Preparation and in-Situ Spectroscopic Characterization of Molecularly Dispersed Titanium Oxide on Silica

Tungsten Oxide Dispersed on Silica as Robust and Readily Available Oxo/Imido Heterometathesis Catalyst

Continuing our investigation of catalytic oxo/imido heterometathesis as novel water-free method for C=N bond construction, we report here the application of classical transition metal oxides dispersed on silica (MOx/SiO2, M=V, Mo, W) as cheap, robust and readily available alternative to the catalysts prepared via Surface Organometallic Chemistry (SOMC). The oxide materials demonstrated activity in heterometathetical imidation of ketones, WO3/SiO2 being the most efficient. We also describe a new well-defined supported W imido complex (≡SiO)W(=NMes)2(Me2Pyr) (Mes=2,4,6-Me3C6H2, Me2Pyr=2,5-dimethylpyrrolyl) and characterize it with SOMC protocols, which allowed us to identify the position of W on the oxo/imido heterometathesis activity scale (Mo Collapse

Key Words

• imido
• imines
• metathesis
• supported catalysts
• tungsten

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MESH Headings Collapse

Grants

• 19-73-10163 Russian Science Foundation
• 075-00277-24-00 Ministry of Science and Higher Education of the Russian Federation

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Affiliation(s)

Nikolai S Bushkov A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Andrey V Rumyantsev A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia Chemistry Department, Moscow State University, Vorob'evy Gory, 1, 119992, Moscow, Russia
Anton A Zhizhin A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Tatyana V Strelkova A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Roman A Novikov N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky prosp., 47, 119991, Moscow, Russia
Evgenii I Gutsul A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Rina U Takazova A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Dinara K Kitaeva A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Nikolai A Ustynyuk A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Pavel A Zhizhko A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia
Dmitry N Zarubin A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str., 28, 119334, Moscow, Russia

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Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments

In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VOx) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VOx species appeared very clear at 30-100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ/operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.

Redox Dynamics of Active VO x Sites Promoted by TiO x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Titania-supported vanadia (VO _x /TiO₂) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V₂O₅/15% TiO₂/SiO₂ catalyst (consisting of submonolayer VO _x species anchored onto a TiO _x monolayer, which in turn is supported on SiO₂) during the oxidative dehydrogenation of ethanol. The VO _x species in 5% V₂O₅/15% TiO₂/SiO₂ show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO _x species supported on SiO₂ with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V₂O₅/15% TiO₂/SiO₂ is kinetically coupled to the formation of a V⁴⁺ intermediate, while the formation of V³⁺ is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VO _x species (V⁵⁺, V⁴⁺, and V³⁺) in the 5% V₂O₅/15% TiO₂/SiO₂ catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti⁴⁺/Ti³⁺ couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiO _x is rather complex. TiO _x species might be involved in a fast electron transport between VO _x species and might affect the electronic structure of VO _x , thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.

Research Progress on the Applications of Electrospun Nanofibers in Catalysis

During the last two decades, electrospinning has become a very popular technique for the fabrication of nanofibers due to its low cost and simple handling. Nanofiber materials have found utilization in many areas such as medicine, sensors, batteries, etc. In catalysis, these materials also present important advantages, since they present a low resistance to internal diffusion and a high surface area to volume ratio. These advantages are mainly due to the diameter–length proportion. A bibliographic analysis on the applications of electrospun nanofibers in catalysis shows that there are two important groups of catalysts that are being investigated, based on TiO2 and in carbon materials. The main applications found are in photo- and in electro-catalysis. The present study contributes by reviewing these catalytic applications of electrospun nanofibers and demonstrating that they are promising materials as catalysts, underlining some works to prove the advantages and possibilities that these materials have as catalysts. On one hand, the possibilities of synthesis are almost infinite, since with coaxial electrospinning quite complex nanofibers with different layers can be prepared. On the other hand, the diameter and other properties can be controlled by monitoring the applied voltage and other parameters during the synthesis, being quite reproducible procedures. The main advantages of these materials can be grouped in two: one related to their morphology, as has been commented, relative to their low resistance and internal diffusion, that is, their fluidynamic behavior in the reactor; the second group involves advantages related to the fact that the active phases can be nanoscaled and dispersed, improving the activity and selectivity in comparison with conventional catalytic materials with the same chemical composition.

