Research ArticleAcid-Alkali-Treated Natural Mordenite-Supported PlatinumNanoparticles (Pt/MORn-H-OH) for Efficient CatalyticOxidation of Formaldehyde at Room Temperature

Xiaoya Gao,1 Qian Guo,1 Yuan Zhou,1 Dedong He ,2 and Yongming Luo 1

1Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China2Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, China

Correspondence should be addressed to Dedong He; dedong.he@qq.com and Yongming Luo; environcatalysis222@yahoo.com

Received 19 December 2018; Revised 12 February 2019; Accepted 21 February 2019; Published 3 April 2019

Academic Editor: Tomislav Bolanca

Copyright © 2019 Xiaoya Gao et al. ,is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

In this work, a series of natural mordenite-supported platinum (Pt) catalysts were prepared by a facile two-step method, namely,treatment of natural mordenite and then the loading of Pt nanoparticles. ,e acid-alkali-treated natural mordenite-supported Ptsamples (1% Pt/MORn-H-OH) exhibited the highly enhanced catalytic oxidation activity of formaldehyde (HCHO) at roomtemperature. XRD results showed that the crystalline phase of the mordenite did not change significantly in 1% Pt/MORn-H-OHcatalyst. However, the acid-alkali treatment endowed the Pt particles excellent dispersion with the smallest diameter of 2.8 nm in ahigh loading content, which contributed to the optimal catalytic activity of 1% Pt/MORn-H-OH.

1. Introduction

Formaldehyde (HCHO) pollution has recently attractedpublic concern because of its carcinogenicity and terato-genicity [1, 2]. Among all the investigated processes, catalyticoxidation is regarded as one of themost efficient methods forHCHO removal [3, 4]. So far, supported noble metal cat-alysts show superior HCHO catalytic activity at roomtemperature [5–7]. Choosing suitable supports is importantfor the catalytic oxidation process of HCHO. Zeolites areexcellent supports in catalytic oxidation of HCHO [8–10].For example, Park and coworkers found that HCHO cat-alytic activity for the noble metal catalysts supported in betazeolite is much higher than that in TiO2-H (Hombikat),TiO2-P (P25), and Mn-CeO2 [9]. In general, zeolites can beobtained from freshly prepared sodium aluminosilicate gelby hydrothermal treatment [11]. However, the increasingconsumption of zeolites calls has called cheaper raw ma-terials to synthesize them. From this point of view, naturalmordenite is a valuable natural resource for the synthesis ofzeolites. As a matter of fact, the cost of extracting “industrialmaterial” from natural mordenite is only 1–5% of synthetic

zeolite. Hence, natural mordenite is promising in thepractical application [12, 13].

However, natural mordenite also needs a certain acidand alkali treatment to regulate the silicon-aluminum ratioand specific surface area [14, 15]. Generally, acid-treatments can remove impurities such as SiO2, Al2O3,and Fe2O3 in the surface and pores of mordenite. At thesame time, the acid solution can dissolve the amorphoussubstances in the pores and change the large cations withsmall proton by ion exchange, which increases the poresize, specific surface area, and adsorption capacity. Inaddition, acid treatment changes the surface properties ofthe zeolite molecular sieve [16], thus optimizing theproperties of the mordenite material. Meanwhile, alkalinetreatment is also a general method to generate partialdissolution of the mordenite structure due to silicon ex-traction [17]. ,erefore, this study mainly focused on thecatalytic oxidation of HCHO and researched the catalyticperformance of the acid-alkali-pretreated naturalmordenite-supported Pt-based catalysts. ,e purpose is toexplore the possibility of using natural mordenite as a Ptcatalyst support to remove HCHO at room temperature.

HindawiJournal of ChemistryVolume 2019, Article ID 4582137, 5 pageshttps://doi.org/10.1155/2019/4582137

2. Experimental

2.1. Catalyst Preparation. Firstly, the purchased naturalmordenite particles (Si/Al� 6.6, according to the XRF data;from Yanshan, Yunnan) were milled in a planetary ball millto ensure the particle size at about 1–2 μm, and the acquiredpowder was named as MORn. ,en, the MORn underwentacid treatment or alkali treatment. In the typical acid and/oralkali treatment process, 10 g of MORn was, respectively,placed into 100mL HNO3 (0.1M) or 30mL NaOH (0.2M)solution, operated with reflux, and stirred at 125°C for 10 hor 65°C for 30min, respectively. After washing and filtrationtreatment, the obtained samples were dried at 80°C over-night and named as MORn-H (acid-treated natural mor-denite zeolite), MORn-OH (alkali-treated natural mordenitezeolite), andMORn-H-OH (sequentially acid-treated, alkali-treated natural mordenite zeolite), respectively.

