The Preparation of Photocatalytic Porous Magnesium Oxychloride Cement-Based Materials and Its De-NO_x Performance

Abstract

Porous magnesium oxychloride cement (PMOC) has a high specific surface area formed by interlocking whiskers, which can be used as a promising photocatalyst substrate for the photocatalytic removal of atmospheric pollutants. In this paper, magnesium oxychloride cement (MOC) was used as matrix and TiO₂ as catalyst to prepare MOC blocks. Plant-based protein was used as a foaming agent to form the layered porous structure suitable for supporting TiO₂ particles, which effectively increased the surface area of light radiation and TiO₂ adhesion area in photocatalytic porous magnesium oxychloride cement (PPMOC). It was found that the addition of the foaming agent can increase the adsorption capacity of MOC to TiO₂. The vacuum-immersion loading method can effectively support TiO₂ on the surface of PMOC. The photocatalytic performance of PPMOC can be improved by multiple loading, while higher porosity of PMOC would reduce the loading surface of matrix to TiO₂ particles, which might decrease the photocatalytic efficiency. As can be observed in PPMOC specimens, when the porosity of PPMOC is less than 60%, increasing the porosity can improve the photocatalytic efficiency, while when the porosity is higher than 60%, increasing the porosity decreased the photocatalytic efficiency due to the reduction of the loading surface. The excellent nitrate selectivity of PPMOC also shows good application potential in the field of catalytic degradation of nitrogen oxides.

1. Introduction

Photocatalytic cement-based materials have attracted a lot of attention in recent years [1,2]. This type of material combined with the scale application feature of building materials and the purification function of photocatalysts (e.g., TiO₂ photocatalyst) provide us a chance to eliminate the harmful gases in our living environment, including indoor and outdoor harmful gas, such as NOx, HCHO, and so on. Due to these advantages, many new materials, technologies, and methods are developed to promote the application of photocatalysts and photocatalytic cement-based materials and have obtained good results for research and application [3,4].

As a building material, magnesium oxychloride cement (MOC) has the advantages of high mechanical strength, fast setting, strong abrasive resistance, excellent fire resistance, and superb bonding power [5,6,7,8]. However, when magnesium oxychloride cement is in contact with water for a long time the mechanical properties are significantly reduced, which makes it difficult to expand its application in structural materials [9,10,11,12,13]. For the photocatalyst application potential, magnesium oxychloride cement has its own advantages. Magnesium oxychloride cement was used to replace wood and was widely used in decorative materials (imitation ivory, door handles, and stone carvings), floors, walls, grinding wheels, and even tombs. The hydrated product of MOC, the basic magnesium chloride whisker, is cross-linked to form a porous network structure that gives it a high specific surface area, which makes it suitable for use as a photocatalyst carrier for interior decoration materials. In the previous works, the basic magnesium chloride whisker loaded TiO₂ photocatalyst illustrated a great photocatalytic performance with a 62% degradation ratio of gaseous toluene and the reaction rate of 25.3 × 10⁻⁷ mol/m² min [14]. The cheapness and stability of the photocatalytic cycles make it a photocatalytic material for air purification in the building decorative field. Furthermore, for the thermal stability of this type of material, due to the limitations of cement-based matrices, the highest using temperature is probably less than 100 °C but still enough for the common use in the buildings.

On the other hand, increasing the surface area of photocatalytic cement-based materials by a foaming agent is a common modification method to improve the loading capacity of photocatalytic supporting materials, which is better to enhance the photocatalytic efficiency. After MOC foaming, the specific surface area is greatly increased due to the formation of a multi-layer pore structure. Foamed MOC becomes an excellent architectural decoration material. Additionally, foamed MOC presents good mechanical properties and the compressive strength of foamed MOC is higher than that of foamed ordinary Portland cement with a similar porosity. As reported by our previous work, the compressive strength can achieve up to ~5 MPa with about 75% porosity, meanwhile, the increase of pores is also conducive to TiO₂ adhesion [15,16,17]. With its extremely high specific surface area and the special pore structure formed by the interlacing of whiskers, porous magnesium oxychloride cement (PMOC) provides an excellent condition for the loading of TiO₂ or other photocatalysts and enhances their photocatalytic reaction process. Based on the above, in this work, the effects of PMOC with different porosity and TiO₂ photocatalysts loading times on the NOx degradation performance were studied. The properties and photocatalytic performance of prepared samples were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), stereo microscope, and NO_x photocatalytic performance test. The nitrate selectivity and photon efficiency of NO, NO_x, and NO₂ was calculated and analyzed to evaluate its photocatalytic performance. We hope the research can provide a new method for the promotion of photocatalytic material applications.

