Surface study of graphite-supported Ru–Co and Ru–Ni bimetallic catalysts

Applied Catalysis A: General 275 (2004) 257–269 www.elsevier.com/locate/apcata

Surface study of graphite-supported Ru–Co and Ru–Ni bimetallic catalysts M. Cerro-Alarco´na,b, A. Maroto-Valienteb, I. Rodrı´guez-Ramosb,*, A. Guerrero-Ruiza a

Facultad de Ciencias, Departamento de Quı´mica Inorga´nica y Quı´mica Te´cnica, UNED, C/ Senda del Rey, no 9, 28040 Madrid, Spain b Instituto de Cata´lisis y Petroleoquı´mica, CSIC, C/ Marie Curie, no 2, Campus de Cantoblanco, 28049 Madrid, Spain Received 29 June 2004; received in revised form 21 July 2004; accepted 23 July 2004 Available online 8 September 2004

Abstract Several graphite-supported Ru–Co and Ru–Ni bimetallic catalysts were comparatively studied by X-ray diffraction (XRD) and CO adsorption microcalorimetry. The correlation of the results thus obtained from these techniques with those on the n-butane hydrogenolysis reaction test, provides useful information about the type and distribution of the surface active sites. The CO adsorption microcalorimetry technique provides, in some cases, information that allows identification and quantification of the surface sites of the metallic nanoparticles. Thus, these data may be used to identify specific catalytic properties (activity and selectivity) in the n-butane/hydrogen test, and how such properties are modified on the different surface sites. Also, comparison with the parent monometallic catalysts allows to envisage the contribution of each individual metal to the total surface composition or else of the formation of alloys or bimetallic surface clusters. The results so obtained suggest the formation of surface Ru–Co and Ru–Ni alloys on these bimetallic catalysts, their contribution to the surface composition depending on the relative atomic ratio (Ru/M) and on the pre-treatment (reduction) conditions. # 2004 Elsevier B.V. All rights reserved. Keywords: Carbon-supported metal catalysts; Bimetallic catalysts; Surface alloys; CO adsorption; Microcalorimetry; Butane hydrogenolysis

1. Introduction The main goal of heterogeneous catalysis is to provide novel, less energy consuming, intrinsically safer and cleaner catalytic processes. This means less waste production and fewer by-products. For fulfilling this objective, fundamental and applied research is needed concerning: (i) the discovery of novel or improved catalyst formulations and (ii) a better understanding of physical and chemical surface phenomena underlying the catalytic transformation [1]. Hence, fundamental basis has to be established. On the catalytic hydrogenations field, stereoselective hydrogenation of aromatic compounds is a very important task in the production of fine chemicals. Such is the case of the selective hydrogenation of 4-acetamidophenol (Paracetamol) to yield the trans form of 4-acetamidocyclohex* Corresponding author. Tel.: +34 915854765; fax: +34 915854760. E-mail address: [email protected] (I. Rodrı´guez-Ramos). 0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.07.039

anol, the first step in the synthesis of the pharmaceutical ingredient Ambroxol [2–4]. The information about catalyst compositions and employed conditions for the synthesis of said trans intermediate is very scarce, and mainly patents are found in which no specifications are available, as far as basic research is concerned. For that reason, and aiming to contribute to the state-of-the-art at a fundamental level, we have recently reported our results on the hydrogenation of Paracetamol over carbon-supported Ru, Co and Ni based catalysts and graphite-supported Ru–Co and Ru–Ni bimetallic catalysts [5]. The stereoselectivity on this hydrogenation reaction, as far as the cis and trans isomers ratio is concerned, has been seen to depend on the active metal, the carbon support, the relative Ru/M atomic ratio of the bimetallic catalyst and the pre-treatment conditions (metal particle size) [5]. Thus, this structure/composition sensitivity requires a detailed characterisation of the metal particles and, therefore, of the surface active sites. This task would enable the very ambitious goal (in the heterogeneous

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catalysis field) of establishing structure/activity–selectivity relationships and would allow new catalysts design in a rational manner. Hence, studies of the surface chemistry enables understanding on how the surface sites are modified, for instance by addition of a second metal or by the pretreatment conditions. Among the many techniques available for surface characterisation of heterogeneous catalysts, CO chemisorption followed by microcalorimetry seems to be an adequate approach. This technique provides more accurate adsorption heat values than those obtained from other techniques, and an energetic surface site distribution as a function of coverage may, thus, be obtained. Moreover, the effect of additives on the adsorption properties of metal catalysts may be investigated by CO adsorption microcalorimetry [6,7]. In this sense, this technique can provide quantitative information of the surface structure and/or composition of bimetallic catalysts. In this work we have performed a surface analysis of graphite-supported mono- and bimetallic catalysts using CO adsorption microcalorimetry. Furthermore, the samples were evaluated in the n-butane hydrogenolysis reaction test, which has proven to be a very useful and effective tool for surface characterisation due to the structure sensitive character of this reaction [8,9]. The final aim is to determine the nature of the surface sites on metal clusters supported on a high surface area graphite, including the presence of edge, corner or step atoms, or the type of exposed crystalline planes, and to determine the formation or not of bimetallic clusters or surface alloys on the bimetallic catalysts. This approach will help us to further correlate catalysts structure and catalytic properties on the stereoselective hydrogenation of Paracetamol.

