PtâRu Bimetal Alloy Loaded TiO2 Photocatalyst and Its Enhanced

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Pt-Ru Bimetal Alloy Loaded TiO Photocatalyst and Its Enhanced Photocatalytic Performance for CO Oxidation Tingting Zhang, Shuting Wang, and Feng Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12251 • Publication Date (Web): 20 Apr 2016 Downloaded from http://pubs.acs.org on April 27, 2016

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The Journal of Physical Chemistry

Pt-Ru Bimetal Alloy Loaded TiO2 Photocatalyst and Its Enhanced Photocatalytic Performance for CO Oxidation Tingting Zhang, Shuting Wang and Feng Chen* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China * Feng Chen: Tel: +86-21-64253056, E-mail: [email protected]

1. Tingting Zhang, [email protected], Institute of Fine Chemicals, East China University of Science and Technology. 2. Shuting Wang, [email protected] Institute of Fine Chemicals, East China University of Science and Technology. 3. Feng Chen, [email protected], Institute of Fine Chemicals, East China University of Science and Technology.

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Pt-Ru Bimetal Alloy Loaded TiO2 Photocatalyst and Its Enhanced Photocatalytic Performance for CO Oxidation Tingting Zhang, Shuting Wang and Feng Chen* Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China * Feng Chen: Tel: +86-21-64253056, E-mail: [email protected]


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Abstract Pt-Ru alloy loaded TiO2 (RP-AH) photocatalyst with superior photocatalytic performances was prepared in this work. Firstly, RuO2/TiO2/Pt ternary photocatalyst (RP) is calcined in air to epitaxially spread RuO2 nanoclusters on the surface of TiO2 (RP-A), which insures the close contact of RuO2 with the Pt nanoparticles nearby. Then, RuO2 is reduced into metallic Ru to form Pt-Ru alloy via calcining in H2 atmosphere. XRD, TEM, XPS and CV are applied to verify the formation of Pt-Ru alloy nanoparticles, while the photocurrent and Mott-Schottky plots suggest that RP-AH is a p-type semiconductor. The RP-AH catalyst shows a superior photocatalytic activity to those of RP, RP-A and RP-H (Ru/TiO2/Pt) in CO oxidation under UV irradiation. CO of a concentration of 1000 ppm was completely photocatalytically oxidized into CO2 with RP-AH in 120 min. Either Pt-Ru alloy or p-type semiconductor property of RP-AH plays an effective role in improving the photocatalytic performance of RP-AH. Academically, Pt-Ru alloy clusters favor the adsorption of CO and O2 as well as promote the separation of the photogenerated charges in RP-AH; meanwhile, the high hole mobility due to the p-type semiconductor property of RP-AH also benefits the reactivity of holes in oxidizing CO into CO2 with O−(a).



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Introduction Carbon monoxide (CO) is toxic to humans and animals for its high affinity with the hemoglobin.1 As CO would disable the blood’s ability for oxygen delivery, CO exposure should not exceed 25 ppm in 8 h or 50 ppm in 4 h.2 Massive amount of carbon monoxide are emitted into the atmosphere year by year, mainly from transportation, power plants, and industrial and domestic activities. Besides, CO is not very soluble in water (23 mL CO (g) / L H2O (l) or 0.026 g CO (g) / L H2O (l)), which limits its elimination from air by aqueous treatments. Therefore, it is of great importance to transform CO into harmless CO2 by gaseous treatments, among which catalytic oxidation of CO is suggested as a promising way to eliminate CO in air. TiO2 photocatalysis is a comparatively promising candidate for the gaseous oxidation of CO.3 Nevertheless, the relatively low photocatalytic activity of the bare TiO2 restricts its utilization in the practical applications. Accordingly, enormous efforts have been dedicated to the modification of the TiO2 to enhance the photocatalytic oxidation rate and efficiency, among which noble metal (Pt, Au, Ag, Pd, etc)–loaded catalysts have been frequently used.3-5 Noble metal deposition can inhibit the recombination of photo-generated electrons and holes on the surface of the grains and thus realize the separation of oxidation and reduction reaction.6, 7 As we all know, Pt–loaded catalysts have played very important role in photocatatytic CO oxidation. For example, Jiao et al.8 deposited Pt nanoclusters onto an epitaxial RuO2 spread TiO2 photocatalyst. Epitaxial RuO2 captures the photogenerated holes, while Pt nanoclusters traps the photogenerated electrons, and


