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SiO2 solids: adsorption of hydrogen and carbon monoxide
Applied Catalysis A: General, 98 (1993) 45-59 Elsevier Science Publishers B.V.. Amsterdam
45
APCAT A2494
Complementary study by calorimetry and infrared spectroscopy of alkali metal doped Pd/SiO, solids: Adsorption of hydrogen and carbon monoxide Monique Gravelle-Rumeau-Maillot, Martin and H&ne Praliaud
Veronique Pitchon, Guy Antonin
Znstitut de Recherches sur la Catalyse, Laboratoire Propre du CNRS, Conventionnd k Wniversitk Claude Bernard Lyon Z, 2 Avenue Albert Einstein, 69626 Villeurbanne Ckdex (France) (Received 12 June 1992, revised manuscriptreceived 20 January 1993)
Abstract Alkali metal (Li, Na, K) promoted Pd/SiOa catalysts were preparedby adding nitrates to the Pd/ SiOaprecursorand subsequenthydrogenreduction. The initial heats of adsorption of carbon monoxide and of hydrogenat 298 K and at 473 K and the variations of the differentialheats with coveragewere measuredby calorimetry.The changes caused by the presenceof the alkali: decreasein the initial heat of hydrogen adsorption at 298 K in the presence of lithium and increase in the initial heat of carbon monoxide adsorption at 298 K in the presenceof potassiumand sodium are very small (ca. 10% ). These results,and the conclusions deducedfrom the infraredspectraof adsorbedcarbon monoxide on the same samples,are compared. They are discussedassumingthat alkali cations interact with the oxygen atom of adsorbedcarbon monoxide. Calorimetryclearlyshows that adsorption heats of hydrogenand carbon monoxide do not change appreciably when alkali cations are present on palladium although infrared spectroscopy of adsorbed carbon monoxide and catalytic properties are modified. Thus initial adsorption heats determinedby calorimetryand catalytic activities are not directly related.
Keywords: alkali promotion; calorimetry; carbon monoxide adsorption; hydrogen adsorption; palladium; silica-supported palladium
INTRODUCTION
The catalytic and physico-chemical -properties of metals are significantly affected by the addition of alkali metal promoters [ 11. In our laboratory we have studied model catalysts consisting of palladium supported on silica promoted with alkali compounds [ 2-61. After reduction, the alkali is present in an ionic form [ 21, associated with counteranions (oxCorrespondence to: DrH. Praliaud, Institut de Recherches sur la Catalyse, Conventionnk B l’Universiti Claude Bernard Lyon I, 2 Avenue Albert Einstein, 69626 Villeurbanne CBdex, France. Tel. (+33)72445337, fax. (+33)72445399.
6926-660X/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
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M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
ygen, hydroxyl or mainly silicate) and located partly on the support and partly on the palladium particles, thus giving rise to a partial decoration of the metal surface by the alkali compound [ 31. In the presence of alkali the X-ray photoelectron binding energies of metallic palladium are lowered, the decrease being larger in the presence of sodium and potassium than in the presence of lithium [ 21. The variations in binding energy are not related to changes in the metal particles size; they have been assigned either to an initial chemical shift due to the electron-enrichment of the palladium surface atoms or to a modification of the relaxation process. Furthermore, the addition of Na, K or Cs leads to important changes in the infrared spectrum of carbon monoxide adsorbed on palladium with the appearance of a new band at low wavenumbers in the range 1825-1700 cm-’ according to the alkali and to the coverage [ 41. The smaller the ionic radius of the alkali (Na), the lower is the carbon monoxide wavenumber for the new band which was assumed to be due to an interaction between the oxygen atom of adsorbed carbon monoxide and Na, K or Cs ions. The case of the lithium-doped solids is complex, since the interaction is not clearly increased in spite of the smaller ionic radius of lithium [ 41. The rate of formation of methanol from syngas at atmospheric pressure catalyzed by metallic palladium is strongly increased in the presence of lithium while it decreases in the presence of potassium and caesium following the sequence: Li > Na > unpromoted solid > K > Cs. This sequence parallels the concentration of free hydroxyl groups on the support, suggesting that the synthesis of methanol proceeds via a formate intermediate resulting from carbon monoxide insertion into hydroxyl groups [ 51. It has also been shown that the catalytic features, i.e. the rates and the kinetics parameters, of ethane hydrogenolysis and benzene hydrogenation over palladium are strongly altered by the presence of alkali [ 61. The reactions rates are decreased by factors varying between 5 and 20. The alkali induces an increase of the activation energy of the ethane hydrogenolysis from around 230 to 300 kJ mol-l, the formation of cyclohexene and a modification of the orders towards benzene and hydrogen during the benzene hydrogenation. These observations suggest that the alkali acts as an electronic modifier strengthening or weakening the palladium-adsorbate bonds. Thus, it appears desirable to examine to what extent the addition of alkali modifies the energetic8 of the adsorbate-palladium metal surface systems. As a general rule the presence of alkali changes the binding energies of hydrogen and carbon monoxide on metals and the sticking probabilities [ 7,8]. These energies have generally been deduced from temperature-programmed desorptions except in the work of Dry et al. [ 91 in which the initial heats of adsorption at low coverages were determined calorimetrically. For single-crystal faces promoted with vapodeposited alkali metals [8,10-121 and for supported metals promoted with alkali ions [9,13] the addition of potassium to Fe [&lo], Ni [ 111, Pd [ 121 or Ru [ 131 leads to the appearance of more tightly bound states
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47
of carbon monoxide. The carbon monoxide adsorption heat increases by around 15-20 kJ mol-l [9,10], but there is some discrepancy about the variation of the heat of hydrogen adsorption on alkali-promoted iron solids. For Dry et al. [9] the presence of K,O lowered the initial heat of hydrogen adsorption on magnetite by 25 kJ mol-l but other investigators found that potassium enhanced the hydrogen binding strength on iron by about 8-10 kJ mol-’ [ 8,14,15]. In the present study the heats of adsorption of carbon monoxide and of hydrogen at 298 K and under conditions close to the conditions of the catalytic reaction for the formation of methanol from syngas, i.e. at 473 K, were measured and discussed. EXPERIMENTAL
Preparation of the solids The Pd/SiO, precursor was prepared by the ion-exchange technique. The silica support (Aerosil from Degussa 200 m2 g-l) was contacted with 2 in a basic solution. After washing and drying the solid was Pd(NH,),(OH) calcined at 573 K (heating rate 0.2 K min-’ ) in an oxygen flow. A part of the solid was then added to a solution of alkali nitrate (lithium, sodium, potassium or caesium) and stirred. The solution was evaporated to dryness under reduced pressure in a rotary evaporator. The palladium content in Pd/Si02 is 2.2 wt.-% and the atomic alkali-to-palladium ratios are 2 for Pd-Li/SiO,, 2.4 for Pd-Ma/Si02, 1.8 for Pd-K/SiO,. Another lithium doped solid with an alkali/ Pd ratio equal to 7.4 was also prepared since it has been shown that, during the hydrogenation of carbon monoxide, the catalytic activity for the formation of methanol is promoted by lithium and increases with the lithium content [ 51. In this paper the solids are designated by the atomic alkali-to-Pd ratios. The reduction was carried out overnight in a flow of hydrogen (3-5 dm3 h- ’ ) by linearly raising the temperature (2 K min-‘) from 298 to 673 K. Under these conditions nitrate ions are completely decomposed and (or) reduced into nitrogen and palladium is reduced to its metallic form.
Particle size measurements The reduced catalysts, subsequently air-contacted, were examined in a JEOL 100 K electron microscope with a 0.3 nm resolution. The specimens were prepared by sedimentation on a carbon-coated copper grid of an aerosol of the catalyst powder in order to avoid any dissolution of alkali metal compounds [ 31. Particle size distributions, counting between 200 and 350 particles, were narrow enough to allow the determination of the mean diameter. The average surface diameter, Cn& /Enid F, ( ni number of particles with diameter ranging
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M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
between c&-M/2 and di +dd/2, dd = 0.5 nm), comparable to the mean diameter deduced from chemisorptions [ 161, was calculated. For the Pd/SiO, solid the particles are homogeneously distributed on the Aerosil particles and are situated in the size range 1.0-2.5 nm. The average surface diameter is equal to 2.0 nm. The presence of the alkalis induced moderate increases in the particles sizes distributions and in the average surface diameters. For the Pd-Na (2.4) and Pd-K (1.8) solids the particles are situated in the size range 1.0 and 3.0 nm and the average surface diameters are equal to 2.3 and 2.2 nm respectively. For the Pd-Li (2.0) solid the particles are situated between 1.0 and 4.5 nm, the surface diameter being equal to 3.1 nm. The results expressed in palladium dispersion are listed in Table 1. For the PdLi (7.4) solid the mean surface diameter is equal to 3.5 nm.
