Identification of individual 4-methylpyridine molecules physisorbed and chemisorbed on TiO2(110)-(1 x 1) surface by STM

We succeeded in identifying geometries of 4-methylpyridine (4-MP) molecules adsorbed on a TiO₂(110)-(1 x 1) surface based on sequential STM imaging. Characteristics in mobility, topographic height, and location allowed us to distinguish three adsorption states at room temperature: a chemisorbed state with the upright geometry (A₁), a flat-lying state localized at specific sites (A₂), and a flat-lying physisorbed state mobile over the surface (B). The concentration of A₁ and A₂ species was restricted at an order of 0.01 ML. The A₂ state was related to the adsorption at oxygen vacancies resident on the vacuum-annealed surface. The present study demonstrates the promising ability of STM to identify the adsorption geometry of small probe molecules, 4-methylpyridine in the present case, and to provide atom-level information on the origin of acidic property of oxide surfaces. This revised version was published online in July 2006 with corrections to the Cover Date.     

Imaging of low temperature induced SMSI on Pd/TiO2 catalysts

Pd/TiO₂ catalysts were found to enter an strong metal support interaction (SMSI) state after reduction at temperatures as low as 473 K. This was identified as a significant loss in the CO uptake as monitored by low temperature FTIR. Electron microscopy provides direct evidence of the presence of ordered, reduced titanium oxide layers over palladium (SMSI state) for Pd/TiO₂ catalysts following reduction at temperature 623 K. The crystal phase was identified as Ti₄O₇ and this phase, once formed, was found to be stable even after exposure to atmospheric conditions.     

Propene Adsorption on Gold Particles on TiO2(110)

The adsorption of propene on rutile TiO₂(110) and on gold islands dispersed on TiO₂(110) [Au/TiO₂(110)], both at 120 K, has been studied using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and He⁺ low energy ion scattering spectroscopy (LEIS). Propene adsorbs on both TiO₂(110) and Au/TiO₂(110), with desorption peak temperatures at low coverage of 190 and 240 K, respectively. When only 16% of the TiO₂(110) surface is covered by gold islands [16% Au/TiO₂(110)], moderate propene doses populate both the 240 and 190 K TPD peaks, in that order. The 190 K peak, seen also without Au, is due to propene bound to bare Ti sites. The 240 K peak is attributed to propene adsorbed to Ti sites at the edges of gold islands. Tiny doses of propene to the 16% Au/TiO₂(110) surface give this a 240 K TPD peak but no 190 K feature, showing that the propene is mobile on TiO₂(110). A TPD feature at 150 K, which is more prominent at higher Au coverages and higher propene doses, is due to propene bound only to metallic Au islands. Propene desorption shows additional intensity at 265-310 K when the gold islands are only one atom thick, due to propene adsorbed on 2D Au islands or at Ti sites near their edges.     

Efficient synthesis of titania nanotubes and enhanced photoresponse of Pt decorated TiO2 for water splitting

We investigated the effect of HMT (hexamethylenetetraamine) on the anodic growth of TiO₂ nanotube arrays. The tube length increases to 4.3 μm with HMT concentration increasing to 0.04 mol·L⁻¹. Adsorption of HMT on the TiO₂ surface is shown to markedly decrease the chemical dissolution rate of tube mouth, resulting in longer nanotube length. Furthermore, Pt nanoparticles were successfully deposited on the surface of TiO₂ nanotubes by ac electrodeposition method. The TiO₂/Pt composites were characterized by field emission scanning electron microscope (FESEM), X-ray photoelectron spectra (XPS), and photoelectrochemistry. An enhancement in photocurrent density has been achieved upon modification of TiO₂ nanotubes with Pt nanoparticles.     

