Tailoring Ceria-Based Nanocatalysts in Steam Reforming Processes

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1		Marco Aurélio Suller Garcia	--	4936	2023-08-25 20:09:29	\|
2	format correction	Dean Liu	-1 word(s)	4935	2023-08-28 04:15:59	\|

This entry is adapted from the peer-reviewed paper 10.3390/hydrogen4030033

Ceria-based nanostructures, employed as catalytic supports for noble and non-noble metals, are well-known for their remarkable activity in steam-reforming reactions, exceptional resistance to degradation, and thermal stability.

ceria shape-control nanoengineering Ni Co Pt

1. Introduction

Energy is vital in driving economic growth as all production operations and activities rely on it as a fundamental input. Hence, with the expansion of economies, energy demand has a corresponding rise, with renewable energy resources emerging as promising options for sustainable energy production ^[1]. In this scenario, energy sources such as solar, wind, hydro, geothermal, and biomass play a significant role as they are naturally replenished within a human timescale ^[2]. However, it is crucial to recognize that these energy sources are not directly manageable as their primary energy cannot be stored.

Although some strategies are being studied to overcome potential offer shortages, hydrogen (H₂) has emerged as part of the solution due to its chemical energy carrier properties ^[3], contributing significantly to decarbonizing dominant sectors and facilitating the achievement of net-zero CO₂ emissions by 2050 ^[4]. However, H₂ is predominantly found in compounds rather than in its free form, requiring the utilization of technologies that require substantial energy consumption for its obtaining ^[5]. In addition to the technical challenges associated with H₂ production, other undeniable concerns must be addressed, which include storage, transportation, safety, the integration of H₂ into the energy matrix, and the impact on greenhouse gas emissions ^[6]. Therefore, high costs, high energy consumption, and technical barriers present obstacles to the widespread adoption of H₂ as an energy reservoir.

Considering the challenges mentioned and the present circumstances, H₂ has not been widely regarded as a viable solution on a large scale due to persistent economic concerns and environmental impacts. However, its consumption is essential for fertilizer production in the ammonia industry, oil refining, methanol production, and iron and steel manufacturing ^[7]^[8]^[9]. Thus, attaining technological advancements and achieving economies of scale are crucial to making H₂ economically feasible. Technologies such as photochemical water splitting, bio-photolysis, photo- and dark-fermentation, and thermochemical and biomass-to-energy processes have attracted attention toward producing green H₂ that can help mitigate global temperature increases within the 2 °C limit ^[10]^[11]^[12]. Nevertheless, these technologies are still maturing for commercial viability.

Currently, the most prominent methods for H₂ production are electrochemical water splitting (electrolysis) and steam-reforming reactions, with steam–methane reforming being the dominant process ^[13]. The primary factor behind the limited utilization of electrolysis is the high cost associated with electricity, a crucial requirement for its operation; reforming reactions do not require such input. The non-utilization of electricity is especially relevant for sectors such as the production of chemicals and certain segments of the transportation industry where alternative decarbonization methods may not be feasible. In the long run, attention will also be directed toward addressing portions of the heat market. However, a recent study stated that direct H₂ technologies always increase the energy system’s cost and cannot be considered large-scale solutions for heating and transport, at least for now, and such sectors are critical for replacing the global carbon-based energy matrix ^[14]. Thus, it is crucial to continue reducing the cost of H₂ production, and many efforts are being directed toward steam-reforming reactions due to its stage of development.

