Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH₃) synthesis applications where AlN would be intentionally hydrolyzed to produce NH₃ and aluminum oxide (Al₂O₃), which could be subsequently reduced in nitrogen (N₂) to reform AlN and reinitiate the NH₃ synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH₃ yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH₃, and NH₃ desorption. We found activation barriers (<i>E</i>_a) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al–N bonds required to accumulate protons on surface N atoms to form NH₃. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (<i>E</i>_a = 331 kJ/mol) and mediated proton diffusion (<i>E</i>_a = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH₃ generation from AlN (<i>E</i>_a = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal–nitrogen bonds and, thus, more-facile NH₃ liberation

Stabilizing Ni Catalysts by Molecular Layer Deposition for Harsh, Dry Reforming Conditions

To inhibit sintering of ∼5 nm supported Ni particles during dry reforming of methane (DRM), catalysts were stabilized with porous alumina grown by ABC alucone molecular layer deposition (MLD). The uncoated catalyst continuously deactivated during DRM at 973 K. In contrast, the DRM rates for the MLD-coated catalysts initially increased before stabilizing, consistent with an increase in the exposed nickel surface area with exposure to high temperatures. Post-reaction particles were smaller for the MLD-coated catalysts. Catalysts with only 5 MLD layers had higher DRM rates than the uncoated catalyst, and a sample with 10 MLD layers remained stable for 108 h

Growth of Pt Particles on the Anatase TiO₂ (101) Surface

Growth of Pt_<i>n</i> (<i>n</i> ≤ 37) clusters on the defect-free TiO₂ anatase (101) surface has been studied using ab initio pseudopotential calculations based on density functional theory. Several initial configurations for clusters of 1, 2, 7, 10, and 37 atoms were relaxed to determine the most stable structures. All final optimized structures are three dimensional, suggesting that formation of island-like particles is favored over planar monolayers, as verified experimentally using Pt atomic layer deposition and high-resolution transmission electron microscopy. Diffusion barriers of a single Pt adatom on TiO₂ were calculated to understand the mobility of Pt atoms on the TiO₂ surface. Activation barriers of 0.86 and 1.41 eV were calculated for diffusion along the [010] and [101̅] directions, respectively, indicating that Pt atoms are relatively mobile along the [010] direction at moderate temperatures. The energy barriers for a Pt atom to escape from an 11- and a 37-atom Pt cluster on (101) anatase are predicted to be 1.38 and 2.12 eV, suggesting that particle coarsening occurs by Ostwald ripening and that Ostwald ripening of deposited Pt particles is limited by atom detachment from particles as small as several tens of atoms

First-Principles Analysis of Cation Diffusion in Mixed Metal Ferrite Spinels

Ferrite spinels are metal oxides used in a wide variety of applications, many of which are controlled by the diffusion of metal cations through the metal oxide lattice. In this work, we used density functional theory (DFT) to examine the diffusion of Fe, Co, and Ni cations through the Fe₃O₄, CoFe₂O₄, and NiFe₂O₄ ferrite spinels. We apply DFT and crystal field theory to uncover the principles that govern cation diffusion in ferrite spinels. We found that a migrating cation hops from its initial octahedral site to a neighboring octahedral vacancy via a tetrahedral metastable intermediate separated from octahedral sites by a trigonal planar transition state (TS). The cations hop with relative activation energies of Co ≈< Fe < Ni; the ordering of the diffusion barriers is controlled by the crystal field splitting of the diffusing cation. Specifically, the barriers depend on the orbital splitting and number of electrons which must be promoted into the higher energy t_2g orbitals of the tetrahedral metastable intermediate as the cations move along the minimum energy pathway of hopping. Additionally, for each diffusing cation, the barriers are inversely proportional to the spinel lattice parameter, leading to relative barriers for cation diffusion of Fe₃O₄ < CoFe₂O₄ < NiFe₂O₄. This results from the shorter cation-O bonds at the TS for spinels with smaller lattices, which inherently possess shorter bond lengths, and consequently higher system energies at their more constricted TS geometries

