A handy preparation of Pt-based bulk catalysts for indoor formaldehyde purification

This paper mainly studies the preparation of Pt-based bulk combustion catalysts using cordierite honeycomb ceramics as the substrate, and chemical plating technology to directly load precious metal Pt on the sensitized and activated ceramics. Formaldehyde was used as the reaction gas for performance testing of the combustion catalytic activity of the catalyst. The XRD and SEM results have shown that the Pt particles were loaded on the face of cordierite. The effect of formaldehyde purification is significantly affected by factors such as the initial concentration of formaldehyde in the closed space, environmental humidity, and catalyst calcination temperature.


Introduction
Air is a necessity for human survival, air quality is closely linked to human health, and a clean atmospheric environment is a hot topic in recent years.During the process of industrial construction in our country, a large amount of exhaust gas emitted from industrial production and life contaminates our living environment, in which formaldehyde inside the room seriously endangers our physical health and threatens our life safety.Formaldehyde is a highly volatile organic compound, colorless and irritating gas.Indoor formaldehyde mainly comes from building materials, indoor decoration materials and fuel combustion [1] .The toxic effects of formaldehyde in humans are mainly characterized by respiratory toxicity, immunotoxicity, neurotoxicity, cardiovascular toxicity, and carcinogenicity.The great harm of formaldehyde has attracted great attention from various governments [2] .The mean concentration of formaldehyde in newly decorated residential dwellings was 0.11 mg/m3, which is higher than the formaldehyde content of the "Indoor Air Quality Standard" (GB/T18883-2002).The main methods to treat formaldehyde include biodegradation, physical or chemical absorption and catalytic combustion [3] .Among the above methods, catalytic combustion has become the most promising method to treat formaldehyde due to its low energy consumption, high treatment efficiency and no secondary pollution.Therefore, researchers have done a lot of research on catalytic combustion technology, and related issues have become a hot spot of environmental catalysis in recent years.With the increasing demands of human beings on the living environment, the cleanness of the indoor air environment has received great attention, and various air cleaners have entered into people's production life.The development of an efficient and rapid purification device for air purification has become an urgent need.Catalytic oxidation is the essence of catalytic combustion technology, and the core is to develop efficient catalysts.Among the numerous research directions, high activity and stability are two of the most studied aspects.However, lowering the catalytic reaction temperature is an important factor for indoor formaldehyde purification.Huang's group [4] studied the Pt-based catalyst Pt/Ti, which showed 100% formaldehyde removal by Pt/TiO 2 catalyst at room temperature.Liu's group [5] explored the dual-support Pt/TiO 2 catalyst, and it showed a much more excellent degradation efficiency than the single Pt/TiO 2 catalyst.Yu's group [6] loaded TiO 2 nanoparticles on graphene to promote formaldehyde absorption and improve the degradation efficiency of photocatalysis technology.Minsu's group [7] prepared MnOx/TiO 2 as a catalyst, with a formaldehyde removal rate of up to 100%, achieving complete and effective purification of formaldehyde at room temperature.Lu's group [8] synthesized a series of Ag-K/MnO 2 nanorods using a conventional wet initial impregnation method and found that they exhibited a 100% formaldehyde conversion efficiency at a low temperature of 60 ℃.Hassan H's group [9] used the microwave method to deposit active components Pt and Ru on graphene support and applied the catalyst to the electrochemical oxidation treatment of formaldehyde.Wang's group [10] prepared a unique Pt/MnOx-CeO 2 catalyst showing excellent efficiency in the catalytic degradation of formaldehyde at 25℃.Qu's group [11] loaded Ag onto the SBA-15 carrier to achieve catalytic degradation of formaldehyde, and it expressed a complete degradation at 100℃.Li's group made Au/Fe 2 O 3 to achieve catalytic degradation of formaldehyde, and it expressed a complete degradation at 80℃.As people's demand for air quality continues to increase, various indoor or outdoor air purifiers are emerging.Through the invention and creation of researchers, there are many types of air purifiers on the market.The first air purification method might be using formaldehyde scavengers to capture formaldehyde and purify organic waste gas through a six-layer purification network.The second air purification method might be composite formaldehyde purification based on photocatalytic degradation.Based on the above purification devices, the device is mainly divided into an air supply system, a gas treatment system, and a waste gas and liquid discharge system.The main part of the device is the gas treatment system, which relies on different types of purifiers, including adsorption-type activated carbon and adsorbents, TiO 2 for photocatalytic degradation, and catalysts for catalytic oxidation.Designers have developed various catalysts based on differences in appearance, main structure, and experimental methods, but each has its shortcomings: (1) some purifiers cannot completely remove formaldehyde and can only perform physical separation or physical adsorption; (2) some purifiers require large amounts of energy, such as the use of ultraviolet light; (3) some devices are too complex in structure and difficult to disassemble and clean; (4) some devices are prone to secondary pollution.To address these issues, it is of great significance to develop a formaldehyde purification device that can completely purify formaldehyde, does not cause secondary pollution, is easy to operate and move, has low energy consumption, efficient processing, and an appearance that is compatible with the environment.

