Effect of support on the performance of PtRu-based catalysts in oxidative steam reforming of ethanol to produce hydrogen

Oxidative steam reforming of ethanol (OSRE) to produce hydrogen has been investigated over a series of supported PtRu catalysts, with different supports. Bimetallic PtRu-based catalysts were prepared by the impregnation method using H₂PtCl₆ and RuCl₃ as precursors. Six supports (reducible oxides of ZrO₂, CeO₂, and Co₃O₄, and irreducible oxides of ZnO, Al₂O₃, and NiO) were chosen to fabricate bimetallic catalysts. The catalytic performance of the OSRE reaction in the series of PtRu-based samples was evaluated using a fixed-bed flow reactor under atmospheric pressure. In front reaction, the catalyst was pre-activated by reduction under 200°C for 3 h. The gas hourly space velocity was adjusted at 66,000 h⁻¹, and the optimal molar ratios of the H₂O/EtOH and O₂/EtOH feeds were 4.9 and 0.44, respectively. The results indicated that the PtRu supported on the ZrO₂ and CeO₂ exhibited superior catalytic performance in the OSRE reaction under a low temperature (a T_R of approximately 320°C) for producing the main products of H₂ and CO₂ with lower CO and CH₄ by-products. Also, it was quite stable during a long time evaluation; the maximum Y_H2 maintained at 4.5–4.2, and the CO distribution approached 3.3–3.5 mol% around 84 h test at 340°C over the PtRu/ZrO₂ catalyst.

1 Introduction

Hydrogen is a versatile element that is often used as a raw material for gasoline (the processes of hydrotreating and hydrocracking), chemistry (the synthesis of NH₃ and CH₃OH), foodstuff processing (hydrogenation of fats and oils), steel, and the manufacturing of electronics (Ramachandran and Menon, 1998; Armor, 1999). However, current industry does not produce hydrogen as an energy carrier or as a fuel for generators. In order to support sustainable global economic growth as well as reduce air pollution and the greenhouse effect, it is urgent to adopt hydrogen as an energy carrier; however, several technical hurdles must be conquered to manufacture and transfer the amount of hydrogen needed to reach a hydrogen-based economy.

Hydrogen can be manufactured from oxygenates, such as various ethanol reforming processes (Goldemberg, 2007; Ni et al., 2007; de la Piscina and Homs, 2008; Rabenstein and Hacker, 2008; Zanchet et al., 2015; Li et al., 2016) including the partial oxidation of ethanol (POE), steam reforming of ethanol (SRE), and oxidative steam reforming of ethanol (OSRE). The OSRE reaction is the incorporation of partial oxidation and steam reforming processes. Moreover, the OSRE reaction can lessen the energy of the SRE process and lower the rate of coke formation. Thermodynamic analysis of the production of hydrogen from ethanol by catalytic processes has found that the OSRE process possesses many advantages in terms of heat management and reforming efficiency (Rabenstein and Hacker, 2008).

Besides choosing a single active component (a noble or non-noble metal) as the reforming catalyst, multi-component catalysts have been reported in various catalytic reactions (Bi et al., 2007a; Alayoglu et al., 2008; Pereira et al., 2008; Koh et al., 2009; Pereira et al., 2010). Pereira et al. (Pereira et al., 2010) stated that the K-promoter enhances the catalytic behavior of CoRh/CeZr under the OSRE process to produce hydrogen, in which the addition of Rh facilitates the reducibility of Co. and enhances the catalytic activity. The same group also reported the effect of adding Ru and Rh to the Co./SiO₂ catalysts and found a synergistic effect between Co and Ru (or Rh) in the OSRE reaction (Pereira et al., 2008). In particular, bimetallic PtRu catalysts have been widely used as CO tolerant for polymer electrolyte fuel cells (PEFCs) (Alayoglu et al., 2008). An investigation on the catalytic ability of PtRu-based catalysts for hydrogen production in reforming reactions confirmed that PtRu/ZrO₂ is an excellent catalyst for low temperature OSRE reaction (Bi et al., 2007a). A nano-sized PtRu/Al₂O₃ originated from the organometallic clusters, which exhibited a high capability to manufacture hydrogen under the SRE reaction (Koh et al., 2009).

