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Catalysts 2020, 10(1), 105; https://doi.org/10.3390/catal10010105

Article
Low-Temperature Catalytic CO Oxidation Over Non-Noble, Efficient Chromia in Reduced Graphene Oxide and Graphene Oxide Nanocomposites
1
Science Department, College of Basic Education, Public Authority of Applied Education and Training (PAAET), P.O. Box 23167, Safat 13092, Kuwait
2
Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait
3
Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
4
Electrochemistry Laboratory, Physical Chemistry Department, National Research Centre, Dokki, P.O. 12622, Giza, Egypt
*
Authors to whom correspondence should be addressed.
Received: 3 December 2019 / Accepted: 8 January 2020 / Published: 11 January 2020

Abstract

:
Herein, bare chromia nanoparticles (Cr2O3 NPs) and chromia supported on reduced graphene oxide (rGO) and graphene oxide (GO) hybrids were synthesized, followed by characterization by means of FESEM, Raman spectroscopy, TGA, XRD, TEM/HRTEM, XPS and N2 sorptiometry. The investigated bare Cr2O3 and the hybrids (Cr2O3/rGO and Cr2O3/GO) were employed as catalysts for low-temperature CO oxidation. Compared with the other catalysts, the results revealed efficient catalytic activity using Cr2O3/GO, which was attributed to its higher surface area together with the mixed oxidation state of chromium (Cr3+ and Cr>3+). These are important oxidation sites that facilitate the electron mobility essential for CO oxidation. Moreover, the presence of carbon vacancy defects and functional groups facilitate the stabilizing of Cr2O3 NPs on its surface, forming a thermally stable hybrid material, which assists the CO oxidation process. The Cr2O3/GO hybrid is a promising low-cost and efficient catalyst for CO oxidation at low temperatures. The higher activity of graphene oxide supported Cr2O3 NPs can provide an efficient and cost-effective solution to a prominent environmental problem.
Keywords:
chromium oxide NPs; CO oxidation; Cr2O3/carbonaceous hybrid; catalysis

1. Introduction

Carbon monoxide (CO) is a toxic and highly poisonous gas and is considered a silent killer that contributes indirectly to different environmental and health problems [1]. The main sources of CO are gases emitted from petroleum, chemical and petrochemical industries, and automobile emissions [1]. To overcome its toxicity, CO catalytic oxidation into less harmful carbon dioxide (CO2) is an important strategy. Many researchers have employed different noble metals such as Au, Pt and Pd as potential catalysts for CO oxidation; however, the issues of cost, poor stability, and sensitivity to impurities for deactivation limit their industrial applications [2]. Recently, low-temperature CO oxidation on non-noble metal catalysts has been an important research objective in order to overcome high cost and realize efficient results [3,4,5].
Metal oxide catalysts are regarded as promising materials for industrial and environmental CO oxidation reactions [6]. Their abundance, low cost and pronounced chemical stability make them a promising alternative to noble-metal based catalysts [7,8,9]. Studies that have hitherto been carried out show that transition metal oxides produce catalysts with high CO oxidation activity. Promoting the CO oxidation activity of oxide-based transition metals is still a point of enquiry, and further research is seeking to accomplish this objective. Chromia (Cr2O3) is among the transition metal oxides that have been extensively researched [10]. Many researchers pay great attention to its physical and chemical characteristics [11,12,13]. This oxide demonstrated pronounced behavior in redox reactions, including the CO oxidation reaction [14,15].
Cobalt oxide was reported to have nearly the same catalytic activity as noble metal catalysts for CO oxidation [16]. Nevertheless, because of the high surface energy of cobalt oxide’s nanoparticles, it is ready to aggregate, which may lead to a reduction in catalytic activity. Supported metal nanoparticles are commonly used in heterogeneous catalytic processes, which are critical to various chemical reactions and can be used to solve the problem of individual metal oxides by protecting the in situ grown nanocatalysts from agglomeration [17]. This important supportive role improves the catalytic performance of metal nanoparticles (NPs).
Graphene and graphene oxide showed high surface-to-volume aspect ratios. Moreover, because of the presence of a carbon vacancy defect structure and different functional groups, metal ions can be easily adsorbed onto graphene’s surface, forming thermally stable composite materials [18]. rGO and GO are widely used to support metallic nanoparticles in different applications due to their pronounced specific area and high stability [19]. On the basis of their unique properties, graphene and graphene oxide should be used as support materials for different heterogeneous catalysts [20,21]. Wang et al. studied the use of rGO-Co3O4 catalysts for CO oxidation, and the results show remarkable activity and stability for the oxidation of CO using 30% rGO [22]. Wen et al. reported using an rGO-supported Cr2O3 catalyst for an oxygen reduction reaction (ORR), proving that hybrid Cr2O3/rGO has pronounced catalytic performance with high stability for ORR [23]. Xia et al. investigated Cr2O3 NPs with a rGO hybrid as an efficient catalyst for electrochemical N2-to-NH3 conversion [24]. Up to the best of our knowledge, however, reports on the catalytic application of metal oxide hybrids with carbonaceous materials for CO oxidation are rare.
Consequently, the present first-of-a-type investigation studies the activity of unsupported and supported Cr2O3 NPs on carbonaceous compounds (rGO and GO) as a catalyst for CO catalytic oxidation at low temperatures. Obtained hybrids of Cr2O3/rGO and Cr2O3/GO showed pronounced catalytic activity for the oxidation of CO at low temperatures.

