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A TPR and TPO study of bimetallic Ru-Au catalysts
Journal
of Molecular
Catalysis,
25 (1984)
351
- 366
351
A TPR AND TPO STUDY OF BIMETALLIC G. R. TAUSZIK*, Montedipe
Ru--Au
CATALYSTS
G. LEOFANTI
S.p.A.,
Research
Center
of
Bollate,
Via
San Pietro
50,
20021
Bollate
MI
Lucia
39,
(Italy)
and S. GALVAGNO C.N.R., Institute for Energy 98013 Pistunina ME (Italy)
Transformation
and
Storage,
Via Salita
Santa
Summary Temperature-programmed reduction (TPR) and oxidation (TPO) have been used to study a series of bimetallic Ru-Au catalysts supported on MgO and SiOZ. Metal-metal and metal-support interactions have been found which modify the nature of the species formed after impregnation and drying, and the oxidizability after reduction. The presence of gold in the MgO-supported catalysts lowers the reducibility of Ru, so that metallic gold can form before Ru is reduced to a significant extent. On silica, however, the reduction of both metals occurs in the same temperature. range. These findings support the previous hypotheses concerning the genesis of the Ru-Au bimetallic particles/aggregates and the resulting surface composition.
Introduction Supported Ru-Au catalysts have attracted our attention for several reasons. First of all, these metals are almost immiscible in the bulk state, so that it appeared of interest to inquire into the possibility of their interaction in the dispersed state, in analogy to the Sinfelt Ru-Cu and Os-Cu catalysts [ 1 - 41. Furthermore, gold is peculiar for being the most noble metal in the bulk state, while supported gold catalysts can display an activity comparable to that of platinum [5]. Finally, metal-support interactions were found in both gold [6 - 91 and ruthenium [ 10,111 monometallic catalysts. These factors prompted an investigation on a series of Ru-Au samples supported on MgO and SiOZ, which are expected to yield metal-support interactions of different strengths. The oxygen exchange between 12C0 and the hydrogenation of cyclopropane, the hydrogenolysis of ethane ‘4C02) and propane, and several physicochemical techniques were used to characterize the samples [ 12 - 161. On both supports, a Ru-Au interaction was *Author to whom correspondence 0304-5102/84/$3.00
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
358
detected, probably in the form of bimetallic particles and/or aggregates. Contrary to the general tendency of the Group Ib metals to cover the Group VIII metals, an unexpected Ru surface enrichment was found in the magnesia-supported samples, while on silica the surface composition was close to that of the bulk. These results were tentatively correlated to the way in which the mono- and bimetallic particles form and grow on MgO and, respectively, on SiO,, during the reduction by hydrogen [ 161. Nevertheless, most of the data concerning these catalysts were collected either before or after the reduction treatment [12 - 161. More recently, we have performed temperature-programmed experiments in order to investigate the critical phenomena which occur during the reduction step. An initial study by thermal analysis pointed out the great chemical and morphological modifications which take place on the MgO support when the catalysts are heated up to the temperature used for the reduction (673 K) [17]. This paper reports the results of temperature-programmed reduction (TPR) [18] and temperature-programmed oxidation (TPO) experiments, which were performed on the MgO- and Si02-supported samples, in order to obtain information on the chemical properties of Au and Ru during and after the catalyst reduction.
Experimental The catalysts studied were prepared by co-impregnation of the supports (MgO, Carlo Erba RPE-ACS, or SiOz, Davison 951) with aqueous solutions of RuC1,*HzO and HAuCl,.3H,O. These solids were dried at 383 K. Further details on the preparation are reported in refs. 14 and 16. The dried catalysts were submitted to the following experimental sequence: a first TPR (TPR-l), cooling, TPO, cooling, and a second TPR (TPR-2). The temperature range explored (from ambient to 775 K) was approximately the same used in previous work [12 - 161 for the reduction of the dried catalysts. The experiments were performed in a typical gas chromatographic apparatus at a heating rate of 20 K min-‘. The following mixtures of purified gases were used: Hz (36 vol.%) in Ar for TPR; O2 (20 vol.%) in He for TPO. A cold trap and a tube filled with supported NaOH (Merck) were used to block water and, respectively, CO, released from the samples [ 171. The calibration of the consumption of hydrogen during TPR was accomplished by injections of known amounts of hydrogen in a flow of Ar + H, as well as by the reduction of known amounts of pure oxides. This procedure was not repeated for TPO, as the oxygen uptakes could be indirectly estimated from the consumption of Hz during TPR-2.
Results and discussion The results of TPR and TPO experiments carried out on the Ru-Au/ MgO samples are reported in Table 1. The Ru-richest samples RlOO and
359 TABLE 1 Approximate peak temperature (T,) MgO catalysts Sample
Ru
Rua
and consumption
Aua
Ru + Au (at.%)
of hydrogen (CH) for the Ru-Au/
TPR-1
TPO
TPR-2
2)
2)
590 740 595 735 580 575 n.d. n.d.
495
0.8
485
0.5
485 495 n.d. n.d.
0.2 0.1 -
CHb 2)
RlOO
100
0.45
-
R089
89
0.34
0.04
R064 R047 ROlO ROOO
64 47 10 0
0.21 0.12 0.03 -
0.12 0.14 0.23 0.17
425,450 600, 675 430,470 615,675 700 705 675 680
0.6 2.8 0.6 5.0 2.8 1.6 0.3 0.3
CHb
ag-at metal (g cat)-’ X 103. bg-at H (g cat)-’ x 103. n.d. = not detected
300
500
Fig. 1. Some examples of TPR-1 spectra: A, Ru-Au/MgO; magnified by a factor of 2 with respect to those of A).
