Karl E. Spear Material Sciences Department Pennsylvania State University University Park, 16802
I
I
Chemical Transport Reactions A relevant a r e a of research
Research on chemical transport reactions exemplifies many scientific principles, yields important scientific information, and is of current relevance to modern technology. A particular advantage of this area of research is that the scope of the problems and the required equipment and supplies can he readily adjusted to fit the needs and resources of all colleges and universities. Research problems that are both interesting and significant can be easily formulated for both undergraduate and graduate students. The purpose of this paper is to introduce this exciting and relevant area of research to a broader spectrum of the chemical world and to outline the basic experimental aspects of the research that are needed to initiate such studies. The scientific principles applicable to these studies are also discussed, as are examples of the scientific and technological uses of chemical transport reactions. In short, it is hoped that this paper will provide the incentive for more chemistry departments to initiate investigations in this important area of solid state chemistry. Schafer (1) has given excellent discussions of both the experimental and theoretical aspects of chemical transport reactions, provides examples of their special applications, and reviews many previous studies involving these reactions in his book, "Chemical Transport Reactions." This book should be consulted for more details and references. Definitions and Notation
A chemical transport reaction is a reaction in which a condensed phase reacts with a gas phase to form adusively vapor phase reaction products, which in turn undergo the reverse reaction a t a d i e r e n t location in the system with the resulting reformation of the condensed phase. The simplest form of such a reaction is i A(cond.1
+ k B(g)
$j
C(g)
(1)
where i moles of condensed A react with k moles of gaseous B to form j moles of gaseous C a t one location in the system, and then the reverse reaction occurs a t another location because of a shift in the chemical equilibrium, and condensed A is deposited. The change in the equilibrium conditions required for the reversal of such a heterogeneous reaction may he achieved by temperature differences, pressure differences, or activity differences in the solids. Examples are given later. Transport through a temperature gradient is the most common method used. A chemical transport reaction may appear to he one of sublimation, but the condensed substance being transported does not have an appreciable vapor pressure a t the applied temperatures-it is chemically trans-
ported. As a rule, the transported materials yielded by such reactions are in the form of well-defined crystals. For clarification, the following conventions are followed when writing chemical transport processes (1) The condensed substance to be transported is always written on the left aide of the chemical equation, as in reaction (1) above. (2) Transport is often accomplished by applying a temperature gradient; Tz always denotes the hotter temperature, and T I the cooler temperature-or, T; > T I . (3) An arrow is used to denote the direction of trans~ o r in t a tem~eratureeradient. For exam~le. -t TzdenoGs transport from T Ito T z .
h
Typical Examples
The condensed phase systems whose transport by means of chemical transport reactions have been studied the moat extensively include the element metals, oxides, chalcogenides, halides, and oxyhalides. Investigations have also been reported on nitride, phosphide, arsenide, antimonide, carbide, silicide, germanide, stannide, and boride systems, as well as a few intermetallicsystems. The migration, or transport of condensed substances by chemical transport reaction mechanisms hm been known for a long time. Back in 1852, Bunsen (3) noted that the HC1 in volcanic gases can cause FezOa to migrate. The chemical transport reaction involved is Fe,Oa(s)
+ 6KCl(g) = 2FeCldg) + 3HnO(g)
(2)
and the transport direction is from Tz -t T I . More specifically, in laboratory experiments the starting oxide was a t 1000°C and transported material was a t 800°C. Predictions of the transport direction in a temperature gradient, and calculations of the extent of trans~ortare discussed later. ~ t i r t i n gabout 1925, van Arkel and de Boer (3) and later workers developed the well-known "iodide process" for the purification of metals. The general reaction types involved are
