4-nitrophenol

Articlepubs.acs.org/JPCC Kinetic Analysis of the Catalytic Reduction of 4‑Nitrophenol by Metallic Nanoparticles Sasa Gu, Stefanie Wunder, Yan Lu, and Matthias Ballauff* Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner-Platz 1, 14109 Berlin, Germany Robert Fenger and Klaus Rademann Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany Baptiste Jaquet Institute for Chemical and Bioengineering, ETH Zürich, Wolfgang-Pauli-Strasse 10/HCI F123 ETH Zürich, CH-8093 Zürich, Switzerland Alessio Zaccone Physik-Department and Institute of Advanced Study, Technische Universität München, 85748 Garching, Germany S Supporting Information * ABSTRACT: We present a study on the catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by sodium borohydride (BH4−) in the presence of metal nanoparticles in aqueous solution. This reaction which proceeds via the intermediate 4-hydroxylaminophenol has been used abundantly as a model reaction to check the catalytic activity of metallic nanoparticles. Here we present a full kinetic scheme that includes the intermediate 4-hydroxylaminophenol. All steps of the reaction are assumed to proceed solely on the surface of metal nanoparticles (Langmuir−Hinshelwood model). The discussion of the resulting kinetic equations shows that there is a stationary state in which the concentration of the intermediate 4-hydroxylaminophenol stays approximately constant. The resulting kinetic expression had been used previously to evaluate the kinetic constants for this reaction. In this stationary state there are isosbestic points in the UV/vis-spectra which are in full agreement with most published data. We compare the full kinetic equations to experimental data given by the temporal decay of the concentration of Nip. Good agreement is found underlining the general validity of the scheme. The kinetic constants derived from this analysis demonstrate that the second step, namely the reduction of the 4-hydroxylaminophenol is the rate-determining step. ■ INTRODUCTION nanoparticles or for a given type of nanoparticle immobilized in different carrier systems. In recent years, the reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) by borohydride (BH4−) in aqueous solution has become such a model reaction that meets all criteria of a model reaction.10 It can be monitored easily with high precision by UV−vis spectroscopy.10−13 This is due to the fact that Nip has a strong absorption at 400 nm and the decay of this peak can be measured precisely as the function of time. Moreover, the reaction rate is small enough so that the conversion Metallic nanoparticles (NP) have been the subject of intense research during the recent years because of their potential use in catalysis.1−6 It is now well-established that even inert metals such as gold may become active catalysts when divided down to nanoscale.7,8 Very often, nanoparticles are attached to a suitable colloidal carrier for easier handling and in order to avoid potential hazards.9 However, these carrier systems may impede the activity of the nanoparticles for a given reaction. Comparing the catalytic activity of nanoparticles bound in various systems hence requires a model reaction, that is, a well-controlled reaction without side reactions.10 Kinetic data and rate constants obtained from such a reaction can be compared for different © 2014 American Chemical Society Received: June 18, 2014 Revised: July 22, 2014 Published: July 24, 2014 18618 dx.doi.org/10.1021/jp5060606 | J. Phys. Chem. C 2014, 118, 18618−18625 the reaction starts and after an intermediate period a stationary state is reached that may last for many minutes. we have three compounds that adsorb and desorb during the reaction cycle. no traces of azoxybenzene and the following products are found. namely 4-aminophenol (Amp). In the final step.19. a rearrangement of the surface atoms seems to be necessary to create catalytically active sites as. The present analysis aims at a quantitative understanding of the entire kinetics starting just after the delay time and the subsequent transition to the stationary state. Here we present a full kinetic analysis of the reduction of Nip in the presence of metal nanoparticles. The induction