2. Experimental 2.1. Catalyst preparation The commercial high surface area graphite HSAG-300 (Lonza Ltd.) with specific surface area of 299 m2/g was used as precursor material of the support. This material was pretreated in N2 at 1173 K prior to metal loading in order to eliminate oxygen functional groups present at the surface. The final material, hereinafter named H, shows a specific surface area of 297 m2/g and no textural or structural changes are induced by such treatment [10]. Monometallic MxH (M = Ru, Co, Ni; x = wt.% metal loading) and bimetallic samples Ru2M0 yH (M = Co, Ni; (2 + y) = wt.% total metal loading) were prepared by the incipient wetness technique by (co)impregnation of the carbon material with the corresponding metal salt/s water solution (Ru(NO)(NO3)3, Co(NO3)26H2O, Ni(NO3)26H2O). The X-ray diffraction (XRD) patterns were recorded using the conventional powder method with a Seifert XRD ˚ ; 40 kV, 3000P diffractometer, using Cu Ka (l = 1,540598 A

40 mA) radiation, a secondary beam graphite monochromator, and a Ni filter to eliminate the Kb component. Typically, the patterns were recorded in the 2-theta (2u) Bragg angle range from 48 to 808 in steps of 0.028. 2.2. CO adsorption microcalorimetry measurements The CO chemisorption isotherms were determined volumetrically and the evolved heats in each pulse were measured simultaneously by means of a Tian Calvet heatflow calorimeter (Setaram C-80 II) operated isothermally at 330 K and connected to a glass vacuum-dosing apparatus. Doses of approximately 2  1017 molecules of the probe gas were introduced into the system to titrate the surface of the metal catalysts. Both calorimetric and volumetric data were stored and analysed by microcomputer processing. The apparatus has been described in detail elsewhere [11]. Previous to these experiments, the catalysts were first in situ reduced under hydrogen flow at 673 or 773 K for 2 h, outgassed overnight at the same temperature, and cooled to 330 K. The metal dispersions were calculated from the total CO uptake at the monolayer (Nads), considered to be attained when the evolved heat falls to the physisorption field (40 kJ/mol), and assuming a M:CO = 1:1 stoichiometry [12,13]. The mean crystallite sizes were calculated from dispersion values, assuming spherical metal particles, using the equation dCO(nm) = v/D (v = 1.32, 0.99 and 1.01 for Ru, Co and Ni, respectively) for monometallic catalysts [14], and considering that both metals are equally dispersed on the surface of the bimetallic catalysts. 2.3. Butane–H2 reaction The n-butane/H2 test reaction was performed in a glass tubular reactor, operated at atmospheric pressure and using the adequate quantity of catalyst so as to avoid secondary reactions (total conversion below 15%). The reaction was carried out at 473 K over reduced catalysts. Typically, samples were treated at 673 or 773 K for 2 h under a continuous 40 cm3/min H2 flow. After reduction, the catalyst was cooled to the reaction temperature before introducing a H2/n-C4H10 mixture (10/1 ratio). The flow rates of these reactants were controlled by mass flow controllers (Brooks 5850 TR). The effluent gases were analysed by gas chromatography (Varian CP-3380) equipped with a flame ionisation detector (FID). The catalytic activity per gram of metal was calculated from the equation A¼

Cfn-C4 H10 44:67 ðmmolÞ  60 gM s 100WM

where C is the n-butane converted percentage; fn-C4 H10 , the n-butane flow rate (cm3/min) through the reactor, and; WM, the weight (g) of metal in the catalyst. Also, the activity per active centre or turnover frequency (TOF) was calculated

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following the expression TOF ¼

44.58, respectively) corresponding to the (1 1 1) plane arrangement of metallic cobalt and nickel, respectively. On the other hand, for a reduction treatment at 773 K the peaks corresponding to the (2 0 0) and (2 2 0) planes were found too. It is important to point out that all these peaks are overlapped with those corresponding to the graphite support, so dXRD values in Table 1 assume an error since a substraction had to be done for the determination of the metal diffraction peak width. No diffraction peaks, different to those of the graphite support, where detected for monometallic Ru2H nor for Ru– Co and Ru–Ni bimetallic catalysts suggesting that the (bi)metallic particles on these catalysts are well dispersed over the surface of the graphitic support (d < 5 nm, below the detection range of the XRD technique), or else the formation of amorphous phases can be considered too. Only Ru2Co2H, when reduced at 773 K, showed a diffraction peak different to those of the support (2u ca. 45.68) that cannot be ascribed to individual Ru or Co metal particles. These results suggest the presence of Ru–Co alloy bimetallic particles (with a mean particle size equal or bigger than 5 nm) on Ru2Co2H. Furthermore, the TPR profiles of the Ru–Co and Ru–Ni bimetallic catalysts [5] showed the simultaneous reduction of the two metals, supporting the formation of Ru–Co and Ru–Ni alloys on these catalysts.

A 1 ðs Þ Nads

where Nads is the number of surface metal centres obtained from the CO chemisorption measurements. Ethane selectivity (SE), that is, the percentage of n-butane that is converted to ethane among the total n-butane converted via the hydrogenolysis reaction, was calculated from the equation: SE ¼

2C2 100 ð%Þ Ch

where Ch = C1 + 2C2 +3C3, and Ci values were calculated as mole percentage of hydrogenolysis products. The fragmentation factor the hydrogenolysis reaction P of P was defined as j ¼ Ci =ð iCi =4Þ. This parameter characterises the depth of hydrogenolysis and represents the number of hydrocarbon fragments per decomposed molecule. Hence, a value of 2 reflects the splitting of one single bond, whereas j > 2 shows multiple fragmentation. When the isomerization reaction (C4i) took place, as well as the hydrogenolysis one, reaction selectivities were calculated as follows: Hydrogenolysis : Isomerization :

Sh ð%Þ ¼ Si ð%Þ ¼

259

Ch 100 Ch þ 4C4i

3.2. CO adsorption volumetric and calorimetric measurements

4C4i 100 Ch þ 4C4i

3.2.1. Monometallic catalysts Table 1 shows the CO adsorption amount (Nads, mmol/ gcat), dispersion (DCO, %) and mean particle size (dCO, nm) values determined from the CO chemisorption measurements for Ru, Co and Ni samples reduced at 673 and 773 K. Also, initial CO adsorption heats (qads)0 are shown to enable comparison among catalysts. This value is determined by extrapolation to zero coverage of the chemisorption heat curves thus obtained from the adsorption microcalorimetric experiments. In Table 1 it may be observed that the metal particles (dCO) are larger for Co and Ni samples than for Ru,

where Ch = C1 + 2C2 + 3C3, and Ci and C4i are mole percentage of products.

