t-butyl nitrite (TBN) and t-butyl alcohol (TBA) reactions on clean and O-covered Rh

Article abstract of DOI:10.1021/jp004539v

The reactions of t-butyl nitrite (TBN) and t-butyl alcohol (TBA) on clean and O-covered Rh foils were investigated using time-of-flight mass spectrometry temperature-programmed desorption and Auger electron spectroscopy. Although t-butoxy groups are formed when TBN and TBA dissociate on clean Rh and when TBA dissociates on O-covered Rh, there are significant differences in the thermal behaviors of these systems. For TBN on clean Rh, there is evidence that a monolayer decomposes to form NO and two oxygen-containing fragments, t-butoxy and a stabilized oxametallacycle. The proposed oxametallacycle decomposes at 350 K to acetone, whereas t-butoxy, relatively stable in the presence of N and NO, decomposes to isobutene at 500 K. For TBA dosed onto an O-covered surface, the O-H bond breaks between 200 and 300 K to form t-butoxy. This occurs in competition with TBA desorption. The t-butoxy is stable only up to 380 K where, assisted by O, it decomposes to acetone and butene via a transient form of the oxametallacycle involved in TBN decomposition. Combustion reactions to form water, carbon monoxide, and carbon dioxide compete. For a monolayer of TBA dosed onto a clean Rh surface, H₂, H₂O, and CO, but no acetone or butene, are observed TPD, and carbon remains after TPD.

Full text of DOI:10.1021/jp004539v

J. Phys. Chem. B 2001, 105, 3587-3593
3587
t-Butyl Nitrite (TBN) and t-Butyl Alcohol (TBA) Reactions on Clean and O-Covered Rh
X.-M. Yan, C. Kim, and J. M. White*
Department of Chemistry and Biochemistry, Center for Materials Chemistry and Texas Materials Institute,
UniVersity of Texas, Austin, Texas 78712
ReceiVed: December 20, 2000; In Final Form: March 7, 2001
The reactions of t-butyl nitrite (TBN) and t-butyl alcohol (TBA) on clean and O-covered Rh foils were
investigated using time-of-flight mass spectrometry temperature-programmed desorption and Auger electron
spectroscopy. Although t-butoxy groups are formed when TBN and TBA dissociate on clean Rh and when
TBA dissociates on O-covered Rh, there are significant differences in the thermal behaviors of these systems.
For TBN on clean Rh, there is evidence that a monolayer decomposes to form NO and two oxygen-containing
fragments, t-butoxy and a stabilized oxametallacycle. The proposed oxametallacycle decomposes at 350 K to
acetone, whereas t-butoxy, relatively stable in the presence of N and NO, decomposes to isobutene at 500 K.
For TBA dosed onto an O-covered surface, the OsH bond breaks between 200 and 300 K to form t-butoxy.
This occurs in competition with TBA desorption. The t-butoxy is stable only up to 380 K where, assisted by
O, it decomposes to acetone and butene via a transient form of the oxametallacycle involved in TBN
decomposition. Combustion reactions to form water, carbon monoxide, and carbon dioxide compete. For a
2 2
monolayer of TBA dosed onto a clean Rh surface, H , H O, and CO, but no acetone or butene, are observed
TPD, and carbon remains after TPD.
1
. Introduction
which, on Rh surfaces, is relatively strongly bound and which,
upon thermal activation, can be dissociated to form adsorbed
atomic N and O.
Surface alkoxy species (RO(a)) are common intermediates in
heterogeneously catalyzed partial oxidation reactions, e.g.,
methanol is catalytically oxidized to formaldehyde on Ag.
â-C-H (â to the surface) bonds are generally 4-7 kcal/mol
weaker than normal C-H bonds, and there is good evidence
that the initial dissociation of alkoxy species typically involves
The mechanism and kinetics describing the decomposition
of t-butoxy coadsorbed with atomic oxygen on O-covered
1
6
Rh(111) have been reported. Isobutene forms, indicating
CsO bond cleavage. Contrasting with tertiary forms, primary
alkoxy species dehydrogenate and decarbonylate (except meth-
2
â-C-H cleavage. t-Butoxy, which has no â-C-H bonds, is
10-14
oxy) via either aldehyde or oxametallacycle intermediates.
Secondary alkoxy species form ketones.
typically more stable than primary and secondary alkoxy species,
e.g., on Cu(100), t-butoxy is stable up to 500 K, but methoxy
starts to dehydrogenate at 350 K.3
1
0,15
In the present work, we use time-of-flight mass spectrometry
temperature-programmed desorption (TOFMS-TPD) and Auger
electron spectroscopy (AES) to investigate (CH3)3CONO (TBN)
as a potential source of t-butoxy on a Rh foil. For comparison
with TBN, (CH3)3COH (TBA) was dosed onto both clean and
O-covered Rh surfaces. Both TBA and TBN dissociate to form
The thermal desorption chemistry of t-butoxy has been studied
4
on both relatively inert transition metal surfaces, e.g., Ag(110)
and Cu(110), and more active transition metal surfaces, e.g.,
Rh(111)
5
6
-8
9
and Pt(111). Illustrating significant differences,
t-butoxy on O-covered Ag(110) forms t-butanol, isobutene
oxide, isobutene, acetone, water, and carbon dioxide, but on
O-covered Cu(110), it forms only isobutene and water. On more
active transition metal surfaces such as Pt and Rh and in the
absence of coadsorbed oxygen, dihydrogen, water, carbon
monoxide, and surface carbon typically dominate. On O-covered
Rh(111), C-H cleavage is less extensive, and some isobutene
is formed.
