Hyrdogen bronzes from VMoO5.5 : An X-ray diffraction investigation

Mat. Res. B u l l . , Vol. 20, p p . 575-581, 1985. P r i n t e d in the USA. 0025-5408/85 $3.00 + .00 C o p y r i g h t (c) 1985 Pergamon P r e s s Ltd.

HYRDOGEN BRONZES FROM VMo05. 5 : AN X-RAY DIFFRACTION

INVESTIGATION

C. Ancion, G. Poncelet and J.J. Fripiat x Groupe de Physico-Chimie Min4rale et de Catalyse, Universit4 Catholique de Louvain, Place Croix du Sud i, B-1348 Louvain-la-Neuve, Belgium M

CNRS-CRSOCI, rue de la F~rollerie F-45045 Orl~ans C4dex, France

i,

(Received March 14, 1985; Communicated b y S. Amelinckx) ABSTRACT Hydrogen bronzes obtained by spillover of hydrogen onto Pt particles deposited on the surface of VMoO5. 5 have been examined by X-ray diffraction. The results show that H 3 .4VMoO.~ ~- is crystalline and that no intermediate crystalline phase is ~ormed durlng the insertion of hydrogen in the host lattice. Oxidation of the bronze restores the initial oxide phase. Upon back-titration with H 2 at 60°C, the bronze phase reappears (indicating the reversible character of these bronzes) at a much higher rate than when the bronze is formed for the first time.

Introduction In a previous study (i), we reported that the mixed vanadium molybdenum oxide, with composition VMo05. 5, impregnated with I% Pt and exposed to molecular hydrogen, yielded, through hydrogen spillover onto the Pt particles, a crystalline hydrogen bronze, HxVMO05. 5 where 2.9 ~ x 6 3.4. The starting oxide, obtained by thermal decomposition at 400°C of an oxalic precursor, is different from the crystalline phases V9Mo6040 reported by Jarman et al. (2) and V2MoO 8 of Freundlich et al. ( 3 ) . However, the unit cell parameters of VMoO5. 5 are close to those of the 8-phase that Jarman et al. (4) obtained upon calcining at 500°C the solid residue resulting from the evaporation to dryness of a solution containing the ammonium salts of vanadium and molybdenum. The composition of this E-phase, (V0.5Mo0.5)308 or VMo05.33, is close to VMoO5. 5. It was also reported that in the presence of ethylene as hydrogen accepting molecule, 35% of the hydrogen content of H3.4VMo05. 5 could be recovered as ethane at 120°C, through hydrogen reversal spillover (5,6). The present paper will mainly focus on the results of a X-ray diffrac575

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tion study carried out with the aim of i) establishing whether intermediate crystalline phases develop during the formation of the bronze, ii) examining the phase modifications when the bronze is exposed to oxygen and iii) controlling the reversible character of the bronze phase. Experimental Material : The preparation procedure of the oxide phase, VMoO5.5,from ammonium metavanadate and ammonium heptamolybdate dissolved in oxalic acid, has been detailed elsewhere (I). The solid precursor is calcined at 400°C for 48 h and yields a very well-crystallized oxide. The compilation of the interplanar distances and the unit cell parameters are given in ref. (1). The mixed oxide was impregnated with the necessary volume of a 0.2 M hexachloroplatinic acid solution in order to load the solid with i% Pt. The slurry was then freeze-dried. ~ Z ~ [ _ ~ £ ~ _ ~ ! [ ~ : The X-ray diffraction patterns a Seifert Scintag PAD II instrument provided with a PDP II sample could be outgassed and heated in a high temperature were scanned between 15 and 40 °2@ within 8 minutes, using

were recorded with computer. The camera. The spectra CuK~ radiation.

Results and Discussion Phase__tr_ansfo_rm_at!on_dur!n~

the formation of the bronze

The Pt-impregnated oxide was first outgassed at 200°C for 2 h in order to decompose the platinum salt (7). Then, molecular hydrogen (300 Tort) was introduced in the camera and the diffractograms were recorded at different times during the formation of the bronze at 60°C. Fig.

i shows the X-ray diagrams recorded between

FIG.

15 and 40 °2@. As can

I

X-ray diagrams recorded during the insertion of H 2 at 60°C in Pt/VMoO5. 5. a : initial oxide, and after 3h30 (b), 4h45 (c),6h50 (d), 10hl0 (e), 15h (f) and 24h (g).

