Catalysis Today, 4 (1966) 71-96 71 Elsevier Science Publishers B.V., Amsterdam-Printed in The Netherlands
CARBON MONOXIDE AND OXYGEN CHEMISORPTION Ni-W/A1203 HYDROTREATING CATALYSTS
Jean-Claude DUCHET, Jean-Claude LAVALLEY,
AND FUNCTIONALITIES OF SULPHIDED
SaYd HOUSNI, Driss OUAFI, Jean BACHELIER, Mahjoub LAKHDAR, Amar MENNOUR, Daniel CORNET
Laboratoire Catalyse et Spectrochimie - U.A.CNRS.0414 I.S.M.Ra, Universite de Caen - 14032 CAEN CEDEX (FRANCE)
ABSTRACT
Carbon monoxide and oxygen chemisorption was used to investigate the nature and number of surface sites present over sulphided Ni-W/Al 0 catalysts with comoositions varvina over a wide ranae (O-36% WO,: O-B% NiO?.'The aas uotakes were measured by the pulse method at 2j3 K (fo?'CO) and 333 K (for 0 j. The binding of CO on the surface was further studied by FTIR spectrosco y. B The significance of the results in terms of the different functions of the catalysts, namely hydrogenolysis of C-S and C-N bonds and hydrogenation, is discussed. In the pulse experiments, the amounts of CO chemisorbed on the single-component Ni or W/Al 0
c?r3
catalysts are negligible. On the promoted catalysts, the amounts chemis bed are much more important; they can be related to the activities for both C-S and C-N hydrogenolysis, and thus follow the promoter effect. The correlation accounts for the effects of sulphiding, calcination temperature and deactivation with running time, all of which modify the number of active sites, but not their intrinsic activity. The IR spectra_pf CO adsorbed on the promoted catalysts are characterized by a band at 2090 cm which persists after evacuation, whereas the bands recorded over unqomoted Ni or W sulphide do not persist under these conditions. The 2090 cm band is assigned to CO adsorbed on highly reduces W ions influenced by neighbouring Ni ions, and the intensity of the signal is a function of both caialyst composition and sulphiding procedure. In addition, a new band at 1830 cm is recorded over the tungsten-rich promoted catalysts. Oxygen chemisorption measurements provide complementary data. The oxygen uptake measures the number of promoter (Ni) ions in the Ni-W phase, but does not account for all of the HDS sites as CO adsorption is not completely hampered by pre-adsorbed oxygen. However, it appears that oxygen is able to reach the sites responsible for hydrogenation of the pyridine ring. Thus, combining CO and 0
a satisfactory correlation with the hydroge ation $ chemisorption measurements provides activity. The nature of the sites remains unclear, however.
INTRODUCTION
In the search for more active and more selective sulphide catalysts of the CO-MO type, an important feature is the characterization of the active sites.
The most popular method for taking up this challenge is afforded by chemisorption of gaseous probes which is expected to titrate the active sites and elucidate their chemical nature.
In their pionneering work, Tauster et al. [l] correlated the oxygen capacity of unsupported MoS2 catalysts with their hydrodesulphurization (HDS) activity.
Since then, a number of similar correlations have been established for several
0920-5661/66/$09.10 0 1966 Elaevier Science Publishers B.V.
types of unsupported and supported sulphide catalysts (mainly CO-MO and Ni-Mo).
Various catalytic activities obey such correlations, e.g., HDS [l-8], olefin hydrogenation [9] and CO hydrogenation [lo]. However, several studies have shown that the validity of such relationships is restricted to narrow families of samples. It was concluded that there is a lack of selectivity of the oxygen probe, which indistinctly counts sites with different specific activities [9, 11-131. As oxygen adsorption occurs on the edges of the layered MoS2 entities [14], it is considered to be a qualitative means of investigating the dispersion of the active phase. In fact, as reported by Burch and Collins for unsupported MoS2 [13], oxygen adsorption would not occur all along the edges. Therefore, some specificity in adsorption may reveal different kinds of active sites. This adsorption is irreversible but, unfortunately, the interaction with the catalyst surface remains unknown as the adsorbed oxygen species cannot be examined by spectroscopic techniques.
In contrast, nitric oxide and carbon monoxide adsorptions can be followed by IR spectroscopy. Adsorption of NO is attractive, as it enables the promoter, Co or Ni [15], and MO surface ions to be distinguished [15-16). As a counterpart the IR spectra for the mixed catalyst are complex: each adsorbed species gives rise to a doublet band with a narrow spacing, so that overlaps are frequent.
Moreover, adsorption of NO is not very sensitive to the state of the catalyst, either reduced or sulphided. Nevertheless, Topsbe and Tops$e derived a correlation between the HDS activity of Co-Mo/A1203 catalysts and the intensity of the IR band of adsorbed NO [15]. The nature of the adsorption sites, probably located at edge positions, is still under debate: MO*+ or Mo4+ oxidation states have been suggested in the case of the single-component Mo/A1203 sulphide catalysts [17,18]; the interpretation is more difficult for promoted samples.
The CO probe has received much less attention, probably owing to the weakness of the IR signal of coordinated species. The few results pertaining to the sulphided catalysts are promising, however. Peri [17] reported CO adsorption on reduced and sulphided Mo/A1203 catalysts and concluded that there is a lower oxidation degree of MO ions in the latter instance. Similarly, Delgado et al.
