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Catalysts 2013, 3(1), 88-103; doi:10.3390/catal3010088

Article
Coating SiO2 Support with TiO2 or ZrO2 and Effects on Structure and CO Oxidation Performance of Pt Catalysts
Mi-Young Kim 1, Jae-Soon Choi 1,*, Todd J. Toops 1, Eun-Suk Jeong 2, Sang-Wook Han 2, Viviane Schwartz 3 and Jihua Chen 3
1
Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, Knoxville, TN 37932, USA; E-Mails: kimm@ornl.gov (M.-Y.K.); toopstj@ornl.gov (T.J.T.)
2
Department of Physics Education and Institute of Fusion Science, Chonbuk National University, Jeonju 561-756, Korea; E-Mails: eunsuk1986@hanmail.net (E.-S.J.); shan@jbnu.ac.kr (S.-W.H.)
3
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA; E-Mails: schwartzv@ornl.gov (V.S.); chenj1@ornl.gov (J.C.)
*
Author to whom correspondence should be addressed; E-Mail: choijs@ornl.gov; Tel.: +1-865-946-1368; Fax: +1-865-946-1354.
Received: 10 November 2012; in revised form: 13 November 2012 / Accepted: 29 January 2013 /
Published: 6 February 2013

Abstract

: In this work, we studied how TiO2 and ZrO2 coatings enhance the CO oxidation performance of SiO2-supported Pt catalysts under conditions relevant to automotive emissions control. SiO2 was coated with metal oxides TiO2 or ZrO2 by sol-gel method and the subsequent Pt loading was done by incipient wetness method. The prepared catalysts Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 were compared with Pt/SiO2 and Pt/Al2O3 in fresh, sulfated, and hydrothermally aged states. The structure of the catalysts was characterized with BET, XRD, TEM, EDS, EXAFS, XANES, TPD and TPR to help interpret the CO oxidation performance. Higher dispersion, stability, and oxidation state of Pt were achieved on TiO2-SiO2 and ZrO2-SiO2 supports due to stronger metal-support interaction leading to superior CO oxidation performance compared to Pt/SiO2 and Pt/Al2O3. TiO2 and ZrO2 coatings introduced surface acidity but negligible basicity, which is a likely reason for the weak and low adsorption of SO2. The results suggest that the sol-gel coating of SiO2 with metal oxides could be an attractive strategy for designing automotive oxidation catalysts with enhanced performance such as low-temperature activity, sulfur tolerance, and hydrothermal stability.
Keywords:
platinum; SiO2; TiO2; ZrO2; surface coating; sulfur tolerance; hydrothermal stability; CO oxidation; diesel oxidation catalysts

1. Introduction

Diesel oxidation catalysts (DOCs) have been extensively studied due to their various roles in the emission control systems of diesel engine vehicles. For example, DOCs such as Pt/Al2O3 catalyze the oxidation of carbon monoxide (CO), nitric oxide (NO), unburned hydrocarbons (HC) and the soluble organic fraction (SOF) of particulates [1,2,3,4,5,6,7,8,9,10,11]. Despite the successful commercial implementation of DOCs, however, performance loss due to hydrothermal aging is still an important challenge [4,5]. Indeed, DOCs are often exposed to temperatures above 800 °C created to regenerate downstream diesel particulate filters (DPFs) and/or desulfate NOx control devices. Sulfur poisoning of DOCs themselves is another significant technical barrier [2,3].

Alumina is a most widely used support for DOCs due to good thermal stability and interaction with Pt-group metals. Its high reactivity with SOx, however, can lead to the formation of Al2(SO4)3 with resultant performance degradation [2]. Non-basic metal oxides including Ta2O5, Nb2O5, WO3, SnO2, V2O5, TiO2, SiO2 have been studied as supports for Pt to mitigate the adsorption of acidic sulfur oxides (SOx) [9,10]. Kröcher et al. investigated the adsorption and desorption of SOx on commercial DOCs [2]. Their work showed that catalysts with higher SiO2 content required lower desulfation temperatures as SiO2 formed few sulfates. Based on these results, the authors suggested the use of metal-oxides less sensitive to SO2 than Al2O3 to mitigate sulfur poisoning.