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therefore the photocatalytic oxidation of CO was dramatically promoted. Meanwhile, Hwang et al.9 discovered that stabilized active oxygen species generated on Pt/TiO2 surface play an extremely vital role in enhancing the CO photooxidation rate. However, to be economically feasible, as well as environmentally friendly, it is of importance to reduce the dosage of Pt used in the practical applications of the Pt-containing catalysts. The PtM (M denotes the transition metal) bimetallic nanostructures are recently suggested a desirable way to reduce the consumption of expensive Pt metal. A proper PtM structure can well maintain the inherent properties of the Pt component, or even show a superior performance than the monometallic Pt.10-12 For instance, the PtRu bimetallic system has been applied in fuel cell electrocatalysis13, 14, water–gas shift reaction (WGSR)15, 16, CO oxidation17, 18 and so on. Lin et al.17 conducted a comparative study of CO electro-oxidation on different catalysts using in situ FTIR spectroscopy. Polycrystalline Pt, Ru and PtRu (50: 50) alloy were used as electrode material, among which PtRu alloy presents the highest reactivity toward the COad oxidation due to a loose ad-layer structure at the alloy17. Alayoglu et al.18 utilized PtRu nano-alloy to remove CO from hydrogen through selective oxidation and its catalytic property is better than monometallic mixtures of nanoparticles and pure Pt particles. In principle, the formation of Pt–Ru bonds in bimetallic nanostructure could modify the chemical properties of the bonded metals through electronic or ligand effects19. In conclusion, substituting a single noble metal with bimetal, a valid approach to develop high effective carbon monoxide oxidation catalysts, should deserve more research. Very recently, some works have revealed that



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the inherent conductivity of TiO2 nanomaterials changes from n-type to p-type after annealing treatment.20, 21 Due to advantages of the correct band alignment, high hole mobility22, 23 and stability in corrosive electrolyte,22 the p-type semiconductors can been applied in environmental modification20, 24, energy recovery20 and photoelectric conversion22, 25, 26. Generally, highly mobile holes are thought to be beneficial for the oxidation reaction27-29, including CO oxidation8, 24; therefore, a p-type TiO2 modified with PtRu alloy is prepared on purpose in this work for optimizing its photocatalytic application in oxidizing the carbon monoxide. A surface modification method was adopted in this work to synthesize RuO2/TiO2/Pt ternary photocatalysts with RuO2 nanoclusters of ca. ~ 2 nm size. Then, a RuO2 epitaxial layer was obtained by calcining RuO2/TiO2/Pt at 400 oC in air8, 30 to improve the possibility of the direct interaction between RuO2 and Pt nanoparticles. RuO2 can be reduced to the Ru metal by either chemically exposing to the reducing agents (such as CO, H2, and methanol) at temperature above 420 K, or being heated in a vacuum to 850-1000 K31. Therefore, RuO2-containing samples were further treated under H2 atmosphere at 400 oC to obtain metallic ruthenium. During the period of reduction, the interaction between Pt and RuO2 converts to the one between Pt and Ru; consequently, Pt-Ru bimetallic system is obtained. The formation of Pt-Ru bimetal, as well as its role on photocatalytic oxidation of CO is thus investigated hereinafter.

Experimental Section Materials synthesis Synthesis of RuO2 loaded TiO2 (RuO2/ TiO2)


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TiO2 (P25, Evonik Degussa) was from commercial sources. A hydrothermal method was used to prepare RuO2/TiO2 catalysts just as previously reported8, 30. 1.0 mL of RuCl3 aqueous solution (0.4 g/L) was mixed with 1.0 g of TiO2 using 74 mL of water as the solvent. Then the mixture was transferred into a 100 mL autoclave and heated at 180 oC for 10 h. The as-formed powders were separated by centrifugation, washed several times with deionized water, and dried at 60 oC under vacuum overnight to obtain the RuO2/TiO2 catalyst (nominal RuO2 content: 0.02 wt%) which was denoted as R hereinafter (R means the binary catalysts merely contain RuO2). Synthesis of RuO2/TiO2/Pt RuO2/TiO2/Pt photocatalysts were prepared by a photo-reduction method8. 0.5 g of RuO2/TiO2 nanocomposite was mixed with 2.65 mL of H2PtCl6 solution (1.0 g/L) followed by the addition of 100 mL of methanol/H2O solution (25% v/v methanol/H2O). The suspension was magnetically stirred for 3 h under UV (300 W high pressure Hg lamp, Shanghai Yaming Light) illumination. After irradiation, the samples were centrifuged, washed and then dried at 60 oC under vacuum overnight to obtain RuO2/TiO2/Pt (nominal Pt content: 0.2 wt%) which was denoted as RP (RP means the ternary catalysts contain both Pt and RuO2). Synthesis of Pt/TiO2 Pt/TiO2 photocatalysts were also prepared by a photo-reduction method (nominal Pt content: 0.2 wt%). The obtained Pt/TiO2 was denoted as P (P means the binary catalysts merely contain Pt). Synthesis of Pt-Ru alloy/TiO2



