Infrared Spectroscopy of Ammonia on Iron: Thermal Stability and the

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Infrared Spectroscopy of Ammonia on Iron: Thermal Stability and the Influence of Potassium P. Iyngaran, D. C. Madden, D. A. King, and S. J. Jenkins* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. S Supporting Information *

ABSTRACT: We report on reflection−absorption infrared spectroscopy investigations, supported by first-principles density functional theory, into the surface chemistry of NH3 on Fe{111}. Ammonia is found to adsorb intact at low temperature and predominantly to desorb intact at around room temperature. Some of the adsorbed molecules dissociate if the surface is held at temperatures a little below the desorption temperature, transiently producing NH2 (directly observed) and ultimately NH and/or N (both inferred). Preadsorbed potassium is found to substantially weaken the interaction of ammonia with the surface, to induce a change in the ammonia adsorption geometry, and to promote both ammonia desorption and dissociation.

I. INTRODUCTION Synthesis of ammonia over an iron catalyst, via the Haber− Bosch process, is reputedly responsible for feeding up to onesixth of the world’s population (artificial fertilizer production) as well as providing feedstock for the manufacture of myriad commodity and fine chemicals (dyestuffs, cleaning agents, pharmaceuticals, etc.). At the same time, however, ammonia synthesis is reckoned to account for around 1% of global energy production, and efforts to optimize the efficiency of this century-old reaction have never been more urgent. In the broadest terms, ammonia synthesis over iron involves four distinct steps: (i) dissociative adsorption of molecular hydrogen, (ii) dissociation of adsorbed molecular nitrogen, (iii) stepwise hydrogenation of adsorbed atomic nitrogen by adsorbed atomic hydrogen, and (iv) desorption of ammonia. The first of these steps is generally considered to be essentially trivial, but in principle any of the remaining three could be ratedetermining under different conditions. Results obtained from high-area catalysts under industrially relevant conditions1−6 provided the first strong evidence that dissociative adsorption of nitrogen was often rate-determining, augmented later by ultrahigh-vacuum, single-crystal work from the group of Ertl.7−11 Somorjai12 subsequently demonstrated that selfpoisoning by ammonia could be critical under conditions where a high partial pressure of the product led to significant blocking of sites for nitrogen dissociation, and computational work from Norskov13 has successfully modeled a realistic volcano curve for ammonia synthesis activity on a variety of metals on the basis of a similar mechanism. More recent work on this key industrial reaction includes kinetic modeling14,15 and discussion of oxygen as a poison16 or alkali metals as promoters.17,18 Of particular interest, however, are a number of studies that shed light on the degree of © XXXX American Chemical Society

nitridation likely to occur at iron surfaces under realistic synthesis conditions. Yeo et al.,19 for instance, present firstprinciples density functional calculations showing a rather high activation barrier for penetration of adsorbed nitrogen adatoms into the subsurface region. If the dissociative adsoption of nitrogen is the rate-determining step for the overall reaction, then we must expect rather low surface coverage of nitrogen adatoms (since their rate of consumptionconversion to ammoniais relatively high compared with their rate of production) and hence a very low rate for absorption into the substrate. But those surface science experiments showing the clearest evidence for dissociative nitrogen adsorption being rate-determining are precisely those which were carried out on deliberately prenitrided surfaces, likely to be somewhat passivated against further nitrogen adsorption and dissociation. Indeed, Kandemir et al.20 report no nitridation of iron surfaces under realistic reaction conditions, calling into question the strict relevance of studies conducted on prenitrided surfaces. The precise mechanism of ammonia synthesis thus appears, as much as ever, to be a topic that will reward reinvestigation. The well-known promoter effect of potassium for this reaction has generally been interpreted as being due either to a direct enhancement of dissociative nitrogen adsorption or to enhanced desorption of ammonia (or some combination of both). Very recently, our own work, carried out under conditions where neither nitrogen adsorption nor ammonia desorption can be rate-limiting, has shown that the presence of potassium also enhances the rate of hydrogenation for adsorbed atomic nitrogen, to a degree that is consistent with the overall Received: March 31, 2013 Revised: May 9, 2014

