Chapter 7. Surface reactivity: geometry and electronic structure

Transcription

1 146 Chapter 7. Surface reactivity: geometry and electronic structure 7.1 Introduction As described in previous chapters, the Ir(210) surface structure can be modified at will and in situ: the atomically rough planar Ir(210) can be converted to a clean nano-faceted surface, and facet size can be controlled. For these reasons Ir(210) is a most convenient substrate for studying structure reactivity and particle size effects in catalytic chemical reactions on surfaces. The amazing effect of catalysis by surfaces is demonstrated in many systems. One of the most striking is the simple process of water formation: 2H 2(g) + O 2(g) 2H 2 O (g). The free energy for this reaction is around 230 kj/mol, yet a stoichiometric mixture of O 2 and H 2 gasses may be stored indefinitely in a glass container without any detectable signs of water formation. If, however, a piece of high-area platinum gauze is added to the mixture, the reaction occurs spontaneously and explosively. The accelerating (or enabling) effect of catalysts was first noticed in the early 18 th century by Kirchhoff and Davy, and since that time catalytic reactions have become of central importance in the chemical industry. 1 The usefulness of catalysis is often not only in the fact that it allows a system (of reactants) to achieve its most thermodynamically stable state, but in the ability to steer reactions into pathways that are otherwise not probable and to yield products that can not be obtained in simple full conversion reactions. The usual term used for this ability of catalysts to stimulate the formation of particular products is selectivity. 1 Industrial applications aside, photosynthesis and enzymatic reactions, processes essential for life as we know it, are catalytic reactions.

2 147 When the reactants and catalyst are in different phases (e.g. gaseous or liquid reactants over a solid catalyst), the process is described as heterogeneous catalysis. Heterogeneous catalysis accounts for around 80 % of catalytic application in today s industry. Under UHV conditions most of the gas in a vacuum system is adsorbed on the walls (see Section 2.1) and mean-free-paths of species in the gas phase are so long that gas-phase collisions occur only very rarely. It is immediately obvious that reactions studied under such conditions will be that of the background gasses on suitably chosen substrates and therefore inherently heterogeneous. The importance of transition metals in catalysis stems from the facts that these elements and their compounds (e.g. oxides, sulfides, carbides) are outstandingly active as catalysts and are used in most surface catalytic processes. The reasons for the sometimes striking effect of catalysts on chemical reactions are not completely understood, but can be plausibly explained. For example, water is not formed at room temperature in a O 2 /H 2 mixture because dissociation of either gas, a necessary step in the reaction, requires energy far higher than that thermally available [1]. The same is true for the gas phase reaction of the corresponding atomic species. On the surface of platinum, H 2 and O 2 dissociate and react without activation. The role of surfaces in catalytic reactions is then to dissociate otherwise very stable molecules and lower activation energies for reactions on the surface. In particular, d-electrons and bonds in transition metals are thought to play a crucial role by contributing to bonding of atoms and molecules at surfaces and by mixing with s and p electronic states to provide a wealth of states that can donate and accept electrons and open or facilitate reaction pathways [2]. If the availability of degenerate electronic states is responsible for catalytic action (especially bond breaking), it is plausible (somewhat obviously) that catalytic activity must de-

3 148 pend on the material used as catalyst and (not so obviously) that not all sites/atoms of a particular surface are equally active in this respect. In the latter case, macroscopically, a surface with higher density of active sites should exhibit higher overall activity. This structure sensitivity has been demonstrated in many systems. One of the most important catalytic reactions, ammonia synthesis, exhibits an almost order of magnitude difference in rate over different iron surfaces [3]. Additionally, the very size of catalyst particles can affect the rate of chemical reactions [4]. This is explained, in part, by differences in the surface structure of particles of different sizes and the dependence of electronic properties on particle size. There is, however, an additional effect that depends very much on the microscopic details, especially surface diffusion, of the overall process. This can be understood by considering a reaction in which intermediates diffuse on the surface before reacting with each other. The presence of steps, edges or grain boundaries in the diffusion paths can significantly affect reaction rates [5]. Structure sensitivity and size effects have been demonstrated in ethylene formation from acetylene on the planar and faceted Pd/W(111) surface [6]. In this study, TPD spectra of C 2 H 2 /Pd/W(111) show product desorption temperatures that depend not only on the substrate crystal orientation, Pd/W(211) and Pd/W(111), but also on the size of facets formed on the Pd/W(111) surface upon annealing. The ultimate goal of surface science studies of heterogeneous catalysts is to understand how they function on the atomic scale. This includes identification of active sites, the effects of structure and composition on reactivity and selectivity and, ultimately, new catalyst design. Desirable properties of a catalyst depend on two factors directly related to the elementary steps of adsorption and desorption. The reactants (and intermediates, if applicable) must

