Surface structural information carried by desorbing reaction products

Progress in Surface Science 82 (2007) 435–477 www.elsevier.com/locate/progsurf

Review

Surface structural information carried by desorbing reaction products Tatsuo Matsushima

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Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan

Commissioning Editor: P.A. Thiel

Abstract Recent progress in angle-resolved measurements of desorbing surface reaction products is reviewed. The angular and velocity distributions of desorbing products with hyper-thermal energy deliver the most direct structural information of the product formation site. These distributions yield the orientation of the intermediate species emitting the product as well as the shape of the product formation site. This method works well even when the overall reaction rate is controlled by reactant adsorption or when the interaction between adsorbed species is obscured in kinetic studies under steady-state conditions. For its application, however, information about the reaction mechanism is requisite because the method is directly linked to the reaction itself. Analysis of the product emission in NO reduction on palladium and rhodium as well as the product formation site and its switchover in CO oxidation on platinum is exemplified.  2007 Elsevier Ltd. All rights reserved. Keywords: Spatial distribution; Desorption dynamics; Reaction site; Intermediate analysis; Catalysis; Nitrogen oxides; Nitrous oxide; Carbon monoxide; Rhodium; Palladium

Contents 1. Chemical kinetics and dynamics of surface reactions. . . . . . . . . . . . . . . . . . . . . . . . . 436 2. Structure-informative desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437

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Tel./fax: +81 29 874 1508. E-mail address: [email protected]

0079-6816/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.progsurf.2007.07.001

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3. 4. 5. 6. 7.

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Collimated desorption and surface structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy partitioning on an energetically flat surface . . . . . . . . . . . . . . . . . . . . . . . . Presentation of angular and velocity distributions . . . . . . . . . . . . . . . . . . . . . . . . . Apparatus for angle-resolved measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dissociative desorption. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. N2O decomposition on Rh(1 1 0), Pd(1 1 0) and Ir(1 1 0) . . . . . . . . . . . . . . . . . 7.3. N2O decomposition on Rh(1 0 0). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. N2O adsorption structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. DFT and STM work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. NEXAFS work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N2O reduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Pd(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Rh(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. Inclined N2 emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NO reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Rh(1 0 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3. Pd(1 1 0) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4. Surface-nitrogen removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5. Branching of surface-nitrogen removal and inhibition by oxygen . . . . . . . . . . Associative desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Site cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3. Changes in structures and distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Chemical kinetics and dynamics of surface reactions Reaction mechanisms on solid surfaces were originally examined from the viewpoint of chemical kinetics, but most of the deduced mechanisms were excluded in the 1960s after re-examination by surface science techniques, including vibration and photoelectron spectroscopies [1–4]. Currently, it is well known that chemical species are not always homogeneously distributed, even on clean and defect-free surfaces [5]. Thus, the validity of a simple rate expression tacitly assuming a unique reactivity is limited. In chemical kinetics, the interaction between adsorbed species has been treated by means of a mean-field approximation [6]. In some cases, it has been more finely dealt with by using Monte Carlo simulations in the framework of lattice gas models [7]. Nevertheless, surface chemical kinetics still remains in a rather passive position, i.e., it just describes the reaction rate in a phenomenological way. This limited contribution, compared with that in gas-phase reactions, is largely due to the limited knowledge of both energy partitioning in a reaction event and the potential energy surface (PES) around the reaction site [8]. We still do not have a suitable method for directly obtaining structural information about the reaction site through the reaction itself.

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In fact, no structural information is provided by chemical kinetics since the expression of the reaction rate (events/s) does not involve structural information. A surface chemical reaction must be characterized with respect to not only chemical kinetics but also with respect to reaction dynamics, since a large amount of energy is partitioned during an event. Structural information will be found in reaction dynamics dealing with energy partitioning as the shape of PES, as is usually the case in gas-phase reactions, i.e., the PES is analyzed from the spatial and energy distributions of emitted molecules in state-resolved ways [8]. The observed PES in general depends on the energy content of emitted products. This situation does not vary in chemical reactions on solid surfaces. At present, the observed structural information is simply classified into either the orientation of dissociating intermediates or the shape of the reaction site because of the lack of the azimuth and desorption angle dependence of the internal energies of desorbing products. The shape of the reaction site includes the symmetry and local facet orientation of the reaction sites. Nevertheless, this approach will provide the most direct structural information of the product formation site. On the other hand, surface spectroscopies, such as surface vibration or photoelectron spectroscopy, yield indirect information on the reaction site since they are based on the signal from surface species after energy dissipation, i.e., before and/or after the reaction event. This is because the reaction is completed in a very short period, on the order of a picosecond, and the subsequent energy relaxation of nascent products is on the order of a pico- to nano-second on solid surfaces [9,10]. Only the desorption process yielding products with hyper-thermal energy can be studied at the energy partitioning level, providing potential elucidation of structures of reaction sites [11]. 2. Structure-informative desorption Structure-informative desorption dynamics is possible even for thermal reactions on solid surfaces when the product has hyper-thermal energy. Such dynamics become available in the angle-resolved (AR) analysis of desorbing reaction products, i.e., surface-structural information is provided from crystal azimuth dependence of the flux and translational and internal energies because the product has already left the surface. This angle-resolved product desorption analysis yields the orientation of parent molecules directly emitting products for dissociative desorption as well as the shape of product formation sites for associative desorption [11]. Such surface-structure analysis from desorbing molecules was attempted for the associative desorption of hydrogen adatoms on various metal planes in 1971–1974 [12,13]. However, no crystal azimuth dependence has ever been found for desorbing hydrogen. Hydrogen might be too small as a probe molecule; H(a) atoms must move far from their adsorption sites because of the small atomic distance in H2 molecules (0.074 nm) compared with the distance between adsorption sites (a few tenths nm). Eventually, the H2 desorption place will be far from the surface where the surface corrugation becomes weak [13]. The promise of structure-informative desorption dynamics re-emerged in 1989 when a remarkable anisotropy was found in the spatial distribution of desorbing product CO2 in the CO oxidation on Pd(1 1 0) [14]. In the 1989 study, the focus was on product emission processes involved in thermal reactions, providing the site symmetry. This is different from electron-stimulated desorption ion angular distribution (ESDIAD), in which the orientation of ruptured bonds is analyzed from the spatial distribution of emitted fragments [15,16].

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A surface chemical reaction shows more or less its own sensitivity toward surface structures. The term ‘‘structure-sensitive reaction’’ was once used to classify reactions with rates that were extremely dependent on local atomic configurations, with differences of a few orders of magnitude [17]. However, this classification has never provided information on the site structures. A similar structural information loss will happen even when remarkable differences are found in the energy of desorbing products among surfaces with different structures. Recently, large differences were found in the vibrational temperatures of desorbing product CO2 in both the CO + O2 and NO + CO reactions between Pd(1 1 1) and Pd(1 1 0) through chemiluminescence analysis in a non-AR way [18,19]. The expected anisotropy of the internal energy would be directly related to the structure of either the transition state of the CO2 formation or the product formation site. Recent studies of both NOx reduction and CO oxidation, which largely share the product emission on well-known three-way catalysts for automobile exhaust gases, have illustrated the remarkable advantages that are possible through simultaneous analysis from both the kinetic and dynamic viewpoints. In fact, the steps to remove surface nitrogen have been difficult to analyze by other means because of the presence of several fast pathways after the slow NO dissociation and the remarkable inhomogeneity of surface-nitrogen reactivity [20,21]. Ordinary kinetic work under steady-state reaction conditions is not informative for these removal pathways. On the other hand, the angular and velocity distributions of desorbing nitrogen-containing products yield information on these pathways in the course of catalyzed NO reduction whenever any step becomes rate-determining. This is because neither distribution involves the reaction rate and both distributions are always related to the desorption process. The steady-state oxidation rate of CO on noble metals is determined by either CO adsorption or O2 dissociation, depending on the CO/O2 pressure ratio and the surface temperature, and not by the fast CO2 formation process [22]. Nevertheless, the spatial distribution of desorbing CO2 sharply changes at the kinetic transition point, showing the shape of the CO2 formation site. This catalytic reaction has been one of the prototypical reactions in surface chemistry, and it has led to new concepts in understanding surface reactions. Currently, the product desorption dynamics is of renewed interest for this reaction because the transition state structure can be analyzed through the internal energy of desorbing product CO2 [23]. 3. Collimated desorption and surface structure For AR desorption measurements, emitted products must be analyzed both after AR procedures and before collisions with reactants or other products because their momentum would be changed even by one collision. The distance from a sample surface to a detector must be much shorter than the mean free path of products, i.e., the product molecules in a definite desorption angle range must be separately detected from the molecules either scattered from the chamber walls or affected by collisions by other molecules. The AR-product analysis work is actually limited to pressures below 0.1 Pa. Only products whose desorption is completed before energy flows to the surface can provide structural information. The time interval for this energy flow can be of the same order as the relaxation time of vibrationally excited surface species. The relaxation time for adsorbed CO is on the order of 1012 s. For example, it is 2 · 1012 s on Pt(1 1 1) at 300 K [24]. The molecule vibrates about a hundred times before relaxing, i.e., nascent

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products can maintain high energy immediately after formation. The product being desorbed, however, will be trapped by attractive dispersion forces exerted from the surface or chemical adsorption forces if it is not repulsively desorbed. It will be desorbed after complete thermalization once it is trapped on the surface because the heat of adsorption, which reaches about 20 kJ mol1 even due simply to dispersion forces, yields a surface residence time on the order of 109 s at around room temperature [25]. The detailed balance principle (DBP), also known as microscopic reversibility, sometimes is invoked in order to derive desorption parameters from adsorption phenomena or vice versa. Its use, however, must be limited to the special case described below. The ‘‘principle’’ is valid only at equilibrium. The equilibration must be established even in a local event, and with each constituent step. For example, the flux of molecules leaving the surface must be balanced with the incident flux. The departing molecules can involve desorption from both chemical and physical adsorption states, and even molecules that are scattered without trapping. These restrictions stand in contrast to the common belief that DBP holds for subsets of particles that belong to each step, such as those adsorbing and desorbing, and even for differential fluxes, e.g., in a special direction or special energy state (translational, vibrational, and rotational state). When this principle is used for non-equilibrium adsorption/desorption systems, two additional assumptions have been tacitly introduced. (1) The forward process (for example, adsorption) is just the reverse of the other process, e.g., desorption. In other words, the system consists of a single step, and the transition state is common. (2) The dynamics of each process is valid under conditions far from equilibrium. The above assumptions are not derived from equilibrium conditions, but they are worthwhile for an adsorption– desorption system consisting of a single step, since Assumption (2) is not so seriously affected by changes in reaction conditions, for example by adsorbate coverage changes. In fact, the adsorption/desorption of hydrogen has been discussed within these assumptions as reviewed by Hodgson [26]. No surface-structure information was found in hydrogen desorption dynamics, i.e., no surface corrugation effect appears in their spatial distribution, and their desorption parameters were then correlated to their adsorption parameters without surface-structural information [27]. Within the applicability of DBP, the desorption dynamics – including spatial and energy distributions – can be consistently described by the surface temperature and coverage. This has been unfortunate for the development of structure-informative desorption dynamics because of the limited approach to surface structures. Adsorption, however, generally involves several elementary steps, such as surface diffusion, localization at a site, and energy dissipation. On the other hand, sometimes product desorption is localized on the atomic scale and its direction is limited in a narrow angle range, i.e., adsorption is not necessarily the reverse of desorption. This review describes several examples in which the spatial distribution of desorbing products is strongly related to aspects of surface atom arrangements, including the orientation of the reactant. 4. Energy partitioning on an energetically flat surface For a system consisting of gas and a surface in equilibrium, the incident flux toward a unit surface area follows the cosine distribution since gas-phase molecules are not localized and their velocity is isotropic. This flux must be balanced to that of molecules leaving from the unit area, i.e., DBP is valid; the density of leaving molecules is constant (Fig. 1), and

