TiO2-Based Photocatalytic Geopolymers for Nitric Oxide Degradation

This study presents an experimental overview for the development of photocatalytic materials based on geopolymer binders as catalyst support matrices. Particularly, geopolymer matrices obtained from different solid precursors (fly ash and metakaolin), composite systems (siloxane-hybrid, foamed hybrid), and curing temperatures (room temperature and 60 °C) were investigated for the same photocatalyst content (i.e., 3% TiO2 by weight of paste). The geopolymer matrices were previously designed for different applications, ranging from insulating (foam) to structural materials. The photocatalytic activity was evaluated as NO degradation in air, and the results were compared with an ordinary Portland cement reference. The studied matrices demonstrated highly variable photocatalytic performance depending on both matrix constituents and the curing temperature, with promising activity revealed by the geopolymers based on fly ash and metakaolin. Furthermore, microstructural features and titania dispersion in the matrices were assessed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) analyses. Particularly, EDS analyses of sample sections indicated segregation effects of titania in the surface layer, with consequent enhancement or depletion of the catalyst concentration in the active sample region, suggesting non-negligible transport phenomena during the curing process. The described results demonstrated that geopolymer binders can be interesting catalyst support matrices for the development of photocatalytic materials and indicated a large potential for the exploitation of their peculiar features.


Introduction
The photocatalytic oxidation (PCO) technology gained great attention in recent years thanks to the possible applications in both energy production (e.g., hydrogen generation by water splitting [1] or photovoltaic generation with Graetzel cells [2]) and pollution control (as advanced oxidation process for polluted air [3] and water [4] treatment). Ambient applications typically involve the development of photocatalytic devices for air or water active treatment or photocatalytic materials to be placed in the target environment with large surface installations. This latter application needs the development

Photocatalytic Specimens Preparation
All samples were prepared in glass Petri dishes (diameter 9.0˘0.1 cm, exposed area 63.5˘1 cm 2 ).

Metakaolin (MK and MK60) and Fly Ash (FA and FA60) Geopolymer-Based Samples
Concerning to MK and MK60 synthesis, the alkaline activating solution was prepared by dissolving solid sodium hydroxide into the sodium silicate solution. The solution was then allowed to equilibrate and cool for 24 h. The composition of the obtained solution can be expressed as Na 2 O¨1.34SiO 2¨1 0.5H 2 O. Meanwhile, in the case of the preparation of FA and FA60 specimens, the activating solution was obtained by means of mixing of the sodium silicate solution with a sodium hydroxide solution (15 M). Moreover, in this case, the solution was left to equilibrate and cool for 24 h. Its composition can be expressed as Na 2 O¨0.7SiO 2¨1 0.5H 2 O. For both sets of samples, the raw materials (metakaolin for MK and MK60 and fly ash for FA and FA60, respectively) were incorporated into the activating solution (with a liquid-to-solid ratio of 1.4:1 by weight for metakaolin-based samples and 0.66:1 for fly-ash-based samples respectively) and mixed with a mechanical mixer for 10 min at 800 rpm. Finally, the photocatalyst (3% by weight with respect to geopolymer paste) was added to the freshly prepared geopolymer suspension and quickly incorporated by controlled mixing (5 min at 1000 rpm).

Hybrid Siloxane-Metakaolin Geopolymer Samples (HS and HS60)
Hybrid polysiloxane-geopolymer samples were prepared by incorporating 10% by weight of a commercial oligomeric dimethylsiloxane mixture into the freshly prepared metakaolin-based geopolymer suspension under mechanical stirring, when the polycondensation reaction of both the geopolymer and dimethylsiloxane had already started but were far from completion. Moreover, in this case, the photocatalyst (3% by weight) was added to the freshly prepared polysiloxane-geopolymer paste and quickly incorporated by controlled mixing (5 min at 1000 rpm).

