Trends in the adsorption and reactivity of hydrogen on magnesium silicate nanoclusters

Abstract

We study nanoclusters of Mg-rich olivine and pyroxene (having (MgO)6(SiO2)3 and (MgO)4(SiO2)4 compositions) with respect to their reactivity towards hydrogen atoms, using density functional calculations. Ultrasmall silicate particles are fundamental intermediates in cosmic dust grain formation and processing, and are thought to make up a significant mass fraction of the grain population. Due to their nanoscale dimensions and high surface area to bulk ratios, they are likely to also have a disproportionately large influence on surface chemistry in the interstellar medium. This work investigates the potential role of silicate nanoclusters in vital interstellar hydrogen-based chemistry by studying atomic H adsorption and H2 formation. Our extensive set of calculations confirm the generality of a Brønsted-Evans-Polanyi (BEP) relation between the H2 reaction barrier and the 2Hchem binding energy, suggesting it to be independent of silicate dust grain shape, size, crystallinity and composition. Our results also suggest that amorphous/porous grains with forsteritic composition would tend to dissociate H2, but relatively Mg-poor silicate grains (e.g. enstatite composition) and/or more crystalline/compact silicate grains would tend to catalyse H2 formation. The high structural thermostability of silicate nanoclusters with respect to the heat released during exothermic H2 formation reactions is also verified.

H2 Formation on Cosmic Grain Siliceous Surfaces Grafted with Fe+: A Silsesquioxanes‐Based Computational Model

“…When compared to other siliceous models with the Mg−Si−O centers involved in the H 2 synthesis, the Fe‐POSS system favorably shifts all the energies (from thermodynamic potentials to barrierless kinetics) toward the H 2 formation because of the Fe catalytic center.…”

“…Though not receiving the same attention as ice and carbonaceous‐based surfaces, the theoretical analysis of siliceous surfaces like crystalline Mg 2 SiO 4 ( forsterite ), Fe 2 SiO 4 ( fayalite ), olivine and Mg‐silicates nano‐clusters has been considered . The net results showed that although H 2 formation is possible, reaction mechanisms are a function of the crystalline surfaces (001, 010, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed …”

“…[2,[8][9][10] Such am odel, basically al owtemperature heterogeneous phase catalysis, has been expanded to include not only the formation of molecular hydrogen but also many of the chemical specieso bserved in space. [11,12] In the last decade the experimental analysiso fg rain surfaces with differentc hemical make-up has produced an impressive amount of models and data [10] spanning from amorphous solid water, [13,14] to amorphous carbon, [15] graphite, [16] oxidized graphite, [14] polyaromatic hydrocarbons (PAHs), [17] siliceous nanoclusters [18,19] ( % 10 %o ft he total mass fraction of the interstellar siliceous grain population [20] ), crystalline anda mor-phouss ilicates [21][22][23][24][25] with the last attracting an increasing interesta si tr epresents one of the main cosmic dust constituents.…”

“…[26,33,34] Though not receiving the same attention as ice and carbonaceous-based surfaces, [16] the theoretical analysis of siliceous surfaces like crystalline Mg 2 SiO 4 (forsterite), [35][36][37] Fe 2 SiO 4 (fayalite), [38] olivine [39] and Mg-silicates nano-clusters has been considered. [18,19,40] The net results showedt hat although H 2 formation is possible, reaction mechanisms are af unction of the crystalline surfaces (001,0 10, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed. [18,19,[35][36][37][38][39][40] Recently,o ne of us hass uggested that the role of transition metals (TMs) in astrochemistry shouldr eceive greater attention from the scientificc ommunity [41,42] following research threads dating back to the early 1980s.…”

“…[18,19,40] The net results showedt hat although H 2 formation is possible, reaction mechanisms are af unction of the crystalline surfaces (001,0 10, 110), atoms and defects (metal cation vacancies) where the adsorption/reaction sites are placed. [18,19,[35][36][37][38][39][40] Recently,o ne of us hass uggested that the role of transition metals (TMs) in astrochemistry shouldr eceive greater attention from the scientificc ommunity [41,42] following research threads dating back to the early 1980s. [43][44][45][46][47][48] In the last two decades, an increasing attention of the TM involvementi na stro/cosmochemicalp rocesses was developed [28,28,[49][50][51][52][53] coupled to observational/laboratory spectral characterization efforts in the search of gas-phase inorganic astrochemicalsa s[ FeCN], [54] [FeCO], [55] [FeCO] + [49] and Fe-PAHs.…”