4 149 bind to the catalyst substrate strongly enough in order to stay on the surface sufficiently long to have a satisfactory probability of reaction, but weakly enough to be mobile and diffuse on it and collide with their reaction counterparts. The final products, on the other hand should be weakly bonded and desorb upon formation to free the catalyst surface for further adsorption and reaction. This chapter summarizes results regarding TPD studies of hydrogen desorption, acetylene thermal decomposition and ammonia decomposition on planar and faceted Ir(210) surfaces. Ammonia decomposition on iridium is of particular interest due to the feasibility of ammonia as a hydrogen storage medium and the favorable decomposition conditions on iridium [7-9]. The electronic structure of the several iridium surfaces prepared in these experiments was studied using HRSXPS. A section is also devoted to the results of these experiments and the effects of electronic structure on surface reactivity are discussed. 7.2 Experimental The desorption experiments described in this chapter were performed in the reaction chamber described in Section 2.2. The sample was cleaned using the procedure described in the same section, with flashes to a slightly lower temperature of 1700 K. Faceted surfaces with three average facet sizes were prepared as described in Chapter 6. Surface cleanliness for planar and faceted surfaces was verified by AES and TPD. LEED was used to determine the surface structure. As described in Section 4.7, a clean faceted surface can be prepared by reacting the oxygen adsorbate with CO or H 2 at relatively low temperatures (550 K and 400 K, respectively). In the context of the experiments described in this chapter, it is noteworthy that neither of these

5 150 two reactions (CO 2 and H 2 O formation), although exothermic, occurs in the gas phase at these temperatures. The fact that a clean faceted surface can be prepared by CO oxidation and water formation shows that the faceted Ir(210) surface is catalytically active for these conversions. Research purity oxygen, hydrogen, acetylene and ammonia were deposited on the sample by introducing gas in the background. TPD spectra were obtained as described in Section 3.3. A linear background was subtracted from some spectra in order to better compare the integrated areas under the curves; in other cases a deliberate offset was introduced to better distinguish between spectra for different C 2 H 2 and NH 3 exposures. 7.3 Hydrogen adsorption and desorption Hydrogen is a constituent of many compounds that are of interest in studying catalysis on surfaces, most notably hydrocarbons and ammonia. In many reactions of these compounds, such as decomposition and dehydrogenation, it is also a major desorption product. To be able to interpret desorption spectra of hydrogen-containing molecules, the adsorption and desorption of hydrogen on the Ir(210) surface and its derivatives must be studied first. By comparing hydrogen desorption spectra from adsorbed hydrogen and from adsorbed hydrocarbons, a distinction can be made between reaction-rate limited desorption-rate limited processes. TPD spectra of H 2 following deposition of several amounts of molecular hydrogen are shown in Figure 7.1. The temperature range in these and other figures is deliberately wide in order to allow for easy comparison between other spectra in this chapter. For adsorption at 300 K (Figure 7.1b), a single desorption peak around 400 K can be observed for the lowest exposure. As exposure is increased, this peak grows with the formation of a shoulder at a slightly

6 151 lower temperature (around 350 K). A comparison of the integrated surface areas under the 0.05 L and 3.0 L peaks, shows a ratio of around 2.7, considerably smaller than the exposure ratio of 60, indicating that coverage for the 3 L peak nears saturation (see uptake curves in Chapter 3). The spectra for 100 K deposition (Figure 7.1a) show the development of an additional peak at around 270 K for doses higher than 0.5 L, an indication of a second binding site for H/Ir(210). A common and, in the context of the experiments described in the following sections an important feature of both sets of spectra, is that all of the hydrogen desorbs from the surface at just above 500 K. H2 signal [arb. units] H 2 exposure: 0.5L 0.1L 0.05L a H2 signal [arb. units] H 2 exposure: 3.0L 1.0L 0.5L 0.1L 0.05L b Sample temperature [K] Sample temperature [K] Figure 7.1. TPD spectra of H 2 from clean planar Ir(210). Deposition performed at (a) 100K (b) and 300K. Hydrogen adsorbs dissociatively, in atomic form, and recombines to desorb as H 2. TPD spectra of hydrogen deposited on the clean faceted Ir(210) surface are shown in Figure 7.2. Deposition was performed at room temperature. The two coverages shown, correspond to saturation and around 58 % of saturation. For the lower coverage there is a single peak (b) in the spectrum at 440 K. For saturation coverage, a distinct low temperature peak (a) is formed also at 340 K. There are several differences between the spectra for the faceted surface and the spectra for the planar surface shown in Figure 7.1b. The most obvious is the

