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. 2024 Mar 20;2(6):245–255. doi: 10.1021/prechem.4c00004

Atomically Precise Design of PtSn Catalyst for the Understanding of the Role of Sn in Propane Dehydrogenation

Hui Ye †, Lina Cao †,*, Minghui Gu †, Han Nie †, Qingqing Gu ‡, Bing Yang ‡, Yunxing Bai †, Qinxue Nie †, Weixin Huang †, Junling Lu †,*
PMCID: PMC11504697  PMID: 39474203

Abstract

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Propane dehydrogenation (PDH), an atom-economic reaction to produce high-value-added propylene and hydrogen with high efficiency, has recently attracted extensive attention. The severe deactivation of Pt-based catalysts through sintering and coking remains a major challenge in this high-temperature reaction. The introduction of Sn as a promoter has been widely applied to improve the stability and selectivity of the catalysts. However, the selectivity and stability of PtSn catalysts have been found to vary considerably with synthesis methods, and the role of Sn is still far from fully understanding. To gain in-depth insights into this issue, we synthesized a series of PtSn/SiO2 and SnPt/SiO2 catalysts by varying the deposition sequence and Pt:Sn ratios using atomic layer deposition with precise control. We found that PtSn/SiO2 catalysts fabricated by the deposition of SnOx first and then Pt, exhibited much better propylene selectivity and stability than the SnPt/SiO2 catalysts synthesized the other way around. We demonstrate that the presence of Sn species at the Pt-SiO2 interface is of essential importance for not only the stabilization of PtSn clusters against sintering under reaction conditions but also the promotion of charge transfers to Pt for high selectivity. Besides the above, the precise regulation of the Sn content is also pivotal for high performance, and the excess amount of Sn might generate additional acidic sites, which could decrease the propylene selectivity and lead to heavy coke formation. These findings provide deep insight into the design of highly selective and stable PDH catalysts.

Keywords: Propane dehydrogenation, PtSn catalysts, sintering, coke formation, acidity

1. Introduction

Propylene is a platform molecule for the synthesis of a variety of downstream petrochemicals, including polypropylene, acrylonitrile, propylene oxide, acrolein, acetone, and various high-value chemicals/intermediates.1 Fluid catalytic cracking (FFC) and steam cracking of naphtha and light diesel are the traditional routes for propylene production. However, these routes have appeared to hardly satisfy the rapid growth in market demand for propylene in recent years.2−4 New approaches for propylene production, such as propane dehydrogenation (PDH), methanol-to-olefins (MTO), and the Fischer–Tropsch-to-olefins process, have been widely developed.1 Among these approaches, PDH is particularly attractive for its atom-economic process.

1. 1

where the products of propylene and H2 are both value-added chemicals, along with superior selectivity and production yield of propylene. Moreover, the emergence of a large amount of propane gas from the large-scale exploitation of natural gas and shale gas greatly facilitated the rapid expansion of the PDH process on a large scale.5 At present, the PDH units in industry have utilized two main types of catalysts: Pt-based catalysts (the UOP Oleflex process) and Cr-based catalysts (the ABB Lummus Catofin process), with market shares of 60% and 30%, respectively.6

Pt-based catalysts have garnered significant interest due to their environmental friendliness, high activity, and exceptional propylene (C3H6) selectivity.7,8 However, the PDH reaction is highly endothermic along with an increase in the number of gas molecules (eq 1). According to Le Chatelier’s principle, this implies that high reaction temperature and/or low gas partial pressure are necessitated to achieve reasonable conversions.9 Typically, PDH is operated in the temperature range of 550–750 °C to achieve propane (C3H8) conversions exceeding 50% at 1 bar.10 However, at such high temperatures, the rates of both C–H bond cleavage and undesired C–C bond cleavage are accelerated, leading to adverse side reactions, including hydrogenolysis, cracking, and coke deposition. Therein, the coke deposition could partially or even completely block the active metal sites, leading to a significant decrease in activity. Moreover, high reaction temperature also induces severe sintering of Pt nanoparticles (NPs) mainly through the Ostwald ripening mechanism, which decreases the number of Pt active sites and further causes catalyst deactivation.11,12 The development of highly coking- and sintering-resistant Pt catalysts is of essential importance for the PDH reaction.

Combining Pt with other metal promoters (Sn,13−16 Cu,17,18 Zn,19,20 Fe21 and Ga,22 etc.) to form alloys or Pt-metal composites to divide Pt ensembles is an efficient approach to suppress coke formation and improve the stability of Pt NPs. Compared to other promoters, Sn has been widely used in industry and has received extensive attention in academy.1,23,24 It has been demonstrated that the Sn promoter helps to separate larger Pt ensembles into smaller ones, thereby diminishing coke formation.25−27 In addition, strong charge transfers between Sn and Pt atoms make Pt atoms more electron rich and widen the d-band of Pt, thereby weakening the adsorption energies of propylene/propyl on Pt, which would further stimulate propylene desorption for high selectivity and transfers of coke from Pt metal sites to the adjacent support surfaces for high coking resistance.28−31 For instance, Motagamwala et al. reported that silica-supported Pt1Sn1 alloy NPs with a particle size less than 2 nm showed excellent stability and selectivity to propylene (>99%) at thermodynamically limited conversion levels.32

