Distinctive roles of chemisorbed atomic oxygen and dioxygen in methane catalytic oxidation on Pt{110}

Article abstract of DOI:10.1021/jp0132477

The isothermal oxidation of C_a formed by CH₄ dissociation has been studied on Pt{110} using molecular beam techniques at a surface temperature of 650 K. The reaction products CO and CO₂ are produced in the ratio of 9:1, respectively, for all carbon coverages studied (0.03-0.8 ML). When the O₂ beam is turned on, there is a sharp rise in the rate of CO production (less than 0.1 s) which is attributed for the first time to the surface reaction of C_a with short-lived chemisorbed molecular dioxygen, O_2c. This is followed by a slow rise to a maximum rate that is due to the surface reaction between C_a and adsorbed atomic oxygen, O_a, which accumulates more slowly on the surface. The initial rate of CO formation from O_2c decreases as the C_a precoverage is increased, while the time taken to reach the maximum in the rate from the C_a + O_a reaction increases. The CO rate maximum always occurs earlier than that of CO₂, for all carbon coverages. The rate constant for the C_a + O_2c reaction decreases by a factor of 6 up to a C_a precoverage of 0.2 ML and then stays almost constant up to saturation.

Full text of DOI:10.1021/jp0132477

3
416
J. Phys. Chem. B 2002, 106, 3416-3421
Distinctive Roles of Chemisorbed Atomic Oxygen and Dioxygen in Methane Catalytic
Oxidation on Pt{110}
D. T. P. Watson, J. J. W. Harris, and D. A. King*
Department of Chemistry, Lensfield Road, UniVersity of Cambridge, Cambridge CB2 1EW, United Kingdom
ReceiVed: August 22, 2001; In Final Form: NoVember 19, 2001
The isothermal oxidation of C
beam techniques at a surface temperature of 650 K. The reaction products CO and CO
ratio of 9:1, respectively, for all carbon coverages studied (0.03-0.8 ML). When the O
a 4
formed by CH dissociation has been studied on Pt{110} using molecular
2
are produced in the
beam is turned on,
2
there is a sharp rise in the rate of CO production (less than 0.1 s) which is attributed for the first time to the
surface reaction of C with short-lived chemisorbed molecular dioxygen, O2c. This is followed by a slow rise
to a maximum rate that is due to the surface reaction between C and adsorbed atomic oxygen, O , which
accumulates more slowly on the surface. The initial rate of CO formation from O2c decreases as the C
precoverage is increased, while the time taken to reach the maximum in the rate from the C + O reaction
increases. The CO rate maximum always occurs earlier than that of CO , for all carbon coverages. The rate
constant for the C + O2c reaction decreases by a factor of 6 up to a C precoverage of 0.2 ML and then stays
almost constant up to saturation.
a
a
a
a
a
a
2
a
a
1
. Introduction
While catalytic Ca oxidation has been studied extensively,
very little work has been done on single crystals. In all cases,
the products were found to be CO2 and CO, the selectivity being
a complex function of the surface temperature, Oa coverage,
A detailed understanding of the oxidation of carbon over
transition-metal surfaces is important for several reasons. First,
the surface oxidation of carbon to CO is an important step in
the partial oxidation of methane to syngas (CO and H2 in the
ratio 1:2). Second, coke formation during the oxidation of C2
hydrocarbons leads to catalyst deactivation, a problem which
must be avoided for an efficient route to oxidation.
10
and reaction temperature used. Sch a¨ fer and Wassmuth studied
the oxidation of graphite on Pt{111} over a surface temperature
range of 550-1100 K. They found that oxidation yields CO2
(500-800 K) and CO (700-1100 K) as the desorbing reaction
products. They explained the temperature-dependent selectivity
using a model where O and CO were highly mobile while C
a
Carbon deposition has been investigated over many metals
such as Fe, Ni, and Au.1-3 Somorjai and co-workers found
that over Pt{111} graphite was produced during the decomposi-
tion of ethene at high surface temperatures (greater than 850
a
a
4
atoms were immobile and the mean residence time of COa
decreased with increasing surface temperature. Walker and
11
King studied the oxidation of Ca, formed by CH4 decomposi-
tion, on Pt{110}-(1×2) over the temperature range 550-750
K. They observed that the percentage of CO2 produced in the
CO2+CO product decreases from ∼50% to less than 1% when
the surface temperature is increased from 550 to 750 K, and
that the CO2 peak maximum occurs later than that of CO.
