J . Phys. Chem. 1990, 94, 4991-4997
499 1
Structural Sensitivity of the Standard Pt/SiO, Catalyst EuroPt-1 to H2 and O2 Exposure by in Situ X-ray Diffraction V. Gnutzmann and W . Vogel* Department of Surface Physics, Fritz- Haber-Institut der MPG, Faradayweg 4-6, D- 1000 Berlin 33, West Germany (Received: June 27, 1989; In Final Form: January 10, 1990)
The structure of small platinum particles, in the standard Pt/Si02 catalyst EuroPt-1, of 17 A average size is investigated in situ by X-ray diffraction. For this analysis a numerical fit with Debye functions of a sequence of cuboctahedra build up by 13, 5 5 , 147, ..., atoms is used. After interaction with oxygen the metallic phase was found to coexist with platinum oxide phases PtO,, the major fraction of which exhibits the local atomic arrangement as PtO and a smaller fraction that of Pt304. The maximum amount of Pt oxidized was found to be 65% under 1 bar of oxygen at 300 OC. Under particular conditions (oxidation at 400 OC and cooling to room temperature in oxygen) the particle size distribution of the residual metal is that of the starting sequence of cuboctahedra with their outer shells removed. For all oxidized samples the reduction in 1 bar of H2for T 2 300 K is fast (e40 s), and in each case the initial particle size distribution is restored. Under hydrogen the Pt particles seem to relax to perfect fcc symmetry, but the Debye temperature is lower than that of the bulk metal. After evacuation, the reduced samples exhibit X-ray patterns very similar to those of weakly oxidized catalysts. The particles then show a high static disorder. In this study, the fraction of particles with icosahedral morphology could be restricted to 510 wt %.
Introduction In recent years differential anomalous X-ray scattering with synchrotron radiation has become a promising technique for the study of supported catalysts.l,z Although this is a unique method to discriminate the scattering from the support atoms, there are special problems involved that hinder its applicability. Most of the conventional X-ray work on catalysts focuses on the analysis of parts of the patterns, for instance, the Fourier line profile analysis or the integral intensities of single Bragg peaks. These techniques are effective if the metal dispersion is not too high (less than -40%) but fail for highly dispersed systems. It will be shown in this paper that new analytical tools can provide detailed structural information even in this range of dispersion, especially when the measurements are performed under in situ conditions. The method presented in this work is to fit the experimental diffraction pattern to a series of Debye functions, including thermal diffuse scattering by calibration of the respective weight factors. These functions are calculated for regular polyhedra with fcc symmetry. Germer and White4 first used the comparison between the experimental diffraction pattern and the pattern calculated from models with the Debye formula to obtain information on particle size and structure. Recently Hall et al.s used Debye functions to estimate the relative amount of regular icosahedra, decahedra and cuboctahedra in small unsupported Ag particles between 1 and 4 nm by electron diffraction. We have for the first time applied the Debye function analysis (DFA) to obtain information of the size distribution under in situ conditions. Specifically DFA is used in this work to study the oxidation-reduction behavior of the standard 6.3% Pt/silica catalyst EuroPt-I, which has been characterized in series of papers.610 A review of research in this area has recently been given by McCabe et al.Il
Experimental Section All X-ray diffraction patterns were measured by using a specially designed in situ ce11I2 mounted on a commercial Guinier diffractometer (HUBER) and aligned to the 45' transmission geometry. A conventional X-ray source in combination with a Johansson-type Ge monochromator was used to produce a focused Cu Ka, primary beam, which was monitored for constancy at the entrance slit of the system. The typical measuring time for one pattern was 10 h, with count rates of the order of 150 counts/s for the platinum difference intensity. Catalyst samples were pressed with -4.5 tons/cm2 to 15 X 12 mmz tablets. The 0.3 mm. For this size pd 1 (p thickness of the tablets is d = linear absorption coefficient), and the absorption factors e-pd needed for the usual angular corrections are in the optimal range. The support scattering was found to be practically inde endent of the treatment history for a scattering length 10.3 {-I. All treatments were performed sequentially under static conditions mbar. Commercial Minican gases with initial evacuation to (Messer-Griesheim) were used without further purification.
