La2O3

Applied Catalysis, 73 (1991) 1-15

1

Elsevier Science Publishers B.V., Amsterdam

Preparation and pretreatment effects on metal decoration in Rh/La203 G e o r g e R. G a l l a h e r , J a m e s G. G o o d w i n Jr.* a n d L a s z l o G u c z i 1

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA 15261 (USA), tel. (+ 1 412)6249642, fax. (+ 1-412)6249639 (Received 24 April 1990, revised manuscript received 5 February 1991 )

Abstract

Ethane hydrogenolysisand cyclopropane hydrogenation were used to probe changes in the decorative overlayer in Rh/La2Q resulting from variations in reduction temperature, weight loading, treatment in oxygen followed by low temperature reduction, and method of precursor decomposition. Increasing reduction temperature resulted in an increase in the extent of decoration, especially between 200 ° C and 400 ~C, as evidenced by a suppression of cyclopropane hydrogenation and ethane hydrogenolysis rate. For a series of catalysts with increasing weight loading, reduction at 500 ° C resulted in significant decoration in each case. However, only a slight effect of weight loading may be suggested. An oxidation/ low temperature reduction cycle partially reversed the decorative effect. No difference in decoration was observed due to calcination vs, reduction for precursor decomposition when the catalysts were exposed to the same final reduction temperature. Due to the smaller ensemble size required for its sites, cyclopropane hydrogenation was found to be more sensitive than ethane hydrogenolysisto changes in surface structure for these heavily decorated catalysts. Because of limitations in determining free rhodium surface area, the extent of coverage of the rhodium by the decorative overlayer was only able to be deter mined qualitatively.

Keywords: hydrogenation, hydrogenolysis, lanthanum oxide, metal decoration, pretreatment effects, rhodium.

INTRODUCTION Lanthanide oxides are important promoters and/or supports for many catalysts of commercial interest. Applications include carbon monoxide hydrogenation and automotive exhaust catalysis. For carbon monoxide hydrogenation, interest in La20~-supported noble metals has resulted from the observed selectivity of these catalysts for oxygenate synthesis [1-10]. This ability has been attributed to a metal-support interaction involving the decoration of metal crystallites with some form of lanthana moiety [4,11]. However, questions regarding the origin and modification of the decoration remain. ~Permanent address: Institute of Isotopes, Hungarian Academy of Sciences, P.O. Box 77, H-1525 Budapest, Hungary.

Besides the CO +H2 reaction, hydrogen and carbon monoxide adsorption [4,9,10,12-14], carbon monoxide IR [9,12], and XPS [11,14] have been the primary tools used to explore this decoration. However, interpretation of the results from each of these techniques is somewhat limited. For example, suppression of carbon monoxide chemisorption is generally observed, but carbonate and formate formation with the support species [9,12] may result in an overestimation of metal dispersion by this technique. For hydrogen chemisorption, several studies indicate a suppression of uptake [4,13 ], while others suggest that hydrogen may be spilt over onto the decorative species and perhaps the support [9,12,14]. Negative binding energy shifts have been observed from XPS for the 3d levels of palladium [11] and rhodium [14] suggesting charge transfer and/or modification of the extra-atomic relaxation. However, referencing problems cannot be eliminated as a probable influence. While the effect of La203 on the carbon monoxide hydrogenation characteristics of noble metals has been demonstrated, its use to characterize the decoration phenomenon is limited as a result of the possible partial reversal of the decoration due to the presence of water as a reaction product [5,9]. Hydrogenolysis reactions are known to be sensitive to decorative effects. Suppression of the structure-sensitive ethane hydrogenolysis reaction [15] in Group VIII/Group 1B bimetallic catalysts [16,17] and in supported noble metals in the SMSI state [18-20] have been attributed to break up of the relatively large surface ensembles [21,22 ] required for this reaction by inactive decorating species. For structure-insensitive reactions such as cyclohexane dehydrogenation [23], site activity remains relatively constant upon the addition of decorating species until the active component is almost entirely replaced. Cyclopropane hydrogenation has generally been considered as a classic example of structure insensitivity [24], requiring a small ensemble for its reaction site [25]. However, evidence exists that it is structure sensitive over single crystal nickel [26] and supported ruthenium [27,28]. For rhodium, a four-fold increase in cyclopropane conversion was observed by Wong et al. [29] as dispersion increased from 23% to 91%. The effect of adding inactive iridium in that study paralleled that which would be expected for a structure insensitive reaction. This suggests that if cyclopropane hydrogenation is structure sensitive over rhodium, it is only moderately so. Recently, the sensitivity of these reactions to changes in the decoration in Rh/La203 as a result of varying reduction temperature from 200 ° C to 400 ° C has been found [14]. This paper reports on the application of ethane hydrogenolysis and cyclopropane hydrogenation to explore modifications in the metal-support interaction in Rh/La203 resulting from changes in preparation and pretreatment conditions: precursor decomposition method, metal weight loading, reduction temperature, and treatment in oxygen followed by low temperature reduction.

