Changes in microstructure and catalytic activity effected by redox cycling of rhodium upon CeO2 and Al2O3

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control III

Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.

237

CHANGES IN MICROSTRUCTURE AND CATALYTIC ACTIVITY EFFECTED BY REDOX CYCLING OF RHODIUM UPON CeO2 AND A1203 J. Cunningham, D. Cullinane, F. Farrell, M. A. Morris, A. Datye* and D.Lalakkad*

Chemistry Department, University College, Cork, Ireland *Farris Engineering Center, University of New Mexico, Albequerque, NM 87131-1341, USA ABSTRACT Prior calcination at Tox > 673 resulted in Rhox/CeO2(A) materials having activities for Ro-type o.i.eq. and for R2-type o.i.x, much enhanced relative to that over CeO2 but in which, unlike Rh/AI203, the oxidised Rhodium component could not be detected by HRTEM or XRD. Observations in reducing conditions were likewise consistent with differences in the nature of Rhodium upon/within CeO2 relative to that dispersed on A1203.

INTRODUCTION

Despite the acloaowledged ilnportance of rhodimn as a componem of three way catalytic converters (TWC's), some tmcertahlties remain concenfing changes in its lnicrostructural characteristics and the occurence of various valence states during various stages of TWC operation [1,2]. Controversy also cominues concenfing file question as to whether or not catalysts featuring rhodimn dispersed on ceria are susceptible to inhibition of their activity by "decoration-encapsulation"-type Strong Metal Support hlteraction (SMSI), or alternatively by rhodimn loss fllrough burial as ions or atoms witlfin the oxide support [3,4]. Various experimemal approaches have been adopted ha this study ha efforts to resolve some of these uncertahlties: namely (i) the use of XRD and HRTEM directed at clarification of microstructural changes induced by redox treatmems; (ii) studies by TPR of how ease of rereduction of oxidised rhodimn species depended upon severity of prior oxidation; (iii) utilization of structure-sensitive hydrogenolyses as probe reactions to gain information concerning file SMSI susceptibility of rhodimn hi metallic form when supported on A1203 or CeO~; and (iv) comparisons of the oxygen-hmldling capabilities of file oxidised catalysts using oxygen isotope equilibration and exchange.

238 EXPERIMENTAL Materials In order to minimise possibilities for spurious metal-support effects associated in the literature with use of RhCl3 as precursor of ceria-dispersed rhodium [5], the materials used in the present study were prepared using rhodium(III) acetylacetonate (Aldrich) as precursor. Wet impregnation at 0.5, 2 or 4 wt% onto the surfaces of the oxide powders was achieved from solution in high purity methanol or tetrahydrofuran, after which samples were dried and calcined in 02 for 2 hr at 823 K. Ceria available from Aldrich (CeO2(A), 16 m2m~) or Rhone-Poulenc (CeO2(r.p.),llO m 2 g~), and Al~O3 (Come, 200 m~-g~) were used as supports. Material Characterizations These were made by temperature programmed reduction in 3% H2+97% Argon (TPR), by high resolution electron microscopy (HRTEM) and by powder Xray diffraction (XRD) on aliquots of the materials both in their "as prepared" states and after preoxidations (HTOx) and/or prereductions (HTR). Powder XRD patterns were collected at RT using a Philips MCD diffractometer feattaJng optoelectronic control to ensure precise measurement of Q values. Peak width, positions and areas were evaluated by computer fitthlg of profiles. Particle sizes, where quoted, were calculated using the Scherrer formula. Detailed comparisons were made between patterns for samples after ageings in air at temperatures up to 1423 K and/or after reductions in 3% CO/He up to 573 K. HRTEM was done at the High Temperature Materials Laboratory, Oak Ridge National Laboratory, using a JEOL 4000 EX microscope. Transmission electron microscopy was also done at the University of New Mexico using a JEOL JEM 2000-FX microscope. The samples for TEM were made by simply dipping the holey carbon grid in the powder and shaking the excess off. No solvent was used in any stage of the sample preparation. Elemental analysis was done by energy dispersive spectroscopy (EDS) using a Tracor Northern System. Reactivity Studies Since structure sensitivity in the hydrogenolysis of n-butane over oxidesupported group VIII metals had been reported [6], this was utilised as a suitable catalytic probe reaction with wlaich to compare activities of the catalysts in net reducing conditions. Particular interest attached to the question as to whether prior high temperature reductions (e.g. HTR773 K) would bring about similar strong inhibition of hydrogenolysis activity of Rh/Al203 or Rh/CeO2 after LTR4~. Such reactivity studies were carried out in quartz-tube differential microreactors and samples of the exit gases were analysed by gas chromatographs fitted with appropriate columns and flame ionisation detectors. A reactant-flow composition