2. Shape-Controlled Cerium-Oxide-Based Nanocatalysts with Ni and Co

Low-cost and available non-noble catalysts, particularly Ni-based catalysts, are extensively used in steam-reforming reactions. However, there is a limited understanding in the literature regarding the mechanisms of their active sites and the significance of metal–support interactions in Ni and cerium oxide systems. Furthermore, when such discussions are present, they often refer to specific reaction conditions and substrates, making it challenging to generalize the findings. However, as a general assumption, the combination of highly dispersed metal nanoparticles on oxide surfaces exhibits high reactivity ^[15]. In this context, ceria is crucial in preventing metal sintering by establishing strong metal–support interactions, demonstrating high reducibility. As a result, ceria serves as a vital promoter for maintaining the stability and activity of metal catalysts ^[16]. In low-loaded Ni catalysts supported on CeO₂, the presence of the ceria facilitates strong metal–support interactions, leading to the partial oxidation of Ni atoms in direct contact with the support ^[17]^[18]^[19], bringing significant changes in the chemical and catalytic properties of these systems, particularly in the cleavage of C–H and O–H bonds ^[20]^[21]^[22]^[23]^[24]. Moreover, the high reducibility of the ceria support allows it to act as an oxygen reservoir, enabling unique reaction pathways such as the reverse spillover of oxygen from ceria to metal sites, as discussed before. As a result, the remarkable decoking activity observed in ceria-supported Ni catalysts for methane steam reforming can be attributed to a mechanism involving the supply of oxygen from the support, facilitating the removal of carbon as CO, and water plays a crucial role as one of the reactants, reloading the oxygen vacancies generated during the reverse spillover step ^[25].

However, the scenario can be more complicated depending on the temperature at which the reaction is conducted and the substrate as Ni active species tend to sinter and suffer from carbon deposition, resulting in their deactivation ^[26]. In this context, shape-controlled CeO₂ is one of many options and is highly used to avoid such drawbacks ^[27]. Luo and co-workers proposed a strategic approach to finely adjust the metal–support interaction by employing different shapes of CeO₂—spheres, rods, and cubes—as supports for Ni nanoparticles (Figure 1a–i) ^[28]. This approach also influences the specific surface area exposed by the Ni nanoparticles, metal–support interactions, concentration of oxygen vacancies, and catalytic performance in glycerol steam reforming. The molar ratio of nC₃H₈O to nH₂O was 1:9, with a gas hourly space velocity (GHSV) of 23,700 mL h⁻¹ gcat⁻¹, and the reaction was carried out at a pressure of 1 atm and a maximum temperature of 600 °C.

Figure 1. HRTEM images depict the reduced (a,b) sphere-shaped Ni/CeO₂, (d,e) rod-shaped Ni/CeO₂, and (g,h) cube-shaped Ni/CeO₂ catalysts. In addition, scanning STEM images along with mappings illustrate the distribution of Ni, Ce, and O in the reduced (c) sphere-shaped Ni/CeO₂, (f) rod-shaped Ni/CeO₂, and (i) cube-shaped Ni/CeO₂ samples. TEM images of spent catalysts: (j) sphere-shaped Ni/CeO₂, (k) rod-shaped Ni/CeO₂, and (l) cube-shaped Ni/CeO₂ catalysts. Modified with permission from ^[28].

Interestingly, they observed that the sphere-shaped Ni/CeO₂ catalyst did not change its morphology after the reaction (and promoted higher production of H₂), unlike the rod- and cube-shaped Ni/CeO₂ catalysts. Furthermore, the researchers noted distinct carbon deposition characteristics among the catalysts. The Ni/CeO₂ catalyst supported on CeO₂ spheres exhibited abundant filamentous carbon surrounding the catalyst, while encapsulated carbon was scarce, leaving the remaining facets still available for reactant adsorption (Figure 1j). On the other hand, the catalysts based on rod- and cube-shaped Ni/CeO₂ structures displayed amorphous carbon on their surfaces (Figure 1k,l). This carbon layer covered the metal particles, leading to catalyst deactivation. These experimental findings corroborate the significance of the CeO₂ shape in determining overall catalytic efficiency.

Although specific reaction conditions support morphology, and substrate utilization may highly account for the results, it should be noted that ceria enhances the catalytic activity of Ni through metal–support interactions and plays an essential role in the dissociation of water and prevention of coke formation, as discussed before for methane and glycerol steam reforming, also for ethanol steam reforming ^[25]. Thus, this seems to be the case for steam-reforming reactions.