Extracting Kinetic Information from Complex Gas–Solid Reaction Data

We develop an approach for extracting gas–solid kinetic information from convoluted experimental data and demonstrate it on isothermal carbon dioxide splitting at high-temperature using CoFe₂O₄/Al₂O₃ (i.e., a “hercynite” cycle based on Co-doped FeAl₂O₄) active material. The reaction kinetics equations we derive account for competing side reactions, namely catalytic CO₂ splitting on and O₂ oxidation of doped hercynite, in addition to CO₂ splitting driven by the oxidation of oxygen-deficient doped hercynite. The model also accounts for experimental effects, such as detector dead time and gas mixing downstream of the reaction chamber, which obscure the intrinsic chemical processes in the raw signal. A second-order surface reaction model in relation to the extent of unreacted material and a 2.4th-order model in relation to CO₂ concentration were found to best describe the CO generation of the doped hercynite. Overall, the CO production capacity was found to increase with increasing reduction temperature and CO₂ partial pressure, in accordance with previously predicted behavior. The method outlined in this paper is generally applicable to the analysis of other convoluted gas–solid kinetics experiments

Controlling Nanoscale Properties of Supported Platinum Catalysts through Atomic Layer Deposition

Platinum nanoparticles were grown on alumina by atomic layer deposition using either H₂ or O₂ as the second half-reaction precursor. Particle diameters could be tuned between ∼1 and 2 nm by varying between use of H₂ and O₂ and by changing the number of ALD cycles. The use of H₂ as the second precursor led to smaller Pt particle sizes. Differences in particle size were found to be related to the availability of surface hydroxyl groups, which were monitored via in situ infrared spectroscopy during Pt ALD. Temperature-programmed desorption (TPD) of CO and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) for adsorbed CO were used to characterize sites and coordination numbers of the nanoparticles. As expected, smaller nanoparticles had sites with lower average coordination numbers. The catalysts were evaluated for oxidative dehydrogenation of propane to propylene. Catalysts having the smallest Pt particles with the lowest coordination number (synthesized by one cycle of Pt ALD with H₂) had a C₃H₆ selectivity of 37% at 14% conversion, whereas under the same reaction conditions the selectivity was less than 1% for larger (3.6 nm) commercial Pt catalysts at 9% conversion

Solvent Control of Surface Plasmon-Mediated Chemical Deposition of Au Nanoparticles from Alkylgold Phosphine Complexes

Bottom-up approaches to nanofabrication are of great interest because they can enable structural control while minimizing material waste and fabrication time. One new bottom-up nanofabrication method involves excitation of the surface plasmon resonance (SPR) of a Ag surface to drive deposition of sub-15 nm Au nanoparticles from MeAuPPh₃. In this work we used density functional theory to investigate the role of the PPh₃ ligands of the Au precursor and the effect of adsorbed solvent on the deposition process, and to elucidate the mechanism of Au nanoparticle deposition. In the absence of solvent, the calculated barrier to MeAuPPh₃ dissociation on the bare surface is <20 kcal/mol, making it facile at room temperature. Once adsorbed on the surface, neighboring MeAu fragments undergo ethane elimination to produce Au adatoms that cluster into Au nanoparticles. However, if the sample is immersed in benzene, we predict that the monolayer of adsorbed solvent blocks the adsorption of MeAuPPh₃ onto the Ag surface because the PPh₃ ligand is large compared to the size of the exposed surface between adsorbed benzenes. Instead, the Au–P bond of MeAuPPh₃ dissociates in solution (<i>E</i>_a = 38.5 kcal/mol) in the plasmon heated near-surface region followed by the adsorption of the MeAu fragment on Ag in the interstitial space of the benzene monolayer. The adsorbed benzene forces the Au precursor to react through the higher energy path of dissociation in solution rather than dissociatively adsorbing onto the bare surface. This requires a higher temperature if the reaction is to proceed at a reasonable rate and enables the control of deposition by the light induced SPR heating of the surface and nearby solution