Reactivity of crotonaldehyde and propene over Au/Pd(111) surfaces

The surface chemistry of crotonaldehyde and propene, primary and secondary reaction products in the aerobic selective oxidation of crotyl alcohol, has been studied by temperature-programmed reaction over Au/Pd(111) surface alloys. Gold strongly promotes desorption versus reaction at mole fractions ≥0.3 (crotonaldehyde) and ≥0.8 (CH); only ∼5% of the chemisorbed aldehyde or alkene react over Au-rich alloys. Surprisingly, co-adsorbed oxygen strongly suppresses crotonaldehyde decomposition over both clean Pd(111) and alloy surfaces, while CH combustion, an important undesired side-reaction over unpromoted Pd(111), is also moderated by Au. © the Owner Societies.

A fast XPS investigation of NO-promoted acetylene cyclotrimerisation on Pd{111}

The strong promotion by NO of acetylene cyclotrimerisation to benzene on Pd{1 1 1} has been investigated by fast XPS, supported by LEED and Δφ data. Islands of NO act to compress co-existing islands of acetylene. The resulting increase in acetylene density dramatically lowers the nominal threshold acetylene coverage required for reaction. In addition, the threshold reaction temperature is reduced from 150 to <100 K, and increased lateral interactions within the organic domains promote formation of tilted benzene. Co-adsorption of NO with benzene itself accelerates the desorption of both tilted and flat-lying benzene at low temperatures. These factors act to increase the yield of reactively formed benzene to 100%, completely suppressing carbon deposition that would otherwise result from decomposition of reactant and product.

A fast XPS study of sulphate promoted propene decomposition over Pt{111}

SO2 oxidation has been followed by Fast XPS over Pt{111}. Preadsorbed oxygen reduces the low temperature saturation coverage of SO2 with respect to the clean surface. Heating a mixed O2/SO2 adlayer results in efficient oxidation of both upright and flat-lying SO2 molecules to surface-bound SO4. Sulphate decomposes above room temperature liberating gas-phase SO2 and SO3. Propene adsorbs molecularly at 100 K over clean Pt{111} and dehydrogenates above 250 K to form a stable propylidyne adlayer, which in turn decomposes above 400 K to form graphitic carbon. Preadsorbed surface sulphate enhances the sticking probability of propene via formation of an alkyl-sulphate complex. Thermal decomposition of this complex accounts for low temperature propene combustion and is accompanied by atomic sulpur deposition. Propylidyne forms as on clean Pt but is less reactive undergoing partial oxidation above 450 K with residual surface oxygen.

Optical autocorrelator that uses a surface-emitting second-harmonic generator on (211)B GaAs

An optical autocorrelator grown on a (211)B GaAs substrate that uses visible surface-emitted second-harmonic generation is demonstrated. The (211)B orientation needs TE mode excitation only, thus eliminating the problem of the beating between the TE and TM modes that is required for (100)-grown devices; it also has the advantage of giving higher upconversion efficiency than (111) growth. Values of waveguide loss and the difference in the effective refractive index between the TE(0) and TE(1) modes were also obtained from the autocorrelation experiment.

Optical autocorrelator that uses a surface-emitting second-harmonic generator on (211)B GaAs

An optical autocorrelator grown on a (211)B GaAs substrate that uses visible surface-emitted second-harmonic generation is demonstrated. The (211)B orientation needs TE mode excitation only, thus eliminating the problem of the beating between the TE and TM modes that is required for (100)-grown devices; it also has the advantage of giving higher upconversion efficiency than (111) growth. Values of waveguide loss and the difference in the effective refractive index between the TE(0) and TE(1) modes were also obtained from the autocorrelation experiment.

