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Introduction

Since the automobile amount rise in recent years, the Global has had trouble with environmental problems, air pollution in particular. Carbon monoxide is one of the main air pollutants generated from automotive emission, and catalytic oxidation is considered to be the most efficient method for elimination of carbon monoxide from environment.

Among the catalysts, the transition metal oxides with particular morphologies of micro/nano configuration have fascinated the scientific community in the recent years because of its large specific surface area and outstanding oxygen storage and release capacity [1-2]. All these have been proved to be beneficial as support for CO oxidation [3-6]. As a p-type wide band gap semiconductor, NiO is a very promising material among the transition metal oxides because of its extensive important applications as catalysts [7-9], electrode materials for lithium ion batteries and fuel cells [10,11] and magnetic materials [12]. Thus, precious metals such as gold, platinum and rhodium with high activity and stability supported on transition metal oxides have been widely used in CO oxidation and gas exhaust emissions control [13-15].

Recently, a series of transition metal oxide (ferric oxide, nickel oxide, silica, etc.) supported Pd catalysts have been synthesized and applied for low temperature CO oxidation because they are much stable and relatively less expensive than Au [16–18]. In particular, to solve the continuously rising price and rare reserve of the noble metals problems, Alloying of noble metals with non-noble metals (Fe, Co, Cu etc.) is one of the effective approaches to enhance the catalytic activity and the selectivity and thereby reduce the loading of the noble metals [19-23].

In this paper, novel flower-like nickel oxide spheres were synthesized by hydrothermal method. NiO nanosheets less than 10 nm aggregated together to form the flower-like NiO microspheres support. PdCu nanoparticles were evenly deposited by wet impregnation and in-situ reduction protocol. The resulting materials showed excellent catalytic activity for low temperature CO oxidation especially under ambient conditions.

Experimental Section

Preparation of flower-like NiO(F-NiO)

4g CTAC (cetanecyl trimethyl ammonium chloride) and 7.27g nickel nitrate hexahydrate were dissolved into 20 mL anhydrous ethanol in a 300 mL beaker, stirring until the CTAC is basically dissolved, then 80 mL deionized water at 90 °C was added into the beaker under vigorous stirring conditions. After 10 minutes, urea (1.6 wt%, 100 mL) was dropped into the mixed solution with feeding speed at 2-3 drops per second. After stirred 6 hours at room temperature, the mixture was transferred into an electro-thermostatic blast oven and aged at 90 °C for 4.5 h. The precipitate was centrifuged, washed with deionized water and anhydrous ethanol. The obtained green samples were frozen in a cryogenic refrigerator for 8 hours, and then dried in a freeze-dryer for 24 hours.The fine green Ni(OH)₂·xH₂O powder was annealed under air atmosphere at 400 °C for 2h with a ramp rate of 5 °C per min to get the final product F-NiO.

Synthesis of PdCu/ F-NiO

An aqueous solution of PdCl₂ (3.3 mg/mL, 3.8 mL), 20 mg of CuSO₄·5H₂O, and 50 mg of glutamate were mixed together in 40 mL of ethylene glycol (EG). The pH of the system was adjusted to ca. 11 by the dropwise addition of the 8 wt% KOH/EG solution under vigorous stirring. During the process, the green solution turned into dark blue. Subsequently, 30 mg of F-NiO were added, and the solution was stirred for 2 h to obtain a homogeneous suspension. Upon completion, the suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The autoclave was sealed, heated at 160 °C for 6 h, and air-cooled to room temperature. Finally, the product was collected by filtration and washed several times with deionized water. The obtained product was dried at 40°C under vacuum for 8 h. The catalyst thus obtained was denoted as PdCu/ F-NiO. PdCu catalysts supported on F-NiO with Pd:Cu nominal atomic ratios of 1:0, 1:1, and 0:1 were prepared under similar conditions and denoted as 2 wt% Pd/ F-NiO, 2 wt% Cu / F-NiO, and 2 wt% Pd₁Cu₁/ F-NiO, respectively.

Catalyst Characterization

The catalyst’s crystallographic structure was analyzed using X-ray diffraction (XRD) (X’Pert3 Powder instrument. (PANalytical B.V.) operated at 45 kV and tube current of 40mA). The field-emission scanning electron microscopy (FE-SEM) images were conducted on a SU8010 system. The transmission electron microscope (TEM) images were obtained using a JOEL JEM-2100 operated at 200 kV. A drop of ethanol-dispersed nanoparticles was placed on a carbon-coated copper grid and left to dry. The catalysts’ surface area detailed in the Brunauer–Emmett–Teller technique (BET) was measured by an ASAP2460 instrument, which was also used to obtain a nitrogen adsorption–desorption isotherm and pore diameter distribution pattern. Elemental analysis of the samples was performed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES Thermo Scientific iCAP 6500 ICP Spectrometer, (Waltham, MA, USA).

