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.     

EXAFS characterization of Ti/Al2O3 supports and Co/Ti/Al2O3 catalysts

The structure of Ti/Al₂O₃ supports (0–14 wt% Ti) and Co/Ti/Al₂O₃ catalysts (3 wt% Co) was examined by EXAFS. The results indicated that the Ti was present primarily as a highly dispersed surface phase. The Ti EXAFS results indicated that the Ti species were octahedrally coordinated. Evidence of Ti—Ti interactions was found for all loadings (2–14 wt% Ti) suggesting that the Ti surface species are present as small clusters of TiO₂.The Co EXAFS results showed evidence for several structurally different Co surface phases as a function of Ti loading. Evidence of a Co species interacting with the Ti surface phase was observed for the 3% Co/2% Ti-3%Co/6%Ti catalysts. At the highest loadings studied, 3%Co/8%Ti and 3%Co/14%Ti, evidence was found for a CoTiO₃-like phase.     

Ultradeep Hydrodesulfurization of Dibenzothiophene (DBT) Derivatives over Mo-Sulfide Catalysts Supported on TiO2-Coated Alumina Composite

The present paper reviews in detail the different studies now being conducted by our research team concerning the ultradeep hydrodesulfurization (HDS) of dibenzothiophene (DBT) derivatives over Mo/TiO₂ and Mo/TiO₂–Al₂O₃ catalysts. First, a detailed characterization of Mo/TiO₂ (P-25 Degussa, 50 m²/g) catalysts prepared by equilibrium adsorption technique shows that Mo- species are highly and uniformly dispersed on the surface of titania up to 6.6 wt% MoO₃ loading. Above this value, some aggregation of Mo occurs, leading to the formation of bulk MoO₃. Below 6.6 wt% MoO₃ loading, the Raman spectroscopy data of the calcined samples show that the supported Mo-species possess a highly distorted octahedral MoO₆ structure. TiO₂–Al₂O₃ composites were prepared by chemical vapor deposition (CVD) using TiCl₄ as a precursor. Using several characterization techniques, we demonstrated that the support composite presents a high dispersion of TiO₂ over -Al₂O₃ without forming precipitates up to ca. 11 wt% loading. Moreover, the textural properties of the composite support are comparable to those of alumina. Under the present sulfidation conditions (673 K, 5%H₂S/95%H₂), Mo-species supported on TiO₂ are better sulfided than on alumina, as demonstrated using XPS. This can be attributed to the relatively lower interaction between Mo-species and titania. The state of sulfide species supported on the composite support can be considered as a transition state between TiO₂ and Al₂O₃. However, at relatively higher TiO₂ loadings (ca. 11 wt%), Mo/TiO₂–Al₂O₃ catalysts exhibit sulfidability similar to that of Mo/TiO₂. The HDS tests conducted in both the laboratory and in industry show that sulfide catalysts supported on TiO₂–Al₂O₃ (ca. 11 wt% TiO₂) are more active than those supported on TiO₂ or Al₂O₃.     

Transition metal loaded TiO2 for phenol photo-degradation

Photocatalytic degradation of phenol under both UV radiation and visible light, using TiO₂ (Degussa P-25) and TiO₂ loaded with some transition metal ions (Co, Cu, Fe and Mo) was examined. From the series of metal loaded catalysts, Mo/TiO₂ was the most efficient one. In the presence of Mo, neither TiO₂ anatase/rutile fraction nor its pore size diameter has been affected. However, Mo made its surface more acidic. The percentage of phenol degradation reached under visible light was significantly lower than that under UV radiation due to the lower degree of light absorption by the catalyst surface. From the series of studied catalysts, 2 wt% Mo/TiO₂ was the most efficient one. The synergetic effect between S_BET, mean pore size diameter, catalyst agglomerate size, band gap, ZPC and the type of Mo_xO_y species on TiO₂ surface, depending on Mo loading, created its photocatalytic performance.     

Increased promotion by deposition of Co acetylacetonate over sulphided Mo/TiO2 hydrodesulphurization catalyst

CoMo/TiO₂ catalysts prepared by deposition of Co acetylacetonate on presulphided Mo/TiO₂ catalyst in methanol exhibited higher promotion of hydrodesulphurization activity than catalysts prepared by conventional impregnation of Mo/TiO₂ by a solution of Co nitrate.     

