1024
Langmuir 1996,11, 1024-1032
A Catalysis Related Electrochemical Study of the V205/ TiOz(Ruti1e)System Jerzy Haber* and Pawel Nowak Polish Academy of Sciences, Institute of Catalysis and Surface Chemistry, ul. Niezapominajek 1, 30239 Krakdw, Poland Received July 26, 1994. I n Final Form: December 20, 1994@ Cyclic voltammetry was used to study properties of the (110)surface of a rutile (TiO2) monocrystal and its interactionwithvanadium(\')speciesat conditionssimulatingtypicalpreparation procedures for catalysts. It was found that vanadium enters the crystal lattice of rutile as a tetravalent ion and cannot be electrochemically oxidized to pentavalent but can easily be reduced to the trivalent oxidation state. A second vanadium species may also appear on the surface of rutile, which may be both reduced from +5 through +4 to +3 and oxidized from +3 through +4 to +5 oxidation state.
Introduction Titania supported vanadium oxide catalysts have been extensively studied because they are widely used for selective oxidation ofo-xylene,' ammoxidation of different alkylaromatics,2 selective oxidation of olefins,S selective reduction of NO by N H s , ~and in many other catalytic processes. Despite the fact that redox transformations of vanadium species a t the titania surface play an important role in the catalytic behavior of this system,l most of the published studies addressed mainly the structural properties of titania supported vanadium catalyst,5p6assuming that vanadium exists a t the surface in the +5 oxidation state. Recently Centi et suggested that in real catalytic conditions, for the surface coverages close to one monolayer, most of vanadium species exist a t the titania surface as V(IV). Formation oftetravalent vanadium and its incorporation into the crystal lattice of titania support during catalytic reaction was observed by Gpsior et aL9 Ggsior and Grzybowska-$wierkosd0studied the oxidation state of vanadium in real catalytic conditions and found that in the steady state of the catalytic oxidation of o-xylene, a significant part of vanadium exists always in the V4+oxidation state. Electrochemistry offers great possibilities of studying the redox behavior of surface species. So, we have undertaken the electrochemical investigations of the interaction of the (110) surface of a rutile monocrystal with the solutions of vanadium(V), or with the solid V2O5 during heating rutile with Vz05 powder, as well as the transformations of the obtained surface species formed by thermal treatment. Rutile was used, because of all crystallographic modifications of titanium dioxide only rutile may be obtained in the form of large monocrystals of good electrical conductivity. It must be however noticed
that contrary to the opinionslong existing in the literature, recent studies by De0 et al.ll showed no difference either in the catalytic activity or in the form of surface vanadium species between rutile and other titanium dioxide polymorphs, for the surface coverages lower than one monolayer. Two types of experiments were performed. The rutile monocrystal was either immersed in the vanadium(V) solution and left there for some time, to obtain the adsorbed vanadium layer, or covered with the Vz05 powder and heated 750 K for some time, to form the monolayer by thermal spreading.12J3 This treatment was intended to model the interaction of rutile with vanadium species during the preparation of the catalyst by impregnation or thermal spreading. After the treatment with vanadium, the rutile monocrystal was transferred to the electrochemical cell and cyclic voltammetry experiments were performed to register the redox transformations of vanadium species present a t the surface.
Abstract published in Advance A C S Abstracts, March 1,1995. (1)Bond, G. C.J . Catal. 1989,116,531. (2)Cavani, F.;Parrinello, F.; Trifiro, T. J . Mol. Catal. 1987,43,117. (3)Bond, G. C.; Sarkany, A. J.;Parfitt, G. D. J.Catal. 1979,57,476. (4)Bjorklund, R. B.; Odenbrand, C. U. J.; Brandin, J. G. M.; Andersson, L. A. H.; Liedberg, B. J . Catal. 1989,119,187. (5) Deo, G.; Wachs I. E. J . Phys. Chem. 1991,95,5889. (6)Vuurman, M. A,; Wachs, I. E.; Hirt, A. M. J . Phys. Chem. 1991, 95,9928. (7) Centi, G.; Giamello, E.; Pinelli, D.; Trifiro, T. J . Catal. 1991,130, 220. (8) Centi, G.; Pinelli, D.; Trifiro, T.; Ghoussoub, D.; Guelton, M.; Gengembre, L. J . Catal. 1991, 130,238. (9)Ggsior, I.;Ggsior, M.; Grzybowska, B.; Koztowski, R.; Stoczyf~ski, J. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1979,27,829. (10)Ggsior, M; Grzybowska-Swierkosz, B. In Vanadia Catalysts in Processes of Oxidation of Aromatic Hydrocarbons; Grzybowska-&vierkosz, B.; Haber, J.;Eds.; Polish Scientific Publishers: Krakow, 1984; p 133.
further treatment. In the experiments on adsorption ofvanadium species from solution, the crystal was simply immersed in the solution of ammonium vanadate (NH4V03)of various concentrations at pH = 4.0 & 0.1, containing additionally sodium sulfate (Na2S04)of the concentration 0.5 mol dm+, and kept there for the predetermined period of time. It was then withdrawn from
@
Experimental Method The synthetic rutile monocrystal was obtained by the Vernouil14 method from high-purityTi02 prepared by the hydrolysis of analytical reagent grade TiC4. It was next X-ray oriented and cut to expose (110)surface. The conductivityof stoichiometric Ti02 is very low. It may however be increased by reduction of T i 0 2 in hydrogen atm05phere.l~ So the crystal,originallyoxygendeficient due to the high-temperature treatment during its preparation, was additionally reduced by heating in flowing hydrogen during 3 h at 1000K, which is far above the Tammann temperature of T i 0 2 . 1 6 The exposed surface area of the crystal in electrochemicalexperiments was 0.2 cm2. Before the experiments, the crystal was hand-polished on 1-pm diamond paste, cleaned on filter paper, and rinsed in water, acetone,water, ethyl alcohol,and finally water (3times with each solvent). The crystal was then mounted in the electrochemical cell or subjected to
(11)Deo, G.; Turek, A. M.; Wachs, I. E.; Machej, T.; Haber, J.;Das, N.; Eckert, H.; Hirt, A. M. Appl. Catal. A 1992,91,27. (12)Haber, J.; Machej, T.; Czeppe, T. Surb Sci. 1985,151,301. (13)Ggsior, M.; Haber, J.; Machej, T. Appl. Catal. 1987,33,1. (14)Anthony, A. M.; Colognes, R. In Preparatory Methods in Solid State Chemistry; Hagenmuller, P., Ed.; Academic Press: New York, 1972;p 157. (15)Khader, M. M.; Kheiri, F. M.-N.; El-Anadouli, B. E.; Ateya, B. G. J . Phys. Chem. 1993,97,6074. (16)Davidson, A.; Che, M. J . Phys. Chem. 1992,96,9909.