While high surface area SiO2 is relatively inert toward SOx, SiO2-supported catalysts suffer from sintering of Pt particles due to the weak interaction between Pt and SiO2. It was recently shown that the dispersion and stability of SiO2-supported Pt can be greatly enhanced by incorporating a layer of TiO2, ZrO2, CeO2 or V2O5 [12,13,14,15]. Interaction between Pt and the supports was enhanced by the formation of Pt-O-M (M: Ti, Zr, Ce, V) linkages which keep Pt particles from coarsening at elevated temperatures. These oxide-coated catalysts showed excellent performance in the oxidation of carbon monoxide, methane and propane.

Building upon the knowledge gained in the previous research [12,13,14,15], the aim of this study was to further understand the effects of metal-oxide coatings (TiO2, ZrO2) on the structure and catalytic performance of Pt/SiO2. In particular, the sulfur tolerance aspect of these metal-oxide-coated Pt/SiO2 catalysts has not been addressed in the previous work. This paper presents data obtained from detailed characterization of catalysts and explains how metal-oxide coatings improve the CO oxidation performance of Pt/SiO2 in fresh, sulfated, and hydrothermally aged states, even outperforming the widely used Pt/Al2O3.

2. Results and Discussion

2.1. Catalyst Morphology and Pt Dispersion

Table 1 lists the composition and BET surface area of supports and Pt catalysts (refer to Section 3 for the synthesis procedure). The surface area of Al2O3 and SiO2 was, respectively, 160 and 195 m2/g before the Pt impregnation and reduced to 142 and 176 m2/g after. The TiO2-coated Pt catalyst had a surface area of 193 m2/g which is almost identical to that of SiO2, indicating that a thin layer of TiO2 was formed on SiO2 surfaces. By contrast, the ZrO2-coated Pt catalyst possessed a significantly lower surface area of 118 m2/g. This substantial reduction could be explained by a higher loading obtained with ZrO2 (ca. 27–28 wt% of Zr) than with TiO2 (ca. 6–7 wt% of Ti). In addition to coating SiO2 surfaces, some ZrO2 was present as stand-alone ZrO2 particles as confirmed by the X-ray diffraction (XRD) patterns of the fresh catalysts (see the peaks at 2θ of 28.2°, 30.2°, 31.4°, 35.2°, 50.2° and 60.0° in Figure 1). These ZrO2 particles are likely to have blocked some of SiO2 micropores thereby reducing BET surface area. The target Pt loading was 1 wt% and actual values determined by ICP and EDS are summarized in Table 1.

Table Table 1. Composition and BET surface area of the supports and Pt catalysts.

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Table 1. Composition and BET surface area of the supports and Pt catalysts.
Samples Composition (wt%)SBET (m2/g)
PtTiZr
ICP/EDSICP/EDSICP/EDS
Al2O3---160
SiO2---195
Pt/Al2O30.60/0.83--142
Pt/SiO20.83/1.12--176
Pt/TiO2-SiO20.94/0.995.85/7.02-193
Pt/ZrO2-SiO20.88/0.59-27.3/28.3118

X-ray diffraction patterns were recorded for each of the samples to investigate both the phases present and their crystallinity (Figure 1). The diffraction peaks of Pt particles appear at 2θ of 39°, 46°, and 67°, while those of Al2O3 are found at 2θ of 32°, 37°, 39°, 45°, 61° and 66° [14]. Our Pt/Al2O3 sample showed diffraction peaks corresponding to Al2O3; the peaks near 2θ of 39° are too small to distinguish between Al2O3 and Pt. Pt/SiO2 exhibited peaks attributable to Pt particles in addition to a broad peak at 2θ of 22° corresponding to amorphous SiO2 structure. Pt/TiO2-SiO2 did not present any diffraction peaks relative to Pt particles; a small peak at 2θ of 25° corresponds to an anatase-type TiO2. Pt/ZrO2-SiO2 did not exhibit any Pt-attributable peak either. The absence of Pt peaks on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 in fresh state suggests that incorporating TiO2 and ZrO2 on SiO2 considerably improved Pt dispersion likely due to a stronger Pt and support interaction.