239 ratio of He:H2:C4H10 equal to 20:20:1 sccm was used. Between runs the removal of any carbonaceous deposit was effected by flowing (Hz+He) over catalysts to regenerate them. Experiments aimed at comparing the oxygen-handling activity of preoxidised aliquots of the materials, wlfilst ensuring fllat surfaces remained in an oxidised condition throughout (e.g. Rh"+ox/CeO~), were carried out in a recirculatory reactor system under low pressures of an (16Oz+~80~)equimolar mixture.

RESULTS AND DISCUSSION TEM

Photo (a) in fig. 1 shows the low magnification transmission electron micrograph of Rh/AI203 in its precalcined state. The rhodium particles (shown by arrows) have a rounded morphology and are seen tmifol~y all over the catalyst and the average particle size is ))300 A. EDS confirmed a uniform distribution of rhodium in this catalyst. Photo (b) in fig. 1 shows a high magnification picture of the same catalyst. The lattice spacings on the large particle (shown by the arrow) indicate that the rhodium in this catalyst is present in file form of Rh203. Treatment of this catalyst in 1-12reduced the Rh203 to metallic Rh accompanied by a small decrease in particle size, but otherwise the distribution of particle sizes or the morphology of the catalyst was unchanged. Photo (c) of fig. 1 shows a high magnification view of Rh/CeO2 in the oxidised state. In this case too, EDS confmned the presence of rhodium in amounts similar to those seen on Rh/A1203. However, there is a distinct absence of a particulate rhodium phase on tiffs catalyst, both in the oxidized catalyst which is shown, and the same catalyst reduced in 1-12at 473 K. Tiffs agrees with our previous study [7], where it was shown that on a CeO2 support, Pt was difficult to observe in the TEM, at least in the fresh catalyst, but in that case, the Pt particles could be readily seen after several oxidation-reduction cycles. Murrell et al have previously demonstrated the existence of a dispersed oxidised phase of Pt on Pt/CeO~ [8]. It is possible that such a dispersed oxidised phase of Rhodium here formed on or within CeO2 thereby making the oxidised Rhodium component difficult to detect. [9]. XRD 4% Rh/CeO:: No peaks due to rhodium or its compounds were visible after ageing acac-derived samples at 823, 923 and 1023 K. After a 1123 K trealanent however, peaks readily ascribable to Rh203 could be easily detected (plot a, fig. 2). Between 1123 K and 1423 K the peaks sharpened consistent with average particle size having increased from ca. 300 to 650 A. Other noteworthy points to emerge from particle size measurements included: Firstly, that below 1123 K no rhodia-related peaks were detectable, thereby indicath~g sizes below the X R detection limit even

240

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.

~"

,~

'

~,~~:1

i ~ ~

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. . . . i.'~.~!~:!r,-,-,~,-,.~.:,~i,~.~~!i:?f:...;t ~~.,.~ '~.~'-.,~"t:i:~,:'~:~..:'~. -~-~,~:' .._.., ~:['..~."~ , :."-..-.:.. ~ . . 1.~. '-.'." ~ . ~ - - - ~~,~4~.~:~,~:~.....-.:.~:.: . .~. ., . . , ~ . ...

-iOnm

......

....;..... :-.

1

~,."." ,.~-.i;" ... .......

:!i~,~

, ,,~.., b'~.i,i

FIGURE 1" Transmission electron micrographs of precalcmed materials (a) TEM of Rh d A l~9 s; (b) HR TEM of Rh d A le0 3; (c) HR TEM of Rh o,/Ce O e.

241

A

I

[

I

k) {

......

2

F ~ ~

[ . . . . . . . .

| ...........

40

_

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! 60

I ......... B

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! 20

_

" [degs]

1 .... 2S

t. . . . . . . .

!

