More Like this

1. Selective Catalytic Combustion of Hydrogen under Aerobic Conditions on Na ₂ WO ₄ /SiO ₂

   https://doi.org/10.1002/anie.202412932  

   Kipp, Elijah R; Garcia‐Barriocanal, Javier; Bhan, Aditya (December 2024, Angewandte Chemie International Edition)

   Abstract Na₂WO₄/SiO₂, a material known to catalyze alkane selective oxidation including the oxidative coupling of methane (OCM), is demonstrated to catalyze selective hydrogen combustion (SHC) with >97 % selectivity in mixtures with several hydrocarbons (CH₄, C₂H₆, C₂H₄, C₃H₆, C₆H₆) in the presence of gas‐phase dioxygen at 883–983 K. Hydrogen combustion rates exhibit a near‐first‐order dependence on H₂partial pressure and are zero‐order in H₂O and O₂partial pressures. Mechanistic studies at 923 K using isotopically‐labeled reagents demonstrate the kinetic relevance of H−H dissociation and absence of O‐atom recombination. In situ X‐ray diffraction (XRD) and W L_III‐edge X‐ray absorption spectroscopy (XAS) studies demonstrate, respectively, a loss of Na₂WO₄crystallinity and lack of second‐shell coordination with respect to W⁶⁺cations below 923 K; benchmark experiments show that alkali cations must be present for the material to be selective for hydrogen combustion, but that materials containing Na alone have much lower combustion rates (per gram Na) than those containing Na and W. These data suggest a synergy between Na and W in a disordered phase at temperatures below the bulk melting point of Na₂WO₄(971 K) during SHC catalysis. The Na₂WO₄/SiO₂SHC catalyst maintains stable combustion rates at temperatures ca. 100 K higher than redox‐active SHC catalysts and could potentially enable enhanced olefin yields in tandem operation of reactors combining alkane dehydrogenation with SHC processes. 

   more » « less
2. Improving alkane dehydrogenation activity on γ-Al ₂ O ₃ through Ga doping

   https://doi.org/10.1039/d0cy01474e  

   Abdelgaid, Mona; Dean, James; Mpourmpakis, Giannis (November 2020, Catalysis Science & Technology)

   null (Ed.)

   Nonoxidative alkane dehydrogenation is a promising route to produce olefins, commonly used as building blocks in the chemical industry. Metal oxides, including γ-Al 2 O 3 and β-Ga 2 O 3 , are attractive dehydrogenation catalysts due to their surface Lewis acid–base properties. In this work, we use density functional theory (DFT) to investigate nonoxidative dehydrogenation of ethane, propane, and isobutane on the Ga-doped and undoped (100) γ-Al 2 O 3 via the concerted and stepwise mechanisms. We revealed that doping (100) γ-Al 2 O 3 with Ga atoms has significant improvement in the dehydrogenation activity by decreasing the C–H activation barriers of the kinetically favored concerted mechanism and increasing the overall dehydrogenation turnover frequencies. We identified the dissociated H 2 binding energy as an activity descriptor for alkane dehydrogenation, accounting for the strength of the Lewis acidity and basicity of the active sites. We demonstrate linear correlations between the dissociated H 2 binding energy and the activation barriers of the rate determining steps for both the concerted and stepwise mechanisms. We further found the carbenium ion stability to be a quantitative reactant-type descriptor, correlating with the C–H activation barriers of the different alkanes. Importantly, we developed an alkane dehydrogenation model that captures the effect of catalyst acid–base surface properties (through dissociated H 2 binding energy) and reactant substitution (through carbenium ion stability). Additionally, we show that the dissociated H 2 binding energy can be used to predict the overall dehydrogenation turnover frequencies. Taken together, our developed methodology facilitates the screening and discovery of alkane dehydrogenation catalysts and demonstrates doping as an effective route to enhance catalytic activity. 

   more » « less
3. Propane dehydrogenation over extra-framework In( i ) in chabazite zeolites

   https://doi.org/10.1039/d1sc05866e  

   Yuan, Yong; Lobo, Raul F. (March 2022, Chemical Science)