Transmission electron microscopy (TEM) images in Figure 2 show clear differences in the size of Pt particles among samples. Large Pt particles of 20 nm were observed on Pt/SiO2 with an average particle size of ca. 4 nm. In contrast, uniform and fine dispersion of Pt was achieved on TiO2- and ZrO2-coated SiO2 with an average particle size of less than 3 nm. This value is comparable to that obtained on Al2O3 (< 3 nm). Coating SiO2 with TiO2 and ZrO2, therefore, appears to be an effective way to enhance the interaction between Pt and supports.

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Figure 1. XRD patterns of Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 in fresh and hydrothermally aged state. The large SiO2 peaks observed on all the hydrothermally aged catalysts were due to the quartz powder physically mixed with the catalysts for the reactor evaluation.

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Figure 1. XRD patterns of Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 in fresh and hydrothermally aged state. The large SiO2 peaks observed on all the hydrothermally aged catalysts were due to the quartz powder physically mixed with the catalysts for the reactor evaluation.
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The micro-structural properties determined using extended X-ray absorption fine structure (EXAFS), which reflects the averaged environment around a selected atomic species, confirm the XRD and TEM observations on Pt dispersion and provide more detailed information. The EXAFS data were analyzed using IFEFFIT software package [16] and fitted to the EXAFS theoretical calculations [17]. The EXAFS results are summarized in Table 2. An atom of Pt in a closest packing structure has 12 neighboring Pt atoms as the first nearest neighbors, whereas an atom of Pt in small particles has a smaller Pt coordination number (CN) [18,19]. The reduction in average CN occurs because the surface-to-volume ratio increases as particle size decreases and particles on the surface have lower CNs.

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Figure 2. TEM images of fresh Pt catalysts supported on Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2 or Pt/ZrO2-SiO2; a few representative Pt particles are marked with circles in the images.

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Figure 2. TEM images of fresh Pt catalysts supported on Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2 or Pt/ZrO2-SiO2; a few representative Pt particles are marked with circles in the images.
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For a given catalyst sample, there was a difference in the CN of Pt-Pt atomic pairs between reductive and oxidative gas environments: the CNs for all the samples were higher in H2 flowing condition than O2 flowing condition. This indicates that re-dispersion of Pt particles occurred in O2 flow. It has been reported that Pt particles sintered at high temperature in H2 flow can be re-dispersed by O2 treatment above 400 °C [20,21,22,23,24]. In the H2 flowing condition, the CN of the Pt-Pt atomic pairs of Pt/Al2O3was 10.4 at around R = 2.7 Å and that of Pt/SiO2 was 10.5. This similarity between Pt/Al2O3 and Pt/SiO2 appears to contradict the TEM results (Figure 2) which evidenced the presence of larger particles on SiO2. The apparent discrepancy could be explained by the fact that EXAFS is a bulk, average technique while TEM can discern large particles (e.g., 20 nm Pt/SiO2) present on samples with small average particle sizes (e.g., 4 nm particles for Pt/SiO2 support). On the other hand, the CNs of the Pt-Pt atomic pairs were 5.2 and 5.1 on TiO2-SiO2 and ZrO2-SiO2, respectively. This CN of ca. 5 suggests an average particle size of about 1 nm [19]. This value agrees well with the TEM data further confirming that TiO2 and ZrO2 coatings enhanced Pt dispersion on SiO2. The Pt-Pt distance for Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 were 2.743, 2.743, 2.651 and 2.604, respectively. In a previous study, indeed, the distance of Pt-Pt atomic pairs was shown to decrease from 2.75 to 2.60 Å with decreasing Pt particle size [25]. The shortened distance between Pt and Pt atoms further corroborates the improved Pt dispersion.

Table Table 2. EXAFS results a.

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Table 2. EXAFS results a.
CatalystCondition bAtomic pairCNR (Å)σ22)
Pt/Al2O3H2Pt-Pt10.4(4c)2.743(5)0.011(1)
O2Pt-Pt9.2(4)2.758(3)0.010(1)
Pt/SiO2H2Pt-Pt10.5(2)2.743(1)0.012(1)
O2Pt-Pt8.2(2)2.758(2)0.010(1)
Pt/TiO2-SiO2H2Pt-Pt5.2(2)2.651(3)0.014(1)
O2Pt-O3.7(1)2.001(3)0.003(1)
Pt/ZrO2-SiO2H2Pt-Pt5.1(1)2.604(2)0.012(1)
O2Pt-O3.1(1)2.024(2)0.004(1)

a Determined from the fitted EXAFS spectra of the Pt catalysts at Pt LIII-edge; S02 = 0.86, k3 weighted; CN: coordination number; R: bond length; σ2: Debye–Waller factor. b Measured at 400 °C. c The calculated limits of accuracy in the last reported digit.

