Study on the MOF Frame Pt-TiO₂ Hybrid Photocatalyst and Its Photocatalytic Performance

Abstract

As the efficient catalysts are synthesized by a simple strategy, photocatalysis offers a sustainable route to solve the problem of environmentally sound treatment of industrial dye wastewater. Herein, Pt-doped TiO₂ with MIL-125 as the backbone has been fabricated using thermal synthesis. The photocatalytic efficiency of the synthesized catalyst samples was tested using Rhodamine B (RhB) as the contaminant model. The physicochemical properties of the solid specimens were characterized using XRD, SEM, TEM, BET and EDX techniques. The photocatalytic degradation experiments showed that the photocatalytic degradation of RhB by M-Pt-TiO₂ was 98.97% after 30 min of degradation. The radical capture experiments showed that superoxide radicals and conduction band electrons as reactive oxygen species played a major role in the degradation process. After four cycles, the photocatalytic activity of M-Pt-TiO₂ decreased from 98.4% to 94.9%, indicating that the photocatalyst sample had good photocatalytic stability.

1. Introduction

With rapid social and economic development, the question of how to treat industrial wastewater in a rational and efficient manner has to be addressed [1]. As one of the major types of industrial wastewater, dye wastewater is characterized by complex composition, high chromaticity and its difficulty with biodegradation. Some of the dye wastewater is also highly toxic and has “tritogenic” effects (mutagenesis, carcinogenesis and teratogenesis), which can cause serious harm to the ecological environment and human health. Once a large amount of dye wastewater enters an aquatic ecosystem, it can be very harmful to the ecosystem. Photocatalytic oxidation technology provides a green and sustainable way to purify air and bodies of water. It mainly uses semiconductor materials as a medium to convert light energy into chemical energy and produces oxygen-containing groups with a certain oxidation capacity to achieve the effective degradation of pollutants in water bodies. Due to its high efficiency, safety and environmental friendliness, photocatalytic oxidation is now widely used in industrial wastewater treatment [2,3,4].

Metal-organic frameworks (MOFs) is a generic term for a class of organic–inorganic hybrid materials, which usually form a multidimensional spatial lattice structure linking different metal ions or groups through multidentate organic groups [5,6]. MOFs are characterized by high pore volume, high specific surface area and variable pore size and functional groups and have gained widespread interest in gas adsorption and storage, catalysis and optoelectronics [7,8,9]. MOFs have been widely used in the field of photocatalysis. The band gap of MOFs is closely related to their HOMO-LUMO energy levels and their photo-responsiveness can be improved by modulating the central metal or organic ligands [10,11]. As a typical Ti-based MOF material with photocatalytic activity, MIL-125 uses TiO₂ octahedra as the metal framework and is linked by p-phenylene dimethyl (BDC) organic groups to form a three-dimensional spatial lattice structure with good water/light stability, which has promising applications in photocatalysis [12,13]. However, MIL-125 has a forbidden band width of approximately 3.6 eV and its weak spectral absorption and slow carrier transport limit its catalytic performance [14]. It needs to be compounded with other low-bandwidth materials to effectively enhance its photocatalytic performance. The advantage of MOFs compared to other solid state synthesis precursors of metal oxides is their porosity and long-term orderliness, which provides unique opportunities for the synthesis of unusual metal oxide morphologies. In order to obtain TiO₂ materials, the Ti-containing MOFs (e.g., MIL-125) can be used as sacrificative precursors. Zhao et al. [15] reported a series of TiO_x/C composites, prepared by pyrolyzing MIL-125, which exhibited excellent catalytic activities towards the photodegradation of methylene blue (MB) in aqueous solution. Khaletskaya et al. [16] reported that a series of gold/titania nanocomposites GNP/TiO₂, fabricated through the pyrolysis of GNP/NH2-MIL-125 nanocrystals, has significantly increased the photocatalytic activity for the reduction of CO₂ to CH₄.