0743-746319512411-1024$09.00/00 1995 American Chemical Society
Study of V205ITi02 System
Langmuir, Vol. 11, No. 3, 1995 1025
the solution, washed 3 times with water, and transferred to the electrochemical cell. In the electrochemical measurements the sodium sulfate solution, also 0.5 mol dm-3 and pH = 4.0 0.1, was used. At this pH the surface of rutile should be slightly positively charged, and vanadium exists in the solution as negatively charged decavanadate anionic species;17-19 such conditions should thus facilitate adsorption. The other reason for using the solutionof pH = 4 in the electrochemicalexperiments was the fact that this pH is situated between the pH values of minimum solubility of V205 and V2O4,l8which should minimize the desorption of vanadium species during electrochemical experiments. The 0.5 mol dm-3 NazSO4 solution, used as a base electrolyte, as well as the solutions of ammonium vanadate were acidified or alkalized to the desired pH value with H2S04 or NaOH solutions. Two different samples of V205 were used in experiments simulating the interaction of vanadium pentoxide with rutile surface during thermal spreading. The first was obtained by thermal decomposition of ammonium vanadate. Vanadium pentoxide obtained by thermal decomposition of ammonium vanadate may contain some concentration of tetravalent vanadium. For the present study it was essential to use V205 with as small as possible admixture of tetravalent vanadium. So the other sample ofVzO5was obtained by precipitation from solution. Ammoniummetavanadate was dissolvedin 0.1 mol dm-3 NaOH solution and the solution was acidified to the pH of 3. The fine orange precipitate formed was decanted and washed 3 times with water. It should be however noted, that V205 obtained by precipitation from aqueous solution may contain some sodium. Few droplets of the suspension of V205 in pure water were evaporated on the surface of rutile monocrystal and the crystal was calcined at 750 K for 1 h. The excess of V205 was then brushed off from the surface and the crystal was washed with 0.5 mol dm-3 HzS04 solution during 1h to remove weakly bound vanadium species. Electrochemical measurements were conducted using the measuring system consisting of Frequency Response Analyzer 1250and ElectrochemicalInterface 1286(a digital potentiostat), both Schlumberger-Solartron, and a Hewlett-Packardcomputer with 1091DDMS software (Schlumberger-Solartron) as well as home-written programs. Aone-compartmentcell similar t o that usually used in electrochemicalmeasurements on semiconductor electrodes20was used in the measurements. Saturated calomel electrode (SCE)was used as a reference electrode-all potentials are given versus that electrode. To eliminate the influence of the impedance of the contact between the workingelectrode plug and the sample on the course ofvoltammograms and impedance spectra, the potentiostat was operated in four-electrode mode21 with one reference electrode (reference 1)plug connectedto SCE and the other (reference 2) connected to the so-called potentiommetric contact (a contact made in the point of the sample where no ohmic potential drop occurred). Such a solution was proved22 to be effective in the case of samples which show high resistance and/or non-ohmiccontacts. No soldering of contacts was used to avoid contamination of the sample by diffusion of the soldering medium to other regions during heat treatments. All measurements were performed in the dark, in a constant temperature (25 “C)box, which served at the same time as a Faraday cage. Impedance spectra were taken in the frequency range from 65 535 to 0.1 Hz. Doubly distilled water and analytical grade reagents were used to prepare the solutions. In electrochemicalmeasurements oxygen was removed both from the solution as well as from the cell by flowing through them high-purity argon ( < 1ppm of 0 2 ) . In cyclic voltammetry experiments the occurrence of an
-1.E
-.5
1.E
.5
E.8
1.5
2.8
*
(17)Parks, G. A. Chem. Rev. 1966,65, 177. (18)Israel, Y.; Meites, L. In Encyclopedia ofthe Electrochemistry of Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. VII, Chapter VII-2. (19)Israel, J.;Meites, L. InStandardPotentials in Aqueous Solution; Bard, A. J., Parsons, R., Jordan, J., Eds; Marcel Dekker, Inc: New York, 1985; p 507. (20) Morrison, S. R. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum Press: New York, 1980. (21) Samec, Z.: Marecek, V.: Koryta, J.:Khalil, M. W. J.Electroanal. Chem. 1977,83,393. (22) Peters, E. In Trends in Electrochemistry; Bockris, J. O’M., Rand, D. A. J., Welch, B. J., Eds.; Plenum Press: New York, 1977; p 267.
3888 N
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Q
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-.2t
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.nQ . ” . -.5
,
8.8
E/V
.
. , , .
.5
..
1.8
. . * . 1.5
,
. 2.0
e
v s SCE
Figure 1. First (solid line) and second (dashed line) cycles of the cyclic voltammetry curve for a monocrystal (110) oriented rutile electrode in the solution ofNazSO4(0.5moYdm3,pH 4.0), potential sweep rate 20 mV/s, sweep commenced from the rest potential in cathodic direction,and the Mott-Schottky plot (0) performed at the same conditions, frequency 81 Hz. Electrode was polished before the experiment. electrochemical reaction is usually visualized on the voltammogram as a current peak, and the quantity ofreacted substance may be calculated from the Faraday law, dividing the charge obtained by integration of the area under the peak by the number of electrons exchanged in the reaction and by the Faraday constant. Comparisonof that quantity with the calculated charge required to form a monolayer of adsorbed species is the usual procedure in surface studies. In the case of small area solid electrodes a serious problem is the lack of information of the surface roughness factor (SRF). Fortunately, for a thoroughly polished material of such high hardness as rutile, SRF should be close to 1. The elementary crystallographic unit of rutile contains two Ti atoms on the (110) surface, which leads to the value of 1.67 C/m2 as a monolayer charge (assuming one elementary charge for one surface Ti atom and SRF = 1). In catalytic studies it is usually assumed,23on the basis of the size of vo4 tetrahedron, that to form a monolayer of vanadium(V) species on the surface of a sample with the specific surface area of 10 m2/g the concentration of V206 equal to 1%by weight is required, which leads to the monolayer charge of 1.06 C/m2, assuming the SRF = 1 and one electron reduction of vanadium (i.e.the reduction ofvanadium(V)to vanadium(IV)). Ifvanadium is reduced to lower oxidation state, a multiple charge will be obtained. In the calculation of the charge under voltammetric peaks, connected with electrochemicalreduction or oxidation of adsorbed species, the charge registered for the “clean”electrode surface in the same potential range was always subtracted, to account for the charging of the double layer. This subtraction amounted always to (depending on the potential range considered) 20 to 30%ofthe calculated charge ofvanadium monolayer.
