J. CHEM. SOC. FARADAY TRANS., 1995, 91(3), 507-515 507 Effect of Chlorine on Microstructure and Activity of Pd/CeO, Catalysts Leszek Kqpihski,* Marek Wdcyrz and Janina Okal Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrocfa w , Poland The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO, catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573-973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A rapid decline to zero of the activity of Pd/CeO, catalysts in benzene hydrogenation was observed when the tem- perature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal-support reaction leading to Pd-Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeO,) was observed by HRTEM. Platinum-group metal catalysts (Rh, Pd, Pt) containing ceria as the support or promoter have received much attention because of their applications in automotive emission control and in syngas conversion. Recently, it has been found that the type of metal precursor used (with or without chloride) dras- tically influences the microstructure and chemisorptive or catalytic properties596 of M/CeO, systems. In a previous paper' we showed that in a Pd/ceria catalyst prepared from a PdCl, precursor a tetragonal CeOCl compound was formed during prolonged (20 h) reduction in hydrogen at 900 K. Bernal et aL4 observed the same phase by HRTEM in Rh/CeO, reduced at 973 K. Besides transformation of the support, high-temperature reduction (HTR) of M/ceria cata- lysts induces interaction between metal and support which influences the chemisorption capacity and catalytic activity of the metal.2*6-14 HRTEM studies revealed that HTR at 773 K caused epitaxial orientation of Pt", Rh6*16 and Pd17 par- ticles on CeO, , such that M(111) )) CeO,(ll 1). At higher tem- peratures effects of M-Ce and metal decoration and encapsulation by ceria" were observed. In this work we performed extended studies using XRD, SAD and HRTEM, on the structural evolution of Pd/ceria catalysts prepared from chloride and nitrate precursors, caused by reduction in hydrogen at various temperatures. Additionally, we performed measurements of catalytic activ- ity of the samples in benzene hydrogenation. The results obtained show that chloride dramatically changes the micro- structure of the catalyst over the whole range of reduction temperatures, i.e. 573-973 K. Experimental Catalyst Preparation and Reduction Two Pd/ceria catalysts were prepared using different Pd salts. The Pd(N)/ceria catalyst was obtained by evaporation and drying (at 305 K, overnight) a mixture of suitable amounts of an aqueous solution of Pd nitrate and a 20 wt.% colloidal dispersion of CeO, in dilute acetic acid (Aldrich). Finally, the catalyst was ground in a mortar and stored for further use. The Pd(Cl)/ceria catalyst was prepared in the same way but PdCl, solution was used instead of Pd nitrate. Atomic absorption spectrometry showed that both catalysts con- tained 9 & 0.5 wt.% Pd. Moreover, the latter catalyst con- tained 8 wt.% C1. Prolonged reduction (20 h) of the catalysts in hydrogen even at 973 K did not change these values. For comparison, we also prepared Pd-free samples. One of them (referred to as CeO,) was obtained by drying a colloidal CeO, dispersion at 350 K overnight while the second (Cl/ ceria) was prepared by impregnation of the CeO, sample with sufficient HC1 to yield 8 wt.% C1 after evaporation, and drying. Samples of the catalysts were reduced in a hydrogen flow at atmospheric pressure at temperatures up to 973 K for 20 h. The hydrogen was purified by passing it through a Pd/ asbestos catalyst kept at 570 K and then through columns packed with KCl, P,OS and zeolite. Catalyst Characterization The morphology and structure of the catalyst samples sub- jected to 20 h reduction at various temperatures were investi- gated by XRD, EC and HRTEM. XRD spectra were collected in transmission mode with an STOE powder diffrac- tometer using a curved germanium monochromator and a position-sensitive detector. Corundum powder was added to each sample as an internal standard for precise measurement of lattice parameters and for instrumental peak-broadening correction for the determination of mean crystallite sizes. The mean crystallite size (L,) was calculated using the Scherrer formula L, = kAB,,,(cos 8)-', where k is a constant (assumed to be 1 in this work), 3L = 0.154 nm, the wavelength of Cu- Ka, radiation used, Be,, = (B2 - B;)ll2, the effective FWHM of the XRD peak with a maximum at 28, corrected for instru- mental broadening (BJ assumed to be equal to the FWHM of the 104 peak of the corundum standard (28 = 35.14'). L, and the lattice parameter of CeO, and Pd were calculated from their 11 1 peaks (28 = 28.5" and 40.1", respectively). For CeOCl, L, was calculated from the 110 peak at 30.96", and the lattice parameters from this peak and the 102 and 200 peaks. The intensities of the peaks and positions of their maxima were determined by a profile-fitting procedure, which also enabled deconvolution of overlapping peaks. Electron diffractograms and HRTEM micrographs were obtained with a Philips CM20 microscope equipped with the SuperTwin objective lens (C, = 1.2 mm) at an accelerating voltage of 200 kV. Specimens for TEM were prepared simply by dipping a Cu microscope grid covered with holey carbon in a freshly reduced sample. The grid was then immediately inserted into the microscope. This procedure minimized poss- ible contamination of the sample since no solvents were used and exposure to air was reduced. Downloaded by University of Illinois - Urbana on 19 March 2013 Published on 01 January 1995 on http://pubs.rsc.org | doi:10.1039/FT9959100507 View Article Online / Journal Homepage / Table of Contents for this issue
