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Article

Sulfur-Resistant CeO2-Supported Pt Catalyst for Waste-to-Hydrogen: Effect of Catalyst Synthesis Method

by
Ga-Ram Hong
1,
Kyoung-Jin Kim
1,
Seon-Yong Ahn
1,
Beom-Jun Kim
1,
Ho-Ryong Park
1,
Yeol-Lim Lee
2,
Sang Soo Lee
1,
Yukwon Jeon
1 and
Hyun-Seog Roh
1,*
1
Department of Environmental and Energy Engineering, Yonsei University, 1 Yonseidae-gil, Wonju 26493, Republic of Korea
2
Department of Chemical Engineering, Keimyung University, 1095 Dalgubeol-daero, Dalseo-gu, Daegu 42601, Republic of Korea
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1670; https://doi.org/10.3390/catal12121670
Submission received: 18 November 2022 / Revised: 11 December 2022 / Accepted: 16 December 2022 / Published: 19 December 2022
(This article belongs to the Special Issue Nanocatalysts for Carbon Upcycling)
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Abstract

:
To improve the sulfur tolerance of CeO2-supported Pt catalysts for water gas shift (WGS) using waste-derived synthesis gas, we investigated the effect of synthesis methods on the physicochemical properties of the catalysts. The Pt catalysts using CeO2 as a support were synthesized in various pathways (i.e., incipient wetness impregnation, sol-gel, hydrothermal, and co-precipitation methods). The prepared samples were then evaluated in the WGS reaction with 500 ppm H2S. Among the prepared catalysts, the Pt-based catalyst prepared by incipient wetness impregnation showed the highest catalytic activity and sulfur tolerance due to the standout factors such as a high oxygen-storage capacity and active metal dispersion. The active metal dispersion and oxygen-storage capacity of the catalyst showed a correlation with the catalytic performance and the sulfur tolerance.

Graphical Abstract

1. Introduction

Environmental policies and research worldwide recently aimed to achieve carbon neutrality (net-zero) by 2050. This is because 55% of the world’s population lives in urban areas, and this proportion is expected to increase to 68% by 2050, leading to rapid urbanization. Furthermore, the world’s present population is predicted to increase by 30%, leading to serious overuse of energy and global warming [1]. Therefore, replacing fossil fuels with more environmentally-friendly fuels has received more attention in recent years due to relevant environmental issues and an increasing energy demand. In particular, sustainable renewable energy such as hydrogen is a key factor for achieving net-zero [2].
Hydrogen is considered the most preferred alternative energy because of its numerous advantages, such as zero pollution, sustainability, and ease of storage [3]. However, more than 96% of hydrogen is produced using fossil fuels as a source [3,4]. As an alternative to this, the process of producing hydrogen from various waste resources is being actively studied [2,5,6]. As one primary type of waste, municipal solid waste has great interest worldwide [3,7,8,9]. Municipal solid waste is converted into useful syngas composed of CO and H2 through the waste gasification process, which is one of the representative technologies of waste-to-energy [2,10,11]. By applying a water–gas shift (WGS) reaction (Equation (1)), the waste-derived synthesis gas can be converted into hydrogen [11,12,13].
CO + H2O ↔ H2 + CO2, ∆H = −41.2 kJ/mol
Here, the considerations in applying waste-derived synthesis gas to WGS are different from that in applying the conventional synthesis gas from natural gas. Waste-derived synthesis gas is mainly composed of CO (>38%) [10,11] with a trace amount of sulfur components [14,15]. Therefore, in the design of high-performance WGS catalysts for hydrogen production from waste-derived synthesis gas, it is essential to ensure high CO conversion in excessive CO conditions and sulfur resistance. Since trace amounts of sulfur components are preferentially adsorbed into the active sites of the catalysts to cause rapid deactivation, understanding the correlation between the physicochemical properties of the catalysts and sulfur resistance is emerging as a major research issue [15,16].
Previously, we confirmed that a Pt catalyst using CeO2 as a support exhibited excellent catalytic activity and sulfur resistance in WGS reactions using synthesis gas from waste-containing sulfur components [17]. The optimal amount of Pt loading was then found to enhance the performance of the catalyst [18]. CeO2 is an excellent reducible support as an oxygen buffer that can greatly enhance the reaction rates involving redox steps in the WGS reactions [19,20]. It has been widely reported that CeO2 is easily and reversibly reduced to several CeO2−x stoichiometries when exposed to an oxygen-deficient atmosphere [20]. This unique characteristic of CeO2 results in a high oxygen-storage capacity (OSC). In addition, the presence of Pt can weaken the Ce–O bond, changing the surface properties, which makes the surface CeO2 more reducible [21,22]. These reduction properties in the CeO2-PtOx interfaces are explained by the oxygen reverse spillover phenomenon from CeO2 [21,23]. Due to this synergistic effect, the Pt/CeO2 catalyst has recently been actively studied as a catalyst for the WGS reaction [24,25,26,27].
In the catalyst optimization process, the synthesis method is an important consideration that can improve catalytic performance [28,29,30,31]. As reported by Zhang et al., the Au/CeO2 catalyst prepared by the incipient wetness impregnation (IWI) method for the CO oxidation reaction was compared with the catalyst prepared by the deposition-precipitation method [28]. The catalyst prepared by the IWI method showed excellent catalytic activity by forming smaller and more uniform gold particles on the catalyst surface. Kim et al. reported that the CeO2-supported Co catalyst prepared by the sol-gel (SG) method showed excellent WGS activity due to the high concentration of oxygen vacancies originating from the strong metal–support interaction [29]. Li et al. prepared CeO2–MoO3 catalysts by various synthesis methods for the selective catalytic reduction of NO with NH3 [30]. They claimed that catalysts prepared by the hydrothermal (HT) method showed a larger surface area, highly dispersed nanocrystalline CeO2, good redox ability, and stronger adsorption capacity. Lee et al. prepared catalysts by co-precipitation (CP), IWI, HT, co-impregnation, and sequential impregnation methods for dry reforming using coke oven gas [31]. The catalyst prepared by CP showed improved catalytic performance, which was due to the strong metal–support interaction. Considering the above results, it can be confirmed that the physicochemical properties of the catalyst change according to the synthesis methods. Therefore, the synthesis method is an important factor influencing catalyst performance. In this study, the effect of the synthesis method on the physicochemical properties of the catalyst was investigated by varying the synthesis method (IWI, SG, HT, and CP) of the CeO2-supported Pt catalysts. The factors that improve the catalytic activity and sulfur tolerance in the WGS reaction were investigated, and an optimal synthesis method was derived. The prepared catalysts by various synthesis methods (IWI, SG, HT, and CP) were denoted as PtCe-I, PtCe-S, PtCe-H, and PtCe-C, respectively.