2. Result and Discussion

2.1. Physicochemical Properties

Figure 1 shows the macroscopic structure of PPMOC taken by SEM. After loading, it can be seen that TiO₂ photocatalyst particles attached on its surface. However, the smooth surface is difficult to attach a large amount of TiO₂; Figure 1b shows the pore structures of PPMOC3 after loading. The pore structure has been filled with TiO₂ particles, and the framework structure composed of short rods has been completely wrapped by TiO₂ adhesion forming a dense layer of TiO₂ particle clusters on the surface. The macroscopic stomatal structure can also be clearly observed and shows that TiO₂ spreads over the entire pore structure. From Figure 1c, it can be found that more TiO₂ particles have been loaded on the framework structure compared to that of Figure 1b. This indicates that decreasing the density of PMOC is beneficial to the loading of TiO₂, while large amount of TiO₂ particles can cause agglomerations and form the large clusters. According to our previous reports [15,16], the prepared matrix composition is mainly 5Mg(OH)₂·MgCl₂·8H₂O (phase 5) and contains a certain amount of Mg(OH)₂; the results also indicate the loaded TiO₂(P25) is mainly composed of anatase TiO₂ and is successfully loaded on the PMOC matrix.

The surface changes of the substance before and after TiO₂ loading observed by the stereo microscope are shown in Figure 2. The surface of PPMOC1 in Figure 2a is relatively smooth without uniform pore-like structures. There is a small amount of white powder on the surface, which indicates a small number of TiO₂ particles on the surface after loading. While an obvious TiO₂ loading layer can be seen in PPMOC3 (Figure 2b), some of the smaller pores within the bigger pores also can be observed. On the surface of PPMOC5 (Figure 2c), there is a large amount of TiO₂ in the pores. Some smaller pores within the larger pores are also covered with TiO₂ particles. This suggests that the porous structure provides more attachment points for TiO₂ particles, but too many TiO₂ particles also could result in the decrease of light and gas transmission and diffusion. Accordingly, this process probably is useful to enhance the photocatalytic efficiency of materials. By forming the stack and interlaced network pores between TiO₂ particles and MOC hydrates (phase 5, 5Mg(OH)₂·MgCl₂·8H₂O), the loading of TiO₂ particles is expected to increase the mesoporous structures and absorption volume of the matrix, which is helpful to the diffusion of light and gas and are quite vital to the photocatalytic reaction of system.

2.2. Photocatalytic Performance

2.2.1. Photocatalytic Performance of PPMOC and TiO₂

The curve of Figure 3 presents the concentration differences of NO, NO₂, and NO_x in the process of photocatalytic reaction of the sample. Generally, the NO_x photocatalytic degradation and the evaluation of photocatalytic efficiency have three steps [18,19,20]: (1) before photocatalysis (dark reaction step), the sample should keep in the dark condition to let the gas concentration become stable; (2) photocatalytic reaction step, after the gas concentration becomes stable, start to irradiate the samples and the photocatalytic reaction happens; the NO and NO_x concentration decreases and NO₂ concentration increases due to the generation of NO₂, while all of these concentrations would gradually get to the stable state; this process should remain for approximately 2 to 3 h; (3) after photocatalysis (re-dark reaction step), in this step, the light should be turned off, and the concentrations of NO, NO₂, and NO_x would go back to the initial state, similar to the first step. As an illustration, it can be seen from Figure 3 that the concentrations of NO, NO₂, and NO_x keep the stable level in the initial dark reaction, while NO and NO_x concentrations decreased rapidly and NO₂ gradually generation after being irradiated by the light means the photocatalytic reaction happened. It also can be seen that the NO, NO₂, and NO_x concentration gradually become relatively stable after 1 to 2 h of the photocatalytic reaction, indicating the photocatalytic reaction becomes stable. The degradation of NO and NO_x and the generation of NO₂ is obtained. After the light is turned off, the NO, NO₂, and NO_x concentrations return to the initial states to carry out the re-dark reaction. To obtain the stable data, the experiment was carried out by using a continuous photocatalytic reaction test method; each sample was repeated three times for at least 3 h. By using this method, the repeatability of photocatalytic ability can be further investigated.

