Push–pull mechanism of hydrodenitrogenation over carbide and sulfide catalysts

Effect of Platinum, Gold, and Potassium Additives on the Surface Chemistry of CdI2-Antitype Mo2C

Transition metal carbides are versatile materials for diverse industrial applications including catalysis, where their relatively low cost is very attractive. In this work, we present a rather extensive density functional theory study on the energetics of adsorption of a selection of atomic and molecular species on two Mo terminations of the CdI₂ antitype phase of Mo₂C. Moreover, the coadsorption of CO in the presence of preadsorbed metal atoms and its dissociative adsorption in the absence and presence of preadsorbed Pt and K were investigated. By using CO as a probe to understand the structural/electronic effects of the preadsorption of the metal atoms on the Mo₂C(001) surface, we showed that K further enhances CO adsorption/activation on the surface, in contrast to the precious metals considered. Moreover, it was observed that the presence of both Pt and K stabilizes the transition state for the C-O bond dissociation, lowering the activation barrier for the dissociation of the C-O bond by about 0.3 and 0.4 eV, respectively.

Adsorption and desulfurization reaction mechanism of thiophene and its hydrogenated derivatives over NbC(001) and NbN(001): an ab initio DFT study

Herein, we present periodic DFT-based calculations on the thiophene and its H-derivatives adsorption and reaction pathways over niobium carbide and nitride cubic face-centered (001) surfaces.

Thiophene Adsorption and Activation on MoP(001), γ-Mo2N(100), and Ni2P(001): Density Functional Theory Studies

The adsorption and dissociation of thiophene on the MoP(001), gamma-Mo(2)N(100), and Ni(2)P(001) surfaces have been computed by using the density functional theory method. It is found that thiophene adsorbs dissociatively on MoP(001), while nondissociatively on gamma-Mo(2)N(100) and Ni(2)P(001). On MoP(001), the dissociation of the C-S bonds is favored both thermodynamically and kinetically, while the break of the first C-S bond on gamma-Mo(2)N(100) has an energy barrier of 1.58 eV and is endothermic by 0.73 eV. On Ni(2)P(001) there are Ni(3)P(2)- and Ni(3)P-terminated surfaces. On the Ni(3)P(2)-terminated surface, the dissociation of the C-S bonds of adsorbed thiophene is endothermic, while it is exothermic on the Ni(3)P-terminated surface.

Desulfurization Reactions on Ni2P(001) and α-Mo2C(001) Surfaces: Complex Role of P and C Sites

X-ray photoelectron spectroscopy and first-principles density-functional calculations were used to study the interaction of thiophene, H(2)S, and S(2) with Ni(2)P(001), alpha-Mo(2)C(001), and polycrystalline MoC. In general, the reactivity of the surfaces increases following the sequence MoC < Ni(2)P(001) < alpha-Mo(2)C(001). At 300 K, thiophene does not adsorb on MoC. In contrast, Ni(2)P(001) and alpha-Mo(2)C(001) can dissociate the molecule easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C(x)H(y) fragments and S) from these surfaces. Our experimental and theoretical studies indicate that the rate-determining step in a hydrodesulfurization (HDS) process is the transformation of adsorbed sulfur into gaseous H(2)S. Ni(2)P is a better catalyst for HDS than Mo(2)C or MoC. The P sites in the phosphide play a complex and important role. First, the formation of Ni-P bonds produces a weak "ligand effect" (minor stabilization of the Ni 3d levels and a small Ni --> P charge transfer) that allows a high activity for the dissociation of thiophene and molecular hydrogen. Second, the number of active Ni sites present in the surface decreases due to an "ensemble effect" of P, which prevents the system from deactivation induced by high coverages of strongly bound S. Third, the P sites are not simple spectators and provide moderate bonding to the products of the decomposition of thiophene and the H adatoms necessary for hydrogenation.

Mechanism of Hydrodenitrogenation on Phosphides and Sulfides

The mechanism of hydrodenitrogenation (HDN) of 2-methylpiperidine was studied over a silica-supported nickel phosphide catalyst (Ni2P/SiO2, Ni/P = 1/2) and a commercial Ni-Mo-S/Al2O3 catalyst in a three-phase trickle-bed reactor operated at 3.1 MPa and 450-600 K. Analysis of the product distribution as a function of contact time indicated that the reaction proceeded in both cases predominantly by a substitution mechanism, with a smaller contribution of an elimination mechanism. Fourier transform infrared spectroscopy (FTIR) of the 2-methylpiperidine indicated that at reaction conditions a piperidinium ion intermediate was formed on both the sulfide and the phosphide. It is concluded that the mechanism of HDN on nickel phosphide is very similar to that on sulfides. The mechanism on the nickel phosphide was also probed by comparing the reactivity of piperidine and several of its derivatives in the presence of 3000 ppm S. The relative elimination rates depended on the structure of the molecules, and followed the sequence: 4-methylpiperidine approximately piperidine > 3-methylpiperidine > 2,6-dimethylpiperidine > 2-methylpiperidine. [Chemical structure: see text] This order of reactivity was not dependent on the number of alpha-H or beta-H atoms in the molecules, ruling out their reaction through a single, simple mechanism. It is likely that the unhindered piperidine molecules reacted by an S(N)2 substitution process and the more hindered 2,6-dimethylpiperidine reacted by an E2 elimination process.

Effects of carbon on the stability and chemical performance of transition metal carbides: A density functional study

Density functional theory was employed to study the stabilities and chemical activities of transition metal carbides. Here we take the well-known Mo carbides and Ti carbides as an example. Different kinds of structures including the bulk surfaces [Mo(2)C(001), MoC(001), and TiC(001)] and metcars [Mo(8)C(12) and Ti(8)C(12)] are taken into consideration. Systematic studies show that by raising the C coordination number of the metal atoms in the carbides, in general the stability of the carbides increases (metcars are an exception since they include both high-coordinated and low-coordinated metal atoms.); at the same time, the chemical activities of the carbides decrease due to a downshift of the metal d-band center (ligand effect). Considering the better catalysts those that combine high stability and moderate chemical activity, our results suggest that the catalytic potential of Mo carbide systems should decrease in the following sequence: Mo(8)C(12)>Mo(2)C(001) or MoC(001)>pure Mo(110). In spite of having the largest C/Mo ratio, the metcar appears as the most attractive system. Our studies also indicate that the "magic" behavior of metcars is not unique for Mo carbides. Similar behavior is also observed for Ti carbides. This implies that nanoparticles like metcar species could exhibit better performances than the corresponding bulk metal carbides as catalysts.