5-8
K). Using XPS, AES, and STM, Kvon et al.
have studied
carbon on the Pt{110}-(1×2) surface formed by exposure to
residual gases and ethene decomposition. They found that the
nature of the adsorbed carbon, Ca, was dependent on both Ca
coverage and sample temperature. They observed two different
temperature regimes. At surface temperatures less than 800 K,
Ca was found to induce the (1×2) f (1×1) structural trans-
formation and was extremely reactive to O2. The second type
of carbon observed by Kvon et al. was formed when ethene
was dosed at 1150 K. In this case, they concluded that the Ca
was “diamond-like” and was extremely unreactive to both H2
and O2 below 1100 K. They also studied the adsorption site of
the low-temperature reactive Ca. At coverages, θ, of less than
The underlying mechanism of oxidation is still a matter
of much debate. On Rh foil, using AES and XPS, Mikhailov
1
2
et al. found that the reaction was Langmuir-Hinshelwood,
in which both the O and C atoms were thermally equi-
a
a
1
1
librated with the surface. Walker and King concluded that on
Pt{110}-(1×2) two processes are operative, a Langmuir-
Hinshelwood mechanism between O and C and a reaction
a
a
between C and chemisorbed molecular dioxygen, O , in which
a
2c
∼
0.1 ML, the Ca atoms were thought to be located at “near top
CO formed in the surface reaction at high surface temperatures
is impulsively desorbed. The spatial distribution of the impul-
sively desorbed CO shows a sharp lobe centered at an angle of
32° from the surface normal. The CO translational energy was
estimated from the shape of the distribution to be g135 ( 5 kJ
mol . Kislyuk et al. had previously reported a slightly peaked
distribution along the surface normal on Pt foil. However,
Walker and King also concluded that as the surface temperature
is lowered, between 650 and 500 K, the CO formed in the
surface reaction between O2C and Ca is no longer promptly
desorbed: it is trapped into the chemisorbed COa state.
Pt sites” on the (1×1) surface, while at higher coverages, 0.3
e θ e 1 ML, the Ca dissolVed into the surface via a carbide
state. They also found that even at a Ca coverage of ∼0.1 ML,
the coverage estimated from STM data was significantly less
than that measured using AES, which suggested that some of
the carbon was just beneath the surface. In fact, DFT slab
-1
13
9
calculations performed in our group by Petersen et al. for Ca
on Pt{110}-(1×2) and Pt{110}-(1×1) show that a pseudo-4-
fold site with the Ca atom located just below the surface was
the most stable.
1
0.1021/jp0132477 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/08/2002

Roles of Chemisorbed Atomic Oxygen and Dioxygen
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3417
The second product, CO2, is produced by the reaction of
11
equilibrated adsorbed CO, with Oa. This reaction has been
1
4-16
well studied
on Pt{110}-(1×2). The general consensus is
17,18
that CO2 desorbs with excess energy.
In this paper, we extend the Ca coverage range on Pt{110}
studied by Walker and King11 and find that for all surface carbon
coverages studied, Ca is oxidized to CO and CO2 in a ratio of
9
:1. We proceed to examine the mechanism of Ca oxidation in
detail.
2
. Experimental Details
The experimental apparatus is described in detail elsewhere.19
The sample is mounted centrally on a manipulator in an
Figure 1. CO produced by the isothermal reaction of different C
a
-
1
-
10
precoverages with an 0.014 ML s
O
2
molecular beam at a surface
ultrahigh vacuum chamber with a base pressure of <2 × 10
temperature of 650 K. The carbon precoverages were as shown in the
legend.
mbar. The molecular beam is sourced at a supersonic expansion
from a 50 µm nozzle, skimmed, differentially pumped, and
collimated before entering the sample chamber. An inert King
and Wells (KW) flag just in front of the crystal is used to control
the dosing time and to measure sticking probabilities. A further
flag in the source stage allows the beam to be turned on and
off. The surface temperature (Ts) is monitored by a crystal-
mounted thermocouple referenced to an electronic ice-point and
is regulated by programmed resistive heating. The partial
pressures of up to 16 individual gases can be monitored using
a fixed QMS situated behind the crystal.
