The value of in situ analysis on CO chemisorption, titration and oxidation over supported Pt catalysts using calorimetry, catalytic and micro-FTIR methods is illustrated using silica- and titania-supported samples. Isothermal CO-O and O2-CO titrations have not been widely used on metal surfaces and may be complicated if some oxide supports are reduced by CO titrant. However, they can illuminate the kinetics of CO oxidation on metal/oxide catalysts since during such titrations all O and CO coverages are scanned as a function of time. There are clear advantages in following the rates of the catalyzed CO oxidation via calorimetry and gc-ms simultaneously. At lower temperatures the evidence they provide is complementary. CO oxidation and its catalysis of CO oxidation have been extensively studied with hysteresis and oscillations apparent, and the present results suggest the benefits of a combined approach. Silica support porosity may be important in defining activity-temperature hysteresis. FTIR microspectroscopy reveals the chemical heterogeneity of the catalytic surfaces used; it is interesting that the evidence with regard to the dominant CO surface species and their reactivities with regard to surface oxygen for present oxide-supported Pt are different from those seen on graphite-supported Pt

Self, Valerie A.

Sermon, Paul A.

Vong, Mariana S. W.

Wurie, Alpha T.

English

NASA Technical Reports Server

 90-24606
CALORIMETRY, ACTIVITY AND MICRO-FTIR ANALYSIS OF CO CHEMISORPTION,
TITP_TION AND OXIDATION ON SUPPORTED Pt
Paul A. Sermon, Valerie A. Self, Mariana S.W. Vong, Alpha T. Wurie,
and Nigel D. Hoyle
Department of Chemistry, Brunel University, Uxbridge UB8 3PH, UK
SUMMARY
The value of in-situ analysis on CO chemisorption, titration and oxidation
over supported Pt catalysts using calorimetry, catalytic and micro-FTIR methods is
illustrated using silica- and titania-supported samples.
INTRODUCTION
Catalysis of CO oxidation has been extensively studied (ref. I). The catalysed
reaction is important in the clean-up of car-exhaust gases, but a far more demanding
situation is to be found within the laser field.
CO TEA lasers, which emit ultra-short pulses of ir radiation in essentially
paralle_ beams, contain a high partial pressure of CO 2 (i.e. 1 atmosphere) some of
which during laser operation dissociates
2C02 # 2C0 + 02
with the result that there is localised arcing. The percentage dissociation of 1
atmosphere C02 was shown by Pourbaix in 1948 (ref. 2) to increase with increasing
temperature in the manner below.
% decomposition T(K)
10 2474
1 1841
0.i 1635
0.01 1400
0.001 1225
Originally Pt wire at 1373K was used to catalyse the re-oxidation of the CO
produced. On such surfaces not only are oscillations in the rate seen as a result
of microfaceting (ref. 3), but also moderate temperatures are required. In other
words, the Pt wire was not a particularly good catalyst. More recently Pt/stannic
oxide and Pt/Fercalloy have been reported to be more effective in catalysing the
recombination of CO and 02 in sealed CO 2 lasers (ref. 4).
Previously, the oxidation of CO on Pt surfaces has been followed by molecular
beam (ref. 5), transient (ref. 6) and theoretical (ref. 7) methods, showing that CO
reacts with pre-adsorbed O at 600K rapidly with a first-order collision frequency.
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The reverse reaction is very slow and formation of unreactive surface oxygen may be
involved. Attention here is given to methods of studying and analysing the activity
of Pt-based catalysts in this reaction and to probes which allow rationalisation and
interpretation of their surface properties.
EXPERIMENTAL CATALYSTS
Two silica-supported Pt samples were used. (i) 3% Pt/Si02 on non-porous
Degussa Aerosil 200 was prepared by impregnation to the point of incipient wetness
using an aqueous solution of hexachloroplatinic acid (HpPtCI6; Johnson Matthey).
This was then dried in air at 393K for 16h and reduced in H 2 at 570K. Chemisorption
of oxygen suggested that this had a Pt surface area of 31.7m2/g Pt. (ii) 6.3%
Pt/Si02 EURO-Pt was used as supplied; its metal area estimated by oxygen
chemisorption was 187.7 m2/g Pt and the support exhibited porosity. These
catalysts are denoted $3 and $6.
Titania-supported Pt (metal loading 3%) was prepared by impregnation of non-
porous Degussa P25 titania, drying and reducing. This is denoted T3.
METHODS
Calorimetry of CO-O Titrations (ref. 8)
A sapphire-calibrated Dupont 990 differential scanning calorimeter (DSC) was
used to follow titrations of preadsorbed 0 by CO_ , and preadsorbed CO by O_, , at
- _g) ±nermoKlnetlcconstant temperatures on samples (10-25 mg) of the above catalysts. --_i _g)_.
profiles of heat flow versus titration time were obtained. Reactant gases were
6% CO/N_ (BOC: 99.995% purity from which traces of oxygen and water had been
removed_via passage through Pd/alumina and molecular sieve beds) and 12% 02/N 2
