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Catalytic activity of Cu-based amorphous alloy ribbons modified by cathodic hydrogen charging
Applied Catalysis A: General 283 (2005) 177–184 www.elsevier.com/locate/apcata
Catalytic activity of Cu-based amorphous alloy ribbons modified by cathodic hydrogen charging ´ . Molna´rc,*, K. Hughesd M. Pisareka,b, M. Janik-Czachora, A b
a Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw, Poland c Department of Organic Chemistry, University of Szeged, Do´m te´r 8, H-6720 Szeged, Hungary d Thermo Electron, East Grinsted, England
Received 11 October 2004; received in revised form 17 December 2004; accepted 4 January 2005 Available online 30 January 2005
Abstract Since hydrogen is known to be one of the most efficient embrittling atomic species, cathodic hydrogen charging was used in an attempt to modify the structure, composition, and morphology of Cu-based amorphous alloy ribbons. Various methods of analysis such as X-ray electron microanalysis, SEM, high resolution Auger microanalysis (SAM), with varying lateral resolution and different information depths, as well as measurements of specific surface area, porosity and catalytic tests, were used to follow the changes within the ribbons and at the surface, and their interrelations with catalytic activity. The activity in a test reaction (dehydrogenation of 2-propanol) was enhanced up to conversion levels of 66% for Cu–Ti and of 88% for Cu–Hf, which are much higher than those obtained with all other pre-treatments previously applied. The rather low specific surface area and porosity, as well as a lack of Cu0 both before and after the catalytic test suggest that the generally accepted mechanism for the dehydrogenation of alcohols over copper catalysts with the involvement of Cu0 is not operative here. # 2005 Elsevier B.V. All rights reserved. Keywords: Cu-based amorphous alloys; Devitrification; Cu segregation; Hydrogen charging; Catalytic dehydrogenation; SEM; Auger microanalysis (AES; SAM)
1. Introduction One of the important tasks of materials science is preparation and characterization of new functional materials including new catalysts for technically important reactions. In order to find efficient methods of activation of new materials for catalytic purposes, the mechanism of the catalytic reaction should be understood. Dehydrogenation of alcohols to produce the corresponding carbonyl compounds has attracted considerable attention [1]. Copper, either as pure metal or as supported catalyst has been shown to be highly selective in the process. Most experimental observations including the results of surface science studies [2] and data acquired by catalytic [3,4] and * Corresponding author. Tel.: +36 62 544 277; fax: +36 62 544 200. ´ . Molna´r). E-mail address: [email protected] (A 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.022
kinetic investigations [5,6] are in harmony with the so-called carbonyl mechanism depicted in Scheme 1 for 2-propanol. With respect to the surface active sites involved in the process the results are somewhat contradictory. The rate of dehydrogenation was found to be proportional to the concentration of exposed Cu0 sites [7–9]. In other studies, in contrast, the coexistence of metallic and oxidized copper was shown to be necessary for catalytic activity [10]. In fact, surface oxygen was found to greatly enhance the ability of Cu to dissociatively chemisorb methanol and ethanol in single crystal studies [11–13]. Both the latest results [6] and earlier observations [14] show that the rate-determining step in the dehydrogenation of secondary alcohols is the breaking of the O–H bond. Detection of surface alkoxide species on copper [2,11– 13,15,16] is considered to be a definitive proof that O–H bond breaking occurs prior to the breaking of a C–H bond.
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Scheme 1. Dehydrogenation of 2-propanol over Cu.
This is despite the fact that the a C–H bond in secondary alcohols is somewhat weaker than the O–H bond. It is supposed that this may be due to the favourable orientation of the hydroxyl group on the surface [6]. The catalytic action of the copper catalyst is a result of partial charge transfers between the various entities on the surface and in the adsorbed molecule according to the electronegativity of individual atoms. The carbonyl mechanism, in fact, was originally suggested to interpret dehydrogenation of alcohols on oxide catalysts such as Cr2O3 [1,17] and may be viewed as the weakening of the corresponding three bonds in the aliphatic alcohol through interactions with three surface entities Cu0, Cud+ and Od . However, our recent investigations suggest that the mechanism of the catalytic reaction can be different. This suggestion is based on the following experimental observations: we observed a stable or even increased conversion with time-on-stream, while the specific copper surface area (Cu0) was decreasing [18], we observed an appreciable, stable conversion of 88% over Cu–Hf [19], while the specific surface area of Cu0 was zero, activation of Cu + Ti powders by mechanical alloying followed by oxidation resulted in an improvement of otherwise poor catalytic performance, while the specific surface area (Cu0) was always negligible; the improvement of catalytic efficiency thus was not in harmony with Cu0 surface area data [20], our recent results [21] show that for Cu–Ti conversion reaches 66%, while the segregated Cu layer peels off, and only a minute amount of Cu is expected to be present on the surface, the presence of Cu is necessary to initiate the reaction, as TiO2 itself exhibits no catalytic activity for the reaction concerned. A more appropriate mechanism should involve interaction of the reacting molecule with an active site composed of Cu, Ti, and O surface entities. The aim of this work was to perform appropriate investigations to get further information with the aim of elucidating the most probable mechanism, which may be operative for these alloy systems. High-resolution electron microscopy and local chemical surface analysis are applied to get an insight into the reaction mechanism focusing on the properties of the surface involved in the catalytic process.
