CATALYSIS Synthesis of ultrasmall, homogeneouslyalloyed, … · Synthesis of ultrasmall,...
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CATALYSIS Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supports A. Wong, 1 Q. Liu, 1 S. Griffin, 1 A. Nicholls, 2 J. R. Regalbuto 1 * Supported nanoparticles containing more than one metal have a variety of applications in sensing, catalysis, and biomedicine. Common synthesis techniques for this type of material often result in large, unalloyed nanoparticles that lack the interactions between the two metals that give the particles their desired characteristics. We demonstrate a relatively simple, effective, generalizable method to produce highly dispersed, well-alloyed bimetallic nanoparticles.Ten permutations of noble and base metals (platinum, palladium, copper, nickel, and cobalt) were synthesized with average particle sizes from 0.9 to 1.4 nanometers, with tight size distributions. High-resolution imaging and x-ray analysis confirmed the homogeneity of alloying in these ultrasmall nanoparticles. B imetallic nanoparticles (NPs), anchored on porous carbon or oxide supports with high surface area to prevent agglomeration, have many applications in chemical sensing, bio- medicine, and especially catalysis (1, 2). The most common method to prepare supported metal NPs is impregnation (IMP), in which just enough metal precursor solution is used to fill the pore volume of the support. Although this method is simple and quick, IMP generally yields NPs with large average size, broad size distributions, and, in the case of multiple metals, inhomogeneous alloying. The highest possible metal utilization occurs when all atoms of a NP are accessible to the gas or liquid fluid phase, and this occurs when the particle size is about 1 nm, or clusters of sev- eral dozen atoms (3, 4). Optimal bifunctionality and synergy often occur when the atoms at the NP surface are well mixed (5, 6). We demonstrate a simple method that can be applied to noble and base metals alike to syn- thesize ultrasmall (~1 nm) NPs with homoge- neous alloying. The method is based on strong electrostatic adsorption (SEA), whereby charged metal precursors are strongly adsorbed onto op- positely charged oxide or carbon surfaces by con- trolling the pH relative to the surface point of zero charge (PZC) (7–9). This interaction holds the precursors in place during drying, as opposed to IMP, in which the metal precursors remain and aggregate in solution as drying occurs. The required surface charge is provided by the native OH groups on the surfaces; no explicit surface functionalization is needed. This method involves fewer steps than colloidal methods of NP syn- thesis and eliminates the problematic removal of the capping agents of those methods, because no capping agent is used. Electrostatic adsorption has been demonstrated for a host of single metals on many different sup- ports (7–17 ). Over composite oxide surfaces, pH- controlled adsorption has been used to achieve selective metal adsorption onto one surface com- ponent or the other (18–20). Most recently, there have been preliminary attempts at simultaneous electrostatic adsorption of two metal precursors for the synthesis of bimetallic NPs, involving Pt and Pd (21) or Pt and Co (22); this has been termed simultaneous or co-SEA. This method is schematized in Fig. 1A, which shows an elec- trostatically adsorbed layer of a mixture of hy- drated metal precursors, followed by a thermal reduction in H 2 needed to remove the metal lig- ands, reduce the metals to their zero-valent state, and nucleate the atoms into NPs. We now present a systematic application of this method using a common silica support with a variety of noble and base metal ammine pre- cursor pairs (Pt, Pd, Co, Ni, and Cu ammines) as shown in Table 1. The silica support (Evonik Aerosil 300) has a PZC of 3.6, a value similar to that of other fumed silicas (10, 11, 21). Adsorption of metal ammine precursors was measured as a function of pH for all pairs of metal complexes to determine their relative affinities for the support (23). Uptake surveys for representative noble- noble (Pt-Pd), noble-base (Pd-Cu), and base-base (Ni-Co) metal pairs are shown in Fig. 1B. For the Pt-Pd