Noble metals are generally not involved in chemical reactions and, distributed in a specific form and size, they can improve the photo-response range and photocatalytic reaction rate of catalysts. Thus, noble metals are often used for photocatalytic modification of MOF frameworks. The most commonly used noble metals at this stage are Pt, Pd, Ru, Au, etc. Currently, research on the noble metal Pt is extensive because the outermost D d-orbital electrons of Pt are unfilled with transition states (or active intermediates), creating noble metal catalysts endowed with excellent catalytic activities [17]. Tai et al. [18] reported a series of photocatalysts CdS/Pt/MIL-125 fabricated using γ-ray irradiation and the hydrogen generation rate was measured to 6783.5 μmol/(g·h) under visible-light illumination, much higher than similar non-ternary catalysts.

In this work, we prepared Pt-MIL-125 by a one-step method and, based on this, Pt-TiO₂ with MIL-125 as the backbone was prepared by thermal synthesis. The internal structure, elemental composition and morphology of the catalysts were characterized using XRD, XPS, SEM, EDS and BET. The organic pollutant Rhodamine B (RhB) was selected as the degradation target and the effects of catalyst input and light conditions on the photodegradation ability of the catalyst were investigated. Finally, the photocatalytic degradation mechanism of M-Pt-TiO₂ on the target degradation products was investigated by trapping agent experiments.

2. Experimental Section

2.1. Experimental Reagents

Details of reagents and related parameters required for the experiments are shown in

Table 1. Reagents and their parameters.

Reagents	Specifications	Manufacturer
Ammonium chloride	AR	Hangzhou Gopin Fine Chemicals Co.; Hangzhou, China
Methanol	AR	Hangzhou Gopin Fine Chemicals Co.
Anhydrous ethanol	AR	Hangzhou Gopin Fine Chemicals Co.
Isopropyl alcohol	AR	Shanghai Aladdin Reagent Co.; Shanghai, China
2-Methylimidazole	AR	Shanghai Aladdin Reagent Co.
Tetra-butyl titanate	AR	Sigmar Aldridge, Miamisburg, OH, USA
Dimethylformamide	AR	Shanghai Aladdin Reagent Co.
Chloroplatinic acid	ACS, ≥99.0%	Shanghai Aladdin Reagent Co.
Titanium dioxide (P25)	AR	Hangzhou Gopin Fine Chemicals Co.
Terephthalic acid	AR	Hangzhou Gopin Fine Chemicals Co.
Rhodamine B	AR	Shanghai Aladdin Reagent Co.
EDTA	AR	Shanghai Aladdin Reagent Co.
p-Benzoquinone	Strong, AR	Hangzhou Gopin Fine Chemicals Co.
Deionised water	AR

. In order to meet the next step in the preparation of the experimental material, the reagents in Table 1 need to be pre-treated to some extent, i.e., impurities and unwanted substances are filtered out and the reagents are mixed with an aqueous or ethanol solution to prepare the desired liquid.

2.2. Experimental Instruments

The instruments and parameters used to prepare the photocatalytic composites are shown in

Table 2. Experimental instruments and their parameters.

Instruments	Models	Manufacturer
Ultrasonic cleaners	JP-040S	Shenzhen United Cleaning Equipment Co., Shenzhen, China
Constant temperature and pressure drying ovens	DHG-9035A	Shanghai Heng Yi Technology Co.; Shanghai, China
High-speed centrifuges	H-1850	Hunan Xiang Yi Experimental Instrument Development Co.; Changsha, China
Digital magnetic stirrer	SZCL-24A	Hangzhou Youning Instrument Co.; Hangzhou, China
Muffle Furnaces	KSL-II00XX	Hefei Kepin Materials Technology Co.; Hefei, China

.

2.3. Experimental Methods and Conditions

2.3.1. Preparation of Catalysts

Schematic representation of M-Pt-TiO₂ is shown in Figure 1. First, 0.5 g of terephthalic acid and 0.26 mL of tetra-butyl titanate were added to 10 mL of DMF solution mixed with 1 mL of methanol and stirred for 30 min, after which they were poured into a 20 mL reaction tank and reacted in an oven at 150 °C for 36 h. The reaction solution was removed and centrifuged and the solid part was washed 3 times with DMF and ethanol and dried at 80 °C for 10 h to obtain MIL-125.

Eight grams of MIL-125 were dispersed in 30 mL of ethanolic solution dissolved with 0.5 g of chloroplatinic acid, stirred for 20 min and then centrifuged to dry at 60 °C for 12 h to obtain Pt-doped MIL-125.

