Chemical reactions on solid surfaces play critical roles in many technologies:

1. Catalysts for producing fuels, pollutant removal and chemical production

2. Microelectronics fabrication

3. Electro optic devices

4. Solar energy devices

5. Nanotecnology6. Chemical /

biochemical sensorsAtomic-resolution STM image of alloy surface

from group of Peter Varga, TU Vienna

Adsorption EnergiesMeasured by Microcalorimetry

Adsorption energetics are the most important ingredient in understanding the thermodynamics of surface reactions.

Also provide benchmarks against which to compare theoretical approximations in quantum computational methods.

Molecular adsorption on Pt(111):Intermediates in hydrocarbon dehydrogenation / hydrogenation

reactions over Pt catalystsMolecular Beam Epitaxy:

Metal atoms on Si(100)Metal atoms on polymer surfaces

Proteins on hydroxy apatite powder

FCC(110)

FCC(111)

Single crystal surfaces:• Facilitate detn. of adsorbate structures• Most known structures are on single crystals

Elementary step energy diagram: Water-gas shift on Cu(110)

J. Nakamura and CTC, Faraday Trans. 86 (1990) 2725

Measuring adsorption energies on surfaces of single crystals:

Two older method provide such adsorption energies:1. Temperature-programmed desorption (TPD)2. Equilibrium adsorption isotherms

BUT…

Both fail if any dissociation or change in adsorbatestructure before desorption.

Need: direct calorimetric methodfor measuring adsorption energies on

single crystal surfaces!!

Temperature-ProgrammedDesorption

MassSpectrometry

(TPD)studies of alkane

adsorptionon MgO(100)

Steven L. Tait, Zdenek Dohnálek,CTC, Bruce D. Kay

(expts. done at PNNL)J. Chem. Phys. 122, 164708 2005;

and 125, 234308 2006..

Alkane desorption energies and prefactors from TPD

kd = νd

. exp(-Edes / kBT)

Assumed:Edes varies with coverage, but νd constant.

Steven L. Tait, ZdenekDohnálek, CTC, Bruce D. Kay (done at PNNL)J. Chem. Phys. 122, 164708 2005; and 125, 234308 2006..

First Successful Single-Crystal Adsorption MicrocalorimeterDavid A. King and group, Cambridge Univ.

Rev. Sci. Instr. 62 (1991) 2177.Chem. Rev. 98 (1998) 797; Adv. Catal. 43 (2000).

• Pulse of gas strikes 200 nm thick single crystal’s surface→ Transient temperature rise of ~10 mK detected by infrared optical pyrometry.

• Sticking probability measured by reflected fraction with mass spectrometer.

250

200

150

100

50

Diff

eren

tial h

eat (

kJ/m

ol)

1.00.80.60.40.20.0Apparent coverage (ML)

C

CH3

CH2CH2

CH2 CH2

CHHC

From: W. A Brown, R. Kose andD. A. King, Chem. Rev. 98 (1998) 797.

Ethylene adsorptionon Pt(100)-hex

300 K

Ta b le 6 Ta b le of h yd ro ca rb on a d so r p ti on he a t s, init ia l s t ic k in g a nd M -C bon d

s tr ength s. It ca n be s e en th a t hy d r oc ar bon ads o rp ti on le ads to the fo rm a t ion of

m a n y d iffer ent sp ec ie s on the su rf ac e . In gen er a l , go od ag ree m ent is a ch iev ed

fo r c alc u la te d M -C b on d ener g ie s w h a t ever the n a t u re o f the ads or ba te sp e cie s.