2_ #l

.4

W C

5n

O

I FIGURE 2: Powder X-ray diffractograms of 4% Rh/Ce02 (Part ,4) and 4% Rh/Al20s (Part B) after treatments as follows: Part A: Rh/Ce02 with CeO2features indicated by vertical lines, metallic Rh by O, Rhodia by * and mixed Rhodium-Ceria phase by #: Plot (a) calcined @ 1123K; plot (b) aged @ 1423 K; plot (e) reduced in CO @ 523 K. Part B: Rh/Al20~ with a-Al20s indicated by vertical lines, Rhodia by * and mixed Rhodium-alumina phase by #: plot (d) calcined @ 823 K; only 2Q angles 15~ ~ are displayed since above this the complex y-Al20s (and related structures) is too complex to reveal additional features; (e) full 2Q scan after calcination @ 423 K. Note the change of y-Al20s to ct-Alz03.

242 for this substantial rhodium loading. As may be seen later, whenever metallic Rh was formed from this same loading it could be detected as particles around 50 A. On that basis it appeared that whatever rhodia particles were present atter calcinations below 1123 K must be smaller than 50 ,A,. That conclusion is strongly supported by the above-mentioned HRTEM observations upon Rh/CeO2 (cf. photo _.cof Fig. 1) which gave no evidence for rhodia particles, although ceria (111)lattice planes with spacings of 3.1/k were readily resolved. Secondly, in many experiments it was found that sizes calculated from different reflections spanned a wide range e.g. after calcination at the highest temperature (1423 K) results ranged 650+50 A, whereas after 1123 K calcination the calculated sizes ranged 200-700 A (average 300 A). These results may be rationalised if rhodia particles were polyerystalline assemblies of individual erystallites having morphologies not corresponding to hemispherical. It has been suggested for Rh/AI203 that raft-like particles may be formed [10] and present XRD results for rhodia upon ceria may not be inconsistent with this possibility in respect of individual mieroerystallites. After ageing at 1423 K new peaks are seen in the diffraetogram not attributable to either ceria or rhodia (plot b, fig. 2). Peaks due to Rh metal presumably formed via the high temperature loss of oxygen - are indicated on the plot 2b. This agrees with recent work on rhodia by Carol and Mann [11]. However, other new peaks (also indicated) point to the formation of a completely new phase which has not yet been identified, but might be the Rhodium analogue of a surface-related, mixed phase such as reported previously for Pt upon CeOz [12]. When the 1423 K aged sample was reduced in CO at 523 K and re-oxidized at 573 K complete removal of peaks due to the mixed phase was achieved (but not of peaks due to metal, which rather hacreased in size during the reduction process cf. plot c fig. 2). However, the metallic particles produced by this 523 K reduction with CO appeared resistant to reoxidation at 573 K (cf. TPR data below). XRD 4% Rh/y-Al~O3: After agehlg at 823 K peaks due to y-A1203 were observed together with others attributable to the presence of rhodia (plot d, fig. 2). As a result of ageing at temperatures between 823 K and 1423 K rhodia particle size increased from c_aa.50 (+30) to c~. 400 (• A. Lack of consistency between particle-size calculated from different reflections was evident, consistent with observed rhodia particles being polycrystalline assemblies within which individual crystallites may not be equiaxed. The contrast between this facile detection of rhodia upon A1203 and its non-observance upon the ceria-supported sample indicates that one effect of the ceria was to produce/maintain oxidised Rhoditun ha hyper-dispersed form at temperatures up to 1023 K. However, after calcination at 1123 K the size of the rhodia particles on alumina was about 120+50 A which was smaller than observed for Rh/CeO2 similady-pretreated. After ageing at 1423 K dramatic changes in the pattern occurred with the appearance of new peaks due to metallic Rh, as well as others due to an

243 unidentified phase, and yet others attributable to a-alumina and rhodia (plot e, fig. 2). Yates et al [13] have suggested that during the high temperature processing of Rh/AI203 there is not only some solution of the rhodium oxide to give a surface mixed phase but also an encapsulation ofrhodia particles as the 3' ~ ot alumina phase transformation occurs. In our work the collapse at the g-structure was observed atter ageing at 1400 K, thereby lending support to this hypothesis. However, it should be pointed out that the formation of the mixed phase is not dependent upon support structure collapse, since heating a physical mixture of Rh203 and ~x-A1203at 1223 K in air for 2h rapidly produces the mixed-component phase. Redox Cycles (TPR-T,o,)

