TiO2 Induced by Mild HCHO

Sep 24, 2014 - Sang Wook Han , Myung-Geun Jeong , Il Hee Kim , Hyun Ook Seo ... of strong metal-support interaction: state-of-the-art and what's the n...
22 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/IECR

Strong Metal-Support Interaction in Pt/TiO2 Induced by Mild HCHO and NaBH4 Solution Reduction and Its Effect on Catalytic Toluene Combustion Zebao Rui, Lingye Chen, Huayao Chen, and Hongbing Ji* Department of Chemical Engineering, School of Chemistry & Chemical Engineering, and The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou, 510275 Guangdong, P. R. China S Supporting Information *

ABSTRACT: Strong metal-support interaction (SMSI) in titania supported noble metals has been a subject of many studies due to its importance to many fields of science, in particular to material science and catalysis system. H2 reduction at a high temperature has been commonly considered as the inducement to SMSI in TiO2 supported noble metals. This work, however, demonstrates that SMSI in Pt/TiO2 can occur through mild NaBH4 and HCHO solution reduction processes based on CO chemisorption, transmission electron microscopy, and X-ray photoelectron spectroscopy characterizations. Moreover, the effect of TiO2 crystalline forms on the degree of SMSI in NaBH4 reduced Pt/TiO2 and the performance of the as-reduced catalysts for trace toluene combustion reaction were studied. It was found that the degree of SMSI in Pt/TiO2 drew a significant effect on the catalytic performance. Our discovery provides a new way to control the interaction between noble metals and the TiO2 support as well as their catalytic activities.

1. INTRODUCTION Strong metal-support interaction (SMSI) in titania supported noble metals has been a subject of many studies since the discovery by Tauster et al.1 due to its importance to many fields of science, in particular to material science and catalysis system.1−7 Tauster et al.1 found that when the Group VIII noble metals supported on TiO2 were reduced at temperatures in excess of 775 K complete suppression of hydrogen and carbon monoxide chemisorption occurred, a result which was attributed to SMSI. H2 reduction at a high temperature has been commonly considered as the inducement to SMSI in TiO2 supported noble metals,2−7 which generates oxygen vacancies in the form of coordinative unsaturated cations in the vicinity of active metals and leads to SMSI.2−6 SMSI relates to the catalytic activity and stability of TiO2 supported noble metals.5−7 Kim et al.7 used TiO2-supported Pd−Cu catalysts of different anatase/rutile phase compositions in a nitrate reduction in water. The observed trend of the catalytic activity was correlated to the degree of SMSI over the catalysts. H2 reduction at a high temperature, however, may lead to the sintering of TiO2 support, the phase transition of TiO2 support, and the coarsening of the noble metal particles to some extent.8−10 Moreover, it is difficult to control the noble metal particle size during the high temperature H2 reduction process. Recently, a mild reduction process in a NaBH4 or HCHO solution for the supported metal catalysts attracts increasing attention and shows impressive advantages over a high temperature H2 reduction process.10−12 Regarding the reason for the catalytic performance enhancement with a mild reduction process, different explanations were considered by different groups. Huang et al.10 studied the catalytic oxidation of HCHO over Pt/TiO2 at ambient temperatures. They proposed that well-dispersed and negatively charged metallic Pt nanoparticles and rich chemisorbed oxygen were probably © 2014 American Chemical Society

responsible for the high catalytic activities of the NaBH4 reduced Pt/TiO2 catalysts. Zhang et al.12 attributed the high activity of the NaBH4-reduced Pt/TiO2 samples to the effect of sodium ions. However, few studies focus on SMSI in TiO2 supported noble metals induced by the mild reduction process, which is well-known to relate with the performance of the catalyst. Understanding the intrinsic factors contributing to the enhanced catalytic activity is critical to future TiO2 supported metal catalyst design. Thus, it is necessary to figure out whether SMSI occurs in TiO2 supported noble metals through a mild reduction process and the relationship between the catalytic activities with the degree of SMSI. In this work, Pt/TiO2 with different TiO2 phase forms, i.e., anatase (A-TiO2), rutile (RTiO2), AEROXIDE TiO2 P25, and anatase/rutile mixture with a mass ratio of 3 (AR3), were prepared and reduced by the NaBH4 and HCHO solutions at ambient temperature. Characterization studies using CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were performed to check SMSI in the reduced Pt/TiO2 with different reduction processes and TiO2 crystalline forms. The as-reduced catalysts were then evaluated in a toluene combustion reaction, which was an important route for the removal of the pollutant toluene.13 Finally, the effect of the degree of SMSI on the catalytic performance over the catalysts was discussed. Received: Revised: Accepted: Published: 15879

July 21, 2014 September 18, 2014 September 24, 2014 September 24, 2014 dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

Table 1. Specific Surface Area, Average Pore Size, Pore Volume, Pt Apparent Dispersion, and Particle Size (0.4 wt % Pt) of the Different Samples

a

sample

BET surface area (m2/g)

average pore width (nm)a

pore volume (cm3/g)b

dCO (nm)c

Pt apparent dispersion (%)c

A-TiO2 P25 R-TiO2 AR3 Pt/A-unreduced Pt/A-H2 Pt/A-HCHO Pt/A-NaBH4 Pt/P25-unreduced Pt/P25-NaBH4 Pt/AR3-unreduced Pt/AR3-NaBH4 Pt/R-unreduced Pt/R-NaBH4

267.0 55.3 40.5 189.6 158.0 99.2 83.2 77.1 65.3 47.3 73.6 61.6 33.0 24.4

5.4 12.1 19.2 8.0 13.0 11.0 15.0 13.1 26.1 28.2 15.9 15.9 21.7 22.8

0.37 0.17 0.19 0.38 0.63 0.31 0.31 0.30 0.52 0.41 0.35 0.28 0.21 0.18

1.7 3.8 7.3 4.7 1.6 14.2 1.6 11.6 3.2 6.4

66.6 30.0 15.6 23.9 73.3 8.0 71.0 9.8 35.2 17.7

b

dTEM (nm)d

6.09 ± 1.87 2.88 ± 0.70 2.67 ± 0.65 2.81 ± 0.87 4.02 ± 1.21

c

BJH desorption average pore width. The desorption pore volume for the 1.7−300 nm range of pore diameters. Average Pt particle size and apparent dispersion calculated by CO chemisorptions assuming that the metal particle to be hemisphere. dPt particle size distribution calculated by TEM observation.