Adsorption measurements In order to determine the saturation coverages, hydrogen adsorption experiments were performed up to 150 Torr (1 TOIT= 133.3 Pa) in a classical volumetric apparatus. The reduced solids were outgassed at 623 K for 2 h. The adsorption-back-sorption method [ 17,181 was used to determine chemisorption uptakes of hydrogen at 298 K. A first isotherm gave the total hydrogen uptake due to both adsorption and absorption. An evacuation at 298 K for 20 min was performed in order to decompose the hydride and to leave only chemisorbed hydrogen on the palladium surface. A second isotherm gave the uptake of hydrogen due to bulk hydride formation. The method of Aben [ 191 (adsorption at 343 K) was also used to check the values thus obtained for the volumes of hydrogen irreversibly adsorbed at saturation. Both methods gave TABLE 1 Calorimetric (initial heats) and volumetric data for the adsorptions at 298 K Solid
Disp.
(%o) Pd Pd-Li(2.0) Pd-Na(2.4) Pd-K(1.8)
56 36 49 51
QH, 104 96 104 104
VHz
VI%
(0.5)
(sat)
0.27 0.28 0.26 0.25
0.28 0.31 0.28 0.26
QCO 136 136 146 146
vco
vco
(0.5)
(sat)
0.36 0.27 0.33 0.27
0.40 0.31 0.37 0.27
Disp.: Pd dispersion (% ) deduced from TEM. QHz, QCO: initial heats of adsorption (kJ mol-‘) obtained by extrapolation of the first isotherms. The uncertainty (repeatibility for the heata evolved during the first doses) reaches 5 kJ mol-‘. VH2, VCO: adsorbed volumes (cm’ NTP rn-’ Pd) for an equilibrium pressure of 0.5 Torr (0.5) (first isotherms at 298 K) and at saturation (sat) (irreversibly adsorbed hydrogen and carbon monoxide at 298 K) (1 mz Pd: 1.24.10” atoms).
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comparable results. The dispersions were calculated by assuming a chemisorption stoichiometry of H,&/Pd, = 1 where Pd, is a surface palladium atom. The values thus obtained (Table 1) were in good agreement with the values deduced from transmission electron microscopy (TEM) measurements. The saturation coverages by carbon monoxide were also determined from the quantities of irreversibly adsorbed carbon monoxide at 298 K. Calorimetric measurements The determination of the heats of adsorption was performed at 298 and 473 K using a Calvet-type calorimeter. The calorimetric cell was connected to a vacuum system and to a volumetric apparatus allowing adsorption experiments up to 1.5 Torr. Prior to the adsorptions, the catalysts (between 0.100 and 0.150 g) were reduced in flowing hydrogen as described above, cooled down to room temperature and transferred, in vacua, into the calorimetric cell [ 20231, Adsorbed hydrogen was evacuated at 623 K for 2 h. The adsorptions of hydrogen or carbon monoxide were carried out at 298 or 473 K by introducing successive doses onto the samples. The heat evolved, the quantity of gas adsorbed and the equilibrium pressure were measured at the end of the interaction of each dose, i.e., when the calorimeter returned to thermal equilibrium. In order to detect possible small variations of the initial heats of adsorption due to the presence of alkali, small doses were successively introduced. The equilibrium pressure for the first dose was usually less than 0.04. 10m3Torr (1 Torr= 133.3 Pa). 13 to 26 doses, in the case of hydrogen, and 9 to 17 doses, in the case of carbon monoxide, were required to reach the final equilibrium pressure (ca. 1.5 Torr). After the first isotherm of hydrogen at 298 K the samples were evacuated at 298 K and successive doses of hydrogen were reintroduced as described above in order to determine the possible energy change due to bulk hydride formation [ 241. Similar experiments were performed in the case of carbon monoxide. RESULTS
Adsorption of hydrogen and carbon monoxide at 298 K Adsorption of hydrogen The Figs. 1 and 2 show the variations, for the first isotherms, of the differential heats of ad- (ab) -sorption of hydrogen as a function of the adsorbed volumes for the solids: Pd/SiOa, Pd-Li (2.0) /SiOp, Pd-Na( 2.4) /SiOp. The coverages expressed in fractions of a monolayer (8= 0.5,0= 0.8) and the equilibrium pressures corresponding to some adsorbed volumes are also indicated on the figures. The initial heats obtained by extrapolation to zero coverage are reported in Table 1. They are very close to the heats evolved during the first
M. Gravelle-Rumeau-Maillot et al.jApp1. Catal. A 98 (1993) 45-59
50
kJ.mOl-’
t 1ooL
SO0’ ._ gb
60-
0 ;
40-
P =
20-
tL_ cm3.mmzP
0 1
auat_Aed hydrogen
I c a 13.165
P(Tor 9.10-s
0.1
l&lo-5
1
0.5
0.8
COYW.O#
Fig. 1. Differential heats of adsorption (kJ mol-‘) of hydrogen at 298 K as a function of the quantity of adsorbed hydrogen (cm3 NTP me2 Pd) for the Pd/SiO, solid. The uncertainty on the heats reaches 5 kJ mol-‘.
dose. They are also in good agreement with the values available in the literature for undoped-single-crystal Pd faces, the initial heat for hydrogen adsorption ranging from 88 to 109 kJ mol-’ [ 25,261. During the first hydrogen isotherm at 298 K, the energy changes could be due both to adsorption and absorption of hydrogen. In our case we think that the fraction of initial heat produced by hydride formation is very small because of the following data: (i) During the first doses the equilibrium pressure ranged between 0 and 20-10-5 Torr. At such low pressures we can suppose that the palladium surface is first saturated with chemisorbed hydrogen and that hydrides and hydrogen subsurfaces species are not formed 1241. (ii) The initial heats or the heats evolved during the first dose are close to the values reported in the literature for adsorbed hydrogen [ 25,261. (iii) We have also measured the heats evolved during a second adsorption of hydrogen after evacuation at 298 K in order to try to determine the energy changes due to a possible bulk hydride formation. For Pd/SiOP the initial heat reaches 80 kJ mol-’ and is not altered by the presence of alkalis. This value is very high compared with values reported in the literature for the heat of palladium hydride formation (around 24 kJ mol-’ for the (Yphase and 36-53 kJ
M. Gravelle-Rumeau-Maillot
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51
I
kJ.mol-’
80g 3 e o
GO-
3 % % 2
40-
2Q-
0
0.3
I I I ,
adsorbed 2.10
-5
s.,lo-5
,
0.1
1fs;10-5
--T
0.8
0!5
I I !
hydrogen
4.10-5
7.10-5
0.1 ,
19.10-5 I 0.5
I 0.8
’
p
Pd.Li
CO”.
PfTorr) coverage
Pd _NII
Fig. 2. Differential heats of adsorption (kJ mol-‘) of hydrogen at 298 K as a function of the quantity of adsorbed hydrogen (cm3 NTP me2 Pd) for the Pd-Li(B.O)/SiO, (-_) and PdNa(2.4)/Si02 (---) solids.
