Ammonia Adsorption and Decomposition on Co(0001) in Relation to

Feb 16, 2016 - In order to fundamentally understand cobalt catalyst deactivation in Fischer–Tropsch synthesis (FTS) due to parts per million levels ...
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Ammonia Adsorption and Decomposition on Co(0001) in Relation to Fischer−Tropsch Synthesis A. C. (Ali Can) Kizilkaya,† J. W. (Hans) Niemantsverdriet,† and C. J. (Kees-Jan) Weststrate*,‡ †

Laboratory for Physical Chemistry of Surfaces, Eindhoven University of Technology, 5600MB Eindhoven, The Netherlands Sasol Technology Netherlands BV, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands

‡

ABSTRACT: In order to fundamentally understand cobalt catalyst deactivation in Fischer−Tropsch synthesis (FTS) due to parts per million levels of NH3 in the synthesis gas, the adsorption and decomposition of NH3 on Co(0001) are investigated experimentally under ultrahigh vacuum (UHV) conditions and theoretically using density functional theory (DFT) calculations. NH3 desorbs intact from the surface, between 100 and 270 K. In agreement with this, DFT calculations show that the activation barrier for NH3 decomposition, 105 kJ/mol, is higher than the adsorption energy of NH3, 59 kJ/mol. Neither COad nor Had block the adsorption of NH3. Instead, CO and NH3 form a stable coadsorbed layer. Preadsorbed ammonia negatively affects dissociative H2 adsorption. Electron-induced dissociation produces NHx species on the surface at low temperature. The order of stability is NH(+2 Had) > N(+3 Had) > NH2(+ Had) > NH3. N and NH lower the quantity of CO that can be accommodated on the surface but do not affect the adsorption energy significantly. For FTS, we conclude that (i) NH3 adsorption on cobalt is not inhibited by the other FTS reactants and thus parts per million levels of NH3 can already be detrimental, (ii) due to their high stability, NHx species are most likely responsible for catalyst deactivation.

1. INTRODUCTION

Ammonia does not dissociate on close-packed surfaces of transition metals such as Rh(111),15 Ni(111),30 and several others (see Thornburg et al.31 and references therein) under ultrahigh vacuum-type pressure conditions. More open surfaces such as Ir(110)7 or defect-rich Rh surfaces such as Rh wires or foils,15 on the other hand, catalyze ammonia decomposition. Although ultrahigh vacuum (UHV) type studies indicate that close-packed surfaces are rather inactive for NH3 decomposition, the study of NHx species on the close-packed surface is relevant for applied catalysis, as the higher pressures used in this case will produce a high enough equilibrium coverage of NH3 at the temperatures where the decomposition barrier can be overcome, thus resulting in the production of NHx fragments due to NH3 decomposition on the close-packed surface as well. In the context of Fischer−Tropsch synthesis, where the reactants CO and H2 are present in high concentrations, we aim to address two fundamental questions: (i) how can NH3, being present in a ∼105 times smaller concentration than the reactants, have a significant impact on catalysis, and (ii) what is the mechanism of poisoning by NH3; e.g., is it due to intact ammonia or instead due to NHx species? The adsorption and decomposition of NH3 on Co(0001) is investigated experimentally under well-controlled conditions, i.e., in an ultrahigh vacuum environment. A combination of

The interaction of NH3 with catalyst surfaces is of importance for a number of industrial processes. Ammonia is produced from N2 and H2 in the Haber−Bosch process using Fe- or Rubased catalysts. It is a reactant in the Pt-catalyzed reaction of ammonia with methane in HCN synthesis1 and in the oxidation of NH3 during nitric acid production (Ostwald process)2 where a Pt−Rh gauze is used as the catalyst. The interaction of NH3 with various metal surfaces such as Fe,3 Pt,4−6 Ir,7 Ru,8−10 Ag,11 Cu,12 Ni,13,14 and Rh15,16 has been reported in the literature. Some other fundamental studies have focused on ammonia oxidation, where ammonia takes part as the reactant.17−21 For low-temperature Fischer−Tropsch synthesis (FTS) using cobalt catalysts, ammonia is known as a poison.22 FTS forms the central part of gas-to-liquid (GTL) technology, a sequence of catalytic processes used to produce synthetic fuels from natural gas via the intermediate production of synthesis gas, a mixture of CO and H2. Cobalt-based catalysts are typically used for low-temperature FTS, where the aim is to produce long hydrocarbon chains. It has been reported that parts per million quantities of NH3, typically present in commercially produced synthesis gas,23 act as a poison for the catalyst.24,25 Up to now there have been neither fundamental nor applied studies in the scientific literature about the fundamental reasons behind NH3 poisoning. In the patent literature related to FTS, improved techniques for NH3 separation from the inlet synthesis gas stream are discussed, indicating its importance for applied FTS.26−29 © 2016 American Chemical Society

Received: November 30, 2015 Revised: February 5, 2016 Published: February 16, 2016 3834

