A simple strategy generating hydrothermally stable core-shell platinum

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A simple strategy generating hydrothermally stable coreshell platinum catalysts with tunable distribution of acid sites Houlin Wang, Minghan Liu, Yue Ma, Ke Gong, Wei Liu, Rui Ran, Duan Weng, Xiaodong Wu, and Shuang Liu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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A simple strategy generating hydrothermally stable coreshell platinum catalysts with tunable distribution of acid sites Houlin Wang,a,1 Minghan Liu,b,1 Yue Ma,b Ke Gong,a Wei Liu,a Rui Ran,b Duan Weng,b Xiaodong Wu,*,b and Shuang Liu*,a a

School of Materials Science and Engineering, Ocean University of China, Qingdao 266100,

China b

The Key Laboratory of Advanced Materials of Ministry of Education, School of Materials

Science and Engineering, Tsinghua University, Beijing 100084, China

ABSTRACT: There are critical needs for platinum catalysts with high hydrothermal stability and tunable Pt-acid site proximity, which could not be achieved via traditional methods. Here, we describe a simple strategy (SiO2 alumination combined with controlled removal of capping agent) through which Pt-based core-shell catalysts that tolerant both high-temperature steam and boiling water can be obtained. More importantly, this strategy allows precise control of the distance between acid sites and Pt, thus the interfacial electronic interaction can be cut off without prohibiting the spillover of adsorbed species. This tunable structure not only helps to unravel the mechanism of C3H8 oxidation over acidic Pt catalyst, but also increases the N2 selectivity for NOx selective catalytic reduction. Given that the component of both the “core” and “shell” can be changed easily, this strategy should have wide application in mechanism exploration as well as development of catalysts for various reactions.

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KEYWORDS: Core-shell Pt catalyst, acid catalyst, hydrothermal stability, Pt-acid site distance, NO oxidation, C3H8 oxidation, NH3-SCR

1. INTRODUCTION Metal nanoparticles are one of the most widely applied catalysts that play essential roles in modern chemical industry and environmental protection.1,2 However, the sintering of metal particles at high temperatures can result in severe catalyst deactivation. To solve this problem, core-shell catalysts with metal particles encapsulated by oxide supports have been developed, which exhibit obviously better thermal stability than conventional supported catalysts.2-4 Silica is one of the most common “shell” materials, given that mesoporous SiO2 shell can be obtained easily via colloid chemistry methods.4 Nevertheless, due to the high concentration of silanol groups and the lack of crystallinity, mesoporous SiO2-coated materials are vulnerable to steam and water.5 This weakness restricts the application of core-shell materials in humid situations, such as industrial catalysis,6,7 mobile exhaust after-treatment,8 surface Plasmon resonance (SPR) and drug delivery in body fluid media.9,10 For this reason, it is necessary to develop hydrothermally stable core-shell catalysts to popularize their application. Besides metal particles, the acid sites over catalysts are also of great importance. For example, the key reaction eliminating NOx in diesel vehicle emissions—selective catalytic reduction of NOx with ammonia (NH3-SCR)—relies crucially on the cooperation of acid sites and redox sites.11,12 Meanwhile, bifunctional catalysts for hydrocarbon and CO2 conversion always contain acid sites that work in tandem with active sites (e.g. platinum).13-16 For these catalysts, the proximity of bifunctional sites plays critical roles. For instance, de Jong and co-workers observed that a nanoscale rather than the closest proximity of Pt and acid sites improved the


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isomerization yield during selective hydrocracking of hydrocarbons.14 Moreover, although Gao et al.15 found that closer In2O3-HZSM-5 proximity gave rise to higher target production (C5+) selectivity, both they and Wei et al.16 agreed that the closest proximity led to poison of reactive phases and hereby poor catalytic selectivity. In spite of these observations, there are limited methodologies (selective impregnation, mortar mixing and granule stacking) to alter the distances between active sites and acid sites. Strategies that can control the distance between bifunctional sites precisely at the nanoscale (0–100 nm) have yet to be established. In addition to the reactive sites, one of the most challenging parts of catalytic mechanism exploration comes from the metal-support interface. Given factors like interfacial electron transfer,17 interfacial species18,19 and tandem adsorption/reactions over multi interfaces20 can affect catalytic reactions simultaneously, inconsistent conclusions may be drawn from researches with different focuses. For instance, when dealt with C3H8 total oxidation over platinum catalysts, Apesteguía and co-workers attributed the promotion effect of acidic supports to their hydrocarbon adsorption ability.21-23 This was reasonable since this reaction demonstrated positive order with respect to C3H8. Meanwhile, Yoshida et al. focusing on interfacial electron transfer suggested that the advantage of acidic (electrophilic) supports lay in their ability to change the electron density of platinum, which prevented the highly active metallic Pt0 from being oxidized/deactivated during reaction.24-27 The difference between these two explanations centers on the role of metal-support interface in the reaction. So, developing model catalysts with and without particular interfaces would make mechanism exploration much easier. Here, we report the design and synthesis of a new generation core-shell platinum catalysts. By simply grafting aluminum and manipulating the removal of capping agent, catalysts with high hydrothermal stability and tunable acid site distribution were obtained. Given this strategy


