1. Observation of transient surface-bound intermediates by interfacial matrix stabilization spectroscopy (IMSS) Nina K. Jarrah and David T. Moore Chemistry Department, Lehigh University 69 th ISMS, Univ. Illinois Urbana-Champaign June 19, 2014

2. Au nanocatalysts for CO oxidation chemistry Image by Matthew Neurock, University of Virginia, Yates group (http://www.nanowerk.com/news/newsid=22319.php accessed 140312) 2 Perimeter active site?? Gold nanoparticles (AuNPs) supported on titanium dioxide (TiO 2 ) RT activity (Haruta, 1987) Activity depends on sample prep AuNP size, support choice Au/support contact

3. 3 Pressure and temperature effects on binding dynamic equilibrium Equilibrium CO pressureVacuum (10 -6 torr) non-equilibrium (weakly bound species pumped away) Previous spectroscopic studies on AuTiO 2 Strong IR band observed at 2106 cm -1 Specific sample preparation / reaction conditions required Common preparation via deposition precipitation (DP) method Reported band observed under static pressure conditions Cryogenic spectroscopic studies under vacuum Allows for simpler picture of binding

4. Experimental setup 4 Detector Thin powder film of catalyst sapphire thermal break sample stage: 4K to 700K gas lines IR He ports Helium cryostat

5. 5 Thermal removal of hydrocarbon species Increased CO binding at cationic support sites with temperature (Blue-shifted band at 2180 cm -1 ) CO interactions with pure TiO 2 (CO) cm -1 Assignment 2137Physisorbed CO 2150Ti-OH----CO 2168 – 2179M +q – CO 2180Ti 4+ – CO (free CO) 2143 cm -1 2180 cm -1 Increased Lewis acidity of M +

6. 6 Direct adsorption experiment profile Deposition conditions Gas mix: 0.1% - 10% CO in He Flow rate: 1 sccm (0.1% mix for 1 hour  1.6mmol) Deposition at 40K shows CO binding surface sites Time Temperature Heating to 150K Tracks thermal desorption / pop. transfer Heating above 400K cleans surface / forms AuNPs (de-wetting) TPD Heating rate: 5 K/min 123

7. thermal de-wetting driven by surface free energy minimization 7 In-situ formation of AuNPs HEAT Au film (2-25nm) TiO 2 powder AuNPs (5-10 nm) TiO 2 powder

8. 8 TEM akin to DP samples In-situ formation of AuNPs Before heatingAfter 500KAfter 700K

9. 9 FTIR spectra of CO direct adsorption at 40K Detection of CO binding to Au  broad feature at 2090 cm -1 Thermal “sweet spot” for pretreatment of gold samples at 500K Temperature influences – AuNP population (TEM images) – levels of surface adsorbed species (water, hydrocarbons, carbonates) CO-Ti 4+ Pure TiO 2 CO-Au Au/TiO 2

10. Thermal desorption on different AuTiO 2 samples 10 CO/Au  2090 cm -1 (no shift) No pre-exposure to CO CO/Au  2096 cm -1 (shifts to 2106) Pre-exposure to CO Behavior of CO/gold band Dependent on de-wetting conditions (presence of CO)

11. 11 Matrix-assisted stabilization of weakly-bound species Matrix Over-layer “Caging” effect stabilizes weakly-bound species at surface (formed while annealing) Ar Kr Matrix deposition over CO Direct Adsorption saturates CO binding site population

12. 12 Matrix experiment profile Direct adsorption of CO saturates CO sites Time Temperature Heating to 150K Cleaning step Removing matrix Matrix deposition Stabilizes weakly- bound species / Isolates gases from surface 1 2 4 3 5 Matrix annealing transports gases from matrix to surface / stabilizes formation of weak complexes *Matrix spectra collected at 4K*

13. 13 Matrix-assisted stabilization of weakly-bound species Annealing matrix Population transfer to 2150 cm -1 weakly bound CO, hydrogen bonded to Ti-OH

14. 14 Annealing matrix w/ O 2 2112 band  stabilized by the matrix over-layer persistence above 40K  evidence for matrix- assisted crossing of energy barrier Matrix-assisted stabilization of transient species: CO and O 2 O 2 in Ar Deposition (reactant isolation) Annealing (reactant transport)

15. assignment 12 CO 13 COisotope ratio CO/Au209020361.0265 Transient CO species211220651.0228 physisorbed CO213720901.0225 CO H-bonded to Ti-OH215321051.0228 CO/Ti 4+ 218221341.0225 Isotope studies to verify CO binding sites Isotope ratio suggests 2112 cm -1 is a remote support site 15 12 CO deposition over AuTiO 2 13 CO deposition over AuTiO 2

16. Summary Direct adsorption studies at 40K CO binding on Au sites (2090/2106 cm -1 band) Spontaneous formation of AuNPs in vacuo confirmed by TEM images 16 Transient, weakly-bound complexes are important to isolate and study for the identification of catalytically relevant species Direct adsorption (vacuum) Direct adsorption (eq. pressure) IMMS (matrix isolated) IMSS Isolation of transient species (2112 cm -1 ), observed after annealing matrix w/ O 2 Stabilization of weakly-bound CO at Ti-OH centers (2150 cm -1 )