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J. CHEM. SOC. FARADAY TRANS., 1995, 91(3), 507-515 507
Effect of Chlorine on Microstructure and Activity of Pd/CeO, Catalysts
Leszek Kqpihski,* Marek Wdcyrz and Janina Okal Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 937, 50-950 Wrocfa w , Poland
The evolution of microstructure and activity in benzene hydrogenation of Pd/CeO, catalysts, prepared from Pd chloride or Pd nitrate, upon reduction at 573-973 K has been studied by X-ray diffraction (XRD), selected-area electron diffraction (SAD), high-resolution transmission electron microscopy (HRTEM) and gas chromatography. The crucial role of chloride from the metal precursor in structural transformations of the ceria support has been established and attributed to its ability to form stable cerium oxychloride at temperatures as low as 573 K. A rapid decline to zero of the activity of Pd/CeO, catalysts in benzene hydrogenation was observed when the tem- perature of reduction was increased from 573 to 773 K. This effect could not be explained by sintering, but XRD data indicated that a metal-support reaction leading to Pd-Ce alloy formation was likely to be the cause. At higher temperature (873 K) the coverage of Pd particles by a thin overlayer (probably CeO,) was observed by HRTEM.
Platinum-group metal catalysts (Rh, Pd, Pt) containing ceria as the support or promoter have received much attention because of their applications in automotive emission control and in syngas conversion. Recently, it has been found that the type of metal precursor used (with or without chloride) dras- tically influences the microstructure and chemisorptive or catalytic properties596 of M/CeO, systems. In a previous paper' we showed that in a Pd/ceria catalyst prepared from a PdCl, precursor a tetragonal CeOCl compound was formed during prolonged (20 h) reduction in hydrogen at 900 K. Bernal et aL4 observed the same phase by HRTEM in Rh/CeO, reduced at 973 K. Besides transformation of the support, high-temperature reduction (HTR) of M/ceria cata- lysts induces interaction between metal and support which influences the chemisorption capacity and catalytic activity of the metal.2*6-14 HRTEM studies revealed that HTR at 773 K caused epitaxial orientation of Pt", Rh6*16 and Pd17 par- ticles on CeO, , such that M(111) ) ) CeO,(ll 1). At higher tem- peratures effects of M-Ce and metal decoration and encapsulation by ceria" were observed.
In this work we performed extended studies using XRD, SAD and HRTEM, on the structural evolution of Pd/ceria catalysts prepared from chloride and nitrate precursors, caused by reduction in hydrogen at various temperatures. Additionally, we performed measurements of catalytic activ- ity of the samples in benzene hydrogenation. The results obtained show that chloride dramatically changes the micro- structure of the catalyst over the whole range of reduction temperatures, i.e. 573-973 K.
Experimental Catalyst Preparation and Reduction
Two Pd/ceria catalysts were prepared using different Pd salts. The Pd(N)/ceria catalyst was obtained by evaporation and drying (at 305 K, overnight) a mixture of suitable amounts of an aqueous solution of Pd nitrate and a 20 wt.% colloidal dispersion of CeO, in dilute acetic acid (Aldrich). Finally, the catalyst was ground in a mortar and stored for further use. The Pd(Cl)/ceria catalyst was prepared in the same way but PdCl, solution was used instead of Pd nitrate. Atomic absorption spectrometry showed that both catalysts con- tained 9 & 0.5 wt.% Pd. Moreover, the latter catalyst con- tained 8 wt.% C1. Prolonged reduction (20 h) of the catalysts in hydrogen even at 973 K did not change these values. For
comparison, we also prepared Pd-free samples. One of them (referred to as CeO,) was obtained by drying a colloidal CeO, dispersion at 350 K overnight while the second (Cl/ ceria) was prepared by impregnation of the CeO, sample with sufficient HC1 to yield 8 wt.% C1 after evaporation, and drying.
Samples of the catalysts were reduced in a hydrogen flow at atmospheric pressure at temperatures up to 973 K for 20 h. The hydrogen was purified by passing it through a Pd/ asbestos catalyst kept at 570 K and then through columns packed with KCl, P,OS and zeolite.
