Reaction mechanism between small-sized Ce clusters and water molecules: An ab initio investigation on Ce\u3csub\u3e\u3ci\u3en\u3c/i\u3e\u3c/sub\u3e+H\u3csub\u3e2\u3c/sub\u3eO

Reactions of small-sized cerium clusters Cen (n = 1–3) with a single water molecule are systematically investigated theoretically. The ground state structures of the Cen/H2O complex and the reaction pathways between Cen + H2O are predicted. Our results show the size-dependent reactivity of small-sized Ce clusters. The calculated reaction energies and reaction barriers indicate that the reactivity between Cen and water becomes higher with increasing cluster size. The predicted reaction pathways show that the single Ce atom and the Ce2 and Ce3 clusters can all easily react with H2O and dissociate the water molecule. Under UV-irradiation, the reaction of a Ce atom with a single H2O molecule may even release an H2 molecule. The reaction of either Ce2 or Ce3 with a single H2O molecule can fully dissociate the H2O into H and O atoms while it is bonded with the Ce cluster. The electronic configuration and oxidation states of the Ce atoms in the products and the higher occupied molecular orbitals are analyzed by using the natural bond orbital (NBO) analysis method, from which the high reactivity between the reaction products of Cen + H2O and an additional H2O molecule is predicted. Our results offer deeper molecular insights into the chemical reactivity of Ce, which could be helpful for developing more efficient Ce-doped or Ce-based catalysts. Includes supplementary materials

Mechanistic study of pressure and temperature dependent structural changes in reactive formation of silicon carbonate

The discovery of the silicon carbonate through chemical reaction between porous SiO2 and gaseous CO2 addressed a long-standing question regarding whether the reaction between CO2 and SiO2 is possible. However, the detailed atomic structure of silicon carbonate and associated reaction mechanism are still largely unknown. We explore structure changes of silicon carbonate with pressure and temperature based on systematic ab initio molecular dynamics simulations. Our simulations suggest that the reaction proceeds at the surface of the porous SiO2. Increasing number of CO2 molecules can take part in the reaction by increasing either the pressure or temperature. The final product of the reaction exhibits amorphous structures, where most C atoms and Si atoms are 3-fold and 6-fold coordinated, respectively. The fraction of differently coordinated C (Si) atoms is pressure dependent, and as a result, the structure of the final product is pressure dependent as well. When releasing the pressure, part of the reaction product decomposes into CO2 molecules and SiO2 tetrahedrons. However more than 50% of C atoms are still in 3-fold coordination, implying that stable silicon carbonate may be obtained via repeated annealing under high pressure. The mechanism underlying this chemical reaction is predicted with two possible reaction pathways identified. Moreover, the reaction transition curve is obtained from the extensive simulation, which can be useful to guide the synthesis of silicon carbonate from the reaction between SiO2 and CO2

The reaction pathways of 5-hydroxymethylfurfural conversion in a continuous flow reactor using copper catalysts

The transformation of 5-hydroxymethylfurfural is investigated using supported and bulk copper oxide catalysts. We show that the selectivity to 5-methylfuraldehyde or 2,5-diformylfuran can be controlled by the solvent and the carrier gas. The use of water as the solvent and N2 as the carrier gas led to the highest conversion and most selective pathway to 2,5-diformylfuran. Quasi in situ X-ray photoelectron spectroscopy and H2-TPR measurements revealed that H2O can re-oxidise Cu, significantly enhancing the selectivity to 5-methylfuraldehyde. Subsequent density functional theory calculations revealed more precisely the role of water in the reaction mechanism

