Investigation of Calcination and O2 Plasma Treatment Effects on TiO2

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Investigation of Calcination and O2 Plasma Treatment Effects on TiO2-Supported Palladium Catalysts Yanan Li and Ben W.-L. Jang* Chemistry Department, Texas A&M UniVersity-Commerce, Commerce, Texas 75429-3011

This study focuses on investigating the effects of conventional calcination procedures and O2 plasma treatments on the metal precursor-support interaction (MPSI) of 1 wt % Pd/TiO2. MPSI is likely a strong precursor to the metal-support interaction of supported metal catalysts. The authors identify H2-DSC (H2-differential scanning calorimetry) as a sensitive and effective technique for measuring and differentiating the MPSIs of PdO and palladium nitrate with TiO2. At room temperature, the effect of O2 plasmas on MPSI is stronger than the effect of high-temperature calcination. In addition, O2 plasma treatments can increase the strength of the MPSIs of both supported PdO and palladium nitrate while minimizing the detrimental effects of hightemperature calcinations on the composition of supported metal precursors. 1. Introduction A strong metal-support interaction (SMSI) is an important phenomenon in heterogeneous catalysis that has attracted much attention since first being reported by Tauster et al. in 1978,1 because metals, especially noble metals, supported on reducible oxides show significant differences in catalytic activity, selectivity, and stability after reduction at high temperature.2 Most studies on SMSIs of supported metal catalysts have concentrated on the effect of the final reduction step on the surface properties of the reduced catalysts, such as changes in chemisorption capacity, encapsulation, decoration, and so on.3-5 However, the preparation of industrial supported metal catalysts normally involves procedures such as impregnation, aging, drying, calcination, and reduction to obtain the final catalyst product. It is believed that the metal-support interaction of the final catalyst product will likely be a complex function of the catalyst preparation and pretreatment history, not only the reduction step and the reduction temperature. In fact, some literature reports have demonstrated that the procedures before reduction are important factors determining the final interactions between metals and supports in supported metal catalysts. For example, the strength of the interaction between Rh and Nb2O5 was found to depend strongly on the preparation conditions, namely, the impregnation procedure, Nb/Rh atomic ratio, and calcination temperature.3 Fu et al. reported that the interaction of Pd clusters with TiO2(110) depends strongly on the pretreatment history of the oxide support.4 The encapsulation reaction depends on the oxygen defects of the TiO2 surface, and the encapsulation of metal clusters by TiOx layers is favored by the relatively high surface energies of the metals. However, the oxygen defect distribution of support surfaces is a strong function of catalyst preparation procedures, especially high-temperature calcination and reduction. Therefore, this study investigates the effect of calcination on the metal precursor-support interaction (MPSI) because calcination is normally the step before the reduction procedure and is performed at high temperature. Nonthermal plasma technology has been reviewed recently for applications in catalysis.6,7 The technology has been used to modify catalysts that produce special catalytic properties and change the activity, selectivity, and stability of various reactions. Plasma-inducing special metal-support interactions have also * To whom correspondence should be addressed. E-mail: [email protected].

been reported.8,9 For example, H2 and air radio-frequency (RF) nonthermal plasmas effectively enhance both the activity and stability of supported Ni catalysts for benzene hydrogenation.10 It was proposed that the modified metal precursor-support interaction induced by plasma modifications changes the reduction process of the metal precursor on the support material, which, in turn, results in changes in the structure of the metal particles and the final metal-support interaction. Changes in the shape and structure of Ni particles on H2-plasma-modified Ni/Al2O3 were confirmed by high-resolution scanning transmission electron microscopy.11 The argon-plasma-treated Pd/AlMCM-41 catalyst was reported to exhibit a higher initial activity and a better stability for methane combustion. Plasma treatment improved the dispersion of the PdO particles, which led to a higher initial activity. The better stability of plasma-treated Pdbased catalyst was shown to be closely related to the stronger interaction between palladium oxide and the molecular sieve support.12 With the unusual capacity of nonthermal plasma technology, this study also focuses on investigating the effects of O2 plasma treatments on the interaction between the support and the metal precursor in TiO2-supported palladium nitrate catalyst. TiO2 was chosen as a support in this study because it has been widely reported to be easily induced to have SMSI. Most interestingly, this report describes a new application of an existing technique, namely, H2DSC (differential scanning calorimetry in H2 atmosphere), to detect the slight changes of MPSI of 1 wt % Pd/TiO2. The long-term objective is to use the knowledge of MPSI to advance the fundamental understanding of the unique potential of nonthermal plasma technology in inducing metal-support interactions, including SMSI, for catalyst design and development. 2. Experiments 2.1. Catalyst Preparation. To prepare the catalysts, 1/8-in. titania pellets (from Alfa Aesar) were crushed and sieved through 20-40 mesh sieves and then dried at 200 °C for 10 h, after which they were allowed to cool to room temperature. The supports were impregnated with 1 wt % Pd using Pd(NO3)2 · xH2O (39% Pd, Alfa Aesar) by the incipient wetness technique. The resulting materials were dried at room temperature for 12 h and then dried at 120 °C for 12 h. The obtained samples were divided into three parts. One part without any further treatment was designated as the fresh sample of 1 wt %

