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We thank J. P. Eisenstein, L. Fu, T. Hsieh, P. A. Lee, A. de la Torre,
and L. Zhao for useful discussions. RA-SHG experiments were
supported by the U. S. Department of Energy under grant DE-
SC0010533. Instrumentation for the RA-SHG setup was partially
supported by a U.S. Army Research Office Defense University
Research Instrumentation Program award under grant W911NF-
13-1-0293 and the Alfred P. Sloan Foundation under grant FG-
BR2014-027. D. H. also acknowledges funding from the Institute
for Quantum Information and Matter, a NSF Physics Frontiers
Center (PH Y-1125565) with support of the Gordon and Betty
Moore Foundation through grant GBMF1250. J.-Q. Y. and D.G. M.
were supported by the U. S. Department of Energy, Office of
Science, Basic Energy Sciences, Materials Sciences and
Engineering Division. Z. Y. Z. acknowledges the Center for
Emergent Materials and NSF Materials Research Science and
Engineering Center under grant DMR-1420451. D.H. and
D. H. Torchinsky are inventors on U.S. patent application #14/
705,831 submitted by the California Institute of Technology,
which covers a spectrometer apparatus for the study of the
crystallographic and electronic symmetries of crystals and
methods of using said apparatus. The data that support the plots
within this paper and other findings of this study are available
from the corresponding author upon reasonable request.
Materials and Methods
Figs. S1 to S3
19 October 2016; accepted 20 March 2017
Low-temperature activation of
methane on the IrO2(110) surface
Zhu Liang,1 Tao Li,1 Minkyu Kim,2 Aravind Asthagiri,2 Jason F. Weaver1*
Methane undergoes highly facile C–H bond cleavage on the stoichiometric IrO2(110)
surface. From temperature-programmed reaction spectroscopy experiments, we found
that methane molecularly adsorbed as a strongly bound s complex on IrO2(110) and
that a large fraction of the adsorbed complexes underwent C–H bond cleavage at
temperatures as low as 150 kelvin (K). The initial dissociation probability of methane on
IrO2(110) decreased from 80 to 20% with increasing surface temperature from
175 to 300 K. We estimate that the activation energy for methane C–H bond
cleavage is 9.5 kilojoule per mole (kJ/mol) lower than the binding energy of the
adsorbed precursor on IrO2(110), and equal to a value of ~28.5 kJ/mol. Low-temperature
activation may avoid unwanted side reactions in the development of catalytic
processes to selectively convert methane to value-added products.
The increasing supply of natural gas provides ubstantial motivation for developing catalyt- ic processes that can efficiently and directly transform methane (CH4) to value-added products such as methanol, formaldehyde,
or ethylene. Selective catalytic transformations of
CH4 remain a major challenge in catalysis (1, 2).
A limitation with most existing heterogeneous
catalysts is that initial C–H bond cleavage is rate
controlling (1), so subsequent reaction steps occur
rapidly and are difficult to control. Achieving CH4
activation at low temperature could eliminate this
limitation and allow for its selective oxidation.
However, catalytic materials that can readily activate CH4 at low temperatures (e.g., below 300 K)
have not been reported.
The activation of light alkanes on solid surfaces
can occur by direct and precursor-mediated mech-
anisms (3). In the direct mechanism, the alkane
molecule undergoes C–H bond cleavage during its
initial collision with the surface, and reaction is
activated with respect to the gas-phase energy
level. In the precursor-mediated mechanism, the
alkane first adsorbs intact on the surface, and the
resulting molecularly adsorbed state serves as a
precursor for C–H bond cleavage. Dissociation
by the precursor-mediated mechanism is facile
when the activation energy for C–H bond cleav-
age (Er) is smaller than the activation energy for
desorption (Ed) of the molecularly adsorbed pre-
cursor. Molecular beam experiments show that
CH4 dissociation is activated (Er > Ed) on the
many crystalline transition-metal surfaces that
have been investigated (3). Facile dissociation
(Er < Ed) of CH4 on a solid surface has not been
previously reported, but other light alkanes do
undergo facile activation on certain facets of me-
tallic Ir and Pt (4–7). Prior studies also report
only weak molecular adsorption of alkanes on
many metal oxides, including alkaline-earth ox-
ides, rare-earth oxides, and TiO2 (8–10).
We have reported that specific facets of late
transition-metal oxides, in particular PdO(101),
can promote alkane C–H bond cleavage (11, 12).
The key aspect of these surfaces is the presence
of pairs of coordinatively unsaturated (cus) metal
and oxygen atoms on the surface that promote
the formation and facile C–H bond cleavage of
adsorbed alkane s complexes (11). We have shown
that the cus-Pd sites of PdO(101) datively bond
with alkanes (11–13) and that the resulting mo-
lecularly adsorbed species are analogous to coordi-
nation compounds known as alkane s complexes
(14). The dative interaction with cus-metal sites
facilitates alkane activation by both strengthening
the alkane-surface binding as well as weakening
the Pd-coordinated C–H bonds. The cus-oxygen
atoms also play a central role in alkane C–H bond
cleavage on PdO(101) by acting as H-atom accep-
tors. In situ measurements show that formation
of a PdO(101) layer gives rise to high rates of CH4
oxidation over Pd surfaces under steady-state con-
ditions at elevated pressure, thus demonstrating
that fundamental studies with PdO(101) are di-
rectly relevant for understanding CH4 oxidation
over Pd surfaces under realistic conditions (15).
Density functional theory (DFT) calculations
predict that small alkanes also form strongly bound
s complexes on rutile RuO2 and IrO2 surfaces
(11, 16–20). We have experimentally confirmed the
formation of alkane s complexes on RuO2(110) and
also shown that n-butane undergoes facile C–H
bond cleavage during temperature-programmed
reaction spectroscopy (TPRS) experiments in ultrahigh vacuum (UHV) (19, 20). Dispersion-corrected
DFT calculations predict that the binding energy
of the CH4 s complex on IrO2(110) is greater by
about 40 kJ/mol than the energy barrier for C–H
bond cleavage, so that CH4 activation should occur at high rates on IrO2(110) at temperatures as
low as 100 K (17, 18).
The facile activation of CH4 by the IrO2(110)
surface reinforces earlier studies that show an
unusual ability of iridium to activate hydrocarbon
C–H bonds. As originally reported by Ardntsen
and Bergman (21), cationic Ir(III) complexes are
among the most highly reactive transition-metal
compounds known for promoting C–H bond activation. Further, crystalline surfaces of metallic
Ir exhibit the highest activity toward alkane
C–H bond cleavage among the metal surfaces
that have been investigated (3), and the presence
of low-coordination surface sites strongly enhances
the reactivity of Ir and other metals toward alkane activation (4, 6, 7, 22). A common feature
1Department of Chemical Engineering, University of Florida,
Gainesville, FL 32611, USA. 2William G. Lowrie Chemical and
Biomolecular Engineering, The Ohio State University,
Columbus, OH 43210, USA.
*Corresponding author. Email: firstname.lastname@example.org