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1. Catalytic and modulation excitation visible-NIR spectroscopic insights into the consequences of K promotion and pretreatment on Cu/SiO2 for direct propylene epoxidation

   https://doi.org/10.1016/j.jcat.2025.116242  

   Alzahrani, Hashim A; Zhu, Hongda; Bravo-Suárez, Juan J (May 2025, Journal of Catalysis)

   Systematic fixed-bed reactor studies were performed under propylene epoxidation conditions with O2 (1 atm, 250 °C) to examine the impact of potassium and oxidative/reductive pretreatments on Cu/SiO2 catalysts. At similar propylene conversion and copper particle sizes (2–3 nm), hydrogen pretreatment of K-promoted Cu/SiO2 significantly increased propylene oxide (PO) selectivity, making it the primary product (∼40 %). Characterization of the spent catalysts using XPS and H2-TPR revealed a unique Cu-K interaction following H2 pretreatment and suggested a potential role for subsurface oxygen in the reaction. Activity tests with CuO and Cu2O, both with and without K, showed that K deactivates CuO while enhancing PO/acrolein ratio for Cu2O. Operando Modulation Excitation-Phase Sensitive Detection- Diffuse Reflectance Visible-Near Infrared Spectroscopy (ME-PSD-DRVis-NIR) revealed charge transfer correlations. These correlations suggested that the combined K and H2 increases the population of mixed-valent Cum+ (1 < m < 2) (Cu1+ and Cu2+ ensemble) sites and modulates the strength of oxygen adsorption and activation at these sites, thereby improving PO selectivity and formation rate. These results demonstrate that controlling the copper oxidation state through promoters and pretreatments enhances PO selectivity in direct propylene epoxidation with O2, providing valuable insights for improving catalytic activity and identifying crucial oxygen species. 

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2. Flow Electrolyzer Mass Spectrometry with a Gas‐Diffusion Electrode Design

   https://doi.org/10.1002/anie.202013713  

   Hasa, Bjorn; Jouny, Matthew; Ko, Byung Hee; Xu, Bingjun; Jiao, Feng (December 2020, Angewandte Chemie International Edition)

   Abstract Operando mass spectrometry is a powerful technique to probe reaction intermediates near the surface of catalyst in electrochemical systems. For electrochemical reactions involving gas reactants, conventional operando mass spectrometry struggles in detecting reaction intermediates because the batch‐type electrochemical reactor can only handle a very limited current density due to the low solubility of gas reactant(s). Herein, we developed a new technique, namely flow electrolyzer mass spectrometry (FEMS), by incorporating a gas‐diffusion electrode design, which enables the detection of reactive volatile or gaseous species at high operating current densities (>100 mA cm⁻²). We investigated the electrochemical carbon monoxide reduction reaction (eCORR) on polycrystalline copper and elucidated the oxygen incorporation mechanism in the acetaldehyde formation. Combining FEMS and isotopic labelling, we showed that the oxygen in the as‐formed acetaldehyde intermediate originates from the reactant CO, while ethanol and n‐propanol contained mainly solvent oxygen. The observation provides direct experimental evidence of an isotopic scrambling mechanism. 

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3. Deal–Grove-like thermal oxidation of Si (001) buried under a thin layer of SrTiO3

   https://doi.org/10.1063/1.5097839  

   Guo, Wei; Posadas, A. B.; Demkov, A. A. (February 2020, Journal of Applied Physics)

   Dry oxidation of Si (001) beneath a thin epitaxial SrTiO3 layer has been studied using furnace annealing in flowing oxygen. A 10-nm layer of SrTiO3 is epitaxially grown on Si with no SiO2 interlayer. For such a structure, an annealing temperature of 800 °C was found to be the limiting temperature to prevent silicate formation and disruption of the interface structure. The effect of annealing time on the thickness of the SiO2 layer was investigated. In situ x-ray photoelectron spectroscopy and reflection-high-energy electron diffraction were used to ensure that the quality of SrTiO3 is unchanged after the annealing process. The experimental annealing data are compared with a theoretical oxygen diffusion model based on that of Deal, Grove, and Massoud. The model fits the experimental data well, indicating that oxygen diffusion through the SrTiO3 layer is not the limiting factor. One can therefore readily control the thickness of the SiO2 interlayer by simply controlling the annealing time in flowing oxygen. 

