Disclosure of the Surface Composition of TiO2-Supported Gold

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Disclose the surface composition of TiO2 supported gold-palladium bimetallic catalysts by high-sensitivity low energy ion scattering spectroscopy Yangyang Li, Jun Hu, Dongdong Ma, Yanping Zheng, Mingshu Chen, and Huilin Wan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03839 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Disclose the surface composition of TiO2 supported gold-palladium bimetallic catalysts by high-sensitivity low energy ion scattering spectroscopy Yangyang Li, Jun Hu, Dongdong Ma, Yanping Zheng, Mingshu Chen* and Huilin Wan State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P.R. China ABSTRACT: It is well known that there is a critical relationship between the surface composition and activity for a bimetallic catalyst. However, the surface is normally reconstructed under different conditions, which makes the surface more complicated. In this work, TiO2 supported colloidal PdxAuy-NPs with different atomic ratios were prepared, and treated under oxidation, reduction, and actual catalytic reaction conditions. The surface composition and structure were measured using X-ray photoelectron spectroscopy (XPS) and high-sensitivity low energy ion scattering spectroscopy (HS-LEIS). Phase diagrams of the surface compositions under various conditions as a function of the bulk composition were established. Oxidation induces de-alloying and enrichment of PdO on the surface, while H2 reduction results in realloying with just slight surface enrichment of Pd. Gold can help Pd against oxidation, and enhances its activity for CO oxidation at low temperature.

KEYWORDS: Gold-Palladium bimetallic catalyst, ex-situ XPS, ex-situ HS-LEIS, surface composition, phase diagram, CO oxidation

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INTRODUCTION

Gold-palladium bimetallic catalysts have been found to exhibit better selectivity and activity for many reactions comparing to the supported monometallic catalysts.1-15 Since Au and Pd are completely miscible, Pd-Au catalysts with fine-tuned different ratios have been widely prepared and used for various reactions. Most of those works have correlated the catalytic performance with the bulk composition.16-19 However, a heterogeneous catalyst reaction always takes place on the surface. Hence, it is crucial to measure the surface composition of an alloy catalyst and correlate with its catalytic performance, and if possible, to measure under the reaction condition regarding the surface restructuring. Such restructuring has been evidenced by XPS, STM, TEM, DRIFTS, etc.6,8,10,20-25 Based on the well-defined model surfaces, a phase diagram of the surface composition as a function of the bulk composition for the Pd-Au system has been established using a conventional low energy ion scattering spectroscopy (LEIS). 9,22 LEIS is one of the most powerful techniques to measure the topmost surface composition.26-28 It was found that Au enriches on the surface after annealing at high temperature in ultra-high vacuum.9,22 The surface enrichment of Au has also been observed by STM on PdAu(100) and (110) surfaces.23,24 However, there are “pressure gaps” and “material gaps” between the realistic supported system and the well-defined model system. Moreover, the conventional LEIS is limited to a smooth surface. Thus, little progress has been made about the top surface composition for a supported catalyst. Recently developed highsensitivity low energy ion scattering spectroscopy (HS-LEIS) (Qtac100) can achieve about 3000 times higher sensitivity than the conventional one, which offers possibility to measure the top surface composition for a supported catalyst (a short introduction of the HS-LEIS is shown in the


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Supporting Information).26,29-31 A homemade pretreatment reaction cell was developed and mounted onto the HS-LEIS facility to treat the sample under realistic catalytic reaction conditions and transfer into the analysis chamber under vacuum. In this paper, PdxAuy/TiO2 catalysts were synthesized using a colloidal preparation method, which show a narrow distribution of the particle size and uniform dispersion of Au and Pd characterized with TEM and elemental mapping. The surface compositions under various reaction conditions measured by XPS and HS-LEIS were compared and correlated with their catalytic performance for CO oxidation. It is found that HS-LEIS can provide more convincible information of the top surface compositions than XPS. Under both H2 and O2 conditions, Pd is rich on the surface. EXPERIMENTAL SECTION Materials and Reagents. The bimetallic PdxAuy samples were prepared by an alcohol-reduction method.12,16,32-34 HAuCl4·4H2O (Alfa) and PdCl2 (Alfa) were dissolved in water with a concentration of 1.0 × 10-3 M, and then added to the solution of ethanol/water (1/1, v/v) under vigorous stirring. Poly (N-vinyl-2-pyrrolidone) (PVP K-30 MW55000) with a mole ratio of 20 was added as a surfactant. The mixture was refluxed at 100 °C for 2 hours under the protection of N2. Supported bimetal samples were prepared by adding commercial TiO2 (P25) into the above mixtures with a total metal weight loading of 5 wt%, stirred for 12 hours, then filtered and washed by water and ethanol. The obtained samples were dried at 60 °C for 12 hours, and calcined at 500 °C in air for 2 hours. The bulk compositions of the samples were measured by inductively coupled plasma mass spectrometry (ICP-MS).


