Unveiling Pseudo‐Inert Basal Plane for Electrocatalysis in 2D Semiconductors: Critical Role of Reversal‐Activation Mechanism

Partially occupied orbitals play a pivotal role in enhancing the performance of electrocatalyst by facilitating electron acceptance and donation, thus enabling the activation of molecular bonds. According to this principle, the basal plane of most 2D semiconductors is inert for electrocatalysis because of the fully occupied orbitals at the surface. Here, taking monolayer CrX (X = P, As, Sb) and Cr2PY (Y = As, Sb) as examples and through first‐principles calculations, it is revealed that even with fully occupied surface orbitals, the basal planes exhibit remarkable catalytic activity for the nitrogen oxide reduction reaction (NORR). This leads to the concept of the pseudo‐inert electrocatalyst. The underlying physics behind such pseudo‐inert character can be attributed to the reversal‐activation mechanism: contrary to conventional expectations, the adsorbed NO molecule reversely triggers the activity of the inert basal plane first, and then the basal plane activates NO molecules, forming the intriguing “Reversal Activation‐Transfer‐Donation‐Backdonation” process. This study further predicts that such pseudo‐inert character can demonstrate many distinctive properties, for example, it can introduce a novel type of surface catalysis, one that selectively targets radicals possessing an inherent dipole moment such as NO. The explored phenomena and insights greatly enrich the realms of electrocatalysis and 2D materials.


Introduction
The escalating energy demands and growing chemical consumption are compelling our society to rely more on renewable, ecofriendly, and sustainable alternative energy sources. [1]Among DOI: 10.1002/aenm.202303953various techniques to mitigate this issue, electrocatalysis takes a central role by enabling the conversion of electrical energy into chemical bonds. [2]This process has the remarkable ability to transform exhaust molecules, such as N 2 , [3] NO, [4] and CO 2 , [5] into valuable fuels and chemicals, including NH 3 , CH 4 , and C 2+ products.In electrocatalysis, the electrocatalyst plays a critical role. [6]To date, a wide variety of electrocatalysts has been explored to facilitate these reactions, and substantial improvements have been achieved. [7]The hallmark of an electrocatalyst lies in the existence of a combination of empty and occupied orbitals.Specifically, the active centers should feature partially occupied orbitals, ensuring the acceptance or donation of electrons from adsorbates. [6]Such inherent nature would guarantee the effective adsorption and weaken the stability of the adsorbates, promoting the activation of adsorbates and thereby facilitating effective catalyzing processes. [6]n recent years, two-dimensional (2D) materials have emerged as pioneering frontiers in electrocatalysis, and more and more 2D structures have demonstrated good electrocatalytic activity. [8]his phenomenon can be chiefly attributed to their expansive specific surface area, which potentially supplies plenty of reactive sites. [9]However, it is worth noting that the intrinsic basal plane of most 2D semiconductors is typically composed of fully occupied orbitals.The absence of combination of empty and occupied orbitals in most 2D semiconductors significantly hampers their ability to destabilize adsorbates, and thus limiting their electrochemical activity. [10]Hence, the prevailing belief is that 2D semiconductors with basal plane consisting of full-occupied orbitals are inert for electrochemical reactions, [11] and 2D semiconductors with high basal plane activity toward electrocatalysis have not been reported yet.
In this study, utilizing monolayer CrX (X = P, As, Sb) and Cr 2 PY (Y = As, Sb) as illustrative models and through firstprinciples calculations, we propose that the basal plane of 2D semiconductors, which is composed of atoms with full-occupied orbitals, is able to exhibit high catalytic activity intrinsically.This is in contrast to the generally accepted knowledge, [12] and we introduce the term "pseudo-inert electrocatalyst" to encapsulate this novel concept.We elucidate that the foundation of this pseudo-inert behavior is linked to the reversal-activation mechanism.Specifically, the adsorbed NO molecule reversely triggers the activity of the inert basal plane first, and then the basal plane activates NO molecules, which thereby generates the proposed "Reversal Activation-Transfer-Donation-Backdonation" process.
