Co/MCM41 catalyst in the COProx reaction prepared by supercritical CO2 reactive deposition

https://doi.org/10.1016/j.micromeso.2016.10.006Get rights and content

Highlights

  • The SCFRD method contributes to formation of cobalt oxide and Co(II) occupying Si tetrahedral sites.

  • By ‘tie’ method the outside of the MCM41 particles represents the area easily exchanged.

  • Supercritical CO2 method leads a catalyst with values of CO conversion similar to those obtained by others.

Abstract

Co/MCM41 catalyst with 4.3 wt % Co (Co/sc) has been prepared by supercritical CO2 reactive deposition (‘scfrd’) and characterized by different physicochemical techniques. This synthesized method was compared with others conventional methodologies such as template-ion exchanged (Co/tie) and incipient wet impregnation (Co/iwi) with similar cobalt content. All the samples were studied as catalysts on the CO total oxidation (COTox) and preferential oxidation of CO on H2-rich streams (COProx). Incorporating cobalt with supercritical CO2 leads to a catalyst which produces values of CO conversion similar to those obtained by conventional methods such as incipient wetness impregnation (Co/iwi) or template-ion exchanged (Co/tie). It has been possible to identify different cobalt species present in catalysts depending on their synthesis methods by Temperature-Programmed Reduction (TPR), X-ray Photoelectronic (XPS), Laser Raman (LRS) and X-ray Absorption (XANES/EXAFS) spectroscopic studies. All samples containing a main cobalt species of cobalt (II) coordinated with Si tetrahedral sites form part of mesoporous structure and lesser extent, cobalt orthosilicate on the surface. In addition, Co3O4 species dispersed over the MCM41 support were detected for the Co/iwi and Co/sc catalysts.

Thus, the combination of Co3O4 nanoparticles and Co(II) sites interacting with the siliceous structure, highly dispersed on the surface and inside the mesoporous support obtained by the ‘scfrd’ method resulted in a more active and selective catalyst for the COProx reaction.

Introduction

TheMCM-41 material is one of the most representative members of the mesoporous structures, characterized by a one-dimensional structure of uniform cylindrical mesopores of about 2–10 nm organized in a hexagonal symmetry [1], [2]. The highly ordered pore systems with tunable pore sizes [3], large surface areas and pore volumes, as well as a high density of surface silanols [4] provide excellent opportunities in chemistry, catalyst and separation processes [5], [6].

Many efforts have been devoted to expand the application of mesoporous silica materials as catalysts with tunable properties. In this sense, the introduction of various metal cations provides great potential. Likewise, it has been found that, depending on the preparation method used, the metal ion can be incorporated in the framework of the mesoporous silica matrix or can occupy extra-frame positions [7], [8]. Initially, the incorporation of aluminum into the MCM41 structure was performed to create acidity in the material [9], [10]. In addition, the MCM41 material with V, Fe, Mn, Cs and others metals has been successfully synthesized [11], [12], [13], [14]. Similarly, the incorporation of heteroatoms such as Cu, Zn, Al, B, Ga, Fe, Cr, Ti, V, Co and Sn into the mesoporous silica framework has been extensively investigated [15], [16]. The incorporation of transition metals as cobalt or copper into mesoporous materials is of considerable interest because Co and Cu catalysts are widely used in many processes, particularly those of environmental importance, and they constitute a cheap alternative to the use of noble metals as active centers of catalysts for the oxidation and reduction reactions. In this sense, the preferential oxidation of CO (COProx) has recently raised considerable interest as an alternative route for the removal of CO from hydrogen-rich streams used to PEM fuel cells (Proton Exchange Membrane Fuel Cells) [17]. Before entering the cell, the CO concentration must be less than 10 ppm in order not to deteriorate the cell anode. Therefore, numerous articles on transition metal oxides such as Cusingle bondCe and Cosingle bondCe systems have been published, in which oxides play a fundamental role in redox reactions [18], [19].

On the other hand, some catalysts have been reported which are based on noble metals highly dispersed in mesoporous supports. Wang et al. [20] reported Ru catalysts supported on different mesoporous silica, MCM-41, MCM-48, SBA-15 and KIT-6. The performance of Ru catalysts in the COProx reaction was related to the Ru dispersion and reducibility, and support pore structure. Following this line of research, Huang et al. [21] reported high activity and selectivity of Pt supported on mesoporous substrate (FSM-16) for the PROX reaction.

The behavior of systems CuOsingle bondCeO2 on SBA-15 active and selective in the oxidation of CO has recently been published [22]. This catalyst showed high CO conversion with 5 wt % of copper.