2. Results and Discussion

2.1. Characterization of As-Prepared Samples

Figure 1A,B shows the FESEM micrographs obtained for GO and rGO. It is clear that, after oxidation, the GO and rGO sheets were both crumpled. The reduction of the graphite sheets due to the distortion of their conjugated π-bond system is also clear. However, a flaky layered microstructure was only shown for GO (Figure 1A), whereas in rGO, the surface morphology was relatively close (Figure 1B) due to GO exfoliation during ultrasonic treatment. Raman spectra obtained for GO and rGO are compared in Figure 1C. The two broad peaks therein monitored are the D and G bands, which are the predominant features for carbon-based materials [25]. In the spectrum of GO, the peak at 1589 cm−1 is due to the G band, whereas the peak at 1352 cm−1 is attributed to the D band [25]. The spectrum of rGO shows the G peak at 1582 cm−1 and the D peak at 1348 cm−1. These peak red-shifts are considered indicative of the recovery of sp2 domains [26] and, hence, the reduction of GO to rGO.
TG curve obtained for the as-obtained chromia, i.e., the un-calcined Cr2O3.4H2O, displays two main decomposition steps, as shown in Figure 2. The first step is shown to undertake about a 25% mass loss maximized at 237 °C. This process is consistent with the removal of weakly held water molecules plus the dehydration to Cr2O3.H2O (=2CrOOH). The second step is shown to maximize at 415 °C, giving rise to mass loss of about 11%, which is very close to the dehydration of Cr2O3.H2O into Cr2O3 (10.6%). These results verify the chemical composition of the parent chromium hydroxide compound and emphasize the appropriateness of the calcination temperature regime (≥400 °C), applied for the thermal genesis of the Cr2O3 NPs from the parent hydroxide. Accordingly, we chose the calcination temperature of 600 °C.
Figure 3 compares XRD powder diffractograms measured for Cr2O3 NPs and the hybrids Cr2O3/rGO and Cr2O3/GO. In the case of Cr2O3, the monitored diffraction peaks are quite similar to the peaks filed for α-Cr2O3 in JCPDS card no. 38-1479 [27]. Hence, α-Cr2O3 is the sole XRD-detectable crystalline phase in the calcination products at 600 °C. The average crystallite size determined for the α-Cr2O3 particles was 32 nm. In the case of Cr2O3/GO (Figure 3), the peak at around 2θ = 10.6° is assigned to the graphene oxide nanosheet, whereas the rest of the peaks monitored are indexed to the chromia NPs. As well as this, the XRD data of Cr2O3/rGO (Figure 3) is related to JCPDS no. 38-1479, and the diffraction peak at 2θ = 24.8° could be assigned to the rGO nanosheet. It is obvious that the peak at 2θ = 10.6° has disappeared, confirming the quantitative reduction of GO to rGO [28]. Hence, the XRD patterns of the synthesized hybrids Cr2O3/rGO and Cr2O3/GO reveal their effective preparation.
TEM image obtained for α-Cr2O3 NPs yielded after calcination at 600 °C is shown in Figure 4 A. It displayed well defined cubic-shaped nanoparticles of chromia. The average particle size for the obtained Cr2O3 NPs at 600 °C is 47 nm, which is higher than the XRD-determined average crystallite size (32 nm). This may reveal that the Cr2O3 NPs are polycrystalline and are formed via coalescence and/or aggregation of the primary crystallites. Figure 4B,C show the TEM images of the Cr2O3/rGO and Cr2O3/GO hybrids, respectively. The images clearly reveal the distribution of relatively non-aggregating chromia NPs on the GO and rGO surfaces. The HRTEM image for Cr2O3 calcined at 600 °C is shown in Figure 4D. The image shows well-resolved lattice fringes with d-spacings of 0.264 nm for rhombohedral crystallites. Accordingly, the surface-abundant crystallite facet is 104 [23]. These findings are in good agreement with the XRD results, which indicate that the (104)-facet is the strongest reflecting crystal plane, with a d-spacing value of 2.61 (in Å).
