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1. T. Zambelli, J. Wintterlin, J. Trost, G. Ertl, Science 273,1688–1690 (1996).

2. J. K. Nørskov, T. Bligaard, J. Rossmeisl, C. H. Christensen,Nat. Chem. 1, 37–46 (2009).

3. R. A. Van Santen, Acc. Chem. Res. 42, 57–66 (2009).4. G. A. Somorjai, J. Carrazza, Ind. Eng. Chem. Fundam. 25,

63–69 (1986).5. I. Lee, F. Delbecq, R. Morales, M. A. Albiter, F. Zaera, Nat.

Mater. 8, 132–138 (2009).6. V. R. Stamenkovic et al., Science 315, 493–497 (2007).7. M. Behrens et al., Science 336, 893–897 (2012).8. J. Greeley et al., Nat. Chem. 1, 552–556 (2009).9. A. S. Bandarenka et al., Angew. Chem. Int. Ed. 51, 11845–11848

(2012).10. L. Dubau et al., J. Mater. Chem. A 2, 18497–18507 (2014).11. P. Strasser et al., Nat. Chem. 2, 454–460 (2010).12. M. Escudero-Escribano et al., J. Am. Chem. Soc. 134,

16476–16479 (2012).13. H. A. Gasteiger, S. S. Kocha, B. Sompalli, F. T. Wagner, Appl.

Catal. B 56, 9–35 (2005).14. J. Suntivich et al., Nat. Chem. 3, 647 (2011).15. I. E. L. Stephens, A. S. Bondarenko, U. Grønbjerg,

J. Rossmeisl, I. Chorkendorff, Energy Environ. Sci. 5,6744–6762 (2012).

16. J. K. Nørskov et al., J. Phys. Chem. B 108, 17886–17892(2004).

17. I. E. L. Stephens et al., J. Am. Chem. Soc. 133, 5485–5491(2011).

18. F. Calle-Vallejo et al., Chem. Sci. 4, 1245 (2013).19. F. Calle-Vallejo, J. I. Martínez, J. M. García-Lastra, E. Abad,

M. T. M. Koper, Surf. Sci. 607, 47–53 (2013).20. J. D. Baran, H. Grönbeck, A. Hellman, J. Am. Chem. Soc. 136,

1320–1326 (2014).21. H. Li, Y. Li, M. T. M. Koper, F. Calle-Vallejo, J. Am. Chem. Soc.

136, 15694–15701 (2014).22. F. Calle-Vallejo, M. T. M. Koper, A. S. Bandarenka, Chem. Soc.

Rev. 42, 5210–5230 (2013).23. F. Viñes, J. R. B. Gomes, F. Illas, Chem. Soc. Rev. 43,

4922–4939 (2014).24. J. Kleis et al., Catal. Lett. 141, 1067–1071 (2011).25. G. Mpourmpakis, A. N. Andriotis, D. G. Vlachos, Nano Lett. 10,

1041–1045 (2010).26. Materials and methods are available as supplementary

materials on Science Online.27. F. Calle-Vallejo, J. I. Martínez, J. M. García-Lastra,

P. Sautet, D. Loffreda, Angew. Chem. Int. Ed. 53, 8316–8319(2014).

28. P. Strasser, S. Koh, J. Greeley, Phys. Chem. Chem. Phys. 10,3670–3683 (2008).

29. A. A. Topalov et al., Angew. Chem. Int. Ed. 51, 12613–12615(2012).

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31. B. D. B. Aaronson et al., J. Am. Chem. Soc. 135, 3873–3880(2013).

ACKNOWLEDGMENTS

We received funding from The Netherlands Organisation forScientific Research (NWO), Veni project 722.014.009; theEuropean Union’s FP7/2007-2013 program, grant 303419(PUMA MIND); the Cluster of Excellence RESOLV (EXC 1069)funded by Deutsche Forschungsgemeinschaft, Helmholtz-Energie-Allianz (HA-E-0002), and SFB 749; and the Cluster of ExcellenceNanosystems Initiative Munich. We thank Stichting NationaleComputerfaciliteiten (NCF), Institut du Développement et desRessources en Informatique Scientifique, Centre InformatiqueNational de l’Enseignement Supérieur (project 609, GENCI/CT8),and Pôle Scientifique de Modélisation Numérique for CPUtime. We thank A. Pathan, M. Schmuck, and A. Zychma(RUB) for supporting measurements. The DFT data appear intables S1 to S3.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/350/6257/185/suppl/DC1Materials and MethodsFigs. S1 to S22Tables S1 to S3References (32–40)