-
-
-
Theoretical Methods In this analysis we are dealing with the difference intensity, Le., the measured intensity of the pure substrate is subtracted from that of the catalyst by use of suitable scaling factors. This difference intensity includes interference terms arising from possible geometrical correlations between surfaces of the metal to the substrate material. It will be shown that a noticeable modification of the intensity versus that of a free particle is to be expected only if a strong bonding gives rise to an epitaxial growth of the metal phase to the substrate surface. The catalysts under investigation represent an assembly of nanometer crystallites, occasionally coexisting with its oxides. The intensity of these small atomic aggregates arranged in random orientations can be calculated by use of the Debye function13 N
(1) Liang, K.S . ; Laderman, S. S.; Sinfelt, J . H . J . Chem. Phys. 1987, 86, 2352. (2) Georgopoulos, P.; Cohen, J . B. J. Catal. 1985, 92, 21. (3) Yacaman, M . J.; Dominguez, J. M . J . Cafal. 1980, 64, 213. (4) Germer, L. H.; White, A. H . Phys. Rev. 1941, 60, 447. (5) Hall, B. D.; Fliieli, R.; Monot, R.; Borel, J.-P.Z. Phys. D: At., Mol. Clusters 1989, 12. 97. (6) Bond, G . C.; Wells, P. B. Appl. Catal. 1985, 18, 221. (7) Bond, G . C.; Wells, P. B. Appl. Catal. 1985, 18, 225. (8) Geus. J. W.; Wells, P. B. Appl. Cafal. 1985, 18, 231. (9) Bond, G . C.; Gelsthorpe, M . R. Appl. Cafal. 1987, 35, 169. (IO) Frennet, A.; Wells, P. B. Appl. Coral. 1985, 18, 243.
0022-3654/90/2094-499 1$02.50/0
IN(^) =
C f J m [sin (2*brnm)1/(2*brnm) n.m=l
(1)
weref, andf, are the atomic scattering amplitudes of atoms n and m . In this analysis we have used thefvalues calculated by Cromer and Mann.14 r,,, is the distance of pairs of atoms n and (11) McCabe, R . W.; Wong, C.; Woo, H . S. J . Catal. 1988, 1 1 4 , 354. (12) Gnutzmann, V.; Vogel, W . Z. Phys. D At., Mol. Clusters 1989, 12, 591. (13) Debye. P. Ann. Phys. 1915, 46, 809.
C 1990 American Chemical Society
4992
Gnutzmann and Vogel
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 *,'06r-----
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D (81 Figure 2. Mass fractions from DFA for reduced EuroPt-1: (a) computed for a sequence of seven cuboctahedra; (c) for a sequence of seven cubes; (b) for a mixture of both cluster shapes. The respective R values of the fit and the number of atoms per cluster are given in the Figure.
Figure 1. Comparison of the Debye functions INI,IN2 of differently shaped Pt clusters with fcc symmetry. (In case of N1> N2the function belonging to N 2 has been multiplied with N 1 / N 2 ) : (a) 172-atom cube (---) and 147-atom cuboctahedron (-); (b) 586-atom truncated octahedron (---) and 561-atom cuboctahedron (-); (c) 684-atom sphere (-) and 674-atom sphere cap (---), cut from a 2046-atom sphere; (d) 309-atom cuboctahedron (-) and 309-atom icosahedron (---). Bragg peaks are indicated in Figure 1 b.
m;b = 2 sin 29/X is the length of the vector in reciprocal space;
X is the wavelength and
0 the Bragg angle. This function can be computed for aggregates containing some IO3 atoms corresponding to fcc platinum crystallites 5 4 0 8, in size. We have done this for four types of regular polyhedra, namely
cubes
N = 14, 63, 172, 365, 666, 1099, ..., atoms
truncated octahedra N = 38, 201, 586, ..., atoms cuboctahedra N = 13, 55, 147, 309, 561, 923, ..., atoms icosahedra N = 13, 55, 147, 309, 561, 923, ..., atoms
For cuboctahedra, for instance, the respective numbers Ns of exposed atoms are 12,42,92, 162, 252, 362. While cubes expose only (100) planes, cuboctahedra and truncated octahedra expose atoms in the (100) and (1 11) planes by a ratio of 2.1 and 1:3, respectively. Icosahedra expose solely (1 11) planes. Debye functions are in fact rather insensitive to the particle shape, as far as the wide-angle X-ray scattering (WAXS) is concerned. This is shown in Figure 1 for different shapes of fcc clusters containing about the same number of atoms. Small differences are seen for a cube ( N = 172) versus a cuboctahedron ( N = 147) in the range of the 111/200 Bragg peaks (Figure la), while a truncated octahedron ( N = 586) and a cuboctahedron ( N = 561) are nearly indistinguishable (Figure Ib). The same is true for an irregular polyhedron with a near-spherical shape of equal volumn. Figure I C shows the Debye function of a sphere ( N = 684) and sphere cap ( N = 674). The sphere cap is cut parallel to a (1 1 1 ) plane from a larger sphere with N = 2046. For this nonglobular particle of nearly the same volume, a shape effect is clearly visible. Partjcdarely, the "shape factor", Le., the Fourier transform of the shape function, produces oscillations (14) Cromer, D. T.; Mann, J. B. Acta Crystallogr., 1968, A24, 321. (15) Endell, J. Kolloid-Z. 1948, 1 1 1 , 19.
