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Preparation of Cr2O3-promoted copper catalysts on rice husk ash by incipient wetness impregnation
Applied Catalysis A: General 288 (2005) 53–61 www.elsevier.com/locate/apcata
Preparation of Cr2O3-promoted copper catalysts on rice husk ash by incipient wetness impregnation Feg-Wen Chang *, Wen-Yao Kuo, Hsien-Chang Yang Department of Chemical and Materials Engineering, National Central University, Chungli 32001, Taiwan Received 9 September 2004; received in revised form 6 April 2005; accepted 12 April 2005 Available online 23 May 2005
Abstract Rice husk ash (RHA) was utilized as the support material for manufacturing Cr2O3-promoted copper catalyst by incipient wetness impregnation. With constant copper loading at 15 wt%, the effects of Cr content varying from 0 to 5 wt% on surface properties and catalytic activity were investigated. In addition to RHA, commercial silica gel was also used as catalyst support for comparison to study the effect of support material. Surface characterizations were examined extensively by XRD, TPR, SEM, N2 sorption, and H2–N2O titration, while catalytic activities were studied using ethanol dehydrogenation. The results indicate that copper dispersion is enhanced by the initial increase in Cr2O3 promoter content up to 2 wt%, while it then deteriorates gradually upon further increase in promoter content. It has been suggested that an optimal Cr content around 2 wt% not only enhances catalytic activity but also retards catalyst deactivation. Generally speaking, catalyst deactivation results predominantly from copper sintering. Despite the lower BET surface area, RHA is superior to commercial silica gel as a candidate for catalyst support in this work, because the surface of the former may possess more unique pores, while the majority of surface pores on the latter are interconnected and thus can be clogged easily. # 2005 Elsevier B.V. All rights reserved. Keywords: Rice husk ash; Silica support; Chromia promoter; Copper catalyst; Ethanol dehydrogenation; Incipient wetness impregnation
1. Introduction Copper, one of the most widely used metals in the history of civilization, is an important material in modern industry. Copper used as a catalyst is frequently applied in reactions, such as methanol synthesis [1,2], gas shift reaction [3,4], and alcohol dehydration/dehydrogenation [5,6]. Its utility is, however, often limited by rapid deactivation. One of the probable reasons for deactivation of copper catalyst is sintering due to the inherently low melting point of bulk copper. Thus, instead of bulk copper, the supported copper catalyst has become prevalent in practice, and searching for an adequate support is extremely necessary for distributing copper particles to avoid or, at least, retard the occurrence of sintering. In addition to playing the role of an inert material * Corresponding author. Tel.: +886 3 4227151x34202; fax: +886 3 4252296. E-mail address: [email protected] (F.-W. Chang). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.04.046
for increasing the degree of metal dispersion, promoting thermal stability and modifying mechanical properties, a catalyst support may also serve as an active species in some reacting systems. Consequently, to meet the demand for the application in copper catalysts, silica, owing to its abundance in nature as well as the superiority of both its physical and its chemical characteristics, becomes an excellent candidate for a catalyst support in industry. Silica-supported copper catalyst has recently received much attention, above all, in the catalysis of alcohol dehydrogenation and/or dehydration [7–9]. In comparison with the commercial silica gel, rice husk ash (RHA), composed of extremely amorphous silica, was first adopted by Chang et al. [10–14] as a support in both nickel and copper catalysts and was found to exhibit better performance in both CO2 hydrogenation and ethanol dehydrogenation. Conventionally, rice husk, the predominant byproduct in the milling process of domestic agriculture, used to be either discarded or incinerated. Such
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treatment will not only result in environmental pollution but also in waste of resources. Therefore, for both industrial and environmental purposes, it makes sense to utilize RHA, which can be extracted from rice husk by a series of thermal–chemical processes including acid leaching, pyrolysis, and carbon removal. As is well known, the properties of supported metal catalysts usually show a strong dependence on both the preparation routes and the thermal (activation) treatments [15]. As usual, the preparation routes of supported-metal catalysts can be categorized into three types: namely, incipient wetness impregnation, deposition-precipitation, and ion exchange. Among the preparation routes, incipient wetness impregnation is the one most frequently adopted in industry due to its simplicity and convenience. However, its utility is still limited by poor metal dispersion. To enhance metal dispersion, an inert material termed as ‘‘textural promoter’’ is therefore introduced during preparation to separate the metal particles from contact with one another so that the coalescence of metal particles can be minimized. Such a textural promoter is required to have a relatively high melting point and to have somewhat smaller particles than those of the active metal species. Owing to its high melting point (2708 K) and fine particle size, chromia (Cr2O3) was found to be a good textural promoter [16]. However, the dependence of chromia content on the surface properties of the promoted copper catalyst as well as on the mollification of deactivation rate is seldom documented. Some chromia-promoted copper catalysts supported on RHA (denoted as Cu/Cr/RHA) and those on commercial silica gel (denoted as Cu/Cr/SiO2) have been prepared for comparison in this work to investigate the effects of chromia content on both the surface properties and the catalytic activity in ethanol dehydrogenation.
