NO Chemisorption and Reactions on Metal Surfaces: A New

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J. Phys. Chem. B 2000, 104, 2578-2595

FEATURE ARTICLE NO Chemisorption and Reactions on Metal Surfaces: A New Perspective Wendy A. Brown Department of Chemistry, UniVersity College London, 20 Gordon Street, London, WC1H 0AJ, U.K.

David A. King* Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K. ReceiVed: August 31, 1999; In Final Form: NoVember 11, 1999

NO adsorption on metal surfaces has been studied extensively due to its important role in many catalytic processes. In the past, it was recognized that the tendency for a metal surface to dissociate NO depends on its position in the periodic table, but little was understood about the dissociation process itself. Recent experimental and theoretical studies have shown that this view is oversimplified. In addition to the distinction between molecular and dissociative adsorption on surfaces, the structure of NO on metal surfaces has also been the subject of detailed study and debate. It is therefore timely to update our ideas about NO adsorption and reactivity. Here we examine the structure of NO on surfaces, in particular the break-down of vibrational assignments and we look at how these assignments can be improved. The NO dissociation process is also discussed along with the formation, and reactions, of the NO dimer, (NO)2, on some metal surfaces.

1. Introduction The adsorption of NO on metal surfaces is of considerable scientific and technological interest. The catalytic reduction of the NOx species to N2 and O2 is one of the important reactions that takes place in the three-way car exhaust catalyst. NO is also an important byproduct in the ammonia oxidation process. The adsorption and reactions of NO on well-defined singlecrystal metal surfaces are therefore very important and take us some way to beginning to understand real catalytic systems on a fundamental level. In addition to its practical applications, NO is interesting from a purely scientific point of view. The dissociation energy of NO1 (630 kJ mol-1) is much lower than that of the similar molecule, CO1 (1076 kJ mol-1), and hence there is a greater probability of finding both molecular and dissociated NO species on a surface. Because of this, NO shows a tendency to undergo reactions on surfaces and species such as (NO)2, N2O, and NO2, as well as NO and N and O atoms, have all been identified on metal surfaces. NO reactions on surfaces can also exhibit oscillatory behavior, particularly in the case of reactions on metals such as Rh and Pt. Oscillatory reactions such as the reactions of NO and CO, NO and H2 and NO and NH3, have been reviewed by Imbihl,2 by Nieuwenhuys and co-workers,3 and by Gruyters and King.4 NO, unlike CO, has an unpaired electron in its 2π* orbital and this has led to the notion that amphoteric bonding for NO on a surface is a useful consideration. NO can either donate its 2π* electron to the surface or it can accept electron density from the surface into the half filled 2π* orbital, and should therefore show a very wide variety of chemistry on surfaces, adsorbing in many different binding geometries. We will see

that this concept is of limited use, and may even be misleading, in a consideration of NO adsorption on metals. A rather simple view of NO adsorption emerged from the conclusions reached by Rhodin and co-workers5 that the susceptibility of NO to dissociation, when adsorbed on a surface, varied systematically with the position of the substrate in the periodic table. This correlation follows the earlier tabulations of Bond6 showing that adsorption heats for O2, N2, CO, and CO2 fall monotonically as a function of Periodic Group number, i.e., from left to right. A high heat implied dissociation. This is illustrated in Figure 1, where metals to the left of the solid line were expected to dissociate NO and those to the right should only show molecular adsorption, at 300 K. In many cases these rules do hold, but recent studies of NO adsorption on various surfaces have shown that the situation is more complex. For example, whether a surface dissociates NO often depends on surface temperature, surface coverage, crystal plane, and the concentration of surface defects. In addition, recent studies of NO adsorption on Cu and Ag surfaces which, by the rule in Figure 1, should only be molecular, have shown that reaction products such as N2O are formed at low temperatures (∼100 K), while at 300 K there is no reactivity on Ag.7-18 For both metals, NO dissociation should not occur, and hence there was much confusion about the exact mechanism of N2O formation on these surfaces. In addition, in recent studies of NO adsorption on Mo{110}19-22 it has been suggested that dinitrosyl species are formed. This surface should dissociate NO at all coverages and under all conditions. The chemistry of NO adsorption on metal surfaces is therefore much more complex than was originally thought. In addition to the distinction between molecular and dissociative adsorption on surfaces, the detailed structure of NO

10.1021/jp9930907 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/02/2000

Feature Article

Figure 1. A diagram showing the tendency of different metal surfaces to dissociate NO depending on the position of the substrate in the periodic table. Adapted from ref 5.

on metal surfaces has also caused much debate in recent years. Many gas-surface interactions have analogues in organometallic and coordination chemistry. This analogy was first proposed by Muetterties23,24 and he and others then developed this idea in more recent work.25-28 The most important idea to come out of this analogy was the use of vibrational frequencies from carbonyl and nitrosyl compounds to provide the basis for the assignment of CO and NO vibrational modes on surfaces to adsorption at a site with a particular geometry. In particular, NO was classified into adsorption in 3-fold, 2-fold bridging, atop or bent configurations, and each of these gave rise to different vibrational frequencies. In recent years, this idea has been shown to be unreliable. For the particular case of NO adsorption on Ni{111}, vibrational studies29,30 suggested that NO was adsorbed in a 2-fold bridge site, and both bent and upright species were suggested as a function of coverage. However, diffraction based structural studies31-34 showed that NO occupies 3-fold sites over the whole coverage range. Recent density functional theory (DFT) calculations35,36 have also shown disagreement with conclusions from the earlier vibrational studies for NO adsorption on other surfaces. These examples all show that it is necessary to update our ideas about NO adsorption, and this is the subject of this review. It is not a complete review of NO chemistry, but rather highlights the new perspectives that have been gained in NO adsorption in recent years. A detailed review of NO adsorption up to 1984 was given by Bridge and Lambert.37 Topics dealt with here include the structure of NO on surfaces (and in particular the break-down of vibrational structural assignments), the NO dissociation process and the formation, and reactions, of the NO dimer, (NO)2, on metal surfaces. 2. NO Surface Structure Molecular structure on a surface includes the preferred site, the axial geometry, interatomic spacings and metal atom relaxations and is important in a consideration of molecular reactivity at surfaces. In the past, the main route to surface structural information for NO was from vibrational studies, particularly reflection absorption infrared spectroscopy (RAIRS) or electron energy loss spectroscopy (EELS). More recently, the advent of faster computers and improved theoretical techniques has meant that detailed low energy electron diffraction (LEED) analyses of adsorbate structures have become routinely available. The use of a relatively new structural technique, photoelectron diffraction (PED), has also led to detailed structural assignments for many systems. Finally, DFT calculations are also now in use to determine which surface structure is the most stable, providing a direct comparison with experiment, and providing unique information concerning bonding and electronic structure. Many different NO adsorption geometries are observed. This leads to a broad range of N-O stretching frequencies, ranging from 1200 to 1900 cm-1. With such a broad range, it is often rather difficult to assign bands to a particular adsorption site.

J. Phys. Chem. B, Vol. 104, No. 12, 2000 2579 In the past the following inorganic metal complex nitrosyl frequencies were used to aid vibrational site assignments: 3-fold nitrosyls,38,39 1320-1545 cm-1; 2-fold nitrosyls,40,41 480-1545 cm-1; linear atop,40 1650-2000 cm-1; bent atop,40,42 15251700 cm-1. It is clear from the above list that there is much overlap between the frequencies observed for different species, and this can make it rather difficult to accurately assign vibrational frequencies to a particular NO species. Widely varying vibrational frequencies are seen for adsorption on similar sites, as illustrated by the data shown in Table 1.7-10,12,15,19-22,29,30,36,43-65 This shows the observed vibrational frequencies and site assignments of adsorbates on the various metal surfaces that are discussed in detail in this article. It is not a complete listing, but rather illustrates the wide variety of assignments and frequencies observed. No indication is given here about the accuracy of these assignments; many are discussed later. These problems in site assignment from vibrational frequencies demonstrated a need to revise the methodology. The first case demonstrating this was the adsorption of NO on Ni{111}, which was studied using a wide variety of techniques over the years.29-34,43-46,66-80 The main controversy arose from the assignments of vibrational bands identified in a RAIRS study by Erley29 and in an earlier EELS study by Ibach and co-workers.30 In the RAIRS study, NO adsorption on Ni{111} was studied at 85 K. Previous work66,69 had shown that, at temperatures below 300 K, NO adsorbed molecularly and a c(4 × 2) LEED pattern was observed at the saturation coverage of 0.5 ML. The RAIRS study29 showed that at very low coverages there is a single band in the infrared spectrum, at 1460 cm-1. With increasing coverage a high frequency shoulder is observed, which develops into a band at 1475 cm-1. Further increasing the NO exposure shifts the band at 1475 cm-1 to higher frequency, until it grows to dominate the spectrum. Since the low-frequency band saturates before the population of the second state begins, the two bands were assumed to be due to different adsorbed species. The 1460 cm-1 band was assigned to NO adsorption on defect sites. At higher coverages still, the NO band shifts to 1513 cm-1 and a new band grows in at 1543 cm-1. With increasing exposure, this band shifts to higher frequency and increases in intensity and at the same time the intensity of the low frequency band decreases. At saturation the only band in the spectrum is at 1585 cm-1. These results were in agreement with the earlier lower resolution EELS study30 where the authors proposed two adsorption states to account for their data. In agreement with this, Erley29 assigned the low-frequency band at 1475-1513 cm-1 to NO adsorbed in a bent 2-fold site and the band at 15431585 cm-1 to upright NO in a 2-fold bridge site. These assignments were made by comparison with the vibrational frequencies of nitrosyl compounds with known structures. However, in an electron stimulated desorption ion angular distribution (ESDIAD) study, Netzer and Madey69 found only upright NO at 80 K, and only at 150 K was evidence found for bent (or tilted) NO. In an X-ray photoelectron spectroscopy (XPS) study, Breitschafter et al.68 also found no evidence for bent NO. A more recent surface extended X-ray absorption fine structure (SEXAFS) study by Aminopirooz and co-workers31 provided stark disagreement with the vibrational site assignments. Their study showed that NO was adsorbed in 3-fold hollow sites in the c(4 × 2) NO structure at a coverage of 0.5 ML, despite the assignments of the bands seen in RAIRS29 to adsorption on a 2-fold bridge site. In addition, they showed that