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Characterization techniques. HS-LEIS spectra were obtained using Ion-TOF Qtac100. In order to minimize the damage to the surface, helium was selected as the ion source with a kinetic energy of 3 keV, an ion flux of 6000 pA·m-2, and a spot size of 2 mm × 2 mm. More details about HS-LEIS can be found in the Supporting Information and Figure S1.26,29 XPS spectra were acquired at room temperature by an Omicron Sphera II Hemispherical energy analyzer with monochromated AlKα radiation (hv = 1486.6 eV) which was operated at 15 kV and 300 W. The pass energy is 35 eV, and the base pressure of the analysis chamber is 5.0 × 10-9 mbar. Samples were pressed into a pellet and fixed to a stainless steel sample holder, then transferred into the ex-situ chamber to treat under various conditions. XRD were carried out using a Rigaku Ultima IV X-ray diffractometer with a Cu Kα (35 kV, 15 mA) source. TEM was performed using a TECNAI F-20 transmission electron microscope. RESULTS AND DISCUSSION For the as-prepared 5 wt% Pd0.97Au/TiO2, TEM images (Figure 1a) show high dispersion of nanoparticles with a narrow particle size distribution and an average size of 2.1 ± 0.4 nm. After calcination at 500 °C in air, the average particle size increases to 6.2 ± 1.5 nm (Figure 1b). The elemental maps of Au and Pd (Figure 1c) show that the distributions of Pd and Au are mostly spatially coincident, demonstrating the homogeneous dispersion of Au and Pd or the formation of alloy particles.33,35,36 The elemental maps also show some highly dispersive Pd on the support surface. XRD patterns (Figure 1d) confirm the formation of a PdxAuy alloy for the 5 wt% Pd0.97Au/TiO2.37 A weak Pd(111) peak is observed for the Pd6.8Au/TiO2 and Pd14.5Au/TiO2 . And peaks corresponding to Au can be seen for the PdAu10/TiO2.


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Figure 1. TEM images of the 5 wt% Pd0.97Au/TiO2 (a) as-prepared and (b) after calcination at 500 °C in air; (c) HAADF-STEM image and STEM-EDX elemental maps for the 5 wt% Pd0.97Au/TiO2 after calcination; (d) XRD patterns (Au, PDF#04-0784; Pd, PDF#72-0710) of various Pd/Au ratio catalysts after the reduction at 500 °C in H2. To understand the nature of metal-support interactions, the oxidation and reduction of a 5 wt% Pd/TiO2 sample at various temperatures were conducted and measured by HS-LEIS and XPS. HS-LEIS spectra (Figure 2a) show that there are only O, Ti, and Pd three main peaks, and no obvious C peak. XPS Pd 3d5/2 peak can be deconvoluted into two peaks, 335.2 and 337.0 eV corresponding to Pd0 and Pd2+, respectively (Figure 2b).16,38 The results reveal that Pd is gradually oxidized as the oxidation temperature increases, and completely oxidized at about 350


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°C. By careful evaluation of the relative intensity of Pd to the surface Ti and O from both the HS-LEIS and XPS results (Figure 2c), it can be found that the surface concentration of Pd first increases and then decreases as the oxidation temperature increases from 100 to 500 °C, with a maximum at 350 °C. The changes of the surface concentration of Pd were further measured with a fresh sample by several oxidation (350 °C)–reduction (300 °C) cycles using HS-LEIS, as shown in Figure 2d. Note that evidenced by XPS, PdO is easily reduced at about 100 °C under H2 or CO, but at 225 °C under high vacuum treatment (Figure S2). It is obvious that about 2.5 times higher surface concentration of Pd is achieved under the oxidation condition than the reduction condition. This demonstrates that oxidized palladium can better disperse on the TiO2 support surface than the metallic one below 350 °C, leading to an obvious increase of the overall interface contact area. Although it had been evidenced the enlargement of the volume for PdO compared with that for the metallic Pd,39 the surface Pd intensity is found to decrease significantly upon the oxidation of a bulk Pd surface as measured by HS-LEIS, since oxygen atoms also locate on the surface. Hence, the significant increase of the Pd concentration for the 5 wt% Pd/TiO2 after oxidation concludes the much better dispersion of PdO on TiO2.


