Abstract Using attenuated total‐reflectance infrared spectroscopy (ATR/FTIR), the concentration of deuterated polybutadiene near the surface of a flat zinc selenide crystal was followed as it was replaced by ordinary polybutadiene by flow and diffusion. Experiments were performed in the melt, both below (M ∼ 1,500) and above (M ∼ 15,000) the entanglement threshold. The decay profiles agree well with a finite‐element simulation of the system. In contrast to previous investigations of C 16 's, the decay profiles of both unentangled and entangled polybutadienes are consistent with a uniform diffusivity in the near‐wall region.

Promotion of a Rh/SiO/sub 2/ catalyst with lanthana results in a partial coverage of the supported Rh crystallites with LaO/sub x/ islands. These moieties reduce the capacity of Rh to adsorb CO but have little effect on the chemisorption of H/sub 2/ because of spillover of H atoms from the exposed Rh sites onto the surface of the LaO/sub x/ islands. At elevated temperatures, the LaO/sub x/ islands promote the dissociation of CO. It is proposed that this process occurs preferentially at Rh sites located along the perimeter of the LaO/sub x/ islands. Support for this interpretation is provided by infrared spectra of CO adsorbed on lanthana-promoted Rh/SiO/sub 2/. 40 references.

LSMO

Abstract The hcp phase of Mo 2 C has been prepared by carburization of Mo and is shown to have a BET surface area of 10‐30 m 2 /g, and pores 30 Å in diameter.

The reduction and reoxidation of submonolayer coverages of TiO2 deposited onto MCM-48 were investigated. The deposited TiO2 was characterized by Raman and UV−visible spectroscopy. Raman spectra show that Ti atoms are bonded to the silica support by Ti−O−Si bonds and that crystalline TiO2 is not formed. The results of the Raman and UV−visible spectroscopy suggest that the dispersed TiO2 is present as two-dimensional oligomeric structures. Reduction in H2 at 923 K produces Ti3+ cations observable by EPR (g = 1.932), suggesting the formation of oxygen vacancies. The fraction of Ti that could be reduced increased with TiO2 surface concentration. This observation is attributed to the ease with which O atoms can be removed from the TiO2 overlayer as the size of the titania patches increases. The amount of oxygen removed during reduction was quantified by pulsed reoxidation. It was observed that the temperature required for complete reoxidation decreased with increasing surface coverage of the silica support by TiO2. This trend is explained with a proposed model of the reoxidation process, in which the rate limiting step is the migration of peroxide species through or between the deposited TiO2 patches. A linear correlation was established between the intensity of the EPR signal for Ti3+ and the amount of oxygen removed from TiO2/SiO2. This relationship was then used to determine the oxygen vacancy concentration present on the surface of TiO2/SiO2 after temperature-programmed oxidation of methanol.

Efficient identification of transition states is important for understanding reaction mechanisms. Most transition state search algorithms require long computational times and a good estimate of the transition state structure in order to converge, particularly for complex reaction systems. The growing string method (GSM) [B. Peters et al., J. Chem. Phys. 120, 7877 (2004)] does not require an initial guess of the transition state; however, the calculation is still computationally intensive due to repeated calls to the quantum mechanics code. Recent modifications to the GSM [A. Goodrow et al., J. Chem. Phys. 129, 174109 (2008)] have reduced the total computational time for converging to a transition state by a factor of 2 to 3. In this work, three transition state-finding strategies have been developed to complement the speedup of the modified-GSM: (1) a hybrid strategy, (2) an energy-weighted strategy, and (3) a substring strategy. The hybrid strategy initiates the string calculation at a low level of theory (HF/STO-3G), which is then refined at a higher level of theory (B3LYP/6-31G∗). The energy-weighted strategy spaces points along the reaction pathway based on the energy at those points, leading to a higher density of points where the energy is highest and finer resolution of the transition state. The substring strategy is similar to the hybrid strategy, but only a portion of the low-level string is refined using a higher level of theory. These three strategies have been used with the modified-GSM and are compared in three reactions: alanine dipeptide isomerization, H-abstraction in methanol oxidation on VOx/SiO2 catalysts, and C–H bond activation in the oxidative carbonylation of toluene to p-toluic acid on Rh(CO)2(TFA)3 catalysts. In each of these examples, the substring strategy was proved most effective by obtaining a better estimate of the transition state structure and reducing the total computational time by a factor of 2 to 3 compared to the modified-GSM. The applicability of the substring strategy has been extended to three additional examples: cyclopropane rearrangement to propylene, isomerization of methylcyclopropane to four different stereoisomers, and the bimolecular Diels–Alder condensation of 1,3-butadiene and ethylene to cyclohexene. Thus, the substring strategy used in combination with the modified-GSM has been demonstrated to be an efficient transition state-finding strategy for a wide range of types of reactions.

Surface interactions of H2, CO and CO2 with the perovskite-type oxide LaMnO3 have been studied by temperature-programmed desorption (t.p.d.) and i.r. spectroscopy. A t.p.d. desorption peak of H2 at 355–360 K, which increases in intensity with increasing reduction temperature of the oxide (Tr), is assigned to molecular adsorption of H2 on reduced manganese sites (Mnn+, n≈ 2). CO adsorption yielded t.p.d. peaks of CO and CO2. A peak of CO at 473 K (for oxidized LaMnO3) associated with a carbonate group and peaks at 360–395 K, 540–550 K and 773–800 K (for reduced LaMnO3) associated with linear and bridged CO species adsorbed on Mnn+ ions were observed. A very wide CO2 desorption peak at 473 K and tail centred at 773 K (oxidized LaMnO3) are associated with monodentate and bidentate carbonates interacting with Mn3+. CO2 adsorption yielded t.p.d. peaks of CO2 at 345–385 K and at 540–665 K whose intensity decreased and increased, respectively, with Tr. These are associated with monodentate and bidentate carbonates, respectively, interacting with reduced sites of manganese or La3+. Detection of bands at ca. 2900 cm–1 in the i.r. spectrum obtained after CO + H2 adsorption, the appearance of new CO desorption features at 570 K and above 860 K, and the detection of a new H2 desorption peak at 770–785 K in the t.p.d. spectra obtained after CO–H2 or H2–CO adsorptions suggest decomposition of an oxygenated species formed by interaction of CO and H2 adsorbed on the same adsorption centre.