2580 J. Phys. Chem. B, Vol. 104, No. 12, 2000

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TABLE 1: Vibrational Frequencies and Site Assignments for NO Adsorption on Various Different Single Crystal Metalsa system

technique

Ag{111}

EELS

atop frequency (cm-1)

bridge frequency (cm-1)

1863 1750

3-fold frequency (cm-1) 1274-1282 1153

RAIRS

Ni{111}

RAIRS

Pd{110}

RAIRS RAIRS Theory EELS EELS RAIRS

1730

Theory Theory EELS EELS EELS RAIRS

1719 1812 1735 1735 1710 1700-1720

RAIRS RAIRS EELS EELS RAIRS

1690-1710 1700-1725

Pd{111} Pt{111}

Pt{110}

Theory Rh{111}

Cu{111}

1681 1680 1820

1760 1730 (1x2) 1800 (1x1) 1955

Theory EELS EELS

1812

EELS RAIRS

1823

1830

1475-1513 1543-1581 1578 1584 1474 1490-1600 1494-1549 1629-1661 1624 1660 1672 1672-1707 1667 1516-1613 1575 1490 1475-1500 1476 1476-1498 1516 1610 1575 1600 1819 1765 1750 1644 1480-1630 1515 1590-1635 1533

Theory

Cu{110} Mo{110} Co{101h0}

a

Theory EELS RAIRS

1409 1807

EELS+ RAIRS RAIRS

1860 1720 1857 1790-1800 1826-1857

1163 1484 1605-1631

1640-1700

other

10

1865; 1788 (NO)2 2229; 1250 (N2O) 1460 (defects)

[9,15]

[29]

1688 ((NO)2)

[43] [44] [45] [46] [30] [47]

1471; 1449 1581

1456 1588; 1574

1820-1840 1609-1634 (defects) 1718 (NO)2 1350

1670 (defects)

1626 1544; 1554

1520-1560 1610 1779-1908 1335-1471 1432

ref

2242 (N2O)

[48] [36] [49] [50] [51] [52] [53] [54] [55] [56] [5760] [60] [36] [61] [62]

1274 (N2O) 1780 (NO)2 1830 (NO)2

[12] [7] [63]

1780 (NO)2 1850 (NO)2 1821 (dinitrosyl) 1440-1460

[64] [29] [8] [1922] [65]

Only the systems included in this article are included, and no indication can be given about the accuracy of the site assignments.

the NO was adsorbed upright on the surface at all coverages. They therefore concluded that the vibrational assignments made in the RAIRS study29 were incorrect. They assigned their SEXAFS data to an equal occupation of fcc and hcp 3-fold hollow sites. Asensio and co-workers32 performed a PED study of NO adsorption on Ni{111} which also contradicted the conclusions from the RAIRS experiments. They performed their study under three different conditions: low coverage (correlated with the 2-fold tilted NO species in RAIRS29), a coverage of 0.5 ML where the c(4 × 2) NO structure is formed (correlated with a 2-fold upright NO species from RAIRS29), and NO adsorption on an O precovered surface. A RAIRS investigation of the latter phase44 had previously shown a band at ∼ 1800 cm-1 which was assigned to atop NO. Their PED data showed that NO was

adsorbed in a 3-fold hollow site in every case. This was in agreement with the SEXAFS study31 but in disagreement with the vibrational studies.29,30 Theoretical fits to the data suggested that NO was adsorbed only in fcc, and not hcp, sites at all coverages investigated. LEED experiments33,34,80 were performed which supported the observations of the SEXAFS and PED studies, but showed occupation of both fcc and hcp 3-fold sites. However, more recent PED investigations at low coverage76 have shown equal occupation of the fcc and hcp 3-fold hollow sites by NO, in agreement with the LEED studies. The proposed c(4 × 2) structure for NO on Ni{111}, containing NO adsorbed in fcc and hcp 3-fold hollow sites, is shown in Figure 2. Theoretical calculations on the same adsorption system45 have also supported these recent findings. These cluster calcula-

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J. Phys. Chem. B, Vol. 104, No. 12, 2000 2581 TABLE 3: Calculated Vibrational Frequencies ωN-O and Adsorption Energies Ead for NO Adsorbed at Various Sites on Pd Clusters which Represent the Pd{110} Surface48 site atop bridge 3-fold

Figure 2. Proposed c(4 × 2) overlayer structure of NO adsorbed on Ni{111} as determined by LEED33,34,80, and PED32 experiments. NO occupies 3-fold hollow sites, in contradiction to the conclusions of vibrational studies.29,30

TABLE 2: Calculated Adsorption Energies Ead, and Bond Lengths for NO Adsorbed on Pt{111} at 0.25 ML Coverage35 adsorption site

dN-O (Å)

dN-Pt (Å)

Ead (kJ mol-1)

fcc hollow hcp hollow bridge atop

1.225 1.221 1.214 1.187

2.045 2.086 1.972 1.837

169 160 138 62

tions showed that the 3-fold site is the most strongly bound adsorption site for NO on Ni{111} and, in addition to this, calculated vibrational frequencies show good agreement with the experiments. It was found that NO is adsorbed with the molecular axis almost perpendicular to the surface at all coverages and the infrared band at 1475 cm-1 was reattributed to NO adsorption in 3-fold sites. They also concluded that the band at 1513 cm-1 was due to NO adsorbed in 3-fold sites but the shifted frequency was attributable to NO interacting strongly with other NO molecules. Strong dipole coupling raises the vibrational frequency. This observed break-down in vibrational assignments is not unique to the NO on Ni{111} adsorption system. For the adsorption of NO on Pt{111}, RAIRS54 and EELS51,55 measurements showed two different vibrational modes on the surface as a function of coverage. At low coverage, one vibrational band occurs with a frequency in the range from 1476 to 1498 cm-1. At higher coverage,54 this band is replaced by another, ranging from 1700 to 1725 cm-1. The saturation coverage was thought to be 0.25 ML, since a p(2 × 2) LEED pattern was seen at saturation. These vibrational modes were assigned, on the basis of nitrosyl vibrations, to the adsorption of a 2-fold bridged NO species at low coverage replaced by an atop NO species at high coverage. The switch is completely reversible. LEED data80 for saturation coverages of NO on Pt{111}, where the p(2 × 2) LEED pattern was observed, again provide conflicting site assignments to those derived from RAIRS and EELS. Materer et al. found that the LEED-IV data could only be fitted with NO molecules in 3-fold fcc hollow sites. The experiment was not performed at coverages below saturation, so no structural information for lower coverages was obtained. However, these findings are in stark contrast to the earlier conclusions from vibrational spectroscopy, again suggesting a breakdown of vibrational site assignments. The LEED findings for NO on Pt{111} are supported by a recent DFT calculation.35 This gives the parameters shown in Table 2 for the adsorption of NO in various sites on Pt{111} at a coverage of 0.25 ML for the p(2 × 2) structure. The fcc and

cluster Pd5NO Pd5NO Pd4NO Pd8NO (rigid) Pd8NO (relaxed) Pd5NO gas phase

Ead (kJ mol-1)

calculated ωN-O (cm-1)

