The nature of loading-dependent reaction barrier over mixed RuO2

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The nature of loading-dependent reaction barrier over mixed RuO2/TiO2 catalysts Hao Li, Shenjun Zha, Zhi-Jian Zhao, Hao Tian, Sai Chen, Zhongmiao Gong, Weiting Cai, Yanan Wang, Yi Cui, Liang Zeng, Rentao Mu, and Jinlong Gong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00797 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Catalysis

The nature of loading-dependent reaction barrier over mixed RuO2/TiO2 catalysts Hao Li,1,2 Shenjun Zha,1,2 Zhi-Jian Zhao,1,2 Hao Tian,1,2 Sai Chen,1,2 Zhongmiao Gong,3 Weiting Cai,1,2 Yanan Wang,1,2 Yi Cui,3 Liang Zeng,1,2 Rentao Mu,1,2* and Jinlong Gong1,2* 1

Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; 2

Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China;

3

Vacuum Interconnected Nanotech Workstation, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China.

ABSTRACT: The mixed-metal oxides are one of the most frequently used catalysts in chemical industry, because the superior catalytic reactivity can be achieved by taking advantage of the synergetic effects of their parent oxides. However, the interfacial electronic interactions between metal oxides remains unclear due to their structural complexity. This paper describes the modulation of catalytic performance of mixed RuO2/TiO2 catalysts via adjusting the loading amount of RuO2. We show that at very low loadings, the majority of RuO2 catalysts can be anchored at the defective sites of TiO2 substrates. Spectroscopic studies combined with density functional calculations indicate that electron transfers from defective sites of substrates to RuO2, causing an increase in the apparent reaction barrier of CO oxidation. As the loading of Ru increases, RuO2 starts to appear on the terrace of TiO2, and the apparent reaction barrier of CO oxidation decreases. At the medium loading of ~0.75 wt%, the lowest apparent reaction barrier is achieved. The phenomenon can be attributed to the electron transfer from RuO2 to the terrace of TiO2. Further increasing the loading of Ru, the apparent reaction barrier rises again. When the loading of Ru is more than 2 wt%, the apparent reaction barrier is found to be very comparable to that over RuO2 catalysts supported on inert substrate, indicating that the electronic effect of TiO2 is isolated underneath thick RuO2 overlayers. The demonstrated loading-electron transfer-reaction barrier relationship at RuO2/TiO2 catalysts provides an insight into the interfacial interaction between mixed oxides, and can be readily extended to many other catalytic systems.

KEYWORDS: Interfacial catalysis, electron transfer, mixed oxide catalysts, CO oxidation, reaction barrier The interfacial structure of heterogeneous catalysts is of great importance in a variety of catalytic processes, such as Pt/FeO and Au/TiO2 in catalyzing CO oxidation, Cu/ZnO in activating CO2, and Au/CeO2 in water-gasshift reaction.(1-6) Tauster has reported the strong metalsupport interactions (SMSI) over metal/TiO2 systems (metal = Ru, Rh, Pd, Os, Ir, and Pt), where the encapsulation of reduced TiO2-x on metal nanoparticles has been observed after high-temperature reduction.(7, 8) In general, the SMSI has been considered to be beneficial to the dispersion and stabilization of metal catalysts.(6, 9) More importantly, the interfacial structures often exhibit superior reactivity, as many reactions occur at interface via a bi-functional mechanism.(10, 11) In addition, the electron transfer may also occur, which consequently improves the catalytic reactivity.(12) It is, therefore, not surprising that the interfacially electronic interaction for heterogeneous catalysts have been receiving ample attention, in particular on transition metal/oxides system.(13, 14) For example, the studies of model catalytic system as well as reactivity test over supported Au catalysts have

shown that the charged Au bilayer on TiO2 performs excellent CO oxidation reactivity.(2, 3) Similarly, Pt/CeO2 catalysts have also shown strong dependence of reactivity on electronic perturbation.(9, 15) The large electronic perturbations for Pt on CeO2 have been observed, which significantly enhances the reactivity of Pt to dissociate water.(15) Furthermore, by employing the synchrotronradiation photoelectron spectroscopy, scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, the electrons transfer between Pt and CeO2 has been quantified. Approximately 0.1 electron per Pt atom can be transferred to CeO2 at the particle size of Pt is ~1.5 nm.(12) In addition, the modification of electronic structure for oxides has also been studied on inversed model systems. For example, the FeO presents strongly interfacial interaction with Pt substrate. As a result, the FeO/Pt shows superior reactivity for catalyzing CO oxidation at low temperature.(16-18) Similarly, the ultrathin MgO on Ag(100) shows low activation barrier for CO oxidation as the adsorption energy of CO is low.(19, 20) Due to the interfacial electrons transfer, ultrathin TiO2 films