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The RuO2/TiO2/Pt composites (RP) were further calcined at 400 oC for 10 h in air, and followed by pretreatment under H2 atmosphere at 400 oC for 3 h, during which some Pt-Ru alloy nanoparticles were formed on the surface of TiO2. The ternary sample calcined in air was denoted as RP-A (A means the air calcined ternary catalysts), and the sample further treated under H2 atmosphere was denoted as RP-AH (AH means the ternary catalysts have been calcined in air followed by in flowing hydrogen). The RP composites were also directly treated at 400 oC for 3h under H2 atmosphere, which was denoted as RP-H (H means the ternary catalysts was calcined in H2). For comparison, the binary catalysts, both RuO2/TiO2 (R) and Pt/TiO2 (P) composites, were also calcined at 400 oC for 10 h in air, and followed by pretreatment under H2 atmosphere at 400 oC for 3 h. Accordingly, the RuO2/TiO2 sample calcined in air was denoted as R-A, and the sample further treated under H2 atmosphere was denoted as R-AH. The Pt/TiO2 sample calcined in air was denoted as P-A, and the sample further treated under H2 atmosphere was denoted as P-AH. Table 1. The nomenclature of catalysts. Catalysts

RuO2/TiO2/Pt

RuO2/TiO2

Pt/TiO2

As-prepared

RP

R

P

Air calcined

RP-A

R-A

P-A

Air &H2 calcined

RP-AH

R-AH

P-AH

H2 calcined

RP-H

/

Synthesis of ep-RuO2/TiO2/Pt


/

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RuO2/TiO2 composites were calcined at 400 oC for 10 h in air to obtain ep-RuO2/TiO2 (R-A). And ep-RuO2/TiO2/Pt (denoted as ep-RP) samples were prepared with the same procedure as RuO2/TiO2/Pt except that ep-RuO2/TiO2 was used instead of RuO2/TiO2. Since the Pt nanoclusters prefer to deposit on the reduction sites of TiO2 rather than on ep-RuO2 in this case, it implies that the possible contact between the Pt nanoparticles and the RuO2 would be mostly avoided in the as-prepared sample ep-RP. In this way, the effect of the interaction of Pt nanoparticles with RuO2 can be identified with a comparative study.

Materials characterization The crystalline phase of products was examined by powder X-ray diffraction (XRD). Diffraction patterns of the samples were performed using an X-ray diffractometer (Rigaku Ultima IV) operating in the reflection mode with CuKa radiation. UV-vis spectrophotometer (Shimadzu UV-2600) was used to analyze UV-vis diffuse reflectance spectra of samples to observe their optical properties. BET specific surface area was determined by nitrogen adsorption at 77.3 K (Micromeritics ASAP 2020). High resolution transmission electron microscopy (HRTEM) was investigated using a JEOL JEM 2100F instrument operated at 200 kV. To prepare the HRTEM specimens, the powder samples were first dispersed ultrasonically in absolute ethanol in a centrifuge tube for nearly 10 min. After standing for 10 min, one drop of the supernatant suspension was dropped onto a carbon film supported copper grid and allowed to dry in air before the specimen was transferred into the microscope. The instrument employed for XPS studies was a Thermo Fisher ESCALAB 250Xi system with Al Kα radiation (photon energy 1361 eV), and



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calibrated internally by C1s binding energy (BE) at 284.6 eV from the carbon contaminants. The electrochemical test was measured in a standard three-electrode system with a CIMPS photo-electrochemical workstation (Zahner Co, Germany), using the prepared sample as the working electrode, a Pt wire as the counter electrode and saturated calomel electrode (SCE) as a reference electrode. The transient photocurrent test was taken in 0.5 M Na2SO4 aqueous solution. A 35 W Metal halide lamp (GE Lighting Co., Ltd) was used as the light source. The working electrode was prepared by coating the sample powders on the FTO glass via copper foil tape with conductive adhesive (3M). During the cyclic voltammetry (CV) test, 0.3 M H2SO4 aqueous solutions were employed as the electrolyte. The Mott–Schottky measurement was carried out in 0.5 M Na2SO4 aqueous solution. A 10 mV sinusoidal excitation signal was employed to interrogate the capacitance. The single-frequency is at 1 kHz. Electrochemical Impedance Spectroscopy (EIS) measurement was performed under open-circuit voltage. A sinusoidal ac perturbation of 10 mV was applied to the electrode over a frequency range of 100 kHz-100 mHz. 2.5 mM K3Fe(CN)6/K4Fe(CN)6 (1:1) and 1.0 M KCl aqueous solution were used as the electrolyte. In the tests of CV, EIS and Mott–Schottky, the working electrode was prepared by coating the sample powders on the FTO directly.

