Metal-Catalysed Reactions of Hydrocarbons / 03-Chemisorption and Reactions of Hydrogen

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Figure 3.5. Potential profiles through the surface illustrating (a) attractive and (b) repulsive interactions at sites adjacent to a hydrogen atom.

chemisorbed on it. We now turn our attention to the nature of the adsorbed phase, concentrating mainly on the metals of Groups 8 to 10 and thinking first of ‘geometric aspects’, particularly the location and disposition of the adsorbed atoms.

In order to try to summarise and make a little sense of the very extensive literature, it is necessary to start with a few basic considerations. (1) The presence of one chemisorbed hydrogen atom may influence the tendency for adjacent sites to accept another: this influence may be either positive or negative, that is to say the two atoms may either attract or repel each other (see Figure 3.5). Thus on certain surfaces the atoms are observed by LEED to adopt an expanded array (Figure 3.6), signifying a repulsive interaction extending over as many as five or six atomic diameters (>3 nm); this must take place through the metal rather than directly between the atoms. On the fcc(110) surface, which has two-fold symmetry, the interaction may be anisotropic, being repulsive across the rows and attractive along them (see Figure 3.6c). Easy surface diffusion is needed to allow hydrogen atoms to move from their initial positions where they occupy a precursor state (Figure 3.7) to positions of greatest stability, but this happens at all but the lowest temperatures because the activation energy for diffusion is only about one tenth that for desorption75 (typically 7–10 kJ mol −1).

(2) We saw in Chapter 2 that the (110) faces of the fcc 5d metals (Ir and Pt) undergo spontaneous reconstruction in the clean state to give a ‘missing row’ (MR) structure: this also happens with the same surfaces of nickel, rhodium87 and palladium when hydrogen atoms are present in high enough coverage and when the temperature is sufficiently elevated. These adsorbate-induced reconstructions are seen with many other atoms, (O, S, alkali metals) and serve to remind us of the dynamic character of a metal surface. Passage of palladium single crystal and films through the hydride phase results in the appearance of (111) facets.88 Reaction models based on static surfaces are surely over-simplified, and similar structural modifications may be expected with small particles when hydrogen is

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Figure 3.6. Maps of expanded arrays of adsorbed hydrogen atoms at low coverages caused by repulsive (a and b) or repulsive + attractive interactions (c).

(a)Ni(111)c(2 × 2)2H at <270 K: θ = 0.67 (note both types of tetrahedral hole are used).

(b)Pd(110)c(2 × 2) at <260 K: θ = 0.5.

(c)Ni(110)c(2 × 2) at <200 K.

present. The effects produced by sulfur and alkali metal atoms may have relevance to the phenomena of poisoning and of promotion (Section 2.7). Although neither copper nor gold retains a significant coverage by hydrogen atoms at 300 K,76 in their presence their (110) surfaces are both affected; hydrogen also enhances the diffusion of platinum atoms in channels on the reconstructed (110) surface.89

Figure 3.7. Calculated initial position of adsorption of a hydrogen molecule on an fcc(110) surface, and movement of atoms on dissociation to octahedral sites.355

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These changes must result in a lowering of the surface energy, but it is not clear whether this stabilisation is caused by the presence of the chemisorbed atoms, or whether they simply facilitate it by weakening metal-metal bonds in the surface.1 Spontaneous restructuring in the case of iridium and platinum but not with the 3d and 4d metals was attributed (Section 1.22) to the greater energy advantage that resulted, i.e. to their higher sublimination heats: this would suggest that the former explanation is the more probable. On the more densely packed fcc(100) and (111) surfaces, where chemisorption produces no changes parallel to the surface, it partly counteracts the ‘skin’ effect (Section 1.2.2), and M M bond lengths normal to the surface relax towards their bulk values; however with the (5 × 20) hexagonal reconstruction of the Pt(100) surface hydrogen chemisorption restores the original square lattice.90 Some examples of the structures formed by hydrogen atoms on fcc metals at low coverage are shown in Figure 3.6. These reveal an important characteristic of the chemisorbed phase, namely, that the stablest position of the atom is where it makes most contacts with the metal atoms, i.e. where the 1s electron overlaps most with the emergent orbital lobes or where its wave function integrates most completely with the unfilled surface electron band. These positions are therefore four-fold sites on (100) surfaces and three-fold sites on (111) and (110) surfaces (Figure 3.6). The small size of the hydrogen atom relative to that of a metal atom means that the electron interaction is extensive, and indeed on the (100) surface the atom almost disappears into the surface structure91 (Figure 3.8).

