Indian Journal of ChemistryVol. 29A, November 1990. pp. 1101-1105

Studies on hydrogen storage alloy, Mg2Ni: catalytic activity and XPSt

V Padmasubhashini, I A P S Murthy, M P Sridhar Kumar & C S Swamy"

Department of Chemistry, Indian Institute of Technology, Madras 600 036

Catalytic activity of hydrogen storage alloy, Mg2Ni and modified Mg2Ni(O-Mg2Ni)has been stud-ied using decomposition of 2-propanol as test reaction. Besides dehydrogenation activity, Mg2Nishows .some activity towards the formation of condensation products, viz. mesityl oxide, isobutyl me-thyl ketone and diisobutyl ketone. XRD of spent Mg2Ni indicate disproportionation of Mg2Ni toMgO, Ni and NiO under reaction conditions, while XP spectra show coke formation on its surface.On 0-Mg2Ni the activity is reproducible with only acetone as the product. The activity, selectivityand stability have been explained based on the difference in their surface composition as revealed byXPS studies.

Hydrogen storage intermetallic compounds haveattracted attention as catalysts in recent years dueto their higher activity as compared to that of theconventional supported metal catalysts 1·3. Theiractivity has been attributed to the highly dispersedstate of the active species generated during theactivation of the alloy. When oxygen containingreactants are used, bulk decomposition of the ~-loy occurs, resulting in the formation of a sup-ported system?". In the present paper, catalyticactivity of fresh and modified forms of the in-termetallic Mg2Ni has been investigated using2-propanol decomposition as test reaction.

Materials and MethodsThe intermetallic Mg2Ni was obtained from

Ergenics Corporation, USA. Modified Mg2Ni(0-Mg2Ni) was prepared by the oxidation (02) of thealloy at 773 K for 10 hr followed by reduction(H2) at 673 K for 10 hr in a tubular furnace. Thesurface areas of fresh and spent catalysts were de-termined by the BET method using nitrogen asadsorbate at 77 K. The sample (1 cc, particlesize 75-106 microns) was placed between twoquartz wool plugs in the reactor. No pressure wasapplied for packing the catalyst but it was gentlytapped. It was ensured that the packing allowedfree passage of activating gas as well as reactantvapours.Decomposition of 2-propanol was carried out

to assess the activity of the catalysts, each expe-riment lasting 30 min. The conversions were cal-

tPaper presented in the National Workshop on SpectroscopicMethods in Heterogeneous Catalysis held at Bombay during20-22 December 1989.

culated by analysing the liquid products collec-ted during the steady state. Product analysis wascarried out in a gas chromatograph (Chromato-graph Instruments Corporation, model ACD)using Carbowax 20 M column with a thermalconductivity detector. Gaseous products wereanalysed in an Orsat apparatus. X-ray diffracto-grams (XRD) were obtained with a Phillips (mo-del PW 1130) diffractometer using CuKa radiation(A.= 1.5418 A). X-ray photoelectron spectra (XPS)were recorded in an ESCALAB Mark IT (VGScientific) spectrometer using MgKa radiation(hv = 1253.6 eV) at a base pressure of 5 x 10-9mbar. C(ls) peak was taken as internal standardcorresponding to the binding energy (BE) 285.0eY. The samples were scanned in the as receivedform and after etching with argon ions at 5 x 10 - 6mbar for 300 sec. Electron spectroscopy was car-ried out in a Cambridge Stereoscan 180 scanningelectron microscope.

Results and Discussion

Studies on Mg2NiDecomposition of 2-propanol was carried out

in the temperature range of 443-483 K. Alongwith acetone in the liquid products, mesityl oxide,diisobutyl ketone and isobutyl methyl ketone weredetected. The results along with the experimentalconditions are given in Table 1. It can be seenfrom Table 1 that the conversions ranged between40 and 70 mole %. The conversions could not bereproduced within 1 mole % of 2-propanol whenthe reaction was repeated under the same condi-tions. Thus, non-reproducibility refers to the in-