14 April 2015; accepted 2 September 201510.1126/science.aab3501

SURFACE CHEMISTRY

Identification of active sites in COoxidation and water-gas shift oversupported Pt catalystsKunlun Ding,1 Ahmet Gulec,2 Alexis M. Johnson,1 Neil M. Schweitzer,3

Galen D. Stucky,4 Laurence D. Marks,2 Peter C. Stair1,5*

Identification and characterization of catalytic active sites are the prerequisites for anatomic-level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Indirect evidence in recent reports suggests thatplatinum (Pt) single atoms are exceptionally active catalytic sites. We demonstratethat infrared spectroscopy can be a fast and convenient characterization method withwhich to directly distinguish and quantify Pt single atoms from nanoparticles. In addition,we directly observe that only Pt nanoparticles show activity for carbon monoxide (CO)oxidation and water-gas shift at low temperatures, whereas Pt single atoms behave asspectators. The lack of catalytic activity of Pt single atoms can be partly attributedto the strong binding of CO molecules.

Low-temperature catalytic conversions of car-bon monoxide (CO) to carbon dioxide (CO2)via oxidation and water-gas shift (WGS) re-actions are integral to several importantprocesses, including the removal of CO from

hydrogen gas (H2) for fuel cell applications (1, 2)and emission control in automobiles with cata-lytic converters (3). Supported noblemetal–basedcatalysts,mainly platinum (Pt) and gold (Au), havebeen intensively studied for the reactions duringthe past decade because of their excellent activityandstability at low reaction temperatures.However,the reactionmechanisms are still highly debated,especially regarding the active site structures—single atoms (4–10) versus small Pt and Au nano-particles (NPs) (11–16). For instance, there havebeen disagreements on the promotional effectsof alkali cations in CO oxidation (17–20) andWGS (5, 10, 16, 21) reactions over supported Ptand Au catalysts. Some researchers attributedthe promotional effects to the increased disper-sion and activity of Pt andAu single atoms (5, 10);others correlated it to the increased activity ofPt and Au NPs (16–21). These differences inreaction mechanisms and identification of activesites may have arisen from conclusions drawnfrom techniques—such as microscopy or x-ray ab-sorption spectroscopy—that provide either statis-tically limited information or sample-averagedinformation, respectively. A direct observationof the catalytic performance of specific siteshas long been lacking.

Site-specific techniques that provide statistical-ly sufficient information on site identificationandquantification aswell as the activity evaluationof specific sites would make substantial progresstoward resolving these discrepancies. Infrared (IR)spectroscopy of CO on supported noble metal cat-alysts is widely used because of its sensitivity tothe atomic and electronic structures of the bind-ing sites (22–24). Work done by Green et al. ontitaniumdioxide (TiO2)–supportedAuNPs showedthat IR spectroscopy could identify the active sitein CO oxidation (25) and that CO oxidation oc-curred within a zone at the perimeter of Au NPssurrounded by a TiO2 surface. Here, we show thatIR spectroscopy with CO as a probe molecule candifferentiate and quantify both Pt single atomsand NPs. We confirm the coexistence of Pt singleatoms and NPs in many conventional catalystsand show that only the CO molecules adsorbedon Pt NPs can react at low temperatures upon O2

or H2O exposure. Thus, the active sites in CO oxi-dation andWGS reactions are present on theNPsbut not on single atoms.CO molecules were adsorbed on a series of Pt

catalysts with varying ratios of Pt single atoms toNPs in order to investigate the correspondingchanges to the IR absorption bands. Mesoporouszeolite HZSM-5 [silicon/aluminum (Si/Al) ratioof 62; morphology shown in fig. S1] (26) was cho-sen as the catalyst support because Al atoms arestrictly isolated in the zeolite framework (27), pro-viding isolated binding sites for Pt. The meso-porous structure is introduced so as to facilitatethe diffusion of an organometallic Pt precur-sor, trimethyl(methylcyclopentadienyl)platinum(MeCpPtMe3), to the Al sites. Pt was loaded onthemesoporous HZSM-5 via either solution graft-ing at room temperature or vapor deposition atelevated temperatures (26).The IR spectra of adsorbed CO on four Pt/