Preparation of Pt-doped monolithic catalyst
The preparation of Pt-based bulk catalyst (Pt/CHC) is achieved by using cordite honeycomb ceramics as the substrate and covering the active component Pt on the surface of the substrate through chemical plating technology.In a common situation, the cordite ceramic block is cut into two sizes: Φ 5×25 mm and Φ 42×50 mm.The ceramics are boiled and soaked in a 10 wt% HNO 3 solution for about 20 minutes.After the pores are free of obvious impurities, they are removed and rinsed with distilled water and dried at 120℃ for 3 hours before use.Then, the above ceramic cylinder is covered with insulating tape and soaked in a sensitizing solution prepared by mixing 10 g/L SnCl 2 and 30 ml/L 38 wt% concentrated HCl for 10 minutes, then taken out and cleaned with distilled water.After complete sensitization, the ceramic is immersed in an activation solution prepared by mixing 0.1g/L PdCl 2 and 1ml/L 38 wt% concentrated HCl for 10 minutes, and then taken out and rinsed with distilled water for use.The chemical plating solution is prepared by mixing 0.05 -0.5g/L H 2 PtCl 6 ꞏ6H 2 O, 80 ml/L 25 wt% ammonia water, and 13.5 g/L ammonium chloride, with a pH of approximately 10.After that, the prepared chemical plating solution is placed in a constant temperature water bath (30℃).When the plating solution temperature is balanced with the water bath temperature, the activated ceramic is placed in the plating solution, observing the reaction status.The temperature program is controlled and batches of reducing agent hydrazine hydrate (12 mol/L N 2 H 4 ꞏH 2 O) synchronously are added with the temperature rise, as shown in Table 1.The reducing agent is added in 5-6 batches to maintain the progress of chemical plating.After the reaction is completed at 80℃, it is taken out and rinsed with distilled water, naturally air dry, then dried at 60℃ for 3 hours according to the subsequent experimental requirements for calcination.The whole electroless plating method was shown in Figure 1.
Table 1 Procedure of heating-up.

Calculation of Pt load capacity
The main study in this paper is the Pt-based catalyst, and its active component Pt content is calculated as Formula 1. Catalysts with different Pt contents are represented as (x) Pt/CHC, where x is the Pt loading amount, such as 0.24 wt%Pt/CHC.The Pt loading amounts involved in the experiment were 0.12%, 0.18%, 0.24%, 0.3%, 0.5%, and 1.0%.% 100 capacity t carrier m Pt : the mass of Pt added to the chemical plating solution.

Material characterization
The results of X-ray diffraction (XRD) patterns were obtained by the machine of the ULTIMA-ⅢX diffractometer (SHIMADZU, Japan) using CuKα radiation.The imagines of scanning electron microscopy (SEM) were taken by using the machine of the S-4800 (HITACHI, Japan) electron microscope operated at 15 kV.

Calculation of formaldehyde concentration
The QC-2BI dual-channel atmospheric sampler was used to sample the air in the study.The samples were bubbled and dissolved in small bubble collection tubes and then colorized with ammonium iron (II) sulfate.The absorbance was detected and the formaldehyde content was calculated by using a standard curve.Finally, the formaldehyde concentration was calculated based on the formaldehyde content.The formaldehyde concentration in the air is calculated according to formula 2. C = (A-A 0 )×B g /V 0 C: air formaldehyde concentration, mg/m 3 ; A: the absorbance of the sample solution; A 0 : absorbance of blank solution; B g : calculation factors obtained from standard curves, μg/Abs; V 0 : sampling volume under standard state, L.

