J. Chem. SOC., Faraday Trans. I , 1984, 80, 1891-1901

Spectroscopic Study of the Adsorption of Carbon Monoxide on Solutions of Nickel Oxide and

Magnesium Oxide Part 2.-Samples Pretreated with Hydrogen

B Y ADRIAN0 ZECCHINA,* GIUSEPPE SPOTO, SALVATORE COLUCCIA AND EUGENIO GUGLIELMINOTTI

Istituto di Chimica Fisica della Universita di Torino, Corso M. D'Azeglio 48, 10125 Torino, Italy

Received 6th September, 1983

MgO-NiO solid solutions can be reduced by H, in two steps: in the first (temperature range 673-1093 K) mainly the Ni2+ ions located on the surface are involved, while in the second (T > 1093 K) those located in the bulk are also reduced.

The dimensions of the Ni particles formed during the reduction process increase with the temperature of reduction and with the nickel concentration on the surface. The particles formed at low temperatures are easily destroyed by CO with the formation of mononuclear and polynuclear carbonyls ; particles formed at higher temperature are more resistant to CO attack.

The carbonyls formed on reduced solid solutions upon adsorption of CO are the same as those found on the unreduced (standard) samples, the only difference being the absence in the first case of the oxidized products (adsorbed carbonates).

In order to fully prove the conclusions of our preceding paper,l in this contribution the adsorption of CO on solid solutions prereduced in hydrogen is described in detail. In particular the following items are taken into consideration: (i) the stability of Ni2+ ions emerging on the various surface configurations ((100) faces, edges, steps and corners) towards H, reduction, (ii) the stabilizing effect of the MgO matrix versus bulk reduction, (iii) the interaction of CO with Nio obtained by previous reduction with hydrogen and (iv) the effect of CO pressure and metal loading on the aggregation state of the dispersed nickel.

EXPERIMENTAL All the relevant details have been reported in Part 1.'

RESULTS AND DISCUSSION

EFFECT OF H, REDUCTION ON THE! REFLECTANCE SPECTRUM OF THE! SOLID SOLUTIONS

The effect of the H, pretreatments at increasing temperatures is illustrated in detail in fig. 1 (MN5). The relevant results are as follows. (i) Reduction at 673 K does not cause any modification of the intensity of the d-d bands for Ni2+ ions located in the bulk, and only produces a small intensity decrease in the 5000 cm-l peak associated with Ni2+ ions in the square-pyramidal configuration (i.e. the (100) face). (ii) Reduc- tion at 773 K still leaves the Ni2+ ions of the bulk completely unaffected while causing

1891 62-2

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online / Journal Homepage / Table of Contents for this issue

1892 ADSORPTION OF CO ON NiO-MgO SOLID SOLUTIONS

I ./-..

,.' '. i '. '. '. f

i i i i i ; \

\. ! \.

'. '.

.-.' '.

\ \

1

..-*.* *...

... . ...... *.. ...

'. \

\.

1 3 2 1

wavenumber/ 1 O4 cm-'

Fig. 1. Reflectance spectra of MN5 at increasing degrees of reduction: (-) standard solid solution, (....) reduced in H, at 673 K for 1 h, (---) reduced in H, at 773 K for 1 h and

(-.--) reduced in H, at 973 K for 1 h.

Table 1.

% of reduced surface

temperature/K Ni ions

673 13 773 19 973 70

1093 146

the partial erosion of the 5000 cm-l peak. (iii) The complete erosion of the 5000 cm-l peak is only achieved by reduction at 973 K. (iv) An extremely broad and featureless absorption (ca. 20000 cm-l wide) centred at (25-23) x lo3 cm-l gradually develops, with increasing degree of reduction and becomes extremely intense on samples treated at 973 K. These results clearly indicate that the Ni ions located on the edges (steps) and corners are reduced first (673 K) followed by the 5-coordinated square-pyramidal ions of the (100) facelets (773-973 K).

