Preparation of Pt-containing bimetallic and trimetallic catalysts using continuous electroless deposition methods

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Preparation of Pt-containing bimetallic and trimetallic catalysts using continuous electroless deposition methods ⁎

Gregory Tate, Adam Kenvin, Weijian Diao , John R. Monnier

⁎

Department of Chemical Engineering, University of South Carolina, Columbia, SC, 29208, USA

A R T I C LE I N FO

A B S T R A C T

Keywords: Electroless deposition (ED) Bimetallic catalysts Trimetallic catalysts Platinum Gold Copper

Electroless deposition (ED) is a highly effective method for synthesis of bimetallic catalysts. However, limitations from ED bath stability often exist with this method using the conventional batch method of preparation. Continuous ED is a synthesis procedure in which the secondary metal salt and reducing agent are added separately at controlled rates such that the rates of electroless deposition and addition to the ED bath are similar. Thus, ED reagent concentrations do not accumulate to a level where spontaneous reduction of the secondary metal salt to nanoparticles occurs. Pt@Au/SiO2 and Pt@Cu/SiO2 bimetallic catalysts have been synthesized by controlled deposition of Cu2+ and AuCl4− salts on a 2% Pt/SiO2 to demonstrate the benefits of continuous deposition. Continuous ED has been extended to deposition of two metal salts onto a third supported metal surface to presumably form a bimetallic shell layer of uniform composition. A series of different Pd@Pt-Cu/C catalysts has been made by simultaneous deposition of Cu2+ and PtCl62- salts on carbon-supported Pd. The transformation of ED from a batch to continuous process has permitted greater control over the ED reaction and solved problems with bath instability of highly reducible metal salts. Formation of trimetallic catalysts is also very straightforward using continuous ED methods.

1. Introduction Bimetallic catalysts are of interest to the industrial and academic communities due to their unique properties. However, despite the large volume of literature surrounding this class of catalysts, methods for their preparation remain relatively limited. The most commonly used methods for large, industrial scale preparations are co-dry impregnation or sequential dry impregnation (DI) [1]. These methods, however, lack the ability to direct the location of the two metal salts. Because the effectiveness of bimetallic catalysts relies on positioning both metallic components in proximal contact with each other, the catalysts prepared by these methods are often not bimetallic, and in fact, may consist of monometallic particles of each metal as well as bimetallic particles of unknown and widely-varying compositions. To effectively prepare these catalysts, a method must be used that ensures that only bimetallic particles are generated. Monnier [2–5] has developed the method of electroless deposition (ED) for the formation of true bimetallic catalysts, which is a variation of the method of electroless plating, often used for plating surfaces with metallic films when electroplating cannot be used. This methodology has been used to prepare a wide variety of bimetallic particles for use as both chemical and electrochemical catalysts, including Pd@Cu [6,7], Pt@Ag [8], Pd@Pt [9], Co@Pt [3], ⁎

Pd@Ag [7,10,11], Pd@Au [7,10], Ru@Pt [2,12,13], Pt@Ru [12], Ni@ Pt, Ir@Pt, and Ir@Ag [14]. Electroless deposition makes use of the catalytic activity of a core metal to oxidize an aqueous phase reducing agent to provide the site for deposition of a second, reducible salt [15,16]. A schematic of electroless deposition is shown in Fig. 1. This scheme shows activation of a suitable reducing agent (RA) on a core metal surface (A) providing an active site (denoted for convenience as Hads) where the second metal salt Bn+ is deposited as a metal atom. The number of metal surface sites (A) is typically determined by H2 chemisorption and is used to calculate fractional monolayers or multiples of complete layer assuming a deposition stoichiometry of B : A = 1. Because activation of the reducing agent on a pre-existing metal site is required, reduction and deposition on the catalyst support does not occur. Repetition of this cycle results in the deposition of more atoms of metal B to form either a shell of B, or, if autocatalytic deposition of Bn+ on metal B occurs, a three-dimensional aggregate of metal B. This is, of course, possible since in most cases the reducible metal salt is also catalytic for activation of reducing agents. Thus, catalytic vs autocatalytic deposition is controlled by the kinetics of deposition of each metal salt. The most important of these kinetic expressions are shown in Table 1. The first equation represents the unwanted thermal reduction of the

Corresponding authors. E-mail addresses: [email protected] (W. Diao), [email protected] (J.R. Monnier).

https://doi.org/10.1016/j.cattod.2018.12.041 Received 6 July 2018; Received in revised form 12 December 2018; Accepted 19 December 2018 0920-5861/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Tate, G., Catalysis Today, https://doi.org/10.1016/j.cattod.2018.12.041

