Publications

We report the energetics and properties of the β, α, R, γ, λ, and δ polymorphs of MnO<sub>2</sub> within density functional theory, comparing the performance of the recently introduced SCAN functional with that of conventional exchange-correlation functionals and experiment. We find that SCAN uniquely yields accurate formation energies and properties across all MnO<sub>2</sub> polymorphs. We explain the superior performance of SCAN based on its satisfaction of all known constraints appropriate to a semilocal exchange-correlation functional and its accurate representation of all types of orbital overlap.

We present observations of SN 2021csp, a unique supernova (SN) which displays evidence for interaction with H- and He- poor circumstellar material (CSM) at early times. Using high-cadence spectroscopy taken over the first week after explosion, we show that the spectra of SN 2021csp are dominated by C III lines with a velocity of 1800 km s$^{-1}$. We associate this emission with CSM lost by the progenitor prior to explosion. Subsequently, the SN displays narrow He lines before metamorphosing into a broad-lined Type Ic SN. We model the bolometric light curve of SN 2021csp, and show that it is consistent with the energetic ($4\times10^{51}$ erg) explosion of a stripped star, producing 0.4 M$_\odot$ of 56Ni within a $\sim$1 M$_\odot$ shell of CSM extending out to 400 R$_\odot$.

The metallostannylene Cp*(^<i>i</i>Pr₂MeP)(H)₂Fe-SnDMP (<b>1</b>; Cp* = η⁵-C₅Me₅; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*(^<i>i</i>Pr₂MeP)HFe[Sn(H)DMP] intermediate, serves as a robust platform for exploration of transition-metal main-group element bonding and reactivity. Upon one-electron oxidation, <b>1</b> expels H₂ to generate the coordinatively unsaturated [Cp*(^<i>i</i>Pr₂MeP)Fe═SnDMP][B(C₆F₅)₄] (<b>3</b>), which possesses a highly polarized Fe-Sn multiple bond that involves interaction of the tin lone pair with iron. Evidence from EPR and ⁵⁷Fe Mössbauer spectroscopy, along with DFT studies, shows that <b>3</b> is primarily an iron-based radical with charge localization at tin. Upon reduction of <b>3</b>, C-H bond activation of the phosphine ligand was observed to produce Cp*HFe(κ²-(<i>P,Sn</i>)═Sn(DMP)CH₂CHMePMe^<i>i</i>Pr) (<b>5</b>). Complex <b>5</b> was also accessed via thermolysis of <b>1</b>, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H₂ from <b>1</b> to produce a stannylyne intermediate, Cp*(^<i>i</i>Pr₂MeP)Fe[SnDMP] (<b>A</b>), is likely rate-determining. Evidence indicates that the production of <b>5</b> proceeds through a concerted C-H bond activation. DFT investigations suggest that the transition state for this transformation involves C-H cleavage across the Fe-Sn bond and that a related transition state where C-H bond activation occurs exclusively at the tin center is disfavored, illustrating an effect of iron-tin cooperativity in this system.

Sum frequency generation (SFG) vibrational spectroscopy in combination with cyclic voltammetry (CV) was used to investigate the oxidation of carbon monoxide (CO) in acetonitrile (CH3CN)/water (H2O) solutions at the platinum (Pt) electrode surface. CO oxidation on Pt was studied as a function of water concentration and electrode potential. In electrolyte solutions containing small concentrations of water, a preoxidation wave was observed at +900 mV vs SCE whereby a small amount of the adsorbed CO was oxidized. This preoxidation wave was followed by the main oxidation of CO on Pt at approximately +1700 mV vs SCE. Spectroscopy and voltammetry data in these essentially nonaqueous electrolytes indicate an oxidation intermediate of CO to CO2 that was nearly parallel to the Pt electrode surface.

The effect of acidic properties of mesoporous zeolites on the control of product selectivity during the hydrogenative isomerization of methylcyclopentane has been investigated. A series of mesoporous zeolites with controlled acidic properties were prepared by postdealumination process with hydrochloric acid under hydrothermal conditions, and the resultant zeolites used for supporting colloidal Pt nanoparticles (NPs) with a mean size of 2.5 nm (± 0.6 nm). As compared to the pure Pt NPs supported on catalytically inert mesoporous silica (MCF-17) as the reference catalyst that can produce isomers most selectively (∼80%), the Pt NPs supported on mesoporous zeolites produced C6-cyclic hydrocarbons (i.e., cyclohexane and benzene) most dominantly. The type and strength of the Brönsted (B) and Lewis (L) acid sites of those zeolites with a controlled Al amount are analyzed by using FT-IR after the adsorption of pyridine and NH3 temperature-programmed desorption measurements, and they are correlated with the selectivity change between cyclohexane and benzene. From this investigation, we found a linear relationship between the number of Brönsted acid sites and the formation rate for cyclohexane. In addition, we revealed that more Lewis acidic zeolite having relatively smaller B/L ratio is effective for the cyclohexane formation, whereas more Brönsted acidic zeolite having relatively larger B/L ratio is effective for the benzene formation.