around the 000 peak ( b I0.3 A-'), the so-called small-angle X-ray scattering (SAXS). These are strong for the sphere but completely smoothed for the sphere cap. To the contrary, the SAXS intensity of a non-fcc icosahedron and a cuboctahedron is similar, but deviations are strong in the WAXS domain as shown in Figure Id for Pt30s clusters. Unfortunately for the Pt/SiO, catalysts the SAXS information is masked by the strong pore scattering produced by the support. In real catalysts a more or less strong departure from this regularly shaped morphology will occur. This situation is expressed by a continuous distribution function H(D), were dH(D) is the number fraction of crystallites with sizes between D and D dD. However, for practical reasons the numerical calculations were performed by a limited series k = I , ..., n of the above-mentioned regular polyhedra of sizes Dk containing N k atoms. It will be shown that this replacement does not affect the intensity function significantly. The shellwise growth of these polyhedra induces a near-linear diameter increase. (We used the sphere equivalent 3, a. = 3.9236 A is the lattice diameter D = a o ( 3 N / 2 ~ ) 1 /where constant of platinum.) The intensity function I(b) for the fit of the experimental diffraction patters contains the Debye functions INk(b)of the kth cluster:
+
I(b) = CHk{ZNk(b)e-2M + N S ( 1 - e-2M)} + C k
(2)
The second term contains the temperature diffuse scattering, which will be treated in the uncoupled harmonic oscillator approach:
The so-called Debye parameter B is related to the mean-square linear amplitude by B = 8 n ( u X 2 ) .The factor e-2M produces a progressive damping of the Debye functions. B,,, stands for the static local atomic displacements as first treated by Huang for binary alloys.16 For B,,,, = 0 the exponential e-2M is the usual Debye- Waller temperature factor containing the characteristic temperature E). The constant C corrects for errors in the scaling of the difference intensity. Compton scattering will be neglected, since the contribution t o p is 51%for platinum and b 5 1 A-'. The parameters Hk,B, and C are used as free parameters. For the numerical calculations the maximum number of Hkparameters (16) Huang, K. Proc. R. Soc. London 1947, A190. 102.
The Journal of Physical Chemistry, Vol. 94, N o . 12, 1990 4993
Sensitivity of EuroPt-l to H2 and O2 Exposure TABLE I: X-ray Results of EuroPt-1 from DFA run no.
pretreatment
meas condtn
av Pt metal diams," A
oxide cont, wt % PtO Pt304
DN
DS
DM
dispersb dx, a/C
Debye param B, A'
12.5 12.8 12.2
14.7 14.5 13.3
16.1 16.0 14.1
65.0 65.7 70.0
4.19 1.93 4.14
12.8 13.4 11.5 12.9 11.5 12.5 10.9 12.2 11.4 10.5
15.4 17.0 14.4 15.5 14.9 15.5 15.1 14.5 15.0 15.8 14.3
18.0 19.7 17.6 18.4 18.4 18.4 18.1 17.4 17.8 19.8 18.1
62.5 58.3 64.3 62.0 62.3 61.8 62.9 63.8 63.0 60.0 63.8
1.57 2.23 2.92 1.61 0.81 1.12 1.58 2.98 2.03 1.69 0.82
12.9
16.6
20.2
58.7
1.56
Nonsintered EuroPt-l 1 5
8 34 32 51 47 74 75 76 79 80 84 86 101
as received 300 O C , 1 bar, H2 300 OC, 1 bar, 0,
vac, 20 a.p.C a.p.C
300 "C, I bar, H2 300 OC, 2 mbar, O2 300 O C , 1 bar, O2 300 "C, 1 bar, H2 24 O C , 1 bar, H, 150 OC, 1 bar, H2 300 O C , I bar, H2 300 O C , 1 bar, 0, 300 "C, 1 bar, O2 400 O C , vac, 80 h 300 OC. 1 bar, 0,
a.p.C a.p.c a.p.C a.p.' a.p.C a.p.C a.p.C a.p.c vac 300 "C a.p.C 20 "C, 1 bar, H,
300 "C, 1 bar, CO
Second Sintered EuroPt-l 300 "C, 1 bar, H2 0 0
28.5
O C
0 51.0
7 0 8.5
First Sintered EuroPt-l 0 0 33 53.5 0 0 0 0 47 47 29
0
7 6.5 0 0 0 0 5.5 6.5 7
10.8
0
"The platinum diameters are calculated for spherical particles and averaged by number (DN), by surface (Ds), and by mass (DM).bThe dispersion dx is obtained by counting the number of surface atoms of the cuboctahedra. Obscured atoms of faces contiguous with the support are not considered.