period t0 which 20 s in this case is marked with the black arrow.20 Hence. the reduction of Nip by BH4− is catalyzed by many other noble metals such as Pt.The Journal of Physical Chemistry C Article we have modeled this stationary state in terms of an apparent reaction rate kapp (see Figure 1). is the notable exception. namely the reduction of nitrobenzene is a well-studied reaction. Typical time dependence of the absorption of 4-nitrophenolate ions at 400 nm.21 These authors state that the catalysis of the reduction of Nip in the presence of gold nanoparticles is affected by a soluble species leaching from the metal nanoparticles.19 Ag. there is a delay time in which no reaction takes place. respectively.27 Recently. Following ref 19. 4-hydroxylaminophenol (Hx) and 4-aminophenol (Amp). and camp be the actual concentrations of Nip. we have analyzed the kinetics of this reaction in great detail.1021/jp5060606 | J. Let cNip. and n is the Langmuir− Freundlich exponent. The coverage θHx and θBH4 of 4-hydroxylaminophenol. can be conveniently monitored over several minutes. we have Figure 1. phenylhydroxylamine is reduced to aniline.15−17 we formulate the reaction in terms of the direct route15. The work of Nigra et al. 18618−18625 . which is the rate-determining step.org/10. Moreover. cHx.24 Ru. n was set to 0. There is an adsorption/desorption equilibrium for all compounds in all steps. one must postulate soluble species of all these metals with similarly high catalytic activity. as the function of time. Since the classical work by Haber. 118. The goal of this work is to develop a kinetic model for the entire dependence of the concentration of Nip on time and the comparison of this model with the experimental data given in ref 20.27 has given clear evidence that the reaction is proceeding at the surface. C 2014. e. θNip = (KNipcNip)n 1 + (KNipcNip)n + KHxcHx + KBH4c BH4 (1) where KNip. Its reduction to the final product. The analogous reaction.25.14 the various intermediates are well-known:15 In the so-called direct route nitrobenzene is reduced to nitrosobenzene and then to phenylhydroxylamine. The surface coverage θNip of Nip is modeled in terms of a Langmuir− Freundlich isotherm.g. The induction period t0 was related to a surface restructuring of the nanoparticles before the catalytic reaction starts..doi. The blue portion of the line displays the linear section. This compound is the first stable intermediate.19 Hence.16−20 This finding has been explained by a strong adsorption of all intermediates to the surface of the nanoparticles. and applies to reductions catalyzed by other nanoparticles as well. We consider the direct route15 that takes place on the surface.5.19. and KBH4 are the Langmuir adsorption constants of the respective compounds. the intermediates nitrosobenzene and phenylhydroxylamine react to form azoxybenzene which is reduced subsequently to aniline. Proposed mechanism (direct route) of the reduction of 4-nitrophenol by metallic nanoparticles: In step A. and of Amp. The concentration of this soluble gold species must be very low and its catalytic activity in turn very high.16 shown in Figure 2. Additional evidence is given in recent experimental work. At first. Phys. and of 18619 dx. namely 4-nitrophenol (Nip). We20 and others25 demonstrated that this rate constant can be fully evaluated in terms of a Langmuir−Hinshelwood kinetics: Both reactants. The first stable intermediate is the 4-hydroxylaminophenol as is well borne-out of the studies done on nitrobenzene. no byproducts can be detected. 4-hydroxylaminophenol. Practically all published studies agree on this point and assume that the catalysis takes place on the surface of the nanoparticles.22. The model is general. the recent study by Mahmoud et al. Chem. In our previous work.23 Pd. The only prerequisite is that all steps take place at the surface.28 Two intermediates may be Figure 2. We assume furthermore that all three compounds compete for a fixed number of surface sites on the surface of the nanoparticles. 