1101

l'd)I.\".1 ('1111\1.SIT ..\. !':O\'FI\!BER I'NO

CatalystTable I - Product distribution for the decomposition of 2-propanol onMg~Ni and O-Mg~Ni

Reaction Contact Mole % of liquid productstemp. (time)(K) (s) Acetone IMK MO

443 1.54 30.6 6.2 1.8

463 1.46 38.2 14.6 2.5473 2.38 42.1 19.0 2.1473 1.43 51.7 13.7 1.4

473 0.89 52.4 11.5 0.9453 0.83 7.1

453 1.51 10.1

453 1.91 12.3

473 0.79 7.4

473 1.15 9.8

473 1.10 13.7

533 0.76 11.9

533 1.02 18.2

533 1.62 24.2

OIK

trace

2.0

5.91.1

0.7

* Results were not reproducible. IMK = Isobutyl methyl ketone; MO = mesityl oxide; OIK = diisobutyl ketone.

consistency in the conversion of alcohol (typicallythe variation was - 5% in this case) under similarconditions. However, the results presented areonly indicative of the activity of the intermetallic.The reactions were associated with rise in tem-perature. Since the dehydrogenation of 2-propa-nol is an endothermic reaction (~H = - 66.5 kJ/mol at 32rC)1, the rise in temperature may bedue to some other process such as oxidation ofthe intermetallic as will be clear from the discus-sion in the sequel.

After a few experimental runs, back pressuredeveloped in the reactor within 5 min from thestart, which prevented the free flow of feed. Exa-mination of the spent catalyst showed that it hadlost its metallic lustre and had become a pellet-like block. This explains the hindrance of the feedflow and back pressure during experiments. In or-der to identify the phases present on the surfaceand in the bulk, XPS and XRD of the spent catal-yst were recorded. Scanning electron micrographswere taken to examine the morphology 'of the ca-talysts.

XRD pattern of fresh Mg2Ni showed only theintermetallic phase of Mg2Ni, while those of thespent Mg2Ni catalyst showed Ni, NiO and MgOalong with traces of Mg2Ni. The surface area ofthe catalyst increased from 0.5 m2/g to 23.4 m2/gwhen exposed to reaction conditions. SEM pic-

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tures of the fresh and spent Mg2Ni (shown in Fig.1) indicates an overall decrease in the particlesize. From these observations it is clear that thespent catalyst is different from the fresh one. Suchtransformation is due to the disintegration andoxidation of the alloy in which an oxygen contain-ing reactant (2-propanol) is involved. Since oxida-tion is an exothermic process, it accounts for thetemperature rise during the reaction. Since it is al-so associated with gain in mass and volume, ex-pansion of the catalyst bed occurs. The oxidisedintermetallic particles thus form a pellet-likemass consisting of fine particles' of oxides of Mgand Ni. The SEM pictures indicate the same.

XP spectra of spent MgjNi are shown inFigs 2 and 3. The features are given inTable 2. Signal due to Ni(2p) observed at855.7 eV was weak but it improved significantlyafter etching the surface for 300 see with Ar +

ions. The surface composition was calculated us-ing the formula" (1 )

... (1)

where I is the area under the peak and 0, thephotoionization cross section. The ratio Ni/Mgcalculated using this formula was found to in-

PADMASUBHASHINI et al.: STUDIES ON HYDROGEN STORAGE ALLOY. Mg,Ni

(q) Fre sh

(b) Spent

Fig. I - Electron micrographs of MgNi catalyst.

crease from 0.013 to 0.036 after etching. The up-ward shift of BE by - 2 eV indicates that Ni ex-ists in + 2 state. The satellite peaks observed at- 6 eV confirms this"!",

Mg(ls) peak (Fig. 2) was observed at 90.0 eV,which is 1.2 eV away on the higher BE side ofthat of metallic Mg. This indicates that Mg existsin + 2 state II. The Mg( 1s) peak is partially asym-metric (a shoulder on the higher BE side can beseen), which is assigned to its hydroxide, viz.Mg(OHh (see ref. 12). This shoulder decreasesupon sputtering indicating that the hydroxide spe-cies is confined to the surface layers.