2.2. Characterization of Catalysts. The morphologies and metal nanoparticle size distribution of the catalysts were observed by a JEOL 2100F transmission electron microscopy (TEM). The phase purity and crystal structure of the catalysts were examined by X-ray diffraction (XRD, D-MAX 2200 VPC), using monochromatic Cu Kα radiation at a scanning rate of 10°/min and a step size of 0.02°. The Brunauer−Emmett− Teller (BET) surface area, pore volume, and pore size distribution of the samples were measured with a Micromeritics ASAP 2020 instrument using adsorption of N2 at 77 K. Prior to adsorption analysis, the catalysts were degassed in a flowing N2 at 300 °C for 3 h. CO chemisorption was performed using a Micromeritics ASAP 2020C automated system. Before CO chemisorption, around 0.1 g of fresh calcined catalyst was evacuated to 10−6 mm Hg at 110 °C for 30 min; then, it was reduced under flowing H2 at 200 °C for 30 min. The catalyst was evacuated again at 200 °C for 30 min to desorb any H2. The chemisorption analysis was carried out at 35 °C. A CO/Pt average stoichiometry of 1 has been assumed for the calculation of dispersion. X-ray photoelectron spectra (XPS) were recorded on a ESCALAB 250 spectrometer (Thermo Fisher Scientific, Al Kα, hν = 1486.6 eV) under a vacuum of ∼2 × 10−7 Pa. Charging effects were corrected by adjusting the main C 1s peak to a position of 284.8 eV. 2.3. Testing of Catalysts. The catalytic tests were performed using a continuous-flow quartz fixed-bed reactor (i.d. = 7 mm) under atmospheric pressure, as described in our previous work.13 N2 from a cylinder was flowed into a saturator that was filled with toluene aqueous solution (≥99.5%) and then mixed with the volatilized gas and the gas mixture of N2 and O2 from another cylinder. The molar ratio of N2/O2 in the final inlet gas was 4. The inlet toluene concentration was about 300 ppm, which could be controlled by controlling the temperature of saturator and the flow rate of N2 through the saturator. Approximately 200 mg of catalyst with particle sizes in the range of 200−450 μm was packed. Activity test experiments were performed at a constant GHSV (gas hourly space velocity) of 30000 mL h−1 g−1. GHSV is defined as the total flow rate of gas at STP per unit weight of catalyst. Toluene conversion was calculated by measuring the toluene disappearance by GC1 (GC-950, FID) and CO2 evolution by GC2

2. EXPERIMENTAL PROCEDURE 2.1. Preparation of Catalysts. The commercial titania supports with different crystalline structures used are anatase TiO2 (A-TiO2, 99%, Alfa Aesar) and AEROXIDE TiO2 P25 (≥99.5%, Degussa AG). To obtain rutile TiO2 (R-TiO2) with a high surface area, isolation of rutile TiO2 particles from P25 by treatment with a 10% HF solution was adopted.14,15 A-TiO2 and R-TiO2 mixture with a mass ratio of 3:1 (AR3) was dispersed in water, sonicated for 30 min, filtrated, and dried at 120 °C for 6 h for use. TiO2 supported Pt catalysts were prepared by a deposition precipitation method. H2PtCl6 (Alfa Aesar) solution was added drop by drop into the slurry of TiO2 powder under vigorous stirring at 70 °C. After stirring for 1 h, the pH of the solution was adjusted to 8 with NaOH aqueous solution. Then deposition precipitation was allowed to continue for 2 h under vigorous stirring at 70 °C. The TiO2 powder was subsequently washed with deionized water to remove chlorine ions that might affect catalytic activity. Finally, the powder was dried in air at 120 °C overnight, followed by calcinations at 500 °C for 6 h. According to the Inductively Coupled Plasma (ICP, IRIS (HR)) element analysis, the amount of platinum supported on each support is ca. 0.4 wt %. The as-prepared catalysts were denoted as Pt/A (for Pt/A-TiO2), Pt/R (for Pt/ R-TiO2), Pt/AR3, and Pt/P25 for simplicity. The reduced Pt/ TiO2 catalysts were prepared by H2 stream reduction at 500 °C for 1 h or with NaBH4 solution or HCHO solution impregnation method, denoted as Pt/TiO2−H2, Pt/TiO2− NaBH4, and Pt/TiO2−HCHO, respectively. In a typical NaBH4 reduction process, a NaBH4 aqueous solution with a volume of 5 mL as reducing agent was quickly and thoroughly mixed with 0.5 g of catalyst (NaBH4/Pt = 20, molar ratio) under supersonic vibration. When the excess aqueous solution was evaporated, the suspension was dried in air at 120 °C for 6 h for use. In a HCHO reduction process, about 0.5 g of as-calcined catalyst was mixed with 50 mL of deionized water and 5 mL of HCHO solution (∼35 wt % HCHO) for about 1 h under reflux conditions and vigorous stirring at 70 °C. Then, the suspension was separated and dried in air at 120 °C for 6 h for use. The adsorbed excess HCHO can be evaporated or oxidized during the drying process.16 15880

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

Figure 1. TEM images of Pt/TiO2 samples with different reduction methods and different TiO2 polymorph reduced with NaBH4: (a) Pt/A-H2, (b) Pt/A-HCHO, (c) Pt/A-NaBH4, (d) Pt/R-NaBH4, (e) Pt/P25-NaBH4, and (f) Pt/AR3-NaBH4. Insets show EDS analysis for (a) and the Pt diameter distributions for (b−f).

(GC7890II, FID), which was equipped with a nickel catalyst based methanizer to enable FID to detect CO2. All conversions were calculated by X toluene =

ln Y = ln(k r0mcat ) −

(2)

where

nCO2 /7 nCO2 /7 + ntoluene

Ea RT

(1)

Y=

where nCO2 and ntoluene are the molar flow (mol/s) of CO2 and C7H8 at the outlet of the reactor. For the experimental measurements, we measured 3 to 5 conversion data at each temperature with around 15 min interval time for each datum. The conversion datum adopted shows less than 10% difference with other measured data at each temperature. Toluene conversions could thus be measured for the different catalysts as a function of temperature. From these data, under the quite reasonable assumption of a plug-flow regime and neglecting the pressure drop across the reactor and the overall variation of the mole flow rate (small C7H8 feed concentration), the following equation can be obtained through a simple mass balance17,18

⎛y ⎞ ln⎜⎜ in ⎟⎟ RT0 ⎝ yout ⎠ φ0

(3)

