On the Nature of Extra-Framework Aluminum Species and Improved Catalytic Properties in Steamed Zeolites.

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O5Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al1O5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites

Unveiling Highly Sensitive Active Site in Atomically Dispersed Gold Catalysts for Enhanced Ethanol Dehydrogenation

Abstract: Developing a desirable ethanol dehydrogenation process necessitates a highly efficient and selective catalyst with low cost. Herein, we show that the “complex active site” consisting of atomically dispersed Au atoms with the neighboring oxygen vacancies (Vo) and undercoordinated cation on oxide supports can be prepared and display unique catalytic properties for ethanol dehydrogenation. The “complex active site” Au−Vo−Zr3+ on Au1/ZrO2 exhibits the highest H2 production rate, with above 37,964 mol H2 per mol Au per hour (385 g H2   h−1) at 350 °C, which is 3.32, 2.94 and 15.0 times higher than Au1/CeO2, Au1/TiO2, and Au1/Al2O3, respectively. Combining experimental and theoretical studies, we demonstrate the structural sensitivity of these complex sites by assessing their selectivity and activity in ethanol dehydrogenation. Our study sheds new light on the design and development of cost‐effective and highly efficient catalysts for ethanol dehydrogenation. Fundamentally, atomic‐level catalyst design by colocalizing catalytically active metal atoms forming a structure‐sensitive “complex site”, is a crucial way to advance from heterogeneous catalysis to molecular catalysis. Our study advanced the understanding of the structure sensitivity of the active site in atomically dispersed catalysts

APPLICATIONS OF IN SITU MAGNETIC RESONANCE SPECTROSCOPY FOR STRUCTURAL ANALYSIS OF OXIDE-SUPPORTED CATALYSTS

Nuclear Magnetic Resonance (NMR) stands as an invaluable non-destructive technique to probe catalytic materials. The rise of in situ NMR has enabled detailed structural analysis of materials under tightly controlled conditions that are relevant for the chemistry of interest. Herein, the utilization of such technology has been described as it pertains to supported oxide catalysts—in particular, supported vanadium oxide materials. Metal oxides are notoriously challenging to characterize due to the distribution of species (monomer, dimer, polymer, and bulk oxides) they possess, often under the same conditions, as well as their sensitivity to the chemical environment surrounding the active center. 51V NMR is extensively used to understand the structure of vanadia-based catalysts under different environmental conditions. Dehydrated structures are analyzed for silica, titania, and titania/silica supports where the active structures for emissions controls applications are proposed. Vanadia materials under hydrated conditions are also considered where dramatic changes in the surface species towards both less support-coordinated structures and oligomers are present. Dry materials which have previously experienced harsh hydrothermal treatment or have undergone reaction cycles are shown to redisperse on the surface, including dispersion of the bulk V2O5 oxide phase after a single redox cycle of ethanol oxidative dehydrogenation. Finally, the interactions of water and other small molecules with the acid sites of MFI zeolite under strictly controlled environments are explored by monitoring both the interacting chemical constituent and the active site of the oxide. In each case, a firm control over the environment of the materials has enabled the observation of the catalysts under relevant conditions to better understand the nature of the active species. These studies represent a foundation for the wealth of information in situ NMR is capable of providing to the understanding of catalytic systems

Thermal perturbation of NMR properties in small polar and non-polar molecules

Water is an important constituent in an abundant number of chemical systems; however, its presence complicates the analysis of in situ 1H MAS NMR investigations due to water's ease of solidification and vaporization, the large changes in mobility, affinity for hydrogen bonding interactions, etc., that are reflected by dramatic changes in temperature-dependent chemical shielding. To understand the evolution of the signatures of water and other small molecules in complex environments, this work explores the thermally-perturbed NMR properties of water in detail by in situ MAS NMR over a wide temperature range. Our results substantially extend the previously published temperature-dependent 1H and 17O chemical shifts, linewidths, and spin-lattice relaxation times over a much wider range of temperatures and with significantly enhanced thermal resolution. The following major results are obtained: Hydrogen bonding is clearly shown to weaken at elevated temperatures in both 1H and 17O spectra, reflected by an increase in chemical shielding. At low temperatures, transient tetrahedral domains of H-bonding networks are evidenced and the observation of the transition between solid ice and liquid is made with quantitative considerations to the phase change. The 1H chemical shift properties in other small polar and non-polar molecules have also been described over a range of temperatures, showing the dramatic effect hydrogen bonding perturbation on polar species. Gas phase species are observed and chemical exchange between gas and liquid phases is shown to play an important role on the observed NMR shifts. The results disclosed herein lay the foundation for a clear interpretation of complex systems during the increasingly popular in situ NMR characterization at elevated temperatures and pressures for studying chemical systems

High-Temperature and High-Pressure In situ Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy.