According to the curve in Figure 3, the photonic efficiency and nitrate selectivity of photocatalytic samples to NO_x degradation can be calculated as shown in Figure 4. It can be clearly seen that PPMOC has stronger photocatalytic power than that of pure P25 because PPMOC has a multilayer pore structure, which can provide a lot of loading points for TiO₂ particles. The PPMOC3 has lower NO₂ generation efficiency and higher photon degradation efficiency of NO and NO_x, which means it has higher nitrate selectivity. The multilayer pore structure of PPMOC also can improve the specific surface area, light utilization ratio, and gas diffusion rate, which improves the photocatalytic efficiency.

2.2.2. Effect of Porosity and Loading Concentration on Photocatalytic Efficiency

Figure 5 and Figure 6 show the photocatalytic removal efficiency of different porosity samples under loading once and loading twice in TiO₂ solution. It is found that the NO and NO_x conversion rate increases with increasing sample porosity but decreases in excessive high porosity. As can be calculated, the porosity increases from ca. 5% to ca.54% resulting in the NO photonic efficiency increase from 0.185% to 0.722%, while it decreases to 0.576% as porosity increases to ca. 66%. These results indicate that the adsorption capacity of TiO₂ on the surface of the MOC matrix is gradually increasing with the increase in porosity. However, as the porosity continues to increase, the NO and NO_x conversion efficiency decreases gradually. The photon efficiency of NO₂ also decreased slightly when PPMOC2 reached its peak, from 0.232% to 0.144%. The maximum photon efficiency of NO₂ is reached in PPMOC4 samples (0.263%). This is resultant from the fewer sites on the surface of MOC that can be used to adhere for TiO₂ in the early stages. As the porosity increases, the increase in the number of perforated structures and the short rod-like structure provide more attachment points for TiO₂ particles. Thus, improving the utilization rate of light energy and increasing the amount of catalyst leads to an increase in the photocatalytic efficiency. Nevertheless, the high porosity combined with the low TiO₂ loading concentration would result in the reduction of photocatalytic efficiency because of the low loading surface and TiO₂ loading amounts.

Figure 6 shows the NO_x photonic efficiency for the PPMOC after loading twice. After loading twice, the NO and NO_x photonic efficiency increases comprehensively compared to loading once. The highest photon efficiency of NO and NO_x were found in the PPMOC3 sample, which was 1.00% and 0.717%, respectively. It is explained that the surface loading of PMOC matrix has not reached saturation after loading once and the pore structure is not fully filled. In this case, the twice loading can provide a large amount of TiO₂ particles to the substrate at low porosity, thereby increasing the photonic efficiency. At the same time, the maximum conversion efficiency of the twice loading samples are higher than that of the maximum efficiency of the once loading samples. Therefore, it is concluded that higher porosity can adsorb more TiO₂ and NO_x molecules, and thus can improve the photocatalytic efficiency. However, excessive high porosity will result in a decrease in the effective surface area, which is harmful to photocatalytic efficiency. It is worth noting that the generation efficiency of NO₂ is quite similar after the twice loading of TiO₂ on PMOC. Generally, the NO₂ generation efficiency is highly related to the NO degradation efficiency and nitrate selectivity of materials. In this case, the lower NO degradation efficiency would result in the relative NO₂ generation efficiency, as shown in PPMOC1 in Figure 5, while, in Figure 6, it can be observed that the NO degradation efficiency is much higher in comparison to that of the first loading samples meaning the similar NO₂ generation efficiency is caused by other reasons. In this experiment, the PMOC matrix was used as the substrate of TiO₂ particles. Due to the alkaline environment, the hydrates of this matrix (phase 5 and Mg(OH)₂) probably have high reactivity with NO₂ and NO₃⁻, which could promote the photocatalytic reaction process of NO to NO₂ and NO₂ to NO₃⁻, thus balancing the generation efficiency of NO₂, which is actually good for the nitrate selectivity.