Evidence for strong metal-support interaction (SMSI) in Rh/TiO2 system

Rh(1 wt%)/TiO₂ samples were prepared by both incipient wetness and ion-exchange methods and were characterised by temperature programmed desorption (TPD), electron spin resonance (ESR), mass spectrometry (MS) and hydrogen chemisorption. The incipient wetness sample was found to be more favourable for the onset of SMSI state. The reduction of Ti⁴⁺ to Ti³⁺ during hydrogen spillover or due to the lattice oxygen (O^2–) deficiency seemed to be responsible for the SMSI state. A mechanistic pathway is proposed to explain the onset of SMSI behaviour.     

Prediction of a highly sensitive molecule sensor for SOx detection based on TiO2/MoS2 nanocomposites: a DFT study

First principles calculations within density functional theory have been carried out to investigate the adsorptions of SO_x (x?=?1, 2) molecules on TiO₂/MoS₂ nanocomposites in order to fully discover the gas sensing capabilities of TiO₂/MoS₂ composite systems. The van der Waals interactions were included to obtain the most stable geometrical structures of TiO₂/MoS₂ nanocomposites with adsorbed SO_x molecules. SO_x molecules preferentially interact with the doped nitrogen and fivefold coordinated titanium sites of the TiO₂ anatase nanoparticles because of their higher activities in comparison with the other sites. The results presented include structural parameters such as bond lengths and bond angles and energetics of the systems such as adsorption energies. The variation of electronic structures are discussed in view of the density of states and molecular orbitals of the SO_x molecules adsorbed on the nanocomposites. The results show that the adsorption of the SO_x molecule on the N-doped TiO₂/MoS₂ nanocomposite is energetically more favorable than the adsorption on the undoped one, implying that the nitrogen doping helps to strengthen the interaction of SO_x molecules with TiO₂/MoS₂ nanocomposites. These calculated results thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites in the removal and sensing of harmful SO_x molecules.     

Photoassisted mineralization of remazole red F3B over NiO/TiO2 and CdO/TiO2 nanoparticles under simulated sunlight

In this work, a series of titania-supported NiO and CdO materials were synthesized by a modified sol-gel process. The prepared photocatalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM). The activities of titania-supported NiO and CdO photocatalysts for photocatalytic degradation of Remazole Red F3B (RR) dye, under simulated sunlight, were investigated. The photocatalytic mineralization of an RR dye solution over various NiO-x/TiO₂ and CdO-x/TiO₂ photocatalysts under simulated sunlight was investigated. It was worthy noticing that the photocatalytic activity of titania improved using the prepared catalysts. The prepared TiO₂, NiO-5/TiO₂, and CdO-2/TiO₂ photocatalysts exhibited higher photocatalytic activity under simulated sunlight than did commercial TiO₂. The prepared photocatalysts were stable after photocatalytic degradation of the dye. The observed photocatalytic mineralization of the dye was 51 and 71% over NiO-10/TiO₂ and CdO-2/TiO₂ after 180 min of irradiation, respectively. Juxtaposing a p-NiO-5/TiO₂ semiconductor provided a potential approach for decreasing charge recombination. The prepared photocatalystsNiO-5/TiO₂ and CdO-2/TiO₂ are promising composites for the solar detoxification of textile wastewater.     

Influence of flame conditions on the dispersion of Pd on the flame spray-derived Pd/TiO2 nanoparticles

The Pd/TiO₂ nanoparticles containing 5 wt.% Pd were synthesized by one-step flame spray pyrolysis (FSP) under different flame conditions. As revealed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) results, the average particle sizes of Pd/TiO₂ were increased from 9.7 to 24.6 nm with increasing the precursor concentration and the feed flow rate as well as reduction of the O₂ dispersing gas during FSP synthesis. Although the BET surface area and %anatase phase content were decreased with increasing Pd/TiO₂ particle size, %Pd dispersion as determined from the amounts of CO chemisorption were higher on the larger size FSP-made Pd/TiO₂ nanoparticles. It is suggested that the shorter residence time in flame and/or the lower combustion energy (enthalpy density) resulted in more coverage of Pd surface by the formation of Ti-O groups, rendering lower CO chemisorption ability of the smaller size Pd/TiO₂.     