M-Pt-TiO₂ was obtained by placing Pt-doped MIL-125 in an alumina crucible which was placed in a muffle furnace and sintered at 530 °C for 30 min. The furnace was cooled at a rate of 2 °C/min.

M-TiO₂ was prepared by MIL-125 using the same method.

2.3.2. Characterization of Catalysts

The catalyst was subjected to physical phase analysis, diffraction pattern indexing and grain size determination by X-ray diffraction (XRD; Bruker, D8-ADVANCE, 40 kV, 40 mA, Cu Ka, Billerica, MA, USA). Scanning electron microscopy (SEM; Hitachi, S-4800, Tokyo, Japan) was used to observe the surface morphology and loading of the catalyst carriers and transmission electron microscopy (TEM; FEI-Tencai G2 F20S-TWIN, Hillsboro, OR, USA) was used to characterize the microscopic morphology as well as the two-dimensional geometry of the catalysts using electrons as the illumination beam. A gas adsorption analyzer and N₂ adsorption-desorption isotherm curves were used to characterize the specific surface area and pore structure of the solid catalysts. Fourier transform infrared spectroscopy (FTIR) was used to detect changes in the molecular structure and functional groups in the range of 400–4000 cm⁻¹. Solid UV–Vis was used for the characterization of the catalyst surface light absorption capacity at 200~800 nm; X-ray photoelectron spectroscopy (XPS) was used to characterize elemental composition, valence distribution and elemental content of the material.

2.3.3. Photocatalyst Performance Testing

The molecular and structural formulae of Rhodamine B (RhB) are shown in

Table 3. Molecular formulae, structural formulae and molecular weights of target degradate.

Name	Molecular Formula	Structural Formula	Molecular Weight
Rhodamine B (RhB)	C28H31C1N2O3		479.01

. As a classical water-soluble cationic dye, RhB has large molecular weight and structures such as heteroatomic benzene rings and nitrogen-containing groups that are not easily degraded by conventional means, so is well suited as a model compound of dye wastewater for photocatalytic degradation. In this paper, the photocatalytic performance of samples was determined using RhB as the target degradant.

A 40 W UV lamp (wavelength 254 nm) was used as the light source for the photocatalytic degradation of RhB. During the experiment, 50 mg of the sample was added to 100 mL of RhB solution at a concentration of 5 ppm and stirred for 30 min in the dark. Then, the light source was switched on and 3 mL of the sample solution was taken at 5 min intervals for measurement.

The RhB concentration was measured by UV spectrophotometry: the light absorption intensity of the supernatant of the above 5 mL solution was measured using a UV spectrophotometer at 554 nm according to Lambert–Beer’s law,

$$A=\lg (1/T)=\epsilon \times b\times c$$

(1)

where A represents the absorbance of the sample and T is the transmission ratio (transmittance), which is the intensity of the outgoing light (I) over the intensity of the incoming light (I₀). ε represents the molar absorption coefficient, b represents the thickness of the cuvette when light is transmitted through it and c represents the molar concentration of the degradation solution. The absorbance of the sample is proportional to its concentration and therefore, the degradation rate of the degradate can be obtained from the following equation:

$$X=(C_0-C_t)/C_o\times 100\%$$

(2)

where X represents the degradation rate, C₀ is the initial concentration of the degradation solution and C_t represents the molar concentration of the solution at different times.

3. Results and Discussion

3.1. Characterisation of M-Pt-TiO₂ Photocatalysts

3.1.1. X-ray Diffraction Pattern

As seen from Figure 2, the diffraction peaks of MIL-125 are consistent with the simulated MIL-125 peaks, demonstrating the successful synthesis of MIL-125 by the solvothermal method. No characteristic peaks of MIL-125 were observed for the M-TiO₂ or M-Pt-TiO₂ curves, indicating that the MOF structure was completely decomposed. The diffraction peaks at 25.48, 37.96 and 48.32 correspond to the (101), (004) and (200) crystallographic planes of anatase phase TiO₂, respectively and 27.64 and 54.48 correspond to the (110) and (211) crystallographic planes of rutile phase TiO₂, respectively [19]. This indicates that the TiO₂ in the sample is a composite state of anatase and rutile phases and, after the calcination process, the internal structure is that of MIL-125. In the M-Pt-TiO₂ curve, the diffraction peaks at 39.94 and 46.44 correspond to the (111) and (200) crystallographic planes of the face-centred cubic crystal Pt (JCPDS no. 87-0644), respectively, demonstrating the successful loading of Pt nanoparticles onto TiO₂.

