Carbon catalyst surface contamination

A further serious factor determining the apparent gap between model studies with well-defined clean surfaces and the conditions encountered with real catalysts is the fact that the latter are never prepared in a way that the degree of surface contamination is well controlled. Elements such as sulfur or carbon might always be present in considerable concentrations on the surface so that the chemical nature of the catalytically active surface could possibly be quite different. 

The possible surface contaminations were carefully followed by Auger-XPS analysis. Similarly, as with the copper catalyst described earlier (Section III) the Cu/ZnO binaries were free from alkali metals, iron, chlorine, and sulfur, and contained only small amounts of carbon after the use in catalytic reactor (39). The latter result indicates that reactants, intermediates, and the product are adsorbed with moderate strength, a feature that is desirable for all efficient catalysts. 

The systems described above were complex om the point of view of the various components of the mixture [78] or experimental conditions [48, 132, 133]. The elevated temperature in the study of Dalai and coworkers [132] was the likely the reason for the complexity of surface reaction and formation of SO2 and CO2. Both of these gases create problems form the point of view of secondary contamination (SO2 to be adsorbed on carbon requires an adsorbent of special features) and exhaustion of the capacity of the catalysts (surface oxidation revealed in formation of CO2). 

Metal acetyl acetonate decomposition on alumina goes along with the formation of carbonaceous surface compounds Pt particles which were deposited in N2 are masked by adsorbed CO. Furthermore, detectable amounts of carbon are deposited on the catalyst surface when decomposing the adsorbed metal acetyl acetonate in N2 [4, 5]. The contaminations are removable by an additional air treatment of the samples as applied in catalyst preparation (see above). 

Electrocatalytic reactions occur on catalyst surfaces. The catalyst surface structure and chemically bonded or physically absorbed substances on the catalyst surface exert strong influences on catalyst activity and efficiency. X-ray photoelectron spectroscopy (XPS) (also known as electron spectroscopy for chemical analysis (ESCA), auger emission spectroscopy (AES), or auger analysis) is a failure analysis technique used to identify elements present on the surface of the sample. For instance, this can be used to identify Pt and carbon surface chemical species that may present histories of chemical reactions or contamination in the catalyst layer. AES and XPS can also provide depth profiles of element analysis. Wang et al. [41] studied XPS spectra of carbon and Pt before and after fuel cell operation. They observed a significant increase in O Is peak value for each oxidized carbon support, the result of a higher surface oxide content in the support surface due to electrochemical oxidation. However, sample preparation in AES and XPS analysis is critical because these methods are very sensitive to a trace amount of contaminants on sample surfaces, and detect as little as 2-10 atoms on the sample surface. 

However, the situation can change significantly in the presence of contaminant species. For example, the presence of small amounts of carbon monoxide in the fuel stream can cover >90% of the anode catalyst surface [23], and result in significant anode overpotentials and decreased cell performance [24]. 

This failure mechanism can have significant impact on the ability of the anode to tolerate adsorbed contaminants. Similar to the impact of carbon corrosion on the cathode, the reduced electrochemically active catalyst surface area becomes very sensitive to the presence of contaminants. This is very important, for example, for operation on reformate where even small amounts of carbon monoxide can result in significant performance loss. 

The class of chemicals designated as VOCs includes a wide range of carbon-based molecules of sufficient vapor pressure to be present in the air, such as aldehydes and ketones. The most common VOC is methane, the primary component of nafural gas. There are various sources, both natural and human, of VOCs. The response of the fuel cell to VOCs will vary significantly, depending on the molecules in question, but can be significant. For example, benzene and toluene at the ppm level have both been found to significantly affect performance, with the dominant effect believed to be due to adsorption on the catalyst surface resulting in kinetic losses. A semiem-pirical model for toluene contamination based on kinetic losses is described in chapter 3, section 3.8. 

Fig. 5 (a) Mitigation by fluid circulation in the cathode compartment leading to ion exchange of foreign cations X> from the ionomer and contaminant X desorption from the catalyst and carbon components, (b) Mitigation by imposed potential changes at the cathode leads to contaminant X desorption or the creation of a product X, removed from the catalyst surface. BP bipolar plate, GDE gas-diffusion electrode... 

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

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