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THE STRUCTURE ANH ACTMTY OF

CATALYTKAUY ACTIVE SOLIDS

FIFTH TECHNICAL REPORT

Project NR 057 143

Contract N7 onr-45003

Period Covered: 1 October 1952 to 30 September 1953

CM#r fftv#sn0i9for P. W. SHWUOD

THOMAS FREUND KftMMHTCa AttOCVCPfO

M.J. HULATT

PAUL I. JACOESON jtanarffi Atslstoat LOUlSf MOORE In—wt Astiff anf

Tho Department off Chemistry Northwestern University

Evanston, Illinois

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The Department of Chemistry

northwestern University

Bvanston, Illinois

H

THE STRUCTURE AND ACTIVITY OF

CATALYTICALLI ACTIVE SOLIDS

Fifth Technical Report

Project HR 057 U3

Contract W7 our - 45003

Period Covered s 1 October 1952 to 30 September 1953

by

P. V* Selvoodj, Chief Investigator

Thomas Freund, Research Associate

N» Jo Kulstt, Research Associate

Paul E. Jacobson, Research Assistant

Louise Moore, Research Assistant

f

s la

SUMMLBI

This report is in two parts* Part I is a manuscript to be submitted to the Editor of the Journal of the American Chemical Society* This represents com- pletion of a phase of our Contract work started four Tears ago on an attempt to measure thermal Migration in a supported copper catalyst system. The conclusion reached is that thermal migration, or sintering, doee not occur in such a catalyst system*

Part II of this report represents the first phase of a magnetochemieal study of poisoning and, in general, the influence of adsorbed gases, on supported metal catalyst systems* The principal conclusion is that carbon monoxide promotes the migration of Iron atoms in a supported iron-alumina catalyst*

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piSTBipPTrow frJST TOR TECHNICAL REPORTS I

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1 TABIX OF CGHTBNTS

Title Page i

Summary ii

Distribution Idst iii

Table of Contents riii

Page

Part I* Thermal Migration in Supported Copper Catalysts 1

Introduction 1 Experimental Methods 2 Tig* 1 - Faraday Balance 3 Flgo 2 - Field and Field Tinea Gradient 5 Fig. 3 " 7 Fig-. 4 - 7 Results 9 Ftg« 5 * Susceptibility Isotherm for Comer on Alumina ID Fig. 6 - Susceptibility vs. Reciprocal Field 12 Fig. 7 - Susceptibility TS. Reciprocal Field 12a Tkble I 13 Discussion of Results 14 fable II 16

Hart II. The Effect of Carbon Monoxide on the Magnetic Properties of Iron Supported on alumina

__ Introduction 2 Preparation and Purification of Materials 2 apparatus 3 Calibration 5 Table I 6 Table II 7 Experimental Technique 7 Results 3 Table III 10 Discussion 12 References 13 Fig. 1 14 Fig. 2 2A Fig. 3 15 Fig. U 16 Fig. 5 17 Kg. 6 17 Fig. 7 17 Fig. 8 17

\ Contribution from the Chemical Laboratory of Northwestern University| L —J

THERM&L MIGRATION IN SUPPORTED COPPER CATALYSTS

by Paul Eo Jaeobson and Po Wo Selwood

(l) This work was initiated under contract with the Office of Naval Research and continued on the Union Carbide and Carbon Fellowship in Physical Chemistry, receipt of which is gratefully acknowledgedo This is the fourteenth paper from this Laboratory on the susceptibility isotherm and related subjectso The thirteenth appeared in This Journal 76s000 (1954) o Inquiries should be addressed to Po ¥o Selwocdo

Development of a more sensitive magnetic susceptibility balance of the Fara- day type has made possible extension of observations on supported cupric oxide, and of supported copper9 on gamma alumina lown to copper concentrations of less than 0.5 TMT> cento It has been confirmed that thermal inactlvation (sintering) of this system is not accompanied by copper particle aggregation.. It has also been shown that supported cupric oxide is structurally different from a mechanical mixture of crystalline cupric oxide plvfB cupric ions at infinite magnetic dilution*

The word "sintering* is often used in discussions on contact catalysis « It

implies a change brought about by heat,, and attended with a loss of activity and

of specific surface© Sintering is generally found to occur above the Tammantt

temperature„ and far below the melting points of the catalyst components« Tims

Shfifchter et al*

• •aes**Mvmmiiv

(2) Ao Be Shekhterj, Ao Io Echeistova,, and I« I* Tret»yakov, Chemo Absto44, 915 (19501 j 45, 3216 (1951)«

have reported microcrystal growth in supported metal catalysts such as platinum on