The performance of OSRE not only depends on the active metals; the other key factor is the nature of the support. To enhance the catalytic behavior of supported metal catalysts, support has influenced the catalytic performance (Llorca et al., 2002; Furtado et al., 2009; Youn et al., 2010; Wang and Spivey, 2015; Zhang et al., 2015; Tóth et al., 2016; Hsia et al., 2019). The effect of the support’s (ZnO, MgO, ZrO₂, TiO₂, and Al₂O₃) acidity on the OSRE activity of supported nickel catalysts was explored by Youn et al. (Youn et al., 2010), in which the intermediate acidity of Ni/Ti_0.2Zr_0.8O₂ was identified as the optimal catalyst. The evaluation of the support materials (Al₂O₃, Nb₂O₅, and Ce_0.6Zr_0.4O₂) of a bimetallic NiCu-based catalyst on the SRE reaction showed that the NiCu/Ce_0.6Zr_0.4O₂ could achieve the best performance for hydrogen production (Furtado et al., 2009).

In our previous work, a PtRu/ZrO₂ catalyst modified with Na in an OSRE indicates active at 300°C and begets little CO at 340°C (Wang et al., 2011). Here in, the OSRE reaction was further studied over series PtRu-based catalysts. Six supports (reducible oxides of ZrO₂, CeO₂, and Co₃O₄, and irreducible oxides of ZnO, Al₂O₃, and NiO) were picked to fabricate supported bimetallic catalysts. The goal of this effort was to find an excellent and durable catalyst that could be applied to the on-board reforming of ethanol at low temperatures.

2 Experimental

2.1 Preparation of catalysts

Six supports (ZrO₂, CeO₂, Co₃O₄, ZnO, Al₂O₃, and NiO) were chosen to prepare the supported bimetallic catalysts. The Al₂O₃ was purchased from Merck (γ-form, 142 m²⋅g⁻¹), while the others were self-synthesized. The ZrO₂ (130 m²⋅g⁻¹) was prepared using the sol-gel method (Bi et al., 2007a), the CeO₂ (106 m²⋅g⁻¹) was prepared using a simple reduction-oxidation method (Siang et al., 2010), the Co₃O₄ (102 m²⋅g⁻¹) and NiO (104 m²⋅g⁻¹) were prepared by the precipitation-oxidation method (Wang et al., 2005a; Lai et al., 2006), and the ZnO (89 m²⋅g⁻¹) was obtained through the thermal decomposition of Zn(OH)₂·2ZnCO₃·xH₂O (Stream) in the air (Chiou et al., 2012), respectively. The supported PtRu catalysts were prepared by the impregnation method using an aqueous solution of H₂PtCl₆ (Merck) and RuCl₃ (Strem) as precursors (the loading of each component was 1.5 wt%). The as-prepared sample went through drying at 110°C and calcination at 400°C for 4 h in the air, after which the fabricated samples were smashed to 60–80 mesh and kept as fresh catalysts (both metal content and surface area are listed in Table 1).

TABLE 1

TABLE 1. Surface area and metal content of series PtRu-based catalysts.

2.2 Physicochemical characterizations

Both the Pt and the Ru contents in the fresh catalysts were ascertained by the measurement of ICP-MS (Perkin-Elmer). The surface area (S_BET) was measured with the physisorption using a Micromeritics ASAP 2010 instrument. First, the catalysts were pre-outgassed in a vacuum for 3 h at 110°C, then, the S_BET was measured by the N₂ adsorption at 77 K. The phases and crystalline structures were surveyed with X-ray diffraction (XRD) using a MAC Science MXP18 diffractometer with Cu K_α1 radiation (λ = 1.5405 Ǻ) at 30 mA and 40 kV. The particle size was observed using transmission electron microscopy (TEM) images from a JEM-2010 transmission electron microscope (JEOL) with a 200 kV acceleration voltage. During the TPR experiment, a 50 mg sample located in the reactor and was heated at a ramping rate of 7°C⋅min⁻¹ from RT to 500°C under a reducing gas (10% H₂/N₂) with a flow rate of 10 ml⋅min⁻¹. The consumption of H₂ was detected continuously using a thermal conductivity detector (TCD).

