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A study of thermal aging of PtAl2O3 using temperature-programmed desorption spectroscopy
JOURNAL
OF
CATALYSIS
110,285-297
(1988)
A Study of Thermal Temperature-Programmed DONALD Physical Chemistry Department,
D.
BECK
Aging of Pt/A1203 Using Desorption Spectroscopy AND
CONSTANCEJ. CARR
General Motors Research
Received
June 8, 1987; revised
Laboratories, October
Warren, Michigan 48090-9055
27, 1987
The metal dispersion of 1.0% Pt/AlZ03 was characterized using temperature-programmed desorption (TPD) of Dz in an ultrahigh vacuum apparatus. On the nonaged catalyst desorption of DZ gives rise to two peaks, one which can be identified as weak desorption from the AIZO, support surface and the other as D2 desorption from the supported Pt particle surfaces. The Pt/A1r03 sample was thermally aged in 5% Oz/Nz or in 5% HZ/N2 at a selected aging temperature for several hours. At intervals as small as I min. the Pt dispersion was obtained by D2 TPD. For aging temperatures of 700, 800, and 9OO”C, the Pt dispersion decreased rapidly within the first few minutes of treatment and then more gradually afterward, consistent with the change in the dominant sintering mechanism from particle migration to interparticle transport. For sintering temperatures of 700, 800, and 9OO”C, the D2 TPD peak shape changed as a function of aging time; this change suggests that faceting of the Pt particles occurs during sintering. The data also indicate that aging in an oxygen environment does not redisperse Pt/A1203, contrary to other reports. For all aging temperatures, sintering was less severe in the H2 environment than in the 01 environment. The aging data from both environments were fitted to a simple kinetic sintering model. D 1988 Academic PXSS, IK.
volves transport of metal atoms between particles anchored on the support. Experimentalists have empirically fitted exposed noble metal area (from chemisorption measurements) versus aging time data to a simple power law relation, (19)
INTRODUCTION
When supported metal catalysts are subjected to elevated temperatures for long periods of time, sintering of the supported metal can occur (I, 2). This process causes the exposed surface area of the supported metal to decrease, generally resulting in a change in the catalytic activity (3-10). Thus, the industrial importance of sintering has prompted a number of studies with the purpose of gaining a better understanding of this phenomenon. Some investigations have attempted to correlate chemisorption measurements with X-ray diffraction or electron microscopy in an effort to elucidate the mechanism(s) responsible for the decrease in exposed metal area as a function of aging time (8, 9, 11-29). As a result of that work, two mechanisms were proposed. One mechanism involves metal particle migration on the support surface, wherein particles agglomerate upon collision. The other proposed mechanism in-
_-=dS dr
kS”,
(1)
where S is the supported metal area, k is a rate constant, and n is the sintering order. Researchers have attempted to fit an entire data set to a single value of n (where 2 5 n I 16) and to correlate n with one particular sintering mechanism (2). However, some studies have shown that the sintering order is not necessarily constant over time indicating that sintering may involve sequential steps (9, II, 14). Under certain conditions, the sintering rate can change dramatically from fast to slow within the first few hours of thermal aging (6). However, in that and other studies, the catalyst was characterized at intervals in the aging treatment of 285 0021-9517/88 Cowrkht All rights
0
1988 hv Academic
of reproduction
in any form
$3.00
Press.
Inc.
reserved.
BECK
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AND
not less than 30 min. Thus, in rapidly sintering samples, the sintering kinetics cannot be well defined, particularly within the first hour when the decrease in dispersion can be dramatic at high sintering temperatures. A better understanding of sintering mechanisms requires better time resolution in the acquisition of kinetic data. The work presented here utilizes an apparatus that allows a catalyst to be characterized by temperature-programmed desorption (TPD) of D2 before and after aging treatments of several minutes duration. The supported metal area can be calculated using the area of the D2 TPD spectrum (20). EXPERIMENTAL Materials
The 1% Pt/A1203 catalyst used in this study was prepared by treating 100 g of W. R. Grace low-bulk-density alumina spheres (3.2 mm diameter, 110 m*/g, 1.1 ml/g pore volume) with a solution of 1.713 g [Pt(NH3)JC12 (57.7% Pt by ignition) in 100 ml of water. The sample was air-dried overnight in a hood and then calcined in flowing air (1 SCFH) for 2 h at 500°C. Portions of this lot of catalyst were ground to provide powdered catalyst for the TPD experiments. Conventional static hydrogen chemisorption measurements gave an as-prepared Pt dispersion of 0.67. For the purpose of this report, dispersion is defined as the ratio of the number of exposed noble metal atoms to the total number of metal atoms and is expressed in fractional form. The gases used in this study were 5% 02/ N2, 5% H2/N2, and D2 (99.7% CP grade) obtained from Scott Specialty Gases. The D2 was liquid nitrogen trapped before use. Apparatus
The experiments were performed in an ultrahigh vacuum system consisting of two stainless-steel chambers connected by a gate valve (Fig. 1). The sample preparation chamber was pumped by a 50 liter/s turbomolecular pump and was equipped with an ionization gauge, two capacitance manome-
CARR
ters (1 and 1000 Tot-r full scale), and a gas dosing apparatus. The analysis chamber was used for thermal desorption experiments and was equipped with an Extranuclear mass spectrometer and a liquid nitrogen trapped diffusion pump. The powder sample was mixed with H20 to form a slurry which was coated and air-dried on the center 5-mm section of a 1.O-mm-diameter molybdenum or tungsten wire loop. A Chromel-Alumel thermocouple junction, used for measurement of the sample temperature, was tack-welded to the center of the wire loop prior to application of the sample. The sample mixture was applied to the wire loop to form a thin uniform catalyst bed on the support wire. The sample was removed from the wire after the experiments were completed and was weighed using an electrobalance. Typical samples used in these experiments weighed on the order of 1 mg or less. The sample support wire was suspended between two copper leads of a feedthrough unit. Samples were heated to a maximum of 1000°C by applying dc current across the copper leads and cooled to a minimum of - 150°C by contacting the copper leads to a liquid nitrogen reservoir, located above the copper lead vacuum feedthrough. The sample support wire was stationary within the preparation chamber. The base pressure within both chambers was 5 x 10V9 Torr after bakeout, and typical working pressure after dosing was 2 x 10e8 Tot-r. Temperature-programmed desorption experiments were carried out using the following procedure. The sample was cooled to a predetermined temperature, usually - 14o”C, and exposed to D2 at 1 x lop3 Torr for a chosen length of time, with the gate valve open to the turbomolecular pump. Dz was chosen over H2 for TPD because the ambient partial pressure in the analysis chamber at 4 amu was much lower than that at 2 amu. At the end of the dose, the preparation chamber was pumped out and the valve between preparation and analysis chamber was opened. A TPD spectrum was
THERMAL
LNS
BAlli
287
AGING OF Pt/Al,O,
-
THERMOCOUPL
.E LEADS LEADS
MASS
BPECTROMETER VlEWPORl
SAMPLE
LEAK (ON
SUSPENDED
VALVES OTHER
FOR GAS
ON WiRE
INLET
PORT)
CRYOTRAP DlFf UB10N
PUMP
TURBOMOLECULAR
PUMP
B
THERMOCOUPLE
JUNCTION \
I -
COATINQ
OF CATALYST
Aging Treatments
Catalyst samples were aged in a reducing environment using the following procedure.
LOOP
PARTICLES
FIG. 1. Vacuum apparatus for temperature-programmed matic of apparatus; (B) detail of sample support.
obtained by resistively heating the sample support wire at a linear rate of 5”U.s or less using a temperature programmer. The partial pressures of desorbing gases were measured by the mass spectrometer during the heating program and recorded as a function of time and temperature. The measurement of up to eight masses in thermal desorption was obtained by interfacing the mass spectrometer with a PDP 1l/O3 computer.