[19] suggested that Mo*+-CO complexes are formed on sulphided Mo/A1203 samples;
in this way the concentration of unsaturated sites may be estimated. At the same time, our group found that CO adsorption on the sulphided Co-Mo/A1203 catalysts with low MO contents is distinct from that occurring on the single-component Co and MO sulphide samples [20]. The signal at 2065 cm -1 is attributed to an interaction between the two elements in the mixed sulphide phase, and adsorption is thought to involve MO ions in a low oxidation state. The adsorption sites are identified with HDS active sites as the intensity of the 2065 cm -' line and the catalytic activity follow similar trends. With CO-MO catalysts with a high MO content site heterogeneity leads to a more complex situation [21].
73 Other probes such as H2S [13], CO2 [22] and H2 [23] have been tried but not developed further.
Although the survey of the literature does not prove a universal relationship between chemisorption data and catalytic activity, much can be learnt about sulphided surfaces by performing adsorption studies. This paper deals with oxygen and carbon monoxide chemisorption, performed on Ni-W/Al203 hydrotreating catalysts. Quantitative data were obtained by the pulse technique and compared with the activities measured for the different HDS, HDN and HN functionalities of the Ni-W couple. FTIR spectroscopy was used to study the coordination of carbon monoxide on surface sites. Oxygen adsorption was further examined through its influence on CO adsorption.
EXPERIMENTAL Catalysts
The alumina-supported Ni-W/Al203 catalysts belong to the GS series described in other papers in this series and elsewhere [24]. They are listed in Table 1 with their code numbers (GSi), their compositions expressed as weight of WO3 and NiO per 100 g of catalyst and their atomic ratio, a = Ni/(Ni+W). The whole series covers a wide range of concentrations: constant W (21% WO3) and variable Ni (O-8% NiO), or variable W (O-35% WO3) and constant Ni (3% NiO). In addition, a catalyst with a composition optimized for HDN reactions (GSg: 4.6% NiO, 36%
W03) was studied. A few other samples, either single- (Ni, Co, W) or multi-component (Co-W) were also examined.
After pore volume impregnation and calcination in air at 773 K, the catalysts were standardly presulphided according to an industrial procedure involving hydrogen and a liquid phase under 6 MPa total pressure. The liquid feed consisted of cyclohexane (78% by weight), toluene (20%) and dimethyl disulphide (DMDS) (2%). The temperature was raised stepwise from 423 to 623 K and maintained at 623 K for 2 h. The catalysts were then transferred to sealed flasks where they were stored under a layer of liquid heptane. Some catalysts were directly activated in a flow of H2-H2S (15%) for 4 h at various temperatures (673-723 K). Sulphur analyses of the mixed samples were in close agreement with the theoretical composition WS2+Ni3S2, but lower degrees of sulphidation were obtained with single-component W and Ni catalysts.
TABLE 1
Composition, chemisorption and activity data for the Ni-W/A1203 catalysts
Constant W
4 1
1.5 3.2 20.8 20.9 0.18 0.32
Constant Ni
2 5
3.1 3.0 0 11.2 1 0.45 Catalysts
GS3 0 20.9
0
6 8.2 22.3 0.53
8 7 9
3.3 2.1 4.4 35.2 6.7 36.3 0.23 0.49 0.28 NiO %
wo3 % a q Ni/(Ni+W)
Chemisorption NO (1) O/Ni
0.13 0.57 1.23 0.28 0.29
1.55 0.11 1.23 1.95 0.45 1.84 0.14 0.03 0.30 0.44 0.16 0.31
0.44 0.12 0.27 0.23 0.14 0.27 0.04 0.03 0.07 0.05 0.05 0.05
NC0 (2)
CO/N1
0.05 0.18 0.25 0.09 0.06
Catalytic activities
kHDS (3) 0.8 10.0 12.2 12.3 0.1 8.4 12.8 4.2 14.1
0 1.1
'HDN (4) 1 1.3 - 1.2
0
‘HN (5) 0.2 0.7 1.4 1.5 0.05 0.8 2.9 - 3
Sulphidation (%)
degree 74 104 103 96 28 103 95 - 92
Oxygen uptakes (1D-4g-atom O,g-') Carbon monoxide uptakes (10 mol cg g::) Thiophene HDS rate constants (1.h .g )
Piperidine ring opening initial rates (tmnol.Dt~~.g;~) Pyridine hydrogenation initial rates (mmo1.h .g )
Catalytic activities
The catalytic activities of the sulphided samples were determined by running several test reactions representative .of different functionalities: thiophene hydrodesulphurization (HDS) (673 K, lo5 Pa), pyridine hydrogenation (HN) and piperidine hydrodenitrogenation (HDN) (33.105 Pa, 573 K). The tests were carried out in the presence of H2S, from thiophene for the HDS test
(0.03 < PH2s/PH2 < 0.07, depending on the conversion), or from DMDS for the other reactions (pH2S/pH2 = 0.02). Activities are expressed either by
pseudo-first order rate constants, kHDS (l.h-'.g-l), or initial rates, roHDN (mmol.h-'.g-l),
"HN' under steady-state conditions. All of the presulphided catalysts were treated again with a sulphiding agent (H2S or DMDS) in the reactor itself before the activities were measured.