2. Results and Discussion

The XRD patterns for CeO2-supported Pt catalysts prepared by various synthesis methods are shown in Figure 1. The patterns of all prepared samples exhibited peaks attributable to the face-centered cubic fluorite structure of CeO2 (JCPDS No. 34-0394) [32]. In the case of the PtCe-H and PtCe-C catalysts, Pt (111) diffraction peaks were observed at a 2θ of 39.8° (JCPDS No. 04-0802) [33]. This indicates that the metal crystallites on the CeO2 supports were either largely formed or have high crystallinity. Conversely, when using the PtCe-I and PtCe-S catalysts, clear diffraction peaks of Pt nanoparticles were not observed because they are small and well dispersed over the support. The improved dispersion has a positive effect on enhancing the catalytic activity for the WGS reaction [20,34].
Table 1 summarizes the physicochemical properties of PtCe catalysts prepared by various synthesis methods. The specific surface area of the samples shows the following order: PtCe-C (132 m2/g) > PtCe-I (107 m2/g) > PtCe-H (78 m2/g) > PtCe-S (67 m2/g). Figure S1 shows the N2 adsorption–desorption isotherms of PtCe catalysts prepared by various synthesis methods. All the catalysts presented a type IV isotherm with a hysteresis loop, which indicates the mesoporous capillary condensation. According to the IUPAC classification, PtCe-I and PtCe-H exhibited an H1 hysteresis loop [35,36]. H1 hysteresis indicates the presence of cylindrical pores having a uniform size or shape. PtCe-S exhibited an H3 hysteresis loop type, whereas PtCe-C exhibited an H2 hysteresis loop type [35,37]. Figure S2 shows the pore distribution of PtCe catalysts prepared by various synthesis methods. As discussed for the isotherms, the PtCe-I catalyst exhibited mesopore distribution. The PtCe-H catalyst contained mesopores and even macropores. The PtCe-S catalyst showed a non-uniform distribution of pores with mesopores close to micropores and up to macro pores. The PtCe-C catalyst showed a distribution of relatively small-sized mesopores compared to the PtCe-I catalyst.
Synthesis methods changed the specific surface area as well as the Pt0 dispersion. As confirmed by CO chemisorption analysis, the PtCe-I catalyst showed the highest value of 89.9% and the PtCe-C catalyst showed the lowest value of 2.4%. The PtCe-S and PtCe-H catalysts showed a Pt0 dispersion of 53.4% and 36.2%, respectively. Metal dispersion is generally defined as the ratio of the number of active sites exposed on the catalyst surface, and only the atoms exposed on the surface can participate in the catalyst surface reaction. In the WGS reaction, the CO2 product generates through the adsorption of CO on the active Pt particles of the catalyst surface during the reaction process [21]. Therefore, it is expected that the PtCe-I catalyst with the highest Pt0 dispersion would show high WGS activity.
Concerning H2-TPR (Figure 2), two peaks related to the redox properties of the CeO2-supported Pt catalysts were observed. During the reduction process, the first intense peak centered at low temperatures (<200 °C) included the reduction of oxidized Pt species, as well as partial reduction of O species between Pt nanoparticles and CeO2 interfaces [21,38,39]. The second peak located at high temperatures over 300 °C is ascribed to the reduction of CeO2 [28,38]. The reduction behaviors of the catalysts were clearly different depending on the catalyst synthesis method. The first reduction peak of the PtCe-I catalyst appeared at the lowest temperature among the tested samples. The reduction temperature of the catalytic active species (CeO2-PtOx) was as follows: PtCe-S (181 °C) > PtCe-C (111 °C) > PtCe-H (93 °C) > PtCe-I (75 °C).
To compare the reducibility of the catalyst in detail, the H2 consumption of the catalyst was also calculated and is shown in Table 1. The hydrogen consumption of each catalyst was calculated with the first peak between 50 and 300 °C. The PtCe-I catalyst showed the highest H2 consumption compared to the others. In addition, as the lattice oxygen of CeO2 has reversely spilled over at lower temperatures, the second reduction peak area of the PtCe-I catalyst was markedly decreased [21,38]. The reduction behaviors of CeO2-PtOx interfaces are related to the reaction between hydrogen and reversely spilled-over oxygen from CeO2 [21,23,39]. According to the results of the above analysis, the oxygen reverse spillover amount of CeO2-supported Pt catalysts is different depending on the synthesis method. The oxygen reverse spillover further modifies the redox properties of CeO2-supported Pt catalysts [23,40]. The WGS reaction is widely known to operate through a redox mechanism in the high-temperature range [23,41]. In detail, CO is adsorbed to active sites and oxidized by mobile oxygen species on the catalysts [21]. Then, the defect oxygen sites are reoxidized by water adsorbed on the oxide surface and generate hydroxyl groups [40]. Therefore, the redox property of the catalyst is a main factor that affects the activity of the WGS catalyst. Consequently, the PtCe-I catalyst with excellent redox properties and high H2 consumption is expected to exhibit relatively high catalytic activity for the WGS.