3. Results and discussion 3.1. Catalysts characterisation Table 1 shows the mean metal particle sizes (dXRD) determined from the XRD patterns. For Co and Ni samples reduced at 673 K, a sole peak was found (2u = 44.38 and

Table 1 Surface properties of graphite-supported Ru–Co and Ru–Ni bimetallic samples, reduced at 673 and 773 K as determined by CO chemisorption Catalyst

Ni5H Ru2Ni2H Ru2H Ru2Co0.5H Ru2Co1H Ru2Co2H Co5H

M (%)

4.0 4 2.0 2.5 3 4 4.7

Reduced at 673 K

Reduced at 773 K

Nads (mmol/gcat)

DCO (%)

dCO (nm)

dXRD (nm)

qads0

45 65 79 95 100 105 80

6.6 12 39 34 27 19 10

15 9.4 3.4 3.6 4.3 5.7 9.8

6.9a n.d n.d. n.d. n.d. n.d. 6.5a

125 135 140 135 115 110 107

n.d., Metal diffraction peaks not detected. a Calculated from the (1 1 1) face diffraction peak. b Calculated from the (2 0 0) face diffraction peak. c Calculated from the (2 2 0) face diffraction peak.

(kJ/mol)

Nads (mmol/gcat)

DCO (%)

dCO (nm)

dXRD (nm)

qads0(kJ/mol)

25 24 58 25 37 28 16

3.6 4.5 29 9.0 10 5.2 2.0

28 25 4.6 13 12 21 50

12b n.d. n.d. n.d. n.d. n.d. 35c

125 116 145 110 145 150 120

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independent of the reduction temperature. These would be in agreement with XRD results if we consider the absence of amorphous phases in the Ru2H sample, and that the lack of XRD peaks (corresponding to Ru particles) is due to a good dispersion of the metal over the surface of the support (d < 5 nm, as determined by CO chemisorption). When reduced at 773 K, the catalysts suffer sintering of the metal particles, in general, but among them Co5H experiences the most acute sintering, or else stoichiometry of CO adsorption changes. The latest is better understood if we take in mind that if the CO adsorption stoichiometry is M:CO = 1:1 then Nads equals the total surface active sites. But if other stoichiometries occur (i.e. bridged, M:CO = 2:1, or subcarbonyl, M:CO = 1:2, species) then dCO values (calculated considering M:CO = 1:1) would be bigger and smaller, respectively, than the real ones. This effect on the mean metal particle sizes will be considered later (Fig. 1). Fig. 1A–C show the CO differential heats versus coverage plots for monometallic Ru2H, Co5H and Ni5H, comparatively for reduction treatments at 673 and 773 K. Ru2H reduced at 673 K (Fig. 1A) shows a CO initial adsorption heat of ca. 140 kJ/mol, in agreement with other studies carried out with other carbon-supported Ru catalysts [7,15,16]. Also, an heterogeneous surface site distribution is observed for this catalyst. The population of sites with such a high CO (initial) adsorption heat is small, and a continuous but slow decay of the differential heats (from 115 to 105 kJ/ mol) is observed until u  0.6. Further increase in the CO coverage produces a sharp fall in the adsorption heat values. When reduced at 773 K, Ru2H shows a parallel microcalorimetric profile, but it can be appreciated that this increase of the reduction temperature induces the formation of slightly stronger surface sites for CO adsorption. This effect can be interpreted considering that when sintering takes place the electron density of the surface metal atoms increases. This is so since the probability of finding high index planes and, thus, highly coordinated sites increases. Since CO adsorption microcalorimetry is a measure of the metal–carbon bond strength, and taking into account that this M–C bond may have a p double bond participation (by p back-donation from the metal atoms to antibonding molecular orbitals of the CO molecule), the moderate increase of the adsorption heats can be considered reasonable. Also, changes in particle structure and/or morphology, and generation of different adsorption centres may be taking place when increasing the reduction temperature on this sample. The information on the CO adsorption heats for the different Ru crystallographic planes that is available in the literature is very scarce and sometimes change from one author to another. Values found for Ru (100) are ca. 105– 120 kJ/mol [17] and 124 kJ/mol [18], while a 160 kJ/mol [19] value is found for the (0 0 1) plane, and 105–120 kJ/mol for the (1 1 0) face arrangement [17]. These data seem to indicate that Ru particles in the Ru2H catalyst reduced at 673 and 773 K expose mainly (1 0 0) and (1 1 0) planes. Moreover, the slight increase in the CO adsorption heats

