Effect of Pt Particle Size and Valence State on the Performance of Pt

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The Effect of Pt Particle Size and Valence State on the Performance of Pt/TiO Catalysts for CO Oxidation at Room Temperature 2

Geo Jong Kim, Dong Wook Kwon, and Sung Chang Hong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02945 • Publication Date (Web): 27 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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The Journal of Physical Chemistry

The Effect of Pt Particle Size and Valence State on the Performance of Pt/TiO2 Catalysts for CO Oxidation at Room Temperature Geo Jong Kim, Dong Wook Kwon, Sung Chang Hong* Department of Environmental Energy Engineering, Graduate School of Kyonggi University, 946 San, Iui-dong, Youngtong-ku, Suwon-si, Gyeonggi-do 443-760, Republic of Korea

ABSTRACT: A series of Pt/TiO2 catalysts with various Pt particle sizes and valence states were prepared and tested for CO oxidation at room temperature. Field-emission transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the activity of the Pt/TiO2 catalyst was influenced by the particle size and valence state of the catalyst particles. Excellent CO oxidation activity was observed at room temperature using highly dispersed, small metallic Pt particles. Increasing the Ptmetallic/Pttotal ratio resulted in an increase of turnover frequencies. According to the Fourier-transform infrared spectroscopy results, the linear CO species that was adsorbed on metallic Pt sites at 25 °C reacted with atmospheric O2 and was easily desorbed. However, linear CO species adsorbed on PtOx (x ≥ 2) sites was only desorbed at temperatures ≥ 100 °C, confirming the lack of CO oxidation activity at room temperature with ionic Pt catalysts.

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1. Introduction Catalytic oxidation of carbon oxide (CO) is a very important research area in the last two decades because of its role in various catalysis applications, such as air purification, three-way car exhaust treatment, and pollution control devices for reducing industrial and environmental emissions.1–3 Numerous studies on catalytic oxidation of CO have been conducted using noble metal catalysts such as Au, Pt, and Pd,4–9 as well as transition metals with low catalytic activity, such as Cu, Nb, and Co,10–13 In recent years, it has been reported that promotion effects are caused by adding transition metals such as Mo, Co, Mn and Ce in precious metal, Pt, catalyst.14– 17

Among various heterogeneous catalysts, noble metals possess excellent catalytic activity for

CO oxidation at low temperature, especially Au-based catalysts. However, they have decreased stability at low temperatures and in the presence of water vapor. Recent studies on CO oxidation using platinum as the active metal, accompanied by reducible oxides such as TiO2, have been conducted at relatively low temperatures (60–100 °C). In addition, according to Tomita et al., Pt/Fe-containing alumina catalysts prepared and treated with water under moderate conditions exhibited extremely high CO oxidation activity at low temperature. The presence of Fe had a large positive effect on the CO oxidation activity of the catalysts, presumably because the formation of effective contacts between the PtNPs and iron oxides was optimized after water-pretreatment. However, this paper was focused on evaluating activities of the catalysts and it did not discuss about the roles of active metal Pt, nor mention analysis on the characteristics of the catalysts.18 However, these Pt-based catalysts showed low activity at room temperature (25 °C). Moreover, investigations on the catalyst’s active site for CO oxidation are presently incomplete. 19,20


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In general, it has been reported that CO oxidation activity of noble metal catalysts is strongly dependent on the size of the active metals.22,23 It is believed that a particle size within the nanometer range (generally below 5 nm) is an important factor for high catalyst activity. Boubnov et al. performed a CO oxidation reaction by controlling the particle diameter of Pt, which is the active metal in a Pt/Al2O3 catalyst, in the range of 1–10 nm under various pretreatment conditions.24 They obtained excellent CO oxidation results with catalyst particles measuring 2–3 nm in diameter. However, it was also reported that CO oxidation typically occurs at relatively low temperatures (60–100 °C), with no mention of the factors influencing the efficiency of CO oxidation at room temperature. In addition, CO oxidation also depends on either the type of catalyst support used, or the interface between the active metal and catalyst support. The properties of the catalyst support are an important factor in facilitating the CO oxidation reaction.25,26 In addition, when noble metalbased catalysts are prepared using reducible oxides (TiO2, Fe2O3, or CeO2), it imparts a positive effect on the activity of the catalysts due to the strong metal-support interaction (SMSI) effect.27,28 Liu et al. reported that both Pt/FeOx and Pd/FeOx catalysts have excellent catalytic activity for CO oxidation, they were dependent on the magnitude of the SMSI effect and the particle size of the catalyst metal, also, large amount of oxygen absorbed onto FeOx support in the presence of Pt, Pd.8,21 However, their investigation focused on the role of the support (FeOx) instead of the active metal (Pt), which is influenced by the SMSI effect and having the right active metal properties could increase CO oxidation activity. Lastly, the oxidation state of the noble metal catalysts is a very important factor affecting CO oxidation activity. Seo et al. have examined the effect of Pt metal’s valence state on CO oxidation.29 It was shown that superior CO oxidation could be achieved using a Pt/TiO2 catalyst