(BOC: 99.995% purity).
Calorimetry of Catalysed CO Oxidation
Previously calorimetry has shown that the rate of heat generation in
exothermic reactions is proportional to the rate of catalysed reaction (ref. 9).
DSC was used here in CO oxidation with 6% CO/N 2 and 6% 02/N 2 reactant gases over
catalyst samples (20-30 mg) during temperature programming at 10K/min in the range
293-848-293K. Silica was used in the reference pan. A septum was placed at the
DSC outlet and from this samples were taken and injected onto a Pye 104 gas
chromatograph with a HWD and a silica-gel column at 373K. This separated CO 2 and
gave a response which was linear with CO 2 concentration in the relevant range.
Catalysis of CO Oxidation
Studies of the rates of CO oxidation over samples (0.1g) of the catalysts were
followed in a micro-reactor with gc-ir analysis of products. Reactant streams of
CO/Oo/No were passed at 40 cm3/min through catalyst samples (preheated in flowing
N 2 (_0 _m3/min) for 30 min at 393K) as these were heated and cooled at 2K/min.
298
FTIR Microspectroscopy (ref. i0)
Infrared has long been used to probe the nature of CO adsorbed on metal
surfaces (ref. ii). The samples were placed in an FTIR cell within an IRPLAN
(Spectra-Physics) IR microscope linked to a Perkin Elmer 1710 FTIR through which
reactant gases could flow as follows:
(i) in chemisorption 6% CO/N 2 flowed at 23 cm3/min while heating to 425K at
5K/min before cooling to room temperature
3
(ii) in CO oxidation where 6% CO/N o (21.4 cm /min) and 6% O2/N 2 (21.1 cm3/min)
flowed while heating at 425K _t 5K/min and holding iso_he_mally for 2h before
cooling to room temperature.
RESULTS
Pt/SiO 2
Titrations of CO-oxygen at 373K over $6 were entirely repeatable in sustained
titration cycles and thermokinetic profiles were identical to those shown in
Figure i, suggesting that the titrations were entirely reversible at this
temperature. Interestingly the shapes of the two titrations are different, but it
is not yet possible to suggest that the titration with the induction period and
lower symmetry is not involving a Langmuir- Hinshelwood mechanism. However, it is
tempting to associate the induction period of low heat flux with chemisorption of
the gaseous titrant on the preadsorbed monolayer. Interestingly, the induction
period on $6 when O is titrating the CO-covered surface decreases as the2(g)
temperature ot titratlon rises:
T(K): 293 323 373 423 473 523 573
t(s) 330 168 80 48 0 0 0
At 373K the heat liberated in the two titration steps on $6 is not identical (i.e.
227OJ were liberated per g Pt in CO titration of preadsorbed O and 1260J per g Pt
were liberated in 02 titration of preadsorbed CO).
Now consider what calorimetry can reveal about the catalysed CO oxidation
reaction. Figure 2 shows that as the temperature of $3 increases, DSC shows a
light-off at 473K with a maximum heat flow at 573K of about 75 mJ/s. Bearing in
mind the relationship of heat flow and rates of reaction (ref. 9) it might be
expected that the catalysed rate would also follow a similar profile. However,
this would suggest that the rate of reaction would decrease to a low value at about
773K. This low rate of heat flow is interesting at high temperature and requires
further investigation. On decreasing temperature the same type of profile was noted
but displaced to a lower temperature than that seen with increasing temperature
(i.e. hysteresis is shown). On decreasing temperature oscillations appear close to
the point where extinction of the reaction might occur.
Comparison in Figure 3 of DSC-thermokinetic profiles with measured rates during
CO oxidation over $3 during temperature programmed analysis confirms the difference
between DSC and catalytic analysis especially at highest temperatures of analysis.
299
There is the suggestion that at these hi_her temperatures after light-off the
reaction does not proceed entirely on the catalyst surface.
Sustained analysis of the catalysis of CO oxidation on $6 (see figure 4)
revealed substantial activity-temperature hysteresis loops where activity was stable
with reaction time. The hysteresis may in part be aggravated by support porosity
(not seen in $3). At high P02 rates of oxidation are higher than in Figure 3.
Turning now to the micro-FTIR-spectrometry it is seen in Figure 5 that different
parts of the top surface of a bed of $6 during CO chemisorption and CO oxidation
under isothermal conditions show different spectral characteristics. It is known
that average metal surface atom coordination affects the nature of adsorbed CO (ref.
12) and results are consistent with CO on low index crystallographic planes (ref.
13):
-1
(i) 1890 -+9 cm bridge-bound CO
-i
(ii) 2082 -+i cm linearly-bound CO.
Thus the two areas show little bridge-bound CO in chemisorption, when CO coverage
should be almost complete, but a greater proportion of this species during CO
