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Article

Steam Treatment Promotion on the Performance of Pt/CeO2 Three-Way Catalysts for Emission Control of Natural Gas-Fueled Vehicles

by
Xi Liu
,
Yuankai Shao
,
Xiaoning Ren
,
Anqi Dong
,
Kaixiang Li
,
Bingjie Zhou
,
Chunqing Yang
,
Yatao Liu
and
Zhenguo Li
*
National Engineering Laboratory for Mobile Source Emission Control Technology, China Automotive Technology & Research Center Co., Ltd., Tianjin 300300, China
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(1), 17; https://doi.org/10.3390/catal14010017
Submission received: 26 November 2023 / Revised: 21 December 2023 / Accepted: 22 December 2023 / Published: 25 December 2023
(This article belongs to the Special Issue Catalysts for Mobile Source: Low-Carbon and Pollution Emission Control)
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Abstract

:
Three-way catalyst (TWC) is the mainstream technology for stoichiometric natural gas vehicle gas emission purification to meet the China VI emission standard for heavy-duty vehicles. Due to the high price of Pd-Rh TWC widely used at present, it is of great significance to develop cheaper Pt-only catalysts as substitutes. However, there are few studies on Pt-only TWC, especially for natural gas vehicles. It remains a formidable challenge to develop Pt-only TWC with excellent activity and stability. In this study, we significantly improved the catalytic performance of Pt/CeO2 TWC through thermal treatment, especially steam treatment at 800 °C, and used XRD, TEM, H2-TPR, and XPS techniques to investigate how Pt/CeO2 can be activated via these treatments. Our results suggested that after these treatments, CeO2 crystallites sintered slightly, while platinum particles remained highly dispersed. Moreover, these treatments also weakened the Pt-CeO2 interaction, promoted the formation of oxygen vacancies in CeO2 support, and generated a new type of active surface oxygen in the vicinity of Ptδ+, thus improving the activity of the catalyst. After 800 °C steam treatment, the T50 of CH4 and NO decreased by 31 and 36 °C, respectively. The results obtained in this study provide implications for the synthesis of efficient Pt-based catalysts.

