The novel catalysts of truncated polyhedron Pt nanoparticles supported on three-dimensionally ordered macroporous oxides (Mn, Fe, Co, Ni, Cu) with nanoporous walls for soot combustion

https://doi.org/10.1016/j.apcatb.2013.03.019Get rights and content

Highlights

  • 3DOM TMO-supported truncated polyhedron Pt nanoparticle catalysts were firstly synthesized.

  • The mesopores and Pt nanoparticles are highly dispersed on the inner walls of uniform macropores.

  • 3DOM Pt/TMO catalysts with nanoporous walls exhibited high catalytic activity for soot combustion.

  • The nanopores on the walls can remarkably improve the catalytic selectivity to CO2 formation.

  • The role of strong Pt–TMO interaction in catalytic soot combustion reaction was systematically investigated.

Abstract

Three-dimensionally ordered macroporous (3DOM) transition metal oxides (TMO) (Mn, Fe, Co, Ni, Cu) with nanoporous walls were synthesized by the surfactant (P123)-assisted colloidal crystal template (CCT) method, and 3DOM TMO-supported truncated polyhedron Pt nanoparticle catalysts were prepared by the gas bubbling-assisted membrane reduction (GBMR) method. All the catalysts possessed well-defined 3DOM nanostructure with small interconnected pore windows (∼80 nm). The nanopores with the size of ∼6 nm and truncated polyhedron Pt nanoparticles with the sizes of 2.9–3.5 nm are highly dispersed on the inner walls of uniform macropores. 3DOM Pt/TMO catalysts with nanoporous walls exhibit the large total pore volume (∼3.4 ml g−1), the high porosity (>91%) and surface area (36–40 m2 g−1). 3DOM structure improves the contact efficiency between catalyst and soot, and the strong metal (Pt)–support (TMO) interaction is favorable for the improvement of reducibility and for increasing the amount of active oxygen species. Among all of 3DOM Pt/TMO catalysts with nanoporous walls, Fe- and Co-based catalysts with the moderate reducibility exhibited higher catalytic activity for diesel soot combustion in contrast to the others. For instance, the T50 of 3DOM Pt/Fe2O3 and Pt/Co3O4 catalysts are 358 and 351 °C, respectively. And the nanopores on the inner walls of 3DOM Pt/TMO catalysts remarkably improve the SCO2 for soot oxidation, and it is nearly 100% in a wide temperature range (200–450 °C). The excellent performance of the catalysts for soot combustion might be due to the factors including unique 3DOM structure with nanoporous walls, highly active component (Pt), large surface area, and the moderate reducibility of the materials. The materials consist of 3DOM supports with nanoporous walls and metal nanoparticle active sites may be the potential practical applications in the catalytic oxidation of diesel soot particles.

Introduction

Soot particles emitted from diesel engines as a typical particulate matter (PM) can cause acute human health and environmental problems [1]. This calls for efficient treatment systems for the exhaust gas from diesel engines [2]. The combination of traps and oxidation catalysts in the continuously regenerating particulate trap (CRT) appears to be one efficient after-treatment technique for diesel engines [3], [4]. The key challenge is to find effective catalysts for soot combustion that operates at low temperatures. Noble metals [5], [6], transition metal oxides [7], [8], alkaline metal oxides [9], [10], perovskite-like type oxides [11], [12] and rare earth oxides [13], [14] have exhibited good catalytic performances for diesel soot combustion. In particular, transition metal oxides (Mn, Fe, Co, Ni, Cu) are one of the key components in auto-exhaust treatment catalysts [7], [8], [13]. To design and synthesize high-performance materials in terms of catalytic activity, understanding the properties affecting catalytic performance is of great importance. The catalytic performance for soot combustion is affected by two factors: the contact between soot particles and the catalyst, and the intrinsic activity of the catalyst [4], [10], [15]. For conventional catalysts with smaller pore sizes (<10 nm) or no pore, it is difficult for soot particles (>25 nm) to enter the inner pores of these catalysts with high surface area, and only those in external surfaces of catalysts are available for soot combustion. Thus, the catalytic activity for soot combustion may be restricted by the poor soot/catalyst contact and/or limited active sites. One way to increase the contact between solid reactant and catalyst is through fabricating macroporous nanostructure materials [16].