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provided controllable distance between the core material and surface sites, it could be widely extended to elucidate the role of catalyst interfaces in different reactions and help develop practical materials with high catalytic selectivity.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Characterizations. Aluminosilicate (SiO2-Al2O3) is one of the most common acidic supports for industrial catalysis. Recently, it was observed that mesoporous SiO2Al2O3 could be obtained by aluminating SiO2 materials via post-grafting methods.28-30 Inspired by these works, a four-step strategy was developed to synthesis acidic platinum core-shell catalysts (Scheme 1): (1) preparing Pt nanoparticles using polyvinylpyrrolidone (PVP) as the capping agent, (2) SiO2 polymerization around the Pt cores adapting the stöber method,3,4,20 through which Pt@SiO2 core-shell structure was formed, (3) removal of the capping agent by calcinations to produce the mesoporous Pt@SiO2 catalyst (Pt@Si) and (4) fabricating acidic Pt@SiAl catalyst by impregnating Al(NO3)3 onto the SiO2 mesopore walls (Figure 1A). The key strategy leading to tunable acid sites distribution lay in the sequence of capping agent removal and aluminum grafting: if one reverses Step (3) and Step (4), it will be impossible for aluminum to enter the mesopores filled with PVP. Consequently, Pt@Si@SiAl catalyst with an acidic SiO2-Al2O3 outer shell and platinum core separated by a SiO2 layer can be obtained (Figure 1B). Based on this strategy, further thickening or thinning the SiO2 shell by varying the TEOS addition in Step (2) can result in catalysts with tunable distance between Pt and acid sites. See Figure 1C for instance, Pt@Si-t@SiAl with thicker SiO2 isolated layer than Pt@Si@SiAl was synthesized readily by doubling the TEOS addition.


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Scheme 1. Synthesis of Pt@Si, Pt@SiAl and Pt@Si@SiAl in this study.

Figure 1. Elemental mapping of (A) Pt@SiAl, (B) Pt@Si@SiAl and (C) Pt@Si-t@SiAl (thickened SiO2 shell) with energy dispersive X-ray spectroscopy (EDS) showing Pt (yellow), Si (green) and Al (red) signals. Notably, overlapping of the Si and Al signals may result in the formation of orange/yellow-like signals. 2.2. Catalyst Stability Tests. To evaluate the hydrothermal stability of the catalysts, they were heated to 800 °C in 10% H2O/air and maintained under this condition for 10 h. As indicated in


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Figure 2 and Figure 3A (detailed information in Table S1), severe sintering of Pt nanoparticles and cross-linking between SiO2 occurred over Pt@Si-A. This evidenced the poor protective ability of SiO2 shell towards steam at high temperature.5-8 Notably, this SiO2 cross-linking led to collapse of the mesopores (2–4 nm, see Figure 3B), resulting in a drastically decreased Brunauer−Emmett−Teller surface area (from 198 m2/g to 83 m2/g, see Table S1). What’s worse, most of the unsintered Pt nanoparticles (< 6 nm) were buried inside the collapsed SiO2 network and thus isolated from the external environment. As plotted in Figure 3C, Pt@Si-A released little CO2 during the CO titration test, suggesting its restricted accessibility for gaseous reactant. With the introduction of aluminum (Si/Al ratio of ~9:1), two major structural changes occurred. On one hand, as shown in Table S1, partial blockage of the mesopores resulted in reduced surface area (122 m2/g and 134 m2/g for Pt@SiAl and Pt@Si@SiAl, respectively). On the other hand, as indicated by the NMR results (Figure S1), aluminum was built into SiO2 shell to form aluminosilicate phases. According to previous studies, this might reduce the number of free silanol groups (anchoring sites for Al) and protect siloxane (Si−O−Si) bonds from hydrolysis,31,32 which therefore improved the hydrothermal stability of the Al-containing catalysts. From Figure 2, it is clearly that Pt@SiAl, Pt@Si@SiAl and Pt@Si shared similar morphology, but only the former two catalysts remained their original core-shell structure after ageing. The preservation of mesoporous structure (2–4 nm, Figure 3B) conferred Pt@SiAl-A and Pt@Si@SiAl-A relatively large surface area (> 100 m2/g, Table S1) and high amount of exposed Pt atoms (Figure 3C). Notably, due to inevitable vapor-mediated Pt atomic ripening,1 there were still some core-shell particles possessing larger Pt cores and Pt-free hollow outer shells in Pt@SiAl-A and Pt@[email protected] In spite of this, the porous SiO2-Al2O3 shell inhibited surfacemediated Ostwald ripening of Pt and thus prevented its severe sintering.2,33,34 In addition, the


