Catalysis and photoelectrochemistry on ruthenium disulphide surfaces

Materials

14 (1986)

Chemistr.v and Physics.

REVIEW

113

113 - 12 1

PAPER

CATALYSIS

AND PHOTOELECTROCHEMISTRY

ON RUTHENIUM

DISULPHIDE

SURFACES

A.J. McEVOY Ecole Polytechnique

Institut de Chimie Physique, 1015 Lausanne (Switzerland)

F&&rale

de Lausanne,

ABSTRACT

The effective catalytic behaviour of ruthenium disulphide for the hydrodesulphurisation (HDS) of organic sulphur compounds has been established. As a semiconductor electrode in a photoelectrochemical cell, the n-type material functions as a stable photoanode for oxygen evolution. As an electrocatalyst on cadmium sulphide colloids, it promotes the photochemical dissociation of H2S, and the associated evolution of hydrogen is then observed at a high rate. The present work reviews current knowledge of the catalytic and electrocatalytic behaviour of this compound.

INTRODUCTION The demands made on the catalysts stantly more severe as the industry taining

significant

amounts

current use are based kel-promoted

of sulphur

on transition

molybdeneum

in particular,

of effective

technologies

catalysts:

hydrogen,

and photoelectrochemical

catalysis

minimises

sulphides,

energy

such as Co2NiSk,

for the production trochemistry,

0254-0584/86/$3.50

supported

promoted

kinetics

(HDS)

[I,;?].

cells

for the desired

of

In electrolysis,

effects.

as cathode

electrolysis

A recent

for the production

of solar energy.

have been investigated in alkaline

RuS2

in

and nic-

for their future on the

systems

losses due to overpotential

of hydrogen

catalytically

are dependent

electrolytic conversion

cobalt-

on alumina.

disulphide,

con-

con-

The catalysts

typically

of hydrodesulphurisation

by the use of ruthenium

Two other energy-related development

sulphides

have become

'sour' crudes,

compounds.

sulphides,

for improvement

refining

increasingly

and nitrogen

metal

or tungsten

study has shown the potential catalysts,

used in petroleum processes

Transition

metal

electrocatalysts [3]. In photoelecreaction,

e.g.

0 Elsevier Sequoia/Printed in The Netherlands

114 water splitting population

otherwise

undesirable provides

to evolve hydrogen

competing

tensively

investigated

heterogenous

reduction

gies, in addition

processes,

and prospective

to the theoretical

there is at present

significant

interest

motivation

dioxide

[4] it has been ex-

colloidal

particles

in

that the ox-

[5-81. The investigation

of the

as an alternative

was therefore

applications

for

as a film on semiconduc-

itself a semiconductor,

for anodic charge transfer

With these present

interface

there is now evidence

processes

disulphide,

carrier

Since ruthenium

surface

with semiconductor

systems. However,

cathodic

of ruthenium

electrode

either deposited

or associated

photochemical

ide also promotes properties

such as photocorrosion.

oxygen evolution

as a catalyst,

surfaces,

deplete the minority

at the semiconductor/electrolyte

reactions

a very efficient

tor electrode

catalyst

available

and oxygen,

initiated

in energy-related

in clarifying

[g].

technolo-

catalytic

mechanisms

for the study of the ruthenium

di-

chalcogenides.

PHYSICAL

PROPERTIES

It has long been known that ruthenium structure

disulphide

adopts the pyrite,

[lo-121. A somewhat more recent confirmation

measurement

of the lattice parameters,

rite structure

is basically

a unit cell of edge length

C-2 type

of the structure,

was that of Sutarno

with

et al. [131. The py-

cubic (space group Pa 3) with four formula units to 5.61 8, located

Ru:

(4)

000; 0 ; $; 2 0 ;; ; ; 0.

s :

(SC)

+(uuu;

as follows:

;+u, s-u, ii; ;, ;+u, a-u; i-u, U, ;+u)

where u = 0.39 The sulphur

atoms are therefore

interpenetrating

that of the ruthenium

the pyrite structure, For ruthenium magnetic

in pairs in a F.C. cubic sublattice

atoms. By analogy with the prototype

FeS2, an oxidation

state of +2 is assigned

(d7 s') this implies a d6 ion in the crystal

properties

and associated

each filled by two electrons splitting

placed

low-spin

with opposed

from the upper, empty, d-levels

of

to the cation.

