堇青石表面化学气相沉积二氧化钛的负载机理及负载动力学

杨眉; 刘良文; 郑珩; 卢静祥; 王静怡; Yang Mei; Liu Liangwen; Zheng Heng; Lu Jingxiang; Wang Jingyi

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Loading Mechanism and Loading Kinetics of TiO₂ on Cordierite Surface by Chemical Vapor Deposition PDF

- ORCID：
Yang Mei ¹
✉
- ORCID：
Liu Liangwen ¹
- ORCID：
Zheng Heng ²
- ORCID：
Lu Jingxiang ¹
- ORCID：
Wang Jingyi ¹

1. School of New Energy and Materials, Southwest Petroleum University, Chengdu 610500, China； 2. State Key Laboratory of Industrial Vent Gas Reuse, Southwest Research & Design Institute of Chemical Industry, Chengdu 610225, China

Updated：2022-03-30

Abstract

In order to explore the loading mechanism and loading kinetics of TiO₂ on cordierite, the acid-etched cordierite was used as the matrix and the TiO₂ loading test was conducted by the chemical vapor deposition (CVD) method. The scanning electron microscope, energy disperse spectroscope, X-ray diffraction, and Brunauer-Emmett-Teller (BET) specific surface area measurement were used to characterize the TiO₂ on cordierite surface and determine the loading speed at different temperatures. The results show that the surface of modified cordierite with TiO₂ mainly consists of (211)-oriented and (200)-oriented anatase-TiO₂ which is the octahedron and cube with BET specific surface area of 78.80 m²·g^-1, an average pore diameter of 9.80 nm, and a bimodal distribution. The loading process is the diffusion and adsorption of TiCl₄ and O₂ towards the cordierite matrix. TiCl₄ decomposes and Ti⁴⁺ enters the matrix lattice under the high oxygen potential to form TiO₂ nuclei. After the preferential orientation and epitaxial growth, the loading deposition rate equation is $V = 6807 e x p (- \frac{7255}{T}) P_{T i C l_{4}}^{0}$ , where T is the loading temperature and $P_{T i C l_{4}}^{0}$ is the partial pressure when TiCl₄ is in the gas phase.

Keywords

catalyst; SCR; CVD; TiO₂; loading kinetics

Science Press

A large amount of NO_x in the exhaust gas of coal-fired power plants has a serious damage to the environment and human health^[

1,2]. Currently, the selective catalytic reduction (SCR) with a wide reaction temperature range, high denitration efficiency, good selectivity, and good operational safety is an effective way to control NO_x pollution, and it is also a commonly used flue gas denitration method with the technical core of catalysts^{[Reference 3

Baidu Scholar}3]. The metal oxide catalysts have been widely investigated in recent years, and the vanadium oxide attracts much attention^{[Reference 4-9}4-9].

The commonly used commercial catalysts are monolithic V₂O₅-WO₃/TiO₂, and the content of TiO₂-carrier accounts for more than 80%^[

10]. V₂O₅ is the main active component of SCR denitration reaction, which needs to be loaded on a suitable carrier surface; otherwise, the SCR denitration efficiency will be negatively influenced. The activity and selectivity of V₂O₅ are also sensitive to the carrier. Only a few oxides, such as TiO₂, Al₂O₃, and SiO₂, have good surface dispersion. Most researches focus on the addition of different elements into the V₂O₅-WO₃/TiO₂, such as the alkali metals, alkaline earth metals, and other elements (As, Pb, Zn, S, P)^{[Reference 11-18}11-18]. TiO₂ is usually loaded on the surface of a low-cost porous matrix to reduce the total content of TiO₂ in the carrier^{[Reference 19-21}19-21], and the loading method is sol-gel method. However, this method has low loading efficiency of TiO₂, uneven dispersion, and poor bonding to the matrix. The TiO₂ film or TiO₂ particle treated by chemical vapor deposition (CVD) for photocatalysts has the advantages of high bonding strength, uniform dispersion, and large Brunauer-Emmett-Teller (BET) specific surface area^{[Reference 22

