1118
Langmuir 1988, 4 , 1118-1122
space-filling model of ethylene shown in Figure 7, each ethylene molecule occupies about 21 A2 on the surface. At OBi = 0.33, with a Bi radius of 1.86 A,' there should be no Pt ensemble available that is large enough for ethylene adsorption, even though some Pt surface remains uncovered. However, we observe ethylene adsorption until Bgi 2 0.37. This general phenomena has also been observed for larger cyclic hydrocarbons such as cyc10hexane'~J~ and b e n ~ e n e ' ~adsorbed ?'~ on B i / P t ( l l l ) surfaces. LEED observations indicate that the bismuth adatoms are fairly mobile on the surface, even at temperatures as low as 150 K.' Also, TPD shows clearly that the heat of adsorption (and, by inference, the activation energy of diffusion) is reduced as the Bi coverage is increased. The work function change with increasing bismuth coverage indicates that the bismuth adatoms, which are slightly positively charged at low coverage, depolarize and become more metallic-like at higher coverages. Thus, at higher OBi, the Pt-Bi chemical bond and Bi-Bi lateral repulsive interactions are weakened. It is possible that the ethylene-metal interaction is strong enough to displace the bismuth adatoms laterally a t high bismuth coverages, thus forcing the bismuth to segregate and leaving larger Pt ensembles available for adsorption and reaction. These effects obviously become more important with adsorbates that are more strongly chemisorbed than ethylene. Conclusions Coadsorbed bismuth does not substantially alter the
chemisorption bond strength of molecular ethylene to Pt(ll1) or affect the decomposition reaction mechanism. The activation energies for the elementary dehydrogenation steps in this mechanism are also not affected. No chemisorption is observed on complete layers of bismuth. Finally, no significant changes are observed in the vibrational frequencies of adsorbed ethylene upon coadsorption with bismuth. Thus, bismuth adatoms appear to be ideal probes for determining hydrocarbon reaction site size requirements on Pt surfaces. Preliminary consideration of the site-blocking influence of coadsorbed bismuth indicates that an ensemble of four contiguous Pt atoms is required to chemisorb ethylene, and an ensemble of six Pt atoms is required to dehydrogenate ethylene to ethylidyne (CCH3). A similar ensemble size requirement is also indicated for the subsequent decomposition of CCH3. Acknowledgment. Support for this work was provided by the US.Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division, through Grant DEFG02-86ER13473. Support by the US.Department of Energy, through the Morgantown Energy Technology Center Phosphoric Acid Fuel Program, is gratefully acknowledged by M.T.P. We thank Dr. A. F. Voter and Professor C. T. Campbell for useful discussions and for making available data prior to publication. Registry No.
Bi,7440-69-9;C2H4, 74-85-1; Pt,7440-06-4.
Synthesis and Catalytic Activity of N-Oxide Surfactant Analogues of 4- (Dimethylamino)pyridine Alan R. Katritzky,*it Bradley L. Duel1,j Danuta Rasala,t Barry Knier,§ and H. Dupont Dursts Department of Chemistry, University of Florida, Gainesville, Florida 32611, and Applied Chemistry Branch, Chemical Division, Research Directorate, U.S. Army Chemical Research and Development Center, Aberdeen Proving Ground, Maryland 21010-5423 Received September 28, 1987. I n Final Form: January 22, 1988
Surfactants of different charge types containing the 4-(N,N-dialkylamino)pyridine1-oxidemoiety have been synthesized and tested as catalysts for the hydrolysis of active carboxylic and phosphorus esters in the presence of cetyltrimethylammonium chloride. Their catalytic activity is compared with that of a series of simple 4-(dimethy1amino)pyridine 1-oxides. Introduction We have previously reported1 studies of the catalytic activity of surfactants incorporating the 4-(N,N-dialkylamino)pyridine moiety for the hydrolysis of fluorophosphonate and carboxylate esters. All these compounds catalyzed the hydrolysis of p-nitrophenyl hexanoate, but disappointingly none efficiently accelerated that of fluorophosphonate (Soman), for which hydrolysis was increased over the background by a factor of only 1.1-1.2. In an attempt to improve the catalytic efficiency of these derivatives for the hydrolysis of fluorine-phosphorus bonds, we turned our attention to the related N-oxides. Recent studies have shown that, although less basic than t
University of Florida. U S . Army Chemical Research and Development Center.
