Dynamics of CO oxidation: A model with two oxygen forms

Dynamics of the CO oxidation NO reduction reaction on thin MgO (100) supported Pd clusters has been studied by quantum chemical molecular dynamics (QCMD). Chemical changes, change of adsorption states, and structural changes were investigated. We observed that CO adsorption occurs molecularly, whereas NO adsorption occurs first molecularly on the Pd cluster, but it is then dissociatively adsorbed on Pd cluster after interacting with CO. Oxygen atoms resulting from the NO dissociation react with adsorbed CO and form CO2 that desorbs from the Pd cluster, whereas the remaining N atom diffuses in the lattice of the Pd cluster. NO dissociation has been found to be the key step for this reaction. Furthermore, from the molecular dynamics and electronic structure calculations, we have identified a number of consequences for the interpretation and modeling of diffusion experiments demonstrating the coverage. As far we know, this is the first QCMD based simulation of the dynamics of CO oxidation, NO reduction, and ...

The Dynamics of Oxygen Storage in Ceria–Zirconia Model Catalysts Measured by CO Oxidation under Stationary and Cycling Feedstream Compositions

The dynamics of surface chemical reactions: infrared chemiluminescence of the product CO 2 desorbed from metal surfaces

The dynamics of CO oxidation on Pt deduced from translational, rotational and vibrational excitation in product CO 2

Electro-oxidation of CO adsorbed on a polycrystalline Pt electrode in the potential region of hydrogen absorption is examined by fast time-resolved surface-enhanced infrared absorption spectroscopy coupled to voltammetry or chronoamperometry. Oxidation dynamics at a weak preoxidation peak around 0.5 V (vs RHE) and the main oxidation peak around 0.7 V observed in stripping voltammetry are focused. IR spectra show that the shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. It is also shown that CO on terraces becomes very mobile in the main oxidation region after some amount of CO being oxidized via a nucleation-and-growth mechanism and that terrace CO is oxidized faster than CO adsorbed at step edges. The result is interpreted in terms of a Langmuir−Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species (most likely OH) adsorbed at steps. The oxidation mechanism is essentially identical to that proposed in some earlier studies, but more convincing spectroscopic evidence of the mechanism is presented.

Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the reaction dynamics of the photooxidation of CO on the anatase TiO2(101) surface in real time. Our time-resolved soft X-ray photoemission spectroscopy results provide evidence of ultrafast timescales and, coupled with theoretical calculations, clarify the mechanism of oxygen activation that is crucial to unraveling the underlying processes for a range of photocatalytic reactions relevant to air purification and self-cleaning surfaces. The reaction takes place between 1.2 and 2.8 (±0.2) ps after irradiation with an ultrashort laser pulse leading to the formation of CO2, prior to which no intermediate species were observed on a picosecond time scale. Our theoretical calculations predict that the presence of intragap unoccupied O2 levels leads to the formation of a charge-transfer complex. This allows the reaction to be initiated following laser illumination at a photon energy of 1.6 eV (770 nm), taking place via a proposed mechanism involving the direct transfer of electrons from TiO2 to physisorbed O2.

ABSTRACTIn this article, comonomer effects in copolymerization of ethylene and 1‐hexene with four MgCl2‐supported Ziegler‐Natta catalysts using either ethylene or 1‐hexene as the main monomer were investigated. It was found that no matter which monomer was used as the main monomer, the polymerization activity was significantly enhanced by introducing small amount of comonomer. In copolymerization with ethylene as the main monomer, the strength of comonomer effects was much stronger in active centers producing low‐molecular‐weight polymer than those producing high‐molecular‐weight polymer. In copolymerization with 1‐hexene as the main monomer, the number of active centers ([C*]/[Ti]) was determined, and the propagation rate constants (kp) were calculated. Deconvolution of the polymer molecular weight distribution into Flory components were made to study the active center distribution. Introduction of small amount of ethylene caused marked increase in the number of active centers and decrease in average chain propagation rate constant. Introducing internal electron donor in the catalyst enhanced not only the number of active centers but also the chain propagation rate constant. In copolymerization of 1‐hexene with small amount of ethylene, the internal donor weakened the comonomer effects to some extent and changed the distribution of comonomer effects among different types of active centers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41264.

