Elucidating the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y by transient XAS

Imbao, Jerick; Bokhoven, Jeroen A. van; Clark, Adam H.; Nachtegaal, Maarten

doi:10.1038/s41467-020-14982-x

Cited by 30 publications

(40 citation statements)

References 41 publications

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“…Computational interrogation of ion mobility is challenging because adsorption free energies in zeolites are drastically underestimated by the standard “harmonic vibrational” assumptions that pervade the computational heterogeneous catalysis literature. ,− Metadynamics and other enhanced-sampling MD simulation techniques, which are ubiquitous for exploring complex free energy surfaces in the biological simulation community, are being adapted to model reactions and species mobility in zeolites. ,− These mechanistic insights are also transferable to other chemistries, as ethene-solvated mobile Ni ions have been implicated in ethene dimerization in Ni-AFI by kinetic measurements and AIMD . Moreover, H 2 O-solvated Ag, Cu, Pd, and Fe sites in zeolites have been computed and observed or inferred by experiment, ,,,− which may provide conceptual extensions to cases wherein the molecules responsible for metal ion solvation and mobilization do not serve as reactants in the catalytic cycle. The solvation and mobilization of metal ions thus appears to be a general phenomenon of metal-zeolite catalysis under certain reaction conditions, characterized by a dynamic active-site motif that can reversibly interconvert among structures with different nuclearities to mediate distinct elementary steps during steady-state catalytic turnover.…”

Section: Discussionmentioning

confidence: 99%

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

et al. 2020

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Conspectus Copper-exchanged chabazite (Cu-CHA) zeolites are catalysts used in diesel emissions control for the abatement of nitrogen oxides (NO x ) via selective catalytic reduction (SCR) reactions with ammonia as the reductant. The discovery of these materials in the early 2010s enabled a step-change improvement in diesel emissions aftertreatment technology. Key advantages of Cu-CHA zeolites over prior materials include their effectiveness at the lower temperatures characteristic of diesel exhaust, their durability under high-temperature hydrothermal conditions, and their resistance to poisoning from residual hydrocarbons present in exhaust. Fundamental catalysis research has since uncovered mechanistic and kinetic features that underpin the ability of Cu-CHA to selectively reduce NO x under strongly oxidizing conditions and to achieve improved NO x conversion relative to other zeolite frameworks, particularly at low exhaust temperatures and with ammonia instead of other reductants. One critical mechanistic feature is the NH3 solvation of exchanged Cu ions at low temperatures (<523 K) to create cationic Cu–amine coordination complexes that are ionically tethered to anionic Al framework sites. This ionic tethering confers regulated mobility that facilitates interconversion between mononuclear and binuclear Cu complexes, which is necessary to propagate SCR through a Cu2+/Cu+ redox cycle during catalytic turnover. This dynamic catalytic mechanism, wherein single and dual metal sites interconvert to mediate different half-reactions of the redox cycle, combines features canonically associated with homogeneous and heterogeneous reaction mechanisms. In this Account, we describe how a unified experimental and theoretical interrogation of Cu-CHA catalysts in operando provided quantitative evidence of regulated Cu ion mobility and its role in the SCR mechanism. This approach relied on new synthetic methods to prepare model Cu-CHA zeolites with varied active-site structures and spatial densities in order to verify that the kinetic and mechanistic models describe the catalytic behavior of a family of materials of diverse composition, and on new computational approaches to capture the active-site structure and dynamics under conditions representative of catalysis. Ex situ interrogation revealed that the Cu structure depends on the conditions for the zeolite synthesis, which influence the framework Al substitution patterns, and that statistical and electronic structure models can enumerate Cu site populations for a known Al distribution. This recognition unifies seemingly disparate spectroscopic observations and inferences regarding Cu ion structure and responses to different external conditions. SCR rates depend strongly on the Cu spatial density and zeolite composition in kinetic regimes where Cu+ oxidation with O2 becomes rate-limiting, as occurs at lower temperatures and under fuel-rich conditions. Transient experiments, ab initio molecular dynamics simulations, and statistical models relate these sensitivities to the m...