4-nitrophenol (Nip) is first reduced to the nitrosophenol which is quickly converted to 4-hydroxylaminophenol (Hx). namely 4-nitrosophenol and 4-hydroxylaminophenol.25 and alloys. namely Nip and BH4− must be adsorbed on the surface to react. KHx. no follow-up products are considered. corners or edges on the surface. the theory presented in refs 19 and 20 only models the decay rate of Nip. identified. from which kapp is taken. However.13. Recent work has clearly revealed that in the presence of gold particles as catalyst the reduction proceeds only along the direct route.26 Hence. In the condensation route.20 Figure 1 shows a typical absorbance spectra measured ■ KINETICS In analogy to the well-studied case of the reduction of nitrobenzene. The presence of isosbestic points in the UV−vis spectra measured at different time gives clear evidence that Nip is fully reduced to the final product Amp.20 However. This kinetic model has met with gratifying success when compared to experimental data. takes place in step B.19. however.15−17 Thus. Subsequently. All reactions take place at the surface of the particles. Figure 2) termed A and B: First Nip is reduced to 4-hydroxylaminophenol in step A. it is assumed that the adsorption equilibrium between the solution and the surface of the catalyst is established quickly. eq 11). 18618−18625 . Here the concentration of 4-hydroxylaminophenol is approximately constant. This can be seen directly from the fact that the initial rate of reaction as determined from the tangent in Figure 1 for t > t0 is much larger than the tangent in the stationary state. it will more and more compete with nitrophenol for the surface places of the (5) In this approximation. The reduction of the latter compound is done in step B. The black line shows the concentration of 4-nitrophenol as the function of time whereas the red dashed line corresponds to the concentration of 4-hydroxylaminophenol. Moreover. where 4-nitrophenol is reduced to 4-hydroxylaminophenol. respectively. a strong adsorption of Amp on the surface of the particles would strongly influence the kinetics of the reaction. This is not observed in any study so far. that is. In principle. In case of strong adsorption. it is evident that kA ≫ kB. Given these assumptions and prerequisites. This procedure leads to the concentration of nitrophenol cNip as the function of time that can directly be compared to experimental data. A full solution of the kinetic problem thus defined consists in the simultaneous solution of eq 3 and 4 which must be done numerically. This can be argued from the fact that its formation slows down the rate of reaction while not accumulating in solution. Figure 2) and may be approximated by eq 7.org/10. Stage II. Figure 2) and decay (step B. Chem. cf. ranging from t0 to a time ts: Here cHx ≈ 0 and − 18620 dcNip dt ≈ kaS (KNipcNip)n KBH4c BH4 [1 + (KNipcNip)n + KBH4c BH4]2 (6) dx. the reaction rate should decrease markedly since more and more places are blocked by Amp. Phys. As a tacit assumption in the entire LH kinetics. in first approximation a simple solution may be found: After the initial state. The decay rate in this stage is approximated by kapp. However. Moreover. 4-hydroxylaminophenol is formed rather quickly but its further reduction in step B is much slower.II that can be approximated by eq 8. This concentration equals the decay of the concentration of 4-nitrophenol at time tS. the stationary concentration cHx. 118. the reaction rate for step A follows as − dcNip dt = kaS Figure 3. it is evident that 4-hydroxylaminophenol is very strongly adsorbed to the surface of the nanoparticles. at least in the final state when the concentration of Amp has risen. (KNipcNip)n KBH4c BH4 [1 + (KNipcNip)n + KHxcHx + KBH4c BH4]2 ⎛ dc ⎞ = ⎜ Hx ⎟ ⎝ dt ⎠source (3) nanoparticles and thus slow down the rate of reaction. The early stage I. cf. the total number of adsorbed molecules is much smaller than the total number of molecules of a given species in solution. the full rate equation for the generation and decay of the intermediate 4-hydroxyaminophenol is given by ⎛ dc ⎞ ⎛ dc ⎞ dcHx = ⎜ Hx ⎟ − ⎜ Hx ⎟ ⎝ dt ⎠source ⎝ dt ⎠decay dt = kaS − kbS (KNipcNip)n KBH4c BH4 [1 + (KNipcNip)n + KHxcHx + KBH4c BH4]2 KHxcHxKBH4c BH4 [1 + (KNipcNip)n + KHxcHx + KBH4c BH4]2 (4) dcHx =0 dt Equations 3 and 4 now constitute a set of coupled rate equations that allows us to discuss the