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ChemComm Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/chemcomm Page 1 of 5 ChemComm Journal Name RSC Publishing COMMUNICATION Microemulsion Flame Pyrolysis for Hopcalite Nanoparticle Synthesis: A new Concept for Catalyst Cite this: DOI: 10.1039/x0xx00000x Preparation a a a ,a,b Received 00th January 2012, T. Biemelt , K. Wegner , J. Teichert and S. Kaskel * Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/ Manuscript A new route to highly active hopcalite catalysts via flame is characterised by the generation of combustible aerosols, spray pyrolysis of an inverse microemulsion precursor is containing volatile metal-organic precursors dissolved in a fuel. However, as a major drawback compared to often used metal reported. The nitrate derived nanoparticles are around 15 nm nitrates, metal-organic precursors usually exhibit high prices or can in diameter and show excellent conversion of CO at ambient only be prepared at lab-scale level. Hence, it is of great interest to conditions, outperforming commercial reference hopcalite combine the benefits of inexpensive metal nitrates and the FSP- materials produced by co-precipitation. process, since the relatively low number of synthesis steps is advantageous. At the same time applying a scalable 12 and Carbon monoxide is a gas with toxic effects on humans when industrially established process contamination with alkali metal ions 1 inhaled at concentrations as low as 100 ppm. Especially miners, is avoided. firefighters or soldiers can be confronted with lethal amounts. Considering these requirements for the production of hopcalite by Accepted Hence, it is of special importance to improve protection systems for FSP, we used the platform of thermodynamically stable and the decontamination of air. A common way to remove CO from spontaneously forming inverse microemulsions. They can be breathing air is used in respirator masks containing active filter understood as a tool box with variable composition but well defined media based on either physisorption, chemisorption, or catalytic microstructure. Moreover, they contain monodisperse water droplets removal. In the case of catalytic toxic gas conversion by oxidation smaller than 10 nm in diameter, which should lead to homogeneous with ambient O 2, many catalytically active components such as doping effects and nanosized particles, simultaneously ensuring 2 3 oxide supported noble metals or transition metal oxides are sufficiently high nitrate concentrations in a non-polar organic established. Although catalyst research for the oxidation of CO was solvent. Metastable emulsions, consisting of large micelles with revived with the discovery of Haruta et al., who found gold diameters in the micrometer range, were used for the generation of nanoparticles dispersed on α-Fe 2O3 to be very active even at single and mixed oxides in the emulsion combustion method, but 4 temperatures as low as -70 °C , gold catalysts are prone to poisoning resulted in micron-sized, usually hollow and broad size-distributed 13 and thus the more robust well-known copper manganese mixed spheres. 5 oxide “Hopcalite” is still the commercial catalyst of choice for this For the preparation of nanostructured stable microemulsions, reaction. manganese and copper nitrate (molar ratio 2:1) were dissolved in 6 ChemComm Hopcalite has been produced by sol-gel processes , supercritical deionised water and dispersed in n-heptane with the help of the non- 7 8 anti-solvent precipitation , redox methods , or most commonly co- ionic surfactant MARLOPHEN NP-5. The turquoise solutions were precipitation including several partly time consuming preparation fed to a nozzle, generating micelle containing n-heptane/O 2 aerosols, steps such as: dissolving the precursors, precipitation, ageing, which were instantaneously ignited by an annular surrounding 9 filtering, washing, drying and subsequent calcination . This supporting flame. A glass fibre filter was used to separate the black procedure causes contamination with sodium ions resulting from nanoparticles from the exhaust gas stream (more detailed Na 2CO 3-containing precipitation solutions, which is known to information on hopcalite synthesis is available in the experimental 10 reduce the catalytic activity to some extent. Furthermore it is part of ESI). crucial to adjust synthesis parameters such as aging time and Two inverse microemulsions differing in the total metal content calcination temperature thoroughly for receiving the optimal were prepared. Aiming at the formulation of precursors with 9,11 performing