EXPERIMENTAL

Preparation Catalysts were prepared by incipient wetness impregnation of La203 (Alfa Products) with various concentrations of aqueous solutions of Rh (NO~) 3"2H20 (Alfa Products) in order to achieve the desired weight loading. Weight loadings of 0.3, 1.5 and 3.0% rhodium were used. The impregnated samples were dried overnight in air at 90 ° C. The standard pretreatment involved reducing the catalysts by heating in 100 cm3/min of flowing hydrogen (UHP grade, Linde) at 2 ° C / m i n from 25°C to 500°C then holding at 500°C for 4 h. These conditions were chosen since previous studies had indicated that they are sufficient to fully decompose lanthanum carbonates [ 14,30-32 ] and to fully convert La (OH)3 and LaOOH to La203 [30-33]. The carbonates and hydroxides are present after impregnation or exposure to air. For treatments such as those used in this study, support surface areas of 10-20 m2/g have been observed [33]. The effect of reduction temperature was explored by reducing samples from the same impregnation of 1.5% Rh/LaeO3 using the conditions described above except that final reduction temperatures of 200, 400 and 600 ° C were used. The effect of an oxidation/low temperature reduction cycle was investigated using a 1.5% Rh/La203 which had originally undergone the standard 500°C reduction. Before oxidation, the sample was reduced following the standard treatment and purged in 100 cm3/min of flowing helium (UHP grade, Linde) at 500 ° C for 1 h before cooling to 400 ° C. The sample was then oxidized in 60 cm3/min of flowing oxygen (UHP grade, Linde) at 400 °C for 90 min, purged for 15 min in 100 cm3/min of flowing helium, and cooled to 25°C. Following reduction by heating in 100 cm3/min of hydrogen at 2 ° C / m i n to 200°C and holding there for 4 h, the catalyst was studied directly using ethane hydrogenolysis or cyclopropane hydrogenation. In order to determine the effect of the decomposition method on the metalsupport interaction, samples from the same impregnation of 3% R h / L a 2 Q were decomposed in two additional ways. The first involved heating the catalyst in 100 cm3/min of flowing air (hydrocarbon free, Linde ) from 25 to 350 ° C, holding at that temperature for 2 h, cooling to 25 ° C, and purging with helium before applying the standard reduction pretreatment at 500 ° C. This method was chosen to explore the effect of calcination vs. direct reduction on the metalsupport interaction. A second sample underwent a two-stage reduction by heating in 100 cm3/min of flowing hydrogen at 2 ° C / m i n from 25 to 300 °C and holding there for 4 h then heating at 2 ° C from 300 to 500 ° C and holding there for 4 h. This treatment was chosen to allow complete reduction of the rhodium before conversion of the support species to LaeO3.

Characterization Static hydrogen chemisorption was performed on all catalysts except those which had been decomposed via the 350 °C calcination/500° C reduction, the 300 ° C/500 ° C reduction, and the 500 ° C reduction/400 ° C oxidation/200 ° C low temperature rereduction treatment. Hydrogen chemisorption was performed in a glass high-vacuum gas volumetry system in which an ultimate vacuum of 10-6 Torr (1 Torr = 133.3 Pa) was attained. Each sample was rereduced at its original reduction temperature in 0.5 atm (1 a t m = 101.32 kPa) of hydrogen for 2 h then evacuated at that temperature for 1 h before cooling to 25 ° C under dynamic vacuum. Catalysts were exposed to approximately 300 Torr of hydrogen and allowed to equilibrate before the total adsorption isotherm was measured. The sample was then evacuated at 25°C for 30 min after which the reversible isotherm was measured. The linear portions of each isotherm were extrapolated to zero pressure to determine uptakes, and the difference between the total and reversible isotherms was taken as the irreversibly adsorbed hydrogen. X-ray diffraction was carried out on air-passivated samples on a General Electric XRD-5 diffractometer using Cu K a radiation.