DOI: 10.1021/acs.jpcc.5b11609 J. Phys. Chem. C 2016, 120, 3834−3845

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previously.34 The top chamber (∼2 × 10−10 mbar) is equipped with a mass spectrometer (Balzers Prisma QMA200), LEED optics (OCI vacuum engineering), an X-ray source and electron energy analyzer (Vacuum Generators), a sputter gun for sample cleaning (OCI vacuum engineering), and a Kelvin probe (KP Technology Ltd.). The lower chamber ( 0.12 ML a second desorption state is observed in the TPD data [Figure 1(a)]. The work function data shown in Figure 1(b) match the desorption spectra, indicating a linear correlation between the relative work function and θNH3. This correlation can then be used to quantitatively study the increase of θNH3 as a function of NH3 exposure using work function

measurements, shown in Figure 1(c). The sticking coefficient (slope of the curve) for 0 ML < θNH3 < 0.12 ML remains constant, while it levels off for θNH3 > 0.12 ML. This corresponds with the two desorption states (θNH3 < 0.12 ML and θNH3 > 0.12 ML) found with TPD and TP-WF. The sticking coefficient of ammonia appears to be rather high, comparable to that of CO (∼0.8).33 Our data do not allow a precise determination of the sticking coefficient. However, on Pt(100) a value of 0.95 was reported using a more accurate experimental technique,42 in line with our findings. The IR absorption spectrum [Figure 1(d)] at 90 K shows only a single absorption band, shifting from 1160 to 1120 cm−1 with increasing ammonia coverage. This band corresponds to the symmetric NH3 bending mode of on-top adsorbed NH3.30 The NH stretching bands were not resolved in the experiment, possibly due to the high noise level in the spectral region where these peaks are expected (3000−3500 cm−1). On Ni(111) Xu et al.43 also report that the NH stretching band of chemisorbed NH3 has a much smaller intensity than the NH3 deformation mode. 4.2. TPD Analysis. The adsorption energy of ammonia was obtained from the experimental data by using the desorption/ work function measurement for a low coverage of ammonia (0.062 ML), as this value would be most representative of the adsorption energy of an ammonia molecule in isolation, i.e., at the low coverage limit. Different methods are available to obtain the adsorption energy from experimental data.44 We have used Redhead45 and Chan−Aris−Weinberg (CAW).46 These empirical methods, Redhead and CAW, yielded the adsorption energy of ammonia for the low coverage limit as 67 and 65 kJ mol−1, respectively. The CAW method also yielded a prefactor of 1.2 × 1013 s−1. For the Redhead analysis, a heating rate of 1 K/s and a prefactor (k0) of 1 × 1013 s−1 were used in the calculations. For the CAW analysis, the peak width at 3/4 of the maximum intensity, 20 K, was taken. 3836

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Figure 2. TPD and TP-WF spectra (a,b) after adsorption of NH3 on 0.5 ML Had and (c,d) adsorption of H2 (100 L, 90 K) on an NH3-precovered surface, respectively. The inset in (b) shows θNH3 as a function of NH3 dose, comparing the H-saturated surface with the clean surface. Inset in (d): θH vs θNH3 (at 90 K) for the two coadsorption experiments (Had first vs NH3 first).

4.3. Coadsorption of NH3 with H2. In the context of catalyst poisoning, the influence of NH3ad on the dissociative adsorption of H2 and on the stability of Had as well as the effect of Had on the adsorption and stability of ammonia are of interest. The coadsorption of ammonia and hydrogen (Figure 2) was explored in two different ways: (i) In the first set of experiments the surface was presaturated with Had at low temperature (100 L H2 at T < 150 K), which produces a coverage of 0.5 ML Had.32,47 NH3 was dosed afterward onto the hydrogen-precovered surace. (ii) In second set of experiments, the surface was first exposed to varying doses of NH3. Afterward, the NH3-covered surface was exposed to a hydrogen dose of 100 L. For both experiments the desorption of H2 was probed with the mass spectrometer [Figure 2(a,c)], while θNH3 was derived from the TP-WF measurements [Figure 2(b,d)]. The hydrogen desorption spectrum after dosing NH3 onto 0.5 ML Had is identical to the hydrogen desorption from the clean surface [Figure 2(a)], and we find no indications that Had is strongly influenced by NH3ad. The work function data [Figure 2(b)] show that 0.25 ML of NH3 (the saturation coverage on the clean surface) can adsorb on a hydrogencovered surface. In addition, adsorption of NH3, e.g., the NH3 sticking coefficient, is not affected by the presence of surface hydrogen, as shown in the inset of Figure 2(b). The only noticeable difference induced by Had is a ∼30 K downward shift of the NH3 desorption temperature. For the case of 0.06 ML NH3 on 0.5 ML Had, a downward shift of ∼30 K corresponds to a decrease in the adsorption energy of ammonia from ∼67 to ∼58 kJ mol−1 (Redhead equation). The situation is different when ammonia is dosed first. Figure 2(c) shows that the quantity of Had after exposure to 100 L H2 strongly decreases as the initial NH3 coverage increases. The inset in Figure 2(d) compares the quantity of adsorbed hydrogen after dosing 100 L H2 as a function of NH3 coverage for the two cases: hydrogen dosed first or ammonia dosed first.

Similar to the previous experiment, the NH3 desorption temperature is somewhat shifted to lower temperature but only for those cases where significant quantities of Had were coadsorbed with NH3. 4.4. Coadsorption of NH3 with CO. The influence of NH3 on the adsorption of CO and vice versa is also of interest in the context of cobalt catalyst poisoning. To better understand why parts per million concentrations of NH3 can have a significant impact on the catalyst we explore both how NH3ad affects CO adsorption and how the adsorption of NH3 is influenced by COad. 4.4.1. CO Adsorption on an NH3-Covered Surface. Figure 3 shows the results that were obtained after dosing 10 L CO at 90 K to a Co(0001) surface that was precovered with different quantities of NH3. The CO desorption spectra show that adsorption of CO is only mildly affected by the presence of preadsorbed NH3. The main effect appears in the lowtemperature desorption feature (around 240 K), which decreases in size and shifts to lower temperature with increasing NH3 coverage. For the highest ammonia coverage used in this study, 0.2 ML, around 0.5 ML CO could be coadsorbed, compared to 0.67 ML CO in the absence of NH3.33 The inset in Figure 3(c) shows the correlation between NH3 precoverage and the quantity of CO that was adsorbed after dosing 10 L. The coadsorption of CO has a strong effect on the ammonia desorption spectrum, as shown in Figure 3(b), where the ammonia desorption spectra of 0.2 ML NH3 both in the absence (blue) and in the presence of coadsorbed CO (red) are compared. Although the quantity of NH3 that desorbs is the same in both cases, the spectral shape is significantly affected by the presence of COad. In the presence of (∼0.5 ML) COad, only 0.04 ML NH3 desorbs around 183 K, whereas the majority (0.16 ML) desorbs around 260 K. A further indication of a special interaction between coadsorbed NH3 and CO is apparent from the work function 3837