20
The Pt sample, 11 mm diameter by 1 mm thick, was cut to
within 0.1° of the {110} plane. Initial cleaning of the crystal
was achieved by repeated cycles of ion sputtering, annealing,
and oxygen treatment. Routine cleaning consisted of annealing
at 1240 K, exposure to oxygen for 5 min while cooling from
Figure 2. CO
precoverages with an 0.014 ML s
temperature of 650 K. The carbon precoverages were as shown in the
legend.
2
produced by isothermal reaction of different C
a
-
1
O
2
molecular beam at a surface
1
100 to 950 K, and annealing for 15 min at 950 K. This
procedure yields a clean Pt{110}-(1×2) surface which gives a
sharp LEED pattern and oxygen thermal desorption spectra that
are in good agreement with the literature.21 It should also be
noted that Auger analysis performed during earlier studies found
no evidence for impurities such as calcium, sulfur, and silicon,
and the above cleaning procedure removed carbon which
remained on the surface after experiments. The CH4 and O2
used were > 99.9995% and 99.995% pure, respectively, as
quoted by the suppliers (Messer (U.K.) Ltd.). The supersonic
methane beams used in these experiments to prepare the CHa
adlayers had a composition of 8% CH-92% He, and the nozzle
was heated to 790 K. The oxygen beam used was produced
using a room-temperature nozzle. All coverages are quoted in
14
-2
monolayers (ML) (1 ML ≡ 9.22×10 molecules cm ) and
were determined using the spinning rotor gauge which measures
absolute beam fluxes and by comparison of TPD areas with
earlier studies.2
Figure 3. Reactive O
reaction of adsorbed carbon with a O
temperature of 650 K. The O beam flux was 0.014 ML s , and the
2
sticking probability during the isothermal
2
molecular beam at a surface
2,23
-1
2
a
C precoverages were as shown in the legend.
3
. Results and Discussion
maximum while the CO2 maximum occurs after that of the CO,
the details of which will be discussed later.
Carbon buildup can become a problem during the oxidation
of higher weight hydrocarbons on metal surfaces. To gain some
insight into this important problem, we have now studied the
oxidation of Ca on Pt{110}-(1×2) at high coverages at a surface
temperature of 650 K. The Ca adlayers were prepared by dosing
the He-seeded CH4 beam for different amounts of time at a
surface temperature of 650 K. The temporal profiles of the two
products, CO and CO2, are shown in Figures 1 and 2,
respectively, whereas the corresponding O2 reactive sticking
probabilities are shown in Figure 3. For the lowest Ca coverage
We deal with the CO production first. As the Ca precoverage
increases, the extent of the height of the initial sharp rise in the
CO pressure (the prompt process) decreases, and the time taken
to reach the maximum increases. In fact, for the highest Ca
coverage (0.83 ML), the prompt rise is small, and it takes 340
s for the CO production rate to reach a maximum. It should be
noted that for the highest coverages studied, not all the Ca could
be reacted off at 650 K, and the surface temperature had to be
increased to greater than 1000 K to remove totally the Ca by
oxidation.
(
0.03 ML), the CO2 and CO peak shapes are identical to those
11
reported by Walker and King, whose experiments were
performed with a Ca coverage of 0.01 ML. There is a sharp
initial rise in the CO pressure followed by a slow rise to the
The temporal profiles of the CO2 production are presented
in Figure 2. The peak maximum for each Ca precoverage occurs
at a later time than the corresponding CO peak maximum. The

3418 J. Phys. Chem. B, Vol. 106, No. 13, 2002
Watson et al.
not equilibrated with the surface before desorption, at a surface
temperature of 650 K. On the other hand, CO produced by the
Langmuir-Hinshelwood reaction of Ca with Oa has excess
-1
energy of 20 kJ mol and is equilibrated with the surface before
desorption or reaction with Oa to produce CO2, which explains
the delayed maximum in CO2 production.