7 152 appearance of two peaks for desorption from the faceted surface (compared to only one peak for desorption from the planar surface). Considering the structure of the faceted surface, this can be interpreted as desorption from the (110) and {311} facets individually. In studies on fcc Rh(110) and Rh(311) [10], surfaces similar to those of Ir(110) and Ir(311) 2, the hydrogen desorption temperature was found to be around 50 K higher for H/Rh(311) than for H/Rh(110). The ratio of integrated peak areas is in qualitative agreement with the total surface area ratio of the (110) and {311} facets (Section 4.8.1). In view of this and a study of desorption of H/Ir(110) [11], peak (a) can be attributed to desorption from the (110) facets, and peak (b) to desorption from the {311} facets. For fractional coverage, only the high H2 signal [arb. units] 100 b-high coverage a-low coverage ample temperature [K] Figure 7.2. TPD spectra of H 2 from clean faceted Ir(210) with 14nm average facet size. Heating rate is 5K/s. binding sites corresponding to peak (b) are populated. As coverage is increased, sites that give rise to peak (a) are populated as well. Not unexpectedly, this indicates a relatively unimpeded diffusion process of hydrogen on the faceted surface at room temperature. In this process hydrogen molecules adsorb and dissociate at random positions on the surface and then, through diffusion, populate binding sites in order of decreasing binding energy. The atoms recombine to desorb as molecules upon heating. 2 Apart from the identical (fcc) crystal structure, Rh and Ir have very similar lattice constants: pm and 383.9pm, respectively.

8 153 Additional differences can be found in the peak shapes. Compared to the desorption peaks from the planar surface, peak (b) in Figure 7.2 for the faceted surface is considerably narrower. For the lowest exposure on the faceted surface FWHM p = 42 K; on the planar surface, for peak (b), FWHM f = 86 K. Since the Ir(210) surface is atomically rough, and the {311} and (110) are relatively smooth, this difference can be explained in terms of the number of inequivalent high-coordination adsorption sites on each of these surfaces. For an adsorbate/substrate system with more than one kind of adsorption site, desorption occurs from the lowest binding energy sites first, then the next lowest etc. For a surface with several inequivalent binding sites and relatively small binding energy difference between sites, the temperatures of corresponding desorption peaks are closely spaced and there can be substantial overlap. Under such conditions, desorption peaks are difficult to resolve and can have the appearance of a single wide peak. For a surface or assortment of surfaces (such as a faceted surface) with a small number of inequivalent adsorption sites well separated in binding energy, desorption peaks can be resolved. The desorption peak on the (210) surface is wide due to the presence of several high-coordination adsorption sites and the tendency of hydrogen atoms to preferentially adsorb there, rather than at low-coordination, atop sites. Desorption peaks from the atomically smoother (110) and {311} surfaces are relatively narrow and well resolved. In summary, the adsorption/desorption processes of hydrogen on planar and faceted Ir(210) exhibit structure sensitivity. 7.4 Acetylene decomposition TPD spectra show that H 2 strongly dominates as a desorption product following the adsorption of acetylene on clean planar Ir(210) (at 100 K and 300 K) and heating. For both