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Figure 3. (a) CO desorption after dosing CO (10 L, 90 K) onto NH3ad with different coverages. (b) NH3 desorption of 0.2 ML NH3 without and with postdosing CO to saturation (140 K), respectively. (c) Work function measurements corresponding to the desorption data shown in (a). The inset shows the CO coverage as a function of the NH3 precoverage. A heating rate of 1 K s−1 was used in all cases.

Figure 4. (a) CO TPD data after postdosing NH3 on a CO-saturated (0.67 ML) surface at 90 K. (b) NH3 TPD after dosing NH3 to saturation on a 0.67 ML CO-precovered surface at 140 K. (c) Work function measurements corresponding to the TPD data shown in (a). A heating rate of 1 K s−1 was used in all experiments.

data. Both CO and NH3 contribute to the work function. For CO the ΔΦ = 1961 meV ML−1, whereas for NH3 it is −8940 meV ML−1. However, for the coadsorbed system the work function is not simply the sum of the individual contributions of the two adsorbates. For example, 0.2 ML NH3 alone produces a ΔΦ of −1870 meV, but the coadsorption with 0.5 ML CO (which on its own causes a work function shift of +980 meV) produces a work function of +202 meV, instead of −890 meV, which would be expected in case the contributions of the adsorbates could simply be added. Thus, in addition to the significant effect that CO has on molecular NH3 desorption, the work function data indicate that NH3 desorption is strongly affected by the presence of coadsorbed CO. This complexity makes a detailed interpretation of the work function data for the different combinations of θCO and θNH3 difficult. 4.4.2. NH3 Adsorption on a CO-Covered Surface. Ncontaining poisons such as ammonia are typically present at parts per million levels in the reaction gas during FTS under industrial conditions, which contains CO (and H2) at much higher partial pressures. As NH3 interacts much weaker with the Co(0001) surface than CO (67 vs 90−120 kJ mol−1) it is surprising that parts per million levels of ammonia affect the catalyst so much, as one would think that NH3 loses in the competition with the (i) much more abundant (105−106 times more) and (ii) more strongly adsorbing CO. We therefore studied the adsorption of NH3 on a surface that is covered by 0.67 ML CO, the CO saturation coverage. Figure 4 shows the result of a series of experiments in which the surface was precovered with 0.67 ML COad and subsequently exposed to different quantities of NH3. Quantitive

evaluation of the CO desorption spectra [Figure 4(a)] shows that the quantity of CO after dosing NH3 at 90 K remains constant at 0.67 ML in all cases. Ammonia can still adsorb despite the high CO precoverage. Like in the previous series where NH3 was dosed first, the low-temperature CO desorption peak is affected most by the presence of coadsorbed NH3, as it shifts to lower temperatures and broadens. The ammonia desorption spectrum [Figure 4(b)] is similarly affected as when NH3 is adsorbed first, with a broad lowtemperature desorption peak and a sharp desorption peak around 260 K. Quantification of the TPD peak areas shows that the saturation coverage of NH3 in the presence of 0.67 ML CO is ∼0.34 ML, adding up to a coverage of ∼1 ML at 90 K. Around half of the NH3 desorbs between 150 and 240 K as well as 0.17 ML CO, leaving ∼0.16 ML NH3 and 0.5 ML CO on the surface at that temperature, which desorbs around 260 K, like in the previous series where NH3 was dosed first. The work function data [Figure 4(c)] are again difficult to interpret due to the nonlinear concentration dependence for the coadsorbed system. However, for the highest NH3 dose (1 L) it can be noted that the WF change shows a good match with the NH3 desorption spectrum [Figure 4(b)], with a slow increase between 150 and 240 K due to the first NH3 desorption peak and a faster increase around 260 K, due to the second NH3 desorption peak. In short, the experimental data show that CO does not inhibit ammonia adsorption at all. Instead, CO enhances the quantity of ammonia that can be adsorbed and appears to relieve the repulsive interactions between the adsorbed NH3 molecules, as discussed later on. 3838

DOI: 10.1021/acs.jpcc.5b11609 J. Phys. Chem. C 2016, 120, 3834−3845

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The Journal of Physical Chemistry C 4.4.3. Infrared Absorption Measurements of NH3 and CO Coadsorption. RAIRS measurements were performed to obtain more information on the coadsorption of NH3 and CO, shown in Figure 5. For both situations, i.e., “NH3 first” or “CO first”,

