IR spectra of hydrogen absorbed on solids

363

JournalofMolecularStructure,218 (1990)363-368 Elsevier

Science

Publishers

IR SPECTRA

B.V., Amsterdam

OF HYDROGEN

-Printed

ADSORBED

in The Netherlands

ON SOLIDS

G. BUSCA lstituto di Chimica, Facolta di Ingegneria, P.le Kennedy, 16129 GENOVA (Italy)

Universita

di Genova

SUMMARY. The IR spectra of the adsorbed species arising from adsorption of hydrogen and deuterium on different solids (ZnO, Cr,O,, CoCr,C,, Zn-Cr mixed oxide catalysts and CuZnO-ZnCr,O, methanol synthesis catalysts)are discussed in relation to those of metalhydride complexes. Linear, I*,-bridging and multiply bridging species are formed on the different surfaces. INTRODUCTION Hydrogen reaction

adsorbs on several solids and consequently

with several

kinds of substrates

pounds, carbon monoxide belong to very different

and dioxide,

structures

it may become activated toward

(unsaturated

hydrocarbons,

etc.). Solid catalysts

carbonylic

for hydrogenation

com-

reactions

(1) such as: i) pure metals and alloys (e.g. Pt and its

alloys); ii) pure and mixed oxides (e.g. Zn and Cr pure and mixed oxides); iii) metal / metal oxide complex solids (e.g. Cu /metal oxide based catalysts); iv) metal sulphides (e.g. MoS,). As a part of our investigations hydrogenation adsorbed

hydrogen

technique.

concerning

the mechanisms

of heterogeneously-catalyzed

reactions, we have attempted to detect and characterize on some active catalysts,

We summarize

spectroscopically

using the FT-IR transmission/absorption

here some of our results and we attempt

to draw some

conclusions. EXPERIMENTAL The IR spectra have been recorded by a Nicolet 5ZDX FT-IR spectrometer. powders

were pressed into self-supporting

heating under evacuation othercatalysts),

disks and activated

(ZnO) and by heating in hydrogen

before adsorption

experiments.

The catalyst

in the IR cell by simple

followed

by evacuation

(all

ZnO was Kadox 15 (New Jersey Zinc Co.),

Cr,O, was from Degussa (Hanau, West Germany),

while all other catalysts were prepared

by coprecipitation. RESULTS AND DISCUSSION a) jadsorDtionOaoxides: When ZnO activated

by evacuation

at r.t. a very sharp band immediately

002%2860/90/$03.50

at 630 K is put into contact with H, gas (100 Torr)

appears, centered at 1709 cm-’ (Fig. 1 ,a). This band,

0 1990Elsevier

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364

first observed by Eischens, Pliskin and Low (2), isassigned zinc hydride species. It is associated is admitted,

tothe Zn-H stretching ofterminal

to a new vOH band, at 3490 cm-‘. If D, instead of H,

similarly shaped bands appear near 1230 cm-’ (vZnD) and 2580 cm-’ (vOD).

This justifies the hypothesis

that an heterolytic

dissociation

of the H, molecule

occurs on

active Zn-0 sites at the surface of this oxide giving ZnH and OH groups. Note that the vOH band formed by hydrogen

dissociation

does not correspond

to those of residual surface

hydroxy- groups arising from water dissociation. If a sample stoichiometric

of

a-Cr,O,

(previously

reduced

to destroy

excess

oxygen

giving

a

sample) is put into contact with H, at r.t. a split sharp band is formed at 1714,

1697 cm-l (Fig. 1 ,b). Also in this case if D, is adsorbed instead of H, a similar band is formed but displaced

at lower frequencies

of a factor v(H)/v(D)

confirms they are due to metal-hydrogen vOD bands also apparently deuteroxy

(-deuterium)

grow but they correspond

of 1.38 (1240, 1229 cm-‘). This stretchings.

groups residual from (heavy) water dissociative

2000

1600

1600

wavenumbers

1400 cm

In this case vOH and

to those due to surface hydroxy and adsorption.

1200 _f

Fig. 1. FT-IR spectra of adsorbed species arsising from H, adsorption at r.t. on ZnO (a), Cr,O, (b) and a Zn-Cr mixed oxide activated at 790 K (c) and 640 K (d).

b) Hvdroaen

adsorotion

on mixed oxides: CoCr&J

and Zn-Cr oxides.

