3.1. Effect of Si doping on morphology and thermal stability of Al2O3 nanosheet
Figure 1 showed SEM images of Si doped Al2O3 with different Si sources and different Si doping amounts, it can be found that all synthesized Al2O3 presented three-dimensional flower-like structures assembled by nanosheets and the doping of Si did not destroy the morphology of Al2O3 (Wu et al 2014). However, the thickness of Al2O3 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 (Na2SiO3) significantly reduced the thickness of Al2O3 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 Al2O3 nanosheets was about 60 nm (Fig. 1a and b). Our previous work (Wu et al 2014) indicated that Al2O3 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 showed in Fig. 1d-h. It can be found that the thickness of Al2O3 nanosheets obviously thinned with the increase of the Si content, for examples, 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 Al2O3 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 Al2O3 nanosheets was further investigated by XRD and specific surface area of the samples calcined at different temperatures, and the results were listed Fig. 2. XRD patterns shown in 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 SiO2 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 θ- and α-phase while the γ phase was still maintained after calcining at 1000°C for 3.6SiAlNS-Y and 7.2SiAlNS-Y samples and only θ phase formed even at 1200°C (α-phase was not observed). Thereby, XRD results showed that Si doping effectively suppressed the high-temperature phase transformation of Al2O3 nanosheets and enhanced its thermal stability, which attributed to the fact that Si occupied the holes of the AlO4 tetrahedron in Al2O3 and inhibited the O2− reconstruction to hcp (Mardkhe et al 2015). Meanwhile, the presence of SiO2 could isolate Al2O3 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 100.5 and 56.3 m2∙g− 1 after calcining at 1000°C and 1200°C, respectively, while the specific surface area of the 0SiAlNS-1000 and 0SiAlNS-1200 samples decreased from 64.2 m2∙g− 1 sharply to 20.1 and 12.8 m2∙g− 1. This result further confirmed that the doping of Si improved the thermal stability of Al2O3 nanosheets, which was critical in catalysis reactions occurred at high temperature.
3.2. Characterization of Pd supported on Al2O3 nanosheets
Al2O3 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 2f showed XRD patterns of Pd supported on Al2O3 nanosheets. Only γ phase Al2O3 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 Al2O3 nanosheets surface. Interestingly, it can be observed from Fig. 2a-b and 2e-f that the peaks corresponding to SiO2 phase of the Pd supported on 7.2SiAlNS catalysts disappeared, which indicated that the surface SiO2 was re-doped into the defective spinel structure of γ-Al2O3 during the secondary calcination process, or the partial etching of Si residue on Al2O3 surface occurred due to the low pH value (about 3.0) during the loading of Pd.
Table 1
The Pd content of the Pd/XSiAlNS-500-T catalysts, as well as T90 and reaction rate (r, at 225°C) for C3H8 combustion
Catalyst | Pd particle size (nm)a | Pd dispersion (%)b | T90 (℃) | r×107 (mol/g/s)c | Ea (kJ/mol) |
Pd/0SiAlNS-500-500 | 13 | 11.6 | 280 | 106 | 80.8 |
Pd/3.6SiAlNS-500-500 | 4.3 | 34.3 | 278 | 371 | 72.4 |
Pd/7.2SiAlNS-500-500 | 4.6 | 37.2 | 282 | 205 | 80.2 |
Pd/0SiAlNS-500-800 | 19 | 4.1 | 302 | 111 | 82.6 |
Pd/3.6SiAlNS-500-800 | 9.7 | 16.6 | 278 | 242 | 94.8 |
Pd/7.2SiAlNS-500-800 | 5.2 | 35.7 | 278 | 205 | 101.4 |
a Detected by HRTEM. |
b Calculated based on CO chemisorption at 30 ℃. |
c at 225 ℃. |
The states of Pd on Al2O3 nanosheets such as particle size, dispersion, valence and redox ability were further characterized by HRTEM, CO pulse, CO-DRIFT, XPS and H2-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 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 Al2O3 but also improved the resistance to Pd sintering. An isolation effect of SiO2 was responsible for the better resistance to sintering of Pd/7.2SiAlNS, since a small portion of SiO2 (when the high content of SiO2 was doped) aggregated to form SiO2 phase and migrated to the Al2O3 surface at high temperature and prevented the intergranular sintering of PdOx (Dai et al 2018). Additionally, as shown in Table 1, the CO pulse tests further confirmed that a higher Pd dispersion was observed on Si doped Al2O3 nanosheets, and the sintering of Pd particles also was suppressed and more significant with the increasing of Si content.