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prepared by a reduction treatment. However, their report was limited to an investigation of the reduction temperature. In addition, they did not correlate the efficiency of the CO oxidation reaction to the properties of the metallic Pt species. In this study, the reaction of CO oxidation at room temperature was evaluated using Pt/TiO2 catalysts, prepared by a heat treatment process at various temperatures. We investigated the influence of the SMSI effect on Pt particle size and the effect of heat treatment on Pt valence state. We also studied the relationship between Pt particle size and valence state of the catalysts, and their effect on the CO oxidation reaction. In addition, we analyzed the surface characteristics of the catalysts to determine the key factors affecting the CO oxidation reaction.

2. Experiments 2.1 Catalyst preparation. The catalyst used in this study was a Pt/TiO2 catalyst, which was manufactured by loading the platinum using commercialized TiO2 supports, which is commercial anatase TiO2 supplied by Cristal global Co. (Cristal global Co., 100% anatase phase, surface area at BET = 288 m2 g-1). The Pt/TiO2 catalyst, which was labeled “C,” was calcined at 400 °C in an air atmosphere and subsequently, the catalyst “(R)-xoo” was prepared by reducing the calcined catalyst using H2 (H2/N2=30/70 as vol.%) gas at 300–700 °C for 1 h. Here, xoo indicates the temperature of reduction treatment. First, the platinum content in TiO2 was calculated based on the desired composition; then, this amount of platinum hydroxide [Pt(OH)2; SNS Co.] was dissolved in distilled water that was heated to 60 °C in order to increase the solubility of the metal. Secondly, the calculated amount of TiO2 was slowly added to the solution with stirring. This slurry mixture was then stirred for more than an hour, followed by an evaporation process at 70 °C using a rotary vacuum evaporator (Eyela Co. N-N series). Thereafter, the solid was dried at 103 °C for 24 h and the catalyst was calcined at 400 °C for 4 h in an air atmosphere. Finally, the reduced catalyst was


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obtained by reducing the calcined catalyst at 300–700 °C using 30 vol% H2 gas (mixed with N2 gas) for 1 h. 2.2 Catalytic acivity measurement. A fixed-bed reactor was used in the investigation of CO oxidation at room temperature. The gas inlet was fabricated using a stainless steel tube wrapped with a heating band and maintained at a constant temperature of 80 °C. The reactor was a continuous-flow fixed bed reactor made of a quartz tube (6-8 mm in internal diameter and 600 mm in height) and quartz wool was used to fix the catalyst bed. The temperature of the reactor was controlled by filling the outside layer of the reactor with water, and a K-type thermocouple was installed in contact with the catalyst bed to measure the operating temperature. In addition, moisture was supplied by introducing moistened N2 gas using a bubbler, and water was circulated at a constant temperature (40 °C) using a circulator placed outside of the bubbler (i.e. a double-jacket design) in order to maintain a constant supply rate. The relative humidity (RH) was between 45% and 55%, measured using a humidity equipment temperature meter (CENTER 310 series, Center Technology Co.). A non-dispersive infrared gas analyzer (ZKJ-2, Fuji Electric Co.) was used to measure the respective concentrations of the unreacted CO (reactant) and the oxidized CO2 (product). Catalytic activity measurements were carried out in a fixed-bed reactor at atmospheric pressure with 0.25–0.125 g of catalyst (40–50 mesh). Subsequently, gas mixtures containing 500 ppm CO and 21 vol% O2 mixed with N2, or 1,000 ppm CO and 21 vol% O2 mixed with N2, were fed into the reactor with a space velocity (SV) range of 60,000–120,000 cc·g-1cat·h-1. 2.3 Catalyst characterizations. The catalysts were characterized in terms of their dispersion and crystalline size by CO chemisorption at 25 °C. The catalyst sample, which was activated in a H2 gas (H2/N2=10/90 as vol.%) flow at 250 °C for 30 min, was cooled to 25 °C and saturated with pulses of CO (CO/He = 10/90 as vol.%).