To assess the surface electronic properties of the CeO2-supported Pt catalysts, the reduced samples were analyzed by XPS (Figure 3). The reduction conditions were the same as the pretreatment (400 °C with 5% H2/N2) applied to the reaction. Figure 3A shows the XPS Pt 4f spectra of the CeO2-supported Pt catalysts. Through the reduction process, PtOx was reduced to metallic Pt0. The corresponding Pt0 peaks are displayed at 74.3 (Pt 4f5/2) and 71.0 eV (Pt 4f7/2) [42,43]. The PtCe-I and PtCe-S catalysts clearly showed Pt0 peaks. Conversely, the PtCe-H and the PtCe-C catalysts showed a weak Pt0 peak. It can be considered that Pt is not sufficiently exposed on the catalyst surface in the PtCe-H and PtCe-C catalysts [44]. This trend is consistent with the Pt dispersion results. This same result was also shown in the poisoned samples (Figure S3).
From the XPS analysis results, the O 1s and Ce 3d spectra can provide the surface concentration of oxygen vacancies. In the O 1s spectra (Figure 3B), the peaks deconvoluted into three main peaks: 529.4, 530, and 531.5 eV [45,46]. Each deconvoluted peak represents: 529.4 eV (lattice oxygen (OL)); 530 eV (defect oxygen (OD)); and 531.5 eV (adsorbed oxygen species (OA)), respectively [45,46]. In particular, the oxygen vacancy value of the catalyst can be assumed through the OD peak shown in color at 530 eV [46]. As the number of defect oxygen species in CeO2 increases, the content of Ce3+ surface defects, leading to the formation of oxygen vacancies [47]. This oxygen vacancy serves as a site for mobile oxygen that participates in the redox mechanism of the WGS reaction, thereby accelerating the reaction [40]. It has been reported that the oxygen vacancy of the PtCe catalyst is related to the reduction of CeO2 through oxygen reverse spillover [23,40]. Table 2 shows the surface atomic concentration of the OD calculated from the deconvoluted results. The surface atomic concentration of OD in the catalysts shows the order of PtCe-I (22.18%) > PtCe-C (18.31%) > PtCe-S (11.66%) > PtCe-H (10.35%). Thus, it is expected that the PtCe-I catalyst exhibits high activity.
The Ce 3d spectra (Figure 3C) of the samples exhibited multiple peaks related to Ce3+ and Ce+4 at 882–916.4 eV. The peaks were deconvoluted into 10 peaks (v0, v, v′, v″, v‴, u0, u′, u, u″, and u‴) from five doublets of the spin-orbit split, respectively [48,49]. Among the peaks, the two colored doublets (v0, u0, and v′, u′) are related to Ce3+, which correspond to the states of Ce 3d94f1 O 2p6 and Ce 3d94f2 O 2p5, respectively [48]. The formation of Ce3+ ions is ascribable to the reduction of the CeO2 support, indicating the generation of oxygen vacancies [50,51]. Specifically, the movement of mobile oxygen occurs at oxygen vacancies in the CeO2 lattice of the catalyst [40]. Thus, the tendency of Ce3+ was expected to follow the trend of OD. The ratio of Ce3+ was calculated by Equation (2) figure based on the deconvoluted XPS Ce 3d spectra.
Ce 3 +   ( % ) = { ( A v 0 + A v ′ + A u 0 + A u ′ ) / ∑ A i }   ×   100
where ∑ A i is the total area of the original Ce 3d peak before deconvolution. Table 2 displays the Ce3+ concentration values obtained in the calculation. As expected, the results followed the same trend as the OD, in the order PtCe-I (32.21%) > PtCe-C (31.04%) > PtCe-S (29.57%) > PtCe-H (28.19%).