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on Ag(112) also show improved reactivity for NOx reduc-

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tion reaction.(21)

Figure 1. (A) The dependence of CO conversion on reaction temperature over RuO2/a-TiO2 and RuO2/r-TiO2. The Ru loading and calcined temperature of catalysts are 2 wt% and 300 °C respectively. (B) The Arrhenius plots for CO oxidation over the RuO2/TiO2 with different Ru loadings. 0.01A/118 represents that the RuO2/a-TiO2 with 0.01 wt% Ru loading has the Ea of 118 kJ/mol, while the 0.01R/96 indicates that the RuO2/r-TiO2 with 0.01 wt% Ru loading has the Ea of 96 kJ/mol. (C) The dependence of Ea values on Ru loadings.

Although the understanding of interfacially electronic interaction between noble metals and reducible supports has made a great progress, the comprehension over mixed-metal oxides have not been well developed due to their structural complexity and diversity of oxygen species.(22) Among the mixed metal catalysts, one of the most frequently investigated systems are RuO2-TiO2 mixed catalysts, which are widely applied in many important reactions, such as photocatalytic water splitting, CO oxidation, dehydrogenation of HCl, NH3, and CH3OH, and electrocatalysis.(23-28) The RuO2-TiO2 mixed catalysts are also rather interesting as the rutile RuO2 is isostructural with rutile TiO2.(29, 30) A detailed understanding of the interfacial interaction between RuO2 and TiO2 should yield further insight into its catalytic properties and factors that control it. Depositing RuO2 on TiO2(110) surface, the structurally well-defined RuO2/TiO2(110) interface can be formed since the two oxides have the same rutile structure and very similar lattice parameters.(29, 30) However, the exact nature of the interfacial interaction between mixed oxides remains somewhat controversial.(31)

(1% CO, 0.5% O2, He balance) is carried out. The light-off curves of CO conversion depended on pre-calcined temperature are presented in Figure 1A and S1. For both catalysts, the best reactivity can be observed after the precalcined temperature at 300 °C. On RuO2/r-TiO2, the 100% CO conversion occurs at 182 °C. In contrast, the RuO2/a-TiO2 shows worse performance, in which the 100% CO conversion takes place at 227 °C. As the calcined temperature further increases, the CO oxidation reactivity over RuO2/r-TiO2 and RuO2/a-TiO2 catalysts become worse. When the calcined temperature is increased to 750 °C, the 100% CO conversion on RuO2/r-TiO2 is observed at a much higher temperature, 286 °C, which is very similar with that on RuO2/a-TiO2.

This paper describes the relationship between electron transfer and catalytic performance over RuO2/TiO2 catalysts. Employing spectroscopic characterizations combined with DFT calculations, we show that as the loading of Ru increases, the apparent reaction barrier of CO oxidation firstly decreases and then rises again. At the medium loading of Ru at ~0.75 wt%, the lowest apparent reaction barrier is seen. Spectroscopic studies and theoretical simulations provide a detailed mechanistic understanding for the nature of loading-dependent reaction barrier in mixed oxide catalysts. RuO2 catalysts supported on rutile TiO2 (RuO2/rTiO2) and anatase TiO2 (RuO2/a-TiO2) are prepared by wetness impregnation method, and the loading mount of Ru is controlled at ~2 wt%. Before reactivity test, the RuO2/TiO2 catalysts are calcined in air at different temperatures. Subsequently, the reactivity to CO oxidation

Figure 2. XRD patterns of (A) RuO2/r-TiO2 and (B) RuO2/aTiO2 with 2 wt% Ru loading calcined at different temperatures. The Figures on right in (A) or (B) show the enlarged XRD patterns.

