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

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Journal Article

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

Author(s):

Berner, H.; Oesterreicher, H.; Ensslen, K.; Schlapbach, L.; Schlapbach, L.

Publication Date:

1982

Permanent Link:

https://doi.org/10.3929/ethz-b-000423131

Originally published in:

Zeitschrift für physikalische Chemie 132(1), http://doi.org/10.1524/zpch.1982.132.1.075

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In Copyright - Non-Commercial Use Permitted

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Zeitschrift furPhysikalischeChemie^NeueFolge,^{Bd. 132,S. 75-84}(1982)

© byAkademischeVerlagsgesellschaft, ^{Wiesbaden 1982}

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,WestGermany and

L.

Schlapbach

Laboratorium fiirFestkorperphysik,ETH, CH-8093Zurich,Switzerland

(ReceivedJuly^{26, 1982)}

Metalhydride / Surfacesegregation/ Catalysis /Methanation

Thecompound ^{ThRh forms} ThRhH3A under ¹bar H2 ^{pressure at room} temperature.

Thermodynamic analysis yields^AH°=

—

58.7kJ/mole H2and AS"⁼

—

128J/K ^moleH2.^A

methanation reaction is observed whenThRhH3[isexposed^{toCO. A}preferentialreaction of CO with thehydrogenstored in the metalhydriderather than with thehydrogenin the ambient gas phase ^{is shown} by ^{use of}hydrogen-isotopes. ^XPS experiments ^ofThRh and ThRhHz

demonstrate surfacesegregation^intoTh02 ^{and Rh}clusters,^whenexposed^{to02 orCO.}

Die intermetallischeVerbindung^{ThRhbildetbeieinem}H2-Gleichgewichtsdruck^{von1 bar}

undZimmertemperatur^dasHydrid ThRhH3dessen Bildungsenthalpie^{AH"= —58.7kJ/mol} H2 und Bildungsentropie ^AS°=-128J/K ^mol H2 betragt. ^Wird ThRhH3, einer CO-

Atmosphareausgesetzt,^{ist die}Entstehung^vonMethan nachzuweisen. Mit Hilfe^vonWasser-

stoffisotopen ^wird gezeigt, daB bei dieser Reaktion der Wasserstoff, ^{der im} Metallhydrid gespeichert^ist,gegeniiberdem Wasserstoff^ausderGasphase bevorzugt eingebaut^{wird. XPS-} Untersuchungen^vonThRh undThRhHz zeigen^beiZugabe^von02bzw. CO eine Oberflachen- zersetzung,^{bei der}Th02- und Rh-Teilchenentstehen.

Introduction

Metals andintermetallic

compounds consisting

^{ofrareearthsoractinides}

incombinationwith transition metalscanabsorb

large

^amountsof

hydrogen.

1GuestProfessor, permanentaddressatUC-SanDiego,La Jolla where work issupported by

agrantfrom theDepartment^ofEnergy, ^BasicEnergy^Sciences

Onestep ^{of this}processis the dissociation of the

H2

^moleculeatthe surface.

As shown

by

^the

example

^of

LaNi5 [1]

^a

segregation

^{of therare}earth 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 a}conventional 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}support^thanLa,U^orZr.

In addition to a

study

^ofthe

hydriding

behaviour of ThRh and the reaction^{of ThRh}

hydride

^with

^CO,

^{XPSwasused to}

help understanding

^of

the surface processes.

Experimental

^details and results

1. Material

ThRhwasprepared by melting^thecomponentstogether^{on awatercooled}copperhearth of

anarcfurnace under argonatmosphere.^{The buttonwas}flippedand remelted several times^to

improve homogeneity.

The orthorhombiccrystal^structure[7]^wasverifiedby analysis^ofX-ray powder diagrams (Guinier^{camera,Cu-A'a}radiation).^Therewasalso evidence ofTh02presence, but^notof Thor

Rh.

2. Hydriding

ThRhwashydrided^{in thereactorofa}high-pressurestainless steelapparatuswith various reservoirs of known volume and facilities^toworkatdifferentpressure ranges.Thetemperature

was monitored by ^{a NiCr —Ni} thermocouple ^{in the reactor bed. For} the variation of the temperature,^anexternal heaterwasused.