when Ru2H is reduced at 773 K may be due to a change in the CO adsorption stoichiometry. In fact, various different CO adsorbed species have been detected by IR spectroscopy on supported Ru catalysts [20], and references therein. On SiO2-supported Ru catalysts, namely bridged (Ru2(CO)) and linear (Ru–CO) species over metallic Ru, and adsorbed CO over partially oxidized Ru (Rud+–CO) species are found [21]. Hence, considering the chemical nature of our graphite support, the present results suggest the presence of bridged and linear CO species adsorbed on our supported Ru particles, and that an increase of the bridged CO species population over Ru sites may be taking place when increasing the reduction temperature of Ru2H, in agreement with the increase in the mean metal particle size. The formation of bridged CO species would imply ‘‘real’’ d values slightly lower than the dCO values calculated for this Ru2H sample reduced at both studied temperatures (Table 1). Catalyst Co5H (Fig. 1B) shows two constant CO heat values (plateaux), which are indicative of two homogeneous surface site distributions, that is, of two different types of surface Co active sites for CO chemisorption. FT-IR of adsorbed CO experiments carried out over Co/SiO2 [22,23] reveal different adsorbed CO species over the supported Co particles: adsorbed CO on metallic cobalt (Co0), carbonyl species Co(CO)x (x > 1) favoured over edge particle-located centres of metallic cobalt, bridged CO on big metal particles, and adsorbed CO over multi-fold centres. At this point we have to assume that the active centres that show the highest differential adsorption heats (the strongest from the view point of energy interaction with the probe molecule CO) are first occupied. Once these same family sites are saturated (corresponding to one plateau), adsorption proceeds over the following family sites with a lower adsorption heat. Therefore, the microcalorimetric profiles of Co5H suggest the presence of Co0–CO linear species (plateaux at ca. 106

4 kJ/mol) and carbonyl species (Co(CO)x) for plateaux at ca. 83 kJ/mol. Therefore, the apparition of different adsorbed species may be related to the presence of different surface sites on the metal particles (corners and edges, and plane sites, where different adsorption modes are favoured). The fact that this Co5H catalyst shows very similar profiles and calorimetric values for both reduction treatments suggests the same structure and morphology of the metal particles, even though the mean particle sizes are very different (Table 1). Moreover, the coincidence in calorimetric profiles obtained rule out the possibility of a change in the adsorption stoichiometry when the reduction temperature is increased from 673 to 773 K as previously suggested at the beginning of Section 3.2.1. And hence, the increase in the dCO value for Co5H reduced at 773 K (Table 1) seems to be only due to metal sintering. As the formation of carbonyl species seems possible for this Co5H catalyst reduced at 673 and 773 K, real d values would be slightly higher than those calculated (dCO) considering a M:CO=1:1 stoichiometry (Table 1).

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261

Fig. 1. Differential heats of CO adsorption at 330 K as a function of surface coverage for: (A) Ru2H (&, &); (B) Co5H (~, ~) and (C) Ni5H (*, *).

On the other hand, Ni5H shows CO initial adsorption heats (Table 1 and Fig. 1C) of ca. 125 kJ/mol, independent of the reduction temperature, in agreement with other studies carried out by other research groups for Ni powder (ca. 120 kJ/mol) [24]. In fact, this value fits well with those found in the literature corresponding to the Ni (1 0 0) face arrangement: 125 [25], 120 [26] and 123 2 kJ/mol [27]. Such heats may be ascribed to linearly adsorbed CO over metallic Ni (Ni0–CO) as previously suggested for Ni powder

[28]. Since this initial CO adsorption heat value decreases slightly on increasing the CO coverage, a small contribution of carbonyl species (Ni(CO)x; x = 2 or 3) could also be possible. The formation of these last species have been previously suggested for alumina and silica-supported Ni catalysts [29]. Bridged CO species can also be formed over Ni (Ni2(CO)), but CO adsorption heats over the (1 0 0) face will account for 140–150 kJ/mol [30]. No important differences in the surface site distribution are observed

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when increasing the reduction temperature (Fig. 1C), thus suggesting that morphology and structure of Ni particles (generated at both reduction temperatures) are very similar. As the formation of carbonyl species over Ni sites seems possible from the calorimetric results, real d values would be slightly higher than the calculated dCO values in Table 1. 3.2.2. Bimetallic catalysts reduced at 673 K The decrease in metallic dispersion values (DCO, Table 1) along the Ru–Co series (and in the Ru–Ni sample) from Ru2H to Co5H (Ni5H) indicates the higher CO capacities of the bimetallic samples in comparison with monometallic Co (Ni). Moreover, the addition of Co or Ni to the Ru catalyst decreases the CO initial adsorption heats (Table 1), thus suggesting that the strongest active sites weaken along the series. This effect being higher for increasing second metal loading, so that, for the Ru–Co series, Ru2Co2H nearly reaches a (qads)0 value like that of Co5H. The similarity among the adsorption heats of a bimetallic catalyst and those of the monometallic catalyst with the highest qads indicates that there are no effects that directly alter the strongest chemisorption sites. The possible mechanisms through which the chemisorption bond or surface site is modified in a bimetallic particle include [31]: electronic effects, structural effects (aggregates), or structure sensitivity induced by surface segregation. This last refers to the possibility that the metal with the less energetic sites (in our particular case, Co and Ni) preferentially populates edges, corners and other metallic (Ru) low coordinated sites. In this sense, adsorption over those segregated sites would be less energetic (weaker). Also, structural effects due to formation of bimetallic Ru–Co and Ru–Ni clusters or aggregates is possible, since these two metals are miscible and a micro-mixture at the surface may take place. Also, electronic effects (also called ligand-effect) could be responsible of the CO adsorption heat variations, and would be due to an electronic transfer from or to the ruthenium atoms. The adsorption heat is a measure of the bond strength, and since electrons are involved in bonds, any influence of the base metal (Co, Ni) on the electron transfer would alter their values [31]. For these reasons, the CO adsorption microcalorimetry results of the graphite-supported bimetallic Ru–Co and Ru–Ni catalysts are shown with those of the parent monometallic catalysts (Figs. 2 and 3). The calorimetric profiles of the individual metals are different and may be used as a fingerprint. Thus, a comparison of the differential adsorption heats provides information about the relative contribution of each metallic component to the total adsorption [32]. Ru2Co0.5H (Fig. 2A) shows intermediate differential adsorption heat values, among Ru2H and Co5H, almost for the entire u range. The profile starts at ca. 135 kJ/mol to sharply decrease until a plateau is reached at ca. 110 kJ/mol for u  0.1–0.4. Then, heats fall to the physisorption field. Ru2Co1H (Fig. 2B) shows a microcalorimetric profile [15] typical of more heterogeneous sites, since no plateaux are

