The Formation of ThRh Hydride and the Synthesis of Methane by its Reaction with CO

© byAkademischeVerlagsgesellschaft,

The

Formation

of ThRh

_Hydride

and the

_Synthesis

of Methane

_by

its Reaction with CO

By

H. _Berner, H.

_{Oesterreicher1,}

K. Ensslen

Fakultat fiirPhysik,UniversitatKonstanz, D-7750Konstanz,West_Germany

and L.

_Schlapbach

Laboratorium fiirFestkorperphysik,ETH, CH-8093Zurich,Switzerland

(ReceivedJuly26, 1982)

Metal_{hydride / Surface}segregation/ Catalysis /Methanation

Thecompound ThRh forms ThRhH3A under 1bar H2 pressure at room _temperature. Thermodynamic analysis yieldsAH°=

—

58.7kJ/mole H2and AS"=

—

128_J/K moleH2.A methanation reaction is observed whenThRhH3[isexposedtoCO. A_preferentialreaction of CO with the_hydrogenstored in the metalhydriderather than with thehydrogenin the ambient

gas phase is shown _by use ofhydrogen-isotopes. XPS experiments ofThRh and ThRhHz

demonstrate surfacesegregationinto_Th02 and Rhclusters,when_exposedto02 orCO.

Die intermetallische_VerbindungThRhbildetbeieinem_{H2-Gleichgewichtsdruck}von1 bar und_{Zimmertemperatur}das_{Hydrid ThRhH3dessen Bildungsenthalpie}AH"= —58.7kJ/mol

H2 und Bildungsentropie AS°=-128J/K mol H2 betragt. Wird ThRhH3

, einer

CO-Atmosphareausgesetzt,ist die_EntstehungvonMethan nachzuweisen. Mit Hilfevon

Wasser-stoffisotopen wird gezeigt, daB bei dieser Reaktion der _Wasserstoff, der im Metallhydrid gespeichertist,gegeniiberdem WasserstoffausderGasphase bevorzugt eingebautwird.

XPS-Untersuchungenvon_{ThRh undThRhHz zeigenbeiZugabe}von_{02bzw. CO eine}

Oberflachen-zersetzung,bei derTh02- und Rh-Teilchenentstehen.

Introduction

Metals andintermetallic

_compounds

_consisting

ofrare_earthsoractinides

incombinationwith transition metalscanabsorb

large

amountsof

hydrogen.

1

GuestProfessor, permanentaddressatUC-SanDiego,La Jolla where work issupported by

76 H. Berner,H.Oesterreicher, K. Ensslen and L._Schlapbach

One_step of thisprocessis the dissociation of the

_H2

moleculeatthe surface. As shown

_by

the

_example

of

_{LaNi5 [1]}

a

segregation

of therareearth element

atthe surface takes

_{place, binding}

the

_{oxidizing impurities}

and

_keeping

the transition metal in the metallicstate. Inthiswaysitesare

created,

whichare

abletodissociate molecules. On the other hand these

compounds

candeliver

large quantities

ofatomic

_hydrogen

from the bulk.

_They

therefore should be

good

candidates as

catalysts

for

hydrogenation

reactions.

Wallaceetal.

_[2

—

4] investigated

various

_compounds

ofthistypeintheir

unhydrided

statewith

_regard

totheir

_{catalytic activity}

in the

_NH3

and

CH4

synthesis. They

found,

that under the

_synthesis

gas CO and

H2

the

com-pounds decompose

into

_{crystallites consisting}

of the transition metal and ofanoxide of therare

earth/actinide

which showedatleastinone case

higher

activity

than aconventional oxide

supported catalyst

of thesame elements.

Soga

etal.

_[5]

studied the

_{hydrogenation}

of

_ethylene

over

LaNi5

and

found,

that the rate of

_{hydrogenation}

over the

hydride

was

_larger

than over the

unhydrided

material. Oesterreicheretal.

_[6]

showed the

_catalytic

formation ofwater, when

_hydrides

ofintermetallic

_compounds

were

exposed

toair.

Inthis workwestudied the

CH4 synthesis

from carbon monoxideover

ThRh

_hydride.

We choose ThRhas amodel

compound

_type

"rareearth or

actinide/transition

metal" for several reasons:

a)

the material is

_expected

to form a

hydride,

which is even stable at

temperatures

of 200

-300°

C.

b)

Rhodium could become

_{catalytically}

active in clustersize on the

surface,

if the

_{expected segregation}

ofThorium occurs.

c)

Thorium is

_{reported [4]}

toforma moreactive_supportthanLa,UorZr.