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ACS Catalysis Thermo-gravimetric analysis (TGA) and X-ray diffraction (XRD) experiments are conducted to study the structural changes of fresh catalysts after calcined treatments (Figure 2 and S2). The TGA experiments show that the mixture of support and precursor of RuO2 starts to lose weight from about 300 °C, suggesting that the RuO2 can be formed when the calcined temperature is higher than 300 °C. While there is no diffraction peak of RuO2 can be detected on rutile TiO2 substrate after the calcined treatment at 300 °C, indicating RuO2 is dispersed very well. In contrast, the (110) and (101) diffraction peak of RuO2 is found at 14.0o and 35.1o on anatase TiO2 substrate after the same calcined treatment.(30) TEM study also shows that RuO2 is dispersed better on the r-TiO2 substrate (Figure S3). Moreover, X-ray photoelectron spectroscopy (XPS) studies show that no Cl ions can be detected after the calcination at 300 °C (Figure S4). So the worse reactivity over RuO2/a-TiO2 can be attributed to the aggregation of RuO2. Although the lattice match can make RuO2 form ultrathin structures on rutile TiO2, the calcined treatment at 450 °C still induces the slight aggregation of RuO2. When the calcined temperatures are 600 and 750 °C, the similar diffraction peaks are found on RuO2/r-TiO2 and RuO2/a-TiO2, which consequently present similar reactivity. Catalytic properties can be extremely distinct, depending on the surface structure of catalysts those may range from nanoparticles of different shapes and sizes to polycrystalline films to single crystals. More importantly, all these systems have a certain fraction of steps, edges and corners, which are believed to be active sites for a variety of catalytic reactions.(4) Also, these defective sites of oxide substrates can often act as nucleation sites for metal atoms and are important in catalytic activity of metal clusters.(5, 32)

Figure 3. XRD patterns of (A) RuO2/r-TiO2 and (B) RuO2/aTiO2 with different Ru loadings. The Figures on right in (A) or (B) show the enlarged XRD patterns.

Similar to the deposition of metal clusters on oxide substrates, we try to control the nucleation sites for RuO2 by simply adjusting its loading amount with respect to TiO2 substrate (Figure S5).(29) As expected, the reaction temperature for complete CO oxidation decreases as the loading of Ru increases (Figure S6). It should be noted that at a small amount of Ru loading, ~0.3 wt%, 100% CO conversion occurs at 270 °C with RuO2/r-TiO2, which is very comparable to that with RuO2/a-TiO2. Because most of RuO2 should be anchored at defective sites and dispersed very well on various substrates when the loading is very low.(29) At a medium loading of about 1 wt % Ru on r-TiO2, 100% CO conversion takes place at 226 °C, which is 31 °C lower than that over RuO2/a-TiO2. XRD measurements show that the diffraction peaks of RuO2 start to appear on a-TiO2 when the loading of Ru is higher 1 wt% (Figure 3). While only weak diffraction peaks of RuO2 can be observed on r-TiO2 as the loading of Ru is increased to higher than 2 wt% (Figure 3). It is important to note that the spent RuO2/TiO2 catalysts show similar XRD patterns as those from fresh samples (Figure S7), demonstrating there should be no structural change of spent RuO2. This result indicates that lattice match between RuO2 and rutile TiO2 can promote the dispersion of RuO2.(30, 33) However, when the loading of Ru is more than 2 wt%, the reactivity between RuO2/r-TiO2 and RuO2/a-TiO2 becomes similar. For example, the 100% CO conversion on RuO2/a-TiO2 and RuO2/r-TiO2, with loading of 5 wt% Ru are found at 203 and 189 °C respectively shown in Figure S6. To further investigate the catalytic reactivity depended on loading amount of Ru, we also analyze the kinetics of CO oxidation reaction. The conversion of CO is controlled lower than 20% to exclude the diffusion effect.(34) It is interesting to see that the lowest apparent reaction barrier (Ea) of CO oxidation is observed on the RuO2/rTiO2 catalyst with a medium loading amount of Ru (Figures 1B and 1C). Over this catalyst with 0.75 wt% Ru the Ea is determined to be 41 ± 2 kJ/mol. In contrast, when the loading amount of Ru is 0.05 wt%, the Ea on RuO2/r-TiO2 is much higher. As the loading amount of Ru increases from 0.05 to ~0.75 wt%, the Ea is found to gradually decrease from 106 ± 12 to 41 ± 2 kJ/mol. Continually increasing the loading amount of Ru, the Ea starts to rise again. For example, over the RuO2/r-TiO2 catalyst with 5 wt% Ru the Ea is determined to be ~80 kJ/mol, which is close to that for RuO2 catalysts supported on inert substrate.(35, 36) Reactivity data from RuO2/a-TiO2 catalysts are also included in Figure 1B and 1C for comparison. It can be seen that the RuO2/a-TiO2 catalyst with 0.05 wt% Ru has a very comparable Ea of 119 ± 12 kJ/mol to that for RuO2/rTiO2 catalyst with 0.05 wt% Ru. Similar to the trend in Ea for RuO2/r-TiO2, the Ea for RuO2/a-TiO2 decreases to about 60 kJ/mol as the loading amount of Ru increasing from 0.05 to 0.35 wt%. Further increasing the loading amount of Ru on a-TiO2, we find the Ea increases to ~80 kJ/mol as well. Raman spectra are acquired from RuO2/r-TiO2 and RuO2/a-TiO2 catalysts, and used to determine the elec-