adsorption surveys, no adsorption of metals occurred near the PZC. As the pH was increased, the OH groups of the silica became deprotonated, which allowed the cationic Pt/Pd ammine precursors to adsorb. At lower pH values, Pd had a greater affinity than Pt to the support; the opposite was true at higher pH values. Maximum adsorption of 1.3 mmol/m 2 occurred at an optimum pH (pH ≈ 11.5) followed by a decrease in uptake as more base was added. This decrease in uptake is attributed to the strong ionic strength in solution that creates a double- layer screening effect (9); the metal complexes were prevented from experiencing the surface charge. A similar trend was observed in the Pd- Cu adsorption survey, but the maximum adsorp- tion occurred at a higher pH value. Adsorption uptake surveys of Ni- and Cu-containing solutions were kept above a pH of 11 to prevent precipita- tion. The base-base metal pair of Ni-Co (Fig. 1B) was consistent with the other uptake surveys. In all cases, the maximum adsorption of com- plexes for complete monolayer coverage for the bimetallic systems was ~1.0 to 1.3 mmol/m 2 . This result indicates that all of the metal complexes adsorbed with the same number of hydration sheaths and were of similar size during adsorp- tion (12). The Co surveys with Pd, Ni, and Cu (Fig. 1B and fig. S1) revealed competitive adsorption, in that the Co ammine favored the silica surface. The high affinity of Co ammines for silica is known and ultimately results in the formation of a new phyllosilicate phase (24, 25). Thus, the Co adsorp- tion was independent of pH in these experiments. The relative concentration of Co was lessened to allow the adsorption of the other metal to achieve an atomic ratio of 1:1. All other metal pairs were adsorbed onto the support in a 1:1 atomic ratio at the monolayer capacity of the precursors; this was achieved by controlling the metal precursor concentrations in solution (table S1). Temperature-programmed reduction (TPR) was performed to characterize the intimacy of alloying of the metals in the nascent NPs. We compared monometallic and bimetallic NPs pre- pared by SEA and IMP (see tables S1 and S2 for synthesis parameters). TPR profiles for the Pd-Cu series (Fig. 1C) showed that the monometallic Pd in the IMP sample exhibited a low reduction temperature of ~100°C but that the monometal- lic Pd in the SEA sample required 150°C for re- duction, indicating a much stronger interaction with the silica support. The TPR for Cu IMP re- duction peaks began with a shoulder at ~190°C and had a sharp peak at 230°C. The broad shoul- der of the IMP profile suggests a wide particle size distribution in which some of the larger particles were more easily reduced. The TPR for the Cu SEA profile contains a sharp peak reduction peak at 220°C and a broader peak around 500°C. Among the bimetallic NPs, the Pd-Cu co-SEA NPs (Fig. 1C, center) showed two distinct peaks. The first peak at ~180°C was from the reduction of a Pd-Cu alloy, and the second peak at 400°C was the monometallic Cu peak at 500°C shifted to a lower temperature. We attribute the shift to lower temperature to hydrogen spillover from Pd to unreduced Cu complexes, indicating that there must be close proximity of Pd-Cu atoms in the co-SEA–derived NPs. The bimetal Pd-Cu IMP NPs contained multiple reduction peaks between 80° and 250°C with distinct peaks that corresponded to the nonshifted monometallic Pd and Cu. Thus, unalloyed Pd or Cu existed in the IMP NPs. Further evidence of hydrogen spillover in co- SEA bimetallic NPs is observed in the Pd-Co TPR profiles (Fig. 1C, right). Although Co complexes are very difficult to reduce over silica and require temperatures of 700°C (20), alloying Pd-Co en- abled us to observe hydrogen spillover–assisted reduction of Co cations by Pd at a temperature of RESEARCH Wong et al., Science 358, 1427–1430 (2017) 15 December 2017 1 of 4 1 Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA. 2 Research Resource Center (East), University of Illinois, Chicago, IL 60607, USA. *Corresponding author. Email: [email protected] on June 6, 2020 http://science.sciencemag.org/ Downloaded from
Transcript of CATALYSIS Synthesis of ultrasmall, homogeneouslyalloyed, … · Synthesis of ultrasmall,...