Carbon monoxide and oxygen chemisorptions
Dynamic oxygen and carbon monoxide uptakes were measured by a pulse method in the catalytic reactor according to a previously described procedure [4]. The sulphided samples were purged in a flow of helium at 673 K for 2 h and then submitted to oxygen or carbon monoxide injections at 333 and 273 K, respectively, with He as the carrier gas. Results are given in g-atom 0.g and mol.CO.g-' (NCo).
-' (NO)
Infrared spectra were recorded with a Nicolet MX-1 FTIR spectrometer. The catalysts were pressed into self-supporting wafers and activated (oxidic samples) or reactivated (presulphided samples) by three successive treatments for 1 h, 1 h and 12 h at 673 K with 150 Torr of H2-H2S (10%) and finally evacuated (10B5 Torr). Then CO was introduced into the catalyst cell at room temperature (final pressure 10 Torr). The bands of the adsorbed CO species (total adsorption) were obtained by subtracting the spectra recorded after and before CO introduction. Species remaining after evacuation represent the irreversible adsorption. In some experiments, 10 Torr of oxygen were first admitted into the cell .at room temperature, then evacuated before CO chemisorption was performed.
RESULTS
Oxygen chemisorption
Conditions of measurement. The amount of oxygen taken up in pulse experiments on a particular Ni-W/A1203 catalyst (GS8) is plotted in Fig. 1 for various temperatures.
The continuous increase in the amount chemisorbed is disturbed by a pseudo-plateau between 323 and 353 K, as previously noted for Ni-Mo/A1203 catalysts [25]. Under such conditions, saturation is rapidly attained. The first pulse accounts for about 80% of the total uptake; the third and fourth injections are adsorbed less than 0.5%. At lower or higher temperature, saturation is much more difficult to obtain. Our oxygen measurements performed at 333 K are then considered as achieving surface oxidation of the sulphide. The short contact times of the oxygen pulses do not yield any sulphate species, as was checked by IR spectroscopy, which is very sensitive towards such species
WI.
T(k)
Fig. 1: Temperature dependence of oxygen consumption by the sulphided GS8 Ni-W/A1203 catalysts.
Oxygen uptakes. Dynamic oxygen chemisorption measurements are reported in Table 1. The amount of oxygen taken up by the unpromoted tungsten sulphide phase (GS3, 21% WO3) is very small. The same trend is also observed at higher W loadings, where the O/W stoichiometries range from 0.085 to 0.015, far below the corresponding values for sulphided MO catalysts [27]. As under our conditions oxygen uptake is thought to saturate the surface, such low values reveal some specificity of the probe. Therefore, oxygen adsorption is unlikely to occur all along the edges of the WS2 slabs on the W/A1203 samples.
The single component Ni/A1203 sample (GSZ) is also weakly reactive towards oxygen pulses, yielding an O/Ni ratio of less than 0.03. This is in contrast to previous studies of an Ni series [3]. However, as the GS sample is much less active for thiophene HDS, the difference may be attributed to a lack of dispersion. Indeed, the preparation steps were not kept identical in both instances.
Oxygen chemisorption is more important over the mixed Ni-W catalysts, and exceeds considerably the sum of the uptakes expected from the individual Ni and W components. Provided that the composition parameter a of the catalyst does not exceed 0.45, the No value increases with increasing Ni content and more moderately with W content. Ignoring tungsten, the O/Ni ratio remains almost constant (close to 0.3). For a values above 0.45 a decrease in the O/Ni stoichiometry is observed (GS6, GS7).
Then, neglecting the adsorption on the single-component catalysts, it appears that oxygen is able to detect the build-up of the mixed sulphide phase. In that sense, oxygen chemisorption can be considered as a measure of the fraction of the active surface which, at constant W coverage, grows as the amount of promoter increases. However, detection of the promoter does not mean detection of the active sites. This is clearly evidenced by inspection of Table 2: oxygen uptake is essentially the same whether or not thiophene HDS has been run over the samples. As the HDS activity drastically decreases with running time, it is clear that the oxygen molecule is not a selective probe to count HDS active sites. Hence all the promoter ions do not function similarly and no general correlation can hold when a wide range of samples are measured.
TABLE 2
Oxygen and carbon sulphided catalysts
monoxide uptakes by fresh and thiophene-run Ni-W/A1203
Fresh Thiophene-run Oxygen
No (10-4g-atom O.g-') GSl 1.33 1.23
GS9 1.80 1.84
Carbon monoxide
NC0 (1O-4 mol.CO.g-') GSl 0.39 0.25
GS9 0.45 0.27
HDS of thiophene
kHDS (l-h-l-g-') GSl 19.1 12.2
This situation is exemplified in Fig. 2, which further indicates that oxygen does not properly count the sites responsible for hydrogenation. It confirms similar observations reported by other workers for promoted Co(Ni)-Mo/A1203 catalysts [2].
Fig. 2: Oxygen uptake hydrogenation catalysts.
