Microcalorimetry of O2 and NO on flat and stepped platinum surfaces

Surface Science 603 (2009) 1360–1364

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Microcalorimetry of O2 and NO on flat and stepped platinum surfaces Vittorio Fiorin, David Borthwick, David A. King * Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK

a r t i c l e

i n f o

Article history: Available online 14 January 2009 Keywords: Single-crystal adsorption calorimetry Sticking probability Heat of adsorption Oxygen Nitric oxide Stepped platinum surfaces Pre-exponential factor

a b s t r a c t Using the single-crystal adsorption calorimeter (SCAC), coverage-dependent heats of adsorption and sticking probabilities are reported for O2 and NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} at 300 K. At low coverage, oxygen adsorption is dissociative for all Pt surfaces. The highest initial heat of adsorption is found on Pt{2 1 1}, with a value of 370 kJ/mol, followed by those on Pt{4 1 1} (310 kJ/mol) and Pt{1 1 1} (300 kJ/ mol). We attribute this relatively large difference in the dissociative heat of adsorption at low coverage to the step character of the {2 1 1} surface. Initial sticking probabilities, so, are similar for the three surfaces, 0.22 on Pt{1 1 1}, 0.17 on Pt{2 1 1} and 0.18 on Pt{4 1 1}, rapidly decreasing as the oxygen coverage increases. For nitric oxide, the initial heats of adsorption are very similar and consistent with either dissociative or molecular adsorption, with values of 182 kJ/mol on Pt{1 1 1}, 192 kJ/mol on Pt{2 1 1} and 217 kJ/mol on Pt{4 1 1}. The so value is virtually identical for all three systems, with values ranging from 0.82 to 0.85, suggesting that the initial sticking probability is insensitive to the surface structure and adsorption is intrinsically precursor mediated. SCAC data are also used to evaluate pre-exponential factors, m, for first-order desorption at high coverage where adsorption is non-dissociative. Values of 3  1018, 6  1018 and 2  1018 s1 for O2, and 4  1019, 6  1017 and 2  1020 s1 for NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1}, respectively, are found. These unexpectedly high values are rationalised in terms of conventional transition state theory entropy changes. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Studies of the interactions between diatomic molecules and stepped surfaces are of great importance both from a fundamental point of view and in applied heterogeneous catalysis. Catalytic reactions on metal surfaces often occur at defect sites, which can be in the form of steps or vacancies, for which the activation barrier for a reaction may be low [1]. It is also well known that real catalytic surfaces are very different from the well-defined singlecrystal surfaces that are used in ultra-high vacuum environments. One way of bridging this so called ‘materials gap’ is to systematically investigate the effect of surface structure on adsorption and reaction by increasing its degree of complexity. Here we report the energetics and sticking probabilities of oxygen and nitric oxide adsorption on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} using single-crystal adsorption calorimetry (SCAC). The fcc{2 1 1} plane is one of the simplest stepped surfaces, consisting of two-atom wide {1 1 1} terraces separated by a single-atom step of {1 0 0} orientation. The fcc{4 1 1} plane is rather more complex: within the unit cell there are two terraces of {1 0 0} character, one being two-atom wide and the other one-atom wide separated by a single-atom step of {1 1 1} orientation. * Corresponding author. Tel.: +44 1 223 336 338; fax: +44 1 223 336 362. E-mail address: [email protected] (D.A. King). 0039-6028/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.08.034