The OSC measurements of the catalysts were performed by an H2-O2 pulse reaction (Table 2). The OSC value of the catalysts provides information on the reactive and available oxygen atoms related to the WGS redox mechanism [20,41]. The OSC intimately relates to the sulfur tolerance of the catalyst according to previous studies [17,18]. The catalyst with the higher OSC showed high sulfur tolerance [17,18]. In particular, the CeO2 support contributes to the redox reaction by providing mobile oxygen due to the unique characteristics (e.g., rich oxygen vacancies and ease of change between Ce3+ and Ce4+) to promote redox [19,50,52]. A platinum-based catalyst supported on a reducing oxide such as ceria can exhibit superior redox properties due to its high OSC [21]. This explained the generation of oxygen vacancies through oxygen reverse spillover according to previous reports [18,23]. The trend of the oxygen-storage capacity of PtCe catalysts showed the following order: PtCe-I (4.68 × 10−4 gmol/gcat) > PtCe-C (4.11 × 10−4 gmol/gcat) > PtCe-S (1.85 × 10−4 gmol/gcat) > PtCe-H (1.45 × 10−4 gmol/gcat). This result indicated that the oxygen-storage capacity of the catalyst can be changed by controlling the synthesis method. The PtCe-I catalyst exhibited the best redox properties which originated from the high OSC of the catalysts. This was demonstrated by showing the same trend in the H2-TPR, XPS, and H2-O2 pulse reaction. Thus, the PtCe-I catalyst is expected to exhibit high WGS activity and sulfur resistance among the prepared samples.
To investigate the structural information about the lattice disorder and defects of the prepared catalysts, Raman spectroscopy was conducted, and the results are shown in Figure 4. In all cases, a band at approximately 460 cm−1, corresponding to the F2g triple degenerate vibrational mode of the fluorite structure (face-centered cubic) of the CeO2, was observed [53,54]. Since this mode is very sensitive to the vibrations of oxygen around the Ce ions, changes in the state of Ce ions or oxygen lattice have a significant effect on this mode [55]. The peak of the corresponding mode changes to be broader and asymmetric due to the local structure distortion of CeO2. This distortion causes defects in the CeO2 structure, which are related to the oxygen vacancies. Thus, the broadening and asymmetry of the main peak at 460 cm−1 are related to the oxygen vacancies in the CeO2 structure [50,54,56]. The PtCe-I catalyst had a noticeably broadened peak, followed by the PtCe-C catalyst, which had a weak and broad peak. The PtCe-S and PtCe-H catalysts showed relatively sharp and clear peaks. This trend agreed with the OSC results in the XPS and H2-O2 pulse reaction.
Figure 5 shows the TEM images and EDS mapping results of all reduced samples. The extent and distribution of Pt species exposed on the surface varied according to the synthesis method. This suggests that the Pt dispersion is affected by the synthesis method, which is consistent with the trends in the CO-chemisorption (Table 1) and XPS results (Figure 3A). In addition, TEM images and particle size distribution results of reduced and poisoned CeO2-supported Pt catalysts are exhibited in Figure S4. The TEM images of all the reduced and poisoned catalysts showed a mixed morphology of particles with irregular shapes. It is difficult to clearly distinguish Pt particles in both the reduced and poisoned TEM images. The PtCe-I catalyst showed relatively smaller particle sizes compared to other catalysts.
As shown in Figure 6A, the CO conversion of the CeO2-supported Pt catalysts showed a clear difference depending on the synthesis method. Each catalyst was tested under two conditions, and the results are shown for each section. First, the intrinsic catalytic activity for the WGS reaction using waste-derived synthesis gas without sulfur components was confirmed. In the next step, the sulfur tolerance of the catalyst was evaluated for 100 h with an injection of H2S (500 ppm). Sulfur contained in the feed gas is adsorbed on the catalyst surface and acts as a poison that decreases the catalyst activity [15,16]. As can be seen from TEM and EDS mapping images in Figure S5, sulfur was adsorbed to the catalyst. As expected from the characterization results, the PtCe-I catalyst exhibited the highest catalytic activity. In particular, the catalyst prepared by the impregnation method not only showed a high CO conversion of 90%, but also maintained a high catalytic activity without significant deactivation, even after sulfur injection. The PtCe-S catalyst exhibited the second highest intrinsic catalytic activity and sulfur tolerance. Conversely, the PtCe-H catalyst showed lower intrinsic catalytic activity than the previous two catalysts and was more rapidly deactivated in the presence of sulfur. In the case of PtCe-C catalyst, the lowest CO conversion was observed. Here, the intrinsic catalytic activity in sulfur-free condition of the catalysts was most affected by Pt0 dispersion. The PtCe-I catalyst showed the highest Pt0 dispersion, followed