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Deuterium tracer experiments on the reactions of some metal hydrides with aqueous oxidizing agents
J. inorg,nucl.Chem., 1969,VoL 31, pp. 613 to 623. PergamonPress. Printedin Great Britain
DEUTERIUM TRACER EXPERIMENTS ON THE REACTIONS OF SOME METAL HYDRIDES WITH AQUEOUS OXIDIZING AGENTS R. A. S U T U L A * and J. B. H U N T The Department of Chemistry, The Catholic University of America, Washington, D. C. 20017
(First received 24 May 1968; in revised form 16 A ugust 1968) Abstract-When Call2 reacts with D20 solutions, slightly more than two-thirds of the evolved hydrogen is HD, and hydrolysis is the only reaction occurring when such oxidizing agents as Ag ÷, Ce 4÷, Cr2Or = and MnO4- are present. In the reaction of MgH2 with D20 and DCI solutions, H D is again the major product and the proportion of H D increases significantly with increasing acidity. Aqueous Ag ÷ is reduced by MgH~ with most of the hydride hydrogen being released as hydrogen gas without exchange with the solvent. Cerium hydrides in the composition range CeH~.j~-CeH2.96 react with solutions of non-oxidizing acids to evolve hydrogen gas, the major isotopic species of which contains one atom from the hydride and one atom from the solvent. When the cerium hydrides undergo hydrolysis in solutions of low acidity, most of the hydride hydrogen is lost to the solvent by exchange. Oxidation of cerium hydrides by Ag +, NO3- and H~O2 is more rapid than hydrolysis, and most of the evolved hydrogen is derived from the hydride. INTRODUCTION
SruDms of the hydrolysis reactions of the hydrides of active metals have revealed two extremes of behavior. The alkali metal hydrides N a i l [ l ] and LiH[2] react with D20 to yield HD as the principal gaseous product, implying intimate contact between solvent and hydride hydrogen. The other extreme of behavior is the reaction of the metallic hydride, UH3 with c a . 10 M DC1, which yields principally H2 and D2 rather than HD, implying a mechanism not involving direct contact between solvent and hydride hydrogen [3, 4]. At least three distinct types of behavior have been observed in the reactions of metal-hydrides with aqueous solutions containing oxidants other than the hydrogen ion, as exemplified by the behavior of various hydrides toward the powerful oxidizing agent, eerie sulfate. When MgH2 is placed in contact with solutions of Ce(SO4)2, only hydrolysis is observed[5]. The Ce 4+ ion is reduced by UH3 but only the metal of the hydride is oxidized, the hydride hydrogen being released without significant isotopic exchange with the hydrogen of water[4]. In contrast to this latter result, only the hydrogen of palladium hydride[6] is oxidized by Ce 4+. *Based on the M.S. Thesis of Raymond A. Suture, The Catholic University of America, May 1968. H. Beutler, G. Brauer and H. O. Junger, Naturwissenschaften 24, 347 (1936). I. Wender, R. A. Friedel and M. Orchin, J.Am. chem. Soc. 71, 1140 (1948). U. Agarwala, J. B, Hunt and H. Taube, J. chem. Phys. 32, 1567 (1960). J. B. Hunt and H. Taube, lnorg. Chem. 3, 1431 (1964), T. N. D y n o v a , Z. K. Sterlyadkina and N. G. Eliseeva, Russ. J. lnorg. Chem. (English translation) 6, 768 (1961). 6. F. A. Lewis and A. R. Ubbelohde,J. chem. Soc. 1710(1954). 613
I. 2. 3. 4. 5.
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R . A . S U T U L A and J. B. H U N T
Such a wide variation in behavior is not surprising in view of the large differences in properties of the hydrides cited as examples. Unfortunately, it is not certain whether the examples cited represent behavior typical of any particular class of hydrides. The available information is simply too sparse to allow any broad generalizations about the reactions of metal hydrides with aqueous solutions. The present paper represents the beginning of an attempt to obtain systematic information about such reactions. The hydrides chosen for study are cerium hydrides of various H/Ce ratios, calcium hydride and magnesium hydride. Of particular interest are the stoichiometry of reactions in which metal ions compete with the proton as oxidizing agents for the hydrides and the fate of hydride hydrogen in such reactions, i.e. whether hydride hydrogen is oxidized to H ÷, exchanges with the solvent or is transferred quantitatively to the gas phase. EXPERIMENTAL Materials and preparations. Calcium hydride, Call2 and magnesium hydride (labeled 86% MgH~) were used as obtained from Metal Hydrides, Inc. Cerium hydride was prepared by combination of the elements, as follows. Very pure hydrogen gas (H~ or D2 was prepared by the thermal decomposition of uranium hydride, as described elsewhere [4]. The purity of the gas, and the isotopic purity when D2 was used, was determined by analysis in a Consolidated 21-620A mass spectrometer. Cerium metal (99'9%, Lindsay or United Mineral and Chemical Corp.) was stored under mineral oil in an evacuated desiccator to prevent oxidation. The metal used in hydride preparation was rinsed in trichlorethylene to remove the oil, cleaned with 1 M HNO3 to remove the oxide coating, and dried in acetone prior to weighing. The weighed sample was placed in the reaction vessel which was connected to both a high vacuum line and the source of purified hydrogen. The vessel was evacuated to 10-5 mm Hg and hydrogen gas was then admitted. For hydrides of the approximate composition CeH~ a reaction temperature of ca. 400°C was used, whereas hydrides of the approximate composition Celia were prepared at room temperature. The absorption of hydrogen by the metal was followed by means of a gas burette which served also as a manometer. The reaction was allowed to continue until the absorption of hydrogen ceased, the pressure of hydrogen being maintained at ca. 1 atmosphere by adjustment of the height of mercury in the gas burette. The composition of the hydride sample was calculated from the weight of metal used and volume of hydrogen absorbed, assuming hydrogen to be an ideal gas. In some cases hydride~composition was verified from the volume of gas evolved in the acid hydrolysis of the hydride sample, as discussed below. Gaseous DC 1 was prepared by the reaction of benzoyl chloride with D20 and was dissolved in D20 at 0°C to give DC1 solutions[7]. Solutions of NaOD were prepared by the decomposition of sodium peroxide in D20 and were stored in a polyethylene bottle in a desiccator to prevent absorption of COy To prevent the dilution of D~O by crystal water of such salts as AgC104 and Na~EDTA, weighed samples of the salts were wet with D20 and dried in vacuo several times prior to dissolution in D20. Experimental procedure. The reaction vessels for the dissolution reactions were 500 ml roandbottom flasks fitted with a male 24/40 ground glass joint. The vessel was capped with a female 24[40 joint to which was sealed a stopcock for evacuation of the vessel. Samples of CeH2were prepared in situ from cerium metal in a sidearm of the reaction vessel. Some samples of CeD_3 were prepared on a large scale and then weighed out and transferred to the reaction vessel in a CO~-filled dry bag. Samples of CaH~ and MgH~ were weighed in the atmosphere. The procedure for the dissolution reactions was as follows: the reaction vessel containing the hydride sample and a Teflon-coated stirring bar was evacuated to ca. 10 -5 mm Hg. About 25 ml of the appropriate aqueous solution was placed in a bulb sealed to the stopcock of the reaction vessel. The solution was freed of dissolved gases by means of the vacuum line and then quickly forced into the reaction vessel under atmospheric pressure, care being taken to admit no air. The dissolution 7. R. H. Herber (Ed.), Inorganic Isotopic Syntheses, pp. 49-51. Benjamin, New York (1962).
