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TiO2 catalysts in low-temperature CO oxidation
Journal of Catalysis 368 (2018) 163–171
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Titania-morphology-dependent dual-perimeter-sites catalysis by Au/ TiO2 catalysts in low-temperature CO oxidation Dan Li a, Shilong Chen a, Rui You a, Yuanxu Liu b, Min Yang a, Tian Cao a, Kun Qian a, Zhenhua Zhang a, Jie Tian c, Weixin Huang a,⇑ a Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China b School of Pharmacy, Anhui University of Chinese Medicine, Anhui Academy of Chinese Medicine, Hefei 230012, China c Engineering and Materials Science Experiment Center, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 9 July 2018 Revised 5 September 2018 Accepted 26 September 2018
Keywords: Nanocatalysis Au/TiO2 Facet In situ DRIFTS Adsorption Diffusion Surface reaction
a b s t r a c t Au/TiO2 catalysts for low-temperature CO oxidation exhibit dual-perimeter-sites catalysis in which CO molecules adsorbed at both Au and TiO2 surfaces diffuse to Au–TiO2 perimeters and are oxidized into CO2. Via a comprehensive in situ DRIFTS study of CO adsorption and oxidation on Au/TiO2 catalysts with different TiO2 morphologies predominantly exposing {0 0 1}, {1 0 0}, and {1 0 1} facets, we report that such dual-perimeter-sites catalysis depends sensitively on TiO2 morphology and facets. Adsorbed CO molecules remain stable on TiO2 surfaces up to 223 K. The migration of CO molecules from TiO2 to Au–TiO2 perimeter sites depends on the catalytic activity of Au–TiO2 perimeter sites. Dual-perimetersites catalysis occurs within Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts with catalytically active Au– TiO2 perimeter sites that exhibit large coverage gradients between CO molecules adsorbed at Ti and Au sites, facilitating the migration of CO molecules from TiO2 to the Au–TiO2 perimeter sites, but seldom within Au/TiO2{0 0 1} catalysts with catalytically inactive Au–TiO2 perimeter sites that exhibits few coverage gradients between CO molecules adsorbed at Ti and Au sites. These results provide novel insights into the structure–activity relationship in Au/TiO2 catalysts for low-temperature CO oxidation. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Since Haruta’s discovery of its high catalytic activity in catalyzing low-temperature CO oxidation [1], Au/TiO2 catalyst has been a representative system of Au catalysis and attracted extensive studies [2–9]. Previous researches demonstrate a very complex Au catalysis in low-temperature CO oxidation. Generally the Au– TiO2 perimeter sites of Au/TiO2 catalysts are considered as the active sites [10–12], while both experimental and theoretical calculation results prove that low-coordinated Au atoms on Au nanoparticles with appropriate electronic structures are also catalytically active [13–16]. A recent coverage-dependent microkinetic analysis of CO oxidation catalyzed by Au/TiO2 nanocatalysts shows that the dominant kinetic pathway, activated oxygen species, and catalytic active sites all depend strongly on both temperature and oxygen partial pressure [17]. Behm’s group proposed a highly stable atomic oxygen species, most likely surface lattice oxygen at the perimeters of Au nanoparticles, as the active oxygen ⇑ Corresponding author. E-mail address: [email protected] (W. Huang). https://doi.org/10.1016/j.jcat.2018.09.032 0021-9517/Ó 2018 Elsevier Inc. All rights reserved.
species for CO oxidation above 253 K over Au/TiO2 catalysts [18– 21]. The active CO species had been considered to be adsorbed CO on the Au surface until Yates and co-workers observed the diffusion of adsorbed CO molecules from the TiO2 surface to Au–TiO2 interface and their participation in CO oxidation between 110 and 130 K. They further proposed a dual-perimeter site on Au/TiO2 catalysts [2,22,23]. The dual-perimeter sites of Au/TiO2 catalysts in lowtemperature CO oxidation reveals unrecognized roles of the TiO2 support in low-temperature CO oxidation. TiO2 structures can not only modify structures of supported Au nanoparticles but also directly affect adsorbed CO species in low-temperature CO oxidation. Recently, oxide nanocrystals with uniform and well-defined morphologies have been synthesized and used successfully both for understanding the fundamental catalytic mechanism and for optimizing the catalytic performance [24–29]. Uniform TiO2 nanocrystals with different morphologies were synthesized [30– 33] and TiO2-morphology-dependent Au–TiO2 interaction was observed in Au/TiO2 catalysts [34–36]. In this paper, via a comprehensive in situ DRIFTS study of CO adsorption and oxidation on Au/ TiO2 catalysts with different TiO2 morphologies, we report TiO2-
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morphology-dependent dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation, in which the TiO2 morphology affects the intrinsic oxidative reactivity of CO molecules adsorbed both on the TiO2 surface and on the Au surface.
2. Experimental All chemical reagents with analytical grade as were purchased from Sinopharm Chemical Reagent Co. Synthesis of anatase TiO2{0 0 1} nanocrystals [31]: Ti(OBu)4 (25 mL) and 40 wt% HF (aq) (3 mL) were mixed under stirring at RT. (Caution: Hydrofluoric acid (HF) is extremely corrosive and a contact poison, and it should be handled with extreme care! Hydrofluoric acid solution is stored in Teflon containers in use.) The solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The white precipitate produced was collected by centrifugation, washed repeatedly with EtOH and ultrapure H2O, and dried at 70 °C for 12 h. The acquired powder was dispersed in 0.1 M NaOH (aq) (700 mL), stirred for 24 h at RT, centrifuged, and washed repeatedly with ultrapure H2O until the aqueous solution had a pH of 7–8. Synthesis of anatase TiO2{1 0 0} and TiO2{1 0 1} nanocrystals [30]: TiCl4 (6.6 mL) was added dropwise into 0.43 m HCl (aq) (20 mL) at 0 °C. After being stirred for an additional 0.5 h, the solution was added dropwise into 5.5 wt% NH3 (aq) (50 mL) under stirring at RT. Then an appropriate amount of 4 wt% NH3 (aq) was used to adjust the pH of the solution to between 6 and 7, after which the system was stirred at RT for 2 h. The precipitate produced was filtered, washed repeatedly with ultrapure H2O until no residual Cl 1 could be detected, and dried for 12 h at 70 °C to acquire Ti(OH)4. Then Ti(OH)4 (2.0 g) and (NH4)2SO4 (0.5 g) (for synthesis of TiO2{1 0 0}) or NH4Cl (0.2 g) (for synthesis of TiO2{1 0 1}) were dispersed in a mixture of ultrapure H2O (15 mL) and iPrOH (15 mL) under stirring at RT and the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The white precipitate obtained was collected and washed repeatedly with ultrapure H2O. Au/TiO2 catalysts with a calculated Au/TiO2 weight ratio of 0.45% were prepared by the conventional deposition–precipitation method, employing HAuCl4 as the Au precursor. Typically, the desired amounts of HAuCl4 (aq), TiO2 (1.0 g), and ultrapure H2O (50 mL) were co-added into a three-necked flask and mixed under stirring at 60 °C for 15 min. An appropriate amount of NH3 (aq) was added to adjust the pH value to between 7 and 7.5, after which the system was stirred at 60 °C for 1 h. The solid was then filtered, washed several times with ultrapure H2O, dried at 60 °C under vacuum for 12 h, and calcined for 2 h in a quartz tube at 300 °C at a rate of 2 °C/min under Ar gas flow (30 mL/min) at atmospheric pressure. The loadings of Au in Au/TiO2 catalysts were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). Powder X-ray diffraction (XRD) patterns were recorded in the 2h range 20–80° on a Philips X’Pert Pro Super diffractometer with Cu Ka radiation (k = 0.15406 nm) operating at 40 kV and 50 mA. TEM, STEM, and HRTEM were performed with a JEOL JEM-2100F instrument at an acceleration voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized AlKa (h = 1486.7 eV) as the excitation source, and the likely charge of samples was corrected by setting the C1s binding energy of the adventitious carbon to 284.8 eV. In situ DRIFTS measurements of CO adsorption and CO oxidation were performed on a Nicolet 6700 FTIR spectrometer equipped with an in situ low-temperature and vacuum DRIFTS
reaction cell (Harrick Scientific Products) to enhance the chemisorption with minimum interference of gas-phase molecules. The DRIFTS spectra of CO adsorption and CO oxidation were measured with 256 scans and a resolution of 4 cm 1 using a MCT/A detector. A quantity of catalyst (50 mg) was loaded onto the sample stage of the reaction cell. Prior to adsorption experiments, the samples were evacuated with a base pressure of 0.01 Pa. The samples were cooled to the desired temperature, at which their spectra were taken as the background spectra, and then CO was admitted into the reaction cell to desired pressure via a leak valve and the DRIFTS spectra were recorded after the chemisorption reached steady state. CO oxidation experiments were carried out similarly to CO chemisorption experiments; however, 1% CO/air was admitted into the reaction cell to 800 Pa directly via a leak valve. Subsequently the DRIFTS spectra were collected when the reaction gas pressure reached 800 Pa. The whole reaction processes were monitored by a DRIFTS spectrometer using a series measurement function with a temporal resolution of 97 s. The spectrum labeled 0 s represents the first spectrum in which the reaction gas pressure reaches 800 Pa. For CO oxidation reaction evaluation, desired amounts of catalysts without any pretreatment were placed in a quartz tube reactor. The reaction gas mixture (1% CO and 99% dry air) was fed at a rate of 30 mL/min. The catalyst was heated to the desired reaction temperatures at a rate of 2 °C/min and then kept for 30 min to reach the steady state. The composition of the effluent gas was detected with an online GC-14C gas chromatograph equipped with a 5A column, and the CO conversion was calculated from the change in CO concentrations in the inlet and outlet gases.