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protection effect of aluminum also worked well in boiling water. As illustrated in Figure S2, SiO2 shell was dissolvable in boiling water,9 while such dissolution was suppressed effectively after aluminum grafting. In sum, the Al-containing catalysts were stable towards both hightemperature steam and boiling water, which improved their application opportunities greatly.

Figure 2. TEM images and Pt particle size distributions of (A) Pt@Si, (B) Pt@SiAl and (C) Pt@Si@SiAl catalysts before (-1) and after (-2) the hydrothermal ageing.


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Besides the contribution to structural stability, the other important role of aluminum is to generate Si–(OH)–Al species and thus improve acidity of the catalysts. According to the NH3TPD results in Figure 3D (corresponding quantitative analysis shown in Figure S3 and Table S2), there were obviously more acid sites on Pt@SiAl and Pt@Si@SiAl than on Pt@Si. The NMR (Figure S1) and IR spectra (Figure S4) further confirmed the abundance of surface Brønsted and Lewis acid sites for the Al-containing samples. After the hydrothermal treatment, structural collapse eliminated most of the acid sites over Pt@Si-A. Contrarily, even with possible dealumination,32 Pt@SiAl-A and Pt@Si@SiAl-A retained about half of their original acid sites, whose amount was 4–5 times higher than those of Pt@Si-A (Table S2).

Figure 3. (A) XRD patterns, (B) Pore size distribution, (C) normalized CO2 production during CO titration and (D) NH3 concentration during NH3-TPD of the fresh (solid) and aged (open) Pt@Si (blue, square), Pt@SiAl (red, cycle) and Pt@Si@SiAl (green, triangle) catalysts.


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2.3. Catalytic Performance Tests. The structural and acidic changes that derived by aluminum grafting may cause crucial influences on catalytic behavior. Pt-based diesel oxidation catalysts (DOC) play indispensible roles in diesel aftertreatment. These catalysts often undergo harsh working conditions like high-temperature steaming.1 Therefore, NO and C3H8 oxidation— the two most important catalytic reactions occurred over DOC—were chosen to evaluate the catalysts’ activity in this work. As shown in Figure 4, after hydrothermal ageing, Pt@Si-A became inert for NO and C3H8 oxidation due to structural collapse and Pt sintering. On the contrary, Pt@Si@SiAl-A and especially Pt@SiAl-A showed high catalytic activities (Table S3). These results reconfirmed the catalysts’ superior hydrothermal stability. Interestingly, for the fresh Pt@SiAl catalyst, in despite of its relatively small surface area (Figure 3B) and limited exposed Pt atoms (Figure 3C) comparing with Pt@Si, it exhibited outstanding catalytic performance in both the reactions (maximum NO conversion of 89.5% at 185 °C, 50% C3H8 conversion at 218 °C). These activities were considerably better than those of Pt@Si (maximum NO conversion of 71.4% at 227 °C, 50% C3H8 conversion at 364 °C). Given metallic platinum (Pt0) is the main active phase responsible for both NO and C3H8 oxidation,27,35 the superiority of Pt@SiAl could probably be owing to its high acidity. According to the theory proposed by Yoshida et al., the 5d band in platinum donated electron density to the acidic (electrophilic) aluminosilicate shell of Pt@SiAl instead of oxygen, thus depressed the formation of inactive PtOx and benefited the reactions.24-26 The nature of such Pt-acid site electronic interaction was theoretically evidenced by Treesukol et al..36 Nevertheless, although Pt@Si@SiAl possessed higher acidity (Figure 3D, Table S2) and more exposed Pt atoms (Figure 3C) than Pt@SiAl, its oxidation activities were obviously lower. Specifically, Pt@Si@SiAl


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demonstrated no better NO oxidation activity compared with the chemically neutral Pt@Si. The activity gap between Pt@SiAl and Pt@Si@SiAl suggested that, there should be more factors other than catalysts’ acidity that ruled the reactions.

