Applied Catalysis B: Environmental 242 (2019) 358–368 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Enhanced ceria nanoflakes using graphene oxide as a sacrificial template for T CO oxidation and dry reforming of methane Shawn C. Rooda,b, Huseyin B. Ahmetc, Anais Gomez-Ramond, Laura Torrente-Murcianoe, ⁎ Tomas R. Reinac, Salvador Eslavab, a Centre for Doctoral Training in Sustainable Chemical Technologies, University of Bath, Claverton Down, Bath, BA2 7AY, UK b Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, UK c Department of Chemical Engineering, University of Surrey, Guilford, GU2 7XH, UK d Departament d'Enginyeria Química, Biològica i Ambiental, Universitat Autònoma de Barcelona, Bellaterra, 08193, Spain e Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS, UK A R T I C LE I N FO A B S T R A C T Keywords: The development of novel fabrication methods to produce ceria catalysts with good high-temperature stability is Ceria critical for their implementation across a range of different applications. Herein, graphene oxide flakes are used Dry methane reforming as a sacrificial template in the synthesis of ceria particles to replicate the graphene oxide’s two-dimensionality. CO oxidation While performing the synthesis without graphene oxide results in large agglomerations of ceria crystallites, the Graphene oxide addition of graphene oxide during the synthesis results in ceria nanoflakes (< 10 nm) replicating the graphene Template oxide morphology. This novel shape limits the diffusion of atoms at high temperature to a two-dimensional plane which is translated into a low sintering degree and consequently, an enhanced thermal stability. In this way, the ceria flakes are capable of maintaining high surface areas after calcination at high temperatures (> 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 me- thane 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 cat- alytic performance at high temperatures. 1. Introduction deactivation in this reaction [12,13]. Recently, ceria’s redox cycle has been utilized in the thermochemical splitting of water, using con- Ceria-based materials are well-known for their use as a catalyst or centrated solar energy to reduce cerium ions to Ce3+ at high tem- catalyst support in a variety of environmentally sensitive applications, peratures, followed by reoxidation with water molecules to produce including automotive catalysis, VOC oxidation, solid oxide fuel cells, hydrogen, syngas, or hydrocarbons [5,14]. Another recent application steam reformation for hydrogen production, photocatalysis, and ther- for ceria is roomtemperature dehydrogenation of formic acid, where it mochemical water splitting [1–8]. Ceria’s ability to easily cycle be- demonstrated greatly improved activity compared with other supports tween 4+ and 3+ oxidation states allows it to act as an oxygen storage [15]. Ceria morphology at the nanoscale has also been shown to have medium - for example, storing and releasing oxygen in response to the an effect on the olefin selectivity on the CO2 hydrogenation reaction to varying air-to-fuel ratio in a vehicle’s exhaust to optimize catalytic hydrocarbons [16]. converter performance [9]. Ceria has also been utilized as a catalyst Many ceria applications involve heterogeneous surface-catalyzed support or promoter in the dry reforming of methane (DRM) for syngas reactions, so the ability to synthesize high surface area ceria is critical production, where it enhances activity [10,11]. The use of ceria in DRM to improve performance. Surface areas over 200 m2 g−1 are often re- as a catalyst support for nickel, instead of other supports such as Al2O3 ported for nanostructured ceria materials synthesized via a variety of or SiO2, also inhibits coke formation, otherwise a significant source of methods such as hydrothermal or sol-gel, and surface areas of Abbreviations:GO, graphene oxide ⁎ Corresponding author. E-mail address: s.eslava@bath.ac.uk (S. Eslava). https://doi.org/10.1016/j.apcatb.2018.10.011 Received 29 June 2018; Received in revised form 2 October 2018; Accepted 6 October 2018 Available online 09 October 2018 0926-3373/ © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/). S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 345 m2 g−1 has been obtained in ceria aerogel materials [17–19]. The until the pH of the supernatant matched that of the original distilled morphology of nanoceria and the nature of its exposed crystal planes water (typically after 16 washing cycles). Then, un-exfoliated graphite can also affect catalytic activity [6,20–22]. However, in numerous ap- particles were separated with two further low-speed (< 1000 rpm) plications, ceria must also present a high thermal stability to maintain a centrifugation cycles. Finally, the GO suspension was freeze-dried at useable surface area even when exposed to high temperatures. The next −60 °C and stored at room temperature. generation of low-temperature solid oxide fuel cells still have a tem- Ceria nanoflakes were prepared using a room temperature pre- perature range of up to 650 °C, while the ceria reduction step in ther- cipitation reaction [38]. A suspension of GO in deionized water (3 wt. mochemical water splitting generally involves temperatures greater %) was prepared. 