Time-Resolved Studies of Ethylene and Propylene Reactions in Zeolite H‑MFI by In-Situ Fast IR Heating and UV Raman Spectroscopy

The conversion of ethylene and propylene absorbed in zeolite H-MFI was studied using UV-Raman spectroscopy. To observe early stage reaction intermediates, an infrared laser was used as a fast heating source. Alkyl substituted naphthalenes and fluorenes, which have been previously suggested as hydrocarbon pool species, were observed regardless of the olefin reagent. Conjugated dienes were formed from propylene but not observed for ethylene at short reaction times. Conventional heating in a furnace was used to force the reaction to completion. For propylene sheet-like polyaromatic hydrocarbons were formed immediately. For ethylene cyclic dienes, conjugated olefins, and ultimately sheet-like polyaromatic hydrocarbons were formed at progressively higher reaction temperatures. The results show that the polyaromatic species implicated as deactivating coke in zeolite catalysts can be formed by conversion of polyenes

Alternative Low-Pressure Surface Chemistry of Titanium Tetraisopropoxide on Oxidized Molybdenum

Titanium tetraisopropoxide (TTIP) is a precursor utilized in atomic layer depositions (ALDs) for the growth of TiO₂. The chemistry of TTIP deposition onto a slightly oxidized molybdenum substrate was explored under ultrahigh vacuum (UHV) conditions with X-ray photoelectron spectroscopy. Comparison of the Ti­(2p) and C­(1s) peak areas has been used to determine the surface chemistry for increasing substrate temperatures. TTIP at a gas-phase temperature of 373 K reacts with a MoO_<i>x</i> substrate at 373 K but not when the substrate is at 295 K, consistent with a reaction that proceeds via a Langmuir–Hinshelwood mechanism. Chemical vapor deposition was observed for depositions at 473 K, below the thermal decomposition temperature of TTIP and within the ALD temperature window, suggesting an alternative reaction pathway competitive to ALD. We propose that under conditions of low pressure and moderate substrate temperatures dehydration of the reacted precursor by nascent TiO₂ becomes the dominant reaction pathway and leads to the CVD growth of TiO₂ rather than a self-limiting ALD reaction. These results highlight the complexity of the chemistry of ALD precursors and demonstrate that changing the pressure can drastically alter the surface chemistry

Etheric C–O Bond Hydrogenolysis Using a Tandem Lanthanide Triflate/Supported Palladium Nanoparticle Catalyst System

Selective hydrogenolysis of cyclic and linear ether C–O bonds is accomplished by a tandem catalytic system consisting of lanthanide triflates and sinter-resistant supported palladium nanoparticles in an ionic liquid. The lanthanide triflates catalyze endothermic dehydroalkoxylation, while the palladium nanoparticles hydrogenate the resulting intermediate alkenols to afford saturated alkanols with high overall selectivity. The catalytic C–O hydrogenolysis is shown to have significant scope, and the C–O bond cleavage is turnover-limiting

Precursor Nuclearity Effects in Supported Vanadium Oxides Prepared by Organometallic Grafting

Despite widespread importance in catalysis, the active and selective sites of supported vanadium oxide (VO_<i>x</i>) catalysts are not well understood. Such catalysts are of great current interest because of their industrial significance and potential for selective oxidation processes.− However, the fact that the nature of the active and selective sites is ambiguous hinders molecular level understanding of catalytic reactions and the development of new catalysts. Furthermore, complete structural elucidation requires isolation and characterization of specific vanadium oxide surface species, the preparation of which presents a significant synthetic challenge. In this study, we utilize the structural uniformity inherent in organometallic precursors for the preparation of supported vanadium oxide catalysts. The resulting catalysts are characterized by UV−visible diffuse reflectance spectroscopy (UV−vis DRS), X-ray absorption spectroscopy (XAS), UV-Raman spectroscopy, and H₂-temperature programmed reduction (H₂-TPR). Significant structural and reactivity differences are observed in catalysts prepared from different organometallic precursors, indicating that the chemical nature of surface vanadia can be influenced by the nuclearity of the precursor used for grafting

Direct Spectroscopic Evidence for Isolated Silanols in SiO_<i>x</i>/Al₂O₃ and Their Formation Mechanism

The preparation and unambiguous characterization of isolated Brønsted-acidic silanol species on silica–alumina catalysts presents a key challenge in the rational design of solid acid catalysts. In this report, atomic layer deposition (ALD) and liquid-phase preparation (chemical liquid deposition, CLD) are used to install the SiO_<i>x</i> sites on Al₂O₃ catalysts using the same Si source (tetraethylorthosilicate, TEOS). The ALD-derived and CLD-derived SiO_<i>x</i> sites are probed with dynamic nuclear polarization (DNP)-enhanced ²⁹Si–²⁹Si double-quantum/single-quantum (DQ/SQ) correlation NMR spectroscopy. The investigation reveals conclusively that the SiO_<i>x</i>/Al₂O₃ material prepared by ALD and CLD, followed by calcination under an O₂ stream, contains fully spatially isolated Si species, in contrast with those resulting from the calcination under static air, which is widely accepted as a postgrafting treatment for CLD. Insight into the formation mechanism of these sites is obtained via in situ monitoring of the TEOS + γ-Al₂O₃ reaction in an environmental diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) cell. Upon calcination, the DRIFTS spectra of SiO_<i>x</i>/Al₂O₃ reveal a signature unambiguously assignable to isolated Brønsted-acidic silanol species. Surprisingly, the results of this study indicate that the method of preparing SiO_<i>x</i>/Al₂O₃ catalysts is less important to the final structure of the silanol sites than the post-treatment conditions. This finding should greatly simplify the methods for synthesizing site-isolated, Brønsted-acidic SiO_<i>x</i>/Al₂O₃ catalysts

