Hydrogen adsorption on ruthenium

Reaction Kinetics and Catalysis Letters, Vol. 2, No.. 1-2, 117-122 (1975)

H Y D R O G E N A D S O R P T I O N ON R U T H E N I U M

C. D. ZAKUMBAEVA and L. B. SHAPOVALOVA

Institute of Organic Catalysis and Electrochemistr3~, Academy of Sciences, �9 Kazakh SSR, Alma-Ata, USSR

Received March 15, 1974

Th~ adsorption of hydrogen on a ruthenized platinum electrode has been

studied. On the potontiodynamic curve two peaks are observed, corre-

sponding to weakly and strongly bonded hydrogen. The adsorption of the

two forms of hydrogen increases wlta increasing temperature. Hydrogen

adsorption on ruthenium obeys the ~umkin isotherm for a uniformly hete-

rogeneous surface. The differential'heats of adsorption at constant volume

have been calculated. The heat of adsorption decreases gradually from

10 kcal /mol ( 0 = 0.05) to 3 .2 kcal/mol ( 0 = 0.7).

B p a ~ o T e n p o s o ~ M n O C b u 3 y q e H H e a n c o ~ O ~ H M s o n o p o n a Ha pyTeHHpOB~H--

HOM n~aTHHOBOM 3HeKTDOne. H~ I, ~ "KpHBOR pyTeHH~ Ha0~na~TCH ~Ba

~KCHMyMa, COOTBeTCTByI0t~He c~a0o- H Rpe~OCBR3aHHOMy Bo~opo~y.

C ySe~H~eHHeM TeMnepaTyp~ Bo3pacTaeT a~cop01~H~ o0eHx ~OpM BO~O ~

pona. TerIJ1oTa ancop511HH BOnOpona paBHOMepHo yMeHb~aeTcR OT IO

R.a~/MOn5 (O H = 0,05) do 3,2 .Ra~/~o~b (O H - 0,7).

In contrast to platinum, relatively few investigations of the electrochemical

and adsorptive properties of ruthenium have been m a d e / 1 - 5 / , probably owing

to the tendency of ruthenium towards rapid oxidation and dissolution in electro-

lyte solutions. This is why the position of ruthenium in the series of metal ca-

talysts, for hy~ogermtion, dehydrogenation and hydrogenolysis has not yet been

determined.

In the present work, the adsorption of hydrogen has been studied on a

ruthenized platinum electrode at 20-60 ~ in i N sulfuric acid under helium,

11"]

ZA KUMBA EVA, SHAPOVA LOVA : HYDROGEN ADS ORPTION

Potentiodynamic I, @-curves were recorded with a P-5827 and a KCP-4 point

recorder, at a potential scan rate of 8.8 mV/sec, in the potential ranges of

0 .050-0.25 and 0 .028-0.25 V. Under these conditions the Ru-H system proved

to be practically reversible. At more anodic potentials, owing to early adsorp-

tion of oxygen, the electrode is reduced considerably more slowly. After every

cycle, the electrode was subjected to prolonged cathodic polarization until

complete surface reduction

The ruthenized electrode catalyst was prepared as described ea r l i e r /6 / .

Ruthenium was precipitated on a platinum plate (1 x 1 cm) from a 1% solution

of ruthenium nitrosochloride. The precipitation of ruthenium is continued for

2 hrs at 45 ~ then for additional 12 hrs at room temperature. The pretreat-

merit of the electrode included prolonged cathodic polarization and washing in

air with doubly distilled water. 1 N sulfuric acid was prepared from a standard

solution and doubly distilled water and was subsequently suf~jected to additional

purification for 2.5 hrs under helium, on a large platinized platinum gauze,

at a potential of +0.5 V (vs.nhe).

There are two characteristic peaks in the hydrogen region of the I, ~-

curves for the ruthenium electrode catalyst (Fig. 1). The first peak corresponds

to weakly, the second to strongly bonded hydrogen. The amounts of weakly and

strongly bonded hydrogen increase with the temperature. Under these conditions,

the extent of hydrogen adsorption on ruthenium in sulfuric acid increases by

6-7%. The region of hydrogen adsorption on ruthenium ends at 0.2 V. Owing

to early oxidation of the ruthenium surface, the boundary between the regions

of hydrogen and oxygen adsorption is indistinct. It should be noted that, with

increasing temperature, the second peak on the I, ,p-curve, signifying the

118

ZAKUMBAEVA, SHAPOVALOVA: HYDROGEN ADSORPTION

Fig. 1.

