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Alkaline earth oxides are leading candidates as NOx (x = 1, 2) sorbents; however, the understanding of adsorption and reaction of NOx on these surfaces is currently limited. In this paper, Mg(001) terrace cluster models and density functional theory (DFT) are used to characterize the adsorption properties of neutral and charged NO, NO2, and NO3. Neutral NO and NO2 molecules weakly adsorb on the MgO terrace, while neutral NO3 adsorbs more strongly through oxidation and significant acid−base coordination with the surface. Chemisorbed nitrite can be formed either from adsorption of Lewis acidic NO⁺ on Lewis base surface sites (Os^2-) or from adsorption of Lewis basic NO2^- on Lewis acid surface sites (Mgs²⁺). Similarly, chemisorbed nitrate can form from NO2⁺ adsorption on Os^2- sites or NO3^- adsorption on Mgs²⁺ sites. These charged adsorbates are not expected to exist in isolation on the MgO surface but rather are expected to form oppositely charged, cooperatively bound pairs with enhanced adsorption energy over the isolated neutrals. While results are demonstrated for adsorption on MgO, they have application to NOx adsorption on a wide range of oxide surfaces.

An extensive search for stable chemisorbed SO2 configurations on the Pt(111) surface has been performed using first-principles DFT−GGA calculations. The most energetically stable configurations, η²-Sb,Oa and η³-Sa,Oa,Oa at fcc sites, where η² and η³ mean that the number of atoms of the adsorbate coordinated to surface atoms are two and three, respectively, and the subscripts a and b stand for the atoms on atop sites and bridge sites, respectively, are consistent with experimental observations. It is found that strong sulfur−metal bonds are essential in stabilizing the molecular SO2 binding to the Pt(111) surface. The lateral dipole−dipole interactions among chemisorbed SO2 moieties are shown to be responsible for the strong coverage effect of the SO2 binding energy to the surface. These strong lateral interactions do not greatly affect the molecular structures or relative binding energy differences among different binding configurations. The projected density of states and the induced density of states are studied in detail to explain binding effects.

The chemisorption of molecules on metal oxide surfaces is generally considered to occur by either an acid/base or a redox mechanism, with the former dominating on nonreducible and insulating oxides such as MgO. NO, NO2, and the less familiar NO3 are atypical adsorbates in that their most potent Lewis acidic and basic forms are generated by one-electron oxidation or reduction of the parent molecules. In this work, first-principles density functional theory supercell calculations are used to probe the adsorption chemistry of the nitrogen oxides on an undefected MgO(001) surface. The isolated adsorbates are found to physisorb (NO, NO2) or weakly chemisorb (NO3) to the MgO terrace. In contrast, adsorbate partners located on neighboring surface acid (Mgs) and base (Os) sites form strongly chemisorbed products with features characteristic of nitrite (NO2^-) and nitrate (NO3^-). The origin of this new class of “cooperative” chemisorption is shown to be electron transfer between two NOx species to generate Lewis acid (NOx⁺) and base (NOx^-) pairs that strongly chemisorb to the MgO surface and that are further stabilized by lateral electrostatic attraction. The relatively low ionization potentials and large electron affinities of the NOx molecules are key to enabling the cooperative effect on MgO. Even more pronounced cooperative effects are expected for NOx adsorption on more basic or acidic oxides, including those used for NOx remediation. The effect is also likely to have a role in the heterogeneous chemistry of other odd-electron adsorbates, including the halogen oxides and HOx radicals.

In order to evaluate the effect of size on the adsorption energies on small Pt particles, we have performed a series of DFT-GGA calculations on Pt clusters of varying sizes, between 3 and 25 atoms, and a Pt(111) slab, with and without O and S adsorbates. We have found a significant variation in the energy of adsorption as a function of cluster size. In particular, energies of adsorption on the Pt10 cluster showed the largest deviation from those on the slab surface. Structural and energetic properties compare excellently to published experimental data. The Pt4 clusters with and without adsorbates were found to have reduced symmetry due to the Jahn−Teller effect. A simple rule between the cohesive energy and the size of small close-packed platinum particles is derived and confirmed by our first-principle calculations, the extrapolation of which excellently matches both experimental and computational bulk cohesive energies.