No (lo-4 g.at.0.g-l)
versus thiophene HDS rate constant (circles) and pyridine initial rate (squares) for the sulphided GS Ni-W/A1203
Carbon monoxide chemisorption by pulse experiments
The GS series of catalysts was also submitted to CO chemisorption carried out by the same pulse technique but operated at 273 K. The alumina carrier is inactive under these conditions. The results are given in Table 1.
The unpromoted W catalyst (GS3) adsorbs very little CO, about 10 times less than an Mo/A1203 catalyst with the same metal coverage. With the promoted Ni-W samples, CO adsorption is much higher and varies with catalyst composition, but the CO/Ni ratio is not constant as Ni increases within the series 3-4-l-6. This indicates that CO is not.as sensitive as oxygen to the presence of the promoter.
In fact, considering a given Ni-W/A1203 catalyst (GSl, GS9), we observe that the CO uptake decreases sharply from the freshly sulphided sample to the quasi-stabilized catalysts which had been used in the thiophene hydrodesulphurization reaction for 15 h (Table 2). We observe a similar decrease for the reaction rate. Therefore, CO appears to be more closely related to the presence of HDS active sites than to the amount of promoter. This is not the case for oxygen. A general correlation can be drawn between CO uptakes and the rate constants for thiophene HDS on the GS series (Fig. 3).
0.2 0.4 NC0 (lo-4 Ial01 Cog-I)
Fig. 3: Correlation between CO uptake and thiophene HDS rate constant
(circles) and piperidine ring openingg initial rate (squares) for the sulphided GS Ni-W/A1203 catalysts. Open symbols: non-standard
catalysts.
Two samples of the series (GS2 and GS6) deviate from a straight line. In fact, IR studies of CO adsorbed at low temperature (260 K) on the single-component Ni/A1203 catalyst indicated some nickel carbonyl formation, and also on the GS6 sample characterized by a high Ni/W ratio. Therefore, not all of the added nickel is expected to promote the tungsten phase and the amount of CO measured by the pulse method is overestimated on these samples.
Interestingly, the correlation holds for catalysts that have been activated under conditions widely different from the standard. Thus, sulphiding by an H2S/H2 gas mixture at 673 K and atmospheric pressure results in both a smaller CO uptake and a lower HDS activity. Also, as reported in another paper in these series [28], the GSl samples sulphided at different temperatures (673-923 K) shows a maximum at 773 K for both activity and CO chemisorption. Similarly, CO chemisorption readily detects the influence of calcination and gas-phase sulphiding temperatures; the activity of the GS9 sample with respect to HDS is optimum when calcined at 673 K and sulphided at 773 K. The general correlation is further extended to samples with a different Ni-W composition (35-46% W03 and 1.5-4% NiO) sulphided in the gas phase, and also applies to catalyst deactivation during the reaction.
The CO adsorption by the catalytic Ni-W surface is closely connected with the number of HDS active sites. As a unique turnover number (0.2 s-l) holds for all the promoted samples, it is concluded that different pretreatments of the catalysts may affect the number of the HDS sites but not their intrinsic activity. A promoter factor of six can be deduced from the turnover numbers calculated for the W and Ni-W series. Similarly, deactivation kills some of these sites.
This behaviour within the Ni-W couple contrasts with the Ni-Mo or CO-MO systems, for which the correlation held only for restricted families of samples (i.e., with low MO content [4]).
Considering the other functionalities of the Ni-W catalysts, Fig. 3 emphasizes the correlation between the activity for C-N bond rupture (evaluated by the initial rates of piperidine ring opening) and CO uptakes. However, it is clear that the hydrogenating power of the mixed sulphides, as measured by pyridine ring saturation in the presence of H2S, cannot be detected by the CO probe. The results are displayed in Fig. 4. Thus carbon monoxide appears to be specific for the hydrogenolysis sites of the sulphide catalysts.
O-.2 0.4 NC0 (lOdmol CO.g-')
Fig. 4: CO uptake versus pyridine hydrogenation initial rate.
81
Infrared study of adsorbed CO
In contrast to oxygen, CO adsorption is detected by IR spectroscopy. The signal depends on both the nature of the active phase (Ni, W or Ni-W) and its state, oxidic or sulphided.
The y-alumina carrier shows no signal of adsorbed CO unless it is activated at 723 K. Then the weak signal centred at 2198 cm -I (Fig. 5) readily disappears on evacuation at room temperature. Pretreatment of the carrier with 10% H2S/H2 at 723 K leads to the same conclusion.
OS-4 GS-3
,218s
OS-2
Fig. 5: Infrared spectra of CO adsorbed on oxidic GS Ni-W/A1203 catalysts;
Pco=lOO Torr.
Oxidic samples (calcined state). The spectral features of CO adsorbed on the oxidic catalysts are shown in Fig. 5. Compared with alumina, the oxidic Ni/A1203 catalyst (GS2) shows an important but reversible chemisorption, characterized by a main band at 2185 cm -1 with a shoulder at 2195 cm -1 . These indicate that CO is adsorbed on different Ni species. Nl .2+ ions located at the surface are likely to function as coordination centres as, according to Primet et al., a band at 2195 cm -1 is observed on Ni2+/Si02 [29]. The two bands would correspond to different environments of the Ni 2+ ions.