Quantitative measurements (carried out with a Sartorius microbalance on an MN 10

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

A. ZECCHINA, G. SPOTO, S. COLUCCIA AND E. GUGLIELMINOTTI 1893

08

Of

h

8

0 4

0.2

( 4 3 2 1

wavenumber/ lo4 cm-'

Fig. 2. Effect of CO contact (p = 10 Torr) on the reflectance spectrum of an MN5 sample reduced at 673 K: (-) sample reduced at 673 K, (---) immediately after CO contact

and (-.-.-) 12 h later.

sample) of the weight loss caused by hydrogen reduction and of the successive oxygen uptake (which also titrates the amount of nickel present after reduction) gives the results reported in table 1. Only at 1093 K does a reduction process involving the bulk nickel ions occur, while at lower temperatures the reduction is confined to the surface, in agreement with the spectroscopic results. The last result is in agreement with the literature data2* and confirms the stabilizing effect of the MgO matrix in preventing the reduction of nickel ions located in internal positions.

The surface nature of the transition at 5000 cm-l is totally confirmed. The broad absorption centred at ca. 25000 cm-l, which grows with the degree of reduction, is undoubtedly associated with reduced nickel and is due to collective excitations of free electrons in small metal particle^^-^ as already found for the Ni(CO)4-Mg0,6 Fe(CO),-Mg07 and M(CO),-MgO (M = Cr, Mo, W)* systems. The frequency of the broad absorption moves from 26000 to 23000 cm-l on passing from the low- to high-temperature reduced samples : this fact is probably associated with an increase in the particle dimensions, as already found for other ~ys tems.~-~

EFFECT OF co ADSORPTION AND DESORPTION ON THE REFLECTANCE SPECTRUM OF SAMPLES REDUCED AT 673 K

Upon dosing with CO ( p = 10 Torr; 1 Torr x 133 Nm-2), the broad absorption associated with small metal particles immediately disappears while an absorption increase is observed at 25000 cm-l (fig. 2). If the CO is left to stand in contact for several hours a further absorption increase in the 29000-1 8 000 cm-l range is observed which can be attributed to the growth of a component at 22000-23000 cm-l. This component is reversible: in fact upon outgassing at room temperature it tends to disappear with a parallel increase in the peak at 25000 cm-l (fig. 3). The process is totally reversible: in fact if CO is again allowed in contact with the sample, the peak

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

1894 ADSORPTION OF co ON NiO-MgO SOLID SOLUTIONS

4 3 2 1

wavenumber/ 1 O4 cm-'

Fig. 3. Effect of CO desorption at room temperature: (-) MN5 contacted with CO for 12 h; (---) and (-.---) after pumping CO for increasing time at room temperature.

at 25000 cm-l decreases in intensity while that at 22000 cm-l increases in a pro- portional way. The 25000 cm-' and 22000-23000 cm-l absorptions have already been assigned (see Part 1) to dinuclear Ni2(CO),L2 (L = 02-) and mononuclear [(CO)3NiC02]2- Mg2+ species, respectively. As a consequence it is inferred that a clustering-declustering process also occurs on reduced samples.

The disappearance of the broad absorption associated with metallic particles when CO is dosed at sufficient pressures ( p = 10 Torr), followed by the formation of the 25 000 cm-l peak (dinuclear species) and then by that at 22 000-23 000 cm-l (mono- nuclear species) is due to the following consecutive processes :

+co +co +CO Ni particles a Ni,(CO), L, Ni2(CO),L2 [(C0)3NiC02]2- Mg2+

-co -co -co (1 \ polynuclear (x > z) dinuclear mononuclear

(L = surface 02-), which can be reversed by lowering the CO pressure. Taken in one direction, the consecutive processes described in reaction (1) represent

the disaggregation pathway of a Ni particle under CO with the formation (as a final product) of adsorbed Ni(CO),. Taken in the opposite direction it represents the process of aggregation of adsorbed carbonylic species under decarbonylating conditions.

In previous paper^^-^ only the second aspect of the problem has been taken into consideration. Note that, while the passage from mononuclear to dinuclear species is achieved simply by lowering the CO pressure at room temperature (fig. 3), the aggregation into metal particles is obtained only at 373-473 K under a dynamic vacuum (fig. 4). In fact under these conditions all the CO is given off and the broad absorption associated with the metal particles is restored (vide infra).

Note that, although in both cases metal-metal bonds are present, the absorptions associated with metallic particles and the dimeric species (both centred at ca.

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

A. ZECCHINA, G. SPOTO, S. COLUCCIA AND E. GUGLIELMINOTTI 1895

4 3 2 1

wavenumber/ 1 O4 cm-'

Fig. 4. Effect of CO desorption at 373-473 K : (-) sample pumped off at room temperature, (....) after outgassing at 373 K for 10 min and (----) after outgassing at 473 K

for 10 min.