3. Results and discussion As shown in Fig. 1A1–C1, as-synthesized anatase TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals respectively enclosed with dominant {0 0 1}, {1 0 0}, and {1 0 1} facets all exhibit quite uniform morphologies. TiO2{0 0 1} nanocrystals have a size distribution of 40–60 nm, TiO2{1 0 0} nanocrystals have a length distribution of 20–50 nm and a width distribution of 10–15 nm, and TiO2{1 0 1} nanocrystals have a size distribution of 15–30 nm. The lattice fringes resolved in the HRTEM images all arise from those of anatase TiO2. These microscopic results agree with previous results [30,32]. According to the previously proposed procedure [35], the percentages of {0 0 1} facets in TiO2{0 0 1}, {1 0 0} facets in TiO2{1 0 0}, and {1 0 1} facets in TiO2{1 0 1} nanocrystals were all estimated to be around 80%. The BET specific surface areas of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals are 102, 99, and 108 m2/g, respectively. Fig. 2A1–C1 show in situ DRIFTS spectra of CO adsorption on TiO2 nanocrystals at a CO pressure of 200 Pa at various temperatures. CO stretch vibrational peaks at 2178–2182 cm 1 for CO adsorbed at Ti5c sites of TiO2 (denoted as CO-Ti(IV)) [36] were observed at 153 K, and the corresponding features of 13CO-Ti(IV) species were also observed on TiO2{1 0 0} and TiO2{1 0 1} nanocrystals, due to the large numbers of Ti5c sites on TiO2 {1 0 0} and {1 0 1} surfaces (Fig. S1 in the Supporting Information) and subsequent strong CO vibrational features. The CO-Ti(IV) vibrational feature gradually decreases with increasing adsorption temperature and finally disappears at 223 K; meanwhile, vibrational features between 1975 and 2075 cm 1, with peaks at 2013/2034/2058, 1997/2055, and 2002/2050 cm 1 respectively for TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals, appear and gradually increase. Moreover, these features grow when the sample temperature decreases from 223 K to 153 K, while the feature of CO-Ti(IV) is much weaker than that on fresh TiO2 nanocrystals at 153 K. These observations demonstrate that partial surface
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Fig. 1. Representative TEM images with inserted HRTEM images and schematic morphology illustrations of (A1) TiO2{0 0 1}, (B1) TiO2{1 0 0}, and (C1) TiO2{1 0 1}, and HAADF-STEM images with Au particle size distributions of (A2) Au/TiO2{0 0 1}, (B2) Au/TiO2{1 0 0}, and (C2) Au/TiO2{1 0 1}.
reduction of all TiO2 nanocrystals by CO occurs at 173 K and above to form surface Ti(III) sites at which CO adsorbs (denoted as CO-Ti (III)), resulting in the observed vibrational features appearing between 1975 and 2075 cm 1. The different vibrational features might correspond to CO-Ti(III) species at Ti(III) sites with different local environments. CO-Ti(III) species exhibit lower wavenumbers and better stability than CO-Ti(IV) species. Both can be attributed to the stronger electron donation to the 2p antibonding orbital of adsorbed CO from Ti(III) than from Ti(IV), leading to enhanced TiACO bonding and weakened CAO bonding. Similar vibrational features were previously observed for CO adsorption on reduced Au/TiO2 surfaces [37–39], but were not clearly assigned. Inferred from the attenuation of CO-Ti(IV) vibration features at 153 K, 55.4%, 57.6%, and 38.4% of Ti(IV) is reduced by CO exposure at 223 K on TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystal surfaces, respectively. These results suggest that the anatase TiO2 surface exhibits surface oxygen species capable of reacting with CO at temperatures as low as 173 K, which is important in lowtemperature CO oxidation on TiO2-involved surfaces. This surface oxygen species is likely O2 formed by O2 adsorption at surface oxygen vacancies on as-synthesized TiO2 nanocrystal surfaces, as evidenced by our previous results [33]. Such a surface reduction reaction requires effective CO adsorption and stable O2 species on TiO2 and thus likely does not occur at high temperatures. Weak vibrational bands of CO2 adsorbed onto TiO2 were indeed observed during CO adsorption processes (Fig. S2). CO adsorption on various TiO2 nanocrystals without surface reduction was comprehensively studied by in situ DRIFTS spectra with CO pressures up to 650 Pa at 123–153 K (Figs. S3–S5). The CO-Ti(IV) vibrational features grow as the CO pressure increases and the adsorption temperature decreases. Integrated CO-Ti(IV) vibrational peak areas were plotted as a function of CO pressure
and adsorption temperature to establish the CO adsorption isotherms on various TiO2 nanocrystals (Fig. 2A2–C2). The isotherm curves at low CO coverages were found to be adequately fitted linearly (Fig. S6), from which the isosteric plots at an integrated CO-Ti (IV) vibrational peak area of 0.7 were derived (Fig. 2A3–C3). The isosteric plots could be adequately fitted linearly. From the slopes of fitted lines, the adsorption heat (DHads) of CO adsorption at Ti (IV) sites of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals was derived via Clausius–Clapeyron analysis [40] as 4.2 ± 0.2, 3.3 ± 0.3, and 3.2 ± 0.3 kJ/mol, respectively, which indicates that CO adsorption at the Ti(IV) site of anatase TiO2 nanocrystals is facet-dependent. TiO2{1 0 0} and TiO2{1 0 1} nanocrystals exhibit surface Ti(IV) sites with higher densities but slightly less CO binding strength than for TiO2{0 0 1} nanocrystals. The low adsorption heats of CO(a) on TiO2 surfaces are likely due to their large coverages under the adsorption conditions and subsequent strong repulsive interactions. Au/TiO2 catalysts with a calculated 0.45% Au/TiO2 weight ratio were prepared with TiO2 nanocrystals as supports, and ICP-AES analysis results show that the actual Au loading is 0.45, 0.42, and 0.40 wt% in Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. All Au/TiO2 catalysts display diffraction patterns of anatase TiO2 (JCPDS card No. 89-4921) in the XRD patterns (Fig. 3A) and no Au diffraction peaks, and XPS characterization results (Fig. 3B) show that all Au/TiO2 catalysts exhibit a single Au4f component with Au4f7/2 binding energy 83.0 eV, a typical value for metallic Au supported on TiO2 [34]. The Au/TiO2{0 0 1} catalyst exhibits a much lower Au4f peak intensity and thus a lower surface-to-bulk ratio of Au nanoparticles than the other two catalysts. Meanwhile, all Au/TiO2 catalysts exhibits a Ti2p3/2 binding energy of 458.4 eV and an O1s binding energy oft 529.6 eV arising from TiO2.
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Fig. 2. (A1–C1) In situ DRIFTS spectra of CO adsorption on TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals at 153, 173, 193, and 223 K, and then at 153 K again (P = 200 Pa). (A2–C2) DRIFTS isotherms of CO–Ti(IV) species TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals. (A3–C3) isosteric plots of CO–Ti(IV) species of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals. The dotted lines in figures (A2–C2) indicate CO coverage corresponding to an integrated CO vibrational peak area of 0.7, selected to derive the isosteric plots.
