Yttrium-modified alumina as support for trimetallic PtSnIn catalysts with improved catalytic performance in propane dehydrogenation

Fuel Processing Technology 146 (2016) 48–55

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Yttrium-modified alumina as support for trimetallic PtSnIn catalysts with improved catalytic performance in propane dehydrogenation Liu-Liu Long, Wan-Zhong Lang ⁎, Xi Yan, Ke Xia, Ya-Jun Guo ⁎ The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China

a r t i c l e

i n f o

Article history: Received 1 October 2014 Received in revised form 2 February 2016 Accepted 8 February 2016 Available online 20 February 2016 Keywords: Propane dehydrogenation Propylene Modified alumina Yttrium

a b s t r a c t The yttrium(Y)-modified γ-Al2O3 was used as support for trimetallic PtSnIn catalysts (PtSnIn/xY–Al) and applied in propane dehydrogenation reaction. The PtSnIn/xY–Al catalysts were characterized by several state-of-art techniques such as XRD, BET, NH3-TPD, TEM, H2-TPR, and XPS measurements. The results show that the catalytic performances of PtSnIn/xY–Al catalysts for propane dehydrogenation reaction are clearly improved. The NH3-TPD curves demonstrate that Y-modified γ-Al2O3 could greatly weaken the acidity of the support. The PtSnIn/0.6Y– Al catalyst has the smallest Pt particle size of 16 nm and homogenized particle distribution. XPS and H2-TPR techniques verify that the loading of Y influences the state of metals and its interactions with support. In this study, the initial propane conversion and propylene selectivity attain above 50% and 97% for PtSnIn/0.6Y–Al catalyst, respectively. However, with excessive Y loading, the agglomerations of Pt particles and more metallic (Sn0) are found, which leads to the loss of catalytic activity. © 2016 Elsevier B.V. All rights reserved.

1. Introduction It is well known that propylene is an important raw material for the production of polypropylene, acrolein, polyacrylonitrile and acrylic acid [1]. Now, propylene is mainly obtained as by-product of gasoline by thermal or catalytic cracking processes. Recently, the catalytic dehydrogenation of propane to produce propylene has acquired increasing importance due to the growing demand of propylene [2]. However, the catalytic dehydrogenation of propane is an endothermic process, which requires a relatively high reaction temperature to obtain high propylene yield. Therefore, the undesirable reactions such as cracking, hydrogenolysis and coke deposition are inevitable [3,4]. Platinum(Pt)-based catalysts are the most widely studied catalysts in propane dehydrogenation reaction. Thus, modifying Pt supported catalysts by doping the additional metals to achieve better catalytic performance seems to be a promising route [5–7]. The bimetallic platinum– tin (Pt–Sn) supported catalysts are widely applied in light alkane transformation processes [8–13]. The role of Sn was proposed to modify the catalytic properties of Pt via electronic and ligand effects [14,15]. Furthermore, many researches were dedicated to enhancing the catalytic performances of Pt–Sn supported catalysts by doping additional promoters, including alkali metals (Li, K, Na), alkaline earth metals (Mg, Ca) and so on [16–21]. However, the studies about the effects of support

⁎ Corresponding authors. E-mail addresses: [email protected] (W.-Z. Lang), [email protected] (Y.-J. Guo).

http://dx.doi.org/10.1016/j.fuproc.2016.02.012 0378-3820/© 2016 Elsevier B.V. All rights reserved.

on catalysts' behaviors were also considered to be an interesting topic [22–29]. The support cannot only influence the final states of active metals but also impact the catalytic performances of the catalysts [30,31]. Alumina (γ-Al2O3) has been regarded as the most popular catalyst support due to its excellent thermal stability, desirable textural properties and cheapness [22]. The most important is that alumina carrier has superior capability to maintain a high degree of Pt dispersion, which is essential to get high catalytic activity and selectivity for catalytic dehydrogenation reaction [32,33]. Nevertheless, the specific surface area of alumina tends to significantly decrease at high temperature [34] and its strong acidity easily causes side reactions and coke formation in dehydrogenation reaction process [35]. To overcome these problems, to modify alumina framework by incorporating some metal species is considered as a potential method. Due to the unique properties, rare earth metals (La, Ce, Y etc.) are proposed as the excellent promoters to improve the catalytic performances [2,36–40]. Vu et al. [41] investigated the influence of La-, Ceand Y-doped alumina on the formation and stability of Pt–Sn supported catalysts. They found that La-, Ce-, and Y-doped alumina could increase Pt dispersion and decrease the reduction temperature of Pt–Sn species. In addition, Pt–Sn/La–Al and Pt–Sn/Ce–Al catalysts could improve the catalytic performances and stability. However, the influences of Y loading on the catalytic performances of the Pt supported catalysts for propane dehydrogenation had not been considered. Graf et al. [42,43] compared the differences of Pt, Rh and Pd supported on yttrium-stabilized zirconia in steam reforming reaction of methane, ethane and ethylene. Wang et al. [44] reported Y