3 mL of this 3 wt. % GO suspension in water was than 1200 °C. Automotive catalysts can be exposed to vehicle exhaust added to 600 mL of deionized water, and aqueous ammonia solution temperatures of 850 °C, and close-coupled automotive catalysts can (30%) was added to adjust the pH to 11.0. 150 mL of 0.05 mol L-1 reach 1050 °C [23,24]. Dry reformation of methane often requires cerium nitrate in deionized water was added dropwise to the GO/am- temperatures in excess of 800 °C due to the highly endothermic nature monia solution under stirring. The solution was left to stir for 3 h under of the reaction [25,26]. ambient conditions. Next, three cycles of centrifugation (4000 rpm for To synthesize nanostructured ceria of various morphologies, nu- 10 min) and redispersion in distilled water were performed to wash the merous templating agents have been reported, such as metal-organic product. The wet powder was freeze-dried at −60 °C. Untemplated frameworks and carbon microspheres [20,27,28]. Carbon nanotubes, ceria particles were prepared using the same procedure, without the ZnO nanotubes, and Ag nanowires have been used as templates to addition of GO prior to pH adjustment. synthesize ceria nanotubes, while polystyrene and silica spheres have After synthesis, the product was calcined at 400 °C for 3 h to remove been used to synthesize hollow ceria nanospheres [29–33]. However, the GO. For CO oxidation experiments and characterization, different these methods often involve either an additional chemical treatment samples were prepared by further calcination undertaken at 600, 800, step to remove the template and full removal is usually difficult to 900, and 1000 °C for 3 h. Calcination was performed in air under static achieve. conditions. The use of graphene oxide (GO) as a template offers an attractive For dry reformation of methane, the ceria catalysts calcined at alternative route for synthesizing high surface area nanostructured 400 °C were dispersed in ethanol with a corresponding amount of Ni ceria catalysts. GO consists of a two-dimensional monolayer of carbon (NO3)2·6H2O to achieve a loading of 10 wt. % Ni. After stirring for atoms, analogous to graphene, but interrupted by a range of oxygen- 30 min, the solvent was removed by drying under vacuum in a rotary containing groups [34]. evaporator. The product was further dried in an oven overnight at GO’s surface functional oxygen-containing groups allow it to be 80 °C. Finally, the dried powder was calcined at 700 °C for 4 h in air easily dispersed in a variety of solvents, unlike graphene, enabling its under static conditions. utilization in processes involving liquid suspensions or solutions [35]. Powder x-ray diffraction (XRD) was performed with either a Bruker The use of GO as a templating agent allows its two-dimensional struc- D8-Advance or X’Pert Pro PANalytical instrument using Cu Kα radia- ture to be replicated. For instance, the synthesis of manganese oxide tion. Samples were measured in the 2 theta range of 20-90° with a step nanoflakes has been reported by mixing a potassium permanganate size of 0.0164° and 0.25 s per step, or 5–80° with a step size of 0.05° and solution into a suspension of GO, resulting in the in situ replacement of step time of 0.16 s per step. Crystallite size was calculated using the carbon atoms with manganese [36]. Titanium oxide nanoflakes have Scherrer equation and the unit cell parameter was calculated using been synthesized by anchoring Ti16O16(OEt)32 clusters onto GO sus- Bragg’s law. Nitrogen adsorption experiments were performed with a pended in tetrahydrofuran, followed by calcination at 450 °C to de- Micromeritics 3Flex Surface Characterization Analyzer. Samples were compose the GO, which resulted in a two-dimensional titania structure degassed under vacuum at 200 °C for 90 min. Specific surface areas replicating the GO shape [34]. were calculated using the Brunauer-Emmett-Teller (BET) method and In this work, the use of GO as a sacrificial template for the synthesis pore size was calculated using the Barrett-Joyner-Halenda (BJH) of ceria nanoflakes via a precipitation reaction is reported. It is shown method on the desorption curves. that a highly two-dimensional structure is achieved. Due to improved Temperature-programmed reduction was performed with a textural properties, these ceria nanoflakes demonstrate better anti-sin- Micromeritics AutoChem II 2920. 