9 154 deposition temperatures (shown in Figure 7.3.), the spectra show a single wide peak at around 410 K for the lowest exposure. As exposure is increased, this peak shifts to lower temperature, to a minimum of around 350 K before shifting again to near the original temperature for the highest exposure. Additional, less prominent peaks, also appear with exposure increase and are especially apparent for low temperature deposition. The presence of hydrogen in the C 2 H 2 /Ir(210) hydrogen desorption spectra above 500 K (as in the spectra in a b H2 signal [arb. units] 100 3L 1L 0.7L 0.5L 0.3L Sample temperature [K] 700 H2 signal [arb. units] 3L 2L 1L 0.7L 0.5L 0.3L Sample temperature [K] Figure 7.3. TPD spectra of hydrogen for acetylene adsorption on planar Ir(210) at 100K and 300K Figure 7.1) indicates that the dissociation process is rate-limiting. The broadening of the peaks and the appearance of additional features indicates a stepwise decomposition process in which partially dehydrogenated intermediates decompose as temperature is increased. The existence of intermediates is seen in high-resolution electron energy loss spectroscopy (HREELS) experiments [12]. For low initial acetylene exposures (0.5 L) at 90 K the surface species are dominated by CCH (acetylide). For high exposures (3 L), acetylide and ethylidyne (CCH 3 ) coexist on the surface.

10 155 TPD spectra of hydrogen following the exposure to 3 L of acetylene on clean planar and H2 signal [arb. units] clean planar clean faceted clean faceted Ir(210) are shown in Figure 7.4. The spectra are very similar for temperatures below 500 K. This is confirmed in HREELS measurements following acetylene deposition Sample temperature [K] Figure 7.4. TPD spectra of H 2 from clean planar and clean faceted Ir(210) surface for a H 2 C 2 dose of 3L. at 90 K as well as measurements performed after subsequent anneals of the sample to 300 K, 400 K and 500 K. The same loss features are present in the spectra for planar and faceted surfaces upon deposition. With temperature increase, features in the spectra also vary in intensity, appear and disappear in similar fashion for both surfaces indicating the presence of the same hydrocarbon fragments in the course of thermal decomposition. For temperatures in excess of 500 K, TPD spectra do show differences. Instead of the H2 differential signal [arb. units] S Sample temperature [K] Figure 7.5. Differential TPD spectrum (S diff ) for H 2 desorption from the clean planar and clean faceted Ir(210) surface for a H 2 C 2 dose of 3L. broad feature in the 500 K-750 K range present in the planar surface spectrum, two peaks (at around 550 K and 650 K) appear in the desorption from the clean faceted surface. The peak position above 500 K shows that, in the case of the faceted surface, desorption is also reaction-rate limited. These differences are seen in the desorption spectra in Figure 7.4, but can be estimated better (quali-

11 156 tatively if not quantitatively) in the differential spectrum in Figure 7.5. To calculate this spectrum, background is subtracted from the two original spectra (S pln and S fac ) and so that their intensities at the lowest (before sample heating) and highest (after H 2 desorption is complete) temperature are approximately equal to zero. The integrated area below both curves is then calculated and the overall intensities normalized so that the integrated areas are identical, and finally the normalized curves are subtracted (S diff = S facn S plnn ) and the result smoothed by 5 nearest-neighbor averaging. The interpretation of this spectrum is relatively straightforward: positive values correspond to a higher H 2 desorption rate from the faceted than from the planar surface. For T < 500 K decomposition is mostly faster on the faceted surface, but above that temperature the situation is reversed. These changes in H 2 desorption rate are an indication of structure sensitivity in thermal decomposition of acetylene on iridium. Reasons for this effect can be sought in the differences in stability (or nature) of some intermediates on the planar and faceted surface. 7.5 Ammonia decomposition The motivation for studying ammonia decomposition on iridium partially stems from the feasibility of ammonia as a hydrogen storage medium for on-site hydrogen generation in applications such as fuel cells [7-9]. While ammonia decomposes on many metallic surfaces, the decomposition on iridium is unique in what follows the process. Iridium does not form nitrides, and the desorption temperature of nitrogen from iridium is relatively low, so there is little potential for self-poisoning [13]. Desorption spectra following ammonia adsorption on Ir(210) are shown in figure Figure 7.6. Molecular hydrogen and nitrogen account for around 97 % of the desorption