Fig. 1A2–C2 present representative high-angle annular dark field (HAADF)-STEM images and derived Au particle size distributions of various Au/TiO2 catalysts. Au nanoparticles mainly have spherical morphology and exhibit an average size distribution of 4.1 ± 1.3 nm in Au/TiO2{0 0 1}, 1.8 ± 1.1 nm in Au/TiO2 {1 0 0}, and 2.6 ± 1.3 nm in Au/TiO2{1 0 1}. Generally a strong metal–support interaction facilitates the formation of fine supported metal particles. Therefore, the Au–TiO2 interaction follows the order Au–TiO2{1 0 0} > Au–TiO2{1 0 1} > Au–TiO2{0 0 1}. Fig. 4A1–C1 show in situ DRIFTS spectra of CO adsorption on various Au/TiO2 catalysts with a CO pressure of 200 Pa at various temperatures. Besides CO-Ti(IV) species, CO adsorbed at Au0 sites (denoted as CO–Au0) with vibrational features of 2104/2109 cm 1 [41,42] was observed at 123 K; meanwhile, by comparing the I2130/2131cm13 CO/12CO ratio, the vibrational features 1/I2181cm-1 ratios with the of Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts at 2130/2131 cm 1 should be contributed largely by 13CO(a)–Ti(IV) species and to a lesser degree by CO adsorbed at Aud+ sites (denoted as CO–Aud+) [17]. The observations of CO–Aud+ species on Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts could be related to the presence of ultrafine supported Au nanoparticles whose lattice contracts and charge
at the Au atom site decreases relative to bulk Au [16,43]. The vibrational features of CO–Au0 and CO–Aud+ species do not change when the adsorption temperature increases to 153 K, while those of CO– Ti(IV) species weaken. This demonstrates that CO–Au0 and CO– Aud+ species are more stable than CO–Ti(IV) species. The vibrational features of CO–Ti(IV) species keep weakening with further increase of the adsorption temperature and finally disappear at 223 K. The vibrational features of CO–Ti(III) species resulting from surface reduction of TiO2 appear at 173 K and grow with increasing temperature; meanwhile, new vibrational features at 2070–2078 cm 1 that could be assigned to CO adsorbed at Aud sites (denoted as CO–Aud ) [41,42] emerge at the expense of those of CO–Au0 and CO–Aud+ species. These observations suggest that charge transfer occurs from reduced TiO2 surfaces to supported Au nanoparticles, forming the Aud species in Au/TiO2 catalysts. Such charge transfer decreases the number of Ti(III) sites on the reduced TiO2 surfaces of Au/TiO2, as evidenced by the intensity of CO–Ti(III) species. CO adsorption has been widely used to probe the Au species in supported Au catalysts, and our results for CO adsorption-induced change of Au speciation in Au/TiO2 demonstrate that CO adsorption should be carried out at temperatures
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Fig. 3. (A) XRD patterns and (B1) Au4f, (B2) Ti2p, and (B3) O1s XPS spectra of various Au/TiO2 catalysts.
Fig. 4. In situ DRIFTS spectra of CO adsorption at 123, 153, 173, 193, and 223 K, and then at 123 K again (P = 200 Pa), and a comparison of in situ DRIFTS spectra of CO adsorption at 123 K on fresh sample and the sample subjected to CO adsorption at 223 K on (A1, A2) Au/TiO2{0 0 1}, (B1, B2) Au/TiO2{1 0 0}, and (C1, C2) Au/TiO2{1 0 1} catalysts.
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low enough to prevent surface reduction in order to acquire reliable Au speciation. The vibrational feature of CO–Au0 species on Au/TiO2{0 0 1} catalysts significantly weaken at 223 K, while those on Au/TiO2{1 0 0} and Au/TiO2{1 0 1} do not change, indicating the weakest interaction of CO with Au0 sites of the Au/TiO2{0 0 1} catalyst. A noteworthy observation is that the spectrum region of CO–Au vibrational features exhibits a broad background signal whose intensity increases with CO adsorption temperature, which seriously hampers the analysis of vibrational features within this region. Such a broad background IR signal during CO adsorption on Au/TiO2 catalysts was previously observed and proposed to be related to the reversible partial reduction of TiO2 at the Au-TiO2 interface, which leads to an increase in surface disorder or a roughening of TiO2 particles and a subsequent decrease in IR transmission through the catalyst [44]. We found that the background spectra almost vanished when Au/TiO2 catalysts were subjected to CO adsorption at 223 K and then cooled down to 123 K again for CO adsorption (Fig. 4A2–C2). Such observations demonstrate that surface disorder or roughening of TiO2 particles by CO adsorption is sensitively temperature-dependent and facilitated at high temperatures. Inferred by the attenuation of CO–Ti(IV) vibration features shown in Fig. 4A2–C2, about 32.2, 42.9, and 28.1% of surface Ti(IV) is reduced by CO exposure up to 223 K on Au/ TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. The formation of Aud species from Au0 species on Au/ TiO2{0 0 1}, the formation of Aud and Au0 species from Aud+ species on Au/TiO2{1 0 0}, and the formation of Aud species from Aud+ species on Au/TiO2{1 0 1} can also be clearly identified. Employing the same approach as in TiO2, we measured the CO adsorption isotherms on various Au/TiO2 catalysts and analyzed the isosteric plots (Fig. S7), from which the adsorption heat (DHads) of CO adsorption on Ti(IV) sites of Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts was derived respectively as 4.5 ± 0.6, 3.1 ± 0.2, and 2.4 ± 0.2 kJ/mol. These data do not vary much from those on bare TiO2 nanocrystals. Catalytic activity of Au/TiO2 catalysts in CO oxidation was evaluated between 303 and 343 K. A Weisz–Prater analysis and a Mears analysis [45–47] in the Supporting Information demonstrate the absence of mass and heat transfer limitations under the adopted reaction conditions. The catalytic activity follows the order Au/TiO2{1 0 0} > Au/TiO2{1 0 1} Au/TiO2{0 0 1} (Fig. S8), consistent with previous results [35]. Based on the calculated Au numbers at the perimeter sites (Supporting Information and Table S1), the turnover frequency (TOF) at 70 °C is 2.0, 2.5, and 2.8 s 1 for Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. The dual-perimeter-sites catalysis of Au/TiO2 catalysts occurs in CO oxidation only far below room temperature [2,22,23]. We thus examined dual-site CO oxidation on various Au/TiO2 catalysts below 213 K, employing in situ time-resolved DRIFTS spectra. Figs. 5–7 show DRIFTS spectra at various temperatures and reaction times during CO oxidation on Au/TiO2{0 0 1}, Au/TiO2{0 0 1}, and Au/TiO2{1 0 1} catalysts and derived temporal evolutions of integrated vibrational peak areas of observed CO–Ti(IV) and CO– Au species. Vibrational features of CO–Ti(III) and CO–Aud species were not observed, suggesting that the concentrations of surface Ti (III) sites on Au/TiO2 catalysts under the employed CO oxidation conditions should be too low to be probed by CO adsorption with DRIFTS. This could be attributed to the facile adsorption of oxygen at surface oxygen vacancy sites created by CO reduction. Similar results were also observed during CO oxidation on TiO2 nanocrystals (Fig. S9). As shown in Figs. 5–7, evolution of adsorbed CO species on Au/ TiO2 catalysts under CO oxidation conditions is complex. This is due to the dynamic nature of a chemisorption process initiated