CO adsorption on the oxidic W/A1203 samples leads to a reversible single band near 2000 cm -1 similar to that observed on the support itself. However, the signal is slightly shifted from 2198 to 2208 cm-l owing to increasing introduction of W.
With the mixed oxidic Ni-W/A1203 catalysts, a single reversible signal is observed, the position of which depends on the composition: at low a (GS4), the band observed at 2202 cm-l resembles that recorded on W samples; it tends towards the Ni signal (2195 cm-') at high a values (GS5).
Sulphided samples. Sulphiding the catalysts leads to major changes in the IR spectra of adsorbed CO.
With Ni/A1203 catalysts (3-8% NiO) sulphided by a gas-phase procedure, CO adsorption gives rise to several bands in the ranges 2200-1900 and 1900-1100 cm -1 . The spectrum of the 3.6% NiO sample is shown in Fig. 6a.
cm-l
’ 22bo ’ ’ ’ l$oo ’ ’ ’ 1400 ’ ’ ’
Fig. 6: Infrared spectra of CO adsorbed on sulphided Ni/A1203 catalysts:
P =lOO Torr.
(gp 3.6% NiO/Al O3 sulphided in the gas phase ib) GS2 sulphided in the liquid phase
----: CO adsorbed on oxygen-covered sample).
83
Apart from the weak bands at 2200, 2137 and 2155 cm-', attributed to A13+-CO, Ni+-CO and Ni+-(CO)2, respectively [30-321, the first region exhibits a main reversible signal at 2085 cm -l. This band can undoubtedly be assigned to CO bonded to Ni atoms reduced to the zero-valent state [31]. Their presence is confirmed by a band observed at 1920 cm -1
, attributed to bridge Ni2+C0 species.
Simultaneously, the second region of the spectrum shows irreversible bands at 1230, 1440, 1480 and 1655 cm-l. These are specific hydrogencarbonate species, and indicate some CO2 formation. This fact again suggests the presence of Ni atoms in a reduced state, as metallic nickel is known to catalyse the dismutation reaction 2 CO -CO 2 +C.
The spectrum is fairly insensitive to the temperature of the gas-phase sulphiding in the range 673-873 K. It is also reproduced for the Ni/A1203 catalyst GS2 presulphided in the liquid phase, but with a lower intensity (Fig.
6b). The occurrence of metallic nickel suggested by the IR results is surprising as we are dealing with a sulphided sample, and one may question whether or not the CO probe is able to reduce Ni ions under our chemisorption conditions.
Although such a possibility cannot be firmly excluded, our IR results are in close agreement with XPS studies on the same GS2 catalyst [33], which also indicate a fraction of nickel in a zero-valent state on the bare sulphided catalyst in the absence of CO.
The presence of metallic nickel on the sulphided GS2 sample is further confirmed by adsorbing CO at 260 K instead of at room temperature. A new band is recorded at 2050 cm -1 , the intensity of which increases with increasing CO pressure (Fig. 7).
Fig. 7: Infrared spectra of adsorbed species resulting from CO
adsorption at 260 K on a 3.6% NiO sulphided Ni/A1203 catalysts.
cm-l
‘tdoo * ‘2&o’
the surface. This compound is completely desorbed at room temperature, which makes the phenomenon unobservable under these conditions.
Preadsorbing oxygen at room temperature on the sulphided sample almost totally hinders subsequent CO adsorption on the Ni" site (Fig. 6). The ensuing spectra show a weak band at 2190 cm -1 similar to that observed on the oxidic catalyst. Moreover, the preadsorption of oxygen significantly increases the formation of hydrogencarbonate species. In that event, the reoxidized catalytic surface might react with CO during the subsequent CO adsorption experiment, yielding CO2.
The W/A1203 catalyst (GS3, 23% W03) sulphided with a gas phase can adsorb CO provided that the sulphidation temperature exceeds 573 K. The spectra in Fig. 8 show bands centred at 2200, 2110 and 2060 cm -1 .
2200 2000
Fig. 8: Infrared‘spectra of CO adsorbed on W/Al 0 catalysts; P
effect of sulphiding conditions: gas-phgsg procedure, tEP =lO Torr;
peratures from 673 to 873 K; liquid-phase procedure, 623 K.
The reversibility of all bands as CO is pumped off is consistent with the dynamic adsorption measurements. The signal at 2200 cm -1 is similar to that recorded on the oxidic catalyst and indicates that a fraction of W is unaffected by sulphiding. This is in agreement with the incomplete sulphiding indicated by
XPS [33] and sulphur content analysis (Table 1). The relative intensities of the two other signals vary with sulphiding temperature so that the high-frequency band (2110 cm-') becomes converted into the low-frequency band (2060 cm-') when the temperature increases from 673 to 873 K. By analogy with the sulphided Mo/A1203 catalysts for which the low-wavenumber CO signal was attributed to adsorption on molybdenum with a low oxidation degree [19, 201, it is likely that a high sulphiding temperature strongly reduces the W catalyst. Indeed, the sulphiding process is also able to create highly reduced species on Ni/A1203 catalysts.
With the GS3 sample presulphided in the liquid phase at 623 K, the band at 2060 cm-l has the highest relative intensity. This procedure increases the fraction of reduced sites. An equivalent gas-phase sulphoreduction requires a temperature as high as 873 K.