HZSM-5 samples with different loadings (Fig. 1A)reveal two sets of CO absorption bands centered

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1Department of Chemistry, Northwestern University,Evanston, IL 60208, USA. 2Department of Materials Scienceand Engineering, Northwestern University, Evanston, IL60208, USA. 3Department of Chemical and BiologicalEngineering, Northwestern University, Evanston, IL 60208,USA. 4Department of Chemistry and Biochemistry, Universityof California, Santa Barbara, CA 93106, USA. 5ChemicalSciences and Engineering Division, Argonne NationalLaboratory, Argonne, IL 60439, USA.*Corresponding author. E-mail: pstair@northwestern.edu

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Fig. 1. Spectroscopic and microscopic identification of Pt single atoms and NPs on HZSM-5 and SiO2 (Al-doped). (A) IR spectra of CO adsorbed ondifferent Pt/HZSM-5 after the desorption processes. (B and C) Time-dependent IR spectra of CO adsorbed on (B) 2.6 wt % Pt/HZSM-5 and (C) 0.5 wt %Pt/HZSM-5 during the desorption process. (D and E) Time-dependent IR spectra of CO adsorbed on (D) 2.6 wt % Pt/HZSM-5 and (E) 0.5 wt % Pt/HZSM-5during the oxidation process. (F) HAADF image of 0.6 wt % Pt/SiO2 (Al-doped) with magnified image inserted and arrows to mark the single atoms.

Fig. 2. Identification and quantification of Pt single atoms and NPs on conventional supports. (A and B) IR spectra of CO adsorbed on wet-impregnated(A) Pt/SiO2 and Pt/Al2O3 and (B) Pt/TiO2 and Pt/ZrO2 upon O2 exposure. (C and D) HAADF images of wet-impregnated 1 wt % Pt/SiO2. Scale bars, 20 nm (C)and 2 nm (D). Arrows indicate the single atoms. (Inset) Magnified image. (E) IR spectra of CO adsorbed on impregnated Pt/SiO2 before and after O2 exposure.The difference spectrum shows the CO removed by O2; Kubelka-Munk unit is used for quantification. (F) CO2 signal monitored by means of MS during the TPOprocess of the preadsorbed CO on Pt/SiO2.

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at 2115 cm−1 and 2070 to 2090 cm−1. Adsorbed COwas not present on the bare zeolite HZSM-5 sam-ple under our experimental conditions (fig. S2),indicating that these two peaks originate fromCOmolecules adsorbed on two different Pt species.We assigned the IR peak at 2070 to 2090 cm−1 toCO molecules linearly adsorbed in an a-top ge-ometry on Pt0 atoms on single-crystal or NP sur-faces (28–30). The relative intensity of the IR peakat 2070 to 2090 cm−1 increased with the Pt load-ing,which is consistentwith the increasingamountof Pt NPs on the zeolite support as observed withtransmission electronmicroscopy (TEM) (figs. S3and S4). In addition, the redshift of the IR peakat 2070 to 2090 cm−1 during desorption (Fig. 1Band fig. S5) correlated with changes in dipole-dipole coupling between CO molecules on a Ptcrystal surface (28–30).A 0.5weight percent (wt%)Pt/HZSM-5 sample,

prepared from room-temperature solution graft-ing, displayed several COpeaks in the polycarbonylregion (Fig. 1C) (31, 32). These peaks gradually de-creased in intensity as the temperature increasedduring the desorption process (Fig. 1C), and a newpeak at 2115 cm−1 emerged and eventually becamethe only peak in the spectrum. The peak trans-formation was reversed upon reexposure to CO(fig. S6). These trends indicate that cationicPtd+-polycarbonyl species formed initially uponCO exposure, before transforming into Ptd+-monocarbonyl upon purging and heating. The cat-ionic nature of grafted Ptwas confirmed bymeansof x-ray photoelectron spectroscopy (XPS) (fig. S7).

