SOLID

ELSEVIER Solid State Ionics 101-103 (1997) 775-780

STATE Iowa

Comparison between electrical conductivity properties and catalytic activity of nickel molybdate

G&u-d Thomasa’*, Magali Sautel”, Akim Kaddourib, Carlo Mazzocchiab

“E.N.S. Mines de Saint Etienne, Centre de Recherche sur les Solides et leurs Applications (CRESA), 158 Cows Fauriel, 42023 St Etienne Cedex, France

bDipartimento di chimica Industriale e Ingegneria Chimica, Politecnico di Milano, P.za L. Da Vinci 32, 20133 Milano, Italy

Abstract

Nickel molybdates obtained by calcination of precursors were found to be effective catalysts for the oxy-dehydrogenation

of propane into propene. Various preparation and calcination conditions lead to different powders of nickel molybdate characterized by atomic absorption spectroscopy and powder X-ray diffraction. Their defect properties, studied by electrical conductivity, can be linked to catalytic activity. It can be assumed that some of the molybdenum atoms and electrons in

interstitial position in nickel molybdate compounds are responsible for n-type conductivity. Some catalytic tests were carried out and allow to conclude that free electrons in nickel molybdates could make easier the propane adsorption or its reaction

with oxygen.

Keywords: Electrical conductivity; Catalysis; Defect; Non-stoichiometry

Materials: NiMoO,

1. Introduction

Nickel molybdate has been proved to be a good

candidate for the catalysis of many reactions of

oxy-dehydrogenation and especially in the trans- formation of propane into propene [l]. Studies on catalytic properties have been widely extended but

only a limited number of them deals with physical properties and structure defects in solids.

Different ways of preparation have been used to

lead to solid catalysts exhibiting different atomic compositions: a starting product in which the Ni/Mo ratio is equal to 1 and other products presenting a

*Corresponding author

Ni/Mo ratio different from 1 after addition of

molybdenum or nickel into the initial material.

Nature and concentration of crystal defects can be

changed by such additions and analysed thanks to electrical conductivity measurements [2,3]. It is known that catalytic activity of nickel molybdates

depends on their polymorphic variety [5] and also on

both nature and number of their defects. In this paper we aim to compare the dependence of conductivity on oxygen pressure with theoretical considerations

on the point defect thermodynamics and to consider the nature of the predominant point defects for each

nickel molybdate sample in order to explain the variations of catalytic activity as a function of the preparation route.

0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PII SO167-2738(97)00293-Z

776 G. Thomas et al. I Solid State tonics 101-103 (1997) 775-780

2. Experimental

2.1. Preparation

A Mettler KC1 reactor calorimeter was used for

the preparation of the ammonia precursor with a constant stirring speed of 100 ‘pm. The starting

materials were nickel nitrate hexahydrate and am-

monium molybdate tetrahydrate (Fluka). To one

volume of a solution of ammonium molybdate (0.25

mol 1-l) maintained at 90°C was instantaneously added 7 volumes of a solution of nickel nitrate (0.25 mol 1-l) in order to obtain a Ni/Mo ratio equal to 1.

The pH fell from 4.85 to 3.96 in 30 min. The yellow precipitate formed was filtered in thermoregulated conditions, washed with hot distilled water and dried

at 120°C for 15 h.

solution of molybdenum oxalate or nickel acetate with the appropriate cationic concentration. Molybdenum oxalate was prepared from ammonium molybdate and oxalic acid. Nickel acetate (Prolabo) was quite pure and cobalt free. After drying, the Mo-

or Ni-enriched samples were calcined at 550°C for 4 h in order to achieve both removal of oxalic or acetic

ions and internal diffusion of impregnating cations.

The amount of added molybdenum with respect to the initial composition was 1, 2, 3, 5, 7 and 10 at%

respectively and the amount of added nickel was 1, 2, 5, 7, 10 and 30 at% respectively. These new

samples underwent the same thermal treatment as the starting product.