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Fig. 1. Schematic representation of electroless deposition. Commonly used reducing agents (RA), primary metals (A), and secondary metals (B) are also shown. Both metals A and B can typically be used interchangeably.

a monometallic catalyst and soluble metal salt(s) and a liquid phase, organic reducing agent, all in the proper concentrations. We have chosen this latter methodology used by those in the electroless plating literature for several reasons. Firstly, not all metals are capable of dissociative H2 chemisorption at room temperature (for example Au is not capable of H2 chemisorption, but is catalytically active for activation of hydrazine; secondly, the first row VIII metals (Fe, Co, and Ni) and Cu and Ag in the Group 1B metals are passivated in air and aqueous environments; the presence of a strong reducing agent in the ED bath keeps these surfaces in a reduced and catalytically-active state. Finally, addition by a microprocessor-controlled syringe pump of a solution of a reducing agent provides precise control over the concentration of reducing agent in the continuous ED bath. In this communication we use only hydrazine (N2H4) at basic conditions, since it is a “clean” reducing agent forming mainly N2 and H2O as the oxidation byproducts. For N2H4 at basic conditions, the catalytic oxidation can be written as

Table 1 Equations governing ED. Rthermal rxn = ko exp(-Ea/RT) [CRA]x [CM salt]y Rcat dep = ko’ exp(-Ea’/RT) [CRA]a [CM salt]b [CA sites]c Rautocat dep = ko” exp(-Ea”/RT) [CRA]d [CM salt]e [CM sites]f Rcat dep, M2 = ko’’’ exp(-Ea/RT) [CRA]a [CM2 salt]b [CA sites]c

(1) (2) (3) (4)

reducible metal salt in the ED bath. Since all ED baths are and must be thermodynamically unstable (positive redox potential of the RA and secondary metal ion), they must be kinetically stable for a long enough period of time to be used during ED. Thus, the primary catalytic surface must induce kinetic instability to give catalytic deposition (Eq. (2)) or autocatalytic deposition (Eq. (3)). The deposition of two salts simultaneously requires the addition of another equation to describe the deposition of the additional metal (M2) salt (Eq. (4)). Reduction potentials of the secondary metal ions and oxidation potential of N2H4 used in this work are shown in Table 2. The value of potentials in Table 2 are standard reduction potentials at 298 K, 1 atm pressure, and 1 M concentration of reducing agent (N2H4, in this study). Standard reduction potentials are used as a guideline to understand how to choose an appropriate reducing agent and precursor salts. In addition, reduction potentials calculated at reaction conditions of 298 K, 1 atm pressure, pH 9, and the actual concentrations of N2H4 in our experiments are included in Table 2 to better reflect the continuous ED bath environment. The reduction potentials change with pH and N2H4 concentrations, but the general trend and kinetic and thermodynamic favorability do not change at our experimental conditions and there is a large thermodynamic force favoring chemical reduction. Reduction can also be achieved by bubbling H2 through an aqueous slurry containing a monometallic catalyst to saturate the metal surface with chemisorbed H atoms and then adding a soluble reducible salt, as demonstrated by Barbier [17–19] or by combining an aqueous slurry of

N2 H4 + 4OH− → N2 + 4H2 O + 4e− with Eo (at pH 11) = + 1.16V.

The designation of the reducing species on the core metal can be arbitrarily expressed as M-H, where the adsorbed H is not defined as either an adsorbed H atom or adsorbed H−. In either case, the oxidation products are H+ and one or two e−. It is likely there is no single reducing species, since Group IB metals (Cu, Ag, and Au) are unable to catalytically dissociate H2 at room temperature. Because of this, little detail [15,16,20] is typically given to describe the oxidation of RA on metal surfaces. It is also known that reducing agents such as N2H4 are notoriously inefficient and much H2 is produced at basic conditions; bubbling of H2 during ED is always observed; thus, excess reducing agent is added during electroless plating [15]. Indeed, for hydrazine, although theoretically four electrons are available for metal reduction, a RA : Mz+ molar ratio ≥ 2 is typically used. To explain the generation of H2 during ED with hydrazine, Eq. (5) can be broken down into two separate equations: N2H4 → N2 + 2H2

Table 2 Standard reduction potentials (1 M concentrations) for Au and Cu cations relevant to this study. The oxidation potential for hydrazine is also shown. Because standard conditions were not used for ED experiments, the adjusted potentials are also shown for pH 9 and ambient conditions. also shown in lower portion [15,21,22]. Half reaction

Standard Potential

Potential at pH = 9

AuCl4− + 3e− > Auo + 4Cl− Cu2+ + 2e− > Cuo [PtCl6]2− + 4e− > Pto + 6Cl− N2H4 + 4OH− > N2 + 4H2O + 4e−