Foamed Hybrid Siloxane-Metakaolin Geopolymer Samples (FHS and FHS60)
Hybrid polysiloxane-geopolymer samples were prepared as described in Section 2.2.2. Afterwards, the photocatalyst (3% by weight with respect to the geopolymer paste) was added to the freshly prepared geopolymer composite paste and quickly incorporated by controlled mixing (5 min at 1000 rpm). Finally, silicon powder (0.03% by weight) was added as a foaming agent, and the system was mixed for a further 5 min at 1000 rpm. In this way, an inorganic foaming process can be induced thanks to the gas evolution (hydrogen) during the consolidation of the geopolymer mixture, as reported in the literature [34].

Curing Treatments
As soon as prepared, an initial set of metakaolin-and fly-ash-based specimens (MK; HS; FA) was cast in the Petri dishes and cured in >95% relative humidity conditions at room temperature for 7 days and left for another 21 days in air at room temperature. A second set of samples (MK60; HS60; FA60) was cast in glass Petri dishes and cured in the same relative humidity conditions at 60˝C for 24 h and then kept still in >95% relative humidity conditions at room temperature for another 6 days. Afterwards, the specimens were kept for another 21 days in air at room temperature.
A different curing treatment was reserved for the foamed siloxane-metakaolin-based photocatalytic samples: an initial set of specimens (FHS) was cast in the Petri dishes and cured in >95% relative humidity conditions at room temperature for 7 days and left for another 21 days in air at room temperature. A second set (FHS60) of samples was cast in glass Petri dishes and cured in the same relative humidity conditions at room temperature for 24 h and then at 60˝C for another 24 h. Afterwards, the specimens were kept still in >95% relative humidity conditions at room temperature for another 6 days, and kept for another 21 days in air at room temperature.
All the metakaolin-based samples started solidifying in a few minutes. At the same time, while FA specimens presented a setting time of about 12 h, FA60 samples started solidifying within about 8 h.

Cement-Based Reference Sample (OPC)
A reference photocatalytic cement paste sample was prepared as follows: 5.40 g of titanium dioxide (P25) were suspended in 60 g of deionized water, and 120 g of white Portland cement powder (chloride content 0.02%, w/w, sulfate content expressed as SO 3 2.49%, w/w) were then added. The paste was mechanically mixed using the following procedure: 60 s at low speed, 30 s at high speed, 90 s pause with no mixing, and finally 60 s at high speed. The paste was then poured in the Petri dish and treated with 30 flow table cycles. The sample was allowed to settle for 7 days into a curing chamber (20˝C, >90% RH) and then equilibrated into an environmental chamber (23˝C, 50% RH) until constant weight was achieved. During settling and weight equilibration, the sample was exposed in dark conditions to unfiltered laboratory air. The titanium dioxide content of the sample OPC is 3% (as a titania-cement paste weight ratio).

SEM Analysis
SEM analysis was carried out by means of a Phenom Pro X Microscope (Phenom-World B.V., Eindhoven, The Netherlands) on the surface and fracture surfaces of the samples, without further treatments. The acceleration voltage was in the range 5-15 kV. The energy dispersive X-ray spectrometer (Phenom-World B.V., Eindhoven, The Netherlands) has the following specifications: silicon drift detector, thermoelectrically cooled (LN 2 free); the X-ray window has ultra-thin silicon nitride (Si 3 N 4 ) operating with Mn Kα ď 137 eV energy resolution. EDS (Phenom-World B.V., Eindhoven, The Netherlands) analyses were carried out both on the surface and at different depths, along the section of each sample. The corresponding titanium content (see Table 3) was reported as the average of the four samples.