One of the main obstacles to the use of alkaline-earth-oxide-based NOx adsorbents for emissions control is the ready poisoning of these materials by SOx. To shed light on the mechanisms of poisoning, density functional theory (DFT) calculations and infrared spectroscopy experiments are used to study SOx adsorption on MgO as a model alkaline earth oxide. DFT plane-wave, pseudopotential results are presented for SOx adsorption on MgO(001) terraces and steps. Both SO2 and SO3 are found to exhibit both weakly bound physisorbed and strongly bound chemisorbed forms, with minimal activation barriers separating the two. Chemisorption is dominated by interactions between Lewis acidic sulfur and Lewis basic oxide anion sites to form surface sulfites and sulfates from SO2 and SO3, respectively. Within the generalized gradient approximation to DFT, the SO2 adsorption energy ranges from 25 kcal mol^-1 on terrace sites to 46 kcal mol^-1 on lower-coordinated step edge sites; analogous SO3 adsorption energies range from 49 to 76 kcal mol^-1. Surface sulfite is readily observed upon exposure of calcined MgO powder to SO2 and produces vibrational signatures consistent with that calculated for chemisorbed SO2. Sulfation is accomplished by exposure of the same powders to SO2 and O2 at elevated temperatures. Comparisons to calculated vibrational spectra indicate that both chemisorbed SO2 and SO3 are present under these conditions; further heating induces formation of a bulklike sulfate that is difficult to remove. The results demonstrate the high reactivity of SOx toward metal oxides and illustrate the challenges in developing sulfur-resistant oxide-based NOx traps.

We present a more detailed account of our recently reported [Hass, K. C.; Schneider, W. F.; Curioni, A.; Andreoni, W. Science 1998, 282, 265] first-principles molecular dynamics investigation of the static and dynamical behavior of adsorbed H2O on α-Al2O3 (0001). Al-terminated surfaces with varying degrees of H2O coverage are modeled using large periodic supercells. A predicted large relaxation of the clean surface agrees well with previous density functional theory calculations. Both molecular and dissociative H2O adsorption modes are identified, with the latter favored by ~10 kcal mol^-1. Complementary Al8O12 cluster results are shown to be unreliable because of their finite lateral extent. Constrained dynamical calculations of free-energy barriers indicate that the dissociation rate is very high, even in the absence of defects, but differs by 3 orders of magnitude for two equally exothermic pathways (proton transfer being more favorable across a six-membered ring than to the nearest O site). Unconstrained simulations at intermediate H2O coverages exhibit (1) spontaneous unimolecular and (2) H2O-mediated dissociation events, as well as (3) the diffusion and hydrogen bonding of physisorbed H2O and (4) an additional proton transfer reaction between adsorbed H2O and OH species. An experimentally observed decrease in H2O binding energies with coverage is explained in terms of a separation into defect-dominated, intrinsic (0001) terrace, and “hydrogen-bonding” regimes, with reasonable quantitative agreement throughout. Calculated O−H vibrational frequencies are consistent with known trends on aluminas but indicate a discrepancy between experimental observations for α-Al2O3 (0001) and models based on simple hydroxylation. Simulations for high H2O coverages suggest the possibility of more complicated behavior, including the interchange of adsorbed and lattice oxygen and the etching of surface Al. A “fully”-hydroxylated α-Al2O3 (0001) surface in which each surface Al is replaced by three protons to give uniform OH-termination, as in aluminum hydroxides, is the most likely result of prolonged exposure. Results for this surface confirm its anticipated stability, provide a reasonable explanation of observed vibrational spectra, and reveal a complex, dynamical structure with extensive intraplanar hydrogen bonding.

Effects of the support environment on the existence of Cu ion pairs in Cu-exchanged ZSM-5 catalysts are examined using density functional theory. Results for the molecular and electronic structures of O- and O2-bridged Cu oxocations ([CuOCu]²⁺ and [CuO2Cu]²⁺) are presented, including two distinct isomers for the latter. Both types of oxocations are predicted to be strongly bound for conditions likely to occur in the zeolite. The zeolite framework is represented by a variety of cluster models, including a previously established single-T-site model and larger multi-T-site models specific to particular binding sites in ZSM-5. With the largest models, bent Cu−Ox−Cu structures are found with Cu−Cu distances consistent with X-ray absorption data for Cu−ZSM-5. Implications for catalytic chemistry, including a proposed pathway for O2 formation and subsequent desorption from oxidized Cu sites in the zeolite, are discussed.

Explicit atom (EA) and united atom (UA) models are compared for the description of the structure and dynamics of an alkane film adsorbed on an Al-terminated α-Al2O3 (0001) surface. Ab initio calculations for an Al2O3···CH4 cluster are used to parameterize the EA model. One UA model is derived by explicit averaging of the EA potential, while the second is rescaled from the first to reproduce the EA octane adsorption energy. The appropriate scale factor is found to be sensitive to relaxation of the α-Al2O3 surface. The three models are compared in constant NVT molecular dynamics simulations of an octane monolayer (21 molecules) adsorbed on an 80% relaxed α-Al2O3 (0001) surface. Overall, the UA models reproduce the EA structural predictions well, the primary difference being an underestimation of the average gauche angle and overestimation of the trans fraction in the UA simulations. The agreement between dynamic predictions is less satisfactory; the scaled UA model reproduces the EA trans ↔ gauche interconversion rate reasonably well, but fails to reproduce the overall rotational or translational dynamics. UA models must be used with caution when applied to alkane dynamics on a corrugated surface.

