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1. 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. 

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2. Carbon Free and Noble Metal Free Ni 2 Mo 6 S 8 Electrocatalyst for Selective Electrosynthesis of H 2 O 2

   https://doi.org/10.1002/adfm.202104716  

   Xia, Fan ; Li, Bomin ; Liu, Yiqi ; Liu, Yuzi ; Gao, Siyuan ; Lu, Ke ; Kaelin, Jacob ; Wang, Rongyue ; Marks, Tobin J. ; Cheng, Yingwen ( August 2021 , Advanced Functional Materials)

   Electrocatalytic two‐electron reduction of oxygen is a promising method for producing sustainable H₂O₂but lacks low‐cost and selective electrocatalysts. Here, the Chevrel phase chalcogenide Ni₂Mo₆S₈is presented as a novel active motif for reducing oxygen to H₂O₂in an aqueous electrolyte. Although it has a low surface area, the Ni₂Mo₆S₈catalyst exhibits exceptional activity for H₂O₂synthesis with >90% H₂O₂molar selectivity across a wide potential range. Chemical titration verified successful generation of H₂O₂and confirmed rates as high as 90 mmol H₂O₂g_cat⁻¹h⁻¹. The outstanding activities are attributed to the ligand and ensemble effects of Ni that promote H₂O dissociation and proton‐coupled reduction of O₂to HOO*, and the spatial effect of the Chevrel phase structure that isolates Ni active sites to inhibit OO cleavage. The synergy of these effects delivers fast and selective production of H₂O₂with high turn‐over frequencies of ≈30 s⁻¹. In addition, the Ni₂Mo₆S₈catalyst has a stable crystal structure that is resistive for oxidation and delivers good catalyst stability for continuous H₂O₂production. The described Ni‐Mo₆S₈active motif can unlock new opportunities for designing Earth‐abundant electrocatalysts to tune oxygen reduction for practical H₂O₂production.

    

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3. Stability and activity of titanium oxynitride thin films for the electrocatalytic reduction of nitrogen to ammonia at different pH values

   https://doi.org/10.1039/D3CP01330H  

   Chukwunenye, Precious ; Ganesan, Ashwin ; Gharaee, Mojgan ; Balogun, Kabirat ; Adesope, Qasim ; Amagbor, Stella Chinelo ; Golden, Teresa D. ; D’Souza, Francis ; Cundari, Thomas R. ; Kelber, Jeffry A. ( July 2023 , Physical Chemistry Chemical Physics)

   The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN – with no oxygen content upon deposition – is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions – predominantly Ti IV oxide – upon exposure to ambient, as determined by ex situ X-ray photoelectron spectroscopy (XPS). XPS, with in situ transfer between electrochemical and UHV environments, however, demonstrates that this Ti IV oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N 2 adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N 2 will not bind to Ti IV centers due to a lack of π-backbonding. Ex situ XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination. 

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4. Hydrogen bonding steers the product selectivity of electrocatalytic CO reduction

   https://doi.org/10.1073/pnas.1900761116  

   Li, Jingyi ; Li, Xiang ; Gunathunge, Charuni M. ; Waegele, Matthias M. ( April 2019 , Proceedings of the National Academy of Sciences)

   The product selectivity of many heterogeneous electrocatalytic processes is profoundly affected by the liquid side of the electrocatalytic interface. The electrocatalytic reduction of CO to hydrocarbons on Cu electrodes is a prototypical example of such a process. However, probing the interactions of surface-bound intermediates with their liquid reaction environment poses a formidable experimental challenge. As a result, the molecular origins of the dependence of the product selectivity on the characteristics of the electrolyte are still poorly understood. Herein, we examined the chemical and electrostatic interactions of surface-adsorbed CO with its liquid reaction environment. Using a series of quaternary alkyl ammonium cations (${\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$,${\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$,${\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$, and${\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$), we systematically tuned the properties of this environment. With differential electrochemical mass spectrometry (DEMS), we show that ethylene is produced in the presence of${\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$and${\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$cations, whereas this product is not synthesized in${\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$- and${\mathrm{b}\mathrm{u}\mathrm{t}\mathrm{y}\mathrm{l}}_4{\mathrm{N}}^{+}$-containing electrolytes. Surface-enhanced infrared absorption spectroscopy (SEIRAS) reveals that the cations do not block CO adsorption sites and that the cation-dependent interfacial electric field is too small to account for the observed changes in selectivity. However, SEIRAS shows that an intermolecular interaction between surface-adsorbed CO and interfacial water is disrupted in the presence of the two larger cations. This observation suggests that this interaction promotes the hydrogenation of surface-bound CO to ethylene. Our study provides a critical molecular-level insight into how interactions of surface species with the liquid reaction environment control the selectivity of this complex electrocatalytic process.

    

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5. Direct Evidence of Local pH Change and the Role of Alkali Cation during CO 2 Electroreduction in Aqueous Media

   https://doi.org/10.1002/anie.201912637  

   Zhang, Fen ; Co, Anne C. ( December 2019 , Angewandte Chemie International Edition)

   We report, for the first time, utilizing a rotating ring‐disc electrode (RRDE) assembly for detecting changes in the local pH during aqueous CO₂reduction reaction (CO₂RR). Using Au as a model catalyst where CO is the only product, we show that the CO oxidation peak shifts by −86±2 mV/pH during CO₂RR, which can be used to directly quantify the change in the local pH near the catalyst surface during electrolysis. We then applied this methodology to investigate the role of cations in affecting the local pH during CO₂RR and find that during CO₂RR to CO on Au in an MHCO₃buffer (where M is an alkali metal), the experimentally measured local basicity decreased in the order Li⁺> Na⁺> K⁺> Cs⁺, which agreed with an earlier theoretical prediction by Singh et al. Our results also reveal that the formation of CO is independent of the cation. In summary, RRDE is a versatile tool for detecting local pH change over a diverse range of CO₂RR catalysts. Additionally, using the product itself (i.e. CO) as the local pH probe allows us to investigate CO₂RR without the interference of additional probe molecules introduced to the system. Most importantly, considering that most CO₂RR products have pH‐dependent oxidation, RRDE can be a powerful tool for determining the local pH and correlating the local pH to reaction selectivity.

    

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