Oxidative cleavage of 2-methylpropene on Ir(111): double bond scission during the formation of acetone

surface s c i e n c e

ELSEVIER

Surface Science 383 (1997) 173-202

Oxidative cleavage of 2-methylpropene on Ir( 111 ): double bond scission during the formation of acetone S.G. Karseboom, J.E. Davis, C.B. Mullins * Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, USA Received 24 October 1996; accepted for publication 3 February 1997

Abstract

High resolution electron energy loss spectroscopy (HREELS) and temperature-programmed reactive desorption (TPRD) techniques have been used to study the oxidation of n-bound and di-cr-bound 2-methylpropene (isobutylene) on the Ir( 111 ) surface. For surface temperatures of less than 80 K, isobutylene adsorbs into a n-bound state on the I r ( l l l ) - ( 2 x 1)O surface and into a di-~-bound state on the bare It( 111 ) surface. Isobutylene which is n-bound reacts with oxygen upon heating to yield gaseous t-butanol, acetone, water and dihydrogen as well as adsorbed acetate, rl2(C,O)-acetone, and possibly formate by 330 K. Isotopic labeling experiments indicate that gaseous acetone is not produced from an allylic intermediate. The oxidation of n-bound isobutylene is proposed to proceed through sequential -O(CH3)2CCH 2- and -O(CH3)2CCH20- species, t-Butanol is produced from the former species while acetone, acetate, water, dihydrogen and formate are produced from the latter. Di-cr-bound isobutylene species are not thought to participate in the formation of acetone and acetate since no detectable amounts of adsorbed acetone or acetate and little gaseous acetone were produced from the direct oxidation of di-g-bound isobutylene. © 1997 Elsevier Science B.V.

Keywords: Alkenes; Catalysis; Chemisorption; Electron energy loss spectroscopy; Iridium; Low index single crystal surfaces; Oxidation; Thermal desorption spectroscopy

1. Introduction

The heterogeneously-catalyzed, partial oxidation of olefins is an important branch of the petrochemical industry, and as such, much effort has been directed at understanding the mechanisms of these conversions. Several different reactions have been examined, but selective oxidations, such as the ammoxidation and epoxidation reactions, have been the focus of this effort [1,2]. Although some mechanistic aspects of these two reactions remain unclear, the influence of * Corresponding author. Fax: + 1 512 471 7060; e-mail: [email protected] 0039-6028/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0039-6028 ( 9 7 ) 0 0 1 6 0 - X

allylic hydrogens on product distributions is fairly well understood. During the oxidation of olefins containing aUylic hydrogens, two competing processes occur: direct reaction at the double bond and abstraction of an allylic hydrogen to form a ~-allyl species. In both the ammoxidation reaction and the epoxidation reaction, the latter pathway dominates. For example, the ammoxidation of 3-13C-propene produces acrylonitrile (CH2CHCN) with the 13C almost evenly divided between the CH2CH and CN portions of the molecule [3] as would be expected if the symmetric ~-allyl (C3H5) were an intermediate in the reaction. Other mechanisms that would be expected to produce similar results, such as rapid double bond

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S.G. Karseboom et al. / Surface Science 383 (1997) 1 7 3 2 0 2

migration prior to oxidation and initial protonation to give CH3CHCH3+, can be ruled out based upon studies of the closely related propylene to acrolein (CH2CHCHO) reaction [4,5]. The preferential formation of rt-allyl species is also thought to occur when allylic hydrogen-containing olefins replace ethylene in the silver-catalyzed epoxidation reaction [6]. In this case, however, the n-allyl species are predominantly converted into the combustion products COz and H20. The preference for n-allyl formation and subsequent combustion may be seen by comparing typical yields of ethylene oxide and propylene oxide, produced from oxidation over a silver catalyst. The epoxidation of ethylene, which contains no allylic hydrogens, yields ,-,45% ethylene oxide [7] ( ~ 80% if promoted catalysts are used [8]) while the same process with propylene yields ~4% propylene oxide [7]. Unfortunately, the preference for 7r-allyl formation in epoxidation reactions effectively limits the usefulness of silver to the production of ethylene oxide. In sharp contrast to the ammoxidation and epoxidation reactions is the oxidative cleavage reaction on supported iridium catalysts [9,10]. Partial oxidation products derived from direct reaction at the double bond are produced in yields greater than 30% even when the olefin contains allylic hydrogens. This reaction is even more unusual because the products are formed from the complete scission of the double bond. Scheme 1 presents the products derived from a trisubstituted olefin and from a terminal, disubstituted olefin. Scission fragments containing two alkyl substituents are converted into ketones, while those containing one substituent are converted into carboxylic acids (possibly through an aldehyde intermediate). Methylene fragments are converted into C Q and H20. Little is known about the oxidative cleavage reaction at the molecular level [9,10]. Oxidation of 1-'4C-propene at atmospheric pressures yields acetic acid that is virtually non-radioactive. Since no isotopic mixing occurs, symmetric intermediates such as rc-allyl and CH3CHCH3 + as well as rapid double bond migration prior to oxidation are precluded. On the other hand, rc-allyl may be an intermediate in the total combustion of propylene; deuterium substitution on the methyl group of

RI.

R3

~ H

[Ol Ir

Ra>

O+O

v

Rz

R2

R~

<

OH

R4

R I c H [O]ir-R5 H R)

O +

C02 H20

a 1, R 2, a 3, a 4, R 5 = ~ k y l Scheme 1. Products of olefin oxidation on supported iridium catalysts at atmospheric pressures. The products are formed from the scission of the olefins' double bond.

propylene reduces the rate of reaction to CO2 and HzO by 70% while reducing the rate to acetic acid by only 20%. Such a large isotope effect for the rate of combustion is consistent with the abstraction of an allylic hydrogen in the rate-limiting step. To our knowledge, no studies employing ultrahigh vacuum ( U H V ) techniques, which allow the use of many powerful spectroscopic tools such as high resolution electron energy loss spectroscopy (HREELS) and temperature programmed reactive desorption (TPRD) techniques, have been performed which examine the oxidative cleavage reaction. The incomplete understanding of the mechanistic aspects of this reaction coupled with its unique nature, direct olefinic double bond reaction and scission in the presence of allylic hydrogens, has prompted us to investigate it further. Herein we report the oxidative cleavage of isobutylene, an olefin containing six allylic hydrogens, to acetone on I r ( l l l ) under ultra-high vacuum ( U H V ) conditions. TPRD in combination with HREELS has been used to probe the mechanisms of acetone formation. Selectivities for acetone formation will be presented and possible intermediates in the conversion will be discussed.

2. Experimental A more complete description of the UHV apparatus appears elsewhere [11]. In brief, all

S. G. Karseboom et aL / Surface Science 383 (1997) 173-202

experiments were conducted in an UHV apparatus with a base pressure of ~ 2 . 5 x l 0 - 1 ° T o r r and equipped with a quadrupole mass spectrometer (QMS), an Auger electron spectrometer (AES), a high resolution electron energy loss spectrometer (HREELS) and an ion gun for sputtering. The Ir( 111 ) sample, approximately 1 mm thick and 10 mm in diameter, was polished and was verified to be aligned within +0.4 ° of optimal using Laue backscattering techniques. Cooling and resistive heating were achieved through two 0.5 mm diameter tantalum wires spot-welded to the side of the sample and attached to an electrical feedthrough in thermal contact with a liquid nitrogen reservoir. By pumping on this reservoir, sample temperatures lower than 70 K could be attained. Control of cooling and heating was performed by a custom Windows application capable of executing linear temperature ramp sequences while multiplexed to the QMS mass filter, the QMS signal multiplier, the ion gauge controller output and a thermocouple spot-welded to the side of the sample [12]. A typical cooling time from 1550 to 80 K was 3 min. Sample cleaning employed a combination of oxygen doses and Ar sputter/anneal cycles. The sample initially was cleaned in 10-v Torr of oxygen at 1200 K for several minutes, after which it was subjected to several sputter/anneal cycles. This process was repeated until all surface impurities were below AES detection limits. Between experiments, surface carbon was removed by dosing with 0 2 and then flashing in vacuo to 1550 K. Surface cleanliness was routinely verified by monitoring the amount of desorbing oxygen and by the lack of CO desorption features (indicative of surface carbon oxidation during the flash). On occasion it became necessary to sputter the surface of the sample to remove impurities which had accumulated, presumably from diffusion from the bulk of the sample. During experiments, the sample was positioned outside of the HREELS magnetic shielding while being dosed with gas and while being heated. With the sample in dosing/heating position, the angle made between the sample surface normal and the QMS ionizer was ~45 ° and the overall distance was approximately 10inches. Exposures were

175

made by backfilling the chamber and quantified by integrating ion gauge pressures with respect to time. Reported doses have been corrected for chamber background pressure but not for ion gauge sensitivity. A heating rate of 10 K s-1 was used in all experiments. After dosing/heating was complete during the HREELS experiments, the sample was lowered rapidly into position for the collection of an energy loss spectrum. Spectra were collected with surface temperatures Ts<80 K using a 4--6 eV primary electron beam energy and with the energy analyzer either rotated into the specular beam (on-specular) or 12~ towards the sample normal with respect to the specular beam (off-specular). Typical on-specular resolutions and count rates were 40-50 cm -1 (FWHM) and greater than 100 kcps, respectively. Experiments employing HREELS and TPRD techniques probed the interaction of several compounds with two oxygen-modified Ir(ll 1) surfaces. The first of these surfaces was prepared by exposing the bare Ir(111 ) surface to greater than 24L of oxygen at Ts<100K and then briefly heating the resulting surface to 530 K to dissociate and/or desorb any adsorbed molecular oxygen [13] and to order the resultant oxygen adatom overlayer. The oxygen used was either ultra-pure carrier grade (1602) from Air Products or 95-98% 1802 from Cambridge Isotope Labs. The absolute coverage of oxygen adatoms when the surface was prepared in this manner was determined to be 0.50monolayers after comparing the time-integrated oxygen desorption rate from this surface to that of an It(111) surface exposed to a saturation dose (greater than 75 L) of oxygen at Ts = 300 K. The oxygen coverage on this latter surface has been established previously as 0.50 monolayers and corresponds to a (2 x 1) overlayer as illustrated in Fig. la [14]. The HREEL spectrum of the I r ( l l l ) (2 x 1)160 surface displayed a strong loss feature at 550 cm 1, which has previously been assigned to the v(Ir-O) mode of oxygen adatoms [15, 16], as well as a smaller feature at ~775 cm 1. The latter feature, which had a peak height ratio of ~0.02 relative to the 550 cm -1 feature, could not be attributed to contamination in the source

176

S.G. Karseboom et al. /Surface Science 383 (1997) 173-202 A

Q

B

IridiumAtom

• OxygenAtom

Fig. 1. Position of oxygen adatoms on the Ir(111 ) surface for: (a) a (2 x 1) overlayer and (b) a (2 x 2) overlayer. The (2 × 1) oveflayer corresponds to a coverage of 0.50 monolayers while the ( 2 x 2 ) overlayer corresponds to a coverage of 0.25 monolayers.

oxygen, to oxygen preferentially adsorbing onto some surface contaminant or to the adsorption of contaminants from the background during the collection of the spectra. One possibility is that this feature results from subsurface oxygen or a surface oxide [ 15] which was produced during the annealing step, although this feature was also observed in HREEL spectra of the Ir( 111 ) surface after exposure to oxygen at Ts = 300 K; this latter method of preparation has been reported previously to produce only a 550cm -1 feature [ 15,16 ]. Another possibility is that the 775 cm- 1 feature arises from the parallel movement of oxygen adatoms on the surface. As pointed out by Rahman et al. [17], the adatoms in the (2x 1) overlayer have Cs symmetry which causes both the vertical vibration and one of the parallel vibrations to be dipole-active. If the 775 cm-1 feature does correspond to a parallel vibration, then the ratio of the 775 to 550 cm- 1 frequencies provides information about the depth at which the oxygen adatoms reside in their three-fold sites. Based upon the nearest-neighbor force constant model [18], the oxygen adatoms would be located on the order of ~0.8 A above the Ir( 111 ) plane (drawn through the center of the Ir atoms). The second oxygen-modified surface was prepared similarly to the manner in which the Ir( 111 )(2 x 1)O surface was prepared, except that a 2.4 L dose of oxygen was used. The resultant oxygen adatom coverage on the second surface was ,-~0.25 monolayers. At this coverage, oxygen might be present as: (1) a (2 x 1) overlayer interspersed with areas of bare metal, (2) as a true (2 x2)

overlayer (Fig. lb) or (3) as a combination of the two. To avoid confusion with the Ir( 111 )-(2 x 1)O surface (0o=0.50monolayers), the surface with 0o ~0.25 will be referred to as the I r ( l l l ) Oadatom surface in subsequent text. As was the case for the Ir(lll)-(2 x 1)160 surface, two loss features, one at 550 cm -~ and one at --,765 cm -1, were observed in the HREEL spectrum obtained of the Ir(111)-16Oa~to m surface. The ratio of 765 to 550 cm -~ peak heights was --~0.02. Compounds examined on these two oxygenmodified Ir(111) surfaces were: isobutylene-do (99%, Aldrich Chemical), isobutylene-d6 (98% D dimethyl-, Cambridge Isotope Labs), acetone-do (99.5%, EM Science), acetone-d6 (99% d6, Sigma Chemical and Aldrich Chemical), acetic acid (99.7%, EM Science), formic acid (>88%, EM Science), t-butanol (>99.0%, EM Science), isobutylene oxide (98+%, TCI America), deuterium oxide (99.9% D, Isotec), deuterium (99.8% D, Cambridge Isotope Labs) and 2,3-dimethyl2-butene (98%, Aldrich Chemical). Solid and gaseous compounds were used as received while liquid compounds were subjected to several freezepump-thaw cycles prior to use. To ensure proper purity, the QMS spectrum of each compound was compared to spectra reported in the literature whenever possible. For those experiments that employed formic acid, which apparently degraded in the stainless steel leak valve lines over time, a fresh supply of material was loaded from a glass reservoir (containing liquid formic acid) prior to the start of each experiment. The interaction of two of these compounds, isobutylene and oxygen, on the heated I r ( l l l ) surface was found to produce gaseous t-butanol and gaseous acetone as will be presented and discussed in detail later. However, the identification of the two compounds will be presented here. t-Butanol desorption was positively identified by employing a combination of tests. Because of interference by other desorbing species, only two ions in the TPRD spectrum of the I r ( l l l ) (2 × 1)O/isobutylene prepared surface were used in the identification: one with a mass:charge ratio (m/q) of 31 and one with a m/q of 59. The ratio of re~q=31 to re~q=59 ions in our experiments was ,--0.34, which compares well to the measured

