The Catalytic Oxidation of Formaldehyde by Feox-Mno2-Ceo2 Catalyst: Effect of Iron Modiﬁcation

catalysts

Article The Catalytic Oxidation of Formaldehyde by FeOx-MnO2-CeO2 Catalyst: Effect of Iron Modiﬁcation

Yaxin Dong , Chenguang Su, Kai Liu, Haomeng Wang, Zheng Zheng, Wei Zhao and Suhong Lu *

College of Chemistry and Chemical Engineering, Xi’an Shiyou University, Xi’an 710065, China; [email protected] (Y.D.); [email protected] (C.S.); [email protected] (K.L.); [email protected] (H.W.); [email protected] (Z.Z.); [email protected] (W.Z.) * Correspondence: [email protected]; Tel.: +86-156-1922-8512

Abstract: A series of FeOx-MnO2-CeO2 catalysts were synthesized by the surfactant-templated coprecipitation method and applied for HCHO removal. The inﬂuence of Fe/Mn/Ce molar ratio on

the catalytic performance was investigated, and the FeOx-MnO2-CeO2 catalyst exhibited excellent catalytic activity, with complete HCHO conversion at low temperatures (40 ◦C) when the molar ratio

of Fe/Mn/Ce was 2/5/5. The catalysts were characterized by N2 adsorption and desorption, XRD, H2-TPR, O2-TPD and XPS techniques to illustrate their structure–activity relationships. The result revealed that the introduction of FeOx into MnO2-CeO2 formed a strong interaction between FeOx- MnO2-CeO2, which facilitated the improved dispersion of MnO2-CeO2, subsequently increasing the surface area and aiding pore development. This promotion effect of Fe enhanced the reducibility and produced abundant surface-active oxygen. In addition, a great number of Oα is beneﬁcial to the 3+ intermediate decomposition, whereas the existence of Ce favors the formation of oxygen vacancies on the surface of the catalyst, all of which contributed to HCHO oxidation at low temperatures.

Citation: Dong, Y.; Su, C.; Liu, K.; Wang, H.; Zheng, Z.; Zhao, W.; Lu, S. Keywords: formaldehyde; FeOx-MnO2-CeO2; catalytic oxidation; catalytic activity The Catalytic Oxidation of

Formaldehyde by FeOx-MnO2-CeO2 Catalyst: Effect of Iron Modiﬁcation. Catalysts 2021, 11, 555. https:// 1. Introduction doi.org/10.3390/catal11050555 As a common indoor air pollutant, formaldehyde (HCHO), mainly from building and decoration materials, causes strong irritation, teratogenicity and carcinogenicity, and Academic Editor: Angeliki it is therefore harmful to human health. In 2004, the World Health Organization identi- A. Lemonidou ﬁed formaldehyde as a group I carcinogen for humans [1]. Hence, from the perspective of protecting human health and the air environment, it is vital to reduce and eliminate Received: 3 April 2021 formaldehyde. Many technologies have been proposed for indoor HCHO removal, such Accepted: 25 April 2021 Published: 27 April 2021 as adsorption [2], photocatalytic oxidation [3], plasma degradation [4] and catalytic oxidation [5]. Among these, catalytic oxidation of HCHO has been proven to be the most

Publisher’s Note: MDPI stays neutral economical and effective technology, because the process involves the complete conversion with regard to jurisdictional claims in of HCHO into CO2 and H2O without by-products at low or even room temperature [6]. published maps and institutional afﬁl- The key factor of this method lies in the selection and preparation of effective catalysts to iations. eliminate HCHO. Two types of catalysts for the catalytic oxidation of HCHO have been applied in the reaction process, namely supported noble metals and transition metal oxides [7]. Comparing both types of catalysts, supported noble metal catalysts exhibit superior catalytic properties for HCHO elimination at low or even room temperature. However, due to the scarce Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. resources and high costs of noble metals, many researchers are shifting their attention to This article is an open access article the development of inexpensive and effective transition metal oxide catalysts. It has been distributed under the terms and reported that [MnOx][8], [Co3O4][9], [CeO2][10] and [CuO] [11], being characteristic rich conditions of the Creative Commons oxygen species and exhibiting rapid electron transfer, are promising alternative catalysts for Attribution (CC BY) license (https:// the catalytic oxidation of HCHO. For example, a MnOx-CeO2 catalyst was prepared by the ◦ creativecommons.org/licenses/by/ redox reaction method and it achieved 100% HCHO conversion at 100 C[12]. Additionally, 4.0/). Co3O4-CeO2 was synthesized by a citric acid sol-gel method, and it fully oxidized HCHO

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at a temperature as low as 80 ◦C[13]. According to the literature [14], the use of metal oxides with promoters for the synthesis of trimetal oxide catalysts can increase the amount of surface-active oxygen species and, thus, significantly enhance the catalytic oxidation processes. Jiang et al. [15] reported that Mn-Cu-Ce mixed oxide networks had higher catalytic activity than Mn-Cu and pure Mn due to the inclusion of Cu, which improved the oxygen activation and transport ability. Lu et al. [16] discovered that the addition of MnOx to Co3O4-CeO2 resulted in high dispersion and easy reducibility of a MnOx-Co3O4-CeO2 catalyst at lower temperatures. However, only a few reports exist on ternary mixed oxide catalysts for the elimination of formaldehyde, and these have failed to address some vital research questions. In an attempt to expand the general knowledge of the application of ternary mixed oxide catalysts for the removal of formaldehyde, it is necessary to develop multi-component metal oxide catalysts for the low-temperature catalytic oxidation of formaldehyde. Therefore, we report the development of Fe-Mn-Ce ternary mixed oxide catalysts for the purification of formaldehyde by the surfactant-templated coprecipitation method. The influence of the Fe/Mn/Ce molar ratio on the structure-activity relationship of the FeOx-MnO2-CeO2 catalysts was investigated.