Dosing t-butyl alcohol (TBA) is one route to adsorbed
t-butoxy. However, on clean coinage metal surfaces, bond
breaking occurs with low probability, whereas on active metals,
it is not selective. Predosing oxygen typically facilitates the
selective formation of t-butoxy by promoting O-H bond
breakage in TBA. An alternative t-butoxy source, t-butyl nitrite
6
t-butoxy. Consistent with the Rh(111) results, an oxametalla-
cycle is proposed as a transient intermediate in the decomposi-
tion of t-butoxy synthesized from TBA on O-covered Rh.
Isobutene and acetone appear in the TPD results, both with peaks
at 380 K. Although the same products desorb, peak temperatures
differ significantly for TBN; acetone and TBA desorb with peaks
at 350 K, whereas isobutene and additional TBA desorb at 500
K. To account for these results, we propose that dissociation
leads to two coexisting intermediates, an oxametallacycle and
t-butoxy.
2
. Experimental Section
A detailed description of the instrument used here can be
1
6
found in another paper. The ultrahigh vacuum chamber, base
(
TBN), has a weak internal NsO bond [(CH3)3COsNO, ∼171
-
10
-1
pressure of 2 × 10
Torr, was equipped for time-of-flight
kJ mol ] that readily cleaves. Whereas TBA dissociation on
O-covered metals leads to OH, TBN dissociation leads to NO,
mass spectrometry (TOFMS) and Auger electron spectroscopy
(
AES). A thin Rh foil (Alfa, 0.8 × 0.8 cm, 0.025 mm thick)
was mounted on a tungsten wire loop and cleaned initially with
a few cycles of sputtering with Ar at 500 K for 10 min,
*
Corresponding author. E-mail: jmwhite@mail.utexas.edu. Fax: 512-
+
4
71-9495. Tel.: 512-471-3704.
1
0.1021/jp004539v CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/07/2001

3588 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Yan et al.
Figure 1. TOFMS-TPD (4.0 K s-1) of 80 Torr s (3.4 × 10 molecules)
of tert-butyl nitrite (TBN) dosed onto clean Rh at 100 K. As indicated
in the text and by extrapolation in Figure 2, 1 monolayer coverage is
defined as a dose of 110 Torr s. The pressure used to calculate the
dose is that behind a calibrated leak, not the local pressure at the
substrate.
15
Figure 2. Integrated TPD peak areas of indicated species found in
TOFMS-TPD of tert-butyl nitrite (TBN) dosed onto clean Rh at 100
K as functions of dose. The extrapolated linear region of the TBN peak
area intersects the intensity axis at 110 s, which is taken as an
operational definition of a monolayer dose (see text).
annealing in 2 × 10-8 Torr O2 at 800 K for 5 min, and annealing
in a vacuum at 1400 K for 2 min. The surface cleanliness was
verified by AES.
the full TOFMS data set (Figure 1). From bottom to top, these
traces are assigned, on the basis of fragmentation pattern
analysis, as follows: 2 amu, H2; 18 amu, H2O; 28 amu, CO
and N2; 30 amu, TBN and NO; 56 amu, isobutene; 58 amu,
TBN and acetone; 59 amu, TBN and TBA; and 88 amu, TBN.
Clearly, TBN dissociates thermally on clean Rh to yield an array
of products. After TPD to 900 K, the carbon level is barely
detectable by AES; the C/Rh atomic ratio is <0.05. As shown
in Figure 2, the peak areas for TBN, two TBA peaks, acetone,
and isobutene are all dose-dependent.
TBA (Alfa, 99.9%) and TBN (Aldrich, 90%) were used after
several freeze-pump-thaw purification cycles. Judging from
a residual gas analysis (RGA), the TBN source contains 3%
TBA. Similarly, the TBA source contains 0.4% H2O. These
compounds were dosed via a calibrated capillary leak built into
a toggle-controlled sampling valve. With a fixed pressure (y
Torr) behind the capillary, the dose can be stated in terms of
the product y × t, where t is the dose time in seconds. In these
13
terms, 1 Torr s of TBN equals (4.2 ( 1.0) × 10 molecules
-1
s
delivered to the system. During dosing, the chamber pressure
Returning to Figure 1, there are sharp peaks at 145 K. The
was unknown because the ion gauge was turned off to avoid
decomposition by electrons, especially of TBN. Oxygen (Spec-
tra, research grade) was dosed by backfilling through a separate
1
45 K TBN peak is attributed to multilayer sublimation; its
intensity does not saturate (not shown), and the peak temperature
9
is consistent with multilayer desorption on Pt(111). The absence
-
8
leak valve to a chamber pressure of 5 × 10 Torr.
of a distinct monolayer desorption feature indicates that none
of the TBN in contact with Rh desorbs. On the basis of a detailed
fragment analysis of all (many not shown) masses with peaks
at 145 K, we conclude that small amounts of impurity TBA
desorb with TBN. Because TBA dosed alone sublimes at 170
K (Figure 4), apparently mixing small amounts of TBA with
TBN alters the desorption properties of TBA.