As pretreatment condition
used was 16, i.e., I O Debye functions for the metal phase and 6 for two oxides and/or icosahedra. One common Debye parameter was used for all metal clusters, and a second one in common to the oxide clusters. A nonlinear least-squares parameter fitting routine by Baumeister and Marquardt" was used for solving 2. As an example Figure 2a,c shows the weight fractions versus D of a reduced EuroPt-l catalyst for sequences of seven cuboctahedra and seven cubes, respectively (cf. run 34 in Table I). (Weight fractions are calculated from the primarily obtained number fractions H k of the fit by multiplication with N k and renormalization.) Cubes are less favored against cuboctahedra according to the related R factors of the fit ( R = C A 2 y / n ) . Nevertheless the resulting distributions are very similar and so are the Debye parameters B = 1.57 (1.62) A*, the mass mean diameters DM = 18.0 (1 8.9) A, and the X-ray dispersions dx = 62.5% (63.8%) (values given in brackets are those for cubes). The X-ray dispersions are calculated according to
dx = C N k S H k / C N k H k
(4)
which is equivalent to the weight average of the dispersion N S / N of individual clusters. Figure 2b shows the same calculation for cubes plus cuboctahedra. The latter are clearly dominating in the mass distribution. Following these arguments we have used the cuboctahedra sequence throughout. There are indications from TEM3,23that nanocrystallites of platinum type supported catalysts often have cuboctahedral morphology. Confidence tests have been performed to analyze the validity of the DFA method. An artificial powder pattern of a quasicontinuous distribution of spherically shaped Pt crystallites (diameter increments 1 A) was calculated for a log-normal type distribution (LND) of the diameters D. In the following we prefer to use the mass distributons M ( D ) D 3 H ( D ) ,since (i) X-ray intensities are sensitive to the masses of crystallites and (ii) the integral line width is equal to the reciprocal mass mean diameter D H . Figure 3 shows the powder pattern of such an assembly with a mass mean diameter D , = 17 A, a value close to that observed for EuroPt-1. Two Debye parameters were chosen for the test patterns, namely, (a) B = 1 (1.04) A2 and (b) B = 3 (2.98) A'. The values in parentheses are the return values of the DFA routine. The mass mean diameters were reproduced to better than 3% in all cases. The inset to the figure show the pregiven lognormal distribution (solid line) compared to the Gauss-smeared point functions of
-
(17) Marquardt,
D.M . J . Sor. Indian Appl. Marh.
1963, 11, 431.
0
1
2
3
1
5
6
I
8
9
113
b Ipi'l
Figure 3. Confidence test of the DFA method. An artificial powder pattern of spheres is calculated and fitted by use of eq 2. Diameters of the spheres are distributed log-normally as shown in the inset of the figure (-) (diameter increments = 1 A; mass mean diameter DM = 17 A). The primarily obtained discontinuous size distributions of the fit (bars) are Gauss smeared and are shown in the insets (---). The fit has been made for two Debye parameters: (a) B = 1 (1.04) A2 and (b) B = 3 (2.98) A2. Values in parantheses are return values of the fit. The pattern (a) is plotted with an offset as indicated by the horizontal line. For pattern (b) the thermal diffuse contribution is indicated (---).
cuboctahedra indicated by vertical bars (dashed curve). Transformation of the discontinuous distribution primarily obtained from DFA to a continuous one was achieved by multiplication of each point function with a Gaussian of unit weight. We reasonably assumed a constant relative width a O / D of each Gaussian. AD/D was chosen so as to produce an oscillation-free curve and is in the order of ( D 2- D l ) / D l . These smoothed distributions agree excellently with the pregiven LNDs. Similar results of confidence tests were obtained for a variety of LNDs and average sizes up to DM = 50 A and even for mixtures of metal and oxide phases. We have performed model calculations to estimate the influence of interference terms. A 63-atom platinum cube is rigidly attached on top of the (001) surface of a cluster consisting of 32 SiOz molecules in 2 X 2 X 1 0-christobalite lattice cells15(lattice spacing 7.13 A). The distance to the (001) platinum plane was selected as 2.0 A, which is the Pt-0 distance in platinum oxides. The
4994
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 EuroPt
-I
( a s receivedl
-r
. _ ~ . .
.
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12
-.-"Bragg angle 3 ^r
5 ,
~
I "
1-
~
Figure 4. X-ray pattern of as-received EuroPt-1. The difference intensity after subtraction of the support scattering (---) is compared with Debye functions of @-PtO,,Pt,O,, and PtO (-). Each oxide cluster consisted of 2 X 2 X 2 lattice cells.
Debye function of this bicluster was compared to the sum of the separate (infinitely apart) clusters. The difference of these functions, which is equal to the interference terms, deviates by less than 1 % in the average and is beyond the limits of accuracy of the work presented here. It is perhaps useful to note that the formation of bonds to the substrate may disrupt the surface of the metal particles and thus markedly change the Debye function of the latter, but this must not be mistaken for interference terms.