+
Me(s) n/2 Idg) = Me1.W Meb) C n I@)= Me1.W
(3) (4)
with the relative importance of each reaction depending on the temperatures used and equilibrium relationships between molecular and atomic iodine. The transport occurs from T I to Tzr with the usual experimental arrangement involving the decomposition of the metal iodide on a hot filament. Rolston (4) has presented an extensive review of this subject. The well-known Mond-Langer process (6) for the preparation of pure nickel utilizes the chemical transport reaction Volume 49, Number 2, February 1972
/
81
which proceeds in the forward direction a t about 50°C and in the reverse d i c t i o n a t 180-200°C. Many sulfides can be transported with the use of iodine as a transporting agent according to the reaction (6) Me@)
+ I&)
= MeIdg)
+ '/nSdg)
(6)
where for Me equal to Fe, Zn, Cd, or Mn, the transport occurs from approximately 90&800°C. A different type of process that can also be considered as a chemical transport reaction involves the preparation of NbO from the metal and NbzOs in the presence of hydrogen (7). Oxygen is transported and the formation of NbO occurs according to the reactions
I n the absence of a transporting agent, and with the use of similar experimental conditions as used with hydrogen, reaction between the metal and pentoxide did not occur for all practical purposes. The above processes are not chemical transport reactions according to the definition given previously, but a chemical transport of oxygen occurs with a gradient in the partial free energy of oxygen as the driving force. The reaction may be written as O(in NbOl+,)
+ H d g ) = HnO(g)
(10)
where the reaction proceeds in the forward direction a t the locations of material with oxygen-rich compositions, and in the reverse direction a t the locations of material with metal-rich compositions. Experimental Aspects Typical Conditions and Charncterization Methods
The temperatures and total pressures employed in studies of chemical transport reactions most commonly fall in the respective ranges of 300-1200°C and 0.1-5 atm. Some studies, of course, have been carried out a t much higher and lower temperatures and pressures. Transport through a temperature gradient is the most common type of experiment. The AT values may range from as low as 5'C to several hundred degrees. Typical times for a n experiment range from a few days to weeks. The usual method for identifiying the solid reactants and products is through the use of X-ray diraction techniques. The capillaries used in powder cameras need only a few milligrams of material to obtain good diffraction patterns. Although other physical and chemical property measurements may he used in identification and characterization of the solids, X-ray methods are highly desirable. However, an X-ray unit is more than an order of magnitude more expensive than the rest of the equipment needed for investigating chemical transport reactions. Typical Furnace Design
Achieving the desired temperatures and gradients is most often accomplished by using a resistance heated tuhe furnace. A good quality two-zone furnace of the type typically used in chemical transport experiments 82
/
Journal o f Chemical Education
Figure 1. Crorr section and end-view of typicol hvo-zone transport furnose. Lener. designate: A, Aivmino or mullite tube, l in. id X 2 6 in. long, which has o continuous furnace winding of 0.040 in. A-1 Konthd wire. 1 in. a t each end of the tube is free of winding; the flnd 2 in. of winding at aosh end hove 12 turnr/in., the center 20 in. has 8 turnrjin. The windings are covered with Alvndum or a rimilor ceramic cement. B, Center electrical tap on windings server 0 9 the neutral for both halves of two-zone furnace. C, End electricol taps on windings ore the hot lines for each zone. D, A suitable insulating moterial wch or Vermiculite. E, Furnace shell is on 8 in. diameter stove-pipe, 2 4 in. long. F, End ploter are in. thick b y 8 in. diameter Trondte. One end piece olro server as on indoting board for the electricol leod-ins. G, Ammeter, 10 amp oc. H, Powerrtot, 1 10 V, 10 omp.