Pt/MORn catalysts were synthesized by the impregna-tion method. First, 2 g of MORn was added into an aqueoussolution of H2PtCl6·6H2O to ensure the loading amount ofPt was 1 wt.% and stirred at room temperature for 2 h. ,en,the solution was placed in a rotary evaporator and evapo-rated under vacuum at 65°C. ,e obtained solid was dried at110°C overnight and calcined in air at 300°C for 2 h. Finally,it was reduced in H2 at 300°C for 2 h with the heating rate of5°C/min. ,e reduced gray solid was denoted as Pt/MORn,Pt/MORn-H, Pt/MORn-OH, or Pt/MORn-H-OH depend-ing on the pretreatment conditions of natural mordenite.

2.2. Catalyst Characterization. ,e X-ray diffraction (XRD)analyses were obtained using a Ultima IV-type powder X-raydiffractometer with Cu Ka radiation (operated at 30mA and40 kV), and the measurements were performed in the rangeof 5–65°. ,e transmission electron microscope (TEM)images of the catalysts were characterized on a HitachiHT7700 electron microscopy (from JEOL Corporation). ,eX-ray photoelectron spectroscopy (XPS) results were carriedout on a Escalab 250Xi X-ray photoelectron spectrometer, bycalculating the C 1s peak to the binding energy of 284.6 eV.

2.3. Catalytic Activity Tests. Catalyst activity tests for HCHOoxidation were measured in a continuous flow fixed bedreactor system with 0.1 g of catalyst. ,e reaction temper-ature was ranged from room temperature to 125°C. In thetypical activity tests, the vaporization of the para-formaldehyde was injected into the reactor, accompanied bysupplying O2 and He as carrier gas, with a total gas flow rateof 50mL/min (the gas mixtures contained 300 ppm ofHCHO, 20 vol.% of O2, and balanced by He; the relativehumidity was 50%). Products and reactants were analyzed byonline GC, which was equipped with a TCD.

3. Results and Discussion

Figure 1 shows the catalytic activity of the as-preparedsamples. It can be seen that 1% Pt/MORn exhibited ex-tremely poor catalytic activity on HCHO oxidation with aconversion rate lower than 10% at room temperature. With

the increase of temperature, the increase of the conversionrate was very slow and Pt/MORn only achieved a conversionrate of 20.8% at 125°C. After the removal of impurities andsurface modification by nitric acid solution, the conversionrate of HCHO can exceed 50% at 25°C and reach 100% at75°C. ,e catalytic activity of alkali treatment on the catalysthas also significantly improved and can reach 100% con-version at about 125°C. ,e optimal catalytic activity wasfound in the sample of 1% Pt/MORn-H-OH, which canoxidize HCHO completely at room temperature. ,en,characterization on structure and morphology was per-formed to understand the underlying mechanism for theimproved catalytic activity of 1% Pt/MORn-H-OH.

Figure 2 shows the XRD patterns of the as-preparedsamples. ,e crystalline phase of the mordenite did notchange significantly after the treatment by acid or alkali.Notably, 1% Pt/MORn and 1% Pt/MORn-H showed obviousPt peaks at 2θ� 39.7° and 45.8°, which can correspond to the(111) and (200) planes of Pt, respectively [18]. In contrast, Ptpeaks of 1% Pt/MORn-OH and 1% Pt/MORn-H-OH wereweaker, indicating the better dispersion of Pt particles in thetwo samples.

Figure 3 shows the TEM image of the as-preparedcatalysts. ,e average particle size of Pt particles on the1% Pt/MORn surface was estimated to be 4.3 nm, while theaverage particle diameters of Pt particles of 1% Pt/MORn-H,1% Pt/MORn-OH, and 1% Pt/MORn-H-OH were 3.0 nm,3.1 nm, and 2.8 nm, respectively.,e size of Pt particles has agreat influence on the catalytic oxidation activity of catalyst,and small Pt particles always exhibit a better catalytic activity[19, 20], which is consistent with the highest HCHO con-version rate of 1% Pt/MORn-H-OH. Besides, from Figure 3,it is seen 1% Pt/MORn-H and 1% Pt/MORn-H-OH samplesdemonstrated better dispersion of Pt particles than the 1%Pt/MORn, which is well consistent with the result of XRD.

To further illustrate the effects of acid and alkali treatmentof the natural mordenite on catalyst activity of HCHO oxi-dation, XPS of Pt on the varied mordenite samples (Pt/MORn) was analyzed. As shown in Figure 4, Pt can be

HCH

O co

nver

sion

(%)

100

80

60

40

20

0

20 40 60 80 100 120Temperature (°C)

a

b

d

c

e

Figure 1: Catalytic activities of the as-prepared catalysts: (a)MORn; (b) 1% Pt/MORn; (c) 1% Pt/MORn-H; (d) 1% Pt/MORn-OH; (e) 1% Pt/MORn-H-OH.

