Adsorption of phosphorus acids on alumina

Adsorption of phosphorus acids on alumina

Surface Science 203 (1988) 72-88 North-Holland, Amsterdam 12 ADSORPTION OF PHOSPHORUS R.D. RAMSIER, P.N. HENRIKSEN Depurtment of Physics and Ins...

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Surface Science 203 (1988) 72-88 North-Holland, Amsterdam

12

ADSORPTION

OF PHOSPHORUS

R.D. RAMSIER,

P.N. HENRIKSEN

Depurtment of Physics and Institute Akron, OH 44325, USA Received

26 January

1988; accepted

ACIDS ON ALUMINA and A.N. GENT

of Polymer Science. The University OJ Akron,

for publication

26 April 1988

Vibrational spectra of several phosphorus acids adsorbed on oxidized aluminum surfaces have been measured using inelastic electron tunneling spectroscopy. From analysis of the spectra, it is concluded that these acids chemisorb on alumina by a condensation reaction in which resonance stabilized structures are formed involving deprotonated hydroxilic and phosphoryl oxygen atoms. These structures are symmetric PO, groups in the case of phosphinic acid, whereas for phosphonic acids, they are symmetric PO, groups. in the spectrum of the hydration inhibitor N[CII,P(O)(OH),], adsorbed on alumina, unexpected bands attributable to P-H motions appeared. It is therefore inferred that this material dissociates upon adsorption. The various phosphonic acids produced in this decomposition subsequently adsorb as symmetrical tridentate species.

1. Introduction Penetration of moisture into adhesively bonded aluminum structures can and often does lead to deterioration and ultimate failure of the bonds. Methods of retarding moisture penetration into these systems are therefore desirable [l]. One successful method consists of treating the oxidized aluminum (alumina) surface with an “hydration inhibitor” prior to application of the adhesive. In a comprehensive study of the use of hydration inhibitors to improve the durability of aluminum joints, Davis et al. [2] found that alumina surfaces treated with certain amino phosphonates exhibited a much higher resistance to hydration than untreated surfaces. There it was proposed that phosphonic acids adsorb on alumina by a condensation reaction involving hydroxyl groups of the acid and those on alumina, resulting in the formation of P-O-Al bonds. As part of a general program on chemisorption on alumina surfaces, we have carried out a detailed study of the adsorption mechanism of phosphorus acids using inelastic electron tunneling spectroscopy (IETS). This technique has been used successfully to determine the adsorption mechanism for a wide variety of compounds on alumina [3,4], including carboxylic acids [5]. Because of the structural similarities between carboxylic and phosphorus oxy-acids, 0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

R. D. Ramsier et al. / Adsorption of phosphorus acids on alumina Table 1 Phosphorus

73

acids used as adsorbates

Phosphinic a) Phosphonic ” Methylphosphonic ‘) Hydroxymethylphosphonic b, Aminomethylphosphonic ‘) Nitrilotris(methylene)triphosphonic

H, P(O)(OH) HP(O)(OH), CH,P(O)(OH), (OH)CH, P(O)(OH), NH,CH2P(O)(OH), ‘)

NlCH,W)(OW,I,

a) Aldrich Chemical Co., Milwaukee, WI. ‘) Alpha Products, Danvers, MA. ‘) Sigma Chemical Co., St. Louis, MO.

