Kinetic study of the reaction of Ir(a4F9/2) with CH4, O2, and N2O

Article abstract of DOI:10.1021/jp9722723

The gas-phase reactivity of ground-state Ir(a⁴F_9/2) with CH₄, O₂, and N₂O is reported. Iridium atoms were produced by the photodissociation of [Ir(CO)₂(acac)] and detected by laser-induced fluorescence. The reaction rate of the a⁴F_9/2 state with CH₄ is very slow and temperature-dependent. The methane reaction is pressure-independent indicating a bimolecular reaction. The bimolecular rate constant from 398 to 498 K is described in Arrhenius form by (7±5)×10^-11 exp(-37±3 kJ/mol/RT) where the uncertainties represent ±2σ. The reaction rates of the a⁴F_9/2 state with O₂ and N₂O are pressure-dependent, indicating adduct formation. The limiting low-pressure third-order, k₀, and limiting high-pressure second-order, k_∞, room-temperature rate constants with O₂ in nitrogen buffer are (4.8±1.6)×10^-30 cm⁶ s^-1 and (3.6±0.4)×10^-12 cm³ s^-1, respectively. For N₂O, k₀ and k_∞ are (2.2±0.5)×10^-33 cm⁶ s^-1 and (5.9±0.8)×10^-15 cm³ s^-1, respectively. A lower limit for the activation energy for the abstraction of an oxygen atom from N₂O to produce IrO is estimated at 45 kJ/mol.

Full text of DOI:10.1021/jp9722723

J. Phys. Chem. A 1997, 101, 9377-9381
9377
Kinetic Study of the Reaction of Ir(a4F9/2) with CH4, O2, and N2O
Mark L. Campbell†
Chemistry Department, United States NaVal Academy, Annapolis, Maryland 21402
ReceiVed: July 14, 1997; In Final Form: September 30, 1997X
The gas-phase reactivity of ground-state Ir(a4F9/2) with CH4, O2, and N2O is reported. Iridium atoms were
produced by the photodissociation of [Ir(CO)2(acac)] and detected by laser-induced fluorescence. The reaction
rate of the a4F9/2 state with CH4 is very slow and temperature-dependent. The methane reaction is pressure-
independent indicating a bimolecular reaction. The bimolecular rate constant from 398 to 498 K is described
in Arrhenius form by (7 ( 5) × 10-11 exp(-37 ( 3 kJ/mol/RT) where the uncertainties represent (2σ. The
reaction rates of the a4F9/2 state with O2 and N2O are pressure-dependent, indicating adduct formation. The
limiting low-pressure third-order, k0, and limiting high-pressure second-order, k∞, room-temperature rate
constants with O2 in nitrogen buffer are (4.8 ( 1.6) × 10-30 cm6 s-1 and (3.6 ( 0.4) × 10-12 cm3 s-1,
respectively. For N2O, k0 and k∞ are (2.2 ( 0.5) × 10-33 cm6 s-1 and (5.9 ( 0.8) × 10-15 cm3 s-1, respectively.
A lower limit for the activation energy for the abstraction of an oxygen atom from N2O to produce IrO is
estimated at 45 kJ/mol.
Introduction
Experimental Section
Gas-phase transition-metal (TM) chemistry is an intriguing
field of study due to the high multiplicities of the atomic ground
states and the large number of low-lying metastable states. The
early gas-phase work of TMs focused on the reactions of the
cations due to the ability of mass spectrometric methods to
selectively control the kinetic energy and electronic state of the
reactant ion and to sensitively detect the products. An
understanding of how the low-lying M+ electronic states govern
chemical reactivity is now fairly well established.1-5
Pseudo-first-order kinetic experiments ([Ir] , [oxidant]) were
carried out in an apparatus with slowly flowing gas using a
laser photolysis/laser-induced fluorescence (LIF) technique. The
experimental apparatus and technique have been described in
detail elsewhere.20 Briefly, the reaction chamber is a stainless
steel reducing four-way cross with attached sidearms and a
sapphire window for optical viewing. The reaction chamber is
enclosed within a convection oven (Blue M, Model 206F) for
temperature dependence experiments.
