Pinliang Ying

Reduction property and catalytic activity of Ce1−XNiXO2 mixed oxide catalysts for CH4 oxidation

Abstract Ce 1− X Ni X O 2 oxides with X varying from 0.05 to 0.5 were prepared by different methods and characterized by XRD and TPR techniques. Ce 0.7 Ni 0.3 O 2 sample prepared by sol–gel method shows the highest reducibility and the highest catalytic activity for methane combustion. Three kinds of Ni phases co-exist in the Ce 1− X Ni X O 2 catalysts prepared by sol–gel method: (i) aggregated NiO on the support CeO 2 , (ii) highly dispersed NiO with strong interaction with CeO 2 and (iii) Ni atoms incorporated into CeO 2 lattice. The distribution of different Ni species strongly depends on the preparation methods. The highly dispersed NiO shows the highest activity for methane combustion. The NiO aggregated on the support CeO 2 shows lower catalytic activity for methane combustion, while the least catalytic activity is found for the Ni species incorporated into CeO 2 . Any oxygen vacancy formed in CeO 2 lattice due to the incorporating of Ni atoms adsorbs and activates the molecular oxygen to form active oxygen species. So the highest catalytic activity for methane combustion on Ce 0.7 Ni 0.3 O 2 catalyst is attributed not only to the highly dispersed Ni species but also to the more active oxygen species formed.

UV RESONANCE RAMAN SPECTROSCOPIC IDENTIFICATION OF TITANIUM ATOMS IN THE FRAMEWORK OF TS-1 ZEOLITE

Framework titanium atoms in titanium-substituted silicalite (TS-1) can be identified by UV resonance Raman spectroscopy since the associated Raman bands at 1125, 530, and 490 cm(-1) (see figure) are observed only when the charge transfer transition associated with the framework Ti atoms is excited by a UV laser. Thus, framework Ti atoms can be distinguished from nonframework Ti atoms and other defect sites. This method can be applicable to identifying transition metal atoms in the frameworks of other molecular sieves.

From Molecular Fragments to Crystals: A UV Raman Spectroscopic Study on the Mechanism of Fe-ZSM-5 Synthesis

The nucleation process of iron-exchanged zeolite Fe-ZSM-5, from the assembly of distorted tetrahedrally coordinated iron species and silicate rings in the precursor to the final Fe-ZSM-5 crystals, as well as variations in the coordination environment of iron, were studied by UV resonance Raman spectroscopy and complementary techniques.The entire sequence of crystallization events of Fe-ZSM-5 was monitored by UV Raman spectroscopy in combination with HRTEM, UV/Vis spectroscopy, X-ray diffraction patterns, and periodic DFT calculations. Fe-ZSM-5 was synthesized by an organic-free method to avoid signal interference from the organic template in Raman spectra. Framework iron atoms with resonance Raman bands at 516, 1115, and 1165 cm(-1), and a Raman band at 1016 cm(-1) are detected for Fe-ZSM-5. In the early stage of Fe-ZSM-5 synthesis, the precursor contains iron atoms in distorted tetrahedral coordination and five- and six-membered silicate rings. Nucleation by aggregation of the precursor species was monitored by UV Raman spectroscopy based on the resonance Raman effect, and confirmed by periodic DFT calculations. Evolution of iron species on the surface and in the bulk phase was monitored by UV Raman spectroscopy with excitation at 244 and 325 nm, as well as HRTEM. Nucleation takes place first in the core of the amorphous particles, and crystalline nuclei with Fe-ZSM-5 structure are formed in the core by consuming the amorphous shell. Finally the amorphous particles are completely transformed into Fe-ZSM-5 crystals.

In situ FT-IR spectroscopic studies of CO adsorption on fresh Mo2C/Al2O3 catalyst

The surface sites of supported molybdenum carbide catalyst derived from different synthesis stages have been studied by in situ FT-IR spectroscopy using CO as the probe molecule. Adsorbed CO on the reduced passivated Mo2C/Al2O3 catalyst gives a main band at 2180 cm(-1), which can be assigned to linearly adsorbed CO on Mo(4+) sites. The IR results show that the surface of reduced passivated sample is dominated by molybdenum oxycarbide. However, a characteristic IR band at 2054 cm(-1) was observed for the adsorbed CO on MoO3/Al2O3 carburized with CH4/H2 mixture at 1033 K (fresh Mo2C/Al2O3), which can be assigned to linearly adsorbed CO on Mo(δ+) (0 < δ < 2) sites of Mo2C/Al2O3. Unlike adsorbed CO on reduced passivated Mo2C/Al2O3 catalyst, the IR spectra of adsorbed CO on fresh Mo2C/Al2O3 shows similarity to that on some of the group VIII metals (such as Pt and Pd), suggesting that fresh carbide resembles noble metals. To study the stability of Mo2C catalyst during H2 treatment and find proper conditions to remove the deposited carbon species, H2 treatment of fresh Mo2C/Al2O3 catalyst at different temperatures was conducted. Partial amounts of carbon atoms in Mo2C along with some surface-deposited carbon species can be removed by the H2 treatment even at 450 K. Both the surface-deposited carbon species and carbon atoms in carbide can be extensively removed at temperatures above 873 K.

In Situ UV Raman Spectroscopic Studies on the Synthesis Mechanism of Zeolite X

have been used to studythe formation mechanism of zeo-lites. Most studies were performed byusing ex situ tech-niques,namelybyfrequentlyremovingaliquotsofthereac-tion mixture andanalyzing thesamplesafter quenching thereaction. However, microporous zeolite-type materials areusually synthesized under hydrothermal conditions, and theneedforsamplequenchingandworkupmaycausedramaticand undeterminable structural changes.

