Modification of Pt surfaces

Crztiys& T&y, 10 (1991) 251-258

Elaevier Science Publishers B.V., Am&&am

MOIXFICATION OF Ft SURFACES, The mechanismof the impmvement of mrorming catalysts V.Ponec, Gorlaeus Laboratories, L&den University P.O.Box 9502, 230 RA I.&den, The Netherlands

Abstract Reactions of n-hexane on three bimetallic (alloy) systems: Pt-Re. Pt-Ir, Pt-Co have been studied with sulphur-free and sulphided catalysts. A shift is observed in the selectivity pattern from “metallic: (on sulphur free catalysts) to “acid cataIysed” reactions (on s~ph~ modified catalysts), which is explained by the shift in the formation of certain intermediates of the reaction.

Naphtha reforming, au imporumt industrial process is sways performed with bimetallic catalysts: Pt-Re, Pt-Ir or R-Sn (l-3). The just mentioned pahs form solution-alloys in a broad (Re) or in the whole composition range (Ir) or, they form one or mom intermetallic compounds (4,s). It has been known that with Pt-Re and Pt-Ir catalysts a modification of the catalysts by sulphur is absolutely required to achieve the desired selectivity (6): a low hy~~noly~s and a high isomerisation and ~hy~li~tion. Further, the following has been aheady establish Pure R/Al& or bimetallics on A&O, work ~bi-~ction~y~: oleflnes formed on 1) the metallic component are converted on the acid centres of the carrier into carbenium ions and these, in their turn, switch on isomerlsation and &hy~yc~tion reaction steps. Pmducts of the latter conversions migrate back to the metallic component and leave the surface as (mostly) saturated aliphatic compounds or as aromat (7-g). A part of the products (for example that of aromaties) is formed also on the metallic component (10,ll) Large areas of the metallic as well as of the oxidic surface are covered at steady 2) state operation by carbonaceous (II-lean) layers. Bimetallic, when compamd with monometallic catalysts, are less deteriorated by this selfpoisoning, either because less deposits are formed or the deposits are less harmful. It is known that the deposit formation on the metallic component is mom detrimental than that on the support. Alloying can suppress the f~tion of deposits and their conversion into the inactive graphite (12-15). Sulphur changes considerably the selectivity pattern of the mono- or bimetallic 3) catalysts. The beneficial effects of bimetallics R-Re and Pt-Ir am only present when the metallic component has been modified by sulphur (6,9,16). The primary purpose of this communication is to review, to compare and to discuss the results on three biitalhc systems from severaI papers pub~sh~ by the I.,eiden group and draw attention to mainly those points of comparison which have not been (sufficiently) discussed in the papers on individual topics. Therefore, the reader is kindly requested to look for the experimental &tails in the original papers (17-22). ~ZO-5861/91/$03.50

0 1991 Elsevier Science Publishers B.V. AH rights reserved.

261

EzcpsimntaI te!chnit?m used ~~titnmus flow aids lvsacter, 1 bar tatat assure) has been described in several papers (e.g. 23). For the pqsration and the charactetisation of catalysts see: pt-Re (19); P&Ix (zo); pt-Co (22);. these catalysts Iwe been used in obtaining the d&t&in figure 1 and la. CaMysts used b obtain data summarized by the figures 2 and 3 are described in ref. (24). The stand& pmcedum of sushi ~cati~n (by ~~hene) and pyridine ~0~~ am described in (19) and (19+24), respectMy. AU ex~n~ reactk Snkures ~ex~~~~); ~~ is 626X.

described below are with II-hexaue and star&& if not stated otkwise, the smdard temperatum for