In O2 flow, EXAFS demonstrated that O atoms existed at 2.0 Å from a Pt atom whereas the Pt atoms at 2.7 Å nearly disappeared on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2. This finding indicates that most of the Pt particles were oxidized in the O2 flowing condition at 400 °C. In fact, it has been known that small Pt particles tend to be easily oxidized [26,27]. The presence of the Pt-O atomic pairs on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 could also be due in part to the formation of Pt-O-M (M: Ti, Zr) bonds at the interface between Pt particles and TiO2 and ZrO2, which stabilized small Pt particles [14,15,16,17,28,29]. Compared to the data obtained in H2 flow, those collected in O2 flow provide information on the chemical state and dispersion of Pt particles more relevant to interpreting CO oxidation performance.

2.2. Redox Properties of Supported Pt

The Pt LIII-edge X-ray absorption near edge structure (XANES) spectra obtained in H2 and O2 flows are shown in Figure 3. The peaks near 11567 eV in the XANES spectra, namely white lines, are mainly the result of electron transition from 2p2/3-band to 5d-band in the X-ray absorption process [30,31]. The peak intensity of the white lines is proportional to the 5d–band vacancy. Furthermore, it can also depend on the size and morphology of Pt particles. There were significant differences in the XANES spectra depending on the flowing gas type. The XANES spectra obtained in H2 flow were practically identical for all catalysts and the reference Pt foil. This similarity means that the contribution of particle size and morphology to the white line intensity is negligible in a reductive gas environment. By contrast, the XANES spectra obtained in O2 flow varied considerably with the type of catalyst support. While the white-line intensity for Pt/Al2O3 and Pt/SiO2 increased slightly, that of Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 increased dramatically when exposed to the O2 flow. These results indicate that the Pt atoms dispersed on the TiO2- and ZrO2-coated SiO2 had a greater affinity towards oxygen leading to higher oxidation states of Pt.

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Figure 3. Pt LIII-edge XANES spectra obtained at 400 °C for Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, Pt/ZrO2-SiO2, and Pt foil in H2 and O2 flow.

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Figure 3. Pt LIII-edge XANES spectra obtained at 400 °C for Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, Pt/ZrO2-SiO2, and Pt foil in H2 and O2 flow.
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2.3. Surface Acidity, Basicity and Sulfur Tolerance

It has been reported that high acidity and low basicity induce high sulfur tolerance of catalytic materials [2,9,10,32,33]. The acidic and basic properties of the Pt catalysts studied here were assessed using the temperature programmed desorption (TPD) of NH3 and CO2, respectively. The TPD profiles varied significantly for the different catalyst supports as shown in Figure 4. The area under the TPD curve is directly proportional to the amount of NH3 or CO2 desorbed from the surface, and therefore provides a relative measure of the number of surface acidic or basic sites. In addition, the temperatures of the desorption peak maxima (Tm) provide information about the relative strength of the acidic or basic sites.

For NH3, Pt/Al2O3 had both the largest desorption peak and highest Tm around 250 °C, while Pt/SiO2 exhibited negligible NH3 desorption. Coating SiO2 with TiO2 generated surface acidity as manifested by a desorption peak near 190 °C. ZrO2 coating also led to surface acidity with an NH3 desorption peak (150–250 °C) slightly larger than that of the TiO2-coated catalyst. The amount and strength of surface acidic sites estimated from NH3-TPD were in the order of Pt/Al2O3 > Pt/ZrO2-SiO2 > Pt/TiO2-SiO2 > Pt/SiO2. These results are in line with an earlier report that TiO2-SiO2 and ZrO2-SiO2 mixed oxides possess strong acidity, while the individual oxides TiO2 and ZrO2 show relatively weak acidity [34].

