1 3
Topics in Current Chemistry (2019) 377:24
acetaldehyde using visible light [86] as well as the production hydrogen from biomass molecules using Sn-grafted Ru:TiO 2 photo-catalysts [87]. A more elaborated analysis considers the measurement of the rate of radical (hydroxyl) species normalized per rate of photon absorption [83]. Such an observable can be
obtained from joined EPR and optical measurements and is shown to be proportional to the ratio between the rate constants of creation of hydroxyl-type radicals and the electron–hole recombination process. This is a measure of the rate at
which such species are available at the surface of the photo-catalyst. In Fig. 12,
this observable (called EPR parameter) is conformed against the reaction rate for
different series of catalysts. An additional measurement of the rate of hydroxyl
availability at the surface using a kinetic approach is also conformed (so-called
kinetic parameter) with the reaction rate in Fig. 12. The two different experimental measurements allow to validate each other and, at the same time, render a
robust procedure to check if hydroxyl-radical species can be kinetically significant in photo-catalytic reactions. The application of this methodology in Fig. 12
concerns the use of binary ceria–titania and carbon nitride–titania materials to
investigate UV and sunlight triggered photo-oxidation of acetaldehyde [88] and
toluene [89, 90], as well as quaternary MnOx-carbon nitride/ceria-titania powders used in toluene photo-elimination [91]. Both UV and visible illumination
conditions are considered. Results in Fig. 12 show that binary and quaternary
titania-based materials and their (single phase or component) counterparts display linear relationships between the rate of surface hydroxyl availability and
the reaction rate, yielding a demonstration that the mechanism is hole-mediated
and that composite materials has physical effects influencing activity by increasing the hydroxyl-related rate. This provides experimental evidence of the type of
mechanism and informs about the potential key kinetic steps.
The study of intermediates and, in general, of the reaction mechanism is typically followed with infrared spectroscopic using micro-kinetic schemes. The
photo-oxidation of 2-propanol is a frequent subject, showing always the production of acetone as a key intermediate [92]. The surface of titania-based catalysts has a rather high coverage of 2-propanol, which limits the reaction rate
and also complicates the progression of the reaction from acetone to generate further oxidized carbon-containing species [60, 93, 94]. Figure 13 displays
results concerning a thorough analysis of the photo-oxidation of acetaldehyde
using titania catalysts having or not sulfate surface groups [95]. Panels A1 and
A2 exemplify the evolution of the IR signal corresponding to the target pollutant in a step-wise, cycled experiment consisting in gas dosing, purging in synthetic air, and UV irradiation. A drifting background (as a consequence of the
conduction band electrons) was subtracted to isolate the IR peaks, which were
subsequently modeled within a micro-kinetic analysis. Acetaldehyde, crotonaldehyde (depending on the surface properties of titania, only appearing in absence
of sulfate species), other intermediates such as formates and CO 2 (panels C1 and
C2 of Fig. 13) were analyzed using a (sequential) mechanistic scheme such as
CH 3 CHO ads → CH 3 (CH) 2 CHO ads → HCOO ads → CO 2 . The modeling describes
the spontaneous formation of crotonaldehyde in the bare titania surface, which is
hindered by interaction of the acetaldehyde with sulfate groups. Such groups also
185
Reprinted from the journal
Topics in Current Chemistry (2019) 377:24
acetaldehyde using visible light [86] as well as the production hydrogen from biomass molecules using Sn-grafted Ru:TiO 2 photo-catalysts [87]. A more elaborated analysis considers the measurement of the rate of radical (hydroxyl) species normalized per rate of photon absorption [83]. Such an observable can be
obtained from joined EPR and optical measurements and is shown to be proportional to the ratio between the rate constants of creation of hydroxyl-type radicals and the electron–hole recombination process. This is a measure of the rate at
which such species are available at the surface of the photo-catalyst. In Fig. 12,
this observable (called EPR parameter) is conformed against the reaction rate for
different series of catalysts. An additional measurement of the rate of hydroxyl
availability at the surface using a kinetic approach is also conformed (so-called
kinetic parameter) with the reaction rate in Fig. 12. The two different experimental measurements allow to validate each other and, at the same time, render a
robust procedure to check if hydroxyl-radical species can be kinetically significant in photo-catalytic reactions. The application of this methodology in Fig. 12
concerns the use of binary ceria–titania and carbon nitride–titania materials to
investigate UV and sunlight triggered photo-oxidation of acetaldehyde [88] and
toluene [89, 90], as well as quaternary MnOx-carbon nitride/ceria-titania powders used in toluene photo-elimination [91]. Both UV and visible illumination
conditions are considered. Results in Fig. 12 show that binary and quaternary
titania-based materials and their (single phase or component) counterparts display linear relationships between the rate of surface hydroxyl availability and
the reaction rate, yielding a demonstration that the mechanism is hole-mediated
and that composite materials has physical effects influencing activity by increasing the hydroxyl-related rate. This provides experimental evidence of the type of
mechanism and informs about the potential key kinetic steps.
The study of intermediates and, in general, of the reaction mechanism is typically followed with infrared spectroscopic using micro-kinetic schemes. The
photo-oxidation of 2-propanol is a frequent subject, showing always the production of acetone as a key intermediate [92]. The surface of titania-based catalysts has a rather high coverage of 2-propanol, which limits the reaction rate
and also complicates the progression of the reaction from acetone to generate further oxidized carbon-containing species [60, 93, 94]. Figure 13 displays
results concerning a thorough analysis of the photo-oxidation of acetaldehyde
using titania catalysts having or not sulfate surface groups [95]. Panels A1 and
A2 exemplify the evolution of the IR signal corresponding to the target pollutant in a step-wise, cycled experiment consisting in gas dosing, purging in synthetic air, and UV irradiation. A drifting background (as a consequence of the
conduction band electrons) was subtracted to isolate the IR peaks, which were
subsequently modeled within a micro-kinetic analysis. Acetaldehyde, crotonaldehyde (depending on the surface properties of titania, only appearing in absence
of sulfate species), other intermediates such as formates and CO 2 (panels C1 and
C2 of Fig. 13) were analyzed using a (sequential) mechanistic scheme such as
CH 3 CHO ads → CH 3 (CH) 2 CHO ads → HCOO ads → CO 2 . The modeling describes
the spontaneous formation of crotonaldehyde in the bare titania surface, which is
hindered by interaction of the acetaldehyde with sulfate groups. Such groups also
185
Reprinted from the journal