J . Chem. SOC., Faruduy Trans. 1, 1987,83, 1651-1665 Is0 thermal Tit ration of Supported Platinum Part 1 .-CO-0, Titration Marie A. Martin Luengo, Paul A. Sermon” and Alpha T. Wurie Department of Chemistry, Brunel University, Uxbridge UB8 3PH The rate and extent of titrations of preadsorbed oxygen by gaseous CO and of preadsorbed CO by gaseous 0, upon oxide-supported Pt vary with the titration temperature; results suggest that 423 K is close to the optimum and is to be preferred to previously used ambient conditions. The consumption of titrant CO and the total amount of heat liberated in the primary titration processes are directly proportional to the Pt surface area available to chemisorb CO and may thus be used to estimate Pt dispersions rapidly and reproducibly. However, they require careful calibration with traditional chemisorptive methods because of great temperature sensitivity if absolute metal surface areas are to be estimated.Importantly, the ratio of enthalpies of the two primary titration processes (QCOTo/QOTco) appears to be very sensitive to the average Pt particle size, possibly as a result of the presence of different types of Pt adsorption site thereon. The thermokinetic profiles dQ/dt us. t provide an insight into the kinetics of such titrations, which may be relevant to the catalysis of CO oxidation. The isothermal titrations of oxygen chemisorbed upon surface Pt atoms by gaseous hydrogenf (HT,), alkenes, such as cyclohexene (AT,) or carbon monoxide3 (COT,) can be represented by: 4n(Pt,,-0) + (2 + 4n) H, + 4(Pt,,-H) + 4nH20 2n’(Pt,,-O) + (1 + 2n’) CO + (Pt,,,-CO) + 2n’CO, 2n”(Pt,,-O) + + (PtncHfl-)x- c) + 2n”/nH(Pt,,-H)+ 2n”H,O + (2 - 2n” - n“/n,) H,.The reverse titrations of preadsorbed hydrogen by gaseous oxygenf (OT,) or alkenes, such as ethene, pent-1-ene or cyclohexene (AT,) and titration of preadsorbed CO by gaseous oxygen (OT,,) are then given by: 4(Pt,,-H) + (1 + 2n) 0, + 4n(Pt,,-O) + 2H,O (Pt,,,-CO) + (n’ +t) 0, + 2n’(Ptn,-O) + CO, 2n”’(Pt,,-H) + (1 + n”‘)O+ (Ptnc.,,-)n -0 + n’” 0 where nH2, no,, nco, nCH and nCH“ are the average numbers of surface Pt atoms adsorbing one molecule of H,, O,, CO, benzene and cyclohexane, irrespective of the real extent of adsorbate dissociation. n is nH2/n02, n’ is nco/nop, n” is nCHu (for benzene adsorption)/n,, and n”’ is nCHllt (for cyclohexene adsorption)/nHg.The number of surface Pt atoms adsorbing a hydrogen atom and an oxygen atom (z.e. nH and no) are taken to be a half of nH2 and no,. Each n will take a varying value, depending upon the adsorbate-titrant coverage, partial pressure and temperature. Thus nH2 and nco may be 2 o r 1. HT, and OT, titrations have been widely used to estimate dispersions of supported and unsupported Pt,l* but observed ratios of the extents of chemisorption to titration 16511652 CO-0, Titration on Pt (e.g. qHC : qoc : qHT,) vary between 2: 2 : 6 and 2: 1 : 4. nH2 and no, vary with Pt particle size and the concentration of Pt step and terrace sites. The extent of titration is also certainly affected by the presence of (i) surface inhomogeneities, (ii) the fraction of reversibly held hydrogen, (iii) the fraction of chemisorbed oxygen which is unreactive towards hydrogen and (iv) the presence of contaminants.However, the uncertainty concerning whether the product water is retained upon Pt (stabilised by fractional coverages of oxygen and interfering with titrations) or transferred to the support or desorbed may be equally important.l? Isothermal AT, and AT, titrations2 have been used far less extensively to estimate Pt dispersion and still involve uncertainty regarding the extent of hydrocarbon retention on the catalyst, since the hydrocarbon titrant can also be held on the surface as an ill-defined surface intermediate. Isothermal COT, and OT,, have been the least-used titrations for estimating metal surface areas, OT,, being very rarely used.Langmuir showed5 that both COT, and OT,, titrations proceed at and above 293 K on unsupported Pt in a reversible manner. How- ever, Akhtar and Tompkins6 carried out the CO titrations of 0 adatoms on Pt film at 195 K and found that only 20-50% of the chemisorbed oxygen was so titrated; it was therefore assumed that the titration was too slow at this temperature. Nevertheless, the COT, was apparently used successfully at 300 K to characterise unsupported, carbon- supported and alumina-supported Pt (with product CO, being evolved and removed using a side-arm at 77 K).3 It was also used to characterise the dispersion of carbon- supported Pt at 303 K7 Interestingly, the reverse OT,, titration was preferred for characterisation of alumina-supported Ir at 423 K (rather than 300 K and the number of CO, molecules produced was directly related to the Ir surface area).