The presence of oxygen vacancies on the catalyst surface is an important feature that can be tailored to enhance redox properties and oxygen mobility. Bearing that in mind, Ni et al. conducted a study to examine the correlation between Ni and CeO₂ in Ni/CeO₂ catalysts and its impact on the CO₂ reforming of CH₄. They employed various synthesis methods, including hydrothermal and precipitation techniques, and compared the results with a commercial counterpart. The CeO₂ nanostructures obtained through these methods exhibited different shapes, although the control over the shape was not precise. Interestingly, the researchers observed variations in the oxygen vacancies between the different synthesis approaches after immobilizing Ni⁰ on the supports. They found that the material prepared through hydrothermal synthesis (the material with a higher shape control) exhibited higher efficiency, which they attributed to improved Ni⁰ dispersion and the presence of a more significant number of oxygen vacancies ^[29].

According to density functional theory (DFT) and experimental studies, it has been observed that the chemical activity of various planes in CeO₂ follows the sequence (110) < (100) < (111), suggesting that the (110) planes of CeO₂ are more prone to the formation of oxygen vacancies compared to (100) and (111) planes ^[30]^[31]^[32]. Thus, Wang et al. specifically controlled the shape of CeO₂-based Ni catalysts and obtained nanorods, nanocubes, nanooctahedra, and nanoparticles of ceria, exposing mainly (110)/(100), (100), (111), and (111)/(100) facets, and used such materials for methane dry reforming. They found that CeO₂-based nanorods used for Ni immobilization presented higher concentrations of oxygen vacancies than the other three ceria materials ^[33]. Once again, it is crucial to emphasize the need for further research on the data related to hydrogen production. In-depth studies in this area can enhance performance and enable the development of more tailored catalysts to meet specific requirements.

The Ce³⁺/Ce⁴⁺ redox cycle enables CeO₂ to undergo a reversible transformation into a nonstoichiometric oxide, leading to its high OSC and enhanced oxygen mobility ^[34]. Tu et al. prepared paper-structured catalysts with dispersed CeO₂–Ni flowers for dry reforming of methane and observed that the high oxygen storage capacity and oxygen mobility of their material contributed to the partial removal of coke formation on the catalyst particles during the reforming process, allowing for an improved catalytic activity compared to other Ni-based catalyst systems with higher Ni contents without the CeO₂ flowers ^[35]. Based on their findings, it can be inferred that the shape of CeO₂ has a significant impact on its overall OSC. It is commonly understood that rod- or cube-shaped CeO₂ structures possess higher OSC than spherical particles in other CeO₂-based catalytic systems than those discussed here ^[32]^[36]^[37], suggesting the shape effect. The same trend appears to hold for steam-reforming reactions as well. Further examples will support this observation.

Qian and co-workers studied attractive core-shell catalysts (Ni in the core and alumina and ceria in the inner and outer shells, respectively) that provided a high hydrogen yield in the steam reforming of acetic acid. They prepared, with different Ni/Ce ratios, alumina over ceria catalysts (designated as Ni@AlxCe, x = 05, 10, 30), and the same was performed with ceria added to the alumina shell but with just one Ni/Ce ratio, designated as Ni@Al10Ce-a (Figure 2) ^[38]. They started the preparation by obtaining Ni nanoparticles using PVP as a stabilizer, NiCl₂·6H₂O as a metal precursor, and NaBH₄ as a reductant that was added after the mixture was ultrasonicated for 15 min; then, Ce(NO₃)₃·6H₂O (or Al₂O₃) was added to the suspension and underwent 1 h of ultrasonication and the Ni nanoparticles were coated with an alumina (or ceria) shell. The authors attributed the enhanced catalytic activity of the Ni@Al10Ce catalyst to the mobility of oxygen within the ceria lattice, allowing it to migrate to the surface of the Ni catalyst and react with the dissociated acetic acid, associated with optimized reaction conditions. Additionally, they attributed the minimal formation of coke to the oxygen storage property of ceria. The mobile oxygen released from ceria could migrate to the surface of Ni nanoparticles, where it reacted with the coke precursor or the already formed coke. The diffusion of acetic acid and water from the bulk solution to the surface of nickel nanoparticles, where dissociation took place, was attributed to the pathway provided by the alumina coating, having a better function at a specific ratio.