2. Results and Discussion 2.1. Characterization of the Catalysts 2.1.1. N2 Adsorption and Desorption

The N2 adsorption/desorption isotherms and pore size distributions of the Fe-Mn- Ce ternary mixed oxides are presented in Figure1A, B and C, while the specific surface area (SBET), pore diameter and pore volume are summarized in Table1. All the catalysts exhibited the type IV isotherm with H3 hysteresis loops in the relative pressure (P/P0) range of 0.6–1.0; the pore structure of the sample caused agglomeration in the capillary, indicating the existence of mesoporous structures [17,18]. It was noted that the FeOx- MnO2-CeO2-2:5:5(FMC-2:5:5) catalyst significantly increased the amount of adsorbed N2, which indicates that the catalyst had large specific surface area and pore volume. This observation is supported by the data in Table1. The specific surface area and pore volume 2 −1 3 −1 of FMC-2:5:5 catalyst were 99 m g and 0.4 cm g , respectively. The pure CeO2, 2 −1 Fe3O4 and MnO2 presented similar results for surface area (22 m g ) and pore volume of 3 −1 2 −1 (0.1 cm g ), while the SBET values and pore volume of MnO2-CeO2 increased to 73 m g and 0.2 cm3 g−1, respectively. This is consistent with the investigation by Zhu et al. [12], which reported that the interaction between MnO2 and CeO2 could restrain the structure growth of mixed oxide and thus increase the specific surface area. The addition of an Fe promoter further increased the specific surface area and pore volume of the FMC catalyst; while the Fe content increased to 3 mol (FMC-3:5:5), the SBET value of the sample decreased. This indicates that while the introduction of Fe enhanced the dispersion of the FMC catalyst, excessive Fe may lead to the agglomeration of particles and decrease the specific surface area. Meanwhile, the average pore diameter also increased in comparison with that of MnO2-CeO2, which may be due to the formation of developed pores promoted by Fe incorporation into MnO2-CeO2. This is confirmed by the pore size distribution curve shown in Figure1C. However, when the Fe content increased to 3 mol (FMC-3:5:5), the SBET value decreased, indicating that excessive Fe may cause particle agglomeration and decrease the specific surface area. Generally, higher values of SBET and pore volume are beneficial to the adsorption of HCHO and could provide more reactive sites, thus improving the catalytic performance [19]. Catalysts 2021, 11, 555 3 of 14 Catalysts 2021, 11, x FOR PEER REVIEW 3 of 14

Figure 1. 1. (A(A) N)N2 adsorption2 adsorption isotherms, isotherms, (B) N (2B adsorption)N2 adsorption isotherms isotherms of CeO2 ofand CeO (C)2 poreand size (C) dis- pore size tributiondistribution of (a of) CeO (a)2 CeO, (b) 2Fe,(3bO)4, Fe(c)3 OMnO4,(c2,) ( MnOd) MnO2,(2-CeOd) MnO2, (e2)-CeO FMC-1:5:5,2,(e) FMC-1:5:5,(f) FMC-2:5:5 (f )and FMC-2:5:5 (g) and FMC-3:5:5.(g) FMC-3:5:5.

Table 1. Specific surface area, average pore diameter and pore volume of the catalysts. Table 1. Speciﬁc surface area, average pore diameter and pore volume of the catalysts. SBET Pore Volume Catalyst S Average Pore DiameterAverage (nm) Pore Pore Volume Catalyst (m2·g−1) BET (cm3 g−1) (m2·g−1) Diameter (nm) (cm3 g−1) CeO2 20 9.5 0.1 CeO 20 9.5 0.1 Fe3O4 2 22 25.9 0.2 Fe3O4 22 25.9 0.2 MnO2 22 29.1 0.2 MnO2 22 29.1 0.2 MnO2-CeO2 73 10.6 0.2 MnO2-CeO2 73 10.6 0.2 FMC-1:5:5FMC-1:5:5 87 87 16.6 16.6 0.3 0.3 FMC-2:5:5 99 12.2 0.4 FMC-3:5:5 82 11.6 0.3

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FMC-2:5:5 99 12.2 0.4 FMC-3:5:5 82 11.6 0.3 Catalysts 2021, 11, 555 FMC-2:5:5 99 12.2 0.4 4 of 14 2.1.2.FMC-3:5:5 XRD Patterns 82 11.6 0.3 The crystal phase of FMC ternary mixed oxide catalysts was characterized by the X- 2.1.2. XRD Patterns 2.1.2.ray diffraction XRD Patterns (XRD) technique and the results are shown in Figures 2 and 3. As observed The crystal phase of FMC ternary mixed oxide catalysts was characterized by the X- fromThe Figure crystal 2, phasethe CeO of FMC2 showed ternary sharp mixed and oxide intense catalysts peaks was at characterized 2θ = 28.7°, by33.1°, the X-ray47.6° and ray diffraction (XRD) technique and the results are shown in Figures 2 and 3. As observed diffraction56.4°, which (XRD) are assigned technique to and the thecubic results fluorite are shownstructure in Figuresof CeO22 and[20].3 .The As observedpure Fe2O3 ex- from Figure 2, the CeO2 showed sharp and intense peaks at 2θ = 28.7°,◦ 33.1°,◦ 47.6°◦ and fromhibited Figure the 2typical, the CeO α-Fe2 showed2O3 pattern sharp (Joint and intenseCommittee peaks on at Po 2θwder= 28.7 Diffraction, 33.1 , 47.6 Standardsand 56.4°,◦ which are assigned to the cubic fluorite structure of CeO2 [20]. The pure Fe2O3 ex- 56.4No.33-0664), which [21]. are assigned MnO2 exhibited to the cubic its main ﬂuorite peak structure at 28.6° of and CeO several2 [20]. weak The pure peaks Fe 2atO 337.3°, hibited the typical αα-Fe2O3 pattern (Joint Committee on Powder Diffraction Standards exhibited44.8°and 55.5°, the typical which -Feare2 Oall3 attributedpattern (Joint to α Committee-MnO2 (JCPDS on Powder No.44-0141) Diffraction [22]. From Standards the XRD No.33-0664) [21]. [21]. MnO 2 exhibitedexhibited its its main main peak peak at 28.6° 28.6◦ and several weak peaks at 37.337.3°,◦, spectra of the different2 Fe contents (Figure 3), it can be seen that the MnO2-CeO2 and FMC 44.8°and◦ 55.5°,◦ which are all attributed to α-MnO2 (JCPDS No.44-0141) [22]. From the XRD 44.8 and 55.5 , which are all attributed to α-MnO2 (JCPDS No.44-0141) [22]. From the catalysts still retained the cube structure of CeO2 and displayed four diffraction peaks spectra of the different Fe contents (Figure 3), it can be seen that the MnO2-CeO2 and FMC XRD spectra of the different Fe contents (Figure3), it can be seen that the MnO 2-CeO2 belonging to the plane facet (111), (200), (220)2 and (311), respectively. Compared with catalystsand FMC still catalysts retained still the retained cube thestructure cube structure of CeO ofand CeO displayed2 and displayed four diffraction four diffraction peaks CeO2, the intensity of MnO2-CeO2 decreased steadily, suggesting that the Mn species en- belongingpeaks belonging to the toplane the planefacet (111), facet (111),(200), (200),(220) (220)and (311), and (311),respectively. respectively. Compared Compared with tered the CeO2 lattice to form a Mn-Ce solid solution. The diffraction peak of the FMC withCeO2 CeO, the2 intensity, the intensity of MnO of MnO2-CeO2-CeO2 decreased2 decreased steadily, steadily, suggesting suggesting that thatthe Mn the Mnspecies species en- catalyst became broader and weaker than that of the MnO2-CeO2 catalyst, leaving only the teredentered the the CeO CeO2 lattice2 lattice to toform form a aMn-Ce Mn-Ce solid solid solution. solution. The The diffraction diffraction peak peak of of the FMCFMC peak related to CeO2 to be detected, thus indicating that Fe existed in highly dispersed or catalyst became became broader broader and and weaker weaker than than that that of of the the MnO MnO2-CeO2-CeO2 catalyst,2 catalyst, leaving leaving only only the peaktheamorphous peak related related formto CeO to [23]. CeO2 to 2beTheto detected, be cerium detected, -basedthus thus indicating solid indicating solution that that Fe formed existed Fe existed asin highlythe in highly active dispersed dispersed center or of ox- amorphousorygen amorphous promoted form form the[23]. [23transport The]. The cerium cerium-based of-based reactive solid oxyg solid solutionen solution species, formed formed while as the as the theactive highly active center centerdispersed of ox- of Fe ygenoxygenand Mn promoted promoted mixed oxidesthe the transport transport as an active of of reactive reactive center oxyg had oxygenen stronger species, species, activation while while the the ability highly highly for dispersed HCHO; Fehence, andthe FMCMn mixed catalyst oxides exhibited as an active active better center center catalytic had had strongerperformance stronger activation [24]. ability for HCHO; HCHO; hence, the FMC catalyst exhibited better catalytic performance [[24].24].