10.1021/ie100749r  2010 American Chemical Society Published on Web 07/30/2010

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Pd/TiO2. The second part was thermally calcined at 200, 300, 400, or 500 °C for 3 h. The third part was treated by nonthermal RF oxygen plasmas as discussed in section 2.2. 2.2. Plasma Treatment. Plasma treatments were carried out in a custom-designed 360° rotating RF plasma system. The details of the system and the apparatus schematic are described in a previous publication and are summarized here.10 The system, housed in a Faraday cage, consisted of three major components: a power supply, a reaction chamber, and a gas delivery and pumping system. It was versatile for either short pulses or continuous waves of plasma generation with various gases. A custom-designed matching network was built to tune and minimize the reflectance power. Power wattage was measured by the combination of a watt meter and an oscilloscope. The 360° rotational reaction chamber was made of a glass tube, 2-in. i.d. × 18-in. length, connected with two fluidic valves. Typically, 0.5 g of catalyst was loaded into the plasma chamber at a pressure of 400 mTorr for oxygen plasma treatments. The duration of O2 plasma treatment was set at 15, 30, or 45 min with a continuous waveform of 130 W. 2.3. Catalyst Characterization. The amounts of nitrate on supported Pd samples after oxygen-plasma treatment and calcination were semiquantitatively measured by Fourier transform infrared (FTIR) transmission analyses using a Thermo Nicolet Nexus 4700 system. The sample was pulverized and then diluted in dried KBr to obtain a homogeneous mixture with a 1:10 sample-to-KBr ratio. The pellets were quantitatively measured to 0.1 mg, and the nitrate peak heights were recalculated based on the weight of pellets. The thermal analysis of samples was carried out using a TA Instruments Q10 DSC apparatus. Samples of approximately 6-7 mg were weighed with a precision of 0.1 mg. The procedure for each H2-DSC run was as follows: heat from 40 to 120 °C in ultra-high-purity (UHP) N2, hold at 120 °C for 30 min, cool with equilibration from 120 to -50 °C, switch the gas from UHP N2 to 5 vol % H2/N2, and then heat from -50 to 300 °C in 5 vol % H2/N2. The temperature scanning rate for all cycles was 15 °C/min. All measurements were repeated at least three times.

Figure 1. FTIR spectra of 1 wt % Pd/TiO2: (a) conventional sample; calcined at (b) 200, (c) 300, and (d) 400 °C; and treated in O2 plasma for (e) 15, (f) 30, and (g) 45 min. Table 1. Percent NO3- Removal of 1 wt % Pd/TiO2 Catalysts

3. Results and Discussion 3.1. FTIR Spectroscopy. Very little information is available in the literature on the calcination and/or reduction of supported metal nitrate catalyst.13,14 According to the limited literature reports, the decomposition temperature of unsupported palladium nitrate is ∼450 K (∼177 °C), and its reduction temperature is ∼375 K (∼102 °C).13 Supported metal nitrate will normally decompose at a temperature that is similar to or slightly lower than that of its unsupported counterpart.14 Therefore, it is expected that the fresh sample of TiO2-supported palladium nitrate will be partly decomposed after being dried at 120 °C for 12 h. However, to calculate, semiquantitatively, the percentage of supported nitrate groups removed by calcination and O2 plasma treatment, the nitrate concentration of the fresh 1 wt % Pd/TiO2 after drying was used as the reference.15 Figure 1 summarizes the FTIR spectra of fresh, calcined, and O2-plasma-treated 1 wt % Pd/TiO2, with calcination temperatures from 200 to 500 °C and plasma treatment times from 15 to 45 min. The peak around 1380 cm-1 is for the nitrate functional group of supported palladium nitrate, and the peak decreases with increasing calcination temperature. From Table 1, the percentage nitrate removal efficiency increased quickly from 37% at 200 °C to 95.7% at 300 °C and then to 100% at