CO oxidation test

The catalytic activity for CO oxidation was carried out infixed-bed quartz tubular reactor (length 220 mm, i.d. 9 mm) containing 100 mg of catalyst under a steady-state condition, involving a feed stream with a fixed composition of CO 2%, O₂ 18%, and N₂ 80% by volume, where N₂ was used as diluent. The sample was pretreated in an air stream at 200 °C for 3 h and then cooled to reaction temperature. After that, the mixed gases were switched on. The reaction was carried out with the same space velocity of 30000 mL g^-1 h^-1. The composition of the gas exiting the reactor was monitored by online gas chromatography (GC-9160). The conversions were determined on the basis of CO consumption and CO₂ formation. All the catalytic results were repeated three times at a given temperature.

Results and Discussion

Figure 1 shows the XRD patterns of F-NiO. The XRD pattern of the as-prepared flower-like NiO spheres shows well-defined diffraction feature of the face centered cubic (fcc) NiO phase. The characteristic diffraction peaks at 2θ of 37.2°, 43.3°, 62.9°, 75.4° and 79.4° could be assigned to (111), (200), (220), (311) and (222) crystal planes respectively. The positions of the corresponding peaks are in good agreement with the reported data (JCPDS Card No. 47-1049). The XRD pattern of F-NiO indicates that the Ni(OH)₂·xH₂O precursor are decomposed completely to F-NiO under the experiment conditions.

Figure 2:

FE-SEM micrographs of F-NiO the F-NiO was characterized by FE-SEM-(field emission scanning electron microscopy) test, and the results are shown in Figure 2. From Figure 2a and 2c’ we can see that the as-prepared NiO are the well-dispersed flower-like microspheres with average size of 1.94 μm and narrow size distribution (Figure 2b). As shown in Figure 2d the monodispersed NiO microspheres were formed by densely packed irregular assembling nanometer sheets with a thickness of about 9nm

In order to further explore the inner structure of F-NiO microspheres, a series transmission electron microscopy (TEM) observations were collected. The results are shown in Figure 3. It clearly reveals that the monodisperse configuration flower-like NiO microspheres are made of the nanometer piece of interlaced assemble micron grade spherical flowers (Figure 3a). From Figure 3b and 3c, we can see that the surface structure of the nanosheet forming the NiO micron flower is not smooth and dense, but a nanosheet with a cavity structure of about 9nm in thickness. The lattice spacing in Figure 3d (d=0.30 nm, which represents the (200) crystal surface of NiO) once again proves that the calcined sample is nickel oxide. In addition, we further studied the crystal phase of NiO micron flowers with clear monodispersed configuration, and the results are illustrated in Figure 3d, which is the selective electron diffraction result of NiO flower-like structure in Figure 3d. From the test results of single point dispersion, we analyzed the NiO flower-like configuration as a single crystal system.

The specific surface area and porosity of the as-prepared flower-like NiO nanocrystals were determined using N₂ adsorption/desorption analysis. The N₂ adsorption/desorption isotherms and the corresponding pore size distribution plots for F-NiO nanocrystals are shown in Figure 4. The BET surface area of the F-NiO nanocrystals is as high as 328.6m²/g. According to the IUPAC classification, the obtained isotherm (Figure 4a) can be recognized as a type IV N₂ adsorption/desorption isotherm, which is the typical characteristics of mesoporous materials. Furthermore, the F-NiO nanocrystals have an obvious strong and narrow peak at about 37.1 nm calculated by Barrett–Joyner–Halenda (BJH) analysis using the desorption branch of the isotherm (Figure 4b). The results could indicate that F-NiO nanocrystals with a large BET surface area and the hierarchical architectures are beneficial to enhance the catalytic ability of catalyst.

X-Ray diffraction (XRD) results were used to demonstrate the formation of the PdCu alloy phase. Figure 5 shows the XRD patterns of Cu/F-NiO, Pd/F-NiO, and Pd₁Cu₁/F-NiO, respectively. From the XRD patterns of Figure 5, A series sharp diffraction peaks (111), (200), (220), (311) and (222) are fitted well with standard PDF card of NiO (no.47-1049). The standard peak located at 42.3° is also assigned to (111) plane of Cu in the XRD pattern for Cu/F-NiO (Figure 5a). A weak peak of catalyst with 2 wt% loading contents emerges at 2θ = 40° (Figure 5b), which corresponded to (111) diffraction bands of palladium with face-centered cubic. After loaded with palladium and copper, it can be observed that there is a slightly positive shift of the XRD peaks of Pd₁Cu₁/F-NiO compared with those of the Pd/ F-NiO (Figure 5c). The peak of PdCu alloy (111) plane lies between the Pd (40.1°) and Cu (42.3°) and the calculated lattice parameter of PdCu alloy (0.217 nm) lies between the Pd (0.225 nm) and Cu (0.209 nm), which is close to the standard d value of PdCu alloyed (111) plane distance. These results show the formation of the PdCu alloy phase. Similar phenomena, the shift of the diffraction peaks and the change of the lattice parameters during the formation of the alloy, have also been reported.