by the PtCe-S and PtCe-H catalysts. However, the PtCe-C catalyst showed a remarkably low active species dispersion. This indicates that there are relatively few active species present on the catalyst surface that provide sites for the reaction. Thus, it can be considered that this result affects the lower catalytic performance. All other catalysts except the PtCe-C catalyst showed 100% CO2 selectivity during the reaction (Figure 6B). On the other hand, the PtCe-C catalyst exhibited methanation (CO + 3H2 → CH4 + H2O), a major side reaction of the WGS reaction [57]. This is because the site for adsorption of the reactants was not sufficiently provided due to the remarkably low Pt dispersion of the PtCe-C catalyst. The numerical values related to the WGS reaction results of the catalysts are summarized in Table S1. To understand the intrinsic catalytic activity of the catalyst more clearly, turnover frequency (TOF) was calculated from the CO-chemisorption analysis and the WGS reaction results (Table 3). It is interesting that the PtCe-I catalyst exhibits similar TOF values to PtCe-S and PtCe-H catalysts, even when excluding the effect of Pt dispersion. This indicates that other catalytic properties such as OSC influence the intrinsic activity of CeO2-supported Pt catalysts.
It has been previously reported that the OSC of the catalyst is a critical factor along with the dispersion of Pt in the WGS reaction [17,18]. In the WGS reaction, oxygen vacancies occurring as mobile oxygen in the lattice of CeO2 oxidize the reactant CO to produce the product CO2. Subsequently, oxygen is stored in the oxygen vacancies from H2O dissociation. The higher the OSC value of the catalyst, the higher the retention of mobile oxygen that promotes the redox mechanism of the WGS reaction. Therefore, the high OSC value of the catalyst is related to excellent WGS reaction activity. Among the prepared samples, the PtCe-I catalyst exhibited the highest OSC value in the H2-TPR, XPS (ratio of OD and Ce3+), and H2-O2 pulse reaction. Additionally, it is reported that the mobile oxygen derived from CeO2 reacts with the attached sulfurs on the active sites of the catalytic surface (S + mobile O → SO2) and causes a regeneration of the catalytic performance [18]. Therefore, the OSC value is considered to be the most important factor in evaluating the sulfur tolerance of the catalysts. The PtCe-I catalyst showed the lowest activity loss and thus the highest sulfur tolerance. However, the PtCe-C catalyst showed low catalytic activity despite the relatively high OSC values. This is because the PtCe-C catalyst has a significantly lower number of activated Pt on the surface of the catalyst than that of the other catalysts, and thus the intrinsic activity of the catalyst is low. Nevertheless, OSC is an important factor in sulfur tolerance given the low decrease values of CO conversion in the PtCe-C catalyst. For this reason, the PtCe-I catalyst appears to exhibit high catalytic activity and superior sulfur tolerance due to the excellent redox properties and dispersion of the Pt active species on the catalytic surface.
The stability of the PtCe-I catalyst was further evaluated in sulfur-injection shut-down/re-start cycles (Figure 7). The cycle test was conducted 4 times in 100 h. The PtCe-I catalyst maintained high catalytic activity and sulfur resistance even during multiple poisoning and regeneration operations. Furthermore, it was shown that the catalytic activity of the catalyst was almost recovered at every cycle.
In Figure 8, we summarize the characterization results and the catalytic performance of the CeO2-supported Pt catalysts prepared by various synthesis methods. The results showed a strong relationship between the physicochemical properties and the catalytic performance of the catalyst. The intrinsic catalytic activity followed the trend of active metal dispersion and, in part, also followed the OSC trend. The OSC of the catalysts affects sulfur tolerance of the catalyst. The decreased rate of the CO conversion (green color) shows the same trend as the OSC data (blue color) confirmed from H2-O2 pulse reaction. As a result, the PtCe-I catalyst shows the highest intrinsic catalytic activity among the prepared catalysts. The PtCe-I catalyst also showed the lowest activity loss and thus the highest sulfur tolerance. As a result, it can be concluded that the active metal dispersion and oxygen-storage capacity of the catalyst are the key factors in the catalytic activity and the sulfur tolerance of the catalyst.

