Results of GO on the construction and texture of catalysts
The scanning electron microscope (SEM) photographs present that every one samples of Co3O4, CoGO50 and CoGO100 have the hexagonal plate-like morphology (Fig. 1A,C,E). Famous that the precursor of Co3O4, i.e., CoOOH, additionally possesses the hexagonal nanoplate morphology, suggesting that the form of Co3O4 was nicely maintained upon the calcination (Determine S1). The common sizes of CoGO20, CoGO50 and CoGO100 nanoplates (~ 150–250 nm) are barely smaller than that of Co3O4 (~ 300 nm) (Figs. 1 and S2). This means that GO addition not solely protects the plate form of Co3O4 but in addition confines the expansion of Co3O4 crystals. The transmission electron microscope (TEM) photographs of single nanoplate of Co3O4, CoGO50 and CoGO100 show that the thickness of CoGO50 is thinner than others, which can contribute to the confinement impact of GO flakes within the preparation (Fig. 1B,D,F). As well as, no graphene layers had been discovered from each CoGO50 and CoGO100, suggesting that GO has been consumed within the calcination of catalysts.
The high-resolution TEM (HRTEM) picture of CoGO50 exhibits an interplanar spacing of 0.287 nm, similar to the cubic Co3O4 (220), and an interplanar spacing of 0.467 nm, assigned to the cubic Co3O4 (111) airplane (Fig. 1G). The microstructure of CoGO50 is nicely in line with that of Co3O4 (Determine S3). Persistently, no lattice fringe of graphic carbon was noticed from the HRTEM picture of CoGO50 (Fig. 1G). The corresponding selected-area electron diffraction (SAED) sample reveals the standard diffraction spots of hexagonal Co3O4, suggesting a excessive crystallinity of CoGO50 (Fig. 1H).
The crystalline construction of the catalysts was measured by X-ray diffraction (XRD), as proven in Fig. 2A. Earlier than calcination, CoGO50 precursor displays the crystal construction of CoOOH (JCPDS PDF#07-0169). As well as, no alerts of GO had been detected from CoGO50 precursor, suggesting that GO is extremely dispersed on the CoOOH floor with ultrathin thickness. The patterns of Co3O4, CoGO50 and CoGO100 present the identical diffraction peaks at 2θ = 19.0°, 31.3°, 36.9°, 38.5°, 44.8°, 55.7°, 59.4°, 65.2°, and 77.3°, that are assigned to (111), (220), (311), (222), (400), (422), (511), (440), and (533) lattice planes of face cantered cubic Co3O4 (JCPDS PDF# 42-1467), respectively23. Equally, no peaks of GO (~ 12°) or graphitic carbon (~ 25°) had been noticed from CoGO50 and CoGO100, indicating that GO was decomposed within the calcination or the quantity of residual GO/GO-derivatives is past the detection restrict24.
The construction of samples was additional investigated by Raman spectroscopy (Fig. 2B). The spectrum of GO comprises a G band at 1595 cm−1, arising from the first-order scattering of sp2 carbon atoms in a 2D hexagonal lattice, and a D band at 1343 cm−1, ascribed to the vibrations of carbon atoms in airplane terminations of disordered graphite25,26. For the CoGO50 precursor, the attribute D and G bands of GO with weak intensities had been additionally noticed (Fig. 2B inset), indicating that the floor lined GO is ultrathin. That is in line with the XRD outcomes that GO is extremely dispersed on the CoOOH floor. The spectra of CoGO50 and CoGO100 submit peaks at 467 cm−1 (Eg), 515 cm−1 (F1g1), 613 cm−1 (F2g1) and 679 cm−1 (A1g1)27, that are the identical as that of Co3O4 spinel construction28. This reveals that the majority construction of Co3O4 was nicely maintained for the samples of CoGO50 and CoGO100. Furthermore, no alerts of graphtic D and G bands had been noticed from each CoGO50 and CoGO100, suggesting the quantity of GO or rGO is under the detection restrict.
The interior construction and floor space of the samples had been measured by nitrogen adsorption isotherms (Fig. 2C). All of the samples confirmed the IV-typed sorption isotherm with H3-typed hysteresis loop within the relative stress vary of P/P0 = 0.5–1.0, suggesting a mesoporous construction for these samples29,30. The pore sizes calculated by the Barrett–Joyner–Halenda (BJH) technique present a unimodal distribution centred at ~ 12.7 nm for these samples (Fig. 2D), which is in line with the TEM observations. For CoGO50 and CoGO100, the Brunauer–Emmett–Teller (BET) particular floor areas are 39.7 and 43.2 m2 g−1, respectively, that are increased than that of Co3O4 (31.5 m2 g−1, Desk S1). This means that the addition of GO will increase the BET floor space via forming smaller and thinner nanoplates. Furthermore, the samples of CoGO50 and CoGO100 additionally exhibit bigger pore volumes (0.31 and 0.28 cm3 g−1) than that of Co3O4 (0.23 cm3 g−1), suggesting that the existence of GO layer promotes the formation of pore through the calcination.