mol-’ for the bulk p phase) [ 241. Apparently some adsorbed hydrogen has been removed during the evacuation at 298 K and readsorbed. So, even if the hydride was formed, it was not possible to determine its heat of formation under our conditions. The heat produced by the adsorption of the first dose of hydrogen and the initial heat of hydrogen adsorption are slightly lowered by the addition of a small quantity of lithium [solid Pd-Li (2.O)/SiO,]. This trend has been confirmed by the study of the Pd-Li (7.4) /SiO, solid. For this sample, the initial heat of hydrogen adsorption is 93 kJ mol-‘, i.e., the addition of lithium results in a decrease of 11 kJ mol-l. The observed variations are very small but significant if we consider that the uncertainty of the determinations of the heats evolved during the first doses is equal to 5 kJ mol-‘. This uncertainty was deduced from the repeatibility of the experiments. No significant variations are observed for the sodium and potassium doped solids. The differential heats of hydrogen adsorption decrease when the coverage increases, first slowly and then more strongly for coverages higher than onehalf monolayer (8 between 0.5 and 0.8). This decrease is practically not perturbed by the presence of the alkali. When the equilibrium pressure increases
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M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
the absorption of hydrogen can be no longer considered as negligible and it adds its effect to the adsorption. The integral heat deduced from graphical integration over the range of 8 between 0 and 0.8 and due to both hydrogen adsorption and hydrogen absorption is not altered by alkali addition. Adsorption of carbon monoxide
The variation of the differential heat of carbon monoxide adsorption with the adsorbed volume is illustrated in Fig. 3 for the case of the Pd/SiO, and PdK (1.8) /SiOp solids. Initial heats and adsorbed volumes at 0.5 Torr and at saturation can be compared in Table 1. As a first approximation, initial heats thus observed compare well with corresponding heats of carbon monoxide adsorption on unpromoted single-crystal palladium faces which vary from 142 to 167 kJ mol-’ [ 26-321. The initial heat of carbon monoxide adsorption over Pd-K/SiO, and Pd-Na/SiOz is increased by 10 kJ mol-l in contrast with the case of the lithium-promoted catalysts, lithium addition resulting in an almost unchanged initial heat. Furthermore it can be noticed that a marked decrease of the differential heat of carbon monoxide adsorption when the coverage increases occurs for coverkJ.mol-’
t__. 140-
120-
IOO-
20-
L_ cm3.ni-2Pd
0
adsorbed
CO
Fig. 3. Differential heats of adsorption (kJ mol-‘) of carbon monoxide at 298 K as a function of the quantity of adsorbed carbon monoxide (cm3 NTP rn-’ Pd) for the (-) Pd/SiO, and (---) Pd-K ( 1.8)/Si02 solids.
M. Gravelle-Rumeau-Maillot
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ages in excess of half a monolayer (Fig. 3). For Pd/SiO, the coverages reach 0.5 and 0.8 for 0.20 and 0.32 cm3 CO/m2 Pd; for Pd-K (1.8) these values correspond to 0.14 and 0.22 cm3 CO/m2 Pd. Let us recall that a surface of 1 m2 contains 1.27. 101’Pd, atoms. The decrease in the carbon monoxide adsorption heat is more pronounced for the doped solids than for the alkali-free catalyst. The presence of Li, Na or K leads to a decrease in the amount of adsorbed carbon monoxide per unit area as deduced from electron microscopy. For instance in the presence of potassium the amount of adsorbed carbon monoxide decreases by 75% but the amount of adsorbed hydrogen does not vary. This is attributed to a loss of access of carbon monoxide to metal particles. A similar behaviour was reported for the case of potassium doped Ni/SiO, solids [33] and was attributed to a more or less complete decoration of metal particles with a thin film of alkali compounds (mainly silicates) [ 341. It was also shown that the loss of access was more prononced for carbon monoxide than for hydrogen, probably owing to the larger size of carbon monoxide. Adsorption of hydrogen and carbon monoxide at 473 K
In order to avoid possible hydride formation and to simulate more closely the conditions of carbon monoxide hydrogenation [ 51 the heats of adsorption have also been measured at 473 K and successive adsorptions have been performed at 473 K on Pd/SiO,, Pd-Li (2.0) /SiO, and Pd-Na (2.4) reduced at 673 K and evacuated at 623 K. Figs. 4 and 5 show the differential heats of adsorption of hydrogen and car-
i
kJ.mol-’
cm3.m-2Pd
I
0.2 adsorbed
*
hydrogen
Fig. 4. Differential heats of adsorption (kJ mol-‘) of hydrogen at 473 K as a function of the quantity of adsorbed hydrogen (cm3 NTP m-* Pd) for the Pd/Si02 solid.
54
M Gravelle-Rumeau-Maillot et al.fAppl. Catal. A 98 (1993) 45-59
t
k J .mol-’
ttL_ cm3.mm2Pd
_1
0. adsorbed
0.2
1 co
Fig. 5. Differential heats of adsorption (kJ mol-‘) of carbon monoxide at 473 K as a function of the quantity of adsorbed carbon monoxide ( cm3 NTP rn-’ Pd) for the Pd/SiOz solid. TABLE 2 Calorimetric heats evolved during the first dose and volumetric data for the adsorptions at 473 K Solid
QH,
W.
QCO
vco
Pd Pd-Li(2.0) Pd-Na(2.4)
90 88 89
0.14
138
0.22
0.14
136
0.24
0.14
136
0.23
Pd” Pd-Li”
87 87
0.035 0.075
QH,, QCO: heats of adsorption evolved during the first dose of gas (kJ mol-‘) VH,, VCO: adsorbed volumes (cm3 NTP m-’ Pd) for an equilibrium pressure of 0.5 Torr. “Metal surfaces are partially precovered with carbon monoxide. The experimental sequence is the following: carbon monoxide 473 K, vacua 473 K, hydrogen 473 K.
bon monoxide on Pd/SiOz. For lithium- and sodium-promoted catalysts the curves obtained are very similar to those of Figs. 4 and 5. Table 2 reports the adsorption heats evolved during the first doses of adsorbed gases and the adsorbed volumes obtained for an equilibrium pressure of 0.5 Torr. The adsorp-
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tion heats do not vary significantly upon addition of alkali. The same trend (no variation) is observed for the integral heats deduced from graphical integrations in the range of coverage O-0.8. Data of Table 2 do not confirm the trends observed at 298 K; they do not infirm them either. The quantities of hydrogen or carbon monoxide chemisorbed and the adsorption heats are not modified if the evacuation of hydrogen (either after reduction, or after hydrogen chemisorption) is performed at 473 K instead of 623 K. These experiments show that an evacuation at 473 K is sufficient to remove most of chemisorbed hydrogen. In contrast, when the surface of palladium and Pd-Li (2.0) catalysts is saturated with carbon monoxide, an evacuation at 473 K does not lead to a zero coverage. As a matter of fact, a subsequent carbon monoxide chemisorption on the metal surface evacuated at 473 K gives rise to smaller heats and adsorbed quantities. This is in agreement with infrared spectroscopy which reveals the presence of adsorbed carbon monoxide after an evacuation at 473 K [ 41. When hydrogen is allowed to react with the metal surface partially precovered by carbon monoxide (carbon monoxide saturation followed by evacuation at 473 K), the heat of adsorption is 87 kJ mol-l (Fig. 6, Table 2) a value comparable to the heat measured on a clean surface. However the quantity of adsorbed hydrogen and the integral heat evolved is superior for the Pd-Li (2.0)/ SiOa solid than for the Pd/SiO, one. This will be discussed later on.
t
kJ.mol-’
A
i
L 0.05
adsorbed
cm? m-2 Pd a-0.1
hydrogen
0
0.05 adsorbed
0.1 hydrogen
Fig. 6. Differential heats of adsorption (kJ mol-‘) of hydrogen at 473 K as a function of the quantity of adsorbed hydrogen for Pd/SiO* (A) and Pd-Li(2.0)/SiO, (B) solids partially precoverdd with carbon monoxide by the following treatment: carbon monoxide saturation at 473 K and pumping at the same temperature.
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M. Graoelle-Rumeau-Maillot et al.jApp1. Catal. A 98 (1993) 45-59
DISCUSSION
At room temperature we did not observe significant trends by comparing integral heats; thus we mainly consider initial heats when interaction between adsorbed species can be neglected as well as palladium hydride formation at room temperature. Table 1 shows that, at 298 K, the decrease of the initial heat of hydrogen adsorption is observed only in the presence of lithium and is pronounced when the lithium content is high enough, i.e. for the Pd-Li (7.4 )/SiO, solid. The initial heat is 13% smaller than the corresponding value obtained on unpromotecl Pd/SiO,. We suggest that metal decoration by the alkali compound [3] could explain this decrease: decoration by lithium would affect selectively the most energetic sites of palladium towards chemisorption. This speculative hypothesis would deserve verification. This hypothesis, however, does not provide a satisfying explanation to account for the alkali-induced change in the initial heat of carbon monoxide chemisorption at room temperature; no variation with lithium, increase of 7% in the presence of potassium and sodium. Infrared spectroscopy may help us in better understanding these observations. From infrared spectroscopy and volumetric measurements it is known that the adsorption of carbon monoxide does not occur on silica and on silica-containing alkali ions. The infrared spectrum of carbon monoxide irreversibly adsorbed at 298 K on the Pd/SiO, solid free from alkali, reduced at 673 K and evacuated at 623 K shows bands at 2088,1965,1910 (shoulder) cm-‘, assigned to linearly bonded carbon monoxide (L species) and to bridged bonded carbon monoxide on the (100) (B, species) and (111) (Bz species) faces of the palladium crystallites [4,35,36], respectively. As the evacuation temperature is increased the position of the bands is coverage-dependent with a downward frequency shift. After evacuation at 373 K the IR bands are situated at 2060 (L species), 1915 (B, species), and 1815 (shoulder) ( Bz species) cm- ‘. The linearly bonded carbon monoxide disappears under vacua at 473 K. At that temperature the two bridged bonded species absorb at 1895 and 1815 cm-‘, respectively. Under vacua the B1 species disappears at 573 K and the B, one at 623 K. Just before disappearance, at 573 K, the B2 species gives a small band at 1775 cm-‘. In presence of potassium or sodium the infrared spectrum of carbon monoxide irreversibly adsorbed at 298 K is strongly modified. The bands due to carbon monoxide adsorbed on unperturbed palladium disappear or are very small [ 41. For instance for the Pd-K (1.8) solid reduced at 673 K they are situated at 2050 and 1955 cm-‘. A new band appears at low wavelengths, at 1825 cm-l for the Pd-K (1.8) solid and at 1810 cm-l for the Pd-Na(2.0) solid. This new band is coverage dependent. An evacuation at 473 K, just before disappearance, results in a frequency shift of 125 cm-’ for the potassium-doped solid and of 110 cm-’ for the sodium-doped one.
