Structures and Energetics of NH3 Adsorption and Decomposition at

Jun 6, 1996 - Using density functional theory (DFT) and a cluster surface model, we have investigated the structures and energetics for NH3 adsorption...
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J. Phys. Chem. 1996, 100, 9800-9806

Structures and Energetics of NH3 Adsorption and Decomposition at Nb(100) Surface: A Density Functional Study Hansong Cheng,* David B. Reiser, Paul M. Mathias, Kenneth Baumert, and Sheldon W. Dean, Jr. Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195-1501 ReceiVed: September 7, 1995; In Final Form: March 27, 1996X

Nitriding is a chemical process that corrodes metal surfaces. Understanding the reaction mechanisms presents a considerable challenge both theoretically and experimentally. Using density functional theory (DFT) and a cluster surface model, we have investigated the structures and energetics for NH3 adsorption and decomposition processes at the on-top and 4-fold-hollow sites of the Nb(100) surface. We show that the preferred adsorption mode for NH3 is the on-top site, that NH2 can reside on both modes, and that other decomposition fragments will most likely fall into the hollow site. Comparison of the results calculated by both local DFT and gradient-corrected DFT is made at both adsorption modes, which shows that the local DFT calculations overestimate the binding energies of the nitriding species considerably, although both calculations yield the same trends. It was found that the decomposition process at the on-top site is essentially an energy-uphill process while at the hollow site it is a down-hill process. The theoretical results provide useful physical insight into the nitriding mechanism at transition metal surfaces and will facilitate materials development of nitriding-resisting products.

I. Introduction Molecular chemisorption on transition metal surfaces and clusters has been an area of increasing interest in surface science and chemical physics.1-4 The intensive research activities are strongly motivated by the possibility to understand the underlying reaction mechanisms of many heterogeneous catalytic processes. Ammonia adsorption on a variety of metal surfaces, in particular, has been most extensively studied both theoretically and experimentally due to the importance of its catalytic synthesis from N2 and H2. For example, Ertl and co-workers and Hu¨ttinger et al. carried out ultraviolet photoelectron spectroscopic (UPS) experiments to measure the valence band structure of NH3 and its fragmentation species at several transition metal surfaces and also obtained thermal desorption spectra for the surface reactions.5-8 Ertl et al. showed that the ammonia synthetic reaction catalyzed by an iron surface is not an energetically favorable process. Recent experiments by Roberts and co-workers showed that at high temperature NH3 can be readily dissociated at transition metal surfaces.5-7 Dissociation becomes increasingly difficult as the d-electron bands of these metals are filled with more electrons. Theoretically, the advances in density functional theory has made it possible to study the chemisorption phenomena at transition metal surfaces and clusters from first principle. There has been intensive effort focusing on understanding the nature of NH3 chemisorption at a variety of metal surfaces.10-13 Recently, Neurock and co-workers have constructed a reaction energy diagram for ammonia oxidation by Cu(111) surface by employing a DFT approach.14 Our interest in NH3 chemisorption and decomposition is derived from a different perspective other than NH3 catalytic synthesis. Our main concern lies in understanding of the mechanism of the corrosive processes that occur in our ammonia reactors. As a consequence of the corrosion, NH3 is dissociated into adatoms which subsequently form nitrides with the metal oxides in the stainless steel materials and eventually corrode X

Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(95)02623-2 CCC: $12.00

the reactors. In general, chemical corrosion is an extremely complicated process that usually involves a chain of reactions. At each step of the series, the reaction can be strongly dependent upon the physical and chemical properties of the materials on which the reaction occurs. This is particularly the case for a nitriding process in which ammonia is adsorbed and subsequently decomposed into atomic nitrogen and atomic hydrogen on metal surfaces at high temperature. Ideally, nitriding can be divided into a three-step decomposition process: NH3 is initially adsorbed on a metal surface; it is then decomposed into fragments of H and NH2 which is further dissociated into NH and eventually into N. Furthermore, it has been observed in many experiments that the extent of nitriding is materialdependent. Structures and chemical reactivities of metals play a key role in determining the nitriding-resisting properties of the materials. In cases of alloys, the chemical compositions can also be an important factor. It is thus a formidable task to investigate the detailed reaction mechanism and, indeed, for years studies on chemical corrosion have been based almost entirely on empirical experience. In a previous paper,15 we employed a hybrid approach combining local density functional calculations (LDF) and extended Hu¨ckel tight-binding method (EHTB) to study the nitriding processes at Ni(100), Fe(100), and Cr(100) surfaces. Using the structural parameters calculated by LDF for NH3 and its decomposition fragments on small clusters, we carried out band structure calculations using the EHTB method and evaluated the decomposition energies at these surfaces. In the present work, we continue our investigation on nitriding mechanism at transition metal surfaces. It is understood that the corrosion process at real stainless steel surfaces may differ considerably from that at pure metal surfaces. Nevertheless, we argue that studies of nitriding at clean surfaces should yield useful physical insight into the reactive process at the surfaces of the reactors, as demonstrated in our previous paper. There are reports that stainless steels containing small amounts of Nb (type 347) perform better in ammonia service than similar stainless steels without Nb (type 304).16 As a first step in trying © 1996 American Chemical Society

