XAS and DFT investigation of atomically dispersed Cu/Co alloyed Pt local structures under selective hydrogenation of acetylene to ethylene

Abstract

In this study, we focused on the semihydrogenation of acetylene (SHA) in an ethylene-rich stream to produce clean ethylene (devoid of acetylene that poisons the ethylene polymerization catalysts) as an incentive to increase polymer production. Two alloyed Pt catalysts namely PtCu and PtCo were synthesized via functionalization with oxalate ligands and supported on ZSM-5 (PtCuOx/Z and PtCoOx/Z), characterized, and tested for the SHA under a typical industrial condition. While there are instances of single-atom Pt alloyed with Cu in the PtCuOx/Z catalyst, nanoclusters of alloyed Pt and Co prevailed in PtCoOx/Z according to the EXAFS and HAADF-STEM results. Threshold energies (E0) of 11566.8 and 11563.8 eV obtained from the first derivative of the XANES spectra were observed for PtCuOx/Z and PtCoOx/Z, respectively under experimental conditions. Accordingly, the high E0 implied a higher unoccupied density of Pt d-orbital which is a key indicator for enhanced SHA activity leading to a higher ethylene yield (YC2H4) of 87.6 % over PtCuOx/Z compared to 62.9 % over PtCoOx/Z at 200 °C. These results were confirmed by a DFT computation which showed that Pt in the catalysts contains more electrons in PtCuOx/Z than in PtCoOx/Z, thus shifting the d-band center downward of the Fermi level in the former than the latter which allows only a weakly π-bonded C2H4 that desorbs before the secondary reactions leading to high YC2H4, while PtCoOx/Z favor higher YC2H6. This result showed that alloying Pt with Cu is more promising in synthesizing industrial relevant SHA catalyst.