The applications of infrared laser pulses to surface vibrational spectroscopy

/

AA PT PA LL E IY DSS C I A:GENERAL

ELSEVIER

Applied Catalysis A: General 160 (1997) 153-168

The applications of infrared laser pulses to surface vibrational spectroscopy K. Domen*, C. Hirose Research Laborato~ of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan

Abstract

Several applications of infrared laser pulses to the study of the kinetics and dynamics of surface chemical reactions are briefly summarized. Specific emphasis is placed on results obtained by the authors' group using methods made possible by recent progress in the generation of intense and frequency-tunable infrared laser pulses. This progress has enabled us to carry out vibrational spectroscopic measurements of the species adsorbed on insulator materials as well as on metal surfaces, providing us with a unique and powerful tool for the study of surface reactions. Keywords: Sum frequency generation; Laser surface; Vibrational spectroscopy

1. Introduction

One of the most important characteristics of surface chemical reactions is their catalytic nature driven thermally at relatively low temperatures. The selectivity of such catalytic reactions is often widely different on each specific surface and is highly controlled in practical catalysts. In order to understand the details of the kinetics and dynamics of the reactions, it is essential to probe the behavior of the chemical species on the surfaces which are incorporated in the reaction. The applications of the laser spectroscopy for this purpose are still limited at the moment, but the potential capability of the method and the recent advance of the laser technique manifests its promising future. Various techniques of laser spectroscopy have been extensively and successfully applied to the investiga*Corresponding author. 0926-860X/97/$17.00 ~'~ 1997 Elsevier Science B.V. All rights reserved. P I I S0926-860X(97)001 33-6

tion of chemical reactions in gas or liquid (condensed) phases, but the application to the reactions on solid surfaces has been rather limited. One of the major reasons for this has been the difficulty to probe the adsorbed species with enough sensitivity. In the past applications to the surface, laser pulses were most commonly used as excitation sources utilizing their intensity as well as their short time duration. One field to which laser spectroscopy has been successfully applied to surface reaction is the study of surface photochemistry. The adsorbed molecules are excited by laser pulses and the internal energy distribution as well as translation energy of desorbing species are analyzed by spectroscopy or by mass analyzer. Several extensive reviews on these subjects are available [1-4]. We focus here on recent advances in vibrational spectroscopies of surfaces species using frequencytunable infrared laser pulses. Several examples will be presented with an emphasis on work conducted by the

154

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

authors' group. Textbooks and reviews are available for the extensive understanding of the basic principles and the scope of the art [5-10]. The vibrational spectroscopies applied to the surface species are divided into two classes: sum frequency generation (SFG) which is a second order nonlinear spectroscopy and linear absorption spectroscopy. The former is used to study species on optically flat surfaces such as fused quartz and single crystal surfaces of metals, semiconductors and insulators. The latter method is applicable to the adsorbates on flat metal surfaces when used in reflection-absorption mode and to various powder samples when used in transmission mode in essentially the same way as conventional infrared spectroscopy except for the use of extremely short (duration time of l0 12_ l0 -s s) pulses. Frequency-tunable infrared sources with such duration time enable us to investigate the molecular kinetics and dynamics of reactions on various kinds of surfaces with basic and practical interest for the understanding of catalysis.

2. Generation of frequency-tunable infrared laser pulses Relatively recent progress in the laser technique has made intense, frequency-tunable laser pulses available to researchers in catalysis and surface science for application in the molecular spectroscopy of surface species. As the generation of tunable infrared laser pulses is essential to the spectroscopies described in this article, various methods of generation reported so far are first summarized. The tuning range of the

available infrared pulses covers almost all of the mid-infrared region, namely, from 500 to 4000 cm -j (20-2.5 ~tm) [11-19]. Typical examples along with the tuning ranges of each method are summarized in Table 1. Infrared pulses between ca. 1000 and 4000 cm-l are now commercially available. The layouts of the home-made systems we have been using for the generation of picosecond (30 ps) pulses are shown in Figs. 1 and 2. The system depicted in Fig. 1 is rather simple and has been used by many groups although its operation is restricted to the wavelength shorter than 4l,tm. Tunable infrared pulses are generated from the fundamental (1.06 ~tm) output of a mode-locked Nd : YAG laser by optical parametric generation and amplification (OPG/OPA) in LiNbO3 crystal pair with the tuning range lying between 2550 and 4000 cm -1 and with a fluence of ca. 200 ~J/pulse. The system shown in Fig. 2 is used to generate the pulses of longer wavelengths, namely from 7.7 to 2.5 ~m, or equivalently, from 1300 to 4000 cm -1. The final output is obtained by difference frequency generation (DFG) in a AgGaS2 (AGS) crystal which is driven by the 1.06~tm beam and by the frequency-tuned nearinfrared beam. The latter beam is generated by the OPG/OPA in BaB204 (BBO) crystal pair, where the SHG output (532 nm beam) of the Nd : YAG laser is used as the excitation source, and the generated near-infrared pulses are frequency-tuned using a grating which selects the wavelength of the return beam going back through the BBO crystals for OPA. A fluence of 80 ~tJ/pulse with the frequency width of 3.3 cm 1 is typically available at 2000 cm -1 using this system.