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2. Experimental

The morphologies of PtSnIn/xY–Al catalysts were detected by a transmission electron microscopy (TEM; JEOL JEM2010). The temperature-programmed reduction (TPR) measurements for the PtSnIn/xY–Al catalysts were carried out in a programmable temperature system. About 0.2g sample was filled in the quartz reactor. Before analysis, the sample was flowed by dry argon (15mL·min− 1) at 500°C for 1h and then the baseline was stabilized in the gas flow at 25°C for 60min. Subsequently, the quartz reactor was heated in a temperature-programmed furnace from room temperature to 800°C at the rate of 10°C·min− 1. Finally, a TCD (thermal conductivity detector) cell was used to monitor and record the consumption of H2. The X-ray photoelectron spectra (XPS) of PtSnIn/xY–Al catalysts were investigated on Perkin–Elmer PHI 5000C ESCA using Al Kα radiation. All samples were reduced in a hydrogen flow at 580°C for 2.5h. The binding energies (BE) were calibrated using the C1s level at 284.8eV as an internal standard.

2.1. Catalyst preparation

2.3. Propane dehydrogenation reaction

Y-modified alumina (Y–Al) was synthesized by the co-precipitation method using aqueous solutions of metal nitrates and aqueous ammonia. Generally, NH4OH (28%) was added quickly to the determined amounts of Y(NO3)3 and Al(NO3)3 in aqueous solution. The mixture was maintained at 60°C and continually stirred for 3h, where the precipitation occurred. After filtrated and washed, the sample was dried at 120°C for 10h and then calcined in air at 600°C for 4h to achieve Y-modified alumina (labeled as Y–Al). Pure alumina was similarly synthesized without the addition of Y species. Then, Pt, Sn and In were loaded onto supports using sequential impregnation method. In short, Sn and In were deposited on Y–Al material by coimpregnating SnCl2 and In(NO3) 3 ethanol mixed solution at room temperature for 12h, followed by drying as the same as that mentioned above. Afterwards, the materials were calcined in air at 550°C for 4h. Finally, Pt species were introduced by using H2PtCl 6 as precursor. After dried and calcined, the catalysts were gotten. Unless otherwise specified, the loadings of Pt, Sn and In of PtSnIn/xY–Al catalysts were 0.3, 0.6 and 1.5wt.%, respectively. All samples were labeled as PtSnIn/xY–Al, where x represents the mass percentage of Y in the catalysts.

The propane dehydrogenation reaction was performed in a quartz fix-bed reactor (8mm in diameter. The catalyst sample (0.3g) was placed in a quartz reactor and reduced with H2 at 580°C for 2.5h. Afterwards, the feed gas including H2, C3H8 and Ar (H2:C3H8:Ar molar ratio= 7:8:35, WHSV=3.3h−1) was fed to the reactor. A gas chromatography (GC, SP-6980) equipped with a FID detector and an AT-PLOT PORA-Q capillary column was employed to analyze the gas compositions. The propane conversion, propylene selectivity and other by-product selectivities were defined as follows:

added Pd/Ce–Zr/Al2O3 can remarkably increase the dispersion of Pd particles and enhance the catalytic activity of the catalysts for propane total oxidation reaction. Our previous work demonstrated that the trimetallic PtSnIn catalysts supported on a commercial Al 2 O 3 exhibited highly catalytic performances [45]. The high activity (propane conversionN 41%) and perfect propylene selectivity (N 96%) could be obtained in propane dehydrogenation reaction. In this paper, the Y-doped alumina was first self-synthesized, and then used as supports for trimetallic PtSnIn catalysts for propane dehydrogenation reaction. The enhanced catalytic activity of PtSnIn/xY–Al was obtained in propane dehydrogenation reaction compared to the bare PtSnIn/Al catalysts. The structure–performance relationships of the catalysts were analyzed by several state-of-art characterizations including XRD, BET, NH3-TPD, TEM, H2-TPR, and XPS techniques.

