Oxygen Vacancy-Assisted Coupling and Enolization of Acetaldehyde on CeO₂(111)

The temperature-dependent adsorption and reaction of acetaldehyde (CH₃CHO) on a fully oxidized and a highly reduced thin-film CeO₂(111) surface have been investigated using a combination of reflection–absorption infrared spectroscopy (RAIRS) and periodic density functional theory (DFT+U) calculations. On the fully oxidized surface, acetaldehyde adsorbs weakly through its carbonyl O interacting with a lattice Ce⁴⁺ cation in the η¹-O configuration. This state desorbs at 210 K without reaction. On the highly reduced surface, new vibrational signatures appear below 220 K. They are identified by RAIRS and DFT as a dimer state formed from the coupling of the carbonyl O and the acyl C of two acetaldehyde molecules. This dimer state remains up to 400 K before decomposing to produce another distinct set of vibrational signatures, which are identified as the enolate form of acetaldehyde (CH₂CHO¯). Furthermore, the calculated activation barriers for the coupling of acetaldehyde, the decomposition of the dimer state, and the recombinative desorption of enolate and H as acetaldehyde are in good agreement with previously reported TPD results for acetaldehyde adsorbed on reduced CeO₂(111) [Chen et al. <i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 3385]. The present findings demonstrate that surface oxygen vacancies alter the reactivity of the CeO₂(111) surface and play a crucial role in stabilizing and activating acetaldehyde for coupling reactions

Adsorption and Reaction of Acetaldehyde on Shape-Controlled CeO₂ Nanocrystals: Elucidation of Structure–Function Relationships

CeO₂ cubes with {100} facets, octahedra with {111} facets, and wires with highly defective structures were utilized to probe the structure-dependent reactivity of acetaldehyde. Using temperature-programmed desorption (TPD), temperature-programmed surface reactions (TPSR), and <i>in situ</i> infrared spectroscopy, it was determined that acetaldehyde desorbs unreacted or undergoes reduction, coupling, or C–C bond scission reactions, depending on the surface structure of CeO₂. Room-temperature FTIR indicates that acetaldehyde binds primarily as η¹-acetaldehyde on the octahedra, in a variety of conformations on the cubes, including coupling products and acetate and enolate species, and primarily as coupling products on the wires. The percent consumption of acetaldehyde ranks in the following order: wires > cubes > octahedra. All the nanoshapes produce the coupling product crotonaldehyde; however, the selectivity to produce ethanol ranks in the following order: wires ≈ cubes ≫ octahedra. The selectivity and other differences can be attributed to the variation in the basicity of the surfaces, defects densities, coordination numbers of surface atoms, and the reducibility of the nanoshapes

One-dimensional supramolecular surface structures: 1,4-diisocyanobenzene on Au(111) surfaces

One-dimensional supramolecular structures formed by adsorbing low coverages of 1,4-diisocyanobenzene on Au(111) at room temperature are obtained and imaged by scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. The structures originate from step edges or surface defects and arrange predominantly in a straight fashion on the substrate terraces along the 〈11-0〉 directions. They are proposed to consist of alternating units of 1,4-diisocyanobenzene molecules and gold atoms with a unit cell in registry with the substrate corresponding to four times the lattice interatomic distance. Their long 1-D chains and high thermal stability offer the potential to use them as conductors in nanoelectronic applications. © the Owner Societies

Supports and Modified Nano-particles in Designing Model Catalysts

In order to design catalytic materials, we need to understand the essential causes for material properties resulting from its composite nature. In this paper we discuss two, at first sight, diverse aspects: a) the effect of the oxide-metal interface on metal-nanoparticle properties and b) the consequences of metal particle modification after activation on the selectivity of hydrogenation reactions. However, those two aspects are intimately linked. The metal-nanoparticles electronic structure changes at the interface as a catalyst is brought to different reaction temperatures due to morphological modifications in the metal and, as we will discuss, those changes the chemistry leading to changes in the reaction path. As the morphology of the particle varies, facets of different orientation and size are exposed which may lead to a change in surface chemistry as well. We use two specific reactions to address those issues in some detail. To the best of our knowledge the present paper reports the first observations of this kind for well-defined model systems. The changes of the electronic structure of Au nanoparticles due to their size and interaction with a supporting oxide are revealed as a function of temperature using CO2 activation as a probe. The presence of spectator species (oxopropyl) as formed during an activation step of acrolein hydrogenation, strongly controls the selectivity of the reaction towards hydrogenation of the unsaturated C-O vs. the C-C bond on Pd(111) when compared with oxide supported Pd nanoparticles.Fil: Obrien, C. P.. US Army Research Laboratory; Estados UnidosFil: Dostert, K. H.. Fritz-Haber Institut der Max-Planck Gesellschaft; AlemaniaFil: Hollerer, M.. University of Graz; AustriaFil: Stiehler, Christian. Fritz-Haber Institut der Max-Planck Gesellschaft; AlemaniaFil: Calaza, Florencia Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina. Fritz-Haber Institut der Max-Planck Gesellschaft; AlemaniaFil: Schauermann, S.. Fritz-Haber Institut der Max-Planck Gesellschaft; AlemaniaFil: Shaikhutdinov, S.. Fritz-Haber Institut der Max-Planck Gesellschaft; AlemaniaFil: Sterrer, M.. University of Graz; AustriaFil: Freund, H. J.. Fritz-Haber Institut der Max-Planck Gesellschaft; Alemani