Catalyst Characterization
The morphology and structure of the catalyst samples sub- jected to 20 h reduction at various temperatures were investi- gated by XRD, EC and HRTEM. XRD spectra were collected in transmission mode with an STOE powder diffrac- tometer using a curved germanium monochromator and a position-sensitive detector. Corundum powder was added to each sample as an internal standard for precise measurement of lattice parameters and for instrumental peak-broadening correction for the determination of mean crystallite sizes. The mean crystallite size (L,) was calculated using the Scherrer formula L, = kAB,,,(cos 8)- ' , where k is a constant (assumed to be 1 in this work), 3L = 0.154 nm, the wavelength of Cu- Ka, radiation used, Be,, = (B2 - B;)ll2, the effective FWHM of the XRD peak with a maximum at 28, corrected for instru- mental broadening (BJ assumed to be equal to the FWHM of the 104 peak of the corundum standard (28 = 35.14'). L, and the lattice parameter of CeO, and Pd were calculated from their 11 1 peaks (28 = 28.5" and 40.1", respectively). For CeOCl, L, was calculated from the 110 peak at 30.96", and the lattice parameters from this peak and the 102 and 200 peaks. The intensities of the peaks and positions of their maxima were determined by a profile-fitting procedure, which also enabled deconvolution of overlapping peaks.
Electron diffractograms and HRTEM micrographs were obtained with a Philips CM20 microscope equipped with the SuperTwin objective lens (C, = 1.2 mm) at an accelerating voltage of 200 kV. Specimens for TEM were prepared simply by dipping a Cu microscope grid covered with holey carbon in a freshly reduced sample. The grid was then immediately inserted into the microscope. This procedure minimized poss- ible contamination of the sample since no solvents were used and exposure to air was reduced.
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The catalytic activity for benzene hydrogenation of the samples subjected to 20 h reduction treatment at 573, 673 or 773 K was measured at atmospheric pressure with a feed composition of C,H, : H, = 1 : 9. Typically, 0.2-0.3 g of catalyst was loaded into the glass reactor. Since samples of the catalysts were exposed to air while they were transported into the reactor additional in situ reduction in hydrogen flow at 573 K for 16 h was applied before testing. The reactor emuent was analysed by the INCO 505 M gas chromato- graph with a TCD detector equipped with 3.7 m long 15% Carbowax 20M/Gas-Chrom Q1 columns. The bracketing technique was employed in the kinetic runs. In a typical experiment, hydrocarbon vapour mixed with H, was passed over the sample for 5 min prior to sampling. The benzene vapour flow was then stopped and the hydrogen flow contin- ued for 15 min prior to another reaction period. Most experi- ments were conducted over the temperature range 395-5 13 K. Conversion was kept below 15% to avoid heat and mass- transport effects.
1000 1 8 8 1
Results Microstructure of the Catalysts
Fig. 1 shows the XRD spectra of all as-prepared (unreduced) samples studied in this work. The convention used for the description of peaks that occur in this and other XRD spectra is shown in the caption for Fig. 2. Parameters charac- terizing the structure of the samples are collected in Table 1. It appears from Fig. 1 that the spectra of all the starting samples are very similar and contain only peaks from CeO, and the corundum standard. The mean crystallite size (L,) of CeO, (5-6 nm) agrees with the TEM estimate (5-8 nm) (Table 1, unreduced samples). The lattice parameter of CeO, (0.544-0.545 nm) is slightly bigger than that of the CeO, standard (0.5411 nrn),,' therefore we prefer to describe this phase as CeO, - ,, .
A full account of our study on the evolution of the micro- structure of Pd(N)/ceria catalyst reduced in H, at 573-973 K
( d ) 500 I 1 I I I
25 30 35 40 45 50 28/degrees
Fig. 1 XRD spectra of as-prepared (unreduced) catalysts and sup- ports studied in this work. For description of diffraction peaks see caption for Fig. 2. (a) Pd(N)/CeO, , (6) Pd(CI)/CeO,, (c) CI/CeO,, (d) CeO, .
is to be published ~eparately'~ and here we present only some additional results. Fig. 2 shows a series of XRD spectra of Pd(Cl)/ceria samples reduced in H, at 573-973 K for 20 h. The structural parameters calculated from Fig. 2 are pre- sented in Table 1 and compared with values obtained for Pd(N)/ceria, Cl/ceria and CeO,. Note that the data for Pd(N)/ceria and CeO, given in Table 1 represent the 'stabilized' values obtained from spectra recorded 1 h after exposure to air. It will be shown later that these samples
Table 1 Summary of XRD and TEM results on the structural evolution of Pd/CeO, catalysts and bare supports during reduction in H,
Fig. 2 Evolution of XRD spectra of Pd(Cl)/ceria due to reduction in hydrogen. A, CeO,-, (fluorite structure type), a = 0.541-0.547 nm, JCPDS no. 34-394; A, CeO, (distorted CeO,), a x 0.557 nm (pseudo-cubic cell); m, Pd (fcc), a = 0.389 nm, JCPDS no. 5-681; 0, Pd* = Pd, -zCe, (fcc), a x 0.397 nm; +, CeOCl (tetragonal), a = 0.408 nm, c = 0.685 nm; 0, corundum (internal standard), JCPDS no. 10-173. (a) Pd(Cl)/CeO,, fresh; (b) reduction in H,, 573 K; (c) reduction in H,, 673 K; (d) reduction in H,, 773 K; (e) reduction in H, , 873 K; (f) reduction in H, , 973 K.