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their velocity distribution, described by a Maxwellian form is isotropic. These come from thermal equilibrium and not from the dynamics for adsorption and desorption. The myth of the ‘‘principle’’ in the field of adsorption–desorption dynamics was probably born from the assumption in the well-known Willigen model [28]. He derived his famous equation for the angular distribution of desorbing hydrogen from adsorption dynamics, in which incident molecules with energy surmounting a threshold energy barrier can adsorb, and DBP was assumed for the reverse process, i.e., in his model, desorbing molecules hold kinetic energy corresponding to the value above the barrier height. Within this model, the principle may be valid because the desorption step is just the reverse of adsorption and no structure is assumed in the repulsive potential, i.e., the potential energy surface (PES) has a structure only along the surface normal and there are no structures parallel to the surface plane; in other words, this is a flat barrier model. This model describes the intrinsic characters of repulsive desorption, yielding the following well-known equation. It was derived for molecules whose translational energy is high enough to surmount the threshold energy barrier for adsorption and not for desorbing molecules. IðhÞ=Iðh ¼ 0Þ ¼ fe þ cos2 ðhÞg expfe tan2 hÞg=ðe þ 1Þ cosðhÞ

ð1Þ

where I(h = 0) and I(h) are the number of desorbing molecules directed along the surface normal and at an angle h away from the normal. e = Ea/kTS, where Ea is the activation barrier height for dissociative adsorption, k is the Boltzmann constant, and TS is the surface temperature. According to this model, the mean kinetic energy of desorbing molecules should increase with increasing h value, as hEðhÞi ¼ 2kT S þ kT S fðe2 = cos4 hÞ=ð1 þ e= cos2 hÞg

ð2Þ

This quantity steeply increases with increasing h, being inconsistent with the observed behavior, i.e., the observed translational energy usually either decreases with increasing

Fig. 1. (a) Desorption flux from a unit area and cosine distribution on 3-D coordinates. (b) Angular distribution of desorbing product 15N2O in a steady-state 15NO + CO reaction on Pd(1 1 0). PNO = PCO = 5 · 103 Pa, and at 600 K. Ref. [66].

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desorption angle or remains invariant [29–31]. It would be necessary to determine which of the assumptions used by Willigen creates the discrepancy. He assumed: (1) flat and repulsive barrier potential, i.e., no PES corrugation parallel to the surface plane; (2) frozen internal energy; and (3) detailed balance. No serious problems may be induced by the flat barrier potential energy surface when no anisotropic behavior is found in the desorption dynamics, such as associative hydrogen desorption. DBP may be acceptable since this adsorption/desorption dynamics is controlled by only a single step. The assumption of frozen internal energy modes is not valid for repulsive desorption. The internal energy of desorbing molecules as well as their translational energy varies as a function of the desorption angle depending on energy partitioning. Indeed, the translational energy of desorbing H2 changes differently with increasing desorption angle on Cu(1 1 1) and Ni(1 1 1) [29–31]. Non-AR state-resolved analysis of desorbing H2 (or D2) on Cu(1 1 1) shows facile energy partitioning into the translational, rotational and vibrational energies in repulsive desorption, i.e., in general, slow (or fast) D2 molecules show high (or low) rotational and vibrational energies [32–34]. Very recently, Yamanaka successfully performed internal energy measurements of desorbing product CO2 during CO oxidation on Pd(1 1 1), with AR procedures [23]. The rotational energy increases sharply with increasing desorption angle, indicating the occurrence of energy partitioning in repulsive desorption. The experimental observation of the decreasing [35] or constant translational energy is easily predicted in this energy partition model, i.e., the molecules that receive high kinetic energy are collimated along the surface normal, whereas the molecules with high rotational energy fail to receive high kinetic energy, yielding broad angular distributions. This shows that assumption (2) above causes serious problems. This has not been noticed because of the lack of experimentally observed angle dependence of internal modes in thermal reactive desorption. AR measurements of the internal energy have advantages over non-AR state-selective ones because the former can extend to surface-structure analysis through desorbing products. The angle dependence of the rotational energy is likely to depend on the crystal azimuth since the translational energy depends on the crystal azimuth [36]. Both the translational and the rotational modes are activated in the repulsive desorption from their quenched states. The energy partition is sensitive to the orientation of the product immediately before desorption, i.e., the repulsive force exerted from the reaction site is efficiently converted either into the translational mode when the molecule is upright or parallel to the surface plane, or into the rotational modes when it is inclined.

5. Presentation of angular and velocity distributions The desorption flux shows a cosine distribution when the molecule is equilibrated at the surface temperature, i.e., the normalized flux;

IðhÞ ¼ cos h;

ð3Þ

where h is the desorption angle (polar angle). This distribution yields a simple sphere on three-dimensional (3-D) polar coordinates as shown in Fig. 1a. The angular distribution in the plane along a definite crystal azimuth is in a simple circular form (Fig. 1b). In fact, this distribution is frequently found for desorbing molecules in thermally activated reactions.

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Even in a reaction system far from equilibrium, desorbing products frequently follow this distribution, indicating that the desorption process is in partial equilibrium with the surface. The velocity distribution of desorbing products in this case follows a Maxwellian distribution at the surface temperature, i.e., the desorbing products contain neither structural nor energy information, aside from the surface temperature. In fact, the observation of collimated (non-cosine) product emission from metal surfaces has been limited to reactive desorption. Even then, a desorbing product with hyper-thermal energy appears only in some exothermic processes. Except for reactions with atom beams [37–39], collimated desorption is classified into two groups. One is in associative desorption such as 2H(a) ! H2(g) [12,13], 2N(a) ! N2(g) [40–46], CH3(a) + H(a) ! CH4(g) [47], C(a) + O(a) ! CO(g) [48,49], and CO(a) + O(a) ! CO2(g) [50]. Their reverse (dissociative) processes commonly require significant activation energy, indicating that this energy barrier for adsorption accelerates the departing products [51,52]. The other is in dissociative desorption such as N2H4(a) ! N2(g) + 4H(a) [53] and N2O(a) ! N2(g) + O(a) [54]. Such collimated product desorption is useful for either site analysis or mechanistic studies even if it is not rate-determining. The remarkable difference between equilibrium and collimated desorption is in the shape of the potential energy versus the distance from the surface. For equilibrium desorption, the energy barrier does not show a maximum above the vacuum energy level (the zero kinetic energy level of product molecules), i.e., adsorbed species in an attractive potential energy field leaves the surface toward vacuum. On the other hand, for collimated desorption, the molecule must be influenced by repulsive forces toward vacuum, i.e., there must be a potential energy maximum above the vacuum energy level [55]. Departing molecules that surmount this maximum are then repulsively accelerated toward the vacuum. The velocity of the molecules that experience repulsive forces is enhanced along the force direction, yielding collimated desorption along the same direction. The normalized flux distribution is approximately represented by a power series of the cosine of the angle shift from the collimation position: IðhÞ ¼ cosn ðh  h0 Þ

ð4Þ

The sharpness parameter n is larger than unity for repulsive desorption and becomes unity for equilibrium desorption. The collimation angle (the maximum flux position), h0, is not necessarily at the surface normal. This power function is useful for presenting the sharpness of the angular distribution. However, it should be noted that no physical reasoning has been given for this equation. In many cases, the angular distributions are bimodal, including two (or more) components with different degrees of sharpness. The total intensity I is expressed by a linear combination of two (or more) power functions: IðhÞ ¼ ð1  xÞ cosn ðh  h1 Þ þ x cosm ðh  h2 Þ

ð5Þ

where x is the fraction of the second component. It is noteworthy that each component in a bimodal form must be examined by the other method, i.e., in terms of the velocity distribution as shown in Section 9.3, because no physical basis has been given for the power series. In general, the angular distribution becomes sharper with increasing n value. In the flat barrier model, this sharpness must be determined by the barrier height and the surface temperature. However, the height would be underestimated from experimentally observed angular distributions because the energy from repulsive forces is partitioned into translational and internal modes as well as being transferred to the surface. Except in

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hydrogen desorption, a large part of the potential energy of the activated state is transferred to the surface [56]. The partitioning into both translational and internal modes always takes place on solid surfaces. Eventually, only a part of the potential energy is converted into a translation mode affecting the angular distribution. Furthermore, different heights would be derived at different crystal azimuths when the angular or velocity distribution shows anisotropy. The activation energy barrier for adsorption is generally reduced when internally excited molecules are adsorbed [32,34,57]. Thus, the activation barrier height must be examined directly from the energy determination of both translational and internal forms as functions of the desorption angle. It cannot be estimated from the sharpness of the angular distributions. Nevertheless, the collimation angle can be determined from the angle dependence of the flux or velocity. The velocity distribution of desorbing molecules is described by a Maxwell distribution at the surface temperature (TS) when desorption takes place after being thermalized to the surface temperature [55]. f ðvÞ ¼ v3 expðmv2 =2kT S Þ

ð6Þ

where f(v) is the distribution function, v is the velocity of molecules, and m is the mass of molecules. The mean kinetic energy of desorbing molecules, hEiT, under this distribution is calculated by the following equation. hEiT ¼ 2kT S