Study of the local structure of supported nanostructural platinum catalysts

The possibility of controlling the state of platinum deposited on the support surface via minor changes in the catalyst preparation procedure is demonstrated using a series of highly dispersed Pt/γ-Al₂O₃ catalysts with different particle size of the active component. Dispersity, local structure and electronic state of supported platinum were examined by a combination of high resolution transmission electron microscopy and X-ray absorption spectroscopy (EXAFS/XANES). It was shown that various platinum species can be obtained on the surface of the support: bulk or surface Pt(II) or Pt(IV) oxides, mixed metal-oxide structures, bulk particles of metallic platinum, and two-dimensional surface Pt⁰ particles strongly interacting with the support.     

Fluoride modification of Mo/Al2O3 catalysts: Characterization of the changes induced in support and Mo phases

The characterization of fluoride-modified Mo/Al₂O₃ catalysts was performed in order to investigate on the effect that low levels of fluoridation of the alumina support (0-2.0 wt.%) cause on the support itself and on the supported Mo oxide and sulfide phases. Fluoride-modified Al₂O₃ supports and Mo/Al₂O₃ catalysts where characterized by nitrogen physisorption, scanning electronic microscopy (SEM-EDX), isoelectric point (IEP), Fourier transform infrared spectroscopy (FT-IR), infrared spectroscopy of adsorbed CO₂ (IR-CO₂), and temperature programmed reduction (TPR). The dispersion of the sulfided catalysts was estimated by dynamic NO chemisorption. The results indicate that the different hydroxyl types present on the alumina surface react to a different extent with fluoride and that it is the most basic hydroxyl groups that are titrated first.The consumption of the alumina OH by F⁻, inhibits, during the deposition of Mo, the formation of tetrahedral molybdenum oxide species in strong interaction with the support, leading to an increased number of polymeric octahedral Mo surface species. The NO adsorption results put in evidence a drop in the dispersion of the MoS₂ phase present in the sulfided samples.     

光驱动多孔无定形TiO2的形成机制与光催化性能的研究

以钛酸正丁醇和乙二醇为原料,采用溶剂热法合成了钛乙二醇盐(TG)前躯体,在高压汞灯照射下制备出无定形TiO2。利用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)、紫外-可见吸收光谱(UV-Vis)、表面光电压谱(SPS)、N2吸附-脱附对所得材料进行了结构和性能的表征。借助X射线吸收精细结构(XAFS)对无定形TiO2的形成机制进行了分析,并通过硝基苯的还原反应考察了材料的光催化性能。结果表明:在紫外光驱动无定形TiO2的形成过程中,中心元素Ti4+的配位环境发生变化,由八面体结构转变为四面体结构;由于特殊的孔道结构使得多孔无定形TiO2显示出较好的光催化活性。     

X-ray absorption near edge structures (xanes) of Cu/ZnO/Al2O3 Co shift catalysts

X-ray absorption near edge structures (XANES) of some Cu/ZnO/Al₂O₃CO shift catalysts have been measured at high resolution using synchrotron radiation, and compared to reference compounds Prior to reduction. CuO and ZnO are present, after reduction ZnO and highly dispersed Cu, no indication was found for Cu₂O or the presence of spinels. The results are qualitatively confirmed by preliminary analysis of EXAFS spectra.     

Effect of the support calcination temperature on selective hydrodesulfurization of TiO2 nanotubes supported CoMo catalysts

TiO₂ nanotubes (TiO₂-NTs) were synthesized by the hydrothermal method. Co and Mo active components were supported on a series of the as-prepared TiO₂-NTs samples which were calcined at different temperatures. The effects of support calcination temperature of CoMo/TiO₂-NTs catalysts on their catalytic performance were investigated for selective hydrodesulfurization (HDS). The samples were characterized by means of the scanning electron microscopy (SEM), the transmission electron microscopy (TEM), N₂ adsorption-desorption, X-ray diffraction (XRD), Raman spectroscopy and H₂ temperature-programmed reduction (H₂-TPR). The experimental results revealed that TiO₂-NTs support calcined under 500 °C can maintain the nanotubular structure with higher surface area and pore volume. Meanwhile, the obtained supported CoMo/TiO₂-NTs catalysts exhibited weak metal-support interaction, more octahedral Mo⁶⁺ species and high catalytic performance in selective HDS.     