3. Materials and Methods

3.1. Catalyst Synthesis

CeO2-supported Pt (98 wt% CeO2 support, 2 wt% Pt) catalysts were prepared by various synthesis methods (IWI, SG, HT, and CP). For catalyst synthesis, stoichiometrically-calculated Ce(NO3)3·6H2O (99%, Sigma Aldrich, St. Louis, MO, USA) and [Pt(NH3)4](NO3)2 (50% Pt basis, Sigma Aldrich, St. Louis, MO, USA) were used as precursors. The theoretical Pt loading was fixed at 2 wt%. Table S2 shows the actual Pt content of the catalysts determined by inductively-coupled plasma-optical emission spectrometry (ICP-OES). The details are provided in each section.

3.1.1. Incipient Wetness Impregnation Method

In the case of the PtCe-I catalyst, the pure CeO2 support was prepared by the precipitation method before the Pt impregnation process. The precursors of Ce were dissolved in deionized water and heated to 80 °C. The mixed solution was treated with dropwise addition of 15 wt% KOH (95%, Samchun Chemicals, Siheung, Republic of Korea) solution until the pH reached 10.5 at 80 °C. Next, the mixture was aged with magnetic stirring for 72 h at 80 °C. The aged solution was washed with deionized water and filtered. The precipitate was dried in an oven overnight at 100 °C and calcined under an air atmosphere at 500 °C in a furnace for 6 h. The Pt precursor dissolved in deionized water (0.5 mL) was impregnated by dropwise addition into the calcined CeO2 support. The impregnated mixture was dried and calcined under the same conditions as for pure CeO2.

3.1.2. Sol-Gel Method

The sol-gel method was used to synthesize the PtCe-S catalyst. Stoichiometric quantities of Pt, Ce precursors, and citric acid were dissolved in 20 mL of deionized water, and the solution was then stirred at 80 °C for 24 h. The gel was formed by citric acid acting as a chelating agent during the slow evaporation of water. The generated gel was heat-treated at 100 °C for 4 h to obtain a swollen mass. This product was then calcined at 500 °C in furnace for 6 h with air.

3.1.3. Hydrothermal Method

To obtain the PtCe-H catalyst, stoichiometric quantities of Pt and Ce precursors were dissolved in 60 mL of deionized water and stirred with a magnetic stirrer at room temperature. After complete dissolution, the pH was adjusted to 10 at 80 °C with an injection of an aqueous 15 wt% NaOH (98%, Daejung Chemicals Co., Pyeongtaek, Republic of Korea) solution. Then, the precipitated solution was heated in a hydrothermal autoclave at 180 °C for 24 h. After this heat treatment, the solution was rinsed and filtered with deionized water to remove impurities. The solid product was dried in an oven overnight at 100 °C and calcined at 500 °C in a furnace for 6 h under an air atmosphere.

3.1.4. Co-Precipitation Method

The PtCe-C catalyst was prepared by the co-precipitation method. The co-precipitation method was carried out using the same process as the precipitation method of the support in the incipient wetness impregnation method section. However, the PtCe-C catalyst was prepared by dissolving the Pt and Ce precursors in deionized water all at once. The mixed solution was heated to 80 °C, and then titration, aging, washing, and filtration processes were sequentially performed. The filtered mixture was then dried and calcined under the conditions described above.

