Pt/CeO₂–ZrO₂–Bi₂O₃/γ-Al₂O₃ (Pt/CZB/Al₂O₃) catalysts for the catalytic combustion of toluene, which is one of the volatile organic compounds (VOCs), were prepared by the wet impregnation method in the presence of polyvinyl pyrrolidone (PVP). X-ray powder diffraction, transmission electron microscopy, and BET specific surface area measurement using N₂ adsorption have been used to characterize the catalysts. The catalytic test was conducted from room temperature in a flow of 900 ppm of toluene in air and gas hourly space velocity (GHSV) of 8000 h⁻¹. The catalytic activity was evaluated in terms of C₇H₈ conversion and the gas composition after the reaction was analyzed using two gas chromatographs with a flame ionization detector (FID) and a thermal conductivity detector (TCD). The Pt/CZB/Al₂O₃ catalysts are specific for the total toluene oxidation and CO and any toluene-derivative compounds were not detected as by-products. The specific surface area of the catalysts was increased by the addition of PVP in the preparation process. By the optimization of the amount of platinum, complete oxidation of toluene was realized at a temperature as low as 120 °C on a 7 wt%Pt/16 wt%Ce_0.64Zr_0.15Bi_0.21O_1.895/γ-Al₂O₃ catalyst.

A series of manganese oxides differing in the structure, composition, average manganese oxidation state and specific surface area have been used in the total oxidation of volatile organic compounds (VOC). Ethanol, ethyl acetate and toluene were chosen as models of VOC.

Among the manganese oxides tested, cryptomelane (KMn₈O₁₆) was found to be very active in the oxidation of VOC. The performance of cryptomelane was significantly affected by the presence of other phases, namely, Mn₂O₃ and Mn₃O₄. Temperature-programmed experiments combined with X-ray photoelectron spectroscopy (XPS) show that the mobility and reactivity of the oxygen species were significantly affected, explaining the catalytic performances of those samples. Mn₃O₄ improves the catalytic performance due to the increase of the reactivity and mobility of lattice oxygen, while Mn₂O₃ has the opposite effect. These results show that there is a correlation between the redox properties and the catalytic performance of the manganese oxides.

Temperature-programmed surface reactions (TPSR) after adsorption of toluene or ethanol, in addition to reactions performed without oxygen in the feed, show that lattice oxygen is involved in the VOC oxidation mechanism. The conversion level was found to be influenced by the type of VOC, the reactivity into CO₂ increasing in the following order: Toluene < Ethanol < Ethyl Acetate. The type of VOC is particularly important, as each VOC affects the reduction of the catalyst and, consequently, the incorporation rate of oxygen from the gas phase. Toluene decreases the oxygen mobility, so there is a slower incorporation rate of oxygen in the lattice, which explains the lower conversions observed.

The influence of the ZrO₂ support modification by Y₂O₃ and the presence of ethylenediamine (“en”) during the preparation of Co/ZrO₂ were studied and compared with a reference catalyst conventionally prepared by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. The effect of the en/Co molar ratio (x = 1–3) was studied. Activation of cobalt species was followed by differential thermal and thermogravimetric analyses (DTA/TG) analyses and by specific surface area measurements which evidence the complete cobalt precursor decomposition at 450 °C, whatever the support composition and the en/Co molar ratio. The addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase in the catalytic activity of the activated solids for the toluene deep oxidation as compared to the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion (X-ray diffraction (XRD)) and also in terms of Co-support interaction (H₂-temperature-programmed reduction (TPR)). The generated cobalt species were reducible at much lower temperatures and were more active in the toluene total oxidation. Finally, an efficient catalyst was produced combining the modifications of the support by yttrium oxide and of the precursor (use of ethylenediamine).

The modification by MnO_x significantly enhanced the activity and stability of Co₃O₄ for the preferential oxidation of CO in H₂-rich stream. The MnO_x-promoted Co₃O₄ catalyst with a Mn/Co molar ratio of 1/8 exhibited the best performance; CO conversion was 56% at 40 °C and increased to >90% as the temperature was raised to >75 °C. This catalyst could provide 100% CO conversion at 125–175 °C. CO conversions of 97% and 100% remained unchanged for 100 h at 100 and 125 °C, respectively, whereas the deactivation was observed over Co₃O₄ alone. Our characterizations using XRD, XPS and TEM indicated that the modification of Co₃O₄ with MnO_x caused the formation of smaller Mn_xCo_3−xO₄ solid-solution particles. The O₂-TPD, CO-TPD, and pulse reaction studies suggest that the incorporation of MnO_x into Co₃O₄ increased the amounts of reactive oxygen species and adsorbed CO species over catalyst surfaces and enhanced the regeneration ability of the reduced catalyst by O₂. All these enhancements are proposed to be responsible for the improved catalytic performance of the MnO_x-promoted Co₃O₄.