3.1.2. SEM and TEM Analysis

As shown in Figure 3, the MIL-125 prepared in Figure 3a had a circular cake particle shape with a diameter of approximately 500 nm. Figure 3b,c show the catalysts prepared by calcination and the original shape of the MIL-125 precursor was kept intact from the microtopography. The particle size of M-TiO₂ in Figure 3b is reduced compared with that of MIL-125 before treatment, while defects and clefts appear on the surface, indicating that, after calcination treatment, the internal structure of MIL-125 has a certain degree of shrinkage, which results from the thermal decomposition of the framework of organic ligands serving as the supporting crystal structure. There are obvious defects on the surface of catalyst particles compared with the former two in Figure 3c; meanwhile, nano Pt particles attachment on the surface can be observed and the average diameter was measured as 24 nm.

HRTEM images showed that the interplanar spacing was 0.353 nm, corresponding to the (101) interplanar distance of TiO₂ nanoparticles [20]. It is clear from Figure 4 that the dark part is Pt and the light part is TiO₂ because the platinum atoms are heavier than Ti and O atoms. At the same time, it can be observed that there is no obvious interface between the nano Pt particles and TiO₂, which demonstrates that the catalysts prepared by calcination have tighter binding between different elements.

Figure 5 is the mapping summary diagram and elemental profiles of O, Ti and Pt of the M-Pt-TiO₂ catalyst, in which the elements are distributed uniformly. Figure 5 shows that Ti, O and Pt elements are randomly distributed and the Ti element distribution is not uniform and shows a stacking phenomenon, which is because the nano-TiO₂ primary particles are small and have a high specific surface energy and tend to form larger, metastable particles at high temperature; thus, some agglomeration appears. The newly formed sheets of TiO₂ separates O₂ from the unreacted species, thus preventing further oxidation by Ti₃C₂, so that the TiO₂ crystals are mostly distributed in the Ti₃C₂ two-dimensional sheet structure, causing the Ti elements in the TiO₂ elements to pile up between the layers.

3.1.3. FTIR Infrared Spectroscopy

Fourier infrared spectroscopy was used to detect the functional groups contained in the samples. In Figure 6, the absorption peaks at 1439 and 1536 cm⁻¹ seen in the FT-IR curve (a) of MIL-125 correspond to the symmetric stretching vibration of carboxylate groups and the vibration bands in the range of 400–800 cm⁻¹ are attributed to the stretching vibration of Ti-O and Ti-O-Ti clusters. The absorption peaks at 1400–1600 cm⁻¹ completely disappeared from the FT-IR curves of M-TiO₂ and M-Pt-TiO₂ in Figure 6b,c. This indicates that carboxylate groups were completely decomposed in MIL-125 due to high-temperature calcination. The MIL-125 framework collapse at high temperature also brings about the strengthening of the Ti-O stretching vibration at 500 cm⁻¹ and the decrease of M-Pt-TiO₂ relative to the M-TiO₂ curve strength originates from the effect of the Pt loading on the Ti-O bonding energy.

3.1.4. BET Analysis

From Figure 7, both M-TiO₂ and M-Pt-TiO₂ exhibited type IV isotherms according to the Brunauer-Deming-Deming-Teller classification and the pore size was mainly mesoporous and macro-porous [21,22], as known from the shape of the hysteresis loop, with discrete hysteresis loops (H3 type) in the range of higher relative pressure (P/P₀, 0.47–1.0), implying the existence of slit-like pores and capillary condensation in the mesopores and large hysteresis between adsorption and desorption branches, confirming the presence of mesoporous pores. Combined with SEM, it is known that this is related to the stacking of the 3D cake-shaped structure of MIL-125 and the collapse of calcination at high temperature to form pore channels. The specific surface area and average pore size of MIL-125, M-TiO₂ and M-Pt-TiO₂ are shown in

Table 4. BET and pore size of samples.