Only recently have the reactions of the gas-phase neutral TM
atoms received considerable attention. The most complete
studies have been performed on the reactions with small alkenes
and alkanes.6-19 The majority of atoms in the 3d, 4d, and 5d
series have now been studied and indicate the ground and low-
lying states of the TM dramatically influence the reaction
kinetics.12
The kinetics of the reactions of TM atoms with oxygen-
containing oxidants has recently been an active field of
study.20-29 The cumulative data reported thus far indicate the
electronic state is a very important factor in the dynamics of
these reactions. For example, for both the abstraction and
termolecular association channels with O2, TMs with s1dn-1
configurations have been found to be more reactive than their
s2dn-2 counterparts.23-27
In this paper we report a kinetic study of the 6s25d7 a4F9/2
state of iridium with methane, oxygen, and nitrous oxide. The
reporting of these reactions for iridium will further update the
database of TM reactions and allow comparison of iridium’s
kinetic behavior with other TMs. Two surprising results will
be reported for iridium. In contrast to the 3d metals with s2dn-2
configurations, iridium is unusually reactive with O2 in the
termolecular association reaction. Iridium is also the first
reported TM to react with N2O through a termolecular mech-
anism. This study also reports on the temperature dependence
of the reaction of iridium with methane which will allow for a
comparison of the experimentally determined activation energy
with theoretically calculated barriers reported previously.11
Iridium atoms were produced by the 248 nm photodissociation
of dicarbonyl(acetylacetenato)iridium(I) [Ir(CO)2(acac)] using
the output of an excimer laser (Lambda Physics Lextra 200).
Iridium atoms were detected via LIF using an excimer-pumped
dye laser (Lambda Physics Lextra 50/ScanMate 2E) with a KDP
doubling crystal tuned to the z6G11/2° r a4F9/2 transition at
292.479 nm.30 The reported lifetime of the z6G11/2° state is 69
ns.31 LIF signals from this state were strong even at high total
pressures, indicating collisional quenching of this state was not
a problem. The dye laser beam (diameter ∼ 4 mm) was
centered within the cross-sectional area swept by the counter-
propagated photolysis beam (diameter ∼ 12.5 mm), and the
fluorescence was detected at 90° to the laser beams with a three-
lens telescope imaged through an iris. A photomultiplier tube
(Hamamatsu R375) was used in collecting the LIF which was
subsequently sent to a gated boxcar sampling module (Stanford
Research Systems SR250), and the digitized output was stored
and analyzed by a computer.
The iridium precursor was entrained in a flow of nitrogen
buffer gas. The precursor carrier gas, buffer gas, and reactant
gases flowed through calibrated mass flow meters and flow
controllers prior to admission to the reaction chamber. Each
sidearm window was purged with a slow flow of buffer gas to
prevent deposition of iridium and other photoproducts. Total
flows were between 500 and 3800 sccm. Pressures were
measured with MKS Baratron manometers, and chamber
temperatures were measured with a thermocouple. The delay
time between the photolysis pulse and the dye-laser pulse was
varied by a digital delay generator (Stanford Research Systems
DG535) controlled by a computer. The trigger source for these
experiments was scattered pump laser light incident upon a fast
† Henry Dreyfus Teacher-Scholar.
X Abstract published in AdVance ACS Abstracts, November 1, 1997.
S1089-5639(97)02272-X This article not subject to U.S. Copyright. Published 1997 by the American Chemical Society

9378 J. Phys. Chem. A, Vol. 101, No. 49, 1997
Campbell
Figure 2. Typical Ir(a4F9/2) decay curves with added O2 or N2O. T )
296 K. (a) P(O2) ) 0.102 Torr, Ptotal ) 10 Torr, P(CH4) ) 4.0 Torr, τ
) 200 µs; (b) P(N2O) ) 11.5 Torr, Ptotal ) 200 Torr, P(CH4) ) 4.2
Torr, τ ) 383 µs. The solid lines through the data are exponential fits.
The inset is a ln plot of the data.
Figure 1. Typical Ir(a4F9/2) decay curves with added CH4. T ) 473
K, Ptotal ) 50.0 Torr, P(SF6) ) 15 Torr. (a) P(CH4) ) 15.6 Torr, τ )
450 µs; (b) P(CH4) ) 5.30 Torr, τ ) 1060 µs. The solid lines through
the data are exponential fits. The inset is a ln plot of the data.
photodiode. LIF decay traces consisted of 200 points, each point
averaged for four laser shots.
