Langmuir 1985, 1, 600-605
600
Thermally Activated Magnesium Oxide Surface Chemistry. Adsorption and Decomposition of Phosphorus Compounds Shaw-Tao Lin and Kenneth J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 660506 Received May 22, 1985 Organophosphates (RO),P=O, organophosphites (RO)3P,and organophosphines R3P adsorb strongly on thermally activated MgO and CaO. High concentrations are adsorbed ranging from one (CH3CH20)3P/foursurface MgO moieties to one (CH30)3P/160surface MgO moieties. Infrared studies indicate that phosphates adsorb very strongly through the P=O bond (the P=O bond is destroyed upon adsorption) accompanied by net electron density loss from the RO groups. Phosphites adsorb through the phosphorus atom with a net electron density gain by the RO groups. Phosphines adsorb less strongly through the phosphorus atom. Heating of adsorbed organophosphorus compounds leads to facile elimination of ethers, alcohols, alkenes, and other organics. Metathesis-like C-C bond forming processes also take place. Mechanisms for these processes are discussed and probably involve nucleophilic substitution at phosphorus and carbon, as well as proton abstraction/carbanion rearrangement processes. The amounts of organophosphorus compounds adsorbed and decomposed by thermally activated MgO are very large; the reactions are essentially stoichiometric. For example, 1 mol of activated MgO decomposes 0.5 mol of (CH3CH20)3P.In general organophosphorus compounds are highly labilized on these activated surfaces.
Introduction Ordered metal oxide surfaces may serve as models for microcrystalline metal oxide surface chemistry in some instances.'s2 For example, an oxidized Mg(001) surface behaves similarly to microcrystalline thermally activated MgO when B r ~ n s t e dacids are involved.' Thus, C2H2, CH,COOH, CH30H, C2H60H, i-C3H70H, and H20, all dissociatively chemisorb (H+X-) on the ordered surface and the microcrystalline On the other hand surface chemistry that is unusual and even remarkable and has not been observed and/or would not be expected on ordered, clean metal oxide surfaces can occur on microcrystalline metal oxide surfaces. Examples are CO telomerization/reduction,6 reduction of various organic molecules to radical anions: and radical abstraction reaction^.^ Both CO and C 0 2 adsorb on thermally activated MgOs but not on an ordered MgO surface.l This information coupled with our earlier reports5 on the higher activity of "defective porous MgO" lends credence to our previous conclusions that surface defectsg are responsible for CO telomerization/reduction6 and reduction of adsorbed org a n i c ~ .It~ is also apparent that such defects can be present with varying activities and in high concentrations (monolayers of radical anions can form, for example, with C6H5N02" and (NC)2C=C(CN2).5 (1) Martinez, R.;Barteau, M. A. Langmuir, submitted for publication. (2) Templeton, M.K.; Weinberg, W. H. J. Am. Chem. SOC.1985,107, 774-779. (3) Tanabe, K."Solid Acids and Solid Bases"; Academic Press: New York, 1970. (4) Gamone, E.; Stone, F. S. Proc. Znt. Congr. Catal. 8th, 1984 1984, 3,441-452. (5) Morris, R. M.; Klabunde, K. J. Znorg. Chem. 1983, 22, 682-687. (6) Morris, R. M.; Klabunde, K. J. J. Am. Chem. SOC.1983, 105, 2633-2639. (7) Driscoll, D. J.; Martir, W.; Wang, J. X.; Lunsford, J. H. J. Am. Chem. SOC.1986,107,68-63. (8) Klabunde, K. J.; Kebe, R. A,; Morris, R. M. Adu. Chem. Ser. 1979, No. 179, 140-161. (9) For excellent dlwurdonr of rolid-atata defects nnd their propertiee/che&try, me: Hendemon,B.;We&, J. E."Defeebin the AlkaUne Earth Oxides";Halsted Press: New York, 1977. Sondor, E.; Sibley, W. A. 'Point Defects in Solide";Crawford, J. H., Slifken, L. M., E&.; Plenum Press: New York, 1972, p 201. Rees, A. L. G. "Chemistry of the Defect Solid State"; Muthuen and Co., Landon and Wiley: New York, 1954.
