samples had been equilibrated (prior to injection) with the same partial water vapor pressure as prevailed in the calorimeter. When this was not done, a correction was necessary, by adding or subtracting a A T obtained in a “blank experiment.” The “blank” was a comparable gas phase which had the appropriate water content in the absence of the unknown. Our preference for using partially filled calorimeters in the analytical applications of gas enthalpimetry was predicated by the finding that a large reservoir of a supernate saturated in water vapor assured the continued maintenance of steady state conditions. Any undersaturated o r supersaturated gas, which may perchance have entrained into the supernate, was thus certain to attain genuine saturation equilibrium within the calorimeter. The range of analytical applicability of the technique described in this paper has a lower limit of 1 COz (or SOz)in a given sample, if a precision and accuracy on the order of 2 z is desired. This restriction is related to the following con-
z
siderations : the smallest temperature change measurable within 2% with our instrumentation was on the order of 0.005 “C;injection of sample must be completed within two minutes, because of limitations in adiabaticity; it was found empirically that kinetics of equilibration restricted volumes of injectable gas to one half the volume of the liquid in the calorimeter. Recently, thermometric methods have been successfully adapted to continuous automatic process control (25). Similar potentialities of gas enthalpimetry for monitoring air pollution are readily apparent.
RECEIVED for review September 19, 1968. Accepted November 8, 1968. Supported by Public Health Service Grant No. HE-02342 from the National Heart Institute. (25) T. R. Crompton and B. Cope, ANAL.CHEM.. 40,274 (1968).
Determination of Metallic Nickel in Catalysts and Ceramic Materials by Differential Thermal Analysis Jiii Macak and Jir‘i Malecha Department of Coke and Gas Technology, Institute of Chemical Technology, Prague, Czechoslocakia
A reliable differential thermal analysis method is presented for evaluating Ni catalysts containing from 0 up to 10% Ni on refractory supports. This method determines the temperature difference between the reactor for catalytic reactions and that with inert SiOz packing (quartz glass) when both are placed in an accurately temperature-controlled metal block. A gas mixture containing first, the reducing and then the oxidizing gases passes through both reactors (quartz glass) under equal conditions of time and flow. It is possible on the basis of the amount of heat evolved as indicated by DTA, to determine not only the catalytic activity of the investigated sample but also the amount of the catalytically active metallic Ni. The present paper offers an application of this method to studying the steam reforming of hydrocarbons. THE APPLICABILITY of the DTA method for the evaluation of catalysts and its advantages in this respect have been pointed out by Patrikeev ( I ) . Stone and Rase ( 2 ) had constructed a differential thermal analyzer to measure the activity of fluid cracking catalysts by means of the heat of chemisorption of catalytic poisons. The sample weight used by the authors was 0.15-0.22 gram of catalysts. In the above mentioned study, Stone and Rase had modified the DTA method, which had been designated later by Stone (3) as one of the five practical DTA techniques. In this case the operating pressure and temperature remains constant, whereas the gas composition around the test sample is changed. This procedure involves therefore an abrupt change of gaseous environment in the sample holder that can be programmed, if necessary, in a cyclic mode. Although the author (3) as(1) V. V. Patrikeev, “Metody IzuEenija Katalizatorov,” Vol. 5 of “Problerni KinZtiki i Kataliza,” 1st ed., Ed. by Acad. Sci. of the USSR, Moscow 1948, p 198.