by collisions of reactants on solid surfaces [48]. The CO adsorption process reaches an equilibrium quickly on bare TiO2 {0 0 1}, {1 0 0}, and {1 0 1} surfaces; in contrast, both the diffusion of adsorbed CO between Au and Ti sites and the oxidation of adsorbed CO occur on Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} surfaces, resulting in variation of adsorbed CO species at both Ti and Au sites prior to steady state. The CO–Ti(IV) species on TiO2 nanocrystals do not exhibit obvious reactivity to produce CO2 under the employed CO oxidation conditions (Fig. S9) and they remain on all TiO2 nanocrystal surfaces up to 193 K (Fig. 4); thus the variations of CO–Ti(IV) species on Au/TiO2 catalysts result from the CO adsorption process and CO migration from Ti to Au–TiO2 perimeter sites. CO oxidation occurs mainly at Au–TiO2 interfaces and the CO–Au species remain on all Au/TiO2 catalysts up to 223 K (Fig. 4); thus the variation of CO–Au species on Au/TiO2 catalysts results from the CO adsorption process, the CO migration process from Ti to Au, and the CO oxidation process. Meanwhile, O2 adsorption can alter the electronic structure of Au nanoparticles to enhance CO adsorption [49,50] and the decrease of CO partial pressure in the in situ DRIFTS batch reactor with CO oxidation also affects the CO adsorption behavior. On Au/TiO2{0 0 1} catalyst (Fig. 5), CO adsorption at the Ti(IV) sites quickly reaches steady state at 153 and 173 K, and few CO– Ti(IV) species could be observed at higher temperatures. CO adsorption at the Au sites quickly reaches steady state at 153 K but not at elevated temperatures. The CO–Au species grows with the reaction time at 173 and 193 K, initially increases, reaches a local maximum, and then decreases at 213 K. These observations suggest that CO oxidation should occur on Au/TiO2{0 0 1} catalyst only at 213 K. However, vibrational features of adsorbed CO2 were observed at temperatures up to 193 K but not at 213 K. Thus, the desorption of adsorbed CO2 seems to be the rate-limiting step in CO oxidation and effectively occurs only at 213 K. Similar results were also observed during the CO + O(a) reaction on Au(9 9 7) at low temperatures [51]. The inactivity of the Au/TiO2{0 0 1} catalyst in CO oxidation up to 193 K also indicates high coverage of CO–Au species and lack of enough coverage gradient between CO–Ti(IV) and CO–Au species to drive the migration of adsorbed CO from the Ti(IV) sites to the Au sites [52–54]. The observed growth of CO–Au species with time results from O2-adsorption-enhanced ability of Au nanoparticles to adsorb CO, while the observed later decrease at 213 K results from the decrease of CO partial pressure in the in situ DRIFTS batch reactor with CO oxidation proceeding. On Au/TiO2{1 0 0} catalyst (Fig. 6), the CO–Ti(IV) species initially does not change but then decreases with reaction time at 153 and 173 K, keeps decreasing at 193 K, and could not be observed at 213 K. The CO-Au species does not vary with the reaction time at 153 and 173 K, initially does not vary but then decreases at 193 K, and keeps decreasing at 213 K. Meanwhile, the adsorbed CO2 species initially forms at 153 K and then decreases, indicating the occurrence of CO oxidation and partial CO2 desorption. The adsorbed CO2 species decreases with the reaction temperature and becomes invisible at 213 K. Thus, the consumption of CO–Au species due to CO oxidation can be effectively compensated for by the migration of CO species from Ti sites to Au sites and the enhanced CO adsorption due to O2 coadsorption at 153 and 173 K, but cannot do so at 193 and 213 K due to the accelerated CO oxidation rate, the decreased gasphase CO pressure, and the decreased CO adsorption at the Ti sites and subsequent CO migration rate. The decrease of CO–Ti(IV) species is due to the migration to Au sites to participate CO oxidation and the decreased gas-phase CO pressure. On the Au/TiO2{1 0 1} catalyst (Fig. 7), the CO–Ti(IV) species initially does not change but then decreases with reaction time at 153 K, keeps decreasing at 173 and 193 K, and could not be observed at 213 K. The CO–Au species does not vary with the reac-
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Fig. 5. (A1–A4) The evolution of adsorbed CO and CO2 species during the CO oxidation on Au/TiO2{0 0 1} catalysts at 153, 173, 193, and 213 K. (B, C) The evolution of integrated peak areas of CO–Ti(IV) and CO–Au species on Au/TiO2{0 0 1} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
Fig. 6. (A1–A4) The evolution of adsorbed CO and CO2 species during CO oxidation on Au/TiO2{1 0 0} catalysts at 153, 173, 193, and 213 K. (B, C) The evolutions of integrated peaks area of CO–Ti(IV) and CO–Au species on Au/TiO2{1 0 0} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
tion time at 153 K and keeps decreasing at elevated temperatures. Meanwhile, the adsorbed CO2 species initially forms at 153 K and then decreases greatly, indicating the occurrence of CO oxidation and partial CO2 desorption. The adsorbed CO2 species decreases with the reaction temperature but is visible at 213 K without the presence of both CO–Ti and CO–Au species. This implies that adsorbed CO2 on Au surface at 213 K results to the adsorption of gas-phase CO2 produced by CO oxidation in the in situ DRIFTS reactor. Thus, the consumption of the CO–Au species due to CO oxidation can be effectively compensated for by the migration of the CO
species from Ti sites to Au sites and the enhanced CO adsorption due to O2 co-adsorption at 153 K, but not at elevated temperatures due to the accelerated CO oxidation rate, the decreased gas-phase CO pressure, and the decreased CO adsorption at the Ti sites and subsequent CO migration rate. The decrease of CO–Ti(IV) species is due to migration to Au sites to participate CO oxidation and the decreased gas-phase CO pressure. The above observations reveal that the dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation sensitively depends on TiO2 morphology and facets. Such depen-
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Fig. 7. (A1–A4) The evolution of adsorbed CO and CO2 species during CO oxidation on Au/TiO2{1 0 1} catalysts at 153, 173, 193, and 213 K. (B, C) The evolution of integrated peaks area of CO–Ti(IV) and CO–Au species on Au/TiO2{1 0 1} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
dence mainly results from the coverage gradients between CO molecules adsorbed at Ti and Au sites required for driving efficient migration processes. TiO2 surfaces do not catalyze CO oxidation, and CO coverage on TiO2 surfaces is determined by CO adsorption. Our results show that CO adsorption on TiO2 surfaces effectively occurs up to 193 K; thus the contributions of CO molecules adsorbed on the TiO2 surface contribute to catalytic activity of Au/TiO2 catalysts only below 193 K. Au–TiO2 perimeter sites catalyze CO oxidation, and CO coverages on Au surfaces are determined by both CO adsorption and the oxidation reactivity of adsorbed CO. Au–TiO2 perimeter sites of Au/TiO2{1 0 0} and Au/ TiO2{1 0 1} catalysts are catalytically active in catalyzing CO oxidation even at 153 K; thus CO coverage on Au surfaces is below saturation, resulting in formations of large coverage gradients between CO molecules adsorbed at Ti and Au sites, sufficient to drive the migration of CO molecules adsorbed onto TiO2 to the Au–TiO2 perimeter sites to participate in CO oxidation. However, Au–TiO2 perimeter sites of the Au/TiO2{0 0 1} catalyst are poor in catalyzing CO oxidation up to 193 K; thus CO coverage on Au surfaces is almost saturating and there are few coverage gradients between CO molecules adsorbed at Ti and Au sites; CO molecules adsorbed onto TiO2 are not able to migrate to the Au–TiO2 perimeter sites to participate in CO oxidation. Besides the coverage gradients, the strength of adsorbed CO binding with TiO2 surfaces also affects the migration process. Among TiO2 {0 0 1}, {1 0 0}, and {1 0 1} facets, CO adsorption onto {0 0 1} facets exhibits the highest adsorption heat and thus the strongest interaction; thus CO adsorbed onto TiO2{0 0 1} facets should be less mobile than that on TiO2 {1 0 0} and {1 0 1} facets. These results greatly deepen fundamental understanding of the dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation.
