8/13/2019 S Subramanian-Adsorption of Chloroplatinic Acid and
Chloroiridic Acid on Composite Oxides
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1436 Langmuir 1991, 7,1436-1440
Adsorption of Chloroplatinic Acid and Chloroiridic Acid
onComposite OxidesS. Subrammiant and J. A. Schwarz'
D epar tmen t o Chemical Engineering and Materials Science,
Syracuse University,Syracuse, New York 13244Received October 1,
1990. I n F inal Form: December 19, 1990
Th e objective of the present s tudy is to investigate he
adsorption of chloroplatinicacid and chloroiridicacid on alumina ,
titania, and com posite oxides formed from alumina and titania. The
adsorption resultswere analyzed by using the L angmuir equation. Th
e adsorption kinetics and adso rption equilibriumamounts indicate
tha t the metal ions have a higher affinity for alumina comparedto
titania; the differenceis more s ignificant in th e case of Ir. It
was also observed th at the presence of titania inhibits
aluminumdissolution when the TiOzAl2Oa supports are broug ht in
contact with acidic impreg nation solutions. Theadsorption results
suggest that the metal ion can be selectively partitioned to alum
ina present in a & Os--Ti02 com posite oxide. This selective
adsorption process was explored by using tempe rature-pro gram
medreduction. Our results suggest th at th e me tal weight loading
determ ines the selectivity of this pa rtitioning.Introduction
Supp orted platinum and iridium catalysts are used insystems of
ind ustrial intere st such as, automobile catalyticconverters an d
reformers.lP2 A critical step in th e prep-aration of supported
catalysts is th e impregnation of th eactive ingredient onto th e
support. Scientific inter est inthe impregnation step
developedafter itwa s demonstratedthat certain acids and
saltscontributed to the distributionof the active ingred ient
within a porou s catalyst pelletS3p4Th e investigation of th e
adsorption process during thewet impreg nation of H2PtCLJA1203
becam e the subjec tof interest in sub sequ ent studies.b** T he im
pregnationste p for several other precursor/support systems has
beenstudieda7-l2 Bu t no information is reported on th eadsor ption
of HzIrCh. Also, th e adsorption of catalyticprecursors on more
complex suppor ts such as compositeoxides has not been studied t o
date.Composite oxides form a class of supports for thepotentia l
design of supp orted m etal catalysts with uniqueproperties.13-18
The strength, number, and type of acid
* To whom correspondence should be addressed.'Present address:
Ford Motor Co., P.O. Box 2053, 2162 SRL,(1) Kummer, J. T. h o g .
Energy Combust. Sci . 1980,6, 177.(2).Sinfelt, J. H. Bimetallic C
atalysts: Discoveries, Conce pts, and(3) Maatman, R. W.; Prater, C.
D. Ind. Eng. Chem. 1957,49, 253.(4) Maatman, R. W. Ind. Eng. Chem.
1969,51,913.5 )Santacasaria, E.; Canna, S.;Adami, I. Ind. Eng.
Chem. Ro d. Res.(6) Santacesaria, E.; Ga lli, C.; Canna,F.React.
Kinet. Catal . Lett .(7) Chen, H. C.; Anderson,R.B. In d . Eng.
Chem. Rod .Res. Deu. 1973,12, 122.(8) Chen, H . C.; Anderson, R. B.
J. Catal . 1976,43,200.(9) Cervello, J.; Garcia dela Banda , J.
F.;Hermana, E .; Jimenez, J. F.Chem. Eng. Technol. 1967,48,520.(10)
Cervello, J.; Chou, T. S.; Summers, J. C.; Potter, N . M.
InPreparation of Catalysts I . Studies in Surface Science and
Catalysis;Delmon, B., Grange, P., Jacobs, P ., Po ncelet, G., Ed.;
E lsevier: Amstar-dam, 1976; Vol. 2, p 251.11)Hegedus, L. L.; Chou,
T. S ;Summers, J. C.; Potter, N. M. InPrepar ation of Catalys ts II
. Studies i n Surface Science and Ca talysis;Delmon, B., Grange,
P., Jacobs, P ., Poncelet, G., E&.; Elsevier: Am-sterdam, 1979;
Vol. 3, p 171.(12) Komiyama, M.; Merrill,R. P.; Harnsberger, H. F.
J. Catal. 1980,63, 35.(13) Jin, T.; Hattori, H.; Tanabe, K. Bull.
Chem. Soe. Jpn. 1982,55,2279.
Dearborn, MI 48121.Apphcationu; John Wiley: New York, 1983.