2.3 Activity test of catalysts

A fixed-bed flow reactor was chosen to evaluate the performance of the PtRu-based catalysts towards the OSRE reaction under atmospheric pressure. A 100-mg sample was placed in a quartz tubular reactor (4 mm i. d.) and guarded with glass-wool plugs. The reactor was twined with heating tape and the temperature was controlled using a thermocouple (1.2 mm i. d.) placed in the center of the reactor bed. Prior to the reaction, the catalyst was pre-activated by reduction at 200°C for 3 h. The catalytic activity of the series of supported PtRu catalysts towards OSRE was executed by tuning the molar ratio of the H₂O/EtOH (0.8–13) and O₂/EtOH (0.32–0.61), which was executed between 280°C and 600°C under atmospheric pressure. The outlet gas was detected by both gas chromatography (GC), one with an MS-5A column (for the separation of H₂, O₂, CH₄, and CO), and the other with a Porapak Q column (for the separation of C₂H₅OH, H₂O, CH₃CHO, CH₃COOC₂H₅, and CO₂). Based on the ethanol conversion (X_EtOH), selectivity of carbon monoxide (S_CO) and yield of hydrogen (Y_H2) to evaluate the catalytic performance as follows:

${\mathrm{X}}_{\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}}\left(\%\right)=\raisebox{1ex}{$\left[\mathrm{n}{\left(\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}\right)}_{\mathrm{i}\mathrm{n}}–n{\left(\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.{\left(\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}\right)}_{\mathrm{i}\mathrm{n}}\times 100\%(1)$

${\mathrm{S}}_{\mathrm{C}\mathrm{O}}\left(\%\right)=\raisebox{1ex}{${\mathrm{n}\left(\mathrm{C}\mathrm{O}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}$}\!\left/ \!\raisebox{-1ex}{$\left[\mathrm{n}{\left({\mathrm{C}\mathrm{H}}_4\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}+\mathrm{n}{\left(\mathrm{C}\mathrm{O}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}+\mathrm{n}{\left({\mathrm{C}\mathrm{O}}_2\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]$}\right.\times 100\%(2)$

${\mathrm{Y}}_{\mathrm{H}2}=\raisebox{1ex}{$\mathrm{n}{\left({\mathrm{H}}_2\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}$}\!\left/ \!\raisebox{-1ex}{$\left[\mathrm{n}{\left(\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}\right)}_{\mathrm{i}\mathrm{n}}–n{\left(\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}\right)}_{\mathrm{o}\mathrm{u}\mathrm{t}}\right]$}\right.(3)$

