Multiplicity in the partial oxidation of propylene over a copper-molybdena catalyst

Measurement of rates of formation of acrolein and acetaldehyde in the partial oxidation of propylene over a copper molybdate catalyst were made under steady-state and transient modes. These measurements disclosed a rate of product formation multiplicity apparently attributable to the oxidation state of the catalyst and not to surface phenomena usually thought to be the source of hysteresis in catalytic systems.     

Mass spectral characterization of the thermal degradation of poly(propylene oxide) by electrospray and matrix-assisted laser desorption ionization

The thermal degradation of poly(propylene oxide) (PPO), M_n = 2000, can be characterized by electrospray (ESI) and matrix-assisted laser desorption ionization (MALDI). ESI and MALDI spectra of partially degraded PPO provide strong support for the thermal degradation pathway previously suggested by Griffiths et al. and Lemaire et al. Although these pathways differ in detail, it is not possible to distinguish between them from the masses of the resultant degradation species. Gel permeation chromatography data indicate that both mass spectrometric methods emphasize the presence of low-mass material, particularly in the degraded samples. This is attributed to the different sensitivities of the two techniques and some in situ fragmentation during mass spectrometric analysis.     

Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces

Temperature-programmed desorption (TPD) and Density Functional Theory (DFT) were used to investigate the reactions of oxametallacycles derived from ethylene oxide on clean and oxygen-covered Ag(110) surfaces. Ethylene oxide ring-opens following adsorption at 250 K on both clean and O-covered Ag(110) to form a stable oxametallacycle. On the clean Ag(110) surface, the oxametallacycle reacts to reform the parent epoxide at 280 K during TPD, while the aldehyde isomer, acetaldehyde, is observed at higher oxametallacycle coverages. In the presence of coadsorbed oxygen atoms, a portion of the oxametallacycles dissociate to release ethylene. However, of those that react to form oxygen-containing products, the fraction forming ethylene oxide is similar to that on the clean surface. The acetaldehyde product of oxametallacycle reactions combusts via formation of acetate species; the acetates react to form CO₂ at temperatures as low as 360 K on the O-covered surface. No evidence was observed for other combustion channels. This work provides experimental evidence for the connection of oxametallacycles to combustion via acetaldehyde formation as well as to ring-closure to form ethylene oxide.     

Adsorption, desorption and thermal oxidation of 2-CEES on nanocrystalline zeolites

In this study, FTIR spectroscopy and flow reactor measurements are used to investigate the adsorption, desorption and thermal oxidation of the mustard gas simulant, 2-chloroethyl ethyl sulfide (2-CEES), on nanocrystalline zeolites. Adsorption, desorption and thermal oxidation of 2-CEES on nanocrystalline NaZSM-5 are compared to that on nanocrystalline silicalite-1, the purely siliceous form of ZSM-5. It is shown here that NaZSM-5 is more reactive toward 2-CEES compared to silicalite. Furthermore, it is shown that the total 2-CEES adsorption capacity increases as the nanocrystalline particle size decreases suggesting that adsorption occurs on the external surface as well as in the internal pores of zeolite. Both zeolites effectively adsorb 2-CEES, suggesting that 2-CEES adsorption and subsequent thermal oxidation may be a viable method for the decontamination of mustard gas.     

Angular distribution of reactive carbon dioxide desorption on a rhodium (110) surface

The angular distribution of reactive CO₂ desorption was studied on Rh (110) by means of angle-resolved thermal desorption. Four CO₂ formation peaks appear in the range of 170–500 K. The distribution of the desorption collimated along the surface normal becomes sharp with increasing reactant coverages. No azimuth dependence is found in the distribution.     

The mechanism of CO oxidation over Cu(110): Effect of CO gas energy

The effect of the temperature of gas phase CO upon the kinetics of the oxygen titration reaction: CO_g +O_a CO_2,g, has been studied. It is found that the reaction's rate is independant of CO gas temperature between 300 and 623 K. The activation energy (6.5 kcal/mole), dependence upon CO pressure (first-order), and independence upon oxygen coverage for 0.1 _o 0.4 are all independant of the CO gas phase temperature. This result rules out any Eley-Rideal type mechanism whereby CO reacts directly from the gas phase with an oxygen adatom without first being accommodated to the surface temperature in an absorbed state. The result is instead interpretable in terms of a Langmuir-Hinshelwood mechanism.Camille and Henry Dreyfus Foundation Teacher-Scholar Fellowship.     

Low temperature CO oxidation on Au(111) and the role of adsorbed water

This paper presents results of an investigation of low-temperature CO oxidation and the role of moisture on an atomic oxygen covered Au(111) surface by employing molecular beam scattering techniques under ultrahigh vacuum (UHV) conditions. The effect of atomic oxygen precoverage on CO oxidation was examined at sample temperatures as low as 77 K. Prompt CO₂ production was observed when the CO beam impinges on the sample followed by a rapid decay of CO₂ production in all cases. At oxygen precoverages above 0.5 ML, CO₂ production decreases with increasing oxygen precoverage primarily due to the decrease in CO uptake. CO oxidation at 77 K goes through a precursor mediated reaction mechanism, where CO is in a precursor or trapped state and oxygen atoms are in a chemisorbed state. The role of adsorbed water was studied by using isotopically labeled water [H ₂ ¹⁸ O] to distinguish the oxygen species from that used in oxygen atom exposures [¹⁶O]. Evidence is presented that shows activated water or OH groups formed from water can directly participate in oxidizing CO on an atomic oxygen covered Au(111) surface.     