NH3 Adsorption and Decomposition at Nb(100) Surface to understand this behavior, we have investigated the dissociation behavior of ammonia on a pure Nb surface. Recently, several studies on small Nb clusters have shown that Nb can be highly reactive with H2, H2O, and a variety of hydrocarbon compounds.17,18 Goodwin and Salahub employed a density functional approach to study the binding energies and bond dissociation energies for small Nb clusters and achieved good agreement with experimental results.19 To understand the role of Nb in alloy surfaces in nitriding process, we utilized the DFT approach to study the chemisorption structures and energetics of the nitriding species. It should be stressed that it is not our intention to seek for rigorous description of a nitriding process in the present work. Instead, we utilize a simple surface model to estimate the NH3 adsorption binding energy and the bond dissociation energies of the fragments for the decomposition process, hoping to gain physical insight into the nitriding mechanism. Information about the chemisorption structures and the decomposition energetics should be useful to enhance our knowledge on the microscopic processes in the chemical corrosion system. Section II describes the physical models we employed to simulate various chemisorption patterns as well as the computational procedure. Results and discussions are presented in Section III. Section IV summarizes the conclusions that can be derived from this work. II. Models and Computational Method To study an adsorption system at a metal surface, it is usually convenient to confine the process in a relatively small portion of the surface on which the adsorption occurs. The metal surface structure is then modeled by employing appropriate clusters.14,15 The calculated adsorption binding structures on these clusters then yield information about reaction mechanisms. The size of the clusters may often affect the accuracy of the modeling results, as shown by Whitten.20 For a nitriding system on Nb(100) surface, the situation is much more complicated. First, electronic structure calculations for systems involving transition metal atoms are known to be substantially difficult mainly due to a large number of unpaired electrons in the metal and the relatively small energy gap between the occupied orbitals and the virtual orbitals. Moreover, for a niobium atom, the electronic configurations of 4d45s1, 4d55s0, and 4d35s2 are nearly degenerated. Consequently, there is a large sampling space of configurations in the electronic structure calculations for a sizable Nb cluster, and the computational complexity increases rapidly with the system size. Finally, a nitriding process often involves many steps of reactions, including adsorption, bond dissociation, and diffusion. Accordingly, a physically “correct” cluster model of the metal surface would have to be large enough to accommodate the reactive processes as well as the fragment species. It is therefore apparent that it would be exceedingly difficult to describe a full nitriding system quantum-mechanically without making physically reasonable simplifications of the surface model. One way to simplify the problem may be to use less rigorous quantum-mechanical methods, such as the extended Hu¨ckel theory21 and ZINDO method,22 which allow calculations for relatively large transition metal systems. The approach of using the extended Hu¨ckel theory to study nitriding processes on Fe, Ni, and Cr surfaces was described in our previous paper.15 In the present study, we take an alternative approach by employing a rigorous theoretical method, the density functional theory, for adsorption of the decomposition species on small metal cluster systems which accommodate only one fragment species each time. The bond dissociation energies are then evaluated based

J. Phys. Chem., Vol. 100, No. 23, 1996 9801

Figure 1. Nb clusters used to model Nb(100) surface: (a) on-top site; (b) 4-fold hollow site.