Here k0r is the reaction rate constant (mol Pa−1g−1 s−1), mcat is the catalyst mass in the reactor (g), Ea is the activation energy (J mol−1), R is the gas constant, T is the temperature in K, T0 is the standard temperature (273.15 K), yin and yout are the toluene molar fraction at the reactor inlet and outlet, respectively, and φ0 is the feed flow rate in normal conditions (m3 s−1). By plotting ln Y vs 1/RT, estimates of the apparent activation energy and the pre-exponential kinetic constant can be easily obtained by the least-squares fitting method. 15881

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

3. RESULTS 3.1. Structural Properties. Table 1 lists the specific surface area, average pore size, and pore volume of the different samples. In general, the BET surface area and the pore volume of the as-prepared catalysts decrease after reduction. In comparison with Pt/A-H2 and Pt/A-HCHO, Pt/A-NaBH4 holds lower BET area and pore volume. It was reported that metallic Pt0 in the TiO2 system could catalyze the sintering of the support,19 reduction of TiO2, and the migration of partially reduced TiOx support fragments onto the metal,20 which probably was the reason for surface area decrease during the high temperature H2 reduction process. As for the wet reduction process, some dissolution of the TiO2 support in the HCHO or NaBH4 aqueous solution during the reduction process and its further deposition at the surface of the TiO2 may cause partial pore mouth blockage and BET area decrease. Moreover, the residual sodium ion was found to have a negative effect on the BET surface area due to its plugging into the small pores, which resulted in a decrease in BET surface area of the NaBH4 reduced samples.10 The results also indicate that the support polymorph has a significant effect on the morphology of Pt/TiO2−NaBH4 samples. In the case of Pt/R-NaBH4, whose support is isolated from P25, the BET area, average pore size, and pore volume are lower than those corresponding values for Pt/P25-NaBH4. As expected, the BET area, average pore size, and pore volume of Pt/AR3-NaBH4 are ranged between those of Pt/A-NaBH4 and Pt/R-NaBH4. Pt particle size and dispersion of the different samples were calculated by both CO chemisorption and TEM observation, as shown in Figure 1 and listed in Table 1. For the unreduced samples, the Pt particles are too small to be observed by the TEM images (not listed here). Small and comparable average Pt particle sizes are measured by CO chemisorption for these unreduced samples except for Pt/R which holds a large measured Pt particle size probably due to its small surface area. The reduction process has a significant effect on the dispersion of Pt particles. The Pt dispersion deceases, while Pt particle size increases during all the reduction processes. The TEM images and Pt particle size distribution of Pt/TiO2 samples with different reduction conditions and different TiO2 polymorph reduced with NaBH4 are compared in Figure 1. Approximately 500 Pt nanoparticles were calculated to obtain the histogram of metal particle size distribution for each sample. It can be found that small and homogeneous Pt nanoparticles uniformly present on all the reduced catalysts. The Pt particle size measured from both TEM observation and CO chemisorption measurement has a sequence of Pt/A-HCHO > Pt/A-NaBH4 >Pt/A-H2, while the Pt dispersion holds a reverse trend. It should be noted that in the case of the Pt/A-H2 catalyst, Pt particles are too small to be clearly observed, and the presence of Pt particles is demonstrated by the EDS analysis, as shown in Figure 1a. It was suggested that partially reduced oxide species TiOx produced by H2 reduction could migrate onto the metal particles to destroy large ensembles of the metal.9 At the same time, the metal particle size growth by Ostwald ripening was reported in the solution in the absence of any surfactant.21,22 Thus, the Pt particle size measured from TEM observation shows that Pt/A-H2 holds smaller average Pt particle size than those over Pt/A-HCHO and Pt/A-NaBH4. The results further indicate that the Pt particle growth phenomenon is more significant during the HCHO solution reduction process than that in the NaBH4 solution reduction process. The properties

of the support also show an important effect on the Pt particles dispersion. As observed from the TEM images, the Pt particle size over Pt/R-NaBH4 (∼4.02 nm) is larger than that over Pt/ A-NaBH4 (∼2.88 nm) due to the lower surface area of Pt/RNaBH4. Although the BET areas of Pt/AR3-NaBH4 and Pt/ P25-NaBH4 are a little smaller than that of Pt/A-NaBH4, the Pt particle sizes over these three samples are comparable probably due to the larger pore size over Pt/AR3-NaBH4 and Pt/P25NaBH4 leading to a better Pt dispersion. Interestingly, the Pt particle size measured by CO chemisorption is much larger than the corresponding Pt particle size calculated by TEM characterization for all the NaBH4 reduced samples and the samples with H2 and HCHO reduction. Especially, the measured Pt particle sizes by CO chemisorptions over Pt/ AR3-NaBH4 and Pt/P25-NaBH4 are as large as 11.6 and 14.2 nm, while the corresponding observed values from TEM images are only ∼2.81 nm and ∼2.67 nm, respectively. A CO/ Pt average stoichiometry of 1 has been assumed for the calculation of Pt apparent dispersion and Pt particle size during CO chemisorption experiments. The large measured Pt particle size is associated with the low CO sorption amounts.3 Figure 2 shows some characteristic XRD patterns for the reduced Pt/TiO2 catalysts. The characteristic peaks of PtO2 or

Figure 2. XRD patterns of Pt/TiO2 samples with different reduction processes and different TiO2 polymorphs.

Pt are too weak to be detected during the XRD characterization for all the samples, indicating a good Pt dispersion over TiO2 supports. Pt/A-NaBH4, Pt/A-HCHO, and Pt/A-H2 consist only of pure anatase (JCPDS file no. 21-1272). Pt/P25-NaBH4, on the other hand, consists of both anatase and rutile (JCPDS file no. 65-0190). While Pt/R-NaBH4, whose support is isolated from P25 with HF solution treatment, contains only rutile. The characteristic peaks of the anatase phase completely disappear. This result suggests that the anatase is efficiently dissolved, which is in agreement with the results reported in the literature.14,15 As expected, AR3 supported Pt catalyst contains both anatase and rutile phases. 3.2. XPS Characterization. XPS has been utilized to address the possible change in electronic properties of the components.5,23 The surface chemical states of the Ti, Pt, and O elements are shown in Figure 3 and listed in Table 2. It is known that the Pt 4f7/2 binding energies (BE) of Pt0, Pt2+, and Pt4+ are around 71.1, 72.4, and 74.2 eV, respectively.24 15882