1. Introduction

Natural gas vehicles (NGVs) have received increasing attention in recent years because they emit less CO2, a potent greenhouse gas, than gasoline and diesel vehicles [1,2]. However, in order to meet stringent emission regulations, pollutants such as CO, nitric oxides (NOX), and unburned CH4 must be removed from natural gas vehicle emissions. Three-way catalyst (TWC), named for its simultaneous catalytic purification of hydrocarbons (HCs), NOx, and CO, was first applied to gasoline vehicles in the 1970s. So far, it remains the core technology for gasoline vehicle gas emission control. For the same purpose, stoichiometric natural gas vehicles are also equipped with TWC for catalytic purification of their main gas pollutants: CH4, NOX, and CO [3,4]. The main components in TWC are precious metals such as platinum, palladium, or rhodium as active species and inorganic oxides such as Al2O3 and ceria-based oxide as support [5,6]. As the main HC from NGV exhaust emissions is CH4, which is more difficult to oxidize than most HCs, more than three times the precious metal load is required to fully purify CH4 for natural gas vehicles compared to conventional TWC used in gasoline vehicles. Due to the excellent catalytic performance of Pd-Rh-based TWC, contemporary commercial TWC used in natural gas vehicles generally comprises Pd and a relatively small amount of Rh [7,8]. However, the rising price of Pd and Rh in recent years has hindered the application of Pd-Rh TWC [9]. In the past five years, the price of Pt has been lower and more stable than that of Pd and Rh [10,11]. Therefore, it is promising to develop Pt-only TWC with excellent activity and stability.
As the literature reports [12,13], Pt sinters readily to form large particles at elevated temperatures because Pt reacts with oxygen to form volatile PtO2 with significant vapor pressure at 800 °C, which facilitates the transport of Pt. Carrillo et al. [14] reported that decreasing the O2 concentration or alloying Pt with Pd to lower the vapor pressure of PtO2 can slow down the sintering rate of Pt. Further systematic investigation of various Pt catalysts shows that the sintering of Pt can be inhibited by controlling the Pt-oxide-support interaction [15,16]. Shinjoh et al. [17] suggested that the Pt-O-M (M: cation of oxide) bond formed between Pt and support acts as an anchor to suppress the Pt sintering and that the strength of the Pt-oxide-support interaction is well correlated with the electron density of oxygen in the support oxide. Several studies have shown that a Pt/ceria-based catalyst could maintain Pt stability against sintering. Kunwar, et al. [18] discovered that ceria can trap Pt ions, thereby restraining vapor phase transport, while alumina and MgAl2O4 spinel are unable to trap the Pt ions, leading to the formation of large Pt particles. Nagai, et al. [19] reported that Pt in a Pt/ceria-based catalyst does not sinter after aging at 800 °C in air but sinters in a Pt/Al2O3 catalyst. Kunwar, et al. [20] further investigated the sinter-resistance mechanism of Pt/ceria-based catalysts and showed that the reaction of mobile PtO2 with undercoordinated cerium cations present at CeO2(111) step edges allows Pt to achieve a stable square planar configuration.
Many studies have shown that high-temperature treatment could lead to the migration and agglomeration of Pt particles, resulting in the deterioration of the catalyst. Recently, some literature reports that Pt-based catalysts could be activated by treatment at specific atmospheres and temperatures. For example, Nie, et al. [21] suggested that single-atom Pt/CeO2 can be activated via steam treatment at 750 °C, thereby improving its low-temperature CO oxidation performance. Hatanaka, et al. [22] found that treatment with a Pt/ceria-based catalyst at 800 °C under a flow of air can effectively redisperse Pt agglomerates. However, comprehensive studies are required to further understand the behavior of Pt/CeO2 treated at high temperatures.
In this study, we examined the effect of thermal treatment and steam treatment on the performance of Pt/CeO2 three-way catalysts for emission control of natural gas-fueled vehicles and systematically investigated the nature of the performance changes of these treated catalysts by nitrogen adsorption-desorption, X-ray diffraction (XRD), transmission electron microscope (TEM), pulsed CO chemisorption experiment, hydrogen temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS).