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Section: Discussionmentioning

confidence: 99%

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

et al. 2020

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“…The Fourier-transformed (FT) k 2 -weighted EXAFS spectra of Pd K-edge for PdCu/Z-5 and Pd/Z-5 in the R-space showed similar peaks to that of PdO at 1.52 Å and 2.98 Å in the Rspace. The peak at 1.52 Å corresponded to the first shell of PdÀ O, whereas the peak at 2.98 Å was assigned to the PdÀ OÀ Pd structure for PdO, [18] respectively. Based on fitting of the EXAFS results (Table 1), the coordination number of the first PdÀ O shell for PdCu/Z-5 was 3.7 with an average bond length of 2.02 Å.…”

Section: Resultsmentioning

confidence: 99%

Tandem Catalysis for Selective Oxidation of Methane to Oxygenates Using Oxygen over PdCu/Zeolite

Lin

Huang

et al. 2022

Angewandte Chemie

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Selective oxidation of methane to oxygenates with O2 under mild conditions remains a great challenge. Here we report a ZSM‐5 (Z‐5) supported PdCu bimetallic catalyst (PdCu/Z‐5) for methane conversion to oxygenates by reacting with O2 in the presence of H2 at low temperature (120 °C). Benefiting from the co‐existence of PdO nanoparticles and Cu single atoms via tandem catalysis, the PdCu/Z‐5 catalyst exhibited a high oxygenates yield of 1178 mmol g−1Pd h−1 (mmol of oxygenates per gram Pd per hour) and at the same time high oxygenates selectivity of up to 95 %. Control experiments and mechanistic studies revealed that PdO nanoparticles promoted the in situ generation of H2O2 from O2 and H2, while Cu single atoms not only accelerated the activation of H2O2 for the generation of abundant hydroxyl radicals (⋅OH) from H2O2 decomposition, but also enabled the homolytic cleavage of CH4 by ⋅OH to methyl radicals (⋅CH3). Subsequently, the ⋅OH reacted quickly with the ⋅CH3 to form CH3OH with high selectivity.

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“…In PCA processing, the maximum variance among samples is set as the objective function, and then the feature vectors representing all the samples will be obtained. After that, the samples will be projected to the new feature vector for the preliminary judgment of origin identification [61]. Figure 4a visualizes the first three PC scores of 417 chestnut clusters into three-dimensional scatter plots.…”

Section: Pca Score Plotmentioning

confidence: 99%

Identification of Geographical Origin of Chinese Chestnuts Using Hyperspectral Imaging with 1D-CNN Algorithm

Jiang

et al. 2021

Agriculture

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The adulteration in Chinese chestnuts affects the quality, taste, and brand value. The objective of this study was to explore the feasibility of the hyperspectral imaging (HSI) technique to determine the geographical origin of Chinese chestnuts. An HSI system in spectral range of 400–1000 nm was applied to identify a total of 417 Chinese chestnuts from three different geographical origins. Principal component analysis (PCA) was preliminarily used to investigate the differences of average spectra of the samples from different geographical origins. A deep-learning-based model (1D-CNN, one-dimensional convolutional neural network) was developed first, and then the model based on full spectra and optimal wavelengths were established for various machine learning methods, including partial least squares-discriminant analysis (PLS-DA) and particle swarm optimization-support vector machine (PSO-SVM). The optimal results based on full spectra for 1D-CNN, PLS-DA, and PSO-SVM models were 97.12%, 97.12%, and 95.68%, respectively. Competitive adaptive reweighted sampling (CARS) and a successive projections algorithm (SPA) were individually utilized for wavelengths selection, and the results of simplified models generally improved. The contrasting results demonstrated that the prediction accuracies of SPA-PLS-DA and 1D-CNN both reached 97.12%, but 1D-CNN presented a higher Kappa coefficient value than SPA-PLS-DA. Meanwhile, the sensitivities and specificities of SPA-PLS-DA and 1D-CNN models were both above 90% for the samples from each geographical origin. These results indicated that both SPA-PLS-DA and 1D-CNN models combined with HSI have great potential for the geographical origin identification of Chinese chestnuts.

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Elucidating the mechanism of heterogeneous Wacker oxidation over Pd-Cu/zeolite Y by transient XAS

Cited by 30 publications

References 41 publications

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

Solvation and Mobilization of Copper Active Sites in Zeolites by Ammonia: Consequences for the Catalytic Reduction of Nitrogen Oxides

Tandem Catalysis for Selective Oxidation of Methane to Oxygenates Using Oxygen over PdCu/Zeolite

Identification of Geographical Origin of Chinese Chestnuts Using Hyperspectral Imaging with 1D-CNN Algorithm

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