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1. Platinum and cobalt intermetallic nanoparticles confined within MIL-101(Cr) for enhanced selective hydrogenation of the carbonyl bond in α,β-unsaturated aldehydes: synergistic effects of electronically modified Pt sites and Lewis acid sites

   https://doi.org/10.1039/D0CY02082F  

   Zahid, Muhammad ; Li, Jiang ; Ismail, Ahmed ; Zaera, Francisco ; Zhu, Yujun ( April 2021 , Catalysis Science & Technology)

   Precious metals have been shown to play a vital role in the selective hydrogenation of α,β-unsaturated aldehydes, but still suffer from challenges to control selectivity. Herein, we have advanced the design of catalysts made out of Pt–Co intermetallic nanoparticles (IMNs) supported on a MIL-101(Cr) MOF (3%Pt y %Co/MIL-101(Cr)), prepared by using a polyol reduction method, as an effective approach to enhance selectivity toward the production of α,β-unsaturated alcohol, the desired product. XRD, N 2 adsorption–desorption, FTIR spectroscopy, SEM, TEM, XPS, CO adsorption, NH 3 -TPD, XANES and EXAFS measurements were used to investigate the structure and surface properties of our 3%Pt y %Co/MIL-101(Cr) catalysts. It was found that the Co-modified 3%Pt y %Co/MIL-101(Cr) catalysts can indeed improve the hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL), reaching a higher selectivity under mild conditions than the monometallic Pt/MIL-101(Cr) catalysts: 95% conversion of CAL with 91% selectivity to COL can be reached with 3%Pt3%Co/MIL-101(Cr). Additionally, high conversion of furfural (97%) along with high selectivity to furfural alcohol (94%) was also attained with the 3%Pt3%Co/MIL-101(Cr) catalyst. The enhanced activity and selectivity toward the unsaturated alcohols are attributed to the electronic and geometric effects derived from the partial charge transfer between Co and Pt through the formation of uniformly dispersed Pt–Co IMNs. Moreover, various characterization results revealed that the addition of Co to the IMPs can promote the Lewis acid sites that facilitate the polarization of the charge-rich CO bonds and their adsorption via their oxygen atom, and also generate new interfacial acid sites. 

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2. Stability and kinetics of iron‐terephthalate MOFs with diverse structures in aqueous pollutant degradation

   https://doi.org/10.1002/aic.18205  

   Moore, Samuel C. ; Sarazen, Michele L. ( August 2023 , AIChE Journal)

   Synthesized iron‐terephthalate metal–organic frameworks (MOFs), MIL‐101 and MOF‐235, with contrasting morphologies are examined to elucidate the role of structural arrangement in catalytic aqueous pollutant degradation. MIL‐101 demonstrates a larger pseudo‐first order rate constant than MOF‐235 (3.5 ± 0.2 mol_Fe⁻¹·s⁻¹vs. 0.84 ± 0.07 mol_Fe⁻¹·s⁻¹) toward oxidation of methylene blue (MB) dye with excess hydrogen peroxide at ambient temperature, likely due to intrinsic differences in ligand coordination at their metal nodes. However, despite continued activity upon reuse, both MOFs undergo structural alterations resulting in formation of leached species active for MB degradation that have been obfuscated in previous studies. Detailed stability testing andex situcharacterization of recovered catalyst, examinations that remain underreported in Fe‐MOF studies for pollutant oxidation, indicate that water plays a prominent role in the breakdown of these frameworks. Collectively, this work informs the interpretation and use of common Fe‐MOFs for aqueous applications, relating material changes to observed reaction phenomena.

    

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3. Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites

   https://doi.org/10.1073/pnas.1813849115  

   Snyder, Benjamin E. R. ; Bols, Max L. ; Rhoda, Hannah M. ; Vanelderen, Pieter ; Böttger, Lars H. ; Braun, Augustin ; Yan, James J. ; Hadt, Ryan G. ; Babicz, Jr., Jeffrey T. ; Hu, Michael Y. ; et al ( November 2018 , Proceedings of the National Academy of Sciences)

   A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.

    

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4. 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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5. Photolytic reaction of dissolved organic matter and bromide ions promote the formation of manganese oxides

   Gao, Zhenwei ; Liu, Jing ; Skurie, Charlie ; Zhu, Yaguang ; Jun, Young-Shin ( March 2022 , American Chemical Society National Meeting)

   Manganese (Mn) oxide solids widely exist in nature, serving as both electron donors and acceptors for a variety of redox reactions. Previous studies have highlighted the adsorption of dissolved organic matter (DOM) on Mn oxides, as well as the reduction of Mn oxides by DOM. Here, we show the underappreciated roles of photolytic reactions of DOM in Mn2+(aq) oxidation and its consequential formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2) can be generated. Among them, we found that O2•− was responsible for Mn oxidation. In addition, in the presence of bromide ions (Br−), the photolytic reactions between DOM and Br− formed reactive bromide radicals and facilitated the oxidation of Mn2+(aq) to Mn oxide solids. Moreover, the composition of DOM affected its oxidative capability. When DOM contained more aromatic functional groups, we observed more oxidation of Mn2+ to Mn oxides. These new findings advance our knowledge of natural Mn2+ oxidation and Mn(III/IV) oxide formation, as well as the hitherto overlooked oxidative role of DOM in the oxidation of metal ions in surface water under sunlight illumination. 

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