hopcalite catalyst. preferably small micelles, we used the work of Henle et al. as a A homogeneous distribution of oxide forming ions on the atomic starting point and applied formulations with Rw = 2 -1 14 level is also achieved by the flame spray pyrolysis (FSP) process. It (Rw = n(H 2O) · n(surfactant) ). These resulted in precursor This journal is © The Royal Society of Chemistry 2012 J. Name ., 2012, 00 , 1-3 | 1 ChemComm Page 2 of 5 COMMUNICATION Journal Name solutions with very small micelle sizes in the range of 5 nm for larger spheres, visible especially in Fig 1b). Nevertheless, besides 0.05 M as well as 0.10 M overall metal concentrations. Micelle small either amorphous or crystalline nanoparticles with diameters and properties of the FSP derived particles are approximately 10 nm in diameter, larger spherical particles ca. summarised in Table 1. Injecting these precursor solutions into the 50 nm in diameter were observed by TEM investigations (Fig. 1c FSP nozzle resulted in two voluminous powders (powder density and d). For the amorphous nanoparticles chain-like structures were -3 ρdump = 0.15 g cm ) denoted by ME-1 and ME-2. prominent. Analysing the two differently sized types of particles in ME-1 using electron dispersive X-ray spectroscopy (EDX) revealed Table 1: Properties of microemulsion derived FSP-hopcalites with differences in Mn:Cu ratios of 1.52 for areas with small particles and -1 respect to the initially applied overall metal concentrations. an elevated ratio of 9.34 mol mol for the large spheres. Thus, some segregation in physical binary oxide mixtures occurred during the a b c d cmetal dmicelle SBET dBET Mn/Cu pyrolysis process. This can be explained by an inhomogeneous 2 -1 -1 ME- (M) (nm)(PDI) (m g ) (nm) (mol mol ) temperature profile of the flame including sharp temperature 1 0.10 4.7 (0.067) 97 ddd 11.3 d 1.96 ± 0.01 gradients causing different particle formation mechanisms in dependence on the trod flame-path during pyrolysis. Moreover, 2 0.05 4.7 (0.034) 114 ddd 9.6 d 1.93 ± 0.02 Mn(NO 3)2 and Cu(NO 3)2 show divers behaviour during heating. a determined by dynamic light scattering While Mn(NO ) decomposes at ~140 °C, Cu(NO ) is more stable b 15 3 2 3 2 calculated from multipoint BET-method for 0.05 < p/p 0 < 0.2 c -1 against decomposition as reasoned from a melting temperature of dBET = 6 · (S BET · ρhopcalite ) assuming mono-disperse spherical primary particles with - 16 ρhopcalite = 5.485 g cm ³ 256 °C. These differences may result in a delayed decomposition d detected by ICP-OES especially for precursor droplets, not passing the hottest spot of the flame exhibiting temperatures higher than 2000 °C 18 . Fig. 1a) shows sample ME-1, consisting of two types of particles. Nitrogen physisorption experiments at 77 K revealed IUPAC Type II The main fraction is characterised by very small partly agglomerated isotherms indicating non-porous nanoparticulate catalysts (Fig. S2). chain-like nanoparticles ca. 10 - 15 nm in diameter. Additionally, Applying the multipoint BET-method, surface areas of 97 and larger spheres with diameters ranging from 50-300 nm were 114 m2 g-1 were extracted for ME-1 and ME-2, respectively. The observed. These two types of particles result from different slightly dissimilar specific surface areas result from varying metal formation mechanisms within the spray flame. On the one hand, ion concentrations within the water phase of the two inverse precursor evaporation and subsequent vapour combustion occurs microemulsions. Both precursors consisted of identical micelle sizes Manuscript forming small mono-disperse nanoparticles. On the other hand, as confirmed by dynamic light scattering measurements (DLS). precipitation of the precursor at the aerosol droplet surface and Dynamic viscosities and refractive indices, resulting from different subsequent calcination resulting in poly-disperse, larger and metal salt concentrations (Table S1), were taken into account for sometimes hollow spheres is possible.17 For catalytic applications these experiments. Identical micelle diameters imply differences in the first route of the two mentioned formation pathways is preferred, ion densities within the micelles, inducing a higher degree of due to the fact that the catalytically available surface area for those agglomeration for nanoparticles derived from ME-1 compared to particles is higher compared to the precipitation/calcination pathway ME-2 and favouring the precipitation/calcination particle generation derived spheres. pathway caused by reduced energy supply for fast decomposition of the nitrate precursors, as well.