+1.0 V +0.34 V +0.718V +1.16 V

+0.820 V +0.065V +0.541V +0.865V

(5)

2H2 + 4OH

−

(6)

→ 4H2O + 4e

−

and combined with the reduction of Cu 2Cu

2+

+ 4e

−

→ 2Cu

(7) 2+

to give 2Cu

o

o

(8)

the combination of Eqs. (6)–(8) gives the overall redox Eq. (9). N2H4 + 4OH− + 2Cu2+ → N2 + 4H2O + 2Cuo

(9)

After selection of appropriate reducible salts and a RA to minimize thermal instability, the kinetic expressions in Table 1 indicate the best way to restrict thermal reduction and control the rates of catalytic and 2

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difference of what was added by syringe pump and the amount of metal salt in solution.

autocatalytic deposition is to operate at lower temperatures and minimal essential concentrations of RA and the reducible metal salt. For deposition of Aun+ salts, which are highly unstable in the presence of virtually all RA, it is necessary to use highly stable Au(CN)2− salts (E°red = -0.60 V) which are highly toxic and are used only by commercial electroless plating companies [16,20]. One of the goals of this study was to develop a stable ED bath that used AuCl4− as a stable Au source for bimetallic catalysts. Further, since most previous work in our laboratories have used batch methods [7], the concentrations of RA and metal salts are front-loaded into the bath, meaning bath instabilities are highest even before ED begins. To circumvent this and to make ED more attractive for large scale use, continuous ED becomes much more essential. By adding controlled concentrations of a metal salt and RA from two independent syringe pumps at rates comparable to the rate of ED, neither metal salt nor reducing agent concentration accumulates in the ED bath, which minimizes thermal reduction. It also permits more options during ED. For example, reducing agents can be changed part way during ED to change the relative rates of catalytic vs autocatalytic deposition. Thus, the RA might be changed from N2H4 to formaldehyde (HCHO) at a particular deposition time to shift RA activation to a Cu or Au surface rather than at the surface of a platinum group metal, since HCHO is more rapidly activated on Cu than Pt surfaces [22]. In this communication we detail the development of continuous ED for the deposition of highly unstable AuCl4− on Pt surfaces. We also describe the use of closed-loop deposition using a UV–vis spectrometer as a continuous monitor for deposition of reducible metal salts. Finally, to further demonstrate the versatility of continuous ED we show initial data for the co-deposition of two metal salts, Cu2+ and PtCl62- on a Pd surface, simply by adding a third syringe pump for addition of the second salt. For the co-deposition of Cu and Pt, results suggest formation of a uniform distribution of both Cu and Pt in the deposited layer to give a true alloy composition.

2.2. Catalyst characterization Catalysts were characterized by hydrogen chemisorption and powder x-ray diffraction (XRD). Prior to chemisorption, all catalyst samples were reduced in situ at 300 °C in 10% H2/bal Ar. Temperatures were ramped at 10 °C/min and held for 1 h before being cooled to 40 °C in flowing Ar. Pulse chemisorption analyses were performed using a Micromeritics Autochem 2920 Analyzer. For determination of Pt coverage by Au for Pt@Au catalysts, the reduction in H2 uptake was used to determine fractional coverage of Pt by Au atoms, assuming deposition occurred at a 1:1 atomic ratio of Au to Pt [23]. The same methodology was used for the Pt@Cu series of catalysts to determine Cu coverage of Pt. However, as shown in the Results and Discussion section, diffusion of Cu into the Pt bulk made it difficult to determine actual coverage of Cu on Pt [24,25]. Powder x-ray diffraction (XRD) was performed using a Rigaku Miniflex-II outfitted with a high sensitivity D/teX Ultra silicon strip detector and a Cu-Kα radiation source (k = 1.5406 Å). Scans were run from 20°-70° 2θ values. XRD deconvolution and background stripping employed fityk 0.9.8 software [26]. The Scherrer equation was used to estimate average particle size from XRD analysis [27,28]. 2.3. Synthesis of Pt@Au/SiO2 bimetallic catalysts The base Pt catalyst used for ED of KAuCl4•xH2O (Alfa Aesar 99% metal basis) was a commercial 2 wt% Pt/SiO2 supplied by BASF, with a dispersion of 28.5% giving an average particle size of 4.0 nm; particle size and dispersion of Pt particles were determined by H2 chemisorption. The Pt/SiO2 catalyst was added to 150–190 ml DI water and the pH adjusted to 9–9.5 by a NaOH solution. The reducing agent selected for this reaction was dilute, aqueous hydrazine (N2H4) (Aldrich 35%wt in H2O) also adjusted to pH 9 – 9.5. Basic pH conditions were used to prevent strong electrostatic adsorption of AuCl4− on the SiO2 surface and also because N2H4 is a stronger RA at basic conditions [22]. The recirculation pump was started and spectra from 200 to 800 nm were recorded (every 200 s) before syringe addition of N2H4 or AuCl4−. Ethylenediamine (en) (Alfa Aesar 99%) was also added to the AuCl4− solution before being added to the syringe-pump at a molar ratio of [en]/[AuCl4−] = 5:1 to help ensure stability of AuCl4− upon exposure to N2H4. Ethylenediamine chelates with Au3+ [29,30] to lower the reduction potential from +1.0 V to +0.31 V (Table 1). When ED was conducted, both syringe pumps were activated and pumping continued for 60 min at a molar addition ratio of [N2H4]/ [Au3+] = 2:1. After 60 min, the syringe pumps were stopped and the ED bath allowed to recirculate for another 60 min. During the 2 h time of experiment, aliquots of the ED bath were taken in 5, 10, or 15 min intervals and analyzed by AA over the course of the reaction to monitor the concentration of AuCl4− remaining in the ED bath; analysis was performed by either AA or ICP-OES. After the reaction period was completed, the catalyst was allowed to settle, then decanted to remove excess liquid. It was then washed with excess DI water before being dried at ambient conditions until it was free flowing. Finally, it was dried in a vacuum oven at 120 °C for several hours before storage in a bottle.