Baidu Scholar}22]. Woods et al^{[Reference 23

Baidu Scholar}23] applied CVD method on Ti(NMe₂)₄ and O₂ as the precursors at 250~300 °C, and the TiO₂ film was obtained after annealing at 600 °C. Wang et al^{[Reference 24

Baidu Scholar}24] reported that TiN and TiO₂ are directly deposited on the surface of 310S stainless steel with the dimension of 10 mm×10 mm×0.9 mm by atmospheric pressure CVD method. Kuo et al^{[Reference 25

Baidu Scholar}25] reported that the crystalline titanium oxide films with a thickness of 0.09~0.55 μm are prepared below 500 °C by CVD method with a mixture of titanium tetrachloride (TiCl), carbon dioxide (CO₂), and hydrogen (H₂).

Although the preparation of TiO₂ by CVD has been widely studied, the loading mechanism and loading kinetics of TiO₂ by CVD on cordierite matrix are still unclear. This research investigated the loading mechanism and loading kinetics of TiO₂ on the surface of honeycomb ceramics of modified cordierite, and provided theoretical basis for further CVD application.

1 Experiment

The honeycomb cordierite ceramic was used, and its main chemical composition is MgO (17.2wt%), Al₂O₃ (30.2wt%), and SiO₂ (51.1wt%). The cordierite was acid-treated (HNO₃) at 110 °C for 6 h. Then the modified cordierite was cut into small specimens of 20 mm×20 mm×30 mm as the matrix material for CVD. The physical properties of the modified cordierite are shown in Table 1.

Table 1 Properties of modified cordierites with and without TiO₂

Specimen	BET specific surface area/m²·g^-1	Total porous area/m²·g^-1	Total pore volume/cm³·g^-1	Average pore diameter/nm
Modified cordierite	46.18	37.32	0.0447	4.82
Modified cordierite with TiO₂	78.80	72.76	0.2018	9.80

The titanium tetrachloride (TiCl₄) and O₂ were used as the reaction precursors for CVD treatment. TiCl₄ was vaporized at 35 °C with a carrier gas of N₂. Then N₂ for dilution was added to control the gas flow and flow rate. The flow of N₂ carrier gas, N₂ for dilution, and O₂ was 500, 1000, and 80 mL/min, respectively. The pressure was maintained at the standard atmospheric pressure, the reaction time was 10 min, and the loading temperature (T) was 450 °C.

The BET specific surface area and pore structure of the catalyst were measured by the 1990-type N₂ physical adsorption instrument. TESCAN VEGA2 variable vacuum scanning electron microscope (SEM) was used to observe the morphology of specimens. INCA Energy 350 X-ray energy disperse spectrometer (EDS) was used to analyze the composition. The Rigaku D/max-3C X-ray diffractometer (XRD) was used for structure and phase analysis. The loading speed was calculated through the specimen mass before and after loading by an electronic balance. The metallographic microscope (DME-300M) was also used for microstructure observation. The bonding strength between the TiO₂ layer and the acid-modified cordierite was tested on the specimens of 20 mm×10 mm×5 mm, and the bonding force was measured by MFT-4000 surface scratch.

2 Results and Discussion

2.1 Characterization of TiO₂-loaded cordierite

The properties of TiO₂-loaded cordierite are shown in Table 1. All TiO₂-loaded cordierite specimens have a porous surface with BET specific surface area of 78.80 m²·g^-1, and the total porous area is 72.76 m²·g^-1. The total pore volume is 0.2018 cm³·g^-1. The pore size distribution of TiO₂-loaded cordierite is shown in Fig.1. Most TiO₂-loaded cordierite has the pore size of 3~25 nm, and the peak pore size is 9~11 nm. The average pore size is 9.80 nm, and the pore size distribution has the bimodal characteristic, which provides a good structure condition for the loading of active components.

Fig.1 Pore size distribution of TiO₂-loaded cordierite

SEM morphologies of the cordierite matrix with and without TiO₂ are shown in Fig.2. The surface microstructure changes due to the macroscopic properties of the modified cordierite matrix.

