1. Introduction
Nowadays, the quality of air, both in indoor and outdoor environments, is an extremely important concern. Furthermore, the COVID-19 emergency has pointed to the necessity of clean air to discourage virus infection. Among the air pollutants, volatile organic compounds (VOCs) include many of the most dangerous substances for both human health and the environment. Different strategies were employed to remove VOCs from the air, and an innovative and sustainable solution is represented by solar photocatalytic or photothermo-catalytic oxidation [
1,
2]. Compared to the most used catalytic or non-catalytic VOCs combustion, the photocatalytic process allows one to exploit solar irradiation with green advantages to work at milder conditions using renewable energy [
2]. However, the performance of the photocatalysts is much lower compared to the catalysts employed for the thermocatalytic removal of VOCs [
3], and for these reasons, the multi-catalytic approach of the photothermo-catalysis is a fascinating way to obtain high VOCs removal values of thermocatalysis but at lower temperatures, increasing, at the same time, the energy efficiency of the process. To design performing photothermo catalysts, different properties are required [
4,
5]. Analogously to photocatalysis, it is necessary to have a semiconductor material that, after solar irradiation, is able to generate photoelectrons and photoholes in its conduction (CB) and valence (VB) bands, respectively. It should have redox properties activated with the temperature; in this way, the superficial/mobile oxygens of the catalyst or of the support can participate in the oxidation of VOCs increasing the overall activity [
3,
6]. Finally, the photothermo catalysts should be resistant to both long-time irradiation and heating. The preparation of mixed oxides or composites is the best and easiest way to combine all of these features. Indeed, with the formation of a suitable heterojunction, it is possible to exploit solar irradiation, decreasing the bandgap (E
g) of the main semiconductor oxide, profiting from both the photocatalytic activity of the principal oxides and the thermocatalytic activity of the hosted oxide. Moreover, the introduction of host ions in the lattice of the main oxide allows one to create defects and oxygen vacancies that favour VOCs oxidation [
7,
8].
The TiO
2-CeO
2 composites showed promising performance in the photo-thermal approach for both VOCs removal and CO
2 reduction [
3,
9]; however, one of the side effects of the current pandemic situation is the crisis of raw materials exportation, and as a consequence, in 2020, titanium featured in the EU critical raw materials list [
10]. Considering that up until now, TiO
2 is the most studied and applied semiconductor, both in academia and in industrial research focused on photocatalytic applications, the exploration of unconventional non-critical (photo)catalysts is highly required.
In this work, we have investigated the photothermo-catalytic properties of MnO
x-ZrO
2 mixed oxides, with the aim of finding new and sustainable alternatives to the most common TiO
2-based photocatalysts, and without the addition of noble metal co-catalysts, usually used in the catalytic and photocatalytic removal of VOCs [
11], to obtain even more environmentally friendly catalysts, in the end.
Manganese oxide exists in four stable forms (MnO, MnO
2, Mn
3O
4 and Mn
2O
3), and all of them own a semiconductor electronic structure characterized by the partially filled d orbitals which permit the electronic d-d transitions under UV or visible light irradiation [
12]. Based on the preparation method, it is common to obtain a non-stoichiometric oxide or a mixture of different MnO
x oxides with the +II, +III and/or +IV oxidation states. The high mobility/reducibility of manganese oxide lattice oxygens is particularly useful for VOCs removal [
13,
14], whereas the redox properties of MnO
x can be particularly advantageous for the photothermo-catalytic oxidation of VOCs, as well as its low bandgap (in the range 2.0–3.5 eV depending of the crystalline structure [
12,
15]) that can allow a more efficient use of solar radiation.
Zirconium oxide (zirconia) was largely used as a support of several noble metal-based catalysts used for the thermocatalytic oxidation of VOCs, due to its high stability, thermal resistance, and ionic conductivity [
16,
17]. Furthermore, it is a large bandgap semiconductor (E
g of about 5.0 eV [
18] or lower depending to the zirconia synthesis). Therefore, its coupling with a lower bandgap semiconductor (as MnO
x) can be a performing and fascinating strategy to reduce the odds of charge recombination (a common reason for photocatalysts deactivation) and to synergistically exploit both the thermal stability and the redox properties of MnO
x and ZrO
2 [
19,
20] together with their photocatalytic features.