The temporal profiles obtained at the lowest carbon coverages
(
Figures 1 and 2) are identical to those obtained by Walker and
1
1
King. However, to determine whether our experimental data
obtained at higher Ca precoverages are consistent with their
mechanism, we consider the following set of equations which
describe the processes taking place at the surface in more detail:
2
Figure 4. Initial reactive sticking probability of an O beam of 0.014
-
1
ML s as a function of C
a
coverage relative to saturation at a surface
RF02
ka
8 2Oa
2
O (g) y z O2c
9
(1)
temperature of 650 K. The solid line is s
0
(1- θ
C
/θsat) .
2
kd
-
1
O2 flux (0.014 ML s ) used in these experiments was
kCO
*
11
C + O
8 CO * + Oa
(2)
(3)
(4)
(5)
(6)
considerably higher than that used by Walker and King, which
results in measurable CO2 production as soon as the KW flag
is raised. The time-dependent reactive O2 sticking probability
is shown in Figure 3. From run to run, the initial O2 sticking
probability decreases with increasing Ca precoverage while the
time-dependent sticking probability increases to a maximum as
the Ca is removed during the reaction and then decreases to
about 0.03, for all Ca precoverages. The decrease is attributed
to an increasing surface coverage of Oa. The maximum in the
O2 reactive sticking probability coincides with the maximum
in the CO2 production rate for a given Ca precoverage. The
magnitude of the maximum reactive sticking probability is also
interesting. For the lowest two Ca precoverages, the maximum
is ∼0.225, which is the sticking probability on the clean surface
a
2c
g
kCOg*
8 CO + Oa
C + O
a
2c
a
kCO
a
C + Oa
8 COa
kdes
COa 8 COg
k
CO2
CO + Oa
8 CO (g)
2
a
Here, R and FO are the O2 trapping probability and O2 flux,
2
1
2
at 650 K. This implies that the total surface coverage (Ca,
COa, and Oa) is very low at this stage. For the two higher Ca
precoverages, the maximum sticking probability is reduced to
respectively, and COg* is the translationally hot gas-phase
product of the Ca + O2c reaction. The first step (eq 1) involves
the dissociation of O2 via chemisorbed molecular dioxygen, O2c,
which can react with Ca to produce CO which impulsively
desorbs (eq 2) or at surface temperatures lower than 650 K can
be trapped and equilibrated to produce COa (eq 3). Any O2c
which dissociates before reacting or desorbing forms Oa which
reacts with Ca to produce COa (eq 4). The COa can then either
desorb or be oxidized to CO2 via two competing pathways (eqs
0
.19 and 0.175, respectively. When oxidation is complete at
these precoverages, there is still some unreactive graphitic
carbon left. This acts as a site blocker and explains the reduced
maximum sticking probability.
Figure 4 shows the initial reactive sticking probability as a
function of the Ca precoverage. The sticking probability
2
decreases according to s0(1 - θC/θsat) , where s0 is the initial
5
and 6).
sticking probability on the clean surface at 650 K and θC/θsat is
the carbon coverage relative to the saturation Ca coverage at a
surface temperature of 650 K.
The above equations can be used to obtain the following
expression for the rate of change of θO2c, θOa and θCa,
respectively:
The lowering of the initial O2 sticking probability with
increasing Ca precoverage shows that Ca acts as a site blocker
to O2 adsorption and that O2 must first chemisorb, either
molecularly or dissociatively, before reaction with Ca can take
place. If the reaction occurred via an Eley-Rideal mechanism
involving a collision of a gaseous O2 with Ca, the initial rate
would increase as the Ca precoverage increases; in fact the
opposite is true. Walker and King21 have shown that on Pt{110}-
dθO2c
) R(θ , θ )F - kdθO2c - kaθO2c - kCO*θO2cθCa
Ca
O
O2
a
dt
(
7)
dθOa
dt
)
2kaθO2c + kCO*θO2cθCa - k θ θ - kCO2θCOaθOa
CO C
Oa
a
(
1×2) dissociative oxygen adsorption occurs via chemisorbed
(8)
molecular dioxygen, O2c. Therefore, the reaction to produce CO
may occur by reaction of Ca with either or both types of
chemisorbed oxygen.