0743-7463/85/2401-0600$01.50/0
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We believe the CO Ce02-/3- reaction on thermally activated MgO or CaO is a good indicator of the presence of highly defective sites. This reaction can be blocked by Br0nsted acid adsorption on normal and defect oxide sites, which suggests that CO is first weakly absorbed on normal sites and then migrates to defective sites. OR
H
I
Mg-9-Mg-0
I
I
I
I
0-Mg-0 Mg-0-
l
-1 ROH
1
1
1
I
I
I
0-Mg-0-H
I
I
Mg-0
l
49-0
Mg-0-
1
OR
Mg-0-Mg-0
not active toward CO
defect model
Ico
I co I Mg-0-Mg--b
I 0
I I Mg 0 I I I Mg-0-Mg
co I Mg-0-Mg-0,
-I co
l
l
I,+ I
I
O-Mg-0-
Mg-0-Mg-0
\
To/-'
Since CO can function both as a Lewis base or Lewis acid (empty p orbital and back-?r-bonding is possible) we decided to investigate molecules that possess similar properties but which could be manipulated through change in substituents. We chose to investigate the adsorption and decomposition of a series of phosphorus compounds on thermally activated MgO and CaO, and we report the results herein.
Results Adsorption Amounts. A series of phosphines, phosphites, and phosphates were allowed to adsorb on thermally activated MgO (700 "C activation) a t 25,75,125, and 175 OC. Table I lists the compounds employed and the amounts adsorbed. General trends indicate that most of the phosphites adsorb in larger amounts than the phosphate, and that the phosphine studied was least adsorbed. Note that eubatituent effects are important. If we consider (10) Klabunde, K. J., Kaba, R. A.; Morris, R. Inorg. Chem. 1978, 17, 2684-2685.
0 1985 American Chemical Society
Thermally Activated Magnesium Oxide Surface Chemistry
Langmuir, Vol. 1, No. 5, 1985 601
Table I. Amounts of Phosphorus Compounds Adsorbed at Various Temperatures on Thermally Activated MgO (700 OC Activation)" 25 OC 75 OC 125 "C 175 "C mol/g of MgO g/g of MgO mol/g of MgO g/g of MgO mol/g of MgO g/g of MgO mol/g of MgO g/g of MgO compound x 10-5 x 10-2 x 10-6 x 10-2 x 10-5 x 10-2 x 10-5 x 10-2 (CH&H20)3P=O 24.1 4.39 8.79 1.60 2.26 0.411 1.36 0.248 C13P=0 negligible (CHBO)BP 1.65 0.334 (CH3CHzO)J' 63.3 10.5 24.9 4.14 19.4 3.22 8.01 1.33 (4.28)* (0.71)b (CH3CHzCH225.7 5.34
0h.P ((CH3)2CH0)3P (ClCH&H20)3P (CFSCHZO)BP (CBHS)(CH&H2-
018
(CH&H2)2(CH30)P (CH3CHz)J' PC13 PC15 CBHbN02' CO'
24.7 47.5 20.7 49.4
5.14 12.8 6.80 9.77
18.4
2.20
11.7 negligible negligible 10.8 1.64
1.38
3.70
1.22
small
negligible
1.33 0.045
"All MgO samples were activated at 700 O C for 14 h under vacuum. bThis sample of MgO was activated at 300 OC for 14 h under vacuum.
'Reference 10.