Whilst there is a substantial body of literature pertaining to O2 and NO adsorption on low-Miller-index Pt surfaces, less attention has been paid to the adsorption of simple diatomic molecules on stepped metal surfaces, especially on Pt{4 1 1}. Oxygen on Pt{2 1 1} was investigated using temperature programmed desorption (TPD) [2,3], density functional theory (DFT) methods [4], and near edge X-ray absorption fine structure (NEXAFS) at 110 K to determine the angle of orientation of the adsorbed molecule [5]. Both the TPD and DFT studies found that the reactivity of the steps of the {2 1 1} surface is significantly higher than that of Pt{1 1 1} either for the molecular or the dissociative state [2–4]. As already mentioned, surprisingly, there is only one study of oxygen on Pt{4 1 1}, which deals with CO oxidation at relatively high O2 pressure (103 mbar). The authors report a substantial increase of CO oxidation rate due to a lower activation energy for oxygen dissociation associated with step sites compared with that on Pt{1 1 1} [6]. Adsorption and dissociation of NO have been studied on a number of stepped Pt surfaces, including {2 1 1} [7–11], {4 1 1} [11], {4 1 0} [12] and {5 3 3} [13]. A general conclusion one can draw from these studies is that step sites enhance the reactivity of NO [14], and that steps and/or terraces of {1 0 0} character are particularly active for NO dissociation even at room temperatures. This is supported by a recent XPS study on Pt{1 0 0} [15], which reports that at temperature as low as 250 K, NO adsorbs both molecularly and dissociatively, and that at 375 K adsorption is only

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dissociative. It is worth noting that the above-mentioned stepped surfaces contain either steps or terraces of {1 0 0} character. Our previous SCAC work on the stepped Ni{2 1 1} and Pt{2 1 1} surfaces revealed that steps may be important in some aspects of adsorption and reaction, but less crucial in others [16–18]. For example, in the case of NO on Ni{2 1 1}, steps are a determinant as to whether the molecule dissociates or not. The initial heat of adsorption of 400 kJ/mol drops considerably to 160 kJ/mol once the steps are saturated with adatoms, and NO adsorption becomes molecular [17]. However, when the coverage-dependent heat of adsorption is compared with that for Ni{1 0 0} hardly any difference between the two surfaces is observed [17]. Steps of the Ni{2 1 1} surface also play an important role towards oxygen dissociative adsorption and subsequent surface oxidation: the initial heat of adsorption of 620 kJ/mol at the steps is considerably higher than that for any low-Miller-index nickel surface, and at higher coverage oxidation is found to begin at step sites acting as nucleation points [18]. Thermodynamically, however, the enthalpy change at the onset of the oxidation is a relatively constant value for both the flat and stepped Ni surfaces [18]. In the case of CO, a significant increase of the initial adsorption energy is found on Pt{2 1 1} compared with Pt{1 1 1}, while little variation is found between the Ni{2 1 1} surface and the low-Miller-index Ni surfaces [16]. Here we elucidate further the role of steps by extending these studies to a more complex stepped surface. In addition we use the SCAC data to evaluate pre-exponential factors, m, for first-order desorption at 300 K saturation coverage, and find considerably higher values than normally assumed, i.e. 1013 s1.

means of Auger electron spectroscopy (AES), and the structure by low energy electron diffraction (LEED). 3. Results and discussion 3.1. O2 on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} The heat of adsorption and sticking probability for O2 on Pt{1 1 1}, {2 1 1} and {4 1 1} are shown in Fig. 1. In order to aid comparison between the three surfaces, data are plotted as a function of oxygen atom surface density: 1 ML is defined as 1 oxygen atom per (1  1) unit cell and corresponds to 1.31, 0.53 and 0.31  1015 O atoms/cm2 for the {1 1 1}, {2 1 1} and {4 1 1} surfaces, respectively. The three heat curves are indicative of dissociative adsorption and show some common characteristics: an almost constant heat value at low coverages and a decline at coverages greater than 0.2  1015 O atoms/cm2. This decline is more pronounced for the Pt{1 1 1} surface, indicating stronger lateral adatom repulsions compared with the stepped surfaces. This is consistent with the formation of a p(2  2) structure at 300 K on Pt{1 1 1} [22]; no ordered structures are found on the stepped Pt surfaces. The magnitude of the next-neighbour oxygen adatom lateral repulsions on Pt{1 1 1} for the p(2  2) structure (at 0.25 ML or 0.33  1015 molecules/cm2) can be estimated to be 12.5 kJ/mol. This is done by considering one oxygen adatom surrounded by six other neighbours in a p(2  2) structure and the fact that at 0.25 ML the heat