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Figure 2. HS-LEIS spectra (a) and XPS Pd 3d spectra (b) for the oxidation of the 5 wt% Pd/TiO2 at various temperatures. (c) Surface concentration of Pd measured by HS-LEIS and XPS as a function of the oxidation temperature. (d) Surface concentration of Pd measured by HS-LEIS with cycle oxidation at 350 °C and reduction at 300 °C. For the 5 wt% Pd0.97Au/TiO2, the binding energy of Pd 3d5/2 after reduction at 500 °C in H2 is 334.7 eV, which is slightly lower than that for the 5 wt% Pd/TiO2 (Figure 3a). A negative shift of Au 4f7/2 was also observed (Figure 3b) as compared with the 5 wt% Au/TiO2.37 Such shift may result from the size effect and the electronic effect of the alloy.40-42 Upon oxidation, Pd2+ species appears at 336.6 eV and co-exists with Pd0. Even oxidized at 500 °C, there still exists about 40% metallic Pd. Compared with complete oxidation at 350 °C for the 5 wt% Pd/TiO2, the alloying effect resists the oxidation of Pd. At higher oxidation temperature, the binding energy


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of Au 4f7/2 shifts to a higher value by 0.3 eV, which may result from the de-alloying effect owing to the oxidation of Pd(Au) to PdO(Au). HS-LEIS spectra were also collected for the 5 wt% Pd0.97Au/TiO2, and the peaks of O, Ti, Pd, Au were all evidenced (Figure 3c). The atomic ratios of Pd/Au were evaluated from both the HS-LEIS and XPS data (Figure 3d). It is interesting to find that the Pd/Au ratios evaluated from the XPS data are very close to the bulk values measured by ICP, which is consistent with literature reported results.22,43 But the ones obtained from HS-LEIS are obviously higher than the bulk values. The maximum surface ratio of Pd/Au obtained after oxidizing at 450 °C by HSLEIS is almost ten times higher than the bulk value, which suggests the formation of a PdO shell on the alloy particle surfaces. Such enrichment of Pd on the surface is also confirmed by the depth-profile measurement (Figure S3). Note that the related sensitivity of Pd/Au was estimated to be 0.86 from measuring the high-purity polycrystalline Pd and Au foils, respectively. Such comparisons also demonstrate that HS-LEIS can provide more reliable information of the top surface composition for a supported nanocatalyst than does XPS. Although by tuning the photon energy in a synchrotron-based XPS, the surface sensitivity is still limited to 2-3 atomic layers of the surface.44,45


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Figure 3. XPS spectra of Pd 3d (a), Au 4f (b), and HS-LEIS spectra (c) for the 5 wt% Pd0.97Au/TiO2 reduced at 500 °C and then oxidized at various temperatures. (d) Atomic ratio of Pd/Au evaluated from the XPS and HS-LEIS peak area. The dash line indicates the bulk ratio measured by ICP. The PdxAuy bimetal catalysts with different bulk compositions were measured by the HSLEIS to evaluate a quantity correlation between the surface composition and bulk composition. Unsupported PdxAuy alloys had also been prepared using the precursor mixtures calcined at 500 °C for comparison. XPS Pd 3d spectra of the unsupported PdxAuy alloys (Figure S4) indicate that Pd is almost completely oxidized to Pd2+ except for the PdAu5 with a small amount of metallic


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Pd. Such results also confirm the resistance to be oxidized for Pd with alloying of Au. The phase diagrams of the surface Pd as a function of the bulk compositions obtained from the HS-LEIS are plotted in Figure 4. It had been reported that Au prefers to enrich on the surface as annealing in vacuum for the PdxAuy bimetal films on Mo(110), as seen from the green line in Figure 4.9 On the planar oxide model surfaces, Au was also found to be rich on the surface of PdAu nanoparticles.46 But it is slightly Pd-rich on the surface after the reduction in H2, and significantly Pd-rich on the surface after the oxidation in air. Such effects of the atmosphere on the surface composition were also found for the PdAu/SiO2 by alkali acetate.47 And the TiO2 support has less effect on such enrichments. This is different from that of the PtCux/TiO2, where TiO2 enhances the dispersion of CuOx on the support surface resulting in a significant increase of the Cu/Pt ratio compared to the unsupported ones.26

Figure 4. Phase diagrams of surface atomic percentage of Pd/(Pd+Au) (HS-LEIS) as a function of the bulk value for both supported and unsupported PdxAuy catalysts after the oxidation at 450 °C and reduction at 500 °C. The black asterisk shows for the 5 wt% Pd0.97Au/TiO2 under CO/O2/He (1:1:98) at 200 °C. The data obtained from the model surfaces under UHV annealing (ref. 9) are re-generated and plotted for comparison.


