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Article

Flower-like Co3O4 Catalysts for Efficient Catalytic Oxidation of Multi-Pollutants from Diesel Exhaust

by
Zihao Li
1,
Xianhuai Chen
1,
Jinghuan Chen
2,
Huazhen Chang
3,
Lei Ma
1,* and
Naiqiang Yan
1
1
School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
2
Key Laboratory of Microbial Technology for Industrial Pollution Control, School of Environment, Zhejiang University of Technology, Hangzhou 310014, China
3
School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(5), 527; https://doi.org/10.3390/catal12050527
Submission received: 18 April 2022 / Revised: 1 May 2022 / Accepted: 5 May 2022 / Published: 7 May 2022
(This article belongs to the Special Issue Catalytic Materials: Elimination of Environmental Pollutants)
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Abstract

:
Nowadays, the oxidation activity at the low-temperature regime for Co3O4 catalysts needs to be improved to meet the stringent regulation of multi-pollutant diesel exhaust. Herein, nanoflower-like Co3O4 diesel oxide catalysts (DOCs) were fabricated with the addition of a low-content Pt to trigger better catalytic activities for oxidizing multi-pollutants (CO, C3H6, and NO) emissions by taking advantage of the strong-metal supporting interaction. Compared to the conventional DOCs based on Pt/Al2O3, the as-synthesized Pt/Co3O4 catalysts not only exhibited better multi-pollutants oxidation activities at the low temperature but also obtained better resistance toward NO inhibition. Moreover, Pt/Co3O4 catalysts showed exceptional hydrothermal durability throughout long-term tests in the presence of water vapor. According to the XPS and H2-TPR results, Pt promoted low-temperature catalytic activity by increasing the active surface oxygen species and reducibility due to the robust synergistic interaction between metallic Pt and supporting Co3O4. Meanwhile, TGA curves confirmed the Pt atoms that facilitated the desorption of surface-active oxygen and hydroxyl radicals in a low-temperature regime. Furthermore, instead of probing the intermediates during CO and C3H6 oxidation for Pt/Co3O4 catalysts, which included carbonates, formate, and acetate species, in situ DRIFTs experiments also revealed C3H6 oxidation mainly took place over metallic Pt sites.

Graphical Abstract

1. Introduction

Compared to gasoline engines, diesel engines provide higher fuel efficiency and better durability, along with much lower production of carbon dioxide (CO2) [1], which enlarges the automobile market for diesel vehicles. However, the multi-pollutants of carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and diesel particulate matter (PM) emitted from the diesel engines cause severe environmental problems [2]. Diesel oxide catalysts (DOCs) are applied as the upstream element in the automobile after-treatment systems to purify the multi-pollutant emissions. As the first module in the vehicle after-treatment systems, DOCs not only convert CO and HC into CO2 but also play a critical role in oxidizing nitrogen monoxide (NO) to nitrogen dioxide (NO2) [3]. Due to its robust oxidizing ability, NO2 generated from DOCs facilitates the passive regeneration of the diesel particulate filter (DPF) and promotes the selective catalytic reduction of NOx with ammonia (NH3-SCR) by triggering the “fast” SCR pathway in the downstream system [4,5,6]. As a result, DOCs are the crucial part governing the holistic catalytic performances of the vehicle after-treatment systems. Typically, commercial DOCs apply platinum group metals (PGMs), such as Pt and Pd, as the dominant components due to their promising catalytic oxidation activities of CO and HC. However, the strong adsorption of CO and HC over the PGMs usually poisons the active sites of DOCs, which prohibits the activation of O2 and leads to poor catalytic activity at the cold-start stage [7]. Thus, better low-temperature catalytic oxidation performances are required for DOCs to accomplish 90% conversion at 150 °C of vehicle multi-pollutants emissions set by U.S. DRIVE [8].
In past decades, spinel-structured cobalt oxide (Co3O4) demonstrated excellent catalytic oxidation activities for various pollutants such as methane and volatile organic compounds [9]. Specifically, Co3O4 nanorods exposing adequate active sites of Co3+ accomplished the complete conversion of CO at −77 °C [10]. Therefore, we have developed Co3O4 nanorods as a potential non-PGM catalyst for diesel exhaust treatment. According to our previous studies, Co3O4 nanorods fabricated via the SBA-15 template exhibited excellent catalytic performances and hydrothermal stability toward NO and propane oxidation as a rival to traditional platinum DOCs [11,12]. Meanwhile, the as-synthesized Co3O4 nanorods also performed great catalytic activities for multi-pollutants diesel emissions [13]. However, the low-temperature catalytic activities of C3H6 and NO oxidation could still be improved over Co3O4 catalysts.
Triggering the strong metal–support interaction (SMSI) between noble metal and reducible metal oxide support is a reliable method to elevate low-temperature reactivity [14]. Kaper et al. claimed that the room-temperature CO oxidation could be achieved by the strong interaction between Pt and CeO2 [15]. Moreover, Christopher et al. indicated that the isolated Pt supported by TiO2 obtained better reactivity than nanoparticle Pt because of the larger interfacial area between isolated Pt atoms and supporting TiO2, which resulted in the strong synergistic interaction [16]. Moreover, the oxygen-induced transformation at the interface of Pt encapsulated by Fe3O4 contributed to the better catalytic activity toward CO oxidation [17]. Other than the SMSI effect, the catalytic activities might be affected by the morphology of Co3O4 (nanorods, nanoparticles, nanosheets, and nanocages) [11,18,19], which indicated that the specific structures might be a feasible factor governing the catalytic activity. Recently, Chen et al. claimed that flower-structured Co3O4 exposing both (110) and (112) plans possessed better catalytic activity of toluene oxidation than the Co3O4 nanorods [20].
Herein, a nanoflower-like Co3O4 (coded as Co3O4-F) was prepared via the hydrothermal method to oxidize simulated diesel exhaust. Approximately 0.8 wt.% Pt was loaded to further improve the reactivity of C3H6 and NO oxidation. Initially, the catalytic performances were carried out for various catalysts of Co3O4-F, Pt/Co3O4-F, and other conventional DOCs, to compare the catalytic activities toward multi-pollutants diesel exhaust. Meanwhile, the feasibility of Pt/Co3O4-F as DOCs was approved during the hydrothermal durability tests under simulated diesel emissions. Moreover, the morphological properties were investigated via SEM, XPS, and N2 adsorption–desorption isotherms. The results of XPS, H2-TPR, and N2-TGA were also presented in the literature to illustrate the SMSI effect between Pt and supporting Co3O4-F. Lastly, in situ DRIFTs experiments not only probed the specific surface intermediates during the reactions but also elucidated the function of Pt atoms in promoting the catalytic oxidation activity.

