Bonding of Ammonia and Its Dehydrogenated Fragments on Ru

Article pubs.acs.org/JPCC

Bonding of Ammonia and Its Dehydrogenated Fragments on Ru(0001) Sabine Maier,†,‡ Ingeborg Stass,†,§ Jorge I. Cerda,∥ and Miquel Salmeron*,†,⊥ †

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Physics, University of Erlangen-Nürnberg, Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany § Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany ∥ Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC, Cantoblanco, 28049 Madrid, Spain ⊥ Department of Materials Science and Engineering, University of California−Berkeley, Berkeley, California 94720, United States ‡

ABSTRACT: The adsorption and dissociation of ammonia on Ru(0001) was studied by scanning tunneling microscopy (STM), density functional theory (DFT), and STM contrast simulations. Various NHx (with x = 0−2) species were formed by controlled STM tip manipulation. Each species shows a characteristic imaging contrast in STM measurements, changing from a protrusion for NH3 and NH2, to a depression for NH and N. The adsorption sites of each species determined from the STM images and their contrast agree well with DFT calculations and STM image simulations. Ammonia was found to interact with hydrogen atoms present on the surface, leading to the formation of triangular-shaped NH3 + 3H complexes. At submonolayer coverage, ammonia dimers were also identified and their formation and dissociation were observed upon tip manipulation.

1. INTRODUCTION The synthesis of ammonia from N2 and H2, the so-called Haber−Bosch process, is one of the most important and extensively studied industrial catalytic processes, because, among other things, it has facilitated the production of synthetic fertilizers on an industrial scale and, therefore, dramatically increased global agricultural productivity.1 Recently, the catalytic ammonia decomposition chemistry has attracted attention owing to the high hydrogen storage capacity of ammonia and the possibility to produce hydrogen without toxic or greenhouse gases as byproducts,2 making ammonia a potential candidate as an energy carrier for fuel cells.3 Because ammonia dehydrogenation is thermally activated, it requires heating or energy input from some other process. For both synthesis and dissociation, ruthenium is one of the most active catalysts.4−7 The decomposition and adsorption of ammonia on Ru(0001) has been the objective of several previous surface science studies. Experiments showed that ammonia reversibly adsorbs on Ru(0001) in the temperature range of 80−300 K.8,9 Above 320−400 K or under electron bombardment, dehydrogenation and formation of H2 and N2 occurs as a concurring reaction.9−11 There has been also a considerable theoretical effort to calculate dissociation12 and hydrogenation13−17 mechanisms of ammonia on Ru(0001) as well as activation barriers. There is a general consensus that N2 dissociative adsorption is the rate-determining step in NH3 synthesis,16,17 whereas for NH3 decomposition, both N−H bond cleavage and recombinative desorption of N2 have been proposed as the ratedetermining step.10,18,19 First-principles calculations of reaction © 2012 American Chemical Society

mechanisms depend on well-defined adsorption sites of the NHx (x = 0−3) species as starting geometries. Despite this, a definitive experimental determination of the adsorption sites and geometries is still missing for some of the species, including NH2 and NH. This is likely due to the fact that the former is not stable at room temperature,20 and the latter can easily be dissociated by electrons when using electron-based spectroscopies.21 Through atomically resolved imaging and controlled STM tip manipulation, we can generate all the intermediate products and characterize their adsorption properties. To our knowledge, no other study exists today that reveals the local adsorption properties of all the NHx species on Ru(0001) or other close-packed transition metals. Only the dissociation products N22,23 and H24,25 have been identified and imaged previously on Ru(0001) using STM. Here, we report scanning tunneling microscopy experiments and density functional theory calculations combined with STM image simulations that unambiguously establish the adsorption sites and the dissociation pathways of ammonia on Ru(0001). The STM experiments include all N−H bond cleavage reactions induced by controlled tip manipulation that result in the formation of each of the NHx (x = 0−2) intermediates on the surface. Each species is identified through its distinct STM contrast and the adsorption site. Received: September 5, 2012 Revised: November 2, 2012 Published: November 8, 2012 25395