Comparisons between the TPR profiles at 10 K min~ from aliquots of rhodium-loaded and rhodium-free CeO2 and A1203 supports in their 'as prepared' condition (i.e. after calchaation at 823 K) demonstrated that a reduction feature onsetting at ca. 330 K and exhibiting a maximum at 390 K originated from the rhodia component. Following that initial (and subsequent) TPR runs up to TR-873 K, aliquots were retained in-situ whilst being cooled to 295 K in 3% Hz/Ar, after which they were exposed to a flow of pure, pre-dried 02 for 2h at a reoxidation temperature, T~, before being cooled to 295 K in Oz, flushed with 3% Hz/Ar and another TPR run made in standard conditions. Repetition of that sequence at progressively higher Try0, yielded, for each of the daodium-loaded materials, a set of TPR profiles from which the following features emerged concerning the extent to which 2h at various T~ restored the H2-reduced rhodium component to oxidised form(s) reducible in the range 350-430 K: (i) no such reoxidation was detectable after T,ox@ 373 K, whereas 18%, 46% and 100% of flint ultimately attahmble was achieved by T~x @ 473, 573 and 673 K respectively; (ii) wlfilst position of T~x of the main TPR feature shitted progressively from 350 K after T~ox@ 473 to 430 K after Tro~@ 1073 K, continued existence of a lower-temperature shoulder at ca. 373 K (which became clearly resolved atter T~ox@ 1073 K) suggested the continued co-existence of a minor, more easily reduced form of rhodia (possibly surface or near surface) together with the major and more diffictdt to reduce fonn (possibly wiflain bulk) achieved by Tr~ ---773 K. A set of such profiles similarly compiled for 4% Rh/AI203 again appeared consistent with two distinguishable fonns of dlodium alter Trox -> 973 K. However in that material the more easily reducible fonn ( T ~ ~ 373 K) was the major component after Tro~< 873 K - an observation which may be correlated with the TEM and XRD results above demonstrating the existance of sizeable rhodia particles upon that material after ageing at such temperatures. Another point made clear by the sets of TPR profiles for Rh/CeO, and Rh/A1203 is flint, irrespective of T~oxvalues within the

244 range 673 ~ 1023 K, subsequent exposure to H2 at TR >__450 K sufficed for rapid conversion of the rhodium content to its reduced form:

Oxygen-handling Properties of Preoxidised Materials Advantages in the use of an equimolar, isotopicaUy non-equilibrated (i.n.eq) mixture of 1602 ~-- 1802 to probe the oxygen-handling properties of oxidic materials include capabilities to detect: (a) activity of the surfaces at 295 K for Ro-type homophase isotopic equilibration (o.i.eq) as per eqn (0), via the intermediaey of very weakly held four-oxygen-atom surface species [14]; and (b) activity at higher temperatures for thermally activated heterophase oxygen isotope exchange (o.i.x) of types Rt and R2 as per eqns. 1 and 2, possibly via the intermediacy of strongly bonded O- and O ~species respectively [15]. Ro-type

P('SO,)o.5+ p(1602)0.5 ~ P('60'sO)0.5 + P('602)o.z5 + P('sO00.2,

Eq. (0)

R,-type

'sOz(g) "[- 1602-(S) ~

160"O(g) + 'sO2(s)

Eq. (1)

R2-type

1802(g) + 21602"(S) ~

1602(g) "~- 21802"(S)

Eq. (2)

The tL,-process was promoted at 295 K by preoxidised samples of ex Rh.ac.ac. Rhox/CeO2(A) samples, but not by the rhodium-free supports nor by ex-RhCl3 materials. Thermal activation was necessary to bring about o.i.x., as evidenced by onset of variations in partial pressures of ~802, ~602 and ~60~sO during upward ramping of reactor temperature. Over rhodium-free CeO2, no tL,-type o.i.eq. occurred, with the result that when o.i.x, did onset at ca. 773 K it operated on the i.n.eq, mixture and resulted hi increases m/z = 32, equivalent decreases at rn/z = 36, but with very little increase at m/z = 34. Those changes are consistent with operation of an R2 or place-exchange mechanisms involving vacancy-pair creation and removal. Figure 3 illustrates the thermally activated changes hi isotopic composition of the gas phase over Rhox/CeO2 and demonstrates onset of o.i.x, at ca. 523 K, i.e. ca. 250 ~ below that over CeO2 alone. Since the Rh/CeO:(A) material already exhibited Ro-type activity at 295 K, operation of o.i.eq, had effeeted a change to the isotopically equilibrated composition as per eqn. O during the period of hlcrease from 295 to 523 K. Slopes of the relative rates of change away from that composition, evident in fig. 3 as a result of the onset of o.i.x, at 523 K, are consistent with operation of a mainly R2-type o.i.x, process. This observation that oxidised species within or upon RhJCeO2 made such o.i.x, possible at much lower temperature than for Rhodia-free Ceria is qualitatively very similar to reports of o.i.x, promotion over Sr2§ relative to La203 [16], and a similar

245 interpretation seems appropriate, viz. in terms of a promoting effect of an enhanced concentration of oxygen vacancies caused by incorporation of lower valent cationic dopants.

