Application of Aerosol Techniques to Study the Catalytic Formation of Methane on Gasborne Nickel Nanoparticles Alfred P. Weber,* Martin Seipenbusch, and Gerhard Kasper Institut fu ¨ r Mechanische Verfahrenstechnik und Mechanik, UniVersita ¨ t Karlsruhe (TH), D-76128 Karlsruhe, Germany ReceiVed: April 25, 2001; In Final Form: July 13, 2001 “Aerosol catalysis” is shown to be a powerful tool for investigating the catalytic properties of freshly formed nanoparticles in situ and without substrate interference. The method is first outlined conceptually, followed by an illustrative application to the catalytic formation of methane on a nickel nanoaerosol. Reaction order and activation energy were found conform with generally accepted values from supported Ni catalysts. The TOR decreases strongly during the first 10 s as the reaction proceeeds toward a steady value. The decrease correlates with a buildup of about 0.3 monolayer equivalents of carbon on the particle surface measured by TGA and a decline in particle photoelectric activity observed via measurement by aerosol photoemission spectroscopy (APES). APES is shown to be capable of detecting the progressive degradation of the freshly formed particle surface due to a heterogeneous surface reaction on a millisecond time scale. Furthermore, it was possible to induce order-of-magnitude changes in TOR via defined changes in particle morphology, induced by aerosol restructuring techniques preceding exposure to the catalytic reaction. 1. Introduction In contemporary parlance, the “active phase” of a catalyst consists of nanoparticles. To preserve their high degree of dispersion and also to provide a suitable form of packaging and exposure in a chemical reactor, these nanoparticles are usually supported on much larger granules of an “inert” material such as Al 2 O 3 or MgO. Experiments under real reaction conditions are usually done with such materials. However, supported catalysts are not ideal for investigating the fundamental proper- ties of nanoparticles or materials because the various multistep methods for their preparation usually result in a loss of control over the size distribution and morphology of the nanophase and it also becomes difficult to separate the influence of the support material and other extraneous factors. Research has thus resorted to a variety of sophisticated tools for probing and modifying pure catalyst surfaces, many of which require ultrahigh vacuum to understand how molecules interact with catalysts. Rapid advances during the past decade for preparing and handling “clusters” or “nanoparticles” have also opened up new avenues to study catalysts. Today, there we have an almost unlimited array of possibilities for making very well-defined particles from a few nanometers in diameter upward, especially by liquid-phase processes. This has also spurred fundamental investigations of unsupported colloidal catalysts. Schmid et al., 1 for example, demonstrated how variations in size of ligand- stabilized metal clusters correspond to systematic changes in catalytic properties. The aerosol routesoften called gas-phase synthesis or chemi- cal vapor synthesis in order to stress analogies with surface coating by CVDsprovides another conceptually interesting approach to study unsupported as well as supported catalysts. For one, various types of techniques are available for on-demand production of well-defined nanoparticles in flow reactors, with high purity, and over a wide range of pressures. Aerosol conferences, workshops, and the pertinent archival literature give ample testimony to that effect. Second, one can proceed without removing these freshly generated particles from the gas stream to initiate a specific chemical reaction by mixing them in a second flow reactor with the required educt molecules and then continue to study its progress downstream. Since the reactor axis is in effect the reaction coordinate, one has a high degree of control over the size, morphology, and surface properties of the particulate phase. This represents some compensation for the fact that aerosols cannot be stabilized or manipulated as elegantly as colloids by means of electrical bilayers. More importantly, however, aerosol science has at its disposal a range of on-line tools for characterizing the particles, which offer unique ways to correlate particle properties with chemical kinetics and under actual process conditions, i.e., also at atmospheric or higher pressures. “Aerosol catalysis” may therefore eventually contribute to closing the so-called pressure gap between UHV methods of investigation and the real process and become another tool for catalyst development. Despite its potential, the aerosol route with all its possibilities has so far not been used broadly, and pertinent literature is scarce. Glikin 2,3 has made extraordinary claims with regard to the effects of the aerosol state on the catalytic activity for the total oxidation of acetic acid over an iron oxide catalyst. However, it is doubtful whether the aerosols they used were sufficiently characterized and quantitatively understood. Nev- ertheless, these claims have contributed to sparking our own interest. 4 The aim of this article is to demonstrate the possibilities of the aerosol technique by investigating a well know reaction, namely, the so-called methanation reaction over a Ni catalyst, i.e., the formation of methane from carbon monoxide and hydrogen. * Corresponding author. E-mail: alfred.weber@ciw.uni-karlsruhe.de. FAX: 0049 721 608 6563. 8958 J. Phys. Chem. A 2001, 105, 8958-8963 10.1021/jp0115594 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/06/2001