Figure 2.2. XRD XRD patterns patterns of of ( a ()a CeO ) CeO2,,( (2b,b ())b Fe Fe) Fe22OO23O3, ,((3c,c) ()MnOc MnO) MnO2.2 . 2.

Figure 3. XRD patterns of (a) MnO2-CeO2,(b) FMC-1:5:5, (c) FMC-2:5:5 and (d) FMC-3:5:5.

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Figure 3. XRD patterns of (a) MnO2-CeO2, (b) FMC-1:5:5, (c) FMC-2:5:5 and (d) FMC-3:5:5.

2.1.3. H2-TPR 2.1.3. H2-TPR The H2-TPR test was used to observe the redox properties of CeO2, Fe3O4, MnO2 and The H2-TPR test was used to observe the redox properties of CeO2, Fe3O4, MnO2 MnO2-CeO2 catalysts and the results are illustrated in Figure 4(A) and (B). According to and MnO2-CeO2 catalysts and the results are illustrated in Figure4A,B. According to the the literature, CeO2 has only one weak reduction peak at around 500◦ °C, which is ascribed literature, CeO2 has only one weak reduction peak at around 500 C, which is ascribed 4+4+ 2 toto the the reduction reduction of of Ce Ce speciesspecies on onthe the surface surface [25]. [25 The]. TheTPR TPRspectrum spectrum of CeO of CeO in this2 in work this iswork consistent is consistent with the with literature, the literature, with the with observation the observation of small of peaks small at peaks 494 °C at 494(see◦ FigureC (see ◦ 4B).Figure The4B). Fe The2O3 exhibited Fe 2O3 exhibited three peaks, three a peaks, reduction a reduction peak centered peak centered at 393 at°C, 393 andC, two and over- two ◦ lappingoverlapping peaks peaks at 577 at and 577 and645 °C, 645 relatingC, relating to the to reductions the reductions of Fe of2O Fe3 to2O Fe3 to3O Fe4, Fe3O34O,4 Fe to3 OFeO,4 to n+ n+ FeOFeO, to FeO Fe, to respectively Fe, respectively [26]. [Mn26]. Mn is easilyis easily reduced reduced from from MnO MnO2 → Mn2 →2OMn3 →2O Mn3 →3OMn4 from3O4 ◦ ◦ 200–500from 200–500 °C. TheC. MnO The2 MnOspectrum2 spectrum displayed displayed an obvious an obvious reduction reduction peak at peak 310 °C at 310with Ca slightwith a shoulder slight shoulder at 336 °C. at 336 The◦ C.shoulder The shoulder peak is peak related is related to the easily to the easilyreduced reduced surface surface man- ganesemanganese oxides oxides and andthe themain main peak peak represents represents the the reduction reduction of of MnO MnO22 toto Mn2O3.. Another Another ◦ peak located at 423 °CC is assignedassigned toto thethe reductionreduction ofof MnMn22OO33 to Mn3O44 [27].[27]. By By contrast, contrast, thethe incorporation incorporation of of Ce Ce resulted resulted in in the the reduction reduction peak peak shifting shifting towards towards low low temperatures. temperatures. The decrease decrease in reduction reduction temperature indica indicatestes that the interactions between manganese andand cerium cerium promoted promoted the reduction of Mn sp speciesecies through the formation of Mn-Ce solid solution,solution, and and thus thus increased increased the mobility of oxygen species [[28].28].

FigureFigure 4. 4. ((AA))H H22-TPR-TPR profiles proﬁles of of (a) (a) CeO CeO2, 2(b), (b) Fe Fe3O34O, (c)4, (c)MnO MnO2 and2 and (d) (d)MnO MnO2-CeO2-CeO2, (B2),( EnlargedB) Enlarged H2- TPR profiles of (a) CeO2. H2-TPR proﬁles of (a) CeO2.

AsAs displayed displayed in in Figure Figure 55,, thethe TPRTPR proﬁlesprofiles ofof thethe FMCFMC samplessamples werewere categorizedcategorized intointo twotwo peaks, peaks, labeled labeled as α and β, respectively. These are similarsimilar toto thethe MnOMnO22-CeO2 peak,peak, althoughalthough the position changed. The The low-te low-temperaturemperature reduction peak (<500 °C)◦C) shifted slightly towards a high temperature, and a new reduction peak appeared in the high- temperature region (>500 ◦C), which is attributed to the reduction of FeO to metallic Fe.

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Catalysts 2021, 11, 555 6 of 14 slightly towards a high temperature, and a new reduction peak appeared in the high- temperature region (>500 °C), which is attributed to the reduction of FeO to metallic Fe. This implies that a strong redox interaction was generated between Mn-Ce and Fe, and This implies that a strong redox interaction was generated between Mn-Ce and Fe, and the oxygen species in the system exhibited strong mobility. From FMC-1:5:5 to FMC-3:5:5, the oxygen species in the system exhibited strong mobility. From FMC-1:5:5 to FMC-3:5:5, the peak value of H2 consumption first decreases and then increases at low temperatures. the peak value of H consumption ﬁrst decreases and then increases at low tempera- At the same time, it 2also shows that the added amount of co-active component Fe does tures. At the same time, it also shows that the added amount of co-active component Fe not necessarily conform to the rule of “more is better”. According to Figure 5 and Table 2, does not necessarily conform to the rule of “more is better”. According to Figure5 and the order of low-temperature reduction capacity is FMC-2:5:5 > FMC-3:5:5 > FMC-1:5:5 > Table2, the order of low-temperature reduction capacity is FMC-2:5:5 > FMC-3:5:5 > FMC- MnO2-CeO2. Such results suggest that the appropriate amount of Fe can improve the re- 1:5:5 > MnO -CeO . Such results suggest that the appropriate amount of Fe can improve the duction behavior2 of2 the FMC catalyst. FMC-2:5:5 shows the first reduction peak at 314 °C, reduction behavior of the FMC catalyst. FMC-2:5:5 shows the ﬁrst reduction peak at 314 ◦C, lower than FMC-1:5:5 (318 °C) and FMC-3:5:5 (327 °C), and the FMC-2:5:5 sample has the lower than FMC-1:5:5 (318 ◦C) and FMC-3:5:5 (327 ◦C), and the FMC-2:5:5 sample has the highest hydrogen consumption. A A similar similar phen phenomenonomenon was was reported reported by by Lu et al. [[16],16], where they discovered that the inclusion of MnOx enhanced the dispersion of Co3O4 and where they discovered that the inclusion of MnOx enhanced the dispersion of Co3O4 and CeO2, causing the reduction temperature to move slightly towards lower temperatures. CeO2, causing the reduction temperature to move slightly towards lower temperatures. In conclusion,In conclusion, both both Brunauer-Emmett-Teller Brunauer-Emmett-Teller (BET) (BET) and and XRD XRD results results proved proved that that the the addition addition of FeOx promoted the dispersion of MnOx and CeO2. of FeOx promoted the dispersion of MnOx and CeO2.