on the adsorption binding energies of the fragments calculated for the small clusters. The present study involves several approximations. First, the Nb(100) surface is simplified with 9-atom cluster models as shown in Figure 1, where part (a) is for adsorption at the ontop mode and part (b) is for adsorption at the 4-fold-hollow mode. It is understood that clusters at this size range may not be large enough to include all the surface effect on the adsorbents, which will consequently affect the accuracy of the calculated adsorption binding energies. However, on the other hand, they certainly contain the metal atoms that are most influential on the adsorption binding structures of the nitriding species at the Nb(100) surface. We thus expect that the DFT calculations with the small metal clusters would yield information on the adsorption geometries and the relative trend of binding energies. Furthermore, we neglect the interactions between any two decomposition species in the nitriding process and focus on the adsorption binding energies based on which we derive the bond dissociation energies. While the van der Waals force among the adsorbed species and the nonbonding interactions between the adsorbed species and the neighboring metal clusters are also important and, in some cases, can be substantial, the strong chemisorption of the nitriding species certainly is the driving force that contributes mostly to the overall system energy. The small clusters of course do not accommodate any of the reactive processes, i.e. they cannot be directly used to model the bond dissociation processes on Nb(100). However, under the above assumption, the structures and energetics of these processes can be readily estimated based on the adsorption energies calculated for the small clusters. To further simplify the problem, the cluster geometries are taken from the crystal structure of Nb(100) surface. Throughout the calculations, the cluster structures are fixed and only the fragment species are allowed to move. In reality, lattice relaxation can be an important process upon the adsorption of the nitriding species. However, this is most likely to be a secondary effect on the bonding structures for a nitriding system since chemisorption is apparently the dominant factor that determines both the adsorption structures and the energetics. The rigid surface model allows us to take advantage of the lattice symmetry in the calculations. Several previous studies for a few other transition metal systems have shown that NH3 adsorbs only at the on-top mode with N atom heading toward the surfaces and H atoms pointing away from the metals and that the molecular rotation with respect to the 3-fold axis virtually costs no energy penalty. We have a observed similar phenomenon in the present study. For NH3 in the on-top mode on Nb(100), a Cs point group symmetry was utilized for partial

9802 J. Phys. Chem., Vol. 100, No. 23, 1996 optimization of the adsorption geometry. For the decomposition species, NH2, NH, N, and H, both on-top mode and 4-foldhollow mode, are examined. Except for NH2, for which the C2V symmetry is applied, all other species exhibit C4V symmetry. The calculations were carried out by using the DFT methods provided by the DMol package.23 Double numerical basis functions are employed to solve the Kohn-Sham equation.24 To enhance the accuracy, polarization functions are also added to the basis set. Core electrons are frozen to reduce the computational cost while still maintaining reasonable accuracy. The LDF calculations utilize the Vosko-Wilk-Nusair local correlation functionals.25 It is understood that LDF often overestimates the binding energies, and gradient corrections may be necessary to improve the energetics. However, computationally, full geometry optimization using the nonlocal DFT approach is extremely intensive, and the SCF convergence becomes exceedingly difficult for such complicated systems. Instead, we employed Becke’s gradient-corrected exchange and Lee-Yang-Parr’s gradient-corrected correlation functional to perform single point energy calculations using the optimized adsorption structures obtained in LDF calculations. The unrestricted Kohn-Sham scheme is employed throughout the calculations to treat the electronically open-shell systems. SCF convergence is enhanced by first smearing a small amount of electron density into the virtual orbitals and then systematically reducing the smear value until the converged SCF energies become invariant. All calculations were done on our IBM/ R6000 390 workstations. III. Results and Discussions III.1. Chemisorption Structures. Geometry optimization was first performed for NH3 and its decomposition fragments on the Nb(100) surface under the constraint of point group symmetry by using LDF method. The optimized structures are shown in Figure 2, where we use the black balls to denote the top layer atoms, shaded balls for the second layer atoms, large white balls for the N atom, and small white balls for the H atoms. For NH3, it was found that NH3 only adsorbs at the on-top site. At the 4-fold-hollow site, a reaction path calculation was also performed by forcing the N atom to gradually approach the second layer while optimizing the Cartesian coordinates of the three H atoms. This results in spontaneous dissociation of the N-H bonds and, consequently, NH3 cannot be stabilized at the hollow site. This is in agreement with the previous study on NH3 chemisorption on other transition metal surfaces.26,27 Detailed structural analysis indicates that the main reason that the 4-fold-hollow mode is not the preferred adsorption site for NH3 is due to the mismatch of NH3 size and the relatively small pore of the hollow site of the surface. While both N-H bonds and the bond angle ∠HNH are considerably rigid in NH3 and its van der Waals radius is about 2.04 Å, the pore size of the 4-fold hollow is about 1.965 × 1.96 Å2, too small for NH3. Here we have used the van der Waals radii rh ) 1.10 Å, rN ) 1.50 Å and rNb ) 1.33 Å to estimate the van der Waals volume of NH3 and the pore size.28 It is understood that in reality, upon the adsorption of NH3, the surface atoms near the adsorption site will slightly adjust their position, i.e. open a larger pore, to maximally accommodate the gas-phase species. However, we do not expect that the lattice relaxation is significant enough to alter the adsorption pattern. This is also consistent with the results obtained in the thermal desorption spectroscopic measurement for NH3 adsorption on Fe(100).29 The chemisorption structures of NH2 shown in Figure 2 are distinctively different from that of NH3. It can be stabilized at