'Ir
is schematically shown in Figure 1, and can be built for about $150 maximum in materials, with two powerstats and two ammeters making up about $70 of the cost. The dimensions and materials given are suitable for a wide variety of situations. The furnace should last for a long time a t temperatures of about 1200°C and lower when wound with 0.040-in. A-1 Kanthal wire. The closer spacing of the windings near each end of the furnace helps to smooth out the temperature profile. Winding the wire on the furnace tube is most easily accomplished with the use of a machinist's lathe. The furnace tuhe may be fitted a t each end, for example, on a cone-shaped wooden piece that is tightened into the lathe chuck. The wire is wound on the tuhe while the lathe is hand-turned. Apparatus
I n many cases, Pyrex, Vycor, or silica glass apparatus can be used. When reaction with the glass occurs, ceramic or metal liners are often used to protect the glass; which still provides protection from the atmosphere. Reaction vessels used in studying chemical transport reactions can he divided into two classes, open and closed systems. When the heterogeneous gas-solid reactions are rapid, and the reverse reaction proceeds rather extensively, then open tubes and extremely simple flow techniques are often used. With such reactions, most of the condensed phase that was initially incorporated into the gas phase by reaction with the transporting agent is deposited during the reverse reaction process. I n other cases, closed diffusion or convection tubes are used. Some of these simple arrangements are discussed below with the general transport reaction given by eqn. (1) serving as an example.
Figure 2. A constont pressure, Row arrangement in which the transport of Air) from T i - T j is depicted.
Figure 2 shows a schematic of a constant pressure, flow experiment in which gaseous B reacts with condensed A a t a temperature T, to form the gaseous product C. This product plus unreacted B pass into the portion of the system a t T,where the reverse reaction occurs, and A is deposited. The transport may he from T1 T2or from TZ T I . I n such a ayatem, a portion of B and C gases are lost out the exit of the system. Flow rates of 2-10 1 of STP gas per hour are commonly used. The flowrates and lengths of reaction zones used depend on the rates of the heterogeneous reactions. Ideally, thermodynamic equilibrium is reached in each reaction zone. Reactions which result in a decrease in the number of gas molecules [k > j in eqn. (I)] can be reversed by decreasing the total pressure of reactive species. This can be accomplished by outflow through a nozzle, or as depicted in Figure 3, by dilution with an inert gas after the initial reaction of A(s) with B(g). With these
-
prepared in the tubes before inserting the transporting agent and sealing. Sealing the ampules a t the constrictions is experimentally much easier. Transport yields in closed tubes can he increased considerably if convection occurs. Typical systems require total pressures of about 3 atm or greater and tube diameters larger than 20 mm. The tube is placed in a sloping position with the hot end downwards, as is shown in Figure 5.
-
Figure 3. A constant temperature, Row arrmngement in which the reversal of the transport reoction ir soured b y o pressure gradient.
pressure gradient methods, a transport can he accomplished under isothermal conditions. However, temperature and pressure gradients can be used simultaneously to enhance the transport effect. By far the most widely used arrangement is the closed diffusion tube. A typical arrangement is shown in Figure 4. If the transporting agent is solid at room temperature, the usual method for preparing such a tube is to insert the solid reactants into the tube, evacuate, and then seal it off. The end of the tube containing the reactants may he cooled during evacuation if the transporting agent is volatile, as is the case with iodine. A common method for preparing the ampules when the transporting agent is gaseous a t room temperature (C12, HC1, etc.) is to simply purge the system containing the substance to be transported with an atmosphere of transporting agent and seal off the tube. Constrictions (capillaries) are usually
Figure 4. Cloged diffu3ion tube in which AI.1 i s transported from TiT y p i ~ ddimensions ore 10-20 mm diameter b y 10-20 cm in length.
Tj.
-
Gas motion by thermal convection in the transport of A(s1 from Tube diameterj should b e larger thon 20 mm, and total pressure several atmo3pherer. Figwe
T2
I
TI.