Preadsorption of oxygen on the sulphided W/A1203 GS3 catalyst results in a lower intensity of the two CO bands at 2110 and 2060 cm-I. The absorbance at 2200 cm-l seems unaffected, which is in line with its assignement to the oxidic fraction of W remaining upon sulphidation.
The behaviour of the Ni-W/A1203 GSl sample (3% NiO, 23% W03) sulphided by H2S/H2 was studied in detail. The surface of a sample sulphided at 673 K can chemisorb CO in three different ways, according to the IR signals at 2127, 2090 and 2073 cm-l (Fig. 9). For higher sulphiding temperatures (from 673 to 873 K), the side bands are less intense while the central band increases sharply. The new fact is that part of the main signal at 2090 cm -1 is irreversible; the part which persists on evacuation increases with sulphiding temperature.
Although the band at 2090 cm -1 nearly coincides with that observed on sulphided Ni/A1203 samples (ca.2085 cm-'), the presence of metallic nickel on the Ni-W system seems unlikely for the following reasons: (i) on the promoted catalyst no Ni(C0)4 is formed as CO is' chemisorbed at low temperature; and (ii) NiO species are not detected by XPS analysis. Hence this IR signal characterizes the Ni-W sulphide phase in which the promoter and the tungsten probably strongly interact, like Co and MO in the "CoMoS" phase [34]. The two other bands of the spectrum (2127 and 2073 cm-') may correspond to CO coordinated with the unpromoted tungsten phase. Their decrease with increasing sulphiding temperature is then consistent with a more complete association between Ni and W atoms.
However, the observed 15 cm -1 shift relative to the single-component W catalyst would indicate that although not fully incorporated into the promoted phase, some W atoms in the mixed Ni-W catalysts are influenced by nickel. Some precursor state of the Ni-W-S active phase could exist.
.-.. . .
b
a n GSl
cm-l
. . . . . +-
2200 2000 2200 I . . I 2000 r 1
Fig.9: Infrared spectra of CO adsorbed on the GSl Ni-W/Al 0 catalyst sulphided in the gas phase (a) and liquid phase (b?.3PCo=10 Torr ( ----: irreversible species).
The whole spectrum is more intense on liquid phase sulphiding, but the band positions remain unchanged.
In order to ascertain that the band at 2090 cm-I reflects a promoter effect in the Ni-W samples, a Co-W/Al203 catalyst (3% COO, 11% WOJ) was sulphided and subjected to CO chemisorption. With a sulphiding temperature of 673 K the IR spectrum consists of a 2080 cm -' band, with a 2110 cm-l side-band and a shoulder towards lower wauenumbers. For a sample sulphided at 773 K these side-bands disappear, leading to one symetrical band, at 2075 cm-', partly irreversible. As a sulphided Co/A1203 sample adsorbs CO reversibly giving a Co-CO band at 2055 cm -1 , there is no doubt that the signal at 2075 cm -1 characterizes new adsorption sites, probably created at the surface of the Co-W-S entities. Then, with the Ni-W couple, the CO signal at 2090 cm -1 is also attributed to
"promoted" sites and not to Ni/A1203-like nickel sites. Finally, it is worth
mentioning that the tungsten phase can undergo complete promotion by cobalt more easily than by nickel.
Influence of the catalyst composition. Fig. 10 shows the spectra recorded for the series of promoted Ni-W GS catalysts sulphided by the liquid-phase procedure. The main features described for the GSl sample are modified considerably as the catalyst composition changes, although the reversible band at 2125 cm -1 is recorded for all Ni-W compositions. Unpromoted W sites are always present, increasing with increasing W/N1 ratio (GS5 < GSl < GS8); the tungsten phase in also incompletely promoted in the series GS4, 1, 6, at constant W and with variable amounts of Ni.
t
/ a.**** . . . . . .
AA
_;a
. . . . . . ..( a...;
. . . .-*-~~ **.. . . . ...* . _.. ** 5..OS 6
A
. . . . . . . . j.--0.. .-._
-....*.
’ 2ioo 2600
j
**-*.-... . . . .._ _.__ . . . . 2
cm-l t 2foo I I I I
1900 I
Fig. 10: Infrared spectra of CO adsorbed on the GS Ni-W/Al 0 catalysts sulphided in the liquid phase; effect of catalyst 23 c mposition.
. irreversible species).
Fig. 11:
AA
[
cm-1 -
t * I 1
2200 ’ 2000
Effect of preadsorbed oxygen on CO adsorption on the sulphided GSl Nr-W/Alz03 catalyst).
( -: bare surface)
( ..----_-: oxygen pre-covered surface)
( --*-.-: irreversible species after oxygen adsorption)
This phenomenon cannot be attributed to a lack of nickel in the case of samples with high a. Indeed, the catalysts GS5, 7 and 6 give rise to nickel carbonyl formation on CO chemisorption at low temperature. The second band at 2075 cm-', which would confirm the presence of unpromoted tungsten, is not observable owing to the close proximity of the 2085 cm -1 signal.