the (CH3CH20),P as the standard it can be seen that adding a C1 or CF3 decreases the amount adsorbed. (Complete substitution by C1 leads to complete inactivity toward MgO, e.g., PCl,, PC15, and Cl,PO). And if larger or smaller R groups are involved less still is adsorbed. It would appear that a delicate balance of basicity and steric effects are important. Thus, C1 and CF, substitution lowers basicity (and lowers adsorption) while CH3CH2CH2 and (CH3)2CHsubstitution increases steric problems in approach to the MgO surface (and lessens adsorption). On the other hand the smaller but less basic (CH30),P is absorbed much less efficiently. As R groups are attached directly to phosphorous (C6H5 and progressively CH,CH,) adsorption is progressively less efficient, probably reflecting increased steric problems. The decreasing amounts adsorbed with MgO temperature increase reflects the fact that a t least some of the phosphorous compounds are weakly adsorbed without chemisorption/bond breaking. More than one type of surface interaction is apparent. For example, by holding the 75 "C (CH,CH,O),P/MgO sample for 4 h a t lo4 torr and then cooling to 25 "C, 0.69 X g could be additionally adsorbed (giving a total of 2.29 X lo4 g absorbed/g MgO). It is apparent from adsorption/desorption experiments that some of the molecules chemisorb irreversibly and their presence can affect the readsorption of molecules weakly adsorbed earlier in the history of the sample. This is a type of surface site poisoning that must be due to migration of chemisorbed fragments to semireactive sites freed up where the previously adsorbed phosphite molecule desorbed. I t is interesting to note that once the MgO surface is exposed to a phosphite the surface is totally poisoned toward CO adsorption. Furthermore, lower MgO activation temperature leads to much less adsorption of (CH3CH20),P; note 300 vs. 700 OC in Table I. It is illustrative to compare the amounts of phosphorus compounds adsorbed with CeH5N02 and CO. Nitrobenzene is adsorbed in monolayer concentrations on activated MgO which corresponds to about 1C6H5NO2--/25 surface MgO moieties.'O Carbon monoxide adsorbs as C606,-- with approximately 1 C6063--/1000surface MgO moieties, although at least 6-fold more CO is adsorbed than
other nonparamagnetic species.6J0 Quantitative studies of CO adsorption showed that (9.9 f 3.2) X 10l8molecules of CO/g of MgO were adsorbed,'O which corresponds to 1.64 X mol/g of MgO or 0.045 X g/g of MgO and can be compared with the values given in Table I. Knowing that a monolayer of C6H5No2is adsorbed we can further calculate that 1000/25 X 6 = 6.6 times as much C6H5N02adsorbs as CO, which leads to the conclusion that 10.8 X 10" mol of C6H5No2/gof MgO or 1.33 X g/g of MgO is adsorbed. These calculations and the comparisons in Table I show that all the organophosphorus compounds are adsorbed in substantial amounts and approximate or exceed monolayer coverages in all cases. Infrared Studies of Adsorbed Species." Adsorption of (CH3CH20)3Pcauses significant changes in the IR (Figure 1A). Characteristic CH3 and CH2bending modes for the free phosphite a t 1382 and 1440 cm-' changed to two broad bands at 1370 and 1430 cm-' upon adsorption." Bands due to the P-OC moiety a t 1160 and 1030 cm-' fused to a single peak centered at 1080 cm-'. An additional P-OC band shifted from 915 to 885 cm-' upon adsorption. When the phosphite/MgO sample was baked a t 220 "C under vacuum the bands due to CH3CH2disappeared, a strong vp=o band appeared at 1285 cm-', and a very strong broad absorption from 900-1200 appeared. These results combined with decomposition studies discussed later would indicate that the initial adsorption of (CH3CH20),P is through the phosphorus atom and upon heating organic moieties are lost leading to chemisorbed P=O species. Subsequent exposure of the sample to CO yielded no spectral changes, which indicates that CO was not adsorbed. Adsorption of (CH3CH20),P=0 caused drastic changes in the IR spectrum (Figure 1B). The strong vw band a t 1245 cm-' disappeared upon adsorption. Also the bands due to P-OCH2 changed: ~ p + - c H ~ 1145 1150 cm-'; vp+-(c) 810,790 850,835 cm-'; qP)+c 1010,960 br 1030 cm".'' A strong interaction is definitely present between the adsorbed phosphate and MgO. After baking the sample at 220 "C under vacuum a possible vp+ band appeared (1320 cm-') and a u p M H z
-
-
-
(11)Carbridge, D. E. C. Top. Phosphorous Chem. 1969, 6 , 235-365.
