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Influence of carrier doping on catalytic performance of titanium dioxide supported platinum
Applied Catalysis, 46 (1989) 297-312 Elsevier Science Publishers B.V., Amsterdam -
297
Printed in The Netherlands
Influence of Carrier Doping on Catalytic Performance of Titanium Dioxide Supported Platinum EDMOND C. AKUBUIRO Department of Chemical Engineering, Drexel University, Philadelphia, PA 19104 (U.S.A.) and XENOPHON E. VERYKIOS* and THEOPHILOS IOANNIDES Institute of Chemical Engineering and High Temperature Chemical Processes, Department of Chemical Engineering, University of Patras, Patras (Greece) (Received 30 July 1988, revised manuscript received 22 September 1988)
ABSTRACT The effects of metal-support interactions, induced by doping titanium dioxide supports with cations of higher valency, were investigated under carbon monoxide hydrogenation and oxidation conditions over platinum catalysts. A significant decrease in turnover frequencies was observed as a result of doping of the carrier with cations of higher valency (Ta5+, Sb”+, We+). The activation energies of methanation and water-gas shift reactions were found to be unaffected by doping whereas that of carbon monoxide oxidation increased by up to 45%. The effects of dopants on kinetics parameters were found to depend on the concentration of the doping cation in the tits nium dioxide matrix. The phenomena observed are interpreted in terms of long-range electronic interactions at the metal-support interface, involving electron transfer from the doped support to the metal particles.
INTRODUCTION
Various forms of interaction between catalyst carriers and supported metal crystallites which influence the morphology of the active phase, its adsorptiondesorption characteristics or its activity and selectivity properties under various reactions have been reported. Early work by Schwab [l] and Solymosi [ 21 attributed the origin of metal-support interactions of certain catalytic systems to the electronic state of the carrier, evoking the theory of metal-semiconductor contacts. As a result of doping of the carrier with altervalent cations, the kinetic behaviour of supported metal crystallites was observed to change. In the literature of the last decade, the issue of metal-support interactions was dominated by the concept of strong metal-support interaction (SMSI) [ 3-51. It was subsequently shown that the phenomena collectively called SMSI, in a
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strict sense, are not due to any interaction between the metal and the carrier which alters the intrinsic properties of metal atoms. Auger surface analysis showed that the observed alterations in the chemisorptive and catalytic properties of SMSI catalysts are due to migration of Tif), species to the surface of supported metal crystallites [6-8]. Recently, this geometric explanation has also been coupled with an electronic factor operating concominantly [9,10]. In previous publications [11,12] the effects of altervalent cation doping of titanium dioxide carriers on the adsorptive properties of highly dispersed platinum crystallites were discussed. It was shown that small ( < 40 A) platinum particles supported on titanium dioxide doped with higher valency cations (Sb 5 + , Ta 5 + , W 6 + ) lose a major fraction of their ability to chemisorb hydrogen, oxygen and carbon monoxide. Normal chemisorption capacity was observed when the carrier was doped with equal (Ge4+) or lower (Mg 2+, K+) valency cations. Fourier transform IR (FTIR) studies showed significant shifts in the vibrational frequency of carbon monoxide adsorbed on higher valency doped catalysts, which were attributed to weakened chemisorption of carbon monoxide on platinum owing to higher saturation of platinum d-orbitals. Electrical conductivity measurements [13] detected dramatic changes in the electronic state of the doped carriers. Electrical conductivity of higher valency doped catalysts was found to be approximately three orders of magnitude higher than that of undoped catalysts, and the activation energy of electron conduction to be significantly reduced. Transmission electron microscopic (TEM) analysis showed that the observed phenomena cannot be attributed to reduced platinum dispersion in the doped catalysts, while ESCA showed no surface enrichment of the carriers with the dopant. The phenomena observed were interpreted in terms of long-range electronic interactions at the metal-support interface, involving electron transfer from the doped carrier to the supported metal particles. A model of electron transfer, based on the physics of metalsemiconductor contacts, showed the amount of charge transferred to be significant, and to depend strongly on the size of the metal crystallites and their surface density (number of metal crystallites per unit area of carrier). In this paper, the effects of altervalent cation doping of titanium dioxide carriers on the catalytic properties of supported platinum crystallites in the carbon monoxide-hydrogen and -oxygen reactions are discussed. Strong experimental evidence that the phenomena described in this and in previous publications [11,12] cannot be attributed to diffusion of the doping cations to the surface of the platinum crystallites is also presented. A major contribution in this field is the recent work of Solymosi et al. [14], who showed enhanced activity of rhodium supported on higher valency doped titanium dioxide in carbon monoxide- and carbon dioxide-hydrogen reactions. The results of these studies and the careful characterization of the catalytic systems involved offer strong evidence that the theories of metal-support interactions, first presented
299
by Schwab and Solymosi, do indeed represent physical reality to a considerable extent, at least in certain systems and under certain restrictive conditions. EXPERIMENTAL
The preparation and characterization of the doped supports and catalysts have been described in previous papers [ 11-131. For kinetics studies a fixedbed, single-pass, plug-flow reactor, consisting of a U-shaped, 1 cm I.D. stainless-steel tube, operated in the differential mode was used. The reactor was immersed in a constant-temperature, fluidized sand-bath for maintaining nearly isothermal conditions during the course of the reaction. Axial temperature profiles were determined by a number of thermocouples inserted in a thermocouple well in the center of the reactor tube. Temperature variations along the reactor length were determined to be less than i 0.5 K. Feed flowrates were measured and controlled by high-precision thermal mass flow meters and control valves. The feed stream was thoroughly mixed and preheated to the reaction temperature in a preheating section of the reactor, which was filled with inert alumina pellets. Analysis of the reaction mixture was performed by means of a gas chromatograph which was connected to the reactor apparatus via a gas sampling valve. The gas chromatograph was equipped with thermal conductivity and flame ionization detectors, temperature programming capability and a reporting integrator. The composition of the feed mixture was determined before and after kinetic experiments by bypassing the reactor and directing the mixture into the gas chromatograph. The gases used were all of ultrahigh purity and they were further purified by passing them through a DEOXO purifier and a molecular sieve trap. Kinetic experiments on carbon monoxide hydrogenation were performed at atmospheric pressure in the temperature range 518-598 K. Prior to kinetic experiments, a pretreatment schedule consisting of a nitrogen flow for 30 min, followed by a hydrogen flow for 1 h and a nitrogen flow for 30 min to purge the system, at the reaction temperature, was followed. The feed gas was hydrogencarbon monoxide (3:l) . Activity measurements were made in ascending temperature sequences, and a bracketing technique was used between each measurement [ 151. This technique involves switching off the feed gas and replacing it with a hydrogen purge for 20 min so as to regenerate the catalyst to its original activity. Measurements were taken only after steady-state conditions had been achieved. After determination of rates at the highest temperature, the reactor was cooled to the first temperature and new rate measurements were conducted. The nearly identical results obtained in all instances indicate that no substantial alterations of the catalytic surfaces occurred during the process of activation energy determination. Carbon monoxide hydrogenation carried out on the carriers (without metal content) showed no measurable conversions up to the upper limit of the reaction temperatures employed.