25000 cm-l but with definitely different widths) have an entirely different origin. In the first case the absorption is associated with a collective excitation (Mie-type surface plasmon) involving the overall free-electron population of the particle, while in the second case the absorption is monoelectronic in nature (a normal molecular-type transition between two discrete electronic levels).

During the clustering process from dimeric species to metallic particles, trimeric, tetrameric and polymeric species should be formed to some extent as intermediates. It should be interesting to investigate this problem experimentally with the aim of studying the gradual transformation of the molecular-type monoelectronic transitions characteristic of low-nuclearity aggregates into the collective (Mie-type) ones typical of particles having metallic character. This is a fundamental problem which has been already taken into consideration in specifically designed optical studies in cryogenic mat rice^.*-^ On the surface, we observe (fig. 4) that by outgassing at 373 K the band associated with the dimeric species decreases while that associated with the particles increases. Apparently this implies that aggregates of intermediate nuclearity are absent and that the dimeric species are directly transformed into metallic particles because of the high surface mobility at 373 K. As it will be shown in the following, this conclusion is only partially true, because positive indications of the presence of such intermediates will be obtained by means of i.r. spectroscopy (vide infra). Indeed a close observation of the spectrum reported in fig. 4 (b) indicates that the 25 000 cm-l band has an asymmetric shape on the low-frequency side which could be ascribed to PO*

transitions in low-nuclearity clusters.

EFFECT OF co ADSORPTION AND DESORPTION ON THE REFLECTANCE SPECTRUM OF SAMPLES REDUCED AT 773 K

After H, treatment at 773 K the Ni2+ ions (5-fold coordinated, square pyramidal) emerging on the common {loo} faces of the microcrystals have partially been reduced

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

1896 ADSORPTION OF CO ON NiO-MgO SOLID SOLUTIONS

wavenumber/ 1 O4 cm-’

Fig. 5. Effect of CO adsorption on the reflectance spectrum of the MN5 sample reduced at 773 K: (-) standard MN5 solid solution (reference spectrum), (---) MN5 after reduction at 773 K

and (-.---) after CO contact (p = 10 Torr) (spectrum taken afer 30 min).

to metallic nickel, with segregation of larger particles. CO contact on samples thus treated ( p = 10 Torr) causes the disappearance (with reduced speed) of the broad absorption at 25000cm-l (fig. 5) , with the appearance of the narrower transition associated with dimeric species. However, unlike the previous case, the tendency to form mononuclear [(CO),NiC0J2- Mg2+ species as a final product is greatly reduced. This fact is probably due to the enhanced tendency for the formation of aggregated (di- and poly-nuclear) structures when both the concentration of reduced nickel present on the surface and the reduction temperature are increased. This hypothesis will find support from the i.r. data (vide infra). Successive outgassing at 373-473 K restores the initial situation (result not shown for the sake of brevity), thus indicating that under more severe decarbonylating conditions the metallic particles are re-formed.

EFFECT OF CO ADSORPTION AND DESORPTION ON THE REFLECTANCE SPECTRUM OF MN5 SAMPLES REDUCED AT 973 K

When CO ( p = 10 Torr) is put into contact with samples reduced at 973 K (i.e. samples where nearly all the surface nickel ions have been reduced) no spectral change is initially observed (result not reported for sake of brevity). However, if CO is left in contact for several hours, the broad and strong absorption associated with the metallic particles shows some tendency to decrease in intensity with the usual formation of the narrower components associated with the dimeric species. The slowness of the process indicates that the largest nickel particles formed by reduction of the solid solution at high temperature show enhanced resistance to CO attack with respect to the smaller ones generated at lower reduction temperatures.

I.R. SPECTRA OF co ADSORBED ON REDUCED MN5 SAMPLES 673 K

identical to those illustrated in fig. 2-4 are illustrated in fig. 6-8. The i.r. spectra obtained on an MN5 sample reduced at 673 K in conditions

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

A. ZECCHINA, G. SPOTO, S. COLUCCIA AND E. GUGLIELMINOTTI 1897

8

\ ?.

I . I

’ 1603 1400 1 m xxx) 21co m 1933 wavenumber/cm -’

Fig. 6. Infrared spectrum of CO adsorbed on an MN5 sample recuced at 673 K: (---) immediately after CO contact (10 Torr) and (--) 12 h later.