Results Debye Function Analysis. It could be clearly shown that P-Pt02 does not fit the observed intensity function. Figure 4 shows the total and the difference intensity of the partly oxidized as-recieved EuroPt-1. The latter is compared to Debye functions of P-PtO,, Pt304,and PtO. The structure data are taken from refs 18, 19, 20, respectively. The diffuse maximum of P-PtO, around 9 15' is not found in the experimental pattern. This is in agreement with the findings of Georgopoulos and Cohen.* Figure 5 shows a series of X-ray patterns (bottom) of EuroPt-I measured at 300 O C . Before these measurements the catalyst was exposed to 1 bar of oxygen at 450 OC. As discussed below this oxygen exposure induced a small sintering. The following treatments of the same sample are performed under nonsintering conditions. We were able to prove that the crystallite size distributions under hydrogen after intermediate oxidation at maximum 300 OC are completely reversible. (The mean-square deviation of the mass mean diameters of five such treatments performed between runs 34 and 76 is only 18.2 f 0.25 A). This is shown in Figure 5a,d (cf. Table I, runs 34, 47). The bar graphs in the top row give the mass fractions versus diameter of the sequence of cuboctahedra. The corresponding Debye functions are fitted to the experimental intensity (smooth lines in the bottom row). At 300 OC apparently the greatest platinum mass fraction is represented by the 55-atom cuboctahedron. As shown in the foregoing section, the X-ray patterns can equally well be ascribed to a continuous distribution obtained by a smoothing of the discontinuous distribution with Gaussians of
-
(18) Muller, 0.;Roy, R. J . Less-Common Mer. 1968, 16, 129. (19) Moore, Jr., W. J.; Pauling, L. J . Am. Chem. Soc. 1941, 63, 1392. (20) Siegel, S.; Hoekstra, H. R.; Tani, B. S. J . Inorg. Nucl. Chem. 1969, 31, 3803.
Gnutzmann and Vogel constant relative width. These respective continuous distributions are plotted in the second row of Figure 5. The error bands (solid lines) are related to the confidence limits of the fitting routine. In Figure 5b,c, the influence of treatment in oxygen at 2 and 1013 mbar is shown, respectively (runs 32, 51). Besides the cuboctahedra a sequence of Debye functions belonging to oxide clusters had to be added in order to fit these patterns. In the bottom drawings the intensity contributions to Pt metal (long dashed). PtO (dash-dotted), and Pt304 (dashed) to the total intensity are shown. The fraction of Pt304was always small, between 4 and 8 wt %. The main oxide fraction is PtO, which combined with the Pt304 amounted to 40 and 60 wt % in Figure 5b,c, respectively. Clusters of 16, 54, and 128 PtO molecules or 8, 27, and 64 lattice cells were included in the calculations. The smallest cluster size (diameter -9 A) dominates, as indicated by the dark bars in the top figures. At 400 "C the 54-molecule clusters contributed about one-third to the PtO mass fraction. The continuous mass distributions in the second row show the growth of the PtO phase (dash-dotted) at the expense of the metal phase (long dashed). At low oxygen pressure (run 32) most of the small metal clusters seem to disappear (Figure Sb), while at 1013 mbar (run 51) the large clusters are also disrupted (Figure 5c). The thermal stability of the oxide formed at 300 O C was tested by successive heating in vacuum (run 80). From room temperature to 400 OC, a residual fraction of -36% oxide was found by DFA (run 84). Successive treatment in 1 bar of H2 at room temperature indicated complete reduction (run 86). Similar to the oxide clusters, the Debye functions of 55, 147, 309-atom icosahedrons2' were used together with the fcc cuboctahedra to fit the X-ray patterns of reduced catalysts. As a result a fraction of 8 wt % of these icosahedra was added without notable improvement of the fit, independent of the applied temperature. When the fraction of icosahedra is forced to higher values, the R value of the fit increases rapidly. Therefore we cannot exclude the presence of particles with icosahedral symmetry, but their mass fraction should be less than 10%. This is in agreement with the majority of EXAFS and TEM observations that exclude the existence of platinum clusters with icosahedral s y ~ n m e t r y . ~ ~ * * ~ The sensitivity of the DFA method is demonstrated by the detection of two steps of sintering. Figure 6 shows three mass distribution functions of platinum particles in the reduced state: (a) after primary reduction of the as-received catalyst (run 5): (b) after oxygen treatment at 450 OC and subsequent reduction (run 34); (c) probably induced by a reduction with C O at 300 OC (run 101). The mass mean diameters increase in the sequence 16.0, 18.0, and 20.2 A. The respective dispersion decreases from 0.657 to 0.625 and 0.587. A temperature-dependent rearrangement in the particle size distribution seems to occur for the sintered EuroPt-1 under hydrogen. From room temperature to 300 "C, the occupation of the first four cuboctahedra changes reversibly. Interestingly, the X-ray dispersion is not altered; cf. Table I, runs 47, 74, 75, 76. This is demonstrated in Figure 7. It shows the number Hk of cuboctahedra in arbitrary units at 300 OC (black bars) and room temperature (open bars). The Hk's are not normalized, since only the total mass - x N k H k is invariant. The two-shell cuboctahedron ( N = 5 5 ) is extremely dominating at 300 O C , while at room temperature the occupation of the one-shell and three-shell cuboctahedra ( N = 13 and 147) increases on account of the two-shell one. The broad, hatched bars show the results of a TEM study reported by Geus and Wellss on EuroPt-1, rereduced at 400 OC. In an EXAFS study Lagarde et aLz4 found the 13-atom fcc structure to be the best model for elemental platinum metal in a reduced Pt/AI,03 catalyst at room temperature.