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

cracking ratio of 0.37 for t-butanol leaked into the apparatus. When ~so 2 was used in place of 1602, these two ions were replaced by ions at m/q= 33 and m/q=61, thus indicating that each contained a single oxygen atom. Based upon this data and cracking patterns reported in the literature, the compounds 1-propanol, 2-propanol, 2-propenal, 2-methylpropanal, 1-butanol, 2-butanol, 2-methyll-propanol and isobutylene oxide can be ruled out. One compound that cannot be dismissed is methyl isopropyl ether. However, the production of this compound is less likely than the production of t-butanol since an oxygen atom would have to be inserted into isobutylene's carbon skeleton instead of onto it. In addition, an ether should be more unstable than a tertiary alcohol under oxidizing conditions. The acetone identification procedure was similar to the t-butanol identification procedure. Once again, only two ions, m/q = 58 and m/q = 43, were used in the identification because of interference from other desorbing species. When 180 2 replaced 1602 in experiments, these two ions were replaced by ions at m/q=60 and m/q=45 indicating that each contained only a single oxygen atom. The ratio of m/q = 58 to m/q = 43 in T P R D experiments was consistently 0.26 as compared to 0.29 for acetone leaked into the apparatus. The experimental ratio improved slightly (to 0.27) after correcting for the desorption of t-butanol which also has a significant fragmentation ion at m/q = 43. Assuming that the m/q= 58 ion is the parent ion for this species and that it contains only carbon, hydrogen and one oxygen atom, its empirical formula must be C3H60. All stable C3H60 compounds except acetone (methyl vinyl ether [58:43 = 1.1], propylene oxide [58:43= 1.3], allyl alcohol [58:43 =8.7], propanal [58:43 =47] and trimethylene oxide [CH2CH2CH20" 58:43=63]) are preLI I' cluded based upon the m/q= 58 to m/q =43 ratio. Acetone production was also confirmed on another oxygen/isobutylene modified Ir( 111 ) surface. This surface was prepared by exposing the bare l r ( l l l ) surface at T+<80 K first to isobutylene and then to greater than 24 L of oxygen. Because of poor signal to noise ratios, the procedure used in identifying the production of gaseous acetone by this surface was limited to the

177

detection of m/q = 58 and 43 ions when 1602 was used and the detection of m/q=60 and 45 ions when 180 2 was used.

3. Results

T P R D and H R E E L S techniques have been used to examine the oxidation of isobutylene on the Ir( 111 )-(2 × 1 )O surface. As will be detailed below, the chemistry of this system is rather complex and involves: (1) the formation of two gaseous partial oxidation products, acetone and t-butanol, as well as several gaseous combustion/dehydrogenation products; and (2) at least four distinct sets of adsorbates, each of which is stable in a different surface temperature regime. The identity of two of the adsorbate sets could only be positively confirmed after investigating experimentally the interaction of acetone and acetic acid with oxygenprecovered Ir( 111 ). These experiments show that acetate and qz(c,O)-acetone exist as stable isobutylene oxidation products between Ts ~ 300 K to Ts~400 K while acetate and small amounts of other species exist from Ts ~ 400 K to Ts > 440 K. Furthermore, the experiments show that rl2(C,O) acetone is oxidized to acetate near 400 K. Thus, the results of the acetate investigations are presented first followed by a discussion of the experiments regarding acetone. This assists the reader in more fully understanding the experimental results regarding the interaction of acetone with the oxygen-precovered Ir(111) surface as well as the experiments regarding isobutylene. Similarly, the results of T P R D and H R E E L S experiments regarding the reactions of per-hydrido and partially deuterated isobutylene with the oxygen-precovered lr( 111 ) surface are presented next. Finally, the results of experiments regarding the interaction of 2,3-dimethyl-2-butene with the lr( 111 )-(2 x 1 )O surface are also discussed. Like isobutylene, 2,3-dimethyl-2-butene is converted into acetone. However, the precursor(s) to acetone are different in the two systems, thus suggesting that isobutylene is not converted into acetone through a (C[-I3)2C intermediate. The decision of whether to study acetic acid and acetone on the Ir( 111)-(2 x 1)O surface and/or on

178

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

the Ir(111)-Oadatom surface depended upon what portion of the proposed isobutylene oxidation mechanism the experiments were expected to support; experiments expected to support the initial stages of isobutylene oxidation were conducted on the Ir( 111 )-(2 × 1 )O surface while those expected to support the final stages were conducted on the Ir( 111 )-Oadatomsurface. Those expected to support intermediate stages were conducted on both surfaces. The rationale for these choices is based upon the amount of oxygen remaining on the I r ( l l l ) surface during the course of isobutylene oxidation. Almost all isobutylene oxidation results presented in this paper were conducted on the I t ( l l 1)(2 × 1)O surface following exposure to 1.0 L of isobutylene at T~ < 80 K. During the initial stages of oxidation on this surface, the oxygen adatom coverage should be close to that on the Ir(111)(2 × 1)O surface without any isobutylene, 0.50monolayers. As oxidation proceeds, the oxygen adatom coverage drops until at the completion of oxidation, ,-~0.20 monolayers remain. This coverage is close to the oxygen adatom coverage of the Ir( 111 )-Oadatomsurface, ,-~0.25 monolayers.

to 0.5 L of acetic acid at Ts<80 K and (2) heating the resulting surface to 440 K. The loss feature at 550cm -1 corresponds to the v ( I r - O ) mode of oxygen adatoms [15,16] while the feature at 2075 cm -1 corresponds to the v(CO) mode of carbon monoxide [19]. All remaining features are consistent with acetate [20,21] and can be assigned as follows: v(Ir--Oaeetate) 335 c m - 1, 6(OCO) 690 cm -1, 8s(fH3) 1335 cm -1, v~(OCO) 1415 cm -1 and v ( f H 3 ) 2985 cm -1. Comparison of the loss features in Fig. 2 to those of acetate on the Pd( 111 )(2 x 2 ) 0 surface reveals many similarities between the two; on the Pd(111)-(2 x 2 ) 0 surface acetate loss features appear at: v(Pd-Oacetate) 305 cm -1, 6(OCO) 675 c m - 1, t~s(CH3) ~ 1300 c m - z, vs(OCO) 1405 cm -1 and v(CH3) 2945 cm -1 [21]. T P R D experiments were conducted to complement the information that was obtained from the H R E E L S experiments. Shown in Fig. 3 is the T P R D spectrum of the Ir(11 l)-Oadatom surface after exposure to 0.5 L of acetic acid at T~< 80 K. Only m/q=2, 18, 28, 44 and 60 ions were monitored during the experiment. Little hydrogenation of surface acetate to form gaseous acetic acid occurred. Instead, the acetate that was present on

3.1. Interaction of acetic acid with Ir( l l l)-O, aa,om Presented in Fig. 2 is the H R E E L spectrum of the Ir( 111 )-Oadatomsurface following ( 1 ) exposure •

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carbon dioxide

(m/q = 44, x3.2)

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C~m/qffi 28, x l . 0 ) ,

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/~ j ~

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water

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(m/q ffi 18, x19)

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hydrogen

(m/q - 2, _x7~)

2985 I

0

500

I000

1500

Energy Loss

2000 ( c m

2500

3000

"1)

Fig. 2. HREEL spectrum of Ir(111)-(2 × 2)0 surface after exposure to 0.5 L of acetic acid-do at T~< 80 K and subsequent heating to 440 K.

,

lOO

i

I

300

I

i

I i

,

I

500 700 T e m p e r a t u r e (K)

i

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900

Fig. 3. TPRD spectrum of the lr( 111)-(2 x 2)0 surface after exposure to 0.5 L of acetic acid-do at T~<80 K. Only masses 2, 18, 28, 44 and 60 were monitored.

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

the surface at 440 K (see Fig. 2) preferentially decomposed/oxidized at slightly higher temperatures to yield gaseous H20 and CO2 (maxima at 520 K). CO (540 K) and H 2 ( ~ 5 6 0 K) were also evolved above 440 K, and might have been the result of the decomposition/oxidation of one or more CxH~O~ surface species. Except for the CO desorption feature near 125 K, which is caused by desorption of CO from the sample holding assembly, all remaining features in Fig. 3 are attributed to the deprotonation of acetic acid to form acetate or to the non-selective decomposition of a portion of the dosed acetic acid.

179

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535

3.2. Interaction of acetone with lr( l l l)-Oad,tom /Ir( l l l ) - ( 2 x I ) 0 Previous studies have shown that acetone binds to metal surfaces either in an I"1'(O) state or in an rl2(C,O) state [22,23]. Only a weak interaction between the oxygen's electron lone pairs and the surface exists in the former, which results in little C = O rehybridization, while relatively strong ~-bonding/~*-backbonding exists in the latter, which results in significant rehybridization. For example, the C = O stretch of q~(O)-acetone-do on the R h ( l l l ) - ( 2 x 2 ) O surface appears at 1665 cm ' while this same stretch appears at 1380 cm a for qz(c,O)-acetone-do on the R h ( l l 1 ) surface [22]. For comparison, the C = O stretch of liquid acetone is at 1710cm -~ [24]. In general, rl'(O)-acetone is favored at low temperatures and on oxygen-precovered surfaces while rlz(C,O)-ace tone is favored at higher temperatures and on bare surfaces. At low temperatures, acetone binds both to the Ir(lll)-O~d~tom surface and to the I r ( l l l ) (2 × 1)O surface in the rl' state. Shown in Figs. 4 and 5a are the spectra of the Ir(lll)-Oad~tom surface after exposure at T s < 8 0 K to 0.5 L of acetone-do and acetone-d6, respectively. The HREELS spectrum of the Ir( 111 )-(2 x 1)O surface after exposure to 0.5 L of acetone-do at Ts<80 K was nearly identical to that shown in Fig. 4a. Loss assignments and comparisons to liquid acetone [24], q'(O)-acetone on R h ( l l l ) - ( 2 × 2 ) O [22], and q'(O)-acetone on Ru(001)-(2 × 2 ) 0 [23] are presented in Table 1. rll(O)-Acetone remained the

P~

0

500 1000 1500 2000 2500 3000

Energy Loss (cm") Fig, 4. H R E E L spectra of the lr( 111 )-(2 × 2 ) 0 surface: (a) after exposure to 0.5 L of aceton-do at T~<80 K and after heating the resulting surface to (b) 300 K and (c) 440 K.

only detectable organic species on the I r ( l l l ) O,d~tom and lr( 111 )-(2 x 1 )O surfaces after heating to 270 K. Heating to 330 K, however, induced changes that were dependent upon the surface being studied.

S. G. Karseboom et al. /Surface Science 383 (1997) 173-202

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500

1000

1500

2000

Energy Loss (cm "1) Fig. 5. H R E E L spectra of the Ir( 111 )-(2 × 2 ) 0 surface: (a) after exposure to 0.5 L of acetone-d0 at T~<80 K and after heating the resulting surface to (b) 300 K and (c) 440 K.

