Acid-Alkali-Treated Natural Mordenite-Supported Platinum Nanoparticles ( Pt / MORn-HOH ) for Efficient Catalytic Oxidation of Formaldehyde at Room Temperature

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 Pt samples (1% Pt/MORn-H-OH) exhibited the highly enhanced catalytic oxidation activity of formaldehyde (HCHO) at room temperature. XRD results showed that the crystalline phase of the mordenite did not change significantly in 1% Pt/MORn-H-OH catalyst. However, the acid-alkali treatment endowed the Pt particles excellent dispersion with the smallest diameter of 2.8 nm in a high loading content, which contributed to the optimal catalytic activity of 1% Pt/MORn-H-OH.


Introduction
Formaldehyde (HCHO) pollution has recently attracted public concern because of its carcinogenicity and teratogenicity [1,2].Among all the investigated processes, catalytic oxidation is regarded as one of the most efficient methods for HCHO removal [3,4].So far, supported noble metal catalysts show superior HCHO catalytic activity at room temperature [5][6][7].Choosing suitable supports is important for the catalytic oxidation process of HCHO.Zeolites are excellent supports in catalytic oxidation of HCHO [8][9][10].For example, Park and coworkers found that HCHO catalytic activity for the noble metal catalysts supported in beta zeolite is much higher than that in TiO 2 -H (Hombikat), TiO 2 -P (P25), and Mn-CeO 2 [9].In general, zeolites can be obtained from freshly prepared sodium aluminosilicate gel by hydrothermal treatment [11].However, the increasing consumption of zeolites calls has called cheaper raw materials to synthesize them.From this point of view, natural mordenite is a valuable natural resource for the synthesis of zeolites.As a matter of fact, the cost of extracting "industrial material" from natural mordenite is only 1-5% of synthetic zeolite.Hence, natural mordenite is promising in the practical application [12,13].
However, natural mordenite also needs a certain acid and alkali treatment to regulate the silicon-aluminum ratio and specific surface area [14,15].Generally, acidtreatments can remove impurities such as SiO 2 , Al 2 O 3 , and Fe 2 O 3 in the surface and pores of mordenite.At the same time, the acid solution can dissolve the amorphous substances in the pores and change the large cations with small proton by ion exchange, which increases the pore size, specific surface area, and adsorption capacity.In addition, acid treatment changes the surface properties of the zeolite molecular sieve [16], thus optimizing the properties of the mordenite material.Meanwhile, alkaline treatment is also a general method to generate partial dissolution of the mordenite structure due to silicon extraction [17].erefore, this study mainly focused on the catalytic oxidation of HCHO and researched the catalytic performance of the acid-alkali-pretreated natural mordenite-supported Pt-based catalysts.e purpose is to explore the possibility of using natural mordenite as a Pt catalyst support to remove HCHO at room temperature.

Experimental
2.1.Catalyst Preparation.Firstly, the purchased natural mordenite particles (Si/Al � 6.6, according to the XRF data; from Yanshan, Yunnan) were milled in a planetary ball mill to ensure the particle size at about 1-2 μm, and the acquired powder was named as MORn.en, the MORn underwent acid treatment or alkali treatment.In the typical acid and/or alkali treatment process, 10 g of MORn was, respectively, placed into 100 mL HNO 3 (0.1 M) or 30 mL NaOH (0.2 M) solution, operated with reflux, and stirred at 125 °C for 10 h or 65 °C for 30 min, respectively.After washing and filtration treatment, the obtained samples were dried at 80 °C overnight and named as MORn-H (acid-treated natural mordenite zeolite), MORn-OH (alkali-treated natural mordenite zeolite), and MORn-H-OH (sequentially acid-treated, alkalitreated natural mordenite zeolite), respectively.
Pt/MORn catalysts were synthesized by the impregnation method.First, 2 g of MORn was added into an aqueous solution of H 2 PtCl 6 •6H 2 O to ensure the loading amount of Pt was 1 wt.% and stirred at room temperature for 2 h.en, the solution was placed in a rotary evaporator and evaporated under vacuum at 65 °C.e obtained solid was dried at 110 °C overnight and calcined in air at 300 °C for 2 h.Finally, it was reduced in H 2 at 300 °C for 2 h with the heating rate of 5 °C/min.e reduced gray solid was denoted as Pt/MORn, Pt/MORn-H, Pt/MORn-OH, or Pt/MORn-H-OH depending on the pretreatment conditions of natural mordenite.