The Formation of ThRh Hydride^{and the}Synthesis^{of Methane 77}

Thesample^wasexposed^toH2 gas(99.999%), additionally purified^from02^and H20 by chemisorption and molecular sieve filtration, respectively, accomplished by passing ^the gas

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

absorbedhydrogen^wascalculated from the_pressuredrop.The first reaction withH2^{(28 bar)}

occurredspontaneously^atroomtemperature^withoutpreviousactivation.Atthistemperature and1barequilibrium H2pressurethehydride ThRhH3^,is formed. Thepressure-composition isotherms,^{shown in}Fig.^1weremeasured for fourtemperaturesby monitoring^theequilibrium H2pressurestarting^fromapoint,^{whereThRh isinthe}hydride phase^andpumping^outknown quantities^ofhydrogengas. Threeof thesedesorptionisotherms showplateaus,^{associatedwith}

the transformation of thehydride (/?) phaseinto the solid solution(a)phase. Thermodynamic parametersderived fromalogarithmic plot^ofplateaupressure^vs. reciprocaltemperature^are AH"⁼ -58.7kJ/mole H2 and AS"⁼ -128J/K ^molr H2.^{The X-ray} pattern of the ThRh

hydridecould^notbe indexedby enlargingthe orthorhombic cell ofThRh,^norbythepresenceof

ThH2^orTh4H15.

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}

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}

gas^tocheck

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 0.75g^was

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

total_pressureand

obtaining

^thesameratios of the

partial

^{pressuresasthose}

ratios determined from the filled in

quantities.

Either the closedsystem^was used

during

^thewhole

experiment

^{orthereactorwasswitchedovertoa}

^single

pass^reactorand thegases

passed

^{overthe 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

therecorded_pressure 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

previous experiment.

We took the detection of¹

%

^of

^CH4

^{relativetothe total}

The Formation of ThRhHydride^{and the}Synthesis^{ofMethane 79} 300-

L,

^200-

•- 100- 1000,

t/1

<

UJ LU o =>

en -J LUD_

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}CH4^{formationfrom COover}ThRhHzduring^{increaseof 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

^{wascarriedout. 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

^tooxygen^at200^°Cfor12 handthe resultofthe

subsequent catalysis

Table1. Distributionof^reactionproducts of^{CO and}D2 ^overThRhHz^at171°C

_J_I_I_I_

CH*

^{CH3D CH2D2 CHD3 CD<}

0 50 100 150 200 250 300

TIME (min)

Fig.3. Formation ofCH4^andC02from CO andH2 ^overthe 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 thematerialdidnottake_upanymore

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.

The Formation of ThRhHydride^{and the}Synthesis^{of Methane SI} 4. Surface

analysis

The ThRh surface^wasstudied

by ^X-ray photoemission

spectroscopy^with

a

VG-Escalab-Spectrometer using ^Mg-Ka

^radiation

^(hv

⁼

^1253.6eV;

^Au

4fV2

^at83.9

^eV,

^{FWHM 1.2}

^eV).

^{The ThRhwas}cleft under^UHVconditions

(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 after_oxygentreatmentare

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.XPSspectra of cleft ThRhexposed^to02 ^{at20 C:}(a)^cleftsample,^Th:Rh=1.6:1;

(b)exposed^totL02,^Th:Rh=2.1:1;(c)exposed^to10LO2,Th:Rh^=2.5:1;(d) exposed^to

100L02,^Th:Rh=2.7:1

Fig.^{5. XPS}spectraof cleft ThRhexposed^{toCOat200 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 ThRhwere}recorded before and after COexposure atroom

temperature

^andat200° C. Some of them^areshown in

Fig.

^{5. The}

same effect

—

enrichment of Th as

ThOz

^and metallic Rh

—

as after

02

exposure is^seen.

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 ofsurface}coverage

by

oxygen,which diffuses from the bulktothe

surface,

^wefollowed the surface concentration of^O and ^C of^a

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 cleft^ThRhwas

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 warmed_uptoremoveall

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}

The Formation of ThRhHydride^{and the}Synthesis^{of Methane 83}

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

Fig. ^{6. XPS}spectra of cleft ThRh and ofThRhHz exposed^toCO:(a)^cleftsample^at20°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 exposed^{to100 L}CO,^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

^isvery

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}

oxidation of

Th,

^the

completely

^oxidized

sample

^wouldnotshow any

CH4

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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-

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