Activity measurements Ethane hydrogenolysis and cyclopropane hydrogenation studies were carried out in a quartz U-tube microreactor heated by a tube furnace controlled with an Omega temperature controller having a chromel-alumel thermocouple in the catalyst bed. A nominal 100 cm3/min, 3 mol-% hydrocarbon, reaction mixture was prepared by diluting a flow of ethane (research grade, Linde) or cyclopropane (99.0%, Matheson) with a stream of hydrogen ( U H P grade, Linde). Hydrocarbon flows were controlled by a needle valve and hydrogen flow was controlled by a Brooks flow controller. Total flow-rate was calibrated with a soap bubble meter. The reactor feed and reaction products were analyzed with an on-line Varian 3700 gas chromatograph equipped with a FID detector and a 30 ft. X 1/8 in., 23% SP-1700 80/100 Chromosorb column. The reactor was operated under differential conditions by keeping total hydrocarbon conversion below 10% and by using 100 mg of catalyst. Products were sampled after 10 min on stream with a 30-min hydrogen bracket between runs at reaction temperature to prevent catalyst deactivation from a build up of carbonaceous species. With the system and technique used in this study, reaction rates were reproducible to within + 6%. RESULTS Hydrogen chemisorption results for catalysts as a function of weight loading and reduction temperature are summarized in Table 1. Comparing catalysts of

TABLE 1 Summary of hydrogen chemisorption results Catalyst

0.3% Rh/La20:~ 1.5% Rh/La~O:~ 1.5% Rh/La~O:~ 1.5% Rh/La20:~ 1.5% Rh/La~O:~ 3% Rh/La2Q

Reduction temperature

H~ uptake

H/Rh

Total (zmol/g cat.)

Irr. (/~mol/g cat.)

Irr.

( :C ) 500 200 400 500 600 500

21.9 24.2 54.3 32.5 28.6 45.9

21.9 16.9 30.8 24.2 25.4 40.7

1.50 0.23 0.42 0.33 0.35 0.28

different rhodium loading that were all reduced at 500 °C, the irreversible H / R h ratio fell from 1.50 to 0.28 as the rhodium loading was increased from 0.3% to 3.0%. The effect of reduction temperature can be seen for 1.5% Rh/La20:~. The H / R h ratio increased from 0.23 to 0.42 as reduction temperature was raised from 200 to 400°C then fell to 0.33 after a 500°C and to 0.35 after a 600 °C reduction. X-ray diffraction was performed on air-passivated samples of the catalysts listed in Table 1. This was done to allow conversion of L a O O H and La~O:~ to La(OH):~ in order to prevent interference from X R D reflections of L a O O H with the major reflections of rhodium. No peaks attributable to rhodium or rhodium oxide were observed suggesting that the particle sizes were less than 4.0 nm. Changes in the support phase as a function of reduction temperature were followed by XRD. This indicated that the as-received support and the support after impregnation were La(OH):~. After 200 °C reduction, the support phase was still La(OH):~. A mixture of La(OH):~, LaOOH, and La~O:~ were observed after 400 °C reduction, and only La20:~ was found after 500 °C and 600 °C reduction. Upon several days exposure to room air after reduction, all supports reverted to La (OH):~. The effect of reduction temperature on cyclopropane hydrogenation and ethane hydrogenolysis activity can be seen from the Arrhenius plots in Figs. 1 and 2 and in Table 2. Arrhenius plots corresponding to cyclopropane hydrogenation and ethane hydrogenolysis activity results in subsequent tables have not been shown for sake of brevity. However, all such plots exhibited linearity similar to Figs. 1 and 2. Listed in Table 2 are rates on a mol s - 1 g ~ rhodium basis along with turnover frequencies based on the total hydrogen uptake. It should be noted that the Arrhenius parameters were used to extrapolate the data for 1.5% Rh/La~O:~,

&

"2" -9 E o

T

cl:

3 1.6

2.0

214

I/Ternpercture"

2.8

5.2

1000 ( 1 / I K )

Fig. 1. Arrhenius plots showing the effect of reduction temperature on cyclopropane hydrogenation over 1.5% Rh/La20a ( ~ ) 200°C; (VT) 400°C; (O) 500°C; ( A ) 600°C.

