Haiqiang Wang

Low-temperature selective catalytic reduction of NO with NH3 over MnCe oxides supported on TiO2 and Al2O3: A comparative study

MnCe oxides were supported on TiO(2) and Al(2)O(3) by an ultrasonic impregnation method and used for selective catalytic reduction (SCR) of NO with NH(3) at low-temperature (80-220 degrees C). MnCe/TiO(2) showed a relatively higher SCR activity than MnCe/Al(2)O(3) at the temperature range of 80-150 degrees C. When the reaction temperature was higher than 150 degrees C, MnCe/Al(2)O(3) exhibited superior SCR activity to MnCe/TiO(2). NH(3) temperature programmed desorption study proved that MnCe/TiO(2) was mainly Lewis acidic, while MnCe/Al(2)O(3) could provide more Brönsted acid sites. These acid sites play an important role in SCR according to in situ diffuse reflectance infrared transform spectroscopy (DRIFT) analysis. The main SCR reaction was a typical Eley-Rideal mechanism on MnCe/TiO(2), which took place between coordinated NH(3)/NH(4)(+) and gas-phase NO. For MnCe/Al(2)O(3), the reaction mainly occurred via another pathway when the temperature exceeded 150 degrees C, which commenced with the adsorption and oxidation of NO and was followed by reaction between NO(2) or NO(2)-containing compounds and NH(3) adspecies. This reaction pathway makes a significant contribution to the improved NO conversion for MnCe/Al(2)O(3) at higher temperature.

The fabrication and characterization of novel carbon doped TiO2 nanotubes, nanowires and nanorods with high visible light photocatalytic activity

Novel carbon doped TiO(2) nanotubes, nanowires and nanorods were fabricated by utilizing the nanoconfinement of hollow titanate nanotubes (TNTs). The fabrication process included adsorption of ethanol molecules in the inner space of TNTs and thermal treatment of the complex in inert N(2) atmosphere. The structural morphology of carbon doped TiO(2) nanostructures can be tuned using the calcination temperature. X-ray diffraction, Raman and Brunauer-Emmett-Teller studies proved that the doped carbon promoted the crystallization and phase transition by acting as nucleation seeds. X-ray photoelectron spectroscopy (XPS) showed that O-Ti-C and Ti-O-C bonds were formed in the nanostructures. Additional electronic states from the XPS valence band due to carbon doping were observed. This evidence indicated the electronic origin of the band gap narrowing and visible light absorption. The differences in chemical and electronic states between the surface and bulk of as-prepared samples confirmed that carbon was doped into the lattice of TiO(2) nanostructure through an inner doping process. The as-prepared catalysts exhibited enhanced photocatalytic activity for degradation of toluene in gas phase under both visible and simulated solar light irradiation compared with that of commercial Degussa P25. This novel fabrication approach can valuably contribute to designing nanostructured photocatalytic materials and modifying various nanotube materials.

Characterization and activity of Pd-modified TiO2 catalysts for photocatalytic oxidation of NO in gas phase

Pd-modified TiO(2) prepared by thermal impregnation method was used in this study for photocatalytic oxidation of NO in gas phase. The physico-chemical properties of Pd/TiO(2) catalysts were characterized by X-ray diffraction analysis (XRD), Brunauer-Emmett-Teller measurements (BET), X-ray photoelectron spectrum analysis (XPS), transmission electron microscopy (TEM), high resolution-transmission electron microscopy (HR-TEM), UV-vis diffuse reflectance spectra (UV-vis DRS) and photoluminescence spectra (PL). It was found that Pd dopant existed as PdO particles in as-prepared photocatalysts. The results of PL spectra indicated that the photogenerated electrons and holes were efficiently separated after Pd doping. During in situ XPS study, it was found that the content of hydroxyl groups on the surface of Pd/TiO(2) increased when the catalyst was irradiated by UV light, which could result in the improvement of photocatalytic activity. The activity test showed that the optimum Pd dopant content was 0.05 wt.%. And the maximum conversion of NO was about 72% higher than that of P25 when the initial concentration of NO was 200 ppm, which showed that Pd/TiO(2) photocatalysts could be potentially applied to oxidize higher concentration of NO.