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Ru loadings.(37-39) Figure 4A and 4B display the Raman

Figure 4. (A, B) The Raman spectra of the RuO2/TiO2 with different Ru loadings. (C) The Eg shift of RuO2/TiO2 as a function of Ru loading.

spectra of pure TiO2 and RuO2/TiO2, and Table S1 lists the position of Raman bands. The Raman peaks at 143, 238, 445, and 609 cm-1 are assigned to B1g, two-phonon scattering process, Eg (planar O-O interaction), and A1g (Ti-O stretch) of pure r-TiO2, respectively.(39, 40) Surface science investigations have shown that when a small amount of RuO2 is deposited on TiO2, the majority of RuO2 should nucleate at the step edges and other defective sites,(29) which are often considered to be electron-rich sites.(9, 41, 42) Given that the electron transfers from electron-rich sites of oxide substrate to supported metal clusters have been extensively observed,(12, 43) we suggest that the excess electrons of TiO2 may transfer to RuO2 as well. However, after loading 0.075 wt% Ru, there is no change in Eg peak of r-TiO2 can be observed. In general, TiO2 has only a small fraction of defective sites.(44) Therefore, it can be understood that RuO2/r-TiO2 with tiny RuO2 does not show clear Raman shift. As the loading amount of Ru is further increased, the Eg peak of r-TiO2 starts to shift until the loading amount of Ru approaches ~2 wt%. For example, the Eg peak shift to 424 cm-1 with the Ru loading of 0.75 wt%. In addition, the shift of Eg peak to lower wavenumber suggests that the electron transfer from RuO2 to the flat terraces of TiO2.(37, 45) More interestingly, it can be seen that the Raman shift between pristine rTiO2 and RuO2/r-TiO2 increases linearly as a function of the Ru loading, when the Ru loading is lower than 0.7 wt% (Figure 4C). This indicates that the interfacial contact should increase till 0.7 wt%, which is in a good agreement with quantitation of XPS shown in Figure 5(A).(46) The intensity ratio of Ru3d to Ti2p (IRu3d / ITi2p) increase linearly with Ru loading when the Ru loading is lower than 0.7 wt%, indicating that the RuO2 is dispersed uniformly on r-TiO2. Subsequently, the slope of the fitting line was found to decrease when the Ru loading is higher than 0.7 wt%, suggesting that the r-TiO2 should be covered completely by ultrathin RuO2 at the Ru loading of 0.7 wt%, and the excess RuO2 start to appear on anchored RuO2.(47, 48) It is noteworthy that this critical Ru loading is well consistent with that (~0.75 wt%) for lowest apparent reaction barrier. The loading amount of monolayer RuO2, according to the Brunauer-Emmett-teller (BET)

surface areas of r-TiO2, is calculate to be 0.4 wt% Ru (Table S2). However, the optimized loading amount of Ru (~0.75 wt%) for lowest apparent reaction barrier is nearly twice than the calculated amount of monolayer thick RuO2. In model systems, STM studies have shown that the several layer thick RuO2 islands can be formed on TiO2(110) at low RuO2 coverage.(23) So we suggest that the 0.75 wt% RuO2 may completely encapsulate the TiO2 substrate. Further increasing Ru loading, the change of Raman shift becomes less significant and eventually reaches a constant value of ~23 cm-1.

Figure 5. (A) The XPS IRu3d / ITi2p ratio versus Ru loadings. (B) The in-situ XPS Ru3d peaks of RuO2/r-TiO2 catalyst with 1 wt% Ru loading.

As a reference, the Raman peaks of pure a-TiO2 and RuO2/a-TiO2 catalysts are also included. The six allowed

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ACS Catalysis modes for a-TiO2 appears at 142 cm-1 (Eg), 195 cm-1 (Eg), 396 cm-1 (B1g), 517 cm-1 (A1g+B1g) and 638 cm-1 (Eg).(49) Different from RuO2/r-TiO2, RuO2/a-TiO2 catalysts show similar peak position as pure a-TiO2 as increasing the loading amount of Ru (Figure 4B and 4C). The Eg of aTiO2 only shifts 4 cm-1 to lower wavenumber after loading 5 wt% Ru. In addition, in-situ XPS investigations were conducted under CO oxidation atmosphere. As shown in Figure 5(B), the binding energies of Ru3d5/2 peaks locate at 280.4 eV, which can be easily assigned to RuO2 structures.(50, 51) After pumping out reactant gases and cooling to room temperature, the Ru3d5/2 peak slightly shifts to higher binding energy with ~ 0.1 eV, indicating the chemical state of RuO2 keeps unchanged.