-
(21) Coordinates of the Icosahedra were kindly supplied by J. Urban, private communication. (22) Joyner, R. W.; Meehan, P. W. Vacuum 1983, 33, 691. (23) Sattler, M. L.; Ross, P. N. Ultramicroscopy 1986, 20, 2 I , (24) Lagarde. P.:Murata, T.; Vlaic. G.; Freund, E.; Dexpert. H.; Bournonville, J P.J . C a r d . 1983, 84, 333.
Sensitivity of EuroPt-l to H2 and O2 Exposure
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4995
20.02 10
20
LO
30
b=2sin3/X (&'I Figure 5. X-ray difference intensities (bottom row) and corresponding mass distributions from DFA (top rows) taken under in situ conditions at 300 OC after a first sintering of EuroPt-1 at 450 ' C in 0,: (a, d) under 1 bar of Hz; (b, c) intermediate oxidation under 2 mbar and 1 bar of O2 Intensity
contribution by Pt metal, long dashed; by PtO, dash-dotted; by Pt304,dashed. Top row: mass contribution of the sequence of cuboctahedra (open bars) and of PtO clusters (black bars). Middle row: Gauss-smeared continuous mass distribution of the three different phases including error bands of the fit. 0 10
--
i 30 -
008
%!
-
C
;" 006
25
-
3
e I
VI
d
001
VI
0
I
0 02
0
0
10
20
D
fa)
30
40
50
Figure 6. Gauss-smeared mass distribution functions of the metal phase at different stages of sintering: run 5 (-); run 34 (---), and run 101 (---). The inset shows the corresponding X-ray patterns.
We have made no efforts to measure size-induced lattice contractions in this work. All the patterns could however be fitted by using the bulk platinum lattice constant (cf. Figure 5, bottom row). We estimate the possible average lattice contraction to 51%. Debye Parameter. The Debye parameters B( T ) of the metallic platinum phase was obtained with good reproducability by the Debye function analysis only for reduced catalysts. Figure 8 shows plots of this parameter as a function of temperature compared to the known bulk data. In the reduced state under 1 bar of hydrogen (filled circles) the slope of B ( T ) is increased against the bulk curve (solid line). This corresponds to a lowering of the Debye temperature to 8 N 147 K compared to the bulk value of 8 = 234 K.25 No static displacement was found under hydrogen, assuming this contribution is independent of temperature. The reduced catalyst behaves anomalously after evacuation. The corresponding Debye parameters are much larger (open circles). However, now the slope is that of bulk platinum. ( 2 5 ) Harris, J. R.; Benczer-Koller,N.; Rothberg, G. M . Phys. Reu. 1965, 137, A I 101.
5
0
10
15
25
20
35
30
DIAI
Figure 7. Reversible variation of the number of cuboctahedra in a reduced catalyst under 1 bar of H2when heated from room temperature (black bars) to 300 O C (open small bars). TEM results of Geus and Wells* on reduced EuroPt-1 are added to the figure as hatched bars.
I
I m
w
x
1-
0
2 00
100
600
TIK)
Figure 8. Debye parameters of reduced EuroPt-1 from DFA versus temperature measured under 1 bar of H2 ( O ) , measured under vacuum (O), and bulk Pt (-).
The oxidized catalysts (runs 5 1,79) exhibit even larger Debye parameters (cf. Table I), especially in the nonsintered state (run
Gnutzmann and Vogel
4996 The Journal of Physical Chemistry, Vol. 94, No. 12, 1990
-.r
75 m
t
s
31
i
l
EUROPT-1
7 1w 3 0 0 * c
;'I;--/---
_ _ - - -+..-_ - - - -_ _ - - -
0
I
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__ 02
0+
05
n
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08
dH
Figure 9. Debye parameters from DFA for a series of Pt/SiO, catalysts'* including EuroPt-l versus dispersion dH. X-ray patterns taken under 1 bar of H2
8). Due to the small fraction of the metallic phase these parameters became uncertain and no definite conclusions could be made, as well as for those belonging to the oxide phase. Figure 9 shows the variation of B ( T ) as a function of the dispersion at 300 OC in H2 for EuroPt-1 and two other Pt/Si02 catalysts (symbols 69, 65) characterized by X-ray diffraction in a previous work.I2 The values at dH = 0 are the calculated bulk values
Discussion Contrary to EXAFS findings of Joyner et a L z 2who claim a nearly complete oxidation of as-received EuroPt- 1, in the present X-ray work a 36% oxidation in the as-received state (run 1) and a maximum oxidation of -60% at 300 OC in 1 bar of O2 for 20 h is obtained (run 8). This value is close to the fraction of metal exposed to the surface. Similar results were found for a series of Pt/Si02 catalysts of lower dispersion investigated previously.I2 In a recent study on supported Pt catalysts, McCabe et al. conclude that the Pt oxidation is highly passivating; the oxide film thickness is less than or equal to one monolayer equivalent of Pt atoms." The Debye function analysis (DFA) is sensitive not only to