J . Phys. Chem. 1990, 94, 8709-8714
8709
Surface Reaction of Dialkyl Phosphite on Alumina and Magnesia Studied by Inelastic Electron Tunneling Spectroscopy Morihide Higo* and Satsuo Kamata Faculty of Engineering. Kagoshima University, Korimoto, Kagoshima 890. Japan (Receiced: March 22, 1990)
Vibrational spectra of dimethyl, diethyl, and diphenyl phosphite on alumina and magnesia surfaces have been measured by inclastic electron tunneling spectroscopy. These dialkyl phosphites (0,O-dialkyl phosphonates) were adsorbed onto A1203 and MgO surfaces from anhydrous benzene solutions by using a spin doping method. The tunneling spectra of phosphorous acid (phosphonic acid) on A1203have also been measured for comparison. Phosphorous acid reacts with surface OH groups of AI,?, by losing its protons and is adsorbed as phosphite anion (phosphonate dianion) onto the surface. Analysis of the tunneling spectra and comparison of the infrared and Raman spectra of the dialkyl phosphites give information about their iiiolccular structures on A1203and MgO and a surface reaction on them. These dialkyl phosphites decompose by a reaction bith the surfacc OH groups on A1203and MgO to give the adsorbed 0-alkyl phosphonate monoanions and phosphonate dianion. The similarity of the tunneling spectra of dimethyl phosphite and that of phosphorous acid suggests that dimethyl phosphite decompose5 rapidly to give preferentially the adsorbed phosphonate dianion on the surfaces.
SCHEME I
introduction
Inelastic electron tunneling spectroscopy (IETS) has been used for studying ;I wide varicty of compounds on the oxide surfaces of ii metal-oxide-metal tunneling junction.! It reveals the vibrationnl spectrum of the oxide surface and the species adsorbed on i t . The tunneling junction can be considered as a good model system for oxide catalysts or electronic devices. The information on the surfriccs and interfaces can be obtained through the analysis of the tunneling spectrum. The high sensitivity and wide spectral range of IETS cnablc us to learn not only about the molecular structure on thc surface but also about the reaction mechanism on it. Compounds of phosphorus have a variety of molecular structures and chemical proper tie^.^.^ Organophosphorus compounds arc used. for example. as oil additives, adhesives, insecticides, etc. I t is, thcrcfore, important and intcrcsting to know the surfacc chcinical propertics of these compound^.^ However, only a few IETS studies on compounds of phosphorus have been carried W e have reported the tunneling spectra of methylphosphonic and dimethylphosphinic acid8 on A1203 and of diethyl phosphate on A1203 and Mg0.93’0 These organophosphorus acids were found to react with the surface O H groups on A1203 and MpO and to be adsorbed as the anions on the surfaces. Phosphorous acid ( 1 ) and dialkyl phosphites (3) are in tautomeric equilibrium with their pentavalent isomers (Scheme I). However. because of the high stability of the P=O groups, they cxist wholly ;is phosphonic acid (2) and 0,O-dialkyl phosphonates (4). r c s p c ~ t i v c l y . The ~ ~ ~ infrared and Raman spectra of phos-
?H :P-OH bH
e
?H ?R H - 7 ~ 0 , :y-OH
1
OH
OR
2
3
e
PR
H-P=O OR
4