We have also determined the chemico-physical and the photocatalytic, thermocatalytic and photothermo-catalytic activities of MnO
x-ZrO
2 oxides in the oxidation of toluene and ethanol, chosen as VOCs models, due to the high toxicity nature of toluene and to the wide use of ethanol as a solvent in many industrial processes and as an octane booster in combustion engines, whose incomplete oxidation can give the emission of dangerous compounds, as acetaldehyde, in the environment [
21].
3. Discussion
The mixed oxides MnO
x-ZrO
2 here investigated showed promising performance in the removal of VOCs in the gas phase, considering the absence of noble metals co-catalysts and an initial VOCs concentration of 1000 ppm. The amount of zirconium oxide added on manganese oxide is a key parameter to improve the catalytic and the photocatalytic performance. The as-synthesized samples showed a comparable optical bandgap, in the range 3.0–3.3 eV (
Table 1), similar to the TiO
2, and able to exploit the UV-A portion of the solar light. The addition of zirconia on manganese oxide led to a slight decrease of the surface area (
Table 1) that, however, did not comprise the catalytic activity of the mixed oxides.
The presence of a small amount of zirconium oxide on MnO
x allowed, as stated by XRD and SEM-EDX, an ionic exchange between Zr
4+ and Mn
2+; this favoured the formation of a synergistic effect between the two oxides, with structural changes in the bulk of MnO
x. These modifications led to increasing the (photo)catalytic activity compared to the bare oxides. Indeed, when reducible oxides, i.e., which own mobile/reducible oxygens, were used for the oxidation of VOCs, the total oxidation to CO
2 is favoured, because these oxygens can participate in the reaction with a Mars–Van Krevelen (MvK) mechanism [
41,
42]. The oxygen vacancies on the surface of the oxide will be subsequently filled by the O
2 present in the gas phase.
This mechanism was boosted up with the photothermo-catalytic approach because the photocatalytic mechanism generated the superoxide (O
2•−) and hydroxyl (OH
•) radicals [
43,
44], that being more reactive of the molecular O
2, increased the rate of the total oxidation of VOCs (reactions a–i,
Figure 6) and the re-filling of the oxygen vacancies, being the mobile oxygens of MnO
x activated by the heating [
13,
45]. For these reasons, the conversion temperatures of both toluene and ethanol oxidation were sensibly lower compared to the thermocatalysis, especially with the MnO
x-5%ZrO
2 sample. In this way, it was possible to exploit a double positive effect: (i) the solar irradiation effect: that allowed the formation of more reactive species, (ii) the thermal effect: that activated the redox mobility of the manganese oxide oxygens [
13,
20,
45].
Photothermo-catalytic mechanism:
- (i)
Solar irradiation effect (VB and CB indicate the valence and the conduction bands):
- (a)
Charge carriers formation: MnOx-5%ZrO2 + hν(solar) → MnOx-5%ZrO2 (hVB++ e−CB)
- (b)
Formation of hydroxyl radical: h+VB + H2O (g) → OH• + H+aq
- (c)
Formation of superoxide radical: e−CB + O2 → O2•−
- (ii)
Thermal effect (Vo = oxygen vacancy)
- (d)
Oxygen from the mixed oxide: MnOx-5%ZrO2 → MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide
- (e)
VOC oxidation: VOC + O2 (g) + O2(g)from oxide CO2 + H2O
- (f)
Oxygen restoring: MnOx-5%ZrO2 (Vo) + 1/2 O2(g) → MnOx-5%ZrO2
- (iii)
Solar photothermal effect
- (g)
MnOx-5%ZrO2 + hν MnOx-5%ZrO2 (Vo) + 1/2 O2(g)from oxide + OH• + O2•−
- (h)
Improved VOC oxidation: VOC + O2 (g) + O2(g)from oxide + OH• + O2•− → CO2 + H2O
- (i)
Oxygen speeded up restoring: MnOx-5%ZrO2 (Vo) + 1/2 O2(g) + O2• → MnOx-5%ZrO2
It is worth noting that the reactions (a–c) and (d–f) are also involved in the solar photocatalysis at room temperature and in the bare thermocatalytic tests, respectively. The multi-catalytic effect (reactions g–i) allowed one to increase the performance and to favour the total oxidation of the employed VOCs to CO2.