Compared with the absorption band from CObonded to Pt0 NPs, the CO band assigned toPtd+-monocarbonyl is less redshifted from the gasphase value (2143 cm−1) because of decreased Ptd-electron back-donation to theCO p* antibondingorbital. At short electron beam exposures, weverified by means of TEM that Pt NPs were notpresent on the 0.5 wt % Pt/HZSM-5 sampleprepared from room-temperature solution graft-ing, as expected for the absence of the 2070- to2090-cm−1 peak (fig. S3). However, the zeolitestructures decomposed in the electron beamwithin1 min, leading to the appearance of Pt NPs. Thepoor stability under electron beamexposure hind-ers the high-angle annular dark-field (HAADF)imaging of Pt species with atomic resolution inzeolites. In order to avoid the degradation of thesupport, we used Al-doped amorphous silica asthe support. A 0.6 wt % Pt/SiO2(Al) sample pre-pared from room-temperature solution graftinggave similar CO IR spectra to that of the 0.5 wt %Pt/HZSM-5 sample (fig. S8). Aberration-correctedHAADF images show that the Pt was dispersedon the surface predominantly as isolated singleatoms (Fig. 1F and figs. S9 and S10).On the basis of the IR, TEM, and XPS results,

we assigned the IR bands centered at 2115 cm−1

and 2070 to 2090 cm−1 to COmolecules adsorbedon Pt single atoms and NPs, respectively, whichallowed us to compare the oxidation activity ofCO molecules adsorbed at different sites and in-vestigate the active species in supported-Pt cata-lyzed CO oxidation. For all the Pt samples studied,

only the CO molecules adsorbed on Pt NPs couldbe oxidized and subsequently desorb as CO2 atreaction temperatures below 100°C. The IR peakat 2115 cm−1, corresponding to CO adsorbed onPt single atoms, remained unchanged under thereaction conditions (Fig. 1, D and E, and fig. S5).This result clearly indicates the superior activityof Pt NPs as compared with single atoms for COoxidation.The coexistence of Pt single atoms andNPswas

also observed in a variety of conventional, sup-ported Pt catalysts. The IR spectra of CO adsorbedon 1 wt % Pt/SiO2, Pt/Al2O3 (g), Pt/TiO2 (anatase),and Pt/ZrO2 (monoclinic) at 100°C—all of whichwere prepared by means of incipient wetnessimpregnation and calcined at 400°C in air—aregenerally composed of two types of CO bands(Fig. 2, A and B). Themajor CO bands centered at2050 to 2080 cm−1 are identical to the band fromCOadsorbedonPtNPs shown inFig. 1A.A redshiftof the major IR bands during argon purging wasobserved as well (fig. S11), implying the nano-particulate nature of the CO adsorption sites. Aswell as the major band, each spectrum containsone or more shoulders on the higher frequencyside. The band positions of these shoulders aresimilar, if not identical, to the band position thatwe have assigned to CO molecules adsorbed oncationic Pt single atoms (within 20 cm−1). Theseshoulder bands have been observed previously(33–35) but were ambiguously assigned to CO ad-sorbed on certain cationic Pt species. On the basisof our study with the model Pt/HZSM-5 system,

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Fig. 3. 12CO-13CO exchangeon Pt/SiO2. (A) Comparisonof the IR peaks of 12CO and13CO adsorbed on Pt/SiO2

before and after O2 exposure.(B) Shift of the IR peaks atdifferent 12CO exposure timeafter 13CO adsorption. Thefour spectra at the bottomwere recorded after O2 expo-sure in order to show the COpeaks related to Pt singleatoms.

Fig. 4. Catalytic activity of Pt single atoms and NPs in WGS. (A and B) Time-dependent IR spectra of CO adsorbed on (A) Pt/SiO2 and (B) Pt-Na/SiO2

upon H2O exposure. (C) Temperature-programmed WGS reaction spectra over Pt/SiO2 and Pt-Na/SiO2.