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dx.doi.org/10.1021/jp409718x | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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It is worth noting that ammonia was dosed directly into the side chamber, with the sample in position for RAIRS experiments, but the pressure was monitored using a gauge located in the main chamber due to a lack of available ports. In this geometry, the measured pressure is considerably less than the pressure pertaining at the sample, and so the effective exposure at the sample will be much higher (possibly 2 or even 3 orders of magnitude higher) than those inferred from our instruments. In effect, exposures reported here should be considered to be merely nominal, although they are nevertheless expected to be reliably proportional to the actual exposures; we emphasize this nominality throughout the remainder of this paper wherever numerical values are reported. Sample temperatures are, of course, directly measured via a thermocouple directly attached to the sample itself, regardless of the sample location; all temperatures reported in this paper are sample temperatures measured in this way. Experiments were performed using research grade ammonia (purity 99.999%) implying that impurity gases introduced from this source should be 5 orders of magnitude below the partial pressure of ammonia. Even allowing for a discrepancy of 3 orders of magnitude between the main chamber and the side chamber, the nominal dosing pressure of 10−8 mbar measured in the main chamber would correspond to a partial pressure of impurity gases at the sample not exceeding 10−10 mbar. This is comparable with the base pressure of our chamber, which is typically in the high 10−11 mbar range, and does not constitute any problem for our methodology. B. Preparation of the Clean Fe{111} Surface. Experiments were performed on a sample cut and polished to within 1° of the {111} plane, supplied by Metals, Crystals and Oxides Ltd. Pretreatment of this particular sample by annealing at 900 K in a hydrogen atmosphere was carried out some years earlier by Escott et al.39,40 and was found to be highly beneficial in removing bulk impurities, particularly sulfur, prior to its first use under UHV conditions. Extensive further cleaning in situ via cycles of Ar+ sputtering (680 K) and annealing (875−950 K) was necessary to reduce surface concentrations of carbon, nitrogen, oxygen, and remaining sulfur. Of these, carbon was found to be the most stubborn contaminant, but titration with oxygen at 320 K followed by annealing at 875 K was effective at bringing it down to acceptable levels (see Supporting Information). Any excess oxygen left on the surface after this last step could then be removed by a final cycle of Ar+ sputtering and annealing. Daily cleaning was then achieved each morning by means of one or more cycles of oxygen treatment, sputtering, and annealing (as described above) to remove any buildup of carbon due to adsorption of residual carbon monoxide overnight. Following this treatment, we believe our Fe{111} surface to be at least as clean as any previously reported in the literature (see Supporting Information) and considerably cleaner than many (where minimal cleaning was undertaken or where the surface was deliberately saturated with nitrogen to prevent subsurface diffusion in subsequent experiments). The LEED pattern obtained from this surface shows clear (1 × 1) character (Figure 1), and although some spots are fairly broad, they are nevertheless consistent with expectations from a clean surface of kinked structural type. Such surfaces, of which the bcc-{111} surface is one, possess top-layer atoms whose coordination number is precisely half that of the bulk material; in consequence, the formation of adatom−vacancy pairs at the surface is approximately thermoneutral, and a higher than

rate enhancement observed in studies of the complete reaction.21 In the present work, we report on infrared vibrational investigations into the surface species found upon ammonia adsorption onto both clean and potassium-precovered Fe{111} surfaces, shedding light on the role of the alkali metal in modifying the ammonia−iron bond. In efforts to identify adsorbed molecular or submolecular species, vibrational measurements can be a powerful tool, whether obtained via high-resolution electron energy loss spectroscopy (HREELS) or by reflection−absorption infrared spectroscopy (RAIRS), and one might have expected to find several such studies in the literature relating to ammonia on iron. While several vibrational spectra of ammonia adsorbed on platinum,22−25 nickel,26 and ruthenium27−29 surfaces have been reported, however, we are aware of only one vibrational study on iron, published across two papers some 30 years ago. The HREELS work of Erley and Ibach revealed a band in the range 1105−1170 cm−1, supporting adsorption onto atop sites at low coverages and temperatures (110−120 K) on the Fe{110} surface, with some suggestion of partial dissociation to form NH2 at higher temperatures (195 K) indicated by a band at 1580 cm−1.30,31 Turning away from vibrational studies, X-ray photoemission spectroscopy (XPS) has provided evidence for complete roomtemperature ammonia dissociation at low coverages on the Fe{111} surface, giving way to a molecularly adsorbed species at higher exposures.32 Formation of NH2 was tentatively concluded to be the cause of an emergent feature in ultraviolet photoemission spectroscopy (UPS) conducted on the Fe{111} and Fe{100} surfaces at 180 K, and some degree of isotopic exchange was also observed in desorption experiments on Fe{111}, though not on the Fe{110} surface.33,34 The {111} and {100} surfaces of iron seem, therefore, to be particularly prone to dissociate ammonia, in comparison with the {110} facet. We note that the {111} and {100} surfaces of bcc materials may both be considered to be “kinked” facets, exposing top-layer atoms whose nearest-neighbor coordination number (i.e., four) is precisely half that of the bulk material; in contrast, the {110} surface of a bcc material possesses a much higher top-layer coordination number (i.e., six) and is therefore best thought of as “flat”.35,36 The reduced coordination number of the kinked surfaces is likely to be very relevant to their high reactivity. Notwithstanding these observations regarding clean surfaces, however, the role of potassium in weakening the ammonia−iron bond has hardly been addressed, and it is this that forms the focus of the present work.

II. EXPERIMENTAL AND COMPUTATIONAL METHOD A. RAIRS Apparatus. The ultrahigh-vacuum (UHV) apparatus used in this work has been described in detail elsewhere.37,38 In brief, the system incorporates a main chamber designed for use only under UHV conditions and a small side chamber capable of operating either under UHV conditions or with ambient gases up to atmospheric pressure. The main chamber is equipped with an ion gun for sputtering, Auger spectrometer for monitoring surface contamination, lowenergy electron diffraction capabilities to assess surface ordering, and a mass spectrometer for detection of desorbing species. Once transferred to the side chamber, the sample geometry allows impingement of an infrared beam at grazing incidence for RAIRS studies. We present each spectrum as a ratio against a reference spectrum obtained from the clean surface immediately before each experiment. B



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Figure 2. Variation in the K250/Fe650 ratio versus potassium dosing time.

