Acid-Modified Sepiolite-Supported Pt (Noble Metal) Catalysts for HCHO Oxidation at Ambient Temperature

Abstract

The critical need to enhance the quality of indoor air leads to the improvement of catalyst activity for the removal of formaldehyde. Sepiolite can be utilized in catalytic reactions for its unique structure, composition and high surface area. The adhesion between sepiolite fibers and the blocked microporous channel (by impurities) demands the activation of natural sepiolite through acid treatment. This treatment successfully produces acid-modified sepiolite Pt-supported samples. The impacts of different acid concentrations, Pt loading content and calcination temperature on catalytic activity for formaldehyde (HCHO) oxidation are studied. The catalytic activity of HCHO is characterized and evaluated by techniques including specific surface area, X-ray diffraction, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy and transmission electron microscopy. The results show the maximum specific area of sepiolite at the optimized 0.06 M acid concentration. Among all the prepared samples, the 0.02Pt/Sep catalyst calcined at 500 °C exhibits the highest catalytic activity for the oxidation of HCHO.

1. Introduction

Formaldehyde (HCHO) is one of the main invisible pollutants in the indoor environment [1]. Reportedly, people spend more than 80% of their life in the indoor environment, and being in an environment containing HCHO for a long time will lead to allergic reactions or even poisoning [2]. Therefore, the development of cost-effective alternatives for the elimination of HCHO from indoor environments is crucial for human health [3,4,5]. The conventional procedures for HCHO exclusion include physical adsorption [6,7,8,9], photocatalytic degradation [10,11,12], plasma purification [13,14,15] and catalytic oxidation [16,17,18]. Among them, catalytic oxidation has been identified as the most efficient approach due to its non-toxic by-products and high activity under relatively mild conditions [19,20,21]. Heretofore, most studies about HCHO catalytic oxidation have focused on supported noble metal and metal oxide catalysts [22].

Supported noble metal catalysts are beneficial for their excellent stability, high activity, recyclability and no secondary pollutants. Meanwhile, their excellent catalytic performance at relatively low temperatures [23] makes them important. Chen et al. [16] optimized Pt/MnO₂-BN with excellent catalytic activity for HCHO at room temperature. Wang et al. [2] found the advantages of nanosized Pt particles and strong metal support interaction between Pt and Fe₂O₃ for the catalytic degradation of HCHO at room temperature. Lei et al. [24] reported that the low-temperature HCHO decomposition of Pt/CTA is obtained via good Pt dispersion and the unique adsorption properties of cellulose triacetate (CTA). Substantially, platinum (noble metal) shows high dispersion and catalytic oxidation activity, which promote the effective decomposition of HCHO at room temperature.

The improvement in the mechanical properties of catalysts and the better surface area for ingredients’ activation is provided by supporting materials [25] such as silica (SiO₂), alumina (Al₂O₃) and titania (TiO₂) [26]. However, the activity still does not compensate for its activation cost. Therefore, the development of more active and sustainable supported catalysts is necessary. Sepiolite is a fibrous mineral with alternating blocks in the direction of the fibrous tunnels. The center of block structure is composed of MgO that is surrounded by two tetrahedral SiO₂ components [27]. Sepiolite promises activity for various catalytic reactions due to its peculiar pore structure [28], special groups on the surface [29], abundant storage, non-toxicity, low cost and interior channels [30]. For example, Karaoglu et al. [31] reported an active basic catalyst obtained via KOH impregnation onto a sepiolite support for biodiesel production. Gao et al. [13] synthesized a NiO/sepiolite hybrid for application in dye removal in wastewater.

Acid treatment can increase the surface area of sepiolite [32], while keeping the original structure unchanged. We treated sepiolite with different concentrations of hydrochloric (HCl) acid and obtained optimized sepiolite, noted as Sep. The Pt nanoparticles were supported on Sep to obtain a Pt/Sep catalyst.

2. Results and Discussion

2.1. Textural Properties of Modified Sepiolite

The BET experiment confirmed the catalyst’s porous structure and determined the surface area and pore size distribution. The N₂ adsorption–desorption isotherms of sepiolite and modified sepiolite, shown in Figure 1a, belongs to type IV isotherms, with a type of H3 hysteresis loop, indicating porous materials [33]. The adsorption performance of sepiolite was significantly improved by acid treatment. The hysteresis loop closed at P₀ = 0.4, indicating small mesopores in the sample. The pore size distribution of the sepiolite and modified sepiolite was approximately located at 3.8 nm, as indicated by the corresponding pore size distribution plots. Moreover, modified sepiolite demonstrated certain numbers of micropores and an increase in mesopores after acid modification.