* Subsequently, there has been disagreement about the titration conditions ~elected.~ The COT, process differs significantly from HT, and AT, titrations in that the product CO, is only weakly held [i.e.the enthalpy of CO, adsorption on Group VIII transition metals is -= 40 kJ mol-1 and thus low compared with that of 0, and CO (which have enthalpies of up to 335 and 230 kJ mol-l on Pt, respectively)]. Therefore CO, desorbs even at 110-109 K and 1.3 mPal0 and is not measurably adsorbed under titration conditions, but rather is rapidly desorbed, thereby minimising uncertainties. This titration is also different in that it involves no greater titrant consumption than direct CO adsorption, unless CO, is intentionally removed from the gas phase. Therefore (without such action) even if COT, goes to completion with all CO, desorbed, the ratio of the extents of titration to chemisorption (i.e.qcoT,/qcoc) should be close to unity. The COT, titration could in principle proceed either via a Langmuir-Hinshelwood or an Eley-Rideal mechanism. However, the OT,, titration is assumed to require dissociative chemisorption of oxygen before reaction and would presumably involve a Langmuir-Hinshelwood mechanism; possibly with a titration induction period. This would be especially so if adsorbed CO blocks the surface and inhibits the rate of OT,,. It may not be surprising therefore that COT, has generally been preferred to OT,, for estimating the dispersion of * Bearing in mind the frequent use of supported metal catalysts, it is important to note that at very high temperatures some oxide supports may also reduce in the presence of the CO titrantll during COT,,, but this is probably no more serious than reduction by H, in HT,.The rates and extents of these COT, and OT,, titrations involving the reaction between CO and oxygen on the surface of oxide-supported Pt are now described. These have been studied because of their apparent simplicity in terms of the greater extent of product desorption. After optimisation, consideration has been given to the accuracy they provide in estimating Pt surface areas in comparison with existing alternative chemisorption methods.M .A . M . Luengo, P. A . Sermon and A . T. Wurie 1653 Table 1. Pt/SiO, catalysts prepared conditions/K (h) silica Pr support (% ) impregnation calcination reduction sample ~~ ~ Shell 2.5 3 Shell 1.5 3 Shell 2.5 3 Shell 1.5 3 Shell 2.5 3 Shell 1.5 3 Davison 923 6 Davison 923 1 Sorbsil 6.2 373 (5) 373 ( 5 ) 373 (2) 373 (2) 273 (5) 273 (5) 298 (4) 298 (i) - 523 (1) - 523 (1) - 523 (1) - 523 (1) 543 (3) 713 (1) 543 (3) 713 (1) - 673 (1) - 673 (1) 378 (24) 673 (4) 2.513731-1523 1.5/373/-1523 2.5 1273 1-1 523 1.5/273/-1523 2.5/373/543/713 1.5 1 373 1 543 17 1 3 Fa IU KU a Described in ref. (12), where the fractional dispersions of supported Pt were found to be 0.265, 0.755 and 1.038 for samples F, I and K using the average extents of CO, 0, and H, chemisorption at ambient temperature.Experiment a1 Catalysts Four silica-supported Pt catalysts used here (denoted F, I and K) have been described previously.12 In addition six Pt/SiO, samples containing 3% Pt were prepared and are indicated in table 1 ; the silicas of controlled porosity (Shell S980-1.5 and S980-2.5) were impregnated with hexachloroplatinic acid (Johnson Matthey; Specpure) aqueous solutions to the point of incipient wetness at 373 or 273 K. Samples containing the acid salt upon the surfaces of the silica were then calcined in air at 534 K (or not; these treatments were designed to produce different Pt dispersions) and reduced in H, at 523 or 713 K. Such samples are denoted in table 1 by the silica used, the temperature of drying, calcination and reduction. Sample K is EuroPt Pt/SiO,; a comparison of titration results suggests no