. Methyl formate results from the interaction between the linear methoxy group and the formate complex located on CuO. The source of CO₂ appearing in the gas phase is the formate complex, and the source of CO is methyl formate. The rates of methoxy group conversion and product formation over supports (ZrO₂, CeO₂, Ce_0.8Zr_0.2O₂) and copper-containing catalysts (5%Cu/CeO₂, 5%Cu/ZrO₂, 2%Cu/Ce_0.8Zr_0.2O₂, 2%Cu/Ce_0.1Y_0.1Zr_0.8) are compared. The dominant process in methoxy group conversion over the supports and copper-containing catalysts is methanol decomposition to H₂ and CO and to H₂ and CO₂, respectively. The methoxy group conversion rate is proportional to the H₂ and CO₂ formation rate and is determined by the concentration of supported copper.     

Influence of ZrO<Subscript>2</Subscript> on the physicochemical properties of the ZrO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> binary system

A series of ZrO₂-TiO₂ mixed oxides with different weight ratios (5, 20, and 30% ZrO₂) were prepared by wet impregnation of TiO₂-P25 Degussa with certain amounts of ZrO(NO₃)₂·6H₂O (Fluka) dissolved in deionised water. The samples were characterized by the XRD, , , , and BET methods. An increase in ZrO₂ content shifted the phase transition temperature (anatase into rutile) toward higher temperatures. X-ray diffraction using an Anton Paar XRK900 reactor chamber indicated that, in the case of samples containing ZrO₂, an additional diffraction peak appeared after cooling down to 25°C. This peak could be attributed to a polymorph of TiO₂ such as in the single crystal of anatase or hexagonal form of TiO₂ which appears in the presence of ZrO₂. Generally, the preparation of dioxide systems can modify the properties of pure compounds or generate new catalytic sites as a result of strong interaction between ZrO₂ and TiO₂ oxides. The binary systems exhibit advantages like strong acidity, extended specific surface area, and high thermal stability in comparison with TiO₂. The article is published in the original.     

Effect of the microstructure of Pt/CeO2-TiO2 catalysts on their catalytic properties in CO oxidation

The microstructure of 2% Pt/CeO₂-TiO₂ catalysts has an effect on their catalytic properties in CO oxidation. The nanostructured catalysts as platinum clusters 0.3–0.5 nm in size are the most active. These clusters are stabilized at crystal boundaries formed by irregularly intergrown anatase particles. The catalyst containing platinum particles 2–5 nm in size is less active because of the decrease in the extent of dispersion of platinum and the change of its electron state.     

Phase formation in mixed TiO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> oxides prepared by sol-gel method

Pure titania, zirconia, and mixed oxides (3–37 mol.% of ZrO₂) are prepared using the sol-gel method and calcined at different temperatures. The calcined samples are characterized by Raman spectroscopy, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption porosimetry. Measurements reveal a thermal stability of the titania anatase phase that slightly increases in the presence of 3–13 mol.% of zirconia. Practically, the titania anatase-rutile phase transformation is hindered during the temperature increase above 700°C. The mixed oxide with 37 mol.% of ZrO₂ treated at 550°C shows a new single amorphous phase with a surface area of the nanoparticles double with respect to the other crystalline samples and the formed srilankite structure (at 700°C). The anatase phase is not observed in the sample containing 37 mol.% of ZrO₂. The treatment at 700°C causes the formation of the srilankite (Ti_0.63Zr_0.37O_x) phase.     