TPD was performed up to 950 K with a ramp rate of 4.0 K
-1
s . The temperature was measured with a type-K thermocouple
spot-welded to the back of the sample. Desorbed species were
monitored repeatedly by TOFMS, with a full mass spectrum
from 1 to 145 amu acquired every 0.3 s. The upper limit exceeds
the parent masses of TBA (74 amu) and TBN (103 amu),
providing a search for up to C8 products. Because every mass
is recorded in every scan, this method records in one experiment
all of the species that do and do not desorb and makes possible
a retrospective analysis for any positive ion within the mass
range recorded.
When peak area is plotted as a function of dose (Figure 2),
it is evident that TBN desorption does not occur below ∼60
15
Torr s (2.6 × 10 molecules dosed) and that an extrapolation
of the linearly increasing region above 150 Torr s intersects
the dose axis at 110 Torr s. Operationally, and somewhat
1
5
arbitrarily, 110 Torr s (4.6 × 10 molecules dosed) is defined
as a monolayer dose. Inspection of Figure 2 indicates that the
reaction productssTBA, acetone, and isobutenesare all satu-
3
. Results and Discussion
1
5
3
.1. TBN on Rh without O(a). After 80 Torr s (3.4 × 10
1
5
molecules) of TBN is dosed on clean Rh at 100 K, the TPD
results are summarized as having eight masses selected from
rated for doses exceeding 200 Torr s (8.4 × 10 molecules)
and that the multilayer TBN peak makes measurable contribu-

TBN and TBA Reactions on Clean and O-Covered Rh
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3589
tions below 110 Torr s. Taken together, this reflects the well-
known tendency of TBN to form 3-dimensional islands.
area ratioss(350 K)/(500 K) TBA and acetone/isobutenes
uncorrected for ionization probabilities, are 5.5 and 11, respec-
tively. Strikingly, the acetone/isobutene ratio is a factor of 20
smaller for TBA dosed on O-covered Rh (see below).
1
7
As the TPD temperature increases above 145 K, a rise in the
9 and 18 amu signals (Figure 1) begins at 180 K, followed by
5
Regarding acetone formation, a C3 structure with oxygen
bound to the central carbon atom, i.e., isopropoxy, is ruled out
because it would lead to some 2-propanol, contrary to observa-
tion. This points to a C4 tertiary carbon structure. Two
a 2 amu onset at 230 K. This is clear evidence for surface
reactions that we take as dominated by cleavage of the internal
N-O bond. Although not obvious in the raw data, Figure 2,
there is a 59 amu peak at 265 K (sketched in), along with clearly
evident peaks near 350 and at 500 K. The 265 K peak contains
no evidence for TBN and is ascribed to desorption-limited TBA
consisting of two parts: impurity TBA dosed with TBN and
TBA formed between 100 and 265 K by the hydrogenation of
t-butoxy.
6,9,24
possibilities emerge: the oxametallacycle 1,
which we favor,
and a structure that involves carbon-rhodium bonding and
retention of O-H, i.e., either HOC(CH3)2sC(a)H2-Rh or HOC-
(CH3)2sC(a)HdRh. The formation of acetone from 1 relies on
weakening of the CsC(a)H2 bond and, in concert, strengthening
of the C-O bond. Forming acetone from the second possibility
is much more complicated; CsC and OsH bond breakage must
be catalyzed, and a transient OsRh bond must be formed.
The 18 and 2 amu TPD profiles reflect low-temperature
activation of the C-H, N-O, and C-O bonds, leading to water
and molecular hydrogen and, by inference, new surface het-
eroatom hydrocarbon intermediates. The H2O desorption is
reaction-limited because the peak lies above 180 K, the
1
8
temperature expected for desorption-limited water evolution.
Disproportionation and hydrogenation of O-H contribute, the
H coming from C-H bonds. Clearly, by 250 K the rate of C-H
bond breaking increases the local concentration of H(a) to a level
that makes recombination to form H2 compete effectively with
both hydrogentation of t-butoxy to form TBA and of OH to
form H2O. The water desorption rate maximizes (320 K) 20-
The 500 K desorption of isobutene in competition with TBA
suggests a second intermediate with the same C4 carbon
backbone as isobutene. Because there is significant H(a), the
absence of isobutane evolution rules against t-butyl, while the
evolution of TBA certainly favors t-butoxy. Further, the high-
temperature evolution of CO (680 K) is consistent with the
presence of O(a) after all of the H(a) is desorbed (>600 K). The
reaction path leading to isobutene likely involves a transition
state 2 in which both the CsO and CsH bonds are elongated.