significant effect of surface chlorine after reduction.One sample of 3% Pt/TiO, was also used and has been described previously.12 Calorimetric Methods of Titration A differential scanning calorimeter (Dupont 990 with a 910 analyser) was used under constant temperature conditions. The catalyst and its support were placed in separate A1 pans on a constantan disc using chromel-constantan thermocouples to measure the differential heat flow as a function of time. A sample (10-25 mg; previously held in air at ambient temperature and therefore exhibiting a chemisorbed monolayer of oxygen) and its support were introduced and their temperature raised to that of the titration in flowing He. Then 6% CO in N, (BOC; 99.995% purity which had been further purified by passage through a Pd/Al,O, catalyst and then a molecular sieve trap) was introduced at 45 cm3 min-l and dQ/dt followed during COT,.After flushing with He for 10 min, 12% 0, in N, was introduced (BOC; 99.995% purity; further purified by passage through a molecular sieve trap; 45 cm3 min-l) and dQ/dt again followed as a function of time during OT,,. To determine the repeatability and reproducibility of COTo-OT,, titrations, several cycles were performed at different temperatures. Areas under the primary thermokinetic peaks were integrated up to the point where d2Q/dt2 < 0.02 mJ s-, to give enthalpies of the primary titration processes. Areas of integration are indicated in fig. 1 by shading. For titrations where dQ/dt > 0 such1654 CO-0, Titration on Pt tlminM . A .M . Luengo, P . A . Sermon and A . T. Wurie 1655 enthalpies are not total titration enthalpies; for this reason the ratios of enthalpies are judged most useful, i.e. QcoTo/QoT,o. Volumetric Methods of Chemisorption The extent of chemisorption of O,, H, and CO (i.e. qoc, qHC and qcoc) on the above Pt catalysts has been measured in a conventional volumetric apparatus12 using the following procedure. A sample (20&300 mg) was evacuated to < 1 mPa and the temperature raised to 423 K over 15 min. 13 kPa H, was then introduced for 30 min, after which the sample was re-evacuated for 1 h while heating to 683-693 K, at which temperature it was held for a further 2 h with evacuation to 1 Pa.Samples were then cooled to ambient temperature for chemisorption measurement. The values of qi corresponding to monolayer capacity were then deduced by extrapolation of the isothermal data to the zero-pressure intercept and also from the gradient of the linear Langmuir isotherm. Pt surface areas were then calculated assuming that nHz, no2 and n,, were 2,2 and 1 , respectively, and that the supported Pt existed as homogeneous cubic particles with one face upon the support. Volumetric Methods of COT, Titration The extent of CO titration of pre-adsorbed oxygen (qCOTo) on the Pt catalysts was also measured in the same volumetric apparatus using the following procedure. A sample (200-300 mg; previously held in air at ambient temperature and hence with its surface covered by adsorbed oxygen) was outgassed to 1 Pa and the temperature raised to 423 K over 30 min.A typical COT, isotherm was then measured at 423 K up to 6.7 kPa CO. Equilibration times were 10-15 min. The value of qCOTo corresponding to monolayer consumption was deduced by extrapolation of isothermal titration data to zero pco or from the gradient of the linear Langmuir isotherm. These values were converted to Pt surface areas assuming no* and n,, were 2 and 1, respectively, and in addition making the other assumptions used in chemisorption. Results Optimisation of COT,-OT,, Titrations using Calorimetry Fig. 1 shows the thermokinetic profiles obtained by differential scanning calorimetry during repeated COT, and OT,, titrations over Pt/SiO, sample K at 373 K using 40 cm3 min-l of either 6 kPa CO in N, or 21 kPa 0, in N, and intermediate flushing.First, it is clear that the profiles of heat flux (dQ/dt us. t ) are repeatable; hence the total extent of heat generation must be reproduced in repeated titrations. Presumably it must follow that the extent of exothermic COT,-OT,, titrations is also repeatable and reversible. Secondly, the shape of these profiles differs in the two titration steps, with that for the COT, more symmetrical (and appearing after a shorter