Cheng et al. both on-top mode and the 4-fold-hollow mode, as will be demonstrated later. While at the on-top mode, the fragment can still rotate freely with respect to its 2-fold axes without costing too much energy, and the NH2 adsorption at the hollow site is considerably rigid since the rotation along the 2-fold axes would result in either a close contact between the H atoms and the surface atoms in the first layer or significant N-H bond stretching. In either case, rotation is not an energetically favorable process. Furthermore, the optimized structure of NH2 at the hollow site shown in Figure 2 seems to suggest that the chemisorption on this mode is a quasi-bond-dissociation process since the bond distance of N-H is much longer than the one in the gas phase (by 0.27 Å). The hollow seems still not large enough to accommodate the decomposition species without causing strong repulsion; on the other hand, adsorption at the hollow site is energetically more favorable as will be shown in the next section. As a compromise, the N-H bond becomes significantly stretched, and the species itself is trapped in a quasidissociation state. Similar to the case of NH2, chemisorption of NH can take place at both on-top and hollow sites with the latter being more energetically favorable. At the hollow site, in particular, there appears no steric effect that would otherwise result in N-H bond stretching since the NH species is now perpendicular to the surface and thus allows the H atom to avoid a direct close contact with the first layer surface atoms. This is evident from the optimized N-H bond distance which remains essentially unchanged. Finally, chemisorption of N and H atoms occurs at both on-top and hollow sites, as expected. It is interesting to compare the calculated bond distances between the atoms directly chemisorbed and the central metal atom that serves as an adsorbent. At the on-top mode, one observes the bond distance of N-Nb bond decreases with the size of the nitriding species, which is consistent with the chemisorption binding energies as will be demonstrated in the next section. On the other hand, at the hollow site, the N-Nb bond essentially remains the same except for NH chemisorption in which the bond is slightly stretched. This should not be surprising since NH2 is in a quasi-dissociation state, and thus the bond distance should be close to that of the N-cluster system; however, the bond of N-H in the NH species is very strong, which gives rise to a slightly longer bond distance of N-Nb. The distance of H-Nb at the on-top mode is shorter than that at the hollow site since in the latter case the H atom is more constrained. III.2. Chemisorption and Decomposition Energetics. Figure 3 displays the chemisorption binding energies calculated for the fragments of the nitriding species at both on-top and 4-fold-hollow modes, where the circles represent the binding energies calculated by local DFT and the dots by gradientcorrected DFT. Here, parts (a) and (b) in Figure 3 are for adsorption at the on-top and hollow modes, respectively. Except for NH3, all nitriding species can be stably chemisorbed on both modes. Adsorption of NH3 at the hollow site will result in direct N-H bond dissociation due to the large van der Waals volume of NH3 and the relatively small pore of the hollow of the Nb(100) surface. At both on-top and hollow sites, it is seen that for the nitriding species NHx (x ) 0-3), the chemisorption binding energies increases very rapidly as the size of chemisorption/decomposition species decreases. This is consistent with the calculated bond distance of N-Nb shown in Figure 2, i.e., the larger the binding energy, the shorter bond distance will be. Furthermore, the binding energies at the hollow site is much larger than that at the on-top site, indicating that the hollow site is a much