shown (the CO adsorption heats fall from initial coverages until monolayer formation). For this catalyst, intermediate heats are found for low surface coverages, whereas, for medium to high coverage values, the profile is similar to that of Co5H. On the other hand, Ru2Co2H (Fig. 2C) shows a narrow plateau at ca. 110 kJ/mol from initial to 0.2 surface coverage, to then show more heterogeneous sites since heat values decrease until the physisorption field is reached. This bimetallic catalyst shows heat values very similar to those of Co5H, in general, for all the u range. For bimetallic Ru2Ni2H (Fig. 3) a microcalorimetric profile characteristic of an heterogeneous surface site distribution is shown. For this catalyst heats are very similar to those of Ni5H, mainly for the 0.0–0.4 surface coverage range. There are two main characteristics on the chemisorption calorimetry results that suggest the formation of surface alloys in bimetallic catalysts [7,15,32]: a) Adsorption heats of the bimetallic sample intermediate respect to those of the parent monometallic catalysts. This is better understood if we keep in mind that if the surface of a bimetallic sample is composed by two segregated metals, then the initial adsorption heats would be similar to those of the monometallic catalyst with the highest adsorption heats (since the CO molecule adsorbs on the most energetic centres) and then, heats would fall more or less sharply (depending on the energetic differences among the two individual metals) as saturation of the stronger sites takes place, and as a consequence of adsorption on weaker sites corresponding to the second metal. b) Variations on the total CO uptake for monolayer formation. Two segregated metals would adsorb the sum of the Nads amounts of both monometallic catalysts (if we assume that dispersion and mean particle sizes of the two metals in the bimetallic catalyst is the same as in the monometallic samples). Following these criteria, the Ru–Co bimetallic samples whose surface may be constituted by a surface alloy, when reduced at 673 K, are Ru2Co0.5H and Ru2Co1H (Fig. 2A– B). And the participation of such Ru–Co alloy to the total surface composition would be higher for Ru2Co0.5H (u  0.0–0.7, nearly for the entire u range). Moreover, for Ru2Co1H, the microcalorimetric profile seems to indicate a Co surface enrichment (u > 0.25), similar to what happens for Ru2Co2H (Fig. 2C), but for the latest it seems that the surface is only composed by Co particles or sites. For Ru2Ni2H (Fig. 3), a surface constituted by Ni particles is also possible, but for this particular catalyst, it is more difficult to assign specific curve fragments to individual metal behaviours. TPR experiments also indicate alloy formation on these samples because only one peak assignable to the simultaneous reduction of the two metallic precursors present in the catalyst is observed [5].

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Fig. 2. Differential heats of CO adsorption at 330 K as a function of surface coverage for: (A) Ru2Co0.5H (^); (B) Ru2Co1H ($) and; (C) Ru2Co2H ( ), reduced at 673 K. Comparatively, results of the parent monometallic catalysts are also shown (&, Ru2H; ~, Co5H).

3.2.3. Bimetallic catalysts reduced at 773 K The effect of Co addition on the Ru catalyst does not follow a specific trend as far as the CO initial adsorption heat values is concerned (Table 1 and Fig. 4). Ru2Co0.5H shows a lower value than those of the parent monometallic samples, while Ru2Co1H and Ru2Co2H show an equal and slightly higher value respect to that of Ru2H, respectively. However, the addition of Co decreases the total CO amount (Nads).

Lower dispersion values and, consequently, bigger metal particle sizes are calculated for increasing Co content on the Ru–Co bimetallic catalysts, probably due to the higher sintering capability of Co, as previously shown by the monometallic samples. However, the lower CO uptakes determined for the bimetallic Ru–Co catalysts may be else due to a change on the CO adsorption stoichiometry. In fact, no XRD peaks could be ascribed to metal particles

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At this point, it is important to point out the coincidence on the adsorption heat values of the plateaux shown by these three bimetallic Ru–Co catalysts reduced at 773 K (ca. 110 kJ/mol). This last, added to the fact that the adsorption heats are intermediate among the monometallic extremes, and considering that the Nads values are very similar too, suggests the presence of a Ru–Co alloy in the surface of these Ru–Co bimetallic catalysts. The contribution of such Ru–Co alloy to the total surface composition based on the CO microcalorimetric profiles would follow the trend: Ru2Co0.5H  Ru2Co1H > Ru2Co2H. The surface of Ru2Co1H would also contain Ru centres (since initial adsorption heats are high and similar to those of Ru2H), while for Ru2Co2H the presence of Co sites is also possible (adsorption heats for medium to high coverages look more to those of Co5H). On the other hand, Ru2Ni2H (Fig. 5) shows a microcalorimetric profile with a plateau at ca. 116 kJ/mol from initial surface coverages up to 0.4. For this bimetallic catalyst, the surface coverage range for which intermediate heats are found is ca. 0.1–0.58. When u > 0.58, the heats, as well as the microcalorimetric profile, are similar to those of Ni5H. From these results and the Nads value (very similar, too, to that of Ni5H) we may deduce that a surface of bimetallic Ru–Ni is formed by alloying, and that surface Ni isolated centres are also present. Fig. 3. Differential heats of CO adsorption at 330 K as a function of surface coverage for Ru2Ni2H ( ) reduced at 673 K. Comparatively, results of the parent monometallic catalysts are also shown (&, Ru2H; * Ni5H).