MnOx/TiO2 composite nanoxides synthesized by deposition-precipitation method as a superior catalyst for NO oxidation

A series of MnO(x)/TiO(2) composite nanoxides were prepared by deposition-precipitation (DP) method, and the sample with the Mn/Ti ratio of 0.3 showed a superior activity for NO catalytic oxidation to NO(2). The maximum NO conversion over MnO(x)(0.3)/TiO(2)(DP) could reach 89% at 250°C with a GHSV of 25,000h(-1), which was much higher than that over the catalyst prepared by conventional wet-impregnation (WI) method (69% at 330°C). Characterization results including XRD, HRTEM, FTIR, XPS, H(2)-TPR, NO-TPD and Nitrogen adsorption-desorption implied that the higher activity of MnO(x)(0.3)/TiO(2)(DP) could be attributed to the enrichment of well-dispersed MnO(x) on the surface and the abundance of Mn(3+) species. Furthermore, DRIFT investigations and long-time running test indicated that NO(2) came from the decomposition of adsorbed nitrogen-containing species.

Enhanced visible light photocatalytic activity of novel Pt/C-doped TiO2/PtCl4 three-component nanojunction system for degradation of toluene in air

C-doped TiO(2) nanoparticles prepared by partial oxidation of TiC were modified with Pt species by impregnation-calcination method in order to enhance the visible light photocatalytic activity. The physicochemical properties of as-prepared samples were characterized by various techniques in detail. The results indicated that a novel Pt/C-doped TiO(2)/PtCl(4) three-component nanojunction system was formed, where C-doped TiO(2) and PtCl(4) behaved as two visible light responsive components, and Pt metal as electron-transfer system. The three-component nanojunctioned photocatalyst system exhibited six times higher visible light activity than that of the pristine C-doped TiO(2) in degradation of toluene in air. The dramatically enhanced activity can be attributed to the increased utilization of visible light, the enhanced charge carrier separation and transfer process. Further more, the band structure and photocatalysis mechanism over the three-component nanojunction system was proposed and discussed. This work may provide new insights into the design of novel multi-component photocatalyst system with efficient visible light activity.

Photocatalytic reduction of NO with NH3 using Si-doped TiO2 prepared by hydrothermal method.

A series of Si-doped TiO2 (Si/TiO2) photocatalysts supported on woven glass fabric were prepared by hydrothermal method for photocatalytic reduction of NO with NH3. The photocatalytic activity tests were carried out in a continuous Pyrex reactor with the flow rate of 2000mL/min under UV irradiation (luminous flux: 1.1x10(4)lm, irradiated catalyst area: 160cm2). The photocatalysts were characterized by X-ray diffraction (XRD), BET, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectrophotometer, transmission electron microscopy (TEM), photoluminescence (PL) and temperature-programmed desorption (TPD). The experiment results showed that NO conversion on Si/TiO2 at 323K could exceed 60%, which was about 50% higher than that on Degussa P25 and pure TiO2. With the doping of Si, photocatalysts with smaller crystal size, larger surface area and larger pore volume were obtained. It was also found that Ti-O-Si bands were formed on the surface of Si/TiO2 and that the surface hydroxyl concentration was greatly increased. As a result, total acidity and NH3 chemisorption amount were enhanced for Si/TiO2 leading to its photocatalytic activity improvement.

Deactivation mechanism of Ce/TiO2 selective catalytic reduction catalysts by the loading of sodium and calcium salts

In this paper, the poisoning effect of alkali and alkaline earth metal on Ce/TiO2 catalysts was investigated for the first time and a deactivation mechanism was proposed. The Ce/TiO2 catalyst was observed to be deactivated seriously by the loading of Na+, K+ or Ca2+ ions. When the Na/Ce, K/Ce or Ca/Ce molar ratio exceeded 0.25, NO conversion of the Ce/TiO2 catalyst at 380 °C decreased from 78% to negligibly low. After subjecting it to a range of analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectra etc., it was found that amorphous ceria was dominant in the Ce/TiO2 sample, and would change to ceria crystals with the loading of Na+ or Ca2+ ions, leading to a poor dispersion of ceria on the surface of TiO2 as well as the transformation of Ce3+ to Ce4+. Both of these led directly to the disappearance of oxygen vacancies in the ceria particles, slowing down the reduction rate of ceria and hence decreasing its rate of oxidation/reduction recycle. We proposed that the enlargement of ceria nanoparticles, the reduced Ce4+/Ce3+ redox cycle rate and the change in the surface acidity could be the three major factors contributing to the decline of selective catalytic reduction (SCR) activity of the Ce/TiO2 catalyst after loading with Na+ or Ca2+ ions.

