Langmuir 1993, 9, 984-992
984
Microcalorimetric Study of the Absorption of Hydrogen by Palladium Powders and Carbon-Supported Palladium Particles R. W. Wunder, J. W. Cobes, and J. Phillips' Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802
L. R. Radovic Department of Material Science, The Pennsylvania State University, University Park, Pennsylvania 16802
A. J. Lopez Peinado DPTO, QCA, Inorganica y Tecnica, F. Ciencias, U.N.E.D.,28040 Madrid, Spain
F. Carrasco-Marin DPTO, QCA, Inorganica, F. Ciencias, Universidad de Cranada, 18071 Granada, Spain Received June 24,1992. In Final Form: January 12,1993
The adsorption and absorption of hydrogen on and in various forms of supported and unsupported palladiumwere studied using heat-flow calorimetry. The supportswere found to have a tremendouseffect on the nature of the hydrogen-palladium system. For palladiumsupportedon silica or alumina,irreversible chemisorption was found to take place and be followed by bulk hydride formation. In contrast, it was found for carbon-supportedparticlea that chemisorptiondoes not take place and that the observedbehavior could be explained by a model using only bulk hydride formation. It was further found that the heat of formation and equilibrium for &hydrideformation are functionsof particle size. It was not clear if surface adsorption takes place on unsupported palladium. Introduction The interaction of palladium with hydrogen is of fundamental interest to two different communities. First, it is a classic field in condensed matter physics because of the unusual phase behavior of hydrides. Palladium hydride is the most thoroughly studied of all hydrides.',* Moreover, hydrides, including palladium hydrides, are of interest as hydrogen storagematerials. Second,the nature of palladium hydride, and how it changes with particle size, has been of interest to the catalysis community for many years. There are two major motivations for this interest. First, for a number of isomerization/hydrogenation reactions palladium has better selectivity and/or activity than any other single metalg5 More information regardingthe nature of the palladiumjhydrogeninteraction could lead to a better understanding of the unusual catalytic properties of palladium. Second, the activity and selectivityof palladium for variousreactions are clearly a function of particle size.6 In this study a true differential calorimeter was used to measure the heat of absorption of hydrogen at 300 K on various carbon, silica, and alumina-supported palladium particlesand on unsupported palladium black and powder.
* To whom correspondence should be addressed.
(1) Lewis, F. A. The Palladium Hydrogen System; Academic Press, London, 1967. (2) Mueller, W. M.; Blackledge, J. P.;Libowitz, G. G. Metal Hydrides; Academic Press: New York, 1968. (3) Bond, G. C.; Well, P. B. Ado. Catal. 1963, 15, 91. (4) Boutonnet, M.; Kizling, J.; Mintsa-Eya,V.; Choplin, A,; Touroude, R.; Maird, G.; Stenius, P. J . Catal. 1987, 103, 95. (5) Massardier, J.; Bertolini, J. C.; Renouprez, A. Int. Cong. Catal. 1988,1222. ( 6 ) Boitiax, J. P., Cmyns, J.; Vaeudevan, S. Appl. Catal. 1983,6, 41.
0743-7463/93/2409-0984$04.00/0
A number of new findings resulted when the values of the differential (function of amount adsorbed, pressure, and/ or H/Pd ratio) heat of hydrogen absorption were compared for forms of palladium investigated. The results were surprising. There is a distinct difference between absorption on carbon-supported palladium and adsorption on silica or alumina-supported samples. On all carbonsupported samples the heat of absorption initiallydeclines, then increases. Moreover, it appears that chemisorption does not take place on these particles. In contrast, when the palladium is supported on alumina or silica, the heat of adsorption simplydeclineswith increasingcoverageand irreversible adsorption, of the type generally associated with chemisorption, takes place. A detailed study of absorption on the carbon-supported particles suggests that the pressure at which the 8-hydride begins to form is a function of particle size. This result, it will be shown, follows not only from the absorption heat data but also from characteristic isotherms combined with a clear understanding of the Gibbs phase rule regarding degrees of freedom in two-component systems. It was also found that the range of pressures over which two phases coexist decreases with decreasing particle size.
