Abstract
The core subject of the present paper represents the interrelated spillover (effusion) phenomena both of the primary oxide and the H‐adatoms, their theory and practice, causes, appearances and consequences, and evidences of existence, their specific properties, and their alterpolar equilibria and kinetic behavior, structural, and resulting catalytic, and double layer charging features. The aim is to introduce electron conductive and d‐d interactive individual and composite (mixed valence) hypo‐d‐oxide compounds, of increased altervalent capacity, or their suboxides (Magnéli phases), as the interactive catalytic supports and therefrom provide (i) the strong metal‐support interaction (SMSI) catalytic effect and (ii) dynamic spillover interactive transfer of primary oxides (M‐OH) and free effusional H‐adatoms for further electrode reactions and thereby advance the overall electrocatalytic activity. Since hypo‐d‐oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. In fact, altervalent hypo‐d‐oxides impose spontaneous dissociative adsorption of water molecules and then spontaneously pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/HxWO3) under cathodic and/or its hydrated state (Pt/W(OH)6), responsible for Pt‐OH effusion, under anodic polarization, this way establishing instantaneous reversibly revertible alterpolar bronze features (Pt/H0.35WO3⇔ Pt/W(OH)6) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured‐type electrocatalysts, even of mixed‐valence hypo‐d‐oxide structures (Pt/H0.35WO3/TiO2/C, Pt/HxNbO3/TiO2/C), have for the first time been synthesized by the sol‐gel methods and shown rather high stability, electron conductivity, and nonexchanged initial pure monobronze spillover and catalytic properties. Such a unique electrocatalytic system, as the striking target issue of the present paper, has been shown to be the superior for substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The main target of the present thorough review study has been to throw some specific insight light on the overall spillover phenomena and their effects in electrocatalysis of oxygen and hydrogen electrode reactions from diverse angles of view and broad contemporary experimental methods and approaches (XPS, FTIR, DRIFT, XRD, potentiodynamic spectra, UHRTEM).
Highlights
The present review study represents the theoretical basis of our achievements and advances in electrocatalysis for hydrogen and oxygen electrode reactions, based on individual and hypo-hyperd-d-interelectronic combinations of intermetallic phases [52], interactively supported upon corresponding hypo-doxides and leading to the reversible interrelating composite nanostructured electrocatalysts of bronze Pt/H0.35WO3 versus its hydrated Pt/W(OH)6 type [1,2,3,4, 20, 21], appearing as the effect of displayed reversible spillover phenomena
A typical example is shown in Figure 3(a) for Mo/TiO2 after reduction under hydrogen at 300◦C and exposure to He flow at room temperature (RT)
Since the initial amount of adsorbed water is fixed, while hydrogen is continuously supplied, the conclusion is very interesting: water molecules undergo spontaneous dissociative adsorption (2a) upon anatase titania [15], and they become the subject of both the membrane flow, ((2b) and (2c)), and Ertl’s [30] autocatalytic mechanism of Pt-OH production (1), the latter being reduced by H-adatoms (Pt-H)
Summary
After the dissociative adsorption of water molecules on hypo-d-oxide support of Pt, the Advances in Physical Chemistry fast interactive effusion of H-adatoms over its hydrated (W(OH)6) surface becomes dramatically sped up even at ambient conditions in the ultimate presence of condensed at least monolayered aqueous precipitate [9, 10] This is significant both for the evidence of the extremely fast spillover widespreading migration and thereby resulting with imposed the reversible substrate reduction. The latter leads to the corresponding form of electrocatalytically active bronze (Pt/H0.35WO3) for cathodic processes, in which nonstoichiometric incorporated hydrogen obeys the same free reactive properties like adsorptive H-adatoms (Pt-H) and is the main source for the electrode or heterogeneous catalytic reaction. The point is that spontaneous dissociative adsorption of water molecules imposes much smaller activation energy for transformation of the resulting hydrated W(OH) into corresponding bronze state (Pt/H0.35WO3), even at ambient temperature, than the initial oxide WO3 and, thereby, dramatically facilitated the overall spillover effect under pronounced wet status
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