2. Results and Discussions

2.1. Catalyst Performance

The three-way catalytic performance of the Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H catalysts was evaluated under simulated exhaust conditions of NGVs. All catalysts were pretreated in simulated gases at 550 °C for 1 h prior to activity measurements. The conversion curves of CH4, NO, and CO for the catalyst samples are presented in Figure 1a–c, and the corresponding light-off temperature (temperature of 50% pollution conversion, T50) and full conversion temperature (temperature of 90% pollution conversion, T90) are summarized in Table 1. Surprisingly, we discovered that thermal treatment, especially steam treatment at 800 °C, activated the Pt/CeO2 catalyst, leading to substantially improved CH4 and NO conversion. As shown in Figure 1a–c and Table 1, the conversion curves of CH4 and NO have the same trend, and the T50 and T90 of both are close over all catalysts. Salaün, et al. [23] reported that in lean burn conditions, the oxidation of methane by oxygen is favored, while in rich conditions, the reduction of NO by methane becomes predominant. The exhaust gas atmosphere of natural gas vehicles is a rich condition, so the reaction with NO is the main pathway of methane conversion. The similar conversion curves of CH4 and NO in Figure 1 confirm this. The T50 of CH4 and NO over Pt/CeO2 is 373 and 386 °C; it decreases by 15 and 16 °C over Pt/CeO2-800A and shifts by 31 and 36 °C to a lower temperature over Pt/CeO2-800H. The light-off temperature is closely related to the low-temperature catalytic activity of the catalyst [24], so the results indicated that the catalyst had better low-temperature activity after 800 °C thermal or steam treatment. The T90 of CH4 and NO in the samples also decreases in the order of Pt/CeO2 > Pt/Ce-800A > Pt/CeO2-800H.
As shown in Figure 2, the reaction involved in the three-way catalyst is very complicated due to the various gas components contained in the exhaust of the natural gas vehicle [25]. In the reaction, NO may be converted to undesired byproducts, including oxidation to NO2 and reduction to N2O and NH3, so the selectivity of NO reduction to N2 needs to be considered. The concentration of NO2, N2O, and NH3 at the reactor exit in the process of the reaction was analyzed online by FT-IR, and the results are shown in Figure 3a–c. It can be observed that NO2 and N2O are formed over all catalysts at about 150–415 °C, with the highest concentrations of 322–372 ppm and 60–66 ppm, respectively, and the temperature range of these byproduct formations is narrower over Pt/CeO2-800H. Figure 3c shows that NH3 is generated at 365–550 °C on the catalysts, with the highest concentration of 90–110 ppm. Since N2 cannot be detected by FT-IR, the selectivity of N2 was calculated indirectly based on the concentration of NO, NO2, N2O, and NH3 at the reactor exit, and the results are shown in Figure 3d. The results show that when the temperature of the catalyst bed is above the light-off temperature, more than 50% of NO is converted to N2.
In order to explore whether further increasing the treatment temperature to 900 °C has the same promotion effect on the performance of the catalyst, the pristine Pt/CeO2 sample was treated in flowing air at 900 °C for 12 h to obtain Pt/CeO2-900A and treated with 10 vol.% H2O in flowing air at 900 °C for 12 h to obtain Pt/CeO2-900H. The performance of these catalysts was evaluated under the same conditions, and the results are shown in Figure S1 and Table S1. The results showed that the activity of the catalysts decreased when the treatment temperature increased to 900 °C. The T50 of CH4 increases from 373 to 388 and 381 °C, and the T50 of NO increases from 386 to 391 and 399 °C for Pt/CeO2-900A and Pt/CeO2-900H, respectively. The T90 of CH4 and NO increases after thermal treatment but remains unchanged after steam treatment at 900 °C. Figure 1c and Figure S1c show that the conversion of CO is 100% over all samples in the whole test temperature range (150–550 °C).

