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

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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)

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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

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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*

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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

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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

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=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

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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)