one may anticipate a similar reaction mechanism [6]. The present work establishes a consistent mechanism for adsorption of phosphorus oxy-acids on alumina. Because the number of bands observed in the spectra increases with the complexity of the compounds, several simple phosphorus acids were employed as model adsorbates. These are listed in table 1. IR absorption spectra of aqueous solutions of all of these compounds were also recorded for comparison with the IET spectra of the compounds in the adsorbed state. In this way, chemical changes upon adsorption could be detected. 2. Experimental Several excellent reviews of all phases of IETS are given in the literature [7,8], therefore, only those details pertinent to this research will be discussed. Samples for IETS were prepared by thermally evaporating high purity (99.999%) aluminum onto clean microscope slides at pressure less than 2 x lo-’ Torr, followed by DC glow discharge oxidation in 85 mTorr of oxygen for about 30 s. This procedure is known to produce an amorphous alumina film about 15 A thick on an aluminum film approximately 900 A thick [9]. The films were then transferred to an adjacent fume hood open to the atmosphere and quickly doped with a solution containing the compound of interest. The room temperature doping procedure consisted of placing several drops of a dilute solution (0.05 wt%) of the compound in water onto the alumina surface, followed by immediate removal of the excess by rapidly spinning the film. The film with adsorbate was returned to the vacuum chamber, which was then evacuated to a pressure of 2 x 10P ’ Torr. Lead counter electrodes were evaporated over the substrate/adsorbate samples to complete the tunnel junctions. The IETS spectrometer used is similar to that described by Oxley et al. [lo]. A Tracer Northern TN-1710 signal averager was used for data collection, all spectra being recorded with the tunnel junctions maintained at a temperature of 4.2 K.

R.D. Ramsier et al. / Adsorption of phosphorus acids on alumina

74

IR absorption spectra were obtained after 256 scans using a Digilab FTS-40 spectrometer with 8 cm- I resolution. Samples were prepared in the standard capillary film technique using AgCl windows in a demountable cell.

3. Results and discussion 3. I. General observations All of the IET spectra presented in this work contain the underlying background of an undoped tunnel junction, shown in fig. 1. The 3610 cm-’ band corresponds to stretching vibrations of surface hydroxyl groups, the 1860 cm-’ band is attributed to valence vibrations of aluminum hydride [ll], and the strong peak at 940 cm-’ to the Al-O stretching mode of the bulk oxide. These are intrinsic features of the IET spectra, and will not be discussed further. The inset of fig. 1 is an idealized representation of the aluminum oxide surface grown under the described experimental conditions. Water vapor is an intrinsic feature of an unbaked vacuum system, therefore aluminum hydride and surface OH groups are to be expected, as well as cationic aluminum, and 02- sites [12-151. It should be noted that no bands appear in fig. 1 in the regions expected for hydrocarbon stretching and deformation bands, indicating a relatively hydrocarbon-free vacuum chamber. Periodically throughout this study, undoped devices were fabricated and the spectrum recorded to ascertain the cleanliness of the vacuum system.

0.1 0

I 600

Fig. 1. IET spectrum

0.2 FNERGY 1600 WAVENUMBER

of an undoped

0.3 (eV), 2400 (cm-‘)

Al/Al-oxide/Pb

0.4 I 3200

tunnel junction

0. 5 I 4000

at 4.2 K.

R. D. Ranker

et al. / Adsorption

ofphosphorusacids

on alumina

15

IR absorption spectra were also recorded for aqueous solutions of the compounds used in this work. The solvent (water) caused large broad absorption bands in the regions 3000-3600 and 550-850 cm-‘, and a strong, well-defined peak near 1650 cm-‘. Therefore, only the lower frequency regions are presented, since the higher frequency regions are dominated by these solvent absorption bands and do not aid in interpreting the IET spectra. Vast quantities of IR data on phosphorus-oxygen compounds have been collected and analyzed in detail by several authors [16-191. The spectra of these compounds are found to exhibit characteristic bands attributable to the P(O)(OH) groups, which will be discussed where pertinent. IET vibrational frequencies can be shifted slightly, with respect to those observed in IR and Raman spectra, due to the adsorption process. Charge redistributions and bond angle variations may occur in the adsorbate in obtaining compatibility with the coordination sphere of the adsorption site. Therefore, both inductive and steric effects can cause changes in the skeletal force constants of the adsorbed species, resulting in small frequency shifts of the corresponding vibrations. 3.2. Phosphinic acid The IR spectrum of phosphinic acid in water (50 wt%) is shown in fig. 2a and the IET spectrum of the same acid adsorbed on alumina is shown in fig. I

I

I

I

I

0

OH j

\\

, H

I

0

Fig. 2. Spectra

I

0.1 I 800

I

I 0.3 ,ENERGY (oV)~

0.2

1600 WAVENUMBER

2400 (cm.‘)

P

\ H

I 0.4

0.:

I 3200

I 4000

of phosphinic acid. (a) IR absorption spectrum of phosphinic acid in H,O wt%), and (b) IET spectrum of phosphinic acid adsorbed on Al-oxide.