Nuclear magnetic resonance (NMR) spectroscopy represents an important technique to understand the structure and bonding environments of molecules. There exists a drive to characterize materials under conditions relevant to the chemical process of interest. To address this, in situ high-temperature, high-pressure MAS NMR methods have been developed to enable the observation of chemical interactions over a range of pressures (vacuum to several hundred bar) and temperatures (well below 0 °C to 250 °C). Further, the chemical identity of the samples can be comprised of solids, liquids, and gases or mixtures of the three. The method incorporates all-zirconia NMR rotors (sample holder for MAS NMR) which can be sealed using a threaded cap to compress an O-ring. This rotor exhibits great chemical resistance, temperature compatibility, low NMR background, and can withstand high pressures. These combined factors enable it to be utilized in a wide range of system combinations, which in turn permit its use in diverse fields as carbon sequestration, catalysis, material science, geochemistry, and biology. The flexibility of this technique makes it an attractive option for scientists from numerous disciplines

Low-temperature (< 200 °C) degradation of electronic nicotine delivery system liquids generates toxic aldehydes

Electronic cigarette usage has spiked in popularity over recent years. The enhanced prevalence has consequently resulted in new health concerns associated with the use of these devices. Degradation of the liquids used in vaping have been identified as a concern due to the presence of toxic compounds such as aldehydes in the aerosols. Typically, such thermochemical conversions are reported to occur between 300 and 400 °C. Herein, the low-temperature thermal degradation of propylene glycol and glycerol constituents of e-cigarette vapors are explored for the first time by natural abundance 13C NMR and 1H NMR, enabling in situ detection of intact molecules from decomposition. The results demonstrate that the degradation of electronic nicotine delivery system (ENDS) liquids is strongly reliant upon the oxygen availability, both in the presence and absence of a material surface. When oxygen is available, propylene glycol and glycerol readily decompose at temperatures between 133 and 175 °C over an extended time period. Among the generated chemical species, formic and acrylic acids are observed which can negatively affect the kidneys and lungs of those who inhale the toxin during ENDS vapor inhalation. Further, the formation of hemi- and formal acetals is noted from both glycerol and propylene glycol, signifying the generation of both formaldehyde and acetaldehyde, highly toxic compounds, which, as a biocide, can lead to numerous health ailments. The results also reveal a retardation in decomposition rate when material surfaces are prevalent with no directly observed unique surface spectator or intermediate species as well as potentially slower conversions in mixtures of the two components. The generation of toxic species in ENDS liquids at low temperatures highlights the dangers of low-temperature ENDS use

Variable Temperature and Pressure Operando MAS NMR for Catalysis Science and Related Materials

The characterization of catalytic materials under working conditions is of paramount importance for a realistic depiction and comprehensive understanding of the system. Under such relevant environments, catalysts often exhibit properties or reactivity not observed under standard spectroscopic conditions. Fulfilling such harsh environments as high temperature and pressure is a particular challenge for solid-state NMR where samples spin several thousand times a second within a strong magnetic field. To address concerns about the disparities between spectroscopic environments and operando conditions, novel MAS NMR technology has been developed that enables the probing of catalytic systems over a wide range of pressures, temperatures, and chemical environments. In this Account, new efforts to overcome the technical challenges in the development of operando and in situ MAS NMR will be briefly outlined. Emphasis will be placed on exploring the unique chemical regimes that take advantage of the new developments. With the progress achieved, it is possible to interrogate both structure and dynamics of the environments surrounding various nuclear constituents (1H, 13C, 23Na, 27Al, etc.), as well as assess time-resolved interactions and transformations.Operando and in situ NMR enables the direct observation of chemical components and their interactions with active sites (such as Brønsted acid sites on zeolites) to reveal the nature of the active center under catalytic conditions. Further, mixtures of such constituents can also be assessed to reveal the transformation of the active site when side products, such as water, are generated. These interactions are observed across a range of temperatures (-10 to 230 °C) and pressures (vacuum to 100 bar) for both vapor and condensed phase analysis. When coupled with 2D NMR, computational modeling, or both, specific binding modes are identified where the adsorbed state provides distinct signatures. In addition to vapor phase chemical environments, gaseous environments can be introduced and controlled over a wide range of pressures to support catalytic studies that require H2, CO, CO2, etc. Mixtures of three phases may also be employed. Such reactions can be monitored in situ to reveal the transformation of the substrates, active sites, intermediates, and products over the course of the study. Further, coupling of operando NMR with isotopic labeling schemes reveals specific mechanistic insights otherwise unavailable. Examples of these strategies will be outlined to reveal important fundamental insights on working catalyst systems possible only under operando conditions. Extension of operando MAS NMR to study the solid-electrolyte interface and solvation structures associated with energy storage systems and biomedical systems will also be presented to highlight the versatility of this powerful technique

