catalysis at the nanoscale has attracted 
 significant attention 
 in the past two decades due to the unique properties of materials 
 that arise at the nanoscale . following 
 the discovery by valden et al .
of catalytically 
 active gold nanoclusters , gold , in particular , has become the basis 
 for novel catalysts due to its activity at the nanoscale .
metal nanocatalysts 
 have found a wide range of applications including co oxidation , carbon nanotube nucleation , alcohol dehydrogenation , and 
 formic acid electrooxidation . in particular , 

nanocatalysts have played an extensive role in fuel cell catalysts 
 by reducing oxygen and oxidizing methanol .
theoretical tools have allowed for the prediction 
 of novel catalysts for oxygen reduction , olefin / paraffin separations , and ammonia synthesis .
bimetallic nanoparticles are particularly 
 useful due to the additional 
 degrees of freedom  composition and structure  that may 
 be adjusted in order to improve catalytic behavior .
bimetallic catalysts 
 frequently display catalytic activity that is higher than either constituent 
 material .
for instance , two epitaxially grown overlayers of pd on 
 au have been predicted to bind co stronger than either pure metal .
similar effects have been exploited in the ni / au 
 bimetallic system in order to design a steam reforming catalyst with 
 a higher efficiency .
the interplay 
 between geometry and electronic structure has been 
 extensively studied both theoretically and experimentally for bimetallic 
 systems in order to determine how these factors affect the catalytic 
 properties of these materials .
the role 
 of electronic structure in metals , specifically the d - band center , was first 
 modeled by newns and later expanded upon 
 with first principles calculations by hammer and nrskov .
this model , which relates the d - band center with binding energy , has been applied to co adsorption 
 on bimetallic systems , hydrogenation 
 of olefins by bimetallics , nitrobenzene 
 hydrogenation by bimetallic nanoclusters , and many other surface reactions .
first - order surface reactions , 
 which are of particular importance 
 for catalysis , are typically constrained to a linear relationship 
 between adsorption energy and reaction rate .
polanyi ( bep ) relation , has been demonstrated to fundamentally constrain many surface reactions .
the bep relation predicts that strong adsorption 
 energies to the surface result in faster reaction rates for first - order 
 surface reactions . in practice ,
the turnover frequency of a 
 catalyst is limited in 
 both the weak binding and strong binding regimes . in the strong binding 
 regime where the bep relation shows the smallest reaction barriers , 
 the catalytic performance is limited by the desorption of products . 
 in the weak binding regime , the reaction rate is limited by the large 
 activation energies predicted by bep .
an optimal catalyst will balance 
 these two regimes such that the barriers are low enough to be overcome 
 and that the adsorbed molecules may diffuse and desorb from the surface .
the balance between these two regimes is described graphically 
 by the so - called volcano plot .
the volcano 
 plot , a scatter plot of turnover frequency as a function of adsorption 
 energy , peaks at the center and avoids both the weak and strong binding 
 limits . by using bimetallic dens ,
we demonstrate that a catalyst for p - nitrophenol ( pnp ) reduction may be designed by tuning 
 den composition and structure in order to bind the product molecule 
 with an adsorption energy closest to the peak in the volcano plot .
pnp , as with other nitrophenols and derivatives , is a common byproduct 
 from the production of pesticides , herbicides , and synthetic dyes .
coinage metals , 
 in particular , have been demonstrated to be excellent catalysts for 
 pnp reduction at the nanoscale .
bimetallic 
 coinage metal nanoparticles have been demonstrated to catalyze pnp 
 reduction with rates that strongly differ from a simple linear interpolation 
 between the rates of the two pure metals .
these types of bimetallic nanoparticles may be easily synthesized 
 in solution with dendrimer encapsulation methods that allow for precise 
 control over particle size and composition .
experimentally , pnp has a strong absorption at 400 nm , which allows for an easy and reliable means 
 to measure reaction rates by uv  vis spectroscopy .
the overall 
 reaction rate is pseudo - first - order when both the metal catalyst and 
 nabh4 are in excess .
the reduction 
 of pnp is , thus , an ideal model system for demonstrating the bep relation 
 for pseudo - first - order reactions and the tunability of bimetallic 
 nanoparticles in order to produce an optimal catalyst .
when nabh4 is in excess , the rate is controlled 
 by the adsorption of pnp .
experimental 
 approach in 
 order to tune the reaction rate of pnp reduction by adjusting the 
 composition of the metal nanoparticle catalysts .
in particular , we 