Table 1 Various systems for tunable infrared pulse generation Systema

Crystal

Range Om)

Duration (ps)

Reference

OPO OPO OPO SRS SRS DFG OPO DFG DFG

LiB305 (LBO) LiNbO3 KTiOPO4 (KTP) H2 (gas) Cs vapor LilO3 Ag3AsS3 AgGaS2 GaSe

0.4-2.4 1.4-4.0 0.6--4.3 2.8-5.4 4.4-5.6 2.5-5.6 1.2-8 1.2-10 4-20

12 6 30 7000 100 18 8 8 0.7

[11] [ 12] [ 13] [14] [15] [ 16] [ 17] [ 18 ] [ 19]

a

OPO: optical parametric oscillation, SRS: stimulated Raman scattering, DFG: difference frequency generation.

K. Domen, C. Hirose/Applied Catalysis A: General 160 (I997) 153-168

i

LiNbO3

•

155

Filters

Nd:YAG laser

I I

1064 nm 35 ps 10 Hz

"-

infrared 2550~4000 cm-1

Fig. 1. Schematic diagram of the picosecond IR source (2550~4000 cm t) using LiNbO~ crystals.

1064 nm 35 ps 10 Hz

K*DP 532 nm

mode-lock I z~ Nd:YAG laser I 1

%

.J

}

OPG/OPA ~J~ / "

1064 nm h

532nm

"

BBO V UI

"~

c=:~

I

i Gr +JL+..~ 4,`'`4

•

~

..". . . . . . . . 5 # ' ~ ............

BBO 1~=:3i '~,'

" ~,0"

:

i! +",~

AGS Filtersi '

~II+

>

infrared 1300~4000 cm 1 Fig. 2. Schematic diagram of the picosecond IR source (1300~4000 cm L) using BaB,O4 and AgGaS2 crystals.

A single free electron laser capable of producing pulses with the tuning range covering the whole region of the mid- and far-infrared has been already applied successfully to SFG experiments [20,21]. Although this may be the ultimate device it is not readily affordable by most researchers.

3. Vibrational sum frequency generation spectroscopy 3. I. Basic principles and characteristics The unique feature of infrared-visible SFG spectroscopy lies in the fact that it enables us to perform vibrational spectroscopy on chemical species located specifically at surfaces and interfaces [6,7]. As mentioned before, the method is applicable to a variety of surfaces including insulator surfaces [22]. Information on the orientation of the surface species can be obtained from the analysis of polarization characteristics of the SFG signal [23-25]. The orientation of the

terminal methyl groups of molecules extended at glass and water surfaces [26,27] and at methanol/air interface [28,29], and that of the surface CH bonds at Hterminated diamond surface [30] have been determined. A conceptual sketch of SFG is shown in Fig. 3. The surface to be probed is irradiated by a visible and an infrared beam simultaneously and the intensity of the sum frequency beam generated in the direction determined by Fresnel's conditions is monitored as a function of the frequency of the infrared beam• The process takes place only at surface layer when the substrate is isotropic. The phase matching condition, which is an important factor in nonlinear optical processes in bulk media, is not relevant since the depth of the layer is negligible with respect to the wavelengths of associated light beams• The intensity of SFG signal, ISFG, is proportional to the square of SFG polarization, PSFG, induced by the incident visible and infrared light [ 11 ], ISFG v~ IPsvGIz, (2j PSFG = •SFG : Evis : E1R,

(1)

156

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

IR

IR

vis ~

SFG

vis

surface

Fxg. 3. Sum-frequency generation by infrared and visible light.

(2) where XSFG is the macroscopic SFG tensor of the probed surface, and Evis and EIR are the electric field . (2) vectors of visible and IR beams, respectively. /Csm, which vanishes in isotropic media, consists of two (2) terms, Z(R2) and XNR ~{23

. (2/ + l(~),

(3)

SFG = •NR

where, the vibration-resonant term ~ ) gets enhancement when the frequency of infrared beam, wiR, coincides with the transition frequency w~ of a vibrational mode of surface species. The nonresonant term ~(2) NR on}the other hand, stays independent of WtR. X~R and ~ are expressed as: ~¢/2/ NR = c(cos ~ + i sin ~) = cexp(i~), Z(R2) : Z

v[b(v2)/(~v - 6OIR -]- iFv)].

(4a) (4b)

In Eqs. (4a) and (4b), c is a constant factor, ~p denotes the relative phase angle of ,.,(z) with respect ~NR to I ~ / b ( 2 ) is the term which contains the product of the vibrational dipole moment and the Raman scattering tensor, and Fv is the homo~:eneous width of the vibrational band v. The sum in Eq. (4b) extends over the vibration bands of all molecules contributing to the SFG signal. The signal intensity ISFG is thus expressed as: (2) 12 ISFG o( ~SFG : {CCOS ~ q- ~

vb(v2)(tOv -,.'lR)/[(COv -- ~,R) 2

+ v2]} 2 + {c sin ~0 + ~ v b [ : ' V v / [ ( W v + r~_,]} 2.