300
Kinetic experiments on carbon monoxide oxidation were conducted at atmospheric pressure in the temperature range 513-573 K. A carbon monoxideto-oxygen ratio of 0.45 was used, with 2% of carbon monoxide in the feed stream, which lies well within the carbon monoxide-inhibiting concentration region [16]. It was found necessary, in carbon monoxide oxidation, to mix the catalyst with similar sized inert alumina particles, prior to packing the reactor, which served as a heat sink for the exothermic heat of reaction. Prior to each kinetic experiment, the pretreatment schedule described earlier was also followed. Carbon monoxide oxidation carried out on the carriers (without metal content) showed no measurable conversion up to the upper limit of the reaction temperatures employed. In a series of preliminary experiments, operating conditions that eliminate external and internal heat and mass transport resistances were determined. Thus, the minimum feed flow-rate and the maximum catalyst particle size that eliminate such transport resistances were determined for each reaction. Kinetic experiments were conducted in operating regions where the interface and intraparticle transport resistances did not influence the kinetic parameters. RESULTS AND DISCUSSION
The catalysts employed in carbon monoxide hydrogenation and oxidation were characterized in terms of room temperature hydrogen, oxygen and carbon monoxide chemisorption capacity and by measurement of their electrical conductivity and activation energy of electron conduction under vacuum, hydrogen, carbon monoxide methanation conditions (8% carbon monoxide, 24% hydrogen, 68% nitrogen), and carbon monoxide oxidation conditions (2% carbon monoxide, 4% oxygen, 94% nitrogen). They were also studied by TEM, ESCA and FTIR of adsorbed carbon monoxide. The results of these characterization procedures have been reported elsewhere [11-13]. For ease of reference, catalyst configurations are designated as x% Pt/Ti0 2 (D), where x indicates the metal content and D the doping material. In all catalysts the concentration of the dopant in the carrier was 1 % by weight, unless indicated otherwise. Variation of the activity of 0.5% Pt/Ti0 2 (D) catalysts with time on-stream at 548 K for the methanation and water-gas shift reactions is shown in Fig. 1 and 2, respectively. For both reactions, the activity of platinum supported on undoped titanium dioxide is significantly higher than that of platinum supported on titanium dioxide doped with cations of higher valency. A small reduction in activity with time on-stream was observed in all instances, indicating that all catalysts, especially the doped ones, are very stable. A summary of the kinetic results of carbon monoxide hydrogenation is presented in Table 1. There is always difficulty in expressing specific activities of catalysts exhibiting anomalous chemisorption behaviour, as chemisorption
301
t
\
0
QS%Pt/TiO2
0
0.5;6pt/riO$Sb*)
0
a5%Pt/Ti02(Tot51
t
Ot, , , , , , , , , ilO 0
40
160
120
80
200
TIME (min)
Fig. 1. Variation of methanation activity with time on-stream of 0.5% Pt/TiOp (D) catalysts at 548 K.
- 76 70
0.5%Pt/TiOp
0
O.sf.F’t/TiO2(
0
O.$FY/TiO.$T&
- ‘5
1
Sti5)
0 x
3-
0
0”
!2 2.
O-
0
40
80
120
160
. 200
168
TIME(min)
Fig. 2. Variation of water-gas shift activity with time on-stream of 0.5% Pt/TiOz at 548 K.
(D) catalysts
302 TABLE 1 Summary of kinetic results of the carbon monoxide-hydrogen reaction at 548 K Catalyst
N cH4’ lo:] (flmol/s.m* Pt)
N co2. 10” (~mol/s*m’
a*
b*
a*
14.5 16.0
110 2.3 1.7
10.0 9.6
82 77 1.0 0.8
Z%Pt/TiO, 2%Pt/TiO,(Sb”+) 2%Pt/TiO,(Ta”+
180 2.9 2.3
0.5% 0.5% 0.5% 0.5%
120 108 1.1 0.9
Pt/TiO, Pt/TiO*(Mg’+) Pt/TiO,(Sb’+) Pt/TiO,(Ta”+ )
0.5% Pt/AI,O,,
16
14.2
Hydrocarbon distribution (%)
ECH,
EC02
11.5 12.0
16.8 17.2 16.8
19.1 19.7 19.4
100 95.3 3.5 90.4 7.1
1.2 2.5
10.0 8.7
16.6 18.2 18.8 18.6
18.2 19.0 19.5 19.9
100 100 95.7 3.6 94.7 4.0
0.7 1.3
17.6
18.8
91.1 7.4
1.5
Pt)
(kcal/mol)
(kcal/mol)
b’
C,
C2
C,
*Based on area obtained by hydrogen adsorption on (a) undoped catalyst and (b) doped catalyst.
techniques are often used to determine the exposed metallic area. Specific rates of methanation and water-gas shift reactions are reported in Table 1 based on areas obtained from hydrogen chemisorption on undoped samples (column a) and areas based on hydrogen chemisorption on doped samples (column b ) . It should be emphasized that TEM analysis has shown that the 0.5% Pt/TiO:! (D) catalysts suffered no measurable reduction in the degree of dispersion of platinum, as compared with the undoped catalysts, whereas the 2% Pt/TiOa (D) catalysts suffered a minor reduction in dispersion [ 111. Hence the true specific activities approach those reported in column a in Table 1 and not those in column b. Specific rates of higher valency doped catalysts are significantly lower than those of undoped or lower valency doped catalysts, differing by a factor of 120 with the 0.5% metal loading and by a factor of 70 with the 2% metal loading. For comparison purposes, the specific activity of a 0.5% Pt/ A1203 catalyst is also shown in Table 1 and it is seen to be almost one order of magnitude lower than that of 0.5% Pt/TiO,. The temperature sensitivity of carbon monoxide hydrogenation and watergas shift reactions is shown in Fig. 3 in the form of Arrhenius plots. Activation energies obtained over various catalysts are also shown in Table 1. It is observed that the activation energies are not significantly affected by the doping process, as the variation of ? 1 kcal/mol (1 cal= 4.1868 J) observed seems to be random. The results for 0.5% Pt/TiOa and 0.5% Pt/A1203 in terms of specific activity and activation energy of the two reactions compare very favorably with those of Vannice and co-workers [ 18-201, who conducted kinetic studies under identical conditions. The distribution of the hydrocarbons formed is dominated by methane, as expected. Nevertheless, small amounts of higher hydrocarbons, particularly
303
0 23
61
O.S/.Pt/TiO~(Ta
-17
l$!
-1911
1
1.71
1
s 1.73
1
’
1.75
’
8
I.77
8
’ 1.79
1
’
I.81
8
l/T X 103(K-'1
Fig. 3. Temperature sensitivity of methanation and water-gas shift reactions. Rates are expressed in pmol/s*m’Pt.
ethane and propane, are also formed over catalysts doped with higher valency cations. Higher hydrocarbons account for only 5-10% of the total hydrocarbons formed. The influence of dopants on the catalytic properties of platinum was also investigated under carbon monoxide oxidation conditions. Variation of the specific activity of 0.5% platinum catalysts with time on-stream at 533 K is shown in Fig. 4. As with carbon monoxide-hydrogen, the activity of platinum supported on pure titanium dioxide is higher than that of platinum supported on higher valency doped titanium dioxide, whereas the stability of doped catalysts is better than that of undoped catalysts. The temperature sensitivity of carbon monoxide oxidation is shown in Fig. 5, in the form of Arrhenius plots. Specific activity and activation energy results are summarized in Table 2. A suppression of specific activity is observed when platinum is supported on carriers doped with higher valency cations (Sb’+, Ta5+, W6+ ), whereas no significant changes are observed when the carrier is doped with lower valency cations (Mg2’, K+ ). The reduction of specific activity in carbon monoxide oxidation is significantly smaller than that in carbon monoxide hydrogenation. For the 0.5% platinum catalysts, a reduction by a factor of 3-5 is observed in
0
80
160
0
O.O6%%/TiO2
0
O.O6%Ft/TiO2tSb?
0
0.06%Pt/Ti02W+6)
240
320
400
TIME (mid
Fig. 4. Variation of carbon monoxide oxidation activity with time on-stream of 0.5% Pt/Ti02 catalysts at 533 K.