Figure 4. Temperature-programmed (10 ºC/min) oxidation curves of (A) NO and (B) C3H8 conversion for the fresh (solid) and aged (open) Pt@Si (blue, square), Pt@SiAl (red, cycle) and Pt@Si@SiAl (green, triangle) catalysts. Reaction conditions: 500 ppm NO/5% O2/N2 balanced (for NO-TPO) or 800 ppm C3H8/2% O2/N2 balanced (for C3H8-TPO); GSHV = 180,000/h. In order to unravel the underlying reaction mechanism, steady-state measurements were performed over the three fresh samples. After excluding the influence of platinum active surface area, Pt@SiAl exhibited significantly higher turnover frequencies (TOFs) for NO oxidation than Pt@Si@SiAl (Figure 5A). Interestingly, the TOFs of Pt@Si@SiAl were always identical to those of Pt@Si. When using C3H8 instead of NO as the reactant, Pt@SiAl still showed the highest TOFs, while the activity of Pt@Si@SiAl turned out to much better than that of Pt@Si (Figure 5B). These results were in good accordance with the temperature-programmed oxidation results (Figure 4, Table S3), which led us to consider possible differences in active phases for NO and C3H8 oxidation in this study. Exposed metallic platinum atoms were the main active sites for catalytic oxidation in this work. However, they could be oxidized from the surface and thus deactivated under oxidizing


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atmosphere.27,35,37 As indicated by the XPS results in Figure 5C (corresponding deconvolution shown in Figure S5), the platinum (pre-reduced at 350 °C in H2 for 30 min) over Pt@Si and Pt@Si@SiAl Pt was only present as Pt2+ after oxidation reactions,37 suggesting the formation of surface PtOx species (or strongly chemisorbed oxygen). Contrarily, Pt@SiAl remained ~40% of its surface platinum sites in a metallic state. It seemed that the acid sites over Pt@SiAl helped inhibit the formation of PtOx, but those over Pt@Si@SiAl did not. The key mechanism behind this difference might lie in the interfaces between Pt and acid sites: Different from Pt@SiAl, the acid sites of Pt@Si@SiAl were blocked away from the Pt core by a SiO2 shell with several nanometers thick (Figure 1B). In this case, no electron transfer between Pt and acid sites was expected,38 thus the aforementioned Pt oxidation-inhibition effect could hardly occur (a scheme illustrating Pt-acid site interaction in different catalysts was shown in Figure S6).24-26 As a consequence, the NO oxidation behavior of Pt@Si@SiAl was similar to that of the chemically neutral Pt@Si. Both of them were suppressed severely due to the oxidation/deactivation of reactive surface Pt0 sites.35 Considering Pt@Si@SiAl showed higher TOF for C3H8 oxidation than Pt@Si, there should be other active phases besides platinum affecting this reaction. According to the results obtained by Apesteguía et al., the adsorption of C3H8 on acidic supports may benefit its deep oxidation.21-23 To gain insights into the affinity of propane on different catalysts, C3H8 chemisorption was measured with DRIFT tests. As shown in Figure 5D, Pt@SiAl and especially Pt@Si@SiAl exhibited remarkably more intense bands at 2920–2960 cm-1 (νas CH3) and 1410–1450 cm-1 (δas CH3) than the chemically neutral [email protected] These results confirmed the correlation between support acidity (Figure 3D, Figure S4) and their propane affinity, which came from the promoted chemisorptions and activation of C3H8 over Brønsted/Lewis acid sites.40,41 As indicated before,


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this improved C3H8 storage provided extra reaction routes for the catalytic oxidation reaction: After activating over platinum, oxygen species can spillover onto the acid sites and react with the absorbed (activated) hydrocarbon species, resulting in excellent activities for both the acidic Alcontaining catalysts.21,22 Given that moleculars’ spillover works within several nanometers,42 the above reaction routes would not be prohibited by the porous SiO2 layer. To summarize, by applying acidic model catalysts (Pt@SiAl, Pt@Si@SiAl) with and without interfaces between Pt and acid sites, it was evidenced that Pt-acid site interaction benefitted the oxidation of both NO and C3H8, while the latter reaction was facilitated additively by the adsorption of C3H8 on acid sites. What’s more, the inner core (Pt nanoparticles) and outer shell (SiO2) of these model catalysts can be easily replaced by other materials, and different materials can grow epitaxially over the SiO2 layer.43,44 This means the above strategy can be applied to build various model catalysts with controlled interfaces, which may therefore help distinguish the roles of interfacial behavior (electron transfer, special reactive phases) and tandem reactions (adsorption and spillover followed by reaction) in different catalytic processes.


