Study of the Mechanism of the Electrochemical Promotion of Rh/YSZ Catalysts for C2H4 Oxidation Via AC Impedance Spectroscopy

Abstract

The electrochemical promotion of Rh/YSZ catalysts for C 2 H 4 oxidation was investigated together with the mechanism of electrochemical promotion via the use of AC impedance spectroscopy. The impedance response consists of two distinct semicircles and provides strong support for the sacrificial promoter mechanism of electrochemical promotion. The impedance analysis of the low frequency part of all spectra show the existence of a finite length Gerischer impedance which corresponds to the effective double layer formed by the spillover of promoting anionic oxygen from the YSZ support to the rhodium/gas interface and its slow consumption there by adsorbed hydrocarbon fragments. AC impedance analysis provides values both for the rate of consumption of the promoting spillover oxygen species as well as for the stability and capacitance the catalyst/gas double layer in various gaseous environments and temperatures.

Theoretical insight into the origin of the electrochemical promotion of ethylene oxidation on ruthenium oxide

“…Furthermore, many highly dispersed electrochemically promoted systems have been established, rendering the practical application of EPOC more attractive 11,12,[21][22][23][24][13][14][15][16][17][18][19][20] . Experimentally, the origin of EPOC is mostly explained by the electrochemically induced promoters (see Figure 1), which have been substantiated by many in-situ and ex-situ techniques: x-ray photoelectron spectroscopy (XPS) 25,26 , in-situ XPS 27 , temperatureprogrammed desorption (TPD) 28,29 , scanning tunneling microscopy (STM), photoemission spectroscopy (PES) 30 , in-situ AC impedance spectroscopy 31,32 , and isotopic exchange 33 . When applying an electrochemical potential across a solid electrolyte on which a metallic catalyst is adsorbed, ions originating from the solid electrolyte, O δin case of YSZ, are (partially) discharged at the three-phase boundary and migrate to the gas exposed catalyst surface to formally form a double layer O δ--δ + (δ + is the mirror charge in the conductor for preserving the neutrality).…”

“…Herein, we rely on the model reaction of ethylene oxidation 31,32,[34][35][36][47][48][49][50] to demonstrate, using ab-initio computations, the effect of electrochemical potential on the surface reactivity. We will elucidate on the one hand the impact of the electrochemical potential on the surface state compared to thermal catalysis and on the other hand, the impact of the potential generated electric field on the reactivity of ethylene and oxygen activation.…”

The Effect of Polarization and Reaction Mixture on the Rh/YSZ Oxidation State During Ethylene Oxidation Studied by Near Ambient Pressure XPS

“…The laser heating is homogenous in an area of 5x5 mm 2 , therefore the temperature distribution is considered uniform in the overall measured cell area. In this setup the two sides of the cell are exposed to the same gas mixture, which corresponds to previously reported experiments on a single chamber reactor [14]. A schematic representation of the cell configuration inside the NAP-XPS chamber is given in Figure 1a.…”

“…ΔΦ=eΔUWR [3][4][5][6][7][8]11]. In some cases, the promoting species can even change the oxidation state of the catalyst [3,[12][13][14]. Thus, both catalytic activity and selectivity can be effected in a pronounced, reversible, and, to some extent, predictable manner [3,6].…”

Electrochemical promotion of carbon supported Pt, Rh and Pd catalysts for H2 oxidation in aqueous alkaline media

“…Thus, under positive current application, OHare electrochemically supplied to the anode electrode/electrolyte interface and finally adsorbed on the catalyst. Labou and Neophytides 4 proposed the reaction scheme of Equations (12) and (13) in order to explain the formation path of OH ad species through the intermediate ionic dipoles (OH − ad ), which are strongly adsorbed on the gas exposed surface of the catalyst forming an effective double layer.…”