2. Experimental methods 2.1. Continuous ED apparatus The ED bath consisted of a beaker set inside a water bath for temperature control. The bath typically contained 200 ml of DI water at pH 9 and either 2 wt% Pt/SiO2 or 5 wt% Pd/C (both catalysts supplied by BASF). Two (three for co-ED of two metal salts) programmable, microcontroller syringe pumps (New Era NE-300) capable of feed rates ranging from 1.2 μl/min to 25 ml/min were used to accurately add the N2H4 reducing agent and metal salt for ED. In the case of co-deposition of a second metal salt, a third syringe pump was added to the system. A closed recirculation loop (Fig. 2) consisting of 1/8” Teflon tubing, a peristaltic pump, and flow through, quartz UV cell permitted the ED bath solution to be continuously circulated (circulation time determined by adjustable pump speed of the peristaltic pump) and the metal salt concentration monitored by UV–vis spectrophotometry. A dip tube in the ED bath terminated with a medium porosity Pyrex frit prevented the supported catalyst slurry from being circulated through the loop. All wetted surfaces were either Teflon, Pyrex, or quartz. The UV–vis spectrophotometer (Shimadzu UV-1800) had a scan range from 200 to 800 nm which permitted detection of all metal salts; scan time was typically 200 s to give an optimized balance of sensitivity and frequency of data collection. Temperature (PID-controlled hot water bath) and pH logging were also incorporated into the control system. In order to determine accuracy of the UV–vis detection system, liquid aliquots of the bath were periodically taken throughout the reaction and analyzed by either atomic absorption (Perkin-Elmer AAnalyst 400) or inductively coupled plasma-optical emission spectroscopy (ICP-OES) (Perkin-Elmer Optima 2000 DV) to confirm metal salt concentrations remaining in the bath. Metal deposition was determined from the

2.4. Synthesis of Pt@Cu/SiO2 bimetallic catalysts The same general procedure used for the synthesis of Pt@Au catalysts was used for preparation of Pt@Cu samples. The same 2 wt% Pt/ SiO2 sample was used for the base catalyst. Cu(NO3)2 • 6H2O (J.T. Baker Analyzed ACS Reagent) and N2H4 were selected for the Cu2+ salt and reducing agent, respectively. No complexing agent was used, since Cu2+ was stable enough (reduction potential of +0.34 V, Table 2) to 3

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Fig. 2. Schematic representation of continuous ED apparatus.

Fig. 3. Stability test for AuCl4−; (a), AuCl4− alone in solution, (b), AuCl4− in presence of N2H4, c) AuCl4− complexed with en at an [en]:[AuCl4−] = 5:1 molar ratio, depressed plasmon formation and reduced baseline creep indicating mitigated Auo NP formation. Same target Au3+ concentration (50 ppm) used in all cases shown here.

the deposition was done at 25 °C. All samples were washed after ED with copious amounts of DI water to remove any residual metal salts and byproducts and then dried at 25 °C.

resist spontaneous reduction in the presence of concentrations of N2H4 used for ED. The molar ratio of N2H4 to Cu2+ used for all ED experiments was 5:1. The pH of the bath was maintained at 8.5–9 by addition of dilute NaOH, and the reactions were run at 25 °C. The pH was rigorously kept ≤ 9 to prevent strong electrostatic adsorption (SEA) of Cu2+ on the SiO2 surface [31,32]. Since cationic Cu was used, the uptake of Cu2+ on the SiO2 surface was a concern. At pH < 9, minimal amounts of Cu2+ were adsorbed, which was confirmed by studying uptake of Cu(NO3)2 on the silica support at reaction conditions (results are shown in Section 3.2.).