C3 H8 conversion ¼ ðC3 H8in −C3 H8out Þ=C3 H8in

ð1Þ

C3 H6 selectivity ¼ C3 H6out =ðC3 H8in −C3 H8out Þ

ð2Þ

Ci selectivity ¼ Ciout =ðC3 H8in −C3 H8out Þ

ð3Þ

where C3H8in and C3H8out are the propane content in feed and exit gases respectively, C3H6out is the propylene content in exit gases, Ciout represents the content of methane, ethane and ethylene in exit gas flow. 3. Results and discussion

2.2. Catalyst characterizations 3.1. Characterizations of the catalysts The X-ray diffraction (XRD) patterns of PtSnIn/xY–Al catalysts were obtained on a Bragg–Brentano diffractometer (Rigaku D/Max-2000) with monochromatic CuKa radiation (λ=1.5418Å) of graphite curve monochromator. The X-ray diffraction data were collected from 10 to 80° with a 2θ scanning rate of 4°/min. The textural properties of PtSnIn/xY–Al catalysts were obtained from N 2 adsorption–desorption measurements at liquid nitrogen temperature on an automatic analyzer (NOVA 4000, Quantachrome, USA). Before adsorption, the catalysts were degassed for 4h under vacuum at 300°C. The BET (Brunauer–Emmett–Teller) surface area of the samples was calculated by BET multi-point method. The porous volumes were calculated by BJH (Barrett–Joyner–Halenda) method using Halsey equation for multilayer thickness [45–47]. The pore size distribution curves were obtained from the desorption branch of isotherms. The temperature-programmed desorption of ammonia (NH3-TPD) measurements were implemented to analyze the acidity of those catalysts. About 0.05g sample was placed into a quartz reactor between two quartz wool plugs. Before NH3 adsorption, the sample was treated by dry argon flow (99.99%, 30mL·min−1) at 400°C for 1h. Then the sample was saturated with NH3 at 120°C. A thermal conductivity detector (TCD) was carried out to detect NH3-TPD profile from 120 to 600°C at a rate of 10°C/min.

As demonstrated in Fig. 1, all samples only show three characteristic peaks at ~ 37.6°, ~ 45.8° and ~66.8° corresponding to γ-Al2O3 [48], implying that γ-Al2O3 structure is formed during the catalyst preparation process. Also, the structure of γ-Al2O3 varies little with the Y loading in the catalysts. The diffraction peaks corresponding to Pt, Sn, In and Y species are not detected, indicating that the contents of these species are too low for detection or they are highly dispersed on the supports [41,45,49,50]. As Fig. 2(a) shows, all samples exhibit the typical irreversible type IV adsorption isotherms with a H1 hysteresis loop as defined by IUPAC [51]. In addition, it can be seen from Fig. 2(b) that the pore size distribution of all the catalysts except PtSnIn/1.2Y–Al has no evident distinction. However, PtSnIn/1.2Y–Al catalyst shows a broader pattern than others. The results imply that the small amount of Y2O3 does not destroy the structure of alumina, which is corresponding to the finding of XRD. However, for excessive Y loading, part Y species may enter into and block the channels of γ-Al2O3. As Table 1 displays, the specific surface area (SBET) slightly decreases after loading PtSnIn, implying that these species have entered into the channels of γ-Al2O3. The similar effects on the textural properties of doping metals to support are also reported by Zhang et al. [52] and Alvarez-Galvan et al. [53]. However, the specific surface area (SBET) varies little as Y loading increases (≤0.6wt.%), which

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L.-L. Long et al. / Fuel Processing Technology 146 (2016) 48–55 Table 1 The textural properties of PtSnIn/xY–Al catalysts.

Fig. 1. XRD patterns of PtSnIn/xY–Al catalysts.

indicates that the small amount of additives cannot change the properties of γ-Al2O3. Nevertheless, a slightly smaller SBET is found in PtSnIn/ 0.9Y–Al and PtSnIn/1.2Y–Al. This phenomenon may be explained that excessive Y may block part channels of γ-Al2O3. As illustrated in Fig. 3, the NH3-TPD curves were semi-quantitatively analyzed by deconvoluting the desorption peaks using the Gaussian deconvolution method. As listed in Table 2, it is clear that the representative PtSnIn/xY–Al catalysts exhibit three desorption peaks. The first peak centered at ~ 222°C can be attributed to weak acid sites. The second peak centered at ~270°C corresponds to medium strength acid sites. Lastly, the third peak with a broad desorption centered at ~340°C is ascribed to typical strong acid sites. Compared with the bare alumina support (Al), the total acidity and the strong acid sites evidently decrease after the impregnation of Pt, Sn and In species (PtSnIn/0.0Y– Al). This result suggests that the loading of Pt, Sn and In can neutralize the acid sites of support, and the decrease of acidity is mainly ascribed to the loss of strong acid sites. The loss of acidity with the addition of Pt, Sn and In is in agreement with the experimental results of Passos et al. [54] and Liu et al. [45]. In addition, the weakest total acidity and the least peak area of strong acid sites are obtained for PtSnIn/0.6Y–Al catalyst, implying that appropriate Y species may further neutralize