catalysts in low-temperature CO oxidation. CO molecules adsorb onto TiO2 surfaces below 223 K and interact with TiO2{0 0 1} facets more strongly than TiO2 {1 0 0} and {1 0 1} facets. The migration of adsorbed CO molecules from TiO2 to the Au–TiO2 perimeter sites depends on the catalytic activity of the Au–TiO2 perimeter sites. Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts with catalytically active Au–TiO2 perimeter sites exhibit large coverage gradients between CO molecules adsorbed at Ti and Au sites, sufficient to drive the migration of CO molecules adsorbed onto TiO2 to the Au–TiO2 perimeter sites, while the Au/TiO2{0 0 1} catalyst with catalytically inactive Au–TiO2 perimeter sites does not. Thus, dual-perimeter-sites catalysis plays a role in Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts but not in the Au/TiO2{0 0 1} catalyst. Acknowledgments This work was financially supported by the National Key R & D Program of Ministry of Science and Technology of China (2017YFB0602205), the National Natural Science Foundation of China (21525313, 91745202), the Changjiang Scholars Program of Ministry of Education of China, the Fundamental Research Funds for the Central Universities of Ministry of Education of China (WK2060030017) and Collaborative Innovation Center of Suzhou Nano Science and Technology.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2018.09.032. References
4. Conclusions In summary, employing various types of anatase TiO2 nanocrystals as supports, we have successfully demonstrated TiO2morphology-dependent dual-perimeter-sites catalysis of Au/TiO2
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Titania-morphology-dependent dual-perimeter-sites catalysis by Au/ TiO2 catalysts in low-temperature CO oxidation Dan Li a, Shilong Chen a, Rui You a, Yuanxu Liu b, Min Yang a, Tian Cao a, Kun Qian a, Zhenhua Zhang a, Jie Tian c, Weixin Huang a,⇑ a Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Chemical Physics, University of Science and Technology of China, Jinzhai Road 96, Hefei 230026, China b School of Pharmacy, Anhui University of Chinese Medicine, Anhui Academy of Chinese Medicine, Hefei 230012, China c Engineering and Materials Science Experiment Center, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 9 July 2018 Revised 5 September 2018 Accepted 26 September 2018
Keywords: Nanocatalysis Au/TiO2 Facet In situ DRIFTS Adsorption Diffusion Surface reaction
a b s t r a c t Au/TiO2 catalysts for low-temperature CO oxidation exhibit dual-perimeter-sites catalysis in which CO molecules adsorbed at both Au and TiO2 surfaces diffuse to Au–TiO2 perimeters and are oxidized into CO2. Via a comprehensive in situ DRIFTS study of CO adsorption and oxidation on Au/TiO2 catalysts with different TiO2 morphologies predominantly exposing {0 0 1}, {1 0 0}, and {1 0 1} facets, we report that such dual-perimeter-sites catalysis depends sensitively on TiO2 morphology and facets. Adsorbed CO molecules remain stable on TiO2 surfaces up to 223 K. The migration of CO molecules from TiO2 to Au–TiO2 perimeter sites depends on the catalytic activity of Au–TiO2 perimeter sites. Dual-perimetersites catalysis occurs within Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts with catalytically active Au– TiO2 perimeter sites that exhibit large coverage gradients between CO molecules adsorbed at Ti and Au sites, facilitating the migration of CO molecules from TiO2 to the Au–TiO2 perimeter sites, but seldom within Au/TiO2{0 0 1} catalysts with catalytically inactive Au–TiO2 perimeter sites that exhibits few coverage gradients between CO molecules adsorbed at Ti and Au sites. These results provide novel insights into the structure–activity relationship in Au/TiO2 catalysts for low-temperature CO oxidation. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Since Haruta’s discovery of its high catalytic activity in catalyzing low-temperature CO oxidation [1], Au/TiO2 catalyst has been a representative system of Au catalysis and attracted extensive studies [2–9]. Previous researches demonstrate a very complex Au catalysis in low-temperature CO oxidation. Generally the Au– TiO2 perimeter sites of Au/TiO2 catalysts are considered as the active sites [10–12], while both experimental and theoretical calculation results prove that low-coordinated Au atoms on Au nanoparticles with appropriate electronic structures are also catalytically active [13–16]. A recent coverage-dependent microkinetic analysis of CO oxidation catalyzed by Au/TiO2 nanocatalysts shows that the dominant kinetic pathway, activated oxygen species, and catalytic active sites all depend strongly on both temperature and oxygen partial pressure [17]. Behm’s group proposed a highly stable atomic oxygen species, most likely surface lattice oxygen at the perimeters of Au nanoparticles, as the active oxygen ⇑ Corresponding author. E-mail address: [email protected] (W. Huang). https://doi.org/10.1016/j.jcat.2018.09.032 0021-9517/Ó 2018 Elsevier Inc. All rights reserved.
species for CO oxidation above 253 K over Au/TiO2 catalysts [18– 21]. The active CO species had been considered to be adsorbed CO on the Au surface until Yates and co-workers observed the diffusion of adsorbed CO molecules from the TiO2 surface to Au–TiO2 interface and their participation in CO oxidation between 110 and 130 K. They further proposed a dual-perimeter site on Au/TiO2 catalysts [2,22,23]. The dual-perimeter sites of Au/TiO2 catalysts in lowtemperature CO oxidation reveals unrecognized roles of the TiO2 support in low-temperature CO oxidation. TiO2 structures can not only modify structures of supported Au nanoparticles but also directly affect adsorbed CO species in low-temperature CO oxidation. Recently, oxide nanocrystals with uniform and well-defined morphologies have been synthesized and used successfully both for understanding the fundamental catalytic mechanism and for optimizing the catalytic performance [24–29]. Uniform TiO2 nanocrystals with different morphologies were synthesized [30– 33] and TiO2-morphology-dependent Au–TiO2 interaction was observed in Au/TiO2 catalysts [34–36]. In this paper, via a comprehensive in situ DRIFTS study of CO adsorption and oxidation on Au/ TiO2 catalysts with different TiO2 morphologies, we report TiO2-
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morphology-dependent dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation, in which the TiO2 morphology affects the intrinsic oxidative reactivity of CO molecules adsorbed both on the TiO2 surface and on the Au surface.
2. Experimental All chemical reagents with analytical grade as were purchased from Sinopharm Chemical Reagent Co. Synthesis of anatase TiO2{0 0 1} nanocrystals [31]: Ti(OBu)4 (25 mL) and 40 wt% HF (aq) (3 mL) were mixed under stirring at RT. (Caution: Hydrofluoric acid (HF) is extremely corrosive and a contact poison, and it should be handled with extreme care! Hydrofluoric acid solution is stored in Teflon containers in use.) The solution was then transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The white precipitate produced was collected by centrifugation, washed repeatedly with EtOH and ultrapure H2O, and dried at 70 °C for 12 h. The acquired powder was dispersed in 0.1 M NaOH (aq) (700 mL), stirred for 24 h at RT, centrifuged, and washed repeatedly with ultrapure H2O until the aqueous solution had a pH of 7–8. Synthesis of anatase TiO2{1 0 0} and TiO2{1 0 1} nanocrystals [30]: TiCl4 (6.6 mL) was added dropwise into 0.43 m HCl (aq) (20 mL) at 0 °C. After being stirred for an additional 0.5 h, the solution was added dropwise into 5.5 wt% NH3 (aq) (50 mL) under stirring at RT. Then an appropriate amount of 4 wt% NH3 (aq) was used to adjust the pH of the solution to between 6 and 7, after which the system was stirred at RT for 2 h. The precipitate produced was filtered, washed repeatedly with ultrapure H2O until no residual Cl 1 could be detected, and dried for 12 h at 70 °C to acquire Ti(OH)4. Then Ti(OH)4 (2.0 g) and (NH4)2SO4 (0.5 g) (for synthesis of TiO2{1 0 0}) or NH4Cl (0.2 g) (for synthesis of TiO2{1 0 1}) were dispersed in a mixture of ultrapure H2O (15 mL) and iPrOH (15 mL) under stirring at RT and the mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and kept at 180 °C for 24 h. The white precipitate obtained was collected and washed repeatedly with ultrapure H2O. Au/TiO2 catalysts with a calculated Au/TiO2 weight ratio of 0.45% were prepared by the conventional deposition–precipitation method, employing HAuCl4 as the Au precursor. Typically, the desired amounts of HAuCl4 (aq), TiO2 (1.0 g), and ultrapure H2O (50 mL) were co-added into a three-necked flask and mixed under stirring at 60 °C for 15 min. An appropriate amount of NH3 (aq) was added to adjust the pH value to between 7 and 7.5, after which the system was stirred at 60 °C for 1 h. The solid was then filtered, washed several times with ultrapure H2O, dried at 60 °C under vacuum for 12 h, and calcined for 2 h in a quartz tube at 300 °C at a rate of 2 °C/min under Ar gas flow (30 mL/min) at atmospheric pressure. The loadings of Au in Au/TiO2 catalysts were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). Powder X-ray diffraction (XRD) patterns were recorded in the 2h range 20–80° on a Philips X’Pert Pro Super diffractometer with Cu Ka radiation (k = 0.15406 nm) operating at 40 kV and 50 mA. TEM, STEM, and HRTEM were performed with a JEOL JEM-2100F instrument at an acceleration voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 high-performance electron spectrometer using monochromatized AlKa (h = 1486.7 eV) as the excitation source, and the likely charge of samples was corrected by setting the C1s binding energy of the adventitious carbon to 284.8 eV. In situ DRIFTS measurements of CO adsorption and CO oxidation were performed on a Nicolet 6700 FTIR spectrometer equipped with an in situ low-temperature and vacuum DRIFTS
reaction cell (Harrick Scientific Products) to enhance the chemisorption with minimum interference of gas-phase molecules. The DRIFTS spectra of CO adsorption and CO oxidation were measured with 256 scans and a resolution of 4 cm 1 using a MCT/A detector. A quantity of catalyst (50 mg) was loaded onto the sample stage of the reaction cell. Prior to adsorption experiments, the samples were evacuated with a base pressure of 0.01 Pa. The samples were cooled to the desired temperature, at which their spectra were taken as the background spectra, and then CO was admitted into the reaction cell to desired pressure via a leak valve and the DRIFTS spectra were recorded after the chemisorption reached steady state. CO oxidation experiments were carried out similarly to CO chemisorption experiments; however, 1% CO/air was admitted into the reaction cell to 800 Pa directly via a leak valve. Subsequently the DRIFTS spectra were collected when the reaction gas pressure reached 800 Pa. The whole reaction processes were monitored by a DRIFTS spectrometer using a series measurement function with a temporal resolution of 97 s. The spectrum labeled 0 s represents the first spectrum in which the reaction gas pressure reaches 800 Pa. For CO oxidation reaction evaluation, desired amounts of catalysts without any pretreatment were placed in a quartz tube reactor. The reaction gas mixture (1% CO and 99% dry air) was fed at a rate of 30 mL/min. The catalyst was heated to the desired reaction temperatures at a rate of 2 °C/min and then kept for 30 min to reach the steady state. The composition of the effluent gas was detected with an online GC-14C gas chromatograph equipped with a 5A column, and the CO conversion was calculated from the change in CO concentrations in the inlet and outlet gases.
