Si-doped Al2O3 nanosheet supported Pd for catalytic combustion of propane: effects of Si doping on morphology, thermal stability, and water resistance

Catalytic combustion of propane as typical light alkanes was important for the purification of industrial VOCs and automobile hydrocarbon emissions. Si-doped Al2O3 nanosheet was synthesized by a hydrothermal method, and effects of Si content on the morphology and thermal stability of Al2O3 were investigated. The doping of SiO2 could tune the thickness of Al2O3 nanosheets and significantly improve its thermal stability, the θ phase was still maintained, and the specific surface area was as high as 56.3 m2 g−1 after calcination at 1200 °C. And then the Si-doped Al2O3 nanosheets were used as support of Pd catalysts (Pd/Si-Al2O3 nanosheets) for catalytic combustion of propane, especially Pd/3.6Si-Al2O3 nanosheets, which presented high activity, stability, and resistance to sintering and H2O due to the promotion of Si on the thermal stability of Al2O3 and the stabilization (dispersion, isolation, and strong interaction) of PdOx species.


Introduction
Volatile organic compounds (VOCs) were tightly controlled due to their toxicity and involvement in the formation of photochemical smog, and catalytic combustion was recognized the most efficient technique to eliminate VOC emission (He et al. 2019;Liu et al. 2016;Han et al. 2017). Light hydrocarbons (HCs) such as methane, ethane, and propane were considered to be extremely difficult to be oxidized due to their high stability of molecular structures, which were produced from automotive exhaust or industrial processes, such as natural gas vehicle (NGV), liquefied petroleum gas (LPG) vehicles, and stationary power source (Siegl et al. 1999;Lai et al. 2009;Lin et al. 2020;Rahman 2019). Among HCs, propane was commonly selected as the model reaction to investigate the catalytic activity for unburnt HC oxidation in gasoline vehicles exhaust and other VOCs in industrial processes. At present, different catalytic materials were employed to investigate the catalytic combustion of propane; Co 3 O 4 and Rubased catalysts were identified as the most active materials for propane oxidation Hu et al. 2018;Okal and Zawadzki 2009;Liu et al. 2009). Liu found that nanocrystalline Co 3 O 4 catalysts synthesized by a low-temperature liquid-phase complexation method with different small molecular carboxylic acids presented a high activity (the conversion of propane reached 90% at 250°C) and stability (at least 40 h) ). Cai also confirmed that Co 3 O 4 /γ-Al 2 O 3 was a high-active for propane combustion and developed a facile reduction-passivation approach to produce more abundant active cobalt oxides (Cai et al. 2020). Hu reported the complete oxidation of propane on Ru/CeO 2 could be achieved below 200°C, due to the rich oxygen storage capacity of CeO 2 host, which can provide a large amount of active oxygen to RuO x for propane oxidation (Hu et al. 2018). However, the high temperature of vehicles exhausted even above 1050°C was a tremendous challenge for Co 3 O 4 and Ru-based catalysts; thus, the supported noble metals were still the most promising candidate due to their high thermal stability (Rahmati et al. 2020;Yang et al. 2019;Khairudin and Mohammadi 2021).
However, the enhanced resistance to sintering of noble metals was still desired, and the highly stable support with the strong metal-support interaction (SMSI) such as stabilized γ-Al 2 O 3 was one of the feasible strategies (Busca 2014). Generally, the bulk doping or surface modification such as SiO 2 (Zheng et al. 2016), TiO 2 (Chen et al. 2019), CeO 2 (Cargnello et al. 2012), La 2 O 3 (Jing et al. 2019;Zhou et al. 2014), MgO , and NiO 2 (Fang et al. 2015) was considered an effective strategy to improve high temperature stability of γ-Al 2 O 3 to obtain the stabilized γ-Al 2 O 3 . Masakuni Ozawa studied the existence of a strong interaction between the La and Al 2 O 3 by characterizing the surface, structural, and chemical properties of La-doped γ-Al 2 O 3 (Ozawa et al. 2016). However, the introduction of La into the γ-Al 2 O 3 supports reduced the electron cloud density of the Pd particles and weakens the adsorption of C-H, which affected the water resistance and stability of the Pd-based catalyst ). Our previous work indicated that phosphorous-modified γ-Al 2 O 3 nanosheet presented both enhanced thermal stability of γ-Al 2 O 3 and resistance to sintering of PdO x ; PdO x supported P-modified Al 2 O 3 calcined at 1000°C still exhibited a superior catalytic activity for propane combustion due to the re-dispersion of PdO x particles and the SMSI at high temperature (Wu et al. 2014). However, the surface modification of phosphorous led to the occupation of the coordinatively unsaturated Al 3+ centers and the highly active adsorption sites, which restrained the loading of Pd and the further enhancement of catalytic activity and stabilization of Pd species.