2.2.3. Effect of Porosity on Nitrate Selectivity

Figure 7 and Figure 8 show the nitrate selectivity of the prepared samples. It is clear that the porosity has no direct effects on nitrate selectivity, and the overall nitrate selectivity is slightly improved after loading twice. As shown in Figure 8, although the nitrate selectivity of P25 is about 40%, the overall nitrate selectivity has been greatly improved when P25 was loaded in PMOC. The nitrate selectivity exceeds 70% after twice TiO₂ loading. Meanwhile, the adsorption capacity of nitrogen oxides has been enhanced as well. This probably can be explained by Equations (1)–(8) [21,22]. Adsorbed water molecules react with photogenerated charges to generate ^●OH_ads and ^●O₂⁻_ads radicals, in which NO_ads would be oxidized to NO_2ads and HNO_3ads. In this case, the absorbing capacity is beneficial to the HNO_3ads formation, thus high porosity samples present better nitrate selectivity. Furthermore, Mg(OH)₂ in PPMOC also can be involved in the reaction; Mg(OH)₂ can fix HNO_3ads into NO₃⁻ in PPMOC, thereby reducing NO₂ emissions and greatly improving nitrate selectivity as well. For pure P25 and low porosity PPMOC samples, the photocatalytic oxidation of NO would generate the NO_2ads and HNO_3ads. However, since there is no substance that can fix these species, the generated NO_2ads would be released, while in high porosity PPMOC, the generated NO_2ads and HNO_3ads can react well with Mg(OH)₂ to form NO₃⁻, thus improving the nitrate selectivity.

TiO₂ + hv → TiO₂ (h⁺ + e⁻)

(1)

TiO₂ (h⁺) + H₂O_ads → TiO₂ + ^●OH_ads + H⁺

(2)

TiO₂ (e⁻) + O_2ads → TiO₂ + ^●O₂⁻_ads

(3)

NO_ads + ^●OH_ads → HNO_2ads + ^●OH_ads →NO_2ads + H₂O

(4)

NO_ads + ^●O₂⁻_ads + H⁺ → NO_2ads + ^●OH

(5)

NO_ads + ^●O₂⁻_ads + H⁺ → HNO_3ads

(6)

NO_2ads + Mg(OH)₂ → Mg(NO₃)₂ + H₂O

(7)

HNO_3ads + Mg(OH)₂ → Mg(NO₃)₂ + H₂O

(8)

As shown in Figure 9, the photocatalytic reaction process for PPMOC to NO-NO₂-NO_x can be concluded. On one hand, the active radicals (^●OH_ads and ^●O₂⁻_ads), generated by the TiO₂ photocatalyst participated in the photocatalytic reaction to transfer the NO to NO₂ and NO₃⁻; on the other hand, the PMOC matrix also can participate in the degradation reaction due to its specialized hydrates, such as alkalized phase 5 (5Mg(OH)₂·MgCl₂·8H₂O) and Mg(OH)₂. These hydrates can transfer and solidify the generated NO₂ and NO₃⁻ molecules, further promoting the degradation reaction of the system, thus presenting a high photocatalytic efficiency.