Reactions of some metal hydrides with aqueous oxidizing agents
615
reaction was allowed to proceed at room temperature with vigorous stirring. After the reaction was complete or the evolution of gas ceased, the vessel was attached to a vacuum line, and the evolved gases were transferred by means of an automatic Toeppler pump to the gas measurement apparatus. Analyses. The hydrogen gas evolved in the dissolution reactions was purified by repeated cycling through a liquid nitrogen trap and then transferred to a bulb of known volume fitted with a manometer. The quantity of hydrogen was calculated from its volume (corrected for the volume of the manometer) and pressure, assuming hydrogen to be an ideal gas. The accuracy of the gas measurement apparatus and the efficiency of gas transfer were tested by reacting weighed samples of magnesium ribbon with dilute HCI and transferring the evolved hydrogen to the gas measurement apparatus. The measured and calculated quantities of hydrogen agreed to within 0.2 per cent. The purity of the evolved hydrogen was tested by scanning the gas for N2, 02, COs, HaO and HC] in the mass spectrometer. The isotopic composition of the evolved hydrogen, i.e. the relative amounts of Ha, Da and HD, was determined in a mass spectrometer as well. To eliminate any affect of isotopic fractionation at the leak, peak heights for the various masses wer¢ determined as a function of time and extrapolated back to time of admission of the sample to the mass spectrometer. To eliminate the contribution of Ha + to the mass 3 peak, the ratio D J H D was determined at different pressures and extrapolated to zero pressure [8]. The isotopic ratios so determined were accurate to within 1 per cent, as shown by the analysis of known mixtures of Da and Ha. For example, a sample prepared to contain 36.9 mole per cent D2 and 63.1 mole per cent H._,analyzed as 37.1 mole per cent Da and 62.9 mole per cent H2. Solutions of the various acids were standardized by titrations to a phenolphthalein end-point with sodium hydroxide solutions previously standardized against potassium acid phthalate. The reaction product Ce '~÷ was determined by oxidizing it to Ce 4÷ with sodium bismuthate, removing the excess sodium bismuthate by filtration, and titrating the filtrate to a ferroin endpoint with a standard ferrous solution[9]. The reaction product Fe 2+ was determined by titration against a standard ceric sulfate solution to a ferroin endpoint. RESULTS AND DISCUSSION
In the discussion which follows, the word "hydrogen" will be used to designate all chemical species of atomic number one, without regard for isotopic mass. "Moles of hydrogen evolved" will then mean the total number of moles of hydrogen of all isotopic species. The formulae H2, D~ and H D will be used to designate the particular isotopic species. The term "hydride hydrogen" will refer to hydrogen of any isotopic species contained in, or derived from, the metal hydride; thus the term "hydride hydrogen" used in reference to CeD3 would mean the isotopic species D. The reaction of Call2 with D20 solutions. The hydride Call2 is hydrolyzed rapidly and quantitatively in D~O. Typical tracer results for this reaction are recorded in Table 1. If it is assumed that the hydride was stoichiometric CaH~ and that no isotopic exchange with the solvent occurred, then the hydrolysis reaction may be written as: CaHz + 2DzO ~ Ca(OD)2 + 2(H) + 2(D) i.e. per are the
(1)
2 g-atoms of isotopic species H and two of isotopic species D would be released
mole of the hydride. It is seen from Table 1 that the proportions of H and D the same within experimental error (1-2 per cent), which proves both that hydride was stoichiometric and that the reaction occurs without exchange*
*Since the solvent was present in large excess in all the experiments reported here, exchange with the solvent would have increased the proportion of solvent-type hydrogen in the gas phase. 8. 1. Friedman, Geochim. cosmochim. Acta 4, 89 (1953). 9. A. I. Vogel, A Textbook of Quantitative Inorganic Analysis, p. 325. Wiley, New York (1961). 10. E. D. Hughes, C. K. lngold and C. I. Wilson, J. chem. Soc. 493 (1934).
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R.A. SUTULA and J. B. HUNT Table 1. Tracer results for the reaction of Call2 with D 20 solutions Composition of evolved gas Medium
% H2
% HD
% D2
DzO 8 M DCI 0.1MNaOD 3 M AgNO3
15 13 15 13
70 73 69 73
15 15 16 14
with the solvent, confirming the observation of H u g h e s et a/.[10] I f exchange had occurred, or if the h~ydride had contained free metal, then the ratio H / D would have b e e n less than 1 : 1. It is seen also that the proportions of the various isotopic species in the evolved hydrogen is essentially independent of the acidity of the medium. T h e proportion of H D is smaller than was found for the alkali metal hydride N a i l [ 1], but it is considerably larger than the 50 per cent expected for an equilibrium mixture of the isotopic species in the gas phase. A t t e m p t s to find a metal ion oxidizing agent which would compete with the proton of water in the reaction with Call2 were unsuccessful. N o reduction of silver ion occurred when the hydride reacted with 3 M AgNO3, as implied by the tracer result of T a b l e 1 and substantiated b y the fact that no free silver was found in the reaction residue. Likewise, hydrolysis was the only reaction when the medium was 0.1 M Ce(SO4)2, 0.1 M KsCr2OT, 0.1 M KMnO4 or 3 M KaFe(CN)6. Reactions of MgH2 with D20 solutions. T r a c e r and stoichiometric results for the reactions of magnesium hydride with some D,,O solutions are given in Table 2. As noted earlier, the hydride used in these experiments was not stoichiometric MgH2, but rather a commercially available mixture of MgH2 and Mg metal. T h e stoichiometry of the hydride is established b y the reaction with 0.3 M DC1 (see T a b l e 2) in which the hydride was c o n s u m e d completely. Without regard for isotopic species, the reaction of the impure hydride m a y be written as: 2 M g H x + 4 H + ~ 2 Mg ~+ + ( 2 + x ) H ~
(2)
Table 2. Results of reactions of MgH2 with D20 solutions Medium
Hydrogen evolved, (Moles/g-atom Mg)
DzOa 0.5 M DCIb 0.3 M DC1c 0.5 M AgNO3c 0.2 M Na~EDTA a alncomplete after 1 week. bComplete in 5 min. cComplete in 10 rain. dComplete in 24 hr.
1.25 -1.66 0.68 1.70
Composition of evolved gas %Hz 19.0 7.1 7.8 42.6 15.2
%HD
%D2
47.7 66.9 63.7 54.6 49.8
33.3 26.0 28.5 2.8 35.8
D/H 1.33 1.46 1"52 0-43 1.52
Reactions of some metal hydrides with aqueous oxidizing agents
617
Since 1.66 moles of hydrogen were evolved per gram-atom of magnesium in the acid hydrolysis reaction, it can then be shown that x = 1.32. The hydride was then 66 mole-per cent MgH2-34 mole-per cent Mg. If the reaction of MgH~.a2 with D20 occurred according to Equation (2) and without isotopic exchange with the solvent, then the ratio D / H expected for the evolved hydrogen is 1.51. It is seen from Table 2 that the ratio D / H is very close to 1.51 for the reactions with DC1 solutions and with the E D T A solutions (pH - 7). The lower D / H ratio observed for the incomplete reaction with DeO can be explained by assuming that the MgH~ of the hydride sample reacted more rapidly than did the Mg metal impurity. Consistent with this explanation is the low yield of hydrogen in this experiment. It is seen also from Table 2 that the proportion of H D formed in the hydrolysis of MgH2 increases markedly as the acidity of the medium increases. This trend is reflected most accurately in a comparison of the amounts of H D and He. since part of the D2 arose from the hydrolysis of Mg metal. The ratio HD/H2 increased from 2.5 to 9.4 as the medium was changed from DeO to 0.5 M DCI. The reaction of MgHe with the AgNOa solution gave a residue of pure silver metal. The Mg impurity was consumed almost exclusively by reaction with Ag + rather than by hydrolysis, as indicated by the very low proportion of De evolved. If the hydride were consumed only by reaction with Ag ÷, rather than by hydrolysis, then the expected stoichiometry would be MgHl.a2 + 2Ag + ~ 2Ag + Mg z+ + 0"66 He.
(3)
The amount of hydrogen evolved was very nearly that predicted by Equation (3). However, about 28 per cent of the H originally contained in the hydride was lost to the solvent. Reactions of cerium hydride. In Table 3 are recorded typical results for the reactions of hydride samples of the approximate composition CeH~ with D20 solutions. It is expected that the reaction of a cerium hydride sample with hydrogen ions will obey the stoichiometry: CeH~ + 3H + ~ (3 + x)/2 He + Ce 3+.
