Electrochimica Acta 52 (2007) 6195–6205 Mechanism of hydrogen adsorption/absorption at thin Pd layers on Au(1 1 1) Hugues Duncan, Andrzej Lasia ∗ D´ epartement de Chimie, Universit´ e de Sherbrooke, Sherbrooke, Qu´ ebec, Canada J1K 2R1 Received 22 December 2006; received in revised form 21 March 2007; accepted 23 March 2007 Available online 30 March 2007 Abstract Hydrogen adsorption and absorption at thin palladium deposits of 0.8–10 monolayers (ML) on Au(1 1 1) was studied in 0.1 M H 2 SO 4 and HClO 4 using cyclic voltammetry, ac voltammetry, and impedance spectroscopy in the absence and in the presence of poison, crystal violet. Hydrogen adsorption on palladium is more reversible in sulfuric acid than in perchloric acid but it occurs at potentials 30 mV more positive in latter. The charge-transfer resistance exhibits a minimum at ∼0.27 V versus RHE and decreases with increasing in Pd deposit thickness in both acids. Adsorption capacitance at 0.8 ML Pd reaches maximum at the same potential. At other deposits the pseudo-capacitance starts to increase at lower overpotentials indicating the beginning of absorption, even at 2 ML Pd. The double layer capacitance is similar for all the deposits in sulfuric acid and it has a sharp maximum at 0.27 V versus RHE. In perchloric acid a broad maximum is observed. Crystal violet inhibits hydrogen adsorption but makes hydrogen absorption more reversible. The results suggest a fast direct hydrogen absorption mechanism that proceeds in parallel with slower hydrogen adsorption and indirect absorption. © 2007 Elsevier Ltd. All rights reserved. Keywords: Kinetics and mechanism of hydrogen adsorption and absorption; Thin Pd layer; Impedance spectroscopy; Cyclic voltammetry 1. Introduction Absorption of hydrogen in palladium lattice is of great fun- damental interest. However, there is still discussion of the mechanism of hydrogen absorption. Two mechanisms were proposed: (a) indirect absorption, in which the hydrogen goes through an adsorption step [1,2] and (b) direct absorption mech- anism [3–7]. These two mechanisms and their consequences were discussed in our earlier papers [7–9]. Besides, there is always presence of the underpotential hydrogen at the same potential range as hydrogen absorption and the separation of the adsorption and absorption processes is difficult. Moreover, at less positive overpotentials (0 < η < 70 mV) there is an appear- ance of the overpotentially deposited hydrogen and formation of hydrogen in the solution, according to the Nernst equation [7]. In our recent study [10] the effect of crystal violet on the hydrogen adsorption and absorption in Pd membranes and deposits on gold was investigated. It was shown that crystal vio- ∗ Corresponding author. Tel.: +1 819 821 7097; fax: +1 819 821 8017. E-mail address: a.lasia@usherbrooke.ca (A. Lasia). let blocks the hydrogen adsorption but enhances the kinetics of hydrogen absorption, evidenced by the increased reversibil- ity of cyclic voltammetric peaks of hydrogen absorption and a decrease of the charge-transfer resistance. The results suggested the possibility of a direct hydrogen insertion in Pd [7,10]. It was recommended that studies of 1–10 monolayers of palladium on Au(1 1 1) by cyclic voltammetry and impedance spectroscopy could help resolve the absorption mechanism. These studies were performed in the present paper in sulfuric and perchlo- ric acid. First studies of Pd deposition on monocrystals were carried out by Attard and Bannister [11] and Clavilier et al. [12], where Pd was deposited on Pt(1 1 1). Kolb and collabora- tors have studied the initial stages of Pd deposition on Pt(1 1 1), Au(1 1 1) and (1 0 0), and Rh(1 1 1) [13–18] and the electrochem- istry of these thin layers [14,19]. These studies suggested that Pd forms one UPD layer and further layers on Au(1 1 1) remain pseudo-morphic up to ∼10 ML. However, no impedance or other kinetic measurements of hydrogen adsorption were performed on those deposits and no other kinetic information is available. Studies of very thin Pd layers should help resolve the hydrogen absorption mechanism and the kinetics of hydrogen adsorption reaction. The separation of these two processes is not possible 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.03.068