Catalytic degradation of formaldehyde
In the experimental section, the Pt-based bulk catalyst was prepared by the chemical plating method.The catalyst exists in the form of a cylinder, with a size of Φ42 mm × 50 mm and a volume of V≈70 ml.This size of catalyst can be well matched with the air supply device in the purification device.The working range of the fan can be well controlled under voltage control.After starting the purification device, the gas hourly space velocity (GHSV) is between 0 -550000h -1 .In theory, this purification device can circulate the gas in the sealed box 20 times per hour when it is used in a sealed box.
After the formaldehyde purification device was installed, a series of experiments should be conducted to test its performance.In the testing process, the main method used to periodically detect the simulated cabin air during the operation of the purification device uses formaldehyde detection technology and thereby obtains the optimal effect of this purification device.The activity of the formaldehyde purification device is an important indicator for evaluating its purification effect, and the purification activity can be expressed based on the catalytic purification efficiency.

Results and Discussion
To explore the effect of Pt loading amount and calcination temperature on the crystal phase of the catalyst, XRD was utilized, and the XRD results were shown in Figure 2. As shown in Figure 2, the intensity of the main peak in the blank cordierite significantly decreased after chemical plating, indicating that the active component could be well dispersed on the surface of the substrate.The characteristic peak of Pt (111) in the JCPDS card of platinum (04-0802) was found to be 2θ = 39.761.However, it was observed from the graph that there were no obvious Pt diffraction peaks below 0.24%, which indicates that the Pt content is too low and the surface dispersion is relatively uniform.To further investigate the effect of calcination temperature on the crystal structure of the catalyst, XRD tests were performed on catalysts prepared at different calcination temperatures, and the results were shown in Figure 3.As shown in Figure 3, there was no significant difference in the XRD patterns obtained at different calcination temperatures.Comparing the characteristic peaks at 2θ = 39.761 and 2θ = 41.465,there was no obvious change in peak intensity.This is because, under low loading conditions, the Pt particles obtained by chemical plating technology have a small particle size (50 -100 nm) and are uniformly dispersed, which cannot form a good crystal structure on the surface that can reflect the relationship between activity and calcination temperature well.Therefore, other characterization techniques are needed.We used the electron microscope to analyze the as-prepared samples to investigate the distribution of Pt on cordierite, and the results were shown in Figure 4.As shown in Figure 4, there were no Pt particles in the blank cordierite.However, compared with pictures b) and c), it was obvious that there were more Pt particles observed in c) than in b), which indicates that Pt can be loaded on the cordierite after chemical plating treatment.During the loading process, due to the different lengths of the channels, the concentration distribution of the plating solution is uneven at the two ends and middle of the channels, resulting in uneven growth of Pt particles on the carrier.The Pt content is high at both ends of the carrier, while the Pt component inside the channel is relatively small.To further verify the purification effect of the catalyst on formaldehyde at different concentrations, we conducted research experiments using a simulated formaldehyde purification test device.The result of the formaldehyde simulation purification device at the initial stage was shown in Figure 5.The curve represents the change in formaldehyde content over time during the purification process.The condition for obtaining the curve is room temperature, with a catalyst of 0.12 wt% Pt/CHC calcined at 350℃ and GHSV of 350000h -1 .The formaldehyde concentration was obtained by naturally releasing formaldehyde dilution solution for 12 hours.From the overall analysis of the figure, it can be seen that when the initial concentration of formaldehyde is within the range of 2-36 mg/m 3 , the concentration of formaldehyde decreases with time under the action of the purification device, indicating a good purification effect on formaldehyde.After 4 hours, the formaldehyde content decreased to a relatively stable concentration, with a purification rate of about 50%.At the same time, it can be observed that the decrease in formaldehyde concentration is more significant at higher concentrations, and the purification device can still play a certain purification role at lower initial concentrations.Compared with the previous researchers who achieved a 100% formaldehyde purification rate at room temperature with 1% Pt, the advantage of the current study is that the Pt content is only 0.12%.Considering the factors of cheaper product price and easier experimental implementation, the development of this purification device has good prospects and is worth further research.To further verify the purification effect of catalysts prepared at different calcination temperatures on formaldehyde, a simulated formaldehyde purification test device was used, and the results were expressed in Figure 