SiO 2 was doped in γ-Al 2 O 3 as an inert component; it can not only inhibit the phase change of γ-Al 2 O 3 at high temperature, but also avoid the weakening of the competitive adsorption of C-H by Pd particles caused by the doping of La (Mardkhe et al. 2015). Here, Si was doped into Al 2 O 3 by a dynamic hydrothermal method in the presence of tetrapropylammonium hydroxide (TPAOH); effects of Si on morphology and thermal stability of Al 2 O 3 were firstly investigated. Al 2 O 3 nanosheet was successfully synthesized, and the Si doping obviously thinned its thickness and improved the thermal stability. Sequentially, the synthesized Al 2 O 3 nanosheet was used as support of PdO x catalyst for catalytic combustion of propane; physicochemical properties of Pd/Si-Al 2 O 3 nanosheets were characterized by XRD, H 2 -TPR, XPS, CO pulses, and in situ CO adsorption DRIFTS; and effects of Si content, high temperature aging, and H 2 O were investigated.

Synthesis of Si-doped Al 2 O 3 nanosheets
A series of different contents of Si-doped alumina nanosheets were prepared through a dynamic (60 rpm) hydrothermal method (Wu et al. 2014). In a typical experiment, 2.5 g of polyethylene glycol with average molecular weight of 600 (PEG 600) and 0.902 g of sodium nitrate (NaNO 3 , ≥99.0%) were dissolved in 54 ml of distilled water and stirred for 10 min. Then, 15.3 g of tetrapropylammonium hydroxide solution (TPAOH, 25% in H 2 O) and 3.12 g of aluminum isopropoxide (AIP, >98%) were added to the mixture and stirred for 1 h, and next the given mass percentage of tetraethyl orthosilicate (SiC 8 H 20 O 4 , liquid, 99.9%, TEOS) was added dropwise; other Si sources such as sodium metasilicate, silica sol, and water glass also were investigated and the expected Si content (SiO 2 ) was 3.6 wt.%. The mixed solution was stirred at room temperature for 24 h, and then transferred to a 100 mL polytetrafluoroethylene hydrothermal kettle and hydrothermally heated under dynamic conditions of 150°C (rotating at a speed of 60 rpm) for 48 h. Finally, the white precipitate was cooled to room temperature, filtered, washed, dried, and then calcined at 500, 800, or 1000°C for 4 h, respectively. The prepared Si-doped Al 2 O 3 nanosheets were marked as XSiAlNS-Y (X denoted the mass content of Si; Y meant the calcination temperature).

Preparation of supported palladium catalysts
The preparation of the XSiAlNS-500 supported Pd catalyst was achieved by an excessive impregnation method by adjusting the pH value of the impregnation solution. Specifically, 1 g of the calcined XSiAlNS-500 was ultrasonically dispersed in 200 mL of deionized water for 30 min, and HNO 3 was used to adjust the pH to 3~3.5. Then, 0.5 mL (the weight content of Pd was expected to be 1%) of palladium precursor (20 mg/mL H 2 PdCl 4 ) was diluted in 200 mL of deionized water (pH = 3), and then was dropped into the XSiAlNS-500 suspension at a speed of 1 mL/min through a peristaltic pump. After stirring for 24 h, the suspension was filtered, washed, dried, and calcined at 500 or 800°C for 4 h. The prepared sample was remarked as Pd/XSiAlNS-500-T (T meant the calcination temperature).