Apparent Density and Open Porosity Determination
The hydrostatic weighing technique for apparent density and open porosity measurements was carried out by means of a balance OHAUS-PA213 provided by Pioneer. The samples were dried in an oven at 110˝C for 12 h and weighed after cooling at room temperature (weight of dry sample: m d ). Afterwards, the specimens were placed in an empty desiccator and kept in a vacuum for 30 min. Later, the desiccator was filled with water, and the samples were kept immersed for 2 h in a vacuum and then weighed (weight of soaked sample: m s ). Finally, the samples were weighed when immersed in water at atmosphere pressure (soaked immersed sample: m i ). Apparent density (D) and open porosity (P) can be expressed according to the following: (2)

Photocatalytic Activity Characterization
The photocatalytic activities were measured with a dedicated experimental system based on a previously described apparatus for the measurement of the photocatalytic degradation of volatile organic compounds [35].
Briefly, the computer-controlled system ( Figure 1) comprises an air generator based on digital mass-flow controllers (model 5850S, Brooks Instrument, Hatfield, PA, USA), a stirred flow photochemical reactor installed inside an irradiation chamber, and a chemiluminescence NO/NO 2 analyzer (model 200E, Teledyne, San Diego, CA, USA). The stirred flow photoreactor ensures the uniform reactant concentrations at the sample surface even at a high conversion factor. This allows for the avoidance of both the errors due to the longitudinal concentration gradient on the catalyst surface (that is characteristic of laminar flow reactors) and the error propagation in the calculated reaction rate due to a low conversion operation (i.e., differential conditions).
The sample photocatalytic activity can be expressed as degradation rate according to the following: where r is the degradation rate (mol¨m´2¨s´1), C and C 0 are the equilibrated photoreactor pollutant concentrations with and without irradiation respectively (mol¨m´3), Q is the photoreactor volumetric air flow rate (m 3¨s´1 ), and A is the exposed sample area (m 2 ). In order to ensure that all the measurements were carried out at the predefined NO concentration independently from the sample activity, a specifically developed constant-concentration analytical method was used [35]. This method works to reach the desired reactor internal NO concentration (under UV irradiation) by modulating the inlet pollutant flow in a successive approximation trial ( Figure 2). After the reaching and the confirmation of the desired target concentration, the UV source is turned off, and the concentration in dark conditions is measured after equilibration.
photochemical reactor installed inside an irradiation chamber, and a chemiluminescence NO/NO2 analyzer (model 200E, Teledyne, San Diego, CA, USA). The stirred flow photoreactor ensures the uniform reactant concentrations at the sample surface even at a high conversion factor. This allows for the avoidance of both the errors due to the longitudinal concentration gradient on the catalyst surface (that is characteristic of laminar flow reactors) and the error propagation in the calculated reaction rate due to a low conversion operation (i.e., differential conditions). The sample photocatalytic activity can be expressed as degradation rate according to the following: where r is the degradation rate (mol·m −2 ·s −1 ), C and C0 are the equilibrated photoreactor pollutant concentrations with and without irradiation respectively (mol·m −3 ), Q is the photoreactor volumetric air flow rate (m 3 ·s −1 ), and A is the exposed sample area (m 2 ). In order to ensure that all the measurements were carried out at the predefined NO concentration independently from the sample activity, a specifically developed constant-concentration analytical method was used [35]. This method works to reach the desired reactor internal NO concentration (under UV irradiation) by modulating the inlet pollutant flow in a successive approximation trial ( Figure 2). After the reaching and the confirmation of the desired target concentration, the UV source is turned off, and the concentration in dark conditions is measured after equilibration. This is particularly important in the case of the comparison of samples with very different activities because, according to (3), operating with a fixed inlet NO concentration will result in very different internal reactor concentrations and, consequently, in reaction rate values measured at substantially different conditions. The use of a flow photoreactor works to take all the concentration measurements in steady-state conditions following the equilibration of the sample with the target pollutant in the reactor internal atmosphere.

