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1. Atom-by-atom electrodeposition of single isolated cobalt oxide molecules and clusters for studying the oxygen evolution reaction

   https://doi.org/10.1073/pnas.2002168117  

   Jin, Zhaoyu; Bard, Allen J. (May 2020, Proceedings of the National Academy of Sciences)

   We report an electrodeposition protocol for preparing isolated cobalt oxide single molecules (Co₁O_x) and clusters (Co_nO_y) on a carbon fiber nanoelectrode. The as-prepared deposits are able to produce well-defined steady-state voltammograms for the oxygen evolution reaction (OER) in alkaline media, where the equivalent radius (r_d) is estimated by the limiting current of hydroxide oxidation in accordance with the electrocatalytic amplification model. The size of isolated clusters obtained from the femtomolar Co²⁺solution through an atom-by-atom technique can reach as small as 0.21 nm (r_d) which is approximately the length of Co–O bond in cobalt oxide. Therefore, the deposit was close to that of a Co₁O_xsingle molecule with only one cobalt ion, the minimum unit of the cobalt-based oxygen-evolving catalyst. Additionally, the size-dependent catalysis of the OER on Co_nO_ydeposits shows a faster relative rate on the smaller cluster in terms of the potential at a given current density, implying the single molecular catalyst shows a superior OER activity. 

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2. B‐MWW Zeolite: The Case Against Single‐Site Catalysis

   https://doi.org/10.1002/anie.201914696  

   Altvater, Natalie R.; Dorn, Rick W.; Cendejas, Melissa C.; McDermott, William P.; Thomas, Brijith; Rossini, Aaron J.; Hermans, Ive (February 2020, Angewandte Chemie International Edition)

   Abstract Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3−x/2)(x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using¹¹B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO₃units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO₃units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle. 

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3. Sub‐6 nm Fully Ordered L 1 ₀ ‐Pt–Ni–Co Nanoparticles Enhance Oxygen Reduction via Co Doping Induced Ferromagnetism Enhancement and Optimized Surface Strain

   https://doi.org/10.1002/aenm.201803771  

   Wang, Tanyuan; Liang, Jiashun; Zhao, Zhonglong; Li, Shenzhou; Lu, Gang; Xia, Zhengcai; Wang, Chao; Luo, Jiahuan; Han, Jiantao; Ma, Cheng; et al (March 2019, Advanced Energy Materials)

   Abstract Engineering the crystal structure of Pt–M (M = transition metal) nanoalloys to chemically ordered ones has drawn increasing attention in oxygen reduction reaction (ORR) electrocatalysis due to their high resistance against M etching in acid. Although Pt–Ni alloy nanoparticles (NPs) have demonstrated respectable initial ORR activity in acid, their stability remains a big challenge due to the fast etching of Ni. In this work, sub‐6 nm monodisperse chemically orderedL1₀‐Pt–Ni–Co NPs are synthesized for the first time by employing a bifunctional core/shell Pt/NiCoO_xprecursor, which could provide abundant O‐vacancies for facilitated Pt/Ni/Co atom diffusion and prevent NP sintering during thermal annealing. Further, Co doping is found to remarkably enhance the ferromagnetism (room temperature coercivity reaching 2.1 kOe) and the consequent chemical ordering ofL1₀‐Pt–Ni NPs. As a result, the best‐performing carbon supportedL1₀‐PtNi_0.8Co_0.2catalyst reveals a half‐wave potential (E_1/2) of 0.951 V versus reversible hydrogen electrode in 0.1mHClO₄with 23‐times enhancement in mass activity over the commercial Pt/C catalyst along with much improved stability. Density functional theory (DFT) calculations suggest that theL1₀‐PtNi_0.8Co_0.2core could tune the surface strain of the Pt shell toward optimized Pt–O binding energy and facilitated reaction rate, thereby improving the ORR electrocatalysis. 

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4. 10 ⁶ -fold faster C–H bond hydroxylation by a Co ^III,IV ₂ (µ-O) ₂ complex [via a Co ^III ₂ (µ-O)(µ-OH) intermediate] versus its Fe ^III Fe ^IV analog

   https://doi.org/10.1073/pnas.2307950120  

   Li, Yan; Abelson, Chase; Que, Lawrence; Wang, Dong (December 2023, Proceedings of the National Academy of Sciences)

   The hydroxylation of C–H bonds can be carried out by the high-valent Co^III,IV₂(µ-O)₂complex2asupported by the tetradentate tris(2-pyridylmethyl)amine ligand via a Co^III₂(µ-O)(µ-OH) intermediate (3a). Complex3acan be independently generated either by H-atom transfer (HAT) in the reaction of2awith phenols as the H-atom donor or protonation of its conjugate base, the Co^III₂(µ-O)₂complex1a. Resonance Raman spectra of these three complexes reveal oxygen-isotope-sensitive vibrations at 560 to 590 cm⁻¹associated with the symmetric Co–O–Co stretching mode of the Co₂O₂diamond core. Together with a Co•••Co distance of 2.78(2) Å previously identified for1aand2aby Extended X-ray Absorption Fine Structure (EXAFS) analysis, these results provide solid evidence for their “diamond core” structural assignments. The independent generation of3aallows us to investigate HAT reactions of2awith phenols in detail, measure the redox potential and pK_aof the system, and calculate the O–H bond strength (D_O–H) of3ato shed light on the C–H bond activation reactivity of2a. Complex3ais found to be able to transfer its hydroxyl ligand onto the trityl radical to form the hydroxylated product, representing a direct experimental observation of such a reaction by a dinuclear cobalt complex. Surprisingly, reactivity comparisons reveal2ato be 10⁶-fold more reactive in oxidizing hydrocarbon C–H bonds than corresponding Fe^III,IV₂(µ-O)₂and Mn^III,IV₂(µ-O)₂analogs, an unexpected outcome that raises the prospects for using Co^III,IV₂(µ-O)₂species to oxidize alkane C–H bonds. 

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5. Octahedral spinel electrocatalysts for alkaline fuel cells

   https://doi.org/10.1073/pnas.1906570116  

   Yang, Yao; Xiong, Yin; Holtz, Megan E.; Feng, Xinran; Zeng, Rui; Chen, Gary; DiSalvo, Francis J.; Muller, David A.; Abruña, Héctor D. (December 2019, Proceedings of the National Academy of Sciences)

   Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB 2 O 4 /C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo 2 O 4 /C, CoMn 2 O 4 /C, and CoFe 2 O 4 /C. CoMn 2 O 4 /C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo 2 O 4 /C was shown to have a unique Co-Mn core–shell structure. ELNES spectra indicate that the Co in the core is predominantly Co 2.7+ while in the shell, it is mainly Co 2+ . Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells. 

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