Experimental Section Apparatus. The experiments were carried out in a differential calorimeter which permits simultaneous gathering of heat and kinetic data. The system, described in detail elsewhere,%ontains three main parts, all held at the same temperature during adsorption studies: a gas dosing volume, a sample cell, and a sample preparation reactor. (7) Gatte R. R.; Phillips, J. Langmuir 1989,5, 758. (8)Gatte,R. R. Ph.D.Thesis,The PennsylvaniaState University,1988.
Q 1993 American
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Absorption of Hydrogen by Palladium The gas dosing volume consists of two Baratron pressure heads and an 'extra" dead space, isolated from the sample chamber by a metal valve. The rate of absorption can be determined, with virtually no instrument transfer function, by monitoring the pressure with the Baratron gauges. In previous studies of adsorption on supported metal particles, the Baratron heads alone composed the dead volume. The heats released during hydrogen absorption in palladium are relatively small and the amounts of gas absorbed comparatively large; thus, for this study an extra dead volume of about 50 cm3 was attached with an O-ring joint. This allowed larger amounts of gas absorption with each dose, resulting in larger heat signals and, consequently, smaller errors. The sample cell is a simple cylindrical Pyrex vessel which is about 4 cm in diameter and 0.5 cm high. It is situated between two thermopiles such that the heat released by any process taking place within the sample cell travels through these thermopiles to large, metal, constant-temperature heat sinks. The sample preparation reactor is a quartz tube of small bore attached directly to the sample cell. This tube rests inside a tube furnace such that pretreatments can be performed at high temperature prior to transferring the sample to the cell. Sample transfer is done by tilting the entire instrument, which causes the sample to transfer by gravity. There are errors of precision involved in measurements of both the heat and the amount adsorbed. Given the deviation in readings on the Baratron gauges, it is clear that the measurement of the amount adsorbed has a potential variation of 5%, for the smallest pressure differences measured. In the experiments reported here, the smallest heat measured was 10 mcal, although in general the heats measured were at least 2 or 3 times larger (as much as 20 times larger during hydride formation). Since the error involved in the measurement of heat is about 1 mcal, the maximum error in the heat measurements was about 10%. There are also absolute errors, which are difficult to quantify. For this particular study these errors are considered to be of less importance than the errors of precision, because this study focuses on comparison between different samples studied using the same instrument. However, studies with electrical heat probes suggest that the absolute values of the heats are within 5 % of the true values in all cases. The absolute errors in the amounts adsorbed are also considered to be small. Placing different "known volumes" on the O-ring joint allowed volume measurements to be made. The volume measurements made showed very good agreement with the known values. Thus, errors in the absolute value of the amount adsorbed are considered to be less than 5 % in all cases. The various errors compound such that a conservative estimate is that the heats of adsorption are correct to within 20%. Samples. Palladium black samples were obtained from Strem Chemicals. The palladium black has a reported surface area of 23 mz/g, which corresponds to a mean (initial) particle diameter of 22 nm. As discussed below, palladium powders and blacks sinter at very low temperatures. Given that these were treated at 650 K prior to adsorption studies, it can be safely assumed that the particles were much larger than the intially reported values. The supplier claims typical impurity of 200-300 ppm platinum with other metallic impurities being undetectable. Two different palladium powders were used. The first was obtained from Aldrich Chemical and is listed as 99.9+% pure with a mean diameter of 1.0-1.5 mm. This particle size corresponds to a surface area of 0.3-0.2 m2/g. The second palladium powder sample was Johnson Matthey Grade 1listed as 99.999% pure. It was previously tested in another lab using argon BET and was found to have a surface area of 0.837 m2/g. The carbon-supported palladium samples were prepared by incipient wetness impregnation using an aqueous solution of Pd(NHs)4(NO&. The pH of the solution was adjusted to either 5.67 or 9.02 to vary the degree of electrostatic interaction between the cationic catalyst precursor and the carboxyl groups on the nitric-acid-treated carbon black (Monarch 700, C a b ~ t )A. ~similar procedure was used to prepare the samples supported on Grafoil (Union Carbide), a high surface area (22 m2/g)graphitic carbon, (9)Solar, J. M.; Leon y Leon, C. A.; Oaseo-Asare, K.; Radovic, L. R. Carbon 1980,28,369.
