Enhanced Ceria Nanoflakes using Graphene Oxide as a Sacrificial Template for CO Oxidation and Dry Reforming of Methane

Abstract The development of novel fabrication methods to produce ceria catalysts with good high-temperature stability is critical for their implementation across a range of different applications. Herein, graphene oxide flakes are used as a sacrificial template in the synthesis of ceria particles to replicate the graphene oxide’s two-dimensionality. While performing the synthesis without graphene oxide results in large agglomerations of ceria crystallites, the addition of graphene oxide during the synthesis results in ceria nanoflakes ( 400 °C) which results in improved catalytic performance for the oxidation of carbon monoxide. This resistance versus sintering has also a beneficial effect when ceria flakes are used as catalytic support of nickel particles. Improved metal dispersion and high metal-support interaction leads to lower sintering during the dry reforming of methane than similarly prepared un-templated ceria nickel catalysts. These results demonstrate the advantage of using graphene oxide as a sacrificial template for the production of sintering-resistant catalysts with good catalytic performance at high temperatures.


Introduction
Ceria-based materials are well-known for their use as a catalyst or catalyst support in a variety of environmentally sensitive applications, including automotive catalysis, VOC oxidation, solid oxide fuel cells, steam reformation for hydrogen production, photocatalysis, and thermochemical water splitting. [1][2][3][4][5][6][7][8] Ceria's ability to easily cycle between 4+ and 3+ oxidation states allows it to act as an oxygen storage medium -for example, storing and releasing oxygen in response to the varying air-to-fuel ratio in a vehicle's exhaust to optimize catalytic converter performance. [9] Ceria has also been utilized as a catalyst support or promoter in the dry reforming of methane (DRM) for syngas production, where it enhances activity. [10,11] The use of ceria in DRM as a catalyst support for nickel, instead of other supports such as Al2O3 or SiO2, also inhibits coke formation, otherwise a significant source of deactivation in this reaction. [12,13] Recently, ceria's redox cycle has been utilized in the thermochemical splitting of water, using concentrated solar energy to reduce cerium ions to Ce3+ at high temperatures, followed by reoxidation with water molecules to produce hydrogen, syngas, or hydrocarbons. [5,14] Another recent application for ceria is room-temperature dehydrogenation of formic acid, where it demonstrated greatly improved activity compared with other supports. [15] Ceria morphology at the nanoscale has also been shown to have an effect on the olefin selectivity on the CO2 hydrogenation reaction to hydrocarbons. [16] Many ceria applications involve heterogeneous surface-catalyzed reactions, so the ability to synthesize high surface area ceria is critical to improve performance. Surface areas over 200 m 2 g -1 are often reported for nanostructured ceria materials synthesized via a variety of methods such as hydrothermal or sol-gel, and surface areas of 345 m 2 g -1 has been obtained in ceria aerogel materials. [17][18][19] The morphology of nanoceria and the nature of its exposed crystal planes can also affect catalytic activity. [6,[20][21][22] However, in numerous applications, ceria must also present a high thermal stability to maintain a useable surface area even when exposed to high temperatures. The next generation of low-temperature solid oxide fuel cells still have a temperature range of up to 650 °C, while the ceria reduction step in thermochemical water splitting generally involves temperatures greater than 1200 °C.
Automotive catalysts can be exposed to vehicle exhaust temperatures of 850 °C, and close-coupled automotive catalysts can reach 1050 °C. [23,24] Dry reformation of methane often requires temperatures in excess of 800 °C due to the highly endothermic nature of the reaction. [25,26] To synthesize nanostructured ceria of various morphologies, numerous templating agents have been reported, such as metal-organic frameworks and carbon microspheres. [20,27,28] Carbon nanotubes, ZnO nanotubes, and Ag nanowires have been used as templates to synthesize ceria nanotubes, while polystyrene and silica spheres have been used to synthesize hollow ceria nanospheres. [29][30][31][32][33] 10.1016/j.apcatb.2018. 10.011 4 However, these methods often involve either an additional chemical treatment step to remove the template and full removal is usually difficult to achieve.
The use of graphene oxide (GO) as a template offers an attractive alternative route for synthesizing high surface area nanostructured ceria catalysts. GO consists of a two-dimensional monolayer of carbon atoms, analogous to graphene, but interrupted by a range of oxygen-containing groups. [34] GO's surface functional oxygen-containing groups allow it to be easily dispersed in a variety of solvents, unlike graphene, enabling its utilization in processes involving liquid suspensions or solutions.