the corresponding pyridines, pyridine N-oxides are much more effective catalysts in acyl- and sulfonyl-transfer rea c t i o n ~ . ~In, ~particular, 4-(N&-dimethylamino)pyridine 1-oxide (DMAP 1-oxide) (2a) was approximately 50 times more active than DMAP (la) itself in the cleavage of sulfur-halogen bonds in sulfonyl halides, and the 1-(sulfony1oxy)pyridinium intermediates thus formed were 3-17 times more reactive than the 1-sulfonylpyridinium analogues toward aminolysis with aryl amine^.^,^ Thus, al(1) (a) Katritzky, A. R.; Duell, B. L.; Seiders, R. p.; Durst, H. D. Langmuir 1987, 3, 976. (b) Katritzky, A. R.; Duell, B. L.; Knier, B. L.; Durst, H. D. Langmuir 1988, 4, 192. (2) Savelova, V. A,; Belousova, I. A.; Litvinenko, L. M. J. Org. Chem. USSR (Engl. Transl.) 1981,17, 1333. (3) Belousova, I. A,; Savelova, V. A.; Litvinenko, L. M.; Matvienko, V. N. J . Org. Chem. USSR (Engl. Transl.) 1979, 15, 1759.
0743-7463/88/2404-1118$01.50/0 0 1988 American Chemical Society
Langmuir, Vol. 4,No. 5, 1988 1119
N-Oxide Surfactant Analogues of DMAP though the 4-(N,N-dialkylamino)pyridinecatalysts previously reported' showed low hydrolytic activity versus fluorophosphonate, it was conceived that the corresponding N-oxides might be significantly more active. Therefore, 1-oxides of the 4-(N,N-dialkylamino)pyridinecatalysts of several charge types have been prepared. The compounds investigated comprised five simple pyridine 1-oxides (2a-e) and four surfactant N-oxides (3a-c, 4). R
bb +T
1
lb=2b
R=NMe, R=NEt,
I C = 2C
R
la=2a
N(CH,),
I d = 2d
R N(CH,), le = 2e R = N(CH,CH,),CHMe
-0 1
L
b
+i
a Kinetic analysis of 2a against PNPDPP gave essentially background hydrolysis at concentrations between 0.5-8 mM. The analysis program gave unreliable fits to these data. *The solubility in aqueous buffer of 3c is limited about 0.75 mM, even in 0.005 M CTAC. Thus all concentrations used to determine the kz were between 0.08 and 0.8 mM. This insolubility limits the usefulness of 3a in any aqueous catalytic process. 4-Substituent = 44 1-carboxymethyl-1-decylpiperazine).
N\ ' 3a
R=H
3b 3c
-0 3
Table I. Kinetic Results for 4-(Dialky1amino)pyridines and 4-(Dialkylamino)pyridine 1-Oxides kCAT,M-' s-l at [CTAC] a/0.001 a/0.005 compd 4-substituent substrate M CTAC M CTAC PNPH 1.96 1.32 NMe2 la PNPH 3.23 2.32 lb NEt 2.51 2.96 IC 5.21 5.22 Id (4.57) 13.2 9.35 N(CH2CH2)&HMe PNPH le 0.12 0.11 NMe, PNPH' 2a 0.28 0.19 PNPH 2b 0.32 0.23 PNPH 2c 0.52 0.34 PNPH 2d 1.60 1.31 PNPH 2e 29.2 25.3 PNPH 3a 0.11 0.07 PNPDPP 3a 25.4 22.4 PNPH 3b PNPDPP