Results Interactionof Rutile Surfacewith the Atmosphere during Thermal Treatment. Figure 1 presents the cyclic voltammetry curve of the rutile electrode in 0.5 mol dm-3 Na2S04 solution at the pH 4.0. The electrode was polished and washed and then mounted in the cell, and the deoxidized Na2S04 solution was poured into the cell. The rest potential ofthe electrode was registered for some time and the cyclic voltammetry sweep was commenced in the cathodic direction, starting from the rest potential. First two cycles are shown (the consecutive sweeps were always similar to the second one)together with the MottSchottky (M-S) plot, which was determined afterward, by measuring the impedance of the electrode at different potentials. The M-S plot was linear in a wide range of (23) Bond, G. C.; Briickman, K Faraday Discuss. Chem. SOC.1982, 72, 235.
Haber and Nowak
1026 Langmuir, Vol. 11, No. 3, 1995 potentials, except the region from -0.5 to +0.5 V, which may be ascribed to the influence of surface state^,^^^^^ as well as to the fact that in this potential range the space charge layer (SCL) is no longer the depleted one. For rutile electrode a t the pH = 4.0 the position of the conduction band edge (CBE) is reported to occur in the potential range from -0.3 to -0.5 V vs SCE.20,26-2sThe flat band potential (FBP), determined from the intercept of the M-S plot with the abscissa, was very close to the CBE (see Figure 1). The free charge carrier (FCC) Concentration, estimated from the M-S plot was rather high (-8 x loz4m-3). An independent estimation of the FCC concentration was made on the basis of the measurement of dc conductivity performed by the fourelectrode method in the direction perpendicular to the tetragonal c-axis. Assuming after Bransky and TannhauserZ9the electron mobility in hydrogen reduced rutile to 1.1 x m2 V-l s-l, the FCC concentration was calculated to be 2 x loz4m-3, in reasonable agreement with the value estimated from the M-S plot. Some frequency-dispersion of both the FBP and FCC concentrations determined from M-S plot was observed, which was also reported in the l i t e r a t ~ r eand ~ ~ascribed , ~ ~ to the influence of surface states (this surface state being most probably the Ti3+ defects), so the above given values of FBP and FCC are only the rough estimation; however it may be stated that the investigated sample is a “strongly n-type” semiconductor, probably close to degeneracy. It is exactly what one may expect for a rutile monocrystal reduced in hydrogen atmosphere. Assuming that each oxygen vacancy introduces two FCC (electrons), one may estimate the deviation from stoichiometry (oxygendeficit) to be less than 0.01 atomic % which is within the possible range of deviation from ideal stoichiometry in rutile.30On the anodic side of FBP there was no current up to +2 V, where the oxygen evolution reaction started. The current “loop”in the potential range from -0.5 to -1.0 V was due to the hydrogen evolution reaction (HER).28 The shape of the voltammetric curves and the impedance spectra of the electrode (not shown) were similar to those presented in the literature for n-type rutile electrode^.^^,^^,^^ When the electrode was heated during 1h in air a t the temperature of 750 K (-100 K below the Tammann temperature) and then subjected to electrochemical investigations without polishing, the slope of the M-S plot (not shown) grew by afactor of 10,indicating that the concentration of electrons a t the surface diminished, a t the same time the intercept of the M-S plot with the potential axis moved in anodic direction. The frequency dispersions of FBP and FCC were no longer observed. Evidently, heating of the sample in air resulted in annihilation of oxygen vacancies a t the surface, which lowered the concentration of Ti3+defects and electrons in the surface layer. It is worthwhile to mention that the current was diminished in the whole investigated range (especially in the HER range), which may be ascribed to the decrease of the concentration of FCC (electrons). (24)Siripala, W.; Tomkiewicz, M. J. Electrochem. Soc. 1982,129, 1240. (25) Frank, N.S.; Bard, A. J. J . Am. Chem. SOC.1975,97,7427. (26)Memming, R. In Progress in Surface Science; Davison, S . G., Ed.; Pergamon Press: New York, 1986; Vol. 17, p 1. (27)Bolts, J. M.; Wrighton, M. S. J . Phys. Chem. 1976,80,2641. (28) Finklea, H. 0.Insemiconductor Electrodes; Finklea, H . O., Ed.; Elsevier: Amsterdam, 1988; p 43. (29) Bransky, I.; Tannhauser, D. S . Solid State Commum. 1969,7, 245. (30) Dennis, P.F.; Freer, R. J . Mater. Sei. 1999,28,4804. (31) Janssen, M. J . G.; Stein, H. N. J . Colloid Interface Sci. 1987, 117,10. (32) Noufi, R.N.; Kohl, P. A.;Frank, S. N.; Bard, A. J. J . Electrochem. SOC.1978,125, 246.
-1
i : Iil
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E/V vs SCE
-.1 --
-.2
Figure 2. First (solid line), second (dashed line), and third (dotted line) cycles ofthe voltammetric curve for a monocrystal (110) oriented rutile electrode in the solution of NazSO4 (0.5 mol/dm3,pH 4.01, potential sweep rate 20 mV/s, sweep commenced from the rest potential in cathodic direction. Electrode was heated in argon atmosphere for 1h at 750 K.