Nonetheless, the catalytic performance of PtSn catalysts has been found to vary greatly with synthesis methods. For example, Deng et al. prepared a series of Pt–Sn/SiO2 catalysts with a particle size of 2.2–3.3 nm by the sequential impregnation method and investigated the effect of reduction methods on the catalytic performance of Pt–Sn/SiO2 catalysts in PDH. They demonstrated that the formation of Pt3Sn alloy NPs via direct reduction is highly active (initial C3H8 conversion of ∼27%) and selective (>99%); in contrast, PtSn alloy NPs generated via the calcination reduction method showed very poor activity (initial C3H8 conversion of ∼4%) along with and a slightly lower C3H6 selectivity (95%).33 Kaylor et al. also reported that, compared to those prepared by incipient wetness impregnation, the 1Pt0.6Sn/SiO2 catalyst prepared by the polyol synthesis method exhibited considerably higher catalyst stability without significant changes in metal particle size (3.1 nm vs 3.4 nm) after catalyst regeneration.34 The optimum PtSn ratio also varies in different literature. Apparently, how the Sn promoter improves the stability and selectivity of PtSn catalysts is still far from fully understanding.

In this work, we precisely synthesized PtSn clusters with a size of ∼1 nm on a SiO2 support using atomic layer deposition (ALD) by varying the sequence of Pt and SnOx deposition and their contents (Scheme 1). Very interestingly, we found that the deposition sequence had a markable impact on the activity, selectivity, and stability of the PtSn catalysts in the PDH reaction. The PtSn/SiO2 catalysts fabricated by first depositing SnOx and then Pt, exhibited a much higher C3H6 selectivity and stability than the SnPt/SiO2 catalysts synthesized the other way around. We revealed that the presence of Sn species at the Pt-SiO2 interface is of essential importance for achieving a high catalytic performance in terms of stability against sintering and selectivity. Nonetheless, the excess amount of Sn might create additional acidic sites on catalyst supports, which would generate heavy coke formation and lower the C3H6 selectivity.

Scheme 1. Schematic Illustration of the Precise Synthesis of SnPt/SiO2 (Up) and PtSn/SiO2 (Down) Catalysts Using ALD by Switching the Sequence of Deposition.

Scheme 1

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2. Experimental Section

2.1. Catalyst Synthesis

2.1.1. Materials

Trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3) was purchased from Laajoo. Dibutyl tin diacetate (DBTDA) (CH3CO2)2Sn[(CH2)3CH3]2 was purchased from Aladdin Industrial Corporation, and the SiO2 support (with a BET surface area of 600 m2/g) was bought from Sigma-Aldrich. Ultrahigh purity Ar (99.999%), N2(99.999%), O2 (99.999%), 10% O2 in Ar, and 10% H2 in Ar were obtained from Nanjing Special Gases. All chemicals were used as received without further purification.

2.1.2. SnOx ALD

SnOx ALD was performed in a viscous flow ALD reactor (ACME (Beijing) Technology Co., Ltd.) with a carrier gas of ultrahigh-purity N2 at a flow rate of 150 mL min–1 at 250 °C. DBTDA and O2 were used as precursors.35 The DBTBA precursor contained in a sealed stainless-steel bubbler was heated to 70 °C to obtain a sufficient vapor pressure. The ALD timing sequences were 200, 240, 400, and 240 s for the DBTBA exposure time, N2 purge time, O2 exposure time, and N2 purge time, respectively (200–240–400–240). SnOx ALD was carried out on the SiO2 support for different cycles. The resulting samples are denoted as Snx/SiO2, and here x represents the Sn loading in weight percentage. Therein, the Sn0.3/SiO2 and Sn0.6/SiO2 samples with low Sn loadings were synthesized by reducing the DBTBA exposure time in one SnOx ALD cycle.

2.1.3. Pt ALD

Pt ALD was performed in the same ALD reactor at 150 °C using MeCpPtMe3 and ultrahigh purity O2 as precursors.36 The MeCpPtMe3 precursor container in a sealed stainless-steel bubbler was heated to 65 °C to obtain a sufficient vapor pressure. The timing sequence was 120–240–200–240 in second.

2.1.4. Synthesis of PtSn/SiO2 and SnPt/SiO2 catalysts

PtSn/SiO2 catalysts were synthesized on the SiO2 support by sequentially performing SnOx ALD for different cycles at 250 °C, followed by one cycle of Pt ALD at 150 °C. The resulting samples are denoted as PtxSny/SiO2, where x and y represent the loadings of Pt and Sn in weight percentage, respectively. SnPt/SiO2 catalysts were prepared in the inverse order by sequentially performing one cycle of Pt ALD at 150 °C, followed by SnOx ALD for different cycles at 250 °C. As a control experiment, Pt ALD was also performed on the bare SiO2 substrate for one cycle under the same ALD conditions, which is denoted as Pt0.5/SiO2, with a Pt loading of 0.5 wt %.