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The lowering of the vC0 frequency may result either from a direct interaction or from an electron enrichment of the metallic phase. Based on the fact that the smaller the size of the alkali cation (Na,K,Cs), the lower the carbon monoxide stretching frequency, we have already suggested [ 41 that the oxygen atom of adsorbed carbon monoxide interacts with the electric field of alkali cation, the order of interaction being Na+ >K+ > Cs+. The increase in the heat of carbon monoxide adsorption resulting from sodium or potassium addition can be tentatively ascribed to the interaction between the oxygen of carbon monoxide and the alkali ions. This model would give a rough estimate of the energy of the electrostatic interaction; using data of sodium and potassium promoted catalysts, the interaction would be of the order of 10 kJ mol-l. A similar effect, even more pronounced, is to be expected for lithium-promoted catalysts. However lithium addition does not result in a significant variation of the carbon monoxide adsorption heat. This could be the result of a compensating effect: as for the case of hydrogen chemisorption, lithium addition would mask selectively the most active sites towards carbon monoxide chemisorption. The variations of initial heats of carbon monoxide and hydrogen chemisorptions here observed, are very small. At this stage of our discussion we can discuss the correspondence between the stretching frequency vC-0 and the strength of the metal M-CO bond. The initial heat of adsorption or the bond strength is proportional to the strength constant of M-CO, i.e. to the M-CO mode frequency. Consequently it is inversely proportional to the S-0 frequency [37]. For various single metal surfaces of Cu, Ni, Ru, Pt, a linear relationship was found between (v M-CO) 2 and the initial adsorption energy of carbon monoxide [ 381. On palladium single crystals the decrease in the heat of carbon monoxide adsorption when the coverage increases is reflected in the increase in the C-O stretching frequency (29-32,36,39]. In fact, in the literature, for palladium, the initial heats of carbon monoxide adsorption show only a small variation, never exceeding 15%, with surface structure [40]. For instance the initial heat of carbon monoxide adsorption is equal to 142 kJ mole1 on a Pd(ll1) face and to 152 kJ mol-l on a Pd(lOO) face (variation of 7%). At the same time, for low coverages, the CO stretching frequencies are observed at 1823 and 1895 cm-l; so the variation is also relatively small (4%). These variations are of the same order of magnitude as the variations reported in the present work. For palladium-supported solids, temperature-programmed desorption studies of carbon monoxide [ 41,421 and measurements of the integral heats of adsorption of hydrogen [ 241 and carbon monoxide [43] have been reported. Variations have been found to occur for small particles having an electron-deficient character, when the size drops below 3 or 4 nm. In our case, for an alkali-to-Pd ratio close to 2, the mean particle size increases from 2 nm to 2.2,2.3,3.1 nm adding K, Na and Li, so the modification is very moderate. At 473 K the initial or integral heats evolved during hydrogen or carbon
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M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
monoxide adsorption are no more perturbed by the presence of the alkali. Furthermore there is no initial heat of interaction of hydrogen with preadsorbed carbon monoxide at 473 K suggesting that adsorbed carbon monoxide and hydrogen do not mutually interact on the metal surface. This observation agrees with the idea that, in methanol synthesis from syngas [ 51, adsorbed carbon monoxide would react with hydroxyl groups of the lithiated support to give formates rather than with adsorbed hydrogen to make a formyl intermediate. For the Pd-Li solid the increase of the quantity of adsorbed hydrogen and of the integral heat of adsorption could be due to the hydrogenation of the formate intermediate leading thus to further heat evolvement. CONCLUSION
It can be concluded that alkali addition to Pd/SiO, leads to very small variations of the initial heats of carbon monoxide and hydrogen adsorptions while the carbon monoxide adsorption mode is strongly modified as evidenced by infrared spectroscopy. Furthermore major changes occur in rates, selectivities, activation energies and orders of reactions. The large decreases in carbon monoxide stretching frequencies and the modifications of the catalytic properties suggest changes in the palladium-adsorbate bond strengths. From this work, it seems that no relationship exists between adsorption heats determined by calorimetry and catalysis. It could be suggested that the calorimetric method gives an average value for several adsorbed states and that temperature-programmed desorptions would be more suitable to determine changes in the binding energies of the various states. Let us also recall that both theory and experiments indicate no strong variation (around 15%) of the initial adsorption energy with the surface orientation. The modifications of catalytic properties are perhaps related to variations in sticking probabilities. The very small variations observed here at room temperature, decrease of about 13% of the initial heat of hydrogen adsorption in presence of lithium and increase of 7% of the initial heat of carbon monoxide adsorption in presence of sodium or potassium, have been tentatively interpreted in terms of (i) metal decoration by the lithium promoter and (ii) interaction between the oxygen atoms of adsorbed carbon monoxide and alkali cations. ACKNOWLEDGEMENT
The authors fully acknowledge Dr. Pierre Charles Gravelle for his help in fruitful discussions.
M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
59
REFERENCES 1 2 3 4 5
6 7 8 9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
W.D. Mross, Catal. Rev., Sci. Eng., 25 (1983) 591. V. Pitchon, M. Guenin and H. Praliaud, Appl. Catal., 63 (1990) 333. V. Pitchon, P. Gallezot, C. Nicot and H. Prahaud, Appl. Catal., 47 (1989) 357. V. Pitchon, M. Primet and H. Prahaud, Appl. Catal., 62 (1990) 317. V. Pitchon, H. Praliaud and G.A. Martin, in A. Holmen, K.J. Jens and S. Kolboe (Editors), Natural Gas Conversion (Studies in Surface Science and Catalysis, Vol. 61), Elsevier, Amsterdam, 1991, p. 265. V. Pitchon and H. Praliaud, C.R. Acad. Sci. Paris, S&e II, 304 (15) (1987) 885. G.A. Somorjai, Catal. Rev. Sci. Eng., 23 (1981) 189. J.B. Benziger and R.J. Madix, Surf. Sci., 94 (1980) 119. M.E. Dry, T. Shingles, L.J. Boshoff and G.J. Oosthuizen, J. Catal., 15 (1969) 190. G. Broden, G. Gafner and H.P. Bonzel, Ned. Tijdschr. Vacuumtech., 16 (1978) 160. M.P. Kiskinova, Surf. Sci., 111 (1981) 584. A. Berko and F. Solymosi, Surf. Sci., 171 (1986) L498. R.D. Gonzalez and H. Miura, J. Catal., 77 (1982) 338. G. Ertl, S.B. Lee and M. Weiss, Surf. Sci., 111 (1981) L711. D. Weatherbee, J.L. Rankin and C.H. Bartholomew, Appl. Catal., 11 (1984) 73. H. Dexpert, P. Gallezot and C. Leclerq, in B. Imelik and J.C. Vedrine (Editors), Les techniques physiques d’etude des catalyseurs, Technip, Paris, 1988, p. 655. J.E. Benson, H.S. Hwang and M. Boudart, J. Catal., 30 (1973) 146. M. Boudart and H.S. Hwang, J. Catal., 39 (1975) 44. P.C. Aben, J. Catal., 10 (1968) 224. P.C. Gravelle, Adv. CataI., 22 (1972) 191. J.J. Prinsloo and P.C. Gravelle, J. Chem. Sot. Faraday Trans. 1,76 (1980) 512. J.J. Prinsloo and P.C. Gravelle, J. Chem. Sot. Faraday Trans. 1,76 (1980) 2221. J.J. Prinsloo and P.C. Gravelle, J. Chem. Sot. Faraday 1,78 (1982) 273. Pen Chou and M.A. Vannice, J. Catal., 104 (1987) 1. R.J. Behm, K. Christmann and G. Ertl, Surf. Sci., 99 (1980) 320. H. Conrad, G. Ertl and E.E. Latta, Surf. Sci., 41 (1974) 435. G. Doyen and G. Ertl, Surf. Sci., 69 (1977) 157. G. Doyen and G. Ertl, Surf. Sci., 43 (1974) 197. T. Engel and G. Ertl, Adv. Catal., 28 (1979) 1. J.C. Tracy and P.W. Palmberg, J. Chem. Phys., 51 (1969) 4852. G. Ertl and R. Koch, Z. Naturforsch, 25a (1970) 1906. H. Conrad, G. Ertl, H. Kuipers and J. Segner, Surf. Sci., 107 (1981) 207. G.A. Martin and H. Praliaud, Catal. Lett., 9 (1991) 151. P. Moral, H. Praliaud and G.A. Martin, React. Kinet. Catal. Lett., 34 (1987) 1. F.M. Hoffmann, Surf. Sci. Rep., 3 (1983) 107. A.M. Bradshaw and F.M. Hoffmann, Surf. Sci., 72 (1978) 513. G. Blyholder, J. Phys. Chem, 68 (1966) 2772. J.C. Bertolini and B. Tardy, Surf. Sci., 102 (1981) 131. S. Ishi, Y. Ohno and B. Viswanathan, Surf. Sci., 161 (1985) 349. J. Siera, F. Rulten and B.E. Nieuwenhuys, Catal. Today, 10 (1991) 353. S. Ladas, H. Poppa and M. Boudart, Surf. Sci., 102 (1981) 151. E. Gillet, S. Channakhone and V. Matolin, J. Catal., 97 (1986) 437. Pen Chou and M.A. Vannice, J. Catal., 104 (1987) 17.