28/degrees
underwent fast reoxidation in air. Pd(Cl)/ceria and Cl/ceria samples reduced at 973 K did not show this effect and spectra recorded 3 min and 1 h after exposure to air were practically identical. From Fig. 2 and Table 1 it appears that with increasing temperature of Pd(Cl)/ceria reduction both support and Pd underwent structural transformation. At 673 K peaks from a new phase became visible at positions char- acteristic of tetragonal CeOC1.3 The lattice parameters of CeOCl (a = 0.408 nm, c = 0.685 nm) did not change with temperature of reduction, but the width of the XRD peaks decreased, indicating severe sintering, especially above 773 K. The width and position of the CeO,-, 111 peak (28.5") remained nearly constant up to 873 K, though its relative intensity decreased systematically (ZP,&e02-, and ICeOC1/ICe02-, increased). However, the width of the CeO, -,, 220 peak (47.3') increased suddenly at 773 K, suggesting its splitting into two components. Since this effect might also concern the 11 1 peak, the method of calculation of L, applied in this work may give false results, viz. underestimation of the crystallite size. Reduction at 973 K caused a drastic change in
the ceria peaks which became sharp and shifted towards lower 26 (L, increased to 20 nm and the lattice parameter expanded by 2.3% to 0.557 nm). At the same time a further decrease of the relative intensity of the ceria 111 peak was observed. Evolution of the Pd 11 1 peak indicates that even at 573 K severe sintering of the metal occurred (L, = 11 nm). The Pd lattice parameter (0.389 nm) was very close to the standard value (0.3890 nrn).,' At 673 K further sintering occurred (L, = 21 nm) with no change in lattice parameter. At 773 K, a hump began to form at the low-angle side of the Pd 111 peak, causing apparent broadening of the peak and a shift of its maximum. At 873 and 973 K Pd was transformed into a new Pd* phase characterized by the lattice parameter 0.398 nm which was 2.3% larger than that of pure Pd. The L, of this phase was 29 nm.
Effect of Pd and C1 on ReductioMxidation Properties of Ceria
We have already mentioned that Pd(N)/ceria and CeO, samples reduced at 973 K underwent fast reoxidation upon exposure to air at room temperature. Fig. 3 and Table 2 show the progress of this process. The spectrum acquisition took 3 min. It is evident that two distinct phases of ceria exist in both samples, one referred to as CeO, with lattice param- eter a = 0.555-0.556 nm, and a second referred to as CeO, - , , with a lattice parameter that changes slowly with time of exposure to air from 0.544 to 0.541 nm. The intensity
A A 0 1 3 ! (a ) A A I l 1 1 ~ ~ * ' " ' 1 1 ~ ~
24 26 28 30 32 34 36 30 28/degrees
. . I I I 1 ' 1 I 1 1 I I I 1 I 1 I I
24 26 20 30 32 34 36 38 28ldegrees
Fig. 3 Reoxidation of reduced (973 K) A, Pd(N)/ceria and CeO, samples upon exposure to air for A, (a)-(@: 3, 12, 19, 25, 32, 38, 49 and 63 min and for B, (a)-@): 3, 10, 16,22,28,43,49 and 63 min
ratio of CeO, and CeO,-, 111 peaks (Table 2) for Pd(N)/ ceria at the beginning of experiment was twice as large as that for CeO,. However, it decreased with time much faster than the ratio for CeO, and after 63 min the ratio was zero for Pd-containing samples and small but measurable for CeO, . As we stated before, there was no visible change in the XRD spectra of Pd(Cl)/ceria and Cl/ceria samples reduced at 973 K and exposed to air for a short time (1 h).
Fig. 4 compares the XRD spectra of Pd(Cl)/ceria and Cl/ ceria reduced at 973 K. It appears (cf. Table 1) that although the samples have the same C1 content, the relative amount of CeOCl formed during reduction is much lower for Cl/ceria. Moreover, there is no indication of formation of the Pd(N)/ ceria phase in this sample. The L, of ceria and CeOCl was, however, similar in both samples. To check the stability of the CeO,, Pd* and CeOCl phases in air we recorded the XRD spectra of Pd(N)/ceria and Pd(Cl)/ceria samples reduced at 973 K again after 6 months (Fig. 5). For CeO, [cf: Fig. 3(a)] the only additional changes on this long time-frame were some broadening of the Pd* peak and occurrence of a very weak Pd peak, while for Pd(Cl)/ceria [cf. Fig. 2(f)] the changes were more pronounced, especially in the support. Most of the CeO, was transformed into CeO,-, and the intensities of the CeOCl peaks were strongly reduced. It is interesting that the width of the new CeO,-, peak was nearly identical to that of the original CeO, peak. Changes of the metal phase were small and similar to those observed for Pd(N)/ceria.