2.2. Characterization

2.2.1. Element Composition

The Pt and Sn loadings of the catalysts were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Thermo Fisher iCAP 7400); therein, all of the samples were dissolved in hot aqua regia. Ultraviolet–visible (UV–vis) spectroscopy measurements were conducted on a Shimadzu DUV-3700 spectrometer (University of Science and Technology of China) using Barium Sulfate (BaSO4) as a reference. The samples were loaded into a powder holder with a UV window, where the thicknesses of the samples were greater than 1 mm.

2.2.2. Morphology

Transmission electron microscopy (TEM) measurements were performed on a JEOL-2010F instrument operated at 200 kV to characterize the morphology of the catalysts. All of the as-prepared catalysts were pretreated at 550 °C at a flow rate of 25 mL min–1 of 10% H2 in Ar for 1 h before characterization. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) at 200 kV (JEM ARM200F) was also conducted to observe the more details of the optimized samples. The Pt particle size distribution was obtained by counting more than 100 Pt NPs from the HAADF-STEM images at different locations using ImageJ software. Energy dispersive X-ray spectroscopy (EDS) measurements were also performed on the same equipment.

2.2.3. DRIFTS

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) CO chemisorption measurements were performed on a Nicolet iS10 spectrometer equipped with a mercury cadmium telluride (MCT) detector and a high-temperature reaction cell (Praying Mantis Harrick) with CaF2 windows. Thirty mg of catalyst was placed in the cell. The catalyst was first calcined in 10% O2 in Ar at 200 °C for 1 h, followed by a reduction in 10% H2 in Ar at 200 °C for another 1 h. Then the catalyst was cooled to room temperature under a continuous flow of Ar, after which the background spectrum was collected. Next, the catalyst was exposed to 10% CO in Ar at a flow rate of 25 mL min–1 at room temperature for 30 min until saturation. After that, the catalyst was purged with Ar for 1 h to remove the gas-phase CO, and the DRIFT spectrum was collected with 256 scans at a resolution of 4 cm–1. In the following, we further reduced the catalysts at 400 or 550 °C, and the DRIFT spectrum was recorded at room temperature, according to the process of catalyst degradation at 200 °C H2.

2.2.4. TGA

In situ thermogravimetry analysis (TGA) measurements were conducted on a TGA550 instrument (TA Instruments) equipped with an evolved gas analysis furnace to quantify coke formation during PDH. After loading into the furnace, the sample was heated to 550 °C at a heating rate of 10 °C min–1 in 10% H2 in Ar (25 mL min–1) and then isothermally maintained for 1 h to reduce the sample. Then, PDH gas consisting of 20% propane and 20% H2 balanced with Ar was introduced into the sample at a flow rate of 25 mL min–1 for about 5 h at the same temperature. The change in sample weight with time during PDH was recorded.

2.2.5. TPO

Temperature-programmed oxidation (TPO) was carried out by using a Micromeritics AutoChem II 2920 chemisorption analyzer, which was connected to a HIDEN QIC-20 mass spectrometer (MS). The catalysts were pretreated at 500 °C for 1 h under flowing Ar (30 mL min–1). After cooling to 40 °C, a flow rate of 30 mL min–1 of 10% O2 in He was introduced to the reactor, and the sample temperature was increased linearly from 40 to 700 °C at a heating rate of 10 °C min–1 to burn the coke. The CO2 in the reactor outlet was monitored and recorded online by MS.

2.2.6. NH3-TPD

Temperature-programmed desorption of ammonia (NH3-TPD) experiments were performed on a Quantachrome CHEMBET3000 analyzer with a thermal conductivity detector (TCD). Therein, 0.05 g of sample was pretreated at 550 °C for 1 h under 10%H2 in Ar flow (20 mL min–1), and then cooled to 100 °C in Ar, followed by an Ar purge for 1.5 h. Afterward, the sample was saturated with 2% NH3 in Ar at 100 °C for 1 h, followed by Ar purging until the baseline became stable. Next, the sample was heated to 500 °C at a heating rate of 10 °C/min. The signal of desorbed ammonia was monitored using a TCD analyzer.

2.3. Catalyst Evaluation

The PDH reaction was conducted in a quartz-bed flow reactor with an inner diameter of 10 mm at atmospheric pressure. The amount of as-synthesized catalyst (the mesh number of the catalysts in reactions was about 600 mesh) was 0.05 g, which was diluted with 1 g of 200 mesh quartz chips. Before the reaction test, all of the catalysts were reduced in 10% H2 in Ar at 550 °C for 1 h. Then, a PDH reaction stream of 20% C3H8 and 20% H2 with Ar as the balance gas at a flow rate of 25 mL min–1 was introduced into the reactor to start the reaction. In the high temperature reaction at 600 °C, the Pt0.5Sn0.6 catalyst was reduced in 10% H2 in Ar at 600 °C for 1 h, and then the reaction was started with a PDH reaction stream of 50% C3H8 and 50% H2 at a flow rate of 25 mL min–1. The gas products were analyzed by an online gas chromatograph (A90, Panna Instrument) equipped with a flame ionization detector and a capillary column (ValcoPLOT VP–alumina–KCl; 50 m × 0.53 mm).