XPS spectra for Cr(2p) and O(1s) obtained for Cr2O3 NPs after calcination at 600 °C are shown in Figure 5A,B. Cr(2p) electron BE values monitored at 586.1 and 587.8 eV are attributed to Cr 2p1/2, and the values monitored at 576.5 and 578.6 eV are assigned to Cr 2p3/2, respectively [29]. The lower BE of 576.5 eV corresponds to the Cr3+ state [30]. The surface is also partially oxidized in order to form Cr>3+ ions, as previously reported by Fahim et al. [12]. The band at 578.6 eV is assigned to the higher oxidation state Cr6+ (CrO3 species) [29]. The O(1s) spectrum monitors a broad peak deconvoluted into three subpeaks at binding energy 530, 531 and 532.4 eV. The peaks at 530 and 531 eV are assignable to lattice oxygen, and this is in accordance with previously reported work [31,32]. The peak located at around 532.4 eV was attributed to surface oxygen species and/or adsorbed surface hydroxyls [32].
XPS spectra for the hybrids Cr2O3/rGO and Cr2O3/GO illustrate the presence of the chromium, oxygen and carbon spectra of the investigated hybrids. The chromium spectrum shows the same deconvoluted peaks obtained in the case of Cr2O3 (Figure 5C,D). In the C 1s spectrum of Cr2O3/GO (Figure 5E) there are also three peaks at BEs of 289.1 for the O−C=O, 285.8 for C−OH, and 284.6 eV for C−C species. In the case of Cr2O3/rGO (Figure 5F), there was one more peak at 286.2 eV, consequent to the epoxide (C−O−C) bond. The C−C bonds in the case of rGO are more intense than in GO. The intensity of the C−O peak also decreased further in the case of reduced rGO than in GO. The dramatic decrease of the peak’s intensity in the oxygen species and the sharp increase of the C–C peak at 284.6 eV in the case of Cr2O3/rGO hybrid support the reduction process of GO to rGO. As well as this, the decrease in the Cr/C atomic ratio of Cr2O3/rGO (0.26), compared with Cr2O3/GO (0. 34), is related to a higher carbon percentage in rGO than GO.
Moreover, the surface elemental analysis indicated that the carbon-to-oxygen atomic ratio (C/O) in the case of GO and rGO is 2.2 and 3.1, respectively. Increasing the C/O ratio for rGO confirms the partial removal of oxygen through the reduction process, which is in line with the XRD results.
The Brunauer–Emmett–Teller (BET) surface area of the prepared nanoparticles of Cr2O3 NPs at 600 °C and the hybrids Cr2O3/rGO and Cr2O3/GO were measured. N2 adsorption-desorption isotherms of the examined materials are displayed in Figure 6. It is obvious that the isotherm obtained for bare Cr2O3 is a Type IV isotherm with a H1 type hysteresis loop, which implies a largely mesoporous surface [33]. Comparatively, the other two isotherms (hybrid catalysts) look like Type IV with a H3 type hysteresis loop that could be attributed to ink-bottle-like pores of varying radius, often produced by agglomerates or compacts of spheroidal particles of non-uniform size. The surface area values measured for Cr2O3 and the hybrids rGO-Cr2O3 and GO-Cr2O3 were found to be 39, 112 and 144 m2/g, respectively. Moreover, the corresponding mean pore volumes therefrom derived are 0.26, 0.64 and 0.93 cm3g−1, respectively.