Figure 2 presents the voltammetric curve registered with the electrode which was previously heated during 1 h a t 750 K in argon atmosphere (no polishing after heating). Note that significant anodic currents start to flow a t the potentials much below the valence band edge (which is located a t the potential -+2.75 V).20,26-28 This may be interpreted as a creation of the donor states a t the electrode surface, which are oxidized during the anodic sweep. The anodic charge during the first anodic halfcycle, for the potential range from 0 to +1.5 V was equal to 0.97 C/m ,which is close to the charge of one monolayer (in the consecutive sweeps the anodic charge in this range was negligibly small in comparison with the charge in the first sweep). It must be however remembered that at room temperature the diffusion of both titanium and oxygen in rutile should be rather slow, so the annihilation of defects located more deeply than in the first monolayer seems to be improbable. Currents in the region of higher anodic potentials may be assigned to oxygen evolution mediated by the surface states created during heating. Note also enhanced current in the range of HER. Simplest explanation of the observed behavior of the electrode is the assumption that on heating in oxygen-free environment generation of oxygen vacancies and equivalent number of Ti3+ surface states took place, which mediated the electrochemical reactions occurring a t potentials within the band gap of the rutile semiconductor, including the reaction of reoxidation of these surface states themselves. On the other hand recent STM investigationsof the surface of rutile monocrystal showed33that the surface of reduced rutile is usually corrugated. So, the enhancement of current in the whole potential range after heating in a nonoxidizing atmosphere may probably be partially ascribed to the roughing of the surface. After the electrode was repolished, the shape of voltammogram and M-S plot from Figure 1 were always restored, which means that the stoichiometry changes generated by heating during 1h a t 750 K were restricted to the outermost surface layer only. (33) Zhong, Q.; Vohs, J. M. Bonnell, D. A.Surfi Sci. 1992,274,35.
Langmuir, Vol. 11, No. 3, 1995 1027
Study of V~05lTi02System
Assignment of Potentials of the Possible Redox Processes. In the case of a metal electrode, electrons may be exchanged between the electrode and any redox couple in the solution (or a t the surface). In the case of a semiconductor electrode, the exchange of electrons is possible only if the redox potential of the investigated redox couple assumes a favorable position versus semiconductor band edges on the energy s ~ a l e . ~For ~ ,an ~ ~ , ~ ~ n-type semiconductor the free flow of electrons in both €4vs SCE directions is possible only if redox potential of the redox couple is located above the CBE on the electron energy scale. For redox couples of the potential slightly below the CBE, cathodic processes are possible, whereas anodic processes are kinetically hindered. For the redox couples, which show the redox potential in the range of band gap, far below the CBE, neither cathodic processes nor anodic ones can occur. So, the knowledge of the position of redox potentials, corresponding to the possible redox processes of vanadium species, on the electron energy scale is of great importance for the interpretation of results. The standard electrode potential for the redox couple I V5+N4+a t the pH between 3 and 3.5 is +0.48 V (vs SCE) I I -.3 and the corresponding value for the V4+Wfcouple is +0.24 V.19 Note however that the standard values are given for Figure 3. First (solid line) and second (dashed line) cycles of the unit activities of the considered species. In the case the cyclic voltammetry curve for a monocrystal (110) oriented of vanadium, which in aqueous solutions forms many rutile electrode in a solution of Na2S04 (0.5 mol/dm3,pH 4.0), different complexes (also with sulfate ions, which were potential sweep rate 20 mV/s, sweep commenced from the rest potential in cathodic direction. Electrodewas polished,washed, present in the investigated solutions), the real values and contacted 1h with the solution of NH4V03 0.01 mol/dm3 measured in solution may differ significantly from the (pH 4.0). standard ones. Unfortunately not much may be learned from the literature on that problem. As already mentioned direction)will proceed with high overpotential, which was vanadium species form (especially in solutions of circumreally observed in voltammetric experiments on the rutile neutral pH) different complexesand both the polarograms electrode.36 and cyclic voltammograms of the solutions of vanadium Adsorption of Vanadium Species from Aqueous salts show usually many waves (or maxima) which may Solutions. Figure 3 shows the cyclic voltammetry curve be ascribed to vanadium in the same oxidation state, but for the sample which was contacted during 1h with 0.01 differently complexed.ls It makes the interpretation very mol dm-3 solution of ammonium metavanadate (pH 4.0). difficult. From the data of Filipovic et al.34the potential After the sample was withdrawn from the solution, it was of 0.29 V may be estimated for the redox couple V5+N4+ washed 3 times with water and dried on filter paper and a t pH 4. D ~ c r e measured t~~ in equimolar solution of the cyclic voltammetry curve was registered. During the pentavalent and tetravalent vanadium a t pH 4 the redox first cathodic half-cycle a current peak appeared on the potential of 0.13 V (the reason for the observed difference curve ((21’). The corresponding anodic peak (All was is different composition of the solutions). Present auobserved on the anodic half cycle and again the cathodic t h o r measured ~ ~ ~ the redox potential of the V5+N4+ couple peak (Cl”) was observed on the second cathodic half-cycle. in the same solution as that used in electrochemical The positionofthe Cl“wasshiRedbyO.06Vinthecathodic experiments. The redox potential of ammonium metadirection in comparison with Cl’. The range -0.6 to +O. 1 vanadate and vanadyl sulfate solution in equimolar V was chosen for the calculation of charge in the first concentrations (5 x mol dm-3) was +0.229 V, cathodic half-cycle, and the range -0.6 to +0.5 for the measured with a n Au electrode. In cyclic voltammetry consecutive cathodic cycles. The charge for anodic process experiment^^^ performed on the Au electrode in this was calculated by integration in the range from -0.5 to solution, a pair of peaks was observed with the anodic f 0 . 5 V. peakat -0.24Vandcathodiconeat -0.1OV. Themidpeak It may be noticed that the charge in the first cathodic position is significantly shifted in the cathodic direction half cycle was always significantlyhigher than the charge in comparison with the measured rest-potential in that in both the anodic and cathodic half cycles of consecutive solution; however in potentiodynamic conditions the sweeps. Similar behavior was observedin the experiments overpotential for the cathodic process and the anodic one performed a t other conditions (time of adsorption and may differ significantly. Another pair of peaks, much less concentration). The lower charge in anodic half-cycle may distinctive, with the midpeak position at -0.14 V was be explained by the lower rate of electron transfer for a n visible too. The pair of peaks a t -0.14 V might be ascribed anodic process in comparison with the cathodic one on an to the redox couple V4+N3+. n-type semiconductor (see previous section), but the The redox potential of the V4+N3+redox couple is thus difference between the cathodic charges in the first and close to the CBE-this redox transformation will probably the consecutive sweeps needs closer examination. For 14 proceed easily, a t least in cathodic direction. The redox experiments the mean value of the ratio of the charge potential of the V5+N4+redox couple is however located registered in the first cathodic half-cycle to the charge of much deeper within the band gap of rutile and one can the second cathodic half-cycle (the third and the consecuexpect that this redox transformation (especially in anodic tive cycles were similar to the second) was found to be -1.8. The reaction of the reduction of vanadium(V1 ions (34)Filipovic, I.; Hahl,Z.; Gasparac, Z.; Klemencic, V. J.Am. Chem. in aqueous environment is known18 to be sluggish and SOC.1964,-76, 2074. usually occurs with significant overpotential. Usually the (35)Ducret, L.-P. Ann. Chim. (Paris) 1951, Ser. 12, 6, 727. consecutive steps of reduction are not well resolved, and (36) Haber, J.; Nowak,P. Cutul. Lett. 1994, 27, 369.