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

Table 2. XPS Data for the Reduced 0.4 wt % Pt/TiO2 Catalysts binding energy (eV)

surface atomic ratio

samples

Pt 4f7/2

OII(OI)a

Ti2P

(OII/OI)a

Pt/Ti

Pt/A-H2 Pt/A-HCHO Pt/A-NaBH4 Pt/P25-NaBH4 Pt/AR3-NaBH4 Pt/R- NaBH4

70.65 70.51 69.95 70.58 70.37 70.38

532.2(530.2) 531.8(530.2) 531.5(529.7) 531.5(529.7) 531.5(529.7) 531.3(529.7)

458.9 458.9 458.4 458.4 458.5 458.3

0.22 0.14 0.16 0.15 0.16 0.15

0.0026 0.0005 0.0015 0.0010 0.0016 0.0021

OII/OI was calculated from the corresponding areas of fitted peaks done by XPSPEAK 4.1 with Shirley background. a

respectively, with a peak BE of 69.95 eV. Similarly, the main peaks of O 1s (OI) and Ti 2p3/2 on the reduced catalysts are found at a value lower than 530.2 and 490 eV, which are the main peaks of O 1s and Ti 2p3/2 over the PtO/TiO2 sample, respectively.10 In addition, a significant shoulder peak of O 1s (OII) appears at 532.2 eV over Pt/A-H2, 531.8 eV over Pt/AHCHO, and 531.2−531.5 eV over the NaBH4 reduced samples, respectively, as shown in Figure 3. The reduction process has a significant influence on the surface Pt atom and active oxygen (OII) concentrations. For example, the Pt/Ti and OII/OI surface atom ratios of Pt/A-H2 are 0.0026 and 0.22, respectively, while they are changed to 0.0015 and 0.16 for Pt/A-NaBH4. 3.3. Catalysts Activity Test. Figures 4 and 5 present the relationship between the toluene conversions and reaction temperatures over Pt/A catalysts with different reduction processes and the NaBH4 reduced Pt/TiO2 samples with different TiO2 crystalline forms. For the Pt/A catalysts with different reduction processes, the activity has a sequence of Pt/ A-NaBH4 > Pt/A-HCHO > Pt/A-H2 > Pt/A-unreduced. TiO2 polymorph also has an important influence on the activity of Pt/TiO2 catalysts reduced with NaBH4. The largest activity difference among these samples appears at T = 150 °C with a sequence of Pt/A-NaBH4 > Pt/AR3-NaBH4 > Pt/R-NaBH4 > Pt/P25-NaBH4. By proper treatment of the conversion data, the Arrhenius plots and the Ea values could be evaluated for each sample.17,18 By the way, estimates of k0r could also be drawn from the intercept of best-fitting lines with the Y-axis of Figure 3b. Owing to the wide range of temperatures and conversion analyzed in the experimental runs, the determination of the intercept (strictly related to k0r ) of the best fitting model lines is affected by serious extrapolation errors: a low variation (minor error) concerning the slope (Ea) entails a large variation (major errors) on the intercept (k0r ).17,18 From the slope of the resulting linear plots we obtain Ea values around 88 kJ/mol, 85 kJ/mol, 75 kJ/mol, and 119 kJ/mol for Pt/A-unreduced, Pt/AH2, Pt/A-HCHO, and Pt/A-NaBH4, respectively. Although estimates of k0r are affected by severe errors and not as accurate as Ea values, the pre-exponential factor k0r measured over Pt/Aunreduced is clearly lower than that obtained over Pt/ANaBH4, suggesting that the number of active sites over Pt/ANaBH4 is higher. In the same way, the Arrhenius plots and the Ea values are evaluated for the NaBH4 reduced Pt/TiO2 samples with different TiO2 crystalline forms, as listed in Table 3. The activation energies for these samples are in the range of 97.6−119.3 kJ/mol and have a sequence of Pt/ANaBH4 > > Pt/AR3-NaBH4 > Pt/R-NaBH4 > Pt/P25-NaBH4. These results indicate the behavior differences among the Pt/

Figure 3. O 1s peak in the XPS of the Pt/TiO2 samples with different reduction processes and different TiO2 polymorphs: (a) Ti 2p, (b) Pt 4f, and (c) O 1s.

However, it is negatively shifted to a lower BE of 69.95−70.65 eV over all the reduced catalysts, which is far from the BE of Pt2+ and Pt4+ but close to the BE of Pt0, indicating that Pt nanoparticles are mainly reduced into the metallic state.10 Here, XPS peak was fitted by XPSPEAK 4.1 with Shirley background for each sample. For example, the start BE and end BE of the fitted Pt 4f7/2 peak of Pt/A-NaBH4 are 72.65 and 68.1 eV, 15883

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

Table 3. Weisz-Prater Criterion Number and Kinetic Parameters for Toluene Combustion over 0.4 wt % Pt/TiO2 Catalysts NW−P (T = 200 °C)

sample Pt/A-unreduced Pt/A-H2 Pt/A-HCHO Pt/A-NaBH4 Pt/P25-NaBH4 Pt/AR3-NaBH4 Pt/R-NaBH4

1.7 6.1 8.3 9.5 4.4 7.9 5.5

× × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2

k0r (mol Pa−1g−1 s−1) (1.49 (1.23 (3.86 (3.72 (2.18 (7.68 (8.24

± ± ± ± ± ± ±

1.33) 1.07) 2.59) 3.71) 2.17) 7.56) 8.20)

× × × × × × ×

101 101 100 108 105 104 104

Ea (kJ mol−1) 88 85 75 119 98 101 99

± ± ± ± ± ± ±

6 5 3 19 18 8 11

μm) employed. In addition, the overall variation of the reactant mole flow rate may also be neglected due to the small toluene feed concentration. Second, the effect of inner and outer diffusion can be ignored due to the small, well-proportioned particle size and the thin catalyst bed and well-proportioned particle size. Saracco and Specchia17 and Rui et al.18 made the similar assumptions and regressions in their work for the purpose of comparison.