B, Ru-Au/SiOz
(spectra
R089 (i.e. the samples with Ru/(Ru + Au) = 100 and, respectively, 89 at.% were characterized by complex TPR-1 spectra (Fig. 1A). Below 500 K, a medium-in~nsity peak was preceded by a very weak signal, which suggests
360
I
I
300
500
700
K
Fig. 2. TPR-1 spectra of pure Ru compounds and of a physical mixture of RuOz with a MgO reference sample (see text). the presence of more than one easily reducible species, differing either in the chemical nature or in the strength of interaction with the support. Under our experimental conditions, pure RuCl,-Hz0 was reduced at ca. 525
K and pure RuO, at ca. 475 K (Fig. 2). Considering that physical changes could shift the reduction temperature in the supported samples, it seems reasonable to attribute the reduction signals found below 500 K to compounds deriving from the hydrolysis of the precursor chloride, such as RuO, and/or ruthenium hydroxide(s). This assignment is consistent with that proposed for a Ru/MgO sample prepared starting from a different precursor compound [19,20]. The intensity of the signals below 500 K was too low to account for the reduction of all the Ru present. On the contrary, the intensity of the strong overlapped peaks above 500 K, which varied widely from sample to sample, was too high with respect to the metal contained in the catalysts. The signal intensity was not increased by the presence of H,O and CO2 released from the samples [ 171. In fact, experiments carried out on certain MgO reference samples (MgO impregnated with Hz0 or dilute HCl), which released Hz0 and CO* in amounts quite comparable with those of the catalysts [17], gave flat TPR spectra. Furthermore, very strong TPR signals in the 575 - 725 K region were recorded when the MgO reference samples were just physically mixed with a Pt/A120s catalyst or with unsupported RuO, (Fig. 2). In the latter case, the high temperature peaks were even stronger and were preceded by the reduction signal of the oxide. All this suggests that Ru reduced below 500 K can cause hydrogen spillover to the MgO support. Spillover phenomena from Pt/A1203 to MgO have already been described [21]. As a complete reduction of ruthenium appeared possible after treatment with hydrogen at 673 K [12,14], it is very likely that the proposed spillover is accompanied by the reduction of some remaining ruthenium compounds. In any case, the fact that the reduction of Ru cannot be completed at the reduction temperature of RuCl,
361
indicates that a strong interaction exists between the support and some Ru compounds. Just one strong peak at co. 700 K was found in the TPR-1 spectra of the samples with Ru/(Ru + Au) - 50 at.%, namely R064 and R047 (Fig. 1A). The absence of a significant reduction signal below 500 K, like those found for RlOO and R089, cannot be attributed to the lower absolute concentration of Ru: in fact, a signal at 465 K was very evident in the spectrum of a Ru/MgO sample containing less Ru than R064. It must be therefore concluded that the presence of HAuCl, in the impregnation solution significantly modifies the nature of the Ru compounds which are formed in the dried catalysts. This confirms through direct evidence a suggestion previously inferred from the diffuse reflectance spectra of these samples [ 141. The intensity of the high temperature signal seems excessive with respect to the amount of Ru and Au. It is therefore likely that in these samples there also was a spillover of hydrogen from the metal(s) to the support. The structure of the TPR-1 spectrum in the high temperature region was much simpler for these catalysts than for those at higher Ru/ (Ru + Au) ratios. Probably the spillover phenomena, and consequently the structure of the TPR-1 spectrum, can change depending on whether or not some Ru was already reduced at lower temperature. The existence of great HZ consumptions due to spillover makes it difficult to detect the reduction of Ru and Au: the possible signals could coincide with that due to spillover or be lost in the large low-temperature tail which characterizes the peaks recorded for R064 and R047. In any case, the colour changes of the samples and the absorption of oxygen in the subsequent TPO experiments indicate that at least part of the metal content was reduced during TPR-1. Moreover, the TPR-1 spectra of Au/MgO (ROOO) and of a bimetallic sample with 90 at.% Au (ROlO) exhibited a signal only at high temperature (cu. 680 K, Fig. 1A). Unlike those found for the previously mentioned samples, these signals were low and did not account for all metal(s) present. It seems therefore unlikely they could be due to spillover. This would indicate that large spillover phenomena can be originated by Ru but not by Au. More probably, the 680 K signals correspond to some gold compound which was made difficult to reduce owing to a strong interaction with the support. However, this does not mean that metallic gold can form in these samples only at high temperature. In a previous study [ 141, small amounts of metallic gold were found in Au/MgO even after drying at 383 K. In fact, immediately after drying, the sample turns yellow, but after some days a violet nuance appears, indicating that metallic gold forms by photodecomposition of gold compounds. Therefore, for this work, we have used a freshly-prepared Au/MgO catalyst and have followed the effect of heating not only by TPR, but also through the colour changes of the solid. The violet nuance first appears between 423 and 473 K, even in inert gas. Furthermore, in the diffuse reflectance spectrum of the Au/MgO sample heated at 473 K it was already possible to distinguish a weak band at cu. 540 nm, which is characteristic of finely dispersed metallic Au on MgO [ 141. This