Role of steam in partial oxidation of propylene over a Pd/SDB catalyst

Step-response studies of propylene partial oxidation with oxygen over a hydrophobic Pd/SDB catalyst were conducted at 1000 kPa and 185°C in a fixed-bed reactor. CO₂ was found to be the only oxidation product when the feed contained only propylene and oxygen. CO₂ formation was significantly suppressed by addition of steam to the feed, and this addition leads to the formation of partial oxidation products: acrolein and acrylic acid. A competitive reaction mechanism involving water molecules is proposed to explain the significant influence of steam concentration on the rate of propylene oxidation and product selectivity.     

Kinetic Monte Carlo modeling of CO desorption and adsorption on Pd(110) surface

Desorption and adsorption of carbon monoxide on Pd(110) is modeled and simulated, aiming at gaining atomic level understanding of experimentally observed rates. The model parameters are fitted to reproduce the temperature programmed desorption spectra and molecular beam surface scattering data. Desorption turns out to be best described as thermally activated, the activation energy depending on the detailed nearest neighbor site occupation configuration. For a good fit, the adsorption induced surface reconstruction needs to be included in the model. Also, desorption needs to be modeled with a precursor state included. However, surface diffusion was not found to be essential. With these ingredients the coverage dependent sticking coefficient can be successfully simulated in the temperature range from 300 to 500 K. Furthermore, the experimentally observed saturation coverage—temperature dependence is correctly predicted from the balance between simultaneous adsorption and desorption.     

Further interlayer desorption studies of CSH(1)

X-ray diffraction of CSH(1) samples with a calcium to silicon ratio of 1.25 and 1.50 after room temperature desorption showed a step-like decrease from 12.5Å to 10.0Å and 12.4Å to 9.9Å respectively. The step-like decrease is not as well defined in CSH(1) with a higher calcium to silicon ratio. A distinct 0.6Å basal spacing decrease occurs as the relative humidity drops from 100 to 35 percent.     

Shedding light on catalytic ignition: coverage changes during CO oxidation on Pd(110)

Catalytic ignition or ‘light-off’ has been studied for a reaction of importance in automobile exhaust emissions, namely, CO oxidation over Pd, a metal which has become an important component of car catalysts. We present kinetic and spectroscopic studies to show that light-off is associated with a step change in surface composition of reactants as the surface is heated. Fast XPS using synchrotron radiation of high brightness reveals that the surface is dominated by CO at low temperature and the coverage suddenly switches during light-off to an oxygen-atom-dominated situation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Differential water sorption studies on Kevlar 49 and as-polymerised poly (p-phenylene terephthalamide): adsorption and desorption isotherms

We have conducted differential water vapour sorption experiments on Kevlar^TM 49 at 30°C over a series of water vapour pressures from 0 to 90% of saturation, and on the as-polymerised form of the material at 30°C, 45°C and 60°C over a series of water vapour pressures of 0-60%, 0-25% and 0-15%, respectively. The equilibrium isotherms obtained for both samples show a distinct hysteresis-type behaviour. For Kevlar^TM 49, the hysteresis loop can be divided into two regions, namely above 30%, which is indicative of the presence of microvoids, and below 30%, which suggests inclusion of water into the intimate structure of the surface layer of the polymer crystallites, in a process known as intercalation.     

The study of dielectric relaxation in propylene glycol-poly(propylene glycol) mixtures

Dielectric complex permittivity of propylene glycol (PG), poly(propylene glycol) (PPG-2000) and their mixtures with concentration of 25, 50 and 75 vol% of PG were measured in the frequency range 10 MHz-4 GHz at 25°C using time domain reflectometry (TDR). For these molecules and their mixtures, only one frequency independent dielectric loss peak was observed. The relaxation for these systems is described by a single relaxation time using Debye model. The large value of observed relaxation time for PG molecules shows the formation of molecular clusters. It is found that the relaxation time for PG-PPG mixtures is smaller in comparison to the relaxation times of PG and PPG molecules, and it linearly increases with the concentration of the PG in the mixtures. The values of relaxation times of PG-PPG mixtures are discussed particularly with respect to the solvent (PG) behaviour, which can be assigned to unaffected, loosely affected and tightly bound solvent and also with respect to the PPG chain coiling. As a peculiar feature the observed relaxation time is direct evidence of the interchange of solvent-solvent to solvent-polymer interaction.     

