Ozone catalytic oxidation of low-concentration formaldehyde over ternary Mn-Ce-Ni oxide catalysts modified with FeO_x

Abstract

Manganese oxide-based catalysts have attracted extensive attention due to their relatively low cost and remarkable performance for removing VOCs. In this research, we used the Pechini method to synthesize manganese-cerium-nickel ternary oxide catalysts (MCN) and evaluated the effectiveness of catalytic destruction of formaldehyde (HCHO) and ozone at room temperature. FeO_x prepared by the impregnation method was applied to modify the catalyst. After FeO_x treatment, the catalyst represented the best performance on both HCHO destruction and ozone decomposition under dry conditions and exhibited excellent water vapor resistance. The as-prepared catalysts were next characterized via H₂-temperature programmed reduction (H₂-TPR), temperature programmed desorption of O₂ (O₂-TPD), and X-ray photoelectron spectroscopy (XPS), and the results demonstrated that addition of FeO_x increased Mn³⁺ and Ce³⁺ concentrations, oxygen vacancies and surface lattice oxygen species, facilitated adsorption, and redox properties. Based on the results of in situ diffuse reflectance infrared Fourier transform spectrometry (DRIFTS), possible mechanisms of ozone catalytic oxidation of HCHO were proposed. Overall, the ternary mixed-oxide catalyst developed in this study holds great promise for HCHO and ozone decomposition in the indoor environment.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11356-022-24543-y.

Introduction

In the winter of 2019, a novel coronavirus disease (COVID-19) was detected in China and rapidly spreading as a global pandemic. China was the first country which was determined to have lockdown human and industrial activities, and the consequence was enormous. Specifically, the lockdown results in people spending more time in the indoor environment, causing them to be highly exposed to indoor air pollutants (Du et al. 2020). Especially, people might have to prepare their meals at home rather than eating out. Jung et al. (2021) pointed out that a significant amount of formaldehyde (HCHO) was generated in typical home cooking procedures. Additionally, smoking was also an important source of indoor formaldehyde. During the lockdown, the frequency of nonsmokers exposed to second-hand smoke may also increase. Long-term exposure to low-concentration HCHO may increase the prevalence of nasopharyngeal cancer and leukemia (Nielsen et al. 2017). During the past few decades, researchers have developed effective methods such as adsorption, photocatalysis, and nonthermal plasma for removing HCHO from gas streams. However, these technologies have encountered some challenges, such as low efficiency, the formation of unwanted products, and catalyst deactivation. To solve these bottlenecks, ozone catalytic oxidation (OZCO) with the advantages of low cost, high removal efficiency, and less byproduct formation has been developed (Zhu et al. 2017a). With strong oxidation capability of ozone and oxygen radicals, HCHO could entirely oxidize into harmless water and CO₂ at room temperature. However, ozone is a double-edged sword (Li et al. 2018). A previous study indicated that exposure to low-concentration ozone (~ 51 ppb) may cause respiratory illness and cardiovascular mortality (Cao et al. 2019). Consequently, simultaneous removal of HCHO and ozone is essential to protect human health and the environment when OZCO is applied.

Noble metal catalysts show good catalytic performance, but they are expensive and tend to be deactivated (Bao et al. 2020). Among the transition metal oxides, MnO_x, CeO₂, and NiO-based oxides as potential alternatives have attracted increasing attention due to their low cost and high thermal stability. Zhang et al. (2019) prepared MnCeO_x catalysts by the Pechini method; Mn is introduced into the CeO₂ structure to form Mn-Ce solid solution, which revealed excellent catalytic activity. Solid solutions are alloy phases in which solute atoms are dissolved into solvent lattices while remaining solvent types. The phase boundaries of bimetallic interfaces can provide abundant lattice mismatches, distortions, and defects, which will provide abundant oxygen vacancies and active sites (Wang et al. 2018). Gong et al. (2019a) synthesized heterostructure Ni/NiO nano-catalysts via solgel method and displayed excellent decomposition rate for ozone since NiO promotes the transfer of active oxygen to the surface. However, these catalysts have low humidity resistance; thus, it should be further improved for real application in the field. FeO_x is a common MnO_x doping material due to its high specific surface area (S_BET), great stability, and high capability for electron transfer. Zhang et al. (2014) indicated that addition of FeO_x into Mn₂O₃ reveals excellent activity toward NO removal thanks to its high S_BET and abundant Mn³⁺, Fe³⁺, and chemisorbed oxygen species. Besides, strong interaction between Fe and Mn facilitated the catalyst durability and humidity resistance (Lian et al. 2015, Wang et al. 2020b). At present, there are few reports on the simultaneous destruction of HCHO and ozone for indoor air quality management.

In this study, we synthesized a series of novel MnCeNiO_x catalysts by the simple Pechini method, and then modified it by FeO_x to further improve the catalytic activity for HCHO destruction at room temperature. It was found that the addition of FeO_x contributed to a better performance in HCHO oxidation. The FeO_x-MnCeNiO_x samples were characterized by various techniques, including N₂ physisorption, XRD, SEM, XPS, H₂-TPR, O₂-TPD, and TGA. The high performance was correlated with a high ratio of Mn³⁺/Mn⁴⁺ and Ce³⁺/Ce⁴⁺ and abundant surface-adsorbed oxygen species. The effects of initial O₃ concentration and water vapor with the catalytic performance were also investigated to demonstrate fundamental insights into the superior activity of the FeO_x-MnCeNiO_x catalysts. Based on in situ DRIFTS results, a viable mechanism of formaldehyde decomposition is presented. This study provides a high-activity, economical, environment-friendly, and simple catalytic method for the degradation of low-concentration HCHO. This work offers a valuable route to simultaneously catalytic destruction of HCHO and ozone for indoor air quality environmental management.