Several nanocrystalline Co₃O₄ catalysts were investigated for their activity and selectivity during the oxidation of 1,2-dichloroethane, which was selected as a model chlorinated volatile organic compound. A wide number of synthesis routes starting from cobalt(II) nitrate were examined, namely calcination of the precursor salt, solid-state reaction, precipitation and sol–gel. The catalysts prepared by precipitation decomposed the chlorinated feed at the lowest temperatures. Activity was observed to be chiefly governed by a small crystallite size which may give rise to more easily accessible active sites (oxygen –O⁻ or O²⁻– species), which were not present on the more highly crystalline Co₃O₄ catalysts. Additionally, surface Lewis acidity played a relevant catalytic role. Interestingly, the behaviour of some of the nanocrystalline oxides was superior to that of supported noble metal catalysts and other bulk oxide catalysts. Conversion to deep oxidation products was complete (CO₂, HCl and Cl₂), and no appreciable deactivation with time on stream was noticed.

Highly ordered mesoporous crystalline MoO₂ materials with bicontinuousIa3d mesostructure were synthesized by using phosphomolybdic acid as a precursor and mesoporous silica KIT-6 as a hard template in a 10% H₂ atmosphere via nanocasting strategy. The prepared mesoporous MoO₂ material shows a typical metallic conductivity with a low resistivity (0.01Ω cm at 300 K), which makes it different from all previously reported mesoporous metal oxides materials. Primary test found that mesoporous MoO₂ material exhibits a reversible electrochemical lithium storage capacity as high as 750 mA h g⁻¹ at C/20 after 30 cycles, rendering it as a promising anode material for lithium ion batteries.

Acetalization of glycerol with various aldehydes has been carried out using mesoporous MoO₃/SiO₂ as a solid acid catalyst. A series of MoO₃/SiO₂ catalysts with varying MoO₃ loadings (1–20 mol%) were prepared by sol–gel technique using ethyl silicate-40 and ammonium heptamolybdate as silica and molybdenum source respectively. The sol–gel derived samples were calcined at 500 °C and characterized using various physicochemical characterization techniques. The XRD of the calcined samples showed the formation of amorphous phase up to 10 mol% MoO₃ loading and at higher loading of crystalline α-MoO₃ on amorphous silica support. TEM analyses of the materials showed the uniform distribution of MoO₃ nanoparticles on amorphous silica support. Raman spectroscopy showed the formation of silicomolybdic acid at low Mo loading and a mixture of α-MoO₃ and polymolybdate species at high Mo loadings. Moreover the Raman spectra of intermediate loading samples also suggest the presence of β-MoO₃. Acetalization of glycerol with benzaldehyde was carried out using series of MoO₃/SiO₂ catalysts with varying MoO₃ loadings (1–20 mol%). Among the series, MoO₃/SiO₂ with 20 mol% MoO₃ loadings was found to be the most active catalyst in acetalization under mild conditions. Maximum conversion of benzaldehyde (72%) was obtained in 8 h at 100 °C with 60% selectivity for the six-membered acetal using 20% MoO₃/SiO₂. Interestingly with substituted benzaldehydes under same reaction conditions the conversion of aldehydes decreased with increase in selectivity for six-membered acetals. These results indicate the potential of this catalyst for the acetalization of glycerol for an environmentally benign process.

The dispersion of MoO₃ onto the surface of mesoporous silica MCM-41 by means of thermal spreading was studied using different characterization techniques. The XRD results showed that MoO₃ dispersed on the surface of MCM-41 and transformed into dispersed Mo oxide species after thermal treatment of the mixtures of MCM-41 and MoO₃ at 773 K for 24 h. The critical dispersion capacity of MoO₃ on the surface of MCM-41 was MoO₃/MCM-41 of 0.26 g/g, i.e. 1.6 μmol/m² of Mo/MCM-41. The ordered porous structure of MCM-41 was destroyed when the loading of MoO₃ exceeded the critical dispersion value. The surface area and pore volume of the treating mixtures decreased and there was a small reduction in pore size when the loading of MoO₃ was below the critical dispersion capacity, which suggested that the dispersed Mo oxide species was in the pores of MCM-41. A broad band near 957 cm⁻¹ emerged in the laser Raman spectra of the thermally treated mixtures, which was the evidence of dispersed Mo oxide species and was assigned to polymolybdates. The broadening full width at half-maximum (FWHM) and the loss of resolution for the Mo 3d doublet in the X-ray photoelectron spectra for the treated mixtures gave better evidence of the presence of Mo oxide species on the surface of MCM-41 than was found for the physical mixture.

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.