Heating the Ir(111)-(2 × 1)O/ql(O)-acetone prepared surface to 330 K lead almost exclusively to desorption. Only a few, small loss features were observed besides those attributable to oxygen adatoms and adsorbed CO. Because of poor signal to

noise ratios, it was impossible to identify positively the one or more species responsible for these features. Since TPRD experiments conducted with this surface revealed that 20-25% of the total desorbing acetone was evolved above 330 K, at least a portion of these species are thought to be intact acetone, bound to the surface in an 111(O) and/or an lqe(c,o) state. On the lr( 111 )-O,datom surface, 11l(O)-acetone was converted into one or more new species between 270 and 330 K. HREELS spectra of the new species are presented in Fig. 4b (non-deuterated) and Fig. 5b (deuterated). Based upon studies that have examined the interaction of acetone with the bare R h ( l l l ) [22] and Ru(001) [23] surfaces, the loss features in the two spectra are indicative of rlZ(C,O)-acetone. Assignments are provided in Table 2. A few of the weak loss features in the two figures cannot be assigned readily to rl2(C,O) acetone, oxygen adatoms, or CO: (1) the 1610-1620cm -1 features in Figs. 4b and 5b are attributed to rll(O)-acetone that either remained on the surface after heating to 330 K or was produced from the adsorption of acetone from the background during the collection of the spectrum and (2) the small loss features at 1480 and 1405 cm- 1 in Fig. 5b may result from a contaminant or from some species that is produced in competition with rl2(C,O)-acetone-d6 . The latter feature may correspond to the vs(OCO) mode of surface acetate-d3 based upon the results to be presented in the next paragraph. Between 355 and 390K, rlz(C,O)-acetone is converted into acetate. Figs. 4c and 5c show the H R E E L spectra of the Ir( 111 )-Oaaatom surface after (1) exposure to 0.5 L of acetone at Ts<80 K and (2) heating the resulting surface to 440 K. As can be seen, Fig. 4c is essentially the same as the spectrum of acetate formed from acetic acid (Fig. 2). A similar conversion of adsorbed acetone into surface acetate has been observed on Rh ( 111 )( 2 × 2 ) 0 [22]. Mode assignments are presented in Table 3 along with comparisons to acetate-do from Fig. 2 and H3COONa/CD3COONa [25]. Carbon-hydrogen stretching features in Figs. 4c and 5c are presumed to be lost in the background noise; only a small amount of acetate is produced, and the carbon-hydrogen stretching features of

S.G. Karseboom et al. / Surface Science 383 (1997) 173 202

181

Table 1 q~(O)-Acetone assignments Mode

CH3COCH 3 liquid [24]

Rh(111)[22]

Ru(001 )[23]

lr(lll)

8(CCC )

488 530 785 995/1091 1220 1361/1410 1710 2924/2964

n.r. n.r. n.r. 930 1240 1350/1410 1665 2980

515 540 780 825 900 n.r. 1395 1 4 4 0 1690 2950

425 n.r.

n(C O) v~(CCC) p(CH3) v~,(CCC) 8(CH3) v~(C =O) v(CH3)

CD3COCD 3 liquid[24]

408 480 690 945/1080 764/884 1240 1245 1350/1415 1005/1092 1630 1707 2915/2985 2222/2255

Rh(111)[22]

Ru(001 ) [23]

lr(lll)

n.r. 340 n.r. 715 1270 1015 1660 2230

495 505 715 720 920 1260 1030 1 0 4 5 1665 1675 2210 2240

350 n.r. 720 720/905 1275 1020/1100 163(I 2250

Table 2 q2(C,O)-Acetone assignments Mode

Q'H3COCH 3 liquid[24]

R h ( l l l ) [22]

Ru(001)[23]

8(CCC ) n(C=-O) p(CH3) vs(CCC )

488 530 995/1091 785 1220 1361-1410 1710 2924/2964

n.r. 600 990/ 800 1260 1380 1380 /3000

655 980/1170/1370 1300 -/2955

Vas(CCC) 8(CH3) v~(C=O) v(CH3)

Ir(lll)

CDsCOCD 3 liquid[24]

~ 335 680 975~ 760 1180/1385 1275 -/2960

408 480 764/884 690 1245 1005/1092 1707 2222/2255

Rh(lll)[22]

470 600 800/ 1230 1100 1360 2200/2220

Ru(001)[23]

Ir(lll)

610 820/820 880/1075 1275 2220/2220

~ 345 640 720/ 720 n.r. 885,'1070 1260 2230/2230

Table 3 Surface acetate assignments Mode

CH3COO [25]

v(lrOa~et,te) p(OCO) 8(OCO) v(CC ) p(C H,~) 8(CH3t v(OCO) v(CH3)

471/621 650 926 1020/1050 1344/1442 1413/1556 2935/3010

Fig. 2: CH3COOH on I r ( l l l ) - ( 2 x2)O annealed to 440 K

Fig. 4c: CH3COCH 3 on l r ( l l l ) - ( 2 ×2)O annealed to 440 K

335 n.r./n.r, 690 -/ 1335/1415 1415/ 2985

320 n.r,/n.r. 680 -/1335/1400 1400/-

acetate are weak compared to other acetate features (see Fig. 2). In addition, the feature at 890 cm-1 in Fig. 5c, while assigned to the v(CC) mode of acetate-d3, might belong to some other adsorbate since the v(CC) mode is not observed in the HREEL spectrum of acetate-d o prepared either from acetic acid-d o (Fig. 2) or from acetone-

CD3COO [25]

419/526 619 883 832/940 1085/1039 1406/1545 211/2264

Fig. 5c: CD3COCD 3 on l r ( l l l ) - ( 2 x 2)O annealed to 440 K 320 n.r./n.r. 660 890 1085,, 1410~

do (Fig. 4c). Finally, the small feature near 1610 cm -1 in Figs. 4c and 5c may result from the adsorption of acetone from the background during the scan or could correspond with acetate's vas(OCO) mode. Although rlz(C,O)-acetone was reported to be converted into surface acetate between 355 and

182

S. G. Karseboomet at, / Surface Science 383 (1997) 173~02

390 K in the last paragraph, the ultimate conversion of acetone into acetate on oxygen-precovered lr( 111 ) surfaces is considerably more complex. In particular, heating the Ir(111)-Oadatom/aCetone prepared surface to 330 K produced three different sets of product(s) during nominally identical experiments: rl/(C,O)-acetone exclusively, a mixture of rlZ(C,O)-acetone and acetate, and acetate exclusively. The latter two sets of products were observed infrequently and could have resulted from slight changes in the initial oxygen adatom coverage. The possible conversion of ql(O)acetone into acetate without going through an rlZ(C,O)-acetone intermediate, given these results, cannot be precluded, but the conversion of rl2(C,O)-acetone-d6 to surface acetate-d3 was verified. TPRD experiments of CH3C16OCH3 oxidation on the Ir( 1l 1)-(2 × 2)160 surface were conducted in conjunction with the HREELS experiments. A 0.5 L dose of acetone was used in these experiments as well. Hydrogen desorption maxima appeared at 265 and 300 K, water desorption maxima appeared at ~305, ~370 and 525 K, CO desorbed near 535 K, CO2 desorption maxima appeared at 440 and 525K, and acetone desorption maxima appeared near 300 and 360 K. The second acetone desorption feature was considerably smaller than the first and appeared as a shoulder in the spectrum. This feature probably resulted from the desorption of q2(C,O)-acetone given the temperature at which it appeared. The 525 K H 2 0 , 525 K CO2 and 535 K CO features were very similar to those in the TPRD spectrum of acetic acid on Ir( 111 )-Oadatom(Fig. 3), although a H 2 desorption feature near 560 K was observed in the latter spectrum. Additional TPRD experiments that employed C H 3 C 1 6 O C H 3 w e r e conducted on the I r ( l l l ) 18Oadatom surface. The exchange of oxygen between the surface and the adsorbed acetone was indicated by the evolution of both C H 3 C 1 6 O C H 3 and CH3CXSOCH3 and is suggestive of an (CH3)2C (180-) (160-) intermediate. Similar results have been observed on the R h ( l l l ) - ( 2 x l ) 1 8 0 surface [26]. For a 0.5L exposure of C H 3 C 1 6 0 C H 3 , approximately equal amounts of

C H 3 C 1 6 O C H 3 and C H 3 C l S O C H 3 desorbed from the Ir(111 )-18Oadatom surface.

3.3. Interaction o f lsobutylene-do with Ir( l l l )(2 x 1) O, T P R D

The TPRD spectrum of the Ir(111)-(2 × 1)O surface following exposure to 1.0 langmuir ( 1 L = 10 - 6 Torr" s) of isobutylene-do at Ts < 80 K is presented in Fig. 6. Only acetone, t-butanol, CO, CO2, water, dihydrogen, oxygen and isobutylene were detected during a mass search from m/q = 2 to re~q= 90. These species can be divided into three categories: partial oxidation products (acetone, t-butanol), combustion/dehydrogenation products (CO, CO2, H20, H2) and reactants (02, isobutylene). The first partial oxidation product, acetone, desorbs near Ts=305 K. The evolution is firstorder with respect to acetone and is reactionlimited rather than desorption-limited. TPRD measurements of the evolution of acetone-do and acetone-d6 from an I r ( l l l ) - ( 2 × 1)O/isobutylenedo prepared surface that was postadsorbed with a t-butanol

(m/q = 59, x160) ..........~. ?-" L'--- i ---"---~ ....."~;.,~i ......... ~

~

acetone ~_ ~ ~ _ ~ -~ (m/qf58, x160) carbon dioxide (m/q = 44, x6.1) isobutylene (m/q = 41, x3.0)

"~

~ ]

g~

t..q

oxygen (m/q = 32, x l . 0 ) ~ /

"~ co

(m/q. 28, ~.O)

water (m/q = 18, x40.) hydrogen 100

400

700 1000 T e m p e r a t u r e (K)

1300

Fig. 6. TPRD spectrum of the Ir(lll)-(2 × 1)O surface after exposure to 1.0 L of isobutylene-do at Ts < 80 K. H2, H20, CO, 02, (CH3)2CCH2, CO2, CH3COCH3, and (CH3)3COH were the only species that desorbed in detectable amounts.

S.G. Karseboom et al. / SurJace Science 383 (1997) 173~02

small amount ( ~ 0.1 L) of acetone-d6 revealed two distinct desorption maxima for the acetones. The acetone-d6 desorption maximum appeared at 265 K while the produced acetone-d0 desorption maximum appeared at 305 K. Identical results were obtained from an Ir(111)-(2 × 1)O/acetoned6 prepared surface postadsorbed with isobutylenedo. If the production and evolution of acetone-do were desorption-limited, acetone-do would have evolved from the surface at or slightly below the temperature at which the adsorbed acetone-d6 evolved, assuming that both acetones were bound similarly to the surface. Additionally, the leadingedges and desorption maxima for acetone and the lowest temperature hydrogen desorption feature in the TPRD spectra are nearly identical (Fig. 6), suggesting that both are produced from the decomposition/reaction of a common intermediate. Since HREEL spectra indicate that no carboncontaining species remained on the I r ( l l l ) (2 × 1)O prepared surface at the end of a TPRD experiment (to be presented shortly), the selectivity of irreversibly-adsorbed isobutylene to gaseous acetone could be calculated from the relative amounts of the different carbon-containing products that desorbed from the surface (irreversiblyadsorbed isobutylene is the portion of the adsorbed isobutylene that undergoes reaction rather than desorbing when the surface is heated). For an initial 1.0 L dose of isobutylene, the selectivity of irreversibly-adsorbed isobutylene to gaseous acetone was 4.7+0.4% (average and standard deviation from four experiments) after correcting for the differing ion gauge sensitivities and measured cracking patterns of the products. A similar value, 4.0 + 0.4%, was obtained from an H atom balance of desorbing products. For larger isobutylene doses, the selectivity to acetone was comparable, but for 0.25 and 0.50 L doses, the selectivity was about 7%. The evolution of t-butanol, like the evolution of acetone, is a first-order process. Based upon isobutylene-d6 (D dimethyl-) oxidation experiments to be presented later, t-butanol evolution is reactionlimited; the desorption maximum of produced (CD3)z(CH3)COH occurred 10K lower than the desorption maximum of produced

183

(CD3)2(CH3)COD. Such an isotope effect is consis-

tent with the protonation/hydrogenation of an oxygen atom on a t-butanol precursor (such as tbutoxide) as the rate-limiting step in t-butanol evolution. The selectivity of irreversibly-adsorbed isobutylene to gaseous t-butanol is lower than the selectivity to gaseous acetone; both C atom and H atom balances of desorbing products indicated that the selectivity was about 1% for isobutylene doses between 0.25 and 5.4 L. Several dehydrogenation and/or combustion products also desorb from the I r ( l l l ) (2 × 1)O/isobutylene prepared surface. CO2 desorption maxima appear at 450 and 520 K in the TPRD spectra. The lower temperature feature appears in the region where adsorbed CO was oxidized to gaseous CO2 on the Ir(11 l)-Oadatom surface while the higher temperature feature results from the decomposition of surface acetate (via HREELS, presented later). CO desorbs in two temperature regimes, one centered near 130 K and one centered near 560 K. The lower temperature regime is attributed to CO desorption from the sample holding assembly while the higher temperature regime probably corresponds to the production-limited oxidation of carbon-containing adsorbates. The H20 and H 2 desorption spectra are characterized by a series of incompletely resolved peaks. For H20 , desorption maxima occur at 305, 375, ~440 and 520 K, and for H2, desorption maxima occur at 305, 375 and 520 K. As mentioned previously, the evolution of H 2 and acetone near 305 K is probably limited by the decomposition/reaction of a common intermediate. To help support this hypothesis, a TPRD experiment was conducted on the Ir( 111 )-(2 x 1)O surface with coadsorbed isobutylene and deuterium. The D 2 desorption maximum (145 K with a tail extending past 700 K) was well below the first H 2 desorption maximum (305 K) as would be expected if the desorption of H2 were limited by the decomposition of some intermediate. However, it is possible that some or all of the D 2 desorption feature was an experimental artifact given that it extended from 145 K to greater than 700 K. A similar experiment, using D2O instead of D2, was conducted to determine if water evolution was also reaction-limited. Once again the non-deuterated