[35] The use of GO as a templating agent allows its two-dimensional structure to be replicated. For instance, the synthesis of manganese oxide nanoflakes has been reported by mixing a potassium permanganate solution into a suspension of GO, resulting in the in situ replacement of carbon atoms with manganese. [36] Titanium oxide nanoflakes have been synthesized by anchoring Ti16O16(OEt)32 clusters onto GO suspended in tetrahydrofuran, followed by calcination at 450 °C to decompose the GO, which resulted in a two-dimensional titania structure replicating the GO shape. [34] In this work, the use of GO as a sacrificial template for the synthesis of ceria nanoflakes via a precipitation reaction is reported. It is shown that a highly two-dimensional structure is achieved. Due to improved textural properties, these ceria nanoflakes demonstrate better anti-sintering behavior and higher catalytic activity compared with untemplated ceria particles produced in a GO-free precipitation synthesis. The catalytic activity of both materials (templated and un templated) is tested for CO oxidation for eventual application in automotive catalysis. The use of templated ceria flakes as a catalyst support for nickel nanoparticles in dry reformation of methane (DRM) is also examined. In both cases, the flakes offer enhanced catalytic activity compared with untemplated ceria particles. Templating ceria with GO offers a potential way for the synthesis of catalysts with better performance for high-temperature applications such as automotive catalysis or dry reforming of methane.

Experimental
GO was prepared by oxidizing and exfoliating graphite with a modified Tour et al. synthesis method. [34,37] 24 g of 100-500 µm natural graphite flakes (Aldrich) were added to a concentrated acid mixture (3 L H2SO4 : 0.3 L H3PO4) under vigorous stirring. 144 g of KMnO4 was added gradually. The reaction mixture was vigorously stirred for 18 h at 50 °C. Next, the mixture was cooled to room temperature and 1.72 L of 2 wt. % aqueous H2O2 was added dropwise to stop the oxidation reactions.
The resulting suspension of GO was washed by repeated centrifugation and re-dispersion in distilled water until the pH of the supernatant matched that of the original distilled water (typically after 16 10.1016/j.apcatb.2018.10.011 5 washing cycles). Then, un-exfoliated graphite particles were separated with two further low-speed (<1000 rpm) centrifugation cycles. Finally, the GO suspension was freeze-dried at -60 °C and stored at room temperature.
Ceria nanoflakes were prepared using a room temperature precipitation reaction. [38] A suspension of GO in deionized water (3 wt. %) was prepared. 3 mL of this 3 wt. % GO suspension in water was added to 600 mL of deionized water, and aqueous ammonia solution (30%) was added to adjust the pH to 11.0. 150 mL of 0.05 mol L-1 cerium nitrate in deionized water was added dropwise to the GO/ammonia solution under stirring. The solution was left to stir for 3 h under ambient conditions. Next, three cycles of centrifugation (4000 rpm for 10 minutes) and redispersion in distilled water were performed to wash the product. The wet powder was freeze-dried at -60 °C. Untemplated ceria particles were prepared using the same procedure, without the addition of GO prior to pH adjustment.
After synthesis, the product was calcined at 400 °C for 3 h to remove the GO. For CO oxidation experiments and characterization, different samples were prepared by further calcination undertaken at 600, 800, 900, and 1000 °C for 3 h. Calcination was performed in air under static conditions. Renishaw inVia Raman Microscope. A 532 nm green laser was used, and spectra were obtained with 2-10 s exposure time at 1-10% laser power. Thermogravimetric analysis (TGA) after the DRM reactions was carried out in a TA Instruments Discovery TGA, Q50. Approximately 3 mg of sample was combusted under 40 mL min -1 of air low from 50 to 900 °C at 10 °C min -1 .
Catalytic activity tests for CO oxidation were performed in a U-shaped quartz tube reactor (10 mm ID) at atmospheric pressure. In a typical experiment, 10 mg of ceria catalyst was dispersed in a 4 cm 3 catalytic bed consisting of silicon carbide particles. The catalytic bed was secured at both ends with quartz wool. The reactant feed consisted of 2000 ppm each CO and O2 in nitrogen, with a total flow rate of 50 mL min -1 , achieving a weight hourly space velocity (WHSV) of 300 L g -1 h 1 . The catalyst was tested from room temperature to 500 °C, and the outlet gas was measured with a Fuji Electric ZRH Infrared Gas Analyzer and a Hiden mass spectrometer. Statistical analysis: each data point reported for CO oxidation represents the average of ten continuously logged data points during testing. Therefore, error bars reported in Figure 6 show instrument error.
