3.2 N2 adsorption, activation, selective hydrogenation behavior, and the underlying ’capture-charge distribution’ mechanism.
Now, we obtained the structural properties that keep constant induced high detectivity but intrinsic properties largely various homonuclear DAC induced high sensitivity system, and it is essential to investigate how the N2 adsorption, activation, and hydrogenation behavior under the intrinsic-properties-only determined system. For the N2 capture process, we divided it into two capture pathways, as depicted in Figure 2 (c); for the N2 molecule that parallels the plan, each side of the N2 molecule exhibits the same sensitivity to the active site, while the higher active TM site tends to capture the free N2 molecule successively, resulting N2molecule tilts to interact with a stronger side of the TM site. Once the N2 molecule is captured by the adsorption stronger one with end-on configuration, charges quickly transfer between the N2 molecule and the active site, following the ’donation-backdonation’ mechanism. Subsequently, the rest of the active site continues to capture the distal N atom until forming a side-on configuration since its high activity as well. During this process, the charges redistribute again, and an asymmetric charge distribution forms. For the vertical configuration N2 molecule capture process, things get simpler than the parallel one but still follow three capture steps: the proximal N atom is captured by the higher active TM site. It forms the end-on configuration first, while the charge distribution also follows the ’donation-backdonation’ mechanism. However, the distal N atom may not act with the rest of the active site since the N atom is located above the substrate. To relieve the activity of the rest active site, a weak interaction between the captured proximal N atom and the uncaptured TM active site further enlarges the charge asymmetric redistribution in the N2 molecule and accelerates the hydrogen process. The dynamic ’capture-charge distribution-recapture-charge redistribution’ mechanism occurred in both parallel and vertical N2 capture processes corresponding to side-on and end-on configurations.
Meanwhile, charge redistribution asymmetry tends to enlarge gradually under the synergistic effect of atomic diversity-free induced high detective, but sensitive intrinsic properties vary in homonuclear DAC systems. For the hydrogenation process, estimating which side tends to hydrogenate priorly for the side on configuration is controversial. Conventionally, it is necessary to calculate and compare the Gibbs free energy changes of both first hydrogenation sites and select the optimal one. At the same time, this strategy may be imperfect due to the lack of charge distribution analysis. For example, the interaction between proton and distributed charges is vital for the hydrogenation process but lacks consideration in structural optimization. In other words, selecting a desirable side for a proton attack is subjective. It could also get energy and force minimum to obtain optimal hydrogenated configuration due to restarted charge self-consistent calculation. In contrast, the final results may be inaccurate once the selected site is occupied by positive charges (see Figure S4 ).
Thanks to a mass of data collected in our study and the reasonable ’capture-charge distribution-recapture-charge redistribution’ mechanism, several typical charge distribution pictures are shown to select the correct hydrogenate site. As can be seen in Figure S5 (a), after the stronger active TM site sensitively captures the N2 molecule, charges from the N2molecule donate to the TM site and then back donate to the N2 molecule, where the adsorbed N atom act as charges carrier and packaged by positive charges, thus avoid proton’s attack, and among all N2 captured systems, Cr, Fe, Co, Ru, Re, Os in site1; Fe, Pd, Os, Ir in site2; Co, Cu in site3; Mn, Co, W, and Re in site4 belong to this type of N2capture(See in Figure S6 ). For the second adsorption type, which could be regarded as the rest TM active, tends to attract the distal N atom (see in Figure S5 (b)), positive charges still distribute around the bonded N atom, thus avoiding proton’s attack, where Mn, W in site1; Co, Ni, Rh, W, Re in site2; Cr, Ni, Mo, Pd, Ir, Pt in site3; V, Ni, Cu, Ru, Rh, Pd, Os, Pt in site4 show this type of N2 capture configuration (Figure S6 ). After bonding with the second TM site under the ’capture-charge distribution-recapture-charge redistribution’ mechanism, it is clearly shown that the initial bonded N atom still acts as a charge transporter enveloped by positive charges (Figure S5 (c), (d)). And systems such as Co, Cr, Mn, Ir in site1; Fe, W, Ir in site2; Cr, Fe, Co, Mo, Ru, Rh, W, Os, Re, Ir, Pt, Au in site3; V, Cr, Mn, Fe, Co, Mo, W, Re, Os in site4 exhibit this kind of N2 capture configuration. Noticeably, an unexpected N2 captured configuration with negative charges contained in both sides of the N2 molecule may be observed in our homonuclear dual TM atom catalyst system, which can be explained by the little variety-tuning for the homonuclear DAC and the weak adsorption energy of N2, thus are not considered in our researches, as plotted in Figure S5 (e).
In total, the bonded proximal N acts as a charge transporter enveloped by positive charge in both side-on and end-on, as considered in our study, demonstrating that the proton can’t attack the proximal N atoms, thus certain the proton prefers to attack the distal N atom in both side on and end on configurations. Nevertheless, the Gibbs free energy changes of both N2 molecule sides attacked by proton are comparatively calculated, although the charge distribution is not considered. As depicted in Figure 2 (d), nearly all the captured N2 systems with side-on configuration exhibit lower Gibbs free energy changes for the distal site than the proximal one, reasonably meet the results of the charge distribution analysis while may mislead the correct hydrogenate site for W2@site2, Re2@site3, Mn2@site4, and Re2@site4. However, it disclosed fantastic information when concentrating on their structural properties. Taking Mn2@site1 as an example (Figure S4 (a)), the bond lengths between TM-N are 1.87 Å and 1.99 Å, respectively, and can be elongated to 1.94 and 2.08 Å after hydrogenation, where the elongated values increased by 0.07 and 0.09 Å, respectively. That’s to say, it is hard to hydrogenate the proximal N site hydrogenation while easily hydrogenating the distal one, meeting the ’capture-charge distribution-recapture-charge redistribution’ mechanism and further confirmed by the rest of the systems’ structural properties summarized in Table S6 .