 show that the bep relation accurately represents this type of multistep , 
 pseudo - first - order reaction and that bimetallic dens can be used to 
 approach the peak in the resulting volcano plot .
this systematic study 
 demonstrates that theoretically predicted adsorption energies onto 
 simple bimetallic surfaces provide a simple and accurate predictor 
 for catalytic reaction rates in pseudo - first - order reactions .
we note 
 that when the lattice constants of the constituent metals of the bimetallic 
 nanoparticles are similar , the resulting properties are essentially 
 a linear combination of the properties of the two pure metals . in 
 contrast ,
the synthesis of aucu , pdcu , and 
 ptcu dens was reviewed by yeung and crooks .
preparation of ptau bimetallic particles by 
 cocomplexation in g6-oh dendrimers has not been previously reported , 
 but the uv  vis spectra presented here match those for bimetallic 
 ptau dens prepared by lang et al . using 
 galvanic exchange .
nabh4 was purchased 
 from emd , 
 haucl4 was purchased from acros organics , cuso4 was purchased from mallinckrodt , and both k2pdcl4 and k2ptcl4 were purchased from alfa 
 aesar .
all solutions were prepared with water acquired from a barnstead nanopure 
 diamond uv deionization system , > 18.2 mohmcm ( denoted as 
  nanopure 
 water  ) . g6-oh
pamam dendrimer ( 13.22% w / w in methanol ) was 
 a donation from dendritech , inc .
each dendrimer solution 
 was prepared by evaporating the methanol from 5.46 l of g6-oh 
 pamam dendrimer in a 20 ml vial ( i - chem ) with septum cap .
the dendrimer 
 was hydrated with enough nanopure water to result in a final volume 
 of 10 ml and a final dendrimer concentration of 1 m after the 
 addition of metal salt , the ph adjustment , and the addition of reductant . 
 for a 55 molar excess of metal - to - dendrimer , 5.5 l of 0.1 m 
 metal salt was added .
complexation time was dependent on the metal salt 
 used : haucl4 was complexed for < 10 min , cuso4 for 15 min , k2pdcl4 for 3060 min , 
 and k2ptcl4 for 72 h. dendrimer 
metal 
 complexes were reduced with a 10 molar excess of nabh4 to 
 metal ( except au , for which only 12 molar excess was used ) 
 by adding freshly prepared 0.2 m nabh4 .
the ph of the reduced 
 nanoparticle solution was adjusted to < 8 using 0.1 m hcl .
the dens were characterized via 
 uv  vis spectroscopy prior to and immediately after reduction .
bimetallic dens were synthesized as described above , except only 
 a 28 molar excess of each metal was added ( 2.8 l of 0.1 m metal 
 salt ) .
the longer complexing metal salt was added first , the ph was 
 adjusted to 67 , and then at the appropriate time before reduction , 
 the second metal salt was added .
vis spectra were 
 collected using a spectral bandwidth of 1.0 nm with an agilent 8453 
 uv  vis diode array spectrophotometer and agilent chemstation 
 software .
fresh 0.2 m nabh4 was 
 prepared 10 min before each set of kinetic trials .
polystyrene cuvettes 
 were prepared by adding 2.35 ml of nanopure and 0.250 ml of 600 m 
 pnp . an ocean optics visible spectrophotometer was set to monitor 
 the absorbance at 400 nm ( the max for pnp ) and 600 
 nm ( for background ) by sampling three times per second .
the spectrophotometer 
 was placed on a stir plate , and a microstir bar was added to the reaction 
 cuvette . immediately after initiating data collection , 0.750 ml of 
 0.2 m nabh4 was added to the cuvette using a micropipet 
 ( gilson ) and 0.600 ml of 1 m den solution was added to the 
 cuvette via gastight syringe .
the minimum 
 absorbance at 400 nm was subtracted from all absorbances to correct 
 for background absorption from the dens .
the natural log of the absorbance 
 at 400 nm was plotted against time , and the steepest part of the curve 
 was fit with a line , the negative slope of which was considered the 
 apparent rate constant , kapp .
the rate 
 constant is considered apparent because previous work by other groups 
 has shown that the observed rate constant depends on the concentration 
 of sodium borohydride as well as the starting concentration of pnp .
all calculations 
 were performed 
 by density functional theory ( dft ) within the vienna ab initio simulation package . the kohn 
sham single - electron 
 orbitals for the valence were expanded in a plane - wave basis with 
 an energy cutoff of 300 ev .
the exchange - correlation ( xc ) energy was treated by the pw91 implementation 
 of the generalized - gradient approximation .
the interpolation 
 of the xc term was performed using the method of vosko et al .
adsorption was considered on the 
 ( 100 ) face of a 79-atom nanoparticle in a truncated octahedron geometry , 
 which was found to be the most stable for pt by both total x - ray scattering 
 experiment and dft calculations .
this 
 face is the most undercoordinated and taken to be the most active 
 for catalysis .
the pnp molecule was then allowed to adsorb onto the 
 surface such that the total force per atom was less than 0.01 ev /  . 