-9 13

-lO

E -12

5

-13

-14

-1.60

,

3

1.65

~

1

1.70

,

I

1.75

~

]

1.80

r

I

1.85

I

--

1.90

1 / T e r n p e r c t u r e * 1000 ( l / K )

Fig. 2. Arrhenius plot showing the effect of reduction temperature on ethane hydrogenolysis over 1.5% Rh/La2Q: ( ~ ) 200°C; ([q) 400oc; (O) 500°C; (A) 600°C.

3% Rh/SiO2, and the La203 blank. This was done to provide a common basis for comparison and should not necessarily be interpreted as the true activity of these samples at these conditions. Comparing cyclopropane results at 200 ° C, the specific rate was a factor of 50 and the TOF over two orders of magnitude higher for a reduction temperature of 200 °C vs. 400 ° C. Even lower rates were observed for higher reduction temperatures, although the decrease was much smaller in magnitude. For ethane hydrogenolysis at 300 ° C, the specific rate fell by nearly a factor of 2 and TOF dropped four-fold for a 400°C-reduced sample vs. one reduced at 200 ° C, then remained nearly unchanged for cata-

TABI,E 2

Impact of reduction temperature on ethane hydrogem)lysis and cych)propane hydrogenation activity Catalyst

1.5% Rh/La20:~ 1.59; Rh/La~Oa 1.5°S Rh/I,a~O:~ 1.5% Rh/La~O:, La20:~ Blank 3% Rh/SiO~

Reduction temperature (::C)

Cyclopropane activity at 200 C

Ethane activity at 300: C

Specific rate (×10~,mols-~g-~Rh)

Specific rate ( X l 0 6 m o l s ~g-~Rh)

200 400 500 600 400 400

84(//j 17 5.1 8.:/ 0.11004/~'; 12 900 t~

TOF" ( × 1 0 a s l) 2600 ~ 22 12 8 17 0()0/~

12.9 7.9 6.4 6.3 0.0()(12/~';' 1044

TOF '~ (Xl0:~s -~) 4.0 1.0 1.5 1.6 138J~

"Based on total hydrogen uptake. r~xtrapolated via Arrhenius data. ;'Ratedata in units {}fmol s ~g t catalysl.

'FABLE 3

Effect of weight loading on cych}propane hydrogenation and ethane hydrogenolysis activity Catalyst

Reduction lemperature { C)

0.3% Rh/I,a20:, 500 1.5% Rh/La.20:~ 500 3% Rh/La20~ 500

Cych}propane activity at 200 C

Ethane activity at 325 C

Specific rate (X10r'mols ~g ~Rh)

'POF" Specific rate (Xl0:~s ~) (×10r'mols ~g-~Rh)

TOF" (×10:~s t)

27.6 5.1 1.6

19 12 5

6.6 7.8 10.8

9.6 3.4 3.3

"Based on total hydrogen uptake.

lysts reduced at 500°C and 600 ° C. Also shown in Table 2 are results for R h / Si02 and a La~O:~ blank from a previous study [ 14] for comparison purposes. For cyclopropane, the Rh/SiO~ was 6 times more active on a TOF basis and 15 times more active on a specific rate basis than the most active, 200 ° C-reduced Rh/La~O:~ catalyst. For ethane hydrogenolysis, the SiO2-supported catalyst was over 30 times more active on a TOF basis and over 80 times more active on a specific rate basis than the 200 ° C-reduced, La2Q-supported catalyst. The rate data for the La20:, blank are reported on a mol s-1 g-1 catalyst basis. Comparing that sample to the least active, 600 ° C-reduced Rh/La20:~ catalyst on a per gram catalyst basis, the Rh/La~O:~ is over three orders of magnitude more active for cyclopropane hydrogenation and over four orders of magnitude more active for ethane hydrogenolysis than the La203 blank. The effect of weight loading on the activity for cyclopropane hydrogenation and ethane hydrogenolysis can be seen in Table 3. Again, activities are expressed on both a mol s - ~ g - 1 rhodium and a TOF basis. Comparing cyclopropane results at 200 ° C, the observed trend was a decrease in cyclopropane spe-