&3u€5wG5 SLV wsmperauures of a rev najsnz&a degresso

This kind of investigation lends itself to magnetochemical study because the

magnetic susceptibility of many catalyst components is strongly dependent en part-

icle size in the range of sizes often encountered in active catalystso A prelimin-

ary study of supported copper by this method' mm,, mm <m, «*> M<»W<^*aca«>0-J

(3) Po W. Selwood end No So Dallas9 This Journal, 70, 2145 (1948).

failed to reveal any surface migration of copper atoms leading to aggregatione Bat

the development of apparatus for measuring susceptibilities with about one hundred

fold incr )ase of sensitivity,, and of conducting reductions and oxidations in situ.

suggested a ^investigation of this problems

Experimental Methods

Magnetic Susceptibility MeasurementSo~The method used S$ essentially that of Fara-

day* The sample hangs in a strong field which possesses a gradient in the verti-

cal dlrection0 The sample is suspended from a silica spiral in a manner similar

to a MoBain-Bakr sorption balance* Deflection of the sample on application of the

magnetic field is observed with a micrometer microscope» The silica spiral and

the surrounding glass tubing are attached to the mechanical stage from a micro*

scope* The stage is adapted for convenient vertical motion of the whole system

relative to the magneto The apparatus is shown diagrammatically in Fig* lo

The field and gradient were obtained by an electromagnet with poles cut in

the manner suggested by Sueksmitho*

CO Wo Sucksmithj Phil* Mag** 89 158 (1929)*

One pole face in profile is shown in Pigo 2» In an effort to increase sensitiv-

ity the angle 0 shown as 15® was increased from the 5* recommended by Sucksmith*

• compromise of 10* has since been found to give good sensitivity with more con-

venience* The minimum pole interspace is lol cm* Maximum fields of 20,000 oer-

steds were obtained*

The force acting on the small sample in this apparatus is given by

f • m7H^H/<d4 where m is the mass of the sample,, ^ the susceptibility of the

jsample per gram9 H is the field strength, and oH/&s the field gradiento It is,

therefore j, necessary to obtain HOE/OS in a region where this product is uni-

form and large* Most inorganic solids contain traces of ferromagnetic impurities*

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It Is necessary, for this reason, also to c-btain H as a function of current in

the magnet9 so that extrapolation may be mad-? to zero reciprocal fieldo

The product Ed H/6 s was obtained by calibration with pure cane sugar, the [

-6 5 | susceptibility of which was taken as -Oo566 x30 per gram©

(5) This value is based on studies by Dro Carl Pitha in this laboratory;, and. is in agreement with the average of the three principal susceptibilities given in the International Critical Tabieso

Correction must,, of course9 be made for the iiaraagnetism of the glass or silica

bucket used to hold the samples The sample moves on application of the field,

hence the procedure followed is to apply the desired fie]d$ move the sample to

a fixed position as observed In the micrometer microscope, remove the field, and

observe extension of the silica splralo This operationf extended to complete

vertical mapping of H^H/^s , is conveniently done with the aid of the mechani-

cal stage referred to aboveo The results are shown in Figo 2o

The field, H, alone may be obtained by the Quincke method, using nickel chlor-

ide solution as a calibrating agento A glass or, better„ silica rod mounted im

the fashion of the Gouy method will also serve to obtain the field as a function

of current as at the working position indicated in Figo 2«

The samples used ranged in weight from 20 to 40 mgo The several silica

spirals used averaged in sensitivity 0o2 mme extension per onto Overall precis"

ion with the catalyst samples used was 0«1 to 0o6 per cent of the measured sus-

ceptibility « ill measurements, except a few noted below, were made at room temp-

erature, near 25•« The surrounding atmosphere was nitrogen which had been passed

over copper turnings at 609®. then dried ovor magnesium purehiorateo

This apparatus is convenient for carrying out reactions such as oxidation

and reduction Jn sltuo The mechanical stage permits horizontal motion of the

sampleo A small tubular furnace may be placed around the sample for such heat

treatment as may be dosired0 The atmosphere surrounding the sample may readily

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be changed through flexible tubing attached to the apparatus ae shove, in Fig* 1.

