New synthesis of nanosized Cu–Mn spinels as efficient oxidation catalysts

Abstract

Copper manganese nanoscale oxides were prepared using a novel method with low cost natural biopolymer precursor, alginate, and their catalytic activity was evaluated for the complete oxidation of toluene as a model VOC. Depending on the Cu/Mn ratio the following oxides were formed: Mn₃O₄, Cu_1.5Mn_1.5O₄, CuO or a mixture of these phases, as detected by XRD. The nanocrystals aggregate in small spheres (≲1 mm), which retain the shape of the parent gel and present homogeneous composition as characterized by TEM and SEM-EDX. The best performance for toluene combustion was observed for the cubic spinel Cu_1.5Mn_1.5O₄ nanoparticles (∼10 nm), which was able to completely oxidize toluene at ∼240 °C. For the pure oxides the activity order: Cu_1.5Mn_1.5O₄ > Mn₃O₄ > CuO, correlates with the TPR results as the catalyst with the higher redox potential shows higher activity. For comparison the TiO₂ supported oxides were also studied and alginate issued Cu_1.5Mn_1.5O₄ mixed with TiO₂ presented similar activity as the Cu–Mn oxide deposited on TiO₂ by incipient wetness impregnation however the overall activity of supported oxides was lower than for the unsupported Cu_1.5Mn_1.5O₄. The method used for the synthesis of copper manganite has great potential as it can be extended to other manganite spinels of general formula AMn₂O₄ and presents the advantage of easy catalyst forming.

Introduction

Spinel oxides AB₂O₄ have attracted since long time scientific and technological interest resulting in many important applications ranging from electronics, optics, magnetism, catalysis to energy storage and conversion. Manganite spinels AMn₂O₄ represent such multifunctional materials in which adjusting the quantity and the chemical identity of A²⁺ permits to reach a wide range of properties. The copper manganite Cu_xMn_3−xO₄ being an example of such a system presents a distinguished feature of having a flexible valence in Cu^1+/2+ and Mn^3+/4+ giving rise to its particular properties.

The Cu–Mn–O system is well known and important industrially as a versatile and effective oxidation catalyst since 1920 [1], [2]. Historically, mixtures of manganese and cupric oxides have been studied as CO oxidation catalysts towards the end of World War I and were given the general name of “Hopcalites”, referring to Johns Hopkins University and the University of California, the alma maters of its inventors [1], [3], [4], [5]. Since then, mixed copper manganese oxides have been studied extensively as oxidation catalysts for room temperature CO oxidation [1], [6], [7], [8], [9], [10] and at elevated temperatures (200–500 °C) for combustion of a wide range of volatile organic compounds (VOCs) including hydrocarbons [11], [12], [13], [14], hydroxy, halide [15], [16] and nitrogen containing compounds [17]. Musick and Williams have classified 35 substances susceptible to hopcalite-catalysed decomposition [18]. Because of its catalytic abilities, Hopcalite is still a catalyst of choice in respiratory protection for many applications including military, scuba diving, fire fighting, mining and space. Cu–Mn oxide catalysts have been shown also efficient in reactions such as water–gas shift [19], [20], [21], NO reduction with CO [22], direct NO decomposition [23], selective oxidation of ammonia to N₂ [24] and more recently in the catalytic steam reforming of methanol [25], [26], [27], [28], oxidative methanol reforming [29], preferential CO oxidation [30], [31] as well as in selective oxidation of toluene to benzoic acid [32] and benzyl alcohol to benzaldehyde [33]. In reduced state the Cu–Mn systems were used for methanol [34] and DME [35] synthesis.

Copper-doped manganese spinels have also attracted significant attention for their potential applications in electrochemical systems [36], [37], [38], electrocatalytic oxygen reduction [39] and as absorbers in solar thermal collectors [40] or as adsorbents for air analysis [41]. The importance of the Cu–Mn–O system has led to extensive studies of its phase equilibria [42], [43], [44], [45] and structural details of different phases as well as surface studies as model system for the design of oxidation catalysts [46].

Conventionally the Cu–Mn oxides are prepared by either coprecipitation [9], [14], high temperature ceramic method, or wet impregnation followed by thermal decomposition for supported oxides. In recent years, there has been a considerable research focusing on alternative preparation methods including: sol–gel [30], redox-precipitation [8], [37], soft reactive grinding [27], synthesis under supercritical water conditions [47], reverse microemulsion [13], and the combustion method for formation of Cu–Mn oxide layers on the surface of Al metal foam [28]. The main focus of new preparation methods was to enhance textural properties. However it is also important to take into account the environmental impact of the material precursor [27], [48].

Recently we reported a novel method for synthesis of single oxides using polysaccharide (alginate) precursor as biopolymer auxiliary providing nanoscale oxide dispersion [49]. Biotemplating is an effective strategy to obtain morphology-controllable materials with structural specificity, complexity, and related unique functions [50], [51]. The potential of natural polysaccharides as precursors for materials applied in catalysis, adsorption and remediation has been recently reviewed [52]. The attractiveness of the applications of polysaccharides as materials stems both from their availability from renewable resources and their intrinsic properties. Most polysaccharides are obtained from biomass wastes or from purposely grown biomass not in competition with food resources. Alginate, a natural block polysaccharide with carboxylic functions, is extracted from the cell walls of several brown algae. It is well known to have a high affinity to divalent cations and has been studied for adsorption of heavy metal ions from waste water streams from mineral operating industries such as mining, electronic and metal plating [53].

In this paper we demonstrate for the first time experimental method for the preparation of nanocrystalline manganite spinel Cu_xMn_3−xO₄ from ionotropic alginate gel. The materials obtained were tested for complete toluene oxidation. Toluene was chosen as a model VOC, as it can be considered a representative of BTEX, a major group of pollutants in indoor air [54] and has been classified as one of eight representative indoor VOCs by ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) in a test method for determining the efficiency and capacity of gas-phase air cleaning systems for indoor air applications. Moreover total oxidation of toluene [12], [13], [14] has been widely studied using Cu–Mn oxides obtained by different methods and this literature background will serve us as a reference for comparison of the preparation method proposed in the present work.