NH3 Adsorption and Decomposition at Nb(100) Surface

J. Phys. Chem., Vol. 100, No. 23, 1996 9803

Figure 2. Chemisorption structures of NH3 and its decomposition species at on-top site and 4-fold hollow site.

preferred adsorption mode. Salahub et al. studied N atom adsorption on a few transition metal surfaces by using DFT method and observed similar phenomena.26 We found that the hollow site is a preferred mode even for larger species like NH. For NH2, however, adsorption on both sites are almost equally favorable with the hollow site being a dissociative chemisorption mode. We believe the reason for the hollow site being energetically more favorable for relatively small fragments is due to the electrostatic interaction between the decomposition species and the surface atoms. While at the on-top mode, NHx is attracted mostly by only one Nb atom, it can readily interact with five Nb atoms, four of which are first layer atoms, at the

hollow site. As a consequence, the bond distance of N-Nb at the hollow site becomes even longer than that at the on-top mode. Figure 3 indicates that both LDF and nonlocal DFT yield the same trends of adsorption binding energies. However, the LDF calculations overestimate the adsorption binding energies considerably, and the nonlocal corrections result in smaller binding energies, as expected. In particular, one sees that the gradient correction yields much smaller binding energies at the hollow site for NHx (x ) 0, 1, 2) compared with the binding energies at the on-top site. This is due to the fact that the adsorption geometries calculated by LDF are in general not at

9804 J. Phys. Chem., Vol. 100, No. 23, 1996

Cheng et al. indicate that NH2 adsorption at the hollow site will result in dissociative adsorption. The discrepancy between the LDF and nonlocal DFT calculations in the optimized geometries seems unusual since most DFT calculations suggest that the gradient corrections contribute only minor changes to the LDF geometries.30 The main cause for the discrepancy in the present case may be mainly attributed to the rigid lattice approximation employed in our calculations. While the LDF calculations overestimate the binding, which results in relatively shorter bond distances, the nonlocal corrections will give rise to a relatively more relaxed structure. However, the rigid lattice model does not allow the surface atoms to move and thus to better accommodate the adsorbate. Furthermore, the N-H bonds in NH2 are lengthened in the nonlocal DFT calculations. Consequently, the dissociative adsorption mode, in which H atoms become adatoms adsorbed at the nearby bridge sites, becomes energetically more favorable. Using the assumptions stated in Section II, we can estimate the N-H bond dissociation energies from the calculated chemisorption binding energies shown in Figure 3. By neglecting the van der Waals interactions between any two neighboring nitriding species, one can simplify the surface reactions as the following:

NHx-Nbn + Nbn f NHx-1-Nbn + H-Nbn

Figure 3. (a) Calculated adsorption binding energies at the on-top mode. Open circles: local DFT; dots: nonlocal DFT. (b) Calculated adsorption binding energies at the hollow site. Open circles: local DFT; dots: nonlocal DFT.

the energy minima in the nonlocal DFT calculations, and the calculated energies are very sensitive in the present case to the geometries of the adsorbates which are tightly restrained by five adjacent surface atoms. In fact, we performed the gradientcorrected DFT geometry optimization for NH2 at the hollow site using the initial geometry obtained at the LDF level. The optimized adsorption structure differs considerably from the LDF geometry. While the N atom still sticks to the central atom in the second layer with a bondlength 2.18 Å, the two H atoms are now adsorbed at the “bridge” sites of the first layer. Consequently, the bond distance of N-H becomes 1.76 Å and the bond angle of H-N-H is 137.5°. We anticipate that the adsorption binding energies at the hollow site in geometryoptimized nonlocal DFT calculations should be somewhat smaller than the ones calculated by LDF but much larger than the energies calculated by single-point energy nonlocal DFT calculations. We wish to point out the major difference of the NH2 adsorption geometries at the hollow site optimized by LDF and nonlocal DFT methods. The LDF results suggest that NH2 can be indeed adsorbed at the hollow site with a relatively long N-H bondlength, which results in N-H bond dissociation. On the other hand, the results of gradient-corrected DFT calculations

