observation of transient surface-bound intermediates by interfacial matrix stabilization...
Post on 16-Jan-2016
218 Views
Preview:
TRANSCRIPT
Observation of transient surface-bound intermediates by interfacial matrix stabilization spectroscopy (IMSS)
Nina K. Jarrah and David T. MooreChemistry Department, Lehigh University
69th ISMS, Univ. Illinois Urbana-ChampaignJune 19, 2014
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
Perimeteractivesite??
Gold nanoparticles (AuNPs) supported on titanium dioxide (TiO2)• RT activity (Haruta, 1987)• Activity depends on sample prep
• AuNP size, support choice• Au/support contact
3
Pressure and temperature effects on binding
dynamic equilibrium
Equilibrium CO pressure Vacuum (10-6 torr)
non-equilibrium (weakly bound species pumped
away)
Previous spectroscopic studies on AuTiO2
• 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 vacuumAllows for simpler picture of binding
Experimental setup
4
Detector
Thin powder film
of catalyst
sapphire thermal break sample stage:
4K to 700Kgas lines IR
He portsHelium
cryostat
5
Thermal removal of hydrocarbon species
Increased CO binding at cationic support sites with temperature (Blue-shifted band at 2180 cm-1)
2700 2800 2900 3000 3100
Wavenumber ( cm-1)
unheated sample 400K 500K 600K 700K
0.005 A
2000 2100 2200-0.005
0.000
0.005
0.010
0.015 unheated sample 400K 500K 600K 700K
Abs
orba
nce
Wavenumber (cm-1)
CO interactions with pure TiO2
(CO) cm-1 Assignment
2137 Physisorbed CO
2150 Ti-OH----CO
2168 – 2179 M+q – CO
2180 Ti4+ – CO
(free CO)2143 cm-1
2180 cm-1
Increased Lewis acidity of M+
6
Direct adsorption experiment profile
Deposition conditionsGas mix: 0.1% - 10% CO in HeFlow rate: 1 sccm (0.1% mix for 1 hour 1.6mmol)
Deposition at 40K
shows CO binding surface sites
Time
Tem
pera
ture
4K
TPD Refscans
400-700K
40K60K
90K120K
RT Heat
Deposition
Heating to 150K Tracks thermal desorption / pop. transfer
Heating above 400K
cleans surface / forms AuNPs (de-
wetting)
TPDHeating rate: 5 K/min
1
2
3
thermal de-wettingdriven by surface free energy
minimization
7
In-situ formation of AuNPs
HEAT
Au film(2-25nm)
TiO2 powder
AuNPs(5-10 nm)
TiO2 powder
8
TEM akin to DP samples
In-situ formation of AuNPs
100 nm 100 nm100 nm
Before heating After 500K After 700K
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)
2000 2100 2200
0.000
0.002
0.004
0.006
0.008
0.010 No pre-heating 400K 500K 600K 700K
Abs
orba
nce
Wavenumber (cm-1)
CO-Ti4+
Pure TiO2
2000 2100 2200
Wavenumber (cm-1)
CO-Au
Au/TiO2
Thermal desorption on different AuTiO2 samples
10
2000 2100 2200
Wavenumber ( cm-1)
40K 60K 90K 120K 150K
0.001 APhysisorbed CO
CO/Ti4+
CO/AuTiO2
CO/Au 2090 cm-1 (no shift)
No pre-exposure to CO
2000 2100 2200
Wavenumber ( cm-1)
40K 60K 90K 120K 150K
Physisorbed CO
CO/Ti4+CO/AuTiO2
0.001 A
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
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
CODirect
Adsorption saturates CO binding site population
12
Matrix experiment profile
Annealing
Matrix Dep TPD
CO Dep
RT Heat
Direct adsorption of
COsaturates CO sites
Time
Tem
pera
ture
Heating to 150K
Cleaning step
Removing
matrix
Matrix deposition
Stabilizes weakly-bound species /Isolates gases from surface
1
24
3
5
Matrix annealing
transports gases from matrix to
surface / stabilizes
formation of weak complexes
*Matrix spectra collected at 4K*
13
Matrix-assisted stabilization of weakly-bound species
2050 2100 2150 2200 2250
Wavenumber (cm-1)
Deposition
0.001 A
2050 2100 2150 2200 2250
Ti-OH----CO
Wavenumber (cm-1)
30K Annealing
0.001 A
Annealing matrixPopulation transfer to 2150 cm-1 weakly bound CO, hydrogen bonded to Ti-OH
14
Annealing matrix w/ O2
• 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 O2
AuAu
Au
Au
Krypton
O2 in Ar
Deposition (reactant isolation)
AuAu
Au
Au
Krypton
Annealing (reactant transport)
2000 2050 2100 2150 2200 2250
Wavenumber ( cm-1)
CO/AuTiO2
CO/Ti4+
Deposition
Physisorbed CO
0.001 A
2000 2050 2100 2150 2200 2250
Wavenumber ( cm-1)
CO/AuTiO2
CO/Ti4+
Deposition Annealing at 32K
Physisorbed CO
0.001 A
2112
2000 2050 2100 2150 2200 2250
CO/AuTiO2
CO/Ti4+
Wavenumber ( cm-1)
Deposition Annealing at 32K Annealing at 43K
Physisorbed CO
0.001 A
2112
assignment 12CO 13COisotope
ratio
CO/Au 2090 2036 1.0265
Transient CO species 2112 2065 1.0228
physisorbed CO 2137 2090 1.0225
CO H-bonded to Ti-OH 2153 2105 1.0228
CO/Ti4+ 2182 2134 1.0225
2000 2050 2100 2150 2200 2250
Wavenumber ( cm-1)
deposition annealing at 32K
2182
2153
2112
2090
2137
0.001 A
Isotope studies to verify CO binding sites
Isotope ratio suggests 2112 cm-1 is a remote support site
2000 2050 2100 2150 2200 2250
Wavenumber ( cm-1)
deposition annealing at 32K annealing at 37K
2065
2134
2105
2090
2036
0.001 A
15
=12𝜋 √ 𝑘𝜇
12CO deposition over AuTiO213CO deposition over AuTiO2
SummaryDirect 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
AuAu
Au
Au
AuAu
Au
AuAu
Au
Au
Au
Direct adsorption (vacuum)
Direct adsorption (eq. pressure)
IMMS(matrix isolated)
IMSS• Isolation of transient species (2112 cm-1), observed after
annealing matrix w/ O2
• Stabilization of weakly-bound CO at Ti-OH centers (2150 cm-1)
top related