184

S. G. Karseboom et al. / Surface Science 383 (1997) 173-202

species desorbed at a significantly higher temperature (first maximum at 305 K) than its deuterated counterpart (maxima at 160 and 275 K, probably corresponding to the desorption of molecular water and water formed from the disproportionation of surface hydroxyl, respectively [27]). Although the latter result indicates that the evolution of water is reaction-limited, it remains unclear whether the evolution of water, hydrogen and acetone near 305 K is limited by the decomposition/reaction of a single species. Unlike hydrogen and acetone, water begins to desorb by 230 K and the change in its rate of desorption is less influenced by surface temperature. However, these differences could be caused by an additional pathway that produces water below 305 K, such as disproportionation of surface hydroxyl, which does not produce hydrogen or acetone. Further information that is relevant to resolving the apparent differences between acetone and hydrogen desorption at 305 K and water desorption at 305 K will be presented when the TPRD results of isobutylene-d6 (D dimethyl-) oxidation are presented. A portion of the dosed oxygen and isobutylene also desorbs from the surface. Oxygen desorption is characterized by a broad feature centered near 1050 K. For an initial 1.0 L dose of isobutylene, about 40% (0.20monolayers) of the starting adsorbed oxygen desorbs. Given the significant amounts of unreacted oxygen coupled with the lack of HREELS loss features once TPRD is complete, it appears that all surface carbon is incorporated into some volatile species: either isobutylene, t-butanol, acetone, CO or CO2. Isobutylene desorbs from the surface with a maximum at 165 K for doses of 1 L or less. At higher doses a multilayer desorption feature is also observed (maximum at 1l0 K). Isobutylene desorption features above 165 K in Fig. 6 are likely due to desorption from the sample holding/cooling assembly. Based upon the amounts and compositions of the H-containing products that are formed from the portion of the adsorbed isobutylene that does not desorb from the surface, ~ 30% of the H atoms originally contained in this irreversiblyadsorbed isobutylene leave the surface by 330 K while 60% leave by 440 K.

3. 4. Interaction of lsobutylene-do with Ir( l l 1)(2 × 1) O, TPRD TPRD experiments using isobutylene-d6 (D dimethyl-) were also conducted. In these experiments, the production of deuterated t-butanols, acetones, dihydrogen and water was monitored. For the experiments that examined the production of deuterated t-butanols and deuterated acetones, an Ir( 111 )-(2 × 1)1so surface was used and ions with a m/q from 63 to 68 were monitored. The use of 180 allowed for the detection of t-butanols and acetones that might otherwise have been obscured by the desorption of isobutylene-d6 (MW of 62 amu) from the sample holding/cooling assembly. Since the base peak (and also the peak with the highest value of the m/q) in the mass spectrum of t-butanol is derived from the loss of a methyl group, t-butanols that could be detected by monitoring these ions include: (CD3)2(CH3)ClSOH (m/q = 64, 67 in a 2:1 ratio assuming that there is equal probability of losing any one of the three methyl groups during ionization), (CD3)z(CH3)ClSOD (m/q=65, 68 2:1 ), (CD3)2(CH2D)ClSOH (m/q = 65, 67 2:1 ) and (CD3)2(CHeD)ClSOD (m/q = 66, 68 2:1). Other tbutanols such as (CDzH)(CD3)(CHzD)ClSOH (m/q = 64, 65, 66 l: 1:1 ) also would produce ions in the m/q--63 to 68 range, but the production of these species is improbable unless t-butanol precursor molecules are capable of rapid intermolecular H/D exchange. While this possibility cannot be precluded, no such exchange is observed for tbutoxide on the R h ( l l l ) - ( 2 × 1)O surface [28]; this system may behave similarly to the Ir( 111)(2 × 1)O/isobutylene system because ( 1) Rh lies in the same column of the periodic table as Ir and would be expected to have properties similar to Ir and (2) the intermediate responsible for t-butanol formation should be similar in structure to t-

butanol/t-butoxide. Several deuterated acetones can also be detected in the re~q=63 to 68 region including: CD3ClSOCH3 (m/q=63), CD3ClSOCH2 D (m/q= 64), CD3ClSOCD2H (m/q=65) and CD3C18OCD3 (re~q=66). Other acetones also would produce ions in the re~q=63 to 68 region, but the production of these acetones is unlikely unless the

185

S.G. Karseboom et al. / Surface Science 383 (1997) 173 202

precursor(s) to acetone are capable of rapid H / D exchange. During the oxidation of isobutylene-d6 on the Ir( 111 )-(2 x 1 )1sO surface, ions at m/q=64 (peak maximum at 290 K), 65 (300 K), 66 (310 K), 67 (290 K ) and 68 (290-300 K ) were detected. The ratio of the time-integrated desorption rates of these ions was approximately 5.1:4.4:10:2.7:1.0, respectively. No m/q=63 desorption feature was observed in the 280 to 320 K region. Based upon these ratios as well as the desorption maxima, the m/q = 64 and 67 ions are attributed to the desorption of (CD3)z(CH3)ClSOH, the re~q=65 and 68 ions are attributed to the desorption of (CD3)2(CH3)CXSOD, and the re~q=66 ion to the desorption of CD3C18OCD3. The m/q=64 to m/q = 67 ratio of 1.9 is very close to the statistical ratio for random methyl group scission during the ionization of (CD3)2(CH3)ClSOH, and in addition, the desorption maxima for the two ions (290 K ) is the same as the desorption maximum for t-butanol-d0 produced from the oxidation of isobutylene-do. The ratio of m/q=65 to m/q=68 ions, 4.4, is not particularly close to the statistical ratio for random methyl group scission of (CD3)2(CH3)ClSOD during ionization but could be due to the poor signal to noise ratio of the m/q=68 ion. The peak maxima of the m/q=65 and 68 ions (300 K, 290-300 K ) are higher than the desorption maximum of t-butanol-do produced from isobutylene-do (290 K ) or of (CD3)2(CH 3) ClSOH produced from isobutylene-d6 ( 2 9 0 K ) . This result would be expected if the evolution of t-butanol were limited by the protonation/ hydrogenation of an oxygen atom on some tbutanol precursor such as adsorbed t-butoxide. The remaining ion, m/q = 66, has no companion m/q -- 68 ion of sufficient intensity or at the appropriate temperature to be accounted for by the desorption of (CD3)2(CH2D)CXSOD. Since the m/q = 66 ion has about the same desorption maximum as acetone-do produced from isobutylene-do (310 K vs 305 K), it is assigned to the desorption

of CD3CI8OCD3. T P R D not only was used to identify the tbutanols and acetones produced during isobutylene-d6 oxidation, but also to quantify them. After correcting for ion gauge sensitivities

and ion cracking patterns, the relative amounts of CD3C18OCD3, (CD3)2(CH3)CISOH and (CD3)2 (CH3)C18OD produced from a 1.0 L dose ofisobutylene-d6 are about 5.0:1.5:1.0, respectively. Absolute selectivities could be obtained by combining these numbers with data from H / D atom balances of gaseous species produced during T P R D experiments employing an Ir( 111 )(2 x 1 )160 surface exposed to 1.0 L of isobutylened6 at T, < 80 K. The absolute selectivities of irreversibly-adsorbed isobutylene-d6 to CD3COCD 3, (CD3)2(CH3)COH and (CD3)z(CH3)COD were estimated to be 12, 3.6 and 2.4%, respectively. Approximately two-thirds of the acetone and three quarters of each of the t-butanols are evolved by 330 K while the remainder is evolved by 440 K. Besides acetone and t-butanol, the production of dihydrogen (H 2, HD, D2) and water (H20, HDO, D2O ) was monitored and quantified during the oxidation of isobutylene-d6. Fig. 7 shows the T P R D spectrum of dihydrogen and water evolving from an Ir( 11 l )-(2 × 1 )160 surface after exposure to 1.0 L of isobutylene-d6 at less than 80 K. Although none of the spectra have been corrected

I

100

300

I

I

500 700 T e m p e r a t u r e (K)

~ /

900

Fig. 7. TPRD spectrum of the lr( l 11)-(2 x 1)O surface after exposure to 1.0 L of isobutylene-d6(D dimethyl-) at Ts < 80 K showing the rn/q = 2, 3, 4, 18, 19 and 20 ions. The ~165 K, m/q=18 feature results from the desorption of isobutylene-d6.

186

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

for ion gauge sensitivities or ion cracking patterns, all of the values reported below have been. As can be seen, the individual spectra differ considerably not only from the spectra of H2 and H20 produced from the oxidation of isobutylene-do (Fig. 6) but also from one another. The desorption of species containing H atoms is favored at low temperatures with the desorption of species containing D atoms being favored at higher temperatures. This trend is most striking for water desorption: the H to D ratio for water desorbing below 285 K is 6.0 + 0.8, for that desorbing between 285 and 330 K it is 2.5+0.1, for that desorbing between 330 and 440 K it is 0.66 +0.03, while for that desorbing above 440K it is 0.23 + 0.09. The ratios for the desorption of dihydrogen are 0.50+0.03, 0.25 +0.01, 0.23 +0.01 and 0.11+-0.02 over the same ranges. The relatively large uncertainties for the desorption data at Ts<285 K and Ts>440 K are associated with the poor signal to noise ratios of these features in these regimes. Since only small amounts of water and dihydrogen evolve below 285 K, the H:D ratios of the dihydrogen and water that evolve at or below 330 K are nearly identical with the ratios over the 285 to 330 K temperature range; the H:D ratio for dihydrogen evolving at or below 330 K is 0.26+-0.01 while the H:D ratio for water evolving below 330K is 2.5+0.1. When the relative amounts of desorbing dihydrogen, water, t-butanol and acetone are considered, the H:D ratio of products that desorb below 330 K is 0.44+0.02, for those that desorb between 330 and 440 K it is 0.35 +-0.02, and for those that desorb above 440 K it is 0.14+-0.03. The H/D composition of adsorbed products was also calculated as a function of surface temperature. Below 230 K, where no H/D-containing products have started to desorb from the surface, there should be six D atoms for every two H atoms on the surface (isobutylene-d6=[CD3]2CCHj. Using six D atoms and two H atoms as a basis, the H/D composition of adsorbed products at 330 K is 4.0 D atoms and 1.0 H atoms while at 440 K it is 2.3 D atoms and 0.37 H atoms. These numbers reflect the differing selectivities to acetone and t-butanol that are observed during the oxidation of isobutylene-d6 and isobutylene-do; during

the oxidation of isobutylene-d0, approximately 5.6 of the starting 8 H atoms (isobutylene-do= [CHa]2CCH2) remain on the surface to 330 K while 3.2 remain to 440 K. As a check on the accuracy of the ratios and quantities reported above, the combined H:D ratio for all H/D-containing products that desorbed during isobutylene-d6 oxidation was calculated. This number should be equal to the H:D ratio in isobutylene-d6, 0.33. The calculated value is 0.30+0.03, in good agreement with the expected value. Replacing isobutylene-d0 with isobutylene-d6 as the reactant revealed other aspects of the isobutylene oxidation mechanism besides the preferential desorption of products derived from the = C H 2 portion of the molecule at low temperatures. In particular, a very slight isotopic shift was observed for the acetone desorption maximum as well as for the lowest temperature dihydrogen desorption maximum (305 K for isobutylene-d0 and 310 K for isobutylene-d6). This result is consistent with the hypothesis that the rate-limiting step in dihydrogen/acetone evolution is the reaction/ decomposition of a common intermediate. The isotopic composition of the produced dihydrogen and acetone provides insight into this intermediate. Since deuterium is incorporated into dihydrogen (HD and D2) and the only acetone that is produced in detectable amounts is CD3COCD3, either (1) the intermediate must have more than six deuterium atoms per molecule or (2) competing with the pathway that leads to CDaCOCD 3 is another pathway which yields species that rapidly decompose/react to form gaseous HD and De. Unlike the desorption of dihydrogen and acetone, the lowest temperature desorption maximum of water remained at 305 K when isobutylene-d6 replaced isobutylene-d0 as the reactant. Since there is no isotopic shift for water desorption but there is for acetone and dihydrogen desorption, it raises questions as to whether or not the water is produced from the intermediate that gives rise to acetone and dihydrogen. 3.5. Interaction of isobutylene-do and isobutylene-d 6 with Ir(111)-(2 x 1)O, HREELS HREELS experiments were conducted to determine what adsorbed species were produced

187

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202 , i

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54O -,540

0

500

1000

1500

2000

2500

3000

Energy Loss (cm "x)

0

500

1000

1500 2000

2500

3000

Energy Loss (cm "1)

Fig. 8. HREEL spectra of the Ir( 111 )-(2 × 1)O surface: (a) after exposure to 1.0 L of isobutylene-d0 at Ts < 80 K and after heating the resulting surface to (b) 230, (c) 270, (d) 330 and (e) 440 K. Very weak loss features also appear at 875 and 1080cm-1 in spectrum (d), and 940, 1040, 1155, 1600, 2920 and 3040 cm-1 in spectrum (e).