Catalytic combustion of propane
The catalytic combustion reaction of propane was carried out in a continuous flow microreactor composed of Ushaped quartz tubes (inner diameter = 4 mm) under atmospheric pressure. 200 mg catalyst (60-80 mesh) was placed at the bottom of the reactor between the two layers of quartz wool. Then, the feed gas containing 0.1 vol.% C 3 H 8 , 20 vol.% O 2 , and balanced with Ar was passed through the reactor at a flow rate of 50 mL/min with the gas hourly space velocity (GHSV) being 15,000 ml g −1 h −1 . The catalyst was ramped from 100°C to 400°C at the heating rate of 5°C/min, and composition of effluent gases was analyzed on-line by a gas chromatograph (GC-2060) equipped with a flame ionization detector (FID).

Characterization of the catalyst
The morphology of the catalyst was tested on a Hitachi S-3400N Scanning Electron Microscope (SEM), operating with an acceleration voltage of 15 kV. The powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus diffractometer with Cu Kα radiation (λ = 0.15406 nm, operated at 40 mA and 40 kV). The diffractograms were recorded within the 2θ range of 10 to 80°with a 2θ step size of 6°and a step time of 1 min. The specific surface area (S BET ) of the sample was tested by the Micromeritics ASAP 2020 analyzer. Before the test, the catalyst was pretreated in a vacuum environment at 200°C for 8 h, and then tested at −196°C. The specific surface area of the catalyst was obtained by the BET (Brunauer-Emmett-Teller) formula. The composition, content, and valence of the surface elements of the catalyst were detected by ESCALAB 250 X-ray electron spectrometer. The X-ray source was monochromatic Al Kα palladium (1486.6 eV), and the binding energy was corrected by carbon C1s (284.8 eV).
The Pd dispersion was determined by CO pulses and tested on Autochem 2920 II chemical adsorption analyzer. First, the catalyst (50 mg) was reduced with 50 mL/min 10% H 2 /N 2 mixture at 200°C for 0.5 h, and then cooled to 30°C in flowing He. Several pulses of CO were introduced until no more adsorption was observed. The Pd dispersion was calculated from the amount of CO chemisorption by assuming a stoichiometric ratio of CO/Pd=1/1. H 2 -TPR was detected by a TPDRO 1100 chemical adsorption apparatus equipped with a thermal conductivity detector (TCD). 10 vol% H 2 /N 2 flow passed through 50 mg of sample at a flow rate of 40 mL/min; the temperature was ramped linearly from 30°C to 500°C at 10°C/min. The actual hydrogen consumption of the catalyst was calculated using the H 2 consumption of copper oxide as a reference.
In situ CO adsorption DRIFTS was detected on a Nicolet 6700 Fourier Transform Infrared Spectrometer with 64 scans and a resolution of 4 cm −1 . First, the catalyst (20 mg) was pretreated with 50 mL/min 20 vol.% O 2 /Ar at 300°C for 1 h, and then purged with 50 mL/min Ar at 150°C for 0.5 h. After cooling down to room temperature and collecting background, CO adsorption was carried out under 50 mL/min 20 vol.% for 0.5 h.