During catalytic reaction Mg gets oxidised,evolving a large amount of heat (601 k.l/rnol)!',which accounts for the temperature rise duringthe reaction. Since the surface energy 'of Mg (790

//

Table 2 - XPS Data of Mg,Ni and O-Mg,Ni samples (BE/eY)

Region Mg,Ni O-Mg,Ni

As received Etched As received Etcheu

Mg Is l)O.6 l)O.5 l)O.5 l)O.S

Ni 2p X55.0 XS5.0 X55.5 X55.4

X6I.3 X6I.1

o Is 53 I.l) 531.7 530.5 530.7

n!,,1nMg 0.013 0.036 0.153 0.I5X

'-19 2s

.•c.,C

b1173

d_____ 1423

1585

~943

85 90 95Binding energy t ev

100

Fig. 2 - XP (Mg(2s)) spectra of (a) as received MgNi, (b)Mg~Ni etched for 300 sec. (c) as received O-Mg~Ni and (d)

O-Mg~Ni etched for 300 sec.

m.l/m-) is much less than that of Ni (2450 ml/rn")(see ref. 14), Mg segregates to the surface andshields nickel. This is evidenced from the lowconcentration of nickel on the surface. The resultis in accordance with a previous report" on theXPS study of air-exposed Mg2Ni. With each ex-pcrimental run fresh amounts of the alloy dispro-portionate, thereby changing the surface of thecatalyst. This explains the non-reproducible acti-vity. The heat evolved in the process leads tocracking of the reactant as a result of which cokeis formed on the surface.

I \OJ

I,\D!:\J\ J CIII~1. SIC :\. J\O\TMBFR 1')')(1

.•C::Jo~

Ni Zp

Q 451

865850 855Binding energy leV

Fig. ,,-XP (Ni(2p)) spectra of (a ) as received Mg.Ni, (b)Mg,J\i etched for ,,00 sec. (e) as received O-Mg,Ni and (d)

O-Mg,Ni etched for "O() sec.

The formation of condensation products in thedecomposition of 2-propanol has been reportedon a number of mixed oxide systems lf as well ason alkaline earth metal oxides 17. On the oxidisedintermetallic compound LaNi5, the condensationability was attributed to the basic nature of La~01and a mechanism was proposed for the formationof condensation product:'. Zhang et al." proposedthat basic OH - ions were the active sites for al-dol addition of acetone on solid base catalysts.The formation of these condensation products issimilar to aldol condensation which is normallypromoted by acids or bases through the formationof a carbocation or a carbanion. The initial stepsin these reactions involve either a proton additionor proton abstraction by OH -. In the presentcase, the latter seems to be made possible by thepresence of Mg(OH):. The shoulder observed inMg( Is) peak corresponding to Mg(OH): in the XPspectrum (Fig. 2a) supports this mechanism.

Studies on O-Mg:NiDecomposition of 2-propanol was carried out

in the temperature range of 473-533 K. Acetone\\ as the only product observed with the convcr-

II().f

sions ranging between 7 and 24%. The resultswere reproducible and are given in Table 1. TheXRD patterns of the fresh and the spent catalystswhich showed lines corresponding to Ni, NiO andMgO were nearly the same. Their surface areasalso did not vary appreciably ( - 8 m2/g), indicat-ing that the catalyst did not undergo any appreci-able change during the experiment.

XP (Ni(2p)) spectra of O-Mg2Ni are given inFig. 3 along with those of Mg2Ni. The spectrashow peaks centered at - 855 eY. Also, a satellitepeak can be seen at 6 eV away from the mainpeak. The intensities of Ni(2p) peaks are muchgreater in the case of 0-Mg2Ni, as compared tothose of Mg~Ni. Moreover the NilMg ratio ismuch higher for O-MgcNi (Table 2) indicating thatthe concentration of nickel on the surface of0-Mg2Ni is quite high. The upward shift ofNi(2p) BE shows that Ni is in + 2 state. XP(Mg(1s)) spectra given in Fig. 2 (c and d), aresymmetric unlike those in the case of Mg2Ni. Thisshows that Mg exists in only one environment, i.e.as MgO.