ABSTRACTThe microstructure of the isotactic polypropylene obtained with various MgCl2‐supported catalyst systems at high polymerization temperature of 70–100°C is investigated by discussing the intrinsic relation between the different types of active centers and the polymerization temperatures via gel permeation chromatography, temperature rising elution fractionation, and 13C NMR. For the MgCl2/TiCl4/di‐n‐butyl phathalate‐AlEt3/external donor and MgCl2/TiCl4/2,2‐diisobutyl‐1,3‐dimethoxypropane‐AlEt3 catalyst systems, the differences in the isotactic productivity of polymers obtained at different polymerization temperatures mainly result from the variation of both the activity of the different isospecific active centers and the stability constants of the complex of catalyst/donor. The reaction rate of high isotactic active centers reaches maximum at 85–90°C, and this effect contributes to both the highest isotacticity and the narrowest molecular weight distribution. For the MgCl2/TiCl4/phthalate ester‐AlEt3 catalyst system, the isotacticity of polypropylene remains approximately constant in the temperature range of experiments, which could be ascribed to elution of phthalate ester after the activation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42487.

Mechanistic study on hydrogen effects in propylene polymerization with MgCl2-supported Ziegler-Natta catalysts: New evidences from active center counting and reaction kinetics

Novel MgCl2-supported Ziegler-Natta (Z-N) catalysts prepared using a new one-pot ball milling method can effectively control the amounts of Ti-loading in the catalysts. Complex GPC data on polypropylene synthesized by these novel catalysts were analyzed using the method of fitting the molecular weight distribution (MWD) curves with a multiple Flory-Schulz function. It was found that multiple active centers exist in these novel catalysts. Detailed study of the effects of the Ti-loadings in the catalysts on the distribution of the active centers showed that the Ti-loadings in the novel MgCl2-supported Z-N catalysts might affect the proportion of each type of active centers; and might be the main factor responsible for the effect of the Ti-loadings on the microstructure, the molecular weight and molecular weight distribution width of the resultant polymer, the catalytic activity and polymerization kinetics.

Supported titanium-magnesium catalyst for ethylene polymerization

Cationic copolymerization of β-propiolactone and 1,3-dioxolane

Two catalysts, denoted as catalyst 1 [silica/MAO/(nBuCp) 2ZrCl2] and catalyst 2 [silica/nBuSnCl 3/MAO/(nBuCp)2ZrCl2] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene-1-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive self-nucleation and annealing (SSA)], and 13C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. 13C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter- and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, nE MPDSC-GT and n E MPNMR-Flory, as well as the weight-average lamellar thicknesses, Lwav DSC-GT and Lwav SSA DSC, were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo- and copolymers increased linearly as a function of LMPDSC-GT. This indicates that the homo- and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene-α-olefin linear low-density polyethylenes (LLDPEs). © 2013 American Chemical Society.

The production of adipic acid (AA) from the ring-opening hydrogenolysis (hydrodeoxygenation - HDO) of tetrahydrofuran 2,5-dicarboxylic acid (THFDCA) by metal-metal oxide catalysts (Pt-MOx/TiO2 and Pt/MOx) was evaluated. Correlations between AA yield, or products distribution, and the type of active centers in the catalyst surface were established by studying the type (MOx: WOx, MoOx, and ZrOx), the amount of metal oxide (MOx:Pt molar ratio), and the reaction temperature (180ºC, 220ºC, and 240ºC) in Pt-MOx/TiO2 and Pt/MOx. The activity results show that only the catalysts that contain both Pt and Mo produced AA at 220ºC, with yields of 24% for Pt-MoOx/TiO2 (4 wt% Pt, 1:1 Mo:Pt molar ratio) and 34% for 4 wt% Pt/MoO3. At 180ºC, these catalysts exhibited high selectivity towards reducing the carboxyl group. Computational studies using DFT suggest that this behavior is due to the preferential interaction between the oxygen vacancy (O-VAC) in MoO3 and the CO bond in the carboxyl group in 2-hydroxyadipic acid (2-HAA), the first observed intermediate from the HDO of THFDCA. Coverage of this adsorption configuration decreases at temperatures above 200ºC, where the interaction between the alcohol group in the 2 C position in 2-HAA and the Brønsted acid site (BAS) on the surface of MoO3 is preferred. This latter interaction could promote the acid-catalyzed dehydration of the alcohol group to obtain AA. Increasing the amount of Mo to 5:1 Mo:Pt in Pt-MoOx/TiO2 and decreasing the Pt content to 0.1 wt % in Pt/MoO3 decreases the selectivity to alcohols, increasing the AA yield to 55% at 220ºC and 65% at 240ºC for both catalysts. This enhancement in performance is consistent with the findings from the DFT studies. To our knowledge, this is the highest yield reported for AA production from THFDCA or FDCA by metal-metal oxide catalysts.

Specific behaviour of cycloolefins in ring-opening polymerisation