dθCa
)
- kCO*θO2cθCa - k θ θ
CO Ca Oa
(9)
dt
Walker and King11 have shown that two processes are
operative during the oxidation of Ca at low coverages: in the
first, Ca reacts with the short-lived but highly mobile O2c to
produce CO which at 650 K impulsively desorbs in a sharp
lobe at an angle of 32° to the surface normal (the initial sharp
rise); The second is a Langmuir-Hinshelwood (LH) reaction
between Ca and Oa (the slower rise to a maximum). They found
that the CO produced via the reaction of Ca with O2c has excess
The trapping probability of O2 into the molecular chemisorbed
state is a function of both θC and θO and is expressed as R-
a
a
(θC ,θO ).
a
a
Chemisorbed molecular oxygen, O2c, desorbs and dissociates
at 160 K. The lifetime of O2c on the surface is given by τ )
τ0exp(Ed/RTs) where Ed, the desorption energy, is ∼60 kJ
-
1 21
mol . Assuming that 1/τ0 is the vibrational frequency of the
-
1
-124
translational energy greater than 135 kJ mol , and is therefore
particle-surface bond (about 860 cm
), the lifetime of O2c is

Roles of Chemisorbed Atomic Oxygen and Dioxygen
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3419
about 2 ns at 650 K. Therefore, at any time significantly longer
than this dθO /dt ) 0 and θO is given by
2c
2c
R(θ ,θ )F
O2
C
O
a
a
θO2c
)
(10)
k + k + kCO*θ
Ca
d
a
We have shown earlier that CO production is best described as
a combination of a fast process and a slower LH reaction
between Ca and Oa. Therefore, the total rate of gaseous CO
produced, rCOTOT is given by
rCOTOT ) rCO* + rCO
(11)
Figure 5. Surface O
with an O molecular beam at a surface temperature of 650 K. The C
precoverage was 0.18 ML, and the O
The inset shows the corresponding product and O
a a
and C coverages during the reaction of carbon
2
a
where rCO* and rCO are the rate of CO production by via the
impulsive and LH mechanisms, respectively. Since the pumping
speed is high, rCOTOT is proportional to CO pressure measured
by the fixed QMS, PCOTOT. The QMS response to different gases
has been calibrated and can therefore be directly related to the
production rate. The rate of gaseous CO formation via each
pathway is given by
-
1
2
beam flux was 0.014 ML s
uptake profiles.
.
2
rCO* ) kCO*θO2cθCa
(12)
(13)
rCO ) kCOθO θ - k θCOaθOa
C
CO2
a
a
This equation was obtained by assuming that as soon as CO is
produced by the LH mechanism it either desorbs immediately
or reacts with another Oa to produce CO2 (Ca oxidation is the
rate-determining step for gas-phase CO production).
Figure 6. Surface O
with an O molecular beam at a surface temperature of 650 K. The C
precoverage was 0.83 ML, and the O beam flux was 0.014 ML s .
a a
and C coverages during the reaction of carbon
2
a
1
Using mass balance, we determined θC and θO during the
reaction of carbon with an O2 molecular beam at Ca precover-
ages of 0.18 and 0.83 ML, as shown in Figures 5 and 6,
respectively. First, for the low Ca precoverage experiment, the
O2c coverage, which is given by eq 10, initially rises to the
steady-state value (less than 0.1 s), stays almost constant, and
then decreases as the Oa coverage builds up. Since the
production of impulsive CO is proportional to the product of
the O2c and the Ca coverage, the production rate very rapidly
reaches the maximum at t = 0 and decreases at a similar rate
to that of the decrease in carbon coverage. On the other hand,
the Oa coverage initially rises slowly, with a sharper rise after
-
2
less than 1 µs). Therefore, CO is produced exclusively via the
impulsive mechanism, and the initial sharp rise in the CO
pressure, PCO, is given by
rCO ) cPCO ) kCO*θO2cθCa
(14)
where c is a constant related to the pumping speed of the system.
