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1. Revealing the Mechanistic Details for the Selective Deoxygenation of Carboxylic Acids over Dynamic MoO 3 Catalysts

   https://doi.org/10.1021/acscatal.3c01053  

   Gomez, Laura A. ; Bavlnka, Caleb Q. ; Zhang, Tianhao E. ; Resasco, Daniel E. ; Crossley, Steven P. ( June 2023 , ACS Catalysis)

   The selective activation of renewable carboxylic acids could enable the formation of a variety of highly valuable renewable products, including surfactants, valuable dienes, and renewable hydrogen carriers. A kinetic study is performed to enhance understanding of the selective deoxygenation of carboxylic acid on promoted MoO3 at mild temperatures. Although several studies indicate that deoxygenation of oxygenated biomass-derived compounds on MoO3 occurs via a reverse Mars−van Krevelen mechanism, this study suggests that the deoxygenation of pentanoic acid (PA) on an oxygen vacancy can also be explained by a Langmuir−Hinshelwood mechanism. A detailed analysis of the experimental data indicates that the incorporation of Pt on MoO3 shifts the reaction order with respect to hydrogen from 1 to 0.5 at a low partial pressure of PA. We reveal that the rate-determining step (RDS) shifts upon the incorporation of Pt from H2 dissociation to H addition to adsorbed acid molecules. We further illustrate how the RDS can shift as a function of PA coverage. The inhibition effect of PA and its possible causes are discussed for both MoO3 and 0.05 wt % Pt/MoO3 catalysts. Here, we decouple promoter effects from the creation of highly active sites located at the Pt/MoO3 interface. The nature of the active site involved upon Pt incorporation is also studied by separating Pt from MoO3 at a controlled distance using carbon nanotubes as hydrogen bridges, confirming that the kinetically relevant role of Pt is to serve as a promoter of the MoO3. 

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2. Spatial Diffusion of Hydrogen Atoms in Normal and Para-Hydrogen Molecular Films at Temperature 0.7 K

   https://doi.org/10.1007/s10909-024-03053-w  

   Sheludiakov, S. ; Wetzel, C. K. ; Lee, D. M. ; Khmelenko, V. V. ; Järvinen, J. ; Ahokas, J. ; Vasiliev, S. ( February 2024 , Journal of Low Temperature Physics)

   We report on electron spin resonance studies of H atoms stabilized in solid H2 films at temperature 0.7 K and in a magnetic field of 4.6 T. The H atoms were produced by bombarding H2 films with 100 eV electrons from a radiofrequency discharge run in the sample cell. We observed a one order of magnitude faster H atom accumulation in the films made of para-H2 gas with a small ortho-H2 concentration (0.2% ortho-H2 ) as compared with those made from normal H2 gas content (75% ortho-H2 ). We also studied the influence of ortho-H2 molecules on spatial diffusion of H atoms in solid H2 films. The spatial diffusion of H atoms in both normal and para-H2 films is faster than the diffusion obtained from the measurement of H atom recombination. The rate of spatial diffusion of H atoms in para-H2 films was slower in comparison with that in the normal H2 films. We discuss possible explanations of these observations. 

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3. Secondary organic aerosol formation from camphene oxidation: measurements and modeling

   https://doi.org/10.5194/acp-22-3131-2022  

   Li, Qi ; Jiang, Jia ; Afreh, Isaac K. ; Barsanti, Kelley C. ; Cocker III, David R. ( January 2022 , Atmospheric Chemistry and Physics)