entire kinetics of the reaction. n = 1. C 2014. This equation follows directly from the fact that SθNip is proportional to the number of all adsorbed molecules in the system while θBH4 denotes the conditional probability to find an adsorbed surface hydrogen atom near to an adsorbed Nip molecule. When its concentration rises quickly in the early stage of the reaction. Idealized time dependence of the concentration of 4-nitrophenol and definition of the different stages of the reaction (see the discussion of eqs 5 to 10). Hence. we may postulate a stationary state in which The intermediate 4-hydroxylaminophenol thus generated is further reduced to the final product Amp in step B and its rate of decay may be formulated through ⎛ dc ⎞ −⎜ Hx ⎟ ⎝ dt ⎠decay = kbS KHxcHxKBH4c BH4 [1 + (KNipcNip)n + KHxcHx + KBH4c BH4]2 = dcamp dt Hence. Hence.stat of this compound follows from the balance of its generation (step A.The Journal of Physical Chemistry C Article borohydride. In the following we give a brief qualitative discussion of these equations: First of all. the reaction kinetics after t0 may be divided into two regimes depicted schematically in Figure 3: (i) Early regime. are formulated using the classical Langmuir isotherm.doi. The present model does not consider the adsorption/ desorption equilibrium of the final product 4-aminophenol. is mainly determined by step A of the reaction (see Figure 2) while the concentration of the final product 4-aminophenol is still small. the rate of reaction of Nip follows as − dcNip dt ⎛ dc ⎞ = kappcNip = kaSθNipθBH4 = ⎜ Hx ⎟ ⎝ dt ⎠source (2) where S denotes the total surface of all nanoparticles in the solution. is the stationary state characterized by kapp.1 given by eq 6. Hence.1021/jp5060606 | J. The reaction is now modeled in two steps (cf. we disregard this possibility in the present model. that is. In this idealized picture. starting at time tS (cf. adsorption on the surface of the catalyst does not shift the concentration in the system in a detectable way. 18618−18625 . the tangents of the absorbance as the function of time. C 2014. that is. The solid lines refer to the fits by the kinetic model. Fit of the concentration of Nip as the function of time by the numerical solution of eq 3 and 4. − dcNip dt = kaS = (KNipcNip)n KBH4c BH4 ⎡ n ⎢⎣1 + (KNipcNip) 1 + ( ka kb ) ⎤ + KBH4c BH4 ⎥ ⎦ kapp . I = kaS 2 (KNip)n (cNip)n − 1KBH4c BH4 [1 + (KNipcNip)n + KBH4c BH4]2 (9) where tS is the time where stationary state starts (see Figure 3). the constants kapp. the amount of aminophenol generated per unit time is exactly given by the decay rate of nitrophenol. Phys. In this stationary state. are given for the two limiting cases by (ii) Stationary state (t > ts) in which cHx is approximately constant (see Figure 3): eq 5 leads to the condition that cHx .doi.0. dcamp dt (II) Stationary state for t > ts: (8) 18621 dx. the condition for the isosbestic point is restored.1021/jp5060606 | J.The Journal of Physical Chemistry C Article Figure 4. Chem.org/10. no isosbestic point can be expected since the spectra of 3 compounds varying with time are superimposed. Hence. The experimental data have been taken from ref 20 and refer to a temperature of 10 °C (data points with error bars). If the stationary concentration of the 4-hydroxylaminophenol is small. stat = ka(KNipcNip)n kbKHx (7) (I) Early regime from t0 to ts: Thus. 118. In this regime. The concentration of Nip was normalized to the respective starting concentration cNip. th as the function of time for a given values of KNip.0. The concentration of Nip was normalized to the respective starting concentration cNip.0 being the concentration of nitrophenol at t = 0. Changing one parameter at one time. In all cases.th match the corresponding experimental data sets cNip. stat cNip . The calculation is repeated until most of calculated data of cNip. The Matlab routines were used to calculate the theoretical Nip concentration cNip.stat should be given in good approximation by the amount of nitrophenol that has reacted at t = ts. Now the reduction of 4-hydroxylaminophenol becomes the rate-determining step. I = [1 + (KNipcNip . 