for x ) 3, 2, 1, where Nbn stands for the metal cluster. From Figure 3, one sees that the H atom prefers the hollow site as its adsorption mode. We thus use the hollow-mode energy of the H-Nbn cluster to estimate the dissociation energies for the above reactions. Utilizing the chemisorption binding energies shown in Figure 3, one can readily derive the N-H bond dissociation energies at each step of the surface reactions. The results are displayed in Figure 4, where “ad” refers to the adsorbed species. Here a and b are for the decomposition at the on-top mode and the hollow mode, respectively. It is seen from Figure 4 that both LDF and the gradientcorrected calculations yield the same trends of decomposition energetics, although their absolute values differ considerably. The calculated dissociation energies of NH3 into NH2 at both chemisorption modes indicate that the first step of reaction should be relatively slow. At the on-top mode, the calculations suggest that nitriding in general is not an energetically favorable process, and the decomposition will become increasingly difficult, even though the fragmentation species can each be strongly chemisorbed at this site. At the hollow site, upon the dissociative chemisorption of NH2 further dissociation into NH and H will become a rapidly energy down-hill process due to the unstable quasi-dissociative state of NH2. NH can also readily dissociate into the adatoms at the hollow site, which is consistent with the large chemisorption binding energies for the nitriding species. This indicates that ammonia and its fragments will most likely dissociate into smaller species at the hollow site, and these decomposition species will further diffuse into the bulk through the channels consisting of hollow atoms to form nitrides and to eventually corrode the metal. We note that the off scale dissociation energies of the nonlocal DFT calculations shown in Figure 4b are mainly due to the underestimation of adsorption binding energy of NH2 at the hollow site. III.3. Population Analysis. Our DFT calculations show that the energy bands of Nb(100) surface near the Fermi level are composed mostly by the 4d orbitals of Nb with a considerable number of unpaired electrons which give rise to the magnetism of the metal. The estimated average magnetic moment of the Nb cluster is about 1.14 mB. There are empty bands composed

NH3 Adsorption and Decomposition at Nb(100) Surface

J. Phys. Chem., Vol. 100, No. 23, 1996 9805 TABLE 1: Mulliken Population Analysis of Gas-Phase NH3 and the Nitriding Species upon Chemisorption on the Metal Surface on-top species

atom

gas-phase

charge

transfer

charge

transfer

NH3

N H1 H2 H3 N H1 H2 N H N H

-1.18 0.39 0.39 0.39 -0.73 0.37 0.37 -0.35 0.35 0.00 0.00

-1.42 0.50 0.50 0.53 -1.17 0.42 0.42 -0.92 0.42 -0.64 -0.18

0.11

-1.09 0.01 0.01 -1.23 0.50 -0.98 -0.54

-1.07

NH2 NH N H

Figure 4. Calculated decomposition energies of the elementary processes at the on-top mode (a) and the hollow site (b). Line with a dot: LDF calculations; line with a rhombus: gradient-corrected DFT calculations.

of mainly 4d orbitals nearby the Fermi level with a band gap less than 0.5 eV. This type of electronic structure of a metal surface should be easily accessible for gas molecules to transfer electrons to or to gain electrons from the metal. On the other hand, NH3 has an electron lone pair on the N atom that occupies an orbital symmetry toward the Nb surface. It is thus expected that a charge transfer process from the gas molecule to the metal surface should occur. Similar analysis can also be applied to the fragmentation species except the H atom in which the 1s orbital may turn to attract an electron from the metal. It should be pointed out that both geometry optimized Nb clusters and bulk are found to be nonmagnetic.19 The magnetism is due to the cluster model of Nb surface and thus is an artifact. To examine the spin polarization effect on the chemisorption properties, we performed a nonpolarized calculation for NH3 at the on-top site. The calculated adsorption binding energy is 34.14 kcal/mol at the LDF level, about 2.70 kcal/mol higher than the spin-polarized result; at the nonlocal DFT level, the calculated NH3 chemisorption binding energy is 16.79 kcal/mol, about 1.72 kcal/mol lower than the spinpolarized one. In either case, we expect that the artifact due to the cluster magnetism will not significantly affect the reaction energetics, particularly the trends. We performed Mulliken population analysis in our DFT calculations to examine the charge-transfer process taking place