The signal at 2085 cm -1 is also observed for every composition and part of it resists evacuation. According to the above-mentioned studies on GSl, this indicates an intimate Ni-W association. However, with catalysts GS5, 6 and 7 with high Ni/W ratios, CO adsorption on non-interacting Ni species may also occur, giving a band at the same position, but reversible. Then only the irreversible part of the 2085 cm -1 signal can be used to evaluate the relative amounts of the corresponding "promoted" sites within the series of GS samples. A maximum is found for catalyst GSl. The concentration of such sites decreases at higher W loadings (GS8) and the irreversible part of the signal is ultimately cancelled with GS9 catalyst.
In contrast, samples GS8 and 9 are characterized by the appearance of a new band, totally irreversible at 1830 cm-l. This band is not attributed to any carbonate species, although prolonged contact of the sample with CO produces co2. It would correspond better to CO strongly bonded to the surface, such as a bridging CO species on to metallic clusters. This adsorption is more likely to occur on ensembles of metal atoms or ions which can be realized on the more heavily charged catalysts.
Effect of oxygen preadsorption. Fig. 11 shows that the total CO adsorption on the promoted GS1 Ni-W samples is sensitive to oxygen preadsorption. For catalysts with moderate W loadings, oxygen almost totally suppresses the irreversible part of the promoted band at 2085 cm -l. The other signals, i.e., reversible bands at 2085 and 2125 cm -1 , do not seem affected. At higher W concentration (GS9), oxygen preadsorption totally hinders the irreversible adsorption of CO characterized by the band at 1830 cm -1 .
DISCUSSION
The sulphided Ni-W/A1203 catalysts appear suitable for studies of the activity-chemisorption relationship as they afford the useful advantage over the other pairs (Ni-, CO-MO) that the W/A1203 sample is nearly inactive according to the catalytic tests and insensitive to pulses of oxygen and carbon monoxide.
This enables the observed uptakes to be related to the promoted active sites on the mixed catalysts.
Specificity of O2 and CO probes
Following Topsde et al. [34], we assume that the promoter atoms create active sites on the edges of the layered Mo(W)S2 slabs. The importance of the edge
by the geometric model developed by Kasztelan et al. 1351. Finally, Chianelli [36] obtained convincing evidence for oxygen adsorption occurring on the edges of an MoS2 crystal.
With the Ni-W/A1203 catalysts, it is unlikely that adsorption is restricted to corner atoms, as was suggested by Burch and Collins [13]. Electron microscopy performed by Breysse et al. on catalysts GSl and GS9 [ZB] indicated the lateral size of the slabs to be about 40 i the values for oxygen uptake then lead to unrealistic O/corner stoichiometries, ranging from 5 to 7 depending on the pretreatment of the samples. On the other hand, we obtain 0.5-0.6 oxygen atom chemisorbed per edge metal site, whereas a 1:l stoichiometry would be expected if the 02 molecule dissociates and 2:l if adsorption is associative.
Several possibilities may account for the low O/edge ratio: (i) first, recalling that the adsorption sites are associated with the promoted phase, there might be incomplete decoration of the WS2 slabs by the promoter; (ii) alternatively, if completion does occur, not all the Ni atoms associated with the tungsten phase are active. Both possibilities would be consistent with the XPS observation of Bonnelle et al. [33] that even though the catalysts appear completely sulphided into (WS +Ni3S2), a significant fraction of W6+
2+ species is
observed together with some W species. Additional W-O-Al linkages are thought to be responsible for these unexpected W6+. According to the first hypothesis, they might hamper the build-up of the mixed Ni-W phase with the corresponding edge W atoms; alternatively, they might prevent the nickel functioning as a promoter. The presence of unpromoted W ions suggested by the IR spectra of adsorbed CO (bands at 2127 and 2073 cm-') would favour the former proposal, although the observed shift in wavenumbers compared with the single-component W/A1203 catalyst is intringuing.
In every instance, oxygen is selective for promoted edge sites. Therefore, it cannot measure the dispersion of the sulphide phase, as reported for the CO-MO couple [9], but, more interestingly, it could indicate the extent of the promoted Ni-W-S phase, provided that the O/promoter stoichiometry has been determined.
The specificity of the oxygen probe is further suggested by combined oxygen and carbon monoxide adsorption. Oxygen preadsorption lowers the overall CO chemisorption and, from the IR results, CO reversibly adsorbed on the Ni-W surface is hardly sensitive to the preadsorption of oxygen. Hence O2 and CO do not cover the same range of sites. Therefore, both probes are necessary to describe the catalytic surface and their connection with the catalytic activities will now be discussed.
91
Carbon monoxide chemisorption and hydroqenolysis activity
Heterogeneity in HDS sites. The relationship between the activity for C-S or C-N bond rupture and the amount of carbon monoxide adsorbed at 273 K on the Ni-W/A1203 catalysts is striking (Fig. 3). The correlation holds remarkably well for samples differing widely in composition, calcination and sulphiding procedures and also in extent of deactivation. Obviously, some fortuitous coincidence can be ruled out and one must admit that CO specifically detects hydrogenolysis sites on the these catalysts.
Anion vacancies are generally identified as the HDS active sites; they are indeed appropriate for CO adsorption, which we found to be poisoned by pyridine.
IR spectra distinguish several types of coordinated CO with differences in binding strength on the Ni-W/A1203 catalysts. Probably only the more strongly bound species are counted by the chromatographic method; the weakest, if present during the pulse, will be eluted later. Hence the pulse technique is not expected to measure the total CO capacity but only the "irreversible" species revealed by IR and also some of the reversible species.
