Figs. 3 and 4 show Pt species located randomly
on the ceria surfaces (not embedded in the ceria),
with no preference for specific facets.
Atom trapping should be broadly applicable
as a method for preparing single-atom catalysts.
The approach requires a supply of mobile atoms
and a support that can bind the mobile species.
Conditions that are conducive to Ostwald ripening,
which normally is implicated in the degradation
of catalysts (3), are ideal because mobile species
are continually being generated. In our work, at
the aging temperature of 800°C in air, mobile
PtO2 is rapidly emitted; the estimated lifetime is
only a few seconds for a 5-nm Pt crystallite (24).
Surface species such as hydroxyls and carbonates,
which could prevent the trapping of mobile species, would have desorbed at high temperatures,
providing a clean surface for the formation of
covalent metal oxide bonds that are needed to
stabilize single atoms. Trapping of atoms provides a plausible explanation for the role of ceria
in slowing the rates of Ostwald ripening and may
help to explain how other supports modify the
rates of catalyst sintering.
REFERENCES AND NOTES
1. G. W. Graham et al., Catal. Lett. 116, 1–8 (2007).
2. M. H. Wiebenga et al., Catal. Today 184, 197–204 (2012).
3. T. W. Hansen, A. T. Delariva, S. R. Challa, A. K. Datye,
Acc. Chem. Res. 46, 1720–1730 (2013).
4. T. R. Johns et al., J. Catal. 328, 151–164 (2015).
5. J. A. Kurzman, L. M. Misch, R. Seshadri, Dalton Trans. 42,
6. C. B. Alcock, G. W. Hooper, Proc. R. Soc. London Ser. A 254,
7. G. Cavataio et al., SAE Int. J. Fuels Lubr. 2, 204–216 (2009).
8. Y.-F. Yu-Yao, J. T. Kummer, J. Catal. 106, 307–312 (1987).
9. J. G. McCarty, K.-H. Lau, D. L. Hildenbrand, Stud. Surf.
Sci. Catal. 111, 601–607 (1997).
10. C. Carrillo et al., J. Phys. Chem. Lett. 5, 2089–2093
11. G. B. McVicker, R. L. Garten, R. T. K. Baker, J. Catal. 54,
12. Y. Nagai et al., J. Catal. 242, 103–109 (2006).
13. J. A. Farmer, C. T. Campbell, Science 329, 933–936 (2010).
14. J. H. Kwak et al., Science 325, 1670–1673 (2009).
15. B. Qiao et al., Nat. Chem. 3, 634–641 (2011).
16. W.-Z. Li et al., Nat. Commun. 4, 2481 (2013).
17. T. R. Johns et al., ChemCatChem 5, 2636–2645 (2013).
18. H.-X. Mai et al., J. Phys. Chem. B 109, 24380–24385
19. S. Agarwal et al., ChemSusChem 6, 1898–1906 (2013).
20. T. Wu et al., J. Phys. Chem. Lett. 5, 2479–2483 (2014).
21. A. Bruix et al., Angew. Chem. Int. Ed. 53, 10525–10530
22. A. Neitzel et al., J. Phys. Chem. C 120, 9852–9862 (2016).
23. Z. L. Wang, X. Feng, J. Phys. Chem. B 107, 13563–13566
24. See supplementary materials on Science Online.
25. P. J. Berlowitz, C. H. F. Peden, D. W. Goodman, J. Phys. Chem.
92, 5213–5221 (1988).
26. R. Kopelent et al., Angew. Chem. Int. Ed. 54, 8728–8731
27. M. Cargnello et al., Science 341, 771–773 (2013).
28. M. Moses-DeBusk et al., J. Am. Chem. Soc. 135, 12634–12645
29. K. Ding et al., Science 350, 189–192 (2015).
30. F. Dvořák et al., Nat. Commun. 7, 10801 (2016).
Supported by NSF GOALI grant CBET-1438765 (J.J., H.X., S.R.C.,
A.K.D.), General Motors Global R&D (G.Q., S.O., and M.H. W.),
U.S. Department of Energy grant DE-FG02-05ER15712 (A. T.D., E.J.P.,
A.K.D., X.I.P.H., and Y. W.), and the Center for Biorenewable Chemicals
funded by NSF grant EEC-0813570 (H.X., H.P., and
A.K.D.). This work made use of the JEOL JEM-ARM200CF at the
University of Illinois at Chicago. We thank A. Nicholls for
recording the AC-STEM images and D. Kunwar for assistance
in catalyst preparation.