2.2. Characterization

This precursor prepared in stoichiometric con- The analysis of the nickel and molybdenum

ditions was calcined at 550°C for 15 h to form the concentrations was carried out by inductively cou-

low temperature a-phase of nickel molybdate called pled plasma-atomic emission spectroscopy (ICP-

below starting or stoichiometric sample. The intrinsic AES) within a 2% error. X-ray diffraction patterns

composition of this product was modified by addition were obtained by a Siemens D5000 diffractometer

of molybdenum or nickel. The additives were intro- with Cu Ko radiation. They were collected in the

duced by chemical impregnation using an aqueous range 10-60” 28. Specific surface areas were mea-

gas flow = 6 l/h

I I

Mesurement and acquisition of the potential difference u (V)

Fig. 1. Experimental set-up for electrical conductivity measurements.

G. Thomas et al. / Solid State Ionics 101-103 (1997) 775-780 171

sured according to the standard BET method with a

Micromeritics ASAP 2000 sorptometer using nitro- gen as the adsorbate.

The electrical conductivity was measured at 550°C

with the set-up described in Fig. 1. The powder was cold pressed and sintered at 550°C for 2 h. Then gold

(400 nm) was deposited by sputtering onto both sides

of the pellets. Two movable gold electrodes were

used as electrical contacts. Samples were maintained

at 500°C under a controlled flow of gas mixture at the atmospheric pressure. The conductance was

measured at constant voltage (2 V), first after a treatment in nitrogen for 12 h, then after each plateau

at constant partial oxygen pressure from lo-’ to 1 atm respectively.

2.3. Catalytic tests

Catalytic tests were carried out as described in ref.

[l] using 0.25 g of catalyst in the following con- ditions:

at 500°C with the low temperature o-phase (prod-

uct having never been transformed into B-phase); at 500°C and 530°C with the high temperature B-phase obtained by heating the a-phase for one

hour at 675°C then cooling at the working

temperature; at 500°C with the a’-phase obtained by cooling

the B-phase at room temperature before heating at

the working temperature.

The gas mixture was composed of 18 ~01% O,, 15

~01% C,H, and 67 ~01% He with a flow rate of 15 1 h-’ under atmospheric pressure.

3. Experimental results and discussion

3.1. Chemical composition and sur$ace area

ICP-AES analysis showed that the amount of

nickel atoms in the MO-rich compounds was not

modified by addition of molybdenum and the amount of molybdenum atoms in the Ni-rich compounds was

almost constant after addition of nickel. In other words, preparation and pretreatment preserved the Ni/Mo ratio defined initially. This ratio was of 0.98

100 0.85 0.9 0.95 1 1.05 1.1

Ni/hb e~ticnral

Fig. 2. Dependence of the surface area on the NiMoO, com-

position.

instead 1 for the initial sample, clearly identified as

nickel molybdate by XRD at room temperature.

The same, that is a nickel molybdate structure,

was found for all Ni-enriched samples and for the

MO-enriched samples except for the sample enriched at 10% which appeared to be a mixture of NiMoO,

and a small amount of MOO, (around 5 mol%). Scanning electron microscopy brought another evi- dence of some molybdenum oxide grains among

nickel molybdate particles in the 10% and 7% MO

enriched compound. To summarize, for 0.95 < Ni/ MO < 1.3, the Ni- and MO-enriched samples con-

tained only one phase, for Ni/Mo < 0.95, a two

phase region appeared with MOO, and NiMoO, and

for Ni/Mo > 1.3, no clear conclusion can be given

but the limit of solubility of Ni in NiMoO, seems to

be not yet reached. The surface area of the catalysts was found to be

weakly affected by the change in composition of the Ni-rich and MO-rich ol-NiMoO, (Fig. 2). In the

opposite, the surface areas of MO-rich a’-NiMoO, rapidly decreased with increasing molybdenum con-

tent [4] (Fig. 2).

