AN INVESTIGATION OF THE SYSTEM: PbO-Cr203

by

WELDON ERNEST SCHAEFER, B.S. w

A THESIS

IN

CHEMISTRr

Submitted to the Graduate Faculty of Texas Technological College in Partial Fulfillment of

the Requirements for the Degree of

MASTER OF SCIENCE

Approved

Dean of the Graduate School

May, 1961

LIBRARY

ACKNOWLEDGEMENTS

I wish to thank The Robert A. Welch Foundation for their

generous financial support in the form of a Research Fellowship

during the period in which this investigation was carried out.

I am deeply indebted to Dr. Arthur L. Draper for his direc­

tion of this thesis and to the other members of my committee, Dr.

Samuel H. Lee and Dr. Earl H. Gilmore, for their helpful critism.

Appreciation is gratefully acknowledged to Dr. Edward Sturm and

the Department of Geology for the use of their set of the Ameri­

can Society for Testing Materials X-Ray Diffraction Index.

11

TABLE OF CONTENTS

ACKNCMLEDGEMENTS ii

LIST OF TABLES iv

LIST OF FIGURES v

I. INTRODUCTION 1

II. HISTORICAL 2

III. EXPERIMENTAL PROCEDURES $

Preparation of PbO-Cr20^ Gels 5

Heat Treatment of PbO-CrgO^Gels 9

Analysis by X-ray Diffraction 9

IV. EXPERIMENTAL RESULTS 11

V. DISCUSSION OF EXPERIMENTAL RESULTS 20

Crystallization Inhibition of CrgO^ 20

Compound Formation in the PbO-Gr20~ System 21

Other Effects of Heating in Nitrogen 23

VI. CONCLUSIONS 25

LIST OF REFERENCES 27

0.11

LIST OF TABLES

Table Page

I. References to the X-ray Diffraction Patterns Used for Identification of Conpounds h

II. Preparation of PbO-Cr203 Gels 8

III. Compoiinds Formed in the PbO-Cr203 System 11

IV. X-ray Diffraction Patterns of the Identified CoiT jounds Formed in the Pb0-Cr203 System 12

V. X-ray Diffraction Patterns of the Unidentified

Compounds Formed in the PbO-Cr203 System l5

VI. Compound Formation in Air l6

VII. Compound Formation in a Nitrogen Atmosphere 18

IV

LIST OF FIGURES

Figure Page

1. Yields of PbO-Cr203 Gels 7

2. The PbO-Cr203 System Heated in Air 17

3 . The Pbo-Cr203 System Heated in Nitrogen 19

V

I. INTRODUCTION

The physical and structural properties of mixed oxides may

be altered by changes in the method of preparation or of treatment

of samples. Many times, compound formation or changes in catalytic

properties are observed in mixed oxide systems because of these al­

terations. Compound formation results from solid state reactions

which may occur when the samples are heated. Changes in catalytic

behavior usually result from an alteration of the structure or an

increase in the surface area of a catalyst by the addition of a

small amount of another oxide. Mixed oxides are made by preparing

a known oxide-catalyst with another oxide-catalyst or another oxide.

In the present investigation, the lead monoxide-chromic oxide sys­

tem was studied for such behavior.

For the investigation of the system, Pb0-Cr20-^, mixed oxide

samples were prepared in intimate mixtures by coprecipitation. The

prepared samples were heated at various elevated temperatures in a

systematic manner. X-ray diffraction patterns were taken of the

samples to identify the confounds formed in the system and to de­

tect any structural alterations which might occur. The results ob­

tained from the investigation of the PbO-CrgO^ system are discussed

in the following sections.

1

II. HISTORICAL

Physical properties, structural characteristics, and conpound

formation in the system, PbO-Cr203, have been incompletely studied.

Only portions of the system have been investigated, although the

pure oxides of lead have been widely studied.

Jaeger and Germs (1) studied the system, PbO-PbCrOi . Their

samples were physical mixtures of PbO and PbCrOi , heated to elevated

temperatures. For lead chromate, they reported three forms: an

o(-PbCrOi was found to exist in the temperature range, from room

temperature to approximately 700° C; a/5-PbCrOi existed in the

teirperature range, 700-780° C^ and, a T-form was obtained at tem­

peratures above 780° C. Two forms of Pb^CrOg were observed. An

QC-Pb^CrOg was found in the temperature range from room temperature

to approximately 750* C, and f> -Pb[ CrOg was observed in the tem­

perature range, 750-820° C. Also, two forms of PbyCrgO^^, were re­

ported at the mole ratio of 5PbO:2PbCrOi . The «<- and ^ -forms of

PboCr20-, were found in the same tenperature ranges as the corre­

sponding *K - and ^ -Pb^CrOg. At 50 mol^ PbCrOi , only one form of

PbpCrOK was detected, existing from room temperature to 921° C.

Hicks (2) studied the solid state reactions of mixtures of

PbO and PbCrOr in a fused salt medium of KNO3 and NaNOo. Freezing-

point curves were used to determine compound formation and phase

transitions. In addition to Pb2CrO^ and Pb^CrOg as reported by

Jaeger and Germs (1), Hicks (2) reported the formation of Pb3CrO^

3

and Pb^rj^O-LY ^^ ^^® PbO:PbCrO|^ mole ratios of 2:1 and 1:1;, respectively.