(4)
If hydride hydrogen is transferred quantitatively to the gas phase in the reaction of CeHx with D +. then the evolved hydrogen should exhibit the isotopic ratio D/H---3/x. According to this criterion the reaction of Cell2 with 0.3 M DCI occurred with little loss of hydride hydrogen. For stoichiometric Cell2 the evolution of 2.50 moles of hydrogen per gram-atom of Ce is expected, whereas 2.59 moles were evolved in this reaction. Thus we calculate that the hydride was CeH2.1s. For such a hydride a ratio D / H = 1.33 is expected for no exchange. If only 3 per cent of the hydride hydrogen were lost to the solvent by exchange, then the ratio D / H would be 1.45, as observed. In contrast to the reaction with the DCI solution, the results for the much slower reaction of CeHe with DeO can be rationalized only by assuming that considerable exchange between solvent and hydride hydrogen occurred. If the hydride sample had been consumed completely without exchange, then 3.0
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R . A . S U T U L A and J. B. H U N T
g-atoms of solvent hydrogen, i.e. H, would have been contained in the evolved hydrogen, whereas c a . 4.2 g-atoms of H were evolved, although dissolution was incomplete. Oxidation of Cell2 by AgNO3 was found to be much more efficient that hydrolysis of the hydride, as was evident from the velocity of the reaction and the tracer results. It is seen from Table 3 that the quantity of hydrogen evolved Table 3. Stoichiometry and tracer results for reactions of Cell2 with D20 solutions
Medium
0.5 0.1 0.2 0.2
Hydrogen evolved (Moles/g atom Ce)
D20 a M DCI n M AgNO3 c M NaNOa a M AgCIO4 e
2.35 2.59 0.91 0"82 1"08
Composition of evolved gas %H2
%HD
%D2
6"0 7.3 93"2 73"6 72"0
15.2 67"0 6.3 20"9 19"5
78.7 25.7 0.4 5"5 8'4
D/H 6"3 1.45 0.04 0" 19 0"22
"Incomplete after 1 week. bComplete in 5 min. cComplete in 15 rain. dlncomplete after 48 hr. eComplete in 6 hr.
in the reaction with the AgNO3 solution was slightly less than expected only from the release of hydride hydrogen and that the evolved hydrogen was derived almost exclusively from the hydride. The solution which remained at the end of the reaction was slightly acidic, which can be explained only by assuming that some oxidation of hydride hydrogen to hydrogen ion occurred. The latter assumption would account also for the evolution of less than one mole of hydrogen per mole of the hydride. During the purification of the hydrogen evolved in this reaction a small amount of blue solid collected in the liquid nitrogen trap. This was identified as NO from its mass spectrum. The presence of NO implies that some reduction of the NO3- ion by the hydride occurred. The results of the reaction of Cell., with the NaNO3 solution (see Table 3) confirm the reduction of NO3- by the hydride. In this case the only oxidants present were NO3- and the hydrogen ion of water. The major constituent of the evolved hydrogen was H2, a result which can be explained only by assuming that oxidation of the hydride by NO3-, rather than hydrolysis, was the principle reaction. The reaction of the hydride with the A g C I O 4 solution was slower than the reaction with the AgNO3 solution, and a greater proportion of solvent hydrogen was present in the evolved gas. Curiously, the quantity of hydride hydrogen evolved in the reactions with AgNO3 and AgCIO4 were essentially identical, about 0.87 mole in each case. Apparently, no reduction of the C104- ion occurred, since no turbidity due to AgC1 was apparent. In Table 4 are presented results for experiments in which each of three cerium
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Reactions of some metal hydrides with aqueous oxidizing agents Table 4. Stoichiometry and isotopic course of reactions of CeD~ with H~O solutions
Medium
Hydrogen evolved (Moles/g. atom Ce)
Composition of evolved hydrogen %H2
%HD
%D~
H/D
Remarks
CeD2.5o H20 0-3 M HCI 0-15 M H2SO4 0.3 M NaOH 0.3 M AgNO3 0.3 M AgCIO4 0.3 M NaNO3 0-3 M FeCI3 0-3 M Ce(IV)"
2.76 2.75 2.75 2.80 0.98 1.24 0-87 1.56 2.21
77-3 21-0 30-2 77.8 3-2 23.6 19.7 11.9 21.9
15-7 71-8 59-3 15.1 13.3 20-0 37.1 31-6 53-6
7.0 7-2 10.5 7.1 83.6 56-3 43.3 56-5 24-5
5.74 1.32 1.49 5.83 0.11 13.51 0.62 0.38 0-95
6.3 5.3 67.9 79.7 75.3 25-5
1.39 7.36 0.25 0.14 0.19 0.89
14.8 7.5 7.5 53.0 76.5 50.9 28.7 25.2
1-42 4.13 4.16 0.43 0.17 0.49 0.96 0.94
Complete in ca. 48 hr Complete in 13 min Complete in 4 min Complete in 40 hr
Incomplete Complete in 5 min
CED2.38 0.3 M HCI H20 0.2 M HzOz 0.3 M AgNO3 0.3 M AgCIO4 0.15 M Ce(IV) '~
2.68 -0-43 0.81 0-76 2.00
22.7 81.4 8.1 4.6 7.6 19-7
71.0 13.3 24.1 15.7 17.2 54.8
CeD2.3o 0.3 M HC1 H20 0-3 M NaOH 0.3 M FeCI3 0-3 M AgNO3 0.3 M AgC104 0.3 M NaNO.~ 0' 15 M Ce(IV) ~
2.64 2-57 2.67 1.57 0.90 1-11 0.84 1-78
32.1 68.5 68.8 12-9 5-0 16-9 27.0 22-3
53.1 24.0 23.7 34-1 18-5 32.1 44-3 52.5
Incomplete
"Solution in 1 M H2SO4.
deuteride preparations were divided into several samples and then reacted with a variety of solutions in water of normal isotopic abundance. As observed earlier by Kost [ 11], increasing the H/Ce ratio resulted in more rapid hydrolysis, so that the CeD2.5o sample was consumed completely in about two days even in very basic solutions. The hydrolysis reactions, i.e. the reactions with water and with HCI, H2SO4, and N a O H solutions, serve to confirm the stoichiometry of the hydrides used in the reactions of Table 4. Based on Equation (3), the hydrolysis of CeD2.50 is expected to yield 2.75 moles of hydrogen gas, compared to 2-75-2.80 moles found per mole of the hydride. Some exchange of hydride hydrogen with the solvent occurred in the hydrolysis reactions of Table 4, even in the reactions with HCI and H 2 S O 4 solutions. For I 1. M. E. Kost, Russ. J. inorg. Chem. 2, 2689 (1957).
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R.A.
S U T U L A and J. B. H U N T
example, if no exchange had occurred in the hydrolysis reactions of CeD2.50, then an H / D ratio of 1.20 would have been observed. From the observed H / D ratios for the CeD2.50 sample, we calculate that about two-thirds of the hydride hydrogen was lost to the solvent by exchange in the reactions with water and with the N a O H solution, about 11 per cent was lost by exchange in the reaction with the H2SO4 solution, and slightly less than 5 per cent exchanged in the reaction with the HCI solutions. The same trend of increased exchange in the hydrolysis at low acidities is seen for the other hydrides cited in Table 4. In an attempt to ascertain whether the extensive loss of hydride hydrogen to the solvent at low acidities was inherent in the hydrolysis mechanism or whether it arose from bulk exchange between the solid hydride and the solvent, a sample having the approximate composition CeD2.re was placed in contact with a phosphate buffer at pH = 7, and periodically the evolved hydrogen was removed completely for isotopic analysis. The first sample of gas, removed about 1.5 min after mixing and representing 11 per cent of the total hydrogen evolved, exhibited a ratio H / D = 0.56. The ratio H / D increased for succeeding samples, being 3.1 for the second sample, which was collected 3.5 min after mixing and represented 22 per cent of the total gas evolved. For the third sample, collected 12 min after mixing and containing 48 per cent of the total hydrogen, the ratio H / D was 178, and for the final 19 per cent collected after 2 hr, the ratio H / D was 285. The significance of these results will be discussed below. The reactions of the hydride samples of Table 4 with silver salt solutions were qualitatively quite similar to those of the lower hydride of Table 3. In the reactions with AgCIO4 and AgNO3 the bulk of the evolved hydrogen came from the hydride. With AgNO3 some hydride hydrogen was lost to the solvent, presumably by oxidation to hydrogen ion, since less hydrogen was evolved than predicted if the metal were oxidized and the hydrogen from the hydride set free. Some oxidation of hydride hydrogen occurred also when H202 and AgCIO4 were the oxidizing agents. Oxidation of the hydride samples by H202 and FeCI3 also competed effectively with hydrolysis, as indicated by the low H / D ratio. The H / D ratio may be used to set a lower limit on the fraction of the hydride sample which was consumed by reaction with the oxidizing agent, rather than by hydrolysis. For example, the lowest possible H / D ratio would be observed if oxidation of the hydride by Fe 3+ were expressed by Equation (5). Reaction according to Equation (5) would 3Fe 3+ + CeDx ~ 3Fe 2+ + Ce 3+ + x(D)
~5)
result in the evolution of x g-atoms D and no H, per mole of hydride consumed. The competing hydrolysis reaction would result in the evolution of 3 g-atoms H and x g-atoms D per mole of hydride consumed, assuming no exchange. The fraction, f, of the hydride consumed by reaction with Fe z+ would then be f = 1-(x/3)(H/D)
(6)
where H / D is the ratio of these isotopes observed for the evolved gas (Table 4). Using Equation (6) and the data of Table 4 we calculate that at least 68 per cent of the CeD2.50 sample was consumed by oxidation by Fe 3+. Similarly, we calculate
Reactions of some metal hydrides with aqueous oxidizing agents
621
