Next Article in Journal
Nickel(BiPhePhos)-Catalyzed Hydrocyanation of Styrene—Highly Increased Catalytic Activity by Optimized Operational Procedures
Next Article in Special Issue
Investigation into the Exciton Binding Energy of Carbon Nitrides on Band Structure and Carrier Concentration through the Photoluminescence Effect
Previous Article in Journal
Groundwater Bioremediation through Reductive Dechlorination in a Permeable Bioelectrochemical Reactor
Previous Article in Special Issue
Prominent COF, g-C3N4, and Their Heterojunction Materials for Selective Photocatalytic CO2 Reduction
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:

Comparing the Photocatalytic Oxidation Efficiencies of Elemental Mercury Using Metal-Oxide-Modified Titanium Dioxide under the Irradiation of Ultra-Violet Light

Institute of Environmental Engineering, National Sun Yat-Sen University, No. 70, Lian-Hai Road, Kaohsiung 80424, Taiwan
Author to whom correspondence should be addressed.
Catalysts 2024, 14(3), 209;
Submission received: 7 February 2024 / Revised: 16 March 2024 / Accepted: 18 March 2024 / Published: 20 March 2024
(This article belongs to the Special Issue Application of Photocatalysts in Air Pollution)


Since the signing of the Minamata Convention in 2013, attempts have been primarily focused on reducing the emission of elemental mercury (Hg0) from coal-fired power plants (CFPPs). The most cost-effective measure for controlling the emission of mercury involves oxidizing Hg0 to mercury oxides, which are then removed using wet flue gas desulfurization (WFGD). Thus, novel photocatalysts with the best properties of photocatalytic ability and thermal stability need to be developed urgently. In this study, titanium dioxide (TiO2)-based photocatalysts were synthesized through the modification of three metal oxides: CuO, CeO2, and Bi2O3. All the photocatalysts were further characterized using X-ray diffraction, X-ray photoelectron spectroscopy, photoluminescence, and ultraviolet-visible spectrometry. The photocatalytic oxidation efficiencies of Hg0 were evaluated under an atmosphere of N2 + Hg0 at 100–200 °C. The photocatalytic reactions were simulated by kinetic modeling using the Langmuir–Hinshelwood (L–H) mechanism. The results showed that Bi2O3/TiO2 exhibited the best thermal stability, with the best oxidation efficiency at 200 °C and almost the same performance at 100 °C. L–H kinetic modeling indicated that photocatalytic oxidation reactions for the tested photocatalysts were predominantly physical adsorption. Additionally, the activation energy (Ea), taking into account Arrhenius Law, decreased dramatically after modification with metal oxides.

1. Introduction

According to the United Nations Environment Programme, the estimated mercury emissions from coal-fired power plants (CFPPs) exceed 30% of total anthropogenic mercury emissions [1]. Mercury (Hg) is recognized as a metallic pollutant with detrimental effects on human beings and ecosystems due to its biochemical properties of bioaccumulation and biomagnification [2]. After the signing of the Minamata Convention in 2013, the control of mercury emissions from CFPPs was treated as the primary target for reducing elemental mercury (Hg0) emission into the atmosphere. Currently, constructed CFPPs have to adopt the best available control technologies and the best environmental practices according to the Minamata Convention on Mercury [3].
The typical mercury species normally consists of three basic forms: elementary, oxidized, and particulate. Among these, Hg0 accounts for the largest proportion [4]. Particulate mercury can be removed by particle collectors such as the electrostatic precipitator (ESP) and fabric filter. Oxidized mercury can be either adsorbed on a fly ash surface and then removed by particle collectors, or dissolved in an absorbent and removed using wet flue gas desulfurization (WFGD) [5]. Hg0 is highly volatile in ambient air and insoluble in water. It can be adsorbed by activated carbon (AC) or carbon black or further oxidized to mercury oxides and then dissolved via WFGD [6]. Using adsorbents such as AC requires the installation of extra air pollution control devices, which require high maintenance and involve high costs [7].
As a result, previous studies proposed an economical method for reducing mercury emissions by adding oxidative catalysts in selective catalytic reduction (SCR) to progress the potential catalytic oxidation of Hg0 [8]. However, there is an urgent need to resolve several obstacles in the catalytic oxidation process in SCR. The optimal temperature for existing SCR catalysts is 300–400 °C, which, unfortunately, is a fatal defect in the photocatalytic oxidation of Hg0 as its optimal operating temperature is below 100 °C [9]. Moreover, to avoid installing extra heating devices, SCR devices are commonly installed upstream of particle collectors (i.e., ESP), which mostly contain high-concentration particulate matter causing a masking effect over the surface of catalysts and reducing their catalytic activity and lifetime [10]. Therefore, it is crucial to develop a low-temperature SCR catalyst operating at 100–200 °C to achieve the goal of moving the SCR behind the particle collector [11].
TiO2 is the most commonly used photocatalyst due to its advantages of non-toxicity, high oxidation activity, good chemical and thermal stabilities, and low cost. However, the operating temperature of pristine TiO2 is too low for direct use in industrial applications. Thus, numerous methods have been developed to modify TiO2 by employing metal oxide [12], graphene [13], metal–organic frameworks [14], and zeolite [15]. Among these, the modification of metal oxide is the simplest method for effectively enhancing TiO2 activity by retarding the recombination of photo-induced electron/hole pairs, decreasing the energy bandgap, and/or increasing light absorptivity. The metal modification can be classified as either single metal atom doping with Au, Ag, and Pt or metal oxide modification with CuO2, MnO2, Fe2O3, CeO2, and Bi2O3 [16,17]. In particular, the radius of Cu2+ is 0.073 nm, which is close to that of Ti4+ (0.068 nm). Thus, Cu might enter the TiO2 molecular structure, which is beneficial for separating the photo-induced electron/hole pairs [18]. Wu et al., (2015) reported that TiO2 did not exhibit any photocatalytic activity on Hg0 under visible light. The modification of 1.25 weight % CuO had 57.8% photocatalytic oxidation efficiency of Hg0 under sunlight and 85% under UV light [19]. Another potential modifying material is CeO2; the oxygen vacancies in CeO2 can form active sites and capture either Hg0 or oxygen atoms from the gas steam. Additionally, CeO2/TiO2 shows strong thermal stability at high temperatures of 160–250 °C. This is mainly due to the conversion of the valence states of Ce(II)O2 to Ce(III)2O3 (2 CeO2 + Hg0 → Ce2O3 + HgO). The consumed oxygen can be recovered by Ce3+, which captures O2 molecules from the gas stream, accelerating the oxidation of Hg0 [20]. Li et al. (2011) reported that 1.5% CeO2/TiO2 could reach 90% at 250 °C related to the weakly bonded oxygen and chemisorbed oxygen on Ce3+ [21]. TiO2 responds solely to UVA, thus its overall light absorptivity is relatively low because it does not respond to visible light. Therefore, adding visible-light-responsive additives to TiO2 might potentially enhance the catalytic oxidation of Hg0. With its low energy band gap (2.85 eV), Bi2O3 is one of the potential materials for increasing the light adsorption ability [22].
In this study, we chose three metal oxides (CuO, CeO2, and Bi2O3) to synthesize TiO2-based photocatalysts with the aim of enhancing the photocatalytic oxidation of Hg0 at higher temperatures. This study analyzed the characteristics of self-prepared photocatalysts and conducted photocatalytic oxidation experiments to evaluate the photocatalytic oxidation efficiencies of Hg0 in an N2 + O2 atmosphere. Furthermore, the pros and cons of three metal-oxide-modified TiO2 photocatalysts were also evaluated.