Figure 5. H2--TPR profiles proﬁles of (a)(a) FMC-1:5:5,FMC-1:5:5, (b)(b) FMC-2:5:5FMC-2:5:5 and and (c) (c) FMC-3:5:5. FMC-3:5:5.α αrefers refers to to the the reduction reduc- peaktion peak at 250–350 at 250–350◦C; β °C;refers β refers to the to reduction the reduction peak peak at 350–450 at 350–450◦C. °C.

Table 2.2. HydrogenHydrogen consumptionconsumption ofof FMCFMC samplessamples withwith differentdifferent ratiosratios ofof Fe/Mn/Ce.Fe/Mn/Ce.

H2 Consumption (mmol/g) Samples H2 Consumption (mmol/g) Samples α β α β MnO2-CeO2 0.20 0.63 MnOFMC-1:5:52-CeO2 0.200.38 0.631.28 FMC-1:5:5 0.38 1.28 FMC-2:5:5FMC-2:5:5 0.32 1.65 FMC-3:5:5FMC-3:5:5 0.30 1.39

2.1.4. O2-TPD 2.1.4. O2-TPD The O2-TPD profiles of the FMC catalysts were tested to investigate the mobility of The O2-TPD proﬁles of the FMC catalysts were tested to investigate the mobility of the surfacesurface oxygenoxygen species.species. It has been reportedreported that chemicallychemically adsorbedadsorbed oxygenoxygen speciesspecies (such(such as O O22−− andand O− O) are−) aredesorbed desorbed at temperatures at temperatures below below 500 °C, 500 and◦C, the and desorption the desorption peaks peaksof lattice of oxygen lattice oxygen occur above occur 500 above °C [29]. 500 As◦C[ pr29esented]. As presentedin Figure 6, in due Figure to the6, weak due to signal the weakof the signaloxygen of desorption the oxygen peak desorption in CeO2, peakno oxygen in CeO desorption2, no oxygen peak desorption was characterized peak was in characterizedthe test temperature in the testrange. temperature A small desorption range. A small peak desorption detected for peak Fe3O detected4 around for 200–500 Fe3O4 around 200–500 ◦C indicates the existence of a small amount of chemically adsorbed ◦ oxygen species. There are two obvious desorption peaks of MnO2 at 268 and 626 C,

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Catalysts 2021, 11, 555 7 of 14 °C indicates the existence of a small amount of chemically adsorbed oxygen species. There are two obvious desorption peaks of MnO2 at 268 and 626 °C, which are assigned to sur- whichface oxygen are assigned and lattice to surface oxygen oxygen species, and respectively. lattice oxygen After species, the fusion respectively. of MnO2-CeO After2, thethe ◦ fusionintensity of MnOof the2 -CeOdesorption2, the intensitypeak at around of the desorption300 °C decreased, peak at meaning around that 300 theC decreased,concentra- meaningtion of surface-active that the concentration oxygen decreased; of surface-active the intensity oxygen of the decreased; desorption the intensitypeak at 500–700 of the desorption°C increased peak significantly, at 500–700 ◦meaningC increased that signiﬁcantly, the concentration meaning of thatlattice the oxygen concentration increased, of latticewhich oxygenmay be increased, the consequence which mayof MnO be thex transition consequence through of MnO the xlosstransition of lattice through oxygen the at losshigh of temperatures lattice oxygen [30]. at highThe desorption temperatures temper [30].ature The desorptionof the FMC temperature catalyst series of moved the FMC to ◦ catalyst200–500 series °C with moved the introduction to 200–500 C of with FeO thex to introductionMnO2-CeO2. ofThe FeO Ox2 desorptionto MnO2-CeO peak2. The shifted O2 desorptionto low temperatures peak shifted and to the low signals temperatures became st andronger, the signalsindicating became that Fe stronger, enhanced indicating the mo- thatbility Fe of enhanced the surface the oxygen mobility species of the and surface effectively oxygen activated species andOlatt effectivelyto Oads. This activated phenomenon Olatt tois Oconsistentads. This phenomenonwith the X-ray is consistentphotoelectron with spectroscopy the X-ray photoelectron (XPS) results. spectroscopy Compared (XPS) with results.FMC-1:5:5 Compared and FMC-3:5:5, with FMC-1:5:5 it is obvious and that FMC-3:5:5, FMC-2:5:5 it is exhibited obvious that the largest FMC-2:5:5 O2 desorption exhibited thepeak largest area in O 2thedesorption low-temperature peak area region, in the indi low-temperaturecating that the region,sample indicatinghad the largest that thede- samplesorbed hadoxygen the content. largest desorbed According oxygen to Table content. 3, the Accordingdesorption toO2 Table content3, the of FMC-2:5:5 desorption (1.89 O 2 contentmmol/g) of is FMC-2:5:5 higher than (1.89 that mmol/g) of FMC-1:5:5 is higher (1.68 than mmol/g) that of FMC-1:5:5and FMC-3:5:5 (1.68 (1.71 mmol/g) mmol/g). and FMC-3:5:5FMC-2:5:5 (1.71had abundant mmol/g). surface-active FMC-2:5:5 had oxygen abundant species. surface-active It is general oxygen knowledge species. that Itsur- is generalface oxygen knowledge exhibits that higher surface mobility oxygen than exhibits lattice higher oxygen, mobility and this than can lattice facilitate oxygen, the cata- and thislytic can oxidation facilitate reaction. the catalytic oxidation reaction.

