Evolution of Pt and Pd species in functionalized UiO-67 metal-organic frameworks

Abstract

Functionalization of metal-organic frameworks (MOFs) with noble metals is a promising way for producing new versatile catalysts that will combine the outstanding porosity and specific surface area of MOFs with high catalytic activity of metals. Here, we present a comparative study of two metal-organic frameworks with UiO-67 topology, functionalized with palladium and platinum moieties. The initial structure of all studied samples contained palladium or platinum atoms grafted into MCl2bpydc (M = Pd, Pt) linkers of MOFs. The materials were further activated by heating in inert and H2-containing atmospheres. Both Pd- and Pt- functionalized materials exhibited high thermal stability upon heating in these atmospheres. The evolution of Pt and Pd species during the activation procedure was monitored by in situ time-resolved X-ray absorption near-edge structure (XANES) spectroscopy. We applied multivariate curve resolution alternating least squares (MCR-ALS) approach to XANES to unravel the intermediates which can be formed during the activation procedure. For UiO-67-Pd, only simple one-step transformation from PdCl2bpydc to Pd nanoparticles (NPs) was observed. For UiO-67-Pt, two additional intermediate states were observed, which behave differently depending on the activation procedure. Theoretical calculation of XANES spectra allowed us to suggest the 3D-atomic structures corresponding to each of the pure spectra determined by MCR-ALS. In addition, reaction enthalpies for different possible reaction routes were calculated within a density functional theory approach. Based on the experimental and theoretical results showed that Pd nanoparticles (NPs) tend to be formed in UiO-67-Pd samples irrespective of the activation procedure, while either Pt NPs or isolated PtII active sites, grafted in the MOF framework may be formed in UiO-67-Pt samples depending on the activation temperature and atmosphere.