Solid reaction layers

Perhaps the most common use for REELS is to monitor gas—solid reactions that produce surface films at a total coverage of less than a few monolayers. When Eq is a few hundred eV, the surface sensitivity of REELS is such that over 90% of the signal originates in the topmost monolayer of the sample. A particularly powerfiil application in this case involves the determination of whether a single phase of variable composition occurs on the top layer or whether islands occur that is, whether... 

The characteristic feature of solid—solid reactions which controls, to some extent, the methods which can be applied to the investigation of their kinetics, is that the continuation of product formation requires the transportation of one or both reactants to a zone of interaction, perhaps through a coherent barrier layer of the product phase or as a monomolec-ular layer across surfaces. Since diffusion at phase boundaries may occur at temperatures appreciably below those required for bulk diffusion, the initial step in product formation may be rapidly completed on the attainment of reaction temperature. In such systems, there is no initial delay during nucleation and the initial processes, perhaps involving monomolec-ular films, are not readily identified. The subsequent growth of the product phase, the main reaction, is thereafter controlled by the diffusion of one or more species through the barrier layer. Microscopic observation is of little value where the phases present cannot be unambiguously identified and X-ray diffraction techniques are more fruitful. More recently, the considerable potential of electron microprobe analyses has been developed and exploited. 

Since the interposition of a barrier layer diminishes the effective contact between reactants, the nucleation step in solid + solid reactions is Usually completed very rapidly at temperatures conveniently used in studies of the bulk reaction and, accordingly, the rate processes are often deceleratory throughout. In addition to the progressive diminution in rate... 

The above rate equations were originally largely developed from studies of gas—solid reactions and assume that particles of the solid reactant are completely covered by a coherent layer of product. Various applications of these models to kinetic studies of solid—solid interactions have been given. 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes... 

Fig. 20. Schematic representation of the solid + solid reaction A + B -> AB in which constituents of the relatively mobile reactant (A) are transported to the outer surfaces of the product phase (AB) and rate is controlled by diffusion of constituents of A and/ or B across the barrier layer AB.

Product layer diffusion Many fluid-solid reactions generate ash or oxide layers that impede further reaction. 

Sol-gel technique has been used to deposit solid electrolyte layers within the LSM cathode. The layer deposited near the cathode/electrolyte interface can provide ionic path for oxide ions, spreading reaction sites into the electrode. Deposition of YSZ or samaria-doped ceria (SDC, Smo.2Ceo.8O2) films in the pore surface of the cathode increased the area of TPB, resulting in a decrease of cathode polarization and increase of cell performance [15],... 

It should be noted that in the case of the reaction of a fluid with a nonporous solid, the chemical reaction step and the mass transport step are connected in series. This makes the analysis much simpler as compared to the case of a porous solid. In reactions of nonporous particles there can essentially be two cases one which shows absence of a solid product layer, and the other which shows its presence. 

A reaction in which a solid product layer is formed can be represented as... 

The performance of a reactor for a gas-solid reaction (A(g) + bB(s) -> products) is to be analyzed based on the following model solids in BMF, uniform gas composition, and no overhead loss of solid as a result of entrainment. Calculate the fractional conversion of B (fB) based on the following information and assumptions T = 800 K, pA = 2 bar the particles are cylindrical with a radius of 0.5 mm from a batch-reactor study, the time for 100% conversion of 2-mm particles is 40 min at 600 K and pA = 1 bar. Compare results for /b assuming (a) gas-film (mass-transfer) control (b) surface-reaction control and (c) ash-layer diffusion control. The solid flow rate is 1000 kg min-1, and the solid holdup (WB) in the reactor is 20,000 kg. Assume also that the SCM is valid, and the surface reaction is first-order with respect to A. 

When a solid-solid reaction starts, a layer of product is formed between the two reactant phases and a single phase boundary converts into two different phase boundaries as... 

The organic layer is separated and washed twice with a total of 200 mL of water. The aqueous reaction layer is extracted five times with a total of 600 mL of chloroform. The organic extracts are washed twice with a total amount of 250 mL of water. The combined organic layers are dried with magnesium sulfate and evaporated to dryness. The residual solid product is carefully dried under reduced pressure to yield 48-50 g (2 100%) (Note 8). 

Gas—solid reactions, being essentially two-dimensional, are readily poisoned by any additive which displaces the reagent gas from the surface. For instance, H2O or H2S will partly displace CO by forming a chemisorbed layer of oxygen or sulphur on a metal so that, once more, it is essential to employ the commercial reagents when obtaining data on the rate of gas—solid reactions for design purposes. 

Reactions between certain solids possessing layered structures (e.g. graphite, silicates, metal dichalcogenides such as TaS2) and Lewis bases such as ammonia and pyridine, forming intercalation compounds, are discussed in the next section. 

Heterogeneous reactions of the type A+B = AB can, in principle, occur in two ways. 1) The product molecule AB is formed from A and B in the surrounding solvent or immediately at the surface of the AB crystal. These AB molecules are then added to the crystal on its external surface. This is additive crystal growth. 2) The solid product AB forms between A and B and separates the reactants spatially. Further reaction is possible only via (diffusional) transport across the reaction layer AB. This is reactive crystal growth [H. Schmalzried (1993)]. The moving AB interfaces in additive crystal growth are inherently unstable morphologically (see Chapter 11). 

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