The path to this configuration from t-butoxy would involve
CsO elongation, allowing for transient bonding of the tertiary
carbon to Rh while a γ-C-H bond stretches to its breaking
5
0 K below the maximum desorption rates of H2 (345 K) and
TBA (350 K). There is a maximum in the 58 amu signal at 350
K due to acetone desorption. The next-higher-temperature
desorption peak (400 K) occurs in the 30 amu spectrum. It is
broadly distributed and assigned to desorption-limited NO
because this range is typical for dosed NO.
Moving to higher temperature (460 K), there is a peak in the
28 amu spectrum that is attributed to CO and a strong shoulder
in the 2 amu (H2) spectrum that is ascribed to further C-H
bond breaking. Overlapping these and peaking at 500 K, there
are strong, nearly coincident desorptions of isobutene (56 amu)
and TBA (59 amu). The CO peak occurs in the range where
CO normally desorbs from Rh,19 but the overlap with the
acetone, isobutene, and hydrogen desorptions, as well as the
relatively high onset temperature (400 K), make it likely that
this CO desorbs in a reaction-limited process. The CO desorption
continues up to about 600 K (vertical line), where N2 desorption
begins.
2
5,26
point.
By 600 K, all desorption signals, except for 28 amu, had
returned to background levels, i.e., there were no more C-H
bonds to be broken. The high-temperature 28 amu peaks are
ascribed to CO formed by combining adsorbed C with residual
To summarize, all submonolayer TBN decomposes and can
be described in terms of two C4 intermediates, t-butoxy and an
oxametallacycle 1, accompanied by NO. NO partially dissociates
to N(a) and O(a). N(a) leaves the surface as N2 at 800 K, and O(a)
as H2O or CO. We propose that the oxametallacycle 1
dehydrogenates selectively to form acetone at 350 K, whereas
t-butoxy remains to 500 K, where it decomposes to isobutene
via the proposed intermediate 2 and other CxHy fragments, most
of which react with O(a).
adsorbed O (680 K) and to N atom recombination (800 K).
Dissociation of NO on Rh is well-documented.2
0-23
As noted
above, AES after the samples were heated to 900 K indicated
barely detectable amounts of C as the only contribution other
than Rh.
Returning to the dose dependence (Figure 2), it is noteworthy
that the 500 K TBA and butene intensities rise and saturate
together. Similarly, the 350 K TBA and acetone peaks parallel
each other. Taken with the other data, these results can be
described by a reaction model that involves dissociation of
weakly held [designated by subscript (p)] (CH3)3CONO(p) into
two intermediates. Figure 2 also indicates that the onset
coverages differ, but only slightly, for the two pairs of product
peaks. Peaks at 350 K, but not 500 K, are evident for a dose of
3.2. TBA on Rh without O(a). For comparison, we briefly
examined TBA chemistry on clean and O-covered Rh foils. On
clean Rh up to monolayer coverage, most of the TBA
decomposes, and the TOFMS-TPD signal is dominated by H2,
CO, and small amounts of H2O and CO2 (see Figure 3 for a
1
5
dose of 60 Torr s, 2.5 × 10 molecules). No hydrocarbons or
other oxygen-containing species desorb up to 950 K; in
particular, there is no acetone or isobutene desorption. After
TPD, AES shows only C, and the atomic ratio, C/Rh, is
0.15, more than 3-fold higher than its value after TPD of TBN
on Rh.
2
5 Torr s, whereas for doses of 50 Torr s and more, all of the
areas rise monotonically until saturation is reached at 200 Torr
s. Accounting for the scaling factors in Figure 2, the two peak

3590 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Yan et al.
Figure 4. TOFMS-TPD (4.0 K s- ) of 60 Torr s (2.5 × 10 molecules)
of TBA dosed onto O-covered Rh at 100 K. The amount of preadsorbed
Ã(a) is 0.66 of saturation coverage. A monolayer dose of TBA is 70
Torr s. The pressure used to calculate the dose is that behind a calibrated
leak, not the local pressure at the substrate.
1
15
Figure 3. TOFMS-TPD (4.0 K s-1) of tert-butyl alcohol (TBA) dosed
15
onto clean Rh at 100 K. The dose was 60 Torr s (2.5 × 10 molecules).
As operationally defined in the text, 1 monolayer dose is 70 Torr s.
The pressure used to calculate the dose is that behind a calibrated leak,
not the local pressure at the substrate.