2. Results and Discussion

2.1. Characterization of Photocatalysts

Figure 1 illustrates the nitrogen adsorption–desorption isotherms of the photocatalysts. It shows that there is no overlap between the adsorption and desorption curves of all photocatalysts presenting type IV isotherm species according to the International Union of Pure and Applied Chemistry isothermal classification. The type IV isotherm, with a capillary effect resulting in the hysteresis loop, is usually observed in mesoporous materials [23]. A more detailed examination of the hysteresis loop revealed that TiO2, CeO2/TiO2, and CuO/TiO2 had a type H2 loop that contributed to the complex pore structure, with the pores in the shape of ink bottles [24]. The desorption curve of CuO/TiO2 showed a steep desorption branch at a lower relative pressure region, indicating a smaller pore diameter of CuO/TiO2. Further, Bi2O3/TiO2 showed a H3-type hysteresis loop with a lower limit of the desorption branch located at the cavitation-induced P/P0. This phenomenon showed that the average pore diameter of Bi2O3/TiO2 was larger than those of TiO2, CeO2/TiO2, and CuO/TiO2. Table 1 summarizes the specific surface areas (SSAs) and average pore diameters of the photocatalysts. The SSAs of the photocatalysts were in the order TiO2 > Bi2O3/TiO2 > CeO2/TiO2 > CuO/TiO2, while the average pore diameters were in the order Bi2O3/TiO2 > CeO2/TiO2 > TiO2 > CuO/TiO2. Because Bi2O3/TiO2 had the highest SSA among the three metal-oxide-modified photocatalysts, it could have more active sites over the surface of inner pores to adsorb the reactant (Hg0). Additionally, it had a larger pore diameter, which could more easily move the desorbed product (HgO) out of the photocatalyst through its inner pore passages. As a result, Bi2O3 had better photocatalytic oxidation efficiency for Hg0 compared to the other two photocatalysts.
The morphologies of the prepared TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 were observed using FE-SEM as illustrated in Figure 2. The results of FE-SEM show that TiO2 and metal-oxide-modified TiO2 presented as nanoparticle agglomerations due to calcination during the preparation process. The EDS analysis depicts the atomic partition of each atom. The results show that the actual stoichiometry was not with the design in the synthesis process. This could be attributed to the limitation of EDS, in which the detection area was relatively small [25]. Figure 3 presents the mapping analysis of the photocatalysts, which shows that various atoms were well distributed. Figure 4 presents the TEM analysis of the photocatalysts, showing the overlap between nanoparticles, creating a darker region. The crystal size of nanoparticles could be further estimated. As shown in Figure 4, the diameters of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 were 15.5, 16.98, 20.15, and 21.27 nm, respectively.
Figure 5 presents the XRD patterns of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2. TiO2 had several characteristic peaks at 2θ = 25.39°, 3.7.84°, 48.09°, 53.98°, 62.79°, 70.34°, and 75.10° that were consistent with the Anatase TiO2 from the American Mineralogist Crystal Structure Database [26]. The crystallite sizes of the photocatalysts were calculated using the Scherrer equation [27]. Table 2 presents the results of crystallite sizes in the order CeO2/TiO2 > Bi2O3/TiO2 > CuO/TiO2 > TiO2. Additionally, Figure 5 clearly shows the relatively low or undetected characteristic peaks of CuO, CeO2, and Bi2O3. Particularly, the characteristic peaks of CuO were not observed, while those of CeO2 and Bi2O3 had relatively low intensity because the amounts of these metal oxides were too low to form crystals, with most of the photocatalysts in the amorphous state [28]. This might be attributed to the non-uniformly dispersed metal oxides over the Anatase TiO2. Additionally, the atomic radius of Cu (ϕ = 0.14 nm) was smaller than that of Ti (ϕ = 0.187 nm), potentially causing two adverse effects [29]. First, excess Cu could enter the TiO2 lattice, thus decreasing the crystallinity of TiO2 and reducing the intensity of (101) facets of TiO2. Second, CuO could easily block the inner pores of TiO2, reducing the adsorptive capacity; this was consistent with the analytical BET results. Furthermore, overwhelmed metal oxides can accumulate over the surface of TiO2, resulting in a decrease in light absorptivity and photocatalytic activity [19].
The photocatalysts absorbed incident light energy to excite electrons from the valence band to the conduction band. This left a hole in the valence band and an electron in the conduction band, thus creating electron/hole pairs. Photo-induced electrons and holes can further react with O2 and H2O to produce reactive radicals O2 and ·OH, respectively, with high redox activity [30,31]. Therefore, the lifetime of the photo-induced electron/hole pair was the most important parameter for assessing the photocatalytic activity of the photocatalysts. Figure 6 presents the PL patterns of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2. It demonstrates that TiO2 has the highest PL intensity, which was mainly attributed to the rapid recombination of electron/hole pairs, meaning that electrons could not be transited from Ti3+ to O [32]. However, as TiO2 was modified with CuO, CeO2, and Bi2O3, the PL intensity decreased gradually, suggesting that the addition of metal oxides could effectively retard the recombination of electron/hole pairs and thus extend the lifetime of active species. For all the tested photocatalysts, the emission bands were observed mainly in the range of visible light (λ = 428–604 nm). This could be attributed to the recombination of excited electrons with oxygen vacancies on the photocatalysts. On the other hand, a small peak appeared in the range of UVA light, which is attributed to near-band emissions and the recombination of electron and hole pairs [33].
Figure 7 illustrates the XPS spectra of Ti 2p over TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2. It shows two characteristic peaks at around 458 and 464 eV in the pristine TiO2, attributed to Ti 2p3/2 and Ti 2p1/2, respectively, also indicating that Ti appears in Ti4+. After modification with metal oxides, the electron cloud around Ti decreased and the oxidation state decreased, leading to a shift of the peaks to lower binding energy [34]. Figure 8 depicts the characteristic peaks of metal oxides in the photocatalysts. Cu had two characteristic peaks at 933.8 and 953.4 eV, labeled Cu 2p3/2 and Cu 2p1/2, respectively, with a binding energy gap of 19.6 eV, indicating that Cu was mainly in the form of Cu2+ instead of Cu1+ and Cu0 [35]. Ce 3d spectra were classified into Ce 3d3/2 and Ce 3d5/2, labeled u and v, respectively. The peaks u1 and v1 corresponded to Ce4+, while the peaks u, v, u2, v2, u3, and v3 corresponded to Ce3+ [36]. The Bi 4f spectra showed that Bi was present as Bi0, with two characteristic peaks at 154.9 and 160.2 eV, and Bi3+, which exhibited two characteristic peaks at 157.2 and 162.5 eV [37]. Different valence states provide strong oxygen storage capacity, which can capture oxygen from the atmosphere and oxidize Hg0 to Hg2+. Figure 9 presents the O 1s spectra of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2. Herein, Oα at a lower binding energy is assigned to lattice oxygen, while Oβ at a higher binding energy is the chemisorbed oxygen and surface hydroxyl group, which has been reported to have higher activity to conduct oxidation reactions [38,39]. The integration area showed an increased proportion of Oβ with the modification of metal oxides, further increasing Hg0 oxidation efficiency. Additionally, the increased binding energy compared with that of TiO2 indicated an increased oxidation state (O2− to O), which could enhance the photocatalytic oxidation of Hg0 [40].