MO WIRE
desorption (TPD) experiments. (A) Sche-
A “fresh” sample installed in the preparation chamber was heated at 500°C under vacuum for 1 h, then at 450°C in the presence of 1 Torr of 5% 0JN2 for 1 h, and then in the presence of 1 Torr of 5% Hz/N2 for 300 s. A D2 TPD spectrum of the “clean” fresh catalyst was then obtained and repeated to check for reproducibility. After TPD, the sample was cooled to - 150°C and exposed to 1 Torr of 5% Hz/N2. The catalyst temperature was rapidly increased to a preselected aging temperature and held at that temperature for 1 min (or longer), followed by rapidly decreasing the sample
288
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Exposure
lime
(8)
G -100
0
200
100
300
Temperature
400
600
600
(‘Cl
FIG. 2. TPD of D, adsorption at - 150°C on 1% Pt/A1203 for varying dose times at 1 x lo-) Torr. (A) 1000 s; (B) 260 s; (C) 80 S; (D) 50 s; (E) 40 s; (F) 20 s; (G) 10 s. Inset: Area of the /3 TPD peak as a function of dose time at i X 1O-3 Torr.
temperature to - 150°C and removing the H2 by flash heating (heat at 5OWs to 500°C and then immediately cool to - 15O’C). The D2 TPD of the “aged” catalyst was then obtained. Aging treatments followed by DZ TPD were repeated until an accumulated aging time of about 4 h was obtained. Catalysts aged in an oxidizing environment were cleaned by heating to 450°C in the presence of 02 and then Hz according to the procedure described above. A D2 TPD spectrum of the clean catalyst was obtained, and then the sample was exposed to 1 Ton- of 5% 02/N2 at - 150°C. The catalyst temperature was rapidly increased to a preselected temperature for 1 min or longer
and then rapidly decreased to - 150°C. The sample was then exposed to 1 Torr of 5% Hz/N2 for 300 s at 450°C in order to remove oxygen chemisorbed on the metal surface. Two D2 TPD spectra of the aged catalyst were obtained consecutively to check for reproducibility, and aging treatments followed by two D2 TPD measurements were repeated until an accumulated aging time of about 4 h was obtained. RESULTS
AND DISCUSSION
TPD Experiments
About 1 mg of 1% Pt supported on alumina was coated on a MO support wire using the slurry-drying technique described
THERMAL
previously. The sample was outgassed at 500°C and cleaned at 450°C by exposure to O2 and HZ. TPD experiments were performed in which the exposure of the sample at - 150°C to adsorbate gas (Dz) varied from 10 to 1000 s at 1 x 10m3 Torr. The resulting TPD spectra, Fig. 2, display two peaks. The low-temperature peak (cw)has a peak temperature (T,) equal to -125°C and its intensity reaches a maximum as a function of increasing dose time. The high-temperature peak (/3) is broad and its Tp decreases as a function of increasing dose time. A maximum in the /3 peak intensity occurs as
0
289
AGING OF Pt/AI,O,
100
a function of dose time (inset, Fig. 2) and a limiting value of Tp = 75°C is eventually observed. In subsequent experiments involving Pt/AI,O, , adsorbate doses resulting in a saturation coverage of the /3 state were used (i.e., 1 x 1O-3 Torr for 100 s). A TPD experiment using a DZ dose of 100 s at 1 X 1O-3 Torr was also carried out using about 1 mg of the blank alumina support. A single peak was observed at Tp = - 125°C in Fig. 3C which corresponds to the (Y peak observed using a 1% Pt/A1203 sample (Fig. 3A). A l-cm* Pt polycrystalline foil sample, previously cleaned in the vacuum chamber
200
Temperature
300
400
500
f%)
FIG. 3. TPD of D2 (exposure: - 150°C for 100 s and 1 X IO-? Ton) from various surfaces: (A) 1 mg of 1% Pt/AlZ03; (B) l-cm2 Pt polycrystalline foil; (C) 1 mg A&O,; (D) sample support wire alone,
BECK
AND
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peak indicates that deuterium absorbs dissociatively on the Pt surfaces. The relative Pt surface area was calculated from the area of the p peak assuming a constant chemisorption stoichiometry of D/ Rutface = 1. While this assumption may not be valid for the surfaces of very small Pt clusters or for some crystal plane boundaries on the surface of larger particles, it represents a useful approximation to the actual mean chemisorption stoichiometry for the total Pt surface in the samples studied here. The relative dispersion was then related to the area of the p peak as AT-p-peak, Areap-peA, -100
0
100 Tompereture
200
300
400
600
PC)
FIG. 4. TPD of Dz (exposure: - 150°C for 100 s at 1 X 1O-3 Torr) from 1% Pt/A1203 after various aging treatments: (A) fresh catalyst; (B) aged in 1 Torr of 5% 021 N2 for 1 h at 700°C: (C) same as that in (B) except aged for 2 h; (D) same as that in (B) except aged for 3 h; (E) same as that in (B) except aged for 4 h.
unknown known
=
DiSperSionunknown
Dispersionknown ’
(2)
The dispersion of the “cleaned” fresh Pt/ A&O, was measured by a conventional chemisorption technique and found to be 0.67 (average of three measurements). The area of the p peak for the “cleaned” fresh Ptl AllO3 was therefore taken to be representative of this dispersion. Aging in Oxygen
using the procedure described by Norton and Richards (21), was given a similar D2 dose. The resulting TPD spectrum, Fig. 3B, consists mainly of a broad signal which is somewhat resolved into three peaks, in agreement with the literature (22-24). Although this broad feature occurs at a lower temperature than that of the j3 peak observed in Fig. 3A, its width suggests a closer correspondence to the p peak than to the (Y peak. An identical TPD experiment conducted in the absence of any sample (i.e., using the bare support wire) resulted in a spectrum with a small peak at low temperature, Fig. 3D. The results of these TPD experiments suggest that the (Y peak which is observed with Al2O3 and 1% Pt/Al,O, and is first order can be assigned to the weak adsorption of deuterium on the A&O3 support and the p peak can be assigned to deuterium adsorbed on supported Pt metal surfaces. The second-order behavior of the /3
The effect of thermal aging on the Pt/ A1203 dispersion in an oxidizing environment is shown in Fig. 4. Figure 4A shows the D2 TPD of the cleaned fresh 1% Pt/ Alz03 catalyst after a Dz dose of 1 x 10e3 Torr at -150°C for 100 s. The sample was then aged in 1 Torr of 5% 02/N2 for 1 h at 700°C evacuated at - 15O”C, and cleaned by exposure to 1 Torr 5% HZ/N2 at 450°C for 300 s. TPD after a deuterium dose of 1 X 10m3 Tot-r at -150°C for 100 s is shown in Fig. 4B. The area of the /3 peak is smaller than that in Fig. 4A, indicating a decrease in the dispersion of the supported Pt. Also, upon aging in oxygen, the /3 peak splits into two peaks. The sample was then aged in 1 torr of 5% 02/Nz for another l-h period at 7OO”C, evacuated at -150°C and cleaned by exposure to 1 Torr 5% HZ/N2 at 450°C for 300 s. The D2 TPD, using the same dose as that described earlier, is shown in Fig. 4C. The overall area of the p peak is smaller than that in Fig. 4B. After a third and fourth
THERMAL
AGING
FIG. 5. Metal dispersion versus aging time at varying temperatures in 1 Torr 5% 02/N2: TPD measurements were obtained at l-h intervals. Solid points: the aging experiments at 600 and 800°C were repeated beginning with a “fresh” sample in both cases.
hour of aging in 1 Torr 5% 02/N2 at 7Oo”C, the area of the /3 peak decreased further (Figs. 4D and 4E, respectively). Splitting of the p peak was also observed when the catalyst was aged at 800 and 900°C; the splitting of the /3 Dz TPD peak is discussed later in this report. The aging experiments using 1% Pt/Al,O, were repeated for other treatment temperatures (600, 800, 900°C). Again, aging treatments of 1 h duration were used until an accumulated aging time of 4 h was obtained. The Pt dispersion as a function of aging time for several aging temperatures is shown in Fig. 5. The experiments were shown to be reproducible by duplicating the aging experiments at 600 and 800°C (solid points, Fig. 5) with fresh samples. These data indicate that for the same aging time, the dispersion decreases with increasing aging treatment temperature, which is consistent with studies reported in the literature (5, 25). Note that for aging at 700, 800, and 9Oo”C, most of the decrease in the dispersion occurs within the first hour of thermal aging. Upon aging at 6OO”C, the decrease in the chemisorption capacity occurred primarily within the first 2 h of treatment.