2 Journal of Chemistry

divided into Pt0 and Pt2+. Also, Table 1 exhibits the peakposition and peak area obtained after charge correction andpeak fitting. By comparing the Pt0 and Pt2+ peak areas of thePt 4f7/2 peaks, it was found that the Pt contents varied greatlyon different catalysts. ,e HCHO catalyst activity was

increased with the increase of Pt content, and the most ef-ficient catalyst activity was obtained in 1.0% Pt/MORn-H-OHsamples with significantly enhanced Pt0 and Pt2+ content. It ispreviously reported that Pt acts as an active site in the HCHOcatalytic oxidation reaction, and its content is closely related

Figure 3: TEM images of the as-prepared catalysts: (a) 1% Pt/MORn; (b) 1% Pt/MORn-H; (c) 1% Pt/MORn-OH; (d) 1% Pt/MORn-H-OH.

Inte

nsity

(a.u

.)

10 20 30 402θ (°)

50 60

Pt (111) Pt (200)

e

d

c

b

a

Figure 2: XRD patterns of the as-prepared catalysts: (a) MORn; (b) 1% Pt/MORn; (c) 1% Pt/MORn-H; (d) 1% Pt/MORn-OH; (e) 1% Pt/MORn-H-OH.

Journal of Chemistry 3

to the catalytic activity of the catalyst [21, 22]. ,is is con-sistent with the effect of Pt content on catalytic activity in thispaper. ,us, it can be concluded that the acid-alkali-treatednatural mordenite favors the most efficient loading of Pt,thereby facilitating the catalytic oxidation of HCHO at roomtemperature. Besides, it is noted that one of the basic strategies

for the support improvement was to increase the number of-OH by choosing supports treated with acid or alkali to obtainhydroxyl density [23] since hydroxyl groups (-OH) on thesupport surface can capture HCHO through H-bonding andalso assist Pt dispersion in catalyst preparation [6, 7].

4. Conclusions

In this paper, a series of natural mordenite-supported Ptcatalysts were successfully synthesized. Activity results in-dicated that treatment conditions of the natural mordenitecan influence the HCHO catalytic oxidation significantly.And, the acid-alkali-treated natural mordenite-supported Ptsamples (1% Pt/MORn-H-OH) exhibited the optimal cat-alytic activity, which can be benefited from the excellentdispersion of Pt particles, small particle diameters, and highloading content of Pt particles, as illustrated from thecharacterization measurements. Moreover, acid-alkali-treated natural mordenite-supported Pt samples showedincreased hydroxyl density, which was beneficial for

Pt2+

Pt0

78 76 74 72BE (eV)

70

(a)

Pt2+ Pt0

78 76 74 72BE (eV)

6870

(b)

Pt2+ Pt0

78 76 74 72BE (eV)

70

(c)

Pt2+ Pt0

78 76 74 72BE (eV)

6870

(d)

Figure 4: XPS spectra of the as-prepared catalysts: (a) 1% Pt/MORn; (b) 1% Pt/MORn-H; (c) 1% Pt/MORn-OH; (d) 1% Pt/MORn-H-OH.

Table 1: XPS results of the series of 1.0% Pt/MORn catalysts.

Catalysts Pt species Peak area (BE)a

(Pt 4f7/2)Sum of peak area

(Pt 4f7/2)1.0% Pt0 37.0 (71.2) 93.2Pt/MORn Pt2+ 56.2 (71.7)1.0% Pt0 97.0 (70.9) 161.5Pt/MORn-H Pt2+ 64.5 (71.8)1.0% Pt0 87.4 (71.0) 138.0Pt/MORn-OH Pt2+ 50.6 (71.7)1.0% Pt0 131.0 (71.1) 223.6Pt/MORn-H-OH Pt2+ 92.0 (71.9)aPeak area was estimated from XPS measurements, and the relative bindingenergy (eV) is shown in parenthesis.

4 Journal of Chemistry

capturing HCHO through H-bonding and also assisting Ptdispersion in catalyst preparation.

Data Availability

No data were used to support this study.

Conflicts of Interest

,e authors declare that they have no conflicts of interest.

Acknowledgments

,e authors gratefully acknowledge the financial support ofthe National Natural Science Foundation of China(U1402233 and 21667016) and the High-Level ScientificResearch Foundation for Talent Introduction of KunmingUniversity of Science and Technology (10978172).

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