As exposure is increased, other structures appear in which the hydrogen atoms are ever more densely packed (Figure 3.9). At low temperature on the unreconstructed Ni(110) surface, four other structures follow that shown in Figure 3.6c, their coverages being 0.5, 0.67, 1.0 and 1.5; this last is shown in Figure 3.9b. At exposures giving coverages between these values, two phases will co-exist. On unreconstructed Rh(110) there is a similar sequence of phases, but the final one, formed only above 120 K under an atmosphere pressure of hydrogen, corresponds to a coverage of 2.0. The location of hydrogen atoms is also obtained by high-resolution electron-energy-loss spectroscopy (HREELS),5,17,92 which detects M H vibrations (subject to certain impact selection rules as well as the more familiar dipole selection rules).

There is much evidence to suggest that the M H bond is essentially covalent in character, but that the hydrogen atom is slightly the more electronegative,1,93 carrying a charge of about 0.1e−: charge is therefore withdrawn from the metal, and the work function is increased, provided the centre of charge density is above the image plane of the metal. This is not so with Pt(110) and (111) and Ir(110), where the low location of the atom causes the sense of the work function change ϕ to be reversed. Work function changes are complex quantities, since the perturbation of the electronic structure of the metal by relaxation or restructuring alters its chemical potential, and hence its contribution to the work function: firm conclusions about the polarity of the M H bond are therefore not easy to derive, although changes in the slope of ϕ versus coverage reflect the occurrence of different phases,1

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Figure 3.8. Model showing hydrogen atoms chemisorbed on the (100) face of an fcc metal (the large sphere shows the location of a metal atom in the next layer).

or the existence of step sites (e.g. Pt(997), which are occupied first and with an increase in ϕ.17 However, after proper calibration, measurement of ϕ provides a handy means of estimating surface coverage. The covalent nature of M H bonds is confirmed by numerous estimates of their length by LEED structural studies; values equate closely to the sum of their covalent radii if the hydrogen atom radius is taken as 40–60 pm.

Figure 3.9. Arrangements of hydrogen atoms at high coverage: (a) Pd(100)(2 × 2) at θ = 1;

(b) Ni(110)(2 × 1)p2mg.

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Ultraviolet photoelectron spectroscopy (UPS), in either the angle-integrated or angle-resolved (ARUPS) mode, reveals changes in the metal’s electronic band structure caused by the chemisorption of hydrogen atoms.1 New resonance states at 5–10 eV below the Fermi level are regularly seen, and these 1s derived states extend over more than 4 eV, confirming their free-electron-like form. Close to the Fermi surface there is a suppression of d-band states, showing their participation in the bonding and in the case of nickel there may be magnetic consequences as well. Although the picture of a delocalised M H bond is clearly consistent with the atoms occupying a three or four-fold site, it needs a modicum of mental gymnastics to square this with the conclusion that the bond is ‘essentially covalent’. Perhaps the safest conclusion to draw from the proliferation of results obtained by all these different techniques is that expressed in the old adage: what you see depends on where you look.

Thus far we have been concerned almost exclusively with low index surfaces of single crystals of pure metals, for good reason. In the older work on metal powders24 and films,26,27,29 their polycrystalline nature forbade the drawing of conclusions about surface geometry, and their doubtful cleanliness was a continual worry. Only the work on FEM and FIM produced results of comparable importance, although these techniques are now rarely used.26,34 There have however been a number of studies of stepped surfaces. The behaviour of chemisorbed hydrogen on Ni(997), which is a stepped version of Ni(111), is dominated by the plane areas, but the order-disorder transition from the (2 × 2)2H phase to the randomly located lattice gas was raised by about 40 K by the presence of the steps, which stabilise the ordered structure.