Experimental

Synthesis of Mn_xCeNiO_x catalysts

All reagents were of analytical grade and without purification. Porous Mn_xCeNiO_x catalysts were prepared by the Pechini method. Stoichiometric amounts of Mn(NO₃)₂•6H₂O, Ni(NO₃)₇•6H₂O and Ce(NO₃)₃•6H₂O were dissolved in deionized water under ultrasonication to obtain a homogeneous solution and placed in a beaker with magnetic stirrer; then citric acid and ethylene glycol were added to the solution as chelators. Afterwards, the solution was stirred continuously at 80 °C until it formed a stabilized pale green gel. After drying at 100 °C overnight, it turned into a puffy and porous dry solid. Finally, the fresh MnCeNiO_x catalysts were obtained by calcination at 500 °C in air for 6 h at a rate of 10 °C/min. The final products are named as MCN and M₂CN based on varying Mn/Ce atomic ratios (Mn/Ce = 1:1, 2:1), respectively. Pure MnO₂, CeO₂, and NiO was synthesized by the same Pechini method.

Synthesis of FeO_x-Mn_xCeNiO_x catalysts

FeO_X were prepared following a previous work (Chen et al. 2014). In brief, stoichiometric amounts of Fe(NO₃)₃•9H₂O were dissolved in deionized water under ultrasonication to obtain a homogeneous solution. Afterwards, Na₂CO₃ was dropwise added into the solution under vigorous stirring at 60 °C until the pH value stabilized at 8. After aging for 4 h, MCN was soaked in the resulting precipitate and dried at 60 °C for 18 h. Then, the final product was kept at 200 °C for 4 h to obtain fresh FeO_x-MnCeNiO_x catalyst which is denoted as F-MCN. Pure FeO_x was synthesized by the same method. The catalysts prepared were then ground and sieved to a particle size of 30–70 mesh before conducting catalyst characterization and activity test. The catalysts obtained were then characterized accordingly as described in the supplementary information.

Activity test

The schematic of the experimental setup for evaluating HCHO removal via OZCO is shown in Fig. S1. The inlet HCHO concentration was controlled by adjusting N₂ flow rate and water bath temperature. Water vapor was produced by passing N₂ through a water-containing bottle, and before entering the reactor, the relative humidity (RH) was measured by a humidity sensor (Center 310 RS-232, Taiwan). HCHO was measured by Fourier transform-infrared spectroscopy (Thermo Nicolet 6700, USA). The concentration of outlet CO₂ was analyzed by CO₂ Analyzer (Thermo 41C, USA), and the outlet CO concentration was measured by the Testo flue gas analyzer (Testo 350, Germany). Ozone was generated by a commercial ozone generator (Dean’s OW-K2/A-O, Taiwan), and the outlet ozone concentration was analyzed using an ozone analyzer (ITRI CMS/HOA, Taiwan). The equations for calculating HCHO mineralization rate and ozone decomposition are shown in supporting information.

Results and discussion

Crystal structures and morphology analysis

The crystal structures of the original MCN, M₂CN, and FeO_x-treated MCN samples were examined by XRD, as presented in Fig. 1. The diffraction peaks at 2θ = 28°, 33°, 47°, and 56° can be classified as (110), (200), (220), and (311) planes of fluorite-type CeO₂ (PDF#43–1002). The position of the diffraction peak is slightly shifted, and the diffraction angle becomes larger compared with the pure CeO₂ (Zhang et al. 2019), indicating that Mn ions are added into the CeO₂ lattice to form Mn-Ce solid solution. In addition, no obvious peaks of Mn oxides are found in MCN and F-MCN catalysts, which means Mn-Ce solid solution was formed or Mn ions were well dispersed over the catalyst surface. The formation of solid solution will induce more surface defects and surface oxygen species, which is conducive to the adsorption and decomposition of O₃ and HCHO molecules under the catalyst surface. The appearance of the peak at 35° is attributed to Mn₃O₄ (PDF#13–0162) as the content of the Mn ratio is increased. Moreover, the peaks of 37°, 43°, 63°, and 75° were the characteristic peaks of NiO (PDF#47–1049). The FeO_x modified catalyst exhibited weak and broad diffraction lines at 33° and 36°, which could be ascribed to Fe₅HO₈ ·4H₂O (PDF#16–0653) or ɑ-Fe₂O₃ (PDF#33–0664) (Daniells et al. 2005; Hutchings et al. 2006). Among them, the diffraction peak at 35°could be attributed to Fe₃Mn₃O₈. Chen et al. (2011) indicated the Fe₃Mn₃O₈ could greatly enhance the catalytic activity on NO_x removal. The strength of the peaks was enhanced by FeO_x modification, revealing that the crystallinity increased with the modification. Generally, the average grain size (D) of the catalyst is given by the following Scherrer’s formula [51].

$$D=\frac{K\times \lambda }{\beta \times cos\theta }$$ 1

Fig. 1.

XRD patterns of MCN, M₂CN, and F-MCN, respectively

where K is a constant (0.89); λ is the X-ray wavelength (0.154056 nm); β is the half-peak width; and θ is the diffraction angle. The as-calculated grain sizes of MCN, M₂CN, and F-MCN are 5.175 nm, 7.1226 nm, and 4.815 nm, respectively. According to previous studies, the F-MCN catalyst has the smallest particle size, which can have a larger S_BET (Bariki et al. 2020).

