Supporting Information

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1 Supporting Information Catalytic Reduction of 4-Nitrophenol: A Quantitative Assessment of the Role of Dissolved Oxygen in Determining the Induction Time Eredzhep Menumerov, Robert A. Hughes, and Svetlana Neretina,,* College of Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, United States Center for Sustainable Energy at Notre Dame, Notre Dame, Indiana, 46556, United States * sneretina@nd.edu 1. EXPERIMENTAL SECTION (i) Chemicals. Au, Ag, and Pd nanoparticles were synthesized using hydrogen tetrachloroaurate(iii) trihydrate (HAuCl4 3H2O), silver nitrate (AgNO3), and palladium nitrate (Pd(NO3)2), respectively (Sigma Aldrich). 4-Nitrophenol and sodium borohydride (NaBH4), were sourced from Fluka. All aqueous solutions were prepared using deionized (DI) water derived from a Milli-Q system (18.2 MΩ cm at 25 C). Gas purging utilized ultra-high purity Ar and O2 (Airgas). All chemicals were used as received. (ii) Gas Purging Procedures. The dissolved oxygen content within aqueous solutions of 4-NP, NaBH4, and the various catalysts was reduced by purging each solution with Ar at a flowrate of 250 ccm. During this procedure the liquids were placed in a Parafilm-capped vial in which openings allowed for (i) the insertion of a 1 /16 th of an inch diameter Teflon tube through which Ar flowed into the liquid and (ii) an exhaust through which excess gas escaped. The small opening used for the exhaust resulted in the buildup of a slight overpressure of Ar gas above the surface of the purging liquid that limited the backflow of air and caused the Parafilm to bulge. Procedures put in place to maintain the dissolved oxygen content at low levels included (i) purging the solutions immediately before use, (ii) maintaining an Ar atmosphere over the surface of the purged solution whenever possible, and (iii) limiting the amount of turbulence created when combining reactants through slow injections where the pipette tip is held below the surface of the liquid. The necessity for these procedures and their effectiveness was ensured through control experiments that monitored the dissolved oxygen concentration as they were carried out. The induction time obtained for the Pd catalyst was shorter than those obtained for Au and Ag at 8.3 mg L 1 (i.e., the ambient dissolved oxygen concentration). In order to prolong the induction times, and, hence, obtain the t0 vs ct=0 dependence shown in Figure 2d, one of the initial 4-NP solutions was purged with O2 gas to obtain a data point at a concentration of 10 mg L 1. The procedures used for O2 purging were the same as those used for Ar. The data shown in Figure 4 required that the reactants contained within a cuvette be purged with O2 gas while spectroscopically monitoring the reaction. For these experiments a Teflon tube was inserted into the cuvette through which O2 gas was flowed at a rate of 50 ccm. (iii) Nanoparticle Synthesis. Aqueous solutions of HAuCl4 3H2O (9 ml, 144 µm) and NaBH4 (10 ml, 13 mm) were prepared and then exposed to the aforementioned Ar purging procedure for a 15 min duration. Au nanoparticle synthesis occurred at room temperature by combining the entire HAuCl4 3H2O solution with 1 ml of the NaBH4 solution. The final Au nanoparticle concentration is 130 µm. Ag nanoparticles (100 µm) were prepared similarly by combining aqueous AgNO3 (9 ml, 111 µm) with NaBH4 (1 ml, 100 mm). Catalytic reactions utilizing the Au and Ag nanoparticles occurred within an established 2 h window of stability. Immediately prior to these 1