Figure 6. O -TPD proﬁles of (a) CeO , (b) Fe O , (c) MnO , (d) MnO -CeO , (e) FMC-1:5:5, (f) FMC- Figure 6. O22-TPD profiles of (a) CeO22, (b) Fe33O4, (c) MnO2, (d) MnO22-CeO22, (e) FMC-1:5:5, (f) FMC- 2:5:52:5:5 andand (g)(g) FMC-3:5:5.FMC-3:5:5.

TableTable 3. 3.Oxygen Oxygen desorption desorption of of FMC FMC samples samples with with different different ratios ratios of of Fe/Mn/Ce. Fe/Mn/Ce.

Samples Desorbed Desorbed OO22(mmol/g) (mmol/g) 2 2 MnOMnO2-CeO-CeO2 1.451.45 FMC-1:5:5 1.681.68 FMC-2:5:5 1.891.89 FMC-3:5:5 1.71 FMC-3:5:5 1.71

2.1.5.2.1.5. XPSXPS ToTo furtherfurther evaluate evaluate the the surface surface elemental elemental composition composition and and chemical chemical states states of the of FMC the catalysts,FMC catalysts, XPS measurements XPS measurements were performedwere perfor andmed the and results the results are shown are shown in Figure in Figure7 and 7 Tableand Table4. Two 4. obviousTwo obvious peaks peaks located located at 651.8 at and651.8 640.3 and eV640.3 are eV observed are observed in Figure in Figure7A, and 7A, theseand these peaks peaks were were assigned assigned to the to signal the signal of Mn of 2p1/2 Mn 2p1/2 and Mn and 2p3/2, Mn 2p3/2, respectively. respectively. The spin The orbital splitting energy of Mn 2p was 11.5 eV, which is close to that of MnO2 (11.7 eV) [31]. The Mn 2p3/2 peak for MnO2 and the FMC ternary mixed oxides were deconvoluted into

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spin orbital splitting energy of Mn 2p was 11.5 eV, which is close to that of MnO2 (11.7 eV) [31]. The Mn 2p3/2 peak for MnO2 and the FMC ternary mixed oxides were deconvoluted into three peaks at binding energies of 640.5 eV, 641.2 eV and 642.3 eV, corresponding to the Mn2+, Mn3+ and Mn4+ species, respectively [32]. However, the absence of a satellite peak at +5 eV from the Mn 2p3/2 peak indicates that Mn2+ was not present while Mn3+ and Mn4+ co-existed [33]. In addition, Table 4 shows that the concentration of Mn4+ (54.81%) in the MnO2-CeO2 samples was lower than those of the FMC samples (56.28–57.63%). These results indicate that the structural distribution of Mn and Ce was more homogeneous with the addition of Fe. From Table 4, the order of surface Mn4+ concentration is as follows: MnO2-CeO2 < FMC-3:5:5 < FMC-1:5:5 < FMC-2:5:5, suggesting that the FMC-2:5:5 catalyst possessed the largest Mn4+ concentration. The high-valence state of Mn species could enhance the redox properties of the catalyst, and the high content of Mn4+ will be favorable to the oxidation reaction [34]. Figure 7B shows that the O 1s spectra in the binding energy range of 525.8–534.0 eV were broad with shoulder peaks. This may have resulted from the overlapping signals of different oxygen species. The peak corresponding to the higher Binding Energy (BE) (531.4 eV) is associated with the surface-absorbed oxygen, such as, O22−,O2− and O−, and it is denoted as Oα, while the peak corresponding to the lower BE (527.5 eV) is related to surface lattice oxygen, and denoted as Oβ [35]. It is known that Oα ions are electrophilic, and a large amount of Oα is beneficial to the decomposition of intermediates [33]. The FMC catalysts have a higher Oα/(Oα + Oβ) ratio than MnO2-CeO2, implying that the promotional effect of Fe between Mn and Ce enhanced the concentration and mobility of adsorbed oxygen on the surface. In addition, the relative concentration of Oα species in- Catalysts 11 2021, , 555 creased with increasing amount of Fe, with the highest Oα/(Oα + Oβ) value of 66.76% 8 of 14 achieved for FMC-2:5:5, indicating that the sample had the most abundant surface-adsorbed oxygen species. Ce is a rare earth metal element located in the lanthanide series. During the X-ray three peaks at binding energies of 640.5 eV, 641.2 eV and 642.3 eV, corresponding to the excitation process, multiple electrons are excited at the same time, thus complicating the Mn2+, Mn3+ and Mn4+ species, respectively [32]. However, the absence of a satellite peak peak shape. In the Ce 3d spectra of CeO2 (Figure 7C), four pairs of spin-orbit coupling at +5 eV from the Mn 2p3/2 peak indicates that Mn2+ was not present while Mn3+ and were assigned to two orbits, Ce 3d3/2 and Ce 3d5/2, which were divided into eight peaks 4+ 4+ and labeledMn co-existed as u and v, [33 respecti]. In addition,vely. The Table peak4 ushows (880.6 thateV), theu″ (887.0 concentration eV), u‴ (896.3 of Mn eV), (54.81%)v in (899.1the eV), MnO v″ 2(905.7-CeO 2eV)samples and v‴ was (914.7 lower eV) thanare characteristic those of the FMCof Ce4+ samples, while u’ (56.28–57.63%). (883.3 eV), These and resultsv′ (901.7 indicate eV) are that assigned the structural to Ce3+ [36]. distribution Ce3+ is closely of Mn related and Ce to was oxygen more vacancies; homogeneous with 4+ hence,the when addition Ce3+ appears of Fe. Fromon the Table surface4, theof CeO order2, oxygen of surface vacancies Mn areconcentration usually formed is in as follows: orderMnO to observe2-CeO 2the< FMC-3:5:5charge conservation < FMC-1:5:5 [37].The < FMC-2:5:5, calculated suggesting Ce3+/(Ce3+ + that Ce4+ the) ratios FMC-2:5:5 of the catalyst samplespossessed are presented the largest in Table Mn 4+4, andconcentration. the table shows The that high-valence the value for state FMC-2:5:5 of Mn specieswas could 25.79%,enhance which the is redoxhigher propertiesthan the recorded of the catalyst, values andfor the the FMC-1:5:5 high content (23.29%) of Mn and4+ will FMC- be favorable 3:5:5 to(21.26%) the oxidation catalysts. reaction The formation [34]. of oxygen vacancies and defects helps to improve the catalytic performance of HCHO oxidation [38].

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Figure 7. XPS spectra of (a) MnO2-CeO2, (b) FMC-1:5:5, (c) FMC-2:5:5 and (d) FMC-3:5:5. (A) Mn Figure 7. XPS spectra of (a) MnO2-CeO2, (b) FMC-1:5:5, (c) FMC-2:5:5 and (d) FMC-3:5:5. (A) Mn 2p, (B) O 1s, (C) Ce 3d. Oα represent the surface-absorbed oxygen, Oβ represent the surface lattice 2p, (B) O 1s, (C) Ce 3d. Oα represent the surface-absorbed oxygen, Oβ represent the surface lattice oxygen. The peak u (880.6 eV), u’ (883.30 eV), u″ (887.0 eV),00 u‴ (896.3 eV),000 v (899.1 eV), v′ (901.7 eV), 0 v″ (905.7oxygen. eV) and The v peak‴ (914.7 u (880.6eV). eV), u (883.3 eV), u (887.0 eV), u (896.3 eV), v (899.1 eV), v (901.7 eV), v00 (905.7 eV) and v000 (914.7 eV). Table 4. Surface elemental composition of FMC catalysts.