Upon CO adsorption ( p = 10 Torr) the following features are observed (fig. 6). (i) The carbonylic complexes of reduced nickel (B, C and D) already observed on the unreduced solid solutions are readily formed. (ii) The bands due to surface carbonates are absent, because the system is already in a reduced state after the treatment in H,. (iii) The bands due to dinuclear and low-nuclearity complexes C and D are more intense in the initial stages and then decline. (iv) The mononuclear carboxylic complexes B reach their maximum concentration after prolonged exposure to CO.

These results definitely confirm that the mononuclear (B), dinuclear (C) and polynuclear (D) species involve reduced nickel because they arise in standard solid solutions through formation of surface carbonates or without the appearance of an oxidized counterpart on the reduced samples. If the sample reduced at 673 K is contacted with 0.5 Torr CO only (instead of 10 Torr as before), the interesting observation is made that the metallic particles are still disaggregated, but only the dinuclear (C) and polynuclear species (D) are formed as final product. This result (although not described in detail) must be kept in mind because the Ni particles formed by reduction at high temperatures (973 K) will show entirely different behaviour (uide infra).

The surface concentration of dinuclear species (C) and polynuclear (D) species can be enhanced at the expense of the mononuclear species (B) by lowering the CO pressure at room temperature (fig. 7) in exactly the same way found for standard solid solutions. The i.r. spectra are indeed very similar, the only differences lying in the slightly larger initial intensity of the dimeric (C) and polymeric (D) species and in the absence of the peak at 2068 cm-l (Ni2+-C0 adducts; species’A). It must also be recalled that spectra identical to those reported in fig. 6 and 7 can be obtained by directly dosing Ni(CO), on MgO outgassed at 1073 K.7

If the reflectance experiments do not provide adequate information about the existence of low-nuclearity intermediates in the clustering process, the same does not

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

1898 ADSORPTION OF co ON NiO-MgO SOLID SOLUTIONS

I

/- I 2xx) 2m xco 1900 -1 1603 M a 3 1x0 loo0

Fig. 7. Effect of CO desorption at room temperature: (-) as in fig. 6 and (---) and (- - - .-) after outgassing at room temperature for increasing times.

wavenum berlcm-'

hold for the infrared experiments. In fact it can be seen from fig. 8 that upon outgassing at 373 K, in addition to the peak at 1963 cm-l already assigned to low-nuclearity surface carbonyls of unknown structure (species D), another peak appears at 1936 cm-1 (species E) which was not present (or was very weak) on the samples only outgassed at room temperature.

As species E are observed under more drastic decarbonylation conditions and originate from the dimeric species C , we infer that their nuclearity is higher than that of species C and D so that they represent further intermediates of the clustering process.

773 K The i.r. spectra in the 2200-1800 cm-l region of CO (10 Torr) adsorbed on MN5

samples reduced at 773 K are not illustrated in detail because they are very similar to those found on incompletely diffused solid solutions (fig. 6 of Part 1). Unlike the previous case, the peaks of the dinuclear species (C) and polynuclear species (D) now have high relative intensity: this result indicates that the disaggregation process of the nickel particles does not proceed completely up to the mononuclear species B. This result is similar to that found on standard concentrated solid solutions.

A plausible explanation is as follows : monomeric species B [adsorbed Ni(CO),] are bonded to the most reactive Mg2+02- pairs of the surface (i.e. those on the edges and steps)? as a consequence their surface concentration is very limited. The strength of the interaction with the surface is sufficiently high as to prevent agglomeration and is consequently the true cause of their mononuclear nature. When the coverage of reduced nickel increases (as on the 773 K reduced system), the excess of nickel necessarily form metal-metal bonds (owing to the smaller interaction with the surface) and so dimeric and polymeric structures are favoured.

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

A. ZECCHINA, G. SPOTO, S. COLUCCIA AND E. GUGLIELMINOTTI 1899

wavenum berlcm-'

Fig. 8. Effect of CO desorption at 373473 K: (-) last spectrum of fig. 7, (....) after outgassing for 10 min at 373 K and (---) after outgassing for 10 min at 473 K.

Under constant CO pressure, the proportions of mononuclear, dinuclear and polynuclear structure are a function of the quantity of reduced nickel existing on the surface. This conclusion, together with that concerning the effect of the CO pressure, indicates that the aggregation state of the nickel on the MgO surface is a maximum for low CO pressures and high nickel loadings.