Catalytic activity tests for the dry reforming of methane were performed in a tubular quartz reactor (10 mm ID) at atmospheric pressure. Catalysts were reduced in H2 atmosphere (10 vol. % H2 balanced in N2) at 850 °C for 1 hour prior to the activity tests. In a typical experiment, 100 mg of Niceria catalyst was loaded in the reactor, supported on a bed of quartz wool. The reactant feed consisted of a 1/1/6 ratio of CH4/CO2/N2, with a total flow rate of 100 mL min -1 , achieving a WHSV of 60 L g -1 h -1 .
Reactants and products were monitored using an on-line gas analyzer (ABB AO2020), equipped with both IR and TCD detectors. The catalyst was tested between 550 and 850 °C. Long-term dry reforming studies were performed at a constant temperature of 800°C for 20 hours.

Results and Discussion
Ceria particles and nanoflakes were prepared via a room temperature precipitation synthesis in the absence and presence of GO respectively, followed by a calcination in air at 400 °C to remove the GO template. An SEM micrograph of the GO used is shown in Figure 1. Prior to calcination, the nanoflakes are a brown powder, while after calcination at 400 °C, the product is yellow, similar to other ceria syntheses ( Figure S1). The untemplated ceria particles were bright yellow both pre-and post-calcination.
The GO-templated ceria nanoflakes were much more loosely packed than the untemplated ceria particles.
Without any further treatment, the nanoflakes had a bulk density of 0.34 g mL -1 , while the untemplated particles had a bulk density of 0.65 g mL -1 .    Subsequent calcinations in air at 600, 800, 900, or 1000 °C to assess thermal stability also show differences between GO-templated ceria flakes and untemplated ceria particles. While both the ceria nanoflakes and particles show crystallite growth with increasing calcination temperature, the ceria nanoflakes retained their two-dimensional morphology even at high calcination temperatures (1000 °C).
Powder XRD patterns of the calcined ceria particles and nanoflakes crystallite sizes show that both are very similar. Figure 3 shows that the diffraction peaks get narrower and taller with increasing calcination temperature, indicating a growth in crystallite size. However, this trend is more prominent for the untemplated ceria particles compared to the GO-templated ceria nanoflakes for any calcination temperature. Crystallite size calculations using the ceria (111) peaks clearly show this trend in Table 1.
When calcined at 400 °C, the ceria particles and flakes have a similar crystallite size, slightly smaller for the flakes (6.7 and 6.0 nm, respectively). But upon calcination at higher temperatures, crystallite size increases more quickly for the untemplated particles than for the GO-templated nanoflakes. This observation indicates that the two dimensional high aspect ratio arrangement of crystallites of the GOtemplated nanoflakes replicating the GO morphology limits the diffusion of atoms at high temperature to a two dimensional plane which is translated into a low sintering degree. In contrast, diffusion of atoms at high temperature in untemplated particles takes place in three dimensions, favoring the sintering.  have been used to synthesize ceria nanoparticles dispersed on the surface of graphene oxide for the oxidation of uric acid, degradation of methylene blue, and removal of arsenic species from water. [39][40][41] However, because calcination at 400 °C is sufficient to completely remove GO from such composite materials, there is quite a low upper temperature limit for such catalysts to be useful. In contrast, the GOtemplated ceria flakes retain useful properties at much higher temperatures.
Both the ceria particles and templated flakes were analyzed with Raman spectroscopy, shown in Figure 4. In both cases, ceria's characteristic F2g band at approximately 466 cm -1 , attributable to the vibrational characteristics of oxygen atoms surrounding cerium in the fluorite crystal structure, is clearly visible. [42] An additional defect band at 595 cm -1 is also visible in both samples calcined at 400 °C. This defect band can be attributed to the presence of Ce 3+ related defects in the ceria lattice, or to defects caused by crystallite size effectsin general, in pure ceria it can be related to oxygen vacancy concentration due to non-stoichiometry of the CeO2 lattice. [43,44]. Shifts in the 466 cm -1 band can be attributed to differences in oxygen vacancy levels or lattice contraction, and this would also be indicated in a difference in intensity in the 595 cm -1 band. [45][46][47] While the GO templated and untemplated samples calcined at 400 °C appear to be similar, the 595 cm -1 band disappears in the untemplated ceria particle spectrum at calcination temperatures > 600°C, while it is retained in the GO-templated ceria spectrum even after treatments at 1000 °C, although its magnitude is considerably decreased.