3 Results and discussion

3.1 Characterization of catalysts

Figure 1A indicates the XRD patterns of the six bimetallic PtRu catalysts. Except for the PtRu/Al₂O₃ and PtRu/ZnO catalysts, which exhibited a very faint RuO₂ phase, no significant diffraction peak of PtRu (111) was observed in all catalysts. This result demonstrated that the nanoparticles of the bimetallic PtRu were smaller than 5 nm. The crystal phase of metal oxides is conspicuous, i.e. the unique diffraction peaks of PtRu/NiO at 37.6^o, 43.6^o, 63.2^o, and 75.7^o, corresponding to the (111), (200), (220) and (311) planes, respectively, were attributed to the cubic NiO (JCPDS 04-0835); the clear diffraction peaks of PtRu/Co₃O₄ at 31.5^o, 37.1^o, 38.8^o, 45.1^o, 55.9^o, 59.6^o and 65.5^o, corresponding to the (220), (311), (222), (400), (422), (511) and (440) planes, respectively, were ascribed to the spinel Co₃O₄ (JCPDS 43-1003); the noticeable diffraction peaks of PtRu/ZrO₂ at 30.4^o, 35.4^o, 50.4^o and 60.4^o, corresponding to the (111), (200), (220) and (311) planes, respectively, were associated with the tetragonal ZrO₂ (JCPDS 79-1769); the distinct diffraction peaks of PtRu/ZnO at 31.9^o, 34.6^o, 36.4^o, 47.8^o, 56.8^o, 63.1^o, 68.2^o and 69.3^o, corresponding to the (100), (002), (101), (102), (110), (103), (112) and (201) planes respectively, were well indexed to the hexagonal ZnO (JCPDS 79-2205); the bright diffraction peaks of PtRu/CeO₂ at 28.5^o, 33.1^o, 47.5^o and 56.4^o corresponding to the (111), (200), (220) and (311) planes, respectively, were confirmed the cubic CeO₂ (JCPDS 34-0394); the bright diffraction peaks of PtRu/Al₂O₃ at 37.3^o, 45.9^o and 66.9^o, corresponding to the (311), (400) and (440) planes, respectively, were assigned to the γ-Al₂O₃ (JCPDS 10-0425). Figures 1B, C present the TEM images of the PtRu/CeO₂ and PtRu/ZrO₂, in which the average particle sizes of the PtRu were around 3.3 and 1.7 nm for both catalysts, respectively. The reducibility of the bimetallic PtRu catalysts was evaluated using temperature programmed reduction experiments (the TPR profile is listed in Figure 2). According to the literature (Trovarelli, 1996; Lin et al., 2003; Wang et al., 2005a; Wang et al., 2005b; Lai et al., 2006; Siang et al., 2010), the reduction of NiO occurs around 300°C (Wang et al., 2005b; Lai et al., 2006), the reduction of Co₃O₄ occurs around 200–400°C (Lin et al., 2003; Wang et al., 2005a), and the reduction of the surface capping oxygen ions of CeO₂ occurs above 400°C (Trovarelli, 1996; Siang et al., 2010). The results showed that in addition to the reduction of the active species (the bimetallic oxide reduced at below 200°C for all catalysts and the RuO₂ reduced at around 200°C–300°C for the PtRu/Al₂O₃ and PtRu/ZnO catalysts) (Bi et al., 2007a), the NiO, Co₃O₄, and the surface oxygen of the CeO₂ supports could also be reduced at temperatures below 500°C.

FIGURE 1

FIGURE 1. XRD patterns and TEM images of series PtRu-based catalysts: (A) XRD profile (B) TEM image of PtRu/CeO₂ catalyst (C) TEM image of PtRu/ZrO₂ catalyst.

FIGURE 2

FIGURE 2. TPR profiles of series PtRu-based catalysts.