2. Experimental 2.1. Materials Amorphous alloy ribbons: Cu65–Hf35, Cu60–Ti40, 2– 4 mm wide and 40 mm thick, freshly cast, or partially devitrified, were used. 2.2. Electrochemical pretreatment and investigation Amorphous samples were hydrogen charged in order to increase their catalytic activity for the dehydrogenation of 2propanol. Cathodic charging in 0.5 M H2SO4 at i = constant was applied during time intervals from 24 to 100 h. Low current density (not higher than 2 mA/cm2) and prolonged, but, controlled charging time were applied to produce subtle effects. This procedure seemed to be most promising, as the investigations by Gebert and co-workers [22,23] show that even very small amounts of hydrogen, introduced at a current density as low as 1 mA/cm2, already produce distinct and well resolved crystallization, thus inducing segregation of small, but yet detectable amounts of the components of the pretreated amorphous alloys. The H/M ratio was determined by elemental analysis. Details are given elsewhere [19]. 2.3. Specific surface area (BET) and porosity measurements An ASAP 2010 sorptometer (Accelerated Surface Area and Porosimetry System, MICROMETRITICS) was used to estimate specific surface area and porosity from Ar adsorption isotherm. The cathodically modified samples were carefully degassed in vacuum at 300 8C before Ar adsorption, then cooled to room temperature. Eventually Ar was admitted to the chamber, and the isotherm was measured at room temperature. ASAP 2010 V4.00 D computer program was used for the estimation of BET specific surface areas and of BJH cumulative surface area of pores and an average pore diameter. All the corresponding data are presented with respect to the total weight of the modified ribbons. 2.4. Microscopic and surface analytical investigations The samples were examined with an optical microscope and a scanning electron microscope (HITACHI 3500N). A Microlab 350 (Thermo Electron) was used to determine the morphology and chemical composition of
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the surface, utilising high resolution SEM, Auger electron spectroscopy (AES) and Auger mapping functions of the Microlab. This equipment provides lateral resolution for AES mapping of 12 nm and depth information of a few monolayers. The following surface corrosion/devitrification products were analysed: (a) the surface oxide zone on both sides of the ribbon, (b) the morphology and composition of the segregated copper layer, the surface underneath the copper layer on the top of the ribbons modified due to hydrogen charging and a subsequent air exposure, (c) carefully polished cross sections of the surface zone of the ribbon. To obtain such a cross-section, a piece of ribbon was mounted into epoxy to protect the peculiar structure of the wheel side and then carefully polished to reveal the structure of the cross-section. 2.5. Catalytic test Dehydrogenation of 2-propanol was chosen to test the changes in the catalytic activity of the cathodically pretreated Cu-based ribbons. About 10 mg of the as received (aged) or electrochemically pre-treated ribbon samples was loaded into a glass micro reactor. 2-Propanol was fed by a micro feeder into a stainless steel evaporator, where it was mixed with hydrogen (99.999%). Then the gas mixture (2propanol/hydrogen = 0.018) was introduced into the reactor, which was maintained at 300 8C. The total flow rate was 10 ml min 1. The reactor temperature was controlled to an accuracy of 0.5 8C by using a microprocessor-based controller (Selftune plus, LOVE Controls). The effluents
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sampled by a pneumatic gas sampling system were analysed by GC (Shimadzu 8A equipment, thermal conductivity detector, CWAX 20 M column, 100 8C, 30 ml min 1 flow rate of hydrogen carrier gas). Calculations were made using a Data Apex Chromatography Station for Windows 1.5. The specific surface area of Cu0 was measured with the aid of N2O titration [24,25], based on the reaction of nitrous oxide with Cu0 species using the GC pulse method.
