The Origin of Electrochemical Promotion in Heterogeneous Catalysis:  Photoelectron Spectroscopy of Solid State Electrochemical Cells

Abstract

Electropumping of Na from or to a Na-′′ alumina solid electrolyte contacted with a thin film porous copper electrode results in fully reversible transport of Na to or from the vacuum-exposed Cu surface. The extent of pumping is controlled by the potential of the catalyst film (V WR ), measured with respect to a reference electrode. The time constants of these spill over and reverse spill over processes are short compared with 1 min. Photoelectron microscopy suggests that the spatial distribution of Na is fairly uniform. Over an extended range of catalyst potential (∆V WR ∼ 1 V), both the Na coverage (ϑ Na ) and the Cu work function (φ) scale linearly with V WR . This is the same regime over which the rate and nitrogen selectivity of the Cu-catalyzed CO+NO reactions are greatly increased. The maximum Na coverage achieved by electro-pumping is ∼0.06 monolayer, commensurate with the corresponding catalytic response of the system. At sufficiently high positive values of V WR , this quantity becomes uncoupled from ∆φ, ϑ Na , and the catalytic behavior. The possible origin of this uncoupling effect is discussed and a consistent explanation offered for the phenomenon of electrochemical promotion.

A Review of Surface Analysis Techniques for the Investigation of the Phenomenon of Electrochemical Promotion of Catalysis with Alkaline Ionic Conductors

“…This shift is numerically equal to the change in catalyst potential, strongly suggesting that this emission arises from the underlying solid electrolyte, whose electrostatic potential differs from that of the Rh film by the change in V WR . This interpretation was confirmed by the experimental results obtained by grazing exit synchrotron photoemission, where the signal from the electrolyte vanished [46]. As also stated in the other referenced studies, the spectral behaviour was reversible and reproducible as a function of V WR , consistent with the reversible and reproducible catalytic response observed during the electrochemical promotion of the Rh catalyst for the NO reduction reaction with both CO and C 3 H 6 [41].…”

“…The +1 V spectrum corresponds to the clean (unpromoted) sample, while increasingly negative values of V WR correspond to increasing amounts of electropumped Na + on the catalyst surface. As typically observed in these studies [35,38,[40][41][42]44,46], the Na 1s emission comprises two components. The first one exhibits invariant binding energy (BE) and its intensity increases with decreasing V WR , i.e., as Na + ions are electropumped to the catalyst.…”

“…In two of the previously mentioned studies [38,46], scanning photoelectron microscopy (SPEM) was also used to in situ analyze the surface of Cu [46] and Pt [38] catalysts promoted by Na + and K + , respectively. For instance, Figure 7 shows the results obtained on this latter work for the alkali-promoted acetylene hydrogenation reaction.…”

Solid electrochemical mass spectrometry (SEMS) for investigation of supported metal catalysts under high vacuum

“…Surface analysis techniques, e.g. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) [1,2], scanning photoelectron spectroscopy (SPEM) [3][4][5], photoelectron emission microscopy (PEEM) [4][5][6][7], scanning tunneling microscopy (STM), are usually coupled to electrochemistry to investigate electrochemical deposition and adsorption at the electrode while spectroscopic techniques, e.g. infrared-spectroscopy (IR), gas chromatography (GC) [8,9], mass spectrometry (MS) [8][9][10][11], are coupled to monitor (qualitatively and quantitatively) the products of an electrochemical reaction.…”

Electrochemical Promotion by Potassium of Rh-Catalysed Fischer–Tropsch Synthesis at High Pressure

“…This in situ control of promoter concentrations provides a unique means for studying the mechanisms of heterogeneously catalysed reactions [3]. EP is implemented (figure 1a) by interfacing a thin film catalyst with a solid electrolyte (in the present case K b'' alumina, a K + conductor) and by varying the overpotential of the catalyst, electrochemically pumping the promoter species from/to the solid electrolyte to/from the catalytically active surface [4,5].…”