Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195 Pt Chemical Shift Anisotropy

Abstract

International audienceTechniques thatcancharacterize themolecular structures ofdilute surfacespeciesarerequired tofacilitate therationalsynthesis andimprovement of Pt-based heterogeneous catalysts. 195Ptsolid-state NMRspectroscopy couldbean idealtoolforthistaskbecause 195Ptisotropic chemical shiftsandchemical shift anisotropy (CSA)arehighlysensitive probesofthelocalchemical environment andelectronic structure. However, thecharacterization ofPtsurface-sites is complicated bythetypicallowPtloadings thatarebetween 0.2and5wt%and broadening of 195Ptsolid-state NMRspectrabyCSA.Here,weintroduce asetof solid-state NMRmethods thatexploitfastMASandindirectdetection usinga sensitive spynucleus( 1Hor 31P)toenabletherapidacquisition of 195PtMAS NMRspectra.Wedemonstrate thathigh-resolution wideline 195PtMASNMRspectracanbeacquired inminutestoafewhoursfor aseriesofmolecular andsingle-site PtspeciesgraftedonsilicawithPtloadingofonly3-5wt%.Low-power, long-duration, sidebandselective excitation, andsaturation pulsesareincorporated into t1-noiseeliminated dipolarheteronuclear multiple quantum coherence, perfectechoresonance echosaturation pulsedoubleresonance, or J-resolved pulsesequences. Thecomplete 195PtMAS NMRspectrum isthenreconstructed byrecording aseriesof1DNMRspectrawheretheoffsetofthe 195Ptpulsesisvariedin increments oftheMASfrequency. Analysisofthe 195PtMASNMRspectrayieldsthe 195Ptchemical shifttensorparameters. Zeroth orderapproximation densityfunctional theorycalculations accurately predict 195PtCStensorparameters. Simpleandpredictive orbitalmodelsrelatetheCStensorparameters tothePtelectronic structure andcoordination environment. Themethodology developed herepavesthewayforthedetailedstructural andelectronic analysisofdiluteplatinum surface-sites