2. Experimental 2.1. Pretreatment of raw materials Rice husk was washed thoroughly with distilled water to remove the adhering soil. The material was then dried, leached by chloric acid, pyrolyzed and carbon was removed, thus yielding rice husk ash, as reported in our previous work [10–14,17–20]. During acid leaching, the dried rice husk with soil previously removed was refluxed with 3N HCl in a glass round-bottomed flask at 373 K for 1 h. This material was then filtered and washed repeatedly with warm distilled water until the filtrate was free from acid and then dried at 373 K for 24 h. Pyrolysis was performed in the atmosphere of nitrogen at 1173 K for 1 h, with rice husk placed in a tubular reactor made of quartz. Then the black crude material obtained from pyrolysis was heated in an air furnace at 1173 K for 1 h to remove the carbon content, thus yielding the white ash with amorphous silica higher than 99%. Such ash was used as a
support for catalyst preparation. We designated this ashsupported system as RHA. 2.2. Catalyst preparation Rice husk ash (153 m2/g, denoted as RHA) and commercial silica gel (385 m2/g, Merck, denoted as SiO2) were used as the catalyst supports. Before being impregnated with copper nitrate trihydrate (Cu(NO3)23H2O > 99.5%, Merck) and chromium nitrate (Cr(NO3)39H2O > 99.5%, Merck), the support was preheated at 393 K to remove the adsorbed water. Then the required amount of an aqueous solution of Cu(NO3)23H2O and Cr(NO3)39H2O was slowly added to the support, with thorough stirring at room temperature. Copper loading for all catalysts was maintained at 15 wt%, with chromium content at 0, 1, 2, 3, 5 wt%, respectively. The precursor was then dried at 373 K for 24 h, and consequently calcined in air at 723 K. We designated the ash-supported and silica gel-supported copper catalysts prepared in this system as Cu/Cr/RHA and Cu/Cr/SiO2, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Schimadzu, Model XD-5) using Cu ˚ , from 5 to 808 at Ka radiation of a wavelength of 1.54006 A a rate of 0.058/s. Temperature–programmed reduction (TPR) was performed in a micro-reactor made of a U-shaped quartz tube, surrounded by a furnace with a programmed heating system. Prior to the test, a sample of 50 mg of catalyst precursor was dried in an argon flow at a rate of 60 ml/min at 373 K. A reducing gas composed of 5% H2 plus 95% Ar was employed at a flow rate of 30 ml/min, with a heating ramp of 5 K/min from 303 to 773 K for the reduction. H2 consumption was detected with a thermal conductivity detector (TCD) of which the signal was analyzed by a computer-aided integrator. Scanning electron microscopy (SEM) with the aim of investigating the surface texture of the catalyst precursors was carried out with a scanning microscope (Hitachi, S-3500). The surface properties including specific surface area and average pore diameter were obtained using an ASAP 2000 apparatus from nitrogen adsorption at 77 K after degassing at 473 K. H2–N2O titration involving two reduction processes is widely employed in the measurement of metal surface area for copper catalyst [8]. First, 40 mg of calcined precursor was placed in the microreactor (described above, in TPR measurement), purged in the reducing gas of 5% H2/95% Ar at a flow rate of 60 ml/min, with a heating ramp of 10 K/min from ambient to 573 K and kept at this temperature for 2 h. The temperature was then maintained at 333 K. Pure N2O gas was allowed to flow through the fixed bed for 1 h at
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60 ml/min. The reduced copper, after reacting with nitrous oxide, was believed to give cuprous oxide. The later step, H2–N2O titration, was performed from ambient temperature to 1073 K. The operation system of evaluating the amount of H2 consumption is the same as that of TPR. The analysis of carbon content was performed with an element analyzer (Perkin Elmer, Model 240C).
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as the promoter in all the catalyst precursors can be expected, since the weight percentage of Cr-promoter is not above 5 wt%, which is under the detection limit of XRD analysis. Moreover, this may imply the fairly good dispersion of chromium species among CuO particles over the support. 3.2. TPR
2.4. Ethanol dehydrogenation
The calcined catalyst precursors have been examined by XRD analysis to ascertain the phase composition of the catalyst surface. The precursors supported on rice husk ash have exhibited similar patterns, as shown in Fig. 1, indicating the presence of amorphous SiO2 and CuO crystallites, in agreement with our previous work [14]. The absence of any diffraction peak capable of indicating chromium species
The temperature-programming reduction (TPR) profiles have been depicted in Fig. 2. The onset temperature of reduction is generally located at ca. 450 K, while the termination temperature varies with the promoted Cr content. The unpromoted Cu/RHA precursor exhibiting a similar yet slightly narrow reduction peak centering at 518 K may reveal copper species in the form of bulk CuO [14]. The 1 wt% Cr-promoted Cu/Cr/RHA poses an unsymmetrical reduction peak centered at ca. 520 K with a long ’’tail’’ terminated at ca. 630 K, which should probably be assigned to the reduction of some copper species rather than chromium species, since it has been documented that chromia will not be reduced until the temperature is raised to ca. 773 K during a TPR process [21]. Moreover, from the viewpoint of weight percentage, it seems that the 1 wt% Cr promoter cannot display a reduction peak with such appreciable intensity. The copper species that is reduced at higher temperature than bulk CuO might be attributed to the smaller CuO particles with a certain degree of interaction with the chromium species used as the promoter. The 2 wt% Cr-promoted Cu/Cr/RHA precursor shows a narrower unsymmetrical reduction peak with the onset temperature delayed until ca. 470 K and the reduction vanishing at ca.
Fig. 1. XRD patterns of the calcined Cu/Cr/RHA catalyst precursors.
Fig. 2. TPR profiles of the calcined Cu/Cr/RHA catalyst precursors. (ramp rate, 10 K/min).
Dehydrogenation was carried out in the U-shaped microreactor at 523 K. For each run, 50 mg of fresh catalyst previously reduced with 5% H2/95%Ar at 573 K for 2 h were used. To keep the flow with a steady mole fraction of ethanol vapor before the entrance to the reactor, nitrogen was employed as the carrier gas, flowing at a rate of 15 ml/ min through an ethanol saturator maintained at 298 K. The products were sampled using a six-port valve and were analyzed by gas chromatography with a TCD.
3. Results and discussion 3.1. XRD
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616 K. It is notable for the maximum of the reduction peak, instead of being located at 520 K, to shift toward a higher temperature at ca. 535 K. This might imply the existence of a phase composed of well-dispersed CuO particles surrounded by fine chromia particles with a certain degree of interaction, so the crystallite growth of CuO may be interrupted by chromia and the probability of the formation of larger bulk CuO crystallites may be reduced. However, for the Cu/Cr/ RHA precursors, further increase in promoted Cr percentage to 5 wt% seems to contrarily give more agglomerates of bulk CuO, as revealed by the major reduction peak located at ca.520 K again. 3.3. SEM To investigate the influence of Cr-promoted content on the crystallite growth of CuO, we employed SEM to examine the general feature of calcined catalyst precursors. The images of Fig. 3a correspond to the unpromoted Cu/RHA precursor, while those of Fig. 3b–d correspond to the 1, 2, and 5 wt% Cr-promoted Cu/Cr/RHA precursors, respectively.
The regions with comparatively high brightness are assigned to CuO particles along with or without chromia, while the rest of the area with much weaker brightness indicates the support surface. The unpromoted Cu/RHA possesses CuO particles with larger average crystallite size, indicating the obvious aggregation of CuO particles when copper loading is as high as 15 wt%. In the 1 wt% Cr-promoted Cu/Cr/RHA precursor with the same copper loading at 15 wt%, the crystallites composed of CuO, probably along with chromia, seem to have smaller average size than the unpromoted Cu/ RHA precursor. More uniformly dispersed particles, as previously described in TPR results, can be observed in the 2 wt% Cr-promoted Cu/Cr/RHA precursor. This observation implies that the aggregation of CuO may be mollified when chromia is intruded into CuO phase. Owing to its inherently high melting point (2708 K), chromia may play the role of spacer among the CuO particles to interfere with the crystal growth of CuO, since chromia cannot fit into the CuO lattice [22]. However, when the Cr-promoted content is further increased to 5 wt%, the dispersion of CuO-chromia mixture seems to deteriorate, as revealed by the aggregation
Fig. 3. SEM images (5000x) of calcined Cu/Cr/RHA catalyst precursors: (a) unpromoted; (b) 1 wt% Cr-promoted; (c) 2 wt% Cr-promoted; (d) 5 wt% Cr-promoted.