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we assigned these bands to cationic Pt singleatoms. Similar to the CO oxidation behavior thatwe observed on Pt/HZSM-5 samples, the COmolecules adsorbed on Pt NPs were quickly oxi-dized and removed upon O2 exposure, as indi-cated by the rapid drop in themajor CO bands at2050 to 2080 cm−1 (Fig. 2, A and B, and fig. S11).Meanwhile, the bands related to CO on Pt singleatoms remained unchanged regardless of the iden-tity of the support. Thus, the IR technique can beused for identifying the two Pt species on manyconventional metal oxide supports in addition tothemodel Pt/HZSM-5 system. The coexistence ofPt single atoms and NPs in Pt/SiO2 was furtherconfirmed by aberration-corrected HAADF imag-ing, as shown in Fig. 2, C and D, and fig. S12.Microscopy canprovide only statistically limited

information about the population of different Ptspecies. In this regard, IR spectroscopy can be usedas a tool for site quantification, with a knowledgeof the ratio of IR extinction coefficients for COmolecules adsorbed on Pt single atoms and NPs.To obtain this number, we quantified the respec-tive population of Pt single-atom and NP-relatedCO adsorption sites in Pt/SiO2 using temperature-programmed oxidation (TPO) coupled with massspectrometry (MS) (26). As shown in Fig. 2F (fullspectrum and extended discussion given in figs.S13 and S14), the first sharpCO2 peak is associatedwith the CO molecules adsorbed on Pt NPs. Thesecond broader CO2 peak, which extends from150°C to 350°C, is from the CO molecules thatwere adsorbed on Pt single atoms, as evidencedfrom TPO-IR spectra (fig. S15). According to ourcalculations, the CO molecules adsorbed on Ptsingle atoms andNPs correspond to ~11 and 10%,respectively, of the total Pt atoms in the 1 wt %Pt/SiO2 catalyst. From the peak area ratio derivedfrom TPO-MS and IR spectra (Fig. 2, E and F), weobtained a ratio of IR extinction coefficients forCO molecules adsorbed on Pt single atoms andNPs equal to ~0.12. This ratio can then be usedfor the quantification of CO adsorption sites byusing IR spectroscopy.The temperature-programmed reaction (TPRx)

spectra fromCOoxidationwithO2undercontinuous-flow conditions over Pt/SiO2 and Pt/Al2O3 areshown in fig. S16. The activation energies weremeasured to be 90 and 95 kJ/mol for Pt/SiO2 andPt/Al2O3, respectively, which is in good agreementwith the experimental and theoretical values re-ported for supported Pt NPs (30). The TPRx-IRspectra of Pt/SiO2 (fig. S17) show negligible COoxidation activity below 150°Cbecause the surfaceof Pt NPs is dominated by CO adsorption in thepresence of CO and O2. The Pt NP–related COpeak completely disappeared when the reactiontemperature was increased to 160°C, accompa-nied by a constant formation of CO2 (fig. S17). Incontrast, the Pt single atom–related CO peak didnot change substantially until the reaction tem-perature was above 200°C (figs. S15 and S17).The rapid rise of CO oxidation activity is clearlyassociated with the abrupt change of CO cover-age on Pt NPs, which further confirms that theactive sites of supported Pt in CO oxidation are PtNPs but not single atoms.

There are two possible explanations for the dis-tinctly different reactivity of CO molecules ad-sorbed on the Pt NPs and single atoms. One isthe size dependence toO2 activation by Pt clusters.Small clusters have lower d-band centers, resultingin less electron back-donation to the antibondingorbital of O2 molecules and thus less efficient O2

activation (12).Another possible contribution to the observed

reactivity may be the binding strength of the COmolecule to the different Pt species.We conducteda series of 12CO-13CO exchange experiments on the1 wt % Pt/SiO2 catalyst to probe the respectivebinding strengths of CO molecules on the Pt NPand single-atom sites. The IR bands of 13CO ad-sorbed on Pt single atoms and NPs both redshift~50 cm−1 compared with those of 12CO (Fig. 3A),which is consistent with reported values (28, 29).After the 13CO adsorption reached an equilibriumstate at 100°C, 12CO was introduced to exchangewith the preadsorbed 13CO. The IR peak related toPt NPs shifted back to 2080 cm−1 in less than 30 s,indicating that the preadsorbed 13CO on Pt NPswas rapidly replaced by 12CO. In contrast, the13COmolecules adsorbed on Pt single atoms wereonly partially replaced by 12CO, even after 18 min(Fig. 3B and fig. S18). The stronger binding ofCO to Pt single atoms than to Pt NPs resultedin substantially lower catalytic activity for COoxidation.We also examined the WGS reaction over the