Sample Mn4+/(Mn3+ + Mn4+) Oα/(Oα + Oβ) Ce3+/(Ce3+ + Ce4+) MnO2-CeO2 37.81% 58.77% 20.78% FMC-1:5:5 43.51% 59.42% 23.29% FMC-2:5:5 47.63% 66.76% 25.79% FMC-3:5:5 46.28% 62.83% 21.26%

2.2. Catalytic Activity

Figure 8 reveals the catalytic activity of pure FeOx, CeO2, MnO2 and the FMC catalysts for HCHO oxidation at different temperatures (20–120 °C), and it can be seen that pure FeOx, CeO2 and MnO2 exhibited low catalytic activity even at temperatures as high as 120 °C, with MnO2 achieving the highest HCHO conversion rate of 80% of this catalyst category. After combining MnO2 and CeO2, the catalytic activity of MnO2-CeO2 obviously improved, reaching a conversion rate of 93% at 100 °C. Furthermore, the introduction of FeOx into the MnO2-CeO2 resulted in the enhanced catalytic performance of the FMC catalysts. Notably, the catalytic activity of the FMC catalyst initially increased with increasing iron content, and then decreased. With low Fe content addition, the small amount of surface compounds will contribute little to the catalytic activity. As the Fe content gradually increases, the amount of surface compounds also increases, which is beneficial to the oxidation reaction of formaldehyde, thereby enhancing the catalytic activity. An optimum catalytic performance is achieved when the amount of surface compounds reaches the

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Table 4. Surface elemental composition of FMC catalysts.

4+ 3+ 4+ 3+ 3+ 4+ Sample Mn /(Mn + Mn ) Oα/(Oα + Oβ) Ce /(Ce + Ce )

MnO2-CeO2 37.81% 58.77% 20.78% FMC-1:5:5 43.51% 59.42% 23.29% FMC-2:5:5 47.63% 66.76% 25.79% FMC-3:5:5 46.28% 62.83% 21.26%

Figure7B shows that the O 1s spectra in the binding energy range of 525.8–534.0 eV were broad with shoulder peaks. This may have resulted from the overlapping signals of different oxygen species. The peak corresponding to the higher Binding Energy (BE) 2− − − (531.4 eV) is associated with the surface-absorbed oxygen, such as, O2 ,O2 and O , and it is denoted as Oα, while the peak corresponding to the lower BE (527.5 eV) is related to surface lattice oxygen, and denoted as Oβ [35]. It is known that Oα ions are electrophilic, and a large amount of Oα is beneﬁcial to the decomposition of intermediates [33]. The FMC catalysts have a higher Oα/(Oα + Oβ) ratio than MnO2-CeO2, implying that the promotional effect of Fe between Mn and Ce enhanced the concentration and mobility of adsorbed oxygen on the surface. In addition, the relative concentration of Oα species increased with increasing amount of Fe, with the highest Oα/(Oα + Oβ) value of 66.76% achieved for FMC-2:5:5, indicating that the sample had the most abundant surface-adsorbed oxygen species. Ce is a rare earth metal element located in the lanthanide series. During the X-ray excitation process, multiple electrons are excited at the same time, thus complicating the peak shape. In the Ce 3d spectra of CeO2 (Figure7C), four pairs of spin-orbit coupling were assigned to two orbits, Ce 3d3/2 and Ce 3d5/2, which were divided into eight peaks and labeled as u and v, respectively. The peak u (880.6 eV), u00 (887.0 eV), u000 (896.3 eV), v (899.1 eV), v00 (905.7 eV) and v000 (914.7 eV) are characteristic of Ce4+, while u0 (883.3 eV), and v0 (901.7 eV) are assigned to Ce3+ [36]. Ce3+ is closely related to oxygen vacancies; 3+ hence, when Ce appears on the surface of CeO2, oxygen vacancies are usually formed in order to observe the charge conservation [37].The calculated Ce3+/(Ce3+ + Ce4+) ratios of the samples are presented in Table4, and the table shows that the value for FMC-2:5:5 was 25.79%, which is higher than the recorded values for the FMC-1:5:5 (23.29%) and FMC-3:5:5 (21.26%) catalysts. The formation of oxygen vacancies and defects helps to improve the catalytic performance of HCHO oxidation [38].

2.2. Catalytic Activity

Figure8 reveals the catalytic activity of pure FeO x, CeO2, MnO2 and the FMC catalysts ◦ for HCHO oxidation at different temperatures (20–120 C), and it can be seen that pure FeOx, ◦ CeO2 and MnO2 exhibited low catalytic activity even at temperatures as high as 120 C, with MnO2 achieving the highest HCHO conversion rate of 80% of this catalyst category. After combining MnO2 and CeO2, the catalytic activity of MnO2-CeO2 obviously improved, ◦ reaching a conversion rate of 93% at 100 C. Furthermore, the introduction of FeOx into the MnO2-CeO2 resulted in the enhanced catalytic performance of the FMC catalysts. Notably, the catalytic activity of the FMC catalyst initially increased with increasing iron content, and then decreased. With low Fe content addition, the small amount of surface compounds will contribute little to the catalytic activity. As the Fe content gradually increases, the amount of surface compounds also increases, which is beneﬁcial to the oxidation reaction of formaldehyde, thereby enhancing the catalytic activity. An optimum catalytic performance is achieved when the amount of surface compounds reaches the maximum for a certain Fe content. When this value is exceeded, the Fe particles will agglomerate and cover part of the active center, which will weaken the interactions between the Fe particles and Mn-Ce, thus discouraging the enhancement of the catalyst’s performance, therefore exhibiting no promotional effect on the catalytic activity. Hence, it can be inferred that the order of catalytic activity of these seven catalysts is FeOx < CeO2 < MnO2 < MnO2-CeO2 < FMC- 3:5:5 < FMC-1:5:5 < FMC-2:5:5. FMC-2:5:5 exhibited the highest catalytic activity, achieving Catalysts 2021, 11, x FOR PEER REVIEW 10 of 14

Catalysts 2021, 11, x FOR PEER REVIEWmaximum for a certain Fe content. When this value is exceeded, the Fe particles will10 ofag- 14

glomerate and cover part of the active center, which will weaken the interactions between the Fe particles and Mn-Ce, thus discouraging the enhancement of the catalyst’s performance,maximum therefore for a certain exhibiting Fe content. no promotional When this effe valuect on is the exceeded, catalytic the activity. Fe particles Hence, will it canag- beglomerate inferred and that cover the orderpart of of the catalytic active center,activity which of these will seven weaken catalysts the interactions is FeOx < between CeO2 < MnOthe Fe2