a

sample

NO3- removala (%)

fresh sample calcined at 200 °C for 3 h calcined at 300 °C for 3 h calcined at 400 °C for 3 h calcined at 500 °C for 3 h O2 plasma for 15 min O2 plasma for 30 min O2 plasma for 45 min

37.0 95.7 100 100 10.9 15.2 23.9

Determined by FTIR spectroscopy.

400 °C and above. O2 plasmas, although operated at room temperature, also showed significant capacities in removing nitrate groups from the TiO2 surface, with values of 10.9%, 15.2%, and 23.9% after 15-, 30-, and 45-min treatments, respectively. However, their capacities are much lower than the capacity of high-temperature calcination. 3.2. H2-DSC. To the authors’ knowledge, there is currently no effective experimental procedure to measure and differentiate the reduction of small amounts (less than 1 wt %) of supported PdO and palladium nitrate simultaneously. In situ X-ray diffraction (XRD), H2 thermogravimetric analysis (TGA), or H2 temperature-programmed reduction (TPR) can theoretically provide these measurements, but they are all hampered by the sensitivity issue for the detection of small quantities. DSC is a well-known technique for detecting enthalpy changes during heating/cooling process. It is normally performed under inert atmosphere, such as N2, at temperatures below the decomposition temperature of the material tested. In this study, the DSC system under 5% H2/N2 atmosphere, namely, H2-DSC, was

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Figure 2. H2-DSC results for fresh and calcined 1 wt % Pd/TiO2. Table 2. Peak Temperatures of the Reduction of 1 wt % Pd/TiO2 Catalysts during H2-DSC

a

sample

Tmax1 (°C)

Tmax2 (°C)

fresh sample calcined at 200 °C for 3 h calcined at 300 °C for 3 h calcined at 400 °C for 3 h calcined at 500 °C for 3 h O2 plasma for 15 min O2 plasma for 30 min O2 plasma for 45 min

2.7 ( 0.9 7.4 ( 1.4 12.5 ( 0.6 15.5 ( 0.4 24.7 ( 0.1 19.9 ( 1.1 28.9 ( 1.9 34.1 ( 1.1

42.7 ( 0.2 43.6 ( 0.3 NPa NPa NPa 54.5 ( 1.2 61.6 ( 2.3 68.4 ( 0.9

NP ) negligible peak.

identified as an effective and sensitive tool for the measurement of small quantities of supported PdO and palladium nitrate through the heat evolved. H2-DSC is also effective for differentiating the strength of the interaction between support and the supported metal precursors and is therefore used to characterize the reduction of supported Pd precursors, including PdO and palladium nitrate on TiO2. Figure 2 presents the H2-DSC results for fresh and calcined 1 wt % Pd/TiO2. As shown in Figure 2, the fresh sample exhibits two distinct peaks at 2.7 and 42.7 °C. Chou et al. reported that blank PdO can be completely reduced at 280 K (7 °C) by TPR using a subambient-temperature routine.16 On the other hand, unsupported palladium nitrate is reduced at 375 K (102 °C), as reported by Shanmugam et al.13 Therefore, the first and second peaks of H2-DSC analyses are assigned to the hydrogenation of supported PdO and palladium nitrate, respectively. Tmax1 and Tmax2 are peak temperatures of the two exothermic peaks of PdO and palladium nitrate of each H2-DSC result. The Tmax1 and Tmax2 values of the samples with different calcination temperatures and O2 plasma treatments are summarized in Table 2. Because H2-DSC tests were run in scanning mode, the equilibrium of hydrogenation was never reached. In addition, the heat released during the hydrogenation of palladium nitrate heavily depends on the final products formed, such as N2, NH3, and NO. Therefore, the sizes of the H2-DSC peaks are not discussed here. From Figure 2 and Table 2, H2-DSC of the sample calcined at 200 °C also shows two distinct peaks for the fresh sample. However, the PdO peak (Tmax1) shifts to a higher temperature at 7.4 °C, with the palladium nitrate peak (Tmax2) being relatively unchanged at 43.6 °C. On the other hand, the increase of the