phorous acidI4-l5and its a n i ~ n ’ have ~ - ~ been ~ measured, and their molecular structures were clarified. The infrared and Raman as well as their bands spectra of diniethyl and diethyl assignments have been obtained. Infrared spectra of metal salts of some dialkyl phosphites were measured.20,21 Rates and mechanisms of hydrogen22and oxygen2, exchange and hydroly,iS23-26 of dialkyl phosphites were studied. Adsorption and reaction propertics of dialkyl phosphites with iron surfaces have been examined.27 In our previous papers,9-” we reported the tunneling spectra of phosphorous acid on A1203and dimethyl phosphite on AI,03 and MgO; their molecular structures and the interaction with the surfaces were studied. It was found that dimethyl phosphite decomposes on the oxides and is adsorbed as the anion on the surfaces. In this study, the tunneling spectra of dimethyl, diethyl, and diphcnyl phosphite adsorbed on AI2O3and MgO from benzene solutions have been obtained. The analysis of the tunneling spectra of these dialkyl phosphites and comparison with the tunneling spectrum of phosphorous acid give information about the surface chemical properties of these organophosphorus compounds on A1203 and MgO. Experimental Section
( I ) H m n i a . P. K . Tunneling Specrroscopy: Plenum: New York. 1982. ( 2 ) Bellamy. L. J . The Infrared Spectra of Complex Molecules: Chapman and Hall: 1.ondon. 1975. ( 3 ) Enislcy. J.: Hall. D. The Chemi.rtr.13of Phosphorus; Harper and Row: London. 1976. ( 4 ) Ekerdt. J . G.; Klabunde. K . J.; Shapley. J . R.; White, J . M.: Yates. J . T. J. Phys. Chem. 1988. 92, 6182. ( 5 ) Templeton, M. K.; Weinberg. W. H . J. Am. Chem. Soc. 1985. 107. 97. 774. (6) White. H. W . : Crowder. C.D.: Alldredge. G. P. J . Electrochem. Soc. 1985. 132. 773. ( 7 ) Higo. M.; Owaki, Y.; Kamata. S. Chem. Lett. 1985. 1309. (8) Kamata, S.; Higo. M.; Mizutaru. S.: Owaki. Y . Rep. Asahi Glass Found. Ind. Technol. 1986. 49. 191. ( 9 ) Kamata. S.; Higo. M.: Owaki. Y.: Hayashi. H . Rep. Asahi G1as.s Found. Ind. Technol. 1987. 5 1 . 263. (IO) Higo. M.; Owaki. Y.; Kamata, S. Chem. Lett. 1987. 1567. ( I I ) Higo. M.: Owaki. Y.; Kamata. S. Chem. Lerl. 1987, 2009. ( I 2) Henriksen. P. N.: Gent. A . N.: Ramsier. R. D.; Alexander. J . D. SurJ Inrerface Anal. 1988. 11, 283. ( I 3 ) Ramsier. R. D.: Henriksen. P. N.; Gent. A . N . Surf. Sei. 1988. 203. 72.
0022-3654/90/2094-8709$02.50/0
The method of junction preparation and the apparatus for measuring the tunneling spectrum have been described in detail c l s ~ w h c r c . ~Aluminum *~~~ (99.999%’) or magnesium (99.99%) (14) Leuchs, M.: Zundel, G . Can. J . Chem. 1979, 57, 487. (15) Brun, G.; Maurin, M. C. R . Acad. Sei., Ser. C 1970, 271, 294. (16) Tsuboi, M. J. Am. Chem. Sot. 1957, 7 9 , 1351. ( I 7) Ahlijah, G.E. B. Y.; Mooney, E. F. Spectrochim. Acra, Parr A 1966, 22, 547. ( 1 8 ) Meyrick, C . I.: Thompson, H . W. J . Chem. Soc. 1950. 225. (19) Nyquist, R. A . Spectrochim. Acta, Parr A 1969, 25, 47. (20) Daasch, L. W. J. Am. Chem. Soc. 1958, 80, 5301. (21) Smith, T. D. J . Inorg. Nucl. Chem. 1960, 15. 95. (22) Hammond, P. M. J . Chem. Soc. 1962, 1365. (23) Samuel. D.: Silver, B. L. J . Phys. Chem. 1968, 72, 1809. (24) Hammond, P. M. J . Chem. Soc. 1962, 2521. (25) Francina, A.; Lamotte, A,; Merlin, J.-C. C.R . Acad. Sei., Ser. C 1968, 266, 1050. (26) Westheimer, F. H.: Huang, S.: Covitz, F. J . A m . Chem. Soc. 1988, 110, 181. 2993. (27) Forbes, E. S.: Battersby, J. A S L E Trans. 1974, 17, 263.
0 1990 American Chemical Society
Higo and Kamata
The Journal of Physical Chemistry, Vol. 94. No. 24. 1990
8710
Ii ! I
/ i*."" A'2°3
.
0
I
b
v MgO
100
200
300
400
0
500ElmeV I
I
1000
2000
3000
LOOO Slcm-'
Figure 1 . Tunneling spectra of the undoped A120, and MgO junction nicasurcd a( 4.2 K .