3.2 Catalytic activity of oxidative steam reforming of ethanol reaction

3.2.1 Effect of feed molar ratio

For the purpose of verification, the optimal feeds on the molar ratio of H₂O/EtOH and O₂/EtOH, the PtRu/ZrO₂ sample has been chosen to tune both H₂O/EtOH and O₂/EtOH molar ratio, separately. First, a regular H₂O/EtOH molar ratio of 4.9 was transmitted to the reactor to adjust the O₂/EtOH molar ratio (0.32, 0.44, and 0.61, respectively). All situations revealed a complete conversion at and above 300°C. The distribution of products and the hydrogen yield are displayed in Table 2, which shows that the diminishing of the O₂/EtOH ratio inclines to grow in number the amount of by-products (CO and CH₄) and lessened the hydrogen yield from 280°C to 320°C. However, the raising of the O₂/EtOH molar ratio inclines to facilitate the oxidation of hydrogen and reduced the hydrogen yield. Based on the tuning of O₂/EtOH, 0.44 was the most appropriate molar ratio. Under this circumstance, both the Y_H2 improved and the CO reduced with the raising of the reaction temperature (T_R). Second, a definite O₂/EtOH molar ratio of 0.44 was delivered to the reactor to regulate the H₂O/EtOH molar ratio (0.8, 2.2, 4.9, and 13, respectively). The product distribution and hydrogen yield are exhibited in Table 3, which reveals that the diminishing of the H₂O/EtOH ratio inclines to boost the POE reaction, while the raising of the H₂O/EtOH ratio stimulates produce more CO side-product. According to the regulating of the H₂O/EtOH, 4.9 was the most appropriate molar ratio for the OSRE reaction, as it enhanced the hydrogen yield and reduced the CO distribution manifestly from 280°C to 320°C.

TABLE 2

TABLE 2. Effect of O₂/EtOH for OSRE reaction with various temperatures over the PtRu/ZrO₂ catalyst under a molar ratio of H₂O/EtOH = 4.9.

TABLE 3

TABLE 3. Effect of H₂O/EtOH for OSRE reaction with various temperatures over the PtRu/ZrO₂ catalyst under a molar ratio of O₂/EtOH = 0.44.

3.2.2 Effect of support

The literature (Mavrikakis and Barteau, 1998; Cavallaro et al., 2003; Fierro et al., 2005; Diagne et al., 2002) has reported that maintaining the OSRE process at a low temperature can preclude methanation (at an adequate temperature of around 500°C) and initiate large amounts of CO (at an adequate temperature of around 700°C). Based on the above consequents, the optimized feeds of the O₂/EtOH and H₂O/EtOH on the OSRE reaction were 0.44 and 4.9, respectively. Keeping this reforming condition, six bimetallic PtRu-based catalysts have been evaluated. Table 4 summarizes the conversion and distribution of products, and Figures 3, 4 gather up the Y_H2 and S_CO over the series of PtRu-based catalysts for the OSRE reaction under various temperatures. The curves in Figures 3, 4 were drawn based on the fitting algorithm of the B-spline curve brought into the experimental data. In order to discuss the uncertainty analysis for numerical data, each sample was evaluated three times separately and the results indicated that the error was within ±1%–5%. Apparently, a synergistic effect emerged between the metal oxide supports and PtRu active species. The ethanol could be converted completely at lower temperatures for the reducible oxides (CeO₂ and ZrO₂ ≈ 280°C, Co₃O₄ > 300°C) supported catalysts with only C₁ species (CH₄, CO, and CO₂), while the ethanol could be converted completely at higher temperatures for the irreducible oxides (Al₂O₃ > 350°C, NiO >400°C, and ZnO >450°C) supported catalysts with C₁ species and a trace amount of undesirable C₂H₄ and CH₃CHO species. Although the catalytic activity of the PtRu/Co₃O₄ catalyst was bright and the S_CO was lower, the Y_H2 was also lower than 3, and the long-time reaction impel a deactivation by the progressive deposited carbon. The Y_H2 was high for the PtRu/NiO catalyst, while the T_R approached a high temperature that initiated the decomposition of ethanol and increased the S_CO as the T_R moved above 500°C. Both the PtRu/Al₂O₃ and the PtRu/ZnO catalysts possessed a lower Y_H2 and higher S_CO. Bi et al. (Bi et al., 2007b) suggested that the reducible oxides could offer the lattice oxygen (O_L) to assist the oxidation of adsorbed CO (CO_ad), and the irreducible oxides lacked the lattice oxygen which made the oxygen molecules had to be adsorbed on the metal surface (O_ad). Since the O_L is more effective than O_ad for oxidation of CO_ad, therefore, a high S_CO produced easily from the desorption of CO_ad over the irreducible oxides, and the O_ad could integrate the adjacent adsorbed hydrogen to lower the Y_H2. Additionally, both the CeO₂ and the ZrO₂ can improve the dispersion of the active phase (Srinivas et al., 2003; Biswas and Kunzru, 2008), and possess large amount of surface oxygen vacancies that can restore/release oxygen (Hori et al., 1998; Silva et al., 2005) to prevent deactivation by the deposited coke and were active in the water gas shift (WGS) reaction (Gutierrez et al., 2011) to elevate the Y_H2 and lower the S_CO. Both the Co₃O₄ and the NiO have been reported to exhibit the performance C−C bond cleavage on reforming of ethanol and a high Y_H2, however, both deactivated easily via the sintering and/or carbon deposition over the catalyst surface (Abdelkader et al., 2013; Sekine et al., 2009; da Silva et al., 2011). The Al₂O₃ has low mobile oxygen vacancies (Duprez, 1997), there is a limited number of oxygen intermediates available for CO oxidation, and the acidic support can stimulate the dehydration of ethanol to ethylene (Fatsikostas and Verykios, 2004), which pursued a high S_CO and low Y_H2 production. Based on the demonstration and comparison of these reported literatures, in this study, both the PtRu/ZrO₂ and the PtRu/CeO₂ presented well-dispersed and excellent OSRE catalysts to produce hydrogen under low temperatures. The maximum Y_H2 approached 4.5 and the S_CO was 3.3 mol% at 340°C for the PtRu/ZrO₂ catalyst, while the Y_H2 was 4.0 and the CO distribution was 2.8 mol% for the PtRu/CeO₂ catalyst, respectively.