Materials. Dicarbonyl(acetylacetonato)iridium(I) (Strem,
99%), O2 (MG Industries, 99.8%), N2O (MG Industries,
electronic grade, 99.999%), CH4 (Linde, ultrahigh purity grade,
99.99%), SF6 (Air Products, 99.9%), and N2 (Potomac Airgas,
Inc., 99.998%) were used as received.
Data Analysis and Results
The decay rates of the ground state (a4F9/2) of iridium as a
function of reactant pressure were investigated as a function of
temperature and total pressure. In the absence of a quencher
gas, the decay profiles showed growth (i.e., nonexponential
behavior) at the beginning of the decays. We attribute this
behavior to the relaxation of electronically excited states to the
ground state. Two polyatomic gases, CH4 and SF6, were found
to efficiently relax the excited electronic states at room
temperature without reacting with the ground state. Methane,
however, could not be used at elevated temperatures due to
chemical reaction. Methane appeared to be the better quencher
at room temperature based on the appearance of the iridium
temporal profiles in the absence of reactant. Thus, for the study
of the O2 and N2O reactions at room temperature, approximately
4 Torr of methane was added as a quencher gas. Variations in
the methane pressure from 4 to 19 Torr gave rate constants equal
to within experimental uncertainty. Thus, methane pressures
of 4 Torr or greater appear to sufficiently relax the excited states.
Rate constants measured at elevated temperatures used SF6 as
the quencher gas.
Figure 3. Typical plots for determining k2nd for Ir(a4F9/2) + CH4
illustrating the large dependence of the bimolecular rate constants on
temperature. The solid line for each set of data is a linear regression
fit from which k2nd is obtained.
determined by adjusting the range of the linear regression
analysis; i.e., the range did not include the beginning of the
decay when growth was present. Decays were typically
analyzed after a delay of up to one reaction lifetime and included
data for a length of two to three reaction lifetimes. Lifetimes
of Ir(a4F9/2) without oxidant were typically 2000 µs or greater,
indicating diffusion out of the detection zone is inconsequential
in these experiments.
Once the excited states have relaxed to the ground state, the
loss of ground-state iridium is described by the first-order decay
constant, k1:
The second-order rate constant is determined from a plot of
1/τ vs reactant number density. Typical plots for obtaining
second-order rate constants are presented in Figures 3 and 4;
the slope yields the observed rate constant. The relative
uncertainty (i.e., the reproducibility) of the second-order rate
constants is estimated at (20% based on repeated measurements
of rate constants under identical temperature and total pressure
conditions. The absolute uncertainties are conservatively
estimated to be (40% and are based on the sum of the statistical
scatter in the data, uncertainty in the flowmeter and flow
k1 ) 1/τ ) kd + k2[oxid]
(1)
where τ is the first-order time constant for the removal of iridium
under the given experimental conditions, kd is the loss term due
to diffusion out of the detection zone and reaction with the
precursor and precursor fragments, and k2 is the second-order
rate constant. Typical decay profiles are shown in Figures 1
and 2. A time constant, τ, for each decay profile was determined
using a linear least-squares procedure. Time constants were

Reaction of Ir with CH4, O2, and N2O
J. Phys. Chem. A, Vol. 101, No. 49, 1997 9379
Figure 4. Typical plots for determining k2nd for Ir(a4F9/2) + O2,
indicating the dependence of the second-order rate constants on total
pressure. The solid line for each set of data is a linear regression fit
from which k2nd is obtained.
Figure 5. Arrhenius plot for the collisional disappearance of Ir(a4F9/2
with CH4. Error bars represent (40% uncertainty.
)
TABLE 1: Second-Order Rate Constants (k2nd, 10-15 cm3
s-1) for Ir(a4F9/2) + CH4 at 50 Torr
temp (K)
k2nd
temp (K)
k2nd
398
423
448
1.0
2.2
3.7
473
498
5.7
9.9
controller readings (5%) and the total pressure reading (1%),
uncertainties due to incomplete gas mixing, and uncertainties
due to incomplete relaxation of the excited electronic states to
the ground state.