Fig.2 SEM morphologies (a, b) and EDS analysis (c) of modified cordierites without (a) and with (b, c) TiO₂; EDS results of point 1 in Fig.2c (d)

Before loading TiO₂, the surface of the modified cordierite matrix has many macropores and a few micropores. After loading TiO₂, the surface of the modified cordierite matrix still has many large pores, whereas the number of micropores is greatly increased. The entire surface is evenly covered by TiO₂, and many ravines are filled. The large increase in micropores causes the increase in BET specific surface area, and the change in linear expansion coefficient is also closely related to the uniform coverage of TiO₂. The surface of cordierite matrix consists of agglomerated particles, thereby forming a clump-like structure. The diameter of the clumps varies from 1 μm to 2 μm. There are many small micropores between the particles. Some large pores with the diameter of 4~6 μm are formed, which is in good agreement with the pore size distribution in Fig.1.

The phase components of TiO₂-loaded cordierite at 450 °C were analyzed by XRD and EDS. XRD pattern is shown in Fig.3. The main phases are the cordierite matrix (Mg₂Al₄Si₅O₁₈) and anatase-TiO₂. The peaks of anatase-TiO₂ are at 2θ=25.381°, 37.80°, 48.049°, 62.688°, and no obvious crystal plane of (011) orientation can be observed. The main elements are O and Ti, suggesting that anatase-TiO₂ is successfully loaded on the cordierite matrix by CVD method. Anatase-TiO₂ is a tetragonal crystal, and its unit cell model mainly has three crystal plane orientations: (211), (200), and (211). The unit cell appears as an octahedron when all crystal planes are (211)-oriented, and the final crystal grains also appear as an octahedron. When the crystal planes are all (200)-oriented, the unit cell appears as a cube with the final cubic crystal grains. It can be inferred from the octahedral and cubic morphologies of anatase-TiO₂ that the loaded TiO₂ has the crystal orientations of mainly (211) and (200). After the formation of TiO₂ crystal nuclei on the surface of cordierite matrix during CVD process, the growth of TiO₂ crystal grains suffer the orientation elimination and adjustment, forming the preferred (211) and (200) orientations. The final grain growth method is epitaxial growth, and the TiO₂ crystal covers the surface of cordierite matrix.

Fig.3 XRD pattern of TiO₂-loaded cordierite

A trace amount of C can be observed in Fig.2d, because the matrix contains impurities, which diffuse to the surface during CVD process. The bonding strength results are shown in Fig.4. The bonding strength between TiO₂ and the modified cordierite matrix reaches 28 N.

Fig.4 Bonding force of TiO₂-loaded cordierite

2.2 Thermodynamic calculation of CVD method

The reaction system was TiCl₄, O₂, and carrier gas N₂ in this research, and the reaction is as follows:

TiCl₄ (g)+O₂ (g)→TiO₂+2Cl₂ (g)

(1)

According to the thermodynamic formula, the free energy of reaction is as follows:

Δ_r

H_{m}^{θ} (298 K) = - 180 k J / m o l

(2)

Δ_r

G_{m}^{θ} (298 K) = - 162.5 k J / m o l

(3)

where ∆_rH $_{m}^{θ}$ (298 K) is the standard molar enthalpy change of Eq.(1) at 298 K; ∆_rG $_{m}^{θ}$ (298 K) is the molar Gibbs free energy of Eq.(1) at 298 K. It can be seen from the calculation results at room temperature (298 K), the thermodynamics does not restrict the reaction, but the kinetic factor is the main factor controlling the entire reaction process.

2.3 Analysis of loading mechanism and loading kinetics

The mixture gas of TiCl₄ and O₂ accompanied by carrier gas is diffused into the cordierite matrix during the first stage of CVD process. Excess O₂ mainly plays a role in maintaining the oxygen potential. TiCl₄ decomposes on the cordierite matrix: TiCl₄→Ti⁴⁺+4Cl^-. Then Cl^- agglomerates to form Cl₂, thereby leaving cordierite matrix and the reaction system.