that at least 91 per cent of the CeDe.5 sample was consumed by oxidation by AgNO3 and at least 21 per cent of the CeD2.~0 sample was consumed by oxidation by Ce(IV). In the reaction of CED2.38 with H2OR at least 80 per cent of the hydride sample was consumed by reaction with the oxidizing agent. Only lower limits may be set because both oxidation of hydride hydrogen to hydrogen ion and isotopic exchange would tend to increase the H / D ratio. In Table 5 are tabulated data for some reactions of hydride samples prepared in situ and containing higher D/Ce ratios than the hydrides of Table 4. The results for the hydrolysis reactions, i.e. the reactions with H20 and H2SO4 solutions, are indicative of the precision of the gas measuring apparatus. The D/Ce ratio was calculated from the quantity of D2 consumed in the preparation, as indicated by the gas burette. The evolution of 2.96 and 2.98 moles of hydrogen was expected, as compared to 2.93 and 2-94 moles found, in the reactions with H~O and 0.5 M H.,SO4, respectively. The tracer results for the hydrolysis reactions are quite similar to those for lower cerium hydrides. A small amount, ca. 13 per cent, of hydride hydrogen was lost by exchange with the solvent in the reaction with the H~SO4 solution, and most of the hydride hydrogen was lost in the reaction with H20. In the reactions with Ce(IV) and FeC13 the quantities of the oxidizing agents consumed are quite consistent with the hydride composition and the quantity of the hydrogen evolved. In the reaction with Ce(IV) we calculate from the composition of the hydride and the quantity of Ce 4÷ consumed that 2.63 moles of hydrogen should have been evolved, whereas 2.64 moles were evolved, per gatom Ce. For the reaction with FeCI3 the evolution of 1.87 moles of hydrogen is expected, whereas the evolution of 1,84 moles was observed. It is seen also that oxidation by Ce(IV) did not compete very effectively with the hydrt~lysis reaction, as was found also for the reactions of lower cerium hydrides (see Table 4). G e n e r a l discussion. The most severe limitation on the understanding of reactions occurring at solid-solution interfaces is that the nature of the surface at which the reaction occurs is usually unknown. Although this limitation applies largely to the reactions investigated here, the tracer results do provide some information of a general nature about the solution-solid interface. If in the dissolution of a hydride, hydrogen atoms from the hydride and hydrogen atoms from the reduction of hydrogen ion were released at a common surface, then the molecular hydrogen formed would be an equilibrium mixture of the isotopic species H D , H2 and D2. A 1 : 1 mixture of D and H would contain 50% H D and 25 % each of H2 and D2. A non-equilibrium mixture could arise in two ways: in the case of a saline hydride, direct reaction of hydride ions in the lattice with hydrogen ions in solution, without intermediate formation of hydrogen atoms, would lead to the formation of hydrogen molecules containing one atom from the hydride and one atom from the solvent. A mechanism of this sort apparently operates for some alkali metal hydrides, since the dissolution of N a i l or LiH in D20 leads primarily to the evolution of HD[1,2]. A non-equilibrium mixture of isotopic species might arise also if the oxidation and reduction half-reactions occurred at different sites, as has been suggested to explain the formation of H,, and Dz in preference to H D in the reaction of UH3 with dilute DCi solutions [4].
R. A. S U T U L A and J. B. H U N T
622
Table 5. Stoichiometry and isotopic course of reactions of cerium deuteride samples with H20 solutions
Medium H20 0'5MHzSO4 0"19M Ce(IV) 0.28 M FeCI3
D/Ce 2"93 2"96 2.94 2.81
Hydrogen evolved (moles/gatomCe) 2'93 2'94 2.64 1.84
Reagent consumed, (moles/g. atomCe) --0.677 2.06
%H2 % H D %D2 H/D 79'8 13"4 6"8 26"1 62'0 11"9 38-7 43.8 17.5 12.7 33.4 53.9
6"40 1'33 1.54 0.42
The interpretation of the tracer results may be complicated by isotopic exchange between the hydride, hydrogen atoms or hydrogen molecules and the solvent. Since the solvent is present in great excess in the type of experiment described here, true isotopic equilibrium would favor the formation of molecular hydrogen consisting primarily of the isotope present in the solvent. In the hydrolysis reactions of Call2 (Table 1) and MgH2(Table 2) in D20 solutions, the proportions of H D formed were in excess of the equilibrium value. A reasonable explanation for this observation is that a major path for these reactions involves direct reaction between hydrogen ions of the solvent and hydride ions in the lattice. Likewise, hydrolysis of cerium hydrides in acidic solutions led to the evolution of H D in excess of equilibrium proportions, indicating that a major pathway for these reactions as well involves direct contact between hydride and solvent hydrogen. Because extensive hydrogen exchange with the solvent occurred during the hydrolysis of cerium hydrides in solutions of low acidity, the tracer results for such conditions reveal essentially nothing about reaction mechanisms. It is interesting to note that, whereas the proportion of H D formed in the hydrolysis reactions of Call2 (Table 1) was essentially independent of acidity, the proportions of H D formed in the hydrolysis of MgH2 (Table 2) increased markedly with increasing acidity. Both of these hydrides are saltlike, the only obvious qualitative difference between them being that Call2 has the fluorite structure, whereas MgH2 has the rutile structure[12]. The insensitivity of the tracer results for Call2 to acidity suggests that the water molecule is the oxidizing agent for Call,,, whereas the oxidation of MgH2 may occur both by a path involving water and a path involving the hydrogen ion as the oxidant. There is no obvious explanation for the observation that the Ag + ion is reduced by MgH2 and not by Call2. The tracer results for the reactions of oxidizing agents (other than H20 or H ÷ with cerium hydrides allow the rejection of one possible mechanism for these reactions, namely the possibility that the oxidizing agents are reduced by hydrogen atoms or molecules generated in the hydrolysis reaction. Such a mechanism would result in a much larger proportion of solvent hydrogen in the evolved gas than is observed. We conclude therefore that the oxidizing agents react directly with the hydrides. The inefficiency of the powerful oxidant Ce(IV) in competing with the hydrolysis reaction of cerium hydrides is probably due to 12. T. R. P. Gibb, Prog. inorg. Chem. 3, 315 (1962).
Reactions of some metal hydrides with aqueous oxidizing agents
623
the high acidity in which Ce(IV) must be used (1 M H2SO4). The other oxidizing agents were used in solutions of much lower acidity. The oxidation of some hydride hydrogen to hydrogen ion, which was observed in the reactions of both magnesium and cerium hydrides with Ag +. may involve silver-hydrogen complexes such as AgH ÷, which has been postulated by Webster and Halpern[13] to explain the catalysis by Ag + of the oxidation of H., by a variety of oxidizing agents and also to explain the oxidation of H2 by Ag +. The observation which is most difficult to explain is the extensive isotopic exchange between the hydride and solvent hydrogen which occurred in the hydrolysis of cerium hydrides at low acidities. It is likely that exchange was extensive under such conditions because dissolution was slow and the hydride was in contact with the solvent for long times, and it was shown that the extent of exchange increased for later hydrogen samples when several samples were taken during a given hydrolysis reaction. At least two possible explanations may be offered for this increase in the extent of exchange with time of contact between the hydride and the solvent. The change in isotopic composition may reflect a change in reaction mechanism with time, perhaps due to the development of an insoluble oxide or hydroxide layer on the hydride surface.* An alternative explanation is that the bulk hydride undergoes isotopic exchange with the solvent. If we assume that water molecules have direct access to the hydride surface, as must be assumed to account for the evolution of H D in greater-thanequilibrium proportions, then exchange between the solvent and the hydride may be understood. A water molecule colliding with the hydride surface could use one of its lone pairs of electrons to pick up a proton from the surface of the hydride lattice while depositing one of its own protons to take the place of the one taken from the hydride. The exchange process would be completed by diffusion of the hydrogen atom from the surface to the interior of the hydride particle [14]. Acknowledgement-The authors acknowledge gratefully the support of this research by the Office of Naval Research Contract Nonr-2249(11 ). *The authors are grateful to a referee for the suggestion of this explanation. 13. A. H. Webster andJ. Halpern, J. phys. Chem. 61, 1239 (1957); Trans. Faraday Soc. $3, 51 (1957). 14. The rate of self-diffusion of hydrogen in cerium hydrides has apparently not been measured. However, proton self-diffusion occurs at moderate temperatures in the similar lanthanum hydrides of composition LaH>~. (D. S. Schreiber and R. M. Cotts, Phys. Rev. 131, 1118 (1963).