2. Results and Discussion

2.1. Catalytic Activities and Stability

The catalytic oxidation activities of CO, C3H6, and NO were measured over various catalysts, and the results were summarized in Figure 1. For both CO and C3H6 oxidation, Co3O4-F and Pt/Co3O4-F exhibited better catalytic activities than commercial Pt/Al2O3 catalysts. Although Pt/Al2O3 catalysts reached optimal NO conversions at a lower temperature, the maximum conversions of NO oxidation still decreased in the sequence of Pt/Co3O4-F (79.4%) > Co3O4-F (78.9%) > Pt/Al2O3 (71.6%). Since water vapor was a common component in diesel exhaust and could deactivate catalysts by blocking the active sites [21,22], high hydrothermal stability was requested for DOCs. Consequently, a long-term durability test was carried out under a simulated diesel exhaust mixture with 5% H2O at 260 °C to investigate the hydrothermal durability of the Pt/Co3O4-F catalysts. As shown in Figure 1d, the Pt/Co3O4-F catalysts maintained a nearly 100% CO conversion through 43-hour on-stream experiments. Meanwhile, no significant activity loss was observed in C3H6 oxidation, in which the conversion slightly dropped from 99.3% to 93.1%. According to the light-off results from Figure 1c, the Pt/Co3O4-F catalyst was supposed to achieve an approximately 74% NO conversion under a dry simulated exhaust atmosphere at 260 °C. Yet, the conversion of NO decreased to 8% on the stream, which could be due to the rapid reaction between formed NO2 and H2O to generate HNO3 [23]. Fortunately, NO conversion in the presence of water vapor was relatively stable throughout the long-term hydrothermal durability tests. Overall, the catalytic performance toward multi-pollutant diesel emissions on Pt/Co3O4-F was sufficiently stable under the simulated exhaust condition. To further compare catalytic performance, T50 and T90, representing the temperatures of 50% and 90% conversion for specific pollutants, respectively, were applied. As listed in Table S1, the catalytic activities of as-synthesized Pt/Co3O4-F catalysts outperformed the previously reported Pt/Al2O3 catalysts and commercial Pt-Pd/Al2O3 DOCs toward multi-pollutants exhaust, which proved that the Pt/Co3O4-F catalyst could be a feasible choice for DOCs.
To better understand the influence of CO, C3H6, and NO in simulated reactions, light-off tests were conducted under different atmospheres with single reactant or a mixture of CO and C3H6. As shown in Figure 2a, the Pt/Co3O4-F and Co3O4-F catalysts, which reached 90% conversion (T90) at approximately 100 °C, performed much better single CO oxidation activities than the conventional Pt/Al2O3 catalysts. Even though the catalytic activity of single C3H6 oxidation on Pt/Al2O3 catalysts outperformed Pt/Co3O4-F and Co3O4-F (Figure 2b), the presence of CO extremely suppressed the C3H6 oxidation activity over Pt/Al2O3 catalysts, which was demonstrated in Figure 2d. On the contrary, there was no apparent interference between C3H6 and CO over Pt/Co3O4-F and Co3O4-F catalysts (Figure 2c,d). Notably, NO was a severe inhibitor of catalytic oxidation reactions [13]. Consistent with the previous literature, NO intensively prohibited the CO oxidation activities among Pt/Co3O4-F, Co3O4-F, and Pt/Al2O3 catalysts. However, Pt/Co3O4-F and Co3O4-F exhibited better resistance on NO toward C3H6 oxidation. In contrast with Pt/Co3O4-F catalysts, NO acutely suppressed the activity of C3H6 oxidation over Pt/Al2O3 catalysts. Therefore, Pt/Co3O4-F and Co3O4-F catalysts obtained better catalytic oxidation activities and possessed superior resistance toward NO compared to the conventional Pt/Al2O3 catalysts.
The addition of Pt also boosted the catalytic oxidation activities of NO and C3H6 in the low-temperature regime by taking advantage of SMSI between Pt and reducible Co3O4. Compared to Pt supported on the inert SiO2, Pt/Co3O4-F exhibited dramatically better catalytic oxidation performances among CO, C3H6, and NO oxidations, which illustrated that the synergistic interactions between Pt and Co3O4 facilitated the oxidation process. According to the previous studies, SMSI generally occurred at the interface between noble metal and supporting reducible metal oxide [24]. Therefore, the physical mixing of Pt/SiO2 and Co3O4-F catalysts (Pt/SiO2 and Co3O4-F) were prepared and tested in different reactions. In comparison to Pt/Co3O4-F catalysts, Pt/SiO2 and Co3O4-F catalysts displayed much lower catalytic activities toward CO, C3H6, and NO oxidations, which implied that SMSI only took place once Pt was in close proximity to the supporting Co3O4-F. Moreover, the Pt/SiO2 and Co3O4-F catalysts demonstrated better activities than the Pt/SiO2 catalysts, which could be due to the extraordinary oxidation ability of Co3O4-F.