ð7Þ

The translational temperature, ThEi, is calculated from the relation ThEi = hEiT/2k. When desorption shows an angular distribution sharper than the cosine form, i.e., when the desorption is repulsive, this translational temperature becomes higher than the surface temperature. The observed velocity is not simply described by a Maxwellian distribution at the surface temperature. The observed signals are forced to fit the modified Maxwellian form [55], 2

f ðvÞ ¼ v3 expfðv  v0 Þ =a2 g

ð8Þ

where f(v) is the modified Maxwellian distribution function, vo is the stream velocity and a is the width parameter. The stream velocity mostly determines the peak position of the distribution, and the width parameter shows the distribution width. This distribution function is widely used to describe the observed velocity distributions. However, it should be noted that there is no physical justification to demonstrate that the velocity distribution should follow Eq. (8). The distribution varies depending on the experimental conditions, for example, the desorption angle [55]. The modified distribution function is useful to deconvolute and/or smooth out the observed velocity distribution curves. 6. Apparatus for angle-resolved measurements For surface-structure analysis, the AR measurements of desorbing products or fragments from solid surfaces were first reported in both thermal and electron-stimulated reactions in 1974 [12,58]. Thereafter, the latter quickly developed into the technique known as electron-stimulated desorption ion angular distribution (ESDIAD) [15,16,59]. On the other hand, the remarkable anisotropy of the former was first reported about 15 years later in the spatial distribution of desorbing CO2 in the CO oxidation on Pd(1 1 0) [14]. This delay is mostly due to the weak anisotropy of the spatial distribution as well as

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the difficulties in making AR measurements of both the translational and internal energies in thermal reactions [55,60]. The angle dependence of the rotational energy of emitted molecules was once examined in the ESDIAD of adsorbed NO on Pt(1 1 1) [61–64], but no noticeable correlation was found between the relative population of rotational states (J) of desorbing NO and the desorption angle. In this kind of ESDIAD work, AR measurements can be performed in a single ultra-high vacuum chamber. With the use of short laser pulses or electron beams, the signal due to molecules directly moving from the surface to the detector could be distinguished in time-resolved measurements because of the delayed arrival of molecules scattered from the chamber wall. This can be extended to AR internal-energy measurements by introducing a two-dimensional microchannel plate detector into the chamber. On the other hand, for AR measurements of desorbing products in thermal reactions, at least three chambers must be combined because at least two slits should be operative between the sample surface and the analyzer, and high-speed pumping is requisite between the two slits [60]. This is common in different AR measurements such as angle-resolved temperature-programmed desorption (AR-TPD), angle-resolved steady-state desorption (AR-SSD), and modulated molecular beams (MMB) scatterings. The principle behind the apparatus for AR desorption measurements is drawn in Fig. 2. The flux and the translational and internal energies of desorbing products must be analyzed using AR procedures. AR-state-selective desorption measurements have not been successful yet due to insufficient product density, but they will be performed in the near future using angleresolved resonance-enhanced multi-photon ionization techniques (AR-REMPI [26,65]) for a steady-state reaction. The apparatus is an ultra-high vacuum system composed of a reaction chamber, a chopper housing and an analyzer, which are separately pumped. The angle-resolution performance was first numerically evaluated by Kobayashi and Tuzi [60]. An apparatus with a one-slit system cannot yield reliable AR signals because it becomes difficult to separate the molecules that arrive directly through surface reactions from those which are emitted into the reaction chamber and scattered from the chamber wall and then penetrate the analyzer. Only about 1/104 of the desorbed species are transmitted to the ionizer/detector in the analyzer at an angular resolution of one-degree for desorption in a cosine form. In the figure, a very large pumping rate, about 7 m3/s, can be established in the chopper housing by a copper plate cooled to around 25 K or a Ti getter. The reaction chamber is a conventional ultra-high vacuum vessel with a high pumping rate. It has standard facilities for surface analysis, such as low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy optics and a quadruple mass spectrometer (QMS) [11,66]. The chopper house has a narrow slit facing the reaction chamber and contains a cross-correlation chopper blade for time-of-flight (TOF) measurements [67]. The blade has a double sequence of 255 slot elements with an equal width of 1.0 mm distributed in a pseudo-random sequence. This TOF system has a high time resolution and a high transmission of molecules (50%). For internal energy measurements after AR procedures, the optics for infrared chemiluminescence collection or the laser light for REMPI should be focused on the product flow after the second slit. The product density there is reduced to about 0.1% of that a few mm from the surface [18,26]. Recently, Yamanaka constructed an apparatus for IR emission detection of desorbing product CO2 using AR procedures. A meaningful signal

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Fig. 2. Principle behind the apparatus of angle-resolved product desorption measurements incorporating time-offlight techniques. The reaction chamber is an ordinary UHV apparatus with standard facilities for surface analysis and a mass spectrometer. Either the chopper house or the reaction chamber must be evacuated with a large pumping rate. A pseudo-random chopper blade for cross-correlation time-of-flight techniques is drawn. Ref. [11].

was detected after the background emission was reduced to 0.05% of that without cooled shielding around the optics [23]. 7. Dissociative desorption 7.1. Introduction In dissociative repulsive desorption, the orientation of parent molecules is preserved in the spatial distribution of desorbing fragments in a way similar to ESDIAD. Many studies on the collimated fragment desorption from surface molecules have been reported in ESDIAD, in which electrons or photons impinge on ad-molecules [15,16,59]. The resultant fast-desorbing fragments are collimated along the ruptured bond axis, yielding structural information on the adsorbed parent molecules. In the thermal decomposition of adsorbed

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molecules, however, the released fragment has been believed to be quickly thermalized to the surface temperature before emission probably because these species interact with attractive forces due to their chemical or physical adsorption potential. Two exceptional dissociations have been reported, i.e., a sharp N2 emission in N2H4 decomposition on Ir(1 1 1) [53] and N2O decomposition on Pd(1 1 0) [54], Rh(1 1 0), Ir(1 1 0) and Rh(1 0 0) [68–71]. In the former N2H4 decomposition, the product N2 peaked at around 290 K and 440– 500 K in AR-TPD after N2H4 exposure at 260 K. The N2 desorption at 290 K collimated steeply along the surface normal. However, the reliability of the AR signal is not clear since the performance of the angle-resolution in an apparatus with a single slit is very poor for non-time-resolved measurements. Repeating this decomposition using an apparatus with two slits would be worthwhile. Thus, the inclined N2 emission in N2O decomposition on the above surfaces was the first example to show collimated fragment desorption in thermal decompositions on metal surfaces. The N2O decomposition is sensitive to surface structure [72,73]. Desorption of adsorbed N2O is completed below 120 K without dissociation on a (1 1 1) plane of Pt, Ir, Ni, Ag and Rh [74–78]. On the other hand, on open surfaces such as the (1 1 0) planes of Ni, Pd, Rh, and Ir and stepped Ni(5 5 7), N2O dissociates below 100 K [68–70,79–81]. The reactivity of a (1 0 0) plane depends on the kind of metal, i.e., N2O dissociates on Rh(1 0 0) and Ni(1 0 0) and not on Pd(1 0 0) [71,72]. Adsorbed N2O is largely decomposed into N2(g) and O(a) in the subsequent heating when clean Pd(1 1 0), Rh(1 1 0), Ir(1 1 0) or Rh(1 0 0) is exposed to N2O at low temperatures. There are three to five N2 desorption peaks in the temperature range of 70–200 K. AR-TPD combined with TOF is useful to characterize this N2O(a) decomposition. N2O has been frequently proposed to be the intermediate in the deNOx process at low temperatures in the d-N2 formation pathway of N(a) + NO(a) ! N2O(a) ! N2(g) + O(a) [82,83]. At high temperatures, the surface-nitrogen removal proceeds as 2N(a) ! N2(g). In fact, N2O is one of the byproducts in NO reduction on metal catalysts. The participation of the intermediate N2O in this d-N2 formation has not been confirmed yet because of the absence of spectroscopic evidence of adsorbed N2O in the course of NO reduction [84,85]. This was, however, confirmed in AR-product desorption measurements in steady-state NO reduction, as described in the following sections. 7.2. N2O decomposition on Rh(1 1 0), Pd(1 1 0) and Ir(1 1 0) In AR-TPD work with N2O on Rh(1 1 0), N2 desorption shows five peaks at around 70 K (b5-N2), 95 K (b4-N2), 115 K (b3-N2), 140 K (b2-N2), and 160 K (b1-N2) [86]. b1-N2 yields a cosine distribution, indicative of desorption from trapped N2, whereas the others sharply and commonly collimate along 69 ± 3 off normal into the [0 0 1] direction. The distribution of b4-N2 desorption is shown in Fig. 3. Sharper desorption collimated at around 27 is also found in b3-N2 and b2-N2 when the surface is oxidized [87]. Transient N2 desorption is also induced even at 60 K when N2O is introduced to clean Rh(1 1 0) (Fig. 4). This desorption sharply collimates at 65 ± 3 off normal towards the [0 0 1] direction. Similar inclined N2 desorption is also observed in thermal N2O decomposition on Pd(1 1 0), Ir(1 1 0), and Rh(1 0 0). The above different desorption temperatures are induced by deposited oxygen. The adsorption heat of N2O is increased in the presence of a small amount of oxygen adatoms,

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yielding a different activation energy for dissociation [88]. Dissociation is retarded at high oxygen coverage. These phenomena were confirmed on oxygen-pre-covered Rh(1 1 0). The sum of the b4-N2 and b5-N2 peaks, which always appear together, show a maximum in the small coverage range of pre-adsorbed oxygen [89]. On the other hand, the sum of b2-N2 and b3-N2 (not separated) increases slowly and shows a maximum at around a relative oxygen coverage of 0.08 (Fig. 5). The initial increase in the former indicates that its formation is already affected by oxygen, consistent with N2O dissociation at around 55 K. A typical N2 TPD spectrum from Pd(1 1 0) is shown in Fig. 6a. N2 desorption shows four peaks in the range of 110–150 K. b1-N2 at around 150 K appears as a shoulder and becomes evident at high N2O exposures [68]. On the other hand, b4-N2 at around 110 K is clearly seen only at HN2 O < 0:10 in the AR form. The b3-N2 peak at 123 K is enhanced above HN2 O ¼ 0:05. The b2-N2 formation showing a cosine distribution is more than half of the total N2 desorption. Three of them (b1-, b3-, and b4-N2) commonly emit N2 in an inclined way collimated at 43–50 (Fig. 6b). The translational temperature of b1-N2 at 140 K reaches about 1900 K [90].

Fig. 3. (a) AR-TDS spectra of 15N2 from Rh(1 1 0) exposed to 15N2O at 55–60 K. HN2 O ¼ 0:15. The signal was corrected by subtraction of the 15N2O fragmentation. The curves were deconvoluted into five Gaussian peaks. Angular distributions of desorbing 15N2 in the plane along the [0 0 1] direction at HN2 O ¼ 0:15, (b) b1-N2, and (c) b4-N2. Ref. [86].