indicates that some of the gold compounds formed after impregnation and drying can yield the metal by thermal decomposition, without any significant absorption of hydrogen. The TPO spectra of RlOO and R089 (which were run immediately after TPR-1) showed two broad weak signals, at cu. 590 and 740 K (Fig. 3A) respectively. Just one, even weaker, TPO signal at cu. 580 K could be detected for R064 and R047. Thus, two distinct oxidation processes characterized the samples richest in Ru, while only the lower temperature process occurred in the samples with Ru/(Ru + Au) - 50 at.%. In this connection, it may be of interest to recall that, according to the EXAFS analysis performed on the catalysts reduced by Hz at 673 K [ 141, Ru was still more oxidized in R064 than in RlOO. It is possible that the behaviour during TPO is correlated to the different oxidation states existing in the catalyst after reduction. In any case, these TPO results once more stress that the properties of the bimetallic Ru-Au/MgO catalysts are not purely additive with respect to those of the monometallic samples. No TPO signal was found for the catalysts ROlO and ROOO which are richest in Au. This is consistent with the low concentration of Ru in ROlO and with the absence of a bulk oxidation of Au particles. Oxygen adsorption on the Au surface occurs above 473 K [22], but the uptake expected for these samples [ 141 is too low to be detected by TPO. Similarly, no signal was found when ROlO and ROOO were submitted to a second temperature-programmed reduction (TPR-2), performed immediately after TPO. All the other samples yielded just one reduction signal at ca. 490 K, which can be reasonably attributed to RuO*. Assuming this stoichiometry and keeping in mind that the metal dispersion was low in these samples [ 141, it can be concluded from the intensities of the TPR-2 signals that oxidation during TPO and subsequent reduction were not restricted to the surface of the metal particles, but did not actually involve all the ruthenium contained in the catalysts. This suggests that, in the same sample, there are particles with a different oxidizability, possibly as a consequence of differences in size, metal-support interaction and/or average oxidation state. In fact, for RlOO and R089 there is an apparent correspondence between the consumption of hydrogen during TPR-2 and that measured in the low-temperature portion of TPR-1, which could indicate that the Ru particles oxidized during TPO and then reduced during TPR-2 are mainly those formed below 500 K during TPR-1. Furthermore, a heterogeneity of the metal particles of RlOO and R089 with respect to oxidation could account for the existence of two oxidation processes yielding the same product. No significant spillover of hydrogen from the metal to the support was found during TPR-2. This suggests that a tight connection exists between the spillover and the other phenomena which occur on MgO during heating, i.e. decomposition of hydrates and carbonates and an increase in surface area. Even if the nature of the sites on which hydrogen is adsorbed under our conditions is not clear (cf. ref. 23), it seems quite possible that
363 TABLE
2
Approximate peak temperature SiOz catalysts Sample
Ru __-
Rua
( Tp) and consumption
Aua
Ru + Au (at.%)
of hydrogen
TPR-1
TPO CHb
TKP)
RSlOO
100
0.38
-
RS091
91
0.33
0.03
RS062
62
0.18
0.12
RS048
48
0.16
0.18
RS014 RSOOO
14 0
0.04
0.24 0.24
445 > 525 455 > 535 455, > 560 455, 630, 470, 480
(CH)
for the Ru-Au/
TPR-2 CHb
T, (R)
490 495 700 510
1.4 0.4 1.6 1.0 0.9 0.5 0.8 1.0 0.5 0.2
455
470
1.3
460
470
1.4
460
470
0.8
465
475
0.6
530 660 770
470 V.W.
0.1
ag-at metal (g cat)-’ X 103. bg-at H (g cat)-’ x 103. V.W. = very weak
the spillover can take place only on the fresh surface developed by thermal decomposition. Obviously, this was the case for TPR-1, but not for TPR-2. Only when RlOO and R064 were exposed to the air after TPO, which allowed a partial re-hydration of the support, did their TPR-2 also exhibit a signal at cu. 570 K, possibly due to spillover. The TPR and TPO spectra of the SiO+upported samples (Table 2) were significantly different than those of the corresponding MgO-supported catalysts. The TPR-1 of RSlOO (where the number 100 indicates the Ru/ (Ru + Au) atomic percentage) was characterized by a strong peak at cu. 445 K, followed by a broad weaker signal above 520 K (Fig. 1B). A similar spectrum has already been reported for Ru/SiO, [ 191; the high temperature signal has been correlated to the presence of chloride ions from the precursor RuCl,. The 445 K peak, which was also found for Ru/MgO, is probably due to ruthenium hydroxide(s) or to similar products of the hydrolysis of RuCls. The Au/SiOz RSOOO sample gave only a weak TPR-1 signal at cu. 480 K (Fig. 1B). It happens that for Au/MgO, metallic gold can likewise form by thermal decomposition without consumption of hydrogen: in fact, upon heating RSOOO in inert gas, it was possible to detect the colour changes due to the formation of the metal between 375 and 425 K. Among the bimetallic samples, RS091 gave a TPR-1 spectrum very similar to that of RSlOO. Significantly different spectra were achieved for RS062 and RS048: the peak at cc. 455 K was followed respectively by a shoulder or by a second peak at cu. 490 K (Fig. 1B). At the same time, the intensity in the 700 K region increased, resulting in a very evident signal for RS048. This spectral trend did not continue when the gold content was
