Vibrat ional Spectral Studies of Solut ions at Elevated Temperatures and Pressures . III. A Furnace A s s e m b l y for Raman Spectral Studies to 300°C and 15 MPa.

D . E. I R I S H , T. J A R V , * a n d C. I. R A T C L I F F E Guelph-Waterloo Centre for Graduate Work in Chemistry, Waterloo Campus, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

The design of a furnace assembly for maintaining aqueous solutions at temperatures up to 300°C and controlled pressures of up to 15 MPa is described. The apparatus is being used to monitor the chemical processes taking place in high-tempera- ture aqueous solutions. Spectra of the Zn(NOa}2/H20 system and the HC1/H20 system are presented as examples. Index Headings: Raman spectroscopy; High-temperature fur- nace; Solution spectroscopy at high temperatures and pres- sures; Techniques, spectroscopic; High-temperature aqueous solutions.

INTRODUCTION

High-temperature aqueous solutions have been receiv- ing more attention in recent years because of their im- portance in fossil fuel-fired power plants, nuclear power plants, and geothermal energy extraction) Raman spec- troscopy has provided particularly lucid glimpses of the various constituents of solutions, their geometries, pop- ulations, and interactions. 2 In an earlier publication, 3 we described an apparatus for holding samples at tempera- tures up to 150°C and its application for the study of the temperature dependence of Raman band parameters for aquated cations, 3'4 indium nitrate solutions, 4 and mercury nitrate solutions? In addition to having a limited maxi- mum temperature, the apparatus did not allow for the measurement or control of the pressure. We describe here a new apparatus which we are using to obtain Raman spectra at temperatures up to 300°C and 15 MPa. Liquid samples can be placed under the final desired pressure, the temperature can be increased, and the pressure decreased to maintain the pressure constant, in such a way that the sample is held at a point in the field of the phase diagram. By this means the vapor phase is entirely eliminated and problems with gas or bubbles in the line are avoided. One can then study the Raman spectra at constant P and variable T. The Raman spec- trometer system consists of the Jarrell-Ash 25-100 com- puterized system described previously. ~

I. EXPERIMENTAL

A. F u r n a c e a n d A n c i l l a r y P r e s s u r e Line. The de- scription of the apparatus is conveniently divided into three parts: the cell in which the sample is placed (Fig. 1, part CC), the pressure generation and measurement equipment (Fig. 1, part AA), and the furnace assembly (Fig. 2).

Received 5 August 1981. * Present address: Ontario Hydro, Kipling Complex, 800 Kipling Ave-

nue, Toronto, Ontario M82 5S4, Canada.

Samples were placed in cells constructed from Corning Pyrex glass capillary tubing (Fig. 1, part PC). Details of construction are given below. Only the bottom section of the cell was heated by the furnace. The temperature stability suggests that mixing by convection is negligible probably because of the small bore. A vertical configu- ration was adopted to facilitate cell attachment and removal, application of pressure, displacement of trapped air or bubbles, and the positioning of the furnace. The nickel connecting section (Fig. 1, CP) was attached to the high-pressure coupler (HPC) to attain the correct sample cell length. A sample cell holder (Fig. 1, part BB) was mounted on top of the Jarrell-Ash sample compartment (H). The cell gland was fastened to the coupler C. Set screw SS held C in place and also allowed the sample cell some vertical movement for alignment. The pressure generation and measurement equipment is illustrated in part AA of Fig. 1. The working pressure fluids were bubble-free distilled water and oil. Isolators I1 and I2 separated the oil and water phases with Teflon pistons. VENTi and VENT2 were used to bleed trapped air from the distilled water lines during the priming of the pres- sure apparatus.

The high-pressure gauges (U.S. Gauge) G1 and G2 had measurement ranges of 0 to 70 ± 0.5 and 0 to 21 ± 0.1 MPa, respectively. Valve #1 (V0 was used to isolate gauge #2 (G2) when the pressure to be measured ex- ceeded 21 MPa. To generate the pressures an oil ram (R) with a maximum pressure output of 68.947 MPa (10 000 psi) was used. Valve #2 (V2) was used solely as an oil filler port for the pressure ram. Valve #3 (V3), when opened, allowed a manual external pressure to be applied to the system with the syringe (S) to exclude air when either the sample cell (part CC) or the test plug (D) was attached to the coupler (C).