Preparation and Analysis- of Materials*—The substances investigated were alumina,

euprio oxide, and coppero So-called » gamma"-alumina used throughout the study was

prepared as described by Eischens and Selwoodo*

(6) Ro Po Eisohens and P. ¥« Selwood, This Journal, 69, 1590 (19-47).

The specific surface area of this alumina as measured by nitrogen adsorption

(BoEȴo) was about 250 m2* The magnetic susceptibility versus reciprocal field

for this substance is shown in Fig* 3, together with corresponding £e.ta for four

commercial aluminas* The apparent susceptibility is markedly dependent on the

presence of paramagnetic and„ especially, of ferromagnetic impurities* It also

depends on the water content* For instance, an oven-dried alumina with a sus-

ceptibility of -0*378 (xl0°") per gram was ignited at 500* for three hours- The

susceptibility became -0*357* the change being roughly proportional to the water

loste

In the interest of high precision it was necessary that the copper used in

this study should be as nearly free from ferromagnetic Impurity as possible*

The expedient of quenching the sample from a high temperature and thus forcing

C. Po copper generally shows strong evidence of ferreaagnetisa, and this

may even be true of samples which seem to be spectroscopically free from iron*

nickel, or cobalto Magnetically pure copper can sometimes be prepared directly

from selected sheet copper, or it may be purified by repeated electrolysis*7

the ferromagnetic impurity into solid solution is obviously not practical here j

because the catalytically active surface would be destroyed*

(7) H* Morris and P. ¥• Selwood, This Journal, 62, 2245 (1943)-.

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1 convenient alternative method1 is to dissolve the copp-ar in nitric acid, to rorrn

w w «7. «» Wat. roomomo

(8) Developed in this Laboratory by Dr0 Fred N0 HUlo

the tetrammine complex by the addition of excess ammonium hydroxide, and to add a

dilute solution of aluminum nitrateo The aluminum hydroxide formed apparently

eeprecipitates ferromagnetic~forming impurities because, after the precipitate ef

aluminum hydroxide is filtered, the solution may be neutralized and the precipi-

tate of cupric hydroxide filtered;, washed, ignited,, and reduced to form magnetically

pure coppero Seme data related to these procedures are shown in Figo U°

Supported cupric oxide was obtained in the usual manner by impregnation ef

gamma alumina with cupric nitrate solutiono The mixture is filtered, dried, and

ignited at 390* for twenty-four hourso The amount of copper present in the prod-

uct say be varied at will by varying the concentration of the cupric nitrate so-

lution

Analysis of supported cupric oxide samples was done by dissolving the samples

in sulfuric acid, then electroplating the copper on to weighed platinum electrodes*

The supported cupric oxide samples gave an x-ray diffraction pattern for

cupric oxide, in addition to that for gamma alumina, at copper concentrations in

excess of 7o5 per cento As previously reported, the sample colors ranged from

yellow-green at the lowest concentrations studied ( ^^ 0o5^ Cu ) through blue"

green to gray at the highest concentrations ( ^ 20^ Cu)o Samples reduced 1B

hydrogen were jet black at all copper concentrations except the highest*

Details concerning the procedure used in reducing the above supported oxides

are given belovo

ai

those of intermediate cotceentratiene It uas also noted that cooling the reduced

samples in hydrogen produced the same final susceptibility as flushing and cool-

Results are reported first for the "susceptibility isotherm" of supported

cupric oxide, then for reduced supported copper;, then for samples which hare been

reduced, sintered, and reoxidizedo Finally, some data, are given at lower tempera-

tures o

A typical isotherm for the supported cupric oxide system on alumina at 23* is

shown in Figo 5« This is similar in general form to that previously reported for

this system by Selwood and Dallas, but the copper concentration limit is extended

down to lo42 per cento The plateau of susceptibility per gram of cupric ion at low

concentrations is confirmede It should be emphasized that this is the only system,

of some eight or ten studied in this Laboratory, is which a. plateau suggesting in-

finite magnetic dilution is foundo It was also found that the exact shape and

height of the isotherm depend, as expected, on the purity and formulation of the

support, and to tome degree on the magnetic purity of the solution used for impreg-

nationo The limiting susceptibility per gram of cupric ion at infinite dilution

corresponds to a magnetic moment of lo7 Bohr magneton, in excellent agreement with

the apia-only moment for the one unpaired electron in the cupric ion«

Reduced copper-alumina samples were prepared by reduction with hydrogen at

550* to 650* for eight to ten hourso All reductions were carried out in situ in

the magnetic balanceo Figo 5 includes the susceptibility per gram of reduced cop-

per supported on alumina down to a copper concentration of O0A&8 per cent* Most . *

of these reduced samples gave susceptibilities which were independent of field ') li-

strengthe A typical sample containing 5o43 per cent of copper had a susceptibility \i -6 I of 0.775 (x 10 ) per gram of catalyst sample in the oxidized statec This was

