A sintering-resistant Pd/SiO₂ catalyst by reverse-loading nano iron oxide for aerobic oxidation of benzyl alcohol

Abstract

An efficient and simple method, depositing nano iron oxide onto the surface of Pd/SiO₂, to construct sintering-resistant Pd nanocatalysts was presented. The introduction of nano iron oxide not only promoted the sintering-resistance of Pd nanoparticles, but also increased the boundary between the Pd and iron oxide. The resultant catalysts exhibited enhanced catalytic activity for aerobic oxidation of benzyl alcohol.

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The selective aerobic oxidation of benzyl alcohol to benzaldehyde is of fundamental and technological importance for the chemical industry, since benzaldehyde is commercially significant as a versatile intermediate in the manufacture of fine chemicals.¹ It is also employed as a probe reaction in laboratory to study the catalytic mechanisms of different catalysts. Among numerous catalysts, noble metal nanoparticle catalysts are extensively studied in this reaction, such as Ru/HAP,² Au/Al₂O₃,³ Pd/HAP,⁴ AuPd⁵ and AuCu.⁶ However, the high surface energy of noble metal nanoparticles leads to easy agglomeration and sintering during calcination and catalytic reactions, resulting in a dramatic decrease in activity and selectivity.⁷ To avoid this drawback, considerable efforts have been made by depositing metal nanoparticles onto various solid supports,⁸ or burying metal nanoparticles into various solid shells.⁹

To improve the sintering-resistance of noble metal nanoparticles, an effective method is employed by introducing another component, especially another non-precious metal oxide, such as TiO₂,¹⁰ La₂O₃, Nb₂O₅ (ref. 11) or CeO₂.¹² Significant progress has been made by P. C. Stair that Al₂O₃ overcoating of supported metal nanoparticles effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts.¹³ There after a number of research efforts have been devoted to so-called reverse-loading catalysts. Recently, iron oxides were reported as efficient additives to noble metal catalysts.¹⁴ For instance, after adding FeO_x as additive, Au nanoparticles exhibited strong sintering-resistance in Au/HAP even after being calcined at 600 °C. Similarly, the catalytic activity of Pt/CNT for benzyl alcohol aerobic oxidation was remarkably improved by decorating FeO_x on Pt nanoparticles. Our previous results showed that nano iron oxide could promote the catalytic performance of Au nanoparticles for the aerobic oxidation of alcohols.¹⁵

Herein, we report a simple and efficient procedure to improve the anti-sintering ability of the Pd nanoparticle by depositing small amounts of FeO_x onto the surface of Pd/SiO₂ catalysts. The FeO_x modified Pd catalysts exhibit excellent stability against sintering and high activity for benzyl alcohol oxidation.

Pd/SiO₂ was prepared via an incipient wetness impregnation method. SiO₂ was impregnated with the aqueous solution of PdCl₂. After drying at 60 °C for 24 h, the sample was subjected to calcination in air. As shown in Fig. 1a and b, after calcination at 400 °C, Pd nanoparticles on SiO₂ surface aggregated into large particles, and some Pd particles even aggregated up to 10 nm. The lattice fringe can be observed clearly in the HRTEM image (Fig. 1b, inset), and the lattice fringe space is 0.224 nm, corresponding to the (111) facet of metallic Pd. The size distribution of Pd nanoparticle is broad, with a mean diameter of ∼5 nm (Fig. 1a, inset).

Fig. 1 TEM images of (a and b) Pd/SiO₂, (c and d) FeO_x/Pd/SiO₂-0.5, and (e and f) FeO_x/Pd/SiO₂-10. The inset of (a) and (c) is the particle size distribution of the corresponding sample.

FeO_x was deposited onto the Pd/SiO₂ by impregnating aqueous solution of Fe(NO₃)₃·9H₂O before calcination at 400 °C. FeO_x/Pd/SiO₂ catalysts with different content of iron oxide (calculated by the amounts of Fe₂O₃) were synthesized, as summarized in Table S1.† As shown in Fig. 1c and d, in FeO_x/Pd/SiO₂-0.5, FeO_x is highly dispersed as uniform nanoparticles with similar particle size compared to Pd nanoparticles. The nanoparticle size distribution was counted including both nano-FeO_x and Pd nanoparticles (Fig. 1c, inset). The average diameter of nanoparticles is ∼2.9 nm, much smaller than Pd/SiO₂. The lattice fringe of FeO_x nanoparticles can be seen clearly on the HRTEM image (Fig. 1d). The lattice fringe space is 0.270 nm, corresponding to the (104) planes of α-Fe₂O₃. When FeO_x coverage is high, like FeO_x/Pd/SiO₂-10 (Fig. 1e and f), the surfaces of catalysts are covered by FeO_x layer. Since Pd nanoparticles are completely sheltered by FeO_x, Pd and FeO_x nanoparticles are rarely observed.

XPS measurements were conducted to further investigate the surface structure and electronic properties of these Pd nanoparticle catalysts. The results of Pd/SiO₂ and FeO_x/Pd/SiO₂ are illustrated in Fig. 2. Pd/SiO₂ and FeO_x/Pd/SiO₂-0.5 catalysts show typical doublet of Pd 3d core level bands. For FeO_x/Pd/SiO₂-10, the doublet of Pd 3d core level bands almost disappear. It indicates Pd nanoparticles are almost buried under FeO_x, in agreement with the former TEM results. These results suggest that by introducing of appropriate amounts of FeO_x while ensure the exposure of Pd nanoparticles, the sintering of Pd nanoparticles can be efficiently inhibited.

