4.5.4Chemisorption and catalysis: macroeconomics, macromolecules and microscopy

At the other end of the same scale, but also driven by the need to understand and improve industrial processes, are the catalytic industry and the emerging sensor market. This sector provides the second type of answer to the question posed in the previous section. Here we typically are interested in relatively weak chemisorption, since although we want the atoms or molecules to stick on the surface long enough to react at moderate temperature, we also want the reaction products to desorb, and leave the catalyst surface free for the next molecules to arrive. If this doesn't happen the catalyst is said to be poisoned.

As mentioned already in section 2.4.4, there are three major types of catalyst that are the subject of intense study: these are (single crystal) metal and oxide catalysts, and supported metal catalysts, where small metal particles (SMP) are suspended, typically on oxide surfaces. Examples of SMP catalysts are Pt, Pd and/or Rh dispersed on polycrystalline alumina, zirconia and/or ceria; a selection of these form the principal components of the catalytic converters in car exhaust pipes, converting partially burnt

hydrocarbons, CO and NOx (nitrous oxides) into CO2, N2 and H2O. The role of the catalyst is traditionally de®ned as promoting reactions, while not itself being changed in

the process. But the present view is that SMP catalysis is a highly dynamic process, in which the particles move, change shape and eventually coalesce, at the same time as enabling the reactions between the adsorbed species and subsequent desorption to take place. In other words, the whole system may behave like a giant molecule with almost biological properties. This behavior is reminiscent of the changes which take place in

hemoglobin during breathing in (uptake of O2) and out (giving oV CO2); even the sizes of the two types of structure are similar, around 2±5 nm diameter for SMPs and 5.5

nm for hemoglobin.

This picture of the interactive substrate is essentially the opposite of the inert substrate invoked in section 4.3 for physisorption, and is one of the reasons why catalysis is considered a diYcult topic scienti®cally.2 As in the case of breathing, we should not let a minor diYculty of understanding get in the way of continuing the practice. Catalyst-based industry is worth billions of pounds/dollars annually, and is central to the production of all petroleum and pharmaceutical products. And in addition, diVraction and imaging tools (and a lot of determination and patience) have been instrumental in ®nding out what we know about SMPs as well as hemoglobin. It took Perutz 23 years before he drew blood on the famous molecule (Perutz 1964, Stryer 1995). We probably need a similarly patient attitude to catalysis.

The literature on SMPs in the context of catalysis is extensive, and there have been some successes. Campbell (1997) gives a review with a `surface processes' viewpoint. A

2Of course, the inert substrate is not strictly true for physisorption either. Measurement of the stress caused by adsorbing Xe and other gases on thin graphite shows that at low coverage, the substrate tends to wrap around the adsorbate, and at higher coverage the adsorbate bends the substrate in the other direction (Beaume et al. 1980).

136 4 Surface processes in adsorption

(a)

(c)

(b)

Figure 4.15. Epitaxial Pd particles on MgO: (a) TEM overview of particles after some coalescence has occurred; (b) higher magni®cation view of particles with diVerent shapes numbered 1±3; (c) transmission diVraction pattern, giving epitaxial orientation of all such islands (after Henry et al. 1991, 1992, reproduced with permission).

combination of microscopy and diVraction to characterize the particles, and mass spectrometry to measure desorption products has been usefully employed by the group of Claude Henry in Marseille (not the other (William) Henry, who worked during the ®rst third of the nineteenth century). For the case of Pd/MgO(001), they characterized the particle density, sizes and shapes and epitaxial orientation by TEM and THEED (Henry et al. 1991,1992), as shown in ®gure 4.15. In parallel, they used a chopped molecular beam to deliver CO to the sample at a given temperature, and a mass

described in chapter 1, ®gure 1.7. The major facets in the equilibrium shape are {111}, followed by {100} and {110}. However, exposure3 to ,100 L O2 in 100 s is suYcient to increase the size of the {100} at the expense of the {111} facets, and by 104 L the crystal is bounded by greatly enlarged {100} faces. AES shows that we are dealing with ML quantities of oxygen on the surface, not more. This exposure, however, has little eVect on the tabular {111} crystals shown in ®gure 4.17(d).