- ~,R) 2

(5)

~(2) NR which originates presumably from the surface electronic resonance shows up in a rather unpredict-

able fashion. Typically, the SFG signals from single crystalline MgO (1 0 0) surface gave rise to a significant contribution to ~(~ after the adsorption/ desorption cycle of formic acid was repeated, and the interference effect between the nonresonant and resonant nonlinear susceptibilities had to be taken into account in the analysis of spectral profile to derive the molecular orientation [24,25,31]. The formulas for the derivation of surface orientation of adsorbates from their SFG spectra is also included. The orientation analysis starts by referring to the transformation coefficients relating the tensor components in molecule-fixed coordinate system to those in a laboratory-fixed coordinate system. The general expressions in terms of Euler's angle have been tabulated in Ref. [24] and the matters to be specified in the analysis have been summarized in Ref. [25]. In the case of vibrational resonance by the stretching vibration of diatomic species, three components of molecular SFG tensor flSF in moleculefixed axis system, namely, ~s~c, fls~,., and flSF are nonvanishing when the axis of C a rotation symmetry is taken as the c axis of the molecule-fixed (abc) system. They are related to the Raman tensor a¢ and to the vibrational transition vector /~ by flSFac= flS~c=C%a#c and fls~.=CO~cc#cwhere c is a constant. The orientation of a molecule on the surface is specified by the tilt angle 0 and by the azimuth angle 0, the angle between the molecular c axis and the surface normal (taken as the z axis of surface-fixed coordinate system) and the angle of the surface projection of the c axis with respect to the surface-fixed x axis, respectively. The following expressions of the tensor components in surface-fixed system are taken from Table 2 of Ref. [25]:

157

K. Domen, C. H i r o s e / A p p l i e d Catalysis A: General 160 (1997) 1 5 3 - 1 6 8

[sps] combination

Sites

Edge (defects)

Terrace

Amount SFG back ground Adsorption Formate type C - H stretch Decomposition

< 10% Yes Rapid Bridge or bidentate 2850~2870 cm -I ~700 K

>90% No Slow Unidentate 2920 cm i ~570 K

/3vrv. = - (~) 1 (/#,.cc -/3..~) sin30(cos k - cos 3 ~ ),

(lOa) flycy ~- ( l ) ( / 3 c c c - - /3aac)( cOS O -- COS30)(1 -

[pps] combination

/3xx., = -(J)(~c,-c - /3aac)Sin30( 3c°s X + COS3~)

/3.rzv. z

~..,. sin 0 cos X,

/3_~, = ~,~r = ( ½ ) ( / 3 ~ . - / 3 . . , . ) ( c o s

sin ~ + sin 3 \ )

+ 3..,. sin 0 cos y,

[ppp] combination

-

cos2x), (10b)

3x~, = (1)(3,','~ - / 3 ~ ) s i n 3 0 (

-

(9c)

/3~,~ = (/3,-cc --/3~.c)(sin 0 -- sin30)sin X,

Table 2 Adsorption and decompositiion of formic acid on MgO (1 0 0)

~I-~Lr)" = _ _ ( l ) ( ~ c c c

( 11 a) __ / 3 . a c ) ( C O S 0 - -

cos30)sin 2X, (lib)

(6a)

3=y = (3c,-c -/3..c) (sin 0 - sin 30) sin ~ +/3..,. sin 0sin k.

0 - cos30)

× (1 + c o s 2 x ) ,

(llc)

(6b) [pss] combination

/3::.~ = -(t4,.,.,. - / 3 . . , . ) ( s i n 0 - sin30)cos X

-/'4..,. sin 0 cos X,

(6c)

/3x>,,. =

- (¼) ( /3.v -/3.ac)sin30( c°s \ - cos 3~(), / 12a)

/3xx: = (½)(/~,.~c -/3..c)(COS 0 - cos30)(1 + cos 2X) + J..,. cos O,

(6d)

/3~y = ( 1 ) ( / 3 c c , . -

' ~ a . c ) ( C O S O -- COS30)(1 --

cos2k), (12b)

l~-.~: =/3~= = - (/3~,.c -/3aoc) (sin 0 -- sin30)cos X, 3::: = ( / ~ - - / 3 . . , ) c°s30 +/3aa~ COS 0,

(6e)

[sss] combination

(6f)

/3yyy = (1)(/3ccc -- /3aac)Sin30( 3sin X -- sin 3X)

+/3aac sin 0 sin ~(.

(13)

[spp] combination

/3,,x~ = (¼)(/3~cc - / 3 . . c ) si n 3 0 (si n X + sin3x),

(7a)

/3y:x = b,.~: = - (½)(/3e,.~. - 3..~)(cos 0 - cos 3O)sin 2X, (7b)

3,,:: = (/3,.,.,. - / 3 . . , . ) (sin 0 - sin30)sin X,

(7c)

[ssp] combination /3y~,.~= -(¼)(/3,.c~ - / 3 . . c ) s i n 3 0 ( c°s X - cos 3X) -

13..~ sin 0cos ~,

~8a)

/3yy: = (½)(/3cc,' - - ~ a . c ) ( C O S 0 -- COS30)(1 -- COS

+ 3..~ cos O,

2X) (8b)

[psp] combination

3,-yx = (¼)(/3,- : - / 3 . ~,' )si n 3 0 (si n x

+ sin 3X),

(9a)