(50

76

R.D. Ramsier

et al. / Adsorptron

of phosphorus

acids on alumina

2b. Several significant differences exist between the two spectra (besides the solvent absorption peak at 1651 cm-’ in the IR spectrum). Specifically, the strong peaks at 979 and 1185 cm-’ in the IR spectrum, attributed to P-OH and P=O stretching vibrations [20], respectively, are absent in the IET spectrum. In the IR spectrum (fig. 2a) PH, stretching, deformation and rocking motions produce peaks at 2408, 1067, and 811 cm-’ [20], respectively. A small concentration of ionized species in the aqueous solution contributes to the unlabeled shoulders at 1140 and 1040 cm-’ [20], corresponding to asymmetric and symmetric PO; stretching vibrations, respectively. Unresolved bands contributing to the high frequency side of the 1651 cm-’ water peak and to the low frequency side of the 2408 cm-’ PH, peak are attributed to characteristic P(O)(OH) absorptions [16-191. The small peak at 434 cm-’ may be due to P=O deformations, raised in frequency by hydrogen bonding as suggested by Thomas [21]. The large peak at 2391 cm-’ in fig. 2b corresponds to unresolved asymmetric and symmetric PH, stretching vibrations [22-251, and the shoulder at 1073 -’ to PH, deformation [22-241. The PH, twisting and rocking motions Ezduce peaks at 915 and 817 cm-’ [22-241, respectively, while PO, asymmetric stretching and deformation vibrations produce the bands observed at 1142 and 474 cm-’ [22-24,261, respectively. The strong peak at 1026 cm-’ contains contributions from both PH, wagging and PO, symmetric stretching motions [22-241. No conclusive assignment of the weak shoulder at 568 cm-’ has been made. Absence of P=O and P(OH) bands in the IET spectrum (fig. 2b) and presence of PO, bands makes it likely that phosphinic acid is adsorbed on alumina as a symmetric hypophosphite (H,PO,) anion formed by a condensation reaction in which the deprotonated hydroxilic oxygen forms a resonance stabilized structure with the phosphoryl oxygen. Such an anion would bond to exposed aluminum sites, as shown schematically in fig. 3a. Support for this proposed mechanism comes from comparison of the IET spectrum with existing literature on vibrational spectra of this anionic structure.

H

H

Fig. 3. Proposed

bonding

of phosphorus

acids to alumina.

R. D. Ram&-r et al. / Aakorption of phosphorus acids on alumina

77

Low intensity of the OH stretching band near 3610 cm-’ in fig. 2b is consistent with the proposed condensation mechanism. Surface hydroxyls displaced by the adsorbing species will combine with freed acidic protons to form water, which is subsequently removed from the surface by evaporation. 3.3. Phosphonic acid (PA) The IR absorption spectrum of PA in water (50 wt%) is shown in fig. 4a, accompanied by the IET spectrum of PA adsorbed on alumina in fig. 4b. The most notable difference between the spectra (apart from the characteristic water peak at 1654 cm -’ in the IR spectrum) is again the complete absence of the P=O stretching band at 1179 cm-’ [20,27] in the IET spectrum of the adsorbed species. In addition, bands attributable to P(OH), deformation, and symmetric and asymmetric stretching modes are present at 536 [27], 946 and 1026 cm-i [20,27], respectively, in fig 4a but not in fig. 4b. In fig. 4a, characteristic solvent bands are evident, as well as CO, peaks at 2346 and 2366 cm-‘, due to a poorly purged spectrometer. A peak at 2441 cm-’ is attributed to P-H stretching [20,27,28], with P-H bending vibrations remaining unresolved, but undoubtedly contributing to the 960-1050 cm-’ to a P=O region [27,29]. The small peak at 426 cm-’ may again be attributed deformation [21,27].