2.2. Catalyst Characterization

In order to explore how the thermal treatment and steam treatment promote the CH4 oxidation and NO reduction activity of Pt/CeO2, the physical and chemical properties of the catalyst are analyzed through a series of experiments.
According to ICP-OES analysis, the content of Pt in Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H was 2.11, 2.00, and 2.03 wt%, respectively (Table 2), which was close to the theoretical value of 2 wt%. The texture parameters of the samples are summarized in Table 2. It can be seen that the BET surface area of the Pt/CeO2 (44 m2/g) decreases slightly compared with the CeO2 support (51 m2/g) and remains almost unchanged after 800 °C thermal or steam treatment.
The structure of the phases and compositions of the catalysts were identified by XRD analysis. The XRD diffraction patterns recorded on the samples are shown in Figure 4. The broad XRD lines appear to be associated with the characteristic reflections of CeO2 (JCPDS 34-0394) [26]. The crystallite size of CeO2 has been calculated from the Debye-Scherrer equation by using CeO2 (111) reflection at 2θ = 28.5°, and the estimates are reported in Table 2. It can be seen from Table 2 that the size of CeO2 crystallite in Pt/CeO2 catalyst (7.2 nm) and CeO2 support powers (7.0 nm) are almost the same and increase from 7.2 to 11.1 and 10.8 nm after 800 °C thermal treatment and steam treatment, respectively, indicating that both treatments lead to the sintering of CeO2 crystallite. In addition, compared with Pt/CeO2, the XRD peaks of Pt/CeO2-800A and Pt/CeO2-800H shift to a lower 2θ value, revealing the expansion of the CeO2 crystal lattice. It can be inferred from the literature [27,28,29] that the thermal treatment and steam treatment introduce more oxygen vacancies and accompanying Ce3+. Peaks belonging to platinum species in Pt/CeO2 were not identified, indicating that the platinum particles are highly dispersed on the CeO2 support or that the size of the platinum crystal is out of the detection limit (below 3–4 nm) of the XRD instrument [30,31]. Following the 800 °C thermal treatment or steam treatment, no peaks attributable to platinum species are observed via XRD, showing that the CeO2 support has a good stabilizing effect on platinum. In addition, it is worth noting that when the temperature of thermal treatment and steam treatment rises to 900 °C, the CeO2 crystallite grows to 19.7 and 20.2 nm, and the characteristic peaks of platinum (JCPDS 04-0802) appear (Figure S2), indicating that these treatments lead to significant sintering of the CeO2 support and agglomeration of platinum particles. This might be the main factor leading to the decrease in catalytic activity.
As reported in the literature [22], the agglomerated Pt can be redispersed under certain oxidatively heated conditions to reverse the effects of sintering and regenerate spent Pt/Al2O3-based reforming catalysts. In order to analyze whether the Pt particles in the Pt/CeO2 catalyst are redispersed after thermal or steam treatment at 800 °C, the average size of platinum particles and the distribution of elements for these samples were determined by TEM. According to literature reports [21], the presence of platinum nanoparticles is readily visible in STEM mode. However, no platinum nanoparticles are found on the HAADF-STEM images and EDS elemental maps of all samples (Figure 5). The results showed that the Pt species in these samples were highly dispersed without significant difference, and it was unclear whether the Pt particles in the catalyst were redispersed after thermal or steam treatment.
To further analyze the dispersion of Pt in the catalysts, pulsed CO chemisorption experiments were examined, and the results are shown in Table 2 and Figure 6. As both the CeO2 support and Pt species in the catalysts adsorb CO, the experiments are usually performed at low temperatures to exclude the adsorption of CO on CeO2. However, we found that even if the adsorption experiment was carried out at −78 °C, as reported in the literature [8,32], the adsorption of CO on the CeO2 support could not be eliminated. Therefore, this method can only characterize the CO adsorption sites on the catalyst [33]. As seen in Table 2 and Figure 6, the CO uptake amount of Pt/CeO2 is 163.23 μmol/gcat, which is larger than the theoretical value of the Pt uptake (102.56 μmol/gcat), because the support CeO2 also adsorbs CO. After thermal treatment and steam treatment at 800 °C, the amount of CO uptake decreases to 96.17 and 73.46 μmol/gcat, respectively, indicating that the CO adsorption sites on the catalyst decrease after these treatments. The results of XRD and TEM experiments have shown that the Pt particles in Pt/CeO2-800A and Pt/CeO2-800H are highly dispersed, so it is speculated that the decrease of CO adsorption sites is mainly caused by the sintering of CeO2 and the slight agglomeration of Pt particles.
The reducibility of oxygen species in TWC plays a critical role in the oxidation and reduction mechanisms participating in the reactions, influencing the catalytic performance significantly. The reducibility of all samples was determined by H2-TPR, and the profiles are shown in Figure 7. The H2-TPR profile of CeO2 support shows two peaks: the first low temperature signal located at 400 °C is assigned to the reduction of nanocrystalline ceria and the surface-capping oxygen of bulk-like large ceria crystal, while the high-temperature signal at 727 °C is attributed to the removal of the bulk oxygen of large ceria crystal [33,34,35,36]. The pristine Pt/CeO2 sample exhibits two hydrogen consumption peaks; the peak α located at 100 °C is ascribed to the reduction of Pt oxide species, while the peak η centered at 375 °C is assigned to the reduction of nanocrystalline ceria and the surface oxygen of a large CeO2 crystal distant from Pt [15,37]. Compared to Pt/CeO2, the peak α of Pt/CeO2-800A and Pt/CeO2-800H shifts to 125 and 129 °C, respectively, possibly due to the slight aggregation of Pt particles [16,38]. The peak η of Pt/CeO2-800A and Pt/CeO2-800H shifts to a lower temperature compared with Pt/CeO2, which may be due to the expansion of the CeO2 crystal lattice confirmed by XRD, which produces active surface oxygen that is reducible at a lower temperature [8,39]. Notably, an extra peak β is observed over Pt/CeO2-800A and Pt/CeO2-800H at 180 and 192 °C, respectively, which is likely caused by a new type of active surface oxygen in the vicinity of Ptδ+ generated during thermal or steam treatment [21], and it is clear from Figure 5 that there is more of this active surface oxygen over Pt/CeO2-800H, resulting in its prominent performance.
The surface elemental composition and atomic ratio of the samples were analyzed by XPS. The surface elemental relative contents were calculated from Pt 4f and Ce 3d core level spectra, and the results are listed in Table 3. The results demonstrated that the content of Pt or Ce among these samples was almost the same. The Pt 4f and Ce 3d XPS peaks of the samples are shown in Figure 8. The Pt 4f spectra were deconvoluted into two pairs for all samples. The peaks at 72.65–72.68 eV and 75.98–76.08 eV are attributed to Pt2+ species, and the peaks at 74.47–74.62 eV and 77.80–77.94 eV are associated with Pt4+ species [40]. The ratio of Pt4+ in the total Pt species was calculated based on the corresponding peak areas and listed in Table 3. It can be noted from Table 3 that the surface Pt4+ content of Pt/CeO2-800A and Pt/CeO2-800H was reduced compared with Pt/CeO2, which can be explained by the weaker interaction between Pt and CeO2 support after thermal treatment and steam treatment [41]. As shown in Figure 8d–f, the spectra of Ce 3d are deconvoluted into eight peaks; the peaks labeled u′ and v′ arise from Ce3+, while the others (marked as u, u″, u′′′, v, v″, and v′′′) are assigned to Ce4+ species [41,42]. According to the literature [33], the coexistence of Ce3+ and Ce4+ is a signal of oxygen vacancies, which is closely related to the redox property, and the oxygen vacancies associated with Ce3+ species near the noble metals are generally considered to be the active sites for NO reduction. As listed in Table 3, the surface Ce3+ content increases in the order of Pt/CeO2 < Pt/CeO2-800A < Pt/CeO2-800H. The higher proportion of Ce3+ over Pt/CeO2-800H contributes to the improved synergistic conversion efficiency of NO and CH4.