2.2. Effects of Different Modifying Materials on Photocatalytic Efficiency of Hg0

Figure 10 illustrates the photocatalytic oxidation efficiencies of Hg0 in an N2 + Hg0 atmosphere at reaction temperatures ranging from 100 to 200 °C for 2 h using TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2. The photocatalytic oxidation efficiencies of Hg0 for TiO2, CuO/TiO2, and CeO2/TiO2 decreased with reaction temperature. However, an opposite trend was observed for Bi2O3/TiO2. The photocatalytic oxidation efficiencies of Hg0 for TiO2 was 91% at 100 °C and then dropped to 36% at 200 °C, showing a dramatic decrease of nearly 60% as reaction temperatures increased from 100 to 200 °C. However, after modifying using Bi2O3, the photocatalytic oxidation efficiency of Hg0 improved. To clarify the contribution of the photocatalytic and the traditional catalytic oxidation, we conducted continuous experiments, first without UVA to let the catalyst react as a traditional thermal catalytic reaction. The catalytic oxidation efficiency of Hg0 reached its maximum after 30 min, as shown in Figure 11. As a catalyst, TiO2 had less than 5% of the catalytic oxidation efficiency of Hg0 without the irradiation of UVA. By modifying TiO2 with metal oxides, the oxidation efficiency of Hg0 increased significantly. CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 had the best catalytic oxidation efficiencies of Hg0 at 100 °C, which were 15%, 32%, and 52%, respectively. Secondly, we turn on UVA to allow the materials to undergo the photocatalytic reaction. The photocatalytic reaction efficiencies of Hg0 increased dramatically for metal-oxide-modified TiO2. The results showed that, for all the tested catalysts, photocatalysis contributed more than classical thermal catalysis. A previous study reported that TiO2 modified with metal oxide could yield more surface oxygen [41]. These findings concurred with the XPS analytical results obtained in this study. Equations (1)–(3) show the production of surface chemisorbed oxygen (Oβ) through the chemical reaction of metal oxides. Metal oxides can promote photocatalytic reactions, with their effectiveness in the order Bi2O3 > CeO2 > CuO. At 200 °C, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 had performances of 47, 71, and 89%, respectively. Furthermore, Bi2O3 exhibited excellent thermal stability, retaining almost the same oxidation efficiency at temperatures between 100 and 200 °C.
2 CuO → Cu2O + Oβ
2 CeO2 → Ce2O3 + Oβ
Bi2O3 → 2 Bi0 + 3 Oβ

2.3. Kinetic Modeling of the Photocatalytic Oxidation of Hg0

The L–H kinetic mechanism was applied to simulate the photocatalytic oxidation efficiency of Hg0 [42]. In the context of a heterogeneous photocatalytic reaction, a decrease in KHg0 with the rise in the reaction temperature signified a chemical reaction characterized as physical adsorption. In contrast, an increase in KHg0 with reaction temperature characterized the chemical reaction as chemisorption, where adsorbed molecules form chemical bonds and adhere to the surface of the adsorbents [43].
As depicted in Table 3, an increase in reaction temperature led to an elevation in the kr and a decrease in the value of KHg0. This trend suggests that at higher reaction temperatures, the adsorption efficiency of Hg0 on the surface of photocatalysts diminishes. Based on the aforementioned outcomes, in this study, we inferred that physical adsorption predominantly governed the photocatalytic reactions of Hg0 for all the photocatalysts prepared. We further calculated the activation energy (Ea) according to Arrhenius Law. The respective activation energies of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 were 9.24, 2.54, 1.49, and 1.53 kcal/mol. These results indicate that the modification of metal oxides on TiO2 could effectively reduce the reaction barrier and improve the photocatalytic oxidation efficiency of Hg0 [44].