Ultra-Low Amounts of Palladium (0.005-0.05 Wt% Pd) Supported on Titania: Remarkable Low-Temperature Activity for NO Reduction with CO and Structure-Function Property Relationships in Methane Oxidation

Atomically dispersed Pd +2 cations with ultra-dilute loading of palladium (0.005-0.05 wt%) were anchored on anatase titania and characterized with FTIR, microscopy and catalytic tests. CO infrared adsorption produces a sharp, narrow mono-carbonyl Pd(II)-CO band at ~2,130 cm-1 indicating formation of highly uniform and stable Pd+2 ions on anatase titania. The 0.05 wt% Pd/TiO2 sample was evaluated for methane combustion under dry and wet (industrially relevant) conditions in the presence and absence of carbon monoxide. Notably, we find the isolated palladium atoms respond dynamically upon oxygen concentration modulation (switching-on and switching off). When oxygen is removed from the wet methane stream, palladium ions are reduced to metallic state by methane and catalyze methane steam reforming instead of complete methane oxidation. Re-admission of oxygen restores Pd+2 cations and switches off methane steam reforming activity. Moreover, 0.05 wt% Pd/TiO2 is a competent CO oxidation catalyst in the presence of water steam with 90% CO conversion and TOF ~ 4,000 hr-1 at 260 ⁰C. More importantly, we find that diluting 0.05 wt% Pd/titania sample with titania to ultra-low 0.005 wt% palladium loading produces a remarkably active material for nitric oxide reduction with carbon monoxide under industrially relevant conditions with >90% conversion of nitric oxide at 180 ⁰C (~460 ppm NO and 150 L/g*hr flow rate in the presence of >2% water steam) and TOF ~6,000 hr-1. Pd thus outperforms state-of-the-art rhodium containing catalysts with (15-20 times higher rhodium loading; rhodium is ~ 3 times more expensive than palladium). Furthermore, palladium catalysts are more selective towards nitrogen and produce significantly less ammonia relative to the more traditional rhodium catalysts due to lower Pd amount nd lower water-gas-shift activity. Our study is the first example of utilizing ultra-low (0.05 wt% and less) noble metal (Pd) amounts to produce heterogeneous catalysts with extraordinary activity for nitric oxide reduction. This opens up a pathway to study other Pd, Pt and Rh containing materials with ultra-low loadings of expensive noble metals dispersed on titania or titania-coated oxides for industrially relevant nitric oxide abatement.</p

PdO self-assembly on zeolite SSZ-13 with rows of O3Al(IV)OH selectively incorporated in PdO(101) facets for moisture-resistant methane oxidation

We describe an efficient way to prepare moisture-tolerant methane (hydrocarbon) combustion catalysts based on PdO nanoparticles supported on siliceous SSZ-13 zeolite. Only zeolites with high Si/Al ratios >15 are hydrophobic enough to exclude the Pd from the micropores while forming well-faceted PdO nanoparticles. Simultaneously, during self-assembly mobile Al hydroxo species get incorporated into the as-formed PdO nanoparticles. For the first time, we reveal selective incorporation of rows of O3Al(IV)-OHbridging aluminum hydroxo-species into the (101) facets of PdO nanoparticles that form during thermal self-assembly in Pd/SSZ-13 using state-of-the-art atomically-resolved HAADF-STEM imaging, solid-state NMR, DFT calculations and reactivity measurements. The Al+3-OH moieties form atom-thin rows in place of tri-coordinate Pd ions Pd+2 in Pd1O3 on (101) facets: these tri-coordinate Pd1+2O3 are responsible for C-H bond dissociation of methane and hydrocarbons during catalytic methane oxidation. However, on unmodified or non-zeolite supported PdO nanoparticles in the presence of water vapor from engine exhaust, water competes with methane by forming a deactivated Pdtetra(OH)(H2O)Pdtetra site with two water molecules on contiguous 3-coordinate Pd, which is not active for C-H bond activation. When Al-OH moieties are present in place of some tri-coordinate Pd1O3 sites, water dissociation becomes kinetically unfavorable due to disruption of Pdtetra(OH)(H2O)Pdtetra species formation. Consequently, our catalytic measurements reveal a significantly more stable performance of such catalysts in methane combustion in the presence of water vapor. Our findings provide an unprecedented atomic-level insight into structure-property relationships for supported PdO materials in catalytic methane oxidation and offer a new strategy to prepare moisture-tolerant Pd-containing methane combustion catalysts for green-house gas mitigation by selectively doping atomically thin rows of non-precious metal into specific facets of PdO