364
further increased: the TPR-1 of RS014 was like that of RSOOO, except for the presence of a second weak peak (Fig. 1B). Therefore, it seems unlikely that the peak at cu. 490 K, which becomes gradually evident for RS062 and RS048, is simply a ‘gold signal’ superimposed on a ‘ruthenium spectrum’ like that of RSlOO. It is instead suggested that the spectral features of RS062 and RS048 could be due to species deriving from an interaction between Ru and Au compounds, which is evident from TPR-1 only when the elements are present in comparable amounts. The TPR-1 spectra showed that the fraction of Ru reduced below 500 K was significantly higher in the SiO,-supported samples than in those on MgO. Also, the weak TPR-1 signal of Au occurred at a much lower temperature on SiO,. These facts show that the compounds formed after impregnation and drying are stabilized by an interaction with MgO, which does not exist or is much weaker on SiOZ. Some of the TPO spectra of the Ru-Au/SiOz catalysts are reported in Fig. 3B. Unexpectedly, two TPO signals were found even for the Au/SiOz RSOOO sample. The intensity of these signals was comparable to those of the other samples and cannot be attributed to the sole chemisorption of oxygen: in fact, RSOOO has a gold dispersion [16] even lower than that of the Au/MgO catalyst [ 141, which gave no TPO signal. A tentative explanation is that oxygen can dissolve at high temperature into the Au/SiO, particles, at least to a depth of some monolayers from the surface. The TPO spectra of RSlOO and RS091 showed just one peak at cu. 455 K, i.e. at a significantly lower temperature with respect to the MgO-supported catalysts. In the bimetallic samples with a lower Ru/(Ru + Au) atomic fraction, this peak was gradually replaced by a weak signal at cu. 530 K. The presence of this signal, which is not typical either of Ru/SiOz or of Au/SiO*, confirms that a metal-metal interaction exists in the reduced samples. Over the entire composition range, the above TPO spectra were significantly different in shape and intensity than those of the Ru--Au/MgO samples (Fig. 3), which shows that also the behaviour with respect to oxidation is strongly influenced by the nature of the support. This appears also from the amounts of H2 consumed during TPR-2, which were always higher than those of the corresponding MgO samples (cfi Tables 1 and 2). Therefore, the Ru particles formed during TPR-1 are oxidized more easily and to a greater extent on SiO, than on MgO, even if the product formed during TPO probably is still RuO, (as shown by the sharp TPR-2 peaks at 470 K). Except for RS014, where the concentration of Ru is very low, the intensity of the TPR-2 peaks was very consistent with those of the respective TPR-1 peaks at cu. 450 K. Therefore, as on MgO, the fraction of Ru oxidized during TPO and then reduced during TPR-2 seems to originate mainly from those compounds which are easier to reduce during TPR-1. Only a very weak, broad signal resulted in the TPR-2 of RSOOO between 340 and 440 K: it is possible that, during the replacement of the He + O2 mixture with Ar + H,, oxygen was released without consumption of H2 or the reaction with H, had already occurred at room temperature and was therefore not recorded.
365
RSOOO
Fig, 3. Some examples of TPO spectra: A, Ru-AuiMgU of 5 with respect to those of B); B, Ru-Au/SEO~,
(spectril magnified by a factor
Conclusions TPR and TPU studies have added further interesting details to the ~formation available on bimetallic Ru-Au catalysts. The reducibility of the compounds formed after drying changes sign~i~~tly on going from the MgO to the SiO, support and from the mono- to bimetallic samples. For the catalysts supported on MgO, it has been shown that metallic gold forms even at low temperature, while the reduction of Ru is shifted to higher temperatures by the presence of Au. These findings strengthen the proposed model t16], according to which Au particles formed at low temperature can act as nucleation centers for Ru, thus yielding bimetallic particles with a Ru surface enrichment. In the SiUrsupported samples, instead, gold and most of the ruthenium can both be reduced at low temperature. This, together with the easier sintering of Au on silica, is consistent with the proposed formation of random aggregates of Ru and Au particles [16 3. This way, the surface composition would not be controlled by the thermodynamic equilibrium, according to which an Au enrichment was expected, but by the mechanism by which these particles form during the catalyst preparation, The behaviour of the reduced catalysts during the oxidation experiments was significantly affected by the nature of the support. Re-oxidation was easier on silica and involved a greater fraction of the metal. Also, the addition of gold to Ru/MgO and to Ru/SiO, modified the TPO spectra, but the trends observed when increasing the Au fraction were different. This is evidence of the differences in the Ru-Au-support interactions existing between the l&C- and the SX&supported catalysts.
366
Acknowledgment The experimental work of Mr. G. Macario is gratefully acknowledged.