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 Al2O3 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 2087 cm− 1, which were attributed to the linear adsorption of CO on Pdσ+(0 < σ < 1) and Pd0 (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 2018). These results indicated that the high Pd dispersion was confirmed (HRTEM and CO pulse) and the doping of Si led to the Pd electrons transfer to Si or suppressed the reduction of PdOx by CO. In addition, some weak bands at 1976, 1962, 1924 and 1930 cm− 1 assigned to the bridged adsorption of CO on PdOx, and the bands at 1976 and 1962 cm− 1 corresponding to the adsorption of CO on the stepped PdOx 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 PdOx 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 were mainly located in the range of the Pd0-CO characteristic adsorption peak (2060–2100 cm− 1) (Dai et al 2018); 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 were showed in Fig. 4c-d. The characteristic peaks assigning to Pd2+ 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 et al 2019). The Pd0 was not detected, which indicated that Pd0 species determined by CO-DRIFT was possibly ascribed to the reduction of PdOx by CO due to the good redox ability of the supported PdOx (Hoflund et al 2003). Therefore, the redox performance of Pd/XSiAlNS-500-T catalysts was evaluated by H2-TPR and 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 PdHx species appeared at 60–80 ℃, which indicated that PdOx species had been easily reduced at low temperature (at 40 ℃) due to the high dispersion of PdOx. However, the peak temperature and intensity were varied with the Si content. Specifically, the amount of PdHx species (the intensity of the negative peak) decreased with the increase of Si content, which was attributed to the smaller formation enthalpy of PdHx when H atoms combined with Pd in the bulk phase (Delogu et al 2010). In addition, the highest decomposition temperature of PdHx 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 2003). After Pd/XSiAlNS catalysts were aged at 800 ℃, 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 ℃, while only a negative peak from the decomposition of PdHx 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 Al2O3, the obvious sintering and aggregation of PdOx particles due to the weak metal-support interaction was the dominant factor, while the encapsulation and segregation of PdOx particles owing to the migrated SiO2 to the surface of Al2O3 was responsible for the more difficult reduction of Pd/7.2SiAlNS-500-800 (the sintering of PdOx particles at high temperature was not observed) (Nampi et al 2010).
3.4. 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% C3H8 and 20% O2 in Ar at a space velocity of 15,000 ml∙g− 1∙h− 1. Almost silent effect on catalytic activity was observed and T50 (the temperature achieved 50% conversion of C3H8) of all the Pd/XSiAlNS-500-500 catalysts was about 226 ℃ (Fig. 6b). It is meaningful for Pd decorated Al2O3 catalysts because the presence of trace Si in Al2O3 generally was considered to be poisoning to supported noble metals such as three-way catalysts (TWCs) for the control of vehicle exhaust pollution,[40] which was possibly ascribed to the high dispersion of PdOx 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 Al2O3 but higher than the isoelectric point of SiO2, thus Pd was considered to be preferentially adsorbed on Al2O3). 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 T90 of Pd/3.6SiAlNS-500-800 and Pd/7.2SiAlNS-500-800 was equivalent to that of the fresh catalysts while the T90 of Pd/0SiAlNS-500-800 increased from 280 to 302°C. Table 1 clearly indicated that the doping of Si suppressed the aggregation of PdOx particles. Nevertheless, when considering the T50 (Fig. 6b), the high temperature aging caused the slight loss of activity of Pd supported Si-doped Al2O3 catalysts and T50 increased by 4–10°C, but Pd/3.6SiAlNS still presented the best performance (only increasing of 4°C) while Pd supported pristine Al2O3 catalyst increased by 30°C. In short, the doping of Si did not suppress catalytic activity of Pd/γ-Al2O3 catalysts instead promoted the resistance-sintering, which was attributed to the high dispersion of the PdOx particles and the improvement of the thermal stability of Al2O3 due to the Si doping. However, it should be noted that performances of Pd/γ-Al2O3 catalysts were not continuedly improved after the much Si was doped (Pd/7.2SiAlNS), because the introduction of much Si led to the formation of SiO2 phase, which could prevent the aggregation of PdOx particles but also weaken the interaction between the PdOx particles and the Al2O3 support, and then brought the intergranular sintering of the PdOx particles (Dai et al 2018).
Effects of H2O on catalytic combustion of propane (catalytic activity and stability) over Pd/XSiAlNS-500-500 catalysts were comparatively investigated, and the results are showed in Fig. 7. The presence of H2O obviously suppressed catalytic combustion of propane on all catalysts, which was ascribed to the competitive adsorption of propane and H2O on PdOx active sites or the transformation of the instable PdOx 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/Al2O3 catalysts especially for Pd/3.6SiAlNS-500-500. For example, T90 of the Pd/0SiAlNS-500-500 increased by 73 ℃ (from 278 ℃ to 351 ℃) while T90 of Pd/3.6SiAlNS-500-500 (from 278 ℃ to 322 ℃) only increased by 45 ℃. It could be speculated that the increasing of the hydrophobicity and stabilization of PdOx particles (the strong interaction between PdOx and Si doped Al2O3) 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 T90 increased from 322 ℃ to 341 ℃. The aged experiments and the corresponding characterization results confirmed that the PdOx particles on the Pd/3.6SiAlNS-500-500 catalyst showed a better stability, thus the deactivation from the transformation of the instable PdOx 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.% H2O) at 300 ℃ were evaluated and showed in Fig. 7b. The three catalysts presented almost same conversion of propane under dry conditions. After 3 vol.% H2O was introduced, the conversion of propane rapidly declined but differences in these catalysts. The doping of Si retarded the inhibition of H2O 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 86–96%) was detected on Pd/3.6SiAlNS-500-500 after H2O was switch off, which indicated that the effect of H2O was reversible. Even after running for 50 h, only a slight deactivation of Pd/0SiAlNS-500-500 (from 85–82%) and Pd/7.2SiAlNS-500-500 (from 89–88%) catalysts was observed, while the Pd/3.6SiAlNS-500-500 catalyst still presented an increased conversion of propane by 10% (from 86–96%). 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 PdOx with high stability and SMSI in the presence of H2O (Nie et al 2017; Zhao et al 2017).