Fig. 9. HREEL spectra of the Ir( 111 )-(2 x 1 )O surface: (a) after exposure to 1.0 L of isobutylene-d6 at Ts < 80 K and after heating the resulting surface to (b) 230, (c) 270, (d) 330 and (e) 440 K. Very weak loss features also appear at 1170 and 1280cm i in spectrum (c), and at 865. 965 and l195cm 1 in spectrum (d).

d u r i n g the o x i d a t i o n o f i s o b u t y l e n e - d o a n d i s o b u tylene-d6. T h e results o f those e x p e r i m e n t s are p r e s e n t e d in Figs. 8 a n d 9 w h i c h s h o w t h e v i b r a -

t i o n a l loss s p e c t r a o f t h e species p r e s e n t o n the I r ( 1 1 1 )-(2 × 1)O s u r f a c e i m m e d i a t e l y after exposure to 1.0 L o f i s o b u t y l e n e at Ts < 80 K a n d after

188

S.G. Karseboom et al. / Surface Science 383 (1997) 173~02

briefly heating this surface to 230, 270, 330 and 440 K. These spectra will be discussed separately, but the loss features near 540 and 2070 cm-1 in each will not. The feature near 540 cm 1 is the v(lr-O) mode of oxygen adatoms [15,16] while the feature near 2070 cm-~ is the v(CO) mode of adsorbed CO [19]. Surface CO could be "produced" not only from the oxidation of carboncontaining adsorbates but also from the adsorption of CO from the background during the scan. Figs. 8a and 9a show HREELS spectra of the Ir( 111 )-(2 x 1)O surface immediately after exposure to isobutylene-do and isobutylene-d6, respectively, at T~< 80 K. Comparison of these spectra to the Raman spectra of solid isobutylene-do and solid isobutylene-d6 [29] reveals that isobutylene is present on the I r ( l l l ) - ( 2 × 1)O surface in a n-bound state, similar to that seen for olefins adsorbed on metal surfaces such as A g ( l l 0 ) [30] and P d ( l l l ) [31]. Loss feature assignments are presented in Table 4 along with comparisons to the Raman assignments of solid isobutylene. n-Bound isobutylene was also observed on the Ir( 111 )-(2 x 1)O surface following exposure to a 0.25 L dose of isobutylene at Ts < 80 K. The intensity of the vs(CCC) and v(C=C)related features of isobutylene can be used in elucidating the orientation of the adsorbed isobutylene molecules. Since the contribution of dipolescattering to the intensity of such features is proportional to the square of the net dynamic dipole moment normal to the surface, these features

should be weak if the molecule was lying parallel or nearly parallel with the surface. While the first of these is indeed weak, the second appears with moderate intensity. However, the coupling of the v ( C = C ) vibration with charge transfer between isobutylene and the surface could be contributing to the intensity of the second feature, and as such it seems most appropriate to consider isobutylene to lie parallel or nearly parallel to the I r ( l l l ) (2 × 1)O surface. In contrast, isobutylene is at an inclination to the oxygen-precovered Ag(110) surface as evidenced by a strong vs(CCC) feature in the HREEL spectrum of that surface [30]. Competing with isobutylene desorption near T~=165 K is another process which converts n-bound isobutylene into one or more new adsorbed species between 110 and 180K. This/these species remain(s) stable to surface temperatures greater than 230 K. Fig. 8b (derived from isobutylene-do) and Fig. 9b (derived from isobutylene-d6) show the vibrational spectra of the new species. These species are intermediates in the formation of rl2(C,O)-acetone and acetate, and further consideration and identification will be left for Section 4. However we present our rationale of the exclusion of other possible species here. Based upon the absence of certain loss features in Fig. 8b and/or 9b, it appears that there are no appreciable amounts of lql(O)-acetone, rl2(C,O) acetone, formic acid, rll(O)-formaldehyde, monodentate-bonded formate, bridging/bidentatebonded formate, hydroxyl or water present. One

Table 4 n-Bound isobutylene assignments Mode

(CH3)2CCHzsolid[29 ]

6(CCC) y(CzC-- ) v(C C) y(=CH2) p(=CH2) p(CH3) ~(CH3) 6(=CH2) v(C=C) v~(CH3) vas(CH3) vs(=CH2)

389 438 809/1271 889 958 983/1071 1381/ 1446 1421 1653 2898/2916 2956 2988/3077

Ir( 111 ) 435 760~ 890 -/1065 1395/1 445 1445 1630 2915/2915 2915 3040/3040

(CDs)2CCH2solid [29 ] 324 378 721/1278 890 916 789/900 1083/1059 1402 1640 2040/2120 2197 2980/3073

Ir( 111 ) . 375 735/1270 880 -/880 1040/1040 1415 1630 -/2195 2195 3020/3020

S.G. Karseboom et al, / Surface Science 383 (1997) 173 202

of the strongest loss features in the spectra of both ql(O)-acetone and r12(C,O)-acetone is due to the v(CO) mode of acetone. This mode appears as a feature near 1630 cm-~ for rll(O)-acetone (Figs. 4 and 5a) and as a feature near 1275cm ~ for qZ(C,O)-acetone (Figs. 4b and 5b). No such features appear in Figs. 8b and 9b. Likewise, the v ( C ~ O ) mode of both adsorbed formic acid and rl~(O)-formaldehyde would be expected to produce a feature between 1625 1720cm -1 [32,33] but there is no such feature in Figs. 8b or 9b. The absence of a feature between 1290 and 1340cm- 1 in Figs. 8b and 9b rules out the possibility of significant amounts of bridging, monodentate- and bidentate-bonded formate-do or formate-d~; each of these species would be expected to have a strong v~(OCO) feature in this region based upon the interaction of formic acid with oxygen-precovered metal surfaces such as Pd(100) [32], Ag(110) [33] and P d ( l l l ) [34]. Finally, although not shown in Fig. 8b, there are no water or surface hydroxyl-related loss features [27] in the 3200 to 3600 cm ~ region of the H R E E L spectrum. The species present on the surface at 230 K (Figs. 8b and 9b) is/are converted into one or more new species by 270 K. The vibrational spectra of this/these new species are presented in Fig. 8c (derived from isobutylene-do) and Fig. 9c (derived from isobutylene-do). These species are also rlZ(C,O)-acetone and acetate precursors, and their further consideration will be left for Section 4. However~ it is clear that no detectable amounts of rll(O)-acetone, Tl2(C,O)-acetone, formic acid, Bl(O)-formaldehyde, monodentate formate or bridging/bidentate-bonded formate are present. In addition, there are no water or surface hydroxylrelated features in the 3200 to 3600 cm-~ region of the HREEL spectrum (data not shown). The absence of detectable amounts of rlx(O)-acetone and rl2(C,O)-acetone at 270 K, the temperature at which acetone starts to desorb from the Ir(111 )(2 x 1)O/isobutylene surface, further supports the hypothesis that acetone evolution during the oxidation of isobutylene is reaction-limited rather than desorption-limited. By 330 K, when acetone evolution from the Ir( 111 )-(2 x 1 )O/isobutylene prepared surface is nearly complete, a new set of species is present on

189

the surface. The vibrational spectra of these species are presented in Fig. 8d (derived from isobutylenedo) and Fig. 9d (derived from isobutylene-d6). After comparing these spectra to Fig. 2 (surface acetate-do from acetic acid-do), Figs. 4b/5b (q2-acetone) and Figs. 4c/5c (surface acetate from rlZ-acetone) these new species are identified as surface acetate and rlZ(C,O)-acetone. Loss feature assignments are presented in Table 5. The rl2(C,O)-acetone and acetate produced from isobutylene-do were treated as being fully deuterated in these assignments since the H:D ratio of species present on the surface at 330 K is only about 0.25. Additionally, several small loss features in the two figures are not assigned to either acetate or acetone and are thought to be associated with minor oxidation products, one of which may be surface formate as discussed in the following paragraph. The H R E E L spectrum of the lr( 111 )-Oadatom surface after ( 1 ) exposure to 0.6 L of formic aciddo at Ts < 80 K and (2) briefly heating the resulting surface either to 330 or to 440 K was dominated by four strong loss features located at 380, 780, 1330 and 550 cm i The first three of these features are consistent with formate's v(lr--Orormate), ~(OCO) and vs(OCO) modes [33,34] while the fourth corresponds to the Ir-O stretch of oxygen adatoms [15,16]. In Fig. 8d, features at 405, 760 and 1340 cm-1 could be assigned to the three formate-do modes mentioned above. However, the 760 cm-I feature could also be assigned to the vs(CCC) mode of rl2(C,O)-acetone-d0 (Fig. 4b) while the 1335 cm 1 shoulder could be assigned to the 6~(CH3) mode of acetate-do (Figs. 2 and 4c). In addition, the feature at 760 cm ~ may be the one that is observed in this region following the formation of the (2 x 1) oxygen atom overlayer. In Fig. 9d, the loss features at 375, 760 and 1305cm ~ could be assigned to the three bridging/bidentate-bonded formate modes. Heating the rl2(C,O)-acetone/acetate mixture from 345 to 440 K caused the t]2(C,O)-acetone to disappear, leaving only acetate and small amounts of other species (Figs. 8e and 9e). The disappearance of rl2(C,O)-acetone features was accompanied by an increase in the intensity of acetate features, thus suggesting that at least a portion of the

S. G. Karseboom et aL / Surface Science 383 (1997) 173 202

190

Table 5 Loss feature assignments for rl;(C,O)-acetone and surface acetate produced from isobutylene Fig. 8d Feature (cm- l)

Acetone-do mode

Acetate-d0 mode

Fig. 9d Feature (cm - 1)

Acetone-d6 mode

Acetate-d6 mode

315 680 760 975 l 175 1275 1340 1405 2970

B(CCC) n(C=O) v,(CCC) p(CH3) 8~(CH3) v(C = O) {~as(CH3) ~as(CH3) v(CH3)

v(lr-O) ~5(OCO) -

300 660 760 1050 1250 1400 2240

6(CCC) n(C=O) v,(CCC) ~(CDa) v(C = O) v(CD3)

v(Ir-O) 6(OCO)

~s(CH3) vs(OCO)/8.s(CH3) v(CH3)

rl2(C,O)-acetone was converted into surface acetate like on the Ir(111 )-Oad~tom/aCetone prepared surface. Acetate loss features are assigned as in Table 3. While the loss feature near 895cm -1 in Fig. 9e is assigned to acetate's v(CC) mode and is observed in the HREEL spectrum of acetate-d3 produced from q2(C,O)-acetone-d6 (Fig. 5c), this feature may belong to some other species since no comparable feature is observed in the HREEL spectrum of acetate-do produced either from acetic acid-do (Fig. 2) or from rl2(C,O)-acetone-do (Fig. 4c). Several additional loss features also appear in Figs. 8e and 9e and are likely related to small amounts of other isobutylene oxidation products. One of these, the loss feature near 1300 cm-1 in Fig. 9e, appears in the region where the vs(OCO) mode of formate would be expected to appear. One can determine an upper limit on the selectivity of irreversibly-adsorbed isobutylene-d6 to surface acetate at T~=440 K by assuming that any surface hydrogen present at this temperature is incorporated into acetate. By making use of this assumption and the fact that about 35% of the hydrogen from the irreversibly-adsorbed isobutylerie-d6 remains on the surface at T~=440 K, one arrives at a selectivity of 90%. A similar calculation for acetate produced from isobutylene-do yielded a selectivity of 105%. This value is clearly too high since (1) about 12% of the irreversibly-adsorbed isobutylene-d6 is converted into gaseous acetone and another --~6% is converted into gaseous tbutanols and (2) small amounts of other organic species appear to coexist with acetate at T~=

t3(CD3) v,(OCO) v(CD3)

440 K. Nonetheless, it indicates that a very large proportion of the irreversibly-adsorbed isobutylene is converted into acetate by Ts = 440 K.

3.6. Interaction of oxygen with Ir(111) /isobutylene The interaction of oxygen with Ir( 111 )-isobutylene prepared surfaces was studied using HREELS and TPRD techniques. Fig. 10 shows the HREEL spectrum of the bare Ir( 111 ) surface after exposure to 1.0 L of isobutylene-do at Ts<80 K (Fig. 10a) and after exposure to 1.0 L of isobutylene-d6 at T s < 8 0 K (Fig. 10b). Since no carbon-carbon double bond stretching feature is observed in either 346

v

265~

,hlLIk

3.°

1035

9o01,1

0

500

1000

1500

2000

2500

3000

Energy Loss (cmq)

Fig. 10. H R E E L spectra of the bare Ir( 111 ) surface after exposure to: (a) 1.0 L of isobutylene-d6 at Ts<80 K and (b) 1.0 L of isobutylene-d6 at T~<80K. Very weak loss features also appear at 1175, 1235 and 1350 cm -1 in spectrum (b).