4. DISCUSSION 4.1. On SMSI in Pt/TiO2. It is found that the Pt particle size over Pt/A-H2, Pt/A-HCHO, and NaBH4 reduced Pt/TiO2 samples measured by CO chemisorption are all much larger than the values detected by TEM observation and XRD detection. This phenomenon indicates the suppression of CO chemisorption occurs over these reduced samples, which is not due to Pt particle size growing but to a chemical interaction between the noble metal and the support (or SMSI) in the reduced samples.2−7 The suppression of CO chemisorption for Pt/TiO2 reduced by H2 at a high temperature due to an influence of SMSI has been frequently reported.2−7 Here, we show that the reduction with HCHO and NaBH4 solution under mild conditions can also lead to the suppression of CO sorption or SMSI over Pt/TiO2. SMSI is usually explained in terms of partial charge transfer.5−7,25 The negative BE shift of Pt4f7/2, O1s, and Ti2P happen not only over the high temperature H2 reduced sample but also over the mild HCHO and NaBH4 solution reduced samples in comparison with those over the unreduced Pt/TiO2. Moreover, the BE negative shift of Pt4f7/2 over the HCHO and NaBH4 solution reduced samples is larger in comparison with the shift value over Pt/A-H2, implying strong SMSI exists in the mild HCHO and NaBH4 solution reduced samples. The electron transfer from TiO2 to Pt particles is the reason for the negatively charged Pt particles.26 The driving force for charge transfer is the difference in the work function between Pt (5.6 eV)10 and the reduced TiO2 or, alternatively, the difference in the electrochemical potentials.26 This phenomenon may become more intense for smaller Pt particles due to the decreased coordination number of surface Pt atoms and the close contact between Pt particles and the support.26 Meanwhile, oxygen vacancies and the negatively charged Pt show enhanced capacity of O2 adsorption. The chemisorbed oxygen is confirmed by a significant shoulder peak of O 1s (OII) on all the reduced Pt/TiO2 catalysts. The prechemisorbed oxygen (OII) in turn can draw a negative effect on the BE negative shift of Pt4f7/2 due to the formation of Pt−O and its electron accepting property. In our previous study, we found that a small Pt4f7/2 BE negative shift was accompanied by a large

Figure 4. Comparison of Pt/A samples reduced under different conditions for the oxidation of trace toluene in simulated air: (a) toluene conversion vs temperature and (b) Arrhenius plots.

Figure 5. Comparison of the different Pt/TiO2 samples with different TiO2 polymorphs reduced with NaBH4 and Pt/A-H2 for the oxidation of toluene.

TiO2 samples with different TiO2 crystallites and different reduction conditions. It should be noted here the kinetic data derived from the dynamic light-off tests in this work are not the intrinsic values. However, these values are still capable of comparing the performance among the samples in this work. First, the assumptions made for the calculation are reasonable. For example, a plug-flow regime may be assumed, and the pressure drop may be neglected across the reactor due to the thin catalyst bed and well-proportioned particle size (200−450 15884

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

surface OII concentration.18 Thus, although the Pt particle size over Pt/A-HCHO is larger than that over Pt/A-H2, the former has a larger measured BE negative shift of Pt4f7/2 probably due to its less prechemisorbed oxygen (OII) concentration. Among the Pt/A-H2, Pt/A-HCHO, and Pt/A-NaBH4 samples, Pt/ANaBH4 holds the largest BE negative shift of Pt4f7/2. The Pt/ANaBH4 and Pt/A-HCHO have a comparable OII/OI ratio (or surface OII concentration), which is much lower than that over Pt/A-H2. However, the presence of the Na−O(OH)x-Pt interaction was expected over Pt/A-NaBH4 due to the residual of sodium ions.12,27,28 The Pt−O(OH)x-Na interaction may migrate the negative effect of the Pt−O bond on the electron accepting ability of Pt particles and lead to a large BE negative shift of Pt4f7/2. In combination with the positive effect of a small Pt particle, the measured BE negative shift of Pt4f7/2 over Pt/ANaBH4 is larger than that over Pt/A-HCHO. The electron transfer from TiO2 to Pt particles is also dependent on the TiO2 crystalline structure. Considering that R-TiO2 is more thermodynamically and structurally stable than A-TiO2,29 the work function of reduced A-TiO2 is expected to be lower than that for reduced R-TiO2 under the same reduction conditions. Thus, the charge transfer in Pt/A-NaBH4 is more significant than that in Pt/R-NaBH4. As expected, the BE down-shift of Pt 4f or SMSI over Pt/AR3-NaBH4 is between those over Pt/A-NaBH4 and Pt/R-NaBH4. The effect of the TiO2 phase structure found here is consistent with reported results.7,29 Li et al.29 made a comparative investigation into SMSI for anatase and rutile titania supported palladium catalyst by EPR using CO as probe molecules. It was discovered that even prereduced by H2 at lower temperature resulted in SMSI for an anatase titania supported palladium catalyst but not for a rutile titania supported palladium catalyst. A similar trend was also found for the high temperature H2 reduced Pt/ TiO2 catalysts in our recent work.18 The small BE down-shift of Pt 4f (or poor SMSI) over Pt/P25-NaBH4 is probably because of the special crystalline composition in P25.30 It was found that P25 contained not only anatase and rutile TiO2 but also a small amount of amorphous phase,30 which might lead to a different SMSI behavior over Pt/P25-NaBH4 in comparison with the mixed anatase/rutile phase TiO2 supported catalyst. It should be noted that the surface OII/OI ratio and the residual Na+ over these NaBH4 solution reduced samples with different TiO2 crystalline structures are comparable, and their effects on the measured Pt4f7/2 BE values comparison among the Pt/ TiO2−NaBH4 samples with different TiO2 structures can be neglected. 4.2. On the Performance of Pt/TiO2. 4.2.1. Effect of the Catalyst Morphology. The reactivity measurements here are reported as light off curves, which are subject to the mass transfer limitations. Alternatively, the morphology of the catalysts may influence their activities. Takeguchi et al.31 reported that pore structure was important for toluene to diffuse effectively near the catalysts’ metals, and the light-off temperatures depended on not only the catalytic activities but also the catalyst structure because of low concentration of toluene. In this work, the Weisz-Prater Criterion (NW−P) was used to estimate the influence of pore diffusion on reaction rates,32,33 which was described in the Supporting Information. It was reported that if the criterion NW−P ≤ 0.3 for reactions with an order of 2 or less was satisfied, pore diffusion limitations could be excluded.33 Because an apparent less 2order has been previously reported for the toluene oxidation over Pt-based catalysts,34,35 and the calculated NW−P (T =