(shown in the inset of Figure 5) with 0.25 ML NH3 adsorbed on-top and the majority of CO adsorbed in hollow sites. 4.5. NHXad Decomposition. Although ammonia does not decompose on the close-packed surface under UHV conditions it is still of interest to study the chemistry of NHx species on the flat surface, for three reasons: (i) In the presence of NH3(g), θNH3 is the result of the adsorption−desorption equilibrium at a given temperature. As a result, even when the barrier for NHx dissociation is higher than the desorption barrier dissociation can still take place, potentially resulting in the formation of NHx species. (ii) Dissociation can proceed on defect sites which are present in significant quantities on the surface of a catalyst nanocrystallite.49 Diffusion from the defects sites onto the close-packed terrace sites would then produce NHx on Co(0001), the situation explored here. (iii) NHx species can also be derived from HCN present in commercial synthesis gas in small concentrations,23 where dissociation of this molecule would produce Nad, which can further hydrogenate to other NHx. Low-energy electrons (100−300 eV) can induce dissociation of adsorbed ammonia, as has been shown on, for example, different Pt surfaces6,50 and on Ir(111).21 In this way NHx + (3 − x)Had are produced on the surface at low temperature. The thermally induced chemistry of NHx can then be studied by temperature-programmed techniques to provide information about the relative stabilities of the different NHx species as well as to experimentally determine the activation barriers associated with the different dehydrogenation steps. We have used this approach to produce NHxad on a Co(0001) surface at 90 K, by exposing the Co(0001) surface covered by 0.25 ML NH3 to a defocused electron beam (E = 100 eV) for 10 min. The thermal behavior of this system was then followed by TPD and TP-WF measurements to determine the chemical nature and dehydrogenation temperature of the NHx surface species produced by the electron bombardment. Figure 6(a) shows the hydrogen desorption spectrum after irradiation of 0.25 ML NH3ad. A reference spectrum for hydrogen desorption from the clean surface is added, as this is known to be equal to 0.5 ML.32,47 Figure 6(b) shows the corresponding work function data, together with the TP-WF reference for 0.25 ML NH3 without irradiation. The hydrogen desorption spectrum of the irradiated NH3ad layer shows a first desorption peak between 300 and 400 K, equal to 2/3 of the total quantity, and a second peak, 1/3 of the total intensity, centered around 440 K. Quantitative analysis using the 0.5 ML Had reference spectrum shows that the amount of hydrogen that desorbs from the irradiated NH3ad layer equals 0.3 ML H; e.g., irradiation leads to decomposition of 0.1 ML NH3ad. On the basis of the intensity ratio we attribute the low-temperature H2 desorption peak to desorption of Had formed in the first two decomposition steps which occur either during e-bombardment or at T < 350 K during heating. The second, reaction-limited H2 desorption peak around 440 K is then attributed to decomposition of NH to N + Had, followed by immediate desorption of surface hydrogen as H2(g). Electron irradiation causes a decrease of the work function [Figure 6(b)], assigned to (partial) decomposition of NH3 into NH2, NH, or a mixture of both. In the absence of direct spectroscopic identification of the NHx surface species, we instead try to deduce which NHx is most likely produced by the electron bombardment. During heating the first significant work function change is observed between 200 and 300 K, where it overlaps with the WF assigned to molecular NH3

Figure 5. RAIRS measurements for the coadsorbed NH3/CO system on Co(0001). The spectra were recorded using a resolution of 4 cm−1. Adsorption was performed at 140 K, to avoid multilayer adsorption of NH3. The inset shows the proposed coadsorption structure at 140 K.

the spectra were recorded at 140 K to avoid bi/multilayer adsorption of NH3. The spectrum shown in blue represents a pure NH3ad layer, with θNH3 = 0.21, with an adsorption band at 1120 cm−1 (symmetric N−H bending). The spectrum shown in red was taken after dosing CO to saturation afterward. It is clear that NH3ad is strongly affected by COad, as the NH3 symmetric deformation mode shows a large shift, from 1120 to 1290 cm−1, indicating a strong disturbance of the NH3 adsorbate by the presence of CO, potentially forming an adsorbate complex. In addition the absorption intensity of the peak decreases due to the presence of COad. Similar strong shifts of the NH3 deformation mode have been reported for a coadsorption study of NH3 and CO on Ni(111) as well.48 The spectrum shown in green was obtained after dosing NH3 to saturation on a CO-saturated surface. It looks practically identical to the red spectrum, with a slightly higher intensity for the CO and NH3 peaks due to the higher coverages of CO and NH3 in the latter case, as found by quantification of the TPD data [Figure 4(a)]. The vibrational frequency of the C−O stretching band is not much affected by the presence of NH3, but the presence of ammonia strongly affects the site occupancy of CO. Three CO reference spectra on the clean surface are shown, for 0.33 ML (only adsorption on top), 0.50 ML (top and 3-fold adsorption), and 0.67 ML (saturated, top, and bridge adsorption). For the coadsorbed systems the spectral region between 2100 and 1750 cm−1 shows absorption bands corresponding to top (2029 cm−1) and 3-fold hollow adsorption (1832−1790 cm−1). The peak due to adsorption in hollow sites is the most intense component that is seen, in addition to a minor peak at the top position. On the basis of this information we tentatively propose a rather densely packed coadsorption layer at 140 K 3839

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Figure 7. Low-energy electron diffraction image (∼100 eV) of (a) clean Co(0001) and (b) Co(0001) ∼ 0.1 ML N surface.

