Phosphorus on platinum — Field electron emission microscopy studies

Surface Soence 166 (1986) L131-LI35 North-Holland, Amsterdam

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S U R F A C E S C I E N C E LETTERS P H O S P H O R U S O N PLATINUM MICROSCOPY STUDIES

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FIELD ELECTRON E M I S S I O N

M. M U N D S C H A U and R. V A N S E L O W Department of Chemtstry and Laboratory for Surface Studws, Unwerstty of Wtsconsm - Mdwaukee, Milwaukee, 14ilsconsm 53201, USA

Received 16 September 1985; accepted for pubhcat~on 28 October 1985

Phosphorus deposited onto a clean Pt FEM tip (at RT) reacts readily with the emitter surface below 630 K. Compound clusters are formed on top of a coherent compound layer. Cluster formation, growth, and disappearance below 890 K indicate a high compound mobdity. The coherent compound layer shows the development of {221} planes, an enlargement of the (110} areas and ring structures around {100}. The coherent compound layer breaks up only upon heating to about 1670 K. The observations are discussed briefly in connection with Pt-phosptude chemistry and P poisoning of Pt surfaces

Phosphorus is one of the most tenacious poisons for Pt-containing automotive exhaust catalysts and petroleum refining catalysts [1-6]. Despite the importance of P poisoning, to our knowledge no fundamental studies have been published of P adsorption on Pt surfaces using the well controlled conditions of U H V along with ultrapure materials and single crystal surfaces. In the present letter we report observations of P adsorption on Pt surfaces using the field electron emission microscope (FEM). The F E M allows the simultaneous observation of all major crystallographic areas, including the high Miller index surfaces, which are believed to be important in m a n y catalytic reactions [7]. The F E M can, therefore, provide information on possible poison selectivity. Also, the submicron-size emission tip apex area is similar in size to larger Pt catalyst particles. This technique has been employed earlier to study the adsorption of S on Pt [8,9] and Ni [10] surfaces. Sulfur is another well-known catalyst poison. The F E M system used for the present studies has been described in detail previously [11]. The emitters were prepared from a Pt wire (Materials Research Corporation; 99.999% purity). Semiconductor grade red P (Atomergic Corporation; 99.9999% purity) was placed in a 14 cm long by 1 cm wide Pyrex glass tube sidearm which had been degassed in U H V by heating it close to the softening point. The entire F E M system was then baked at 640 K except for the sidearm base, which held a P lump of about 0.1 cm 3. The P was kept at low 0039-6028/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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M Mundschau, R Vanselo~t, / P on Pt - F E M studv

temperatures during the bakeout in order to avoid premature P c o n t a m m a n o n of the sample. (Red phosphorus has a vapor pressure of only 1.7 × 10 10 Torr at 298 K which increases to 7.7 × 10 5 Torr already at 400 K [12].) After the