In addition to a

study

ofthe

hydriding

behaviour of ThRh and the

reactionof ThRh

_hydride

with

CO,

XPSwasused to

help understanding

of

the surface processes.

Experimental

details and results

1. Material

ThRhwasprepared by meltingthe_componentstogetheron awatercooled_copperhearth of

anarcfurnace under argonatmosphere.The buttonwasflippedand remelted several timesto

improve homogeneity.

The orthorhombiccrystalstructure_[7]wasverifiedby analysisofX-ray powder diagrams

(Guiniercamera,Cu-A'aradiation).Therewas_{also evidence ofTh02presence, but}notof Thor

Rh.

2. _Hydriding

ThRhwashydridedin thereactorofahigh-pressurestainless steel_apparatuswith various reservoirs of known volume and facilitiesto_workatdifferentpressure ranges.Thetemperature

was monitored _by a NiCr —Ni _thermocouple in the reactor bed. For the variation of the temperature,anexternal heaterwasused.

The_samplewasexposedto_{H2 gas(99.999%), additionally purified}from₀₂and _{H20 by}

chemisorption and molecular sieve filtration, respectively, accomplished by passing the _gas

through a commercially availablecartridge (Oxisorb, Messer Griessheim). The amount of

absorbed_hydrogenwascalculated from thepressuredrop.The first reaction withH2(28 bar) occurred_{spontaneously}atroom_temperaturewithoutpreviousactivation.Atthis_temperature

and1bar_{equilibrium H2pressure}the_{hydride ThRhH3},is formed. Thepressure-composition

isotherms,shown in_Fig.1weremeasured for four_{temperaturesby monitoring}theequilibrium

H2pressurestartingfroma_point,whereThRh isinthe_{hydride phase}and_pumpingoutknown

quantitiesof_{hydrogengas. Three}of these_desorptionisotherms show_plateaus,associatedwith the transformation of thehydride (/?) phaseinto the solid solution(a)phase. Thermodynamic

parametersderived froma_{logarithmic plot}of_{plateaupressure}vs. reciprocal_temperatureare

AH"= -58.7kJ/mole H2 and AS"= -128J/K molr H2.The X-ray _pattern of the ThRh

hydridecouldnotbe indexedby enlargingthe orthorhombic cell ofThRh,norbythepresenceof

ThH2or_Th4H15.

3.

_Catalysis

3.1.

_Apparatus

The

_catalysis

_experiments

were

performed

inthe

hydriding

apparatus

by

useof

corresponding

parts

of itas a

single

passreactor

_(volume

5

_cm3)

or as a

78 H. Berner, H.Oesterreicher, K. Ensslen and L._Schlapbach

closed

_system.

This contained several chambers for

_filling

in knownamounts

of different gases. In addition to CO

_{(99.97%), H2 (99.999%)}

and

_D2

(99.4 %)

required

for the

_{CH4 synthesis,}

Ar

_(99.997

_%)

wasusedas areference

gastocheck

_changes

inthe

_homogeneity

ofthegasmixture and in the

partial

pressures ofthe

components.

A

_quadrupole

mass

_spectrometer

(Balzers,

QMS)

built

together

with a

separate

high

vacuum station was used. The

_components

of the gas were

quantitatively

analysed by considering

the known ratios of

_singly

and

_doubly

ionized and

_fragmentary

molecules and

_by

also

_including

a factor for the

probability

of

ionization,

which isdifferentfordifferent_gases

_[8,9].

Since the

peak

atthe atomicmass

unity

(amu)

16is

composed

not

only

of

CH4

but also

of CO and

_H20,

the

_peak

atamu15

(CH3-fragment)

wasusedtodetect the presence of

CH4.

3.2. Procedure

AThRh

_piece

of about 0.75gwas

hydrided

atroom

_temperature,

cooled

to

—

50 °C and after

_removing

thegaseous

hydrogen, exposed

toCO and Ar. Then thereactorwasheatedto100°

Cand

_part

ofthe

_hydrogen

inthe

_hydride

allowedtodesorb. This

_temperature

washeld for about 12h whilethegases

were circulated in the closed

system,

driven

by

convection,

to reach

equilibrium

and

_homogeneity.