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Field-emission transmission electron microscopy (FE-TEM) images were recorded on a JEM2100F microscope (JEOL Co.) operating at 200 kV. Samples for the FE-TEM measurements were prepared by suspending ultrasonically treated catalyst powder in ethanol and placing a drop of the suspension on the Cu grid. X-ray photoelectron spectroscopy (XPS) analysis was conducted using the ESCALAB 210 spectrometer purchased from VG Scientific and Al Kalpha wavelength (1486.6 eV) was used as the excitation source. After the catalysts were dried at 100 °C for 24 h to remove the moisture content, they were analyzed without surface sputtering and etching in order to maintain a vacuum pressure of 10-12 mmHg in the XPS instrument. The binding energy and intensity of Ti, O, Pt, and C in the specimens were analyzed through wide spectrum scanning. In-situ diffuse reflectance infrared (IR) spectroscopy (DRIFTS) analysis used in this study was performed with FT-IR 660 Plus (JASCO Co.) and a Diffuse Reflectance (DR) 400 accessory was used for solid reflectance analysis. A CaF2 window was used as a plate for DR measurement and the spectra were collected using a Mercury Cadmium Telluride (MCT) detector. Using a rod, all of the catalysts used in the analysis were ground in the sample pan of an in situ chamber with a temperature controller installed. To exclude the influence of moisture and impurities, a sample was pre-processed with Ar at a flow rate of 50 cc·min-1 at 150 °C for 1 h, and maintained in a vacuum state using a vacuum pump. In order to collect the spectra of the catalysts, a single-beam spectrum of the pre-processed sample was measured as a background, and all analyses were performed via auto scanning at a resolution of 4 cm-1. CO temperature programed oxidation (TPO) measurements were performed using 30 mg of the catalyst sample with a total flow of 50 cc·min-1. Before the TPO measurements, the catalyst was pre-treated with 5 vol% H2 gas (mixed with N2) at 300 °C for 0.5 h and subsequently, cooled


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to 25 °C. The samples were then treated with 1 vol% CO mixed with N2 for 0.5 h. Adsorbed CO was desorbed using N2 gas flow for at least an hour before starting the TPO experiments. During the TPO experiments, the evolution of CO2 gas (m/z=44) was continuously monitored using a quadrupole mass spectrometer (QMS 422, Pfeiffer-Vacuum, Germany) while increasing the reaction temperature to 450 °C at a rate of 10 °C min-1 under 5 vol% O2 gas mixed with N2.

3. Results and Discussion 3.1. Characterization of Pt/TiO2 catalysts. FE-TEM images of the Pt/TiO2 catalysts are shown in Fig. 1. The heat treatment of the catalysts produced different particle sizes. In the case of the Pt/TiO2 (C) catalyst (heat-treated at 400 °C in air atmosphere), the average particle size was 16.7 nm, which was the largest of those measured. On the other hand, the average particle size of the Pt/TiO2 (R)-600 catalyst (heat-treated at 600 °C in H2 atmosphere) was only 2.2 nm, which was the smallest measurement obtained. The particle size of Pt decreases with an increasing reduction temperature used in the catalytic reaction. However, in the case of the Pt/TiO2 (R)-700 catalyst, a large average particle size (4.4 nm) was formed. We assumed that this was due to particle agglomeration caused by thermal shock at an extremely high temperature. Kim et al. reported that a Pt peak could be observed in the XRD pattern of a Pt/TiO2 catalyst that was prepared via heat treatment at temperatures ≥ 700 °C; characterized by the agglomeration of the active Pt metal.30 Their observation is confirmed by the FE-TEM analysis presented in this study. Subsequently, we examined the active particle diameter and metal dispersion by CO chemisorption analysis and the results are shown in Table 1. The active particle diameter obtained by the CO chemisorption analysis was similar to the FE-TEM results. The active