Figure 1. (1 × 1) LEED pattern (at 160 eV) obtained from the clean Fe{111} surface.

normal degree of atomic-scale surface roughness is naturally to be expected.35,36 C. Preparation of Potassium Precovered Surfaces. Potassium was deposited on our sample in the main chamber by means of a dispenser (SAES Getters) containing potassium chromate and reducing agents. Resistive heating of the dispenser activates chemical reduction of the chromate, producing a source of potassium vapor. Degassing of the dispenser prior to deposition was found to be essential to avoid contamination of the sample with coemitted carbon monoxide, carbon dioxide, water, and hydrogen. Measurement of the potassium flux by mass spectrometry established that constant emission of the alkali metal was achieved after about 3 min of heating the dispenser, so we always kept the sample away from the line-of-sight dosing position until such time had elapsed. Uptake of potassium on the sample, after completing the induction period of the source, was monitored by means of Auger electron spectrometry (AES). Specifically, the 250 eV AES feature provided the most reliable indicator of potassium coverage (since the 275 eV feature overlaps with a carbon feature at 270 eV), and we took the ratio of its peak-to-peak height with that of the 650 eV ironderived AES feature as our measure of relative coverage. We found the buildup of adsorbed potassium to be almost independent of surface temperature, consistently saturating at a K250/Fe650 ratio of around 2.5 (see Figure 2). At the lower ratio of 0.34, we obtained a (3 × 3) LEED pattern (see Figure 3a) after briefly annealing to 580 K, and we follow Ertl and coworkers10 in assuming that this corresponds to a coverage of 0.11 ML. Further assuming the K250/Fe650 ratio to be proportional to coverage, we infer that saturation of potassium on this surface occurs at 0.81 ML, in good agreement with the value of 0.83 ML deduced in earlier work.10 In addition, we also found a higher coverage (2 × 2) LEED pattern (see Figure 3b) briefly mentioned in our earlier publication21 but otherwise unreported in the literature. This pattern was weak upon initial dosing but strengthened upon annealing to 580 K. The corresponding K250/Fe650 ratio of 0.93 would suggest a nominal

Figure 3. LEED patterns (at 170 eV) obtained after dosing with potassium: (a) the (3 × 3) pattern corresponding to a K250/Fe650 ratio of 0.34 and (b) the (2 × 2) pattern corresponding to a K250/Fe650 ratio of 0.93. C



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coverage of 0.30 ML, in tolerable agreement with the plausible expectation of a 0.25 ML coverage for such an overlayer. Potassium coverages reported in the results presented below were similarly estimated by measuring the same AES ratio and assuming a direct linear proportionality. D. Density Functional Theory. To assist in the interpretation of features in our RAIR spectra, we have performed a number of first-principles density functional calculations. These were carried out using the CASTEP computer code,41 which invokes periodic boundary conditions in all three dimensions. Within the surface plane, we worked with a (2 × 2) unit cell, consistent with surface coverage of 0.25 ML for adsorbed species. In the surface normal direction, an artificial periodicity equivalent to 24 Fe{111} layers was imposed. The surface was then modeled by means of a ninelayer slab, whose uppermost six layers (and any adsorbed species) were permitted to relax according to the calculated forces. Electronic wave functions were represented by a summation over plane waves up to a kinetic energy cutoff at 340 eV and the Brillouin zone sampled using a (3 × 3 × 1) Monkhorst−Pack mesh. Vibrational calculations were then performed on well-converged groundstate geometries by the finite displacement method (i.e., construction of a massweighted force constant matrix through systematic shifts in atomic positions, followed by diagonalization to obtain frequencies and displacement modes). For the clean surface, we determined a contraction of 10% in the outermost interlayer spacing (relative to the theoretically predicted bulk spacing of 0.83 Å) and of 19% in the second interlayer spacing, partly balanced by a 10% expansion in the third. The fourth and fifth interlayer spacings show 2% contraction and 2% expansion, respectively. In calculations for the potassium-covered surface, we assumed a coverage of 0.25 ML and an adsorption site in the hollows of the clean surface. A buckling of 0.07 Å was then found between the outermost layer Fe atoms adjacent to the adatoms (three per unit cell, lying relatively low) and those furthest from them (one per unit cell, lying relatively high). In the second layer, the buckling was larger, at around 0.14 Å, with the atoms closest to the adatom (again, three per cell) lying relatively high and those furthest away (one per cell) lying relatively low. In the third layer, one atom per cell was situated immediately beneath the adatom, and this was found to lie 0.03 Å higher than the other atoms in that layer. The mean interlayer spacings showed contractions of 16%, 19%, and 3% in the first, second, and fourth interlayer spacings, respectively, with expansions of 12% and 1% in the third and fifth. These relaxations are, if anything, a little more pronounced than for the clean surface.

Figure 4. RAIR spectra in the range 900−2000 cm−1 with increasing exposure (L ≡ “nominal langmuirs”) of ammonia at 112 K on clean Fe{111}. The final spectrum was obtained at an ambient pressure of 1 × 10−8 mbar of ammonia.