The role of water in the initial steps of methanol oxidation on Pt(2 1 1)

We report results of quantum-chemical calculations within the framework of density functional theory for the oxidation of methanol on the (2 1 1) face of a platinum single crystal. Similar to the reaction pathway on the low indexed (1 1 1) crystal face water plays an important role as found from energy minimization calculations: the adsorption of methanol on charged and uncharged surfaces is strongly enhanced by the formation of a hydrogen bond to a coadsorbed water molecule. The methanol is adsorbed via a methyl hydrogen atom preceding scission of one of the CH-bonds as the first reaction step. In the presence of additional water, e.g., from a liquid phase, the onset of the oxidation reaction is favored by a coadsorbed neighboring water molecule which forms a hydrogen bond with the methanol OH-group. At a minimum number of adjacent water molecules (n≥2) the CH-bond as well as the OH-bond are cleaved on charged surfaces. The protonic charge stemming from the dissociation of the methanol hydroxyl group is delocalized inside the aqueous cluster and formaldehyde is formed as an intermediate product.     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

The role of cysteine oxidation in the thermal inactivation of T4 lysozyme

Wild-type T4 lysozyme contains unpaired cysteine residues atpositions 54 and 97. To investigate the role these residuesplay in the thermal inactivation of the wild-type, we constructeda double mutant with these cysteines replaced with valine andserine. This molecule, T4 lysozyme (C54V/C97S), is more stablethan the wild-type to inactivation at 70°C at pH 6.5 and8.0. Guanidine hydrochloride reactivation experiments and SDS-PAGEon the inactivated products show that the wild-type is susceptibleto varying degrees of oxidative damage, depending on bufferconditions, while the cysteine-minus mutant inactivates onlyby other pathways. The products of thermal, oxidative inactivationof the wild-type are disulfide-linked oligomers. The dependenceof inactivation rate on temperature suggests that the formationof these aggregates depends on prior thermal unfolding of theT4 lysozyme molecule.     

Catalytic and electrocatalytic oxidation of propane on V-Mg-O and V-Mg-O (Ag) catalysts

Vanadium magnesium oxide catalysts prepared in this work were found active in selective oxidation of propane to propene. A selectivity as high as 79% was obtained at 10% conversion (813 K). No oxygenated or C₂ products were detected and the catalysts were found to undergo no change in activity over many weeks of operation. Under electrochemical pumping of oxygen (EOP) towards the catalyst (with oxygen present in the feed gas), both conversion and selectivity were found to increase slightly as external current increased, indicating the effect of electrical current can be exhibited by an oxide catalyst. However, in the absence of oxygen in the feed gas, EOP could lead to an even higher selectivity: 84 and 86.9% respectively for a 24 V-Mg-O and a 24 V-Mg-O (Ag) (1/2) catalyst. The overall results obtained suggest that electrochemically supplied oxygen is more selective towards C₃H₆. Mechanisms of both catalytic and electrocatalytic oxidation of propane were tentatively suggested, with surface oxygen ion vacancy identified as active surface species and the rate determining step involving heterolytic splitting of the C₃H₈ molecule to form a surface bonded C₃H ₇ ^– ion and a surface hydroxyl ion. The higher selectivity towards C₃H₆ in case of EOP was explained on this basis. While mixing with Ag powder was found to improve significantly the electrocatalytic performance of vanadium magnesium oxide, its role appears to be non-chemical: it simply gives rise to a larger area of the gas/catalyst/Ag electrode interface.     

Aliphatic poly(propylene dicarboxylate)s: Effect of chain length on thermal properties and crystallization kinetics

Polyesters based on 1,3-propanediol glycol and aliphatic dicarboxylic acids with different chain length were synthesized by melt polycondensation, obtaining samples characterized by high and comparable M_n. The polymers were subjected to molecular and thermal characterization. All polymers showed a good thermal stability, even though depending on the chain length. At room temperature all the polymers appeared as semicrystalline materials; the effect of the chain length was a lowering in the T_g value, an odd-even fluctuation for T_m and an increase of the crystallization rate. A comparison of the X-ray data revealed that the polymers with odd carbon number per repeat unit, show similar patterns, different from those of samples with even carbon atoms number. Multiple endotherms were evidenced in melt isothermally crystallized samples, due to melting and recrystallization processes. By applying the Hoffman-Weeks' method, the of the samples was derived. Lastly, the presence of an interphase was not evidenced.     

Controlling reaction selectivity in the oxidation of methanol at Cu(110) surfaces

When deuterated methanol (CD₃OH) and dioxygen mixtures are coadsorbed at Cu(110) surfaces at room temperature and above, the main reaction product can be switched between formaldehyde and formate by changing the mixture composition. For oxygen-rich mixtures formaldehyde is the major product but when methanol is in 5 : 1 excess only formate is produced. It is proposed that the reaction mechanism is controlled by the microscopic structure of the surface: when the reactants are adsorbed sequentially, separate islands of oxygen and methoxy develop which inhibit further oxidation but when reactants are coadsorbed a homogeneous mixture of adsorbates is produced creating a facile pathway to formate. This revised version was published online in July 2006 with corrections to the Cover Date.