This type of surface movement is typically mediated by mass transfer surface diVusion, where adatoms and/or vacancies have to be both created at steps and move to the next one, under the driving force of surface energy reduction. In this case the distance moved r in a given time scales as (Dt)1/4 (Mullins 1957, Nichols & Mullins 1965, Bermond & Venables 1983). Since we are seeing eVects at the ,1 mm scale in 100 s in the example shown, the same eVects on the 10 nm scale would take place in an estimated 1 ms. However, nothing happens to the {111} tabular Pb crystals of a similar size. This indicates both how face-speci®c these arguments can be, and also that there may be severe nucleation barriers before the reactions can take place. In this example, {111} crystallites exhibit a nucleation barrier to melting (Spiller 1982, Métois et al. 1982). Similarly, there can be substantial barriers to incorporation of diVusing adatoms on perfect crystals, which is the reason why such tabular crystals are formed during vapor deposition and can co-exist with the equilibrium forms (Bermond & Venables, 1983).

A recent case of weak chemisorption which has been studied using low temperature STM is O2/Pt(111) (Winterlin et al. 1996, Zambelli et al. 1997). The initial chemisorbed O2 appears as pairs of atoms, some two±three atom spacings apart. It was shown that the presence of already adsorbed atoms catalyzed the breakup of O2 arriving later, leading to the formation of linear chains and then networks. This system shows interesting nonlinearity, which are characteristic of many such reactions, and also anisotropy, even though the O atoms are adsorbed in symmetric three-fold hollow sites. This may be due to stresses, both caused and relieved by adsorption, and the possibility that adsorption can change the reconstruction of the substrate. The input of calculations to the discussion of what is going on is at an interesting stage (Feibelman, 1997).

One of the most fascinating phenomena is the occurrence of spaceand time-depen- dent reactions which have been observed in real time by photo-electron emission microscopy (PEEM), as shown in ®gure 4.18. The reactions can be periodic or chaotic in time, and spatial patterns evolve on the surface, often resembling spiral waves. The original work by the Ertl±Rotermund group in Berlin (Rotermund et al. 1990, Jakubith et al. 1990, Nettesheim et al. 1993, Ertl 1994) showed that the reaction between CO and O2 to produce CO2, on a Pt(110) substrate, proceeds at the boundary between two adsorbed phases, one primarily CO and the other primarily O; this reaction was followed by TV observation in real time with a typical length scale of 10±50 mm, at CO pressure up to a few 1024 mbar.

There are many reasons why one would want to follow such reactions at higher pressures, in order to simulate the conditions of real catalysts. Optical observation is

3One langmuir (L), the unit of exposure to a gas, is equal to 1026 Torr´s; do not confuse 1 ML51 Torr´s with the symbol for a monolayer (ML).

140 4 Surface processes in adsorption

Figure 4.18. PEEM pictures of the spatio-temporal reaction CO1O2 to produce CO2 on a Pt(110) substrate, at T5448 K and partial pressures ,4´1024 mbar. The darker areas show adsorbed O (work function change Df50.5 V) and the lighter areas adsorbed CO (Df50.3 V relative to Pt), with the reaction proceeding at the moving boundary between the phases (Nettesheim et al. 1993, reproduced with permission).

advantageous, even if the spatial resolution is limited. A development of ellipsometry from the same group (Rotermund et al. 1995, Rotermund 1997) has observed the same reactions at CO pressures .5´1022 mbar, and at higher T,550 K. The reactions have been identi®ed as being associated with the following features. These are: (a) oxygen needs two adjacent Pt sites to chemisorb, which suppresses O-adsorption at high CO coverage; and (b) CO lifts the 231 reconstruction which is present, both on the clean and O-covered surfaces (Eiswirth et al. 1995). The coupling of these reactions has been modeled with three non-linear coupled rate-diVusion equations, for the local concentrations of CO, O covered and 131 uncovered structures (the areas of 231