band broadens, but drops only slightly further in frequency, settling at around 1090 cm−1. Little change in the δs band is observed when spectra are taken under an ambient ammonia pressure of 1 × 10−8 mbar, indicating that the highest exposures in the sequential dosing experiments (i.e., 0.055 langmuir) achieve saturation coverage of the surface. In addition, a lowintensity peak emerges at 3200 cm−1 in the highest exposure spectra (see Figure 5), once again seen previously in the work of Escott,39 which could conceivably be interpreted as arising from the symmetric stretch mode of ammonia, labeled νs; it is equally possible, however, that this feature may be an artifact due to the formation of ice on the infrared detector. At any rate, the quality of our spectra in this frequency range, specifically the strongly sloping background, discourages us from attempting to interpret possible stretch modes. All further discussion in this paper will concern only modes in the frequency range below 2000 cm−1. The gas-phase frequency of the ammonia umbrella mode is 968 cm−1, so the 1140 cm−1 band associated with this mode on the surface is strongly blue-shifted by 172 cm−1 at low exposure, and the red-shift upon increased exposure actually represents a reversion toward the gas-phase frequency. It is reasonable, therefore, to conclude that the formation of a chemical bond between ammonia and the surface (essentially a dative covalent bond involving the nitrogen lone pair) is responsible for the low-exposure blue-shift and that gradual weakening of that bond with increased exposure is responsible for the subsequent red-shift. Similar behavior has previously been noted in HREELS measurements of ammonia adsorption on Fe{110}30,31 at 110 K, where the observed low-coverage blue-shift to a frequency of 1170 cm−1 was interpreted as indicating electron transfer from adsorbate to surface, while a relative red-shift to a frequency of 1105 cm−1 upon increasing

III. RESULTS A. Adsorption of Ammonia on Clean Fe{111}. Ammonia was dosed onto the clean Fe{111} surface at 112 K, and RAIR spectra, averaged over 100 scans, were recorded with increasing exposure at a resolution of 4 cm−1 (see Figure 4). At very low exposures, an absorption band was observed at 1140 cm−1, attributed to the symmetric deformation (i.e., umbrella) mode of ammonia, labeled δs and found previously in unpublished work by Escott.39 The intensity of this band grows, and its frequency is red-shifted, with increasing exposure; the feature remains sharp up to a nominal exposure of around 0.01 langmuir, at which stage it is centered at a frequency just over 1100 cm−1. At nominal exposures beyond 0.01 langmuir, the D



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Figure 6. RAIR spectra with increasing exposure (L ≡ “nominal langmuirs”) of ammonia at 112 K on the Fe{111} surface precovered with 0.1 ML of potassium.

With 0.2 ML potassium precoverage, the RAIR spectra (see Figure 7) at lowest ammonia exposures show the umbrella

Figure 5. RAIR spectra in the range 2000−4000 cm−1 with increasing exposure (L ≡ “nominal langmuirs”) of ammonia at 112 K on clean Fe{111}. The final spectrum was obtained at an ambient pressure of 1 × 10−8 mbar of ammonia.

coverage indicates a reversal of that change. On that surface, a second layer of ammonia could be formed at higher exposures, giving rise to an additional strongly blue-shifted band at 1190 cm−1, but formation of multilayers at even higher exposure produced only a band at 1095 cm−1, broadly consistent with the frequency of 1060 cm−1 known from solid ammonia. Clearly, the spectral features associated with chemisorbed ammonia may readily be distinguished from those due to purely physical interactions. Broadening of the umbrella mode peak at coverages approaching saturation of the first layer (in both our own experiments and those of Erley and Ibach30,31) likely indicates an increasing role for hydrogen bonding between ammonia molecules or at least an enhanced degree of intermolecular interaction of one sort or another. B. Adsorption of Ammonia on Potassium Precovered Fe{111}. Further experiments were performed to study ammonia adsorption on a potassium precovered surface, prepared as described in the section on experimental methodology above with adatom coverages of 0.1, 0.2, and 0.3 ML. Dosing conditions for ammonia (i.e., temperature and pressure) were the same as for experiments on the clean surface, as were the procedures adopted for spectroscopy. With 0.1 ML potassium precoverage, the RAIR spectra (see Figure 6) for the lowest ammonia exposures show the umbrella mode, δs, at a frequency of 1093 cm−1, which is substantially lower than the frequency of 1140 cm−1 observed on the clean surface. Once again, this band remains sharp and is gradually red-shifted down to around 1077 cm−1 up to 0.01 langmuir nominal exposure, beyond which the frequency remains fairly constant and the band broadens slightly.


mode, δs, at a frequency of just 1083 cm−1, red-shifting still further to around 1059 cm−1 at just over 0.01 langmuir nominal exposure. As before, the band broadens significantly at higher exposures. These changes are readily apparent in plots of the frequency, intensity, and width of the umbrella mode band for 0.0, 0.1, and 0.2 ML precoverages of potassium (see Figure 8). A consistent explanation is that potassium destabilizes ammonia, leading to (i) an umbrella mode frequency much closer to the gas-phase value, (ii) a lowered saturation intensity of the umbrella mode band, and (iii) slightly reduced broadening of that band, which might indicate some impediment to the formation of intermolecular hydrogen bonds. With 0.3 ML potassium precoverage, the RAIR spectra (see Figure 9) are conspicuously devoid of evidence for the ammonia umbrella mode, δs, across the entire range 1000− E