69.6 18.8 103.1 196.1 221.7 123.3

1719 1707 1673 1672 1456 1905

hcp 3-fold hollow sites have adsorption energies very close to one another, and these sites are more stable than any of the others calculated. The atop site is the least stable. The calculation provides strong support for the findings of the LEED experiments. These two examples, NO on Ni{111} and Pt{111}, demonstrate that using nitrosyl vibrational frequencies to aid vibrational analysis can lead to incorrect assignments. From these two examples we reach an unexpected conclusion. For Ni{111}, very clear changes occur in the vibrational spectra over the whole range of coverage up to saturation, and yet structural studies show that only 3-fold sites are occupied over the whole coverage range. The question then arises as to what causes the dramatic alteration to the vibrational spectra. Similarly, on Pt{111} two vibrationally distinguishable species apparently exist, with the higher coverage species completely replacing the lower coverage species as the NO exposure is increased. In this case, LEED at saturation coverage80 and DFT calculations35 again show that only a 3-fold site is occupied. Various theoretical studies have been performed to try to shed some light on these problems. Pe´rez-Jigato et al.48 performed DFT cluster calculations simulating the adsorption of NO on Pd{110}. The aim of the study was to investigate the influence of the adsorption site on the N-O stretching frequency. Previous data,47,81 had shown that NO adsorption on Pd{110} was always molecular at 300 K. From RAIRS data47 a vibrational band at 1644 cm-1 at low coverage was assigned to NO adsorption in a 2-fold bridge site on the unreconstructed (1 × 1) surface. This was consistent with near edge extended X-ray absorption fine structure (NEXAFS) studies82,83 which showed that NO was adsorbed upright on the surface in a 2-fold bridge site. The aim of this paper was to test these RAIRS assignments and DFT was used to calculate the vibrational frequency for NO at different adsorption sites on model clusters which represented the Pd{110} surface. The calculated vibrational frequencies and adsorption energies for NO in each of the available sites on Pd{110} are shown in Table 3.48 The experimental vibrational frequency of 1644 cm-1 is very close to the calculated frequency for the bridge site and this site also has the highest adsorption energy. In this case, it was therefore concluded that the vibrational assignments were correct. However, the calculated frequency differences between the atop, bridge and hollow site adsorption shown in Table 3 are quite small and hence it is clear that the vibrational frequency alone cannot necessarily be used to determine the adsorption site for NO on a metal surface. Further DFT calculations36,84 were performed for NO adsorption on Pd and Rh {111} and {100} surfaces. For the {111} surfaces, the aim of the calculations was to attempt to provide new theoretical frequency ranges that could be used to aid the interpretation of vibrational spectra on similar metal surfaces in the future. For Rh{111} and Pd{111}36 three different coverages of NO were investigated: 0.33 ML corresponding


Feature Article TABLE 4: Calculated Anharmonic Vibrational Frequencies, ωN-O, for NO on Pd{111} and Rh{111} for the Different Structures Investigated for Each of the (x3 × x3)R30°, c(4 × 2) and (2 × 2) Structures36 structure

site

Pd{111} ωN-O (cm-1)

Rh{111} ωN-O (cm-1)

(x3 × x3)R30°

atop bridge hcp 3-fold fcc 3-fold bridge + bridge fcc + hcp atop + 2 bridge atop + fcc + hcp

1812 1667 1588 1574 1688 1643 1800 + 1650 1801 + 1573

1812 1644 1544 1554 1683 1608 1798 + 1664 1845 + 1562

c(4 × 2) (2 × 2) Figure 3. Proposed 0.75 ML NO overlayer structure for NO on Pd{111} and Rh{111}. The (2 × 2) surface unit cell contains two 3-fold and one atop NO molecule. The structure was derived from structural studies87,88 and DFT calculations.36,84

to a (x3 × x3)R30° structure, 0.50 ML corresponding to a c(4 × 2) surface structure, and 0.75 ML corresponding to a (2 × 2) overlayer structure. Both of these systems had been previously characterized using vibrational techniques. On Pd{111}, no LEED pattern was seen at a coverage of 0.33 ML and the observed vibrational bands were assigned to NO adsorbed at a bridge site.49,85 The same site was suggested for the c(4 × 2) structure, formed at a coverage of 0.5 ML. The 0.75 ML (2 × 2) structure was thought to be a mixture of atop and bridge sites.49,85 For Rh{111},62 a 2-fold bridging species had also been assigned from vibrational spectra. This finding was now questioned by a temperature programmed static secondary ion mass spectrometry (TPSSIMS) study which suggested that NO is adsorbed at 3-fold hollow sites.86 Similarly, for the high coverage (2 × 2) structure of NO on Rh{111}, in early EELS work a structure with one atop and two bridge sites was deduced62 while a recent PED study gives a geometry with one atop and two 3-fold hollow sites.87 From the DFT calculations,36,84 it was concluded that, for both Pd{111} and Rh{111}, the 3-fold hollow fcc and hcp sites are the most stable for the 0.33 ML (x3 × x3)R30° model structure. On Pd{111}, the fcc hollow site was slightly more stable, and on Rh{111} the hcp hollow site was more stable. In both cases, the bridge site was 0.2-0.3 eV less stable than the 3-fold sites, in good agreement with the calculations for Pt{111}35 described earlier. For the 0.50 ML c(4 × 2) structure, a model with two NO molecules, one on an hcp hollow site and one on an fcc hollow site, was the most favorable combination investigated. The c(4 × 2) structure with two NO molecules in bridge sites was 0.2 eV per NO molecule higher in energy. The most stable (2 × 2) structure on both the Pd and Rh surfaces is with two 3-fold (one fcc and one hcp) and one atop NO molecule coexisting on the surface, as shown in Figure 3. For all of the coverages investigated the calculated structures are in disagreement with the original interpretation of the vibrational spectra, and in agreement with the more recent structural studies.87,88 For all of the structures just described, Lofredda et al.36 also calculated vibrational frequencies. The results are shown in Table 4. For Pd{111} at low coverage (the (x3 × x3)R30° structure) the calculated frequency for the most stable fcc 3-fold hollow site is 1574 cm-1, while that for the bridge site is 1667 cm-1. The experimental value50 is 1570 cm-1, showing good agreement with the frequency calculated for the fcc 3-fold hollow site. Similar good agreement was found for Rh{111}. At higher coverages, the experimental vibrational frequencies also agree well with the frequency calculated for the most stable adsorption sites (see Table 4). The theoretical calculations show

unequivocally that reassignment of the observed vibrational modes is required. We conclude that vibrational band assignment based on analogies to inorganic nitrosyl compounds have been misleading, and should be abandoned for NO. One aim of the theoretical studies of Pe´rez-Jigato and co-workers48 and of Lofredda and co-workers36 was to redefine the frequency ranges used for assignments in vibrational spectroscopy and to attempt to produce new, theoretical, frequency ranges that could be used to aid the interpretation of vibrational spectra on similar metal surfaces in the future. The favored frequency ranges which should be used, albeit with caution,48 in future N-O vibrational band assignments for adsorption on metal surfaces are the following: atop NO, 1700-1850 cm-1; 2-fold NO, 1640-1710 cm-1; 3-fold NO, 1450-1645 cm-1. On the basis of this new assignment range from Pd{110} and {111} and Rh{111} calculations, we can now reexamine the data discussed above for other surfaces. For NO on Ni{111},29,30 the bands observed at 1460 cm-1 at low coverage29 and that at 1475 cm-1 at slightly higher coverage29 are now reassigned to adsorption in 3-fold sites. The high coverage band at 1543-1585 cm-1 can also be assigned to the 3-fold hollow site. These reassignments bring the vibrational studies into excellent agreement with the SEXAFS,31 PED,32 and LEED studies33,34,80 for this adsorption system, all showing adsorption in the 3-fold site on Ni{111}. For Pt{111}, a band at 1476 to 1498 cm-1 at low coverage51,54,55 is now reassigned to NO in 3-fold sites. However at higher coverage, this band at low coverage on Pt{111}54 is replaced by a band in the range from 1700 to 1725 cm-1. This would now be assigned to adsorption in either a bridge or an atop site; this feature straddles the overlap region between atop and bridge sites in the above tabulation. However, LEED data for this system80 are supported by DFT calculations35 in showing that, at high coverage, NO is still adsorbed in 3-fold sites. This illustrates the caution that must still be used in vibrational assignments for NO on metal surfaces. It is clear that the best way forward for NO site assignments is to use a combination of techniques, including theory, to derive the correct structure for NO on a metal surface. This is demonstrated again in the following example. Many surfaces undergo reconstructions which can lead to difficulties with structural and site assignments. Clean Pt{110} is one such surface: it is reconstructed to a (1 × 2) missing row phase. The reconstruction is lifted on adsorption of NO at temperatures g230 K. NO adsorption on Pt{110} has been studied using a variety of techniques.56-60,89-95 Adsorption is mainly molecular and only a small amount of dissociation, possibly on defect sites, is observed. Gorte and Gland56 performed an EELS, temperature programmed desorption (TPD), and LEED study of NO adsorption on Pt{110}. Two

