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The use of titania- and iron oxide-supported gold catalysts for the hydrogenation of propyne
Applied Catalysis A: General 291 (2005) 230–237 www.elsevier.com/locate/apcata
The use of titania- and iron oxide-supported gold catalysts for the hydrogenation of propyne Jose Antonio Lopez-Sanchez, David Lennon * Department of Chemistry, Joseph Black Building, The University of Glasgow, Glasgow G12 8QQ, Scotland, UK Received 25 October 2004; received in revised form 21 January 2005; accepted 24 January 2005 Available online 4 June 2005
Abstract Propyne hydrogenation over titania-supported and iron oxide-supported gold catalysts has been investigated under pulse-flow conditions. The two catalysts exhibit different catalytic profiles. The Au/TiO2 catalyst displayed complete selectivity to propene and progressive deactivation, whereas Au/Fe2O3 exhibited selectivities and deactivation patterns dependent on ageing, catalyst pre-treatment and reaction temperature. These results suggest significant differences in the active surfaces of these two catalysts, due to the interaction of the gold metal with the support. The activity of gold catalysts can be modified by the selection of the oxide support and pre-treatment to produce catalysts completely selective to the partially hydrogenated product. # 2005 Elsevier B.V. All rights reserved.
1. Introduction In the last few years, gold catalysis has generated a great amount of interest [1]. This has been the result of two main findings: gold is the most active catalyst for ethyne hydrochlorination [2–4], and it is also effective for carbon monoxide oxidation at low temperatures [5,6]. Because of these findings and their industrial importance, the potential of gold catalysts for CO oxidation and selective oxidation reactions is now the subject of intensive research. As a consequence, progress has been made in the understanding of the effect of the many variables that govern catalytic activity in oxidation reactions, such as gold particle morphology and catalyst preparation method [1,7]. However, relatively little work has been carried out in assessing the applicability of gold in hydrogenation reactions and the variables that define the catalyst activity [1]. Gold catalysts have been reported to hydrogenate a range of olefins, such as ethene [8], propene [9,10], pent-1-ene [11], cyclohexene [12,13], 1,3-butadiene [14,15], ethyne [16,17], propyne [18], but-2-yne [19], phenylacetylene [20] and aromatic compounds such as toluene and naphthalene * Corresponding author. Tel.: +44 141 330 4372; fax: +44 141 330 4888. E-mail address: [email protected] (D. Lennon). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.048
[21]. Moreover, early work on gold films examined the hydrogenation of acetone to iso-propanol [22], although more recently, supported gold catalysts were reported to be inactive for the same reaction [23]. Favourable results are reported in the hydrogenation of a,b-unsaturated aldehydes to the corresponding unsaturated alcohol: acrolein (2propenal) [24–26] and crotonaldehyde (2-butenal) [15,27,28], as well as the selective hydrogenation of unsaturated ketones in the liquid phase [29,30]. These results are encouraging and suggest that gold catalysis could have application in a number of selective hydrogenation reactions, normally the domain of metals such as palladium or platinum [31]. Inspection of the gold hydrogenation literature indicates that there still remains some uncertainty on the catalyst specifications that favour hydrogenation activity. For example, in contrast to the particle size dependency established for oxidation reactions [7], Okumara et al. report the hydrogenation of 1,3-butadiene to be almost insensitive to the particle size and nature of the support [15]. This contradicts an earlier study by Buchanan and Webb that established strong support effects for the same reaction [14]. Other discrepancies arise in the study of crotonaldehyde hydrogenation with gold catalysts. For example, Bailie et al. proposed that sites active for the selective formation of
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crotyl alcohol on Au/ZnO catalysts were associated with large gold particles [27], whereas Zanella et al. observed favourable crotyl alcohol selectivities of ca. 60–70% for gold particles having diameters below 4 nm for a series of Au/TiO2 catalysts [28]. However, it is acknowledged that the different support materials could be playing a substantial role in perturbing the surface chemistry of the crotonaldehyde conversion in these two cases [28]. Against this background, it was decided to investigate a relatively simple hydrogenation reaction over some wellcharacterised supported gold catalysts. Recognising the success and benefits of the EuroPt-1 exercise [32], the World Gold Council (WGC) has recently made available a number of supported gold catalysts to provide standardisation in an area that has a range of catalyst preparative techniques available [33]. Examples of recent investigations on WGC reference catalysts are studies by Corma and co-workers [34] and Dumestic and co-workers [35] in CO oxidation reactions, and by Galvagno and co-workers [30] on hydrogenation reactions. Galvagno et al. found the reference catalyst Au/Fe2O3 to exhibit lower surface area, activity and selectivity than similar Au/Fe2O3 prepared in their laboratories in the liquid phase hydrogenation of a,bunsaturated ketones to a,b-unsaturated alcohols. This communication examines the hydrogenation of propyne over Au/TiO2 and Au/Fe2O3 catalysts, using pulse-flow methods. The Au/TiO2 and Au/Fe2O3 catalysts were prepared by deposition–precipitation and coprecipitation, respectively, and have been reported elsewhere [36,37]. Catalysts prepared by co-precipitation and deposition–precipitation contrast with catalysts utilised in the early hydrogenation studies where an impregnation method was used [19]. The newer preparative procedures are thought to yield narrower particle size distributions than those attainable via wet impregnation [1]. The support is also reported to play an important role in hydrogenation reactions [14,15,24,29,30], although this role might be dependent on the reagent, being for some reactions practically negligible [15,30]. This role can be the one providing a source of hydrogen to the metal sites [14], or interacting with gold to produce more selective electron-rich gold particles [24].