The BET specific surface area and pore volume of the sepiolite after acid treatment with different concentrations are given in

Table 1. The BET specific surface area and pore volume of sepiolite after acid treatment with different concentrations.

C(HCl)/mol L−1	0	0.04	0.06	0.08
Specific Surface Area (m2/g)	23.995	24.853	25.581	23.374
Pore Volume (cm3/g)	0.032	0.040	0.040	0.039

. Results showed an increase in the specific surface area of sepiolite with acid treatment, which then decreased with the acid concentration. The acid treatment converted the the Si-O-Mg-O-Si bond into a Si-OH- bond. Noticeably, the pore size and surface area increased with the internal framework destruction of sepiolite, opening internal channels and stripping fibers at different angles. The acid concentration of 0.06 M maximized the specific surface area, with 25.581 m²/g, and provided more active sites and interfaces for HCHO adsorption. Sometimes, HCl corrodes the sepiolite seriously and decreases the specific surface area. Sepiolite modified by 0.06 M HCl (noted as Sep) provided the largest specific surface area and was chosen for subsequent studies.

2.2. The Influence of Calcination Temperature on Material Structure

The hybrid sample prepared with 0.02 M chloroplatinic acid-impregnated solution (noted as 0.02Pt/Sep) was selected for further heat treatment. The samples heat-treated at 400, 500, 600 and 700 °C are denoted as 0.02Pt/Sep-400, 0.02Pt/Sep-500, 0.02Pt/Sep-600 and 0.02Pt/Sep-700, respectively. The XRD patterns in Figure 2 show the exact phase composition, with mainly sepiolite, calcite and quartz for all samples. The XRF results showed that the major components of sepiolite are CaO, SiO₂ and MgO, with a composition of 53.479%, 32.529% and 13.523%. However, the XRD patterns of 0.02Pt/Sep-700 showed the disappearance of the characteristic peaks of CaCO₃ located at 2θ = 18.1°, 28.7° and 34.1°, corresponding to Ca(OH)₂ [34]. This suggested the decomposition of CaCO₃ to form CaO, according to the following chemical reaction:

$${\mathrm{CaCO}}_3 \stackrel{\mathrm{High} \mathrm{Temperature}}{\to } \mathrm{CaO}+{ \mathrm{CO}}_2$$

2.3. The Influence of Pt Loading on Structure and Morphology

The XRD analysis examined the crystal structure of samples. Figure 3a displays the characteristic peaks of Sep localized at 2θ = 26.6°, 29.3°, 42.5° and 44.5°. The XRD patterns of the 0.02Pt/Sep samples indicated the unchanged structure and lack of diffraction peak of Sep after Pt loading, reflecting the evenly dispersed Pt nanoparticles. The XRD patterns of samples loaded with different concentrations of platinum are shown in Figure 3b. The absence of diffraction peaks of platinum in all samples is mainly due to the low content and even dispersion of Pt nanoparticles.

The functional groups present on the surfaces of sepiolite, Sep, 0.02Pt/Sep-fresh and 0.02Pt/Sep-used were determined by FTIR spectroscopy (Figure 4). The peaks after acid treatment were found at 1661 and 3442 cm⁻¹, which were ascribed to δ (H₂O) and the stretching vibration absorption peak of the OH group, respectively. There is a hydrogen bond between HCHO and the hydroxyl group [35], which can facilitate the adsorption of HCHO. Thus, the oxidation procession was accelerated. The similar characteristic peaks for Sep and 0.02Pt/Sep show the unaffected structure after Pt nanoparticle loading. The peaks attributed to H₂O and OH⁻ disappeared after HCHO exposure, which shows their involvement in the reaction (oxidation process), and the formation of a new peak at 2967 cm⁻¹ is ascribed to the formate (CHO₂⁻) species [27] on the sample surface as an intermediate.