Figure 2. HRTEM images of (a) Ni@Al10Ce and (b) Ni@Al10Ce-a with low magnification. (c) Ni@Al10Ce and (d) Ni@Al10Ce-a with high magnification. STEM images with cerium EDS mappings and overlaid mappings of the (e) Ni@Al10Ce and (f) Ni@Al10Ce-a catalysts reduced at 600 °C. The green and blue spots represent Ni and Al elements, respectively. Modified with permission from ^[38].

Also, using the synergy between CeO₂ and Al₂O₃, Lai et al. prepared Ni−CeO₂−Al₂O₃ hybrid nanoparticle clusters as catalysts for hydrogen production via low-temperature steam-methane reforming ^[39]. The authors obtained a significant metal–support interface generation in the hybrid nanostructure using aerosol-based evaporation-induced self-assembly, in which the addition of Al₂O₃ was responsible for the surficial area increase, suppression of Ni sintering, and lowering of coke formation. Tu et al. developed a novel two-step hydrothermal process to produce flower-like nanostructures comprised of ceria–zirconia with an OSC of 536 µmol O₂ g⁻¹, which was twice the OSC of pure flower-like CeO₂ (284 µmol O₂ g⁻¹) ^[40]. By forming a solid solution with zirconia, the function of ceria as a support material for Ni catalysts in the dry reforming of methane was enhanced at 750 °C, with an initial methane conversion of 88.4%. The authors concluded that the flower-like catalyst’s petal-shaped structure improved the Ni metal’s sintering resistance and enhanced coking resistance. It is important to highlight that the manipulation of oxygen vacancies (and OSC) has been achieved through doping methods ^[41]^[42]. However, to the best of researchers' knowledge, no studies have been conducted on the control or rational effect of the different morphology or shape of CeO₂-based catalysts in this regard. Moreover, researchers feel that such studies can provide interesting insights if they include experimental data and DFT calculations to support their findings.

Extensive research has been conducted on using Co as a catalyst (or co-catalyst) for steam-reforming reactions ^[43]^[44]^[45]^[46], Its combination with CeO₂ structures has shown a synergistic effect, enhancing the active surface area, inhibiting sintering, and facilitating oxygen storage. The catalytic properties have long been studied as CeO₂-morphology-dependent for Co-based catalysts ^[47]. However, the search indicated a recent trend of focusing on ethanol steam reforming and more profound knowledge regarding this substrate; thus, researchers will focus on the recent aspects of the interaction of Co and ceria, beyond simply speaking of a specific morphology and including the effect of ceria facets. Interestingly, the catalysts’ activity and selectivity are influenced by the acid-base properties of the oxide support, as discussed in Noronha’s excellent publication ^[48]. Thus, choosing the support correctly is of high priority, and ceria properties are remarkable. For example, Ferencz et al. reported a notable influence of the support on product selectivity during ethanol reforming for Co-based catalysts. Due to ceria’s basic nature, hydrogen production was accompanied by the generation of aldehyde and crotonaldehyde as byproducts. Therefore, the acidity/basicity properties of the support play a crucial role in determining the products’ formation ^[49]. In the same way, the oxidation state of the active species also can play a pivotal role in the activity and selectivity of the process ^[50].