Identification of Piwil2-Like (PL2L) Proteins that Promote Tumorigenesis

PIWIL2, a member of PIWI/AGO gene family, is expressed in the germline stem cells (GSCs) of testis for gametogenesis but not in adult somatic and stem cells. It has been implicated to play an important role in tumor development. We have previously reported that precancerous stem cells (pCSCs) constitutively express Piwil2 transcripts to promote their proliferation. Here we show that these transcripts de facto represent Piwil2-like (PL2L) proteins. We have identified several PL2L proteins including PL2L80, PL2L60, PL2L50 and PL2L40, using combined methods of Gene-Exon-Mapping Reverse Transcription Polymerase Chain Reaction (GEM RT-PCR), bioinformatics and a group of novel monoclonal antibodies. Among them, PL2L60 rather than Piwil2 and other PL2L proteins is predominantly expressed in various types of human and mouse tumor cells. It promotes tumor cell survival and proliferation in vitro through up-regulation of Stat3 and Bcl2 gene expressions, the cell cycle entry from G0/1 into S-phase, and the nuclear expression of NF-κB, which contribute to the tumorigenicity of tumor cells in vivo. Consistently, PL2L proteins rather than Piwil2 are predominantly expressed in the cytoplasm or cytoplasm and nucleus of euchromatin-enriched tumor cells in human primary and metastatic cancers, such as breast and cervical cancers. Moreover, nuclear PL2L proteins are always co-expressed with nuclear NF-κB. These results reveal that PL2L60 can coordinate with NF-κB to promote tumorigenesis and might mediate a common pathway for tumor development without tissue restriction. The identification of PL2L proteins provides a novel insight into the mechanisms of cancer development as well as a novel bridge linking cancer diagnostics and anticancer drug development

Unraveling Crystalline Structure of High-Pressure Phase of Silicon Carbonate

Although CO2 and SiO2 both belong to group-IV oxides, they exhibit remarkably different bonding characteristics and phase behavior at ambient conditions. At room temperature, CO2 is a gas, whereas SiO2 is a covalent solid with rich polymorphs. A recent successful synthesis of the silicon-carbonate solid from the reaction between CO2 and SiO2 under high pressure [M. Santoro et al., Proc. Natl. Acad. Sci. U.S.A. 108, 7689 (2011)] has resolved a long-standing puzzle regarding whether a SixC1−xO2 compound between CO2 and SiO2 exists in nature. Nevertheless, the detailed atomic structure of the SixC1−xO2 crystal is still unknown. Here, we report an extensive search for the high-pressure crystalline structures of the SixC1−xO2 compound with various stoichiometric ratios (SiO2∶CO2) using an evolutionary algorithm. Based on the low-enthalpy structures obtained for each given stoichiometric ratio, several generic structural features and bonding characteristics of Si and C in the high-pressure phases are identified. The computed formation enthalpies show that the SiC2O6 compound with a multislab three-dimensional (3D) structure is energetically the most favorable at 20 GPa. Hence, a stable crystalline structure of the elusive SixC1−xO2 compound under high pressure is predicted and awaiting future experimental confirmation. The SiC2O6 crystal is an insulator with elastic constants comparable to typical hard solids, and it possesses nearly isotropic tensile strength as well as extremely low shear strength in the 2D plane, suggesting that the multislab 3D crystal is a promising solid lubricant. These valuable mechanical and electronic properties endow the SiC2O6 crystal for potential applications in tribology and nanoelectronic devices, or as a stable solid-state form for CO2 sequestration

Reaction mechanism between small-sized Ce clusters and water molecules: An ab initio investigation on Ce\u3csub\u3e\u3ci\u3en\u3c/i\u3e\u3c/sub\u3e+H\u3csub\u3e2\u3c/sub\u3eO

Reactions of small-sized cerium clusters Cen (n = 1–3) with a single water molecule are systematically investigated theoretically. The ground state structures of the Cen/H2O complex and the reaction pathways between Cen + H2O are predicted. Our results show the size-dependent reactivity of small-sized Ce clusters. The calculated reaction energies and reaction barriers indicate that the reactivity between Cen and water becomes higher with increasing cluster size. The predicted reaction pathways show that the single Ce atom and the Ce2 and Ce3 clusters can all easily react with H2O and dissociate the water molecule. Under UV-irradiation, the reaction of a Ce atom with a single H2O molecule may even release an H2 molecule. The reaction of either Ce2 or Ce3 with a single H2O molecule can fully dissociate the H2O into H and O atoms while it is bonded with the Ce cluster. The electronic configuration and oxidation states of the Ce atoms in the products and the higher occupied molecular orbitals are analyzed by using the natural bond orbital (NBO) analysis method, from which the high reactivity between the reaction products of Cen + H2O and an additional H2O molecule is predicted. Our results offer deeper molecular insights into the chemical reactivity of Ce, which could be helpful for developing more efficient Ce-doped or Ce-based catalysts. Includes supplementary materials