cific rate and TOF as rhodium weight loading was increased. For ethane hydrogenolysis at 325 ° C, the specific rate fell by a factor of nearly 3 as loading increased from 0.3% to 1.5%, then remained unchanged as loading was further increased to 3.0%, while TOF increased slightly as weight loading was increased. Cyclopropane and ethane activity over Rh/SiOe (see Table 2) was much greater t h a n over the most active Rh/La203 catalyst. The least active R h / L a z Q catalyst was over three orders of magnitude more active t h a n the L a 2 Q blank for both cyclopropane hydrogenation and ethane hydrogenolysis on a mol s - ' g - 1 catalyst basis. A 90-min t r e a t m e n t in oxygen at 400°C followed by rereduction at 200°C for a catalyst originally reduced at 500 °C resulted in an increase in cyclopropane hydrogenation and ethane hydrogenolysis specific rate as shown in Table 4. The rate of cyclopropane reaction at 160°C over the treated catalyst was greater by an order of magnitude t h a n t h a t observed over the catalyst reduced at 500 ° C. The treated catalyst was, however, still a factor of 20 less active t h a n the catalyst originally reduced at 200 ° C. For ethane hydrogenolysis at 300 ° C, the treated catalyst was more active by a factor of 4 t h a n the 500°C-reduced sample. In contrast to the cyclopropane results, the treated catalyst was also more active t h a n the 200 ° C-reduced sample by a factor of 2. Cyclopropane hydrogenation was used to study the effect of precursor decomposition method on the decoration phenomenon and the results are summarized in Table 5. No difference was observed in cyclopropane activity between a catalyst t h a t was directly reduced at 500 ° C, one t h a t underwent a twostage reduction at 300 °C and 500 ° C, and one t h a t was first calcined at 350 °C then reduced at 500 ° C. The reaction of cyclopropane with hydrogen can proceed via three pathways: ( 1 ) ring opening to propane, (2) a simple hydrogenolysis to ethane and methane, and (3) a double hydrogenolysis to methane only. Under differential conTABLE4 Impact of oxidation/lowtemperature reductioncycleon 1.5% Rh/LaeO3 Catalyst treatment

Cyclopropaneactivity at 160°C (× 10~mol s -1 g-' Rh)

Ethane activity at 300°C ( × 106 mol s -1 g-' Rh)

500 °C Reduction

0.74

6.4

500°C Reduction/ 400 °C Oxidation/ 200°C Reduction

9.56

26.1

200°C Reduction

219~

'~Extrapolatedvia Arrhenius data.

12.9

9 TABLE 5 Effect of precursor decomposition method on cyclopropane hydrogenation activity over 3% Rh/ I,a~O3 Catalyst treatment

Cyclopropane activity at 200 C ( × 10'~mol s-1 g 1Rh)

500°C Reduction/4 h

1.78

350°C Calcination/2 h 500 C Reduction/4 h

1.78

300 ~C Reduction/4 h 500°C Reduction/4 h

1.83

ditions, only ring opening was observed over Rh/SiO2 while some simple hydrogenolysis was observed over the La20:csupported rhodium. In all cases this simple hydrogenolysis accounted for less than 10% of the reaction products. It appeared to be a result of the high reaction temperature required to observe activity over the La~Oa-supported catalysts rather than the catalysts themselves, since the same product distributions were observed over Rh/SiO2 when the reaction was run at the temperatures where the La203-supported catalysts exhibited activity. DISCUSSION