High-resolution XPS spectra of Pt 4f of the 0.02Pt/Sep sample are shown in Figure 5. The spectrum of Pt 4f can be fitted to Pt 4f7/2 (71.04 and 74.51 eV) and Pt 4f5/2 (72.54 and 75.74 eV) [36], because of their spin orbit interaction, matching with Pt0 and Pt ions (Pt²⁺ and Pt⁴⁺), separately.

Figure 6 shows the O-1s XPS spectra of Sep and 0.02Pt/Sep. The presence of O in [SiO₄] tetrahedra and [Mg(Al)O₆] octahedra (532.1 eV) [37] is confirmed by the peaks in Figure 6a. The binding energy of O-1s in the 0.02Pt/Sep sample is shifted lower relative (531.86 eV) to Sep (Figure 6b). It indicates the electron density around the O atom due to Pt nanoparticle loading and proves the chemical bond between Sep and Pt, rather than the physical adsorption. The peak at 532.8 eV was attributed to water adsorption.

Figure 7 shows the Pt concentration’s effect on the morphology of Sep. The micrographs in Figure 7a,e,i prove the Pt nanoparticles’ distribution over Sep and show the fibrous structure of Sep. Meanwhile, for Pt nanoparticles attached to the fibers, the fibrous morphology was still retained. The nanoparticles’ size distribution on the fibers was very narrow, with a size of approximately 2.77 nm in diameter for the 0.01Pt/Sep sample. The 0.02Pt/Sep sample showed Pt nanoparticles with a 3.16 nm size distribution. Moreover, agglomeration is observed with the increase in Pt loading, as in the case of the 0.05Pt/Sep sample. The crystalline Pt nanoparticles on the above-mentioned three samples are depicted in Figure 7b,f,j. The measured lattice spacing associated with the (111) plane is 0.2285, 0.2253 and 0.2315 nm, respectively. Furthermore, the EDS maps corresponding to 0.01Pt/Sep, 0.02Pt/Sep and 0.05Pt/Sep are shown in Figure 7d,h,l, in which Mg and Si are associated with Sep, indicating the successful loading of platinum nanoparticles on the Sep surface.

2.4. HCHO Catalytic Oxidation

2.4.1. Effect of Pt Loading

The catalytic oxidation of HCHO by the Pt/Sep catalysts with different Pt loadings at ambient temperature is shown in Figure 8 (the values of 10, 20, 30, 40, 50 and 60 correspond to concentrations of 0.01 M to 0.06 M of chloroplatinic acid solution). The 0.02Pt/Sep catalyst exhibited the highest HCHO oxidation efficiency (around 52%), whereas the 0.01Pt/Sep catalyst could partly oxidize the HCHO, by 47%. However, the adsorbed HCHO was hardly oxidized to CO₂ due to the shortage of Pt nanoparticles. Moreover, the 0.03Pt/Sep, 0.04Pt/Sep, 0.05Pt/Sep and 0.06Pt/Sep catalysts possess certain catalytic ability for the oxidation of HCHO to CO₂, but it is still lower than the efficiency of 0.02Pt/Sep. This is because excessive loading can cause the agglomeration of Pt nanoparticles [38], by which the reactive sites and active surface were reduced and the Sep nanofibers’ pores and tunnels were also blocked. This confirms the optimized Sep acid treatment with 0.02 M solution.

2.4.2. Effect of Calcination Temperature

Figure 9 shows the catalytic oxidation of HCHO with the 0.02Pt/Sep catalyst obtained at different calcination temperatures. Among the catalysts calcined at 400, 500, 600 and 700 degrees, the catalyst calcined at 500 degrees had the best performance, and the formaldehyde degradation rate was 52%. A suitable temperature is beneficial to the reduction of the platinum precursor and the improvement of the catalytic performance of the catalyst, whereas higher temperatures (600 and 700 °C) destroy the basic structure of Sep nanofibers. Furthermore, a higher temperature leads to grain growth [39], which reduces the catalytic active sites, resulting in lower HCHO reduction. Therefore, 500 °C is regarded as the optimal calcination temperature for the 0.02Pt/Sep sample.