Dev. 1977, 16, 41.1977,6, 301.
sites in a com posite oxide can be controlled by
varyingcomposition.13-ls Dum esic an d co-w orkers have reponon t h
e acidic DroDerties of a se ries of mod el bina ry oxic
tad?Sformed by m ou&ing a m etal oxide at low
concentrationsonto the surface of a second metal oxide. Since the
physicalproperties of th e newly formed adso rbent ar e
differentfrom either of the pure components, we would expect tha
ttheir adsorption properties during impregnation wouldalso be
different from tha t of the pur e phases. Inparticular, the
partitioning of th e supported metal betweenth e two phases of a
com posite oxide would be controlledby th e adsorption kinetics on
each phase.
Th e objective of the p resent s tudy is to investigate
theadsorption of chloroplatinic an d chloroiridic acid on purean d
composite oxides formed from alumina and titania.Tita nia is of the
strong metal-support interaction (SM SI)type and thus provides a
unique anchor for catalyticHowever, pure titania is a low surface
areasup por t and alumina-titania composite oxides provide
aconvenient method for increasing the surface area of thesupported
titania phase.23Th e rate of adsorption of the hexachlorinate Pt
and Irions (MCh2-, where M = Pt or Ir) on AlzOs, TiO2,
andTiOz/Al203 composite oxides was measured andt he resultswere
analyzed by using the Langm uir equation.s Tem -perature-programmed
reduction ( TP Rd ) was used toassess the distribution of the m
etal species in th e compositeoxide supported precursors. Th e
trans ient an d equilib-rium adsorption da ta and T PR d results
suggest that , atrelatively low metal weight loadings, the
selective ad-sorption of th e hexachlorinate ion on alum ina in a
TiO2-A1203composite oxide is achieved.
(14) Tanabe, K.; Itoh, M.; Morahige, K.; Hattor i, H.
InPreparationof Catalysts; Delmon, B., Jacobs, P. A., Po ncelet,
G.,Ede.;Elsevier:Amsterdam, 1976; p 65.(15) Shiba ta, K.; Koyoura,
K.; Kitagawa, J.; Samiyoahi, T.; Tanabe ,K. Bull. Chem. SOC. pn.
1973,46, 2986.(16) Connell, G.; Dumesic, J. A. J. Catal. 1986, 101,
103.(17) Connell, G.; Dumesic, J. A. J. Catal. 1986, 102, 216.(18)
Connell, G.; Dumesic, J. A. J. Catal. 1987, 105, 285.(19) Tauster,
S. J.; Fung,S. C. J. Catal. 1978, 55, 29.(20) Tauster,S. J.;Fung, .
.; arten, R. L. . Am. Chem. Soc. 1978,(21) Vannice, M. A.; Garten,
R. L. J. Catal. 1979,56, 236.(22) Vannice, M. A,;Twu, C. C.; Moon,
S. H. J. Catal. 1983, 79, 70.(23) McVicker, G.B.; Ziemiak, J. J. J.
Catal. 1985,95,473.
105, 170
0743-7463/91/2407-1436 02.50/0 1991 American Chemical
Society
8/13/2019 S Subramanian-Adsorption of Chloroplatinic Acid and
Chloroiridic Acid on Composite Oxides
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Adsorption of Acids on Composite Oxides Langmuir, Vol 7 , No.
7,1991 1439-I
m 4 6 800Tempemlua(I0Figure 3. Temperature-programm ed eduction
profile for 0.7Ir/&03, 0 .7 Ir/TiOz, and 0.7 Ir/0.87 A120s-Ti02
pre-cursors.
Table IV. TPRd Peak Temperature and HydrogenConsumption
ValuesMetal Loading: 0.7
hydrogenconsumed,system peak temp, K rcmol/a of
precursorIr/Al*Os 352,479,600-800 118.2Ir/A1203-Ti02 355, 375,4
93,60 0-800 122.4Ir/TiOz 379,484,600-800 129.9
Metal Loading: 3.0hydrogenconsumed,system peak temp, K pmollg of
precursor
Ir/A120s 379,495,600-800 349.6Ir/ALOs-TiOz 340-380,477,600-800
356.4Ir/TiOz 353, 462,600-800 348.9tion on titania. T he
aforementioned considerations sug-gest that the metal component may
be directed toselectively adsorb on t he alumina present in th e
compositeoxide.T h e 0.87% A1203-Ti02 com posite is of par ticu
larinteres t. Her e Al2O3, th e compon ent on which adsorptionis
favored, is present as the secondary phase. One mayconsider th e
following "idealized" scenario assuming th atmetal will be d
irected toward Ti 02 only after A1203 reachesadsorption equilibrium
with th e metal. At extremely lowme tal loadings, th e m etal ions'
prefere nce for A1203 willdirect t he metal exclusively toward
A1203. For example,a t a 0.05 ota l m etal loading, th e weight
loading on A1203will be only 5.75% , if all the available metal is
pres ent onA1203. At relatively high metal loadings, A1203 will
be"saturated" w ith the m etal and th e remaining availablemetal
will be present on TiOz. For example, a t a 3 totalme tal loa ding,
th e weigh t loading on A1203wll be 344.82if all the available
metal is present on A1203. A metalloading of 344.85 is unrealistic.