S y s te m I n iti a lhea t (k J

m ol -1 )

I n iti a ls tic k i n g

p ro bab ility

S u rf aces p ec i es

N u m be r o fM -C b o nd s

M -C s i n gl eb o n d ene rgy

(k J m ol -1 )C 2H 4/ P t{110 }33 ,34 235 0.85 ≡ C- C H 2-

≡ C- C H 3-C H 2-C H 2-

432

229239257

C 2H 4/ P t{111 }35 195 0.67 ≡ C- C H 3= C H -CH 3

32

238250

C 2H 4/ P t{100 }-he x 33

213 0.79 q uad -σ C 2H 2≡ C- C H 3

d i- σ C 2H 4

432

223230249

C 2H 4/ P t{100 }-(1x1 )33

305 0.69 q uad -σ C 2H 2d i- σ C 2H 4

42

246253

C 2H 4/ P d {100 }31 73 (s te ad ys t a te

ads or p t ionon ly)64 ± 3

0.75 _ _ _

C 2H 4/ N i{100 }31 203 0.81 ≡ C H≡ C C H

3š

inter ac ti on

20574 -189

C 2H 2/ P d {100 }31 112 0.83 d i- σ C 2H 2 2 171C 2H 2/ N i{100 }31 264 0.81 ≡ C H

≡ C C H3š

inter ac ti on

20451 -165

From: W. A Brown, R. Kose and D. A. King,Chem. Rev. 98 (1998) 797.

King group’s results starting to reveal systematics of C-M surface bond strengths

Single Crystal Adsorption MicrocalorimeterStuckless et al., J. Chem. Phys. 107 (1997) 5547.

Rev. Sci. Instr. 69 (1998) 2427.Follows the design of D. A. King

But different method of heat detection:Detector : 9μm thick, 4 mm wide pyroelectric ribbon, (β-PVDF),

flexible, w/ 50 nm NiAl coating on both sides for measuring V.

Advantages:• More sensitive so we can use thicker single crystal samples (up to 80 μm so far).• Works also at low temperatures (down to 100 K so far).• Can pretreat samples to > 2000 K.

UHV Chamber with AES, LEED and QMS

Thermal Reservoir

V

PulsedMolecular

Beam

ThinSample

PyroelectricRibbon

QuadrupoleMass Spectrometer

Sample

In Contact

ApproachingContact

PVDFRibbon

Principle of adsorption microcalorimetry on single crystal surfaces

Single-crystal

Approach Contact In Contact

Molecular Beam Pulses

Stuckless et al., Rev. Sci. Instr. 69 (1998) 2427

PyroelectricVoltage Signal(after preamp)

Pyroelectric ribbon

MolecularBeam

V

Mass Spectrometer

Benzene adsorption on Pt(111):Enthalpy of adsorption (ΔHads) at 300 K

First calorimetric measurement for any aromatic hydrocarbon on any metal crystal face.Since benzene completely dissociates below the desorption temp. at < 0.6 ML,

heats were not previously known.

H. Ihm, H. Ajo, M. Gottfried, P. Bera and C. T. Campbell, J. Phys. Chem. 108 (2004) 14627

Heat vs. n for planar aromatic hydrocarbons: CnH3+n/2

M. Gottfried, E. Vestegaard, P. Bera and C.T. Campbell, J. Phys. Chem. (2006)

Pt(111)

ΔHad,integral = 28 kJ/mol x #C atoms

←Very similar DFT energy also reported for benzene by Neurockgroup: JPC 109 (2005) 2064

Slope = 28 kJ/mol per C = C-Pt bond energy

DFT- Sautet

V

Henry Ajo

Hyeran Ihm

Cyclohexene dehydrogenation mechanismon Pt(111)

200200--240 K240 K240240--300 K300 K

>300 K>300 K

Intactc-C6H10

π-allyl c-C6H9 benzene

want adsorption energies

Rodrigquez, J.A. et al. J. Catal. 1989, 115, 500.Henn, F. C. et al. J. Phys. Chem. 1992, 96, 5965.

Recent changes to the detector design

Lead Region

100 μm thick Kapton electrically insulating

Single crystalPt(111) 1 μm thick

After chemically etched front and back of pyroelectric ribbon

Reduced dimensionto increase contact with

area of highest intensity of heats

PVDF polymer ribbonBefore entire ribbon area coated by metal shorts along edges

Approaching Contact

20 nm Al Coating

Improvements in Signal to Noise Ratio Allow Effective Measurements at 100 K

Raw signal for 50 – 1 μJ light pulses

AFTER improvements:

Average of 50 light pulses

BEFORE improvements:

Cyclohexene on Pt(111) at 100 K

Ole Lytken, Wanda Lew and CTC, in prep.

