Adsorption of acrolein, propanal, and allyl alcohol on Pd(111): a combined infrared reflection–absorption spectroscopy and temperature programmed desorption study

Atomistic-level understanding of the interaction of α,β-unsaturated aldehydes and their derivatives with late transition metals is of fundamental importance for the rational design of new catalytic materials with the desired selectivity towards C=C vs. C=O bond partial hydrogenation. In this study, we investigate the interaction of acrolein, and its partial hydrogenation products propanal and allyl alcohol, with Pd(111) as a prototypical system. A combination of infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) experiments was applied under well-defined ultrahigh vacuum (UHV) conditions to obtain detailed information on the adsorption geometries of acrolein, propanal, and allyl alcohol as a function of coverage. We compare the IR spectra obtained for multilayer coverages, reflecting the molecular structure of unperturbed molecules, with the spectra acquired for sub-monolayer coverages, at which the chemical bonds of the molecules are strongly distorted. Coverage-dependent IR spectra of acrolein on Pd(111) point to the strong changes in the adsorption geometry with increasing acrolein coverage. Acrolein adsorbs with the C=C and C=O bonds lying parallel to the surface in the low coverage regime and changes its geometry to a more upright orientation with increasing coverage. TPD studies indicate decomposition of the species adsorbed in the sub-monolayer regime upon heating. Similar strong coverage dependence of the IR spectra were found for propanal and allyl alcohol. For all investigated molecules a detailed assignment of vibrational bands is reported

Isolation of high quality graphene from Ru by solution phase intercalation

2013 AIP Publishing LL

Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix

While nanoparticles are being pursued actively for a number of applications, dispersed atomic species have been explored far less in functional materials architectures, primarily because composites comprising dispersed atoms are challenging to synthesize and difficult to stabilize against sintering or coarsening. Here we show that room temperature oxidation of Au–Sn alloys produces nanostructures whose surface is terminated by a reducible amorphous oxide that contains atomically dispersed Au. Analysis of the oxidation process shows that the dispersal of Au in the oxide can be explained by predominant oxygen anion diffusion and kinetically limitedmetalmass transport, which restrict phase separation due to a preferential oxidation of Sn. Nanostructures prepared by oxidation of nanoscale Au–Sn alloys with intermediate Au content (30–50%) show high activity in a CO-oxidation probe reaction due to a cooperative mechanism involving Au atoms as sites for CO adsorption and reaction to CO2 embedded in a reducible oxide that serves as a renewable oxygen reservoir. Our results demonstrate a reliable approach toward nanocomposites involving oxide-embedded, atomically dispersed noble metal species

Alloy oxidation as a route to chemically active nanocomposites of gold atoms in a reducible oxide matrix

While nanoparticles are being pursued actively for a number of applications, dispersed atomic species have been explored far less in functional materials architectures, primarily because composites comprising dispersed atoms are challenging to synthesize and difficult to stabilize against sintering or coarsening. Here we show that room temperature oxidation of Au–Sn alloys produces nanostructures whose surface is terminated by a reducible amorphous oxide that contains atomically dispersed Au. Analysis of the oxidation process shows that the dispersal of Au in the oxide can be explained by predominant oxygen anion diffusion and kinetically limitedmetalmass transport, which restrict phase separation due to a preferential oxidation of Sn. Nanostructures prepared by oxidation of nanoscale Au–Sn alloys with intermediate Au content (30–50%) show high activity in a CO-oxidation probe reaction due to a cooperative mechanism involving Au atoms as sites for CO adsorption and reaction to CO2 embedded in a reducible oxide that serves as a renewable oxygen reservoir. Our results demonstrate a reliable approach toward nanocomposites involving oxide-embedded, atomically dispersed noble metal species

Selective Partial Hydrogenation of Acrolein on Pd: a Mechanistic Study

Identifying the surface processes governing the selectivity in hydrogenation of α,β-unsaturated carbonyl compounds on late transition metals is crucial for the rational design of catalytic materials with the desired selectivity toward C=C or C=O bond hydrogenation. The partial selective hydrogenation of acrolein on a Pd(111) single crystal and Fe₃O₄- supported Pd nanoparticles under well-de fined UHV conditions was investigated in the present study as a prototypical reaction. Molecular beam techniques were combined with infrared reflection − absorption spectroscopy (IRAS) and quadrupole mass spectrometry (QMS) in order to simultaneously monitor the evolution of surface species and the formation of the final gas-phase products under isothermal reaction conditions as a function of surface temperature. Over a Pd(111) single crystal, acrolein is hydrogenated at the C=O bond to form the desired reaction product propenol with nearly 100% selectivity in the temperature range between 250 and 300 K, while over Pd/Fe₃O₄, selective hydrogenation of the C=C bond to form propanal occurs. We found that the high selectivity toward C=O bond hydrogenation over Pd(111) is connected to the initial modification of the catalytic surface with a dense monolayer of an oxopropyl surface species. This strongly bound oxopropyl layer is formed on the pristine Pd crystal in the induction period from half-hydrogenation of the C=C bond in acrolein. Subsequently deposited acrolein molecules adsorb via the C=O bond and form a half-hydrogenated reaction intermediate propenoxy species, which is attached to Pd via a C-O single bond. The evolution of the surface concentration of the propenoxy intermediate monitored spectroscopically was found to closely follow the propenol formation rate detected in the gas phase. At temperatures higher than 300 K on Pd(111) and on Pd nanoparticles supported on Fe₃O₄, decarbonylation of acrolein occurs, leading to accumulation of CO and strongly dehydrogenated carbonaceous species on the surface. This process prevents formation of well-ordered overlayers of oxopropyl species required for selective C=O bond hydrogenation, resulting in only minor nonselective hydrogenation of acrolein. At temperatures below 250 K on Pd(111), only a small fraction of the initially adsorbed acrolein is converted into the oxopropyl species, yielding a partially modified surface and thus rather unselective formation of both propanal and propenol products

Kooperative Bildung einer langreichweitig geordneten Wasserschicht auf der Fe₃O₄(111)-Oberfläche

Die ersten Elementarschritte in der Adsorption von Wasser auf der Magnetit(111)-Oberfläche und die atomare Struktur der Wasser/Metalloxid-Grenzfläche werden kontrovers diskutiert. Hier werden experimentelle Befunde präsentiert, die durch Infrarot-Reflexions-Absorptions-Spektroskopie und temperaturprogrammierte Desorption erhalten wurden. Unterstützende DFT-Rechnungen zeigen, dass Wassermoleküle an Fetet- und O-Atomen dissoziativ unter Bildung zweier Hydroxo-Spezies adsorbieren. Diese wirken als Anker für weitere Wassermoleküle, wobei Dimere entstehen. Diese ordnen sich zu einer definierten (2×2)-Überstruktur an. Die Entstehung dieser geordneten Wasserschicht erfolgt durch eine kooperative Wechselwirkung zwischen den Dimeren und wird durch den Aufbau eines 2D-Netzwerks über H-Brücken angetrieben

Isolation of high quality graphene from Ru by solution phase intercalation

2013 AIP Publishing LL