3. Results and discussion As shown in Fig. 1A1–C1, as-synthesized anatase TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals respectively enclosed with dominant {0 0 1}, {1 0 0}, and {1 0 1} facets all exhibit quite uniform morphologies. TiO2{0 0 1} nanocrystals have a size distribution of 40–60 nm, TiO2{1 0 0} nanocrystals have a length distribution of 20–50 nm and a width distribution of 10–15 nm, and TiO2{1 0 1} nanocrystals have a size distribution of 15–30 nm. The lattice fringes resolved in the HRTEM images all arise from those of anatase TiO2. These microscopic results agree with previous results [30,32]. According to the previously proposed procedure [35], the percentages of {0 0 1} facets in TiO2{0 0 1}, {1 0 0} facets in TiO2{1 0 0}, and {1 0 1} facets in TiO2{1 0 1} nanocrystals were all estimated to be around 80%. The BET specific surface areas of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals are 102, 99, and 108 m2/g, respectively. Fig. 2A1–C1 show in situ DRIFTS spectra of CO adsorption on TiO2 nanocrystals at a CO pressure of 200 Pa at various temperatures. CO stretch vibrational peaks at 2178–2182 cm 1 for CO adsorbed at Ti5c sites of TiO2 (denoted as CO-Ti(IV)) [36] were observed at 153 K, and the corresponding features of 13CO-Ti(IV) species were also observed on TiO2{1 0 0} and TiO2{1 0 1} nanocrystals, due to the large numbers of Ti5c sites on TiO2 {1 0 0} and {1 0 1} surfaces (Fig. S1 in the Supporting Information) and subsequent strong CO vibrational features. The CO-Ti(IV) vibrational feature gradually decreases with increasing adsorption temperature and finally disappears at 223 K; meanwhile, vibrational features between 1975 and 2075 cm 1, with peaks at 2013/2034/2058, 1997/2055, and 2002/2050 cm 1 respectively for TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals, appear and gradually increase. Moreover, these features grow when the sample temperature decreases from 223 K to 153 K, while the feature of CO-Ti(IV) is much weaker than that on fresh TiO2 nanocrystals at 153 K. These observations demonstrate that partial surface
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Fig. 1. Representative TEM images with inserted HRTEM images and schematic morphology illustrations of (A1) TiO2{0 0 1}, (B1) TiO2{1 0 0}, and (C1) TiO2{1 0 1}, and HAADF-STEM images with Au particle size distributions of (A2) Au/TiO2{0 0 1}, (B2) Au/TiO2{1 0 0}, and (C2) Au/TiO2{1 0 1}.
reduction of all TiO2 nanocrystals by CO occurs at 173 K and above to form surface Ti(III) sites at which CO adsorbs (denoted as CO-Ti (III)), resulting in the observed vibrational features appearing between 1975 and 2075 cm 1. The different vibrational features might correspond to CO-Ti(III) species at Ti(III) sites with different local environments. CO-Ti(III) species exhibit lower wavenumbers and better stability than CO-Ti(IV) species. Both can be attributed to the stronger electron donation to the 2p antibonding orbital of adsorbed CO from Ti(III) than from Ti(IV), leading to enhanced TiACO bonding and weakened CAO bonding. Similar vibrational features were previously observed for CO adsorption on reduced Au/TiO2 surfaces [37–39], but were not clearly assigned. Inferred from the attenuation of CO-Ti(IV) vibration features at 153 K, 55.4%, 57.6%, and 38.4% of Ti(IV) is reduced by CO exposure at 223 K on TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystal surfaces, respectively. These results suggest that the anatase TiO2 surface exhibits surface oxygen species capable of reacting with CO at temperatures as low as 173 K, which is important in lowtemperature CO oxidation on TiO2-involved surfaces. This surface oxygen species is likely O2 formed by O2 adsorption at surface oxygen vacancies on as-synthesized TiO2 nanocrystal surfaces, as evidenced by our previous results [33]. Such a surface reduction reaction requires effective CO adsorption and stable O2 species on TiO2 and thus likely does not occur at high temperatures. Weak vibrational bands of CO2 adsorbed onto TiO2 were indeed observed during CO adsorption processes (Fig. S2). CO adsorption on various TiO2 nanocrystals without surface reduction was comprehensively studied by in situ DRIFTS spectra with CO pressures up to 650 Pa at 123–153 K (Figs. S3–S5). The CO-Ti(IV) vibrational features grow as the CO pressure increases and the adsorption temperature decreases. Integrated CO-Ti(IV) vibrational peak areas were plotted as a function of CO pressure
and adsorption temperature to establish the CO adsorption isotherms on various TiO2 nanocrystals (Fig. 2A2–C2). The isotherm curves at low CO coverages were found to be adequately fitted linearly (Fig. S6), from which the isosteric plots at an integrated CO-Ti (IV) vibrational peak area of 0.7 were derived (Fig. 2A3–C3). The isosteric plots could be adequately fitted linearly. From the slopes of fitted lines, the adsorption heat (DHads) of CO adsorption at Ti (IV) sites of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals was derived via Clausius–Clapeyron analysis [40] as 4.2 ± 0.2, 3.3 ± 0.3, and 3.2 ± 0.3 kJ/mol, respectively, which indicates that CO adsorption at the Ti(IV) site of anatase TiO2 nanocrystals is facet-dependent. TiO2{1 0 0} and TiO2{1 0 1} nanocrystals exhibit surface Ti(IV) sites with higher densities but slightly less CO binding strength than for TiO2{0 0 1} nanocrystals. The low adsorption heats of CO(a) on TiO2 surfaces are likely due to their large coverages under the adsorption conditions and subsequent strong repulsive interactions. Au/TiO2 catalysts with a calculated 0.45% Au/TiO2 weight ratio were prepared with TiO2 nanocrystals as supports, and ICP-AES analysis results show that the actual Au loading is 0.45, 0.42, and 0.40 wt% in Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. All Au/TiO2 catalysts display diffraction patterns of anatase TiO2 (JCPDS card No. 89-4921) in the XRD patterns (Fig. 3A) and no Au diffraction peaks, and XPS characterization results (Fig. 3B) show that all Au/TiO2 catalysts exhibit a single Au4f component with Au4f7/2 binding energy 83.0 eV, a typical value for metallic Au supported on TiO2 [34]. The Au/TiO2{0 0 1} catalyst exhibits a much lower Au4f peak intensity and thus a lower surface-to-bulk ratio of Au nanoparticles than the other two catalysts. Meanwhile, all Au/TiO2 catalysts exhibits a Ti2p3/2 binding energy of 458.4 eV and an O1s binding energy oft 529.6 eV arising from TiO2.
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Fig. 2. (A1–C1) In situ DRIFTS spectra of CO adsorption on TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals at 153, 173, 193, and 223 K, and then at 153 K again (P = 200 Pa). (A2–C2) DRIFTS isotherms of CO–Ti(IV) species TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals. (A3–C3) isosteric plots of CO–Ti(IV) species of TiO2{0 0 1}, TiO2{1 0 0}, and TiO2{1 0 1} nanocrystals. The dotted lines in figures (A2–C2) indicate CO coverage corresponding to an integrated CO vibrational peak area of 0.7, selected to derive the isosteric plots.
