Atom-by-atom　control　of　a　catalyst　surface　is　a　central　yet　challenging　topic　in　heterogeneous　catalysis,　which　enables　precisely　confined　adsorption　and　oriented　approach　of　reactant　molecules.　Here,　exposed　surfaces　with　either　consecutive　Pd　trimers　(Pd3)　or　isolated　Pd　atoms　(Pd1)　are　architected　for　Pd2Ga　intermetallic　nanoparticles　(NPs)　using　reactive　metal-support　interaction　(RMSI).　At　elevated　temperatures　under　hydrogen,　in　situ　atomic-scale　transmission　electron　microscopy　directly　visualizes　the　refacetting　of　Pd2Ga　NPs　from　energetically　favorable　(013)/(020)　facets　to　(011)/(002).　Infrared　spectroscopy　and　acetylene　hydrogenation　reaction　complementarily　confirm　the　evolution　from　consecutive　Pd3　to　Pd1　sites　of　Pd2Ga　catalysts　with　the　concurrent　fingerprinting　CO　adsorption　and　featured　reactivities.　Through　theoretical　calculations　and　modeling,　we　reveal　that　the　restructured　Pd2Ga　surface　results　from　the　preferential　arrangement　of　additionally　reduced　Ga　atoms　on　the　surface.　Our　work　provides　previously　unidentified　mechanistic　insight　into　temperature-promoted　RMSI　and　possible　solutions　to　control　and　rearrange　the　surface　atoms　of　supported　intermetallic　catalyst.　Copyright　©　2022　The　Authors,　some　rights　reserved;