Results and disc~stin Figure la collect8 da& on the sek&ities, faw thm@ groups of lotions: hy~~n~~is, isomerisation and cycl~ati~ (both C, C& &air& with bimetallic systems as indicated (5% pt 1~~ nsed with the PI-IT and Pt-Ci2 tie% 2% Pt iu Pt-Re, with nhexane as the feed). Conclution from these data is equally straightforward and trivial when a metal which itself is a go& hy~g~~~is catalyst (Re, IT, 0~) is added to Pt, s&&v&y for by~og~no~y~ ~tin~us~y increases and that of ~~~~don decreases with increasing ~o~en~atio~ of the second ekment (Re, Ir, C%) in Pt. Si tbe ~~1~~ of the surface ~~i~o~ af the catalysts used is nat known in suffiti~t deta& it is ~~sib~ to speak about adze (or absence thereef) of proper&~ of ~~i~~ ~~e~~. The data in fi la gain relevancy when they 8t~ compared with those in figme lb, concerning the swlphur modified catalysts. H&S we observe a just opposite trend: in all cases isometisation increases with increasing content of the hydrogenalydcaHy active metal. The mentioned increase in isomerisation is steep with Pt-Re and P&Co and less pronounced with Pt-k. Let us mention in this respect that tbe biing strength in su~ph~~s of Re and Co is much higher than that in Ir and Pt (which are both abler (ZS). A fkrtber im~t point is that when Pt-Re or inert Si& act.&@ is left sfter (instead of A&$&)no IBM Pt-k a part of the metallic activity sur+=kes. It is likely, that alI panes just rne~ti~~ in this paragraph are inter&&M. An important ~ti~~~ portion on the w@k@ of various cstalysts is &%a&& when instead of selectivities the rates sre compared. This compar&on has been done with a se&es af home-made catalysts and two cumme~ial catalysts (Europt3 and a Pt-Re, Ketjen catalyst). E the activity (the rate at &XlK) of each given su~~~-~ catalyst is put equal ta 100, a pietrue is obtained with the same but now ~~~~ ~a~~ys~~ as shown in figure 2 Tn all cases, tbe rate of b~~~y~s is suppressed veq stmq&. The rate of Safe by safe with the cata@& ~~~~ is ia a mosf plus which shows a b&h selectivity to isomerkation befare s~~~~tion. On the other ban& with catalysts which anz of the lowest S(isomerisation) b&Yoresulphidation (Ir, Pt-Re), the rate of isomtisation is eveu higher on sulphided catalysts than an the sulphur-free ones. These facts and the ~ow~~~e (X8) that with neohexane o&y 2,? ~e~y~bu~e is prcdueed by Sophie catalysts (NB this is the only product which can be produe& aIso by a carbenium ion intermedia&e) have Iead to a suspicion that in spite of the xelatively

253

low temperatum (620K) the acid-catalysed reactions must play an important role with sulphided catalysts. To establish the possible role of acid centres in the overall conversion of hydrocarbons, poisoning by pyridine has been used (19,24). When the steady state of the catalysts was achieved after about 20 hours (state “A”), pyridine was administemd in pulses, until a new steady state was achieved (state “B”). Then, the reaction was continued without pyridine, for about 4 hours (state “C”), a period which was followed by reduction in pure Hz at 62OK. for about 5 hours (state “D”). When the reaction was restarted thereafter with the standard reaction mixture and at the standard temperature, a new steady state was fiially achieved, designed as state “II”. The data obtained in the just described way am evaluated in the following manner. An idea is followed, put forward in the paper by Robschlager et al.(26), that pyridine poisons both the metallic as well as the acidic sites of the bifunctional catalyst, but under suitably chosen temperatums the first mentioned poisoning is reversible and the latter mentioned one - ixrevcrsible. By this criterion the following picture of the importance of acid cenues catalysed reactions (see fig.3). With sulphur-free catalysts, the activity in the final state “E“ is 90% or more of the steady state activity at “A”. The poisoning can thus be considered as reversible and the activity of the sulphur-free catalysts is (by the applied criterion) almost purely due to the metallic sites. With the sulphur containing catalysts, the situation is quite different. In some cases hydrogenolysis is still poisoned reversibly but with the two commercial catalysts, the poisoning of hydrogenolysis is to a large extent irreversible. Isomerisation is in all cases mom influenced by the irreversible pyridine poisoning than hydrogenolysis and with the two commekal catalysts (Euro Pt3 and the Pt-Re catalyst), which are specially acldifkd, the suppression of isomerimtion is most dramatic. Comparing of sulphur-free with the sulphided catalysts reveals a shift in the selectivities; from reactions catalysed by metallic sites to reactions catalysed by acidic sites (19,20,22). Let us mention that speculations on the existence of such a shift demonstmted by figs.l-3 above, can be found aheady in some earlier literature (e.g.16). The shift in selectivities caused by sulphur deserves some more comment. Reading the literature one does not find indications that irreversibly bound sulphu (the one which played a role in our experiments) would incmase the number or acid strength of the alumina hound acid sites. Our pmliminary experiments with Pt-ReJAtio, confum this conclusion (27). However, sulphur certainly decmases the number of sites active in “metallic”-reactions ((de-)hydrogenation, most of hydrogenolysis and cyclisation, a part of inaction) and yet the activity in i~~ti~ increases in some cases after sulphidation (fig.2). It means that these catalysts produce after sulphidation tgp~~ of those intermediates which lead to isomerisation products, on account of the intermediates of other elementary reactions. We speculate that this shift in selectivity in the intermediate formation should be due to something like shown by the very schematic picture in schemeI.