Name	Specific Surface Area (m2.g−1)	Average Pore Size (nm)
MIL-125	1366.937	/
M-TiO2	34.288	47.856
M-Pt-TiO2	41.199	47.988

. MIL-125 possesses a larger specific surface area, but the specific surface areas of both M-TiO₂ and M-Pt-TiO₂ which are 34.288 and 41.199 m²/g, respectively, decrease greatly due to the high-temperature collapse effect. Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) were used to analyze the specific surface area and pore size distribution of M-TiO₂ and M-Pt-TiO₂, result shown in Figure 7d. It can be seen that the loading of Pt improves the pore size distribution of M-TiO₂ and in the range of 75–100 nm, the pore size distribution of M-Pt-TiO₂ is much higher than that of M-TiO₂, can be attributed to the loading dispersion of Pt, not only does not cause blockage to the TiO₂ mesopores but also improves the pore size distribution of the catalyst and increases the specific surface of the catalyst, thus exposing more active sites for photocatalytic reactions, which is beneficial to the further improvement of the photocatalytic activity of the catalyst.

3.1.5. Solid UV–Vis

From Figure 8, the UV–Vis curves of the samples modified by TiO₂ showed a larger redshift than the original samples due to the formation of Ti³⁺ and Ov in the range of 0.75–1.18 eV below the CB of TiO₂, indicating a significant change in the absorption wavelength. As the energy change caused by the surface effect of Pt is greater than that caused by the spatial effect, the optical band gap of the catalyst particles is reduced, which shows a certain degree of redshift in the M-Pt-TiO₂ curve compared to the M-TiO₂ curve and a large improvement in the intensity of light absorption in the visible region. In addition, the M-Pt-TiO₂ catalyst absorbs visible light more strongly than M-TiO₂, which further improves the visible light catalytic performance.

The forbidden band width (Eg) of a semiconductor is negatively correlated with the absorbance and the performance of a semiconductor photocatalyst depends, in many cases, on the width of the forbidden band; the narrower the forbidden band width, the greater the redshift in the absorbance spectrum. Therefore, the forbidden band width (Eg) of a catalyst can be calculated by extrapolating the UV–vis absorption spectrum to the Kubelka-Munk function. The calculated Eg of M-Pt-TiO₂ is 3.14 eV, which is narrower than the forbidden band width of M-TiO₂ and thus also makes the electron leap easier.

3.1.6. XPS Analysis

The XPS spectrum of M-Pt-TiO₂ with C 1s, O 1s, Ti 2p and Pt 4f for its chemical composition, and elemental chemical states is shown in Figure 9. The double peak 2p of Ti is located at 458.4 and 464.15 eV and assigned to the Ti (IV) oxidation state in the Ti-O cluster, typical of the MIL-125 structure. The O 1s diagram at 529.8, 529.5 and 530.9 eV corresponds to the Ti-O clusters, C=O clusters and hydroxyl groups in the MIL-125 backbone and TiO₂, respectively. The characteristic double peaks of Pt 4f are located at 70.5 and 73.8 eV, indicating the presence of Pt₀ in the catalyst. The peaks at 284.7 eV and 288. 2 eV in the C 1s spectrum correspond to the O–C=O bond. All the above results confirm the successful synthesis of M-Pt-TiO₂ materials.