CATALYSIS
Synthesis of ultrasmall,homogeneously alloyed, bimetallicnanoparticles on silica supportsA. Wong,1 Q. Liu,1 S. Griffin,1 A. Nicholls,2 J. R. Regalbuto1*
Supported nanoparticles containing more than one metal have a variety of applicationsin sensing, catalysis, and biomedicine. Common synthesis techniques for this type ofmaterial often result in large, unalloyed nanoparticles that lack the interactions betweenthe two metals that give the particles their desired characteristics. We demonstrate arelatively simple, effective, generalizable method to produce highly dispersed, well-alloyedbimetallic nanoparticles. Ten permutations of noble and base metals (platinum, palladium,copper, nickel, and cobalt) were synthesized with average particle sizes from 0.9 to1.4 nanometers, with tight size distributions. High-resolution imaging and x-ray analysisconfirmed the homogeneity of alloying in these ultrasmall nanoparticles.
Bimetallic nanoparticles (NPs), anchored onporous carbon or oxide supports with highsurface area to prevent agglomeration, havemany applications in chemical sensing, bio-medicine, and especially catalysis (1, 2). The
most commonmethod to prepare supportedmetalNPs is impregnation (IMP), in which just enoughmetal precursor solution is used to fill the porevolume of the support. Although this method issimple and quick, IMP generally yields NPs withlarge average size, broad size distributions, and,in the case of multiple metals, inhomogeneousalloying. The highest possible metal utilizationoccurs when all atoms of a NP are accessible tothe gas or liquid fluid phase, and this occurswhenthe particle size is about 1 nm, or clusters of sev-eral dozen atoms (3, 4). Optimal bifunctionalityand synergy often occur when the atoms at theNP surface are well mixed (5, 6).We demonstrate a simple method that can be
applied to noble and base metals alike to syn-thesize ultrasmall (~1 nm) NPs with homoge-neous alloying. The method is based on strongelectrostatic adsorption (SEA), whereby chargedmetal precursors are strongly adsorbed onto op-positely charged oxide or carbon surfaces by con-trolling the pH relative to the surface point ofzero charge (PZC) (7–9). This interaction holdsthe precursors in place during drying, as opposedto IMP, in which the metal precursors remainand aggregate in solution as drying occurs. Therequired surface charge is provided by the nativeOH groups on the surfaces; no explicit surfacefunctionalization is needed. This method involvesfewer steps than colloidal methods of NP syn-thesis and eliminates the problematic removalof the capping agents of those methods, becauseno capping agent is used.
Electrostatic adsorptionhas beendemonstratedfor a host of single metals on many different sup-ports (7–17 ). Over composite oxide surfaces, pH-controlled adsorption has been used to achieveselective metal adsorption onto one surface com-ponent or the other (18–20). Most recently, therehave been preliminary attempts at simultaneouselectrostatic adsorption of two metal precursorsfor the synthesis of bimetallic NPs, involving Ptand Pd (21) or Pt and Co (22); this has beentermed simultaneous or co-SEA. This methodis schematized in Fig. 1A, which shows an elec-trostatically adsorbed layer of a mixture of hy-drated metal precursors, followed by a thermalreduction in H2 needed to remove the metal lig-ands, reduce themetals to their zero-valent state,and nucleate the atoms into NPs.We now present a systematic application of
this method using a common silica support witha variety of noble and base metal ammine pre-cursor pairs (Pt, Pd, Co, Ni, and Cu ammines)as shown in Table 1. The silica support (EvonikAerosil 300) has a PZC of 3.6, a value similar tothat of other fumed silicas (10, 11, 21). Adsorptionof metal ammine precursors was measured as afunction of pH for all pairs of metal complexes todetermine their relative affinities for the support(23). Uptake surveys for representative noble-noble (Pt-Pd), noble-base (Pd-Cu), and base-base(Ni-Co) metal pairs are shown in Fig. 1B. For thePt-Pd adsorption surveys, no adsorption of metalsoccurred near the PZC.As the pHwas increased, the OH groups of the
silica became deprotonated, which allowed thecationic Pt/Pd ammine precursors to adsorb. Atlower pH values, Pd had a greater affinity than Ptto the support; the opposite was true at higherpH values. Maximum adsorption of 1.3 mmol/m2
