Wang, Wenxi; Cao, Zhen; Elia, Giuseppe Antonio; Wu, Yingqiang; Wahyudi, Wandi; Abou-Hamad, Edy; Emwas, Abdul-Hamid M.; Cavallo, Luigi; Li, Lain-Jong; Ming, Jun(ACS Energy Letters, American Chemical Society (ACS), 2018-11-06)[Article]
Sulfurized polyacrylonitrile (SPAN) is the most promising cathode for next-generation lithium–sulfur (Li–S) batteries due to the much improved stability. However, the molecular structure and reaction mechanism have not yet been fully understood. Herein, we present a new take on the structure and mechanism to interpret the electrochemical behaviors. We find that the thiyl radical is generated after the cleavage of the S–S bond in molecules in the first cycle, and then a conjugative structure can be formed due to electron delocalization of the thiyl radical on the pyridine backbone. The conjugative structure can react with lithium ions through a lithium coupled electron transfer process and form an ion-coordination bond reversibly. This could be the real reason for the superior lithium storage capability, in which the lithium polysulfide may not be formed. This study refreshes current knowledge of SPAN in Li–S batteries. In addition, the structural analysis is applicable to analyze the current organic cathodes in rechargeable batteries and also allows further applications in Al–S batteries to achieve high performance.
Wimmer, Florian P.; Caporaso, Lucia; Cavallo, Luigi; Mecking, Stefan; Falivene, Laura(Macromolecules, American Chemical Society (ACS), 2018-06-07)[Article]
The copolymerization of ethylene (E) with allyl ethyl ether (AEE) by [di(2-dianisyl)phosphine-2-yl]benzenesulfonato Pd(II) as a catalyst is investigated by DFT calculations and compared with the copolymerization of E with diallyl ether (DAE). For AEE, both 1,2- and 2,1-monomer insertions lead to a very stable O-Chelate product (a five-membered and a four-membered ring, respectively) that hinders any further ethylene insertion. As for DAE, a first 2,1-insertion (favored by 1.8 kcal mol vs the 1,2-insertion) leads to the four-membered O-Chelate product that easily evolves to the most stable intermediate with the second DAE C=C bond coordinated to the metal promoting the following 1,2-insertion. The 2,1 + 1,2 DAE insertion product, bearing a five-membered cyclic unit, is stabilized by a β-agostic interaction that easily opens in favor of E coordination and insertion. Based on the proposed copolymerization mechanism, the stereochemistry of the E/DAE copolymer is studied and the experimental microstructure explained. Finally, [di(2-anisyl)phosphine-2-yl]benzenesulfon(methyl)amido Pd(II) species showing a greater regioselectivity toward a first DAE 2,1-insertion (Δ ΔG of -3.6 kcal mol) are suggested to be a promising catalyst.
Le Quéméner, Frédéric; Basset, Jean-Marie; Barman, Samir; Merle, Nicolas; Aljuhani, Maha A.; Samantaray, Manoja K; Saih, Youssef; Szeto, Kai C.; De Mallmann, Aimery; Minenkov, Yury; Huang, Kuo-Wei; Cavallo, Luigi; Taoufik, Mostafa(ACS Catalysis, American Chemical Society (ACS), 2018-07-10)[Article]
The catalytic oxidation of cis-2-butene and propylene with molecular oxygen in the presence of a well-defined surface coordination compound, (≡SiO)2Mo(═O)2, affords acetaldehyde. Using a cis-2-butene/O2 feed at 350–400 °C, the reaction yields a conversion of approximately 10% and an acetaldehyde selectivity of approximately 70%. This performance is maintained up to an experimental time of 20 h in a continuous flow reactor. The Mo(bis-oxo) surface compound was fully characterized by multiple spectroscopic techniques as well as surface microanalysis. The results from quantum mechanics calculations indicate that the reaction proceeds via [2 + 2] cycloaddition/cycloelimination steps with the formation of metalla-oxacyclobutane intermediates, analogous to the Chauvin mechanism in olefin metathesis.