However, the formation of PbyCr202_3 was not observed as reported by

Jaeger and Germs (1),

Wagner, Haug, and Zipfel (3) prepared a red, plate-like sub­

stance from the reaction of lead chromate with alkali. It was assumed

that the compound was a mixture of amorphous Pb2(OH)2CrOL»nH20 with a

few crystals of Pb2(0H)2Cr0|^. Wagner and Schirmer (1;) prepared a com­

pound which they called Pb2Cr0^ by reacting PbCrO^ with KOH. The struc­

ture of Pb2(0H)2Cr0^ was determined by de Wolff (5) from x-ray diffrac­

tion data. Recently, further investigations by de Wolff (6) have cast

some doubt as to the true formula of the reported compound, Pb2(0H)2Cr0^,

A monoclinic and an orthorhombic form of PbCrO|^ have been re­

ported. The monoclinic PbCr0[^ structure was determined by de Wolff

(5). The orthorhombic form of PbCr0[^ was first reported by Quittner,

Sapgir, and Rassudowa (7). Collotti, Conti, and Zocchi (8) deter­

mined the structure of the orthorhombic form of PbCr0|^.

The oxides of lead have been studied extensively by several

workers. Consequently, a large volume of x-ray diffraction data is

available for the various lead oxides. A compilation of the refer­

ences listing the x-ray diffraction patterns of the various oxides of

lead is given in Table I, together with the references containing x-ray

diffraction data for the other compounds studied in this investigation.

TABLE I.—References to the X-ray Diffraction Patterns Used for Identification of Compounds

Substance References

PbCrO^ 5,7

Pb2(OH)2CrO^ 5

Cr203 10

5PbO-2H20 9

PbO(red) 11,16,17,20

PbO(yellow) 12,16,18,19,20

Pb 2 O3 13,16,20,21,22,25,27

?h^Ol^ Ii;,l6,20,21,22,23,2ii,25,26

PbOg 15,16,21,22,23

III. EXPERIMENTAL PROCEDURES

The experimental procedures used for the study of the PbO-Cr20^

system are coprecipitation of the mixed oxide gels, heat treatment of

the prepared samples, and analysis of the heated sanples by the use

of x-ray powder diffraction techniques. These procedures are de­

scribed below.

The Preparation of the Pb0-Cr203 Gels

The lead nitrate used in the preparation of the gels was crys­

talline, chemically pure, Baker and Adamson reagent and was obtained

from the General Chemical Division, Allied Chemical and Dye Corpora­

tion, New York, New York. The chromium(ic) nitrate used in the pre­

paration of the mixed oxide gels was Cr(N0o)^'9H20. The reagent was

crystalline, chemically pure, "Baker»s Analyzed"; it was obtained

from the J. T. Baker Chemical Company, Phillipsburg, New Jersey.

The PbO-Cr20o mixed oxide gels were prepared by coprecipita­

tion of the hydroxides of lead(II) and chromium(III). The hydro­

xides were precipitated as gels by rapidly mixing sodium hydroxide

and aqueous lead(II) nitrate and chromium(III) nitrate solutions,

all IN in strength. The composition of the resultant gels was deter­

mined by calculating the volumes of nitrate solutions required to

yield the desired mol fraction in a particular mixed oxide. The re­

quired volumes of filtered lead(II) nitrate solution and chromium(III)

nitrate solution were mixed thoroughly in a 50 ml beaker. The beaker

containing the nitrate solutions was placed inside a 12-oz screw-cap

bottle. A stirring rod of the proper length with a rubber policeman

at each end was used to keep the beaker at the bottom of the bottle.

The stoichiometric equivalent of sodium hydroxide required to pre­

cipitate the metallic hydroxides was placed in the bottle. The

sealed bottle containing the beaker of aqueous nitrate solution was

quickly inverted and shaken, thereby effecting a rapid, homogeneous

coprecipitation of the mixed oxide gel of the predetermined conposi-

tion.

To detennine the effectiveness of the precipitation and to

ascertain the closeness of the actual yields to the predicted yields,

one set of air-dried Pb0-Cr203 mixed oxide gels was weighed and com­

pared with the theoretical yields. The results are shown in Figure

1. It will be noted that the actual yields vary from theoretical

yields, indicating a difference in the effectiveness of the precipi­

tation of the lead and chromium hydroxides. The chromium appears

more effectively precipitated than the leadj in fact, it exceeds

100^ yield. The high water adsorption capacity of the Cr203 causes

this discrepancy, so that the Cr20o yield is also below the theore­

tical value. Thus, it is seen that some of each oxide is lost in

the precipitation, but the effect on the composition is small. These

compositions, of course, could be checked by chemical analysis. In

view of the new corr jounds detected in this system, a careful deter­

mination of the composition at each point will be necessary when

structural and chemical identification are effected. For the pur­

pose of this study in delineating the possible existence of new

m 0)

C5

r^ O

CsJ

O I

O

£ o

O •H

•H

sniBvta *Pi9fi la^oj,

8

compounds and detecting any crystallization inhibition which might occur

in the system, the error in oonposition was considered negligible.