multilayer

submonolayer

Θ= 0.25 ~ 1 cyclohexeneper 4 Pt atoms

Δ Had,initial = 120 kJ / mol

ΔHad = 40.2 kJ/ molc.f. heat of sublimationΔHsub = 49.7 kJ / mol

DFTa

Cyclohexene on Pt(111)

λ TPD peak B

γ TPD peak

α TPD peak

integral heat of adsorption

differential heat of adsorption:

DFTa

A Morin, C., Simon, D., Sautet, P., Surf. Sci. 2006, 600, 1339-1350.B Rodrigquez, J.A.; Campbell, C.T. J. Catal. 1989, 115, 500.

O. Lytken, W. Lew and CT Campbell, in prep.

Multilayer

100 K

Average sigma bond energybetween C atom and Pt surface atom,

from initial heat of adsorption

253 kJ / molcdi-σ ethylene/Pt(100)

257 kJ / molcdi-σ ethylene/Pt(110)186 kJ / moldi-σ cyclohexene / Pt(111)

C W.A. Brown; D.A. King Chem.Rev. 98, 1998,797.

DFTa

Lower bond energy due to:Higher Pt atom coordination?Steric repulsion?

Comparison of integral heat of adsorption to state-of-the-art quantum theory

113 kJ/mol (Θ = 0.11)DFT slab B83 kJ/mol (Θ = 0)DFT cluster A

127 kJ/mol (Θ = 0)110 kJ/mol (Θ = 0.11)

Microcalorimetry (present study)

Cyclohexene on Pt(111)Method

A Saeys, M. et al. Surf. Sci. 2006,600, 3121-3134. B Morin, C. et al. Surf. Sci. 2006, 600, 1339-1350.

Note: same DFT slab method by same group gets ΔHads for benzene on Pt(111) too low by 84 kJ/mol, but that is only 14 kJ/mol per C atom -Role of dispersion forces missed in DFT?

One M-C bond energy goes a long waytoward predicting other energies

Linear correlation of slope 1.0 between carbon-carbon bond strength and the corresponding hydrogen-carbon bond strength for the same ligand.

H. Gross, CT Campbell and DA King, Surface Sci. 572 (2004) 179.

(J. Bercaw group)

Also a linear correlation of slope 1.0 between metal-carbon bond strengthand the corresponding hydrogen-carbon bond strength for the same ligand

King’s Pt-C σ bond energy of ~250 kJ/mol for a sp3-hybridized primary C atom (i.e., di- σ C2H4) on Pt(100) implies:

↓ Pt-C bond energies203 on Pt(100)207237250279299303391

H. Gross, CT Campbell and DA King, Surface Sci. 572 (2004) 179.

Predicted

Average sigma bond energybetween C atom and Pt surface atom,

from initial heat of adsorption

253 kJ / molcdi-σ ethylene/Pt(100)

257 kJ / molcdi-σ ethylene/Pt(110)186 kJ / moldi-σ cyclohexene / Pt(111)

C W.A. Brown; D.A. King Chem.Rev. 98, 1998,797.

DFTa

Lower bond energy due to:Higher Pt atom coordination?Steric repulsion?→13 kJ/mol of the difference due to secondary vs primary C

Cyclohexene dehydrogenation mechanismon Pt(111)

200200--240 K240 K240240--300 K300 K

>300 K>300 K

Intactc-C6H10

π-allyl c-C6H9 benzene

want adsorption energies

Rodrigquez, J.A. et al. J. Catal. 1989, 115, 500.Henn, F. C. et al. J. Phys. Chem. 1992, 96, 5965.

These large molecules comparable to those used in Chemical Vapor Deposition, or CVD.

→ Can extend to studies of CVD intermediates on semiconductor surfaces.

Molecular Beam Epitaxy (MBE)for

Microelectonics Fabrication:

1. Metals on Si(100)2. Metals on Polymers

Calorimetric adsorption signalsPb / multilayer Pb / Mo(100)

* Metal pulses: ~2.5 % ML, 100 ms long, every 2 s.* Heat calibration: light pulses of known energy.* Metal flux by quartz crystal microbalance.* Sticking probability by mass spectrometry.→ Absolute heats per mole adsorbed (±1-2 %)→ Pulse-to-pulse std. dev. <1.5 kJ/mol possiblePulse-shape analysis: Stuckless et al., Sensors and Actuators B62 (2000) 13.