induction period and with less subsequent tailing) than that for OT,,. The ratio of the times to maximum dQ/dt (tOT,o/tCOTo = 6.4) might suggest that OT,, and COT, proceed via Langmuir- Hinshelwood and Eley-Rideal mechanisms, respectively. However, there is also further information from the OT,, thermokinetic profiles.Thus the main OT,, thermokinetic peak is preceded by a period when dQ/dt is positive but small; it is tempting to associate this period with the dissociative chemisorption of 0, on the CO-covered surface prior to titration. Maximum rates of titration have been observed13 in residual gas analysis, Fig. 1. Repeat COT,-OT,, titrations of Pt/SiO, (K) at 373 K measured as a rate of heat flux by differential scanning calorimetry as a function of time. A: (a) OTcol, (b) OTcoz, (c) OT,,,. B: (a) COT,,, (b)COT,,.1656 CO-0, Titration on Pt 6 P k M TIK Fig. 2. Extents of primary heat liberation in COT, (0) and OT,, (0) titrations of Pt/SiO, (F) at temperature between 323 and 523 K. Table 2. Induction periods before dQ/dt > 0 for OT,, titrations on Pt/SiO, ~~ titration induction titration titration induction temp./K period/s temp./K no.period/s 293 330 323 1 323 168 323 2 373 80 323 3 423 48 323 4 473 0 323 5 523 0 423 1 573 0 423 2 3 4 5 165 315 840 1340 1410 24.0 22.5 18.0 15.0 13.5 fast i.r. response, Auger and radiotracer studies which have suggested that on Pt both COT, and OT,, proceed by Langmuir-Hinshelwood mechanisms, even if the CO titrant in COT, is only a weakly held precursor. This being so, then the rate of both isothermal COT,, and OT,, titrations should be proportional to a function of the fractional coverages of oxygen and CO [i.e. f ( ~ , , O , ) ] and hence should be a maximum at intermediate 6,, and 6, at intermediate titration times as shown in the dQ/dt us.t profiles here. However, the profiles in fig. 1 suggest that the titrations may not be complete under the conditions used here; this is particularly true when tailing is significant. Langmuir showed5 that qCOT, and qOTCo titres increased during repeated titrations upon un- supported Pt foil as the temperature was raised from 293 to 473-603 K. Thus a complete and stoichiometric titration may be unlikely9 at the arbitrarily chosen partial pressures and temperatures selected in fig. 1. Certainly others have found6 that the titrations are incomplete at 195 K. Fig. 2 shows the integrated areas of the primary steps in COT, and OT,, titrations over sample F Pt/SiO, at different temperatures; such areas of thermokinetic profiles are measured by integration of dQ/dt us.t plots to the pointM . A . M . Luengo, P. A . Sermon and A . T. Wurie Table 3. Enthalpies (in J per g Pt) of COT, and OT,, titrations on Pt/SiO, at 373 K Pt/SiO, (I) Pt/SiO, (K) run Q C O T ~ Q o T ~ ~ Q C O T ~ Q o T ~ ~ 1 630 2240 2270 1260 2 660 2200 2740 1260 3 670 2170 2920 1240 4 670 2140 3030 1240 5 680 2100 3090 1230 +36 -2 overall + 8 -6 change % 1657 where d2Q/dt2-< 0.02 mJ s-, (see the shaded areas in fig. 1). Such areas correspond to the enthalpies of these primary titration processes, but in the presence of thermokinetic profile tailing these necessarily cannot be the total titration enthalpy. From fig. 2 it is clear that the total extent of primary heat generation (to the point where d2Q/dt2 < 0.02 mJ s - ~ ) and hence the extent of both primary COT, and OT,, titration must increase significantly with increasing temperature.The greatest extents were seen at ca. 473 K. As the titration temperature is increased (i) qcoc and Oco will decrease with CO desorption before titration, liberating free surface Pt atoms * available for 0, chemi~orptionl~ in OT,, and (ii) qoc will increase with the eventual bulk oxidation of Pt particles to the 2 + state,14 which can then only weakly adsorb C0.15 Thus a maximum in the extent of COT,-OT,, titrations is to be expected at temperatures intermediate between 423 K and ambient. The titration rate should naturally rise with temperature. It would also be expected that the induction period in OT,, should decrease as the titration temperature rises.Table 2 shows that this is indeed the case. It therefore appears that a titration temperature of ca. 423 K is optimum; a value well above the ambient conditions often chosen in the The