In Table II, the pipetted volumes of IN nitrate solutions and

IN sodium hydroxide are given for the preparation of the mixed oxide

gels at the indicated compositions,

TABLE II.—Preparation of Pb0-Cr203 Oels

moljS PbO

100 98 9^ 92 90 89 86 82 80 79 75 71 70 67 60 57 50 i;6 UO 33 30 20 18 10

0

xno1$ CrgOo

0 2 5 8

10 11 Ik 18 20 21 25 29 30 33 ho h3> 50 5ii 60 67 70 80 82 90

100

ml of Pb(N03)2

20.0 38.0 18.0 3i4.0 18.0 16.0 30.0 m.o 16.0 26.0 12.0 22.0 lU.O 10.0 12.0

8.0 10.0

6.0 8.0 U.o 6.0 I4.O 2.0 2.0 •N • •

ml of Cr(N03)3

. «

3.0 3.0 9.0 6.0 6.0

15.0 9.0

12.0 21.0 12.0 27.0 18.0 15.0 2ii.O 18.0 30,0 21,0 36.0 21;.0 1|2,0 U8.0 27.0 5U.0

1 60.0

ml of NaOH

20.0 Ul.O 21.0 1;3.0 2ii.0 22.0 15.0 23.0 28.0 li7.0 2U.0 ii9.0 32.0 25.0 36.0 26.0 Uo.o 27.0 UU.o 28.0 i;8,0 52.0 29.0 56.0

1 60.0

The freshly precipitated gels were centrifuged, washed with

distilled water, and centrifuged again. After each washing, the

supernatant liquid was tested for the presence of nitrate ion by

the use of the nitrate brown-ring test. The results of the tests

were negative after three washings, indicating the absence of nitrate

ion. However, the washing process was repeated until the sanples be­

gan to peptize. Approximately five washings were required to initiate

the peptization of the samples.

The washed mixed oxide gels were dried in air either at room

temperature or in an oven at 105° C; one set of mixed oxide gels was

dried in a nitrogen atmosphere at 105° C, The dried gels were used

for the subsequent heat treatments.

Heat Treatment of PbO-Cr203 Gels

Portions of 10-50 mg of the dried PbO-Cr203 gels were heated in

air and in nitrogen at various temperatures between 105 and 800° C

steps. A portion of fresh sample was used for each heating. A muffle

furnace was used to heat the samples in air at a temperature constant

to within 10° C for 21; hours. A tube furnace was used for the heatings

in nitrogen.

The nitrogen used for an inert atmosphere was passed through two

traps. The first trap contained glass wool to remove any physical im­

purities, and the second trap contained calcium chloride to remove any

water vapor present in the nitrogen. After leaving the traps, the ni­

trogen was allowed to flow over the sample in the tube furnace. The

samples heated in nitrogen were also allowed to cool in nitrogen.

Analysis by X-ray Diffraction

After cooling, the heat-treated samples were examined by x-ray

diffraction powder methods. The x-ray diffraction patterns of

10

approximately 100 samples were obtained in the course of the study of

the PbO-Cr203 system. Standard Debye-Scherrer x-ray diffraction pic­

tures were taken with a General Electric Camera mounted on a Philips

Electronic X-ray Generator. Copper Kc radiation, obtained by filter­

ing with a thin sheet of nickel, was used throughout the investigation.

The distance in cm on the x-ray diffraction film between the

center of the unreflected x-ray beam and the center of the particular

line was measured with a Norelco Illuminating and Measuring Device.

This distance was multiplied by U, the camera factor, to obtain the

angle Q of the Bragg equation, so that d/n values could be calculated

from the Bragg equation:

' s m ^

where d is the distance between parallel planes of the crystal, n is

the order of reflection, X is the wavelength of the incident radia­

tion, and 9 is the angle of reflection.

The relative intensities, I/IQJ of the diffraction lines were

estimated visually using the darkest line as the standard. The esti­

mated intensities were not corrected for beam absorption since the

latter is due to the geometry of the mounted sarr5)le.

IV. EXPERIMENTAL RESULTS

A list is given in Table III of the distinct con5)ound8 observed

in the Pb0-0r203 system, their composition, ten^jerature of formation,

temperature of decomposition, and identity where possible. The eleven

coiT^jounds listed were those detected as basic coirponents of the x-ray

diffraction pictures for the samples of the Pb0-Cr203 system. The

patterns of previously reported compounds were Identified by conpari-

son with the x-ray diffraction patterns given in the references in

Table I. These corrparisons are given in Table IV. The x-ray diffrac­

tion patterns of the previously unreported compounds are given in

Table V.

TABLE III,—Compounds Formed in the Pb0-Cr203 System

Cmpd

A B

C

D

E

F

Q

H

J

K

L

Identity

PbO(red) Cr203

PbjOj^

FbO(yeilljaw)

PbCrO|^

PbgCrO/

?

7

?

?

?