(2) R . L. Stone and H. F. Rase, ANAL.CHEM., 29, 1273 (1957). (3) R. L. Stone, ibid.,32, 1582 (1960). 442
ANALYTICAL CHEMISTRY
sumes that such a modified DTA method is appropriate for the quantitative determination of oxidizable or reducible material, he has made use of this method only for studying the thermal dependence of oxidation-reduction of manganeseoxide-containing ceramic body by gas cycling with CO, and 0 2 . Crandall and West ( 4 ) had also studied a similar problem concerning the oxidation of a mixture of Co. namely with ceramic material. It was not possible to ascertain the amount of C o with an accuracy corresponding to quantitative determination with their laboratory equipment and analytic method. The total Ni content in catalysts is generally determined after a complete dissolution of the sample (in acids or by alkaline fusion) by gravimetric, colorimetric, or polarographic methods. Modern methods of instrumental analysis may also be used, such as spectral or roentgenographic ones. The unreactedlamount of Ni in the support-the portion of which may be assumed to take an active part in accelerating the reaction of hydrocarbon with water vapor-can generally be determined by extraction of the catalysts with mineral acids. This determination method has been described in the work of LejbuS (5). This author used the mixture of hydrochloric and nitric acids for extraction, which, in her opinion ensures complete separation of nickel oxide from Ni-spinel. Nispinel results from the reaction between a-A1203and NiO. A similar determination method (with HC1) has been used by Marsh and Wylde (6) for determining free Ni. They ascer-
(4) W. B. Crandall and R. R. West, Am. Ceram. SOL-.Bull., 35, 66 (1956). ( 5 ) A. G. LejbuS, B. G. Ljudkovskaja, A. N. Gruzincova, A. C. LichaEeva, E. V. Janikina, and A. M. Goldman, Chem. Znd. (Russian), No. 2, 16 (1961). (6) J. D. F. Marsh and J. H. Wylde, Gas Council Research Cornmunication No. 98, 1963.
Table I.
Survey of Catalyst and Results of Ni Determinations
Catalysts no. 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17
Ni content, Support A A A A A A A B B B B B B B B B B
At of Ni oxidation,
73
Ar of CH4 conversion, "C
0.6 1.0 1.7 2.7 4.1 6.5 8.4 1.9 4.7 6.8 0.3 0.5 0.8 1.6 3.3 4.6 7.3
1.5 8.7 35.5 38.0 40.0 43,8 46.7 42.0 44.7 50.0 8.5 29.2 44.7 49.0 50.0 54.5 52.0
6.2 9.0 13.5 21.0 28.5 45.0 57.0 17.0 37.2 50.5 3.0 3.5 7.5 10.5 20.8 35.0 49.0
1
EXPERIMENTAL Material. Nitrogen was a pure gas containing traces of O2and Ar, whereas hydrogen was technically pure, free from 02. Natural gas of Czechoslovak origin contained (besides pure CHI) 1.65% nitrogen, 0.8% CO?, and 0.8% ethane. The hydrogen was passed through a column filled with active carbon and pumice impregnated with a concentrated K O H solution to remove any traces of H2S. In preparing the model samples of the catalysts, two supports were employed. One was aluminum oxide manufactured in Czechoslovakia under the name of "alumogel." After annealing i t contained almost pure a-A120a. (Chemical composition of the support A : 0.41% S O 2 , 1.85% MgO, 0.14% Fe203,97.6% Al2O3; specific surface 1.8 mZ/g after calcination at 1400 "C and 1 hour.) The other was a support for the Ni catalysts prepared by impregnation (The chemical composition of the support B: 16.85% SO,, 0.76% CaO, 7.51% MgO, 0.05% NiO, 1.28% Fe and 71.76% A1203; the weight loss by ignition 0.96%; specific surface 11.7 m2,'g, the free volume of pores 0.34 cm3/g) and its chemical composition is more complicated. Before use the "alumogel" was calcined at 1400 "C. In this way the y-modification was converted to a-Aln03, which was monitored by X-ray diffraction. The grain sizes of the support and of the catalysts used were 0.5-1 mm. The
- 9
1
5
7
catalyst was prepared by saturation of the supports in the active component solutions (in an aqueous solution of nickel nitrate at 0 "C in a water-pump vacuum for 1 hour), O n removing the excess liquid by suction and drying at 100 "C in a dry-box, the Ni-nitrate was decomposed in the stream of nitrogen using a temperature increase of 2 "C/min up to the maximum temperature 600 "C. The prepared sample was maintained at the latter temperature of 600 "C for 30 minutes, and subsequently cooled in Nz. The survey of the catalysts used together with the active component content are given in Table I. As an auxiliary substance for filling the reactors, quartz glass ground to the particle sizes of 0.5-1 mm and 1.25-2 mm was used. Before using in the reactor, it was boiled in nitric acid, then thoroughly washed in redistilled water until there were no traces of catalytically active ions and calcinated at 800 "C in an electric furnace. Apparatus. To determine the temperature differences between the reactor containing active material and that containing inert SiO2 packing, the following equation can be derived based on the thermal balance of the volume element of the catalytic bed. The heat consumption (evolution) in the volume of the catalyst per unit of time may be written approximately as : QR
(AHTO)FRX
=
(1)
For the heat supply into the catalytic bed of the reactor, the following equation can be derived from the thermal balance: Qp = T
- To Z
Cpi
Fi f CY(T- To)
+
Under the steady dynamic state, the equilibrium condition requires: (3 1
= QR
Qp
Neglecting the heat contribution by radiation due to small temperature differences inside and outside the reactor, the following final equation is obtained: At
=
H T " x FR F , Cpi CY
(4)
- To)
(5)
+
where At
=
(T
and the condition of the same arrangement of the reactor and the same feed being observed we can write: At
(7) Suzanne Vignes and Francois Jeannot, Compte Rendu du 82" Congrks de 1'Industrie du Gaz, Paris 1965, p 314. (8) Heikichi Saito, Sci. Rept. Tokohu Imp. Univ. 1 6 3 7 (1927).