3. Results and discussion Our own recent results have shown that the procedure developed by Gebert and co-workers [19] was efficient in transforming Cu–Hf amorphous alloys into stable, efficient (90% conversion) and selective (95%) catalysts. Furthermore, after hydrogen charging the Cu–Ti or Cu– Hf samples were exposed to air for a prolonged time to desorb hydrogen. This allowed the second element to undergo oxidation thereby enhancing segregation phenomena. Figs. 1 and 2 show the effect of hydrogen charging on catalytic activity of Cu based amorphous alloys, as well as on their BET specific surface area estimated for the total weight of the ribbon. Although BET areas are not necessarily relevant to catalysis, the rather small values (a few m2/g for both systems) are indicative of a small concentration of active sites. It is interesting that for Cu–Hf both the specific surface area and conversion grow with hydrogen charging. At the same time, the Cu0 specific surface area of the working catalyst after 6 h-on-stream was one order of magnitude smaller than that after hydrogen charging. In contrast, BET areas are smaller and decreasing with hydrogen charging for Cu–Ti, nevertheless, the catalytic
Fig. 1. BET specific surface area (estimated with respect to a total weight of the modified ribbon) vs. time of hydrogen charging of Cu65–Hf35 amorphous alloy at i = 1 or 2 mA/cm2 in 0.1 M H2SO4. Corresponding conversion data are also given.
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Fig. 2. BET specific surface area (estimated with respect to a total weight of the modified ribbon) vs. time of cathodic hydrogen charging at i = 0.1 M H2SO of Cu–Ti amorphous alloys. Corresponding conversion data are also given.
activity is growing. It is important to emphasize again that for this sample surface Cu0 could not be detected, even after hydrogen treatment. It may be argued that the sensitivity and accuracy of N2O titration are not high enough to measure the surface concentration of Cu0 accurately. However, for Cu– Zr and Cu–Hf alloys and powders in earlier studies [20,26] activities could be correlated with Cu0 concentration measured by the same method. Furthermore, even if small amounts of undetected Cu0 are present on Cu–Ti this definitely cannot account for activities, which are comparable to those determined for Cu–Zr and Cu–Hf. Additional data acquired by surface analytical techniques are presented below to the further characterisation of the working catalysts. Inspection of the average pore diameter, however, reveals the following
1 mA/cm2 in
which is about 5% of the total thickness of the amorphous ribbon. Apparently, hydrogen concentration is maximised at the thin surface layer as hafnium hydride, which then blocks further entry of hydrogen into the bulk. This type of blocking effect by hydrogen is known for other metals [27]. Hydrogen charging, therefore, affects primarily the surface layer, which is relevant to catalysis. Naturally, the specific surface area and porosity also change locally; thus the corresponding ‘‘average’’ figures given in Fig. 1 should be, in fact, multiplied by a factor of about 20. A closer look at the surface of the hydrogen-treated samples with a high resolution Auger Microanalyser (Microlab 350) confirmed that a new surface developed upon cathodic hydrogen charging (Fig. 4). Local Auger spectra shown in Fig. 4b (P1) and Fig. 4c (P2) reveal that the
for Cu–Hf it grows from 2.6 to 6.8 nm with hydrogen charging time varying from 24 to 100 h, for Cu–Ti it oscillates between 8.6 and 7.2 nm for the hydrogen charging time varying from 27 to 96 h. It is clear that the catalytic activity of Cu–Hf increases with hydrogen charging time because of development of specific surface area and porosity. In contrast, it is still unclear what is the decisive factor responsible for an increase in catalytic activity of Cu–Ti, although a similar unique behaviour of the Cu–Ti system was already observed [21,26]. Fig. 3 shows a cross section of a Cu65–Hf55 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 for 74.5 h. While the average amount of hydrogen in the material was rather low (H/M = 0.8), the cross section reveals that its local content could have been large. The well visible devitrified layer is only 1 mm,
Fig. 3. A typical cross-section of Cu65–Hf35 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1M H2SO4 during 74.5 h. The modified surface layer is only about 1 mm thick, which corresponds to about 5% of the total thickness of the original Cu–Hf ribbon.
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Fig. 4. (a) SEM image showing typical morphology of Cu65–Hf35 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 100 h. Local survey Auger spectra at two points, as indicated in the SEM image; (b) substrate (P1) and (c) Cu island (P2).