2 catalysis measurements, the nanoparticle solutions were diluted to a 5 µm concentration using Arpurged water. A window of stability could not be established for Pd nanoparticles due to the lack of a measurable LSPR peak. This necessitated that each catalysis measurement be carried out using freshly prepared nanoparticles. They were synthesized by combining aqueous Pd(NO3)2 (9.9 ml, 5 µm) with NaBH4 (0.1 ml, 5 mm) and then allowing 10 min for the nanoparticles to form. (iv) Nanoparticle Stability. The stability of the Au and Ag nanoparticles was assessed by monitoring the LSPR peak. For these measurements freshly prepared nanoparticles were diluted to a concentration of 5 µm, after which 3 ml was pipetted into 1 cm path-length cuvette. The LSPR peak of the nanoparticle was then monitored for a 2 h duration. There were no significant changes to the spectra over this time interval, indicating stability in nanoparticle size and a resistance to aggregation. (v) 4-NP Catalysis. In situ monitoring of the dissolved oxygen concentration required modifications to the sample compartment of the spectrometer (Figure S1a). Custom-made cuvettes, which are large enough to accommodate the physical dimensions of the detector, were fabricated from Borofloat 33 floated borosilicate flat glass (Specialty Glass Products Willow Grove, PA, USA). These 38 ml cuvettes have a 2.5 cm path-length and a height of 63 mm. The mounts for both the sample and reference cuvettes were 3D-printed out of black ABS plastic. The optical sensor (Vernier Software and Technology Beaverton, OR, USA) for measuring the dissolved oxygen concentration is based on a luminophore whose luminescence is quenched when exposed to dissolved oxygen, a process that is reversible. The sensor requires no stirring, is unaffected by H2 gas, and is highly selective to dissolved oxygen. It did, however, prove necessary to wrap the lower portion of the probe with Teflon tape (Figure 2a) in order to prevent crosstalk between the optical detection systems of the dissolved oxygen probe and UV vis spectrometer. When inserted into the cuvette, the sensing element is 5 mm above the spectrometer s optical beam. During experiments, the sample compartment is covered with a black foamboard lid over which an optically dense fabric is placed (Figure S1b), measures taken to prevent stray light from entering. An easily raised and lowered black rubber gasket provides the access needed to pipette chemicals into the cuvette, quickly close the access point, and then begin spectroscopic monitoring. When experiments were being carried out that required a dissolved oxygen content below the detection limit, it proved necessary to maintain an Ar gas pressure over the purged reactants. As an example, Figure S2 shows a schematic of the experimental configuration used to obtain the data shown in Figure 4. The cuvette is capped with Parafilm through which two Teflon tubes are inserted. The first tube, which does not penetrate the surface of the liquid, maintains an Ar overpressure above the liquid. The second tube, which penetrates the surface of the liquid, provides the capability to purge reactants with O2 gas. A hole in the Parafilm allows for the injection of reactants and acts as an exhaust through which gases flow out of the cuvette. The catalytic reduction of 4-NP was assessed using a 50 µm 4-NP solution obtained by sequentially pipetting aqueous solutions of (i) 4-NP, (8 ml, 150 µm) (ii) NaBH4, (8 ml, 15 mm) and (iii) catalyst (8 ml, 5 µm) where the dissolved oxygen content of the 4-NP solution was varied using the aforementioned gas purging techniques. Multiple pipettes were used for this procedure in order to facilitate the uninterrupted transfer of reactants to the cuvette. After each experiment the sensing element of the dissolved oxygen detector was cleaned by submerging it in 1% nitric acid for 1 h and then 5 times rinsing it in DI water. Between experiments the sensor was kept saturated by storing it in DI water. 2

3 (a) (b) Figure S1. Images of (a) the modifications made to the spectrometer s sample compartment that allow for the real-time monitoring of the dissolved oxygen content in an aqueous solution contained within a cuvette and (b) the same compartment when it is covered with an opaque fabric. Figure S2. Schematic showing the experimental configuration used to (i) maintain an Ar overpressure over the reactants contained within a cuvette and (ii) bubble O2 gas through the reactants. (vi) H2 Gas Evolution Experiments. The data presented in Figure 3b shows a plot of the cumulative volume of the H2 gas released during the catalytic reduction of 4-NP as function of time. These measurements were carried out using a pneumatic trough setup where the evolving gas displaces water contained within a graduated column. These measurements utilized the same reactant concentrations and volumes as the catalysis measurements. (vii) Instrumentation. Transmission electron microscopy (TEM) images were obtained using either a JEOL JEM 1400 or JEOL JEM 2011 TEM. The samples were prepared by depositing a drop of freshly prepared nanoparticle solution onto a TEM grid (carbon Type A on 400 mesh Cu Ted Pella, Inc.). TEM images were taken immediately after the sample dried. A Jasco V 730 Spectrophotometer was used to monitor the catalytic reduction of 4-NP. 2. CHARACTERIZATION OF Pd NANOPARTICLES Figure 1c f shows TEM images and the nanoparticle size distribution for Au and Ag nanoparticles. Figure S3 presents the equivalent data for Pd nanoparticles. 3