0.1 g of each sample was heated tering behavior and higher catalytic activity compared with un- under 50 mL min-1 of 5% H2 in argon to 900 °C at a rate of 10 °C min−1. templated ceria particles produced in a GO-free precipitation synthesis. The outlet gas was analyzed to quantify the amount of hydrogen con- The catalytic activity of both materials (templated and un templated) is sumed with a thermal conductivity detector. Scanning electron micro- tested for CO oxidation for eventual application in automotive catalysis. scopy (SEM) was performed with an LEO 1525 field emission gun The use of templated ceria flakes as a catalyst support for nickel na- scanning electron microscope. Transmission electron microscopy (TEM) noparticles in dry reformation of methane (DRM) is also examined. In was performed with a JEOL JEM1200EXII or JEOL JEM-2100Plus mi- both cases, the flakes offer enhanced catalytic activity compared with croscope. EDX analysis was performed with an Oxford Instruments untemplated ceria particles. Templating ceria with GO offers a potential large area EDX detector integrated into the TEM. Raman analysis was way for the synthesis of catalysts with better performance for high- performed with a Renishaw inVia Raman Microscope. A 532 nm green temperature applications such as automotive catalysis or dry reforming laser was used, and spectra were obtained with 2–10 s exposure time at of methane. 1–10% laser power. Thermogravimetric analysis (TGA) after the DRM reactions was carried out in a TA Instruments Discovery TGA, Q50. 2. Experimental 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. GO was prepared by oxidizing and exfoliating graphite with a Catalytic activity tests for CO oxidation were performed in a U- modified Tour et al. synthesis method [34,37]. 24 g of 100–500 μm shaped quartz tube reactor (10 mm ID) at atmospheric pressure. In a natural graphite flakes (Aldrich) were added to a concentrated acid typical experiment, 10 mg of ceria catalyst was dispersed in a 4 cm3 mixture (3 L H2SO4 : 0.3 L H3PO4) under vigorous stirring. 144 g of catalytic bed consisting of silicon carbide particles. The catalytic bed KMnO4 was added gradually. The reaction mixture was vigorously was secured at both ends with quartz wool. The reactant feed consisted stirred for 18 h at 50 °C. Next, the mixture was cooled to room tem- of 2000 ppm each CO and O2 in nitrogen, with a total flow rate of 50 mL perature and 1.72 L of 2 wt. % aqueous H2O2 was added dropwise to min−1, achieving a weight hourly space velocity (WHSV) of 300 L g−1 stop the oxidation reactions. The resulting suspension of GO was wa- h 1. The catalyst was tested from room temperature to 500 °C, and the shed by repeated centrifugation and re-dispersion in distilled water outlet gas was measured with a Fuji Electric ZRH Infrared Gas Analyzer 359 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 and a Hiden mass spectrometer. Statistical analysis: each data point Powder XRD patterns of the calcined ceria particles and nanoflakes reported for CO oxidation represents the average of ten continuously crystallite sizes show that both are very similar. Fig. 3 shows that the logged data points during testing. Therefore, error bars reported in diffraction peaks get narrower and taller with increasing calcination Fig. 7 show instrument error. temperature, indicating a growth in crystallite size. However, this trend Catalytic activity tests for the dry reforming of methane were per- is more prominent for the untemplated ceria particles compared to the formed in a tubular quartz reactor (10 mm ID) at atmospheric pressure. GO-templated ceria nanoflakes for any calcination temperature. Crys- Catalysts were reduced in H2 atmosphere (10 vol. % H2 balanced in N2) tallite size calculations using the ceria (111) peaks clearly show this at 850 °C for 1 h prior to the activity tests. In a typical experiment, trend in Table 1. When calcined at 400 °C, the ceria particles and flakes 100 mg of Ni-ceria catalyst was loaded in the reactor, supported on a have a similar crystallite size, slightly smaller for the flakes (6.7 and bed of quartz wool. The reactant feed consisted of a 1/1/6 ratio of CH4/ 6.0 nm, respectively). But upon calcination at higher temperatures, CO2/N2, with a total flow rate of 100 mL min−1, achieving a WHSV of crystallite size increases more quickly for the untemplated particles 60 L g−1 h−1. Reactants and products were monitored using an on-line than for the GO-templated nanoflakes. This observation indicates that gas analyzer (ABB AO2020), equipped with both IR and TCD detectors. the two dimensional high aspect ratio arrangement of crystallites of the The catalyst was tested between 550 and 850 °C. Long-term dry re- GO-templated nanoflakes replicating the GO morphology limits the forming studies were performed at a constant temperature of 800 °C for diffusion of atoms at high temperature to a two dimensional plane 20 h. 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. 3. Results and discussion CHN analysis indicates that the GO is eliminated from the templated ceria material after the initial calcination at 400 °C. Pre-calcination, the Ceria particles and nanoflakes were prepared via a room tempera- ceria flakes and untemplated ceria particles contain 1.64 and 0.38 wt. ture precipitation synthesis in the absence and presence of GO respec- % carbon, respectively, confirming the presence of GO. After calcina- tively, followed by a calcination in air at 400 °C to remove the GO tion at 400 °C, the ceria flakes and untemplated ceria particles contain template. An SEM micrograph of the GO used is shown in Fig. 1. Prior to 0.39 and 0.34 wt. % carbon, respectively. These comparable values calcination, the nanoflakes are a brown powder, while after calcination confirm the volatization of GO during the calcination in air