602 Langmuir, Vol. 1, No. 5, 1985
I
,
Lin and Klabunde
n
I
Table 11. Ethers Formed from Phosphorus Compound Decomposition over Activated M a 0 and CaOa
"MgO was activated at 700 "C for 14 h under vacuum. Phosphorus compounds were added in excess (to wet the oxide), and the sealed sample was warmed to 100 O C . After 2 h, volatile compounds were collected and analyzed. *Based on MgO or CaO in the sample tube. e Nonthermally activated MgO. Nonthermally activated MgO heated with phosphite for 2 h at 300 O C . OActivated at 700 O C for 14 h.
c m-'
18
cm-'
cl
1
1600
1200
1400
1000
800
'-I
c m-'
Figure 1. (A) IR spectra of (-) triethyl phosphite, activated MgO, (-1 adsorbed triethyl phosphite, and (- - -) after baking adsorbed triethyl phosphite at 220 "C. (B)IR spectra of triethyl phosphate, (---I activated MgO, (-1 adsorbed triethyl phosphate, (- - -) after baking adsorbed and triethyl phosphate at 220 "C. (C) IR spectra of (.I)triethyl phoephine, -) activated MgO, (-1 adsorbed triethyl phosphine, (-- -) after baking adsorbed (-e-)
(-a)
(-a
triethyl phopshme at 220 OC.
(1145 cm-l) band appeared along with strong bands at 1140,1020,and 840 cm-l. The 840-cm-l band is possibly due to the presence of a P-0-P bond." Triethylphoephine adsorption (Figure IC) does not yield such drastic changes in the IR,suggesting a weaker mode of interaction. A small shift of the vp_c band (1190 1220 a - ' ) was O ~ S W V ~ Similarly . the bands 1300-1400 became broader 1300-1460 cm-l. After baking at 220 OC under vacuum a new band at 1275 cm-' appeared, which could be coordinated P=O.
-
A comparison of all these spectra with those of Mg2P20, and Mg3(P0.J2 showed no simi1arities.l' Decomposition of Phosphorus Compounds by Activated Mg0.12 Phosphites are readily decomposed by activated MgO at 100 OC. For example, if an excess of (CH3CH20),Pand MgO were heated at 100 "C for 2 h, a was formed along with large amount of CH3CH20CH2CH3 some C2Hk On the basis MgO used, the yield of ether was 52% (see Table 11). Since the total MgO/surface MgO molecules is about 9.4,'O this means that if every surface MgO molecule were an active site each one caused the decomposition of approximately 9.4 X 0.52 = 5 phosphite molecules. In reality fewer sites are probably active, and somehow this reaction is operating catalytically. The results in Table I1 also show that nonthermally activated MgO is not effective at 100 OC for phosphite decomposition. Even a t 300 OC nonthermally activated MgO was relatively ineffective. Also note that CaO activated at 700 OC decomposed only 7% phosphite (based on CaO present). Additional volatile decomposition products were identified by GC and GC-MS, and the results are summarized in Table III. Let us examine (CH3CH2O),Pin some detail. Diethyl ether was the major product until high decomposition temperatures were reached where alkenes became major. Ether formation should leave the CH3CHzOP0 moiety on the MgO surface. Total elemental analysis of the remaining oxide sample supported a formulation of C2H6P03(Mg0)1.9. Since a substantial portion of the volatile products are non-oxygenated, especially at higher temperatures, the "extra" oxygen left on the surface is reasonable. Note that the mass balance is close; as discussed earlier if one MgO in 10 each decomposed five phosphite molecules, about two MgO/phosphorus residue remain. Several control experiments showed that activated MgO was necessary for ether formation. Thus, (CH3CH20),P heated a t 100 or 300 "C yielded some ether. In a similar way nonthermally activated CaO at 100 O C did not cause phosphite decomposition, although thermally activated CaO did. Other phosphites such as (CH,O),P and (CF3CH20)3P also yielded ether as major products. However, (CH,CH2CH20),P,(CICHzCHzO)~, and ((CH3)2CH0)3Pyielded mainly alkenes. In the case of (CH3CH2CH20),Ppropene formation was shown not to be due to decomposition of (12) For diecussions of, phoephonu compounds, see, for example: Quin, L. D., Verkade, J., Eds.ACS Symp. Ser. 1981, No. 171. Alyea, E. C., Meek, D. W., Eds. Adu. Chem. Ser. 1982, No. 196.
