We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal-correlation driven adsorption of H2 and CO2 in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO2 adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO2 would suggest. We explain this counter-intuitive result, tracking the behavior to a coincidental hybridization of the O p states with the Ni d states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions. The modular building-block nature of metal organic frameworks (MOFs) and their extraordinary affinity for adsorption of small molecules make these nano-porous materials ideal for technologically important applications. MOFs are used, for example, for gas storage and sequestration [1][2][3][4][5], catalysis [6,7], polymerization [8,9], luminescence [10,11], non-linear optics [12], magnetic networks [13], targeted drug delivery [14], multiferroics [15][16][17], and sensing [18][19][20][21]. The design of novel MOFs with improved properties requires insight into the molecule/MOF interaction. The large unit cells and periodic nature of MOFs make density functional theory (DFT) the prospective tool for a theory exploration. However, both the adsorbate molecule and the MOF's metal centers can carry spin, giving rise to complex magnetic interactions and a molecular-spin response. It is thus crucial that DFT can reliably capture van der Waals (vdW) forces-which govern adsorption in MOFs-in concert with spin effects.Concerning the former, the last decade witnessed the development of DFT descriptions for these forces [22]. Here, the vdW-DF versions [23][24][25][26] stand out by being nonempirical exchange-correlation functionals that are systematic and truly nonlocal extensions beyond LDA [27] and GGA [28] in the electron-gas tradition [22,29,30]. Subsequent developments include variants which differ by their choice of the semi-local exchange [31][32][33][34][35] and related nonlocal correlation functionals that rely on optimizing parameters [36][37][38]. The vdW-DF method and relatives have been successfully applied to numerous materials in general [22,29,39], and to smallmolecule adsorption in MOFs in particular [4,5,[40][41][42][43][44][45][46].Concerning the spin effects, however, a systematic description within the vdW-DF framework is still missing. Such effects can play important roles ...

Water dissociation represents one of the most important reactions in catalysis, essential to the surface and nano sciences [e.g., Hass et al., Science, 1998, 282, 265-268; Brown et al., Science 2001, 294, 67-69; Bikondoa et al., Nature 2005, 5, 189-192]. However, the dissociation mechanism on most oxide surfaces is not well understood due to the experimental challenges of preparing surface structures and characterizing reaction pathways. To remedy this problem, we propose the metal organic framework MOF-74 as an ideal model system to study water reactions. Its crystalline structure is well characterized; the metal oxide node mimics surfaces with exposed cations; and it degrades in water. Combining in situ IR spectroscopy and first-principles calculations, we explored the MOF-74/water interaction as a function of vapor pressure and temperature. Here, we show that, while adsorption is reversible below the water condensation pressure (~19.7 Torr) at room temperature, a reaction takes place at ~150 ˚C even at low water vapor pressures. This important finding is unambiguously demonstrated by a clear spectroscopic signature for the direct reaction using D 2 O, which is not present using H 2 O due to strong phonon coupling. Specifically, a sharp absorption band appears at 970 cm -1 when D 2 O is introduced at above 150 ˚C, which we attribute to an O-D bending vibration on the phenolate linker. Although H 2 O undergoes a similar dissociation reaction, the corresponding O-H mode is too strongly coupled to MOF vibrations to detect. In contrast, the O-D mode falls in the phonon gap of the MOF and remains localized. First-principles calculations not only positively identify the O-D mode at 970 cm -1 but derive a pathway and kinetic barrier for the reaction and the final configuration: the D (H) atom is transferred to the oxygen of the linker phenolate group, producing the notable O-D absorption band at 970 cm -1 , while the OD (or OH) binds to the open metal sites. This finding explains water dissociation in this case and provides insight into the long-lasting question of MOF-74 degradation. Overall, it adds to the understanding of molecular water interaction with cation-exposed surfaces to enable development of more efficient catalysts for water dissociation.2

Supporting InformationABSTRACT: The importance of co-adsorption for applications of porous materials in gas separation has motivated fundamental studies, which have initially focused on the comparison of the binding energies of different gas molecules in the pores (i.e. energetics) and their overall transport. By examining the competitive co-adsorption of several small molecules in M-MOF-74 (M= Mg, Co, Ni) with in-situ infrared spectroscopy and ab initio simulations, we find that the binding energy at the most favorable (metal) site is not a sufficient indicator for prediction of molecular adsorption and stability in MOFs. Instead, the occupation of the open metal sites is governed by kinetics, whereby the interaction of the guest molecules with the MOF organic linkers controls the reaction barrier for molecular exchange. Specifically, the displacement of CO 2 adsorbed at the metal center by other molecules such as H 2 O, NH 3 , SO 2 , NO, NO 2 , N 2 , O 2 , and CH 4 is mainly observed for H 2 O and NH 3 , even though SO 2 , NO, and NO 2 , have higher binding energies (~70-90 kJ/mol) to metal sites than that of CO 2 (38 to 48 kJ/mol) and slightly higher than water (~60-80 kJ/mol). DFT simulations evaluate the barriers for H 2 O CO 2 and SO 2 CO 2 exchange to be ∼ 13 and 20 kJ/mol, respectively, explaining the slow exchange of CO 2 by SO 2 , compared to water. Furthermore, the calculations reveal that the kinetic barrier for this exchange is determined by the specifics of the interaction of the second guest molecule (e.g., H 2 O or SO 2 ) with the MOF ligands. Hydrogen bonding of H 2 O molecules with the nearby oxygen of the organic linker is found to facilitate the positioning of the H 2 O oxygen atom towards the metal center, thus reducing the exchange barrier. In contrast, SO 2 molecules interact with the distant benzene site, away from the metal center, hindering the exchange process. Similar considerations apply to the other molecules, accounting for much easier CO 2 exchange for NH 3 than for NO, NO 2 , CH 4 , O 2 , and N 2 molecules. In this work, critical parameters such as kinetic barrier and exchange pathway are first unveiled and provide insight into the mechanism of competitive co-adsorption, underscoring the need of combined studies, using spectroscopic methods and ab initio simulations to uncover the atomistic interactions of small molecules in MOFs that directly influence co-adsorption.