3. Results and discussion 3.1. Pt@Au/SiO2 Before deciding on the use of N2H4 as the preferred reducing agent for KAuCl4, several reducing agents were tested, including hydrazine (N2H4), dimethylamine borane (DMAB), sodium borohydride (NaBH4), and sodium hypophosphite (NaH2PO2). In each case, thermal instability occurred immediately indicating a complexing agent to stabilize AuCl4− was required; ethylenediamine gave a good balance of stability, yet permitted facile reduction of Au3+ to Auo in the presence of N2H4. Stability tests were conducted at a AuCl4− concentration of 50 ppm (254 μmol/L) at a 2:1 molar ratio of N2H4 to KAuCl4 and a pH value of 9. As shown in Fig. 3a, AuCl4− is stable and shows an increase in intensity as pump time increases to 60 min; each scan line requires 200 s, so in 60 min of pumping, there were 18 scans. The dark blue line at the top of the UV–vis peak at 290 nm is due to continuous overlay of the same peak intensity from 60 to 120 min of scans. When both AuCl4− and N2H4 solutions were pumped into the ED bath (no catalyst present),

2.5. Continuous co-ED synthesis of Pd@Cu-Pt/C bimetallic catalysts Continuous co-ED involves the simultaneous electroless deposition of two salts on a catalytic surface. In these experiments, the base catalyst was changed to a commercial 5 wt% Pd/C (BASF) with Pd particles of 5.2 nm and a dispersion of 21.6%, as determined by H2 titration of O-precovered Pd. Cu(NO3)2 and H2PtCl6 • xH2O (Aldrich ≥99.9 trace metal basis) were used as the reducible salts. The relatively low thermal stability of PtCl62− required addition of en to maintain stability in the ED bath; en at a molar ratio of [en]/[Cu2+ + PtCl62−] = 3:1 was added to the ED bath at the beginning of the experiment, and a molar ratio of [N2H4]/[Cu2+ + PtCl62−] = 5:1 was used for ED. The pH of the solution was maintained at 8.5–9 using a dilute NaOH solution and 4

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Fig. 4. Continuous ED of Au on 2 wt% Pt/SiO2, theoretical target coverage 9.2 ML (target 5.03 wt%, deposited 3.37 wt%) (same amount used in stability tests of Fig. 3.) a) UV–vis spectra taken continuously throughout the reaction. b) reaction dynamics, note the pumping regime to 60 min followed by a batch regime where deposition continues to occur. c) agreement between the concentrations derived from AA spectroscopy and UV–vis intensity.