TABLE 4

TABLE 4. Catalytic performance in the OSRE reaction over series PtRu-based catalysts under O₂/EtOH = 0.44 and H₂O/EtOH = 4.9.

FIGURE 3

FIGURE 3. S_CO of series PtRu-based catalysts for OSRE reaction under various temperatures.

FIGURE 4

FIGURE 4. Y_H2 of series PtRu-based catalysts for OSRE reaction under various temperatures.

Further, in order to ascertain the features and derivations of the OSRE pathways over PtRu-based catalysts, the ethanol conversion along with the distribution of H₂, CO₂, CH₄, and CO products in the outlet stream as a function of the T_R was plotted between 260 and 380°C for both the PtRu/CeO₂ and the PtRu/ZrO₂ catalysts, as shown in Figures 5, 6. Summarizing with Table 4, also, comparing with other studies (Rabenstein and Hacker, 2008; Bi et al., 2007a; Mavrikakis and Barteau, 1998; Hung et al., 2012), the simplified reaction network in the OSRE process over the PtRu-based catalysts was outlined in Scheme 1. The involved intermediate steps included a variety of reactions which depended on the dependency relationships of different functionalities of the catalyst. Only minor ethylene produced over the PtRu/Al₂O₃ catalyst via the dehydration of adsorbed ethanol. All catalysts showed that the ethanol could be converted completely over the whole temperature range, and the consumption of oxygen was complete. The products of the H₂ and CO₂ increased with the T_R. Thus, the performance at relatively low temperatures (<300°C) was mainly due to the partial oxidation of ethanol by consuming the O₂ according to Eq. 4:

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+\raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\mathrm{O}}_2\to 2{\mathrm{C}\mathrm{O}}_2+3{\mathrm{H}}_2(4)$

FIGURE 5

FIGURE 5. Catalytic performance in the OSRE reaction over the PtRu/CeO₂ catalyst under O₂/EtOH = 0.44, H₂O/EtOH = 4.9.

FIGURE 6

FIGURE 6. Catalytic performance in the OSRE reaction over the PtRu/ZrO₂ catalyst under O₂/EtOH = 0.44, H₂O/EtOH = 4.9.

SCHEME 1

SCHEME 1. Reaction pathway for the OSRE reaction over the PtRu-based catalysts.