Photocatalytic performance The photocatalytic oxidation processes of CO were carried out in a closed loop gas-flow system at ambient temperature and pressure. The photocatalytic reactor was made of a horizontal quartz tube with a flange connection. A quartz plate loaded the


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assigned catalyst (5.0 cm × 10.0 cm × 0.2 cm) was laid into the tube for photocatalytic use. The reactor was connected to a circulation pump through an external Teﬂon tubing (o.d. 0.32 cm) to push the gas in the system in dynamic loop. A metal halide lamp (35 W) was fixed 10 cm right above the photocatalytic reactor. The UV light intensity on the surface of the photocatalyst is measured as 3.8 mW/cm2. The CO concentration variation during the reaction was real-time determined by a gas chromatograph (GC2014, Shimadzu) using FID detector and 5A molecular sieve packed column. The closed system was prefilled by pure dry air (99.9995%) and then a desired amount of CO was injected into the system to reach a concentration of 1000 ppmv. When the CO molecules in the system reached equilibrium, the photocatalytic reaction was initiated by turning on the light.

Results and Discussion Characterization of the photocatalyst XRD patterns of TiO2, RP, RP-A and RP-AH

RP-AH

Intensity (a.u.)

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RP-A

RP

TiO2

20

30

40

50

60

70

80

2θ (o)

Fig. 1. XRD patterns of TiO2, RP, RP-A and RP-AH.



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Fig. 1 shows XRD patterns of TiO2 as well as RuO2/TiO2/Pt composites before and after calcination under different atmosphere. The XRD pattern of P25 TiO2 shows the typical signals of anatase (80%, PDF JPCDS. 89-4921) and rutile (20%, PDF JPCDS. 21-1276), which is accordant with results reported in the literatures3, 4, 8. No characteristic peaks for RuO2 and Pt can be found for the series of RuO2/TiO2/Pt composites, which may be attributed to the low amount of RuO2 and Pt. After the calcination at 400 oC in air or further under H2 atmosphere, the signals standing for TiO2 have no change, which indicates that the TiO2 base is stable during the calcination process. What’s more, due to the unobvious variations of SBET (shown in Table. S1), the changes in the photocatalytic activity of ternary photocatalysts should not be attributed to the SBET. Meanwhile, as all the photocatalysts have a similar photo-absorbance in the UV region (< 400 nm, Fig. S1), the possible effect from the light absorption is also denied in this work. HRTEM images of RP, RP-H, RP-A and RP-AH

Fig. 2. HRTEM images of (A) RP, (B, C) RP-H, (D) RP-A. (E, F) RP-AH. Fig. 2A shows the HRTEM image of RuO2/TiO2/Pt. The particles seem inhomogeneous due to the presence of Pt and RuO2 nanoparticles. The distribution


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state of small nanoclusters in the circled parts is the same as that in Fig. S2A, in both cases the small nanoclusters are identified as RuO2 nanoparticles. These nanoclusters disappear in the HRTEM image of RP-A (Fig. 2D), which is generally ascribed to the epitaxial grow of RuO2 nanoparticles along the surface of TiO2 during the calcination in air8. From the inset of Fig. 2A (RP) and Fig. 2B&C (RP-H), the interplanar space of 0.23 nm is a characteristic lattice fringe of (111) of Pt crystals8, which suggests that the Pt-Ru bimetallic nanoparticles haven’t formed by directly calcining RP under flowing H2 atmosphere. However, the marked d-spacing of 0.22 nm in RP-AH (Fig. 2E&F), corresponds to the prominent face-centered-cubic (f.c.c.) {111} lattice fringes of Pt-Ru alloy18. The enlarged HRTEM images of RP, RP-H and RP-AH are also displayed in Fig. S3 to clearly show their lattice fringe spacing. Hence, Pt-Ru bimetal alloy has formed by calcining the ternary composite (RP-A) in H2 atmosphere. Presumably, the RuO2 epitaxial layer facilitates the interaction between RuO2 and Pt. Transient Photocurrent response and Mott–Schottky plot of ep-RP, RP, RP-A, RP-AH and RP-H

Fig. 3. (A) Transient photocurrent response of ep-RP, RP, RP-A, RP-AH and RP-H at a bias voltage of 10 mV vs SCE under the metal halide lamp irradiation. (B) Mott–Schottky plot of ep-RP, RP, RP-A, RP-AH and RP-H measured at a frequency of 1k Hz.