at 400 °C. at 400 °C, the product is yellow, similar to other ceria syntheses (Fig. Visual analysis of the materials support this – before calcination, the S1). The untemplated ceria particles were bright yellow both pre- and GO-templated ceria flakes are dark brown, but after calcination, they post-calcination. The GO-templated ceria nanoflakes were much more are pale yellow, more typical for ceria powders (Fig. S1). Numerous loosely packed than the untemplated ceria particles. Without any fur- examples of GO-ceria composite materials have been reported in the ther treatment, the nanoflakes had a bulk density of 0.34 g mL−1, while literature. For instance, hydrothermal methods have been used to the untemplated particles had a bulk density of 0.65 g mL−1. synthesize ceria nanoparticles dispersed on the surface of graphene Fig. 2 shows TEM images of GO-templated ceria nanoflakes and oxide for the oxidation of uric acid, degradation of methylene blue, and untemplated ceria particles calcined in air at different temperatures removal of arsenic species from water [39–41]. However, because between 400–1000 °C. The difference between the two materials is calcination at 400 °C is sufficient to completely remove GO from such significant - while the GO-templated synthesis provided a two-dimen- composite materials, there is quite a low upper temperature limit for sional arrangement of ceria crystallites, the untemplated synthesis re- such catalysts to be useful. In contrast, the GO-templated ceria flakes sulted in much larger unstructured agglomerations. Typical ceria flakes retain useful properties at much higher temperatures. calcined at 400 °C were a few hundred nanometers in diameter and less Both the ceria particles and templated flakes were analyzed with than 10 nm in thickness, while the size of the untemplated ceria particle Raman spectroscopy, shown in Fig. 4. In both cases, ceria’s character- agglomerations was significantly larger, in the micrometer range. istic F2g band at approximately 466 cm−1, attributable to the vibra- Subsequent calcinations in air at 600, 800, 900, or 1000 °C to assess tional characteristics of oxygen atoms surrounding cerium in the thermal stability also show differences between GO-templated ceria fluorite crystal structure, is clearly visible [42]. An additional defect flakes and untemplated ceria particles. While both the ceria nanoflakes band at 595 cm−1 is also visible in both samples calcined at 400 °C. This and particles show crystallite growth with increasing calcination tem- defect band can be attributed to the presence of Ce3+ related defects in perature, the ceria nanoflakes retained their two-dimensional mor- the ceria lattice, or to defects caused by crystallite size effects – in phology even at high calcination temperatures (1000 °C). 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 le- vels or lattice contraction, and this would also be indicated in a dif- ference in intensity in the 595 cm−1 band [45–47]. 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, al- though 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. For pure ceria, Δω = -γω/(ΔV/V0), where ω is the shift in Raman frequency, γ is the Grüneisen parameter (1.24 for this mode), and ΔV is the volume change from the reference volume. Volume changes of the unit cell are due to expansion of Ce4+ to Ce3+ (ionic radii 0.970 Å and 1.143 Å, respectively), which is only partially offset by the replacement of O2- with oxygen vacancies (ionic radii 1.380 and Fig. 1. SEM micrograph of graphene oxide used as the template for ceria na- 1.164 Å, respectively). Assuming pure ceria with no other contributing noflakes synthesis. The size distribution of GO flakes was approximately defects, the relationship between oxygen vacancy concentration δ and 1–20 μm. change in volume ΔV/V0 is relatively linear: δ = −10(Δa/a0) = 360 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Fig. 2. (above) TEM micrographs of untemplated ceria particles, calcined in air between 400 and 1000 °C. (below) TEM micrographs of GO-templated ceria flakes, calcined in air between 400 and 1000 °C. Two-dimensional morphology is clearly retained for GO-templated ceria after thermal treatment. Fig. 3. Powder X-ray diffraction patterns for (left) untemplated ceria particles and (right) GO-templated ceria flakes, calcined between 400 and 1000 °C. −3.3(ΔV/V0), where (Δa/a0) is the change in the lattice constant. temperature-programmed reduction was performed, shown in Fig. 5. Therefore, δ = 2.66(Δω/ω0) [49,50]. It should be noted that the peak Typically, two TPR peaks are associated with nanostructured ceria - a shift in these samples is quite small (< 4 cm−1), creating a degree of lower-temperature peak associated to readily available surface oxygen uncertainty. Furthermore, depending how far the Raman laser pene- reduction, and a higher-temperature peak representing the reduction of trates through the catalyst materials, this is a measure of oxygen va- the bulk lattice oxygen [6,51]. As shown in Fig. 5a–b, both the GO- cancies at or near the surface of the particles. Nevertheless, the trend is templated and untemplated ceria calcined at 400 and 600 °C clearly clear – untemplated ceria particles show lower levels of oxygen va- show both peaks. However, for the 800 °C calcination, this is greatly cancies than GO-templated ceria nanoflakes. reduced for both samples. Table 2 shows the fraction of peaks in- To further quantify oxygen vacancy levels in the ceria materials, tensities, clearly showing that the ceria flakes appear to retain more 361 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Table 1 (Fig. S2 in supporting information), characteristic of mesoporous ma- Structural properties of ceria particles and GO-templated ceria nanoflakes. terials [52]. The type H2 hysteresis loop displayed is characteristic of a Calcination T Crystallite size BET surface Avg. pore Oxygen non-uniform network of pores, in both size and shape [53]. In both (°C) (nm)a area (m2/g) diameter vacancies (%)c cases, the type IV isotherm is maintained upon calcination at 600 °C. As (cm3/g)b shown in Fig. S2(a), nitrogen adsorption and desorption from un- templated ceria particles calcined at 800 °C and above is negligible. GO- Untemplated ceria particles 400 5.9 116 33.9 2.7 templated ceria nanoflakes calcined at 400 °C show a pore size dis- 600 10.7 18 33.3 1.3 tribution with narrower pore diameters than untemplated ceria parti- 800 35.4 1 0.8 cles, shown in Fig. S3. However, upon calcination at 600 °C, both ma- 900 48.7 <1 0.9 terials show a similar pore size distribution. 1000 56.8 <1 0.6 While both materials experience severe sintering at high tempera- GO-templated ceria flakes 400 5.4 113 29.1 3.3 tures, the GO-templated ceria nanoflakes maintain a higher BET surface 600 7.1 43 38.8 3.2 area compared to untemplated ceria particles after calcination at the 800 21.4 9 49.8 1.4 same temperature (Fig. 6 and Table 1). This improved thermal stability 900 30.5 6 41.5 0.9 of the flake materials is in agreement with the crystallite sizes derived 1000 42.1 6 47.0 1.0 from powder XRD data. a Calculated using Scherrer equation using ceria (111) peak in XRD pattern. The catalytic activity of GO-templated ceria flakes and untemplated b Calculated using Barret-Joyner-Halenda (BJH) method on desorption ceria particles for CO oxidation is shown in Fig. 7. Catalysts were cal- curves. cined in static air at 400, 600, or 800 °C respectively prior to testing. c Calculated from Raman spectra F2g band shift. The GO-templated ceria flakes calcined at 400 °C achieves similar al- though slightly superior activity than the untemplated ceria particles up low-temperature surface reducibility than the untemplated ceria par- to 400 °C reaction conditions. At higher reaction temperatures ticles for all three calcination temperatures in agreement with the (400–500 °C), considerably higher conversions are achieved with the Raman spectra analysis. ceria flakes compared to the particles due to their considerably lower The physical properties of both materials were also characterized by in-situ sintering as shown by the smaller reduction on surface area nitrogen adsorption. Both the GO-templated ceria flakes and un- above 400 °C (Table 1). At 500 °C, the ceria flakes achieve full con- templated ceria particles calcined at 400 °C show a type IV isotherm version of CO to CO2, while the untemplated ceria particles only reach Fig. 4. Raman spectra of (left) untemplated ceria particles and (right) GO-templated ceria flakes, calcined between 400 and 1000 °C. 362 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Fig. 5. TPR profiles for GO-templated ceria flakes and untemplated ceria particles calcined at (a) 400 °C, (b) 600 °C, and (c) 800 °C. Table 2 sacrificial GO results in more thermally stable catalysts. Fraction of peak areas in TPR profiles for untemplated ceria particles and GO- Templated ceria nanoflakes perform comparably to or slightly better templated ceria flakes. than some nanostructured ceria catalysts recently reported in the lit- Calcination T (°C) Surface oxygen peak Bulk oxygen peak (fraction) erature for CO oxidation. At 250 °C, the turnover frequency of these (fraction) ceria nanoflakes for CO oxidation (in terms of μmol of CO converted per g of catalyst) is 19.1 μmol g−1 min−1, while other papers show catalyst Untemplated ceria particles performance corresponding to 14–20 μmol g−1min−1 [54,55]. Other 400 0.64 0.36 600 0.53 0.47 papers report catalysts with conversion rates corresponding to much 800 0.03 0.97 higher TOF values; 53 and 163 μmol g-1 min-1 for solvothermally pre- GO-templated ceria flakes pared ‘cauliflower-like’ ceria and microwave-assisted hydrothermally 400 0.71 0.29 synthesized ceria nanocubes, respectively [56,57]. However, it should 600 0.65 0.35 be noted that variations in reaction conditions can limit comparability; 800 0.06 0.94 in particular, significantly different O2:CO ratios in reaction gas mix- tures. 