Con^osition

mol^ PbO

100 —

100

100

67

80

82

95-92

100

100

95-92

mol^ Cr203

—

100 —

—

33

20

18

5-8 —

•»••

5-8

Temp.of Formation

350° Uoo° 1;00°

500° 160°

160°

25°

105° l6o° 2^0

160°

Temp.of Decomp,

U5o°

600°

720°

160° 0

300 250° 150°

650°

Comment

in N2

in Air

in N2

in N2

^Previously reported as Pb2(0H)2Cr0L(5).

The observed relative intensities given in Tables IV and V were

11

12

not corrected for beam absorption. Thus, the observed relative inten­

sities of the reflections of the longer d/n spacings were found to be

less than the corrected values given for the standard patterns. This

means that the observed relative intensities of the shorter d/n spac­

ings should appear greater in coiT5)arison with the intensities of the

reflections of the longer d/n spacings, as is seen in Table IV.

TABLE IV.—X-ray Diffraction Patterns of the Identified Compounds Formed in the

PbO-Cr203 System

Con jound A

d/n (X) 5.02 3.10 2,79 2,50

1.98 1.86 1.67 1.55 1.51;

1.1;3 l . U o 1.28 1.25 1.22

1.21 1.19 1.11; 1.12 1.07

1.06

lAo 10

100 50 20

20 90 90 20 50

10 80 20 20 50

50 5

10 10 20

10

PbO(red)

d/n (I) 5.018 3.115 2.809 2.510 2.121;

1.988 1.872 1.675 1.558 i.5i;2

1.1;38 l.i;05 1,282 1.256 1,226

1.219 1.1977 l.li;62 1.1232 1.0768

1,0610

(11)

Vio 5

100 62 18

1

8 37 21;

6 11

2 5 2 3 1;

5 < l

2 2 3

2

Compound B

d/n d)

3.63 2,67 2.1;8

2,18

1.82 1,68 1.58 l.i;7

1.1;3 1.30 1.21; 1.21

^Ao 60 60 60

20

30 100

1 60

80 30 10 10

Cr203 (10)

d/n (I)

3.633 2,666 2,1;80 2,261; 2.176

2.0i;8 1,8156 1.672 1.579 1.1;65

1.1;311; 1.2961 1,2398 1.2101 1.1731

Vio 71;

100 96 12 38

9 39 90 13 25

Uo 20 17

7 11;

13 TABLE IV—Continued

Compound C

d/n (X)

6.27

3.38 3.29 3.12

2 .91

2.6U

2.26

1.91

1.83 1.75

1.61;

lAo i

90 3

^0

10

10

1

10

10 90

10

Pb30^ {Ik)

d/n (X)

6.23 3.66 3.38 3.28 3.113

2.903 2.787 2.632 2.1;I;li 2.289

2.260 2.205 2.076 2.032 1.970

1.903 1.887 1.829 1.755 1.729

1.7025 1.6897 1.6i;17 1.6320

lAo 11

3 100

7 19

i;8 hS 30

2 h

8 1 1

12 12

22 < 1 21 30

1

2 2 8

<1

Conpound D

d/n d)

3.06 2.91; 2.71; 2 .51

2.37 2.28

2.02 1.96

1.8^ 1.80 1.72 1.61; 1.60

1.53 1.51 1.1^7 l . i ;0 1.37

1.36 1.33 1.30 1.29 1.25

1.21; 1.20 1.19 1,18 l . l U

1.12 1.10

lAo

70 30 20 5

20 1

30 30

35 30

100 So

1

50 5

100 1 1

30 1

30 30 30

1 20 10 15 10

50 10

PbO(yell

d/n d)

5.893 3.067 2.9l;6 2.71;!; 2.i;93

2.377 2.278 2.203 2.008 1.963

1.850 1.797 1.721; 1.61;0 1.596

1.531; 1.511; l.l;7l; 1.1;08 1.372

1.363 1.325 1.297 1.289 1.252

1.214; 1.203 1.188 1.171; 1.139

1.120 1.102 1.091

.ow) 0-2)

lAo 6

100 31 28

< 1

20 < 1 < 1 12

2

m 11; 15 13 < 1

9 2

11 < 1

1

< 1 1 2 3 2

2 h 3 1; 2

2

CM

CM

lU

TABLE IV~Continued

Compound E

d/n (X)

$.h6 5.12 h.9h 1;.39

3.76

3.1;9

3.28

3.17 3.11 3.01;

2.73 2.67 2.6o 2.56

2,1;7

2.32 2,26

2.09

2.06

2.00

1.98

1.905

1.85

I/Io

30 5

50 60

30

90

100

20 20 80

5 30 30 50

5

30 60

3

3

3

3

3

70

PbCrO^

d/n (X)

5.ii3 5.10 h.96 i;.38 1;.37

3.76 3.72 3.i;8 3.32 3.28

3.21; 3.15 3.09 3.03 3.00

2,710 2.653 2.597 2.51;9 2.510

2.1;60 2.351 2.320 2.252 2.21;3

2.211; 2.151; 2.130 2.120 2.090

2.080 2.056 2.0l;6 2.002 1.988

1.978 1.967 1.900 1.866 1.857

(5)

I/Io

10 6

25 25 13

12 7

SS h

100

5 11 6

6S 30

16 1

11; 18 1

h 3

13 25 9

3 3 1 1

25

3 1; 3 7 3

20 13 8 3 1;

Compound F

d/n (X)

6.1;6 6.32 S.9S

l;.i;2

3.77

3.37 3.21

2.97 2.87 2.82

2.50 2.1;7

2.36

2.31 2.25 2.18

2.10

2.05

1.98 1.91;

1.86

1.77 1.76 1.7U5 1.718

I/Io

20 20 20

1;0

10

100 10

90 10 30

10 15

5

10 30 1

20

30

10 10

80

10 10 5 20

Pb2(0H:^CrO|^ (5)

d/n (X)

6.1;5 6.32 5.96 5.17 h.hh

3.78 3.73 3.51; 3.39 3.23

2.986 2.882 2.8i;0 2.650 3.590 .