8
91
"C
tained the evolution (aging) of the catalysts used under plant conditions. Vignes (7) and her coworkers used a methanolic solution of bromine for extracting Ni. The acid leaching method cannot be applied in the case of MgO and MgOsupports because of their high solubility in acids. It is assumed that it is possible to determine quantitatively the metallic Ni in the catalysts being prepared by means of impregnation with the D T A method. With this view, we have applied the DTA method termed by Stone as dynamic gas cycling method (DTA-DGCM). The advantage of this method is obvious in the case of catalysts when cyclic steam reforming of hydrocarbons is used. It can be anticipated that the evolved quantity of heat in metallic Ni oxidation is considerable. In accordance with (8), it can be expected that under favorable conditions of the process, the whole amount of free metallic Ni will be instantaneously and completely oxidized.
2
=
KX
(6)
According to Equations 2 and 5 the extent of At is a measure of the extent of the reaction or the amount of the oxidized ingredient of the catalysts, provided that the reactor apVOL. 41, NO. 3, MARCH 1969
443
kd Figure 2. Detailed schematic of apparatus 1. Air pump 2. Electromagnetic valve for pressure control
3. Flask for pressure-pulse damping 4. Active charcoal column 5. Flow meter 6. Control valve 7. Electromagnetic valve controlled by programmer
8. Flow meter with electric contacts
proaches adiabatic conditions. The further limiting factors have been discussed in the present authors preceding communication (9). A general simplified diagram of the apparatus is given in Figure 1. Its detailed description including the automatic control system is given in a previous study (9). For the purposes described here, (determination of free Ni), certain modifications have been made, as seen in Figure 2. The apparatus was operated in such a way that the temperature of gas saturation was 71.0 "C, the temperature of the furnace with reactors (sample holder) was 621 "C, the amount of catalysts 0.20 ml. A typical thermogram obtained is given in Figure 3. In this case the operating program of the apparatus involves eight periods, characterized by their time, by dynamic gas composition, and by the gas volume, flowing through the reactors. The metallic nickel oxidation has been performed in air after the reduction. This latter operation has been accomplished in two periods; at first it was the reduction of nickel oxide present by hydrogen (24 liters per 1 hour flow rate, 10 minutes operation time) and afterwards by reducing and simultaneously reacting a gas mixture of 21.6% C H e (natural gas of previously mentioned composition), 45.88% N2 and 32.52% H 2 0 (21.8 liters per (9) Jiii MacAk and Jiii Malecha, Chem. Listy, 61, 368 (1967). 444
ANALYTICAL CHEMISTRY
Electromagnetic valve for changeover of gas stream Saturator of gas by water vapor Differential manometer with electric contacts Container of water for saturation Air thermostat Furnace with reactors Recorder of At and controller of furnace temperature Cold junctions of thermocouples Motors for automatic regulation of control valves (flow control of dynamic gases) 18. Quartzglass reactors 9. 10. 11. 12. 13. 14. 15. 16. 17.
hour, 10 minutes). The air in the oxidation period had a flow rate of 13 liters per hour, for 5 minutes. DISCUSSION AND RESULTS Thermochemistry and Thermodynamics of Ni Reactions. In the reaction system studied the following reactions are of major significance:
NiO
+ Hy
-+
+ H?O Ni + CO + 2 H2
Ni
NiO f C H I +
AH '298 NiO Ni
+ CO
+
l/.