18618−18625 .19.0 which may be approximated through ts − t0 = cHx . The values of ka and kb were changed while keeping KNip. First.doi. I(ts − t0) cNip . It is interesting to compare this result to the previous version of theory that did not take into account explicitly the intermediates. With this simplification we obtained for the stationary state (see eq 3a of ref 19 or eq 5 of ref 20) kapp . n KNip (cNip)n − 1KBH4c BH4 [1 + (KNipcNip)n + KBH4c BH4]2 (10b) which differs from eq 10a only by a factor 1 + ka/kb in the denominator. Routine I calculates cNip. cNip. C 2014. Third. II = kaS Figure 5.exp. the delay time t0 has been subtracted as discussed previously. Thus. ka. Then every theoretical cNip.org/10. Chem. Fit of the concentration of Nip as the function of time by the numerical solution of eq 3 and 4. that is. Evidently. Hence. In this case. Figure 3 summarizes all stages of this kinetic scheme together with temporal evolution of the concentrations of all reactants expected from this model. cNip. These data are compared to the experimental results and the constants are changed until agreement with the experiment is reached. 4-hydroxylaminophenol reacts much faster than 4nitrophenol. KHx. The experimental data have been taken from ref 20 and refer to a temperature of 30 °C (data points with error bars). KHx. Phys. The concentration of Nip normalized to the respective concentration cNip. and the reaction is slowed down due to the additional factor in the denominator of eq 10b. with cNip.20 were used as a ■ RESULTS AND DISCUSSION Figure 4 and 5 display examples of the fits of the experimental data at temperature of 10 °C (Figure 4) and 30 °C (Figure 5) obtained by a simultaneous numerical solution of eq 3 and 4. This routine can only analyze one cNip.0 at t = 0 is plotted as the function of time for different initial concentrations of Nip and BH4−. KHx. kb.0)n + KBH4c BH4]2 kbSKBH4c BH4KHx (11) Equations 7 and 11 give the predictions for the onset of the stationary state. so the values of ka and kb were reoptimized using MatLab routine II (see Supporting Information). was then analyzed by a numerical solution of eq 3 and 4 by two MatLab routines (see the full sheets in the Supporting Information). ■ NUMERICAL SOLUTION OF THE KINETIC EQUATIONS All data analyzed here have been taken from ref 20. eq 10b is a good approximation and the reduction of 4-nitrophenol is the rate-determining step. KBH4. ka. Second.20 The concentration of Nip as the function of reaction time. and n. the error bars of these parameters were also checked by MatLab routine II. all cNip.The Journal of Physical Chemistry C kapp .exp to check whether the value was within the error bars. and n obtained by routine I constant until full agreement was reached. kb and n.1021/jp5060606 | J. There are two limiting cases that can be derived from eq 10a: (i) ka ≪ kb. KBH4. KBH4. 118. In the following we give the details of this procedure. we get for the time ts where the stationary state has been reached cNip . The adsorption constant KHx does not appear in eq 10a because of the stationary state condition of eq 5.th as the function of time was compared to the corresponding experimental data cNip. stat ln = − kapp .th for a given set of values of KNip. ka.0kapp .exp. The parameters from ref.The solid lines refer to the fits by the kinetic model. kb) may be different at different initial reaction concentrations. KBH4.exp data obtained for a given temperature were put into MatLab routine I (see the Supporting Information). II = kaS Article n KNip (cNip)n − 1KBH4c BH4 ⎡ n ⎢⎣1 + (KNipcNip) 1 + ( ka kb )+K ⎤2 BH4c BH4 ⎥ ⎦ (10a) The concept of a stationary state leads immediately to the conclusion that the stationary concentration of 4-hydroxylaminophenol cHx. we assume that the subsequent conversion to the Hx has not taken place to a notable degree.th was compared to the corresponding experimental data cNip. (ii) ka ≫ kb. the reaction rate of steps A and B (ka. To ensure a 18622 dx.exp at one time. and n. the consumption of the Hx intermediate cannot be measured directly and the fit values for kb and KHx are less precise to get from this fit than the other parameters.0 − cHx .exp. kb. first input for KNip. Chem.1 ± ± ± ± 0. Figure 6 gathers the reaction rates of steps A and B