2.3. General Assessment

The investigation clearly showed that the steam treatment at 800 °C significantly improved the catalyst performance of the Pt/CeO2 catalyst for natural gas vehicle emission purification, as did the thermal treatment, but the effect was less. The results of XRD and TEM experiments showed that the CeO2 support could disperse and well anchor Pt, so that the Pt species loaded on CeO2 were highly dispersed, and no obvious agglomeration occurred even after thermal or steam treatment at 800 °C for 12 h. However, the strong Pt-O-Ce bond in Pt/CeO2 calcined at 550 °C over-stabilized the platinum sites, resulting in lower activity of the catalyst [43]. Combined with pulsed CO chemisorption, H2-TPR, and XPS experiments, it could be seen that even if thermal or steam treatment at 800 °C caused CeO2 and a few Pt particles to slightly agglomerate, it also weakened the Pt-O-Ce bond, promoted the formation of oxygen vacancies in CeO2, and especially formed a new type of active surface oxygen in the vicinity of Ptδ+, thus enhancing the catalyst reactivity [44]. Furthermore, the steam-treated catalyst had the best performance due to the maximum oxygen vacancy and active surface oxygen, so steam treatment was the most effective way to activate Pt/CeO2. It was worth noting that when the treatment temperature further increased to 900 °C, the CeO2 support and platinum particles significantly sintered, as confirmed by XRD, and the catalyst activity decreased.
Furthermore, it is necessary to acknowledge the limitations of this work. The conclusions in this study are based on powder catalysts, while in practice TWC operates as a structured catalyst, in which the active phase is deposited over a honeycomb monolith. Properties and, therefore, performance of the powder may change a lot after being supported onto the honeycomb. Thus, this study needs to be further extended to monolith TWC before any real application.