Recently, extensive works have been devoted to the morphology control of mesoporous materials with respect to their use in various applications. In this paper, we used two kinds of mesoporous silica, SBA-15 rods and spheres as hard templates to synthesize morphology-controllable mesoporous metal oxides. By carefully controlling the loading of metal precursors in the mesopores of the hard template, mesoporous Co₃O₄ and CeO₂ with different morphologies, such as micrometer-sized rod, hollow sphere, saucer-like sphere, and solid sphere were conveniently obtained. The structural properties of these materials were characterized by XRD, BET, SEM and TEM. In addition, it is found that the differences observed in the textural properties of the two mesoporous metal oxides nanocasted from the same template can be attributed to the properties of metal precursors and the interaction between metal oxide and SiO₂. Thus-obtained mesoporous metal oxides with such special morphologies may have a potential application in the field of environmental catalytic oxidation.

Rhombohedral chromia with three-dimensionally (3D) ordered mesopore structures were fabricated adopting the ultrasound-assisted nanocasting strategy with 3D ordered mesoporous silica (KIT-6) as hard template and chromium nitrate as metal source. The physicochemical properties of the materials were characterized by the XRD, TGA/DSC, BET, TEM/SAED, XPS, and H₂-TPR techniques, and their catalytic activities were evaluated for the oxidation of typical volatile organic compounds (VOCs), such as formaldehyde, acetone, and methanol. It is found that there were tri-, penta-, and hexavalent chromium ions in the 3D mesoporous structure. Compared to the bulk chromia, the mesoporous chromia materials were larger in surface area (69–124 m²/g) and could be reduced at lower temperatures. Among the chromia catalysts, themeso-Cr-400 one obtained after calcination at 400 °C showed the best performance. Overmeso-Cr-400 at space velocity = 30,000 mL/(g h), formaldehyde, acetone, and methanol conversions achieved 90% at 117, 124, and 130 °C, respectively; the corresponding apparent activation energies were 45.6, 49.7, and 50.8 kJ/mol. It is concluded that the ultrasound treatment was a key step to improve the mesoporosity quality of the chromia materials during the nanocasting process, and the factors, such as low-temperature reducibility, 3D ordered mesoporous architecture, and high-surface area, were responsible for the excellent catalytic performance ofmeso-Cr-400.

Three-dimensionally (3D) ordered and wormhole-like mesoporous iron oxides (denoted as Fe-KIT6 and Fe-CA) were respectively prepared by adopting the 3D ordered mesoporous silica KIT-6-templating and modified citric acid-complexing strategies, and characterized by a number of analytical techniques. It is shown that the Fe-KIT6-400 and Fe-CA-400 catalysts derived after 400 °C-calcination possessed high surface areas (113–165 m²/g), high surface adsorbed oxygen concentrations, and good low-temperature reducibility, giving 90% conversion below 189 and 208 °C for acetone and methanol oxidation at 20,000 mL/(g h), respectively. It is believed that the good catalytic performance of Fe-CA-400 and Fe-KIT6-400 was related to factors such as higher surface area and oxygen adspecies concentration, better low-temperature reducibility, and 3D mesoporous architecture.

Abstract

Zeolite P was synthesized for the first time via a novel water-bathing route at 90 °C using scrubbed diatomite, sodium hydroxide, and aluminum hydroxide as precursor, with SiO₂/Al₂O₃, SiO₂/Na₂O, and H₂O/Na₂O molar ratios of 7.43, 3.81, and 80.00, respectively. The as-fabricated samples were characterized by means of scanning electron microscopy, X-ray diffraction, and nitrogen adsorption measurements. This study showed that (i) treating the diatomite raw material with sodium hexametaphosphate could open the pores in the diatomite via removal of the clay clogged in its pores; (ii) tetragonal mesoporous zeolite P samples with a surface area of 56–60 m²/g could be generated after 6–24 h of water-bathing reaction at 90 °C; (iii) extension of water-bathing reaction time could improve the mesoporous structure of zeolite P; and (iv) Ca²⁺ adsorption capacity of the zeolite P sample was about 300 cmol/kg. Such high-surface-area porous zeolite P could be used as an effective adsorbent for the treatment of water containing calcium and magnesium ions.

Graphical abstract

Zeolite P was synthesized via a novel water-bathing route at 90 °C using scrubbed diatomite, sodium hydroxide, and aluminum hydroxide as precursor. Such high-surface-area porous zeolite P could be used as an effective adsorbent for the treatment of water containing calcium and magnesium ions.

Cobalt catalysts supported on silica were prepared by an impregnating method and characterized using temperature-programmed reduction (TPR). H₂ was used as the reduction agents. The two-stage reduction was observed. Co₃O₄ was reduced to CoO and then reduced to metallic Co. The activation energies for the two reduction steps of cobalt oxide are 94.43 and 82.97 kJ mol⁻¹, respectively. The simulation by reduction models of the TPR patterns presents well-fitting of two-dimensional nucleation according to Avarmi-Erofeev model for the reduction of Co₃O₄ to CoO and unimolecular model for the reduction of CoO to Co.