OF Pt/Al,O,
291
Better definition of the behavior of the Pt dispersion, particularly within the first hour of aging, requires dispersion measurements at shorter aging time intervals, preferably on the order of several minutes or less. The need for better time resolution in the dispersion versus aging time data led to the following series of experiments. A 1% Pt/Al,03 sample was exposed to thermal aging treatments (in 5% OJN2) of 1 min in duration until an accumulated aging time of 10 min was obtained; thereafter treatments somewhat longer than 1 min were used. Between aging treatments, the metal dispersion was obtained from DZ TPD. The results for aging at 600, 700, 800, and 900°C are shown in Fig. 6. In the case of aging at 700, 800, and 9OO”C, the chemisorption capacity decreased rapidly within thefirstfew minutes of thermal aging, and afterward decreased more gradually. The data obtained using l-h aging treatments (from Fig. 5) are superimposed (solid points) on the data using aging treatments of shorter duration in Fig. 6 and show good agreement with the latter. This agreement suggests that the decrease in the dispersion of the supported Pt is not notably affected
FIG. 6. Metal dispersion versus aging time at varying temperatures in 1 Torr 5% OZ/N2. Open points: TPD measurements obtained at intervals
292
BECK AND CARR
FIG. 7. Metal dispersion versus aging time in 1 Torr 5% 02/N* at 600°C for a total of 2 h, and then at 900°C for a total of 2 h. TPD measurements obtained at intervals Cl0 min.
by the frequency of the rapid heating and cooling of the sample before and after aging treatments but instead depends on the accumulated aging time at a particular aging “Fresh” and aged samples temperature. were analyzed by powder X-ray diffraction to investigate changes in the support phase, but no changes were observed, even upon heating at 900°C for 4 h. Therefore, the decrease in the dispersion was not associated with any bulk transformation of the A1203 phase. When the 1% Pt/Al,O, catalyst was thermally aged at 600°C in 5% 02/N* for an accumulated time of 2 h, and then aged at 900°C for an additional 2 h, the result shown in Fig. 7 was obtained. Dispersion was measured by TPD at -(IO-min intervals. In this experiment, the final dispersion is about equal to the dispersion of a “fresh” catalyst which has been thermally aged at 900°C for 2 h. This result shows that aging treatments at relatively low temperature (i.e., 600°C) cannot be used to prevent the dramatic metal sintering which occurs when the catalyst is later aged at higher temperature (i.e., 900°C). Likewise, this result implies that significant thermal damage to the catalyst will occur whenever the catalyst is treated at high temperatures.
Some researchers have claimed that exposure of supported Group VIII transition metals to oxygen at elevated temperature can result in redispersion of the metal on the basis of chemisorption (4) and TEM and XRD studies (26-28). Several redispersion mechanisms have been advanced which require conversion of the noble metal to an oxidized form. This conversion leads to fracture of supported particles or to migration of noble metal oxide molecules away from large crystallites (18, 27, 28). However, redispersion was not evident in the present study. The observation in other studies of an initial increase in the chemisorption capacity as a function of aging (4) can be attributed to incomplete cleaning of the “fresh” catalyst material. A supported metal catalyst which has been stored in air may have metal surfaces which are partially occluded by adsorbed species or their decomposition products. For this reason our “fresh” catalysts were pretreated before any aging experiments were performed, by oxidizing at 450°C for 1 h in order to remove these species, and then were reduced at 450°C to remove residual chemisorbed oxygen. This pretreatment schedule was necessary to obtain reproducible Dz TPD spectra. Aging in Hydrogen
The chemical environment plays an important role in the sintering of supported metal catalysts, and thus catalysts tend to sinter differently in a reducing environment than in an oxidizing environment. The sintering of Pt/Al,O, in a reducing environment was examined by performing the following experiments. A deuterium TPD spectrum was obtained at l-h intervals as a function of aging time in 5% H2/Nz. Typical TPD spectra are shown in Figs. 8A-8E for aging at 700°C. The j3 DZ TPD peak decreases in area as a function of aging time and splits into two peaks. Plots of the metal dispersion (measured from the area of the p peak) versus aging time at l-h intervals are shown in Fig. 9 (solid points)
THERMAL
-100
0
100
Temperature
200
300 PC,
400
AGING
500
FIG. 8. TPD of D2 (exposure: - 150°C for 100 s at 1 x 10m3Torr) from 1% Pt/Al,O, after varying aging treatments: (A) fresh catalyst; (B) aged in 1 Torr of 5% Hz/ N2 for 1 h at 700°C; (C) same as that in (B) except aged for 1 h; (D) same as that in (B) except aged for 3 h; (E) same as that in (B) except aged for 4 h.
for the four aging temperatures. The rate of sintering is most rapid within the first hour of thermal treatment, as is the case for aging in 5% 02/N2. In order to better characterize the sintering kinetics in 5% HZ/N2 particularly within the first hour of thermal aging, the noble metal dispersion was measured after thermal treatments 1 min in length until a total of 10 min of aging was obtained. Thereafter, dispersion was measured after longer thermal treatments. These data for aging at 600, 700, 800, and 900°C are shown in Fig. 9 (open points). When a fresh catalyst was aged for 2 h at 600°C in 5% H2/N2, and then for 2 h at 9Oo”C, the resulting dispersion (not shown) was about equal to that of a fresh catalyst aged for 2 h at 900°C. The observations which can be made from these experiments are mostly similar to the ones noted in the case of aging in 5% 02/N*. First and most importantly, rapid
OF Pt/Al,O,
293
changes in the dispersion occur within the first few minutes of treatment for aging at 700, 800, and 900°C. Second, the change in the metal dispersion as a function of aging in 5% Hz/N2 is independent of how often the dispersion measurements were obtained and only depends on the accumulated aging time at a particular aging temperature. Third, the change in metal dispersion as a function of aging time at 600°C does not exhibit as rapid a decrease as that observed for aging at higher temperatures. Fourth, dramatic thermal damage occurs regardless of when the catalyst is treated at relatively high temperatures (such as at 9OoOC). Fifth, the decrease in the metal dispersion with aging treatment at all temperatures was not reversible. Sixth, splitting of the p TPD peak occurred when the catalyst was aged at 700, 800, and 900°C. The results of these experiments differed from the results obtained for aging in 5% OJN2 in one regard. For all comparable aging conditions, the initial rapid decrease in the dispersion in a H2 aging environment was smaller than that observed in an oxygen aging environment. The change in the shape of the 0 D2 TPD peak as a function of thermal aging time can
FIG. 9. Metal dispersion versus aging time in 1 Torr 5% Hz/N2 at varying temperatures. Open points: TPD measurements taken at intervals (1 h. Solid points: TPD measurements taken at l-h intervals.
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BECKANDCARR
be attributed to the faceting of Pt particles as they grow. In this study, the metal dispersion of the 1% Pt/Al,O, catalyst aged at 900°C decreases from 0.67 (for the fresh catalyst) to co.10 (after aging for 4 h). The mean particle diameter increases from 2 to 15 nm during this 4-h aging schedule, assuming a Pt atom two-dimensional packing density of 1.5 x lOi atoms/cm2 (an average of the low-index surface planes (29) and a hemispherical particle morphology. The change in the estimated particle diameter with thermal aging is similar in range to that reported by Drechsler (30), who showed that an increase in metal particle size in the range from 1 to 10 nm results in the appearance of different types of crystal faces. Also, faceting of Pt supported on alumina films has been observed by electron microscopy when they were thermally annealed (10). The formation of well-defined crystal surfaces on the particles as they grow can lead to multiple peaks in D2 TPD. For example, two resolved Hz TPD peaks have been observed for the (1 lo), (loo), and (211) crystal planes of Pt (29). This study clearly shows that the sintering rate for supported Pt particles is highest during the first few minutes of aging and represents the first kinetic sintering data ever published with a relatively high degree of time resolution. Oxygen appears to be more effective than Hz as a sintering agent. Our results are consistent with those of other researchers who have reported that supported Pt particles either grow larger (32, 32) or sinter at a higher rate (6) when aged in O2 rather than in H2 or N2. Chen and Schmidt (31) have suggested that particle growth is enhanced in the presence of O2 at elevated temperature due to the mobility of PtO, either in the vapor phase or on the support. Other workers have reported experimental results that do not completely agree with these findings. For instance, Hassan and co-workers (5) reported that the surface area of Pt/Al,O, increased when exposed to H2 above 400°C. Soma and coworkers (33) observed that Hz chemisorp-
tion, suppressed after treatment in H2 at 500°C can be recovered by oxidation in O2 at 450°C followed by reduction in H2 at 300°C. The reason for the disagreement in published data remains unclear, and further study of the catalyst preparation, pretreatment, and role of aging environment on catalyst sintering is needed to understand the nature of metal-support bonding. Comparison of Experimental Data with a Sintering Model
Two mechanisms have been proposed in the published literature to explain the sintering kinetics of supported metal particles (I, 2). Mathematical models have been developed on the basis of each mechanism. Numerous publications describe attempts to fit experimental sintering data to a particular model. In one model, the sintering process involves the migration of particles on the support followed by particle fusion upon collision. In this model, the rate of change in the metal dispersion at constant temperature is given by a power law expression of the form given in Eq. (1) (13, 19). An alternate form of this power law can be expressed as (34) (n) log(DOID,) = log t + C,
(3)
where DOis the metal dispersion of the unaged catalyst, D, is the metal dispersion of a catalyst aged at time t, C is a constant, and n is defined as the sintering order. According to this model, sintering data plotted in the form log DO/D, vs log t are linear and provide a value of n. The physical significance of the sintering order has been the subject of debate (35). However, the sintering order may be helpful in identifying the predominant sintering mechanism in light of certain recent studies (discussed later) (9, 36, 37).
