bisulfate dehydration at the vapor/solution interface probed by vibrational sum frequency generation...
TRANSCRIPT
SO2 (g) SO2(g) + H2O
Oxidation
HSO3-
H+/HSO4- /SO4
2-
V.O.C. S.O.A. : Function of Interfacial Species
Mineral Core
SO42-
(ads)
H2SO4
Oxidation
Bisulfate Dehydration at the Vapor/Solution Interface Probed by Vibrational Sum Frequency Generation Spectroscopy
Aaron M. Jubb and Heather C. Allen67th meeting of the International Symposium on Molecular Spectroscopy
June 19, 2012 – Columbus, [email protected]
Raman
• Change in polarizability with
vibration
vis
Stokes
Vibrational Spectroscopies Utilized
IR
vis
IR
sum
v = 0
v = 1
Virtual excited state
vis
Virtual excited state
Anti-
Stokes
Infrared
• Change in dipole moment with
vibration
Vibrational Sum Frequency Generation
• Lack of inversion symmetry
• Vibration must be both IR and Raman active
What role does cation (H+, Na+, Mg2+) identity play for HSO4
- behavior?
• Ion-pair formation?
• Hydration disruption?
How does interfacial behavior compare to bulk behavior?• VSFG versus Raman
Target νSS – SO3 of HSO4-
50
0
x104 1300120011001000900800
Wavenumber (cm-1
)
1
0
0.4 x H2SO4 - Infrared 0.4 x H2SO4 - Raman Component Peaks
Ram
an C
oun
ts (
x104 )
Ab
sorb
ance
A - Infrared
B - Raman
AS - SO3 & AS - SO42-
SS - SO3
SS - SO42-
SS - S-(OH)
Dehydration versus Ion-pairing
Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, 10162-10173. Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), 1189-1193.
Dehydration• Breaks H-bonds
• Shortens S-O bonds• Blue shift for νSS – SO3
Ion-Pairing• Induces polarization • Electron withdrawing• Lengthens S-O bonds• Red shift for νSS – SO3
2-2+ 2-
2+2-
2+
Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)
H2SO4 results correspond well with literature
Blue shifted from bulk frequency of 1035 cm-1
pH 1 Na2SO4 solutions exhibit bimodal spectra
Indicates two distinct populations of HSO4-
molecules at interface
MgSO4 results feature only blue-shifted peak
Entire HSO4- interfacial population disrupted
VSFG: HSO4- Dehydration
0.15
0.00
0.11
0.00
0.15
0.00
1150110010501000950
Wavenumber (cm-1
)
VS
FG
In
ten
sity
(a.
u.)
C - 0.035 x MgSO4 pH 1
B - 0.033 x Na2SO4 pH 1
A - 0.4 x H2SO4
Ion-Pairing Dehydration
A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: 10.1021/jp302585h
Cations Induce Dehydration Within Interface
Walrafen, G.E.; Yan, W.-H.; Chu, Y.C. J. Phys. Chem. A 2002, 106, 10162-10173. Fung, K.H.; Tang, I.N. Appl. Spectrosc. 1992, 46(6), 1189-1193.