Table I sample Pd black Pd powder (99.999%) Pd powder (99.9+ %) Pd/Grafoil Pd/Monarch 700(5.67) Pd/Monarch 700(9.02) Pd/silica Pd/alumina
final helium hydrogen vacuum fip;ure(s) treatment treatment treatment 1h at 300 'C 1 h at 575 K 1 h at 575 K, cool to ambient 1h at 300 O C 1h at 575 K 1h at 575 K, cool to ambient none 4 h a t 650 K 4 hat 650K, cool to ambient 4 h a t 675 K 4 h a t 675 K, none cool to ambient none 4 h at 675 K 4 h a t 675 K, cool to ambient 4 h a t 675 K 4 h a t 675 K, none cool to ambient lhat300'C 1hat575K lhat575K, cool to ambient lhat300'C 1hat575K l h a t 5 7 5 K , cool to ambient
the surface of which consists primarily of basal planes.lOJ1The carbon supported samples had loadings of 2.3% (Grafoil), 3.1 76 (Monarch 700(5.67)),and 2.0% (Monarch 700(9.02)). The silica-supported palladium sample was prepared by another researcher using ion exchange between Pd(NH3)4(N03)z and SiOz. The catalyst had a loading of 0.481 7% Pd by weight. Details on preparation can be found e l s e ~ h e r e . ~ ~ J 3 The alumina-supported palladium sample was a highly dispersed commercialcatalyst of 5.0% palladium by weight obtained from Aldrich Chemical. Procedure. Prior to placement in the sample chamber of the calorimeter, all samples were reduced by hydrogen in the sample preparation reactor of the calorimeter, then treated in vacuum (5 X 10-5Torr) at the same temperature before being cooled to the experimental temperature and transferred to the sample cell. Because of the balanced concerns of sintering the palladium and cleaning the surface, several different preparation temperatures were used. No difference in the quality of the samples was observed due to changes in treatment. All samples were treated at 575 K or higher. When the study was started, supported palladium samples were treated at 675 K and unsupported sampleswere treated at 650 K. Later in the study, it was suggested by other researchers that treatment involving 1 h a t 395 K under flowing helium followed by 1 h at 575 K under hydrogen and 1 h under vacuum at 575 K would be sufficient to remove all surface adsorbed species. This was found to be the case, and later treatments used this more efficient method. This method also permitted direct comparison with studies of hydrogen adsorption on silica- and alumina-supported samples studied in other labs. The treatments of the various samples are summarized in Table I.
Rssulte Unsupported Palladium. The differential heats of adsorption of hydrogen on palladium black are shown in Figure la, and the differential heats of adsorption of hydrogen on palladium powder are shown in Figure 2a. From these data it is clear that on large, bulklike particles the heat of hydrogen absorption initially decreases and then steadily increases, before reaching a plateau. The results obtained in this study contain far less scatter than those presented in earlierreporta;14additionally,the results do agree very closely with those obtained by Artamonov et
d.15
(10) Phillips, J.; Clausen, B.; Dumesic, J. A. J. Phys. Chem. 1980,84, 1814.
Vickers, (11)Bretz, M.; Dash, J. G.; Hickernell, D. C.; McLean, E.0.; 0. E.Phys. Rev. 1973,A8, 1589. (12) Chou, P.; Vannice, M. A. J. Catal. 1987,104,1. (13)Chou, P. Ph.D. Thesis, The Pennsylvania State University, 1986. (14) Lynch, J. F.; Fanagan, T. B. J. Chem. SOC. Faraday Tram 1 1974, 70, 814. (15) Artamatov, S.V.;Zakumbaeva, G. D.;Soklo'skii, D. V. Dolk. Akad. Nauk SSSR 1979,244,123.