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Figure 5. Catalytic performance for (A) NO and (B) C3H8 oxidation of the fresh catalysts during isothermal reactions. Reaction conditions: 500 ppm NO/5% O2/N2 balanced (for NO oxidation) or 800 ppm C3H8/2% O2/N2 balanced (for C3H8 oxidation); GSHV = 180,000/h. (C) Pt 4d spectra of the fresh catalysts after NO oxidation reaction. (D) DRIFT spectra of C3H8 adsorption at 100 ºC over the fresh catalysts, units in Kubelka–Munk. 2.4. Controlling interfaces for application. Gratifyingly, besides mechanism exploration, adjusting the Pt-acid site distance can also contribute to catalytic selectivity. Taking selective catalytic reduction (SCR) of NO with NH3 as an example: 4NH3 + 4NO + O2 → 2N2 + 6H2O

(1)

This reaction plays crucial roles for hazardous NOx elimination, but usually does not work efficiently at low temperatures (< 150 ºC) due to dissatisfied catalytic selectivity.45 Noble metal materials are the earliest studied SCR catalysts, which demonstrate good low-temperature activity but generate undesirable byproduct—N2O.46-48 In this section, we will show that by


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increasing the support acidity as well as separating Pt (the redox site) and Si–(OH)–Al (the acid site), N2O generation over Pt-based catalysts can be effectively inhibited. All the NH3-SCR tests were performed at 100 ºC, at which temperature the reaction is mainly accelerated by the so-called “fast” SCR reaction:45 2NH3 + NO + NO2 → 2N2 + 3H2O

(2)

N2 is the ideal product. However, Reactions (1) and (2) are always paralleled by several sidereactions that give rise to N2O:46,49,50 3NO2 + O2- → 2NO3- + NO (formation of nitrate)

(3-1)

2NH3 + 2NO3- + H2O → 2NH4NO3 + O2- (formation of ammonium nitrate)

(3-2)

NH4NO3 → N2O + 2H2O (decomposition of ammonium nitrate)

(3-3)

NH3 + O2- - 2e- → NH + H2O (oxidation of NH3)

(4-1)

NH + NO + O2- - e- → N2O + OH- (deep oxidation of NH3)

(4-2)

Since the Pt core with strong oxidizing ability promoted the oxidation of both NO and NH3 remarkably, Reactions (3-1), (4-1) and (4-2) would be facilitated over Pt@Si. As a consequence, this catalyst converted most of NO into N2O (~89%) instead of N2 (Figure 6). After introducing acid sites into this system, Pt@SiAl exhibited obviously higher NO conversion (78%) and N2 selectivity (21%) than Pt@Si (68% and 11%). The high NO conversion of Pt@SiAl can be explained by its superior NO oxidation ability (Figure 5A), which provided a continuous supply of NO2 for Reaction (2).45,51 More importantly, the Si–(OH)–Al structure that distributed homogeneously in the SiO2 shell (Figure 1A) played a dual role to improve the catalyst’s N2 selectivity: On one hand, these acid sites could inhibit NO2 adsorption,52 which thus restrained Reaction (3-1) and the following Reaction (3-2). On the other hand, NH3 could adsorb readily on these acid sites without being oxidized (Figure S4). According to the results obtained by Shibata


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et al., the adsorbed ammonia could reduce NOx into N2 without the participant of Pt.51 This provided extra N2 generation routes and therefore increased N2 selectivity. As a consequence, the acidic Pt@SiAl gave rise to less N2O and more N2 compared with the chemically neutral Pt@Si. Interestingly, if one further increases the distance between acid sites and Pt—for example, by using Pt@Si@SiAl instead of Pt@SiAl as the model catalyst—N2O formation can be further depressed. As shown in Figure 6, with average Pt-acid site distances of ~9.5 and ~19.4 nm (Figure 1), the N2 selectivity of Pt@Si@SiAl (~25%) and Pt@Si-t@SiAl (~41%) was about 1.2 and 2 times of that of Pt@SiAl, respectively. Given the NH3 absorbed on acid sites would diffuse onto the redox sites (Pt) during SCR reactions,12 it was suggested that a long Pt-acid site distance could retard this back spillover process42 and hereby inhibit the deep oxidation of NH3 over Pt (Reactions (4-1) and (4-2)). Consequently, Pt@Si-t@SiAl exhibited the highest N2 selectivity. On the basis of the above results, it was supposed that by using sulfur-promoted ZrO2, TiO2 or zeolites with even higher acidity than aluminosilicate as the outer shell, higher N2 selectivity which satisfies practical NOx purification might be achieved. Moreover, given that the distance between platinum and acid sites has been regarded as a crucial factor affecting hydrocarbon selective conversion,13-16 the aforementioned bifunctional systems with tunable distribution of acid sites could be extended to various hydrocarbon conversion reactions.


