Jan Haubrich

Vapour-phase gold-surface-mediated coupling of aldehydes with methanol

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

Selectivity Control in Gold-Mediated Esterification of Methanol†

The Midas touch: The low-temperature transformation of methanol to methyl formate, formaldehyde, and formic acid is promoted by atomic oxygen adsorbed on metallic gold (see picture). The reactions occur with O-containing Au nanoparticles formed on Au(111) upon oxidation with ozone at 200 K; the facile esterification to methyl formate occurs well below room temperature.

Surface-Mediated Self-Coupling of Ethanol on Gold

The transformation of ethanol to its carbonyl compounds, namely acetaldehyde, ethyl acetate, acetic acid, and ketene, occurs on Au(111) with O-containing Au nanoparticles formed as a result of Au atom release upon ozone exposure. The product distribution strongly depends on the surface oxygen coverage. Ethoxy and acetate are identified as two key reaction intermediates during the oxidation of ethanol. The formation of acetaldehyde is due to the deprotonation of ethoxy, which can be further oxidized into acetate. The low-temperature formation of the ester, ethyl acetate, proceeds via the coupling of acetaldehyde with excess surface ethoxy. These reaction pathways appear relevant to heterogeneous processes catalyzed by supported gold nanoparticles, thus providing further insight into the mechanistic origin of gold-mediated oxidation of alcohols.

The Role of Surface and Subsurface Point Defects for Chemical Model Studies on TiO2: A First-Principles Theoretical Study of Formaldehyde Bonding on Rutile TiO2(110)

We report a systematic investigation of the effects of different surface and subsurface point defects on the adsorption of formaldehyde on rutile TiO(2)(110) surfaces using density functional theory (DFT). All point defects investigated--including surface bridging oxygen vacancies, titanium interstitials, and subsurface oxygen vacancies--stabilize the adsorption significantly by up to 56 kJ mol(-1) at a coverage of 0.1 monolayer (ML). The stabilization is due to a decrease of the coordination (covalent saturation) of the surface Ti adsorption sites adjacent to the defects, which leads to a stronger molecule-surface interaction. This change in the Ti is caused by the removal of a neighboring atom (oxygen vacancies) or substantial lattice relaxations induced by the subsurface defects. On the stoichiometric reference surface, the most stable adsorption geometry of formaldehyde is a tilted η(2)-dioxymethylene (with an adsorption energy E(ads)=-125 kJ mol(-1)), in which a bond forms to a nearby bridging O atom and the carbonyl-O atom in the formaldehyde binds to a Ti atom in the adjacent fivefold coordinated lattice site. The η(1)-top configuration on five-coordinate Ti(4+) is much less favorable (E(ads)=-69 kJ mol(-1)). The largest stabilization is exerted by subsurface Ti interstitials between the first and second layers. These defects stabilize the η(2)-dioxymethylene structure by nearly 40 kJ mol(-1) to an adsorption energy of -164 kJ mol(-1). Contrary to popular belief, adsorption in a bridging oxygen vacancy (E(ads)=-86 kJ mol(-1)) is much less favorable for formaldehyde compared to the η(2)-dioxymethylene structures. From these results we conclude that formaldehyde will bind in the η(2)-dioxymethylene structure on the stoichiometric surface as well as in the presence of Ti interstitials and bridging oxygen vacancies. In the light of these substantial effects, we conclude that it is essential to include all the types of point defects present in typical, reduced rutile samples used for model studies, at realistic concentrations to obtain correct adsorption sites, structures, energetic, and chemi-physical properties.