Enhanced photocatalytic activity of efficient magnetically recyclable core-shell nanocomposites on 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) degradation under UV-LED irradiation

The congener polychlorinated biphenyls (PCBs) are one of the of persistent organic pollutant compounds that increase lifestyle-related diseases, such as diabetes, obesity, and cancer. So, 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153), which is one of the most common PCB contaminants in nature, was selected as a model compound to study the photocatalytic degradation of Fe₃O₄@SiO₂@TiO₂ core-shell structure. In this work, Fe₃O₄@SiO₂@TiO₂ nanocomposite was synthesized and characterized using transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), energy-dispersive X-ray (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM) techniques. Then, the effect of parameters such as catalyst dosage, initial concentration of PCB 153, solution pH, amount of H₂O₂, and kind of co-solvent on photocatalytic degradation of PCB 153 by the synthesized nanocomposite was investigated. The high degradation efficiency of Fe₃O₄@SiO₂@TiO₂ nanocomposite, which was 96.5%, was obtained at 4 g/l of the catalysts, 4 ppm of PCB 153, pH 5, 20 mM H₂O₂, 2 h of reaction time, and acetone as a cosolvent. Also, the rate of mineralization for Fe₃O₄@SiO₂@TiO₂ nanocomposite with H₂O₂ and UV-LED irradiation was 75.3% which had a significant efficiency compared to control experiments. Moreover, the mentioned photocatalysts are possible to be reused through exposing to external magnetic field, with insignificant decrease in the catalytic activity even after 6 cycles. The photocatalytic degradation process has an effective and environmental friendly effect on the degradation of organic pollutants.

High Surface Area VOx/TiO2/SBA-15 Model Catalysts for Ammonia SCR Prepared by Atomic Layer Deposition

The mode of operation of titania-supported vanadia (VOx) catalysts for NOx abatement using ammonia selective catalytic reduction (NH3-SCR) is still vigorously debated. We introduce a new high surface area VOx/TiO2/SBA-15 model catalyst system based on mesoporous silica SBA-15 making use of atomic layer deposition (ALD) for controlled synthesis of titania and vanadia multilayers. The bulk and surface structure is characterized by X-ray diffraction (XRD), UV-vis and Raman spectroscopy, as well as X-ray photoelectron spectroscopy (XPS), revealing the presence of dispersed surface VOx species on amorphous TiO2 domains on SBA-15, forming hybrid Si–O–V and Ti–O–V linkages. Temperature-dependent analysis of the ammonia SCR catalytic activity reveals NOx conversion levels of up to ~60%. In situ and operando diffuse reflection IR Fourier transform (DRIFT) spectroscopy shows N–Hstretching modes, representing adsorbed ammonia and -NH2 and -NH intermediate structures on Bronsted and Lewis acid sites. Partial Lewis acid sites with adjacent redox sites are proposed as the active sites and desorption of product molecules as the rate-determining step at low temperature. The high NOx conversion is attributed to the presence of highly dispersed VOx species and the moderate acidity of VOx supported on TiO2/SBA-15.

Grafting metal complexes onto amorphous supports: from elementary steps to catalyst site populationsviakernel regression

We use transition state theory, kernel regression, and population balance modeling techniques to model the grafting of metal complexes onto amorphous catalyst supports.

Ultrathin Silicon Oxide Film-Induced Enhancement of Charge Separation and Transport of Nanostructured Titanium(IV) Oxide Photoelectrode

The development of nanostructured semiconductor electrodes represented by a mesoporous TiO₂ nanocrystalline (mp-TiO₂ ) film is currently bringing great progresses in photoelectrochemical (PEC) devices for solar-to-electricity and solar-to-chemical conversion. Two serious losses can occur in PEC devices: 1) recombination between the conduction band (CB) electrons and valence band (VB) holes in the bulk and at the surface and 2) back reaction or electron trapping by oxidant in the electrolyte solution during transport to the electron-collecting electrode. Thus, the major challenge in common with the nanostructured semiconductor photoanodes is to achieve efficient charge separation and electron transport. In this study, an ultrathin SiO_x layer was formed on both the external and the internal surface of mp-TiO₂ using an original chemisorption-calcination technique employing 1,3,5,7-tetramethyltetrasiloxane as a starting material. The SiO_x surface modification of the mp-TiO₂ photoanode drastically prolongs the mean lifetime of CB-electrons in TiO₂ because of enhanced charge separation and electron transport by the negative charge applied in aqueous electrolyte solution. We have demonstrated that the performance of a one-compartment H₂ O₂ -photofuel cell using mp-TiO₂ as the photoanode is greatly boosted by the surface modification with the SiO_x layer. We anticipate that this methodology is widely applicable to nanostructured metal oxide semiconductor electrodes, contributing to the improvement in the performance of PEC devices.