High sample temperatures were achieved by surround- ing the high-pressure glass cell (part CC) with a specially designed furnace. A cross-sectional view (DD) of the furnace is shown in Fig. 2. Four machinable glass ceramic sections (MGC1, MGC2, MGC3, and MGC4) enclosed the furnace. A Pyrex jacket (PJ, 16 mm o.d.) supported the chromel-C heater wire (W). A silver metal block (SB) inside the furnace minimized the temperature gradients along the sampling region. The remaining volume inside the metal casing (MC) was insulated with a high-tem- perature asbestos-based insulation (AI). A chromel-alu- mel thermocouple (TC) connected to a FLUKE 8800A digital multimeter was used to measure the furnace tem- perature.

The laser beam entered the furnace horizontally, passed through the Pyrex glass jacket (PJ), an entrance

Volume 36, Number 2, 1982 APPLIED SPECTROSCOPY 137

AA

. . . . . j I

Z

)_ O _!

BB

Z

C D

H-/ ~ Li LI'

CC

f G l a n d

r C P

f H P C

f P C

SCALE

IOcm Z (15cm for Pressure Ram)

FIG. 1. The pressure generation and measurement apparatus (part AA), the sample cell holder (part BB), and the glass cell and fittings (part CC).

MG,

SCATTER LIGHT

D

'~' /A,

WATER OUT WATER IN

,9

LASER BEAM

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SECTION VIEW DD 5°m FIo. 2. High-temperature furnace.

port in the silver block (SB), and the Pyrex glass cell (PC), and exited through a hole in the silver block and Pyrex glass wall to strike the back of the sample com- partment. The 90 ° scattered Raman light exited through PC, SB, PJ, and MGC4 to be focused onto the slit assembly by the collection lens.

The pure silver block, used for its excellent heat ca- pacity properties, was highly reflective. As a consequence the Raman spectrum of Pyrex glass from the heavy- walled capillary tubing dominated the spectrum at low counting levels. This problem was eliminated by tarnish- ing the silver with a concentrated solution of ammonium sulphide.

Desired temperatures were generated by a LFE Series 228-10 two-mode temperature controller utilizing solid state circuitry, digital set point, and a temperature meter. The range of the temperature controller was 0 to 1000°C when a chromel-alumel thermocouple was used. The controller was connected to the heater wire (W) of the furnace through plug (HP). The entire outer furnace assembly was cooled with water passing through copper coils (CC) that encircled the metal casing (MC); this prevented heat damage to the external optics.

B. Sample Cell Const ruct ion . Thick-walled Corn- ing Pyrex glass capillary tubing (7.5 mm o.d., 1.5 mm i.d.) was cut into 28-cm pieces. Both ends of each piece were

138 Volume 36, Number 2, 1982

ground on a diamond wheel to remove any nicks that may have occurred when cut. The tubes were thoroughly cleaned with methanol and air dried. One end of the glass tube was sealed and the other end was flamed such that no beveling or rolling developed.

The glass tubes were etched with 52% hydrofluoric acid for approximately 1 h and the outside washed three times with 10% hydrofluoric acid to increase the stability of the Pyrex glass to fracture by applied pressure, s The tubes were rinsed five times with distilled water, three times with methanol, and air dried. Teflon tape was wound around the tubes to reduce abrasion when the glass tubes were handled. The hydrofluoric acid was presumed to etch away small scratches on both the inside and outside of the capillary cell. The fluoride ion may also have a chemical effect on the properties of the Pyrex glass such as lattice stabilization. Pressure tests on com- pleted nonetched Pyrex cells consistently resulted in fracture below 5 MPa (=50 atm). The equivalent cells that were etched could withstand 15 MPa and could be cycled up to 10 times before failure would occur.