High-pressure XPS of crotyl alcohol selective oxidation over metallic and oxidized Pd(111)

Here, we report on the first application of high-pressure XPS (HP-XPS) to the surface catalyzed selective oxidation of a hydrocarbon over palladium, wherein the reactivity of metal and oxide surfaces in directing the oxidative dehydrogenation of crotyl alcohol (CrOH) to crotonaldehyde (CrHCO) is evaluated. Crotonaldehyde formation is disfavored over Pd(111) under all reaction conditions, with only crotyl alcohol decomposition observed. In contrast, 2D Pd5O4 and 3D PdO overlayers are able to selectively oxidize crotyl alcohol (1 mTorr) to crotonaldehyde in the presence of co-fed oxygen (140 mTorr) at temperatures as low as 40 °C. However, 2D Pd5O4 ultrathin films are unstable toward reduction by the alcohol at ambient temperature, whereas the 3D PdO oxide is able to sustain catalytic crotonaldehyde production even up to 150 °C. Co-fed oxygen is essential to stabilize palladium surface oxides toward in situ reduction by crotyl alcohol, with stability increasing with oxide film dimensionality.

Metastable de-excitation spectroscopy and density functional theory study of the selective oxidation of crotyl alcohol over Pd(111)

The extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS) has been utilized to probe the bonding and reactivity of crotyl alcohol over Pd(111) and provide insight into the selective oxidation pathway to crotonaldehyde. Auger de-excitation (AD) of metastable He (23S) atoms reveals distinct features associated with the molecular orbitals of the adsorbed alcohol, corresponding to emission from the hydrocarbon skeleton, the O n nonbonding, and C═C π states. The O n and C═C π states of the alcohol are reversed when compared to those of the aldehyde. Density functional theory (DFT) calculations of the alcohol show that an adsorption mode with both C═C and O bonds aligned somewhat parallel to the surface is energetically favored at a substrate temperature below 200 K. Density of states calculations for such configurations are in excellent agreement with experimental MDS measurements. MDS revealed oxidative dehydrogenation of crotyl alcohol to crotonaldehyde between 200 and 250 K, resulting in small peak shifts to higher binding energy. Intramolecular changes lead to the opposite assignment of the first two MOs in the alcohol versus the aldehyde, in accordance with DFT and UPS studies of the free molecules. Subsequent crotonaldehyde decarbonylation and associated propylidyne formation above 260 K could also be identified by MDS and complementary theoretical calculations as the origin of deactivation and selectivity loss. Combining MDS and DFT in this way represents a novel approach to elucidating surface catalyzed reaction pathways associated with a “real-world” practical chemical transformation, namely the selective oxidation of alcohols to aldehydes.

Hydrodebromination of bromobenzene over Pt(111)

The surface chemistry of benzene and bromobenzene over Pt(111) has been studied by temperature-programmed XPS/MS and NEXAFS. Time-resolved XPS shows that benzene adopts a single chemically distinguishable environment during low-temperature adsorption within the monolayer, with a saturation coverage at θC6H6 = 0.2 ML. Around 20% of a benzene monolayer desorbs molecularly, while the remainder dehydrogenates to surface carbon. Bromobenzene likewise adsorbs molecularly at 90 K, giving rise to two C 1s environments at 284.4 and 285.3 eV corresponding to the C−H and C−Br functions, respectively. The saturation C6H5Br monolayer coverage is 0.11 ML. NEXAFS reveals that bromobenzene adopts a tilted geometry, with the ring plane at 60 ± 5° to the surface. Bromobenzene multilayers desorb at ∼180 K, with higher temperatures promoting competitive molecular desorption versus C−Br scission within the monolayer. Approximately 30% of a saturated bromobenzene monolayer either desorbs reversibly or as reactively formed hydrocarbons. Debromination yields a stable (phenyl) surface intermediate and atomic bromine at 300 K. Further heating results in desorption of reactively formed H2, C6H6, and HBr; however, there was no evidence for either biphenyl or Br2 formation. Pt(111) is an efficient surface for low-temperature bromobenzene hydrodebromination to benzene and HBr. © 2007 American Chemical Society.