To get further insight into the elemental distribution in the as-prepared Pd₁Cu₁/F-NiO composite, EDAX mapping analysis of the Pd and Cu by field emission scanning electron microscopy (FE-SEM) were performed in Figure 6. The EDAX mapping analysis indicates the uniform distribution of Pd and Cu in the Pd₁Cu₁/F-NiO composite (Figure 6b and c). Furthermore, the amounts of Pd and Cu loaded on the support in these catalysts are almost identical to the results from ICP-MS analysis (Table 1).

To further understand the structure of F-NiO sheet and the distribution of PdCu alloy active component. The Pd₁Cu₁/F-NiO sample was performed with TEM analysis (Figure 7). It clearly reveals that the NiO sheets are actually the accumulation of nano-sized particles less than 17 nm (Figure 7b and c). The particle size and dispersion of PdCu nanoparticles could be calculated from HRTEM image, as showing in Figure 7c. The average size of PdCu from 20 particles is about 16.3 nm. Figure 7d indicates that the interplanar distance is 0.217 nm, which match with the calculated lattice parameter of PdCu alloy. These results proved the successful loading of PdCu on the NiO support. The nanosized PdCu alloy particles and the NiO support which has large BET surface area make the Pd₁Cu₁/F-NiO catalyst suitable for CO oxidation.

Figure 8 shows the CO oxidation performance for as-prepared catalysts. The flower-like NiO was also conducted in the same reaction condition for comparison. The results indicate that the pure F-NiO support show certain catalytic activity within the whole reaction temperature. It also shows that F-NiO is beneficial as support for CO oxidation. After loaded with metals, the catalytic performances of the three catalysts show significant differences. It can be seen that when Cu/F-NiO was used as the catalyst, the reaction was difficult to carry out even at 100 °C, with almost no product (4.7%) being obtained. In comparison, the Pd/F-NiO catalyst exhibited obviously enhanced catalytic activity, with the conversion reaching about 30.2% under the same reaction conditions. The above result confirmed the important role of Pd for the CO oxidation. Furthermore, when the Pd₁Cu₁/ F-NiO compound was used as the catalyst, a yield of about 83.1% was achieved even at 25 °C. With a further increment of the temperature to about 150 °C, the yield would reach about 99.2%,much higher than the other counterparts, suggesting that the synergetic effect between Pd and Cu is indeed beneficial for the progress of the reaction. All above results clearly prove that the PdCu alloy content and highly dispersion are all important for CO oxidation. NiO supported PdCu catalyst are suitable for CO elimination especially under ambient conditions.

To better understand the stability of Pd₁Cu₁/F-NiO, the catalyst was recovered after reaction, washed with deionized water, dried at 40°C under vacuum for 8 h and reused in a subsequent new catalytic run. As shown in Figure 9, the conversion towards CO is basically constant in the consecutive runs and only a slight decrease. This verifies that this Pd-Cu based catalyst possesses long-term stability.

Proposed Reaction Process

To better understand the formation of Pd₁Cu₁/F-NiO, a proposed reaction process is shown in Scheme 1. Firstly, F-NiO nanocrystals were successfully prepared through annealing flower-like Ni(OH)₂·xH₂O precursors. Subsequently, Pd₂₊, Cu₂₊ and glutamate were reduced by EG during the hydrothermal reaction to form PdCu nanoparticles supported F-NiO. Finally, the obtained Pd₁Cu₁/F-NiO was used for catalytic oxidation of carbon monoxide, and showed the highest catalytic activity compared with Pd/F-NiO and Cu/F-NiO.

According to many previous results [24-25], the use of intermetallic compounds by contact with active oxides may improve the catalytic properties. So, in this work, Pd₁Cu₁/F-NiO catalysts have higher catalytic oxidation of CO, which could be due to three reasons: (1) flower-like NiO possess a larger surface area, bimodal pore size distribution, higher reducibility, and superior catalytic activity for CO oxidation [26]; (2) particle size of PdCu is smaller, which is beneficial to the reaction; (3) Adding an appropriate amount of Cu can efficiently promote the surface dispersion of Pd and affect the redox properties during the CO oxidation. The particular structure of the PdCu intermetallic surface has an important role in the low kinetic barrier to the weakest adsorption strength of O₂ for the reaction, allowing for an easy approach of the O₂ dissociation and CO absorption [27-28].

Conclusion

In summary, hierarchically porous flower-like NiO crystals ranging from 1to 3μm have been successfully prepared through hydrothermal and pyrolysis process. NiO nanoparticles less than 10 nm aggregate together to form irregular NiO nano sheets and finally compose the flower-like NiO sphere. After decorated with PdCu alloy, the hierarchically porous structure and evenly dispersed PdCu nanoparticles make it one of the excellent catalysts for low temperature CO oxidation especially under ambient condition.

Statements and Declarations

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1904196), and the PhD Start-up Fund of Henan Normal University, China (Grant No. qd14122).