Allen G. Sault

Ethylene oxidation on silver powder: A tap reactor study

Abstract The oxidation of ethylene over silver powder catalysts was studied between 475 and 575 K at pressures of circa 10 Torr under transient conditions that allowed reactions of atomic and molecular oxygen to be distinguished. Dioxygen and d 4 -ethylene were pulsed separately over silver powder in a microreactor with pulse durations of 200 μs, and the products were detected by a multiplexed mass spectrometer. This method, denoted as temporal analysis of products, allows either simultaneous pulsing of the reactants or pulse delays ranging from a few milliseconds to minutes. In these experiments both ethylene oxide and carbon dioxide were detected as products. Ethylene oxide formed instantaneously on the time scale of the reactant pulse, but carbon dioxide formed with a much slower time constant, characteristic of the decomposition of surface carbonate, indicating the importance of secondary interactions of CO 2 with surface oxygen in the kinetics of CO 2 formation. Pulse-probe experiments, in which the catalyst was first loaded with adsorbed atomic oxygen and then reacted with anaerobic ethylene pulses or ethylene-oxygen mixtures, showed that the adsorbed species giving rise to ethylene oxide is atomic, not molecular, oxygen.

The synthesis and characterization of iron colloid catalysts in inverse micelle solutions

We have studied the synthesis and characterization of Fe based nanometer sized clusters formed in inverse micelle solutions. Inverse micelles provide a colloidal sized reaction template in an oleic solvent. Metal salts are solubilized within the interior of inverse micelles, and the addition of a reducing agent initiates nucleation and growth to produce the clusters. Surfactant acts to stabilize the particles. TEM shows that the average particle size is less than 3 nm in diameter. The reduction reaction of iron salts with LiBH4 in inverse micelle solutions results in the formation of FeB, Fe2+BOx (i.e. pyroborate), and α-Fe as determined by Mossbauer spectroscopy, electron diffraction, and X-ray photoelectron spectroscopy (XPS). Oxidation after exposure to air leads to the conversion of the FeB to α-Fe then to an undetermined Fe2+ phase and then finally to Fe3O4. Iron based metals are of interest as active, selective catalysts for a number of hydrogenation reactions including methanation and Fischer-Tropsch synthesis. We report catalytic results of iron clusters in the hydrogenolysis of naphthyl bibenzyl methane (NBM), a model reaction for coal liquefaction. The role of surfactant in the reaction mechanism is determined.

Dissociative adsorption of alkanes on Ni(100): Comparison with molecular beam results

The dissociative adsorption of ethane, propane, and n‐butane on Ni(100) has been investigated at pressures of 0.1–0.001 Torr and temperatures between 350 and 500 K. Activation energies for dissociation are found to be 9.5 kcal/mol for ethane, 3.8 kcal/mol for propane, and 3.1 kcal/mol for n‐butane. Dissociative sticking probabilities increase with increasing carbon chain length. Comparison of the present results with the results of a recent molecular beam study of alkane dissociation on Ni(100) show very poor agreement. The sticking probabilities measured here are all orders of magnitude higher than those predicted from the molecular beam study. The discrepancy is attributed to the behavior of alkane molecules with very low normal kinetic energies, which have sticking probabilities below the limits of detection of the beam experiment. C2 H6 and C2 D6 have identical dissociative sticking probabilities indicating that quantum tunneling of hydrogen is not an important step in ethane dissociation. Dissociatio...

Stochastic Generator of Chemical Structure. 3. Reaction Network Generation

A new method to generate chemical reaction network is proposed. The particularity of the method is that network generation and mechanism reduction are performed simultaneously using sampling techniques. Our method is tested for hydrocarbon thermal cracking. Results and theoretical arguments demonstrate that our method scales in polynomial time while other deterministic network generators scale in exponential time. This finding offers the possibility of investigating complex reacting systems such as those studied in petroleum refining and combustion.

The formation of active species for oxidative dehydrogenation of propane on magnesium molybdates

Pure and mixed magnesium molybdate phases (MoO3, MgMoO4, and MgMo2O7) have been examined for the oxidative dehydrogenation reaction of propane. The results are very sensitive to the stoichiometry and method of preparation. The catalysts exhibiting superior activity and selectivity are characterized by a unique temperature-programmed reduction peak that is not present for the poorly active or selective catalysts. Mixtures of MgMoO4 and MoO3 or MgMoO4 and MgMo2O7, materials that perform poorly by themselves, show significant improvements in performance upon heating. The solid-state interactions leading to these improvements correspond to the appearance of the characteristic reduction peak. The results suggest that the beneficial synergistic effects seen with mixtures of inactive phases are due to formation of a new phase or species, rather than remote communication between phases (e.g., oxygen spillover).

Quantitative analysis of Auger lineshapes of oxidized iron

Abstract A detailed data set of Fe(MNN) Auger lineshapes and O(511 eV)/Fe(LMM) Auger peak ratios for polycrystalline iron is reported as a function of oxygen exposure, and compared to corresponding X-ray photoelectron spectra. The data demonstrate that the Fe(MNN) lineshapes of Fe 3 O 4 and Fe 2 O 3 are extremely similar, and cannot be used to discriminate between these two oxides, except under nearly idealconditions. Other methods of determining iron oxidation states using relative heights of the Fe(LMM) peaks and O(KLL)/Fe(LMM) peak height ratios are critically evaluated. The data also demonstrate that although X-ray photoelectron spectroscopy is generally superior to Auger electron spectroscopy for determining iron oxidation states, the extreme surface sensitivity of the Fe(MNN) Auger peak provides certain advantages for analysis of the initial stages of iron oxidation. A methodology is discussed whereby this data set can be used to separate contributions to the O(511 eV) Auger peak due to iron oxides from contributions due to other oxides in multicomponent samples. In this manner, oxidation-state information can be obtained for individual components of multiple-element samples by Auger electron spectroscopy. In principle, this approach is applicable to transition-metal elements other than iron.