(D)
carbon monoxide oxidation, compared with a factor of 120 in carbon monoxide hydrogenation. Further, in contrast to the carbon monoxide-hydrogen case, the activation energies of carbon monoxide oxidation seem to be affected by the doping process. Activation energies of higher valency doped catalysts are higher by as much as 45%. Titanium dioxide carriers have been shown to exert a significant influence on the activity and selectivity characteristics of certain metals under various reactions, particularly carbon monoxide hydrogenation [ 17-211. The specific activity of platinum for the same reaction has also been shown to vary significantly with the support employed, from the least active Pt/SiO, to the most active Pt/TiO:! [ 17-201. The behavior of platinum is similar in carbon monoxide oxidation. Pt/TiO, has been found to exhibit the highest activity and lowest activation energy requirements compared with platinum supported on other carriers [ 161. In previous papers [11,12] it was shown that platinized titanium dioxide doped with cations of higher valency exhibits a drastically reduced chemisorption capacity for hydrogen, carbon monoxide and oxygen. Carbon monoxide hydrogenation and oxidation are used as probe reactions in the investigation of the effects of altervalent cation doping of carriers on kinetic parameters. These reactions were selected for the following reasons : (i ) the two reactions
305
I
0
L82
0.5%Pt/Ti02(Ta+51
1.85
1.91
I.88
l/T X lo3 (K-l’] Fig. 5. Temperature sensitivity of carbon monoxide oxidation reaction. Rates are expressed in ~mol/s*m’ Pt.
TABLE 2 Summary of kinetic results of carbon monoxide-oxygen reaction at 533 K Catalyst
N co (pm01 CO/s*m*Pt)
E(kcal/mol)
0.06% Pt/TiOs 0.06% Pt/TiOz (K+ ) 0.06% Pt/TiO* (Mg*+ )
109.3
126.3 100.6
16.4 16.6 16.9
0.06% Pt/Ti0,(Sb5’) 0.06% Pt/TiOs (Ta’+ ) 0.06% Pt/TiOz (W’+ )
4.4 6.2 8.2
21.4 23.9 21.1
0.5% Pt/TiO? 0.5% Pt/TiO* (M<‘+ )
271.6 209.2
16.7 17.3
98.4 60.9
22.4 21.0 21.9
0.5% Pt/TiO,(Sb”+) 0.5% Pt/TiOs(Ta5+) 0.5% Pt/TiO, ( W6+ )
90.2
306
are fairly simple and reasonably well understood; (ii) both reactions are insensitive to structure in the regions employed in this study, so potential metalsupport interactions would not be obscured from dispersion effects; (iii) the apparent sensitivity of the kinetic parameters of these reactions to the catalyst carrier is well documented; and (iv) the chemisorption characteristics of all the reactants involved (hydrogen, carbon monoxide and oxygen) are affected by doping of the carrier. The specific rates of the methanation and water-gas shift reactions (Table 1) indicate that the activity of platinum is significantly reduced when the titanium dioxide carrier is doped with cations of higher valency. It must be emphasized at this point that these results are based on hydrogen chemisorption on the undoped samples. TEM analysis of doped samples did not detect any measurable growth of metal particle size [ 111. It was therefore concluded that the degree of dispersion of platinum in the undoped and doped samples is approximately the same. Even if the hydrogen chemisorption suppresion of higher valency doped catalysts is ignored, and specific rates are calculated on the basis of the observed hydrogen chemisorption capacity, the specific activity is still reduced by approximately one order of magnitude (column b in Table 1) . The degree of reduction of the specific catalytic activity of platinum is over two orders of magnitude when the metal loading of the catalyst is 0.5% and approximately a factor of 70 when the metal loading is 2%. This observation is in accordance with the conclusion drawn from chemisorption and FTIR experiments that the effects of dopants depend strongly on the metal crystallite size, decreasing with increasing particle size. This conclusion is also in qualitative agreement with the theory of metal-semiconductor contacts, which has been evoked to explain the phenomena observed in this study. Quantitative conclusions on the effects of metal crystallite size and surface density on the amount of charge transferred between the metal particles and the semiconducting support have been presented in previous papers [ 11,121. The activation energies for the methanation and water-gas shift reactions, given in Table 1, indicate that this kinetic parameter is not affected by the doping process and varies randomly within ? 1 kcal/mol. Activation energies measured in this study are in good agreement with literature values for Pt/ TiO, [ 171 and Pt/Al,O, [ 17,221. The hydrocarbon distribution, also given in Table 1, is dominated by methane, as expected, owing to the low pressure and small conversions employed in this study. Nevertheless, small amounts of higher hydrocarbons, particularly ethane and propane, are also formed over doped catalysts, accounting for approximately 510% of the total hydrocarbons produced. The improved selectivity towards higher hydrocarbons of doped catalysts could be attributed to their suppressed hydrogen chemisorption capacity, as the methanation reaction is more hydrogen demanding. A qualitatively similar pattern of behavior of higher valency doped catalysts is also observed under carbon monoxide oxidation conditions, the kinetic re-
307
sults of which are summarized in Table 2. A reduction of specific activity by over one order of magnitude is observed with the 0.06% platinum catalysts, and a reduction by a factor of 3-5 with the 0.5% platinum catalysts. Differences in specific activity between the 0.06% Pt/Ti02 and 0.5% Pt/TiO:, catalysts are due to the different dispersions of the two catalysts [ 161. In contrast to observations made in the carbon monoxide-hydrogen reaction, the activation energies in the carbon monoxide-oxygen reaction appear to be affected by doping of the catalyst carriers. The activation energies of higher valency doped catalysts are higher than those of the undoped catalysts by as much as 45%. The suppressed activity of platinum catalysts supported on higher valency doped carriers is explained in terms of extended electronic interactions between highly dispersed metal particles and the support, which alter the electronic configuration of surface metal atoms. Details of this model for metalsupport interactions, which was first proposed by Schwab [l] and Solymosi [ 21, and quantitative estimates of charge transfer have been presented in previous papers [ 11,121. In summary, electron transfer from the semiconducting support to the metal particles originates from the requirement that the Fermi energy level of the two solids in contact must be the same at the interface. Doping titanium dioxide with cations of higher valency results in an increased Fermi energy level or a decreased work function. As the work function of platinum is higher than that of the semiconducting supports, electrons are transferred into platinum particles until the Fermi energy levels at the interface are at equal heights. For electrostatic reasons, the charge transferred into the metal particles is distributed between interfacial and surface atoms. The charge transferred into surface platinum atoms tends to saturate their empty d- orbitals and, as a result, these atoms tend to resemble those of their neighbor in the Periodic Table, gold, which is inactive towards the catalytic processes investigated. The kinetic results reported here conform with this model. The suppressed activity of doped catalysts can be correlated and attributed to their reduced chemisorption capacity for the reactant gases. However, the fact that the activation energy of carbon monoxide oxidation is affected by the doping process whereas that of carbon monoxide hydrogenation is not requires a mechanistic explanation. The range of operating conditions employed in this study for carbon monoxide oxidation clearly lies in the region of high and rate-inhibiting surface coverage of carbon monoxide [ 16,231. Rate expressions in this region have been correlated with a Langmuir-Hinshelwood mechanism [ 17,23,24]. The rate-controlling step is considered to be the interaction between an adsorbed carbon monoxide molecule and an adsorbed oxygen atom [ 17,251. This step, written in a manner which accommodates the presence of quasi-free electrons, CO++O--COz++e-
(1)
Hence the rate-controlling step proceeds as a donor reaction. The charge which
has been transferred into the platinum atoms via electronic interactions with the doped carrier offers increased resistance to this step owing to the fact that transfer of electrons into an already saturated bonding orbital is difficult. This enhanced resistance to the rate-controlling step is manifested as increased activation energy of higher valency ( Sb5+, Ta5+, W6+ ) doped catalysts. For carbon monoxide hydrogenation the rate-determining step involves dissociation of the carbon monoxide [ 24,26-301, which occurs readily over some metals but with difficulty over platinum. The rate-determining step for the methanation reaction can be written in the form CO++2H++e--C++H,O+