4 Figure S3. TEM image and histogram of the nanoparticle size distribution for Pd nanoparticles. 3. CONTROL EXPERIMENTS VALIDATING THE DISSOLVED OXYGEN SENSOR Control experiments were carried out that assess whether the dissolved oxygen sensor influences the catalytic reduction of 4-NP. For these studies a 24 ml reaction solution containing 50 μm 4-NP, 5 mm NaBH4, and 1.7 µm Au catalyst was monitored. Figure S4a shows the time evolution of the 400 nm 4-NP absorbance obtained when the sensor is monitoring the reaction and for an identical reaction where the sensor is removed from the cuvette. The overlap in the two datasets indicates that the data is uncompromised by the sensor. A second control experiment assessed whether the detector response was compromised by the presence of dissolved hydrogen. To make this determination the dissolved oxygen content was monitored as an 8 ml solution of 4-NP was purged with Ar gas and compared to an identical solution purged with a gas mixture containing 10% H2 and 90% Ar (Figure S4b). With only small deviations in the two datasets the sensor was, once again, deemed suitable for this study. Figure S4. Comparison of the time evolution of the 4-NP absorbance when (a) the dissolved oxygen sensor is inserted into the cuvette (red) and when it is removed (blue) and (b) the reaction is carried out as the solution within the cuvette is purged with Ar gas (blue) and a 90%/10% Ar/H2 gas mixture (red). 4. THE EXPOSURE OF DEAERATED 4-NP AND NaBH4 TO AIR The dissolved oxygen content of an 8 ml solution of 4-NP contained within a 2.5 cm path length cuvette was reduced below the level of detectability (0.03 mg L 1 ) by purging it with Ar. The uptake of dissolved oxygen within the solution was then monitored after its surface was exposed to air (Figure S5a). Superimposed on the curves are solid circles indicating the critical values of the dissolved oxygen concentration (ccr) required to eliminate the induction period for each catalyst. Figure S5b shows the time-dependent dissolved oxygen concentration when an 8 4

5 ml solution of 10 mm NaBH4 is added to an 8 ml solution of 100 µm 4-NP with a dissolved oxygen concentration of 8.3 mg L 1 where the surface of the liquid is exposed to air for the first 1600 s and then to Ar gas. Note that the dissolved oxygen content only drops below the levels of detectability when the Ar overpressure is present. Both datasets demonstrate the importance of maintaining an Ar atmosphere over the purged solution if the dissolved oxygen concentration is to be minimized. Figure S5. Time evolution of the dissolved oxygen concentration of (a) a deaerated 4-NP solution whose surface is exposed to air at t = 0 s and (b) a 4-NP solution with a dissolved oxygen concentration of 8.3 mg L 1 into which NaBH4 is added. In the latter case, the liquid surface is exposed to air for the first 1600 s and then to Ar gas for remainder of the experiment. 5. ABSORBANCE DATA The left hand column of Figures S6, S7, and S8 show the time-dependent absorbance and dissolved oxygen concentration for the catalytic reduction of Au, Ag, and Pd, respectively, where each graph corresponds to a different starting concentration for the dissolved oxygen. Data points were acquired every second, but error bars are only shown for a few representative data points in order to reduce clutter. The error bars shown for each plot are derived from at least three reactions carried out under identical conditions. The right hand column of each of these figures shows the ln(a/a0) vs time plot from which kapp is extracted. Superimposed on each plot is the curve obtained from least-squares curve fitting where the equation of the fitted curve and the associated standard deviation is provided in the upper left corner. The results presented in Figure 2b d were extracted from this data. 5

6 (i) Kinetic Data for the Au Catalyst Figure S6. (a e) Time-dependence of the 400 nm 4-NP absorbance (blue) and the dissolved oxygen concentration (red) for reactions utilizing Au nanoparticles as the catalyst, where each graph represents a different starting concentration for the dissolved oxygen (DO). (f j) The ln(a/a0) vs time plot (blue) derived from the adjacent absorbance data and the fitted curve (red) from which kapp is extracted. 6