occurred at an optimum pH (pH ≈ 11.5) followedby a decrease in uptake as more base was added.This decrease in uptake is attributed to the strongionic strength in solution that creates a double-layer screening effect (9); the metal complexeswere prevented from experiencing the surface
charge. A similar trend was observed in the Pd-Cu adsorption survey, but the maximum adsorp-tion occurred at a higher pH value. Adsorptionuptake surveys of Ni- andCu-containing solutionswere kept above a pH of 11 to prevent precipita-tion. The base-base metal pair of Ni-Co (Fig. 1B)was consistent with the other uptake surveys.In all cases, the maximum adsorption of com-
plexes for complete monolayer coverage for thebimetallic systems was ~1.0 to 1.3 mmol/m2. Thisresult indicates that all of the metal complexesadsorbed with the same number of hydrationsheaths and were of similar size during adsorp-tion (12). The Co surveys with Pd, Ni, and Cu (Fig.1B and fig. S1) revealed competitive adsorption,in that the Co ammine favored the silica surface.Thehigh affinity of Co ammines for silica is knownand ultimately results in the formation of a newphyllosilicate phase (24, 25). Thus, the Co adsorp-tion was independent of pH in these experiments.The relative concentration of Co was lessened toallow the adsorption of the other metal to achievean atomic ratio of 1:1. All other metal pairs wereadsorbed onto the support in a 1:1 atomic ratio atthe monolayer capacity of the precursors; thiswas achieved by controlling the metal precursorconcentrations in solution (table S1).Temperature-programmed reduction (TPR)
was performed to characterize the intimacy ofalloying of the metals in the nascent NPs. Wecompared monometallic and bimetallic NPs pre-pared by SEA and IMP (see tables S1 and S2 forsynthesis parameters). TPR profiles for the Pd-Cuseries (Fig. 1C) showed that the monometallicPd in the IMP sample exhibited a low reductiontemperature of ~100°C but that the monometal-lic Pd in the SEA sample required 150°C for re-duction, indicating a much stronger interactionwith the silica support. The TPR for Cu IMP re-duction peaks began with a shoulder at ~190°Cand had a sharp peak at 230°C. The broad shoul-der of the IMP profile suggests a wide particle sizedistribution in which some of the larger particleswere more easily reduced. The TPR for the Cu SEAprofile contains a sharp peak reduction peak at220°C and a broader peak around 500°C.Among the bimetallic NPs, the Pd-Cu co-SEA
NPs (Fig. 1C, center) showed two distinct peaks.The first peak at ~180°C was from the reductionof a Pd-Cu alloy, and the second peak at 400°Cwas the monometallic Cu peak at 500°C shiftedto a lower temperature. We attribute the shift tolower temperature to hydrogen spillover fromPdto unreduced Cu complexes, indicating that theremust be close proximity of Pd-Cu atoms in theco-SEA–derived NPs. The bimetal Pd-Cu IMPNPscontained multiple reduction peaks between 80°and 250°C with distinct peaks that correspondedto the nonshiftedmonometallic Pd and Cu. Thus,unalloyed Pd or Cu existed in the IMP NPs.Further evidence of hydrogen spillover in co-
SEA bimetallic NPs is observed in the Pd-Co TPRprofiles (Fig. 1C, right). Although Co complexesare very difficult to reduce over silica and requiretemperatures of 700°C (20), alloying Pd-Co en-abled us to observe hydrogen spillover–assistedreduction of Co cations by Pd at a temperature of
RESEARCH
Wong et al., Science 358, 1427–1430 (2017) 15 December 2017 1 of 4
1Department of Chemical Engineering, University of SouthCarolina, Columbia, SC 29208, USA. 2Research ResourceCenter (East), University of Illinois, Chicago, IL 60607, USA.*Corresponding author. Email: [email protected]
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400°C. TPR profiles of additional metal combi-nations are shown in figs. S2 and S3. These ad-ditional metal combinations exhibited similarresults: The SEA NPs showed greater support in-teraction and commonly required a higher reduc-tion temperature than the IMP NPs. The co-SEANPs often had peaks shifted to lower temper-atures than the monometallic SEA analogs, in-dicative of hydrogen spillover–assisted reductionbetween the twometals. The IMPNPs containedbroader peaks andmore reducible species, whichare consonant with a larger particle size distri-bution and various NP compositions.After reduction in 10% H2 balanced in He for
1 hour at 400°C, x-ray diffraction (XRD) profileswere taken of each material. The XRD patternsof the co-SEA materials (Fig. 2A) showed no dis-tinct features between 30° and 45° 2q, the rangecontaining the most intense diffraction peaks.