a Auo plasmon, formed by thermal decomposition of the ED bath appeared at 550 nm after only 2–3 scans, indicating a high degree of instability of the bath components [33]. The increasing background intensity is due to increasing optical density of the solution, since the Pyrex frit in the ED bath was unable to remove the smallest Auo nanoparticles from the recirculating solution [7,34]. After addition of en to AuCl4− at a 5:1 molar excess and repeating the pumping sequence at the same conditions, the Auo plasmon was strongly diminished in intensity, and the background did not increase as rapidly, indicating retarded formation of Auo nanoparticles. Appearance of a small peak intensity at 550–600 nm occurred only after 10–12 scans, or 30–40 min, meaning that if ED of Au takes place in less than 30 min, there should be essentially no instability. Note the appearance of the growing shoulder at ˜300 nm, indicating that AuCl4- species was increasing in concentration in the ED bath. The test was repeated again, but with the presence of the Pt/SiO2 catalyst to study the dynamics of continuous ED. As seen in the UV–vis spectra of Fig. 4a, the Auo plasmon was completely absent, indicating no spontaneous reduction of AuCl4− to Auo. The intensity of the AuCl4− shoulder also increased with time suggesting that not all AuCl4− was deposited on the Pt surface. The dynamics of AuCl4− is shown in Fig. 4b and shows that at the end of pumping at 60 min, approximately 50% of the Au3+ has been deposited. Gold deposition continued slowly from 60 to 120 minutes due to residual N2H4 left in the bath along with the unreacted AuCl4-. Finally, comparison of the residual AuCl4- amounts left in the bath determined by AA and UV–vis agree extremely well, confirming that UV–vis spectroscopy is an excellent method for monitoring in real time the dynamics of deposition of AuCl4- by N2H4 on platinum group metal surfaces. Electroless deposition was repeated for different weight loadings of Au, ranging from 0.28 to 6.8 theoretical monolayers (ML) of Au (0.16–3.8 wt%). As the weight loadings of AuCl4− increase, the rate of ED slows, as shown in Fig. 5. This is consistent with the results of Ohno, [22] who showed that N2H4 is oxidized more slowly on Au than Pt surfaces. Thus, as Au coverage on Pt increases, and Pt sites decrease, ED slows because N2H4 is activated on a more sluggish Au surface. To determine the extent of coverage of Pt by Au, chemisorption by H2 was conducted on the bimetallic samples. Since Au does not chemisorb H2, the decrease in H2 uptake of the Pt@Au samples is directly related to Au coverage on Pt; the results are shown in Fig. 6. The dashed line shows the theoretical amount of Au required for a monodisperse coverage of Au on Pt, assuming a Au/Pt ratio of one. Monodisperse coverage appears to occur until about 0.5 ML coverage of Au on Pt as also seen in previous work [7]. Gold levels corresponding to ˜12 monolayers (ML) were required to experimentally reach full coverage of Au on Pt. This may be due to either changes in deposition stoichiometry of Au on Pt or the ability of H2 to dissociatively adsorb on Pt sites smaller than those required for ED of Au. More likely, though, is that autocatalytic deposition of AuCl4− on Auo occurs due to the higher surface

concentration of Au sites relative to available Pt sites as illustrated in Table 1. Regardless, continuous ED has been shown to be capable of electroless deposition of a very unstable Au3+ salt, without using the much more stable, but highly toxic KAu(CN)2 salt. Given the importance of bimetallic catalysts containing Au, this is a substantial accomplishment. 3.2. Pt@Cu/SiO2 The copper system differed from gold since the precursor salts were much more stable, enabling more flexibility in experimental design. Regardless, the restricted number of anionic Cu2+ salts made it difficult to find a precursor not subject to SEA at pH conditions above the point of zero charge (PZC) value of most supports. However, from the results of others [31], and the SEA uptake measurements in Fig. 7, we found that Cu(NO3)2 does not undergo SEA at pH ≤ 9. Further, stability tests of an ED bath containing N2H4 and Cu2+ at a 2 : 1 (no en stabilizing agent required) did not undergo spontaneous reduction to Cuo NPs, except at the highest Cu weight loadings that were examined. Of course, if ED occurs during pumping of high concentrations of Cu(NO3)2, the concentration of Cu2+ never accumulates to the same level in the ED bath as encountered in the highest stability test. Results for ED of Cu2+ are shown in Fig. 8 and indicate that virtually all Cu2+ undergoes deposition during the 120 min of deposition time. Similar to high weight loadings of Au deposition, the ED of Cu is slower for 1.64 wt% Cu2+, most likely due to slower activation of N2H4 on the Cu surface [22]. However, it is obvious that over a wide range of loadings Cu2+ can be completely deposited, both catalytically on Pt° and autocatalytically on Cu°. The H2 chemisorption plot in Fig. 9 for the Pt@Cu/SiO2 samples show a large deviation from the theoretical monodisperse values and is quite different from the similar plot in Fig. 6 for Pt@Au/SiO2. Even at 30 ML of theoretical coverage, there is still approximately 20% of the Pt that remains exposed to the surface. Computations from Nørskov [24] have predicted that diffusion of Cuo into the bulk lattice of Pto is favored and forms a true Cu-Pt alloy structure (Cuo is substitutionally placed in the fcc lattice of Pto). To determine whether this is the case for the bimetallic catalysts prepared by ED, XRD analysis of a select number of samples were conducted after both preparation and drying at 25 °C and after reduction in flowing 10% H2/balance Ar for 1 h at 300 °C. Results are shown in Fig. 10. The XRD patterns for the dried only samples indicated that three dimensional aggregates of Cu° were formed on the Pt surface during ED of 1.60 and 5.49 Cu wt% loadings; these underwent facile oxidation to form Cu2O at 25 °C in air. Cu2O was not observed for ED of 0.39 wt% Cu which corresponded to two theoretical ML of Cu. This possibly indicates a stabilizing influence of Cu by the Pt surface since at these coverages of Cu, most of the Cu atoms will be either in contact with or only one atomic layer of Cu from the Pt surface. Since the atomic weights for Cu and Pt are 63.54 and 195.08, respectively, even low weight loadings of 5

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Fig. 5. Reaction dynamic charts for multiple weight loadings of Pt@Au/SiO2 note decreased extent of reaction as Au loading is increased.