O+OH P’ ‘“Z ,““Z

o;l

\ Ii

\

I

,y I

b

:’

Fig. 4. Spectra

_a-.--

JI’ 0.1

0

‘&__

I 600

0.2 FNERGY 1600 WAVENUMBER

of PA. (a) IR absorption spectrum of PA adsorbed

I 0.3 LsV), 2400 (cm-‘)

of PA in Hz0 on Al-oxide.

I 0.4 I 3200

0. 5 I 4000

(50 wt!%), and (b) IET spectrum

78

R. D. Ramsrer et al. / Adsorption

of phosphorus aclds on alumina

In fig. 4b, intense peaks at 1034 and 2428 cm-’ are assigned to bending and stretching modes of the P-H bond [22,30,31]. Characteristic of peaks of this intensity, when the Pb electrode is superconducting, is an undershoot on the high frequency side of a peak, followed by a small overshoot [32]. Small peaks at 1145 and 2540 cm- are attributed to this phenomenon. A weak band at 2057 cm-’ is assigned to the P-H bending overtone, which is also expected to accompany such an intense peak [33]. Weak bands at 1428 and 2890 cm-’ are due to adsorbed atmospheric hydrocarbons; a result of removing the sample from the vacuum chamber for liquid phase doping. Spectra obtained from many similarly prepared IET samples were examined, and no evidence was found for bands attributable to the expected P(O)(OH), modes. However, bands at 460 and 550 cm-’ always appeared. These bands can be attributed to deformation modes associated with a symmetrical phosphite (HPOi-) anion [22,31]. Symmetric and asymmetric stretching modes of such a PO, structure are expected to produce bands at approximately 990 and 1100 cm-‘, respectively [22,30,31], which would fall under the large P-H bending peak and its associated overshoot discussed above. Relative intensities of the P-H peaks suggest that the dipole moment associated with the P-H bond is oriented almost perpendicular to the surface, further substantiating the proposed structure, illustrated in fig. 3b. A high frequency shoulder at 645 cm-’ is probably due to another deformation vibration of the PO, structure. Thus, it seems likely that phosphonic acid is adsorbed as a symmetrical phosphite anion, formed by a condensation reaction in which deprotonated hydroxilic oxygens from a resonance stabilized structure with the phosphoryl oxygen. Such an anion would then bond to an Al+ site, as shown schematically in fig. 3b. It is possible that PA exists in the bidentate form, illustrated in fig. 3c, as an intermediate species. However, we observed no spectral evidence for this, and have concluded that the PO, structure is more stable. It should be noted that the lattice spacing of alumina may permit bridging to two Al+ sites [6] by the bidentate form, and to three Al+ sites by the tridentate. This mechanism of chemisorption would account for the absence of any bands in the IET spectra of the adsorbed acid attributable to P=O or P(OH) vibrations. It would also account for dehydroxylation of the surface, as evidenced by the relative weakness of the 3610 cm-i band in fig. 4b. Moreover, it is consistent with spectra observed for similar acids adsorbed on alumina, reported in the following sections. 3.4. Methylphosphonic

acid (MPA)

The IR absorption spectrum of MPA in water obtained during this study is identical in all pertinent details to those presented by previous authors [34,35], and is not presented here in the interest of brevity.

R. D. Ramsier et al. / Adsorption of phosphorus acids on alumina

0

600

1600 WAVENUMBER

Fig. 5. IET spectrum

2400 (cm”)

of MPA adsorbed

3200

79

4000

on Al-oxide.