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Fig. 4. Isothermal N2O decomposition on clean Rh(1 1 0). AR-N2 signals during the N2O exposure at h = 70 in the plane along the [0 0 1] direction. TS = (a) 60 K and (b) 120 K. P N2 O ¼ 5:3  108 Pa. The steady value of the N2 signal is due to the fragmentation of N2O. The broken curve (1) shows a first-order decay, and the curve (2) describes the rate on the oxygen-affected site. (c) Angular distributions of desorbing N2 in the plane along the [0 0 1] direction. A typical deconvolution is shown for the data at 140 K by broken curves. Ref. [87].

Fig. 5. Oxygen effect on N2O decomposition on Rh(1 1 0); Variation of the amounts of N2 with pre-adsorbed oxygen. Ref. [89].

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Fig. 6. N2O decomposition on Pd(1 1 0). (a) AR-TPD N2 spectrum at HN2 O ¼ 0:08 and h = 55 in the plane along the [0 0 1] direction. The heating rate was 0.6 K/s. A typical deconvolution is shown by dotted curves. The solid line indicates the sum of all the components. (b) Angular distributions of N2 peaks in the normally directed plane along the [0 0 1] direction; s, b3-N2 at TS = 123 K, and d, b4-N2 at TS = 110 K. The ordinate was normalized to the maximum signal of b3-N2 at saturation. b1-N2 at TS = 152 K shows a distribution similar to b3-N2, whereas b2-N2 at TS = 135 K shows a cos(h) form. (c) PCO-dependence of the AR-15N2 signal at h = 43 toward the [0 0 1] direction in a steady-state N2O reduction at TS = 450, 470, 500 and 520 K P N2 O ¼ 4:0  105 Pa. Angular distributions of desorbing N2 in the plane along the [0 0 1] direction are also shown at PCO values indicated by arrows. Refs. [68,101,122].

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7.3. N2O decomposition on Rh(1 0 0) The product N2 emission was also examined in N2O decomposition on Rh(1 0 0). It shows two main peaks at around 80 K (b2-N2) and 105 K (b1-N2) and a shoulder between them [71]. The former sharply collimates far from the surface normal at low N2O coverage and collimates along the surface normal at high coverage (Fig. 7). The b2-N2 desorption splits into three components in the plane along the [0 0 1] direction. Two of them collimate in an inclined way and decrease at high N2O coverage where the normally directed component is enhanced. The inclined component is sharp, and can be approximated in a cos25(h ± 66) form at HN2 O ¼ 0:16. The normally directed desorption is in a cos8(h) form at HN2 O ¼ 0:68. On the other hand, in the plane along the [0 1 1] direction, the AR signal is negligibly small below HN2 O ¼ 0:12 and increases steeply around HN2 O ¼ 0:20. The AR-N2 signal decreases quickly with increasing desorption angle, irrespective of N2O coverage. The other peak (b1-N2) at 100–110 K shows a cosine distribution. Considering the surface symmetry, the inclined N2 desorption shows a four-directional form (Fig. 7). At small N2O coverage, adsorbed N2O is again proposed to decompose after lying along the [0 0 1] or [0 1 0] direction. At higher coverage, N2O is proposed to adsorb upright through the terminal oxygen in aggregated forms [71]. N2O dissociation releases oxygen atoms on the surface, which retard the reaction. In fact, pre-adsorbed oxygen severely suppresses N2O dissociation. Suppression is completed when a p(2 · 2)-O lattice forms on Rh(1 0 0) [71]. 7.4. N2O adsorption structures N2O decomposition emits N2 with hyper-thermal energy collimated far from the surface normal toward the [0 0 1] direction on Pd(1 1 0) and Rh(1 1 0). This was proposed to be due to N2O being oriented along the [0 0 1] direction [54]. This adsorption form was confirmed on Pd(1 1 0) by density-functional theory (DFT) [91], scanning-tunneling microscopy (STM) [92], and near-edge X-ray-absorption fine structure (NEXAFS) [93]. The structure of adsorbed N2O had been studied on Pt(1 1 1), Ir(1 1 1) and Pd(1 1 0) by either high-resolution electron energy-loss spectroscopy (HREELS) or infrared reflection-absorption spectroscopy (IRAS), showing the terminal nitrogen atom interacting with the surface and the molecule adsorbed in a tilted form [74,75,94]. NEXAFS work on Ni(1 1 1), Cu(1 0 0) and Ag(1 1 0) also showed largely inclined forms [77,95]. This species, however, is not the precursor for the N2O dissociation because the terminal oxygen must be deposited on the surface during the event. This disagreement was recently resolved by confirming the presence of form that lies parallel to the surface plane together with the tilted form. The lying form is inactive in vibrational spectroscopy according to the surface-selection rule [96]. 7.5. DFT and STM work DFT calculations in a generalized gradient approximation (GGA) by Kokalj on Pd(1 1 0), Rh(1 1 0) and Rh(1 0 0) predict two stable adsorption forms, i.e., one form lying parallel to the surface along the [0 0 1] direction and the other form tilted with the terminal nitrogen bound to the metal (Fig. 8a and b) [91,97,98]. Adsorbed N2O is mobile above approximately 20 K because of the small adsorption heat. The surface at 14 K was exposed to a small amount of N2O and STM images were observed. Bright elliptical spots

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Fig. 7. N2O decomposition on Rh(1 0 0) (a) Angular distributions of desorbing b2-15N2 in the plane along the [0 0 1] direction at HN2 O = (a) 0.16, (b) 0.32, and (c) 0.68 and (d) along the [0 1 1] direction at HN2 O ¼ 0:32. The ordinate was normalized to the b2-N2 signal at the surface-normal direction at HN2 O ¼ 0:68. The distributions were deconvoluted into two inclined components and a normally directed component, as shown by broken curves. The solid lines indicate their summations. The right column shows a three-dimensional representation of the distributions. Ref. [106].

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Fig. 8. Adsorbed N2O on Pd(1 1 0); (a) Standing and lying-down forms optimized by DFT-GGA at the on-topsites in a p(2 · 2) lattice. (b) Calculated geometries of the two forms. A physisorbed N2 molecule was estimated from the van der Waals’ radius. Distances are in nm. (c) STM image of isolated N2O molecules at 14 K with Vbias = 0.30 V and Iset = 0.18 nA. The proposed orientation and position of adsorbed N2O molecules are inserted. Refs. [91,92].

assigned as N2O monomers are oriented along the [0 0 1] direction and do not move at this temperature (Fig. 8c) [93]. Fig. 9 shows an STM image at 8 K after the surface was exposed to N2O between 90 and 80 K in the course of cooling. N2O adsorbates and defect sites are shown as brighter and darker spots, respectively, than the Pd atoms. The average coverage of N2O was around 0.15 monolayer. The majority of N2O forms small aggregates along the ½1 1 0 direction. Only a relatively small number of N2O monomers are seen. The N2O monomers are observed as dim oval spots with the long axis along the [0 0 1] direction. The spots of N2O monomers show a height of 0.13–0.17 nm. On the other hand, the aggregates are commonly 0.20–0.25 nm high. In the inset of Fig. 9, the molecular structure of N2O

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Fig. 9. Clustered N2O on Pd(1 1 0); (a) Enlarged STM image of aggregated N2O molecules at 8 K with Vbias = 0.05 V and Iset = 0.25 nA. The proposed orientation and position of adsorbed N2O molecules are inserted. The surface was exposed to N2O at 90–80 K in the course of cooling. The average N2O coverage is ca 0.15 monolayer. The part marked by the broken line is expanded. (b) N2O adsorption structures optimized from DFT calculations and their predicted STM images. Ref. [93].

and the lattice of the Pd(1 1 0) substrate are superposed on the STM image. The length of the aggregate along the ½1  1 0 direction is commonly the multiple of the metal–metal nearest-neighbor (nn) spacing along this direction. N2O is too long to align along the ½1 1 0 direction in the nn periodicity. Therefore, each N2O molecule in the aggregate must be oriented along the [0 0 1] or ½00 1 direction or in a tilted way. DFT-GGA calculations yielded several stable cluster forms. The horizontal-bridge form, where horizontal -N2O is alternatively oriented along either the [0 0 1] or ½001 direction and is located on bridge sites, is unstable. On the other hand, the horizontal-on-top form in which horizontal N2O is in on-top sites shows an adsorption energy of 14 kJ mol1. This is smaller than that of monomers in the on-top site, 39 kJ mol1. This small binding is due to the close contact between N2O molecules, 0.28 nm, which is closer than that in gas-phase clusters [99]. More stable clusters were found in tilted forms as shown in Fig. 9c. The terminal nitrogen interacts with the metal and the N2O axis is inclined into the [100] or ½00 1 direction toward the adjacent molecules. The structure is more stable when the N is in on-top sites, yielding an adsorption energy of 30 kJ mol1. The adsorption energy is 27 kJ mol1 when the N is on the bridge sites. The simulated STM images of these structures are also shown. The cluster consisting of the titled forms is about 0.05 nm higher than that of the horizontal ones. The N2O aggregate should split into monomers before the dissociation of N2O. The DFT-GGA-predicted activation barrier for the transition from the tilted to the horizontal N2O form is very low, 4 kJ mol1. The tilted N2O monomer may change its form to horizontal at higher temperatures. The presence of tilted N2O is consistent with observations from vibrational spectroscopy [94]. 7.6. NEXAFS work Careful NEXAFS work with N2O(a) on Pd(1 1 0) at around 60 K showed the remarkable anisotropy of the polarization dependence of p resonance [93] (Fig. 10). This

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Fig. 10. NEXAFS of adsorbed N2O on Pd(1 1 0); (a) Typical NEXAFS spectra of N2O(a) after normalization by the edge jump. The incident angle of the X-ray was at 10 from the surface normal. The peaks at 401 eV and 405 eV are due to the transition from the terminal nitrogen atom (t-N) and that of the central nitrogen (c-N). (b) Polarization dependence of p resonance at 405 eV. E is in the plane along the ½1 1 0 (b) and [0 0 1] (c) directions. The broken and red lines represent the X-ray incident angle dependence for the [0 0 1]-oriented and standing N2O. Ref. [93]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

anisotropy is explained by a mixture of upright and the horizontal forms along the [0 0 1] direction. The p resonance for N2O(a) split into two X-ray-absorption peaks at photon energies of 401 and 405 eV (Fig. 10a). The peak at 401 eV is due to the transition from the 1s state of