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J. Phys. Chem. B, Vol. 104, No. 12, 2000 2583

Figure 4. A schematic diagram showing the various structures that are formed on the Pt{110} surface with increasing coverage of NO. The exact nature of the structure depends on whether the (1 × 2) to (1 × 1) reconstruction is lifted.57 NO molecules are indicated by filled black circles.

species were identified on the surface, assigned to bridged and atop NO species. The bridged species was seen on the surface at low coverage, and with increasing coverage NO was forced to occupy atop sites, with only a small amount of bridged NO remaining at saturation coverage. Similar results were obtained by other workers.89-92,95 However, a reflection high energy electron diffraction (RHEED) investigation,94 in contradiction with other work, suggested that NO adsorbed in many different sites depending on coverage. On the (1 × 2) surface, bridged NO on the top layer was deduced, as well as NO adsorbed on hollow sites of the {111} microfacets. On the (1 × 1) surface, top layer atop sites were said to be occupied in addition to those occupied on the (1 × 2) surface. During the reconstruction it was suggested that many other sites were occupied. This study is in contradiction to all other studies where, in agreement with Gorte and Gland,56 only evidence for atop and bridge site occupation on the top layer Pt atoms was found for both the (1 × 2) and (1 × 1) structures. A photoemission study92 provided support for the same adsorption sites as the EELS data, although it was suggested that both bridge and atop sites were occupied over the whole coverage range. Very recently, detailed RAIRS and DFT studies of NO adsorption on Pt{110} were performed by ourselves over the temperature range from 30 to 400 K in an attempt to gain a better understanding of this adsorption system.57-60 It was found that over the temperature range from 90 to 300 K57 the adsorption of NO is very dependent on temperature and coverage. This is illustrated by the adsorption sequences shown in Figure 4. At 90 K, lifting of the (1 × 2) reconstruction to (1 × 1) is thermally inhibited and adsorption occurs solely on the (1 × 2) surface. In agreement with earlier EELS work,56 at low coverage there is a band at ∼1620 cm-1. With increasing coverage this band is replaced by a band at ∼1760 cm-1 which grows into the spectrum with increasing coverage. By direct comparison with the EELS data and with DFT calculations, these bands were assigned to bridged and atop NO, respectively, on the (1 × 2) surface. The atop site was only occupied at high

coverage when lateral repulsions between neighboring bridged NO molecules became larger than those between atop NO molecules.57 At 300 K, adsorption of NO leads to lifting of the (1 × 2) reconstruction back to the (1 × 1) phase.57-60 Again at low coverage, bridged NO is initially formed which is replaced with increasing coverage by atop NO. The atop species on the (1 × 1) surface has an infrared band at ∼1800 cm-1, whereas that on the (1 × 2) surface is at 1760 cm-1 (see Figure 4) and hence this frequency shift could be used to identify the temperature at which the restructuring begins. When a (1 × 2) surface is saturated with NO at 90 K and then annealed, the (1 × 2) to (1 × 1) restructuring begins at 230 K, but only at 350 K was very good (1 × 1) substrate order achieved. Experiments were also performed at very low temperature58,59 (30 K) where nonequilibrated adsorption occurs on the frozenin (1 × 2) surface. At these very low adsorption temperatures, thermal diffusion of adsorbate species is suppressed, leading to the occupation of metastable adsorption sites corresponding to local minima in the lateral potential energy surface (PES) for adsorption. A combination of experimental RAIRS measurements and DFT calculations allowed the identification of these adsorption sites to be made and the PES for adsorption of NO on Pt{110}-(1 × 2) was determined. Very low NO exposures at 30 K gives rise to four infrared bands58,59 at 1350, 1565, 1600, and 1670 cm-1. Only the band at 1600 cm-1 (attributed to adsorption in the bridge site on the top layer of Pt atoms) is seen at low coverage at higher adsorption temperatures; this is the global minimum in the lateral PES. All the other bands are due to the presence of metastable species. To help with the assignment of these vibrational bands, DFT calculations were performed for a wide range of possible adsorption sites on the Pt{110}-(1 × 2) surface, shown in Figure 5. The results of these calculations are shown in Table 5.60 The vibrational frequencies shown in the table are harmonic vibrational frequencies and have not been corrected for anharmonicity. They are therefore all higher than the experimentally observed frequencies. However, the trends in the frequencies are expected to be reliable


Feature Article

Figure 6. A potential energy surface for the adsorption of NO on Pt{110}-(1 × 2). The labeled adsorption sites are as shown in Figure 5. Reprinted with permission from ref 58. Copyright 1994 Elsevier Science. Figure 5. Possible adsorption sites for an NO molecule on the Pt{110}-(1 × 2) surface. Adapted from refs 58 and 60.

TABLE 5: Calculated Harmonic Vibrational Frequencies ωN-O and Adsorption Energies Ead for NO Adsorption at Various Sites on Pt{110}-(1 × 2) as Shown in Figure 560 adsorption site

Ead (kJ mol-1)

ωN-O (cm-1)

A B C D E F G H I

210 133 166 63 208 156 63 160 160

1818 1626 1765 1955 1750

and these agree well with the DFT calculations for NO on Rh{111} and Pd{111} performed by Sautet and co-workers.36 The calculated adsorption energies for each of the sites shown in Figure 5 are given in Table 5, showing clearly that the top layer bridge site (site A) is the most stable, in good agreement with experiment.56,58,59 Using the results in Table 5, the four infrared bands observed at very low coverage at 30 K were assigned as follows:58 1600 cm-1 band, bridge site on the top layer of Pt atoms (site A, Figure 5); 1350 cm-1 band, 3-fold site (site C); 1670 cm-1 band, adsorption on defects; and the band at 1570 cm-1, bridge site E or H. Having made these assignments it was possible to produce the lateral PES for NO adsorption on Pt{110}-(1 × 2) and this is shown in Figure 6. As expected, site A is the deepest minimum, sites D, F, and G are maxima (explaining why no adsorption occurs into these sites at 30 K), and sites H and C correspond to reasonably deep local minima. In addition to calculating vibrational frequencies and adsorption energies at low coverages on Pt{110}-(1 × 2),60 DFT calculations were also performed for higher NO coverages. The high coverage conversion to atop sites, concluded from vibrational studies,56-59 was not found, but much weaker repulsive interactions were seen between two adjacent atop NO molecules than between two bridged NO molecules. The apparent disagreement between experiment and theory is attributable to entropic effects not accounted for in the DFT calculations. On the (1 × 2) surface, relaxing this atop structure led to the formation of an (NO)x polymer species depicted in Figure 7. This has a considerably lower energy than two adjacent atop

Figure 7. A schematic diagram of the (NO)x polymer species which possibly forms at high coverage on the Pt{110}-(1 × 2) surface. This species is predicted from DFT calculations. Adapted from ref 60.

species, and provides an alternative assignment for the band observed at 1760 cm-1 on the NO saturated (1 × 2) surface. This study shows that it is possible to get detailed information about the structure of NO on a surface from vibrational studies, in conjunction with detailed DFT calculations. It shows the power of combining experiment and theory to give a greater overall understanding of a reaction system. 3. The NO Dissociation Process at Metal Surfaces On many surfaces, whether dissociation occurs depends on surface temperature, structure and coverage. Many surfaces show both molecular and dissociative adsorption, but some surfaces, notably Pd, show only molecular adsorption. By looking at the temperature and coverage regimes in which NO dissociation takes place, it is possible to get an insight into the NO dissociation process. Ni is a surface which is on the traditional borderline between purely molecular and purely dissociative adsorption, as seen in Figure 1. In general, molecular adsorption takes place on Ni at low temperatures, and at higher temperatures both molecular and dissociative adsorption are observed. On Ni{100}, various studies have been performed which have shown that initial NO adsorption is dissociative at temperatures above 200 K and, once a critical coverage has been achieved, subsequent adsorption is molecular.96-106 Adsorption at lower temperatures leads only to molecular adsorption, but heating the surface causes dissociation.96,99-101,103-105,107 It was suggested that a highly tilted, possibly lying down, NO species deduced from XPS data may be the precursor to NO dissociation on Ni{100}.104 TPD studies96,97,101,103 showed only recombinative N2 desorption from the dissociated state. The N2 peaks shifted down in temperature with increasing coverage.97 Molecular adsorption occurs above a critical O + N adatom coverage, and this has been associated with large repulsive lateral interactions between