The conversion of propane was calculated as follows:

2.3. 2

The selectivity and yield of propylene were calculated as follows:

2.3. 3

Here, [C3H8]inlet and [C3H8]outlet are the concentrations of propane at the inlet and outlet of the reactor, respectively; [C3H6]outlet is the concentration of propylene at the outlet of the reactor.

The carbon balance is calculated according to the amount of carbon atoms in the reactant and products based on the following equations:

2.3. 4

where Cout and Cin are the total carbon output and input rate (mol h–1), respectively; cout and cin are the total carbon concentration (mol L–1) in the outlet and inlet gases, respectively; and Qout and Qin are the flow rates (L h–1) of the outlet and inlet gases, respectively.

A first-order deactivation model was used to evaluate the catalyst stability.37,38

2.3. 5

where Coninitial and Confinal represent the conversions measured at the initial and final period of an experiment, respectively; t represents the reaction time (h); and kd is the deactivation rate constant (h–1). A high kd value indicates rapid deactivation, that is, low stability.

3. Results and Discussion

3.1. Synthesis, Structure, and Thermal Stability

We synthesized PtSn catalysts by varying the deposition sequence and Pt:Sn ratios using ALD with precise control (Scheme 1). Pt ALD was performed in a viscous flow stainless tube reactor using MeCpPtMe3 and O2 at 150 °C.36 And SnOx ALD was performed in the same reactor using DBTDA and O2 at 250 °C.35 We systematically varied the ALD deposition sequences of Pt and SnOx, while maintaining consistent deposition amounts for both elements, as shown in Figure 1. In the first synthesis route, the Pt clusters were first deposited on the SiO2 support (denoted as fresh Pt0.5/SiO2, where 0.5 indicates the Pt loading in weight percentage) with OH groups on SiO2 as nucleation sites;39,40 and then Sn species were deposited on the Pt clusters, yielding the fresh Sn0.6Pt0.5/SiO2 catalyst. According to the TEM images, the particle sizes for the fresh Pt0.5/SiO2 and Sn0.6Pt0.5/SiO2 catalysts were 1.0 ± 0.4 and 1.6 ± 0.4 nm, respectively (Figure 1a,d). A slight increase in the particle size of Sn0.6Pt0.5/SiO2 was likely due to the SnOx coating. Conversely, in the second synthesis route, 0.6 wt % Sn species were first deposited on the SiO2 support (denoted as fresh Sn0.6/SiO2, where 0.6 indicates the Sn loading in weight percentage), and then Pt was deposited afterward. The resulting Pt0.5Sn0.6/SiO2 catalyst exhibited a particle size of 1.2 ± 0.2 nm.

Figure 1.

Figure 1

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Microscopic characterization of the Pt0.5/SiO2, Sn0.6Pt0.5/SiO2, and Pt0.5Sn0.6/SiO2 catalysts before and after 550 °C H2 reduction. (a, d, g) Representative TEM/HAADF STEM images of these fresh catalysts; (b, e, h) representative TEM/HAADF STEM images of these catalysts after 550 °C reduction; (c, f, i) the particle size distributions of these catalysts corresponding to (a, b; d, e; g, h).

The thermostability of these three catalysts was first compared. Notably, after H2 reduction at 550 °C, the particle size of Pt0.5/SiO2 slightly increased to 1.6 ± 0.3 nm (Figure 1b,c). Such less pronounced metal aggregation after high temperature reduction is likely contributed to the presence of abundant OH groups on SiO2.41,42 Surprisingly, we found that SnPt particles in the Sn0.6Pt0.5/SiO2 catalyst aggregated from 1.6 ± 0.4 to 2.4 ± 0.7 nm (Figure 1e,f) after 550 °C H2 reduction, which was even more severe than that of Pt0.5/SiO2 after 550 °C H2 reduction. In sharp contrast, the particle size of Pt0.5Sn0.6/SiO2 was maintained at 1.2 ± 0.4 nm after 550 °C H2 reduction (Figure 1h,i). Clearly, the Pt0.5Sn0.6/SiO2 catalyst fabricated by the deposition of SnOx first and then Pt was much more stable than the Sn0.6Pt0.5/SiO2 catalyst synthesized the other way around. Therefore, this result suggests that the presence of SnOx at the Pt-SiO2 interface is likely the reason for the high stability. Here we further measured the Pt dispersions of the Pt0.5Sn0.6/SiO2 and Sn0.6Pt0.5/SiO2 catalysts by using CO chemisorption. We found that the deposition sequence affected the Pt dispersions, where the Pt dispersions were 45% and 24% for Pt0.5Sn0.6/SiO2 and Sn0.6Pt0.5/SiO2 after 550 °C H2 reduction, respectively, in line with their corresponding particle sizes. Clearly, more surface Pt sites were exposed in the Pt0.5Sn0.6/SiO2 catalyst.