2.2. Morphological and Textural Properties

The morphologies of Co3O4-F catalysts were visualized via SEM techniques. As shown in Figure 3, the Co3O4-F catalysts exhibited blooming flower structures with several petal-like nanosheets bundled together. The Pt/Co3O4-F catalysts demonstrated a nanoflower-like structure identical to Co3O4-F.
Moreover, N2 physisorption experiments were performed to investigate the textural properties of Co3O4-F and Pt/Co3O4-F catalysts. As displayed in Figure 4, both Co3O4-F and Pt/Co3O4-F catalysts demonstrated type IV isotherms with the H3 hysteresis loops, suggesting the formation of mesoporous structure on Co3O4-F and Pt/Co3O4-F catalysts [25]. According to Table S2, Co3O4-F and Pt/Co3O4-F catalysts exhibited BET surface areas of 30.4 and 30.2 m2 g−1, respectively. In addition, the pore volumes of Co3O4-F and Pt/Co3O4-F were 0.089 and 0.090 cm3 g−1, respectively, indicating that the deposition of Pt did not influence the surface properties of Co3O4-F.
Furthermore, the XRD profiles of Co3O4-F and Pt/Co3O4-F catalysts are demonstrated in Figure 5. Pt/Co3O4-F exhibited identical patterns in Co3O4-F without any Pt diffraction peaks, suggesting that Pt particles are highly dispersed on the surface of Co3O4-F. In addition, the XRD profiles of Co3O4-F and Pt/Co3O4-F catalysts perfectly matched the standard reference patterns of spinel-structured Co3O4 (PDF#43-1003) with the space group Fd-3m (227). The intensive XRD diffraction peaks, which appeared at 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.4°, 65.2°, and 77.3°, can be ascribed to crystal planes of (111), (220), (311), (222), (400), (422), (511), (440), and (533), respectively.

2.3. Surface Chemical Properties

XPS measurements were carried out to probe the surface valence states of different elements over the Co3O4-F and Pt/Co3O4-F catalysts. Figure 6a demonstrated the XPS spectra of Co 2p3/2 for Co3O4-F and Pt/Co3O4-F catalysts, which were fitted into three intensive peaks and two broad shake-up satellite peaks. For Co3O4-F catalysts, two shake-up satellite peaks at 789.9 and 785.7 eV could be due to Co2+ and Co3+, respectively. Meanwhile, three characteristic peaks at approximately 781.9, 780.6, and 779.4 eV corresponded to Co2+, Co3+, and the combination of Co2+ and Co3+, respectively [26]. As shown in Table 1, the addition of Pt atoms did not significantly affect the Co XPS pattern over Co3O4-F. Moreover, the XPS core-level spectra of O 1s for both catalysts (Figure 6b) were composed of a broad peak at 531.2 eV and a sharp peak at 529.9 eV, ascribed to chemisorbed or surface oxygen and lattice oxygen, respectively [27]. According to the relative area in Table 1, the deposition of Pt particles dramatically elevated the molar ratio of chemisorbed or surface oxygen from 38.6% to 50.1% over Pt/Co3O4-F, compared to Co3O4-F, which indicated the generation of more active surface oxygen species over Pt/Co3O4-F catalysts. These active surface oxygen species might play a critical role in promoting the oxidation occurring over the surface of Pt/Co3O4-F by being directly involved in the reaction [28]. Since the XPS core-level spectra of Co 2p did not show any significant difference between Co3O4-F and Pt/Co3O4-F, the SMSI between Pt and supporting Co3O4-F mainly elevated the amount of active surface oxygen over Pt/Co3O4-F.
As displayed in Figure 6c, Pt 4f core-level spectra demonstrated two intensive peaks at approximately 71.4 and 74.6 eV, which could be due to the binding energies of Pt 4f7/2 and Pt 4f5/2 for metallic Pt species [29]. Meanwhile, the spin-orbit components of Pt 4f core-level spectra could separate well with ∆metal = 3.2 eV [30]. These results proved that the metallic Pt species dominated the surface of the Pt/Co3O4-F catalysts.