This was confirmed

by observing

constant

totalpressureand

_obtaining

thesameratios of the

partial

pressuresasthose

ratios determined from the filled in

_quantities.

Either the closedsystemwas

used

during

thewhole

_experiment

orthereactorwasswitchedovertoa

single

passreactorand thegases

passed

over_{the ThRh}

hydride

_ata constantfeed

rate of1 ml

_{gas/min (standard pressure).}

This ratewas limited

by

the mass

spectrometer,

which was run at 1 •

10~4mbar.

At the end of each

single

experiment

the reactor was

evacuated,

the _gas further

analysed by

mass

spectrometry

and the

_hydride

was

outgassed

at200 °C for about 1h. This

procedure

was

repeated

several times with the same _ThRh

sample.

3.3. Results

In all

_experiments

with

_exposing

ThRh

_hydride

to

CO,

_CH4

wasthe

only

detectable

_product.

_{The presence of}

_H20

wasnot detected either

_during

the

experiment

or

during

evacuation at the end of the

_experiment.

Further a

progressive

decrease ofthe

_hydrogen

storage

capacity

was observed. This

fact,

together

with the absence of oxygen

containing

gaseous

products

and therecordedpressure decrease

during CH4

synthesis

suggest,

that the ThRh

sample

became more and more oxidized.

Ineach successive

_experiment

the time and the

_temperature,

atwhich the

beginning

of the

_CH4

_synthesis

could be

_detected,

was lower than in the

300-L,

200-•- 100-0, 100 t/1 < UJ LU o => en -J LU D_ Ui 50

_H

vH2

0.75 0.60 QMS

-I

0.15 0 o £

-I

0.30

ffi

< i i i—i T—i-—r 0 10 20 30 40 50 60 70 80 TIME (min)

Fig.2. Rate of_CH4formationfrom COoverThRhHzduringincreaseof the_{temperature.H2}

evolves from thehydride.Ar isusedas a_{"feed rate" reference}

amount of the

_analysed

gas as a measure for the onset. In the

virgin

hydrogenation cycle

of ThRh theonsetis found after12hat100"C

_plus

3.5 h at200°

C. Inthe8th

_catalysis

_cycle

itcouldbe

_already

seenafter12 hat95°

C. The total pressure was 1.1 and 1.2

bar,

respectively.

Westudiedmoredetailed the influence of the

desorption

of the

hydrogen

from the metal

_hydride

onthe

CH4

formation.In

Fig.

2thegasmixture and

temperature

course in a

_single

_pass

experiment

are shown. A

_temperature

increase leadstoincreased

hydrogen desorption,

initiating instantaneously

a

higher

rateof

_CH4

formation

_compared

with the fraction ofargon,which isa

reference of thefeedrate

_during

desorption.

Toshow the

_preference

in _the

methane

_synthesis

between the

_hydrogen

evolved from the

hydride

and the ambient

_H2

gas,an

isotope experiment

was_carriedout. ThRhwas

hydrided

and after

_removing

most of _{the gaseous}

_hydrogen,

_exposed

to carbon monoxide and deuterium in the closed

_system.

_During

evacuation of the

reactorthemass

spectrum

showed

peaks

atamu

2,

3and4in the ratio 7:6:8

arising

from

_H2,

HD and

_D2

and

_peaks

atamu12to

20,

which refertoseveral

C

—

H

—

D

_compounds

listedinTable1.

_Although

_{the presence of}more

D2

gas than

H2

gasis

evident,

_CH4

and

CH3D

are dominant over

CD4.

From

these results we

conclude,

that in

forming

methane,

hydrogen

from the

hydride

is

_preferred

over

hydrogen

from thegas

phase.

After7

_experiments

with CO and 10

_{hydriding cycles,}

the ThRh

_piece

had

disintegrated

to

powder

and had increased its

weight by 7%. X-ray

diffraction

_analysis

showed

_{Th02, ThRh3}

and Rh. This

_sample

was then

exposed

to_oxygenat200°

80 H. _Berner,H.Oesterreicher, K.Ensslen and L. _Schlapbach Table1. Distribution_ofreaction_{products of}CO and_D2 over_ThRhHzat171°C

_J_I_I_I_

CH*

CH3D CH2D2 CHD3 CD<

0 50 100 150 200 250 300

TIME (min)

Fig.3. Formation of_CH4and_C02from CO and_H2 over_{the oxidized ThRhsample during}

increaseof thetemperature.H20is alsoformed(seetext).Aris usedas a"feed rate" reference

experiment

is shown in

_Fig.