2.2. Catalytic Activity

Figure 7 demonstrates the catalytic activities for CO oxidation on the Cr2O3 NPs alone and the hybrid catalysts Cr2O3/rGO and Cr2O3/GO in the temperature range of 30−240 °C. T100 is the temperature at which 100% of CO has been oxidized, while T50 is the light-off temperature. As illustrated in Figure 7, the conversion percentage of CO increased with the rise of the catalytic reaction temperature. The light-off temperature of CO oxidation (T50) was achieved at 55, 98 and 115 °C in the presence of Cr2O3/GO, Cr2O3/rGO and Cr2O3, respectively. The Cr2O3/GO hybrid nanocatalyst showed the best complete CO conversion (T100) values at 120 °C.
The catalytic activity was attributed to higher surface area compared with the other catalysts and the mixed oxidation state of chromium (Cr3+ and Cr>3+), which facilitates the electron mobility required for CO oxidation along with graphene oxide support. In Figure 7, the hybrid catalyst containing GO shows the highest CO conversion percent to CO2 due to the different functional groups present on GO compared with rGO, which promote the CO oxidation process. The Cr2O3/GO hybrid nanocatalysts used in this study showed much higher catalytic activity than in previous reports, as shown in Table 1.
The turnover frequency (TOF) for the investigated catalysts was calculated in terms of μmol of CO converted per g of the catalyst at 60 °C to evaluate the intrinsic activity. The TOF of Cr2O3 NPs and the TOFs of Cr2O3/rGO and Cr2O3/GO nanocatalysts were found to be 11, 15 and 24 μmol g−1 min−1, respectively. According to the results, Cr2O3/GO exhibits the best intrinsic activity, which could be related to high surface area and the presence of redox couples (Cr3+/Cr>+3) on chromia surfaces. These are important catalytic oxidation sites that facilitate the electron mobility required for CO oxidation.
In addition, the Cr2O3/GO hybrid catalyst revealed good stability at 240 °C compared with pure Cr2O3, which started to lose its conversion rate after 38 h and decreased up to 73% after 72 h, as shown in Figure 8. The pronounced catalytic activity of Cr2O3/GO with respect to Cr2O3 NPs is believed to be due to its higher surface area of 144 m2/g compared with 39 m2/g in the case of the pure Cr2O3 NPs. The decreased conversion rate is suggested to be a result of the aggregation of the pure Cr2O3 NPs and the reduction of active oxygen on its surface.
In the case of the hybrid catalysts, Cr2O3 is dispersed well in the carbonaceous material, which prevents its aggregation, therefore maintaining its stability at the conversion rate for a long time.
The proposed possible CO oxidation pathways over the Cr2O3/GO hybrid nanocatalyst are as follows: the molecular oxygen is dissociated into reactive oxygen species, and the adsorbed CO binds with active oxygen to form CO2 on the surface of Cr2O3 NPs. XPS results confirmed the presence of the redox couples (Cr3+/Cr>+3) on chromia surfaces, which is significant for the required electron mobility for CO oxidation. The adsorbed CO molecules could be extracting the surface lattice oxygen from Cr2O3 NPs to form CO2 along with surface oxygen vacancy. The CO2 molecules break away from the Cr2O3 surface, and the vacancy is filled with oxygen gas, which could be reacted with CO to form bidentate carbonate, which can then form CO2 again. Moreover, the oxygen vacancy could be filled by the active oxygen species passed through the graphene oxide. In conclusion, compared with Cr2O3, the Cr2O3/GO nanocatalyst shows the highest activity for CO oxidation due to the presence of graphene oxide substrate, which prevents the aggregation of Cr2O3 NPs and delivers more active sites. Its higher activity compared with Cr2O3/rGO is believed to be because of its higher surface area.