The ionic radius of Mg²⁺, Al³⁺, Si⁴⁺, and Ti⁴⁺ in the cordierite matrix is 0.089, 0.053, 0.040, and 0.065 nm, respectively. Because the ion size of Ti⁴⁺ is closer to that of Al³⁺ and Si⁴⁺, the displacement reaction can easily cause Ti to enter the cordierite lattice. After the acid etching modification of cordierite, the alkaline Mg²⁺ and the neutral Al³⁺ are lost on the surface of the cordierite matrix, and the acidic Si⁴⁺ ions are abundant. Si⁴⁺ is mainly distributed in the pore parts of the cordierite matrix with a smaller radius of curvature, indicating that its surface energy is high and it is in an unbalanced state. Due to the existence of a large amount of high-energy silicon dioxide, Ti⁴⁺ ions firstly replace Si⁴⁺, and the lattice oxygen forms a covalent bond with the titanium and oxygen. Excess oxygen in the atmosphere maintains the high oxygen potential to keep Ti⁴⁺ and form the anatase-TiO₂ crystal structure.

During the formation of Ti-O on the cordierite surface, the diffusion of the interface layer in cordierite surface and the deposition of Ti diffusion are involved. The diffusion capacity is affected by temperature and can influence the reaction rate. In addition, the reaction process is affected by the temperature and the partial pressure of the components in the system. Thus, CVD process is controlled by kinetics.

2.4 Establishment of kinetic model

To establish the loading kinetic model, the deposition rate V (g·cm^-2·h^-1) obtained by the loading tests at different loading temperatures T (523.15, 573.15, 623.15, 673.15, 723.15, 773.15, and 823.15 K) is used, as listed in Table 2.

Table 2 TiO₂ loading speeds at different loading temperatures

Loading temperature/K	Mass gain of cordierite matrix/g	Deposition rate, V/ g·cm^-2·h^-1
523.15	0.024	0.008
573.15	0.051	0.017
623.15	0.120	0.040
673.15	0.600	0.200
723.15	1.110	0.370
773.15	1.680	0.560
823.15	2.670	0.890

It can be seen that the deposition rate is increased with increasing the loading temperature, based on the preliminary judgement of a dynamic control process. The relationship between deposition rate and loading temperature is shown in Fig.5, and the fitting equation is as follows:

lnV=8.825 71-7255.721/T

(4)

Fig.5 Kinetics of deposition rate and loading temperature

According to Arrhenius formula:

K = A_{0} e x p (- \frac{Δ E_{a}}{R T}) \to l n (K) = l n (A_{0}) - \frac{Δ E_{a}}{R T}

(5)

where A₀ is the frequency factor; K is the rate constant of activation process; E_a is the activation energy; R is the molar gas constant.

Based on the fitting equation, the average activation energy at 250~550 °C is 60.3 kJ/mol, and the frequency factor A₀=6807/s. From the previous analysis, CVD is mainly composed of three key steps (the influence of other processes on deposition is not considered).

Step 1: TiCl₄ and O₂ are adsorbed, diffused, and migrate on the cordierite matrix. Because O₂ is excessive and maintains the sufficient partial pressure, only the partial pressure of TiCl₄ and the reaction rate are considered:

V_{T i C l_{4}} = H_{T i C l_{4}} (P_{T i C l_{4}}^{0} - P_{T i C l_{4}})

(6)

where V_TiCl4 is reaction rate of TiCl₄; H_TiCl4 is mass transfer coefficient of TiCl₄; P $_{T i C l_{4}}^{0}$ is the partial pressure when TiCl₄ is in the gas phase; P_TiCl4 is the partial pressure of TiCl₄ on the surface of cordierite substrate.

Step 2: based on Eq.(6), the reaction rate of TiO₂ deposition and Cl₂ generation can be obtained, as follows:

V_{T i O_{2}} = k_{f} P_{T i C l_{4}} - k_{c} P_{C l_{2}}

(7)

where V_TiO2 is the net reaction rate of TiO₂ generation; k_f is the positive response coefficient; k_c is the inverse reaction coefficient; P_Cl2 is the partial pressure of Cl₂ at the surface of the cordierite substrate.