Lamers, Marlene; Li, Wenjie; Favaro, Marco; Starr, David E.; Friedrich, Dennis; Lardhi, Sheikha F.; Cavallo, Luigi; Harb, Moussab; van de Krol, Roel; Wong, Lydia H.; Abdi, Fatwa F.(Chemistry of Materials, American Chemical Society (ACS), 2018-11-21)[Article]
Recent progress on bismuth vanadate (BiVO) has shown it to be among the highest performing metal oxide photoanode materials. However, further improvement, especially in the form of thin film photoelectrodes, is hampered by its poor charge carrier transport and its relatively wide bandgap. Here, sulfur incorporation is used to address these limitations. A maximum bandgap decrease of ∼0.3 eV is obtained, which increases the theoretical maximum solar-to-hydrogen efficiency from 9 to 12%. Hard X-ray photoelectron spectroscopy measurements as well as density functional theory calculations show that the main reason for the bandgap decrease is an upward shift of the valence band maximum. Time-resolved microwave conductivity measurements reveal a ∼3 times higher charge carrier mobility compared to unmodified BiVO, resulting in a ∼70% increase in the carrier diffusion length. This work demonstrates that sulfur incorporation can be a promising and practical method to improve the performance of wide-bandgap metal oxide photoelectrodes.
Brzozowska, Aleksandra; Azofra, Luis Miguel; Zubar, Viktoriia; Atodiresei, Iuliana; Cavallo, Luigi; Rueping, Magnus; El-Sepelgy, Osama(ACS Catalysis, American Chemical Society (ACS), 2018-03-30)[Article]
The first example of manganese catalyzed semihydrogenation of internal alkynes to (Z)-alkenes using ammonia borane as a hydrogen donor is reported. The reaction is catalyzed by a pincer complex of the earth abundant manganese(II) salt in the absence of any additives, base or super hydride. The ammonia borane smoothly reduces the manganese pre-catalyst [Mn(II)-PNP][Cl]2 to the catalytically active species [Mn(I)-PNP]-hydride in the triplet spin state. This manganese hydride is highly stabilized by complexation with the alkyne substrate. Computational DFT analysis studies of the reaction mechanism rationalizes the origin of stereoselectivity towards formation of (Z)-alkenes.
Suryanto, Bryan Harry Rahmat; Kang, Colin Suk Mo; Wang, Dabin; Xiao, Changlong; Zhou, Fengling; Azofra, Luis Miguel; Cavallo, Luigi; Zhang, Xinyi; Macfarlane, Douglas R.(ACS Energy Letters, American Chemical Society (ACS), 2018-04-25)[Article]
Renewable energy driven ammonia electrosynthesis by N2 reduction reaction (NRR) at ambient conditions is vital for the sustainability of the global population and energy demand. However, NRR under ambient conditions to date has been plagued with low yield rate and selectivity (<10%) due to the more favourable hydrogen evolution reaction (HER) in aqueous media. Herein, surface area enhanced α-Fe nanorods grown on carbon fibre paper was used as a NRR cathode in an aprotic fluorinated solvent – ionic liquid mixture. Through this design, a significantly enhanced NRR activity with NH3 yield rate of ~2.35 × 10-11 mol s-1 cmGSA-2, (3.71 × 10-13 mol s-1 cmECSA-2) and selectivity of ~32% has been achieved under ambient conditions. This study reveals that the use of hydrophobic fluorinated aprotic electrolyte effectively limits the availability of protons and thus suppresses the competing HER. Therefore, electrode-electrolyte engineering is essential in advancing the NH3 electrosynthesis technology.
Falivene, Laura; Kozlov, Sergey M.; Cavallo, Luigi(ACS Catalysis, American Chemical Society (ACS), 2018-05-08)[Article]
Better catalysts are needed to address numerous challenges faced by humanity. In this perspective, we review concepts and tools in theoretical and computational chemistry that can help to accelerate the rational design of homogeneous and heterogeneous catalysts. In particular, we focus on the following three topics: 1) identification of key intermediates and transition states in a reaction using the energetic span model, 2) disentanglement of factors influencing the relative stability of the key species using energy decomposition analysis and the activation strain model, and 3) discovery of new catalysts using volcano relationships. To facilitate wider use of these techniques across different areas, we illustrate their potentials and pitfalls when applied to the study of homogeneous and heterogeneous catalysts.