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Figure 6. Catalytic performance of Pt@Si, Pt@SiAl, Pt@Si@SiAl and Pt@Si-t@SiAl (thickened SiO2 isolated layer) for NH3-SCR at 100 ºC. Reaction conditions: 500 ppm NO/500 ppm NH3/5% O2/N2 balanced; GSHV = 180,000/h.

3. CONCLUSIONS Based on the strategy of aluminum grafting combined with controlled removal of capping agent, Pt-based catalysts with tunable distribution of acid sites were obtained. By using them as model catalysts, conclusions can be drawn as: (1) SiO2 shells demonstrated low hydrothermal stability, while even a thin layer of aluminosilicate outside the silica could provide considerable resistance towards hightemperature steam and boiling water. (2) Interfaces between Pt and acid sites could inhibit the formation of inactive PtOx under oxidizing atmosphere. After separating Pt and acid sites with SiO2, the electronic interaction was cut off, resulting in easily oxidized/deactivated platinum sites. (3) SiO2 isolation did not block the spillover of adsorbed species. As a result, both metallic platinum sites and C3H8 adsorption on acid sites contributed to the catalytic oxidation of


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propane. Meanwhile, NO oxidation that relied solely on the states of platinum was not benefited by interfacial spillover effect. (4) By decreasing the proximity of Pt and acid sites at the nanoscale, N2 selectivity of lowtemperature NH3-SCR could be improved effectively. More importantly, given both the inner core and the outer shell of the model catalysts can be replaced by other materials (noble metals, transition metal oxides, rare-earth metal oxides, zeolites and so forth), we believe this strategy can be extended widely to different catalytic systems. This may shed light on unambiguous identification of active phases, and provide practical catalysts with high catalytic selectivity.

EXPERIMENTAL METHODS Chemicals. Platinum nitrate solution [Pt(NO3)2, 18.02%], polyvinylpyrrolidone (PVP, Mw = 58,000), ethylene glycol, acetone, ethanol, hexane, ammonia solution (28–30%), tetraethyl orthosilicate (TEOS) and aluminum nitrate [Al(NO3)3·9H2O] were purchased from Aladdin. All chemicals were used as received without further purification. Synthesis of Pt@SiO2 with PVP embedded. To obtain Pt nanoparticles with an average size of 5.2 nm, 0.3 mmol of Pt(NO3)2 and 700 mg of PVP were dissolved in 60 ml of ethylene glycol. The solution was heated to 180 °C and held for 20 min under nitrogen protection and magnetic stirring. After cooling to room temperature, acetone (200mL) was added to form a cloudy suspension, which was separated by centrifugation at 8000 rpm for 5 min. Then, the brown precipitate was collected and washed three times by precipitation/dissolution (redispersed in 50 mL of ethanol and then precipitated by adding 200 mL of hexanes). Finally, the PVP stabilized Pt slurry was dispersed in 30 mL of ethanol before use.


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Silica-coated platinum particles were obtained through a Stöber method: the above 30 mL of PVP stabilized Pt suspension was treated ultrasonically for 30 min. Subsequently, 10 mL of H2O and 2 mL of ammonia aqueous solution (28 wt.%) were dropped into the suspension. After magnetic stirring for 5 min, certain amount of TEOS (200 µL for most cases, 400 µL for a “thickened” silica shell) dissolving in 2 mL of ethanol was rapidly injected, and the reaction continued for 12 h at room temperature. Finally, Pt@SiO2 with PVP embedded was collected by centrifugation and dried at 100 °C overnight before use. Synthesis of acidic core-shell Pt catalysts. Pt@SiO2 catalyst (denoted as Pt@Si) with mesoporous silica shell was obtained by calcination of the PVP-embedded Pt@SiO2 at 500 °C (ramp rate = 2 °C/min) for 6 h in static air to eliminate the capping agent (Figure S7). To obtain Pt@SiO2-Al2O3 catalyst (denoted as Pt@SiAl) with acidic shell, the above PVP-embedded Pt@SiO2 was first calcined at 500 °C for 3 h, after which the powder was mixed with 0.034g Al(NO3)3·9H2O dissolved in 5 mL water, ultrasonicated for 15 min and dried at 100 °C overnight. Finally, a calcination treatment at 500 °C for 3 h resulted in the formation of aluminosilicate shell. The synthesis processes for Pt@SiO2@SiO2-Al2O3 catalyst (denoted as Pt@Si@SiAl) were similar to those of Pt@SiAl, except for that no calcination was performed before Al(NO3)3 impregnation. Hydrothermal treatments. To study the effect of high-temperature steam on catalyst stability, the as-received catalysts were treated in an air flow (450 mL/min, provided by an air pump) containing 10% water (provided by an injector followed by a pre-heater, 2.4 mL/h of water = 50 mL/min water vapor) at 800 ºC for 10 h to obtain the hydrothermally aged catalysts with a suffix of “-A”. To evaluate catalysts’ resistance towards boiling water, the as-received catalysts were treated in distilled water at 100 °C for 24h (water-to-sample ratio: 1 L/g).


