Spherical-like Pd/SiO2 catalysts for n-butylamine efficient combustion: Effect of support property and preparation method
Graphical abstract
Introduction
Nowadays, volatile organic compounds (VOCs) emitted from industrial processes, transportation, house-hold activities have attracted much attention as VOCs are associated with various environmental problems including photochemical smog, stratospheric ozone depletion, and the formation of secondary aerosols [[1], [2], [3]]. The common VOCs include hydrocarbons, oxygen-containing VOCs (OVOCs), nitrogen-containing VOCs (NVOCs), and chlorinated VOCs (CVOCs) [4]. Catalytic combustion is considered as a promising way to dispose VOCs, especially with low concentrations [5]. However, compared with the other VOCs which have been studied widely, NVOCs are rarely studied. In addition to catalytic activity, NOx yield is also an important indicator to evaluate the performance of catalysts in NVOC combustion [6]. N-butylamine is a typical NVOC emitted from various industrial processes such as pesticide production can make great irritations to our skin, eye, and respiratory system [7]. As such, the efficient disposal of n-butylamine is of great importance for public health.
Noble metal supported catalysts (NSCs) have been attracted extensive attentions in catalytic oxidation of VOCs due to their excellent catalytic activity at low temperature, in which the Pd- and Pt-based catalysts are the most studied [8]. Both the support property and preparation method have significant influences on catalytic performance of NSCs [9]. It is reported that the high surface area materials (HSMs) have important applications in supercapacitor [10], gas storage [11], diagnosis or targeting of drugs [12], absorbent materials [13], and catalysis. Amongst, HSMs are often used as catalyst supports to obtain highly dispersed active sites and catalytically active materials considering their high specific surface area (SSA) and developed porosity, that are great meaningful for NSCs [12]. HSMs are a research hotspot in the region of catalysis. Macro-mesoporous ZrO2, TiO2, and ZrO2-TiO2 were synthesized by Tidahy and co-workers [14] and used as supports for Pd-based catalysts. They found that the samples with high SSA are very active in total oxidation of toluene and chlorobenzene. Palma et al. [15] proposed that the catalytic performance of Pt-Ni/CeO2 for ethanol reforming can be enhanced by supporting it on high SSA silica. Compared with Pt, Pd is cheaper and has better hydrothermal stability. In addition to this, the high porosity of HSMs is also beneficial for enhancing the activity of NSCs due to the promotion of mass transfer [[16], [17], [18]]. Hierarchical core-shell Pd-CoAlO-Al microsphere catalysts were prepared by Zhao et al. [16] and adopted in toluene combustion. They demonstrated that the increase of mesoporosity can improve the diffusion of toluene molecular and enhance the catalytic activity. SBA-15 materials with expanded microporosity were synthesized by He and co-workers [17], and results suggested that the high porosity of these materials obviously promoted the activity of Pd supported catalysts in toluene destruction.
Various methods such as in situ synthesis (IS), chemical vapour deposition (CVD), sonochemical preparation (SP), grafting (GA), coprecipitation (CP), and sol-gel process, have been developed with target to prepare catalysts with highly dispersed Pd sites and enhanced anti-sintering ability [[19], [20], [21], [22], [23], [24]]. Lin et al. [19] adopted an in situ encapsulation method to introduce (NH4)2PdCl4 into MCM-41 and indicated that the Pd sites are well-dispersed in MCM-41 mesochannels which can effectively prohibit Pd nanoparticles aggregation through their spatial confinement effect. CVD is considered as a convenient technology to obtain highly dispersed Pd particles, while this technology is often used in the preparation of one-dimensional materials [20]. Gaudino et al. [21] prepared Pd/alumina sphere catalysts through a SP method. Results showed that Pd particles are evenly dispersed over the support. Catalysts with highly dispersed Pd nanoparticles confined in SBA-15 channels were synthesized by He et al. [22] using a GA protocol, and they demonstrated that the prepared catalysts possessed high activity and stability in toluene elimination. Li et al. [25] prepared a series of Pd/Co3AlO catalysts by four different routes, and the sample prepared by a CP method shows higher activity than the other samples in total oxidation of toluene. However, the effects of support property and synthesis protocol of Pd-based catalysts on NVOC destruction are very limited to date.
In this work, two types of spherical-like SiO2 materials with different SSA and porosity were prepared and used as support for Pd nanoparticles which were respectively introduced by the IS, WI, and GA methods. The physicochemical property and catalytic activity of synthesized catalysts were systematically studied to reveal the effects of support property and preparation method on their catalytic performance. Moreover, the destruction mechanism of n-butylamine was proposed based on the results of in situ DRIFTS. It is shown that the catalysts prepared by the GA method exhibit higher activity than the other materials due to superior Pd dispersion; however, the NOx yields of these catalysts are very high. Comparatively, the catalysts prepared by the IS method are appropriate candidates in n-butylamine combustion due to their excellent catalytic activity and low NOx yield. The obtained results can provide a substantial foundation for the design and optimization of catalysts applied in NVOC combustion.
Section snippets
Synthesis of spherical-like silicon
Typically, 1.05 g of cetyltrimethyl ammonium bromide (CTAB) was dissolved in a mixed solution containing 58.8 g of ammonia (25 wt.%), 152.6 g of ethanol, and 51.8 g of deionized water. Then, 3.2 mL of tetraethyl orthosilicate (TEOS) was added drop-wise. After stirring at 65 °C for 1 h, the mixture was aged at ambient temperature for 24 h. Following this, obtained material was filtered and washed by an ethanol-HCl (0.1 M) mixed solution for several times before dried at 80 °C overnight. Finally,
Physicochemical property of synthesized materials
The structure and textual parameters of all prepared materials are shown in Table 1. It is shown that the actual Pd contents of catalysts are similar and close to 0.5 wt.%; however, the Pd content over Pd/S-2IS (0.31 wt.%) is obviously lower than the theoretical loading may be due to the loss of Pd during the IS process. Results from Table 1 also reveal that the dispersion of Pd over catalysts with SP-2 as the support (Pd/S-2IS, Pd/S-2WI, and Pd/S-2GA) is higher than those over SP-1 (Pd/S-1IS,
Conclusions
In summary, a series of Pd/SiO2 catalysts supported on two types of spherical-like SiO2 were prepared by various methods (IS, WI, and GA) and applied in n-butylamine combustion. Pd dispersion is a key factor affecting the catalytic activity. GA method is an efficient protocol obtaining highly dispersed Pd active sites. Pd/S-2GA prepared by the GA method exhibits the best catalytic activity than other materials due to the highest Pd dispersion. NOx yield during n-butylamine decomposition is
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (21677114, 21477095), the National Key Research and Development Program (2016YFC0204201), and the Fundamental Research Funds for the Central Universities (xjj2017170). The valuable comments from the editor and anonymous reviewers are much appreciated.
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These authors are contributed equally to this work.