NSA ¼ SA catalyst =ð1 À yÞ Á 1=SA MCM
ð7Þ
where SA is the specific surface area of the parent Al-MCM-41 or the metal oxide/
Al-MCM-41 composite, and y is the weight fraction of metal oxides in the catalyst.
In other words, NSA reflects the surface area per gram of MCM. If the active
component is distributed at the support surface in the form of a close-packed
monolayer of a given thickness (no pore blocking occurs), the NSA decreases only
as a result of narrowing of the support pores. For example, for a 45 wt% MoO 3
phase loaded on a wide-pore Al-MCM-41 support, the reduction of pore diameter
from 8.3 to 7.7 nm yields an NSA value of 0.93. The results shown in Figure 6.6
speak for themselves, showing that only a slight reduction in the NSA occurs when
sonochemistry is used for the insertion of nanoparticles, or in other words, the
loading of the MSP by large amounts of nanoparticles reduces the SA only slightly,
whereas when other methods are used a considerable reduction in the SA is observed.
The remaining question is what is the role of the ultrasound radiation in the insertion of the nanoparticles into the mesopores? It is clear that the bubble cannot
collapse inside the mesopores because the size that the bubble reaches before its
collapse is estimated to be about 100 mm [105], while the pore diameter is less than
3 nm. Instead, we propose two possible mechanisms. The first is based on microjets and shock waves that result when a bubble collapses near a solid surface. As
already explained, near a solid surface the collapse drives high-speed jets of liquid
into the surface. Since most of the energy is transferred to the accelerating jet, the
jet can reach velocities of hundreds of meters per second. In our case the small
nanoparticles are pushed by these jets into the mesopores and, as a result of their
reaction with the mesoporous support, are anchored to the inner surface of the
mesoporous material. The other possibility is that the solution inside the pores
undergoes a chemical reaction initiated by shock waves forming nanoparticles. The
nanosized products interact with silanols, forming the chemical bonds described
above. The second mechanism sounds a better explanation since it accounts for the
homogeneous spreading of the nanoparticles in the pores.
Another active group in this field is that of Chen and Zhang. In their first paper
[106] they reported on a sonochemical procedure at room temperature for the
preparation of gold nanoparticles loaded in mesoporous silica. They show that the
Au nanoparticles, with a mean size of 3–4 nm in diameter and a fairly narrow size
distribution, are dispersed uniformly within the pores of the silica host. In a second paper they measured the optical absorption of the Au nanoparticles dispersed
within ports of monolithic mesoporous silica after subsequent annealing treatment
[107]. Charge transfer at the interfaces between Au nanoparticles and pore walls is
introduced qualitatively to discuss the red-shift of Mie resonance absorption band
with decreasing the Au particle size. A longer paper [108] gives more details on the
sonochemical reduction of chloroauric acid (HAuCl 4 ) within the pores of silica.
This reduction leads to the insertion of gold nanoparticles into mesoporous silica.
The paper also reports on the optical measurements. In another publication related
to gold inserted into MSP silica [109], they present an HR-TEM showing nearly
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
140
ð7Þ
where SA is the specific surface area of the parent Al-MCM-41 or the metal oxide/
Al-MCM-41 composite, and y is the weight fraction of metal oxides in the catalyst.
In other words, NSA reflects the surface area per gram of MCM. If the active
component is distributed at the support surface in the form of a close-packed
monolayer of a given thickness (no pore blocking occurs), the NSA decreases only
as a result of narrowing of the support pores. For example, for a 45 wt% MoO 3
phase loaded on a wide-pore Al-MCM-41 support, the reduction of pore diameter
from 8.3 to 7.7 nm yields an NSA value of 0.93. The results shown in Figure 6.6
speak for themselves, showing that only a slight reduction in the NSA occurs when
sonochemistry is used for the insertion of nanoparticles, or in other words, the
loading of the MSP by large amounts of nanoparticles reduces the SA only slightly,
whereas when other methods are used a considerable reduction in the SA is observed.
The remaining question is what is the role of the ultrasound radiation in the insertion of the nanoparticles into the mesopores? It is clear that the bubble cannot
collapse inside the mesopores because the size that the bubble reaches before its
collapse is estimated to be about 100 mm [105], while the pore diameter is less than
3 nm. Instead, we propose two possible mechanisms. The first is based on microjets and shock waves that result when a bubble collapses near a solid surface. As
already explained, near a solid surface the collapse drives high-speed jets of liquid
into the surface. Since most of the energy is transferred to the accelerating jet, the
jet can reach velocities of hundreds of meters per second. In our case the small
nanoparticles are pushed by these jets into the mesopores and, as a result of their
reaction with the mesoporous support, are anchored to the inner surface of the
mesoporous material. The other possibility is that the solution inside the pores
undergoes a chemical reaction initiated by shock waves forming nanoparticles. The
nanosized products interact with silanols, forming the chemical bonds described
above. The second mechanism sounds a better explanation since it accounts for the
homogeneous spreading of the nanoparticles in the pores.
Another active group in this field is that of Chen and Zhang. In their first paper
[106] they reported on a sonochemical procedure at room temperature for the
preparation of gold nanoparticles loaded in mesoporous silica. They show that the
Au nanoparticles, with a mean size of 3–4 nm in diameter and a fairly narrow size
distribution, are dispersed uniformly within the pores of the silica host. In a second paper they measured the optical absorption of the Au nanoparticles dispersed
within ports of monolithic mesoporous silica after subsequent annealing treatment
[107]. Charge transfer at the interfaces between Au nanoparticles and pore walls is
introduced qualitatively to discuss the red-shift of Mie resonance absorption band
with decreasing the Au particle size. A longer paper [108] gives more details on the
sonochemical reduction of chloroauric acid (HAuCl 4 ) within the pores of silica.
This reduction leads to the insertion of gold nanoparticles into mesoporous silica.
The paper also reports on the optical measurements. In another publication related
to gold inserted into MSP silica [109], they present an HR-TEM showing nearly
6 Sonochemistry and Other Novel Methods Developed for the Synthesis of Nanoparticles
140