derived from fitting at 10 and 30 °C.4 1. In the Langmuir−Hinshelwood model. such as Hx. the reaction will be slowed down. the model assumes the strict validity of the Langmuir adsorption isotherm which may not be fully valid anymore when going to higher concentration of 4-nitrophenol. at least in the early stage up to conversions of ca. The dashed lines give the average value of the constants. Obviously.g.6 2. Moreover.5 150000 160000 175000 200000 10000 15000 20000 25000 intermediates compete for free places at the surface of Au nanoparticles.9 2.7 11.doi. Moreover. all curves are plotted up to a conversion of 30%.2 9.1021/jp5060606 | J. In general. the agreement of theory and experiment may be regarded as satisfactory. Evidently. Partial hydrolysis of BH4− is an unavoidable side reaction.5 KNip [L/mol] 2700 3700 4600 5200 ± ± ± ± 500 900 1200 1500 KBH4 [L/mol] 30 50 62 86 ± ± ± ± 2 4 6 10 KHx [L/mol] n ± ± ± ± 0.5 0. The rate constants ka and kb scatter around a mean values indicated by a dashed line in Figure 6. C 2014. Table 1 gathers the resulting constants. The solid lines are the fits by theory.9 average kb [10‑5mol/m2 s] 2. Given the various uncertainties of the analysis. Kinetic constants ka and kb obtained from the comparison of theory and experiment for (a. It is clear that the early stage and the transition to the state II which is clearly seen for the data taken at 10 °C (Figure 4) are wellmodeled by the kinetic scheme given in Figure 2. The much larger value of KHx proves that intermediate hydroxylamine is much stronger adsorbed on the surface of the nanoparticles than the other components.5 0. the substituted azoxybenzenes. For this reason the accumulation of Hx slows down the apparent reaction rate when the reaction approaches stage II. b) 10 and (c.9 1. Figure 6 shows that kb is much smaller than ka. The resulting fit parameters are plotted in Figure 6 and are summarized in Table 1. Table 1. and will shift its concentration during the measurements. The results of other fits taken at different concentrations and temperatures are given in the Supporting Information. the adsorption constant of intermediate Hx is considerably greater than that of the other components. KBH4 and KHx is capable of describing the experimental data at a given temperature.4 9. the reactants and 18623 dx.9 3.8 7. 30%.org/10. Phys. The formation of the latter compounds requires the presence of a sufficient concentration of meaningful comparison. a single set of constants KNip. These deviations are clearly seen at longer reaction time. cNip.1 5. respectively. This strong adsorption of Hx on the surface of the particles precludes the formation of other products as e.The Journal of Physical Chemistry C Article Figure 6.6 7. 118. and the reaction can occur only between species adsorbed on the surface. Constants Derived from the Fits of the Measurements at Different Temperatures temp [°C] 10 20 25 30 average ka [10‑4mol/m2 s] 4. in particular at higher temperatures.7 1. It should be noted that the constant kb is derived in an indirect fashion since the experiment measures only the decay of Nip. the reduction of Hx is rate-determining step of the reaction and the accumulation of Hx on the surface slows down the reaction when stage II is reached. 18618−18625 .5 ± ± ± ± 0. d) 30 °C. If most places are occupied by a single species.th deviates from cNip.5 0.exp more for higher Nip concentrations. and Hx ΔH[kJ/mol] ΔS[J/mol K] KNip KBH4 KHx 24 ± 3 150 ± 12 37 ± 2 158 ± 6 10 ± 1 133 ± 3 ΔH ΔS + RT R Table 2 summarized the value of thermodynamic parameters. the concentration of this main intermediate is rising steadily throughout the time 300 s used for the evaluation of the data. 18618−18625 .3 kJ/mol. Summary of Enthalpy and Entropy Values of the Adsorption of Nip.The Journal of Physical Chemistry C Article Figure 7. the adsorption constants KHx of 4-hydroxyaminophenol (c). The enthalpies and entropies for the adsorption of Nip. Calculated concentrations of Nip and Hx as the function of time.1 ± 3. ■ DISCUSSION OF THE STATIONARY STATE ASSUMPTION Figure 8 displays the temporal evolution of the concentration of 4-hydroxylaminophenol as calculated from the numerical solution in comparison to the decay of 4-nitrophenol. BH−4 and Hx can be obtained from the dependence of the adsorption constants on temperature through ln K = − Table 2. the adsorption constants KBH4 of borohydride (b). which ln(cNip) varies linearly with time. All adsorption processes are endothermic. Here the compound with larger adsorption constants has smaller enthalpy.org/10. 