1
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1028 Langmuir, Vol. 11, No. 3, 1995
Haber and Nowak
its final product is vanadium(II1) or vanadium(I1). So the obtained ratio of the charges in the first and the second voltammetric cycles may be interpreted as the reduction of adsorbed vanadium species from the $5 to +3 oxidation state in the first cycle, whereas in the consecutive voltammetric sweeps vanadium ions would be subjected to changes of the oxidation state between +3 and +4. It is well-known from the l i t e r a t ~ r e ~ (and , ~ , references ~~,~~,~~ cited therein) that tetravalent vanadium oxide forms easily solid solutions with rutile and vanadium ion, substituting in rutile crystal lattice the Ti4+ion, cannot be oxidized to 5+ oxidation state. So, one may assume that simultaneously with the reduction of adsorbed vanadium the incorporation of vanadium in the outermost surface layers (maybe the first monolayer) of rutile took place. Neither the position of the peak C1 nor the charge under the peak depended on the sweep rate (in the sweep rate ranges from 0.1 to 0.01V s-l), which suggests that reacting species are located in the surface layer ofthe electrode (no diffusion control). The midpeak position ([&I E A ~ I /was ~) approximately -0.21 V and the difference between the anodic peak and the cathodic peak potentials was 0.12 V, which means that the reaction is not strongly kinetically -.6 -.4 -.2 0 .2 .4 hindered. Note, that for a semiconductor electrode (except E/V v s SCE the case of strong degeneration) some overpotential, due Figure 4. First cathodic going voltammetric half-cycle of the to the potential drop in the SCL (band bending) always cyclic voltammetry curve for a monocrystal (110)oriented rutile occurs, so the reversible behavior (with the position of electrodein a solutionof NazS04 (0.5mol/dm3,pH 4.0), potential anodic peak matching the position of the cathodic one for sweep rate 20 mV/s, sweep commenced from the rest potential in cathodic direction, for (A) electrode contacted 1h with 0.5 a reversible surface reaction) may not be expected. mol/dm3Na~S04solution (pH4.0); (B)electrode contacted with Vanadium is bound to the surface rather weakly-when 0.05 mol/dm3metavanadate solution (pH 4.0) next washed with the electrode after adsorption was introduced to the 0.5 water; (C) electrode contacted 1 h with 0.05moYdm3metamol dm-3 HzS04 solution for 1h, vanadium disappeared vanadate solution (pH 4.0) and next l h with 0.5 mol/dm Nazfrom the surface almost completely (see Figure 4,curve so4 solution (pH 4.0); (D)electrode contacted 1 h with 0.05 D). However 1h contact with the sodium sulfate solution mol/dm3metavanadatesolution (pH 4.0) and next 1h with 0.5 (Figure 4,curve C) removed only approximately 50% of moVdm3 solution. Arrows point to the rest potential. the adsorbed vanadium, indicating that vanadium is bound to the rutile surface stronger than simply by electrostatic (physical) adsorption. Note that after adsorption of vanadium species the rest potential of the rutile electrode measured in pure base electrolyte was higher N than the rest potential of the “clean”rutile electrode and I slightly higher than the potential of the redox couple V5+/ E 0 V4+ (Figure 4,compare curves A and B), which means \ that most of the vanadium was in the 5+ oxidation state. a When, after voltammetric measurement, the electrode was withdrawn from the solution and introduced once again to the vanadate solution and the cyclic voltammetry curve registered, the amount of vanadium a t the surface grew. This is demonstrated in Figure 5 where the charges 0 0 I 2 3 4 5 registered in the first and in the second cathodic halfcycles for consecutive experiments are shown. It is to be Number o f experiments seen from Figure 6 that a second redox system (with a Figure 5. Charge in the first cathodic half-cycle (+), and the higher redox potential) appeared a t the surface after charge in the second cathodic half-cycle (0)in consecutive consecutiveadsorptionsfrom metavanadate solution. Only adsorptiodvoltammetric reduction experiments (see text for the cathodic half-cycle showed well developed peak (C2) details). and no peak was visible on the anodic half-cycle; however the rise in the anodic current a t the potentials more anodic consecutive experiments, but peak C2 grew faster (Figure than the C2 peak is evident-it is probable that the anodic 6). Such results suggest that there are two different reaction is more sluggish than the cathodic one and does species a t the surface: the first, formed mainly after the not lead to the well-developed voltammetric peak. This first adsorption, which undergoes only the V4+N9+redox was confirmed by restricting the potential range in transformation giving rise to the pair of peaks Al/Cl, and voltammetric experiments on the anodic side to +0.5 V. the second, growing on the top of the previous one, being In that case the C2 peak disappeared, which means that able to be reduced consecutively from 5+ to 4+ (peak C2) the species reduced a t the C2 potential was formed a t and further from 4+ to 3+ (peak C1) oxidation state as much higher anodic overpotential than the species reduced well as to be oxidized consecutively to 4+ and to 5+ at C1. Note that both C1 and C2 cathodic peaks grew in oxidation state. The maximum charge ever recorded in the first volta(37) Dyrek, K.; Serwicka, E.; Grzybowska, B. React. Kinet. Catal. mmetric cathodic half-cycle after single vanadate adsorpLett. 1979, 10, 93. tion was equal to approximately 1.0 C/m2, which should (38)Gautron, J.;Lemasson, P.; Poumellec, B.; Marucco, J.-F.Solar Energy Mater. 1983,9,101. be compared with the of 1.06 C/m2 calculated for one
+
Langmuir, Vol. 11, No. 3, 1995 1029
Study of V20.5lTi02 System
'-'
I
A1
II
c1 -.S
-.3
-.I
I
.3
.5
E/V v s SCE