45
APCAT A2494
Complementary study by calorimetry and infrared spectroscopy of alkali metal doped Pd/SiO, solids: Adsorption of hydrogen and carbon monoxide Monique Gravelle-Rumeau-Maillot, Martin and H&ne Praliaud
Veronique Pitchon, Guy Antonin
Znstitut de Recherches sur la Catalyse, Laboratoire Propre du CNRS, Conventionnd k Wniversitk Claude Bernard Lyon Z, 2 Avenue Albert Einstein, 69626 Villeurbanne Ckdex (France) (Received 12 June 1992, revised manuscriptreceived 20 January 1993)
Abstract Alkali metal (Li, Na, K) promoted Pd/SiOa catalysts were preparedby adding nitrates to the Pd/ SiOaprecursorand subsequenthydrogenreduction. The initial heats of adsorption of carbon monoxide and of hydrogenat 298 K and at 473 K and the variations of the differentialheats with coveragewere measuredby calorimetry.The changes caused by the presenceof the alkali: decreasein the initial heat of hydrogen adsorption at 298 K in the presence of lithium and increase in the initial heat of carbon monoxide adsorption at 298 K in the presenceof potassiumand sodium are very small (ca. 10% ). These results,and the conclusions deducedfrom the infraredspectraof adsorbedcarbon monoxide on the same samples,are compared. They are discussedassumingthat alkali cations interact with the oxygen atom of adsorbedcarbon monoxide. Calorimetryclearlyshows that adsorption heats of hydrogenand carbon monoxide do not change appreciably when alkali cations are present on palladium although infrared spectroscopy of adsorbed carbon monoxide and catalytic properties are modified. Thus initial adsorption heats determinedby calorimetryand catalytic activities are not directly related.
Keywords: alkali promotion; calorimetry; carbon monoxide adsorption; hydrogen adsorption; palladium; silica-supported palladium
INTRODUCTION
The catalytic and physico-chemical -properties of metals are significantly affected by the addition of alkali metal promoters [ 11. In our laboratory we have studied model catalysts consisting of palladium supported on silica promoted with alkali compounds [ 2-61. After reduction, the alkali is present in an ionic form [ 21, associated with counteranions (oxCorrespondence to: DrH. Praliaud, Institut de Recherches sur la Catalyse, Conventionnk B l’Universiti Claude Bernard Lyon I, 2 Avenue Albert Einstein, 69626 Villeurbanne CBdex, France. Tel. (+33)72445337, fax. (+33)72445399.
6926-660X/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
46
M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
ygen, hydroxyl or mainly silicate) and located partly on the support and partly on the palladium particles, thus giving rise to a partial decoration of the metal surface by the alkali compound [ 31. In the presence of alkali the X-ray photoelectron binding energies of metallic palladium are lowered, the decrease being larger in the presence of sodium and potassium than in the presence of lithium [ 21. The variations in binding energy are not related to changes in the metal particles size; they have been assigned either to an initial chemical shift due to the electron-enrichment of the palladium surface atoms or to a modification of the relaxation process. Furthermore, the addition of Na, K or Cs leads to important changes in the infrared spectrum of carbon monoxide adsorbed on palladium with the appearance of a new band at low wavenumbers in the range 1825-1700 cm-’ according to the alkali and to the coverage [ 41. The smaller the ionic radius of the alkali (Na), the lower is the carbon monoxide wavenumber for the new band which was assumed to be due to an interaction between the oxygen atom of adsorbed carbon monoxide and Na, K or Cs ions. The case of the lithium-doped solids is complex, since the interaction is not clearly increased in spite of the smaller ionic radius of lithium [ 41. The rate of formation of methanol from syngas at atmospheric pressure catalyzed by metallic palladium is strongly increased in the presence of lithium while it decreases in the presence of potassium and caesium following the sequence: Li > Na > unpromoted solid > K > Cs. This sequence parallels the concentration of free hydroxyl groups on the support, suggesting that the synthesis of methanol proceeds via a formate intermediate resulting from carbon monoxide insertion into hydroxyl groups [ 51. It has also been shown that the catalytic features, i.e. the rates and the kinetics parameters, of ethane hydrogenolysis and benzene hydrogenation over palladium are strongly altered by the presence of alkali [ 61. The reactions rates are decreased by factors varying between 5 and 20. The alkali induces an increase of the activation energy of the ethane hydrogenolysis from around 230 to 300 kJ mol-l, the formation of cyclohexene and a modification of the orders towards benzene and hydrogen during the benzene hydrogenation. These observations suggest that the alkali acts as an electronic modifier strengthening or weakening the palladium-adsorbate bonds. Thus, it appears desirable to examine to what extent the addition of alkali modifies the energetic8 of the adsorbate-palladium metal surface systems. As a general rule the presence of alkali changes the binding energies of hydrogen and carbon monoxide on metals and the sticking probabilities [ 7,8]. These energies have generally been deduced from temperature-programmed desorptions except in the work of Dry et al. [ 91 in which the initial heats of adsorption at low coverages were determined calorimetrically. For single-crystal faces promoted with vapodeposited alkali metals [8,10-121 and for supported metals promoted with alkali ions [9,13] the addition of potassium to Fe [&lo], Ni [ 111, Pd [ 121 or Ru [ 131 leads to the appearance of more tightly bound states
hi. Gravelle-Rumeau-Maillot
et al./Appl. Catal. A 98 (1993) 45-59
47
of carbon monoxide. The carbon monoxide adsorption heat increases by around 15-20 kJ mol-l [9,10], but there is some discrepancy about the variation of the heat of hydrogen adsorption on alkali-promoted iron solids. For Dry et al. [9] the presence of K,O lowered the initial heat of hydrogen adsorption on magnetite by 25 kJ mol-l but other investigators found that potassium enhanced the hydrogen binding strength on iron by about 8-10 kJ mol-’ [ 8,14,15]. In the present study the heats of adsorption of carbon monoxide and of hydrogen at 298 K and under conditions close to the conditions of the catalytic reaction for the formation of methanol from syngas, i.e. at 473 K, were measured and discussed. EXPERIMENTAL
Preparation of the solids The Pd/SiO, precursor was prepared by the ion-exchange technique. The silica support (Aerosil from Degussa 200 m2 g-l) was contacted with 2 in a basic solution. After washing and drying the solid was Pd(NH,),(OH) calcined at 573 K (heating rate 0.2 K min-’ ) in an oxygen flow. A part of the solid was then added to a solution of alkali nitrate (lithium, sodium, potassium or caesium) and stirred. The solution was evaporated to dryness under reduced pressure in a rotary evaporator. The palladium content in Pd/Si02 is 2.2 wt.-% and the atomic alkali-to-palladium ratios are 2 for Pd-Li/SiO,, 2.4 for Pd-Ma/Si02, 1.8 for Pd-K/SiO,. Another lithium doped solid with an alkali/ Pd ratio equal to 7.4 was also prepared since it has been shown that, during the hydrogenation of carbon monoxide, the catalytic activity for the formation of methanol is promoted by lithium and increases with the lithium content [ 51. In this paper the solids are designated by the atomic alkali-to-Pd ratios. The reduction was carried out overnight in a flow of hydrogen (3-5 dm3 h- ’ ) by linearly raising the temperature (2 K min-‘) from 298 to 673 K. Under these conditions nitrate ions are completely decomposed and (or) reduced into nitrogen and palladium is reduced to its metallic form.