, and given its dia-

state, three low energy d-levels

spins separated

due to crystal

field

[lb]. The energy gap is therefore,

de-

fined by d-levels. The diamagnetic a preparation as prepared,

behaviour

of RuS 2, just noted,

reaction has gone to completion is normally

n-type,

ved in certain iron-ruthenium

is a convenient

[15]. The semiconducting

116-181 alth ough p-type behaviour

pyrite

width of the band gap is presently

indicator

solid solutions,

FexRul_xS2

in dispute. Originally

that

crystal,

may be obser[16]. The

a figure of 1.8 ev was

115 accepted, [lg].

based

However,

on diffuse

eV, the experimental crystals.

data being

From the relationship

from the reflectance (hv = Eg + ka2), However,

optical

reflection

derived

from reflectance

between

the absorption

data) and the photon

b) data extend only to photon

energies

below

is also inconclusive.

on this effect using

on the basis

root of quantum theless

1.8 eV is remarkably

sintered

a significant

response

[20] and was interpreted

defects

showing

of energy below

using

in the material.

with the presence

for Eg therefore

the

gap of 1.8

of some 1% of the ruthenium

a significant

of the transition

sin-

However,

followed by a higher

photoresponse

to infrared

[16,211 perhaps though an elevation

tion (hv = 1.4 eV) was obtained lence band edge associated

the square edge. None-

in terms of an energy gap of 1.3 eV

cross section,

eV. It is further noted that by substitution by iron, a semiconductor

deduced

in terms of donor levels close to the

be interpreted

with a low transition

vely by a modification

between

at the red end of the spectrum was observed

could alternatively

a

at single

this gap being

to photons

ob-

[l6,1.7]at single crystal surfaces. An even

persistent

effect

value

and they suggested

from the same laboratory,

energy near the absorption

with structural

finitive

spectra at semiconductor-elecet al. [9] made the initial

linear relationship

Fermi level and associated

(indirect)

edge

region near the previ-

RuS2 electrodes,

in later work

and the photon

spectra,

photoresponse

tered electrodes

(calculated

on.

Guittard

of an apparently

efficiency

in published

stronger

on single

:

[17] the value of 1.85 eV is accepted,

electrodes

as indirect

coefficient

and the spectral

for the energy gap from photoresponse

value of 1.3-1.5 eV. However, crystal

measurements

1.6 eV. That absorption may there-

value of 1.8 eV is not reported

interfaces

servations

Sat@eS

at a figure of 1.3

are not reported;

fore be other than the band gap effect,

Evidence

POW&red

energy close to the absorption

apply to these conclusions

a) the absolute absorption coefficients

trolyte

on

it was deduced that the band gap is indirect.

two reservations

ously reported

measurements

et al. [la] arrives

the recent work of Bichsel

of iron d electrons,

crosssection

awaits further

radia-

of the va-

or alternati-

for the indirect

experimental

sites

evidence,

gap. A dealthough

the lower value now appears more probable.

MATERIAL

PREPARATION

There is a direct

synthesis

route at high temperature

17,18,22,23] or by heating a subsulphide quartz

tube sealed under vacuum.

so the heating

of ruthenium

This reaction

must be slow and progressive.

from the elements

with sulphur

is strongly

Determination

[g,

[13] in a

exothermic

113,181

of the stoichiometry

116 of the material

so prepared

indicates

that the pyrite

tained despite

some sulphur

deficiency

phur excess is used in the synthesis,

produced

Reported

procedures

annealed,

hours

procedure

a 5% sul-

mixture

[24]. To form electrodes, may be sintered,

the

and there[20].

are either chemical vapour trans-

as transport

agent, with a 1040 - 102O'C

across the sealed quartz ampoule being maintained

or by crystallisation

at 800'C

at 1OOO'C in a sulphur atmosphere

for single crystal growth

gradient

[181;

to completion

typically

port with IC13 or an IC13-S2C12 temperature

[15,23]. At SERI, for example,

in the synthesis

after where necessary

can be main-

and the sealed tube is maintained

for three days to bring the reaction microcrystals

structure

[16,17,251

from a tellurium

for 700

or bismuth

[26l

melt. Ambient

temperature

preparation

procedures

[15] using a metathesis

reaction

RuC13 + 2(NH4)HS

RuS2 + HCl + 2NHqC1

This reaction

_I)

is generally

of the type

for RuS2 have also been reported

:

(1)

carried out in a nonaqueous

clusion of oxygen, to avoid any oxide or hydroxide the initial material

sample of RuC13 should also be anhydrous.

has been mentioned

[2] but problems

solvent,

formation

and with the ex-

[2,l5]. Equally,

Use of an 'RuC14' starting

of stability

and handling

of such a

compound may be anticipated. It must be recognised a short-range in contrast bility

of these reactions

order of the RuS2 type. The filtered precipitate

to the stability

is non-zero.