To understand the overall microstructural characteristics differences between as-synthesized catalyst, the N₂-sorption isotherms were presented in Fig. S2, which are well-defined IV-type isotherms with a typical-type H3 hysteresis loop, indicating the presence of mesopores, along with the generated of mesopores which could produce abundant oxygen vacancies to facilitate HCHO oxidation. The S_BET of MCN, M₂CN, and F-MCN are 119, 70, and 140 m²/g, respectively. Compared with the MCN catalyst, introduction of FeO_x increases the S_BET and pore volume, which may be due to the formation of ferrihydrite and hematite under high calcination temperature (Chen et al. 2014). On the other hand, excessive addition of Mn element to catalyst results in a significant reduction of S_BET and pore volume, indicating partial blocking of mesopores by Mn particles.

Further, the structure of MCN, M₂CN, and F-MCN catalysts was investigated by Raman spectra, as presented in Fig. 2. All three as-prepared catalysts displayed a strong peak at 642 cm⁻¹, which is attributed to fluorite F_2g mode of CeO₂. The appearance of a broad band ranging from 500 to 650 cm⁻¹ is observed after the incorporation of Ni and can be attributed to the interaction between Ce and Ni, forming a mixed crystal; thus generation of oxygen vacancies must be accompanied to (Chagas et al. 2016). The band appearance from 500 to 650 cm⁻¹ corresponds to the strong interaction between Ce and Mn to forming the Mn-Ce solid solution. The strong interaction will benefit the formation of oxygen vacancies due to the charge neutrality. The band at 365 cm⁻¹ for M₂CN belongs to the out-of-plane bending modes and symmetric stretching of Mn₂O₃ groups. After FeO_x modification, the appearance of the peak at 310 cm⁻¹ can be observed and attributed to hematite (α-Fe₂O₃). Raman spectra and XRD results suggest that FeO_x has been successfully added to the catalyst to enhance the interaction between Mn-Ce and Ce-Ni mixed crystal, resulting in more oxygen vacancies.

Fig. 2.

Raman spectra of the MCN, M₂CN, and F-MCN catalysts

To further investigate the morphology and microstructure of the as-prepared catalysts, SEM images of MCN, M₂CN, and F-MCN were obtained, as shown in Fig. S3. It was observed that all three catalysts were mainly composed of block morphology with a rough surface and porous structure. And it can be seen that some fine NiO particles adhere to the surface of CeO₂, and these particles have no specific morphology. The diameter of the nanoparticles was 1.5–1.9 μm accompanied by some small pieces with a diameter of 100 nm, besides a portion of the NiO particles agglomerate into spheres with a diameter of 300 nm. Meanwhile, as shown in Fig. S3b, for M₂CN with the excessive doping of Mn ions, some lamellar petals attached to their surface appear, and these petals do not cluster into balls. For F-MCN, as shown in Fig. S3c, the metal oxides were highly dispersed under the surface of catalyst, which may be due to the partial sintering by secondary calcination in the preparation of FeO_x.

Catalyst elemental analysis

The chemical composition of surface elements of MCN, M₂CN, and F-MCN was determined through XPS, as shown in Fig. 3. Fe 2p spectra can be fitted into two peaks at 710.1 and 712.5 eV, which correspond to Fe²⁺ and Fe³⁺, respectively. Moreover, a satellite peak at around 718.0 eV was observed, which can be corresponding to Fe₂O₃ (Liu et al. 2018). Fe³⁺ is beneficial to the formation of surface hydroxyl which could improve the redox property of catalyst (Chen et al. 2014). The new species generated with Fe²⁺ was known as a main reason for the enhancement of chemisorbed oxygen (Wang et al. 2020a). Fe²⁺ and Fe³⁺ were both beneficial to the catalytic decomposition of HCHO.

Fig. 3.

XPS patterns of as-prepared catalyst for a Fe 2p, (ɑ) is MCN, (β) is M₂CN, (γ) is F-MCN. b Mn 2p. c Ce 3d. d O1s, respectively

For the Mn species in the mixed metal oxide, it can be divided into Mn³⁺ (about 641.8 eV) and Mn⁴⁺ (about 642.7 eV), as shown in Fig. 3b. After modification with FeO_x, the Mn³⁺/Mn⁴⁺ ratio increased significantly from 1.23 to 1.33 (Table 1). It is generally recognized that oxygen vacancies will be generated when Mn³⁺ exists in mixed metal oxide due to the maintained charge neutrality, based on the following reaction:

$$4M{n}^{4+}+O^{2-}\to 4M{n}^{4+}+2e^{-}/\square +\frac{1}{2}{O}_2\to 2M{n}^{4+}+2M{n}^{3+}+\square +\frac{1}{2}{O}_2$$

Table 1.