The literature has already shown that the Co content influences the interaction between metal and cerium, favoring the catalytic properties in ethanol steam reforming, and the highest Co content proved to be when the catalyst was prepared by a co-precipitation method under optimized conditions (500 °C, H₂O/EtOH = 12/1) ^[51]. However, metal dispersion plays a crucial role in the catalytic process, as expected in heterogeneous catalysis, and it is generally recognized as an essential factor. For that matter, the preparation method choice and adding additives can be beneficial. Greluk et al. prepared Co/CeO₂ and Ni/CeO₂ catalysts by employing various techniques—the impregnation method, different active phase precursors, impregnation solvents, and the presence of organic additives—in an attempt to enhance the dispersion of the metals (Co or Ni) and promote strong metal–support interactions ^[52]. Adding citric acid to the precursor metal salt solution has proven effective in achieving improved dispersion and smaller metal particle sizes for both catalysts.

Conversely, catalysts prepared using an ammonia solution resulted in larger metal particle sizes. Interestingly, although the authors did not precisely control the shape of the CeO₂ (they obtained rectangular-like shape ceria; Figure 3), the crucial step in the steam reforming of ethanol at 420 °C was using both catalysts is the dehydrogenation of ethanol, which predominantly takes place on the terrace sites of cobalt/nickel. However, the cleavage of the C–C bond is more favorable at the edge/step sites, leading to higher efficiency in the overall process. In the case of Co-based catalysts, apart from size effects, they found that their performance was based on the oxidation state of the Co nanoparticles.

Figure 3. HRTEM images and corresponding FFT patterns of (a) Co/CeO₂ (obtained in the presence of citric acid) and (b) Co/CeO₂ (obtained from ammonia solution). Modified with permission from ^[52].

Following a similar trajectory of exploration, Li et al. recently studied Co/CeO₂ catalysts with an exposure preference of (111) and (100) facets of CeO₂, aiming to understand how to regulate the Co chemical state ^[53]. They discovered that the Co supported on ceria with (111) facets presented a lower oxidation state and displayed an enhanced capability for cleaving C–C bonds, resulting in superior catalytic performance for the ethanol steam-reforming reaction compared to the Co supported on ceria with (100) facets. Using in situ synchrotron radiation photoionization mass spectrometry and DFT calculations, it has been revealed that Co sites in a low oxidation state exhibit a strong preference for carbon chain shortening through C–C cleavage rather than carbon chain lengthening via condensation reactions. This unique property enhances the efficiency of hydrogen production in the ethanol steam-reforming reaction. Additional information regarding DFT calculations provided the knowledge that the O-terminated (111) facet allowed for the stable support of metallic Co, while the O-terminated (100) facet led to the formation of cobalt oxide, controlling the chemical states of the Co sites, indicating that the oxidation state of Co could be regulated by the CeO₂ support with a specific facet exposure (Figure 4).

Figure 4. Side and top views of optimized geometries of Co₁₀ clusters over CeO₂ with the exposure of (111) and (100) facets. Reproduced with permission from ^[53].

One may notice that the Co species may undergo differences during the reaction. Interestingly, Huck-Iriart et al. studied the chemical state of Co in Co-based catalysts during the ethanol steam-reforming reaction by near-ambient pressure XPS (NAP-XPS), extended X-ray absorption fine structure (EXAFS), and X-ray absorption near-edge structure (XANES) ^[54]. They realized that differences in synthesis methods and precursors affected the Co species during the reaction, which is fascinating. Although their studies did not focus on CeO₂-based catalysts, they provided valuable insights that could be applied to investigate supported cobalt on CeO₂, particularly with different morphologies, opening possibilities for future research in this area. Although with another aim, Sohn et al. employed AP-XPS and XANES techniques to observe the effect of Co on the reduction characteristics of ceria under ethanol steam-reforming conditions. The authors found that the reducibility of ceria particles is influenced by their particle size, with smaller particles exhibiting a greater degree of surface reduction. Moreover, the presence of fully oxidized Co nanoparticles on the ceria support hinders the surface reducibility of ceria as the reduction of cobalt oxide phases (Co₃O₄ and CoO) takes priority over the reduction of ceria ^[55].