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the terminal nitrogen atom (t-N) to the 3p* orbital, and the other is due to that of the center nitrogen (c-N). The energy difference of 3.4 eV and the similar absorption intensity of both peaks agree with those of gas-phase N2O, confirming molecular adsorption. The absorption intensity due to the p resonance at 405 eV decreased by about 25% at grazing angles when the X-ray electric vector E was parallel to the plane along the ½1 1 0 direction, whereas when E was in a plane along the [0 0 1] direction, the absorption intensity of the p resonance increased by about 65% with increasing incident angle of the X-ray (Fig. 10b and c). The other p resonance at 401 eV varied in a similar way. The signal at normal incidence was significant for both E directions. The p resonance would vary in the form of sin2v as drawn in the figure when E is in the plane along the [0 0 1] direction and all N2O molecules align in this direction, where v is the angle of X-ray incidence. In this case, only the transition moment that is perpendicular to the surface plane becomes active. The signal decreases in the form of cos2v when the N2O axis is perpendicular to the surface plane. On the other hand, when E is in the plane along the ½1  1 0 direction, the [0 0 1]-oriented molecule yields a constant intensity because both surface-parallel and -perpendicular transition moments become active. The observed signal fits neither the [0 0 1]-oriented nor the normally directed form; however, the coexistence of both forms explains the observed signal. In the figure, the lines are drawn by assuming that the parallel transition moment is 55% that of the perpendicular one so that the remaining signals in both polarization directions can be fitted to the same curve due to the p orbital of the standing N2O. Even at 60 K, the formation of N2O clusters is highly possible at 0.28 ML N2O. Of course, each cluster is unstable and may be converted into monomers or vice versa. On the other hand, the results on Rh(1 1 0) were different [100]. At small N2O exposures, the signal at 402 eV was relatively enhanced and that at 405 eV was weak. The signal at 402 eV comes from trapped N2, i.e., N2O is partly decomposed at 55 K and the product is partly trapped. At higher N2O exposures, both signals became similar, showing that most N2O is adsorbed molecularly. 8. N2O reduction 8.1. Pd(1 1 0) N2O is decomposed on open metal surfaces below 100 K. To keep its decomposition continuous, however, the deposited oxygen must be removed by a reducing reagent such as CO or H2. The catalyzed N2O + CO reaction on Pd(1 1 0) proceeds steadily above 450 K, and N2 desorption collimates sharply along 45 off normal towards the [0 0 1] direction in both the active and inhibited regions (Fig. 6c) [101]. The AR-N2 signal at h = 0 remains negligibly small. This is in contrast to the large fraction of the component with a cosine distribution in AR-TPD work. The N2 flux is approximated by a {cos28(h + 45) + cos28(h  45)} form at 460 K. The distribution is slightly broadened at higher temperatures. On the other hand, CO2 desorption collimates sharply along the surface normal. The three-dimensional presentation of desorbing N2 distribution was constructed on polar coordinates from angular distributions observed at different crystal azimuths (Fig. 11) [102]. The distribution becomes broader with increasing azimuth (/), which is measured from the [0 0 1] direction (Fig. 12). The signal intensity decreases quickly and

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Fig. 11. Spatial N2 distribution on Pd(1 1 0); Angular distributions of desorbing N2 at different crystal azimuths in the steady-state N2O reduction at P N2 O ¼ 4:4  104 Pa, PCO = 0.7 · 104 Pa, and TS = 520 K. The signal was normalized to the maximum value at the collimation angle. The solid curves are simulated by the inserted equations. The resultant N2 distribution is shown on 3-D coordinates in the upper panel. Ref. [102].

is almost suppressed above / = 40. For the anisotropy analysis of inclined N2 desorption, the following transformation from experimental polar coordinates (h, / system) to another polar angle (a, b) system is necessary. According to the rotation defining Eulerian angles, the relation between angles (a, b) and (h, /) is given by cos h = cos a cos b and tan / = tan b/sin a, respectively [103]. a is the longitude measured from the normal direction [110] about the ½ 1 1 0 axis when the polar axis is taken to be parallel to the crystal azimuth ½ 1 1 0, whereas b becomes the longitude shifted from the plane along the [0 0 1] direction when the polar axis is parallel to the [0 0 1] axis.

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Fig. 12. 3D analysis; (a) Definition of the crystal azimuth (/) and desorption angle (h) used in the experiments, (b) Top-view of Pd(1 1 0), and a new angle system (a, b) in the data analysis. The plane at a fixed a value is crosshatched. (c) N2 angular distributions (b-dependence) in the inclined plane at a = 45 for the N2O + CO reaction on Pd(1 1 0) and at a = 41 for the NO + CO reaction. Ref. [102].

The signals estimated at a = 45 for the N2O + CO reaction are shown as a function of b in Fig. 12d. The resultant distribution at a = 45 has been approximated by a cosnb form

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with n = 17 ± 3. In this new three-dimensional way, the N2 distribution for the N2O + CO reaction is approximated by {cos28(a + 45) + cos28(a  45)} cos17(b) at 520 K. The kinetics of the steady-state reaction switched at a critical CO/N2O pressure ratio. The reaction is first order in CO below the transition and negative order in CO above it, as in CO oxidation with O2 (Fig. 6c). Throughout the kinetic transition, the N2 desorption sharply collimates along about 45 off normal. Desorbing N2 showed translational temperatures in the range of 2000–5000 K. Only the (1 · 1) LEED pattern was observed during the steady-state reaction (P N2 O ¼ 4:4  106 Pa, PCO = 0.67 · 106 Pa) between 470 and 700 K, indicating no surface reconstructions. This is reasonable on oxygen-deficient Pd(1 1 0) above 360 K [104]. The surface oxygen becomes significant only above the critical N2O pressure. 8.2. Rh(1 1 0) N2 desorption in both the steady-state N2O + D2 and N2O + CO reactions on Rh(1 1 0) sharply collimates in a two-directional way along 45–80 off normal in the plane along the [0 0 1] direction [105–107]. The steady-state N2O + D2 reaction can be quickly established above around 260 K when the N2O pressure is lower than that of D2, i.e., under reducing conditions. The AR-N2 signal at h = 66 is shown versus TS in Fig. 13. With increasing surface temperature, the reaction rate increases steeply at around 300 K, shows a maximum at around 350 K, and then decreases slowly. In the N2O + CO reaction, the product N2 signal at h = 57 becomes noticeable above 430 K, increases quickly to a maximum at

Fig. 13. Temperature dependence of the steady-state N2O reduction on Rh(1 1 0). d, j, h, AR-15N2 signal at h = 66 in the plane along the [0 0 1] direction in the steady-state N2O + D2 reaction. P N2 O ¼ 4:0  105 Pa, and P D2 ¼ 1:1  104 Pa. , ; AR signals of 15N2 at h = 57 in the plane along the [0 0 1] direction, and n, m, those of 13 CO2 at the surface-normal direction in the steady-state N2O + CO reaction. P N2 O ¼ 3:0  105 Pa, and PCO = 4.7 · 105 Pa. The signals observed in the direction of the increasing surface temperature are designated by closed symbols and those in the downward direction by open symbols. Ref. [106].

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around 520 K, and then decreases slowly at higher temperatures. CO2 desorption shows a simple cosine distribution. The AR-CO2 signals in the normal direction show a temperature dependence similar to that of N2. The large signal difference between CO2 and N2 is mostly due to the difference in their angular distributions. The N2 distribution changed remarkably at 530–450 K in the N2O + CO reaction and at 280–400 K in the N2O + D2 reaction. With decreasing surface temperature below 530 K, the collimation angle in the N2O + CO reaction shifts toward 43 at 445 K (Fig. 14a) [107]. The N2 angular distributions in the N2O + D2 reaction above approximately 440 K are close to those from the N2O + CO reaction above 550 K. Below 340 K, it is sharply collimated along 70–80 off normal. In the range of TS = 306–320 K, the maximum appears at 75–83 off normal (Fig. 14b) [106]. Furthermore, the distribution shows a shoulder indicating the presence of two desorption components. The distribution was deconvoluted into a {cosn(h + hn) + cosn(h  hn)} form with n = 50–55 and hn = 77–79, and a {cosm(h + hm) + cosm(h  hm)} form with m = 15 and hm = 55–60. For the N2O + CO reaction, n = 50–55 and hn = 73– 75, and m = 15 and hm = 43–56. Here, n and m are the sharpness parameters, and hn and

Fig. 14. N2 distribution on Rh(1 1 0); (a) Angular distributions of desorbing 15N2 in the plane along the [0 0 1] direction in (a) the 15N2O + 13CO reaction at different TS values. TS = 445–550 K. P N2 O ¼ 3:0  105 Pa and PCO = 4.7 · 105 Pa. (b) 15N2O + D2 reaction at TS = 309–442 K. P N2 O ¼ 4:0  105 Pa and P D2 ¼ 1:1  104 Pa. Typical deconvolutions are shown by broken curves. The data in the range of 75–90 have significant uncertainty as shown by vertical bars. The signals observed in the direction of the increasing shift from the surface normal (toward the left) are designated by closed symbols, and those in the decreasing shift, by open symbols. Ref. [106].

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hm are the collimation angles. In the N2O + D2 reaction, the former sharp component becomes predominant below 330 K, whereas the latter is the main component at higher temperature, although both components are always noticeable. On the other hand, the sharp component was quickly suppressed in the N2O + CO reaction by adsorbed CO. The velocity distributions of desorbing N2 in both N2O + CO and N2O + D2 reactions are quite similar. The N2 shows very high translational temperatures and commonly shows broad distribution curves. The translational temperature was calculated from the average kinetic energy (hEi) as ThEi = hEi/2k, where k is the Boltzmann constant. The value at TS = 600 K was maximized to 4000 K around the collimation angle in the N2O + CO reaction [105]. It decreased slowly with an increasing shift from the collimation angle, consistent with the presence of two desorption components. The distribution involves a very small amount of the component expected for the Maxwell distribution at the surface temperature. In the N2O + D2 reaction, the translational temperature was estimated to be about 3500 K in the range of 25–80 at TS = 500 K and ca. 3000 K at TS = 300 K. The collimation angle shift in the N2O + CO reaction is due to the effect of adsorbed CO. The nascent N2 is scattered by adsorbed CO or the N2O dissociation yielding N2 emission at large angles is blocked [107]. This effect was confirmed by a sharp CO coverage change in the range of 440-520 K and the absence of this effect in the N2O + D2 reaction [106]. The CO coverage in the steady-state N2O reduction under reducing conditions is equal to that in the absence of N2O, i.e., the adsorption and desorption of CO are in equilibrium in the course of the N2O + CO reaction. The CO coverage is around 1/2 monolayer (ML) at around 460 K, where the reaction is largely suppressed. The value is about 1/4 ML at 500 K, where retardation begins and the collimation angle shift becomes noticeable. The surface under reducing conditions shows a (1 · 1) LEED pattern as expected. 8.3. Inclined N2 emission N2O decomposition is highly exothermic on metal surfaces because of the large energy released in metal-oxygen bond formation and the small heat of N2O adsorption [73]. It is highly endothermic in the gas phase [108]. For the process of N2O(a) ! N2(g) + O(a), the energy (DET), which the product N2 can carry out is given as DET ¼ EN2 ðgÞ þ EOðaÞ  EN2 Oða;TSÞ