Pt/SiO2 catalyst in order to compare the reactivityof O2 and H2O with adsorbed CO and found thatonly CO molecules adsorbed on Pt NPs quicklyreacted with H2O, releasing CO2 (Fig. 4A). TheCO molecules adsorbed on Pt single atoms re-mained intact upon H2O exposure at 100°C, fur-ther emphasizing the lack of activity by the Ptsingle atoms.In order to better understand the origin of

the promotional effect of alkali cations on Ptsingle atoms and NPs, we prepared a Pt-sodium(Na)/SiO2 catalyst with 1 wt % Pt and a Pt/Namolar ratio of 1/3. The WGS performance ofPt/SiO2 and Pt-Na/SiO2 that we have observedconfirms the promotional effect of Na cations(Fig. 4C). HAADF images show a higher dis-persion of Pt in the Pt-Na/SiO2 case as com-pared with the Pt/SiO2 catalyst (fig. S19), whichis in agreement with the literature results (5, 10).The IR spectra show that the addition of Na+

greatly lowered the CO adsorption peak inten-sities, implying that Na+ could reside in andblock CO adsorption sites (36, 37). However, thelinear and bridge CO peaks corresponding to PtNPs of Pt-Na/SiO2 are redshifted 30 and 90 cm−1,respectively, compared with those of the Na-free sample. At a temperature of up to 300°C,the introduction of H2O also only removed COadsorbed on Pt NPs (Fig. 4B and fig. S20). OurIR data show that the promotional effect ofNa+ in WGS reaction mainly originates fromaltering the properties of Pt NPs, not singleatoms. It also appears that alkali metals couldlower the CO surface coverage on Pt NPs. Thus,more sites remain available for the activation ofO2 or H2O (17, 18).

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ACKNOWLEDGMENTS

We acknowledge funding from the U.S. National ScienceFoundation CHE-1058835 (K.D., A.M.J., and P.C.S.) and theNorthwestern University Institute for Catalysis in Energy Processes(ICEP) on grant DOE DE-FG02-03-ER15457 (K.D., A.G., N.M.S., L.D.M.,and P.C.S.). This work made use of the JEOL JEM-ARM200CF inthe Electron Microscopy Service [Research Resources Center,University of Illinois at Chicago (UIC)]. The acquisition of theUIC JEOL JEM-ARM200CF was supported by a MRI-R2 grant fromthe National Science Foundation [DMR-0959470]. This work alsomade use of the Electron Probe Instrumentation Center (EPIC)facility and Keck-II facility of Northwestern University’s Atomic andNanoscale Characterization Experimental Center (NUANCE), whichhas received support from the Materials Research and EngineeringCenter program (NSF DMR-1121262) at the Materials ResearchCenter; the International Institute for Nanotechnology (IIN); theKeck Foundation; and the state of Illinois, through the IIN. Weacknowledge useful discussion with W. Wu (Northwestern University)regarding the infrared data interpretation. All data and images areavailable in the body of the paper or as supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/350/6257/189/suppl/DC1Materials and MethodsFigs. S1 to S20References (38–42)

23 May 2015; accepted 19 August 2015Published online 3 September 201510.1126/science.aac6368

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Identification of active sites in CO oxidation and water-gas shift over supported Pt catalystsKunlun Ding, Ahmet Gulec, Alexis M. Johnson, Neil M. Schweitzer, Galen D. Stucky, Laurence D. Marks and Peter C. Stair

originally published online September 3, 2015DOI: 10.1126/science.aac6368 (6257), 189-192.350Science 

, this issue p. 189Scienceatoms.studies showed that CO reacted at much lower temperatures when adsorbed on nanoparticles versus on isolated metalisolated platinum atoms or nanoparticles dispersed on zeolite and oxide supports. Temperature-programmed desorption

used infrared spectroscopy to identify CO adsorbed onet al.carbon monoxide (CO) oxidation by oxygen or water. Ding Gold and platinum dispersed on metal oxide supports, for example, show remarkable low-temperature reactivity for

Noble metal nanoparticles often exhibit behaviors distinct from atomic and bulk versions of the same material.Comparing active site reactivity

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