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Langmuir, Vol. 9, No. 4, 1993 25
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Figure 1. Absorption of hydrogen in palladium black. (a) Differential heat of absorption. The heat of absorption is initially high, decreases, and then rises again to a steady value as the p-hydride is formed. (b) Isotherm from hydrogen absorption.
In earlier work related to this study, a more detailed insight into the absorption process at very low pressures and H/Pd ratios was desired. A similar experiment was conducted using a larger sample. This allowed more data to be collected over a narrower range of H/Pd ratios and with less scatter as shown in Figure 3. This experiment used 99.99% pure palladium powder supplied by Aldrich Chemicals, with an initial particle size of 1.0-1.5 pm. The data collection was stopped after 40 data points (24 h), well before saturation was reached, as the prinicpal objective was simply to provide a detailed look at absorption at very low H/Pd ratios. The initial "dip" in the heat of absorption can be seen very clearly in Figure 3a. There is also a very small "tail" of hydrogen uptake at almost zero pressure. Carbon-SupportedPalladium. The heats of absorption of hydrogen on Pd/Grafoil are shown in Figure 4a as a function of the H/Pd ratio. Data from two experiments are shown. Between the two experiments, the samples were outgassed at room temperature to a final pressure of 1 X Torr for 10 h. The second experiment gave virtually the same result as the first one. Once again, it is interesting to note that there is initially a decrease in the heat of absorption, followed by an increase. In this case absorption was carried out to the point at which heats once again began to decline. Finally, note that the H/Pd ratio at the point of lowest heat is far higher than it is on the palladium black. In Figure 4b the isotherm for this sample is plotted.
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Figure 2. Absorption of hydrogen into palladium powder. (a) Differential heat of absorption. The heat of absorption for the initial dose was not as high as for the palladium black, but the initial decrease in heat of absorption followed by an increase to the constant heat of hydride formation can be clearly seen here as well. (The last eight heat values were lost to experimental error.) (b) Isotherm from hydrogen absorption. The slight upward slope may be an indication of a distribution in the particle size. The lower equilibrium value (compared to Pd Black in Figure lb) is probably due to lower collection temperature.
There are a number of differences between the isotherm for the small supported particles and the palladium black data. First, the pressure never reaches a plateau in this experiment, though the slope clearly flattens in the pressure region in which hydride formation would be expected. In fact, the final pressure is never the same as the equilibrium pressure of a bulk system. Second, there is no adsorption at zero pressure. That is, the equilibrium vapor pressure rises linearly with increases in the H/Pd ratio (coverage),and the extrapolation of the H/Pd curve clearly passes essentially through zero at zero pressure. Adsorption of hydrogen on the Pd/Monarch 700(5.67) and the Pd/Monarch 700(9.03) samples revealed very similar heat behavior (Figures 5a and 6a). That is, the heat initially decreases, then rises sharply, before falling again. Again, the H/Pd ratio at which high heats are firat observed (approximately 0.1)is higher for these small particles than for the palladium powder (bulk) sample. Also, the value at which "saturation" appears to occur (0.31, as measured by the heats as well as by the uptakes, is less in this case than it is for the bulk sample. The isotherm data reveal that for both samples there is no stable pressure across the two-phase region just as with the Pd/Grafoil sample (Figures 5b and 6b).
Absorption of Hydrogen by Palladium
Langmuir, Vol. 9, No. 4, 1993 987
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Figure3. Absorption of hydrogen in palladium powder collected in the low pressure region. (a) Differential heat of absorption. The sample used exclusively at low H/Pd ratios is shown in comparison to the other unsupported palladium samples to demonstrate the general agreement between all three and the greatly reduced scatter in the low range sample. (b) Isotherm at low H/Pd ratios. It can be easily seen that the zero pressure uptakes are very small indicating little or no surface adsorption. The surface area of these particles is extremely small as well, making it difficult to distinguish surface adsorption, if it occurs at all, from low pressure absorption.