,-' i

changed to -Oo334 in the reduced stateo It was noted that the highest, and es- •'; j ,<

pecially the lowest, concentrations of copper were more difficult to reduce than ,j

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ing in purs nitrogen after reduction hue. be*n completed in hydrogen* Thia suggests

that adsorbed and dissolved hydrogen do not have a measureable effect on the sus-

ceptibility of reduced copper under th6 conditions of this experiment.

Before the results on thermal migration *.va presented the theory of the meth"

od will be outlinedo The cupric ions in highly dispersed cupric oxide have a mag-

netic susceptibility at room temperature of about 25(x 10**°) per gramo The cupric

ions in pure crystalline cupric colds have a susceptibility of about i(z 10 5*

Intermediate degrees of dispersion show Intermediate susceptibilities, although the

exact relationship between susceptibility and dispersion is not knowno

If a highly dispersed copper system is "sinters^ and if iliis brings about an

aggregation of copper atoms, then the decreased dispersion of the copper will be

revealed by a decrease of susceptibilityo The observed relationship of suscepti-

bility to aispersicn occurs for copper only when the copper is in the form of

cupric ions* Hences attempt* to datecst therrsal migration *~ the supported copper

system involve the following steps s (l) measurement of susceptibility in the oxi-

dized state j (2) reduction, (j) sintering„ (4) reoxidation, and (5) measurement of

susceptibility in the reoxidized state** Any decrease of susceptibility occurring

between steps (l) and (5) is evidence of thermal migration to form larger copper

particleso

In the search for evidence of thermal migration there were tried a number of

reduction and sintering conditions as indicated below*

Pigo 6 gives the susceptibilities per gram of samples at 23* against recipro-

cal fieldo The sample contained lt>42 per cent of coppere Reduction in hydrogen

was for ninety minutes at 300*o Reoxidation was performed by slowly heating the

sample to 500* in oxygen over a period of one hour0 No further »sintering" was

done during this particular run* Another trial with reduction conditions changed

to sixtaMB hours at 400" gave similar results* It will be noted that there is no

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evidence for migration© The susceptibility ef the reduces phase, which is in*itided

in Fig* 6, is not necessary for the experiment, but it illustrates the ubiquitous

trace of ferromagnetism as shown by some dependence of susceptibility on field

strengtho

Evidence of diffusion in copper-nickel systems has been obtained as low as

200*, but temperatures above the Tammann temperature were thought more likely to

provide positive thermal migration*) Figo 7 shows the data obtained on a sample

containing 7o29 per cent coppero This sample was reduced and "sintered" in

hydrogen at 550* for one hundred hourso Beoxidation was slot, after this treat-

ments The progress of reozidation was readily followed by the change of color

and by the developing paramagnetismo Results similar to those shown in Fig* 7

were also obtained en samples containing lo42 and 3*35 per cent copper, when

these were subjected to the 550° reduction and sintering treatment*

For reasons described below susceptibilities on several samples were ob-

tained over a range of temperatureo The results are given in Table I**

(9) These data were obtained in this Laboratory by Mr* Stephen Adlers to whom the authors are indebted for this assistance*

Table I

Susceptibilities at 23* per gram of sample for supported cupric oxide on

alumina

Ua4 r»Vi4- fwa-» AA«4 fN«

1*42

9.72

79»9 (pure CuO)

r X ,.6 ISO

298*K* 195*1. 351,

- 0*088 * 0*104 • 1.57

+ 0*811 • 1*26 • 5.23

+ 3o00 • 2*43 • 1.98

u Two observations not directly related to the problem at hand will be mentioned*

The first is that some samples of supported cupric oxide on alumina on prolong-'

ed ignition at temperatures up to 900® gave definite evidence of f6rromagnetism*

This shows that ferromagnetism may be found for certain phases of oxidation state

and environment far almost any transition element in the periodic table*

The second observation is that some samples of supported cupric oxide on

alumina undergo a spontaneous reduction when they are heated to about 500°? If

this heating is done in air the reduction is rapidly followed by a recxidatlon*

The effect is apparently due to some adsorbed reducing gas, or may possibly be

related to release of a trace of water from the gamma alumina at this temperature*