3. Materials and Methods

3.1. Chemicals

The following chemical reagents were used for synthesizing metal-oxide-modified TiO2-based photocatalysts with no further purification, included titanium isopropoxide (ACROS ORGANICS, Waltham, MA, USA, Ti(OCH(CH₃)₂)₄, 98%), copper nitrate (Alfa Aesar, Haverhill, MAs, USA, Cu(NO3)2, 99.5%), cerium nitrate (Alfa Aesar, Ce(NO3)3·6H2O, 99.5%), bismuth nitrate (Alfa Aesar, Bi(NO₃)₃·5H₂O, 99.5%), and isopropanol (Shimakyu’s Pure Chemicals, Osaka, Japan).

3.2. Preparation of Metal-Oxide-Modified TiO2 Photocatalysts

Anatase TiO2 was synthesized using the hydrothermal method [45]. First, 20 mL of titanium isopropoxide and 40 mL isopropanol were stirred for 1 h to form a uniform solution. Next, the solution was transferred to a Teflon container in an autoclave and kept in a furnace at 200 °C for 24 h. After the hydrothermal reactions, the solution was cleaned three times with DI water to remove organic compounds by centrifuging at 7000× rpm for 10 min. The solution was then dried at 80 °C to remove the residual organic compounds and the dry precipitate was further calcined at 450 °C for 4 h to obtain TiO2.
The modification of CuO and CeO2 to TiO2 was further performed using an impregnation method [46]. The molar ratios of CuO/TiO2 and CeO2/TiO2 were 5% and 7%, respectively. Initially, certain amounts of metal oxide precursors were added to the solution of self-prepared TiO2, and DI water, and magnetically stirred for 12 h at room temperature. The solution was centrifuged to remove metal ions and the precipitate was then dried at 80 °C and calcined at 450 °C for 2 h. In this study, the molar ratio of Bi2O3 and TiO2 was 3%. Bi2O3/TiO2 was synthesized using the hydrothermal method [47]. Bismuth nitrate was added to the solution of prepared TiO2 and DI water and magnetically stirred for 1 h at room temperature. The solution was then transferred to the autoclave for a hydrothermal reaction at 180 °C for 24 h and then centrifuged to remove metal ions. The precipitate was further dried at 80 °C and then calcined at 450 °C for 4 h.

3.3. Characterization Analysis of Photocatalysts

Surface characterization analysis of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 was performed using various physicochemical analytical instruments. The morphology and element dispersion of photocatalysts were observed using a field emission-scanning electron microscope (FE-SEM, Zeiss, Oberkochen, Germany, Gemini 450), transmission electron microscope (TEM, Philips, Amsterdam, The Netherland, CM-200 TWIN), and energy-dispersive X-ray spectroscopy (EDS, Zeiss, Germany, Oberkochen, Gemini 450). The specific surface area (SSA) of each photocatalyst was measured using a specific surface area analyzer (SSAA, Micromeritics, Norcross, GA, USA, ASAP2000). The crystallographic structures of the photocatalysts were analyzed using an X-ray diffractometer (XRD, Bruker, Billerica, MA, USA, D8 DISCOVER). The chemical compositions and valence states were analyzed using X-ray photoelectron spectroscopy (XPS, Philips, Amsterdam, The Netherland Hybrid Quantera). The recombination times of photo-induced electron/hole pairs were estimated using photoluminescence (PL, Horiba, Kyoto, Japan, HR800). The band gap of each photocatalyst was analyzed using an ultraviolet-visible spectrometer (UV-Vis, Perkin-Elmer Precisely, Waltham, MA, US, Lambda 850).

3.4. Photocatalytic Activity of Self-Prepared Photocatalysts

For this particular study, a continuous flow photocatalytic reaction system comprising a standard gas generator, a mass flow controller, a mixing chamber, a photocatalytic reaction tube, and a real-time mercury monitor (NIC, EMP-2, measurement range = 0–1000 µg/m3, resolution = 0.1 µg/m3, and response time = 1 s) was established. A standard gas generator with a Hg0 permeation tube released the desired concentration of Hg0, which was heated to 100 °C in an inert gas (N2) and further diluted with N2 and 6% O2 in the mixing chamber. The mixed gas was allowed to flow through the photocatalytic reactor to react with the photocatalysts coated on the surface of glass beads with 2 mm diameters. The photocatalytic reactor consisted of a photocatalytic reaction tube, with a black light of 365 nm wavelength (Sankyo Denki, BB-15W) in the middle of the reactor. The photocatalysts were placed between the reaction tube and the blacklight lamp. The blacklight lamps provided near-UV light of 15 W intensity and 365 nm wavelength. Additionally, the photocatalytic reaction tube was surrounded by a heating tape to maintain the photocatalytic reaction temperatures of 100, 150, and 200 °C.
Finally, the concentration of Hg0 at the inlet and outlet of the photocatalytic reactor was measured at a frequency of 1 plot/s. Calculating the concentration gradient between the inlet and outlet, the photocatalytic oxidation efficiency of Hg0 (ηHg0) was further derived as shown in Equation (4). The mean and standard deviation (Mean ± SD) were further calculated to describe the variation in the experimental data when the data reached the steady state.
η H g 0 = Δ H g 0 H g i n 0 = H g i n 0 H g o u t 0 H g i n 0 × 100 %

3.5. Langmuir–Hinshelwood (L–H) Kinetic Model

In this investigation, a Langmuir–Hinshelwood (L–H) kinetic model was applied to assess the correlation between the photocatalytic oxidation efficiency and the reaction rate of Hg0 across its various inlet concentrations. The adsorption equilibrium of Hg0 on the surface of photothermal catalysts was characterized by employing a Langmuir isotherm [48,49]. By considering the distribution of adsorbates on the surface of photocatalysts, the L–H kinetic model exhibited the reaction kinetics of the oxidation of Hg0. This model helped elucidate the adsorption of Hg0 over the surface of photocatalysts and the activation energy of photocatalytic oxidation of Hg0. By substituting inlet and outlet Hg0 concentrations into Equations (5) and (6), a linear correlation was derived, subsequently determining its intercept (Equation (7)) and slope (Equation (8)). This process allows us to deduce the reaction rate constant (kr) and the equilibrium constant (KHg0) of the photocatalytic reaction of Hg0 using a straight line plot of r−1 (herein, r represents reaction rate) versus CHg0−1.
r = k r ( K H g 0 C H g 0 ) 1 + K H g 0 C H g 0
1 r = 1 k r + ( 1 k r K H g 0 ) ( 1 C H g 0 )
1 k r = i n t e r c e p t
1 k r K H g 0 = s l o p e