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particle diameter of the reduced catalyst was smaller compared to that of the calcined catalyst. In addition, when the reduction temperature was increased, the active particle size was reduced. Metal dispersion of Pt/TiO2 (C) and Pt/TiO2 (R)-600 catalysts were 7.17% (lowest) and 33.38% (highest), respectively. At various reduction temperatures, the order of metal dispersion is as follows: 700 < 300 < 400 < 500 < 600 °C. In particular, when the catalysts were prepared at 500 and 600 °C, the corresponding metal dispersion was rapidly increased to 31.49% and 33.38%. In general, if the Pt/TiO2 catalyst was reduced at temperatures ≥ 500 °C, the shape of Pt particles changed depending on the H2 atmosphere, and the occurrence of the SMSI effect could be detected.32 According to Kim et al., TiO2 could be reduced and was shown to produce the SMSI effect when used together with Pt.31,33,34 The SMSI effect, which is caused by a strong interaction between the reduced Ti3+ sites and the metallic Pt, could produce reduction sites on the TiO2 support. It was reported that the metal was widely dispersed, and the activity of the catalyst was increased if the affinity of the metal and the support was strong.30-34 Work by Huizinga and Prins37 as well as Baker et al.38 showed that the formation of suboxides of titania in the vicinity of supported metal clusters during high temperature reduction causes spreading of the metal. Hunag and Leung39 showed that the compared to the 1% Pt/MgO, the 1% Pt/TiO2 possessed lower BET surface area but better Pt dispersion probably due to its strong metal-support interaction (SMSI).” In the case of the Pt/TiO2 (R)-500 and Pt/TiO2 (R)-600 catalysts (prepared at temperatures ≥ 500 °C), the active Pt metal particles were highly dispersed due to the SMSI effect. On the other hand, active metal agglomeration was caused by high temperature in Pt/TiO2 (R)-700. Active metal agglomeration resulted in decreases in metal dispersion and increases in active particle diameter.


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To elucidate the different oxidation states of the Pt metal that was deposited on the TiO2 support, the XP spectra for Pt 4f photoelectrons are shown in Fig. 2. In Fig. 2a, the binding energy of Pt’s 4f photoelectrons is located at 72.64 eV, and it is deconvoluted into two peaks with binding energies at 72.64 eV and 74.24 eV. The peak at 72.64 eV is attributed to Pt2+ because of the Pt(OH)2 species, whereas the peak at 74.24 eV is attributed to the PtO2 species.40,41 Thus, for the Pt/TiO2 (C) catalyst, the valence state of the active Pt metal exists as Pt4+ (74.24 eV) and Pt2+ (72.64 eV). On the other hand, Figs. 2b and 2c show a major Pt 4f state at 70.9 eV, together with other Pt 4f states at 71.9–72.1 eV and 73.9–74.1 eV. The peak at 70.9 eV is attributed to Pt0 because of the metallic Pt species. Metallic Pt’s 4f binding energy is well known at 70.9–71.2 eV. Thus, as the Pt valence state changes during the reduction process, it was partially reduced to metallic Pt. Figs. 2d and 2e show Pt/TiO2 (R)-500 and Pt/TiO2 (R)-600 with a major Pt 4f state at 70.6–70.9 eV, along with other states at 71.9–72.1 eV and 73.9–74.1 eV. Pt/TiO2 (R)-500 and Pt/TiO2 (R)600 catalysts were negatively shifted to a lower binding energy by approximately 0.3–0.5 eV compared to metallic Pt, whose typical Pt 4f7/2 binding energy is around 70.9 eV. This phenomenon could be attributed to the SMSI effect formed between the reducible TiO2 support and the active Pt metal.42–44 Since the electronic density of the Pt metal increases with the appearance of the SMSI effect, the XPS peaks of Pt were moved to lower energy levels; showing peaks at 70.6 – 70.9 eV. Thus, for the Pt/TiO2 (R)-600 catalyst, the Pt valence state of the active metal was moved to lower energy levels, resulting in Pt2+ (72.64 eV) and metallic Pt (70.62 eV). In the case of the Pt/TiO2 700 catalyst (Fig. 2f), a major Pt 4f state was detected at 70.9 eV, together with another state at 72.3 eV. Even though the Pt/TiO2 (R)-700 catalyst was reduced at temperatures ≥ 500 °C, the XPS peaks were not moved to lower energy levels. This is possibly