FigureFigure 8. The 8. HCHOThe HCHO conversion conversion over over prepared prepared catalysts: catalysts:( (□)) FeOFeOx,(x, (○)) CeO CeO2,(2,4 (△) MnO) MnO2,(2H, ()▼ MnO) MnO2- 2-

CeO2,CeO (◆)2 FMC-3:5:5,,(N) FMC-3:5:5, (►) ( FMC-1:5:5,M ) FMC-1:5:5, (★ (F) )FMC-2:5:5. FMC-2:5:5. Reaction conditions: conditions:# 200 200 ppm ppm HCHO, HCHO, 21 vol% 21 O 2 −1 −1 −1 −1 −1 vol% andO2 and N2 (balance). N2 (balance). The total The flow total rate flow and WHSV rate and were WHSV 30 mL min were and 30 mL 36,000 min mL gandcat 36,000h , respectively. mL gcat h−1, respectively. ◦ Figure 8. TheThe HCHO stability conversion test results over of theprepared FMC-2:5:5 catalysts: catalyst (□) FeO for HCHOx, (○) CeO oxidation2, (△) MnO at 402, (▼C) areMnO2- CeO2,shown (◆) FMC-3:5:5, in Figure 9(►. The) FMC-1:5:5, reaction conditions(★) FMC-2:5:5. were Reaction 200 ppm conditions: HCHO, 21 vol%200 ppm O 2 HCHO,and N2 (bal-21 vol% ance).O2 and It N can2 (balance). be observed The thattotal with flow the rate extension and WHSV of time, were the 30 FMC-2:5:5mL min−1 and catalyst 36,000 performed mL gcat−1 h−1, respectively.with good stability and still maintained complete conversion of HCHO over 68 h.

Figure 9. Stability test of the FMC-2:5:5 catalyst at 40 °C.

◦ FigureFigure 9. Stability 9. Stability test testof the of theFMC-2:5:5 FMC-2:5:5 catalyst catalyst at at 40 40 °C.C.

For the practical application of the HCHO oxidation catalyst, the effect of H2O on the catalytic performance of FMC-2:5:5 was measured under the feed gas adding 5% water. As shown in Figure 10, FMC-2:5:5 was investigated under dry and humid conditions at 230 ◦C ◦ for 24 h. Introducing 5% H2O to the gas stream at 230 C, the formaldehyde conversion increased from 87% to 91% and remained active for 15 h. As the H2O was stopped, the activity was immediately reduced to the original level. This indicates that the existence of water vapor is beneﬁcial to the catalytic oxidation of HCHO. CatalystsCatalysts2021 2021,,11 11,, 555x FOR PEER REVIEW 1111 ofof 1414

Figure 10.10. Effect ofof waterwater vaporvapor on on catalytic catalytic performance performance of of FMC-2:5:5 FMC-2:5:5 at at 30 30◦C °C with with 200 200 ppm ppm HCHO, −1 −1 −1 −1 21HCHO, vol% O212 vol%and N O22 (balance),and N2 (balance), WHSV =WHSV 36,000 = mL 36,000 gcat mLh gcat. h .

3. Materials and Methods 3.1. Catalyst Preparation 3.1. Catalyst Preparation The FeOx-MnO2-CeO2 ternary mixed oxide catalysts were synthesized by a surfactant- The FeOx-MnO2-CeO2 ternary mixed oxide catalysts were synthesized by a surfac- templated coprecipitation method [33]. The different molar ratios of Fe/Mn/Ce were 1:5:5 tant-templated coprecipitation method [33]. The different molar ratios of Fe/Mn/Ce were (referred to as FMC-1:5:5, where the numbers represent the contents of Fe, Mn and Ce), 1:5:5 (referred to as FMC-1:5:5, where the numbers represent the contents of Fe, Mn and 2:5:5 (FMC-2:5:5) and 3:5:5 (FMC-3:5:5). In a typical synthesis such as FMC-2, 2 g CTAB was Ce), 2:5:5 (FMC-2:5:5) and 3:5:5 (FMC-3:5:5). In a typical synthesis such as FMC-2, 2 g first dissolved in 100 mL deionized water to obtain a transparent and clear solution. Then, CTAB was first dissolved in 100 mL deionized water to obtain a transparent and clear Mn(CH3COO)2·4H2O (5 mmol)(Fuchen, Tianjin, China), Ce(NO3)3·6H2O (5 mmol)(Damao, solution. Then, Mn(CH3COO)2·4H2O (5 mmol)(Fuchen, Tianjin, China), Ce(NO3)3·6H2O (5 Tianjin, China) and Fe(NO3)3·9H2O (1 mmol) (Tianli, Tianjin, China) were added to the abovemmol)(Damao, solution inTianjin, succession China) and and the Fe(NO mixture3)3·9H was2O (1 stirred mmol) at (Tianli, 30 ◦C forTianjin, 0.5 h. China) A suitable were added to the above solution in succession and the mixture was stirred at 30 °C for 0.5 h. amount of NaHCO3 solution was added to the mixed solution under vigorous stirring. The obtainedA suitable precipitate amount of was NaHCO aged by3 solution steaming was at 100added◦C to for the 2 h, mixed filtered solution and washed under to vigorous neutral withstirring. deionized The obtained water andprecipitate ethanol. was The aged resultant by steaming solid material at 100 was°C for dried 2 h,overnight filtered and at 80washed◦C to removeto neutral excess with solution, deionized and water then calcinedand ethanol. at 500 The◦C forresultant 3 h under solid air material atmosphere. was Finally,dried overnight the obtained at 80 black °C to powder remove was excess crushed solution, to 40–60 and meshthen calcined particles. at Other 500 °C catalysts for 3 h under air atmosphere. Finally, the obtained black powder was crushed to 40–60 mesh par- with different elemental molar ratios and pure MnO2, CeO2, Fe3O4, MnO2-CeO2 were preparedticles. Other with catalysts the same with procedure. different elemental molar ratios and pure MnO2, CeO2, Fe3O4, MnO2-CeO2 were prepared with the same procedure. 3.2. Characterization 3.2. CharacterizationThe specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method.The specific The BET surface measurements area was werecalculated performed using onthe a Brunauer-Emmett-Teller micromeritics apparatus (BET) (Mi- cromeritics,method. The USA) BET atmeasurements−196 ◦C. Prior were to the perf analysis,ormed theon samplesa micromeritics were degassed apparatus for 4 (Mi- h at ◦ 250cromeritics,C under USA) N2 atmosphere, at −196 °C. andPrior the to porethe analysis, size distribution the samples was determinedwere degassed by thefor Barret4 h at Joyner250 °C under Halenda N2 (BJH)atmosphere, method and using the pore the desorption size distribution branch was of thedetermined nitrogen by adsorption- the Barret desorptionJoyner Halenda data. (BJH) method using the desorption branch of the nitrogen adsorption- desorptionX-ray diffraction data. (XRD) was tested on an X-ray polycrystalline diffractometer (Bruker, Germany)X-ray fitteddiffraction with Cu-Ka(XRD) radiationwas tested source on an (X-rayλ = 1.5406 polycrystalline nm, 40 kV diffractometer and 30 mA). The (Bruker, 2θ of ◦ ◦ ◦ −1 theGermany) angle XRD fitted was with in theCu-Ka range radiation of 10 to source 80 at ( aλ step= 1.5406 rate nm, of 2 40·min kV and. 30 mA). The 2θ of theHydrogen angle XRD temperature-programmed was in the range of 10° to reduction80° at a step (H rate2-TPR) of 2°·min was measured−1. using gas chromatographyHydrogen temperature-programmed (Jiedao GC1690, Hangzhou, reduction China) instrument(H2-TPR) was with measured the reducing using gas gas of −1 ◦ 10chromatography vol% H2/N2 (60 (Jiedao mL·min GC1690,). The Hangzhou, heating temperatureChina) instrument ranged with from the 25 reducing to 700 Cgas at of a heating rate of 10 ◦C·min−1. The amount of H consumption was analyzed by a thermal 10 vol% H2/N2 (60 mL·min−1). The heating temperature2 ranged from 25 to 700 °C at a heat- conductivity detector (TCD) (Jiedao, Hangzhou, China). ing rate of 10 °C·min−1. The amount of H2 consumption was analyzed by a thermal con- ductivityOxygen detector temperature-programmed (TCD) (Jiedao, Hangzhou, desorption China). (O2-TPD) was performed using the same apparatus as H -TPR. A 50 mg sample was first purified with He at 200 ◦C for 1 h to Oxygen temperature-programmed2 desorption (O2-TPD) was performed using the remove surface impurities, followed by cooling to room temperature. Then, the catalyst same apparatus as H2-TPR. A 50 mg sample was first purified with He at 200 °C for 1 h to was made to absorb O (21 vol% O /N ) at room temperature. Thereafter, the sample remove surface impurities,2 followed2 by 2cooling to room temperature. Then, the catalyst