Feature Article

Figure 8. The heat of adsorption of NO adsorbed on Ni{100} as a function of coverage at 300 K. There is a marked change in the slope of the curve at 0.16 ML coverage indicating a change from molecular to dissociative adsorption. Adapted from ref 105.

the N and O adatoms on Ni{100}.101,105,106 This was clearly demonstrated in a study of the calorimetric heat of adsorption of NO on Ni{100}.105,106 The dependence of adsorption heat on coverage is shown in Figure 8. The initial heat of adsorption is 426 kJ mol-1 and is attributable to dissociative NO adsorption. With increasing coverage the heat decreases sharply due to repulsive interactions between N and O adatoms. Monte Carlo simulations of the adsorption heat data105 showed that the pairwise repulsive interaction energy between next nearest neighbor O atoms on the Ni{100} lattice is 40 kJ mol-1, and that between N and O atoms and between N atoms is 100 kJ mol-1. At high coverage, repulsions between adatoms are so large that dissociative adsorption is no longer energetically favorable: above ∼0.16 ML the differential adsorption heat for molecular NO is larger than for adsorbed N and O atoms. Molecular NO is thermodynamically more stable than further dissociated NO at higher coverages. For Ni{110}108,109 and Ni{111}29,31-34,44,45,69,73,75-78,110 initial adsorption is dissociative and subsequent adsorption is molecular at 300 K, as for Ni{100}. For Ni{111}, a molecular layer adsorbed at low temperature was observed to dissociate on heating and low NO coverages dissociated more easily than high NO coverages. These observations for Ni all suggest that NO dissociation has an empty site requirement, suggesting a twostep dissociation mechanism whereby NO first adsorbs, and then reorients to bring the O end into close proximity with the surface, forming a side-on NO precursor, which may lead to N and O adatoms. This requires at least two adjacent empty sites, showing why dissociation is inhibited at high coverages. Similar results have been observed for NO adsorption on Ru,111-129 Ir,130-137 and Rh.61,62,86,87,138-174 On Ru{100}128 NO rapidly dissociates at 373 K to give a c(2 × 4) LEED pattern at lower coverage, followed by a (2 × 1) LEED pattern at high coverage. The O and N adatoms produced in the dissociation process were suggested to occupy separate islands on the surface. Heating the surface to 473 K leads to thermal desorption of N2. At saturation coverage, further dissociative adsorption was not possible and the NO molecule was seen on the surface. Ultraviolet photoelectron spectroscopy (UPS) studies of the same adsorption system gave similar results.129 On Ru{001}, adsorption at room temperature was dissociative initially, and again further adsorption was into a molecular state at higher coverages. A (2 × 2) LEED pattern is formed.111,114 NO and N2 are the main species observed in TPD, and a 3-fold molecular NO species was thought to be the precursor to

J. Phys. Chem. B, Vol. 104, No. 12, 2000 2585 dissociation.111,118,125 The dissociation products were thought to occupy the same surface sites as the 3-fold NO.111,127 Adsorption at temperatures below 200 K was purely molecular. Adsorption of NO on Ir{100} is complex.130,131 The stable clean Ir{100} surface is a (1 × 5) reconstructed phase, where the top layer of Ir atoms adopts a pseudohexagonal structure. Adsorption of NO leads to the lifting of this reconstruction back to the (1 × 1) phase. Detailed investigations130 showed that adsorption on the metastable (1 × 1) surface is dissociative at low coverage at 300 K, forming a p(2 × 2) LEED pattern due to an ordered overlayer of O adatoms. Subsequent adsorption at 300 K is molecular. Adsorption on the (1 × 1) surface at 90 K does not lead to dissociation, and TPD shows that no dissociation occurs on this surface at temperatures less than 200 K.130 The (1 × 5) surface, on the other hand, did not show dissociation at all at 300 or 90 K. This important result is in keeping with the general observation that the stable hexagonal surface phases of Ir{100} and Pt{100} have low dissociative sticking probabilities for diatomics, whereas the metastable (1 × 1) phases of these surfaces are very reactive for the dissociation of O2, H2, and NO. Clean Ir{110} also undergoes a reconstruction to a (1 × 2) phase. At temperatures of 300 K and above, dissociative adsorption was also observed on this surface.132,133 Ir{111} has been studied more extensively than the other low index Ir surfaces.130,133-137 The general consensus is that initial adsorption at 300 K is dissociative and subsequent adsorption is molecular. Scattering studies135,136 showed a marked change in the dynamics at 300 K, caused by the onset of NO dissociation. No translational or vibrational energy effects on dissociation were observed,134 suggesting that coupling to the surface via a molecularly chemisorbed state was required to lead to dissociation. EELS studies134,137 suggested that a species with a vibrational band at 1860 cm-1 (almost certainly an atop species) is the precursor to dissociation. Other bands are also seen at 1480 and 1550 cm-1, but these species simply desorb as molecular NO on heating.134,137 NO adsorbs molecularly on Rh at low temperatures and dissociatively at higher temperatures. On Rh{100}, molecular NO dominates upon adsorption at 100 K, but heating leads to N2 and O2 production in TPD, suggesting dissociation.138,139 For low NO coverages, only dissociation products are seen in TPD but at higher coverages, dissociation is blocked and molecular NO desorption occurs. The surface decomposition of NO adsorbed at low temperatures, to give N and O atoms, occurs at ∼160 K.140 Coadsorption of NO and K141,142,175 reduces the activation energy for the dissociation process, but the rate of dissociation is actually lower due to a decreased preexponential factor. The activation energy for dissociation was thought to be lowered because the presence of K on the surface stabilized a “lying-down” NO species, thought to be the precursor to dissociation.140-144,175 All studies of NO adsorption on Rh{110} show that dissociation occurs at temperatures above ∼170 K, and after a certain coverage of N and O adatoms has been achieved, subsequent adsorption is molecular.148-150,153-155,157-161,167 At low temperatures, molecular NO is always observed. Subsequent heating leads to dissociation and it was found that the fraction of undissociated NO that desorbs increases with increasing initial NO coverage.149 A detailed molecular beam study150 showed that the initial sticking probability for NO dissociative adsorption at 300 K is 0.67. The variation of sticking probability with coverage is indicative of precursor mediated adsorption.158 At low NO coverages, only N2 desorbs at ∼ 650 K, but with

2586 J. Phys. Chem. B, Vol. 104, No. 12, 2000 increasing NO coverage there is a new, low temperature, N2 TPD149,153,159 state. This more weakly bound form of N atoms at high coverages was thought to arise from large repulsive interactions between the adatoms. The dissociation of NO obeys first order kinetics161 with very low kinetic parameters being found. More recent studies158,160,161,163,167 have shown that the N and O adatoms produced by NO dissociation on Rh{110} lead to adsorbate induced restructuring of the surface. The low kinetic parameters measured for NO on Rh{110} probably arise due to these adsorbate induced reconstructions.161 On Rh{111}, as for the other Rh surfaces, again both molecular and dissociative adsorption occurs depending on both temperature and coverage.61,86,169-174 The surface NO dissociation reaction is a first order process.61 After molecular adsorption to saturation coverage, some desorption is required before dissociation can take place, indicating an empty site requirement. Theoretical simulations172 were made which support the notion that NO dissociation was inhibited by NO, N, and O adspecies and that there were strong repulsions between NO and N atoms. This does account both for the decrease in NO dissociation at high coverage and also for a low temperature N2 TPD peak which grows in with increasing coverage of NO.86,172 NO dissociation on Rh{111} was only observed above ∼250 K,61,86 a much higher temperature than that for either Rh{100} or {110}. The uptake of NO onto Rh{111} is precursor mediated, with an extrinsic precursor state.86 Coadsorption of NO and K170,171 led both to increased NO uptake and also to increased amounts of NO dissociation on the surface. At very high K precoverages, the NO molecule was stabilized and no dissociation was observed.170 In all of these cases adsorption at low temperatures is purely molecular. At higher temperatures adsorption is dissociative at low coverage, and subsequent adsorption is molecular. Here it is important to note the conclusion reached by Vattuone, Yeo, and King,105,106 that for NO on Ni{100} no dissociation occurs above an adatom coverage of ∼0.2 ML for thermodynamic reasons: due to adatom interactions the molecule is more stably bound than the adatoms at higher coverages. Nevertheless, consideration should be given to the dissociation mechanism based on the need for vacant sites to facilitate the production of N and O adatoms on the surface. This is interpreted as a two-step mechanism, as already described for Ni. Surfaces on which we expect purely dissociative adsorption to take place (according to Figure 1) include Mo, W, and Co. Mainly dissociative adsorption has also been observed on Re.176-180 For Mo,19-22,181-183 only a very small amount of molecular NO adsorption is seen at high coverages at room temperature and the presence of dinitrosyl species has been suggested on Mo{110} at low temperatures, as discussed in the next section. On W surfaces, complete dissociation takes place at all coverages at 300 K,184-194 although in some cases a small coverage of molecular NO has been characterized at 300 K at very high coverages.186-188 N2O formation has been seen on W surfaces, especially on O or C precovered surfaces.184,185,188 There are no studies of the NO dissociation mechanism on either Mo or W surfaces. Recently, a detailed RAIRS study of NO adsorption on Co{101h0}65 has been performed which has shed some light on the NO dissociation process on this surface. Dissociation is only initiated at surface temperatures g150 K. At temperatures up to ∼270 K, molecular NO is formed at high coverages after dissociation has terminated, but unusually, on heating NO dissociation is always complete.65 TPD spectra showed that the only gas-phase product from NO adsorption was N2; no NO or

