Synthesis, characterization, and catalytic function of novel highly dispersed tungsten oxide catalysts on mesoporous silica

The surface chemistry of the atomic layer deposition of metal thin films

In this perspective we discuss the progress made in the mechanistic studies of the surface chemistry associated with the atomic layer deposition (ALD) of metal films and the usefulness of that knowledge for the optimization of existing film growth processes and for the design of new ones. Our focus is on the deposition of late transition metals. We start by introducing some of the main surface-sensitive techniques and approaches used in this research. We comment on the general nature of the metallorganic complexes used as precursors for these depositions, and the uniqueness that solid surfaces and the absence of liquid solvents bring to the ALD chemistry and differentiate it from what is known from metalorganic chemistry in solution. We then delve into the adsorption and thermal chemistry of those precursors, highlighting the complex and stepwise nature of the decomposition of the organic ligands that usually ensued upon their thermal activation. We discuss the criteria relevant for the selection of co-reactants to be used on the second half of the ALD cycle, with emphasis on the redox chemistry often associated with the growth of metallic films starting from complexes with metal cations. Additional considerations include the nature of the substrate and the final structural and chemical properties of the growing films, which we indicate rarely retain the homogeneous 2D structure often aimed for. We end with some general conclusions and personal thoughts about the future of this field.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Impact of the Local Concentration of Hydronium Ions at Tungstate Surfaces for Acid-Catalyzed Alcohol Dehydration

Tungstate domains supported on ZrO₂, Al₂O₃, TiO₂, and activated carbon drastically influence the hydronium-ion-catalyzed aqueous-phase dehydration of alcohols. For all catalysts, the rate of cyclohexanol dehydration normalized to the concentration of Brønsted acid sites (turnover frequencies, TOFs) was lower for monotungstates than for polytungstates and larger crystallites of WO₃. TOFs were constant when reaching or exceeding the monolayer coverage of tungstate, irrespective of the specific nature of surface structures that continuously evolve with the surface W loading. However, the TOFs with polytungstates and large WO₃ crystallites depend strongly on the underlying support (e.g., WO_x/C catalysts are 10-50-fold more active than WO_x/Al₂O₃ catalysts). The electrical double layer (EDL) surrounding the negatively charged WO_x domains contains hydrated hydronium ions, whose local concentrations change with the support. This varying concentration of interfacial hydronium ions ("local ionic strength") impacts the excess chemical potential of the reacting alcohols and induces the marked differences in the TOFs. Primary H/D kinetic isotope effects (∼3), together with the substantially positive entropy of activation (111-195 J mol^-1 K^-1), indicate that C-H(D) bond cleavage is involved in the kinetically relevant step of an E1-type mechanistic sequence, regardless of the support identity. The remarkable support dependence of the catalytic activity observed here for the aqueous-phase dehydration of cycloalkanols likely applies to a broad set of hydronium-ion-catalyzed organic reactions sensitive to ionic strength.

Magneli-type tungsten oxide nanorods as catalysts for the selective oxidation of organic sulfides

Selective oxidation of thioethers is an important reaction to obtain sulfoxides as synthetic intermediates for applications in the chemical industry, medicinal chemistry and biology or the destruction of warfare agents. The reduced Magneli-type tungsten oxide WO3-x possesses a unique oxidase-like activity which facilitates the oxidation of thioethers to the corresponding sulfoxides. More than 90% of the model system methylphenylsulfide could be converted to the sulfoxide with a selectivity of 98% at room temperature within 30 minutes, whereas oxidation to the corresponding sulfone was on a time scale of days. The concentration of the catalyst had a significant impact on the reaction rate. Reasonable catalytic effects were also observed for the selective oxidation of various organic sulfides with different substituents. The WO3-x nanocatalysts could be recycled at least 5 times without decrease in activity. We propose a metal oxide-catalyzed route based on the clean oxidant hydrogen peroxide. Compared to other molecular or enzyme catalysts the WO3-x system is a more robust redox-nanocatalyst, which is not susceptible to decomposition or denaturation under standard conditions. The unique oxidase-like activity of WO3-x can be used for a wide range of applications in synthetic, environmental or medicinal chemistry.

Tungsten-Based Catalysts for Environmental Applications

This review aims to give a general overview of the recent use of tungsten-based catalysts for wide environmental applications, with first some useful background information about tungsten oxides. Tungsten oxide materials exhibit suitable behaviors for surface reactions and catalysis such as acidic properties (mainly Brønsted sites), redox and adsorption properties (due to the presence of oxygen vacancies) and a photostimulation response under visible light (2.6–2.8 eV bandgap). Depending on the operating condition of the catalytic process, each of these behaviors is tunable by controlling structure and morphology (e.g., nanoplates, nanosheets, nanorods, nanowires, nanomesh, microflowers, hollow nanospheres) and/or interactions with other compounds such as conductors (carbon), semiconductors or other oxides (e.g., TiO2) and precious metals. WOx particles can be also dispersed on high specific surface area supports. Based on these behaviors, WO3-based catalysts were developed for numerous environmental applications. This review is divided into five main parts: structure of tungsten-based catalysts, acidity of supported tungsten oxide catalysts, WO3 catalysts for DeNOx applications, total oxidation of volatile organic compounds in gas phase and gas sensors and pollutant remediation in liquid phase (photocatalysis).