2
30 K in the TPD of 1-propanol14 and 2-propanol15 dosed onto
The TBA desorption spectrum (Figure 3) has three peaks ar
70, 220, and 270 K. The 170 K peak indicates multilayer
clean Rh(111). For TBA, CsO bond scission might be assisted
by the tendency of the tertiary C to bond to Rh. In primary and
secondary alcohols, the R-C can form a C-Rh bond by breaking
a CsH bond, releasing H while maintaining the CsO bond,
albeit weakened. As for TBN, the dihydrogen desorption from
TBA occurs at higher temperatures than water desorption.
Carbon monoxide peaks at 480 K, and there is a weak carbon
dioxide peak at 450 K. Recognizing that the CO desorption peak
temperature is consistent with that found on Rh(100)19 and that
most of the adsorbed oxygen is consumed by hydrogen, it is
still plausible that CO desorption is reaction-limited because it
overlaps with the high-temperature tail of dihydrogen desorption.
Although CO2 can come from the disproportionation reaction
2C(a)O f CO2(g) + O(a), from unidentified residual CxHyOz
species, or from residual atomic oxygen, it is more likely at
this temperature (>400 K) that an oxametallocycle, i.e.,
1
desorption, consistent with reports for TBA on Rh(111) (180
4-9
K), Pt(111) (160 K), Ag (111) (190 K), and Cu(110) (180 K).
The other two peaks come from TBA in contact with Rh. One
monolayer of TBA is defined operationally as the dose (70 Torr
1
5
s, 2.9 × 10 molecules) that saturates the 270 K peak. There
is some multilayer desorption at 170 K before the 270 K peak
saturates. Assuming that the TBA sticking coefficient is constant
during dosing at 100 K, we conclude that 92% of the monolayer
(
as defined above) of TBA dissociates. This result arises from
a comparison of the TBA peak area for a 70-s dose with the
difference in TBA peak areas for two doses in the multilayer
regime that differ by 70 Torr s (not shown). The data of Figure
3
differ from those for single-crystal Rh(111) in that, for the
latter, there is a small TBA peak at 330 K assigned to t-butoxy
6
2
hydrogenation. Apparently, non-(111) sites on this clean Rh
η -acetone, C(a)(CH3)2O(a), is formed transiently and, without
foil lower the activation energy for the decomposition of
t-butoxy.
significant delay, undergoes dehydrogenation, accompanied by
CsC bond breakage and release of CO. From the same
intermediate, some CsO bond scission could release O to form
CO2.
To summarize for TBA dosed onto clean Rh, partial dehy-
drogenation occurs below 230 K, and some CsO bonds break
at 230 K. At higher temperatures, the remaining CxHyOz species
decompose and rearrange to release CO, CO2, H2O, and H2,
leaving only carbon on the surface at 900 K.
The strongest desorption peak of dihydrogen occurs at 310
K. This peak is ascribed to H atom recombination27 and follows
second-order kinetics (not shown). CsH bond cleavage leads
to the tail between 400 and 560 K of the 310 K H2 desorption.
28
This is the typical CsH bond-breaking regime of alkyls on Rh.
Water is evolved with peaks at 215 and 295 K. Water dosed
18
onto Rh(111) desorbs at 180 K, and the desorption shifts to
07 K in the presence of coadsorbed CO.29 We assign the 215
15
2
3.3. TBA on O-Covered Rh. When 60 Torr s (2.5 × 10
K peak as desorption-limited monolayer water coming mostly
from an impurity in TBA, rather than decomposition of TBA.
In the range of 230-380 K, water desorption is reaction-limited
and occurs as the result of CsO and CsH bond scission. In
contrast, there is no evidence for CsO bond scission at or below
molecules) of TBA is dosed onto Rh precovered with 0.66 of
saturation O(a) (based on TPD), the TPD spectrum exhibits the
features summarized in Figure 4. On the basis of a fragmentation
pattern analysis, the peaks are assigned to TBA at 170, 270,
and 380 K; acetone at 380 K; isobutene at 380 K; CO2 at 420

TBN and TBA Reactions on Clean and O-Covered Rh
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3591
K; CO at 420 and 680 K; H2O at 210 and 410 K; and, in very
tiny amounts, H2 between 350 and 450 K. Unlike for TBA on
clean Rh, significant amounts of acetone and isobutene desorb.
Further, the amount of CO2 desorbed is larger, and the amounts
of H2 and CO desorbed are smaller. Significantly, after TPD,
O(a), not C(a), remains (AES not shown).
the falling edge of CO2 overlaps with the leading edge of the
680 K CO peak, indicating, not surprisingly, that surface carbon
is very heterogeneously distributed so that oxidation continues
between 400 and 760 K.
Careful examination of the fragmentation pattern at 380 K
(not shown) indicates that both isobutene and acetone are
desorbed at 380 K. The 56, 58, and 59 amu signals all rise
together, but the falling edge of 56 amu (isobutene-dominated)
is much slower than those of 59 and 58 amu. The relative
intensities of these three signals also vary with dose, reflecting
multiple desorbing species. From an extensive analysis of the
TOFMS-TPD data set, including the TBA fragmentation pattern
at 170 K, we deduce that some TBA desorbs with the acetone
and isobutene. From Figure 4, we then conclude that the
desorption of isobutene, but not acetone, extends through region
III of the CO2 desorption.
