Topics in Current Chemistry (2019) 377:24
1 3
In Fig. 3 we plot results of the e
a,s
variation in perpendicular to the solid surface corresponding to three broadly used photo-catalysts, commercial P25 (ca. 80%
anatase and 20% rutile), pure anatase and graphitic carbon nitride g-C 3 N 4 using a
typical UV illumination source (250–450-nm range, flat intensity) and power (10
mW cm
−2
). Results shown the two extreme values of the absorption coefficient
reported in literature for each of the above mentioned solids [35, 36]. It can be
observed that for low-density powder materials or films, light is essentially absorbed
within the “first” 2 or 3 µm, while in dense powder materials or films it is essentially
confined to the first micron.
For a liquid phase (solid suspension) process, we need to calculate the volumetric
rate of photon absorption ( e
a,v
) at each position of the reaction volume [37]:
The e
a,v
requires solving the radiative transfer equation (RTE) to obtain I λ,Ω (now
the intensity at each point of the in situ cell volume) and the measurement of the
spectral absorption coefficient ( ). Solving the RTE in turn requires the complete
knowledge of the optical properties of the catalyst suspension at the liquid phase;
the spectral absorption coefficient ( ), the spectral scattering coefficient ( ), and
the scattering phase function ( (p(
� → )) ) [37]. The liquid phase differs radically from the gas phase in the point that (liquid phase) scattering of light can occur
within all spatial directions and thus the intensity at each position of the cell cannot be obtained in a straightforward way from the illumination source (and solid)
properties.
In Fig. 4, we plot the e
a,v
decay from an illuminated surface (“light” progressing
along the OX coordinate) in a rectangular cell having two (typical) parallel windows
for spectroscopy and variable distance between them (the OX coordinate). Light
spectral distribution and intensity for on-top illumination are equal to those used in
Fig. 3. To illustrate the situation, four solids, Degussa P25, pure anatase, g-C 3 N 4 , as
well as SnS 2 (all typical photo-catalytic semiconductors) were used. For each solid,
four concentrations (from 0.1 to 2 g l
−1
) are presented, covering the concentrations
typically used in experimental catalytic experiments. Figure 4 shows than the different optical properties of the solids makes than even at relatively low concentrations
(0.5 g l
−1
) some of the samples (P25 and SnS 2 ) render a significant decrease of light.
In any case, for all samples, the optimal concentration of the samples is around 1 g
l
−1
and the decay profile shows that almost all photons are utilized well below 1 mm
away from the illumination surface.
Summarizing, to perform operando or spectro-kinetic studies, we have to be very
careful with the volume and path probe by the beam of each spectroscopy. They
need to match the one fully illuminated. Light penetration is, as a rule of thumb, in
the order of a few (typically less than 5) microns in the gas phase and well below
1 mm in the liquid phase. Here we will describe experiments taking into consideration this point (with specificities for each experimental technique) or mention
the potential weaknesses of others that did not take this point fully into account. Of
course, such a point has peculiarities specific for each spectroscopy, which will be
detailed in subsequent sections.
(2)
e
a,v = ∫
⋅ ∫
=4
I ,Ω
x,
dd
172
Reprinted from the journal
1 3
In Fig. 3 we plot results of the e
a,s
variation in perpendicular to the solid surface corresponding to three broadly used photo-catalysts, commercial P25 (ca. 80%
anatase and 20% rutile), pure anatase and graphitic carbon nitride g-C 3 N 4 using a
typical UV illumination source (250–450-nm range, flat intensity) and power (10
mW cm
−2
). Results shown the two extreme values of the absorption coefficient
reported in literature for each of the above mentioned solids [35, 36]. It can be
observed that for low-density powder materials or films, light is essentially absorbed
within the “first” 2 or 3 µm, while in dense powder materials or films it is essentially
confined to the first micron.
For a liquid phase (solid suspension) process, we need to calculate the volumetric
rate of photon absorption ( e
a,v
) at each position of the reaction volume [37]:
The e
a,v
requires solving the radiative transfer equation (RTE) to obtain I λ,Ω (now
the intensity at each point of the in situ cell volume) and the measurement of the
spectral absorption coefficient ( ). Solving the RTE in turn requires the complete
knowledge of the optical properties of the catalyst suspension at the liquid phase;
the spectral absorption coefficient ( ), the spectral scattering coefficient ( ), and
the scattering phase function ( (p(
� → )) ) [37]. The liquid phase differs radically from the gas phase in the point that (liquid phase) scattering of light can occur
within all spatial directions and thus the intensity at each position of the cell cannot be obtained in a straightforward way from the illumination source (and solid)
properties.
In Fig. 4, we plot the e
a,v
decay from an illuminated surface (“light” progressing
along the OX coordinate) in a rectangular cell having two (typical) parallel windows
for spectroscopy and variable distance between them (the OX coordinate). Light
spectral distribution and intensity for on-top illumination are equal to those used in
Fig. 3. To illustrate the situation, four solids, Degussa P25, pure anatase, g-C 3 N 4 , as
well as SnS 2 (all typical photo-catalytic semiconductors) were used. For each solid,
four concentrations (from 0.1 to 2 g l
−1
) are presented, covering the concentrations
typically used in experimental catalytic experiments. Figure 4 shows than the different optical properties of the solids makes than even at relatively low concentrations
(0.5 g l
−1
) some of the samples (P25 and SnS 2 ) render a significant decrease of light.
In any case, for all samples, the optimal concentration of the samples is around 1 g
l
−1
and the decay profile shows that almost all photons are utilized well below 1 mm
away from the illumination surface.
Summarizing, to perform operando or spectro-kinetic studies, we have to be very
careful with the volume and path probe by the beam of each spectroscopy. They
need to match the one fully illuminated. Light penetration is, as a rule of thumb, in
the order of a few (typically less than 5) microns in the gas phase and well below
1 mm in the liquid phase. Here we will describe experiments taking into consideration this point (with specificities for each experimental technique) or mention
the potential weaknesses of others that did not take this point fully into account. Of
course, such a point has peculiarities specific for each spectroscopy, which will be
detailed in subsequent sections.
(2)
e
a,v = ∫
⋅ ∫
=4
I ,Ω
x,
dd
172
Reprinted from the journal