The thermal behaviours of the catalysts had been investigated by a thermogravimetric-mass analyser (TG-MS) within the temperature vary of 60–1000 °C. As proven in Fig. 3A, the CoGO50 precursor (i.e., GO lined CoOOH) displays weight reduction primarily at ~ 110 °C and ~ 320 °C. Mixed with the mass spectrometry alerts, the load loss at low temperature of 80–120 °C is ascribed to evaporation of adsorption water whereas the load loss centred at 286 °C is contributed to the dehydration response of section transition from CoOOH to Co3O4 in addition to the thermal discount of GO to lowered GO (rGO)17. Moreover, CO2 sign was detected at 319 °C, which displays the deep oxidation of GO/rGO to CO2 and H2O. It confirms that the floor lined GO was decomposed within the calcination of catalyst, according to the observations of TEM and Raman measurements. The decomposition of rGO layer would impression the floor chemistry of Co3O4 if the floor oxygen of Co3O4 is concerned within the oxidation of rGO. The thermal stability of resultant catalysts was examined by TG (Fig. 3B). Besides the desorption of water at low temperature, all of the samples saved secure in weight on the temperature of < 890 °C, suggesting a dependable thermal stability for these catalysts. Then again, TG curves present weight reduction within the temperature ranges of 890–930 °C, which will be attributed to the section transition from Co3O4 to CoO with O2 era (Determine S4). The load loss fee of this step was 6.4%, which is near the theoretical worth of O2 launch (6.6%).
Results of GO on the chemistry of catalysts
The near-surface digital states of the catalysts had been examined by X-ray photoelectron spectroscopy (XPS). The C1s XPS spectra of Co3O4, CoGO50 and CoGO100 show robust sign of C–C (284.8 eV) with weak alerts of C–O (286.1 ± 0.2 eV) and C=O (288.2 ± 0.1 eV)31, as proven in Fig. 4A. The carbon alerts of Co3O4 comes from the contaminations, comparable to conductive tape, which generally has C–C, C–O–C and O–C=O teams31. CoGO50 possesses related C–O and C=O intensities to that of Co3O4, whereas CoGO100 has barely increased C–O and C=O than that of Co3O4 and CoGO50. It signifies that no GO or GO derivatives had been left for CoGO50 upon the calcination, according to the observations of TEM and XRD. As compared, CoGO100 comprises hint GO residues on the floor due to increased GO focus within the preparation.
For the dual peaks of Co 2p, the binding energies (BE) at 781.5 eV and 796.5 eV are attributed to Co2+ whereas BEs at 794.5 eV and 779.0 eV are assigned to Co3+ (Fig. 4B)32. As proven in Desk 1, the floor Co2+/Co3+ ratio of CoGO50 is 0.86, which is considerably increased than that of Co3O4 (0.58) and CoGO100 (0.65). This means that the floor of Co3O4 is lowered by GO and its derivatives within the preparation. Within the calcination of catalyst, the tightly contacted GO could be thermal-reduced firstly by dropping oxygen-containing teams beneath the heating situations. With increased temperature, the rGO tends to be additional oxidized to CO2 and H2O, the place oxygen molecules and floor oxygen of Co3O4 are concerned within the oxidation response (Determine S5). Subsequently, partial of Co3+ atoms on the catalyst floor could be lowered to Co2+ and/or oxygen vacancies could be fashioned on the catalyst floor33.
As proven in Fig. 4C, the O1s spectra of catalysts had been composed by two peaks centred at 529.7 ± 0.1 eV (lattice oxygen, Olatt) and 531.2 ± 0.2 eV (floor adsorbed oxygen, Oadverts)29,33,34. The floor Oadverts/Olatt ratio of CoGO50 is 0.75, which is increased than that of Co3O4 and CoGO100 (Desk 1). As indicated by the Co 2p outcomes, the partial floor oxygen was consumed within the calcination, which can result in the formation of oxygen vacancies33. Moreover, the oxygen vacancies would improve the adsorption and activation of molecular oxygen, leading to excessive Oadverts/Olatt ratio for CoGO50. It’s well-known that prime Oadverts/Olatt displays excessive catalytic exercise for the oxidation of hydrocarbons at low temperature35,36,37. Subsequently, this implies that CoGO50 possesses the best reactivity for methane oxidation, which is nicely in line with the next response take a look at. As well as, the XPS evaluation means that the spent CoGO50 have barely decline within the Oadverts/Olatt (0.69) and Co2+/Co3+ (0.73), that are nonetheless increased than these of CoGO100 and Co3O4 (Determine S6).
The redox properties of catalysts had been investigated by hydrogen temperature-programmed discount (H2-TPR) (Fig. 4D). All catalysts present two discount peaks at 200–300 °C, attributable to the discount of Co3+ to Co2+, and at 350–400 °C, assigned to the discount of CoO to metallic cobalt (Co0)32,38. The discount temperature downshifted from 276 °C of Co3O4 to 214–246 °C of CoGO50. This reveals that CoGO50 comprises extra energetic oxygen species, that are a lot simpler to be lowered. Subsequently, CoGO50 would take excessive oxidative exercise within the catalytic oxidation response.