Selective methanation of CO in the presence of CO<Subscript>2</Subscript> in hydrogen-containing mixtures on nickel catalysts

The screening of commercial nickel catalysts for methanation and a series of nickel catalysts supported on CeO₂, γ-Al₂O₃, and ZrO₂ in the reaction of selective CO methanation in the presence of CO₂ in hydrogen-containing mixtures (1.5 vol % CO, 20 vol % CO₂, 10 vol % H₂O, and the balance H₂) was performed at the flow rate WHSV = 26000 cm³ (g Cat)⁻¹ h⁻¹. It was found that commercial catalytic systems like NKM-2A and NKM-4A (NIAP-07-02) were insufficiently effective for the selective removal of CO to a level of <100 ppm. The most promising catalyst is 2 wt % Ni/CeO₂. This catalyst decreased the concentration of CO from 1.5 vol % to 100 ppm in the presence of 20 vol % CO₂ in the temperature range of 280–360°C at a selectivity of >40%, and it retained its activity even after contact with air. The minimum outlet CO concentration of 10 ppm at 80% selectivity on a 2 wt % Ni/CeO₂ catalyst was reached at a temperature of 300°C.     

Mechanism of CO oxidation in excess H2 over CuO/CeO2 catalysts: ESR and TPD studies

The oxidation of CO with oxygen over (0.25–6.4)% CuO/CeO₂ catalysts in excess H₂ is studied. CO conversion increases and the temperature range of the reaction decreases by 100 K as the CuO content is raised. The maximal CO conversion, 98.5%, is achieved on 6.4% CuO/CeO₂ at 150°C. At T > 150°C, the CO conversion decreases as a result of the deactivation of part of the active sites because of the dissociative adsorption of hydrogen. CO is efficiently adsorbed on the oxidized catalyst to form CO-Cu⁺ carbonyls on Cu₂O clusters and is oxidized by the oxygen of these clusters, whereas it is neither adsorbed nor oxidized on Cu⁰ of the reduced catalysts. The activity of the catalysts is recovered after the dissociative adsorption of O₂ on Cu⁰ at T ～ 150°C. The activation energies of CO, CO₂, and H₂O desorption are estimated, and the activation energy of CO adsorption yielding CO-Cu⁺ carbonyls is calculated in the framework of the Langmuir-Hinshelwood model.     

Activity of Rh/TiO<Subscript>2</Subscript> catalysts in NaBH<Subscript>4</Subscript> hydrolysis: The effect of the interaction between RhCl<Subscript>3</Subscript> and the anatase surface during heat treatment

The reaction properties of Rh/TiO₂ sodium tetrahydroborate hydrolysis catalysts reduced directly in the reaction medium depend on the temperature at which they were calcined. Raising the calcination temperature to 300°C enhances the activity of the Rh/TiO₂ catalysts. Using diffuse reflectance electronic spectroscopy, photoacoustic IR spectroscopy, and chemical and thermal analyses, it is demonstrated that, as RhCl₃ is supported on TiO₂ (anatase), the active-component precursor interacts strongly with the support surface. The degree of this interaction increases as the calcination temperature is raised. TEM, EXAFS, and XANES data have demonstrated that the composition and structure of the rhodium complexes that form on the titanium dioxide surface during different heat treatments later determine the state of the supported rhodium particles forming in the sodium tetrahydroborate reaction medium.     

Effects of precipitants and surfactants on catalytic activity of Pd/CeO<Subscript>2</Subscript> for CO oxidation

A series of precipitants and commercial surfactants (soft templates) were employed to synthesize mesoporous/nano CeO₂ by a hydrothermal method. As-prepared CeO₂ was impregnated with palladium and employed for low-temperature catalytic oxidation of CO. It was found that both soft templates and precipitants had significant effects on the morphology, particle size, crystallinity, and porous structure of the CeO₂, having a significant effect on the surface palladium abundance, molar ratios of surface species, and catalytic activity of the final impregnated Pd/CeO₂. Using ammonia as precipitant could facilitate increased surface palladium abundance and surface molar ratios of PdO/Pd _SMSI , Ce³⁺/(Ce³⁺ + Ce⁴⁺), and O_surface/O_lattice. The catalytic activity of the final Pd/CeO₂ catalysts could be enhanced as well. The optimal P123-assisted ammonia-precipitated Pd/CeO₂ catalyst exhibited over 99% catalytic conversion of CO at 50 °C.