2.4. Reducibility

H2-TPR experiments were performed to investigate the reducibility of Co3O4-F and Pt/Co3O4-F catalysts. As shown in Figure 7, two well-resolved reduction peaks centered at approximately 340 and 422 °C were detected on Co3O4-F catalysts. The relatively weak peak I at 340 °C was the reduction of Co3O4 to CoO (Co3O4 + H2 → 3CoO + H2O). Meanwhile, the second broad peak at a higher temperature corresponded to the reduction process from CoO to metallic cobalt (CoO + H2 → Co + H2O) [31,32]. According to the detailed quantitative results from Table 2, the relative peak area ratio of peak I and peak II was equal to 3.29 for Co3O4-F catalysts, which was slightly larger than the theoretical ratio of 3.0, probably revealing an incomplete reduction of Co3+ to Co2+ during the first reduction peak. Compared to the Co3O4-F catalysts, the reduction processes over Pt/Co3O4-F shifted to much lower temperatures, with the first peak moving to 136 °C and the second peak to 326 °C. The left-shift of the reduction temperature was a typical signal for better reducibility. In addition, the relative area ratio between peak I and peak II decreased to 1.90, which was lower than the theoretical value of Co3O4-F and could be due to the following reasons. In regard to XPS results, SMSI between Pt and Co3O4-F generated active surface oxygen species over Pt/Co3O4-F, which coupled with H2 at a relatively low-temperature range. Moreover, the synergistic interaction between Pt and Co3O4-F caused the complete reduction of Co3O4 to CoO, which increased the H2 consumption of peak I. Meanwhile, the partial reduction of CoO to metallic Co might occur at the overlap area between peak I and peak II, at approximately 200 °C. Furthermore, the H2 spillover might take place at the Pt/Co3O4-F interface, under a low-temperature regime as well. In summary, the significant shift of the reduction temperature indicated that the SMSI effect dramatically improved the reducibility over the surface of the Pt/Co3O4-F catalysts.
The thermogravimetric analysis was conducted under a nitrogen gas atmosphere to investigate the thermal properties of the Co3O4-F and Pt/Co3O4-F catalysts. As demonstrated in Figure 8, the weight losses below 700 °C were 0.79% and 1.17% for Co3O4-F and Pt/Co3O4-F, respectively, which illustrated that the addition of Pt atoms might promote the catalytic oxidation by advancing the desorption of nonstoichiometric oxygen species and surface-adsorbed hydroxyl radicals [33,34]. The weight loss above 700 °C could be due to the loss of lattice oxygen from Co3O4. For pure Co3O4-F catalysts, the weight loss caused by the release of lattice oxygen was approximately 6.2%, which was consistent with the stoichiometric decomposition of Co3O4 (Co3O4 → 3CoO + 1/2O2). On the contrary, the weight loss for Pt/Co3O4-F (5.8 wt.%) was lower than the theoretical value, indicating that the Pt species were relatively stable in the high-temperature regime.

2.5. In Situ DRIFTs

To further probe the reaction mechanism over Co3O4-F and Pt/Co3O4-F catalysts, in situ DRIFTs experiments were carried out under different reactant atmospheres to analyze the surface-adsorbed molecules during the catalytic oxidation processes. As shown in Figure 9, abundant carbonated species located in the range from 1650 to 1200 cm−1 were observed during CO and O2 co-adsorption at different temperatures over the surface of Co3O4-F and Pt/Co3O4-F. For both Co3O4-F and Pt/Co3O4-F catalysts, two intensive peaks at approximately 2114 and 2178 cm−1 could be due to the gaseous CO molecules and CO weakly adsorbed on defect sites on Co3O4, respectively. Moreover, the weak peak around 2340 cm−1 was ascribed to the asymmetric stretching of generated CO2 molecules [35]. As demonstrated in Figure 9a, plentiful bidentate carbonates (1618, 1292, and 1251 cm−1) and free carbonate ions (1436 cm−1) were detected over the surface of Co3O4-F at 30 °C [36]. The carbonate species were assigned as the indispensable intermediates to generate the final product of CO2 since the signal of gaseous CO2 was not detected at 30 °C. Meanwhile, the carbonate species were rapidly consumed, accompanied by the increased intensity of the CO2 peak with the rising temperature. This phenomenon confirmed that the carbonate intermediates formed as the essential intermediates, which could be further oxidized to generate CO2. Over the surface of the Pt/Co3O4-F catalysts (Figure 9b), there was an extra peak at approximately 2093 cm−1, indicating the CO adsorbed on Pt particles. The major carbonates formed over Pt/Co3O4-F catalysts at 30 °C were monodentate carbonates (1324 and 1520 cm−1) and free carbonate ions (1441 and 1398 cm−1), along with a relatively low amount of bidentate carbonates (1259 cm−1) [37]. The monodentate carbonate mainly adsorbed on the Pt species because it was not observed over pure Co3O4-F catalysts. With the increasing temperature, the monodentate and free carbonate ions quickly faded. Additionally, the band position of the bidentate carbonates shifted from 1259 to 1223 cm−1, which implied the surface-active oxygen species involved in the oxidation reaction [13].
Figure 10 exhibited the CO and C3H6 co-oxidation over Co3O4-F and Pt/Co3O4-F catalysts at various temperatures. As shown in Figure 10a, the primary surface substances formed at 30 °C on Co3O4-F were gaseous CO (2113 cm−1) and weakly surface bonded CO (2173 cm−1), accompanied by bidentate carbonates (1637 cm−1) and free carbonate ions (1445 cm−1) formed through CO coupling with surface-active oxygen species. Once the temperature increased to 120 °C, the peak of gaseous CO2 at 2330 cm−1 appeared with the rapid consumption of carbonate species. Simultaneously, the appearance of intensive peaks located at 1540 and 1436 cm−1 could be due to the formation of formate species, which were quickly depleted with the increasing temperature as the core intermediates during C3H6 oxidation [38,39]. Other than surface-adsorbed and gaseous CO molecules, monodentate carbonates (1326 cm−1), carbonate free ions (1419 cm−1), and bidentate carbonates (1609 cm−1) were observed on the surface of Pt/Co3O4-F catalysts at 30 °C presented in Figure 10b. Meanwhile, the intensive peak at 1445 cm−1 could be due to the CH2 scissoring from C3H6 molecules adsorbed on Pt atoms [40]. Regarding CO oxidation, carbonate species quickly faded by reacting with the active oxygen to generate CO2 (2331 cm−1). Meanwhile, the formate species at 1542 cm−1 were initially generated at 120 °C and then decomposed with the increasing temperature (HCO2 → CO + OH-). On the contrary, acetate species (1364 and 1433 cm−1) gradually accumulated with the rising temperature due to their robust thermal stability below 300 °C [38]. Since the signal of adsorbed C3H6 was too weak to be detected over the surface of the Co3O4-F catalysts, this further confirmed that the Pt promoted C3H6 oxidation by working as the active site for the C3H6 adsorption and activation, which was consistent with the results of the catalytic activities.

