No abstract

We have calculated the adsorption of the reducing agent hydrazine (N2H4) on copper surfaces using density functional theory calculations with a correction for the long-range interactions (DFT-D2). We have modeled the perfect and a number of defective Cu(100), (110), and (111) surfaces, which are found in the experimentally produced structures of copper nanoparticles. We have studied adsorption of hydrazine at three types of defects in the surfaces, i.e., monatomic steps, Cu adatoms, and Cu vacancies. Several low-energy adsorption structures for hydrazine on each perfect and defective surface have been identified and compared. Our calculations reveal that hydrazine bridges surface copper atoms, with the molecule twisted from the gauche toward an eclipsed conformation, except on the adatom (100) and vacancy-containing (100) and (110) surfaces, where it adsorbs through one nitrogen atom in gauche and trans conformations, respectively. The strongest adsorption energy is found on the stepped (110) surface, where hydrazine bridges between the copper atoms on the step edge and the terrace, as it stabilizes the low-coordinated copper atoms. Our results show that, although the (110) surface contains a number of low-coordinated atoms that enhance the surface-molecule interactions, the addition of defects on the more stable (111) and (100) surfaces provides sites that enable hydrazine binding to almost the same extent. This study also confirms general observations of surface adsorption trends in terms of d-band center and binding energy as a function of coordination number, i.e., the stronger the molecular adsorption, the higher the d-band shifts at low-coordinated sites.

We have investigated the adsorption of hydrazine (N2H4) and its reactivity on terraces and steps of Cu(111) surfaces by first-principles calculations in order to gain insight into the hydrazine decomposition mechanism. We have investigated different possibilities for the N-N and N-H bond cleavage for any intermediate states by analysing the reaction and barrier energies of each elementary step. We have found that hydrazine dehydrogenation via N-H bond scission is neither energetically nor kinetically favourable on the flat and stepped surfaces, but hydrazine prefers to form NH2via N-N bond decoupling on the Cu(111) with an activation energy below 1 eV. The NH2 molecule reacts fairly easily with co-adsorbed NH2 to form NH3 as well as with N2Hx (x = 1-4) by abstracting hydrogen to produce NH3 and N2 molecules on both the flat and stepped surfaces. We also found that all intermediates except NNH prefer N-N bond breaking as the most likely dissociation pathway, where the amide and imide intermediates produced can be hydrogenated to form NH3 in the presence of hydrogen. NNH is the only intermediate, which prefers to dissociate via a highly exothermic N-H bond breaking process to produce an N2 molecule after overcoming a small barrier energy. We also studied the production of H2 by recombination of hydrogen ad-atoms which, considering the activation energies, is particularly favoured under conditions of moderate temperatures. Our results agree well with experiments suggesting that N2H4 adsorbs dissociatively on copper above ∼300 K leading to N2, NH3 and H2. In general, the lower coordination of the steps is found to lead to higher reactivity than on the flat Cu(111) surface. Furthermore, the calculations show that the influence of step edge atoms is very different for the intra- and intermolecular dehydrogenation mechanisms. They also increase the barrier of N-N decoupling of all the existing species in the reaction.

Density functional Theory (DFT) calculations have been performed to investigate the electronic structure and photocatalytic activity of a hybrid Ag3PO4(111)/g-C3N4 structure. Due to Ag(d) and O(p) states forming the upper part of the valence band and C(p), N(p) and Ag(s) the lower part of the conduction band, the band gap of the hybrid material is reduced from 2.75 eV for Ag3PO4(111) and 3.13 eV for monolayer of g-C3N4 to about 2.52 eV, enhancing the photocatalytic activity of the Ag3PO4(111) surface and g-C3N4 sheet in the visible region. We have also investigated possible reaction pathways for photocatalytic CO2 reduction on the Ag3PO4(111)/g-C3N4 nanocomposite to determine the most favored adsorption geometries of reaction intermediates and the related reaction energies. For CO2 reduction, our findings demonstrate that the Ag3PO4(111)/g-C3N4 heterostructure thermodynamically exhibits a higher selectivity towards CH4 production than that of CH3OH. The CO2 reduction process takes place through either HCOOH * or HOCOH * as an intermediate species, where the highest exothermic reaction energy of-2.826 eV belongs to the hydrogenation of t-COOH * to HCOOH * and the lowest reaction energy of-0.182 eV for hydrogenations of CH2O * to CH2OH * and HCO * to c-HCOH *. Our results from charge density difference calculations of the Ag3PO4(111)/Ag/g-C3N4 revealed that the charge transfer between the Ag3PO4(111) slab and g-C3N4 monolayer occurs through mediation of atomic Ag, thus proposing a Z-scheme mechanism. Moreover, a smaller band gap energy of 0.73 eV is calculated for this ternary nanocomposite due to the mid-gap states of the atomic Ag at the interface. These results provide in depth understanding of the reaction mechanism in the reduction and conversion of CO 2 to useful chemicals via an Ag 3 PO 4 and g-C 3 N 4-based nanocomposite photocatalyst under visible light.