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of the ’’jam-packed’’ clusters toward three dimensions. The pronounced buildup of CuO crystallite growth may result from the mutual incorporation of adjacent clusters, when Cr content is dramatically increased to 5 wt%. As copper loading is maintained at 15 wt% for all the catalysts, the limited space over the support becomes more restricted when chromia is increased over an optimal value and the probability for the aggregation of both CuO and chromia is also increased in the preparation route by incipient wetness impregnation. 3.4. Surface pores Surface pore analysis is an important tool for characterization for porous materials, due to the nearly overwhelming contribution of pores to the total surface area. N2 adsorption isotherms are commonly adopted as a concise access to surface pore analysis since it reflects a statistical concept dealing with the probable integral features of surface texture. It is well known that both the activity and the stability of a supported catalyst usually depend on their surface characteristics, which are substantially related to both the intrinsic properties of the catalytic species and the support as well as to the mutual interactions between them. To clarify the dependence of surface properties on the bare support for the supported catalyst precursors, we have subjected both the rice husk ash and the commercial silica gel to N2 adsorption analysis. N2 adsorption isotherms of the Cu/Cr/ RHA and Cu/Cr/SiO2 catalysts along with their corresponding bare supports are shown in Fig. 4. As one can perceive in Fig. 4a, the N2 adsorption isotherms of the bare RHA support as well as those of the RHA-supported Cu/Cr/RHA precursors belong to type IV of IUPAC classification. The steep increase in N2 adsorption with increasing relative pressure, P/P0, and the N2 adsorbed amount reaching ca. 50 cm3/g at P/P0 = 0.1 suggest the presence of an appreciable amount of micropores on RHA surfaces. The mild increase in N2 adsorption ranging from P/ P0 = 0.1 to 0.95 reveals the presence of mesopores. The hysteresis loop with such a contour ranging from P/P0 = 0.4 to 0.8 is related to the surface pores in nearly cylindrical shape. The 2 wt% Cr-promoted Cu/Cr/RHA precursor, when compared with the bare RHA support, seems to possess plenty of nearly cylindrical pores yet with smaller diameters owing to the slight shift of the loop toward lower relative pressure. More pronounced deformation as depicted in the hysteresis loop corresponding to 5 wt% Cr-promoted Cu/Cr/ RHA precursor may indicate the vast change to some extent in pore size and shape. As depicted in Fig. 4b, the bare SiO2 support along with the SiO2-supported precursors exhibit the adsorption isotherms also identified as type IV of IUPAC classification. However, the isotherm corresponding to the bare SiO2 with pronounced hysteresis loop is conventionally regarded as being related to the surface pore structure of an ‘‘ink bottle’’, in which the neck size is smaller than the interior dimension. Considerable change in pore structure
Fig. 4. N2 adsorption hysteresis loops for calcined catalyst precursors: (a) Cu/Cr/RHA (b) Cu/Cr/SiO2.
might occur, as perceived by the difference in isotherm traces after the SiO2-support is impregnated with the catalytic species. In comparison with the bare SiO2 support, the 2 wt% Cr-promoted Cu/Cr/SiO2 precursor shows a less apparent hysteresis loop, and even more diminution in hysteresis is found in the loop corresponding to the 5 wt% Cr-promoted Cu/Cr/SiO2 precursor. However, just contrary to the conventional idea, it seems difficult to depict the real features of the complicated surface pores with assumptions based on unique pores in simple geometrical shapes. As is well known, there are frequently two mechanisms alternatively responsible for the hysteresis phenomena, namely, the ‘‘unique pore’’ mechanism and the
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‘‘interconnected pore’’ mechanism. The former, from a thermodynamic sense, involves the coexistence of gas and liquid phases within the temperature range near the condensation point. The latter suggests that the delay of desorption may occur in a pore network due to the outmost pores with smaller diameter. The determination of the relative contribution of either mechanism to hysteresis is very difficult. Nevertheless, pore size distributions might still reveal some hints. It is suggested that the ‘‘unique pore’’ mechanism may be neglected for the surface texture with a wide pore distribution [23]. The curves of pore size distribution evaluated from desorption data by utilizing the BJH model are shown in Fig. 5. The bare RHA support
(Fig. 5a) exhibiting a narrower pore size distribution ranging ˚ with its maximum at 30 A ˚ seems to from ca. 20 to 60 A imply the abundance of unique pores on the surface, while the bare SiO2 support (Fig. 5b) possessing a broader ˚ along with a shoulder at 40 A ˚ is distribution from 20 to 85 A probably composed of more interconnected surface pores. 3.5. Copper dispersion The results of copper surface area, along with copper dispersion and copper crystallite size, determined by H2– N2O titration have been detailed in Table 1. Generally speaking, for all the copper catalysts, the copper surface area varies from 15.5 to 35.9 m2/g-Cu, while copper dispersion ranges from 2.6 to 5.6%. The average Cu0 crystallite size ˚ . For the catalysts supported on varies from 186 to 367 A either RHA or SiO2, the copper surface area first increases with Cr promoter content and reaches a maximum at the promoted content of 2 wt% Cr; however, further increase in Cr content deteriorates the copper surface area. For the Crpromoted catalysts on either support, the optimal copper dispersion appears at the promoted content of 2 wt%. The 5 wt% Cr-promoted catalyst exhibits even lower copper dispersion than the corresponding one without Cr promoter. Moreover, it is worth noting that the copper catalysts supported on RHA exhibit higher copper dispersion than the corresponding ones on SiO2, though the BET surface area of the latter is much higher than the former. It can be conceived that a broad or poly-mode size distribution of Cu0 crystallites may be obtained in the preparation of the supported copper catalyst by incipient wetness impregnation. Thus, there may be an appreciable amount of Cu0 crystallites under the average size and, probably these crystallites may even give a portion with dimensions equivalent to the average pore size of the support. On the SiO2 support surface with porous texture probably made up of complex networks with deep interconnected pores, once the network is filled with the impregnated Cu0 crystallites or the pore neck is clogged by the crystallites, the access to the whole network will be ruined and the effective exposed Cu0 surface area will thus be dramatically reduced. This will not only account for the vast transformation of pore texture induced by the copper species distributed on the SiO2 support, but also for the lower copper dispersion on SiO2 than on RHA, despite the higher BET surface area of the bare SiO2 support. In contrast, the bare RHA surface, with narrower size distribution probably composed of unique pores, therefore results in higher copper dispersion. 3.6. Dehydrogenation of ethanol
Fig. 5. Pore size distribution for calcined catalyst precursors: (a) Cu/Cr/ RHA; (b) Cu/Cr/SiO2.