Catalytic combustion of methane over CoGO catalysts
Methane catalytic combustion on Co3O4 and CoGOx (x = 20, 50 and 100) catalysts had been examined in a micro-fixed-bed reactor at 200 to 500 °C. The response was carried out with a feeding situations of 1 vol% of CH4, 10 vol% of O2 and 89 vol% of N2 and GHSV = 30,000 mL g−1 h−1. With temperature growing, as proven in Fig. 5A, the methane conversion on these catalysts elevated from 0 to 100% within the vary of 200 to 500 °C. Nonetheless, CoGOx catalysts exhibit increased actions at low temperature than Co3O4. Intimately, the temperature for 100% conversion of methane (T100) is 500 °C for Co3O4, which considerably decreases to 425, 400 and 450 °C for CoGO20, CoGO50 and CoGO100 catalysts, respectively. Equally, the temperatures for methane conversion of 10% (T10), 50% (T50) and 90% (T90) of CoGOx are a lot decrease than that of Co3O4 (Desk 2). CoGO50 has the best exercise for methane conversion with the bottom T100, T90, T50 and T10 and T100, that are 100 °C, 105 °C, 103 °C and 105 °C decrease than the corresponding values of Co3O4. It demonstrates that GO concerned preparation is considerably enhance the reactivity of CoGO50 via forming floor defects.
The obvious activation power (Ea) of CH4 oxidation over the catalysts was additionally calculated through the Arrhenius plots (Fig. 5B). The order development of Ea for CH4 oxidation on these catalysts is adopted as Co3O4 (86.5 kJ mol−1) > CoGO100 (61.4 kJ mol−1) > CoGO20 (59.9 kJ mol−1) > CoGO50 (53.4 kJ mol−1), which suggests CH4 was most simply activated and oxidized by CoGO50. It’s nicely in line with the reactivities of those catalysts. The reactivity of CoGO50 for methane combustion was in contrast with the reported outcomes over varied catalysts (Desk S2). CoGO50 is extra energetic than the cobalt oxide catalyst and even similar to the noble metal-based catalysts beneath the same situations.
To analyze the temperature sensibility of methane conversion, each Co3O4 and CoGO50 had been comparatively measured by utilizing in-situ diffuse reflectance infrared Fourier rework spectra (DRIFTS) with simulated feeding at completely different temperature (Fig. 6). The band at 2880 cm−1 represents the C–H stretching, which is ascribed to the adsorbed and/or the activated CH4 on the catalyst23,39. The bands at 1560 cm−1 and 1420 cm−1 will be assigned to the uneven and symmetric stretching vibration of intermediate carbonates (νasCO32+ and νsCO32+), respectively40. The band at 3380 cm−1 may very well be assigned to the stretching vibration of hydroxyl, which can mirror the top product of adsorbed water23,39. The band at 2200 cm−1 is ascribed to the top product CO239. For each samples, the intensities of all alerts from CH4 adsorption/activation, intermediates and finish merchandise improve with the response temperature, according to the actual response outcomes. Nonetheless, the DRIFTS spectra of CoGO50 present stronger alerts of intermediates and merchandise at low temperature in in contrast with that of Co3O4 (Fig. 5A,B). The sign of CO2 will be noticed from CoGO50 even at 200 °C, whereas the temperature with seen CO2 peak is 325 °C for Co3O4. That is nicely in line with the actual response exams that CoGO50 have the identical reactivity for methane on the temperature greater than 100 °C decrease than Co3O4.
The consequences of impurities of feed gasoline on the efficiency of CoGO50 had been examined by utilizing water vapor and SO2, respectively. Determine 7A exhibits the steadiness of CoGO50 for methane combustion at 375 and 400 °C. At 375 °C, the CH4 conversion stay secure round 92%. With 10% water vapor feeding, the CH4 conversion decreased quickly and saved secure on 75%. This means that vapor setting has a suppression impact on the CH4 oxidation. On the one hand, water vapor could adsorb and canopy partial of energetic websites of catalyst to kind Co(OH)241. Then again, the oxidation of CH4 to CO2 and H2O could also be thermodynamically restricted because the feeding water is among the merchandise. The conversion will be 100% recovered after switching the vapor off. At 400 °C, the CH4 conversion presents the same change tendency within the situations of introduce/shut-down10% vapor, which first declined from 100 to ~ 85% after which returned to 100%.
The poisoning results of sulphur dioxide on the catalyst was additionally examined over CoGO50, by which 50 ppm SO2 was launched into the response system at 350 °C (Fig. 7B). Upon SO2, the methane conversion quickly decreased from 91 to ~ 65% in 3 h. It was then slowly elevated to ~ 70% within the following 5 h. After switching SO2 off and purging with N2, the CH4 conversion barely elevated to ~ 85% in 4 h after which saved secure, suggesting that greater than 93% of unique reactivity was recovered. This means that SO2 tends to impression the exercise of CoGO50, irreversibly. SO2 can strongly adsorb on the catalyst through the reactions of Co3O4 + SO2 → Co3O4·SO2, which can additional kind sulphates via the reactions of 2Co3O4·SO2 + O2 → 2Co3O4·SO3 or Co3O4·SO3 → Co3(SO4)442.