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distinguish with certainty due to spikes in the spectra associated with water in the beam path. Although the portion of the infrared beam path that lies outside the ultrahigh-vacuum chamber is purged with dry nitrogen to avoid absorption by atmospheric species, transfer of the sample to the correct position for spectroscopy cannot be achieved in our apparatus without disturbing the purge, and up to an hour may then be necessary for the purge to stabilize. For clean surface experiments, it is possible to wait long enough to achieve a stable purge and then to flash anneal the sample to 850 K in order to desorb any impurities that may have built up, but with potassium precoverage such a procedure would also desorb a significant fraction of the adsorbed potassium from the surface. Inevitably, therefore, we must be resigned to spurious features in the range 1400−1800 cm−1 that simply reflect a changing baseline during the experiment as the purge gradually improves in quality. To confirm that the feature at 1610 cm−1 really is due to a surface phenomenon, we performed an additional experiment, with 0.25 ML precoverage of potassium, in which we measured RAIR spectra in an ambient ammonia pressure of 1 × 10−8 mbar. This higher pressure implies a rapid buildup of surface species, so that we could meaningfully record high-exposure spectra immediately after obtaining our background spectrum, rather than after several minutes. In these results, where spikes due to water in the purge are minimized, the band at 1610 cm−1 is very clearly recognizable as a genuine surface feature (see Figure 10). Since this feature is equally clearly not present in the Figure 8. Summary of (a) peak frequency, (b) peak intensity, and (c) peak width, for the umbrella mode band of ammonia on the clean Fe{111} surface. The intensity is taken as the integrated peak area, while the width is the full width at half-maximum.

Figure 10. Two spectra obtained in an ambient pressure (1 × 10−8 mbar) of ammonia on the Fe{111} surface precovered with 0.25 ML of potassium immediately after taking a background spectrum.

spectra obtained from ammonia adsorption on the clean Fe{111} surface, it is evidently not simply a consequence of high surface coverage. More likely, it represents some lowering of the symmetry for ammonia adsorption (rendering this mode compatible with the surface infrared selection rules) that is associated specifically with the interaction between ammonia and potassium adatoms. C. Thermal Stability of Ammonia on Clean and Potassium Precovered Fe{111}. To probe the stability of ammonia on the Fe{111} surface, we carried out temperatureprogrammed experiments with 0.0 and 0.1 ML potassium precovered surfaces, starting from a 2 langmuir (i.e., high nominal exposure) dose of ammonia in every case. Unlike the adsorption experiments described above, each spectrum was recorded as an average over just eight scans, in order to allow sufficient temperature resolution (each scan taking 1 s to


1200 cm−1, consistent with the apparent destabilizing effect of potassium. Interestingly, however, there is some indication of a weak absorption feature at around 1610 cm−1 for the highest exposures, which could be attributable to the antisymmetric deformation mode of ammonia, labeled δa and found in the gas phase at a frequency of 1628 cm−1. Indeed, this feature is arguably present also in the spectra obtained for 0.1 and 0.2 ML potassium precoverages, although in all cases it is difficult to F



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acquire, with 4 s elapsing between the end of each octet and the beginning of the next). In consequence, the spectra are relatively noisy and harder to interpret, and comparison with the statically obtained spectra described above is invaluable. The spectra obtained from potassium-free surfaces upon warming from 150 K at a rate of 1 K s−1 are shown in Figure 11,

Figure 12. Temperature-programmed RAIR spectra of ammonia umbrella mode after dosing 2 langmuirs (nominal) of ammonia at 150 K on the Fe{111} surface precovered with 0.1 ML of potassium.

Figure 11. Temperature-programmed RAIR spectra showing the ammonia umbrella mode after dosing 2 langmuir (nominal) of ammonia at 150 K on the clean Fe{111} surface.

where it can be seen that an initially broad umbrella mode feature at 1090 cm−1 narrows as the temperature rises to around 200 K and then blue-shifts up to around 1140 cm−1 before disappearing at approximately 300 K. With 0.1 ML precoverage of potassium (see Figure 12) the umbrella mode is initially found at about 1087 cm−1, narrows slightly up to around 180 K, and then blue-shifts up to about 1106 cm−1 before disappearing at around 240 K. These results are summarized in Figure 13. Note that disappearence of the ammonia umbrella mode in these experiments may simply correspond to desorption of ammonia, but that it is also possible for ammonia decomposition to play a role. In neither case, however, do we observe any features that might plausibly correspond to the NH2 scissor mode (in the range 1510−1525 cm−1) although spikes due to water in the purge, together with stochastic noise, may obscure sufficiently small features. On the other hand, a conventional temperature-programmed desorption (TPD) experiment after dosing 0.5 langmuir (nominal) of ammonia on the clean surface reveals significant desorption of molecular hydrogen at 345 K (see Figure 14) indicative of substantial dissociation having occurred prior to ammonia desorption (qualitatively similar to early work by Ertl’s group). A small peak corresponding to recombinative N2 desorption appears also to be evident above 800 K, although this is neither clearly defined relative to the noise level nor complete by the highest temperature reached in the experiment; higher temperatures were not probed to avoid risk of passing through the phase transition of iron. It may simply be that surface decomposition

Figure 13. Summary of (a) peak frequency and (b) peak intensity for the umbrella mode band of ammonia on clean and 0.1 ML potassium predosed Fe{111} surfaces.

of ammonia produces mainly N and H adatoms, neither of which would be visible in our RAIR spectra, having no internal vibrational modes. Similarly, the presence of NH would be difficult to confirm, since its stretch mode falls within the frequency range where our spectra are unreliable, and any bend mode would likely have only a small dynamic dipole component perpendicular to the surface. G



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The intensity of our δ(NH2) feature then drops over the next 4 or 5 min, until it is barely perceptible, while that of the NH3 umbrella mode remains more or less constant. The most consistent interpretation of this behavior would appear to be that NH2 is an intermediate in a relatively slow process of dissociation that begins with NH3 and ends with NH, N, or some mixture of the two (both effectively invisible to RAIRS). The process presumably stops either due to blocking of sites for further dissociation by hydrogen adatoms or due to a change in the relative energies of different NHx species as their relative coverages vary. In the presence of 0.1 ML potassium, we worked at a fixed temperature of 240 K, corresponding to the temperature at which the NH3 umbrella mode vanished from the spectra in temperature-programmed RAIRS. Indeed, we see no clear evidence of such a peak in the time-dependent data (see Figure 16) although a weak NH2 scissor mode in the range 1510−

Figure 14. TPD spectra recorded at 4 K s−1 after dosing 0.5 langmuir (nominal exposure) of ammonia on the clean Fe{111} surface.