suggesting that this CO uptake decrease is not due to metal agglomeration but to a variation of the CO adsorption stoichiometry or to alloy formation (amorphous phases or small particles; d < 5 nm). On the other hand, bimetallic Ru2Ni2H shows a lower CO initial adsorption heat if compared to Ru2H and Ni5H (Table 1 and Fig. 5), but closer to that shown by Ni5H (as well as dispersion and mean particle size values). Moreover, Ni addition to the Ru catalyst decreases the total CO amount (Nads), resulting in a very similar value to that of Ni5H. Thus, apparently, Ni is mainly exposed at this bimetallic surface. As for the microcalorimetric profiles, Ru2Co0.5H (Fig. 4A) first shows a plateau at ca. 110 kJ/mol for the 0.0–0.4 surface coverage range. Then, a more heterogeneous surface site distribution is observed. We may say that the CO adsorption heats are intermediate among those of the monometallic catalysts for u  0.15–0.8. Ru2Co1H (Fig. 4B) shows a profile that starts with a sharp fall of the initial adsorption heat to reach a plateau at ca. 110 kJ/mol for the 0.08–0.4 u range. Similarly to what was found for Ru2Co0.5H, the CO adsorption heats are intermediate, among those of the parent catalysts, almost for the entire u range. However, Ru2Co2H (Fig. 4C) showing several plateaux and intermediate heats (in comparison with the monometallics) are only found for u  0.1–0.4.

3.3. n-Butane hydrogenolysis The use of a specific transition metal as a supported catalyst results in an intrinsic activity and selectivity catalytic behaviour, dependent on the surface characteristics, which are related to the size and structure of the supported metal aggregates. These last provide geometric and electronic specific properties, that will affect the final results in the n-butane/hydrogen reaction. 3.3.1. Monometallic catalysts Data for activity, turnover frequency (TOF), reaction selectivities, mole percentage of hydrogenolysis products (Ci) containing i carbon atoms, ethane selectivity (SE) and fragmentation factor values (j), on the n-butane/hydrogen reaction for Ru2H, Co5H and Ni5H, for reduction pretreatments at 673 and 773 K, are summarized in Table 2. Generally the activity trend followed is Ru  Co  Ni, independent of the reduction temperature. Hydrogenolysis of the hydrocarbon molecule is the preferred reaction over Ru2H in agreement with other studies carried out over similar high surface area graphitesupported Ru catalysts [33]. The product distribution obtained for this reaction over Ru2H is mainly methane (C1), which decreases slightly when increasing the reduction temperature. Selectivity towards the various hydrogenolysis products depends on the predominant adsorbed intermediate formed at the catalysts surface. Mainly, there are two accepted mechanisms [34,35] that may proceed with equal probability:

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Fig. 4. Differential heats of CO adsorption at 330 K as a function of surface coverage for: (A) Ru2Co0.5H (^); (B) Ru2Co1H ($) and; (C) Ru2Co2H ( ) reduced at 773 K. Comparatively, results of the parent monometallic catalysts are also shown (&, Ru2H; ~, Co5H).

I) formation of a 1,3-diadsorbed or metallocyclobutane complex (involving one single metal atom), favouring ethane formation by hydrogenolysis and the isomerization reaction, and II) formation of 1,2- and 2,3-diadsorbed complexes (involving two metal atoms), the former will favour methane and propane formation (terminal splitting), while the

latter would lead to ethane formation by central C–C cleavage and further scission. The formation of multibonded species (involving more than two metal atoms) may be associated to this last mechanism, and will lead, mainly, to methane formation by multiple splitting of the n-butane molecule. The product

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Fig. 5. Differential heats of CO adsorption at 330 K as a function of surface coverage for Ru2Ni2H ( ) reduced at 773 K. Comparatively, results of the parent monometallic catalysts are also shown (&, Ru2H; *, Co5H).

distributions, fragmentation factors, j  3.7 and 3.3 (that is, formation of 3 to 4 fragments by splitting of 2–3 bonds), and ethane selectivity values, SE  11 and 23%, shown by Ru2H reduced at 673 and 773 K, respectively, seem to indicate the participation of a mechanism via multibonded species on 3-fold or 4-fold sites (sites constituted by an arrangement of 3 or 4 metal atoms, respectively) of supported-Ru particles and multiple splitting of the n-butane molecule. Co5H leads only to the hydrogenolysis reaction of the nbutane molecule independently of the reduction temperature (Table 2). But the product distribution in this reaction is different depending on the pre-treatment conditions. When reduced at 673 K, mainly methane is produced, in agreement with other results reported for a bulk Co catalyst [36]. These

results suggest the preferential formation of multiple adsorbed species on 4-fold or 3-fold sites. Also, ethane and propane contributions suggest a small participation of 1,2- and 2,3-diadsorbed intermediates, in agreement with the fragmentation factor value (j  3.4). On the other hand, a higher propane contribution may be observed for Co5H when reduced at 773 K, in agreement with other studies carried out over Co catalysts supported on niobium oxides [37,38] and Saran type carbons [36]. This product distribution rules out the possibility of multiple scission of the n-butane molecule what, added to the j value (ca. 2.4), indicates the formation of 1,2-diadsorbed species and further hydrogenolysis of some of the generated propane. Moreover, no ethane is formed. These results suggest that the active sites responsible for ethane production, probably by formation of 2,3-diadsorbed intermediates, present at the surface of Co5H when reduced at 673 K, have disappeared with the reduction treatment at 773 K. Hence, a surface structure re-arrangement is being suggested. The CO adsorption microcalorimetric profiles (Fig. 1B) do not show important modifications of the active surface site distribution of Co5H when changing the reduction temperature, but these results in the hydrogenolysis of n-butane suggest a change in the surface structure and/or morphology of the metal particles that would impose geometric and/or steric hindrances for n-butane multiple and 2,3-adsorption. Since CO is a smaller molecule than n-butane, these variations would not be detected by CO adsorption microcalorimetry. Also, as the reduction treatment at 773 K causes a dramatic sintering of the Co particles (Table 1), another possibility to explain this behaviour is that the number of active sites left for n-butane cleavage would be very scarce. This last refers to the possibility that the Co ensembles generated with a reduction treatment at 773 K do not fit the required specific size and structure for n-butane multiple and 2,3-adsorption. Hydrogenolysis is also the preferred reaction over Ni5H (Table 2) in agreement with previous results over a Ni/Al2O3 catalyst [39]. Also, the product distributions are very similar for both reduction temperatures, and are mainly constituted by methane and propane and a small ethane contribution, and look like those previously reported for Ni catalysts supported on Saran carbons [36] and activated carbon [8]. Thus, these Ci values and the fragmentation factors (j) rule out the possibility of formation of surface intermediates