Feature Article N2O were detected. The absence of NO desorption signifies complete dissociation of NO at all coverages. The influence of temperature on NO dissociation is, however, strongly dependent on the initial coverage of NO. For low NO coverages, dissociation occurs at 150 K, but for surfaces saturated with NO at 100 or 200 K, the NO monomer is stable up to 250 K, as illustrated by the spectra shown in Figure 9. NO adsorbed to only 1 L exposure at 100 K dissociates completely when the temperature is increased to 200 K. However, for a 4 L exposure at 100 K (Figure 9b) the infrared spectra remains unchanged even at 250 K. As expected, the extent of NO dissociation at low exposures depends on the temperature at which adsorption takes place and dissociation was favored at higher adsorption temperatures. Two reaction mechanisms have been discussed to explain NO dissociation on Co{101h0}.65 The first is the two-step process already described, which appears to be relevant for NO dissociation on Ni, Rh, Ru, and Ir. This type of dissociation process is illustrated in Figure 10. The dissociation intermediate is a “side-on” NO species, as seen on Rh{100}140-144 and Ni{100}.104 No side-on NO species was seen on Co{101h0}, although of course it is possible that the species does exist, with a lifetime too short for it to be observed using RAIRS. In fact, coadsorption with K allowed the observation of a side-on NO species on Co{101h0}.65 The other possible NO dissociation mechanism is one in which dissociation is regarded as a first-order unimolecular process. Upright molecular NO is the most stable state on the surface, but given enough thermal excitation the NO molecule tilts or rotates to produce a lying down transition state which can dissociate. Since dissociation is not inhibited at high coverages, but it requires a higher temperature, a strong dependence on coverage of the activation energy for the dissociation process is suggested. This could be due to the inhibition of hindered translational or rotational vibrational motion as the surface adlayer becomes crowded. In conclusion, on Co, Ni, Rh, Ru, and Ir surfaces, adsorption of NO is molecular at low temperatures and dissociative at higher temperatures. On Ni, Rh, Ru, and Ir, the dissociation process is blocked at high coverages, usually attributed to an empty site requirement and indicating a two step dissociation process as shown in Figure 10. On Co{101h0}65 it has been demonstrated that the activation energy for surface dissociation is strongly dependent on surface coverage. 4. Dimers and Their Reactions As discussed already, NO adsorption can lead to dissociation to give Na and Oa on the surface, or to adsorption of the chemisorbed monomer, or to a combination of both of these. In addition to this, NO dimers, (NO)2, can also be formed at the surface. NO dimer formation on transition metal surfaces is important because of the potential for such interactions to lead to N-N bond formation in the catalytic reduction of the NO molecule. The first study to conclusively identify (NO)2 on a surface was by Brundle and co-workers195,196 for the adsorption of NO on Ag{111} using XPS and TPD. NO was adsorbed on the surface at 25 K, and three NO states were identified. The states were assigned to bridge-bonded NO monomers, to NO interacting with Oa and also to physisorbed NO.195 It was concluded that the physisorbed multilayer NO exists as NO dimers, (NO)2, as in solid NO. This was confirmed by a theoretical study196 which showed that intense satellite features seen in the XPS

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Figure 9. RAIRS spectra showing the different affect of heating on (a) low and (b) high NO exposure adlayers formed on Co{101h0} at 100 K.65

Figure 10. Schematic diagram of the two-step NO dissociation process involving a lying down NO species which is the precursor to dissociation. Adapted from ref 65. This process is thought to be active on mnay surfaces including Ni, Rh, Ru, and Ir.

spectra from condensed NO multilayers on Ag{111} could only be explained by the presence of (NO)2. The desorption and decomposition of the adsorbed NO was monitored by XPS and TPD.195 The multilayers of (NO)2 desorbed at 60 K, and N2O formation was seen to occur. There are two possible mechanisms by which N2O can be formed on a surface:

NOa + Na f N2Oa

(1)

NOa + NOa f (NO)2a f N2Oa + Oa

(2)

N2O formation via reaction 1 was excluded by Behm and Brundle on Ag{111} because they did not find any evidence for N adatoms on the surface. They concluded that the

formation of N2O must proceed either directly from two adjacent NO molecules, or via an unstable intermediate such as [O‚‚‚N‚‚‚NO]. In a later study of the adsorption and reactions of NO on Ag{111}, the formation of N2O on the surface was observed at temperatures as low as 80 K.10,12 In addition, studies of NO adsorption on Ag layers on Ru{0001} showed N2O formation at 90 K.197 So et al.10,12 studied the adsorption of NO on the Ag{111} surface using EELS and TPD. At ∼90 K, many vibrational bands were observed in the EELS spectrum. These were assigned to the presence of two types of 3-fold NO monomers, atop NO, adsorbed N and O atoms and also N2O. At saturation, all of these species coexist on the surface. Neither NO2 or (NO)2 were observed at the temperatures and exposures investigated. Since no (NO)2 was identified on the surface, formation of N2O via reaction 1 was proposed, although they could not rule out reaction 2. This was in disagreement with the previous observations of Behm and Brundle.195 The reactivity seen for NO adsorption on Ag{111} at 80 K10 was very surprising, as many scattering studies at higher temperatures (300 K) showed that the interaction potential of NO with the Ag{111} surface is very weak and there is no reaction between NO and the surface.198-204 An ARUPS study


Figure 11. A series of RAIRS spectra showing the formation of N2O from (NO)2 on the Ag{111} surface.9 The (NO)2 is characterized by the band at 1863 cm-1, and on heating to 70-90 K this band disappears and is replaced by bands due to N2O at ∼2230 and ∼1250 cm-1.

also confirmed that NO did not react with Ag{111} at 300 K.205 Later work on the adsorption of NO on Ag{111} resolved this apparent anomaly.9,15,16 Ludviksson and co-workers16 performed a very detailed study of NO adsorption on Ag overlayers on Ru{0001} at 75 K, using different NO isotopes to allow them to conclude whether NO dissociated prior to forming N2O on the Ag surface. A previous study had shown that NO adsorption on Ag/Ru{0001} was identical to that on Ag{111}.197 Detailed TPD studies showed that N2O formation went via an (NO)2 intermediate, in complete agreement with the original studies of Behm and Brundle.195 The anomaly in NO nonreactivity at 300 K and N2O formation at 90 K on Ag{111} was finally resolved by Brown and co-workers.9,15 Combined RAIRS and NEXAFS studies were performed over the temperature range from 40 to 300 K. At 40 K only one band, with a frequency of 1865 cm-1, is seen in the infrared spectrum at low coverages. With increasing coverage, two bands grow to dominate the spectrum, at 1788 and 1863 cm-1. These bands were assigned to the presence of multilayer physisorbed (NO)2 on the surface; the frequencies show good agreement with the gas-phase frequencies for (NO)2. The two bands for physisorbed (NO)2 were assigned to the symmetric and asymmetric stretches of the NO dimer. Heating the surface led to desorption of the dimer multilayers at 60 K, leaving only one band in the infrared spectrum, at 1863 cm-1, assigned to first layer adsorbate. Further heating led to the disappearance of this band and the appearance of two bands at 2229 and 1252 cm-1 (Figure 11), which were assigned to the formation of adsorbed N2O.9 Hence, the species giving rise to a band at 1863 cm-1 converts to N2O with increasing temperature. All that remained was to identify this species. This was achieved by the coadsorption of 14NO and 15NO on the Ag{111} surface.9 If the species was an NO monomer, then coadsorption of an equal mixture of 14NO and 15NO would lead to two bands in the infrared: one due to 14NO and one at a lower frequency due to 15NO. However, if the species was (NO)2, then three bands would be seen due to 14NO14NO, 14NO15NO, and 15NO15NO dimers in the ratio of 1:2:1. Figure 12 shows the results of the coadsorption of an approximately equal mixture of 14NO and 15NO at 90 K. Very clearly there are three bands in the infrared spectrum, and therefore the species which converts into N2O was conclusively shown to be

Feature Article

Figure 12. A spectrum showing the result of the coadsorption of an equal mixture of 14NO and 15NO on Ag{111} at 90 K. Clearly three bands are present in the infrared spectrum indicating the presence of NO dimers, (NO)2.