Figure 6. Secondvoltammetric cycle ofthe voltammetric curve for a monocrystal (110) oriented rutile electrode in the solution of NaZS04 (0.5mol/dm3,pH 4.01, potential sweep rate 20 mV/s. Electrode was polished, washed, and contacted 1 h with the solution of NH3V03 0.05 mol/dm3(pH 4.0) 4 times, after each time the voltammetric curve performed and electrode again subjected to adsorption without polishing. The solid line represents the first experiment, dotted line represents the second experiment,dashed line represents the third experiment, and the dashed-dotted line represents fourth experiment. monolayer. Assuming a two-electron reaction, it would amount to approximately 0.5 monolayer of vanadium a t the surface. The amount ofvanadium at the surface after adsorption is undoubtedly a little higher, because some desorption of vanadium during the time between filling the cell with solution and start of the voltammetric cy'cle evidently occurred (compare Figure 4 curves B and C). The observed maximum surface coverageof approximately 0.5 monolayer corresponds well with the value of -0.4 monolayer calculated from the data of Kera and Mats ~ k a z efor , ~the ~ saturation coverageof the powdered rutile with vanadium by adsorption from aqueous solution a t similar conditions (concentration of ammonium metavanadate -0.05 mol dm3, pH = 2). The V20fliO2 catalyst is usually calcined before use. So,in one experiment the electrode, subjectedto adsorption from the solution of ammonium metavanadate of the concentration 0.05 mol dm-3 (pH 4.0, time of adsorption 1h) was then heated in air a t the temperature of 750 K for 1h. It may be seen (Figure 7, curve B) that vanadium is evidently present at the surface after such treatment, but the quantity is rather low (assuming two-electron reduction the charge registered in the first voltammetric cathodic half-cycle was equivalent to approximately 0.2 monolayer) and the potential of reduction is shifted by -0.1 Vin the cathodic direction. Most probably the reason for this potential shift is the decrease in the concentration of electrons a t the surface during calcination in oxidizing conditions, which slowed down the cathodicprocess. When the same experiment was repeated, with the calcination in argon atmosphere instead of in air, no vanadium was observed at the rutile surface-compare curves D and E from Figure 7. The reduction peak a t approximately -0.55 V, observed for both the sample subjected to adsorption as well as for pure rutile sample, is evidently connected with the HER, not with reduction of vanadium present a t the surface. Note, that this peak is situated on the slope ofthe HER polarization curve and the apparent difference in charges of that peak in curve D and E is rather the consequence of the differences in the slope of the curve, due to the difference in the charge connected with the HER. If proper baseline is taken in charge integration, (39)Kera, Y.; Matsukaze, Y. J. Phys. Chem. 1986,90,5752.
-.6
-.4
-.2
.a
.2
.4
E/V v s SCE
Figure 7. First cathodic going voltammetric half-cycle of the cyclic voltammetry curve for a monocrystal(110)oriented rutile electrode in the solution of NazS04 (0.5 mol/dm3, pH 4.0), potential sweep rate 20 mVIs, sweep commenced from the rest potential in cathodic direction for (A)electrode heated in air for 1 h at 750 K, (B)electrode contacted 1 h with 0.05mol/dm3 metavanadate(pH 4.0) solution and then heated 1h in air, (C) as previously but additionally washed 1 h with 0.5 mol/dm3 HzS04 solution, (D) electrode heated 1h in argon at 750 K, and (E) electrode contacted 1 h with 0.05 mol/dm3metavanadate (pH 4.0) solution and then heated 1h in argon. Arrows point t o the rest potential. the charges of both peaks appear similar. It is evident, in agreement with the mechanism proposed by Centi et al.' that in the nonoxidative environment (argon atmosphere), vanadium is easily reduced to 4+ oxidation state and diffuses into the bulk of rutile, disappearing thus from the surface. In the case of oxidative environment (air) vanadium is kept a t the surface in the 5+ oxidation state, which prevents (at least partially) its diffusion to the bulk of rutile. Vanadium Deposition by Solid-state Wetting (ThermalSpreading). The most popular way to prepare the V2OfliO2 catalyst is impregnation. In that method vanadium-containing solution is mixed with titania, evaporated to dryness, and calcined. Alternatively the vanadidtitania catalyst is obtained simply by mixingV205 with Ti02 and calcination of the mixture. This procedure was simulated by depositing VzO5 suspension on the surface of rutile, evaporation of water, and heating next in either air or argon atmosphere. The voltammetric curve for the sample obtained in this way (V205 precipitated from aqueous solution) and calcined in air is shown in Figure 8. The general shape of the curve changed (compare Figure 1and Figure 8). Especially remarkable is the rise of anodic currents in the whole range of potentials, also inside the band gap of the rutile semiconductor. Similar rise in current in this potential range, in comparison with pure rutile electrode, was observed by Gautron et al.38for the electrode made of the Ti02-VO2 solid solution. For a n n-type semiconductor it may be explained by the creation of the electronic states, located in the band gap, which may mediate the exchange of electrons between the conduction band and the redox systems a t the surface. Accordingto Gautron et this electronic state might be the V4+ions in the crystal lattice
Haber and Nowak
1030 Langmuir, Vol. 11, No. 3, 1995 * 4 T1
A2
0
t
I,’ I V -*6
t
Figure 8. First (solid line) and second (dashed line) cycles of the cyclic voltammetry curve for a monocrystal (110) oriented rutile electrode in the solution of NazS04 (0.5mol/dm3,pH 4.01, potential sweep rate 20 mVIs, sweep commenced from the rest potential in cathodic direction. Electrode was heated 1h in air at 750 K, with V205 precipitate deposited on the surface (see text for details),next washed 1h in 0.5 molldm3H2S04 solution.
6
\
Id
A
i
t-----
.a1
.aa
.
-*- - -- -- -.