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For the 5 wt% Pd0.97Au/TiO2, the activities of the catalytic oxidation of CO are affected significantly by the different pre-treatment conditions (Figure 5a). Among which the pre-reduced catalysts show much higher activity than the pre-oxidized ones, consistent with our previous observation.48-50 And the higher reduction temperature achieves higher activity. During the temperature-programmed reduction by H2, the top surface atomic ratio of Pd/Au shows a gradual decreasing tendency, just being opposite to the oxidation condition, which may correspond to a re-alloying process (Figure S5). These results suggest the promotion of alloying. It is worth to emphasize that the Pd0.97Au/TiO2, after 500 °C H2 reduction and then under CO/O2/He (1:1:98) at 200 °C for 1 hour, results in a similar surface Pd composition as that only under H2 reduction (black asterisk point in Figure 4). The rich of Pd under CO oxidation condition had also been observed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT).8,21 No obvious difference is found for the pre-oxidized catalysts (Figure S6). The different Pd/Au bulk ratios also affect significantly on the activity for CO oxidation (Figure 5b), consistent with previous report.51 The CO turnover frequency (TOF) at 90 °C is plotted as a function of the surface Pd composition estimated from the HS-LEIS in Figure 5c. The TOF per surface Pd based is comparable for the surface Pd composition below 65%, and decreases sharply with further increase of the surface Pd composition. Such sudden change at about 2/3 monolayer of Pd, and regarding a face-centered cubic structure and the most stable (111) plane, suggests that the ensemble effect may play a key role to determine the catalytic performance. Owing to the difficulty of the characterization techniques, the catalytic performances are mostly correlated with their bulk compositions of multicomponent catalysts, although a heterogeneous catalytic reaction always takes place on the outer surface. The present results demonstrate that the


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outermost surface composition of a supported bi-component catalyst can be obtained using HSLEIS and correlate with its catalytic performance.

Figure 5. (a) CO conversion as a function of the reaction temperature for the 5 wt% Pd0.97Au/TiO2 catalyst after pre-oxidized at 500 °C and reduced by H2 at various temperatures. (b) CO conversion as a function of the reaction temperature for the pre-reduced 5 wt% PdxAuy/TiO2 catalysts. (c) CO TOF as a function of the surface atomic percentage of Pd/(Au+Pd) obtained from the HS-LEIS measurements for the pre-reduced 5 wt% PdxAuy/TiO2 catalysts. 60 mg catalyst, CO:O2:He of 1:1:98 with a flow rate of 30 ml/min, reaction temperature of 90 °C. CONCLUSIONS 5 wt% PdxAuy/TiO2 catalysts with different Pd/Au ratios have been successfully prepared by the colloidal method. Using the surface sensitive technique, HS-LEIS, significant differences between the surface and bulk compositions were evidenced, and phase diagrams were established. The oxidation results in the formation of PdO, which prefers to enrich on the surface of the nanoparticles and may disperse on the support surface. This tendency is opposite to that observed under the vacuum treatment, where Au enriches on the surface. The reduction in H2 leads to re-alloying of Pd and Au, but still slightly rich of Pd on the surface. Au significantly promotes the activity for CO oxidation on the Pd-based catalyst. A comparable higher rate is


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observed for the surface Pd composition below 2/3, which demonstrates that the promotion effect of Au on Pd for CO oxidation may mainly be the ensemble effect.

AUTHOR INFORMATION Corresponding Author ∗

E-mail: [email protected]

ASSOCIATED CONTENT Supporting Information The Supporting Information includes more information about HS-LEIS and supplementary experimental data, which is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT We gratefully acknowledge the support received for this work from the National Basic Research Program of China (973 program: 2013CB933102) and the National Natural Science Foundation of China (21273178, 21573180, 91545204). REFERENCES 1.

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Disclosure of the Surface Composition of TiO2-Supported Gold

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