Carbon nanotube and boron nitride nanotube hosted C\u3csub\u3e60\u3c/sub\u3e–V nanopeapods

We investigate electronic and transport properties of a novel form of nanopeapod structure, where the “pod” component is either a carbon nanotube (CNT) or a boron-nitride nanotube (BNNT) while the “pea” component is a chain of C60–V dimers. Compared to the conventional carbon peapod where the “pea” is a chain of C60 fullerenes, marked changes in the electronic structures are found due to the formation of coordination bonds between V and two neighboring C60 molecules. The local spins in the (η6-C60–V)@CNT or (η6-C60–V)@BNNT peapod are coupled via antiferromagnetic (AFM) exchange interaction. In particular, the C60–V chain in BNNT yields a well-defined spin qubit. Density-functional theory calculation suggests that the (η6-C60–V)@CNT peapod is metallic with characteristics of multiple carriers contributed from CNTs, C60, and V. The (η6-C60–V)@BNNT peapod is predicted to be semiconducting with a narrow band gap, and its charge carriers are contributed by the C60–V chain. Evidently, the insertion of a V atom between every two C60 fullerenes can enhance the conductivity of the peapod. Binding H atoms on all the α positions of the pentagons in C60 can further strengthen the V–C60 interaction. Both AFM and FM states of the H-containing peapod are nearly degenerate in energy. The FM state gives rise to a magnetic moment of 3.0 μB per unit cell, three times greater than that of the V–benzene or V–cyclopentadiene multidecker complexes. The binding of H atoms to the C60 however cannot enhance electron transport due to the removal of the π channel of C60. Previous experiments have demonstrated that C60 molecules can enter BNNTs through the open tips of the BNNTs, offering a strategy that the V–C60 dimers may be encapsulated into nanotubes through the open tips of the nanotubes to form M–C60 peapods

Polymorphic Phases of sp³-Hybridized Carbon under Cold Compression

It is well established that graphite can be transformed into superhard carbons under cold compression (Mao et al.<i> Science</i> <b>2003</b>, <i>302</i>, 425). However, structure of the superhard carbon is yet to be determined experimentally. We have performed an extensive structural search for the high-pressure crystalline phases of carbon using the evolutionary algorithm. Nine low-energy polymorphic structures of sp³-hybridized carbon result from the unbiased search. These new polymorphic carbon structures together with previously reported low-energy sp³-hybridized carbon structures (e.g., M-carbon, W-carbon, and Cco-C₈ or Z-carbon) can be classified into three groups on the basis of different ways of stacking two (or more) out of five (A–E) types of buckled graphene layers. Such a classification scheme points out a simple way to construct a variety of sp³-hybridized carbon allotropes via stacking buckled graphene layers in different combinations of the A–E types by design. Density-functional theory calculations indicate that, among the nine low-energy crystalline structures, seven are energetically more favorable than the previously reported most stable crystalline structure (i.e., Cco-C₈ or Z-carbon) in the pressure range 0–25 GPa. Moreover, several newly predicted polymorphic sp³-hybridized carbon structures possess elastic moduli and hardness close to those of the cubic diamond. In particular, Z-carbon-4 possesses the highest hardness (93.4) among all the low-energy sp³-hybridized carbon structures predicted today. The calculated electronic structures suggest that most polymorphic carbon structures are optically transparent. The simulated X-ray diffraction (XRD) spectra of a few polymorphic structures are in good agreement with the experimental spectrum, suggesting that samples from the cold-compressed graphite experiments may consist of multiple polymorphic phases of sp³-hybridized carbon