3.2. Electrical conductivity

The conductivity u obeyed the classical oxygen pressure law according to:

cr = a,Pp, 2’

778 G. Thomas et al. I Solid State tonics 101-103 (1997) 775-780

The exponent p was found to be equal to 0.25, 0.18, 0.09, 0.07, -0.05 and -0.05 for the stoichiometric product and the Mo-enriched samples with 1, 2, 3, 5 and 7% MO respectively (Fig. 3). The positive value of the exponent p characterizes a p-type semicon- ductor behaviour whereas a value equal to 0 is for a n-type semiconductor behaviour. All the Ni-enriched samples also presented a p-type semiconductor be- haviour (Fig. 4) with a value around 0.24 for p.

3.3. Theoretical defect model

This model involves thermodynamical equilibria

between point defects present as well in the stoichio- metric as in the MO- or Ni-enriched compounds. It can be assumed that, in our experimental conditions (T < 550°C) only oxygen can be exchanged between the solid nickel molybdates and the surrounding atmosphere but not nickel and molybdenum. This approach can be justified by the fact that no vapor- ization of nickel or molybdenum oxides was de- tected. It must be emphasized that the conditions for preparation of samples by solid state impregnation and their thermal treatment lead to a metastable system with respect to molybdenum and nickel and to stable thermodynamic equilibrium for oxygen.

a.6 -

v l%Mwich product _ - 2%hWichpmdwt - c -3%Mwichpduct --w--5%Mwichpduct . .+. - 7%Mwkh pmduct

Fig. 3. Electrical conductivity of Mo-rich products versus oxygen pressure.

-5

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 log PoZ (ann)

Fig. 4. Electrical conductivity of Ni-rich products versus oxygen pressure.

G. Thomas et al. I Solid State Ionics 101-103 (1997) 775-780 779

The model proposed for the stoichiometric com-

pound is as follows:

Equilibrium between the crystal and the gaseous oxygen: $[O,] + Vi = 0: + 2h’

Electronic equilibrium: 0 = h’ + e’ Schottky disorder: 0 =Vz, + Vi, + 4Vo

Frenkel disorder for molybdenum: Vc, + Mob. =

Mot0 + Vi Frenkel disorder for nickel: Vi, + Ni;?’ = Niti +

Y Conservation of the total number of molybdenum

atoms: MO, = 1 - [V&J + [Mop.]

Conservation of the total number of nickel atoms:

Ni, = 1 - [V$] + [Ni;.]

Condition of stoichiometry: Ni, = MO, Electroneutrality: [h’] + G[Mo;‘] + 2[V,] +

2[Niz’] = 2[VtJ + [e’] + 6[Vz,l

For the MO-rich compounds, the condition of stoi- chiometry is no more fulfilled and must be replaced by: MO, = 1 +a where a represents the fraction of

molybdenum atoms added by chemical impregna-

tion. In the same way, for Ni-rich compounds, the condition of stoichiometry is replaced by the equa-

tion Ni, = 1 +a with a being the fraction of addition

of nickel in the Ni-rich products. From these various equilibrium conditions, theoretical laws of depen-

dence of conductivity on oxygen pressure can be calculated and compared with the experimental

results (Table 1).

[h’] = 6[V;,].

Considering the Eq. (2), addition of nickel will

change a product rich in nickel vacancies and

interstitial nickel ions in a compound presenting

interstitial nickel ions and molybdenum vacancies. In

both cases the Ni-rich products will be enriched in

molybdenum vacancies.

3.4. Catalytic results

Knowing that the 1% MO-rich compound posses- Fig. 5 compares the absolute rates of formation of ses electron holes and nickel vacancies as major propene and CO, from propane measured at 500°C

point defects, it is clear that two particular cases of

electroneutrality must be considered for the stoichio- metric nickel molybdate:

[h’] = 2[V;J + 6[V;,], (1)

2[Niy’] = 2[VLi]. (2)

Following a logical way of evolution in the solid disorders with the addition of molybdenum, it can be

proposed that the products enriched with 2%, 3%,

5% and 7% of molybdenum present molybdenum

ions in interstitial position and electrons responsible

for n-type conductivity. The experimental value of the exponent p around 0 confirms this assumption.