Between 360 and 460 K, the acetone/isobutene ratio remains
constant to within 15%, pointing to a common transition state
formed from t-butoxy. The oxametallacycle 1 proposed for TBN
dissociation is satisfactory but reacts quickly (does not ac-
cumulate); it has the same carbon skeleton as isobutene, and
its oxygen is bound in a position favorable for acetone formation.
The absence of isobutene at 380 K in the case of TBN, as well
as its presence for TBA on O-covered Rh, can be accounted
for on the basis of the local chemical potential of O(a). In the
absence of O(a), i.e., for TBN on Rh, the activation energy
required for CsO bond breakage exceeds that for CsRh
cleavage, whereas in the presence of O(a), i.e., for TBA on
O-Rh, these energies become more nearly equal.
Compared to the results for TBN on clean Rh (Figure 1), the
temperature and intensity differences of the acetone and butene
desorption peaks are most noteworthy. On TBN, butene
desorption peaks 120 K higher (500 versus 380 K), whereas
acetone desorption peaks slightly lower (350 versus 380 K).
As noted above, the product distributions differ strongly: for
TBA on O-covered Rh, the acetone/butene peak area ratio is
0.5, whereas for TBN on Rh, this ratio is 20 times higher.
In Figure 4, there are TBA peaks at 170, 270, and 380 K. As
for TBA on clean Rh, the 170 K peak is attributed to multilayer
sublimation, and the 270 K peak to monolayer TBA desorption
in the presence of dissociated fragments. Using the procedure
described above, we determined that 68% of monolayer TBA
dissociates compared to 92% on clean Rh. The 380 K TBA
peak is not found for TBA dosed onto clean Rh. In agreement
6
studies on O-covered Rh(111), this peak is ascribed to the
hydrogenation of t-butoxy. The 270 K peak area is 5 times larger
than the 380 K peak area, indicating that most t-butoxy species
decompose below 350 K.
Interestingly and unlike for TBN and TBA on clean Rh
(Figures 1 and 3), there is very little dihydrogen desorption for
TBA on O-covered Rh (Figure 4). On the other hand, the CO2
desorption is higher, and the H2O evolved is shifted to higher
temperatures than for the other two cases. These differences
reflect the higher chemical potential of O(a) compared to that
for TBA or TBN dosed onto Rh without O(a). There is enough
oxygen to (1) promote alcoholic O-H bond breakage to form
To summarize, for TBA dosed onto O-covered Rh, TBA
desorption competes with t-butoxy formation. t-Butoxy remains
up to 350 K and reacts to form acetone, isobutene, and TBA
via the short-lived oxametallacycle 1. Combustion reactions
forming CO2 and H2O compete.
(
2
CH3)3CO(a) and O(a)H; (2) promote C-H bond breakage below
70 K, thereby enhancing the rehydrogenation of neighboring
4
. Discussion
.1. Reaction Paths. With the foregoing results in hand, we
t-butoxy groups; (3) favor formation of CO2 rather than CO;
and (4) allow for the high-temperature oxidation of C(a), leaving
O(a), rather than C(a), at 800 K. It is noteworthy that no CO2
desorbs during the TPD of TBN (Figure 1), indicating that, in
that case, there is never sufficient O(a) to form CO2 in
competition with other reactions.
4
turn to proposed reaction paths for TBN on Rh and TBA on
O-covered Rh (schematically depicted in Figures 5 and 6). As
indicated schematically in Figure 5, we speculate that, prior to
reaching 200 K, adsorbed TBN passes through a transition state
with a geometry intermediate between the cis and trans forms
of TBN. Any such path will move the external O closer to the
H of CH3. Tilting the CONO plane toward the Rh surface then
brings an intermediate that can relax along one of two paths.
The first involves passage through a cis-like configuration to
form the oxametallacycle 1, whereas the other passes through
a trans-like configuration to form t-butoxy. Both dissociation
products are located in environments that contain adsorbed and
dissociated NO. Likely, the C-H bond cleavage required to
form 1 is assisted by the C-H bond being brought into close
proximity with the external O of TBN. We propose that 1 is
stable, coexists with t-butoxy, accumulates to spectroscopically
observable concentrations, and rearranges to desorb acetone and
TBA beginning at ∼300 K. Further, because the local environ-
ment contains NO and N, t-butoxy does not begin to rearrange
until much higher temperatures (∼500 K), i.e., after neighboring
NO has desorbed, opening metal sites for γ-C-H activation.
Unlike for 1, only γ-H exists in t-butoxy, and its activation
transiently forms a structure (2) that is similar to 1. At these
elevated temperatures, it does not accumulate because C-O
cleavage is activated. Alternatively, t-butoxy activation could
occur via the intermediate complex 3 in analogy to the well-
Our results (Figure 4) are consistent with the hydrogen for
the 210 K water peak coming from OsH, rather than CsH,
6
dissociation. Although the onset (180 K) of H2O desorption is
nearly the same for TBN as for the other two cases, there is no
peak at 210 K. This is not surprising because there are no
OsH bonds in TBN, so very little H(a) is available at these low
temperatures.
