Catalytic Chemistry of the Nano-confined Systems

In the present presentation, the unique characters of the catalysis with the nano-confined system will be demonstrated, and the emphasis will be laid on the electron properties derived from the quantum well states in the nano-systems and the synergetic confinement effects between the encapsulated metallic nanoparticle and carbon nanotubes. The effect of electron quantum confinement on the catalytic activities of 2D ultra-thin metal films is explored by comparing the work function change and the initial reaction rate of atomically flat films of different thickness on silicon surfaces, using complementary microscopy and spectroscopy techniques. The obvious oscillations of the oxidation rate of lead films are observed, which are attributed to be a manifestation of the Fabry-Pe´rot interference modes of electron de Broglie waves (quantum well states) in the films. The modulation of the electron density of states near the Fermi level opens a new demission for tuning the catalytic performance of metal systems via size- and thickness-dependent quantum size effects, which will be illustrated through two examples. Coordinatively unsaturated iron species, especially those in low valent states, are pivotal for the activation of oxygen-containing reactants. As a result, highly active oxygen species are generated which are essential for the selective oxidation reactions. In the present, we will demonstrate a new concept to construct a unique high active Fe-oxo species on noble metal surfaces through a structural confinement derived by the strange interaction. The Fe atoms at the edges of the structures show the nature of the coordinatively unsaturated low-valent state, i.e. FeO1-x, at which the adherence of molecular oxygen is remarkably enhanced, and the dissociation of the adsorbed dioxygen presents a barrierless character. The preferential formation of desiccative oxygen species at the boundary of Fe-oxo inland modifies essentially the adsorption dynamics between the carbon monoxide and oxygen, which causes a dramatically change of the catalytic performance towards oxidation of carbon monoxide on the surface. An integrated test of the real catalyst with 1 kw PEM fuel cell for 1000 hours shows that under the operating condition, with 20% water steam, 30ppm of CO in the reforming hydrogen can be completed removed. Carbon nanotubes (CNTs) have well defined hollow interiors and exhibit unusual mechanical and thermal stability as well as electron conductivity. This opens intriguing possibilities to introduce other matter into the cavities, which may lead to nanocomposite materials with interesting properties or behaviour different from the bulk. In the present talk, two unique properties concerning the redox and catalysis of the CNT-encapsulated metals and metal oxides will be addressed. The autoreduction of the encapsulated Fe2O3 is significantly facilitated inside CNTs with respect to the outside nanoparticles, and it becomes more facile with decreasing CNT channel diameter as evidenced by temperature programmed reaction, in situ XRD and Raman spectroscopy. The oxidation of encapsulated metallic Fe nanoparticles on the other hand is retarded in comparison to that of the outside Fe particles as shown by in situ XRD and gravimetrical measurements with an online microbalance. A striking enhancement of the catalytic activity of Rh particles confined inside nanotubes for the conversion of CO and H2 to ethanol has been found. The overall formation rate of ethanol inside nanotubes exceeds that on the outside of the nanotubes by more than an order of magnitude, although the latter is much better accessible. We attribute these unique behaviours’ of transition metal nanoparticles inside CNTs to a particular electronic interaction of the encapsulates with the interior CNT surface. In the same way, a distinct enhancement of the CNT encapsulated Fe species to the F-T process has also been revealed, which is attributed the favourable formation of the reduced iron species, e.g. iron carbides, inside the channel of CNTs. The results have been highlighted by C&E News and Nature China, respectively