Fig. 1A2–C2 present representative high-angle annular dark field (HAADF)-STEM images and derived Au particle size distributions of various Au/TiO2 catalysts. Au nanoparticles mainly have spherical morphology and exhibit an average size distribution of 4.1 ± 1.3 nm in Au/TiO2{0 0 1}, 1.8 ± 1.1 nm in Au/TiO2 {1 0 0}, and 2.6 ± 1.3 nm in Au/TiO2{1 0 1}. Generally a strong metal–support interaction facilitates the formation of fine supported metal particles. Therefore, the Au–TiO2 interaction follows the order Au–TiO2{1 0 0} > Au–TiO2{1 0 1} > Au–TiO2{0 0 1}. Fig. 4A1–C1 show in situ DRIFTS spectra of CO adsorption on various Au/TiO2 catalysts with a CO pressure of 200 Pa at various temperatures. Besides CO-Ti(IV) species, CO adsorbed at Au0 sites (denoted as CO–Au0) with vibrational features of 2104/2109 cm 1 [41,42] was observed at 123 K; meanwhile, by comparing the I2130/2131cm13 CO/12CO ratio, the vibrational features 1/I2181cm-1 ratios with the of Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts at 2130/2131 cm 1 should be contributed largely by 13CO(a)–Ti(IV) species and to a lesser degree by CO adsorbed at Aud+ sites (denoted as CO–Aud+) [17]. The observations of CO–Aud+ species on Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts could be related to the presence of ultrafine supported Au nanoparticles whose lattice contracts and charge
at the Au atom site decreases relative to bulk Au [16,43]. The vibrational features of CO–Au0 and CO–Aud+ species do not change when the adsorption temperature increases to 153 K, while those of CO– Ti(IV) species weaken. This demonstrates that CO–Au0 and CO– Aud+ species are more stable than CO–Ti(IV) species. The vibrational features of CO–Ti(IV) species keep weakening with further increase of the adsorption temperature and finally disappear at 223 K. The vibrational features of CO–Ti(III) species resulting from surface reduction of TiO2 appear at 173 K and grow with increasing temperature; meanwhile, new vibrational features at 2070–2078 cm 1 that could be assigned to CO adsorbed at Aud sites (denoted as CO–Aud ) [41,42] emerge at the expense of those of CO–Au0 and CO–Aud+ species. These observations suggest that charge transfer occurs from reduced TiO2 surfaces to supported Au nanoparticles, forming the Aud species in Au/TiO2 catalysts. Such charge transfer decreases the number of Ti(III) sites on the reduced TiO2 surfaces of Au/TiO2, as evidenced by the intensity of CO–Ti(III) species. CO adsorption has been widely used to probe the Au species in supported Au catalysts, and our results for CO adsorption-induced change of Au speciation in Au/TiO2 demonstrate that CO adsorption should be carried out at temperatures
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Fig. 3. (A) XRD patterns and (B1) Au4f, (B2) Ti2p, and (B3) O1s XPS spectra of various Au/TiO2 catalysts.
Fig. 4. In situ DRIFTS spectra of CO adsorption at 123, 153, 173, 193, and 223 K, and then at 123 K again (P = 200 Pa), and a comparison of in situ DRIFTS spectra of CO adsorption at 123 K on fresh sample and the sample subjected to CO adsorption at 223 K on (A1, A2) Au/TiO2{0 0 1}, (B1, B2) Au/TiO2{1 0 0}, and (C1, C2) Au/TiO2{1 0 1} catalysts.
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low enough to prevent surface reduction in order to acquire reliable Au speciation. The vibrational feature of CO–Au0 species on Au/TiO2{0 0 1} catalysts significantly weaken at 223 K, while those on Au/TiO2{1 0 0} and Au/TiO2{1 0 1} do not change, indicating the weakest interaction of CO with Au0 sites of the Au/TiO2{0 0 1} catalyst. A noteworthy observation is that the spectrum region of CO–Au vibrational features exhibits a broad background signal whose intensity increases with CO adsorption temperature, which seriously hampers the analysis of vibrational features within this region. Such a broad background IR signal during CO adsorption on Au/TiO2 catalysts was previously observed and proposed to be related to the reversible partial reduction of TiO2 at the Au-TiO2 interface, which leads to an increase in surface disorder or a roughening of TiO2 particles and a subsequent decrease in IR transmission through the catalyst [44]. We found that the background spectra almost vanished when Au/TiO2 catalysts were subjected to CO adsorption at 223 K and then cooled down to 123 K again for CO adsorption (Fig. 4A2–C2). Such observations demonstrate that surface disorder or roughening of TiO2 particles by CO adsorption is sensitively temperature-dependent and facilitated at high temperatures. Inferred by the attenuation of CO–Ti(IV) vibration features shown in Fig. 4A2–C2, about 32.2, 42.9, and 28.1% of surface Ti(IV) is reduced by CO exposure up to 223 K on Au/ TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. The formation of Aud species from Au0 species on Au/ TiO2{0 0 1}, the formation of Aud and Au0 species from Aud+ species on Au/TiO2{1 0 0}, and the formation of Aud species from Aud+ species on Au/TiO2{1 0 1} can also be clearly identified. Employing the same approach as in TiO2, we measured the CO adsorption isotherms on various Au/TiO2 catalysts and analyzed the isosteric plots (Fig. S7), from which the adsorption heat (DHads) of CO adsorption on Ti(IV) sites of Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts was derived respectively as 4.5 ± 0.6, 3.1 ± 0.2, and 2.4 ± 0.2 kJ/mol. These data do not vary much from those on bare TiO2 nanocrystals. Catalytic activity of Au/TiO2 catalysts in CO oxidation was evaluated between 303 and 343 K. A Weisz–Prater analysis and a Mears analysis [45–47] in the Supporting Information demonstrate the absence of mass and heat transfer limitations under the adopted reaction conditions. The catalytic activity follows the order Au/TiO2{1 0 0} > Au/TiO2{1 0 1} Au/TiO2{0 0 1} (Fig. S8), consistent with previous results [35]. Based on the calculated Au numbers at the perimeter sites (Supporting Information and Table S1), the turnover frequency (TOF) at 70 °C is 2.0, 2.5, and 2.8 s 1 for Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} catalysts, respectively. The dual-perimeter-sites catalysis of Au/TiO2 catalysts occurs in CO oxidation only far below room temperature [2,22,23]. We thus examined dual-site CO oxidation on various Au/TiO2 catalysts below 213 K, employing in situ time-resolved DRIFTS spectra. Figs. 5–7 show DRIFTS spectra at various temperatures and reaction times during CO oxidation on Au/TiO2{0 0 1}, Au/TiO2{0 0 1}, and Au/TiO2{1 0 1} catalysts and derived temporal evolutions of integrated vibrational peak areas of observed CO–Ti(IV) and CO– Au species. Vibrational features of CO–Ti(III) and CO–Aud species were not observed, suggesting that the concentrations of surface Ti (III) sites on Au/TiO2 catalysts under the employed CO oxidation conditions should be too low to be probed by CO adsorption with DRIFTS. This could be attributed to the facile adsorption of oxygen at surface oxygen vacancy sites created by CO reduction. Similar results were also observed during CO oxidation on TiO2 nanocrystals (Fig. S9). As shown in Figs. 5–7, evolution of adsorbed CO species on Au/ TiO2 catalysts under CO oxidation conditions is complex. This is due to the dynamic nature of a chemisorption process initiated
by collisions of reactants on solid surfaces [48]. The CO adsorption process reaches an equilibrium quickly on bare TiO2 {0 0 1}, {1 0 0}, and {1 0 1} surfaces; in contrast, both the diffusion of adsorbed CO between Au and Ti sites and the oxidation of adsorbed CO occur on Au/TiO2{0 0 1}, Au/TiO2{1 0 0}, and Au/TiO2{1 0 1} surfaces, resulting in variation of adsorbed CO species at both Ti and Au sites prior to steady state. The CO–Ti(IV) species on TiO2 nanocrystals do not exhibit obvious reactivity to produce CO2 under the employed CO oxidation conditions (Fig. S9) and they remain on all TiO2 nanocrystal surfaces up to 193 K (Fig. 4); thus the variations of CO–Ti(IV) species on Au/TiO2 catalysts result from the CO adsorption process and CO migration from Ti to Au–TiO2 perimeter sites. CO oxidation occurs mainly at Au–TiO2 interfaces and the CO–Au species remain on all Au/TiO2 catalysts up to 223 K (Fig. 4); thus the variation of CO–Au species on Au/TiO2 catalysts results from the CO adsorption process, the CO migration process from Ti to Au, and the CO oxidation process. Meanwhile, O2 adsorption can alter the electronic structure of Au nanoparticles to enhance CO adsorption [49,50] and the decrease of CO partial pressure in the in situ DRIFTS batch reactor with CO oxidation also affects the CO adsorption behavior. On Au/TiO2{0 0 1} catalyst (Fig. 5), CO