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Characterizations. Sample morphologies were observed through transmission electron microscopy (TEM) on a JEOL 2100 with an accelerating voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) mapping was carried out with an FEI Titan X 80−300 transmission electron microscope equipped with a spherical aberration (Cs) corrector for the objective lens working at 300 kV. Powder X–ray diffraction (XRD) patterns were performed on a diffractometer (D8 ADVANCE, Bruker, Germany) employing Cu–Ka radiation (λ = 0.15418 nm). Surface area and Barrett–Joyner–Halenda (BJH) pore-size distribution of the catalyst were obtained by N2 physisorption experiments with a JW–BK122F analyzer at -196 ºC. Elemental analysis by ICP-AES was performed using an Agilent 725.

27

Al and

29

Si MAS NMR spectra

were recorded on a JEOL JNM-ECZ600R spectrometer with static field strength of 14.1 T and reference samples of 1M Al(NO3)3 and TMS, respectively. X–ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 Xi system equipped with monochromatic Al Kα (1486.6 eV) X–ray source. The binding energy of C 1s (284.8 eV) was used as an internal standard. Due to the overlapping between Al 2p and Pt 4f spectra, Pt 4d spectra were chosen to investigate the chemical states of platinum. Moreover, CO titration was applied to measure the exposed sites of platinum. Ammonia temperature-programmed desorption (NH3-TPD) was performed to investigate the acidity of the materials. Diffuse reflectance infrared Fourier transform (DRIFT) was used to elucidate different types of acid sites as well as to detect the catalysts’ affinity towards propane. The corresponding experiment details can be found in Supporting Information. Catalytic Reactions. All the reactions were carried in a fixed–bed quartz reactor with the effluent gases (mainly NO, NO2, N2O, C3H8, CO, CO2 and NH3) monitored continuously by a MKS Multigas 2030 infrared analyzer. Prior to each test, the catalyst was pretreated at 350 °C in hydrogen for 30 min to ensure PtOx species were reduced into metallic Pt0.


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Before each temperature-programmed oxidation (TPO) test of NO or C3H8, 50 mg of catalyst (40–60 mesh) diluted with 300 mg of silica pellets were sandwiched by quartz wool. Then, reactant mixtures of 500 ppm NO/5% O2/N2 (for NO-TPO) or 800 ppm C3H8/2% O2/N2 (for C3H8-TPO) were introduced at a total flow rate of 500 mL/min (corresponding to a space velocity of 180,000/h). Finally, the temperature was raised to 500 °C at 10 ºC/min. In this study, NO2 and CO2 were the only products in all the NO-TPO and C3H8-TPO tests, respectively. The outlet NO2 and C3H8 were recorded to calculate the catalytic conversions of NO and C3H8. The used catalysts (denoted with “-s”) were collected and put into XPS tests. Steady-state measurements were performed with procedures similar to those of the TPO tests, except that NO oxidation was measured at 80 °C and 100 °C, while C3H8 oxidation was measured at 150 °C and 170 °C. Diffusion limitations were ruled out by varying GHSV between 180,000/h and 360,000/h. In all the cases, NO/C3H8 conversions were lower than 10%. The turnover frequency (TOF) values were derived by the number of exposed Pt active sites (Pts) and the number of NO/C3H8 molecules converted (see Supporting Information for details). NH3-SCR activities of the catalysts were evaluated at 100 °C. 50 mg of catalyst (40–60 mesh) diluted with 300 mg of silica pellets were sandwiched by quartz wool. Then, reactant mixtures of 500 ppm NO/500 ppm NH3/5% O2/N2 were introduced at a total flow rate of 500 mL/min (corresponding to a space velocity of 180,000/h). The NO conversion, N2O selectivity and N2 selectivity after the reaction was stable for 20 min at each temperature were calculated according to Formulas (5), (6), and (7), respectively: NO conversion (%) =