BET surface areas, pore volumes, average pore sizes, and elemental compositions of as-prepared catalysts

Catalyst	SBET (m2/g)	SBETa (m2/g)	Dpore (nm)	Dporea (nm)	Vpore (cm3/g)	Vporea (cm3/g)	Surface atom ratio			Fe/Mn atom ratio		Mn/Ce/Ni atom ratio
							Mn3+/Mn4+	Ce3+/Ce4+	Oads/Olatt	ICP-OES	XPS	ICP-OES	XPS
MCN	119	113	4.66	4.68	0.14	0.13	1.23	0.34	0.22	-	-	1:1.09:0.98	1:1.07:0.96
M2CN	70	59	6.11	8.25	0.11	0.09	1.57	0.27	0.18	-	-	2:1.08:1.12	2:0.97:0.99
F-MCN	140	136	4.36	5.1	0.16	0.12	1.33	0.63	0.35	0.901	0.88	1:1.03:1.07	1:0.95:0.92

^a: Used catalyst

where □ represents the oxygen vacancy site (Li et al. 2018).

Surface-active oxygen species derived from oxygen vacancies can directly participate in and become conducive to HCHO oxidation (Ye et al. 2022). Meanwhile, the addition of FeO_x could induce the enhancement of Mn⁴⁺ amount (Wang et al. 2020b). Mn⁴⁺ are more conducive to the decomposition reaction over Mn-based catalysts (Chang et al. 2018). As a result of the formation of Fe–Mn solid solution, the incorporation of Fe affected the Mn ion electron state, thus leading the binding energy to exhibit a declining tendency.

Regarding the spectra of Ce 3d, five peaks at 881 eV, 883 eV, 899 eV, and 902 eV could be ascribed to Ce³⁺, while characteristic peaks of Ce⁴⁺ present at 882 eV, 888 eV, 897 eV, 900 eV, 907 eV, and 916 eV (Jiang et al. 2020). The atomic ratios of Ce³⁺/Ce⁴⁺ over MCN and F-MCN are 0.34 and 0.63, respectively (Table 1). In general, the oxygen vacancies will be generated when the Ce⁴⁺ transforms into nonstoichiometric Ce³⁺.

$$4C{\text{e}}^{4+}+O^{2-}\to 4C{\text{e}}^{4+}+2e^{-}/\square +\frac{1}{2}{O}_2\to 4C{\text{e}}^{4+}+2C{\text{e}}^{3+}+\square +\frac{1}{2}{O}_2$$

Therefore, the relative concentration of oxygen vacancies was usually proportional to the concentration of Ce³⁺ cations (Li et al. 2020).

The Ni 2p spectra of as-prepared catalysts were divided into three peaks at 852 eV, 857 eV, and 870 eV which were attributed to Ni⁰ (Song et al. 2017), respectively (Fig. S4). And the peak at 878 eV and 880 eV is corresponding to Ni²⁺ (Fang et al. 2019), indicating the existence of both NiO and metallic Ni in catalysts.

The O 1 s spectrum shows that the peaks at binding energies of 529 eV corresponded to lattice oxygen, while those at 531 eV was corresponding to the surface adsorbed oxygen species (Li et al. 2020), as shown in Fig. 3e. The atomic ratios of O_ads/O_latt over MCN and F-MCN are 0.22 and 0.35, respectively (Table 1). Similarly, surface-adsorbed oxygen species usually adsorb on oxygen vacancy. These results illustrate that the F-MCN catalyst prepared possesses higher Mn³⁺/Mn⁴⁺, Ce³⁺/Ce⁴⁺ ratios, and extremely abundant surface-adsorbed oxygen species compared with MCN and M₂CN, suggesting that modification with FeO_x increases the content of oxygen vacancies.

The Fe, Mn, Ce, and Ni contents of as-synthesized catalysts were investigated by ICP-OES, as shown in Table 1. For MCN and M₂CN, the ICP results demonstrate that the Mn, Ce, and Ni ratio is close to the theoretical value but not identical, while considering the experimental errors, such a deviation is acceptable. For F-MCN_, after FeO_x treatment, the intensities of Mn, Ce, and Ni signals were greatly reduced, and simultaneously a new signal of Fe appeared, which proved that Fe entered the catalyst and four elements are almost uniformly distributed in the catalysts. The strong interaction of Fe generated the reduction of Mn, Ce, and Ni ratio. The uniform distribution of the four elements will facilitate the formation of Fe–Mn and Mn-Ce solid solutions.

Chemisorption measurements

The H₂-temperature programmed reduction profiles of MCN, M₂CN, and F-MCN catalysts are measured to evaluate their reducibility, as presented in Fig. 4a. The H₂-TPR profile of MCN could be divided into two principal reduction peaks, i.e., a little peak at 264 °C and the other stronger peak at 417 °C. The lower-temperature peak may be attributed to the reduction of MnO₂/Mn₂O₃ to Mn₃O₄ (Liu et al. 2017), in which phase was also detected in XRD results. The higher-temperature peak was corresponding to the reduction of NiO particles to Ni⁰ (Chagas et al. 2016). Meanwhile, Mn₃O₄ will further reduce to MnO at 350–400 °C, thus enhancing H₂ consumption in higher-temperature peak (Ma et al. 2020). Besides, there were no obvious peaks of CeO₂ observed, which indicates that the reduction of Ce⁴⁺ accompanied with the reduction of Mn³⁺. The results demonstrated that the mobility of O atoms was increased when Mn ions were induced into the CeO₂ lattice (Zhang et al. 2019). After addition of FeO_x, the catalyst reveals three principal reduction peaks. The lower-temperature demonstrates two reduction peaks located at 240 and 285 °C, which were divided into surface oxygen species and MnO₂ reduction. The higher-temperature peak was attributed to the reduction of Fe and Ni species (Song et al. 2017). The reduction peaks of Ni species moved towards lower temperature due to the addition of FeO_x generated additional oxygen vacancies and facilitated the redox of catalysts. In addition, the peak area for F-MCN was significantly larger than that for MCN and M₂CN catalysts, which enhancement may be due to the synergistic effect between Mn and Fe oxides, which can produce oxygen defects and structural distortion (Chen et al. 2019). Moreover, it should be noted that the addition of FeO_x reduces the reduction temperature from 264 to 240 °C. However, the reduction temperature over M₂CN obviously shifts to a higher temperature (305 °C) due to the excessive addition of Mn. Lower reduction temperature means a higher activity of oxygen vacancies, thus enhancing the reactivity of surface oxygen species (Jiang et al. 2020). Therefore, it is reasonable to deduce that the reactivity of oxygen vacancies increased in the following sequence: F-MCN > MCN > M₂CN.

