Coupling of Acetaldehyde to Crotonaldehyde on CeO_2–x(111): Bifunctional Mechanism and Role of Oxygen Vacancies

Abstract

Selective C–C coupling of oxygenates is pertinent to the manufacture of fuel and chemical products from biomass and from derivatives of C1 compounds (i.e., oxygenates produced from methane and CO2). Here we report a combined experimental and theoretical study on the temperature-programmed reaction (TPR) of acetaldehyde (AcH) on a partially reduced CeO2–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 (CH2CHO) is readily produced on CeO2–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. The proposed mechanism indicates that CrA desorption is rate-limiting while C–C coupling is facile.

Revealing the Adsorption and Decomposition of EP-PTCDI on a Cerium Oxide Surface

“…10 Encouragingly, oxygen vacancies can reduce the potential barrier to facilitate on-surface reactions and, therefore, have different implications. For instance, the C–C coupling reaction is activated by oxygen vacancy dimers, 11 the NO molecules can be converted into N 2 via the catalysis of surface vacancies. 12…”

CheKiPEUQ Intro 1: Bayesian Parameter Estimation Considering Uncertainty or Error from both Experiments and Theory**

“…This system was previously presented in reference, [27] though a nonlinear activation energy dependence was not included in that work and will be included here as the last model considered. During the experiment in which the data was collected, ethanal was adsorbed on CeO 2 (111) at below 200 K under ultrahigh vacuum conditions and the system was then subjected to a temperature ramp of 2 K s −1 [98–100] . Ethanal molecules desorbed molecularly in the temperature range before 325 K (by Eq.…”

“…For these three models, CPE and BPE were performed with the same sequence of steps: Step 1) Start with the initial parameters guess, which is also the mean of the prior, and perform one of several grid‐search variations to find the optimum grid‐point. For all three models and each state therein, the initial guess for each activation energy for desorption is 41.5 kJ/mol and for each pre‐exponential is 10 13 s −1 [27,99, 101] . During the Step 1 grid‐search, each grid‐point is either taken directly or a local optimization is performed, as described in the next paragraph.…”

Elucidating the Mechanism of Ambient-Temperature Aldol Condensation of Acetaldehyde on Ceria

“…More recently, a double-ramp procedure was proposed that did produce CrH in the TPD of AcH on partially reduced CeO 2 (111). 35 We proposed oxygen vacancy dimers to be the active site for C–C coupling in UHV, the formation of which was made possible through the double ramp procedure.…”

“… 28 They exhibit primarily (111) facets, which facilitates comparison with previous works based on CeO 2 (111) thin film surfaces. 31 , 32 , 35 Infrared spectroscopy, in its many variations, has proven to be a powerful tool for elucidating the nature of reaction intermediates and mechanisms in heterogeneous catalysis. 36 We use in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations to demonstrate conclusive evidence for the ambient-temperature formation of CrH on CeO 2 (111).…”