Results and discussion
Effect of Si doping on morphology and thermal stability of Al 2 O 3 nanosheet Figure 1 shows SEM images of Si-doped Al 2 O 3 with different Si sources and different Si doping amounts; it can be found that all synthesized Al 2 O 3 presented three-dimensional flower-like structures assembled by nanosheets and the doping of Si did not destroy the morphology of Al 2 O 3 (Wu et al. 2014). However, the thickness of Al 2 O 3 nanosheets varied as different Si sources (3.6 wt.% Si). The thickness of the prepared nanosheet without adding Si was about 80 nm, while the addition of TEOS or sodium metasilicate (Na 2 SiO 3 ) significantly reduced the thickness of Al 2 O 3 nanosheets and the average thickness was about 30 nm ( Fig. 1c and f). When silica sol and water glass were used as Si sources, the average thickness of Al 2 O 3 nanosheets was about 60 nm ( Fig. 1a and b). Our previous work (Wu et al. 2014) indicated that Al 2 O 3 nanosheets with regular leaf-like architecture could be synthesized by a novel intercalation-swelling-exfoliation pathway and then its morphology/thickness could be further adjusted via the dissolution-growth induced by phosphate. Here, similar roles of the Si with phosphate were observed. Sequentially, the effect of Si content using TEOS as Si source was investigated and is shown in Fig. 1d-h. It can be found that the thickness of Al 2 O 3 nanosheets obviously thinned with the increase of the Si content; for example, the thickness of 0SiAlNS-500 and 1.8SiAlNS-500 was about 100 nm and 70 nm, while the thickness of the 3.6-7.2SiAlNS-500 samples drastically decreased to about 30 nm. Additionally, the morphology of Al 2 O 3 nanosheets also varied with the Si content; an evolution from irregular nanosheets of 0SiAlNS-500 and 1.8SiAlNS-500 to the flower-like architecture assembled by regular leaf-like nanosheets of the 3.6-7.2SiAlNS-500 samples was observed, but too much Si (7.2 wt.% Si) would lead to the accumulation of the leaf-like nanosheets.
Effect of Si content on thermal stability of synthesized Al 2 O 3 nanosheets was further investigated by XRD and specific surface area of the samples calcined at different temperatures, and the results are listed in Fig. 2. XRD patterns shown  Fig. 2 illustrated that the crystal phase of XSiAlNS-Y evolved in the sequence of γ→δ→θ→α in the ranges of 500 and 1200°C, but the characteristic peaks of SiO 2 appeared when the content of Si increased to 7.2 wt.% (7.2SiAlNS-Y). In particular, all 0SiAlNS-Y and 3.6SiAlNS-Y samples only presented a γ phase after calcined at 500°C or 800°C, with the further increasing of the calcined temperature to 1000 and 1200°C; the phase of 0SiAlNS-Y transformed into θand α-phase while the θ phase was still maintained after calcining at 1000°C and 1200°C for 3.6SiAlNS-Y and 7.2SiAlNS-Y samples (α-phase was not observed). Thereby, XRD results showed that Si doping effectively suppressed the high-temperature phase transformation of Al 2 O 3 nanosheets and enhanced its thermal stability, which attributed to the fact that Si occupied the holes of the AlO 4 tetrahedron in Al 2 O 3 and inhibited the O 2− reconstruction to hcp (Mardkhe et al. 2015). Meanwhile, the presence of SiO 2 could isolate Al 2 O 3 particles and prevent its aggregation. Additionally, the specific surface area of the XSiAlNS-Y calcined at different temperatures was determined. The 3.6SiAlNS sample presented a better resistance to sintering and the specific surface area still up to 144.1, 125.2, 100.5, and 56.3 m 2 g −1 after calcining at 500, 800, 1000, and 1200°C , respectively, while the specific surface area of the 0SiAlNS-800, 0SiAlNS-1000, and 0SiAlNS-1200 samples decreased from 64.2 m 2 g −1 sharply to 18.3, 20.1 and 12.8 m 2 g −1 . In addition, the specific surface area changes of 0SiAlNS-Y and 7.2SiAlNS-Y with the increase of the firing temperature (500, 800, 1000, 1200°C) are 64.2, 18.3, 20.1, and 12.8 m 2 g −1 and 46.3, 37.1, 27.6, and 19.8 m 2 g −1 , which can be analyzed that excessive Si doping can also suppress the decrease of the specific surface area of Al 2 O 3 materials. This result further confirmed that the doping of Si improved the thermal stability of Al 2 O 3 nanosheets, which was critical in catalysis reactions occurred at high temperature.

Characterization of Pd supported on Al 2 O 3 nanosheets
Al 2 O 3 nanosheets (XSiAlNS-500) with different Si content calcined at 500°C were further investigated as supports of Pd, and its physicochemical properties were evaluated. Figure 2e and f show XRD patterns of Pd supported on Al 2 O 3 nanosheets. Only γ phase Al 2 O 3 was detected in all samples and no characteristic diffraction peaks of PdO or Pd appeared even after calcining at 800°C, indicating that Pd presented a high dispersion on Al 2 O 3 nanosheet surface. Interestingly, it can be observed from Fig. 2a-b and e-f that the peaks corresponding to SiO 2 phase of the Pd supported on 7.2SiAlNS catalysts disappeared, which indicated that the surface SiO 2 was re-doped into the defective spinel structure of γ-Al 2 O 3 during the secondary calcination process, or the partial etching of Si residue on Al 2 O 3 surface occurred due to the low pH value (about 3.0) during the loading of Pd.