DEUTERIUM TRACER EXPERIMENTS ON THE REACTIONS OF SOME METAL HYDRIDES WITH AQUEOUS OXIDIZING AGENTS R. A. S U T U L A * and J. B. H U N T The Department of Chemistry, The Catholic University of America, Washington, D. C. 20017
(First received 24 May 1968; in revised form 16 A ugust 1968) Abstract-When Call2 reacts with D20 solutions, slightly more than two-thirds of the evolved hydrogen is HD, and hydrolysis is the only reaction occurring when such oxidizing agents as Ag ÷, Ce 4÷, Cr2Or = and MnO4- are present. In the reaction of MgH2 with D20 and DCI solutions, H D is again the major product and the proportion of H D increases significantly with increasing acidity. Aqueous Ag ÷ is reduced by MgH~ with most of the hydride hydrogen being released as hydrogen gas without exchange with the solvent. Cerium hydrides in the composition range CeH~.j~-CeH2.96 react with solutions of non-oxidizing acids to evolve hydrogen gas, the major isotopic species of which contains one atom from the hydride and one atom from the solvent. When the cerium hydrides undergo hydrolysis in solutions of low acidity, most of the hydride hydrogen is lost to the solvent by exchange. Oxidation of cerium hydrides by Ag +, NO3- and H~O2 is more rapid than hydrolysis, and most of the evolved hydrogen is derived from the hydride. INTRODUCTION
SruDms of the hydrolysis reactions of the hydrides of active metals have revealed two extremes of behavior. The alkali metal hydrides N a i l [ l ] and LiH[2] react with D20 to yield HD as the principal gaseous product, implying intimate contact between solvent and hydride hydrogen. The other extreme of behavior is the reaction of the metallic hydride, UH3 with c a . 10 M DC1, which yields principally H2 and D2 rather than HD, implying a mechanism not involving direct contact between solvent and hydride hydrogen [3, 4]. At least three distinct types of behavior have been observed in the reactions of metal-hydrides with aqueous solutions containing oxidants other than the hydrogen ion, as exemplified by the behavior of various hydrides toward the powerful oxidizing agent, eerie sulfate. When MgH2 is placed in contact with solutions of Ce(SO4)2, only hydrolysis is observed[5]. The Ce 4+ ion is reduced by UH3 but only the metal of the hydride is oxidized, the hydride hydrogen being released without significant isotopic exchange with the hydrogen of water[4]. In contrast to this latter result, only the hydrogen of palladium hydride[6] is oxidized by Ce 4+. *Based on the M.S. Thesis of Raymond A. Suture, The Catholic University of America, May 1968. H. Beutler, G. Brauer and H. O. Junger, Naturwissenschaften 24, 347 (1936). I. Wender, R. A. Friedel and M. Orchin, J.Am. chem. Soc. 71, 1140 (1948). U. Agarwala, J. B, Hunt and H. Taube, J. chem. Phys. 32, 1567 (1960). J. B. Hunt and H. Taube, lnorg. Chem. 3, 1431 (1964), T. N. D y n o v a , Z. K. Sterlyadkina and N. G. Eliseeva, Russ. J. lnorg. Chem. (English translation) 6, 768 (1961). 6. F. A. Lewis and A. R. Ubbelohde,J. chem. Soc. 1710(1954). 613
I. 2. 3. 4. 5.
614
R . A . S U T U L A and J. B. H U N T
Such a wide variation in behavior is not surprising in view of the large differences in properties of the hydrides cited as examples. Unfortunately, it is not certain whether the examples cited represent behavior typical of any particular class of hydrides. The available information is simply too sparse to allow any broad generalizations about the reactions of metal hydrides with aqueous solutions. The present paper represents the beginning of an attempt to obtain systematic information about such reactions. The hydrides chosen for study are cerium hydrides of various H/Ce ratios, calcium hydride and magnesium hydride. Of particular interest are the stoichiometry of reactions in which metal ions compete with the proton as oxidizing agents for the hydrides and the fate of hydride hydrogen in such reactions, i.e. whether hydride hydrogen is oxidized to H ÷, exchanges with the solvent or is transferred quantitatively to the gas phase. EXPERIMENTAL Materials and preparations. Calcium hydride, Call2 and magnesium hydride (labeled 86% MgH~) were used as obtained from Metal Hydrides, Inc. Cerium hydride was prepared by combination of the elements, as follows. Very pure hydrogen gas (H~ or D2 was prepared by the thermal decomposition of uranium hydride, as described elsewhere [4]. The purity of the gas, and the isotopic purity when D2 was used, was determined by analysis in a Consolidated 21-620A mass spectrometer. Cerium metal (99'9%, Lindsay or United Mineral and Chemical Corp.) was stored under mineral oil in an evacuated desiccator to prevent oxidation. The metal used in hydride preparation was rinsed in trichlorethylene to remove the oil, cleaned with 1 M HNO3 to remove the oxide coating, and dried in acetone prior to weighing. The weighed sample was placed in the reaction vessel which was connected to both a high vacuum line and the source of purified hydrogen. The vessel was evacuated to 10-5 mm Hg and hydrogen gas was then admitted. For hydrides of the approximate composition CeH~ a reaction temperature of ca. 400°C was used, whereas hydrides of the approximate composition Celia were prepared at room temperature. The absorption of hydrogen by the metal was followed by means of a gas burette which served also as a manometer. The reaction was allowed to continue until the absorption of hydrogen ceased, the pressure of hydrogen being maintained at ca. 1 atmosphere by adjustment of the height of mercury in the gas burette. The composition of the hydride sample was calculated from the weight of metal used and volume of hydrogen absorbed, assuming hydrogen to be an ideal gas. In some cases hydride~composition was verified from the volume of gas evolved in the acid hydrolysis of the hydride sample, as discussed below. Gaseous DC 1 was prepared by the reaction of benzoyl chloride with D20 and was dissolved in D20 at 0°C to give DC1 solutions[7]. Solutions of NaOD were prepared by the decomposition of sodium peroxide in D20 and were stored in a polyethylene bottle in a desiccator to prevent absorption of COy To prevent the dilution of D~O by crystal water of such salts as AgC104 and Na~EDTA, weighed samples of the salts were wet with D20 and dried in vacuo several times prior to dissolution in D20. Experimental procedure. The reaction vessels for the dissolution reactions were 500 ml roandbottom flasks fitted with a male 24/40 ground glass joint. The vessel was capped with a female 24[40 joint to which was sealed a stopcock for evacuation of the vessel. Samples of CeH2were prepared in situ from cerium metal in a sidearm of the reaction vessel. Some samples of CeD_3 were prepared on a large scale and then weighed out and transferred to the reaction vessel in a CO~-filled dry bag. Samples of CaH~ and MgH~ were weighed in the atmosphere. The procedure for the dissolution reactions was as follows: the reaction vessel containing the hydride sample and a Teflon-coated stirring bar was evacuated to ca. 10 -5 mm Hg. About 25 ml of the appropriate aqueous solution was placed in a bulb sealed to the stopcock of the reaction vessel. The solution was freed of dissolved gases by means of the vacuum line and then quickly forced into the reaction vessel under atmospheric pressure, care being taken to admit no air. The dissolution 7. R. H. Herber (Ed.), Inorganic Isotopic Syntheses, pp. 49-51. Benjamin, New York (1962).