Dehydration• Breaks H-bonds
• Shortens S-O bonds• Blue shift for νSS – SO3
Ion-Pairing• Induces polarization • Electron withdrawing• Lengthens S-O bonds• Red shift for νSS – SO3
2-2+ 2-
2+2-
2+
Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)
12001150110010501000
Wavenumber (cm-1
)
20
0
x102
25
0
x102
AS - SO42-
SS - SO3
A - Na2SO4 pH 1
B - MgSO4 pH 1
~ 0.034 x 0.017 x 0.009 x
Ram
an C
ount
s (x
102 )
Bulk solution HSO4- νSS – SO3
frequency increases with:
• Concentration
• H+ < Na+ < Mg2+
Na2SO4 exhibits higher νSS – SO3 intensity than MgSO4
• CIP formation between SO42- and Mg2+
inhibiting HSO4- formation – Kester et
al. 1972
Raman: HSO4- Bulk Dehydration
2-2+ 2-
2+2-
2+
Solvent-Separated Ion Pair (SSIP) Solvent-Shared Ion Pair (SIP) Contact Ion Pair (CIP)
A.M. Jubb and H.C. Allen, J. Phys. Chem. C, 2012, DOI: 10.1021/jp302585h
HSO4- νSS – SO3 frequency increases:
• At vapor/solution interfaces
• With concentration
• H+ < Na+ < Mg2+
νSS – SO3 frequency blue-shifts indicate HSO4- dehydration
First time VSFG has been used to directly interrogate cation effects on HSO4
- at vapor/solution interfaces
Implies acid catalyzed aerosol reactions may be inhibited by interfacial cations
Summary
Acknowledgements
50
0
x104 1300120011001000900800
Wavenumber (cm-1
)
1
0
0.4 x H2SO4 - Infrared 0.4 x H2SO4 - Raman Component Peaks
Ram
an C
ount
s (x
104 )
Ab
sorb
ance
A - Infrared
B - Raman
AS - SO3 & AS - SO42-
SS - SO3
SS - SO42-
SS - S-(OH)
25
0
x103
1300120011001000900800
Wavenumber (cm-1
)
0.4
0.0
0.033 x Na2SO4 pH 1 0.017 x Na2SO4 pH 1 0.009 x Na2SO4 pH 1
A - Infrared
B - Raman
Ram
an C
oun
ts SS - SO42-
AS - SO42-
SS - SO3
Ram
an C
ount
s (x
103 )
AS - SO3
Abs
orb
ance
28
0
x103
1300120011001000900800
Wavenumber (cm-1
)
0.45
0.00
0.035 x MgSO4 pH 1 0.017 x MgSO4 pH 1 0.009 x MgSO4 pH 1
SS - SO42-
SS - SO3
AS - SO42-
Ram
an C
oun
ts (
x103 )
A - Infrared
B - Raman
Ab
sorb
ance
AS - SO3
22
0
x103
120011001000900800
Raman Shift (cm-1
)
-1000
0
1000
0
1 2
0.033 x Na2SO4 pH 1 Fit Lorentzian Peak Components Fit Residual
A
B
C
Ram
an C
oun
ts
0.14
0.00
VS
FG
In
ten
sity
(a.
u.)
1150110010501000
Wavenumber (cm-1
)
0.033 x Na2SO4 pH 1
0.033 x Na2SO4 pH 7
0.14
0.00
VS
FG
In
ten
sity
(a.
u.)
1150110010501000
Wavenumber (cm-1
)
0.035 x MgSO4 pH 1
0.035 x MgSO4 pH 7
Table 1. Curve-Fitting Results for VSFG Spectra
Non-Resonant Terms
Peak 1
Peak 2
Concentration B ϕ ων (cm-1) Aν Γ (cm-1) ων (cm-1) Aν Γ (cm-1)
H2SO4 0.4 x 0.03 0.01 1050 6.9 26.0 N/A N/A N/A
Na2SO4 0.033 x 0.05 0.03 1048 1.7 8.6 1067 3.8 19.7
MgSO4 0.035 x 0.02 0.00 N/A N/A N/A 1069 7.0 22.4
Solution Concentration
(x)
ω
(cm-1)
I
(counts)
FWHM
(cm-1)
Area
H2SO4 0.4 1036 ± 0 3.7 (± 0.05)
x 105
51 ± 1 3.0 (± 0.07)
x 107
Na2SO4 0.009 1053 ± 1 711 ± 47 27 ± 3 29906 ± 2643
0.017 1053 ± 2 880 ± 137 19 ± 5 26152 ± 6697
0.033 1054 ± 2 1020 ± 215 20 ± 8 32705 ± 11947
MgSO4 0.009 1052 ± 2 475 ± 79 18 ± 5 13181 ± 3741
0.017 1055 ± 3 427 ± 127 19 ± 10 12806 ± 6307
0.035 1059 ± 6 402 ± 250 19 ± 22 12175 ± 12768