Langmuir, Vol. 1, No. 5, 1985 603
Thermally Activated Magnesium Oxide Surface Chemistry
Table 111. Composition of Products from Phosphorus Compound Decompositions by MgO at Various Temperaturesn temp, OC
ClCHECHz (100) CFz=CFH (15), CFSCHFOH (259, CF3CHZOCHzCF3 (86), CFBCHZOH (100).
ORatios of products are given, not yields.
intermediate (CH3CH2CH2)20or ((CH3)2CH)20since control experiments with these ethers over MgO yielded no propene. The mixed system (CH3CH2)2(CH30)P yielded mainly diethyl ether. Triethylphosphine did not yield any ether showing oxygen was not scavenged off the MgO surface. Also note that the phosphate (CH3CH2O),P=O yielded no ether. When a mixture of (CH,O),P and (CH3CH20)3Pwere allowed to react over MgO, no CH3CH20CH3was observed indicating the decomposition reaction must be intramolecular. A final experiment where adsorbed (CH3CH20)3Pwas treated with D20 showed that recovered phosphite did not contain deuterium.
Discussion Amounts Adsorbed. Knowing the surface area of thermally activated MgO to be about 140m2/g and knowing the amount of phosphite adsorbed allow determination of coverage. From Table I the amounts of C6H5NO2adsorbed (monolayer coverage by anion radicals) vs. CO vs. (RO)3P can be compared. It is apparent that a t least monolayer coverage takes place. If we calculate the number of surface MgO molecules vs. (RO)3P molecules adsorbed the valuea shown in Table IV are found. These data suggest very crowded coverage and in the cases of (CH3CH20)3Pand (C6H5)(CH3CH20)2P more than monolayer coverage is probable. Mode of Adsorption. Adsorption of (CH3CH20)3Pon MgO caused a slight shift in C-H bending modes to lower energy and the v p a bands became very broad and shifted to lower energy. The CH3CH2groups remained intact.
Table IV. Amounts of Phosphorus Compounds Adsorbed Compared with Surface MgO Moieties no. of surface MgO moieties phosphorus compd /adsorbed molecule" (CHnCH20)nP=O 10 (cH,o),P 160 (CHSCHZO)~~ 4 (CH&H&H20)3P 10 ((CH~)ZCHO)BP 10 (ClCHzCHz0)3P 5 (CF~CHZO)~~ 12 (CGH~)(CH~CHZO)~P 5 (CH&Hz)z(CH@)P 14 (CHSCHZ)BP 25 C13P=0 large PC13 large PC1, large "Based on 1 C ~ H ~ N O molecule/25 Z surface MgO molecules.
These results suggest a coordination through phosphorus accompanied by a net gain in electron density: nEt C' -ElOEt -C2H,
i,d.\ s.y
Upon heating R and RO groups were lost and adsorbed P=O species remained. Adsorption of (CH3CH20),P=0 occurred through the P=O bond and a net electron loss is suggested by the
604 Langmuir, Vol. 1, No. 5, 1985
Lin and Klabunde
slight band shifts to higher energy. Upon baking R and RO groups were completely lost and P=O groups were OEt o-P\
0
I O,Et
0-P
oH,.l
Interestingly, the nucleophilic attack a t phosphorous apparently does not occur with (CH3CH20),P=0, and no ether is formed. Strong coordination through the P=O group must prohibit the RO elimination reactions: OR
0
II
0-P/
OR OR
0 0
'P-0
II
OR
I/
0-P
I
-0
\o
OR FHZCH3
Mg-0-
formed possibly accompanied by formation of a POP moiety. Adsorption of (CH3CH2),P on activated MgO is much weaker than for the phosphate or phosphites. Band shifits to higher energy suggest net electron density loss to the surface. Baking caused loss of R groups and the formation of adsorbed phosphorus with a small amount of P=O formation.