4. Conclusions

In this study, three metal oxides (CuO2, CeO2, and Bi2O3) were employed to modify TiO2 in order to enhance the photocatalytic oxidation efficiency of Hg0. While the modification of metal oxides might have blocked the pore structure of anatase TiO2, it significantly reduced the PL intensity, which benefitted photocatalytic oxidation reactions. Moreover, the catalysts under modification exhibited a higher partition of chemisorbed oxygen. At 200 °C, these two enhancements could effectively improve the photocatalytic oxidation efficiency of Hg0 in the following order: Bi2O3/TiO2 > CeO2/TiO2 > CuO/TiO2 > TiO2. Based on the comparison of catalytic (light off) and photocatalytic (light on) oxidation experiments of Hg0, we revealed that Bi2O3/TiO2 was the best composite to photocatalytically oxidize Hg0 at 100–200 °C compared with CeO2/TiO2 and CuO/TiO2. The innovative findings obtained in this study were mainly attributed to the fact that Bi2O3/TiO2 had a higher specific surface area, which could enrich more chemisorbed oxygen on its surface, compared with CuO/TiO2 and CeO2/TiO2. The simulation of the L–H kinetic model revealed that the overall Hg0 photocatalytic reaction was dominated by physical adsorption. Furthermore, another novelty of this study was to inter-compare the activation energy (Ea) based on the Arrhenius Law. It showed that the activation energies of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2 were 9.24, 2.54, 1.49, and 1.35 kcal/mole, respectively, and the energy barriers were reduced significantly by metal oxide modification. We thus concluded that Bi2O3/TiO2 was the best photocatalyst because it had the highest photocatalytic oxidation of Hg0.

Author Contributions

C.-S.Y. planned and guided the experimental study; J.-R.Z. performed the research and analyzed/summarized the experimental data. The manuscript was written by J.-R.Z. and proofread by C.-S.Y., with contribution from the co-author. All authors have read and agreed to the published version of the manuscript.


This study was performed under the auspices of the Ministry of Science and Technology (MOST) of ROC (Taiwan), which financially funded the research project (MOST 111-2221-E-110-009). The authors are grateful to MOST for its constant financial support.

Data Availability Statement

Raw data are available from the corresponding author upon request.

Conflicts of Interest

The authors declare no conflicts of interest.


ACactivated carbon
Bi2O3/TiO2Bi2O3 modified TiO2
CeO2/TiO2CeO2 modified TiO2
CHg0concentration of gas-phase Hg0
CFPPscoal-fired power plants
CuO/TiO2CuO modified TiO2
Eaactivation energy
EDSenergy-dispersive X-ray spectroscopy
ESPelectrostatic precipitator
FE-SEMfield emission-scanning electron microscope
KHg0equilibrium constant of photocatalytic reaction of Hg0
krreaction rate constant of photocatalytic reaction of Hg0
Oαlattice oxygen
Oβchemisorbed oxygen
SCRselective catalytic reduction
SDstandard deviation
SSAspecific surface area
SSAAspecific surface area analyzer
TEMtransmission electron microscope
UV-Visultraviolet-visible spectrometer
WFGDwet flue gas desulfurization
XPSX-ray photoelectron spectroscopy
XRDX-ray diffractometer


