Synthesis of alumina supported Pd–Cu alloy nanoparticles for CO oxidation via a fast and facile method

Abstract

Supported Pd–Cu alloy nanoparticles have been successfully synthesized via a fast, facile and environmentally friendly method by simultaneous reduction of Pd(NO₃)₂ and Cu(NO₃)₂. The catalyst contains Pd–Cu nanoparticles with higher alloying degree, smaller particle size, higher ratio of Pd⁰, Cu⁺, and chemisorbed oxygen species, and therefore exhibits enhanced CO oxidation activity.

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Catalytic oxidation of carbon monoxide (CO) has received considerable attention due to its wide applications in automotive exhaust gas after-treatment and CO preferential oxidation for proton exchange membrane fuel cells (PEMFC).^1–5 Among the noble metal catalysts, less expensive supported Pd-based catalysts have drawn much research attention from economic point view.^6,7 Previous studies have demonstrated that synthesis of size-, shape-, and composition-controlled bimetallic nanoparticles is of significance to enhance the CO oxidation activity.^8–11 For example, bimetallic alloy of Pd with Cu not only exhibits higher catalytic activity for CO oxidation, but can also reduce the cost of the catalyst significantly.¹² However, it still remains a challenge for synthesizing bimetallic Pd-based nanoparticles that have a narrow size-distribution and well controlled composition.¹³

Compared with conventional methods, cold plasma method is an environmentally friendly and energy efficient method for preparing supported metal catalysts, and has been recently used for preparing supported Pd-based catalysts.¹⁴ The supported metal catalysts prepared by cold plasma exhibited smaller size and controllable structure of metal particles, and thereby exhibiting remarkably enhanced activity and properties. In previous studies, we developed a dielectric barrier discharge (DBD) cold plasma technology at atmospheric pressure for synthesizing supported catalysts and found that it is highly efficient for preparing high performance supported Pd catalysts.¹⁵ It will be an interesting work if high performance bimetallic catalysts can be fabricated by DBD cold plasma and the relationship between the structure and performance of bimetallic catalysts can be disclosed. However, to the best of our knowledge, no work has been conducted.

In this study, inert alumina supported high performance Pd–Cu alloy nanoparticles were successfully fabricated by the fast, facile and environmentally friendly DBD cold plasma method at atmospheric pressure, and the reasons for the high performance were analyzed.

The wide-angle XRD patterns of Al₂O₃, Pd/Al₂O₃–C (C: conventional thermal reduction), Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P (P: plasma reduction) samples are shown in Fig. 1. The Pd peaks at 40.1° and 46.6° for Pd/Al₂O₃–C sample are assigned to the (111) and (200) planes of face-centered cubic (fcc) structure of metallic Pd (JCPDS card, File no. 46-1043), respectively. Interestingly, from the XRD analysis, we can also see that the diffraction peaks of the Pd–Cu bimetallic nanoparticles in Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P display an fcc structure, which are similar to that of pure metallic Pd nanoparticles. However, the peaks shifted to higher 2theta values without forming any discrete Cu reflections, which indicates that the Pd–Cu nanoparticles are bimetallic alloy rather than a physical mixture of separately nucleated monometallic Pd and Cu nanoparticles.¹⁶

Fig. 1 The wide-angle XRD patterns of Al₂O₃, Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P.

XPS spectra of (a) Pd3d, (b) Cu2p and (c) O1s in Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P were collected and shown in Fig. 2. XPS spectra of Pd3d (Fig. 2a) in Pd/Al₂O₃–C can be fitted with two peaks corresponding to metallic Pd and Pd²⁺, while a new peak corresponding to Pd⁴⁺ can be obtained in the Pd–Cu/Al₂O₃ samples due to the existence of Cu. From Fig. 2a, we can also see that binding energies (BE) of Pd3d core levels for Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P shift to higher BE. The obvious positively shifts may be ascribed to the strong electronic interaction between Pd and Cu atoms due to the formation of alloy,^17,18 leading to downshift in the d-band center of Pd atom in the Pd–Cu/Al₂O₃ catalysts. This is consistent with the XRD results. Interestingly, the Pd3d core level for Pd–Cu/Al₂O₃–P shifts to a higher BE than that for Pd–Cu/Al₂O₃–C, which indicates that Pd–Cu nanoparticles exhibits a higher alloying degree.^17,18 As shown in Fig. 2b, XPS spectra of Cu2p can be deconvoluted into three peaks at 932.4 eV, 932.5 eV and 934.1 eV, attributed to Cu⁰, Cu⁺¹⁹ and Cu²⁺.¹² The compositions of Pd and Cu in the samples are summarized in Table 1. Pd predominantly exists in low oxidation state (Pd⁰ and Pd²⁺) and Cu in high oxidation states (Cu⁺ and Cu²⁺) for Pd–Cu/Al₂O₃–P.

