Temperature Dependence of Growth of Surface Oxide Films on

Chapter 23

Downloaded by UNIV MASSACHUSETTS AMHERST on October 6, 2012 | http://pubs.acs.org Publication Date: February 28, 1997 | doi: 10.1021/bk-1997-0656.ch023

Temperature Dependence of Growth of Surface Oxide Films on Rhodium Electrodes 1

Francis Villiard and Gregory Jerkiewicz

Département de chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada

Surface oxides on Rh electrodes were formed by anodic polarization at potentials, E , between 0.70 and 1.40 V, RHE, with an interval of 0.05 V for polarization times, t , up to 10 000 s and at temperatures, T, between 278 and 348 K. This procedure results in thin films having their charge density, q , of less than 1260 μC cm , thus their thickness, X, of up to 2 M L of Rh(OH) . Cyclic-voltammetry, C V , p

p

-2

ox

3

reveals one states, OC1, in the oxide reduction profiles. Increase of Τ leads to augmentation of the oxide thickness but it does not influence its surface state; thermodynamics of their reduction are not affected by Τ variation. Plots of q versus log t or 1 / q versus log t for a wide range of Τ and E allow one to discriminate between the logarithmic and the inverse-logarithmic oxide growth kinetics. Two kinetic regions are observed in the oxide formation plots, each one giving rise to a distinct growth mechanism. Oxides having X ≤1ML of RhOH are formed according to the logarithmic kinetics and the process is limited by the rate of the place exchange between the Rh surface atoms and the electroadsorbed O H groups. Formation of oxides having X between 1 ML of RhOH and 2 ML of Rh(OH) ox

p

ox

p

p

3

follows the inverse-logarithmic kinetics and the process is limited by the rate of escape of Rh from the metal into the oxide. Theoretical treatment of the data in the region corresponding to X>1 ML of RhOH leads to determination of the potential drop across the film and the electric field within the oxide layer, the latter being of 10 V m . 3+

9

-1

Surface oxide films on various transition metals can be formed by application of electrochemical techniques or by exposure to an oxidizing atmosphere and the extent of surface oxidation, thus the oxide thickness, is affected by the oxidation conditions. 1

Corresponding author © 1997 American Chemical Society

In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

324

SOLID-LIQUID ELECTROCHEMICAL INTERFACES

In the case of electroformation of oxides, the extend of surface oxidation depends on the nature of the metals, the polarization conditions (polarization potential, current density or time, E , i and t , respectively) and the electrolyte composition and pH (1-20). The oxide formed on an electrode surface markedly affects anodic Faradaic electrode processes at the double-layer by: (i) afFecting the reaction energetics; (ii) changing electronic properties of the metal electrode; (iii) imposing a barrier to the charge transfer; (iv) affecting the adsorption properties of the reaction intermediates and products (14-17). Knowledge of the chemical and electronic state of the surface oxide is of major importance in electrocatalysis since it determines the electrocatalytic properties of the surface at which various anodic Faradaic reactions take place. At noble metals, the growth of submonolayer and monolayer oxides can be studied in detail by application of electrochemical techniques such as cyclicvoltammetry, C V (11-20) and such measurements allow precise determination of the oxide reduction charge densities. Complementary X-Ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), infra-red (IR) or ellipsommetry experiments lead to elucidation of the oxidation state of the metal cation within the oxide and estimation of the thickness of one oxide monolayer (12,21-23). Coupling of electrochemical and surface-science techniques results in meaningful characterization of the electrified solid/liquid interface and in assessment of the relation between the mechanism and kinetics of the anodic process under scrutiny and the chemical and electronic structure of the electrode's surface (21-23). Rhodium, Rh, like other noble metals, forms surface oxides upon anodic polarization even in the region of water stability, thus below the thermodynamic reversible potential of the oxygen evolution reaction, E ^ = 1.23 V , SHE (1-20). In aqueous H S 0 solution, the oxide growth on Rh commences at 0.55 V , R H E (reversible hydrogen electrode), and up to ca. 1.40 V, RHE, a complete monolayer (ML) of Rh(OH) is formed as revealed by coupled C V and XPS measurements (21). Polarization at potentials between 3.00 and 4.00 V, RHE, leads to formation of oxides comprising two various species, namely Rh(OH) and RhO(OH). Surface oxides on Rh were formed in aqueous H S 0 solution by application of potentiostatic polarization at E up to 2.4 V, RHE, for polarization times up to 10 s. The data demonstrated that these conditions result in thin films having their thickness of less than 4 equivalent M L of R h ( O H ) Subsequently, kinetics of the oxygen evolution reaction, OER, on pre-oxidized Rh electrodes were evaluated and a relation between the oxide thickness and the kinetic parameters of the OER was established (24,25). Some of the most recent data on electrochemical processes refer to Rh single-crystal electrodes and the experiments were focused on the adsorption of hydrogen, small inorganic/organic species, and interaction of anions with the substrates; they showed that a great deal of information may be related to surface specific parameters (26-31). In this paper, the authors demonstrate the first data on the temperature dependence of formation of monolayer oxides at Rh in aqueous H S 0 solution. The growth of Rh surface oxides was accomplished by potentiostatic polarization at 0 . 7 0 ^ E ^ 1.40V, RHE, for t < 1 0 s. Extensive studies at 278 RhOH + H + e