3. Materials and Methods

3.1. Preparation of Catalysts

The Co3O4 hierarchical nanoflower was synthesized by the hydrothermal method according to previous literature [20]. Typically, 0.8100 g of 2-methylimidazole (2-MIM) from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China) was dispersed in 25 mL of deionized water. Meanwhile, 0.2989 g of cobalt acetate tetrahydrate (Co(CH3COO)2⋅4H2O) from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China) was dissolved in 25 mL of deionized water. The solutions were vigorously stirred for 10 min. Then, Co(CH3COO)2⋅4H2O solution was poured into 2-MIM solution and stirred for 5 min. Later, the mixed solution was aged at 125 °C for 1 h in a 100 mL Teflon-lined autoclave. After cooling to room temperature, the product was centrifuged and washed with deionized water at least four times. Finally, the product was dried at 70 °C overnight and calcined at 400 °C for 2 h ramped with 1 °C min−1 to obtain Co3O4-F catalysts.
0.8 wt.% Pt/Co3O4-F catalysts were prepared by the incipient wetness impregnation method with Pt(NH3)4(NO3)2 (Sigma-Aldrich, Darmstadt, Germany) as the Pt precursor. Typically, 0.0205 mmol Pt(NH3)4(NO3)2 was dissolved in 30 mL deionized water, and then 0.5 g as-prepared Co3O4-F catalysts were dispersed into the solution. The mixture was intensively stirred for 10 h, and then the deionized water was evaporated at 70 °C. Subsequently, the mixture was dried at 100 °C overnight and then calcined at 400 °C for 6 h with a ramping rate of 1 °C min−1 to fabricate Pt/Co3O4-F catalysts. Meanwhile, inductively coupled plasma-optical emission spectrometry (ICP-OES) experiments were carried out on Avio 500 (PerkinElmer Inc., Waltham, MA, USA) to investigate the elemental concentrations of platinum. As determined by the ICP-OES experiments, the platinum content of the Pt/Co3O4-F catalysts was 0.72 wt.%. In comparison, 2 wt.% Pt/Al2O3 and 2 wt.% Pt/SiO2 catalysts were also prepared in agreement with our recent work [13,41].