Three-dimensionally ordered macroporous (3DOM) metal oxides with uniform pore size (>50 nm) and well-defined structure have drawn much attention in the field of heterogeneous catalysis [17]. The macroporous structures could not only permit soot particles to enter their inner pores, but also could allow the soot particles easily transfer through the structure and less diffusion resistance to access the active sites. Thus, the contact between soot and catalyst will be improved remarkably, and 3DOM metal oxides exhibited better catalytic performance for diesel soot oxidation in comparison with disordered macroporous and nanoparticles samples [18], [19]. However, their catalytic performances are also limited by their intrinsic activity, i.e., the catalytic combustion of diesel soot particles occur at higher temperatures (ca. 350–500 °C) than the exhaust gas temperatures (ca. 150–400 °C) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In the mechanism of catalytic combustion for diesel soot particles, the reaction pathways for catalyzing soot combustion can be divided into two parts: one is that active oxygen species over the surface of the catalysts directly oxidize soot particles; the other one is that NOx acts as an intermediate to facilitate the indirect catalytic soot oxidation [20]. Therefore, it is also important to improve the intrinsic activity of 3DOM oxide-based catalysts, i.e., enhanced the activation capability of the catalysts for gas reactant (O2 and NO).

Noble metal nanoparticles are possible choices for enhancing the intrinsic catalytic activity of 3DOM metal oxides [21], [22]. The origins of the noble metal-supports synergistic effect have been proposed in the catalysis literature [23], [24], [25]. In our previous works, 3DOM oxides-supported gold (Au) catalysts exhibited high catalytic activities for soot combustion, but their thermal stabilities are poor [26], [27]. Oxides-supported platinum (Pt) catalysts are still the best catalysts in the literature and also the only commercial catalysts for soot combustion [5], [6], [28]. The catalytic performance of Pt nanocrystals can be finely tuned by their shape and the strong metal-support interactions (SMSI), which determines surface atomic arrangement and coordination [29]. Nevertheless, all of the mentioned 3DOM transition metal oxides possessed low surface areas due to no mesopores or nanovoids on the walls, leading to the lower adsorption and activation capability for O2 and NO. In recent years, 3DOM oxides with mesoporous or nanoporous walls have been synthesized and exhibited super catalytic performance for the volatile compounds oxidation and formaldehyde [16], [30], [31]. However, the preparation and catalytic application of 3DOM metal oxides with mesoporous or nanoporous walls for catalytic soot oxidation have not been reported in the literature. Moreover, the catalysts of truncated polyhedron Pt nanoparticles bounded by high-index facets over 3DOM transition metal oxide with nanoporous walls are potential route to enhance their catalytic activities for soot oxidation.

In the present work, a series of 3DOM transition metal oxides (TMO: Mn, Fe, Co, Ni, Cu) with nanoporous walls were synthesized by the colloidal crystal template (CCT) method. During the preparation process, a surfactant (P123) was introduced into the metal precursor solution for the generation of mesoporous or nanoporous walls, which is beneficial for the improvement of physicochemical property. And 3DOM TMO-supported truncated polyhedron Pt catalysts were designed and synthesized by the gas bubbling-assisted membrane reduction (GBMR) method using poly (N-vinyl-2-pyrrolidone) (PVP) as a stabilizer [20]. The novel catalysts of truncated polyhedron Pt nanoparticles supported on 3DOM TMO with nanoporous walls exhibited high catalytic activity for soot combustion. The composition and structure effects of 3DOM TMO with nanoporous walls on the catalytic activity of 3DOM Ptn/TMO catalysts for soot combustion were systematically investigated.