Physical Characterization
A wide set of photocatalytic AAM samples was prepared incorporating a commercial titanium dioxide photocatalyst into several matrices with various compositions. Mix design and curing conditions are recalled in Table 2.  Catalytic activity measurement with the successive approximation process. At the third iteration the pollutant inlet flow required to reach the target concentration C is found; the UV source is then turned off, and the concentration C 0 is measured. This is particularly important in the case of the comparison of samples with very different activities because, according to (3), operating with a fixed inlet NO concentration will result in very different internal reactor concentrations and, consequently, in reaction rate values measured at substantially different conditions. The use of a flow photoreactor works to take all the concentration measurements in steady-state conditions following the equilibration of the sample with the target pollutant in the reactor internal atmosphere.

Physical Characterization
A wide set of photocatalytic AAM samples was prepared incorporating a commercial titanium dioxide photocatalyst into several matrices with various compositions. Mix design and curing conditions are recalled in Table 2.

Photocatalytic Activity
The photocatalytic activity was studied for all samples measuring the NO degradation in air at ambient concentration. For all measurements, both the NO and NO x degradation rates were reported (NO x rate r is calculated as the algebraic sum of NO and NO 2 values).
In order to measure the catalytic activity in consistent conditions throughout the study, all measurements were carried out operating at constant NO concentration as previously described (i.e., the NO concentration C inside the irradiated reactor was the same for all samples within 3% tolerance). The obtained photocatalytic activities of the AAM samples were reported in Figure 3. These measurements were carried out using ambient NO concentration (75 ppb nominal, 3.045 µmol¨m´3 at 27˝C, 1 atm) at 27˘0.2˝C, 50%˘5% RH and 700˘10 mL¨min´1 air inlet flow. The 400˘10 µW¨cm´2 UV-A irradiance was obtained with four 9-W Philips PL-S/10 UV-A fluorescent lamps (all errors are 1 σ estimated repeatability). Before the activity measurement, the samples were equilibrated for more than 30 days in dark conditions at 23˝C, 50% RH.
The samples demonstrate very differentiated photocatalytic activities depending on aluminosilicate source and on curing conditions, with NO degradation rate values spanning from about 3 nmol¨m´2¨s´1 to zero (no measurable activity). All samples demonstrate lower NO x degradation rate in comparison to the NO value, indicating that the NO oxidation was not complete and in these conditions, some NO 2 was desorbed from the samples before mineralization. All the samples cured at 60˝C demonstrate a remarkably lower activity than the corresponding samples cured at RT. In some cases, the samples cured at 60˝C does not demonstrate any measurable NO degradation activity. The sample FA based on fly ash and cured at RT demonstrates the best performance with a twofold NO degradation rate compared with the metakaolin-based sample MK. The hybrid sample (HS) demonstrates a remarkable smaller catalytic activity (about 30% of the MK sample NO degradation rate). The obtained photocatalytic activities of the AAM samples were reported in Figure 3. These measurements were carried out using ambient NO concentration (75 ppb nominal, 3.045 μmol·m −3 at 27 °C, 1 atm) at 27 ± 0.2 °C, 50% ± 5% RH and 700 ± 10 mL·min −1 air inlet flow. The 400 ± 10 μW·cm −2 UV-A irradiance was obtained with four 9-W Philips PL-S/10 UV-A fluorescent lamps (all errors are 1 σ estimated repeatability). Before the activity measurement, the samples were equilibrated for more than 30 days in dark conditions at 23 °C, 50% RH. The samples demonstrate very differentiated photocatalytic activities depending on aluminosilicate source and on curing conditions, with NO degradation rate values spanning from about 3 nmol·m −2 ·s −1 to zero (no measurable activity). All samples demonstrate lower NOx degradation rate in comparison to the NO value, indicating that the NO oxidation was not complete and in these conditions, some NO2 was desorbed from the samples before mineralization. All the samples cured at 60 °C demonstrate a remarkably lower activity than the corresponding samples cured at RT. In some cases, the samples cured at 60 °C does not demonstrate any measurable NO degradation activity. The sample FA based on fly ash and cured at RT demonstrates the best performance with a twofold NO degradation rate compared with the metakaolin-based sample MK. The hybrid sample (HS) demonstrates a remarkable smaller catalytic activity (about 30% of the MK sample NO degradation rate). The further addition of an expanding agent (metallic silicon) on sample FHS shows appreciable improvement, but the activity of this sample is lower than that of the metakaolin sample MK. The sample FA activity (Figure 4) was then compared with the ordinary Portland cement reference sample OPC using a lower irradiance (120˘5 µW¨cm´2 UV-A) in order to avoid an excessive conversion rate for the latter sample. The photocatalytic activity of the sample FA is significantly lower than the activity of the reference sample OPC, but it can nevertheless be considered in the same order of magnitude (about a factor two difference). This result is particularly interesting considering the room for further optimization given by the characteristic variety of AAM materials and the highly differentiated photocatalytic activities demonstrated in the present work. The further addition of an expanding agent (metallic silicon) on sample FHS shows appreciable improvement, but the activity of this sample is lower than that of the metakaolin sample MK. The sample FA activity (Figure 4) was then compared with the ordinary Portland cement reference sample OPC using a lower irradiance (120 ± 5 μW·cm −2 UV-A) in order to avoid an excessive conversion rate for the latter sample. The photocatalytic activity of the sample FA is significantly lower than the activity of the reference sample OPC, but it can nevertheless be considered in the same order of magnitude (about a factor two difference). This result is particularly interesting considering the room for further optimization given by the characteristic variety of AAM materials and the highly differentiated photocatalytic activities demonstrated in the present work.