clusters are of pure Cu. Copper is probably slightly oxidised, as O (510 eV) signal is well distinguishable in the spectrum. The substrate, on the other hand, is oxidized Hf. These results coincide with that found for Cu–Hf amorphous alloy aged in air, that is, the samples consist of Cu particles supported on HfO2. Apparently Hf hydride formation is only an intermediate step forcing Cu to migrate up to the surface and segregate. Then, the hydride decomposes and Hf undergoes oxidation. No Cu signal is visible there suggesting that HfO2 layer is sufficiently thick to quench Cu signals from the amorphous material underneath. Fig. 5 shows high-resolution Auger maps for Cu60–Ti40 amorphous alloy after hydrogen charging at i = 1 mA/cm2
in 0.1 M H2SO4 for 98 h. Here one can see a picture, which is completely different than that in Fig. 4. A rather flat Cu surface layer (Cu segregated at the surface during cathodic hydrogen charging) covers a part of the surface. This peculiar type of Cu segregation resulted in partial peeling off the Cu surface layer thus exposing the oxide support underneath. A closer look at both areas after tilting of the sample revealed a surprising picture (Fig. 6). The Cu surface layer (Fig. 6a, upper left) is all cracked and contains micro-voids, which may be easy path for hydrogen penetration during continuation of the cathodic pretreatment or during catalytic test reaction. The substrate image (Fig. 6b, upper right)
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Fig. 5. SEM image (upper left) and the corresponding Auger maps of Cu, Ti, and O for a Cu–Ti amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 98 h. Depth information is about a few monolayers. SEM image shows a segregated Cu layer (left) and a part of substrate on the surface where the Cu layer peeled off already (right). Auger maps show that the layer surface is Cu, partly oxidized (note some O signal in this region) while the substrate surface contains mostly Ti and O (titanium oxide).
reveals small clusters on the surface with diameters of 20–100 nm. It is, however, obvious that smaller ones, below the resolution limit of the Microlab 350, are also present there. Local Auger spectra, taken at the clusters and at the substrate, presented below the SEM images (Fig. 6c and d) reveal the following features: the clusters consist of Cu, O and some amount of Ti, the corresponding peak to peak heights ratios in the differential spectrum are Cu(LMM)/Ti(MNN) = 3.0, O(KLL)/Cu(LMM) + Ti(MNN) = 0.6, the background consists of Ti and O and a small amount of Cu; the corresponding peak to peak height ratios in the differentiated spectrum are Cu(LMM)/Ti(MNN) = 0.7, O(KLL)/Cu(LMM) + Ti(MNN) = 1.0 Thus, it is clear that all three components (Cu, Ti, O) at different proportions are present both on the surface and in
the clusters. The latter are enriched in Cu, which certainly is important for their catalytic efficiency. Moreover, the substrate is much more oxidized than the clusters. While Cud+ and Od centres play their role according to the previously proposed mechanism (Scheme 1, steps 1 and 2), the identity of the actual surface species in step 3 to split off a second hydrogen atom needs further consideration. In fact, the actual surface site responsible for the removal of this hydrogen was not discussed in earlier studies. The only exception is a work comparing the characteristics of Cu and ZnO in the dehydrogenation of ethanol [28]. Here, a Cu site is suggested to participate in the final step as depicted in Scheme 1. The unique behaviour of Cu–Ti suggests that ionic Ti centres may play a role in the final step of dehydrogenation. The complexity of the actual catalyst surface is a strong indication that a mechanism similar to that occurring on oxides requiring the close proximity of all three components may be operative here. The main feature
Scheme 2. Suggested mechanism of dehydrogenation of 2-propanol over Cu–Ti.
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Fig. 6. (a) High resolution SEM image of a Cu layer after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1M H2SO4 during 98 h. The image demonstrates details of a typical morphology with small cracks developed in the process of hydrogen charging. (b) High resolution SEM image of a typical morphology of a Cu layer support after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 98 h. The picture shows very small metal clusters on the oxide support. (c) Local spectra on a cluster (P1): before sputtering and after sputtering for 5s or 10s; the clusters are enriched in Cu, yet, some Ti and O are also detected there. (d) Local Auger spectra of a substrate (P2): before sputtering, after sputtering for 5s or 10s; the substrate is strongly enriched in Ti and O, but, yet some Cu is also detected there.
of this mechanism is the removal of the b hydrogen by a metal ion centre (Scheme 2), which is similar to an enolictype mechanism [1]. Then, the final product is formed as a result of a simultaneous hydride transfer as suggested in
[28]. Alternatively, a-hydrogen removal by a surface Ti site may also be conceivable. Both suggestions are based on the known tendency of Ti to form stable hydrides readily.
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4. Conclusion Interdisciplinary research of Cu–Hf and Cu–Ti catalysts modified by cathodic hydrogen charging provided a new insight into the catalytic mechanism of dehydrogenation of 2-propanol. Accordingly, a modification of the previously suggested mechanism is proposed.
Acknowledgements Surface characterizations were performed using Microlab 350 located at the Centre for Physical Chemistry of Materials of the Institute of Physical Chemistry, PAS and Faculty of Materials Science and Engineering, Warsaw University of Technology. This work was financially supported by grant KBN 4T08C 02524 and by the Institute of Physical Chemistry, Polish Academy of Sciences.