cvapor;itcd from ;I molybdenum boat on a clean glass slide Torr ( 1 to form three strips ( I mm wide) a t a pressure of Torr = 133.322 Pa). The surfaces of the strips were oxidized in an oxygcn dc glow discharge (400-800 V. 75 mTorr, 5 mA, 30-45 s) in a bell jar. Dimethyl and diethyl phosphite (Tokyo Kasei, >98o/r) were purified b) u c u u t i i distillation (22.7 OC/3.0 mmHg and 30.9 "C/ I .6 mmHg. respectively). Diphenyl phosphite (Tokyo Kasei) wiis uscd without further purification. These dialkyl phosphites wcrc adsorbed onto thc alumina and magnesia surfaces from the solutions of anhydrous benzene (0.1-200 mg/mL) by using a spin doping mcthod in thc tcmperaturc range 17-24 "C. Phosphorous acid (Wako Chemicals, 99.4%) was adsorbcd onto thc alumina surfacc froin aqueous or mcthanol solutions (0.2-1 .O mg/mL). The infrared spectra of these dialkyl phosphites and phosphorous acid were measured with a Shimadzu FTIR-4200 and compared with those previously reported. The junctions were completed with an evaporated Pb (99.999%)) cross ctrip ( I mm wide). Resistances for the measured junctions wcrc in thc rnngc 80-1 580 Q. The tunneling spectrum was obtaincd by nicasuring thc second derivative of the tunneling current through the junction ;it 4.2 K. A 500-Hz ac modulation signal of 3 m d 4 mV,, was applied to the alumina and magnesia junction. rcspectively. and thc sccond harmonic signal was detected with a lock-in amplil'icr (hF LI-574A). A typical trace time of the spcclrum W;IC 60 min with ;I 3-s time constant of the lock-in amplifier. Thc pcnk positions were obtained by averaging more than four spectra and corrcctcd by -1 meV (-8 cm-I) owing to thc cncrgy gap of the superconducting Pb electrode.' The accuracy was cstimated t o be f 4 or f8 cm-' in the frequency range 250-2000 or 2000-4000 cm-I. respectively. The resolution was estimated to be 20-24 cm-'. I n the case of the measurement of the tunneling spectra of phosphorous acid, the junction was kept above the liquid-helium level u t about 7 K to quench the superconductivity of the Pb electrode. A small ferrite magnet (5-mm diameter) of about 1300 G was also used to quench the superconductivity during the mcasurement at 4.2 K . A normal sinusoidal wave modulation signal at 0 eV showed the quench of the superconductivity. w;15
Results
Tittineling Spectra of Alumina and Magnesia. The tunneling spectra of the undoped AI-AI2O3-Pb and Mg-MgO-Pb junctions are shown in Figure I . The peaks at 300 and 930 cm-' of the tunnclinp $pectrum of thc alumina junction are assigned to the ( 1 8 ) Higo. U.:Mi7utaru. S.: Kamata. S. Bull. Chem. SOC.Jpn. 1985.58, 2960. (29) Higo. M.: Mizutaru. S . : Kamata. S. Bull. Chem. SOC.Jpn. 1989.62. 1829.
0 0
100
1000
200
300 2000
,
400
500ElmeV
LOOO C/cm-'
3000
Figure 2. Tunneling spectra of phosphorous acid on AI20, doped from an squcouc solution. The spectra were recorded at 4.2 and about 7 K.
TABLE I: Vibrational Frequencies (cm-I) and Mode Assignments for Phosohorous Acid on ALO, Measured by IETS I ETS this work
Ramsier et ai."
3600 H br 2940 v u br 2529 w 2420 i h ,025 b N
3610 2890 2540 2428 2057
I440 vw 1118 w
I428 1145
1023 vs
I034
m
940 645 550 460
937 630 560 456 284
br w br vw br m N
I Rb
Ramanb
(K,HPO, soiutionj
(Na,HPO, sol;tion)-
assignment
u(OH) (surface) 2315 m
2330 s
1085 vs br 1027 vw br 979 m
I100 vw 1032 m 993 5
567 m 465 m br
550 vw 459 m
contamination bump' IJ(PH) overtone of 6(PH) contamination bump' IJ,,(PO,~-) B(PH) vr(POj2-) ~('410) 1
6,(P0,2') 6,,(P032-) AI phonon
ii Reference 13. Referencc 16. Due to superconductivity. vs, very sLrong; s, strong; m, medium; H , weak; vw, very wcak; sh, shoulder; br, broad; i f . stretching; 6. bending or deformation.
