Topics in Current Chemistry (2020) 378:3
1 3
the extensive use of acids for the leaching recovery process is still a problem. Nonetheless, acid leaching processes have shown high efficiency during Ti recovery [55].
The extensive use of selective catalytic reduction (SCR) systems has generated
large amounts of spent/waste SCR catalysts. The generation of this type of waste
will continue and will be vital in the coming years with the implementation of the
Euro 6 diesel emission standards. Elemental concentrations in spent SCR catalysts
are not a constant variable; however, considerable concentrations of heavy metals
such as Ti
4+
, V
5+
, Fe
3+
, W
6+
and Ca
2+
have been common [56]. As a representative
example, an economical approach for recovering titanium and regenerating TiO 2
photocatalysts from spent SCR catalysts has been reported. Elemental analysis of
the SCR catalyst waste carried out by total reflection X-ray fluorescence identified
a remarkably high concentration of TiO 2 (68.5 wt%), while other major compounds,
namely SiO 2 , SO 3 , Al 2 O 3 , and WO 3 , showed concentrations of 10.3, 6.45, 5.47, and
4.67 wt%, respectively. Minor components (below 2 wt%) including Fe 2 O 3 CaO,
V 2 O 5 , K 2 O, Na 2 O, MgO, P 2 O 5 , Nb 2 O 5 , ZrO 2 , As 2 O 3 , and SrO were also measured.
The high TiO 2 concentration in the waste-catalysts makes them especially interesting for recovery of TiO 2 species. In addition, some of the minor entities (Fe 2 O 3 ,
V 2 O 5 , Nb 2 O 5 , ZrO 2 ) have shown a beneficial photocatalytic response by modification of the optical and electronic properties of the titania counterpart [5, 6]. The proposed titanium recovery scheme and the further production of TiO 2 is described in
Fig. 2a. As can be seen, the protocols followed three general steps: (1) NaOH molten
salt decomposition, (2) water leaching to separate Na 2 TiO 3 and NaOH recycling,
and (3) hydrothermal reaction to regenerate titania. During the leaching process, two
treatments (1.0 mol/L HCl or 0.5 mol/L H 2 SO 4 ) were used, and hence two samples
A and B were obtained. The morphology of the samples differed considerably, with
sample A presenting spherical nanoparticles, while sample B exhibited numerous
rod-shaped structures (Fig. 2b, c). Interestingly, both samples presented iron entities
in the structure, which was confirmed by energy-dispersive X-ray (EDX) mapping
data (Fig. 2b, c) and X-ray photoelectron spectroscopy (XPS) analysis (not shown).
Such a small concentration of iron, which according to XPS data was present as
Fe 2 O 3 , would confer enhanced absorption of visible irradiation. Degradation of rhodamine B (RhB) and methylene blue (MB) were used to demonstrate the photocatalytic properties of samples A and B under visible illumination conditions. Sample A
showed better photocatalytic performance with both dyes, outperforming the activity of P25 commercial TiO 2 references (Fig. 2d, e) [56].
An important limitation of waste-derived photocatalysts is that in many examples, an uncontrolled structure is obtained [3]. In fact, the poor reproducibility of
the synthesis is one of the main drawbacks of waste-derived materials. Such a situation is especially important in photocatalytic materials when minor compositional
variations drive very different catalytic results [57]. However, if the waste source
Fig. 2 a Schematic representation of titanium recovery from spent SCR materials. b, c Scanning electron
microscopy (SEM) and EDX mapping analysis. d, e RhB and MB photodegradation results [56]. f, g
XPS and TGA analysis of produced waste anodic electrolyte. h Scheme of the extraction of the waste
anodic electrolyte. i, j Photocatalytic degradation of HAs by TiO 2 and UV irradiation. k, l Photocatalytic
degradation of bentazone by N-doped TiO 2 under visible irradiation [58]
▸
4
Reprinted from the journal
1 3
the extensive use of acids for the leaching recovery process is still a problem. Nonetheless, acid leaching processes have shown high efficiency during Ti recovery [55].
The extensive use of selective catalytic reduction (SCR) systems has generated
large amounts of spent/waste SCR catalysts. The generation of this type of waste
will continue and will be vital in the coming years with the implementation of the
Euro 6 diesel emission standards. Elemental concentrations in spent SCR catalysts
are not a constant variable; however, considerable concentrations of heavy metals
such as Ti
4+
, V
5+
, Fe
3+
, W
6+
and Ca
2+
have been common [56]. As a representative
example, an economical approach for recovering titanium and regenerating TiO 2
photocatalysts from spent SCR catalysts has been reported. Elemental analysis of
the SCR catalyst waste carried out by total reflection X-ray fluorescence identified
a remarkably high concentration of TiO 2 (68.5 wt%), while other major compounds,
namely SiO 2 , SO 3 , Al 2 O 3 , and WO 3 , showed concentrations of 10.3, 6.45, 5.47, and
4.67 wt%, respectively. Minor components (below 2 wt%) including Fe 2 O 3 CaO,
V 2 O 5 , K 2 O, Na 2 O, MgO, P 2 O 5 , Nb 2 O 5 , ZrO 2 , As 2 O 3 , and SrO were also measured.
The high TiO 2 concentration in the waste-catalysts makes them especially interesting for recovery of TiO 2 species. In addition, some of the minor entities (Fe 2 O 3 ,
V 2 O 5 , Nb 2 O 5 , ZrO 2 ) have shown a beneficial photocatalytic response by modification of the optical and electronic properties of the titania counterpart [5, 6]. The proposed titanium recovery scheme and the further production of TiO 2 is described in
Fig. 2a. As can be seen, the protocols followed three general steps: (1) NaOH molten
salt decomposition, (2) water leaching to separate Na 2 TiO 3 and NaOH recycling,
and (3) hydrothermal reaction to regenerate titania. During the leaching process, two
treatments (1.0 mol/L HCl or 0.5 mol/L H 2 SO 4 ) were used, and hence two samples
A and B were obtained. The morphology of the samples differed considerably, with
sample A presenting spherical nanoparticles, while sample B exhibited numerous
rod-shaped structures (Fig. 2b, c). Interestingly, both samples presented iron entities
in the structure, which was confirmed by energy-dispersive X-ray (EDX) mapping
data (Fig. 2b, c) and X-ray photoelectron spectroscopy (XPS) analysis (not shown).
Such a small concentration of iron, which according to XPS data was present as
Fe 2 O 3 , would confer enhanced absorption of visible irradiation. Degradation of rhodamine B (RhB) and methylene blue (MB) were used to demonstrate the photocatalytic properties of samples A and B under visible illumination conditions. Sample A
showed better photocatalytic performance with both dyes, outperforming the activity of P25 commercial TiO 2 references (Fig. 2d, e) [56].
An important limitation of waste-derived photocatalysts is that in many examples, an uncontrolled structure is obtained [3]. In fact, the poor reproducibility of
the synthesis is one of the main drawbacks of waste-derived materials. Such a situation is especially important in photocatalytic materials when minor compositional
variations drive very different catalytic results [57]. However, if the waste source
Fig. 2 a Schematic representation of titanium recovery from spent SCR materials. b, c Scanning electron
microscopy (SEM) and EDX mapping analysis. d, e RhB and MB photodegradation results [56]. f, g
XPS and TGA analysis of produced waste anodic electrolyte. h Scheme of the extraction of the waste
anodic electrolyte. i, j Photocatalytic degradation of HAs by TiO 2 and UV irradiation. k, l Photocatalytic
degradation of bentazone by N-doped TiO 2 under visible irradiation [58]
▸
4
Reprinted from the journal