Scanning Electron Microscopy Analysis
Because of UV radiation penetration and the reactant diffusion limits, the heterogeneous photocatalytic degradation of airborne pollutant is governed by surface processes. In order to study the catalyst distribution in the AAM matrices, a series of SEM analyses was carried out on the surfaces of AAM specimens that showed the most interesting photocatalytic activities (MK and FA

Scanning Electron Microscopy Analysis
Because of UV radiation penetration and the reactant diffusion limits, the heterogeneous photocatalytic degradation of airborne pollutant is governed by surface processes. In order to study the catalyst distribution in the AAM matrices, a series of SEM analyses was carried out on the surfaces of AAM specimens that showed the most interesting photocatalytic activities (MK and FA samples). Particularly, the SEM images of the surface of the examined samples were reported in Figure 5. This figure shows that a pristine (i.e., without photocatalyst) metakaolin-based geopolymer sample ( Figure 5A) is characterized by a compact morphology revealing some unreacted kaolinite crystals. The sample MK ( Figure 5B) shows a lesser compact surface structure when compared with the pristine geopolymer, with the presence of pores of a different size, uniformly distributed. SEM images of sections of the MK and FA samples have been also carried out ( Figure 6). While the metakaolin-based sample MK ( Figure 6A) shows a poorly compact morphology, quite similar to that one analyzed on surface of the sample, the morphology of the FA sample ( Figure 6B) is dominated by the presence of unreacted fly ash particles that are well dispersed in the geopolymer matrix. The uneven morphology of the FA sample is due to the limited reactivity of the fly ash particles and causes the non-completeness of the geopolymerization reaction. The pristine fly-ash-based geopolymer ( Figure 5C) is characterized by a very disaggregated morphology, typical of this kind of geopolymers [36][37][38]. The specimen FA ( Figure 5D) shows a complex morphology where it is not possible to clearly identify the presence of pores, but the surface structure appears rather uneven with the presence of small domains, and some appear spheroidal.
SEM images of sections of the MK and FA samples have been also carried out ( Figure 6). While the metakaolin-based sample MK ( Figure 6A) shows a poorly compact morphology, quite similar to that one analyzed on surface of the sample, the morphology of the FA sample ( Figure 6B) is dominated by the presence of unreacted fly ash particles that are well dispersed in the geopolymer matrix. The uneven morphology of the FA sample is due to the limited reactivity of the fly ash particles and causes the non-completeness of the geopolymerization reaction.
SEM images of sections of the MK and FA samples have been also carried out ( Figure 6). While the metakaolin-based sample MK ( Figure 6A) shows a poorly compact morphology, quite similar to that one analyzed on surface of the sample, the morphology of the FA sample ( Figure 6B) is dominated by the presence of unreacted fly ash particles that are well dispersed in the geopolymer matrix. The uneven morphology of the FA sample is due to the limited reactivity of the fly ash particles and causes the non-completeness of the geopolymerization reaction. In order to assess the titania distribution on the sample, the relative titanium content on the surface and at different depths along the specimen section was measured with SEM/EDS (Table 3). In order to assess the titania distribution on the sample, the relative titanium content on the surface and at different depths along the specimen section was