Highly selective oxidation of benzene to phenol with air at room temperature promoted by water

Phenol is one of the most important fine chemical intermediates in the synthesis of plastics and drugs with a market size of ca. $30b1 and the commercial production is via a two-step selective oxidation of benzene, requiring high energy input (high temperature and high pressure) in the presence of a corrosive acidic medium, and causing serious environmental issues2-5. Here we present a four-phase interface strategy with well-designed Pd@Cu nanoarchitecture decorated TiO2 as a catalyst in a suspension system. The optimised catalyst leads to a turnover number of 16,000-100,000 for phenol generation with respect to the active sites and an excellent selectivity of ca. 93%. Such unprecedented results are attributed to the efficient activation of benzene by the atomically Cu coated Pd nanoarchitecture, enhanced charge separation, and an oxidant-lean environment. The rational design of catalyst and reaction system provides a green pathway for the selective conversion of symmetric organic molecules

Temperature control in molecular dynamic simulations of non-equilibrium processes

Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NV T ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NV T thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF2 molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO2 surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic force field. We discuss the differences in behaviour of these systems due to applied thermostatting, and show that in some cases a qualitatively different physical behaviour of the simulated system can be obtained if an equilibrium thermostat is used

Continuum and atomistic description of excess electrons in TiO2

The modelling of an excess electron in a semiconductor in a prototypical dye sensitised solar cell is carried out using two complementary approaches: atomistic simulation of the TiO2 nanoparticle surface is complemented by a dielectric continuum model of the solvent–semiconductor interface. The two methods are employed to characterise the bound (excitonic) states formed by the interaction of the electron in the semiconductor with a positive charge opposite the interface. Density-functional theory (DFT) calculations show that the excess electron in TiO2 in the presence of a counterion is not fully localised but extends laterally over a large region, larger than system sizes accessible to DFT calculations. The numerical description of the excess electron at the semiconductor–electrolyte interface based on the continuum model shows that the exciton is also delocalised over a large area: the exciton radius can have values from tens to hundreds of Ångströms, depending on the nature of the semiconductor (characterised by the dielectric constant and the electron effective mass in our model)

A Uniform Algorithm for All-Speed Shock-Capturing Schemes

There are many ideas for developing shock-capturing schemes and their extension for all-speed flow. The representatives of them are Roe, HLL and AUSM families. In this paper, a uniform algorithm is proposed, which expresses three families in the same framework. The algorithm has explicit physical meaning, provides a new angel of understanding and comparing the mechanism of schemes, and may play a great role in the further research. As an example of applying the uniform algorithm, the low-Mach number behaviour of the schemes is analyzed. Then, a very clear and simple explanation is given based on the wall boundary, and a concise rule is proposed to judge whether a scheme has satisfied low-Mach number behaviour

PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors

Methane activation by photocatalysis is one of the promising sustainable technologies for chemical synthesis. However, the current efficiency and stability of the process are moderate. Herein, a PdCu nanoalloy (~2.3 nm) was decorated on TiO2, which works for the efficient, stable, and selective photocatalytic oxidative coupling of methane at room temperature. A high methane conversion rate of 2480 μmol g-1 h-1 to C2 with an apparent quantum efficiency of ~8.4% has been achieved. More importantly, the photocatalyst exhibits the turnover frequency and turnover number of 116 h-1 and 12,642 with respect to PdCu, representing a record among all the photocatalytic processes (λ > 300 nm) operated at room temperature, together with a long stability of over 112 hours. The nanoalloy works as a hole acceptor, in which Pd softens and weakens C-H bond in methane and Cu decreases the adsorption energy of C2 products, leading to the high efficiency and long-time stability

Nanopatterning of a Covalent Organic Framework Host-Guest System

We have used a boroxine-based COF as a template for C60-fullerene self-assembly on graphite. Local removal of the COF by STM based nanomanipulation creates nanocorrals that may host other species

Theory of defects arising from hydrogen in silicon and diamond

EThOS - Electronic Theses Online ServiceGBUnited Kingdo

How TiO2 crystallographic surfaces influence charge injection rates from a chemisorbed dye sensitiser

High-energy metal oxide surfaces are considered to be promising for applications involving surface–adsorbate electron transfer, such as photocatalysis and dye-sensitised solar cells. Here, we compare the efficiency of electron injection into different TiO2 anatase surfaces. We model the adsorption of a carboxylic acid (formic acid) on anatase (101), (001), (100), (110) and (103) surfaces using density functional theory calculations, and calculate electron injection times from a model dye into these surfaces. We find that the different positions of the conduction band edge of these surfaces determine the rate of electron injection (which is faster for the surfaces with lower-lying conduction band, among them the most stable (101) surface). However, if the dye's injection energy is enforced to be at a fixed energy deep inside each surface's conduction band, then several anatase surfaces, such as the synthetically achievable (001) surface, show rates of injection comparable or faster than the (101) surface. Moreover, because of their higher-lying conduction bands, these minority surfaces are likely to offer higher open-circuit voltages in dye-sensitised solar cells. Therefore, synthetically accessible high-energy anatase surfaces, such as (001)-oriented nanostructures, may be promising candidates for use in dye-sensitised solar cells

Modeling the self-assembly of benzenedicarboxylic acids using Monte Carlo and molecular dynamics simulations

We present a theoretical modeling Study of self-assembly Of Molecules into two-dimensional (2D) hydrogen-bonded networks. We compare two computational techniques, molecular dynamics (MD) and Monte Carlo (MC) Calculations, in order to find out whether these computational techniques are efficient in modeling the process of self-assembly and in predicting the ordered supramolecular structures. Terephthalic acid, isophthalic acid, and phthalic acid, which have been widely studied experimentally, are used as test systems. According to both computational techniques, terephthalic acid molecules form ordered Structures made of parallel molecular chains (same as observed in experiment) at high Surface coverage, whereas less symmetric isophthalic and phthalic acid molecules mostly form disordered arrangements of zigzag chains or long isolated zigzag chains. Both computational techniques reproduce the supramolecular Structure of terephthalic acid, the most symmetric of these molecules. However, MD simulations are more robust, whereas MC simulation results are very dependent oil the choice of the starting Structure