Fig. 5 is the IET spectrum of MPA on alumina, which closely resembles that previously published by Templeton and Weinberg [6]. Absence of the expected P(O)(OH), bands in fig. 5 again leads to the conclusion that MPA exists on the aluminum oxide surface in a symmetrical PO, configuration. Spectral evidence for this proposed structure is given by the weak but resolved bands at 525, 1065 and 1160 cm-‘. These correspond, respectively,, to PO, deformation, and symmetric and asymmetric stretching vibrations of the adsorbate [6,34-361. The low intensity of the P-O stretching bands has been attributed to weak electron-vibration coupling constants associated with these vibrations [6]. Asymmetric and symmetric stretching and deformation bands of the methyl group are assigned in table 2, without discussion, by comparison with the existing literature [6,34-361. A small shoulder at 2855 cm-’ is attributed to C-H stretching vibrations and deformation overtones, and a peak at 911 cm-’ falls in the region expected for the methyl rocking motion [6,34]. The relatively strong peak at 790 cm -* is assigned to the P-C stretching mode [6,34-361. The strength of this peak suggests that the dipole moment associated with the P-C bond has a large component perpendicular to the surface, further substantiating the proposed symmetrical structure of adsorbed MPA, shown schematically in fig. 3b. These interpretations are generally consistent with the conclusions drawn by Templeton and Weinberg [6]. In the present work, however, no spectral evidence for a bidentate form of adsorbed MPA was observed. These slight discrepancies between published spectra for identical compounds may be due to differences in sample handling and tunnel junction fabrication procedures.

80

R.D. Ranker

et al. / Aakorption of phosphorus acids on alumina

Table 2 IETS vibrational data, obtained at 4.2 K, of phosphorus acids chemisorbed on plasma-grown aluminum oxide, and FT-IR vibrational data, obtained at room temperature, of phosphorus acids diluted in water Phosphinic (FT-IR)

Phosphinic (IETS)

PA (FT-IR)

Band position (cm-‘)

Assignment

Band position (cm-‘)

Assignment

Band

b(P=O) PH, rock v(P-OH)

474 568 817 915 1026 1073 1142 2391

WO,) ?

434 811 979 1067 1185

8(PH,) V(p=o)

1651 2408

Hz0 rs.a(PH,)

PA (IETS) Band position (cm-‘)

(cm-i)

PH, rock PH 2 twist GPO,)/PH,wag S(PH,) I* %.(PH,)

MPA (IETS) Assignment

Band position (cm-‘)

Assignment

426 536 946 1026 1179 1654 2346 2366 2441

S(F=O)

W’UW,) ~s(WW 2) ~,(WW 2) v(P=O) H2O co* co2

v(P-H)

HMPA (IETS) Assignment

460 550 645

UP%) %(PO,) 6(PO,) (?)

525 790 911

VP%) v(P-C) CH, rock

940 1034 1145 1428 2057

v(Al-0) 6(P-H) See text 8(CH,) 2 x 6(P-H)

1065 1160 1315 1425 1855

&(PO,) rJa(PO,) 8, (CH,) %(CH,) v(Al-H)

2428 2540 2890 3610

v(P-H) See text p(CH,) WH)

2855 2928 2995

u/WCH) (?) QCH,) r~a(CHs)

Band position (cm-‘) 392

Assignment

LY(OCP)

556 777 875 938 1052 1210 1312 1432 1860

S,(PO,) u(P-C) CH, rock/y (CO-H) v(Al-0) v(C-OH)/v(PO,) CH, twist CH, wag 6(CH,)/6(CO-H) v(Al-H)

2842 2898

V&H,) [email protected]=2)

AMPA (IETS)

NTMPA (FT-IR)

NTMPA (IETS)

Band position (cm-‘)

Band position (cm-‘)

Band position (cm-‘)

500 603 766 855 940 1030

Assignment

? ? v(P-C) CH, rock/ NH, wag v(Al-0) v(C-N)

1331

CH 2 wag/ twist

1436 1625

VCH,) WNH,)

464 556 588 715

Assignment

8(p=o) WWW2) ? 7

365 556 790 871

757 824 946

Y(P-C) CH, rock v,(P(OH),)

1032 1105 1218

1028 1079

ra(P(OH),) v(C-N)

1315 1428

Assignment

?