The measured initial reactive sticking probability, s , multiplied
r
by the O flux is equal to the rate at which O is consumed
2
2c
2
3
2 s. This is consistent with the maximum rate of the
Langmuir-Hinshelwood Ca + Oa reaction occurring after about
0 s, as observed (see inset Figure 5). Because the surface
temperature is much higher than the desorption temperature of
which is approximately equal to s (1 - θ /θ ) , as shown in
Figure 4.
0
C
sat
3
*
2
O2
srF ) kaθ
O2c
+ kCO θO2cθ = s (1 - θ ) F
O2
Ca
0
Ca
(15)
2
5
CO, 525 K, CO desorbs very rapidly once it has formed. In
order for CO2 to form from COa and Oa, there must be a large
surface oxygen coverage. Thus, the rate of CO2 formation only
becomes significant once the coverages of Oa increase. The
coverage curves are therefore consistent with the idea that we
have two competing mechanisms taking place.
Substituting this into eq 7 and applying the steady-state
approximation gives
kd(θO2C) ) R(θ )F ) srF O2
(16)
C
O2
a
The behavior is quite different for the high coverage experi-
ment, as shown in Figure 6. Again the maximum in the
impulsive CO formation rate occurs at t ) 0. The initial rate is
significantly less due to the reduced steady-state O2c coverage
Since the θC dependence of R and sr are the same and the
trapping probability is related to the sticking probability
according to R ) s(1 + kd/ka), the above expression rearranges
to give the following expression for θO2c:
(Ca acts as a site blocker). The Oa coverage, however, rises
slowly to a maximum with a curvature typical of a clean-off
reaction and shows that the reaction with atomic oxygen
dominates when high Ca precoverages are used.
srF
O2
θO2c
)
(17)
ka
4
. Variation of k*CO with Ca Coverage
At the beginning of the reaction (as t f 0s), θO is very small,
whereas θO2c has already reached its steady-state value (after
Inserting this into eq 14 and rearranging gives the final
expression for the coverage dependence of kCO*, in terms of
a

3420 J. Phys. Chem. B, Vol. 106, No. 13, 2002
Watson et al.
Figure 7. Plot of PCO/(θ
C
s
r
F
O2) versus θ
C
for the reaction of C
a
with
ref
O
2c at a surface temperature of 650 K.
Figure 8. Experimental (solid lines) and modeled (dashed lines)
product profiles for the isothermal reaction of C
beam, at a surface temperature of 650 K. The products are CO (major
product) and CO
a 2
with an O molecular
TABLE 1: Parameters Used in the Model
rate (s-1
(kJ mol-1
ν (s-1
24
2
.
reaction
flux
+ O
)
E
a
)
1
)
O
2
F
0.014 ML s-
0
58.6
158.8
calcd.
we use the parameter obtained by Walker and King11 for low
precoverages on the same surface. All of the parameters used
in the model are shown in Table 1.
It should be noted that in this Langmuir-Hinshelwood model
the Oa is thermally equilibrated with the surface before reaction
with the Ca adlayer takes place. It is a mean field model and so
no local information about the structural dependence of the
reaction can be included.
C
a
a
f CO
a
k
k
k
CO
CO2
des
a
b
c
9
CO
CO
a
+ O
a
f CO
2
(g)
2 × 10
1
6
a
f CO(g)
2 × 10
a
11 b
From previous works.14,26 c From previous
From previous work.
works.2
5,27,28
PCO, sr, and θCa all of which can be determined experimentally:
Four coupled differential equations are used to describe the
variation of the number of free sites and adsorbate coverages:
kaP
CO
*
kCO
)
(18)
θ s F
r O2
C
a
dθO
2
)
2sF (θ*) - k θ θ - kCO2θCOθO
(23)
(24)
(25)
A plot of PCO/(θCsrFO ) versus θC is shown in Figure 7. This
O
CO C O
2
2
dt
gives kCO*/ka as a function of θC . Assuming that ka is
a
*
independent of coverage, this implies that kCO decreases sharply
dθC
dt
to a coverage of 0.2 ML and remains essentially constant above
about 0.4 ML. The most likely reason for this is the Ca-induced
) -kCOθCθO
5
-8
(
1×2) f (1×1) transformation. At the lowest coverage, the
dθCO
dt
structure is entirely (1×2). However, as the coverage increases,
Ca stabilizes the (1×1) structure which may result in an
increased activation energy. In fact, the activation energy only
needs to increase by ∼3.5 kJ mol to explain the reduced rate
) kCOθ θ - kCO2θCOθ - k θ
C O O d CO
dθ*
dt
-
1
) -2sF * + kCOθCθO + k θ + 2kCO2θCOθO (26)
O2θ
d CO
constant.