Reactions of some metal hydrides with aqueous oxidizing agents
615
reaction was allowed to proceed at room temperature with vigorous stirring. After the reaction was complete or the evolution of gas ceased, the vessel was attached to a vacuum line, and the evolved gases were transferred by means of an automatic Toeppler pump to the gas measurement apparatus. Analyses. The hydrogen gas evolved in the dissolution reactions was purified by repeated cycling through a liquid nitrogen trap and then transferred to a bulb of known volume fitted with a manometer. The quantity of hydrogen was calculated from its volume (corrected for the volume of the manometer) and pressure, assuming hydrogen to be an ideal gas. The accuracy of the gas measurement apparatus and the efficiency of gas transfer were tested by reacting weighed samples of magnesium ribbon with dilute HCI and transferring the evolved hydrogen to the gas measurement apparatus. The measured and calculated quantities of hydrogen agreed to within 0.2 per cent. The purity of the evolved hydrogen was tested by scanning the gas for N2, 02, COs, HaO and HC] in the mass spectrometer. The isotopic composition of the evolved hydrogen, i.e. the relative amounts of Ha, Da and HD, was determined in a mass spectrometer as well. To eliminate any affect of isotopic fractionation at the leak, peak heights for the various masses wer¢ determined as a function of time and extrapolated back to time of admission of the sample to the mass spectrometer. To eliminate the contribution of Ha + to the mass 3 peak, the ratio D J H D was determined at different pressures and extrapolated to zero pressure [8]. The isotopic ratios so determined were accurate to within 1 per cent, as shown by the analysis of known mixtures of Da and Ha. For example, a sample prepared to contain 36.9 mole per cent D2 and 63.1 mole per cent H._,analyzed as 37.1 mole per cent Da and 62.9 mole per cent H2. Solutions of the various acids were standardized by titrations to a phenolphthalein end-point with sodium hydroxide solutions previously standardized against potassium acid phthalate. The reaction product Ce '~÷ was determined by oxidizing it to Ce 4÷ with sodium bismuthate, removing the excess sodium bismuthate by filtration, and titrating the filtrate to a ferroin endpoint with a standard ferrous solution[9]. The reaction product Fe 2+ was determined by titration against a standard ceric sulfate solution to a ferroin endpoint. RESULTS AND DISCUSSION
In the discussion which follows, the word "hydrogen" will be used to designate all chemical species of atomic number one, without regard for isotopic mass. "Moles of hydrogen evolved" will then mean the total number of moles of hydrogen of all isotopic species. The formulae H2, D~ and H D will be used to designate the particular isotopic species. The term "hydride hydrogen" will refer to hydrogen of any isotopic species contained in, or derived from, the metal hydride; thus the term "hydride hydrogen" used in reference to CeD3 would mean the isotopic species D. The reaction of Call2 with D20 solutions. The hydride Call2 is hydrolyzed rapidly and quantitatively in D~O. Typical tracer results for this reaction are recorded in Table 1. If it is assumed that the hydride was stoichiometric CaH~ and that no isotopic exchange with the solvent occurred, then the hydrolysis reaction may be written as: CaHz + 2DzO ~ Ca(OD)2 + 2(H) + 2(D) i.e. per are the
(1)
2 g-atoms of isotopic species H and two of isotopic species D would be released
mole of the hydride. It is seen from Table 1 that the proportions of H and D the same within experimental error (1-2 per cent), which proves both that hydride was stoichiometric and that the reaction occurs without exchange*
*Since the solvent was present in large excess in all the experiments reported here, exchange with the solvent would have increased the proportion of solvent-type hydrogen in the gas phase. 8. 1. Friedman, Geochim. cosmochim. Acta 4, 89 (1953). 9. A. I. Vogel, A Textbook of Quantitative Inorganic Analysis, p. 325. Wiley, New York (1961). 10. E. D. Hughes, C. K. lngold and C. I. Wilson, J. chem. Soc. 493 (1934).
616
R.A. SUTULA and J. B. HUNT Table 1. Tracer results for the reaction of Call2 with D 20 solutions Composition of evolved gas Medium
% H2
% HD
% D2
DzO 8 M DCI 0.1MNaOD 3 M AgNO3
15 13 15 13
70 73 69 73
15 15 16 14
with the solvent, confirming the observation of H u g h e s et a/.[10] I f exchange had occurred, or if the h~ydride had contained free metal, then the ratio H / D would have b e e n less than 1 : 1. It is seen also that the proportions of the various isotopic species in the evolved hydrogen is essentially independent of the acidity of the medium. T h e proportion of H D is smaller than was found for the alkali metal hydride N a i l [ 1], but it is considerably larger than the 50 per cent expected for an equilibrium mixture of the isotopic species in the gas phase. A t t e m p t s to find a metal ion oxidizing agent which would compete with the proton of water in the reaction with Call2 were unsuccessful. N o reduction of silver ion occurred when the hydride reacted with 3 M AgNO3, as implied by the tracer result of T a b l e 1 and substantiated b y the fact that no free silver was found in the reaction residue. Likewise, hydrolysis was the only reaction when the medium was 0.1 M Ce(SO4)2, 0.1 M KsCr2OT, 0.1 M KMnO4 or 3 M KaFe(CN)6. Reactions of MgH2 with D20 solutions. T r a c e r and stoichiometric results for the reactions of magnesium hydride with some D,,O solutions are given in Table 2. As noted earlier, the hydride used in these experiments was not stoichiometric MgH2, but rather a commercially available mixture of MgH2 and Mg metal. T h e stoichiometry of the hydride is established b y the reaction with 0.3 M DC1 (see T a b l e 2) in which the hydride was c o n s u m e d completely. Without regard for isotopic species, the reaction of the impure hydride m a y be written as: 2 M g H x + 4 H + ~ 2 Mg ~+ + ( 2 + x ) H ~
(2)
Table 2. Results of reactions of MgH2 with D20 solutions Medium
Hydrogen evolved, (Moles/g-atom Mg)
DzOa 0.5 M DCIb 0.3 M DC1c 0.5 M AgNO3c 0.2 M Na~EDTA a alncomplete after 1 week. bComplete in 5 min. cComplete in 10 rain. dComplete in 24 hr.
1.25 -1.66 0.68 1.70
Composition of evolved gas %Hz 19.0 7.1 7.8 42.6 15.2
%HD
%D2
47.7 66.9 63.7 54.6 49.8
33.3 26.0 28.5 2.8 35.8
D/H 1.33 1.46 1"52 0-43 1.52
Reactions of some metal hydrides with aqueous oxidizing agents
617
Since 1.66 moles of hydrogen were evolved per gram-atom of magnesium in the acid hydrolysis reaction, it can then be shown that x = 1.32. The hydride was then 66 mole-per cent MgH2-34 mole-per cent Mg. If the reaction of MgH~.a2 with D20 occurred according to Equation (2) and without isotopic exchange with the solvent, then the ratio D / H expected for the evolved hydrogen is 1.51. It is seen from Table 2 that the ratio D / H is very close to 1.51 for the reactions with DC1 solutions and with the E D T A solutions (pH - 7). The lower D / H ratio observed for the incomplete reaction with DeO can be explained by assuming that the MgH~ of the hydride sample reacted more rapidly than did the Mg metal impurity. Consistent with this explanation is the low yield of hydrogen in this experiment. It is seen also from Table 2 that the proportion of H D formed in the hydrolysis of MgH2 increases markedly as the acidity of the medium increases. This trend is reflected most accurately in a comparison of the amounts of H D and He. since part of the D2 arose from the hydrolysis of Mg metal. The ratio HD/H2 increased from 2.5 to 9.4 as the medium was changed from DeO to 0.5 M DCI. The reaction of MgHe with the AgNOa solution gave a residue of pure silver metal. The Mg impurity was consumed almost exclusively by reaction with Ag + rather than by hydrolysis, as indicated by the very low proportion of De evolved. If the hydride were consumed only by reaction with Ag ÷, rather than by hydrolysis, then the expected stoichiometry would be MgHl.a2 + 2Ag + ~ 2Ag + Mg z+ + 0"66 He.
(3)
The amount of hydrogen evolved was very nearly that predicted by Equation (3). However, about 28 per cent of the H originally contained in the hydride was lost to the solvent. Reactions of cerium hydride. In Table 3 are recorded typical results for the reactions of hydride samples of the approximate composition CeH~ with D20 solutions. It is expected that the reaction of a cerium hydride sample with hydrogen ions will obey the stoichiometry: CeH~ + 3H + ~ (3 + x)/2 He + Ce 3+.