%PO,) P(P-C)

8(CH,)

\

SPW ~a(PO,) WH,) See text

R. D. Rams&

et al. / Adsorption

ofphosphorusacids

81

on alumina

Table 2 (Continued) AMPA

NTMPA

(IETS)

NTMPA

(FT-IR)

(IETS)

Band position (cm-.‘)

Assignment

Band position (cm-‘)

Assignment

Band position (cm-‘)

Assignment

1855 2840 2928 3274 3323 3621

v(Al-H)

1179 1332 1436 1649 2340

v(P=O) CH, wag

1860 2428 2799 2855 2936 1 3620

v(Al-H) v(P-H)

Again, attributable condenses eluted as cm-’ is sequesters

GCH,) v,(CH,) vs(NH,) v,(NH,) v(OH)

S(CH,) H,O v(PO-H)

v(CH,) see text v(OH)

absence of any bands in the spectrum of MPA adsorbed on alumina to P=O or P(OH) groups leads one to conclude that MPA at the surface, forming a symmetrical PO, structure, with water the reaction byproduct. The lack of spectral structure near 3610 evidence that MPA actively displaces adsorbed OH groups and the exposed surface sites.

3.5. Hydroxymethylphosphonic

acid (HMPA)

The IET spectrum of HMPA on alumina is presented in fig. 6. Symmetric and asymmetric stretching bands of the CH, group are assigned at 2842 and 2898 cm-’ [37,38], respectively. Peaks at 1312 and 1432 cm-’ are also attributed to the CH, group, being assigned to wagging and deformation contributions vibrations [37,38], respectively, the latter possibly containing

I 0.1 0

I 600

I 0.2 FNERGY

I 0.3 (eV),

1600 WAVENUMBER

Fig. 6. IET spectrum

of HMPA

2400 (cm-‘)

adsorbed

I 0.4

O.!

I 3200

I 4000

on Al-oxide.

82

R. D. Rumsrer et al. / Adsorptron

ofphosphorus

acids on alumina

from the in-plane CO-H deformation. Bands at 938 and 1860 cm-’ are those intrinsic to IET spectra. The relative strength of the 777 cm-’ peak, assigned as the P-C stretching motion [37,38], would suggest once again that a large component of the associated dipole moment lies perpendicular to the surface. This configuration could result from a reaction mechanism similar to that proposed for MPA, which would leave the CH,OH group intact. The 556 cm-’ PO, deformation band [37,38] is further spectral evidence for the proposed structure, illustrated again by fig. 3b. The CH, twisting motion produces the 1210 cm-’ band [37,38], and the broad band at 1052 cm-’ contains contributions from both PO, and C-OH stretching vibrations [37,38]. Both the out of plane CO-H deformation and the CH, rocking motion contribute to spectral structure in the 875 cm-’ region [37,38]. A small peak at 392 cm-’ is tentatively attributed to skeletal O-C-P deformations [37,38]. Note, in addition, that fig. 6 also lacks structure in the 3610 cm-’ region, indicating that HMPA competes successfully with OH groups for active sites on the alumina surface. White et al. [39] also studied the adsorption of HMPA on alumina and proposed a condensation reaction, followed by formation of both P-O-Al and C-O-Al bonds. Several differences exist between the HMPA spectra observed in the present study and that presented by White et al. [39]. One must again conclude that different preparation techniques may yield quite different reaction mechanisms. This may be due to morphological variations in oxide layers grown under different environments. However, the present band assignments and bonding mechanism of HMPA are consistent with the proposed symmetrical PO, structure for the adsorption of phosphonic acids, illustrated by fig. 3b. This mechanism of adsorption differs significantly from that proposed by White et al. [39]. 3.6. Aminomethy[phosphonic

acid (AMPA)

The IET spectrum of AMPA adsorbed on alumina is shown in fig. 7. The large 3621 cm-’ band indicates that a significant number of hydroxyl groups remain on the surface following adsorption of AMPA, contrary to the conclusions drawn from previous IET spectra. However, there is again no spectral evidence for the existence of P=O or P(OH) groups in fig. 7. These observations may be accounted for by assuming that AMPA adsorbs in the symmetrical PO, configuration proposed for phosphonic acids, and schematically represented by fig. 3b. This assumption is supported in the following discussion. Bands at 1625, 3274 and 3323 cm-’ may be attributed to NH, deformation, and symmetric and asymmetric stretching vibrations [40,41], respectively, modes normally observed in IR spectra of salts of and not to the NH:

R. D. Ramsier et al. / Adsorprion

0

600

of phosphorus

1600 WAVENUMBER

Fig. 7. IET spectrum

of AMPA

2400 (cm -‘)

adsorbed

acids on alumina

3200

83

4000

on Al-oxide.

