React. Kinet. Catal. Lett., Vol. 53, No. i, 183-189 (1994)

RKCL2503

O2-ADSORPTION AND (02-H2)-TITRATION ON ELECTRON

DEFICIENT PLATINUM IN REFORMING CATALYSTS

A.S. Belyi, D.I. Kiryanov, M.D. Smolikov. E.V. Zatolokina,

I.E. Udras and V.K. Duplyakin

Omsk Department of Boreskov Institute of Catalysis RAS,

644040 Omsk, Russia

Received February 8, 1994 Accepted March 28, 1994

Behavior of supported Pt in 02 chemisorption and

(O2-H2) titration has been studied. In Pt/Si02,

Pt/~-AI203 and Pt/7-AI203 Pt ~ catalysts are charac-

terized by the following stoichiometric coefficients

of oxygen adsorption (X), hydrogen adsorption (Y) and

oxygen-hydrogen titration (Z) X:Y:Z=I:I:I.5. The co-

efficients differ for Pt-CI/7-AI203 reforming cata-

lysts. This is explained by the presence of elec-

tron deficient platinum (Pt ~) with a coefficient

ratio of X:Y:Z=0.5:2:I.5.

We have found recently [1-3] that a fraction of platinum

surface atoms in reduced Pt/alumina catalysts is capable of

forming strong coordinate bonds with ~-donor water molecules

(Pt~). Such capability characterizes the central atom in nu-

merous complexes of Pt(II) and Pt(IV) and is not typical for

metallic platinum [4]. The same property was used to determine

the number of Pt a atoms in reduced catalysts [3-4]. The de-

crease of oxygen consumption due to water preadsorption served

as a criterion for Pt ~ content. This platinum resembles Pt(II)

in chlorides by its charge [4]. An attempt to investigate the

role of Pt ~ in chemisorption and titration on Pt/AI203 is

presented.

Akad4miai Kiad6, Budapest

BELYI et al.: REFORMING CATALYSTS

EXPERIMENTAL

Adsorption measurements were carried out in a glass set-

up according to [2]. The catalysts were reduced in hydrogen

(i0 kPa) at 823 K for 2-3 hours and evacuated at 773 K to 10 -4

kPa. Oxygen was adsorbed at 293 K and 02 pressure of 0.4 kPa.

Water effect on oxygen adsorption was investigated on freshly

reduced catalysts kept under water vapor at 239 K and PH20=0.1

kPa for an hour before oxygen introduction. Oxygen consumption

was expressed by the OCH20 value (at/at). Water effect was ex-

pressed by (OC-OH20)/OC=~OC/OC.

Oxygen tis was carried out under the same conditions,

after hydrogen adsorption at 423 K and i0 kPa [2].

Pt content varied from 0.03 wt.% (A-3) to 0.59 wt.% (B-l)

on SiO 2 (A-2), ~-AI203 (A-3) and y-Al203 (A-l, B, C and D

series). [Pt(CO) 2] n (sample A-l), Pt(NH3)4CI 2 (A-2) and

H2PtCI 6 (A-3, B, C and D series) were used as initial reagents.

Catalyst compositions and adsorption properties are given in

Tables 1 and 2.

The average Pt particle size in reduced catalysts was

checked by TEM (JEM-100CX, resolution 0.3 nm).

RESULTS AND DISCUSSION

Adsorption measurements (Table i) show that the catalysts

can be divided into three large groups (A, B and C) with re-

spect to the Pt state.

The first group (A) is characterized by the lack of ad-

sorbed water effect on oxygen adsorption (OC and OC H 0 values 2

coincide). Relative values of oxygen consumption durlng oxygen

titration (OT) and chemisorption (OC) are characterized by the

constancy of OT/OC ratio equal to 1.50.

The second group (B) is characterized by a weak water

effect on oxygen consumption (AOC/OC = 0.2-0.33). OT/OC ratios

are considerably higher in the range of 1.93-2.05.

The strongest water effect on chemisorption characterizes

the third group (AOC/OC changes from 0.4 to 0.61, OT/OC ranges

from 2.09 to 2.43).

184

BELYI et al. : REFORMING CATALYSTS

J,J 01

r-~

-IJ rd 0

.,--t 4a

,.q

0

0 -,-I .;J

O

0

,-~ 0 .Q -,-I t~ 4J

o

0 -;.-I 4J -~-I

0

0 0

t~ 0

O .El 0

r..) o

Et o

o m 0 .~1~

0 C

~ 0 0

4~

o

4J

0

,-4

,--4

-~-t ~J ,lJ

0

r.~

0

r~

O0 0 ~-I ' ~ L~ L~

0 0 0

0 0 0

0 0 0

~.~ r ~ ~D

0 0 E.)

0 ~-I 0% P'~ 0%

ix') O

r~

0 ~= cJ U 2: ~J

4J 4J (~

L.~

I I I I

0

~ 0

I I I

0 % ~ 0 0 0 %

0 0 0 0 0

~ 0 ~

0 0 0 0 0

d d d g d

0 0 0 0 0

~0 ,--4 t ~ 4~ = _- = =

C~

' ~ 0 0 0

~ l O I I I U

0 0

0 ~ ~ l O l l l

0 ~ 0 ~

O m l O O

0 0 0 0 0

~ 0 0

O O I O 0

0 0 0 0 0

d g g d d

~ ~ 0

g l O m I

� 9

O o o o o

~o ,-4 L) 4J = = = .-

O O l l O

0 0 0 0

o o ~ ~ o l ~ o ~ ~ ~

~ o u ~ •

I I I I I ! 1 1 1 1

185

BELYI et al.: REFORMING CATALYSTS

All the three groups (see Fig. i) follow similar rules.