References 1 J. H, Sinfelt, J. Cat& 29 (1973) 308. 2 J. H. Sinfelt, Y. L. Lam, J. A. Cusumano and A. E. Barnett, J. Catal., 42 (1976) 227. 3 J. H. Sinfelt and J. A. Cusumano, in J. J. Burton and R. L. Garten (eds.), Advanced Materials in Catalysis, Academic Press, New York, 1977. 4 C. R. Helms and J. H. Sinfelt, Surf Sci., 72 (1978) 229. 5 D. Y. Cha and G. Parravano, J. Catal., 18 (1970) 200. 6 I. W. Bassi, F. W. Lytle and G. Parravano, J. Cotal., 42 (1976) 139. 7 S. Galvagno and G. Parravano, J. Cat& 55 (1978) 178. 8 G. Cocco, S. Enzo, G. Fagherazzi, L. Schiffini, I. W. Bassi, G. Vlaic, S. Gaivagno and G. Parravano, J. Phys. Chem., 83 (1979) 2527. 9 J. Schwank, S. Galvagno and G. Parravano, J. Catal., 63 (1980) 415. 19 A. Bossi, F. Garbassi, A. Orlandi, G. Petrini and L. Zanderighi, in B. Delmon, P. Grange, P. Jacobs and G. Poncelet (eds.), Preparation of Catalysts, Vol. 2, Elsevier, Amsterdam, 1979, p. 405. 11 A. Bossi, F. Garbassi, G, Petrini and L. Zanderighi, J. Chem. Sot., Faraday Trans. 1, 78 (1982) 1029. 12 J. Schwank, G. Parravano and H. L. Gruber, J. CataL, 61 (1980) 19. 13 S. Galvagno, J. Schwank and G. Parravano, J, Cotaf., 61 (1980) 223. 14 1. W. Bassi, F. Garbassi, G. Vlaie, A. Marzi, G. R. Tauszik, G. Cocco, S. Galvagno and G. Parravano, J, Catal., 64 (1980) 405. 15 G. R. Tauszik, F. Garbassi and A. Marzi, Gazz. Chim. Ital., 110 (1980) 443. 16 S. Galvagno, J. Schwank, G. Parravano, F. Garbassi, A. Marzi and G. R. Tauszik, J. C&i., 69 (1981) 283. 17 G. Leofanti, M. Solari, G. R. Tauszik, F. Garbassi, S. Galvagno and J. Schwank, Appt. Catal., 3 (1982) 131. 18 N. W. Hurst, S. J. Gentry, A. Jones and B. D. McNicol, Catal. Rev. Sci Eng., 24 (1982) 233, 19 R. Bert&, A. Boasi, F. Garbassi and G, Petrini, in B. Miller (ed,), Proc. 7th Int. CO& on Thermal Analysis, Vol. 2, Wiley, New York, 1982, p, 1224. 20 A. Bossi, A. Cattalani, F. Garbassi, G. Petrini and L. Zanderighi, J. Thermal Anal., 26 (1983) 81. 21 M. Lacroix, G. Pajonk and S. J. Teichner, React. Kinet. Catal. Lett., 12 (1979) 369, 22 T. Fukushima, S. Galvagno and G. Parravano, J. Catal., 57 (1979) 177. 23 S. Coluccia and A. J. Tenth, in T. Seyama and K. Tanabe (eds.), New Horizons in Catalysis, Elsevier, Amsterdam, 1981, p. 1154.
of Molecular
Catalysis,
25 (1984)
351
- 366
351
A TPR AND TPO STUDY OF BIMETALLIC G. R. TAUSZIK*, Montedipe
Ru--Au
CATALYSTS
G. LEOFANTI
S.p.A.,
Research
Center
of
Bollate,
Via
San Pietro
50,
20021
Bollate
MI
Lucia
39,
(Italy)
and S. GALVAGNO C.N.R., Institute for Energy 98013 Pistunina ME (Italy)
Transformation
and
Storage,
Via Salita
Santa
Summary Temperature-programmed reduction (TPR) and oxidation (TPO) have been used to study a series of bimetallic Ru-Au catalysts supported on MgO and SiOZ. Metal-metal and metal-support interactions have been found which modify the nature of the species formed after impregnation and drying, and the oxidizability after reduction. The presence of gold in the MgO-supported catalysts lowers the reducibility of Ru, so that metallic gold can form before Ru is reduced to a significant extent. On silica, however, the reduction of both metals occurs in the same temperature. range. These findings support the previous hypotheses concerning the genesis of the Ru-Au bimetallic particles/aggregates and the resulting surface composition.
Introduction Supported Ru-Au catalysts have attracted our attention for several reasons. First of all, these metals are almost immiscible in the bulk state, so that it appeared of interest to inquire into the possibility of their interaction in the dispersed state, in analogy to the Sinfelt Ru-Cu and Os-Cu catalysts [ 1 - 41. Furthermore, gold is peculiar for being the most noble metal in the bulk state, while supported gold catalysts can display an activity comparable to that of platinum [5]. Finally, metal-support interactions were found in both gold [6 - 91 and ruthenium [ 10,111 monometallic catalysts. These factors prompted an investigation on a series of Ru-Au samples supported on MgO and SiOZ, which are expected to yield metal-support interactions of different strengths. The oxygen exchange between 12C0 and the hydrogenation of cyclopropane, the hydrogenolysis of ethane ‘4C02) and propane, and several physicochemical techniques were used to characterize the samples [ 12 - 161. On both supports, a Ru-Au interaction was *Author to whom correspondence 0304-5102/84/$3.00
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
358
detected, probably in the form of bimetallic particles and/or aggregates. Contrary to the general tendency of the Group Ib metals to cover the Group VIII metals, an unexpected Ru surface enrichment was found in the magnesia-supported samples, while on silica the surface composition was close to that of the bulk. These results were tentatively correlated to the way in which the mono- and bimetallic particles form and grow on MgO and, respectively, on SiO,, during the reduction by hydrogen [ 161. Nevertheless, most of the data concerning these catalysts were collected either before or after the reduction treatment [12 - 161. More recently, we have performed temperature-programmed experiments in order to investigate the critical phenomena which occur during the reduction step. An initial study by thermal analysis pointed out the great chemical and morphological modifications which take place on the MgO support when the catalysts are heated up to the temperature used for the reduction (673 K) [17]. This paper reports the results of temperature-programmed reduction (TPR) [18] and temperature-programmed oxidation (TPO) experiments, which were performed on the MgO- and Si02-supported samples, in order to obtain information on the chemical properties of Au and Ru during and after the catalyst reduction.
Experimental The catalysts studied were prepared by co-impregnation of the supports (MgO, Carlo Erba RPE-ACS, or SiOz, Davison 951) with aqueous solutions of RuC1,*HzO and HAuCl,.3H,O. These solids were dried at 383 K. Further details on the preparation are reported in refs. 14 and 16. The dried catalysts were submitted to the following experimental sequence: a first TPR (TPR-l), cooling, TPO, cooling, and a second TPR (TPR-2). The temperature range explored (from ambient to 775 K) was approximately the same used in previous work [12 - 161 for the reduction of the dried catalysts. The experiments were performed in a typical gas chromatographic apparatus at a heating rate of 20 K min-‘. The following mixtures of purified gases were used: Hz (36 vol.%) in Ar for TPR; O2 (20 vol.%) in He for TPO. A cold trap and a tube filled with supported NaOH (Merck) were used to block water and, respectively, CO, released from the samples [ 171. The calibration of the consumption of hydrogen during TPR was accomplished by injections of known amounts of hydrogen in a flow of Ar + H, as well as by the reduction of known amounts of pure oxides. This procedure was not repeated for TPO, as the oxygen uptakes could be indirectly estimated from the consumption of Hz during TPR-2.