To study the catalytic activity of catalysts in ethanol dehydrogenation, we have plotted the turnover frequency (TOF) as a function of Cr-promoted content for both the fresh and aged (for 6 h) copper catalysts supported on RHA and SiO2, as shown in Fig. 6. The reaction temperature is
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Table 1 Properties of copper crystallites of the reduced catalyst precursors Catalyst
Cr content (wt%)
Copper surface area (m2/g-Cu)a
Dispersion (%)
˚ )a Average copper crystallite size (A
Cu/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2
0 1 2 3 5 0 1 2 3 5
18.2 26.8 35.9 21.2 17.1 15.5 22.8 27.3 18.9 16.2
3.4 4.3 5.6 3.3 2.7 2.8 3.5 4.2 2.9 2.6
302 225 186 315 345 367 293 245 354 364
a
H2–N2O titration method.
maintained at 523 K. As is well known, although dehydration is frequently accompanied by dehydrogenation during the catalytic decomposition of alcohols, it has been deduced that the selectivity of acetaldehyde yielded in ethanol dehydrogenation is nearly 100% at the temperature below 573 K. In other words, the selectivity of ethene yielded in ethanol dehydration can be neglected in this work [24,25]. As revealed by Fig. 6, for the reaction catalyzed by the RHA-supported catalysts, the initial TOF generally ranges from 1.02 102 to 1.10 102 s1 without drastic difference, while the TOF at 6 h of time on stream diverges from 6.93 103 to 9.88 103 s1 due to a variety of deactivation rates. In comparison with the unprompted Cu/ RHA catalyst, the first increase in the promoted Cr content from 1 to 2 wt%, not only enhances the initial catalytic activity, but also retards deactivation. It is striking that the increase in Cr content up to 3 wt% starts to deteriorate the activity, and the TOF is generally close to that corresponding to the unpromoted Cu/RHA catalyst. The 5 wt% Cr-promoted
Cu/Cr/RHA catalyst exhibits an even lower activity, as well as more dramatic deactivation, than the unpromoted Cu/RHA catalyst. By and large, for the SiO2-supported catalysts, the effects of promoted Cr content on both the initial activity and deactivation rate show trends similar to those corresponding to the RHA-supported catalysts. However, on the basis of the same promoted Cr content, it is notable for the copper catalyst supported on RHA to exhibit a higher initial activity as well as lower deactivation rate than that on SiO2. This may be because the majority of the surface pores on the RHA support are, as previously described, in the type of unique pores, while those on the SiO2 support are in an interconnected network, of which the tortuosity may readily induce pore blocking due to the migration of copper particles during thermal activation or catalytic reaction. Once the neck of pore network is blocked, a quantity of copper particles will loose the immediate access to external support surface, and therefore, the number of exposed copper particles will be dramatically decreased. In contrast, the simple texture with unique pores on RHA surface tends to result in higher copper dispersion and lower sintering rate. Thus, RHA is superior to commercial silica gel as a candidate for the catalyst support in this work. 3.7. Catalyst deactivation
Fig. 6. TOF of ethanol dehydrogenation vs. promoted Cr content.
As is well known, poisoning, coking and sintering are the most probable reasons accounting for catalyst deactivation. Poisoning may first be excluded from the possibility of leading to catalyst deactivation in this work, since the feed reactant is free from any component capable of causing poisoning. Therefore, coking and sintering will probably be responsible for catalyst deactivation. To elucidate the relative contribution of coking to catalyst deactivation, we have analyzed the carbon content for the 5 wt% Crpromoted Cu/Cr/RHA catalyst, which has been given the most pronounced deactivation in the corresponding series. From 3 to 6 h of time on stream, the carbon content varies from 0.35 to 0.41 wt%. Owing to both the carbon content and its variation with time being hardly appreciable, coking or namely, carbonaceous deposit, does not make a pronounced contribution to catalyst deactivation, even if it cannot be completely excluded from the contribution.
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Fig. 7. Copper surface area vs. promoted Cr content for Cu/Cr/RHA catalysts.
Therefore, the authors tried to investigate sintering, which may be the predominant factor responsible for catalyst deactivation in this work. To examine the degree of sintering, we have subjected all the catalysts, both fresh and aged for 6 h, to the measurement of copper surface area as depicted in Fig. 7. After 6 h on stream, a drastic loss in copper surface area can be found in both the unpromoted material and in 5 wt% Cr-promoted copper catalysts. Sintering is therefore evidenced to be predominantly responsible for deactivation. The maximum loss in copper surface area reaches as high as 63%. However, the loss can be minimized to 33%, corresponding to 2 wt% Cr-promoted Cu/Cr/RHA catalyst, indicating the optimal promoted content of chromia which has effectively retarded the deactivation of copper catalyst. To describe the phenomena of catalyst deactivation more precisely, the authors try to correlate catalytic activity with duration on stream. It has been reported that the commonly used decay law for sintering is second order with respect to the present activity [26]. The deactivation rate, rd, can thus be expressed as a function of activity, a, as follows: rd ¼
da ¼ kd a2 dt
Fig. 8. Plot of (1/a1) vs. t for Cu/Cr/RHA catalysts.
linear fitting coincides with the assumption of the secondorder deactivation model. The slope of each line thus obtained from the plot, equivalent to the deactivation rate constant, can be regarded as a criterion to discriminate the efficiency in retarding catalyst deactivation. For all the catalysts supported on RHA, the variation in deactivation rate constant kd with the promoted Cr content is shown in Fig. 9. It can be deduced that the optimal promoted Cr content for the copper catalyst in this work is around 2 wt%, for this value not only gives a high initial activity, but also a low deactivation rate.
(1)
where, a is defined as the ratio of the TOF at t (time on stream) to the initial TOF, and kd is the deactivation rate constant. Eq. (1) can be integrated to yield 1 1 ¼ kd t a
(2)
where, the initial condition is given by a = 1 at time t = 0. As shown in Fig. 8, the plot of (1/a1) versus t showing fairly
Fig. 9. Deactivation rate constant kd vs. promoted Cr content for Cu/Cr/ RHA catalysts.
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4. Conclusion In the calcined Cu/Cr/RHA precursors, chromia as the promoter may be well-dispersed among CuO particles over the RHA support, as revealed by the XRD results. The results of TPR and SEM have shown that an initial increase in Cr content to 2 wt% gives an optimal dispersion of CuO particles, while further increase in Cr content to 5 wt% just deteriorates CuO dispersion. A similar trend for the variation in Cu0 particle dispersion with Cr content in reduced catalyst has been found from H2–N2O titration. The majority of surface pores on the RHA support are unique pores, while those on the silica gel are interconnected, as evidenced by surface pore analysis. Cr content around 2 wt% has been suggested to be optimal for the catalytic ethanol dehydrogenation, as it not only enhances catalytic activity, but also retards catalyst deactivation, which results predominantly from copper sintering. Catalyst deactivation in this work follows generally the second-order model. Despite the lower BET surface area, RHA is superior to commercial silica gel as a candidate for the catalyst support in this work, since the majority of surface pores on the former are, as previously described, unique pores, while those on the latter are interconnected and can be clogged easily.