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Catalytic activity of Cu-based amorphous alloy ribbons modified by cathodic hydrogen charging ´ . Molna´rc,*, K. Hughesd M. Pisareka,b, M. Janik-Czachora, A b
a Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw, Poland Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw, Poland c Department of Organic Chemistry, University of Szeged, Do´m te´r 8, H-6720 Szeged, Hungary d Thermo Electron, East Grinsted, England
Received 11 October 2004; received in revised form 17 December 2004; accepted 4 January 2005 Available online 30 January 2005
Abstract Since hydrogen is known to be one of the most efficient embrittling atomic species, cathodic hydrogen charging was used in an attempt to modify the structure, composition, and morphology of Cu-based amorphous alloy ribbons. Various methods of analysis such as X-ray electron microanalysis, SEM, high resolution Auger microanalysis (SAM), with varying lateral resolution and different information depths, as well as measurements of specific surface area, porosity and catalytic tests, were used to follow the changes within the ribbons and at the surface, and their interrelations with catalytic activity. The activity in a test reaction (dehydrogenation of 2-propanol) was enhanced up to conversion levels of 66% for Cu–Ti and of 88% for Cu–Hf, which are much higher than those obtained with all other pre-treatments previously applied. The rather low specific surface area and porosity, as well as a lack of Cu0 both before and after the catalytic test suggest that the generally accepted mechanism for the dehydrogenation of alcohols over copper catalysts with the involvement of Cu0 is not operative here. # 2005 Elsevier B.V. All rights reserved. Keywords: Cu-based amorphous alloys; Devitrification; Cu segregation; Hydrogen charging; Catalytic dehydrogenation; SEM; Auger microanalysis (AES; SAM)
1. Introduction One of the important tasks of materials science is preparation and characterization of new functional materials including new catalysts for technically important reactions. In order to find efficient methods of activation of new materials for catalytic purposes, the mechanism of the catalytic reaction should be understood. Dehydrogenation of alcohols to produce the corresponding carbonyl compounds has attracted considerable attention [1]. Copper, either as pure metal or as supported catalyst has been shown to be highly selective in the process. Most experimental observations including the results of surface science studies [2] and data acquired by catalytic [3,4] and * Corresponding author. Tel.: +36 62 544 277; fax: +36 62 544 200. ´ . Molna´r). E-mail address: [email protected] (A 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.022
kinetic investigations [5,6] are in harmony with the so-called carbonyl mechanism depicted in Scheme 1 for 2-propanol. With respect to the surface active sites involved in the process the results are somewhat contradictory. The rate of dehydrogenation was found to be proportional to the concentration of exposed Cu0 sites [7–9]. In other studies, in contrast, the coexistence of metallic and oxidized copper was shown to be necessary for catalytic activity [10]. In fact, surface oxygen was found to greatly enhance the ability of Cu to dissociatively chemisorb methanol and ethanol in single crystal studies [11–13]. Both the latest results [6] and earlier observations [14] show that the rate-determining step in the dehydrogenation of secondary alcohols is the breaking of the O–H bond. Detection of surface alkoxide species on copper [2,11– 13,15,16] is considered to be a definitive proof that O–H bond breaking occurs prior to the breaking of a C–H bond.
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Scheme 1. Dehydrogenation of 2-propanol over Cu.
This is despite the fact that the a C–H bond in secondary alcohols is somewhat weaker than the O–H bond. It is supposed that this may be due to the favourable orientation of the hydroxyl group on the surface [6]. The catalytic action of the copper catalyst is a result of partial charge transfers between the various entities on the surface and in the adsorbed molecule according to the electronegativity of individual atoms. The carbonyl mechanism, in fact, was originally suggested to interpret dehydrogenation of alcohols on oxide catalysts such as Cr2O3 [1,17] and may be viewed as the weakening of the corresponding three bonds in the aliphatic alcohol through interactions with three surface entities Cu0, Cud+ and Od . However, our recent investigations suggest that the mechanism of the catalytic reaction can be different. This suggestion is based on the following experimental observations: we observed a stable or even increased conversion with time-on-stream, while the specific copper surface area (Cu0) was decreasing [18], we observed an appreciable, stable conversion of 88% over Cu–Hf [19], while the specific surface area of Cu0 was zero, activation of Cu + Ti powders by mechanical alloying followed by oxidation resulted in an improvement of otherwise poor catalytic performance, while the specific surface area (Cu0) was always negligible; the improvement of catalytic efficiency thus was not in harmony with Cu0 surface area data [20], our recent results [21] show that for Cu–Ti conversion reaches 66%, while the segregated Cu layer peels off, and only a minute amount of Cu is expected to be present on the surface, the presence of Cu is necessary to initiate the reaction, as TiO2 itself exhibits no catalytic activity for the reaction concerned. A more appropriate mechanism should involve interaction of the reacting molecule with an active site composed of Cu, Ti, and O surface entities. The aim of this work was to perform appropriate investigations to get further information with the aim of elucidating the most probable mechanism, which may be operative for these alloy systems. High-resolution electron microscopy and local chemical surface analysis are applied to get an insight into the reaction mechanism focusing on the properties of the surface involved in the catalytic process.