To further understand the role of Sn species at the Pt-SiO2 interface, we synthesized a series of Snx/SiO2 samples (where x indicates the Sn loading in weight percentage) by performing SnOx ALD for different cycles. Taking advantage of the self-limiting nature of sequential surface reactions and the steric hindrance between chemisorbed Sn precursors, Sn species were conformally deposited on a SiO2 support, where the number of ALD cycles gives precise control over the coverage of Sn species on the SiO2 support.26,35 ICP-AES analysis revealed that the Sn loadings in these samples showed a linear increase with the number of ALD cycles (Figure 2a), suggesting precise control of Sn species coverage. The UV–vis spectrum of the Sn0.6/SiO2 samples showed a main absorption band with a maximum absorbance below 200 nm corresponding to Sn4+ in tetrahedral coordination (Figures 2b and S1), suggesting that the Sn species in Sn0.6/SiO2 are likely atomically dispersed on SiO2. As the Sn contents increased from 0.6 to 4.2 wt %, two additional signals at 248 and 290 nm occurred. The band at 248 nm is still a matter of debate, which was attributed to distorted tetrahedral and pentacoordinated framework Sn sites but also to small extra-framework SnO2 domains. The broad absorption around 290 nm has been assigned to hexa-coordinated polymeric Sn–O–Sn type species.43−48 After 550 °C H2 reduction, the absorption peak at 248 and 290 nm did not appear either, supporting that atomically dispersed SnOx species are highly stable against sintering under H2 reducing conditions even at high temperature.

Figure 2.

Figure 2

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(a) Sn loadings in ncSnOx/SiO2 as a function of ALD cycles, determined by ICP; (b) UV–vis of the as-prepared Snx/SiO2 samples as well as the Sn0.6/SiO2 sample after 550 °C H2 reduction; (c) HAADF-STEM images of as-prepared Sn0.6/SiO2 and (d) Sn0.6/SiO2 after 550 °C H2 reduction.

Aberration-corrected HAADF-STEM measurements further showed that Sn species in the fresh Sn0.6/SiO2 sample were atomically dispersed on the SiO2 support (Figure 2c,d), while disordered SnOx clusters further confirmed the high stability of the atomically dispersed SnOx species, in line with the UV–vis results (Figure 2b). It is clear that there are strong interactions between the SnOx species and SiO2, which can be attributed to the formation of Sn–O–Si covalent bonds.49,50 As a consequence, SnOx species with high stability endows it possible to further stabilize Pt clusters in Pt0.5Sn0.6/SiO2 against sintering as a structural stabilizer at the Pt-SiO2 interface.

3.2. Electronic Properties

CO has been often utilized as a probe molecule to investigate the electronic properties of noble metal catalysts.51−54 Here, in situ DRIFTS of CO chemisorption measurements of the Pt0.5/SiO2, Sn0.6Pt0.5/SiO2, and Pt0.6Sn0.5/SiO2 catalysts were carried out at room temperature to track the changes in the electronic structure of the Pt and PtSn particles after H2 reduction at different temperatures. We found that after H2 reduction at 200 °C, the Pt0.5/SiO2 catalyst showed a strong peak centered at about 2072 cm–1, along with a weak one at ∼1800 cm–1, as shown in Figure 3a,b, corresponding to linear- and bridge-bonded CO on Pt metal, respectively.52,53 As the reduction temperature increased, the intensity and position of the linear- and bridge-bonded CO peaks on Pt0.5/SiO2 remained relatively unchanged. While, for the Sn-promoted catalysts, the intensity of the linear-bonded CO peak for both Sn0.6Pt0.5/SiO2 and Pt0.5Sn0.6/SiO2 declined and the bridge-bonded CO peak disappeared as the reduction temperature increased (Figure 3c–f), suggesting that the Pt ensembles were separated by Sn species. According to the literature,55 PtSn alloys are likely formed after high temperature reduction. It is worth noting that after H2 reduction at 550 °C, we observed a small blue shift of the linear CO peak from 2070 to 2072 cm–1 on Sn0.6Pt0.5/SiO2 (Figure 3c), but a red shift from 2072 to 2066 cm–1 on Pt0.5Sn0.6/SiO2 (Figure 3e). The blue shift on Sn0.6Pt0.5/SiO2 is likely attributed to the considerable aggregations of SnPt particles according to TEM (Figure 1d-f), because larger particles often show more pronounced lateral interactions between chemisorbed CO molecules.51 While the red shift on Pt0.5Sn0.6/SiO2 implies electron transfer from Sn to Pt to strengthen the Pt(5d)–CO(2π*) bonding through π back-donation, thus leading to a lower CO vibrational frequency,52 since there was no visible particle size change for Pt0.5Sn0.6/SiO2 (Figure 1g–i). The appearance of electron transfer from Sn to Pt on Pt0.5Sn0.6/SiO2 after high-temperature reduction consists well with the literature.56

Figure 3.

Figure 3

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DRIFT spectrum of CO chemisorption at 25 °C of (a, b) Pt0.5/SiO2, (c, d) Sn0.6Pt0.5/SiO2, and (e, f) Pt0.5Sn0.6/SiO2 catalysts after H2 reduction pretreatment at different temperatures. (b), (d), and (f) are the enlargements of the blue shadows on the right in figures (a), (c), and (e), respectively.