this bonding geometry is shown for a sample pd / au random alloy in 
 figure 1 .
for each bimetallic system , ten random 
 50/50 alloys by composition were generated ; geometries that significantly 
 distorted from the truncated octahedron were removed .
chosen binding geometry 
 for pnp chemisorption on a 79-atom pd / au 
 random alloy nanoparticle .
at least 8  of vacuum was included to separate periodic 
 images 
 in order to prevent spurious image  image interaction .
for density 
 of states ( dos ) calculations , the bulk lattice constants calculated 
 by wang et al . were used for a cell that 
 consisted of five layers of the 3  3 face - centered cubic unit 
 cell .
the brillouin zone for the unit cell was integrated using a 
 4  4  1 monkhorst 
adsorption energies were 
 determined by calculating the difference 
 between the total energy of the relaxed nanoparticle  pnp chemisorbed 
 complex and the total energies of the two constituent pieces : the 
 nanoparticle and the pnp molecule .
thus , the adsorption energy represents 
 the energy difference between taking the pnp molecule from infinite 
 distance and allowing it to chemisorb onto the nanoparticle surface . 
 the binding energy , which has the opposite sign from the adsorption 
 energy , represents the energy required to separate the bound pnp molecule 
 from the nanoparticle and move it to infinite distance .
in order to accurately compare reaction rates 
 for different catalysts , 
 a systematic and uniform method for both synthesizing the catalyst 
 and maintaining similar reaction conditions is required . through an 
 encapsulation synthesis route using the g6-oh pamam dendrimer , we 
 are able to reliably synthesize nanoparticles of similar size and 
 with known composition .
this synthetic route produces nanoparticles 
 of either pure metals or bimetallic random alloys with a 50/50 composition . 