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As shown in Fig. 3A, for the photocatalysts of RP, ep-RP and RP-H, the positive photocurrent response is observed when the light is turned on, indicating that these catalysts are typical n-type semiconductors. Nevertheless, after RP was calcined in air, the obtained RP-A gives a negative photocurrent response, which suggests RP-A as a p-type semiconductor25,

26, 32-34

. What’s more, after further calcined under H2

atmosphere, RP-AH also gives a negative photocurrent and shows up as a p-type semiconductor. In Fig. 3B, the negative slopes of Mott–Schottky plot indicates p-type behavior of RP-A and RP-AH, consistent with the photocurrent analysis (as shown in Fig. 3A).35 Meanwhile, the positive photocurrent response and the positive slopes of Mott–Schottky plot both imply that RP, RP-H and ep-RP are typical n-type semiconductors. Generally, under the UV irradiation, the excited electrons would transit from the valence band to the conductive band of TiO2, which influences the apparent Fermi level (EF*) of TiO2 and make it be an n-type semiconductor. However, when TiO2 particles contact with other semiconductor or metallic nanoclusters, an interfacial carrier transfer occurs, which induces a possible shift of the Fermi level. For instance, when Au nanoparticles’ sizes are 3, 5, and 8 nm, the apparent Fermi level of TiO2/Au system were measured as -290, -270 and -250 mV, respectively.

36

The shift of Fermi

level to the relative positive potential is the result of a different electron accumulation levels via the carriers transfer at the TiO2/Au interface. The as-prepared RuO2/TiO2/Pt (RP) composite (Fig. 3A) exhibits as an n-type semiconductor under UV irradiation. Nevertheless, after it is calcined in air, a close contact between the RuO2 epitaxial layer and Pt arises, which alters the work function of the Pt nanoclusters. The transient photocurrent response and Mott-Schottky plot of RP-A suggest that the


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newly formed contact between RuO2/Pt and TiO2 shifts the apparent Fermi level (EF*) toward positive side that is the valence band (VB) edge of TiO2, which alters the RP-A from n-type to p-type semiconductor.20 The XPS fine spectra of Pt 4f7/2 hereinafter also suggest that the contact between RuO2 and Pt induces a remarkable change in the chemical state of Pt clusters. After the further calcinations in flowing H2, RP-AH is still presented as a p-type semiconductor. It is worth to note that holes play a predominant role for the photocatalytic oxidation8,

24

. A p-type semiconductor with high hole-mobility22,

23

should show

advantage in photocatalytically oxidizing CO. Therefore, the photocurrent response and Mott-Schottky results imply that RP-A and RP-AH possibly possess more excellent performances than RP for photocatalytic oxidation. XPS fine spectra of Ru 3d5/2 region and Pt 4f7/2, 4f5/2 core level of RP, RP-A and RP-AH

C: 284.6 eV

RP-AH RP-A RP

Ru0 Ru0-Oad

surface Ru bulk Ru

280

B: Pt 4f

RP-AH RP-A RP

bulk Pt 2+ Pt0 Pt

Intensity (a.u.)

A:Ru/C

Intensity (a.u.)

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285

290

Binding Energy (eV)

70

72

74

76

78

Binding Energy (eV)

Fig. 4. XPS fine spectra of (A) Ru 3d5/2 region, and (B) Pt 4f core level of RP, RP-A and RP-AH.

To gain an in-depth understanding of the chemical properties of ternary catalysts, the chemical states of Ru and Pt element were carefully characterized with the XPS technique. Fig. 4A presents the XPS fine spectra of Ru 3d5/2 in various ternary



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catalysts. For sample RP, the emission at 280.2 eV is related to the surface Ru(IV) cations, while that at 281.1 eV is ascribed to bulk-Ru(IV) in RuO2.31 It has been reported that RuO2 nanoclusters can epitaxially grow along the surface of TiO2 after calcinations in air at 400 oC.8, 30 As for RP-A, the relative content of bulk-Ru(IV) (bulk-Ru/total Ru) is obviously reduced comparing to that in RP, which means that more surface Ru(IV) cations have generated, that’s, the RuO2 epitaxial layer has formed on the surface of TiO2. However, after further treatment in H2 at 400 oC for 3 h, the XPS signals of the Ru 3d5/2 shift to lower binding energies for RP-AH. According to the literature reports, the strong peak around 279.6 eV corresponds to lattice metallic Ru0 atoms, while another one at 280.6 eV belongs to Ru0-Oad (surface Ru0 atoms chemically adsorbed with oxygen).31, 37, 38 Thus it shows RuO2 has been reduced into metallic ruthenium atoms in RP-AH. Similarly, the nearly identical changes can be observed between binary (RuO2-TiO2) composite of R, R-A and R-AH (as shown in Fig. S4), which confirms that RuO2 has been reduced into metallic Ru0 under the H2 calcination. The XPS fine spectra of Ti 2p (Fig. S5) show that the binding energy of Ti 2p does not have any obvious change among all photocatalysts, which suggests that the Ti element is presented as Ti4+ throughout.