oxidation when the CO coverage is lower.
Pt/Ti02
Results over T3 in Figures 6 and 7 should be compared with those in Figures 1
and 4 for Pt/silica. First, the thermokinetic profiles are different, with much
enhanced induction periods and diminished total heat flows. Second, the activity-
temperature hysteresis is absent and as expected closer to non-porous $3 than $6.
DSC of CO oxidation on 3% Pt/Ti02 under the same conditions as in Figure 2 showed a
light-off at 498K.
DISCUSSION
Isothermal CO-0 and 02-CO titrations have not been widely used on metal
surfaces (ref. 14) and may be--complicated if some oxide supports are reduced by CO
titrant (ref. 15). However, they can illuminate the kinetics of CO oxidation on
metal/oxide catalysts (ref. 16) since during such titrations all O and CO coverages
are scanned as a function of time. There are clear advantages in-following the
rates of the catalysed CO oxidation via calorimetry and gc-ms simultaneously. At
lower temperatures the evidence they provide is complementary. CO oxidation and its
catalysis of CO oxidation have been extensively studied (ref. i), with hysteresis
(ref. 17) and oscillations apparent, and the present results suggest the benefits of
a combined approach. Silica support porosity may be important in defining activity-
temperature hysteresis.
FTIR microspectroscopy reveals the chemical heterogeneity of the catalytic
surfaces used; it is interesting that the evidence with regard to the dominant CO
surface species and their reactivities with regard to surface oxygen for present
oxide-supported Pt are different from those seen on graphite-supported Pt (ref. 9).
300
It is clear using these techniques that Pt supported upon reducible oxides
behaves differently in this reaction to Pt upon a more unreactive support.
CONCLUSIONS
It is expected that the application of these analytical approaches to this
ancient yet intriguing catalytic reaction will lead to a new understanding of the
catalytic process and more effective catalysts.
ACKNOWLEDGMENTS
The financial support of MSWV, VAS and NDH from SERC, for ATW from the Sierra
Leone Government, and from Johnson Matthey for NDH are gratefully acknowledged.
REFERENCES
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301
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302
Figure i
J
I 1 I I
Thermokinetic profiles during titration of pre-adsorbed O by CO(g)
and preadsorbed CO by 02(g ) on Pt/SiO 2 ($6).
dQ
d7
I I I I I I I I I I
323 373 4_!3 473 523 573 62_ 673 723 773 T(K)
Figure 2 The rmokinetlc profiles for CO oxidation (Pco:PO2:PN_ = 23;23:714; Pto " = I01 kPa) over
44 mg Pt/SiO 2 during temperature programming at lOK_min in the range _-848-323K.
Oscillations set in at the point at which exctinctlon of the reaction was imar,inent.
ORIGINAL PAGE IS
OF POOR QUALITY
303
dO/dr
Fisure 3
700
6OO
500
3O0
200
loo
t i _'Az_,_ A_ I
373 473 573 673 773 873
T(K)
Comparison of thermokinetic (_ ,&) and catalytic (O,O) results for
CO oxidation over Pt/SiO 2 during temperature programmed investigation.
Conditions as in Figure 3. O ,_ denote increasing temperature and
• ,A decreasing temperature.
Fisure 4
304
% oxidation
/
5o /
/
/o _0
o I I
273 323 373 423 T(K)
% oxidation of CO (when Pco:Po :P,, = 15:120:520; Ptotal is 101 kPa; flow rate2 2
40 cm3/mln) during programming 27_-433-273K at IK/mln with increasing (O) and
decreasing (Q) temperature over Pt/SiO 2. At high temperature there is additional
desorptlon of adsorbed CO 2. The temperature width of the hysteresis loop at 50%
conversion is 6OK.
oRIGINAL PAGE IS
OIF pOOR QUALITY
2,2657
2.2382
2.2107
2.1832
2.1557
2.1282
2.1007
2.0732
2.0557
2.0182
1,9407
CO
0
1.9032
1.8877
1.8722
1.8567
1.8412
1.8257
1.8102
1.7392
1.7617
2.0626
2,0509
2.0392
2.015B_-
2.0041
\
t,
I
i
I
I
i
CO 2
(0/o_
1.;4e2 1.9s07 I I l l
-1 -1
cm cm
Figure 5 Fisure 5 (continued)
Reflectance FTIR of two areas of the surface of a bed of Pt/S,0 2
during CO chemisorption and CO oxidation.
2.2345
_'.2075
-- %180%
_t 2"1535
2.12_5
2.0995
-- 2.0725
-- 2.04%5
-- 2.0185
-- L.9915
1.9645
ORIGINAL PAGE IS
OF POOR QUALITY
305
_mJ/s
_T co + o
.......... [ ...... [ .............
1 2 3 4 t (_ln)
Figure 6 Thermokinetic profiles during titrations of preadsorbed O by CO(_)
and preadsorbed CO by 02( ) on Pt/Ti02. The former show_ a grea_er
contribution from-the pre§cursor step to that for Pt/SiO 2 (seen in
Figure I).
306
ORIGINAL PAGE IS
OF POOR QUALITY
Fisure 7
% oxidation
100
5O
333 353 373 393
I
413 T(K)
% oxidation of CO under the conditions in Figure 4 over Pt/TiO 2.
O and Q denote data with increasing and decreasing temperature.
The catalysed reaction occurs at a faster rate at lower temperatures
over this catalyst than Pt/SiO 2 (see Figure 4) and without activity-T
hysteresis seen when temperatures decreased.
3O7



Calorimetry, activity, and micro-FTIR analysis of CO chemisorption, titration, and oxidation on supported Pt

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