Fig. 2. Nitrogen adsorption–desorption isotherms (a) and the corresponding pore size distribution curves (b) of PtSnIn/xY–Al catalysts.

Catalyst

SBET (m2g−1)

VP (cm3g−1)

Dpore (nm)

Average Pt particle size(nm)

Al PtSnIn/0.0Y–Al PtSnIn/0.3Y–Al PtSnIn/0.6Y–Al PtSnIn/0.9Y–Al PtSnIn/1.2Y–Al

275.798 271.773 272.806 274.685 265.783 257.446

0.416 0.445 0.395 0.469 0.378 0.366

4.356 4.362 4.383 4.343 4.351 4.359

/ 23 / 16 19 /

the acid sites of support. Samed et al. [55] proposed that Y2O3 had a moderate basicity, which might be the reason for the loss of acid sites over PtSnIn/xY–Al catalyst (≤0.6wt.%). Nevertheless, for PtSnIn/1.2Y– Al sample, an increase of acidity is found. Zhang et al. [1] had pointed out that La3+ cations are also weak Lewis acid sites, which can interfere in density of the acid sites. It can be guessed that excessive Y3+ cations may also have the similar effects,which lead to the increase of catalyst acidity. To analyze the influence of Y on the distribution of metallic particle, the representative catalyst samples were subjected to TEM experiments [50]. As shown in Fig. 4 and Table 1, PtSnIn/0.0Y–Al catalyst exhibits a wide distribution and the largest average size of Pt particles (23nm). However, for the PtSnIn/0.6Y–Al catalyst, the Pt species is well dispersed on the support and the average particle size decreases to 16nm, suggesting that doping suitable amount of Y in PtSnIn/xY–Al catalysts is beneficial to form Pt particles with small size and uniform distribution. Shi et al. [56] demonstrated that Pd5Y/Al2O3 showed smaller metal particle size compared to Pd/Al2O3, which was related to the strong interaction between Pd and support. Possibly, a suitable Y content can hinder Pt particles from aggregating in preparation process, which promotes the interaction between Pt and support to get the better distribution of metallic particles, which was revealed for La species [1]. However, for PtSnIn/1.2Y–Al catalyst, the relatively bigger average particle size of metal of 19nm are found. The results can be explained that high Y loading covers on Al2O3 surface and leads to some Pt species strongly to interact with Y and the increased metal particles. As shown in Fig. 5, all catalysts show two reduction peaks with a maximum at about 368°C or even lower temperature (peakI) and about 520°C (peak II). The peak I may be ascribed to the reduction of oxide species such as Pt oxide easily being reduced and the peak II may be attributed to the reduction of the species in stronger interaction with the support and/or the reduction of the trimetallic system(Pt, Sn and In) of PtSnIn/xY–Al catalysts [45,49]. As can be seen, the intensity of peak I slightly increases and shifts to lower temperature at 335°C

Fig. 3. The NH3-TPD profiles of PtSnIn/xY–Al catalysts.

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Table 2 The results of NH3-TPD of PtSnIn/xY–Al catalysts. Sample

Al PtSnIn/0.0Y–Al PtSnIn/0.6Y–Al PtSnIn/1.2Y–Al

Tm(°C) I

II

III

222 222 226 224

264 271 285 270

336 343 348 343

Total area per g-catalyst (g−1) 3186 2438 2368 2742

for PtSnIn/0.3Y–Al and PtSnIn/0.6Y–Al catalysts, implying that the presence of Y promotes the reduction of Pt oxide species. The similar findings were reported by Yue et al. about the effects of Y on the reduction of PdO [38]. However, the excess of Y content (≥ 0.9wt.%) leads to an obvious change in the TPR profile. The two peaks are overlapped and difficult to discriminate, which indicates that the excessive Y may facilitate Sn and/or In species to be reduced easily. It may be adverse for the dehydrogenation reaction [1]. In addition, based on the previous work [45,49], it demonstrated that Pt species existed in a