Based on the concept of pseudo-inert electrocatalyst, we predict that intriguing phenomena like the completely exothermic NORR processes could occur in such pseudo-inert systems.

Results and Discussion
Figure 1a,b illustrates the crystal structures of monolayer CrX and Cr 2 PY, respectively.Monolayer CrX features a square lattice with P4/NMM symmetry, and its unit cell contains two Cr atoms and two X atoms.In contrast, monolayer Cr 2 PY also displays a square lattice but with P4MM symmetry, containing one P atom, one Y atom, and two Cr atoms within its unit cell.The optimized lattice constants of these systems are shown in Table S1 (Supporting Information), which are well consistent with the previous reports. [13]To characterize the bonding character, the electron location functions (ELF) of these systems are analyzed, with corresponding results depicted in Figure 1c,d and Figure S1 (Supporting Information).Obviously, the electrons are mainly localized around the X atom and almost absent around the Cr atom, indicating a typical ionic bonding character for the Cr─X bonds.Based on the definition of cohesive energy(E coh ), the more positive value corresponds to a more stability.E coh of monolayer CrP, CrAs, CrSb, Cr 2 PAs and Cr 2 PSb are calculated to be 2.787, 2.515, 2.163, 2.654, and 2.350 eV atom −1 , respectively (see Table S1, Supporting Information).Notably, these values are significantly lower than those of graphene (≈7.9 eV atom −1 ) [14] and H-MoS 2 (≈5.1 eV atom −1 ), [15] but compare to that of phosphorene (≈3.5 eV atom −1 ), [16] suggesting their good thermodynamic stability.Figure S2 (Supporting Information) shows their phonon spectra, which have no imaginary frequency in the whole Brillouin zone, indicating their dynamical stability.The corresponding results of the AIMD simulations are shown in Figure S3 (Supporting Information).Except monolayer CrSb, the other four systems are thermally stable.Therefore, in the following, we analysis focuses exclusively on monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb.Figures S4 and S5 (Supporting Information) show their band structures.Density-functional theory (DFT) calculations with the PBE+U functional predict monolayer CrP, CrAs, and Cr 2 PSb as narrow band-gap semiconductors with bandgaps of 0.035, 0.059, and 0.069 eV, respectively.Intriguingly, we find that monolayer Cr 2 PAs exhibits moderate bandgap semiconductor characteristics with a bandgap of 0.145 eV.To obtain more reliable bandgaps, we applied the HSE06 hybrid functional, which predicts these systems to be moderate bandgap semiconductors with bandgaps of 0.536, 0.427, 0.595, and 0.520 eV, respectively, for monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb.
The valence electronic configuration of an isolated Cr atom is 3d 5 4s 1 .The occupation of isolated Cr-3d orbitals is shown in .By donating three electrons to the neighboring P atoms, the valence electronic configuration of Cr becomes 3d 3 4s 0 , which would yield a magnetic moment of 3 μ B .Our firstprinciples calculations confirm that monolayer CrP indeed is spin-polarized, and the magnetic moment is calculated to be ≈3 μ B per Cr atom, which is mainly localized on the Cr atoms (see Figure 2b).These results are confirmed by the partial density of states (PDOS) of Cr-3d orbitals shown in Figure 2c.For an isolated P atom, the valence electronic configuration is 3s 2 3p 3 .The interaction between s and p orbitals results in four degenerate sp 3 hybrid orbitals (see Figure 2a).Upon accepting three electrons from Cr atom, the four degenerate sp 3 hybrid orbitals of P are fully occupied.Generally speaking, as illustrated in Figure 2d, such full-occupied orbitals tend to be stable and less inclined to donate or accept electrons, especially at low temperatures.Physically, to activate adsorbates, an electrocatalyst should possess the ability to disrupt their stability.This involves the capacity to accept or donate electrons, meaning there should be partially occupied orbitals within the electrocatalyst. [17]According to this principle, the surface P atoms with full-occupied orbitals seem to be inert for adsorbates.These observations also extent to monolayer CrAs, Cr 2 PAs and Cr 2 PSb (see Figures S6-S8, Supporting Information).