3.2. Catalytic Oxidation Activity Tests

Reactant gases including 5% CO/N2 (99.999%), 1% C3H6/N2 (99.999%), 1% NO/N2 (99.999%), high-purified O2 (99.999%), and high-purified N2 (99.999%) were purchased from Shanghai Weichuang Standard Gas Analytical Technology Co., Ltd (Shanghai, China). Moreover, mass flow controllers from Beijing Sevenstar Electronics Co., Ltd (Beijing, China) were applied to precisely manipulate the gas flow rate.
The catalytic activity tests were carried out in a fixed-bed quartz tube reactor sealed by quartz wool. The quartz sand (Innochem, Beijing, China), which was pretreated at 800 °C for 6 h in the static air, was used as the diluent. Before each test, all the samples were pretreated in pure N2 (100 mL min−1) at 300 °C for 1 h with a ramping rate of 2 °C min−1 to remove any impurities and moisture. All the light-off experiments were performed with a ramp rate of 2 °C min−1. For single CO or C3H6 light-off tests, reactant gases contained 4000 ppm CO or 1000 ppm C3H6, 10% O2 balanced by N2. For simulating the diesel emissions, a flowing mixture containing 4000 ppm CO, 1000 ppm C3H6, 500 ppm NO, and 10% O2 in the presence or absence of 5% H2O balanced with N2 was purged into the reactor. Deionized water was injected into the reactor by the high-pressure syringe pump followed by the complete vaporization isothermal at 150 °C. The total flowrate of all reactions was fixed at 200 mL min−1, corresponding to a weight hourly space velocity (WHSV) of 240,000 mL g−1 h−1. The concentration of reactant gases and products, including CO, C3H6, NO, and NO2, was measured by an Antaris IGS Gas Analyzer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The conversions of CO, C3H6, and NO were calculated by Equations (1)–(3).
X C O = C C O , i n − C C O , o u t C C O , i n × 100 % ,
X C 3 H 6 = C C 3 H 6 , i n − C C 3 H 6 , o u t C C 3 H 6 , i n × 100 % ,
X N O → N O 2 = C N O 2 , o u t − C N O 2 , i n C N O , i n × 100 % ,

3.3. Characterizations

Powder X-ray diffraction (XRD) profiles of catalysts were measured on a Bruker-AXS D8 Advance X-ray diffractometer (Bruker Corporation, Billerica, MA, USA) using Cu Kα radiation (λ = 0.15406 nm) at a tube voltage of 40 kV and a tube current of 30 mA, under an ambient atmosphere. The scan angle was recorded in the range from 5° to 90° with a step size of 0.02°, and it accumulated at a rate of 5° min−1. The scanning electron microscopy (SEM) experiments were conducted via FEI Nano nova 450 (FEI, Marietta, GA, USA) to visualize the morphologies of the catalysts.
The N2 adsorption–desorption isotherms were performed on a Micromeritics ASAP 2460 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA) using N2 as the carrier gas. Before the measurement, all the samples were degassed at 300 °C under a vacuum for 8 h. The Brunauer–Emmett–Teller (BET) equations were applied to calculate the surface area in the relative pressure (P/P0) between 0.05 to 0.30. The Barrett–Joyner–Halenda (BJH) of the desorption curves were utilized to determine the pore size distributions.
X-ray Photoelectron Spectroscopy (XPS) tests were performed on a Kratos Axis Ultra DLD instrument (Shimadzu Corporation, Kyoto, Japan) using a monochromatic Al source with a working voltage and current of 14 kV and 8 mA, respectively. The carbon impurity with C1s at 284.8 eV was applied to calibrate the binding energies of all measured elements.
A temperature-programmed reduction by H2 (H2-TPR) was conducted on an AutoChem II 2920 chemisorption analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Typically, 60 mg catalysts were loaded into the U-shape quartz tube. Before each test, the samples were pretreated in a flow of 50 mL min−1 Ar at 300 °C for 1 h to remove any impurities and moisture. After the system cooled down to 50 °C in the Ar atmosphere, 10% H2/Ar (50 mL min−1) was introduced to the system. Once the baseline was stable, the temperature was raised from 50 to 900 °C ramping with 10 °C min−1. The consumption of H2 was continuously monitored by the thermal conductivity detector.
A thermo-gravimetric analysis (TGA) was carried out on a Differential Scanning Calorimetry-Mass instrument (Mettler Toledo, Greifensee, Switzerland). Before the experiments, all the catalysts were pretreated at 300 °C in a flow of N2 (40 mL min−1) to remove impurities and moistures. The tests were conducted under N2 flow (40 mL min−1) with the temperature rising from 50 to 900 °C at a ramping rate of 10 °C min−1.
The in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTs) studies were performed on a Nicolet 6700 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) equipped with an MCT/A detector cooled by liquid nitrogen. To increase the absorbance intensity, Co3O4-based catalysts were diluted to a weight ratio of 10% by physically grinding with KBr powder in an agate mortar. Then, the fine ground catalyst powder was filled into the reaction chamber with BaF2 windows embedded in the dome of the sample cell. Prior to the experiments, samples were pretreated in a pure N2 flow (100 mL min−1) at 300 °C for 1 h to remove the impurities and moistures. The background spectrum was recorded once the chamber cooled to the desired temperature and deducted from the sample spectrum at the same temperature during the reaction.