3.2. Catalytic Reaction

The PtCe catalysts prepared by various synthesis methods were tested at 400 °C for evaluation of catalytic performance and sulfur tolerance. A total of 30 mg of catalyst was loaded in a microtubular quartz reactor (I.D. 4 mm). The WGS reaction was performed in a fixed-bed reactor at ambient pressure. Prior to the catalytic test, each catalyst was reduced in situ at 400 °C to form the active phase (PtOx → Pt0). The reactant gas produced through waste gasification consisted of CO (39.70 vol%), CO2 (21.50 vol%), CH4 (2.35 vol%), H2 (27.05 vol%), and N2 (9.40 vol%). For the WGS reaction, steam was injected at an H2O/C ratio of 2.0, and the gas hourly space velocity (GHSV) was set to 46,000 h−1. To confirm the catalytic performance in the WGS reaction using waste-derived synthesis gas, the intrinsic catalytic activity of the CeO2-supported Pt catalysts was evaluated under the above conditions without the injection of sulfur. Then, to account for the sulfur components contained in the waste-derived syngas, a gas containing sulfur was injected. A 1.0% H2S/Ar gas was injected at a sulfur component ratio of 500 ppm in the total inlet gas. After the WGS reaction, sulfur components and moisture contained in the reaction gas, which cause corrosion and failure of the gas detector, were removed. The compositions of gas products was analyzed by a micro gas analyzer. The detailed procedures and information for the reaction condition such as reaction apparatus, pre-reduction treatment, and sulfur-trapping process after the reaction are reported in our previous work [17]. The CO conversion rate, CO2 and CH4 selectivity, and TOF were calculated using the following formulas (Equations (3)–(6)):
CO conversion (%) = ([CO]in − [CO]out)/[CO]in × 100
CO2 selectivity (%) = ([CO2]out − [CO2]in)/(([CH4]out − [CH4]in) + ([CO2]out − [CO2]in)) × 100
CH4 selectivity (%) = ([CH4]out − [CH4]in)/(([CH4]out − [CH4]in) + ([CO2]out − [CO2]in)) × 100
TOF (s−1) = (([CO]in − [CO]out) × ABM × F))/(D × W × XM)
where ABM is the atomic weight of metal (M), F is the total flow rate (mol/s), D is the metal dispersion, W is the mass of catalyst (g), and XM is the metal content (gmetal/gcat).

3.3. Characterization

The metal concentration of prepared catalysts was analyzed by ICP-OES (IRIS-Thermo Jarrell Ash Co., Waltham, MA, USA). The physicochemical properties of the catalysts were investigated by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) analysis, CO pulse chemisorption, H2-temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), H2-O2 pulse reaction, Raman spectroscopy, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The detailed experimental procedure for all of the above analyses except Raman spectroscopy are mentioned in our previous study [17]. In this study, Raman spectra were recorded using a Horiba Jobin Yvon-LabRam Aramis spectrometer (Longjumeau, France) with a 532 nm excitation line of an Nd-YAG laser.

4. Conclusions

The CeO2-supported Pt catalysts prepared by various methods were tested for the WGS reaction in a waste-upgrading process. Results showed that the PtCe-I catalyst exhibited the highest Pt dispersion and oxygen-storage capacity. The PtCe-I catalyst also showed superior catalytic activity with high sulfur tolerance compared with the other prepared catalysts. The dispersed Pt on the surface provides a site where CO is adsorbed for a reaction in the redox mechanism of the WGS reaction. The mobile oxygen of the catalyst reacts with CO to promote the WGS reaction and reacts with sulfur to assist the regeneration of the catalytic performance. Through these results, we conclude that the Pt dispersion of the catalyst and the oxygen-storage capacity are important factors for the catalytic performance and sulfur tolerance of the WGS reaction. In conclusion, we found that the optimal catalytic synthesis method is the incipient wetness impregnation method in the CeO2-supported Pt catalysts for the WGS reaction using waste-derived synthesis gas.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121670/s1, Figure S1: Adsorption-desorption isotherms of CeO2-supported Pt catalysts prepared by various synthesis methods; Figure S2: Pore distribution of CeO2-supported Pt catalysts prepared by various synthesis methods; Figure S3: XPS Pt 4f spectra of poisoned CeO2-supported Pt catalysts prepared by various synthesis methods; Figure S4: TEM images and particle size distribution results of CeO2-supported Pt catalysts prepared by various synthesis methods; Figure S5: TEM images and EDS mapping results of poisoned CeO2-supported Pt catalysts prepared by various synthesis methods; Table S1: Numerical values of catalytic performance at initial (0 h) and poisoned (15 h) points of CeO2-supported Pt catalysts prepared by various synthesis methods; Table S2: The actual Pt contents of as-synthesized catalysts determined by ICP-OES.