Influences of various Pt dopants over surface platinized TiO2 on the photocatalytic oxidation of nitric oxide

Various surface platinized TiO(2) were prepared by four different preparation methods and investigated with respect to their behaviors in UV photocatalytic oxidation of nitric oxide. The physicochemical properties of the Pt modified TiO(2) were investigated by X-ray diffraction analysis, X-ray photoelectron spectrum analysis, transmission electron microscopy, and photoluminescence spectra. From the experimental results, it was found that new electronic states were observed above the valence bands of PtOx-TiO(2) and PtClx-TiO(2). And the lifetime of electrons and holes was found prolonged in the PtOx-TiO(2) catalysts. The activity tests showed that the dopants existed as metallic Pt and platinum chloride had little contribution to the photocatalytic oxidation of NO in gas phase. However, the dopant which existed as PtOx could improve the NO photocatalytic oxidation efficiency and the reaction rate. The photocatalytic activity of the 0.05 at% PtOx-TiO(2) was nearly three times higher than that of the pure Degussa P25 with an inlet NO concentration of 200 ppm.

Marked enhancement of photocatalytic activity and photochemical stability of N-doped TiO2 nanocrystals by Fe3+/Fe2+ surface modification

N-doped TiO(2) (N-TiO(2)) nanocrystals with anatase and rutile mixed phases were prepared by partial oxidation of TiN. The samples were further modified by Fe-ions through incipient wetness impregnation method. The as-prepared samples were characterized by XRD, TEM, XPS, Raman, EPR, UV-vis DRS, and PL in detail. The results indicated that Fe mainly existed as Fe(3+)/Fe(2+) ions on the catalyst surface. The addition of small amounts of Fe-ions to N-TiO(2) nanocrystals caused several times enhancement of the photocatalytic activity under visible, UV and UV-vis light irradiation in degradation of gaseous toluene. The optimized Fe-ions content in this investigation was 0.02 wt.%. EPR and PL clearly showed that Fe(3+)/Fe(2+) redox cycle facilitated electron/hole charge separation, and contributed to the enhanced photocatalytic performance. Moreover, the photochemical stability of N-TiO(2) nanocrystals under visible light was improved due to the stabilization of nitrogen atoms in TiO(2) lattice by surface Fe-ions modification. The N-doped TiO(2) nanocrystals without Fe-ions modification suffered from a gradual deactivation due mainly to the loss of lattice-nitrogen during the photocatalytic reaction. The way to modification of nonmetal-doped TiO(2) nanomaterials brought new concept in enhancing the photocatalytic performance from the viewpoint of practical application.

Polyethyleneimine functionalized protonated titanate nanotubes as superior carbon dioxide adsorbents

In this paper, protonated titanate nanotubes (PTNTs) were modified with polyethyleneimine (PEI) by wet impregnation method for CO(2) adsorption. Their micro-morphology and structural properties were characterized by a range of analytical techniques, including XRD, TEM, SEM, N(2) adsorption etc. Experimental results revealed that the functionalized PTNTs with 50 wt.% PEI loaded exhibited a high CO(2) adsorption capacity of 130.8 mg/g-sorbent at 100°C. Only a minor loss of its capacity was observed after five consecutive adsorption-desorption runs. The PEI was existed both in the internal and external mesoporous pores of PTNTs via chemical combination between amino group and enriched protons, which accounted for their good thermal stability at elevated temperatures. The results present herein imply that the PEI modified PTNTs could be appealing materials for capturing CO(2) from power plant flue gas.