Results and discussion The results of TPR and TPO experiments carried out on the Ru-Au/ MgO samples are reported in Table 1. The Ru-richest samples RlOO and
359 TABLE 1 Approximate peak temperature (T,) MgO catalysts Sample
Ru
Rua
and consumption
Aua
Ru + Au (at.%)
of hydrogen (CH) for the Ru-Au/
TPR-1
TPO
TPR-2
2)
2)
590 740 595 735 580 575 n.d. n.d.
495
0.8
485
0.5
485 495 n.d. n.d.
0.2 0.1 -
CHb 2)
RlOO
100
0.45
-
R089
89
0.34
0.04
R064 R047 ROlO ROOO
64 47 10 0
0.21 0.12 0.03 -
0.12 0.14 0.23 0.17
425,450 600, 675 430,470 615,675 700 705 675 680
0.6 2.8 0.6 5.0 2.8 1.6 0.3 0.3
CHb
ag-at metal (g cat)-’ X 103. bg-at H (g cat)-’ x 103. n.d. = not detected
300
500
Fig. 1. Some examples of TPR-1 spectra: A, Ru-Au/MgO; magnified by a factor of 2 with respect to those of A).
B, Ru-Au/SiOz
(spectra
R089 (i.e. the samples with Ru/(Ru + Au) = 100 and, respectively, 89 at.% were characterized by complex TPR-1 spectra (Fig. 1A). Below 500 K, a medium-in~nsity peak was preceded by a very weak signal, which suggests
360
I
I
300
500
700
K
Fig. 2. TPR-1 spectra of pure Ru compounds and of a physical mixture of RuOz with a MgO reference sample (see text). the presence of more than one easily reducible species, differing either in the chemical nature or in the strength of interaction with the support. Under our experimental conditions, pure RuCl,-Hz0 was reduced at ca. 525
K and pure RuO, at ca. 475 K (Fig. 2). Considering that physical changes could shift the reduction temperature in the supported samples, it seems reasonable to attribute the reduction signals found below 500 K to compounds deriving from the hydrolysis of the precursor chloride, such as RuO, and/or ruthenium hydroxide(s). This assignment is consistent with that proposed for a Ru/MgO sample prepared starting from a different precursor compound [19,20]. The intensity of the signals below 500 K was too low to account for the reduction of all the Ru present. On the contrary, the intensity of the strong overlapped peaks above 500 K, which varied widely from sample to sample, was too high with respect to the metal contained in the catalysts. The signal intensity was not increased by the presence of H,O and CO2 released from the samples [ 171. In fact, experiments carried out on certain MgO reference samples (MgO impregnated with Hz0 or dilute HCl), which released Hz0 and CO* in amounts quite comparable with those of the catalysts [17], gave flat TPR spectra. Furthermore, very strong TPR signals in the 575 - 725 K region were recorded when the MgO reference samples were just physically mixed with a Pt/A120s catalyst or with unsupported RuO, (Fig. 2). In the latter case, the high temperature peaks were even stronger and were preceded by the reduction signal of the oxide. All this suggests that Ru reduced below 500 K can cause hydrogen spillover to the MgO support. Spillover phenomena from Pt/A1203 to MgO have already been described [21]. As a complete reduction of ruthenium appeared possible after treatment with hydrogen at 673 K [12,14], it is very likely that the proposed spillover is accompanied by the reduction of some remaining ruthenium compounds. In any case, the fact that the reduction of Ru cannot be completed at the reduction temperature of RuCl,
361
indicates that a strong interaction exists between the support and some Ru compounds. Just one strong peak at co. 700 K was found in the TPR-1 spectra of the samples with Ru/(Ru + Au) - 50 at.%, namely R064 and R047 (Fig. 1A). The absence of a significant reduction signal below 500 K, like those found for RlOO and R089, cannot be attributed to the lower absolute concentration of Ru: in fact, a signal at 465 K was very evident in the spectrum of a Ru/MgO sample containing less Ru than R064. It must be therefore concluded that the presence of HAuCl, in the impregnation solution significantly modifies the nature of the Ru compounds which are formed in the dried catalysts. This confirms through direct evidence a suggestion previously inferred from the diffuse reflectance spectra of these samples [ 141. The intensity of the high temperature signal seems excessive with respect to the amount of Ru and Au. It is therefore likely that in these samples there also was a spillover of hydrogen from the metal(s) to the support. The structure of the TPR-1 spectrum in the high temperature region was much simpler for these catalysts than for those at higher Ru/ (Ru + Au) ratios. Probably the spillover phenomena, and consequently the structure of the TPR-1 spectrum, can change depending on whether or not some Ru was already reduced at lower temperature. The existence of great HZ consumptions due to spillover makes it difficult to detect the reduction of Ru and Au: the possible signals could coincide with that due to spillover or be lost in the large low-temperature tail which characterizes the peaks recorded for R064 and R047. In any case, the colour changes of the samples and the absorption of oxygen in the subsequent TPO experiments indicate that at least part of the metal content was reduced during TPR-1. Moreover, the TPR-1 spectra of Au/MgO (ROOO) and of a bimetallic sample with 90 at.