figure 2 shows scanning transmission electron 
 microscopy ( stem ) images and size distribution histograms for the 
 synthesis of au , pd , and pd / au nanoparticles .
as seen in figure 2 , dens of au , pd , and 
 pd / au have very similar average diameters on the order of 1 nm . the 
 dens are spherical in shape ; given the size distributions and shape , 
 the nanoparticles contain 55 atoms . as noted previously , the 
 experimental conditions are nearly uniform for each pure metal and 
 bimetallic nanoparticle synthesis route . because of these tight experimental 
 controls , we can directly measure catalytic properties of the resulting 
 particles under both uniform synthesis and reaction conditions .
the reduction of pnp in the presence of nabh4 is fast 
 when performed in solution with coinage metal catalysts .
the overall 
 reaction of interest is shown in scheme 1 . 
 the full catalytic reaction , however , is a multistep process that 
 is not characterized by a single saddle point from which an arrhenius 
 rate may be extracted . in the presence of excess nabh4 and 
 sufficient catalyst ,
the reaction is pseudo - first - order and dependent 
 only upon the concentration of pnp in solution .
the role of the metallic 
 catalyst is to bind the pnp molecule through the two oxygens of the 
 nitro group .
this adsorbed geometry is shown in figure 1 , where a single molecule binds to two atoms of a ( 100 ) surface 
 and creates a pentagonal m  o  n  o  m cyclic 
 intermediate .
pnp is a strong visible absorber with a maximum 
 absorbance at 400 
 nm .
as shown in figure 3 , the reduction of 
 pnp to p - aminophenol ( pap ) is evidenced by a decrease 
 in absorbance at 400 nm and a new absorbance growing in at 315 nm 
 associated with formation of pap . because the reaction is pseudo - first - order 
 in the presence of excess nabh4 , the slope of a plot of 
 the natural log of the absorbance at 400 nm yields the apparent reaction 
 rate , kapp .
this process for determining 
 apparent rates is shown in figure 3 for the 
 case of the pd / cu alloy .
thus , we show that the reaction rates can 
 be easily determined by ultraviolet  visible ( uv  vis ) 
 spectroscopy . stem images of dens ( a ) g6-oh(au55 ) , ( b ) g6-oh(pd55 ) , and ( c ) g6-oh(pd28au28 ) and corresponding 
 size distributions .
the average diameter of the au , pd , and pd / au 
 nanoparticles was 1.4  0.7 , 1.0  0.3 , and 1.1  0.3 
 nm , respectively .
images were collected on a jeol 2500se stem . because of the sole dependence 
 of the rate on the concentration 
 of pnp in solution , the initial adsorption of the pnp molecule to 
 the nanoparticle surface directly correlates with the rate constant 
 for the full reaction .
binding of organic molecules to metal surfaces 
 has been extensively modeled beginning with newns and later expanded upon by hammer and nrskov . because p - nitrophenol is insoluble in water ,
the resulting nitrophenolate is then reduced via sodium borohydride 
 in the presence of the metal catalyst .
the 
 hammer  nrskov model identifies the center of the d - band of the metal surface as the primary controlling factor 
 in chemisorption strength .
when the adsorbate interacts with the metal d - band , the adsorbate state overlaps with the metal states 
 and is split off into bonding and antibonding interactions . as the d - band becomes farther below the fermi energy , the antibonding 
 states are increasingly populated , and the overall chemisorption strength 
 weakens .
a scatter plot of the d - band against the 
 adsorption energy is shown in figure 4 . from figure 4 , a general trend of increasing 
 adsorption strength with a d - band center closer to 
 the fermi energy
a further trend , however , is that adsorption 
 energy decreases as one moves down columns of the periodic table . 

this effect has been noted by hammer for 
 oxygen adsorption onto noble metals . for the coinage metals ,
the d - band is nearly filled , so many antibonding states are 
 populated .
the population of bonding and antibonding states is net 
 repulsive , which increases with orbital size as one moves down a column . 

this increasingly repulsive interaction for the coinage metals accounts 
 for the weaker adsorption onto pt and pd rather than cu , despite the 
 differences in d - band centers . when disparate 
 metals are alloyed , charge rearranges due to the 
 disparate fermi levels that are brought into contact .
because of charge 
 rearrangement , noble metal atoms become less noble and reactive metal 
 atoms become less reactive . in addition , alloying 
 of metals from different rows on the periodic table has been demonstrated 
 to activate surfaces by introducing localized electronic states .
for instance , small amounts of au alloyed into 
 previously inert ag(111 ) surfaces allow it to bind ethylene .
the 5d electrons of the au atoms overlap poorly with the 4d of the ag surface and create localized states where ethylene 
 may bind .
( a ) shows the decreasing absorbance at 400 nm corresponding to decreasing 
 pnp concentration ; spectra are shown at 1 s intervals .
( b ) shows the 
 plot of the natural log of the absorbance at 400 nm versus time ; data 
 points are separated by 0.5 s intervals .
the slope in ( b ) yields the 
 apparent rate constant kapp = 0.12  
 0.03 ( s ) for this trial .
g6-oh(pd28cu28 ) dens were used for this trial . because of the direct dependence of the reaction rate constant 
 on the binding energy , our goal is to optimize the reactivity of bimetallic 
 dens by combining metals which are inactive due to strongly binding 
 the reactants with metals that are inactive due to weakly binding 
 the reactants .
this relationship is typically demonstrated via a volcano 
 plot , which is a scatter plot of the reaction rate in the presence 
 of the catalyst against the adsorption energy of the reactant to the 
 catalyst . in principle , overly strong binding renders the adsorbate 
catalyst 
 so strongly bound that a reaction can not occur . in the regime of overly 
 weak binding , the catalyst has no interaction with the reactant .
we consider chemisorption onto the ( 100 ) face of the truncated 
 octahedron np as a representative site for chemisorption onto the 
 synthesized nps in solution .
chemisorption onto the ( 100 ) face is 
 possible in both truncated octahedron and cuboctahedron geometries ; 
 effects from undercoordination are likely to be smaller than 0.1 ev . in figure 5 , we present 
 a volcano plot of calculated adsorption energies of the pnp molecule 
 onto the various nanoparticle catalysts plotted with the experimentally 
 measured reaction rates for pnp reduction . in this plot ,
, 
 au has a low reaction rate constant due to its weak binding , whereas 
 cu has a very low rate constant due to its overly strong binding . 