3. Conclusions

In this work, the effect of thermal treatment and steam treatment on Pt/CeO2 in three-way catalytic reactions was investigated. XRD, TEM, and pulsed CO chemisorption experiments indicated that the thermal or steam treatment at 800 °C led to the sintering of CeO2 and a slight agglomeration of Pt particles. H2-TPR and XPS characterization techniques further demonstrated that these treatments introduced more oxygen vacancies in CeO2 support and, especially, formed a new type of active surface oxygen in the vicinity of Ptδ+, thus improving the activity of the catalyst. Furthermore, when the treatment temperature increased to 900 °C, XRD results suggested that the CeO2 support and platinum gains grew significantly, resulting in a decrease in catalyst activity. The results obtained in this study provide implications for the synthesis of efficient Pt-based catalysts.

4. Experimental Section

4.1. Preparation of Catalysts

All catalysts were prepared using the incipient wetness impregnation method. Firstly, the commercial CeO2 powders were impregnated with the aqueous solution of Pt(NO3)2 (Pt loading was 2 wt% relative to CeO2) and dried overnight. Then, the impregnated powders were further subjected to drying at 120 °C and calcination in a muffle furnace at 550 °C for 3 h (heating rate of 3 °C/min) to obtain the pristine catalyst sample Pt/CeO2.
To facilitate thermal or steam treatment, the pristine Pt/CeO2 powers were pressed into disks, crushed, and sieved to yield particles of 0.25–0.38 mm in diameter, which were loaded as a packed bed in a quartz tube flow reactor. The Pt/CeO2 particles were then treated in flowing air at 800 °C for 12 h (heating rate of 5 °C/min) to obtain Pt/CeO2-800A. In the same way, the pristine Pt/CeO2 particles were treated with 10 vol.% H2O in flowing air at 800 °C for 12 h to obtain Pt/CeO2-800H.

4.2. Catalyst Characterization

The content of platinum in the catalysts was measured by inductively coupled plasma mass spectrometry (ICP-MS, Thermo Scientific; iCAP-QS, Waltham, MA, USA).
In order to investigate the texture properties of the samples, nitrogen adsorption-desorption experiments were carried out on an adsorption analyzer (Micromeritics, ASAP2460, Norcross, GA, USA) at −196 °C. Prior to the experiment, the samples were degassed in a vacuum at 300 °C for 3 h to remove impurities from the surface. The specific surface area was obtained using the Brunauer-Emmett-Teller (BET) method.
The crystal structure of the samples was characterized by an X-ray diffractometer (Rigaku, Smartlab-SE, Tokyo, Japan). The light source adopts Cu Kα radiation (λ = 0.15406 nm), the X-ray tube flow was 40 mA, and the tube pressure was 40 kV. The scanning range of the diffraction angle was 10–90°, and the step size was 2°. The measured data were analyzed by MDI Jade 6.0 software.
To observe the particle size of Pt and the distribution of metals in the samples, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping spectra of the catalysts were acquired by a transmission electron microscope (FEI, Talos F200S, Hillsboro, OR, USA) with HAADF and energy dispersive spectrometer (EDS) detectors. Before the test, the sample powder was dispersed in ethanol, and then the obtained suspension was dropped into carbon films coated on copper mesh grids and dried in calm air.
The metal dispersion of the samples was estimated by a pulsed CO chemisorption experiment on a quartz U-tube reactor. Prior to chemisorption, 200 mg of samples were reduced in a flow of pure H2 (25 mL/min) at 450 °C for 1 h. The heating rate was 10 °C/min. Then, the samples were cooled to room temperature (about 30 °C) in flowing water, and the CO pulses were injected into the catalyst samples while monitoring the effluent with a thermal conductivity detector (TCD) until the thermal conductivity detector (TCD) signal became constant. One pulse normally contains 6.77 μmol of CO.
The H2-TPR technique was performed to analyze the reducibility of the samples using a chemisorption analyzer (Micromeritics, Autochem II 2920, Norcross, GA, USA). Prior to experiments, 100 mg of samples were first treated in the flow of Ar (20 mL/min) from room temperature to 450 °C (heating rate of 10 °C/min) and held for 1 h. Following cooling down to room temperature, the atmosphere was switched to 10 vol% H2/Ar mixture gas (10 mL/min), and the samples were heated up to 600 or 900 °C at a ramp of 10 °C/min. The signals of H2 uptake were recorded by TCD.
The composition and chemical state of the elements over the catalyst surface were determined by an X-ray photoelectron spectrometer (Thermo Scientific, K-Alpha, Waltham, MA, USA) equipped with a monochromatic Al Kα source operating at 1486.7 eV. The peak of C 1 s at 284.8 eV was used as an internal standard for the correction of charging effects. The fitting process of the XPS spectra was performed on Casa XPS 2.3.19 software using a Gaussian-Lorentzian function with the aid of Shirley background.