3.3. Catalytic Performance

When testing for PDH at 550 °C, as shown in Figure 4a, we found that, for the Pt0.5/SiO2 catalyst, the initial C3H8 conversion was high about 36%, but decreased rapidly to 19% in the first 2 h, and then gradually declined to 10% after 48 h of reaction. Therein, the C3H6 selectivity was only about 76%, in line with the literature.57 With the presence of Sn, Sn0.6Pt0.5/SiO2 showed a slightly lower initial conversion of 30%, but it became more stable, where the C3H8 conversion decreased to 13% after reacting for 48 h. Surprisingly, this Sn0.6Pt0.5/SiO2 catalyst showed a C3H6 selectivity of only 87%, lower than the values (∼92% or even higher) for PtSn catalysts reported in the literature.57,58 In sharp contrast, the Pt0.5Sn0.6/SiO2 catalyst fabricated by the deposition of SnOx first and then Pt, exhibited a much better stability and C3H6 selectivity. Therein, the C3H8 conversion was initially about 32% and could still maintain at 25% after 48 h reaction, meanwhile the C3H6 selectivity was also remarkably high about 98%, which both were much better than that of Pt0.5/SiO2 and Sn0.6Pt0.5/SiO2. The much higher C3H6 selectivity of Pt0.5Sn0.6/SiO2 than that of Sn0.6 Pt0.5/SiO2 is likely due to the more electron rich Pt in Pt0.5Sn0.6/SiO2 according to the DRIFTS results (Figure 3e). This is because electron-enriched Pt could stimulate the desorption of propylene intermediates for high C3H6 selectivity.59−63 Nonetheless, we noticed that the Pt0.5Sn0.6/SiO2 catalyst had a lower intrinsic activity than that of Sn0.6Pt0.5/SiO2 because these two catalysts showed a similar initial activity while Pt0.5Sn0.6/SiO2 had a considerably higher Pt dispersion.

Figure 4.

Figure 4

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(a) Catalytic performance of the Pt0.5/SiO2, Sn0.6Pt0.5/SiO2, and Pt0.5Sn0.6/SiO2 catalysts in the PDH reaction. Conversion of C3H8 and selectivity to C3H6 as a function of time on stream for PtSn/SiO2 catalysts. Reaction conditions: temperature, 550 °C; gas feed, C3H8: H2: Ar = 5:5:15, with balance Ar for total flow rate of 25 mL min–1; WHSV = 11.8 h–1. (b) TPO results for the 48 h used catalysts. HAADF-STEM images of the used catalysts after 48 h reaction (c, d, e); (f, g, h) the particle size distributions of (c, d, e).

Coke deposition is a key factor for the deactivation of PDH catalysts.1,64 Here, TPO measurements were performed on these used catalysts after 48 h reaction to investigate the coke formation by monitoring CO2 signals (m/e = 44) through coke combustion using MS. We found that the TPO curve of Pt0.5/SiO2 showed two CO2 peaks located at about 468 and 574 °C (Figure 4b), corresponding to coke accumulated on the Pt metal and the SiO2 support, respectively.65−67 The much higher intensity of the CO2 peak at 574 °C than the one at 468 °C suggested that there were significant amount of coke formed on the SiO2 support. For Sn0.6Pt0.5/SiO2, the CO2 peaks were located at about 487 and 519 °C and had a considerably smaller intensity. The downshift of the high-temperature peak to 519 °C suggested that the coke on the support was near the metal sites.8,65 Surprisingly, there was only one peak at about 478 °C for the Pt0.5Sn0.6/SiO2 catalyst. Quantitative analysis of the CO2 peak areas of the three curves showed that the Pt0.5Sn0.6/SiO2 catalyst had the smallest amount of coke, about 4.4 and 2.5 times lower than those of Pt0.5/SiO2 and Sn0.6Pt0.5/SiO2, respectively (Table S1). The much lower coke formation on Pt0.5Sn0.6/SiO2 than on Sn0.6Pt0.5/SiO2 suggested that the deposition sequence of Pt and Sn on the supports plays an important role.

HAADF-STEM was performed to characterize the morphology of these used Pt0.5/SiO2, Sn0.6Pt0.5/SiO2, and Pt0.6Sn0.5/SiO2 catalysts. After 48 h of PDH reaction, the particle size of Pt0.5/SiO2 increased slightly to 1.9 ± 0.3 nm (Figures 4c,f and S2), suggesting that coking was the major reason for the deactivation of the Pt/SiO2 catalyst. Surprisingly, we found that, compared to the used Pt0.5/SiO2 catalyst, the SnPt particles in the used Sn0.6Pt0.5/SiO2 catalyst aggregated more severely to 3.0 ± 1.4 nm (Figure 4d,g). In sharp contrast, there was negligible metal aggregation in Pt0.5Sn0.6/SiO2 after 48 h of PDH reaction, with the particle size preserved at 1.4 ± 0.2 nm (Figure 4e,h). The distinctly different catalytic performance of the Sn0.6Pt0.5/SiO2 and Pt0.6Sn0.5/SiO2 catalysts unambiguously confirms that the presence of SnOx species at the Pt-SiO2 interface is essential for improving the high stability.