The states of Pd on Al 2 O 3 nanosheets such as particle size, dispersion, valence, and redox ability were further characterized by HRTEM, CO pulse, CO-DRIFT, XPS, and H 2 -TPR. HRTEM images in Fig. 3 indicated that the doping of Si obviously reduced the size of Pd particles and promoted its dispersion. The average particle size (Table 1) decreased from 13 nm (Pd/0SiAlNS-500-500) to 4.3 nm (Pd/3.6SiAlNS-500-500) and 4.6 nm (Pd/7.2SiAlNS-500-500). Moreover, HRTEM images of the Pd supported catalysts at 800°C revealed that the size of Pd particles in the Pd/0SiAlNS-500-800 Fig. 3 TEM images of Pd/0SiAlNS-500-500 (a), Pd/3.6SiAlNS-500-500 (b), Pd/7.2SiAlNS-500-500 (c), Pd/0SiAlNS-500-800 (d), Pd/3.6SiAlNS-500-800 (e), and Pd/7.2SiAlNS-500-800 (f) and Pd/3.6SiAlNS-500-800 catalysts increased from 13 and 4.3 nm to 19 and 9.7 nm while the sintering of Pd particles in Pd/7.2SiAlNS-500-800 was not observed, which implied that the introduction of Si not only enhanced the thermal stability of Al 2 O 3 but also improved the resistance to Pd sintering. An isolation effect of SiO 2 was responsible for the better resistance to sintering of Pd/7.2SiAlNS, since a small portion of SiO 2 (when the high content of SiO 2 was doped) aggregated to form SiO 2 phase and migrated to the Al 2 O 3 surface at high temperature and prevented the intergranular sintering of PdO x (Dai et al. 2018a, b). Additionally, as shown in Table 1, the CO pulse tests further confirmed that a higher Pd dispersion was observed on Si-doped Al 2 O 3 nanosheets, and the sintering of Pd particles also was suppressed and more significant with the increasing of Si content. Table 1 also shows the relationship between lnr and 1000/T in the C 3 H 8 catalytic combustion reaction. According to its slope, the activation energy of the catalyst can be calculated. It can be found that the activation energy of all Pd/XSiAlNS-Y-T catalysts was 70~105 kJ mol −1 . The activation energy sequence of Pd/ XSiAlNS-500-500 catalyst was as follows: Pd/3.6SiAlNS-500-500 (72.4 kJ/mol) <Pd/7.2SiAlNS-500-500 (80.2 kJ/ mol) ≈ Pd/0SiAlNS-500-500 (80.8 kJ/mol). Pd/3.6SiAlNS-500-500 has a higher reaction rate and lower activation energy, which was consistent with the law of activity. The law of T 90 for Pd/XSiAlNS-500-500 was opposite to Ea, which may be caused by different reaction paths. Figure 4a-b displays IR spectra of CO adsorption (CO-DRIFT) on Pd/XSiAlNS-500-T catalysts. Based on the intensity of CO adsorption peaks, it can be found that the adsorption of CO on Pd/0SiAlNS catalysts was notably less than that on Si doping Al 2 O 3 nanosheets, which was attributable to the lower Pd dispersion (HRTEM and CO pulse). Moreover, some obvious differences in wavenumber and the number of adsorption peaks were observed; Pd/0SiAlNS-500-500 catalyst mainly showed two CO adsorption peaks at 2115 cm −1 and 2086 cm −1 , which were attributed to the linear adsorption of CO on Pd σ+ (0<σ<2) and Pd 0 , respectively (Murata et al. 2017), while the CO adsorption peaks on Si-doped catalysts shifted to the high wavenumber (up to 2139 cm −1 ) besides the peak at 2086 cm −1 , which were attributed to the linear adsorption of CO on Pd + (Pd with higher valence) (Dai et al. 2018a, b). These results indicated that the high Pd dispersion was confirmed (HRTEM and CO pulse) and the doping of Si led to the Pd electron transfer to Si or suppressed the reduction of PdO x by CO. In addition, some weak bands at 1976, 1962, 1924, and 1930 cm −1 assigned to the bridged adsorption of CO on PdO x , and the bands at 1976 and 1962 cm −1 corresponding to the adsorption of CO on the stepped PdO x were also observed (Jbir et al. 2016). For all the samples aged at 800°C , the intensity of bands decreased compared with fresh samples, which may be caused by the sintering of the Pd particles or the disappearance of the corners and edges of the Pd particles (Ding et al. 2016), and consistent with HRTEM