2.5. Catalyst Oxidation Mechanism

Figure 10 demonstrates a possible mechanism for the catalytic oxidation of HCHO with the 0.02Pt/Sep sample. Firstly, HCHO, H₂O and O₂ are adsorbed onto 0.02Pt/Sep. Moreover, O₂ is excited by the Pt nanoparticles to oxygen radicals. Later, the oxygen radicals oxidize HCHO and H₂O to dioxymethylene (DOM) and OH⁻ ions, respectively. DOM reacts with -OH to create formate species, which further react with OH⁻ to form CO₂ and H₂O. The OH⁻ ions on the sepiolite surface are conductive to the adsorption of HCHO throughout the mechanism. The catalyst’s wettability plays a critical role in the desorption and transfer of water [40], by the current water forming reaction:

$$\mathrm{HCHO}+{ \mathrm{O}}_2 \stackrel{ }{\to }{ \mathrm{CO}}_2+{ \mathrm{H}}_2\mathrm{O}$$

Therefore, the physical and chemical properties of the sepiolite support enhance the catalytic oxidation of HCHO.

3. Materials and Methods

3.1. Catalyst Preparation

3.1.1. Sepiolite Acid Treatment

Sepiolite powder (Mg₈ (H₂O)₄ [Si₆O₁₆]₂ (OH)₄·8H₂O, Aladdin, Shanghai, China) (1 g) was added to 20 mL HCl (37%, Aladdin, Shanghai, China) with different concentrations (0.04 M, 0.06 M and 0.08 M) at 25 °C for 3 h. Later, the samples were separated, washed and dried in an oven at 75 °C for 12 h. The resulting acid-treated sepiolite samples were named sepiolite-0.04, sepiolite-0.06 and sepiolite-0.08, according to their corresponding 0.04, 0.06 and 0.08 M acid concentration treatments, respectively.

3.1.2. Catalyst Preparation

The acid-modified sepiolite samples were stirred at room temperature for 1 h in chloroplatinic acid solution for the purpose of vacuum impregnation. Then, the samples were filtered, rinsed three times with deionized water and dried at 60 °C for 12 h. Finally, the samples were calcined under N₂/H₂ flow at 500 °C for 2 h to obtain the Pt/Sep catalyst.

3.2. Characterization

The X-ray fluorescence spectroscopy instrument from RIGAKU ZSX Priums (Tokyo, Japan) was employed to evaluate the composition of sepiolite samples. The surface area, pore volume and pore size distributions of the catalysts were measured with an ASAP 2020 micromeritics instrument (Micromeritics, Norcross, GA, USA). The XRD patterns of powdered samples were obtained with a PANalytical X’ Pert PRO, Almelo, Holland. FTIR spectra analysis was carried out on Spectrum One (PerkinElmer, Waltham, MA, USA), and XPS spectra of the samples were obtained, where XPS results were recorded on a PHI 5000 VersaProbe III electron spectroscope (ULVAC-PHI, Chigasaki, Japan). Morphological analysis of samples was carried out via transmission electron microscopy (TEM, Model FEI Tecnai G2 F30, Portland, OR, USA) and chemical analysis was performed using ICP-OES (Agilent725-Es, Santa Clara, CA, USA).

3.3. Catalytic Activity Tests

A box with a legitimate opening and closing mechanism with a connecting metal rod was filled with 1 g of Pt/Sep catalyst and enclosed in an airtight chamber. Then, 10 mL HCHO was rapidly volatilized when added to the heated aluminum foil. The constant signals on the attached detector (PN-2000-CH2O; range: 0–500 ppm; accuracy: 0.1 ppm) showed the complete and even dispersion of HCHO in the airtight chamber, and the connecting rods of the lid helped to evenly expose the Pt/Sep in HCHO gas. Finally, the variation in HCHO concentration was recorded by the attached detector.

4. Conclusions

We prepared Pt/Sep catalysts through the impregnation route, where Pt nanoparticles were formed by calcination and combined with the Sep, having a higher specific area after acid treatment for the oxidation of HCHO at very low loading (0.16 wt.%). The acid-treatment with 0.6 M HCl solution maximized the surface area to 25.581 m²/g. The sites for HCHO oxidation were increased by acid treatment, as illustrated by the characteristic band of the hydroxyl group in the FTIR results. The oxidation results with different Pt loading content and calcination temperatures were discussed. The results showed the enhanced and optimal catalytic properties for sepiolite impregnated with 0.02 M solution and at a 500 °C calcination temperature, demonstrating 52% efficiency.