3,-,,: = fl~yx = -(½)(/3~,.,. - / 3 . a c ) ( c o s 0 - cos30)sin 2X, (9b)

In Eq. (6a)-Eq. (13) the superscript SF is dropped for the sake of brevity and the tensor components are classified according to the combination of polarization directions of the sum-frequency, visible, and infrared beams with the polarization combinations being denoted by bracketed letters written in the order stated above. When the molecules are oriented isotropically on the surface, that is, either randomly in 3< or in an equivalent of C,, (n>3) rotational symmetry, the terms containing X average out to zero and only components in [ppp], [ssp], [sps], and [pss] combinations remain. If the number of molecules with an azimuth angle equals that of the molecules oriented by -- k. the terms containing sin(my) vanish and only the components for the above-cited polarization combinations remain. As for the values of/3~.c and/3 ...... the ratio ~ a a J 3 c c c is introduced as a parameter, that is, r= /3~c/ [4,...= a../a~,., and r for a diatomic molecule is related to the the Raman depolarization ratio p by

158

K. Domen, C. Hirose /Applied Catalysis A: General 160 (1997) 153-168

p=(3/4)/[1+1.25 (l+2r)Z/(l-r)2]. It is possible to estimate r from Raman spectrum when the nature of the vibration band under investigation remains similar to that of the band in the free molecule. We have used procedure in the analysis of the SFG spectrum of formate/MgO (1 0 0) system. Knowledge of the susceptibility tensor alone is not enough for accurate estimation of molecular orientation even for surfaces made of uniformly oriented molecules as we need the coefficients relating the source polarization to the electric field of SFG beam and the Fresnel coefficients relating the source polarization to the product of the surface electric fields of the visible and the infrared beams.

ethylidyne H~H I

C

d ct2~

/ i

3.2. Applications 3.2.1. C2H4 on Rh (1 1 1) and Pt (1 1 1) The observation of the SFG spectra of ethylidyne, which was formed on the Rh (1 1 1) surface by the coadsorption of C2H4 and NO, was the first example in which the technique was used for the investigation of solid surfaces relevant to catalysis [32]. The work was carried out by the group which pioneered vibrational SFG spectroscopy. The ex situ measurement was carried out in an N2 atmosphere at 300 K and the spectral region where the CH stretching bands are located was measured. The peak due to the CH stretching mode of ethylidyne (2863 cm -~) as well as other unassignable peaks were observed with a rather high nonresonant background. Recently, the behavior of C2H 4 on the Pt (1 1 1) surface was investigated by SFG to reveal that, for the substrate temperature lying between 202 and 243 K, the peak by the CH2 symmetric stretching mode of di-~r bonded ethylene was clearly present at 2906 cm - l but its intensity decreased as the temperature exceeded 243 K [33]. A peak due to the CH stretching band of an intermediate species was observed at about 2957 cm -1 at substrate temperatures between 257 and 296 K, that is, before the ultimate peak of ethylidyne appeared at 2889 cm 1. The peak was ascribed to ethylidene and/or ethyl species. More interestingly, the same authors measured the SFG spectra of the Pt (1 1 1) surface kept under the condition of catalytic reaction with 110 Torr of H2 and 35 Torr of C2H 4 and at the temperature of 295 K (Fig. 4) [34]. The vibrational peak of the 7r-bonded ethylene was observed in addi-

2850

I I -

di-o-ethylene H H H.~_Cr,,H j

!

~-ethylene

#.,c=c#

•

1

I

2900 2950 3000 w a v e n u m b e r / cm-1

3050

Fig. 4. SFG spectrum of C2H4 and hydrogen on Pt (1 1 1) at 110 Torr H2 and 35 Tort C2H4at 295 K [34].

tion to that of ethylidyne. The band of the di-cr bonded ethylene was also observed under the reaction condition but the signal intensity was inversely correlated with that of the ethylidyne peak. On the other hand, the peak of the 7r-bonded ethylene was always present when the surface was exposed to ethylene gas indicating that the ~r-bonded ethylene is the reaction intermediate involved in the catalytic hydrogenation of ethylene. The peak intensity of the 7r-bonded ethylene was weak because of the so-called surface selection rule for metal surfaces. As stated in the previous section, the SFG tensor is proportional to the product of the vibrational dipole moment and the Raman tensor, and the electric field of an infrared light beam at a metal surface is close to being perpendicular to the surface. Thus, as in infrared reflection absorption spectroscopy (IRAS), only the vibrational modes having nonzero vertical-to-surface component of their dynamic dipoles are capable of interacting with the infrared beam to become detectable by SFG. Successful observation of adsorbed species on the Pt (1 1 1) single crystal surface under catalytic reaction condition attests the potential value of the technique. One may wonder, however, if the quantitative investigation of the behavior of the 7r-bonded ethylene must be strongly influenced by the observation of the signals due to the vibrational bands having larger vertical

K. Domen, C. Hirose /Applied Catalysis A: General 160 (1997) 153-168

~-C2H 4 H~

di-o-C2H 4

c~H

"~

H

-=-

H I

3090

~-C2H 4 J~"

H~

,.,.H

~,

2908 954

AR/R I =0.02 % t_

I

3100

~

AR/R [ =0,2 % I

~

3000

I

~

2900

I

,.

