Topics in Current Chemistry (2019) 377:24
1 3
In absence of reactants, Pt and Ir showed changes in the interaction between the
metallic state (obtained during the preparation stage of the materials) and the support under UV illumination [46]. The metallic state was also detected under operando conditions and assumed to be responsible for visible-light-driven hydrogen
photo-reforming from methanol in monometallic Au and Cu as well as bimetallic
AuCu catalysts. Optimum activity for the bimetallic system was achieved in cases
where an effective interaction (alloying) between the two metals takes place [47].
Another study of Pt but in oxidized form (PtO) was carried out during the water
splitting process. Although the work uses co-catalyst concentrations far from those
required for a photo-catalytic process, it showed changes in both Pt-O bond distance
and coordination number under reaction conditions. Such changes likely relate to a
modification of the interaction with the support. Metallic Pt was not detected or generated under reaction conditions [48]. Recently, Pt-based materials were analyzed by
absorption and emission spectroscopies under photo-thermal conditions for CO oxidation. As occurs with some of the previous reports, Pt appears more oxidized under
illumination and together with the combined effect of temperature controlling noble
metal surface CO coverage drive to a significant increase in activity with respect to
dark (thermal-alone) conditions [49].
As mentioned, other experimental techniques focusing on analyzing the solid
response under photo-catalytic conditions concerns the use of Raman or XPS. These
two techniques are only occasionally used in the field of photo-catalysis. Raman has
been utilized to study the face-dependent response of the K 3 B 6 O 10 Br materials used
in chlorophenol degradation under visible light [50] as well as the response of a
core–shell CdS-TiO 2 structure for hydrogen photo-production under both UV and
visible light illumination conditions [51]. The first contribution (see Fig. 7) follows
the evolution of the (211), (110), and (101) facets of a K 3 B 6 O 10 Br single crystallite
(having all these faces) under reaction conditions, showing that only the first suffers
evolution and at the same time provides higher photo-catalytic activity, a fact likely
related to the higher OH density (in turn promoted by the higher presence of K) and
surface interaction with the pollutant [50]. Raman showed, on the other hand, that
the oxide layer of the CdS-TiO 2 catalyst(s) confers stability to the system under reaction conditions concerning hydrogen photo-production, inhibiting the typically corrosion process suffered by the sulphide [51]. A few other examples of using Raman
consider the analysis of the gas or liquid phase–catalyst interface and will be presented in following pages.
XPS studies concentrate on the analysis of the surface and near-surface regions
of the materials in the field of photo-catalysis. As mentioned, XPS can sample
different depths of the solid depending of the energy of excitation, the usual Mo,
Mg, or Al K-alpha sources allow sampling a few nanometers of typical photocatalysts [30, 31]. A nice work described the response of titania, alumina, and
silica materials modified with 5 wt% of uranyl nitrate and tested in the oxidation
of acetone vapor under visible light. The use of XPS (Fig. 8) was able to show
that the surface uranium species was slowly reduced from U
6+
to the U
5+
on the
Al 2 O 3 and SiO 2 surfaces but an easier reduction to the U
4+
state was observed
on the TiO 2 surface. The catalyst based on TiO 2 revealed a substantially higher
activity compared to other oxides, which was ascribed to the easy redox handling
178
Reprinted from the journal
1 3
In absence of reactants, Pt and Ir showed changes in the interaction between the
metallic state (obtained during the preparation stage of the materials) and the support under UV illumination [46]. The metallic state was also detected under operando conditions and assumed to be responsible for visible-light-driven hydrogen
photo-reforming from methanol in monometallic Au and Cu as well as bimetallic
AuCu catalysts. Optimum activity for the bimetallic system was achieved in cases
where an effective interaction (alloying) between the two metals takes place [47].
Another study of Pt but in oxidized form (PtO) was carried out during the water
splitting process. Although the work uses co-catalyst concentrations far from those
required for a photo-catalytic process, it showed changes in both Pt-O bond distance
and coordination number under reaction conditions. Such changes likely relate to a
modification of the interaction with the support. Metallic Pt was not detected or generated under reaction conditions [48]. Recently, Pt-based materials were analyzed by
absorption and emission spectroscopies under photo-thermal conditions for CO oxidation. As occurs with some of the previous reports, Pt appears more oxidized under
illumination and together with the combined effect of temperature controlling noble
metal surface CO coverage drive to a significant increase in activity with respect to
dark (thermal-alone) conditions [49].
As mentioned, other experimental techniques focusing on analyzing the solid
response under photo-catalytic conditions concerns the use of Raman or XPS. These
two techniques are only occasionally used in the field of photo-catalysis. Raman has
been utilized to study the face-dependent response of the K 3 B 6 O 10 Br materials used
in chlorophenol degradation under visible light [50] as well as the response of a
core–shell CdS-TiO 2 structure for hydrogen photo-production under both UV and
visible light illumination conditions [51]. The first contribution (see Fig. 7) follows
the evolution of the (211), (110), and (101) facets of a K 3 B 6 O 10 Br single crystallite
(having all these faces) under reaction conditions, showing that only the first suffers
evolution and at the same time provides higher photo-catalytic activity, a fact likely
related to the higher OH density (in turn promoted by the higher presence of K) and
surface interaction with the pollutant [50]. Raman showed, on the other hand, that
the oxide layer of the CdS-TiO 2 catalyst(s) confers stability to the system under reaction conditions concerning hydrogen photo-production, inhibiting the typically corrosion process suffered by the sulphide [51]. A few other examples of using Raman
consider the analysis of the gas or liquid phase–catalyst interface and will be presented in following pages.
XPS studies concentrate on the analysis of the surface and near-surface regions
of the materials in the field of photo-catalysis. As mentioned, XPS can sample
different depths of the solid depending of the energy of excitation, the usual Mo,
Mg, or Al K-alpha sources allow sampling a few nanometers of typical photocatalysts [30, 31]. A nice work described the response of titania, alumina, and
silica materials modified with 5 wt% of uranyl nitrate and tested in the oxidation
of acetone vapor under visible light. The use of XPS (Fig. 8) was able to show
that the surface uranium species was slowly reduced from U
6+
to the U
5+
on the
Al 2 O 3 and SiO 2 surfaces but an easier reduction to the U
4+
state was observed
on the TiO 2 surface. The catalyst based on TiO 2 revealed a substantially higher
activity compared to other oxides, which was ascribed to the easy redox handling
178
Reprinted from the journal