An annealing

that the product

of the pyrite RuS2 crystal.

Only a featureless

in H2S at a temperature

lise this product.

can only be obtained

to crystallise,

diffraction

after annealing

pattern

and the density

at 850'~ [15]. The structure

of ruthenium

fore be questionable;

there was for a time the suggestion

a charge-neutralised

E.S.C.A. species

studies have established

free elemental E.S.C.A. tainty).

sulphur

features,

formed by low temperature

co-colloid

(-2), and the presence

of ruthenium

the oxidation

of some sulphate

is obtainable. to stabi-

of 6.23

diamagk 0.03

and stoichio-

alone must there-

that the product was

and sulphur. However.

state of the sulphur contamination,

(due to the small chemical

its oxidation

pattern

suscepti-

but a well-defined

metry of any sulphide

merely

can be pyrophoric,

The magnetic

greater than 25O'C is necessary

At 350°C it begins

netic solid giving the pyrite

X-ray diffraction

show, at best, only

our

(dimer)

but certainly

shifts of the ruthenium

state is more difficult

to specify with cer-

no

117 One advantage sulphide

of the metathesis

deposits

cles of other semiconductors exposed

procedure

is that ruthenium

or colloidally

suspended

di-

parti-

27,28 . Here the surface or colloid is

e.g. C&3

to RuC13 or K2RuC16

ion introduced,

preparation

may also be laid on surfaces

in aqueous

solution

for example, by bubbling

, which then reacts with sulphide

H2S or as a NaHS solution.

ELECTROCHEMISTRY In the initial work with sintered gen evolution current

in darkness with a surprisingly

density

trolyte,

of 4.5 ITLO. cm

-2

trode surface was only slightly

cathodic

anodic dark current

terface.

with an essentially

In an alternative

perspective

RuS2 electrode

[16,171. While hydrogen

electrode

dark reaction

surface

Mott-Schottky tigations,

RuS2 electrodes

effects

close to the H20/02

:an be obtained

redox potential The chemical

tal electrode

beha-

more conventional

beha-

potentials

in dark-

[16,17,18I.

dependent

associated

Linear

potential

at -0.48 V

with the observed is however

state recombinatLon

a significant

of the surface

Only conis

photocurrent

has been explained

t28l.

on-

subject to

with this photocurrent

The fact that a continuous

states defining

inves-

on light intensity.

anodic potential

and photocorrosion

'd' nature of the electron

of that

potential.

in the single-crystal

a flat-band

A fur-

and the sintered

is the reduction

does it rise to represent

reaction

with surface

is very stable,

The non-ideal

hydrogen

This photoresponse

only at this relatively

of competition

to the pure

elicited

is compatible

and is not linearly

rerified as oxygen evolution

of deep donor states at

at cathodic

(l/C2 vs V) have been obtained indicated

in-

evident.

electrolytes,

of which

surface

solid-electrolyte

both on the single-crystal

(pH 1). This value

of that

The ef-

under anodic bias is no longer observed.

for anodic photoeffects.

tinous photocurrent.

the basis

is, however,

observed

the extrapolation

strong transient

but it was suppres-

of the oxidised

semiconductor.

just anodic of the reversible

(NHE) in acid electrodes set potential

in the occured

already mentioned.

[171 the charging

in oxygen-saturated

plots

potential,

ohmic contact

is still evolved

ness, the dark oxygen evolution

oxygen at potentials

the elec-

by a hysteresis

leading to a 'metallic'

may give a degenerate

Later work on single-crystal

ther cathodic

hydrogen

oxygen evolution

semiconductor,

these anodic potentials

viour

in a 0.1 M KOH elec-

The onset of an anodic photocurrent

and associated

viour of the sintered

was observed (a

sweep in the anodic sense prior to the appearance

fect may be associated with the underlying

[9,20] oxy-

Under these conditions

as evidenced

oxidised,

of the reversible

a potential

low overpotential

difference).