At higher temperatures attack at carbon probably becomes more prevalent and alkenes and water are formed. A third type of reaction, that of proton abstraction, may OR
/
OR
/
CH2=CH,
Et P
0
II
also be important although this reaction must not lead to stable carbanionic species since D20 addition did not yield deuterated phosphites. The proton abstraction reaction is very likely to occur since even methane13 and neopentane14 undergo this reaction over thermally activated MgO. This process could explain C2H4 formation from ethylphosphines, ethyl phosphites, or ethyl phosphates. Thus, the simple reaction products can be explained. Note, however, that C3, C4, and C5 alkenes and alkanes were also produced in some reactions, especially at higher temperature (see Table 111). Explaining these products necessitates the formation of C1 fragments, probably carbenes. A possible mode of formation of surface carbene is '
q/p\xp Mechanism of Decomposition. Templeton and Weinberg2 have recently reported on the decomposition of phosphonate esters (R0)2(R)P=0 over A1203 surfaces. They identified two important reactions, (1)nucleophilic substitution a t phosphorus and (2) nucleophilic substitution a t the alkyl carbon of an alkoxy. Both of these reactions must also take place on the MgO samples. At lower temperatures the formation of ethers from phosphites may occur as R,
'0 P-0-C
Mg-0
I
R\
/R
\
..J
/
H2 -C 0-Mg-0
H3
I
1
LJ
0-Mg-0-Mg-0-Mg
I
-q ' $f R -0
t
-CH2Cti3
At higher temperatures the CH2 group may be attacked more frequently and vigorously:
At higher temperatures the presence of CH2 and C2H, could lead to propene formation. An alternative though similar explanation may involve metathesis of C2H4/C4H8, and this possibility will be investigated further. Last, the unusual fiiding that (CH3CH&(CH30)Pyields mainly (CH3CH2)z0as a decomposition product suggests that R and RO groups are labilized on the MgO surface. It is difficult to rationalize this result without further study. It does serve to illustrate the extreme labilization phosphorous compounds undergo on these active oxide surfaces. Amounts Decomposed. Decomposition of the phosphorus compounds yielded volatile hydrocarbons, ethers, and alcohols. Products containing phosphorus were not released. These results coupled with elemental analyses of the oxide residues show that the phosphorus remained with the MgO, and large amounts of phosphorus compounds were decomposed, on the order of five mole(13) (14)
Utiyama, M.; Hattori, H.;Tanabe, K.J . Catal. 1978,53,237-242. Unpublished results of M. F. Hoq from this laboratory.
Langmuir 1985,1,605-607 cules/surface MgO moiety. This suggests that the active sites are catalytic and that the phosphorus residues are mobile and move away from the active site so as not to poison it, and thereby must move into the bulk, or the MgO matrix is continually disrupted and fresh active sites are exposed (a stoichiometric reaction). These decomposition reactions are essentially stoichiometric, and in the most favorable cases about one phosphorus molecule is decomposed for every two MgO molecules present in the bulk or on the surface.
Experimental Section Materials. MgO and CaO were purchased from ROC/RIC and are listed as 99.9949999% pure. The procedures for washing, drying, and thermally activating have been described previously! Carbon monoxide (Linde, 99.999%) and deuterium gas (Stohler, 99.7%) were passed through a -196 OC trap just prior to adsorption. Deuterium oxide (Merck, 99.8%) was freeze-thaw d e g d before use. Phosphorus compounds were purchased from Strem Chemical Co. and were freeze-thaw degassed just before use. IR Spectroscopy. A Perkin-Elmer 1330 equipped with a CDS 0.d.) were Data Station 1300was used. The MgO pellets (15" prepared from dry, non-activated MgO at 2000 psig. They were placed in a special cell described earlieru where they were activated under vacuum at 700 OC for 14 h. After it was cooled to room temperature the pellet was exposed to several torr of pressure of the desired phosphorus compounds for several minutes. The excess phosphorus compound was pumped out down to torr, the pellet guided into the NaCl window section of the apparatus, and IR spectra taken. (15) Matauo, K.; Klabunde, K. J. J. Org. Chem. 1982,47, 843-848.