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

spectrum and Fig. 10a is very similar to the spectrum of di-ry-bound isobutylene-do on Pt( 111 ) [35], we conclude that isobutylene also binds to the bare I r ( l l l ) surface in a di-cr-bound state. Loss feature assignments and comparisons to di-~bound isobutylene-d0 on Pt(111) are presented in Table 6. Di-G-bound isobutylene was also observed on the lr( 111 ) surface following a 0.25 L exposure of isobutylene, but, unlike the surface that was exposed to 1.0 L of isobutylene, the postadsorption of oxygen at Ts < 80 K slightly altered the bonding of the isobutylene. In particular, the intensities of the 6(CCC) features were attenuated. However, there was no gross rehybridization to rt-bound isobutylene since no v(C--C)-related features were observed in the 1500 to 1700 cm 1 region of the spectrum. The amounts of gaseous acetone produced during the oxidation of di-~-bound isobutylene were considerably smaller than those produced during the oxidation of rt-bound isobutylene. Fig. I 1 shows a plot of gaseous acetone production versus isobutylene exposure for the I r ( l l l ) (2 x 1 )O surface (~-bound isobutylene) and for the Ir(l 1 l) surface postadsorbed with greater than 24 L of oxygen (di-~-bound isobutylene). In the best case, corresponding to a 0.25 L exposure of isobutylene, the yield of gaseous acetone from di-cy-bound isobutylene was one-fifth of that from g-bound isobutylene. The low yields of acetone from the oxidation of di-c~-bound isobutylene also extended to adsorbed acetone. Heating the Ir(111) surface to 330 K after exposure to (1) either 0.25 or 0.50L of

(CH3)2CCH: Pt( 111)[35]

t;(CCC ) vs( MC ) v,~( MC 1 v(CC) & p(CH3) co(CH2) 6(CH3) v(CH3) v(CH 2)

270, 330 460 600 800, 1000 1090 1240 1400/1470 2930 2930

Ir( 111 )

265,430 n.r. 570 830, 1040

1360/1425 2925 2925

0

=I 0 I~

0

z

•

Pi-Bound Isobutylene

o

Di-Sigma-Bound Isobutylene

i

i°° 0

°

1 2 3 4 5 Isobutylene Dose (Langmuirs)

o-~ 6

Fig. I 1. Gaseous acetone production from the oxidation of the rt-bound and di-ey-bound isobutylene-do as a function of initial isobutylene exposure at Ts < 80 K.

isobutylene-d6 at Ts < 80 K and (2) greater than 24 L of oxygen at T~<80 K failed to produce detectable amounts of rlz(C,O)-acetone or rll(O)-acetone. Furthermore, no surface acetate was observed. Since little or no gaseous acetone, adsorbed acetone or surface acetate is produced directly from di-~-bound isobutylene, it is improbable that di-cy-bound isobutylene is an intermediate in the conversion of rt-bound isobutylene to these products. 3.7. Interaction of 2,3-dimethyl-2-butene with IR (111)-(2 x 1) 0

Table 6 Di-er-bound isobutylene assignments Mode

191

(CD3)2CCH2 lr(lll) 235, 345 515 595 735, 865

1035/1035 2200 2950

The interaction of 2,3-dimethyl-2-butene (tetramethylethylene or TME) with the Ir( 111 )-(2 x 1 )O surface was studied using both HREELS and TPRD techniques, but the scope of these experiments was extremely limited. Based upon Scheme l, TME should be converted into acetone. T P R D experiments conducted with the I r ( l l l ) - ( 2 x 1)O surface after exposure to 1.0 L of TME at Ts< 80 K confirmed the production of gaseous acetone, but the desorption maximum was at 345 K, approximately 40 K

192

S. G. Karseboom et al. / Surface Science 383 (1997) 173-202

higher than the desorption maximum of acetone produced from the oxidation of isobutylene on the I r ( l l l ) - ( 2 × 1)O surface. Furthermore, the amount of acetone produced from a 1.0 L dose of T M E was only about 50% of that produced from a 1.0 L dose of isobutylene. H R E E L S experiments were conducted on the Ir( 111 )-(2 × 1)O surface immediately after exposure to 1.0 L of T M E at Ts < 80 K and after briefly heating this surface to 230, 270 and 330 K. The results of these experiments are presented in Fig. 12. Since isobutylene adsorbed onto the Ir( 111 )-(2 × 1)O surface at Ts < 80 K in a re-bound state, it is reasonable to assume that T M E would also. As such, the loss features in Fig. 12a are assigned (tentatively) as follows: 335cm -1 ~ t ( C 2 C = ), 550 cm 1 v(lr_Oadatom), 765 c m vs(CCC), 1085 crn -1 p(CH3), 1370cm -1 5~(CH3), 1450cm -1 5a~(CH3), 1655 cm -1 v ( C - - C ) and 2910 cm -~ v(CH3). Competing with the evolution of T M E from the surface near 200 K was another process which converted T M E into one or more new adsorbed species (Fig. 12b). The new species was/were already present at 200 K and was/were thermally stable; there are few changes in the H R E E L spectrum of the Ir(111)-(2 × 1)O/TME prepared surface from 230 to 330 K (Fig. 12b-d). This stability is in sharp contrast to the case of isobutylene oxidation where three distinct sets of species were identified on the Ir(111)(2 × 1 )O/isobutylene prepared surface between 230 and 330 K.

' [ ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' 550

-P~t

t"t

w

r3~

1370 1450

3351

765 4. Discussion The iridium-catalyzed oxidation of olefins at atmospheric pressures is unusual for two reasons: (1) direct reaction at the double bond competes effectively with the abstraction of allylic hydrogens and (2) the majority of partial oxidation products are derived from the scission of the double bond (Scheme 1). For example, the oxidation of unlabeled CH3CHCH 2 produces acetic acid in greater than 30% yield [9], and the oxidation of CH3CH~4CH2 produces acetic acid that is virtually non-radioactive [10]. In contrast, commercially important reactions such as the ammoxidation and

~

I

2910

JL 0

500

1000 1500 2000 2500 3000

Energy

L o s s ( c m "1)

Fig. 12. HREEL spectra of the Ir(ll 1)-(2 × 1)O surface: (a) after exposure to 1.0 L of 2,3-dimethyl-2-buteneat Ts<80 K and after heating the resulting surface to (b) 230, (c) 270 and (d) 330 K. epoxidation reactions proceed via allylic hydrogen abstraction to form ~-allyl species. Unfortunately, this preference effectively limits the usefulness of the epoxidation reaction to the production of ethylene oxide.

S. G. Karseboom et al. /Surface Science 383 (1997) 173 202

As we have shown in this paper, the iridiumcatalyzed oxidative cleavage of olefins also proceeds under UHV conditions. Gaseous acetone and rl2(C,O)-acetone are produced during the oxidation of isobutylene on Ir(l 11)-(2 x 1)O. Like the atmospheric pressure studies, this acetone is derived predominantly from the scission of the double bond; the only gaseous acetone that is produced in detectable quantities from (CD3)2CCH 2 is (CD3)2CO while the H:D ratio of the adsorbed products of (CD3)zCCH 2 oxidation, including rlZ(C,O)-acetone, is about 0.25. An additional similarity is that acetate is a byproduct of isobutylene oxidation under UHV conditions while acetic acid is a byproduct of isobutylene oxidation at atmospheric pressures. Although there are many similarities between the oxidative cleavage of isobutylene under UHV conditions and the oxidative cleavage of isobutylene at atmospheric pressures, there are also some differences. One difference is that t-butanol and possibly surface formate are byproducts under UHV conditions while propionic acid, methacrolein and various C4 acids are byproducts at atmospheric pressures [ 10]. A second difference is that the apparent selectivity of isobutylene-do to gaseous acetone is about 4-5% under UHV conditions, well below the 17% selectivity achieved at atmospheric pressures [10]. However, the actual yield of acetone under UHV conditions is greater than 4-5% because not all of the produced acetone desorbs from the surface; some is converted into surface acetate at T~> 345 K. The mechanism of isobutylene oxidation is quite complex. In the remainder of this section we will present the mechanism that we believe to be the most consistent with the experimental data. In developing this mechanism we have made several assumptions. In particular we have considered that: (1) the evolution of acetone, dihydrogen and water near 305 K is limited by the decomposition/reaction of a common intermediate, which is quite reasonable in the case of acetone and dihydrogen desorption but less so for water desorption; (2) the oxidation sequence that leads from n-bound isobutylene to rlz(C,O)-acetone and surface acetate proceeds through at least two sequential sets of intermediates; and (3) the gas-

193

eous t-butanol, acetone-d6, dihydrogen and water that are evolved near 300 K are produced either directly or indirectly from the intermediates that give rise to rlZ(C,O)-acetone and surface acetate. The second of these assumptions arises from the high selectivity of irreversibly-adsorbed isobutylene to rl2(C,O)-acetone and surface acetate. Because of the high selectivity, many of the loss features in the H R E E L spectra of the I t ( I l l ) (2× 1)O/isobutylene prepared surface at 230 K (Figs. 8b/9b) and 270 K (Figs. 8c/9c) must belong to intermediates involved in the formation of rlZ(C,O)-acetone and acetate. The exception to this would be if the precursors to these species had only weakly dipole-active vibrational modes, however this is not the case since no new loss features were observed in H R E E L spectra collected in the off-specular direction. Comparison of the spectra taken after heating to 230 K (Figs. 8b and 9b) to those after heating to 270 K (Figs. 8c and 9c) reveals that there are few loss features that are common to both sets of spectra. The paucity of common loss features suggests that the conversion of rt-bound isobutylene, present on the surface at Ts < 80 K, to rl2(C,O)-acetone and acetate, present on the surface at Ts= 330 K, involves at least two sequential series of intermediates that are stable enough to be observed using HREELS. Since neither set of spectra correspond to di-c>bound isobutylene (Fig. 10), any mechanism proposing this species as an intermediate would have to involve three or more distinct series of intermediates. However, since little gaseous acetone and no detectable amounts of adsorbed acetone or surface acetate are produced directly from di-cs-bound isobutylene, it would seem that this species is not an intermediate in the conversion of n-bound isobutylene to these products. The mechanism that we propose for the partial oxidation of n-bound isobutylene-d, is illustrated in Schemes 2 and 3. The partial oxidation of n-bound isobutylene-do is proposed to proceed in a similar manner. This mechanism fulfills the requirement that the conversion of n-bound isobutylene to rl2(C,O)-acetone and surface acetate involves at least two sequential sets of intermediates, and it is also in accord with the formation of

194

S.G. Karseboom et al. / Surface Science 383 (1997) 173-202

CD3 ,,,,~,~......~,,LI-I

~-BoundIsobutylene To T. • 150K

L CD3

CD3H

O~Amet~]ele, 1 From < 150K To > 230K +H

[

+O

t CD 3.

CD 3

O~CH3

I

CD3 H CD3 ~ - ~ H O O

I I

t-Buto~dde By ~ 240K

l

(CD3)2(CHs)COH (g) (CD3)2(CH3)COD (g) Between 240K and 440K

Diohte, 2 By 270K

1

CD3COCD3 (g) ~2(C,O)-Acetone Surface Acetate Surface Formate Between 270K and 440K

Scheme2. Mechanismproposed in the conversionof n-bound isobutylene-d6into the observedpartial oxidationproducts.

the observed gaseous t-butanols, acetone, dihydrogen and water. The first step in the mechanism is the addition of oxygen to the tertiary carbon of n-bound isobutylene to afford the oxametallocycle, 1 (of Scheme 2), by 180 K. As depicted in the scheme, reaction may proceed via the nucleophilic attack of oxygen at the tertiary carbon. Since relatively few loss features are associated with this species (Figs. 8b and 9b), it may be bound to the surface with close to Cs symmetry. Tentative loss feature assignments (Figs. 8b/9b) are as follows: 8(CCC) (-/375 cm-1), v(CC) (750/765cm-1), p(CH3) (840, 1090cm-1/705,765cm-1), v(CO) (955/

960 cm-1), co(CH2) ( 1220/1165 cm-1), 8(C-H) (1380, 1455cm-'/1040 cm -1 ) and v(C-H) (2985 cm- 1/2240, 2935 cm- '). These assignments are somewhat artificial since substantial vibrational coupling among modes with the same symmetry and similar frequencies would be expected. 1, or a similar species, has been proposed as an intermediate in several other systems including the conversion of propylene to acetone on oxygen-precovered Rh(111) [26], the conversion of isobutylene to tbutanol on oxygen-precovered Rh(111) [36] and the conversion of t-butanol to isobutylene and isobutylene oxide on oxygen-precovered A g ( l l 0 ) [37]. Additionally, numerous examples of olefin oxidation mechanisms involving oxametallocycle intermediates appear in the organometallic literature [38] and several stable oxametallocycle species have been prepared [39]. The preferential formation of 1 over its structural isomer, -(CD3)2CCH20-, is consistent with the observed preferential formation of gaseous t-butanol over gaseous isobutanol and is consistent with a thermodynamically-driven reaction; the heat of formation of gaseous t-butoxide has been calculated to be about 6.8 kcalmol 1 more exothermic than the heat of formation of isobutoxide while a similar difference exists between the heats of formation of gaseous t-butanol and isobutanol [40]. Between 230 and 270 K, 1 is consumed in two competing reactions, one ultimately leading to gaseous t-butanol and the other to gaseous acetone, r12(C,O)-acetone, surface acetate and possibly surface formate. The first of these pathways is minor compared to the second. In the minor pathway, 1 is hydrogenated at the carbon alpha to the surface to form (CD3)2(CH3)O-. The subsequent addition of an H or D atom to (CD3)2(CH3)O- near Ts = 290 K yields gaseous (CD3)2(CH3)OH and (CD3) 2 (CH3)OD. Since a kinetic isotope effect is observed, the rate-limiting step in the evolution of t-butanol would be the addition of H or D to (CD3)2(CH3)O-. The absence of detectable amounts of (CD3)2(CH2D)OH and (CD3)2(CH2D)OD during TPRD experiments suggests that the species providing the hydrogen in the step that converts 1 into t-butoxide must be a good source of H atoms