well-ordered crystallites but also to more or less amorphous phases and to very small atomic aggregates. We have estimated the possible range of the weight fraction of platinum oxide as determined by DFA for the as-received EuroPt-l. In Figure 10b,c the amount of oxide used to fit the X-ray patterns was artificially fixed at 61 and 100 wt %, while Figure 10a shows the free parameter fit given in Table I (run l ) . The inset of Figure 10a shows the corresponding reliability value R of the fit as a function of the oxide content, indicating a reasonable error of f10 wt '7c in the platinum oxide fraction. Present X-ray findings prove the existence of an oxide phase PtO,, with the local arrangement of platinum ions mostly as in PtO. The platinum ions are arranged not in 2-fold coordinated linear chains along a preferred direction as in P-Pt0218but in square planar arrays as in Pt0.20 (According to the difference in the scattering factors the diffracted intensity of platinum oxide depends effectively on the positions of the platinum ions.) The oxide particles PtO, were found to be extremely small, Le., on the order of 16 platinum ions. An estimation of the additional oxygens needed for a PtO cluster of 2 X 2 X 2 lattice cells to saturate the dangling bonds leads to a stoichiometry of x 2 2. In a TPR study Bond et aL9 have found a ratio Pt:O = 0.9 for the as-received EuroPt-1. According to the smallness of p ( 0 ) againstP(Pt), the relative X-ray intensity of the oxide phase gives roughly the fraction of (Pt", PtIV) atoms, which is 36% (Table I, run 1). Assuming x = 2, we end at a ratio Pt:O = 0.72, which comes close to the TPR result, and the maximum ratio would be Pt:O = 1.2 for highly oxidized EuroPt-l (run 8). In PtO platinum is in the Pt" state. If however the additional oxygens form chemical bonds to platinum, than the major fraction should be in the Ptrv state, but this can only be speculative. The mass-weighted size of 17 A of the platinum crystallites in EuroPt-l is too small to allow for an ordered surface oxide, the crystal facets exposing only a few substrate atoms This is
-
I cl
Figure 10. X-ray pattern of the as-received EuroPt-1 fitted with a fixed amount of platinum oxide of 61 and 100 wt % (b, c). The free parameter fit gives 36 wt ?k platinum oxide. Contributions of the three phases to the total intensity are marked as in Figure 3. The inset gives the R values of the fit as a function of weight fraction of platinum oxide.
supported by TEM observations. Smith et a1.26observed poorly crystallized layers surrounding particles of Pt in oxidized Pt/A1203 model catalysts. Foger et aL2' occasionally observed Ir particles enveloped by fine-grained polycrystalline oxide in silica-supported Pt/Ir catalysts exposed to an oxygen-containing atmosphere. The reversibility of the catalyst between the oxidized and reduced states is consistent with this metal-core/oxide-shell picture. Moreover, reduction in hydrogen was found to be complete within less than 40 s.I2 The process is accelerated by its autocatalytic nature. According to our results complete reduction of reoxidized EuroPt-l is already achieved at room temperature (run 86). This implies that the H2 uptake at plus ambient temperatures observed by TPR9." is solely due to hydrogen chemisorption. We have for the first time observed a size-selective oxidation within one catalyst batch: Low-pressure oxidation mainly attacks the small clusters, Le., decreases the dispersion (run 34/32), while oxidation at 1 bar attacks the large clusters, thereby increasing the dispersion of the residual metal phase (run 34/51). The relaxation of reduced EuroPt-1 in hydrogen to the undisturbed fcc symmetry as found in this study was first observed by GallezotZ8on platinum aggregates encaged in Y-type zeolite. Manninger et al.29observed a considerable decrease of lattice strain in platinum blacks in hydrogen by a X-ray line-broadening study. Subsurface hydrogen was observed on single-crystal surfaces by Eberhardt et aL30 as well as on dispersed Pt black by Paal et aL3I and might induce this relaxation. Additionally we observed a lowering of the Debye temperature to 147 K compared to = 234 K. This lowering accounts for the net decrease of the near-neighbor coordination number K for particles as small as in EuroPt-1. (For a 55-atom cuboctahedron K is lowered to K (26) Smith, D. J.; White, D.; Baird, T.; Fryer, J. R. J . Card.1983, 81, 107.
(27) Foger, K.; Jaeger, H. J . Card. 1981, 70, 53. (28) Gallezot, P. Zeolites 1982, 2, 103. (29) Manninger, I.; Paal, Z.; Tetenyi, P. Z.Phys. Chem. 1985, 143, 247. (30) Eberhardt, W.; Greuter, F.; Plummer, E. W. Phys. Rev. Lert. 1981, 46, 1085. (31) Paal, 2.;Thomson, S. J .