473.15 K) number for all the catalysts is much less than 0.3 (as listed in Table 3), the mass transfer limitations or the effect of physical properties difference on the catalytic performance among these catalysts, such as pore size and pore volume, can be negligible. Such estimation is also supported by the experimental results. Although Pt/P25-NaBH4 and Pt/RNaBH4 hold the larger pore size in comparison with Pt/ANaBH4 and Pt/AR3-NaBH4, they show lower activity for toluene oxidation. Except for the pore structure, the Pt particle size difference among these samples may also lead to the catalytic performance difference. It was reported that the turnover frequencies for toluene oxidation was higher for larger Pt crystallites than those for small Pt crystallites size.34 The improvement of the reaction at large Pt crystallites could be mainly related to the higher rate of surface reaction and the lower activation energy of the oxygen chemisorption rate constant obtained for large Pt crystallites than for the small Pt crystallites.34 Such an effect is related to the Pt−O bond strength, which depends on the coordination of Pt atoms present preferentially at the surfaces with either small or large Pt crystallite sizes.36,37 Loosely held chemisorbed oxygen predominates on larger crystallites due to preferably exposed Pt atoms with lower coordinative unsaturation present at planes and terraces. This loosely held oxygen is more reactive in complete oxidation of hydrocarbons.34,38 In this work, however, although the Pt crystalline sizes over the NaBH4 reduced samples are between those over Pt/A-HCHO and Pt/A-H2, all the NaBH4 reduced samples present larger Ea values (∼97−120 kJ/mol) than those H2 and HCHO reduced samples. Moreover, although the Pt particle size distributions over Pt/A-NaBH4, Pt/AR3-NaBH4, and Pt/ P25-NaBH4 are similar, there is still an obvious activity difference among these samples with the toluene conversions of 94.4%, 85.1%, and 47.2% at T = 150 °C, respectively. It was observed that the correlation between the catalytic activity and the metal dispersion depended on the type of VOCs, the noble metal, and the support.39 Papaefthimiou et al.40 studied the oxidation of benzene and ethyl acetate over Pt-based catalysts, and they found that the turnover frequency (TOF) strongly increases with the particle size on Pt/Al2O3; on a Pt/TiO2 catalyst this correlation was found to be rather weak compared to the Pt/Al2O3 catalyst. Thus, either because of the small Pt crystallize size difference among these samples or the difference in the nature of the support, no obvious correlation between the Pt crystallize size and the performance of the catalysts can be found, though the oxidation of toluene over Pt supported catalysts is recognized to be structure sensitive over Pt/Al2O3.34 4.2.2. Effect of SMSI. As aforementioned, the difference of the SMSI in Pt/A catalysts with different reduction treatments and Pt/TiO2−NaBH4 with different TiO2 phase structures leads to large differences in support oxygen vacancies (Ti 2p peak shift) and Pt electronic property (BE negative shift of Pt 4f7/2). The charge was further transferred from negatively charged Pt to the chemisorbed oxygen. Meanwhile, the chemisorbed oxygen was activated during the charge transfer.10 This probably accounts for the high activity for catalytic oxidation of toluene over the reduced samples with SMSI in comparison with the unreduced sample. There are two types of oxygen chemisorption sites, i.e., oxygen precovered sites (OII) and the sites for oxygen and hydrocarbon competitive adsorption.18 As discussed above, the stronger the SMSI (larger negative shift of Pt 4f), the less the density of OII and the more sites for the competitive adsorption 15885

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

down-shift of Pt 4f electrons or the degree of SMSI. In our previous work,13 trace toluene oxidation was performed under the same reaction conditions with this work over NaBH4 solution reduced 0.25 wt % Pt/Al2O3. The measured Pt particle size (∼4.8 nm) and dispersion (∼23.5%) by CO chemisorption over the Pt/Al2O3 were close to those over Pt/ A-NaBH4. Turnover frequency (TOF) at 150 °C, which was calculated by normalizing toluene consumption (mol s−1) to the mole of surface Pt metal atoms (roughly calculated based on CO chemisorption results), over the Pt/A-unreduced, 0.25 wt % Pt/Al2O3, Pt/A-HCHO, and Pt/A-NaBH4 are 0.0002 s−1, 0.007 s−1, 0.01 s−1, and 0.02 s−1, respectively. Since both the NaBH4 reduced Pt/Al2O3 and Pt/A-NaBH4 catalysts used in activity evaluation are small particles and the Pt particle size over both catalysts is close, the effect of the catalyst morphology on their catalytic performance is similar. In addition, the effect of residual sodium and boron containing species on the catalytic performance over both catalysts are parallel due to the same reduction procedure. The large difference in the catalytic activity between the NaBH4 reduced Pt/Al2O3 and Pt/A-NaBH4 is mainly due to the SMSI difference in NaBH4 reduced Pt/Al2O3 and Pt/A-NaBH4. The large activity improvement of Pt/A-HCHO and Pt/ANaBH4 in comparison with Pt/A-unreduced further demonstrates the function of the mild NaBH4 and HCHO solution reduction process. Whatever, the mild NaBH4 and HCHO solution reduction leads to negatively charged metallic Pt nanoparticles, rich chemisorbed oxygen and SMSI in Pt/TiO2, and finally the good catalytic performance of the reduced Pt/ TiO2 for toluene oxidation.

of toluene and oxygen. In addition, the SMSI also influences the competitive adsorption of toluene and oxygen. The strong SMSI (or the Pt particle with a high electron density) facilitate the chemisorption of oxygen (OIII) over toluene. Since the controlling step of toluene oxidation is considered as the adsorption of molecular oxygen,35,41 the density of chemisorbed oxygen (OII + OIII) relates to the pre-exponential factor, and the bond strength of Pt−O affects the activation energy of toluene oxidation. Meanwhile, the electron density over Pt (BE negative shift of Pt 4f7/2) can strengthen the bond of Pt−O. For Pt/A catalysts with different reduction treatments, the degree of SMSI (BE negative shift of Pt 4f7/2) has a sequence of Pt/A-NaBH4 > Pt/A-HCHO > Pt/A-H2, and the toluene oxidation activity holds the same sequence over these catalysts. The high apparent activity of Pt/A-NaBH4 with the strong SMSI should be attributed to the high pre-exponential factor (or high density of total chemisorbed oxygen (OII + OIII)).34 The preferable adsorption of O2 (OIII) over toluene on the more negative Pt particles provides this possibility. Similarly, the degree of SMSI over Pt/TiO2−NaBH4 (BE negative shift of Pt 4f7/2) with different TiO2 crystalline forms hold the same sequence with the toluene oxidation activity and activation energy as Pt/A-NaBH4 > Pt/AR3-NaBH4 > Pt/RNaBH4 > Pt/P25-NaBH4. Except for the above analysis, the promotion effect of the residual sodium has been reported in the literature.12,27,28 Zhang et al.12 showed that the addition of alkali-metal ions to Pt/TiO2 catalyst stabilized an atomically dispersed Pt− O(OH)x−alkali-metal species on the catalyst surface and significantly promoted the activity for the HCHO oxidation by changing the reaction pathway and lowering the activation energy. The difference in this work is that the sodium promotion draws a negative effect on the reaction activation process, i.e., increasing the reaction activation energy. The Pt− O(OH)x-Na interaction over the NaBH4 solution reduced samples, on one hand, may make it difficult to activate OII due to the Na−O(OH)x bond and, on the other hand, can promote the electron density over Pt (or the BE negative shift of Pt4f7/2) of the reduced sample; while the negatively electronic property of Pt can increase the activation energy (or strengthen the Pt− O (OIII) bond) and the pre-exponential factor (or promote the chemisorption of oxygen (OIII) over toluene) of the toluene oxidation reaction. Thus, although the NaBH4 solution reduced samples hold the high activation energies, they still have high apparent activities due to the large pre-exponential factor, as listed in Table 3. In other words, if the electronic property of Pt and the chemisorbed oxygen are taken as the index of SMSI in the reduced Pt/TiO2, the effect of the residual sodium on the reaction can be attributed to its effect on the SMSI. Although the effect of residual boron containing species on the performance of the catalyst has not been clarified, it was found that no promotional effect of B to the Pt/TiO2 catalyst in the BOx/Pt/TiO2 catalyst.42 Huang et al.10 attributed the high activity of the NaBH4 reduced Pt/TiO2 catalysts for HCHO oxidation to the well-dispersed and negatively charged metallic Pt nanoparticles and rich chemisorbed oxygen by neglecting the effect of residual boron containing species. The phenomenon that the degree of SMSI in the Pt/TiO2 can lead to changes in the catalytic activity and stability has been frequently reported.4−7 Lewera et al.5 reported on the changes of electronic properties of nanosized Pt over Pt/TiO2/ C and Pt/WO3/C. Both systems exhibited increased activity toward oxygen reduction reaction, which was correlated to BE