ð9Þ

where EN2(g), EO(a), and EN2O(a;TS) are the potential energies of N2(g), O(a), and the transition state of N2O(a) dissociation, respectively. By assuming 400–500 kJ mol1 as the bond energy of O-Metal [109], the available energy was estimated to be 240–340 kJ mol1 because the dissociation of N2O(g) ! N2(g) + O(g)(3P) is endothermic (about 160 kJ mol1) and the heat of adsorption of N2O on both Pd(1 1 0) and Rh(1 1 0) is close to the activation energy of N2O(a) dissociation. In this process, the released energy mostly comes from metal–O bond formation. A recent molecular dynamic (MD) simulation by Kokalj predicts that the N–O bond in the [0 0 1]-oriented N2O breaks first, leading to breakage of the nascent N2-metal bond and affecting the trajectory of desorbing N2 by deposited oxygen, i.e., the nascent N2 is first emitted closely parallel to the surface plane because it receives repulsive forces from the counter product O(a) and weak attractive forces from metal atoms [73]. The position of the nascent N2 was estimated to be slightly further from the surface than that of

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Fig. 15. A scattering model of the nascent N2 by (a) hydrogen and (b) adsorbed CO on Rh(1 1 0). A side view of the configuration of N2O, D, CO, and nascent N2. The broken circles are drawn from the van der Waals’ radii. The inserted numbers stand for distances in nm units. Ref. [107].

chemisorbed N2. This predicted parallel emission was confirmed in the N2O + D2 reaction on Rh(1 1 0) [106]. In the above steady-state N2O reduction on Rh(1 1 0), two desorption components collimated at 43–60 (hereafter, referred to as the 60 component) and 73–79 (referred to as the 79 component) were derived. The latter sharp 79 component was preferentially observed at 270–320 K. No scattering effect by adsorbed hydrogen was confirmed because the angle of ca. 79 is determined by the van der Waals’ radius of the N2 itself and the position of the N2 part in the adsorbed N2O (Fig. 15a). The collimation angle observed below 320 K is likely to be the limiting value on a clean surface. Hydrogen (actually deuterium) atoms were proposed to be located on threefold hollow sites in the trough running along the ½1  1 0 direction [110,111]. Their height is insufficient to scatter the nascent N2. The N2 receives repulsive forces from the counter oxygen atoms and the surface metal atoms on the nearest atom row (Site 1). On the other hand, CO stands on either on-top or bridge sites, providing a large scattering cross section (Fig. 15b). The nascent N2 emitted parallel to the surface plane will be scattered, yielding the collimation angle shift toward the surface normal. With increasing surface temperature, the share of the 60 component increases and overcomes the sharp component at around 340 K. At higher temperatures, the 60 component is predominant. Two dissociation channels may have different activation energies, i.e., a smaller activation energy is assigned for the 79 component formation which begins at lower temperatures.

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9. NO reduction 9.1. Introduction No direct confirmation of the intermediate N2O has been successful in the course of the catalyzed NO reduction by surface spectroscopy [84,85] probably because the adsorbed amount is extremely small due to a small heat of adsorption [68,69,72]. Furthermore, the active (surface parallel) form for dissociation must be insensitive toward vibrational spectroscopy. Thus, this intermediate has not been identified in the course of the catalyzed NO reduction although it was already proposed in the 1970s, and it has been considered to be formed through a side reaction [20]. AR-product desorption analysis has shown the presence of active intermediates that can emit N2 in the plane along the [0 0 1] direction. This indicates that d-N2 formation involves intermediate N2O oriented along the [0 0 1] direction as NO(a) + N(a) ! N2O(a) ! N2(g) + O(a) [83]. The angle-integrated (AI) N2O signal is negligibly small on Rh(1 0 0) and Rh(1 1 0) under UHV conditions, indicating rapid decomposition of this intermediate. On the other hand, a large amount of N2O is formed in NO reduction on Pd(1 1 0), which provided Ikai and Tanaka a stage for their pioneer work [112,113]. Ikai and Tanaka found that, in the course of heating in their AR-TPD work of NO and H2 or CO on Pd(1 1 0), the N2 peak at 490 K involved desorption collimated at 38 off normal toward the [0 0 1] direction and desorbing N2 in the other peak at around 600 K collimated at the surface normal [112]. Furthermore, they confirmed that the reaction of 14 N(a) with 15NO(a) emitted product 14N15N in an inclined way, while the associative nitrogen (14N2) desorption collimated along the surface normal [113]. Their observations strongly suggest that the d-N2 formation pathway contributes to the inclined N2 desorption. However, they argued that the desorption-mediated reaction, i.e., inclined N2 desorption mediated by NO desorption, was not involved in a catalytic cycle [114]. Later, our group reported that desorbing N2 from N2O decomposition, and a steady-state NO + CO, NO + H2, or N2O + CO reaction on Pd(1 1 0) as well as Rh(1 1 0) and Rh(1 0 0) yielded identical angular and velocity distributions [115]. In fact, the intermediate N2O is easily formed from the reaction of N(a) with NO(a) on platinum metals [116–118]. 9.2. Rh(1 0 0) The steady-state NO + D2 reaction becomes noticeable above approximately 500 K on Rh(1 0 0) when the NO pressure is lower than that of D2, i.e., under reducing conditions [106]. No hysteresis was found in the heating-cooling cycle under this condition (Fig. 16). The AR-N2 signal in the normal direction increased steeply at around 750 K and reached a steady value above 900 K, whereas the AR signal at 70 off normal toward the [0 0 1] direction became noticeable at around 500 K followed by a sharp increase above 750 K. Below 750 K, the AR signal at 70 is higher than that in the normal direction. The N2 desorption sharply collimates at around 70 off normal toward the [0 0 1] direction and is accompanied with a normally directed component in a cos2–2.5h form. The latter desorption is highly enhanced at higher temperatures and becomes predominant. The d-N2 formation pathway highly contributes to the total N2 emission below about 700 K. The N2 distribution was extensively studied in the steady-state NO + CO reaction on Rh(1 1 0). It is mostly collimated along the surface normal. The presence of the inclined

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Fig. 16. NO reduction on Rh(1 0 0); Surface temperature dependence of AR-15N2 signals at the surface normal and 70 off normal toward the [0 0 1] direction in a steady-state 15NO+D2 reaction. The latter signals were normalized to the surface area for measurements in the surface-normal direction. P15NO = 1.1 · 103 Pa, and P D2 ¼ 2:7  103 Pa. The signals observed in the direction of the increasing surface temperature are designated by closed symbols and those in the downward direction, by open symbols. The data in the range of 400–800 K were expanded in the inset. The angular distributions of desorbing 15N2 at TS = 664 K and 760 K are shown at the bottom. A typical deconvolution was drawn by broken curves. Ref. [106].

N2 emission was strongly suggested at around 470 K; however, its intensity was weak [66]. On this surface, NO is dissociated even at room temperature and the associative process of nitrogen adatoms becomes significant at around 450 K. Thus, the observation of d-N2 formation is obscured by an extensive overlap with the b-N2 formation at lower temperatures. The N2O formation becomes detectable at NO pressures above 10 Pa, where AR desorption measurements are not available [119]. On the other hand, on Rh(1 0 0), a high activation energy of b-N2 formation, about 200 kJ mol1, shifts the contribution above 750 K, and the d-N2 formation can then be observed in a wide temperature range [120].

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9.3. Pd(1 1 0) The steady-state NO + D2 (or CO) reaction on Pd(1 1 0) becomes observable above 500 K, peaks around 530 K and decreases slowly at higher temperatures (Fig. 17) [101,121,122]. The total formation ratio of N2/N2O was about 4 at PNO = 4.7 · 103 Pa with a (NO:CO = 1:1) mixture. The product N2 is merely desorbed in the inclined way below 530 K (Fig. 17b and c). The CO2 desorption is sharply collimated along the surface normal. Desorbing N2O always showed a cosine distribution. The spatial distributions of N2, N2O and CO2 were insensitive to the surface temperature and the gas composition below 530 K. On the other hand, the distribution of N2 above 550 K is sensitive to these

Fig. 17. NO reduction on Pd(1 1 0); TS dependence of (a) AI and (b) AR signals of products at the collimation positions in a steady-state 15NO + D2 reaction at PNO = 6.7 · 104 Pa with the pressure ratio of 15NO/D2 = 2. Angular distributions of desorbing 15N2 at (c) 530 K and (d) 640 K. Typical deconvolutions shown by broken curves are based on the velocity distribution analysis. The ordinate was normalized to the maximum at 530 K. Ref. [121].

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Fig. 18. NO reduction by CO on Pd(1 1 0); (a)PCO dependence of the AR-15N2 signals at h = 0 and 40 at 6.7 · 104 Pa of 15NO at TS = 640 K. (b,c) The angular distributions of desorbing 15N2 in the plane along the [0 0 1] direction are shown at the PCO values indicated by the arrows. The intensity on the polar coordinates was normalized to the value in (c). Typical deconvolutions are drawn by broken-line curves. (d) Velocity distribution of N2 at 640 K, at h = 0. PNO = 6.7 · 104 Pa; and PCO=2.7 · 103 Pa. The average kinetic energy is indicated in hi in temperature units. Typical deconvolutions are drawn by broken-line curves. Ref. [123].

quantities because the b-N2 formation becomes significant (Fig. 17b and d). The N2 signal at the surface normal is enhanced at higher TS or higher PCO values (Fig. 18) [123]. The angular distribution was deconvoluted into three components by referring to velocity distribution analysis, i.e., the flux of the thermalized component to the surface temperature yields the signal level of the cosine component in the angular distribution (Fig. 18c,d). The flux of the remaining fast component peaked at the normal direction and at around 40 off normal. The normally directed component in a cos5(h) form comes from the b-N2 formation (2 N(a) ! N2(g)) and the inclined component comes from the intermediate N2O decomposition. The three-dimensional distribution of desorbing N2 was also constructed from angular distributions at different crystal azimuths in the way described above. b dependences at a = 41 from the inclined N2 desorption at 550 and 640 K are very similar to those in the N2O reduction, indicating an identical emission process [102]. The N2 distribution for the NO + CO reaction has the form of {cos22(a + 41) + cos32(a  41)}cos17(b) at 550 K (Fig. 12d). The normally directed N2 desorption is approximated by a cos7a cos2b form. The distribution is commonly sharper in the plane along the [0 0 1] direction. Similarly, the fast CO2 component is approximated as a cos13a cos4 b form, common for the N2O + CO and NO + CO reactions. This anisotropy is very close to that in the CO + O2 reaction on Pd(1 1 0) [11].