3.4. Sn Content on Catalytic Performance

Since the Pt0.5Sn0.6/SiO2 catalyst was fabricated by the deposition of SnOx first and then Pt, it exhibited a much better stability and C3H6 selectivity than the Sn0.6Pt0.5/SiO2 catalyst synthesized the other way around. Here, we further synthesized a series of Pt0.5Sny/SiO2 catalysts with various Sn contents ranging from 0.3 to 2.0 wt % by precisely controlling the number of ALD cycles, followed by Pt deposition. Meanwhile, a Sn2.0/SiO2 catalyst with the absence of Pt was also evaluated as a reference sample. We found that the Sn2.0/SiO2 catalyst exhibited low C3H8 conversions (below 5%) and poor C3H6 selectivities (below 50%) in the temperature range of 500–600 °C (Figure S3). The catalytic performance of the resulting Pt0.5Sny/SiO2 catalysts in the PDH reaction is depicted in Figure 5. The initial C3H8 conversions for the Pt0.5/SiO2, Pt0.5Sn0.3/SiO2, Pt0.5Sn0.6/SiO2, Pt0.5Sn1.1/SiO2, and Pt0.5Sn2.0/SiO2 catalysts were 36%, 34%, 32%, 31%, and 29%, respectively (Figure 5a). It appears that a higher Sn content caused a slight decrease in the initial C3H8 conversion, which is likely attributed to the gradual decrease of the number of exposed Pt active sites with the increase of Sn species.71 Much higher C3H8 conversions over Pt0.5Sny/SiO2 than over Sn2.0/SiO2 suggest that those Sn species on the SiO2 support with no contact with Pt had a trivial contribution to the catalytic activity of Pt0.5Sny/SiO2, and PtSn alloy particles are the major active sites. The C3H6 selectivities of the above catalysts were 76%, 93%, 98%, 92%, and 91%, respectively, showing a volcano trend on the Sn contents, where the Pt0.5Sn0.6/SiO2 catalyst exhibited the highest selectivity. After 48 h of reaction at 550 °C, these catalysts deactivated to different degrees, and the C3H8 conversions were about 10%, 12%, 25%, 22%, and 16% for Pt0.5/SiO2, Pt0.5Sn0.3/SiO2, Pt0.5Sn0.6/SiO2, Pt0.5Sn1.1/SiO2, and Pt0.5Sn2.0/SiO2, respectively. Meanwhile, the C3H6 selectivities for the above catalysts slightly varied to 80%, 95%, 99%, 96%, and 94%, respectively, preserving the volcano trend on the Sn content, where the Pt0.5Sn0.6/SiO2 catalyst still showed the highest selectivity (Figure 5b). The deactivation constants kd (h–1) during the 48 h reaction for the above catalysts were 0.033, 0.028, 0.006, 0.010, and 0.016, showing an inverse volcano trend on the Sn content, further demonstrating the high sensitivity of the catalytic performance on the Sn content. Therein, as the Sn content was higher than 0.6 wt %, the decrease in C3H6 selectivity along with more coke formation might be likely attributed to the presence of selectivity of Sn2.0/SiO2 (Figure S3). Consequently, the Pt0.5Sn0.6/SiO2 catalyst with the moderate amount of Sn content achieved the highest C3H6 selectivity along with the highest stability or lowest deactivation constant at the same time.

Figure 5.

Figure 5

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Catalytic performance of PtSn/SiO2 catalysts in the PDH reaction. (a) Conversion of C3H8 and selectivity to C3H6 as a function of time on stream for PtSn/SiO2 catalysts. Reaction conditions: temperature, 550 °C; gas feed, C3H8: H2: Ar = 5:5:15, with balance Ar for total flow rate of 25 mL min–1; WHSV = 11.8 h–1; (b) deactivation constants (kd) of the catalysts in (a); (c) The regeneration test of the Pt0.5Sn0.6/SiO2 catalyst. Here the catalyst regeneratioin was carried out by calcination in 2% O2 in Ar for 15 min at 500 °C, followed by reduction in 10%H2 in Ar for another 1 h at 550 °C; (d) conversion of C3H8 and selectivity to C3H6 as a function of time on stream for Pt0.5Sn0.6/SiO2. Reaction conditions: temperature, 600 °C; gas feed, C3H8: H2 = 15:15, total flow rate of 30 mL min–1; WHSV = 17.7 h–1.

More importantly, the optimized Pt0.5Sn0.6/SiO2 catalyst showed a remarkable regeneration capability after removing accumulated coke combustion at 500 °C for 15 min (Figure 5c). Increasing the reaction temperature to 600 °C, the industrial reaction temperature,1 the Pt0.5Sn0.6/SiO2 catalyst still preserved the excellent performance, by preserving the C3H8 conversion over 29% and 95% C3H6 selectivity after 35 h (Figure 5d), where the kd was only 0.008, among the lowest values in the literature (Figure S4). Therefore, all of the above results revealed that the Pt0.5Sn0.6/SiO2 was a robust catalyst with high performance in the PDH reaction, holding great potential for practical applications.