aluminum phonon and the vibrational mode of the aluminum oxide, r e ~ p e c t i v e l y . The ' ~ ~ ~broad ~ ~ ~ peak at 3610 cm-' is assigned ~~~~ to the stretching mode of the surface OH g r o ~ p s . ' * *The broadness and asymmetric tailing toward lower frequencies are typical features of hydrogen-bonding hydroxyl groups. The very weak peak at about 2900 cm-' is due to the C H stretching mode of a hydrocarbon contamination; however, the weakness of this peak indicates that the amount of contamination is negligible. The peaks at 430 and 640 cm-l of the tunneling spectrum of the magnesia junction are due to the vibrational modes of Mg0.'.28.29 Thc peak a t 260 cm-' arises from the magnesium phoThe strong peak at 3650 cm-I and the shoulder at 3590 cm-' arc caused by the stretching vibrations of the surface O H groups. These peaks indicate two types of OH groups on the magnesia surface: an isolated O H and a hydrogen-bonding OH group. Thcrc is no peak near 2900 cm-I, and the junction shows no organic contamination. Tunneling Spectra offhosphorous Acid. The tunneling spectra of phosphorous acid adsorbed on the A1203surface measured at 4.2 K (thc Pb electrode is superconducting) and at about 7 K (the electrode is normal) are shown in Figure 2. The peak positions in the spectrum meausrcd at 4.2 K, and thcir assignments are given
Surface Reaction of Dialkyl Phosphite on A1203and MgO
The Journal of Physical Chemislry, Vol. 94, No. 24, 1990 8711 T A B L E II: Vibrational Frequencies (cm-') and Mode Assingments for Dimethyl Phosphite on A120, and M p O Measured by IETS
I ETS A1203
MgO
3638 w br
3635 m br
2973 2921 2829 2387
-
0 0 ~
~ _ _ _
100 ~
_
1000
200 _
300
400
500ElmeV
m w
m vs
2971 2920 2829 2376
m
3000
4000 Slcm-'
m
in Tablc 1 along with those by Ramsier et aI.l3 I n this table are also shown the peak positions and the assignments of the infrared and Ramiin spectrum of thc phosphite anion (phosphonatc dianion) i n an aqueous aoIution.l6 When the Pb electrode is superconducting, it gives narrower and larger amplitude peaks than when it is normal.' However, the superconducting Pb electrode gives rise to an undershoot on the high-energy side of a very strong peak with a following small bump.3o The undershoot and the bump may be confused with an iidditional structure and a peak of a tunneling spectrum. The effects of zupcrconductivity arc not as obvious with broad peaks The bumps at 2529 and or whcrc ;I number of pcaks 114X cm-' in the spectrum measured at 4.2 K are due to this phcnomcnon. Though peak widths increase in the spcctrum mciiaurcd a t Libout 7 K due to thcrnial smearing of the Fermi no such undershoots or the bumps. s u r f ~ x . ' i~t ~shows ' The tunneling spcctrum of phosphorous acid shows the very strong pcaks owing to the stretching (v) and bending (6) motion of the PH group at 2420 and 1023 cm-I, respectively. The u(PH) band in thc infrared and Raman spectrum of the phosphite anion shifts to ;i lowcr frcqucncy ;it iibout 100 cm-' from that in the tunneling spectrum of phosphorous acid on AI2O3.This peak shift may be duc to ti diffcrcncc of thc interaction between the P032group and the metal cations because the barium salt of phosphite anion gives the v(PH) band Lit 2410 cm-I.l6 The tunneling spectrum has ;I weak pcak of the surface OH vibration at around 3600 c n - I . The infrared spectrum of phosphorous acid has a strong Y (p-0)( 1 180 c d ) and a broad u(POH) band (-2900 cm-1).14,15 Thc tunncling spectrum. however, has no corresponding peaks, but it docs show the weak and broad symmetric and asymmetric dcformntionnl ( 6 , kind ALL%)peaks of the POj2- group a t 560 and 456 em-I. respectively. The V ~ , ( P O ~peak ~ - ) i s considered to be wcak in the tunneling spcctrum and may be obscured by the Undershoot of thc vcry strong h(PH) peak at 1023 cm-I. The u ~ ( P O ~peak. ~ - ) which exists at 979-993 cm-' in the infrared and Ronion spcctrum of the phosphite a n i ~ n . ' ~seems - ' ~ to be involved in the b(PH) pcnk. Phosphorous acid is found to be adsorbed as the phosphite anion (phosphonate dianion) onto the A1203surface. I t is concluded that phosphorous acid reacts with the surface O H groups of the alumina by losing the protons of the PO(OH)> group and is adsorbcd ;is the anion a t the Lewis-acid sites (AI') on the (30)Magno. R.:Adler. J. G.J. Appl. f h y s . 1978. 49, 5571. ( 3 1 ) Hippr. K . W.: Pcter, S. L. J . fh,vs. Chenl. 1989. 93, 5717
2852 2438 I462 1449 1283 sh 1266 I I84
s
1446 m
1448 m
1182m
1154 m
1184m 1162 m
1003 s
1047 sh 1003 s
-
1081
Raman' (liquid)
I251 I182
1000 979
1079 I042 999 975
820
81 9
770 sh 765
775 sh 760 542
I055
801 m
assignment
2954 2920 2849 2440 I460
u(Al0)
936 sh 797 m
Figure 3. Tunneling spcctra of dimethyl phosphite on A1201 and MgO
doped from anhydrous benzene solutions. The spectra were measured at 4.1 K .