The particle size distributions of the various supported Pd samples are shown in Figure 7. Note that the dispersions for the Pd/Monarch 700(5.67) (Figure 7b) and the Pd/Monarch 700(9.03) (Figure 7c) samples are quite high, and that the particles on the Grafoil (Figure 7a) are relatively large. This is consistent with the finding that oxygen functionalgroups on the surface of the carbon black act as anchoring sites for the catalyst precursor, especially at high pH values, thus reducing the degree of palladium sintering.9J6 The particle sizes were determined after the experiments were conducted and therefore include the influence of sintering. In order to determine the influence of adsorption onto the support, a study of hydrogen adsorption was carried out on both the unloaded Grafoil and the Monarch carbons, which were pretreated at precisely the same conditions given as the supported palladium samples. It was found that virtually no hydrogen was adsorbed and negligible heat was evolved. Silica-SupportedPalladium. The differential heat of adsorption and equilibrium pressure data collected for (16) Carraeco-Marin, F.; Solar,J. M.; Radovic, L. R. International Carbon Conference, Paris,France (1990).
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Figure 4. Absorption of hydrogen in palladium supported on Grafoil. (a) Differential heat of absorption. The heat of absorption follows the general pattern of behavior for unsupported palladium. (b) Isotherm for hydrogen absorption. The flattening of the isotherm between H/Pd of 0.1 and 0.6 corresponds to the rise and fall in the heat of absorption seen in Figure 4a. This is indicative of hydride formation.
the Pd/SiOz sample are given in Figure 8. In this case, hydrogen adsorption was performed twice: once immediately after treatment and a second time after the sample sat under approximately 1 atm hydrogen pressure overnight and then was evacuatedfor 30 min. Both isotherms/ differential heat curves are qualitatively different from those obtained for both the unsupported and the carbonsupported samples. Three significant differences are immediately evident. First, there is considerable adsorption at zero pressure. Second, outgassing at room temperature does not restore the sample to its original condition. That is, following outgassing at room temperature, the heat of adsorption curve, as well as the isotherm, is not the same as following the initial reduction. The main difference being greatly reduced adsorption at zero pressure following outgassing. Third,the heat of adsorption decreasesmonotonically with coverage. There is no evidence of a minimum in the curve. There are three distinct zones that can be seen in both the heat and pressure charta. Initially, adsorption is very strong and exothermic,with the equilibrium pressure being essentially zero. After that, the absorption becomes less exothermic and there is a significant vapor pressure. Finally, adsorption, and the corresponding heat, ceases to occur as the pressure is increased. When the sample is exposed to vacuum, the initially adsorbed hydrogen
Wunder et al.
Langmuir, Vol. 9, No.4, 1993 14
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Figure 5. Absorption of' hydrogen in palladium supported on
Monarch 700 carbon prepared at a pH of 5.67. (a) Differential heat of adsorption. The shape of the curve is the same as for the Grafoil-supported sample, except that the hydride is apparently saturatedat a H/Pd ratio of about 0.3. (b) Isotherm for hydrogen absorption.
apparently does not come off, and the second curves are shifted to the left. Alumina-SupportedPalladium. Differential heat of adsorption and equilibrium pressure data collected for alumina-supported palladium are shown in Figure 9. Results are very similar to silica-supported palladium and not at all like any of the carbon-supported palladium. Clearly, the support has a major effect on the behavior of the palladium.
Discussion Gibbs Phase Rule. In order to fully understand the analysis of the data given below, it is critical to understand the Gibbs phase rule. Simplystated, any thermodynamic system at equilibrium has a limited number of degrees of freedom. In the case of a two-component pure system, if three phases are present, then only one thermodynamic variable can be changed. This rule is obeyed in the case of palladium black. Indeed, the heat of absorption shows a change in character when the pressure reaches approximately 18 Torr, or H/Pd ratio of 0.002. It begins to increase and then stabilizes when the H/Pd ratio reaches about 0.08. The behavior beyond H/Pd = 0.08 is identical to that of bulk samples. That is, the pressure is constant and the heat of adsorption is constant, until the H/Pd ratio is about 0.5. The fact that the heat output stabilizes when the pressure is just greater than 18 Torr is not surprising as
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Figure 6. Absorption of hydrogen in palladium supported on Monarch 700 carbon prepared at a pH of 9.02. (a) Differential heat of absorption. This sample behaved very similar to the Monarch 700(567)sampledespitebeing over 4 times as dispersed. This indicatesthat the influence of particlesize may have a lower limit. (b) Isotherm for hydrogen absorption.
at this pressure bulk samples begin to form the 8-hydride phase. In this situation, three phases (gaseous hydrogen, cy-hydride, and &hydride) are in equilibrium in a twocomponent (hydrogen and palladium) system. Therefore, according to the Gibbs phase rule, there is one degree of freedom. Temperature is fixed, removing that one final degree of freedom,thus all other intensiveproperties (heats of formation, phase change, etc.) must be constant. In contrast, initially (H/Pd < 0.02 Torr) the heat of formation declines monotonically, which means that the thermodynamic properties are not all fixed. This is consistent with the gradual equilibrium formation of a second phase (the cy-hydride) in a two-phase region. The pressure increase and changing heat of adsorption at H/Pd = 0.5 are also not surprising. This is the behavior expected when the system crosses into another two-phase (&hydrideand hydrogen)region. It is interestingto note that experiments with small Pd particles which found no evidence of the formation of cy-hydride using EXAFS also observed no increase in hydrogen absorption over a narrow pressure range in their isotherm." The behavior of the absorption process into the supported particles is surprisingly different from that of the bulk material. For example,there is not adistinct pressure reached at which two phases of a single pure material are (17) Moraweck, B.; Cluguet, G.; Renouprez, A. J. Chim. Phys. 1986, 83.
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Figure 8. Adsorption on silica-supported palladium. (a) Differential heat of adsorption. The heat data shows three distinct zones of surface adsorption, bulk adsorption, and negligible physisorption. (b) Isotherm for hydrogen adsorption. The equilibrium pressure displays the three zones of uptake as well. The difference between the curves after saturation is considered a valid measurement of dispersion and, in this case, is nearly identical to the initial zero pressure uptake for the sample.
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Figure 7. Particle size distribution for the carbon-supported palladium particles used in this study: (a) Grafoil; (b) Monarch 700(567); (c)Monarch700(902). Particle size was measured using TEM and collected for a sample of over lo00 particles in each case. Dispersion was determined by adding up the total surface atoms calculated for all the particles in each size category and dividing it by the total atoms calculated in the same fashion.
in equilibrium. Thisobservationhas frequentlybeen made in previous work.12J8 Yet it has never been noted that this observation contradicts the Gibbs phase rule. Several explanations for the observation have been advanced. For example, it has been suggested that two phases coexist over a range of pressures. This is not a valid explanation as it requires too many degrees of freedom for the system. (18) Ahen,
P.C. J . Calal. 1968, IO, 224.
One possible explanation for the form of the isotherms, observed both in the present work and in previous studies, is that as the particles get smaller the pressure at which the two phases are in equilibrium becomes higher. Thus, in the event that a particle size distribution exists, experimentally one will observe a range of pressures over which two phases appear to coexist. It must be understood, however, that each single particle will have a specific pressure (not a range of pressures) at which the two phases coexist. Adsorption. In several previous s t ~ d i e sit~has ~ ~been *~ suggested that hydrogen adsorbs on palladium surfacesof all types concomitantly with the formation of a-hydride in the bulk at low pressures (ca. C1 Torr). The evidence collected in the present study suggests that (surface) adsorption at 300 K only takes place on the surface of palladium supported on silica or alumina, and not on the surface of carbon-supported palladium and possibly not on bulk palladium. It might appear that the suggestion that there is little or no surfaceadsorption on bulk samples contradicts too much data collected in previous work to merit serious consideration. This is disputed below. Single crystal palladium and small supported palladium particles have been studied by numerous researchers in (19) Sermon, P. A. J. Catal. 1972,24, 460. (20) Benson, J. E.; Hwang,H.S.;Boudart, M.J . Catal. 1973,30,146.
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Total Sorption, HiPd Atomic Ralio
Figure 9. Adsorption on alumina-supported palladium. (a) Differentialheat of adsorption. This sample exhibited behavior very similar to the silica-supported sample and very different from the carbon-supportedsamples. (b) Isotherm for hydrogen adsorption. the past regarding their ability to adsorb and absorb hydrogen. The methods employed are different, and the findings indicate different behavior as well. Studies of single crystal palladium using temperature programmed desorption (TPD) of samples exposed to hydrogen at various temperature typically produce spectra with three desorption peaks.21 The first to come off, usually referred to as the a-peak, is associated with "subsurface hydride" and desorbs between 150and 200 K. The second peak is associated with chemisorbed hydrogen and comes off between 300 and 400 K. The final peak is associated with bulk hydrogen and comes out at temperatures above 400 K. The relative sizes, or even existence, of the these peaks is strongly affected by the dosing conditions as observed by Gdowski et With a Pd(111) surface, the initial a-peak appeared when the exposure temperature was raised from 80 to 90K, reached a maximum at 115 K, and disappeared again when the exposure temperature was raised above 150 K. The "subsurface hydride" is not regarded by these researchers as simply a-hydride. They suggest that the hydrogen is trapped near the surface by the freezing of the sample before it can diffuse. This would make the a-peaka kinetic phenomena rather than a thermodynamicphase. Finally, they emphasize that above 200 K surface and bulk palladium are in equilibrium. (21) Guo, X.; Hoffman, A.; Yaks, J. T., Jr. Surf.Sci. 1988,203, L672. (22) Gdowski, G. E.; Felter, T. E.; Stulen, R. H. Surf. Sci. 1987,181, L147.
Other work on Pd(l11) surfaces at temperatures above -220 K saw no evidence of an c ~ - p e a k . ~Konvalinka h~~ and S c h ~ l t e did n ~ TPD ~ studies with palladium supportsd on activated carbon and found similar peaks which they also assigned to subsurface hydride and chemisorbed hydrogen. However, their conditions were muchdifferent. The samplewas exposed at 232 K to hydrogen in an argon carrier for repeated doees until the sampleuptake indicated saturation. The observed TPD peaks were at much higher temperatures, with maxima at 293 and 407 K. These temperatures are more in line with the TPD studies of Conrad et al.Z6 who dosed Pd(ll0) and Pd(ll1) surfaces at room temperature. In addition to their TPD work, Conrad et aLZ6found LEED evidence for ordered adsorbed atomic hydrogen. The LEED patterns disappeared when the sample was heated to 373K and re-formed when the samplewas cooled under vacuum. This was attributed to surface hydrogen leaving and re-forming from bulk hydrogen. Later work on the Pd(100) surface2' found that the LEED pattern disappeared at temperatures around 300 K. In all cases, they emphasized that surface and bulk hydrogen are in complete equilibrium. Recent ion scattering studies28 conducted at 300 K clearly indicate the presence of deuterium on the surface at a few of the possible surface sites. This study also indicated that there was no long range order for the surface deuterium at 300 K. It should be noted however, that hydrogen and deuteriumhave different heats of adsorption and surface diffusion m e c h a n i ~ m s ~and * 3 ~ are likely, therefore, to exhibit different adsorption behavior. In sum, studies of bulk palladium suggest that surface hydrogen desorbs between 300 and 400 K and the bulk hydrogen desorbs at significantly higher (>400 K) temperatures. In the catalysis community, the desorption of hydrogen from palladium is assumed to take place in a different order than that indicated by the above TPD and LEED work. A common technique for determining the surface area of supported palladium particles relies on the idea that chemisorption of hydrogen on the palladium surface is irreversible at room temperature while bulk hydrogen can be completely removed by simple exposure to high vacuum. In the "back-sorption" technique, the palladium sample is exposed to repeated doses of hydrogen until it is completely saturated, then the sample is exposed to vacuum at room temperature for a period of time typically between 20 and 60 min. At the end of the outgassing step, the sampleis again dosed with hyrogen until it is saturated. The isotherms generated by the two dosing procedures are parallel and separated by the amount of hydrogen that was not removed by the outgassing and corresponds to a 1:lsurface coverage of hydrogen atoms. This method has been used successfully for many years.12J3J8120Sermon'g modifiedthe method by preadsorbingoxygen,taking the water produced into account, and extrapolating the adsorption and desorption curves to zero pressure. Lynch and Flanigan31used this modified technique but disagreed (23) Noordermeer, A.; Kok, G. A.; Nieuwenhuys,B. E. Surf. Sci. 1986,
165, 375.
(24) Kok, G. A.; Noordermeer A.; Nieuwenhuys,B. E. Surf.Sei. 1983, 135,65. (25) Konvalinka, J. A.; Scholten, J. J. F. J. Catal. 1977,48, 374. (26) Conrad, H.; Ertl, G.; Lath, E. E. Surf. Sci. 1974, 41, 435. (27) Behm, R. J.; Chrietmann, K.; Ertl, G. Surf. Sci. 1980,99, 320. (28) Bastaez, R.; Felter, T. E.; Ellis, W. P. Phys. Reu. Lett. 1989,63, 558. (29) Nace, D. M.; Aston, J. G. J. Am. Chem. SOC.1957, 79,3619. (30) Nace, D. M.; Aston, J. G. J. Am. Chem. SOC.1967, 79,3627. (31) Lynch, J. F.; Flanagan, T. B. J. Phys. Chem. 1973, 77, 2628.
Absorption of Hydrogen by Palladium
Langmuir, Vol. 9, No. 4, 1993 991
Table I1
sample Pd powder
figure(s) 2
(99.999%)
Pd powder (99.9+ % )
3
Pd/Grafoil
4
predicted zero-pressure intercept 0.025
(argon BET) o.oO09 (particle size before sintering) 0.04
observed zero-pressure intercept 0.0007 0.005
-0
(“EM) Pd/Monarch 700(5.67)
5
0.19
-0
(TEM) Pd/Monarch 700(9.02) Pdhilica
6
a
0.89
-0
(TEM) 0.44-0.49
-0.5
(hydrogen back sorption) Pd/alumina
9
0.24
-0.25
(hydrogen back sorption)
with Sermon’s conclusion that bulk absorption began as soon as the surface reached monolayer coverage. Instead, they postulated the existence of weakly adsorbed “C-type” surface hydrogen that formed after monolayer coverage and was the precursor to absorbed hydrogen. In summary, it would appear that there is disagreement between studies of bulk behavior and supported particle behavior as to whether hydrogen in the bulk or on the surface desorbs first, and at what temperature bulk hydrogen desorbs. Also, the bulk studies indicate equilibrium between bulk and surface hydrogen phases, whereas supported palladium studies suggest one can be decomposed (bulk) without affecting the other (surface). In the present workrepeatedly, for both types of carbonsupported samples, there was scant evidence of (surface) adsorption. All the data for these samples can be coherently explained if it is assumed that only (bulk) absorption takes place. This behavior is consistent with that anticipated from bulk studies, as described above. In contrast, it is very clear that on both the silica-supported and alumina-supported samples there was significant adsorption at very low pressures, of a type consistent with the observation of many previous studies of silica-, alumina-,and t itania-supported palladium particles.12~18120 There are severalargumentswhich lead to the conclusion that no adsorption took place on either bulk or carbonsupported samples. First, there was no significant hydrogen uptake at or near zero Torr equilibrium pressure. In all cases studied in the present work, both the absolute value and the extrapolation to zero-pressure intercept are virtually zero. For example, if there were significant surface adsorption (generallyclaimed to be Pd/H = 1for small particles), the zero-pressure intercept on the H/Pd axis for the Pd/Monarch 700(5.67) sample would be in the vicinity of 0.2. From Figure 6b it is clearly seen to be
