1 3
Topics in Current Chemistry (2019) 377:24
the surface or consumed with concomitant formation (positive bands) of carbon
dioxide (Fig. 10c, h). There is also parallel formation of several carboxylate moieties (positive bands at Fig. 10e, j). The more interesting point of the study is
the appearance of carbon monoxide (Fig. 10d, i), which appears strongly correlated with the activity of the materials. Larger intensity of the carbon monoxide
signal(s) leads to larger activity, evidenced by the behavior of the gas-phase carbon dioxide signal(s) concomitantly detected in the infrared spectra. The adsorption of the CO species seems relatively weak, as they decrease under a subsequent dark period. CO is likely formed from the carboxylate species detected by a
water gas-shift type reaction with hydroxyl entities from the support. The positive
or negative role of CO on the reaction depends on its degree of interaction with
the metal, the efficiency of its handling by the metal-support interface (and thus
its removal from the metal surface) but also relates to the nature of the carboxylate (or other) species from which it evolves. The key issue of the study is that the
alcohol molecule is activated by a series of steps (which concomitantly lead to
carbon-containing products and hydrogen ions leading to hydrogen at the metal
surface) up to the production of surface formates and that the evolution of such
species occurs with a critical involvement of the metal–support interface through
a water gas shift-type step. This step would control kinetically the activity of the
two noble metal P25 but also anatase-based noble metal promoted materials [75,
76, 78].
The transformation of carbon dioxide is of importance in the controlling of global
warning. The reduction of this molecule can render useful chemicals such as carbon
monoxide, methane, and, in optimal conditions, methanol, providing a nice route
to synthesize valuable chemicals within the context of a circular economy. Infrared
studies look into such a reaction, with particular emphasis in the analysis of the CO 2
activation step(s) using titania-, carbon nitride-, sulphide-based and others catalysts
[79–82]. As shown in Fig. 11, titania materials are able to adsorb carbon dioxide in
several different forms in the presence of water. Monodentate, bidentate y chelating
brigde carbonates are usually formed but are commonly considered as expectator
species without kinetic significance. Apart from that, the negatively charged CO 2
entity as well as formate ions (also connected with some surface species detected in
Fig. 11) are considered true intermediates of different reaction products formed in
the photo-reduction reaction. In fact, the first is described as directly related to the
production of methane in titania-based materials; a linear relationship was presented
between the intensity of IR bands of such entities (negatively charged carbon dioxide) and the rate of methane production [82]. On the other hand, the second, formate
species is described by a combination of infrared and theoretical results as the intermediate leading to CO production in SnS x -based photo-catalysts [81].
Fig. 10 DRIFTS spectra for the in situ analysis of the methanol photo-transformation occurring under
reaction conditions: upper panel, P25-5Pt (0.5 wt% Pt onto P25 prepared by chemical deposition with
a metal/reductant ratio of 5); lower panel, P25-10Pt (0.5 wt% Pt onto P25 prepared by chemical deposition with a metal/reductant ratio of 10) samples. Difference spectra obtained during saturation with the
alcohol:water mixture (upper zone of each panel), under reaction-illumination conditions (middle part)
and subsequently at dark conditions (bottom part) are presented. Reproduced with permission from Ref.
[75]
▸
183
Reprinted from the journal
Topics in Current Chemistry (2019) 377:24
the surface or consumed with concomitant formation (positive bands) of carbon
dioxide (Fig. 10c, h). There is also parallel formation of several carboxylate moieties (positive bands at Fig. 10e, j). The more interesting point of the study is
the appearance of carbon monoxide (Fig. 10d, i), which appears strongly correlated with the activity of the materials. Larger intensity of the carbon monoxide
signal(s) leads to larger activity, evidenced by the behavior of the gas-phase carbon dioxide signal(s) concomitantly detected in the infrared spectra. The adsorption of the CO species seems relatively weak, as they decrease under a subsequent dark period. CO is likely formed from the carboxylate species detected by a
water gas-shift type reaction with hydroxyl entities from the support. The positive
or negative role of CO on the reaction depends on its degree of interaction with
the metal, the efficiency of its handling by the metal-support interface (and thus
its removal from the metal surface) but also relates to the nature of the carboxylate (or other) species from which it evolves. The key issue of the study is that the
alcohol molecule is activated by a series of steps (which concomitantly lead to
carbon-containing products and hydrogen ions leading to hydrogen at the metal
surface) up to the production of surface formates and that the evolution of such
species occurs with a critical involvement of the metal–support interface through
a water gas shift-type step. This step would control kinetically the activity of the
two noble metal P25 but also anatase-based noble metal promoted materials [75,
76, 78].
The transformation of carbon dioxide is of importance in the controlling of global
warning. The reduction of this molecule can render useful chemicals such as carbon
monoxide, methane, and, in optimal conditions, methanol, providing a nice route
to synthesize valuable chemicals within the context of a circular economy. Infrared
studies look into such a reaction, with particular emphasis in the analysis of the CO 2
activation step(s) using titania-, carbon nitride-, sulphide-based and others catalysts
[79–82]. As shown in Fig. 11, titania materials are able to adsorb carbon dioxide in
several different forms in the presence of water. Monodentate, bidentate y chelating
brigde carbonates are usually formed but are commonly considered as expectator
species without kinetic significance. Apart from that, the negatively charged CO 2
entity as well as formate ions (also connected with some surface species detected in
Fig. 11) are considered true intermediates of different reaction products formed in
the photo-reduction reaction. In fact, the first is described as directly related to the
production of methane in titania-based materials; a linear relationship was presented
between the intensity of IR bands of such entities (negatively charged carbon dioxide) and the rate of methane production [82]. On the other hand, the second, formate
species is described by a combination of infrared and theoretical results as the intermediate leading to CO production in SnS x -based photo-catalysts [81].
Fig. 10 DRIFTS spectra for the in situ analysis of the methanol photo-transformation occurring under
reaction conditions: upper panel, P25-5Pt (0.5 wt% Pt onto P25 prepared by chemical deposition with
a metal/reductant ratio of 5); lower panel, P25-10Pt (0.5 wt% Pt onto P25 prepared by chemical deposition with a metal/reductant ratio of 10) samples. Difference spectra obtained during saturation with the
alcohol:water mixture (upper zone of each panel), under reaction-illumination conditions (middle part)
and subsequently at dark conditions (bottom part) are presented. Reproduced with permission from Ref.
[75]
▸
183
Reprinted from the journal