6.As shown in Figure 8, the calcination temperature affects the chemical state of Pt in the Pt-based catalyst, and different valence states have a significant impact on the purification effect of formaldehyde.The above figure shows that the change in calcination temperature does indeed affect the effectiveness of the catalyst.Under the same initial concentration of 36 mg/m 3 , the catalysts obtained by calcination at 350℃ and 500℃ had similar changes in formaldehyde content in the first two hours of formaldehyde purification.However, as the reaction proceeded, the catalyst calcined at 350℃ showed significantly better performance than that calcined at 500℃.In addition, when the initial concentration was around 10 mg/m 3 , they exhibited similar regular changes.This not only indicates the influence of calcination temperature on the catalyst but also reflects that in the initial stage of the reaction, the active sites provided by both catalysts meet the requirements of the reaction.As the reaction proceeds, the decrease in active sites of the catalyst calcined at 500℃ is more severe, which is insufficient to provide the required reaction.As we all know, water in the environment has a positive effect on promoting the catalytic oxidation of formaldehyde.The reason for this is that the presence of water vapor helps to form active oxygen and accelerates the degradation process of CO to CO 2 .Another reason is that the participation of water can hydrolyze residual Pt-Cl bonds, thereby slowing down the deactivation of the active component Pt.It can be seen that, unlike other catalytic reactions, the presence of water in the catalytic purification of formaldehyde does not have a negative impact.Referring to the previous research, Figure 7 is a graph of formaldehyde purification obtained under the same catalyst conditions of 0.12 wt% Pt/CHC, with the same GHSV value, and room temperature.It can be seen from the figure that the purification effect of using formaldehyde solution (formaldehyde mass fraction ≈ 38%) as the release source for formaldehyde is significantly better than that of using diluted formaldehyde solution (formaldehyde mass fraction < 1%) as the release source.From a thermodynamic perspective, the system atmosphere created by the former has relatively less water, that is, a relatively low relative humidity.Under these conditions, water plays a key role in formaldehyde purification, and high water content enhances the purification effect of formaldehyde.Therefore, water is a key breakthrough in studying formaldehyde purification and requires further exploration and research.To further explore the catalyst's regeneration performance, a thermal regeneration test was conducted, and the results were shown in Figure 8.As shown in Figure 8, the purification effect of the freshly prepared catalyst on formaldehyde reached 60% in the first use, but it decreased in the second use.After drying at 120℃ for 1 hour, the purification effect of the third and fourth uses slightly improved but was still worse than that of the first use.After calcination at 350℃ for 2 hours, the purification effect of the fifth and sixth uses was similar to that of the first use.It can be seen that the effectiveness of the used catalyst decreases with subsequent uses, but after high-temperature calcination, its purification effect can recover to a level close to that of the first use.This is mainly because the used catalyst has some deactivation of the active component, resulting in a decrease in the active sites provided by the catalyst.After high-temperature calcination, the active sites of the catalyst are partially restored, indicating that the reduced purification effect is due to the change in the ratio of Pt m /Pt O caused by the reduction of some oxidized platinum to metallic platinum after catalytic combustion reaction.The results in the figure showed that the device could recover the activity of the catalyst through repeated high-temperature regeneration during use, and the catalyst had a long service life, which met the requirement of multiple uses.

Conclusion
In this paper, we focused on Pt-based catalysts to verify the feasibility of using this catalyst for formaldehyde purification.The as-prepared material was detected by using XRD and SEM detection AMCE-2023 Journal of Physics: Conference Series 2713 (2024) 012079 methods, and it was found that Pt particles were loaded on the cordierite.The performance of catalytic combustion of formaldehyde at room temperature in a closed space was tested, which involved simulating the purification device and the purification environment, as well as evaluating the formaldehyde purification activity.Through the test, the treatment effect is significantly affected by factors such as the initial concentration of formaldehyde in the closed space, environmental humidity, and catalyst calcination temperature.By the above preparing Pt catalysts with a simple method and conducting formaldehyde degradation performance tests in simulated indoor environments, creative ideas can be provided for developing novel indoor air purifiers, further expanding the application of catalytic combustion technology in indoor formaldehyde purification.

TemperatureFigure 1 .
Figure 1.Flow chart of catalyst preparation by electroless plating method.
Figure 7. Different concentrations of formaldehyde solution.

Figure 8 .
Figure 8.The curve of formaldehyde conversion to time.