The profiles of CO2-TPD showed that Pt/Al2O3 had significant basicity as well with a peak at 150 °C. On the other hand, Pt/SiO2 had no desorption confirming its inertness in terms of surface basicity. Contrary to the surface acidity, coating with TiO2 or ZrO2 did not generate a significant amount of surface basicity as confirmed by near zero CO2 desorption from Pt/TiO2-SiO2 and a minor desorption from Pt/ZrO2-SiO2 at around 150 °C. The relative amounts of NH3 and CO2 desorbed are summarized in Table 3.

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Figure 4. NH3 and CO2 TPD profiles for Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 catalysts.

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Figure 4. NH3 and CO2 TPD profiles for Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 catalysts.
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Our data in Table 3 suggest that Pt/SiO2 should be the least reactive toward SO2 adsorption due to its inertness followed by Pt/TiO2-SiO2 and Pt/ZrO2-SiO2. To confirm this conjecture, we performed H2 temperature-programmed reduction (TPR) of the sulfated catalysts. The TPR-desulfation data are compiled in Figure 5, Table 3. As expected, Pt/Al2O3 adsorbed the most sulfur species, which were also more stable (higher release temperature), while sulfur adsorption/desorption was negligible on Pt/SiO2. Relatively small amount of sulfur species of weak adsorption strength were present on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 catalysts.

Table Table 3. Acidic and basic sites of Pt catalysts and amount of desorbed sulfur during temperature-programmed reduction (TPR).

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Table 3. Acidic and basic sites of Pt catalysts and amount of desorbed sulfur during temperature-programmed reduction (TPR).
Catalyst Relative amount of acidic sitesRelative amount of basic sitesAmount of desorbed sulfur (μmol/gcat)
Pt/Al2O311726
Pt/SiO20037
Pt/TiO2-SiO20.350171
Pt/ZrO2-SiO20.630.08368
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Figure 5. Sulfur release profiles during H2-TPR of the sulfated Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2.

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Figure 5. Sulfur release profiles during H2-TPR of the sulfated Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2.
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2.4. Hydrothermal Stability of Pt Particles

XRD peaks attributable to Pt particles (2θ = 39°, 46°, and 67°) were observed on all the catalysts after a 2-h hydrothermal aging at 800 °C preceded by a TPR desulfation to 800 °C (Figure 1). The significant growth of Pt peaks (XRD) compared to the fresh state indicates significant sintering of Pt particles. The average Pt particle sizes were estimated from diffraction peaks using the Scherrer equation: Pt/Al2O3 18.4 nm, Pt/SiO2 44.4 nm, Pt/TiO2-SiO2 13.6 nm, and Pt/ZrO2-SiO2 12.3 nm. Relative changes in peak intensity (i.e., extent of Pt particle growth) were, however, dependent on the support type. Pt/SiO2 exhibited the highest/sharpest diffraction peaks, indicating significant sintering of Pt particles. Pt/ZrO2-SiO2 was the most resistant to Pt sintering as evident from the smallest diffraction peaks. This observation is consistent with a previous study which reported that ZrO2-coating enhances the stability of Pt/SiO2 at high temperatures under an oxidizing environment [14]. Our results also show that Pt dispersion was maintained on ZrO2-SiO2 even during sulfation and high-temperature desulfation in reducing conditions.