and CO pulse results. Moreover, it could be found that the bands at high wavenumber on the aged sample disappeared, corresponding to higher valence Pd species, which was probably attributed to the decomposition of PdO x at high temperature. Specifically, the CO linear adsorption of the Pd/7.2SiAlNS-500-800 (2092 cm −1 ) and Pd/0SiAlNS-500-800 (2069 cm −1 ) samples was mainly located in the range of the Pd 0 -CO characteristic adsorption peak (2060-2100 cm −1 ) (Dai et al. 2018a, b); however, the main CO linear adsorption band of the Pd/ 3.6SiAlNS-500-800 sample was at 2127 cm −1 with a weaker linear adsorption peak of 2022 cm −1 (attributed to the smaller Pd nanoparticles with lower electron density) (Rades et al. 1996).
The Pd 3d XPS spectra of the Pd/XSiAlNS-500-T catalyst are shown in Fig. 4c-d. The characteristic peaks assigning to Pd 2+ at 336.4, 336.5, 336.6, and 336.8 eV were observed on all catalysts and independent of the Si content and calcination temperature (Kusumawati and Sasaki 2019). The Pd 0 was not detected, which indicated that Pd 0 species determined by CO-DRIFT was possibly ascribed to the reduction of PdO x by CO due to the good redox ability of the supported PdO x (Hoflund et al. 2003). Therefore, the redox performance of Pd/ XSiAlNS-500-T catalysts was evaluated by H 2 -TPR and is shown in Fig. 5. For all the Pd/XSiAlNS-500-500 catalysts, the reduction peak was not observed but a negative peak attributing to the decomposition of PdH x species appeared at 60-80°C, which indicated that PdO x species had been easily reduced at low temperature (at 40°C) due to the high dispersion of PdO x . However, the peak temperature and intensity were varied with the Si content. Specifically, the amount of PdH x species (the intensity of the negative peak) decreased with the increase of Si content, which was attributed to the smaller formation enthalpy of PdH x when H atoms combined with Pd in the bulk phase (Delogu 2010). In addition, the highest decomposition temperature of PdH x in the Pd/ 3.6SiAlNS-500-500 sample also proved that the particle size of the Pd/0SiAlNS-500-500 sample was relatively large, which was consistent with the TEM results. However, it can be observed that the lower decomposition temperature of the Pd/7.2SiAlNS-500-500 sample was caused by the reduction of the metal-support interaction caused by the Si on the surface (Murata et al. 2017;He et al. 2014). After Pd/XSiAlNS catalysts were aged at 800°C, obvious differences were observed compared with the fresh catalysts. Pd/0SiAlNS-500-800 and Pd/7.2SiAlNS-500-800 catalysts presented a reduction peak at about 60°C, while only a negative peak from the decomposition of PdH x species on Pd/3.6SiAlNS-500-800 catalyst was detected. The results revealed that the former two were more difficult to be reduced, but might be ascribed to different factors. For the Si undoped Al 2 O 3 , the obvious sintering and aggregation of PdO x particles due to the weak metal-support interaction was the dominant factor, which was consistent with the analysis results of TEM and CO-Infrared. However, the encapsulation and segregation of PdO x particles owing to the migrated SiO 2 to the surface of Al 2 O 3 was responsible for the more difficult reduction of Pd/7.2SiAlNS-500-800 (the sintering of PdO x particles at high temperature was not observed) (Nampi et al. 2010). In addition, by XPS analysis, the Pd σ+ (0<σ<2) valence of Pd/3.6SiAlNS-500-800 PdO x was higher, but there was no positive peak of PdO x decomposition in the H 2 -TPR test, which did not mean that there is no PdOx on Pd/3.6SiAlNS-500-800. After analyzing the results of CO-infrared detection, Pd/3.6SiAlNS-500-800 has a strong interaction between the metal and the carrier, which led to a higher temperature of the H 2 reduction peak of PdO x and coincides with the decomposition peak of PdH x , so a very weak PH x decomposition peak is observed in the Pd/ 3.6SiAlNS-500-800.

