P. Raghunath

Computational study on the reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces

This study investigates the adsorption and reactions of H2O2 on TiO2 anatase (101) and rutile (110) surfaces by first‐principles calculations based on the density functional theory in conjunction with the projected augmented wave approach, using PW91, PBE, and revPBE functionals. Adsorption mechanisms of H2O2 and its fragments on both surfaces are analyzed. It is found that H2O2, H2O, and HO preferentially adsorb at the Ti5c site, meanwhile HOO, O, and H preferentially adsorb at the (O2c)(Ti5c), (Ti5c)2, and O2c sites, respectively. Potential energy profiles of the adsorption processes on both surfaces have been constructed using the nudged elastic band method. The two restructured surfaces, the 1/3 ML oxygen covered TiO2 and the hydroxylated TiO2, are produced with the H2O2 dehydration and deoxidation, respectively. The formation of main products, H2O(g) and the 1/3 ML oxygen covered TiO2 surface, is exothermic by 2.8 and 5.0 kcal/mol, requiring energy barriers of 0.8 and 1.1 kcal/mol on the rutile (110) and anatase (101) surface, respectively. The rate constants for the H2O2 dehydration processes have been predicted to be 6.65 × 10−27 T4.38 exp(−0.14 kcal mol−1/RT) and 3.18 × 10−23 T5.60 exp(−2.92 kcal mol−1/RT) respectively, in units of cm3 molecule−1 s−1.

Effect of Roaming Transition States upon Product Branching in the Thermal Decomposition of CH3NO2

The kinetics for the thermal unimolecular decomposition of CH3NO2 and its structural isomer CH3ONO have been investigated by statistical theory calculations based on the potential energy surface calculated at the UCCSD(T)/CBS and CASPT3(8, 8)/6-311+G(3df,2p) levels. Our results show that for the decomposition of CH3NO2 at pressures less than 2 Torr, isomerization to CH3ONO via the recently located roaming transition state is dominant in the entire temperature range studied, 400-3000 K. However, at higher pressures, the formation of the commonly assumed products, CH3 + NO2, becomes competitive and at pressures higher than 200 Torr the production of CH3 + NO2 is exclusive. The predicted rate constants for 760 Torr and the high-pressure limit with Ar as diluent in the temperature range 500-3000 K, producing solely CH3 + NO2, can be expressed respectively by kd(760)(CH3NO2) = 2.94 × 10(55)T(-12.6) exp(-35500/T) s(-1) and kd(∞)(CH3NO2) = 5.88 × 10(24)T(-2.35) exp(-31400/T) s(-1). In the low pressure limit, the decomposition reaction takes place exclusively via the roaming TS producing internally excited CH3ONO, giving rise to both CH3O + NO and CH2O + HNO with the second-order rate constant kd(0)(CH3NO2) = 1.17 × 10(31)T(-10.94)  exp(-32400/T) cm(3) molecule(-1) s(-1). For CH3ONO decomposition, a new roaming transition state connecting to the CH2O + HNO products has been located, lying 6.8 kcal/mol below the well-known four-member ring tight transition state and 0.7 kcal/mol below CH3O + NO. The rate constants predicted by similar calculations give rise to the following expressions for the thermal decomposition of CH3ONO in He: kd(760)(CH3ONO) = 8.75 × 10(41)T(-8.97) exp(-22600/T) s(-1) and kd(∞)(CH3ONO) = 1.58 × 10(23)T(-2.18) exp(-21100/T) s(-1) in the temperature range 300-3000 K. These results are in very good agreement with available experimental data obtained under practical pressure conditions. The much different branching ratios for the formation of CH3O + NO and CH2O + HNO in the decomposition of both CH3NO2 and CH3ONO are also given in this work.

Computational investigation of O2 reduction and diffusion on 25% Sr-doped LaMnO3 cathodes in solid oxide fuel cells.

The oxygen reduction reaction (ORR) and diffusion mechanisms on 25% Sr-doped LaMnO(3) (LSM) cathode materials as well as their kinetic behavior have been studied by using spin-polarized density functional theory (DFT) calculations. Bader charge and frequency analyses were carried out to identify the oxidation state of adsorbed oxygen species. DFT and molecular dynamics (MD) results show that the fast O(2) adsorption/reduction process via superoxide and peroxide intermediates is energetically favorable on the Mn site rather than on the Sr site. Furthermore, the higher adsorption energies on the Mn site of the (110) surface compared to those on the (100) surface imply that the former is more efficient for O(2) reduction. Significantly, we predict that oxygen vacancies enhance O(2) reduction kinetics and that the O-ion migration through the bulk is dominant over that on the surface of the LSM cathode.

Quantum chemical elucidation of the mechanism for hydrogenation of TiO2 anatase crystals

Hydrogenation of TiO2 is relevant to hydrogen storage and water splitting. We have carried out a detailed mechanistic study on TiO2 hydrogenation through H and∕or H2 diffusion from the surface into subsurface layers of anatase TiO2 (101) by periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U). Both H atoms and H2 molecules can migrate from the crystal surface into TiO2 near subsurface layer with 27.8 and 46.2 kcal∕mol energy barriers, respectively. The controlling step for the former process is the dissociative adsorption of H2 on the surface which requires 47.8 kcal∕mol of energy barrier. Both hydrogen incorporation processes are expected to be equally favorable. The barrier energy for H2 migration from the first layer of the subsurface Osub1 to the 2nd layer of the subsurface oxygen Osub2 requires only 6.6 kcal. The presence of H atoms on the surface and inside the subsurface layer tends to promote both H and H2 penetration into the subsurface layer by reducing their energy barriers, as well as to prevent the escape of the H2 from the cage by increasing its escaping barrier energy. The H2 molecule inside a cage can readily dissociate and form 2HO-species exothermically (ΔH = -31.0 kcal∕mol) with only 26.2 kcal∕mol barrier. The 2HO-species within the cage may further transform into H2O with a 22.0 kcal∕mol barrier and 19.3 kcal∕mol exothermicity relative to the caged H2 molecule. H2O formation following the breaking of Ti-O bonds within the cage may result in the formation of O-vacancies and surface disordering as observed experimentally under a high pressure and moderately high temperature condition. According to density of states analysis, the projected density of states of the interstitial H, H2, and H2O appear prominently within the TiO2 band gap; in addition, the former induces a shift of the band gap position notably towards the conduction band. The thermochemistry for formation of the most stable sub-surface species (2HO and H2O) has been predicted. These results satisfactorily account for the photo-catalytic activity enhancement observed experimentally by hydrogenation at high temperatures and high pressures.

Ab Initio Chemical Kinetic Study for Reactions of H Atoms with SiH4 and Si2H6: Comparison of Theory and Experiment

The reactions of hydrogen atom with silane and disilane are relevant to the understanding of catalytic chemical vapor deposition (Cat-CVD) and plasma enhanced chemical vapor deposition (PECVD) processes. In the present study, these reactions have been investigated by means of ab initio molecular-orbital and transition-state theory calculations. In both reactions, the most favorable pathway was found to be the H abstraction leading to the formation of SiH(3) and Si(2)H(5) products, with 5.1 and 4.0 kca/mol barriers, respectively. For H + Si(2)H(6), another possible reaction pathway giving SiH(3) + SiH(4) may take place with two different mechanisms with 4.3 and 6.7 kcal/mol barriers for H-atom attacking side-way and end-on, respectively. To validate the calculated energies of the reactions, two isodesmic reactions, SiH(3)+CH(4)-->SiH(4)+CH(3) and Si(2)H(5)+C(2)H(6)-->Si(2)H(6)+C(2)H(5) were employed; the predicted heats of the formation for SiH(3) (49.0 kcal/mol) and Si(2)H(5) (58.6 kcal/mol) were found to agree well with the experimental data. Finally, rate constants for both H-abstraction reactions predicted in the range of 290-2500 K agree well with experimental data. The result also shows that H+Si(2)H(6) producing H(2)+Si(2)H(5) is more favorable than SiH(3)+SiH(4.).

Novel asymmetrical single- and double-chiral liquid crystal diads with wide blue phase ranges

In this study, two series of novel asymmetrical single- and double-chiral liquid crystal diads using a central linker to link two different mesogenic cores were successfully synthesized. The effects of the position of chiral centers and the number of aromatic rings on the mesomorphic and electro-optical properties were investigated. We found that diads III-N (where N = A, B, C and D) have better mesophasic properties than II-N (where N = A, B, C, D and E). Compared with III-C with a chiral center at the terminal alkoxyl chain, diad III-D exhibited the widest temperature range of BPI (ca. 31 °C) as the chiral center was introduced to the central linker. Besides III-D, BPs were also observed in diads II-B and III-B with a chiral center at the central spacer. According to our experimental results and molecular modeling, the mesomorphic properties and temperature ranges of BPs will be affected by the values of biaxiality and dipole moment, along with the bent shape of molecular geometry. Therefore, we demonstrated the first example of asymmetrical single- and double-chiral liquid crystal diads involved chiral centers located at the central linker to exhibit BPs, including the mesophases of BPI and BPIII, which might offer new BP single-components to the future applications of eutectic liquid crystal mixtures with wide BP ranges.

Photodissociation Dynamics of Benzaldehyde (C6H5CHO) at 266, 248, and 193 nm

The photodissociation of gaseous benzaldehyde (C(6)H(5)CHO) at 193, 248, and 266 nm using multimass ion imaging and step-scan time-resolved Fourier-transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6-311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C(6)H(5)+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol(-1) dominates. The C(6)H(6)+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C(6)H(5)CHO, ν(3) (v=1) of HCO, and levels v≤2, J≤43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol(-1) and vibrational energy of approximately 6 kJ mol(-1). Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C(6)H(5)+HCO channel becomes the only major channel with a branching ratio of 0.82±0.10 and an increased proportion of the slow component; IR emission from levels ν(1) (v=1) and ν(3) (v=1 and 2) of HCO and v≤2, J≤43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical-theory predicted results based on the computed singlet and triplet potential-energy surfaces.

Computational and experimental studies on the effect of hydrogenation of Ni-doped TiO2 anatase nanoparticles for the application of water splitting

We have studied theoretically and experimentally the effect of Ni-doping in TiO2 nanoparticles (NPs) on hydrogenation. The doped NPs can be hydrogenated readily in a much shorter time at T 80 kcal mol−1 increase in exothermicities.

Hydrogen-bonded effects on supramolecular blue phase liquid crystal dimeric complexes

In this study, a series of novel asymmetric hydrogen-bonded (H-bonded) dimeric complexes D/P and D/P* (proton donors D = A, A*, AF and AF*) were synthesized and self-assembled by appropriate molar ratios of H-donors (A, A*, AF and AF*) and H-acceptors (P and P*). In addition, the influences of the lateral fluoro-substituent of H-donors, the number (along with the position) of chiral centers and the molar ratio of H-donors and H-acceptors on the mesophasic behaviours (e.g., BPs) of asymmetric H-bonded dimeric complexes are investigated. Interestingly, the blue phase (i.e., BPI) was observed in complexes A*/P, A*/P*, AF*/P and AF*/P* containing at least a chiral center in H-donors (A* and AF*), including the widest BPI ranges of complexes AF*/P* (ca. 6 °C and 13 °C for 1 : 1 and 3 : 1 mol, respectively). For the first time, the hydrogen-bonded effects on supramolecular blue phase LCs are compared with their analogous covalent diads. Based on our theoretical calculation, we discovered that the bend angle plays an important role in manipulating the existence of the blue phase, which is preferred to appear at the bend angle within 132.1–152.9°. Hence, owing to inappropriate bend angles, both H-bonded dimeric complex A/P* and covalent diad A*-P (with bend angles of 162.0° and 126.5°, respectively) did not possess any blue phase.

Ab initio chemical kinetics for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the related unimolecular decomposition of Si3H8 under a-Si/H CVD conditions.

The kinetics and mechanisms for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the related unimolecular decomposition of Si3H8 have been investigated by ab initio molecular orbital theory based on the QCISD(T)/CBS//QCISD/6-311++G(d,p) method in conjunction with quantum statistical variational Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. For the barrierless radical association processes, their variational transition states have been characterized by the CASPT2//CASSCF method. The species involved in the study are known to coexist under CVD conditions. The results show that the association reaction of SiH2 and Si2H6 producing Si3H8 occurs by insertion via its lowest-energy path forming a loose hydrogen-bonding molecular complex with 8.3 kcal/mol binding energy; the reaction is exothermic by 55.0 kcal/mol. The chemically activated Si3H8 adduct can fragment by several paths, producing SiH4 + SiH3SiH (-0.7 kcal/mol), Si(SiH3)2 + H2 (-1.4 kcal/mol), and SiH3SiH2SiH + H2 (-1.4 kcal/mol). The predicted enthalpy changes as given agree well with available thermochemical data. Three other decomposition channels of Si3H8 occurring by Si-H or Si-Si breaking were found to be highly endothermic, and the reactions take place without a well-defined barrier. The heats of formation of Si3H8, SiH2SiH, Si2H4, i-Si3H7, n-Si3H7, Si(SiH3)2, and SiH3SiH2SiH have been predicted and found to be in close agreement with those available data in the literature. The product branching rate constants for SiH2 + Si2H6 and SiH3 + Si2H5 reactions and the thermal unimolecular decomposition of Si3H8 for all low-energy paths have been calculated with multichannel variational RRKM theory covering varying P,T conditions typically employed in PECVD and Cat-CVD processes for hydrogenated amorphous silicon (a-Si/H) film growth. The results were also found to be in good agreement with available kinetic data. Our kinetic results may be employed to model and control very large-area a-Si/H film growth for a new generation of solar cell applications.