Oxygen vacancy quantities can be calculated via correlation with a shift in the main F2g band at ~466 cm -1 . [48,49] Results are shown in Table 1 To further quantify oxygen vacancy levels in the ceria materials, temperature-programmed reduction was performed, shown in Figure 5. Typically, two TPR peaks are associated with nanostructured ceria -a lower-temperature peak associated to readily available surface oxygen reduction, and a higher-temperature peak representing the reduction of the bulk lattice oxygen. [6,51] As shown in Figure 5a-b, both the GO-templated and untemplated ceria calcined at 400 and 600 °C clearly show both peaks. However, for the 800 °C calcination, this is greatly reduced for both samples.   The physical properties of both materials were also characterized by nitrogen adsorption. Both the GO-templated ceria flakes and untemplated ceria particles calcined at 400 °C show a type IV isotherm ( Figure S2 in supporting information), characteristic of mesoporous materials. [52] The type H2 hysteresis loop displayed is characteristic of a non-uniform network of pores, in both size and shape. [53] In both cases, the type IV isotherm is maintained upon calcination at 600 °C. As shown in Figure S2(a), nitrogen adsorption and desorption from untemplated ceria particles calcined at 800 °C and above is negligible. GO-templated ceria nanoflakes calcined at 400 °C show a pore size distribution with narrower pore diameters than untemplated ceria particles, shown in Figure S3. However, upon calcination at 600 °C, both materials show a similar pore size distribution. While both materials experience severe sintering at high temperatures, the GO-templated ceria nanoflakes maintain a higher BET surface area compared to untemplated ceria particles after calcination at the same temperature ( Figure 6 and Table 1). This improved thermal stability of the flake materials is in agreement with the crystallite sizes derived from powder XRD data. The catalytic activity of GO-templated ceria flakes and untemplated ceria particles for CO oxidation is shown in Figure 7. Catalysts were calcined in static air at 400, 600, or 800 °C respectively prior to testing. The GO-templated ceria flakes calcined at 400 ºC achieves similar although slightly superior activity than the untemplated ceria particles up to 400°C reaction conditions. At higher reaction temperatures (400-500 ºC), considerably higher conversions are achieved with the ceria flakes compared to the particles due to their considerably lower in-situ sintering as shown by the smaller reduction on surface area above 400ºC (Table 1). At 500 ºC, the ceria flakes achieve full conversion of CO to CO2, while the untemplated ceria particles only reach approximately 80% conversion. GO-templated ceria flakes also outperformed the untemplated particles when calcined at 600 °C -at this calcination temperature the GO-templated ceria flakes are capable of maintaining a high surface area and high concentration of their initial surface oxygen concentration, critical for the CO oxidation reaction. In Figure S4, this is shown in terms of turnover frequency (TOF) calculations at 450 °C. These results demonstrate that templating ceria with sacrificial GO results in more thermally stable catalysts.
Templated ceria nanoflakes perform comparably to or slightly better than some nanostructured ceria catalysts recently reported in the literature for CO oxidation. At 250 °C, the turnover frequency of these ceria nanoflakes for CO oxidation (in terms of μmol of CO converted per g of catalyst) is 19  CO oxidation is a surface-catalyzed reaction, proceeding via the Mars-Van Krevelen mechanism.
[58] Nevertheless, higher surface areas do not necessarily correlate with better catalytic performance, because, among other reasons, the nature of the exposed ceria crystal planes can influence activity. [59] In any case, prevention of excessive sintering is very important, as demonstrated with these ceria samples -where GO templating leads to smaller crystallite sizes and larger surface area upon calcination at 400-1000 °C, resulting in more active and thermally stable catalysts. [6,60,61] In addition, CO oxidation over ceria-based catalysts is rather sensitive to oxygen vacancy concentration in such a way that the greater the population of oxygen defects, the better the CO oxidation performance. [62,63] The observed catalytic trends correlate perfectly with the Raman and TPR experiments, which show how ceria nanoflakes retain a higher population of oxygen defects after thermal treatment compared to ceria nanoparticles and thus improved reducibility, exhibiting improved oxidation activity.