3.

_Hydrogen

was admitted inthe gaseousstate, because thematerialdidnottakeupanymore

hydrogen.

Thedecreaseof CO

combinedwiththe increase in

_CH4

_implies,

that

_CH4

is formed from CO and

not from

_C02,

which was also

produced.

Water was detected in

larger

amounts

_during

evacuation and therefore not marked in the

_figure.

The

X-ray

diffraction

_analysis

showed that the material consists of

_Th02

and Rh. The last

_experiment

carried out without the ThRh

_sample

or its

decomposed products

confirmed,

that no

products

_at reaction conditions were formed

by experimental

artefacts.

4. Surface

_analysis

The ThRh surfacewas_studied

_by

X-ray

_{photoemission}

_spectroscopywith a

VG-Escalab-Spectrometer using

Mg-Ka

radiation

(hv

=

1253.6eV;

Au

4fV2

at83.9

eV,

FWHM 1.2

_eV).

The ThRhwas_{cleft under}UHV_conditions

(2

—

4-

_10~10mbar)

and after

_analysing

the clean surface the

_samples

were

exposed

in _{situ to}

_increasing

_{doses of}

₀₂

_and

_CO,

_{respectively.}

_The

photoelectron

energy distribution curves of the core levels Th

4/7/2i5/2,

Rh

_3J5/2

_3/2, O 1_s1/2 and C 1

_s1/2

were measuredto evaluate the chemical

state and the concentration ratios within the escape

depth

of the

photo-electrons

(~20A).

The chemical state was determined

by comparison

to

known

_spectra.

Theconcentration ratio wascalculated from the

integrated

peak height

divided

_by

theoretical cross sections

[10].

In

_Fig.

4the

_spectra

ofcleft ThRh before and afteroxygentreatmentare

shown. The clean

_sample

contains metallic Th

_[11,12]

and metallic Rh

[13,14].

With

_increasing

amounts _{of oxygen,}

_{increasing intensity}

of the

chemically

shifted Th

_4/peaks

is observed. This is

_{explained by}

formation of

Th02 [12]

in

_agreement

with the increase and

_position

ofthe oxygen

peak.

TheRh

_peak

shifts 0.2 eVtolower

_{binding energies,}

which could be

_explained

by

the formation of small Rh clusters with a

high

amount of

_top

surface

atoms. The C

_spectra

do not

_{change during}

_exposure to

_02.

The

con-centration Th:Rh

changes

from 1.6:1 of the cleft

sample

to2.7:1 after 100L

02 (1

L= 1

Langmuir

= 1•

10~6Torr

s).

Therefore

₀₂

exposure leads to

enrichment of Th at the surface in the form of

_Th02.

The Rh remains metallic.

330 335 340 345 350 305 310 280 285 290 530 535

BINDING ENERGY (eV| _[Ep=0]

-Fig.4.XPS_spectra of cleft ThRhexposedto₀₂ at20 C:_(a)cleftsample,Th:Rh=1.6:1;

(b)exposedtotL_02,Th:Rh=2.1:1;(c)exposed_to10LO2,Th:Rh=2.5:1;(d) exposed_to

82 H. Berner,H.Oesterreicher, K. Ensslen and L._Schlapbach

Fig.5. XPS_spectraof cleft ThRh_exposedtoCOat200 C:_(a)cleftsample,Th:Rh=1.5:1;

(b) exposed to10LCO,Th:Rh= 1.6:1;

(c)

exposed to1 000 L CO,Th:Rh=1.8:1

Further the

_spectra

of ThRhwererecorded before and after CO_exposure

atroom

_temperature

andat200° C. Some of themareshown in

Fig.

5. The

same effect

—

enrichment of Th as

ThOz

and metallic Rh

—

as after

02

exposure isseen.

However,

ittakes 1000LCOto

_produce

thesameamountof

Th02

as with

only

1L

02.

The C

_spectra

show new small contributions at

energies

between 282 and 285

_eV,

_especially

at200

°C,

which could

_originate

fromdissociatedCO

_[15].