O2 /Pt{111} O2 /Pt{211} O2 /Pt{411}

400 300 200 100

0 0.0

0.2

0.4

0.6

0.8 15

1.0

1.2

2

O surface density ( x10 atoms/cm )

b 0.25 Sticking probability

The single-crystal adsorption calorimeter (SCAC) has been described in detail previously [19,20] and here only a brief account of the experimental technique is given. Experiments were performed in an ultra-high vacuum chamber with a base pressure lower than 5  1011 Torr. Oxygen and nitric oxide were dosed at room temperature using a pulsed supersonic molecular beam. Each pulse, which consists of approximately 2  1012 molecules, is 50 ms in duration at a repetition period of 2.5 s. The intensity of the molecular beam is absolutely calibrated after each experiment by using a spinning rotary gauge. The Pt single-crystal surfaces are in the form of a thin film (200 nm thick) in order to achieve a measurable change in temperature upon adsorption. The heat of adsorption is deduced by the temperature change, which is remotely monitored via infrared radiation using a mercury cadmium telluride (MCT) infrared detector. Typically, the initial adsorption of a single gas pulse gives a temperature rise of the order of 0.1 K at the single crystal thin film. In conjunction with heat measurements, the sticking probability is measured as a function of coverage using the King and Wells method [21], by measuring the portion of the incident gas pulse that is reflected from the surface. At saturation coverage, when the amount of desorbing species between pulses equals the incremental amount adsorbed in the preceding pulse, steady state is reached and the measured sticking probability and heat of adsorption are referred as ‘steady state sticking coefficient’ and ‘steady state heat’, respectively, at room temperature. As explained later, this regime is used to extract pre-exponential factors for desorption by equating the amount adsorbed during a pulse and the amount desorbed between pulses. Cleaning of the Pt surfaces consisted of cycles of gentle argon ion sputtering at discharge currents below 8 lA and annealing at 700 K. Oxygen treatment, using the molecular beam, was also used to completely remove further contaminants with the surface subsequently annealed. The cleanness of the surface was checked by

Heat of adsorption (kJ/mol)

a 2. Experimental

O2 /Pt{111} O2/Pt{211} O2/Pt{411}

0.20 0.15 0.10 0.05 0.00 0.0

0.2

0.4

0.6

0.8 15

1.0

1.2

2

O surface density ( x10 atoms/cm ) Fig. 1. (a) Coverage-dependent heat of adsorption for O2 on Pt{1 1 1}, {2 1 1} and Pt{4 1 1}, adsorbed at 300 K. (b) Coverage-dependent sticking probability for O2 on Pt{1 1 1}, {2 1 1} and Pt{4 1 1}, adsorbed at 300 K.

V. Fiorin et al. / Surface Science 603 (2009) 1360–1364

3.2. NO on Pt{1 1 1}, Pt{2 1 1} and Pt{4 1 1} The coverage-dependent NO heats of adsorption and sticking probabilities on flat and stepped platinum surfaces are shown in Fig. 2. The coverage is defined in terms of NO molecule surface density: 1 ML is defined as 1 NO molecule per (1  1) unit cell and corresponds to 1.31, 0.53 and 0.31  1015 NO molecules/cm2 for the {1 1 1}, {2 1 1} and {4 1 1} surfaces, respectively. The initial sticking probability, s0, is virtually identical for all three systems, with values in the range 0.82–0.86, suggesting that the initial sticking probability is insensitive to the surface structure. In keeping with this, previous SCAC studies of NO adsorption

a 240 Heat of adsorption (kJ/mol)