In the second proposed model, sintering involves primarily three steps: (i) loss of metal atoms from a crystallite, (ii) migration of metal atoms on the support, and (iii) capture of metal atoms by larger crystallites (17). Based on surface free energy argu-
THERMAL
r
AGING
I
B I 900c
-1.5
-1.0
-0.6
0 Lop
(Aping
0.6
1.0
Time)
FIG. 10. Plot of log D,lD, versus log t for varying aging temperatures in I Torr 5% Oz/N2 _ For each case, the change in slope is identified with a change in the dominant sintering mechanism (see text).
ments, smaller crystallites will lose metal atoms to larger ones at a particular temperature. The sintering rate in this model is based on the Kelvin equation and can be expressed as (16, 17) -dDldt
= kD” exp(mD),
our experimental data were fitted to the power law (Eq. (3)) and qualitatively compared to other published studies involving the fitting of sintering data to that model. A plot of log Do/D, versus log t was prepared using the areas of the /3 peaks in D2 spectra for varying aging times up to 10 h at 700, 800, and 900°C in 5% 02/N2 (Fig. 10) and 5% Hz/N2 (Fig. 11). In these cases, the data for a particular sintering temperature do not fit a single sintering order. Rather, the sintering order shifts from a high value to a low one, usually after 1 to 2 h of aging. The 600°C data (not shown) exhibit abnormally high sintering order values; thus the particle migration model may be inappropriate for the 600°C aging data. The change in sintering order as a function of sintering time and/or temperature has been observed in other studies. Harris et al. (9) measured the change in the particle size distribution of aged catalysts using transmission electron microscopy (TEM), and using the particle migration model they found sintering or-
(4)
where D is the metal dispersion, k is a rate constant, 12is a constant dependent on the mode of transport (n = 5 when transport involves two-dimensional diffusion of atoms on the support, it = 3 when transport is in the vapor phase), and m is a factor dependent on particle morphology, metalsupport and metal-vapor interfacial energies, metal-support contact angle, metal molar volume, and temperature. At constant temperature, m is approximately constant. Each model has limitations and may represent a simplified approach to the modeling of sintering behavior. Certainly, the best way to identify a dominant sintering mechanism involves direct observation of catalyst particle sintering using electron microscopy. However, some insight to the identification of the sintering mechanism can be gained by fitting experimental sintering data to one of the developed models or a modification of one of them. In this report,
295
OF Pt/Al,O,
I
I
1
0.8
I-t=4
P 0
/ 0.6
-L ;
0.4
3
0.2
0 -1.6
-1.0
-0.5 Loo
(Aglnp
0
0.5
time)
FIG. 11. Plot of log Do/D, versus log t for varying aging temperatures in I Torr 5% Hz/N,.
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BECK AND CARR
ders n = 13 at sintering times of 2 h or less, and n = 6 at sintering times greater than 2 h. In a more recent study, Harris (36) confirmed using TEM that the sintering order changes to a lower value with increasing sintering time. In both cases, the microscopy indicated that the atomic migration mechanism dominated particle growth during the low-sintering-order period. The change in sintering order was attributed to a change in the dominant sintering mechanism. Kuo et al. (37) used X-ray diffraction to measure metal particle size as a function of aging and came to a similar conclusion. Similar changes in sintering order with aging time and temperature have been observed in the study of the aging of Ni/A1203 (21, 14). In those studies, the shift in sintering order from high to low values was attributed to a change in the dominant sintering mechanism. Because our observations are generally in agreement with all of these cited studies, we believe that the change in sintering order indicated in Figs. 10 and 11 coincides with a change in the dominant sintering mechanism. We also suggest that particle migration and coalescence dominate during the first hour of aging, whereas atomic migration dominates thereafter. The change in the slope of the 02 aging data generally occurs in the same time range as that of the H2 aging data, indicating that the aging environment does not significantly influence when the mechanism changes. The break in the sintering order does occur sooner during aging at 900°C than at 800 or 700°C. The cause for the change in the dominant sintering mechanism has been attributed by Kuo ef al. to the influence of the support pore structure (37). The change in the sintering mechanism has also been attributed to collapse of the support (24). Assuming that the mean metal particle size can be estimated from the metal dispersion, our data imply that the pore structure of the support probably does not influence when the sintering mechanism changes, because the breakpoints in sintering order at 700, 800, and
900°C (Figs. 10 and 11) occur at different values in the metal dispersion. We have observed that the BET surface area of the sample decreased after aging at 900°C in either 5% Hz/N2 or 5% Oz/N2 for 1 h. Therefore, support collapse may play a significant role in the mechanism at higher aging temperatures, particularly given that the change occurs sooner during aging at 900°C. In the case of aging at 800 and 900°C the sintering orders in the oxygen environment were generally lower than those in hydrogen, This trend has also been observed by other researchers (II, 14), who suggested that the atomic transport mechanism is more dominant in an oxidizing aging environment. Consistent with this observation is the suggestion that particle growth in O2 at relatively high temperatures involves some conversion of Pt to PtO, which is mobile either in the vapor phase or on the support. However, because our study did not involve direct observation of metal particle growth and morphology changes (for instance, with TEM), the cause for the abrupt change in the sintering rate with aging time remains unclear. The application of the particle migration model to experimental sintering data is useful for obtaining qualitative information about the mechanism(s) active in sintering, particularly since researchers have correlated direct observation by TEM of particle behavior as a function of sintering time and temperature with one of the sintering models. However, we have noted the disagreement in dispersion versus aging time data reported by some investigators. Thus the information about sintering kinetics obtained using either model may be questionable. In order to better understand sintering mechanisms and kinetics, further study is needed, particularly by direct observation of supported metal particle sintering with electron microscopy. SUMMARY
The sintering of 1% Pt/A1203 has been investigated using temperature-pro-
THERMAL
AGING
grammed desorption of DZ in an ultrahigh vacuum apparatus. When the catalyst was thermally aged in 5% OJNZ or 5% Hz/N2 at temperatures of 700°C or higher, the exposed metal surface area decreased rapidly during the first few minutes of treatment and then more gradually afterward. The same trend in the sintering rate change as a function of time was observed for aging in H2/N2, although the initial rate of sintering was lower in the reducing environment. The overall decrease in the metal surface area was dependent on the aging temperature for both aging environments. Sintering of the supported metal was accompanied by a shape change in the D2 TPD peak, which is attributed to the faceting of Pt particles as they grow. Redispersion of Pt/Al,O, in O2 was not observed in these experiments. For sintering temperatures of 700, 800, and 9Oo”C, the order of sintering abruptly decreased as a function of aging time indicating a change in the dominant sintering mechanism. ACKNOWLEDGMENTS The authors thank Mike D’Aniello for the chemisorption results and for helpful comments and suggestions, and Jack Johnson for performing the X-ray diffraction analysis.
I. 2. 3. 4. 5. 6.
7. 8.
REFERENCES Wanke, S. E., and Flynn, P. C., Catal. Rev. Sci. Eng. 12, 93 (1975). Hughes, R., “Deactivation of Catalysts.” Academic Press, London, 1984. Bond, G. C., Surf Sci. 156, 966 (1985). Fiedorow, R. M. J., and Wanke, S. E., J. Catal. 43, 34 (1976). Hassan, S. A., Khalil, F. H., and El-Gamal, F. G., J. Catal. 44, 5 (1976). Furhman, 2. A., and Parravano, G., “Proceedings, 6th International Congress on Catalysis, London, 1976” (G. C. Bond, P. B. Wells, and F. C. Tomkins, Eds.), Vol. 2, p. 686. The Chemical Society, London, 1976. Chu, Y. F., and Ruckenstein, E., J. Catal. 55, 281 (1978). Smith, D. J., White, D., Baird, T., and Fryer, J. R., J. Catal. 81, 107 (1983).