2.565 2.511 2.U79 2.1;60 2.372

2.315 2.269 2.187 2.132 2.111;

2.058 3.015 1.992 1.950 1.871;

1.867 1.859

I/Io

13 15 9 2

13

8 3 5

100 11

80 V 20

35 2 h

h 11 13 5 7

8 15B 6 5 9

17 1 5 1; 3

17 3

1.829 1 2 1.818 1.791;

1.778 1.770 1.753 1.726 ,

1 1

8 h 3

11

15

TABLE v.—X-ray Diffraction Patterns of the Unidentified Compounds Formed in the Pb0-Cr203 System

Compound

d/n (A)

1;.1;8 1;.23 3.60 3.28 2,62

2.21 1.86 1.70 1.59 1.53

1.1;8 1,20 1.15

0

I/Io

15 15 90 90 100

10 5 20 5 5

5 5 5

Con?)ound

d/n (X)

5.89 3.23 3.05 2.92 2.79

2.73 2,36 2,00 1.88 1.81;

1.79 1.71 1.67 1.63 1.59

1.52 1.1;7 1.1;0 1.20 1.18

1.17

H 1 I/Io

1 5

100 15 10

10 15 5B 5 5

10 lOB 5B lOB 1

lOB 15B

1 1 1

1

Compound J

d/n (A)

9.03 7.96 6.65 5.1;7 U.87

1;.57 3.1;1 3.23 3.16 3.06

2.76 2.71 2.01 1.91; 1.81;

1.68 1.65 1.62 1.60 1.51;

1.1;7 1.38 1.36 1.30 1.27

1,26

I/Io

60 5 10 3 10

5 5

100 5

100

30 50 10 50 15

9S 25 20 90 25

25 10 25 25 25

Con5)ounc

d/n (X) 8,U2 5.16 1;.1;3 1;.23 3.31;

3.26 2,98 2.81 2.1;6 2.1;2

2.31 2.20 2.0lt 1.95 1.90

1.77 1.71 1.67 1.60

I K

I/Io

100 1 10 1

100

80 90 9S 2 1

1 1 10 20 25

20 10 30 30

.

Corr5)ound

d/n (X)

8,23 7.i;8 6,28 3.39 3.23

3.15 3.09 2,98 2.89 2,80

2,68 2,51 2,37 2,26 2,06

1.98 1.95 1.87 1.85 1.79

1.73 1.70 1.67 1.65 1.61

1.60 1.57 1.55 1.1;7 1.1;5

l.Uo I.3I; 1.29 1.28 1.27

1.25 1.22 1.21 1.20

L

I/Io

10 5 2 15 100

50 50 5 25 10

80 1 1 2 5

60 60 Uo Uo 1

70 10 1

30 Uo

15 10 10 10 5

lOB 20B 5B 15B 15B

lOB I5B 15B 5B

16

The identification of the samples heated in air is summarized

in Table VI. Compounds formed in the Pb0-Cr20^ system are listed by

composition at the temperature of heat treatment. Figure 2 is a dia­

gram constructed from the x-ray diffraction data of the sair^les heated

in air at various temperatures5 it approximates a phase diagram.

TABLE VI.—Compound Formation in Air

moljgl PbO

100 9S 92 90 89 82 75

67

57

1;6

33

18

0

mol^ Cr203

0 5 8

10 11 18 25

33

1;3

51;

67

82

100

io5°c

K K+(G?)

G+K Q 0

Amor­phous Amor­phous Amor­phous Amor­phous Amor­phous Amor­phous

200°C

J Jf(a?)

Amor­phous

Amor­phous

300°C

J Lf

F F

F+E

E

E

E

Amor­phous

1;00°C

L LfF(+)

5oo°c

C+(A) A+

E

B

600°C 700°C

D A+

F F

F+E

E+F(t)

E+B

E+B

B+E

B+E

B

800°C

F

F F F

The identification of the samples heated in a nitrogen atmos­

phere is summarized in Table VII. Figure 3 is a diagram constructed

from the x-ray diffraction data of the samples heated at various tem­

peratures in a nitrogen atmosphere.