605
GC-MSAnalyses. A Finnigan 4021 GC-MS system with an INCOS data system and Finnigan 9610 gas chromatograph was used. Volatile compounds from phosphorus compound decompositions were separated on a l/* in. X 25 f t stainless steel column packed with 20% bis(Bmethoxyethy1) adipate on Chromasorb P-AW SO/€@at 40 OC and 25 mL/min helium gas flow rate. Mass spectra were taken at 20-70 eV where the optimum was selected to maximize the molecular ion for each compound. Elemental Analyses. The (CH3CH20)3P/Mg0residue was analyzed after baking at 220 OC for 2 h and vacuum removal of volatiles. Anal. Calcd for CzH6P03(Mg0)l.9:C, 13.01; H, 2.73; P, 16.78; Mg, 25.02. Found C, 12.89;H, 2.58; P, 16.91;Mg, 25.22. Adsorption of Phosphorus Compounds. Thermally activated oxide (1.0 g) was exposed to a known amount (PV measurements) of phosphorus compound until the pressure in the closed system (of known volume) was constant for 30 min. The amount adsorbed was then calculated by PV measurements. Decomposition of Phosphorus Compounds. In these experiments excess phosphorus compound was condensed onto a 1.0-g MgO sample, and the sample was isolated from vacuum and warmed to the desired temperature for a prescribed time (usually 2 h). Afterward the sample was cooled to -78 OC, and volatiles were vacuum evaporated and trapped in a -196 OC trap. They were diluted with p-xylene with cyclopentane as an internal standard and analyzed by GC and GC-MS as described above. Acknowledgment. This work was supported by the Army Research Office under Grant DAAG29-84-K-0051. Registry No. MgO, 1309-48-4; CaO, 1305-78-8; (CH3CH2O),P=O, 78-40-0; (CH3CH2)3P,554-70-1; (CH,O),P, 121-45-9; (CHsCH20)3P,122-52-1;(CH&H2CH20)3P, 923-99-9; ((CH&CHO)3P, 116-17-6;(C1CH2CH20),P,140-08-9;(CF3CH20),P,37069-4; (CBHb)(CH3CH20)2P, 1638-86-4; (CH&H2)2(CH3O)P, 13506-71-3.
Photoluminescent Response of Palladium-Cadmium Sulfide and Palladium-Graded Cadmium Sulfoselenide Schottky Diodes to Molecular Hydrogen Michael K. Carpenter, Hal Van Ryswyk, and Arthur B. Ellis* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received March 27, 1985 The bulk photoluminescence (PL) of Schottky diodes constructed from P d and n-type CdS (Pd-CdS) and from n-type, graded CdS,Sel, (0 Ix I1;2: = 1at surface) (Pd-CdS,Sel,) is sensitive to molecular Hz. For the Pd-CdS diode, exposure to Hz (31, N2/Hz mixture) significantly enhances the PL intensity 510 nm) relative to the intensity in air; by use of a dead-layer model, the of edge emission (Amm corresponding reductions in depletion width and Schottky barrier height can be estimated. For the Pd-CdS,Sel, diode, Hz changes the spectral distribution; PL from this material, color coded to spatially resolve e--h+ pair recombination, indicates the depth over which the electric field is reduced in the semiconductor. These phenomena demonstrate optically coupled chemical sensing of hydrogen.
-
Introduction The bulk photoluminescence (PL) of semiconductorshas proven to be a useful probe of Schottky barrier characteristics in both semiconductor/metal diodes' and photoelectrochemicalcells (PEGS)? In particular, the electric field in the semiconductor can be estimated from PL intensity using a dead-layer model: Electron-hole (e--h+) (1) Hollingsworth, R. E.;Sites, J. R. J.Appl. Phys. 1982,53,6367 and references therein. (2) Hobeon, W. S.; Ellis, A. B. J. Appl. Phys. 1983, 54, 6956 and references therein.
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pairs formed within a distance from the interface on the order of the depletion width do not contribute to PL. While the importance of surface properties on bulk PL has been described?~~ their interrelationship has not been examined experimentally in detail. In choosing a system for such studies, our interest was drawn to Schottky diodes constructed with Pd because of the known sensitivity of their current-voltage (i-V) properties to gaseous H,.In general, Schottky barriers (3) Mettler, K. Appl. Phys. 1977, 12, 75. (4) Stephens, R. B. Phys. Rev. B. 1984,29, 3283.
0 1985 American Chemical Society