BaF2 window in:

BaF2 window out:

Fast:Edes = 77 kJ/mol←

Slow: Edes = 83 kJ/mol

↓

Assuming ν = 4x1016 s-1:

gas

at “magicangle”

Ag coverage / ML0 1 2 3 4 5 6

Hea

t of A

dsor

ptio

n / k

J/m

ol

200

240

280

320

360

400

Ag / Si(100)-2x1300 K

Ag / Si(100)-2x1300 K

oooooooooooooooooooooooooooooooooooooooooooooooooo

D.E. Starr et al.,Phys. Rev. Letters87, 106102, 2001.

First calorimetric measurements of ΔHad on any Si single crystal surface.Energetic fingerprint for lattice mismatch: admetal islands induce strain in substrate surface nearby, leading to decrease in Eads with island size (and island -island repulsion)! •

Spontaneous self-assembly of equal-size and nearly equally-spaced quantum dots when growing mismatched thin films,

e.g. Ge on Si(100)

Ross, Tersoff, Tromp, PRL 80 (1998) 984.

Polymer sample platen Spin-coated PMMA area:~8 mm diameter. Heats:calibrated with light pulses of known energySensitivity:~ 450 V/Jabs.Oven radiation:Pb: ~ 0.3 μJ/pulse Ca:~ 0.14 μJ/pulse

Detector: 9 μm thick pyroelectric β-poly(vinylidenefluoride) (PVDF) sheet.Face-to-face voltage pulse ∝ heat input

Heats of adsorption of metals on polymers: Ca on polymethylmethacrylate (PMMA)

0 1 2 3 4 5 60

100

200

300

400

500

600

700

800

900

0.0 0.2 0.4 0.6 0.8 1.0400

500

600

700

800

900

Hea

ts [k

J/m

ol]

Ca coverage [ML]

ΔHsub = 177.8 kJ/mol

Heats vs coverage for Ca adsorption on pristine PMMA at 300 K

Ca coverage [ML]

Hea

ts [k

J/m

ol]

Subsurface Ca-ester complexes form first (ISS):• Clean surface of PMMA exposes mostly –CH3 and –CH2 groups (low surface energy), esters are subsurface.

• Heat of 720 kJ/mol at minimum. Increases to 780 kJ.mol max: attractive interactions.• Saturates after 2-3 layers, but only after ~1 layer when electron-damaged PMMA.

3D solid Ca particles grow on top of surface, in kinetic competition, dominate above 1.5 ML:• Since these nucleate at defects, much more competitive on e-damaged surface.

JF Zhu, P. Goetsch, N. Ruzyckiand CTC, JACS (in press)

Ca(g) + 2 R-COOCH3(s) → (R-COO)2Ca(s) + CH3-CH3(g)ΔHrxn = -855 kJ/mol when R = methyl

vacuum

Protein Adsorption:Energetics

Calorimetric adsorption energy of statherin (a salivary protein that inhibits nucleation / growth of hydroxyapatite on teeth) onto hydroxyapatite powder in H2O

R. Goobes, G. Goobes, CT Campbell and PS Stayton, Biochemistry 45 (2006) 5576.

Prior structural studies of statherin and statherin fragments adsorbed onto hydroxyapatite by high-resolution

solid-state NMR inDrobny / Stayton labs at UW:

N-terminal domain is α-helical and contains residues that are close to HAP

surface.JM Gibson, V Raghunathan, JM Popham, PS

Stayton, and GP Drobny, JACS. 127 (2005) 9350.

• Heat of adsorption of protein exceedingly small (< ~few hydrogen bonds)!• Two sites- one populated at high coverage has ~0 heat of adsorption)• ΔG0 of adsorption (~-40 kJ/mol) dominated by large positive ΔS0 (>97 J K-1 mol-1),

due to the loss of organized water that hydrates protein and mineral surface.