To confirm the inert character of their basal plane, we explore the potential of these systems for N 2 reduction reaction (NRR) toward NH 3 .A crucial step for NRR is the effective adsorption and activation of N 2 , which has decisive influence on the subsequent reactions. [18]Here, we consider two typical configurations for N 2 adsorption, namely N 2 -side and N 2 -end configurations, as shown in Figure 3a.Our results show that, for all these four systems, both the N 2 -side and N 2 -end configurations transform into the physisorption configuration (see Figure 3b; Figure S9a, Supporting Information) after optimization.The adsorption energies (ΔG *N2 ) are calculated to be 0.53, 0.54, 0.54, and 0.47 eV, respectively, for monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb.This prevents the effective adsorption and activation of N 2 on these systems, indicating that they are inert for NRR.These results are expected from the analysis above that the basal plane exhibits full-orbital nature.However, in the following, we show that all these systems actually are pseudo-inert electrocatalysts, which have excellent performance for NO reduction reaction (NORR) toward NH 3 .
As the chemisorption of NO is the prerequisite for NORR, [19] we first study the properties of NO adsorption on these systems.Here, we select P atom as active sites and three typical configurations for NO adsorption, that is, N-end, O-end and NO-side, are considered (see Figure 3c).The corresponding adsorption energies (ΔG *NO ) of these three configurations on monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb are summarized in Figure 3d.Obviously, among these three typical configurations, the NO-side configuration is the most stable one, on which we will focus in the remainder.As shown in Figure S10 (Supporting Information), we also investigate the other adsorption sites, and find that the P atom is the most preferable adsorption sites for all these four systems.The effective adsorption of NO is the beginning of the NORR, [19] which plays an important role in subsequent steps.The calculated ΔG *NO for these four systems are negative (see Figure 3e), and the N─P bond length are calculated to be ≈2 Å, indicating the formation of bond between NO molecule and the catalyst's surface (see Figure S9b, Supporting Information), followed by activation of the catalyst.This is in sharp contrast to the general knowledge that basal plane with full-occupied orbitals is inert for adsorbates.To describe this intriguing phenomenon, we introduce the concept of "pseudo-inert".Additionally, considering that protons may preferentially adsorb on the surface of the catalysts and block the NO adsorption, we further explore the hydrogen evolution reaction (HER) performance.The Gibbs free energies changes of HER process (ΔG *H ) on monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb are calculated to 0.19, 0.32, 0.16, and 0.11 eV, respectively.(see Figure 3e) The positive ΔG *H would significantly inhibit the HER process.Besides, we find the catalyst's surfaces to be repulsive to competing molecules (such as NH 3 ).As shown in Figure S11 (Supporting Information), the adsorption free energies of NH 3 on monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb are 0.36, 0.35, 0.33, and 0.35 eV, respectively.
Recently, some exploration of intrinsic mechanisms in electrocatalytic systems have been reported. [20]To elucidate the underlying physics behind this pseudo-inert behavior, we first investigate the discrepancy in the intrinsic properties between N 2 and NO. Figure 3f,g shows the electron occupation of molecular orbitals of N 2 and NO, respectively.NO, owing to its half-occupied  * 2p orbital, exhibits greater activity compared to the inert N 2 .Figure 3h shows the polarized electron distribution of NO, which facilitates electron transfer to the basal plane, in contrast to nonpolarized N 2 (see Figure 3i).Upon contacting with the basal plane, NO, influenced by the joint effects of the half-occupied orbitals and the asymmetric electron cloud distribution, reversely activates the pseudo-inert basal planes of monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb.Conversely, because of the absence of such distinctive joint effects, the reversal-activation is forbidden for the N 2 adsorption.It should be noted that the change in electrochemical potential would influence orbital occupation, which might affect the mechanism as it relies on the electronic occupancy of orbitals.