Pt 4f core level X-ray photoelectron spectra of RP, RP-A and RP-AH are shown in Fig. 4B. The Pt 4f7/2 signal at 71.8 eV corresponds to the metallic Pt0 interacting with TiO2, whereas the signal at 70.8 eV stands for the bulk Pt metal that does not directly interact with TiO2-base.39, 40 The Pt 4f7/2 peak at 73.5 eV is assigned to Pt2+.41, 42

As shown in Fig. 4B, the peaks on behalf of bulk Pt metal have shifted a little bit


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throughout the work. Table 2. The binding energies and relative contents of different Pt species calculated from the Pt 4f7/2 fine XPS spectra. sample

RP

RP-A

RP-AH

Pt species

BE of Pt4f7/2 (eV)

Relative contents (%)

Bulk Pt

70.6

51

Pt0

71.8

24

Pt2+

73.5

25

Bulk Pt

70.4

44

Pt0

71.7

30

Pt2+

73.4

26

Bulk Pt

70.8

54

Pt0

71.8

25

Pt2+

73.4

21

Table 2 summarizes the content variation of the Pt compotent in the various ternary composites. The relative content (Pt0/ΣPt) of the Pt0 component changes from 24% to 30% and the relative content (bulk Pt/ΣPt) of the bulk Pt component changes from 51% to 44% after air calcinations, which means the bulk Pt closely interacts with the RuO2 epitaxial layer. While further calcining in flowing H2, the amount of Pt2+ theoretically drops, which agrees with the observed relative content of 21% for RP-AH. Comparing RP with RP-AH, we can find that the Pt 4f7/2 signal of bulk Pt



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shifts from 70.6 eV to 70.8 eV, while the main Ru 3d5/2 signals of Ru atoms shifts from 280.2 eV to 279.6 eV. This indicates that there would be chemical environment changes for the corresponding ruthenium and platinum atoms; consequently Pt-Ru bimetal alloy has formed eventually.18, 43 Additionally, the binding energies of Ru 3d5/2 and Pt 4f7/2 (Fig. S6, Table S2) do not show any shift between RP and RP-H, which attests the PtRu alloy is absent in RP-H. Cyclic voltammetry curves of pure Pt, Pt-Ru alloy, RP-A and RP-AH

Fig. 5. CV curves of (a) pure Pt; (b) Pt-Ru alloy; (c) RP-A; (d) RP-AH in 0.3 M H2SO4 (aq) at room temperature with a scan rate of 20 mV/s between -0.1 and +1.5 V. (*The experimental details for the preparation of pure Pt and Pt-Ru alloy nanoparticles are described in support information Fig. S7) CV curves of ternary composites were also measured to observe the redox potential changes (Fig. 5), with which the redox properties of Pt clusters can be analyzed. For comparison, the CV curves of pure Pt clusters and Pt-Ru alloy nanoclusters were also measured as shown in Fig. 5. Pure Pt (Fig. 5a) exhibits two reduction peaks at nearby 0.2 V and 1.0 V. When Pt and Ru interact to give Pt-Ru alloy (Fig. 5b), the corresponding reduction peak at 0.2 V shifts to 0.3 V, while the


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peak at 1.0 V becomes unobvious. A very similar phenomenon can be observed between CV curves of RP-A (Fig. 5c) and RP-AH (Fig. 5d). After the formation of Pt-Ru alloy, the peak appears at 0.13 V in Fig. 5c shifts to 0.28 V in Fig. 5d. In a word, the Pt component in RP-AH exhibits an identical property as that in Pt-Ru alloy with the CV observation. Therefore, it suggests that Pt-Ru bimetal alloy has formed in RP-AH. On the other hand, the CV curve of RP-H (Fig. S8) doesn’t show any shift for the redox peaks comparing to those of metallic Pt, which suggests that there is Pt but not PtRu alloy in the RP-H. Photocatalytic performance of the photocatalysts

A [CO]/[CO]0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

100

100

80

80

60

60

40

40 TiO2 RP ep-RP RP-A RP-H RP-AH

20 0 0

30

20 0 60

90

120

Time (min)

150

0

240

480

Time (min)

Fig. 6. (A) Photocatalytic oxidation of CO with TiO2, RP, ep-RP, RP-A, RP-H and RP-AH samples. (B) The reactivity recycling test of RP-AH.