The x-ray diffraction pattern of a 50-50 mol^ physical mixture

of PbO(red) and Cr203 heated in air at 1;00°C for l5 hours contained

only the diffraction lines of PbCrOh and a small amount of unreacted

17

73

o •H -p •H CQ

s -P

•g O

O

o

•b^ f A O O O CM

O

R

- p • H

O • H bD <D ;^

e w

| 3 CU

- ^

CO

o

o CO

e- -o

f A O

+ ,CM

o

•LA

O CM

1 A

CM + O

pL, pU,

o

•LfN

2 O

CM

P H

c5

o

O l O

C5 !^

o o

T3

5j

o NO

o l A

f A

o CM

O

H O

O

O f A

O CM

o H

o o oo

O o r—

o O NO

o o l A

o o -::t

o o (A

o o CM

o o H

o o O JO H P.

OQ 9J:nq.ej:a(iuaj^

•H

c •H

X ) (U

-P CO 0)

6 (D

+^ W >s

CQ

f A

o CM

O I

o

(D ,c! E-i

9 I •

CM

bO •H

18

PbO and Cr20o. The x-ray diffraction pattern of an identical sanple

heated in nitrogen at 1;00° C for l5 hours contained only the lines of

corT5)ound F.

The x-ray diffraction pattern of a sanple of PbCrOi heated at

800 C in air for 2^ hours was identified as the pattern of compound

F,

TABLE VII,—Compound Formation in a Nitrogen Atmosphere

mol^ PbO

100 98 95 92 90 89 86 82

75 67

0

mol^ Cr203

0 2 5 8 10 11 lU 18

25 33

100

105^0

K

H+K

K+(?)

Amop-phous

160°C

J

J+H

F+J

F

F+E E

200°C

J

H

H+F

Amor­phous

300°C

J

H+L

F+J(?)

Amor­phous

1;00°C

A A+J L+H L+F L+F F+L F

F

F+E E

Amor­phous

B

5oo°c

D+A

B+E

600°C

D+A

+A(+)

700°C

D+A

+A F+H(?)

F+L

800°C

F

F F

19

o •H +> • H CQ O

i-o o

O O

O •H •P •H 03

-P

^3

o o

CO

O •H bD

-P fn

E 03 •H r3 H O , -d H ft

& 05

O

fA O

o

o o O o NO "lA ^

OQ Qanq.Bj8diu8j,

8 8 fA

O O CM

03

O Xi

e-o

fA

Q CM

H O

O ON

O CO

O

o NO

fA O

CM

o o •LA

o

\ A

2 o CM

PU|

1 1

- "3 + ^ \

•LA - ^

? "2 O + O

CM iP

•LA i-A

U TD O + ft

CM B

£ "

•^ 1 <D 73 k b

t Is"-" 1 IP4 1

1

• 1 no 1

1*1 U 1

•LA

"2 O +

CM

£ •-3

TD

t 1 t)

W TJ

i-o

w TD

& 1"

«

, •

• •

•

« W 1 X ) T3

1 §•* i-1 1^ ^

o fA

O CM

o o o H

-P

x> -p

0) K S (D

+J

n CO

fA O CM

u o

I O

e I I •

fA

bD •H

V, DISCUSSION OF EXPERIMENTAL RESULTS

The results of the investigation of the PbO-Cr20^ system are

discussed below. An inhibition of the crystallization of Cr20n was

observed upon doping with 10 mol^ PbO. Formation of numerous dis­

tinct confounds was observed; some were previously unreported.

PbCrO| was observed to form at a PbO:Cr203 mole ratio of 2:1.

Pb2CrO^, reported as Pb2(OH)2CrO|^ by de Wolff (5), was observed to

exist throughout a wide range of temperatures and compositions.

Mechanisms are suggested for the formation of both PbCrOi and Pb2CrO,-.

Pb30|^ and compound G were detected only in the samples heated in air

and con jound H was found only in the samples heated in nitrogen.

Crystallization Inhibition of CrgO^

It is seen in Figures 2 and 3 that chromia crystallizes when

heated to UOO C, However, the ciystallization of Cr^O^ is inhibited

by the presence of 10 mol^ PbO until the CrgO^ is heated to 500° C.

The composition range of inhibition of the crystallization, frequently

called mutual protection, is illustrated by the peak in the dotted

line enclosing the amorphous region in Figures 2 and 3.

Inhibition of the crystallization of a catalyst by mutual pro­

tection tends to keep the crystallite size small. The surface area

varies inversely with the crystallite size, and it is also a measure

of the catalytic activity of a catalyst. Therefore, the inhibition

of the crystallization of a catalyst by mutual protection permits

the use of the catalyst at higher operating temperatures without loss

20

21

of surface area due to the crystallization of the catalyst. Thus

the occurrence of the phenomenon of mutual protection is frequently

indicative of the in^jrovement of the potential catalytic activity

of a catalyst.

Compound Formation in the Pb0-Cr203 System

In the course of this investigation, the formation of numerous,

distinct compounds was observed (Table H I ) . Many of these con5)ound8

were reported previously and were consequently identified by x-ray

diffraction methods. However, compounds G, H, J, K, and L, previously

unreported, were not identified. The x-ray diffraction patterns of

these compounds are given in Table V.