dx.doi.org/10.1021/jp308835x | J. Phys. Chem. C 2012, 116, 25395−25400

The Journal of Physical Chemistry C

Article

Figure 1. Top: STM images of ammonia molecules on Ru(0001) (bright spots in (a)), and the products of successive dehydrogenation reactions (b−d) induced by applying voltage pulses of 3 V between the STM tip and the molecule for a duration of 1 ms. Starting with an intact ammonia molecule (a), the voltage pulse leads to a partial dehydrogenation to NH2 (b), which appears as a weak protrusion in the image. Additional pulses produce successively NH (c) and N (d), both appearing as depressions with N being the deepest one, around −25 pm. Middle: Calculated DFT adsorption geometries of NHx (x = 0−3) species on Ru(0001). N and NH adsorb on 3-fold hollow hcp sites, NH2 on a bridge site, and NH3 at an on-top site. Bottom: Height profiles along the encircled NHx species (black line) and calculated height profiles (gray line) based on the DFT adsorption geometries shown above. The apparent height of the NHx species increases with increasing x, ranging from −40 pm for N to 50 pm for NH3. STM image parameters are 30 mV and 252 pA, and calculation parameters are V = 40 mV and I = 100 pA.

process, only the molecule and the first two substrate layers of the slab were allowed to relax, leaving the rest of atoms in their bulk positions. For the STM image simulations, we employed the GREEN code, which goes beyond the usual Tersoff−Hamann approach by explicitly considering the tip at the same atomistic level as the surface. The formalism, including an improved description of the Hamiltonian matrix elements almost at the ab initio level, has been described in detail elsewhere.28,29 The sharp metallic tip was modeled by a one-atom ended Pt(100)-oriented pyramid stacked on a Pt(100) surface, that is, a semi-infinite tip electrode. The other electrode comprises the molecule on top of the Ru(0001) surface. The elastic current flowing from tip to sample was calculated by adjusting the tip−sample distance at each pixel until the desired current value was obtained. All simulations were performed assuming a constant tunneling current of 100 pA at a +50 mV sample bias voltage, which represent typical tunneling conditions used in the experiments.

2. METHODS The ruthenium crystal was initially cleaned by cycles of argon ion sputtering (1 keV, pressure: 3 × 10−5 Torr) and annealing at 1600 K. Several heating cycles between 800 and 1800 K in a partial oxygen atmosphere of 4 × 10−8 Torr were performed subsequently to deplete the subsurface region from carbon bulk impurities. The oxygen was then removed by flashing the sample to 1700 K in ultrahigh vacuum. Sample cleanliness was monitored with both Auger electron spectroscopy and STM. At the end of the cleaning cycle, surface contaminants (O, C, and H) were determined to be present to less than 1% of a monolayer. A home-built low-temperature STM25 in an adjacent chamber was used to image and manipulate the ammonia molecules on Ru(0001) at a base pressure below 2 × 10−11 Torr. All STM images presented in this paper are acquired in the constant current mode at a sample temperature of around 7 K using electrochemically etched Pt−Rh (80− 20%) tips. Ammonia molecules (99.99% anhydrous ammonia, Sigma-Aldrich) were dosed in situ at sample temperatures of 7 K through a nozzle pointing toward the surface. Density functional theory calculations were performed with the SIESTA code26 under the generalized gradient approximation (GGA)27 for the exchange-correlation part. Isolated species were modeled by slab geometries containing up to eight Ru layers along the [0001] direction with a p(4 × 4) unit cell. Norm-conserving pseudopotentials of the Troullier−Martins type were employed to describe the core electrons, while the atomic orbital (AO) basis set was represented by double-ζ polarized (DZP) numerical orbitals strictly localized using a confinement energy of 100 meV. Real-space three center integrals were computed over a 3D grid with a resolution of 200 Ry. The adsorption structures were relaxed until forces on the atoms were smaller than 0.05 eV/Å. In the minimization

3. RESULTS AND DISCUSSION 3.1. NH3 Dissociation and Identification of NH x Species on Ru(0001). Figure 1a shows an STM image of intact ammonia molecules deposited at 7 K on Ru(0001). Ammonia molecules appear as bright protrusions with an apparent height of 48−58 pm in both filled-state and emptystate STM images, mostly unaffected by the applied bias and current. The image contrast is comparable to that observed in other STM studies of ammonia on other metal surfaces.30,31 NH2 and NH adsorbed on ruthenium can be formed by dissociation of the ammonia molecule. Early thermal desorption studies showed that ammonia adsorption at surface temperatures of ∼100 K does not result in molecular dissociation and that it desorbs molecularly upon heating.8,9 25396