[NO]in -[NO]out × 100% [NO]in

(5)

N 2 O selectivity (%) =

2 × [N 2 O]out × 100% [NO]in +[NH 3 ]in -[NO]out -[NH 3 ]out

(6)


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 [NO 2 ]out + 2 × [N 2 O]out N 2 selectivity (%) = 1 [NO]in +[NH 3 ]in -[NO]out -[NH 3 ]out

  × 100% 

(7)

ASSOCIATED CONTENT Supporting Information. Details of catalyst characterizations, TOF calculation, Tables S1 to S3 and Figures S1 to S7. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (S. Liu) * E-mail: [email protected] (X. Wu)

Author Contributions 1

H. Wang and M. Liu contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENT The authors would like to acknowledge the National Natural Science Foundation of China (Grant No. 51702304), the China Science and Technology Exchange Center (Grant No. 2016YFE0126600), the National Key R&D Program of China (Project 2017YFC0211202 and 2017YFC0211102), the Natural Science Foundation of Shandong Province (Grant No.


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ZR2017BEM006), the Postdoctoral Science Foundation of Shandong Province (Grant No. 201601009), China Postdoctoral Science Foundation (Grant No. 2015M580607 and 2017T100516) and the Fundamental Research Funds for the Central Universities.

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Table of Contents (TOC)


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TOC for manuscript 85x43mm (300 x 300 DPI)


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Scheme 1. Synthesis of Pt@Si, Pt@SiAl and Pt@Si@SiAl in this study. 178x103mm (300 x 300 DPI)


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Figure 1. Elemental mapping of (A) Pt@SiAl, (B) Pt@Si@SiAl and (C) Pt@Si-t@SiAl (thickened SiO2 shell) with energy dispersive X-ray spectroscopy (EDS) showing Pt (yellow), Si (green) and Al (red) signals. Notably, overlapping of the Si and Al signals may result in the formation of orange/yellow-like signals. 194x150mm (300 x 300 DPI)


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Figure 2. TEM images and Pt particle size distributions of (A) Pt@Si, (B) Pt@SiAl and (C) Pt@Si@SiAl catalysts before (-1) and after (-2) the hydrothermal ageing. 177x266mm (300 x 300 DPI)


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Figure 3. (A) XRD patterns, (B) Pore size distribution, (C) normalized CO2 production during CO titration and (D) NH3 concentration during NH3-TPD of the fresh (solid) and aged (open) Pt@Si (blue, square), Pt@SiAl (red, cycle) and Pt@Si@SiAl (green, triangle) catalysts. 154x161mm (300 x 300 DPI)


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Figure 4. Temperature-programmed (10 ºC/min) oxidation curves of (A) NO and (B) C3H8 conversion for the fresh (solid) and aged (open) Pt@Si (blue, square), Pt@SiAl (red, cycle) and Pt@Si@SiAl (green, triangle) catalysts. Reaction conditions: 500 ppm NO/5% O2/N2 balanced (for NO-TPO) or 800 ppm C3H8/2% O2/N2 balanced (for C3H8-TPO); GSHV = 180,000/h. 241x90mm (300 x 300 DPI)


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Figure 5. Catalytic performance for (A) NO and (B) C3H8 oxidation of the fresh catalysts during isothermal reactions. Reaction conditions: 500 ppm NO/5% O2/N2 balanced (for NO oxidation) or 800 ppm C3H8/2% O2/N2 balanced (for C3H8 oxidation); GSHV = 180,000/h. (C) Pt 4d spectra of the fresh catalysts after NO oxidation reaction. (D) DRIFT spectra of C3H8 adsorption at 100 ºC over the fresh catalysts, units in Kubelka–Munk. 158x136mm (300 x 300 DPI)


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ACS Catalysis

Figure 6. Catalytic performance of Pt@Si, Pt@SiAl, Pt@Si@SiAl and Pt@Si-t@SiAl (thickened SiO2 isolated layer) for NH3-SCR at 100 ºC. Reaction conditions: 500 ppm NO/500 ppm NH3/5% O2/N2 balanced; GSHV = 180,000/h. 264x197mm (300 x 300 DPI)


A simple strategy generating hydrothermally stable core-shell platinum

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