(4)
If hydride hydrogen is transferred quantitatively to the gas phase in the reaction of CeHx with D +. then the evolved hydrogen should exhibit the isotopic ratio D/H---3/x. According to this criterion the reaction of Cell2 with 0.3 M DCI occurred with little loss of hydride hydrogen. For stoichiometric Cell2 the evolution of 2.50 moles of hydrogen per gram-atom of Ce is expected, whereas 2.59 moles were evolved in this reaction. Thus we calculate that the hydride was CeH2.1s. For such a hydride a ratio D / H = 1.33 is expected for no exchange. If only 3 per cent of the hydride hydrogen were lost to the solvent by exchange, then the ratio D / H would be 1.45, as observed. In contrast to the reaction with the DCI solution, the results for the much slower reaction of CeHe with DeO can be rationalized only by assuming that considerable exchange between solvent and hydride hydrogen occurred. If the hydride sample had been consumed completely without exchange, then 3.0
618
R . A . S U T U L A and J. B. H U N T
g-atoms of solvent hydrogen, i.e. H, would have been contained in the evolved hydrogen, whereas c a . 4.2 g-atoms of H were evolved, although dissolution was incomplete. Oxidation of Cell2 by AgNO3 was found to be much more efficient that hydrolysis of the hydride, as was evident from the velocity of the reaction and the tracer results. It is seen from Table 3 that the quantity of hydrogen evolved Table 3. Stoichiometry and tracer results for reactions of Cell2 with D20 solutions
Medium
0.5 0.1 0.2 0.2
Hydrogen evolved (Moles/g atom Ce)
D20 a M DCI n M AgNO3 c M NaNOa a M AgCIO4 e
2.35 2.59 0.91 0"82 1"08
Composition of evolved gas %H2
%HD
%D2
6"0 7.3 93"2 73"6 72"0
15.2 67"0 6.3 20"9 19"5
78.7 25.7 0.4 5"5 8'4
D/H 6"3 1.45 0.04 0" 19 0"22
"Incomplete after 1 week. bComplete in 5 min. cComplete in 15 rain. dlncomplete after 48 hr. eComplete in 6 hr.
in the reaction with the AgNO3 solution was slightly less than expected only from the release of hydride hydrogen and that the evolved hydrogen was derived almost exclusively from the hydride. The solution which remained at the end of the reaction was slightly acidic, which can be explained only by assuming that some oxidation of hydride hydrogen to hydrogen ion occurred. The latter assumption would account also for the evolution of less than one mole of hydrogen per mole of the hydride. During the purification of the hydrogen evolved in this reaction a small amount of blue solid collected in the liquid nitrogen trap. This was identified as NO from its mass spectrum. The presence of NO implies that some reduction of the NO3- ion by the hydride occurred. The results of the reaction of Cell., with the NaNO3 solution (see Table 3) confirm the reduction of NO3- by the hydride. In this case the only oxidants present were NO3- and the hydrogen ion of water. The major constituent of the evolved hydrogen was H2, a result which can be explained only by assuming that oxidation of the hydride by NO3-, rather than hydrolysis, was the principle reaction. The reaction of the hydride with the A g C I O 4 solution was slower than the reaction with the AgNO3 solution, and a greater proportion of solvent hydrogen was present in the evolved gas. Curiously, the quantity of hydride hydrogen evolved in the reactions with AgNO3 and AgCIO4 were essentially identical, about 0.87 mole in each case. Apparently, no reduction of the C104- ion occurred, since no turbidity due to AgC1 was apparent. In Table 4 are presented results for experiments in which each of three cerium
619
Reactions of some metal hydrides with aqueous oxidizing agents Table 4. Stoichiometry and isotopic course of reactions of CeD~ with H~O solutions
Medium
Hydrogen evolved (Moles/g. atom Ce)
Composition of evolved hydrogen %H2
%HD
%D~
H/D
Remarks
CeD2.5o H20 0-3 M HCI 0-15 M H2SO4 0.3 M NaOH 0.3 M AgNO3 0.3 M AgCIO4 0.3 M NaNO3 0-3 M FeCI3 0-3 M Ce(IV)"
2.76 2.75 2.75 2.80 0.98 1.24 0-87 1.56 2.21
77-3 21-0 30-2 77.8 3-2 23.6 19.7 11.9 21.9
15-7 71-8 59-3 15.1 13.3 20-0 37.1 31-6 53-6
7.0 7-2 10.5 7.1 83.6 56-3 43.3 56-5 24-5
5.74 1.32 1.49 5.83 0.11 13.51 0.62 0.38 0-95
6.3 5.3 67.9 79.7 75.3 25-5
1.39 7.36 0.25 0.14 0.19 0.89
14.8 7.5 7.5 53.0 76.5 50.9 28.7 25.2
1-42 4.13 4.16 0.43 0.17 0.49 0.96 0.94
Complete in ca. 48 hr Complete in 13 min Complete in 4 min Complete in 40 hr
Incomplete Complete in 5 min
CED2.38 0.3 M HCI H20 0.2 M HzOz 0.3 M AgNO3 0.3 M AgCIO4 0.15 M Ce(IV) '~
2.68 -0-43 0.81 0-76 2.00
22.7 81.4 8.1 4.6 7.6 19-7
71.0 13.3 24.1 15.7 17.2 54.8
CeD2.3o 0.3 M HC1 H20 0-3 M NaOH 0.3 M FeCI3 0-3 M AgNO3 0.3 M AgC104 0.3 M NaNO.~ 0' 15 M Ce(IV) ~
2.64 2-57 2.67 1.57 0.90 1-11 0.84 1-78
32.1 68.5 68.8 12-9 5-0 16-9 27.0 22-3
53.1 24.0 23.7 34-1 18-5 32.1 44-3 52.5
Incomplete
"Solution in 1 M H2SO4.
deuteride preparations were divided into several samples and then reacted with a variety of solutions in water of normal isotopic abundance. As observed earlier by Kost [ 11], increasing the H/Ce ratio resulted in more rapid hydrolysis, so that the CeD2.5o sample was consumed completely in about two days even in very basic solutions. The hydrolysis reactions, i.e. the reactions with water and with HCI, H2SO4, and N a O H solutions, serve to confirm the stoichiometry of the hydrides used in the reactions of Table 4. Based on Equation (3), the hydrolysis of CeD2.50 is expected to yield 2.75 moles of hydrogen gas, compared to 2-75-2.80 moles found per mole of the hydride. Some exchange of hydride hydrogen with the solvent occurred in the hydrolysis reactions of Table 4, even in the reactions with HCI and H 2 S O 4 solutions. For I 1. M. E. Kost, Russ. J. inorg. Chem. 2, 2689 (1957).
620
R.A.
S U T U L A and J. B. H U N T
example, if no exchange had occurred in the hydrolysis reactions of CeD2.50, then an H / D ratio of 1.20 would have been observed. From the observed H / D ratios for the CeD2.50 sample, we calculate that about two-thirds of the hydride hydrogen was lost to the solvent by exchange in the reactions with water and with the N a O H solution, about 11 per cent was lost by exchange in the reaction with the H2SO4 solution, and slightly less than 5 per cent exchanged in the reaction with the HCI solutions. The same trend of increased exchange in the hydrolysis at low acidities is seen for the other hydrides cited in Table 4. In an attempt to ascertain whether the extensive loss of hydride hydrogen to the solvent at low acidities was inherent in the hydrolysis mechanism or whether it arose from bulk exchange between the solid hydride and the solvent, a sample having the approximate composition CeD2.re was placed in contact with a phosphate buffer at pH = 7, and periodically the evolved hydrogen was removed completely for isotopic analysis. The first sample of gas, removed about 1.5 min after mixing and representing 11 per cent of the total hydrogen evolved, exhibited a ratio H / D = 0.56. The ratio H / D increased for succeeding samples, being 3.1 for the second sample, which was collected 3.5 min after mixing and represented 22 per cent of the total gas evolved. For the third sample, collected 12 min after mixing and containing 48 per cent of the total hydrogen, the ratio H / D was 178, and for the final 19 per cent collected after 2 hr, the ratio H / D was 285. The significance of these results will be discussed below. The reactions of the hydride samples of Table 4 with silver salt solutions were qualitatively quite similar to those of the lower hydride of Table 3. In the reactions with AgCIO4 and AgNO3 the bulk of the evolved hydrogen came from the hydride. With AgNO3 some hydride hydrogen was lost to the solvent, presumably by oxidation to hydrogen ion, since less hydrogen was evolved than predicted if the metal were oxidized and the hydrogen from the hydride set free. Some oxidation of hydride hydrogen occurred also when H202 and AgCIO4 were the oxidizing agents. Oxidation of the hydride samples by H202 and FeCI3 also competed effectively with hydrolysis, as indicated by the low H / D ratio. The H / D ratio may be used to set a lower limit on the fraction of the hydride sample which was consumed by reaction with the oxidizing agent, rather than by hydrolysis. For example, the lowest possible H / D ratio would be observed if oxidation of the hydride by Fe 3+ were expressed by Equation (5). Reaction according to Equation (5) would 3Fe 3+ + CeDx ~ 3Fe 2+ + Ce 3+ + x(D)
~5)
result in the evolution of x g-atoms D and no H, per mole of hydride consumed. The competing hydrolysis reaction would result in the evolution of 3 g-atoms H and x g-atoms D per mole of hydride consumed, assuming no exchange. The fraction, f, of the hydride consumed by reaction with Fe z+ would then be f = 1-(x/3)(H/D)
(6)
where H / D is the ratio of these isotopes observed for the evolved gas (Table 4). Using Equation (6) and the data of Table 4 we calculate that at least 68 per cent of the CeD2.50 sample was consumed by oxidation by Fe 3+. Similarly, we calculate
Reactions of some metal hydrides with aqueous oxidizing agents
621
