Browsing by Author "Kuo, Chun-Te"

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    18.1% single palladium atom catalysts on mesoporous covalent organic framework for gas phase hydrogenation of ethylene
    Kuo, Chun-Te; Lu, Yubing; Arab, Pezhman; Weeraratne, K. Shamara; El-Kaderi, Hani; Karim, Ayman M. (2021-07-21)
    Noble metal single-atom catalysts maximize metal utilization and offer opportunities to design heterogeneous catalysts at the molecular scale. Mesoporous covalent organic frameworks provide an ideal support to stabilize metal single atoms with specific ligand configuration similar to a homogeneous catalyst In this work, a high loading of single Pd atoms, 18.1 wt %, on mesoporous imine-linked covalent organic framework was synthesized, characterized, and evaluated for ethylene hydrogenation. X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and diffuse-reflectance infrared Fourier transform spectroscopy of adsorbed CO demonstrate that the Pd is atomically dispersed with a highly homogeneous local coordination. The Pd single atoms are active for hydrogenation of ethylene to ethane at room temperature. The study demonstrates that mesoporous COFs provide a large number of identical metal binding sites that are good candidates for immobilizing metal single atoms and their use in gas-phase catalytic applications.
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    Elucidating the Effect of Ion Exchange Protocol on the Copper Exchange Efficacy, Copper Siting, and SCR activity in Cu-SSZ-13
    Koishybay, Aibolat; Umhey, Charles; Kuo, Chun-Te; Groden, Kyle; McEwen, Jean-Sabin; Karim, Ayman M.; Shantz, Daniel F. (Wiley-VCH, 2023-05)
    The influence of the copper ion exchange protocol on SCR activity of SSZ-13 is quantified. Using the same parent SSZ-13 zeolite, four exchange protocols are used to assess how exchange protocol impacts metal uptake and SCR activity. Large differences in the SCR activity, nearly 30 percentage points at 160 degrees C at constant copper content, are observed for different exchange protocols implying that different exchange protocols lead to different copper species. Hydrogen temperature programmed reduction on selected samples and infrared spectroscopy of CO binding corroborates this conclusion as the reactivity at 160 degrees C correlates with the intensity of the IR band at 2162 cm(-1). DFT-based calculations show that such an IR assignment is consistent with CO adsorbed on a Cu(I) cation within an eight-membered ring. This work shows that SCR activity can be influenced by the ion exchange process even when different protocols lead to the same metal loading. Perhaps most interesting, a protocol used to generate Cu-MOR for methane to methanol studies led to the most active catalyst both on a unit mass or unit mole copper basis. This points to a yet not recognized means to tailor catalyst activity as the open literature is silent on this issue.
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    Structure Sensitivity in the Subnanometer Regime on Pt and Pd Supported Catalysts
    Kuo, Chun-Te (Virginia Tech, 2020-10-29)
    Single-atom and cluster catalysts have been receiving significant interest due to not only their capability to approach the limit of atom efficiency but also to explore fundamentally unique properties. Supported Pt-group single atoms and clusters catalysts in the subnanometer size regime maximize the metal utilization and were reported to have extraordinary activities and/or selectivities compared with nanoparticles for various reactions including hydrogenation reactions. However, the relationship between metal nuclearity, electronic and their unique catalytic properties are still unclear. Thus, it is crucial to establish their relations for better future catalyst design. Ethylene hydrogenation and acetylene hydrogenation are two important probe reactions with the simplest alkene and alkyne, and they have been broadly studied as the benchmark reactions on the various catalyst systems. However, the catalytic properties and reaction mechanism of those hydrogenation reactions for metal nuclearitiy in the subnanometer regime is still not well understood. In this study, we applied different characterization techniques including x-ray absorption fine structure (XAFS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy(XPS), diffuse reflectance infrared spectroscopy (DRIFTS), calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the structure of Pt/TiO2 and Pd/COF single-atom catalysts and tested their catalytic properties for hydrogenation reactions. In order to develop such relations, we varied the nuclearity of Pt supported on TiO2 from single atoms to subnanometer clusters to larger nanoparticles. For acetylene hydrogenation, Pt in the subnanometer size regime exhibits remarkably high selectivity to ethylene compared to its nanoparticle counterparts. The high selectivity is resulted from the decreased electron density on Pt and destabilization of C2H4, which were rationalized by X-ray photoelectron spectroscopy and calorimetry results. On the other hand, the activity of H2 activation and acetylene hydrogenation decreased as Pt nuclearity decreased. Therefore, our results show there's a trade-off between activity and selectivity for acetylene hydrogenation. Additionally, the kinetics measurements of ethylene hydrogenation and acetylene hydrogenation were performed on Pt/TiO2 catalysts, and they found to be structure sensitive for both reactions, which the reaction orders and activation energy changes as particles size change. The activity of ethylene hydrogenation decreases, and activation energy increase from 43 to 86 kJ/mol, as Pt nuclearity decreased from an average size of 2.1 nm to 0.7 nm and single atoms. The reaction orders in hydrocarbons (ethylene and acetylene) were less negative on subnanometer clusters and single atoms in contract to nanoparticles. The results imply that hydrocarbons, ethylene and acetylene species, do not poison the catalyst on Pt in the subnanometer size regime, and hydrogen activation turn to competitive adsorption path with surface hydrocarbons species. Moreover, single atom Pd supported on imine-linked covalent organic framework was synthesized, characterized by a various of techniques including X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO, and evaluated its catalytic properties for ethylene hydrogenation. The XAS results show that Pd atoms are isolated and stabilized by two covalent Pd–N and Pd-Cl bonds. DRIFTS of CO adsorption shows a sharp symmetrical peak at 2130 cm−1. The Pd single atoms are active for hydrogenation of ethylene to ethane at room temperature. The reaction orders in C2H4 and H2 were 0.0 and 0.5 suggesting that ethylene adsorption is not limiting while hydrogen forms on Pd through dissociative adsorption.