calcination temperature to 300 °C results in a large sharp peak of PdO (Tmax1) at 12.5 °C, whereas the palladium nitrate peak is negligible. The negligible palladium nitrate peak in the H2DSC of this sample is due to the efficient conversion of palladium nitrate to PdO at this temperature, 300 °C, which matches well with the FTIR results. Further increases of the calcination temperature to 400 and 500 °C shift Tmax1 to higher temperatures of 15.5 and 24.7 °C, respectively, and eliminates the palladium nitrate peaks. Similarly, the FTIR results show complete removal of palladium nitrate on TiO2 surfaces. The H2-DSC results for O2-plasma-treated samples, including 15-, 30-, and 45-min treatments, and the fresh sample are compared in Figure 3. All three O2-plasma-treated samples show two distinct peaks, for PdO and palladium nitrate. The Tmax1 and Tmax2 values of O2-plasma-treated samples are also summarized in Table 2. With 15 min of O2 plasma treatment on the fresh sample, Tmax1 and Tmax2 shift significantly, from 2.7 and 42.7 °C to 19.9 and 54.5 °C, respectively. With 30 min of O2 plasma treatment, Tmax1 and Tmax2 further shift to 28.9 and 61.6 °C, respectively. Tmax1 and Tmax2 continue to shift to higher temperatures of 34.1 and 68.4 °C, respectively, after 45 min of O2 plasma treatment. Although Tmax1 and Tmax2 are the peak temperatures in the reductions of PdO and palladium nitrate in H2-DSC, they are considered as indicators of the strength of the MPSIs between the Pd precursors and the TiO2 support. Higher Tmax1 and Tmax2 values indicate stronger MPSIs between the TiO2 support and the Pd precursors, so it takes high temperatures to carry out the reductions. It is possible that the shift to a higher temperature could be caused by the particle size of the metal precursors. However, H2-DSC tests on a physical mixture of palladium nitrate and titania, as shown in Figure 4, do not show any temperature difference of the PdO peak (first peak) between samples calcined at 200 and 500 °C. The results rule out particle size effect on shifts of the peak temperature in H2-DSC. As shown in Table 2, Tmax1 increases with calcination temperature from 200 to 500 °C. This suggests that the strength of MPSIs between PdO and the TiO2 surface increases at higher calcination temperature. However, the interaction between palladium nitrate and the TiO2 support cannot be determined at calcination temperatures of 300 °C and above because of the effective

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Figure 3. H2-DSC results for fresh and O2-plasma-treated 1 wt % Pd/TiO2.

Figure 4. H2-DSC results for the physical mixture of palladium nitrate and TiO2 calcined at 200 and 500 °C.

conversion of palladium nitrate to PdO at these temperatures. To our knowledge, this is the first quantitative analysis demonstrating that the interactions (MPSIs) between supported PdO and the TiO2 surface increase with increasing calcination temperature. On the other hand, 15 min of O2 plasma treatment at room temperature shifts both Tmax1 and Tmax2 of fresh 1 wt % Pd/ TiO2 to 19.9 and 54.5 °C, respectively. The shift of Tmax1 is larger than that for the sample calcined at 400 °C, indicating that 15 min of O2 plasma treatment at room temperature can induce stronger MPSIs on 1 wt % Pd/TiO2 than calcination at 400 °C for 3 h. With 30 min of O2 plasma treatment, the induced MPSI is larger than that for the calcination at 500 °C for 3 h. Comparison of the Tmax1 and Tmax2 values of fresh, 200 and 500 °C calcined, and 30-min O2-plasma-treated 1 wt % Pd/TiO2 is easily seen in Figure 5. Plenty of literature reports show that O2 plasma is very effective in modifying the structure of surfaces including metal oxides.17-19 The results in this study suggest that O2 plasmas could significantly alter the structures of the TiO2 surface and PdO with short treatment times and at room