A previously reported density-functional-theory-based model of NO decomposition in Cu-exchanged zeolites (Schneider, W. F.; et al. J. Phys. Chem. B 1997, 101, 4353) is extended to consider more generally the Cu-zeolite catalyzed chemistry of nitrogen oxides. The catalyst active site is considered to be an isolated, zeolite (Z)-bound Cu ion, which can exist in either a reduced (Z^-−Cu(I)) or an oxidized (Z^-−Cu(II)−O^-) state. Three different cluster models are used to study the affinity of ZCu and ZCuO for gaseous molecules (e.g., NO, NO2, or N2O), the structures and vibrational spectra of the stable complexes thus formed, and the possible reactivity between active sites and gaseous species. The reduced and oxidized states are found to react with nitrogen oxides via two types of O atom transfer reactions, one in which ZCu adds an O atom to form ZCuO, and the other in which ZCuO adds an O atom to form ZCu + O2 via a dioxygen (ZCuO2) intermediate. Potential energy surfaces for several key reactions are explored, and the results combined into a mechanistic model which can be used to rationalize much of the known catalytic chemistry of nitrogen oxides on Cu zeolites.

A small cluster model proposed earlier to examine bound Cu ions and their interaction with CO and NO in zeolites [Schneider, W. F.; Hass, K. C.; Ramprasad, R.; Adams, J. B. J. Phys. Chem. 1996, 100, 6032] is used to study Cu-bound dinitrosyl complexes. The possibility of a single-step, symmetric, concerted reaction occurring between the two nitrosyl ligands to form either a N−N bond or free N2 and O2 is addressed. Density functional theory is used to predict molecular and electronic structures and binding energies. N-down dinitrosyl binding to Cu⁰, Cu⁺, and Cu²⁺ can be represented as [Cu(I)−(NO)2^-], [Cu(I)−(NO)2], and [Cu(I)−(NO)2⁺], respectively, with the dinitrosyl moiety closely resembling the free NO dimer, and having a long N−N bond (≈2.8 Å). Dinitrosyl species bound to Cu through the O display two distinct binding modes, one resembling the N-down dinitrosyl binding, again with a long N−N bond (≈2.0 Å), and the other similar to hyponitrite binding to a metal atom, displaying a short N−N bond (≈1.2 Å). The single-step, symmetric, concerted decomposition reaction of NO in the vicinity of Cu ion sites in zeolites is forbidden by orbital symmetry and is anticipated to have a comparable or higher activation barrier than the same reaction in the gas phase. Metastable hyponitrite complexes, on the other hand, display N−N coupling and may be precursors for a multistep decomposition of NO in the presence of Cu-exchanged zeolites.

First-principles quantum calculations are potentially of great value for assessing the plausibility of proposed catalytic reaction mechanisms. The present analysis of reaction pathways and energetics for key elementary steps in the catalytic decomposition of NO to N2 + O2 by Cu-exchanged zeolites yields novel insights into this widely studied reaction. We find evidence for a pathway involving two successive O-atom transfers to an isolated, zeolite-bound Cu⁺ center, initiated by formation of a short-lived and difficult to detect isonitrosyl intermediate, and yielding sequentially N2O and Cu-bound O followed by N2 and Cu-bound O2. The calculations allow us to identify a complete catalytic cycle with reasonable energetics.

Local spin density functional theory calculations of vibrational frequencies were performed for small Cu-containing complexes in an effort to assess models of exchanged Cu ion sites in zeolites and to help interpret infrared spectroscopy results. Model complexes consisted of Cuⁿ⁺ (n = 0−2) ions with varying coordination to water ligands and to more realistic fragments of zeolites. Calculated CO and NO vibrational frequencies for Cu-bound mono- and dicarbonyl and mono- and dinitrosyl species lie in ranges consistent with experimentally observed frequencies and confirm earlier assignments. Our results show a clear linear correlation between bond length and frequency for both carbonyl and nitrosyl complexes. The (nominal) oxidation state of Cu in these complexes is the most important factor in determining CO and NO frequencies and bond lengths, with the local coordination of Cu and the presence of explicit countercharges producing secondary effects.