Researchers used La₂O₃ as a promoter for Co/CeO₂ and Ni/CeO₂ catalysts for the conversion of hydrogen at a temperature of 500 °C, obtaining a complete conversion of ethanol in 21 h ^[56]. However, the Co/CeO₂ catalyst prepared with a La/Co molar ratio equal to 0.1 by co-impregnation proved selective during ethanol steam reforming, showing hydrogen and carbon dioxide selectivity of 94% and 88%, respectively. Although this showed good conversion, methane and carbon monoxide were produced in low amounts, and trace level acetaldehyde was detected among the reaction products. Incorporating La in the structure resulted in forming smaller particle sizes of Co and good dispersion, improving the catalytic activity; however, there was no control of ceria. Thus, it is crucial to understand the effect of promoters, like La and other lanthanides as Zr ^[57], but there is a lack of studies associating the doping process with shape controlling. The doping of Co into the lattice of CeO₂ was studied for the auto-thermal reforming of acetic acid. Although it was unrelated to ethanol reforming, the authors’ findings may be interesting for other substrates and open up further possibilities.

As a final example considering Co-based catalysts and showing the importance of the shape control of ceria, Hu et al. prepared Co-Ce-O by different hydrothermal methods; however, the material that offered the best response was based on ceria nanorods ^[58], which exhibited a stable hydrogen yield of 2.69 mol-H₂/mol acetic acid and a H₂ production rate of 332.3 mmol-H₂/(g_cat·h), even at 600 °C. The acetic acid conversion was initially close to 98.6% and remained stable. In addition, the authors obtained CO₂ and CO selectivity of 69.2% and 29.9%, respectively, with no records of by-products. Its greater reactivity concerning the other structures was related to controlling the material’s morphology, exhibiting a greater surface area than the others. It is important to emphasize that a comprehensive understanding of the catalysts’ surface processes requires further research. Therefore, a combination of theoretical and experimental studies becomes essential. For example, Varga et al. observed a strong dissolution of Co into ceria and a certain extent of Co encapsulation by ceria through XRD, XPS, and LEIS (low energy ion scattering) ^[59]. These observations can significantly influence the material’s performance, indicating the need for further investigations in this area.

3. Shape-Controlled Cerium-Oxide-Based Nanocatalysts with Pt

Despite their high value, noble metals are commonly used as catalysts for steam-reforming reactions due to their high catalytic activity, resistance to catalyst poisoning, thermal stability, and ability to control selectivity ^[60]. Research is ongoing to explore alternative non-noble metal catalysts, as discussed before; however, the fine-tuning of ceria as a support for noble metals can be used to improve metal–support interactions, avoiding high concentrations of the metal on the support, improving efficiency and resistance to deactivation. Thus, the influence of the noble metal used is important, and deeper knowledge regarding its effect is still the focus of much research ^[60]. In this context, Vayssilov et al. observed that the electron transfer is favorable on ceria supports regardless of their morphology, while oxygen transfer requires nanostructured ceria in close proximity to Pt. These findings provide valuable insights into the formation mechanism of Pt–O species on ceria and the structure-activity dependence of ceria-based catalysts ^[61].

Thus, through the reduction of nanoparticle size, catalytic activities can be enhanced while reducing costs. However, smaller noble nanostructures tend to aggregate during catalytic processes, and significant efforts have been made to develop sinter-resistant, single-site catalysts. Jones et al. conducted research using various CeO₂ nanostructures to identify the most effective facet for trapping Pt, forming highly thermally stable, single-atom Pt on CeO₂ nanostructures ^[62]. Atomically dispersed Pt was observed on CeO₂ nanocubes and polyhedra, and, after various catalytic cycles at temperatures of up to 300 °C, the Pt on CeO₂ nanorods remained atomically dispersed, showing CeO₂’s role in suppressing catalyst sintering and offering insights for the design of single-atom nanocatalysts. A recent study by Wang et al. showed that steam treatment had a significant impact on catalytic properties by accelerating the formation of active surface lattice oxygen near Pt single atoms ^[63]. This treatment led to a substantial enhancement in catalytic performance. Unfortunately, researchers could not find similar studies for steam-reforming reactions, but they prove to be promising to the field.