1400

1.0x10 -5 Torr

I

,

1200

I

1000

wavenumber / cm -1

Fig. 5, IRAS spectra of 7r-adsorbed C2H4 on Pt(1 1 1) surface at various equilibrium pressures at 112 K [35,36].

4

(a): SFG

component of the dynamic dipole moments and bands such as the out of plane CH bending mode should meet this requirement. Actually, the generation of infrared beams with an output power large enough for SFG at the wavelengths of such vibrational bands is not a simple matter, although the techniques have been developed as listed in Table 1. In this respect, IRAS serves as a complementary technique as seen from Fig. 5 which shows the IRA spectrum of the 7r-bonded ethylene on Pt (1 1 1) as observed under 10 6 Tort of C2H4 at 112 K [35,36]. We have seen that both SFG and IRA spectroscopies serve as powerful tools for studying the vibrational spectra of adsorbed molecules on metal surfaces, but the band-to-band sensitivities can be very different as the optical processes leading to the signals differ from each other. As an typical example, the SFG and IRA spectra of a multi-layer of HCOOH on the Pt (1 1 0) surface are compared in Fig. 6 [37]. In the IRA spectrum, the observed CH stretching

C-H stretch

~d --.- 3 ~"

O-H stretch

.E_ 2 (.9

combination

1

n

0

• (b): IRAS

1.5

=

O k~

1.0

°

\ o,5

0

3200

I

!

30'o0

28'00

|

'1

2600

•

2400

wavenumber / cm-1 F i g . 6. S F G a n d I R A S s p e c t r a o f m u l t i - l a y e r o f H C O O H

159

o n P t (1 1 0)-( 1 x 2 ) s u r f a c e at 158 K and 1801 (1 1 = 1 T o r t s) [37].

160

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

band was much weaker than the hydrogen-bonded OH stretching band. The SFG spectrum, on the other hand, gave much stronger peaks for the CH stretching band due presumably to the larger contribution of the Raman tensor. It should be thus noted that there are cases when the SFG signal is more distinct than the IRAS signal, or vice versa, and the comparison of the two spectra enables us to obtain a finer insight of the structure and orientation of surface species. 3.2.2. HCOOH on MGO (1 0 O) [38-40] One of the great advantages of vibrational SFG spectroscopy was the application to the investigation of chemical processes taking place on the surfaces of single crystalline oxides, a task which is usually difficult to achieve by other techniques such as IRAS and HREELS (high resolution electron energy loss spectroscopy) because of the nonconducting nature of such surfaces. We describe below the results of our SFG and TPD (temperature programmed desorption) observations of the adsorption and decomposition reaction of formic acid HCOOH on a MgO (1 0 0) surface [38]. MgO is known to be a very active catalyst for certain base-catalyzed reactions, and the catalytic activity is believed to be associated with coordinatively unsaturated adsorption sites on the surface [39,40]. Adsorption and reaction of formic acid on the single crystalline surface have been extensively investigated, and it has been shown that formic acid dissociates heterolytically to formate species on the surface and that the formate on the surface under ultra high vacuum conditions decomposes at elevated temperature exclusively to water and carbon monoxide. The predominance of the dehydration reaction on the surface under UHV conditions has been postulated [39,40]. However, our TPD observation revealed the occurrence of the desorption of carbon dioxide on the aged surface, i. e., after repeated cycles of adsorption/ thermal desorption cycles, although the surface retained a well-defined pattern in the LEED image. The detection of carbon dioxide (the product of dehydrogenation reaction), in addition to that of CO (the product of dehydration reaction) indicated that the adsorption/reaction sites changed during the experiment. The feature introduced by the aging of the surface was thus investigated by combining the TPD and SFG observations.

MgO(100) crystal ~

a

a film

wire

IR Vis

Fig. 7. A sketch of the MgO ( 1 0 0) sample used for the SFG and TPD measurements.

Fig. 7 is a sketch of the MgO (1 0 0) sample used for the SFG and TPD measurements. The in-air cleaved sample surface was polished fiat using diamond pastes and Ta metal was sputter-deposited on the back while leaving the central part of 5 mm diameter void of Ta. The sample was electrically heated to 1200 K by passing a current through Ta wires spotwelded to the metal coating at the sides of sample piece. Examination by AFM indicated that the fresh sample had a flat (1 0 0) surface with the average surface roughness of a few ,~ in height. The roughness was enhanced as the cycles of adsorption and thermal decomposition of formic acid were repeated. TPD measurement revealed that the formic acid adsorbed on the MgO (1 0 0) surface at room temperature to form surface formate which decomposed to CO+H20 at 560 K and to CO+H20 and CO2+H2 at 700 K. This desorption of H20 was also observed to start at the temperature lower than 560 K, the feature was not seen in the past [39], indicating that the surface oxygen atoms of the studied MgO (l 0 0) frame were involved in the low temperature desorption of water. The TPD spectra of CO from the formate-covered surface with various degree of coverage are shown in Fig. 8. The coverage after the dosing of the HCOOH/ Ar mixture (6 vol%, 1 × 10 -6 Torr) for 240 s was about 10% of the total number of surface Mg atoms. It was shown that the sticking probability was higher at the initial stage of adsorption and relatively large part of