2. Experimental 2.1. Materials Amorphous alloy ribbons: Cu65–Hf35, Cu60–Ti40, 2– 4 mm wide and 40 mm thick, freshly cast, or partially devitrified, were used. 2.2. Electrochemical pretreatment and investigation Amorphous samples were hydrogen charged in order to increase their catalytic activity for the dehydrogenation of 2propanol. Cathodic charging in 0.5 M H2SO4 at i = constant was applied during time intervals from 24 to 100 h. Low current density (not higher than 2 mA/cm2) and prolonged, but, controlled charging time were applied to produce subtle effects. This procedure seemed to be most promising, as the investigations by Gebert and co-workers [22,23] show that even very small amounts of hydrogen, introduced at a current density as low as 1 mA/cm2, already produce distinct and well resolved crystallization, thus inducing segregation of small, but yet detectable amounts of the components of the pretreated amorphous alloys. The H/M ratio was determined by elemental analysis. Details are given elsewhere [19]. 2.3. Specific surface area (BET) and porosity measurements An ASAP 2010 sorptometer (Accelerated Surface Area and Porosimetry System, MICROMETRITICS) was used to estimate specific surface area and porosity from Ar adsorption isotherm. The cathodically modified samples were carefully degassed in vacuum at 300 8C before Ar adsorption, then cooled to room temperature. Eventually Ar was admitted to the chamber, and the isotherm was measured at room temperature. ASAP 2010 V4.00 D computer program was used for the estimation of BET specific surface areas and of BJH cumulative surface area of pores and an average pore diameter. All the corresponding data are presented with respect to the total weight of the modified ribbons. 2.4. Microscopic and surface analytical investigations The samples were examined with an optical microscope and a scanning electron microscope (HITACHI 3500N). A Microlab 350 (Thermo Electron) was used to determine the morphology and chemical composition of
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the surface, utilising high resolution SEM, Auger electron spectroscopy (AES) and Auger mapping functions of the Microlab. This equipment provides lateral resolution for AES mapping of 12 nm and depth information of a few monolayers. The following surface corrosion/devitrification products were analysed: (a) the surface oxide zone on both sides of the ribbon, (b) the morphology and composition of the segregated copper layer, the surface underneath the copper layer on the top of the ribbons modified due to hydrogen charging and a subsequent air exposure, (c) carefully polished cross sections of the surface zone of the ribbon. To obtain such a cross-section, a piece of ribbon was mounted into epoxy to protect the peculiar structure of the wheel side and then carefully polished to reveal the structure of the cross-section. 2.5. Catalytic test Dehydrogenation of 2-propanol was chosen to test the changes in the catalytic activity of the cathodically pretreated Cu-based ribbons. About 10 mg of the as received (aged) or electrochemically pre-treated ribbon samples was loaded into a glass micro reactor. 2-Propanol was fed by a micro feeder into a stainless steel evaporator, where it was mixed with hydrogen (99.999%). Then the gas mixture (2propanol/hydrogen = 0.018) was introduced into the reactor, which was maintained at 300 8C. The total flow rate was 10 ml min 1. The reactor temperature was controlled to an accuracy of 0.5 8C by using a microprocessor-based controller (Selftune plus, LOVE Controls). The effluents
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sampled by a pneumatic gas sampling system were analysed by GC (Shimadzu 8A equipment, thermal conductivity detector, CWAX 20 M column, 100 8C, 30 ml min 1 flow rate of hydrogen carrier gas). Calculations were made using a Data Apex Chromatography Station for Windows 1.5. The specific surface area of Cu0 was measured with the aid of N2O titration [24,25], based on the reaction of nitrous oxide with Cu0 species using the GC pulse method.