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Acknowledgement The authors express their thanks to the National Science Council of Taiwan for its financial support under Project NSC 90-2214-E008-017.
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Preparation of Cr2O3-promoted copper catalysts on rice husk ash by incipient wetness impregnation Feg-Wen Chang *, Wen-Yao Kuo, Hsien-Chang Yang Department of Chemical and Materials Engineering, National Central University, Chungli 32001, Taiwan Received 9 September 2004; received in revised form 6 April 2005; accepted 12 April 2005 Available online 23 May 2005
Abstract Rice husk ash (RHA) was utilized as the support material for manufacturing Cr2O3-promoted copper catalyst by incipient wetness impregnation. With constant copper loading at 15 wt%, the effects of Cr content varying from 0 to 5 wt% on surface properties and catalytic activity were investigated. In addition to RHA, commercial silica gel was also used as catalyst support for comparison to study the effect of support material. Surface characterizations were examined extensively by XRD, TPR, SEM, N2 sorption, and H2–N2O titration, while catalytic activities were studied using ethanol dehydrogenation. The results indicate that copper dispersion is enhanced by the initial increase in Cr2O3 promoter content up to 2 wt%, while it then deteriorates gradually upon further increase in promoter content. It has been suggested that an optimal Cr content around 2 wt% not only enhances catalytic activity but also retards catalyst deactivation. Generally speaking, catalyst deactivation results predominantly from copper sintering. Despite the lower BET surface area, RHA is superior to commercial silica gel as a candidate for catalyst support in this work, because the surface of the former may possess more unique pores, while the majority of surface pores on the latter are interconnected and thus can be clogged easily. # 2005 Elsevier B.V. All rights reserved. Keywords: Rice husk ash; Silica support; Chromia promoter; Copper catalyst; Ethanol dehydrogenation; Incipient wetness impregnation
1. Introduction Copper, one of the most widely used metals in the history of civilization, is an important material in modern industry. Copper used as a catalyst is frequently applied in reactions, such as methanol synthesis [1,2], gas shift reaction [3,4], and alcohol dehydration/dehydrogenation [5,6]. Its utility is, however, often limited by rapid deactivation. One of the probable reasons for deactivation of copper catalyst is sintering due to the inherently low melting point of bulk copper. Thus, instead of bulk copper, the supported copper catalyst has become prevalent in practice, and searching for an adequate support is extremely necessary for distributing copper particles to avoid or, at least, retard the occurrence of sintering. In addition to playing the role of an inert material * Corresponding author. Tel.: +886 3 4227151x34202; fax: +886 3 4252296. E-mail address: [email protected] (F.-W. Chang). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.04.046
for increasing the degree of metal dispersion, promoting thermal stability and modifying mechanical properties, a catalyst support may also serve as an active species in some reacting systems. Consequently, to meet the demand for the application in copper catalysts, silica, owing to its abundance in nature as well as the superiority of both its physical and its chemical characteristics, becomes an excellent candidate for a catalyst support in industry. Silica-supported copper catalyst has recently received much attention, above all, in the catalysis of alcohol dehydrogenation and/or dehydration [7–9]. In comparison with the commercial silica gel, rice husk ash (RHA), composed of extremely amorphous silica, was first adopted by Chang et al. [10–14] as a support in both nickel and copper catalysts and was found to exhibit better performance in both CO2 hydrogenation and ethanol dehydrogenation. Conventionally, rice husk, the predominant byproduct in the milling process of domestic agriculture, used to be either discarded or incinerated. Such
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treatment will not only result in environmental pollution but also in waste of resources. Therefore, for both industrial and environmental purposes, it makes sense to utilize RHA, which can be extracted from rice husk by a series of thermal–chemical processes including acid leaching, pyrolysis, and carbon removal. As is well known, the properties of supported metal catalysts usually show a strong dependence on both the preparation routes and the thermal (activation) treatments [15]. As usual, the preparation routes of supported-metal catalysts can be categorized into three types: namely, incipient wetness impregnation, deposition-precipitation, and ion exchange. Among the preparation routes, incipient wetness impregnation is the one most frequently adopted in industry due to its simplicity and convenience. However, its utility is still limited by poor metal dispersion. To enhance metal dispersion, an inert material termed as ‘‘textural promoter’’ is therefore introduced during preparation to separate the metal particles from contact with one another so that the coalescence of metal particles can be minimized. Such a textural promoter is required to have a relatively high melting point and to have somewhat smaller particles than those of the active metal species. Owing to its high melting point (2708 K) and fine particle size, chromia (Cr2O3) was found to be a good textural promoter [16]. However, the dependence of chromia content on the surface properties of the promoted copper catalyst as well as on the mollification of deactivation rate is seldom documented. Some chromia-promoted copper catalysts supported on RHA (denoted as Cu/Cr/RHA) and those on commercial silica gel (denoted as Cu/Cr/SiO2) have been prepared for comparison in this work to investigate the effects of chromia content on both the surface properties and the catalytic activity in ethanol dehydrogenation.
2. Experimental 2.1. Pretreatment of raw materials Rice husk was washed thoroughly with distilled water to remove the adhering soil. The material was then dried, leached by chloric acid, pyrolyzed and carbon was removed, thus yielding rice husk ash, as reported in our previous work [10–14,17–20]. During acid leaching, the dried rice husk with soil previously removed was refluxed with 3N HCl in a glass round-bottomed flask at 373 K for 1 h. This material was then filtered and washed repeatedly with warm distilled water until the filtrate was free from acid and then dried at 373 K for 24 h. Pyrolysis was performed in the atmosphere of nitrogen at 1173 K for 1 h, with rice husk placed in a tubular reactor made of quartz. Then the black crude material obtained from pyrolysis was heated in an air furnace at 1173 K for 1 h to remove the carbon content, thus yielding the white ash with amorphous silica higher than 99%. Such ash was used as a
support for catalyst preparation. We designated this ashsupported system as RHA. 2.2. Catalyst preparation Rice husk ash (153 m2/g, denoted as RHA) and commercial silica gel (385 m2/g, Merck, denoted as SiO2) were used as the catalyst supports. Before being impregnated with copper nitrate trihydrate (Cu(NO3)23H2O > 99.5%, Merck) and chromium nitrate (Cr(NO3)39H2O > 99.5%, Merck), the support was preheated at 393 K to remove the adsorbed water. Then the required amount of an aqueous solution of Cu(NO3)23H2O and Cr(NO3)39H2O was slowly added to the support, with thorough stirring at room temperature. Copper loading for all catalysts was maintained at 15 wt%, with chromium content at 0, 1, 2, 3, 5 wt%, respectively. The precursor was then dried at 373 K for 24 h, and consequently calcined in air at 723 K. We designated the ash-supported and silica gel-supported copper catalysts prepared in this system as Cu/Cr/RHA and Cu/Cr/SiO2, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (Schimadzu, Model XD-5) using Cu ˚ , from 5 to 808 at Ka radiation of a wavelength of 1.54006 A a rate of 0.058/s. Temperature–programmed reduction (TPR) was performed in a micro-reactor made of a U-shaped quartz tube, surrounded by a furnace with a programmed heating system. Prior to the test, a sample of 50 mg of catalyst precursor was dried in an argon flow at a rate of 60 ml/min at 373 K. A reducing gas composed of 5% H2 plus 95% Ar was employed at a flow rate of 30 ml/min, with a heating ramp of 5 K/min from 303 to 773 K for the reduction. H2 consumption was detected with a thermal conductivity detector (TCD) of which the signal was analyzed by a computer-aided integrator. Scanning electron microscopy (SEM) with the aim of investigating the surface texture of the catalyst precursors was carried out with a scanning microscope (Hitachi, S-3500). The surface properties including specific surface area and average pore diameter were obtained using an ASAP 2000 apparatus from nitrogen adsorption at 77 K after degassing at 473 K. H2–N2O titration involving two reduction processes is widely employed in the measurement of metal surface area for copper catalyst [8]. First, 40 mg of calcined precursor was placed in the microreactor (described above, in TPR measurement), purged in the reducing gas of 5% H2/95% Ar at a flow rate of 60 ml/min, with a heating ramp of 10 K/min from ambient to 573 K and kept at this temperature for 2 h. The temperature was then maintained at 333 K. Pure N2O gas was allowed to flow through the fixed bed for 1 h at
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60 ml/min. The reduced copper, after reacting with nitrous oxide, was believed to give cuprous oxide. The later step, H2–N2O titration, was performed from ambient temperature to 1073 K. The operation system of evaluating the amount of H2 consumption is the same as that of TPR. The analysis of carbon content was performed with an element analyzer (Perkin Elmer, Model 240C).
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as the promoter in all the catalyst precursors can be expected, since the weight percentage of Cr-promoter is not above 5 wt%, which is under the detection limit of XRD analysis. Moreover, this may imply the fairly good dispersion of chromium species among CuO particles over the support. 3.2. TPR
2.4. Ethanol dehydrogenation
The calcined catalyst precursors have been examined by XRD analysis to ascertain the phase composition of the catalyst surface. The precursors supported on rice husk ash have exhibited similar patterns, as shown in Fig. 1, indicating the presence of amorphous SiO2 and CuO crystallites, in agreement with our previous work [14]. The absence of any diffraction peak capable of indicating chromium species
The temperature-programming reduction (TPR) profiles have been depicted in Fig. 2. The onset temperature of reduction is generally located at ca. 450 K, while the termination temperature varies with the promoted Cr content. The unpromoted Cu/RHA precursor exhibiting a similar yet slightly narrow reduction peak centering at 518 K may reveal copper species in the form of bulk CuO [14]. The 1 wt% Cr-promoted Cu/Cr/RHA poses an unsymmetrical reduction peak centered at ca. 520 K with a long ’’tail’’ terminated at ca. 630 K, which should probably be assigned to the reduction of some copper species rather than chromium species, since it has been documented that chromia will not be reduced until the temperature is raised to ca. 773 K during a TPR process [21]. Moreover, from the viewpoint of weight percentage, it seems that the 1 wt% Cr promoter cannot display a reduction peak with such appreciable intensity. The copper species that is reduced at higher temperature than bulk CuO might be attributed to the smaller CuO particles with a certain degree of interaction with the chromium species used as the promoter. The 2 wt% Cr-promoted Cu/Cr/RHA precursor shows a narrower unsymmetrical reduction peak with the onset temperature delayed until ca. 470 K and the reduction vanishing at ca.
Fig. 1. XRD patterns of the calcined Cu/Cr/RHA catalyst precursors.
Fig. 2. TPR profiles of the calcined Cu/Cr/RHA catalyst precursors. (ramp rate, 10 K/min).
Dehydrogenation was carried out in the U-shaped microreactor at 523 K. For each run, 50 mg of fresh catalyst previously reduced with 5% H2/95%Ar at 573 K for 2 h were used. To keep the flow with a steady mole fraction of ethanol vapor before the entrance to the reactor, nitrogen was employed as the carrier gas, flowing at a rate of 15 ml/ min through an ethanol saturator maintained at 298 K. The products were sampled using a six-port valve and were analyzed by gas chromatography with a TCD.
3. Results and discussion 3.1. XRD
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616 K. It is notable for the maximum of the reduction peak, instead of being located at 520 K, to shift toward a higher temperature at ca. 535 K. This might imply the existence of a phase composed of well-dispersed CuO particles surrounded by fine chromia particles with a certain degree of interaction, so the crystallite growth of CuO may be interrupted by chromia and the probability of the formation of larger bulk CuO crystallites may be reduced. However, for the Cu/Cr/ RHA precursors, further increase in promoted Cr percentage to 5 wt% seems to contrarily give more agglomerates of bulk CuO, as revealed by the major reduction peak located at ca.520 K again. 3.3. SEM To investigate the influence of Cr-promoted content on the crystallite growth of CuO, we employed SEM to examine the general feature of calcined catalyst precursors. The images of Fig. 3a correspond to the unpromoted Cu/RHA precursor, while those of Fig. 3b–d correspond to the 1, 2, and 5 wt% Cr-promoted Cu/Cr/RHA precursors, respectively.
The regions with comparatively high brightness are assigned to CuO particles along with or without chromia, while the rest of the area with much weaker brightness indicates the support surface. The unpromoted Cu/RHA possesses CuO particles with larger average crystallite size, indicating the obvious aggregation of CuO particles when copper loading is as high as 15 wt%. In the 1 wt% Cr-promoted Cu/Cr/RHA precursor with the same copper loading at 15 wt%, the crystallites composed of CuO, probably along with chromia, seem to have smaller average size than the unpromoted Cu/ RHA precursor. More uniformly dispersed particles, as previously described in TPR results, can be observed in the 2 wt% Cr-promoted Cu/Cr/RHA precursor. This observation implies that the aggregation of CuO may be mollified when chromia is intruded into CuO phase. Owing to its inherently high melting point (2708 K), chromia may play the role of spacer among the CuO particles to interfere with the crystal growth of CuO, since chromia cannot fit into the CuO lattice [22]. However, when the Cr-promoted content is further increased to 5 wt%, the dispersion of CuO-chromia mixture seems to deteriorate, as revealed by the aggregation
Fig. 3. SEM images (5000x) of calcined Cu/Cr/RHA catalyst precursors: (a) unpromoted; (b) 1 wt% Cr-promoted; (c) 2 wt% Cr-promoted; (d) 5 wt% Cr-promoted.
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of the ’’jam-packed’’ clusters toward three dimensions. The pronounced buildup of CuO crystallite growth may result from the mutual incorporation of adjacent clusters, when Cr content is dramatically increased to 5 wt%. As copper loading is maintained at 15 wt% for all the catalysts, the limited space over the support becomes more restricted when chromia is increased over an optimal value and the probability for the aggregation of both CuO and chromia is also increased in the preparation route by incipient wetness impregnation. 3.4. Surface pores Surface pore analysis is an important tool for characterization for porous materials, due to the nearly overwhelming contribution of pores to the total surface area. N2 adsorption isotherms are commonly adopted as a concise access to surface pore analysis since it reflects a statistical concept dealing with the probable integral features of surface texture. It is well known that both the activity and the stability of a supported catalyst usually depend on their surface characteristics, which are substantially related to both the intrinsic properties of the catalytic species and the support as well as to the mutual interactions between them. To clarify the dependence of surface properties on the bare support for the supported catalyst precursors, we have subjected both the rice husk ash and the commercial silica gel to N2 adsorption analysis. N2 adsorption isotherms of the Cu/Cr/ RHA and Cu/Cr/SiO2 catalysts along with their corresponding bare supports are shown in Fig. 4. As one can perceive in Fig. 4a, the N2 adsorption isotherms of the bare RHA support as well as those of the RHA-supported Cu/Cr/RHA precursors belong to type IV of IUPAC classification. The steep increase in N2 adsorption with increasing relative pressure, P/P0, and the N2 adsorbed amount reaching ca. 50 cm3/g at P/P0 = 0.1 suggest the presence of an appreciable amount of micropores on RHA surfaces. The mild increase in N2 adsorption ranging from P/ P0 = 0.1 to 0.95 reveals the presence of mesopores. The hysteresis loop with such a contour ranging from P/P0 = 0.4 to 0.8 is related to the surface pores in nearly cylindrical shape. The 2 wt% Cr-promoted Cu/Cr/RHA precursor, when compared with the bare RHA support, seems to possess plenty of nearly cylindrical pores yet with smaller diameters owing to the slight shift of the loop toward lower relative pressure. More pronounced deformation as depicted in the hysteresis loop corresponding to 5 wt% Cr-promoted Cu/Cr/ RHA precursor may indicate the vast change to some extent in pore size and shape. As depicted in Fig. 4b, the bare SiO2 support along with the SiO2-supported precursors exhibit the adsorption isotherms also identified as type IV of IUPAC classification. However, the isotherm corresponding to the bare SiO2 with pronounced hysteresis loop is conventionally regarded as being related to the surface pore structure of an ‘‘ink bottle’’, in which the neck size is smaller than the interior dimension. Considerable change in pore structure
Fig. 4. N2 adsorption hysteresis loops for calcined catalyst precursors: (a) Cu/Cr/RHA (b) Cu/Cr/SiO2.
might occur, as perceived by the difference in isotherm traces after the SiO2-support is impregnated with the catalytic species. In comparison with the bare SiO2 support, the 2 wt% Cr-promoted Cu/Cr/SiO2 precursor shows a less apparent hysteresis loop, and even more diminution in hysteresis is found in the loop corresponding to the 5 wt% Cr-promoted Cu/Cr/SiO2 precursor. However, just contrary to the conventional idea, it seems difficult to depict the real features of the complicated surface pores with assumptions based on unique pores in simple geometrical shapes. As is well known, there are frequently two mechanisms alternatively responsible for the hysteresis phenomena, namely, the ‘‘unique pore’’ mechanism and the
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‘‘interconnected pore’’ mechanism. The former, from a thermodynamic sense, involves the coexistence of gas and liquid phases within the temperature range near the condensation point. The latter suggests that the delay of desorption may occur in a pore network due to the outmost pores with smaller diameter. The determination of the relative contribution of either mechanism to hysteresis is very difficult. Nevertheless, pore size distributions might still reveal some hints. It is suggested that the ‘‘unique pore’’ mechanism may be neglected for the surface texture with a wide pore distribution [23]. The curves of pore size distribution evaluated from desorption data by utilizing the BJH model are shown in Fig. 5. The bare RHA support
(Fig. 5a) exhibiting a narrower pore size distribution ranging ˚ with its maximum at 30 A ˚ seems to from ca. 20 to 60 A imply the abundance of unique pores on the surface, while the bare SiO2 support (Fig. 5b) possessing a broader ˚ along with a shoulder at 40 A ˚ is distribution from 20 to 85 A probably composed of more interconnected surface pores. 3.5. Copper dispersion The results of copper surface area, along with copper dispersion and copper crystallite size, determined by H2– N2O titration have been detailed in Table 1. Generally speaking, for all the copper catalysts, the copper surface area varies from 15.5 to 35.9 m2/g-Cu, while copper dispersion ranges from 2.6 to 5.6%. The average Cu0 crystallite size ˚ . For the catalysts supported on varies from 186 to 367 A either RHA or SiO2, the copper surface area first increases with Cr promoter content and reaches a maximum at the promoted content of 2 wt% Cr; however, further increase in Cr content deteriorates the copper surface area. For the Crpromoted catalysts on either support, the optimal copper dispersion appears at the promoted content of 2 wt%. The 5 wt% Cr-promoted catalyst exhibits even lower copper dispersion than the corresponding one without Cr promoter. Moreover, it is worth noting that the copper catalysts supported on RHA exhibit higher copper dispersion than the corresponding ones on SiO2, though the BET surface area of the latter is much higher than the former. It can be conceived that a broad or poly-mode size distribution of Cu0 crystallites may be obtained in the preparation of the supported copper catalyst by incipient wetness impregnation. Thus, there may be an appreciable amount of Cu0 crystallites under the average size and, probably these crystallites may even give a portion with dimensions equivalent to the average pore size of the support. On the SiO2 support surface with porous texture probably made up of complex networks with deep interconnected pores, once the network is filled with the impregnated Cu0 crystallites or the pore neck is clogged by the crystallites, the access to the whole network will be ruined and the effective exposed Cu0 surface area will thus be dramatically reduced. This will not only account for the vast transformation of pore texture induced by the copper species distributed on the SiO2 support, but also for the lower copper dispersion on SiO2 than on RHA, despite the higher BET surface area of the bare SiO2 support. In contrast, the bare RHA surface, with narrower size distribution probably composed of unique pores, therefore results in higher copper dispersion. 3.6. Dehydrogenation of ethanol
Fig. 5. Pore size distribution for calcined catalyst precursors: (a) Cu/Cr/ RHA; (b) Cu/Cr/SiO2.