In this case, th e metalwill be distributed on T i02 once A1203 tta
ins adsorptionequilibrium with th e metal. It appears th at
selective metalexchange on alum ina may be achieved a t reasonably
lowme tal loadings. For com pleteness, in the 11.4% TiOz-A1203
omposite, A1203 is th e primary component. Sincethe adsorption on
th e primary component is favored, it isnot possible to direct the
metal component toward Ti 02a t reasonable metal loadings.Th e
ratio of the forward adsorption r ate constants onalumina a nd
titania is higher in th e case of iridium (5.96X 1Om3/O.36X 10-3 =
16.56) comp ared t o platinum (4.17X 10-3/1.03X 10-3 = 4.05). The
se kinetic considerationsfavor the selective adsorption process in
the case of iri-dium compared to platinum. Therefore Ir precursors
werechosen for further study. In order to investigate the
2 403 6 COTemperature ( K )
Figure 4. Temperature-programmed eduction profile for
3Ir/A1203,3 Ir/TiO2, and 3 Ir/0.87 AlzOs-Ti02 precursors.selective
exchange process, iridium precursors were char-acterized by TPR d.
It is important to note that theprecursors were no t calcined; th e
changes occurring duringthis high temperature treatmen t are
dependent on thenatu re of the s upp ort and the T PR d profiles of
calcinedprecursors canno t be interpre ted easi1y.m In this
study,the peak temperature obtained during the TP Rd of
driedprecursors was used as a "fingerprint" to identify the
iri-dium distribution in a composite oxide supported
pre-cursor.
Figure 3 show s th e T P R d s pectra of th e Al2O3,
TiOz,and0.87 A1203-Ti02 supp orted Ir precursors. T he
peaktemperatures and hydrogen consumption values are re-ported in
Tab le IV. In these cases th e metal weight loadingwas adjusted to
0.7 5 . At metal weight loadings signif-icantly lower than 0.7
(i.e., 0.05 ), the T PR d s ignalwas insignificant. It is
interesting t o note th at a t the 0.7level weight loading, the
TPRd spectrum for the 0.7%Ir/0.87 A1203-Ti02 prec urso r qualita
tively resem blesthe T PR d spectrum of the 0.7 9
Ir/A1203precursor. Thepeak temperatures for these spectra are
similar. Com-parison of th e reduction spectra for Ir/TiOz an d
Ir/0.87Ti02-Al203 precursors suggests tha t th e peak a t 355 K
inthe case of th e 0.87 A1203-Ti02 supp orted Ir precurso
roriginates from the fraction of Ir distributed on TiOz.Figure 4
shows th e T P R d s pectra of t he TiOz, A1203,an d 0.87%
A1203-Ti02 supp orted Ir precursors with ame tal weigh t loading of
3 . The peak temperatures andhydrogen consum ption values are
reported in Ta ble IV.It is interesting to note that th e TP Rd
spectrum of thecomposite oxide supported precursors is
qualitativelysimilar to th at obtained for th e T i02 supported
precursors.Th e similarity in peak temperatures is striking. Th eam
ount of m etal d istributed on A1203 is negligible incomparison to
th e am oun t of m etal distributed on TiOz.Th is confirms the
assertion tha t, a t relatively high metalweight loadings, the
metal is essentially distributed onTi 02 in th e 0.87% A1203-T i02
composite oxide.These studies illustrate t ha t selective
partitioning ofthe metal species can be achieved if the ion
underconsideration h as a significant affinity for one of th e
oxides.The variable that determines the selectivity of theexchange
process is th e metal weight loading. T he conceptof selectively
adsorbing metal ions on one of the com-poents of a composite oxide
can be extende d t o cations aswell as anions. T he selective
adsorption process is ofparticular interest in low metal weight
loading systemswhere the metal may be selectively distributed on
thesecondary phase (th e minor com ponent on a weight basis)of th e
com posite oxide.
(30)
ubramanian,S.PbD.Dissertation,SyracuseUniversity,SyracueeNY,
989.
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