Highly ordered mesoporous silica film nanocomposites containing gold nanoparticles for the catalytic reduction of 4-nitrophenol

Abstract

We report that transparent mesostructured silica/gold nanocomposite materials with an interpore distance of 4.1 nm, as-synthesized from a templated sol-gel synthesis method using discotic trinuclear gold(I) pyrazolate complex, were successfully utilized for the fabrication of thin film mesoporous silica nanocomposites containing gold nanoparticles. The material exhibited a highly ordered hexagonal structure when subjected to a thermal hydrogen reduction treatment at 210 °C. In contrast, when the material was subjected to calcination as a heat treatment from 190 to 450 °C, the thin film nanocomposites showed an intense d ₁₀₀ X-ray diffraction peak. Moreover, gold nanoparticles inside the thin film nanocomposites were confirmed by the presence of the d ₁₁₁ diffraction peak at 2θ = 38.2°, a surface plasmon resonance peak between 500-580 nm, and the spherical shape observed in the transmission electron microscope images, as well as the visual change in color from pink to purple. Interestingly, by simply dipping the material into a reaction solution of 4-nitrophenol at room temperature, the highly ordered structure of the as-fabricated silica/gold nanoparticle thin film composite after thermal hydrogen reduction at 210 °C resulted in an improved catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol compared to the material calcined at 250 °C. Such catalytic activity is due to the presence of gold nanoparticles of smaller size in the silicate channels of the highly ordered mesoporous film nanocomposites.

Keywords: 4-nitrophenol reduction; catalyst; gold nanoparticle; mesoporous silica; nanocomposite; thermal hydrogen reduction.

Figure 1

Schematic for the synthesis of [Au₃Pz₃]_C10TEG from C₁₀TEGPzH and the fabrication of [Au₃Pz₃]_C10TEG/silica_hex through a sol–gel synthesis with tetrabutyl orthosilicate (TBOS) as a silica source, followed by the synthesis of [AuNPs]_cal/silica_hex or [AuNPs]_red/silica_hex by calcination or thermal hydrogen reduction. The synthetic scheme is adapted from [25].

Figure 2

TGA thermogram of [Au₃Pz₃]_C10TEG.

Figure 3

a) XRD diffractogram; the inset is a photograph of the material. b) TEM image; the inset shows the FFT (top) and auto-correlation images (bottom) of [Au₃Pz₃]_C10TEG/silica_hex.

Figure 4

XRD patterns in the small-angle region of a) [AuNPs]_cal/silica_hex films after calcination and b) [AuNPs]_red/silica_hex films after the thermal hydrogen reduction at (a) 190, (b) 210, (c) 230, (d) 250 and (e) 450 °C, where the inset graphs shown the XRD peaks of the maximum intensity of d₁₀₀ versus temperature.

Figure 5

TEM images of a) [AuNPs]_cal/silica_hex films at 250 and b) 450 °C and as well as c) [AuNPs]_red/silica_hex films at 210 and d) 250 °C. The insets show the corresponding FFT (top) and auto-correlation images (bottom).

Figure 6

XRD patterns for the wide-angle region of a) [AuNPs]_cal/silica_hex and b) [AuNPs]_red/silica_hex at (a) 190, (b) 210, (c) 230, (d) 250 and (e) 450 °C.

Figure 7

a) TEM images of [AuNPs]_cal/silica_hex films at 250 and b) 450 °C as well as c) [AuNPs]_red/silica_hex films at 210 and d) 250 °C. The insets show the corresponding FFT patterns (top) and auto-correlation images (bottom).

Figure 8

Absorption spectra of a) [AuNPs]_cal/silica_hex and b) [AuNPs]_red/silica_hex films at (a) 190, (b) 210, (c) 230, (d) 250 and (e) 450 °C. The insets show representative photography images for the as-synthesized films after both heat treatments.

Figure 9

UV–vis absorption spectral changes for the reduction of 4-NP at 20 min intervals over a) [AuNPs]_cal/silica_hex film at 250 °C with b) a corresponding graph of ln A/A₀ versus time within 160 minutes. c) The [AuNPs]_red/silica_hex film at 210 °C with d) a corresponding graph of ln A/A₀ versus time within 140 minutes.