973 K After reduction at 973 K the amount of reduced nickel reaches an even larger value,

because nearly all the surface nickel ions have been reduced. As a consequence the formation of isolated mononuclear [(CO)3Ni(C02)]2- Mg2+ species is greatly un- favoured upon CO adsorption, because the number of reduced nickel atoms is much larger than the number of active Mg2+02- pairs: the presence of clusters containing metal-metal bonds is thus favoured.

The presence of metallic nickel particles on samples reduced at 973 K in H, and their reduced tendency to break up under CO and to form mononuclear isolated species is illustrated in fig. 9. In this figure curve 1 is the spectrum of adsorbed CO taken under 0.5 Torr CO after 1 h contact. The position of the bands and their broadness allows us to assign them immediately to CO adsorbed on polycrystalline nickel characterized by high heterogeneity of the dimensions and shapes. The spectrum does not change with time, thus indicating that under this CO pressure no disaggregation process takes place. This result is different with respect to previous observations obtained on samples reduced at lower temperatures. The explanation is as follows: on the last samples the Ni particles have much reduced dimensions, because the amount of reduced nickel is lower (while the available MgO surface is unchanged or higher) and the lower treatment temperature has prevented excessive sintering. As

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

1900 ADSORPTION OF CO ON NiO-MgO SOLID SOLUTIONS

C

B R

h a 2103 m 1900 waven urn b er /c m

Fig. 9. Infrared spectrum of CO adsorbed on an MN5 sample reduced at 973 K: (1 ) after contact with 0.5 Torr CO (spectrum taken after 12 h); (2H4) after successive contact with 10 Torr CO

(spectra taken after 15, 30 and 45 min).

a consequence we infer that the smallest particles are attacked by CO under a CO pressure which is inactive with respect to the largest particles.

If the CO pressure is increased to 10 Torr the disaggregation process starts slowly. Spectra 2-4 were taken at increasing contact times and show that after an initial increase in the broad absorption at 2100-1900 cm-l due to CO absorbed on nickel particles, the narrow peaks of the dimeric species (C) and polymeric species (D) gradually develop. After prolonged contact time the mononuclear species (B) are also observed. However, the proportion of C and D species is always much greater than that of B, in agreement with the high concentration of Ni at the surface.

The experiment illustrated in fig. 9 is also interesting because it allows a clear distinction between chemisorption and disaggregation phenomena.

Finally, it is worth mentioning that reduced solid solutions containing < 5 % nickel never produce (upon CO attack) significant amount of gaseous or physically adsorbed Ni(CO),. We interpret this fact in terms of the ability of the MgO surface to stabilize mononuclear species through the formation of [(CO)3Ni(C02)]2- Mg2+ complexes and polynuclear ones by insertion of surface ligands L (where L = 02-).

This work was carried out with financial support from the Consiglio Nazionale delle Ricerche, Progetto Chimica Fine e Secondaria.

A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 1984,80, 1875. M. H. Tikkanen, B. 0. Rossell and 0. Wiberg, Acta Chem. Scand., 1963, 17, 513.

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online

A. ZECCHINA, G. SPOTO, S. COLUCCIA AND E. GUGLIELMINOTTI 1901

M. H. Tikkanen, B. 0. Rossell and 0. Wiberg, Acta Chem. Scand., 1963, 17, 521. M. Moskovits and J. H. Hulse, J . Chem. Phys., 1977, 66, 3988. G. A. Ozin, Faraday Symp. Chem. Soc., 1980, 14, 7. G. A. Ozin, Catal. Rev. Sci. Eng., 1977, 16, 191.

Clusters, ed. J. Bourdon (Elsevier, New York, 1980), p. 165. E. Guglielminotti and A. Zecchina, J . Mol. Catal., submitted for publication. E. Guglielminotti and A. Zecchina, J . Chim. Phys., 1981, 78, 891.

’ E. Guglielminotti, A. Zecchina, F. Boccuzzi and E. Borello, in Growth and Properties of Metal

(PAPER 3/ 1562)

Publ

ishe

d on

01

Janu

ary

1984

. Dow

nloa

ded

by S

tate

Uni

vers

ity o

f N

ew Y

ork

at S

tony

Bro

ok o

n 22

/10/

2014

22:

25:0

9.

View Article Online