3. Experimental Section

3.1. Materials

Chromium nitrate nonahydrate (≥99.99%), sodium hydroxide (≥98%), graphite (fine powder extra pure), sodium nitrate (≥99.0%), sulfuric acid (99.999%), potassium permanganate (≥99.0%), hydrogen peroxide (30 wt% in H2O) and hydrazine hydrate (RG) were purchased from Sigma-Aldrich, Taufkirchen, Germany.

3.2. Preparation of GO and rGO

GO was synthesized according to the modified Hummer’s method [36], wherein a 2 g portion of graphite powder was placed in a 250 mL beaker, then 1.0 g of NaNO3 and 46 mL of sulfuric acid were added with vigorous stirring in an ice bath at 0 °C.Then, 6 g of potassium permanganate was placed slowly onto the mixture at <20 °C. After 5 min we removed the ice bath, followed by heating out at 35 °C for 30 min. Subsequently, a 92 mL aliquot of deionized water was poured slowly onto the mixture while increasing the temperature to 98 °C and stirring continuously. This was maintained for another 30 min. Subsequently, a 280 mL portion of hot water (60 °C) and 30 mL of hydrogen peroxide (H2O2) aqueous solution (30%) were added until there was no bubble evolution. Eventually, the mixture was centrifuged for 10 min (7200 rpm) with an aqueous solution of HCl (5%) to remove excess of manganese salt; the final powder was then washed using warm water until a reddish-brown color was obtained from the black color. Finally, GO was ultrasonically dispersed in water to form a stable solution of GO.
For the synthesis of rGO, 0.1 g of GO was sonicated in 20 mL H2O, followed by adding 100 mL of hydrazine hydrate and heating it using a microwave at 1000 W in two 30 s cycles. A black color was obtained, confirming the complete reduction of GO to rGO [22].

3.3. Preparation of Bare Chromia Nanoparticles (Cr2O3 NPs)

Cr2O3 NPs were synthesized as follows: chromium nitrate was mixed with NaOH solution by stirring at 100 °C for one hour in order to obtain a dark green precipitate of Cr(OH)3. The precipitate was centrifuged and then washed with H2O to remove the NaNO3. Finally, the precipitate was dried at 110 °C overnight, accompanied by calcination in air at 600 °C for 5 h to obtain Cr2O3 NPs.

3.4. Preparation of the Hybrid Catalysts (Cr2O3/GO and Cr2O3/rGO)

In a 100 mL round bottom flask, 0.25 g of GO or rGO was dispersed in 75 mL of THF. Subsequently, 1.0 g of the prepared Cr2O3 NPs was added to the suspension and stirred continuously for 30 min. Then, the mixture was refluxed for 24 h at 70 °C, accompanied by centrifuge and washing using THF. Finally, the prepared hybrids were thermally treated at 150 °C for 4 h in a hot air oven before use in order to improve crystallinity. The hybrid materials were prepared while maintaining the support material at 25%, corresponding to Cr2O3 NPs.