The use of hydrogen chemisorption as a characterization tool for Rh/La203 is complicated by the ability of this system to spill hydrogen onto the decorating species as well as the support [9,12,14]. Considering the results for hydrogen uptake as a function of weight loading, the H / R h value of 1.5 for the 0.3% Rh/La~O:~ suggests that either the stoichiometry for hydrogen adsorption in this system is greater than unity [34] or that a significant portion of the adsorbed hydrogen is being spilled over onto the decorating species and/or the support. Increasing the weight loading would be expected to result in a decrease in dispersion. The decline in the H / R h ratio with increasing weight loading is consistent with such an observation. However, increased decoration and hydrogen spillover with increased rhodium weight loading have also been reported for La20:~-supported metals [9,11,12]. As a result, the rhodium dispersion effects cannot be separated from possible changes in the decorative layer that may have also taken place as the rhodium content changed. X-ray diffraction results indicated that average particle size was less than 4.0 nm (dispersion > ca. 30% ) in all cases. The increase in hydrogen uptake with increasing reduction temperature between 200 ° C and 400 ° C is consistent with previously reported results [ 14 ]. As

10

reported there, a 200 °C reduction was sufficient to fully reduce rhodium species to Rh ° as indicated by XPS results. An increased hydrogen uptake concurrent with a decreased carbon monoxide uptake and rhodium dispersion was observed in that study as reduction temperature was raised. That suggested that the extent of decoration and its ability to accept spillover hydrogen was also increased, possibly associated with the decomposition of La2 (C03)3 [ 14,3032 ] and dehydration of La (OH) 3 to LaOOH and La203 [ 11,14,30-33 ] which also occur in that temperature range. Dehydration is only completed at higher temperatures prompting the study of higher reduction temperatures. The decline in the H / R h ratio for catalysts reduced at 500 °C and 600 °C may be due to a decline in rhodium dispersion expected from the higher temperature treatment. However, this effect cannot be separated from changes in the over layer which could affect the extent of hydrogen spillover. Again, XRD results indicated that average particle size was less than 4.0 nm (dispersion > ca. 30% ) for all of these catalysts. In addressing the results of the hydrogenolysis reactions, both decorative and dispersion effects must be considered. For the structure-sensitive ethane hydrogenolysis reaction, an increase by a factor of 20 has been observed in rate per m 2 rhodium as rhodium dispersion was increased from 0.4% to 94% [15]. For supported rhodium, decoration of the active rhodium surface by small amounts of an inert species resulted in a dramatic decline in ethane hydrogenolysis activity [ 29 ], followed by a more gradual decline as additional inert species was added. This type of an effect is attributable to break-up of the relatively large ensembles required as reaction sites for this reaction (16,21). Results for the reaction of cyclopropane with hydrogen over supported rhodium [29] indicate that this reaction is structure-insensitive. A four-fold increase in rhodium dispersion resulted in a factor of four increase in percent conversion, which would suggest that activity vs. dispersion on a TOF basis is invariant. Decoration by inert species had little effect on TOF until very high amounts were added. This parallels the observations of the effect of decoration on other structure-insensitive reactions [16,21], where high levels of decoration were required to significantly suppress activity due to the small ensembles required for such reactions. A previous study considered the effect of reduction temperature up to 400 ° C [ 14]. In the present case, the temperature range was extended to 500 °C and 600 °C to insure complete conversion of hydroxide support species to La203 [31-33 ]. The large decline in cyclopropane dehydrogenation activity on both a rate per gram rhodium and TOF basis between reduction temperatures of 200 and 400 °C is consistent with earlier observations [14]. It should be noted that both measures of activity suffer from some limitations. Rate per gram rhodium does not account for changes in rhodium dispersion, which is expected to decline with increasing reduction temperature. However, the XRD results indicate that dispersion was greater than ca. 30% in all cases here, so only a maximum factor of 3 to 4 variation in dispersion was possible. The presence