Silica-Related Catalysts for CO2 Transformation into Methanol and Dimethyl Ether

The climate situation that the planet is experiencing, mainly due to the emission of greenhouse gases, poses great challenges to mitigate it. Since CO2 is the most abundant greenhouse gas, it is essential to reduce its emissions or, failing that, to use it to obtain chemicals of industrial interest. In recent years, much research have focused on the use of CO2 to obtain methanol, which is a raw material for the synthesis of several important chemicals, and dimethyl ether, which is advertised as the cleanest and highest efficiency diesel substitute fuel. Given that the bibliography on these catalytic reactions is already beginning to be extensive, and due to the great variety of catalysts studied by the different research groups, this review aims to expose the most important catalytic characteristics to take into account in the design of silica-based catalysts for the conversion of carbon dioxide to methanol and dimethyl ether.

Lower methane combustion temperature on palladium nanoparticles anchored on TiOx subnano-islets in stellate mesoporous silica nanospheres

Highly accessible and active palladium nanoparticles stabilized on TiO_x subnano-islets in monodispersed mesoporous silica nanospheres for methane combustion at low temperature.

Tungsten anchored onto functionalized SBA-15: an efficient catalyst for diastereoselective synthesis of 2-azapyrrolizidine alkaloid scaffolds

We used a novel hybrid catalyst in chemo-, regio-, and diastereoselective multi-component reactions (MCR) for the synthesis of the 2-aza analogue of pyrrolizidine and spirooxindole-2-azapyrrolizidine derivatives. The nanocatalyst, W(iv)/NNBIA-SBA-15 [where NNBIA = N,N'-(ethane-1,2-diyl)bis(2-aminobenzamide)] was synthesized by covalent grafting on chloro-functionalized SBA-15. The synthesis process was followed by the anchoring of WCl6 to catch the desired catalyst. The quality of the catalyst was assessed using different analytical techniques such as X-ray diffraction spectroscopy (XRD), Fourier-transform infrared spectroscopy (FT-IR), N2 adsorption analysis, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), ammonia Temperature Programmed Desorption (TPD), X-Ray photoelectron spectroscopy (XPS) and thermogravimetric, differential thermal analysis (TGA-DTA). The catalyst, W(iv)/NNBIA-SBA-15, with high catalytic performance is a good candidate for the diastereoselective synthesis of 2-azapyrrolizidine alkaloid scaffolds. The catalyst could be recovered for reuse without noticeable loss of activity.

A chemical titration method for quantification of carbenes in Mo- or W-containing catalysts for metathesis of ethylene with 2-butenes: verification and application potential

We introduce and experimentally validate a simple titration method for quantifying the number of carbenes acting as active sites in the metathesis of ethylene with 2-butenes over Mo- or W-containing catalysts.

Nucleation and growth process of atomic layer deposition platinum nanoparticles on strontium titanate nanocuboids

Uniform, well-dispersed platinum nanoparticles were grown on SrTiO₃ nanocuboids via atomic layer deposition (ALD) using (methylcyclopentadienyl)trimethylplatinum (MeCpPt(Me)₃) and water. For the first half-cycle of the deposition particles formed through two sequential processes: initial nucleation and growth. The final particle size after a single complete ALD cycle was dependent on the reaction temperature which alters the net Pt deposition per cycle. Additional cycles resulted in further growth of previously formed particles. However, the increase in size per cycle during additional ALD cycles, beyond the first, was significantly lower as less Pt was deposited due to carbonaceous material that partially covers the surface and prevents further MeCpPt(Me)₃ adsorption and reaction. The increase in particle size was also temperature dependent due to changes in the net Pt deposition. Pt nanoparticles increased in size by 59% and 76% after 15 ALD cycles for reaction temperatures of 200 °C and 300 °C, respectively. There was minimal change in the number of particles per unit area as a function of reaction time, indicating that there was minimal Ostwald ripening or secondary nucleation for the reaction conditions.

Production of p-xylene from bio-based 2,5-dimethylfuran over high performance catalyst WO3/SBA-15

Mesoporous solid acid catalyst WO₃/SBA-15 possessing mainly Lewis acids exhibits high performance for the production of bio-based PX.

The growth mode of ZnO on HZSM-5 substrates by atomic layer deposition and its catalytic property in the synthesis of aromatics from methanol

The resulting ZnO film on zeolites could effectively prevent exposing the HZSM-5 framework to steam during the reaction and regeneraton processes.

Self-metathesis of 1-butene to propene over SBA-15-supported WO3

Self-metathesis of 1-butene on SBA-15-supported WO₃ occurs via isomerization on Brønsted and Lewis acid sites and subsequent metathesis on Si–O–W–OH.