3.5. Characterization of Chromia NPs and Hybrid Catalysts

XPS measurements were performed by a Thermο ESCA Lab 250xi (Waltham, MA, USA) equipped with Mg Kα radiation (1253 eV). The XRD analysis was performed by a D8 Advance diffractometer (Leipzig, Germany) with a Cu target and nickel filter with CuKα radiatiοn (λ = 0.154056 nm). Crystallite size was realized by adopting the line broadening technique and Scherrer’s formula [37]. Raman spectra of GO and rGO were measured using RENISHAW (Oxfordshire, United Kingdom) via a Raman microscope, with an excitation wavelength of 514 nm, a line of an Argon ion laser, and an exposure time of 10 s. TEM data were recorded using a JEOL JEM 1230 microscope (Tokyo, Japan) at 120 kV. HRTEM was performed on a JEM-3010 JEOL ultra high-resolution electron microscope (Tokyo, Japan) operated at 300 kV and 119 μA. FESEM was performed using SEM model (JSM-6300 JEOL, Tokyo, Japan). TG was performed by heating 10 mg of test material at 10 °C/min up to 800 °C in a dynamic (50 cm3/min) atmosphere of air, using a model TA-50 Shimadzu thermal system (Kyoto, Japan) equipped with a workstation. N2 adsorption-desorption isotherms were measured on test samples at liquid nitrogen temperature (−195 °C), using an automatic Micromeritics sorptiometer model ASAP 2010 (Norcross, Georgia, USA) equipped with a degassing platform and online data acquisition and handling system powered with BET-based analytical software for surface area (SBET/m2g−1) determination.

3.6. Catalytic Activity Measurements

The catalytic activity of the investigated catalysts for CO oxidation was tested using a tubular reactor adopting the same method previously reported [32]. The test catalyst powder was weighed out to 300 mg and placed on a G1-porous quartz disc mounted in the middle of a tubular reactor with an inner diameter of 2 cm and a length of 15 cm. The catalyst–bed aspect ratio (length to diameter) is 2 with short catalyst bed length of 1.3 cm. A 300 Torr portion of 1CO + 3O2 was expanded in the reactor at room temperature (RT), followed by a temperature increase in the range of 50–400 °C. 50 μL gas samples were withdrawn at different temperatures for analysis by a CP-GC6890 gas chromatograph (Santa Clara, CA, USA) equipped with a thermal conductivity detector (TCD) and a packed column with PORAPACK Q. The catalytic activity was expressed by the %CO conversion.

4. Conclusions

In summary, bare and supported chromium oxide hybrid nanocatalysts were successfully synthesized via a simple method and utilized for the low-temperature catalytic oxidation of CO. The Cr2O3/GO hybrid nanocatalyst showed the highest activity and stability for CO oxidation. The pronounced stability could be credited to the GO substrate. Cr2O3/GO showed T50 at 55 °C, and the complete conversion of CO was achieved at about 120 °C. This study promotes the development of transition metal oxide catalysts supported on carbonaceous compounds with significant efficiency and stability in low-temperature CO oxidation.

Author Contributions

A.A.A., F.A.S. and M.I.Z. outlined the work plan; A.A.N., M.M. and A.A.A. conducted the experimental part; A.A.N. and M.M. draw the figures and drafted the manuscript. All authors participated in the reviewing and publication processes of the article. All authors have read and agreed to publish this version of the manuscript.

Funding

This research was funded by [Kuwait Foundation for the Advancement of Sciences (KFAS)] grant number [PN17-14SC-01] and the APC was funded by [KFAS].