+
Ni
+ COS
On+ NiO
=
0 . 0 3 kcal/mol
=
4 9 . 3 kcal/mol
AH '298 =
- 9 . 8 kcal/mol
AHoS98= -57.46 kcal/mol
For Ni determinations by the DTA-DGCM method the oxidation of Ni by free oxygen is of special importance. This reaction is very rapid, especially in the case of finely dispersed Ni, and has a important thermal coloration. The equilibrium data for the reduction of NiO by hydrogen and by carbon monoxide have been reported in the work of
0
20
P
7 e Figure 3. Typical "thermogram" curve recorded in course of one oxidation-reduction cycle
Neumann (IO). According to these data the reaction of nickel oxide with hydrogen starts when the partial pressure ratio is pHa/pH20 a t temperatures of 300-900 "C. Provided this condition is observed, the reduction to metallic Ni proceeds to completion. The direction of the NiO reduction by carbon monoxide depends both on the existing partial pressure ratio pco/pcot as well as on the reaction temperature. In forming nickel oxide, reduction proceeds completely a t 400 "C a t partial pressure ratio >, at 900 "C this ratio is 3 10-2. The effect of the hydrogen dissolved or bound in the metallic Ni can be ignored. This has been confirmed by the studies of Kolomaznik (11) and of Zapletal and Soukup (12) who found that at temperatures considered for our tests, 1 g contained 10-20 ml HZas a maximum. This amount is in a given sample virtually always and therefore cannot cause any serious error in the method. Its absolute value is small, well
>
(10) Gustav Neumann, Archiu fir das Eiserzhiitrenwesen, 14, 429 (1940/41).
(1 1) Karel Kolomaznik, Doctorate Thesis, Department of Organic Technology, Institute of Chemical Technology, Prague (1967). (12) Vladirnir Zapletal and Jaroslav Soukup, Department of Organic Technology, Institute of Chemical Technology, personal communication, 1967.
% Ni Figure 4. The dependence of At on the free metallic Ni content of examined samples Part A illustrates this dependence for samples of catalysts on support A ; B illustrates that for support B
below tion.
5z of the value of the reaction enthalpy of Ni oxida-
The Determination of Ni by Oxidation Method. The nickel produced by the reduction from nickel oxide during the reduction period is then reoxidized by oxygen. In the course of the oxidation the temperature in the catalytic bed is registered, the maximum of the function At = f(time) being the desired quantity. The maximum value of the temperature difference between the reactor for catalytic reaction and that with inert Si02 packing is proportional, in agreement with Equation 6, to the amount of Ni in the catalyst sample. It is known that the oxidation of Ni does not proceed strictly according to the stoichiometric formation of NiO. The final form of the compound of Ni and O2 does not-under constant experimental conditions-introduce any errors in the determination, since the quantitative evaluation of the thermo-
Table 11. Oxidizing Determination of Ni Content in Catalysts after Aging Catalyst Used: Sample No. 9, reaction temperature 621 "C,total Ni content 4.7
Conditions for evolution Temperature, "C Time, hours lo00 1 Cycle Cycle lo00 3 Cycle Cycle lo00 529
Ni determined by leaching with HC1, 1.21 1.05 0.30
At for reaction
CHI
+ H 2 0 , "C
At for reaction Ni Os, "C
+
Ni content determined by oxidation,
17 27.5 28.8 11.5 18.5 20.5
3 .O 4.0 4.5
0.40
1 .o
0.12 0.20
0.0
0.0
1.5 3.0
0.50 0.60
0.40 0.0
VOL. 41, NO. 3, MARCH 1969
445
gram is based on the calibration of the apparatus by the samples of known free nickel content. The relationship of At and the Ni content was established by measuring A t in several samples of freshly prepared catalysts, the total Ni content of which was in a free chemically unbound form. This was verified by chemical analyses both by alkaline fusion as well as by the extraction with hydrochloric acid only. Both methods gave concurring data o n the Ni content. A survey of the results obtained is given in Table I. To obtain a set of catalysts after different aging periods, the prepared catalysts were subjected to artificial aging in a stream of oxygen at a temperature of 1000 "C. The results obtained with the catalysts No. 9, containing 4.775 of total Ni are recorded in Table 11. This table shows a remarkable difference between the amount of metallic Ni, determined by acid leaching accompanied by a polarographic determination of Ni in water solution of N H 4 0 H and NHICl by adding triethanol amine (13) and that determined by defined oxidation. The perfect agreement of the catalytic activity of the sample after aging achieved from the height of the peak caused by CHa HzO reaction with the results obtained by the oxidation method (DTA-DGCM) in determining free Ni confirms