.as
K . ,
I
, ia
k4s-l Figure 9. Dependence of the potential of anodic peak A2 (+) and cathodic peak C2 ( x ) on sweep rate in voltammetric experiments with the electrode treated as previously.
of rutile. A pair of peaks (C2 and A2)was observed on the curve (Figure 8). The positions of these peaks depended on the sweep rate (see Figure 9). Extrapolation to infinitely slow sweep rate gave the midpeak position ([Ecz Em]/2) = -+0.24 V, which is in good agreement with our estimation for the V5+N4+redox potential. Closer inspection of the shape of the voltammetric curve shows that besides the redox system located a t approximately +0.24 V, there is another pair of peaks, located a t much more cathodic potential. It is clearly visible in voltammograms performed in reduced potential range and a t higher sweep rates (Figure 10). The second redox system occurred a t the potential close to the potential of the Al/ C1 redox couple observed for the samples after adsorption from aqueous solution. The midpeak position for the C1/ A1 peaks from Figure 10 was -0.14 V, which is -0.07 V more anodic than in the case of experiments with the adsorption from aqueous solutions. It may be thus assumed that in both cases the pair of peaks Al/Cl represents the same redox process (V4+N3+redox transformation of vanadium present in the crystal lattice), but
+
-.6
i
Figure 10. Second cycle of the cyclic voltammetry curve for a monocrystal (110) oriented rutile electrode in the solution of Na2S04(0.5 mol/dm3,pH 4.0). Electrode was heated 1h in air at 750 K, with VzO5 precipitate deposited on the surface (see text for details), and next washed 1 h in 0.5 mol/dm3H2S04 solution.Sweepsperformed at different sweep rates: solid line, 0.1 VIS; dashed line, 0.05 VIS; dotted line, 0.02 VIS; dasheddotted line, 0.01 VIS.
the concentration of vanadium in the surface layer of rutile is higher in the case of calcined sample, which influences the position of voltammetric peaks. Note, that the peak C2, which started to grow up in multiple-adsorption experiments (see Figure 6) occurred a t the potential close to the peak C2 observed in the case of “calcined” sample (Figure lo), representing probably the same process. The charge connected with the observed voltammetric peaks was difficult to estimate. The A2/C2 process was very sluggish, and selection of potential ranges for charge integration was ambiguous-on the other hand the currents not connected with the redox transformation of vanadium evidently grew up (compare Figures 1 and 8) which falsified the charge calculation. As a rough estimation the charge connectedwith both processes taken together (Al/Cl and A2/C2) was 1 monolayer for a 2 electron reaction. Vanadium is relatively strongly bound to the surface-note that the sample was washed, before voltammetric examination, in 0.5 mol dm-3 HzS04 solution for 1 h. When the rutile sample was then subjected to 1 h of boiling in 0.5 mol dmT3HzSO4, the A2/C2 redox system disappeared almost completely from the surface, but the AM21 redox system was still observed, with the charge equivalent to -0.5 monolayer (for 1 electron reaction). However, the polishing of the surface removed the vanadium from the surface completely. It is improbable, that in such a n aggressive environment (boiling 0.5 mol dm-3 HzSO4 for 1h!) the species present in the outermost layer of the surface could survive, because even rutile itself dissolves to some extent at this conditions. So, this observation confirms the interpretation that the Al/Cl pair of peaks is connected with the redox transformation (V4+N3+) of vanadium ions which have diffused into the crystal lattice of rutile. Consequently, the A2/C2 pair of
Study of VZOSJTi02 System N
-2r
-1
Figure 11. Second cycle of the cyclic voltammetry curve for a monocrystal (110) oriented rutile electrode in the solution of NazS04 (0.5 mol/dm3,pH 4.01,potential sweep rate 20 mV/s. Electrode was heated 1h in argon at 750 K, with the vanadium precipitate deposited on the surface (see text for details) and next washed 1h in 0.5 mol/dm3 solution. The solid line represents the electrodejust after cooling to room temperature and the broken line represents the same electrode after an additional 1 h of heating in air at 750 K. peaks may be ascribed to the redox transformation of species present a t the surface in the form of a monolayer (which participated however in the Al/Cl process too). These conclusions are supported by results of the experiment in which the calcination step was performed in argon atmosphere (all other conditions kept the same). In this case the voltammetric peaks were much better resolved (Figure 11). One can observe that the charge connected with the AlIC1 pair of peaks is much higher than the charge connected with the A2/C2 pair. The total charge, connected with both redox couples was estimated to be equal to approximately 2 monolayers (for a two electron reaction). The higher charge may be partially connected with the roughing of the surface, which was postulated for rutile electrode heated in argon atmosphere also in the absence of vanadium species (Figure 2). Also in this case washing in boiling HzS04 removed the pair of peaks A2/C2 almost completely and diminished the pair Al/Cl to approximately 50%. This confirmsthe conclusion that two different species are present at the surface: one which undergoes only the V4+N3-redox transformation and the other which undergoes both the V4+N3+and the V5+N4+redox transformation. Note that annealing in the oxidizing atmosphere (see Figure 11) did not change significantly the proportion of the charge connected with both processes. It means that species which undergo only the V4+N3+redox transformation did not oxidize during the annealing in air a t 750 K. The midpeak position for the A2/C2 process was +0.21 V, again very close to our estimation for the V5+N4+redox potential. The midpeak position for AM21 process was -0.17 V-also close to the potential observed in previous experiments. The observed differences in the shape of voltammetric curves between the sample calcined in air and in argon may be explained assuming that during calcination in argon more vanadium(IV)is dissolved in the crystal lattice of rutile. According to Gautron et al.3svanadium dissolved in the rutile crystal lattice forms a broad band situated
Langmuir, Vol. 11, No. 3, 1995 1031 approximately 0.8 eV below the CBE of rutile. Note that redox potentials of both the Al/Cl and A2/C2 processes are located between the CBE of rutile and this d-band created by vanadium ions. So for rutile samples, saturated to some extent with vanadium, one can expect easy electron exchange with the lattice for redox processes in both cathodic and anodic direction, which is really visible in Figure 11. Evidently, the electrochemical behavior of the sample depends on the amount of vanadium dissolved in the crystal lattice of rutile. For the sample sintered with VzO5 obtained by thermal decomposition of ammonium metavanadate, the voltammetric curves were similar to those presented in Figure 11. In the case of samples obtained by thermal spreading, the rest potential of the sample after its first introduction to the solution was close to the redox potential of V5+N4+ measured in this solution, but the shape of the first voltammetric cycle as well as the comparison ofthe charge registered in the first and in consecutive sweeps suggested that most of vanadium was present at the surface in the V4+oxidation state. The sample used in the experiments was reduced in hydrogen atmosphere before the experiments to