118.1021/jp5060606 | J. For the range in Figure 8. respectively. The activation energy for the reduction of Nip to Hx obtained from an Arrhenius plot of ka. BH4−. is 36. 18624 dx. The initial concentrations of Nip and BH4− are 0. Figure 7 displays that the adsorption constants and reaction rate ka increase with an increasing temperature which can be determined as a function of temperature. the nitroso. After reaching the maximum the concentration of 4-hydroxylaminophenol decreases slowly. Phys. Quite evidently. Chem. C 2014. The ΔH and ΔS of the adsorption process of Nip and BH4− are larger than those obtained from previous version of theory20 that did not take into account the intermediates. Dependence of the adsorption constants KNip of Nip (a).04 mM and 5 mM. But clearly the full kinetic scheme as developed here is superior and leads to a more consistent description of all the data and should be preferred for the analysis.and the hydroxylamino compounds which is not the case. the reaction rate of step A ka (d) on the inverse of temperature.doi. the assumption of a stationary state may be regarded as satisfactory. 41. 45−55.Ballauff@helmholtz-berlin. Science 2006. M. (8) Zhang.. Platinum. ■ ASSOCIATED CONTENT S Supporting Information * Tables of parameters from the simulation by MATLAB and optimized ka and kb values and figures showing the fits of the concentration of Nip and kinetic constants ka and kb. Meijboom. Technol. F... 2012. P... Catal. 14.. Rev.. J. 89. P. Langmuir 2013.. Phys. 7404−7407..acs. A. W. Elektrochem. Chem. 116. Marvin. M. (21) Nigra. Y. (2) Astruc. E. Sano. 237−243. Size-Dependent Hydrogenation of p-Nitrophenol with Pd Nanoparticles Synthesized with Poly(amido)amine Dendrimer Templates. R. 2008. 22.. Garlyyev. J. Lopez. Silver Nanoparticle Catalyzed Reduction of Aromatic Nitro Compounds. 2976−2983. Kirmse. Szczerba. (20) Wunder.. Kinetic Analysis of Catalytic Reduction of 4-Nitrophenol by Metallic Nanoparticles Immobilized in Spherical Polyelectrolyte Brushes. G. M. W. Johnston.. B.. Albrecht.. 14. M.. 2013. G. X. E. 117. M. R. Facile Synthesis of Silver Nanoparticles Stabilized by Cationic Polynorbornenes and Their Catalytic Activity in 4-Nitrophenol Reduction. J.doi. and the MATLAB routine. H. Yoshimura.. 4594−4604.. Sasaki.. 22644−22651. Nishio-Hamane. J. Kobayashi. B... 2467−2505. Serna. Nano-Gold Catalysis in Fine Chemical Synthesis.. An isosbestic point is predicted for the stationary state which is observed indeed for the great majority of the published experimental data. Chem. A.. ChemCatChem. Rodenbusch. 13433− 13442. L. B. Shirai. 105. C. A.. 2013..20 was only capable of describing the stationary state. Giacomelli... Chemoselective Hydrogenation of Nitro Compounds with Supported Gold Catalysts. (15) Blaser. N. Albrecht. (26) Kaiser.. J. C 2013. Phys. (6) Carchini. Phys. H.. S.. Phys. R. Identification of Site Requirements for Reduction of 4-Nitrophenol Using Gold Nanoparticle Catalysts. H. A. ■ AUTHOR INFORMATION Corresponding Author *(M. El-Sayed. Chem. K. 2011. Chem. Riesemeier. Chem. 112. N. This material is available free of charge via the Internet at http://pubs. K.. Chem. Chem. 2012. 313. Breu. (12) Esumi. 332− 334. 6487−6495. N. C. N.Z. M. M. Serna. Ballauff. D. the analysis of the stationary state used in earlier analysis of this reaction can be derived directly from this model. Wiley-VCH: New York. Novel Gold Catalysts for the Oxidation of Carbon Monoxide at a Temperature far Below 0 °C. D. Chem. Y. the entire temporal evolution of the concentration of Nip can be described while the earlier approach19. Deng. CONCLUSION A kinetic scheme for the reduction of Nip by BH4− catalyzed by metal nanoparticles in aqueous solution has been presented. Yang. Crystallogr..1021/jp5060606 | J. O. Phys.. (3) Ferrando. 2007. (7) Haruta. Phys. F... (24) Johnson. 