(2)
and that of the water-gas shift reaction CO++H,O++2e--CO,(g)+H,(g)
(3)
Hence both steps proceed as acceptor reactions. Titanium dioxide doped with higher valency cations supplies a small amount of negative charge (free electrons) to the surface platinum atoms without altering the resistance of these rate-controlling steps, as the electrons needed are already available. In other words, the activation energy of the reactions is not affected by a greater availability of electrons for these steps. The significant suppression of activity is due to reduced concentrations of adsorbed carbon monoxide and hydrogen, a phenomenon discussed previously [ 11,121. Charge transfer into the orbital of platinum as a consequence of the large Schottky barrier height and subsequent redistribution of these electrons onto the outermost layer of surface metal atoms hinders the adsorption of both carbon monoxide and hydrogen on surface platinum atoms. This is due to the fact that donation of electrons from the lone electron pair on the carbon atom in carbon monoxide and sharing of an electron by hydrogen with platinum bonding orbitals are required steps for the adsorption of these molecules. However, the donation to or the sharing of an electron with an already saturated dorbital is a difficult process. The degree of suppression of the activity of the two reactions is different. For the same metal loading (0.5%) and metal dispersion, the activity is suppressed by two orders of magnitude in carbon monoxide hydrogenation compared with a factor of 3-5 in oxidation. This observation can be explained by the results of in situ measurements of electrical conductivity [ 131. The electrical conductivity of catalyst samples under carbon monoxide hydrogenation conditions was found to be approximately five orders of magnitude higher than that under oxidation conditions. The reduced conductivity in an oxygen environment is due to scavenging of conduction electrons via the process +0,(g) +Vo2- +2ee-02-
(4)
where Vo2- represents doubly charged oxygen vacancies. A lower conductivity
of the carrier (in carbon monoxide oxidation) indicates a reduced Schottky barrier height and a smaller amount of charge transferred into the platinum crystallites and thus a smaller effect of the dopant on the catalytic activity of the platinum particles. If the electron transfer model is correct, the amount of charge transferred into the platinum particles and its effect on kinetic parameters should be proportional to the dopant concentration in the carrier, which defines the height of the barrier (for quantitative details see ref. 12 ). To test this concept, a number of catalysts were prepared with various dopant concentrations and their activities were determined under carbon monoxide hydrogenation conditions. The titanium dioxide carrier employed in the preparation of these catalysts was doped with Ta20s at concentrations up to 2 wt.-%. The results are presented in Fig. 6 in terms of turnover frequencies of the methanation and watergas shift reactions as a function of the dopant concentration in the carrier. It is apparent that the activity of supported platinum particles is indeed a function of dopant concentration or, alternatively, of the electron state of the carrier. As expected, the activity decreases with increasing amount of charge transferred into the platinum particles, very drasticaly initially and leveling off when the dopant concentration exceeds 1 wt.-%. This correlation between suppression of catalytic activity and electron state of the carrier provides strong supporting evidence for the electron transfer model. A serious criticism of the theory of electronic interaction at the metal-semi-
Dopant
(1.~1~0~) Concentration,
IWt.%
)
Fig. 6. Variation of turnover frequency of methanation and water-gas shift reactions with dopant content of the carrier.
310
conductor interface which has been employed to explain the phenomena observed in this study is the fact that certain (although not all) of the observations reported in this and previous papers can be explained by a process of diffusion of the dopant to the surface of the metal particles. This process would render the metallic surfaces inaccessible to chemisorption and catalytic action. This hypothesis was addressed by ESCA, which failed to detect any segregation of dopants to the surface of the catalysts [ 111. Other arguments against this hypothesis have also been provided: (i ) diffusion of a cation incorporated into the crystal matrix of a solid is expected to be an extremely slow process with significant activation energy requirements; it is unlikely that this process could occur at the low temperatures and short periods of time employed in this study; (ii) the lower valency cations would be expected to diffuse to the surface of metal particles with the same ease as the higher valency cations. Nevertheless, no alterations in the chemisorptive and catalytic properties were detected when the carriers were doped with cations of lower valency (K+, Mg+ ). To address this issue further, mixed oxide carriers were prepared and used as supports for platinum catalysts and tested under carbon monoxide hydrogenation conditions. Thus, titanium dioxide was mixed with 1 wt.-% of Ta,O, or W03 and processed in a manner identical with that used for doping. The essential difference is that the mixed oxides were heated at 673 K for 5 h instead of the 1173 K required for the doping process to occur. No doping of titanium dioxide would be expected at this low temperature. The resulting material was platinized with a metal content of 0.5%. Kinetic results obtained with these catalysts under carbon monoxide hydrogenation conditions are summarized in Table 3. It is apparent that in the absence of doping, foreign cations diffuse to the surface of platinum particles and they exert a strong positive influence on their activity. The degree of platinum surface coverage with foreign species is not known, but is probably very small. Thus, Ta,O, and, to a smaller extent, WO, function as promoters of platinum in carbon monoxide hydrogenation. It is observed in Table 3 that the turnover frequencies of the methanation and water-gas shift reactions increase by a factor of approximately 3 when the carrier is mixed with a small amount of Ta20, whereas they increase only slightly when the carrier is mixed with WO,. TABLE 3 Effects of mixed oxide supports on carbon monoxide-hydrogen activity at 548 K support
NcH4.103 (~mol/s~m* Pt)
Nco; lo3 (,umol/s~m* Pt)
TiOs TiO, + l%Ta,O, TiO*+ l%WO,
120 307 130
82 259 96
311
Thus, if the doping cations were diffusing to the surface of platinum particles under reaction conditions, their effect on turnover frequencies would have been positive in contrast to the large negative influences observed in this study. These results offer strong evidence that the dramatic reduction in the catalytic activity of higher valency doped catalysts cannot be attributed to diffusion of the dopants to the surface of the metal. The origin of this and the other phenomena observed in this investigation must be different. Based on evidence presented previously [ 11-131, the results of this study are interpreted in terms of long-range electronic interactions at the metal-semiconductor boundary layer. It must be stated that a number of workers, including one of the referees of this paper, doubt that the transfer of electrons over the metal-semiconductor interface can be large enough to change the properties of a substantial number of metal atoms and that the idea of a long-range electrostatic influence of the transferred charge is supported by solid-state theory. Obviously, further work on this theoretical problem is required. A phenomenological discrepancy is apparent between the results reported in this study on carbon monoxide hydrogenation on Pt/Ti02 (D) and those reported by Solymosi et al. [ 141 on Rh/TiO, (D). This discrepancy can be explained by considering the different electron structures of the two metals and the different rate-controlling steps in the reaction sequence over the two metals. The reduction in the activity of Pt/TiO* (D ) catalysts is primarily due to reduced populations of adsorbed carbon monoxide and hydrogen. As a result, no changes in activation energy are observed. The enhanced activity of Rh/Ti02 (D ) is probably due to weakened metal-carbon bonds and thus an enhanced rate of carbon hydrogenation. It is for this reason that the apparent activation energy over Rh/Ti02 (D) is lower than that over Rh/TiO,. These kinetic alterations of the Rh/Ti02 (D) catalysts reported by Solymosi et al. [ 141 have also been confirmed in our laboratory.