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4. In Situ DDR-UV-Vis and Raman spectroscopic study of titania promoting effect on silica supported tungsten catalysts for enhanced propylene metathesis

   https://doi.org/10.1016/j.cattod.2024.115138  

   Patil, Bhagyesha S; Torres-Velasco, Alejandra; Alzahrani, Hashim A; Pratt, Skylar; Bravo-Suárez, Juan J (March 2025, Catalysis Today)

   Tungsten based catalysts supported on silica (zWOX/SiO2) and silica promoted by titania (zWOX/yTiOX/SiO2) were studied for their catalytic activity towards propylene metathesis. The catalysts were prepared by a simple incipient wetness impregnation method using a large pore SiO2 of intermediate surface area (∼50 m2/g). Catalytic activity studies carried out in a fixed-bed reactor (723 K, 101 kPa propylene) indicated that propylene conversion increased with increasing W loading in zWOx/SiO2 catalysts (z = 0.5−6 W/nm2). It was shown that the catalytic activity of a poorly WOX dispersed 6WOX/SiO2 catalyst could be enhanced and maximized by an optimum titania promotion of 2 wt% TiO2 (∼3 Ti/nm2). In situ differential diffuse reflectance (DDR) UV-Vis spectroscopy at reaction conditions showed that TiOX domain size increased with increases in titania loading from isolated TiOX to TiOX clusters to TiO2 nanocrystals. The UV-Vis results also evidenced the existence of highly dispersed isolated WOX species, WOX clusters, and WO3 nanoparticles in the 6WOX/yTiOX/SiO2 (y = 0.5−6 wt% TiO2 or ∼0.7–9 Ti/nm2) catalysts. In situ DDR-UV-Vis, Raman, and mass spectrometry during propylene metathesis, and catalyst oxidation and reduction revealed the reasons for an optimum amount of titania promoter in 6WOX/2TiOX/SiO2. They were the result of a balanced interplay between two factors: (1) enhanced WOx species dispersion due to the presence of a trimeric TiOX cluster and (2) absence of catalyst deactivation (present at high TiO2 loadings) due to the trimeric TiOX cluster poor reactivity towards coke formation. 

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5. Low tone bias during perception of period doubling

   https://doi.org/10.1121/10.0015927  

   Huang, Yaqian (October 2022, The Journal of the Acoustical Society of America)

   Period-doubled voice consists of two alternating periods with multiple frequencies and is often perceived as rough with an indeterminate pitch. Past pitch-matching studies in period-doubled voice found that the perceived pitch was lower as the degree of amplitude and frequency modulation between the two alternating periods increased. The perceptual outcome also differed across f0s and modulation types: a lower f0 prompted earlier identification of a lower pitch, and the matched pitch dropped more quickly in frequency- than amplitude-modulated tokens (Sun & Xu, 2002; Bergan & Titze, 2001). However, it is unclear how listeners perceive period doubling when identifying linguistic tones. In an artificial language learning paradigm, this study used resynthesized stimuli with alternating amplitudes and/or frequencies of varying degrees, based on a production study of period-doubled voice (Huang, 2022). Listeners were native speakers of English and Mandarin. We confirm the positive relationship between the modulation degree and the proportion of low tones heard, and find that frequency modulation biased listeners to choose more low-tone options than amplitude modulation. However, a higher f0 (300 Hz) leads to a low-tone percept in more amplitude-modulated tokens than a lower f0 (200 Hz). Both English and Mandarin listeners behaved similarly, suggesting that pitch perception during period doubling is not language-specific. Furthermore, period doubling is predicted to signal low tones in languages, even when the f0 is high. 

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