has dropped to 150 kJ/mol from an initial value of 300 kJ/mol (i.e. (300  150)/(6  2), the factor 2 in the denominator takes into account the heat being per oxygen molecule). Interestingly, the {2 1 1} surface exhibits the highest heat values compared with the {4 1 1} and {1 1 1} surfaces, and, in particular, the initial heat of adsorption on Pt{2 1 1} is approximately 70 kJ/mol higher than that for the other two surfaces. From a number of studies [23– 26] it is known that, on stepped Pt surfaces, oxygen initially adsorbs dissociatively at the step atoms and subsequently on terraces. Here, it is the character of the step that determines the strength of the adsorption. Steps on Pt{2 1 1} are {1 0 0}-type, whereas those on Pt{4 1 1} are {1 1 1}-type. The initial adsorption heats on {1 1 1} and {4 1 1} are very similar, while on {2 1 1} it is higher by about 70 kJ/mol, strongly supporting the conclusion that adatom bonding is strongest on {1 0 0} steps. This is supported by a DFT study by Feibelman et al. who looked at two types of Pt step, and found the adsorption energy of oxygen on the {1 0 0}-type to be some 40 kJ/mol higher than that on the {1 1 1}-type of step [27]. Our integrated adsorption heats show good agreement with the DFT calculations. The O2 sticking probabilities for the three surfaces are rather similar. However, on Pt{2 1 1} a ‘clean-off’ effect is evident at the lowest coverages: an initial increase in the sticking is followed by a decrease until the steady state regime. This effect has been observed before by Winkler et al. on Pt{1 1 1} and Pt{2 1 1} [2]: they found that oxygen was reacting with impurities, which were below the AES detection limit, producing CO and water. In the present work, if this effect is ignored then the initial sticking probability, s0, is highest on Pt{1 1 1} followed by Pt{2 1 1}, and the lowest on Pt{4 1 1}. The values of s0 and the shapes of the curves, which appear to be very similar, may be suggestive of direct activated dissociative adsorption. However, compared with other systems that exhibit activated dissociative adsorption, such as methane and ethane on Pt{1 1 0}  (1  2) for which the initial sticking probabilities are of the order of 106 and 103 [28,29] respectively, a value of 0.2 in this case is significantly higher. Indeed previous DFT studies reported an activation barrier of 0.18 eV for oxygen dissociation on Pt{1 1 1} from two molecular chemisorbed states [30,31]. In keeping with this, we note that in our experiments the oxygen molecular beam possesses a normal translational energy of about 0.09 eV (9 kJ/mol) which may contribute to surmount the barrier, hence explaining the relatively high s0 values. Since the initial sticking probability is also very similar on the three Pt surfaces, we conclude that steps are unimportant in promoting adsorption and dissociation compared with the flat surface, at the beam energy used here. As the coverage increases, s(h) drops considerably on the three Pt surfaces: oxygen adatoms passivate the surface for O2 dissociation at h = 0.2  1015 atoms/cm2. We can also rule out the possibility of strong lateral repulsions as the reason for the drop in sticking probability because the heat of adsorption does not change as the coverage increases up to 0.2  1015 atoms/cm2. At this density oxygen adsorption is spread out over the entire surface and islands are not formed.

NO/Pt{111} NO/Pt{211} NO/Pt{411}

220 200 180 160 140 120 100 0.0

0.2

0.4

0.6

0.8

15

1.0 2

NO surface density ( x10 molecules/cm )

b NO/Pt{111} NO/Pt{211} NO/Pt{411}

0.8

Sticking probability

1362

0.6

0.4

0.2

0.0 0.0

0.2

0.4

NO surface density ( x10

0.6 15

0.8

1.0 2

molecules/cm )

Fig. 2. (a) Coverage-dependent heat of adsorption for NO on Pt{1 1 1}, {2 1 1} and Pt{4 1 1}, adsorbed at 300 K. (b) Coverage-dependent sticking probability for NO on Pt{1 1 1}, {2 1 1} and Pt{4 1 1}, adsorbed at 300 K.