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E.C. Akubuiro and X.E. Verykios, J. Phys. Chem. Solids, (1988) in press. F. Solymosi, I. Tombacz and J. Koszta, J. Catal., 95 (1985) 578. M.A. Vannice and C. Sundhaker, J. Phys. Chem., 88 (1984) 2429. E.C. Akubuiro and X.E. Verykios, Appl. Catal., 14 (1985) 215. R.K. Herz and S.P. Marin, J. Catal., 65 (1980) 281. M.A. Vannice, C.C. Twu and S.H. Moon, J. Catal., 79 (1983) 70. M.A. Vannice and R.L. Garten, J. Catal., 56 (1979) 236. M.A. Vannice S.Y. Wang and S.H. Moon, J. Catal., 71 (1981) 152. B. Imelik, C. Naccache, G. Condurier, H. Praliand, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine (Eds.), Studies in Surface Science and Catalysis, Vol. 11, Metal-Support and Metal Additive Effects in Catalysis, Elsevier, Amsterdam, 1982. J. Sarkany and R.D. Gonzalez, Appl. Catal., 4 (1982) 53. E. McCarthy, J. Zahradnik, G.C. Kuczynski and J.J. Carberry, J. Catal., 39 (1975) 29. J.A. Rabo, A.P. Risch and M.L. Poutsma, J. Catal., 53 (1978) 295. T. Engel and G. Ertl, Adv. Catal. Relat. Subj., 28 (1979). S.V. Ho and P. Harriot, J. Catal., 64 (1980) 272. T. Mori, H. Masudo and Y. Murakami, J. Phys. Chem., 87 (1983) 3648. V. Ponec, Catal. Rev., 18 (1978) 151. A.T. Bell, Cata. Rev., 23 (1981) 203. P. Biloen and W.M.H. Sachtler, Adv. Catal., 30 (1981) 165.
297
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Influence of Carrier Doping on Catalytic Performance of Titanium Dioxide Supported Platinum EDMOND C. AKUBUIRO Department of Chemical Engineering, Drexel University, Philadelphia, PA 19104 (U.S.A.) and XENOPHON E. VERYKIOS* and THEOPHILOS IOANNIDES Institute of Chemical Engineering and High Temperature Chemical Processes, Department of Chemical Engineering, University of Patras, Patras (Greece) (Received 30 July 1988, revised manuscript received 22 September 1988)
ABSTRACT The effects of metal-support interactions, induced by doping titanium dioxide supports with cations of higher valency, were investigated under carbon monoxide hydrogenation and oxidation conditions over platinum catalysts. A significant decrease in turnover frequencies was observed as a result of doping of the carrier with cations of higher valency (Ta5+, Sb”+, We+). The activation energies of methanation and water-gas shift reactions were found to be unaffected by doping whereas that of carbon monoxide oxidation increased by up to 45%. The effects of dopants on kinetics parameters were found to depend on the concentration of the doping cation in the tits nium dioxide matrix. The phenomena observed are interpreted in terms of long-range electronic interactions at the metal-support interface, involving electron transfer from the doped support to the metal particles.
INTRODUCTION
Various forms of interaction between catalyst carriers and supported metal crystallites which influence the morphology of the active phase, its adsorptiondesorption characteristics or its activity and selectivity properties under various reactions have been reported. Early work by Schwab [l] and Solymosi [ 21 attributed the origin of metal-support interactions of certain catalytic systems to the electronic state of the carrier, evoking the theory of metal-semiconductor contacts. As a result of doping of the carrier with altervalent cations, the kinetic behaviour of supported metal crystallites was observed to change. In the literature of the last decade, the issue of metal-support interactions was dominated by the concept of strong metal-support interaction (SMSI) [ 3-51. It was subsequently shown that the phenomena collectively called SMSI, in a
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
298
strict sense, are not due to any interaction between the metal and the carrier which alters the intrinsic properties of metal atoms. Auger surface analysis showed that the observed alterations in the chemisorptive and catalytic properties of SMSI catalysts are due to migration of Tif), species to the surface of supported metal crystallites [6-8]. Recently, this geometric explanation has also been coupled with an electronic factor operating concominantly [9,10]. In previous publications [11,12] the effects of altervalent cation doping of titanium dioxide carriers on the adsorptive properties of highly dispersed platinum crystallites were discussed. It was shown that small ( < 40 A) platinum particles supported on titanium dioxide doped with higher valency cations (Sb 5 + , Ta 5 + , W 6 + ) lose a major fraction of their ability to chemisorb hydrogen, oxygen and carbon monoxide. Normal chemisorption capacity was observed when the carrier was doped with equal (Ge4+) or lower (Mg 2+, K+) valency cations. Fourier transform IR (FTIR) studies showed significant shifts in the vibrational frequency of carbon monoxide adsorbed on higher valency doped catalysts, which were attributed to weakened chemisorption of carbon monoxide on platinum owing to higher saturation of platinum d-orbitals. Electrical conductivity measurements [13] detected dramatic changes in the electronic state of the doped carriers. Electrical conductivity of higher valency doped catalysts was found to be approximately three orders of magnitude higher than that of undoped catalysts, and the activation energy of electron conduction to be significantly reduced. Transmission electron microscopic (TEM) analysis showed that the observed phenomena cannot be attributed to reduced platinum dispersion in the doped catalysts, while ESCA showed no surface enrichment of the carriers with the dopant. The phenomena observed were interpreted in terms of long-range electronic interactions at the metal-support interface, involving electron transfer from the doped carrier to the supported metal particles. A model of electron transfer, based on the physics of metalsemiconductor contacts, showed the amount of charge transferred to be significant, and to depend strongly on the size of the metal crystallites and their surface density (number of metal crystallites per unit area of carrier). In this paper, the effects of altervalent cation doping of titanium dioxide carriers on the catalytic properties of supported platinum crystallites in the carbon monoxide-hydrogen and -oxygen reactions are discussed. Strong experimental evidence that the phenomena described in this and in previous publications [11,12] cannot be attributed to diffusion of the doping cations to the surface of the platinum crystallites is also presented. A major contribution in this field is the recent work of Solymosi et al. [14], who showed enhanced activity of rhodium supported on higher valency doped titanium dioxide in carbon monoxide- and carbon dioxide-hydrogen reactions. The results of these studies and the careful characterization of the catalytic systems involved offer strong evidence that the theories of metal-support interactions, first presented
299
by Schwab and Solymosi, do indeed represent physical reality to a considerable extent, at least in certain systems and under certain restrictive conditions. EXPERIMENTAL
The preparation and characterization of the doped supports and catalysts have been described in previous papers [ 11-131. For kinetics studies a fixedbed, single-pass, plug-flow reactor, consisting of a U-shaped, 1 cm I.D. stainless-steel tube, operated in the differential mode was used. The reactor was immersed in a constant-temperature, fluidized sand-bath for maintaining nearly isothermal conditions during the course of the reaction. Axial temperature profiles were determined by a number of thermocouples inserted in a thermocouple well in the center of the reactor tube. Temperature variations along the reactor length were determined to be less than i 0.5 K. Feed flowrates were measured and controlled by high-precision thermal mass flow meters and control valves. The feed stream was thoroughly mixed and preheated to the reaction temperature in a preheating section of the reactor, which was filled with inert alumina pellets. Analysis of the reaction mixture was performed by means of a gas chromatograph which was connected to the reactor apparatus via a gas sampling valve. The gas chromatograph was equipped with thermal conductivity and flame ionization detectors, temperature programming capability and a reporting integrator. The composition of the feed mixture was determined before and after kinetic experiments by bypassing the reactor and directing the mixture into the gas chromatograph. The gases used were all of ultrahigh purity and they were further purified by passing them through a DEOXO purifier and a molecular sieve trap. Kinetic experiments on carbon monoxide hydrogenation were performed at atmospheric pressure in the temperature range 518-598 K. Prior to kinetic experiments, a pretreatment schedule consisting of a nitrogen flow for 30 min, followed by a hydrogen flow for 1 h and a nitrogen flow for 30 min to purge the system, at the reaction temperature, was followed. The feed gas was hydrogencarbon monoxide (3:l) . Activity measurements were made in ascending temperature sequences, and a bracketing technique was used between each measurement [ 151. This technique involves switching off the feed gas and replacing it with a hydrogen purge for 20 min so as to regenerate the catalyst to its original activity. Measurements were taken only after steady-state conditions had been achieved. After determination of rates at the highest temperature, the reactor was cooled to the first temperature and new rate measurements were conducted. The nearly identical results obtained in all instances indicate that no substantial alterations of the catalytic surfaces occurred during the process of activation energy determination. Carbon monoxide hydrogenation carried out on the carriers (without metal content) showed no measurable conversions up to the upper limit of the reaction temperatures employed.