Peak area per g-catalyst (g−1) and corresponding fraction I

II

III

557.6 (17.5%) 624.1 (25.6%) 1049.0 (44.3%) 521.0 (19.0%)

923.9 (29.0%) 938.6 (38.5%) 495.0 (20.9%) 1000.8 (36.5%)

1704.5 (53.5%) 875.2 (35.9%) 824.0 (34.8%) 1220.2 (44.5%)

reduced state in the PtSnIn/xY–Al catalysts after reduction at 580°C for 2.5h. The specified data about the reducibility of Sn and In oxides will be futher disscussed in the following section. As can be seen from Fig. 6, the Sn3d5/2 XPS spectra of PtSnIn/xY–Al catalysts can be deconvoluted into three peaks at ~ 485.7, ~ 486.6 and ~ 487.4eV, corresponding to different Sn species [45]. Generally, the component at lower binding energy (~485.7eV) corresponds to the reduced Sn species either in zerovalent Sn (Sn0) or in alloy state (SnPtx); while the peaks at ~486.6 and ~487.4eV are assigned to oxidized species

Fig. 4. The TEM images of PtSnIn/xY–Al catalysts: (a) PtSnIn/0.0Y–Al; (b) PtSnIn/0.6Ca-Al; (c) PtSnIn/1.2Y–Al.

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L.-L. Long et al. / Fuel Processing Technology 146 (2016) 48–55 Table 3 Summary of XPS results PtSnIn/xY–Al catalysts. Catalyst

PtSnIn/0.0Y–Al

PtSnIn/0.6Y–Al

PtSnIn/1.2Y–Al

Fig. 5. The H2-TPR profiles of PtSnIn/xY–Al catalysts.

of Sn. However, it is difficult to discriminate between Sn2+ and Sn4+ due to their similar binding energies [10,57]. On examining the peak percentage given in Table 3, it can be observed that the percentage of Sn0 (23%) of PtSnIn/0.6Y–Al catalyst is less than that (34%) of PtSnIn/ 0.0Y–Al catalyst. Obviously, this finding suggests that suitable Y doping is beneficial to keep the nonmetallic state of Sn species, which is advantageous to propane dehydrogenation reaction [58,59]. Nevertheless, an increase in the percentage of reduced Sn species (29%) for the PtSnIn/ 1.2Y–Al sample compared to PtSnIn/0.6Y–Al sample is found, implying that excessive Y loading weakens the interactions between Sn species and support.

Binding energy(eV) Sn3d5/2

In3d5/2

485.8 (34%) 486.7 (38%) 487.5 (28%) 485.7 (23%) 486.6 (42%) 487.4 (35%) 485.8 (29%) 486.7 (38%) 487.5 (33%)

444.2 (30%) 445.2 (70%) 444.1 (26%) 445.1 (74%) 444.2 (29%) 445.3 (71%)

As shown in Fig. 7, all catalysts show two peaks of In3d5/2 XPS spectra at ~ 444.2 and ~ 445.2eV. The peak at lower binding energy may correspond to the reduced In phase, either in zerovalent In (In0) or in alloyed (InPtx) state; while the one at higher binding energy is assigned to the oxidation state of In [60]. It can be observed that the percentage of In0 (26%) of PtSnIn/0.6Y–Al catalysts are also less than that (30%) of PtSnIn/0.0Y–Al catalyst. However, an increase in the percentage of reduced In species (29%) is found for the PtSnIn/ 1.2Y–Al sample compared to PtSnIn/0.6Y–Al sample. These results indicate that the addition of Y over PtSnIn/xY–Al catalysts may have the similar effects on the state of In as Sn. 3.2. Catalytic performances As shown in Fig. 8(a), PtSnIn/0.0Y–Al catalyst exhibits the worst reaction activity and stability among all samples, and the propane conversion markedly decreases from 47.4% to 36.4% after 2.5h reaction. However, after doping 0.3–0.6wt.% Y species, the catalytic activity and stability are enhanced. The initial conversions of propane for PtSnIn/

Fig. 6. Sn3d5/2 XPS spectra of PtSnIn/xY–Al catalysts.

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Fig. 7. In3d5/2 XPS spectra of PtSnIn/xY–Al catalysts.

