Abstract
Combining advantages of homogeneous and heterogeneous catalysis by incorporating active species on a solid support is often an effective strategy for improving overall catalyst performance, although the influences of the support are generally challenging to establish, especially at a molecular level. Here, we report the local compositions, and structures of platinum species incorporated into covalent triazine framework (Pt-CTF) materials, a solid analogue of the molecular Periana catalyst, Pt(bpym)Cl2, both of which are active for the selective oxidation of methane in the presence of concentrated sulfuric acid. By using a combination of solid-state 195Pt nuclear magnetic resonance (NMR) spectroscopy, aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), important similarities and differences are observed between the Pt-CTF and Periana catalysts, which are likely related to their respective macroscopic reaction properties. In particular, wide-line solid-state 195Pt NMR spectra enable direct measurement, identification, and quantification of distinct platinum species in as-synthesized and used Pt-CTF catalysts. The results indicate that locally ordered and disordered Pt sites are present in as-synthesized Pt-CTF, with the former being similar to one of the two crystallographically distinct Pt sites in crystalline Pt(bpym)Cl2. A distribution of relatively disordered Pt moieties is also present in the used catalyst, among which are the principal active sites. Similarly XAS shows good agreement between the measured data of Pt-CTF and a theoretical model based on Pt(bpym)Cl2. Analyses of the absorption spectra of Pt-CTF used for methane oxidation suggests ligand exchange, as predicted for the molecular catalyst. XPS analyses of Pt(bpym)Cl2, Pt-CTF, as well as the unmodified ligands, further corroborate platinum coordination by pyridinic N atoms. Aberration-corrected high-angle annular dark-field STEM proves that Pt atoms are distributed within Pt-CTF before and after catalysis. The overall results establish the close similarities of Pt-CTF and the molecular Periana catalyst Pt(bpym)Cl2, along with differences that account for their respective properties. (Figure Presented).
Original language | English |
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Pages (from-to) | 2332-2340 |
Number of pages | 9 |
Journal | ACS Catalysis |
Volume | 6 |
Issue number | 4 |
DOIs | |
State | Published - Apr 1 2016 |
Funding
The solid-state 195Pt NMR measurements were supported by the U.S. National Science Foundation, under Grant No. MSNCHE-1059108, and were conducted using the Central Facilities of the UCSB Materials Research Laboratory, which was supported by the MRSEC Program of the National Science Foundation (under Award No. DMR-1121053). L.C.J. thanks the NSF-IOSE-PIRE Program (No. 0968399) and the NSF ConvEne IGERT Program (No. NSF-DGE 0801627) for fellowship support. XAS characterization was carried out at beamline C, HASYLAB at DESY, Hamburg. The authors are grateful for Dr. E. Welter for support. TEM work was performed through a user project supported by ORNL's Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Financial support by the "ENERCHEM" project house of the Max Planck Society is gratefully acknowledged. Part of this cooperation was initiated within the framework of a Max Planck Society-UCSB research partnership. R.P. acknowledges financial support from the DFG (PA1689/1-1) and Aachen-California Network of Academic Exchange (ACalNet) supported by the DAAD and financed by the German Federal Ministry of Education and Research. T.Z. is grateful for a Kekulé scholarship of the Fonds der Chemischen Industrie.
Keywords
- Periana catalyst
- atomic dispersion
- methane oxidation
- solid analogue vs molecular catalyst
- solid-state Pt NMR