Direct photo-oxidation of benzene to phenol over Ti/Si binary oxide prepared by sol-gel method

Titanium-silicon (Ti/Si) binary oxides having a varying Ti content were prepared using the sol-gel method and used as photocatalysts. The photo-oxidation of benzene to phenol was carried out using Ti/Si binary oxide catalysts in the presence of benzene, water and gaseous oxygen. The amounts of benzene used hardly affected the phenol yield. On the other hand, the addition of sulfuric acid into reaction solution led to the improvement of phenol formation. The H₂O₂ formation rate seemed to be related to the phenol yeilds. The photocatalytic reactivity of Ti/Si binary oxide having different TiO₂ contents was investigated and it was found to be dramatically enhanced in the range of lower TiO₂ contents. XANES, ESR and XRD spectroscopic investigations of these Ti/Si binary oxide catalysts indicated that Ti species were highly dispersed in SiO₂ matrices and existed in a tetrahedral coordination. The photo-irradiation of catalysts having tetrahedral Ti species seemed to effectively lead to the formation of H₂O₂ as an intermediate.     

Interpretation of the difference of optimal Mo density in MoS<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MoS<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> HDS catalysts

The first part of this paper deals with the morphology of the MoS₂ phase and its oxide precursor, the MoO₃ phase, mainly from a geometrical point of view. After giving a brief review of the literature describing the structure of these compounds, Mo densities in both phases were calculated along various crystallographic planes. Further, using structural models recently proposed by others, Mo densities in MoS₂ were also calculated in the case of an epitactic growth on γ-Al₂O₃ and TiO₂ model surfaces. Then, the calculated Mo densities were compared with experimental results (Mo density when HDS activity is maximal) previously obtained for catalysts constituted of MoS₂ supported on a low SSA TiO₂, a high SSA TiO₂ and a conventional γ-alumina. It was suggested that either on alumina or titania the MoS₂ phase is growing as (100) MoS₂ planes. However, while on the alumina the optimal MoS₂ phase might be constituted of dispersed MoS₂ slabs covering only a part of the alumina surface (2.9–3.9 Mo atoms/nm²), on titania the optimal MoS₂ phase might be constituted of a uniform MoS₂ monolayer (5.2 atoms/nm² for the high SSA titania, which is equal to the Mo density of a perfect MoS₂ (100) plane). This difference may originate in the creation of a 'TiMoS' phase enhancing the S atoms mobility over Mo/TiO₂-sulfided catalysts. Indeed, while in the case of a γ-alumina carrier the active sites (labile S atoms) are located on the edge of MoS₂ slabs making the ratio Mo_edge/Mo_total a crucial parameter for the catalytic performances, in the case of a titania carrier the labile sulfur atoms might be statistically distributed all over the TiMoS active phase. Further, the higher Mo density observed over the high SSA titania (5.2 atoms/nm²) when compared to that over the low SSA titania (4.2 atoms/nm²) was supposedly due to the pH-swing method advantageously used to prepare the former carrier. Indeed, this method allows giving a solid with enhanced mechanical properties providing a good stability to the derived catalysts under experimental conditions. In addition, this TiO₂ carrier exhibits a great homogeneity, with a surface structure substantially uniform, which might be adequate for a long-range growth of (100) MoS₂ slabs.     

Pt LIII-edge XANES and EXAFS studies on γ-Al2O3 supported Pt catalysts

Pt L_III-edge XANES and EXAFS were employed to investigate the nature of Pt/γ-Al₂O₃, Pt−Sn/γ-Al₂O₃ and Pt−Fe/γ-Al₂O₃ catalysts. The results indicated that Pt species on these catalysts were all in the oxidized states before reduction, and in the metallic states after reduction. The dispersity of the Pt species on the catalysts was very high after reduction. The electronic properties of the highly dispersed Pt species were different from that of the bulk Pt in large crystallites. An interaction between Pt and the metal-oxide modified γ-Al₂O₃ support is proposed. The interaction improved the dispersity of the Pt species on the catalysts and is thought to be the reason for the enhanced activity and selectivity for dehydrogenation reactions over these catalysts.     