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

161

(a)

="

/I

exposure t,me

\\

, 240

8

~J/~,~.-,.._....,,~

~._-"--

120 15 ° ~

I

300

I

I

I

500 700 temperature / K

I

I

I

900

Fig. 8. TPD spectra of CO from HCOOH-adsorbed MgO (1 0 0) surface at various exposure times. The exposure to formic acid was carried out at 300 K under the atmosphere of 1 x 10 6 Torr of 6 v/v% HCOOH/Ar mixture.

ffl

r-

(c)

c-

U_

(d) adsorbed species decomposed at 700 K when the coverage was rather low and that the increase of the coverage led to the growth of the desorption peak at 560 K. The SFG spectra of the 120 s-dosed surface are shown in Fig. 9. The measurement was made for the [ppp] and [ssp] polarization combinations giving the results shown in Fig. 9(a) and (b), respectively. For the sake of comparison, we show in Fig. 9(c) and (d) the SFG signals from a chemisorbed film of n-alkyltrichlorosilane on quartz for the [ppp] and [ssp] polarization combinations, respectively, and the main peaks in Fig. 9(c) and (d) are the symmetric and the degenerate CH stretching modes of terminal methyl group. One may notice that the S/N ratios of the signals from MgO surface are considerably lower than that obtained from the fully covered chemisorbed film. As mentioned earlier, the coverage of formate on the MgO surface was about 10% and the poor S/N ratio must be due to the low coverage since the intensity of the vibrational SFG signal is proportional to the square of the density of adsorbate. Actually, the results indicate that the intensity of SFG signal can be used to monitor the coverage of surface species having the coverage of less than 10%.

I

2700

I

I

I

|

I

I

2900 3100 wavenumber / cm -1

Fig. 9. Comparison of the SFG spectra of formate on MgO (1 0 0) surface at 300 K and exposure for 120 s (aJ and (b) in vacuum and chemisorbed n-alkyltrichlorosilane films (C1,) on quartz plate in air (c) and (d) [38]. The traces (a) and (c) were measured using the p-polarized visible beam while traces (b) and (d) using s-polarized visible beam. The infrared beam was p-polarized in all cases.

The profiles of the vibrational peaks in Fig. 9(a) and (b) are broad and asymmetric suggesting that the band is overlapped by several vibrational bands and thus indicating the presence of several surface species. The SFG spectra observed on the surface after various dose times are compared in Fig. 10. It is seen that the increase of the signal intensities with dose time does not conform with the squares of the coverage esti-

162

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

(a)

240 s

(b)

24O s

120 s

120 s

30 s

30 s

15s

15s

5 d ~9 r-" t-"

II

co

,

2700

2900 w a v e n u m b e r / cm -1

31 O0

2700

I

J

I

,

I

,

2900 3100 w a v e n u m b e r / cm -1

Fig. 10. Exposure dependence of SFG spectra of formate on MgO (1 0 0) surface at 300 K. The spectra (a) and (b) were measured by p- and s-polarized visible beams, respectively.

mated from the TPD signal shown in Fig. 8. We repeated the measurements several times and found that the peaks at 2850 and 2870 c m - ] were saturated at an early stage of the dosing while the 2920 cm -] peak continued to grow gradually. The analysis was only semiquantitative, however, because of the weak signal intensity. It should be pointed out that the band

at 2910 cm - t showed up only when the s-polarized visible beam was used. The features described above as a whole suggested that several adsorption sites existed on the surface and that the number of such sites changed as the adsorption/desorption cycles were repeated. In this respect, we also noted that the SFG spectra observed for the

K, Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

(a)

[63

(b)

>.,

0

(D 2850 2870

LO

2870 2850 / 2920

LL

co 0

0 I

2700

I

I

I

I

I

I

2900

I

3100 2700

wavenumber / cm -1

I

I

t

t

I

i

2900

3100

wavenumber / c m -1

Fig. I 1. Calculated and observed SFG signals of HCOOH/MgO (0 0 l) systems obtained by the exposure for 120 s. The spectra (a) and (b) were measured by p- and s-polarized visible beams, respectively. Open circles and solid lines indicate the results of the SFG observation and theoretical calculations. The deconvoluted bands are shown by the bottom traces.

s-polarized visible beam were accompanied by a vibrationally nonresonant and coverage-dependent background signal as seen in Fig. 10(b), while no such background signal was present on the spectra observed for the p-polarized beam. When the observed spectra were deconvoluted using Eq. (5) and three resonance peaks were obtained at 2850, 2870, and 2920 cm J as shown in Fig. 11. The solid curves (2 2 overlayed represent ZsFc while the curves shown in the lower halves are the values of IZ~l 2 and of the three bands. One should be reminded that the SFG is a coherent process and that the cross-terms of the BG and resonant terms in the absolute squares of the sum affect the spectral feature considerably as demonstrated is by the unequal height of the signal level at either side (the side of constructive interference at lower frequency and the side of destructive interference at higher frequency of the peak). The frequencies are lower than the value for the CH stretch band of formic acid but are close to the frequencies of formate species. The formates on solid surface are postulated to take one of the three forms, namely: bridging; bidentate; or unidentate forms (see Fig. 12). The correlation between the types of adsorption and the vibrational frequencies, observed by HREELS or IR, for the formate on oxides and semiconductor