Fig. 4.

a H₂-TPR, b O₂-TPD profiles of MCN, M₂CN, and F-MCN catalysts

The oxygen desorption behavior was investigated by O₂-TPD, as presented in Fig. 4b. The peaks could be divided into three parts; i.e., below 300 °C was assigned to the desorption of physically adsorbed oxygen, the middle-temperature peak (350–600℃) is assigned to chemisorbed surface active oxygen, and the high-temperature peak (650–900℃) attributed to the desorption of lattice oxygen species (Gong et al. 2019b). Compared with MCN and M₂CN, the catalyst modified with FeO_x exhibits the most intensity at 200–600℃. Meanwhile, the peak intensity of M₂CN is obviously less than that of MCN; it illustrates that excessive doping of Mn is unfavorable for the formation of oxygen vacancies (Zhang et al. 2020). The results indicated that between Fe and Mn, there exists a strong interaction, resulting in surface defects. Therefore, addition of Fe into MCN could enhance surface adsorbed oxygen content and surface lattice oxygen became more mobile, which is in good agreement with the XPS results. Previous studies have proven that the existence of abundant surface active oxygen species facilitates the destruction of VOC molecules, ozone molecules, and the deep oxidation of intermediates (Gong et al. 2019b). In comparison with MCN and M₂CN catalysts, the F-MCN catalyst possesses rich chemisorbed surface-active oxygen species with higher reducibility, mobility, and reactivity due to the strong interaction between Fe and Mn. Therefore, the F-MCN catalyst has a great advantage in HCHO removal.

Thermo-catalytic activity

During the performance of as-synthesized catalysts for HCHO oxidation at different temperatures, the removal activity was investigated with the initial HCHO concentration of 15 ppm, total gas flow rate of 1200 sccm, and GHSV of 15,000 h⁻¹, as shown in Fig. 5. CO was not detected in our experimental condition. For all three catalysts evaluated, it is observed that the HCHO conversion is less than 50% as the operating temperature is below 50 °C, and the HCHO conversion increases with increasing temperature rapidly above 50 °C. Complete conversions of HCHO over MCN and M₂CN catalysts were achieved at 150 °C, while MCN exhibited a higher activity at low temperatures. Catalytic oxidation of HCHO over F-MCN exhibited 100% conversion at 100 °C, which is 50 °C lower than those of MCN and M₂CN under HCHO complete conversion. Certainly, catalytic performance of F-MCN is better than that of MCN and M₂CN, suggesting that addition of FeO_x could enhance the catalytic conversion of HCHO. Besides, the results indicate there is also a striking improvement of catalytic HCHO conversion when a small amount of Mn is added into the catalysts; however, doping too much Mn may lead to the activity decrease.

Fig. 5.

HCHO conversion as a function of reaction temperature over MCN, M₂CN, and F-MCN catalysts, respectively

Catalytic degradation of HCHO was obeyed by the Mars van Krevelen model, and the apparent activation energy (E_a) could be calculated by Eq. (2),

$$lnk=-\frac{E_a}{\mathit{RT}}+lnA$$ 2

where R is the molar gas constant; T is the temperature at reaction; and A is the pre-exponential factor.

The Arrhenius plots for HCHO conversion of less than 20% over the as-synthetized catalysts are shown in Fig. S5. Under thermo-catalytic conditions, the correlation coefficients of MCN, M₂CN, and F-MCN catalysts calculated by linear regression equations are 0.996, 0.991, and 0.990, respectively, showing that the experimental results meet the assumptions. Meanwhile, the calculated activation energies of MCN, M₂CN, and F-MCN catalysts for HCHO oxidation are 33 kJ/mol, 34 kJ/mol, and 29 kJ/mol, respectively. The results illustrate that the E_a values for the F-MCN catalysts are reduced with the addition of FeO_x, while the M₂CN catalyst showed the highest E_a value. Meanwhile, as shown in Table S1, compared with other non-noble metal catalysts, F-MCN shows a reasonable E_a value, which was close to that of noble metal catalysts. In order to better evaluate the impact of modification on catalysts, we introduced the concept of the specific reaction rate (r_cat (mol/(g·s)) which was defined as the moles of HCHO being converted per second per gram of catalyst (Du et al. 2018). The r_cat was used to evaluate the intrinsic activity of a catalyst. Based on the activity data, we calculated the specific reaction rates (r_cat) of three catalysts for HCHO oxidation. The results show that the specific reaction rates of F-MCN, M₂CN, and MCN are 3.33 × 10 ⁻⁶, 1.47 × 10 ⁻⁶, and 2.07 × 10 ⁻⁶ mol/(g·s), respectively. In addition, we can use the molar amount of the Mn (active metal) to determine the TOF_Mn. The results show that the TOF_Mn of F-MCN, M₂CN, and MCN are 2.85, 2.57, and 2.77 × 10 ⁻² s ⁻¹, respectively. The results confirm that modification of FeO_x is the main reason for improving the degradation efficiency of HCHO after excluding the effect of specific surface area.