The very broad peaks observable in some pat-terns, with respect to the metal-free supports,were the result of highly dispersed NPs. Thisinterpretation was later confirmed by electronmicroscopy. XRD patterns of the co-IMP NPs,reduced at the same conditions (Fig. 2B), showedstrong XRD peaks for all of the IMP metal com-binations. The intense XRD peaks between 30°and 45° 2qwere indicative of large NPs (i.e., x-raydiameters dXRD > 5 nm). Moreover, the non-Gaussian shape of the peaks, containing shoul-ders and sharp centers, suggests variable phasecompositions and a wide particle size distribu-tion, respectively.Electron microscopy Z-contrast images of the
co-SEA NPs (Fig. 2C) show that they existed asclusters ~1.0 to 1.3 nm in diameter with narrowparticle size distributions (standard deviationswere about 20 to 30% of average particle size).
This finding is consistent with the absence ofpeaks in the high-sensitivity x-ray diffractometer,whichhas a size limit of ~1 nm (26). In comparison,the microscopy images in Fig. 2D showed the co-IMPNPs tohave broader particle size distributionsand overall larger particle sizes (~5 to 25 nm). Adifference inNP shapewas also observed betweenthe two synthesis methods. The co-SEA NPs pro-duced more regular spherical particles, whereasmore frequent irregular shapes and particle ag-glomeration were observed in the co-IMP NPs(Table 2; see fig. S4 for particle size distributions).SEA with single metals typically yields particlesin the size range 1 to 2 nm (10); with co-SEA,the NPs were about the same size or smaller.Although the 1-nm NPs were too small for in-
dividual NP elemental mapping, we were able touse energy-dispersive x-ray spectroscopy (EDXS)in spot mode on multiple individual bimetallicNPs produced by co-SEA. These measurementscan help to determine composition consistencyby comparing the nominal weight ratios to theactual weight ratios determined through x-rayanalysis. TheNPswere prepared near a 1:1 atomicratio; thus, the weight percent (wt %) and weightratio of the two metals will vary depending ontheir molecular weights. Table S3 shows thespot analysis of the Pt-Pd NPs, which have anexpected weight ratio of 0.65 Pt:0.35 Pd. Theresults showed that the Pt-Pd NPs were consist-ent with the expected ratio. In all cases (tables S4to S12), the co-SEA NPs were bimetallic, withmost individual NPs being near the predictedwt % for each metal (23).EDXSmapping was performed over the larger
IMP NPs to determine the degree of alloying(figs. S5 and S6). Incomplete alloying occurredin most of the co-IMP bimetallic NPs, and theextent of alloying varied from particle to particle.The segregation of themetals was independentof the predicted structure obtained from phasediagrams, where most metal combinations doform alloys. This incomplete alloying agrees withXRD data of the IMP NPs, which showed multi-ple phase compositions.Finally, atomically resolved Z-contrast images
were taken to individually characterize the co-SEA NPs (Fig. 3). Careful examination of theseimages revealed a speckling effect, where ele-ments of higher atomic number are brighterand those of lower atomic number are dimmer.This phenomenon could be best observed in thesamples in which the two metals have large dif-ferences in atomic number. Figure 3, A and B,shows the clearest speckling of the Pt-Co andPt-Cu NPs; this is direct visual evidence of a well-mixed alloy. Other combinations of Pd-Ni, Pd-Cu,and Pt-Pd (Fig. 3, C to E) with lower differencesin Z also showed speckling, although less pro-nounced. The speckling effect has been reportedmost clearly in the case of solution-derived andsomewhat larger Pt/Pd and other bimetallic NPs(27). No specklingwas observed in the Cu-CoNPsbecause of the very small Z difference and poorcontrast; however, EDXS spot analysis of this sam-ple (table S11) reveals that this as well as the othertwo base metal combinations, Ni-Cu and Ni-Co
Wong et al., Science 358, 1427–1430 (2017) 15 December 2017 2 of 4
Table 1. Compositions (weight percent) of bimetallic NPs supported on silica.