on the Pt{1 1 0} and Pt{1 0 0}-hex surfaces have also found initial sticking probabilities of 0.87 and 0.86, respectively [32]. In contrast with the oxygen case, at low coverage the sticking probability of NO hardly changes with coverage, which indicates that adsorption occurs via a precursor mechanism that is not influenced by the nature of the final adsorption site, in terms of its geometry and/or coordination number of the adsorption site. All three sticking probability curves in Fig. 2 trace the same path up to a coverage of 0.4  1015 molecules/cm2, which corresponds to 0.3 ML for Pt{1 1 1}, 0.75 ML for Pt{2 1 1}, and 1.3 ML for Pt{4 1 1}. Conversely, the presence of step atoms has a significant effect on the energetics of adsorption. On a clean surface, the highest heat is found on the {4 1 1} surface (217 kJ/mol), followed by that on Pt{2 1 1} (192 kJ/ mol), with the lowest on Pt{1 1 1} (182 kJ/mol). NO adsorption on Pt{1 1 1} proceeds molecularly via sequential filling of fcc-hollow and atop sites, yielding a saturation coverage of 0.5 ML at this surface temperature [33]. Initially, adsorption at fcc-hollow sites results in the formation of a p(2  2) overlayer structure up to a coverage of 0.25 ML (or 0.32  1015 molecules/ cm2), after which adsorption switches to atop sites forming a p(2  2)-2NO overlayer structure of 0.5 ML coverage (or 0.64  1015 molecules/cm2) [33–35]. Our calorimetric data are consistent with this picture: a clear break in the heat of adsorption can be seen at around 0.32  1015 molecules/cm2 or 0.25 ML, which indicates two distinct adsorption regimes, the second continuing up to 0.5 ML. It is therefore possible to estimate the magnitude of the next-near-neighbour lateral repulsions at

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Table 1 Values of the pre-exponential factor, m, and entropy of desorption DSà for O2 and NO on flat and stepped Pt surfaces, derived from the SCAC data using Eq. (1) and compared with those derived from conventional transition state theory (CTST) using Eq. (2). O2

m (s1)

DSà (J K1 mol1)

NO

m (s1)

DSà (J K1 mol1)