Ozone catalytic oxidation activity

Three catalysts were investigated to compare their performance for HCHO oxidation with [O₃]/[ HCHO] ratio = 4. As shown in Fig. 6(a), the HCHO oxidation efficiency decreased in the following order: F-MCN > MCN > M₂CN. Nearly 88% of the HCHO was degraded over F-MCN, while the HCHO conversion rates achieved with MCN and M₂CN reached 70% and 65%, respectively. Notably, no CO was detected in the exhaust gas of three catalysts, while CO₂ was the major C-species. Previous studies indicate that decomposition of O₃ was the first step for the ozone catalytic oxidation process, so the outlet O₃ concentration was monitored [15,27]. Under dry conditions, it is exhibited that all three catalysts show excellent performance and complete decomposition of O₃ is achieved and maintained within the 8-h test duration.

Fig. 6.

Catalytic performance of HCHO over as-synthesized catalysts with [O₃]/[HCHO] = 4. Conditions: HCHO,15 ppm; total flow rate, 1,200 sccm; O₃, 60 ppm; GHSV,15,000 h⁻¹; temperature, 25 °C. a Dry air; b RH = 70%

In the real environment, however, there exists a significant amount of water vapor which may inhibit the catalytic activity; therefore, the test was carried out with RH = 70% to explore the humidity effect, as shown in Fig. 6b. The results indicate that F-MCN and MCN exhibit significant O₃ decomposition rate, illustrating good humidity resistance. The efficiency of HCHO conversion achieved with F-MCN is up to 100%. Obviously, the rate of HCHO conversion to CO₂ achieved with F-MCN catalyst increases along with the presence of water vapor. The abundant oxygen vacancies are very efficient active sites to enhance the water vapor decomposition to surface hydroxyl groups and assist the regeneration of catalyst.

However, ozone conversion rate decreased sharply to nearly 60% over M₂CN, and HCHO mineralization rate further decreases to 40%, which emphasizes that O₃ decomposition rate which is the first step for OZCO process. According to the results of catalyst characterization, the excessive Mn content blocks the surface pores to inhibit oxygen vacancies. Besides, excessive moisture may hinder the exposure of active sites, resulting in competitive adsorption of water molecules under the surface of catalyst, reducing the efficiency of ozone decomposition. It causes the reduction of the surface oxygen species (e.g., O•, •O₂⁻) from ozone decomposition, which is not beneficial for HCHO oxidation. For better analyzing the mechanism of catalyst deactivation, the unit water content (U) to estimate the competitive adsorption can be calculated by the following formulas:

$$\text{U}=\frac{\text{W}}{\text{S}}$$ 3

where W is the water content in the airflow and S is the S_BET of catalyst. The unit water content value of F-MCN, MCN, and M₂CN can be approximately calculated to be 0.1, 0.17, and 0.2 g/m², respectively. The results show that competitive adsorption occurs when the weight of water on the catalyst is greater than 0.2 g per m². It is noteworthy that after being treated with FeO_x, the humidity resistance is enhanced and excellent HCHO and O₃ conversion rates are simultaneously achieved.

For a better understanding of the kinetics of catalytic process, the degradation data of HCHO could be fitted with the pseudo-first-order kinetics, based on Eq. (4).

$$In\left(\frac{C_0}{C_t}\right)=k_{\mathit{app}}\times t$$ 4

where t is the time of reaction, C₀ and C_t were the HCHO concentrations at time 0 and t, and k_αpp is the apparent first-order rate constant (min⁻¹). As shown in Fig. S6, the calculated k value of F-MCN is 2.12*10^–1 min⁻¹ which is significantly higher than that of MCN and M₂CN. The high decomposition efficiency and rate constant of F-MCN revealed the addition of FeO_x promotes its superior reactivity, i.e., the active oxygen species, as manifested from the previous characterization. Furthermore, the poor activity of M₂CN adequately suggested that the catalytic performance was highly dependent on its morphological structure, physical, and chemical properties.

In order to explore the OZCO performance of F-MCN with different O₃/HCHO ratios, experimental tests were carried out at 15 ppm HCHO, ozone concentration varying from 30 to 60 ppm, and RH = 70% condition, with the results being shown in Fig. 7. As the [O₃]/[HCHO] ratio is controlled at 4, F-MCN catalyst reveals excellent conversion for HCHO and maintains a 100% during the 8-h test period. When [O₃]/[HCHO] ratio is reduced to 3, the conversion rate of HCHO decreases slightly, but still remains at 95%. As the [O₃]/[HCHO] ratio is dropped to 2, although the catalyst does not show deactivation after 8 h of reaction, the efficiency gradually decreases to 65%. In summary, in the OZCO test with F-MCN, the catalyst exhibits excellent catalytic activity; when [O₃]/[HCHO] ratio is reduced to 3, it still maintains a high conversion rate of HCHO (average of 95%). In the absence of a catalyst, no significant product was observed in the reactor even in the presence of ozone, implying that the ozone catalytic oxidation of formaldehyde was triggered by the F-MCN catalyst.

Fig. 7.

