1 3
Topics in Current Chemistry (2019) 377:24
3.1 Operando Spectroscopy
Studies focusing on photo-catalytic solid samples at reaction conditions were mostly
performed using X-ray absorption spectroscopy (XAS) with a limited number considering the use of Raman and XPS. Both X-ray absorption near edge (XANES) and
X-ray extended absorption fine structure (EXAFS) have been used to follow the evolution of a catalyst under the simultaneous influence of light and reactants.
XAS studies using pump and probe procedures were mostly aimed at understanding charge carrier capture and transfer processes. In the case of titanium [38] and
tungsten [39] oxides, the photoabsorption process occurs with partial reduction of
(mostly surface) cation species form their initial Ti
4+
and W
6+
chemical states, followed by a local structural distortion in the tungsten case. The studies thus established a way to analyze the trapping of excited electrons in the solids. Also, the
charge transfer process between semiconductors and metals has been analyzed using
pump and probe XAS. The process of charge transfer between gold and titania under
visible light excitation was the objective of several works using X-ray absorption
and emission techniques. In particular, they analyzed the electron transfer and capture occurring after visible light excitation in materials having metallic gold species
deposited over titania. A rather interesting study pointed out the key role played by
Ti surface centers in the vicinity of the metal particles. Such Ti cations behave as
long-lived trapping charge centers allowing the transfer of charge from the noble
metal particles to titania after light excitation of the metal component with visible
light [40]. The same subject was investigated in other publications that analyzed
electronic details concerning the formation of electron–hole pairs after light excitation in gold as well as the subsequent transfer to the titania semiconductor [41]. A
similar procedure was carried out to investigate Cu doping of a InP nanostructure
material, detecting the capture of a hole by copper after light excitation, relaxation
differences between bulk and surface copper species, and the annihilation of charge
localized at copper with initially trapped electrons [42].
XAS has also been utilized in situ to study the behavior of different photo-catalysts during reaction. A recent contribution considers the Co-Ru-UIO-67(bpy)
photo-catalyst, based on a metal organic framework platform with incorporated
molecular photosensitizer (bpy; bipyridine) and “active” Co and Ru species. The
XAS analysis (Fig. 5) showed that the reduction of Co
2+
to Co
1+
with electrons
coming from the sensitizer (bpy) has a relevant role in driving activity in hydrogen photo-production under visible light. This process occurs after an induction
period, which suggests that a local reordering around or connected with Co centers
is required to achieve photo-activity [43]. Although MOFs and other novel systems
are analyzed using XAS, most results reported correspond to oxide or metal parts of
composite photo-catalysts. Studies of non-noble metal atoms like Cu or Ni showed
the dominant presence of oxidized phases, which evolves in a limited quantity (typically the fraction of atoms evolving is well below 10%) to a reduced state. This has
been analyzed for the photo-oxidation of 2-propanol [44] as well as the reduction of
CO 2 in presence of water [45]. However, a recent contribution taking into account
the matching of spectroscopic and illuminated sample volumes uncovered a more
complex evolution of the non-noble metal component for titania-based materials
175
Reprinted from the journal
Topics in Current Chemistry (2019) 377:24
3.1 Operando Spectroscopy
Studies focusing on photo-catalytic solid samples at reaction conditions were mostly
performed using X-ray absorption spectroscopy (XAS) with a limited number considering the use of Raman and XPS. Both X-ray absorption near edge (XANES) and
X-ray extended absorption fine structure (EXAFS) have been used to follow the evolution of a catalyst under the simultaneous influence of light and reactants.
XAS studies using pump and probe procedures were mostly aimed at understanding charge carrier capture and transfer processes. In the case of titanium [38] and
tungsten [39] oxides, the photoabsorption process occurs with partial reduction of
(mostly surface) cation species form their initial Ti
4+
and W
6+
chemical states, followed by a local structural distortion in the tungsten case. The studies thus established a way to analyze the trapping of excited electrons in the solids. Also, the
charge transfer process between semiconductors and metals has been analyzed using
pump and probe XAS. The process of charge transfer between gold and titania under
visible light excitation was the objective of several works using X-ray absorption
and emission techniques. In particular, they analyzed the electron transfer and capture occurring after visible light excitation in materials having metallic gold species
deposited over titania. A rather interesting study pointed out the key role played by
Ti surface centers in the vicinity of the metal particles. Such Ti cations behave as
long-lived trapping charge centers allowing the transfer of charge from the noble
metal particles to titania after light excitation of the metal component with visible
light [40]. The same subject was investigated in other publications that analyzed
electronic details concerning the formation of electron–hole pairs after light excitation in gold as well as the subsequent transfer to the titania semiconductor [41]. A
similar procedure was carried out to investigate Cu doping of a InP nanostructure
material, detecting the capture of a hole by copper after light excitation, relaxation
differences between bulk and surface copper species, and the annihilation of charge
localized at copper with initially trapped electrons [42].
XAS has also been utilized in situ to study the behavior of different photo-catalysts during reaction. A recent contribution considers the Co-Ru-UIO-67(bpy)
photo-catalyst, based on a metal organic framework platform with incorporated
molecular photosensitizer (bpy; bipyridine) and “active” Co and Ru species. The
XAS analysis (Fig. 5) showed that the reduction of Co
2+
to Co
1+
with electrons
coming from the sensitizer (bpy) has a relevant role in driving activity in hydrogen photo-production under visible light. This process occurs after an induction
period, which suggests that a local reordering around or connected with Co centers
is required to achieve photo-activity [43]. Although MOFs and other novel systems
are analyzed using XAS, most results reported correspond to oxide or metal parts of
composite photo-catalysts. Studies of non-noble metal atoms like Cu or Ni showed
the dominant presence of oxidized phases, which evolves in a limited quantity (typically the fraction of atoms evolving is well below 10%) to a reduced state. This has
been analyzed for the photo-oxidation of 2-propanol [44] as well as the reduction of
CO 2 in presence of water [45]. However, a recent contribution taking into account
the matching of spectroscopic and illuminated sample volumes uncovered a more
complex evolution of the non-noble metal component for titania-based materials
175
Reprinted from the journal