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9. Harris, P. J. F., Boyes, E. D., and Cairns, J. A., J. Cafal. 82, 127 (1983). i;: Harris, P. J. F., A@. Catal. 16, 439 (1985). Kim, K.-T., and Ihm, S.-K., J. Catal. 96, 12 (1985). 12, Ruckenstein, E., and Dadyburjor, D., Thin So/id Films 55, 89 (1978). 13. Ruckenstein, E., and Pulvermacher, B., J. Catal. 29, 224 (1973). 14. Bartholomew, C. H., and Sorensen, W. L., J. Catal. 81, 131 (1983). 15, Chen, J. J., and Ruckenstein, E., J. Catal. 69, 254 (1981). 16. Flynn, P. C., and Wanke, S. E., .I. Catal. 34, 390 (1974). 17. Flynn, P. C., and Wanke, S. E., J. Catal. 34, 400 (1974). ,8, Handa, P. K., and Matthews, J. C., AlChE J. 29, 717 (1983). 19. Ruckenstein, E., and Pulvermacher, B., AIChE J. 19, 356 (1973). 20. Falconer, J. L., and Schwarz, J. A., Catal. Rev. Sci. Eng. 25, 141 (1983). 21, Norton, P. R., and Richards, P. J., Surf. Sci. 41, 294 (1974). 22. Kawasaki, K., Kadama, T., Miki, H., and Kioka, T., Surf. Sci. 64, 349 (1977). 23. Winterbottom, W. C., Surf. Sci. 36, 195 (1973). 24. Ko, C. S., and Gorte, R. J., Surf. Sci. 155, 296 (1985). 25. Fiedorow, R. M. J., Chahar, B. S., and Wanke, S. E., J. Catal. 51, 193 (1978). 26. Lieske, H., and Volter, J., J. Phys. Chem. 89, 1841 (1985). 27. Ruckenstein, E., and Hu, X. D., Langmuir 1,756 (1985). 28. Dadyburjor, D. B., in “Catalyst Deactivation” (B. Delmon, and F. G. Froment, Eds.), p. 341. Elsevier, Amsterdam, 1980. 29. Lu, K. E., and Rye, R. R., Surf. Sci. 45, 677 (1974). 30. Drechsler, M., NATO Adv. Sci. Inst. Ser. B. 86, 405 (1983). 31. Chen, M., and Schmidt, L. D., J. Catal. 55, 348 (1978). 32. White, D., Baird, T., Fryer, J. R., Freeman, L. A., Smith, D. J., and Daz, M., J. Catal. 81, 119 (1983). 33. Kunimori, K., Ikeda, Y., Soma, M., and Uchijima, T., J. Catal. 79, 185 (1983). 34. Wynblatt, P., and Gjostein, N. A., Bog. So/id State Chem. 9, 21 (1975). 35. Lee, H. H., J. Catal. 63, 129 (1980). 36. Harris, P. J. F., J. Catal. 97, 527 (1986). 37. Kuo, H. K., Ganesan, P., and deAngelis, R. J., J. Catal. 64, 303 (1980).
OF
CATALYSIS
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(1988)
A Study of Thermal Temperature-Programmed DONALD Physical Chemistry Department,
D.
BECK
Aging of Pt/A1203 Using Desorption Spectroscopy AND
CONSTANCEJ. CARR
General Motors Research
Received
June 8, 1987; revised
Laboratories, October
Warren, Michigan 48090-9055
27, 1987
The metal dispersion of 1.0% Pt/AlZ03 was characterized using temperature-programmed desorption (TPD) of Dz in an ultrahigh vacuum apparatus. On the nonaged catalyst desorption of DZ gives rise to two peaks, one which can be identified as weak desorption from the AIZO, support surface and the other as D2 desorption from the supported Pt particle surfaces. The Pt/A1r03 sample was thermally aged in 5% Oz/Nz or in 5% HZ/N2 at a selected aging temperature for several hours. At intervals as small as I min. the Pt dispersion was obtained by D2 TPD. For aging temperatures of 700, 800, and 9OO”C, the Pt dispersion decreased rapidly within the first few minutes of treatment and then more gradually afterward, consistent with the change in the dominant sintering mechanism from particle migration to interparticle transport. For sintering temperatures of 700, 800, and 9OO”C, the D2 TPD peak shape changed as a function of aging time; this change suggests that faceting of the Pt particles occurs during sintering. The data also indicate that aging in an oxygen environment does not redisperse Pt/A1203, contrary to other reports. For all aging temperatures, sintering was less severe in the H2 environment than in the 01 environment. The aging data from both environments were fitted to a simple kinetic sintering model. D 1988 Academic PXSS, IK.
volves transport of metal atoms between particles anchored on the support. Experimentalists have empirically fitted exposed noble metal area (from chemisorption measurements) versus aging time data to a simple power law relation, (19)
INTRODUCTION
When supported metal catalysts are subjected to elevated temperatures for long periods of time, sintering of the supported metal can occur (I, 2). This process causes the exposed surface area of the supported metal to decrease, generally resulting in a change in the catalytic activity (3-10). Thus, the industrial importance of sintering has prompted a number of studies with the purpose of gaining a better understanding of this phenomenon. Some investigations have attempted to correlate chemisorption measurements with X-ray diffraction or electron microscopy in an effort to elucidate the mechanism(s) responsible for the decrease in exposed metal area as a function of aging time (8, 9, 11-29). As a result of that work, two mechanisms were proposed. One mechanism involves metal particle migration on the support surface, wherein particles agglomerate upon collision. The other proposed mechanism in-
_-=dS dr
kS”,
(1)
where S is the supported metal area, k is a rate constant, and n is the sintering order. Researchers have attempted to fit an entire data set to a single value of n (where 2 5 n I 16) and to correlate n with one particular sintering mechanism (2). However, some studies have shown that the sintering order is not necessarily constant over time indicating that sintering may involve sequential steps (9, II, 14). Under certain conditions, the sintering rate can change dramatically from fast to slow within the first few hours of thermal aging (6). However, in that and other studies, the catalyst was characterized at intervals in the aging treatment of 285 0021-9517/88 Cowrkht All rights
0
1988 hv Academic
of reproduction
in any form
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Inc.
reserved.
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AND
not less than 30 min. Thus, in rapidly sintering samples, the sintering kinetics cannot be well defined, particularly within the first hour when the decrease in dispersion can be dramatic at high sintering temperatures. A better understanding of sintering mechanisms requires better time resolution in the acquisition of kinetic data. The work presented here utilizes an apparatus that allows a catalyst to be characterized by temperature-programmed desorption (TPD) of D2 before and after aging treatments of several minutes duration. The supported metal area can be calculated using the area of the D2 TPD spectrum (20). EXPERIMENTAL Materials
The 1% Pt/A1203 catalyst used in this study was prepared by treating 100 g of W. R. Grace low-bulk-density alumina spheres (3.2 mm diameter, 110 m*/g, 1.1 ml/g pore volume) with a solution of 1.713 g [Pt(NH3)JC12 (57.7% Pt by ignition) in 100 ml of water. The sample was air-dried overnight in a hood and then calcined in flowing air (1 SCFH) for 2 h at 500°C. Portions of this lot of catalyst were ground to provide powdered catalyst for the TPD experiments. Conventional static hydrogen chemisorption measurements gave an as-prepared Pt dispersion of 0.67. For the purpose of this report, dispersion is defined as the ratio of the number of exposed noble metal atoms to the total number of metal atoms and is expressed in fractional form. The gases used in this study were 5% 02/ N2, 5% H2/N2, and D2 (99.7% CP grade) obtained from Scott Specialty Gases. The D2 was liquid nitrogen trapped before use. Apparatus
The experiments were performed in an ultrahigh vacuum system consisting of two stainless-steel chambers connected by a gate valve (Fig. 1). The sample preparation chamber was pumped by a 50 liter/s turbomolecular pump and was equipped with an ionization gauge, two capacitance manome-
CARR
ters (1 and 1000 Tot-r full scale), and a gas dosing apparatus. The analysis chamber was used for thermal desorption experiments and was equipped with an Extranuclear mass spectrometer and a liquid nitrogen trapped diffusion pump. The powder sample was mixed with H20 to form a slurry which was coated and air-dried on the center 5-mm section of a 1.O-mm-diameter molybdenum or tungsten wire loop. A Chromel-Alumel thermocouple junction, used for measurement of the sample temperature, was tack-welded to the center of the wire loop prior to application of the sample. The sample mixture was applied to the wire loop to form a thin uniform catalyst bed on the support wire. The sample was removed from the wire after the experiments were completed and was weighed using an electrobalance. Typical samples used in these experiments weighed on the order of 1 mg or less. The sample support wire was suspended between two copper leads of a feedthrough unit. Samples were heated to a maximum of 1000°C by applying dc current across the copper leads and cooled to a minimum of - 150°C by contacting the copper leads to a liquid nitrogen reservoir, located above the copper lead vacuum feedthrough. The sample support wire was stationary within the preparation chamber. The base pressure within both chambers was 5 x 10V9 Torr after bakeout, and typical working pressure after dosing was 2 x 10e8 Tot-r. Temperature-programmed desorption experiments were carried out using the following procedure. The sample was cooled to a predetermined temperature, usually - 14o”C, and exposed to D2 at 1 x lop3 Torr for a chosen length of time, with the gate valve open to the turbomolecular pump. Dz was chosen over H2 for TPD because the ambient partial pressure in the analysis chamber at 4 amu was much lower than that at 2 amu. At the end of the dose, the preparation chamber was pumped out and the valve between preparation and analysis chamber was opened. A TPD spectrum was
THERMAL
LNS
BAlli
287
AGING OF Pt/Al,O,
-
THERMOCOUPL
.E LEADS LEADS
MASS
BPECTROMETER VlEWPORl
SAMPLE
LEAK (ON
SUSPENDED
VALVES OTHER
FOR GAS
ON WiRE
INLET
PORT)
CRYOTRAP DlFf UB10N
PUMP
TURBOMOLECULAR
PUMP
B
THERMOCOUPLE
JUNCTION \
I -
COATINQ
OF CATALYST
Aging Treatments
Catalyst samples were aged in a reducing environment using the following procedure.