Figure 8 summarizes the results that were obtained, for 0.10 ML N, 0.04 ML N, and 0.04 ML NH, respectively. The LEED

Figure 6. (a) Desorption of hydrogen after dosing 0.25 ML NH3 at low temperature followed by irradiation with ∼100 eV electrons and (b) corresponding work function measurements (heating rate 1 K s−1). The lower graph (shown in black) shows the derivative of the work function data, to indicate where changes occur on the surface.

desorption observed in the absence of dissociation. In addition, the derivative of the TP-WF data looks very similar to an NH3 desorption spectrum (see Figure 1). We thus assign the increase of the work function between 200 and 300 K to desorption of NH3 rather than to an NHx decomposition step. In this interpretation the ΔΦ observed between 200 and 300 K after e-bombardment corresponds to desorption of 0.13 ML NH3. Combined with the observation from TPD that ebombardment leads to the decomposition of 0.1 ML, the sum of the two pathways, molecular desorption and decomposition, adds up to 0.23 ML, close to the initial θNH3 of 0.25 ML. We further elaborate on this topic in the Discussion section. No N2 desorption was found in the temperature region explored (up to 630 K), indicating that the nitrogen is strongly bound and does not leave the surface easily. The LEED image after heating to 630 K, shown in Figure 7, shows a weak (2 × 2) pattern, which is assigned to (small islands of) N atoms in a p(2 × 2) structure, with a local coverage of 0.25 ML. 4.5.1. Effect of NHxad on CO and H2 Adsorption. As both NH and N can be selectively produced on the surface, we investigated how both species influence adsorption of CO. In addition, the influence of N on the adsorption of hydrogen was investigated, as well as the possible rehydrogenation of Nad. NH was produced by heating the irradiated surface to 400 K, to desorb all free hydrogen produced in the first two decomposition steps and leave only NH on the surface, whereas heating to 630 K produces a partial coverage with Nad atoms.

Figure 8. (a) TPD after adsorbing a saturation dose of CO on different quantities of preadsorbed N(H)ad. The % values indicate the relative quantity of CO present at 320 K during heating. (b) Hydrogen desorption after exposing a 0.1 ML N-covered surface to 100 L H2 at 150 K. A heating rate of 1 K s−1 was used.

images indicate that islands with local coverage of 0.25 ML N(H) are formed. For an overall coverage of 0.1 ML this translates to a 40% coverage of the surface by islands of N, leaving 60% of the surface empty. For a coverage of 0.04 ML N(H) this corresponds to a N-island coverage of 16%, leaving 84% of the surface free of adsorbates. The shape of the CO desorption peaks [Figure 8(a)] obtained for the coadsorbed N(H)/CO system is only slightly different from the reference desorption spectrum in the absence of coadsorbates. In particular the low-temperature CO desorption peak is shifted to slightly lower temperatures due 3840

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imentally observed 0.25 ML saturation coverage of ammonia on Co(0001). In addition, the relative stabilities of NHx species and the activation barriers of the different NHx decomposition steps were calculated. Figure 10 shows that the order of stability for

to Nad/NHad, whereas the shape of the CO desorption spectrum for T > 250 K remains unaffected. However, a quantitative evaluation shows that all desorption peaks are lowered in intensity. The CO coverage at 320 K, where only on-top CO is present, is reduced to 60% (compared to the clean surface value) due to the presence of 0.1 ML N, a number that matches with the estimated area of the clean surface. A similar match is found for N(H) coverages of 0.04 ML. At 90 K, around 78% (0.52 ML) of the clean surface saturation coverage can be accommodated, a value that does not match the island coverage. This point is discussed in more detail later on. Figure 8(b) shows that when 0.1 ML N is present on the surface and H2 is dosed (100 L) at low temperature (150 K) the area of the hydrogen desorption peak is reduced by only 10% compared to the clean surface value, which must mean that the hydrogen also adsorbs between the Nad atoms present as islands. We tentatively attribute the low-temperature shoulder at 280 K to the influence of Nad on the adsorption of hydrogen. All hydrogen desorbs below 400 K, and no indications are found for hydrogenation of atomic nitrogen during the experiment. This would have been evident from the TPD spectrum, as NH decomposition produces a hightemperature H2 formation peak around 440 K which is wellseparated from the desorption peak due to 2 Had → H2 (g). 4.6. DFT Modeling. The origin of the experimentally observed strong coverage dependence of the NH3 desorption temperature was investigated by calculating the NH3 adsorption energy using DFT for four different coverages, 0.5 ML, 0.25 ML, 0.111 (1/9) ML, and 0.0625 (1/16) ML, the results of which are shown in Figure 9.

Figure 10. Potential energy diagram for the NH3ad (0.25 ML) decomposition sequence on Co(0001), where the superscript * denotes adsorbed states. All reported energies correspond to the most stable adsorption site. Minimum energy calculations for NHx and H coadsorbed are performed such that the H atom is far away from NHx and they do not interact. Zero-point energy corrections are included in the data.

NHx is NH + 2 Had > N + 3 Had > NH2 + Had > NH3. The decomposition barriers for NHx vary significantly with the value of x, where NH decomposition has the highest dissociation barrier of all, around 110 kJ mol−1. The decomposition of NH3ad is almost as difficult (105 kJ mol−1), while NH2 decomposition is relatively easy, with a barrier of only 67 kJ mol−1. It can be seen that the transition state for NH3 decomposition is much less stable than NH3(g), even when we use the highest calculated NH3 adsorption energy for the low coverage limit (assuming that the dissociation barrier is not coverage-dependent).