However, other studies show that the shape-control of ceria is important in Pt-based catalysts. For example, CeO₂ nanorods have been studied as a support for Pt nanoparticles for the steam reforming of various substrates. Claudio-Piedras et al. conducted a study on Pt/CeO₂ nanorod (hydrothermal method) catalysts for methanol steam reforming ^[64]. They discovered that the choice of Pt precursor (Pt(NH₃)₄(NO₃)₂), (CH₃-COCHCO-CH₃)₂Pt, and H₂PtCl₆.6H₂O) immobilized onto the ceria by the incipient impregnation method and that synthesis solvent influenced the catalyst’s performance, indicating that further research is required to establish a common trend. Additionally, the impact of ceria shape on catalytic properties suggests that there is still much to explore in this area.

Studying the catalytic properties of Pt/CeO₂ nanorods (hydrothermally prepared) for the steam reforming of methanol, Papavasiliou et al. used the H₂PtCl₆.6H₂O as the metal precursor for the immobilization of Pt by wet impregnation and deposition–precipitation methods. The latter technique provided highly dispersed Pt (mean particle size of less than 1 nm) over the support, which was correlated by the authors due to oxygen vacancies (although no characterizations were provided to prove such a relationship). Interestingly, the catalyst with just 0.3 wt.% Pt strongly interacted with the support surface, in agreement with the STEM, HR-TEM, XRD, and Raman analyses, resulting in an efficient material for the steam reforming of methanol reactions ^[65].

Verykios and co-workers gave researchers an overview of the catalyst performance of differently shaped CeO₂ supports as carriers of Pt nanoparticles for low-temperature steam reforming of methanol ^[66]. Comparing CeO₂ bulk (PPT), nanocubes (NC), nanorods (NR), and flower-like (FL) structures (Figure 5), the Pt/CeO₂–PPT catalyst demonstrated the highest activity for the dehydrogenation of ethanol to acetaldehyde. On the other hand, the Pt/CeO₂–FL and Pt/CeO₂–NC catalysts exhibited a higher rate of decomposition of acetaldehyde into CH₄ and CO. The ethanol dehydrogenation activity followed the order PPT > NR > NC > FL, while Pt/CeO₂–NR presented the best stability and Pt/CeO₂–PPT deactivated quickly. XPS analysis showed the presence of Ce³⁺ and Pt⁰ on the Pt/CeO₂–NR catalyst after a reaction at 300 °C. However, the Pt/CeO₂–PPT catalyst exhibited a lower degree of reduction related to the accumulation of acetate species, which can be attributed to the imbalance between the decomposition rate of acetate species to CH_x species on Pt^δ+ and their desorption as CH₄. The authors believe that this was caused by the lower metal–support interface of the Pt/CeO₂–PPT catalyst, which had a lower Pt dispersion. The blockage of the Pt–support interface by these CH_x species leads to catalyst deactivation. Curiously, nanorods are the focus of great interest, probably due to their best performances.

Figure 5. SEM images for the (A) CeO₂–PPT, (B) CeO₂–NR, (C) CeO₂–NC, and (D) CeO₂–FL catalysts. Reproduced with permission from ^[66].