In Fig. 2,A the spectra of a pressed disk of pure CoCr,O, are shown after activation

(a)

and after contact with H, (b) and D, (c) gases. Sharp bands at 1762 and 1660 cm-’ and a broader one near 1605 cm-’ are observed

after contact with H,. They are all sensitive to

isotopic labeling, being detected, with the same shape and relative intensity, at 1276, 1200 and 1 160 cm.’ when D, is adsorbed (see in Fig. 2,B the difference spectra). A fourth intense band is observed just below 1000 cm-’ (990 cm-‘) only after H, adsorption,

being very likely

shifted below the cut-off limit of our sample (abot 800 cm-‘) upon D, adsorption,

while also

365

in this case a vOD band (2694 cm-‘) superimposed grows upon D, adsorption. hydrogen

adsorption

to those of “normal” deuteroxy

groups

The observed bands are similar to those reported previously for

on a Co-Cr mixed oxide catalyst having an higher cobalt content (3).

I

r Ib

L

cl

I n II IL\ I : I \ ,, 1:.....

3000

2200

600

1400

1800

1'

t

1400

‘I

wavenumbers Fig. 2 - A: FT-IR spectra of CoCr,O, after activation (b) and D, (c); B: ratios b/a (b) and c/a (c). Contrary

to the Co compound,

detectably

hydrogen

cointaining

anexcessofzn

causes the formation

the stoichiometric

after the same preteatment

spine1 ZnCr,O,

(4). However,

at r.t. of H,

does not adsorb

if a sample

is prepared

with respect to the 1:2Zn:Crstoichiometry,contactwith

of IR bands assignable

to Zn-H stretchings.

strong (Fig. 1,d) and shows a main maximum cm-’ aftercontact

than on ZnO. Moreover,

structure (4)) this band is very

having a half-heigth

with the above

width of about 60

of this sample is carried out in stronger band another

observed at 1450-l 390 cm-’ (1065-l 025cm-’ for D, adsorption)

broad absorption

out. This band is also observed on ZnO if appropriate c) Hvdroaen

adsorotion

Zn-H-Zn stretching

on metal/metal

pretreatments

is carried

are carried out and has

of bridging hydrides

oxide catalvsts:

is also

This band is very likely due

to species formed on sites where hydroxy groups are bonded if a milder activation been assigned to the asymmetric

near 1815

with DJ. This band is placed at much higher frequencies

it is also much broader,

(Fig. 1 ,c), together

H,again

In a sample containing

at 1788 cm-‘, with a shoulder

cm-‘, with respect to 15 cm-’ for ZnO. If activation conditions

cm

-4

at 790 K (a) and adsorption

Zn and Cr in a ratio of 1 :I (that shows to XRD a spinel-type cm-l (1315,1295

600

iooo

Cu-Zn-Cr

(5).

methanol

svnthesis

catalvsts. Hydrogen

adsorption

and non-stoichiometric

similar to those observed adsorption

on reduced

ZnCr,O,

phases,

Cu-Zn-Cr

catalysts (consisting of copper, ZnO

from XRD analysis)

does not produce

on ZnO, Cr,O, and Zn-Cr mixed oxides.

is carried out near 520 K a very broad band is observed

bands

On the contrary

in the region 1100-900

if

366

2200

f400

600

wavenumbers

cm-'

Fig. 3 - FT-IR spectra of a Cu-Zn-Cr methanol synthesis catalyst after activation at 590 K (a) and adsorption of hydrogen at 520 K (b). (c) ratio b/a in absorbance scale. The “continuous” absorption in all the IR range and the decrease of the sharp skeletal bands near 1000 and 920 cm-’ (negative bands in the ratio c) are due to electronic phenomena (Ref. 6). cm-‘. This is evident in the ratioed spectrum, Fig. 3,c, for a catalyst containing

25 % Cu, 50

% Zn and 25 % Cr (atom percent), that is very active in methanol

The result is

similar to that already reported for a less active catalyst containing This band is not observed

after D, adsorption,

that this fenomenon

much less copper (6).

being shifted below the cut-off limit of the

sample (near 850 cm-‘). The conditions underwhich known observation

synthesis.

we detect H, adsorption

is activated

on Cu-containing

agree with the

catalysts

(6).

DISCUSSION The contact of activated catalyst surfaces with hydrogen or deuterium the appearance

gases results in

of IR bands due to adsorbed species. Some of them are placed in the typical

regions of OH and OD stretching bands. Other are instead clearly related to other species, i.e. surface hydride species. The isotopic confirms these assignments. by comparison

shift when deuterium is used instead of hydrogen

The nature of the surface hydride species may be investigated

of their spectroscopic

features with those of metal-hydride

complexes

(7,8)

and of the species arising from hydrogen adsorption on metals, discussed by Sheppard and co-workers

(9,lO).