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 Fig. 6. (a) BET surface area vs. calcination temperature and for GO-templated nanoflakes retain a higher population of oxygen defects after thermal ceria flakes and untemplated ceria particles. treatment compared to ceria nanoparticles and thus improved re- ducibility, exhibiting improved oxidation activity. approximately 80% conversion. GO-templated ceria flakes also out- In addition to bare ceria nanoflakes and particles, nickel-loaded ceria nanoflakes and particles were synthesized as catalysts for the dry performed the untemplated particles when calcined at 600 °C - at this calcination temperature the GO-templated ceria flakes are capable of reforming of methane. Nickel is a common low-cost alternative to noble metal catalysts for this application. While noble metals such as Pt, Pd, maintaining a high surface area and high concentration of their initial surface oxygen concentration, critical for the CO oxidation reaction. In or Rh typically retain higher stability, activity and resistance to coke formation than non-noble alternatives, the interaction between the Fig. S4, this is shown in terms of turnover frequency (TOF) calculations metal and support can play a large role in modifying metal dispersion at 450 °C. These results demonstrate that templating ceria with and electronic effects – and thus overall catalyst performance Fig. 7. Catalytic activity for CO oxidation of GO-templated ceria flakes and untemplated ceria particles, calcined at (a) 400 °C, (b) 600 °C, and (c) 800 °C. 363 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Fig. 8. Powder XRD patterns of Ni-ceria catalysts, pre- and post-reaction. (a) Untemplated Ni-ceria particles. (b) GO-templated Ni-ceria flakes. [26,64–66]. Here, the effect of using GO-templated ceria nanoflakes vs. nanoflake TPR profile, but not the NiO/ceria particle sample, attributed untemplated ceria particles as a support for nickel is explored. to readily available surface oxygen in the ceria flakes. Powder XRD patterns of the Ni-ceria samples are shown in Fig. 8. TEM micrographs of the fresh Ni-loaded ceria flakes and particles After nickel deposition and calcination at 700 °C, but pre-reaction, NiO are shown in Fig. 10a–b. EDX analyses and maps (Figs. S6–8) show that is visible in the diffraction pattern. Using the Scherrer equation, the the smaller (< 10 nm) crystallites are primarily ceria, while the larger sizes of the ceria crystallites are calculated to be 6.6 and 5.7 nm on (20–40 nm) crystallites are nickel (NiO according to XRD). These ceria templated ceria flakes and untemplated ceria particles, respectively, crystallite sizes match those calculated using Scherrer equation analysis while the size of NiO particles is calculated to be 20 and 24 nm, re- using the ceria XRD patterns in Fig. 3 (∼6 nm for both materials, spectively. These values are broadly similar, given the limitations of Table 1), and Ni-loaded ceria XRD patterns in Fig. 8. In Fig. 10a, of the Scherrer equation calculations, indicating that both templated Ni-ceria fresh Ni-loaded ceria flakes, NiO particles can be seen clustering around flakes and untemplated Ni-ceria particles initially have similar crys- the edges of ceria crystallite agglomerations. However, for the fresh tallite sizes for both the ceria and NiO components of the catalyst. untemplated Ni-loaded ceria particles, shown in Fig. 10b, while some Additionally, TPR experiments were conducted on the Ni-deposited NiO particles appear to be similarly deposited on the edge of ceria ceria nanoflakes and particles, shown in Fig. 9. The TPR profiles are crystallite agglomerations, much of the visible nickel is not deposited similar to the ones of the bare ceria materials (Fig. 5) with the addition on ceria. These un-supported NiO particles were evenly dispersed of low temperature peaks (< 400 °C), attributable to NiO reduction. across the TEM sample grid, a phenomenon not observed with the Ni- The lowest temperature (∼200 °C) peak can be attributed to the re- ceria flake sample. This observation suggests that NiO particles have a duction of larger “free” NiO not bound to the ceria support, while the better interaction with the ceria support when supported on ceria flakes higher-temperature peak (at ∼325 °C) is attributed to NiO bound to the than ceria particles in agreement with the TPR data. Surface defects surface of ceria [67,68]. For this main peak, the NiO-ceria nanoflake such as oxygen vacancies are known to be preferential nucleation sites TPR profile shows a shift to higher reduction temperatures compared for metal particle formation [73,74]. Therefore, the ceria flakes pro- with the NiO-ceria particle sample. There are two possible explanations mote better nickel dispersion and stronger Ni-ceria interaction than the for this. The size of NiO particles can affect the reduction temperature, ceria particles, in agreement with the higher surface oxygen vacancy so this is possibly indicative of different NiO particle sizes on the ceria concentration of the nanoflakes, seen in Raman and TPR analysis. nanoflake sample [69,70]. Alternatively, metal-support interactions can Catalytic activity of the Ni-loaded ceria particles and flakes for the influence the reduction temperature as well. In the case of nickel-ceria, dry reforming of methane (DRM) is shown in Fig. 11a–b. In both cases, stronger interaction between NiO and the ceria support results in higher conversion of CH4 is lower than conversion of CO2 across the tem- reduction temperatures [71,72]. XRD analysis showed that the NiO perature range tested, a common observation in DRM because methane crystallite sizes in the nanoflake and untemplated particle systems were activation is the rate-limiting step [75]. For CO2, conversion levels similar, so the shift to a higher reduction temperature is likely due to range from approximately 20% at 550 °C to 90% at 850 °C, while CH4 differences in NiO-ceria interaction in the two catalysts. A shoulder conversion levels are less than 5% and 60–65% at 550 and 850 °C, re- extending from approximately 450 to 500 °C is visible for the NiO/ceria spectively. The ratio of H2 to CO produced ranges from 0.5 to 0.8 across the same temperature range. While this is promising, conversion levels do not appear to be as high as other nickel-based DRM catalysts re- cently reported in the literature. However, with papers reporting wide variation in Ni loading (1 to 63.5%), reduction temperatures, space velocity and gas ratios, results are not always directly comparable [76]. The Ni supported on ceria flakes and particles perform similarly, al- though there are small differences. In 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 Fig. 11c–d. In contrast with the short-term tem- perature curve experiment shown in Fig. 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 approximately 5 h, while Fig. 9. TPR profiles for nickel-deposited GO-templated ceria flakes and un- the Ni-ceria flakes demonstrated much higher stability for both CH4/ templated ceria particles. CO2 conversion and maintaining a high H2/CO ratio. This can also be 364 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Fig. 10. TEM micrographs of Ni-ceria cata- lysts, both fresh and post-methane reforming reaction. Dark and light areas are Ni and ceria, respectively. (a) Fresh Ni-ceria flake catalyst after calcination at 700 °C. (b) Fresh Ni-ceria particle catalyst after calcination at 700 °C. (c) Post-20 h, 800 °C reaction Ni-ceria flake cata- lyst. (d) Post 20 h, 800 °C reaction Ni-ceria particle catalyst. seen in the TOF values (Fig. S5) – for both CH4 and CO2 conversion over particles is reduced by 66% and 89%, respectively. Although only small the 20-hour reaction, the activity of the Ni-ceria particles decreases improvements in activity are initially achieved with the ceria flakes significantly more than the activity of the Ni-ceria flakes. After 20 h, the compared with ceria particles, the main benefit of utilizing GO-tem- Ni-ceria flakes experience a reduction in activity of 18% for CO2 con- plated ceria flakes is the improved thermal stability and resistance to version and 40% for CH4 conversion, while the activity of Ni-ceria sintering that is due to the stronger nickel-ceria interaction of the ceria Fig. 11. (a) Catalytic activity of Ni-loaded ceria flakes and particles for the dry reforming of methane, showing both CH4 and CO2 conver- sion. (b) The ratio of H2 to CO produced as a function of temperature. (c) Long-term cata- lytic activity of Ni-loaded ceria flakes and par- ticles at 800 °C for the dry reforming of me- thane, showing both CH4 and CO2 conversion. (d) The ratio of H2 to CO produced as a func- tion of time at 800 °C. 365 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 flakes. (Fig. 10c) showed that this was a network of filamentous carbon. Nickel sintering and carbon deposition are the two major causes of Carbon deposition has often been reported to be a significant factor in catalyst deactivation of Ni based catalysts for dry methane reforming nickel catalyst deactivation [78–80]. However, the type of carbon de- [77]. Post-reaction analysis was undertaken to determine the sig- posit - encapsulating, filamentous, or other - affects methane reforming nificance of these factors. Post-reaction powder XRD patterns are shown catalysts in different ways, if at all, and the relationship between cat- in Fig. 8. Compared with the pre-reaction patterns, nickel is now pre- alytic activity and quantity of carbon deposited on the catalyst is not sent as Ni instead of NiO. Similarly to the pre-reaction NiO-ceria sam- always straightforward [81]. While the formation of amorphous carbon ples, the ceria crystallite sizes are similar in the spent catalysts – 34.4 can coat and deactivate catalysts, filamentous carbon growth can lead and 33.1 nm for flakes and particles, respectively. In contrast, the Ni to structural modifications within the catalyst (such as forced separa- metal particles deposited on the post-reaction ceria particles are cal- tion of the nickel and ceria crystallites) which reduces activity [82]. culated to be 56 nm, compared with 43 nm for post-reaction ceria na- Nevertheless, not all filamentous carbon growth appears to be harmful noflakes. This is an indication that while ceria nanoflakes do not en- to catalytic activity. Additionally, it should be underlined that carbon tirely prevent nickel sintering, they do hinder sintering in comparison formation is difficult to avoid for Ni-based materials during methane to nickel particles deposited on untemplated ceria, and shows that the reforming reaction and the formation of carbon (soft or hard) is an Ni particles are better