100

8.0

<I

E 7 6 . 0

4.0

2.0

~ 5

1 - 2 0 ~

/ / 2 2-30~ 1 3-40~

4-50oC 5-600C

0 0.025 0.'25 4 cv)

Potentiodynamic I, ~-curves for hydrogen adsorption on Ru/Pt in 1 N

I-X2SO4 solution ($r = 0. 025 V)

amount of hydrogen which is most strongly bonded to ruthenium, shifts to more

negat ive potentials.

The amount of adsorbed hydrogen at various surface coverages ( 0 ) was

determined by graphical integration of the I, C-curves. The calculations are

based on the assumption that O H is 1 at a reversible hydrogen potential. The

equilibrium pressure of molecular hydrogen was determined from the Nernst

equation,

obtained,

(Fig. 2).

taking into account the electrolyte 's vapour pressure. From the data

the isotherms of hydrogen adsorption on ruthenium were calculated

The l inearity of the curves at medium coverages shows that this re -

119


0.8

0

O4

(12

i

8 6 4 2 0 -~g PH2

Fig. 2. Isotherms for hydrogen adsorption on a Ru/Pt electrode in 1 N H,2SO 4

(r = 0.025 V)

gion can be described by a logarithmic isotherm. The surface coverage of ru-

thenium decreases with increasing temperature at a given hydrogen pressure.

From the slope of the hydrogen adsorption isotherm, the roughness factor o f the

ruthenium surface is f=2 .88 , this is considerably lower than in the case of

platinum, f= 14. The isosteres for hydrogen adsorption on ruthenium are liliear

(Fig. 3) for all surface coverages, thus allowing the calculation of the differen-

tial heats of hydrogen adsorption at constant volume. The heat of hydrogen

adsorption decreases gradually from 10 kcal/mol (OH= 0.05) to 3.2 kcal/mol

( e H = 0. 7). Extrapolation of the q vs. 0 curve shows that the heat of hydrogen

adsorption at O H = 0 is 11 kcal/mol (Fig. 4). The same figure shows also the

heats of adsorption calculated from the I, r -curves recorded at 0. 025 and 0.05 V

with subsequent determination of the total amount of adsorbed hydrogen by extra-

polation of the er - OH curve. The calculated isosteric heats of adsorption,

determined by two different methods, are in fair agreement. Thus with in-

creasing coverage the heat of hydrogen adsorption decreases from 11 to 3.3

120

ZAKUMBAEVA, SHAPOVALOVA: HYDROGEN AI~ORPTION

Fig. 3.

Fig. 4.

~ 4

2

8=01

. " o, _ , g -0.5 '~' J ~ ~ ~ g =06

I i I i "I 3.4 3d 3.0

( I / T ) 103

lsosteres fox hydrogen adsorpfim on a Ru/Pt electrode in 1 N I-I2S0 4

(r = o. 025 v)

8

~ 4

bL_ t-,f= 0.025V

o4 o'8 9

Differential heats of hydrogen adsorption at constant volume on a Ru/l~

e l e c = o d e in 1 N H,2SO 4 (~'r = 0.0'25 V)

kcal/mol, L e , , the difference between the heats of hydrogen adsorption at

O H = 0 and at 0 H = 0,? is ? .8 kcal/mol. The value q0=0 - q0=0 .? is almost

two times smaller on ruthenium than on platinum. This implies a highly homo-

geneous ruthenium surface.

The literature data on the heats of hydrogen adsorption are Conflicting. In

Ref, / ? / it has been shown that with increasing coverage the heat of hydrogen

adsorption on ruthenium in sulfuric acid decreases from 15. ? to 0.0 kcal /moL

121


For Ru/SiO, 2 in the gas phase, the heat of hydrogen adsorption is ii kcal/mol/8/.

Our results on the heat of hydrogen adsorption are in good agreement with the

data reported in Ref. /9 / .

Thus hydrogen is adsorbed on ruthenium in two forms, one is probably po-

larized molecular hydrogen desorbing from ruthenium at potentials between

0 .0 -0 .07 V, whereas the second form is atomic hydrogen with a desorption

maximum at 0.12 V. Using a potentiodynamic method, we have first detected

the existence of two adsorbed forms of hydrogen on ruthenium.

,It

The authors are indebted to Professor Dr. D. V. Sokolskii, Member of the

Academy of Sciences of the Kazakh SStL for his interest in this work.

REFERENCES

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Zh. Obshch. Khim., 9, 944 (1939).

4. L L Kavtoradze: Zh. Obshch. Khim. 32.__, 1214 (1959).

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(1962); 2_ 56 (1965), ~_, 61 (1965).

7. Yu. A. Podvyazkin: Kinetika i Kataliz, 11__, 1077 (1970).

8. F. B. Goneym: Thesis, MGU, Moscow 1963.

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London 1962.

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Hydrogen adsorption on ruthenium