TEM and ED
Fig. 6 shows the HRTEM micrographs and SAD patterns of unreduced and reduced Pd(Cl)/ceria. Thanks to the high- resolution mode of observation, lattice fringes of CeO, (111)
0 . p ) I 1 I I I 25 30 35 40 45 50
281degrees
Fig. 4 Effect of Pd on CeOCI formation during reduction at 973 K: Pd(Cl)/ceria (a) and Cl/ceria (b)
15004 : A
i5 d0 315 40 45 sb 2 8/d eg rees
Fig. 5 XRD spectra of reduced (973 K) Pd(Cl)/ceria (a) and Pd(N)/ ceria (b) stored in air for 6 months
(0.32 nm) and CeOCl (001) (0.68 nm) lattice planes are visible and enable estimation of the crystallite sizes of these phases. The results are given in Table 1 as hEM. For CeOCl crys- tallites, which exhibited flake-like shapes, two values, measured in perpendicular directions, are given. In all samples, except the unreduced one, the SAD patterns contain in addition to CeO, rings quite strong rings from CeOCI. This means that even at 573 K CeOCl is formed in significant quantities (note that XRD did not reveal CeOCl in this sample). Above 673 K CeOCl rings dominate the SAD pat- terns and CeO, rings are hardly visible. This result is appar- ently in contradiction with the XRD results which showed that even at 973 K ceria was the dominant phase. The expla- nation lies in the inhomogeneity of the Pd(Cl)/ceria samples. In all the reduced samples there were grains consisting nearly exclusively of ceria crystallites. Fig. 7 shows a micrograph and an SAD pattern of such a grain in the sample reduced at 973 K. Comparison of Fig. 7 and 6(f) (at the same magnification) reveals that the ceria crystallites are smaller than those of CeOCl. Owing to the high degree of scattering of electrons by CeO, and CeOCl it is very difficult to identify crystallites of Pd in electron micrographs and we were unable to estimate their mean size by this method. In SAD patterns the identification of Pd was also difiicult since the Pd 111 ring (0.225 nm) lies very close to the CeOCl 112 and 003 rings (0.220 and 0.228 nm, respectively). Fig. 8 shows a micrograph and a microdiffraction pattern of a large Pd particle at the edge of the catalyst grain in the sample reduced at 873 K. Indexing of the microdiffraction pattern showed that the par- ticle was oriented with its [Oll] axis parallel to the electron beam. The HRTEM micrograph of the particle edge (Fig. 9) revealed that the (110) and (111) faces of the particle were covered with a thin, ordered layer of material exhibiting lattice fringes of ca. 0.32 nm.
Fig. 6 HRTEM micrographs and SAD patterns of Pd(Cl)/ceria as prepared (a) and reduced at 573 K (b), 673 K (c), 773 K (4, 873 K (e) and 973 K (I) for 20 h. Note the change in magnification for (e) and (f).
temperature (Ted) its yield exhibited a reversible maximum at Catalytic Activity Metal-free CeO, showed no activity in benzene hydro- 503-508 K. The effect of Ted on the activity of the catalysts is genation over the whole range of temperatures studied, i.e. shown in Table 3, where specific activities in benzene hydro- 393-513 K. For Pd(N)/ceria and Pd(Cl)/ceria, cyclohexane genation and activation energies for this process are sum- was the only reaction product and independent of reduction marized. For both catalysts the activity decreased with
Table 3 Effect of reduction temperature on activity of Pd/CeO, catalysts in benzene hydrogenation
x e d a E*ct activity /kcaI mol-' /pmol s-' g,' sample /K
Pd(N)/CeO 573 8.0 673 10.8 773 -
Pd(C1)/Ce02 573 14 673 12.2 773 12.1
19.6 12.9 0.04
53.2 3.7 0.5
0.0 - CeO 573
a Reduction treatment involved 20 h heating in hydrogen at indi- cated temperature plus 16 h at 573 K inside the reactor.
Fig. 7 HRTEM micrograph and SAD pattern of a ceria-rich grain in Pd(Cl)/ceria reduced at 973 K the activity of Pd(Cl)/ceria. The activation energy did not
change much with Ted and the average value was 12 2 kcal rno1-I for Pd(N)/ceria and 10 & 2 kcal mol-' for Pd(Cl)/ ceria.
increasing Ted from the maximum at 573 K to nearly zero at 773 K. An especially abrupt decrease (by a factor of 14) occurred for Pd(N)/ceria after increasing Ted from 573 to 673 I(. The same change of Ted caused only a 1.5-fold decrease in Discussion
Unreduced Samples
The structural data (Fig. 1 and Table 1) for all four unre- duced samples are very similar and show the presence of only one crystalline phase, CeO,. We assume that both Pd pre- cursors existed in an amorphous state on the support and that they (or HCl) did not cause any noticeable destruction of the ceria crystallites (mean size CQ. 5 nm). This observation agrees with ref. 2 and 22, where it was found that ceria exhibits much higher stability against hydrolysis than other lanthanide metal oxides, e.g. La203 or Sm203.