11

of hydrogen spillover would result in an overestimation of rhodium surface area and an underestimation of TOF. This is further complicated by the observation that hydrogen spillover may increase by over a factor of 2 as reduction temperature increases from 200 ° C and 400 ° C [ 14 ]. In either case though, the magnitude of the decline in cyclopropane activity was larger than can be accounted for by these effects and suggests an increase in extent of decoration. Rate per gram rhodium, TOF, and hydrogen uptake all fell moderately as reduction temperature was increased further to 500°C and 600 °C. Given the uncertainties involved with each of these methods of comparison, these higher reduction temperatures are concluded to have had only minor effect on decoration, if any at all. For ethane hydrogenolysis, the small drop in rate per gram rhodium and TOF for a catalyst reduced at 400°C vs. 200°C is in contrast to the earlier study where no effect of reduction temperature was observed. This could be due to differences in weight loading or to minor variations in preparation between the two studies. It should be noted that the 200 ° C-reduced catalyst had to be run at 275-315 ° C, well above its reduction temperature, in order to observe any activity. The lack of any significant effect of reduction temperature on ethane hydrogenolysis suggests that decoration after even a 275 °C reduction effectively breaks up the ensembies required for this reaction. The large suppression of activity over Rh/La20~ for a reaction requiring a small ensemble as well as one requiring a large ensemble relative to Rh/Si02 is indicative of the extensive decoration that occurs in this system. The fact that activity is greater t h a n over the blank La20.~ indicates that a few unaffected sites remain even after extensive decoration, as has been observed elsewhere for Rh/TiO2 in the SMSI state [35,36]. The effect of reduction temperature on ethane hydrogenolysis is much less t h a n the orders-of-magnitude decline observed in SMSI catalysts [18-20]. This is due to the fact that on La2Q there is a high suppression of rate even after low temperature treatment in hydrogen. Comparing the cyclopropane hydrogenation results as a function of weight loading, activity declined moderately on both a rate per gram rhodium and TOF basis. The decline in rate per gram rhodium is attributable to a loss in rhodium surface area. This in turn may be due to the decrease in dispersion expected upon increasing weight loading a n d / o r to an increase in the extent of decoration with increasing weight loading that has been observed elsewhere [9,11,12]. For structure-insensitive reactions, a decline in TOF is observed at very high extents of decoration where the active component is greatly displaced and the small ensembles for these reactions are broken up. However, the consideration of TOF is again complicated by hydrogen spillover, which has been observed to increase with increasing weight loading [9] for this system. Also to be considered is the possible overestimation of rhodium surface area for the 0.3% Rh/La203 due to hydrogen adsorption stoichiometry greater t h a n unity.

12

Correcting for the spillover effect would tend to increase the TOF for the higher weight loading catalysts relative to those of lower weight loading. Accounting for the adsorption stoichiometry of the lowest weight loading catalyst would tend to increase its TOF relative to the other catalysts. As a result, while a decline in cyclopropane activity due to increased decoration of rhodium with increasing weight loading may be indicated, the effect, if any, appears to be small. This could be due to the 500 °C reduction temperature employed with these catalysts. As noted above, reduction temperatures in this range resulted in highly suppressed activity for cyclopropane hydrogenation. Similar considerations complicate the interpretation of the ethane hydrogenolysis results. Here again, little effect of weight loading is observed, indicating extensive break-up of ensembles in all cases. An order-of-magnitude increase in cyclopropane hydrogenation specific rate and a factor of 4 increase in ethane hydrogenolysis specific rate was observed after a 90 min treatment in oxygen at 400 °C followed by reduction at 200 °C for a sample originally reduced at 500 ° C. It should be noted that as with the 200 °C-reduced catalyst, reaction temperatures greater than 275 °C were required to observe ethane hydrogenolysis activity over the oxygen-treated, 200 ° C-rereduced catalyst. In addition, the condition of the catalyst following a 90 min treatment in oxygen may not represent the final state that might be attained after longer treatment times. Oxidation/low temperature reduction cycles have been shown to reverse metal decoration in SMSI catalysts [ 19,35 ] restoring the orders-of-magnitude suppression of alkane hydrogenolysis activity observed after high temperature reduction. For the non-interacting Rh/SiO2, similar treatments have resulted in an increase of over one order of magnitude for alkane hydrogenolysis [35,37,38] attributed to a roughening and redispersion of metal crystallites. In the present case, the increase in rates suggests an increase in metal sites. This could be due to a decrease in the extent of rhodium decoration without a change in rhodium dispersion, an increase in rhodium surface area through redispersion or roughening without a change in extent of decoration, or the simultaneous decrease in extent of decoration with an increase in rhodium surface. A fourth alternative would be the creation of clean rhodium surface similar to that in Rh/SiO2, perhaps via a fracturing of the original particles. These alternatives are depicted schematically in Fig. 3. Any increase in exposed rhodium would be expected to have a greater effect on cyclopropane hydrogenation than on the ethane hydrogenolysis because of the smaller ensemble size required to constitute a reaction site for the former reaction. An increase in rhodium dispersion without a change in extent of decoration cannot account for the entire effect observed. Since XRD indicated dispersions> 30%, an increase of only 3 to 4 in dispersion is possible. For the structure-insensitive cyclopropane reaction, this would have resulted in a factor of 3 to 4 increase in rate per gram rhodium, while over an order of magnitude