In addition to bare ceria nanoflakes and particles, nickel-loaded ceria nanoflakes and particles were synthesized as catalysts for the dry reforming of methane. Nickel is a common low-cost alternative to noble metal catalysts for this application. While noble metals such as Pt, Pd, or Rh typically retain higher stability, activity and resistance to coke formation than non-noble alternatives, the interaction between the metal and support can play a large role in modifying metal dispersion and electronic effects
Powder XRD patterns of the Ni-ceria samples are shown in Figure 8.  Additionally, TPR experiments were conducted on the Ni-deposited ceria nanoflakes and particles, shown in Figure 9. The TPR profiles are similar to the ones of the bare ceria materials ( Figure   5) with the addition of low temperature peaks (<400 °C), attributable to NiO reduction. The lowest temperature (~200 °C) peak can be attributed to the reduction of larger "free" NiO not bound to the ceria support, while the higher-temperature peak (at ~325 °C) is attributed to NiO bound to the surface of ceria. [67,68] For this main peak, the NiO-ceria nanoflake TPR profile shows a shift to higher reduction temperatures compared with the NiO-ceria particle sample. There are two possible explanations for this.
The size of NiO particles can affect the reduction temperature, so this is possibly indicative of different NiO particle sizes on the ceria nanoflake sample. [69,70] Alternatively, metal-support interactions can influence the reduction temperature as well. In the case of nickel-ceria, stronger interaction between NiO 16 and the ceria support results in higher reduction temperatures. [71,72] XRD analysis showed that the NiO crystallite sizes in the nanoflake and untemplated particle systems were similar, so the shift to a higher reduction temperature is likely due to differences in NiO-ceria interaction in the two catalysts. A shoulder extending from approximately 450 to 500 °C is visible for the NiO/ceria nanoflake TPR profile, but not the NiO/ceria particle sample, attributed to readily available surface oxygen in the ceria flakes.  Table 1), and Ni-loaded ceria XRD patterns in Figure 8. In Figure 10a, of the fresh Niloaded ceria flakes, NiO particles can be seen clustering around the edges of ceria crystallite agglomerations. However, for the fresh untemplated Ni-loaded ceria particles, shown in Figure 10b, while some NiO particles appear to be similarly deposited on the edge of ceria crystallite agglomerations, much of the visible nickel is not deposited on ceria. These un-supported NiO particles were evenly dispersed across the TEM sample grid, a phenomenon not observed with the Ni-ceria flake sample. This observation suggests that NiO particles have a better interaction with the ceria support when supported on ceria flakes than ceria particles in agreement with the TPR data. Surface defects such as oxygen vacancies are known to be preferential nucleation sites for metal particle formation. [73,74] Therefore, the ceria flakes promote better nickel dispersion and stronger Ni-ceria interaction than the ceria particles, in agreement with the higher surface oxygen vacancy concentration of the nanoflakes, seen in Raman and TPR analysis. 18 both cases, the syngas H2/CO ratio is close to 1 at high reaction temperatures, which is the limit imposed by the stoichiometry of the reaction. Longer-term studies of the dry reforming reaction were undertaken at 800 °C, shown in Figure   11c-d. In contrast with the short-term temperature curve experiment shown in Figure 11a, where there were only small differences, in a long-term experiment the difference in activity between the two catalysts is quite dramatic. Untemplated Ni-ceria particles experience severe deactivation after conversion and maintaining a high H2/CO ratio. This can also be seen in the TOF values ( Figure S5)for both CH4 and CO2 conversion over the 20-hour reaction, the activity of the Ni-ceria particles decreases significantly more than the activity of the Ni-ceria flakes. After 20 hours, the Ni-ceria flakes experience a reduction in activity of 18% for CO2 conversion and 40% for CH4 conversion, while the activity of Niceria particles is reduced by 66% and 89%, respectively. Although only small improvements in activity are initially achieved with the ceria flakes compared with ceria particles, the main benefit of utilizing GO-templated ceria flakes is the improved thermal stability and resistance to sintering that is due to the stronger nickel-ceria interaction of the ceria flakes.