Catalytic combustion of propane
The catalytic performance of Pd/XSiAlNS-500-T catalysts was investigated through catalytic combustion of propane, and Fig. 6 displays its light-off curves under the conditions of 0.1% C 3 H 8 and 20% O 2 in Ar at a space velocity of 15,000 ml g −1 h −1 . Almost silent effect on catalytic activity was observed and T 50 (the temperature achieved 50% conversion of C 3 H 8 ) of all the Pd/XSiAlNS-500-500 catalysts was about 226°C (Fig. 6b). It is meaningful for Pd-decorated Al 2 O 3 catalysts because the presence of trace Si in Al 2 O 3 generally was considered to be poisoning to supported noble metals such as three-way catalysts (TWCs) for the control of vehicle exhaust pollution (Mohammad et al. 2010), which was possibly ascribed to the high dispersion of PdO x particles due to the isolation effect of Si (the pH value of the impregnating solution was adjusted to 3.0, lower than the isoelectric point of Al 2 O 3 but higher than the isoelectric point of SiO 2 ; thus, Pd was considered to be preferentially adsorbed on Al 2 O 3 ). However, for the aged catalysts at 800°C (Pd/XSiAlNS-500-800), the doping of Si evidently inhibited the declining of activity and presented a better resistance sintering of high temperature; the T 90 of Pd/3.6SiAlNS-500-800 and Pd/ 7.2SiAlNS-500-800 was equivalent to that of the fresh catalysts while the T 90 of Pd/0SiAlNS-500-800 increased from 280 to 302°C. Table 1 clearly indicated that the doping of Si suppressed the aggregation of PdO x particles. Nevertheless, when considering the T 50 (Fig. 6b), the high temperature aging caused the slight loss of activity of Pd supported Si-doped Al 2 O 3 catalysts and T 50 increased by 4-10°C, but Pd/ 3.6SiAlNS still presented the best performance (only increasing of 4°C) while Pd supported pristine Al 2 O 3 catalyst increased by 30°C. In short, the doping of Si did not suppress catalytic activity of Pd/γ-Al 2 O 3 catalysts but instead promoted the resistance sintering, which was attributed to the high dispersion of the PdO x particles and the improvement of the thermal stability of Al 2 O 3 due to the Si doping. However, it should be noted that performances of Pd/γ-Al 2 O 3 catalysts were not continuously improved after the much Si was doped (Pd/7.2SiAlNS), because the introduction of much Si led to the formation of SiO 2 phase, which could prevent the aggregation of PdO x particles but also weaken the interaction between the PdO x particles and the Al 2 O 3 support, and then brought the intergranular sintering of the PdO x particles (Dai et al. 2018a, b).