Table 2 Catalytic properties of graphite-supported Ru, Co and Ni monometallic catalysts reduced at 673 and 773 K on the n-butane/H2 reaction at 473 K Catalysts

Ni5H Ni5H Ru2H Ru2H Co5H Co5H

Tred (K)

673 773 673 773 673 773

dCO (nm)

15 28 3.4 4.6 9.8 50

Activity (mmol/gM s)

53.4 1.2 4111 2256 136 9.6

TOF (103 s1)

50 2 1270 770 80 30

Product distribution (%) C1

C2

C3

67 58 93 82 89 66

7 5 6 14 6 0

26 37 1 4 5 34

SE (%)

8.8 5.6 11.1 23.0 10.3 0.0

j

2.5 2.2 3.7 3.3 3.4 2.4

Reaction selectivity (%) Sh

Si

99.9 99.8 100 99.8 100 100

0.1 0.2 0.0 0.2 0.0 0.0

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involving more than two Ni atoms. It can be inferred that the formation of 1,2-diadsorbed complexes is the preferred mechanism. In fact, Ni preference towards bonding to terminal C–C bonds has been previously postulated [8,40,41]. Since j values are slightly higher than 2 and a small quantity of ethane is also produced, it may be suggested that further hydrogenolysis of part of the generated propane is taking place (or else a small contribution of 3-fold site adsorption). These results on the n-butane hydrogenolysis over Ni5H would be in agreement with those stemming from CO adsorption microcalorimetry (Fig. 1C), where no important modifications on the surface site distribution was observed when changing the reduction temperature. 3.3.2. Bimetallic catalysts reduced at 673 K When using bimetallic samples, it is important to take in mind that the catalytic properties are dependant, basically, on particle composition and on the relative surface M/M ratio. We may say that the activity values (per gram of metal and TOF) shown by the Ru–Co bimetallic catalysts (Table 3) are intermediate to those of the parent monometallic samples, and since these values decrease with Co loading, it can be deduced that the surface of these catalysts are Co enriched, and that this enrichment is higher along the series. This is in agreement with our CO adsorption microcalorimetry results (Fig. 2), where the similarity of the profiles and of the CO adsorption heats, with those of Co5H, increased along the Ru–Co series with increasing Co loading. Ni addition to the Ru catalyst (Ru2Ni2H) also produces a decrease in both activity values (Table 3), again suggesting a Ni enrichment of the surface, and in agreement with CO adsorption microcalorimetry results. As for selectivity results is concerned, all bimetallic catalysts are highly selective towards the hydrogenolysis reaction (a maximum of 1.5% of isobutane is detected). This reaction selectivity being higher, in the case of the Ru–Co catalysts, for lower Co contents. Considering the product distributions, we can indicate that bimetallic Ru–Co catalysts show a different behaviour to both parent monometallic catalysts and characteristic (typical) of alloyed phases, which are Co enriched along the series. This is better understood if we consider that if a non-cooperative effect would be taking place among segregated Ru and Co atoms, then the main hydrogenolysis product would be

267

methane, since both metals independently favour deep hydrogenolysis of the n-butane molecule (Tables 2 and 3). However, Ru–Co bimetallic catalysts generate an important quantity of ethane (SE), suggesting that deep hydrogenolysis has been impeded (this effect being higher for Ru2Co0.5H). Therefore, the product distribution on the hydrogenolysis reaction and the fragmentation factor values, shown by these Ru–Co bimetallic catalysts reduced at 673 K, suggest the presence of a surface Ru–Co alloy. This last will favour formation of 1,2- and 2,3-diadsorbed intermediates and further hydrogenolysis of ethane and/or propane thus, generated. Also, a small participation of 1,3-diadsorbed intermediates would be favoured, justifying isobutane formation. The alloy contribution to the surface total composition, as determined by the selectivity and fragmentation factor values, would follow the trend Ru2Co0.5H > Ru2Co1H > Ru2Co2H, as already evaluated from the CO microcalorimetric profiles. For bimetallic Ru2Ni2H something similar happens (Table 3). The apparition of ethane quantities higher than those shown by the parent monometallic catalysts suggests the formation of a Ru–Ni surface alloy, and methane and propane distributions suggest the presence of monometallic Ni centres at the surface of this catalyst. This is in agreement with CO adsorption microcalorimetry (Fig. 3). As for the Ru–Ni alloy, it is more difficult to confirm its presence since CO adsorption heats and calorimetric profiles for Ru2H and Ni5H are rather close together, even though TPR results support its presence [5]. 3.3.3. Bimetallic catalysts reduced at 773 K Table 4 shows activity and selectivity results on the nbutane/hydrogen reaction, carried out at 473 K, over the Ru– Co and Ru–Ni bimetallic catalysts reduced at 773 K. Considering the XRD patterns, where no diffraction peaks were detected for these bimetallic catalysts, selectivity results of Co5H reduced at 673 K should be considered for comparison instead of those of the same sample reduced at 773 K (because of the larger metal particles of the latter). Again, addition of Co to the Ru catalyst decreases both activity values (per gram of metal and TOF), the effect being higher for increasing Co contents. The activity behaviour shown by Ru2Co0.5H and Ru2Co1H is intermediate among Ru2H and Co5H. However, the activity values shown by