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9.4. Surface-nitrogen removal In a steady-state NO + CO reaction, three surface-nitrogen removal pathways, i.e., (i) b-N2 formation; 2N(a) ! N2(g), (ii) d-N2 formation; NO(a) + N(a) ! N2O(a) ! N2(g) + O(a), and (iii) N2O(a) ! N2O(g), are commonly operative, and their contribution depends on the kind of metal and the surface plane as well as the surface temperature and the pressures of NO and the reducing reagent. No cosine component was found in desorbing N2 in the steady-state N2O + CO reaction above 400 K. Only the inclined N2 desorption proceeds in the N2O + CO reaction in the wide temperature range studied on Pd(1 1 0) and Rh(1 1 0), 400–800 K. Therefore, the thermalized (cosine distribution) component of N2 in the NO reduction should be formed in the other processes, probably in b-N2 formation, although the cosine component cannot be assigned to a definite process from the angular distribution. The d-N2 formation contributes significantly to the product N2 emission in the range of 450–700 K. At higher temperatures, the N2O decomposition pathway is reduced by decreased amounts of adsorbed NO. At lower temperatures, it is limited by NO dissociation because the N2O formation proceeds in the co-adlayer of N(a) and NO(a) even at around 100 K and adsorbed N2O dissociates at temperatures as low as 70 K. This indicates that the surface-nitrogen removal pathway to N2 through N2O can be extended to lower temperatures by enhancing the NO dissociation. On the other hand, the associative desorption of nitrogen adatoms, 2N(a) ! N2(g), has an activation energy in the range of 120–200 kJ/mol [119,120,124]. Thus, its contribution to N(a) removal is limited at high temperatures. This desorption commonly emits N2 along the surface normal on flat surfaces such as Ru(0 0 1), Cu(1 1 1), Pd(1 1 0), Ag(1 1 1), W(1 1 0) and W(320) [40–46]. The distribution varies from ‘‘very sharp’’ in a cosnh form with n = 75 on Ag(1 1 1) to a cosine form with n = 1 on W(320). The translational temperature of N2 was very high, more than twice that of CO2. The value was maximized to 3700 K at TS = 550 K at around the collimation on Pd(1 1 0) and around 4000 K on Rh(1 1 0). It somewhat decreased with an increasing shift from the collimation angle. In the velocity distribution analysis, the thermalized component described by the Maxwellian form was first subtracted from the observed curves. The resulting fast component usually shows velocity distribution curves that are broader than the Maxwellian form at its average translational temperature. The distribution width is presented by the speed ratio (SR), defined as (hv2i/hvi2  1)1/2/(32/9p  1)1/2 [55]. Here, v is the velocity of N2, hvi is the mean velocity, and hv2i is the mean square velocity. The SR value is unity when the distribution fits the Maxwellian form and frequently becomes smaller for desorbing molecules with hyper-thermal energy because repulsive desorption yields molecules that surmount the activation barrier [55]. For product N2 in the NO + CO reaction on Pd(1 1 0) and Rh(1 1 0), the value was estimated to be 1.1–1.3 at 640 K even at the collimation position of 43. This suggests that the resultant velocity curve involves two or more components. The present time resolution (15–20 ls) in the velocity work is not short enough to separate each component. 9.5. Branching of surface-nitrogen removal and inhibition by oxygen The real catalysts for NO reduction must be active in the presence of gas-phase oxygen although this reaction is highly retarded by oxygen (Fig. 19a–c). AR-product analysis has

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Fig. 19. Oxygen inhibition to NO reduction on Pd(1 1 0); Temperature dependence of the AR-product signals at their collimation angles in a steady-state 15NO + CO reaction. (a) 15N2 at 41 off the surface normal towards the [0 0 1] direction. (b) 15N2 at the surface normal, and (c) 15N2O at the normal direction. The closed circles show the results when the 15NO pressure was 6.7 · 104 Pa and the pressure ratio of 15NO/CO was unity. The open circles show the signals when the pressure ratio of 15NO:CO:O2 = 1:1:1. Branching ratio of surface-nitrogen removal versus temperature; (d) [Inclined N2 emission]/[N2O] and (e) {[inclined N2 emission] + [N2O]}/{[normally directed N2 emission] + [cosine N2 emission]}. Ref. [126].

given a new insight to this inhibition. In the steady-state NO + CO + O2 reaction on Pd(1 1 0), the angular and velocity distributions of the products are very similar to those in the NO + CO reaction, i.e., the reaction pathways do not change by the addition of oxygen. Both N2O(a) and NO(a) dissociations are retarded by the addition of O(a) and the rates of the surface-nitrogen removal processes vary differently. There are three kinds of branching in surface-nitrogen removal processes in the NO + CO reaction. The first is the branching of the intermediate N2O decomposition to desorption. It is defined as Branching I ¼ ½Inclined N2 =½N2 O

ð10Þ

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where [inclined N2] and [N2O] are the amount of each species estimated from their AR-signal intensities and angular distributions [125]. The results at PNO = PCO = 6.7 · 104 Pa are shown as a function of the surface temperature in Fig. 19d [126]. The ratio increases with increasing temperature below 550 K and shows a constant value at around 2 above it. This increase is due to the decreasing blocking by CO(a) through its removal, and the activation energy of the dissociation is not necessarily higher than that of desorption. In fact, the branching ratio decreases below 550 K with increasing temperature when O2 is introduced at 1.3 · 104 Pa. A small amount of adsorbed oxygen can stabilize N2O(a) [88]. The second is the branching in the conversions of N(a) into the associative process of N(a) to N2(g) and the N2O(a) formation as N(a) + NO(a) ! N2O(a). This branching is presented by the ratio of the sum of the inclined N2 and the cosine N2O desorption to that of the normally directed N2 and the cosine N2 desorption as, Branching II ¼ f½inclined N2  þ ½N2 Og=f½normally directed N2  þ ½cosine N2 g ð11Þ where [normally directed N2] and [cosine N2] were again estimated from their signal intensity and angular distributions. The ratios estimated at PNO = PCO = 6.7 · 104 Pa are shown versus the temperature in Fig. 19e. It becomes very large below 550 K, indicating that the reaction proceeds mostly through the intermediate N2O pathways. Above 650 K, the ratio decreases below unity, and the association of N(a) becomes predominant. The third is the branching in the associative process as presented by a ratio of the normally directed N2 to the cosine N2 component. This is always kept at around 0.1, irrespectively of the surface temperature and NO/CO pressure ratio, i.e., its product N2 is mostly thermalized. The first two branching ratios changed in the presence of gas-phase oxygen. Branching II shifts slightly to higher values (Fig. 19e), i.e., the pathway through the intermediate of N2O(a) is somewhat enhanced. On the other hand, branching I increases largely below 550 K, i.e., N2 emission through N2O(a) is largely enhanced below 550 K. This is probably due to the stabilization of adsorbed N2O by surface oxygen. This suggests active NO reduction catalysts working at around room temperature, at which NO is converted into N2O followed by reduction. 10. Associative desorption 10.1. Introduction There are several associative desorption processes yielding products with hyper-thermal energy as described in Section 5. The CO desorption in carbon oxidation and CO2 desorption in CO oxidation show remarkable surface corrugation effects [49,127]. Especially thermal CO oxidation shows an advantage since the product formation is localized at around the oxygen adsorption site because of the high mobility of adsorbed CO. In fact, all the angular distributions of desorbing CO2 reported on different planes are consistent with this idea [127]. Of course, CO2 formation proceeds at different sites in photo-induced CO oxidation where a different mechanism is operative [128–130]. The bulky CO2 product is repulsively desorbed at the moment of formation because the reactants CO(a) and O(a) are located close to the surface and the nascent product then

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appears at the repulsive part of the potential energy curve for CO2 adsorption. The repulsive force exerted from its formation site to the product is strong enough to hold the site information in the angular distributions. The information is classified into three parts, as follows: (I) Coverage effect: The product CO2 desorption on flat surfaces such as Pd(1 1 1), Pt(1 1 1) and Pd(1 0 0), is collimated along the surface normal, and its angular distribution becomes sharper with increasing coverage of the reactants. The kinetic energy of desorbing CO2 is enhanced at higher coverage, showing that the repulsion is also operative from surrounding reactants (Fig. 20a). (II) Site symmetry: The symmetry of the formation site appears in the angular distribution on anisotropic surfaces, such as fcc(1 1 0) and stepped surfaces. The distribution becomes broader in a flatter plane, such as along the ½1 1 0 direction on fcc(1 1 0) rather than along the [0 0 1] direction (Fig. 20b) [36,127].

Fig. 20. Characteristics in repulsive CO2 desorption from the CO(a) + O(a) associative process. (a) Coverage effect on flat planes. Top and side view of (b) non-reconstructed fcc(1 1 0)(1 · 1) and (c) reconstructed fcc(1 1 0)(1 · 2) planes. A long-bridge site in the trough on the (1 · 1) works as the adsorption site for O(a) and emits CO2 along the surface normal. On the (1 · 2) form, a threefold hollow site on the inclining terrace is suitable for O(a) adsorption and emits CO2 into the inclined direction. The dashed ellipses indicate the size of the CO2 molecule drawn by the van der Waals’ radii. Typical spatial distributions showing the symmetry and orientation of the reactive desorption site on Pd(1 1 0)(1 · 1) and Pt(1 1 0)(1 · 2) are shown at the bottom. Ref. [127].

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(III) Local facet orientation: The CO2 desorption collimates closely along the local normal of inclined terraces, showing the CO2 formation on these terraces (Fig. 20c). The facet orientation, described in item III above, is useful for identification of the product formation site. Such CO2 desorption closely parallel to a local normal was first found on Pt(1 1 0)(1 · 2), where three-atom-wide terraces with a (1 1 1) structure incline about 30 from the bulk plane (Fig. 20b) [131]. The product CO2 is desorbed in two-directional ways collimated around 26 off normal toward the [0 0 1] direction. Of course, this CO2 distribution shows the coverage dependence predicted from the coverage effect described in item I above as well as the anisotropy that would be expected from the site symmetry described in item II above. A similarly split distribution of desorbing product CO was found when O2 beams were introduced on a carbon-covered Pt(1 1 0)(1 · 2) surface by Walter and King [49]. The resultant CO flux was in a sharp form of cosn(h-32), where n = 50 ± 5. Differences are noticeable in the collimation angle between CO2 and CO in the above reactions on Pt(1 1 0)(1 · 2). Furthermore, these angles are largely shifted from the local normal of the inclined terraces because an angle of 35.2 is expected for the inclination when the surface is not distorted. Thus, this collimation angle shift in CO oxidation was systematically examined over various stepped surfaces on which (1 1 1) terraces were inclined at different angles [11,127]. The observed collimation angles in AR-TPD work always shift to smaller values that are 25–30% of the local normal of the terrace without distortions. This angle shift is due to the surface smoothing effect of conduction electrons, i.e., the shape of the PES is shifted from that of the geometrical surface structure. The shift disappears when the desorbing CO2 holds higher kinetic energy than that in thermal CO oxidation [129]. Such CO2 with a high kinetic energy can be formed in photo-induced CO oxidation on platinum surfaces. In fact, the resultant CO2 at 193 nm collimated along the local terrace normal on stepped Pt(1 1 2)(1 · 1) and Pt(113)(1 · 2) [128–130]. The translational temperature of this CO2 was 3240 K at the collimation angle, about 2.6 times higher than that in thermal CO oxidation, 1240 K at TS = 230 K. The product CO desorption on Pt(1 1 0)(1 · 2) shows a very sharp distribution suggesting a high translational temperature collimating closer to the local normal of the inclined terrace. 10.2. Site cooperation The CO2 formation site switches depending on the reaction conditions. A reconstructed stepped surface of Pt(113)(1 · 2) is suitable for examining how surface facets with different functions work cooperatively when they are located next to each other [125]. It consists of 3-atom-wide (1 1 1) terraces and 3-atom-wide (0 0 1) facets [132,133]. These facets alternately incline in different directions. The (1 1 1) facet slopes +29.5 from the bulk surface plane (without surface distortions) and the (0 0 1) facet slopes 25.2 in the opposite direction (Fig. 21). On this surface, adsorbed oxygen atoms on the (1 1 1) facet are likely to be more reactive toward CO than those on the (0 0 1) facet, since the adsorption heat is small on the (1 1 1) facet. In other words, the oxygen on (1 1 1) is selectively removed by mobile CO and more dissociated O2 is located on the (0 0 1) facets. Such cooperative interaction between these two facets can be examined through AR-product analysis; i.e., at high oxygen coverage, the product CO2 is expected to be emitted from the (1 1 1) facet. In fact, at CO pressures far below the kinetic transition point, CO2 mostly desorbs along about +22