TABLE 6: Calculated Adsorption Energies Ead and Spin Polarization for an NO Monomer Adsorbed on Ag{111} in Different Sites207 site

Eads (kJ mol-1)

spin polarization

3-fold fcc 3-fold hcp 2-fold bridge atop

65.6 62.7 59.8 45.3

0.90 0.96 0.87 0.95

(NO).29 Furthermore, NEXAFS studies15,206 showed the anticipated splitting of the π* resonance for the dimer species, and the results demonstrate that the dimer is bonded with the N-N axis parallel to the surface with the N-O axis tilted by 30° with respect to the surface normal, providing an explanation for the facile removal of an O atom to produce N2O. This study provided the first conclusive evidence for the presence of (NO)2 on a metal surface in the monolayer regime. It is now generally accepted that N2O formation on Ag{111} occurs via an (NO)2 intermediate. This was recently confirmed in an XPS study of NO and CO coadsorption on Ag{111}.17 The driving force for (NO)2 dimer formation on the Ag{111} surface was recently demonstrated by a DFT slab calculation.207 Table 6 shows the adsorption energies and spin-polarization for NO adsorbed on Ag{111} in various possible sites. The most stable adsorption site for the monomer is the fcc 3-fold hollow site. More interesting is the fact that the NO monomer retains 90% of the spin density of the free molecule: i.e., it has an unpaired electron in its 2π* orbital. Adsorbing two NO molecules, one on an fcc site and one on an adjacent hcp site, resulted in the loss of the spin density for the adsorbed NO and a π bond was seen to form between the two NO molecules i.e., an (NO)2 dimer was formed. The calculations therefore showed that the dimer forms on Ag{111} because the adsorbed monomer retains an unpaired electron. NO dimers have also been observed on Cu surfaces. The unusual activity of Cu surfaces toward NO adsorption was first observed by Roberts and co-workers.13,208,209 On Cu films,208 NO monomers and dissociated N and O atoms were seen. There was no evidence for N2O formation. In contrast, when NO was adsorbed on Cu{100} and Cu{111} surfaces13,209 N2O formed. The presence of adsorbed N adatoms on the surface suggested

Feature Article that N2O was formed via the reaction of NOa with Na, rather than via (NO)2. Similar results were obtained in EELS studies by Wendelken11,14 and by Balkenende and co-workers.210,211 In all cases, large amounts of NO dissociation were seen, and both molecular and dissociated NO coexisted on the surface. N2O formation was always assumed to go via NO dissociation (reaction). So et al.12 also studied the adsorption of NO on Cu{111} and observed small amounts of N2O formation which was again assumed to be via the dissociation of NO. Other studies of NO adsorption on various Cu surfaces have also shown N2O formation.212,213 However, none of these studies provided any evidence for the mechanism of N2O formation on the Cu surface. More recently, evidence has been presented for the presence of (NO)2 on Cu{110},8 and Cu{111}.7 Brown and co-workers performed a combined RAIRS, TPD and molecular beam study of the adsorption of NO on Cu{110}.8 The interaction of NO with Cu{110} was observed to be complex and strongly dependent on temperature and coverage. At low coverages, NO monomers are observed on the surface, followed at higher coverages by the formation of (NO)2, still in the monolayer regime. At the same time, bands due to N2O form in the infrared spectrum. Subsequent adsorption leads to the formation of physisorbed multilayers of (NO)2 at low temperatures.8 Unlike the case for Ag{111}, the direct conversion of (NO)2 to N2O was not observed, but the presence of (NO)2 on the surface was strongly suggestive of the dimer as the intermediate to N2O formation. Evidence for NO dissociation was also obtained, but the molecular beam investigations suggest that N2O formation is exclusively via (NO)2. Unlike the case for Ag{111}, at low coverages the NO monomer is stable with dimer formation only taking over at high exposures. Similar observations were made by Dumas and co-workers7 for the adsorption of NO on Cu{111}. They performed RAIRS experiments over the whole frequency range from 200 to 2500 cm-1. At low temperatures, they observed monomeric NO adsorption, probably in 3-fold hollow sites, as well as dissociative adsorption. They were able to observe an anti-absorption band in their infrared spectrum which could be attributed to the dipole-forbidden hindered rotation of the NO molecule. This is the only example of the observation of a dipole forbidden mode for NO adsorption, although the same workers have made similar observations for CO adsorption.214-216 The theory of this process has been discussed.217-223 N2O forms at high coverages of NO, and (NO)2 is formed at even higher coverages (greater than one monolayer). Again it was assumed that N2O formation occurred via the (NO)2 dimer reaction pathway. Model Hamiltonian calculations224 and two photon photoemission experiments225 for NO adsorption on Cu{111} suggest that as with NO on Ag{111},207 the NO monomer retains its unpaired electron when adsorbed on the Cu surface. This strongly suggests that (NO)2 formation is favored on Cu{111}. By the rules established by Rhodin and co-workers,5 Mo is a metal on which we only expect NO dissociation to occur. It is therefore somewhat surprising that dinitrosyl species form on the surface of Mo{110}.19-22 Queeney and Friend19 performed a detailed investigation of the adsorption of NO on Mo{110} using EELS, RAIRS, and TPD. As expected, the results showed that the main reaction pathway for NO on Mo{110} was dissociation to atomic N and O, even upon adsorption at 90 K. At coverages less than 65% of saturation coverage no other reaction was observed on the surface. At higher coverages however, TPD showed the production of N2O in the gas phase below 300 K, and a small amount of molecular NO was also


Figure 13. Comparison of (a) RAIRS and (b) EELS data obtained for NO adsorption on Mo{110} at 100 K.19 Reprinted with permission from ref 19. Copyright 1999 American Institute of Physics.

Figure 14. The suggested structure of the dinitrosyl species thought to be formed on Mo{110}.19

observed. The reaction pathway to produce N2O only became active at coverages above 65% of saturation coverage. To try to determine the reaction pathway to produce N2O on Mo{110}, Queeney and Friend19 performed TPD studies of isotopically mixed NO overlayers. The aim was to determine whether dissociated adatoms were incorporated into the desorbing N2O. They found that N2O was predominantly formed from the reaction of intact NO molecules - i.e., probably via some sort of dimeric or dinitrosyl intermediate. EELS and RAIRS investigations supported the findings from TPD. EELS spectra at saturation coverage showed vibrational bands in the NO stretch region, as well as low-frequency losses due to the production of N and O adatoms on the surface. Heating the surface caused the atomic losses to grow, indicating that dissociation increases with increasing temperature. The molecular NO species remained on the surface up to 600 K. To determine the exact nature of the species observed at saturation coverage in the EELS spectrum, RAIRS was used. The higher resolution of this technique allowed the identification of additional peaks to those seen in EELS. Figure 13 shows the presence of three infrared bands, rather than the two bands from EELS. The bands were identified by adsorbing mixed overlayers of 14NO and 15NO. The results were attributed to two monomeric NO species, and one dinitrosyl species, like that shown in Figure 14. Both NO molecules are bonded to the same metal atom and no N-N bond is formed, unlike (NO)2 dimers. However, we note that on a flat surface, such as the bcc{110}, this species would be strongly sterically hindered, which raises strong doubts about its existence. The band was assigned to a dinitrosyl rather than an (NO)2 dimer by comparison with observations on molybdenum-based catalysts, which suggested the presence of dinitrosyl species.226 Since N2O formation was shown by TPD to proceed via intact NO molecules, it was assumed that the dinitrosyl species was a possible intermediate in the formation of N2O on Mo{110}. However, on a flat metal surface a dimeric species would be a better assignment. In addition to the formation of a dinitrosyl species, Queeney and co-workers22 do suggest that a dimer also forms on oxidized


Figure 15. A schematic illustration of orbital mixing in NO chemisorption on transition metals, based on DFT slab calculations for NO on Pt{111}.35

Mo{110}, bonded more strongly to the surface than on either Ag{111} or Cu{111}. This dimer is reported to be formed by metal-mediated NO coupling, interacting strongly with the surface via one of its NO molecular components. This (NO)2 species was thought to be less stable than the dinitrosyls formed on the same surface. Vibrational bands at 1871 and 1728 cm-1 were assigned to this dimer species. A normal-mode analysis was performed to test the validity of these assignments and gave good agreement with the observed vibrational modes. Tungsten is another surface on which we expect NO adsorption to be completely dissociative. However, Baldwin and Friend184,185 have shown that, on an oxygen, or carbon,184 precovered W{100} surface, N2O and N2 formation are observed. Again, experiments with mixed isotopes of NO suggested that N2O formation did not occur via the dissociation of NO, but rather via an (NO)2 species. No evidence was seen for an (NO)2 species on the surface and it was therefore concluded to be an unstable intermediate. It was suggested that the presence