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HYDROGEN BRONZES

be seen, there is no noticeable modification of treatment under H 2. With increasing times, the bronze phase begin to appear while those of the sively in intensity. After 24 h, the bronze is

577

the oxide phase after 3h30 reflections typical of the oxide phase decrease progrescompletely formed.

It may be concluded from this figure that there is no transient crystalline phase appearing during the formation of the bronze. This evolution bears some similarities with the one observed for the hydrogen bronze prepared under identical conditions with Pt/MoO 3 (7) for which, as shown by Tinet et a L (8), no intermediate phase between M o O 3 and H1.6MoO 3 could be evidenced. On the contrary, the hydrogen bronze of vanadium, H3.sV205, is completely amorphous (9). The evolution of the intensity (in relative units) of the 001, 400 and 600 reflections of VMo05. 5 and of H3.4VMoO5.5 versus time is shown in Fig. 2.

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FIG. 2. Evolution (in relative units) versus time of the 400 (M), 001 (+) and 600 (0) reflections of VMoO 5 5 (left fig.) and of HxVMOO5. 5 (right fig.) during the insertion of H 2 at 6~°C. These figures clearly illustrate the transformation of the starting oxide phase" into the bronze phase. Fig. 3 compares the evolution of the 400 reflection of VMoO5. 5 when the bronze is prepared with a freshly impregnated oxide, as in Fig. I, and for an oxide which was impregnated with the platinum salt 23 days before being treated under H 2. The difference observed between the two systems cannot be attributed to the starting oxide since samples of the same well-crystallized oxide were taken. This effect suggests that modifications of still unidentified nature occur during the storing of the Pt-impregnated oxide (possible migration of Pt, better dispersion, surface interactions ...). This point still needs to be clarified.

In another set of experiments, a freshly prepared bronze was outgassed at temperatures at which the bronze is still reversible, i.e. in which the hydrogen content can be restored to its initial value upon back-titration with H2 • The X-ray diagrams scanned after outgassing the bronze successively at room temperature, and then at 80°C for Ih40 and at 120°C for i h are shown in Fig. 4.

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100. 80 60,

A.O. 20' 0

10

5

15

20 TIME (h)

FIG. 3. Evolution of the 400 reflection of VMo05. 5 (in relative units) versus time during the insertion of hydrogen at 60°C for a freshly impregnated oxide (open symbols) and for an oxide impregnated 23 days before insertion (full symbols).

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20 FIG. 4. X-ray diffraction patterns recorded during the outgassing : a: initial oxide; b: H3.4VMo05.5 ; c: outgassed at room t~nperature; d: outgassed at 80°C for lh40; e: outgassed at I20°C for lh. Parallel experiments carried out in a glass volumetric apparatus, as described elsewhere (7), showed that upon outgassing at 120°C, nearly 18% of the hydrogen inserted in the host lattice was driven out. Upon back-titration with hydrogen at 60°C, the initial stoichiometry was restored. As shown in this figure, the partial depletion in H 2 from the bronze does not bring about significant modifications in the diffractograms. The intensity of the different reflections is not visibly affected. The only modifications concern the position of several diffraction lines, principally the

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ii0,

600,

510,

HYDROGEN BRONZES

579

111 and 601 reflections.

Table I compares the computed positions of the X-ray peaks corresponding to these reflections for the initial bronze, and after outgassing at 80 and 120°C. TABLE

I.

Computed positions (in °2@) of the 001,110,600,510,111 and 601 reflections after outgassing the bronze at 80 and 120°C.

h

k

1

initial bronze

first outgassing at 80°C for Ih40

0 1 6 5 1 6

0 I 0 1 1 0

I 0 0 0 I 1

23.5292 24.2801 26.3269 32.5175 33.9883 35.4495

23.5134 24.3713 26.2431 32.5426 34.0666 35.3834

slightly crease.

second outgassing at 120°C for lh 23.5169 24.4523 26.1698 32.5633 34.1330 35.3319

difference (%) + + + -

0.05 0.71 0.59 0.14 0.42 0.33

As shown in this table, the values of the ii0 and 111 reflections increase whereas those of the 600 and 601 reflections slightly de-

A freshly prepared hydrogen bronze was brought into contact with air at 120°C and the X-ray diffractograms were recorded at several time intervals. The sequence of the diffraction patterns is illustrated in Fig. 5.