CO shows two peaks in Figure 4. C(a) + O(a) accounts for
the peak at 680 K, whereas that at 420 K is the result of C-CO
bond breakage. The latter occurs in the presence of O(a), and
3
0
CO desorbs in competition with its oxidation to CO2. The
broad CO2 profile can be separated into three parts: (I) 240-
3
80 K, (II) 380-460 K, and (III) 460-600 K (marked in Figure
4
). The CO2 signal rises slowly in I, exhibits a sharp peak in II,
and undergoes a steady decay in III. Parenthetically, the
desorption of CO2 dosed onto clean Rh(111) has a broad peak
at 280 K.31 The evolution of CO2 in region I reflects low-
temperature CsC and CsH bond cleavage. Increasing the initial
oxygen coverage to saturation (not shown) leads to less CO2
desorption because there are fewer Rh sites available for CsC
and CsH bond cleavage. In region II, CsH bond breaking
becomes rapid and there is sufficient oxygen to oxidize both
carbon and hydrogen to form CO2, CO, and H2O. In region III,
3
2
known â-H dehydrogenation. The contributions of these two

3592 J. Phys. Chem. B, Vol. 105, No. 17, 2001
Yan et al.
Figure 6. Proposed reaction paths of TBA on O-covered Rh.
SCHEME 2
Figure 5. Proposed reaction paths of TBN on clean Rh.
SCHEME 1: O-assisted Oxametallacycle Formation
paths will depend on the orientation of t-butoxy and the surface
dehydrogenation activity. Although γ-elimination is still not
well-understood, it is known that CsH bond cleavage is not
the rate-determining step, and a weakened C-O bond, as in 3,
can facilitate γ-elimination from t-butoxy.2
5,26
below 300 K. t-Butoxy remains up to ∼380 K, where coad-
sorbed oxygen facilitates oxydehydrogenation, i.e., the oxam-
etallacycle 1 forms transiently via an oxygen-assisted mechanism
4
(
Scheme 1). From this transient configuration both acetone and
isobutene desorb.
Hydrogenation of t-butoxy to release TBA and combustion
to form CO2 and H2O all compete. Above 550 K, only oxygen
and surface carbon remain. The latter is removed as CO at 680
K, leaving an oxygen residual at 800 K.
The situation for TBA on O-covered Rh is different. Fol-
lowing previous work, we propose, as diagrammed in Figure
4.2. Comparison with TBN on Pt(111). Comparing the data
for TBN on Pt(111) and Rh, there is evidence for t-butoxy
6
9
6
, that TBA reacts selectively to form t-butoxy on O-covered
and the oxametallacycle 1 on both. Outstanding among the
differences is the absence of acetone and butene desorption from
Pt(111). Although the underlying reasons remain to be clarified,
Rh. The surface hydrogen from OsH bond cleavage reacts
exclusively with surface oxygen to form water that desorbs

TBN and TBA Reactions on Clean and O-Covered Rh
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3593
we note that dissociation of NO on Pt is much less likely
compared to its dissociation on Rh and, thus, the transition
configurations of the TBN might differ on the two substrates.
Although the intrinsic dehydrogenation activities of Rh and Pt
are similar, the atomic N and O produced by NO dissociation
on Rh, but not on Pt, might locally passivate Rh atoms and
help stabilize 1.
a fellowship. Support for instrumentation by the Science and
Technology Center Program of the National Science Foundation
(CHE8920120) is gratefully acknowledged.
References and Notes
(
1) Wachs, I. E.; Madix, R. J. Surf. Sci. 1978, 76, 531.
(2) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105,
929.
3) Ihm, H.; Smith, K. C.; Celio, H.; White, J. M. Langmuir 2001, 17,
3
7
To form acetone (C3H6O) from 1, one C and two H atoms
must be removed. The simplest path is to remove methylene
(
86.
(CH2). In a second path, methyl is removed and supplies
(4) Brainard, R. L.; Madix, R. J. J. Am. Chem. Soc. 1989, 111, 3826.
hydrogen for the group left behind (Scheme 2). This path forms
an enol, CH3COHCH2, which can reconstruct to acetone. The
methylene path is preferred because the γ-H elimination in the
formation of the enol is expected to require a higher activation
energy than the methylene path.
(5) Brainard, R. L.; Madix, R. J. Surf. Sci. 1989, 214, 396.
(
6) Xu, X.; Friend, C. M. Langmuir 1992, 8, 1103.
(7) Bol, C. W. J.; Friend, C. M. Surf. Sci. 1996, 364, L364.
8) Xu, X.; Friend, C. M. J. Phys. Chem. 1991, 95, 10753.