that at least 91 per cent of the CeDe.5 sample was consumed by oxidation by AgNO3 and at least 21 per cent of the CeD2.~0 sample was consumed by oxidation by Ce(IV). In the reaction of CED2.38 with H2OR at least 80 per cent of the hydride sample was consumed by reaction with the oxidizing agent. Only lower limits may be set because both oxidation of hydride hydrogen to hydrogen ion and isotopic exchange would tend to increase the H / D ratio. In Table 5 are tabulated data for some reactions of hydride samples prepared in situ and containing higher D/Ce ratios than the hydrides of Table 4. The results for the hydrolysis reactions, i.e. the reactions with H20 and H2SO4 solutions, are indicative of the precision of the gas measuring apparatus. The D/Ce ratio was calculated from the quantity of D2 consumed in the preparation, as indicated by the gas burette. The evolution of 2.96 and 2.98 moles of hydrogen was expected, as compared to 2.93 and 2-94 moles found, in the reactions with H~O and 0.5 M H.,SO4, respectively. The tracer results for the hydrolysis reactions are quite similar to those for lower cerium hydrides. A small amount, ca. 13 per cent, of hydride hydrogen was lost by exchange with the solvent in the reaction with the H~SO4 solution, and most of the hydride hydrogen was lost in the reaction with H20. In the reactions with Ce(IV) and FeC13 the quantities of the oxidizing agents consumed are quite consistent with the hydride composition and the quantity of the hydrogen evolved. In the reaction with Ce(IV) we calculate from the composition of the hydride and the quantity of Ce 4÷ consumed that 2.63 moles of hydrogen should have been evolved, whereas 2.64 moles were evolved, per gatom Ce. For the reaction with FeCI3 the evolution of 1.87 moles of hydrogen is expected, whereas the evolution of 1,84 moles was observed. It is seen also that oxidation by Ce(IV) did not compete very effectively with the hydrt~lysis reaction, as was found also for the reactions of lower cerium hydrides (see Table 4). G e n e r a l discussion. The most severe limitation on the understanding of reactions occurring at solid-solution interfaces is that the nature of the surface at which the reaction occurs is usually unknown. Although this limitation applies largely to the reactions investigated here, the tracer results do provide some information of a general nature about the solution-solid interface. If in the dissolution of a hydride, hydrogen atoms from the hydride and hydrogen atoms from the reduction of hydrogen ion were released at a common surface, then the molecular hydrogen formed would be an equilibrium mixture of the isotopic species H D , H2 and D2. A 1 : 1 mixture of D and H would contain 50% H D and 25 % each of H2 and D2. A non-equilibrium mixture could arise in two ways: in the case of a saline hydride, direct reaction of hydride ions in the lattice with hydrogen ions in solution, without intermediate formation of hydrogen atoms, would lead to the formation of hydrogen molecules containing one atom from the hydride and one atom from the solvent. A mechanism of this sort apparently operates for some alkali metal hydrides, since the dissolution of N a i l or LiH in D20 leads primarily to the evolution of HD[1,2]. A non-equilibrium mixture of isotopic species might arise also if the oxidation and reduction half-reactions occurred at different sites, as has been suggested to explain the formation of H,, and Dz in preference to H D in the reaction of UH3 with dilute DCi solutions [4].
R. A. S U T U L A and J. B. H U N T
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Table 5. Stoichiometry and isotopic course of reactions of cerium deuteride samples with H20 solutions
Medium H20 0'5MHzSO4 0"19M Ce(IV) 0.28 M FeCI3
D/Ce 2"93 2"96 2.94 2.81
Hydrogen evolved (moles/gatomCe) 2'93 2'94 2.64 1.84
Reagent consumed, (moles/g. atomCe) --0.677 2.06
%H2 % H D %D2 H/D 79'8 13"4 6"8 26"1 62'0 11"9 38-7 43.8 17.5 12.7 33.4 53.9
6"40 1'33 1.54 0.42
The interpretation of the tracer results may be complicated by isotopic exchange between the hydride, hydrogen atoms or hydrogen molecules and the solvent. Since the solvent is present in great excess in the type of experiment described here, true isotopic equilibrium would favor the formation of molecular hydrogen consisting primarily of the isotope present in the solvent. In the hydrolysis reactions of Call2 (Table 1) and MgH2(Table 2) in D20 solutions, the proportions of H D formed were in excess of the equilibrium value. A reasonable explanation for this observation is that a major path for these reactions involves direct reaction between hydrogen ions of the solvent and hydride ions in the lattice. Likewise, hydrolysis of cerium hydrides in acidic solutions led to the evolution of H D in excess of equilibrium proportions, indicating that a major pathway for these reactions as well involves direct contact between hydride and solvent hydrogen. Because extensive hydrogen exchange with the solvent occurred during the hydrolysis of cerium hydrides in solutions of low acidity, the tracer results for such conditions reveal essentially nothing about reaction mechanisms. It is interesting to note that, whereas the proportion of H D formed in the hydrolysis reactions of Call2 (Table 1) was essentially independent of acidity, the proportions of H D formed in the hydrolysis of MgH2 (Table 2) increased markedly with increasing acidity. Both of these hydrides are saltlike, the only obvious qualitative difference between them being that Call2 has the fluorite structure, whereas MgH2 has the rutile structure[12]. The insensitivity of the tracer results for Call2 to acidity suggests that the water molecule is the oxidizing agent for Call,,, whereas the oxidation of MgH2 may occur both by a path involving water and a path involving the hydrogen ion as the oxidant. There is no obvious explanation for the observation that the Ag + ion is reduced by MgH2 and not by Call2. The tracer results for the reactions of oxidizing agents (other than H20 or H ÷ with cerium hydrides allow the rejection of one possible mechanism for these reactions, namely the possibility that the oxidizing agents are reduced by hydrogen atoms or molecules generated in the hydrolysis reaction. Such a mechanism would result in a much larger proportion of solvent hydrogen in the evolved gas than is observed. We conclude therefore that the oxidizing agents react directly with the hydrides. The inefficiency of the powerful oxidant Ce(IV) in competing with the hydrolysis reaction of cerium hydrides is probably due to 12. T. R. P. Gibb, Prog. inorg. Chem. 3, 315 (1962).
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the high acidity in which Ce(IV) must be used (1 M H2SO4). The other oxidizing agents were used in solutions of much lower acidity. The oxidation of some hydride hydrogen to hydrogen ion, which was observed in the reactions of both magnesium and cerium hydrides with Ag +. may involve silver-hydrogen complexes such as AgH ÷, which has been postulated by Webster and Halpern[13] to explain the catalysis by Ag + of the oxidation of H., by a variety of oxidizing agents and also to explain the oxidation of H2 by Ag +. The observation which is most difficult to explain is the extensive isotopic exchange between the hydride and solvent hydrogen which occurred in the hydrolysis of cerium hydrides at low acidities. It is likely that exchange was extensive under such conditions because dissolution was slow and the hydride was in contact with the solvent for long times, and it was shown that the extent of exchange increased for later hydrogen samples when several samples were taken during a given hydrolysis reaction. At least two possible explanations may be offered for this increase in the extent of exchange with time of contact between the hydride and the solvent. The change in isotopic composition may reflect a change in reaction mechanism with time, perhaps due to the development of an insoluble oxide or hydroxide layer on the hydride surface.* An alternative explanation is that the bulk hydride undergoes isotopic exchange with the solvent. If we assume that water molecules have direct access to the hydride surface, as must be assumed to account for the evolution of H D in greater-thanequilibrium proportions, then exchange between the solvent and the hydride may be understood. A water molecule colliding with the hydride surface could use one of its lone pairs of electrons to pick up a proton from the surface of the hydride lattice while depositing one of its own protons to take the place of the one taken from the hydride. The exchange process would be completed by diffusion of the hydrogen atom from the surface to the interior of the hydride particle [14]. Acknowledgement-The authors acknowledge gratefully the support of this research by the Office of Naval Research Contract Nonr-2249(11 ). *The authors are grateful to a referee for the suggestion of this explanation. 13. A. H. Webster andJ. Halpern, J. phys. Chem. 61, 1239 (1957); Trans. Faraday Soc. $3, 51 (1957). 14. The rate of self-diffusion of hydrogen in cerium hydrides has apparently not been measured. However, proton self-diffusion occurs at moderate temperatures in the similar lanthanum hydrides of composition LaH>~. (D. S. Schreiber and R. M. Cotts, Phys. Rev. 131, 1118 (1963).