of Au and a-MoC (CO + OH = CO2 + ½H2) was
apt to proceed. Although the reforming process
was facile, it still had a higher barrier than the
first step of the WGS reaction (water dissociation
on partially oxidized a-MoC). Thus, the rate-determining step of the WGS reaction on Au15/
a-MoC is the reforming process, which is in good
agreement with our TPSR observations (Fig. 2).
The interfacial nature and optimum bonding of
this a-MoC–confined Au nanostructure confers
the catalyst with outstanding WGS reactivity at
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This work received financial support from the 973 Project (grants
2017YFB0602200, 2013CB933100, and 2011CB201402), the CAS
Pioneer Hundred Talents Program, and the Natural Science
Foundation of China (grants 91645115, 21473003, 21222306,
21373037, 21577013, and 91545121). The electron microscopy
work was also supported in part by the U.S. Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences, Materials
Sciences and Engineering Division (to W.Z.), and through a user
project at Oak Ridge National Laboratory's Center for Nanophase
Materials Sciences, which is a DOE Office of Science User Facility.
The x-ray absorption spectroscopy experiments were conducted
at the Shanghai Synchrotron Radiation Facility and the Beijing
Synchrotron Radiation Facility. We also acknowledge the National
Thousand Young Talents Program of China, the Shanxi Hundred
Talents Program, and the Fundamental Research Funds for
the Central Universities (grants DUT15TD49 and DUT16ZD224). The
research done at Brookhaven National Laboratory (BNL) was financed
under contract no. DE-SC0012704 with the DOE. Some of the
theoretical calculations were done at the Center for Functional
Nanomaterials on the BNL campus. The Advanced Light Source is
supported by the Director, Office of Science, Office of Basic Energy
Sciences, of the DOE under contract no. DE-AC02-05CH11231.
Y. Y. acknowledges the support of the Advanced Light Source
Doctoral Fellowship. C.J.K. gratefully acknowledges funding from the
U.S. National Science Foundation Major Research Instrumentation
program (grant MRI/DMR-1040229). D.M. thanks L. Peng and
M. Wang for help with 17O NMR experiments. All data are reported
in the main text and supplementary materials. C.S. and X.Z. are
inventors on a patent application (ZL 2015 1 0253637.6) held by
Dalian University of Technology that covers the preparation of
Au/a-MoC. D.M., C.S., and J.A.R. designed the study. X.Z. and S. Y.
performed most of the reactions. W.Z., L.Lu, C.J.K., and L.G.
performed the electron microscopy study. R.G., X. W., P.L., and Z.Z.
finished the DFT calculations. S. Y., W.X., and W.L. performed the
x-ray structure characterization and analysis. S. Y., D.M., W.Z.,
and J.A.R. wrote the paper. Other authors provided reagents,
performed certain experiments, and revised the paper.
Materials and Methods
Figs. S1 to S32
Tables S1 and S5
26 June 2016; resubmitted 12 April 2017
Accepted 9 June 2017
Published online 22 June 2017
Seismic evidence for partial melting
at the root of major hot spot plumes
Kaiqing Yuan1 and Barbara Romanowicz1,2,3*
Ultralow-velocity zones are localized regions of extreme material properties detected
seismologically at the base of Earth's mantle. Their nature and role in mantle dynamics
are poorly understood. We used shear waves diffracted at the core-mantle boundary
to illuminate the root of the Iceland plume from different directions. Through waveform
modeling, we detected a large ultralow-velocity zone and constrained its shape to be
axisymmetric to a very good first order. We thus attribute it to partial melting of a locally
thickened, denser- and hotter-than-average layer, reflecting dynamics and elevated
temperatures within the plume root. Such structures are few and far apart, and they
may be characteristic of the roots of some of the broad mantle plumes tomographically
imaged within the large low-shear-velocity provinces in the lower mantle.
The region of the mantle right above Earth’s core is a boundary layer for mantle convec- tion, and a chemical interface between the silicate mantle and the fluid iron core, where complex dynamic processes are thought to
take place (1). The strong lateral variations in
seismic structure in the bottom ~300 km of the
mantle (the D′′ region) are difficult to explain with
temperature variations alone. Ultralow-velocity
zones (ULVZs) are thin, localized patches (15 to
30 km in height) near the core-mantle boundary
characterized by compressional and shear velocity
reductions (5 to 10% and 10 to 30%, respectively)
(2, 3), along with density increases (up to 10%)
(4), with respect to ambient mantle. Most ULVZs
reported are in regions of D′′ with lower-than-
average shear velocity, imaged by seismic shear-
wave velocity tomography under the central Pacific
and Africa (1, 5), and referred to as large low-shear-
velocity provinces (LLSVPs). The origin of ULVZs
is thought to be either zones of partially molten
silicates (6), possibly enriched in iron from the
core (7), or dense iron-enriched solid heteroge-
neities (8, 9). The nature of ULVZs can clarify our
understanding of the thermal and chemical struc-
ture near the core-mantle boundary. For exam-
ple, some geodynamics simulations have modeled
ULVZs as passive blobs of denser-than-average
material. In this scenario, they would preferen-
tially collect on the edges of the LLSVPs (10).
ULVZs may come in different sizes and shapes,
although most previous studies have reported
lateral extents of no more than 100 to 200 km
(1). Determining their morphology is important
for understanding their nature and dynamics. The
shape of ULVZs is hard to constrain given avail-
able data, so estimates of their possible lateral
extent are often based only on Fresnel zone con-
siderations (11). This has led to the suggestion of a
global correlation between the locations of ULVZs
and hot spot volcanoes (11). However, this link
and its causality have remained hypothetical.
Two examples of unusually large ULVZs (800
to 1000 km in lateral extent) were reported in
the Pacific, one in the vicinity of the Samoa hot
spot (12) and the other in the vicinity of Hawaii
(13). Global tomographic images of the lower
mantle obtained using numerical wavefield
computations (14) showed that these ULVZs
are located at the roots of broad quasi-vertical
low-shear-velocity conduits that extend from the
core-mantle boundary to at least 1000 km depth,
in the vicinity of those hot spot volcanoes that
fall within the boundaries of the LLSVPs (15).
The precise shape of the Samoa ULVZ could not
be constrained, but for the Hawaiian ULVZ, the
azimuthal sampling by shear-diffracted (Sdiff)
phases, afforded by the USArray deployment in
North America, showed that a simple cylindri-
cal model is a good fit to the available data (13).
However, illumination was limited to a particular
1Berkeley Seismological Laboratory, Berkeley, CA 94720,
USA. 2Collège de France, Paris, France. 3Institut de Physique
du Globe, Paris, France.
*Corresponding author. Email: email@example.com