The effect is not accompanied by a "glow phenomenon* or by significant loss of

specific surfaceo

Discussion of Results

The plateau observed for the susceptibility isotherm of'supported cupric oxide

on alumina suggests that at low concentrations of copper in this system the cupric

ions are effectively isolated from each othero Reduction might then be expected

to yield isolated copper atoms, although these would doubtless be highly active

chemicallye Copper atoms have an odd number of electrons^ and should be paramagnetic*

It fallows that reduction of a low concentration copper "alumina catalyst should pro*

duce little or no change of paramagnetismo let even at 1*42 per cent copper, which

is well up on the plateau, reduction lowers the susceptibility per gram of copper

to sere* The conclusion previously reached^ namely that isolated copper atoms are

not formed in these systemss isp therefore, confirmed and extended down to the

lowest copper concentration which it is practical to reach with susceptibility ap-

paratus at present available*

It has also been confirmed that there is no thermal migration leading to in-

creased copper particle size in this systeme "Sintering" as applied to low and

n

15

moderate copper concentrations In a eopp6r~aiumIna catalyst must have a different

meaningo The fact that prolonged sintering leads to some ?ncrease of suscepti-

bility (after reoxidation) suggests that solid solution or compound formation be-

tvson cupric oxide and alumina is the more probable reason why ignition destroys

the activity of this system., This is not to say that sintering in pure copper

powder nay not lead to increased particle size because in any pure, rather than

supported; catalyst the system must be regarded as self-supportedo

Another conclusion to be drawn from the drta concerns the nature of the sup-

ported oxide phaseo In earlier work from this Laboratory it has been assumed that

supported oxides differ qualitatively from pure crystalline oxides by a continu-

ous diminution of the paramagnetic neighborhood surrounding each paramagnetic sup-

ported ion0 There is an alternative view that a supported oxide may consist of a

mixture of ions at infinite magnetic dispersion together with massive crystalline

oxidee According to this view the susceptibility of, say, supported cupric oxide

at any concentration on alumina should be the weighted average of the susceptibility

of cupric ions at infinite magnetic dilution and that of cupric ions in massive

cupric crldee That this view is probably incorrect is shown by the following cal-

eulatlono

Lety.be the susceptibility per gram of cupric ions in cupric oxide at Infin-

ite magnetic dilution, and letX-^be the susceptibility per gran of cupric ions in

massive cupric oxideo Then according to the view presented above, the observed

susceptibility, "Y ,. per gram of cuprl? ions in any catalyst sample should be given

by V " X P * )L(l"P) where p is the fraction of total cupric ions present as

infinitely dispersed ionso This fraction should be independent of the temperature

at which the susceptibility measurements are mad©<>

Taking as an example the sample containing 9o72 per cent copper0 we find the

susceptibilities per gram of cupric ion to be as follows!

_x

16

I Jit I

Susceptibilities per gram ct cupri , jot

Condition aft ^*T[*

infinite magnetic dilution , .5

9o72 per cent catalyst ,

pars cupric oxide* »i3

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"*Fure cupric oxide is anti ferrMagnetk.

Substituting in the abovs eqpa*i> «** *<*: •.,--, (?» ^ ye f^-i ti:>*t *JV '*

ent fraction of cupric ionf prssMW »K iaflKVM .at#r*$es *§* fct i*sf*C, ^ ,,-"

at 195*Kes 0<,44« and at S3*Ko. C»£U , &** 4«»1 %** six? teaj»»t*?* o# SH i

ent degree of dispersion shcv? *b» f V Uej ef % v*m %aUir f _^ss^-*s^ *es»Jj'r

that supported cupric oxide say tec* *ld«r*d #4 * B±TU&$ ^ £45? «»ly ifcir?«r*r

cupric ions and of pure crystal ^ia* < #*i» :xla» Si t£a? r*»ilt# sr J£?

with other catalyst eoncantr&tlsflfa' ttmtm HJWJ t»!»! ^» (KHH *• »tr* iwa

viously expressed that these ruppgr •" # s«*-#lj*t * saw*, ipej^iSjw scsg»> *?<

other paramagnetic elements;, shc» r. 'ffmrtam-t of «:«jt.SiU -« ttt.ci.^.i«

because of a statistically Taryinf.j mnafcetlc 5. $r •>• . .*• «qg«» s*l*g **jfa p«^

magnetic ion«