4.3. Catalytic Activity Evaluation

The catalytic activity of the powder catalysts was measured in a tubular continuous-flow reactor operated at atmospheric pressure. The inlet gas temperature of the catalyst was measured by a thermocouple. The gas was fed using a series of mass flow controllers. The total inlet flow rate was 833 mL/min, corresponding to a space velocity of 108,960 mL/(gcat⋅h). The simulated gases consisted of 1041 ppm CH4, 3934 ppm CO, 969 ppm NO, 3241 ppm O2, 92,949 ppm CO2, 76,900 ppm H2O, and N2 as balance gases. Prior to measurements, the catalyst powers were prepared into small particles of 0.25–0.38 mm, and then 500 mg of catalysts were first treated in the simulated gases from room temperature to 550 °C (heating rate of 10 °C/min) and held for 1 h. As CH4 was essentially non-converted below 150 °C, the treated catalysts were cooled to 150 °C in flowing N2, and then the temperature of the catalysts was increased from 150 to 550 °C (heating rate of 5 °C/min) in flowing simulated gases. Meanwhile, the reaction gas outlet concentration was online analyzed with a Fourier-transform infrared spectrometer (FT-IR, Thermo Scientific, Antaris IGS, US). The conversion of reactant species i (Xi) was calculated by the following equation:
Xi = 100%⋅(Ci,inlet − Ci,outlet)/Ci,inlet
where Ci,inlet, and Ci,outlet are the volumetric concentrations of i species in the inlet and outlet gas mixture, respectively.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14010017/s1. Table S1: Light-off temperature (T50) and full conversion temperature (T90) of CH4, NO, and CO over Pt/CeO2, Pt/CeO2-900A, and Pt/CeO2-900H catalysts Figure S1: Conversion curves of CH4 (a), NO (b), and CO (c) over Pt/CeO2, Pt/CeO2-900A, and Pt/CeO2-900H. Figure S2: Powder XRD patterns of CeO2, Pt/CeO2, Pt/CeO2-900A, and Pt/CeO2-900H.

Author Contributions

Conceptualization, X.L.; Methodology, X.L.; Validation, Y.S.; Formal analysis, X.L. and X.R.; Investigation, A.D.; Resources, C.Y.; Data curation, Y.L.; Writing—original draft, X.L.; Writing—review & editing, Z.L.; Visualization, B.Z.; Supervision, K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the CATARC Automotive Test Center (Tianjin) Co., Ltd. Youth Fund (No. TJKY2324011) and the CATARC Guidance Project (No. 21243409 and No. 23243404).