Oxygen-assisted cross-coupling of methanol with alkyl alcohols on metallic gold

We demonstrate for the first time that selective cross-coupling of methanol with either ethanol or n-butanol occurs below room temperature on metallic gold with no metal oxide support in a reaction sequence that occurs entirely on the surface. The esterification proceeds via activation of the alcohols by adsorbed oxygen and a sequence of reactions that involve both surface-bound alkoxys and hemiacetals as intermediates. The reaction selectivity is dictated by competing β-hydride elimination from the alkoxys. Due to the higher activation energy for β-hydride elimination from methoxy, no formate esters are formed. A molecular-scale mechanism constructed using our results is in excellent agreement with studies of heterogeneous catalysts, providing insight into selectivity control under a broad range of conditions.

In Situ Ambient Pressure Studies of the Chemistry of NO2 and Water on Rutile TiO2(110)

The adsorption of NO(2) on the rutile TiO(2)(110) surface has been studied at room temperature in the pressure range from approximately 10(-8) torr to 200 mtorr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Atomic nitrogen, chemisorbed NO(2), and NO(3) were formed, each of which saturates at pressures below approximately 10(-6) torr NO(2). Atomic nitrogen originates from decomposition of the NO(x) species. For pressures of up to 10(-3) torr, no significant change in the NO(x) surface species occurred, suggesting that environmentally relevant conditions with typical NO(2) partial pressures in the 1-100 ppb range can be modeled by ultrahigh vacuum (UHV) studies. The chemisorbed surface species can be removed by in situ annealing in UHV: all of the NO(x) species disappear around 400 K, whereas the N 1s signal associated with atomic nitrogen diminishes around 580 K. At higher pressures of NO(2) (p(NO(2)) > or = 10(-6) torr), physisorbed NO(2) and adsorbed water, which was likely due to displacement from the chamber walls, appeared. The water coverage grew significantly above approximately 10(-3) torr. Concurrently with co-condensation of water and NO(2), the population of NO(3) species grew strongly. From this, we conclude that the presence of NO(2) and water leads to the formation of multilayers of nitric acid. In contrast, pure water exposure after saturation of the surface with 200 mtorr NO(2) did not lead to a growth of the NO(3) signals, implying that HNO(3) formation requires weakly adsorbed NO(2) species. These findings have important implications for environmental processes, since they confirm that oxides may facilitate nitric acid formation under ambient humidity conditions encountered in the atmosphere.

McMurry Chemistry on TiO2(110): Reductive C═C Coupling of Benzaldehyde Driven by Titanium Interstitials

Selective reductive coupling of benzaldehyde to stilbene is driven by subsurface Ti interstitials on vacuum-reduced TiO(2)(110). A combination of temperature-programmed reaction spectroscopy and scanning tunneling microscopy (STM) provides chemical and structural information which together reveal the dependence of this surface reaction on bulk titanium interstitials. Benzaldehyde reductively couples to stilbene with 100% selectivity and conversions of up to 28% of the adsorbed monolayer in temperature programmed reaction experiments. The activity for coupling was sustained for at least 20 reaction cycles, which indicates that there is a reservoir of Ti interstitials available for reaction and that surface O vacancies alone do not account for the coupling. Reactivity was unchanged after predosing with water so as to fill surface oxygen vacancies, which are not solely responsible for the coupling reaction. The reaction is nearly quenched if O(2) is adsorbed first-a procedure that both fills defects and reacts with Ti interstitials as they migrate to the surface. New titania islands form after reductive coupling of benzaldehyde, based on scanning tunneling microscope images obtained after exposure of TiO(2)(110) to benzaldehyde followed by annealing, providing direct evidence for migration of subsurface Ti interstitials to create reactive sites. The reliance of the benzaldehyde coupling on subsurface defects, and not surface vacancies, over reduced TiO(2)(110), may be general for other reductive processes induced by reducible oxides. The possible role of subsurface, reduced Ti interstitials has broad significance in modeling oxide-based catalysis with reduced crystals.

Molecular imaging of reductive coupling reactions: interstitial-mediated coupling of benzaldehyde on reduced TiO2(110).

We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface-a diolate formed from benzaldehyde coupling on TiO(2)(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to ∼400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.