4
.1. Modeling the Ca + Oa Reaction. To simulate the experi-
where θx and θ* are, respectively, the concentration of species
x relative to the carbon saturation coverage and the fraction of
sites available for adsorption. The system of coupled differential
equations was integrated numerically using routines in the NAG
Fortran Library. This program returned time-dependent cover-
ages which were substituted into the following equation to give
the time-dependent product profiles for PCO, PCO2 and s:
mental observations at high Ca precoverages, the following
elementary reaction steps were assumed:
Fsr
O (g) + 2* 82Oa
(19)
(20)
(21)
(22)
2
kCO
C + Oa 8 CO + /
a
a
rCO ) cPCO ) kdθCO
(27)
(28)
kdes
COa
8 CO + /
g
rCO2 ) cPCO2 ) kCO2θCOθO
kCO2
2
CO + Oa
8 CO (g) + 2/
s ) s (θ*)
(29)
a
2
0
Equation 19 represents O2 dissociative adsorption on the
Pt{110}-Ca surface. For simplicity, it is assumed that each
adsorbate blocks one free site (represented by /) and that the
total number of free sites θ* is equal to (1 - θCOa - θCa -
θCgra - θO ) where θC is unreactive carbon and that each O2
The experimentally determined and corresponding modeled
-
1
profiles for the reaction of 0.83 ML Ca with a 0.014 ML s O2
molecular beam at a surface temperature of 650 K can be seen
in Figure 8. We get good qualitative agreement, in that the CO
pressure rises slowly to a maximum as the reaction proceeds
and then decreases sharply near the end of the reaction and the
CO2 peak maximum occurs after that of CO. There are, however,
some subtle differences between the experimental and modeled
profiles. The initial rate of CO production is significantly larger
in the model. The initial CO production rate can be decreased
to match the initial rates by making kCO smaller. However, this
a
gra
requires two free sites to dissociate. O2 desorption is not included
because at a surface temperature of 650 K it does not occur to
a significant degree. Since the carbon induced (1×2) f (1×1)
structural transformation is not well understood we use the
parameters reported in the literature for COa oxidation (eq
1
4,26
25,27,28
2
2)
and desorption (eq 21).
For the Ca + Oa reaction

Roles of Chemisorbed Atomic Oxygen and Dioxygen
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3421
results in the modeled peak maximum occurring at a much later
time than that determined experimentally. In this model, we
assume however, that kCO is independent of Ca coverage, Ca
does not form islands, and the reaction proceeds entirely via a
Langmuir-Hinshelwood reaction between Ca and Oa. A more
sophisticated model which incorporated all of these effects
should give a much better fit to the data. However, the relatively
good agreement obtained using such a simple model indicates
that the underlying mechanism is essentially correct.
Ca + Oa and CO2 is produced by the competing COa + Oa
reaction. The model gives a reasonable, qualitative fit to the
data, which supports the underlying mechanism.
Acknowledgment. We acknowledge an equipment grant
from the UK EPSRC, a TMR Studentship from the European
Community (D.T.P.W.), and EPSRC/Cambridge Common-
wealth Trust support (J.J.W.H.).
References and Notes
(
1) Goodman, D. W.; Kelley, R. D.; Madey, T. E.; White, J. M. J.
5
. Summary and Conclusion
Catal. 1980, 64, 479.
The isothermal reaction of Ca with an O2 molecular beam
(2) Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1980, 91, 499.
(
(
3) Grossmann, A.; Erley, W.; Ibach, H. Surf. Sci. 1995, 337, 183.