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Only at sample temperatures between 450 and 550 K has dissociation reported to occur.9−11 Here, we succeeded in dissociating the ammonia molecules step-by-step by applying voltage pulses between the STM tip and the surface. For this, the tip was moved over a molecule, like the one marked by a circle in Figure 1a, and voltage pulses of 3 V were applied for 1 ms to dehydrogenate the molecule. The same procedure is applied successively to the resulting fragments (Figure 1b−d) until complete dissociation to N atoms. The corresponding sequence of elementary steps for NH3 decomposition can be written as NH3* + *→ NH 2* + H*

(1)

NH 2* + *→ NH* + H*

(2)

NH* + *→ N* + H*

(3)

Table 1. Summary of Experimental and Calculated Adsorption Sites as Well as Apparent Heights of NHx Species on Ru(0001)a topographic height STM exptl (pm)

STM calcd (pm)

N

−29 ± 2

−35

NH

−10 ± 3

−15

9±3 53 ± 5 90 ± 5

22 50 120

species

NH2 NH3 NH3 dimer

adsorption site STM exptl 3-fold hollow site 3-fold hollow site bridge on-top

DFT hcp hcp bridge on-top on-top

a

STM parameters in the topographic height determination: experiment, ±36 mV, 12 pA; and calculation, 40 mV, 100 pA.

experiment than in the calculation. This is likely due to a larger tip size in the experiment compared to the highly symmetric and sharp Pt pyramid tip used for the calculations. 3.2. Adsorption Site Determination. The adsorption sites of the various NHx species determined from the DFT calculations are summarized in Table 1 and illustrated in Figure 1. NH3 adsorbs on top sites with the N−H bond directions projecting in the direction of the neighboring Ru atoms. NH2 adsorbs on bridge sites with one N−H bond projecting toward the 3-fold fcc site and another toward the hcp site. NH and N adsorb on hcp sites. These calculated adsorption sites agree with previous calculations,17,35,36 except for the NH2 species, where different adsorption sites were reported, namely, bridge17,35 and fcc.36 The calculations reveal also that ammonia is bound to the ruthenium surface via the electron lone pair of the nitrogen atom, with the hydrogen atoms pointing away upward from the surface with the 3-fold axis normal to the surface. The energy barriers for rotation of the NH3 about the surface normal are small, below 50 meV. We could also determine the adsorption sites of the various NHx species experimentally from the atomically resolved STM images. We found that atomic nitrogen adsorbs at a 3-fold hollow site (see Figure 2a), consistent with the DFT calculations as well as earlier STM measurements from the Wintterlin group22,23 and low-energy electron diffraction (LEED) measurements by Schwegmann et al.37 In contrast to the N atoms, it is difficult to image the hydrogenated molecules and the Ru atoms simultaneously because, at the close tip−surface distance needed to obtain atomic resolution of the metal, the tip interacts too strongly with the molecules, inducing their displacement while scanning. However, we could determine the adsorption site by overlaying a lattice-resolved image from a subsequent high-resolution image and using nearby nitrogen atoms as reference (Figure 2b), which, as mentioned above, are not affected by tip proximity. In this way, we confirm the on-top adsorption for NH3 predicted by DFT. To our knowledge, this is the first direct experimental determination of the adsorption site of ammonia on Ru, which has been the subject of disagreement and is still under discussion. Both on-top and 3-fold hollow adsorption sites have been proposed for intact molecules depending on coverage. Benndorf et al.38 suggested that it adsorbs with the nitrogen atom in a 3-fold hollow site based on LEED and electron stimulated desorption ion angular distribution. In contrast, high-resolution electron energy-loss spectroscopy (HREELS) results by Parmeter et al.39 are more consistent with chemisorbed NH3 bonding on top sites. Zhou