‘c’

lc/ Lc/ / \*/ \

c

>c c
scheme I

In scheme I iutermediates multiply bound to the metallic sites am replaced to some important extent by unsaturated intermedktes which am mom ltxx+ely bound, migrate mom easily sod can form on acidic sites carbenium ions which in their turn induce is~~sadon and other acid centms catalysed mactions. Recently (27), hydrogenolysis of ethane has been compamd with hydrogenolysis of n-hexane on Ir/SiO, and Ir + S/%0, catalysts. It appeared, that the activity of Ir iu nhexaue reactions is suppressed by sulphq but it is not eliminated completely (see above). However, the activity in the ethane hydmgenolysis is suppmssed totally by the same stat&& s~h~tion, We coucluded from tbis observation that the m~tiply bound surf&~ inlays derived from the aS binding (ethane cannot do anything else) of the feed molecules require a larger ensemble of metal surface atoms to be formed than the complexes of the ~~~ycl~u~ safe or other, for example x-complexed species (see scheme I). Fiy, a brief remarlc (brief since the poiut has got much attention in the past) on the iiquently postulated electronic or ligand effects of sulphur or of the ‘second component in the bimetallic catalysts. Sulpbur on metals is known to cause a shift iu the fnquency of the IR absorption TV) (28). This is probably best by the stretching vibmtion of the co&e&d explained by the small electric fkld of the negative charge of S located above the metallic surface. The electrostatic field of this charge should cause sucb a shift in v(C!O) and weaken the metal-CO bond strength. By analogy, a similar, but of au opposite sign (strengtheuing) effect could be expected for x-complexed olefines, or for multiply bound (to the metal) species. This “through-the-vacuum” effect can in principle contribute to the overall effect caused by sulphur in the reforming reactions of hydmcarbons, but the main effect is tubby the shift from the mctal-catalysed to the acid catalysed mactious.

255

Analysis of the information available in the litemtum on the solid state physics of the catalytic relevant alloys and biitalliis

revealed (29), that the so-called “electronic” or

“ligand” effects of alloying are difficult to be detected or proven. Also experiments in which the v(CG) was monitored (30) or the mtal-Co

bond sangth was determined by

TPD (31) showed that while the way of bonding changes by alloying (or biitallics farmation), for example, from pmvailingly bridged Co (multiply coordinated) to singly coordinated (linear) CO, there were no signs seen for a considemble “electronic” or “ligand” effect in the adsorption of the individual species. Since the search for a possible ligand effect has been already done with many systems (v(CO) variations by alloying have been determined on Pt-Cu. Pt-Re, Pd-Ag, Pt-Co, Pd-Cu, Ni-Cu. Pt-Sn) and they all showed the likely insignificancy of the detected ligand effect for catalysis, these ideas were not inVOlVed in the explanations suggested above for the functioning of the bimetallic catalysts of naphtha- reforming. Acknowledgement The author acknowledged the profit and pleasure he had fmm the discussion with drs.M.J.Dees, who also contributed in important way to the collection of data presented above.

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30.

31.

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267

0 100

50 Re%

100

Pt-Ir/A1203

S%

I

50I 0

0

1

Qlue

fig.la fig.lb a

(left) (right)

b 1 50 Ir%

1

100

Selectivity in the reactions of n-hexane at 62OK, standard reaction conditions as a function of catalyst composition Catalyst unmodified by sulphur Catalyst modified by sulphur

hydrogenolysis

0 isomerisation 8

iLcyclisation

258

140% 120-j

100

S-FREE]

806040-

20-

63 Pt

c

3

61

Ir Pt-Ir lWT”A

100

Ru

1

Pt Pt-Re 0.3 1WTv Euro Pt 3

PYRIDINE-FREE]

% 80

bifunct. mech.

60 40 20

Pt

IIII

Pt-: lWT%

1 Pt $ -Re 0.3 lWT% Euro Pt 3

rVU'

figunT2 (above)

Relative rates (100 = given catalyst in the sulphur fne state) after sulphidization.

figure 3 (below)

Relative rates (100 = given catalyst after sulphidization) after poisoning by pyridine; the irreversible loss of particular activity indicates the extent of the bifukional mechanism. left - hydrogenolysis

right - isomerisation