Catalytic performance of HCHO over F-MCN with different [O₃]/[HCHO] ratios Conditions: HCHO,15 ppm; total flow rate, 1,200 sccm; O₃, 30–60 ppm; GHSV, 15,000 h⁻¹; RH = 70% at room temperature (25 °C)

Reusability of the F-MCN catalyst

Catalytic recycling of F-MCN

Stability is an important factor in judging the potential value of the catalyst in application. Regarding the stability of catalyst, the HCHO concentration of 15-ppm at [O₃]/[HCHO] = 4 with MCN and F-MCN as catalyst for 6 h and the average value of each test was taken from three experimental runs. As shown in Fig. 8, after 3 cycles, the final HCHO conversion achieved with MCN was significantly decreased to 73%. However, F-MCN demonstrated an excellent stable HCHO conversion of 93% even after 3 cycles. Besides, deactivation and CO weren’t observed or detected for both catalysts. These data indicated that the F-MCN exhibited excellent durability to HCHO and ozone even under relatively long-term test. After modification with FeO_x, the catalyst stability increases, and FeO_x provides favorable corrosion resistance, credit to the strong interaction between the Fe and Mn (Wang et al. 2020b).

Fig. 8.

Stability test of MCN and F-MCN catalyst OZCO of HCHO. Conditions: HCHO,15 ppm; total flow rate, 1,200 sccm; O₃, 60 ppm; GHSV, 15,000 h⁻¹; RH = 70%

Figure S7 exhibited the XRD patterns of fresh and used F-MCN catalyst. After 3 cycles, the position and strength of peak were almost unchanged, which proved that after being modified by FeO_x, the micromorphology of the catalyst has no obvious change during the OZCO tests, indicating that F-MCN had significant corrosion resistance and stability.

SBET analysis

To disclose the influence of carbon deposition on catalyst pores, the S_BET analysis after OZCO reaction was conducted, as shown in Table 1. After reaction, the S_BET of MCN decreased slightly from 119 to 113 m²/g, M₂CN decreased from 70 to 59 m²/g, and F-MCN decreased slightly from 140 m²/g to 136 m²/g. In the reaction process, part of the intermediates and incomplete reaction products accumulated in the mesopores of the catalyst, resulting in the decrease of S_BET. As for the pore size, MCN did not change significantly, while M₂CN and F-MCN increased from 6.11 and 4.2 nm to 8.25 and 5.2 nm, respectively. That is generally ascribed to the carbon deposition on the catalyst which blocks some micropores (small holes), resulting in the increase of macropores proportion. Compared with MCN and M₂CN catalysts, after modification with FeO_x, S_BET changes slightly, indicating less deposition of byproducts on catalyst. The results reveal that F-MCN has good resistance to endure the corrosion.

Thermogravimetry analysis

To compare the species changes and byproducts formation on catalyst surface after OZCO and thermal process, thermogravimetric analysis (TGA) was investigated, and the profiles are presented in Fig. 9. For the fresh catalyst, almost 4.4% weight loss was observed for MCN catalyst, while this value was approximately 5.2% for M₂CN catalyst. After modification with FeO_x, the weight loss was reduced to 3.1%, suggesting better thermal stability. After OZCO and thermo-catalytic process, the weight losses of all catalysts were increased. Additionally, after thermo-catalytic processes, the weight loss is much higher than that of OZCO process for MCN and F-MCN catalysts. In general, a higher amount of byproduct was deposited on the catalyst when operated at room temperature, while the opposite trend was observed in this study. The results verified that the addition of O₃ accelerated the conversion of formate to CO₂ and reduced the formation and adsorption of intermediates and byproducts on catalyst surface. Besides, addition of O₃ resulted in the formation of CO_x species, thereby enhanced CO and CO₂ desorption (Chen et al. 2020a). However, M₂CN catalyst could not utilize ozone completely and then reduces the conversion rate of formate to CO₂ due to its weak O₃ decomposition capacity. Then, the intermediates and byproduct are easy to accumulate on M₂CN surface, and higher weight loss also proves this hypothesis. The above results also verify that catalytic decomposition of O₃ is the first step for OZCO. In summary, the FeO_x-MCN catalyst exhibits excellent thermostability and less byproduct deposition, suggesting a more stable structure is induced after MCN treated with FeO_x.

Fig. 9.

Weight loss curves of the thermogravimetric analysis for a MCN, b M₂CN, and c F-MCN under difference processes

Intermediates and mechanism

Under wet condition, F-MCN exhibited an excellent HCHO conversion rate, while M₂CN HCHO mineralization rate sharply decreased. To shed light on HCHO ozone catalytic oxidation mechanism of F-MCN and catalyst deactivation mechanism over M₂CN, the main intermediates formed were analyzed by in situ DRIFTS. The DRIFTS spectra over the M₂CN and F-MCN catalysts under exposure to HCHO for 3 h were presented in Fig. 10a and b, respectively.

Fig.10.

In situ DRIFT spectra of HCHO interaction with ozone at room temperature under wet air conditions over a M₂CN and b F-MCN samples as a function of time

The absorption bands at 1,360, 1,550, 1,650, and 3,249 cm⁻¹ were observed on the catalysts. According to the previous studies, the characteristic peaks at 1,550 cm⁻¹ were assigned to the asymmetric stretch [ν_as(COO −)] of formate species, and the weak bands at 1,360 cm⁻¹ can be ascribed to monodentate carbonate species (Ji et al. 2020). The peaks of carbonate and formate species were hardly discriminated due to their co-existence within the wide wavenumber range of 1,300–1,500 cm⁻¹ (Guo et al. 2019). Thus, it is speculated that the accumulation of carbonate and formate species factors is occurred.