The formation of PbCrOi was observed at 33 l/3 molj Cr20o in

. 0 samples heated at 150-700 C. The mole ratio of PbO to Cr203 is 2:1

at this composition. The suggested reaction for the formation of

PbCrOi is:

^ 2 3 * ^ 0 > 2 ^^3

2 Cr03 + 2 PbO > 2 PbCrO^.

In order to form the PbCrOi , the CrpO^ must be oxidized to CrO^. It

is postulated that the CrpO^ is oxidized to 2 CrO^ in the presence of

lead oxide at l6o° C. Udy (28) supplied evidence to support this hy­

pothesis. The tenperature of transition of CrgO^ to 2 CrO- was given

as l;6o C. It is possible that in a very intimate mixture with lead

oxide, such as that obtained by coprecipitation, the oxidation of

CrgO^ to 2 Cr03 occurs at temperatures of approximately 150-160 C.

22

In the temperature range, 700-780 C, Jaeger and Germs (1) ob­

served the formation of another form of PbCrOi , which they called

p-PbCrO|^. Nevertheless, in this investigation, the x-ray diffrac­

tion pattern of a 33 1/3 mol^ Cr203 gel heated at 720° C contained

only diffraction lines characteristic of PbCrO, and PhpCrO^. An

identical sanple heated to 800° C yielded only PhpCrO^. Only one

form of PbCrO|^ was observed in this investigation, in agreement with

Hicks (2).

In Figures 2 and 3 it is seen that compound formation occurs

at 20 mol^ Cr203, The compound formed at this composition would be

expected to have the formula PbgCiO^ or PbO'PbCrO, . However, the

x-ray diffraction pattern of the compound formed at 20 mol^ CrpO-

(compound F in Table III) was found to be the same as the x-ray

diffraction pattern reported by de Wolff (5) for Pb2(0H)pCr0j , The

existence of Pbp(OH)pGrOi at temperatures of approximately 150° C

is certainly conceivable at this composition. However, it seems

that a compound such as Pbp(OH)pCrOt would be deconposed to an oxide

by heating at high temperatures. Nevertheless, by using the x-ray

diffraction pattern given by de Wolff (5) for Pb2(OH)2CrO, , it would

appear that Pbp(OH)pCrO, exists at 800 C throughout a -wide conposi-

tion range.

If the conpound formed at 20 mol^ ^ 2 3 ^ ^\iOE)JZTo^, the

transition: o

PbCr0|^ .800_C ^ Pb2(OH)2CrO^

which occurred at 33 1/3 mol^ CrpO-., must be explained. The reported

x-ray diffraction pattern of Pbp(OH)pCrO. was also observed by de Wolff

23,

(6) in a sanple of PbCrO, heated at 900° C,

If instead of Pb2(0H)2Cr0j^, the conpound formed at 20 mol^

Cr203 is Pb2Cr0^, the transition:

PbCrO^ — i 2 2 l c _ ^ Pb2CrO^

may be explained as follows: since Cr03 is quite volatile, even at

200 C, it is possible that PbCrO^^ was partially decorrposed at 800° C

yielding a mole of Pb2Cr0^ and a mole of Cr03 for each two moles of

PbCrO^ heated. The appearance of deposits of a red substance inside

the crucible covers and inside the tube furnace indicated that vola­

tilization of some material occurred duidng the heat treatment of the

samples. For the transition to occur, some chromium must be lost in

the reaction. Therefore, it is concluded that the compound formed

at 20 mol^ Ov^O^^ is PbgCrO^ and not Pb2(0H)2Cr0, . The formation of

PbgCrO^ at 20 mol^ ^ 2 3 agrees with the results of Jaeger and Germs

(1) and of Hicks (2).

A set of PbO-Cr20^ mixed oxide gels was heated in a nitrogen

atmosphere in an effort to discern the origin of the oxidation of

Cr203 to QvOy The formation of PbCrOi and Pb2CrO^ was observed at

33 1/3 moljg and 20 molj^ Cr203, respectively. Therefore, oxidation

of Cr20^ to CrOn also occurred in a nitrogen atmosphere. Thus, the

oxidation of Cr20^ in this case could not be due to molecular oxygen.

Other Effects of Heating in Nitrogen

In some cases the results for heating in an atmosphere of ni­

trogen were found to be different from those obtained for the samples

heated in air. These differences were observed with both physical

21;

mixtures and coprecipitated gels.

Pb30i was found to form when the 100 mol^ PbO gel was heated

in air at 500 C for 21; hours. Heating another portion of the 100

mol^ PbO gel at 500 C in a nitrogen atmosphere produced PbO (yellow).

These observations indicate that some oxidation of PbO occurs in air

but not in nitrogen. Thus, the oxygen for the oxidation of lead

monoxide very likely comes from the air.

Another difference was that conpound G, which appeared only in

the samples dried at room tenperature in air or heated in air at 105° C,

was not detected in the samples heated in nitrogen. Conversely, com­

pound H was found only in the sanples heated in nitrogen. The tenper­

ature range of formation of compound H was found to be from room tem­

perature to approximately 350 C. The compositions of compounds G

and H were not accurately ascertained and further investigation is

necessary to determine their physical and structural properties.