Reactions of silane with the W(110) surface

Reactions of silane with the W(110) surface have been studied using temperature programmed desorption, Auger electron spectroscopy and low energy electron diffraction. At 350 K, silane undergoes complete dissociation on the clean W(110) surface, with an initial dissociation probability of one. The hydrogen atoms produced by silane dissociation are displaced from the surface by silicon atoms during large silane exposures. As the silicon coverage increases, the silane dissociation probability remains 2 0.5 until a monolayer of silicon is deposited. The weak dependence of dissociation probability on silicon coverage suggests that dissociation occurs via a mobile precursor. Once the first monolayer is complete, the silane dissociation probability decreases sharply to less than 0.01. At 120 K, silane undergoes only partial dissociation. The resulting adsorbed species are tentatively identified as silyl (SiHs) groups. The silyl groups decompose during temperature programmed desorption at 200-300 K to evolve H, and leave adsorbed silicon atoms on the surface. Adsorbed silicon atoms form a number of ordered overlayer structures, depending on both the silicon and hydrogen coverages and the temperature to which the overlayers are annealed. Heating silicon overlayers to 1050 K results in reaction between silicon and tungsten to form epitaxial tungsten silicide overlayers. Repeated cycles of silane adsorption followed by annealing to 1050 K result in the epitaxial growth of stoichiometric WSi, layers on the surface.

The mechanism of formate oxidation on Ag(110)

The effects of coadsorbed oxygen atoms and gaseous O2 at pressures up to 0.7 Torr on the stability of the surface acetate intermediate on Ag(110) were investigated. Under an O2 pressure of 0.7 Torr acetate decomposes at temperatures as low as 400 K, which is 200 K lower than the decomposition temperature of acetate in vacuum. This destabilization is due to population of the surface with atomic oxygen when 0.7 Torr O2 is present. Acetate reacts with coadsorbed oxygen atoms via an intermediate which decomposes at 340 K to CO2, water and formate. The formate subsequently decomposes at 400 K to give CO2 and H2. Through the use of isotopic labelling, it was found that the carbon and hydrogen atoms in the formate originate from the methyl group of the acetate, while the two oxygen atoms come from surface atomic oxygen. A mechanism for the reaction of acetate with atomic oxygen is presented in which a surface oxygen atom abstracts a proton from the acetate, and the resulting CH2COO(a) species reacts further with another oxygen atom to form a glycolate (O2CCH2O(a)) intermediate. Nucleophilic attack at the methylene carbon by an oxygen atom, followed by C-C bond scission releases CO2 and forms H2CO2(a), which subsequently loses a hydrogen atom to make formate. The results presented here show that acetate will decompose under commercial ethylene oxidation conditions, and thus cannot be ruled out as an intermediate in the total oxidation of ethylene. Isomerization of ethylene oxide to acetaldehyde, followed by oxidation of the acetaldehyde is a plausible route to acetate formation during ethylene oxidation. In addition, this work demonstrates that phenomena which occur at high pressures can be observed in ultra high vacuum provided the relevant surface species can be formed in vacuum and remain on the surface up to the temperatures at which the high pressure phenomena occur.

Oxidation Reactions of Ethane over Ba-Ce-O Based Perovskites

Abstract Ethane oxidation reactions were studied over pure and Ca-, Mg-, Sr-, La-, Nd-, and Y-substituted BaCeO 3 perovskites under oxygen limited conditions. Several of the materials, notably the Ca- and Y-substituted materials, show activity for complete oxidation of the hydrocarbon to CO 2 at temperatures below 650°C. At higher temperatures, the oxidative dehydrogenation (ODH) to ethylene becomes significant. Conversions and ethylene yields are enhanced by the perovskites above the thermal reaction in our system in some cases. The perovskite structure is not retained in the high temperature reaction environment. Rather, a mixture of carbonates and oxides is formed. Loss of the perovskite structure correlates with a loss of activity and selectivity to ethylene.

Deposition and characterization of highly oriented Mg3(VO4)2 thin film catalysts. 2. Controlled variation of oxygen content

Thin films of magnesium vanadates oriented to expose a single crystalline face, could potentially serve as ideal models for high surface area magnesium vanadate catalysts for oxidative dehydrogenation. The growth of oriented films of one particular magnesium vanadate phase, the orthovanadate (Mg3(VO4)2), has been achieved by rf sputter deposition of the orthovanadate onto Au(111) surfaces. X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy have been used to investigate the structure and composition of the films. The orthorhombic orthovanadate grows epitaxially with the (021) plane oriented parallel to the surface. By varying oxygen flow rates during deposition the stoichiometry of the films can be varied from fully oxidized to highly oxygen deficient. At very low oxygen flow rates or in the complete absence of oxygen, a reduced Mg3V2O6 phase is formed. This reduced phase has a cubic structure and grows with the (100) plane parallel to the surface.