Acknowledgments

The authors acknowledge the support of this study by Kuwait Foundation for the Advancement of Sciences, under Grant Number [PN17-14SC-01] and Kuwait University, Research Administration general facility projects GS 01/01, GS 01/05, GS 02/01, GE03/08 and GS 03/01. Additionally, help from the Nanoscopy Science Center is appreciated.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Typical FESEM images obtained for GO (A) and rGO (B) and their Raman spectra (C).
Figure 1. Typical FESEM images obtained for GO (A) and rGO (B) and their Raman spectra (C).
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Figure 2. TG curve obtained for the as-obtained Cr2O3 tetrahydrate.
Figure 2. TG curve obtained for the as-obtained Cr2O3 tetrahydrate.
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Figure 3. XRD patterns obtained for the bare α-Cr2O3 NPs and the hybrids Cr2O3/rGO and Cr2O3/GO.
Figure 3. XRD patterns obtained for the bare α-Cr2O3 NPs and the hybrids Cr2O3/rGO and Cr2O3/GO.
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Figure 4. TEM micrographs obtained for Cr2O3 NPs (A) Cr2O3/rGO (B) and Cr2O3/GO (C) hybrids. The panel (D) shows HRTEM micrograph obtained for Cr2O3.
Figure 4. TEM micrographs obtained for Cr2O3 NPs (A) Cr2O3/rGO (B) and Cr2O3/GO (C) hybrids. The panel (D) shows HRTEM micrograph obtained for Cr2O3.
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Figure 5. Deconvoluted XPS peaks of (A) Cr(2p) and (B) O(1s) for the crystalline Cr2O3; (C) Cr(2p), (E) C(1s) and (G) O(1s) peaks for Cr2O3/GO; and (D) Cr(2p), (F) C(1s) and (H) O(1s) peaks for Cr2O3/rGO.
Figure 5. Deconvoluted XPS peaks of (A) Cr(2p) and (B) O(1s) for the crystalline Cr2O3; (C) Cr(2p), (E) C(1s) and (G) O(1s) peaks for Cr2O3/GO; and (D) Cr(2p), (F) C(1s) and (H) O(1s) peaks for Cr2O3/rGO.
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Figure 6. N2 adsorption-desorption isotherms for Cr2O3 NPs, and the hybrids Cr2O3/rGO and Cr2O3/GO.
Figure 6. N2 adsorption-desorption isotherms for Cr2O3 NPs, and the hybrids Cr2O3/rGO and Cr2O3/GO.
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Figure 7. Catalytic activity for CO oxidation of Cr2O3 NPs alone and the hybrids Cr2O3/rGO and Cr2O3/GO catalysts in the temperature range of 30−240 °C.
Figure 7. Catalytic activity for CO oxidation of Cr2O3 NPs alone and the hybrids Cr2O3/rGO and Cr2O3/GO catalysts in the temperature range of 30−240 °C.
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Figure 8. The stability test of the Cr2O3 catalyst and the hybrid Cr2O3/GO for CO oxidation at the reaction temperature of 240 °C.
Figure 8. The stability test of the Cr2O3 catalyst and the hybrid Cr2O3/GO for CO oxidation at the reaction temperature of 240 °C.
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Table 1. T100 and T50 for Cr2O3 NPs alone and also for the hybrids Cr2O3/rGO and Cr2O3/GO, compared with different previously reported data on Cr2O3-based catalysts.
Table 1. T100 and T50 for Cr2O3 NPs alone and also for the hybrids Cr2O3/rGO and Cr2O3/GO, compared with different previously reported data on Cr2O3-based catalysts.
MaterialExperimental ConditionsT100, °CT50, °CRef.
Cr2O345–46 g (80 cm3) of catalyst, gas composition, CO~l.2%, O2~1.2%, balance He; gas rate—1400 cm3/min.340265[34]
CuO/Cr2O3 supported on silicaThe feed composition was 2% CO, 2% O2 in He. 0.1 g of catalyst at a flow rate of 100 mL min−1≈500213[35]
CuO/Cr2O3 supported on AluminaThe feed composition was 2% CO, 2% O2 in He. 0.1 g of catalyst at a flow rate of 100 mL min−1≈320233[35]
Cr2O3-4The dose of examined material: 250–300 mg
The mass ratio 1CO to 3O2
20098[32]
Cr2O3-6The dose of examined material: 250–300 mg
The mass ratio 1CO to 3O2
248115[32]
Cr2O3-8The dose of examined material: 250–300 mg
The mass ratio 1CO to 3O2
304132[32]
Cr2O3The dose of examined material: 300 mg
The mass ratio 1CO to 3O2
240114Present
Cr2O3/rGOThe dose of examined material: 300 mg
The mass ratio 1CO to 3O2
21098Present
Cr2O3/GOThe dose of examined material: 300 mg
The mass ratio 1CO to 3O2
12055Present
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