Synchronous Thermal Desorption and Decomposition of Ethanol on Rh{111}

Rh-induced Support Transformation and Rh Incorporation in Titanate Structures and Their Influence on Catalytic Activity

Rh is one of the most effective metals in several technologically important heterogeneous catalytic reactions, like the hydrogenation of CO2, and CO, the CO+H2O reaction, and methane and ethanol transformations. Titania and titanates are among the most frequently studied supports for Rh nanoparticles. The present study demonstrates that the nature of the support has a marked influence on the specific activity. For comparison, the catalytic activity of TiO2 P25 is also presented. It is pointed out that a certain amount of Rh can be stabilized as cation (Rh+) in ion-exchange positions (i.e., in atomic scale distribution) of the titanate framework. This ionic form does not exists on TiO2. We pay distinguished attention not only to the electronic interaction between Rh metal and the titania/titanate support, but also to the Rh-induced phase transitions of one-dimensional titanate nanowires (TiONW) and nanotubes (TiONT). Support transformation phenomena can be observed in Rh-loaded titanates. Rh decorated nanowires transform into the TiO2(B) phase, whereas their pristine counterparts recrystallize into anatase. The formation of anatase is dominant during the thermal annealing process in both acid-treated and Rh-decorated nanotubes; Rh catalysis this transformation. We demonstrate that the phase transformations and the formation of Rh nanoclusters and incorporated Rh ions affect the conversion and the selectivity of the reactions. The following initial activity order was found in the CO2 + H2, CO + H2O and C2H5OH decomposition reactions: Rh/TiO2 (Degussa P25) ≥ Rh/TiONW > Rh/TiONT. On the other hand it is remarkable that the hydrogen selectivity in ethanol decomposition was two times higher on Rh/TiONW and Rh/TiO(NT) catalysts than on Rh/TiO2 due to the presence of Rh+ cations incorporated into the framework of the titanate structures.

Mechanism of C-C and C-H bond cleavage in ethanol oxidation reaction on Cu2O(111): a DFT-D and DFT+U study

The performance of transition metal catalysts for ethanol oxidation reaction (EOR) in direct ethanol fuel cells (DEFCs) may be greatly affected by their oxidation. However, the specific effect and catalytic mechanism for EOR of transition metal oxides are still unclear and deserve in-depth exploitation. Copper as a potential anode catalyst can be easily oxidized in air. Thus, in this study, we investigated C-C and C-H bond cleavage reactions of CH_xCO (x = 1, 2, 3) species in EOR on Cu₂O(111) using PBE+U calculations, as well as the specific effect of +U correction on the process of adsorption and reaction on Cu₂O(111). It was revealed that the catalytic performance of Cu₂O(111) for EOR was restrained compared with that of Cu(100). Except for the C-H cleavage of CH₂CO, all the reaction barriers for C-C and C-H cleavage were higher than those on Cu(100). The most probable pathway for CH₃CO to CHCO on Cu₂O(111) was the continuous dehydrogenation reaction. Besides, the barrier for C-C bond cleavage increased due to the loss of H atoms in the intermediate. Moreover, by the comparison of the traditional GGA/PBE method and the PBE+U method, it could be concluded that C-C cleavage barriers would be underestimated without +U correction, while C-H cleavage barriers would be overestimated. +U correction was proved to be necessary, and the reaction barriers and the values of the Hubbard U parameter had a proper linear relationship.

Competitive C-C and C-H bond scission in the ethanol oxidation reaction on Cu(100) and the effect of an alkaline environment

Direct ethanol fuel cell technology is impeded by inefficient, yet expensive anode catalysts. As such, research on effective and cheap anode catalysts towards complete ethanol oxidation reaction (EOR) is greatly needed. Herein, we report the investigations of the competitive C-C and C-H bond scissions in the EOR involving CH₃CO, CH₂CO, and CHCO species on Cu(100) using density functional theory and transition state theory calculations. The easiest C-C bond cleavage was found in CH₂CO while the most difficult C-H bond cleavage was also found in CH₂CO, both with an activation energy of 1.02 eV. The feasible C-C bond scission may take place in CH₂CO with a rate constant ratio of the C-C to the C-H bond scission at 100 °C of 0.32. Furthermore, in an alkaline environment, the C-H bond scission activation barrier is considerably lowered but the C-C bond cleavage activation barrier is slightly increased for both CH₃CO and CH₂CO species. The reaction of CH₃CO species on Cu(100) under alkaline conditions produces mainly acetic acid with a barrier of 0.49 eV and a rate constant of 4.93 × 10⁵ s^-1 at 100 °C.

Decomposition of ethanol on Pd(111): a density functional theory study

Ethanol decomposition over Pd(111) has been systematically investigated using self-consistent periodic density functional theory, and the decomposition network has been mapped out. The most stable adsorption of the involved species tends to follow the gas-phase bond order rules, wherein C is tetravalent and O is divalent with the missing H atoms replaced by metal atoms. Desorption is preferable for adsorbed ethanol, methane, and CO, while for the other species decomposition is preferred. For intermediates going along the decomposition pathways, energy barriers for the C-C, C(alpha)-H, and O-H scissions are decreased, while it is increased for the C-O path or changes less for the C(beta)-H path. For each of the C-C, C-O, and C-H paths, the Bronsted-Evans-Polanyi relation holds roughly. The most likely decomposition path is CH(3)CH(2)OH --> CH(3)CHOH --> CH(3)CHO --> CH(3)CO --> CH(2)CO --> CHCO --> CH + CO --> CO + H + CH(4) + C.

Chemistry of ethylene glycol on a Rh(100) single-crystal surface

The adsorption and decomposition of ethylene glycol on Rh(100) have been studied with temperature-programmed reaction spectroscopy and reflection absorption infrared spectroscopy. Ethylene glycol adsorbs onto the surface via the hydroxyl groups. At 150 K, both hydroxyl bonds are broken, forming an ethylenedioxy intermediate. At high coverage, a portion of the ethylene glycol molecules dehydrogenate only one hydroxyl bond, forming a monodentate species. These intermediates decompose further, with complete dehydrogenation and simultaneous C--C bond breaking occurring at around 290 K. Hydrogen and carbon monoxide are formed, which desorb at 290 and 500 K, respectively.

Ethanol Adsorption, Decomposition and Oxidation on Ir(111): A High Resolution XPS Study

Ethanol (C(2)H(5)OH) adsorption, decomposition and oxidation is studied on Ir(111) using high-energy resolution, fast XPS and temperature-programmed desorption. During heating of an adsorbed ethanol layer a part of the C(2)H(5)OH(ad) desorbs molecularly, and another part remains on the surface and decomposes around 200 K; these two decomposition pathways are identified, as via acetyl (H(3)C--C=O) and via CO(ad)+CH(3ad), respectively. Acetyl and CH(3ad) decompose around 300 K into CH(ad) (and CO(ad)). CH(ad) decomposes forming C(x) and H(2) around 520 K. In the presence of O(ad) an acetate intermediate is formed around 180 K, as well as a small amount of CH(3ad) and CO(ad). Acetate decomposes between 400-480 K into CO(2), H(2)(/H(2)O) and CH(ad).