effect of Pt dispersion and titration temperature on the repeatability of COT, -OT,, titration was determined with titration cycles on samples I and K Pt/SiO, at 373 K. Table 3 shows that the total extent of the primary titration process in OT,, changed only a little with repeated titrations, but that that for COT, does increase. Enhancements have been seen previously3 as they have also been observed in HT,-OT, ~yc1es.l~ It should also be noted that the ratio of primary titration enthalpies QCOTo/QoTco is very different for these two silica-supported platinum samples. Fig. 2 has also shown that this ratio is very temperature-dependent.In addition, table 2 shows that the induc- tion period for the OT,, titration is large and increases substantially at 323 K with repeated titrations, but that at 423 K it is very small and decreases; the former situation probably arises from a progressive CO blocking of the surface in the titration cycles, which is not as serious at the higher titration temperature where Oco is lower. Again this suggests 423 K as an optimum titration temperature, where extents and kinetics of titrations are reasonable. However, it is important to note that some adsorbate will desorb before t i t r a t i ~ n ; ~ indeed this has been observed16 with OT,,. This temperature was selected for further study; this was also the temperature used for titration on supported Ir.*1658 CO-0, Titration on Pt 3 ( n m N m \ 2 4 6 8 P/ 1 O3 Pa 0 .0 9 1 0.08 t m ru ? 0.07 I M 0 1 2 3 p i ' / 1 O4 Pa Fig. 3. (a) Isotherms of H,, 0, and CO chemisorption at 293 K and COT, titration at 423 K on Pt/SiO, 2.5/373/-/523 (0) and the SiO, support alone (0) measured volumetrically. (b) and (c) Linear Langmuir plots of data in (a).M. A . M. Luengo, P. A . Sermon and A . T. Wurie 1659 (h) o ~ o - - - o - - - - o - o - 2 0 2 4 6 8 Pco/ 1 O3 Pa Fig. 4. COT, titrations at 423 K on different Pt/SiO, samples measured volumetrically, where (a), (b), (c), (d), (e), (f), (g) and (h) denote 2.5/373/543/713, 2.5/373/-/523, 1.5/373/-/523, 2.5/373/-/523, 2.5/273/-/523, 1.5/373/543/713, 1.5/273/-/523/, and Silica 2.5, respectively.COT,, Titrations at 423 K COT, titrations were obtained at 423 K on all six samples of Pt/SiO, prepared on the Shell silica supports of controlled porosity. In addition, one titration was carried out on one sample at 298 K for purposes of comparison. The justification for the selection of 423 K is seen in the fact that the value of qCOTo at 298 K was 25% lower than that obtained at 423 K; presumably this is a reflection of the slower titration kinetics at the lower temperature. Fig. 3 shows the qi us. pi isotherms for the chemisorption of CO, H, and 0, on 2.5/373/---/523 Pt/SiO, at 298 K and also that for the COT, titration on the same catalyst at 423 K. Isotherms on the support alone are also indicated. After subtraction of consumption upon the support alone, data were extrapolated to zero pressure and plotted as linear Langmuir isotherms; in the latter plots good linearity was seen.Titration data were obtained for each Pt/SiO, and are shown in fig. 4. From table 4 it can be seen that there is good correlation between the CO monolayer adsorption and titration Capacities and also between capacities measured at the zero-pressure intercept and from the linear Langmuir gradient. Values of the ratio of titration to adsorption capacities (i.e. ~ ~ o T , / ~ C O c ) are also given in table 4. This ratio should be unity if the COT, goes to completion and all CO, is returned to the gas phase. However, the values of the ratio suggest only 62-100% completion of COT,. Therefore, although the COT,-OT,, titrations are repeatable in continuous titration cycles under isothermal conditions (see table 3 and fig. l), neither titration step appears to go to completion even under the optimised conditions selected here.Hence the results are self-consistent, but require calibration before absolute surface areas of Pt can be assessed by titration. Fig. 5 shows the heat liberated in the primary COT, process (QcoTo) at 498 K on Pt/SiO, samples prepared from Shell silicas of controlled porosity and that this varies linearly with the average Pt particle size (dpt)1660 CO-0, Titration on Pt Table 4. Extents of isothermal adsorbate