Sintering Studies on Platinum Black CatalystsPart 1.-Effect of Pretreatment and Reaction on Particle SizeBY THOMAS BAIRD, ZOLTAN PALL j- AND SAMUEL 3. THOMSON *Chemistry Dept., The University, Glasgow GI2 8QQReceived 20th July, 1972Counter claims appear in the literature on the effects of the particle size, sintering and pre-treat-ment of a catalyst on its activity. We have investigated these by studying a platinum black catalystand the way in which helium, hydrogen, air, oxygen and thermal cycling affect the particle size asseen in the electron microscope. By making a logical sequence of experiments it has been possibleto show that the sintering of the catalyst was most affected by the presence of hydrogen which causedconsiderable growth in crystallite size.It thus seems possible that apparent effects of reaction andthermal treatment may have, in previous work, been caused by pre-treatment in hydrogen.Platinum is one of the most commonly used catalysts for hydrocarbon trans-formations. There are differing views in the literature on the extent to which theparticle size or dispersion of platinum may affect its catalytic activity or selectivity.It was found that several reactions such as hydrogenation of cyclopropane,l de-hydrogenation of cyclohexaneY2 hydrogenation of ben~ene,~ were independent of theparticle size of platinum. These reactions were called by Boudart " facile " reactions.'On the other hand, the so-called " demanding " reactions require specific surfacestructures, the concentration of which is dependent on dispersion and surface area ofplatinum.Such reactions are the hydrogenolysis of neopentane and skeletalisomerization by various mechanism^.^.Platinum catalysts of different dispersion were prepared in different ways.Gault observed different selectivity of platinum catalysts containing different amountsof platinum 5 * ; he explained this phenomenon by the different dispersion of themetal in the catalysts. Poltorak on the other hand found that by increasing theplatinum content of his Pt/SiO, catalysts prepared in different ways, only the numberof Pt-aggregates increased but the size of individual metal clusters (or crystals) did not.It was mainly the method of preparation which affected the activity and selectivityof the catalysts used by him.Boudart and co-workers prepared platinum samplesof different surface area by sintering them at various temperatures between 425 and900°C and found different selectivities for hydrogenation and hydrogenolysis of neo-pentane. Their treatment, however, was not uniform for every sample, and involvedheat treatment in hydrogen and/or vacuum or eventually a cyclic treatment withoxygen and hydrogen. The sintering effect nevertheless was attributed exclusivelyto the thermal treatment.Wilson and Hall investigated the behaviour of alumina- and zeolite-supportedplatinum catalysts heated in hydrogen, with and without oxygen pretreatment, atdifferent temperatures. They observed a change in the surface area of the metal,t permanent address : Institute of Isotopes of the Hungarian Academy of Sciences, Budapest114 POB 77.5T. BAIRD, z.PALL AND s. J . THOMSON 51according to X-ray and electron microscopic measurements, and they deduced that“ heat treatment did cause growth in crystal size ”.On the other hand the decrease of area of silica-supported platinum was notaccompanied by growth of crystal size.8In experiments which may be related to those which have been described it wasfound ’* * that the stoichiometry of hydrogen-oxygen titration, used in the determina-tion of metal surface areas,g* lo was affected by sintering the catalyst at differenttemperatures. It was the hydrogen chemisorption which followed the real metalsurface. This point has been investigated by Akhtar and Tompkins 11* l 2 whofound that at elevated temperatures the chemisorbed oxygen may migrate into thebulk of the platinum and can thus be removed incompletely and slowly by subsequenthydrogen treatment.Hence, it cannot be surprising that a controlled oxygen treat-ment of a sintered reforming catalyst may lead to redistribution of the platinumparticles. Such a phenomenon may cause the different catalytic activities ofplatinum samples which have, or have not, been in contact with 0xygen.l’These studies suggest that the subtle relationships between thermal treatment ofplatinum in different atmospheres on the one hand and its surface and catalyticactivity on the other cannot be regarded as having been elucidated. It is for thesereasons that we decided to carry out these investigations.Supported catalysts are as a rule stable and active and the relative instability ofunsupported platinum black may thus offer better opportunities for the study of itschanges during pre-treatment and reaction.It is also advantageous in that thepossible effects of the support,14 which may play an important part in catalysis, canbe eliminated.Sintering of Pd-black has been observed by heating it in hydrogen at 60-90”C.15One of the direct observations which stimulated