300
Kinetic experiments on carbon monoxide oxidation were conducted at atmospheric pressure in the temperature range 513-573 K. A carbon monoxideto-oxygen ratio of 0.45 was used, with 2% of carbon monoxide in the feed stream, which lies well within the carbon monoxide-inhibiting concentration region [16]. It was found necessary, in carbon monoxide oxidation, to mix the catalyst with similar sized inert alumina particles, prior to packing the reactor, which served as a heat sink for the exothermic heat of reaction. Prior to each kinetic experiment, the pretreatment schedule described earlier was also followed. Carbon monoxide oxidation carried out on the carriers (without metal content) showed no measurable conversion up to the upper limit of the reaction temperatures employed. In a series of preliminary experiments, operating conditions that eliminate external and internal heat and mass transport resistances were determined. Thus, the minimum feed flow-rate and the maximum catalyst particle size that eliminate such transport resistances were determined for each reaction. Kinetic experiments were conducted in operating regions where the interface and intraparticle transport resistances did not influence the kinetic parameters. RESULTS AND DISCUSSION
The catalysts employed in carbon monoxide hydrogenation and oxidation were characterized in terms of room temperature hydrogen, oxygen and carbon monoxide chemisorption capacity and by measurement of their electrical conductivity and activation energy of electron conduction under vacuum, hydrogen, carbon monoxide methanation conditions (8% carbon monoxide, 24% hydrogen, 68% nitrogen), and carbon monoxide oxidation conditions (2% carbon monoxide, 4% oxygen, 94% nitrogen). They were also studied by TEM, ESCA and FTIR of adsorbed carbon monoxide. The results of these characterization procedures have been reported elsewhere [11-13]. For ease of reference, catalyst configurations are designated as x% Pt/Ti0 2 (D), where x indicates the metal content and D the doping material. In all catalysts the concentration of the dopant in the carrier was 1 % by weight, unless indicated otherwise. Variation of the activity of 0.5% Pt/Ti0 2 (D) catalysts with time on-stream at 548 K for the methanation and water-gas shift reactions is shown in Fig. 1 and 2, respectively. For both reactions, the activity of platinum supported on undoped titanium dioxide is significantly higher than that of platinum supported on titanium dioxide doped with cations of higher valency. A small reduction in activity with time on-stream was observed in all instances, indicating that all catalysts, especially the doped ones, are very stable. A summary of the kinetic results of carbon monoxide hydrogenation is presented in Table 1. There is always difficulty in expressing specific activities of catalysts exhibiting anomalous chemisorption behaviour, as chemisorption
301
t
\
0
QS%Pt/TiO2
0
0.5;6pt/riO$Sb*)
0
a5%Pt/Ti02(Tot51
t
Ot, , , , , , , , , ilO 0
40
160
120
80
200
TIME (min)
Fig. 1. Variation of methanation activity with time on-stream of 0.5% Pt/TiOp (D) catalysts at 548 K.
- 76 70
0.5%Pt/TiOp
0
O.sf.F’t/TiO2(
0
O.$FY/TiO.$T&
- ‘5
1
Sti5)
0 x
3-
0
0”
!2 2.
O-
0
40
80
120
160
. 200
168
TIME(min)
Fig. 2. Variation of water-gas shift activity with time on-stream of 0.5% Pt/TiOz at 548 K.
(D) catalysts
302 TABLE 1 Summary of kinetic results of the carbon monoxide-hydrogen reaction at 548 K Catalyst
N cH4’ lo:] (flmol/s.m* Pt)
N co2. 10” (~mol/s*m’
a*
b*
a*
14.5 16.0
110 2.3 1.7
10.0 9.6
82 77 1.0 0.8
Z%Pt/TiO, 2%Pt/TiO,(Sb”+) 2%Pt/TiO,(Ta”+
180 2.9 2.3
0.5% 0.5% 0.5% 0.5%
120 108 1.1 0.9
Pt/TiO, Pt/TiO*(Mg’+) Pt/TiO,(Sb’+) Pt/TiO,(Ta”+ )
0.5% Pt/AI,O,,
16
14.2
Hydrocarbon distribution (%)
ECH,
EC02
11.5 12.0
16.8 17.2 16.8
19.1 19.7 19.4
100 95.3 3.5 90.4 7.1
1.2 2.5
10.0 8.7
16.6 18.2 18.8 18.6
18.2 19.0 19.5 19.9
100 100 95.7 3.6 94.7 4.0
0.7 1.3
17.6
18.8
91.1 7.4
1.5
Pt)
(kcal/mol)
(kcal/mol)
b’
C,
C2
C,
*Based on area obtained by hydrogen adsorption on (a) undoped catalyst and (b) doped catalyst.
techniques are often used to determine the exposed metallic area. Specific rates of methanation and water-gas shift reactions are reported in Table 1 based on areas obtained from hydrogen chemisorption on undoped samples (column a) and areas based on hydrogen chemisorption on doped samples (column b ) . It should be emphasized that TEM analysis has shown that the 0.5% Pt/TiO:! (D) catalysts suffered no measurable reduction in the degree of dispersion of platinum, as compared with the undoped catalysts, whereas the 2% Pt/TiOa (D) catalysts suffered a minor reduction in dispersion [ 111. Hence the true specific activities approach those reported in column a in Table 1 and not those in column b. Specific rates of higher valency doped catalysts are significantly lower than those of undoped or lower valency doped catalysts, differing by a factor of 120 with the 0.5% metal loading and by a factor of 70 with the 2% metal loading. For comparison purposes, the specific activity of a 0.5% Pt/ A1203 catalyst is also shown in Table 1 and it is seen to be almost one order of magnitude lower than that of 0.5% Pt/TiO,. The temperature sensitivity of carbon monoxide hydrogenation and watergas shift reactions is shown in Fig. 3 in the form of Arrhenius plots. Activation energies obtained over various catalysts are also shown in Table 1. It is observed that the activation energies are not significantly affected by the doping process, as the variation of ? 1 kcal/mol (1 cal= 4.1868 J) observed seems to be random. The results for 0.5% Pt/TiOa and 0.5% Pt/A1203 in terms of specific activity and activation energy of the two reactions compare very favorably with those of Vannice and co-workers [ 18-201, who conducted kinetic studies under identical conditions. The distribution of the hydrocarbons formed is dominated by methane, as expected. Nevertheless, small amounts of higher hydrocarbons, particularly
303
0 23
61
O.S/.Pt/TiO~(Ta
-17
l$!
-1911
1
1.71
1
s 1.73
1
’
1.75
’
8
I.77
8
’ 1.79
1
’
I.81
8
l/T X 103(K-'1
Fig. 3. Temperature sensitivity of methanation and water-gas shift reactions. Rates are expressed in pmol/s*m’Pt.
ethane and propane, are also formed over catalysts doped with higher valency cations. Higher hydrocarbons account for only 5-10% of the total hydrocarbons formed. The influence of dopants on the catalytic properties of platinum was also investigated under carbon monoxide oxidation conditions. Variation of the specific activity of 0.5% platinum catalysts with time on-stream at 533 K is shown in Fig. 4. As with carbon monoxide-hydrogen, the activity of platinum supported on pure titanium dioxide is higher than that of platinum supported on higher valency doped titanium dioxide, whereas the stability of doped catalysts is better than that of undoped catalysts. The temperature sensitivity of carbon monoxide oxidation is shown in Fig. 5, in the form of Arrhenius plots. Specific activity and activation energy results are summarized in Table 2. A suppression of specific activity is observed when platinum is supported on carriers doped with higher valency cations (Sb’+, Ta5+, W6+ ), whereas no significant changes are observed when the carrier is doped with lower valency cations (Mg2’, K+ ). The reduction of specific activity in carbon monoxide oxidation is significantly smaller than that in carbon monoxide hydrogenation. For the 0.5% platinum catalysts, a reduction by a factor of 3-5 is observed in
0
80
160
0
O.O6%%/TiO2
0
O.O6%Ft/TiO2tSb?