It can be seen from Table 1 that side productsare not formed on O-MglNi. This is due to theabsence of Mg(OHh, which is responsible fortheir formation. Since the surface of O-Mg2Ni isrich in nickel, the possibility of hydroxide forma-tion becomes less. In conclusion, it can be sug-gested that the oxidation-reduction treatment on0-Mg2Ni results in the formation of a very stablecatalyst (with respect to bulk as well as surface).Surface areas, XRD and XPS studies support thisfact.

AcknowledgementThe authors thank the authorities of Regional

Sophisticated Instrumentation Centre, lIT, Madrasfor the XP spectra. They also thank the CSIR,New Delhi and lIT, Madras for research fellow-ships to two of them (MPS and IAPSM).

ReferencesI Takeshita T. Wallace W E & Craig R S, } Catal. 44

(1973)236.2 Wallace W E. Chemtech. Dee (1982 \ 75'2 and references

therein." Sridhar Kumar M P. Viswanathan B. Swarny C S & Srin-

ivasan V. Indian ) Chern. 28A ( 1989) 19.4 Elatter A. Takeshita T. Wallace W E & Craig R S. Sci-

ence. (IY77) ]()<}".

:'\ Burrault J. Duprez D. Percheron-Guegon A & Achard JC. ) Less Common Metals. 89 (1<}83) 537.

(, Luengo C A. Cabrera A L. McKay H B & Maple M P. )Cawl,47(1977) I.

7 Kirk-Othmer Encyclopaedia or Chemical Technologv. Vol.I<}. p. 202. (John Wiley & Sons. Inc .. New York) (1978).

PADMASUBHASHINI et al.: STUDIES ON HYDROGEN STORA(;E ALLOY. Mg:Ni

8 Powell C J & Larson P E, Appl Surf Sci; I (1978) 186.9 Sharma D D, Kamath P V & Rao C N R, Chern Phys.

73 (1983) 71.10 Wagner C D in Practical surface analysis by Auger and

photoelectron spectroscopy (eds. Briggs D & Seah M P) p.477 (John Wiley, New York) (1983).

II Fuggle J C, Watson L M, Fabian D J & Affrossman S, j

Phys F Metal Phys, 5 (1975) 375.12 Fuggle J C, Watson L M. Fabian D J & Affrossman S,

Surf Sci; 49 (1975) 61.

to the segregation of lanthanum to the surfacewhich has got low surface energy (900 mJ m-2)

compared to copper (1850 mJ m") (ref. 22).Since copper is the surface active site in thesecompounds, the high activity of La2Cu04 (A) canbe attributed to the higher concentration of cop-per on the surface.

The mechanism, of decomposition of N20 in-volves the transfer of the electron from the catal-yst. Therefore, the charge carriers also govern theactivity of the catalyst. The adsorption of N20 re-quires the presence of anion vacancies or F-cen-tres in the catalyst. On the other hand, desorptionof oxygen will give rise to R2 centre on the sur-

13 CRC Handbook of Chemistry and Physics (eRe PressInc .. Florida) (1983).

14 Meidema A R. Z Metallkde. 69 (1969) 455.15 Selvam P. Ph.D. Thesis. Indian Institute of Technology.

Madras India (1987).16 Maraka:ni Y & Onda Y. Kogyo Kagaku Zasshi. 72 (1963)

\033. Ch17 McCaffrey M F. Micka T A & Ross R A, j Phvs em.

76 (1972) 3372.18 Zhang G. Hattori H & Tanabe K, Appl Catal. 36 (1988)

189.

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__ r·- - - -- --~ ---'10(B) which is also an evidence for the presence ofmore F-centres apart from higher concentration ofcopper on the surface.

From the foregoing discussion, it is evident thatLa2Cu04 prepared from oxides exhibits higher ac-tivity compared to that prepared from oxalates.The high activity has been attributed to the pres-ence of higher concentration of copper on the sur-face as well as the presence of more anion vacan-cies.

AcknowledgementWe thank the CSIR, New Delhi for the award

of a scholarship to one of us (1 C). We also thank

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