  1. UNEP. Global mercury assessment 2018. In UN Environment Programme; Chemicals and Health Branch: Geneva, Switzerland, 2019. [Google Scholar]
  2. Cediel Ulloa, A.; Gliga, A.; Love, T.M.; Pineda, D.; Mruzek, D.W.; Watson, G.E.; Davidson, P.W.; Shamlaye, C.F.; Strain, J.J.; Myers, G.J.; et al. Prenatal methylmercury exposure and DNA methylation in seven-year-old children in the Seychelles Child Development Study. Environ. Int. 2021, 147, 106321. [Google Scholar] [CrossRef] [PubMed]
  3. The Lancet. Minamata Convention on mercury: A contemporary reminder. Lancet 2017, 390, 822. [Google Scholar] [CrossRef] [PubMed]
  4. Pirrone, N.; Cinnirella, S.; Feng, X.; Finkelman, R.B.; Friedli, H.R.; Leaner, J.; Mason, R.; Mukherjee, A.B.; Stracher, G.B.; Streets, D. Global mercury emissions to the atmosphere from anthropogenic and natural sources. Atmos. Chem. Phys. 2010, 10, 5951–5964. [Google Scholar] [CrossRef]
  5. Cheng, C.-M.; Cao, Y.; Kai, Z.; Pan, W.-P. Co-effects of sulfur dioxide load and oxidation air on mercury re-emission in forced-oxidation limestone flue gas desulfurization wet scrubber. Fuel 2013, 106, 505–511. [Google Scholar] [CrossRef]
  6. Zhang, Y.; Yang, J.; Yu, X.; Sun, P.; Zhao, Y.; Zhang, J.; Chen, G.; Yao, H.; Zheng, C. Migration and emission characteristics of Hg in coal-fired power plant of China with ultra low emission air pollution control devices. Fuel Process. Technol. 2017, 158, 272–280. [Google Scholar] [CrossRef]
  7. Yang, J.; Na, Y.; Hu, Y.; Zhu, P.; Meng, F.; Guo, Q.; Yang, Z.; Qu, W.; Li, H. Granulation of Mn-based perovskite adsorbent for cyclic Hg0 capture from coal combustion flue gas. Chem. Eng. J. 2023, 459, 141679. [Google Scholar] [CrossRef]
  8. Zhang, Y.; Zhao, Y.; Xiong, Z.; Gao, T.; Xiao, R.; Liu, P.; Liu, J.; Zhang, J. Photo- and thermo-catalytic mechanisms for elemental mercury removal by Ce doped commercial selective catalytic reduction catalyst (V2O5/TiO2). Chemosphere 2022, 287, 132336. [Google Scholar] [CrossRef]
  9. Snider, G.; Ariya, P. Photo-catalytic oxidation reaction of gaseous mercury over titanium dioxide nanoparticle surfaces. Chem. Phys. Lett. 2010, 491, 23–28. [Google Scholar] [CrossRef]
  10. Szymaszek, A.; Samojeden, B.; Motak, M. The Deactivation of Industrial SCR Catalysts—A Short Review. Energies 2020, 13, 3870. [Google Scholar] [CrossRef]
  11. Pu, Y.; Xie, X.; Jiang, W.; Yang, L.; Jiang, X.; Yao, L. Low-temperature selective catalytic reduction of NOx with NH3 over zeolite catalysts: A review. Chin. Chem. Lett. 2020, 31, 2549–2555. [Google Scholar] [CrossRef]
  12. Barreca, D.; Carraro, G.; Gasparotto, A.; Maccato, C.; Warwick, M.E.A.; Kaunisto, K.; Sada, C.; Turner, S.; Gönüllü, Y.; Ruoko, T.-P.; et al. Fe2O3-TiO2 Nano-heterostructure Photoanodes for Highly Efficient Solar Water Oxidation. Adv. Mater. Interfaces 2015, 2, 1500313. [Google Scholar] [CrossRef]
  13. Zheng, J.-R.; Yuan, C.-S.; Ie, I.-R.; Shen, H. S-scheme heterojunction CeO2/TiO2 modified by reduced graphene oxide (rGO) as charge transfer route for integrated photothermal catalytic oxidation of Hg0. Fuel 2023, 343, 127973. [Google Scholar] [CrossRef]
  14. Zhou, Q.; He, K.; Wang, X.; Lim, K.H.; Liu, P.; Wang, W.; Wang, Q. Investigation on the redox/acidic features of bimetallic MOF-derived CeMOx catalysts for low-temperature NH3-SCR of NOx. Appl. Catal. A Gen. 2022, 643, 118796. [Google Scholar] [CrossRef]
  15. Zhang, G.; Song, A.; Duan, Y.; Zheng, S. Enhanced photocatalytic activity of TiO2/zeolite composite for abatement of pollutants. Microporous Mesoporous Mater. 2018, 255, 61–68. [Google Scholar] [CrossRef]
  16. Park, H.; Park, Y.; Kim, W.; Choi, W. Surface modification of TiO2 photocatalyst for environmental applications. J. Photochem. Photobiol. C Photochem. Rev. 2013, 15, 1–20. [Google Scholar] [CrossRef]
  17. Mei, Q.; Zhang, F.; Wang, N.; Yang, Y.; Wu, R.; Wang, W. TiO2/Fe2O3 heterostructures with enhanced photocatalytic reduction of Cr(vi) under visible light irradiation. RSC Adv. 2019, 9, 22764–22771. [Google Scholar] [CrossRef] [PubMed]
  18. Li, L.; Chen, X.; Quan, X.; Qiu, F.; Zhang, X. Synthesis of CuOx/TiO2 Photocatalysts with Enhanced Photocatalytic Performance. ACS Omega 2023, 8, 2723–2732. [Google Scholar] [CrossRef] [PubMed]
  19. Wu, J.; Li, C.; Zhao, X.; Wu, Q.; Qi, X.; Chen, X.; Hu, T.; Cao, Y. Photocatalytic oxidation of gas-phase Hg0 by CuO/TiO2. Appl. Catal. B Environ. 2015, 176–177, 559–569. [Google Scholar] [CrossRef]
  20. Shen, H.; Huang, X.-W.; Ie, I.-R.; Yuan, C.-S.; Wang, S.-W. Enhancement of Oxidation Efficiency of Elemental Mercury by CeO2/TiO2 at Low Temperatures Governed by Different Mechanisms. ACS Omega 2020, 5, 1796–1804. [Google Scholar] [CrossRef] [PubMed]
  21. Li, H.; Wu, C.-Y.; Li, Y.; Zhang, J. CeO2-TiO2 Catalysts for Catalytic Oxidation of Elemental Mercury in Low-Rank Coal Combustion Flue Gas. Environ. Sci. Technol. 2011, 45, 7394–7400. [Google Scholar] [CrossRef]
  22. Zhang, Y.; Lu, J.; Hoffmann, M.R.; Wang, Q.; Cong, Y.; Wang, Q.; Jin, H. Synthesis of g-C3N4/Bi2O3/TiO2 composite nanotubes: Enhanced activity under visible light irradiation and improved photoelectrochemical activity. RSC Adv. 2015, 5, 48983–48991. [Google Scholar] [CrossRef]
  23. Thommes, M.; Kaneko, K.; Neimark, A.V.; Olivier, J.P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K.S.W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051–1069. [Google Scholar] [CrossRef]