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28 FIG. X-ray diffraction a: initial oxide; e: after 3 h.

5.

patterns recorded during the oxidation at 120°C of the bronze b: H3.4VMo05.5 ; c: after 5 min exposure; d: after 20 min and

As can been seen, already after 5 min in the presence of air, the reflections typical of the bronze phase are no longer visible;only those c h a r a c t ~ ristic of the oxide phase are present. Increasing times do not improve significantly the peak intensities, the main modifications bearing on the position

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of the 001 reflection (near 23 °2@) which is slightly shifted towards the value observed in the initial oxide. However, as compared to the initial oxide (Fig. 5a) the peak intensities are smaller in the bronze that has been oxidized, which reflects a loss of crystallinity, possibly due to the highly exothermic effect accompanying the reaction between oxygen and the inserted hydrogen atoms. This may result in surface disturbances.

b

20 FIG. 6. X-ray diffraction patterns recorded on a bronze previously outgassed at 120"C and then oxidized, a: initial oxide; b: H3.4VMoO5.5; c: after 5 min oxidation; d: after 20 h oxidation. Essentially analogous observations are made when the bronze has been partly depleted in hydrogen by outgassing at 120°C before treatment with air. The X-ray diagrams are shown in Fig. 6. After 5 min in air, as was the case in the previous figure, the reflections of the bronze phase have completely vanished but the crystallinity of the oxide phase is significantly less well restored. However, after a prolonged treatment at 120°C (Fig. 6d), the diffractogram is much similar to that of the initial oxide. Reversib_~l~t~ of the bronze It was also interesting to look by X-ray diffraction at the reversibility of the bronze phase. For that purpose, a freshly prepared bronze was treated under air for 3 h at 120°C. After evacuation of the camera, the solid was back-titrated with hydrogen at 60°C. Fig. 7 compares the diffraction patterns recorded after the formation of the bronze (Fig. 7b), after oxidation (Fig. 7c) and after 5 min in contact with H 2 at 60°C (Fig. 7f). Obviously, as far as crystallinity is concerned, completely restored in its initial state.

the bronze phase is

Most important is to note the extreme rapidity at which the bronze is formed, as compared to the time required for the formation of the bronze when the impregnated oxide is treated for the first time with hydrogen. Indeed, as seen earlier (see Figs. i-3), it takes over 20 h to have the complete transformation of the oxide phase into the bronze phase. Several reasons or hypotheses may account for this observation: better

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HYDROGEN BRONZES

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FIG. 7. Reversibility: X-ray diffraction patterns recorded on : a: initial oxide; b: hydrogen bronze; c: after 3 h oxidation at 120°C and d: after 5 min under hydrogen at 60°C. contact between the Pt particles and the surface of the oxide,higher deqree of reduction of the platinum, better dispersion of the platinum. Acknowledgements C.A. is indebted to I.R.S.I.A. for a doctoral grant. The authors gratefully acknowledge Dr. W. Mortier from the Laboratorium voor Oppervlakte Scheikunde, K.U. Leuven, for giving access to the X-ray equipment. References i. C. Ancion, J.P. Marcq, G. Poncelet, D. Keravis, L. Gatineau and J. Fripiat, C.R. Acad. Sc. Paris 296, 1509 (1983). 2. R.H. Jarman, P.G. Dickens and A.J. Jacobsen, Mat. Res. Bull. 17, 325 (1982). 3. W. Freundlich and P. Pailleret, C.R. Acad. Sc. Paris 261, 153 (1965). 4. R.H. Jarman and A.K. Cheetham, Mat. Res. Bull. 17, 1011 (1982). 5. P.A. Sermon and G.C. Bond, Catal. Rev. Sci. Eng. ~, 211 (1973). 6. C. Ancion and G. Poncelet, Acta Chim. Acad. Sc. Hung., article in press. 7. J.P. Marcq, X. Wispenninckx, G. Poncelet, D. Keravis and J. Fripiat, J. Catal. 73, 309 (1982). 8. D. Tinet and J.J. Fripiat, J. Chim. Phys. 76, 867 (1979). 9. J.P. Marcq, G. Poncelet and J.J. Fripiat, J. Catal. 87, 339 (1984).