The interaction between metal and coke influencing selectivity is reminiscent of dopants or promoters; the selectivity improvement from adding Sn or Ge to the Pt catalyst comes at the cost of activity, though not to the same extent as coking. Sn is the most widely-used dopant for Pt, though Ge has been found to produce similar improvements in stability against cok-ing. XPS of subnano PtSn clusters shows a shift to lower binding energies, implying that Sn tends to donate electrons to Pt. However, there have been reports that PtSn catalysts build up more coke than their pure Pt analogues, suggesting that they are not simply resistant to the build-up of coke, but that their activity persists despite partial coking. The contrast between the electronic and steric mechanisms of coking, and the role of doping are represented in Figure .