A HIP (High Pressure Equipment Co.) HF2-HF4 high- pressure adapter with the HF4 end bored out (0.400 in diameter) had a stainless steel plug (0.800 in. long) silver soldered into it. This was drilled and reamed to the diameter of the glass tubing to be used as the Raman cell. The stainless steel section provided for a larger surface area for bonding by epoxy and gave an accurate alignment for the tube.

The open end of the tube was cleaned with 1,1,1- trichloroethane because this solvent left no filmy residue. Pliobond (Good Year Co.) a runny adhesive, was applied with a tiny brush approximately 1 in. from the open end and then brushed toward the open end. The tubes were baked for 2 h at 100°C to cure the adhesive. Then the tube was bonded to the adapter with Delta Bond 152-KA epoxy (Wakefield Engineering Inc., Wakefield, MA) as follows.

Teflon tubing the size of the adapter hole was pulled approximately 6 in. through the adapter. The tubing also fit the glass cell as its purpose was to prevent the epoxy from running into the glass tube and blocking it. With- drawing a little Teflon tubing kept the adapter hole clean as epoxy was placed around the inside of the metal collar and base. Epoxy was then placed on the end of the glass tube and the sidewalls. The total amount of epoxy was kept at a minimum to give a better bond. The glass tube was inserted partway into the metal adapter with a slow 180 ° twist while simultaneously withdrawing the Teflon tubing slightly. The entire assembly was placed upside down in a preparation jig. The final bit of tubing was pushed into the adapter with a slight twist. The surface excess was wiped away leaving a fillet between the glass and metal. The sample cell was left to cure for at least 24 h. The Teflon tubing was removed carefully and a clear hole remained through the glass and metal.

Working at high pressures and high temperatures inev- itably led to breakage of the sample cells. The design allowed for the high-pressure adapter to be rebuilt with a new glass cell. To refit the adapter, the broken glass was removed by soaking the epoxy in methylene chloride. If the fractured glass was difficult to remove, a carbide- tipped 1/4 in. drill bit was used to loosen the glass; care

was taken not to touch the metal. A microshot blaster was used to clean the metal thoroughly. The small hole, if not clean, was run through by hand using a drill bit to dislodge any epoxy which remained. The adapter was degreased with acetone and alcohol and air dried and was then ready for reuse.

Details of procedures for filling the cell and manipu- lating the high pressure line are presented in Ref. 7.

II. RESULTS AND DISCUSSION

Systems studied so far include H20 and D20, the salts Zn(NO3)~, Ni(NO~)2, NiCl2, and the acids HC1, HC104, and H2SO4. Details of the spectra will be published elsewhere. Here we present two examples.

In Fig. 3 Raman spectra of a 5.02 mol kg-' solution of Zn(NOs)2 containing a low concentration of HC104, meas- ured at temperatures in the range 25 to 275°C are pre- sented. A polarized band at 385 cm -1 decreases in inten- sity and increases in frequency to 398 cm -1 as the tem- perature increases. This band is attributed to the sym- metric stretching mode of the hexaaquazinc(II) cation. The loss in intensity is attributed to the replacement of the coordinated water molecules by the nitrate ligand. This interpretation is consistent with changes in the spectrum of the nitrate ions. The v4 deformation mode of solvated nitrate ion at 719 cm-' gives way to a new band at approximately 743 cm -1. Another new band at approx- imately 1038 cm -~ develops intensity beside the 1048 cm -1 symmetric stretching mode, vl, of nitrate ion. And a pair of new lines appears at 1305 and 1492 cm -~ replac- ing the 1328 to 1418 cm -~ doublet of solvated nitrate ion; this is the region of the p3 antisymmetric stretching mode. The weaker doublet at 1640 to 1660 cm -1 arises from the overtone of the infrared-active p2(A2") mode. The dou- bling of all of the bands of nitrate ion suggests that nitrate ions occupy at least two kinds of sites, one with an environment of water molecules and one in contact

5 0 2 m Zn[NO~)~

A- 25'C B - 75 "C C - 125'C

/ \ D-,z5"c E- 275t3

2;o 4o ~o '5~ 7~o

i i i • I i i

1190 1490 1790 950 1040 1130 POSITION (cm-')

FIG. 3. Raman spectra of 5.02 mol kg-' Zn(N03)2 at the indicated temperatures and a constant pressure.