this part of our study was the factthat during a prolonged period of use the surface area of the same platinum blackcatalyst had decreased by a factor of ten, whereas only an approximately twofolddecrease in its activity was observed in the aromatisation of n-hexane.16EXPERIMENTALPlatinum black was produced by reduction of H,PtCI, by formaldehyde in the presenceof concentrated KOH at 20 to 25OC.l’ The catalyst was washed with distilled water forseveral weeks, then filtered and stored in the air.Its surface area (as determined by B.E.T.method using krypton as the adsorbate) was - 8 m2 g-l.Changes in particle sizes were followed by electron microscopy. The electron micro-graphs were taken as follows : specimens suitable for electron microscopy were prepared byallowing a drop of distilled water with the catalyst in suspension to dry down on to carbon-film specimen grids. The suspensions were previously placed in an ultrasonic bath for 5 sto aid dispersion of the solid. Samples prepared in the dry state gave similar results, butthe adherence of the catalysts to the supporting film was poor and the former method waspreferred.The specimens were examined with a Siemens Elmiskop I electron microscopeoperating at 100 kV.The different treatments of the catalyst have been carried out in a flow type reactor. Allthe different gases were passed over the platinum at a flow rate of about 50 ml/min at about1 atm total pressure except For the air treatment which was applied simply by opening thereactor to the atmosphere. Most of the samples were obtained by a uniforni and controlledtreatment of fresh, unused Pt called the standard thermal cycle (s.t.c.). This consisted of aheating period from room temperature to 360°C within 25-30 min.The temperature wasraised within about 10 min to 330-340°C, then the final temperature of 360°C was adjustedduring the rest of the warming up period. The temperature was then kept constant and 3 hafter the start the heating was switched off. The sample was left to cool in helium flow 52 PARTICLE SIZES I N A PLATINUM BLACK CATALYSTthe cooling period was 45-50 min. The catalyst samples were removed from the apparatusafter reaching room temperature.RESULTSThe electron micrograph of the fresh catalyst shows a very finely dispersedplatinum black, fig. 1. On the contrary, the same catalyst kept for 50 h at elevatedtemperature not exceeding 360°C and used for study of ethylene reactions in heliumshowed remarkable sintering which caused the particle size to grow, fig.2. Thesereactions involved injecting ethylene pulses onto the catalyst via the helium carriergas. The products of reaction were ethane and methane. Before reaction thecatalyst was heated in air and subsequently in hydrogen (s.t.c., the exact treatmentis described in the caption of fig. 6); deactivated catalyst was regenerated with airand then hydrogen treatment at reaction temperatures. Regeneration took placeseveral times in the case of the sample shown in fig. 2.X-ray line broadening measurements confirmed that the phenomenon observedwas actually the growth of the average crystallite sizes not just surface aggregationor welding of smaller particles. The average crystallite sizes determined by electronmicroscopic observation and X-ray line broadening were as follows.crystallite sizes/Aas determined byelectron micrography X-ray diffractionfresh Pt SO+ 30 l l S + Sused Pt 500+ 60 500+ 35In order to investigate the causes of this sintering we decided to treat the platinumblack in different ways which corresponded to the different stages through which thecatalyst had passed in the ethylene experiments.The standard thermal cycle wasused in every case : the different treatments to which the catalysts were exposed areshown in the legends to the corresponding electron micrographs (fig. 3-8).Hardly any sintering could be observed in catalysts which had been heated inhelium or in air. Likewise, heating in vacuum produced pictures identical with fig. 3.On the other hand samples which had been in contact with hydrogen showedsintering to a greater or lesser extent.The effect was more marked when platinumblack had been pre-treated in air at elevated temperature, fig. 5 and 6.Surprisingly, the ethylene treatment caused only minor sintering as can be seenin fig. 7. When the catalyst which had been used for ethylene studies had beentreated subsequently by hydrogen, virtually no further sintering was observed, fig. 8.These results make it clear that the sintering process is connected in some waywith the hydrogen treatment. To elucidate this point further, catalyst samples weretreated with hydrogen for 60 min, but under different thermal conditions.It can be seen in fig. 9 that hydrogen at room temperature caused no sintering :hydrogen at room temperature followed by the standard thermal cycle in heliumcaused some sintering, but only to a small, uniform extent, fig.10. When heatingwas carried out in hydrogen in the third case, fig. 1 1 ? a considerable, but not complete,sintering was observed.The treatment of the sample for fig. 6 under the standard thermal cycle representedone of our standard pretreatments of catalysts for the study of hydrocarbon reactions.It is therefore of interest to compare the catalytic activities, in a microcatalyticreactor gas chromatographic system, of treated and untreated catalysts. The resultsare shown in table 1FIG. 1.