% Au (ROlO) exhibited a signal only at high temperature (cu. 680 K, Fig. 1A). Unlike those found for the previously mentioned samples, these signals were low and did not account for all metal(s) present. It seems therefore unlikely they could be due to spillover. This would indicate that large spillover phenomena can be originated by Ru but not by Au. More probably, the 680 K signals correspond to some gold compound which was made difficult to reduce owing to a strong interaction with the support. However, this does not mean that metallic gold can form in these samples only at high temperature. In a previous study [ 141, small amounts of metallic gold were found in Au/MgO even after drying at 383 K. In fact, immediately after drying, the sample turns yellow, but after some days a violet nuance appears, indicating that metallic gold forms by photodecomposition of gold compounds. Therefore, for this work, we have used a freshly-prepared Au/MgO catalyst and have followed the effect of heating not only by TPR, but also through the colour changes of the solid. The violet nuance first appears between 423 and 473 K, even in inert gas. Furthermore, in the diffuse reflectance spectrum of the Au/MgO sample heated at 473 K it was already possible to distinguish a weak band at cu. 540 nm, which is characteristic of finely dispersed metallic Au on MgO [ 141. This
indicates that some of the gold compounds formed after impregnation and drying can yield the metal by thermal decomposition, without any significant absorption of hydrogen. The TPO spectra of RlOO and R089 (which were run immediately after TPR-1) showed two broad weak signals, at cu. 590 and 740 K (Fig. 3A) respectively. Just one, even weaker, TPO signal at cu. 580 K could be detected for R064 and R047. Thus, two distinct oxidation processes characterized the samples richest in Ru, while only the lower temperature process occurred in the samples with Ru/(Ru + Au) - 50 at.%. In this connection, it may be of interest to recall that, according to the EXAFS analysis performed on the catalysts reduced by Hz at 673 K [ 141, Ru was still more oxidized in R064 than in RlOO. It is possible that the behaviour during TPO is correlated to the different oxidation states existing in the catalyst after reduction. In any case, these TPO results once more stress that the properties of the bimetallic Ru-Au/MgO catalysts are not purely additive with respect to those of the monometallic samples. No TPO signal was found for the catalysts ROlO and ROOO which are richest in Au. This is consistent with the low concentration of Ru in ROlO and with the absence of a bulk oxidation of Au particles. Oxygen adsorption on the Au surface occurs above 473 K [22], but the uptake expected for these samples [ 141 is too low to be detected by TPO. Similarly, no signal was found when ROlO and ROOO were submitted to a second temperature-programmed reduction (TPR-2), performed immediately after TPO. All the other samples yielded just one reduction signal at ca. 490 K, which can be reasonably attributed to RuO*. Assuming this stoichiometry and keeping in mind that the metal dispersion was low in these samples [ 141, it can be concluded from the intensities of the TPR-2 signals that oxidation during TPO and subsequent reduction were not restricted to the surface of the metal particles, but did not actually involve all the ruthenium contained in the catalysts. This suggests that, in the same sample, there are particles with a different oxidizability, possibly as a consequence of differences in size, metal-support interaction and/or average oxidation state. In fact, for RlOO and R089 there is an apparent correspondence between the consumption of hydrogen during TPR-2 and that measured in the low-temperature portion of TPR-1, which could indicate that the Ru particles oxidized during TPO and then reduced during TPR-2 are mainly those formed below 500 K during TPR-1. Furthermore, a heterogeneity of the metal particles of RlOO and R089 with respect to oxidation could account for the existence of two oxidation processes yielding the same product. No significant spillover of hydrogen from the metal to the support was found during TPR-2. This suggests that a tight connection exists between the spillover and the other phenomena which occur on MgO during heating, i.e. decomposition of hydrates and carbonates and an increase in surface area. Even if the nature of the sites on which hydrogen is adsorbed under our conditions is not clear (cf. ref. 23), it seems quite possible that
363 TABLE
2
Approximate peak temperature SiOz catalysts Sample
Ru __-
Rua
( Tp) and consumption
Aua
Ru + Au (at.%)
of hydrogen
TPR-1
TPO CHb
TKP)
RSlOO
100
0.38
-
RS091
91
0.33
0.03
RS062
62
0.18
0.12
RS048
48
0.16
0.18
RS014 RSOOO
14 0
0.04
0.24 0.24
445 > 525 455 > 535 455, > 560 455, 630, 470, 480
(CH)
for the Ru-Au/
TPR-2 CHb
T, (R)