The ratio C: O however

_changes

from1 :0.5 for the cleft

_sample

to 1 :1.2 after 1000 L

_CO,

sothat not_{all of the oxygen} canbe

derived from dissociated CO. To

analyse

theamount ofsurfacecoverage

by

oxygen,which diffuses from the bulktothe

surface,

we_{followed the surface}

concentration ofO and C ofa

freshly

cleft

sample

at200 °C over2h. The

carbon

_peak

didnotincreaseatall.Theoxygen

peak

increased40

_%

overthat

in the clean

surface,

where the

(Th

+

Rh)

:0 ratio was 3.2:1.

The cleftThRhwas

hydrided

ina

_separate

chamber oftheUH

_V-system

_at

20 bar

_starting

atroom

_temperature

and cooled downto —100° Cto avoid strong

_{H2 desorption.}

The

_hydride

was

exposed

to CO and afterwards warmeduptoremoveall

hydrogen.

The

powdered hydride

had the lowest of

allmeasuredcontentsof

_impurities

C and Oas seenin

Fig.

6. CO exposure of

that

_partially

_{hydrided sample}

showsnofurther differenceas

compared

tothe

unhydrided

material.

Discussion

The derived

_{thermodynamic properties}

of ThRh

_hydride,

AH0 =

—

58.7

kJ/mole

_H2

and AS°= —128

J/K

•mole

H2,

are

comparable

tothose of similar

_hydrides

such as ThNi and

ThCo,

which are

reported

in the

330 335 340 345 350/305 310 280 265 290 530 535 BINDING ENERGY IeV)-*>

Fig. 6. XPS_spectra of cleft ThRh and of_{ThRhHz exposed}toCO:_(a)cleft_sampleat20°C, Th:Rh=1.6:1; (b) hydrided, cooled to -100°C, Th:Rh=1.4:1; (c) hydride exposed to

10 LCO,Th:Rh=1.5:1;(d)hydride exposedto100 LCO,Th:Rh=1.5:1;(c)hydride

ex-posed to 1000 L CO, Th:Rh=1.6:1;_{(/) hydride exposed} to10000 L CO, Th: Rh= 1.8:1;

(g) hydride warmed up to 20 C, Th:Rh=2.5:1; (h) completely dehydrided,

Th:Rh=2.4:1

literature

_[16],

Thus ThRh

_hydride

is stable

_enough

to be handled without

strong

desorption

at

_temperatures

up to 200 °C. This

_temperature

rangeis

necessary to

_{study CH4 synthesis}

from CO over the metal

hydride.

Withthe enrichmentof Thas

Th02

onthesurfaceand the formation of

metallic

Rh,

when

_exposed

to

_02,

ThRh has the

_expected

surface

_properties

anticipated

from similar

_experiments

with

_{LaNi5 [1],}

where oxidized La and metallic Ni arefound in the ratio 1 :1 on the surface.

When ThRh

_hydride

is

_exposed

to

CO,

CH4

is formed below

tempera-tures of 200°C. The

_{hydrogen evolving}

from the

_hydride

is

_preferred

in

forming CH4, compared

tothe

_hydrogen

in the gas

_phase,

as shown inthe

isotope

experiment.

This fact makesevident that the

_origin

andstate of the

hydrogen

is_very

_important.

This

_preference

appearstobe caused

_by

the

_large

supply

of atomic

_hydrogen

_by

the

_hydride.

In addition new

clean,

i.e.

catalytically

active surfaces may be created

during

hydriding

and

_dehydriding

of the intermetallic

_{compound by progressive}

_{disintegration}

or

segregation.

However the oxygen of the carbon monoxide is used

subsequently

for oxidation of

Th,

although

a

"reducing atmosphere"

is formed

by

dehydrid-ing.

But if the

_CH4

_synthesis

from CO would work

_only

on the basis of

84 H.Berner, H.Oesterreicher, K. Ensslen and L.Schlapbach

synthesis.

This isnotthecase. The

X-ray

pattern

shows,

that Rh

crystallites

have been formed. These cristallites

_together

with

_Th02

canalso

produce

CH4

and

_might

be consideredasthoria

supported

Rh

catalyst.

The formation

of

_CH4

from CO and

_H2

overthe

severely by

CO oxidized

sample

startedat

the lowest

_temperature

of all

_experiments.

This

_extremely

low value was

95°C.

Acknowledgements

We would like tothank Professor E. Bucher for

_support

and interest in this work and Mr. G. G. Baumann for

_help

with the mass

spectrometric

analysis.

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