primary amines [40-431. The NH, modes indicate that the amine group is not involved in bonding to the alumina [5], and that protons liberated during condensation of AMPA have escaped the surface. The frequencies of these modes are also indicative [44] of the absence of strong intermolecular hydrogen bonding between adsorbed species. Therefore, the AMPA surface coverage is less than for the previous acids, with many active sites still occupied by adsorbed hydroxyls. Lower density of AMPA adsorbates may be due to steric repulsions [45] between the functional groups during adsorption, making many potential bonding sites inaccessible. Methylene groups contribute to bands observed at 2928, 2840 and 1436 -‘, assigned as asymmetric and symmetric stretching [46] and deformation ;:I modes, respectively. Methylene wagging and twisting motions produce the broad band at 1331 cm-’ [42,46]. The C-N and P-C stretching vibrations produce peaks at 1030 and 766 cm-’ [42], respectively, and the 855 cm-’ peak contains contributions from both CH, rocking [42,46] and NH, wagging [41] vibrations. Low frequency bands at 500 and 603 cm-’ cannot be assigned conclusively, but are probably due to PO, and P-C-N skeletal deformations. Lack of P(O)(OH), bands in the spectrum of AMPA adsorbed on alumina again leads one to assume an adsorption mechanism involving condensation and the formation of a symmetrical PO, structure on the surface, as shown in fig. 3b. Although this mechanism is consistent with that proposed for other phosphonic acids, saturation coverage of the surface by AMPA is apparently not determined by the number of available reaction sites. Total coverage may be restricted by repulsive effects [45] between the adsorbates. The chemical environment of the amine substituents [44], as evidenced by the observed spectral bands, supports this hypothesis.

84

R. D. Ramsier et al. / Adsorption

3.7. Nitrilotris(methylene)triphosphonic

o/phosphorus

acrds on alumina

acid (NTMPA)

The IR absorption spectrum of NTMPA in water (50 wt%) is presented in fig. 8a, and the IET spectrum of NTMPA adsorbed on alumina is shown in fig. 8b. This compound was found to exhibit the best hydration inhibiting properties of those studied by Davis et al. [2]. Several significant differences exist between the two spectra, indicating a dissociative adsorption mechanism of NTMPA on alumina. In fig. 8a, bands at 946, 1028 and 1179 cm-’ are assigned to symmetric and asymmetric P(OH), and P=O stretching motions [20,27,34], by comparison with fig. 4a. The broad band centered at 2340 cm-’ is characteristic of P(O)(OH) groups [16-191, and is assigned to a PO-H stretching vibration [35]. Methylene rocking, wagging and deformation motions produce the bands at 824, 1332 and 1436 cm-’ [37,38], respectively. The 757 cm-’ peak in fig. 8a is attributed to P-C stretching [34,35,37,38]. P=O and P(OH), deformations produce bands at 464 and 556 cm-’ [21,27], respectively, which are raised slightly in frequency with respect to fig. 4a, probably due to differences in hydrogen bonding. The C-N stretching vibrations are expected to occur in the 1020-1250 cm-’ region [40,41], and are probably contributing to the 1079 cm-’ peak. Conclusive assignment of the small peaks at 588 and 715 cm-’ has not been possible. I

I

I 0.3

I 0.4

0.5

I 3200

I 4000

a

I 0.1

t

0

Fig. 8. Spectra

of NTMPA.

I 600

I 0.2 ,ENERGY 1600 WAVENUMBER

kV+ 2400 (cm -‘)

(a) IR absorption spectrum of NTMPA in H,O spectrum of NTMPA adsorbed on Al-oxide.