Another confirmation of the proposed MvK mechanism was reported in the
Figure S3b. In the phothermo-catalytic oxidation of toluene with the MnO
x-5%ZrO
2 sample, the air (more interesting from a practical point of view) was replaced in the gas mixture with the pure oxygen. It is possible to note that the presence of oxygen led to a beneficial effect for the toluene conversion to CO
2, being the T
90 lower of 25 °C (155 °C) compared to the test with air (180 °C). This can be reasonably ascribed to the easier oxygen restoring on the catalyst surface (reaction i), in an oxygen-rich environment, favouring, in this way, the MvK route.
As stated by the characterization data, the good interaction between the manganese and zirconium oxide (especially at low amount of ZrO
2) improved the photothermo-catalytic mechanism with the redox process on MnO
x that was favoured by the ionic exchange between the zirconium and the manganese ions. On the contrary, an increased amount of zirconium oxide led to a progressive deposition of the hosted oxide on the surface of MnO
x covering, in this way, the surface-active sites of manganese oxide [
35,
36]. For these reasons, the optimal performance was obtained with 5 wt.% of ZrO
2. In this contest, the mobility of the surface oxygens of the MnO
x-5%ZrO
2 sample was favoured by the MnO
x redox properties, and consequently, it is strictly related to its reducibility. Furthermore, the amount of the surface oxygens on MnO
x-5%ZrO
2 was higher compared to the other samples, as detected by XPS. To have a further confirmation of the high reducibility/mobility of the surface oxygens of MnO
x-5%ZrO
2, the H
2-temperature-programmed reduction (TPR) measurements were carried out, and the sample profiles were reported in
Figure 7. In accordance with the literature data [
20,
46], the TPR profiles of the MnO
x-based samples were characterized to broad reduction peaks, due to the occurrence of several reduction processes of the Mn ions. As expected, the MnO
x-5%ZrO
2 sample showed the lowest reduction feature (201 °C) attributed to the reduction of Mn
2O
3 to Mn
3O
4 [
46], 111 °C and 117 °C lower compared to the same reduction feature of Mn
3O
4 and MnO
x-10%ZrO
2, respectively. This reduction peak was also more intense for the MnO
x-5%ZrO
2 compared to the other MnO
x-based samples confirming, as detected by XRD and XPS, the major presence of Mn
3+ ions on MnO
x-5%ZrO
2. The higher temperature reduction signals in the range 300–480° were ascribed to the further reduction of Mn
3O
4 to MnO [
46]. Moreover, in this case, the sample with 5 wt.% of ZrO
2 showed the highest reducibility (i.e., the lowest peak temperature). This is connected to the highest mobility/reducibility of the surface oxygens of MnO
x-5%ZrO
2, which favours the MvK mechanism, and therefore a better VOCs abatement. The reduction temperature of bare ZrO
2 started at a temperature above 500 °C [
47], and for this reason, in our analysis (in the range 50–550°C), its reduction peak was not complete.
Between the photocatalytic, the thermocatalytic and the photothermo-catalytic removal of VOCs, although the solar photocatalytic reaction has the advantages of work at room temperature and that with the MnO
x-5%ZrO
2, it reached a similar activity of the most used TiO
2-based materials (
Table 4); to have a complete VOCs removal, it is necessary to have contextual heating. For this purpose, the solar photothermo-catalysis can be an optimal solution to obtain the good performance of the thermocatalysis, but with an energy saving, due to the lower temperature required for the VOCs conversion. Indeed, the best sample (MnO
x-5%ZrO
2) tested in our experimental conditions showed a decrease of 36 °C and 34 °C of the toluene and ethanol T
90 conversion compared to the thermocatalytic tests favouring in both the reactions; the total oxidation to CO
2 (the T
90 of ethanol conversion to CO
2 was lowered of 205 °C,
Table 5 and
Table 6).
Finally, the stability in the time-on steam toluene removal of MnO
x-5%ZrO
2 was good (
Figure 8, toluene solar photothermo-oxidation) and pointed to the MnO
x-ZrO
2 catalyst being a promising versatile material for application in thermocatalysis, photocatalysis, and photothermo-catalysis.
4. Materials and Methods
4.1. Catalysts Synthesis
Bare manganese oxide was prepared by chemical precipitation with NaOH (1 M) (Panreac Química SLU, Castellar del Vallès (Barcelona), Spain). In particular, a certain amount of manganese (II) chloride tetrahydrate (Sigma-Aldrich, Buchs, Switzerland) was dissolved in demineralized water and heated at 70 °C. After the NaOH was added dropwise until the pH = 10. Successively, the solution was stirred and kept at 70 °C for 2 h. After digestion for 24 h, the slurry was filtered and dried at 120 °C overnight. Finally, the resultant powders were calcined in air at 600 °C for 2 h.