temperature and induce stronger MPSIs of 1 wt % Pd/TiO2. In addition, O2 plasma treatment can continue to increase the MPSIs between palladium nitrate and TiO2 without converting the majority of palladium nitrate to PdO. This is beneficial in terms of inducing stronger interactions between the support surfaces and metal precursors without significantly changing the composition of metal precursors as in the case of hightemperature calcinations, which allows a larger window for the systematic study MPSI and SMSI effects. Based on the FTIR results, the removal of nitrate groups from TiO2 surfaces is fast by calcination, with 100% removal upon calcination at 400 °C. The nitrate removal efficiency of O2 plasma, on the other hand, is lower than that of the hightemperature calcination of TiO2-supported Pd catalyst. Apparently, the removal of palladium nitrate by various radicals and ions produced in O2 plasma at room temperature is kinetically limited. During calcination, palladium nitrate is expected to be decomposed into PdO. However, during O2 plasma treatments, some palladium nitrate can be converted into PdO first, after which the PdO is reduced to Pd metal, or it can be reduced

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Figure 5. H2-DSC results for fresh, 200 and 500 °C calcined, and 30-min O2-plasma-treated 1 wt % Pd/TiO2.

directly to Pd metal. As reported in the literature, supported palladium nitrate can be partially reduced by plasmas using oxidizing gases, including oxygen.15,20 The reduction capacity of O2 plasma can be seen from a comparison of the PdO peaks of the fresh sample and three O2plasma-treated samples in Figure 5. The PdO peaks of three O2-plasma-treated samples are much smaller than the PdO peak of the fresh sample. Although H2-DSC is in scanning mode and the samples never reach equilibrium under H2-DSC, the size of the peak can be considered semiquantitatively proportional to the amount of PdO reduced. With smaller peaks indicating smaller amounts of PdO reduced, these results suggest that some of the PdO is reduced to Pd metal during O2 plasma treatment. In fact, O2 plasma converts palladium nitrate of fresh 1 wt % Pd/TiO2 to PdO while reducing PdO to Pd metal. With smaller PdO peaks for the three O2-plasma-treated samples, this suggests that the reduction kinetics of PdO by O2 plasma is faster than indicated. However, direct reduction of palladium nitrate to Pd metal by O2 plasma cannot be ruled out. 4. Conclusions The authors have identified H2-DSC as a sensitive and effective technique for the measurement of small quantities of supported PdO and palladium nitrate through heat evolved during hydrogenation to guage the interactions between metal precursors and the TiO2 support. The H2-DSC technique can be used to differentiate between the interactions of PdO and palladium nitrate with the TiO2 support surface, an example of MPSI. The strengths of both MPSIs of PdO and palladium nitrate with TiO2 increase with the O2 plasma treatment time. The effect of O2 plasma treatment on MPSI is stronger than that of high-temperature calcinations, which increase the MPSI of only supported PdO on TiO2. On the other hand, hightemperature calcination is much more efficient in removing nitrate groups from TiO2 surfaces than O2 plasma treatment carried out at room temperature. However, O2 plasma treatment can increase the strengths of the MPSIs of both supported PdO and palladium nitrate while minimizing the detrimental effects of high-temperature calcinations on the composition of metal precursors. The results are beneficial in further understanding

