Foils. polycrystalline

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

In principle, one can take the interpretation further and calculate what oxygen concentration profile fits the measurements of the O/Ag ratio best. In fact, Baschenko et al. [39] did this and concluded that the subsurface oxygen resides mainly in the third and fourth atomic layer below the surface. Although the result appears plausible, it should be noted that such calculations are only permitted when the surface satisfies the requirements of lateral homogeneity and absence of roughness discussed above. As the O/Ag experiments were done with polycrystalline foils, one might wonder whether too detailed an analysis is warranted. Anyhow, the work forms a nice illustration of what angle-dependent XPS can achieve on catalytically relevant adsorbate systems. 

For measuring infrared absorption spectra of gases adsorbed on the surfaces of metal single crystals or polycrystalline foils, one uses reflection absorption infrared... 

When the development of dedicated i rared spectrometers for surface studies started some ten years ago, some of them were designed as more or less complete ellipsometers, which in principle are insensitive to the ambient gas phase molecules. Fedyk et al. detected CO adsorbed on an evaporated Cu film at 4 torr, while Golden et al. reported work at 100 torr. More recently, Burrows et used a Fourier transform spectrometer and the polarizer approach above to study the reaction-rate oscillations in the oxidation of CO on a large Pt polycrystalline foil at pressures up to one atmosphere. With this rapid FTIR spectrometer they obtained a time resolution of 0.6 s at a sensitivity of 5% of a full CO monolayer. 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Notwithstanding the capability for measurement of photoemission spectra at pressures up to 1 mbar, the original construction of the gas cell was not suitable for carrying out experiments under catalytic reaction conditions. To provide the short distance between the sample surface and aperture for the exit of electrons (or a short path of the photoelectrons in the zone at the higher pressure), the equipment supplier decreased the dimensions of the gas cell. A section of the cell, together with the standard sample holder and a typical sample, is shown in Figure 4A this design limits the size of the sample so that only thin polycrystalline foils can be... 

The first challenge to surface studies of model ceria surfaces is to produce a suitable Ce02 surface that can be mounted and manipulated within a UHV system. Typically bulk polished or cleaved single crystals, wafers or polycrystalline foils of the desired material are used for surface studies. This approach has been used in the case of Ce02. Ceria single crystals can be obtained commercially, and can be cut and polished to a desired orientation. Several studies on the (111) surface of bulk single crystal Ce02 have been reported." ... 

The power of AES to identify the true nature of the surface of a working catalyst has been demonstrated by Dwyer and Somorjai." They used an apparatus in which the polycrystalline foil could be used as a catalyst for CO/H2 and CO2/H2 reactions at 6 atmospheres pressure. Clean iron at 300 °C gave CH4, 85%, and other C2 to C5 hydrocarbons in small amounts. It rapidly became covered with 1 monolayer of C with a reduction in rate. When multilayers of C had formed, methane alone was produced by H2/CO, but at a further-diminished rate. The H2/CO2 reaction on initially clean Fe produced 97% methane and a marked increase in methanation rate. Both C and O accumulated on the surface during this reaction. The authors point out that in the case of the H2/CO reaction the monolayer carbon may not be the active catalyst. There is one piece of evidence in their work which points to Fe, perhaps in clusters, being the active site. They studied the CO/H2 reaction on pre-oxidized Fe and... 

Cu foils were selected as inert surfaces for these experiments since they do not present significant catalytic activity (3). The rate of surface carbon formation on pure Cu polycrystalline foils was measured with the aid of a microbalance tubular flow reactor. The principles of this equipment were described in a previous paper (4). 

Equation (17) represents very closely the experimental behavior of carbon deposition on Crl polycrystalline foils from mixtures of benzene and hydrogen since ... 

A computer program was used to calculate the gas phase compositions to be used for an experiment. Typically, a preweighted polycrystalline foil was placed in the reactor, heated in hydrogen to the desired reaction temperature, and the reactant gas mixture passed continuously over it. Inlet and exhaust gas samples were taken and analyzed every twenty minutes to insure that the gas composition remained constant. The mass of the foil was recorded throughout the run from start-up through the cooling sequence. 

Both rhodium and iron polycrystalline foils have been used and compared at 6 atm. Again methanation was predominant even at this pressure. Iron was found to be a better methanation catalyst than rhodium, as indicated by Figure 9. The distribution of higher-molecular-weight products from the two metal surfaces are somewhat different as shown in Figure 10. Iron produces hydrocarbon products up to C5 under... 

As the surfaces were heated, the intermediates that were left adsorbed to the surfaces were investigated for the different single-crystal surfaces. For the C/W (111) and CAV(llO) surfaces, the methoxy was found to react without producing any other surface intermediates. The Pt-modified surfaces reacted at a lower temperature than the pure carbides. The methanol on Pt(lll) mostly desorbed from the surface by 300 K [41]. These conclusions from the single-crystal studies were then applied to study polycrystalline foils which are much more realistic surfaces than the idealized single crystals. 

Time spectra of nuclear forward scattering from a-Fe foil with various thicknesses. The polycrystalline foils are in magnetic state where all six transitions are allowed. (Reproduced from Ref. 22 with permission of Springer.)... 

Effects of this type are by no means restricted to uniform and well-defined single crystal planes, but they are also observed with polycrystalline foils as well as with the fine tips used for field ion microscopy (FIM) [23]. There the individual crystal planes have diameters of only a few tens of nm, so that these systems may be regarded as good models for the small particles applied in real catalysis. 

There is a third aspect of spatial effects in catalytic processes which researchers with a bent toward surface science will claim is the most important. This is the fact that spatial uniformity of a catalyst surface is an idealization. In reality, no surface is truly uniform, and there is reason to believe that the natural spatial variations present in all catalytic systems are important in the understanding of behavioral features. Figure 11 is a view taken from a scanning electron microscope of a polycrystalline foil of the type used in the experiments described previously. Various crystal orientations are evident and the scale of these variations is probably too large to justify a general assumption of spatial uniformity in mathematical models, particularly in the unsteady state. Studies with single crystal surfaces have pointed to the possibility that... 

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