Measured rate constants for the a4F9/2 state reacting with CH4
at 50 Torr total pressure from 398 to 498 K are listed in Table
1. Rate constants could not be measured above 498 K due to
thermal decomposition of the precursor. Below 398 K, the rate
constants are too small to be measured reliably using our
methods; i.e., under the experimental conditions employed in
this study, the sensitivity (lower limit) of our apparatus is 1 ×
10-15 cm3 s-1. The rate constants at 498 K were measured at
10, 20, and 50 Torr and are independent of total pressure within
experimental uncertainty. An Arrhenius plot of the rate
constants is shown in Figure 5. The rate constants are described
by
Figure 6. Pressure dependence of the reaction of Ir(a4F9/2) with O2
and N2O in N2 buffer at room temperature. Error bars represent (40%
uncertainty. The solid lines are fits to eq 3.
TABLE 2: Second-Order Rate Constants (k2nd, 10-12 cm3
s-1) for Ir(a4F9/2) + O2 in N2 Buffer at 296 K
total press. (Torr)
k2nd
total press. (Torr)
k2nd
5
7.5
10
20
50
0.70
1.0
1.3
1.7
2.2
75
100
150
200
2.5
2.7
3.1
3.6
k(T) ) (7 ( 5) × 10-11 exp(-37 ( 3 kJ/mol/RT) (2)
where the uncertainties are (2σ.
Measured rate constants for the a4F9/2 state reacting with O2
and N2O in nitrogen buffer at various pressures are listed in
Tables 2 and 3. The rate constants are dependent on total
pressure, indicating termolecular processes. The addition of
methane as a quencher gas raises the possibility that the
quencher gas may affect the rate constants due to the different
efficiency of the stabilization of the adduct by each gas.
However, rate constants measured with varying amounts of
methane (from 4 to 19 Torr at Ptot ) 20 Torr) gave rate constants
equal to within experimental uncertainty. Thus, CH4 and N2
appear to stabilize the adduct with approximately equal ef-
ficiency. Rate constants for these reactions were essentially
temperature invariant up to 498 K, indicating, at most, small
barriers in these processes. The variation with total pressure
of the second-order rate constants at room temperature in N2
buffer are shown in Figure 6. The solid lines through the data
in Figure 6 are weighted fits to the simplified Lindemann-
TABLE 3: Second-Order Rate Constants (k2nd, 10-15 cm3
s-1) for Ir(a4F9/2) + N2O in N2 Buffer at 296 K
total press. (Torr)
k2nd
total press. (Torr)
k2nd
20
50
100
1.2
2.2
3.3
150
200
3.6
4.3
Hinshelwood expression32
k0[M]
1 + k0[M]/k∞
k )
(3)
where k0 is the limiting low-pressure third-order rate constant,
k∞ is the limiting high-pressure second-order rate constant, and
[M] is the buffer gas number density. The values for k0 and k∞
for the reaction of O2 in N2 buffer at room temperature are (4.8
( 1.6) × 10-30 cm6 s-1 and (3.6 ( 0.4) × 10-12 cm3 s-1
,

9380 J. Phys. Chem. A, Vol. 101, No. 49, 1997
Campbell
respectively. The values for k0 and k∞ for the reaction of N2O
in N2 buffer at room temperature are (2.2 ( 0.5) × 10-33 cm6
s-1 and (5.9 ( 0.8) × 10-15 cm3 s-1, respectively. The listed
uncertainties are (2σ.
abstraction mechanism to produce IrO is endothermic by 87
kJ/mol;33,35 thus, the thermodynamic barrier prohibits this
pathway at the temperatures studied here.
The value of the limiting low-pressure third-order rate
constant is large compared to other termolecular reactions with
O2 involving s2dn-2 transition metals. A comparison of the low-
pressure third-order rate constant with that of another group 9
atom, 4s23d7 a4F9/2 Co, indicates the rate constant for iridium
is approximately 400 times greater than the rate constant for
cobalt.27 In fact, the rate constants for iridium are similar in
magnitude to TMs with s1dn-1 configurations. Previous studies
of the 3d transition-metal association reactions with O2 indicate
that TMs with ground-state or low-lying s1dn-1 configurations
are reactive whereas TMs with s2dn-2 ground-state configura-
tions are unreactive.27 This behavior has been explained in
terms of simple molecular orbital concepts. In the approach of
the TM to O2, the nature of the potential in the region of the
onset of orbital overlap will be determined by the interaction
between the s-orbital of the TM atom (which has a larger spatial
extent than the d-orbitals) and the in-plane π*-antibonding
orbital of O2. For a TM atom with an s1 configuration, an
electron pair bond may be formed from the overlap of these
orbitals, as in a radical-radical recombination process. In the
case of an s2 configuration, one of the s-electrons occupies the
antibonding molecular orbital between the TM atom and O2,
which destabilizes the complex and leads to a less attractive or
a repulsive potential.27 Iridium’s relatively large reactivity may
be related to the low-lying 6s15d8 b4F9/2 state only 2835 cm-1
above the ground state.36 It is conceivable that the surface
hopping from the ground state to the s1d8 excited surface is
reasonably efficient, resulting in the unusually fast reaction
compared to other TMs with s2 configurations.