2.5. Catalytic Performance in CO Oxidation

Figure 6 shows the catalytic CO oxidation performance of the four Pt catalysts in fresh, sulfated, and hydrothermally aged states. In addition to the temperature-conversion profiles (Figure 6a–c), the T50% value (“light-off temperature”, defined as the temperature at which 50% conversion of CO is achieved) for each catalyst at different states is summarized using a bar graph in Figure 6d. It is to note that there were some differences in Pt loading (0.6–0.94% by ICP, Table 1) among the catalysts. However, those differences were not significant enough to affect the overall interpretation of the light-off curves. That is, the order of the light-off temperatures remains the same whether or not the CO conversion activity is normalized by Pt loading (results not shown). As expected based on the characterization data, the catalytic activity was sensitive to the type of support and catalyst state. The Pt catalysts supported on TiO2- and ZrO2-coated SiO2 had a higher catalytic activity than Pt/SiO2 and Pt/Al2O3 regardless of treatment type. The T50% was 148 °C on Pt/TiO2-SiO2 and 165 °C on Pt/ZrO2-SiO2 in the fresh state (Figure 6a). Pt/Al2O3 and Pt/SiO2 both had a T50% of around 218 °C. However, it should be noted that the fresh Pt/Al2O3 was intrinsically more active than Pt/SiO2: the former achieved higher CO conversion at temperatures below T50% where the apparent catalytic activity was less affected by heat and mass transfer limitations. The superior activity of TiO2- and ZrO2-coated catalysts agrees well with their higher Pt dispersion. It has been reported that the CO oxidation proceeds via a Langmuir-Hinshelwood mechanism involving reaction between CO and dissociated-O2 adsorbed on Pt surfaces [31,35], and that high oxygen coverage of Pt surfaces is favorable for CO oxidation whereas high CO coverage limits catalytic performance (“self-poisoning”) [36]. Strong interaction between Pt and metal oxide (TiO2 and ZrO2) have been shown to suppress the adsorption of CO [37,38]. Moreover, our XANES results suggest that the interaction between Pt and O was enhanced by TiO2 and ZrO2 coatings as deduced from the increased Pt oxidation state. The weakly adsorbed CO and high O coverage on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 could have contributed to the superior catalytic performance compared to Pt/Al2O3 which also had relatively good Pt dispersion.

The CO oxidation activity of all the catalysts was significantly decreased by sulfation (Figure 6b,d). For instance, the T50% of Pt/SiO2 and Pt/Al2O3 increased to 260 and 240 °C, respectively. Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 also lost performance with a T50% of roughly 200 °C. Nonetheless, the performance advantage of Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 was still maintained after the sulfation, which can be explained in part by their weak interaction with SO2 (Table 3, Figure 5). Pt/TiO2-SiO2 had the lowest T50% in both fresh and sulfated states. Another remarkable observation on Pt/TiO2-SiO2 is its very low initiation temperature as shown in Figure 6a–c. For example, the catalyst achieved 10% conversion below 100 °C in the fresh state. It is also worth mentioning that Pt/SiO2 suffered a significant activity loss even though it adsorbed negligible amount of SO2. This loss might be due to Pt sintering during sulfation. It has been reported that SO2 can facilitate Pt sintering at relatively low temperatures [39]; it is likely that SO2-induced Pt sintering was most pronounced on Pt/SiO2 because of the weak metal-support interaction.

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Figure 6. Catalytic performance of Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 in the oxidation of carbon monoxide with a total flow rate of 200 ml/min (1% CO, 4% O2, 5% H2O, Ar balance); (a) fresh catalysts; (b) sulfated catalysts; (c) hydrothermal aged catalysts and (d) light-off temperatures (T50%).

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Figure 6. Catalytic performance of Pt/Al2O3, Pt/SiO2, Pt/TiO2-SiO2, and Pt/ZrO2-SiO2 in the oxidation of carbon monoxide with a total flow rate of 200 ml/min (1% CO, 4% O2, 5% H2O, Ar balance); (a) fresh catalysts; (b) sulfated catalysts; (c) hydrothermal aged catalysts and (d) light-off temperatures (T50%).
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The catalytic performance further degraded after hydrothermal aging (Figure 6c). Again, Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 outperformed the other catalysts. Unlike the fresh and sulfated states, the T50% of Pt/ZrO2-SiO2 was lower than that of Pt/TiO2-SiO2. This trend is in good agreement with the XRD data (Figure 1) which highlight the superior stability of Pt particles on ZrO2-SiO2. Little activity change was observed on Pt/SiO2 after the hydrothermal aging step; it is likely that an extensive Pt sintering had already occurred during the sulfation step as described above.