7 (ii) Kinetic Data for the Ag Catalyst Figure S7. (a e) Time-dependence of the 400 nm 4-NP absorbance (blue) and the dissolved oxygen concentration (red) for reactions utilizing Ag nanoparticles as the catalyst, where each graph represents a different starting concentration for the dissolved oxygen (DO). (f j) The ln(a/a0) vs time plot (blue) derived from the adjacent absorbance data and the fitted curve (red) from which kapp is extracted. 7

8 (iii) Kinetic Data for the Pd Catalyst Figure S8. (a d) Time-dependence of the 400 nm 4-NP absorbance (blue) and the dissolved oxygen concentration (red) for reactions utilizing Pd nanoparticles as the catalyst, where each graph represents a different starting concentration for the dissolved oxygen (DO). (e h) The ln(a/a0) vs time plot (blue) derived from the adjacent absorbance data and the fitted curve (red) from which kapp is extracted. 8

9 6. INFLUENCE OF NANOPARTICLE SIZE ON kapp, t0, and ccr The nanoparticle size was varied by changing the concentration of NaBH4 into which the metal salt was combined. Figure S9 and S10 show TEM images, the nanoparticle size distributions, and plots of various catalytic descriptors for Ag and Au nanoparticles, respectively. While this procedure is less than satisfactory from the standpoint of manipulating the size over a wide interval, it does demonstrate that the influences of dissolved oxygen on the induction period applies to nanoparticles of varying size and that size-dependencies occur. Figure S11 shows plots of the timeevolution of the dissolved oxygen concentration occurring during 4-NP catalysis when nanoparticles of various sizes are used. It is noted that the synthesis of Pd nanoparticles at higher NaBH4 concentrations proved impractical due to the severe aggregation of nanoparticles. (i) Ag Nanoparticles (a) (b) (c) (d) (e) (f) Figure S9. TEM images and the associated nanoparticle size distributions for Ag nanoparticles synthesized using the aforementioned procedure, but where the AgNO3 solution was combined with NaBH4 concentrations of (a) 10, (b) 50, and (c) 100 mm. The dependence of (d) kapp, (e) t0, and (c) ccr on the nanoparticle diameter for Ag nanoparticles for reactions carried out in the presence (blue, DO=8.3 mg L 1 ) and absence of dissolved oxygen (red, DO<0.03 mg L 1 ). 9

10 (ii) Au Nanoparticles (a) (b) (c) (d) (e) Figure S10. TEM images and the associated nanoparticle size distributions for Au nanoparticles synthesized using the aforementioned procedure, but where the HAuCl4 solution was combined with NaBH4 concentrations of (a) 2.6, (b) 13, and (c) 26 mm. The dependence of (d) kapp and (e) t0 on the nanoparticle diameter for Au nanoparticles for reactions carried out in the presence (blue, DO=8.3 mg L 1 ) and absence of dissolved oxygen (red, DO<0.03 mg L 1 ). Note that a ccr dependence is not shown because all values lie below the detection limit. (a) (b) Figure S11. The time-evolution of the dissolved oxygen content occurring during 4-NP catalysis for (a) Ag and (b) Au nanoparticles of varying diameters. 7. CONTROL EXPERIMENTS COMPARING Ar AND O2 PURGING It was shown in Figure 4a that a 5 s O2 gas purge performed partway through the reaction leads to a sudden rise in the absorbance followed by an induction period. Figure S12 shows the results of a control experiment that confirms that these features can be attributed to the O2 purge. It shows a comparison of the time dependence of the 400 nm absorbance for a reaction where two 5 s O2 gas purges are carried out partway through the reaction (blue curve) and an identical reaction 10

11 where two 5 s Ar purges are carried out (dashed red curve). The experimental configuration used for the latter result is the same as that shown in Figure S2 except that Ar is bubbled through the liquid instead of O2. Note that only the O2 gas purge gives rise to features in the absorbance. It is noted that these experiments, as well as those shown in Figure 4, were carried out in a 1 cm path length quartz cuvette because the larger borosilicate cuvettes strongly attenuate the UV wavelengths needed to monitor the 300 nm 4-AP peak (i.e., Figure 4b). Figure S12. (a) Time-dependent absorbance at 400 nm as Au nanoparticles catalyze the reduction of 4-NP using reactants through which O2 (blue) and Ar (red dashed) gas is bubbled through the reactants for 5 s at times denoted by the black arrows. 11