In Situ FT-IR Study of Photocatalytic Decomposition of Formic Acid to Hydrogen on Pt/TiO2 Catalyst

Abstract The anaerobic photocatalytic decomposition of formic acid to hydrogen on Pt/TiO2 was studied by in situ FT-IR spectroscopy. The molecularly adsorbed formic acid species is transformed to a formate species, and the formate species is transformed to carbonates during this reaction. The addition of water vapor in the reaction system strongly accelerates this photocatalytic reaction and promotes the H2 production efficiency. The possible mechanism of the reaction is proposed.

The effect of oxygen on the aromatization of methane over the Mo/HZSM‐5 catalyst

The aromatization of methane over a Mo/HZSM‐5 catalyst was carried out in the presence of oxygen. It is shown that the addition of a small amount of oxygen is beneficial to improve the durability of the catalyst. UV‐Raman spectra disclose that the carbonaceous deposits formed on the HZSM‐5 are mainly polyolefinic and aromatic, while that on the Mo/HZSM‐5 is mainly polyaromatic. The small amount of O2 added may partly remove the coke deposits on the active sites and keep the catalyst as MoOxCy/HZSM‐5, thus resulting in an improvement of the catalytic performance of the Mo/HZSM‐5 catalyst.

Phase transformation in the surface region of zirconia and doped zirconia detected by UV Raman spectroscopy

The phase evolution of yttrium oxide and lanthanum oxide doped zirconia (Y2O3–ZrO2 and La2O3–ZrO2, respectively) from their tetragonal to monoclinic phase has been studied using UV Raman spectroscopy, visible Raman spectroscopy and XRD. UV Raman spectroscopy is found to be more sensitive at the surface region while visible Raman spectroscopy and XRD mainly give the bulk information. For Y2O3–ZrO2 and La2O3–ZrO2, the transformation of the bulk phase from the tetragonal to the monoclinic is significantly retarded by the presence of yttrium oxide and lanthanum oxide. However, the tetragonal phase in the surface region is difficult to stabilize, particularly when the stabilizer’s content is low. The phase in the surface region can be more effectively stabilized by lanthanum oxide than yttrium oxide even though zirconia seemed to provide more enrichment in the surface region of the La2O3–ZrO2 sample than the Y2O3–ZrO2 sample, based on XPS analysis. The surface structural tension and the enrichment of the ZrO2 component in the surface region of ZrO2–Y2O3 and ZrO2–La2O3 might be the reasons for the striking difference between the phase change in the surface region and the bulk. Accordingly, the stabilized tetragonal surface region can significantly prevent the phase transition from developing into the bulk when the stabilizer’s content is high.

Interaction of methane with surfaces of silica, aluminas and HZSM-5 zeolite. A comparative FT-IR study

Infrared investigations on the interaction of methane with silica, aluminas (η,γ and α) and HZSM-5 zeolite have been carried out. At low temperature (173 K), methane adsorption was observed over these oxides and HZSM-5 zeolite. Our findings featured that the infrared inactiveΝ1 band (2917 cm−1) of a gaseous methane molecule became active and shifted to lower frequencies (2900 and 2890 cm−1) when it adsorbed on the surfaces of these adsorbents. Our results also demonstrate that hydroxyl groups played a very important role in methane adsorption over the acidic oxides and the HZSM-5 zeolite. When interaction between the hydroxyl groups and methane took place, the band shift of the hydroxyl groups varied with different oxides. The strength of the interaction decreased according to the following sequence, Si-OH-Al>Al-OH>Si-OH, which is in accordance with the order of their acidities. At higher temperatures, methane interacted quite differently with various oxides and HZSM-5 zeolite. It has been observed that the hydroxyl groups of silica, γ-alumina and HZSM-5 zeolite could exchange with CD4 at temperatures higher than 773K, while those on η-alumina could exchange at a temperature as low as 573 K. Another interesting observation was the formation of formate species over Al2O3 (both η and γ) at temperatures higher than 473 K. The formate species would decompose to CO2, or produce carbonate at much higher temperatures. Formation of formate species was not observed over silica and HZSM-5 under similar conditions, α-Al2O3 did not adsorb or react with methane in any case.

The visible luminescent characteristics of ZnO supported on SiO2 powder

In situ laser-induced luminescence spectroscopy is used to study the visible luminescent characteristics of ZnO during the preparation process of ZnO supported on SiO2 by the pyrolysis of different Zn precursors in N2 or O2 atmosphere. The excitation source is 325 nm light, which is above the band gap (3.37 eV) of ZnO. In N2 atmosphere, it is shown that green (centered at ca. 520 nm), yellow (centered at ca. 580 nm) and orange (centered at ca. 640 nm) luminescence bands appear for ZnO produced from zinc acetate, zinc hydroxide and zinc nitrate, respectively. After these samples are treated by O2, green band is changed into yellow band and yellow band is changed into orange band. On the other hand, it is also found that the laser irradiation on the sample could alter the luminescent behavior of ZnO produced at the beginning decomposition temperature of the Zn precursors. While this sample is irradiated, the orange band is gradually changed to a yellow band, the luminescent intensity finally increases more than 30 times that at the beginning of irradiation. However, irradiation hardly affects the luminescent properties of ZnO after calcination above 160 °C. The results indicate that the visible luminescence from ZnO is associated with the oxygen vacancies in ZnO, and the electronic state levels responsible for the visible luminescence bands are changing with the density of oxygen vacancies in ZnO. The green, yellow and orange bands are ascribed to the state of ZnO with high density of oxygen vacancies, with moderate density of oxygen vacancies and with less oxygen vacancies, respectively.