OT/OC and OC are linear functions of water effect on oxygen ad-

sorption.

Electron deficient platinum (Pt ~) is known [3,4] to form

o-donor bonds with water molecules preadsorbed on reduced

Pt-CI/y-AI203 catalysts. Presence of such platinum in the cata-

lysts of series B and C can produce the peculiarities men-

tioned.

Oxygen chemisorption on platinum can be expressed by the

equation:

n Pt + OC = n Pt 0 s s x

where OC = xnPts, X is the stoichiometric coefficient of oxygen

adsorption, nPt is the fraction of surface platinum atoms. s

Thus, the oxygen titration equation can be represented

as:

n PtsHy + OT = n PtsO x + 0.5y H20

where OT=nPt s (x+0.5y), y is the stoichiometric coefficient of

hydrogen adsorption.

According to these equations, the relative value of OT/OC

consumption, measured on a particular catalyst, is a function

of the stoichiometric coefficients:

OT/OC = 1 + 0.5 y/x (i)

and does not depend on the platinum dispersity (fraction of

surface atoms).

Experimental dependence (Fig. i) OT/OC = f(AOC/OC) is

well described by the equation:

OT/OC = 1.5 (I + AOC/OC) (2)

The essence of this dependence is that OT/OC, which is

the derivative of oxygen and hydrogen stoichiometric coeffi-

cients is defined by the content of electron deficient plati-

186

BELY et al~ REFORMING CATALYSTS

hum on the oatalysts surface (~OC/OC).

When the active sites are represented by metallic plati-

num only (AOC/OC=0), we have OT/OC = 1.5. Inserting this value

into eq. i, we obtain x=y. Thus, O/Pt ~ = H/Pt ~ = I on metallic

Pt atoms.

When the active sites are represented by Pt ~ only, eq. 2

rearranges to OT/OC = 3. Thus, we obtain 4x = y from eq. i.

Regarding the literature, we have H/Pt ~ = 2, O/Pt ~ = 0.5.

3.0

2.5

2.0

1.0 "b<Z ,m S

%% J %%~,

~OClOC

1.5 0.5

, I , I I I ' ~ I B ' 0 0.2 0./. O.B 1.0

Fig. i. Adsorption properties of platinum versus

the influence of water on oxygen chemisorp-

tion

Experiments (see Fig. i) show that two platinum forms,

Pt ~ and Pt O (their ratio defines OT and OC values),are present

in the catalysts at O <OC/OC <i. If we take these values with-

out reference to the difference of Pt states we inevitably ob-

tain fraction "x" ranging from 0.5-1.0 and "y" ranging from

1.0-2.0. One often sees such conclusions in the literature.

Several important conclusions can be drawn:

i. We can use neither oxygen nor hydrogen chemisorption

to determine quantitatively the number of platinum surface

atoms in the catalysts with various Pt states. We can use them

only if all atoms are Pt ~ or Pt ~

2. O2-H 2 titration appears to be more useful for the pur-

187

BELYI et al.: REFORMING CATALYSTS

pose. It is easy to show that the differences in stoichiometrie

coefficients of oxygen and hydrogen on platinum Pt ~ and Pt ~

atoms predetermine the equality of stoichiometric coefficients

of the titration reaction which can be represented on different

atoms by the equations:

Pt~ + 1.5 0 = Pt~ + 0.5 H20

Pt~ 2 + 1.5 0 = Pt~ 5 + H20

Hence, oxygen-hydrogen titration does not depend on the

platinum state and can be used to obtain the number of sur-

face atoms (Pts). We have substantiated this conclusion with

TEM studies of D series (Table 2). The average Pt particle

size derived from the titration data agrees well with TEM

values under the assumption that Pt particles are spherical.

Table 2

Results of catalysts investigation

Catalyst Pt Support OT Disper- Average particle (wt.%) (at/at) sity size (nm)

(%) chem. TEM

D-I i. 0 y-A1203 I. 18 78 I. 3 i. 5

D-2 1.2 " 1.05 70 1.5 1.5

D-3 1.7 " 0.98 66 1.6 1.5

D-4 2.8 " i. 02 68 i. 5 I. 5

3. The difference in stoichiometry of hydrogen and oxygen

adsorption on Pt ~ and Pt ~ atoms allows to determine them quan-

titatively from OT and OC values. Adsorption measurements in

water vapor are not necessary in this case.

We can use adsorption properties of metallic and electron

deficient platinum to distinguish them quantitatively in sup-

ported catalysts and to study their other chemical and physical

188

BELYI et al.: REFORMING CATALYSTS

properties.

Acknowledgement. This work was carried out with the financial

support of the Russian Fund of Fundamental Investigations.

REFERENCES

i. A.S. Belyi, M.D. Smolikov, V.B. Fenelonov, V.Yu. Gavrilov,

V.K. Duplyakin: Kinet. Katal., 27, 1414 (1986).

2. V.K. Duplyakin, A.S. Belyi, N.M. Ostrovskii, M.D. Smolikov,

E.M. Chalganov, A.I. Nizovskii: Dokl. Akad. Nauk SSSR, Ser.

Khim., 305, 648 (1989).

3. M.D. Smolikov, A.S. Belyi, A.I. Nizovskii, I.E. Smirnova,

A.S. Semikolenov, V.K. Duplyakin: React. Kinet. Catal.

Lett., 3/7, 437 (1988).

4. E.L. Muetterties, R.M. Wexler: Survey of Progress in

Chemistry, p. 62. Elsevier, New York 1983.

189