Progress in Synthesis of Highly Active and Stable Nickel-Based Catalysts for Carbon Dioxide Reforming of Methane

In recent decades, rising anthropogenic greenhouse gas emissions (mainly CO2 and CH4 ) have increased alarm due to escalating effects of global warming. The dry carbon dioxide reforming of methane (DRM) reaction is a sustainable way to utilize these notorious greenhouse gases. This paper presents a review of recent progress in the development of nickel-based catalysts for the DRM reaction. The enviable low cost and wide availability of nickel compared with noble metals is the main reason for persistent research efforts in optimizing the synthesis of nickel-based catalysts. Important catalyst features for the rational design of a coke-resistant nickel-based nanocatalyst for the DRM reaction are also discussed. In addition, several innovative developments based on salient features for the stabilization of nickel nanocatalysts through various means (which include functionalization with precursors, synthesis by plasma treatment, stabilization/confinement on mesoporous/microporous/carbon supports, and the formation of metal oxides) are highlighted. The final part of this review covers major issues and proposed improvement strategies pertaining to the rational design of nickel-based catalysts with high activity and stability for the DRM reaction.

Conversion of dimethyl ether to toluene under an O2 stream over W/HZSM-5 catalysts

The direct conversion of dimethyl ether (DME) to toluene without other aromatics is realized over W/HZSM-5 catalysts with high W contents.

Redox-driven atomic-scale changes in mixed catalysts: VOX/WOX/α-TiO2 (110)

X-ray study of vanadium–tungsten mixed-monolayer-oxide catalysts grown on the rutile α-TiO₂ (110) single crystal surface shows redox behavior not observed for lone supported vanadium or tungsten oxides.

Synthesis of Pd(ii) organometal incorporated ordered Im3m mesostructural silica and its catalytic activity in a water-medium Sonogashira reaction

A novel mesoporous Pd(ii) catalyst exhibits high activity for Sonogashira coupling of aryl halides with phenylacetylene in aqueous medium.

Ordered mesoporous zirconium oxophosphate supported tungsten oxide solid acid catalysts: the improved Brønsted acidity for benzylation of anisole

A series of WO₃ supported on ordered mesoporous zirconium oxophosphate solid acid catalyst was employed in benzylation reaction.

Synthesis of porous carbon supported palladium nanoparticle catalysts by atomic layer deposition: application for rechargeable lithium-O2 battery

In this study, atomic layer deposition (ALD) was used to deposit nanostructured palladium on porous carbon as the cathode material for Li-O2 cells. Scanning transmission electron microscopy showed discrete crystalline nanoparticles decorating the surface of the porous carbon support, where the size could be controlled in the range of 2-8 nm and depended on the number of Pd ALD cycles performed. X-ray absorption spectroscopy at the Pd K-edge revealed that the carbon supported Pd existed in a mixed phase of metallic palladium and palladium oxide. The conformality of ALD allowed us to uniformly disperse the Pd catalyst onto the carbon support while preserving the initial porous structure. As a result, the charging and discharging performance of the oxygen cathode in a Li-O2 cell was improved. Our results suggest that ALD is a promising technique for tailoring the surface composition and structure of nanoporous supports in energy storage devices.

Nanostructured materials for applications in heterogeneous catalysis

In this review, a brief survey is offered on the main nanotechnology synthetic approaches available to heterogeneous catalysis, and a few examples are provided of their usefulness for such applications. We start by discussing the use of colloidal, reverse micelle, and dendrimer chemistry in the production of active metal and metal oxide nanoparticles with well-defined sizes, shapes, and compositions, as a way to control the surface atomic ensembles available for selective catalysis. Next we introduce the use of sol-gel and atomic layer deposition chemistry for the production and modification of high-surface-area supports and active phases. Reference is then made to the more complex active sites that can be created or carved on such supports by using organic structure-directing agents. We follow with an examination of the ability to achieve multiple functionality in catalysis via the design of dumbbells, core@shell, and other complex nanostructures. Finally, we consider the mixed molecular-nanostructure approach that can be used to develop more demanding catalytic sites, by derivatizing the surface of solids or tethering or immobilizing homogeneous catalysts or other chemical functionalities. We conclude with a personal and critical perspective on the importance of fully exploiting the synergies between nanotechnology and surface science to optimize the search for new catalysts and catalytic processes.

Alcohol Dehydration on Monooxo W═O and Dioxo O═W═O Species

The dehydration of 1-propanol on nanoporous WO3 films prepared via ballistic deposition at ∼20 K has been investigated using temperature-programmed desorption, infrared reflection absorption spectroscopy, and density functional theory. The as-deposited films are extremely efficient in 1-propanol dehydration to propene. This activity is correlated with the presence of dioxo O═W═O groups, whereas monooxo W═O species are shown to be inactive. Annealing of the films induces densification that results in the loss of catalytic activity due to the annihilation of O═W═O species.