the metal-support interactions in the final products of supported metal catalysts used widely in various industries. In addition, O2 plasma can carry out the reduction of PdO and the decomposition and reduction of palladium nitrate simultaneously at room temperature. We will therefore pursue further investigation of the reduction/decomposition kinetics and mechanism of nonthermal plasmas on 1 wt % Pd/TiO2 in the future. Acknowledgment The financial support of ARP-THECB and the Welch Foundation (#T-0014) is acknowledged. Literature Cited (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. Strong Metal-Support Interactions. Group 8 Noble Metals Supported on Titanium Dioxide. J. Am. Chem. Soc. 1978, 100, 170. (2) Bowker, M.; Fourre, E. Direct Interactions between Metal Nanoparticles and Support: STM Studies of Pd on TiO2(110). Appl. Surf. Sci. 2008, 254, 4225. (3) Uchijima, T. SMSI effect in some reducible oxides including niobia. Catal. Today 1996, 28, 105. (4) Fu, Q.; Wagner, T.; Olliges, S.; Carstanjen, H. Metal-Oxide Interfacial Reactions: Encapsulation of Pd on TiO2(110). J. Phys. Chem. B 2005, 109, 944. (5) Diebold, U. The surface science of titanium dioxide. Surf. Sci. Rep. 2003, 48, 53. (6) Liu, C. J.; Vissokov, G. P.; Jang, B. W.-L. Catalyst Preparation Using Plasma Technologies. Catal. Today 2002, 72, 173. (7) Jang, B. W.-L.; Liu, C. J.; Hammer, T. Editorial. Catal. Today 2004, 89, 1. (8) Kim, S. S.; Kwon, B.; Kim, J. Plasma Catalytic Methane Conversion over Sol-Gel Derived Ru/TiO2 Catalyst in a Dielectric-Barrier Discharge Reactor. Catal. Commun. 2007, 8, 2204. (9) Li, Y. N.; Xie, Y. B.; Liu, C. J. Enhanced Activity of Bimetallic Pd-Based Catalysts for Methane Combustion. Catal. Lett. 2008, 125, 130. (10) Ratanatawanate, C.; Macias, M.; Jang, B. W.-L. Promotion Effect of the Nonthermal RF Plasma Treatment on Ni/Al2O3 for Benzene Hydrogenation. Ind. Eng. Chem. Res. 2005, 44, 9868. (11) Liu, C.; Zou, J.; Yu, K.; Cheng, D.; Zhan, J.; Ratanatawanate, C.; Jang, B. Plasma Application for More Environmentally Friendly Catalyst Preparation. Pure Appl. Chem. 2006, 78, 1227. (12) Wang, Z. J.; Liu, Y.; Shi, P.; Liu, C. J.; Liu, Y. Al-MCM-41 Supported Palladium Catalyst for Methane Combustion: Effect of the Preparation Methodologies. Appl. Catal. B 2009, 90, 570.

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(13) Shanmugam, Y.; Lin, F. Y.; Chang, T. H.; Yeh, C. T. Thermal Decomposition of Metal Nitrates in Air and Hydrogen Environments. J. Phys. Chem. B 2003, 107, 1044. (14) Nissinen, T.; Leskela, M.; Gasik, M.; Lamminen, J. Decomposition of Mixed Mn and Co Nitrates Supported on Carbon. Thermochim. Acta 2005, 427, 155. (15) Shi, C.; Hoisington, R.; Jang, B. W.-L. Promotion Effects of Air and H2 Nonthermal Plasmas on TiO2 Supported Pd and Pd-Ag Catalysts for Selective Hydrogenation of Acetylene. Ind. Eng. Chem. Res. 2007, 46, 4390. (16) Chou, C. W.; Chu, S. J.; Chiang, H. J.; Huang, C. Y.; Lee, C. J.; Sheen, S. R.; Perng, T. P.; Yeh, C. T. Temperature-Programmed Reduction Study on Calcination of Nano-Palladium. J. Phys. Chem. B 2001, 105, 9113. (17) Lee, C. J.; Lee, S. K.; Ko, D. C.; Kim, D. J.; Kim, B. M. Evaluation of Surface and Bonding Properties of Cold Rolled Steel Sheet Pretreated by Ar/O2 Atmospheric Pressure Plasma at Room Temperature. J. Mater. Process. Technol. 2009, 209, 4769.

(18) Larson, B. J.; Helgren, J. M.; Manolache, S. O.; Lau, A. Y.; Lagally, M. G.; Denes, F. S. Cold-Plasma Modification of Oxide Surfaces for Covalent Biomolecule Attachment. Biosens. Bioelectron. 2005, 21796. (19) Raacke, J.; Giza, M.; Grundmeier, G. Combination of FTIR Reflection absorption spectroscopy and work function measurement for insitu studies of plasma modification of polymer and metal surfaces. Surf. Coat. Technol. 2005, 200, 280. (20) Cheng, D. G.; Zhu, X. L. Reduction of Pd/HZSM-5 Using Oxygen Glow Discharge Plasma for a Highly Durable Catalyst Preparation. Catal. Lett. 2007, 118, 260.

ReceiVed for reView March 29, 2010 ReVised manuscript receiVed June 5, 2010 Accepted June 22, 2010 IE100749R