Interplay between Single and Cooperative H₂ Adsorption in the Saturation of Defect Sites at MgO Nanocubes

Abstract

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H 2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10 −8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O 3C) and Mg at inverse corners (Mg IC) were identified to be the most reactive and to split H 2 irreversibly already in the lowest pressure regime (P H 2 < 10 −3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm −1) in the intermediate range of pressures (10 −3 −1 mbar), along with the appearance of bands at 3605/1225 cm −1 , was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At P H 2 > 1 mbar, fully reversible adsorption was shown to occur at O 4C (either on mono-or diatomic steps) and Mg 3C sites. Another OH/MgH couple (3697/1030 cm −1) that became reinforced at high P H 2 but remained stable upon pumping was correlated to O 3C and Mg IC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H 2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H 2 and other hydrogen-containing molecules, such as CH 4 , on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

Site-Specific Hydrogen Reactivity of Zn0.05Mg0.95O Nanopowders

“…TEM images show that both powders consist of cubically shaped nanoparticles as usually found for pure MgO nanocrystals obtained by CVS. 29 observed, the cubic shape specific of MgO pure nanocrystals is preserved in Zn x Mg 1−x O nanopowders for x < 0.15. 31 This is an important difference regarding the size.…”

“…At this hydrogen pressure, bands at 3610 and 1225 cm −1 pop up in line with those seen on pure MgO upon H 2 adsorption and which were related to cooperative adsorption of stable and less stable bonding. 29 In parallel, bands specific to Zn 0.05 Mg 0.95 O are recorded at 3745, 3610, 1780, 1680, and 1510 cm −1 (see also Figure S2 for the three latter) since the very beginning of the exposure to hydrogen. Their intensity regularly increases up to P H2 = 100 mbar.…”

“…The high values of the hydride frequencies (1510−1780 cm −1 ) indicate the involvement of Zn 2+ ions, since the highest frequencies that we measured for MgH bands are at ∼1400 cm −1 . 29 Finally, at P H2 ≥ 1 mbar (region C), FTIR reveals two prominent and sharp peaks at 3460 and 1325 cm −1 whose intensity promptly increases with the hydrogenpressure, as well as two bands of weak intensity at 1425 and ∼1370 cm −1 . In pure MgO, this set of bands is referred to as reversible chemisorption complex II involving adsorption on O 4C and Mg 3C sites.…”

“…The evolution of spectra upon hydrogen adsorption is in many respects similar to what we observed on pure MgO nanocrystals. 29 Among the first detectable IR bands, which appear up to P H2 = 10 −3 mbar (hereafter labeled region A) are features that in pure MgO are attributed to the irreversible chemisorption complex I related to O 3C (3712 and 3699 cm −1 ) and Mg inverse corners (Mg IC ; 1140, 930−970, and 863 cm −1 ), the latter being a site close to two interpenetrating (100) and (010) faces on MgO(001). 29 The intensities of all these features boost at P H2 > 10 −3 mbar until they reach the saturation limit at P H2 ∼ 1 mbar (region B).…”

“…29 Among the first detectable IR bands, which appear up to P H2 = 10 −3 mbar (hereafter labeled region A) are features that in pure MgO are attributed to the irreversible chemisorption complex I related to O 3C (3712 and 3699 cm −1 ) and Mg inverse corners (Mg IC ; 1140, 930−970, and 863 cm −1 ), the latter being a site close to two interpenetrating (100) and (010) faces on MgO(001). 29 The intensities of all these features boost at P H2 > 10 −3 mbar until they reach the saturation limit at P H2 ∼ 1 mbar (region B). At this hydrogen pressure, bands at 3610 and 1225 cm −1 pop up in line with those seen on pure MgO upon H 2 adsorption and which were related to cooperative adsorption of stable and less stable bonding.…”