S.G. Karseboom et al. / Surface Science 383 (1997) 173.202

CD3 CD3~H

195

H

CD3COCD3 / HCHO

A

CDs H CD3.~....~ H B

~)~JO~.O

[ I

~

CD3COCD3I-OCH,aO"

]

CD3 H CD3...~__/~ H

CD3. ~.~ CD30~~...

c [rl

CD3 D

I

,,,

H

O"~ ~"~) (~ I Ill

I + -OH

o I

=

~L.~ H ~

O ~ [

0 I * -OCD3

CD3 H CD3~__~ o o ¢'[ l CD3

=-

--- CDsCOCD3 / HCOO

H

O O I

~- CD3COOIHCHO

Scheme 3. Possible mechanisms in the conversionof-O(CD3)2CCO- (2) into acetone and acetate. As described in the text, acetate could be formed from acetone in Pathways A-C through an -O(CD3)2CO intermediate. but a poor source of D atoms. Since the H:D ratio of water desorbing below 285 K is 6.0+0.8, the precursor(s) to water may provide this hydrogen (in contrast the H:D ratio of the dihydrogen that desorbs below 2 8 5 K is 0.50+0.03). Surface hydroxyl might be one such precursor even though its concentration on the I r ( l l l ) - ( 2 x l ) O / isobutylene prepared surface at 230 and 270 K is below H R E E L S detection limits. To test the plausibility of this hypothesis, two series of experiments were conducted. In one series the Ir( 111)-(2 x 1 )O surface was (1) exposed to 1.0 L of isobutylene-d0 at Ts<80 K and (2) exposed to -,~20 L of D 2 0 a t T~--180 K, a temperature at which the rate of molecular water desorption and conversion to surface hydroxyl should have been appreciable but not that of surface hydroxyl disproportionation to gaseous water. After dosing, the surface was cooled to less than 80 K to begin the collection of a T P R D spectrum of the m / q = 59, 60 and 61 ions. The first of these ions corresponds to the (CH3)2COH+ cracking ion, the second to the

(CH3)2COD+ and (CH3)(CH2D)OH+ cracking ion(s) and the third to various cracking ions including ( C H 3 ) ( C H 2 D ) O D + . The second series of experiments were conducted identically to the first series except that the surface was not exposed t o D 2 0 while at Ts=180 K. The first series of experiments produced about 50% more t-butanol than the second series with the re~q=60 ion accounting for about two-thirds of the increased production and the m / q = 5 9 ion accounting for the remainder. No m/q=61 ion was detected in either series of experiments. Since statistical methyl group scission during the ionization of (CH3)2(CH2D)OH would be expected to yield m / q = 6 0 and 59 ions in a 2:1 ratio, the increased t-butanol production observed in the first series of experiments is attributed to the formation of (CH3)2(CH2D)OH rather than (CH3)3COD. In support of this, the desorption maxima of all tbutanol-related ions in both series of experiments were at the same temperature, 290 K. These results not only indicate that surface hydroxyl is a plausi-

196

S. G. Karseboom et al. / Surface Science 383 (1997) 173-202

ble source of hydrogen in the conversion of 1 into adsorbed t-butoxide but also indicate that the fraction of 1 that is ultimately converted into gaseous t-butanol may be limited by the availability of surface hydroxyl. The transfer of H to 1 may proceed via a concerted mechanism in which both the Ir-C bond of 1 and the O-H bond of hydroxyl undergo homolytic cleavage. The source of -OH on the Ir(111)-(2 x 1)O/isobutylene-d6 prepared surface may be 1 itself; oxygen adatoms may abstract H atoms from the alpha carbon in some small fraction of 1 molecules via a Bronsted acid-base mechanism. The addition of an H/D atom to the oxygen of (CD3)2(CH3)CO- results in the evolution of (CD3)2(CH3)COH and (CD3)z(CH3)COD near 295K. Since about 1.5 times more (CD3)2 (CH3)COH is produced than (CD3)2(CH3)COD, both the precursor(s) to dihydrogen and the precursor(s) to water may be providing this H/D atom; the H:D ratio of dihydrogen that evolves below 330 K is 0.26_+0.01 while the H:D ratio of water that evolves below 330 K is 2.5_+0.1. Although some 1 is converted into t-butoxide/tbutanol, most is converted into the diolate species, 2. This reaction may proceed via the nucleophilic attack of an oxygen adatom at l's alpha carbon with the accompanying breaking of l's Ir-C bond. Species similar to 2 are intermediates in (1) the oxidative cleavage of alkenes by permanganate ion and (2) the conversion of alkenes to glycols by permanganate ion and by osmium tetroxide [41]. Tentative loss feature assignments for 2 (Fig. 8c/9c) are as follows: 6(CCC) (325, 410 cm 1 / 300, 390 era-l), v(CC) (745/770 era-l), p(CH3) (900/770 era- 1), v(CO) ( 1150/1095 cm- 1), ~(C-H) (1375, 1455 cm -1 / 1035, 1400 cm -1) and v(C-H) (2970 cm -1 / 2240, 2935 cm-1). One particularly unsatisfying aspect of this assignment is that there is (apparently) no v(CO)-related feature for the primary alcoxy carbon of 2. The subsequent reaction of 2 near 305 K results in the formation of q2(C,O)-acetone, surface acetate and possibly surface formate as well as gaseous CD3COCD3, water (H2 O, HDO, D20) and dihydrogen (HD, D2). Some of the possible pathways from 2 to these species are presented in Scheme 3. Although only Pathway D is shown as producing

acetate, it may also be possible to form acetate in the other pathways. If acetone is initially formed into an ql(O) state, an oxygen adatom could attack the carbonyl carbon to form -O(CDa)2CO before it rehybridizes to the more thermodynamically favored rl2(C,O) state. The -O(CD3)zCOspecies, which was hypothesized to be responsible for the exchange of oxygen between adsorbed acetone and the Ir(lll)-Oaa~tom surface, could transfer a methyl group to the surface to form surface acetate. Alternately, O(CD3)2CO- could rearrange to regenerate acetone. In contrast, it is unlikely that acetate would be produced near 305 K in Pathways A, B and C if acetone were formed directly into the q2(C,O) state since rl2(C,O)-acetone was converted into acetate at greater than 345K on the I r ( l l l ) - ( 2 × 1)O/ isobutylene prepared surface. The feasibility of the mechanisms presented in Scheme 3 will be discussed briefly with respect to the constraints imposed by the experimental data. These constraints are that: (1) no detectable amounts of gaseous formaldehyde, methanol or methane are produced during the oxidation of isobutylene; (2) the only gaseous acetone that is observed during the oxidation of (CD3)2CCH 2 is CD3COCD3; (3) little or no formate is observed (via HREELS); and (4) the evolution of acetoned6, water (H20, HDO, D20) and dihydrogen (HD, D2) near 305 K as well as the production of acetate near 305 K is limited by the decomposition/ reaction of a common intermediate in a single reaction. It is unlikely that the third of these constraints could be explained by formate decomposing immediately after formation since formate was stable to greater than 440 K on the Ir(111)Oadatom/fOrmic acid prepared surface. The inclusion of acetate in the fourth constraint is based upon a D atom balance of species desorbing below 330 K. As will be shown in the following paragraph, most of the deuterium that is incorporated into the evolving water and dihydrogen comes from the formation of acetate. One consequence of this is that the desorption of water and dihydrogen should be heavily influenced by the kinetics of acetate formation or by the decomposition kinetics of the D-containing byproducts of acetate formation. Unless these kinetics are nearly identical with

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the kinetics of gaseous acetone-d6 evolution, the production of gaseous water, dihydrogen, acetoned6 and surface acetate should be limited by the decomposition of a common intermediate in a single reaction. Using values from Section 3, approximately 4.0 D atoms (and 1.0 H atoms) are incorporated into adsorbates on the Ir( 111 )-(2 × 1)O/isobutylene-d6 prepared surface at Ts = 330 K (original basis: 6 D atoms and 2 H atoms from irreversibly-adsorbed isobutylene-d6). The remaining 2.0 D atoms are incorporated into the species that desorb below 330 K: CD3COCD3, (CD3)2(CH3)COH , (CD3)2 (CH3)COD, HD, D2, HDO and D20. The former three species account for only 0.8 of these D atoms based upon the selectivity of irreversibly-adsorbed isobutylene-d6 to these compounds (12, 3.6 and 2.4%, respectively), the fraction of each compound that evolves by 330 K ( ~ 0.67, 0.75 and 0.75, respectively), and the number of deuterium atoms per molecule for each compound (6, 6 and 7, respectively). Therefore, the remaining 1.2 D atoms, representing fully 20% of the total deuterium available from the irreversibly-adsorbed isobutylene-d6, are incorporated into the evolving HD, D2, HDO and DzO. Since gaseous acetoned6 has the same number of deuterium atoms per molecule as 2, the reaction pathway(s) that convert(s) 2 into gaseous acetone-d6 cannot be the same as those that supply deuterium for the formation of dihydrogen and water. The same argument holds true for the case of q2(C,O)-acetone production since it is likely that only adsorbed acetone-d6 would be formed from 2. Similarly, the non-selective decomposition of 2 is probably not a major source of deuterium. This statement is based upon the overall selectivity of irreversiblyadsorbed isobutylene-d6 to: (1) surface acetate at 440 K; (2) gaseous acetone-d6, (CD3)2(CH3)COH and (CD3)2(CH3)COD; and (3) non-selective decomposition products. This number should equal 100%. If the non-selective decomposition of 2, and hence the non-selective decomposition of irreversibly-adsorbed isobutylene-d6, were the sole source of deuterium for the dihydrogen and water that are evolved at or below 330K, then the selectivity of irreversibly-adsorbed isobutylene-d6 to the first two sets of products would be limited

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to 86% assuming a best case scenario where: (1) the H/D isotopic composition of surface acetate at 440 K is the same as the average H/D isotopic composition of species adsorbed on the surface at this temperature; and (2) when a molecule of 2 non-selectively decomposes, all of its deuterium is available for the formation of dihydrogen and water. In actuality, the selectivity to the first two sets of products approaches 100% (the calculated selectivity was 110%, slightly above 100% because of the assumptions that went into the calculation of acetate selectivity), thus indicating that the nonselective decomposition of 2 is not a major reaction pathway, and hence is not a major source of deuterium. Unlike the production of acetone and the non-selective decomposition of 2, the conversion of 2 into surface acetate could be a major source of deuterium atoms. If three deuterium atoms are released for the formation of dihydrogen and water for every molecule of 2 that is converted into acetate, the selectivity of irreversiblyadsorbed isobutylene-d6 to acetate at 330 K would only have to be 40% to account for the deuterium that is incorporated into HD, D2, HDO and D20. If two deuterium atoms are released, the selectivity would only have to be 60%. Of the pathways shown in Scheme 3, all have the potential to violate one of the four experimental constraints mentioned previously. Pathway A might be expected to violate the first of the experimental constraints since formaldehyde is produced in this pathway. Pathway B might be expected to violate the first constraint as well since OCH20could rearrange to formaldehyde. There is justification for this since (1) a 180CH~60- species may be responsible for the desorption of HCHlSO from Ag(ll0)-lsO [42] and Cu(110)-180 [43] surfaces that were exposed to HCHI60 and (2) the similar -O(CH3)2C0 species appears be responsible for the exchange of oxygen between acetone and the Ir(111 )-Oadatom surface. Pathway C may violate the third constraint since significant amounts of formate should be produced in this mechanism. However, the other pathways in Scheme 3 also might violate this constraint since the produced HCHO and - O C H 2 0 - in these other mechanisms could decompose to or be oxidized to formate. Pathway D may violate the first con-