0
0.06%Pt/Ti02W+6)
240
320
400
TIME (mid
Fig. 4. Variation of carbon monoxide oxidation activity with time on-stream of 0.5% Pt/Ti02 catalysts at 533 K.
(D)
carbon monoxide oxidation, compared with a factor of 120 in carbon monoxide hydrogenation. Further, in contrast to the carbon monoxide-hydrogen case, the activation energies of carbon monoxide oxidation seem to be affected by the doping process. Activation energies of higher valency doped catalysts are higher by as much as 45%. Titanium dioxide carriers have been shown to exert a significant influence on the activity and selectivity characteristics of certain metals under various reactions, particularly carbon monoxide hydrogenation [ 17-211. The specific activity of platinum for the same reaction has also been shown to vary significantly with the support employed, from the least active Pt/SiO, to the most active Pt/TiO:! [ 17-201. The behavior of platinum is similar in carbon monoxide oxidation. Pt/TiO, has been found to exhibit the highest activity and lowest activation energy requirements compared with platinum supported on other carriers [ 161. In previous papers [11,12] it was shown that platinized titanium dioxide doped with cations of higher valency exhibits a drastically reduced chemisorption capacity for hydrogen, carbon monoxide and oxygen. Carbon monoxide hydrogenation and oxidation are used as probe reactions in the investigation of the effects of altervalent cation doping of carriers on kinetic parameters. These reactions were selected for the following reasons : (i ) the two reactions
305
I
0
L82
0.5%Pt/Ti02(Ta+51
1.85
1.91
I.88
l/T X lo3 (K-l’] Fig. 5. Temperature sensitivity of carbon monoxide oxidation reaction. Rates are expressed in ~mol/s*m’ Pt.
TABLE 2 Summary of kinetic results of carbon monoxide-oxygen reaction at 533 K Catalyst
N co (pm01 CO/s*m*Pt)
E(kcal/mol)
0.06% Pt/TiOs 0.06% Pt/TiOz (K+ ) 0.06% Pt/TiO* (Mg*+ )
109.3
126.3 100.6
16.4 16.6 16.9
0.06% Pt/Ti0,(Sb5’) 0.06% Pt/TiOs (Ta’+ ) 0.06% Pt/TiOz (W’+ )
4.4 6.2 8.2
21.4 23.9 21.1
0.5% Pt/TiO? 0.5% Pt/TiO* (M<‘+ )
271.6 209.2
16.7 17.3
98.4 60.9
22.4 21.0 21.9
0.5% Pt/TiO,(Sb”+) 0.5% Pt/TiOs(Ta5+) 0.5% Pt/TiO, ( W6+ )
90.2
306
are fairly simple and reasonably well understood; (ii) both reactions are insensitive to structure in the regions employed in this study, so potential metalsupport interactions would not be obscured from dispersion effects; (iii) the apparent sensitivity of the kinetic parameters of these reactions to the catalyst carrier is well documented; and (iv) the chemisorption characteristics of all the reactants involved (hydrogen, carbon monoxide and oxygen) are affected by doping of the carrier. The specific rates of the methanation and water-gas shift reactions (Table 1) indicate that the activity of platinum is significantly reduced when the titanium dioxide carrier is doped with cations of higher valency. It must be emphasized at this point that these results are based on hydrogen chemisorption on the undoped samples. TEM analysis of doped samples did not detect any measurable growth of metal particle size [ 111. It was therefore concluded that the degree of dispersion of platinum in the undoped and doped samples is approximately the same. Even if the hydrogen chemisorption suppresion of higher valency doped catalysts is ignored, and specific rates are calculated on the basis of the observed hydrogen chemisorption capacity, the specific activity is still reduced by approximately one order of magnitude (column b in Table 1) . The degree of reduction of the specific catalytic activity of platinum is over two orders of magnitude when the metal loading of the catalyst is 0.5% and approximately a factor of 70 when the metal loading is 2%. This observation is in accordance with the conclusion drawn from chemisorption and FTIR experiments that the effects of dopants depend strongly on the metal crystallite size, decreasing with increasing particle size. This conclusion is also in qualitative agreement with the theory of metal-semiconductor contacts, which has been evoked to explain the phenomena observed in this study. Quantitative conclusions on the effects of metal crystallite size and surface density on the amount of charge transferred between the metal particles and the semiconducting support have been presented in previous papers [ 11,121. The activation energies for the methanation and water-gas shift reactions, given in Table 1, indicate that this kinetic parameter is not affected by the doping process and varies randomly within ? 1 kcal/mol. Activation energies measured in this study are in good agreement with literature values for Pt/ TiO, [ 171 and Pt/Al,O, [ 17,221. The hydrocarbon distribution, also given in Table 1, is dominated by methane, as expected, owing to the low pressure and small conversions employed in this study. Nevertheless, small amounts of higher hydrocarbons, particularly ethane and propane, are also formed over doped catalysts, accounting for approximately 510% of the total hydrocarbons produced. The improved selectivity towards higher hydrocarbons of doped catalysts could be attributed to their suppressed hydrogen chemisorption capacity, as the methanation reaction is more hydrogen demanding. A qualitatively similar pattern of behavior of higher valency doped catalysts is also observed under carbon monoxide oxidation conditions, the kinetic re-
307
sults of which are summarized in Table 2. A reduction of specific activity by over one order of magnitude is observed with the 0.06% platinum catalysts, and a reduction by a factor of 3-5 with the 0.5% platinum catalysts. Differences in specific activity between the 0.06% Pt/Ti02 and 0.5% Pt/TiO:, catalysts are due to the different dispersions of the two catalysts [ 161. In contrast to observations made in the carbon monoxide-hydrogen reaction, the activation energies in the carbon monoxide-oxygen reaction appear to be affected by doping of the catalyst carriers. The activation energies of higher valency doped catalysts are higher than those of the undoped catalysts by as much as 45%. The suppressed activity of platinum catalysts supported on higher valency doped carriers is explained in terms of extended electronic interactions between highly dispersed metal particles and the support, which alter the electronic configuration of surface metal atoms. Details of this model for metalsupport interactions, which was first proposed by Schwab [l] and Solymosi [ 21, and quantitative estimates of charge transfer have been presented in previous papers [ 11,121. In summary, electron transfer from the semiconducting support to the metal particles originates from the requirement that the Fermi energy level of the two solids in contact must be the same at the interface. Doping titanium dioxide with cations of higher valency results in an increased Fermi energy level or a decreased work function. As the work function of platinum is higher than that of the semiconducting supports, electrons are transferred into platinum particles until the Fermi energy levels at the interface are at equal heights. For electrostatic reasons, the charge transferred into the metal particles is distributed between interfacial and surface atoms. The charge transferred into surface platinum atoms tends to saturate their empty d- orbitals and, as a result, these atoms tend to resemble those of their neighbor in the Periodic Table, gold, which is inactive towards the catalytic processes investigated. The kinetic results reported here conform with this model. The suppressed activity of doped catalysts can be correlated and attributed to their reduced chemisorption capacity for the reactant gases. However, the fact that the activation energy of carbon monoxide oxidation is affected by the doping process whereas that of carbon monoxide hydrogenation is not requires a mechanistic explanation. The range of operating conditions employed in this study for carbon monoxide oxidation clearly lies in the region of high and rate-inhibiting surface coverage of carbon monoxide [ 16,231. Rate expressions in this region have been correlated with a Langmuir-Hinshelwood mechanism [ 17,23,24]. The rate-controlling step is considered to be the interaction between an adsorbed carbon monoxide molecule and an adsorbed oxygen atom [ 17,251. This step, written in a manner which accommodates the presence of quasi-free electrons, CO++O--COz++e-
(1)
Hence the rate-controlling step proceeds as a donor reaction. The charge which
has been transferred into the platinum atoms via electronic interactions with the doped carrier offers increased resistance to this step owing to the fact that transfer of electrons into an already saturated bonding orbital is difficult. This enhanced resistance to the rate-controlling step is manifested as increased activation energy of higher valency ( Sb5+, Ta5+, W6+ ) doped catalysts. For carbon monoxide hydrogenation the rate-determining step involves dissociation of the carbon monoxide [ 24,26-301, which occurs readily over some metals but with difficulty over platinum. The rate-determining step for the methanation reaction can be written in the form CO++2H++e--C++H,O+