Feature Article of preadsorbed O or C adatoms inhibited NO dissociation on the surface, thus making the (NO)2 pathway to N2O formation more probable. The above are the only adsorption systems where the (NO)2 dimer has been identified in the submonolayer regime on the surface, either as a stable or an unstable intermediate to N2O formation. However, various other researchers have suggested the presence of (NO)2 dimers on surfaces, both in the physisorbed and the chemisorbed state. Physisorbed multilayer (NO)2 dimers have been identified on Pd{111} at 20 K using both vibrational spectroscopy and UPS.49,227 They have also been identified on Nb{110}228 at 20 K and Pt{111} at 25 K53 using vibrational spectroscopy. All of these were identified by comparison of the observed vibrational frequencies with those of gas and solid phase (NO)2. Chemisorbed (NO)2 dimers, usually at high NO coverages, have been suggested without direct evidence on Pd{100},229 Pd{110},47 and Pt{111}.55 However, the presence of (NO)2 on Pt{111} has since been disproved by Hayden.54 A theoretical study has also suggested that (NO)2 dimer formation on Rh surfaces is viable,230 although there is no current experimental evidence for this. 5. NO Bonding to Metal Surfaces It is useful to summarize the current bonding picture for NO on metals. Over the past few years, theoretical advances and developments in computational capabilities have produced a reliable first principles methodology for calculating surface adsorbate structures (including restructuring of the substrate), energetics and bonding.231-233 In particular, slab calculations using spin DFT with gradient density corrections, deploying a conjugate gradients minimization scheme234 have produced remarkably good agreement with experimentally determined structures and energetics.232 Significantly improved understanding has emerged from these calculations concerning the electronic structure relevant to chemical bond formation at surfaces.231,232 Hu et al.231 demonstrated that the spatial distribution of individual eigenstates can be used to characterize the main bonding features of a chemisorption system. For CO on

Figure 16. Perpendicular cuts through a monomeric NO molecule chemisorbed on an fcc hollow site on Ag{111} from DFT slab calculations, showing (a) the total charge density and (b) the spin density.207 Reprinted with permission from 207. Copyright 1999 Elsevier Science.

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Figure 17. Groups VIA-IB metals showing a summary of the NO adsorption state and site assignment at low coverage. This is an alternative to Figure 1.

transition metals the bonding is dominated by 2π*-d and 5σ-d mixed bonding states, where the 2π* and 5σ molecular orbitals of the molecule have picked up strong d character from the metal states, and the antibonding mixed states lie above the Fermi level and are therefore essentially empty. In particular, Hu et al. noted, following Hoffmann and co-workers235 that, as the transition metal substrate is switched from Cu at the right of the Periodic table to Fe to the left, the d band energy levels shift upward. This results in an increased overlap of metal d states with the CO 2π* orbital, leading to an increased population of Bloch states with 2π-d character and hence a strengthening of the CO-metal bond. This also leads to a weakening of the C-O bond, and therefore an increase in the propensity to dissociate. This underlies the behavior indicated in Figure 1 of this review, for CO and NO. In Figure 15 we show a schematic illustration of the orbital mixing model for NO chemisorption on a metal surface, derived from calculations for NO on a Pt{111} surface.35 For the sampling of mixed orbital states shown, it is clear that the antibonding 2π*-d mixed state lies well above the Fermi level, Ef, and that significant bonding therefore derives from the 2π*-d bonding Bloch states. The strength of this overlap is increased with metals to the left of Pt and decreased with metals to the right, in an analogous fashion to CO. Hence monomeric NO dissociates on W, but not on Ag, in agreement with observation. It is important to note that in the strong overlap case, the chemisorbed NO loses its spin identity.35 From the quantum chemistry viewpoint, we also note no essential difference between the bonding of NO and CO to transition metals, and we conclude that it is misleading to describe the properties of adsorbed NO as “amphoteric”, as a means of distinguishing its properties from CO. In both cases, depending on metal, crystal plane and adsorption site, the direction of net charge flow can be either into the molecule or into the metal. What does

distinguish NO chemisorption from CO is its propensity for binding in 3-fold sites, in the case of weak as well as strong adsorption. In the case of weak bonding a clear distinction with CO does arise, because here the molecule retains almost a full spinunpaired electron (0.9e) in the NO 2π orbital.207 In Figure 16 we show a comparison of the total charge density for adsorbed NO on Ag{111}, in the favored fcc hollow 3-fold site, with the spin density. The latter clearly shows the spatial characteristics of the 2π molecular orbital on NO. When surface dimers are formed, the calculation demonstrates no spin density, due to bond formation between monomers; these π-type orbitals show mixing with the d orbitals. Recent calculations by Pe´rez-Jigato and King236 show that this mixing with metal states strengthens the bonding between NO monomers in the dimer pair compared with that in solid (NO)2. As discussed in this review, these calculations are in full agreement with experimental results. 6. Summary and Conclusions In this review we have discussed some recent studies of NO adsorption on surfaces which have altered our ideas. Structural studies in particular have shown that great care must be taken when assigning vibrational frequencies to a particular adsorption site for NO. DFT calculations also help the study of surface structure. For Ni{111}, RAIRS and EELS data29,30 suggested that NO occupied bridge sites at all coverages, whereas structural studies31-34,80 now show that NO actually occupies 3-fold sites on the surface. Theoretical calculations on the same adsorption system support these findings.45 Other NO adsorption systems that have been investigated theoretically using DFT calculations include Pt{111},35 Pd{110},48 Rh{111} and Pd{111}36,84 and Pt{110}.60 For the {111} surfaces again there is general disagreement with the original experimental vibrational assignments, with the exception


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Figure 18. The melting points of the transition metals, and rare earths, versus atomic number. On this diagram we also show a suggested correlation between the propensity for a metal to dissociate NO, and its melting point.

of Pd{110} where there is good agreement between experiment and theory. In all of these cases the DFT calculations have provided an understanding of the bonding between NO and a metal surface, for the first time. The calculations all show that 3-fold NO has a lower vibrational frequency than 2-fold which has a lower vibrational frequency than atop, as expected. Unexpectedly the frequency ranges for each of these depend on the surface on which adsorption occurs. From these studies we have provided a new basis for assignment, which must nevertheless be used with caution. For Pt{110}, combined experimental and theoretical studies were performed57-60 which together provide a good understanding of the adsorption system, with consistent vibrational assignments and a determination of the lateral PES for NO adsorption throughout the unit cell. The NO dissociation process has also been discussed. For most surfaces where dissociation takes place, the amount of dissociation is highly coverage dependent with dissociation usually becoming inhibited at high coverages. This is either due to a lack of vacant sites or to a thermodynamic switch to molecular NO as the most stable species. At low temperatures, molecular adsorption takes place and at saturation coverage desorption is usually required before dissociation can take place on heating. These observations indicate a vacant site requirement for NO dissociation, in accordance with a possible two-step mechanism for NO dissociation in which NO first adsorbs on the surface, and then reorients to allow the O atom to come into contact with the surface (possibly via a lying down NO precursor state) before dissociation takes place. It is clear, however, that greater attention needs to be paid to the NO dissociation process in order to understand it in detail.

The simple picture represented by the dividing line between dissociative and nondissociative NO adsorption down the Periodic Table, shown in Figure 1, can now be revisited. In Figure 17 we show group VIA-IB metals together with a summary of NO adsorbate state and site assignment at low adsorbate coverages. On all the group VIIIA transition metals, nondissociatively adsorbed NO is formed at low temperatures; and on heating dissociation is initiated at temperatures between 150 and 300 K, with Pt surfaces most resistant to dissociation. On group IB metals, monomeric NO adsorption is weak and nondissociative; but at low temperatures dimers can be formed, and these dimers are reactive, producing for example N2O and O adatoms at temperatures above ∼60 K. In Figure 18 we show a suggestive correlation between the propensity for dissociation of the NO monomer at low coverage and the melting points of the transition metals and rare earth elements. The melting points are a direct measure of the cohesive energies of the elements, and the peaks at Cr, Mo, and W correspond to roughly half occupancy of the d bands for these metals. The higher the metal cohesive energy, the greater is the propensity for NO dissociation. This is correlated both with the strength of the NO metal bond, and hence the correlated weakening of the N-O bond, and the strength of the metal-O and metal-N bonds. The dissociation may be kinetically constrained on some metals, but it may also be thermodynamically excluded. We have also examined the bonding of NO to metal surfaces. From the studies highlighted in this article, and others, it is clear that theoretical calculations are playing an increasing role in our understanding of NO adsorption on surfaces. The interplay

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