APPLIED SPECTROSCOPY 139

with the cation. The postulated process can be simply written

Zn(H20)~ 2+ + yNO~- (aq) ~-- (H20)x Zn(NO3-)y + (6 - x) H20.

The separation of the pair of bands in the p~ region is remarkably large and well-defined at 275°C. Weak fea- tures at 462 and 628 cm -1 in Fig. 3 arise from the perchlorate ion. The increased cation-anion association at 275°C is consistent with the fact that the dielectric constant of water has decreased to approximately 23 at this temperature.

The second example illustrates a weakness of the de- sign. The spectra in Fig. 4 are those of 4.6 mol kg -1 HC1. The bands of water are evident: the stretching modes generate intensity between 3000 and 3700 cm-1; the de- formation mode gives the band at 1640 cm-1; and the librational modes give intensity in the 400 to 600 cm -1 region. On increasing the temperature from 150 to 200°C a broad fluorescence occurred, stretching from 800 to 4000 cm -1. After holding the system at 200°C for 2 hours the fluorescence was found to have increased markedly. This observation is attributed to dissolved silica resulting from attack of the borosilicate Pyrex glass by HC1. Ra- man intensity also increases at -450 cm -1 when glass is attacked. A sample of the solution was analyzed by dc arc emission spectroscopy and was found to have 3 to 5 times as much silica in it as a HC1 blank. Flakes of a clear material came off the inside walls of the tube when the sample was removed. Strong acids are expected to be- come weaker at elevated temperatures. Hydrochloric acid appears to be more analogous to hydrofluoric acid at temperatures above 150°C with regard to its corrosive property towards Pyrex glass. The other acids studied to date have not noticeably attacked the borosilicate Pyrex glass.

Excellent spectra of water were obtained at tempera- tures up to 300°C. Higher pressures and temperatures can be obtained and water was taken to 350°C and 17.5 MPa. Under these conditions the Pyrex glass was strongly attacked. The attack was not immediate as it was possible to obtain a good spectrum during the first

500 1000 1 5 0 2 2 8 0 0 0 5 0 0 3 0 0 0 3 5 0 2 4000

CM-i FIG. 4. Raman spectra of 4.6 mol kg -1 hydrochloric acid at 150°C, 200°C, and following holding for 2 h at 200°C.

FIG. 5. Photograph of the borosflicate Pyrex glass cell after taking water to 350°C and 17.5 MPa for 1 h.

30 min but during the following 30 min the tube became increasingly opaque to the laser beam until only diffuse scattered light was transmitted. Fig. 5 is a photograph of the tube in which the water was heated. The inside surface was coated with a white material and the bore in the hot zone is noticeably enlarged. Thus for aqueous systems and a Pyrex glass cell 300°C appears to be the practical working limit. Experiments with sapphire tubes are in progress.

ACKNOWLEDGMENTS

This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada.

The authors also express their thanks to C. Gendron, J. Selwood, and P. Stilwell of the Science Faculty Workshop.

1. T. W. Swaddle, Chem. Can. 32, (1) 21 (1980). 2. D. E. Irish and M. H. Brooker, in Advances in Infrared and Raman

Spectroscopy, J. H. R. Clark and R. E. Hester, Eds. (Heyden, London, 1976), vol. 2, pp. 212-311.

3. J. T. Bulmer, D. E. Irish, and L. 0dberg, Can. J. Chem. 53, 3806 {1975). 4. D. E. Irish and T. Jarv, Faraday Disc. Chem. Soc., No. 64, 95 (1977). 5. J. T. Bulmer, D. E. Irish, F. W. Grossman, G. Herriot, M. Tseng, and A. J.

Weerheim, Appl. Spectrosc. 29, 506, {1976}. 6. R. K. Williams, Rev. Sci. Instrum. 49, 591 (1978). 7. T. Jarv, Ph.D. thesis, University of Waterloo, Waterloo, Ontario, Canada,

1980.

140 Volume 36, Number 2, 1982