pt and pd fall between these extremes . a scatter plot of the d - band center of the metal 
 surface vs the adsorption strength of p - nitrophenol 
 shows a general increasing binding interaction with a d - band center closer to the fermi energy .
the d - band 
 center is a weighted average of the local dos for a single surface 
 atom .
one effect that is immediately 
 noticeable in figure 5 is that alloying au 
 with either pt or pd results in a stronger 
 binding energy and yields a correspondingly faster reaction rate constant . 

when 
 pd is alloyed with cu , the adsorption energy is much stronger than 
 as compared to pure pd and is followed by a slower reaction rate . 
 somewhat surprisingly , then , is the behavior of pt / cu , which has a 
 higher reaction rate than either pure pt or pure cu .
even worse , au / cu 
 would seem to have an average adsorption energy that sits very near 
 to the peak in the volcano , and yet it has a reaction rate that is 
 very close to pure au .
the solution to this seeming paradox 
 lies in the distribution of 
 the adsorption energies for different configurations of the bimetallic 
 alloys .
cu is much smaller than either pt or au , so the lattice constant 
 mismatch compresses the atoms in the system .
similarly , the 3d cu valence orbitals overlap very weakly with the 5d orbitals of pt and au .
the net result is that configurations 
 in which pnp is bound to two cu atoms is bound quite strongly and 
 is bound weakly when au or pt is present .
the range in the binding 
 energy data is shown in figure 6 , which shows 
 that pd / cu displays a unimodal distribution centered around the average 
 of pd and cu , whereas au / cu displays binding energies that correspond 
 to either binding strongly on two cu atoms or weakly when au is present . a scatter 
 plot of the calculated adsorption energy of pnp on various 
 nanoparticles plotted against the experimentally observed reaction 
 rates .
solid black dots are pure metals , and red dots are random alloys 
 of two metals .
the solid lines are best - fit lines for the strong- 
 and weak - binding regimes .
the dashed lines represent the range of 
 calculated binding energies for various allowed structures that were 
 considered .
in addition , the fact 
 that alloying with cu weakens the adsorption 
 energy on pt and au atoms compared with the pure is also explained 
 by the d - band model .
when a metal with a large lattice 
 constant is compressed by alloying with a smaller metal , the orbital 
 overlap between its atoms increases .
the effect of this compression 
 is that the d - band center shifts away from the fermi 
 level ; this effect results in decreased reactivity at the site of 
 compression .
because of this decrease 
 in d - band center , the larger metal atoms become less 
 likely to form a covalent bond with an adsorbate .
this effect is demonstrated 
 by the fact that two pt atoms bind pnp weaker in the alloy than in 
 the pure nanoparticle .
more pronounced , two au atoms bind pnp in a 
 pure nanoparticle significantly stronger than in the alloy , where 
 the two au atoms are forced closer together .
thus , the effect 
 on the reaction rate is significant . a change 
 in adsorption energy results in an exponential change in the reaction 
 rate .
thus , the system reacts at the rate of its most active component 
 along the volcano plot .
the molecule is too strongly adsorbed on cu , 
 so the reaction rate is controlled by the orders of magnitude faster 
 rate associated with binding at an au  cu two - atom site . in 
 the case of pt / cu ,
two pt atoms bind pnp weaker than two pt atoms 
 in a pure metal nanoparticle ; thus , the reaction rate is faster than 
 pure pt . the ensemble binding energies , represented by the two data 
 points , are less significant to predicting the overall reaction rate 
 than the spread in the data and the maxima and minima of adsorption 
 energies in the set of random alloys .
histograms of the adsorption energies 
 of pnp on nanoparticles of 
 pd / cu and au / cu are shown .
the y - axis of the histogram 
 is the number of nanoparticles in the randomly generated sample of 
 bimetallic alloys of the specified composition , and the x - axis is the binding energy of pnp .
the pd / cu distribution is centered 
 around a single ensemble average for binding energy ; however , the 
 au / cu binding energy has two peaks that correspond to the pure cu 
 site and the au  cu site .
in particular , we show that for metals of similar 
 lattice constants , one can alloy a metal that binds the target molecule 
 too strongly with one that binds it too weakly to yield a more active 
 particle .
in contrast , when the lattice constants are disparate , the 
 results do not follow this trend . instead ,
the resulting adsorption 
 energies are broadly distributed , so the overall ensemble average 
 of binding energy can be completely uncorrelated from the observed 
 reaction rate . in this case
, catalysts with a specific geometry at 
 the binding site are the most active and dominate the reaction rate . 