Combined Magnesia, Ceria and Nickel catalyst supported over γ-Alumina Doped with Titania for Dry Reforming of Methane

This study investigated dry reforming of methane (DRM) over combined catalysts supported on γ-Al2O3 support doped with 3.0 wt. % TiO2. Physicochemical properties of all catalysts were determined by inductively coupled plasma/mass spectrometry (ICP-MS), nitrogen physisorption, X-ray diffraction, temperature programmed reduction/oxidation/desorption/pulse hydrogen chemisorption, thermogravimetric analysis, and scanning electron microscopy. Addition of CeO2 and MgO to Ni strengthened the interaction between the Ni and the support. The catalytic activity results indicate that the addition of CeO2 and MgO to Ni did not reduce carbon deposition, but improved the activity of the catalysts. Temperature programmed oxidation (TPO) revealed the formation of carbon that is mainly amorphous and small amount of graphite. The highest CH4 and CO2 conversion was found for the catalyst composed of 5.0 wt. % NiO-10.0 wt. % CeO2/3.0 wt. %TiO2-γ-Al2O3 (Ti-CAT-II), resulting in H2/CO mole ratio close to unity. The optimum reaction conditions in terms of reactant conversion and H2/CO mole ratio were achieved by varying space velocity and CO2/CH4 mole ratio.

CO Preferential Photo-Oxidation in Excess of Hydrogen in Dark and Simulated Solar Light Irradiation over AuCu-Based Catalysts on SBA-15 Mesoporous Silica-Titania

In this work, SBA-15 silica and silica-titania have been used as supports for photocatalysts based on AuCu alloy (Au:Cu = 1) to be used in the preferential oxidation of CO (CO-PROX) in excess of hydrogen at room temperature and atmospheric pressure both in the dark and under simulated solar light irradiation. To study their textural, structural, chemical and optical properties, the samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), adsorption-desorption of N₂ at -196 °C, 13C and 29Si solid state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy. Titanium was present mainly in the form of titania aggregates, but also as small particles interacting with the SBA support. In both catalysts, the metal alloy nanoparticles displayed an average size of 4 nm as demonstrated by TEM measurements. AuCu/Ti-SBA turned out to be photoactive and selective in the photo-CO-PROX reaction showing the highest activity, with conversion and selectivity towards CO₂ of 80%, due both to the presence of titania incorporated in SBA-15 and to the synergistic effect of Cu when alloyed with Au.

Au and AuCu Nanoparticles Supported on SBA-15 Ordered Mesoporous Titania-Silica as Catalysts for Methylene Blue Photodegradation

The photocatalytic degradation of methylene blue (MB) dye has been performed under UV irradiation in aqueous suspension, employing photocatalysts based on Au (1.5 wt %) and AuCu (Au/Cu = 1, 2.0 wt %), and supported on SBA-15-ordered mesoporous silica, with and without titania (Si/Ti = 3), in order to evaluate the versatility of this mesoporous support in this type of reaction of great impact from the environmental point of view. Samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption-desorption at -196 °C, and X-ray photoelectron spectroscopy (XPS), so as to study their structural, optical, and chemical properties. All the prepared catalysts were found to be active in the test reaction. The bimetallic AuCu-based catalysts attained very high MB degradation values, in particular AuCu/SBA-15 titania-silica sample reached 100% of dye oxidation after the monitored reaction period (120 min).

Study of the structure and characteristics of mesoporous TiO2 photocatalyst, and evaluation of its factors on gaseous formaldehyde removal by the analysis of ANOVA and S/N ratio

This study differs from previous studies of TiO₂/SiO₂ in that 0.5–10 μm microsized TiO₂-rutile based catalysts (TR catalysts) with varying proportions of titanium and silicon were synthesized using a one-step modified hydrothermal method. At Ti/Si = 1/9, a two-dimensional channel-structured catalyst with a morphology resembling that of SBA-15 was obtained. In contrast, at Ti/Si = 3/7 or 5/5, a three-dimensional porous structure was formed, and Ti–O–Si–C bonds appeared. The structure of the TR catalyst transformed due to the decrease in C–Si bond content and the increase in C–C bond content with increasing Ti/Si ratio. The results indicated that the rutile phase was the main crystal phase of the TR catalyst. The small crystal size and large rutile phase content of the mesoporous TR catalyst contributed to the low band gap energy below 3.0 eV. Under 2 × 10 W lamp irradiation with either UVA or visible light, the three TR catalysts showed better formaldehyde (HCHO) removal efficiency than P25. Furthermore, the Taguchi method was employed to evaluate the catalytic factors by analysis of variance (ANOVA) and S/N ratio. The results revealed the contributions of each of the three factors to HCHO removal efficiency over TR catalysts to be as follows: space velocity (62%), Ti ratio (32%), and time on stream (5%). The TR catalyst with Ti/Si = 1/9 showed good HCHO removal efficiency with a high S_BET (787.1 m² g⁻¹) and large pore volume (0.95 cm³ g⁻¹) for a residence time of over 2.29 × 10⁻¹ s under visible light irradiation. Microwave-assisted EG reduction was successfully applied to dope a TR catalyst with nanosized Pt particles in a short synthesis time. After Pt doping, the removal efficiency in the stream improved and stabilized. The Pt particles were Pt⁰ and proved effective for improving the photocatalytic removal of HCHO over the TR catalyst by prolonging the separation time of the electron–hole pairs. Overall, the Pt/TR catalyst is a potential material for pollutant removal and can be easily separated from the pollutant removal system since the catalysts are microsized.