5. CONCLUSIONS HCHO and NaBH4 solution reduction suppresses CO sorption on Pt/TiO2, which is not due to metal agglomeration but to an influence of strong metal-support interaction (SMSI). The crystalline forms of the TiO2 support have great influence on the degree of SMSI. Pt/anatase (A) produces stronger SMSI through treatment by NaBH4 solution in comparison with the NaBH4 reduced Pt/rutile (R), Pt/P25 and Pt/AR3 (anatase/ rutile molar ratio = 3) catalysts. The SMSI effect results in negatively charged metallic Pt nanoparticles and chemisorbed oxygen, which are beneficial to the toluene oxidation activity. The observed trend of the catalytic activity goes along with the degree of SMSI over the catalysts. Our observation, namely that not only Pt crystallite size, TiO2 crystallite structure, and morphology but also the reduction process affect the activity of Pt/TiO2 for VOCs oxidation reaction, should help in guiding the further exploration of practical high-yield TiO2-based and supported catalysts. Especially, the finding that mild HCHO and NaBH4 solution reduction can also lead to SMSI in Pt/ TiO2 would be important in designing a conventional highly active TiO2-based metal catalyst.

■

ASSOCIATED CONTENT

S Supporting Information *

Description of the Weisz-Prater Criterion (NW−P) calculation process. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 20 84113658. E-mail: [email protected]. 15886

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

Notes

(18) Rui, Z. B.; Wu, S. R.; Peng, C.; Ji, H. B. Comparison of TiO2 Degussa P25 with anatase and rutile crystalline phases for methane combustion. Chem. Eng. J. 2014, 243, 254−264. (19) Huang, B.-S.; Chang, F.-Y.; Wey, M.-Y. Photocatalytic properties of redox-treated Pt/TiO2 photocatalysts for H2 production from an aqueous methanol solution. Int. J. Hydrogen Energy 2010, 35, 7699− 7705. (20) Alexeev, O. S.; Chin, S. Y.; Engelhard, M. H.; Ortiz-Soto, L.; Amiridis, M. D. Effects of reduction temperature and metal-support interactions on the catalytic activity of Pt/γ-Al2O3 and Pt/TiO2 for the oxidation of CO in the presence and absence of H2. J. Phys. Chem. B 2005, 109, 23430−23443. (21) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T. Formation of stable platinum nanoparticles by laser ablation in water. J. Phys. Chem. B 2003, 207, 4218−4223. (22) Varghese, N.; Rao, C. N. R. Growth kinetics of platinum nanocrystals prepared by two different methods: Role of the surface. J. Colloid Interface Sci. 2012, 365, 117−121. (23) Corcoran, C. J.; Tavassol, H.; Rigsby, M. A.; Bagus, P. S.; Wieckowski, A. Application of XPS to study electrocatalysts for fuel cells. J. Power Sources 2010, 195, 7856−7879. (24) Tiernan, M. J.; Finlayson, O. E. Effects of ceria on the combustion activity and surface properties of Pt/Al2O3 catalysts. Appl. Catal., B 1998, 19, 23−35. (25) Chen, B. H.; White, J. M. Properties of platinum supported on oxides of titanium. J. Phys. Chem. 1982, 86, 3534−3541. (26) Ioannides, T.; Verykios, X. E. Charge transfer in metal catalysts supported on doped TiO2: A theoretical approach based on metal− semiconductor contact theory. J. Catal. 1996, 161, 560−569. (27) Zhai, Y.; Pierre, D.; Si, R.; Deng, W.; Ferrin, P.; Nilekar, A. U.; Peng, G.; Herron, J. A.; Bell, D. C.; Saltsburg, H.; Mavrikakis, M.; Flytzani-Stephanopoulis, M. Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions. Science 2010, 329, 1633− 1636. (28) Zugic, B.; Zhang, S.; Bell, D. C.; Tao, F.; FlytzaniStephanopoulos, M. Probing the low-temperature water-gas shift activity of alkali-promoted platinum catalysts stabilized on carbon supports. J. Am. Chem. Soc. 2014, 136, 3238−3245. (29) Li, Y.; Fan, Y.; Yang, H.; Xu, B.; Feng, L.; Yang, M.; Chen, Y. Strong metal-support interaction and catalytic properties of anatase and rutile supported palladium catalyst Pd/TiO2. Chem. Phys. Lett. 2003, 372, 160−165. (30) Ohtani, B.; Prieto-Mahaney, O. O.; Li, D.; Abe, R. What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test. J. Photochem. Photobiol., A 2010, 216, 179−182. (31) Takeguchi, T.; Aoyama, S.; Ueda, J.; Kikuchi, R.; Eguchi, K. Catalytic combustion of volatile organic compounds on supported precious metal catalysts. Top. Catal. 2003, 23, 159−162. (32) Weisz, P. B.; Prater, C. D. Interpretation of measurements in experimental catalysis. Adv. Catal. 1954, 6, 143−196. (33) Vannice, M. A.; Joyce, W. H. Kinetics of Catalytic Reactions; Springer: New York, 2005; Vol. 134, pp 63−65. (34) Radic, N.; Grbic, B.; Terlecki-Baricevic, A. Kinetics of deep oxidation of n-hexane and toluene over Pt/Al2O3 catalysts: Platinum crystallite size effect. Appl. Catal., B 2004, 50, 153−159. (35) Barresi, A.; Cittadini, M.; Zucca, A. Investigation of deep catalytic oxidation of toluene over a Pt-based monolithic catalyst by dynamic experiments. Appl. Catal., B 2003, 43, 27−42. (36) Gland, J.; Sexton, G.; Fisher, B. Oxygen interactions with the Pt(111) surface. Surf. Sci. 1980, 95, 587−602. (37) Wang, H.; Tobin, R.; Lambert, D.; Di, M. C.; Fisher, G. Adsorption and dissociation of oxygen on Pt(335). Surf. Sci. 1997, 372, 267−278. (38) Hicks, R. F.; Qi, H.; Young, M. L.; Lee, R. G. Structure sensitivity of methane oxidation over platinum and palladium. J. Catal. 1990, 122, 280−294. (39) Santosa, V. P.; Carabineiroa, S. A. C.; Tavaresb, P. B.; Pereiraa, M. F. R.; Figueiredo, J. L. Oxidation of CO, ethanol and toluene over

