Surface composition changes of CuNi-ZrO2 catalysts during methane decomposition: An operando NAP-XPS and density functional study

Abstract

Bimetallic CuNi nanoparticles of various nominal compositions (1:3, 1:1, 3:1) supported on ZrO2wereemployed for operando spectroscopy and theoretical studies of stable surface compositions under reac-tion conditions of catalytic methane decomposition up to 500◦C. The addition of Cu was intended toincrease the coke resistance of the catalyst. After synthesis and (in situ) reduction the CuNi nanoparticleswere characterized by HR-TEM/EDX, XRD, FTIR (using CO as probe molecule) and NAP-XPS, all indicatinga Cu rich surface, even when the overall nanoparticle composition was rich in Ni. Density functional (DF)theory modelling, applying a recently developed computational protocol based on the construction oftopological energy expressions, confirmed that in any studied composition Cu segregation on surfacepositions is an energetically favourable process, with Cu preferentially occupying corner and edge sites.Ni is present on terraces only when not enough Cu atoms are available to occupy all surface sites.When the catalysts were applied for methane decomposition they were inactive at low temperaturebut became active above 425◦C. Synchrotron-based operando NAP-XPS indicated segregation of Ni on thenanoparticle surface when reactivity set in for CuNi-ZrO2. Under these conditions C 1s core level spectrarevealed the presence of various carbonaceous species at the surface. DF calculations indicated that boththe increase in temperature and especially the adsorption of CHxgroups (x = 0-3) induce the segregationof Ni atoms on the surface, with CH3providing the lowest and C the highest driving force.Combined operando and theoretical studies clearly indicate that, independent of the initial surfacecomposition after synthesis and reduction, the CuNi-ZrO2catalyst adopts a specific Ni rich surface underreaction conditions. Based on these findings we provide an explanation why Cu rich bimetallic systemsshow improved coke resistance.