The deposition of catalytically active nanoparticles on supports with high dispersion is an important and effective strategy for the design of catalysts. Conventional methods such as impregnation techniques often generate agglomerations or large particles with broad size distribution in the mesopores and/or on the external surface of the porous catalyst. The incipient wetness impregnation (‘iwi’) method allows a good control on metal loading but a poor control on metallic dispersion [23], [24]. Thus, it is important to study and analyze different ways to introduce the active phase in the mesoporous support. The MCM41 materials are synthesized in alkaline media by the so-called (S+I) synthesis route [25], where S+ is the cationic surfactant and I is the anionic inorganic silica precursor [26], [27]. The template ion exchange (‘tie’) method is based on the concept that the cationic surfactants in the as-synthesized mesoporous materials can be partially replaced through ion exchange by other inorganic cations. Metal ions (Mδ+) such as Ag+, Mn2+, VO2+, Fe3+ and Cr 3+ have been introduced into MCM41 by the ‘tie’ method [28], [29], [30], [31], [32]. These catalysts were employed for oxidation reactions such as the oxidation of CO [28], methane [31] and short-chain alkanes (ethylene and propylene) [32]. Since there are no cation-exchange sites in the purely siliceous MCM41 after calcination, it is difficult for the metal components to be exchanged as in the exchanged zeolites sites [33], [34].

Furthermore, an interesting method for incorporating active phases on catalyst supports is the reactive deposition using supercritical fluids (‘scfrd’). It is a promising method to deposit nanoparticles and films on inorganic porous supports, polymer substrates and carbon nanotubes [35], [36]. This process involves the dissolution of an organometallic precursor in a supercritical fluid (SCF), the impregnation of the substrate by exposure to this solution, and the subsequent decomposition of the precursor. CO2 is the most commonly used supercritical fluid (scCO2) for material synthesis because it is non-toxic, non-reactive, nonflammable and inexpensive. Under supercritical conditions, CO2 as a solvent has intermediate properties between gases and liquids. The gas-like diffusivity and viscosity of scCO2 are favorable for rapid diffusion and permeation into mesoporous substrates, whereas the liquid-like density allows the dissolution of a wide range of organometallic precursors. The zerosurface tension of scCO2 allows a better penetration and wetting of pores than liquid solvents and avoids the pore collapse which can occur on certain structures. The simple removal from the substrateby controlled decompression is performed without leaving any residue on the support [37], [38], [39].

The goal of the present study is to investigate the cobalt species which are incorporated in the MCM41 catalytic support by different synthesis methods and compare its catalytic properties in the CO Preferential Oxidation reaction. The methods used to incorporate the active phase was (i) template-ionic exchange (‘tie’), (ii) incipient wet impregnation (‘iwi’) and (iii) supercritical CO2 reactive deposition (‘scfrd’) in the total and preferential CO oxidation in reductive atmosphere to purify H2 streams. Spectroscopic methods such as X-ray absorption (EXAFS/XANES), X-ray photoelectron (XPS) and Laser Raman (LRS) together with temperature-programmed reduction (TPR) techniques are used to characterize the nature and local environment of the cobalt sites in these types of materials.

Section snippets

Synthesis of MCM41

The MCM41 support was synthesized following the method used by Szegedi et al. [40]. A solution of surfactant (C16TMABr) was prepared by continuous mixing with de-ionized water and absolute ethanol at room temperature. The pH of the solution was adjusted by adding an aqueous ammonia solution (29 wt %). Then, tetraethylorthosilicate (TEOS) was added dropwise in a couple of minutes. The molar composition of the resulting gel mixture was TEOS:0.3C16TMABr:11NH3:144H2O:58EtOH.

The support thus

Physical and chemical properties of mesoporous materials

Nitrogen physisorption is a routine technique to probe the texture of porous solids. All the synthesized catalysts presented a type IV isotherm (see Fig. S1, supplementary Material) according to the IUPAC nomenclature [48], which is typical for MCM41 mesoporous materials [49], [50].

For all samples, a sharp step in the range of relative pressures between 0.18 and 0.30 is observed, which indicates the filling of rather uniform mesopores by capillary condensation. The position of the pore-filling

Conclusions

Co-MCM-41 with 4.3 wt % of cobalt synthesized using supercritical CO2 was the most active and selective catalyst for the preferential oxidation of CO, with total conversion at 250 °C. The Co deposition by the ‘scfrd’ method contributes with the formation of cobalt oxide nanoparticles (∼20 nm) uniformly distributed on the surface and inside of the support. In addition Co(II) incorporated into the MCM41 framework, occupying Si tetrahedral sites was identified. On the other hand, simpler methods

Acknowledgments

This work was supported by LNLS, Brazil (Project XAFS-18859), UNL (CAI+D 2011), ANPCyT (2013-0354) and CONICET (PIP 112-200801-03079/112-201101-01035). The authors thank ANCPyT for the purchase of the SPECS multitechnique analysis instrument (PME8-2003) and the UV–vis spectrometer (PME 311). Thanks are also given to Fernanda Mori andNuriaNavascuesfor the XPS and TEM measurements, respectively.

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