Addition of nickel decreases the amount of nickel

vacancies present in the stoichiometric product and the electroneutrality condition in Eq. (1) will be- come:

Table I Comparison between experimental and theoretical conductivity dependences

Product

710, 5%, 3% and

2% MO-rich product

1% MO-rich product

Stoichiometric

product

Experimental law

oeelp = q)P “d2’

P,X, =O

P _,=0.18

p,,, = 0.25

Theoretical laws

u,,=uO,Ppd;

Pth=O

pth =0.22

pth =0.25

or Plh = 0.20

Electroneutrality cases

[h’] = [e’]

~[Mo:‘] = [e’]

[h’] =2[V;,]

[h’] =2[V;,] +6[V,,,6’]

6[V;,] =6[Mo;-]

2[Nif’] =2[Vc,]

Ni-rich products P exp =0.24 pth =0.25

or pth =0.22

[h’] = 6[V;,]

[Nif.] =3[Vc,,]

[V”] = 3[V6’ 0 M” ]

780 G. Thomas et al. I Solid State lonics 101-103 (1997) 775-780

0.8

l l

. . . .

0

.

.

. 0

l propene l-l o co2

0.90 0.92 0.94 0.96 0.98 lot-l 1.02 1.04 1.06 1.08 1.10

Controlled atomic composition NilMo

Fig. 5. Evolution of the rate of formation of propene and carbon

dioxide with the atomic composition.

using as catalyst either the P-phase of the starting

product or MO- and Ni-enriched compounds. With the Ni-rich products, it can be observed that the rates of formation of propene and carbon dioxide are quite

equal while the selectivity to propene is enhanced for

the MO-rich catalysts. The MO-rich compounds exhibit a better catalytic

activity with respect to the absolute rates but present

specific areas smaller than those of the Ni-rich

products. The activity per unit of surface area is

proportional to the kinetic constants ki for the formation of propene (in mol Pa-’ s- ’ mp2) pre-

sented in Fig. 6 for different Ni/Mo ratios. It is obvious that the MO-rich products are five times

more efficient than the Ni-rich products. The high values of the kinetic constant of the MO-rich prod-

ucts counterbalance the weak values of their specific

surface areas.

CormolIed atomic composition Ni/Mo

Fig. 6. Evolution of the kinetic constant for the formation of

propene with the atomic composition.

4. Conclusion

A study of electrical conductivity allows to ob-

serve the change in the type of conductivity of nickel molybdates as a function of their Ni/Mo ratio and to identify the disorders responsible for this change. The starting product containing nickel vacancies and

the Ni-rich products containing molybdenum vac-

ancies have a p-type conductivity by electron holes.

A 5% MO addition leads to a n-type conductivity by

free electrons attributed to the formation of intersti-

tial MO-ions. Referring now to the catalytic activity, Mo-rich

compounds are controlled oxidants: therefore it has been shown that the propene selectivity increases while CO, selectivity decreases. This effect may be

tentatively explained by assuming a weaker availa- bility of the lattice oxygens due to the formation of

interstitial MO atoms but a mechanism involving

interstitial MO atoms in the catalyst reduction after

propane adsorption on the sites s, :

2(Mo:‘) + 2(03 + C,H, - o,

= ~(Mo;‘) + 2(OH;) + C,H, - u,,

(MO;‘) + 2(OH;) = ~(Mo;‘) + 0; + (H,OX,)

can also be proposed according to the Mars and Van Krevelen model [6]. These steps are followed by desorption of propene and water vapour. Propane reduces the interstitial molybdenum and is oxidized

in a selective way. The great amount of interstitial

molybdenum ions present in the MO-rich compounds

increases the rate of the oxy-dehydrogenation of

propane and leads to a better selectivity in propene.

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