  24. Zhang, Y.; Wu, K.; Yang, Z.; Ye, G. A reappraisal of the ink-bottle effect and pore structure of cementitious materials using intrusion-extrusion cyclic mercury porosimetry. Cem. Concr. Res. 2022, 161, 106942. [Google Scholar] [CrossRef]
  25. Gnanaseelan, N.; Latha, M.; Mantilla, A.; Sathish-Kumar, K.; Caballero-Briones, F. The role of redox states and junctions in photocatalytic hydrogen generation of MoS2-TiO2-rGO and CeO2-Ce2Ti3O8.7-TiO2-rGO composites. Mater. Sci. Semicond. Process. 2020, 118, 105185. [Google Scholar] [CrossRef]
  26. Yu, R.; Yang, Y.; Zhou, Z.; Li, X.; Gao, J.; Wang, N.; Li, J.; Liu, Y. Facile synthesis of ternary heterojunction Bi2O3/reduced graphene oxide/TiO2 composite with boosted visible-light photocatalytic activity. Sep. Purif. Technol. 2022, 299, 121712. [Google Scholar] [CrossRef]
  27. Scherrer, P. Bestimmung der Größe und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen. Nachrichten Ges. Wiss. Göttingen Math.-Phys. Kl. 1918, 1918, 98–100. [Google Scholar]
  28. Zheng, J.-R.; Yuan, C.-S.; Ie, I.-R.; Shen, H.; Hung, C.-H. Cutting-edge technologies developing for removal of Hg0 and/or NOx—A critical review of graphene-assisted catalysts. Chem. Eng. J. 2023, 474, 145898. [Google Scholar] [CrossRef]
  29. Tuo, Y.-X.; Shi, L.-J.; Cheng, H.-Y.; Zhu, Y.-A.; Yang, M.-L.; Xu, J.; Han, Y.-F.; Li, P.; Yuan, W.-K. Insight into the support effect on the particle size effect of Pt/C catalysts in dehydrogenation. J. Catal. 2018, 360, 175–186. [Google Scholar] [CrossRef]
  30. Gao, C.; Shi, J.-W.; Fan, Z.; Wang, B.; Wang, Y.; He, C.; Wang, X.; Li, J.; Niu, C. “Fast SCR” reaction over Sm-modified MnOx-TiO2 for promoting reduction of NOx with NH3. Appl. Catal. A Gen. 2018, 564, 102–112. [Google Scholar] [CrossRef]
  31. Yang, J.-Z.; Ie, I.-R.; Lin, Z.-B.; Yuan, C.-S.; Shen, H.; Shih, C.-H. Oxidation efficiency and reaction mechanisms of gaseous elemental mercury by using CeO2/TiO2 and CuO/TiO2 photo-oxidation catalysts at low temperatures in multi-pollutant environments. J. Taiwan Inst. Chem. Eng. 2021, 125, 413–423. [Google Scholar] [CrossRef]
  32. Ie, I.-R.; Zheng, J.-R.; Yuan, C.-S.; Shih, C.-H.; Lin, Z.-B. Photo-oxidation and kinetics of gaseous elemental mercury using CeO2/TiO2{001} and {101} photocatalysts in the atmosphere of multiple air pollutants. J. Environ. Chem. Eng. 2022, 10, 107411. [Google Scholar] [CrossRef]
  33. Baran Aydın, E.; Ateş, S.; Sığırcık, G. CuO-TiO2 nanostructures prepared by chemical and electrochemical methods as photo electrode for hydrogen production. Int. J. Hydrogen Energy 2022, 47, 6519–6534. [Google Scholar] [CrossRef]
  34. Cao, Y.; Li, Q.; Li, C.; Li, J.; Yang, J. Surface heterojunction between (001) and (101) facets of ultrafine anatase TiO2 nanocrystals for highly efficient photoreduction CO2 to CH4. Appl. Catal. B 2016, 198, 378–388. [Google Scholar] [CrossRef]
  35. Zhang, S.; Gong, X.; Shi, Q.; Ping, G.; Xu, H.; Waleed, A.; Li, G. CuO Nanoparticle-Decorated TiO2-Nanotube Heterojunctions for Direct Synthesis of Methyl Formate via Photo-Oxidation of Methanol. ACS Omega 2020, 5, 15942–15948. [Google Scholar] [CrossRef] [PubMed]
  36. Tuyen, L.T.T.; Quang, D.A.; Tam Toan, T.T.; Tung, T.Q.; Hoa, T.T.; Mau, T.X.; Khieu, D.Q. Synthesis of CeO2/TiO2 nanotubes and heterogeneous photocatalytic degradation of methylene blue. J. Environ. Chem. Eng. 2018, 6, 5999–6011. [Google Scholar] [CrossRef]
  37. Huang, Y.; Wei, Y.; Wang, J.; Luo, D.; Fan, L.; Wu, J. Controllable fabrication of Bi2O3/TiO2 heterojunction with excellent visible-light responsive photocatalytic performance. Appl. Surf. Sci. 2017, 423, 119–130. [Google Scholar] [CrossRef]
  38. Liu, H.; Su, Y.; Hu, H.; Cao, W.; Chen, Z. An ionic liquid route to prepare mesoporous ZrO2–TiO2 nanocomposites and study on their photocatalytic activities. Adv. Powder Technol. 2013, 24, 683–688. [Google Scholar] [CrossRef]
  39. Li, M.; Wang, P.; Ji, Z.; Zhou, Z.; Xia, Y.; Li, Y.; Zhan, S. Efficient photocatalytic oxygen activation by oxygen-vacancy-rich CeO2-based heterojunctions: Synergistic effect of photoexcited electrons transfer and oxygen chemisorption. Appl. Catal. B Environ. 2021, 289, 120020. [Google Scholar] [CrossRef]
  40. Guo, Z.; Jing, G.; Tolba, S.A.; Yuan, C.-s.; Li, Y.-H.; Zhang, X.; Huang, Z.; Zhao, H.; Wu, X.; Shen, H.; et al. Design and construction of an O-Au-O coordination environment in Au single atom-doped Ti4+ defected TiO2 for an enhanced oxidative ability of lattice oxygen for Hg0 oxidation. Chem. Eng. J. 2023, 451, 138895. [Google Scholar] [CrossRef]
  41. Ie, I.-R.; Zheng, J.-R.; Yuan, C.-S.; Lin, Z.-B.; Shih, C.-H.; Shen, H. Surface characteristics of innovative TiO2{001} and CeO2/TiO2{001} catalysts and simultaneous removal of elemental mercury and nitric oxide at low temperatures. Fuel 2022, 317, 123479. [Google Scholar] [CrossRef]
  42. Zheng, J.-R.; Ie, I.-R.; Wang, S.-W.; Yuan, C.-S.; Shen, H.-Z.; Li, Y.-H. Enhancing the oxidation efficiency of elemental mercury in the presence of NO and SO2 by using Cu(II)O doped TiO2 photothermal catalysts: Kinetics and mechanisms. Fuel 2023, 331, 125850. [Google Scholar] [CrossRef]
  43. Zhang, Q.; Mei, J.; Sun, P.; Zhao, H.; Guo, Y.; Yang, S. Mechanism of Elemental Mercury Oxidation over Copper-Based Oxide Catalysts: Kinetics and Transient Reaction Studies. Ind. Eng. Chem. Res. 2020, 59, 61–70. [Google Scholar] [CrossRef]