4-fold hollow

-0.33 -0.50 -0.64 -0.18

-0.73 -0.98 -0.54

in the chemisorption and the decomposition. It is understood that the Mulliken population analysis is basis-set dependent, and thus the results should be treated with caution. Nevertheless, the analysis often provides useful insight into the charge transfer process and predicts the correct trend of electron movement in a molecule. In Table 1, we show the calculated local DFT Mulliken charges of gas-phase NH3 and the nitriding species upon chemisorption on the metal surface at both ontop and hollow sites. The gross charge transfer from the decomposition species to the metal is also displayed. It is seen from Table 1 that although the net charge of the N atom in NH3 increases upon adsorption at the on-top mode, the overall charge on NH3 drops 0.11, which is largely attributed to the charge transfer from the lone-pair to the 4d, orbitals of the Nb surface. For NH2, the charge transfer process at both adsorption sites becomes reversed by attracting electrons from the metal due to the dangling N-H bond arising from the NH3 decomposition. While at the on-top mode the H atoms are still tightly bonded with N, which results in relatively large charges on the H atoms, the charges on H nearly vanish as NH2 ends up in a quasi-dissociation state at the hollow site. One thus sees much larger electron transfer from the metal to NH2. For the same reason, one observes even heavier charge transfer in NH and N at both chemisorption sites due to the increasing dangling N-H bonds. Finally, as expected, we found that the H adatom attracts a considerable amount of charges from the metal surface, particularly at the hollow site in which the H adatom is nearly embedded into the hollow to maximally take advantage of its fully spherical 1s orbital to attract electrons. IV. Summary We have presented a theoretical approach based on density functional theory to study the nitriding process at Nb(100) surface, aiming to model the corrosion process that occurs on stainless steel materials. Transition metals are important compositions of these materials and mostly responsible for their chemical behavior. The purpose of the present work is to gain physical insight into the corrosion mechanism based on the quantum-mechanical first principle, which will help the new nitriding-resisting materials development. Nitriding is a dissociative chemisorption process that takes a few steps of surface reactions. Due to the inherent complexity of the problem, it is necessary to simplify the model system to allow a feasible study. We first employ a metal cluster to model the Nb(100) surface and examine the chemisorption structures and the binding energies at both the on-top and 4-fold-hollow adsorption sites. We show that NH3 is adsorbed only at the on-top site, NH2 can reside in both modes, while other decomposition fragments will most likely fall into the hollow site. Furthermore, to estimate the dissociation energies at each

9806 J. Phys. Chem., Vol. 100, No. 23, 1996 step of the decomposition process, we made an approximation by neglecting the van der Waals interaction between any two nitriding fragments, which allows us to study the dissociation process by using relatively small surface clusters. The results suggest that while chemisorption is an energetically favorable process for all nitriding species, the decomposition process is site-dependent. At the on-top mode, the decomposition is essentially an energy-uphill process and nitriding is not thermodynamically possible at this site. On the hollow mode, however, the decomposition of NH3 into NH2 and H should initially be slow due to the small energy change of the surface reaction. Once NH2 is formed, the calculation predicts that further decomposition into smaller fragments should readily occur. The small species will subsequently diffuse into the bulk, form nitrides, and eventually degrade the material. The theoretical results also suggest that nitriding will not take place without diffusion of the ammonia fragments into the bulk. We compared the adsorption/decomposition energetics calculated by both geometry-optimized LDF and single-point energy nonlocal DFT. We found that the LDF calculations considerably overestimate the adsorption binding energies. However, the nonminimum energy adsorption structures at the hollow site result in over-corrected binding energies in nonlocal DFT calculations, particularly for NH2 species. In order to improve the energetics, one must perform geometry optimizations using the gradient-corrected methods, which is unfortunately not practical in the present case. Our results show that the trends of adsorption binding energies as well as bond dissociation energies do not change. The present study on nitriding at the Nb(100) surface is a great simplification of such a corrosion process that occurs in reality. We have investigated the role of the Nb metal in catalyzing the NH3 chemisorption and decomposition. More comprehensive treatment should of course deal with the real stainless steel materials. The present study serves as one step of a series of investigations that aim to understand the nature of nitriding at the surfaces of these materials and to provide theoretical guidance on the materials selection to achieve high economic efficiency. References and Notes (1) Ertl, G. Catalysis: Science and Technology 4, Anderson, J. R., Oudart, M., Eds.; Springer-Verlag: Berlin, 1983. Catal. ReV.-Sci. Eng. 1980, 21, 201. Angew. Chem., Int. Ed. Engl. 1990, 29, 1219.

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