In contrast to these temperature-programmed experiments, a series of time-dependent results were obtained with the surface held at a fixed temperature. Spectra were, once again, averaged over 100 scans, implying a time resolution of around 1 min (each spectrum was gathered over 45 s, and a new measurement began every 60 s). For the potassium-free surface, one experimental run (see Figure 15) was carried out

Figure 16. Time-dependent RAIR spectra recorded each minute after dosing 2 langmuirs (nominal) of ammonia on the Fe{111} surface precovered with 0.1 ML of potassium at 240 K. Figure 15. Time-dependent RAIR spectra recorded each minute after dosing 2 langmuirs (nominal) of ammonia on the clean Fe{111} surface at 270 K.

1525 cm−1 indicates that some dissociative adsorption must take place. Once again, that feature grows and then diminishes over time (appearing and vanishing within the space of 8 min), suggesting that it is an intermediate on a pathway leading to NH, N, or both. In the absence of potassium, time-dependent RAIRS data obtained at 240 K show a somewhat slower growth and decline of the NH2 scissor mode, indicating that potassium acts to promote ammonia dissociation as well as ammonia desorption. Note, however, that the frequency of the supposed NH2 scissor mode is not altered by the presence of potassium, suggesting that this species does not itself interact very strongly with the promoter, unlike NH3. D. Calculations for Ammonia on Clean and Potassium Precovered Fe{111}. In our calculations for 0.25 ML ammonia adsorption on the clean Fe{111} surface, we assumed an atop adsorption site but otherwise allowed the geometry to relax freely. After optimization, the Fe−N bond converged to a

at a surface temperature of 270 K, just a little below the temperature at which the umbrella mode disappears from the spectra. In this case, an initially quite intense and broad NH3 umbrella mode peak at 1125 cm−1 narrows and blue-shifts up to 1146 cm−1 over the course of 4 or 5 min, while simultaneously losing intensity, consistent with loss of NH3 coverage. At the same time, however, a clear feature emerges in the range 1510− 1525 cm−1, consistent with accumulation of NH2 on the surface. A band attributed to the NH2 scissor mode, δ(NH2), has been reported in HREELS experiments on Ni{110}, Ru{112̅1}, and Ru{112̅0} surfaces,26−28 in the range 1520− 1530 cm−1; the accompanying NH2 stretch mode in the range 3240−3307 cm−1 is not, however, seen in the present work. H



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to be strongly visible in RAIRS; the umbrella mode, now at 1130 cm−1, would correspondingly be substantially reduced in intensity relative to its strength in the upright geometry. We suggest that at high potassium coverages a greater proportion of adsorbed ammonia is forced to occupy such tilted sites, leading to the observed changes in RAIR spectra.

length of 2.04 Å, retaining a very nearly vertical orientation, while the N−H bonds were all 1.02 Å in length and made angles of 70 ± 1° to the surface normal. These results are very much in line with similar calculations performed for ammonia adsorption on the Fe{211} surface,42 albeit the Fe−N and N− H bonds are respectively about 2% and 1% shorter here; this is not surprising, however, since the outermost atoms in the Fe{211} surface are step atoms, with coordination number five, while those in the Fe{111} surface are kink atoms, with coordination number just four. The calculated phonon spectrum features symmetric and antisymmetric N−H stretch modes at 3501 and 3636/3649 cm−1, respectively, with the umbrella mode at 1111 cm−1 and antisymmetric deformation modes at 1582/1587 cm−1. Agreement with the experimental frequency range for the umbrella mode of adsorbed NH3 on the clean surface (i.e., 1090−1140 cm−1, varying toward the lower end with increasing coverage) is clearly excellent. In the presence of 0.25 ML K in a (2 × 2) phase, three out of every four top-layer Fe atoms are directly adjacent to an alkali metal adatom; a quarter, however, are a little more distant, and anticipating a repulsive adsorbate−adsorbate interaction, we chose initially to place NH3 in an atop position on these. The local bonding geometry of ammonia changed qualitatively very little from that found on the clean surface, but notably the N− Fe bond length was extended to 2.06 Å, indicating a weakening of the interaction. Indeed, the calculated adsorption energy dropped from 1.04 eV on the clean surface to 0.77 eV on the potassium precovered surface, confirming the picture we had deduced based on our experimental results. In addition, the N− H bonds were found to have stretched by 1% from the clean surface geometry, but their angle to the surface normal remained more or less unchanged at 70 ± 1°. Mulliken analysis indicated that the net negative charge on the NH3 molecule increased in magnitude from −0.04e on the clean surface to −0.07e on the potassium precovered surface, consistent with the expected influence of a coadsorbed electron donor. The potassium adatom was found to possess a positive charge of 0.80e in the absence of ammonia, falling to 0.67e when the molecule was coadsorbed. Turning to the vibrational spectra, the coadsorption of potassium leads to a significant red-shift of 63 cm−1 in the calculated umbrella mode, to a frequency of 1048 cm−1. The symmetric and antisymmetric stretch modes are also redshifted, ending up at frequencies of 3486 and 3612/3620 cm−1. Antisymmetric deformation modes at 1578/1582 cm−1 are barely red-shifted and remain consistent with the frequency of 1610 cm−1 attributed in our experiments to a minority species, albeit the vertical orientation of the adsorbed molecule argues against its having a strong intensity in RAIRS. Since no other modes of comparable frequency are found in our calculations, it is clear that observation of these modes in experimental spectra must be associated with a tilted geometry induced by close proximity with potassium adatoms at higher alkali metal coverages. Interestingly, by placing the NH3 molecules in our calculations onto the atop sites directly adjacent to the potassium adatom, we find a metastable geometry (only 0.09 eV less stable than the one discussed above and with a N−Fe bond length of 2.14 Å) in which each molecule is tilted away from the alkali metal at an angle of approximately 45° to the surface normal. The calculated antisymmetric deformation modes for this geometry are found at 1529/1587 cm−1still tolerably close to 1610 cm−1 albeit examination of displacement patterns indicates that only the lower frequency mode is likely