For confirming the reversal activation mechanism, we further study the adsorption properties of O 2 and CO molecules on these systems.Paramagnetic O 2 (Figure S12a, Supporting Information) does not reversely activate the pseudo-inert basal planes of these systems.Our results indeed show O 2 to be inactive, as both the side and end configurations result in the same structure with preserved triplet and with repulsive adsorption energy (ΔG *O2 ), as shown in Figures S13a,d and Figure S14 (Supporting Information).On the other hand, the polarized CO molecule (see Figure S12d, Supporting Information) accumulates electron density around the C atom, promoting favorable adsorption on the catalyst's surfaces (see Figure S13d, Supporting Information).However, since CO lacks half-filled orbital, it does not trigger the pseudo-inert basal planes of our systems, and the corresponding adsorption energies (ΔG *CO ) are close to zero (see Figures S12b,c and S13e, Supporting Information).We conclude that activation of the reversal-activation mechanism requires polarized molecules with partially filled orbitals.In addition, to elucidate whether catalysts of CrX and Cr 2 PY are specialized for the adsorption of NO, we also consider the adsorptions of NO 3 − and NO 2 − .Figure S15 (Supporting Information) shows the corresponding adsorption configurations of NO 3 − and NO 2 − after optimization.The distances between NO 3 − (NO 2 − ) to catalyst's surface are 1.57(1.90),1.80(2.12),1.58(1.90),and 1.63(1.91)Å for monolayer CrP, CrAs, Cr 2 PAs, and Cr 2 PSb, respectively.Besides, the calculated adsorption energies of NO 3 -(ΔG *NO3-)and NO 2 -(ΔG *NO2-)for these four systems are all negative (Figure S16, Supporting Information), which suggests that NO 3 − and NO 2 − can be also effectively absorbed.
To gain deeper insights into the reversal-activation mechanism, we analyzed in detail the activation process of NO on the pseudo-inert basal plane.Our proposed mechanism involves four distinct steps, that is, the "Reversal Activation-Transfer-Donation-Backdonation" process (see Figure 4).In the initial step, NO adsorption reversely activates the inert P sites.For polarized NO molecule, it has the half-occupied  * 2p orbital.When bonding with P atom, the electrons located on sp 3 hybrid orbital of P atom are temporarily transferred to the half-occupied  * 2p orbital of NO molecule.This transfer process realizes a subtle electronic modulation of P atom, which we refer to "Reversal Activation".In fact, the reversal activation is accompanied with adsorption of NO, and they occur simultaneously.In the second step, denoted as "Transfer", the electrons in sp 3 hybrid orbitals of P atom (Psp 3 ) transfer to the d orbital of Cr atom, enabling P atom to have empty orbitals to capture electrons from NO molecule.In the third step, known as "Donation", the empty P-sp 3 orbitals accept the electrons from  orbitals of NO.In the fourth step, "Backdonation" occurs, during which the electrons in P-sp 3 orbitals are backdonated to the antibonding orbital of NO.These processes complete the electron transfer between the pseudo-inert basal plane and NO molecule.
Taking monolayer CrP as an example, we demonstrate that this process is supported by the evaluation of d-band center of Cr (Cr- d ) and the orbital overlap between NO-2p orbitals and P-sp 3 orbitals.Upon adsorbing NO, Cr- d undergoes a shift from −2.42 to −2.56 eV (see Figure 5a).Such a down-shift of Cr- d favors electron transfer into the Cr-d orbitals ("Transfer" step).By focusing on the orbital overlap between NO-2p orbitals and P-sp 3 orbitals (see Figure 5b; Figure S18, Supporting Information), we can see that when adsorbing NO, the NO- orbitals interact with P-sp 3 orbitals below the Fermi level, resulting in occupied sp 3 - orbitals, which relates to the "Donation" process.Meanwhile, NO-* orbitals interact with P-sp 3 orbitals around the Fermi level, and resulting in partially occupied sp 3 -* orbitals, thus confirming the occurrence of the "Backdonation" process.The latter two steps can be further validated by the charge density differences shown in Figure 5c, which reveal significant charge transfer between P atom and NO molecule.Similar conclusions are reached when examining monolayer CrAs, Cr 2 PAs, and Cr 2 PSb, as shown in Figures S17-S21 (Supporting Information).In addition, there are some experimental reports indicating the pseudo-inert character in 2D materials.For example, Han et al. [21] shows that electrophilic addition of Lewis acids (such as AlCl 3 ) to sulfides results in chemical activations of inert base planes in 2D transition metal chalcogenide nanostructures.Besenbacher et al. [22] reports that the basal plane of MoS 2 can adsorb dibenzothiophene.Jia et al. [23] find that the basal plane of TiS 2 nanosheet exhibits efficient electrocatalytic performance for N 2 toward NH 3 under ambient conditions.These experimental works suggest to further explore the "Reversal Activation-Transfer-Donation-Backdonation" mechanism also in other 2D materials.