or titrant consumption on Pt/SiO, samplesa qJpmol per g catalyst sample qHC 4oc 4coc qCOTo qCOTo/qOTco 2.5/373/-/523 16.57 (17.66) 9.81 (10.77) 24.1 1 (24.98) 24.46 (24.72)’ 1.02 0.78 0.62 1.98 9.36 7.29 0.78 0.83 8.84 0.88 1.5/373/-/523 14.31 7.06 27.00 21.04 2.5/273/-/523 10.08 6.44 19.76 12.21 1.5/273/---/523 4.80 2.5/373/543/713 19.31 10.30 29.94 24.93 1.5/373/543/713 5.81 2.09 10.04 ~~ Bracketed data are those derived from fig.3(b) linear-Langmuir isotherms at 290 or 423 K. qi data were obtained from H,, 0, and CO chemisorption at 298 K or COT, titrations at 423 K. * One COT, titration was also carried out on this sample at 298 K, producing a pressure intercept (18.21 pmol per g catalyst) significantly below the value observed in COT, at 423 K (24.46 pmol per g catalyst). Fig. 5. Total extent of heat liberated during COT, titration on different samples of Pt/SiO, at 498 K plotted against Pt particle size, estimated by volumetric measurement of extent of COT, titration at 423 K.M .A . M. Luengo, P. A . Sermon and A . T. Wurie 1661 estimated from volumetric measurement of qCOTo. Similar correlations have been seen7 between enthalpies of HT, titrations and the Pt dispersion in carbon-supported Pt. However, this correlation here suggests that the primary thermokinetic peaks in COT, and OT,, (measured here until d2Q/dt2 < 0.02 mJ s-,) do correspond to titration processes related to the Pt surface alone; it may be that the tailing is involved with a spillover process, in which case it is intriguing to wonder why it is more dominant in one titration step than another. Discussion The above results seem to indicate that after calibration the titration of an oxygen- covered silica-supported Pt catalyst by gaseous CO at 423 K can give reliable estimates of the Pt surface area from the volumetric extent of titration qCOTo or the enthalpy of the primary thermokinetic titration peak QCOTo.The Pt dispersions obtained are self- consistent, but lower than those measured by CO chemisorption at 298 K. This will be so if (i) less CO is chernisorbed at 423 K than 298 K, (ii) not all chemisorbed oxygen can be titrated by CO (either from a kinetic or a thermodynamic point of view) or (iii) product CO, is not totally desorbed, but remains on surface Pt sites (*) inhibiting further titration. Bearing in mind the ease of CO, desorption under titration conditions17 and the change in qcoc with temperature, (i) and (ii) seem most likely.However, the extent of CO chemisorption greatly exceeds the extent of 0, chemisorption (i.e. qcoc 9 2qOc in table 4) and it may be that qoc measured here is suppressed by a significant but variable fraction of unreactive oxygen. This is also reflected in the COT,-OT,, titrations. Nevertheless, under the titration conditions selected here both qCOTo and QCOTo are repeatable, consistent and proportional to the Pt surface area (see fig. 5 and table 4). However, OT,, may be more affected by the presence of unreactive oxygen. COT,-OT,, titrations are the precursors of the continuous catalysis of CO oxidation on Pt. Normally a measurable rate of steady-state CO oxidation is observed at a temperature above the ignition temperature (i.e. 383 K for Pt/Al,O,l*). The steady-state reaction is deemed19 to occur via a Langmuir-Hinshelwood mechanism over a wide range of coverage conditions on Pt surface sites * : co + * $ toads 0, + 2" 2oads Oads + coa& co2, g + 2*.Preliminary equilibria suggest some ti trant desorption before reaction ;9 this is seen as simultaneous CO desorption and liberation of CO, from Pt(l11) at 300-400 K.,* Certainly, CO adsorbs non-dissociatively on Pt at the present titration temperatures,21 while dissociative chemisorption of oxygen is thought,' a prerequisite for reaction. While Coa& inhibits co oxidation, Oads does n0t.l'~ 22 Therefore the incomplete removal of Oads seen here is not predicted. Naturally, surface imperfections affect the rate of CO oxidation on Pt23 (and also the extent of COads inhibition,,), but the induction periods in fig.1 are predicted by a Langmuir-Hinshelwood mechanism, although the temperature-dependence and early titration shoulder are not predicted. It may be that the titrations restructure the Pt surfaces to some extent.24 In future COT,-OT,, titrations