Fig. 2 Pd 3d X-ray photoelectron spectra of the Pd/SiO₂ and FeO_x/Pd/SiO₂ catalysts.

The synthesized catalysts with different FeO_x contents were evaluated by benzyl alcohol liquid phase aerobic oxidation reaction. As shown in Fig. 3a, the conversion of benzyl alcohol within 8 h on Pd/SiO₂ is remarkably low. Compared to Pd/SiO₂ catalysts, the deposition of FeO_x onto Pd/SiO₂ efficiently prevents Pd nanoparticles from sintering and then dramatically improves the catalytic activity in benzyl alcohol oxidation. The added amounts of FeO_x are crucial for the catalytic activity, and the best is 0.5 wt%. Too little amounts of FeO_x are not enough to prevent the sintering of Pd nanoparticles. Excess FeO_x may enshroud the active sites on Pd nanoparticles and block the path of reactants to catalytic active sites. When the content of FeO_x increases to 10 wt%, the corresponding catalyst is almost deactivated. It should be noted that the added amounts of FeO_x does not affect the product selectivity. For all catalysts, the benzaldehyde selectivity is above 90% (Fig. 3b).

Fig. 3 Benzyl alcohol oxidation catalyzed by the Pd/SiO₂ and FeO_x/Pd/SiO₂ catalysts.

For comparison, the catalytic activity of Pd/bulk-FeO_x is relatively lower (Fig. S1†). It is accepted that the boundary between metal and oxide plays an important role in the catalytic reactions.^11,16 The combination of Pd nanoparticles and nano-FeO_x could increase their contact boundaries and also be beneficial to the high activity of FeO_x/Pd/SiO₂-0.5. In N₂ atmosphere, the alcohol conversion on FeO_x/Pd/SiO₂-0.5 dramatically decreases to below 5% (Fig. S1†). It indicates that O₂ is necessary for the catalytic reaction and the conversion in N₂ might be caused by the lattice oxygen of nano-FeO_x, whose mobility can be increased by the adjacent noble metals.^15,17 The alcohol conversion on FeO_x/SiO₂ within 8 h at 100 °C is very limited with alcohol conversion of below 1.7%.

To further study the promotion effect of FeO_x on Pd nanoparticle, FTIR spectra of CO adsorbed on Pd/SiO₂ and FeO_x/Pd/SiO₂ were collected (Fig. 4). The IR bands corresponding to CO adsorbed on Pd can be divided into four modes: linear, compressed-bridged, isolated-bridged and tri-coordinated modes, which are observed at 2100–2050, 1995–1975, 1960–1925 and 1890–1870 cm⁻¹, respectively. The latter three modes constitute the multiply-bound adsorption of CO on Pd,¹⁰ which correspond to compressed-bridged and isolated-bridged modes, respectively. The signal around 1960 cm⁻¹ is assigned to bridge bonded CO mainly adsorbed on defect sites such as particle edges and corners.¹⁸ For Pd/SiO₂, the IR bands between 1990 and 1960 cm⁻¹ are weak. When appropriate amounts of FeO_x is introduced onto Pd/SiO₂ surface, such as FeO_x/Pd/SiO₂-0.5, the IR bands corresponding to CO adsorbed on defect sites of Pd turn to be stronger, especially for the IR bands at 1990 and 1965 cm⁻¹. To further increase the FeO_x amounts, the corresponding IR bands decrease dramatically (FeO_x/Pd/SiO₂-1). Especially, for FeO_x/Pd/SiO₂-10 catalysts, due to little Pd nanoparticles being exposed, the IR bands of CO adsorbed on Pd are weaker. These results suggest that by introducing of appropriate amounts of FeO_x while ensure the exposure of Pd nanoparticles, the sintering of Pd nanoparticles can be efficiently inhibited and then the number of defect sites would increase, which are highly active sites for catalytic reactions and thus improve the activity of Pd nanoparticles.⁴ Sintering-resistant Pd nanoparticles together with the adjacent iron oxide nanoparticles form an excellent catalytic system.

Fig. 4 Infrared spectra of CO adsorbed on Pd/SiO₂ and FeO_x/Pd/SiO₂ catalysts.

Based on the characterizations and catalytic activities of the catalysts, a plausible mechanism of the catalytic reaction on FeO_x/Pd/SiO₂ is proposed in Scheme 1: (1) benzyl alcohol molecules are adsorbed and activated on exposed Pd nanoparticles; (2) the deposition of FeO_x on Pd/SiO₂ facilitates the move of the lattice oxygen of surface Fe–O adjacent to Pd and the reaction with the adsorbed reactants to products, accompanying the oxygen vacancy formed (white square); (3) oxygen molecules are activated at the FeO_x and/or the Pd–FeO_x interface and then react with FeO_x to form lattice oxygen, accomplishing a catalytic circle.

Scheme 1 Reaction schemes for selective oxidation of benzyl alcohol on FeO_x/Pd/SiO₂.

Conclusions

In conclusion, a novel FeO_x/Pd/SiO₂ catalyst has been successfully synthesized. It is found that the reverse loading of FeO_x improves the sintering-resistance of Pd nanoparticles and also increases the defect sites of Pd nanoparticles such as particle edges and corners, which produce more active sites between Pd and FeO_x and lead to a significant enhancement in catalytic performance. The promotion effect of reverse loading FeO_x in FeO_x/Pd/SiO₂ catalyst may provide an efficient method for the development of stable noble metal catalysts.

Acknowledgements

The authors thank the financial supports from the National Science Foundation of China (20673054, 21273107, 91434101), and Sinopec Shanghai Research Institute of Petrochemical Technology.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and extra tables and figures. See DOI: 10.1039/c4ra14498h

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