There is much less information available on geometric aspects of hydrogen chemisorption on bimetallic systems.94,95 Addition of an ‘inert’ metal (of Group 11) to a metal of Groups 8 to 10 not surprisingly lowers the sticking probability, and nickel-aluminium and platinum-tin intermetallic phases seem to be totally inactive.96−98 Most of the work that has been done has had the purpose of characterising the surface composition, and the principal conclusion reached was that the ‘inert’ element acted more or less as a site-blocking agent without very much affecting the reactivity of the active atoms, i.e., no ligand effect was detected. Thus for example while the p(2 × 2)Sn25Pt75/Pt(111) surface will not chemisorb molecular hydrogen, presumably because of an activation energy constraint, the Pt−H bond formed when atomic hydrogen was used had about the same strength as when on Pt(111).94 Similarly on p(2 × 2)NiCu(111), the binding energy of hydrogen atoms residing selectively on nickel sites was not affected by the presence of the copper. A more recent molecular beam study has compared sticking probabilities and desorption characteristics of deuterium on the p(2 × 2)SnPt(111) and (√3 × √3)R30◦SnPt(111) surfaces (see Figure 4.5), and concluded that the absence of a triangle of platinum atoms in the latter led to an increased barrier for dissociation and a decreased sticking probability. Formation of the bimetallic surface

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ruthenium-copper phases by evaporation of the latter onto the former appeared to show that three or four ruthenium atoms were needed to bind one hydrogen atom, but hydrogen atoms migrated from ruthenium to copper sites.

3.2.4. The Chemisorbed State: Energetic Aspects

While the geometric aspects of hydrogen chemisorption reviewed above reveal fascinating details of the surface chemistry of metals, their relevance to the phenomenon of catalysis is not easily perceived. When reactions occur, it is the strengths of chemisorptive bonds that matter, and so although (as we shall see) there are close connections between geometric and energetic factors, it is the latter that can provide numbers that will assist our understanding of catalytic processes.

Once again the detail needs to be prefaced by some general considerations. The process of chemisorption is in essence a chemical reaction: unfortunately, like many chemical reactions, it is not a simple process, as it does not always lead to a well-defined product. The occurrence of the different structures of the hydrogen atom adlayer exemplifies this. Nevertheless the fact that chemisorption takes place means that the Gibbs free energy of the system must decrease, and that because of the loss of translational entropy there has to be a decrease in the system’s heat content: chemisorption is thus of necessity exothermic. All manner of thermodynamic parameters can therefore be ascribed to the process and to the resulting state.4,17,75

If we represent the chemisorption of hydrogen as

H2 + 2M → 2H − M

(3.A)

we can define the equilibrium constant, which is here called the adsorption coefficient b as

1
b = θH/ PH/2	(3.2)

and from its temperature dependence the adsorption free energy, heat and entropy may be derived using the Van’t Hoff isochore and the Clausius-Clapeyron equation. The variation of θH with pressure at constant temperature constitutes an adsorption isotherm; this is an experimental observation and may be modelled by an adsorption equation, the simplest of which is associated with the name of Langmuir and takes the form

θH = b /2	PH/2	/	1 + b /2	PH/2	(3.3)
1	1		1	1

We must note however that this equation rests on simplifying assumptions, namely, that each ‘site’ can acquire only one atom, and that all ‘sites’ are energetically

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Figure 3.10. Dependence of surface coverage by hydrogen atoms on (a) temperature at various fixed pressures (isobars), and (b) on pressure at various fixed temperatures (isotherms). Isosteric heats of adsorption at (say) half-coverage are derived from the pressures giving this coverage at each temperature.