Experiment Pt{1 1 1} Pt{2 1 1} Pt{4 1 1}

3  1018 6  1018 2  1018

109 114 105

Pt{1 1 1} Pt{2 1 1} Pt{4 1 1}

4  1019 6  1017 2  1020

130 95 144

CTST Non-localised Localised

8  1014 6  1016

40 76

Non-localised Localised

1  1014 1  1017

23 80

0.25 ML, (each NO molecule is surrounded by six other molecules), as x = q0  q0.25 ML)/6 = 7 kJ/mol. This value is comparable with that for CO/Pt{1 1 1} at 0.33 ML (5.3 kJ/mol) but lower than that of the O/Pt{1 1 1} p(2  2) overlayer (12.5 kJ/mol). This is not a surprise, since adatoms are more repulsive than admolecules at this separation distance. In the case of Pt{2 1 1}, evidence in the literature for room temperature NO dissociation is varied and contradictory. Recently, Brown and co-workers, using RAIRS, TPD and DFT, have speculated about the possibility that the Pt{2 1 1} surface is capable of dissociating NO at 300 K [11]. However, SCAC values at low NO coverages on Pt{2 1 1} are only 10 kJ/mol higher than those on Pt{1 1 1}, where NO adsorbs molecularly at 300 K. This does not rule out dissociation, if, for example, it occurs on a timescale longer than the 50 ms pulse time, in which the heat data is acquired. However, Masel and co-workers found that a fraction of the NO molecules adsorbed at 300 K on Pt{1 0 0}, Pt{4 1 1} and Pt{2 1 1} dissociate at room temperature, and that the proportion of dissociated molecules is 66% on Pt{2 1 1} and 70% on Pt{4 1 1} [11]. From our previous SCAC work on NO on Pt{1 0 0} at 300 K we concluded an initial heat of adsorption of 190 kJ/mol for h < 0.1 ML, which was attributed to dissociative adsorption at defect sites. At higher coverages (h > 0.1 ML), the heat dropped to 160 kJ/mol, assigned as molecular adsorption. A more recent XPS study confirmed NO dissociation at 300 K [14], so we can ascribe the initial adsorption heat of 192 kJ/ mol for NO on Pt{2 1 1} as dissociative. It is therefore reasonable to associate the first adsorption regime, up to 0.5 ML, with step edge adsorption, consistent with the fact that the {1 0 0} character of the steps is more reactive towards dissociation than the {1 1 1} terraces; the integral heat at 0.5 ML is indeed in good agreement with DFT calculations for dissociative step-bridge adsorption [7,36]. As the coverage increases, a second regime, when step edge sites are saturated, is attributed to molecular adsorption at terrace sites, and due to the similarity between this regime and the second adsorption regime for NO/Pt{1 1 1}, it seems feasible to propose adsorption on the {1 1 1} terraces, with the heat of adsorption agreeing well with the DFT value for terrace-bridge molecular adsorption [33]. The heat curve for NO/Pt{4 1 1} is somewhat harder to interpret due to the lack of literature data available, but it is possible to rationalise some of its features. The high initial heat (214 kJ/mol) favours dissociative adsorption, with the adsorption site likely to be at the step edge. As the coverage increases, the heat initially drops to 195 kJ/mol and then remains fairly constant up to 1 ML (i.e. 0.3  1015 molecules/cm2). By comparison with the heat value on the {2 1 1} surface, it is tempting to conclude that adsorption is dissociative up to 1 ML. The relative amount of dissociated molecules on Pt{4 1 1} (1 ML or 0.31 molecules/cm2) compared with those on Pt{2 1 1} (0.5 ML or 0.26 molecules/cm2) is consistent with the work of Masel and Gohndrone [11]. The second adsorption regime from 0.4  1015 molecules/cm2 appears to show the same behaviour as observed in the zero-coverage limit for NO/Pt{2 1 1}, namely an initial heat of around 194 kJ/mol, decreasing to around

138 kJ/mol. Since this adsorption regime for NO/Pt{2 1 1} was attributed to NO binding at step edge sites, it is reasonable to assign the second regime for NO/Pt{4 1 1} to molecular adsorption at step edge sites. A greater amount of adsorption at step edge sites is possible on Pt{4 1 1} due to the surface having a higher step density than Pt{2 1 1} (0.061 steps Å2 compared to 0.053 steps Å2 for the {2 1 1}). It might be expected that the second regime of NO/ Pt{4 1 1} will be different than the first regime of NO/Pt{2 1 1} due to the {2 1 1} surface being clean and the {4 1 1} surface already containing 1 ML of dissociated NO molecules. However, due to the larger {4 1 1} unit cell, incoming NO molecules will not be as close to the adsorbed N and O adatoms, and so the lateral interactions will affect the adsorption energy to a lesser extent. 3.3. Desorption kinetics and pre-exponential factor for desorption As pointed out by Yeo et al. [37], using SCAC data, the pre-exponential factor m for desorption is readily obtained from the sticking probability and adsorption heat measurements at high coverages. In a pulsed beam experiment a steady state is reached when the amount adsorbed during a 50 ms (t1) pulse is balanced by the amount desorbed in the time (t2) between pulses, which is 2450 ms. Since the amount adsorbed is t1sQ, where Q is the measured beam flux, s is the sticking probability, and the amount desorbed is t2 hmNsexp(q/RT), where q is the heat measured during the adsorption pulse, m is the pre-exponential factor for desorption, Ns is the surface atomic density and h is coverage, equating these amounts adsorbed and desorbed yields:

m ¼ ðt1 =t2 ÞðsQ=hNs Þ expðq=RTÞ

ð1Þ

Experimental values obtained in this way are shown in Table 1. We note that at high coverages the adsorbed states of both O2 and NO are molecular. For O2, values of m are in the region of 1018 s1, while for NO it spans almost three orders of magnitude from 6  1017 to 2  1020 for the three Pt surfaces. Within conventional transition state theory (CTST) [38], the desorption pre-exponential can be expressed as:

m ¼ jðkB T=hÞ expðDSz =RÞ

ð2Þ

Here DSà is the activation entropy for desorption, i.e. the difference in entropy between the transition state and the adsorbed layer, and j (61) is the transmission factor. In evaluating DSà below we have assumed that j = 1. By time reversal symmetry this is justified because it is in effect included in the experimentally determined sticking probability, s, in Eq. (1). From CTST it is possible to generate theoretical values of the pre-exponential factor and desorption entropy based on the partition functions of the adsorbate and the transition state. The major disadvantage of CTST is that interactions between the substrate and the adsorbate, as well as interactions between adsorbed species, are neglected [39]. Thus, similar values of m may be obtained for molecules having very different chemical properties, but with

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similar physical properties. The procedure used to determine the theoretical values has been outlined elsewhere [40], but is summarised here. In one extreme, the transition state may be a twodimensional gas, having two degrees of translational freedom parallel to the surface and three degrees of rotational freedom. The two limiting cases for the adsorbate layer correspond to desorption from a non-localised chemisorbed adlayer which has two degrees of translational freedom parallel to the surface, and desorption from a localised adlayer which has only degrees of freedom due to hindered translations and rotations.1 From CTST, the pre-exponential factor for desorption is given by:

m¼

kB T Q z h Q ad

ð3Þ

à

where Q and Qad are the partition functions of the transition state and adsorbate, respectively. Thus, depending on the choice of the partition function and the approximations used, a range of values for m can be calculated. Calculated values of m and DSà for O2 and NO on the three Pt surfaces are also given in Table 1. Even with the CTST calculation yielding the largest DSà, theoretical values are substantially lower than the values obtained from the experimental data. The implication is that the very high coverage states of O2 and NO that undergo reversible adsorption– desorption at 300 K are (a) highly localised initial states and (b) experience a transition state on desorption which appears to have all three degrees of translational freedom, as in the gas phase. These results clearly require further theoretical analysis. However, at this stage we remark that the experimentally determined values of the pre-exponential factor are considerably higher than those normally assumed in analysis of TPD spectra of surface catalytic reaction. Activation energies for first-order desorption would be considerably underestimated using the usual Redhead analysis with a pre-exponential factor of 1013 s1. This will be discussed in a forthcoming publication. 4. Conclusion Using single-crystal adsorption calorimetry, we have reported heats of adsorption and sticking probabilities as a function of coverage for O2 and NO on flat {1 1 1} and stepped {2 1 1} and {4 1 1} Pt surfaces at 300 K, and show that the presence of step atoms per se is not always sufficient to explain differences in adsorption mechanism and reactivity in comparison with flat surfaces. For O2 we find that adsorption is dissociative for all Pt surfaces and the heat is highest on Pt{2 1 1} by some 70 kJ/mol at all coverages, which is attributed to the {1 0 0} character of the steps. Conversely the sticking probability is hardly affected by the presence of steps and results from a direct activated adsorption for all surfaces considered. For NO, initial adsorption is molecular on Pt{1 1 1}, and dissociative on Pt{2 1 1} and Pt{4 1 1} at coverages below that at which steps are saturated. The amount of dissociated NO is found to be greater on Pt{4 1 1} than on Pt{2 1 1}, due to larger {1 0 0} facets on the {4 1 1} surface compared to the {2 1 1}, and is consistent with previous reports. Pre-exponential values for firstorder desorption, m, for the two sets of systems, directly evaluated from the experimental data, are found to be unexpectedly high: for O2 m is in the region of 1018 s1, and for NO m spans almost three

1 It should be noted that for the localised case we do not treat the adsorbate as totally immobile state as in the model used in Ref. [40].

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