Fig. 2 XPS spectra of (a) Pd3d, (b) O1s and (c) Cu2p in Al₂O₃, Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P.

Table 1 The size of the Pd-based nanoparticles (D_Pd), compositions of Pd and Cu, the ratio of chemisorbed oxygen species (O_chem), 100% CO conversion temperature (T₁₀₀), the apparent activation energies (E_a) and turnover frequencies (TOF) at 155 °C of the samples

Catalysts	DPd (nm)	The composition of Pd (%)			The composition of Cu (%)			Ochem (%)	T100 (^oC)	Ea (kJ mol^−1)	TOF155 °C × 10^3 (s^−1)
		Pd^0	Pd^2+	Pd^4+	Cu^0	Cu^+	Cu^2+
Pd/Al2O3–C	10.0 ± 3.2	79.7	20.3	0	—	—	—	9.8	175	211.3 ± 3.8	59.7
Pd–Cu/Al2O3–C	6.2 ± 1.5	54.5	28.3	17.2	16.9	16.9	66.2	11.6	160	170.3 ± 5.5	136.6
Pd–Cu/Al2O3–P	4.3 ± 0.9	56.9	29.9	13.2	10.0	22.1	67.9	31.1	155	119.3 ± 6.5	408.2

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Fig. 2c presents the O1s spectra. For all samples, O1s spectra can be deconvoluted into three peaks at 530.4, 531.5 and 532.6 eV, ascribed to chemisorbed oxygen species,²⁰ Al–O²¹ and OH,²² respectively. The ratios of chemisorbed oxygen species in O1s for the samples were also summarized in Table 1. More chemisorbed oxygen species are formed on the Al₂O₃ support for Pd–Cu/Al₂O₃–P than Pd/Al₂O₃–C and Pd–Cu/Al₂O₃–C. This is in line with the results of our previous work,^15b which may be ascribed to the fact that the plasma reduction was a low-temperature and fast process (6 min).

For getting more insight into the metal–metal interaction, TPR profiles of the samples were characterized and illustrated in Fig. 3. Negative and positive peaks at around 70 and 470 °C in Pd/Al₂O₃–C correspond to desorption from surface metallic Pd of the H₂ adsorbed at room temperature at the beginning of the analysis,²³ and the reduction of the surface interacted PdO species,²⁴ respectively. However, these peaks disappear and a positive peak is observed in Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P at around 200 °C, ascribed to the reduction of Pd and Cu from the mixed oxide (Pd_xCu_yO) due to the formation of Pd–Cu alloy,²⁵ and the reduction of chemisorbed oxygen species, respectively.^26,27 Furthermore, the positive peak at around 200 °C for Pd–Cu/Al₂O₃–P is more broader than that for Pd/Al₂O₃–C and Pd–Cu/Al₂O₃–C due to the more chemisorbed oxygen species. These are consistent with the results of XRD and XPS.

Fig. 3 TPR profiles of Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P.

Fig. 4 shows the TEM and HRTEM images of the samples, and the corresponding histograms of size distribution of the Pd nanoparticles. The average diameter of the Pd nanoparticles (D_mean) and the standard deviation (STD) were obtained by analyzing numerous TEM images. Pd nanoparticles are 10.0 ± 3.2, 6.2 ± 1.5 and 4.3 ± 0.9 nm, in diameter for Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P, respectively. The smaller size of the bimetallic Pd–Cu nanoparticles was attributed to the formation of high-melting-point Pd–Cu alloy.²⁸ In addition, the size of Pd–Cu nanoparticles in Pd–Cu/Al₂O₃–P is smaller than that for Pd–Cu/Al₂O₃–C due to the low-temperature and fast reduction process. These are consistent with the results of XRD. The insert figures in Fig. 4 show high-resolution TEM (HRTEM) images of two individual particles in Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P, with spots in the Fourier transformed images at 0.22 nm, which is characteristic of a Pd–Cu alloy.²⁹

Fig. 4 TEM images of (a) Pd/Al₂O₃–C, (b) Pd–Cu/Al₂O₃–C and (c) Pd–Cu/Al₂O₃–P, HRTEM images (given as insert), and the corresponding histograms of size distribution of the Pd nanoparticles.

Fig. 5 presents the CO oxidation activity over Pd-based catalysts in the temperature range of 20–200 °C. Under the reaction conditions, the CO conversion increases gradually with the increase in temperature, and a sharp increase can be observed for the Pd catalysts when the temperature is close to T₁₀₀ (100% CO conversion). T₁₀₀ were achieved at 175 °C, 160 °C and 155 °C, respectively, over Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P.

