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.

This study presents the correlation between process parameters and porosity formation in a TiAl6V4 alloy produced by selective laser melting. The porosity is investigated by 2D and 3D methods aiming to identify the mechanisms of void formation, their morphology as well as volume fraction as a function of the energy density. An evident minimum volume fraction is observed between process parameters with significant overheating and insufficient fusion. It is shown that these two marginal regions define two mechanisms of void formation. Two dominant types of voids morphology are identified and examined regarding pore orientation versus their elongation, which together with the curvature distribution analysis allow revealing critical defects.

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.

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We report the first systematic theoretical study of the oxidative self-coupling of methanol to form the ester, methylformate, on atomic-oxygen-covered Au(111) using density functional theory calculations. The first step in the processdissociation of the O−H bond in methanolhas a lower barrier for transfer of the proton to adsorbed oxygen than for transfer of H to gold, consistent with experimental observations that O is necessary to initiate the reaction. The computed barrier for formation of methoxy (CH3O) and OH is 0.41 eV, compared with 1.58 eV calculated for the transfer of H to the clean Au surface. Several different pathways for the ensuing β-H elimination in CH3O(ads) to form formaldehyde have been considered, namely, attack by adsorbed O, OH, or a second CH3O, and transfer to the Au metal. Methoxy attacked by surface oxygen has the lowest calculated barrier, 0.49 eV, and leads to adsorbed H2CO and OH. Subsequent coupling of methoxy and formaldehyde has no apparent barrier in the calculation, consistent with the experimental conclusion that β-H elimination is the rate-limiting step for the overall reaction. With the exception of surface oxygen, all other surface species have low diffusion barriers, suggesting that rearrangement and movement of these species from the preferred adsorption sites to configurations necessary for reactions occur readily, thus contributing to the activity for coupling on gold.

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.

Selective laser melting is a promising powder-bed-based additive manufacturing technique for titanium alloys: near net-shaped metallic components can be produced with high resource-efficiency and cost savings. For the most commercialized titanium alloy, namely Ti-6Al-4V, the complicated thermal profile of selective laser melting manufacturing (sharp cycles of steep heating and cooling rates) usually hinders manufacturing of components in a one-step process owing to the formation of brittle martensitic microstructures unsuitable for structural applications. In this work, an intensified intrinsic heat treatment is applied during selective laser melting of Ti-6Al-4V powder using a scanning strategy that combines porosity-optimized processing with a very tight hatch distance. Extensive martensite decomposition providing a uniform, fine lamellar α + β microstructure is obtained along the building direction. Moreover, structural evidence of the formation of the intermetallic α2-Ti3Al phase is provided. Variations in the lattice parameter of β serve as an indicator of the microstructural degree of stabilization. Interconnected 3D networks of β are generated in regions highly affected by the intensified intrinsic heat treatment applied. The results obtained reflect a contribution towards simultaneous selective laser melting-manufacturing and heat treatment for fabrication of Ti-6Al-4V parts.

The adsorption and thermal decomposition of the α,β-unsaturated aldehyde prenal (3-methyl-2-butenal) have been studied on Pt(111), the Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surface alloys, and the corresponding terminations of the Pt3Sn(111) bulk alloy by means of high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). By comparing the experimental results with extensive theoretical calculations of the multitude of possible adsorption configurations of prenal using density functional theory (DFT), the adsorption configurations actually present on all model catalysts have been identified. This approach, thus, reveals a new way to identify complex, multifunctional molecules adsorbed on model catalyst surfaces. On Pt(111), prenal is strongly adsorbed and decomposes at approximately 300 K. By the aid of density functional theory (DFT), five flat-lying adsorption structures of η2, η3, and η4 hapticity, which exhibit similar adsorption energies E ads between −47 and −59 kJ/mol, have been identified on the surface. The adsorption energy of prenal on the considered Pt−Sn alloys is significantly weaker. On the Pt3Sn and the Pt2Sn/Pt(111) surface alloys, the HREEL spectra recorded at 170 K are essentially assigned to two vertical η-top-(s)-trans configurations (E ads = −39.1 and −30.8 kJ/mol on Pt3Sn and −33.4 kJ/mol on Pt2Sn) adsorbed atop the protruding Sn atoms. Due to the weak adsorption of these structures, the vibrational frequencies are only slightly perturbed as compared to their corresponding gas-phase values. The primary role of tin is a general weakening of the adsorption of prenal on the alloy surfaces. While on Pt(111), flat adsorption configurations are preferred, alloying with tin induces a drastic change in the adsorption geometries to vertical η1-top forms. On the alloy surfaces, generally an oxygen−tin interaction is required to form competitive adsorption structures at all, whereas a coordination from the aldehydic function to a Pt is hardly stable.