Insight into the structural variation of metal organic framework materials upon hydration.

Metal organic framework (MOF) materials in general, and MOF-74 in particular, have promising properties for many technologically important processes. However, their instability under humid conditions severely restricts practical use. We show that this instability and the accompanying reduction of the CO2 uptake capacity of MOF-74 under humid conditions originate in the water dissociation reaction H2O → OH+H at the metal centers. After this dissociation, the OH groups coordinate to the metal centers, explaining the reduction in the MOF's CO2 uptake capacity. This reduction thus strongly depends on the catalytic activity of MOF-74 towards the water dissociation reaction. We further show that-while the water molecules themselves only have a negligible effect on the crystal structure of MOF-74-the OH and H products of the dissociation reaction significantly weaken the MOF framework and lead to the observed crystal structure breakdown. With this knowledge, we propose a way to suppress this particular reaction by modifying the MOF-74 structure to increase the water dissociation energy barrier and thus control the stability of the system under humid conditions.

In situ infrared spectroscopy and ab initio density functional theory (DFT) calculations are combined to study the interaction of the corrosive gases SO 2 and NO 2 with metal organic frameworks M-MOF-74 (M = Zn, Mg, Ni, Co). We find that NO 2 dissociatively adsorbs into MOF-74 compounds, forming NO and NO 3 − . The mechanism is unraveled by considering the Zn-MOF-74 system, for which DFT calculations show that a strong NO 2 −Zn bonding interaction induces a significant weakening of the N−O bond, facilitating the decomposition of the NO 2 molecules. In contrast, SO 2 is only molecularly adsorbed into MOF-74 with high binding energy (>90 kJ/mol for Mg-MOF-74 and >70 for Zn-MOF-74). This work gives insight into poisoning issues by minor components of flue gases in metal organic frameworks materials.

Many novel physical phenomena and fascinating properties emerge when materials are thinned down to two dimension (2D) [1, 2]. Therefore, in the last decade of the 2D materials research, the search for new 2D materials with fascinating physical properties became the scientific mainstream. So far, all known 2D materials have essentially the same atomic structure as a single layer of the corresponding layered bulk form [3, 4].Here we report the first example of a 2D material whose stoichiometry and structure is different from the corresponding layers in the bulk form. In the mechanically exfoliated PdSe2 sample (schematic shown in Fig. 1), we found that the monolayer form of layered PdSe2 differs from what one expects from its bulk counterpart. The monolayer is in a completely new Pd2Se3 phase with unique structure and beautiful symmetry, that has never been reported even as a bulk phase (Fig. 2).Combining the atomic imaging in a scanning transmission electron microscope (STEM) and density functional calculations, we discovered that this new monolayer phase is reconstructed directly from a PdSe2 bilayer through a vertical-interlayer fusion mechanism. We further demonstrate that the interlayer fusion process is triggered by Se deficient conditions in PdSe2 layers, where a substantial reconstruction of the residual under-coordinated Pd atoms pulls the layers towards each other, fusing them into a new Pd2Se3 layer, which inherits the lattice parameters from its parent PdSe2 phase. As evidence, we used electron irradiation to create artificial Se-deficient conditionsion few-layer PdSe2. The new Pd2Se3 is observed to grow along the PdSe2 matrix by in-situ sequential imaging, which verifies our DFT predictions.Our work reveals that the PdSe2 phase expected from bulk PdSe2 is not the intrinsic monolayer phase in the monolayer limit of PdSe2. This is due to the substantial interlayer interaction which leads to the unexpected interlayer fusion. Instead, the as-fused Pd2Se3 phase reported in this work serves as an alternative and the most stable monolayer phase derived from the bulk PdSe2. Our findings represent a breakthrough in understanding another fascinating property in monolayer materials and opens the way for more complex engineering of 2D phases [5].

We report here results of our density functional theory based computational studies of the electronic structure of the Pd-Co alloy electrocatalysts and energetics of the oxygen reduction reaction (ORR) on their surfaces. The calculations have been performed for the (111) surfaces of pure Pd, Pd(0.75)Co(0.25) and Pd(0.5)Co(0.5) alloys, as well as of the surface segregated Pd/Pd(0.75)Co(0.25) alloy. We find the hybridization of dPd and dCo electronic states to be the main factor controlling the electrocatalytic properties of Pd/Pd(0.75)Co(0.25). Namely the dPd-dCo hybridization causes low energy shift of the surface Pd d-band with respect to that for Pd(111). This shift weakens chemical bonds between the ORR intermediates and the Pd/Pd(0.75)Co(0.25) surface, which is favorable for the reaction. Non-segregated Pd(0.75)Co(0.25) and Pd(0.5)Co(0.5) surfaces are found to be too reactive for ORR due to bonding of the intermediates to the surface Co atoms. Analysis of the ORR free energy diagrams, built for the Pd and Pd/Pd(0.75)Co(0.25), shows that the co-adsorption of the ORR intermediates and water changes the ORR energetics significantly and makes ORR more favorable. We find the onset ORR potential estimated for the configurations with the O-OH and OH-OH co-adsorption to be in very good agreement with experiment. The relevance of this finding to the real reaction environment is discussed.