Rational Design of Electrocatalytic Interfaces: Cd UPD Mediated Nitrate Reduction on Pd:Au Bimetallic Surfaces
Authors: Han Qi , Jebaraj Adriel J.J. , Solla-Gullón José , Feliu Juan , Scherson Daniel 
Published: February 2020

INTRODUCTION

Efforts in our laboratories have been focused on the rational design and assembly of bifunctional electrocatalytic interfaces incorporating two electrocatalysts acting in series as a means of improving both the overall activity and specificity of redox processes involving multiple transfer of electrons. This strategy was recently implemented to promote the reduction of NO3
- on Au NP dispersed on the surface of glassy carbon, GC, by adsorbed hemin (Hm), an iron porphyrin, in aqueous electrolytes containing Cd2+.1 This work represents an extension of this previous study in which Hm has been replaced by Pd nanoparticles, NP, co-dispersed with Au NP on GC, a tactic that virtually eliminates the formation of Au|Pd alloys. Impetus for this selection was provided by the much lower overpotential for NO2
- reduction on Pd compared to that on adsorbed Hm, a factor that would lead to overall activities surpassing those found earlier for the Cd(UPD)|Hm system.

EXPERIMENTAL

All electrochemical measurements were performed in an all-glass, three-compartment cell, filled with deaerated (Ar, Airgas, PP300) 0.1 M HClO4 solutions prepared from ultrapure perchloric acid (OmniTrace Ultra) and ultra-high purity water, UPW, generated by a Barnstead system (18.2 MΩ cm). A carbon rod (Alfa Aesar, diameter 6.15 mm) and a Ag/AgCl (Basi, MF2052) were used as counter and reference electrodes, respectively. Experiments were carried out with a GC disk electrode, GCE, (Pine Instruments, disk area, 0.196 cm2) using a commercial rotator (Pine Instruments, Model AFMSRX) and a Metrohm Autolab (PGSTAT302N). Cadmium perchlorate, Cd(ClO4)2 (Sigma-Aldrich, 99.999%), and sodium nitrate, NaNO3, (Sigma Aldrich, ACS reagent) were used as received. Nanoparticles, NP of Au and Pd were synthesized and cleaned using a methodology previously described in which an aqueous solution of the corresponding metallic precursor (HAuCl4 or H2PdCl4) was reduced with ice-cold and freshly prepared NaBH4 solution in the presence of trisodium citrate.2 Stock suspensions of each type of NP were prepared by mixing 1 volume of the original, as received, NP, with 3 volumes of UPW. To prepare either Au or Pd dispersions on the GCE, denoted hereafter as Au|GCE or Pd|GCE, 10 µL of the diluted NP aqueous suspensions were delivered to the GC surface using a micropipette, and then allowed to dry with a gentle stream of Ar. To prepare the co-dispersed Pd and Au NP on a GC disk, Pd:Au|GCE, 500 µL of each dilute NP suspension were mixed in a vial and following sonication, 10 µL of the mixture then delivered on the GCE using a micropipette, and allowed to dry under Ar.

RESULTS AND DISCUSSION

Shown in 1 are cyclic voltammograms recorded with Au|GCE (magenta), Pd|GCE (black) and Pd:Au|GCE (green) in pristine 0.1 N HClO4, displayed features believed to be characteristic of the metals involved. Integration of the characteristic reduction peaks in green in Fig 1 were found to be approximately half of those in either the Au|GCE or Pd|GCE (see magenta and black curves), and thus consistent with the relative amounts of each of the NP in the Pd:Au|GCE. Shown in Panel A, Fig. 2 are cyclic voltammograms obtained with these NP modified GCE surfaces in 0.1 M HClO4 containing 1 mM Cd(ClO4)2 and 2 mM NaNO3. Cursory inspection of these data reveals much higher activity for the Pd:Au|GCE, even though the amount of each NP was decreased to half, compared to the single metal surfaces. The same overall trend was found for measurements performed under forced convection, as shown in Panel B in this figure which reinforces the view that the two catalysts are indeed acting in series, i.e., Cd UPD on Au is responsible for the conversion of NO3
- to NO2
-, whereas Pd reduces NO2
- further to products of lower oxidation states.
REFERENCES

Y. Chen, H. Zhu, M. Rasmussen and D. Scherson, J. Phys. Chem. Lett., 1, 1907 (2010).

C. M. Sanchez-Sanchez, F. J. Vidal-Iglesias, J. Solla-Gullon, V. Montiel, A. Aldaz, J. M. Feliu and E. Herrero, Electrochim. Acta, 55, 8252 (2010).

ACKNOWLEDGEMENTS: This work was supported by a grant from NSF, CHE-1412060.

Figure 1