IV. DISCUSSION The results presented above are consistent with the interpretation that ammonia adsorbs nondissociatively on clean Fe{111} at temperatures substantially below room temperature, in a phase characterized by a sharp umbrella mode peak that is strongly blue-shifted relative to its gas-phase frequency. As the coverage increases, the umbrella mode peak first shifts back toward the gas-phase value, indicating weakening of the ammonia−iron bond, and then broadens substantially at a nearly constant frequency, indicating increasing intermolecular interactions (possibly of a hydrogen-bonded nature). The former effect is entirely consistent with expectations based upon simple electrostatics, as the proximity of neighboring molecules induces mutual depolarisation and thus weakens their dative bonding to the surface. Heating this ammonia-covered surface leads to a gradual narrowing of the umbrella mode peak and a blue-shift in frequency as desorption lowers the coverage. There is marginal evidence for some degree of ammonia decomposition if the temperature is raised relatively quickly (on a time scale of seconds), and it seems likely that the majority of adsorbed molecules desorb intact at around 300 K. Holding the surface temperature at 270 K for a period of minutes, however, does lead to measurable production of transient NH2 (probably en route to more fully dehydrogenated species) at the expense of NH3, although conversion is far from complete. The presence of preadsorbed potassium generally weakens the bond between ammonia and the surface, as indicated by a lessening of the blue-shift observed in the umbrella mode frequency upon adsorption. Once again, simple electrostatic arguments provide an explanation, as the donation of electrons from potassium to iron reduces the surface dipole and impedes the formation of a dative bond by ammonia. This effect increases with increased surface potassium concentration, and indeed the surface appears to be incapable of adsorbing ammonia at potassium coverages above around 0.3 ML. Observation of a spectroscopic feature consistent with ammonia’s antisymmetric deformation frequency is strongly suggestive of a direct interaction between at least some of the molecules and the adatoms, leading to a reorientation of the former relative to the surface normal. Weakening of the bond between ammonia and the surface is further demonstrated by a lowering of the ammonia desorption temperature in the presence of preadsorbed potassium (to 240 K, in the case of 0.1 ML), and once again there is evidence allowing us to infer competition from NH3 dissociation into NH2, NH, and N (even at a temperature where dissociation would be slow in the absence of potassium). It is clear from the above discussion that although ammonia decomposition is a minority process under the conditions probed, it is nevertheless competitive, in terms of rate, with ammonia desorption. That is, only a limited proportion of adsorbed ammonia molecules dissociate, but those that do are able to do so on a time scale that is quite short relative to the surface lifetime of the adsorbate. That potassium appears to facilitate ammonia decomposition to NH2, NH, and possibly N, I