I

(50 wt%), and (b) IET

R. D. Ramsier et al. / Aakorption of phosphorus acids on alumina

85

Kaslina et al. [47] investigated thermal decomposition of NTMPA as a function of temperature and chemical environment and concluded that NTMPA can dissociate by cleavage of P-C and C-N bonds. The byproducts of this decomposition include ammonia, dimethylamine, trimethylamine, methanol, PA and HMPA [47]. These findings are significant to the present work, since bands in fig. 8b at 1032 and 2428 cm-’ can be assigned to P-H bending and stretching vibrations of a surface bound phosphite ion [22,30,31], by comparison with fig. 4b. These bands were observed to vary in intensity from sample to sample, while maintaining the same intensity relative to one another. Consistent with this interpretation are weak shoulders at 556 and 1105 cm-i, assigned to PO, asymmetric deformation and stretching modes [22,31], respectively. Note that in fig. 8b, P-C stretching produces only a small shoulder at 790 cm-’ [6,34-361, identical in frequency to that observed for adsorbed MPA in fig. 5. The breadth of the hydrocarbon bands implies that several types of species are present on the surface, as expected following dissociation of NTMPA [47], but making their conclusive assignment difficult. Here, they will simply be attributed to hydrocarbon deformation (871, 1218, 1315, 1428 cm-‘) and stretching (2799, 2855, 2936 cm-‘) vibrations, respectively, by analogy with previous IET spectra. A small peak at 365 cm-’ is not assigned. Lack of bands in fig. 8b known to occur in IR spectra of metallic complexes of NTMPA [48] lends additional support for the proposed decomposition of this compound on alumina. Medema et al. [49] have shown that adsorption of amines on alumina at room temperature is a slow, activated process. The conditions for adsorption of amines [14] and alcohols [15] on alumina have also been discussed in the literature. It seems likely that the various phosphonic acids produced by decomposition of NTMPA are adsorbed on alumina and either sequester or block the surface sites necessary for adsorption of amine and alcohol moieties. This would account for the lack of bands in fig. 8b attributable to these species. Once again, spectral evidence from NTMPA is in accord with the proposed symmetrical PO, bonding mechanism of phosphonic acids on alumina. However, in this case, the adsorbing molecules appear to dissociate by cleavage of P-C and C-N bonds. This decomposition is probably activated and catalyzed by reactive sites on the surface. Various phosphonic acids produced by decomposition are readily adsorbed at these sites. The amine and alcohol decomposition byproducts may be eluted in gaseous form immediately, or subsequently desorbed in the vacuum chamber during the final phases of sample preparation. The presence of a large OH stretching band at 3620 cm-’ in fig. 8b is consistent with this mechanism. When adsorbing species sterically hinder one another [14,45], displacement of all of the surface hydroxyl groups is unlikely

86

R. D. Ramsrer et al. / Adsorption

of phosphorus acids on alumina

and this band is expected to be appreciable. Additionally, the frequency of this band is similar to that observed for adsorption of AMPA on alumina in fig. 7. This implies that the same types of OH groups reside on the surface [50] and that the adsorption mechanisms are similar.

4. Conclusions

Analysis of the vibrational spectra of several phosphorus acids adsorbed on alumina suggests that these acids adsorb via a condensation reaction. Since no bands attributable to P==O modes were observed, we conclude that the phosphoryl oxygen participates in adsorption. For phosphinic acid, bands due to a symmetrical bidentate structure are evident, whereas for various phosphonic acids, spectral evidence substantiates the formation of symmetrical tridentate species. We propose that phosphinic acid adsorbs at coordinately unsaturated aluminum sites. Such sites exist on alumina and are formed by condensation of the acid with surface OH groups. Decrease in intensity of the OH stretching region of the IET spectrum supports this proposed mechanism. In the case of phosphonic acids, two acidic hydroxyl groups are available, either of which could participate in the adsorption process. No bands associated with P(OH) vibrational modes were observed, whereas bands attributable to symmetric PO, structures were. We conclude that both acidic hydroxyl groups condense with those on the surface to form symmetrical tridentate species. When phosphonic acids with larger functional groups were adsorbed on alumina, spectral evidence for surface bound OH groups was observed. It is possible that intermolecular repulsions render potential bonding sites inaccessible and inhibit complete dehydration of the surface.

Acknowledgments

This work has been supported in part by a grant from the Office of Naval Research (ONR N00014-85-K-0222) and a grant from the Research Faculty Projects Committee of the University of Akron.

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