Wang, Chengming; Maity, Bholanath; Cavallo, Luigi; Rueping, Magnus(Organic Letters, American Chemical Society (ACS), 2018-05-08)[Article]
A new efficient manganese-catalyzed selective C2-alkylation of indoles via carbenoid insertion has been achieved. The newly developed C-H functionalization protocol provides access to diverse products and shows good functional group tolerance. Mechanistic and computational studies support the formation of a Mn(CO)3 acetate complex as the catalytically active species.
Ming, Jun; Cao, Zhen; Wahyudi, Wandi; Li, Mengliu; Kumar, Pushpendra; Wu, Yingqiang; Hwang, Jang-Yeon; Hedhili, Mohamed N.; Cavallo, Luigi; Sun, Yang-Kook; Li, Lain-Jong(ACS Energy Letters, American Chemical Society (ACS), 2018-01-04)[Article]
Graphite anodes are not stable in most noncarbonate solvents (e.g., ether, sulfoxide, sulfone) upon Li ion intercalation, known as an urgent issue in present Li ions and next-generation Li–S and Li–O2 batteries for storage of Li ions within the anode for safety features. The solid electrolyte interphase (SEI) is commonly believed to be decisive for stabilizing the graphite anode. However, here we find that the solvation structure of the Li ions, determined by the electrolyte composition including lithium salts, solvents, and additives, plays a more dominant role than SEI in graphite anode stability. The Li ion intercalation desired for battery operation competes with the undesired Li+–solvent co-insertion, leading to graphite exfoliation. The increase in organic lithium salt LiN(SO2CF3)2 concentration or, more effectively, the addition of LiNO3 lowers the interaction strength between Li+ and solvents, suppressing the graphite exfoliation caused by Li+–solvent co-insertion. Our findings refresh the knowledge of the well-known SEI for graphite stability in metal ion batteries and also provide new guidelines for electrolyte systems to achieve reliable and safe Li–S full batteries.
Bootharaju, Megalamane Siddaramappa; Kozlov, Sergey M.; Cao, Zhen; Shkurenko, Aleksander; El-Zohry, Ahmed M.; Mohammed, Omar F.; Eddaoudi, Mohamed; Bakr, Osman; Cavallo, Luigi; Basset, Jean-Marie(Chemistry of Materials, American Chemical Society (ACS), 2018-03-26)[Article]
The lack of structurally distinct nanoclusters (NCs) of identical size and composition prevented the mechanistic understanding of their structural effects on ion pairing and concomitant optical properties. To produce such highly sought NCs, we designed a new monothiolate-for-dithiolate exchange strategy that enabled the selective transformation of the structure of a NC without affecting its metal atomicity or composition. Through this method, a bimetallic [PtAg28(BDT)12(PPh3)4]4– NC (1) was successfully synthesized from [PtAg28(S-Adm)18(PPh3)4]2+ NC (2) (S-Adm, 1-adamantanethiolate; BDT, 1,3-benzenedithiolate; PPh3, triphenylphosphine). The determined X-ray crystal structure of 1 showed a PtAg12 icosahedron core and a partially exposed surface, which are distinct from a face-centered cubic PtAg12 core and a fully covered surface of 2. We reveal through mass spectrometry (MS) that 1 forms ion pairs with counterions attracted by the core charge of the cluster, which is in line with density functional simulations. The MS data for 1, 2, and other NCs suggested that such attraction is facilitated by the exposed surface of 1. The formation of ion pairs increases the photoluminescence (PL) quantum yield of 1 up to 17.6% depending on the bulkiness of the counterion. Unlike small counterions, larger ones are calculated to occupy ≤90% of the volume near the exposed cluster surface and to make the ligand shell of 1 more rigid, which is observed to increase the PL. Thus, the developed synthesis strategy for structurally different NCs of the same size and composition allows us to probe the structure–property relationship for ion pairing and concomitant PL enhancement.
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