As shown in Fig. 10a, excessive doping Mn ions results in fewer holes and oxygen vacancies, leading to the formate species and carbonate species accumulating on the surface of catalyst. Zhao et al. (2012) indicated that formate species, carbonate species, and other intermediate products are adsorbed and accumulated on the surface of catalysts which cannot be dissociated from the active sites, inducing the reduction of catalytic activity or even deactivation. Similarly, along with the reaction proceeds, the peak at 3,249 cm⁻¹ gradually increased, which illustrates the water vapor formed during the OZCO process also accumulates on the catalyst surface. Chen et al. (2020b) demonstrated that the continuous formation of H₂O results in competitive adsorption. The active sites of the catalyst were firmly occupied by them and could not be desorbed, which was also responsible for a reduction of the catalytic activity.

As presented in Fig. 10b, the intensity of formate and carbonate species on the surface of F-MCN catalyst remained almost unchanged during the reaction; the high reactivity of surface oxygen species enhanced HCHO promptly transformed, which indicated that the adsorption and desorption of formate species were in dynamic equilibrium. Meanwhile, the absorbance peak at 3,249 cm⁻¹ displayed little change; the results illustrate that the surface hydroxyl groups were consumed during the reaction, which could be replenished during the reaction of HCHO and O₃ under the surface of F-MCN. These results demonstrated that the enhancement of surface-active oxygen groups by FeO_x modification could facilitate the intermediate species dissociation and further oxidize them to CO₂ after the action of ozone. In summary, the desorption of both intermediates and products from F-MCN catalyst surface is much faster than that from M₂CN. Based on the above analysis and previous studies (Zhao et al. 2012), the possible OZCO reaction mechanisms of HCHO over F-MCN catalyst were further proposed and demonstrated in Scheme 1.

Scheme 1.

Schematic illustration of the HCHO ozone catalytic oxidation over F-MCN

Process (A)

In this mechanism, O₃ was initially adsorbed and activated via the catalyst; afterwards, O₃ is decomposed by oxygen vacancy into the active oxygen radicals (O• and •O₂⁻) (Wang et al. 2019). The C atoms of HCHO will be attacked by O• and •O₂⁻ under the surface of catalyst and generated to formate species. After that, formate species are further converted into carbonate species, which will quickly decompose into CO₂ and H₂O_(g). Meanwhile, •O₂⁻ further reacts with ozone and releases the oxygen molecule from the catalyst surface to attain the reproduction of surface oxygen vacancy (Zhu et al. 2017b).

Process (B)

Under wet conditions, HCHO molecules firstly adsorb onto OH• groups by hydrogen bonding, while O₃ molecules are adsorbed onto activated oxygen vacancy and decomposed into the active oxygen radicals (O•, •O₂⁻) simultaneously. Afterwards, the surface oxygen species (e.g., O•, •O₂⁻, OH•) oxidizes the HCHO adsorbed on the catalyst to unstable formate species. Finally, the •O₂⁻ further reacts with ozone, would release as the oxygen molecule from the surface to fill oxygen vacancies under the catalyst surface, and then CO₂ and H₂O_(g) rapidly desorb from the catalyst surface.

Because of the existence of the surface OH group, there was almost negligible accumulation of carbonates species under the Process (B). Based on the investigation of the characteristics of catalysts before and after the reaction, it can be found that there is a slight deactivation and micropore blockage of the catalysts. Therefore, it is speculated that in the ozone catalytic oxidation over F-MCN, Process (B) is the predominant mechanism.

Conclusion

The MnCeNiO_x (MCN) was prepared by the Pechini method and further treated with FeO_x via impregnation method (F-MCN) and introduced into the OZCO system to discuss the effect of adding FeO_x for the catalytic degradation of HCHO at room temperature. Under the dry condition, F-MCN catalyst exhibited better performance of HCHO conversion and ozone decomposition than MCN and M₂CN catalysts due to abundant Mn³⁺ and Ce³⁺ which were proportional to the intensity of oxygen vacancies and surface adsorbed oxygen and large S_BET. The excellent redox property is resulted from to the strong interaction between Mn-Ce, Ce-Ni, and Fe–Mn mixed crystal. Besides, under the wet system, the addition of water vapor enhanced the surface hydroxyl radicals, leading to the complete oxidation of HCHO over F-MCN catalyst under room temperature. Hydroxyl radical is regenerated from the interaction between adsorbed water molecules and surface oxygen species produced from the decomposition of ozone on the catalyst surface. This process is effective for the simultaneous decomposition of HCHO and O₃ in indoor air due to water vapor always existing in the real environment. However, additional of water vapor inhibited the decomposition of ozone with M₂CN and then reduces the decomposition of HCHO, which confirms that O₃ catalytic destruction is the first step for OZCO. In addition, after 72-h continuous operation tests, the F-MCN catalyst exhibits excellent stability, and the crystal phase was almost unchanged, indicating FeO_x provides a favorable corrosion resistance. Therefore, F-MCN catalyst revealed high HCHO and ozone removal performance in high RH conditions, demonstrating its great potential for practical applications.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contribution

RY Liu provided and analyzed the test data and wrote the manuscript. MM Trinh and HT Chuang helped revise the manuscript. MB Chang provided conceptual and technical guidance for all aspects of the project. All authors read and approved the final manuscript.

Data availability

All data generated or analyzed during this study are included in this published article and its supplementary information files.

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Competing interests

The authors declare no competing interests.