where “*” and X* denote an empty site and an adsorbed X species on the Ru surface, respectively. This step-by-step dehydrogenation was highly reproducible. Dissociation was achieved for both voltage polarities above a certain voltage threshold, which critically depended on the tip−sample distance. In other words, dissociation is not the direct consequence of excitation of a particular vibration mode (e.g., N−H stretch) by tunneling electrons, but rather an effect of the applied field. This is an important finding that we will present and discuss in detail in a separate paper.32 Here, we focus on the structure and adsorption sites of the various NHx species formed. With longer pulse durations, NH3 could be dissociated directly into N, indicating that the rate-determining step might be attributed to the first N−H bond cleavage (reaction 1) or, alternatively, the barriers for reactions 1−3 are of comparable size. This observation is supported by calculated relative energy diagrams from first-principles studies for the ammonia synthesis on Ru(0001).17,33 The rate-determining step of ammonia decomposition was previously discussed mainly with reference to the N−H bond cleaving as opposed to N2 desorption steps as a function of temperature and pressure.10,18,19,34 Starting with the intact ammonia molecule in Figure 1a, the first voltage pulse leads to a species with a decreased apparent height of 6−12 pm, which we identify as NH2. Successive pulses produce NH and N, respectively, which appear as depressions in the STM images with apparent heights of −7 to −13 pm for NH and −27 to −31 pm for N. Figure 1 (bottom row) displays the corresponding height profiles across the encircled NHx species. The hydrogen released in the reaction is not always visible in the STM images because of its high diffusivity, particularly in the presence of the strong fields used to dissociate the molecules. Additional voltage pulses were applied to the fourth species (N) without any noticeable change of its shape or position. This indicates that, indeed, each pulse splits off one hydrogen atom so that the molecule is completely dehydrogenated after three pulses, leaving behind a N atom on the Ru surface. Nitrogen atoms were identified in a previous STM study23 as depressions of −30 to −40 pm, similar to our value of −27 to −31 pm. We see, therefore, that the apparent height of the NHx species decreases with x, as summarized in Table 1. The apparent heights of all the NHx species are mostly independent of the bias voltage. The measured apparent heights are very well reproduced in the calculated STM images, as seen in Figure 1, corroborating our assignment of the NHx species. The full width at half-maximum of the height profiles, however, is a factor of 2 larger in the 25397



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Figure 3. STM images acquired (a) before and (b) after applying voltage pulses to dehydrogenate the ammonia molecules marked by crosses, leading to dehydrogenation to NH2 (red) and to N (green). Some of the molecules moved to neighboring sites. (c) Expanded view of the central part with an overlaid lattice obtained from subsequent atomically resolved images. Two NH2 species (circled in yellow) are seen adsorbed on bridge sites. The nitrogen atoms and ammonia molecules were used as references to position the lattice, where the nodes correspond to Ru top sites. The contrast of NH2 has the shape of a dumbbell with two lobes aligned perpendicular to the compact lattice direction of the Ru substrate, as shown magnified in (d). (e) Schematic drawing of the NH2 molecule. The dotted circles represent the protrusions of the dumbbell shape of the molecule. STM parameters: (a−d) 28 mV, 195 pA.

Figure 2. (a) STM image of a nitrogen atom recorded at tunneling conditions that resolve the atomic lattice of the ruthenium substrate. The depression in the center corresponds to a N atom produced by tip-induced dissociation of an NH3 molecule. The image reveals the 3fold hollow site as the adsorption site of the N atom. (b) STM image of an NH3 molecule and three nearby nitrogen atoms. The NH3 adsorbs at the on-top site, as shown by the overlaid lattice from a subsequent atomically resolved image and using the nitrogen atoms as reference. The nodes of the lattice in (b) represent Ru top sites. The three nitrogen atoms have been formed by tip manipulation beforehand from ammonia molecules using voltage pulses. (d) STM image showing NH and N (circled at the top) formed after applying a voltage pulse of 2.8 V for 100 ms to dehydrogenate the encircled ammonia molecule in (c). The overlaid Ru lattice, using the N atoms and the NH3 molecule as a reference, shows that the NH occupies a 3fold hollow site. STM parameters: (a) 9 mV, 13.3 nA; (b) 28 mV, 195 pA; (c, d) 30 mV, 254 pA.

perpendicular to the close-packed atomic rows of Ru atoms, as shown in Figure 3d. The simulated image and line profile (Figure 1b) do not reproduce this shape correctly, however, probably due to the particular tip model and/or the small corrugation associated with this species. Finally, the adsorption site for NH is found to be the 3-fold hollow site (Figure 1c), which was determined experimentally using the same procedure as in the previous case, as shown in Figure 2c,d. In this figure, three reference N atoms make it possible to triangulate the exact position of the NH species that was produced by sequential tip-induced dehydrogenation of the NH3 molecule on the top left corner of the image. Staufer et al.42 suggested that NH sits on an hcp site based on a combined DFT and infrared spectroscopy study. In our case, however, both the total energy calculations and the STM simulations confirm the hcp site for NH. 3.3. Ammonia Dimers. Although our studies were performed mostly with very low coverage of ammonia molecules, occasionally we could observe protrusions that appear about twice higher (brighter) than individual ammonia molecules, as in the example of Figure 4. These brighter contrast species could also be formed intentionally by using short voltage pulses (not large enough to dissociate the molecule) in the vicinity of two neighboring individual ammonia molecules (Figure 4a,b). The bright protrusions can be split again into two individual molecules by new voltage pulses. Thus, we identify them as ammonia dimers. As shown in the line profile of Figure 4c, the apparent height of the dimers (85−95 pm) is nearly twice as large as that of an individual ammonia molecule (48−58 pm). The dimer consists of one molecule chemisorbed to the surface via the nitrogen atom (called α-ammonia), and another molecule hydrogen-bonded to the first layer (called βammonia). Figure 4f,g depicts the geometric configuration