Particle size measurements The reduced catalysts, subsequently air-contacted, were examined in a JEOL 100 K electron microscope with a 0.3 nm resolution. The specimens were prepared by sedimentation on a carbon-coated copper grid of an aerosol of the catalyst powder in order to avoid any dissolution of alkali metal compounds [ 31. Particle size distributions, counting between 200 and 350 particles, were narrow enough to allow the determination of the mean diameter. The average surface diameter, Cn& /Enid F, ( ni number of particles with diameter ranging
48
M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
between c&-M/2 and di +dd/2, dd = 0.5 nm), comparable to the mean diameter deduced from chemisorptions [ 161, was calculated. For the Pd/SiO, solid the particles are homogeneously distributed on the Aerosil particles and are situated in the size range 1.0-2.5 nm. The average surface diameter is equal to 2.0 nm. The presence of the alkalis induced moderate increases in the particles sizes distributions and in the average surface diameters. For the Pd-Na (2.4) and Pd-K (1.8) solids the particles are situated in the size range 1.0 and 3.0 nm and the average surface diameters are equal to 2.3 and 2.2 nm respectively. For the Pd-Li (2.0) solid the particles are situated between 1.0 and 4.5 nm, the surface diameter being equal to 3.1 nm. The results expressed in palladium dispersion are listed in Table 1. For the PdLi (7.4) solid the mean surface diameter is equal to 3.5 nm.
Adsorption measurements In order to determine the saturation coverages, hydrogen adsorption experiments were performed up to 150 Torr (1 TOIT= 133.3 Pa) in a classical volumetric apparatus. The reduced solids were outgassed at 623 K for 2 h. The adsorption-back-sorption method [ 17,181 was used to determine chemisorption uptakes of hydrogen at 298 K. A first isotherm gave the total hydrogen uptake due to both adsorption and absorption. An evacuation at 298 K for 20 min was performed in order to decompose the hydride and to leave only chemisorbed hydrogen on the palladium surface. A second isotherm gave the uptake of hydrogen due to bulk hydride formation. The method of Aben [ 191 (adsorption at 343 K) was also used to check the values thus obtained for the volumes of hydrogen irreversibly adsorbed at saturation. Both methods gave TABLE 1 Calorimetric (initial heats) and volumetric data for the adsorptions at 298 K Solid
Disp.
(%o) Pd Pd-Li(2.0) Pd-Na(2.4) Pd-K(1.8)
56 36 49 51
QH, 104 96 104 104
VHz
VI%
(0.5)
(sat)
0.27 0.28 0.26 0.25
0.28 0.31 0.28 0.26
QCO 136 136 146 146
vco
vco
(0.5)
(sat)
0.36 0.27 0.33 0.27
0.40 0.31 0.37 0.27
Disp.: Pd dispersion (% ) deduced from TEM. QHz, QCO: initial heats of adsorption (kJ mol-‘) obtained by extrapolation of the first isotherms. The uncertainty (repeatibility for the heata evolved during the first doses) reaches 5 kJ mol-‘. VH2, VCO: adsorbed volumes (cm’ NTP rn-’ Pd) for an equilibrium pressure of 0.5 Torr (0.5) (first isotherms at 298 K) and at saturation (sat) (irreversibly adsorbed hydrogen and carbon monoxide at 298 K) (1 mz Pd: 1.24.10” atoms).
M. Gravelle-Rumeau-Maillot
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49
comparable results. The dispersions were calculated by assuming a chemisorption stoichiometry of H,&/Pd, = 1 where Pd, is a surface palladium atom. The values thus obtained (Table 1) were in good agreement with the values deduced from transmission electron microscopy (TEM) measurements. The saturation coverages by carbon monoxide were also determined from the quantities of irreversibly adsorbed carbon monoxide at 298 K. Calorimetric measurements The determination of the heats of adsorption was performed at 298 and 473 K using a Calvet-type calorimeter. The calorimetric cell was connected to a vacuum system and to a volumetric apparatus allowing adsorption experiments up to 1.5 Torr. Prior to the adsorptions, the catalysts (between 0.100 and 0.150 g) were reduced in flowing hydrogen as described above, cooled down to room temperature and transferred, in vacua, into the calorimetric cell [ 20231, Adsorbed hydrogen was evacuated at 623 K for 2 h. The adsorptions of hydrogen or carbon monoxide were carried out at 298 or 473 K by introducing successive doses onto the samples. The heat evolved, the quantity of gas adsorbed and the equilibrium pressure were measured at the end of the interaction of each dose, i.e., when the calorimeter returned to thermal equilibrium. In order to detect possible small variations of the initial heats of adsorption due to the presence of alkali, small doses were successively introduced. The equilibrium pressure for the first dose was usually less than 0.04. 10m3Torr (1 Torr= 133.3 Pa). 13 to 26 doses, in the case of hydrogen, and 9 to 17 doses, in the case of carbon monoxide, were required to reach the final equilibrium pressure (ca. 1.5 Torr). After the first isotherm of hydrogen at 298 K the samples were evacuated at 298 K and successive doses of hydrogen were reintroduced as described above in order to determine the possible energy change due to bulk hydride formation [ 241. Similar experiments were performed in the case of carbon monoxide. RESULTS
Adsorption of hydrogen and carbon monoxide at 298 K Adsorption of hydrogen The Figs. 1 and 2 show the variations, for the first isotherms, of the differential heats of ad- (ab) -sorption of hydrogen as a function of the adsorbed volumes for the solids: Pd/SiOa, Pd-Li (2.0) /SiOp, Pd-Na( 2.4) /SiOp. The coverages expressed in fractions of a monolayer (8= 0.5,0= 0.8) and the equilibrium pressures corresponding to some adsorbed volumes are also indicated on the figures. The initial heats obtained by extrapolation to zero coverage are reported in Table 1. They are very close to the heats evolved during the first
M. Gravelle-Rumeau-Maillot et al.jApp1. Catal. A 98 (1993) 45-59
50
kJ.mOl-’
t 1ooL
SO0’ ._ gb
60-
0 ;
40-
P =
20-
tL_ cm3.mmzP
0 1
auat_Aed hydrogen
I c a 13.165
P(Tor 9.10-s
0.1
l&lo-5
1
0.5
0.8
COYW.O#
Fig. 1. Differential heats of adsorption (kJ mol-‘) of hydrogen at 298 K as a function of the quantity of adsorbed hydrogen (cm3 NTP me2 Pd) for the Pd/SiO, solid. The uncertainty on the heats reaches 5 kJ mol-‘.
dose. They are also in good agreement with the values available in the literature for undoped-single-crystal Pd faces, the initial heat for hydrogen adsorption ranging from 88 to 109 kJ mol-’ [ 25,261. During the first hydrogen isotherm at 298 K, the energy changes could be due both to adsorption and absorption of hydrogen. In our case we think that the fraction of initial heat produced by hydride formation is very small because of the following data: (i) During the first doses the equilibrium pressure ranged between 0 and 20-10-5 Torr. At such low pressures we can suppose that the palladium surface is first saturated with chemisorbed hydrogen and that hydrides and hydrogen subsurfaces species are not formed 1241. (ii) The initial heats or the heats evolved during the first dose are close to the values reported in the literature for adsorbed hydrogen [ 25,261. (iii) We have also measured the heats evolved during a second adsorption of hydrogen after evacuation at 298 K in order to try to determine the energy changes due to a possible bulk hydride formation. For Pd/SiOP the initial heat reaches 80 kJ mol-’ and is not altered by the presence of alkalis. This value is very high compared with values reported in the literature for the heat of palladium hydride formation (around 24 kJ mol-’ for the (Yphase and 36-53 kJ
M. Gravelle-Rumeau-Maillot
et al./Appl. Catal. A 98 (1993) 45-59
51
I
kJ.mol-’
80g 3 e o
GO-
3 % % 2
40-
2Q-
0
0.3
I I I ,
adsorbed 2.10
-5
s.,lo-5
,
0.1
1fs;10-5
--T
0.8
0!5
I I !
hydrogen
4.10-5
7.10-5
0.1 ,
19.10-5 I 0.5
I 0.8
’
p
Pd.Li
CO”.