In view of the very low CO uptake on the fairly unreactive W/A1203 samples, the 2125 cm-l signal corresponds to very weak sites that are not catalytically significant. For most of the Ni-W samples with moderate W contents, the main CO signal at 2090 cm -1 covers a range of adsorption site strengths, as revealed by pumping, i.e., the reversible part characterizes sites (I) that are weaker than the irreversible part (sites II). A third kind of sites (III) is also indicated by the IR band at 1830 cm-l on the Ni-W catalysts with high W contents.
The unique correlation observed between HDS activity and CO uptake for the whole series of catalysts indicates that the three adsorption sites are involved in the C-S hydrogenolysis reaction. Candia et al. [37] proposed two categories of HDS sites on Co-Mo/A1203 catalysts: the most active ones are obtained after high-temperature sulphiding, which loosens the interaction between alumina and the CO-MO-S phase. With our Ni-W/A1203 catalysts, improving the sulphidation procedure (gas phase at high temperature or liquid phase) is found to increase the CO signal at 2090 cm -1 and preferentially the irreversible CO species of type II. It may therefore be assumed that the two (I and II) CO sites coexist after moderate sulphiding (673 K) and correspond to Candia et al.'s hypothesis.
Their distribution depends on the Ni-W composition. From IR results, type II sites are maximum for 3% NiO - 21% W03. A third variety occurs at high W content.
Definition of the HDS sites. It is generally agreed that the hydrogenolysis sites are first required to adsorb the reactive molecules and are therefore located on sulphide vacancies. From NMR studies, Ledoux et al. [38] concluded that anion vacancies are borne by the promoter ions; indeed, the nickel and
cobalt sulphides supported by carbon bear a number of such vacancies, by De Beer et al. [39].
However, the surface sites on monometallic (W/A1203) catalysts are
as found
strongly modified on addition of promoter. An electron transfer from the promoter (Ni) to the base metal (MO or W) is substantiated by both theoretical [40] and spectroscopic [34,38,41,42] data. Our IR results obtained using adsorbed CO fully confirm that sites on Ni-W catalysts (band at 2085 cm-') are distinct from those on W/A1203 (band at 2120 cm-'), and point to a high degree of reduction of the metal ions. From the position of the band at 2085 cm-l alone, it is not possible to decide whether the adsorption site is as zero-valent nickel or a low-valency tungsten influenced by a neighbouring nickel, but the results obtained with Co-W catalysts favour the second possibility.
In any event, these highly reduced catalysts possess some metallic character, as emphasized by Vissers et al. [43] and confirmed by us for alkane conversion [44]. The low valency of the metals means that many sulphide anions have been removed form the border of the WS2 patches, where the local structure is distorted from the bulk. As a result, an exceptionally high concentration of metal ions with short mutual distances appears on the edge, and a variety of adsorption centres may be derived.
The Ni-W/A1203 catalyst with a high W loading affords a new situation because, in the absence of sites II, the HDS activity seems almost exclusively achieved by sites III. CO is postulated to be adsorbed on these sites with the same type of bonding as observed on clusters. Therefore, the active phase undergoes structural changes at high W concentration. In fact, after reaching full monolayer coverage of the support, the "CO-MO-S type" phase may plausibly rearrange owing to a three-dimensional build-up.
Finally, the parallel observed between HDS or HDN and CO chemisorption suggests that both reactions proceed through the same functionality of the sulphide catalyst. This is further supported by the lack of significant surface acidity of the catalysts [44], which may therefore not be necessary for the HDN mechanism.
Chemisorption and hydrogenation
It has been reported several times that hydrogenation and bond-cleavage reactions do not operate on the same sites of a Co(Ni)-Mo sulphide catalyst [37,45]. This is also true with the Ni-W couple, as strongly suggested by the chemisorption-activity data, because pyridine hydrogenation does not correlate with either CO or O2 uptake.
Activity data reported in another paper in this series [24] confirm the distinction. On the other hand, our chemisorption studies have proved different specificities of the CO and O2 probes towards the catalytic surface. Indeed,
93
preadsorbed oxygen almost poisons type II or type III CO adsorption sites, depending on the W loading. It is obvious from the high values of the No/NC0 ratio measured on the bare catalysts that other sites of the surface, not detected by CO, are sensitive to oxygen. They may be quantified by some function
(NO - x NCo) that is equivalent to subtracting the contribution of type II (or III) sites from the total oxygen uptake (see Table 3).
TABLE 3
Site specificity for carbon monoxide and oxygen
HDS HN
Site III
CO adsorption ; +
0, adsorption 0 0 0
We expect that the corrected value of No will represent the number of hydrogenation sites. The value for x is at first unknown, as it depends on the distribution of CO between sites I, II and III, and also on the fraction of each sites covered by CO. However, if we assume a catalytic specificity of these sites, x can be calculated from a correlation with the pyridine hydrogenation reaction rate. Fig. 12 shows that a satisfactory straight line is obtained for x = 2.
lo4 (No - 2 NCC)
Fig. 12: Correlation between pyridine hydrogenation initial rate and chemisorption.