Data Availability Statement

Data available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 14 00017 g001
Figure 1. Conversion curves of CH4 (a), NO (b), and CO (c) over Pt/CeO2, Pt/CeO2-800A, and Pt/Ce-800H catalysts.
Figure 1. Conversion curves of CH4 (a), NO (b), and CO (c) over Pt/CeO2, Pt/CeO2-800A, and Pt/Ce-800H catalysts.
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Figure 2. Scheme of reactions involved in the TWC process.
Figure 2. Scheme of reactions involved in the TWC process.
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Figure 3. The curves of the outlet concentration of NO2 (a), N2O (b), NH3 (c), and the selectivity of N2 (d) at different temperatures over Pt/CeO2, Pt/CeO2-800A, and Pt/Ce-800H catalysts.
Figure 3. The curves of the outlet concentration of NO2 (a), N2O (b), NH3 (c), and the selectivity of N2 (d) at different temperatures over Pt/CeO2, Pt/CeO2-800A, and Pt/Ce-800H catalysts.
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Figure 4. (a) Powder XRD patterns of different samples and (b) magnification at 2θ of 27–30°.
Figure 4. (a) Powder XRD patterns of different samples and (b) magnification at 2θ of 27–30°.
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Figure 5. HAADF-STEM images and EDS elemental maps of Pt/CeO2 (a), Pt/CeO2-800A (b), and Pt/CeO2-800H (c).
Figure 5. HAADF-STEM images and EDS elemental maps of Pt/CeO2 (a), Pt/CeO2-800A (b), and Pt/CeO2-800H (c).
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Figure 6. CO uptake amount of Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H samples.
Figure 6. CO uptake amount of Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H samples.
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Figure 7. H2-TPR profiles of CeO2 support (a) and different catalysts (b).
Figure 7. H2-TPR profiles of CeO2 support (a) and different catalysts (b).
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Figure 8. Pt 4f (a–c) and Ce 3d (d–f) XPS spectra of Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H.
Figure 8. Pt 4f (a–c) and Ce 3d (d–f) XPS spectra of Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H.
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Table 1. Light-off temperature (T50) and full conversion temperature (T90) of CH4, NO, and CO over Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H catalysts.
Table 1. Light-off temperature (T50) and full conversion temperature (T90) of CH4, NO, and CO over Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H catalysts.
SamplesT50 (°C)T90 (°C)
CH4NOCOCH4NOCO
Pt/CeO2373386<150393397<150
Pt/CeO2-800A358370<150375379<150
Pt/CeO2-800H342350<150361362<150
Table 2. Physicochemical characteristics of CeO2, Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H samples.
Table 2. Physicochemical characteristics of CeO2, Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H samples.
SamplePt Content a
(wt%)		SBET b (m2/g)CeO2 Crystallite Size c (nm)CO ad. d (μmol/gcat)
CeO2-517.0-
Pt/CeO22.11447.2163.23
Pt/CeO2-800A2.004111.196.17
Pt/CeO2-800H2.034510.873.46
a Obtained from the ICP-OES analysis. b From nitrogen adsorption-desorption experiment results. c Obtained from XRD results based on CeO2(111) diffraction peaks (2θ = 28.5°). d Calculated from pulsed CO chemisorption.
Table 3. Surface elemental compositions derived from the XPS analyses over Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H.
Table 3. Surface elemental compositions derived from the XPS analyses over Pt/CeO2, Pt/CeO2-800A, and Pt/CeO2-800H.
SampleRelative Surface Composition (at.%)Pt2+/Pt (%)Pt4+/Pt (%)Ce3+/Ce (%)Ce4+/Ce (%)
PtCe
Pt/CeO24.4795.5390.669.3427.0472.96
Pt/CeO2-800A4.2695.7493.556.4530.7969.21
Pt/CeO2-800H4.3195.6993.346.6635.3564.65
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Liu, X.; Shao, Y.; Ren, X.; Dong, A.; Li, K.; Zhou, B.; Yang, C.; Liu, Y.; Li, Z. Steam Treatment Promotion on the Performance of Pt/CeO2 Three-Way Catalysts for Emission Control of Natural Gas-Fueled Vehicles. Catalysts 2024, 14, 17. https://doi.org/10.3390/catal14010017

AMA Style

Liu X, Shao Y, Ren X, Dong A, Li K, Zhou B, Yang C, Liu Y, Li Z. Steam Treatment Promotion on the Performance of Pt/CeO2 Three-Way Catalysts for Emission Control of Natural Gas-Fueled Vehicles. Catalysts. 2024; 14(1):17. https://doi.org/10.3390/catal14010017

Chicago/Turabian Style

Liu, Xi, Yuankai Shao, Xiaoning Ren, Anqi Dong, Kaixiang Li, Bingjie Zhou, Chunqing Yang, Yatao Liu, and Zhenguo Li. 2024. "Steam Treatment Promotion on the Performance of Pt/CeO2 Three-Way Catalysts for Emission Control of Natural Gas-Fueled Vehicles" Catalysts 14, no. 1: 17. https://doi.org/10.3390/catal14010017

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