In the last 30 years the world of portable electronics has been powered by Ni-Cd, Ni-MH and then Li-ion batteries, which rapidly became the technology of choice for such applications. More recently, great interest has been given to transport and storage applications. Since concerns have been raised about the future availability of lithium resources, much research has been addressed to alternatives such as Na-ion batteries. Materials for such technology could develop rapidly because of the essential similarity between Li + and Na + intercalation chemistry. A significant fraction of recently proposed electrodes are phosphate-based polyanionic materials which possess stable structural frameworks and whose potential can be tailored thanks to the inductive effect. Among them, Na 3 V 2 (PO 4 ) 2 F 3 is a material attracting great interest. Its crystal structure had been determined from single crystal X-Ray diffraction in 1999 by Le Meins et al. in the tetragonal space group P4 2 /mnm at 298K [1]. Very recently high resolution diffraction techniques led to new insight on Na 3 V 2 (PO 4 ) 2 F 3 , both from the crystal structure and electrochemical points of view, and explaining the significant amount of discrepancies found throughout the literature [2-6], which we believe to be the result of a partial substitution of oxygen for fluorine. Synchrotron radiation powder diffraction data revealed a small but significant orthorhombic distortion in Na 3 V 2 (PO 4 ) 2 F 3 (b/a=1.002). This led to an indexation in the Amam space group and to a new structure preserving the framework, but strongly impacting the sodium distribution [7]. Moreover, a subsequent operando synchrotron radiation XRD allowed to determine the complicated phase diagram observed upon cycling of Na 3 V 2 (PO 4 ) 2 F 3 , consisting of several phase transition between intermediate phases, leading to the fully charged state NaV 2 (PO 4 ) 2 F 3 [8]. Na 3 V 2 (PO 4 ) 2 F 3 has the potential to deintercalate 3 sodium ions, but only two have been experimentally observed to be electrochemically active. The extraction of the third one would require a potential too high, where common electrolytes are not stable. Moreover, Na 3 V 2 (PO 4 ) 2 F 3 actually belongs to a full solid solution Na 3 V 2 (PO 4 ) 2 F 3-2x O 2x [3], where the whole family delivers good performances as positive electrodes for Na-ion batteries, thanks to the active redox couple shifting from V3+/V4 to V4+/V5+. This means a very high degree of flexibility is present in this class of materials. To improve their performances even further, we considered a doping strategy with different transition metals to replace part of the vanadium into the structure, in the attempt at lowering such potential; in this presentation we will show the results of our doping from the predictions of ab initio calculations to the actual synthesis. The best strategies will be presented and the improvement they bring in terms of energy density and/or Na ion kinetics highlighted. References [1] Le Meins, J. M.; Crosnier-Lopez, M. P.; Hemon-Ribaud, A.; Courbion, G. J. Solid State Chem. 1999 , 148 , 260. [2] Chihara, K.; Kitajou, A.; Gocheva, I. D.; Okada, S.; Yamaki, J.-i. J. Power Sources 2013 , 227 , 80. [3] Park, Y.-U.; Seo, D.-H.; Kim, Y.; Kim, J.; Lee, S.; Kim, B.; Kang, K. Adv. Funct. Mater. 2014 . [4] Liu, Z.; Hu, Y.-Y.; Dunstan, M. T.; Huo, H.; Hao, X.; Zou, H.; Zhong, G.; Yang, Y.; Grey, C. P. Chem. Mater. 2014 , 26, 2513. [5] Song, W.; Ji, X.; Wu, Z.; Zhu, Y.; Li, F.; Yao, Y.; Banks, C. E. RSC Advances 2014 , 4 , 11375. [6] Ponrouch, A.; Dedryvere, R.; Monti, D.; Demet, A. E.; MBA, J.-M. A.; Croguennec, L.; Masquelier, C.; Johansson, P.; Palacín, M. R. Energy Environ. Sci. 2013 , 6, 2361. [7] Bianchini, M. ; Brisset, N. ; Fauth, F. ; Weill, F. ; Elkaim, E. ; Suard, E. ; Masquelier, C.; Croguennec, L. Chemistry of Materials ( 2014 ), 26 (14), 4238-4247. [8] M. Bianchini, N. Brisset, F. Fauth, F. Weill, E. Suard, C. Masquelier and L. Croguennec, Chemistry of Materials ( 2015 ), 27(8), 3009-3020.

Abstract For Abstract see ChemInform Abstract in Full Text.

The role of metal layer thickness and resultant plastic constraint in the metal layer during the failure of metal/ceramic layered structures is examined under cyclic and static loading conditions. Crack-growth experiments were conducted on sandwich specimens consisting of 99.999% pure aluminum layers bonded between 99.5% pure polycrystalline alumina with the metal layer thickness varying from 5 to 100 μm. Under cyclic loading, crack growth occurred primarily at the interface separating the two materials; additionally, stable fatigue cracks deviated into the alumina for thin-layered samples at high driving forces. Under monotonically increasing loads, the fracture toughness increased with Al layer thickness, whereas under cyclic loads the threshold driving force for crack growth conversely decreased with increasing layer thickness. Under static loading in a moist environment, interfacial crack growth was never observed at measurable rates (⩾10−9 m/s) for driving forces up to 200 J/m2; however, for thin-layered samples, subcritical cracks did deviate off the interface and grow, sometimes stably, into the alumina. Trends in crack-growth rates and crack trajectories are examined in terms of the level of constraint, loading conditions and environmental influences.