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work has been supported by the Natural Science Foundation of China (21106189 and 21036009) and the Fundamental Research Funds for the Central Universities (12lgpy11).

■

REFERENCES

(1) Tauster, S. J.; Fung, S. C.; Garten, R. L. Strong metal-support interactions. Group 8 noble metals supported on TiO2. J. Am. Chem. Soc. 1978, 100, 170−175. (2) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991. (3) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. Electron microscopy of supported metal particles I. Behavior of Pt on titanium oxide, aluminum oxide, silicon oxide, and carbon. J. Catal. 1979, 56, 390−406. (4) Baker, R. T. K.; Prestridge, E. B.; Murrell, L. L. Electron microscopy of supported metal particles III. The role of the metal in an SMSI Interaction. J. Catal. 1983, 79, 348−358. (5) Lewera, A.; Timperman, L.; Roguska, A.; Alonso-Vante, N. Metal-support interactions between nanosized Pt and metal oxides (WO3 and TiO2) studied using X-ray photoelectron spectroscopy. J. Phys. Chem. C 2011, 115, 20153−20159. (6) Raj, K. J. A.; Prakash, M. G.; Mahalakshmy, R.; Elangovan, T.; Viswanathan, B. Selective hydrogenation of acetophenone over nickel supported on titania. Catal. Sci. Technol. 2012, 2, 1429−1436. (7) Kim, M. S.; Chung, S. H.; Yoo, C. J.; Lee, M. S.; Cho, I. H.; Lee, D. W.; Lee, K. Y. Catalytic reduction of nitrate in water over Pd-Cu/ TiO2 catalyst: Effectof the strong metal-support interaction (SMSI) on the catalytic activity. Appl. Catal., B 2013, 142−143, 354−361. (8) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. Electron microscopy of supported metal particles II.Further Studies of Pt/TiO2. J. Catal. 1979, 59, 293−302. (9) Takanabe, K.; Nagaoka, K.; Nariai, K.; Aika, K. Influence of reduction temperature on the catalytic behavior of Co/TiO2 catalysts for CH4/CO2 reforming and its relation with titania bulk crystal structure. J. Catal. 2005, 230, 75−85. (10) Huang, H.; Leung, D. Y. C.; Ye, D. Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation. J. Mater. Chem. 2011, 21, 9647−9652. (11) Qu, Z. P.; Shen, S. J.; Chen, D.; Wang, Y. Highly active Ag/SBA15 catalyst using post-grafting method for formaldehyde oxidation. J. Mol. Catal. A: Chem. 2012, 356, 171−177. (12) Zhang, C. B.; Liu, F. D.; Zhai, Y. P.; Ariga, H.; Yi, N.; Liu, Y. C.; Asakura, K.; Stephanopoulos, M. F.; He, H. Alkali-metal-promoted Pt/ TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew. Chem., Int. Ed. 2012, 51, 9628−9632. (13) Rui, Z. B.; Chen, C. Y.; Lu, Y. B.; Ji, H. B. Anodic alumina supported Pt catalyst for total oxidation of trace toluene. Chin. J. Chem. Eng. 2014, 22, 882−887. (14) Rui, Z. B.; Huang, Y. F.; Zheng, Y.; Ji, H. B.; Yu, X. Effect of titania polymorph on the properties of CuO/TiO2 catalysts for trace methane combustion. J. Mol. Catal. A: Chem. 2013, 372, 128−136. (15) Ohno, T.; Sarukawa, K.; Matsumura, M. Photocatalytic activities of pure rutile particles isolated from TiO2 powder by dissolving the anatase component in HF solution. J. Phys. Chem. B 2001, 105, 2417− 2420. (16) Chen, H. Y.; Rui, Z. B.; Ji, H. B. Monolith-like TiO2 nanotube array supported Pt catalyst for HCHO removal under mild conditions. Ind. Eng. Chem. Res. 2014, 53, 7629−7636. (17) Saracco, G.; Specchia, V. Catalytic filters for the abatement of volatile organic compounds. Chem. Eng. Sci. 2000, 55, 897−908. 15887

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888

Industrial & Engineering Chemistry Research

Article

TiO2 supported noble metal catalysts. Appl. Catal., B 2010, 99, 198− 205. (40) Papaefthimiou, P.; Ioannides, T.; Verykios, X. E. Performance of doped Pt/TiO2(W6+) catalysts for combustion of volatile organic compounds (VOCs). Appl. Catal., B 1998, 15, 75−92. (41) Chuang, K. T.; Cheng, S.; Tong, S. Removal and destruction of benzene, toluene, and xylene from wastewater by air stripping and catalytic oxidation. Ind. Eng. Chem. Res. 1992, 31, 2466−2472. (42) Ding, Z. X.; Yang, H. Y.; Liu, J. F.; Dai, W. X.; Chen, X.; Wang, X. X.; Fu, X. Z. Promoted CO oxidation activity in the presence and absence of hydrogen over the TiO2-supported Pt/Co-B bicomponent catalyst. Appl. Catal., B 2011, 101, 326−332.

15888

dx.doi.org/10.1021/ie5029107 | Ind. Eng. Chem. Res. 2014, 53, 15879−15888