the result is that two metals that bind too strongly can alloy to 
 create a significantly more active catalyst , even though the average 
 binding energy appears to be unfavorable .
in 
 summary , we have presented a systematic method for the synthesis 
 of uniform pure and bimetallic nanoparticles using a dendrimer - encapsulation 
 synthesis route .
this systematic synthesis method leads naturally 
 into controlled , uniform reaction conditions for measuring catalytic 
 activity of pnp reduction in presence of sodium borohydride . in this 

manner , reaction rates for similar particles under the same reaction 
 conditions can be accurately measured and compared to binding energies 
 from electronic structure calculations . we demonstrate that the reaction 
 rates fall along a standard volcano plot . in order to selectively 
 tune the reaction rate toward the maximum 
 in the volcano plot , we produced several bimetallic catalysts .
for 
 alloys of metals with similar lattice constants , the result was perfectly 
 intuitive  the ensemble binding energy was the average of the 
 two metals . in this case , the reaction rates fell along the volcano 
 plot as expected .
in contrast , metals with dissimilar lattice constants  pt / cu 
 and au / cu  did not fall along the volcano plot as predicted . 

pt / cu catalyzed the reaction much faster than expected from the ensemble 
 binding energy , and au / cu catalyzed it much slower than expected . 

we demonstrated that the reaction rate was not controlled by the ensemble 
 binding energy , but by the binding energy of the most active metal 
 atoms , whose binding energies were quite different from the ensemble 
 average . overall , we have demonstrated a joint experimental  theoretical 
 method for evaluating catalysis by bimetallic and pure dens and analyzed 
 the factors , such as lattice mismatch , that determine whether or not 
 the properties of a bimetallic will be a simple average of the constituents 
 properties .
 we 
 demonstrate that the reduction of p - nitrophenol 
 to p - aminophenol by nabh4 is catalyzed 
 by both monometallic and bimetallic nanoparticles ( nps ) . 
 we also demonstrate 
 a straightforward and precise method for the synthesis of bimetallic 
 nanoparticles using poly(amido)amine dendrimers . 
 the resulting dendrimer 
 encapsulated nanoparticles ( dens ) are monodisperse , and the size distribution 
 does not vary with different elemental combinations . 
 random alloys 
 of pt / cu , pd / cu , pd / au , pt / au , and au / cu dens were synthesized and 
 evaluated as catalysts for p - nitrophenol reduction . 
 
 these combinations are chosen in order to selectively tune the binding 
 energy of the p - nitrophenol adsorbate to the nanoparticle 
 surface . following the brnsted 
  evans  polanyi ( bep ) 
 relation , 
 we show that the binding energy can reasonably predict the 
 reaction rates of p - nitrophenol reduction . 
 we demonstrate 
 that the measured reaction rate constants of the bimetallic dens is 
 not always a simple average of the properties of the constituent metals . 
 
 in particular , dens containing metals with similar lattice constants 
 produce a binding energy close to the average of the two constituents , 
 whereas dens containing metals with a lattice mismatch show a bimodal 
 distribution of binding energies . 
 overall , in this work we present 
 a uniform method for synthesizing pure and bimetallic dens and demonstrate 
 that their catalytic properties are dependent on the adsorbate s 
 binding energy . 
Introduction
Methods
Results
and Discussion
Conclusions