PfTorr) coverage
Pd _NII
Fig. 2. Differential heats of adsorption (kJ mol-‘) of hydrogen at 298 K as a function of the quantity of adsorbed hydrogen (cm3 NTP me2 Pd) for the Pd-Li(B.O)/SiO, (-_) and PdNa(2.4)/Si02 (---) solids.
mol-’ for the bulk p phase) [ 241. Apparently some adsorbed hydrogen has been removed during the evacuation at 298 K and readsorbed. So, even if the hydride was formed, it was not possible to determine its heat of formation under our conditions. The heat produced by the adsorption of the first dose of hydrogen and the initial heat of hydrogen adsorption are slightly lowered by the addition of a small quantity of lithium [solid Pd-Li (2.O)/SiO,]. This trend has been confirmed by the study of the Pd-Li (7.4) /SiO, solid. For this sample, the initial heat of hydrogen adsorption is 93 kJ mol-‘, i.e., the addition of lithium results in a decrease of 11 kJ mol-l. The observed variations are very small but significant if we consider that the uncertainty of the determinations of the heats evolved during the first doses is equal to 5 kJ mol-‘. This uncertainty was deduced from the repeatibility of the experiments. No significant variations are observed for the sodium and potassium doped solids. The differential heats of hydrogen adsorption decrease when the coverage increases, first slowly and then more strongly for coverages higher than onehalf monolayer (8 between 0.5 and 0.8). This decrease is practically not perturbed by the presence of the alkali. When the equilibrium pressure increases
52
M. Gravelle-Rumeau-Maillot et al./Appl. Catal. A 98 (1993) 45-59
the absorption of hydrogen can be no longer considered as negligible and it adds its effect to the adsorption. The integral heat deduced from graphical integration over the range of 8 between 0 and 0.8 and due to both hydrogen adsorption and hydrogen absorption is not altered by alkali addition. Adsorption of carbon monoxide
The variation of the differential heat of carbon monoxide adsorption with the adsorbed volume is illustrated in Fig. 3 for the case of the Pd/SiO, and PdK (1.8) /SiOp solids. Initial heats and adsorbed volumes at 0.5 Torr and at saturation can be compared in Table 1. As a first approximation, initial heats thus observed compare well with corresponding heats of carbon monoxide adsorption on unpromoted single-crystal palladium faces which vary from 142 to 167 kJ mol-’ [ 26-321. The initial heat of carbon monoxide adsorption over Pd-K/SiO, and Pd-Na/SiOz is increased by 10 kJ mol-l in contrast with the case of the lithium-promoted catalysts, lithium addition resulting in an almost unchanged initial heat. Furthermore it can be noticed that a marked decrease of the differential heat of carbon monoxide adsorption when the coverage increases occurs for coverkJ.mol-’
t__. 140-
120-
IOO-
20-
L_ cm3.ni-2Pd
0
adsorbed
CO
Fig. 3. Differential heats of adsorption (kJ mol-‘) of carbon monoxide at 298 K as a function of the quantity of adsorbed carbon monoxide (cm3 NTP rn-’ Pd) for the (-) Pd/SiO, and (---) Pd-K ( 1.8)/Si02 solids.
M. Gravelle-Rumeau-Maillot
et al./Appl. Catal. A 98 (1993) 45-59
53
ages in excess of half a monolayer (Fig. 3). For Pd/SiO, the coverages reach 0.5 and 0.8 for 0.20 and 0.32 cm3 CO/m2 Pd; for Pd-K (1.8) these values correspond to 0.14 and 0.22 cm3 CO/m2 Pd. Let us recall that a surface of 1 m2 contains 1.27. 101’Pd, atoms. The decrease in the carbon monoxide adsorption heat is more pronounced for the doped solids than for the alkali-free catalyst. The presence of Li, Na or K leads to a decrease in the amount of adsorbed carbon monoxide per unit area as deduced from electron microscopy. For instance in the presence of potassium the amount of adsorbed carbon monoxide decreases by 75% but the amount of adsorbed hydrogen does not vary. This is attributed to a loss of access of carbon monoxide to metal particles. A similar behaviour was reported for the case of potassium doped Ni/SiO, solids [33] and was attributed to a more or less complete decoration of metal particles with a thin film of alkali compounds (mainly silicates) [ 341. It was also shown that the loss of access was more prononced for carbon monoxide than for hydrogen, probably owing to the larger size of carbon monoxide. Adsorption of hydrogen and carbon monoxide at 473 K
In order to avoid possible hydride formation and to simulate more closely the conditions of carbon monoxide hydrogenation [ 51 the heats of adsorption have also been measured at 473 K and successive adsorptions have been performed at 473 K on Pd/SiO,, Pd-Li (2.0) /SiO, and Pd-Na (2.4) reduced at 673 K and evacuated at 623 K. Figs. 4 and 5 show the differential heats of adsorption of hydrogen and car-
i
kJ.mol-’
cm3.m-2Pd
I
0.2 adsorbed
*
hydrogen
Fig. 4. Differential heats of adsorption (kJ mol-‘) of hydrogen at 473 K as a function of the quantity of adsorbed hydrogen (cm3 NTP m-* Pd) for the Pd/Si02 solid.
54
M Gravelle-Rumeau-Maillot et al.fAppl. Catal. A 98 (1993) 45-59
t
k J .mol-’
ttL_ cm3.mm2Pd
_1
0. adsorbed
0.2
1 co
Fig. 5. Differential heats of adsorption (kJ mol-‘) of carbon monoxide at 473 K as a function of the quantity of adsorbed carbon monoxide ( cm3 NTP rn-’ Pd) for the Pd/SiOz solid. TABLE 2 Calorimetric heats evolved during the first dose and volumetric data for the adsorptions at 473 K Solid
QH,
W.
QCO
vco
Pd Pd-Li(2.0) Pd-Na(2.4)
90 88 89
0.14
138
0.22
0.14
136
0.24
0.14
136
0.23
Pd” Pd-Li”
87 87
0.035 0.075
QH,, QCO: heats of adsorption evolved during the first dose of gas (kJ mol-‘) VH,, VCO: adsorbed volumes (cm3 NTP m-’ Pd) for an equilibrium pressure of 0.5 Torr. “Metal surfaces are partially precovered with carbon monoxide. The experimental sequence is the following: carbon monoxide 473 K, vacua 473 K, hydrogen 473 K.
bon monoxide on Pd/SiOz. For lithium- and sodium-promoted catalysts the curves obtained are very similar to those of Figs. 4 and 5. Table 2 reports the adsorption heats evolved during the first doses of adsorbed gases and the adsorbed volumes obtained for an equilibrium pressure of 0.5 Torr. The adsorp-
M. Gravelle-Rumeau-Maillot
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55
tion heats do not vary significantly upon addition of alkali. The same trend (no variation) is observed for the integral heats deduced from graphical integrations in the range of coverage O-0.8. Data of Table 2 do not confirm the trends observed at 298 K; they do not infirm them either. The quantities of hydrogen or carbon monoxide chemisorbed and the adsorption heats are not modified if the evacuation of hydrogen (either after reduction, or after hydrogen chemisorption) is performed at 473 K instead of 623 K. These experiments show that an evacuation at 473 K is sufficient to remove most of chemisorbed hydrogen. In contrast, when the surface of palladium and Pd-Li (2.0) catalysts is saturated with carbon monoxide, an evacuation at 473 K does not lead to a zero coverage. As a matter of fact, a subsequent carbon monoxide chemisorption on the metal surface evacuated at 473 K gives rise to smaller heats and adsorbed quantities. This is in agreement with infrared spectroscopy which reveals the presence of adsorbed carbon monoxide after an evacuation at 473 K [ 41. When hydrogen is allowed to react with the metal surface partially precovered by carbon monoxide (carbon monoxide saturation followed by evacuation at 473 K), the heat of adsorption is 87 kJ mol-l (Fig. 6, Table 2) a value comparable to the heat measured on a clean surface. However the quantity of adsorbed hydrogen and the integral heat evolved is superior for the Pd-Li (2.0)/ SiOa solid than for the Pd/SiO, one. This will be discussed later on.
t
kJ.mol-’
A
i
L 0.05
adsorbed
cm? m-2 Pd a-0.1
hydrogen
0
0.05 adsorbed
0.1 hydrogen
Fig. 6. Differential heats of adsorption (kJ mol-‘) of hydrogen at 473 K as a function of the quantity of adsorbed hydrogen for Pd/SiO* (A) and Pd-Li(2.0)/SiO, (B) solids partially precoverdd with carbon monoxide by the following treatment: carbon monoxide saturation at 473 K and pumping at the same temperature.