(
(9) Ihm, H.; Medlin, J. W.; Barteau, M. A.; White, J. M. Langmuir
2001, 17, 798.
10) Davis, M. E.; Suib, S. L. SelectiVity in Catalysis; American
Chemical Society: Washington, D.C., 1993.
(
5
. Summary and Conclusions
(
(
(
11) Houtman, C. J.; Barteau, M. A. Langmuir 1990, 6, 1558.
12) Houtman, C. J.; Barteau, M. A. J. Catal. 1991, 130, 528.
13) Papageorgopoulos, D. C.; Ge, Q.; King, D. A. J. Phys. Chem. 1995,
The TOFMS-TPD investigation of TBA and TBN on clean
and O-covered polycrystalline Rh surfaces leads to the following
conclusions:
9
3
1
9, 17645.
(14) Brown, N. F.; Barteau, M. A. Langmuir 1992, 8, 862.
15) Xu, X.; Friend, C. M. Surf. Sci. 1991, 260, 14.
16) Kim, C.; Yan, X.-M.; White, J. M. ReV. Sci. Instrum. 2000, 71,
505.
(17) Zhao, W.; Kim, C.; White, J. M. Surf. Sci. 2000, 451, 267.
(18) Wanger, F. T.; Moylan, T. E. Surf. Sci. 1987, 191, 121.
19) de Jong, A. M.; Niemantsverdriet, J. W. J. Chem. Phys. 1994, 101,
0126.
20) Brown, W. A.; King, D. A. J. Phys. Chem. B 2000, 104, 2578.
(21) Siokou, A.; van Hardeveld, R. M.; Niemantsverdriet, J. W. Surf.
(
(
(
1) On a clean Rh surface, TBA dehydrogenates nonselec-
tively to form H2, H2O, CO, and surface carbon.
2) On O-covered Rh, TBA dissociates between 200 and 300
(
K by breaking the OsH bond to form t-butoxy. The decom-
position of t-butoxy at 380 K to form and desorb isobutene and
acetone occurs in competition with the hydrogenation of
t-butoxy to reform TBA. Combustion reactions forming CO2
and H2O also participate. Compared to the case for TBA dosed
onto clean Rh, coadsorbed oxygen facilitates the selective
formation of t-butoxy and its subsequent decomposition through
the transient formation of 1.
(
(
Sci. 1998, 402-404, 110.
(
(
22) Bowker, M.; Guo, Q.; Joyner, R. W. Surf. Sci. 1991, 257, 33.
23) Borg, J.; Reijerse, J. F. C.-J. M.; van Santen, R. A.; Niemantsver-
driet, J. W. J. Chem. Phys. 1994, 101, 10052.
(24) (a) Brown, N. F.; Barteau, M. A. J. Phys. Chem. 1996, 100, 2269.
b) Brown, N. F.; Barteau, M. A. Surf. Sci. 1993, 298, 6. (c) Jones, G. S.;
Mavrikakis, M.; Barteau, M. A.; Vohs, J. M. J. Am. Chem. Soc. 1998, 120,
196. (d) Medlin, J. W.; Barteau, M. A.; Vohs, J. M. J. Mol. Catal. 2000,
163, 129.
25) Wiegand, B. C.; Uvdal, P. E.; Serafin, J. G.; Friend, C. M. J. Am.
Chem. Soc. 1991, 113, 6686.
26) Uvdal, P.; Wiegand, B. C.; Serafin, J. G.; Friend, C. M. J. Chem.
Phys. 1992, 97, 8727.
(27) Payne, S. H.; Kreuzer, H. J.; Frie, W.; Heinz, K. Surf. Sci. 1999,
21, 279.
28) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis;
Wiley-Interscience: New York, 1994.
(29) Wanger, F. T.; Moylan, T. E.; Schmieg, S. J. Surf. Sci. 1988, 195,
03.
(
(
3) TBN dissociates on Rh to form NO and two intermediates,
the relatively stable oxametallacycle 1 and t-butoxy. At 350 K,
reacts via CsH bond activation to yield acetone. Unlike the
3
1
(
TBA case, t-butoxy formed from TBN is stable up to 500 K,
where it rearranges via γ-H bond cleavage to form isobutene.
(
(4) The differences in the behavior of the intermediates related
to t-butoxy formation from TBA and TBN are attributed to
differences in the local chemical environments, e.g., H, NO,
OH, and O, that lead to different surface configurations and
reaction paths.
4
(
4
(
(
(
30) Au, C.; Liao, M.; Ng, C. J. Phys. Chem. A 1998, 102, 3959.
31) Dubois, L. H.; Somorjai, G. A. Surf. Sci. 1983, 128, L231.
32) Bochmann, M. Organometallics 1: Complexes with Transition
Acknowledgment. We acknowledge support by the U.S.
Department of Energy, Office of Basic Energy Sciences. One
of us (X.-M. Yan) thanks the Robert A. Welch Foundation for
Metal-Carbon σ-Bonds; Oxford University Press: Oxford, U. K., 1994.