Transporting Agents
By far, the most widely used transporting agents have been the halogens, hydrogen halides, metal halides, and water (or hydrogen-water mixtures). Some other agents used less often are HP, CO, COi, S1, CSdS2, MS,, O,, Moo3, NaCl, NH4CI, etc. The following considerations should be helpful in choosing a transporting agent. 1) Use as small amount of transvorting- agent as possible. The gaseous species involved in the transport reaction should have pressures of a t least a few tom, hut transport can occur-though slowly-if the partial pressure of one species is as low as about lo-= torr. For the use of iodine in a closed system, 0.5-5 mg/ml volume of the system corresponds to respective pressures of 0.16-1.6 atmat 1000°K 2) The transporting agent should be selected on the basis of its small tendency to be incorporated into the crystal lattice of the transported substance. This may be a result of either its chemical properties or its atomic or ionic size. Highly volatile transporting agents may sometimes be removed from the transported substance by applyingvacuum, and perhaps heat. 3) Thermodynamic values for possible transport reactions may dictate the best of several transporting agents. These aspects will be discussed in the next section. Scientific Aspects
Investigating chemical transport reactions provides the researcher with a genuine feeling and understanding of chemical reactions, and how they may be properly written to describe processes that occur. The reaction depicted by eqn. (10) is a good example of a simple, descriptive reaction. The researcher learns to understand and apply thermodynamics to chemical processes. Thermodynamics becomes a powerful tool and a concise descriptive language; it is no longer a group of memorized equations a n d derivations. Reaction rates Volume 49, Number 2, February 1972
/
83
and mechanisms also become reality, and methods for predicting and studying rates and mechanisms become apparent. Simple experiments on chemical transport reactions can make science come alive!
intercept = ASe/R
slope
a
-AHo/R
General Principles
The quantity of material transported by a chemical transport reaction depends on the gas motion, and the partial pressure diierences of the gaseous species between the location of the starting material and that of the transported substance. In relation to gas motion, the magnitude of the partial pressure difference, AP, is of primary importance in the evaluation of the transport properties of a reaction. As a rule, a transport of material will occur if the partial pressure diierence is sizable. In view of this fact, the influence of the enthalpy, AHo, and the entropy, ASo, of a transport reaction upon the partial pressure differences can be examined to determine the extent of transport. This will be done next, and then rate determining steps will be considered. Thermodynamics and Transport Properties
I n discussing the effects of thermodynamic properties on transport properties, a simple transport reaction will be considered. To make the discussion less confusing, an experimental arrangement commonly used will be assumed-a constant pressure reaction vessel heated in a temperature gradient, with the reactant A(s) being initially located at one end of the gradient. The conclusions that can be made from considerations of this simple experimental arrangement and reaction type are also valid for other experimental arrangements and more complicated reaction types. An analysis of the influence of thermodynamics on the transport effect requires the assumption that equilibrium exists in the respective volume spaces surrounding the starting material and the transported substance. Since partial pressure gradients actually exist between these connected volume spaces, the calculated AP values represent upper limits for the actual values. These upper limits for AP are very useful for determining whether or not the transport of a material by means of a particular reaction is thermodynamically possible. Five general rules regarding the influence of the thermodynamic properties of a chemical transport reaction on the transport effect of the reaction are discussed beIow. 1) A reaction with an extreme equilibrium position gives no measurable transport of material. Consider reaction (11) to have an equilibrium constant of 1 0 t 2 a t T I , and lo8 a t T z . For this case, the equilibrium partial pressures of B(g) a t these two temperatures differby about four orders of magnitude, but the A P ( B ) value is of the order of atm for ordinary total pressures of 0.1-5 atm. With such a small pressure difference, the reaction would give no measurable material transport. 2) The direction of transport i n a temperature gradient is determined by the sign of the heat of reaction. If AHo > 0 (endothermic), the tramport direction i s T Z T I ; if AHo < 0 (exothermic), the transport direction i s T I Tz.
--
84
/
Journal o f Chemical Education
Y
-
-10-
0
2
I
I
I
4
6
8
l/T
1
1 O i 2 1 4
I IO-'~~K-'I
Figure 6. Three possible plots of in K, versus 1/T for o transport reoctionr 0 and ASo 0; curre lbl Alsl Blgl = Clgl. Curve la), AH0
+
0 and AS0 > 0; cuwe(c1 A H D = 0 ond AS0 > 0.