(2)
and that of the water-gas shift reaction CO++H,O++2e--CO,(g)+H,(g)
(3)
Hence both steps proceed as acceptor reactions. Titanium dioxide doped with higher valency cations supplies a small amount of negative charge (free electrons) to the surface platinum atoms without altering the resistance of these rate-controlling steps, as the electrons needed are already available. In other words, the activation energy of the reactions is not affected by a greater availability of electrons for these steps. The significant suppression of activity is due to reduced concentrations of adsorbed carbon monoxide and hydrogen, a phenomenon discussed previously [ 11,121. Charge transfer into the orbital of platinum as a consequence of the large Schottky barrier height and subsequent redistribution of these electrons onto the outermost layer of surface metal atoms hinders the adsorption of both carbon monoxide and hydrogen on surface platinum atoms. This is due to the fact that donation of electrons from the lone electron pair on the carbon atom in carbon monoxide and sharing of an electron by hydrogen with platinum bonding orbitals are required steps for the adsorption of these molecules. However, the donation to or the sharing of an electron with an already saturated dorbital is a difficult process. The degree of suppression of the activity of the two reactions is different. For the same metal loading (0.5%) and metal dispersion, the activity is suppressed by two orders of magnitude in carbon monoxide hydrogenation compared with a factor of 3-5 in oxidation. This observation can be explained by the results of in situ measurements of electrical conductivity [ 131. The electrical conductivity of catalyst samples under carbon monoxide hydrogenation conditions was found to be approximately five orders of magnitude higher than that under oxidation conditions. The reduced conductivity in an oxygen environment is due to scavenging of conduction electrons via the process +0,(g) +Vo2- +2ee-02-
(4)
where Vo2- represents doubly charged oxygen vacancies. A lower conductivity
of the carrier (in carbon monoxide oxidation) indicates a reduced Schottky barrier height and a smaller amount of charge transferred into the platinum crystallites and thus a smaller effect of the dopant on the catalytic activity of the platinum particles. If the electron transfer model is correct, the amount of charge transferred into the platinum particles and its effect on kinetic parameters should be proportional to the dopant concentration in the carrier, which defines the height of the barrier (for quantitative details see ref. 12 ). To test this concept, a number of catalysts were prepared with various dopant concentrations and their activities were determined under carbon monoxide hydrogenation conditions. The titanium dioxide carrier employed in the preparation of these catalysts was doped with Ta20s at concentrations up to 2 wt.-%. The results are presented in Fig. 6 in terms of turnover frequencies of the methanation and watergas shift reactions as a function of the dopant concentration in the carrier. It is apparent that the activity of supported platinum particles is indeed a function of dopant concentration or, alternatively, of the electron state of the carrier. As expected, the activity decreases with increasing amount of charge transferred into the platinum particles, very drasticaly initially and leveling off when the dopant concentration exceeds 1 wt.-%. This correlation between suppression of catalytic activity and electron state of the carrier provides strong supporting evidence for the electron transfer model. A serious criticism of the theory of electronic interaction at the metal-semi-
Dopant
(1.~1~0~) Concentration,
IWt.%
)
Fig. 6. Variation of turnover frequency of methanation and water-gas shift reactions with dopant content of the carrier.
310
conductor interface which has been employed to explain the phenomena observed in this study is the fact that certain (although not all) of the observations reported in this and previous papers can be explained by a process of diffusion of the dopant to the surface of the metal particles. This process would render the metallic surfaces inaccessible to chemisorption and catalytic action. This hypothesis was addressed by ESCA, which failed to detect any segregation of dopants to the surface of the catalysts [ 111. Other arguments against this hypothesis have also been provided: (i ) diffusion of a cation incorporated into the crystal matrix of a solid is expected to be an extremely slow process with significant activation energy requirements; it is unlikely that this process could occur at the low temperatures and short periods of time employed in this study; (ii) the lower valency cations would be expected to diffuse to the surface of metal particles with the same ease as the higher valency cations. Nevertheless, no alterations in the chemisorptive and catalytic properties were detected when the carriers were doped with cations of lower valency (K+, Mg+ ). To address this issue further, mixed oxide carriers were prepared and used as supports for platinum catalysts and tested under carbon monoxide hydrogenation conditions. Thus, titanium dioxide was mixed with 1 wt.-% of Ta,O, or W03 and processed in a manner identical with that used for doping. The essential difference is that the mixed oxides were heated at 673 K for 5 h instead of the 1173 K required for the doping process to occur. No doping of titanium dioxide would be expected at this low temperature. The resulting material was platinized with a metal content of 0.5%. Kinetic results obtained with these catalysts under carbon monoxide hydrogenation conditions are summarized in Table 3. It is apparent that in the absence of doping, foreign cations diffuse to the surface of platinum particles and they exert a strong positive influence on their activity. The degree of platinum surface coverage with foreign species is not known, but is probably very small. Thus, Ta,O, and, to a smaller extent, WO, function as promoters of platinum in carbon monoxide hydrogenation. It is observed in Table 3 that the turnover frequencies of the methanation and water-gas shift reactions increase by a factor of approximately 3 when the carrier is mixed with a small amount of Ta20, whereas they increase only slightly when the carrier is mixed with WO,. TABLE 3 Effects of mixed oxide supports on carbon monoxide-hydrogen activity at 548 K support
NcH4.103 (~mol/s~m* Pt)
Nco; lo3 (,umol/s~m* Pt)
TiOs TiO, + l%Ta,O, TiO*+ l%WO,
120 307 130
82 259 96
311
Thus, if the doping cations were diffusing to the surface of platinum particles under reaction conditions, their effect on turnover frequencies would have been positive in contrast to the large negative influences observed in this study. These results offer strong evidence that the dramatic reduction in the catalytic activity of higher valency doped catalysts cannot be attributed to diffusion of the dopants to the surface of the metal. The origin of this and the other phenomena observed in this investigation must be different. Based on evidence presented previously [ 11-131, the results of this study are interpreted in terms of long-range electronic interactions at the metal-semiconductor boundary layer. It must be stated that a number of workers, including one of the referees of this paper, doubt that the transfer of electrons over the metal-semiconductor interface can be large enough to change the properties of a substantial number of metal atoms and that the idea of a long-range electrostatic influence of the transferred charge is supported by solid-state theory. Obviously, further work on this theoretical problem is required. A phenomenological discrepancy is apparent between the results reported in this study on carbon monoxide hydrogenation on Pt/Ti02 (D) and those reported by Solymosi et al. [ 141 on Rh/TiO, (D). This discrepancy can be explained by considering the different electron structures of the two metals and the different rate-controlling steps in the reaction sequence over the two metals. The reduction in the activity of Pt/TiO* (D ) catalysts is primarily due to reduced populations of adsorbed carbon monoxide and hydrogen. As a result, no changes in activation energy are observed. The enhanced activity of Rh/Ti02 (D ) is probably due to weakened metal-carbon bonds and thus an enhanced rate of carbon hydrogenation. It is for this reason that the apparent activation energy over Rh/Ti02 (D) is lower than that over Rh/TiO,. These kinetic alterations of the Rh/Ti02 (D) catalysts reported by Solymosi et al. [ 141 have also been confirmed in our laboratory.
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