490 495 700 510
1.4 0.4 1.6 1.0 0.9 0.5 0.8 1.0 0.5 0.2
455
470
1.3
460
470
1.4
460
470
0.8
465
475
0.6
530 660 770
470 V.W.
0.1
ag-at metal (g cat)-’ X 103. bg-at H (g cat)-’ x 103. V.W. = very weak
the spillover can take place only on the fresh surface developed by thermal decomposition. Obviously, this was the case for TPR-1, but not for TPR-2. Only when RlOO and R064 were exposed to the air after TPO, which allowed a partial re-hydration of the support, did their TPR-2 also exhibit a signal at cu. 570 K, possibly due to spillover. The TPR and TPO spectra of the SiO+upported samples (Table 2) were significantly different than those of the corresponding MgO-supported catalysts. The TPR-1 of RSlOO (where the number 100 indicates the Ru/ (Ru + Au) atomic percentage) was characterized by a strong peak at cu. 445 K, followed by a broad weaker signal above 520 K (Fig. 1B). A similar spectrum has already been reported for Ru/SiO, [ 191; the high temperature signal has been correlated to the presence of chloride ions from the precursor RuCl,. The 445 K peak, which was also found for Ru/MgO, is probably due to ruthenium hydroxide(s) or to similar products of the hydrolysis of RuCls. The Au/SiOz RSOOO sample gave only a weak TPR-1 signal at cu. 480 K (Fig. 1B). It happens that for Au/MgO, metallic gold can likewise form by thermal decomposition without consumption of hydrogen: in fact, upon heating RSOOO in inert gas, it was possible to detect the colour changes due to the formation of the metal between 375 and 425 K. Among the bimetallic samples, RS091 gave a TPR-1 spectrum very similar to that of RSlOO. Significantly different spectra were achieved for RS062 and RS048: the peak at cc. 455 K was followed respectively by a shoulder or by a second peak at cu. 490 K (Fig. 1B). At the same time, the intensity in the 700 K region increased, resulting in a very evident signal for RS048. This spectral trend did not continue when the gold content was
364
further increased: the TPR-1 of RS014 was like that of RSOOO, except for the presence of a second weak peak (Fig. 1B). Therefore, it seems unlikely that the peak at cu. 490 K, which becomes gradually evident for RS062 and RS048, is simply a ‘gold signal’ superimposed on a ‘ruthenium spectrum’ like that of RSlOO. It is instead suggested that the spectral features of RS062 and RS048 could be due to species deriving from an interaction between Ru and Au compounds, which is evident from TPR-1 only when the elements are present in comparable amounts. The TPR-1 spectra showed that the fraction of Ru reduced below 500 K was significantly higher in the SiO,-supported samples than in those on MgO. Also, the weak TPR-1 signal of Au occurred at a much lower temperature on SiO,. These facts show that the compounds formed after impregnation and drying are stabilized by an interaction with MgO, which does not exist or is much weaker on SiOZ. Some of the TPO spectra of the Ru-Au/SiOz catalysts are reported in Fig. 3B. Unexpectedly, two TPO signals were found even for the Au/SiOz RSOOO sample. The intensity of these signals was comparable to those of the other samples and cannot be attributed to the sole chemisorption of oxygen: in fact, RSOOO has a gold dispersion [16] even lower than that of the Au/MgO catalyst [ 141, which gave no TPO signal. A tentative explanation is that oxygen can dissolve at high temperature into the Au/SiO, particles, at least to a depth of some monolayers from the surface. The TPO spectra of RSlOO and RS091 showed just one peak at cu. 455 K, i.e. at a significantly lower temperature with respect to the MgO-supported catalysts. In the bimetallic samples with a lower Ru/(Ru + Au) atomic fraction, this peak was gradually replaced by a weak signal at cu. 530 K. The presence of this signal, which is not typical either of Ru/SiOz or of Au/SiO*, confirms that a metal-metal interaction exists in the reduced samples. Over the entire composition range, the above TPO spectra were significantly different in shape and intensity than those of the Ru--Au/MgO samples (Fig. 3), which shows that also the behaviour with respect to oxidation is strongly influenced by the nature of the support. This appears also from the amounts of H2 consumed during TPR-2, which were always higher than those of the corresponding MgO samples (cfi Tables 1 and 2). Therefore, the Ru particles formed during TPR-1 are oxidized more easily and to a greater extent on SiO, than on MgO, even if the product formed during TPO probably is still RuO, (as shown by the sharp TPR-2 peaks at 470 K). Except for RS014, where the concentration of Ru is very low, the intensity of the TPR-2 peaks was very consistent with those of the respective TPR-1 peaks at cu. 450 K. Therefore, as on MgO, the fraction of Ru oxidized during TPO and then reduced during TPR-2 seems to originate mainly from those compounds which are easier to reduce during TPR-1. Only a very weak, broad signal resulted in the TPR-2 of RSOOO between 340 and 440 K: it is possible that, during the replacement of the He + O2 mixture with Ar + H,, oxygen was released without consumption of H2 or the reaction with H, had already occurred at room temperature and was therefore not recorded.
365
RSOOO
Fig, 3. Some examples of TPO spectra: A, Ru-AuiMgU of 5 with respect to those of B); B, Ru-Au/SEO~,
(spectril magnified by a factor
Conclusions TPR and TPU studies have added further interesting details to the ~formation available on bimetallic Ru-Au catalysts. The reducibility of the compounds formed after drying changes sign~i~~tly on going from the MgO to the SiO, support and from the mono- to bimetallic samples. For the catalysts supported on MgO, it has been shown that metallic gold forms even at low temperature, while the reduction of Ru is shifted to higher temperatures by the presence of Au. These findings strengthen the proposed model t16], according to which Au particles formed at low temperature can act as nucleation centers for Ru, thus yielding bimetallic particles with a Ru surface enrichment. In the SiUrsupported samples, instead, gold and most of the ruthenium can both be reduced at low temperature. This, together with the easier sintering of Au on silica, is consistent with the proposed formation of random aggregates of Ru and Au particles [16 3. This way, the surface composition would not be controlled by the thermodynamic equilibrium, according to which an Au enrichment was expected, but by the mechanism by which these particles form during the catalyst preparation, The behaviour of the reduced catalysts during the oxidation experiments was significantly affected by the nature of the support. Re-oxidation was easier on silica and involved a greater fraction of the metal. Also, the addition of gold to Ru/MgO and to Ru/SiO, modified the TPO spectra, but the trends observed when increasing the Au fraction were different. This is evidence of the differences in the Ru-Au-support interactions existing between the l&C- and the SX&supported catalysts.
366
Acknowledgment The experimental work of Mr. G. Macario is gratefully acknowledged.
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