-Electron micrograph of fresh, un- FIG. 2.-Electron micrographs of the sametreated, finely divided platinum black.Particle catalyst as fig. 1, but maintained at - 360°Csize, about 1008, : magnification x 48 000. for 50 h and used for ethylene reactions inhelium (regenerated several times with air andhydrogen). Particle size, about 500 8, : magni-fication x 48 000.FIG. 3.-Electron micrograph of platinum FIG. 4.-Electron micrograph of platinumblack subjected to helium flow and standard black. Sample heated in air with subsequent 30thermal cycle : magnification x 48 OOO. min in air, then helium : magnification x 48 000.[To face page 5FIG. 5.-Electron micrograph of platinum FIG. 6.-Electron micrograph of platinumblack : sample heated and subsequently held black : sample heated in air and subsequentlyfor 30 min in helium, then 60 min of hydrogen 30 min in air, then flushed with helium, then60min in hydrogen, then helium: magnifi-cation x48 000.flow, then helium : magnification x 48 000.FIG.7.-Electron micrograph of platinum FIG. 8.-Electron micrograph of platinumblack : sample heated in He, then 11 x 0.5 ml black : previous catalyst, fig. 7, kept for 2 daysethylene slugs passed over catalyst (in helium in air, then s.t.c. : heated and kept for 30 mincarrier gas) : magnification x 48 OOO. in He, then 60min H2, then He: Magnifi-cation x 48 000FIG. 9.-Electron micrograph of platinum FIG. 10.-Electron micrograph of platinum :black : sample exposed to He flow for 10 niin, sample exposed to He 10 min, H2 60 min atthen H2 60 min at room temp., He 3 h, room room temp., He and s.t.c.in He : magnificationtemp. : magnification x 48 000. x48 000.FIG. 11 .-Electron micrograph of platinum FIG. 12.-Electron micrograph of platinumblack : sample exposed to He flow for 10 min, black heated to 600°C in air : magnificationH2 10min at room temp., heated to 360°C inH2 over 25 min, maintained in H2 for 25 min,then s.t.c. completed in He : magnificationx48 000.x 48 000T. BAIRD, z. PALL AND s. J . THOMSON 53TABLE 1 .-PRODUCTS FROM ETHYLENE INTERACTIONS WITH TREATED AND UNTREATEDPLATINUM BLACK CATALYSTScatalyst, 0.06 g Pt ; 0.5 ml pulses of ethylene into 60 ml/min He flow; temp. 360°Cproducts/ %retained oxidisedCZH4 species products pulse no. CH4 C2H6treated catalyst3.4 18.8 60.0 17.8 - 12 1.4 6.1 85.6 6.9 -6 1.3 1.5 97.2 I -untreated catalyst1 16.9 33.4 14.5 22.4 12.82 4.2 16.0 70.8 9.06 1.7 1.8 96.5-- -DISCUSSIONThe electron micrographs show clearly and without doubt that the sintering ofour platinum black had been practically completed during the pretreatment and wassomehow connected with the hydrogen treatment.Different mechanisms can beput forward for the sintering. (a) It could be the result of the heat of reactionbetween oxygen and hydrogen on the platinum surface. (b) It could be attributedto the interaction of hydrogen with the catalyst.It is well known that the platinum is covered with a layer of oxygen when it hasbeen in contact with air. The reaction between hydrogen and oxygen can be due tothe removal of this oxygen layer. Before exposure to hydrogen, the catalyst wasalways flushed with helium, thus no atmospheric oxygen was present.If we assumea monolayer of oxygen on the platinum with 1.1 x 1015 surface atom/cm2, the reactionheat would be enough to melt a considerable fraction of the platinum, provided thatthermal conduction were negligible. Since the hydrogen treatment at room tempera-ture and at 360°C resulted in completely different pictures, and the heating ofhydrogen-treated platinum in helium produced a third variant, this assumption maybe rejected, as the only or the main cause of the sintering.Another possible hydrogen + oxygen reaction might have arisen when the hydrogen-treated samples were taken out of the reactor into the open air. Since no pyrophoricphenomena were observed, and what is more, since the catalysts treated with hydrogenunder different conditions, but having the same concluding period of the thermalcycle in helium, showed different pictures, this suggestion, too, can be regarded asfalse.The sintering phenomena must be interpreted in terms of hypothesis (b), that isit must be connected with the hydrogen treatment itself.When platinum surfaces covered with an oxygen monolayer are contacted withhydrogen, the latter will displace the oxygen on the surface.'