Nickel sintering and carbon deposition are the two major causes of catalyst deactivation of Ni based catalysts for dry methane reforming. [77] Post-reaction analysis was undertaken to determine the significance of these factors. Post-reaction powder XRD patterns are shown in Figure 8. Additionally, the ceria flakes are surrounded by a network of carbon nanotubes. In comparison, the Niloaded ceria particle catalyst is shown in Figure 10d. While the large ceria agglomerations seen in Figure   10b are still evident, the nickel is now also present in very large agglomerations being difficult to distinguish any structure at the nanoscale. Furthermore, no carbon nanotubes are visible. While both the Ni-ceria flake and Ni-ceria particle catalysts show sintering of the nickel particles (Figure 10c-d, S9-11), the sintering appears to be more severe for the untemplated Ni-ceria particle catalyst. The nanoflake catalyst shows a wide range of Ni particle sizes, from <100 nm to ~500 nm, while the untemplated particle catalyst only shows 300-500 nm and larger Ni particles.  In summary, the post-reaction analysis indicates that nickel sintering, and not carbon deposition, is responsible for the significant difference in catalyst deactivation in the long-term DRM experiment between the GO-templated and untemplated Ni-ceria catalysts. While both catalysts experienced nickel sintering, the untemplated Ni-ceria particle catalyst suffered from more severe sintering (Figure 10c-d,   S6). While TGA analysis ( Figure 12) showed carbon formation for the Ni-ceria flake catalyst and not the Ni-ceria particle catalyst, the TEM images ( Figure 10c) showed that this was a network of filamentous carbon. Carbon deposition has often been reported to be a significant factor in nickel catalyst deactivation. [78][79][80] However, the type of carbon deposit -encapsulating, filamentous, or other -affects methane reforming catalysts in different ways, if at all, and the relationship between catalytic activity and quantity of carbon deposited on the catalyst is not always straightforward. [81] While the formation of amorphous carbon can coat and deactivate catalysts, filamentous carbon growth can lead to structural modifications within the catalyst (such as forced separation of the nickel and ceria crystallites) which reduces activity. [82] Nevertheless, not all filamentous carbon growth appears to be harmful to catalytic activity. Additionally, it should be underlined that carbon formation is difficult to avoid for Ni-based materials during methane reforming reaction and the formation of carbon (soft or hard) is an indication of catalytic activity. [63] The key is developing Ni-based materials which lead to soft carbon formation, which is possible if Ni sintering is hindered, since large Ni clusters are more prone to forming hard carbon deposits than small Ni clusters. [64,83] In this scenario, Ni-ceria nanoflakes are excellent materials due to their resistance towards metallic sintering.
Overall, GO-templated ceria flakes outperform untemplated ceria particles for both CO oxidation and, with nickel deposition, the dry reforming of methane. Their improved resistance to sintering, higher BET surface area after exposure to high temperatures, and higher surface reducibility leads to a clear stability advantage in high-temperature reactions.

Conclusions
In conclusion, sintering-resistant GO-templated ceria nanoflakes have been successfully synthesized via a room-temperature precipitation reaction. The improved textural properties provided by GO templating result in a ceria catalyst capable of maintaining a higher surface area than untemplated ceria particles when calcined above 400 °C, demonstrating lower crystallite growth and improved resistance to sintering. Furthermore, Raman and TPR analysis show that GO-templated ceria flakes have a higher level of oxygen vacancies than untemplated ceria particles. For these reasons, GO-templated ceria flakes demonstrate improved catalytic activity for CO oxidation. Ceria flakes also inhibit sintering of deposited nickel particles when compared with nickel deposited on untemplated ceria particles. This presents advantages for the dry reformation of methane, where Ni/GO-templated ceria flakes maintains a considerably higher activity for the conversion of both methane and carbon dioxide than untemplated Ni/ceria particles in long-term stability tests.
It should be noted that, while more sintering-resistant than untemplated ceria particles, the nanoflakes do still lose specific surface area with exposure to high temperatures. Future research will include steps to mitigate this behavior furtherfor example, by investigating the applicability of GO templating to ceria-zirconia mixed oxides or the inclusion of other promoters such as Nd, Pr, La, etc.
which are known to improve thermal stability in applications such as automotive catalysis.
The capability to produce thermally stable ceria-based materials which limit high-temperature sintering is critical for ceria's further development across a range of environmentally applications. The use of GO as a sacrificial template offers a potential route for synthesizing thermally stable ceria catalysts, one in which a simple precipitation reaction is utilized and the template easily removed postsynthesis. 10