Catalyst Characterization.
e X-ray diffraction (XRD) analyses were obtained using a Ultima IV-type powder X-ray diffractometer with Cu Ka radiation (operated at 30 mA and 40 kV), and the measurements were performed in the range of 5-65 °.  e transmission electron microscope (TEM) images of the catalysts were characterized on a Hitachi HT7700 electron microscopy (from JEOL Corporation).e X-ray photoelectron spectroscopy (XPS) results were carried out on a Escalab 250Xi X-ray photoelectron spectrometer, by calculating the C 1s peak to the binding energy of 284.6 eV.

Catalytic Activity Tests.
Catalyst activity tests for HCHO oxidation were measured in a continuous flow fixed bed reactor system with 0.1 g of catalyst.e reaction temperature was ranged from room temperature to 125 °C.In the typical activity tests, the vaporization of the paraformaldehyde was injected into the reactor, accompanied by supplying O 2 and He as carrier gas, with a total gas flow rate of 50 mL/min (the gas mixtures contained 300 ppm of HCHO, 20 vol.% of O 2 , and balanced by He; the relative humidity was 50%).Products and reactants were analyzed by online GC, which was equipped with a TCD.

Results and Discussion
Figure 1 shows the catalytic activity of the as-prepared samples.It can be seen that 1% Pt/MORn exhibited extremely poor catalytic activity on HCHO oxidation with a conversion rate lower than 10% at room temperature.With the increase of temperature, the increase of the conversion rate was very slow and Pt/MORn only achieved a conversion rate of 20.8% at 125 °C.After the removal of impurities and surface modification by nitric acid solution, the conversion rate of HCHO can exceed 50% at 25 °C and reach 100% at 75 °C.e catalytic activity of alkali treatment on the catalyst has also significantly improved and can reach 100% conversion at about 125 °C.e optimal catalytic activity was found in the sample of 1% Pt/MORn-H-OH, which can oxidize HCHO completely at room temperature.
en, characterization on structure and morphology was performed to understand the underlying mechanism for the improved catalytic activity of 1% Pt/MORn-H-OH.
Figure 2 shows the XRD patterns of the as-prepared samples.
e crystalline phase of the mordenite did not change significantly after the treatment by acid or alkali.Notably, 1% Pt/MORn and 1% Pt/MORn-H showed obvious Pt peaks at 2θ � 39.7 °and 45.8 °, which can correspond to the (111) and (200) planes of Pt, respectively [18].In contrast, Pt peaks of 1% Pt/MORn-OH and 1% Pt/MORn-H-OH were weaker, indicating the better dispersion of Pt particles in the two samples.
Figure 3 shows the TEM image of the as-prepared catalysts.
e average particle size of Pt particles on the 1% Pt/MORn surface was estimated to be 4.3 nm, while the average 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 a great 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 conversion rate of 1% Pt/MORn-H-OH.Besides, from Figure 3, it is seen 1% Pt/MORn-H and 1% Pt/MORn-H-OH samples demonstrated 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 treatment of the natural mordenite on catalyst activity of HCHO oxidation, XPS of Pt on the varied mordenite samples (Pt/ MORn) was analyzed.As shown in Figure 4, Pt can be

2
Journal of Chemistry divided into Pt 0 and Pt 2+ .Also, Table 1 exhibits the peak position and peak area obtained after charge correction and peak fitting.By comparing the Pt 0 and Pt 2+ peak areas of the Pt 4f 7/2 peaks, it was found that the Pt contents varied greatly on different catalysts.e HCHO catalyst activity was increased with the increase of Pt content, and the most efficient catalyst activity was obtained in 1.0% Pt/MORn-H-OH samples with significantly enhanced Pt 0 and Pt 2+ content.It is previously reported that Pt acts as an active site in the HCHO catalytic oxidation reaction, and its content is closely related to the catalytic activity of the catalyst [21,22]. is is consistent with the effect of Pt content on catalytic activity in this paper.us, it can be concluded that the acid-alkali-treated natural mordenite favors the most efficient loading of Pt, thereby facilitating the catalytic oxidation of HCHO at room temperature.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 obtain hydroxyl density [23] since hydroxyl groups (-OH) on the support surface can capture HCHO through H-bonding and also assist Pt dispersion in catalyst preparation [6,7].

Conclusions
In this paper, a series of natural mordenite-supported Pt catalysts were successfully synthesized.Activity results indicated that treatment conditions of the natural mordenite can influence the HCHO catalytic oxidation significantly.And, the acid-alkali-treated natural mordenite-supported Pt samples (1% Pt/MORn-H-OH) exhibited the optimal catalytic activity, which can be benefited from the excellent dispersion of Pt particles, small particle diameters, and high loading content of Pt particles, as illustrated from the characterization measurements.Moreover, acid-alkalitreated natural mordenite-supported Pt samples showed increased hydroxyl density, which was beneficial for

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