Catalysts 2021, 11, 555 12 of 14

was blown with 40 mL/min He for 30 min to remove the physisorbed O2. Finally, the temperature increased from 25 to 700 ◦C at a ramp of 10 ◦C·min−1 in He stream. X-ray photoelectron spectroscopy (XPS) measurements were conducted on an Es- calab250xi spectrometer (Thermo, USA). Al Ka X-ray line (1653.6 eV) was used for the excitation, and all binding energies were calibrated with the C 1s line (284.6 eV).

3.3. Catalytic Activity Test HCHO oxidation activity test was conducted between 20 and 120 ◦C on a quartz fixed-bed microreactor (Chunlong, Xi’an, China). First, 50 mg of catalyst (40–60 mesh) was packed in a quartz tube (Chunlong, Xi’an, China) with external and internal diam- eters of 6 and 3 mm, respectively. Gaseous HCHO was generated by passing N2 over ◦ paraformaldehyde in a water bath set at 30 C, and then mixed with 21 vol% O2/N2 gas flow to obtain the feed gas of 200 ppm HCHO, 21 vol% O2/N2 (balance). The concentration of formaldehyde was controlled by the weight of paraformaldehyde, the temperature of the constant temperature water bath and the flow rate of the carrier gas. The gas flow rate was controlled by a mass flow meter, and the temperature of the catalyst bed was controlled by tubular resistance furnace and K-type thermocouple. The total flow rate was 30 mL·min−1, −1 −1 corresponding to a gas hourly space velocity (GHSV) of 36,000 mL gcat h . The outlet gas of the reactor was analyzed online by gas chromatograph (GC) equipped with thermal conductivity detector (TCD) and hydrogen flame ionization detector (FID) ((Jiedao, Hangzhou, China)). A catalyst convertor was installed in front of the FID detector, whose function was to quantitatively convert COx into methane in an H2 atmosphere. No other carbon-containing compounds except CO2 in the products were detected for the tested catalysts. Thus, HCHO conversion is equal to the yield of CO2 and calculated as follows:

[CO ] HCHO conversion (%) = 2 out × 100% [HCHO]in

where [CO2]out is the CO2 concentration in the products, and [HCHO]in is the concentration of HCHO in the gas ﬂow.

4. Conclusions

In conclusion, we have successfully developed FeOx-MnO2-CeO2 catalysts with different Fe/Mn/Ce molar ratios, synthesized using the surfactant-templated coprecipitation method and applied for HCHO removal at low temperatures. FeOx-MnO2-CeO2 exhibited excellent catalytic activity at a 2/5/5 molar ratio of Fe/Mn/Ce, and complete conversion of HCHO at a temperature as low as 40 ◦C. Through a series of characterization analyses, it was proven that Fe ions were successfully introduced into MnO2-CeO2. XRD analysis revealed that the introduction of Fe ions promoted the dispersion of MnO2-CeO2, and FeOx-MnO2-CeO2 existed in highly dispersed or amorphous form. Comparatively, BET results show that the FeOx-MnO2-CeO2 catalyst had a larger speciﬁc surface area, and the pores were more developed than those of the MnO2-CeO2 catalyst. The results of H2-TPR and O2-TPD indicate that the strong interactions between iron, manganese and cerium improved the reducibility of the catalyst, and, at the same time, the mobility of oxygen species was enhanced. From the XPS results, it was shown that a large amount of Oα is beneﬁcial to the intermediate decomposition, and the existence of Ce3+ is favorable for the formation of oxygen vacancies on the catalyst surface. All of these play an important role in improving the oxidation activity of HCHO. Therefore, this work demonstrated that the introduction of Fe promoters into binary mixed oxides is an effective strategy for improving the HCHO catalytic performance.

Author Contributions: Methodology, data curation, and writing—original draft. Y.D.; conceptualization, formal analysis. C.S.; investigation, formal analysis. Y.D., K.L. and H.W.; software. Z.Z. and W.Z.; conceptualization, supervision, writing—review and editing. S.L. All authors have read and agreed to the published version of the manuscript. Catalysts 2021, 11, 555 13 of 14

Funding: This research was funded by the Young Talent fund of the University Association for Science and Technology in Shanxi (No. 20180604), the postgraduate Innovative Entrepreneurial Training Program of Xi’an Shiyou University (No. YCS18211014 and YCS19211024), and the college student Innovative Entrepreneurial Training Program of Nation (No. 201910705024). Meanwhile, the modern analysis and testing center of Xi’an Shiyou University provided strong support. Data Availability Statement: Data is contained within the article. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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