12 157 H 2 <l>=14nm N 2 <l>=14nm H2 signal [arb. units] <l>=11nm <l>=5nm planar facet size increase N2 signal [arb. units] facet size increase <l>=11nm <l>=5nm planar Sample temperature [K] 700 a Sample temperature [K] 700 b Figure 7.6. TPD spectra of H 2 and N 2 desorbing from clean planar and clean faceted Ir(210) following ammonia adsorption at 300K. Sample heating rate is 2.5K/s. products, showing that the Ir(210) surface and its faceted derivatives very efficiently catalyze ammonia decomposition. A comparison between the spectra for the planar surface (open circles) and any of the spectra for faceted surfaces (black circles) for both desorption products indicates structure sensitivity for ammonia decomposition on these surfaces. In the case of hydrogen this is manifested by a difference in the peak shape and integrated peak area ratio for the two peaks (370 K and 450 K) in Figure 7.6a. In the case of nitrogen desorption (Figure 7.6b) the differences are somewhat subtle and consist of a change in nitrogen desorption temperature. Since all of the hydrogen desorbs by around 500 K, all ammonia decomposition can be considered complete at that temperature, and the change in N 2 desorption temperature must be due to a change in desorption activation energy a desorption-rate, not reaction-rate limitation. In addition to differences in reaction rates between the planar and faceted surfaces, these rates differ between the faceted surfaces themselves, depending on facet size. For hydrogen desorption the ratio of the two peaks changes from approximately equal for the smallest fac-

13 158 ets, to a broad peak where the two original peaks are no longer distinguishable. Since facets are of identical orientation irrespective of their size (as described in Section 4.4), the observed rate change is an indication of size effects in ammonia decomposition on faceted Ir(210). Nitrogen desorption is also affected by facet size: a difference of the desorption peak temperature of around 30 K can be observed between the surfaces with the smallest and largest facets. The observed effect qualitatively resembles the structure size dependence in reactions of C 2 H 2 /Pd/W(111) [6, 14]. Size effects in chemical reactions can, at least in part, be explained by the different distribution of facet plane and edge sites for surfaces composed of small and large facets respectively. While the facet orientations on surfaces with different facet sizes are identical, the surface morphology is evidently different. Most notably, the length of facet edges per unit surface area decreases with increase in facet size. The adsorption sites (atoms) on the surface can be roughly divided into two groups: those that belong to facet planes and those contained in facet edges. Kinetic Monte-Carlo simulations show that desorption kinetics depend on the spatial distribution of inequivalent adsorption sites on the surface [5, 15, 16]. For example, the reaction/desorption rates from a substrate consisting of two semi-infinite types of surfaces would be very similar to the sum of the spectra of the two surfaces individually. If, however, the two types of surfaces are arranged in small neighboring patches (like planes and edges on a faceted surface), this may no longer be true and desorption spectra for such combined surfaces can, under many realistic conditions, bear little resemblance to the desorption spectra for any of the individual surfaces or their sum. The importance of observing structure sensitivity and size effects on iridium stems, in part, from the fact that the experiments are performed on a clean, unsupported and well character-

14 159 ized metal surface that can be prepared in situ. The preparation procedure is very reproducible and there are no complicating factors (such as substrate support or adsorbates) to contribute to the effects without being adequately accounted for. This makes the faceted iridium surface not only an excellent model catalyst for studying structure sensitivity and size effects, but also one for which these phenomena can be simulated using a relatively small parameter space. 7.6 Electronic properties HRSXPS experiments Differences in electronic structure between the planar and faceted surface could be a contributing factor to the changes on catalytic activity, as seen, for example, in the rate change for acetylene and ammonia decomposition. Binding energy shifts of core electron states of surface atoms (see Section 2.5, surface core-level shifts) reflect chemical environment differences between surface and bulk atoms. In a similar way, core level shifts can be observed between surface atoms located at inequivalent sites in the surface lattice 3. In our studies HRSXPS (described in Section 2.5) was used to probe the core-level shifts of the Ir 4f 7/2 level. The 4f levels in transition metals (e.g. Pt, W, Ir) lend themselves very well for this type of study due to their small line width, comparable to the available instrumental resolution at the U4A beamline at NSLS. Spectra were taken at normal (90º) and grazing (20º) emission, the latter enhancing surface sensitivity as discussed in Sections 2.5 and 4.2. As in the case of other studies of 4f core-level spectroscopy, Doniach-Šunjić line shapes were used to fit the spectral lines in the course of data analysis [17-19]. 3 One example of this would be the difference between the atoms in the topmost layer of the Ir(210) surface with 6 nearest neighbors, the second layer with 9 and the third with 11 nearest neighbors. While they are all surface atoms, their immediate environment differs and so do their core-level shifts.