Nucleation and Growth of Silver Nanoparticles by AB and ABC-Type Atomic Layer Deposition

In this work, we report synthesis strategies to produce Ag nanoparticles by AB-type and ABC-type atomic layer deposition (ALD) using trimethylphosphine­(hexafluoroacetylacetonato) silver­(I) ((hfac)­Ag­(PMe₃)) and formalin (AB-type) and (hfac)­Ag­(PMe₃), trimethylaluminum, and H₂O (ABC-type). In situ quartz crystal microbalance measurements reveal a Ag growth rate of 1–2 ng/cm²/cycle by ABC-type ALD at 110 °C and 2–10 ng/cm²/cycle for AB-type ALD at 170–200 °C. AB-type Ag ALD has a nucleation period before continuous linear growth that is shorter at 200 °C. Transmission electron microscopy reveals that AB-type Ag ALD particles have an average size of ∼1.8 nm after 10 cycles. ABC-type Ag ALD particles have an average size of ∼2.2 nm after 20 cycles. With increasing ALD cycles, ABC-type Ag ALD increases the metal loading while maintaining the particle size but AB-type Ag ALD results in the formation of bigger particles in addition to small particles. The ability to synthesize supported metal nanoparticles with well-defined particle sizes and narrow size distributions makes ALD an attractive synthesis method compared to conventional wet chemistry techniques

Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO₂ on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study

An understanding of the adsorption of CO₂, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO₂ on oxidized, reduced, and platinized TiO₂ nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti³⁺ species, a very weak Lewis acid, along with a splitting of the Ti⁴⁺ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti³⁺ is not observed on reduction of the platinized Ti-NTs, presumably because Pt acts as an electron sink. Exposure of the treated Ti-NTs to CO₂ leads to the formation of differing amounts of bidentate and monodentate carbonates, as well as bicarbonates, where the preference for formation of a given species is rationalized in terms of surface Lewis acidity and or Lewis basicity and the availability of hydrogen. Our data suggest that one source of hydrogen is water that remains adsorbed to the Ti-NTs even after heating to 350 °C and that reduced platinized NTs can activate H₂. Carboxylates, which involve CO₂^– moieties and are similar to what would be expected for adsorbed CO₂^–, a postulated intermediate in CO₂ photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti³⁺/O vacancy formation is observed

Identification of Dimeric Methylalumina Surface Species during Atomic Layer Deposition Using <i>Operando</i> Surface-Enhanced Raman Spectroscopy

<i>Operando</i> surface-enhanced Raman spectroscopy (SERS) was used to successfully identify hitherto unknown dimeric methylalumina surface species during atomic layer deposition (ALD) on a silver surface. Vibrational modes associated with the bridging moieties of both trimethylaluminum (TMA) and dimethylaluminum chloride (DMACl) surface species were found during ALD. The appropriate monomer vibrational modes were found to be absent as a result of the selective nature of SERS. Density functional theory (DFT) calculations were also performed to locate and identify the expected vibrational modes. An <i>operando</i> localized surface plasmon resonance (LSPR) spectrometer was utilized to account for changes in SER signal as a function of the number of ALD cycles. DMACl surface species were unable to be measured after multiple ALD cycles as a result of a loss in SERS enhancement and shift in LSPR. This work highlights how <i>operando</i> optical spectroscopy by SERS and LSPR scattering are useful for probing the identity and structure of the surface species involved in ALD and, ultimately, catalytic reactions on these support materials

Structure-Specific Reactivity of Alumina-Supported Monomeric Vanadium Oxide Species

Oxidative dehydrogenation (ODH) catalysts based on vanadium oxide are active for the production of alkenes, chemicals of great commercial importance. The current industrial practice for alkene production is based on energy-intensive, dehydrogenation reactions. UV resonance and visible Raman measurements, combined with density functional studies, are used to study for the first time the structure–reactivity relationships for alumina-supported monomeric vanadium oxide species. The relationship between the structure of three vanadium oxide monomeric surface species on a θ-alumina surface, and their reducibility by H₂ was determined by following changes in the vanadia’s UV Raman and resonance Raman spectra after reaction with H₂ at temperatures from 450 to 650 °C. The H₂ reducibility sequence for the three monomeric species is bidentate > “molecular”> tridentate. The reaction pathways for H₂ reduction on the three vanadium oxide monomeric structures on a θ-alumina surface were investigated using density functional theory. Reduction by H₂ begins with reaction at the VO bond in all three species. However, the activation energy, Gibbs free energy change under reaction conditions, and the final V oxidation state are species-dependent. The calculated ordering of reactivity is consistent with the observed experimental ordering and provides an explanation for the ordering. The results suggest that synthesis strategies can be devised to obtain vanadium oxide structures with greatly enhanced activity for ODH resulting in more efficient catalysts

Catalysts Transform While Molecules React: An Atomic-Scale View

We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO₂(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the VO vanadyl group. Catalytic tests on the powder-supported VO_<i>x</i> revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts