J. Phys. Chem. B 2001, 105, 5945-5949
5945
A Study on the Formation of Palladium Hydride in a Carbon-Supported Palladium Catalyst Nabin K. Nag* Engelhard Corporation, Chemical Catalyst Group, 23800 Mercantile Road, Beachwood, Ohio 44122 ReceiVed: December 18, 2000; In Final Form: March 20, 2001
The formation and stability of palladium hydride in a well-characterized Pd-on-carbon catalyst have been investigated by using temperature-programmed reduction (TPR), selective carbon monoxide (CO) chemisorption, and X-ray diffraction (XRD). Under experimental conditions that completely eliminated the possible formation of a Pd carbide phase (PdC) it has been found that the stability of the hydride phase and the H:Pd ratio therein decrease as a function of the percent Pd exposed.
Introduction Activated carbon-powder-supported palladium catalysts are widely used in the industries in slurry-phase processes including selective hydrogenation, hydrogenolysis, dehalogenation, etc.1 Palladium has the property of absorbing hydrogen in the crystal matrix to form Pd hydrides even at low hydrogen pressures and room temperatures.2 A lot of work3 has been done to understand the formation of the hydride phases and the possible effect of the hydrides on the activities of Pd catalysts. It is known from these studies that hydride formation may take place in dispersed palladium, carried on supports such as silica, carbon, etc. It has also been reported that the tendency to form the hydride phase decreases with increasing dispersion of Pd in supported catalysts. Thus, Aben4 reported a decrease in the hydrogen content of the hydride phases of a silicasupported Pd catalyst with decreasing Pd crystallite size, indicating a decrease in the hydride formation tendency with increasing Pd dispersion. Boudart and Hwang5 found a similar effect with a Pd/silica catalyst, while Nandi et al.6 found that for Pd/silica catalysts the ease of hydride formation decreases with decreasing Pd particle size. Fagherazzi et al.7 found a similar correlation as reported by Boudart and Hwang.5 Sepulveda and Figoli8 observed with Pd/ silica catalysts that the H:Pd ratio in the hydride phase decreases as a function of Pd dispersion up to 55% and thereafter remains constant at 0.3. Recently, Bonarwoski et al.9 reported that for Pd-on-silica catalysts the H:Pd ratio decreases from about 0.5 to 0.03 as the dispersion increases from 4 to 55%. Carbon, as a catalyst support, poses a complicating situation in studying the formation of the Pd hydride phase. During catalyst preparation and subsequent treatment, C atoms from the support and/or from the organic molecules used for making the catalyst and activity testing may migrate into the Pd lattice to form Pd carbide phases. C in the Pd lattice causes the lattice to expand, and more important, it may suppress the hydride formation tendency. Several authors have studied this phenomenon.10-20 During reactions catalyzed by Pd in the presence of hydrogen there remains the possibility of formation of Pd hydrides. * Fax: (216) 464-5780. E-mail:
[email protected].
Therefore, in these situations some effect of the hydrogen in the hydride phase could be expected on the performance of the Pd catalysts. Detailed discussions can be found in the literature2,3 about the “hydride effect” on Pd-catalyzed reactions. Most notable examples include the enhancement of the rate of hydrogenation of alkynes and suppression of the hydrogenation of cyclohexenes by Pd hydrides.2 More examples may be found in the literature cited above. In general, both positive and negative effects, depending on the type of reaction and experimental conditions, of the hydride phase on the performance of Pd catalysts have been reported. Therefore, studies on the formation of Pd hydrides in Pd, especially in the supported Pd catalysts, are very important. The purpose of the present investigation has been to systematically investigate the formation and stability of Pd hydrides in an activated carbon-powder-supported Pd catalyst. These types of catalysts are widely used in the industries. The effects of Pd dispersion and Pd crystallite size have been studied in details. Temperature programmed reduction (TPR), Pd dispersion by carbon monoxide chemisorption, and X-ray diffraction (XRD) techniques have been used during the investigation. Experimental Section A. Catalysts. A steam-activated wood carbon powder (average particle size of about 20 µm) was used as the support. The physicochemical properties of the carbon were the following: BET surface area, 1150 m2/g; nitrogen pore volume 0.7 mL/g; sulfur and phosphorus were not detected; and total ash content was less than 1 wt %. The catalyst, containing 5 wt % Pd, was prepared by the adsorption of a sodium palladium chloride solution on the carbon powder, taken as a slurry. After adsorption, the catalyst was reduced by a solution of sodium formate. This chemically reduced catalyst was thoroughly washed by deionized water, dried under vacuum, and stored in aerobic conditions for further investigation. B. Sample Treatment. The fresh, vacuum-dried catalyst had high dispersion and contained small, nanosize Pd crystallites. To study the effect of Pd crystallite size on the formation and stability of the hydride phase, the fresh catalyst was heated in argon (Ar) atmosphere at increasingly higher temperatures up
10.1021/jp004535q CCC: $20.00 © 2001 American Chemical Society Published on Web 05/23/2001
5946 J. Phys. Chem. B, Vol. 105, No. 25, 2001
Nag
SCHEME 1
to 700 °C in 100 °C increments (this step will be identified as the “pretreatment” step henceforth). These thermally treated samples, originating from different fresh portions of the same batch of the catalyst, were studied by XRD and used to measure the dispersion of Pd by selective CO chemisorption. The same thermal treatment protocol was applied in situ to a different portion of the fresh catalyst during the TPR experiments. The idea was that the heat treatment done in both the experiments would lead to similar crystallite growth pattern. In this way, the XRD data could be correlated with the quantitative Pd hydride data obtained by the TPR experiments. Experimental limitations did not permit in situ pretreatment of the catalyst during the XRD work. C. XRD. XRD scans were taken on a Bru¨cker D-500 machine using Cu KR radiation (λ ) 1.54 Å). The peak areas and full width at half-maximum (fwhm) values were measured with the help of a software available with the machine. Average crystallite size was measured, wherever possible, by the DebyeScherrer line broadening method using the Pd(111) peak and taking Si as the internal standard for making correction for instrumental line broadening. D. Dispersion Measurements by Selective CO Chemisorption. An Altamira AMI1 machine was used to conduct the selective carbon monoxide (CO) chemisorption experiments. A sample of 100 mg of the fresh, vacuum-dried catalyst was loaded in the sample tube and heated in a flow of He at 120 °C for 30 min. The catalyst was then cooled in He to 30 °C. After this, the temperature was again raised at the rate of 5 °C /min) to 75 °C in a flow of 10% H2 in He and held for 30 min. By this hydrogen treatment any adsorbed oxygen on the reduced catalyst (due to air exposure and handling) was removed, and the Pd surface was cleaned. Having cooled again to 30 °C in He, 100 µL pulses of 10% CO in He were injected at certain intervals until the peak areas reached the saturation point (3-4 pulses were needed in most cases). The amount of CO chemisorbed was determined from the integrated peak areas, and by comparing them with the calibrated area obtained from a 100 µL of the adsorbing gas mixture injected through a sample loop. CO: Pd ratio of 1:1 was used to calculate the % Pd exposed (the dispersion). A spherical geometry of the Pd crystallites was assumed in calculating the average particle diameter from the CO uptake data. E. TPD/TPR Experiments. The same Altamira instrument (AMI1), as used for the CO chemisorption experiments, was
also used to perform the TPR and TPD experiments. The experiment was started as follows: 250 mg of the fresh, vacuum-dried catalyst was taken in the sample tube and heated in flowing Ar (30 mL/min) to 100 °C, and held for 10 min. The tube was then cooled in Ar to 30 °C and then Ar was replaced by the carrier gas for the TPR experiment, which was conducted under the following conditions: carrier gas, 10% H2 in He with a flow rate of 30 mL/min; initial temperature, 30 °C; ramp rate, 15 °C/min; and final temperature, 100 °C. The overall experimental protocol for the pretreatment, first TPR, and subsequent TPR steps is given in Scheme 1 (note that +ve and -ve peaks, as referred to in the scheme, represent hydrogen consumption due to the reduction of the superficial oxide layer and the evolution hydrogen from hydride phase decomposition, respectively). The accuracy in the measurements of the TPR peak temperatures was within (1 °C. Quantification of the hydrogen, consumed or evolved during the TPR cycles, was done by digitally integrating the peak areas and comparing them against that of a standard peak generated by 100 µL of 10% hydrogen in helium injected into the H2/He carrier gas through a sample loop. Results A. XRD. The XRD spectra of the fresh catalyst and various portions of the same after being heated in Ar at various temperatures showed (the spectra are not given here) the growth in Pd crystallites as a function of pretreatment temperature. No changes in the Pd(111) and Pd(200) peak positions were found as a function of the pretreatment temperature. The most important information derived from the XRD data are given in Table 1. As seen in this table, the d spacings of Pd(111) and Pd(200) planes remain unchanged at 2.251 and 1.950 Å, respectively. It is also observed that the Pd(111) peak area increases continuously as a function of the pretreatment temperature. The sharpness of the peak, as reflected by the full width at half-maximum (fwhm), increases continuously as a function of the pretreatment temperature. B. CO Chemisorption. Pd dispersion and average crystallite size data, obtained by selective CO chemisorption, are shown in Figure 1 as a function of the pretreatment temperature. It is observed that the dispersion of Pd remains high and virtually unchanged up to about 300 °C, and then decreases continuously
Formation of Palladium Hydride in a Pd-on-C Catalyst
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5947
TABLE 1: XRD Data pretreatment temp/°C
Pd(111) PK area
30 100 200 300 400 500 600 700
25 65 80 200 225 380 400 450
FWHM Å
2.0 1.62 1.23 0.93 0.55
d(111) Å
ao Å
2.245 2.249 2.245 2.250 2.255 2.256 2.252 2.257 2.251 ( 0.002
3.888 3.895 3.890 3.885 3.893 3.908 3.901 3.901 3.895 ( 0.003
A second peak, representing Pd(200), was observed at 300 °C (and higher) of pretreatment temperature. This peak, like the Pd(111) peak, grew in size as a function of the pretreatment temperature. The d(200) value remained unchanged at 1.95 Å, and the lattice constant at 3.904 ( 0.023 Å. ao is the Pd lattice constant.
Figure 2. TPR profiles of the first cycle (line A), and the second cycle (line B). The peak areas were determined by using a computer software which defined the baselines for this purpose. A drifting baseline for A, covering the large and the small peaks, and a horizontal one for B were observed.
Figure 1. Effect of the pretreatment temperature on the dispersion of Pd and the average Pd crystallite size.
as a function of the pretreatment temperature. It is also observed in Figure 1 that Pd crystallite size remains virtually unchanged up to about 300 °C and then decreases continuously as a function of the pretreatment temperature. C. TPR. The TPR profiles of the very first cycle using a fresh catalyst, and the second cycle are shown in Figure 2, lines A and B, respectively. The TPR profile of the second cycle is representative of the subsequent cycles (see the schematic diagram above) generated from the same sample after having pretreated it in Ar, as elaborated earlier. However, the peak temperatures of the subsequent cycles increased significantly as a function of the pretreatment temperature, especially above 300 °C. The positive peaks in Figure 2, line A, are due to hydrogen consumption by the Pd oxide formed during catalyst storing and handling. The negative peak in Figure 2, line B, is due to the hydrogen that evolves during the decomposition of the hydride phase. It is important to remember here that the source of the hydrogen that formed the hydride phase is the H2/He carrier gas stream that contacts the sample at the commencement of the TPR cycles at 30 °C. It has been confirmed by independent experiments in this laboratory that with this type of C-supported Pd catalysts the hydride phase forms at temperatures around 10 °C and in hydrogen partial pressure of 76 Torr that is used in this work. The hydride decomposition temperature (that is, the maximum of the peak in Figure 2, line
Figure 3. Effect of the average Pd crystallite size on the H:Pd in the hydride phase, and the Pd hydride decomposition temperature.
B) and the H:Pd ratio in the hydride phase as a function of the average crystallite size of Pd are given in Figure 3. It is observed that the both hydride decomposition temperature and the hydrogen content (H:Pd ratio) of the hydride phase increase linearly as a function of the Pd crystallite size. Discussion A. XRD. In this work, any expansion of the Pd lattice due to the presence of interstitial C atoms is ruled out by the XRD data, as discussed below. As documented in Table 1 the d spacing (Pd(111) reflection) and the Pd lattice constant ao remain unchanged at 2.253 ( 0.005 Å and 3.895 ( 0.003 Å,
5948 J. Phys. Chem. B, Vol. 105, No. 25, 2001 respectively, for all the samples. This happens despite the Pd being supported on carbon (an unlimited potential source of C atoms) and pretreated in an inert atmosphere at temperatures up to 700 °C. The above lattice constant compares quite well with other reported numbers: 3.890,10 3.887,14 and 3.890 (JCPDS Card No. 5-681). Any formation of a PdC phase would be reflected by an increase in the lattice constant ao by about 2.6%.10 This did not happen here. It is important to remember that exactly the same heat treatment was performed in situ on the catalyst sample during the TPR experiments. Therefore, the conclusion made above about the absence of a PdC phase would be equally valid for the heat-treated sample used to study the formation and stability of Pd hydride by the TPR experiments. B. TPR. The fresh catalyst shows a TPR profile containing two positive peaks s a large one at 45 °C and a small one at 72 °C. These peaks are due to hydrogen consumption by the oxygen attached to Pd as a superficial Pd oxide layer. Reduced Pd is highly susceptible to atmospheric oxygen. During the present investigation oxidation of Pd took place during storing and handling of the catalyst, because no special precaution was taken to prevent air exposure of the vacuum-dried fresh catalyst during either storing, or loading into the sample tube during the TPR work. From the amount of hydrogen taken up by these two peaks it is estimated that about 10% of the total Pd in the catalyst remained in the “Pd oxide” form under stable ambient conditions. From EXAFS studies, McCaulley19 found the presence of an amorphous Pd oxide layer covering a metallic Pd core in an air-exposed carbon-supported Pd catalyst. McCaulley’s results support the present observation. The total absence of a negative peak (Figure 2, line A) due to hydrogen evolution from the decomposition of a hydride phase that might have formed in the first cycle needs some explanation. It may be that the large positive peak (between 30 °C and 65 °C due to hydrogen consumption) completely masks any hydrogen evolution peak. It is also possible that in the fresh catalyst the Pd core, covered by an oxide Pd oxide layer, is protected from the attack by the hydrogen in the carrier gas mixture. No more can be said about this from the present data. Once the oxide phase is destroyed in the first cycle, the TPR profile changes completely in the second and subsequent cycles. In these cycles a single, well-defined negative peak due to hydrogen evolution is observed. This hydrogen evolves from the decomposition of the hydride phase formed at the beginning of the second TPR cycle. In the first cycle, the Pd surface is completely cleansed of the oxide phase. At the end of the first cycle, when the catalyst is cooled to 30 °C in Ar and the carrier gas switched from Ar to H2/He, Pd instanteneously absorbs hydrogen from the H2/He carrier gas mixture during the 10minute hold period at 30 °C. By the time the heating of the TPR cycle started, hydrogen absorption was already complete, and the hydride phase ready for the right temperature to break down. The partial pressure of hydrogen in the carrier gas mixture was 76 Torr, which was much higher than the minimum 26 Torr (at 25 °C)19 required for the formation of the hydride phase. The hydrogen thus absorbed by the Pd in the catalyst is ejected (the negative peak in Figure 2, line B) at 55-74 °C depending on the Pd crystallite size present in the catalyst in that particular cycle. C. Hydride Phase: Formation and Stability. The main purpose of this work has been to systematically investigate the crystalline growth of Pd in a well-dispersed Pd/carbon catalyst, and the effect of this growth on the formation and stability of the β-Pd hydride phase. The absence of a sharp and well-defined
Nag XRD peak indicates that the Pd in the fresh catalyst consists of very small crystallites of Pd. In addition to this, the presence of a small portion of the Pd in a highly dispersed amorphous phase cannot be ruled out. On heating the fresh catalyst in an inert atmosphere at high temperatures, as done during the pretreatment steps, especially at 300 °C and above, well-defined XRD peaks are generated. The thermal treatment converts the amorphous phase, if any present, to a crystalline phase, and facilitates Pd crystalline growth, generating more and more order in the Pd lattice. As a consequence, the dispersion of Pd decreases and the average crystallite size increases, as shown in Figure 1. As observed in this figure, the effect of pretreatment starts showing up at about 300 °C. At the highest temperature (700 °C) of treatment, Pd dispersion falls down to about 10% from 40%, whereas the average crystallize size, as determined by CO chemisorption, grows from 14 to 106 Å. It is noteworthy that even at the highest temperature (700 °C) of pretreatment the Pd crystallites do not grow too high. Before making conclusions on the formation and stability of the hydride phase, it is important to make sure that the Pd crystal lattice does not contain any C as a Pd carbide phase. As discussed in the Results, section A, no Pd lattice expansion takes place during the present experimental conditions, thus ruling out the possibility of the presence of any PdC phase in the system. It is known that C in the Pd lattice not only expands the latter but also suppresses the hydrogen absorption capacity to form the β-hydride phase. For instance, Krishnankutty and Vannice16 have reported the formation of PdC in a Pd catalyst supported on carbon due to the migration of C into the Pd lattice. Because they used Pd acetylacetonate to prepare the catalyst, the C in the Pd lattice might have come from the organic ligand, as explained by the authors. However, they did not rule out the possibility of the support carbon being the source of the carbon lodged inside the Pd matrix. Many other authors10,14,17,18 have also reported the formation of PdC in supported Pd catalysts. In all these studies it appears that the source of C in PdC is the organic precursors used to prepare the catalyst and/or the chemicals such as vinyl acetate, acetylene, etc., used during the investigation. No such organic compound was used in the present investigation, and most important, no evidence is found for the formation of a PdC phase from the XRD studies. There is unequivocal evidence in the literature, however, that the formation of PdC is quite possible without the presence of any organic reagent. Thus Lamber et al.20 were able to introduce C into Pd lattice by vacuum deposition of Pd on carbon films. This led to an increase in the lattice constant by about 2.8%, and the result is in agreement with findings of others.10 Therefore, it is concluded that the present quantitative data on Pd hydride, obtained from the TPR experiments, are free from any complication due to the possible presence of C in the Pd lattice. As shown in Figure 3, the stability of the hydride phase, as reflected by the temperature of decomposition (that is, the negative peak temperature in Figure 2, line B), increases from 55 °C to 74 °C as a function of the Pd crystallite size. In the same way the H:Pd ratio in the hydride phase increases from about 0.10 to 0.26 as the crystallite size increases from about 24 Å to 106 Å. There is a very good correlation between the hydrogen content of the hydride phase and its stability as shown in Figure 4. It is quite possible that as the order in the Pd lattice increases due to crystalline growth, the electronic nature of the Pd changes. In this background it is tempting to suggest that this change somehow increases the binding force holding the hydrogen in the lattice, thus necessitating higher temperatures
Formation of Palladium Hydride in a Pd-on-C Catalyst
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5949 catalysts with Pd dispersion varying between 4% and 55% Bonarowska et al.9 have recently found similar H:Pd ratios, as found in the present work. Their data are compared with the present data in Figure 5. In any event, the largest Pd particle diameter found in the present catalysts is about 100 Å. Perhaps, this size is not massive enough for the Pd to accommodate a larger amount of hydrogen as found by others. In summary, the present study shows that the H:Pd ratio in the hydride phase, and its thermal stability, increase as the average crystallite size of the Pd increases from 24 to 106 Å in a carbon-powder-supported highly dispersed Pd catalyst. In this system, no Pd carbide phase is formed by the migration of C atoms from the support carbon. Acknowledgment. James De Broux of Engelhard generated the XRD data, and his contribution is acknowledged.
Figure 4. Correlation between H:Pd and the hydride decomposition temperature.
Figure 5. Effect of Pd % exposed on the H:Pd: a comparison between the current results and those reported in ref 9.
for the decomposition of the hydride phase in the larger crystallites. Finally, the H:Pd ratio found in this investigation seems at the first sight to be rather low compared with numbers reported by others.4-8,21 However, from a systematic study on Pd/SiO2
References and Notes (1) Rylander, P. N. Catalytic Hydrogenation OVer Metals; Academic Press: New York, 1967; Rylander, P. N. Catalytic Hydrogenation in Organic Synthesis”; Academic Press: New York, 1979. (2) Maier, W. F. In Catalysis of Organic Reactions; Rylander, P. N., Greenfield, H., Augustine, A. L., Eds.; Marcel Dekker: New York, 1988. (3) For a review consult, Palczewska, W. In Hydrogen Effects in Catalysis; Paal, Z., Menon, P. G., Eds.; Marcel Dekker: New York, 1988; p 373. (4) Aben, P. C. J. Catal. 1968, 10, 224. (5) Boudart, M.; Hwang, H. S. J. Catal. 1975, 39, 44. (6) Nandi, R. K.; Pitchai, R.; Wong, S. S.; Cohen, J. B.; Burwell, R. L., Jr.; Butt, J. B. J. Catal. 1981, 70, 298. (7) Fagherazzi, G.; Benditti, S.; Di Mario, A.; Pinna, F.; Signoretto, M.; Pernicone, N. Catal. Lett. 1995, 32, 293. (8) Sepulveda, J. H.; Figoli, N. S. Appl. Surf. Sci. 1993, 68, 257. (9) Bonarowska, M.; Pielaszek, J.; Juszczyk, W.; Karpinski, Z. J. Catal. 2000, 195, 304. (10) Ziemecki, S. B.; Jones, J. A.; Swartzfager, D. G.; Harlow, R. L. J. Am. Chem. Soc. 1985, 107, 4547. (11) Ziemecki, S. B.,; Jones, J. A. J. Catal. 1985, 95, 621. (12) Kiraly, Z.; Mastalir, A.; Berger, F.; Dekany, I. Langmuir 1988, 14, 1281. (13) Ziemecki, S. B.; Michel, J. B.; Jones, G. A. ReactiVity of Solids 1986, 2, 187. (14) Zaidi, S. A. H. J. Catal. 1981, 68, 255. (15) Zaidi, S. A. H. Appl. Catal. 1987, 33, 273. (16) Krishnankutty, N.; Vannice, M. A. J. Catal. 1995, 155, 312; Ibid. 1995, 155, 327; Ibid. 1998, 173, 137. (17) Stachwiski, J.; Frackiewicz, A. J. Less-Common Met. 1985, 108, 249. (18) Nakamura, S.; Yasui, T. J. Catal. 1970, 17, 366. (19) McCaulley, J. A. J. Phys. Chem. 1993, 97, 10372. (20) Lamber, R.,; Jaeger, N.; Schulz-Eckloff, G. Surf. Sci. 1990, 227, 15. (21) Kiraly, Z.; Mastalir, A.; Berger, F.; Dekany, I. Langmuir 1998, 14, 1281.