Section snippets

Catalyst preparation

Schematic representation of the preparation processing of 3DOM TMO with nanoporous walls supported truncated polyhedron Pt nanoparticles catalysts is shown in Fig. 1, and the synthetic method involving two processes is presented in detail as follows.

The results of XRD characterization

To confirm the formation and phase structures of 3DOM TMO and Pt/TMO catalysts (M: Mn, Fe, Co, Ni, Cu), XRD measurements were carried out, and the results are shown in Fig. 2. By referring to the XRD pattern of standard TMO samples, all the diffraction peaks of 3DOM TMO and Pt/MTO catalysts correspond to a cubic structure of Mn2O3 (JCPDS PDF# 41-1442), a rhombohedral structure of Fe2O3 (JCPDS PDF# 33-0664), a cubic structure of Co3O4 (JCPDS PDF# 78-1970), a cubic structure of NiO (JCPDS PDF#

Discussion

The catalytic combustion of diesel soot particle is a typical heterogeneous catalytic reaction, which takes place at the three-phase boundary among a solid catalyst, a solid reactant (soot) and gaseous reactants (O2, NO). The efficiency of catalyst for solid–solid reaction is strongly influenced by the contact between soot and catalyst in the process of catalysis. In many studies, the catalytic performances for soot combustion are very excellent under tight contact conditions between soot and

Conclusions

A series of 3DOM TMO supports (M: Mn, Fe, Co, Ni, Cu) with nanoporous walls were successfully prepared by the surfactant (P123)-assisted colloidal crystal template method, and 3DOM TMO-supported truncated polyhedron Pt nanoparticle catalysts were synthesized by GBMR method. 3DOM TMO supports with nanoporous walls are highly ordered and the voids are interconnected through the open window with the diameter of 80 ± 5 nm. The nanopores with the average size of ∼6 nm and truncated polyhedron Pt

Acknowledgement

This work was supported by the National Natural Science Foundation of China (Nos. 21177160, 21173270 and 20833011).

References (52)

  • J. Oi-Uchisawa et al.

    Applied Catalysis B

    (2003)
  • A. Setiabudi et al.

    Applied Catalysis B

    (2003)
  • W.F. Shangguan et al.

    Applied Catalysis B

    (1998)
  • J. Liu et al.

    Applied Catalysis B

    (2008)
  • A. Setiabudi et al.

    Applied Catalysis B

    (2002)
  • F.E. López-Suárez et al.

    Applied Catalysis B

    (2008)
  • D. Reichert et al.

    Applied Catalysis B

    (2008)
  • D. Fino et al.

    Catalysis Today

    (2007)
  • M. Sun et al.

    Catalysis Today

    (2011)
  • J. Liu et al.

    Applied Catalysis B

    (2008)
  • J.L. Hueso et al.

    Journal of Catalysis

    (2008)
  • I. Atribak et al.

    Applied Catalysis B

    (2010)
  • Z. Zhang et al.

    Journal of Catalysis

    (2010)
  • G. Mul et al.

    Journal of Catalysis

    (1998)
  • Y. Liu et al.

    Journal of Catalysis

    (2012)
  • G. Zhang et al.

    Applied Catalysis B

    (2011)
  • J. Xu et al.

    Journal of Catalysis

    (2011)
  • H. Laversin et al.

    Journal of Catalysis

    (2006)
  • X. Zi et al.

    Catalysis Today

    (2011)
  • Y. Wei et al.

    Journal of Catalysis

    (2012)
  • K. Hinot et al.

    Applied Catalysis B

    (2007)
  • M. Sadakane et al.

    Journal of Solid State Chemistry

    (2010)
  • B. Liu et al.

    Applied Catalysis B

    (2012)
  • S. Li et al.

    Materials Letters

    (2007)
  • Z. Zhao et al.

    Catalysis Today

    (2004)
  • R. Spinicci et al.

    Catalysis Today

    (1990)
  • Cited by (0)

    View full text