   Indium on silica, alumina and zeolite chabazite (CHA), with a range of In/Al ratios and Si/Al ratios, have been investigated to understand the effect of the support on indium speciation and its corresponding influence on propane dehydrogenation (PDH). It is found that In 2 O 3 is formed on the external surface of the zeolite crystal after the addition of In(NO 3 ) 3 to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H 2 gas (550 °C), indium displaces the proton in Brønsted acid sites (BASs), forming extra-framework In + species (In-CHA). A stoichiometric ratio of 1.5 of formed H 2 O to consumed H 2 during H 2 pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO 2 , 10In/Al 2 O 3 ( i.e. , 10 wt% indium) and bulk In 2 O 3 , in which In 2 O 3 was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H 2 temperature-programmed reduction (H 2 -TPR) profiles, pulse reactor investigations and in situ transmission FTIR spectroscopy. The BASs in H-CHA facilitate the formation and stabilization of In + cations in extra-framework positions, and prevent the deep reduction of In 2 O 3 to In(0). In + cations in the CHA zeolite can be oxidized with O 2 to form indium oxide species and can be reduced again with H 2 quantitatively. At comparable conversion, In-CHA shows better stability and C 3 H 6 selectivity (∼85%) than In 2 O 3 , 10In/SiO 2 and 10In/Al 2 O 3 , consistent with a low C 3 H 8 dehydrogenation activation energy (94.3 kJ mol −1 ) and high C 3 H 8 cracking activation energy (206 kJ mol −1 ) in the In-CHA catalyst. A high Si/Al ratio in CHA seems beneficial for PDH by decreasing the fraction of CHA cages containing multiple In + cations. Other small-pore zeolite-stabilized metal cation sites could form highly stable and selective catalysts for this and facilitate other alkane dehydrogenation reactions. 

   more » « less
4. Insights into how the aqueous environment influences the kinetics and mechanisms of heterogeneously-catalyzed COH* and CH ₃ OH* dehydrogenation reactions on Pt(111)

   https://doi.org/10.1039/C9CP00824A  

   Bodenschatz, Cameron J.; Xie, Tianjun; Zhang, Xiaohong; Getman, Rachel B. (May 2019, Physical Chemistry Chemical Physics)

   Water influences catalytic reactions in multiple ways, including energetic and mechanistic effects. While simulations have provided significant insight into the roles that H 2 O molecules play in aqueous-phase heterogeneous catalysis, questions still remain as to the extent to which H 2 O structures influence catalytic mechanisms. Specifically, influences of the configurational variability in the water structures at the catalyst interface are yet to be understood. Configurational variability is challenging to capture, as it requires multiscale approaches. Herein, we apply a multiscale sampling approach to calculate reaction thermodynamics and kinetics for COH* dehydrogenation to CO* and CH 3 OH* dehydrogenation to CH 2 OH* on Pt(111) catalysts under liquid H 2 O. We explore various pathways for these dehydrogenation reactions that could influence the overall mechanism of methanol decomposition by including participation of H 2 O structures both energetically and mechanistically. We find that the liquid H 2 O environment significantly influences the mechanism of COH* dehydrogenation to CO* but leaves the mechanism of CH 3 OH* dehydrogenation to CH 2 OH* largely unaltered. 

   more » « less
5. Growth of α-Ga ₂ O ₃ on α-Al ₂ O ₃ by conventional molecular-beam epitaxy and metal–oxide-catalyzed epitaxy

   https://doi.org/10.35848/1347-4065/acbe04  

   McCandless, J. P.; Rowe, D.; Pieczulewski, N.; Protasenko, V.; Alonso-Orts, M.; Williams, M. S.; Eickhoff, M.; Xing, H. G.; Muller, D. A.; Jena, D.; et al (March 2023, Japanese Journal of Applied Physics)

   Abstract We report the growth of α -Ga 2 O 3 on m -plane α -Al 2 O 3 by conventional plasma-assisted molecular-beam epitaxy and In-mediated metal–oxide-catalyzed epitaxy (MOCATAXY). We report a growth rate diagram for α -Ga 2 O 3 ( 10 1 ¯ 0 ), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-lane mosaic spread when MOCATAXY is employed for the growth of α -Ga 2 O 3 . Through the use of In-mediated catalysis, growth rates over 0.2 μ m h −1 and rocking curves with full width at half maxima of Δ ω ≈ 0.45° are achieved. Faceting is observed along the α -Ga 2 O 3 film surface and explored through scanning transmission electron microscopy. 

   more » « less