0.55-0.75 V

(1)

-> R h ( O H ) + 2 H + 2 e 0.75-1.40 V

(2)

2

+

RhOH + 2 H 0 2

3

Reduction Rh(OH) + 3 H + 3 e +

3

- » Rh + 3 H 0

(3)

2

A new behavior is presented in Figure 3 and it shows an impact of Τ increase on the oxide growth. The experimental data, which is the main effect presented in the current paper, show that the oxide films increase their thickness when the temperature is raised to higher values. However, the oxide-reduction peak does not shift towards less-positive potentials when the oxide thickness increases (compare it with the behavior reported above on increase of E or t ). Lack of shift of the oxide reduction peak potential indicates that the thermodynamics of the oxide reduction are p

p



330


Figure 4A. Rh oxide growth plots expressed as the reduction charge density, q , versus the polarization potential, E , and log of the polarization time, logtp, for T = 278K; E is between 0.70 and 1.40 V , R H E , and l o g t is between 0 and 4. The data demonstrate linear relations between q and log t for every E = const. o x

p

p

p

o x

p


p


23. VILLIARD & JERKIEWICZ

Surface Oxide Films & Rhodium Electrodes

331

Figure 4B. Rh oxide growth plots expressed as the reduction charge density, q , versus the polarization potential, E , and log of the polarization time, logtp, for T = 348K; E is between 0.70 and 1.40 V , R H E , and l o g t is between 0 and 4. The data demonstrate linear relations between q and log t for every E = const. o x

p

p

p

o x

p


p

332


not affected by temperature and the oxides have the same Gibbs free energy of reduction even though the films becomes thicker when the temperature is raised. The anodic polarization of Rh electrodes at 0.70 < E 1.40 V , R H E , for 1 < t < 10 s and at 278 ^ Τ < 348 Κ results in oxides whose reduction charge density is between 210 and 1260 μΟ cm" , thus in films whose thickness is between 1 M L of RhOH and 2 M L of Rh(OH) . It is worthwhile mentioning that the electroformation of oxide films on Rh proceeds slower than that on Pt under equivalent polarization conditions (15-17). Thus one may conclude that the oxide formation-reduction behavior at Rh differs not only quantitatively but also qualitatively from that at Pt. p

4

p

2


3

Surface-Chemical Composition of R h Oxide Films by X P S . The XPS data on electrochemically formed Rh oxide films are limited to one paper (21). They indicate that the initial electro-oxidation of Rh involves an electroadsorbed O H group, Rh - O H ^ , and the process takes place beyond 0.55 V , RHE. Upon extension of the polarization potential a new surface species, Rh(OH) , is formed; one monolayer of Rh(OH) is grown upon reaching 1.40 V, RHE. At potential higher than 1.40 V , RHE, formation of RhO(OH) commences on top of 3 M L of Rh(OH) (25). The existing XPS data indicate that extension of the polarization potential beyond some 0.8 - 0.9 V , RHE, results in oxide films with Rh in the +3 oxidation state. Finally, the experimental data and results presented in ref. 21 indicate that the OC1 oxidereduction peak corresponds to the surface process shown in equation 3. 3

3

Temperature Effect. The influence of Τ variation on the electrochemical formation of Rh surface oxides has never been investigated before and in this respect the paper represents a new contribution. In the course of research the authors investigated changes of the oxide growth behavior brought about by Τ variation between 278 and 348 Κ for all the E and t values cited above. A representative sample of the results is shown in Figure 3 and the data demonstrate that upon Τ increase (for E = const and t = const) the amount of the oxide formed increases. One of the major observation that arises from these studies is that the Τ increase results in thicker oxide films than those formed at low Τ but the C V oxide-reduction profiles do not change their characteristics and show the same OC1 peak. On the basis of this observation and the above cited XPS data, one may conclude that the same oxide state, namely Rh(OH) , is formed when Τ is raised from 278 to 348 Κ (for 0.70

Temperature Dependence of Growth of Surface Oxide Films on

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