anodic region of the voltammogram. at a potential

RuS2 electrodes

with a Pt counterelectrode,

under a 1.5 V potential

sed during

polycrystalline

on

The single-crysis minimal,

the valence

due

and conduc-

118 tion band edges. In contrast, in the diselenide and ditelluride of ruthenium there is increasing photocorrosion,giving Ru02 and chalcogenate ion, associated with the increasing mixing of ruthenium d and chalcogen p electron states in the band structures of the compounds. For ruthenium disulphide, then, the dark hydrogen evolution, the oxygen-evolving photoeffect and the Mott-Schottky data are presented as evidence for a valence band edge close to the H20/02 redox potential, and the conduction band edge at some -0.5 V (NHE) in darkness; this would imply a band gap of 1.8 eV [l'i']. Photoelectrochemicalexperiments then show that those band edges become unpinned under illumination in the presence of an oxidisable species in aqueous solution; the potential at which a continuous photocurrent is established is displaced snodically with electrolyte redox potential over a wide range, from -0.48 V (NHE) for a sulphide electrolyte to +1.23 V (NHE) for oxygen evolution. The unpinning, and by consequence the anodic onset for photoevolution of oxygen, makes RuS2 an unfavorable candidate for energy conversion by photolysis of water: it also makes credible the narrower band gap (1.3 eV).

PHOTOCHEMICAL CATALYSIS BY RUTHENIUM DISULPHIDE Firely divided cadmium sulphide has been loaded with a ruthenium sulphide by a metathesis reaction (1) in the presence of an aqueous suspension of the CdS particles, followed by drying and annealing in a H2S atmosphere [27,281. For the photochemical dissociation of H2S in aqueous solution, this catalysed CdS suspension is some seven times more effective than Ru02-loaded material [29]. The RuS2 catalyst has the additional advantage of long-term stability. By spectral photoresponse it was confirmed that the CdS was the active semiconductor for photoexcitation, with an onset at 520 nm, hv = 2.4 eV. The function of the RuS2 as a catalyst is therely established, though it is not clear whether it influences preferentiallythe kinetics of the anodic or cathodic reaction

DESULPHURISATIONAND HYDROPROCESSINGREACTIONS These provide the most probable short-term economically significant application, and petroleum industry laboratories have been investigatingRuS2 catalysts for hydrodenitrogenation1301 HDN and hydrodesulphurisation[1,2] HDS of organic compounds for some time. For desulphurisationa systematic preparation of, and a comparison of the catalytic effiency of, a series of metal sulphides have been carried out. Model organosulphur compounds were then treated in an autoclave reactor in the presence of these test catalysts, the products being monitored by mass spectroscopy or gas chromatography.For example, the thermal desulphurisa-

119 tion of tertiary butyl thiol to yield a butene proceeds over an RuS2 catalyst to completion [311 :

(CH~)CSH

*,

(cH~)~c=CH~ + H2s

(2)

RuS2 This by analogy with the dehydration of the corresponding alcohol over a metal oxide catalyst 1321. 2oo"c ~---& 2

(CH2)3COH

(cH~)~c=cH~ + H~O

A useful reference compound for hydrodesulphurisation,HDS, is dibenzothiothene. Among the transition metal sulphides, the catalytic activity for this reaction varies by three order of magnitude. While the sulphides of the first row transition metals show a low and fairly constant activity, those of the second and third row show a significant dependence on location in the periodic table peaking sharply with the sulphides of ruthenium (2nd row) and osmium (3rd row) ('volcano curve', 1,2). During operation of the HDS reaction the ruthenium compound gradually becsme substoichiometricin sulphur. It is generally accepted that sulphur vacancies on the catalyst surface provide the reaction sites for HDS [33],

such vacancies occuring particularly easily in pyrite RuS2, where the

sulphur atoms occur in pairs. However, the interpretationof this catalytic behaviour in terms of the electronic, structural and chemical properties of the material is only now evolving [34], as is consideration of RuS2 as a model for the design of new catalysts of sulphides of base metals with enhanced activation due to selective surface alloying methods [35].

CONCLUSION Ruthenium disulphide is a promising material for the thermal catalytic processing of organic sulphur and nitrogen compounds in petroleum refining. It also has interesting photoelectrochemicalbehaviour, and photochemical catalytic properties. It is a useful model system for the study of catalytic mechanisms. Single crystals, polycrystalline samples and loaded colloids have been prepared and evaluated. However, materials improvement is still required, and characterisation of several physical and chemical parameters of RuS2 is still incomplete.

ACKNOWLEDGEMENTS Contact with Prof. Dr. H. Tributsch and Dr. A. Mackor was promoted by the European Community 'Stimulus'programme, contract STI-005-J-C (CD). Discussions

120 with Prof. J. Cunningham were useful. Prof. M. GrEtzel encouraged this present work, which is currently supported by the Swiss National Fund for Scientific Research.

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