4. Conclusions

Flower-like Co3O4 catalysts were synthesized via the hydrothermal method. The results from SEM, XPS, and N2 adsorption–desorption isotherms confirmed the formation of the uniformly structured nanoflower for both Co3O4 and Pt/Co3O4 catalysts. With the addition of Pt, Pt/Co3O4-F catalysts obtained excellent NO resistance and low-temperature catalytic oxidation performances toward multi-pollutants diesel emissions, which outperformed the commercial platinum DOCs. Moreover, the Pt/Co3O4-F catalysts exhibited the exceptional hydrothermal durability of CO, C3H6, and NO oxidation throughout an on-stream test. Different characterization techniques were carried out to study SMSI between Pt and supporting Co3O4-F. The results of XPS spectra and H2-TPR profiles revealed that Pt boosted the chemisorbed capacity of oxygen on the surface of Co3O4-F and enhanced the reducibility of the catalysts by taking advantage of the SMSI effect between Pt and supporting Co3O4-F, respectively. In addition, the TGA profiles proved that Pt promoted catalytic oxidation activities by increasing the desorption of surface-active oxygen and hydroxyl radicals at a relatively low-temperature regime. Furthermore, in situ DRIFTs spectra uncovered that CO and C3H6 firstly converted to carbonates, formate, and acetate species as the active intermediates and then further oxidized to generate CO2 and H2O as final products over Pt/Co3O4-F. Meanwhile, compared to Co3O4-F catalysts, metallic Pt atoms provided the active site for C3H6 adsorption and activation, which promoted the catalytic oxidation activity for C3H6 oxidation in the multi-pollutants exhaust.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12050527/s1, Table S1: the comparison of catalytic activities of different catalysts toward CO, C3H6 and NO oxidation for multi-pollutant diesel emissions, Table S2: the N2 adsorption–desorption isotherms of Pt/Co3O4-F and Co3O4-F. References [33,42,43,44] are cited in the supplementary materials.