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caused by a weak SMSI effect between Pt and TiO2, due to the agglomeration of adjacent Pt particles during the high temperature reduction process. The oxidation state percentage of Pt (according to the XPS analysis) is shown in Table 2. The table shows the % area occupied by each peak. As the Pt4+ and Pt2+ valence states gradually decreased, the peak of metallic Pt increased accordingly. Pt/TiO2 (R)-300, which was reduced at a relatively low temperature, has the lowest Ptmetallic/Pttotal ratio at 22%. It was thought that the reduction process could not to be easily achieved at 300 °C as the calcination step (i.e. performed prior to reduction) was performed at 400 °C in an air atmosphere. When the reduction temperature was increased, the Ptmetallic/Pttotal ratio also increased according to the area of metallic Pt. In the case of Pt/TiO2 (R)-700, the Ptmetallic/Pttotal ratio was recorded at 67.6%. 3.2. Catalytic activity test. In this study, Pt/TiO2 (C) and Pt/TiO2 (R) catalysts were prepared via a pre-treatment process under calcination and reduction conditions. We performed CO oxidation activity experiments using catalysts prepared at various thermal treatment conditions. The results are shown in Figure 3. “The Pt/TiO2 (R)-600 catalyst showed 100% CO conversion ratio in the SV range of 60,000– 90,000h-1 and 70% CO conversion ratio at an SV of 120,000h-1.” On the other hand, the activity of the Pt/TiO2 “C” catalyst was not evident across the entire SV range tested. Based on the results discussed in section 3.1, two different properties were observed in the Pt/TiO2 “C” and Pt/TiO2 “R” catalysts. Firstly, the physical properties of the active Pt metal, such as its metal dispersion value and the active particle diameter, were changed after the high temperature heat treatment. For noble metal catalysts, the importance of metal dispersion and the active particle diameter has been reported by many previous studies.22–24 Kung et al. claimed that


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smaller active particle diameter was more favorable towards CO oxidation reaction at room temperature because small Au particles have a higher dispersion of the active ingredient than that of larger particles.46 According to the result in this study, the metal dispersion of the Pt/TiO2 “C” catalyst is a low 7.2%, with a large active particle diameter of 16.7 nm. Thus, its physical properties are not appropriate for CO oxidation at room temperature. On the contrary, the Pt/TiO2 “R” catalyst has a high metal dispersion at 33.4% with an active particle diameter of 2.2 nm, resulting in physical properties that are appropriate for CO oxidation at room temperature. Secondly, there was a difference in the valence state of the active Pt metal. The results of the XPS analysis showed that the valence state of Pt in the Pt/TiO2 “C” catalyst is Pt2+ and Pt4+. Kim et al. reported that an efficient formaldehyde oxidation reaction was achieved at room temperature because of the metallic Pt’s valence state.31 The Pt/TiO2 “R” catalyst possesses highly dispersed active particles with a small diameter, in addition to having a metallic Pt’s valence state. Thus, it can be used to achieve an efficient CO oxidation reaction at room temperature. Subsequently, Pt/TiO2 (R)-300, 400, 500, 600, and 700 catalysts were prepared at various temperatures to control the metal dispersion, active particle diameter, and valence state of the Pt metal, all of which are factors affecting the efficiency of the Pt/TiO2 catalysts in an oxidation reaction. The catalytic activity results of the aforementioned catalysts are shown in Fig. 4. The CO conversion gradually improved as the reduction temperature was increased to 600 °C. The Pt/TiO2 (R)-600 catalyst showed 100% reaction activity at the SV of 60,000 and 90,000 h-1. However, the Pt/TiO2 (R)-700 catalyst showed only 90% reaction activity at the SV of 60,000 h1

, which was lower than that of the Pt/TiO2 (R)-600 catalyst.


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In order to establish a relationship between the Pt valence state and the corresponding CO oxidation activity, the ratio of metallic Pt to total Pt (Ptmetallic/Pttotal) is shown in Fig. 5. The Ptmetallic/Pttotal ratio and CO oxidation activity increased as the reduction temperature went up to 600 °C. Thus, it was postulated that the Ptmetallic/Pttotal ratio is correlated to the oxidation efficiency. The reduction of the Pt/TiO2 (R)-700 catalyst at a high temperature resulted in the highest Ptmetallic/Pttotal ratio of 67.6%. However, it showed a lower CO oxidation activity than that of the Pt/TiO2 (R)-600 catalyst (with a Ptmetallic/Pttotal ratio of 57.8%). The high temperature heat treatment of the Pt/TiO2 (R)-700 catalyst led to a high Ptmetallic/Pttotal ratio. At the same time, the metal dispersion value was recorded at 14.6% and the active particle diameter was measured at 7.7 nm due to the agglomeration of active Pt metal particles. In the case of the Pt/TiO2 (R)-600 catalyst, which showed the highest CO oxidation activity, the metal dispersion value was recorded at 33.4%, with an active particle diameter of 3.39 nm. The Pt/TiO2 (R)-600 catalyst particles were highly dispersed and the active metal particles were smaller compared to the Pt/TiO2 (R)-700 catalyst particles. Thus, reaction activity of the CO oxidation was excellent with higher ratio of Ptmetallic/Pttotal. In addition, active metal was highly dispersed. With smaller particle size, the reaction activity was excellent. In addition, the aforementioned results could be further explained through the TOF of the catalysts. Reaction rate and TOF were measured with CO injection concentration of 1,500 ppm at a total flow rate of 500 mL min-1. The conversion rate of CO oxidation was kept between 3 and 12%. The reaction rate and TOF results of the catalysts are shown in Fig. 6 and Table 3. The reaction rate and TOF results of the catalysts are shown in Fig. 6 and Table 3. Reaction rates were calculated using the following equation:


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rCO =

(X × FCO ) W

(1)

Where r is the reactions rate of CO (mol·s-1·g-1cat), FCO is the total flow rate of CO (mol·s-1), W is the mass of catalyst (g), and X is the conversion of CO. These results, along with the metal dispersion measurements, were used to calculate the turnover frequencies (TOFs) of CO, which is defined as the moles of CO converted per surface metal atom per second (s−1):

TOFs =

rCO × AB M D × Xm

(2)

Where ABM represents the atomic weight of metal M, XM is the metal content (gmet/gcat), and D is the metal dispersion.30 CO oxidation activities of catalysts were observed in the following order: Pt/TiO2 (R)600>700>500>400>300. TOFs were observed in the following order: 700>600>500>400>300. With increasing ratio of Ptmetallic/Pttotal, TOF increased. Pt/TiO2 (R)-700 catalyst with the highest ratio of Ptmetallic/Pttotal treated the greatest number of moles in CO per active site. However, due to low metal dispersion, CO oxidation activity was lower than that of Pt/TiO2 (R)-600 catalyst. In other words, CO oxidation activity of Pt/TiO2 catalyst was high with high ratio of Pt metallic/Pttotal and high metal dispersion of Pt. 3.3. The effect of metallic Pt on CO oxidation acitivity using Pt/TiO2 catalysts. The active site of the catalysts has a high Ptmetallic/Pttotal ratio and the active catalysts exist as highly dispersed small particles. To identify how the presence of metallic Pt affects the reaction characteristics of the catalysts, we examined the adsorption and desorption of CO at room temperature using FT-IR analysis. We studied the changes occurring on the surface of the catalysts by injecting 100 ppm of CO into a reaction chamber containing 1 wt% of Pt/TiO2 “R”-


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600 catalyst at room temperature (25 °C) for 30 min. Subsequently, the adsorbed CO species was desorbed using N2 for 10 min, followed by an injection of 21 vol% O2 for 20 minutes. The FT-IR results are shown in Fig 7. Bourace et al. examined the adsorption and desorption of CO on the surface of a Pt/Al2O3 catalyst via FT-IR analysis, and reported that the linear CO species was absorbed on either the metallic Pt or Pt2+, while the bridged CO species was adsorbed in between the two metallic species.45 Representative adsorption peaks of the CO species were observed at 2130–2110 cm-1, 2100–2050 cm-1, and 1850–1840 cm-1. The peak between 2130 and 2110 cm-1 represented the linear CO species, which was adsorbed on Pt2+, whereas the peak between 2100 and 2050 cm-1 represented the linear CO species, which was adsorbed on metallic Pt. Meanwhile, the peak between 1850 and 1840 cm-1 represented the bridged CO species. According to the results in Fig. 7, the linear CO species (resulting from 100 ppm of injected CO) was adsorbed on Pt2+ at 2110 cm-1 and on metallic Pt at 2080 cm-1, 2070 cm-1, and 2050 cm-1. In addition, the peak observed at 1840 cm-1 confirmed the presence of the bridged CO species. After undergoing 10 min of desorption using N2, the peak of the bridged CO species at 1840 cm-1 disappeared, confirming that the bridged CO species was indeed physically adsorbed on the surface of catalyst. As the binding strength was weak, it could be easily desorbed by N2 at room temperature. The linear CO species adsorbed on most of the metallic Pt species was desorbed by injecting 21 vol% of O2 mixed with N2 for 20 min, resulting in a disappearance of the linear CO peak. However, the linear CO species adsorbed on Pt2+ was not desorbed after O2 was injected for 20 min, and the linear CO peak did not disappear. Through the presence of a small peak at 2060 cm1

, we detected the presence of a trace amount of the linear CO species adsorbed on metallic Pt.