~OOO i

! 'ralO

o

lm

a~ T

ma ~

4m C*Q

~

~

70o

FIGURE 3: TPOIX profiles of additional changes m isotopic composition of an o.i.eq, dioxygen mixture over RhJCe02 upon ramping temperature at lO~mm ~.

Hydrogenolysis of n-butane Linear ArrhelfiUS plots were obtained for temperature dependence of steadystate activities of 'fresh' samples of Rh/CeOz and Rh/A1203 in n-butane hydrogenolysis at T~ 423-473 K. These plots indicated closely similar activation energies of 38 and 39 kcal molL respectively, but showed hydrogenolysis rates over RldAlzO3 to be uniformly nine times larger than over Rh/CeO2 at equal T_. Results showing the extent to which Rh/CeO2 hydrogenolysis activity, and the selectivity towards ethane, could be modified by subsequem oxidation reduction cycles at lfigher temperatures arc smnmarised in Table 1. Chmlges in activity arc normalised relative to that observed for the fresh aliquot at the same T~, and data in the Table make clear that whereas oxidation-reduction cycles caused significant reversible chmlges in the

246 ethane selectivity of Rh/AI203, the ethane selectivity of Rh/CeO2 was little affected by such cycles. The reported values for ethane selectivity were recorded under temperatures where the methane/propane product ratio was approximately 1.3, and did not change with catalyst pretreatment. The C~-C3 ratio near unity indicates that hydrogenolysis of the n-C4 involves cleavage of a single C-C bond during the turnover of a n-C4 molecule. Differences in the product distribution can then be related to cleavage of the n-C4, at the central bond versus the terminal bond scission. On Rh/A1203, it is seen that the mole fraction of ethane in the product drops from ~ 0.5 in the flesh state to >>0.18 due to oxidation and this drop is reversed by high temperature reduction whereas no such changes occurred for Rh/CeO2.

Table 1: Selectivity

Effects of HTO~ and HTR upon n-butane Hydrogeneolysis Activity and

Pretreatment Sequence

Nonrmlized Activities Rh/AI203 Rh/CeOz

Ethane Selectivity Rh/AlzO3 Rh/CeO2

1. Fresh (LTR673K)

1.00

1.00

0.484

0.401

2.1st HTOx rnK

0.69

1.82

0.218

0.488

3.1st HTR 773K

0.075

0.058

0.398

0.488

4.2nd HTOx773K

0.78

1.75

0.184

0.441

Those contrasting observations may be understood within the context of observations and interpretations developed in previous studies of relationships between ethane selectivity and dlodium particle size upon A1203 and SiO2 supports [6]. On low weight loading Rh catalysts, where the particles were so highly dispersed that they were invisible by the TEM, oxidation-reduction cycles did not cause any change in either the activity or the ethane selectivity. On higher weight loading catalysts where the rhodium was present as well defined particles, the oxidationreduction cycles caused significant changes in the activity and ethane selectivity that resembled those on macroscopic single crystals [8]. In flaat work it was argued that by virtue of their size, the surface smlcture of the small particles (< 10/~) could not be altered because of hlsufficient room to pack the atoms in different ways. In the present study, the Rh/AI203 with particle size of >>30 nm showed reversible changes in ethane selectivity during n-butane hydrogenolysis, while the Rh/CeO2 catalysts

247 with no detectable particulate Rh phase exhibited no such cycling. This would be consistent with the rhodium particles on the CeO2 support being very highly dispersed and/or having fonr~ed a complex that 'locks' them in a state where they did not undergo significant morphological changes under the pretreatments used in the hydrogenolysis studies.

ACKNOWLEDGEMENTS

Financial support for the research at UCC came in part from Contract SC1-CT910704 with DGXII of The European Commission: that for work performed at The University of New Mexico was provided by The Petroleum Research Fund of The American Chemical Society. High resolution TEM was performed at the High Temperature Materials Laboratory, a user facility supported by the U.S. Department of Energy at Oak Ridge National Laboratory, with additional work being performed at the Electron Microscope Laboratory witlfin the Department of Geology, University of New Mexico.

248 REFERENCES

9

10 11 12 13 14 15 16

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