dispersed on ceria flakes than ceria particles post- indication of catalytic activity [63]. The key is developing Ni-based reaction, in agreement with the TPR results. materials which lead to soft carbon formation, which is possible if Ni TEM micrographs of post-reaction spent Ni-ceria catalysts are sintering is hindered, since large Ni clusters are more prone to forming shown in Fig. 10c–d. After the 20 h stability experiment, the Ni-ceria hard carbon deposits than small Ni clusters [64,83]. In this scenario, Ni- flake catalyst, shown in Fig. 10c, has retained its two-dimensional ceria nanoflakes are excellent materials due to their resistance towards structure. However, the nickel particles have sintered, showing a wide metallic sintering. range of sizes. Additionally, the ceria flakes are surrounded by a net- Overall, GO-templated ceria flakes outperform untemplated ceria work of carbon nanotubes. In comparison, the Ni-loaded ceria particle particles for both CO oxidation and, with nickel deposition, the dry catalyst is shown in Fig. 10d. While the large ceria agglomerations seen reforming of methane. Their improved resistance to sintering, higher in Fig. 10b are still evident, the nickel is now also present in very large BET surface area after exposure to high temperatures, and higher sur- agglomerations being difficult to distinguish any structure at the na- face reducibility leads to a clear stability advantage in high-tempera- noscale. Furthermore, no carbon nanotubes are visible. While both the ture reactions. Ni-ceria flake and Ni-ceria particle catalysts show sintering of the nickel particles (Figs. 10c–d, S9–11), the sintering appears to be more severe 4. Conclusions for the untemplated Ni-ceria particle catalyst. The nanoflake catalyst shows a wide range of Ni particle sizes, from < 100 nm to ∼500 nm, In conclusion, sintering-resistant GO-templated ceria nanoflakes while the untemplated particle catalyst only shows 300–500 nm and have been successfully synthesized via a room-temperature precipita- larger Ni particles. tion reaction. The improved textural properties provided by GO tem- TGA was performed on the post-20 h reaction Ni-ceria catalysts, plating result in a ceria catalyst capable of maintaining a higher surface shown in Fig. 12. The TGA curve of the Ni-ceria flakes shows a sharp area than untemplated ceria particles when calcined above 400 °C, decrease in mass at approximately 500–600 °C, indicative of the pre- demonstrating lower crystallite growth and improved resistance to sence of deposited carbon on the catalyst after reaction. However, the sintering. Furthermore, Raman and TPR analysis show that GO-tem- Ni-ceria particles do not show a similar decrease in mass. This indicates plated ceria flakes have a higher level of oxygen vacancies than un- that significant carbon deposition occurred on the Ni-ceria flakes, but templated ceria particles. For these reasons, GO-templated ceria flakes not the Ni-ceria particles, confirming a different reaction behavior, and demonstrate improved catalytic activity for CO oxidation. Ceria flakes correlates well with TEM post-reaction analysis. also inhibit sintering of deposited nickel particles when compared with In summary, the post-reaction analysis indicates that nickel sin- nickel deposited on untemplated ceria particles. This presents ad- tering, and not carbon deposition, is responsible for the significant vantages for the dry reformation of methane, where Ni/GO-templated difference in catalyst deactivation in the long-term DRM experiment ceria flakes maintains a considerably higher activity for the conversion between the GO-templated and untemplated Ni-ceria catalysts. While of both methane and carbon dioxide than untemplated Ni/ceria parti- both catalysts experienced nickel sintering, the untemplated Ni-ceria cles in long-term stability tests. particle catalyst suffered from more severe sintering (Fig. 10c–d, S6). It should be noted that, while more sintering-resistant than un- While TGA analysis (Fig. 12) showed carbon formation for the Ni-ceria templated ceria particles, the nanoflakes do still lose specific surface flake catalyst and not the Ni-ceria particle catalyst, the TEM images area with exposure to high temperatures. Future research will include steps to mitigate this behavior further – for 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 post-synthesis. Acknowledgements This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) [grant numbers EP/L016354/1, EP/ R512904/1, EP/P008097/1, and EP/L020432/2]. The authors would like to thank the Microscopy and Analysis Suite (MAS) at the University Fig. 12. TGA curves for post-20 h reaction Ni-ceria flakes and particles in air. of Bath for assistance with TEM and Raman characterization. 366 S.C. Rood et al. Applied Catalysis B: Environmental 242 (2019) 358–368 Experimental data is available via the University of Bath Research Data Structure–activity relationships of NiO on CeO2 nanorods for the selective catalytic Archive (DOI: https://doi.org/10.15125/BATH-00514). reduction of NO with NH3: experimental and DFT studies, J. Phys. Chem. 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