measured with SEM/EDS (Table 3). The obtained data indicate uneven catalyst distribution between the surface and the initial layers (up to 600 µm of depth) of the inorganic matrices. Particularly, the fly-ash-based sample cured at room temperature shows higher surface titanium content than metakaolin-based sample cured in the same condition. In addition, both samples cured at 60˝C indicate a surface Ti content lower than the corresponding samples cured at room temperature. Segregation phenomena of titania can be caused by several physico-chemical phenomena, including agglomeration determined by low shear mixing or by particle-to-particle surface interactions. The data suggest a possible convective transport during the casting and curing phase where surface water evaporation can drive local redistribution of unreactive low dimension particles with marked dependence on ambient conditions (e.g., temperature and RH). It is worth pointing out that the samples that show a higher surface amount of TiO 2 also possess a higher catalytic activity. Meanwhile, the samples that have a lower concentration of catalyst on the surface show evident segregation phenomena of TiO 2 in depth (i.e., MK60 and FA60 samples, see Table 3). Moreover, all the studied samples shows a marked decrease in the photocatalytic activity if cured at 60˝C (Section 3.2), and this appears to be reflected by a significant decrease in the surface titania content in the corresponding analyzed sections. These data suggest that titania distribution in the sample surface layers can play a relevant role in the determination of the final sample photocatalytic activity.

Conclusions
An initial comparative assessment of the AAM binders potentials as photocatalyst support matrices was carried out using four different types of AAM: metakaolin geopolymer; fly ash geopolymer; hybrid siloxane-metakaolin geopolymer; and foamed hybrid siloxane-metakaolin geopolymer. The samples was characterized by means of SEM-EDS analysis.
The photocatalytic activity of the samples was evaluated in terms of NO abatement. The photocatalytic activity data show a strong variation depending on the type of binder and the curing process. The highest photocatalytic activity was detected for fly ash-based AAM matrices cured at room temperature. Metakaolin-based AAM matrices also showed promising photocatalytic activity.
A systematic decrease of photocatalytic activity was observed when the same AAM support matrix was cured at higher temperature (60˝C).
EDS data of the studied samples indicate a conspicuous segregation effect depending on AAM matrix and curing temperature, with a notable depletion of the surface titania content for the samples cured at 60˝C. Photocatalytic activity data correlate with the surface titania content measurements suggesting that titania segregation may play a distinct role in the determination of photocatalytic activity.
The described results demonstrate that AAM binders can be interesting photocatalyst support matrices. The high variation of catalytic activity evidenced by the different samples and the inherent variety of AAM binders suggest, moreover, large possibilities in performance enhancement. Particularly, the described results indicate that the optimization of the photocatalyst dispersion by the curing process tailoring and selection of the AAM aluminosilicate precursor-activating solution combination can play a fundamental role in the development of high performance AAM photocatalytic materials. Furthermore, given the good activity of the studied AAM samples in comparison to ordinary Portland cement matrix it is reasonable to expect the future development of high performance photocatalytic AAM materials.