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(8) Bozso, F.; Ertl, G.; Weiss, M. Interaction of Nitrogen with Iron Surfaces: II. Fe(110). J. Catal. 1977, 50, 519−529. (9) Ertl, G.; Weiss, M.; Lee, S. B. The Role of Potassium in the Catalytic Synthesis of Ammonia. Chem. Phys. Lett. 1979, 60, 391−394. (10) Lee, S. B.; Weiss, M.; Ertl, G. Adsorption of Potassium on Iron. Surf. Sci. 1981, 108, 357−367. (11) Ertl, G.; Lee, S. B.; Weiss, M. Adsorption of Nitrogen on Potassium Promoted Fe(111) and (100) Surfaces. Surf. Sci. 1982, 114, 527−545. (12) Strongin, D. R.; Somorjai, G. A. The Effects of Potassium on Ammonia Synthesis over Iron Single-Crystal Surfaces. J. Catal. 1988, 109, 51−60. (13) Dahl, S.; Logadottir, A.; Jacobsen, C. J. H.; Norskov, J. K. Electronic Factors in Catalysis: the Volcano Curve and the Effect of Promotion in Catalytic Ammonia Synthesis. Appl. Catal., A 2001, 222, 19−29. (14) Panczyk, T. Comparative Analysis of Nitrogen Adsorption Kinetics on Fe(100) and Fe(111) Based on Applying the Statistical Rate Theory. J. Phys. Chem. C 2007, 111, 3175−3184. (15) Kozuch, S.; Shaik, S. Kinetic-Quantum Chemical Model for Catalytic Cycles: The Haber-Bosch Process and the Effect of Reagent Concentration. J. Phys. Chem. A 2008, 112, 6032−6041. (16) Kuchaev, V. L.; Shapatina, E. N.; Avetisov, A. K. Mechanism of Oxygen Poisoning of Ammonia Synthesis Catalyst. Russ. J. Electrochem. 2009, 45, 983−995. (17) Arabczyk, W.; Jasinska, I.; Jedrzejewski, R. Iron Catalyst for Ammonia Synthesis Doped with Lithium Oxide. Catal. Commun. 2009, 10, 1821−1823. (18) Kotarba, A.; Holmlid, L. Energy-Pooling Transitions to Doubly Excited K Atoms at a Promoted Iron-Oxide Catalyst Surface: More than 30 eV Available for Reaction. Phys. Chem. Chem. Phys. 2009, 11, 4351−4359. (19) Yeo, S. C.; Han, S. S.; Lee, H. M. Adsorption, Dissociation, Penetration, and Diffusion of N2 on and in bcc Fe: First-Principles Calculations. Phys. Chem. Chem. Phys. 2013, 15, 5186−5192. (20) Kandemir, T.; Schuster, M. E.; Senyshyn, A.; Behrens, M.; Schlögl, R. The Haber-Bosch Process Revisited: On the Real Structure and Stability of “Ammonia Iron” under Working Conditions. Angew. Chem., Int. Ed. 2013, 52, 12723−12726. (21) Iyngaran, P.; Madden, D. C.; Jenkins, S. J.; King, D. A. Hydrogenation of N over Fe111. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 925−930. (22) Gohndrone, J. M.; Olsen, C. W.; Backman, A. L.; Gow, T. R.; Yagasaki, E. Ammonia Adsorption and Decomposition on Several Faces of Platinum. J. Vac. Sci. Technol., A 1989, 7, 1986−1990. (23) Sun, Y.-M.; Sloan, D.; Ihm, H.; White, J. M. Electron-Induced Surface Chemistry: Production and Characterization of NH2 and NH Species on Pt(111). J. Vac. Sci. Technol., A 1996, 14, 1516−1521. (24) Eldad, H.; Jones, J.; Mudiyanselage, K.; Trenary, M. Formation and Hydrogenation of p(2 × 2)-N on Pt(111). Surf. Sci. 2006, 600, 4563−4571. (25) Mudiyanselage, K.; Trenary, M.; Meyer, R. J. Kinetics of NH Formation and Dissociation on Pt(111). J. Phys. Chem. C 2007, 111, 7127−7136. (26) Bassignana, I. C.; Wagemann, K.; Küppers, J.; Ertl, G. Adsorption and Thermal Decomposition of Ammonia on a Ni(110) Surface: Isolation and Identification of Adsorbed NH2 and NH. Surf. Sci. 1986, 175, 22−44. (27) Jacobi, K.; Wang, Y.; Fan, C. Y.; Dietrich, H. Adsorption and Thermal Dehydrogenation of Ammonia on Ru(112̅0). J. Chem. Phys. 2001, 115, 4306−4313. (28) Wang, Y.; Lafosse, A.; Jacobi, K. Stepwise Dehydrogenation of NH3 at the Ru(112̅0) Surface. Surf. Sci. 2002, 507−510, 773−777. (29) Staufor, M.; Neyman, K. M.; Jakob, P.; Nasluzov, V. A.; Menzel, D.; Rösch, N. Density Functional and Infrared Spectroscopy Studies of Bonding and Vibrations of NH Species Adsorbed on the Ru(001) Surface: a Reassignment of the Bending Mode Band. Surf. Sci. 1996, 369, 300−312.

indicating that it must lower the relevant activation barriers to dehydrogenation. Viewed from the opposite perspective, however, the same transition states that are traversed in ammonia dehydrogenation are also those that control hydrogenation in the ammonia synthesis reaction. We thus now have time-resolved infrared spectroscopic evidence for promotion of the surface hydrogenation steps in ammonia synthesis (albeit in time reversal) to augment our previously published timeresolved Auger results.

V. CONCLUSIONS We have conducted extensive infrared experiments, supported by first-principles density functional theory, aimed at elucidating the surface chemistry of ammonia on iron and in particular determining the role of preadsorbed potassium. Importantly, the experimental work has been carried out on truly clean iron samples (rather than ones deliberately presaturated with nitrogen) which we believe more properly represent the likely state of catalyst surfaces under typical Haber−Bosch reaction conditions. These studies reveal a substantial weakening of the ammonia−iron bond in the presence of the alkali metal, concomitant with a reorientation in adsorption geometry and leading to a marked reduction in desorption temperature. Indeed, at sufficiently high coverage, preadsorbed potassium entirely passivates the surface toward ammonia adsorption. Our results additionally demonstrate, however, that decomposition of ammonia on the iron surface is competitive with desorption and also promoted by potassium. In the context of ammonia synthesis, this further supports the notion, proposed in our earlier work,21 that potassium lowers activation barriers for hydrogenation/dehydrogenation of nitrogen-containing surface species and that its relevance to the overall reaction mechanism is not limited to promoting the dissociative adsorption of nitrogen and/or the molecular desorption of ammonia.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Tel +44 1223 336502 (S.J.J.). Notes

The authors declare no competing financial interest.

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REFERENCES

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Infrared Spectroscopy of Ammonia on Iron: Thermal Stability and the

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