Elucidating the Mechanism of Ambient-Temperature Aldol Condensation of Acetaldehyde on Ceria
Not Available
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Engineering topics
Zhao, Chuanlin
Publications and source records attributed to Zhao, Chuanlin.
Coupling of Acetaldehyde to Crotonaldehyde on CeO 2– x (111): Bifunctional Mechanism and Role of Oxygen Vacancies
Selective C–C coupling of oxygenates is pertinent to the manufacture of fuel and chemical products from biomass and from derivatives of C 1 compounds (i.e., oxygenates produced from methane and CO 2 ). Here we report a combined experimental and theoretical study on the temperature-programmed reaction (TPR) of acetaldehyde (AcH) on a partially reduced CeO 2– x (111) thin film surface. The experiments have been carried out under ultra-high-vacuum conditions without continuous gas exposure, allowing better isolation of active sites and reactive intermediates than in flow reaction conditions. AcH does not undergo aldol condensation in a typical TPR procedure, even though the enolate form of AcH (CH 2 CHO) is readily produced on CeO 2– x (111) with oxygen vacancies. We find however that a tailored “double-ramp” TPR procedure is able to successfully produce an aldol adduct, crotonaldehyde (CrA). Using density functional theory calculations and microkinetic modeling we explore several possible C–C coupling pathways. We conclude that the double-ramp procedure allows surface oxygen vacancy dimers, stabilized by adsorbate occupation, to form dynamically during the TPR. The vacancy dimers in turn enable C–C coupling to occur between an enolate and an adjacent AcH molecule via a bifunctional enolate–keto mechanism that is distinct from conventional acid- or base-catalyzed aldol condensation reactions. Here, the proposed mechanism indicates that CrA desorption is rate-limiting while C–C coupling is facile.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Theoretical investigation of dephosphorylation of phosphate monoesters on CeO 2 (111)
The dephosphorylation of neutral model phosphate monoesters on CeO 2 (111), including para -nitrophenyl phosphate and methyl phosphate, has been studied theoretically using self-consistent, periodic density functional theory calculations at the GGA+U-PW91 level. These phosphate monoesters interact strongly with CeO 2 (111) by forming a bond between the P atom and a surface lattice O atom. A surface-assisted hydrolysis mechanism is proposed for the catalytic dephosphorylation of the phosphate monoesters on CeO 2 (111), which involves P-O ester bond scission followed by phosphate hydration and product desorption. The energies of the transition states for the P-O ester bond scission are found to follow a linear scaling relation with respect to the energies of the dissociated fragments reasonably well. A nearly spontaneous transfer of one of the H atoms on the phosphate group to the alkoxide group produces the corresponding alcohol. The hydration of the remaining HPO 3 group has a maximum activation energy of ca. 1.1 eV in vacuo. So, although the nature of the alkoxide group affects the activation of the P-O ester bond, it should not affect the overall catalytic activity of CeO 2 (111) for dephosphorylation because the hydration of the phosphate group is rate-limiting.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Covalent versus localized nature of 4 f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory
We have conducted resonant angle-resolved photoemission spectroscopy of well-defined CeO 2 (111) and c-Ce 2 O 3 (111) model surfaces, revealing distinct f contributions in the valence band of the two compounds. In conjunction with density functional theory calculations, we show that the f contribution in CeO 2 is of a covalent nature, arising from hybridization with the O2p bands. In contrast, c-Ce 2 O 3 exhibits an almost nondispersive f state at 1.3 eV, which is indicative of almost negligible c-f hybridization.
Duchoň, Tomáš↗
Simulated Temperature Programmed Desorption of Acetaldehyde on CeO 2 (111): Evidence for the Role of Oxygen Vacancy and Hydrogen Transfer
The temperature programmed desorption of acetaldehyde adsorbed on partially reduced CeO 2 (111) has been studied in detail using microkinetic modeling based on self-consistent, periodic density functional theory calculations at the GGA-PW91+U level. Previous experimental studies (Chen et al. J. Phys. Chem. C 115: 3385, 2011; Calaza et al. J. Am. Chem. Soc. 134: 18034, 2012) have shown that, whereas on fully oxidized CeO 2 (111) acetaldehyde desorbs molecularly with a peak temperature of 210 K, the polymerization and enolization of acetaldehyde dominate the surface reactivity on partially reduced CeO 2 (111), resulting in acetaldehyde desorption at higher temperatures. Here we propose a comprehensive reaction mechanism that is consistent with the spectroscopic evidence of the identities of the surface intermediates and with the observed desorption activities, including the formation of ethylene and acetylene. Besides acetaldehyde (CH 3 CHO) and its enolate (CH 2 CHO), several other C 2 H x O species are proposed as key intermediates which are not seen spectroscopically, including ethoxy (CH 3 CH 2 O), ethyleneoxy (CH2CH2O), and formylmethylene (CHCHO). Our study suggests that oxygen vacancies play the critical role of activating the carbonyl bond and thereby facilitating b C–H bond scissions in acetaldehyde, leading to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling reactions.
Zhao, Chuanlin↗
Power grid reliability and security
This project has led to the development of a real-time simulation platform for electric power grids called Grid Simulator or GridSim for simulating the dynamic and information network interactions of large- scale power systems. The platform consists of physical models of power system components including synchronous generators, loads and control, which are simulated using a modified commercial power simulator namely Transient Stability Analysis Tool (TSAT) [1] together with data cleanup components, as well as an emulated substation level and wide-area power analysis components. The platform also includes realistic representations of communication network middleware that can emulate the real-time information flow back and forth between substations and control centers in wide-area power systems. The platform has been validated on a realistic 6000-bus model of the western American power system. The simulator GridSim developed in this project is the first of its kind in its ability to simulate real-time response of large-scale power grids, and serves as a cost effective real-time stability and control simulation platform for power industry.
29 ENERGY PLANNING, POLICY, AND ECONOMY↗