Effects of H 2 O on catalytic combustion of propane (catalytic activity and stability) over Pd/XSiAlNS-500-500 catalysts were comparatively investigated, and the results are shown in Fig. 7. The presence of H 2 O obviously suppressed Fig. 6 Light-off curves (a) and T 50 (b) of fresh and aged Pd/XSiAlNS-500 catalysts at 800°C for catalytic combustion of propane catalytic combustion of propane on all catalysts, which was ascribed to the competitive adsorption of propane and H 2 O on PdO x active sites or the transformation of the instable PdO x into Pd(OH) x (Goodman et al. 2017). More importantly, it could be found that the doping of Si enhanced the water resistance of Pd/Al 2 O 3 catalysts especially for Pd/3.6SiAlNS-500-500. For example, T 90 of the Pd/0SiAlNS-500-500 increased by 73°C (from 278°C to 351°C) while T 90 of Pd/ 3.6SiAlNS-500-500 (from 278°C to 322°C) only increased by 45°C. It could be speculated that the increasing of the hydrophobicity and stabilization of PdO x particles (the strong interaction between PdO x and Si-doped Al 2 O 3 ) was responsible for the better water resistance of Si-doped catalysts. Additionally, Pd/7.2SiAlNS-500-500 presented an almost overlapping light-off curve with Pd/3.6SiAlNS-500-500 in the range of lower temperature, but the propane conversion over Pd/7.2SiAlNS-500-500 catalyst was slightly below that of Pd/3.6SiAlNS-500-500 as the temperature increased and T 90 increased from 322°C to 341°C. The aged experiments and the corresponding characterization results confirmed that the PdO x particles on the Pd/3.6SiAlNS-500-500 catalyst showed a better stability; thus, the deactivation from the transformation of the instable PdO x into Pd(OH) x was more restrained. Additionally, the prolonged stability of Pd/ XSiAlNS-500-500 catalysts for catalytic combustion of propane under the alternate dry and humid conditions (3 vol.% H 2 O) at 300°C was evaluated and is shown in Fig. 7b. The three catalysts presented almost same conversion of propane under dry conditions. After 3 vol.% H 2 O was introduced, the conversion of propane rapidly declined but differences in these catalysts. The doping of Si retarded the inhibition of H 2 O on catalytic combustion of propane, and Pd/ 3.6SiAlNS-500-500 still presented the highest catalytic activity, which was consistent with the results from activity tests. More importantly, the conversion of propane could quickly restore the initial conversion and even a higher conversion (from 86% to 96%) was detected on Pd/3.6SiAlNS-500-500 after H 2 O was switch off, which indicated that the effect of H 2 O was reversible. Even after running for 50 h, only a slight deactivation of Pd/0SiAlNS-500-500 (from 85% to 82%) and Pd/7.2SiAlNS-500-500 (from 89% to 88%) catalysts was observed, while the Pd/3.6SiAlNS-500-500 catalyst still presented an increased conversion of propane by 10% (from 86% to 96%). The results re-confirmed that the doping of Si improved the water resistance of the supported Pd catalyst, and the activity promotion of Pd/3.6SiAlNS-500-500 was considered to be related with the possible redispersion of PdO x with high stability and SMSI in the presence of H 2 O (Nie et al. 2017;Zhao et al. 2017).

Conclusions
The Si-doped Al 2 O 3 nanosheets were synthesized by a dynamic hydrothermal method in the presence of TPAOH as precipitating and swelling agent. The influence of different Si sources and Si doping amounts on the structure of alumina was investigated, and it was found that TEOS precursor could significantly tune the thickness of the Al 2 O 3 nanosheets and reduced from 80 nm to 30 nm when the 3.6% Si was doped. Moreover, the doping of Si improved the thermal stability of Al 2 O 3 nanosheets. The θ phase was still maintained after calcination and even the specific surface area was as high as 56.3 m 2 g −1 at 1200°C. Sequentially, the supported Pd catalysts using the Si-doped Al 2 O 3 nanosheets as supports were prepared by an excessive impregnation method with a tailoring pH value. Characterization results of HRTEM, CO pulse, and CO-DRIFT indicated that the doping of Si increased the dispersion of Pd and the formation of stabilized PdO x particles, while the increase in Si doping (7.2%) caused the aggregation of Si to form SiO 2 segregation, weakening the SMSI and leading to the poor dispersion of Pd. Catalytic combustion of propane as typical model reaction for the purification of industrial VOCs and automobile hydrocarbon emissions revealed that the appropriate Si doping (3.6%) promoted the resistance sintering and water resistance of the supported Pd catalysts, which was attributed to the preferable SMSI, the high dispersion of the PdO x particles, and the improvement of the thermal stability of Al 2 O 3 due to the Si doping.