could provide information on the oscillatory kinetics of CO oxidation on catalyst^,,^ since during the titrations all coverages of oxygen and CO are scanned and the rate will be given by d[CO,]/dt or dQ/dt, which will equal kf(Oco Oo), wherefis not a universal or simple function.26 It is also noteworthy that at 373 K the ratio of the primary titration enthalpies QcoTo/QOTco is initially 0.28 for Pt/SiO, (I) with a 75.5% dispersion of Pt and 1.80 for Pt/SiO, (K) containing Pt with a higher Pt dispersion.12 If not is 41662 CO-0, Titration on Pt k A t/min and nco is 1 then scheme (1) is valid, qCOT,/qoC is 1 and Q~~T,/QoT,, is (Qco, -2Qcoc - ~ Q O C ) / ( ~ Q C O , +SQOC - ~ Q c o c ) * Pt,O + 3CO + 2Pt-CO + CO, 2Pt-CO+ 1.50,+ Pt,O+ 2C0,.(1) However, if no, is 4 and n,, is 2 then scheme (2) is relevant, qCOT,/qoC is 1 and QcoT,/QoT,, is (Qco, + Qcoc -iQoc)/(Qco2 + tQoc - Qcoc). Pt,O + 2co + Pt,-CO + co, Pt,-CO + 0, + Pt,O + CO, (2)M . A . M . Luengo, P. A . Sermon and A . T. Wurie 1663 n t/min t/min Fig. 6. COT, and OT,, titration thermokinetic profiles dQ/dt us. t for Pt/TiO, (N) (A), (B) and Pt/SiO, (G) (C), (D) measured at 373 K. For Pt/TiO, the preceding 0, chemisorption in OT,, is far more significant than for Pt/SiO,. A: (a) OTcOl, (b) OT,,,, ( c ) OT,.,.B: (a) COTo1, (b) COT,,. C : (a) OTCol, (6) OT,,,. D: (a) COTo1, (6) COT,,. Here it is assumed that Qi is the enthalpy of chemisorption or the enthalpy of oxidation of CO to CO,, g. (i.e. QcOz is 283 kJ mol-1 of CO oxidised at 298 K). Clearly, the catalyst moves from regime (1) to (2) if it bridge-bonds CO rather than linearly adsorbing it. Qcoc and Qoc are 21-230 and 167-335 kJ mol-l, respectively, on different Pt planes.1° Taking the highest values of Qcoc and Qoc the ratio QcoTo/QOTco is 2.10 and 1.57 for linearly and bridge-bound CO. However, taking the lowest values of Qcoc and Qoc this ratio becomes 0.40 to 0.64. Returning to the data in table 3, there is a suggestion that catalyst I exhibits * surface sites on the larger Pt crystallites which chemisorb CO and 0, with low enthalpies, while sites on the surfaces of the smaller crystallites in sample K chemisorb CO and 0, with higher enthalpies.This could be related to the degree of site coordination, but unfortunately enthalpies of CO and 0, chemisorption are not very different on (1 1 l), (1 10) and (100) planes of Pt10 and differences in QCOTo/QOTco cannot be ascribed to different fractional contributions of these planes to the surfaces of Pt crystallites of different size. Further evidence is required, especially if total rather than primary titration enthalpies can be measured. Nevertheless, the results presented here1664 C0-0, Titration on Pt suggest that this ratio is very sensitive to: (i) Pt particle size and temperature and (ii) continued use and cycling of catalysts.In addition it may be that the support is also important. Compare the results shown in fig. 6 for Pt/TiO, (sample N12 with a Pt dispersion of 12.2%) with those of Pt/SiO, (sample G with a Pt dispersion of 3 1.5% 12). With the titania support the initial low dQ/dt shoulder in OT,, tentatively associated with oxygen adsorption onto the initially CO-covered surface is much more pronounced than when using the silica support. Conclusions The extents of COT, titrations on supported Pt measured as qCOTo or the primary enthalpy change QCOTo can be used to determine Pt surface areas after calibration, but titration at 423 K is preferred to earlier suggestions of ambient ternperat~re.~?~ The enthalpy ratio QCOTo/QoTco appears to probe Pt surface site coordination and particle size, the nature of the support and the extent of restructuring during titration in a manner not shown by the enthalpy ratio in HT,-OT, titrations,,' but is also temperature sensitive.The COT,-OT,, titrations are repeatable and partly reversible ; some surface Oads may remain unreated. Combined thermokinetic-catalytic data may provide more detailed kinetic information on catalysis of CO oxidation in future. Total titration enthalpies are not yet readily determined and absolute values so determined by differential scanning calorimetry28 may be less significant than their ratios and relative values used here. 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