J. C u r d 1973, 30, 96.
Sensitivity of EuroPt-l to H2 and O2 Exposure
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4997
= 7.85 against K = 12 of the bulk.) By LEED measurements Lyon and Somorjai found 8 = 107-1 18 K for different low-index platinum surfaces.32 In a careful Mossbauer study of the AuS5 core in an organometalhc complex Au55(PPh3),2C16Smit et did not observe noticeable decrease of the Debye temperature against the bulk value. Here the encagement by the organic ligands might be of great influence. The discontinuous cuboctahedral cluster distribution is a suitable but not necessarily realistic model for this catalyst. Some arguments for the preference of cuboctahedral shape of the platinum particles may however be raised from this study: (i) The strong occupation of the 55-atom cluster PtSs (more than 90% by number at 300 "C). (ii) Clusters in the proximity of N = 55 appeared to a fraction of less than 3 wt % platinum: the truncated octahedron Pt3*when added to the cuboctahedra and the cube Pt63 when the cuboctahedron Pt55 is added to the sequence of cubes. (iii) The use of cubic shapes lowers the R values for the fit. In an equal mixture of cubes and cuboctahedra the latter are strongly dominating. (iv) When the nonsintered EuroPt-l was heated in oxygen to 400 "C and cooled to room temperature in oxygen, the residual metallic platinum distribution equals the hypothetically expected mass distribution if the outer shells of all the cuboctahedra in the reduced state are removed. The influence of partial pressures on the Debye parameter of the metal phase by evacuating both from the reduced and the oxidized state under 1 bar of H2 and 1 bar of 02,respectively, is strong but no unique. The DFA results after evacuation from the reduced state could be interpreted by two models: (a) 100% metal phase (Le., no allowance is made for an oxide contribution) with a strong increase of dispersion due to a large increase of the 13-atom clusters, plus a large increase of the Debye parameter. In this model the static contribution to the Debye parameter is large, as seen from Figure 8. The extrapolation to zero temperature gives a root-mean-square static displacement ustat= 0.14 A. (b) -38% platinum oxide and a nearly unchanged dispersion. The Debye parameter of the residual metal phase is a little less than in (a). Of course, a mixture of both solutions could also fit the observation. Joyner et a1.22reported similar effects when a reduced EuroPt-1 was heated in vacuum to 520 K. The authors interpreted their EXAFS findings as an interaction with the SiO, surface hydroxyl groups. This type of metal-support interaction would
account for the observed static displacements. These phenomena need further investigation. A new stainless steel cell has been constructed that allows measurements down to 2 X lo-* mbar. By hydrogen chemisorption Frennet and Wellslo have found a dispersion of 65% for EuroPt-1 reduced below 573 K in best agreement with the X-ray dispersion given in Table I for the nonsintered catalyst (run 5 ) . This leads to the important conclusions that (i) most platinum particles must be isolated "single crystals" and (ii) the surface fraction obscured by Pt-SiO, interfaces is very small. It is perhaps worth noting that the frequently applied continuum-theoretical equation d = K/DS ( K = 6 p s / p , ps = surface density, p = volume density), would give a much too large dispersion of 76% in the present case. (From Table I Ds = 14.7 8, and K = 11.2 A for a (loo), (1 IO), ( 1 11) average of ps.) For highly dispersed catalysts the atomistic structure of the particles must necessarily be considered. Candy et al.34observed two maxima in the volume-weighted distribution of particle diameters in a reduced 6 wt % Pt/SiO, catalyst (codename Eurocat), measured by SAXS. This agrees with our primarily obtained distribution of cuboctahedra given in Figure 2, which shows broad maxima at 11 and 29 A. Whether or not this feature pertains after transforming the distribution to a continuous one (Le., assuming irregular particle shapes, cf. Figure 6) depends on the degree of smoothing and implies some ambiguity. Along with the arguments given above, it might well be that the hydrogen-induced relaxation toward perfect fcc symmetry is accompanied by a "shape relaxation" toward c u b ~ c t a h e d r a . ~ ~ No distinct maxima were observed by Geus and Wells,s who have reported the TEM studies of different research groups on EuroPt-1. The sizes of visible particles ranged from 9 to 35 A, in agreement with our results. The hatched bars in Figure 7 show the room-temperature size distribution given by these authors for a specimen rereduced at 400 "C. The differences to our DFA results may partly account for nonimaged ensembles of very small size and/or underestimation of the smallest visible particles. This deficiency is indeed a general problem in usual X-ray work, too. The analytical technique presented here overcomes this problem; its applicability will however be restricted to specially suited systems.
(32) Lyon, H. B.; Somorjai, G. A. J . Chem. Phys. 1965, 44, 3707. (33) Smit, H.H. A,; Thiel, R. C.; de Jongh, L. J.; Schmid, G.; Klein, N . Solid State Commun. 1988, 65, 915.
(34) Candy, J. P.; Fouilloux, P.; Renouprez, A. J. J . Chem. Soc., Faraday Trans. I 1980, 76, 616. (35) Shi, A.-C.; Masel, R. I . J . Catal. 1989, 120, 421.
Acknowledgment. We thank Prof. Z. Paal (Institute of Isotopes, Hungarian Academy of Sciences) for valuable discussions. Registry No. Pt, 7440-06-4.