3. Experimental Section

3.1. Preparation of PMOC and PPMOC

In this study, MOC slurry was prepared by the physical foaming method with a molar ratio of MgO:MgCl₂:H₂O = 5:1:14 as reported in our previous work [15,16,23,24]. MgO and MgCl₂ for the preparation of MOC were purchased from China National Pharmaceutical Group Corporation, which were all analytical reagent grade. Different amounts of plant-based protein foaming agent were added to produce different densities and surface pores of MOC. The preparation process is as follows: 49.18 g of MgO powder and 50.82 g of MgCl₂ aqueous solution with a mass fraction of 27.4% were mixed to form the MOC slurry. The different amounts of foaming were added to the cement slurry and mixed forming the foamed PMOC slurry. The PMOC slurry was cast into the moulds (50 mm × 100 mm) and then cured at a temperature of 20 ± 1 °C and a relative humidity of 60 ± 5% for one day. Finally, the size, density, and porosity of the demould samples were measured.

Table 1. Parameters of PMOC for different porosities.

Name	Amount of Foaming Agent Added mL	Weight g	Length cm	Width cm	Height cm	Density g·cm−3	Porosity
PMOC1	0	45.20	10	5	0.53	1.702	5.4%
PMOC2	50	21.47	10	5	0.52	0.826	54.1%
PMOC3	100	22.07	10	5	0.615	0.718	60.1%
PMOC4	150	23.56	10	5	0.68	0.693	61.5%
PMOC5	200	18.04	10	5	0.59	0.612	66.0%

PPMOC1, PPMOC2, PPMOC3, PPMOC4 and PPMOC5 is the photocatalytic porous magnesium oxychloride cement loaded with TiO₂, respectively.

shows the parameters of PMOC for different porosities. PMOC1 without the foaming agent had a porosity of 5.4% and a density of 1.702 g·cm⁻³. When increasing the foaming agent content, the density of PMOC decreases and the porosity increases gradually. The porosities of PMOC2, 3, 4, and 5 are 54.1%, 60.1%, 61.5%, and 66.0%, respectively. The highest porosity was observed in the PMOC5 samples. High porosity is beneficial to photocatalyst loading and increasing the specific surface area of the photocatalytic reaction. Meanwhile, the porous structure of PMOC is conducive to the adsorption of polluted gas to photocatalytic degradation.

In order to load TiO₂ uniformly on the surface of PMOC, a vacuum negative pressure immersion loading method was used to prepare PPMOC [25]. The schematic image of the preparation apparatus is shown in Figure 10. The preparation route was as follows [15,16]: firstly, the surface of the prepared sample was polished by 60, 240, and 800 sandpaper. Secondly, 5 g of TiO₂ (P25) was dissolved in 500 mL of C₂H₅OH solution (A.R.) (P25 solution). Then, the TiO₂ absolute solution was placed in an ultrasonic dispersion instrument for 20 min to produce a uniform TiO₂ suspension, then the prepared MOC or PMOC sample was added for 5 min. After that, the MOC or PMOC sample soaked TiO₂ suspension was placed in a 0.06–0.08 MPa vacuum chamber for 1 h, and finally the photocatalytic MOC or PMOC samples were obtained after drying the samples in a 40 °C oven for 1 h.

3.2. Characterization

3.2.1. SEM

The microscopic morphology of the sample was analyzed by QUANTA FEG 450 environmental field emission scanning electron microscope (SEM) at an accelerating voltage of 15 KV, which was produced by the FEI (Carl Zeiss Co., Ltd., Oberkochen, Germany).

3.2.2. Stereomicroscope

The macroscopic morphology and pore structures of the prepared MOC, PMOC, and PPMOC samples were also observed by stereo microscope (SZX16, Olympus, Jiayuan Xingye Technology Co., Ltd., Beijing, China).