0.3Y–Al and PtSnIn/0.6Y–Al catalysts are 50.9% and 55.6%, and decrease to 42.0% and 48.7% after 2.5h reaction, respectively. Especially for PtSnIn/0.6Y–Al catalyst, the propane conversion has the minimum decline (6.9%), which is lower than PtSnIn/0.0Y–Al (11.0%) and PtSnIn/ 0.3Y–Al (8.9%) after 2.5h reaction. However, the propane conversion, propylene selectivity and catalytic stability evidently decrease as Y loading further increases to above 0.9wt.%. The initial propane conversions

decrease to 51.0% and 40.8%, and the final conversions after 2.5h reaction only have 41.0% and 24.2% for PtSnIn/0.9Y–Al and PtSnIn/1.2Y–Al catalysts, respectively. As demonstrated in Figs. 8(b) and 9, the propylene selectivity is above 96.5% for all catalysts and varies little in the 2.5h reaction period. Furthermore, for PtSnIn/0.6Y–Al catalyst, the highest propylene selectivity and the lowest by-product selectivities are observed. This phenomenon suggests that the side reactions are restrained to some

Fig. 8. The effects of Y loading of PtSnIn/xY–Al catalysts on the catalytic performances in propane dehydrogenation reaction (reaction conditions: T=600°C; H2:C3H8:Ar (molar ratio)=7:8:35; WHSV=3.3h−1; mcat.=0.3g).

Fig. 9. The major by-product selectivities of the PtSnIn/xY–Al catalysts after reaction for 2.5 h (reaction conditions: T = 600 °C; H2:C3H8:Ar (molar ratio) = 7:8:35; WHSV = 3.3 h−1; mcat. = 0.3 g).

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Fig. 10. The schematic diagram for the effects of Y addition on PtSnIn/xY–Al catalysts.

extent over this catalyst. The suppressed side reactions may be attributed to the least total acidity and the strong acid sites of PtSnIn/0.6Y–Al catalyst, which is presented in Table 2. These results demonstrate that the presence of Y has an obvious impact on the catalytic performances of PtSnIn/xY–Al catalysts. To explain these, it should be firstly noted that the PtSnIn/xY–Al catalysts are bifunctional and the two kinds of active sites (metal particles and acid sites) may collaboratively work [61]. It is assumed that an optimum ratio between the number of active sites and the number of acid sites may exist. As analyzed before, the acidity, especially the strong acid sites of PtSnIn/0.6Y–Al catalyst are less than that of other samples (NH3-TPD results). Meanwhile, the introduction of Y with suitable content cannot only suppress the generation of large particles and improve the uniformity of metal distribution (TEM images) but also inhibit the reduction of SnOx (XPS results). In this side, the better catalytic performances should be obtained due to the optimized matching between the acid sites and the metallic catalytic sites. However, with the further addition of Y, the larger metal particles and more amounts of Sn0 species are found, which leads to the deteriorated catalytic performances for PtSnIn/0.9Y–Al and PtSnIn/1.2Y–Al catalysts. From the other point of view, Pt is the active metal of PtSnIn/xY–Al catalyst for propane dehydrogenation to produce propylene. The cracking product (ethene) is mainly formed from cracking on the carrier. The ethane is mainly formed by hydrogenolysis of propane and by hydrogenation of ethylene; both reactions occur on the metal sites [62]. Furthermore, it is reported that the dehydrogenation and cracking of propane are assumed to proceed through carbonium-ion intermediates [63]. The more acid sites generally promote the subsequent cracking reaction of the initially formed C3 + carbenium ions. Therefore the changes of acidity and metal state should be responsible for the selectivity to propylene. As mentioned above, the decreased catalyst acidity, relatively homogeneous distribution of metal particles and less amounts of Sn0 can be found for PtSnIn/0.6Y–Al. Therefore, the side reactions can be inhibited and the propylene is preferentially produced.

3.3. The mechanism of the effects of Y addition on the catalytic performances As Fig. 10 displays, the Y species in PtSnIn/xY–Al catalysts may work in the following three ways. First, in the catalyst preparation process, some Y species can enter into the channels of alumina and decrease its surface acidity, which is verified by BET and NH3-TPD methods. Second, The Y species formed on the surface of support hinder Pt species from aggregating during the reduction process and remain small Pt particle size. In the third, according to the XPS results, it is deduced that more amounts of Sn can exist in oxidative states over PtSnIn/0.6Y–Al catalysts. The anchoring of Sn oxides species on the support become strong, which is favorable for the location of Pt active sites and the dispersion of Pt species. Pt can anchor Sn oxide surface to form new active sites with “sandwich structure” [64]. These new sites are the main reaction active sites for the dehydrogenation reaction, which results in the higher catalytic properties in the dehydrogenation process.

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