3. Results and discussion Our own recent results have shown that the procedure developed by Gebert and co-workers [19] was efficient in transforming Cu–Hf amorphous alloys into stable, efficient (90% conversion) and selective (95%) catalysts. Furthermore, after hydrogen charging the Cu–Ti or Cu– Hf samples were exposed to air for a prolonged time to desorb hydrogen. This allowed the second element to undergo oxidation thereby enhancing segregation phenomena. Figs. 1 and 2 show the effect of hydrogen charging on catalytic activity of Cu based amorphous alloys, as well as on their BET specific surface area estimated for the total weight of the ribbon. Although BET areas are not necessarily relevant to catalysis, the rather small values (a few m2/g for both systems) are indicative of a small concentration of active sites. It is interesting that for Cu–Hf both the specific surface area and conversion grow with hydrogen charging. At the same time, the Cu0 specific surface area of the working catalyst after 6 h-on-stream was one order of magnitude smaller than that after hydrogen charging. In contrast, BET areas are smaller and decreasing with hydrogen charging for Cu–Ti, nevertheless, the catalytic
Fig. 1. BET specific surface area (estimated with respect to a total weight of the modified ribbon) vs. time of hydrogen charging of Cu65–Hf35 amorphous alloy at i = 1 or 2 mA/cm2 in 0.1 M H2SO4. Corresponding conversion data are also given.
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Fig. 2. BET specific surface area (estimated with respect to a total weight of the modified ribbon) vs. time of cathodic hydrogen charging at i = 0.1 M H2SO of Cu–Ti amorphous alloys. Corresponding conversion data are also given.
activity is growing. It is important to emphasize again that for this sample surface Cu0 could not be detected, even after hydrogen treatment. It may be argued that the sensitivity and accuracy of N2O titration are not high enough to measure the surface concentration of Cu0 accurately. However, for Cu– Zr and Cu–Hf alloys and powders in earlier studies [20,26] activities could be correlated with Cu0 concentration measured by the same method. Furthermore, even if small amounts of undetected Cu0 are present on Cu–Ti this definitely cannot account for activities, which are comparable to those determined for Cu–Zr and Cu–Hf. Additional data acquired by surface analytical techniques are presented below to the further characterisation of the working catalysts. Inspection of the average pore diameter, however, reveals the following
1 mA/cm2 in
which is about 5% of the total thickness of the amorphous ribbon. Apparently, hydrogen concentration is maximised at the thin surface layer as hafnium hydride, which then blocks further entry of hydrogen into the bulk. This type of blocking effect by hydrogen is known for other metals [27]. Hydrogen charging, therefore, affects primarily the surface layer, which is relevant to catalysis. Naturally, the specific surface area and porosity also change locally; thus the corresponding ‘‘average’’ figures given in Fig. 1 should be, in fact, multiplied by a factor of about 20. A closer look at the surface of the hydrogen-treated samples with a high resolution Auger Microanalyser (Microlab 350) confirmed that a new surface developed upon cathodic hydrogen charging (Fig. 4). Local Auger spectra shown in Fig. 4b (P1) and Fig. 4c (P2) reveal that the
for Cu–Hf it grows from 2.6 to 6.8 nm with hydrogen charging time varying from 24 to 100 h, for Cu–Ti it oscillates between 8.6 and 7.2 nm for the hydrogen charging time varying from 27 to 96 h. It is clear that the catalytic activity of Cu–Hf increases with hydrogen charging time because of development of specific surface area and porosity. In contrast, it is still unclear what is the decisive factor responsible for an increase in catalytic activity of Cu–Ti, although a similar unique behaviour of the Cu–Ti system was already observed [21,26]. Fig. 3 shows a cross section of a Cu65–Hf55 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 for 74.5 h. While the average amount of hydrogen in the material was rather low (H/M = 0.8), the cross section reveals that its local content could have been large. The well visible devitrified layer is only 1 mm,
Fig. 3. A typical cross-section of Cu65–Hf35 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1M H2SO4 during 74.5 h. The modified surface layer is only about 1 mm thick, which corresponds to about 5% of the total thickness of the original Cu–Hf ribbon.
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Fig. 4. (a) SEM image showing typical morphology of Cu65–Hf35 amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 100 h. Local survey Auger spectra at two points, as indicated in the SEM image; (b) substrate (P1) and (c) Cu island (P2).