Selective oxidation of crotyl alcohol over Pd(111)

The selective oxidation of crotyl alcohol has been explored over a Pd(111) model catalyst. At low temperatures, the alcohol adsorbs intact with the C=C bond parallel to the surface. Activation likely proceeds through an allyl alkoxide intermediate that follows two distinct reaction channels. Over the clean surface, ∼90% of the alcohol oxidizes to surface bound crotonaldehyde above 200 K, which subsequently all decarbonylates to propene and CO at room temperature. The minor reaction channel involves C-O scission to 2-butene and water. While some of these undesired reactively formed alkene products desorb around 300 K, the majority dehydrogenate to irreversibly bound carbon above 380 K. This latter decomposition pathway is unlikely to be important at the low temperatures utilized in liquid-phase crotyl alcohol oxidation over supported palladium catalysts. Adsorbed CO persists until 430 K and is likely responsible for site-blocking and deactivation of dispersed metallic Pd clusters. Coadsorbed oxygen suppresses crotonaldehyde decarbonylation and promotes its release from the surface. © 2007 American Chemical Society.

Sulfate-enhanced catalytic destruction of 1,1,1-trichlorethane over Pt(111)

The catalytic destruction of 1,1,1-trichloroethane (TCA) over model sulfated Pt(111) surfaces has been investigated by fast X-ray photoelectron spectroscopy and mass spectrometry. TCA adsorbs molecularly over SO4 precovered Pt(111) at 100 K, with a saturation coverage of 0.4 monolayer (ML) comparable to that on the bare surface. Surface crowding perturbs both TCA and SO4 species within the mixed adlayer, evidenced by strong, coverage-dependent C 1s and Cl and S 2p core-level shifts. TCA undergoes complete dechlorination above 170 K, accompanied by C−C bond cleavage to form surface CH3, CO, and Cl moieties. These in turn react between 170 and 350 K to evolve gaseous CO2, C2H6, and H2O. Subsequent CH3 dehydrogenation and combustion occurs between 350 and 450 K, again liberating CO2 and water. Combustion is accompanied by SO4 reduction, with the coincident evolution of gas phase SO2 and CO2 suggesting the formation of a CO−SOx surface complex. Reactively formed HCl desorbs in a single state at 400 K. Only trace (<0.06 ML) residual atomic carbon and chlorine remain on the surface by 500 K.

A fast XPS study of propene decomposition over clean and sulphated Pt{111}

The thermal decomposition of propene over clean and sulphate precovered Pt{111} has been followed by Fast XPS. The saturation propene coverage over the clean surface is 0.21 mL at 90 K. Propene is stable up to 200 K, above which molecular desorption and dehydrogenation result in the formation of a stable propylidyne intermediate adlayer at 300 K. Propylidyne decomposes above 400 K eventually forming graphitic carbon above 800 K. Preadsorbed surface sulphate promotes room temperature propene combustion associated with the decomposition of a thermally unstable alkyl--sulphate complex. Propylidyne also forms as on clean Pt{111}, but is less reactive, its decomposition above 450 K triggering partial oxidation with residual surface oxygen to liberate gas phase CO.

The surface chemistry of nanocrystalline MgO catalysts for FAME production:an in situ XPS study of H2O, CH3OH and CH3OAc adsorption

An in situ XPS study of water, methanol and methyl acetate adsorption over as-synthesised and calcined MgO nanocatalysts is reported with a view to gaining insight into the surface adsorption of key components relevant to fatty acid methyl esters (biodiesel) production during the transesterification of triglycerides with methanol. High temperature calcined NanoMgO-700 adsorbed all three species more readily than the parent material due to the higher density of electron-rich (111) and (110) facets exposed over the larger crystallites. Water and methanol chemisorb over the NanoMgO-700 through the conversion of surface O2 − sites to OH− and coincident creation of Mg-OH or Mg-OCH3 moieties respectively. A model is proposed in which the dissociative chemisorption of methanol occurs preferentially over defect and edge sites of NanoMgO-700, with higher methanol coverages resulting in physisorption over weakly basic (100) facets. Methyl acetate undergoes more complex surface chemistry over NanoMgO-700, with C–H dissociation and ester cleavage forming surface hydroxyl and acetate species even at extremely low coverages, indicative of preferential adsorption at defects. Comparison of C 1s spectra with spent catalysts from tributyrin transesterification suggest that ester hydrolysis plays a key factor in the deactivation of MgO catalysts for biodiesel production.