3. Experimental Section

3.1. Preparation of Pt Catalysts

Amorphous fumed silica (Strem Chemicals, 0.012 μm) was used as a support for the preparation of Pt catalysts. Titanium and zirconium oxides were coated on the SiO2 surface following the procedure described in previous papers [12,13,14,15]. SiO2 was first dehydrated with anhydrous ethanol (90% ethanol with 5% isopropyl alcohol and 5% methyl alcohol as denaturants, Aldrich) and reacted at 80 °C for 3 h with titanium(IV) n-butoxide (98+%, Alfa Aesar) or zirconium(IV) n-propoxide (70% w/w in n-propanol, Alfa Aesar) dissolved in ethanol. TiO2 or ZrO2-coated SiO2 were obtained by removing the non-reacted precursors through washing with ethanol followed by drying at 100 °C and calcining at 500 °C for 2 h. Tetraamineplatinum(II) nitrate (50.0+% Pt basis, Aldrich) was impregnated on TiO2-SiO2 and ZrO2-SiO2 supports by incipient wetness method to 1 wt% Pt loading. After the impregnation, the catalysts were dried at 100 °C in air and treated with hydrogen peroxide (35% w/w aqueous solution, Alfa Aesar) at 60 °C. The Pt catalysts treated with hydrogen peroxide were dried at 100 °C and reduced in a flow of 10% H2 in Ar at 500 °C for 2 h. The prepared catalysts were named as Pt/TiO2-SiO2 and Pt/ZrO2-SiO2. For comparison, Pt catalysts supported on uncoated SiO2 (Pt/SiO2) and Al2O3 (Pt/Al2O3; acidic alumina, pore size 58 Å~150 mesh, Aldrich) were also prepared by incipient wetness method.

3.2. Characterization

The content of Ti, Zr, and Pt of the prepared catalysts was determined using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Optima 4300 DV, Perkin-Elmer, Waltham, MA, USA) and an Energy Dispersive X-ray Spectroscopy (EDS) system (EX-200, Horiba, Kyoto, Japan) attached to a scanning electron microscope (S-4700, Hitachi, Tokyo, Japan). The X-ray diffraction patterns of catalysts were recorded on a powder X-ray diffractometer (X’Pert PRO, PANalytical, Almelo, The Netherlands) operated at 45 kV and 40 mA using CuKα radiation (Kα = 0.154178 nm). A transmission electron microscope (LIBRA-120, Carl Zeiss, Oberkochen, Germany) equipped with a LaB6 filament was employed to examine the dispersion of Pt particles. The acceleration voltage was 120 kV. TEM samples were prepared by dropping ethanol suspension of Pt catalysts on a copper grid. The particle size of Pt was estimated from the digitized TEM photos using image analysis software (Scion Image, Scion Corporation, Frederick, MD, USA). The surface area of catalysts was determined using an automatic volumetric adsorption apparatus (Gemini 275, Micromeritics, Norcross, GA, USA). Surface areas were calculated using the Brunauer-Emmett-Teller (BET) equation.

Temperature-programmed desorption experiments were carried out using a flow reactor system. For each TPD experiment, 0.1 g of catalyst was placed in a U-shaped quartz tube (8 mm I.D.) and pretreated at 600 °C for 0.5 h in an Ar flow. After cooling to 100 °C, the catalyst was exposed to a flow of 0.1% NH3 in Ar for 1 h, then to an Ar flow for a 0.5-h purge. Subsequently, desorption was programmed at 10 °C/min to 600 °C in an Ar flow. The total flow rate was 200 mL/min (STP) for all the steps. The procedure for the CO2 TPD experiments was identical except that the adsorption was done in a flow of 10% CO2 in Ar at 50 °C followed by a 1-h purge. Gas composition was continuously analyzed by a quadrupole mass spectrometer (RGA100, SRS, Sunnyvale, CA, USA). The m/z ratios monitored were 14 for NH3, 18 for H2O, 40 for Ar and 44 for CO2.

The in-situ X-ray absorption spectra (EXAFS and XANES) were recorded at the 9 BM of the Advanced Photon Source. X-ray spectra were monochromated by a double crystal monochromator composed of Si(111) and Si(222) crystals. The X-ray absorption of platinum atoms was measured at the Pt LIII-edge (11,564 eV). The measurements were done in a flow of 3.5% H2 in He (100 mL/min, STP) and a flow of 5% O2 in He (100 mL/min, STP) at 400 °C to study changes in chemical states in reductive or oxidative environment. The X-ray absorption data were processed using an IFFEFIT program based on the FEFF8 theoretical model. A S02 value of 0.86 was used for a curve fitting of EXAFS spectra. The k3-weighted data were fitted in the R range of 1.5–3.3 Å.