The key step of ethanol reforming is the cracking of the C−C bond. The C−C bond can be cracked easily through the use of ruthenium and platinum (Davda et al., 2005; Zhao et al., 2019; Rajabi et al., 2021; Zare et al., 2021). The initial partial ethanol oxidation provided the potential for the dehydrogenation of the ethanol (Eqs 5, 6) and its decarbonylation (Eq. 8) over the PtRu/ZrO₂ and PtRu/CeO₂ catalysts are quicker than PtRu/ZnO and PtRu/NiO (where * is the active site), where the later produced a minor CH₃CHO via desorption of adsorbed acetaldehyde.

${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}+\ast \to {\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}-\ast +{\mathrm{H}}_2(5)$

${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}-\ast \to {\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}-\ast +1/2{\mathrm{H}}_2(6)$

${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}-\ast \to {\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{H}\mathrm{O}+\ast (7)$

${\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}-\ast +\ast \to \mathrm{C}\mathrm{O}-\ast +{\mathrm{C}\mathrm{H}}_3-\ast (8)$

The amount of CH₄ was lower on both PtRu/ZrO₂ and PtRu/CeO₂ catalysts, indicating that the sequent steam reforming of the methyl (Eq. 9) after the splitting of the acetyl group (CH₃CO) was preferential over the formation of methane (Eq. 10).

${\mathrm{C}\mathrm{H}}_3-\ast +{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+5/2{\mathrm{H}}_2(9)$

${\mathrm{C}\mathrm{H}}_3-\ast +\mathrm{H}-\ast \to {\mathrm{C}\mathrm{H}}_4+2\ast (10)$

The S_CO over PtRu/CeO₂ was lower than PtRu/ZrO₂ catalyst, while the CO distribution over PtRu/ZrO₂ declined abruptly and accompanied the increase of CO₂ and H₂ as the T_R upon 300°C. Also, the amount of CH₄ grew and accompanied the decrease of H₂ as the T_R moved above 360°C. In accordance with these observations, the PtRu/CeO₂ catalyst could preferentially oxidize the CO (Eq. 11) in the hydrogen-rich gases at a lower T_R. The WGS (Eq. 12) reaction was favorable compared to the PtRu/ZrO₂ catalyst at temperatures above 300°C for lowering the amount of CO, and the increase of CH₄ coupled with the decrease of H₂ upon 360°C was attributed to the methanation of CO or CO₂ (Eq. 13).

$\mathrm{C}\mathrm{O}-\ast \left({\mathrm{H}}_2\right)+\mathrm{O}-\ast \to {\mathrm{C}\mathrm{O}}_2\left({\mathrm{H}}_2\right)+2\ast (11)$

$\mathrm{C}\mathrm{O}-\ast +{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2+\ast (12)$

$\mathrm{C}\mathrm{O}-\ast +3{\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}+\ast (13)$