In order to explore the influence of various loaded noble metal components on the catalytic activity of ternary photocatalysts, a comparison of their catalytic activity is carried out in this work. The oxidations of CO with RP, RP-A, RP-AH and RP-H have been tested in dark (Fig. S9), which exhibits very little catalytic activity in all cases. Fig. 6A shows photocatalytic oxidation of CO with TiO2, ep-RP, RP, RP-A, RP-AH and RP-H samples. Pure TiO2 exhibits a very limited photocatalytic activity



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while RP-AH reveals the highest activity for the CO oxidation. The conversion of CO reaches 100% in just 120 min with RP-AH, much higher than those of other catalysts. A recycling test of RP-AH (Fig. 6B) for the photocatalytic oxidation of CO shows that the reactivity of RP-AH is very stable after 4 cycles, which verifies the chemical stability of RP-AH for photocatalytic use. The sample of RP-A shows a higher photocatalytic activity than ep-RP. This may be attributed to the contact between RuO2 and Pt after RuO2 nanoparticles epitaxially growing on the surface of TiO2. Besides, the intrinsic electric property of RP-A, p-type semi-conductor, would be also beneficial for its oxidative ability towards CO24. For ep-RP, the epitaxial growth of RuO2 has achieved on the surface of TiO2 before Pt loading; As RuO2 sites are not preferred for photoelectron reduction, the direct connection of photo-deposited Pt clusters with RuO2 seems less possible. However, for RP-A, the epitaxial growth of RuO2 occurs after the Pt loading; therefore, the epitaxial layer may contact with Pt nanoclusters during its 2-D spread. As a result, the conductivity of the composite (RP-A) has been altered from n-type to p-type, which improves its photocatalytic activity of CO oxidation. Comparing with RP-H and RP-AH, the photocatalytic activity of RP-AH is much better than RP-H, which is due to not only p-type semiconductor with some level of oxidative ability, but also Pt-Ru alloy on the sample of RP-AH. The samples of RP-H and RP-AH are both obtained after the calcinations under flowing H2 atmosphere, but the Pt-Ru bimetallic system does not appear on RP-H. As mentioned hereinbefore, a thermal pre-treatment in air is essential to the formation of the Pt-Ru bimetallic alloy.


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100 R-AH

80

[CO]/[CO]0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 P-AH 20 RP-AH

0 0

30

60

90

120

150

Time (min)

Fig. 7. Photocatalytic oxidation of CO with binary catalysts (R-AH and P-AH) and ternary catalyst (RP-AH) samples.

To probe the difference between binary and ternary catalysts which have been calcined both in air and in hydrogen, we have conducted the comparison of their catalytic ability for CO oxidation. As we can see from Fig. 7, R-AH and P-AH convert about 11.1 % and 89.0 % of carbon oxidation in 150 min, respectively. While the conversion of CO of RP-AH reaches 100 % within just 120 min. The significant enhancement in photocatalytic activity was observed here, which illustrates that the Pt-Ru alloy formed in RP-AH catalyst after the secondary calcination plays a significant role in promoting the photocatalytic performance of the catalyst.

Proposed Mechanism of the reaction activity enhancement



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Fig. 8. The schematic preparation process of RP-AH and RP-H. The schematic preparation process of the RP-AH and RP-H photocatalysts is presented in detail in Fig. 8. When RP ternary photocatalyst is calcined in air, RuO2 nanoclusters transform into epitaxial layers on the surface of TiO2; thus, RuO2 epitaxial layers contact with Pt nanoparticles nearby. The as-obtained catalyst of RP-A is a p-type semiconductor with good hole mobility. After the further calcinations under flowing H2 atmosphere, the reduced Ru species from RuO2 prefers to adhere to the contacted Pt nanoparticles to form Pt-Ru alloy nanoparticles. And the eventually prepared sample (RP-AH) still behaves as a p-type semiconductor. However, as shown in Fig. 8, if the photoreduction deposition of Pt nanoparticles is performed after the formation of RuO2 epitaxial layers, the contact between them wouldn’t occur. Therefore, when the catalyst of ep-RP is further reduced in the flowing hydrogen atmosphere, the interaction between the newly-formed Ru clusters and Pt nanoparticles would not be expected. In a word, the Pt-Ru alloy can be prepared in RP-AH due to the close contact between RuO2 and Pt before the reduction of RuO2 to metallic Ru. Similarly, directly reducing RP in the H2 atmosphere also produces RP-H but not the Pt-Ru alloy loaded TiO2, RP-AH.