increase its conductivity. So, the question arises if the structural defects, introduced during the reduction, influenced the process of the interaction of vanadium with the rutile surface. Centi et al.,7 for example, suggested that the presence of Tif3 ions in the crystal lattice of rutile is essential for the process of the reduction of vanadium. At room temperature, this interaction may be neglected. Assuming that the concentration of oxygen vacancies in the surface layer of rutile was equal to the concentration calculated from the M-S plot, the surface concentration of Ti3+defects was less than 1% of a monolayer, which was negligibly small in comparison with the observed surface coverages by vanadium. Note also that a t 750 K in oxidizing environment the concentration of Ti3+at the surface was strongly reduced, as shown by the experiments on the interaction of rutile with gaseous atmosphere during heating. On the other hand in real catalytic conditions catalyst may easily be subjected to the interaction with reductive atmosphere, which leads to the creation of Ti3+defects a t the rutile surface. Results obtained in this work, for the case of vanadium species deposited by thermal spreading at the surface of rutile (110) crystal face, are very similar to the results obtained by Centi et al.'za for powdered rutile samples a t comparable conditions. Those authors also observed two types of vanadium species present a t the surface, both strongly bound to the surface, one present as a monolayer species and the other built in the crystal lattice of rutile. The first species appeared in the quantity close to one monolayer (which may be estimated from Figure 5 of the paper by Centi et al.7),and it was possible to both reduce it to the V3+ and oxidize to the V5+oxidation state. The only difference between this work and the work of Centi et al. is that Centi et al. were not able to reduce nor to oxidize the vanadium built in the rutile crystal lattice and present there as tetravalent, whereas in this work the electrochemical reduction (but not oxidation) of the vanadium built in the rutile crystal lattice occurred easily. This difference may be due to the fact that reduction in the work of Centi et al. was performed by the action of gaseous hydrogen a t relatively low temperature (673 K) which restricted the reaction to the very surface layer only. In the case of electrochemical reduction, the fast diffusion of protons in the crystal lattice of rutile with the possibility of doping rutile with hydrogen even a t room
1032 Langmuir, Vol. 11, No. 3, 1995
temperature40 and formation of OH- ions in the rutile crystal l a t t i ~ eenabled ~ ~ , ~the ~ reduction. Similarbehavior, i.e. the readiness of tetravalent vanadium on the surface of rutile to oxidize to 5+ oxidation state in contrast to the tetravalent vanadium present in the crystal lattice of rutile, was earlier reported by Rusiecka et al.41
Discussion Rutile is a nonstoichiometric semiconductor, and its behavior may be influenced significantly by the changes of stoichiometry, which was indeed observed in our experiments. For short calcination times the changes are restricted to a very thin surface layer only. However it may be expected that during long-term catalytic experiments the properties of the sample may slowly evolve, reaching steady state after some time. Vanadium dissolves in the crystal lattice of rutile in the form of tetravalent vanadium. The amount of vanadium entering the lattice depends on the amount of vanadium a t the surface, on temperature, and on the redox conditions in the experiment. This process may be described by the reaction:
for which the equilibrium will be established
K = [vo2,Ti0z,12Po,
112
where [VO~(T~O,)I is the concentration of vanadium in the crystal lattice of rutile. At low temperature the diffusion of vanadium into the bulk ofthe crystal lattice is extremely slow and the subsurface layer becomes quickly saturated with vanadium. So only a very limited amount of vanadium may be dissolved in rutile a t room temperature. However a t higher temperatures vanadium freely diffuses into the bulk of rutile and the amount of vanadium introduced to the crystal lattice ofrutile depends on oxygen partial pressure (eq 2). Note that according to eq 2, even in oxidizing conditions some vanadium will diffuse into the crystal lattice of rutile. The above presented reasoning concerns of course the case when V205is present a t the surface as a separate phase-in that case the activity of v@5is constant and equal to 1. If the coverage by VzO5 is a submonolayer coverage, eq 2 should include the VzO5 surface coverage too. When a t the constant oxygen pressure the surface coverage diminishes, the subsurface (40) Ginley, D. S.; Knotek, M. L. J. Electrochem. SOC.1979,126, 2163. (41)Rusiecka, M.; Grzybowska, B.; Gqsior, M.AppZ. Cutal. 1984,10,
101.
Haber and Nowak concentration of vanadium diminishes too and consequently less and less vanadium ions can diffuse into the bulk of rutile monocrystal. So some vanadium may survive a t the surface in the oxidative environment even a t relatively high temperature. Vanadium ions which diffused into the bulk of rutile can undergo the V4+N3+but not the V4+N5+redox transformation in electrochemical experiments. Results of the present study indicate that vanadium(V) species present a t the surface are very vulnerable to reduction and a stable monolayer vanadium species a t the surface can exist only if the subsurface layers were saturated by vanadium(W). Namely on the rutile surface, saturated to some extent by tetravalent vanadium, a second kind of vanadium species may be formed, which undergoes both the V5+N4+and V4+N3+redox transformations. The behavior of vanadium present a t the rutile surface in electrochemical experiments was strongly influenced by the presence of vanadium in the crystal lattice of rutile. Electron-exchange processes difficult or impossible between the rutile electrode and vanadium ions in the solution or a t the surface become fast when the subsurface layer of the crystal lattice of rutile contains incorporated vanadium. This is obviously due to changes in the band structure of rutile, caused by vanadium present in the crystal lattice. Similar behavior may be expected for the other redox couples, also in the case of gas phase experiments. One may expect, that the easy exchange of electrons between the d-band created by vanadium incorporated in the crystal lattice of rutile and the redox couples a t the surface will reduce the selectivity of the catalyst to partial oxidation products. It may be thus concluded,that rutile-supported vanadium oxide catalysts are in fact composed of vanadium oxide monolayer supported on the surface of the solid solution of V4+ions in the crystal lattice of rutile, which strongly modify the electronic properties of the solid and hence its redox interactions with adsorbed molecules. The anatase form of Ti02 does not dissolve tetravalent vanadium. This could explain the inferior catalytic properties observed in the case of rutile-supported vanadium oxide catalyst as compared to anatase. On the other hand it has been shown1' that a t submonolayer coverage with vanadium, when vanadium atoms are held a t the surface of rutile and the solid solution is not formed, bothV2Odrutile and VzOdanatase catalyst show the same catalytic properties.
Acknowledgment. The authors wish to thank Mr J. Oblgkowskifrom the Department of Inorganic Chemistry, Academy of Mining and Metallurgy, Krakbw, Poland, for preparing the rutile monocrystal. LA940594H