15 160 The spectra shown in Figure 7.7 are typical for emission from the clean planar Ir(210) surface. While fitting the data with four lines may seem unnecessary, using fewer lines would not be consistent with previously observed line widths (around 300 mev) for iridium [20, 21]. The use of four peaks is also consistent with the geometry of the surface that consists of three layers exposed to vacuum (a,b and c in the model in Figure 7.7). These layers have incomplete coordination, while the fourth and subsequent layers have bulk coordination. Upon closer inspection of Figure 7.7, a decrease in signal/noise ratio can be observed for the grazing emission spectrum, compared to the normal emission one. This is due to the lower overall signal collected in grazing emission experiments. Comparing the normal and grazing takeoff spectra it is also evident that peaks for grazing emission (P 1,2,3 ) have a higher relative intensity compared to the bulk peak. In view of the enhancement of surface features in electron spectroscopy techniques (Section 2.5) this rela- a bulk b bulk P 2 P 2 P 1 P 3 P 1 P Binding energy [ev] Binding energy [ev] Figure 7.7. HRSXPS of Ir 4f 7/2 lines for clean Ir(210) at normal (a) and grazing (b) emission for hν=110ev. The top line in both cases is the experimental data and the bulk and P-peaks represent the best fit. They correspond to bulk emission and emission form three of the surface layers with incomplete coordination, as illustrated in the top and side view of the surface.

16 161 tive intensity increase indicates that they are surface peaks. The binding energy shift toward lower values for surface peaks is consistent with theoretical predictions and experimental evidence for Ir(111) and Ir(100) [21, 22], but in contrast to [20]. By comparing Figure 7.7a and Figure 7.8a, a significant suppression of the surface peak intensity can be observed upon deposition of a saturation coverage oxygen layer. This is not unexpected, since the chemical environment of the surface atoms that produce the surface peaks changes upon deposition and in or around the previously empty nearest neighbor sites, oxygen atoms are adsorbed. The positions of the surface peaks are also affected. The spectrum of a faceted surface prepared by a high temperature flash in oxygen background a b c bulk bulk bulk P 1 P 1 P 2 A P 2 A P 2 P 1 P Binding energy [ev] Binding energy [ev] Binding energy [ev] Figure 7.8. Normal emission HRSXPS for oxygen-covered planar Ir(210) (a), oxygen-covered faceted Ir(210) (b) and clean faceted Ir(210) (c). Figure 7.7b, shows very little change compared to the oxygen-covered planar surface. The spectrum of the clean faceted surface, on the other hand, bears more similarity to that of the clean planar surface than to the spectrum of the oxygen-covered faceted surface. The appearance of the same number of surface peaks (and their similar position) on the clean planar and faceted surfaces is not altogether unexpected. The planar surface has three

17 162 layers of atoms with incomplete coordination: 6, 9 and 11 nearest neighbors for the top, second and third layer, respectively. The (110) surface has two such layers (7 and 11 nearest neighbors), the {311} surfaces also have two (with 7 and 10 nearest neighbors). The three kinds of surface atoms on the faceted surface (grouped by identical number of nearest neighbors) are responsible for the three surface peaks in the Ir 4f 7/2 line. The core-level shift results for the four surfaces are summarized in (Table 7.1). It appears that the presence of oxygen affects core-level spectra more than the surface structure does. More importantly, the differences between the clean planar and clean faceted surfaces, while discernible, are relatively subtle. Table 7.1. Surface core-level shifts for four differently prepared Ir(210) surfaces. Iridium urface SCLS A [mev] SCLS P 1 [mev] SCLS P 2 [mev] SCLS P 3 [mev] Clean planar N/A Oxygen-covered planar N/A Oxygen-covered faceted N/A Clean faceted N/A Discussion Dissociation of molecules on any surface can be a very complicated process. As molecules are adsorbed on the surface they can be partially or completely dissociated, or dissociation may occur in steps, not necessarily all possible at the same temperature or at all adsorption sites. Moreover, the sequence or very existence of some of the steps and intermediates in the dissociation process can depend on surface coverage [12]. In the case of acetylene decomposition, the onset of hydrogen desorption (Figure 7.4) shows a different profile than for desorption of pure hydrogen (Figure 7.1), even though the desorbing H 2 is in both cases a prod-