  44. Gao, F.; Tang, X.; Yi, H.; Zhao, S.; Li, C.; Li, J.; Shi, Y.; Meng, X. A review on selective catalytic reduction of NOx by NH3 over Mn–Based catalysts at low temperatures: Catalysts, mechanisms, kinetics and DFT calculations. Catalysts 2017, 7, 199. [Google Scholar] [CrossRef]
  45. Yang, H.G.; Liu, G.; Qiao, S.Z.; Sun, C.H.; Jin, Y.G.; Smith, S.C.; Zou, J.; Cheng, H.M.; Lu, G.Q. Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets. J. Am. Chem. Soc. 2009, 131, 4078–4083. [Google Scholar] [CrossRef] [PubMed]
  46. Zagaynov, I.V.; Liberman, E.Y.; Prikhodko, O.P.; Kon’Kova, T.V. Catalytic activity of CeO2@TiO2 for environmental protection. New J. Chem. 2024, 48, 2842–2848. [Google Scholar] [CrossRef]
  47. Malligavathy, M.; Iyyapushpam, S.; Nishanthi, S.T.; Pathinettam Padiyan, D. Remarkable catalytic activity of Bi2O3/TiO2 nanocomposites prepared by hydrothermal method for the degradation of methyl orange. J. Nanopart. Res. 2017, 19, 144. [Google Scholar] [CrossRef]
  48. Ishag, A.; Yue, Y.; Xiao, J.; Huang, X.; Sun, Y. Recent advances on the adsorption and oxidation of mercury from coal-fired flue gas: A review. J. Clean. Prod. 2022, 367, 133111. [Google Scholar] [CrossRef]
  49. Wang, C.; Hong, Q.; Ma, C.; Mei, J.; Yang, S. Novel Promotion of Sulfuration for Hg0 Conversion over V2O5–MoO3/TiO2 with HCl at Low Temperatures: Hg0 Adsorption, Hg0 Oxidation, and Hg2+ Adsorption. Environ. Sci. Technol. 2021, 55, 7072–7081. [Google Scholar] [CrossRef] [PubMed]
Figure 1. N2 adsorption–desorption isotherms of photocatalysts.
Figure 1. N2 adsorption–desorption isotherms of photocatalysts.
Catalysts 14 00209 g001
Figure 2. FE-SEM images and EDS results (atomic %) of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Figure 2. FE-SEM images and EDS results (atomic %) of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Catalysts 14 00209 g002
Figure 3. Mapping analysis of photocatalysts (a) CuO/TiO2, (b) CeO2/TiO2, and (c) Bi2O3/TiO2.
Figure 3. Mapping analysis of photocatalysts (a) CuO/TiO2, (b) CeO2/TiO2, and (c) Bi2O3/TiO2.
Catalysts 14 00209 g003
Figure 4. TEM images of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Figure 4. TEM images of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Catalysts 14 00209 g004
Figure 5. XRD analysis of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Figure 5. XRD analysis of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Catalysts 14 00209 g005
Figure 6. PL analysis of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Figure 6. PL analysis of TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Catalysts 14 00209 g006
Figure 7. XPS spectra of Ti 2p over TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Figure 7. XPS spectra of Ti 2p over TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Catalysts 14 00209 g007
Figure 8. XPS analysis of metal components over (a) CuO/TiO2, (b) CeO2/TiO2, and (c) Bi2O3/TiO2.
Figure 8. XPS analysis of metal components over (a) CuO/TiO2, (b) CeO2/TiO2, and (c) Bi2O3/TiO2.
Catalysts 14 00209 g008
Figure 9. XPS spectra of O 1s over (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Figure 9. XPS spectra of O 1s over (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, and (d) Bi2O3/TiO2.
Catalysts 14 00209 g009
Figure 10. Photocatalytic efficiency of Hg0 under an atmosphere of N2 + Hg0 with TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Figure 10. Photocatalytic efficiency of Hg0 under an atmosphere of N2 + Hg0 with TiO2, CuO/TiO2, CeO2/TiO2, and Bi2O3/TiO2.
Catalysts 14 00209 g010
Figure 11. Photocatalytic efficiencies of Hg0 with and without UVA of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, (d) Bi2O3/TiO2. (Dashed line is the time turn on the light).
Figure 11. Photocatalytic efficiencies of Hg0 with and without UVA of (a) TiO2, (b) CuO/TiO2, (c) CeO2/TiO2, (d) Bi2O3/TiO2. (Dashed line is the time turn on the light).
Catalysts 14 00209 g011
Table 1. Specific surface areas and average pore diameters of photocatalysts.
Table 1. Specific surface areas and average pore diameters of photocatalysts.
Type of PhotocatalystSpecific Surface Area (m2/g)Average Pore Diameter (nm)
Table 2. Crystallite sizes calculated using the Debye Scherrer equation.
Table 2. Crystallite sizes calculated using the Debye Scherrer equation.
Crystallite size (nm)11.813.515.0114.2
Table 3. Langmuir–Hinshelwood kinetic parameters for different photocatalysts.
Table 3. Langmuir–Hinshelwood kinetic parameters for different photocatalysts.
Types of
Temperatures (°C)
(m3 μg−1)
(μg m2 min−1)
(kcal mol−1)
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zheng, J.-R.; Yuan, C.-S. Comparing the Photocatalytic Oxidation Efficiencies of Elemental Mercury Using Metal-Oxide-Modified Titanium Dioxide under the Irradiation of Ultra-Violet Light. Catalysts 2024, 14, 209.

AMA Style

Zheng J-R, Yuan C-S. Comparing the Photocatalytic Oxidation Efficiencies of Elemental Mercury Using Metal-Oxide-Modified Titanium Dioxide under the Irradiation of Ultra-Violet Light. Catalysts. 2024; 14(3):209.

Chicago/Turabian Style

Zheng, Ji-Ren, and Chung-Shin Yuan. 2024. "Comparing the Photocatalytic Oxidation Efficiencies of Elemental Mercury Using Metal-Oxide-Modified Titanium Dioxide under the Irradiation of Ultra-Violet Light" Catalysts 14, no. 3: 209.

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop