Behavior of S, SO, and SO3 on Pt (001), (011), and (111) surfaces: A DFT study.

In the hybrid sulfur (HyS) cycle, the reaction between SO2 and H2O is manipulated to produce hydrogen with water and sulfuric acid as by-products. However, sulfur poisoning of the catalyst has been widely reported to occur in this cycle, which is due to strong chemisorption of sulfur on the metal surface. The catalysts may deactivate as a result of these impurities present in the reactants or incorporated in the catalyst during its preparation and operation of the HyS cycle. Here, we report a density functional theory investigation of the interaction between S, SO, and SO3 with the Pt (001), (011), and (111) surfaces. First, we have investigated the adsorption of single gas phase molecules on the three Pt surfaces. During adsorption, the 4F hollow sites on the (001) and (011) surfaces and the fcc hollow site on the (111) surface were preferred. S adsorption followed the trend of (001)4F > (011)4F > (111)fcc, while SO adsorption showed (001)4F > (011)bridge/4F > (111)fcc and SO3 adsorption was most stable in a S,O,O bound configuration on the (001)4F > (011)4F > (111)fcc sites. The surface coverage was increased on all the surfaces until a monolayer was obtained. The highest surface coverage for S shows the trend (001)S = (111)S > (011)S, and for SO it is (001)SO > (011)SO > (111)SO, similar to SO3 where we found (001)SO3 > (011)SO3 > (111)SO3. These trends indicate that the (001) surface is more susceptible to S species poisoning. It is also evident that both the (001) and (111) surfaces were reactive toward S, leading to the formation of S2. The high coverage of SO3 showed the formation of SO2 and SO4, especially on the (011) surface. The thermodynamics indicated that an increased temperature of up to 2000 K resulted in Pt surfaces fully covered with elemental S. The SO coverage showed θ ≥ 1.00 on both the (001) and (011) surfaces and θ = 0.78 for the (111) surface in the experimental region where the HyS cycle is operated. Lower coverages of SO3 were observed due to the size of the molecule.

Role of Intrinsic Factors of Polyimides in Glass Transition Temperature: An Atomistic Investigation.

Polyimides (PIs) are in high demand in the field of active matrix organic light-emitting diode displays because of their excellent heat resistance, chemical stability, and mechanical properties. However, the most critical key to their application is to further enhance their glass transition temperature (Tg), which directly affects the processing temperature of thin-film transistors on the PI films. Therefore, it is of great importance to study the factors that have an influence on the Tg of PIs. To accomplish this goal, PIs derived from pyromellitic acid dianhydride and three sets of isomeric imidazole-based diamines were investigated. The investigation, by computational methods, was to clarify the effect of intrinsic factors associated with the molecular structure of the PIs on their Tg and to construct a structure-Tg relationship for these PIs. For each model system, all-atom molecular dynamics simulations were used to identify and distinguish the effects of chain rigidity, fractional free volume (FFV), cohesive energy density, hydrogen-bonding interactions, and charge-transfer complex interactions on Tg. The results showed that the physical property, chain rigidity, has a direct impact on Tg regardless of the polymer backbone structure. A linear correlation between the increase of FFV and the decrease of Tg was not established due to the existence of hydrogen-bonding interactions, but the tendency was maintained. Furthermore, the formation of hydrogen bonds was found to have an indirect relationship with Tg. That is, the increase of intrachain hydrogen bonds would lead to a decrease in chain rigidity and consequently reduce the Tg value.

Mixing thermodynamics and electronic structure of the Pt1-x Ni x (0 ≤ x ≤ 1) bimetallic alloy.

The development of affordable bifunctional platinum alloys as electrode materials for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) remains one of the biggest challenges for the transition towards renewable energy sources. Yet, there is very little information on the optimal ratio between platinum and the transition metal used in the alloy and its impact on the electronic properties. Here, we have employed spin-polarised density functional simulations with long-range dispersion corrections [DFT-D3-(BJ)], to investigate the thermodynamics of mixing, as well as the electronic and magnetic properties of the Pt1-x Ni x solid solution. The Ni incorporation is an exothermic process and the alloy composition Pt0.5Ni0.5 is the most thermodynamically stable. The Pt0.5Ni0.5 solid solution is highly ordered as it is composed mainly of two symmetrically inequivalent configurations of homogeneously distributed atoms. We have obtained the atomic projections of the electronic density of states and band structure, showing that the Pt0.5Ni0.5 alloy has metallic character. The suitable electronic properties of the thermodynamically stable Pt0.5Ni0.5 solid solution shows promise as a sustainable catalyst for future regenerative fuel cells.

Interaction of H2O with the Platinum Pt (001), (011), and (111) Surfaces: A Density Functional Theory Study with Long-Range Dispersion Corrections.

Platinum is a noble metal that is widely used for the electrocatalytic production of hydrogen, but the surface reactivity of platinum toward water is not yet fully understood, even though the effect of water adsorption on the surface free energy of Pt is important in the interpretation of the morphology and catalytic properties of this metal. In this study, we have carried out density functional theory calculations with long-range dispersion corrections [DFT-D3-(BJ)] to investigate the interaction of H2O with the Pt (001), (011), and (111) surfaces. During the adsorption of a single H2O molecule on various Pt surfaces, it was found that the lowest adsorption energy (E ads) was obtained for the dissociative adsorption of H2O on the (001) surface, followed by the (011) and (111) surfaces. When the surface coverage was increased up to a monolayer, we noted an increase in E ads/H2O with increasing coverage for the (001) surface, while for the (011) and (111) surfaces, E ads/H2O decreased. Considering experimental conditions, we observed that the highest coverage was obtained on the (011) surface, followed by the (111) and (001) surfaces. However, with an increase in temperature, the surface coverage decreased on all the surfaces. Total desorption occurred at temperatures higher than 400 K for the (011) and (111) surfaces, but above 850 K for the (001) surface. From the morphology analysis of the Pt nanoparticle, we noted that, when the temperature increased, only the electrocatalytically active (111) surface remained.

Cyclometallated platinum(II) complexes of benzylidene-2,6-di-isopropylphenylamine containing bidentate phosphines: synthesis, structural properties and reactivity studies.

The reaction of the cyclometallated complex [PtCl(N^C)(dmso)], 1 (N^C represents the cyclometallated Schiff base, benzylidene-2,6-diisopropylphenylamine), with 1,1'-bis(diphenylphosphino)ferrocene, dppf, bis(diphenylphosphino)methane, dppm, or 1,2-bis(diphenylphosphino)ethane, dppe, in a 2 : 1 ratio or an equimolar ratio using acetone as the solvent produced the corresponding binuclear or mononuclear diphosphine platinum complexes. In the case of the mononuclear complexes, the diphosphines act as either a bidentate ligand or a monodentate ligand depending on the size of the bite angle of the diphosphines, while in the case of the binuclear complexes, the diphosphines act as a bridging ligand between the two metal centres. The solid state structures of some of the binuclear as well as mononuclear species are reported. The mononuclear derivatives were found to show different behaviour in solution and in the solid state when compared to the binuclear analogues. This behaviour is also influenced by the nature of the diphosphine ligands employed.

Improved metathesis lifetime: chelating pyridinyl-alcoholato ligands in the second generation Grubbs precatalyst.

Hemilabile ligands can release a free coordination site "on demand" of an incoming nucleophilic substrate while occupying it otherwise. This is believed to increase the thermal stability and activity of catalytic systems and therefore prevent decomposition via free coordination sites. In this investigation chelating pyridinyl-alcoholato ligands were identified as possible hemilabile ligands for incorporation into the second generation Grubbs precatalyst. The O,N-alcoholato ligands with different steric bulk could be successfully incorporated into the precatalysts. The incorporation of the sterically hindered, hemilabile O,N-ligands improved the thermal stability, activity, selectivity and lifetime of these complexes towards the metathesis of 1-octene. A decrease in the activity of the second generation Grubbs precatalyst was additionally observed after incorporating a hemilabile O,N-ligand with two phenyl groups into the system, while increasing their lifetime.

Synthesis, structural characterization and cis-trans isomerization of novel (salicylaldiminato)platinum(II) complexes.

The reaction of cis-[PtCl2(dmso)] with the salicylaldimine ligand, N-(2-hydroxybenzylidene)-2,6-di-isopropylaniline, LA in the presence of sodium acetate in methanol produced both cis- and trans-[PtClLA(dmso)], 1a and 1b. An analogous reaction for the less bulky ligand, N-(2-hydroxybenzylidene)aniline LB produced only cis-[PtClLB(dmso)], 2. The reactions of these dmso complexes with triphenylphosphine also yielded complexes with different geometries depending on the nature of the salicylaldiminato ligand. Thus the cis-trans isomerization of cis-[PtClLA(PPh3)] 3a was investigated both experimentally and computationally, and a tetrahedral transition state was detected in this process. A good agreement of the experimental activation parameters with those determined theoretically using DFT was obtained. LA was also reacted with [PtClMe(cod)] in methanol to yield the corresponding salicylaldiminato complex 6 in which the methyl group is cis to the imine nitrogen. X-ray crystal structures of some compounds obtained are reported.