Fig. 5 (a) Results of CO conversion over Al₂O₃, Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P, and (b) the corresponding apparent activation energies (E_a) and turnover frequencies (TOF) at 155 °C.

Obviously, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P exhibit enhanced CO oxidation activity with the addition of copper.

According to the data taken from Fig. 5a, the Arrhenius plots are shown in Fig. S2,† and good linear relationships are observed. The apparent activation energies (E_a) for Pd/Al₂O₃–C, Pd–Cu/Al₂O₃–C and Pd–Cu/Al₂O₃–P, which could be obtained from the slopes of the plots, were 211.3 ± 3.8, 170.3 ± 5.5 and 119.3 ± 6.5 kJ mol⁻¹, respectively. To get insight into the intrinsic activities of the supported Pd catalysts, the turnover frequencies (TOF) normalized as the number of CO molecules reacting per active site per second were also calculated. The apparent activation energies (E_a) and turnover frequencies (TOF) at 155 °C of the samples are presented in Table 1.

The activation energy has been decreased with the addition of copper, and TOF of Pd–Cu/Al₂O₃–P and Pd–Cu/Al₂O₃–C for CO oxidation are about 6.8 times and 2.3 times as that of Pd/Al₂O₃–C, respectively. Low CO oxidation activity for monometallic Pd catalysts (Pd/Al₂O₃–C) was attributed to the fact that CO adsorption on the Pd surface is too strong, the active sites for oxygen adsorption and activation are blocked, namely, CO poison effect.³⁰ Previous studies have shown that Cu can strongly modify the valence state of Pd by injecting charge into the sp sub-band.¹² This electronic modification means that Pd–Cu alloys may facilitate CO dissociation relative to Pd by converting the nature of the CO interaction with Pd surface centers from covalent to a mixture with an ionic component. In addition, after injecting charge into the sp sub-band of Pd, active Cu⁺ ions will be formed, which may adsorb and activate oxygen.³¹

Interestingly, TOF of Pd–Cu/Al₂O₃–P is about 3.0 times as that of Pd–Cu/Al₂O₃–C, ascribed to the higher alloying degree (Fig. 2a), and smaller particle size of Pd–Cu nanoparticles (Fig. 4). The previous d-band center theory indicated that a change in the adsorbate chemisorption energy scales directly with the change in the metal center of the d-band: lowering of the d-band center (Fig. 2) results in the decrease in interaction strength of the various adsorbates to the substrate.^17,18 In other words, CO adsorption on the Pd surface for Pd–Cu/Al₂O₃–P with higher alloying degree of Pd atom (Fig. 2a) will be not so strong as that for Pd–Cu/Al₂O₃–C. Consequently, there will be more active sites for oxygen adsorption on the Pd surface for Pd–Cu/Al₂O₃–P.

Furthermore, the valence states of Pd and Cu also play an important role in CO oxidation. The ratios of metallic Pd and Cu⁺ in Pd–Cu/Al₂O₃–P are 2.4% and 5.3% higher than that in Pd–Cu/Al₂O₃–C, respectively. Metallic Pd exhibits higher activity than that of PdO and PdO₂, which are stable against reduction by CO.³⁰ As for the valence states of copper, the propensity of Cu⁺ toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enable Cu⁺ to exhibit higher activity than Cu⁰ and Cu²⁺.³¹ Finally, the ratios of chemisorbed oxygen species in Pd–Cu/Al₂O₃–P (31.1%) are much higher than that in Pd–Cu/Al₂O₃–C (11.6%). Chemisorbed oxygen species were thought to be liable to react with CO and may play an important role in CO oxidation.²⁷ For these reasons, Pd–Cu/Al₂O₃–P shows higher CO oxidation activity than Pd–Cu/Al₂O₃–C.

In summary, we have reported a fast, facile and environmentally friendly method for synthesizing high performance supported Pd–Cu alloying nanoparticles on Al₂O₃ support. TOF of Pd–Cu/Al₂O₃–P for CO oxidation is about 3.0 times as that of Pd–Cu/Al₂O₃–C. This method can be regarded as a general approach for synthesizing other high performance supported bimetallic catalysts.

Acknowledgements

This work is supported by National Natural Science Foundation of China (Grant No. 21173028, 11505019), Dalian Jinzhou New District Science and Technology Plan Project (KJCX-ZTPY-2014-0001), the Science and Technology Research Project of Liaoning Provincial Education Department (Grant No. L2013464), and the Scientific Research Foundation for the Doctor of Liaoning Province (Grant No. 20131004).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13813b

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