Transformation of the Support
Our results show that upon reduction in H, the ceria support in Pd(Cl)/ceria catalyst underwent complex transformations induced by the presence of Pd and Cl. Up to 873 K, ceria probably existed as the slightly non-stoichiometric phase Ce02-,,, with y z 0.17, as estimated from the lattice expan- ~ i o n . , ~ The sudden sharpening and shift of the ceria peaks at
Fig. 8
the [Oll] orientation of the Pd particle.
Electron micrograph and microdiffraction pattern of I'd par- 973 K ( ~ i ~ . 2) was, in our opinion, caused by the phase trans- '"1' (upper left-hand corner) '' Pd(C1)/wria at 873 K. Note formation into a more oxygen-deficient phase, most probably
Ce,O,,. The positions of the XRD peaks and the lattice parameter of a pseudo-cubic cell (0.557 nm) fits very well with the reference data.24 Contrary to other oxygen-deficient CeO,-, phases, Ce7012 is stable up to 1300 K and has a very narrow range of compositions in which it exists as a single phase.,' It may be argued whether 973 K is high enough a temperature to cause bulk reduction of ceria to CeO,.,,, which corresponds to Ce,O,,. 11*26*27 It has been reported, however, that for high-surface-area CeO, , partial bulk reduction to Ce02-,, is possible below lo00 K;28*29 moreover, this process may be facilitated by the presence of Ru,~' Ni,31*32 Rh," Pt,33 C034 or Pd.33 Our results for Pd(N)/ceria and CeO, reduced at 973 K (Fig. 3) confirm this, and show the promoting effect of Pd on ceria bulk reduction to Ce7012. From Fig. 3 it is also evident that Ce7012 is very unstable upon exposure to air even at room temperature and is rapidly reoxidized to CeO,, with Pd acting as a promoter of this reaction. Easy reoxidation of Ce7012 at room tem- perature was observed by Ray and and metal- promoted reoxidation of partly reduced ceria in catalytic
Fig.9 Magnified HRTEM image of the edge of the Pd pattick from Fig. 8. Note the presence of the thin crystalline overlayer (lattice fringes of ca. 0-32 nm) on Pd (1 1 1) and (1 10) faces.
systems is also known.’ 17 l9 Two interesting questions arise concerning the Ce7O1, phase: (i) Why is the phase formed in Pd(Cl)/ceria much more stable than that in Pd(N)/ceria or CeO,? (ii) Why is this phase not formed in Cllceria reduced under identical conditions? We believe that C1 is responsible for both these effects. For Pd(Cl)/ceria we propose, in agree- ment with a recent suggestion of Bernal et d.,’ that C1- ions from the metal precursor may be introduced into the ceria lattice in which anion vacancies are created during reduction. The original hypothesis of Bernal et al.,’ that Cl- ions are directly substituted for 0,- ions, seems to be incorrect, since the ionic radius of C1- (0.181 nm) is significantly larger than that of 0’- (0.135 nm).35 Even some expansion of ceria lattice due to formation of anion vacancies (by 2.5% for Ce7012) is not enough to allow such a substitution. This is probably also the reason why cerium oxychloride with a cubic structure does not exist even though cubic cerium oxy- fluoride is known3’ (the ionic radius of F- is 0.136 nm, very close to that of 02-). Summarizing, although it seems evident that C1 stabilizes the Ce701, structure, the mechanism is unknown. The answer to the second question is based on the observation (Fig. 4) that ceria alone is much less effectively reduced than Pd/ceria. We assume therefore that in Cl/ceria all the reduced support is transformed into CeOCl. In Pd(Cl)/ ceria, CeOCl is formed in significant quantities even at 573 K, and with increasing temperature of reduction the process is enhanced (the ratio of intensities of CeOCl: CeO, peaks increases to 0.69 at 973 K, which means that the sample con- tains mostly CeOCl). This could account for the recently reported highly irreversible reduction of ceria in Rh/CeO, catalysts prepared from RhCl, . We expect that most of the irreversibly reduced Ce3+ ions in Rh/Ce02 reduced at 623 K detected in ref. 5 by the magnetic method could in fact be Ce3+ ions in the CeOCl lattice. In another paper4 the same group observed by HRTEM the formation of CeOCl in Rh/CeO, reduced at 973 K but not at 773 K. The reason for this apparent contradiction with our results is probably the amount of chlorine present in the catalyst and therefore the amount of CeOCl formed. In their HRTEM work Bernal et aL4 used 1.9 wt.% Rh catalyst, which contains ca. three times less Cl than ours (assuming no loss of C1 during pretreatment). Moreover, they probably used much shorter reduction times than we did (20 h). We therefore think that the amount of CeOCl, or its crystallinity, was not enough to enable visualization by HRTEM. Le Normand et a1.’ studied Pd/CeO, catalysts containing 8.9 wt.% Pd prepared from Pd(NH3)4C12 precursor and did not find the presence of CeOCl in the XRD spectra of the sample reduced at 773 K, although they observed similar PrOCl and TbOCl phases in Pd supported on Pr,O, and Tb407. The reason is probably the different pretreatment procedure (calcination at 673 K) which caused a loss of C1 (final value 2.6 wt.%).