Cu correspond to high atomic ratios relative to Pt. More interesting, however, is the shift of the Pt(111) peak following reduction at 300 °C. The magnitude of the shift to higher 2θ values and application of Vegard’s law indicate the formation of true Cu-Pt alloys, in agreement with the work of Nørskov [24]. However, even after reduction at 300 °C, Cu2O is present on the surface of the Cu-Pt composition containing 5.49 wt% Cu. The Cu2O appears to exist as an oxide shell on large metallic Cu particles since a large Cu (111) peak is also observed for the 5.49 wt% Cu sample; it is also present as a smaller peak for the 1.60 wt% Cu sample. We can now explain the H2 chemisorption results. Between diffusion of Cuo into the bulk of Pt particles and the preferential formation of large metallic Cu particles on the Pt surface, some of the Pt surface remains exposed to H2 during chemisorption. However, in essence we have used a non-optimized method to prepare Cu-Pt alloys that are rather poorly prepared. If Cu and Pt prefer to co-exist as alloys, they should selectively be prepared as such. In the following section, a controlled method of co-electroless deposition of both Cu2+ and PtCl62− on 5% Pd/C catalysts is described. Fig. 6. H2 chemisorption results for Pt@Au/SiO2 catalysts with different Au loadings.

3.3. Continuous co-ED Pd@Pt-Cu/C The base catalyst was changed from 2% Pt/SiO2 to 5% Pd/C so that Pt deposited during co-deposition could be more easily determined from the base catalyst; the catalysts prepared with this methodology also have a structure (Cu-Pt alloy) that could be tested for direct methanol fuel cell applications. As stated earlier, the transition from deposition of one metal to two metals was accomplished by simply adding a third microprocessor-controlled syringe pump. The reaction dynamics for three different target loading ratios of [Cu]/[Pt] are shown in Fig. 11. For these experiments, the molar ratios of all components were maintained at [N2H4]/[en]/[Cu2+ + PtCl62−] = 5 : 3 : 1. It was necessary to add ethylenediamine to the ED bath to maintain thermal stability of PtCl62− in the presence of N2H4. From Fig. 11, it is obvious that PtCl62− is much more easily reduced than Cu2+, even in the presence of excess ethylenediamine, regardless of the amount of excess Cu2+ pumped into the bath. Only in the case of a target ratio of Cu : Pt = 11.3 : 1 was the percentage of PtCl62− deposited lowered from > 90% to 72%. Thermodynamically, the reduction potential of PtCl62− is indeed more favorable than Cu2+ (+ 0.72 V vs +0.34 V,

Fig. 7. SEA test with Cu(NO3)2 and silica at pH 9.

6

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Fig. 8. Reaction dynamic charts for multiple weight loadings of Pt@Cu/SiO2.

Fig. 9. pulse H2 chemisorption versus weight loading for Cu on 2 wt% Pt/SiO2.

respectively), but the rate differences must be due to kinetic effects. Regardless, both Cu2+ and PtCl62− were co-deposited on the base Pd catalyst. The deposition rates of each salt were also linear over the 60 min pumping period making it possible to form uniform compositions of the two metals during the pumping portion of the ED procedure. Thus, if one wanted to prepare a 1 : 1 bimetallic layer of Cu and Pt, Fig. 11 indicates that pumping 60 μmol Cu2+/g cat. for 60 min while pumping 25 μmol of PtCl62−/g cat. for 60 min should give a bimetallic layer of approximately 20 μmol s each (per g cat.) of Cu and Pt after 60 min. A generalized plot showing expected compositions of Cu : Pt bimetallic layers vs pumped compositions (target ratios) of Cu2+ : PtCl62− is shown in Fig. 12. The linear regression analysis indicates that expected compositions are approximately ½ of what is actually pumped. Plots of these types for many different combinations of reducible metal salts should provide a high degree of control for deposition of bimetallic layers using this straightforward method of preparation. These types of alloy compositions (after the appropriate high

Fig. 10. XRD patterns for several Pt@Cu/SiO2 samples that were, dried only at 25 °C (a) and, after reduction in 10% H2/balance Ar for 1 h at 300 °C (b). For conversion of wt% Cu to theoretical ML coverage, 0.18 wt% Cu = 1.0 ML Cu. Best fit for alloy composition using Vegard’s law is shown after reduction is shown in lower panel.

temperature post-treatment) have been postulated to exhibit superior activities and stabilities for both fuel cell [35–37] and chemical catalysis applications [18,19]. 7