Figure 6. Three calculated structures and the average Bader Charge of surface Ru atoms. The step and terrace sites of rTiO2 are modeled by (451) and (110) facet respectively. The cyan, red, and white mark the Ru, O, and Ti atoms respectively, for clarity, the beneath Ru, O, and Ti atoms are represented by the intersections of cyan sticks, intersections of red sticks, and intersections of white sticks, respectively.

To clarify the electronic interaction between RuO2 and r-TiO2, DFT calculations were conducted. The Figure 6 shows the three models that represent the RuO2 on the step sites of r-TiO2, RuO2 on the terrace sites of r-TiO2, and pure RuO2, respectively. The step sites of r-TiO2 were modeled by (451) facet, and the terrace sites of r-TiO2 were represented by (110) facet.(29, 42) Among the three models, the Bader Charge of Ru atoms on step sites of rTiO2 is the lowest, i.e. +1.40, while that of pure RuO2 is +1.79 demonstrating the electrons transfer from the step sites of r-TiO2 to RuO2. On the contrary, the Ru atoms on terrace sites of r-TiO2 exhibit a more positive Bader Charge, i.e. +1.81, indicating the electrons transfer from RuO2 layer to r-TiO2 terrace. The results are consistent with our Raman results where the RuO2/r-TiO2 with medium or high Ru loadings have the negatively shift of Eg compared with pure r-TiO2.

digo-blue, and red-brown represent the bulk r-TiO2, bulk RuO2, positively and negatively charged RuO2, respectively. In heterogeneous catalysis, the catalytic performance is often optimized by tuning the size, surface and interfacial structure, and components of catalysts. Through these routes, the highly efficient catalysts can be achieved. In contrast, although it has been known that the loading amount of catalysts is of great importance for their catalytic performance, the understanding of the correlation between loading amount of active components and catalytic performance is still lack, especially for mixed oxide catalytic systems. In practice, the structural complexity has made the direct studies of mixed oxide catalysts very challenging. For rutile RuO2 and TiO2, they are isostructural, and consequently can form well-defined interfacial structure, which is a prototypical model system to explore the loading-performance relationship. In the present study, we investigated the catalytic performance of RuO2 on different sites of TiO2 by simply adjusting the loading amount of Ru. By conducting catalytic performance studies, spectroscopic characterizations and theoretical simulations, the effect of the loading amount of Ru on apparent reaction barrier for CO oxidation is revealed. As illustrated in Scheme 1, when a tiny amount of RuO2 is deposited on TiO2 substrate, the majority of RuO2 should nucleate at step edges and other defective sites. In this case, the excess electrons of these defective sites on TiO2 may transfer to RuO2, which shows high apparent reaction barrier for CO oxidation. At the medium Ru loading of ~0.75 wt%, the terrace of r-TiO2 is covered by RuO2 ultrathin structures, on which the lowest apparent reaction barrier of 41±2 kJ/mol is achieved. The Eg peak in Raman spectra for r-TiO2 negatively shift as the loading amount of Ru is increased, indicating the electrons transfer from RuO2 to terrace area of TiO2. When the loading amount of Ru is further increased, the apparent reaction barrier of CO oxidation on RuO2/TiO2 is found to rise again, and eventually reaches a constant at ~80 kJ/mol, which is similar to that on RuO2/SiO2. This indicates that the thick RuO2 overlayers isolate the electronic effect of TiO2 support. The established loading– electron transfer–reaction barrier relationship clearly illustrates the role of site-specifically electronic state of substrate on catalytic performance of mixed oxide systems.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]; [email protected].

Notes The authors declare that they have no competing interests.

ASSOCIATED CONTENT

Scheme 1. Scheme of relationship between the electrons transfer and Ea. The color of RuO2 indicates the charged states of RuO2 and TiO2, where the gray-white, black, in-

Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. More experimental details, characterizations, computational methods, and catalytic results.

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ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (2016YFB0600901), the National Science Foundation of China (21525626, 91645106, 21603159) and the Program of Introducing Talents of Discipline to Universities (No. B06006).

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Table of Contents (TOC) The correlation of catalyst loading–electron transfer–reaction barrier over RuO2/r-TiO2

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