Indian Journal of Chemistry Vol. 43A, March 2004, pp. 464-472 Model studies on segregation and catalytic properties of supported Pt-Rh nanocatalysts Abir De Sarkar & Badal C Khanra' Conde nsed Maller Physics Group, Saha In stitute of Nuclear Physics 1/AF, Bidhannagar, Calcutta 700 064. India E-ma il : badal @cmp.saha.crnet.in Neceived 3 November 2003; revised 9 January 2004 In thi s work. a comprehensive report is presented on model investigations on the surface composition and catalytic activity of supponed Pt - Rh nanocatalys ts and their dependence on the impurit y and the metal-support interactions. Mont e- Carlo ( MC) simulations indicate that sulphur can innuencc the surface composition very strongly; but the influe nce of metal-support interaction on the surface compos ition is in significant. A microkinctic analysis ha s been made to investigate th e role of alloying and impurity on th e CO+ NO. reactions over these nanocatalysls. For th e reaction the alloy particles show an inte rm ediate activity compared to that on Rh and Pt nanoparticles. On th e other hand, for the CO+ NO reaction the all oy parti cl es show no synergistic effect of alloying. The effects of all oy in g on the kinet ics of and CO+ NO reaction s arc found to be identical. Sulphur has been found to reduce the activity of the Pt-Rh nanocawl ys ls for the CO+NO and reaction more through blocking of the active sites on th e surface than through changes in surface composition. The effect of sulphur is much stronger on formation rate than on formation rate. formation takes place more through recombination of N ato ms ( f3 - N 2 ) than through format i on and its subsequent dissociation into and 0 (8 - N 2 ). N, formation rate is found to be the same for CO+NO and CO+N0+0 2 reacti ons. Impl ication s of th ese results are thoroughly di sc usse d. Over the la st three decades, use of Pt· Rh/Ce0 2 -Al p .l formulations as three-way ca t alysts (TWC) in the cata l yt ic convert e rs of automobiles has h elped enormo us ly to control the emission of polluting gases into the environmen{ 7 • The main functions of these catalysts are to convert simultaneously the principal polluting gases like CO, NO and unburnt hydrocarbons (HC) into harmless gases like and Hp. Several factors, such as the legislation for stricter control over em issions, market price of the noble metals and the hi gh temperature durability of the catalysts have l ed to improved formulation like NM/Ce0 2 -Zr0 2 -Alp 3 , where the noble metals (NM) used are genera lly combinat ions of Pt, Pd and Rh. The major improvement in the formulation ofTWC has, however, been in the technology of support preparation. Modern supports like have been shown to have a better control over the oxygen supply for the reactions, high temperature durability (up to 1100 K) a nd the ability to minimize deactivation of the catalysts by sulphur and other impurities present in the fuel 4 " 5 • The developments of these modified catalysts, Pt-Rh/ Ce0 2 -Zr0 2 -AI 2 0 3 were possible through inten sive industrial research. However, our knowledge about how these catalysts work microscopically is stil l poor. On e aspect is very certain about these new three-way catalysts; and that is that the ce ntral component in these formulations is sti ll Pt(Pd)-Rh/Ce0 2 . To understand the new TWC's we have to understand first how Pt- Rh/Ce02 work under various condition s. [n order to understand the role of alloying, impurities in the fuel and met al-s upport interactions on the principal reactions, C0+0 2 , CO+NO and CO+N0+0 2 , we have carried out a series of model studies on the P t-Rh/Ce0 2 system. Some of the results have been published earlier 8 . 12 • ln this report we present the compre hensive results of all our investigations on these systems. Our investigations deal more with the physico-chemical aspects like swface and catalytic prope rti es of the Pt- Rh nanoc atalysts than with the composition of the supports. While these aspects for the supported Rh and Pt catalysts can be studied and understood relatively