[Z~2112

surfaces was examined. The values of 2870 and 2850cm -1 are quite close to 2841 cm -l (the frequency of the CH stretch band of formate ion) and also to the value, 2866 cm ~, of the band observed on MgO powder and were ascribed to a bridging-type formate, but they depart considerably from the values of the formates with unidentate or bidentate types. We thus assigned the two bands to the CH stretch bands of bridge-type formates adsorbed on two different sites of MgO (1 0 0) surface. The difference in the dosetime dependent features of the two vibrational peaks suggested that the adsorption sites, particularly the one which led to the appearance of the 2870 cm - t peak, appeared during the repeated cycles of adsorptiondesorption procedure. The origin of the third band at 2920 cm-t was ascribed to the formate with different type of adsorption as judged from the frequency. The deconvolution revealed that the values o f b~2t (see Eq. (4b)) for the 2870 and 2850 cm ~ bands and thus the number of the species giving the two bands increased at initial stage of adsorption and were saturated rather quickly but the behavior of the value for the 2920 c m - t band differed from them as it kept increasing without indication of saturation. Considering the fact that the 2920 cm-l band was observed only when the visible beam was s-polarized,

164

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) 153-168

,~ 0 ~"

I

"0

bridge

~

0 r'" ""0

.__~.

O

bidentate

C~H

0

unidentate

Fig. 12. Conceptual sketch of the structural models of adsorbed formate on solid surfaces.

gas input

after annealing at 900 K

5

TPD start (300 K) |/

.~_

, ,,."

ID,

time Fig. 13. Changes due to surface condition of vibrational non-resonant SFG signals. The change of ~vib was fixed at 2960 cm 1 and the modification of the surface proceeded from left to right.

we assign it to the CH stretch band of another type of formate. The polarization analysis of the SFG signals indicated that the CH bond of this species was tilted by more than 30 ° from surface normal. We thus infer that the 2920 cm -1 band originated from the unidentate formate while the 2870 and 2850 cm -1 bands originated from bridge type formate as stated above. The deconvolution revealed that the vibrationally (2) nonresonant term INR is significant on the SFG signals with the s-polarized visible beam (Fig. l l(b)), its magnitude was larger on the surface with less coverage, and was magnified as the experiment was repeated. The increase with the repetition of the adsorption/desorption process was indicative of the correlation of its origin with the surface condition. The

BG signal also changed upon adsorption and upon thermal desorption of formic acid as shown in Fig. 13. In Fig. 13 the BG signal at WIR=2960 cm -1, just outside of vibrational resonant region, was monitored following the sequence indicated by the arrow. Firstly, the surface treated by oxygen at 1200 K was exposed to formic acid and the signal intensity decreased sharply. Secondly, the gas phase was evacuated and no change occurred on the SFG signal. Thirdly, the sample temperature was raised gradually to 650 K and a gradual increase of SFG signal was observed. Finally, further heating to 900 K led to the recovery of the SFG signal to its initial intensity. The TPD experiment revealed that the desorption of surface formate causes roughening of the surface. Since the

K. Domen, C. Hirose/Applied Catalysis A: General 160 (1997) i.~3-168

Mg atoms located at the resultant defect sites should have the coordination number smaller than five, the value for the atoms on the stable terrace sites of the MgO (1 0 0) surface, they should be more active for adsorption as discussed in previous publications. The BG signal observed on bare surface may be associated with such detect sites and the signal which remained after the adsorption then originated from other adsorption-inactive defect sites too. When judged from the integrated areas of the TPD signal, the amount of desorbed CO, which should be proportional to the amount of adsorbed formate, did not saturate even after dosing 500 s. The feature closely resembles to the one seen on the SFG signals of the 2920 c m - l band, which we assigned to the unidentate formates adsorbed at terrace sites, while the dominant SFG bands at 2870 and 2850 c m - l were saturated at an early stage of dosing. The latter two bands have been assigned to the formates with bridgetype adsorption. These results are summarized in Table 2.

4. Relaxation dynamics investigated by pumpprobe method Another field in catalysis and surface science which has flourished due to the development of high-powered and frequency-tunable infrared pulse source is the real-time measurement of the dynamics of energy relaxation and/or chemical reactions at surfaces. Laser spectroscopy using short-time pulses has a time resolution of the same order as the duration time of pulses used. In so-called pump-probe spectroscopy, the temporal behavior of molecular species is directly probed by varying the delay time between the pump and probe pulses. The population distribution is forced to change by the pump pulse and the time evolution of the transient change is monitored by the probe pulse which arrives at the spot with a prescribed delay time. So far, this is the only method which allows us to probe the relaxation of vibrational energy on a solid surface on a real time basis [41-54]. However, rapid advances in the development of short pulsed laser sources are such that the infrared pulses which have a duration time as short as 10 ~3 s will become available in near future and advances in spectroscopic techniques to cope with such a high time resolution are going to