LOOP
PARTICLES
FIG. 1. Vacuum apparatus for temperature-programmed matic of apparatus; (B) detail of sample support.
obtained by resistively heating the sample support wire at a linear rate of 5”U.s or less using a temperature programmer. The partial pressures of desorbing gases were measured by the mass spectrometer during the heating program and recorded as a function of time and temperature. The measurement of up to eight masses in thermal desorption was obtained by interfacing the mass spectrometer with a PDP 1l/O3 computer.
MO WIRE
desorption (TPD) experiments. (A) Sche-
A “fresh” sample installed in the preparation chamber was heated at 500°C under vacuum for 1 h, then at 450°C in the presence of 1 Torr of 5% 0JN2 for 1 h, and then in the presence of 1 Torr of 5% Hz/N2 for 300 s. A D2 TPD spectrum of the “clean” fresh catalyst was then obtained and repeated to check for reproducibility. After TPD, the sample was cooled to - 150°C and exposed to 1 Torr of 5% Hz/N2. The catalyst temperature was rapidly increased to a preselected aging temperature and held at that temperature for 1 min (or longer), followed by rapidly decreasing the sample
288
BECK
AND
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Exposure
lime
(8)
G -100
0
200
100
300
Temperature
400
600
600
(‘Cl
FIG. 2. TPD of D, adsorption at - 150°C on 1% Pt/A1203 for varying dose times at 1 x lo-) Torr. (A) 1000 s; (B) 260 s; (C) 80 S; (D) 50 s; (E) 40 s; (F) 20 s; (G) 10 s. Inset: Area of the /3 TPD peak as a function of dose time at i X 1O-3 Torr.
temperature to - 150°C and removing the H2 by flash heating (heat at 5OWs to 500°C and then immediately cool to - 15O’C). The D2 TPD of the “aged” catalyst was then obtained. Aging treatments followed by DZ TPD were repeated until an accumulated aging time of about 4 h was obtained. Catalysts aged in an oxidizing environment were cleaned by heating to 450°C in the presence of 02 and then Hz according to the procedure described above. A D2 TPD spectrum of the clean catalyst was obtained, and then the sample was exposed to 1 Ton- of 5% 02/N2 at - 150°C. The catalyst temperature was rapidly increased to a preselected temperature for 1 min or longer
and then rapidly decreased to - 150°C. The sample was then exposed to 1 Torr of 5% Hz/N2 for 300 s at 450°C in order to remove oxygen chemisorbed on the metal surface. Two D2 TPD spectra of the aged catalyst were obtained consecutively to check for reproducibility, and aging treatments followed by two D2 TPD measurements were repeated until an accumulated aging time of about 4 h was obtained. RESULTS
AND DISCUSSION
TPD Experiments
About 1 mg of 1% Pt supported on alumina was coated on a MO support wire using the slurry-drying technique described
THERMAL
previously. The sample was outgassed at 500°C and cleaned at 450°C by exposure to O2 and HZ. TPD experiments were performed in which the exposure of the sample at - 150°C to adsorbate gas (Dz) varied from 10 to 1000 s at 1 x 10m3 Torr. The resulting TPD spectra, Fig. 2, display two peaks. The low-temperature peak (cw)has a peak temperature (T,) equal to -125°C and its intensity reaches a maximum as a function of increasing dose time. The high-temperature peak (/3) is broad and its Tp decreases as a function of increasing dose time. A maximum in the /3 peak intensity occurs as
0
289
AGING OF Pt/AI,O,
100
a function of dose time (inset, Fig. 2) and a limiting value of Tp = 75°C is eventually observed. In subsequent experiments involving Pt/AI,O, , adsorbate doses resulting in a saturation coverage of the /3 state were used (i.e., 1 x 1O-3 Torr for 100 s). A TPD experiment using a DZ dose of 100 s at 1 X 1O-3 Torr was also carried out using about 1 mg of the blank alumina support. A single peak was observed at Tp = - 125°C in Fig. 3C which corresponds to the (Y peak observed using a 1% Pt/A1203 sample (Fig. 3A). A l-cm* Pt polycrystalline foil sample, previously cleaned in the vacuum chamber
200
Temperature
300
400
500
f%)
FIG. 3. TPD of D2 (exposure: - 150°C for 100 s and 1 X IO-? Ton) from various surfaces: (A) 1 mg of 1% Pt/AlZ03; (B) l-cm2 Pt polycrystalline foil; (C) 1 mg A&O,; (D) sample support wire alone,
BECK
AND
CARR
peak indicates that deuterium absorbs dissociatively on the Pt surfaces. The relative Pt surface area was calculated from the area of the p peak assuming a constant chemisorption stoichiometry of D/ Rutface = 1. While this assumption may not be valid for the surfaces of very small Pt clusters or for some crystal plane boundaries on the surface of larger particles, it represents a useful approximation to the actual mean chemisorption stoichiometry for the total Pt surface in the samples studied here. The relative dispersion was then related to the area of the p peak as AT-p-peak, Areap-peA, -100
0
100 Tompereture
200
300
400
600
PC)
FIG. 4. TPD of Dz (exposure: - 150°C for 100 s at 1 X 1O-3 Torr) from 1% Pt/A1203 after various aging treatments: (A) fresh catalyst; (B) aged in 1 Torr of 5% 021 N2 for 1 h at 700°C: (C) same as that in (B) except aged for 2 h; (D) same as that in (B) except aged for 3 h; (E) same as that in (B) except aged for 4 h.
unknown known
=
DiSperSionunknown
Dispersionknown ’
(2)
The dispersion of the “cleaned” fresh Pt/ A&O, was measured by a conventional chemisorption technique and found to be 0.67 (average of three measurements). The area of the p peak for the “cleaned” fresh Ptl AllO3 was therefore taken to be representative of this dispersion. Aging in Oxygen
using the procedure described by Norton and Richards (21), was given a similar D2 dose. The resulting TPD spectrum, Fig. 3B, consists mainly of a broad signal which is somewhat resolved into three peaks, in agreement with the literature (22-24). Although this broad feature occurs at a lower temperature than that of the j3 peak observed in Fig. 3A, its width suggests a closer correspondence to the p peak than to the (Y peak. An identical TPD experiment conducted in the absence of any sample (i.e., using the bare support wire) resulted in a spectrum with a small peak at low temperature, Fig. 3D. The results of these TPD experiments suggest that the (Y peak which is observed with Al2O3 and 1% Pt/Al,O, and is first order can be assigned to the weak adsorption of deuterium on the A&O3 support and the p peak can be assigned to deuterium adsorbed on supported Pt metal surfaces. The second-order behavior of the /3
The effect of thermal aging on the Pt/ A1203 dispersion in an oxidizing environment is shown in Fig. 4. Figure 4A shows the D2 TPD of the cleaned fresh 1% Pt/ Alz03 catalyst after a Dz dose of 1 x 10e3 Torr at -150°C for 100 s. The sample was then aged in 1 Torr of 5% 02/N2 for 1 h at 700°C evacuated at - 15O”C, and cleaned by exposure to 1 Torr 5% HZ/N2 at 450°C for 300 s. TPD after a deuterium dose of 1 X 10m3 Tot-r at -150°C for 100 s is shown in Fig. 4B. The area of the /3 peak is smaller than that in Fig. 4A, indicating a decrease in the dispersion of the supported Pt. Also, upon aging in oxygen, the /3 peak splits into two peaks. The sample was then aged in 1 torr of 5% 02/Nz for another l-h period at 7OO”C, evacuated at -150°C and cleaned by exposure to 1 Torr 5% HZ/N2 at 450°C for 300 s. The D2 TPD, using the same dose as that described earlier, is shown in Fig. 4C. The overall area of the p peak is smaller than that in Fig. 4B. After a third and fourth
THERMAL
AGING
FIG. 5. Metal dispersion versus aging time at varying temperatures in 1 Torr 5% 02/N2: TPD measurements were obtained at l-h intervals. Solid points: the aging experiments at 600 and 800°C were repeated beginning with a “fresh” sample in both cases.
hour of aging in 1 Torr 5% 02/N2 at 7Oo”C, the area of the /3 peak decreased further (Figs. 4D and 4E, respectively). Splitting of the p peak was also observed when the catalyst was aged at 800 and 900°C; the splitting of the /3 Dz TPD peak is discussed later in this report. The aging experiments using 1% Pt/Al,O, were repeated for other treatment temperatures (600, 800, 900°C). Again, aging treatments of 1 h duration were used until an accumulated aging time of 4 h was obtained. The Pt dispersion as a function of aging time for several aging temperatures is shown in Fig. 5. The experiments were shown to be reproducible by duplicating the aging experiments at 600 and 800°C (solid points, Fig. 5) with fresh samples. These data indicate that for the same aging time, the dispersion decreases with increasing aging treatment temperature, which is consistent with studies reported in the literature (5, 25). Note that for aging at 700, 800, and 9Oo”C, most of the decrease in the dispersion occurs within the first hour of thermal aging. Upon aging at 6OO”C, the decrease in the chemisorption capacity occurred primarily within the first 2 h of treatment.