-3019 sh 2997 2956
ni
~
2000
IR" (solution)
547 w br
655 ni br 554 w br
549
441 w br
457 sh
503 453
,(PO) ? 1ts(P(0)2) '?
-450
6(PoC)/6,(Po32-), 6SP02J 6( P0C)P 6(P03),*da,(P032-), 6as(PO2-)
426 m br 287 w 253 m
246 m
402 378 -228
402 375 220
6(POl)h T(P0C)' AI phonon T(P0C)'
Reference 19. In-plane (POC). 'Out-of plane (POC). plane. 'In the H-P=O plane. 'IndPcrpcndicular to the H-P=O phase. gout-of-phase. hSkeletal deformation. 'Torsion.
surface as are the many organic acids.'~8*9~28~29 This phosphite anion (phosphonatc dianion) is considered to be adsorbed perpendicularly to the surface in a tripod configuration as in the case of methylphosphonic acid, CH3PO(OH)2.7s8 Tutinelitig Spectra of Dimethyl Phosphite. The tunneling spectra of dimethyl phosphite on the alumina and magnesia surface are shown in Figure 3. The peak assignments are given in Table I I with those of the infrared and Raman spectrum of dimethyl phosphite.I9 The tunneling spectra of dimethyl phosphite on A1203 and MgO show the strong u(PH) and y ( P H ) (out-of-plane bending) peaks at about 2380 and 1003 cm-l, respectively. Their pcak intensities are stronger in the spectrum on A1203. The v(PH) peaks shift to a lower frequency about 60 cm-l from those in the infrared and Raman spectrum of dimethyl phosphite probably due to the interaction with the surfaces. The tunneling spectra have the v,(CH,) (2829-2973 cm-I), 6,(CH3) (- 1450 cm-I), and @(CH3)(in-plane bending) and y ( C H 3 ) (out-of plane bending), ( - 1 180 cm-I) peaks. The medium peaks at about 800 cm-' are assigned to the v(P-0) mode. The peak intensities of the surface O H groups of both the oxides decrease owing to their loss from the surfaces. In the case of MgO, the strong u ( 0 H ) peak disappears and a medium and broad peak remains. Though the molecular structure of dimethyl phosphite is different from that of phosphorous acid, the features of the tunneling spectra of dimethyl phosphite are similar to those of the tunneling spectrum of phosphorous acid. The infrared and Raman spectrum of dimethyl phosphite have ; I strongv(P=O) (1251-1266cm-'), medium v,((C-O),) (-1080 cm-I), ~ ~ ~ ( ( c - (1042-1055 0)~) cm-I), P ~ ~ ( P ( O(-820 ) ~ ) cm-I), and us(P(O)J (-760 cm-l) band. The tunneling spectra on both the surfaces. however, have no corresponding peaks. On the other hand, the tunneling spectra show the peaks owing to the v,,(PO~~-) and us( POT) mode at I 154-1 162 cm-I, the 6,( and a,( POT) mode a t about 550 cm-I, and the 6,,(P032-) and 6,,(P02-) mode
The Journal of Phjlsical Chrmisrry, Vol. 94, No. 24, 1990
8712
Higo and Kamata TABLE 111: Vibrational Frequencies (cm-') and Mode Assignments for Diethyl Phosphite on AI,O, and M e 0 Measured bv iETS IETS I R" Ramanh 41107 hlp0 (solution) (liquid) assignment 3641 111 br 3672 ni br v(OH) (curface) -2995 sh V (C H2). C H3) 2957 4 2963 4 2988 2975 < i1(CH2). o(CH,) 2942 s h rj(CH2). 4 C H 3 ) 2918 4 2925 b 2933 2940 < I ~ ( C H * )I ,J ( C H ~ ) 2895 4h 2880 ah 2910 ln(CH2). rs(CH3) 2388 v b 2385 4 2439 2432 \ br I(PH) 1481 h(CH2) I458 6,JC H 3 ) 1446 m I449 in 1445 1445 4 6,,(CH3) 1388 m 1391 m I394 Q ( C H 2)' 1364 m 1367 m 1371 A,(CH,) 1285 w 1292 m 1295 \ h r(CH2) 1275 s h u(P=O) I262 I250 m i ~P=O) ( I)(
=111 J
$
.
N
>
?
I
N
D
-
1
1229 \ w 1161 I
0
I
100
I
I
200
I
300
I
I
400
H
I170 w
I167
1093 sh
I102
I100 m
1004 4
1081 IO52 -1009