56
M. Graoelle-Rumeau-Maillot et al.jApp1. Catal. A 98 (1993) 45-59
DISCUSSION
At room temperature we did not observe significant trends by comparing integral heats; thus we mainly consider initial heats when interaction between adsorbed species can be neglected as well as palladium hydride formation at room temperature. Table 1 shows that, at 298 K, the decrease of the initial heat of hydrogen adsorption is observed only in the presence of lithium and is pronounced when the lithium content is high enough, i.e. for the Pd-Li (7.4 )/SiO, solid. The initial heat is 13% smaller than the corresponding value obtained on unpromotecl Pd/SiO,. We suggest that metal decoration by the alkali compound [3] could explain this decrease: decoration by lithium would affect selectively the most energetic sites of palladium towards chemisorption. This speculative hypothesis would deserve verification. This hypothesis, however, does not provide a satisfying explanation to account for the alkali-induced change in the initial heat of carbon monoxide chemisorption at room temperature; no variation with lithium, increase of 7% in the presence of potassium and sodium. Infrared spectroscopy may help us in better understanding these observations. From infrared spectroscopy and volumetric measurements it is known that the adsorption of carbon monoxide does not occur on silica and on silica-containing alkali ions. The infrared spectrum of carbon monoxide irreversibly adsorbed at 298 K on the Pd/SiO, solid free from alkali, reduced at 673 K and evacuated at 623 K shows bands at 2088,1965,1910 (shoulder) cm-‘, assigned to linearly bonded carbon monoxide (L species) and to bridged bonded carbon monoxide on the (100) (B, species) and (111) (Bz species) faces of the palladium crystallites [4,35,36], respectively. As the evacuation temperature is increased the position of the bands is coverage-dependent with a downward frequency shift. After evacuation at 373 K the IR bands are situated at 2060 (L species), 1915 (B, species), and 1815 (shoulder) ( Bz species) cm- ‘. The linearly bonded carbon monoxide disappears under vacua at 473 K. At that temperature the two bridged bonded species absorb at 1895 and 1815 cm-‘, respectively. Under vacua the B1 species disappears at 573 K and the B, one at 623 K. Just before disappearance, at 573 K, the B2 species gives a small band at 1775 cm-‘. In presence of potassium or sodium the infrared spectrum of carbon monoxide irreversibly adsorbed at 298 K is strongly modified. The bands due to carbon monoxide adsorbed on unperturbed palladium disappear or are very small [ 41. For instance for the Pd-K (1.8) solid reduced at 673 K they are situated at 2050 and 1955 cm-‘. A new band appears at low wavelengths, at 1825 cm-l for the Pd-K (1.8) solid and at 1810 cm-l for the Pd-Na(2.0) solid. This new band is coverage dependent. An evacuation at 473 K, just before disappearance, results in a frequency shift of 125 cm-’ for the potassium-doped solid and of 110 cm-’ for the sodium-doped one.
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The lowering of the vC0 frequency may result either from a direct interaction or from an electron enrichment of the metallic phase. Based on the fact that the smaller the size of the alkali cation (Na,K,Cs), the lower the carbon monoxide stretching frequency, we have already suggested [ 41 that the oxygen atom of adsorbed carbon monoxide interacts with the electric field of alkali cation, the order of interaction being Na+ >K+ > Cs+. The increase in the heat of carbon monoxide adsorption resulting from sodium or potassium addition can be tentatively ascribed to the interaction between the oxygen of carbon monoxide and the alkali ions. This model would give a rough estimate of the energy of the electrostatic interaction; using data of sodium and potassium promoted catalysts, the interaction would be of the order of 10 kJ mol-l. A similar effect, even more pronounced, is to be expected for lithium-promoted catalysts. However lithium addition does not result in a significant variation of the carbon monoxide adsorption heat. This could be the result of a compensating effect: as for the case of hydrogen chemisorption, lithium addition would mask selectively the most active sites towards carbon monoxide chemisorption. The variations of initial heats of carbon monoxide and hydrogen chemisorptions here observed, are very small. At this stage of our discussion we can discuss the correspondence between the stretching frequency vC-0 and the strength of the metal M-CO bond. The initial heat of adsorption or the bond strength is proportional to the strength constant of M-CO, i.e. to the M-CO mode frequency. Consequently it is inversely proportional to the S-0 frequency [37]. For various single metal surfaces of Cu, Ni, Ru, Pt, a linear relationship was found between (v M-CO) 2 and the initial adsorption energy of carbon monoxide [ 381. On palladium single crystals the decrease in the heat of carbon monoxide adsorption when the coverage increases is reflected in the increase in the C-O stretching frequency (29-32,36,39]. In fact, in the literature, for palladium, the initial heats of carbon monoxide adsorption show only a small variation, never exceeding 15%, with surface structure [40]. For instance the initial heat of carbon monoxide adsorption is equal to 142 kJ mole1 on a Pd(ll1) face and to 152 kJ mol-l on a Pd(lOO) face (variation of 7%). At the same time, for low coverages, the CO stretching frequencies are observed at 1823 and 1895 cm-l; so the variation is also relatively small (4%). These variations are of the same order of magnitude as the variations reported in the present work. For palladium-supported solids, temperature-programmed desorption studies of carbon monoxide [ 41,421 and measurements of the integral heats of adsorption of hydrogen [ 241 and carbon monoxide [43] have been reported. Variations have been found to occur for small particles having an electron-deficient character, when the size drops below 3 or 4 nm. In our case, for an alkali-to-Pd ratio close to 2, the mean particle size increases from 2 nm to 2.2,2.3,3.1 nm adding K, Na and Li, so the modification is very moderate. At 473 K the initial or integral heats evolved during hydrogen or carbon
58
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monoxide adsorption are no more perturbed by the presence of the alkali. Furthermore there is no initial heat of interaction of hydrogen with preadsorbed carbon monoxide at 473 K suggesting that adsorbed carbon monoxide and hydrogen do not mutually interact on the metal surface. This observation agrees with the idea that, in methanol synthesis from syngas [ 51, adsorbed carbon monoxide would react with hydroxyl groups of the lithiated support to give formates rather than with adsorbed hydrogen to make a formyl intermediate. For the Pd-Li solid the increase of the quantity of adsorbed hydrogen and of the integral heat of adsorption could be due to the hydrogenation of the formate intermediate leading thus to further heat evolvement. CONCLUSION
It can be concluded that alkali addition to Pd/SiO, leads to very small variations of the initial heats of carbon monoxide and hydrogen adsorptions while the carbon monoxide adsorption mode is strongly modified as evidenced by infrared spectroscopy. Furthermore major changes occur in rates, selectivities, activation energies and orders of reactions. The large decreases in carbon monoxide stretching frequencies and the modifications of the catalytic properties suggest changes in the palladium-adsorbate bond strengths. From this work, it seems that no relationship exists between adsorption heats determined by calorimetry and catalysis. It could be suggested that the calorimetric method gives an average value for several adsorbed states and that temperature-programmed desorptions would be more suitable to determine changes in the binding energies of the various states. Let us also recall that both theory and experiments indicate no strong variation (around 15%) of the initial adsorption energy with the surface orientation. The modifications of catalytic properties are perhaps related to variations in sticking probabilities. The very small variations observed here at room temperature, decrease of about 13% of the initial heat of hydrogen adsorption in presence of lithium and increase of 7% of the initial heat of carbon monoxide adsorption in presence of sodium or potassium, have been tentatively interpreted in terms of (i) metal decoration by the lithium promoter and (ii) interaction between the oxygen atoms of adsorbed carbon monoxide and alkali cations. ACKNOWLEDGEMENT
The authors fully acknowledge Dr. Pierre Charles Gravelle for his help in fruitful discussions.
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