3.2.3 Catalytic stability

Stability is a judgmental feature of any heterogeneous catalyst that decides its efficiency in a catalytic reaction. In order to highlight the discrepancy in the Y_H2 and S_CO over the three reducible oxide-supported PtRu catalysts for the OSRE reaction, the evaluation of the catalyst stability was carried out as a function of the time-on-flux at 340°C, as shown in Figure 7. Except for the PtRu/Co₃O₄ catalyst, in which the activity decayed quickly with the reaction time, the conversion of the EtOH remained complete within the test over PtRu/ZrO₂ and PtRu/CeO₂ catalysts. Under the OSRE reaction, the deactivation of the Co-based catalyst could be attributed to the deposited coke, sintering of the active phase, and the encapsulation of metal sites into the support (da Silva et al., 2010; Espitia-Sibaja et al., 2017). Figure 8 displays the TEM image of the fresh and spent PtRu/Co₃O₄ catalyst. Apparently, the well-dispersed PtRu nanoparticles were somewhat agglomerated and capsulated by the amorphous carbon through the OSRE reaction over the PtRu/Co₃O₄ catalyst. To affirm the amorphous carbon, further analysis with thermal analysis, and the TG/DTG profile of spent PtRu/Co₃O₄ catalyst was listed in Figure 9. Apparently, the deposited carbon (∼6.5%wt loss) could be oxidized around 427°C. Both the PtRu/ZrO₂ and PtRu/CeO₂ catalysts showed preferential durability and exceeded 80 h of stability, whereas the PtRu/Co₃O₄ catalyst was deactivated within 20 h by the deposited carbon. The maximum Y_H2 was maintained at 4.5–4.2 and the CO distribution approached 3.3–3.5 mol% on the PtRu/ZrO₂ catalyst. The maximum Y_H2 stayed at 4.0–3.6 and the CO distribution approached 2.8–3.2 mol% over the PtRu/CeO₂ catalyst, respectively, around 84 h test at 340. Many reactions are associated with the OSRE, the aims to pursue a high Y_H2 while generating minimum CO. Greluk et al. (Greluk et al., 2016) studied the OSRE (H₂O:EtOH:O₂ = 9:1:0.7) using PtKCo/CeO₂ catalyst, the temperature of 420°C was sufficient to achieve complete conversion but only maintained 5 h. Coke deposition that led to the removal of active sites from the catalyst surface was considered to be the major deactivation mechanism. The H₂ distribution decreased from 68% to 60%, and the CO distribution increased from 2% to 2.5% with the increase of process time. Casanovas et al. (Casanovas et al., 2006) investigated the OSRE (H₂O:EtOH:O₂ = 13:1:0.5) using Pd/ZnO catalyst at low temperatures (300°C − 450°C), the ethanol could be converted completely. The H₂ selectivity increased from 37% to 61%, and the CO selectivity decreased from 4.9% to 0.1% with increasing temperature. Palma et al. (Palma et al., 2017) surveyed the OSRE on mesoporous silica supported PteNi/CeO₂ catalysts, which displayed stable behavior for 135 h under 500°C. Therefore, the fabricated Pt-Ru/ZrO₂ showed potential as a candidate OSRE catalyst under low temperatures in this study.

FIGURE 7

FIGURE 7. Durability test for the OSRE reaction over PtRu/ZrO₂, PtRu/CeO₂ and PtRu/Co₃O₄ catalysts: (A) S_CO (B) Y_H2.

FIGURE 8

FIGURE 8. TEM images of PtRu/Co₃O₄ catalyst: (A) Fresh (B) Spent.

FIGURE 9

FIGURE 9. TG/DTG profiles of spent PtRu/Co₃O₄ catalyst.

4 Conclusion

This investigation indicated that the feed molar ratio and kinds of support have influence on the catalytic performance of PtRu-based catalysts. The optimal molar ratios of the H₂O/EtOH and O₂/EtOH feeds were 4.9 and 0.44, respectively for the OSRE reaction. In addition to the synergistic effect emerged between the metal oxide supports and PtRu active species, the reducible oxide supports also could provide the lattice oxygen to promote the oxidation of CO and the water gas shift reaction. Both the PtRu/ZrO₂ and the PtRu/CeO₂ catalysts appeared to be excellent OSRE catalysts to produce hydrogen under low temperatures with lesser distributions toward unwanted CO and CH₄ with complete ethanol conversion. Evidently, the PtRu supported on ZrO₂ and CeO₂ exhibited superior catalytic performance in the OSRE reaction. It was quite stable during a long time evaluation of more than 80 h; the maximum Y_H2 was maintained at 4.5–4.2 and the CO distribution approached 3.3–3.5 mol% around 84 h test at 340°C on the PtRu/ZrO₂ catalyst.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

C-HL: investigation, methodology, software. S-WY: investigation, methodology. C-WT: data curation, conceptualization, writing—original draft preparation. C-BW: resources, supervision, project administration, writing—review and editing.

Acknowledgments

We are pleased to acknowledge the financial support for this study from the Ministry of Science and Technology, Taiwan under contract number of MOST 108-2113-M-606-001-.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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