This study differs from previous studies of TiO₂/SiO₂ in that 0.5–10 μm microsized TiO₂-rutile based catalysts (TR catalysts) with varying proportions of titanium and silicon were synthesized using a one-step modified hydrothermal method.

Probing the Catalytic Activity of Sulfate-Derived Pristine and Post-Treated Porous TiO2(101) Anatase Mesocrystals by the Oxidative Desulfurization of Dibenzothiophenes

Mesocrystals (basically nanostructures showing alignment of nanocrystals well beyond crystal size) are attracting considerable attention for modeling and optimization of functionalities. However, for surface-driven applications (heterogeneous catalysis), only those mesocrystals with excellent textural properties are expected to fulfill their potential. This is especially true for oxidative desulfuration of dibenzothiophenes (hard to desulfurize organosulfur compounds found in fossil fuels). Here, we probe the catalytic activity of anatases for the oxidative desulfuration of dibenzothiophenes under atmospheric pressure and mild temperatures. Specifically, for this study, we have taken advantage of the high stability of the (101) anatase surface to obtain a variety of uniform colloidal mesocrystals (approximately 50 nm) with adequate orientational order and good textural properties (pores around 3-4 nm and surface areas around 200 m²/g). Ultimately, this stability has allowed us to compare the catalytic activity of anatases that expose a high number of aligned single crystal-like surfaces while differing in controllable surface characteristics. Thus, we have established that the type of tetrahedral coordination observed in these anatase mesocrystals is not essential for oxidative desulfuration and that both elimination of sulfates and good textural properties significantly improve the catalytic activity. Furthermore, the most active mesocrystals have been used to model the catalytic reaction in three-(oil-solvent-catalyst) and two-phase (solvent-catalyst) systems. Thus, we have been able to observe that the transfer of DBT from the oil to the solvent phase partially limits the oxidative process and to estimate an apparent activation energy for the oxidative desulfuration reaction of approximately 40 kJ/mol in the two-phase system to avoid mass transfer limitations. Our results clearly establish that (101) anatase mesocrystals with excellent textural properties show adequate stability to withstand several post-treatments without losing their initial mesocrystalline character and therefore could serve as models for catalytic processes different from the one studied here.

A reusable surface-quaternized nanocellulose-based hybrid cryogel loaded with N-doped TiO2 for self-integrated adsorption/photo-degradation of methyl orange dye

Self-integrated adsorption/photo-degradation of methyl orange dye.

Dendritic mesoporous silica–titania nanospheres with enhanced photocatalytic activities

Dendritic mesoporous silica–titania nanospheres (DMSTN) show remarkable performance as a photocatalyst and an adsorbent for photochromic methylene green (MG) dye.

On the dynamical nature of the active center in a single-site photocatalyst visualized by 4D ultrafast electron microscopy

Understanding the dynamical nature of the catalytic active site embedded in complex systems at the atomic level is critical to developing efficient photocatalytic materials. Here, we report, using 4D ultrafast electron microscopy, the spatiotemporal behaviors of titanium and oxygen in a titanosilicate catalytic material. The observed changes in Bragg diffraction intensity with time at the specific lattice planes, and with a tilted geometry, provide the relaxation pathway: the Ti(4+)=O(2-) double bond transformation to a Ti(3+)-O(1-) single bond via the individual atomic displacements of the titanium and the apical oxygen. The dilation of the double bond is up to 0.8 Å and occurs on the femtosecond time scale. These findings suggest the direct catalytic involvement of the Ti(3+)-O(1-) local structure, the significance of nonthermal processes at the reactive site, and the efficient photo-induced electron transfer that plays a pivotal role in many photocatalytic reactions.

Influence of the porous texture of SBA-15 mesoporous silica on the anatase formation in TiO2–SiO2 nanocomposites

TiO₂–SiO₂ composites with 44 wt% of anatase exhibit a better MO degradation rate than those with fully crystallized anatase.

Atomic Layer Deposition from Dissolved Precursors

We establish a novel thin film deposition technique by transferring the principles of atomic layer deposition (ALD) known with gaseous precursors toward precursors dissolved in a liquid. An established ALD reaction behaves similarly when performed from solutions. "Solution ALD" (sALD) can coat deep pores in a conformal manner. sALD offers novel opportunities by overcoming the need for volatile and thermally robust precursors. We establish a MgO sALD procedure based on the hydrolysis of a Grignard reagent.