Author Contributions

Conceptualization, G.-R.H. and K.-J.K.; Data curation, G.-R.H., K.-J.K. and Y.-L.L.; Formal analysis, G.-R.H. and K.-J.K.; Funding acquisition, H.-S.R.; Investigation, G.-R.H.; Methodology, S.-Y.A., B.-J.K. and H.-R.P.; Project administration, H.-S.R.; Resources, G.-R.H. and K.-J.K.; Supervision, S.S.L., Y.J. and H.-S.R.; Validation, G.-R.H.; Visualization, G.-R.H.; Writing—original draft, G.-R.H.; Writing—review and editing, G.-R.H., K.-J.K., S.-Y.A., B.-J.K., H.-R.P. and Y.-L.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2020R1A2B5B01002346 and 2022R1A4A1029632). This research was also supported by “Regional Innovation Strategy (RIS)” through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (2022RIS-005).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01670 g001 550
Figure 1. XRD patterns of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 1. XRD patterns of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
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Figure 2. H2-TPR profiles of CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 2. H2-TPR profiles of CeO2-supported Pt catalysts prepared by various synthesis methods.
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Catalysts 12 01670 g003 550
Figure 3. XPS spectra of reduced CeO2-supported Pt catalysts prepared by various synthesis methods. (A) Pt 4f; (B) O 1s; (C) Ce 3d.
Figure 3. XPS spectra of reduced CeO2-supported Pt catalysts prepared by various synthesis methods. (A) Pt 4f; (B) O 1s; (C) Ce 3d.
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Catalysts 12 01670 g004 550
Figure 4. Raman spectra of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 4. Raman spectra of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
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Catalysts 12 01670 g005 550
Figure 5. TEM images and EDS mapping results of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 5. TEM images and EDS mapping results of reduced CeO2-supported Pt catalysts prepared by various synthesis methods.
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Catalysts 12 01670 g006 550
Figure 6. (A) CO conversion and (B) CO2 and CH4 selectivity as a function of time on stream over CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 6. (A) CO conversion and (B) CO2 and CH4 selectivity as a function of time on stream over CeO2-supported Pt catalysts prepared by various synthesis methods.
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Catalysts 12 01670 g007 550
Figure 7. Sulfur-injection shutdown/restart cycle tests over PtCe-I catalyst.
Figure 7. Sulfur-injection shutdown/restart cycle tests over PtCe-I catalyst.
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Figure 8. Relationship between the physicochemical properties and the performance of CeO2-supported Pt catalysts prepared by various synthesis methods.
Figure 8. Relationship between the physicochemical properties and the performance of CeO2-supported Pt catalysts prepared by various synthesis methods.
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Table
Table 1. Physicochemical properties of CeO2-supported Pt catalysts prepared by various synthesis methods.
Table 1. Physicochemical properties of CeO2-supported Pt catalysts prepared by various synthesis methods.
CatalystS.A.
(m2/g) a		Pore Volume
(cm3/g) a		Pt0 Dispersion
(%) b		Pt0 Particle Size
(nm) b		H2 Consumption
(mmol/g) c
PtCe-I1070.41589.91.058.29
PtCe-S 670.21653.41.775.47
PtCe-H 780.35736.22.603.52
PtCe-C1320.308 2.439.36.23
a Estimated by N2–adsorption/desorption at −196 °C; b Estimated from CO–chemisorption; c Estimated from H2–TPR analysis.
Table
Table 2. Oxygen-storage capacity-related values of CeO2-supported Pt catalysts prepared by various synthesis methods.
Table 2. Oxygen-storage capacity-related values of CeO2-supported Pt catalysts prepared by various synthesis methods.
CatalystOD
(%) a		Ce3+
(%) b		OSC
(10−4 gmol/gcat) c
PtCe-I22.1832.214.68
PtCe-S11.6629.571.85
PtCe-H10.3528.191.45
PtCe-C18.3131.044.11
a Estimated from XPS O 1s spectra. b Estimated from XPS Ce 3d spectra. c Estimated from the H2-O2 pulse reaction.
Table
Table 3. Turnover frequency results over CeO2-supported Pt catalysts prepared by various synthesis methods.
Table 3. Turnover frequency results over CeO2-supported Pt catalysts prepared by various synthesis methods.
CatalystTurnover Frequency (s−1)
PtCe-I0.010
PtCe-S0.015
PtCe-H0.015
PtCe-C0.071
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Hong, G.-R.; Kim, K.-J.; Ahn, S.-Y.; Kim, B.-J.; Park, H.-R.; Lee, Y.-L.; Lee, S.S.; Jeon, Y.; Roh, H.-S. Sulfur-Resistant CeO2-Supported Pt Catalyst for Waste-to-Hydrogen: Effect of Catalyst Synthesis Method. Catalysts 2022, 12, 1670. https://doi.org/10.3390/catal12121670

AMA Style

Hong G-R, Kim K-J, Ahn S-Y, Kim B-J, Park H-R, Lee Y-L, Lee SS, Jeon Y, Roh H-S. Sulfur-Resistant CeO2-Supported Pt Catalyst for Waste-to-Hydrogen: Effect of Catalyst Synthesis Method. Catalysts. 2022; 12(12):1670. https://doi.org/10.3390/catal12121670

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Hong, Ga-Ram, Kyoung-Jin Kim, Seon-Yong Ahn, Beom-Jun Kim, Ho-Ryong Park, Yeol-Lim Lee, Sang Soo Lee, Yukwon Jeon, and Hyun-Seog Roh. 2022. "Sulfur-Resistant CeO2-Supported Pt Catalyst for Waste-to-Hydrogen: Effect of Catalyst Synthesis Method" Catalysts 12, no. 12: 1670. https://doi.org/10.3390/catal12121670

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