Co Ni Cu Pt Pd
Co 0.86 Co 0.94 Ni/0.97 Co 0.70 Cu/0.66 Co 3.2 Pt/0.8 Co 0.7 Co/1.3 Pd.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
Ni 1.84 Ni 0.89 Cu/0.63 Ni 2.6 Pt/0.79 Ni 0.52 Ni/1.2 Pd.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
Cu 2.25 Cu 2.2 Pt/0.94 Cu 1.0 Cu/2.1 Pd.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
Pt 0.95 Pt 1.31 Pt/0.7 Pd.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
Pd 1.27 Pd.. .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. .
Fig. 1. SEA bimetallic adsorption schematic, adsorption density, and temperature-programmedreduction profiles. (A) Metal precursors electrostatically adsorb as a mixed monolayer. Clustersof alloyed NPs form after H2 reduction. (B) Adsorption surveys of noble and base metal pairs depictrelative affinities of metals as a function of pH on the support. (C) TPR profiles (downward peaks indicateH2 consumption) of the monometallic SEA NPs are at a higher temperature, indicating a strongersupport interaction relative to IMP nanoparticles. Bimetallic SEA NPs show improved reducibility fromH2 spillover over the monometallic analogs.
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(tables S9 and S10), are as thoroughly alloyed asthe other samples.Simultaneous SEA can be extended to other
surfaces such as alumina, titania, and carbon, aslong as soluble metal precursors are available; inthe current case, cationic ammine complexeswere adsorbed over a low-PZC silica surface. Overhigh-PZC surfaces such as alumina and carbon,anions such as chloride complexes can be used.The method is limited to the availability of suit-ably charged complexes. A potential drawbackwith the use of silica is the solubility of silica atthe high pHatwhich the adsorption is performed.Silica dissolution during SEA was seen to be ap-preciable above pH 11 and with 24-hour contacttimes (28). Dissolution can be circumvented byworking at a pH of 10.5 or below, where disso-lution is negligible (28), or can be minimized byusing short contact times, as metal adsorptionreaches equilibrium within minutes (4).Another limitation is the amount of metal that
can be applied in one co-SEA application. The nor-mal uptake of charged precursors (~1 mmol/m2,or ~1 complex per nm2) corresponds to a steric
maximum of close-packed complexes retainingone or two hydration sheaths (4). Metal loadingthus depends on the specific surface area of thesupport; for a support such as precipitated silica(200 m2/g), typical metal loadings are limited toa few wt %, whereas for materials with high sur-face area such as mesoporous silica (1000 m2/g),loadings of 10 to 15 wt % are possible in a singleSEA application (10). Higher loadings can beachieved by repeated applications of SEA andreduction [this has been demonstrated for Pt oncarbon (29)]; the reduction transforms the hy-drated, ligated precursors into metal nanoparti-cles and uncovers the support surface, ontowhichmore precursor can be subsequently adsorbed.Other methods for synthesizing highly dis-
persed,well-alloyedbimetallic nanoparticles exist.It is suggested that bifunctional organics thatchelate both a metal and a silica surface, demon-strated for single metals of ruthenium and irid-ium (30), can be applied to mixtures of metals(2). The method appears to be limited to silicaand is also limited to lowmetal loadings becauseof the exothermgenerated in removing the organic
Wong et al., Science 358, 1427–1430 (2017) 15 December 2017 3 of 4
Table 2. Average particle sizes ofbimetallic NPs. STEM sizes are number-
average diameters; XRD sizes are from theScherrer equation.
co-SEA STEM
(nm)
IMP XRD
(nm)
Pt-Pd 1.0 ± 0.2 9.4. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pt-Cu 1.2 ± 0.3 2.8*. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pt-Ni 1.1 ± 0.3 6.9. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pt-Co 0.94 ± 0.3 4.2. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Cu-Co 1.3 ± 0.3 7.3. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Cu-Ni 1.1 ± 0.3 14. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Ni-Co 1.4 ± 0.3 15.0*. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pd-Ni 1.1 ± 0.2 6.4. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pd-Co 1.0 ± 0.2 7.3. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
Pd-Cu 1.1 ± 0.2 14.5. .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ... ... .. ... .. ... ... .. ... ... .. ...