Author Contributions

Conceptualization, L.M.; investigation, Z.L. and X.C.; data curation, Z.L. and X.C.; resources, J.C. and H.C.; writing—original draft, Z.L.; writing—review and editing, L.M. and N.Y.; supervision, L.M.; funding acquisition, L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (22176122), the Shanghai Pujiang Program (20PJ1407000), the special fund of the State Key Joint Laboratory of Environment Simulation and Pollution Control (21K07ESPCT), and the National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2018A12).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The light-off performances of (a) CO, (b) C3H6, and (c) NO oxidation over Co3O4-F, Pt/Co3O4-F, Pt/Al2O3, Pt/SiO2, and Pt/SiO2 and Co3O4-F catalysts. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, 500 ppm NO, 10% O2 balanced with N2 at WHSV = 240,000 mL g−1 h−1. (d) The hydrothermal durability of Pt/Co3O4-F catalysts at 260 °C. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, 500 ppm NO, 10% O2, and 5% H2O balanced with N2 at WHSV = 240,000 mL g−1 h−1.
Figure 1. The light-off performances of (a) CO, (b) C3H6, and (c) NO oxidation over Co3O4-F, Pt/Co3O4-F, Pt/Al2O3, Pt/SiO2, and Pt/SiO2 and Co3O4-F catalysts. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, 500 ppm NO, 10% O2 balanced with N2 at WHSV = 240,000 mL g−1 h−1. (d) The hydrothermal durability of Pt/Co3O4-F catalysts at 260 °C. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, 500 ppm NO, 10% O2, and 5% H2O balanced with N2 at WHSV = 240,000 mL g−1 h−1.
Catalysts 12 00527 g001
Catalysts 12 00527 g002 550
Figure 2. The light-off profiles of (a,c) CO oxidation and (b,d) C3H6 oxidation over Co3O4-F and Pt/Co3O4-F, Pt/Al2O3, Pt/SiO2, and Pt/SiO2 and Co3O4-F catalysts. The reaction conditions were as follows: (a) 4000 ppm CO, 10% O2 balanced with N2; (b) 1000 ppm C3H6, 10% O2 balanced with N2; and (c,d) 4000 ppm CO, 1000 ppm C3H6, 10% O2 balanced with N2. All the reactions were kept at WHSV = 240,000 mL g−1 h−1.
Figure 2. The light-off profiles of (a,c) CO oxidation and (b,d) C3H6 oxidation over Co3O4-F and Pt/Co3O4-F, Pt/Al2O3, Pt/SiO2, and Pt/SiO2 and Co3O4-F catalysts. The reaction conditions were as follows: (a) 4000 ppm CO, 10% O2 balanced with N2; (b) 1000 ppm C3H6, 10% O2 balanced with N2; and (c,d) 4000 ppm CO, 1000 ppm C3H6, 10% O2 balanced with N2. All the reactions were kept at WHSV = 240,000 mL g−1 h−1.
Catalysts 12 00527 g002
Catalysts 12 00527 g003 550
Figure 3. SEM images of Co3O4-F with different scanning scales (a) 10 μm and (b) 20 μm.
Figure 3. SEM images of Co3O4-F with different scanning scales (a) 10 μm and (b) 20 μm.
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Catalysts 12 00527 g004 550
Figure 4. The N2 adsorption–desorption isotherms and pore size distribution for (a,c) Pt/Co3O4-F and (b,d) Co3O4-F.
Figure 4. The N2 adsorption–desorption isotherms and pore size distribution for (a,c) Pt/Co3O4-F and (b,d) Co3O4-F.
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Catalysts 12 00527 g005 550
Figure 5. The XRD profiles of the Co3O4-F and Pt/Co3O4-F catalysts.
Figure 5. The XRD profiles of the Co3O4-F and Pt/Co3O4-F catalysts.
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Catalysts 12 00527 g006 550
Figure 6. The deconvoluted peaks profiles of the (a) Co 2p3/2, (b) O 1s, and (c) Pt 4f X-ray photoelectron spectra for Pt/Co3O4-F and Co3O4-F. Black dots: original data of XPS profiles; Blue lines: deconvoluted peaks of the XPS curves; Red line: fitting curves.
Figure 6. The deconvoluted peaks profiles of the (a) Co 2p3/2, (b) O 1s, and (c) Pt 4f X-ray photoelectron spectra for Pt/Co3O4-F and Co3O4-F. Black dots: original data of XPS profiles; Blue lines: deconvoluted peaks of the XPS curves; Red line: fitting curves.
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Catalysts 12 00527 g007 550
Figure 7. The H2-TPR profiles of Co3O4-F and Pt/Co3O4-F. Black lines: original curves; Green and blue lines: deconvoluted peaks for the first and second reduction peak, respectively.
Figure 7. The H2-TPR profiles of Co3O4-F and Pt/Co3O4-F. Black lines: original curves; Green and blue lines: deconvoluted peaks for the first and second reduction peak, respectively.
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Catalysts 12 00527 g008 550
Figure 8. The thermo-gravimetric analysis profiles of the Co3O4-F and Pt/Co3O4-F catalysts under an N2 atmosphere.
Figure 8. The thermo-gravimetric analysis profiles of the Co3O4-F and Pt/Co3O4-F catalysts under an N2 atmosphere.
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Catalysts 12 00527 g009 550
Figure 9. In situ DRIFTs spectra of CO and O2 co-adsorption over (a) Co3O4-F and (b) Pt/ Co3O4-F. The reaction conditions were as follows: 4000 ppm CO, 10% O2, balanced with N2, and the total flow rate was kept at 100 mL min−1.
Figure 9. In situ DRIFTs spectra of CO and O2 co-adsorption over (a) Co3O4-F and (b) Pt/ Co3O4-F. The reaction conditions were as follows: 4000 ppm CO, 10% O2, balanced with N2, and the total flow rate was kept at 100 mL min−1.
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Catalysts 12 00527 g010 550
Figure 10. The in situ DRIFTs spectra of CO, C3H6, and O2 co-adsorption over (a) Co3O4-F and (b) Pt/Co3O4-F. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, and 10% O2, balanced with N2, and the total flow rate was kept at 100 mL min−1.
Figure 10. The in situ DRIFTs spectra of CO, C3H6, and O2 co-adsorption over (a) Co3O4-F and (b) Pt/Co3O4-F. The reaction conditions were as follows: 4000 ppm CO, 1000 ppm C3H6, and 10% O2, balanced with N2, and the total flow rate was kept at 100 mL min−1.
Catalysts 12 00527 g010
Table
Table 1. The binding energy, FWHM, and relative area derived from the deconvoluted peaks of the Co 2p3/2 and O 1s spectra of the Pt/Co3O4-F and Co3O4-F catalysts.
Table 1. The binding energy, FWHM, and relative area derived from the deconvoluted peaks of the Co 2p3/2 and O 1s spectra of the Pt/Co3O4-F and Co3O4-F catalysts.
SamplesBinding Energy (eV)FWHM (eV)Relative Area (%)Assignment
Co 2p3/2Pt/Co3O4-F779.41.4742.1Co2+ and Co3+
780.61.7227.2Co3+
781.92.1618.1Co2+
785.76.978.9Shake-up satellite
789.92.733.7Shake-up satellite
Co 2p3/2Co3O4-F779.11.4744.4Co2+ and Co3+
780.31.7328.7Co3+
781.62.1415.5Co2+
784.94.986.5Shake-up satellite
789.33.014.9Shake-up satellite
O 1sPt/Co3O4-F529.90.9749.9Lattice oxygen in spinel
531.22.7250.1Chemisorbed and surface oxygen
O 1sCo3O4-F529.50.9461.4Lattice oxygen in spinel
530.92.638.6Chemisorbed and surface oxygen
Table
Table 2. The quantitative results of H2-TPR of the Co3O4-F and Pt/Co3O4-F catalysts.
Table 2. The quantitative results of H2-TPR of the Co3O4-F and Pt/Co3O4-F catalysts.
CatalystsDeconvoluted PeaksPeak Area Ratio of II/I
Peak No.Center (°C)Area
Co3O4-FI281.91.153.29
310.31.31
340.72.73
II392.56.10
431.08.48
460.22.51
Pt/Co3O4-FI110.02.321.90
136.22.60
160.13.01
II256.47.33
318.55.54
353.72.17
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Li, Z.; Chen, X.; Chen, J.; Chang, H.; Ma, L.; Yan, N. Flower-like Co3O4 Catalysts for Efficient Catalytic Oxidation of Multi-Pollutants from Diesel Exhaust. Catalysts 2022, 12, 527. https://doi.org/10.3390/catal12050527

AMA Style

Li Z, Chen X, Chen J, Chang H, Ma L, Yan N. Flower-like Co3O4 Catalysts for Efficient Catalytic Oxidation of Multi-Pollutants from Diesel Exhaust. Catalysts. 2022; 12(5):527. https://doi.org/10.3390/catal12050527

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Li, Zihao, Xianhuai Chen, Jinghuan Chen, Huazhen Chang, Lei Ma, and Naiqiang Yan. 2022. "Flower-like Co3O4 Catalysts for Efficient Catalytic Oxidation of Multi-Pollutants from Diesel Exhaust" Catalysts 12, no. 5: 527. https://doi.org/10.3390/catal12050527

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