3.2. Analysis of the Photocatalytic Degradation Effect of M-Pt-TiO₂

3.2.1. Experimental Results and Analysis of M-Pt-TiO₂ Photocatalytic Degradation of Rhodamine B

The photocatalytic effect of M-Pt-TiO₂ was evaluated by the degradation of Rhodamine B (RhB) under the light condition of a 40 W UV lamp and the results of the degradation efficiency of the catalyst are shown in Figure 10. The self-degradation rate of RhB without the addition of photocatalyst was 18.86%; the photocatalytic efficiencies of M-TiO₂ and TiO₂ (P25) was 84.05% and 89.85%, respectively, under the addition of photocatalyst, while the photocatalytic degradation rate of RhB was 98.97% after adding the M-Pt-TiO₂ catalyst for 30 min. The RhB degradation rate was 73.87%, but excluding the effect of dark adsorption on the experiment, the actual photocatalytic degradation rate was essentially comparable to the self-degradation rate of RhB. The introduction of Pt enhanced the photocatalytic activity compared to commercially available P25 and M-TiO₂. To investigate the photodegradation behaviour of the catalyst, a kinetic analysis of the catalytic reaction was also carried out. The kinetic curve can be calculated from the pseudo-first-rate kinetic equation ln(C₀/C) = kt. The linear relationship between ln(C₀/C) and (t) shows that the photocatalytic degradation reaction follows a pseudo-first-rate reaction, with the M-Pt-TiO₂ photocatalyst having the largest reaction rate constant of 0.0645 min⁻¹. In addition to the reactive radicals generated by the reaction of water with photogenerated electrons under UV irradiation, the generation of carriers as positive holes and negative electrons is also a major part of the photocatalytic process. These generated radicals decompose dye pollutants together with nitrate ions into carbon dioxide, water and inorganic nitrogen. The lower work function of Pt nanoparticles compared to TiO₂ leads to the spontaneous transfer of electrons from the TiO₂ conduction band to the metal Pt nanoparticles, while the Schottky barrier formed by Pt₀ on the surface of the M-Pt-TiO₂ catalyst reduces the complexation of photogenerated electrons with holes [23], i.e., the presence of electron traps inhibits the recombination of photogenerated electron-hole pairs, thus greatly enhancing the photocatalytic reaction efficiency.

Free radical capture experiments (Figure 11) were used to quantify the role in free radical oxidation reactions using EDTA, isopropanol and p-benzoquinone for the capture of vacancies, hydroxyl radicals and superoxide radicals, respectively. The radical capture experiments showed that the photodegradation removal rate decreased from 89.3% to 56.46, 38.04 and 6.14% after the addition of EDTA, isopropanol and p-benzoquinone scavengers, respectively, in the M-Pt-TiO₂ system, reflecting the greater degradation of RhB dyes by superoxide radicals. With this in mind, the degradation mechanism of M-Pt-TiO₂ on RhB is shown in Figure 12. The hole transfer from the titanium dioxide valence band to the Fermi energy level of Pt is accompanied by a reduction in the electron-hole complex and more conduction band electrons and superoxide radicals provide opportunities for the degradation of RhB.

3.2.2. Experimental Analysis of the Recyclability of M-Pt-TiO₂ Photocatalytic Degradation of Rhodamine B

As a green technology, waste disposal is usually not considered in the photocatalytic degradation reaction cycle, so it is necessary to investigate the recyclability of the M-Pt-TiO₂ photocatalyst. Four-cycle degradation test of M-Pt-TiO₂ are shown in Figure 13. After four cycles, the photocatalytic activity of the M-Pt-TiO₂ catalyst for RhB decreased from 98.4% to 94.9%, indicating that the photocatalyst sample has good stability of photocatalytic activity under UV irradiation. In addition, no significant changes were observed in the XRD pattern (Figure 14) after four cycles. The prepared M-Pt-TiO₂ nanocrystals maintained a robust structure during the long cycling process, which was related to the reasonable layered porous structure of the nanocrystals and the strong van der Waals force interactions between neighboring nanocrystals. The slight decrease in photocatalytic activity during recycling may be due to the accumulation of organic intermediates on the catalyst surface that affect dye adsorption. In summary, M-Pt-TiO₂ can be considered a promising photocatalyst for practical applications.

4. Conclusions

In this work, Pt-doped TiO₂ was prepared by thermal synthesis using MIL-125 as the template. XRD and SEM characterization results showed that Pt nanoparticles were uniformly loaded onto the pancake-shaped TiO₂. XPS characterization demonstrated the presence of Pt₀ and BET results showed that the loading of Pt improved the pore size distribution and increased the specific surface of the catalyst. After 30 min of degradation, the photocatalytic degradation of RhB by M-Pt-TiO₂ was 98.97% and the Schottky barrier formed by Pt nanoparticles on the surface of the M-Pt-TiO₂ catalyst inhibited the recombination of photogenerated electron-hole pairs, thus greatly enhancing the photocatalytic reaction efficiency. Radical trapping experiments showed that superoxide radicals and conduction band electrons played a major role in the degradation process. After four cycles, the photocatalytic activity of the M-Pt-TiO₂ catalyst for RhB decreased from 98.4% to 94.9%, indicating that the photocatalytic sample had good photocatalytic stability under UV light irradiation, which was related to the reasonable layered porous structure of the nanocrystals and the strong van der Waals force interactions between adjacent nanocrystals.