5. DISCUSSION Like other close-packed surfaces such as Ru(0001), Rh(111), and other single-crystal surfaces as stated by van Hardeveld et al. 15 , Thornburg et al., 31 and references therein, NH 3 dissociation is difficult on the Co(0001) surface. UHV experiments show molecular desorption rather than dissociation. DFT modeling confirms this notion, as the calculated dissociation barrier of NH3 is 105 kJ mol−1. In other words, under UHV conditions when the temperature is high enough to overcome the activation barrier, there is no NH3ad left on the surface. Similar to observations on Rh(111)15 and Ru(0001)51 metal surfaces, molecular NH3 desorption occurs in a wide temperature range, indicating a strong coverage dependence of the rate constant of desorption (kdes). The expression for the rate constant of desorption in transition state theory is44

Figure 9. Ammonia adsorption energy (with respect to gas-phase ammonia) as a function of ammonia coverage on Co(0001). Each ammonia coverage was simulated with a different unit cell: (i) 0.0625 ML = 1 NH3 molecule on p(4 × 4), (ii) 0.11 ML = 1 NH3 molecule on p(3 × 3), (iii) 0.25 ML = 1 NH3 molecule on p(2 × 2), and (iv) 0.50 ML = 2 NH3 molecules on p(2 × 2).

Figure 9 shows that the adsorption energy of ammonia is strongly coverage dependent, varying from 60 kJ mol−1 for a low coverage to 40 kJ mol−1 for the experimentally observed saturation coverage of 0.25 ML. The calculations performed for 0.5 ML NH3 converged such that one of the two ammonia molecules desorbed from the surface. In other words, it is not energetically favorable to adsorb 0.5 ML of NH3 on the Co(0001) surface, which is in agreement with our exper-

kdes = 3841

k bT q # Ea / RT e h q DOI: 10.1021/acs.jpcc.5b11609 J. Phys. Chem. C 2016, 120, 3834−3845

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The Journal of Physical Chemistry C where kdes = NH3 desorption rate constant (s−1), kb = Boltzmann constant (J K−1), T = temperature (K), h = Planck’s constant (J s), q# = partition function for the transition state, q = partition function for the ground state, Ea = adsorption energy (J mol−1), and R = gas constant (J mol−1K−1). A coverage-dependent kdes can thus be due to both a coverage dependence of the adsorption energy (e.g., due to lateral interactions) or a coverage dependence of the partition functions. For a low NH3 coverage the partition function is similar for both the ground state and the transition state, and q#/q ≈ 1. This translates to a typical pre-exponential factor of 1 × 1013 s−1. Instead, when θNH3 is high the partition function for the ground state decreases as the molecules lose their translational degree or a part of their rotational degrees of freedom, while the mobility and rotational degrees of freedom can still remain high. As a result, q#/q will be significantly higher than 1 as the coverage increases, which translates into a coverage-dependent desorption rate constant even in the case that the adsorption energy is independent of coverage. This explanation was also used for NH3 desorption from Rh(111), where a similar strong coverage dependence is seen.15 This effect can partially contribute to a coverage dependence of kdes, but our calculations also show that the ammonia adsorption energy is strongly dependent on the ammonia coverage. Thus, we conclude that the main reason for the strong coverage dependence of the ammonia desorption rate is the profound dependence of the ammonia adsorption energy on coverage. The repulsive interactions due to the strong dipole of NH3ad are unlikely to play a significant role in the coveragedependent desorption rate since such repulsive interactions start being effective at high coverages (θ > 0.5 ML).52 A detailed study on the exact contribution of both factors is beyond the scope of the present study. Coadsorption studies of NH3 and hydrogen reveal striking differences between experiments where hydrogen vs the case where ammonia is dosed to the surface first. When Had is adsorbed fist, 0.5 ML Had and 0.25 ML NH3ad can coexist on the surface. On the other hand, the amount of Had for a 100 L H2 dose decreases as a function of ammonia coverage. Combining these two findings, we conclude that, in principle, 0.5 ML Had and 0.25 ML NH3 can coadsorb at low temperature, but when ammonia is dosed first this situation cannot be reached with a 100 L H2 dose. In other words, preadsorbed NH3 inhibits the dissociative adsorption of H2, by blocking the ensembles of surface sites that are necessary for dissociation.53 Preadsorbed NH3 has only a minor effect on the subsequent CO adsorption. In the thermal desorption spectra, CO desorption peak positions stay the same, and CO saturation coverage is only slightly lowered. Desorption of NH3ad, on the other hand, is significantly affected by the presence of coadsorbed CO: for high NH3 coverages desorption starts at a significantly higher temperature compared to the NH3 adsorbed without CO. The effect of preadsorbed CO on NH 3 adsorption shows that CO does not block NH 3 adsorption. In fact, up to ∼0.33 ML NH3 can be adsorbed at low temperature on a surface covered with 0.67 ML CO, as opposed to the saturation coverage of 0.25 ML found for the pure NH3ad layer. Work function measurements as well as infrared absorption studies show that NH3 and CO assert a strong influence on each other. The complexity of this coadsorption system has