Efficient Reduction of NO x by H2 Under Oxygen-Rich Conditions over Pd/TiO2 Catalysts: An in situ DRIFTS Study

The H₂/NO/O₂ reaction under lean-burn conditions has been studied by means of in situ DRIFTS, reactor measurements and temperature-programmed desorption with the aim of understanding the very different behavior of Pd/TiO₂ and Pd/Al₂O₃ catalysts. The former deliver very high NO _x conversions (70-80%) with good N₂ selectivity whereas the latter show very low activity. In addition, PdTiO₂ exhibits two distinct NO _x reduction pathways, thus greatly extending the useful temperature range. It is shown that the PdTiO₂ low-temperature channel involves adsorption and subsequent dissociation of NO on reduced (Pd⁰) metal sites. The low activity of PdAl₂O₃ is a consequence of palladium remaining in an oxidized state under reaction conditions. The high-temperature NO reduction channel found with PdTiO₂ is associated with the generation and subsequent reaction of NH _x species.     

The reduction behavior of Mo/TiO2-Al2O3 catalyst

The effect of TiO₂ modified Al₂O₃ surface on the reducibility of MoO₃ has been studied by TPR and XPS. The results show that Mo⁶⁺ in Mo/TiO₂-Al₂O₃ can be reduced to much lower valency, especially at low Mo loading. The influence of the calcination temperature on the reduction of Mo⁶⁺ on Al₂O₃ and TiO₂-Al₂O₃ carriers is different. The data reveals that the reducibility of Mo⁶⁺ on Al₂O₃ slightly decreased, while that on TiO₂-Al₂O₃ increased when the calcination temperature was raised. It is suggested that the stronger tetrahedral site of the Al₂O₃ surface was first occupied by TiO₂ and main octahedral Mo⁶⁺ in polymeric species-; and a small crystalline MoO₃ formed on TiO₂-Al₂O₃, whereas the formation of tetrahedral Mo⁶⁺ species and Al₂(MoO₄)₃ phase was inhibited.     

The effect of Pd/Al2O3 pretreatment on catalytic activity in cyclopentane/deuterium exchange

Big variations in overall activity and product selectivity in the cyclopentane/deuterium exchange reaction were found in effect of various pretreatments of two chlorine‐free Pd/γ‐Al₂O₃ catalysts. The most important changes are observed when severely prereduced (at 600 °C) Pd/Al₂O₃ catalysts have been reoxidised and mildly rereduced: the multiple type of exchange, typical of mildly pretreated Pd catalysts, is replaced by a stepwise mode, and a big increase in catalytic activity occurs. At this state, the Pd/γ‐Al₂O₃ catalysts retain some water (as surface hydroxyls) generated by reoxidation and mild reduction. Deuterium spillover from Pd onto alumina and changes in acidity of alumina are invoked to rationalize the kinetic results. Changes in the state of Pd after various pretreatments, as probed by temperature‐programmed hydride decomposition, can hardly be correlated with changes in the catalytic behaviour in the exchange reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

A Novel Synthesis Route of Butyric Acid from Hydrogenation of Maleic Anhydride over Pd/TiO2 Catalysts

A highly efficient Pd/TiO₂ catalyst for the liquid phase hydrogenation of maleic anhydride has been prepared by sol–gel method, and super critical fluid of ethanol drying (SCFE) was applied. The catalyst exhibited excellent activity and high yield to butyric acid. The structural properties of TiO₂ supported Pd catalyst were investigated by BET, TEM, XRD, XPS and TPR techniques with the aim of finding a correlation between the structure parameters and the catalytic activities.     