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Generally speaking, CO photocataytic oxidation experiences the following mechanism44, 45. O2 → O2 (a)

(1)

CO → CO (a)

(2)

TiO2 + hν → TiO2 (e− + h+)

(3)

O2 (a) + e− → O2− (a) → O (a) + O− (a)

(4)

O− (a) + CO → CO2−

(5)

CO2− + h+ → CO2 (g)

(6)

O− (a) + h+ → O (a)

(7)

CO (a) + O (a) → CO2 (a)

(8)

CO2 (a) → CO2 (g)

(9)

Eqs. 1 and 2 express the chemisorptions of O2 and CO. Previous observation shows that bimetallic alloy owns better adsorption property of CO and O2 than the monometallic nanoparticles43,

46, 47

. Meanwhile, since CO chemisorbs to the metal

oxides with its carbon lone pair electrons, a relatively electrophilic environment of p-type semiconducting oxide would favor the adsorption of CO24,

48

. Hence, the

p-type semiconductor of RP-AH with Pt-Ru alloy nanoparticles plays a positive role in the adsorption of CO and O2 in this work. Under the illumination, an electron transit occurs (Eq. 3). An effective electron transfer between the TiO2 and the loaded metal clusters would benefit the photoefficiency by promoting the charge separation, of which the bimetallic alloy shows advantage to that of monometal as reported (Fig. S10).12 At the surface of metal clusters, the excited electrons reacted with the chemisorbed O2 (a) to produce reactive oxygen species (O

(a)


and O−

(a),

Eq. 4). Then


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

O−

(a)

reacts with CO and h+ successively to form CO2 (Eqs. 5, 6), or CO2

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(a)

is

produced by a chemical reaction between CO (a) and O (a) (Eq. 8). As for the O− (a), it can transform into O (a) by reacting with hole (Eq. 7). Briefly, photoexcited electrons benefit the dissociation of O2 (a) at the surface of metal clusters, while photogenerated holes play a crucial role in the unit reactions for oxidizing CO into CO2 with O−

(a).

The enhanced adsorbability to the substrates (CO and O2), as well as the high hole mobility due to its p-type property, makes RP-AH superior for the photocatalytic oxidation of CO.

Conclusions Pt-Ru alloy nanoparticles loaded TiO2 (RP-AH) was prepared by calcining RuO2/TiO2/Pt ternary composite in air as well as in H2 atmosphere. Transient photocurrent response and Mott-Schottky plots show that RP-AH is a p-type semiconductor. Either Pt-Ru alloy or p-type semiconductor property of RP-AH plays an effective role in photocatalytic oxidation of CO. A remarkable reactivity improvement was observed for RP-AH in photocatalytic oxidation of CO. 1000 ppm of CO was completely photocatalytically oxidized into CO2 with RP-AH in 120 min, while RP and RP-A only gave CO concentration decreases of 13% and 65.5%, respectively. Academically, photoexcited electrons benefit the dissociation of O2(a) at the surface of Pt-Ru alloy clusters; meanwhile photogenerated holes play a crucial role in oxidizing CO into CO2 with O−(a).

Acknowledgement


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This work was supported by the National Nature Science Foundations of China (21177039) and the Innovation Program of Shanghai Municipal Education Commission (13ZZ042).

Supporting Information BET specific surface areas of TiO2 and the ternary photocatalysts; UV-vis DRS spectra of TiO2, RP, RP-A and RP-AH; HRTEM images of R, R-A and R-AH; The enlarged HRTEM images of RP, RP-H and RP-AH; The Ru 3d5/2 XPS fine spectra of R, R-A and R-AH; The Ti 2p XPS fine spectra of RP, RP-A, RP-AH and RP-H; XPS fine spectra of Ru 3d5/2 region and Pt 4f core level of RP-H; XRD patterns of Pt and Pt-Ru alloy nanoparticles and HRTEM image of Pt-Ru alloy nanoparticles; CV curve of RP-H; Control reactions: the concentration variations of CO with various photocatalysts in dark; EIS spectra of RP, RP-A, RP-H and RP-AH.

Notes The authors declare no competing financial interest.

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PtâRu Bimetal Alloy Loaded TiO2 Photocatalyst and Its Enhanced

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