Next, we study the catalytic performance of the pseudo-inert basal planes for NORR toward NH 3 synthesis.In view of the fact that NO prefers to be adsorbed under the NO-side configuration on these systems, we consider six possible reaction pathways for the NO-to-NH 3 conversion, that is, O-distal, O-alternating, N-distal, N-alternating, Mixed-I and Mixed-II (see Figure S22, Supporting Information).There are five protonation steps for lent electrocatalytic activity for NORR, which outperforms most reported electrocatalysts. [24]In addition, taking monolayer CrP as an example, we test the Gibbs free energy pathway of NO-to-NH 3 conversion by HSE06 method (Figure S23  * NH 2 → * NH 3 ) (see Figure 6g,h).It is worth noting that these energetically favorable pathways for monolayer Cr 2 PAs and Cr 2 PSb are exothermic processes, which implies that the NORR on the pseudo-inert basal planes of these two systems can occur spontaneously at room temperature.Therefore, monolayer Cr 2 PAs and Cr 2 PSb are exceptional electrocatalysts for the conversion of NO to NH 3 , even surpassing the performance of monolayer CrP and CrAs.In addition, it can be seen that there is no close relationship between the catalytic performances and their bandgaps.
At last, we discuss why the catalytic performances of monolayer Cr 2 PAs and Cr 2 PSb are superior than that of monolayer CrP and CrAs. Figure S24 (Supporting Information) presents the plane averaged electrostatic potential of the four title catalysts.
For monolayer CrP and CrAs, the electrostatic potential of the two surfaces align with the vacuum energy, giving rise to the electrostatic potential differences (Δ) of 0 eV.This indicates that there is no dipole moment along the out-of-plane direction, which correlates to the existence of the space inversion symmetry for monolayer CrP and CrAs.While for monolayer Cr 2 PAs and Cr 2 PSb, the absence of the space inversion symmetry leads to the electrostatic potentials differing by 0.09 and 0.23 eV, respectively, for monolayer Cr 2 PAs and Cr 2 PSb.Such electrostatic potential difference accelerates the frontier electrons transfer, which enhance their electrocatalytic activity.With these results in hand, we can understand why the catalytic performance of monolayer Cr 2 PAs and Cr 2 PSb are superior than that of monolayer CrP and CrAs.

Conclusion
To summarize, using first-principles calculations, we unveil that the basal plane of 2D semiconductors, comprising atoms with full-occupied orbitals, possesses remarkable intrinsic catalytic activity.We reveal that the physics of the pseudo-inert nature is related to the reversal-activation mechanism.More importantly, we discover a new form of surface catalysis that selectively target radicals with inherent dipole moment, such as NO.In detail, the adsorbed NO molecule reversely triggers the activity of the inert basal plane first, and then the basal plane activates NO molecules, generating the "Reversal Activation-Transfer-Donation-Backdonation" process.Besides, intriguing phenomenon like the completely exothermic NORR processes is demonstrated in such pseudo-inert systems.

Experimental Section
All the spin-polarized density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP). [25]The cutoff energy of plane-wave basis set was 500 eV.The energy and force convergence thresholds were set to 10 −5 eV and 0.01 eV Å −1 , respectively.Reciprocal space was sampled by Monkhorst-Pack k-point scheme with 3 × 3 × 1 grid meshes.Given strong correlation in 3d electrons of Cr atom, DFT+U method was employed, and the U-J value was set to 3 eV. [26]rimme's empirical D3 scheme was applied to account for the London dispersion interaction. [27]A large vacuum slab of ≈20 Å was adopted along the z direction.The phonon dispersion calculations were employed for 3 × 3 × 1 supercell based on the density-functional perturbation theory method as implemented in the PHONOPY code. [28]The ab initio molecular dynamic (AIMD) simulations were carried out at 300 K for 5 ps.