clusters are of pure Cu. Copper is probably slightly oxidised, as O (510 eV) signal is well distinguishable in the spectrum. The substrate, on the other hand, is oxidized Hf. These results coincide with that found for Cu–Hf amorphous alloy aged in air, that is, the samples consist of Cu particles supported on HfO2. Apparently Hf hydride formation is only an intermediate step forcing Cu to migrate up to the surface and segregate. Then, the hydride decomposes and Hf undergoes oxidation. No Cu signal is visible there suggesting that HfO2 layer is sufficiently thick to quench Cu signals from the amorphous material underneath. Fig. 5 shows high-resolution Auger maps for Cu60–Ti40 amorphous alloy after hydrogen charging at i = 1 mA/cm2
in 0.1 M H2SO4 for 98 h. Here one can see a picture, which is completely different than that in Fig. 4. A rather flat Cu surface layer (Cu segregated at the surface during cathodic hydrogen charging) covers a part of the surface. This peculiar type of Cu segregation resulted in partial peeling off the Cu surface layer thus exposing the oxide support underneath. A closer look at both areas after tilting of the sample revealed a surprising picture (Fig. 6). The Cu surface layer (Fig. 6a, upper left) is all cracked and contains micro-voids, which may be easy path for hydrogen penetration during continuation of the cathodic pretreatment or during catalytic test reaction. The substrate image (Fig. 6b, upper right)
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Fig. 5. SEM image (upper left) and the corresponding Auger maps of Cu, Ti, and O for a Cu–Ti amorphous alloy after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 98 h. Depth information is about a few monolayers. SEM image shows a segregated Cu layer (left) and a part of substrate on the surface where the Cu layer peeled off already (right). Auger maps show that the layer surface is Cu, partly oxidized (note some O signal in this region) while the substrate surface contains mostly Ti and O (titanium oxide).
reveals small clusters on the surface with diameters of 20–100 nm. It is, however, obvious that smaller ones, below the resolution limit of the Microlab 350, are also present there. Local Auger spectra, taken at the clusters and at the substrate, presented below the SEM images (Fig. 6c and d) reveal the following features: the clusters consist of Cu, O and some amount of Ti, the corresponding peak to peak heights ratios in the differential spectrum are Cu(LMM)/Ti(MNN) = 3.0, O(KLL)/Cu(LMM) + Ti(MNN) = 0.6, the background consists of Ti and O and a small amount of Cu; the corresponding peak to peak height ratios in the differentiated spectrum are Cu(LMM)/Ti(MNN) = 0.7, O(KLL)/Cu(LMM) + Ti(MNN) = 1.0 Thus, it is clear that all three components (Cu, Ti, O) at different proportions are present both on the surface and in
the clusters. The latter are enriched in Cu, which certainly is important for their catalytic efficiency. Moreover, the substrate is much more oxidized than the clusters. While Cud+ and Od centres play their role according to the previously proposed mechanism (Scheme 1, steps 1 and 2), the identity of the actual surface species in step 3 to split off a second hydrogen atom needs further consideration. In fact, the actual surface site responsible for the removal of this hydrogen was not discussed in earlier studies. The only exception is a work comparing the characteristics of Cu and ZnO in the dehydrogenation of ethanol [28]. Here, a Cu site is suggested to participate in the final step as depicted in Scheme 1. The unique behaviour of Cu–Ti suggests that ionic Ti centres may play a role in the final step of dehydrogenation. The complexity of the actual catalyst surface is a strong indication that a mechanism similar to that occurring on oxides requiring the close proximity of all three components may be operative here. The main feature
Scheme 2. Suggested mechanism of dehydrogenation of 2-propanol over Cu–Ti.
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Fig. 6. (a) High resolution SEM image of a Cu layer after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1M H2SO4 during 98 h. The image demonstrates details of a typical morphology with small cracks developed in the process of hydrogen charging. (b) High resolution SEM image of a typical morphology of a Cu layer support after cathodic hydrogen charging at i = 1 mA/cm2 in 0.1 M H2SO4 during 98 h. The picture shows very small metal clusters on the oxide support. (c) Local spectra on a cluster (P1): before sputtering and after sputtering for 5s or 10s; the clusters are enriched in Cu, yet, some Ti and O are also detected there. (d) Local Auger spectra of a substrate (P2): before sputtering, after sputtering for 5s or 10s; the substrate is strongly enriched in Ti and O, but, yet some Cu is also detected there.
of this mechanism is the removal of the b hydrogen by a metal ion centre (Scheme 2), which is similar to an enolictype mechanism [1]. Then, the final product is formed as a result of a simultaneous hydride transfer as suggested in
[28]. Alternatively, a-hydrogen removal by a surface Ti site may also be conceivable. Both suggestions are based on the known tendency of Ti to form stable hydrides readily.
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4. Conclusion Interdisciplinary research of Cu–Hf and Cu–Ti catalysts modified by cathodic hydrogen charging provided a new insight into the catalytic mechanism of dehydrogenation of 2-propanol. Accordingly, a modification of the previously suggested mechanism is proposed.
Acknowledgements Surface characterizations were performed using Microlab 350 located at the Centre for Physical Chemistry of Materials of the Institute of Physical Chemistry, PAS and Faculty of Materials Science and Engineering, Warsaw University of Technology. This work was financially supported by grant KBN 4T08C 02524 and by the Institute of Physical Chemistry, Polish Academy of Sciences.
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