-1° This process cantake place at room temperature.This reaction obviously does not cause any sintering.If, however, relatively moderate thermal treatment was applied with the hydrogen,it altered the catalyst structure. It can be seen in fig.1 and 6 that this phenomenonis accompanied by the increase of the crystal size and cannot be regarded only as anagglomerating process.The sintering of Pd-black was explained by " meniscus formation " betwee54 PARTICLE SIZES I N A PLATINUM BLACK CATALYSTindividual crystallites. Although the experiments were done in hydrogen, theauthor has not emphasized the importance of its presence.We regard it as obvious that this process required a continuous hydrogen supply.This is confirmed by comparing fig. 5, 1 0 and 11. When there is a hydrogen supplyat higher temperatures, fig. 5 and 1 1, extensive recrystallization occurs, although thiswas not uniform throughout the catalyst : the process took place mainly at the metalsites attacked originally.On the other hand, when the hydrogen layer was formedat room temperature, then heated in helium, this caused only a small, but uniformrecrystallization to the extent which was allowed by the uniform hydrogen layerformed at room temperature.Obviously, the surface hydrogen at higher temperature starts an interaction withthe catalyst which leads to its " bulk " recrystallization. It can be assumed that thismay start by diffusion of hydrogen into the metal.Radioactive tracer experiments carried out with hydrogen gas labelled withtritium clearly show that at 360°C there are at least two types of hydrogen held by theplatinum black (in the absence of gas phase hydrogen). These species can be trans-formed into each other and could be exchanged with gas phase hydrogen.Catalystsamples treated with radioactive hydrogen in ways analogous to those shown in thecaptions of fig. 6, 9, 1 0 and 11 were still radioactive after taking them out into airwhich must have originated from tritium held by surface or sub-surface 1a~ers.l~l2 have shown that at higher temperatures oxygenmay diffuse into the platinum metal. If this was allowed by an oxygen treatment atelevated temperatures (fig. 6), the structure of platinum already loosened to someextent by oxygen, but not to an extent causing visible sintering-see fig. 4, mayrecrystallize more readily under the effect of hydrogen. In this particular case thesintering may be enhanced by the reaction heat between the oxygen, this time in excessof a monolayer, and hydrogen.It must be pointed out that the sintering process may also take place in an oxygenatmosphere, but it requires more severe conditions. Fig.1 2 shows a sample heatedto 600°C in air : severe sintering can be observed.In the case of untreated catalysis it would be expected that the layer of oxygen ontheir surfaces would be susceptible to removal by hydrocarbons. The presence ofcarbon dioxide in the products from the first pulse of ethylene, table 1, was directevidence for this. If we assume an oxygen monolayer on a surface containing1.1 x 1 0 l s atom/cm2, then the amount of carbon dioxide is within a factor of two ofthe calculated amount. No conclusions about the stoichiometry of the layer arepossible.Table 1 also shows, in agreement with previous work, that a considerable part ofthe ethylene which passed over the treated and untreated catalysts was retained.Thus rapid deactivation of the catalyst, table 1, can be accounted for in terms offormation of carbonaceous residues.These residues may well have covered theoriginal platinum grains and prevented their sintering. Fig. 8 can be interpreted interms of surface protection by deposits which inhibited sintering on subsequenttreatment by hydrogen.may be attributed to the slowbuild up of carbonaceous deposits rather than to sintering, for most of the latterprocess must already have taken place during pretreatment.In spite of the fact that the differences in particle size and surface area of thesecatalyst samples remained after they had been in contact with ethylene, the differencesin activity became negligible during subsequent ethylene pulses.This must havebeen caused by the rapid deactivation of the most active sites by hydrocarbon frag-Tompkins and AkhtarThe slow decrease in activity described by PaiT . BAIRD, z. PAAL AND s. J . THOMSON 55inents. This, in turn, suggests that only a fraction of the surface sites actually tookpart in the catalytic reactions.This sintering process under relatively very mild conditions emphasizes that thesystem Pt/H, may behave as a very mobile solid system and also the importance ofthe eventual support in keeping the individual catalyst granulates apart. On theother hand, the unusual activity of hydrogen in transforming the structure of theplatinum metal suggests once more that the hydrogen may play a more active rolethan had been suspected in creating active centres in platinum and other metals.The relationship between different pretreatments, excluding area effects, and catalyticactivity will form the subject of another study.The authors are very grateful to Dr.R. L. Moss of the Warren Spring Laboratory,Stevenage, for the X-ray investigations. One of us (Z. P.) thanks the I.A.E.A. for aFellowship.M. Boudart, A. W. Aldag, J. E. Benson, N. A. Dougharty and C. G. Harkins, J. 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