A study of Co/Mo/Al2O3 hydrodesulphurization catalysts and related model compounds byxps and x-ray absorption spectroscopy (xanes andexafs )

A detailed investigation of sulphided Co/Mo/Al₂O₃ catalysts, their oxide precursors and several model oxides and sulphides of cobalt and molybdenum has been carried out using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy (xanes andexafs). Octahedrally coordinated Co(II) and Mo(IV) are shown to be present in a sulphidic environment on the surfaces of these catalysts. The surface species contain an excess of sulphur, probably involving disulphide linkages. The surface compositions of the catalysts examined conform to the general formula Co¹¹ Mo _2n ^IV (2n + 3)S ₂ ²⁻ (2n -2)S²⁻.     

DFT study of difference caused by catalyst supports in Pt and Pd catalysis of oxygen reduction reaction

Based on an experimental phenomenon that catalytic activity of Pt and Pd for oxygen reduction reaction (ORR) changes with catalyst supports from C to TiO₂, density function theory (DFT) was used to elucidate the cause behind the difference in catalysis caused by catalyst supports. First, factors closely associated with the first electron transfer of the ORR were assessed in the light of quantum chemistry. Then intermediate (atomic oxygen, O) adsorption strength on the catalyst surface was calculated. The results show that, in terms of minimum energy difference, the best orbital symmetry match, and the maximum orbital overlap, TiO₂ does bring about a very positive effect on catalysts Pd/TiO₂ for the first electron transfer of the ORR. Especially, TiO₂ remarkably expands the space size of Pd/TiO₂ HOMO orbital and improves orbital overlap of Pd/TiO₂ HOMO and O₂ LUMO. The analysis of deformation density and partial density of state shows that the strong interaction between Pt and Ti leads to a strong adsorption of intermediate O on Pt/TiO₂, but the strong interaction between Pd and surface O causes positive net charge of Pd and a weak adsorption of intermediate O on Pd/TiO₂. Thus, the ORR can proceed more smoothly on Pd/TiO₂ than Pt/TiO₂ in every respect of maximum orbital overlap and rate delay by intermediate O. The research also discloses that several factors lead to less activity of TiO₂-supported Pt and Pd catalysts than the C-supported ones for the ORR. These factors include the poor dispersion of Pt and Pd particles on TiO₂, poor electric conduction of TiO₂ carrier itself, and bigger energy difference between HOMO of TiO₂-carried metallic catalysts and LUMO of O₂ molecule due to electrons deeply embedded in the semiconductor TiO₂ carrier. Supported by the National Natural Science Foundation of China (Grant No. 20676156), the Chinese Ministry of Education (Grant No. 307021), the National 863 Program (Grant Nos. 2006AA11A141 and 2007AA05Z124), and the Chongqing Sci &Tech Key Project (Grant No. CSTC2007AB6012)     

Oxidative addition of hydrogen in the “reduced” Mo/Al2O3 catalysts

An X-ray photoelectron spectroscopy study of Mo/Al₂O₃ catalysts prepared via [Mo^V ₂O₄(C₂O₄)₂(H₂O)₂]^2- complexes showed that after heating the catalysts with hydrogen in the spectrometer chamber, the position of the Mo3d line shifted to higher values of binding energy. This shift is interpreted as oxidative addition of hydrogen to the surface Mo species. A similar phenomenon was observed for a CO treated catalyst. A temperature-programmed desorption study has shown that hydrogen is strongly bounded to Mo and can only be removed from the catalysts at temperatures as high as 500°C. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of the rhenium additive to VOx/TiO2 and MoOx/TiO2 catalysts on their properties in oxidative dehydrogenation of propane

VO_x/TiO₂ and MoO_x/TiO₂ catalysts with the addition of Re (Re/V or Mo = 0.5) were synthetized and tested in oxidative dehydrogenation of propane and in reduction by propane. XPS measurements showed depletion of the surface in Re. The Re additive does not affect the total conversion of propane, but increases the selectivity to propene. The effect is more pronounced for the MoO_x/TiO₂ catalyst. The increase in the selectivity to propene is accompanied with the increase in the reducibility of the catalysts.     

