Electrochemical activation of an oblique angle deposited Cu catalyst film for H₂ production

Abstract

A novel Cu catalyst film was prepared by oblique angle physical vapour deposition (OAD) on a K-βAl 2 O 3 solid electrolyte (alkaline ionic conductor) for catalytic/electrocatalytic purposes. This technique allowed us to obtain a highly porous and electrically conductive Cu catalyst electrode which was tested in the partial oxidation of methanol (POM) reaction for H 2 production and its catalytic activity was in situ enhanced via electrochemical promotion of catalysis (EPOC). The electropromotional effect was reversible and reproducible, and allowed us to increase both hydrogen and methyl formate production rates by almost three times under optimal promotion conditions (320°C, 2.2 × 10 −7 mol of K + transferred). The observed promotional effect was attributed to a decrease in the Cu catalyst work function as a consequence of the controlled migration of electropositive K + ions which favoured the chemisorption of electron acceptor molecules (O 2 ) at the expense of the electron donor ones (CH 3 OH). Under the reaction conditions these ions formed some kinds of potassium surface compounds as demonstrated by SEM, EDX and XPS post-reaction characterization analyses. The obtained results demonstrate the interest of the used catalyst-electrode preparation technique for the electrochemical activation of non-noble metal catalyst films.

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

“…As also revealed by the EDX analysis taken from different areas of the micrograph (Figures 3c1 and 3c2), oxygen-and potassium-containing surface compounds seemed to be formed on the metal catalyst film during the EPOC experiments, probably in form of some potassium oxides or carbonate molecules. In fact, an excess of these surface species (supplied under high cathodic polarization) could block the Cu active sites causing a decrease in the catalytic reaction rate in agreement with conventional chemical promotion [32]. Furthermore, some nitrogen (in red) was also noticed, homogeneously distributed on the catalyst surface, which was attributed to the K + -promotional effect on the ammonia formation via reaction of hydrogen and nitrogen, both of them present in the gas reaction atmosphere.…”

“…As in the case of the electropromoted Cu catalyst film, a poisoning effect derived from an excess of K + -derived surface species was found on this Pt film [31]. Furthermore, in both mentioned studies [31,32], the obtained electropromotional effect was completely reversible since all the promoter phases were decomposed The presence of potassium carbonate/bicarbonate species was also identified on the surface of a promoted Pt/K-βAl 2 O 3 electrochemical catalyst employed in a methanol partial oxidation reaction [31]. In this study, these kinds of promoter-derived compounds were also detected ex situ by X-ray diffraction analysis after EPOC experiments, as shown in Figure 4.…”

“…Urquhart et al used other K + -conductor solid electrolyte (K-βAl 2 O 3 ) in Fischer-Tropsch reaction studies under both atmospheric [21] and high pressure [22], and de Lucas-Consuegra et al introduced the use of this kind of ion-conducting catalyst support for the electrochemical promotion of Pt in CO [23] and propylene [24] oxidation, as well as in NO x reduction reactions [25,26]. More recent alkaline electrochemical promotion studies on CO 2 hydrogenation [27][28][29][30] and methanol conversion reactions [31][32][33] should also be highlighted. Additionally, in order to understand the mechanism of the phenomenon of electrochemical promotion of catalysis with both anionic and cationic conductors, a wide variety of characterization techniques have been used in the fields of catalysis (e.g., TPD, TPO, or work function measurement), electrochemistry (e.g., cyclic/linear sweep voltammetry or impedance spectroscopy), and surface science (e.g., XPS, UPS, SPEM, or STM) [3].…”

“…Urquhart et al used other K + -conductor solid electrolyte (K-βAl2O3) in Fischer-Tropsch reaction studies under both atmospheric [21] and high pressure [22], and de Lucas-Consuegra et al introduced the use of this kind of ion-conducting catalyst support for the electrochemical promotion of Pt in CO [23] and propylene [24] oxidation, as well as in NOx reduction reactions [25,26]. More recent alkaline electrochemical promotion studies on CO2 hydrogenation [27][28][29][30] and methanol conversion reactions [31][32][33] should Vayenas et al performed the first electrochemical promotion study with alkaline solid electrolyte (Na-βAl 2 O 3 ) in 1991 [8]. From this pioneer work, Na + -conductors have been widely employed in many catalytic systems such as ethylene [9,10], CO [11], propane [12] and propylene oxidation [13], NO reduction [14][15][16], Fischer Tropsch synthesis [17], or hydrogenation of benzene [18] and CO 2 [19].…”

“…For instance, Figure 3 shows the SEM micrograph, along with the corresponding elemental mapping and spectra by EDX, of a selected region of a Cu catalyst film deposited on a K-βAl2O3 pellet (K + -conductor) used for the electrochemical promotion of the methanol partial oxidation reaction [32]. Prior to the surface analysis, the catalyst was subjected to certain reaction conditions and an applied potential, VWR = −0.5 V, such that K + promoter ions were electrochemically supplied to the catalyst surface.…”