OF Pt/Al,O,
291
Better definition of the behavior of the Pt dispersion, particularly within the first hour of aging, requires dispersion measurements at shorter aging time intervals, preferably on the order of several minutes or less. The need for better time resolution in the dispersion versus aging time data led to the following series of experiments. A 1% Pt/Al,03 sample was exposed to thermal aging treatments (in 5% OJN2) of 1 min in duration until an accumulated aging time of 10 min was obtained; thereafter treatments somewhat longer than 1 min were used. Between aging treatments, the metal dispersion was obtained from DZ TPD. The results for aging at 600, 700, 800, and 900°C are shown in Fig. 6. In the case of aging at 700, 800, and 9OO”C, the chemisorption capacity decreased rapidly within thefirstfew minutes of thermal aging, and afterward decreased more gradually. The data obtained using l-h aging treatments (from Fig. 5) are superimposed (solid points) on the data using aging treatments of shorter duration in Fig. 6 and show good agreement with the latter. This agreement suggests that the decrease in the dispersion of the supported Pt is not notably affected
FIG. 6. Metal dispersion versus aging time at varying temperatures in 1 Torr 5% OZ/N2. Open points: TPD measurements obtained at intervals
292
BECK AND CARR
FIG. 7. Metal dispersion versus aging time in 1 Torr 5% 02/N* at 600°C for a total of 2 h, and then at 900°C for a total of 2 h. TPD measurements obtained at intervals Cl0 min.
by the frequency of the rapid heating and cooling of the sample before and after aging treatments but instead depends on the accumulated aging time at a particular aging “Fresh” and aged samples temperature. were analyzed by powder X-ray diffraction to investigate changes in the support phase, but no changes were observed, even upon heating at 900°C for 4 h. Therefore, the decrease in the dispersion was not associated with any bulk transformation of the A1203 phase. When the 1% Pt/Al,O, catalyst was thermally aged at 600°C in 5% 02/N* for an accumulated time of 2 h, and then aged at 900°C for an additional 2 h, the result shown in Fig. 7 was obtained. Dispersion was measured by TPD at -(IO-min intervals. In this experiment, the final dispersion is about equal to the dispersion of a “fresh” catalyst which has been thermally aged at 900°C for 2 h. This result shows that aging treatments at relatively low temperature (i.e., 600°C) cannot be used to prevent the dramatic metal sintering which occurs when the catalyst is later aged at higher temperature (i.e., 900°C). Likewise, this result implies that significant thermal damage to the catalyst will occur whenever the catalyst is treated at high temperatures.
Some researchers have claimed that exposure of supported Group VIII transition metals to oxygen at elevated temperature can result in redispersion of the metal on the basis of chemisorption (4) and TEM and XRD studies (26-28). Several redispersion mechanisms have been advanced which require conversion of the noble metal to an oxidized form. This conversion leads to fracture of supported particles or to migration of noble metal oxide molecules away from large crystallites (18, 27, 28). However, redispersion was not evident in the present study. The observation in other studies of an initial increase in the chemisorption capacity as a function of aging (4) can be attributed to incomplete cleaning of the “fresh” catalyst material. A supported metal catalyst which has been stored in air may have metal surfaces which are partially occluded by adsorbed species or their decomposition products. For this reason our “fresh” catalysts were pretreated before any aging experiments were performed, by oxidizing at 450°C for 1 h in order to remove these species, and then were reduced at 450°C to remove residual chemisorbed oxygen. This pretreatment schedule was necessary to obtain reproducible Dz TPD spectra. Aging in Hydrogen
The chemical environment plays an important role in the sintering of supported metal catalysts, and thus catalysts tend to sinter differently in a reducing environment than in an oxidizing environment. The sintering of Pt/Al,O, in a reducing environment was examined by performing the following experiments. A deuterium TPD spectrum was obtained at l-h intervals as a function of aging time in 5% H2/Nz. Typical TPD spectra are shown in Figs. 8A-8E for aging at 700°C. The j3 DZ TPD peak decreases in area as a function of aging time and splits into two peaks. Plots of the metal dispersion (measured from the area of the p peak) versus aging time at l-h intervals are shown in Fig. 9 (solid points)
THERMAL
-100
0
100
Temperature
200
300 PC,
400
AGING
500
FIG. 8. TPD of D2 (exposure: - 150°C for 100 s at 1 x 10m3Torr) from 1% Pt/Al,O, after varying aging treatments: (A) fresh catalyst; (B) aged in 1 Torr of 5% Hz/ N2 for 1 h at 700°C; (C) same as that in (B) except aged for 1 h; (D) same as that in (B) except aged for 3 h; (E) same as that in (B) except aged for 4 h.
for the four aging temperatures. The rate of sintering is most rapid within the first hour of thermal treatment, as is the case for aging in 5% 02/N2. In order to better characterize the sintering kinetics in 5% HZ/N2 particularly within the first hour of thermal aging, the noble metal dispersion was measured after thermal treatments 1 min in length until a total of 10 min of aging was obtained. Thereafter, dispersion was measured after longer thermal treatments. These data for aging at 600, 700, 800, and 900°C are shown in Fig. 9 (open points). When a fresh catalyst was aged for 2 h at 600°C in 5% H2/N2, and then for 2 h at 9Oo”C, the resulting dispersion (not shown) was about equal to that of a fresh catalyst aged for 2 h at 900°C. The observations which can be made from these experiments are mostly similar to the ones noted in the case of aging in 5% 02/N*. First and most importantly, rapid
OF Pt/Al,O,
293
changes in the dispersion occur within the first few minutes of treatment for aging at 700, 800, and 900°C. Second, the change in the metal dispersion as a function of aging in 5% Hz/N2 is independent of how often the dispersion measurements were obtained and only depends on the accumulated aging time at a particular aging temperature. Third, the change in metal dispersion as a function of aging time at 600°C does not exhibit as rapid a decrease as that observed for aging at higher temperatures. Fourth, dramatic thermal damage occurs regardless of when the catalyst is treated at relatively high temperatures (such as at 9OoOC). Fifth, the decrease in the metal dispersion with aging treatment at all temperatures was not reversible. Sixth, splitting of the p TPD peak occurred when the catalyst was aged at 700, 800, and 900°C. The results of these experiments differed from the results obtained for aging in 5% OJN2 in one regard. For all comparable aging conditions, the initial rapid decrease in the dispersion in a H2 aging environment was smaller than that observed in an oxygen aging environment. The change in the shape of the 0 D2 TPD peak as a function of thermal aging time can
FIG. 9. Metal dispersion versus aging time in 1 Torr 5% Hz/N2 at varying temperatures. Open points: TPD measurements taken at intervals (1 h. Solid points: TPD measurements taken at l-h intervals.