been discussed previously in the literature. On Ni(111), a surface where the nearest-neighbor distance is practically identical to that on Co(0001), Erley48 reports that ammonia adsorbs in a second layer, on top of the first, chemisorbed CO layer. This second layer affects the site occupancy of CO. The presence of a second NH3 layer in this case is confirmed by the presence of NH3-related bands at 1644 and 1466 cm−1, assigned to the asymmetric deformation modes of NH3 in an adsorbed second layer on top of the CO layer. Without going into a detailed explanation of the complex interaction between CO and NH3 we find on Co(0001) (i) a significant blue shift of the NH3 symmetric deformation mode, but much smaller than in the case of NH3 on Ni(111). In addition, we find that (ii) the infrared spectrum after dosing CO onto preadsorbed NH3 is similar to that obtained by dosing CO first, which makes assignment to a double layer unlikely. Instead, we interpret our data as a coadsorption of a dense coadsorption layer of CO and NH3 where both adsorbates are interacting directly with the cobalt surface. Independent of the dosing order the NH3 desorption spectrum is affected in a similar way by the presence of COad. Compared to the CO-free NH3 desorption spectrum the high-temperature peak has become sharper, and the onset of desorption is shifted to higher temperature whereas the temperature where all NH3 has desorbed remains the same. This can be interpreted as a lifting of the repulsive interactions between the NH3 adsorbates due to the presence of COad. It should be noted that the dipole moments of COad and NH3ad are in opposite direction, which would lead to attractive interactions between NH3ad and COad species. As mentioned previously, this cannot be the sole origin of the repulsive interactions between adsorbed NH3 molecules in the absence of COad, as the order of magnitude of dipole−dipole interactions is only significant (e.g., a few kJ mol−1) for θNH3 > 0.5 ML, far more than the experimentally observed saturation coverage. The effect of CO is most likely due to a combination of factors, where modification of the electronic structure by COad most likely plays a role as well. Infrared measurements show that CO is also affected by the coadsorbed NH3, primarily by favoring hollow site adsorption over top site adsorption. It was reported in a review by Over5 that when CO is coadsorbed with an alkali metal a significant charge transfer occurs from the alkali metal to the 2π* orbital of CO. The CO molecules optimize their bonding configuration so that their acceptor character is enhanced by changing the adsorption site to the highly coordinated sites. There is possibly a similar kind of interaction between NH3ad and COad: NH3ad also has an electron-donating character as indicated by the downward shift its adsorption causes in the work function, and COad shifts to 3-fold hollow sites when coadsorbed with NH3ad. Irradiation of an adsorbed NH3 layer produces NHx species at a low temperature. In order to extract kinetic data on the dehydrogenation steps information on the exact composition of the adsorbate is required. The barrier for NH decomposition can be easily derived from the TPD data, where quantitative evaluation of the H2 desorption spectrum shows that NH is present upon heating to 400 K. NH then decomposes around 440 K as evidenced by a high-temperature H2 formation peak. The temperature matches nicely with the calculated barrier of 110 kJ mol−1, which translates to a peak temperature of 452 K using the Redhead equation.45 In the absence of direct spectroscopic information determination of the other dehydrogenation barriers is not as 3842

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Although NH is more stable than N + H according to our DFT calculations, we conclude from our thermal desorption experiment of the coadsorbed N and H system that NH does not form, as the corresponding high-temperature H2 formation peak due to NH decomposition is not observed. In an alternative experiment specifically aimed at hydrogenating Nad, the N-covered surface was cooled in the presence of 1 × 10−5 mbar H2 (not shown here). Also in this case no indications were found for the hydrogenation of N to NH. Looking at the DFT results we find that the barrier to hydrogenate N to NH is rather high, around 98 kJ mol−1. This makes the process rather difficult to observe in a UHV environment, as it requires both a high surface temperature (350−380 K) combined with a significant coverage of hydrogen. A proper assessment of the role of NH3 as a poison for FTS also requires information on the role of undercoordinated sites which are present on a cobalt nanocrystallite and which are considered to play an important role in the activation of the CO molecule55 but are also of importance for the dissociative adsorption of H2.32 However, the present study already contains some important pieces of information. One of the most prominent findings is that neither Had nor COad blocks adsorption of NH3. Translated to FTS conditions this means that weakly adsorbing NH3 present in parts per million levels in the reaction gas can still reach the catalytically active surface despite the overwhelming excess of strongly adsorbing CO and H2 and which are 105−106 times more abundant. However, molecular NH3 does not adsorb particularly strongly on the surface of cobalt: the adsorption energy of 67 kJ mol−1 determined in this study translates to a surface residence time of 10−5−10−6 s at 493 K, the temperature used in an applied study on the influence of NH3 cofeeding on the performance of an alumina-supported cobalt catalyst.25 NHx species, on the other hand, are strongly bound to the cobalt surface, which translates to much longer residence times. In our experiments we find that N hydrogenation does not happen using UHV-type conditions, and theory predicts a sizable barrier of 98 kJ mol−1. Hydrogenation of NH, the most stable species, is even more difficult, and the barrier for this endothermic process is around 120 kJ mol−1. In other words, once formed, NHx species are difficult to remove from the surface at typical FTS temperatures, and strongly hydrogenating conditions are required to drive the endothermic hydrogenation reaction. Thus, NHx species rather than intact NH3 are expected to be responsible for poisoning of FTS catalysts. This hypothesis finds support in a study by Pendyala et al.,25 where an alumina-supported cobalt catalyst deactivated by addition of parts per million levels of ammonia did not recover when the NH3 was removed from the feed but a mild hydrogenation treatment at the reaction temperature, 493 K, did reverse the NH3-induced deactivation, e.g., removed the NHx species from the cobalt surface. In view of our findings on the NHx surface chemistry on a flat cobalt surface we assign the poisonous effect to NHx species. FTS conditions are not hydrogenating enough to remove them from the surface, and only when pure hydrogen is used can those species be removed.