Methodical aspects in the surface analysis of supported molybdena catalysts

Supported molybdena catalysts, with TiO₂, CeO₂ and Al₂O₃ supports, were studied by XPS and ISS. It was found that reliable results are obtained only when samples are calcined and transferred into the ultrahigh vacuum system without further contact with the ambient atmosphere (‘in situ calcination’). This applies also to catalysts that were previously calcined but had been stored in the ambient atmosphere. Supported Mo oxide becomes reduced under x‐ray irradiation during extended XPS data acquisition. A slight decrease of the Mo/support cation intensity ratio as a consequence of this reduction was detected by ISS in MoO₃/TiO₂ and MoO₃/CeO₂, therefore ISS analysis should be performed on freshly calcined samples without prior extended exposure to x‐rays. Because ISS spectra change rapidly due to sputtering, a correct analysis of the surface properties of the supported Mo catalyst requires extrapolation of the trend to the start of the experiment. It was established by this methodology that the surface of a 7% MoO₃/TiO₂ catalyst (5.3 Mo nm^?2) is completely covered by a monolayer of Mo oxide species, and no Ti cation is exposed. In a submonolayer MoO₃/CeO₂ catalyst the exposed support could be detected, as expected, whereas in an MoO₃/Al₂O₃ catalyst with an Mo oxide loading equal to the monolayer coverage a slight exposure of the Al support cation also was noted probably because of the high curvature of the smaller Al₂O₃ particles. Copyright © 2004 John Wiley & Sons, Ltd.     

Mo-Ni-γ-Al2O3体系中Mo、Ni近邻结构的EXAFS研究

运用EXAFS技术对加入P2O5或TiO2的MO-Ni-γ-Al2O3体系中Mo、Ni的K吸收边进行了分析，获得了体系中金属组分原子周围的微观结构信息，从而进一步证实P或Ti的加入有助于含量较高的Mo、Ni在载体表面的高度分散，样品中适量的P有助于形成较多的八面体结构，所有含P的样品中八面体结构含量均高于含Ti的样品，八面体结构Mo-O键上的氧在加氢脱硫反应中较易被硫取代，因而提高反应活性，这是含八面体结构较多的MoNiP样品的加氢脱硫活性优于MoNiTi样品的一个重要原因.     

Cation environment in polyether complexes based on poly(tetramethylene glycol) doped with zinc and cobalt chlorides

X‐ray absorption spectroscopy [extended X‐ray absorption fine structure (EXAFS) and X‐ray absorption near‐edge structure (XANES)], differential scanning calorimetry, and Raman spectroscopy measurements were performed for a series of liquid polyether/salt systems prepared with poly(tetramethylene glycol) (PTMG) and the copolymer poly(tetramethylene glycol‐co‐ethylene glycol) as matrices and zinc and cobalt chlorides as dopants in the concentration range of n = 30–90, where n is the molar ratio of oxygen to metal cation. According to EXAFS, XANES, and Raman results, even in diluted solutions, these complexes exist mostly as undissociated ZnCl₂ and CoCl₂ species, presenting a weak cation–polymer interaction. EXAFS results indicate that for polymer/ZnCl₂ systems, the nearest metal neighbors are only chlorine atoms. However, for polymer/CoCl₂ samples, oxygen is also observed coordinating the metal. Raman spectra do not support any feature related specifically to the cation–polymer interaction. Nevertheless, for both salts the symmetric stretching vibrations are located in frequencies characteristic of salt in solution, which means that the polymer solvating action is effective. Differential scanning calorimetry data show an increase in the glass‐transition temperature for all polymer/salt systems in relation to the pure polymer samples, a consequence of the increase in the macromolecular chain stiffening produced by the presence of the salt. This result corroborates the existence of polymer–salt interactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2572–2580, 2001