18 163 uct of recombination of adsorbed hydrogen atoms. While a small amount of acetylene does appear to (partially) dissociate and release some hydrogen atoms at room temperature, hydrogen desorption rate does not increase as quickly with temperature increase as in the case of hydrogen adsorption, nor does the desorption curve quickly reach a peak at around 400 K. For the planar and faceted surface alike, the hydrogen desorption continues until just above 700 K, clearly indicating that below that temperature the hydrogen is trapped in carbon-containing intermediates and not available for recombinative desorption. In a typical reaction-rate limited process, hydrogen desorption is limited by the rate of hydrogen formation from C 2 H 2 and intermediates on the surface. Although the exact acetylene decomposition steps cannot be identified using TPD only, the width of the desorption peak suggests a stepwise process in which more than one intermediate is formed. Whatever the decomposition pathways, the differences in TPD spectra for the planar and faceted surfaces above 500 K (Figure 7.3) indicates that they differ in rate and possibly in substance. The decomposition of ammonia on planar and faceted Ir(210) can be discussed in similar terms. There are differences in reaction kinetics that are apparent from the TPD spectra. Although the structure of the H 2 TPD spectrum points toward reaction-rate limited desorption, the maximum hydrogen desorption temperature is just over 500 K, similar to that of deposited hydrogen, indicating that all of the ammonia is decomposed at that temperature. This immediately yields the conclusion that nitrogen desorption (Figure 7.6.b) is, for the most part, a process not limited by the preceding decomposition. As in acetylene thermal decomposition, differences between the TPD spectra for the planar and faceted surfaces are proof of structure sensitivity of the reaction. An additional and very interesting effect present in am-

19 164 monia decomposition is reaction and desorption rate dependence on facet size, typical for structure-size dependent processes. Reactivity for bond breaking is often associated with the presence of low coordination atoms on the surface. Such atoms are present on the planar Ir(210) surface: the atoms in the topmost layer have C 6 coordination, corresponding to 6 of the 12 nearest neighbors in an fcc crystal. The {311} and (110) surfaces (Figure 4.15) contain atoms with C 7, not C 6 coordination, that may (but are not necessarily) be less active. Facet edge, corner, pyramid peak atoms and step edge atoms on the restructured (110) facet surface have C 6 coordination (Figure 6.8). From the standpoint of the role of low coordination atoms in bond-breaking, the presence of surface atoms with different coordination can contribute to the structure sensitivity of the reactions. Additional reaction-rate differences may arise from differences in diffusion rates on different surfaces. Generally, in order to dissociate a molecule (or molecular fragment) must adsorb on an active site with an available empty nearest neighbor site (or sites) for the dissociation products. The active site remains blocked until the adsorbate or products diffuse away. The differences in diffusion rates between a rough surface like the fcc (210) and row-structured (110) or (311) surfaces, and especially diffusion barriers such as facet edges can, under these circumstances, contribute to structure sensitivity. The intricate mixture of inequivalent adsorption sites and facet edges also contributes to the size effects observed in the reactions studied here via kinetic mechanisms such as those seen in Monte-Carlo simulations [5, 15, 16]. Finally, purely electronic effects can affect the rate (or very possibility) of chemical reactions, although the differences in core-level electronic structure between the clean planar and faceted surfaces are relatively subtle. It is possible that differences in the va-

20 165 lence levels are more significant, but present data indicate that surface morphology and structure size are responsible for the observed differences in the rates of acetylene and ammonia decomposition on Ir(210). 7.8 Summary TPD experiments were conducted, studying H 2 adsorption and desorption, C 2 H 2 thermal decomposition and NH 3 decomposition on clean planar Ir(210) and on three Ir surfaces consisting of {311} and (110) facets with different average facet size. Hydrogen desorption and acetylene thermal decomposition rates are different on the faceted and planar surfaces, demonstrating structure sensitivity of these processes. Ammonia decomposition also exhibits structure sensitivity but, in addition, the differences in TPD spectra for faceted surfaces of varying facet size indicate that size effects are present in this reaction on the iridium surfaces. Subtle differences in core-level HRSXPS spectra suggest that surface morphology, rather than change in electronic structure is responsible for the observed changes in reaction rates. The straightforward and reproducible in situ preparation procedures for various surfaces, their well characterized properties and lack of need for particle support make Ir(210) an excellent substrate for structure sensitivity and size effects studies in catalytic reactions. 7.9 Acknowledgements The author gratefully acknowledges the leading contribution of Dr. Wenhua Chen of Rutgers University in all of the surface chemistry work presented in this chapter. The presented experiments related to Ir(210) electronic properties were conducted in collaboration with Dr. Michael Gladys, Dr. Gavin Jackson and Dr. Jamie Quinton of Rutgers University.

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