The compound obtained by heating the 50-50 mol^ physical mix­

ture of PbO (red) and Cr20^ in air was found to be different from the

conpound obtained by heating a portion of the same physical mixture

in a nitrogen atmosphere. PbCrOi was obtained by heating this mix­

ture in air at 1;00° C for l5 hours. Pb CrO^ was obtained by heating

a portion of the same physical mixture in a nitrogen atmosphere at

1;00° C for l5 hours. It should also be pointed out that Pb^CrO^ was

not obtained by heating the 50 mol^ ^ 2* 3 °0P^®°^P^"^^^®^ mixed oxide

gel in a nitrogen atmosphere at 1;00° C as it was in the physical mix­

ture. Further investigation of the system is required to explain

this behavior.

VI. CONCLUSIONS

1. The system, PbO-Cr203, has been studied by x-ray diffraction

analysis of mixed oxide gels prepared by coprecipitation and heat

treatment at elevated tenperatures.

2. Pure chromic oxide gel heated at 1;00° G was found to be

crystalline Cr203. However, a sample of chromic oxide gel contain­

ing 10 mol^ PbO did not ciystallize until heated at SOO^ C.

3. Compound formation was observed in the PbO-rich region of

the system.

h. Numerous pre-^iously unreported compounds were observed in

various ranges of tenperature and composition,

5. The compounds formed, as well as their ranges of composition

and tenperature, are not in complete agreement wi.th the results of

previously reported,

6. Only one form of PbCrOr was observed,

7. Pb2CrO^ was found to exist throughout a very -wide range of

composition,

8. A sample (33 l/3 mol^ Cr203), when heated at 300-700° C was

oxidized to PbCrOi , which was converted to PbpCrOj- upon heating to

800° C,

9. PbCrOi was formed by heating a 50-50 mol^ physical mixture

of PbO(red) and CrpO^ in air at i;00° C, while PhpCrO ^ was formed

when a portion of the same physical mixture was heated in an atmos­

phere of nitrogen at 1;00 C.

10, ^ 3* 11 ^® found only in the samples heated in air.

25

26

0 11. At temperatures below 300 C and in the conposition range, 0-30

mol/C Cr203, an unidentified compound (compound G) was observed only in

the sanples heated in air, while another unidentified conpound (com­

pound H) was detected only in the sanples heated in nitrogen.

12. Further investigation of the PbO-Cr203 system is necessary to

identify the previously unreported compounds and to es-bablish the

mechanisms by which these conpounds are formed.

LIST OF REFERENCES

(1) F. M. Jaeger and H. C. Germs, Z. anorg, allgem. Chem., 119, ll;5-73(1921). ^ ^

(2

(3

ih

(5

(6

(7

(8

(9

(10

(11

(12

(13

(111

(15

(16

(17

(18

(19

(20

J. F. G. Hicks, J. Phys. Chem.. 25, 515-60(1921).

H. Wagner, R. Haug, and M. Zipfel, Z. anorg. allgem. Chem.. 208, 2U9-5U(1932).

H. Wagner and H. Schirmer, Z. anorg. allgem. Chem., 222, 21;5-8(1935).

P. M. de Wolff, Technische Physika Dienst, Delft, Holland.

P. M. de Wolff, private communication, April, I96I.

F. Quittner, J, Sapgir, and N. Rassudowa, Z. anorg. Chem., 201;, 315(1932).

G. Collotti, L. Conti, and M. Zocchi, Acta Cryst., 12, l;l6 (1959).

G. L. Clark and W. P. Tyler, J. Am. Chem. Soc, 61, 58(1939).

Swanson, et al., NBS Cir. 539, V0I.V, 1955.

Swanson and Fuyat, NBS Cir. 539, Vol. II, 30(1953).

Swanson and Fuyat, NBS Cir. 539, Vol. II, 32(1953).

A. Baroni, Gazz. chim. ital., 68, 391(1938).

J. D. Hanawalt, H. W. Rinn, and L. K. Frevel, NBS Cir. 539, Vol. 8, 32(1958).

G. F. Olaringbull, Brit. Musevim.

J. A. Darbyshire, J. Chem. Soc, 1932, 211.

R. G. Dickinson and J. B. Firauf, J. Am. Chem. Soc, 1;6, 2l;57(192l;).

A. l^strom, Arkiv. Kemi. Mineral., Geol., 17B, No. 8, 1.

F. Halls and F. Pawlek, Z. nhysik. Chem., 128, 1;9(1927).

A. Bystrom, Arkiv. Kemi. Mineral. Geol., A20, No. 11, 31 pp. (19l;5).

27

28

(21) G. R. Levi and G. Natta, Nuovo cimento, 3, lll;(l926).

(22) G. R. Levi, Nuovo cimento, 1, 335(1921;).

(23) A. Ferrari, M. Nardelli, and L. Gavalca, Gazz. chim. ital.,

85, 11;5-52(1955).

(21;) M. Straumanis, Z. physlk. Chem., B52, 127-30(19l;2).

(25) S, T. Gross, J. Am. Chem. S o c , 63, ll68(19l;l).

(26) S. T. Gross, J. Am. Chem. S o c , (>Sy 1107-10(19l;3).

(27) G. Butler and J. L. Copp, J. Chem. S o c , 1956, 725-35.

(28) Marvin J. Udy, "Chromium," Reinhold Publishing Corporation, New York, N. Y., 1956, p. I3I;.