THERMODYNAMIC CALCULATION OF GLOBAL AND LOCAL SURFACE COMPOSITIONS OFPTRH - A CASE OF SURFACE CLUSTERING EVEN WITH BULK ORDERING

A general theory of surfaces, the MTCIP (modern thermodynamic calculat ion of interface properties) was developed earlier by one of the autho rs. Solving a part of its basic equations in a recent and even more ap plicable second approximation (MTCIP-2A), global surface sublayer comp ositions for several Pt alloys were recently published. The PtRh alloy is of great practical importance due to its catalytical properties. N evertheless, experimental data on its mixing behaviour is not readily available, so that at first corresponding bulk and surface thermodynam ic quantities had to be estimated. A recent chemically resolved scanni ng tunneling microscopy study for PtRh(100) shows surface clustering, in contrast to bulk ordering as predicted from ab initio and other cal culations. Extending the MTCIP-2A approach to include local chemical c ompositions by means of the quasi-chemical theory explains this behavi our. The calculated global surface concentration profiles (for T = 112 0 K), as well as local surface compositions (indicating local clusteri ng) are in agreement with recent experimental results. (C) 1998 Elsevi er Science B.V. All rights reserved.