4) Zi-pu, H.; Ogletree, D. F.; Van Hove, M. A.; Somorjai, G. Surf.
was studied on Pt{110} at surface temperature of 650 K and
over a Ca coverage range of 0.03-0.8 ML. For all coverages,
CO and CO2 were produced in the ratio of about 9:1. There is
an initial sharp rise in the CO pressure which is attributed to
the reaction of Ca with adsorbed molecular oxygen, O2c,
followed by a slow rise to a maximum which is due to the
reaction of Ca with adsorbed atomic oxygen, Oa. This is the
first time that molecular chemsiorbed oxygen has been impli-
cated in the oxidation of adsorbed carbon and, by implication,
in the methane partial oxidation reaction. The height of the initial
sharp rise in CO pressure decreases as the Ca precoverage
increases while the time taken to reach the maximum also
increases. The CO peak maximum occurs earlier than that of
CO2 for all Ca coverages. This is because the CO2 formation
rate is maximized only when the surface atomic oxygen
coverage is large.
Sci. 1987, 180, 433.
(5) Shaikhutdinov, S. K.; Boronin, A. I.; Kvon, R. I. Surf. Sci. 1997,
3
82, 187-192.
(6) Kvon, R. I.; Boronin, A. I.; Shaikhutdinov, S. K.; Buyanov, R. A.
Appl. Surf. Sci. 1997, 120, 239-242.
(7) Kvon, R. I.; Boronin, A. I. Catal. Today 1998, 42, 353-355.
(8) Kvon, R. I.; Koscheev, S. V.; Boronin, A. I. J. Mol. Catal. A. 2000,
1
58, 297-300.
(9) Petersen, M.; Jenkins, S.; King, D. A. Manuscript in preparation.
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(11) Walker, A. V.; King, D. A. J. Chem. Phys. 2000, 112, 1937-1945.
(
12) Mikhailov, S. N.; van den Oetelaar, L. C. A.; Brongersma, H. H.;
van Santen, R. A. Catal. Lett. 1994, 27, 79.
13) Kislyuk, M. U.; Migulin, V. V.; Savkin, V. V.; Vlasenko, A. G.;
Sklyarov, A. V.; Tretyakov, I. I. Kinet. Katal 1990, 31, 1219-1223.
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980, 73, 5862.
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Phys. 1977, 67, 3394.
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18) Ohno, Y.; Matsushima, T. Surf. Sci. 1995, 341, 172.
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Phys. 1993, 99, 1.
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6879.
(22) Watson, D. T. P.; King, D. A., unpublished results..
23) Jackman, T. E.; Davies, J. A.; Jackson, D. P.; Unertl, W. N.; Norton,
(
(
1
(
The time-dependent surface coverages during the isothermal
oxidation at 650 K were obtained using mass balance. By use
of the initial O2 reactive sticking probability, the initial sharp
rise in CO pressure and the Ca coverage, the rate constant was
obtained for the Ca+O2c reaction as a function of the Ca
precoverage. It decreases by a factor of 6 up to a Ca precover-
age of 0.2 ML and then stays almost constant; the change
(
5
(
(
(
-
1
corresponds to an increase of ∼3.5 kJ mol in the activation
energy. This change in the activation energy is attributed to a
lifting of the (1×2) reconstruction of the Pt{110} surface to
Pt{110}-(1×1) by Ca.
(
P. R. Surf. Sci. 1982, 120, 389.
(24) Schmidt, J.; Stuhlmann, C.; Ibach, H. Surf. Sci. 1993, 284, 121.
(25) Hofmann, P.; Bare, S. R.; King, D. A. Surf. Sci. 1982, 117, 245.
26) Hopkinson, A.; King, D. A. Chem. Phys. 1993, 177, 433.
27) von Oertzen, A.; Mikhailov, A.; Rotemund, H. H.; Ertl, G. Surf.
Sci. 1996, 350, 259.
(28) McCabe, R. W.; Schmidt, L. D. Surf. Sci. 1976, 60, 85.
(
(
The profiles for the highest Ca precoverage are quite
successfully modeled using a simple Langmuir-Hinshelwood
reaction mechanism, where CO is produced by the reaction of