et al.40 argued for both, hollow and on-top sites, depending on the coverage. These discrepancies could also be due to damage of the molecules by the energetic electrons used in these techniques. For NH2, our DFT calculations predict an N-bonded molecule at the bridge site with C2v symmetry (Figure 1b). The STM results, shown in Figure 3, are consistent with this prediction. In this experiment, two NH2 species were created by tip-induced dissociation of NH3 molecules (red crosses in Figure 3a). Two other NH3 molecules in the same image were dissociated to N atoms (green crosses in Figure 3b) and will serve as position markers for site identification. Although NH2 species have been previously observed on Ru(0001) by TPD and HREELS as an intermediate of hydrazine decomposition between 220 and 280 K,41 we have not found experimental evidence in the literature for the adsorption site of NH2 on ruthenium, most likely due to the fact that it is unstable at room temperature.20,41 In the STM images, the NH2 molecule appears as a small dumbbell-shaped protrusion (Figure 3b−d) centered on a bridge site and with the line through them 25398



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Figure 4. STM images showing the formation of ammonia dimers on Ru(0001) using voltage pulses. (a, b) A dimer is formed from the two individual ammonia molecules inside the circle in (a) after applying a voltage pulse of −5.75 V for 60 s to the molecule at the position x, which then dissociated to N. The same pulse displaced also a neighboring molecule. (c) Typical cross section of a dimer (red line) and two individual ammonia molecules (black line) revealing an apparent height of about 92 and 48 pm, respectively. Scan parameters for all images: 36 mV, 12 pA. Calculated STM images of an ammonia monomer (d) and dimer (e) on Ru(0001) (same z-scale for both images. STM calculation parameters: 50 mV, 100 pA). The calculated apparent height for the dimer is 120 pm, and that for the monomer is 50 pm. The yellow frame represents the (5 × 5) unit cell used in the calculations. (f) Top view and (g) side view of an ammonia dimer resulting from the DFT calculation. α-Ammonia is connected via the nitrogen atom to the ruthenium top site. β-ammonia is hydrogenbonded to α-ammonia. Only the two upper layers of the ruthenium slab are shown.

Figure 5. (a) STM image showing ammonia molecules (bright spots) and H atoms generated from H2 adsorption or from NH3 dissociation (dark spots). NH3 interacts attractively with hydrogen atoms located at nearby 3-fold sites, to form NH3−nH complexes, with the ammonia surrounded by up to three hydrogen atoms. (b) Expanded view showing an NH3−3H complex, and (c) calculated STM image. (d) DFT geometry of an NH3 molecule surrounded by three hydrogen atoms on Ru(0001). The ammonia adsorbs on a top site, the hydrogen atoms on fcc sites, separated by two lattice constants. All images have the same lattice orientation. STM parameters: (a, b) 53 mV, 21 pA.

ammonia is stable at these scan conditions. The calculated STM image in Figure 5c based on the DFT calculation shown in Figure 5d qualitatively matches the experimental image in Figure 5b.

obtained from DFT calculations. The nitrogen atom of βammonia is 3.38 Å above the surface located near a top site. Similar dimer configurations have been recently discussed on Pt(111)43 and Ni(111).44 In these previous DFT studies, the βammonia was found to adsorb near a bridge site on Ni(111) and near a top site on Pt(111). The calculated height of the dimer in the STM images is 120 pm, a value that exceeds by a factor of 2 the height of a single ammonia molecule and agrees well with the experimental observations. 3.4. Interaction of Ammonia with Hydrogen. Earlier studies of ammonia decomposition showed that, at low temperatures (

Bonding of Ammonia and Its Dehydrogenated Fragments on Ru

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