To assess the catalytic activity, the Gibbs free energy change (ΔG) of each step in the NORR process is obtained by. [29] = ΔE DFT + ΔE ZPE − TΔS + ΔGU + ΔG PH (1) Here, ∆E DFT , ∆E ZPE , and ∆S are the reaction energy difference, zero-point energy difference, and entropy difference, respectively.T refers to 298.15 K. ΔG U = -eU, which is the free energy contribution related to applied electric potential U. ΔG pH = k B T × ln10 × pH, which is the energy correction of pH (pH = 0).The cohesive energy of CrX (Cr 2 PY) is calculated following E coh = 2E Cr + 2E X -E CrX (E coh = 2E Cr + E P + E Y -E Cr2PY ), where E CrX , E Cr2PY , E Cr and E X refer to the total energies (including DFT energy and zero-point energy) of CrX and Cr 2 PY, and the energies of isolated Cr and X atoms, respectively.

Figure 1 .
Figure 1.Crystal structures of monolayer a) CrX (X = P, As, Sb) and b) Cr 2 PY (Y = As, Sb) from top and side views.Electron location function (ELF) maps of monolayer c) CrP and d) Cr 2 PAs, wherein 1.0 and 0.0 refer to the areas with the highest and lowest electron densities, respectively.

Figure 2 .
Figure 2. a) Evaluation of the valence electronic configurations of Cr (P) atom under crystal field theory (sp 3 hybridization).b) Valence electronic configurations of Cr and P atom under distorted tetrahedral geometry of CrP 4 , and spin charge density of monolayer CrP.c) The corresponding partial density of states (PDOS) of Cr-3d orbitals for monolayer CrP.d) Schematics of why surface P atom is inert.

Figure 2a .
Under the distorted tetrahedral coordination environment shown in Figure 2b, the Cr-3d orbitals split into three highlying orbitals (d xy , d xz , d yz ) close in energy and two low-lying orbitals (d x2-y2 , d z2 )

Figure 3 .
Figure 3. a) Two different adsorption configurations of N 2 (N 2 -end and N 2 -side), b) physisorption configurations of N 2 and NO and c) three different adsorption configurations of NO (N-end, O-end, and NO-side), wherein the red, blue and white balls refer to oxygen, nitrogen and hydrogen atoms, respectively.d) Color-map for adsorption energies (ΔG *NO ) of NO under three different adsorption configurations, where the gray shadow refers that the corresponding configuration does not exist.e) Comparison of adsorption energies of NO and N 2 and Gibbs free energies changes of HER process (ΔG *H ), where ΔG *NO refers to the adsorption energy of the most stable NO-side configuration.Orbital hybridization diagrams of the free f) NO and g) N 2 molecules.The PDOS for free h) NO and i) N 2 molecules, the insets in (h) and (i) refer to electron cloud density of molecular orbitals.
Figure 5. a) Schematic diagrams of the evaluation of Cr-d band center and Cr- d on monolayer CrP, CrAs, Cr 2 PAs and Cr 2 PSb before and after NO adsorption.b) PDOS of free NO molecule, monolayer CrP as well as NO-adsorbed CrP, and corresponding projected crystal orbital Hamilton population (-pCOHP) for P-NO interaction in NO-adsorbed CrP.c) Charge density differences for NO-adsorbed monolayer CrP.

Figure 6 .
Figure 6.Gibbs free energy diagrams for NORR process on monolayer a) CrP, c) CrAs, e) Cr 2 PAs, and g) Cr 2 PSb through different pathways, and the pink lines refer to the most favorable pathways.Intermediate structures along the corresponding most favorable pathways for monolayer b) CrP, d) CrAs, f) Cr 2 PAs, and h) Cr 2 PSb.H, N, O, P, Cr, As, and Sb atoms are shown in white, dark blue, red, purple, pink, green, and light blue colors, respectively.