From Isolated Ag⁺Ions to Aggregated Ag⁰Nanoclusters in Silver-Exchanged Engelhard Titanosilicate (ETS-10) Molecular Sieve: Reversible Behavior

Abstract

Recently, it has been shown that Engelhard titanosilicate (ETS-10), a crystalline microporous titanosilicate, is an inverse-shape-selective photocatalyst. The main drawback in the extensive use of this material was its band gap value, located in the UV, that does not allow the use of solar light. In this work, we succeed in shifting the ETS-10 light absorption down to the visible region of the electromagnetic spectrum by introducing Ag + cations inside the ETS-10 channels. Thermal-, chemical-, and UVphototreatments have been applied to Ag-ETS-10 to tune, in a controlled and progressive way, the aggregation of isolated Ag + counterions into metal nanoclusters of increasing nuclearity. This is a direct means to tune the frequency of silver nanoparticle plasmon resonance and thus the light absorption properties of the material. It is further reported that transformation of isolated Ag + ions into aggregated Ag 0 nanoclusters is almost reversible. Cycles of H 2 -chemical reduction/O 2 -chemical oxidation and of UV-photoreduction/vis-photo-oxidation are reported. UV-vis, Ag K-edge extended X-ray absorption fine structure (EXAFS), and Fourier transform infrared (FTIR) spectroscopy of adsorbed CO are the main techniques used to monitor the evolution of the silver aggregation along the different reduction/ oxidation treatments.

Artificial Photosynthesis

“…Recently very interesting observations have been reported on silver exchanged titanosilicates which might become of importance for this research [80]. We also mention a plasmonic photocatalyst consisting of silver nanoparticles embedded in titanium dioxide [81].…”

Integrating efficient filtration and visible-light photocatalysis by loading Ag-doped zeolite Y particles on filtration membrane of alumina nanofibers

“…Small Ag NPs stabilized with the aid of zeolites are active sites of electrontrapping and play a significant role in the photocatalytic and photochemical reactions [11]. Recently, the surface plasmon resonance of silver nanoparticles was employed to develop plasmonic photocatalysts because of their strong absorption of visible light [12][13][14]. Hence, it is possible to utilize sunlight for this target, as visible light (wavelength 400-800 nm) constitutes around 43% of the solar energy received by the earth.…”

Ti‐Based Catalysts and Photocatalysts: Characterization and Modeling

“…Also reported are the spectra of the ETS‐10 sample reduced with NaN 3 (red curve) and that of Ag‐ETS‐10 reduced in H 2 (gray curve). Previously unpublished figure reporting spectra published in Refs . c) HRFD Ti K‐edge XANES spectra of ETS‐10 and of treated ETS‐10.…”

“…Nevertheless, the quantum confinement shifts the band gap in the opposite direction with respect to what is desirable to exploit a large fraction of the solar spectrum. Several routes have been exploited to overcame this drawback, such as: (i) the adsorption of selected dyes which HOMO‐LUMO levels properly matching that of the ‐Ti−O−Ti−O−Ti‐ semiconductor conduction band; (ii) the reduction of a fraction of Ti 4+ atoms of the chain to Ti 3+ via reaction with NaN 3 ; or (iii) the encapsulation of small Ag nanoparticles (NPs) inside the ETS‐10 channels . By exploiting the cation exchange properties of the material, Ag + ‐ETS‐10 has been obtained following a two‐step process: first the protonic form (H‐ETS‐10) has been obtained from the Engelard Na + −K + ‐ETS‐10 by exchanging Na + and K + cations with NH 4 + and successive calcination.…”