Raúl Pérez-Hernández researched Pt/Ni catalysts supported on CeO₂ nanorods for methanol steam reforming. The study revealed that the enhanced catalytic activity was not solely attributed to the synergy between the metals; CO-DRIFTS (CO diffuse reflectance infrared Fourier transform spectroscopy) experiments indicated that the bimetallic PtNi nanoparticles were predominantly Pt-terminated, which contributed to the improved performance. TPD-DRIFTS (temperature-programmed desorption associated with diffuse reflectance infrared Fourier transform spectroscopy) experiments conducted in the presence of methanol demonstrated that methoxy species and C-H groups strongly interacted with the support’s surface oxygen near Pt. This interaction, facilitated by Pt, resulted in the formation of formate species and suggested Pt’s important role as an intermediary in the reaction ^[67].

Based on a sophisticated synthesis approach, Dai et al. created a CeO₂@Pt-Beta yolk-shell (“Beta”—the name of the material refers to the zeolite Beta) catalyst for low-temperature ethanol steam reforming (Figure 6) ^[68].

Figure 6. SEM image of (A) CeO₂; TEM images of (B–D) CeO₂@Pt-Beta catalyst view at different magnifications. (E) The EDX spectrums of the catalyst. Reproduced with permission from ^[68].

To guarantee the superior performance of the prepared catalyst, the authors compared it with the Pt-Beta and CeO₂-Pt-Beta counterparts. They attributed the better performance of the synthesized material to the synergistic effect between the movable CeO₂ core and the Pt nanoparticles and the high dispersion of the metal, which achieved 100% conversion with 67% hydrogen selectivity. They also believe that the interstitial hollow space within the catalyst allows for the high loading of reactant molecules, particularly promoting the reaction of larger molecules such as ethanol and acetaldehyde with the CeO₂ core and metal Pt active sites. Throughout the 20 h test at 350 °C, the catalyst remained stable, without any significant decrease in activity. Once more, it is crucial to emphasize that the synthesis of shape-controlled nanostructures is an actively developing field. Ongoing research and advancements are necessary to gain a comprehensive understanding of the intrinsic processes occurring during reactions, including the significance of doping techniques. This is also pertinent when considering Pt-based catalysts ^[42].

Although extensive research has focused on Pt as a catalyst for steam reforming, leading to a comprehensive understanding of its performance and behavior in this application, the established catalytic response of Pt makes it the preferred choice for researchers and industry professionals. However, other noble metals also are promising candidates for steam-reforming reactions, although there is a relatively limited volume of studies exploring their utilization with shape-controlled CeO₂ nanostructures. For instance, Kolb et al. conducted a study on the impact of ceria oxide supports’ morphology and crystalline facets on the performance of Rh/CeO₂ and Rh/CeO₂/Al₂O₃ catalysts in the oxidative steam reforming of propylene glycol (630 °C, with a steam-to-carbon ratio of 3.5 and an oxygen-to-carbon ratio of 0.15) ^[69]. A hydrothermal method was employed to synthesize well-defined ceria nanorods and nanocubes, which were then deposited onto α-Al₂O₃. TEM images revealed that the ceria nanorods exhibited exposed (110) and (100) planes, while the nanocubes possessed only (100) planes; such structures were used as supports for Rh nanoparticles, providing a large surface area and efficient heat and mass transfer. The superior performance of the Rh catalyst supported on ceria nanorods could be attributed to the improved dispersion of Rh species and the presence of higher surface oxygen vacancies associated with the exposed (100) and (110) crystal planes. Also, the catalyst design allowed Rh nanoparticles to stabilize, prevented sintering, and minimized carbon deposition. Unfortunately, little is known about the shape-controlling of ceria for other noble metals, which is the reason for the emphasis on Pt nanoparticles herein presented.

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Subjects: Nanoscience & Nanotechnology

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Samuel da Silva Eduardo

Jhonatam Pinheiro Mendonça

Pedro Nothaft Romano

João Monnerat Araújo Ribeiro de Almeida

Giovanna Machado

Marco Aurélio Suller Garcia

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Update Date: 28 Aug 2023

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1. Introduction

2. Shape-Controlled Cerium-Oxide-Based Nanocatalysts with Ni and Co

3. Shape-Controlled Cerium-Oxide-Based Nanocatalysts with Pt

References