M Mundschau, R Vanselow / P on Pt - F E M s t u d y

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bakeout, a residual gas pressure of 6 × 10-10 Torr was achieved. The emitter was then flashed until the FEM pattern characteristic of a clean Pt surface appeared (fig. lb). During control heating sequences, neither (012} planes (typical for Si contamination [11]), nor {100} rings (typical for P contamination [11]), nor C islands [13] could be observed. Phosphorus was deposited onto the clean Pt surface (with the emitter at RT) by heating the Pyrex tube until a pressure of about 1 × 10 -6 Torr was registered (fig. lc). The sidearm tube allowed direct line of sight between the emitter apex and the P source. Earlier controls performed by heating the empty sidearm tube to temperatures used to evaporate the P did not show any significant contamination of the emitter. After the deposition of P, the emitter was heated stepwise (without field) for 30 s at increasing temperatures. The emitter was rapidly quenched to room temperature before a picture was taken of the respective surface state. Fig. 1 shows surface changes observed during such a heating sequence. The deposited P (O---3-5) reacts readily with Pt below 630 K. A thin coherent compound layer with clusters on top of it is formed. The clusters can be observed in the high index areas but not on (111) or (100) (fig. ld). Upon further heating, clusters grow by Ostwald ripening. At 810 K most of the larger ones are formed in the stepped vicinal area of (100) (fig. le). At 890 K the high index areas are completely free of clusters (fig. lf). The cluster formation, growth and disappearance from the high index areas below 890 K indicate a high compound mobility. (The existence of pure P clusters is unlikely because of the high P vapor presure in this temperature range (P600~: --- 20 Torr).) One is tempted to compare the cluster compound with the eutectic, Pt/PtsP2 [14] (MP 861 K [15]), which is assumed to form in Pt grain boundaries and to cause the destruction of Pt crucibles and thermocouples [15,16]. However, the face-specific growth of clusters in the area of the (100) (at high P coverages Fig 1 Phosphorus on platinum. After deposition of P on a clean Pt field ermtter (at room temperature), the latter was heated stepwlse for 30 s at increasing temperatures (without apphed electric field) After rapld quendung a FEM picture was taken of the respective surface state (a) Stereograpluc projection of the ermtter crystal. (b) Flashed to 1700 K Clean surface shows well developed {111} and (100} (dark areas), as well as brightly emitting, curved lugh index areas. Dark hnes mark (110) zones. (c) 290 K. Emitter surface of (b) after deposition of 3-5 monolayers of P. (d) 630 K Cluster growth in tugh index areas. (e) 810 K Continued cluster growth. Largest clusters m vlclmty of (100} {221} planes become Vaslble. (f) 890 K Clusters hnuted to {100} vicinity {221} planes clearly developed (g) 960 K. Clusters m {100} vicinity {221} formation only around central (111) (h) 1130 K. Clusters m {100) wcmlty (111) shows hght nng structure {221} formation incomplete. (0 1540 K. {100} nng structure broken up by low ermsslon (111) and (100) zone segments. (111} outline becomes triangular 0) 1590 K. All nng strucutres and plane decorations have disappeared. High index areas show small, low ermsslon spots and enlarged (110} areas (k) 1670 K. Emission from hagh index areas more even. (110} areas smaller than m 0), but stall more pronounced than on clean surface of (b). (1) After anneahng state of (k) to 1030 K, the {100) ring structures and triangular (111} decoraUon reappear.

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clusters can also be observed on the {100} plane itself) hints at PtP z, which shows a very low misfit of only 2.6% for PtP2{100} II Pt{100} with PtP2(ll0 ) II Pt(100). PtP 2 has a pyrite structure (Pa3) [171, while PtsP 2 is monoclinic ( C 2 / c ) [14]. The increased compound mobility could be due to the small cluster size (Kelvin effect). An initial P coverage exceeding the one shown in fig. lc (0 = 3-5) leads to rapid formation of strongly emitting large clusters early in the heating sequence and subsequent emttter destruction dunng imaging. An important feature of the coherent compound layer is the development of the {221} planes, which can be observed already at 810 K (fig. le). The {221 } planes are stable up to 1130 K (fig. lh). Above tlus temperature the {100} ring structure changes its appearance and a triangular shape of the { 111 } can be observed (fig. li). Both of them disappear at 1590 K (fig. 1j). At this temperature the surface still seems to be covered by a thin coherent compound layer, as indicated by the shape of the {111}, the very large {110} areas showing a low emission, and the low emission islands in the high index areas. Only at 1670 K does the compound layer seem to be broken up (fig. lk). However, some residual P adsorption remains, causing a shape change of the {111} and the appearance of low emission {110} areas. A similar thermal stability is known for PtP 2 which shows a macroscoplcally recognizable decomposition only at 1670 K [15]. Once the Pt is contaminated by P, F E M patterns indicative of a clean surface (fig. lb) can no longer be obtained regardless of the anneahng temperature of the emitter. Reheating at lower temperatures leads to a reoccurrence of contamination patterns (e.g., fig. 11) According to the field emission microscopy studies the following features are probably most relevant to P poisoning of Pt surfaces: First, the high cluster compound mobility allows rapid surface diffusion and thus rapid distribution of the phosphade. Second, the formation of an extremely thermally stable P t - P compound layer precludes its removal below the sintering temperatures of most Pt catalysts. Considering the known chemical properties of PtP 2 (e.g., it is unchanged even after 24 h in boiling aqua regia [17]), the compound layer would also be expected to be extremely difficult to remove chemically. The very stable phosphorus adsorption also causes a change in crystal form and habit. The {221} and {110} planes grow at the expense of the high Mdler index reNons and to a certain extent also of the {111 } planes, Some potentially very important catalytic sites are thus ehmlnated or are reduced in number.

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M Mundschau, R Vanselow / P on P t - F E M study

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