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straint and would violate the second and fourth since formaldehyde is produced and acetone-d6 is not. Additionally, the production o f - O C D 3 in Pathway D could violate the first and third constraints since methoxy was hydrogenated to gaseous methanol and oxidized to formate on oxygenprecovered Ir(111) and could also be converted into gaseous formaldehyde if the oxygen coverage was high enough. The production o f - C D 3 during the formation of acetate in Pathways A-C could also violate the first and third constraints based upon results from studies that have examined the interaction o f - C H 3 and oxygen adatoms on Rh(111 ) [44,45]. In these studies, methyl species in high internal energetic states, such as gaseous methyl radicals and - C H 3 formed during the decomposition of species such as CH3I, react with oxygen adatoms by one pathway while accommodated methyl species react by another. The "hot" methyl species reacted with oxygen on R h ( l l l ) to form methoxy, a species that, as mentioned in the preceding paragraph, decomposes/reacts on oxygen-precovered Ir( 111 ) to yield gaseous methanol, surface formate, and, depending upon the oxygen coverage, gaseous formaldehyde. The accommodated methyl species, in contrast, react with oxygen and the R h ( l l l ) surface in a series of hydrogenation/ dehydrogenation steps to yield CO, CO 2, CH 4 (at 215 K), and presumably H 2 and H20. Whether methyl on the Ir(11 I)-(2 × 1)O/isobutylene prepared surface would behave similarly to methyl groups on Rh( 111 ) is unclear. However, if methyl was formed with a relatively low energy on Ir( 111 ), it might not be converted into methane. Methyl is proposed to form on Ir( 111 )-(2 x 1)O/isobutylene prepared surfaces at greater than 270 K, more than 55 K higher than the temperature at which methyl was hydrogenated on Rh( 111 ). Since higher temperatures would be expected to favor dehydrogenation over hydrogenation, methyl might be preferentially converted into CH2 and/or CH on the Ir( 111 )-(2 x 1 )O/isobutylene prepared surface. From the above discussion, we believe that Pathway C is most consistent with the experimental data. This conclusion is based upon the high likelihood that gaseous formaldehyde would have been detected during the oxidation of isobutylene

if one of the other pathways in Scheme 3 were responsible for the conversion of 2 into gaseous acetone, water and dihydrogen as well as acetate and possibly formate. Additionally, the plausibility of Pathway C is supported by studies that examined the interaction of 1,2-ethanedioxy with oxygen-precovered Ag(110) [46]. Through the use of isotopic labeling, these studies found that the rate-limiting step in the conversion of 1,2-ethanedioxy into formate and gaseous formaldehyde was the abstraction of a hydrogen rather than the nucleophilic attack of oxygen at a carbon. Similar to 1,2-ethanedioxy on oxygen-precovered Ag(ll0), a significant ( ~ 2 0 K ) isotope effect should be observed for the evolution of acetone, dihydrogen and water during the oxidation of either ( C D 3 ) 2 C C D 2 o r (CH3)2CCD 2 on Ir(l 11)(2 × 1)O if 2 reacts via Pathway C as suspected. There are some potential inconsistencies with Pathway C, however, but they are not exclusive to Pathway C. The largest difficulty in justifying the feasibility of Pathway C is reconciling the relatively large amount of formate that should be produced by this mechanism with the experimental observation that little or no formate is formed. However, the other pathways in Scheme 3 also suffer from this problem since some of the byproducts of these pathways (HCHO, - O C D 3 and - O C H 2 0 - ) could be converted into formate with high selectivity. Likewise, it is possible that gaseous methane and/or methanol would be produced in Pathway C, which would violate the first of the experimental constraints, but methane and/or methanol could be produced in any of the other pathways from - O C D 3 a n d / o r - C D 3 species. Three alternative mechanisms of isobutylene oxidation to those developed in Scheme 2 and/or Scheme 3 will be presented. While some portions of these mechanisms fit the experimental data as well as the mechanisms developed in the two schemes, other portions suggest that these mechanisms cannot be responsible for the oxidation of isobutylene on the Ir(111 )-(2 x 1)O surface. The first mechanism to be discussed proceeds by the net scission of the carbon-carbon double bond of n-bound isobutylene-d6 to afford the bridging or carbene-like fragments, (CD3)2C and C H 2. Insertion of an oxygen adatom between

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(CD3)2C and the surface could lead to the formation of qz(C,O)-acetone-d6. Insertion of a second oxygen adatom could result in the formation of -O(CD3)2CO-, a potential acetate precursor. Alternately, acetate could be formed from (CD3)2C through sequential ethylidyne (CD3C) and acetyl intermediates. While this mechanism offers the possibility of a relatively simple pathway to acetone, it has many drawbacks, some of which are: ( 1 ) based upon HREEL spectra of the isobutylene oxidation process, the conversion of r~-bound isobutylene into (CD3)2C and CH2 would have to involve at least one isobutylene intermediate which is not di-c~-bound, and this is somewhat difficult to rationalize; (2) the CH2 fragment might be converted into gaseous formaldehyde [47,48], which would be contrary to the experimental results;, and (3) it is unlikely that acetate would be produced near 305 K in this mechanism since rl2(C,O)-acetone would not be expected to be converted into acetate below 345 K and ethylidyne might be expected to undergo dehydrogenation based upon the interaction of this species with Ru(001)-(2 × 2 ) 0 [49]. The TPRD and HREEL studies of TME oxidation cause additional difficulties in justifying the feasibility of this mechanism. If isobutylene-do were cleaved into (CH3)2C and CH2 fragments, TME might be expected to be cleaved into two of the (CH3)2C fragments. Given the differing acetone desorption temperatures, ,-~305 K for acetone produced from isobutylene and 345 K for acetone produced from TME, as well as the qualitative differences between the HREEL spectra of isobutylene-derived adsorbates (Fig. 8) and those derived from TME (Fig. 12), it would seem that the production of acetone from these two olefins does not proceed through a common intermediate. The second mechanism proceeds via the direct conversion of 2 into -O(CD3)2CO- which in turn either loses a methyl group to the surface to form acetate or rearranges to form gaseous and adsorbed acetone. The main difficulty with this mechanism concerns the path by which 2 would be converted into -O(CD3)2CO-. Since bimolecular nucleophilic substitution reactions at tertiary carbon centers proceed very slowly if at all [50], -O(CD3)2CO- would not be expected to be formed

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by the nucleophilic attack of an oxygen adatom at 2's tertiary alcoxy carbon. It is also unlikely that -O(CD3)2CO- would be produced from 2 in a radical mechanism involving an electrophilic oxygen adatom. Studies that have examined the interaction of the electrophilic anion radical Owith alkanes suggest that 2's C-H bonds would be broken in preference to its C-C bonds; in the gas phase [51] and on MgO [52], the interaction of O- with C2H6, C3H8 and n-C4Hlo results in H or H + transfer to O- but does not result in the breaking of C-C bonds. Additionally, it appears that no carbon carbon single bonds are cleaved when O reacts with olefins [53,54]. The production of formaldehyde is another difficulty associated with this mechanism. A third alternative mechanism to those developed in Schemes 2 and 3 is the conversion of the oxametallocycle, 1, not only into t-butanol but also into rlZ(C,O)-acetone, surface acetate and gaseous acetone-d6. The latter species could be formed from 1 through an -O(CD3)2CO- intermediate. Similar to one of the problems associated with the second alternative mechanism, it is difficult to rationalize how -O(CD3)2CO-could be formed directly from 1. In addition, it is difficult to reconcile this mechanism with the experimental observation that there are at least two sequential, non-di-c~-bound isobutylene intermediates involved in the conversion of r~-bound isobutylene to q2(C,O)-acetone, acetate, and gaseous acetone. If 1 is the second of these intermediates, the conversion of ~-bound isobutylene into I must involve at least one non-di-c~-bound isobutylene intermediate. One possible solution to this problem is that the structural isomer of 1, -(CD3)2CH20-, is formed from u-bound isobutylene in a kinetically-driven reaction and that at higher temperatures -(CD3)2CH20- undergoes rearrangement to the more thermodynamically stable 1. This rearrangement might be expected to produce isobutylene oxide (IBO) as a byproduct, since an IBO-like transition state would be envisioned, but no IBO is detected experimentally. This is unlikely to be caused by IBO being unstable to the oxidizing conditions present on the surface since IBO largely desorbed from, rather than reacted with, the Ir( 111 )-(2 x 1 )O surface. This mechanism is more

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in agreement with the spectroscopic evidence than those that proceed via the oxametallocycle/diolate mechanism, however. In particular, the features near 955 cm- 1 in Figs. 8b and 9b could be assigned to the v(CO) mode of-(CX3)2CCH20- while the feature at 1150 cm-1 in Fig. 8c and the feature at 1095 cm -I in Fig. 9c could be assigned to the v(CO) mode of-O(CX3)2CCH2-. For the oxametallocycle/diolate mechanism, the 955 cm-i features were assigned to the v(CO) mode of - O ( C X 3 ) 2 C C H 2- while the others were assigned to the v(CO) mode of the tertiary alcoxy carbon of-O(CXa)2CCH20-. N o particularly suitable feature was present for the v(CO) mode of the primary alcoxy carbon of-O(CX3)2CCH20-. Assigning the 955 cm -1 features to the v(CO) mode of-(CX3)2CCH20- is more appropriate since the v(CO) mode of a tertiary alcoxy carbon would be expected to be closer to l lS0cm -1. However, vibrational coupling could lower this frequency closer to 955 era-1.

5. Summary

As we have shown in this paper, the partial oxidation of olefins to ketones and carboxylic acids on iridium catalysts (Scheme l) also proceeds under UHV conditions. In particular, we have shown that isobutylene can be oxidized to gaseous acetone and rlE(C,O)-acetone on the I r ( l l l ) (2 x 1)O surface. Allylic species are not intermediates in the formation of acetone since the only gaseous acetone that is produced in detectable amounts from (CD3)2CCH 2 is (CD3)2CO. Most, if not all, of this acetone is produced from the oxidation of n-bound isobutylene rather than from the oxidation of di-o-bound isobutylene; the direct oxidation of the latter species produces little gaseous acetone and no detectable amounts of adsorbed acetone. Additionally, the production of acetone does not appear to involve carbene or bridging methylene species since the immediate precursor to acetone is different during the oxidation of isobutylene than it is during the oxidation of 2,3-dimethyl-2-butene. The mechanism that best describes the production of acetone as well as the byproducts t-

butanol, bridging or bidentate-bonded acetate, and possibly bridging or bidentate-bonded formate is illustrated in Scheme 2. This mechanism is consistent with the preferential formation of t-butanol over isobutanol and also is consistent with HREELS data that suggest that the conversion of n-bound isobutylene into acetone and acetate involves at least two sequential sets of intermediates. The first step in this mechanism is the formation of-O(CD3)2CCH2- (1) from the addition of an oxygen adatom to the tertiary carbon of n-bound isobutylene. A portion of 1 is subsequently converted into t-butanol while another portion is converted into -O(CD3)2CCH20- (2). The sequential hydrogenation of 1 leads to the evolution of (CD3)2(CH3)COH at 290K and (CD3)2(CH3)COD near 300 K. Because a kinetic isotope effect is observed, the rate-limiting step in the evolution of t-butanol is believed to be the addition of an H or D atom to adsorbed (CD3)2(CH3)CO-. Surface hydroxyl may be supplying the hydrogen that converts 1 into t-butoxide since no detectable amounts of (CD3)2(CH2D)COH or (CD3)2(CHzD)COD are produced and the H:D ratio of the water that desorbs below 285 K is high (6.0+0.8). TPRD experiments that examined the production of tbutanol from Ir(11 I)-(2 × 1)O/isobutylene/-OD and Ir(111)-(2 × 1)O/isobutylene prepared surfaces support this hypothesis and also suggest that the fraction of 1 that is converted into t-butoxide is limited by the availability of surface hydroxyl; 50% more t-butanol was produced by the former surface with the increased production coming from the formation of (CH3)2(CHED)COH. Competing with the conversion of 1 into tbutanol is the conversion of 1 into 2. The subsequent reaction of this species near 305 K results in the production of gaseous acetone-d6, water (H20, HDO, D20) and dihydrogen (HD, D2) as well as q2(C,O)-acetone, and bridging or bidentatebonded acetate (and possibly bridging or bidentate-bonded formate). Some of the possible mechanisms by which 2 could be transformed into these species are shown in Scheme 3. We believe that Pathway C is most consistent with the experimental data since: (1) this mechanism explains how the production of the above-mentioned species could

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appear to be limited by the decomposition of a common intermediate in a single reaction; and (2) this mechanism, unlike the others in Scheme 3, would be expected to produce little or no formaldehyde, a species which is not observed during the oxidation of isobutylene. Additionally, Pathway C is analogous to the mechanism by which 1,2-ethanedioxy is converted into formate and gaseous formaldehyde on oxygen-precovered Ag(110) [46]. However, there are some potential inconsistencies between Pathway C and other experimental data, although the inconsistencies are not specific to this pathway. The most significant of these is reconciling the observation that little or no formate is produced during the oxidation of isobutylene with the expectation that Pathway C would be expected to produce significant amounts of formate. Additionally, the other byproduct of Pathway C, a methyl species, could undergo subsequent reaction to yield gaseous methanol, methane or possibly formaldehyde, none of which are observed during the oxidation of isobutylene.

Acknowledgements The authors would like to thank E.V. Anslyn, J.G. Ekerdt, J.E. Parmeter and C.G. Willson for helpful discussions. The authors also acknowledge Amoco, Koch Industries and The National Science Foundation Presidential Young Investigator Program for support.

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