When comparing the evolution of ceria support in Pd(C1) and Pd(N) catalysts upon reduction we see dramatic differ- ences (Table 1). Apart from the spectacular transformation of CeO, into CeOCl in Pd(C1) we see also that sintering of the remaining ceria at temperatures up to 873 K is much slower in this catalyst. Another important difference concerns the hindering of the reoxidation of the partly reduced Ce7012 phase in Pd(C1) by chlorine. Finally, in the Pd(Cl)/ceria cata- lyst, contrary to Pd(N)/ceria,I7 no formation of ‘pits’ in ceria crystallites was observed in the samples reduced at 773 and 973 K.
Evolution of Pd During reduction of Pd(Cl)/ceria we observed, in addition to sintering of Pd (growth of L,), its transformation into a new
Pd* phase. As is seen from Table 1, sintering of the metal phase was significant even at 573 K (L, = 11 nm) but up to 773 K, only a small increase in L, (to 14 nm) occurred. This behaviour was also observed for Pd(N)/ceria and is consistent with the literature, where very weak sintering of Pt3, or Rh6 on ceria was observed upon increasing the reduction tem- perature from 673 to 773 K. Rapid growth of Pd crystallites occurred at 873 and 973 K (23 and 29 nm, respectively) simultaneously with transformation of Pd into a new Pd* phase, which also exhibited an fcc structure but with an increased lattice parameter (0.397 nm). The onset of Pd* for- mation could already be seen at 773 K [Fig. 2(d)]. For Pd(N)/ceria very similar behaviour was observed.’ We propose that the Pd* phase could be a solid solution of Ce in Pd, formed as the result of reduction of ceria. Using XRD data for the Pd-Ce binary system37 the Ce content in Pd* can be estimated as 12 atom%, very close to the solubility limit (12.8 atom%). This Ce content is also close to the eutec- tic point (ca. 16 atom%), and therefore the melting point of Pd* is nearly 500 K lower than that of pure Pd.38 This would also account for enhanced sintering of the metal at 873 and 973 K. An important feature of Pd* is its stability. It appears from Fig. 5 that after 6 months only a small fraction of this phase transformed back into ‘normal’ Pd.
To support our hypothesis on Pd-Ce alloying during HTR we must exclude the possibility that the expansion of the Pd lattice parameter by 2.3% observed in this work could be caused by other reasons, e.g. the formation of a P-Pd h~dride,~’ a supersaturated Pd-C solid s o l ~ t i o n ~ ’ - ~ ~ or a Pd-0 s~lution,~’ which bring about very similar structural effects. P-Pd hydride is stable under normal hydrogen pres- sure only up to 430 K39 and in argon it decomposes quickly at room t empera t~ re .~~ Therefore in our experiment it could be formed only during cooling of the samples in hydrogen after HTR. In such a case it should be observed for all reduction temperatures, not only the highest. Moreover, P-Pd hydride is probably even less stable in air than in argon and could not survive 6 months exposure to air. A Pd-0 solid solution characterized by Pd lattice expansion has been observed only once, under very special conditions (CO-0, atmosphere, 553 K)45 which are quite different from ours. We thus reject this possibility. A supersaturated Pd-C solid solu- tion, if formed, could exhibit behaviour similar to that observed in our case, since it is stable at room temperature in air for several rnonth~.~’ However, there are strong indica- tions against this. First, literature data show that a Pd-C solid solution grows in carbon-containing gases (hydrocarbons, CO) in a rather narrow temperature range 400-773 K and decomposes easily in hydrogen at 400-460 K.40-42 It is clear, therefore, that in our case where samples are cooled to room temperature in hydrogen after HTR, there is little chance for Pd-C to survive the treatment. The second point is the magnitude of the Pd lattice expansion, which for Pd-C is larger (2.7%)40-42 than in our work (2.3%). Finally, our own experience with Pd supported on SiO, and Al,O, clearly shows that HTR (973 K) drastically reduces the ability of Pd to form a Pd-C solid s o l ~ t i o n . ~ ~ . ~ ~ On the other hand, the way we performed the HTR treat- ment (flow conditions, low p H z 0 / p H t ratio) favours reduction of the oxide and Pd-Ce alloying. The formation of an inter- metallic Pt,Ce compound in Pt/CeO, catalysts reduced at 117318 or 773 K7 has been reported. HRTEM studies of PtI5 and Rh6,I6 on ceria reduced at 773 K provided no evidence for M-Ce alloying, but note that HRTEM and SAD are not suitable methods to detect the minute changes in lattice parameters that are often caused by the formation of a solid solutions. When comparing the microstructure of Pd in Pd(Cl)/ceria and Pd(N)/ceria, we note also that in the former
the effect of epitaxial orientation of Pd particles on ceria crys- tallites, found in the latter sample reduced at 973 K,17 was not observed. Epitaxial orientation in M/CeO, systems reported in literature6,’ ’*I6 was observed also for chloride- free metal precursors. In Pd(Cl)/ceria reduced at 873 K the Pd (111) and (110) faces were covered with a thin layer of a crystalline substance with lattice fringes of ca. 3.2 nm (Fig. 9). We believe this is the effect of decoration, which is proposed as one of the mechanisms that explain the SMSI behaviour of M/CeO, systems reduced at elevated tempera- t u r e ~ . ~ - ~ ~ ~ ~ ~ * ~ ~ - ~ ~ HRTEM studies showed no decoration up to 773 K,6*15*16 but recently Bernal et al.” reported its appearance at 11 73 K.