Materials and Methods
Figs. S1 to S14
Tables S1 to S3
26 April 2016; accepted 13 June 2016
Tail use improves performance on
soft substrates in models of early
vertebrate land locomotors
Benjamin McInroe,1 Henry C. Astley,1 Chaohui Gong,2 Sandy M. Kawano,3
Perrin E. Schiebel,1 Jennifer M. Rieser,1 Howie Choset,2
Richard W. Blob,4 Daniel I. Goldman1,5†
In the evolutionary transition from an aquatic to a terrestrial environment, early tetrapods
faced the challenges of terrestrial locomotion on flowable substrates, such as sand and
mud of variable stiffness and incline. The morphology and range of motion of appendages
can be revealed in fossils; however, biological and robophysical studies of modern taxa
have shown that movement on such substrates can be sensitive to small changes in
appendage use. Using a biological model (the mudskipper), a physical robot model,
granular drag measurements, and theoretical tools from geometric mechanics, we
demonstrate how tail use can improve robustness to variable limb use and substrate
conditions. We hypothesize that properly coordinated tail movements could have provided
a substantial benefit for the earliest vertebrates to move on land.
During the vertebrate invasion of land, 385 to 360 million years ago, early tetrapods and relatives faced a variety of challenges (1), including locomotion in terrestrial en- vironments. Terrestrial locomotion relies
on interactions between the body and substrate
to generate propulsive forces, but the interaction
between the organism and some substrates may
be complex. Fossil evidence indicates that tetrapods emerged from water in near-shore habitats,
where they likely encountered flowable soft substrates such as sands and muds (2, 3). These
substrates exhibit properties of solids and fluids,
either jamming or yielding (plastic deformation
of the material) depending on how they are
loaded (4) and sloped (5).
The challenge of movement on flowable sub-
strates therefore arises from the complexity of
interactions between the substrate and the or-
ganism. Even on level deformable substrates,
subtle variations in limb morphology (6) and
kinematics (7) can lead to substantial differences
in performance. Furthermore, interactions be-
tween appendages and these substrates leave
local disturbances, which can influence subse-
quent interactions, sometimes leading to deteri-
orating locomotor performance and eventual
total locomotor failure (8). As substrate slope in-
creases, yield forces decrease and downhill ma-
terial flow becomes important, reducing the range
of effective locomotor strategies (5).
The use of an additional locomotor structure
that can be independently coordinated may allow
a greater range of effective behaviors, even in the
absence of derived limb morphology and sophisticated motor patterns. We propose that the tail
could have been a critical locomotor structure for
early tetrapods. In addition to being a primary
driver of aquatic locomotion, tails play major
roles in the propulsion of many modern fishes
during terrestrial locomotion (9–12) and can be
used as inertial reorientation appendages in some
tetrapods (13, 14). Thus, the use of a prominent
tail [as seen in fossil taxa (15–17) (Fig. 1A)] may
have increased locomotor robustness to environmental and kinematic variables.
Evaluating locomotor performance for extinct
taxa is challenging (18, 19), in part because the
sensitivity of locomotion on complex substrates
to kinematics and control strategies cannot necessarily be inferred from range of motion and
morphology (7). Therefore, to test our hypothesis,
we used three complementary modeling methods
(Fig. 1): a model organism, a robophysical model,
and a mathematical model. We made several choices
governing our modeling approaches. In our locomotors, we modeled symmetrical, forelimb-driven
154 8 JULY 2016 • VOL 353 ISSUE 6295 sciencemag.org SCIENCE
1School of Physics, Georgia Institute of Technology, Atlanta,
GA, USA. 2Robotics Institute, Carnegie Mellon University,
Pittsburgh, PA, USA. 3National Institute for Mathematical and
Biological Synthesis, University of Tennessee, Knoxville, TN,
USA. 4Department of Biological Sciences, Clemson University,
Clemson, SC, USA. 5School of Biology, Georgia Institute of
Technology, Atlanta, GA, USA.
*These authors contributed equally to this work. †Corresponding
author. Email: email@example.com