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Fig. 11. Reaction dynamics of continuous co-ED showing simultaneous deposition of Cu and Pt at different ratios of [Cu2+]/[PtCl62−].

from Cuo diffusion into Pt was found for the Pt@Cu/SiO2 system. This observation was intentionally applied for the formation of uniform compositions of Pt and Cu shells on a Pd/C core catalyst by using coelectroless deposition of both Cu2+ and PtCl62− salts. The different, but constant, rates of deposition of Cu2+ and PtCl62− make it possible to prepare predictable Cu-Pt bimetallic layers that can be formed into true alloys using moderate (˜300C) reduction temperatures. Acknowledgements The authors gratefully acknowledge financial support from the National Science Foundation (NSF, CBET 1511615 and NSF, I/UCRC 1464630). The authors would also like to thank Methasit Juthathan, an REU student from Mahidol University, Thailand, for his dedicated experimental work at the beginning of this study. References [1] B.A. Mehrabadi, S. Eskandari, U. Khan, R.D. White, J.R. Regalbuto, A review of preparation methods for supported metal catalysts, Adv. Catal.. (2017) 1–35. [2] K. Beard, M. Schaal, J.V. Zee, J. Monnier, Preparation of highly dispersed PEM fuel cell catalysts using electroless deposition methods, Appl. Catal. B: Environ. 72 (2007) 262–271. [3] J. Tengco, B.T. Mehrabadi, Y. Zhang, A. Wongkaew, J. Regalbuto, J. Weidner, J.R. Monnier, Synthesis and electrochemical evaluation of carbon supported Pt-Co bimetallic catalysts prepared by electroless deposition and modified charge enhanced dry impregnation, Catalysts 6 (2016) 83. [4] A. Wongkaew, Y. Zhang, J.M.M. Tengco, D.A. Blom, P. Sivasubramanian, P.T. Fanson, J.R. Regalbuto, J.R. Monnier, Characterization and evaluation of Pt-Pd electrocatalysts prepared by electroless deposition, Appl. Catal. B: Environ. 188 (2016) 367–375. [5] T.R. Garrick, W. Diao, J.M. Tengco, E.A. Stach, S.D. Senanayake, D.A. Chen, J.R. Monnier, J.W. Weidner, The effect of the surface composition of Ru-Pt bimetallic catalysts for methanol oxidation, Electrochim. Acta 195 (2016) 106–111. [6] M.T. Schaal, A.Y. Metcalf, J.H. Montoya, J.P. Wilkinson, C.C. Stork, C.T. Williams, J.R. Monnier, Hydrogenation of 3,4-epoxy-1-butene over Cu–Pd/SiO2 catalysts prepared by electroless deposition, Catal. Today 123 (2007) 142–150. [7] J. Rebelli, A.A. Rodriguez, S. Ma, C.T. Williams, J.R. Monnier, Preparation and characterization of silica-supported, group IB–Pd bimetallic catalysts prepared by electroless deposition methods, Catal. Today 160 (2011) 170–178. [8] M.T. Schaal, M.P. Hyman, M. Rangan, S. Ma, C.T. Williams, J.R. Monnier, J.W. Medlin, Theoretical and experimental studies of Ag–Pt interactions for supported Ag–Pt bimetallic catalysts, Surf. Sci. 603 (2009) 690–696. [9] M. Ohashi, K.D. Beard, S. Ma, D.A. Blom, J. St-Pierre, J.W.V. Zee, J.R. Monnier, Electrochemical and structural characterization of carbon-supported Pt–Pd bimetallic electrocatalysts prepared by electroless deposition, Electrochim. Acta 55

Fig. 12. Target Cu: Pt pumping ratio vs actual ratio of deposited metals after 120 min. Linear regression of data shown by dashed line.

4. Conclusions Electroless deposition has exhibited a high level of versatility and usefulness for synthesizing bimetallic catalysts for a wide number of applications. However, there are concerns with this process from the inherent instability of the ED bath for some highly reducible metal salts, which is enhanced even further by the high concentrations of both reducing agent and reducible metal salt in the ED bath at the beginning of the batch method of ED. By continuously adding the reagents to the ED bath at rates similar to removal by ED, concentrations of reagents do not accumulate to high levels in the bath, and limitations due to instability are greatly lessened. Using this procedure, Pt@Au/SiO2 bimetallic catalysts were prepared without the use of highly stable (but highly toxic) KAu(CN)2 by using KAuCl4 with ethylenediamine stabilizer as the Au salt. This procedure was also used for the synthesis of Pt@Cu/SiO2 catalysts. Bulk alloy formation after reduction at 300 °C 8

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