To study the catalytic activity of catalysts in ethanol dehydrogenation, we have plotted the turnover frequency (TOF) as a function of Cr-promoted content for both the fresh and aged (for 6 h) copper catalysts supported on RHA and SiO2, as shown in Fig. 6. The reaction temperature is
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Table 1 Properties of copper crystallites of the reduced catalyst precursors Catalyst
Cr content (wt%)
Copper surface area (m2/g-Cu)a
Dispersion (%)
˚ )a Average copper crystallite size (A
Cu/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/Cr/RHA Cu/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2 Cu/Cr/SiO2
0 1 2 3 5 0 1 2 3 5
18.2 26.8 35.9 21.2 17.1 15.5 22.8 27.3 18.9 16.2
3.4 4.3 5.6 3.3 2.7 2.8 3.5 4.2 2.9 2.6
302 225 186 315 345 367 293 245 354 364
a
H2–N2O titration method.
maintained at 523 K. As is well known, although dehydration is frequently accompanied by dehydrogenation during the catalytic decomposition of alcohols, it has been deduced that the selectivity of acetaldehyde yielded in ethanol dehydrogenation is nearly 100% at the temperature below 573 K. In other words, the selectivity of ethene yielded in ethanol dehydration can be neglected in this work [24,25]. As revealed by Fig. 6, for the reaction catalyzed by the RHA-supported catalysts, the initial TOF generally ranges from 1.02 102 to 1.10 102 s1 without drastic difference, while the TOF at 6 h of time on stream diverges from 6.93 103 to 9.88 103 s1 due to a variety of deactivation rates. In comparison with the unprompted Cu/ RHA catalyst, the first increase in the promoted Cr content from 1 to 2 wt%, not only enhances the initial catalytic activity, but also retards deactivation. It is striking that the increase in Cr content up to 3 wt% starts to deteriorate the activity, and the TOF is generally close to that corresponding to the unpromoted Cu/RHA catalyst. The 5 wt% Cr-promoted
Cu/Cr/RHA catalyst exhibits an even lower activity, as well as more dramatic deactivation, than the unpromoted Cu/RHA catalyst. By and large, for the SiO2-supported catalysts, the effects of promoted Cr content on both the initial activity and deactivation rate show trends similar to those corresponding to the RHA-supported catalysts. However, on the basis of the same promoted Cr content, it is notable for the copper catalyst supported on RHA to exhibit a higher initial activity as well as lower deactivation rate than that on SiO2. This may be because the majority of the surface pores on the RHA support are, as previously described, in the type of unique pores, while those on the SiO2 support are in an interconnected network, of which the tortuosity may readily induce pore blocking due to the migration of copper particles during thermal activation or catalytic reaction. Once the neck of pore network is blocked, a quantity of copper particles will loose the immediate access to external support surface, and therefore, the number of exposed copper particles will be dramatically decreased. In contrast, the simple texture with unique pores on RHA surface tends to result in higher copper dispersion and lower sintering rate. Thus, RHA is superior to commercial silica gel as a candidate for the catalyst support in this work. 3.7. Catalyst deactivation
Fig. 6. TOF of ethanol dehydrogenation vs. promoted Cr content.
As is well known, poisoning, coking and sintering are the most probable reasons accounting for catalyst deactivation. Poisoning may first be excluded from the possibility of leading to catalyst deactivation in this work, since the feed reactant is free from any component capable of causing poisoning. Therefore, coking and sintering will probably be responsible for catalyst deactivation. To elucidate the relative contribution of coking to catalyst deactivation, we have analyzed the carbon content for the 5 wt% Crpromoted Cu/Cr/RHA catalyst, which has been given the most pronounced deactivation in the corresponding series. From 3 to 6 h of time on stream, the carbon content varies from 0.35 to 0.41 wt%. Owing to both the carbon content and its variation with time being hardly appreciable, coking or namely, carbonaceous deposit, does not make a pronounced contribution to catalyst deactivation, even if it cannot be completely excluded from the contribution.
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Fig. 7. Copper surface area vs. promoted Cr content for Cu/Cr/RHA catalysts.
Therefore, the authors tried to investigate sintering, which may be the predominant factor responsible for catalyst deactivation in this work. To examine the degree of sintering, we have subjected all the catalysts, both fresh and aged for 6 h, to the measurement of copper surface area as depicted in Fig. 7. After 6 h on stream, a drastic loss in copper surface area can be found in both the unpromoted material and in 5 wt% Cr-promoted copper catalysts. Sintering is therefore evidenced to be predominantly responsible for deactivation. The maximum loss in copper surface area reaches as high as 63%. However, the loss can be minimized to 33%, corresponding to 2 wt% Cr-promoted Cu/Cr/RHA catalyst, indicating the optimal promoted content of chromia which has effectively retarded the deactivation of copper catalyst. To describe the phenomena of catalyst deactivation more precisely, the authors try to correlate catalytic activity with duration on stream. It has been reported that the commonly used decay law for sintering is second order with respect to the present activity [26]. The deactivation rate, rd, can thus be expressed as a function of activity, a, as follows: rd ¼
da ¼ kd a2 dt
Fig. 8. Plot of (1/a1) vs. t for Cu/Cr/RHA catalysts.
linear fitting coincides with the assumption of the secondorder deactivation model. The slope of each line thus obtained from the plot, equivalent to the deactivation rate constant, can be regarded as a criterion to discriminate the efficiency in retarding catalyst deactivation. For all the catalysts supported on RHA, the variation in deactivation rate constant kd with the promoted Cr content is shown in Fig. 9. It can be deduced that the optimal promoted Cr content for the copper catalyst in this work is around 2 wt%, for this value not only gives a high initial activity, but also a low deactivation rate.
(1)
where, a is defined as the ratio of the TOF at t (time on stream) to the initial TOF, and kd is the deactivation rate constant. Eq. (1) can be integrated to yield 1 1 ¼ kd t a
(2)
where, the initial condition is given by a = 1 at time t = 0. As shown in Fig. 8, the plot of (1/a1) versus t showing fairly
Fig. 9. Deactivation rate constant kd vs. promoted Cr content for Cu/Cr/ RHA catalysts.
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4. Conclusion In the calcined Cu/Cr/RHA precursors, chromia as the promoter may be well-dispersed among CuO particles over the RHA support, as revealed by the XRD results. The results of TPR and SEM have shown that an initial increase in Cr content to 2 wt% gives an optimal dispersion of CuO particles, while further increase in Cr content to 5 wt% just deteriorates CuO dispersion. A similar trend for the variation in Cu0 particle dispersion with Cr content in reduced catalyst has been found from H2–N2O titration. The majority of surface pores on the RHA support are unique pores, while those on the silica gel are interconnected, as evidenced by surface pore analysis. Cr content around 2 wt% has been suggested to be optimal for the catalytic ethanol dehydrogenation, as it not only enhances catalytic activity, but also retards catalyst deactivation, which results predominantly from copper sintering. Catalyst deactivation in this work follows generally the second-order model. Despite the lower BET surface area, RHA is superior to commercial silica gel as a candidate for the catalyst support in this work, since the majority of surface pores on the former are, as previously described, unique pores, while those on the latter are interconnected and can be clogged easily.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Acknowledgement The authors express their thanks to the National Science Council of Taiwan for its financial support under Project NSC 90-2214-E008-017.
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