3.3. Evaluation of Catalytic CO Oxidation Performance

The catalytic performance of the prepared Pt catalysts was compared for CO oxidation under atmospheric pressure. A flow reactor equipped with a quadrupole mass spectrometer (RGA100, SRS, Sunnyvale, USA) and a fluorescent SO2 analyzer (100 A UV fluorescence SO2 analyzer, Teledyne API, San Diego, CA, USA) was employed. As previously described in [40], there was a reactor loaded with an oxidation catalyst downstream of the main reactor to oxidize H2S to SO2 because the sulfur analyzer detected only SO2. For each reactor run, a physical mixture of 0.1 g of Pt catalyst and 0.3 g quartz powder was positioned between plugs of quartz wool in a U-shaped quartz tube. Prior to experiments, the catalysts were pretreated in an Ar flow at 400 °C for 1 h. The reaction gas consisting of 1% CO, 4% O2, and 5% H2O in Ar balance was fed into the reactor at a total flow rate of 200 mL/min (STP). The performance was evaluated by continuously increasing the reaction temperature from 60 to 300 °C at 2 °C/min. The effluent gas composition was analyzed with the mass spectrometer. The monitored m/z ratios were 17 for H2O, 28 for CO, 40 for Ar and 44 for CO2. To obtain CO values, the contribution from CO2 fragmentation (m/z 28) was taken into account.

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Figure 7. Sequence of reactor evaluation of catalysts including sulfation, desulfation, and hydrothermal aging steps.

Click here to enlarge figure

Figure 7. Sequence of reactor evaluation of catalysts including sulfation, desulfation, and hydrothermal aging steps.
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Three consecutive CO oxidation runs were performed for each catalyst as shown in Figure 7. After the first run, the catalysts were sulfated at 400 °C for 3 h in a flow of 50 ppm of SO2, 4% O2, 5% H2O, and Ar balance, and evaluated again for CO oxidation. Subsequently, the catalysts were desulfated by temperature-programmed reduction to 800 °C at 10 °C/min in a H2 flow with effluent-gas sulfur analysis. Then, the desulfated catalysts were hydrothermally aged at 800 °C for 2 h. Finally, the hydrothermally aged catalysts were evaluated in CO oxidation.

4. Conclusions

We investigated the impact of TiO2 and ZrO2 coatings on the performance of SiO2-supported Pt catalysts. Key findings in the present study were:

  • Coating SiO2 with TiO2 or ZrO2 via sol-gel method before Pt impregnation led to enhanced dispersion and hydrothermal stability of Pt due to stronger interaction between Pt and supports;

  • TiO2 and ZrO2 coatings increased the oxidation state of Pt in O2 environment;

  • TiO2 and ZrO2 coatings generated acidity but negligible basicity on the catalyst surface, which explains relatively low and weak sulfur uptake on Pt/TiO2-SiO2 and Pt/ZrO2-SiO2;

  • Pt/TiO2-SiO2 and Pt/ZrO2-SiO2 exhibited better CO oxidation performance than Pt/SiO2 and Pt/Al2O3 in fresh, sulfated, and hydrothermally aged states due to the favorable properties brought by metal-oxide coating as described above;

  • Results suggest that the sol-gel coating of SiO2 with metal oxides can be an attractive strategy for designing automotive oxidation catalysts with enhanced performance such as low-temperature activity, sulfur tolerance, and hydrothermal stability;

  • Further research is necessary to further our understanding of the structure and chemistry of TiO2 and ZrO2 coatings; a follow-up study of Pt/TiO2 and Pt/ZrO2 will be particularly helpful. Furthermore, as Pd is another widely used metal component of state-of-the-art DOCs, it would be appropriate to study Pd catalysts to determine if oxide coating has similarly beneficial impact on catalyst performance.

Acknowledgments

This research was sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program, with Ken Howden and Gurpreet Singh as the Program Managers. The contribution of Mi-Young Kim was supported in part by the National Research Foundation of Korea (Grant No.: NRF-2010-357-D00048) and by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administered jointly by the Oak Ridge Institute for Science and Education and the Oak Ridge National Laboratory. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, and at the Advanced Photon Source, which is sponsored at Argonne National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy under Contract No. DE-AC02-06CH11357. The authors would like to thank Prof. Gon Seo at Chonnam National University and colleagues William P. Partridge, Jr. and Josh A. Pihl for useful discussion and technical reviews.

Declaration

This manuscript has been authored by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Conflict of Interest

The authors declare no conflict of interest.

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