equivalent. Despite these limitations, which ought to make the equation unusable, it often gives a fair description of what happens, and more complex equations that avoid the assumptions inherent in the Langmuir equation are rarely used. Because chemisorption has to be exothermic, by Le Chatelier’s Principle the value of b will have to decrease with increasing temperature and the variation of surface coverage with temperature at variable constant pressure is illustrated in Figure 3.10: the curves are isobars and the corresponding plots of coverage against pressure (i.e. the isotherms) are shown in the same Figure. Application of the Clausius-Clapeyron equation in the form

∂ ln PH/2	/∂ T θ = −qi /RT 2	(3.4)
1

leads to qi, the isosteric heat of adsorption, i.e. the heat released at constant θ , and hence by selecting different values of θ the dependence of heat on coverage may be derived. As noted above, work function measurements are often used to estimate coverage. It is also possible to measure heats of adsorption calorimetrically; techniques for doing this with films were pioneered by Beeck25,99−101 and by Trapnell,26 and more recently King has succeeded to do this with single crystals (see Chapter 4). Heat released over a range of coverage yields an integral heat; if successive increments of the gas admitted are small enough, one obtains a differential heat, which is termed a true differential heat − Hads if no work is done in the adsorption. Then

qi = Hads + RT

(3.5)

since RT is the maximum work done in adsorbing one mol of ideal gas at temperature T . Heats of adsorption may also be obtained from the careful analysis of temperature-programmed desorption (TPD) results (see below).

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Heats of adsorption depend on the nature of the metal, the surface geometry,82 the coverage and the perfection of the surface. As concerns the effect of coverage, four aspects are to be noted: (i) in some cases, the heat is constant over a considerable range of coverage3 (e.g. up to θH 0.5 for Rh (110) and to θH 1.0 for Pd(100)); (ii) sometimes there is an initial sharp fall (e.g. Ni(100)) or rise (e.g. Ni(110)) before a constant value is attained; (iii) at higher coverages the heat decreases, but sometimes this only occurs as θH approaches unity (e.g. Pd(100)) and for other surfaces there is a progressive fall from the beginning (e.g. Pt(111)); and (iv) occasionally there is fine structure, i.e. plateaux and intervening decreases, that betoken the existence of different structures. On the whole however the heat is remarkably insensitive to the surface concentration of hydrogen atoms, showing that the magnitudes of the attractive and repulsive interactions are relatively small. An attractive effect at low coverage causes the heat to rise on Ni(110) (Figure 3.6c), while an initial fall is probably due to some step sites or defects: the decreases at high coverage are almost certainly a consequence of repulsive interactions.

Values of heats of adsorption at the flat regions or the initial value where there is no plateau are given in Table 3.1. This shows that for nickel, palladium and iron there is no strong dependence on surface geometry and indeed mean values are all between ca. 95 and 100 kJ mol−1. Analysis of the TPD spectra for deuterium on

TABLE 3.1. Initial Heats of Hydrogen Chemisorption on Single Crystal Faces,3,62,102 Films26 and Powders

					fcc
Metal	(100)				(110)	(111)			Film	Powder
Ni	95				90	95			121–134	85105
Cu			—		—	39a			—	—
Rh			—		92	78			109	—
Pd	99				103	88			113	—
Ag			—		—	15			—	—
Ir			—		77	—			—	—
Pt			79b		75b	71b			89104	90
					bcc
	(100)				(110)	(111)
Fe	100				109	88			134–151	—
					cph
	(1010)				(0001)
Co	80				67				—
Ru	80				80, 120				—	65165
										90188,327

a Cu(311)

b Approximate average values

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Ni(111) by the isostere method gave slightly lower values ( 90 kJ mol−1).102 With cobalt, rhodium, platinum (which is also structure insensitive) and iridium, values are somewhat lower (75−90 kJ mol−1) and for copper and manganese lower still. The heat of adsorption (− Hads) is converted into an M H bond strength DMH by the relation

DMH = 1/2( Hads − DHH)

(3.6)

where DHH is the bond strength of the hydrogen molecule (436 kJ mol−1): in Groups 8 to 10, DMH values lies between 250 and 275 kJ mol−1 and are typical of covalent bonds.103 This equation actually holds only where relaxation or reconstruction of the surfaces makes no contribution to the energy released: loss of strain will be exothermic and will appear in the heat term.