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Fig. 21. Site cooperation on Pt(113); PCO dependence of the steady-state AR-CO2 signal at h = +22 for the (1 1 1) terrace site and h = 20 for the (0 0 1) step site. The vertical broken line indicates a kinetic transition point. The CO2 distributions on three-dimensional polar coordinates are inserted at PCO indicated by arrows. A top view of Pt(113)(1 · 2) surface is shown on the upper panel. Ref. [125].

off normal, close to the local normal of the (1 1 1) facet. On the other hand, CO2 desorption clearly shifts to 20 off normal when the CO pressure is close to the critical value or above it (Fig. 21b [125]. There is a critical CO pressure showing a kinetic transition in CO oxidation. The reaction is first order in CO below the critical pressure (called the active region) and achieves a negative order above it (called the inhibited region). The boundary between them is controlled by the maximum supply of CO(a) and its removal. Such kinetic switching occurs sharply because the reaction of CO(a) with O(a) is very fast at temperatures necessary for steady-state CO oxidation [11]. In the active region, CO(a) does not accumulate, i.e., CO(a) < O(a), whereas in the inhibited region, the O(a) supply is insufficient to significantly affect the coverage of CO(a), i.e., CO(a)  O(a). With increasing CO pressure toward the kinetic transition, the surface begins to lack oxygen, and the remaining O(a)

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is more localized on the O2 dissociation sites. The collimation of CO2 along the [0 0 1] direction at around the kinetic transition indicates that O2 is dissociated on the (0 0 1) facets. This is reasonable since the sticking probability of O2 is close to unity on Pt(1 0 0) and about 0.1 on Pt(1 1 1) [134–136]. CO2 formation takes place mostly on the (1 1 1) facets on platinum-stepped surfaces such as Pt(1 1 2) = [(S)3(1 1 1) · (0 0 1)], Pt(3 3 5) = [(S)4(1 1 1) · (0 0 1)] and Pt(5 5 7) = [(S)6(1 1 1) · (0 0 1)] [127]. Reaction on the (0 0 1) facet is limited to conditions under which O(a) is present in very small amounts. Such cooperation between facets with different functions is likely to take place on small metal particles used on real catalysts because of the coexistence of many surface planes with different structures on a nano-sized particle. The presence of a ‘‘pressure gap’’ was once emphasized as an essential difference in studies between surface science and real catalysis. This difference has become unessential after development of different spectroscopy, especially sum-frequency-generation (SFG) and STM working at high pressures, and no serious differences were found in chemical kinetics over a wide pressure range. This gap was primarily caused, from a technical point of view, by methods using electrons as a probe and not exclusively in surface phenomena. A surface science approach to catalysis should be extended to such a cooperative effect among different facets and not limited to phenomena on a single crystal surface. 10.3. Changes in structures and distributions The angular distribution of desorbing products changes when the surface structure is converted [137,138]. A clean Pt(1 1 0)(1 · 1) surface is reconstructed into the (1 · 2) missing-row form above ca. 300 K [139]. This reconstruction is lifted into a (1 · 1) form when the surface is covered by CO [140]. Oxygen adsorption does not affect this lifting. Thus, the surface is kept in the (1 · 2) form in the active region of CO oxidation (CO(a)  O(a)) and is converted into the (1 · 1) form in the inhibited region (CO(a)  O(a)) [141,142]. This conversion into the (1 · 1) form is completed at half a monolayer (ML) of CO(a). Photo-emission electron microscopy (PEEM) experiments showed that, around the kinetic transition or above it, oxygen adatoms and CO(a) are distributed in separated domains of peculiar patterns [142,143]. This happens when the overall CO coverage is below 0.5 ML and CO(a) forms domains. It has been argued that CO2 is formed between oxygen and CO domains. Of course, this statement does not identify the CO2 formation site, since there are large spaces between domains. Furthermore, CO2 formation is strongly affected by the surface diffusion of reactants, i.e., the reaction rate is much more enhanced when the reactants are highly mixed. No information on the CO2 formation site is provided from kinetic analysis under these conditions. On the other hand, even under such conditions, the angular distribution of desorbing CO2 can provide more information about the formation sites at the atomic level. In the active region, CO2 desorption is split in a two-directional way, indicating CO2 formation on the inclined (1 1 1) terrace. At the kinetic transition, CO2 formation decreases, and its desorption collimates along the surface normal when CO pressure is high enough to reach 0.5 ML of CO(a) [136]. However, when CO pressure is not high enough to yield 0.5 ML, CO2 desorption collimates in a bi-directional way even above the kinetic transition (Fig. 22a and b), indicating the formation of CO2 on the inclined terrace even in the inhibited region; i.e., the surface is covered by separate domains of CO(a) and O(a), consistent with PEEM work. The CO(a) domains are in the (1 · 1) form, whereas

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Fig. 22. CO2 formation sites on Pt(1 1 0)(1 · 2); (a) Angle-resolved CO2 signals observed at h = 25 (s) and h = 0 (d) with CO pressures at constant P O2 and TS. (b) Angular distributions of CO2 in the plane along the [0 0 1] direction are shown on polar coordinates. The signal on the ordinate was normalized. Typical deconvolutions are drawn by broken and dotted curves. (c) An active (1 · 2) domain model. Ref. [138].

the O(a) islands are in the (1 · 2) form [142–144]. The angular distribution sharply changes to the normally directed form when the CO pressure is sufficient to achieve a half-monolayer of CO(a). Experimentally, this was defined as the CO pressure at which the bi-directional desorption changes to the normally directed form [138]. The term for this phenomenon is site-switching (Fig. 22a). Thus, CO2 desorption predominantly takes place on the (1 · 2) domain when both (1 · 2) and (1 · 1) domains coexist. This is expected because of the high reactivity of oxygen on the (1 1 1) terrace (Fig. 21). Site cooperation works once again. In the active region, a clear (1 · 2) LEED pattern, which is merely due to the (1 · 2) domains, appears. With increasing PCO, CO(a) accumulates, and the (1 · 2) reconstruction then converts into the (1 · 1) form. The intensity of the (1 · 2) LEED pattern starts to decrease already in the active region and is diminished to the background level before the site-switching point. Nevertheless, CO2 formation takes place mostly on the (1 · 2) domains until the site-switching point. CO2 formation remains almost invariant although the (1 · 2) area is highly reduced. This is not surprising because of the fast reaction of CO(a) with O(a) on the (1 · 2) domain and the slow supply of CO(a) and O(a). The (1 · 2) domain has high capacity to produce CO2 until its complete transformation to the (1 · 1) form. This is sketched in Fig. 22c.

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11. Summary and future work In the studies of the above associative CO2 or dissociative N2 desorption, the distributions of the flux and translational energy of desorbing products provide structural information about the reaction site, i.e., its shape for associative desorption as well as the orientation of parent molecules for dissociative reactions. The information is still available even if desorption is not rate-determining because desorption dynamics is not controlled by processes before product emission. AR-product desorption analysis will deliver more information when it can be performed at a level of state-selective analysis under steady-state reaction conditions. This next stage becomes important in improving the method because each energy state of desorbing products will show different structural information. The resultant dynamics will provide the most direct site-identification method applicable in the course of catalyzed reactions. In the above desorption dynamics, no effect of the shape of product molecules has been considered because of the lack of internal energy mode measurements in the AR form. The relation of desorption dynamics to site structure will be elaborated by adding the desorption-angle dependence of the internal energies. This dependence indicates that there is facile energy partitioning into the rotational, vibrational, and translational modes in a repulsive desorption event. Furthermore, it delivers the structures of the transition states immediately before desorption when the angle dependence varies with the crystal azimuth. This correlation has not been characterized because the desorption-angle dependence of the internal energy of desorbing molecules has not been studied either experimentally or theoretically. Furthermore, little is known about energy transfer between surfaces and desorbing molecules [145]. Significant energy transfer was already predicted for reactive CO2 desorption, i.e., about half the transferable potential energy of the activated complex was estimated to be transferred back to the surface [56]. Acknowledgements The author thanks Prof. Adolf Winkler (Graz University of Technology, Austria) and Fellow Dr. Kosuke Shobatake (Toyota Physical & Chemical Research Institute, Japan) for their participation in critical discussions, the reviewers for invaluable comments, and Ms. Atsuko Hiratsuka for drawing the figures. The fellowship support of the Alexander von Humboldt Foundation and the hospitality of Professor Eckart Hasselbrink and his group members in Duisburg-Essen University during the preparation of this review are gratefully acknowledged. This work was partly supported by a 1996 Center of Excellence (COE) special equipment program of the Ministry of Education, Sports, and Culture of Japan and by Grant-in-Aid No. 13640493 for General Scientific Research from the Japan Society for the Promotion of Science. References [1] K. Tamaru, Adv. Catal. 15 (1964) 65. [2] G. Ertl, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Springer-Verlag, Berlin, 1983, p. 209. [3] K. Tamaru, in: Dynamic Heterogeneous Catalysis, Academic Press, London, 1978, p. 96. [4] R.P.H. Gasser, in: An Introduction to Chemisorption and Catalysis by Metals, Clarendon Press, Oxford, 1985, p. 178.

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