3.2.3. Photocatalytic Performance Test

The photocatalytic performance of samples was tested by using NO_x gases as model pollutants in a standard gas reactor (ISO 22197-1: 2007). The test procedure is shown in Figure 11. The illumination light source was a 300 W xenon lamp (provided by CEL-HXF300, CeauLight Co., Ltd., Beijing, China; λ_max = 365 nm), and controlling the UV radiation intensity (main wavelength is 365 nm) on sample surface was 1 mW·cm⁻² by the UV-vis intensity test instrument monitor. The gas concentration was 1 ppm ± 0.05 NO gas with the wet air of 50% relative humidity in a 3 L·min⁻¹ flow rate, which was tested by the NO_x monitor (HN-CK5001, Taiyuan Hainachenke Instrument Co., Ltd., Taiyuan, China). The NO, NO₂, and NO_x gas concentration during the photocatalytic reaction process was also tested by the above instrument. The other test procedures can be seen in our previous work [15].

The photonic efficiency (ξ) of the NO₂ generation and the removal of NO and NO_x and the nitrate selectivity (S%) are calculated by Equation (9) and (10), respectively [26,27]:

$$\mathsf{\xi }=\frac{({\mathsf{C}}_{\mathsf{d}} - {\mathsf{C}}_{\mathsf{i}})\mathrm{VP}}{\mathsf{\Phi }\mathrm{ART}}$$

(9)

$$\mathrm{S}\%=\frac{{\mathsf{\xi }}_{\mathsf{NOx}}}{{\mathsf{\xi }}_{\mathsf{NO}}}\times 100$$

(10)

Where, C_d is the gas concentration under dark conditions in ppb. C_i is the gas concentration under light conditions in ppb. V is the rate of gas flow in m³∙s⁻¹. P is the atmospheric pressure in Pa. A is the sample area of reaction in m². R is the gas constant in J∙mol⁻¹∙K⁻¹. T is the absolute temperature in K, and Φ is the photon flux in mol∙s⁻¹∙m².

4. Conclusions

In this study, the TiO₂ loaded PMOC with porosity from ca. 5% to 66% was prepared by using the vacuum negative pressure immersion method. The physiochemical properties, N_Ox photocatalytic efficiencies, and nitrate selectivity were investigated in detail. The conclusions reached are as follows.

The porous structure of PMOC facilitated TiO₂ loading on the matrix surface by the vacuum negative pressure immersion loading method. The framework structure between the internal air holes of PMOC provided an attachment point for TiO₂ particles. Through a SEM and stereo microscope, it can be observed that a large amount of TiO₂ is supported in the pores and frame structure of PMOC, and TiO₂ clusters are also formed. The photocatalytic efficiency of PPMOC is much higher than that of pure P25 because its higher specific surface area improves light utilization and the gas diffusion rate. Meanwhile, the nitrate selectivity of PPMOC is higher than that of pure TiO₂, which shows that PMOC is the great carrier for the photocatalyst.

The effective area of TiO₂ in contact with light was greatly increased, thus increasing the photocatalytic efficiency. The photonic efficiency of NO is up to 0.722% after the once loading of TiO₂ in PPMOC2_. However, the photonic efficiency of NO decreases from 0.722% to 0.576% with increasing porosity. This is because that relative high porosity of PMOC would reduce the loading surface of the matrix to TiO₂ particles, which might decrease the photocatalytic efficiency. After TiO₂ loading twice, the photonic efficiencies of NO, NO_x, and NO₂ are significantly improved. However, the highest photonic efficiency of NO and NO_x appeared in PPMOC3, which were 1.0% and 0.717%, respectively. Excessively high porosity will result in a decrease in the effective surface area, which is harmful to photocatalytic efficiency. This is resultant from the reduction of the effective surface area, which impairs the photocatalytic efficiency of NO and NO_x.

The PMOC substrate can significantly enhance the photocatalytic nitrate selectivity of TiO₂ to NO_x gases, of which the nitrate selectivity can reach to 70%; this is due to the high absorb capacity and phase 5 Mg(OH)₂ reactivity of PPMOC with generated NO₂ and NO₃⁻, which can involve in the photocatalytic reaction process of NO_x and highly reduces the release of generated NO₂ harmful gas.

The excellent model of photocatalytic degradation of nitrogen oxides and high nitrate selectivity make PPMOC a promising material in the field of photocatalytic functional decorative materials, which also expands the application of MOC in functionalization.