Discussion
Ir + CH4. The two most plausible reaction pathways for
the reaction of iridium with methane are the termolecular
insertion channel to produce H-Ir-CH3 and the bimolecular
H2 elimination reaction to produce IrdCH2. Assuming the
reaction has not reached its high-pressure limit at 10 Torr, the
experimental results reported here are consistent with a bimo-
lecular mechanism. Thus, the most likely reaction channel is
the H2 elimination reaction.
Of the three reactions reported here, extensive calculations
have only been performed for the reaction of iridium with
methane. A condensation of the primary findings reported by
Carroll et al follows.11 PCI-80 calculations for iridium reacting
with methane indicate the 6s25d7 electronic state is an important
factor in the dynamics of this reaction. The presence of the
two s electrons makes this state highly repulsive toward
methane, and no energy minimum is found along the potential
energy surface for this state. The attractive surface that
correlates with the strongly bound H-Ir-CH3 insertion product
arises from the low-spin a2P3/2 asymptote which lies 126 kJ/
mol above the ground-state reactants. The quartet and doublet
molecular iridium adducts crossing occurs at an Ir-C distance
of 2.2 Å and is 75 kJ/mol above the ground-state asymptote.
Once the reactants reach the low-spin surface, there is probably
no barrier to formation of the C-H insertion product. The C-H
insertion product is calculated to be 52.7 kJ/mol bound relative
to the ground-state reactants. Elimination of H2 from the
insertion product is calculated to be 66.5 kJ/mol higher in energy
than the reactants. Thus, calculations predict a very slow
reaction at room temperature which was consistent with the
observance of no reaction reported previously.11
Since the crossing for the ground-state surface at 75 kJ/mol
is higher than the endoergicity of the H2 elimination reaction,
reactants that overcome this barrier have the required energy
to go on to eliminate H2. Thus, the calculations indicate once
the energy barrier has been surmounted along the ground-state
surface, reaction should proceed to H2 elimination. The
calculations, however, predict a barrier approximately double
the value determined here experimentally. Thus, qualitatively
the calculations are consistent with experiment in predicting the
H2 elimination mechanism while the quantitative discrepancy
for the size of the barrier is larger than can be accounted for by
experimental error.
The value of k∞ for this reaction is about 1-2 orders of
magnitude smaller than values of k∞ for TMs with s1dn-1
configurations reacting with O2.23 One factor that causes a
depression of the value of k∞ for this reaction is the much smaller
electronic degeneracy of the activated complex compared to the
degeneracies of the reactants. According to transition-state
theory,37 k∞ for a recombination reaction is proportional to the
ratio of the partition functions of the activated complex to the
product of the partition functions of the reactants; i.e., k∞
†
Q(IrO2 )/Q(Ir)Q(O2). Assuming the electronic state of the
activated complex is the 2Σ state observed for the IrO2 insertion
product,34 the electronic contribution to the partition function
†
ratio is approximately 1/15 since Qelect(IrO2 ) ≈ 2, Qelect(Ir) ≈
10, and Qelect(O2) ≈ 3. Thus, the electronic contribution
represents a depression of about an order of magnitude in k∞.
The small value of k∞ may also be due to the less than unity
probability of the s2dn-2 to s1dn-1 surface hopping process.
Confirmation of the importance of the excited-state surface’s
contribution to this reaction awaits accurate calculations.