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We also observed the CO adsorption state on the surface of the catalysts by increasing the reaction temperature during an injection of 20 vol% O2 mixed with N2, in order to identify the desorption characteristics of the CO species that was adsorbed on Pt2+. The results are shown in Fig. 8. The linear CO species (2060 cm-1) that was absorbed on metallic Pt disappeared because it was desorbed at 100 °C. In the case of the linear CO species (2119 cm-1 and 2105 cm-1) that was adsorbed on Pt2+, the peak area gradually decreased as the temperature fell to 200 °C. At 300 °C, the desorption process was complete and no peaks were observed. In the case of the linear CO species that was adsorbed on Pt2+, the desorption process occurred at temperatures ≥ 100 °C, indicating that the adsorption site did not participate in the CO oxidation reaction at room temperature. Fig. 9 shows the CO-TPO results, which confirmed the importance of the metallic Pt species as per previous FT-IR results. The CO-TPO results showed that the linear CO species that was adsorbed on the metallic Pt interacted with O2, leading to the evolution of CO2 as O2 was injected, as shown by the largest peak. In addition, when compared with the FT-IR results, the linear CO species (adsorbed on metallic Pt) was desorbed at 100 °C and CO2 was generated. CO was converted into CO2 as the linear CO species (adsorbed on Pt2+) interacted with O2 at temperatures ≥ 100 °C. As the results of FT-IR and CO-TPO, only Linear CO species absorbed on metallic Pt was desorbed by reacting with atmospheric O2 after it was absorbed on the surface of catalysts. It indicated that it acted as the active site on CO oxidation. As the results of TOF in Fig. 6, it was consistent with results in which TOF increased with increasing Ptmetallic/Pttotal ratio. On the other hand, Linear CO species absorbed on Pt2+ did not react with atmospheric O2 at room temperature. This result is evidence supporting the differences in CO oxidation activity in section


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3.2. Pt/TiO2 (C) catalyst, the valence state of active Pt metal existed as Pt4+ and Pt2+. For this reason, CO oxidation activity was not shown in Pt/TiO2 (C) catalyst. In other words, Pt investigated as the active site of CO oxidation when it existed as metallic Pt. When metallic Pt was highly dispersed, the highest activity was shown.

4. Conclusions In this study, Pt/TiO2 catalysts were prepared at various thermal treatment and they showed a variety of catalytic activities. For catalysts that were prepared at reduction temperatures ≥ 500 °C, the active Pt metal particles are highly dispersed on the surface of the catalyst support due to the SMSI effect. As the ionic Pt species was reduced to the metallic form, excellent CO oxidation and high TOFs were observed at room temperature. CO oxidation DRIFT was examined at room temperature using FT-IR analysis. Linear CO species adsorbed on metallic Pt was desorbed at 25 °C but linear CO species adsorbed on PtOx was unreactive at 25 °C and was still adsorbed on the surface of the catalyst. Based on the results of the TPO analysis, linear CO species adsorbed on PtOx was only desorbed at temperatures ≥ 100 °C and it was readily converted to CO2. Thus, it was concluded that the active site for the CO oxidation reaction at room temperature is on the metallic Pt.

FIGURES Figure 1. FE-TEM analysis of the Pt/TiO2 catalysts pre-treated at various temperatures. (a) Pt/TiO2 (C), (b) Pt/TiO2 (R)-300, Pt/TiO2 (R)-400, Pt/TiO2 (R)-500, Pt/TiO2 (R)-600, Pt/TiO2 (R)-700.


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Figure 2. Pt 4f spectra of the Pt/TiO2 catalysts by XPS analysis. (a) Pt/TiO2 (C), (b) Pt/TiO2 (R)300, Pt/TiO2 (R)-400, Pt/TiO2 (R)-500, Pt/TiO2 (R)-600, Pt/TiO2 (R)-700. Figure 3. Catalytic activities of the Pt/TiO2 catalysts under different pre-treatment conditions. Inlet 500 ppm CO, 21 vol% O2 mixed with N2, RH: 55%, room temperature (25 °C), SV 60,000– 120,000 h-1. Figure 4. Catalytic activities of the Pt/TiO2 catalysts at various temperatures. Inlet 500 ppm CO, 21 vol% O2 mixed with N2, RH: 55%, room temperature (25 °C), SV 60,000 – 120,000 h-1. Figure 5. The correlation between the Ptmetallic/Pttotal ratio and the activities of catalysts. Figure 6. The correlation between the Ptmetallic/Pttotal ratio and the TOF of catalysts. Figure 7. Dynamic changes of in-situ FT-IR spectra of the Pt/TiO2 (R)-600 catalyst as function of time at room temperature. (a) CO adsorption under 1 vol% CO mixed with N2, and (b) CO desorption under N2 or 21 vol% O2 mixed with N2. Figure 8. Dynamic changes of in-situ FT-IR spectra of the Pt/TiO2 (R)-600 catalyst as function of temperature in a gas flow of 21 vol% O2 mixed with N2. Figure 9. Response profiles of the Pt/TiO2 (R)-600 catalyst from a CO-TPO experiment.