Catalytic Activity in Benzene Hydrogenation
Hydrogenation of benzene on supported Pd has been studied intensively49-” and there is general agreement that the reac- tion is structure insensitive, at least for Pd particles 2 I nm.49*50 There are, however, indications that for other sup- ported metals (Rh4’vS2 or PtS3) activity in benzene hydro- genation falls with metal particle size for particles smaller than 1 nm. Strong interactions between Pd and the support were found to modify significantly the activity of Pd in benzene hydrogenation’ and therefore the reaction allows the study of Pd-ceria interactions, neglecting possible effects of sintering. Our results of activity measurements (maximum cyclohexane yield at ca. 505 K, activation energy 12 & 2 kcal mol- ’) are generally consistent with literature data for Pd catalyst^.^'*^' Since the results of activity measurements were reproducible after cycling the temperature of reaction up to 520 K, we conclude that no measurable deactivation of our catalysts by coking took place. The data in Table 3 show a dramatic decrease of activity in benzene hydrogenation [14 fold and 1.5 fold for Pd(N) and Pd(C1) catalysts, respectively] upon increasing the reduction temperature from 573 to 673 K. A further increase of temperature to 773 K completely destroyed the activity of Pd(N)/ceria and nearly so the activ- ity of Pd(C1)fceria. Obviously, the effect of sintering (c j Table 1) cannot account for this behaviour. We believe that the reaction between Pd and ceria leading to reduction of CeO, and diffusion of Ce atoms into the Pd lattice and/or CeO, species onto Pd particles is responsible for the observed deac- tivation. Our XRD data show the onset of Pd-Ce alloying at 773 K for both samples, coinciding with loss in activity. For Pd(Cl)/ceria Pd-Ce alloying and also decoration of Pd par- ticles by partly reduced CeO, species is hindered by chlorine which ‘bonds’ any Ce3+ ions in CeOCl. Only at higher tem- peratures (> 773 K) when the rate of ceria reduction is higher and most C1 is already trapped, may the effects of alloying or decoration operate effectively. Guenin et al.” proposed a similar mechanism to explain the suppressing effect of chlo- ride on SMSI in Ir/CeO, . The effect of metal-support alloy formation during high-temperature reduction as a possible mechanism to explain the sudden decrease of metal activity in benzene hydrogenation was proposed by Moss et aL5’ for Pd/SiO, and later by Meriaudeau et al.’ for Pt/CeO,. We showed also that such alloying hinders the carbonization of Pd43 or NiS4 supported on A1203 and SiO, .
Conclusions
1. Even at moderate reduction temperatures (573 K) a measurable amount of crystalline CeOCl is formed in Pd/ ceria prepared from PdCl, precursors. The presence of
CeOCl may be responsible for the irreversible reduction of Ce4+ to Ce3+ reported in the literature.
2. Reduction at 973 K causes rapid recrystallization of CeO, with its simultaneous transformation into Ce,O, , which, upon exposure to air at room temperature, quickly reoxidizes into CeO,. However, C1 stabilizes the Ce,O,, phase against reoxidation, probably by diffusion into its lattice.
3. Reduction at elevated temperature (>673 K) drastically reduces the activity of Pd/ceria in benzene hydrogenation which cannot be explained by sintering. We showed that the formation of a Pd-Ce alloy is likely to be responsible for this effect. At 873 K or above, decoration of Pd particles by a crystalline overlayer (probably CeO,) was observed by HRTEM.
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