13

---~



(b/

I

j "'c) i

,'- (d)

:

t

i

Fig. 3. Schematic depicting the impact of an oxidation/low temperature reduction cycle on 1.5% Rh/La20:~ originally reduced at 500 c:C: (a) decrease in extent of decoration; (b) redispersion of rhodium and/or roughening of surface; ( c ) combination of (a) and (b); (d) fracturing of original particles.

increase was observed. The creation of clean, Rh/SiO2-1ike surfaces either through reversal of decoration or by fracturing of the metal particles cannot account for the observed results either. Rh/SiO2 had a cyclopropane hydrogenolysis rate of ca. 4100 mol s - 1 g - 1 rhodium at 160 ° C and an ethane hydrogenolysis rate of ca. 1000 mol s - 1g - 1 rhodium at 300 ° C based on extrapolation of previously reported Arrhenius data [14]. In order to achieve the 26.1 mol s I g- 1 rhodium ethane hydrogenolysis rate observed after treatment, the formation of nearly 2% Rh/SiO2-1ike surface would have been required. This surface would be expected to exhibit a cyclopropane hydrogenation activity of nearly 80 mol s - ~ g - ~ rhodium, almost an order-of -magnitude higher than the actual result. This suggests that the effect is due to a combination of opening up free rhodium surface, probably from reversal of decoration, and a roughening or redispersion of rhodium. It is interesting to note that the specific rate for ethane hydrogenolysis following oxidation treatment was above the level for both the 500 ° C-reduced and the 200 ° C-reduced catalysts while the cyclopropane rate after treatment was between those observed for the two reduction temperatures. For the structure-insensitive cyclopropane hydrogenation, the increase in specific rate resulted from the additional rhodium surface exposed by reversal of the extent of decoration and possible redispersion after oxidation treatment. The additional increase in ethane hydrogenolysis rate above the level for a 200 ° C-reduced catalyst may be due to the higher activity of this

14

structure-sensitive reaction on smaller particles which result from redispersion and roughening after the oxidation treatment. Varying the method of precursor decomposition had essentially no effect on the rate of cyclopropane hydrogenation. This suggests that as long as the final reduction temperature remains the same, equivalent decoration results independent of whether calcination or reduction is used to decompose the precursors. SUMMARY

Because of their sensitivity to decorative effects, ethane hydrogenolysis and cyclopropane hydrogenation have been used to probe preparation and pretreatment impacts on the metal-support interaction in Rh/La203. Decoration of the rhodium crystallites appears to be extensively based on the large suppression of activity for both reactions relative to previous observations for Rh/SiO2. For the level of extensive decoration which existed in Rh/La203, cyclopropane hydrogenation proved to be more sensitive to changes in decoration with preparation/pretreatment method than ethane hydrogenolysis due to the smaller ensemble required for the former reaction. Because of limitations in determining free rhodium surface area, the extent of decorative modification could only be qualitatively determined. Increasing reduction temperature between 200 and 600°C appeared to increase the extent of decoration with the major impact occurring between reduction temperatures of 200 to 400 ° C. This is possibly associated with decomposition of La2 (CO3) 3 and/or dehydration of La (OH) 3 known to occur in this temperature range. The 500 °C reduction undergone by the catalysts having different weight loadings resulted in extensive decoration in all cases. As a result, only a slight increase in decoration with increasing weight loading may be inferred. Treatment of a 500 ° C-reduced sample in oxygen at 400 ° C followed by a 200 °C reduction appears to have partially reversed the decoration and may also have resulted in increased rhodium dispersion. Precursor decomposition via calcination or reduction resulted in equivalent extents of decoration as long as each catalyst was exposed to the same final reduction temperature. ACKNOWLEDGEMENTS

The authors would like to acknowledge financial support for this work by NSF (grant no. CBT-8715541 ). Experimental assistance by Ms. Cheryl Gloor and Ms. Judy Matesa and helpful discussions with Dr. Rachid Oukaci are gratefully acknowledged.

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