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Figure 1.


18

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Figure 2.


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Figure 3.


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Figure 4.


21


600

100

700 80

CO conversion (%)

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

Page 22 of 37

500

60

40

400

20 300 0 10

20

30

40

50

60

70

80

Ptmetallic/Pttotal (%) Figure 5.


22

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0.0060 0.0055 0.0050 0.0045 -1

TOFs (s )

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


0.0040 0.0035 0.0030 0.0025 0.0020 0.0015 10

20

30

40

50

60

70

80

PtMetallic/Pttotal (%)

Figure 6.


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Figure 7.


24

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Figure 8.


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Figure 9.


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TABLES Table 1. CO chemisorption properties of 1 wt% Pt/TiO2 catalysts pre-treated various temperatures Pt loading Catalyst

(%)a

Metal dispersion (%)b

Active particle diameter (nm)b

Active particle diameter (nm)c

Pt/TiO2 (C)

1.03

7.17

15.8

16.7

Pt/TiO2 (R)-300

1.02

18.7

6.1

5.9

Pt/TiO2 (R)-400

1.03

19.5

5.8

4.7

Pt/TiO2 (R)-500

1.03

31.5

3.6

2.7

Pt/TiO2 (R)-600

1.02

33.4

3.4

2.2

Pt/TiO2 (R)-700

1.04

14.6

7.7

4.4

a

Measured by ICP analysis. Measured by CO chemisorption. c The average Pt particle diameter calculated from the TEM results.

b


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Table 2. The percentage of area between Pt4+, Pt2+, and metallic Pt in the Pt 4f state calculated by XPS analysis Pt valence state

Pt/TiO2 (C)

Pt/TiO2 (R)-300

Pt/TiO2 (R)-400

Pt/TiO2 (R)-500

Pt/TiO2 (R)-600

Pt/TiO2 (R)-700

Pt2+ + Pt4+

Atom

554

1014

703

660

481

717

Metallic Pt

Atom

-

297

461

536

659

1498

Pt4+/Pttotal

Area %

17.0

36.1

26.5

21.7

16.1

1.1

Pt2+/Pttotal

Area %

83.0

41.3

33.9

33.5

26.1

31.3

Ptmetallic/Pttotal

Area %

22.6

39.6

44.8

57.8

67.6


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Table 3. A summary of the CO oxidation results using the Pt/TiO2 catalysts at 25 °C Catalyst 1% Pt/TiO2 (R)-300 1% Pt/TiO2 (R)-400 1% Pt/TiO2 (R)-500 1% Pt/TiO2 (R)-600 1% Pt/TiO2 (R)-700 a

CO conversion (%) a 4.7 5.2 9.2 12.4 9.5

Reaction rate of CO (mol·s-1·g-1cat.)*105 a 1.39 1.55 2.74 3.69 2.83

TOFs (s-1)*103 a 2.08 2.21 2.44 3.1 5.41

Reaction condition: 25 °C, 1,500 ppm CO, 21 vol. % O2 balanced with N2, 55% R.H., SV 120,000 h-1


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AUTHOR INFORMATION Corresponding Author *

Corresponding

author

phone:

+82-31-249-9733;

fax:

+82-31-254-4941;

e-mail:

[email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This research was supported by a grant (code #: S2047192) from the ‘Small and Medium Business Innovation Development Project’ of the Small and Medium Business Administration, Korea. REFERENCES (1) Schyer, D.R.; Upchurch, B.T.; Sidney, B.D. A Proposed Mechanism for Pt/SnOx-

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Catalysts Prepared from Au Phosphine Complexes and As-precipitated Metal Hydroxides: Characterization and Low-temperature CO Oxidation. J. Catal. 1997, 170, 191–199.


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TOC graphic


37

Effect of Pt Particle Size and Valence State on the Performance of Pt

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