Sharp and relatively observed

strong vMH bands in the region 2200-1500

in the spectra of terminal

ted to a deformation

metal-hydride

complexes.

cm.’ are typically

They are generally

band placed in the 1000-600 cm-’ region (7,8). p,-bridging

are instead responsible

for an asymmetric

stretching

associa-

hydrides (10)

band in the region 1700-l 000 cm-‘,

367

rather broad and weak at r.t., and for a band in the region 1300-800 described asa symmetricstretching

mode. Thefrequencydifference

depends from the M-H-M angle. Athird deformation u,-bridging

hydride species (9) are responsible

two asymmetricstretching bridging

species,

with deformation

between these modes

band is expected at lower frequencies.

in principle for one symmetric

bands falling at lower frequencies

together

cm-’ that can be

modes

and one or

(1200-600 cm-‘) than those of

at even

lower

frequencies.

The

stretching modes are also weak and broad at r.t.. Hydride species multiply bridged or placed in cavities (such as those of intesrtitial

hydrides) are responsible

for broad bands generally

below 1000 cm-’ (11). On these bases we may assign the sharp bands observed (1714,1697

cm-‘), CoCr,O,

cm -I) to MH stretchings CoCr,O,

(1605

cm-‘)

of terminal

also the corresponding

stretching

symmetric

catalysts

(1460-1390

mode of up-bridging

mode

hydrides.

is likely observed

(1815, 1780

bands observed cm-‘) are

on

instead

In the former

case

at 990 cm.‘. The very broad

on the Cu-Zn-Cr methanol synthesis catalyst must instead be assigned

a stretching

mode of multiply bridging hydride species.

deformation

modes of H-bonded

to intense and

in our spectra we do not observe the corresponding

bands. This rules out the possible assignment

Hydride species have been observed

to

Note that in this region also the

MOH groups fall. They also correspond

very broad bands (12). However, stretching

on ZnO (1709 cm-‘), Cr,O,

oxide

hydride species. The broader

and on the Zn-Cr oxide catalyst

assigned to the asymmetric band detected

(1760,166O cm~‘) and Zn-Cr

vOH

of this band to 6MOH modes.

after hydrogen adsorption

on a number of other

oxide materials such as MgO and other alkali metal oxides (13), ThO,, (14), ZrO, (15) MnCr oxide (3), MgO-ZnO (16) and MgO-Co0 terminal

hydrides have been found. On ZrO,and have been detected. species

metal-oxide

dissociation,

on oxide

In the last two cases only

while in the first two cases only bridging

on Mn-Cr oxide (Mn:Cr atom ratio 1:l) both species

As it has been discussed

from hydrogen

heterolytic

surfaces

elsewhere

(3) the formation

may be explained

ionic pairs; and ii) homolytic dissociation,

of hydride

by two mechanisms:

whose driving force is probably the acid-base

reduction of a n-type semiconducting electrons

(17) solid solutions.

hydride species have been observed,

very probably favoured

material with the consequent

i)

strength of surface

availability

by the sligth of quasi-free

near the surface.

Our results on Zn-Cr and Co-Cr oxides seem to indicate that bridging hydrides may be formed together

with terminal

ones on the same surface if couples of sites are available

nearly. This is shown on Zn-Cr oxides by the observation make free sites for bridging hydrides that are “poisoned”

that higher evacuation if activation

shown on Co-Cr oxides through the observation

that the relative

bridging hydrides with respect to that of terminal

ones is stronger on catalysts

intensity of the band of

higher Co content (Co:Cr ratio 1:l with respect to 1:2). The formation hydrides on oxide surfaces is unlikely for obvious geometric The formation interstitial

having an

of multiply bridging

reasons.

of multiply bridging hydrides is instead usual on metals (9-l I), where

metal atoms are nearer each other, and would lead in some cases to hydrogen producing

treatments

is milder. This is also

hydrides (18,19). In the case of metal-metal

absorption

oxide complex

solids a

368

synergistic

effect may be envisaged

between the two phases in hydrogen

adsorption.

As

an exemple, in the case of pure copper metal it has been reported that hydrogen is adsorbed dissociatively

at relatively

high temperature

on low-index

faces. To have fast hydrogen

adsorption atomic hydrogen must be used (20,21). The co-presence to dissociate

hydrogen could favour hydrogen

H,dissociation

on it. Alternatively,

adsorption

of an oxide phase able

on Cu because of the previous

the activating effect of some oxides in the hydrogenating

ability of copper may be related to the ability of these supports to favour the exposition high-index

of

copper faces, or of Cu(l) surface sites.

ACKNOWLEDGEMENTS The authors acknowledges Prof. A. Vaccari (University of Bologna, Italy) for the prepration of several catalysts and F. Guzzo and G. Oliveri for the technical collaboration.

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