   Abstract. While camphene is one of the dominant monoterpenesmeasured in biogenic and pyrogenic emission samples, oxidation of camphenehas not been well-studied in environmental chambers and very little is knownabout its potential to form secondary organic aerosol (SOA). The lack ofchamber-derived SOA data for camphene may lead to significant uncertaintiesin predictions of SOA from oxidation of monoterpenes using existingparameterizations when camphene is a significant contributor to totalmonoterpenes. Therefore, to advance the understanding of camphene oxidationand SOA formation and to improve representation of camphene in air qualitymodels, a series of experiments was performed in the University ofCalifornia Riverside environmental chamber to explore camphene SOA massyields and properties across a range of chemical conditions atatmospherically relevant OH concentrations. The experimental results werecompared with modeling simulations obtained using two chemically detailedbox models: Statewide Air Pollution Research Center (SAPRC) and Generatorfor Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A).SOA parameterizations were derived from the chamber data using both thetwo-product and volatility basis set (VBS) approaches. Experiments performedwith added nitrogen oxides (NOx) resulted in higher SOA mass yields (upto 64 %) than experiments performed without added NOx (up to 28 %).In addition, camphene SOA mass yields increased with SOA mass (Mo) atlower mass loadings, but a threshold was reached at higher mass loadings inwhich the SOA mass yields no longer increased with Mo. SAPRC modelingof the chamber studies suggested that the higher SOA mass yields at higherinitial NOx levels were primarily due to higher production of peroxyradicals (RO2) and the generation of highly oxygenated organicmolecules (HOMs) formed through unimolecular RO2 reactions. SAPRCpredicted that in the presence of NOx, camphene RO2 reacts with NOand the resultant RO2 undergoes hydrogen (H)-shift isomerizationreactions; as has been documented previously, such reactions rapidly addoxygen and lead to products with very low volatility (i.e., HOMs). The endproducts formed in the presence of NOx have significantly lowervolatilities, and higher O : C ratios, than those formed by initial campheneRO2 reacting with hydroperoxyl radicals (HO2) or other RO2.Further analysis reveals the existence of an extreme NOx regime, whereinthe SOA mass yield can be suppressed again due to high NO / HO2 ratios.Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. Theobserved differences in SOA mass yields were largely explained by thegas-phase RO2 chemistry and the competition between RO2+HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidationreactions. 

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4. Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

   https://doi.org/10.1002/aenm.201801155  

   Moreno‐Hernandez, Ivan A. ; Brunschwig, Bruce S. ; Lewis, Nathan S. ( July 2018 , Advanced Energy Materials)

   Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_xlayer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm⁻²of simulated sunlight. The SnO_xlayer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0mKOH(aq) or 1.0mH₂SO₄(aq). Ideal regenerative solar‐to‐O₂(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in 1.0mKOH(aq) with Ni or in 1.0mH₂SO₄(aq) with Pt/IrO_xlayers as OER catalysts. Stable photocurrents for >100 h are obtained for electrodes with patterned catalyst layers in both 1.0mKOH(aq) and 1.0mH₂SO₄(aq).

    

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5. Nitrogen‐Coordinated Single Cobalt Atom Catalysts for Oxygen Reduction in Proton Exchange Membrane Fuel Cells

   https://doi.org/10.1002/adma.201706758  

   Wang, Xiao Xia ; Cullen, David A. ; Pan, Yung‐Tin ; Hwang, Sooyeon ; Wang, Maoyu ; Feng, Zhenxing ; Wang, Jingyun ; Engelhard, Mark H. ; Zhang, Hanguang ; He, Yanghua ; et al ( January 2018 , Advanced Materials)

   Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt‐free and Fe‐free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high‐performance nitrogen‐coordinated single Co atom catalyst is derived from Co‐doped metal‐organic frameworks (MOFs) through a one‐step thermal activation. Aberration‐corrected electron microscopy combined with X‐ray absorption spectroscopy virtually verifies the CoN₄coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half‐wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe‐based catalysts and 60 mV lower than Pt/C ‐60 μg Pt cm⁻²). Fuel cell tests confirm that catalyst activity and stability can translate to high‐performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well‐dispersed CoN₄active sites embedded in 3D porous MOF‐derived carbon particles, omitting any inactive Co aggregates.

    

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