294
BECKANDCARR
be attributed to the faceting of Pt particles as they grow. In this study, the metal dispersion of the 1% Pt/Al,O, catalyst aged at 900°C decreases from 0.67 (for the fresh catalyst) to co.10 (after aging for 4 h). The mean particle diameter increases from 2 to 15 nm during this 4-h aging schedule, assuming a Pt atom two-dimensional packing density of 1.5 x lOi atoms/cm2 (an average of the low-index surface planes (29) and a hemispherical particle morphology. The change in the estimated particle diameter with thermal aging is similar in range to that reported by Drechsler (30), who showed that an increase in metal particle size in the range from 1 to 10 nm results in the appearance of different types of crystal faces. Also, faceting of Pt supported on alumina films has been observed by electron microscopy when they were thermally annealed (10). The formation of well-defined crystal surfaces on the particles as they grow can lead to multiple peaks in D2 TPD. For example, two resolved Hz TPD peaks have been observed for the (1 lo), (loo), and (211) crystal planes of Pt (29). This study clearly shows that the sintering rate for supported Pt particles is highest during the first few minutes of aging and represents the first kinetic sintering data ever published with a relatively high degree of time resolution. Oxygen appears to be more effective than Hz as a sintering agent. Our results are consistent with those of other researchers who have reported that supported Pt particles either grow larger (32, 32) or sinter at a higher rate (6) when aged in O2 rather than in H2 or N2. Chen and Schmidt (31) have suggested that particle growth is enhanced in the presence of O2 at elevated temperature due to the mobility of PtO, either in the vapor phase or on the support. Other workers have reported experimental results that do not completely agree with these findings. For instance, Hassan and co-workers (5) reported that the surface area of Pt/Al,O, increased when exposed to H2 above 400°C. Soma and coworkers (33) observed that Hz chemisorp-
tion, suppressed after treatment in H2 at 500°C can be recovered by oxidation in O2 at 450°C followed by reduction in H2 at 300°C. The reason for the disagreement in published data remains unclear, and further study of the catalyst preparation, pretreatment, and role of aging environment on catalyst sintering is needed to understand the nature of metal-support bonding. Comparison of Experimental Data with a Sintering Model
Two mechanisms have been proposed in the published literature to explain the sintering kinetics of supported metal particles (I, 2). Mathematical models have been developed on the basis of each mechanism. Numerous publications describe attempts to fit experimental sintering data to a particular model. In one model, the sintering process involves the migration of particles on the support followed by particle fusion upon collision. In this model, the rate of change in the metal dispersion at constant temperature is given by a power law expression of the form given in Eq. (1) (13, 19). An alternate form of this power law can be expressed as (34) (n) log(DOID,) = log t + C,
(3)
where DOis the metal dispersion of the unaged catalyst, D, is the metal dispersion of a catalyst aged at time t, C is a constant, and n is defined as the sintering order. According to this model, sintering data plotted in the form log DO/D, vs log t are linear and provide a value of n. The physical significance of the sintering order has been the subject of debate (35). However, the sintering order may be helpful in identifying the predominant sintering mechanism in light of certain recent studies (discussed later) (9, 36, 37).
In the second proposed model, sintering involves primarily three steps: (i) loss of metal atoms from a crystallite, (ii) migration of metal atoms on the support, and (iii) capture of metal atoms by larger crystallites (17). Based on surface free energy argu-
THERMAL
r
AGING
I
B I 900c
-1.5
-1.0
-0.6
0 Lop
(Aping
0.6
1.0
Time)
FIG. 10. Plot of log D,lD, versus log t for varying aging temperatures in I Torr 5% Oz/N2 _ For each case, the change in slope is identified with a change in the dominant sintering mechanism (see text).
ments, smaller crystallites will lose metal atoms to larger ones at a particular temperature. The sintering rate in this model is based on the Kelvin equation and can be expressed as (16, 17) -dDldt
= kD” exp(mD),
our experimental data were fitted to the power law (Eq. (3)) and qualitatively compared to other published studies involving the fitting of sintering data to that model. A plot of log Do/D, versus log t was prepared using the areas of the /3 peaks in D2 spectra for varying aging times up to 10 h at 700, 800, and 900°C in 5% 02/N2 (Fig. 10) and 5% Hz/N2 (Fig. 11). In these cases, the data for a particular sintering temperature do not fit a single sintering order. Rather, the sintering order shifts from a high value to a low one, usually after 1 to 2 h of aging. The 600°C data (not shown) exhibit abnormally high sintering order values; thus the particle migration model may be inappropriate for the 600°C aging data. The change in sintering order as a function of sintering time and/or temperature has been observed in other studies. Harris et al. (9) measured the change in the particle size distribution of aged catalysts using transmission electron microscopy (TEM), and using the particle migration model they found sintering or-
(4)
where D is the metal dispersion, k is a rate constant, 12is a constant dependent on the mode of transport (n = 5 when transport involves two-dimensional diffusion of atoms on the support, it = 3 when transport is in the vapor phase), and m is a factor dependent on particle morphology, metalsupport and metal-vapor interfacial energies, metal-support contact angle, metal molar volume, and temperature. At constant temperature, m is approximately constant. Each model has limitations and may represent a simplified approach to the modeling of sintering behavior. Certainly, the best way to identify a dominant sintering mechanism involves direct observation of catalyst particle sintering using electron microscopy. However, some insight to the identification of the sintering mechanism can be gained by fitting experimental sintering data to one of the developed models or a modification of one of them. In this report,
295
OF Pt/Al,O,
I
I
1
0.8
I-t=4
P 0
/ 0.6
-L ;
0.4
3
0.2
0 -1.6
-1.0
-0.5 Loo
(Aglnp
0
0.5
time)
FIG. 11. Plot of log Do/D, versus log t for varying aging temperatures in I Torr 5% Hz/N,.
296
BECK AND CARR
ders n = 13 at sintering times of 2 h or less, and n = 6 at sintering times greater than 2 h. In a more recent study, Harris (36) confirmed using TEM that the sintering order changes to a lower value with increasing sintering time. In both cases, the microscopy indicated that the atomic migration mechanism dominated particle growth during the low-sintering-order period. The change in sintering order was attributed to a change in the dominant sintering mechanism. Kuo et al. (37) used X-ray diffraction to measure metal particle size as a function of aging and came to a similar conclusion. Similar changes in sintering order with aging time and temperature have been observed in the study of the aging of Ni/A1203 (21, 14). In those studies, the shift in sintering order from high to low values was attributed to a change in the dominant sintering mechanism. Because our observations are generally in agreement with all of these cited studies, we believe that the change in sintering order indicated in Figs. 10 and 11 coincides with a change in the dominant sintering mechanism. We also suggest that particle migration and coalescence dominate during the first hour of aging, whereas atomic migration dominates thereafter. The change in the slope of the 02 aging data generally occurs in the same time range as that of the H2 aging data, indicating that the aging environment does not significantly influence when the mechanism changes. The break in the sintering order does occur sooner during aging at 900°C than at 800 or 700°C. The cause for the change in the dominant sintering mechanism has been attributed by Kuo ef al. to the influence of the support pore structure (37). The change in the sintering mechanism has also been attributed to collapse of the support (24). Assuming that the mean metal particle size can be estimated from the metal dispersion, our data imply that the pore structure of the support probably does not influence when the sintering mechanism changes, because the breakpoints in sintering order at 700, 800, and
900°C (Figs. 10 and 11) occur at different values in the metal dispersion. We have observed that the BET surface area of the sample decreased after aging at 900°C in either 5% Hz/N2 or 5% Oz/N2 for 1 h. Therefore, support collapse may play a significant role in the mechanism at higher aging temperatures, particularly given that the change occurs sooner during aging at 900°C. In the case of aging at 800 and 900°C the sintering orders in the oxygen environment were generally lower than those in hydrogen, This trend has also been observed by other researchers (II, 14), who suggested that the atomic transport mechanism is more dominant in an oxidizing aging environment. Consistent with this observation is the suggestion that particle growth in O2 at relatively high temperatures involves some conversion of Pt to PtO, which is mobile either in the vapor phase or on the support. However, because our study did not involve direct observation of metal particle growth and morphology changes (for instance, with TEM), the cause for the abrupt change in the sintering rate with aging time remains unclear. The application of the particle migration model to experimental sintering data is useful for obtaining qualitative information about the mechanism(s) active in sintering, particularly since researchers have correlated direct observation by TEM of particle behavior as a function of sintering time and temperature with one of the sintering models. However, we have noted the disagreement in dispersion versus aging time data reported by some investigators. Thus the information about sintering kinetics obtained using either model may be questionable. In order to better understand sintering mechanisms and kinetics, further study is needed, particularly by direct observation of supported metal particle sintering with electron microscopy. SUMMARY
The sintering of 1% Pt/A1203 has been investigated using temperature-pro-
THERMAL
AGING
grammed desorption of DZ in an ultrahigh vacuum apparatus. When the catalyst was thermally aged in 5% OJNZ or 5% Hz/N2 at temperatures of 700°C or higher, the exposed metal surface area decreased rapidly during the first few minutes of treatment and then more gradually afterward. The same trend in the sintering rate change as a function of time was observed for aging in H2/N2, although the initial rate of sintering was lower in the reducing environment. The overall decrease in the metal surface area was dependent on the aging temperature for both aging environments. Sintering of the supported metal was accompanied by a shape change in the D2 TPD peak, which is attributed to the faceting of Pt particles as they grow. Redispersion of Pt/Al,O, in O2 was not observed in these experiments. For sintering temperatures of 700, 800, and 9Oo”C, the order of sintering abruptly decreased as a function of aging time indicating a change in the dominant sintering mechanism. ACKNOWLEDGMENTS The authors thank Mike D’Aniello for the chemisorption results and for helpful comments and suggestions, and Jack Johnson for performing the X-ray diffraction analysis.
I. 2. 3. 4. 5. 6.
7. 8.
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