85
7.2 Synthesis of Pt Monolayer Catalysts by Galvanic
Displacement
To synthesize Pt monolayer electrocatalysts, Adzic and co-workers designed an efficient and elegant method based on the underpotential deposition of nonnoble metal
monolayer on core nanoparticles and its galvanic displacement by Pt monolayer [4].
This method facilitates synthesis of Pt ML electrocatalysts to be accomplished by
uniformly placing a monolayer of Pt atoms on metal nanoparticles (Figure 7.2).
Scale-up of the synthesis is possible, and a successful one of 5 kg of catalyst per
batch has been reported [5]. Underpotential deposition (UPD), which involves the
formation of a submonolayer or monolayer of a metal on a foreign metallic substrate at potentials positive to the reversible Nernst potential (See Chap. 5) is applied
to supporting nanoparticles and galvanically displaced by Pt monolayer [4].
Experimental setup for Pt ML deposition on rotating disk electrodes, thin film electrodes, or other small-area electrodes is shown in Fig. 7.3. It consists of two cells
and a secondary compartment with a sliding cover. One cell is a regular electrochemical (EC) cell, and it contains deareated solution for Cu UPD (50 mM
CuSO 4 + 50 mM H 2 SO 4 ). The other cell contains deareated Pt solution (1 mM
K 2 PtCl 4 + 50 mM H 2 SO 4 ). These two cells are connected with a secondary compartment made of Teflon and glass. The two cells and the secondary compartment make
up a closed system. The space above the solutions and the inside of the secondary
compartment is filled with inert (O 2 free) atmosphere (Ar or N 2 ).
The deposition procedure involves two steps. First, the Cu UPD layer is formed
in an EC cell. Then, after removing potential control, the electrode with the Cu UPD
layer on it is transferred to a cell containing Pt solution. The Pt replaces the underpotentially deposited Cu by a simple redox exchange. In order to prevent the oxidation of Cu adatoms in contact with O 2 , the transfer of the electrode from one cell to
the other must be done in O 2 -free atmosphere. This is accomplished by moving the
electrode via a secondary compartment, which is filled with inert gas. The exchange
reaction is influenced by the choice of UPD metal (Cu
2+
vs. Pb
2+
), its valency (Cu
2+
vs. Tl
+
), valency of the deposited metal (Pt
2+
vs Pt
4+
), and the presence of anions and
oxidative agents, such as oxygen and hydrogen ions. A schematic diagram of the
entire process is shown in Fig. 7.2. As an example of the results obtained for a Pt ML
deposition on single-crystal electrodes, the data for an Rh(111) surface are discussed
Pd
Cu
Cu UPD
Pt
Pt
2+
Pt
2–
Cu
2+
Galvanic
Replacement
Cu
Cu upd /PD + Pt
2+
Pt/Pd + Cu
2+
Pt
Pd
Pd
Fig. 7.2 Schematics of the galvanic displacement of a Cu UPD monolayer by Pt. Blue and gray
balls represent Pt and Cu atoms, respectively. The clouds around the balls indicate ions
7.2 Synthesis of Pt Monolayer Catalysts by Galvanic Displacement
7.2 Synthesis of Pt Monolayer Catalysts by Galvanic
Displacement
To synthesize Pt monolayer electrocatalysts, Adzic and co-workers designed an efficient and elegant method based on the underpotential deposition of nonnoble metal
monolayer on core nanoparticles and its galvanic displacement by Pt monolayer [4].
This method facilitates synthesis of Pt ML electrocatalysts to be accomplished by
uniformly placing a monolayer of Pt atoms on metal nanoparticles (Figure 7.2).
Scale-up of the synthesis is possible, and a successful one of 5 kg of catalyst per
batch has been reported [5]. Underpotential deposition (UPD), which involves the
formation of a submonolayer or monolayer of a metal on a foreign metallic substrate at potentials positive to the reversible Nernst potential (See Chap. 5) is applied
to supporting nanoparticles and galvanically displaced by Pt monolayer [4].
Experimental setup for Pt ML deposition on rotating disk electrodes, thin film electrodes, or other small-area electrodes is shown in Fig. 7.3. It consists of two cells
and a secondary compartment with a sliding cover. One cell is a regular electrochemical (EC) cell, and it contains deareated solution for Cu UPD (50 mM
CuSO 4 + 50 mM H 2 SO 4 ). The other cell contains deareated Pt solution (1 mM
K 2 PtCl 4 + 50 mM H 2 SO 4 ). These two cells are connected with a secondary compartment made of Teflon and glass. The two cells and the secondary compartment make
up a closed system. The space above the solutions and the inside of the secondary
compartment is filled with inert (O 2 free) atmosphere (Ar or N 2 ).
The deposition procedure involves two steps. First, the Cu UPD layer is formed
in an EC cell. Then, after removing potential control, the electrode with the Cu UPD
layer on it is transferred to a cell containing Pt solution. The Pt replaces the underpotentially deposited Cu by a simple redox exchange. In order to prevent the oxidation of Cu adatoms in contact with O 2 , the transfer of the electrode from one cell to
the other must be done in O 2 -free atmosphere. This is accomplished by moving the
electrode via a secondary compartment, which is filled with inert gas. The exchange
reaction is influenced by the choice of UPD metal (Cu
2+
vs. Pb
2+
), its valency (Cu
2+
vs. Tl
+
), valency of the deposited metal (Pt
2+
vs Pt
4+
), and the presence of anions and
oxidative agents, such as oxygen and hydrogen ions. A schematic diagram of the
entire process is shown in Fig. 7.2. As an example of the results obtained for a Pt ML
deposition on single-crystal electrodes, the data for an Rh(111) surface are discussed
Pd
Cu
Cu UPD
Pt
Pt
2+
Pt
2–
Cu
2+
Galvanic
Replacement
Cu
Cu upd /PD + Pt
2+
Pt/Pd + Cu
2+
Pt
Pd
Pd
Fig. 7.2 Schematics of the galvanic displacement of a Cu UPD monolayer by Pt. Blue and gray
balls represent Pt and Cu atoms, respectively. The clouds around the balls indicate ions
7.2 Synthesis of Pt Monolayer Catalysts by Galvanic Displacement