the correctness of the oxidation method results. In addition, it was found that the catalysts oxidized a t 1000 "C for 529 hours were no longer active in the CH4 HtO reaction. However extraction yielded as much as 0.3% of free Ni. We assume that the difference mentioned here can be accounted for partly by the nature of the reaction between e-A1203and NiO taking place in a interphase layer. As has been shown in the papers of Reid and other authors (14, 1.5) the formation of Ni-spinel depends upon the diffusion rate of transferred ions. In a reaction layer the thickness is not negligible. The Ni present in this reaction interlayer is apparently more easily reduced than that bound as Ni-spinel or even as a solid solution of Ni-spinel in a-Alz03. This has been proved by the results given in Table I1 denoted as "cycle,'' The experiment was carried out by repeating the operating cycle with the same no. 9 catalysts. Over the course of this procedure, the activity and the amount of Ni determined from the oxidation peak gradually increased. In view of this we may assume that the free Ni determination by acid leaching w 11 in many cases yield higher (and incorrect) results. The present method seems to be the only possible one to determine metallic Ni o n supports which are soluble in mineral acids as MgO and various combinations. In the case of a relatively low Ni content (frequently lower than 3%) even modern instrumental methods (as X-ray or electron diffraction) prove to be inaccurate. The spectral methods (the quantometers) are practically inapplicable in a simple free Ni analysis. On the basis of our experience, it is believed that the
+
+
(13) Charles Degent, Analytical Department, DETN No. l., Gaz de France, personal communication, 1964. (14) J. M. Reid and D. M. Mason, Proc. Operating Sect., American Gas Association CEP-59-3, P-13-9-20 (1959). (15) Jan HlavBf, Scientific Papers from Institute of Chemical Technology, Inorganic and Organic Technology Section 6, 75 (1962).
446
ANALYTICAL CHEMISTRY
method described is potentionally applicable for the determination of easily oxidizable metals in binary or complex systems, where the other constituent is inoxidizable. CONCLUSIONS
It has been found that the dynamic gas cycling modification of the DTA method is appropriate for quantitative free Ni determination in the range of 0-8% of weight. Under the conditions used, the dependence of the peak time of the oxidation on the Ni content turns out to be very near the linear dependence. The industrially produced support B (surface 11.7 mz/g) contains a certain amount of Fe in nonhomogenous distribution, and is the cause for the lower accuracy of results obtained in this case. In the first case of support A , the error amounted to =t4 %; in the second case of complex support B it was more pronounced. The operation of the apparatus is fully automatic and the time required for an determination, including a duplicate run, does not exceed 20 min. The actual oxidation is completed within a few seconds. The apparatus described in this paper is of relatively low sensitivity, capable of registering at up to 1000" C , 0.34 cal of evolved heat with an ensuing temperature difference 1 "C between both the reactors. F o r the design of this apparatus some up-to-date flow calorimeters (such as that made by Beckniann, Ltd.) could be used, after increasing the working temperature of the sample-holder and adjusting the possibility of atmosphere programer. The apparatus described by Stone ( 2 ) is satisfactory provided the material of the sample holder is replaced by a more Chemically and thermically stable one. It seems to be inevitable that quartz glass cells (reactors) should be used. APPENDIX
c,i
=
FE
=
Fi
=
AHTO = K = = =
Qp
QR
To T
=
x
=
C
A
= Y
=
=
specific heat of the component i (cal-mol. grad) feed rate of the reacting substance (mol/hour) feed rate of component i into the reactor (mol/hour) reaction heat at the reaction temperature (cal/mol) proportionality factor (OK) heat supply into the catalytic bed (cal/hour) heat consumption (evolution) in the catalyst (Cali hour) temperature of the furnace with reactors (OK) temperature of the catalytic bed (OK) fraction reacted (mol/mol) overall coefficient of heat transfer by convection (cal/"K. hour) overall coefficient of heat transfer by radiation (Cali O K . hour) ACKNOWLEDGMENT
We are grateful to R. Riedl, professor at the Institute of Chemical Technology, Department of Coke and Gas Technology for his aid and interest in this work and J. PaSek, assistant-professor of the Institute of Chemical Technology, Department of Organic Technology for helpful discussion. RECEIVED for review June 10, 1968. Accepted October 28, 1968.