adsorption at the Ti(IV) sites quickly reaches steady state at 153 and 173 K, and few CO– Ti(IV) species could be observed at higher temperatures. CO adsorption at the Au sites quickly reaches steady state at 153 K but not at elevated temperatures. The CO–Au species grows with the reaction time at 173 and 193 K, initially increases, reaches a local maximum, and then decreases at 213 K. These observations suggest that CO oxidation should occur on Au/TiO2{0 0 1} catalyst only at 213 K. However, vibrational features of adsorbed CO2 were observed at temperatures up to 193 K but not at 213 K. Thus, the desorption of adsorbed CO2 seems to be the rate-limiting step in CO oxidation and effectively occurs only at 213 K. Similar results were also observed during the CO + O(a) reaction on Au(9 9 7) at low temperatures [51]. The inactivity of the Au/TiO2{0 0 1} catalyst in CO oxidation up to 193 K also indicates high coverage of CO–Au species and lack of enough coverage gradient between CO–Ti(IV) and CO–Au species to drive the migration of adsorbed CO from the Ti(IV) sites to the Au sites [52–54]. The observed growth of CO–Au species with time results from O2-adsorption-enhanced ability of Au nanoparticles to adsorb CO, while the observed later decrease at 213 K results from the decrease of CO partial pressure in the in situ DRIFTS batch reactor with CO oxidation proceeding. On Au/TiO2{1 0 0} catalyst (Fig. 6), the CO–Ti(IV) species initially does not change but then decreases with reaction time at 153 and 173 K, keeps decreasing at 193 K, and could not be observed at 213 K. The CO-Au species does not vary with the reaction time at 153 and 173 K, initially does not vary but then decreases at 193 K, and keeps decreasing at 213 K. Meanwhile, the adsorbed CO2 species initially forms at 153 K and then decreases, indicating the occurrence of CO oxidation and partial CO2 desorption. The adsorbed CO2 species decreases with the reaction temperature and becomes invisible at 213 K. Thus, the consumption of CO–Au species due to CO oxidation can be effectively compensated for by the migration of CO species from Ti sites to Au sites and the enhanced CO adsorption due to O2 coadsorption at 153 and 173 K, but cannot do so at 193 and 213 K due to the accelerated CO oxidation rate, the decreased gasphase CO pressure, and the decreased CO adsorption at the Ti sites and subsequent CO migration rate. The decrease of CO–Ti(IV) species is due to the migration to Au sites to participate CO oxidation and the decreased gas-phase CO pressure. On the Au/TiO2{1 0 1} catalyst (Fig. 7), the CO–Ti(IV) species initially does not change but then decreases with reaction time at 153 K, keeps decreasing at 173 and 193 K, and could not be observed at 213 K. The CO–Au species does not vary with the reac-
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Fig. 5. (A1–A4) The evolution of adsorbed CO and CO2 species during the CO oxidation on Au/TiO2{0 0 1} catalysts at 153, 173, 193, and 213 K. (B, C) The evolution of integrated peak areas of CO–Ti(IV) and CO–Au species on Au/TiO2{0 0 1} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
Fig. 6. (A1–A4) The evolution of adsorbed CO and CO2 species during CO oxidation on Au/TiO2{1 0 0} catalysts at 153, 173, 193, and 213 K. (B, C) The evolutions of integrated peaks area of CO–Ti(IV) and CO–Au species on Au/TiO2{1 0 0} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
tion time at 153 K and keeps decreasing at elevated temperatures. Meanwhile, the adsorbed CO2 species initially forms at 153 K and then decreases greatly, indicating the occurrence of CO oxidation and partial CO2 desorption. The adsorbed CO2 species decreases with the reaction temperature but is visible at 213 K without the presence of both CO–Ti and CO–Au species. This implies that adsorbed CO2 on Au surface at 213 K results to the adsorption of gas-phase CO2 produced by CO oxidation in the in situ DRIFTS reactor. Thus, the consumption of the CO–Au species due to CO oxidation can be effectively compensated for by the migration of the CO
species from Ti sites to Au sites and the enhanced CO adsorption due to O2 co-adsorption at 153 K, but not at elevated temperatures due to the accelerated CO oxidation rate, the decreased gas-phase CO pressure, and the decreased CO adsorption at the Ti sites and subsequent CO migration rate. The decrease of CO–Ti(IV) species is due to migration to Au sites to participate CO oxidation and the decreased gas-phase CO pressure. The above observations reveal that the dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation sensitively depends on TiO2 morphology and facets. Such depen-
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Fig. 7. (A1–A4) The evolution of adsorbed CO and CO2 species during CO oxidation on Au/TiO2{1 0 1} catalysts at 153, 173, 193, and 213 K. (B, C) The evolution of integrated peaks area of CO–Ti(IV) and CO–Au species on Au/TiO2{1 0 1} catalysts at the desired temperatures as a function of time during the initial process of CO oxidation.
dence mainly results from the coverage gradients between CO molecules adsorbed at Ti and Au sites required for driving efficient migration processes. TiO2 surfaces do not catalyze CO oxidation, and CO coverage on TiO2 surfaces is determined by CO adsorption. Our results show that CO adsorption on TiO2 surfaces effectively occurs up to 193 K; thus the contributions of CO molecules adsorbed on the TiO2 surface contribute to catalytic activity of Au/TiO2 catalysts only below 193 K. Au–TiO2 perimeter sites catalyze CO oxidation, and CO coverages on Au surfaces are determined by both CO adsorption and the oxidation reactivity of adsorbed CO. Au–TiO2 perimeter sites of Au/TiO2{1 0 0} and Au/ TiO2{1 0 1} catalysts are catalytically active in catalyzing CO oxidation even at 153 K; thus CO coverage on Au surfaces is below saturation, resulting in formations of large coverage gradients between CO molecules adsorbed at Ti and Au sites, sufficient to drive the migration of CO molecules adsorbed onto TiO2 to the Au–TiO2 perimeter sites to participate in CO oxidation. However, Au–TiO2 perimeter sites of the Au/TiO2{0 0 1} catalyst are poor in catalyzing CO oxidation up to 193 K; thus CO coverage on Au surfaces is almost saturating and there are few coverage gradients between CO molecules adsorbed at Ti and Au sites; CO molecules adsorbed onto TiO2 are not able to migrate to the Au–TiO2 perimeter sites to participate in CO oxidation. Besides the coverage gradients, the strength of adsorbed CO binding with TiO2 surfaces also affects the migration process. Among TiO2 {0 0 1}, {1 0 0}, and {1 0 1} facets, CO adsorption onto {0 0 1} facets exhibits the highest adsorption heat and thus the strongest interaction; thus CO adsorbed onto TiO2{0 0 1} facets should be less mobile than that on TiO2 {1 0 0} and {1 0 1} facets. These results greatly deepen fundamental understanding of the dual-perimeter-sites catalysis of Au/TiO2 catalysts in low-temperature CO oxidation.
catalysts in low-temperature CO oxidation. CO molecules adsorb onto TiO2 surfaces below 223 K and interact with TiO2{0 0 1} facets more strongly than TiO2 {1 0 0} and {1 0 1} facets. The migration of adsorbed CO molecules from TiO2 to the Au–TiO2 perimeter sites depends on the catalytic activity of the Au–TiO2 perimeter sites. Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts with catalytically active Au–TiO2 perimeter sites exhibit large coverage gradients between CO molecules adsorbed at Ti and Au sites, sufficient to drive the migration of CO molecules adsorbed onto TiO2 to the Au–TiO2 perimeter sites, while the Au/TiO2{0 0 1} catalyst with catalytically inactive Au–TiO2 perimeter sites does not. Thus, dual-perimeter-sites catalysis plays a role in Au/TiO2{1 0 0} and Au/TiO2{1 0 1} catalysts but not in the Au/TiO2{0 0 1} catalyst. Acknowledgments This work was financially supported by the National Key R & D Program of Ministry of Science and Technology of China (2017YFB0602205), the National Natural Science Foundation of China (21525313, 91745202), the Changjiang Scholars Program of Ministry of Education of China, the Fundamental Research Funds for the Central Universities of Ministry of Education of China (WK2060030017) and Collaborative Innovation Center of Suzhou Nano Science and Technology.
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2018.09.032. References
4. Conclusions In summary, employing various types of anatase TiO2 nanocrystals as supports, we have successfully demonstrated TiO2morphology-dependent dual-perimeter-sites catalysis of Au/TiO2
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