Single-atom　catalysts　have　attracted　attention　because　of　improved　atom　efficiency,　higher　reactivity,　and　better　selectivity.　A　major　challenge　is　to　achieve　high　surface　concentrations　while　preventing　these　atoms　from　agglomeration　at　elevated　temperatures.　Here　we　investigate　the　formation　of　Pt　single　atoms　on　an　industrial　catalyst　support.　Using　a　combination　of　surface　sensitive　techniques　such　as　XPS　and　LEIS,　X-ray　absorption　spectroscopy,　electron　microscopy,　as　well　as　density　functional　theory,　we　demonstrate　that　cerium　oxide　can　support　Pt　single　atoms　at　high　metal　loading　(3　wt　%　Pt),　without　forming　any　clusters　or　3D　aggregates　when　heated　in　air　at　800　degrees　C.　The　mechanism　of　trapping　involves　a　reaction　of　the　mobile　PtO2　with　under-coordinated　cerium　cations　present　at　CeO2(111)　step　edges,　allowing　Pt　to　achieve　a　stable　square　planar　configuration.　The　strong　interaction　of　mobile　single-atom　species　with　the　support,　present　during　catalyst　sintering　and　regeneration,　helps　explain　the　sintering　resistance　of　ceria-supported　metal　catalysts.