It is of interest to compare the results formed with single crystals with those for films obtained chiefly by Beeck99 and by Trapnell26 and others.104 Although similar in form, values of the heat at zero coverage (Table 3.1) are considerably greater, due perhaps to a higher concentration of defects, edges and steps, although the heat liberated with a stepped platinum surface is less than that for the corresponding plane. Values for powders are often below those for single crystals (Table 3.1), and this may be due to lack of cleanliness: electronegative impurities cause the heat for hydrogen to decrease (e.g. O on Ni (100)). Little work appears to have been done with bimetallic surfaces, perhaps for fear of chemisorption-induced restructuring; values for heats of adsorption on powders of the interstitial alloys NiB and NiP are less than those for pure nickel powder.105 Measurements with supported metals and theoretical calculations are considered later.

Extensive work on the nature of chemisorbed hydrogen has been performed using temperature programmed desorption (TPD)1,3,17,26,41,75,106,107. What is done is as follows: after a very small exposure (e.g. 0.01L) the specimen is heated and the TPD spectrum recorded, using, for example, a mass spectrometer. The procedure is repeated on the cleaned surface with successively larger exposures, until finally the coverage becomes as close to saturation as possible. What is typically seen is that the small doses desorb only at relatively high temperatures; then as the dose is increased, a second peak appears at a somewhat lower temperature and ultimately a third (sometime sharp) peak may appear at much lower temperature. Results for the Ni(110) surface showing this behaviour are presented in Figure 3.11; here the three events are coded respectively β2, β1 and α. ‘Open’ surfaces such as the fcc(110) exhibit more peaks than tightly packed surfaces because of the greater variety of phases they can accommodate (see Section 3.23).

The object of the game is then to relate what is seen to a model of what is on the surface. This is quite a complicated exercise and the technique has spawned a large literature on its interpretation; only a brief summary is possible here. There are two issues: (i) to relate the areas under each peak, obtained by deconvolution, in a

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Figure 3.11. Temperature-programmed desorption (TPD) of hydrogen from Ni(110) showing integral and differential plots.

qualitative way to the various surface phases shown for example by LEED: and (ii) in a quantitative way to infer from the values of Tmax what are the activation energies for desorption (Ed),since if there is no activation energy for chemisorption these are equal in size but of opposite sign to the heats of adsorption. Reliable values are however only obtained by rigorous application of the isostere method,102 rather than rough and ready methods based on Tmax.

A note on terminology is in order: the term ‘adsorption’ means ‘chemisorption’ unless prefixed by the word ‘physical’; the term ‘physisorption’ is not much used by people working on catalysis. Purists have pointed out that the word ‘desorption’ is etymologically unsound: the prefix meaning ‘away from’ is dis-. Thus we have associate and dissociate: so the opposite of adsorption should be dissorption; but it is probably too late to do much about it. The solid is referred to as the adsorbent and the gaseous adsorbing species as the adsorptive; this species when adsorbed is referred to as the adsorbate.

Returning to the interpretation of TPD spectra, the early exposures place the hydrogen atoms in the stablest lattice sites (for example, see Figure 3.7) and this continues until the first state (β2) is fully occupied. Further exposure begins to fill the β1 state, and when this is complete the least stable α state is started. TPD reverses this process (see Figure 3.12); desorption at low temperature destroys the α state and leaves atoms forming the β1 state: then further desorption leaves only the β2 state. The various peaks in TPD spectra do not signify a priori heterogeneity of the surface. Hydrogen atoms formed at low coverage will if mobile migrate to occupy sites at which they are most stable, and, as coverage increases, interactions