Table 3 Catalytic properties of graphite-supported Ru–Co and Ru–Ni bimetallic catalysts reduced at 673 K on the n-butane/H2 reaction at 473 K Catalysts

dCO (nm)

Activity (mmol/gM s)

TOF (103 s1)

Ni5H Ru2Ni2H Ru2H Ru2Co0.5H Ru2Co1H Ru2Co2H Co5H

15 9.4 3.4 3.6 4.3 5.7 9.8

53.4 306 4111 1889 533 520 136

50 190 1270 500 160 200 80

Product distribution (%) C1

C2

C3

67 47 93 70 80 90 89

7 32 6 22 15 8 6

26 21 1 8 5 2 5

SE (%)

j

8.8 36.8 11.1 31.9 24.0 14.3 10.3

2.5 2.3 3.7 2.9 3.2 3.6 3.4

Reaction selectivity (%) Sh

Si

99.9 98.5 100 98.7 99.1 99.8 100

0.1 1.5 0.0 1.3 0.9 0.2 0.0

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Table 4 Catalytic properties of graphite-supported Ru–Co and Ru–Ni bimetallic catalysts reduced at 773 K on the n-butane/H2 reaction at 473 K Catalysts

Ni5H Ru2Ni2H Ru2H Ru2Co0.5H Ru2Co1H Ru2Co2H Co5H

dCO (nm)

28 25 4.6 13 12 21 50

Activity (mmol/gM s)

1.2 4.0 2256 179 31 2.5 9.6

TOF (103 s1)

2 7 770 180 20 4 30

Ru2Co2H are even lower than those shown by Co5H (reduced at 673 and 773 K). For this particular catalyst (Ru2Co2H) we first suggested, from the CO adsorption microcalorimetric results (Fig. 4C), that a partial segregation of the metal particles could be possible, however, these activity data rule out this possibility (if partial segregation of individual metals would be taking place then activity values would be similar to those of the most active metal). Thus, these results suggest the presence of a Ru–Co alloy with different intrinsic properties to those of monometallic Ru2H and Co5H. On the other hand, the activity values shown by Ru2Ni2H involve an intermediate behaviour among the monometallic extremes, even though the similarity with those of the Ni counterpart is higher. Again, this fact suggests a surface Ni enrichment, in agreement with CO adsorption microcalorimetry results (Fig. 5). All bimetallic catalysts show a preference towards the hydrogenolysis reaction (<0.5% of isobutane). The product distributions shown by the Ru–Co bimetallic catalysts again suggest the presence of surface Ru–Co alloys that would favour the formation of 1,2- and 2,3-diadsorbed intermediates, and further hydrogenolysis of generated propane (mainly; j > 2.0). Both activity and selectivity results on the n-butane/H2 reaction are in good agreement with those stemming from CO adsorption microcalorimetry (Fig. 4), where Ru–Co surface alloys were detected. On the other hand, the product distribution shown by Ru2Ni2H (Table 4) seems to indicate the presence of a surface Ru–Ni alloy as well as Ni isolated centres or particles. In this particular case, the ethane quantity that is generated looks more like that of Ru2H. However, the low methane selectivity (more similar to that of Ni5H), as well as the TOF value, discards the possibility of Ru isolated particles on the surface of this Ru–Ni sample. Again, these results are in good agreement with those found with CO adsorption microcalorimetry (Fig. 5), TPR [5] and XRD.

Product distribution (%) C1

C2

C3

58 35 82 75 56 69 66

5 38 14 19 26 20 0

37 27 4 6 18 11 34

SE (%)

5.6 40.0 23.0 28.2 32.2 27.7 0.0

j

2.2 2.1 3.3 3.0 2.5 2.8 2.4

Reaction selectivity (%) Sh

Si

99.8 100 99.8 99.6 99.7 100 100

0.2 0.0 0.2 0.4 0.3 0.0 0.0

structures on the metal particles of the studied mono- and bimetallic catalysts, that are mainly dependant on the reduction treatment. On Ru2H, small Ru particles lead to an enhanced hydrogenolysis capability towards methane formation. This behavior diminishes on increasing the Ru mean particle size. The metal particles of Co5H show various types of surface active sites, leading to different CO adsorbed species. Sintering due to the reduction treatment at 773 K leads to the formation of Co aggregates whose specific size and structure impedes multiple and 2,3-adsorption of the n-butane molecule. Ni5H shows very similar active sites, for both reduction treatments, leading to the same kind of specific interactions with the CO probe molecule, and to preferential terminal splitting of the n-butane molecule. The graphite-supported Ru–Co and Ru–Ni bimetallic catalysts have a surface whose structure and properties depend on the relative atomic Ru/M ratio and on the applied reduction temperature. The formation of surface Ru–Co and Ru–Ni alloys over the high surface area graphite has been detected and quantified mainly based in the calorimetric data. The CO adsorption microcalorimetry technique provides, in some cases, information that allows identification and quantification of the surface sites of the metallic nanoparticles. These data have been used to identify specific catalytic behaviours (activity and selectivity) in the nbutane/hydrogen test, and how such catalytic properties are modified when other surface sites are generated (alloy and/ or bimetallic sites).

Acknowledgments MCA would like to thank the Universidad Nacional de Educacio´ n a Distancia (UNED) in Spain for a scholarship Grant (Convocatoria 1999). The financial support of the MC&T of Spain under project MAT 2002-04189-C02 is recognized.

4. Conclusions The surface characterization technique applied in this study, CO adsorption microcalorimetry, along with the catalytic performances in the n-butane/hydrogen test reaction have enabled the detection of multiple sites and surface

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