*2.8 nm is an average of three replicates (3.7,2.5, and 2.3 nm); 15 is an average of 15.9, 15.2,and 13.9 nm.
Fig. 2. Bimetallic NP size characterization after reduction in 10% H2 balanced in He for 1 hour at 400°C. (A and C) XRD patterns andSTEM images of co-SEA bimetallic NPs reveal extremely high dispersion. (B and D) XRD patterns and STEM images of co-IMP bimetallic samplesshow larger particles.
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chelating agent by oxidation (2). Alternatively,in limited instances, supported nanoparticlesmaybe derived from the decomposition ofmetastablebimetallic precursor species such as Keggin-typecomplexes continuing cobalt and molybdenum(31) or organometallic cluster complexes contain-ing platinum and iron (32). Finally, well-alloyed,size- and composition-controlled bimetallic nano-particles can be synthesized in the solution phaseby colloidal methods (33) such as a recent studyof 50 wt %, 4.5- to 6.5-nm PdPt nanoparticles oncarbon (34). The higher achievable weight load-ing of this approach may offset the complexity ofthe synthesis and the difficulty in distributing theNPs into the support. If, on the other hand, ultra-small, homogeneously alloyed bimetallic nano-
particles atmoderateweight loadings are desired,simultaneous SEA provides a more generalizableand straightforward synthesis strategy.
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ACKNOWLEDGMENTS
Supported by NSF grants CBET 1160023 and IIP 1464630. All dataare reported in the main text and supplement.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/358/6369/1427/suppl/DC1Materials and MethodsFigs. S1 to S6Tables S1 to S12
11 August 2017; accepted 13 November 2017Published online 23 November 201710.1126/science.aao6538
Wong et al., Science 358, 1427–1430 (2017) 15 December 2017 4 of 4
Fig. 3. Atomically resolved Z-contrast images showing NP speckling of alloys. (A and B) Pt-Coand Pt-Cu, separated by two rows in the table of elements. (C to E) Pd-Ni, Pd-Cu, and Pt-Pd,separated by one row. (F) Cu-Co, in the same row. Scale bars, 5 nm.
RESEARCH | REPORTon June 6, 2020
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Synthesis of ultrasmall, homogeneously alloyed, bimetallic nanoparticles on silica supportsA. Wong, Q. Liu, S. Griffin, A. Nicholls and J. R. Regalbuto
originally published online November 23, 2017DOI: 10.1126/science.aao6538 (6369), 1427-1430.358Science
, this issue p. 1427Sciencerelative to the surface point of zero charge. Their method was successful for a wide range of metal alloys.strong electrostatic adsorption, in which the metal precursors are strongly adsorbed onto the surface by controlling pH
1-nm-diameter nanoparticles can be made on silica supports. To do this, they exploited∼show that well-mixed bimetallic et al.bimetallic alloys, but typical synthetic methods yield larger nanoparticles where the metals are poorly mixed. Wong
they expose more active surface sites. The properties of metallic nanoparticles can also be improved by creating For applications of nanoparticles in sensing and catalysis, smaller nanoparticles are often more effective because
Dispersing small, bimetallic nanoparticles
ARTICLE TOOLS http://science.sciencemag.org/content/358/6369/1427
MATERIALSSUPPLEMENTARY http://science.sciencemag.org/content/suppl/2017/11/21/science.aao6538.DC1
REFERENCES
http://science.sciencemag.org/content/358/6369/1427#BIBLThis article cites 28 articles, 1 of which you can access for free
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