Step 3: Cl₂ (the initial partial pressure is 0 Pa) is diffused from the deposition zone of the cordierite surface:

V_{C l_{2}} = h_{C l_{2}} (P_{C l_{2}} - P_{C l_{2}}^{0})

(8)

where V_Cl2 is diffusion rate of the product Cl₂ away from the substrate; h_Cl2 is the mass transfer coefficient of Cl₂; P_Cl2 is the partial pressure when Cl₂ is in the gas phase.

When the chemical vapor deposition of TiO₂ is in a stable stage, the system is balanced and the deposition rates of different components are equal:

V_{T i C l_{4}} = V_{T i O_{2}} = V_{C l_{2}} = V

(9)

The deposition rate V is measured value from the tests. Because both TiCl₄ and Cl₂ are diffused through the entire reaction system, their diffusion coefficients are equal. Therefore, Eq.(10~12) can be obtained, as follows:

V = k_{f} P_{T i C l_{4}}^{0}

(10)

k_{f} = A e x p (- \frac{Δ E_{0}}{R T})

(11)

V = e x p (- \frac{Δ E_{0}}{R T}) P_{T i C l_{4}}^{0} = 6807 e x p (- \frac{7255}{T}) P_{T i C l_{4}}^{0}

(12)

where A is the frequency factor; T is the loading temperature and $P_{T i C l_{4}}^{0}$ is the partial pressure when TiCl₄ is in the gas phase. E₀ is the average activation energy at the temperature of 250~550 °C.

A typical first-order reaction rate kinetic equation can be obtained, indicating that CVD process of TiO₂ is a kinetic-controlled process. The establishment of the kinetic equation provides guidance for the process design of CVD in loading TiO₂.

3 Conclusions

1) The surface of modified cordierite with TiO₂ is mainly composed of (211)-oriented and (200)-oriented anatase-TiO₂ which is in octahedron and cube forms with the Brunauer-Emmett-Teller (BET) specific surface area of 78.80 m²·g^-1, an average pore diameter of 9.80 nm, and a bimodal distribution. These components can provide good structure condition for subsequent loading of active components.

2) The chemical vapor deposition (CVD) process of TiO₂ is mainly the diffusion and mass transfer adsorption of TiCl₄ and O₂ to the cordierite matrix. TiCl₄ decomposes and enters the cordierite matrix under high oxygen potential to form TiO₂ nuclei which suffer the preferential orientation and epitaxial growth.

3) The loading reaction of TiO₂ on cordierite is related to the temperature and the partial pressure of the components in the system, and is controlled by kinetics. The activation energy of the reaction is 60.3 kJ/mol, and the deposition rate equation is $V = 6807 e x p (- \frac{7255}{T}) P_{T i C l_{4}}^{0}$ . The kinetic model can provide guidance for further mechanism investigation and CVD process design.

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Introduction

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中文

Loading Mechanism and Loading Kinetics of TiO₂ on Cordierite Surface by Chemical Vapor Deposition PDF

Abstract

Keywords

1 Experiment

2 Results and Discussion

2.1 Characterization of TiO₂-loaded cordierite

2.2 Thermodynamic calculation of CVD method

2.3 Analysis of loading mechanism and loading kinetics

2.4 Establishment of kinetic model

3 Conclusions

References

Home

Introduction

Editorial Board

Submission Guideline

Contact Us

中文

Loading Mechanism and Loading Kinetics of TiO2 on Cordierite Surface by Chemical Vapor Deposition PDF

Abstract

Keywords

1 Experiment

2 Results and Discussion

2.1 Characterization of TiO2-loaded cordierite

2.2 Thermodynamic calculation of CVD method

2.3 Analysis of loading mechanism and loading kinetics

2.4 Establishment of kinetic model

3 Conclusions

References

Loading Mechanism and Loading Kinetics of TiO₂ on Cordierite Surface by Chemical Vapor Deposition PDF

2.1 Characterization of TiO₂-loaded cordierite