easy. A basic question here is whether e-induced dissociation of NH3 at 100 K produces NH2 or NH directly. Of the available experimental data only the work function measurements provide information on the low-temperature surface processes. Earlier we have derived the relationship between θNH3 and ΔΦ. We can do the same for NHad and Nad. After heating to T > 450 K the work function value is identical to the clean surface value, and since the surface at this point is covered with 0.1 ML nitrogen atoms this must mean that Nad does not cause a significant work function shift. The ΔΦ of 257 meV observed between 400 and 450 K in the TP-WF experiment is attributed solely to the disappearance of 0.1 ML NHad. A ΔΦ of 257 meV for 0.1 ML NHad, 2570 meV ML−1, translates to a dipole moment of 0.37 D for NHad on Co(0001). Having established the work function responses of three of the four NHx species, we can quantitatively evaluate the work function data to assess whether electron irradiation at 100 K produces NH2 or NH directly (under the assumption that the measured ΔΦ is the result of a simple addition of the individual contributions of each adsorbate). We previously attributed the work function increase between 100 and 300 K to NH3 desorption. Following this interpretation, the work function change corresponds to desorption of 0.13 ML of adsorbed NH3. Thus, out of the initial ∼0.25 ML NH3 present, around 0.1 ML decomposes to NHx, whereas 0.13 ML desorbs intact. At low temperature after irradiation, the starting work function value (−1500 meV) corresponds rather well to the sum of the contributions from 0.1 ML NH (−257 meV) and 0.13 ML (−1215 meV) NH3. Hence we conclude that electron beam irradiation produces NH directly, similar to what was reported on Pt(111).6,50 It is suggested that the electron energy and exposure time of the electron beam irradiation determine which species are formed. In the case of Pt(410), for example, radiation-induced NH2 formation has been reported.54 Thus, our experimental data cannot be used to derive an activation barrier for the decomposition of NH2 into NH, which according to DFT calculations is 67 kJ mol−1, which translates to a temperature of around 250−270 K. Analyzing the effect of Nad and NHad on adsorption of CO we conclude that at 320 K CO only adsorbs on those parts of the surface that are free of p(2 × 2)-N islands. This picture breaks down when we consider the coadsorbed system at 90 K. In this case 0.52 ML CO can be accommodated on the surface precovered with 0.1 ML Nad, whereas adsorption solely on the clean parts of the sample can only account for 0.4 ML CO (60% of 0.67 ML). Hence, 0.12 ML CO must then adsorb between the N atoms, with a local coverage of 0.25 ML N and 0.3 ML CO. A lower coverage of N (0.04 ML) has a less strong impact on the CO adsorption, as expected. The same low coverage of NH (0.04 ML) has a similar effect as N, so the influence of NH is not much different from that of N. The effect of N on the adsorption of hydrogen [Figure 8(b)] shows somewhat complex behavior that is not completely understood yet. When 0.1 ML N is present on the surface (as p(2 × 2)-N islands covering 40% of the total surface) and hydrogen is dosed (100 L) at low temperature (150 K) the H2 desorption peak is reduced by only 10% compared to the clean surface value, which must mean that the hydrogen adsorbs in the islands of Nad as well. The N-free part of the surface can only accommodate 0.3 ML Had, meaning that the remaining hydrogen has to be adsorbed in between the Nad atoms, creating a local coverage of 0.25 ML Nad and 0.37 ML Had.

6. CONCLUSIONS The adsorption, desorption, and decomposition of NH3, both in the presence and in the absence of coadsorbed hydrogen and CO, have been investigated on the Co(0001) surface, both experimentally under UHV conditions and theoretically using DFT modeling. This study aims to get fundamental insights 3843

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into the detrimental effect of parts per million levels of NH3 on the Co-catalyzed Fischer−Trospch synthesis reaction. NH3 adsorbs molecularly on a top site onto Co(0001), with an experimentally determined adsorption energy of around 65− 67 kJ mol−1. Temperature-programmed desorption experiments indicate that the desorption rate constant is a strong function of ammonia coverage, and DFT calculations reveal that this can be attributed to a large extent to a coveragedependent adsorption energy. In addition a coverage dependence of the pre-exponential factor can play a role as well. Preadsorbed hydrogen slightly weakens the adsorption strength of subsequently dosed ammonia, but the adsorption capacity of the surface for NH3 is not affected by Had. Conversely, preadsorbed ammonia significantly lowers the dissociative sticking coefficient of H2. Substantial amounts of CO can be coadsorbed with NH3, independent of the order of dosing. Work function measurements as well as infrared adsorption experiments indicate significant interactions between the two adsorbates, and thermal desorption experiments show that CO in particular reduces the coverage dependence of the desorption rate constant, by reducing the repulsive interactions between the NH3 adsorbates. DFT calculations show that the activation barrier for NH3 (105 kJ mol−1) is higher than the adsorption energy of NH3 (59 kJ mol−1, DFT), and molecularly desorbed NH3 is the only product observed during heating of an adsorbed NH3 layer in vacuum. NHad species can therefore only be produced at low surface temperature by irradiating a saturated NH3 layer with 100 eV electrons. DFT calculations show that the order of stability is NH + 2 Had > Nad + 3 Had > NH2ad + Had > NH3ad. The activation barriers for NH2 and NH decomposition are computed as 67 and 110 kJ mol−1, where the latter barrier is confirmed by temperature-programmed desorption experiments where NH decomposition is found around 440 K. Both N and NH suppress the adsorption capacity of the surface for CO and to a lesser extent for hydrogen as well. Experimentally Nad was not observed, even when using hydrogen pressures of up to 1 × 10−5 mbar. In line with this, theory predicts a sizable barrier of 98 kJ mol−1 for this process. Translated to FTS, we conclude that (i) NH3 can still access the cobalt surface despite high pressures of H2 and CO, and thus even parts per million levels of NH3 can influence the catalytic surface. (ii) NHx species are much more stable on cobalt than intact NH3 and are most likely responsible for the detrimental effect. Due to their stability, strongly hydrogenating conditions (e.g., pure H2 treatment) are required to reverse the effect of NH3.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +31 40 247 4947. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C.K. thanks the European Graduate School on Sustainable Energy of DTU-TUM-TU/e for providing funding for this project. C.J.W. acknowledges his present employer, Syngaschem BV, for allowing him to spend time working on this manuscript. 3844

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