3.5. Sn Content on Coke Formation

To further understand the underlying reasons for deactivation of these catalysts, in situ TG was performed on the series of PtSn catalysts to quantitatively monitor the coke formation during the PDH reaction. The weight gains owing to coke accumulation on the catalyst surfaces were about 4.6%, 3.1%, 1.3%, 2.0%, and 2.5% for the Pt0.5/SiO2, Pt0.5Sn0.3/SiO2, Pt0.5Sn0.6/SiO2, Pt0.5Sn1.1/SiO2, and Pt0.5Sn2.0/SiO2 catalysts after 5 h PDH reaction, respectively (Figure 6a and Table S2), showing an inverse volcano trend on the Sn content. The weight gains showed a linear correlation with the deactivation constants of these catalysts (Figure 6b), strongly indicating that coke formation is the major factor in catalyst deactivation. Indeed, HAADF-STEM showed that there were no visible metal aggregations on these used Pt0.5Sny/SiO2 catalysts (Figure S5). TPO measurements showed that Pt0.5Sn0.6/SiO2 exhibited one symmetric CO2 peak at about 478 °C, assigned to the coke on the metal.18 While the Pt0.5Sn2.0/SiO2 catalyst with a higher Sn content showed an unsymmetric peak at 557 °C, along with a shoulder at ∼480 °C and a much stronger peak intensity. Clearly, an excess of Sn species might introduce additional sites for coke formation on the support (Figure 6c). We further conducted NH3-TPD measurements on the fresh Snx/SiO2 catalysts. It can be seen from Figure S6 that the number of weak (190–230 °C) and medium (290–330 °C) acidic sites increased with Sn content.26,72−74 After the deposition of Pt, the acidity of the catalysts hardly changed, as shown in Figure 6d. According to previous reports,75−79 the increase of acid sites due to the excess amount of Sn accounts for the increased coke formation.

Figure 6.

Figure 6

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(a) In situ TGA measurements of coke on PtSn/SiO2 catalysts in the PDH. Reaction conditions: temperature, 550 °C; gas feed, C3H8: H2: Ar = 5:5:15, with balance Ar for total flow rate of 25 mL min–1; WHSV = 19.6 h–1. (b) Coke weight gains as a function of the kd for PtSn/SiO2 catalysts. (c) TPO results for the used Pt0.5Sn0.6/SiO2 and Pt0.5Sn2.0/SiO2 catalysts after 48 h PDH reaction. (d) NH3-TPD profiles of Pt0.5Sn0.6/SiO2, Pt0.5Sn1.1/SiO2 and Pt0.5Sn2.0/SiO2 catalysts.

4. Conclusion

In summary, we precisely synthesized a series of PtSn/SiO2 and SnPt/SiO2 catalysts by varying the deposition sequence and Pt:Sn ratios using ALD. In the PDH reaction, we demonstrated that the PtSn/SiO2 catalyst prepared by first depositing SnOx and then Pt, showed better C3H6 selectivity and stability than the SnPt/SiO2 catalyst synthesized using an opposite deposition sequence. Detailed structure characterization showed that the presence of Sn species at the Pt-SiO2 interface is essential for not only the stabilization of PtSn clusters against sintering in the high-temperature PDH reaction but also the promotion of charge transfer to Pt for high selectivity. In addition, the precise regulation of the Sn content was also found to be important for high performance. The excess amount of Sn could create additional acidic sites, leading to a lower selectivity of propylene and heavier coke deposition. These findings provide new insight into the design of highly selective and stable PDH catalysts.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (22102168), the National Science Fund for Distinguished Young Scholars (22025205), the Anhui Natural Science Foundation of China (2108085QB59), Project funded by the China Postdoctoral Science Foundation (BX20190312, 2020M671867), the University of Science and Technology of China Youth Innovation Key Fund (YD9990002015), the Fundamental Research Funds for the Central Universities (WK2060000038, WK3430000005), and the National Synchrotron Radiation Laboratory (KY2340000135). The authors also gratefully thank Associate Professor M. Sun and Dr. H. B. Chong for the ICP-AES measurements and Associate Professor C. Q. Wu for the HAADF-STEM characterization.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.4c00004.

  • Additional figures and data including UV–vis spectra of as-prepared Snx/SiO2 samples, aberration-corrected HAADF-STEM images and corresponding BF images of Pt0.5/SiO2 used catalyst after 48 h PDH reaction, comparisons of the weitht hourly space velocity (WHSV) as a function of deactivation rate over PtSn catalysts, TEM images and the particle size distributions of the used Pt0.5Sn1.1/SiO2 and Pt0.5Sn2.0/SiO2 catalysts after 48 h PDH reaction, NH3-TPD profiles of Snx/SiO2, TPO results, in situ TGA measurements (PDF)

The authors declare no competing financial interest.

This paper was published ASAP on March 20, 2024 with errors in equation 4. The corrected version was reposted on June 11, 2024.

Supplementary Material

pc4c00004_si_001.pdf (573.1KB, pdf)

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