Ir + N2O. Results for the reaction of iridium with N2O
indicate a very weakly bound adduct:
The activation energy reported here for the reaction of iridium
with methane yields a lower limit to the IrdC bond energy in
the IrCH2 product. The formation of methylene and diatomic
hydrogen from methane in the absence of iridium is calculated
to have an endoergicity of 457 kJ/mol.33 Since the endoergicity
of the reaction of iridium with methane can be no larger than
its activation energy (37 kJ/mol), the lower limit to the IrdC
bond energy is thus determined to be 420 kJ/mol. The PCI-80
calculation of the IrdC bond energy (378 kJ/mol) is 10% smaller
than our experimentally determined lower limit.11
Ir(a4F9/2) + N2O [N2]8 IrON2
(5)
where it is not known to which atom(s) the iridium is bound or
the geometry of the adduct. While measurable, the rate
constants are extremely small (near the lower limit of the
apparatus), indicating a very inefficient process. No other
termolecular processes involving a TM with N2O have been
reported previously, although recent experiments in our labora-
tory indicate the reactions of platinum and nickel with N2O are
pressure-dependent. For both platinum and nickel, at a given
pressure, the rate constants of these metals are approximately 2
orders of magnitude larger than those of iridium, while a
Ir + O2. Results for the reaction of iridium with O2 indicate
a termolecular insertion reaction mechanism.
Ir(a4F9/2) + O2 [N2]8 IrO2
(4)
IrO2 has been observed previously from the reaction of laser-
vaporized iridium metal with O2.34 IrO2 was found to be linear
(D∞h) with a 2Σ electronic ground state. The bimolecular

Reaction of Ir with CH4, O2, and N2O
J. Phys. Chem. A, Vol. 101, No. 49, 1997 9381
bimolecular abstraction channel component is also indicated.38,39
The abstraction component is indicated for these metals by a
nonzero intercept in the pressure dependence of the rate
constants and a positive temperature dependence of the rate
constants. There are no such indications of a bimolecular
component for the reaction of N2O with iridium in the
experiments reported here.
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Abstraction reactions of all TM atoms with N2O are exo-
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molecules. Despite this exothermicity, metal atom reactions
with N2O have been observed to have significant energy barriers.
These barriers have been attributed to the requirement of a
nonadiabatic transition along the reaction pathway.40 For
iridium reacting with N2O to produce IrO, the abstraction
reaction is exothermic by 244 kJ/mol.33,35 Experimentally, we
were unable to detect a bimolecular component for this reaction
up to a temperature of 498 K. Assuming a maximum value of
1 × 10-15 cm3 s-1 for the rate constant for the abstraction
channel at 498 K and assuming a preexponential factor of 5 ×
10-11 cm3 s-1, an estimate of the minimum activation energy
for this reaction is calculated to be approximately 45 kJ/mol.
This minimum barrier is similar to the experimentally measured
activation energy for the abstraction reaction of 4s23d7 a4F9/2
Co with N2O (Ea ) 49 kJ/mol).39
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Recently, Fontijn and co-workers have advanced a resonance
interaction model41-44 to predict barriers to reaction and rate
constants for metal atoms reacting with N2O. In this model,
the activation barriers are calculated by taking into account the
ionization potential and sp promotion energy of the metal, the
electron affinity of N2O, and the bond energy of the metal oxide
product. The resonance interaction model predicts the energy
barrier for the reaction of iridium with N2O to be 41 kJ/mol,41
a value slightly smaller than our lower estimate.
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Summary
Sci. Technol. 1994, 101, 59.
We have measured the rate constants as a function of
temperature and pressure for the reaction of ground-state iridium
(a4F9/2) with CH4, O2, and N2O. The reaction of iridium with
CH4 is consistent with a bimolecular H2 elimination reaction.
The reaction is slow with an activation energy of 37 kJ/mol.
The reaction of ground-state iridium with O2 and N2O involves
termolecular processes. For O2, the reaction is rapid compared
to other termolecular reactions involving s2dn-2 transition metals.
The rapidity of the reaction is attributed to a surface crossing
with the low-lying 6s15d8 b4F9/2 electronic state. For N2O, the
termolecular reaction is very inefficient. There is no indication
of the abstraction reaction channel up to a temperature of 498
K, indicating an activation energy of at least 45 kJ/mol.
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Acknowledgment. This research was supported by a Cottrell
College Science Award of Research Corporation. Acknowledg-
ment is made to the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for partial
support of this research.
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