165

provide us with the means to obtain first hand information concerning the unstable reaction intermediates which have not been experimentally identified but only postulated in the past. Such information is crucial for our full understanding of the mechanisms and kinetics of surface reactions. In this section, we intend to introduce the reader to the power of the pump-probe method by describing our recent studies of the relaxation dynamics of vibrational energy of hydroxyl groups in zeolites [47-55]. The SFG spectroscopy described in the previous section has been also used to probe, in pump-probe mode, the vibrational energy transfer from the molecules adsorbed on single crystal surfaces to the substrates [42,44-461. The measurements so far have been restricted to the surfaces of semiconductors where the role of electron-hole pair are of prime importance in the energy relaxation and/ or transfer, and the extension to insulator surfaces, which is hampered by low S/N ratio of tile signal, is greatly anticipated as it will most certainly provide a new breed of information. The experiment was performed on a zeolite surface using the same picosecond infrared laser described above. The v = l ~ 0 relaxation processes of the OH stretching vibration of Bronsted acidic hydroxyl groups in zeolites have been studied by picosecond tunable infrared pulses to confirm that the vibrational energy relaxes into the substrate modes with the population lifetime (F~) of about 100 ps 141,47-54]. The lifetimes, or equivalently the rates of energy transfer, were sensitive to the interaction of the hydroxyl groups with other molecules: the T~ lifetime was decreased and the rate was increased significantly by the formation of the hydrogen bond. Fig. 14 shows an example of the result obtained by the picosecond pump-probe setup where the linear and transient absorption spectra, observed in the region of the OD stretching mode, of a deuterated mordenite are displayed. Shown in Fig. 14(a) is a linear absorption spectrum measured by using the probe pulses alone without the irradiation of pump pulses. Fig. 14(b) is the pump-induced spectral changes represented as a difference spectrum, Ap(~)-Ao(~'), where Ap(,~,) and Ao(-O are the absorbance spectra obtained with and without the simultaneous irradiation of the pump pulses fixed at 2670 cm ~, respectively. The negative peak at 2670 cm ~ is the bleaching signal which resulted from the tr:msient decrease of population

166

K. Domen, C. Hirose/Applied Catalysis A." General 160 (1997) 153-168

(a)

0.018 1

/(3 x / 0 \

D I

I

1.0 0 rt~ J~

~

0 ..Q

0.024 i

(.o,?

2670 cm q

0.012-

vs,._. S,

0.006 -

0.5

o

69

0-0.006 -

0.04

-0.012-0.018-

~" 0.02

-0.024 -

0 t~ c-

-0.026 -

0 m

0

2550

<

cm 1

I

I

I

I

2600

2650

2700

2750

2800

w a v e n u m b e r / cm -1

-0.02

Fig. 15. Transient transmission spectrum of OD of water (D20) adsorbed on zeolite (DM-20) at 123 K for zero delay time between the pump and probe pulses.

-0.04 2550

Cross Correlation of ~.5 Pump and Probe Pulse v j ~

2600 2650 wavenumber / cm -1

2700

Fig. 14. Linear absorption spectrum of the OD stretching band of deuterated mordenite (DM-20) (a) and the transient spectral change caused by the simultaneous irradiation of pumping pulses (b).

difference between v = l and v=0 levels, and the positive peak at 2584 cm -1 is the v = 2 + - I hot band signal made visible as the v= 1 level was populated by the irradiation of pump pulse. Apart from the discussion on the details of the feature, Fig. 14(b) is a clear demonstration that the technique makes it possible to observe the vibrational spectroscopy of the surface species produced transiently on various powder catalysts. We have recently applied this method to a wateradsorbed H-mordenite system in which water molecules primarily adsorb onto BrCnsted acidic hydroxyl groups through hydrogen bonding (see the inset of Fig. 15). The hydrogen-bonded H20 molecule is depicted as having one of the OH bond attached to surface oxygen and the other bond, free of interaction, sticking out into air. The experiment was performed using D20 and D-mordenite instead of H-mordenite

[55]. When the pump pulse was tuned to the peak frequency assigned to the OD stretching band of the free OD bond, an interesting but somewhat puzzling feature arose in the transient spectrum as shown in Fig. 15 (Compare with Fig. 14(b)). An additional absorption peak appeared between the v=2+--1 hot band signal and the v = l , - - 0 bleaching signal. This spectrum seems to show that the vibrational energy transfer occurred within an adsorbed water molecule and, furthermore, that new chemical species was formed as a result of the vibrational excitation. Although further experiments are necessary for the full interpretation of the result, it is emphasized that this type of measurements surely provide us with a new kind of informations on not only the dynamic behavior of the adsorbed species but also on the existence of unstable reaction intermediates which have been postulated but not identified on heterogeneous catalysts.

5. C o n c l u d i n g r e m a r k s

As has been shown in this short article, the application of laser spectroscopy to the surface reaction has

K. Domen, C. Hirose/Applied Catalysis A: General 160 11997) 153-168

begun only recently. Therefore, the examples of the techniques applied to the kinetic study as well as the dynamics of heterogeneous catalysis reactions are still very limited. Recent progress of laser spectroscopy allows us to probe the surface chemical species with high sensitivity and very high time resolution even under catalytic reaction conditions. These characteristics of surface laser spectroscopy will provide valuable information concerning surface reactions which is difficult to access by other techniques.

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