In situ STM study of surface catalytic reactions on TiO2(110) relevant to catalyst design

This paper briefly summarizes our recent work on the characterization of atom-resolved surface images of TiO₂(110) composed of Ti atoms, O atoms, defects and hydroxyls by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM). The paper also presents new kinetic aspects in the catalytic dehydration and dehydrogenation of formic acid on the characterized surface. Switchover of the reaction path from the dehydration to the dehydrogenation occurred by the presence of formic acid undetectable at the surface, where acidic formic acid molecules opened the basic catalysis. In situ STM observation revealed that the dehydrogenation reaction at 400–450 K was strongly suppressed in the vicinity of single-atom height steps. The suppressive effect of step ranged over 2.4 nm into terrace. It is likely that the catalyst with flat surfaces larger than 5 nm is active for the dehydrogenation of formic acid.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

Density Functional Study of the CO Oxidation on a Doped Rutile TiO2(110): Effect of Ionic Au in Catalysis

We used density functional theory to examine whether doping oxides makes them better oxidation catalysts. We studied in detail titania doped with Au and used CO oxidation as a test of the oxidizing power of the system. We show that doping with Au, Ag, Cu, Pt, Pd, Ni reduces dramatically the bond of surface oxygen to titania or ceria, making them better oxidation catalysts. These calculations suggest that it is worthwhile to explore doped oxides as oxidation catalysts.     

Influence of Rutile and Anatase TiO2 Nanoparticles on Polyethylene Properties

Nanocomposites from polyethylene and TiO₂ with different shape and size were prepared by direct mixing and masterbatch dilution, respectively. The mechanical properties of nanocomposites were determined and discussed in relation to the nanofiller dispersion. Moreover, the morphological aspects of polyethylene with and without nanofiller were revealed by means of SEM and WAXD. A better dispersion of the nanoparticles and increased mechanical properties were observed in the case of the masterbatch method. No important differences in mechanical and morphological characteristics of anatase and rutile containing polyethylene composites were observed, except a higher increase of the elastic modulus in case of anatase-containing composites.     

Dehydrogenation of ethylbenzene over TiO2-Fe2O3 and ZrO2-Fe2O3 mixed oxide catalysts

The mixed metal oxides TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ were examined as potential catalysts for the dehydrogenation reaction of ethylbenzene. The acidic and basic properties and surface area, pore volume and pore size distribution of these catalysts were measured. The catalytic activities can be correlated very well with the surface area and the acidity and basicity of ZrO₂-Fe₂O₃ catalysts. However, for TiO₂-Fe₂O₃ catalysts, the surface area, the amount of acidic and basic sites and TiFe₂O₅ crystallinity are all important factors affecting the catalytic activities for ethylbenzene dehydrogenation. A synergistic effect was found for the TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ catalyst system and also for the TiO₂-Fe₂O₃-ZrO₂ system, i.e. the activities of these catalysts can be ranked in the following order: TiO₂-Fe₂O₃-ZrO₂>TiO₂-Fe₂O₃ >ZrO₂>Fe₂O₃>TiO₂. Meanwhile, all of these catalysts showed higher activities than the conventional potassium-promoted iron catalysts.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

Infrared study of crotonaldehyde and CO adsorption on a Pt/TiO2 catalyst

A Pt/TiO₂ catalyst has been subjected to reduction in hydrogen at 473, 573 and 773 K and the various degrees of metal-support interaction (SMSI) confirmed by means of CO and H₂ chemisorption, FTIR of CO and the hydrogenation of crotonaldehyde. Coadsorption of CO and crotonaldehyde were performed to identify the preferred adsorption site and mode of adsorption of the unsaturated aldehyde. Results which appear to suggest shifts to lower frequencies of bands due to adsorbed carbonyls are not due to electronic effects induced by coadsorption, but rather indicate displacement of CO from the weaker bonding sites which eliminate dipole coupling effects between different carbonyl clusters, and consequently removes intensity transfer phenomena leading to enhancement in intensity at lower frequencies. This revised version was published online in August 2006 with corrections to the Cover Date.