A similar procedure was followed for the bare ZrO2. In this case, the zirconyl nitrate hydrate (Fluka, Buchs, Switzerland) and ammonia (as precipitant agent, 25–28%, Sigma-Aldrich, Buchs, Switzerland) were used, following the same procedures reported above, and the same thermal treatments (drying at 120 °C, and calcination at 600 °C for 2 h).
For the MnOx-ZrO2 mixed oxides, the NaOH-driven precipitation was employed using the required stoichiometric amount of zirconyl nitrate hydrate to obtain the chosen nominal concentration in weight percentage (wt.%) of ZrO2. Moreover, in this case, the samples were dried at 120 °C and calcined in air at 600 °C for 2 h.
4.2. Catalysts Characterization
The sample structures were determined through the X-ray powder diffraction (XRD) using a Smartlab Rigaku diffractometer (Rigaku Europe SE, Hugenottenallee 167 Neu-Isenburg 63263, Germany) in Bragg–Brentano mode, equipped with a rotating anode of Cu Kα radiation operating at 45 kV and 200 mA. The surface morphology was examined with field emission scanning electron microscopy (FE-SEM) using a ZEISS SUPRA 55 VP (Carl Zeiss QEC Gmb, Garching b. München, Germany). The composition of the powders was carried out by the energy dispersive X-ray (EDX) analysis using an INCA-Oxford (Oxford Instruments plc, Tubney Woods, Abingdon, Oxfordshire, United Kingdom) windowless detector, and a resolution of 127 eV determined using the half-height amplitude (FWHM) of the Kα of Mn.
The BET surface area values were determined by N2 adsorption–desorption measurements with a Sorptomatic 1990 instrument (Thermo Quest, Milano, Italy). Before the measurements, the catalysts were outgassed overnight at 200 °C.
The UV-vis Diffuse Reflectance spectra (UV-Vis DRS, Diffuse Reflectance Spectroscopy) measurements were performed with a Jasco V- 670 spectrometer (Jasco Europe S.R.L., Cremella, Italy) provided with an integration sphere and using barium sulphate (Fluka, Buchs, Switzerland) as standard. The estimation of the optical band gap of the samples was determined using the Kubelka–Munch function [
26].
The X-ray photoelectron spectroscopy (XPS) was performed with a K-alpha X-ray photoelectron instrument (Thermo Fisher Scientific, Waltham, MA, USA), employing the C 1s peak at 284.9 eV (of adventitious carbon) as reference.
The H
2-TPR (Temperature programmed reduction) profiles of the samples were obtained using a home-made flow equipment (gas-mixture 5 vol.% H
2 in Ar) and a TCD detector, following the procedures reported in ref. [
48].
4.3. Photo, Thermo and Photothermo-Catalytic Oxidation of VOCs
The thermocatalytic removal of VOCs in gas phase and atmospheric pressure was carried out in a fixed bed flow reactor packed with the powder catalysts (0.15 g, 80–140 mesh), using the same experimental conditions described in the ref. [
3]. A heating ramp of 10 °C was used in all the tests from room temperature to 500 °C. To assure a steady-state before the catalytic measurements, the gas mixture (1000 ppm VOCs; 10 vol.% air, rest He) was flowed on the catalyst for 30 min. No substantial contribution due to the adsorption process was detected. The reaction products were analysed by a gas chromatography (Smart-IQ+ Thermo Onix, Thermo Fisher Scientific 168 Third Avenue, Waltham, MA, USA 0245) utilizing a packed column with 10% FFAP on Chromosorb W (from Merck KGaA, Darmstadt, Germany) with a FID (Flame Ionization Detector), coupled with a quadrupole mass spectrometer (VG quadropoles, Fergutec B.V. Dragonder 13 C, 5554 GM Valkenswaard, The Netherlands).
The photocatalytic and the photothermo-catalytic tests were performed with the same instruments described above. The simultaneous irradiation in the phothermo-catalytic tests was made with an artificial solar lamp (OSRAM Vitalux 300 W, 10.7 mW/cm2, OSRAM Opto Semiconductors GmbH, Leibniz, Regensburg Germany). In the photocatalytic tests, a fan located near the reactor allowed us to maintain a constant temperature, avoiding the overheating effects due to lamp emission.