2013. Nanoparticles and Catalysis.. ACS Catal. Chem.. N. E. F.. Lett. 18625 dx. A. and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol. 119. Reinholz. 405−408. M. Langmuir 2013.. 5577−5587. Somorjai. Welz. K. Y. Chem. Dzubiella.. J. Pal. A: Physicochem. Liz-Marzan. Catalytic Activity of Faceted Gold Nanoparticles Studied by a Model Reaction: Evidence for Substrate-induced Surface Restructuring.. 117. Green Chem. C 2010. (28) Haas. L. L. Pal. Phys. Chemistry and Properties of Nanocrystals of Different Shapes.The Journal of Physical Chemistry C ■ Article (10) Herves. Radtke. (25) Antonels. K.org. M. Almora-Barrios. B. Fertitta. 3164−3174. Chem. Jellinek. (18) Layek. 114.. M. Phys. J. Lunkenbein. (16) Corma. Y.. Size and Shape Control of Metal Nanoparticles for Reaction Selectivity in Catalysis. Good agreement between theory and experiment is found. Catalysis by Metallic Nanoparticles in Aqueous Solution: Model Reactions. (19) Wunder. P. Z. Sci. J. The analysis is based on the reaction shown in Figure 2: 4-nitrophenol is first reduced to 4-hydroxylaminophenol which subsequently is reduced to the final product 4-aminophenol. C 2013. Leppert. M.. Notes The authors declare no competing financial interest. A. Lu. L. Gold Nanoparticles Stabilized on Nanocrystalline Magnesium Oxide as an Active Catalyst for Reduction of Nitroarenes in Aqueous Medium at Room Temperature. 2012.. 1262−1272. 1025−1102. 118. M. Chem. 196. 2012. J. 14. Ballauff. Signori.) E-mail address: Matthias. Ballauff. 46. Phys.. A. 247−257. 8814−8820... T. Rev. 2013. C. 108. Synthesis and Catalytic Properties of Silver Nanoparticle−Linear Polyethylene Imine Colloidal Systems. Gabriel. Kantam... Chen. Chem. S. Kümme. Lu. ■ ACKNOWLEDGMENTS A. H. (13) Fenger. J. M. Chem. F. El-Sayed.. C 2012. Wunder. P. Moreover. (22) Santos. Concepcion. H.. gratefully acknowledges financial support of the IAS at TUM via the Moessbauer Fellowship. A. Rev.de. Ha. R. M. S. U. K. Y. Mei. 2012.. Y. Akbashev. F.. Rademann. J. N. J.... J. Domingos. Lu. Rev. Fertitta. Faraday Discuss. 29. A. Eng.. M. Colloid Surf. A Different Reaction Pathway for the Reduction of Aromatic Nitro Compounds on Gold Catalysts. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Stevenson. Cui. Narayanan. R. 312−313.. J... M. 2005. S. Phys. In particular. (5) An. S.org/10. Chem.. M. 1987. ■ REFERENCES (1) Henglein. A Golden Boost to an Old Reaction. 18618−18625 .. Chem. Science 2006. 4225−4234. A. 506−514. 313. Isono. J. Blonski. 2012. Chem. 2002. The kinetic scheme leads to the coupled differential eqs 3 and 4 that upon numerical solution describe the decay of Nip with time. Y. 1512−1524. Catal.. 1898. Booher.. K. M. T.. R. 20... Thünemann. 1861−1873. Albrecht. Y. (23) Baruah. Elias.. Catalytic Activity of Nanoalloys from Gold and Palladium. C. Wanderka. 908−916. Preparation of Well-Defined Dendrimer Encapsulated Ruthenium Nanoparticles and Their Evaluation in the Reduction of 4-Nitrophenol According to the Langmuir−Hinshelwood Approach. 4. (14) Haber. Asp. Small-particle Research: Physicochemical Properties of Extremely Small Colloidal Metal and Semiconductor Particles. 1353−1360. 845−910. (11) Pradhan. Determining the Mechanism of Solution Metallic Nanocatalysis with Solid and Hollow Nanoparticles: Homogeneous or Heterogeneous. T. H. Angew. A. F. X. M. Perez-Lorenzo. E. The Structure of AuPd Nanoalloys Anchored on Spherical Polyelectrolyte Brushes Determined by X-ray Absorption Spectroscopy. Size Dependent Catalysis with CTAB-stabilized Gold Nanoparticles. A. Shi. J. 21886−21893. T.. U. Rev. 2008. (4) Burda. How Theoretical Simulations Can Address the Structure and Activity of Nanoparticles. Lu. S. Ballauff. Polzer.. 162. 9343−9349. Gradual Electrolytic Reduction of Nitrobenzene with Limited Cathode Potential. (9) Kaiser.. R. K. 1989. Angew. T. (17) Corma.. Polzer. Maheswarana. Preparation of PAMAM− and PPI−Metal (Silver. K. 1.. 29. Katz. P. Yamada. Top. J. J. Ballauff. M.... H.. M. C 2014.B.. Langmuir 2004. 3. (27) Mahmoud. Rademann.. Appl. 14. 56. Lu. M... Soc.. Cascade Catalysis of Highly Active Bimetallic Au/Pd Nanoclusters: Structure− function Relationship Investigation Using Anomalous Small-angle Xray Scattering and UV−Vis Spectroscopy. G. Makis. Fenger.