2. Experimental 2.1. Catalyst preparation The 1.5% Au/TiO2 (Lot No. 02-5) and 4.5% Au/Fe2O3 (Lot No. 02-3) catalysts were provided by the World Gold Council as reference catalysts The titania-supported catalyst was prepared by the deposition precipitation method using P-25 (Degussa) powder as the TiO2 support, whereas the Fe2O3-supported catalyst was prepared by co-precipitation from an aqueous solution. The detailed synthesis of these samples has been described elsewhere [36,37]. Character-
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isation data provided by the WGC indicate the gold content to be 1.5 and 4.5% for the titania and iron oxide-supported catalysts, respectively, as indicated by inductively coupled plasma analysis. Transmission electron microscopy (TEM) indicated both catalysts to have well dispersed gold particles; the 1.5% Au/TiO2 catalysts comprises of particles of 3.7 nm average diameter (standard variation 1.5 nm) and 4.5% Au/Fe2O3 3.7 nm (standard variation 0.93 nm). Inspection of the catalyst samples after the catalytic testing described below by transmission electron microscopy indicated no major changes in particle size distributions. 2.2. Reaction testing A pulse-flow microcatalytic reactor system was used throughout this study. The sample of catalyst was supported on a glass sinter in the centre of the reactor (8 mm i.d. down flow), which was placed inside a furnace. Catalyst temperatures were measured by means of a chromel– alumel thermocouple placed alongside the catalyst bed. The standard reduction/activation stage follows a similar reducing pre-treatment to that utilised by Zanella et al. [28]. Approximately 0.220 g of the catalyst sample is reduced in a 25% H2 in He flow (67 ml min 1) while the temperature is raised from ambient temperature to the desired temperature (typically 523 K) at 5 K min 1, then held for 2 h. Experiments showed the performance of Au/ Fe2O3 catalyst to be sensitive to this pre-treatment stage, and so pre-treatment temperatures of 523 and 663 K were examined. The sample was then set to the reaction temperature under a flow of helium. Pulses of reactant gas (78 mmol g 1) of a 3:1 hydrogen/propyne mixture were injected into the helium carrier gas (50 ml min 1), by passing the carrier flow through a sample loop (volume = 5.01 cm3) in the glass line. The sample loop is situated immediately above the catalyst and was previously filled with the desired amount of reactant mixture following a procedure described elsewhere [38]. Thus, each pulse corresponds to an incident propyne pulse of 19.5 mmol propyne g 1. On elution from the catalyst bed the full pulse was analysed by on-line gas chromatography, using a glass column packed with Porapak Type QS 80–100 and a thermal conductivity detector. The amount of gas adsorbed/reacted was determined from the difference between calibration peak areas and the peak areas obtained following injections of pulses of comparable size onto the catalyst. When tests were performed at several temperatures, temperature ramps were typically of 5 K min 1. Helium (BOC, 99% purity) and hydrogen (BOC, 99% purity) were purified using in-line deoxygenating and drying traps. The propyne (BDH, 96% purity) was purified through vacuum distillation prior to use. Propane and propene (Messer Griesheim, 99.95% purity) were used for gas chromatography calibration. Blank experiments performed on the empty reactor at 423, 523 and 623 K displayed no activity. Blank experiments of the catalyst’s
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support materials, titania (TiO2 P-25, Degussa) and ferric oxide (Fe2O3, 99.999%, Aldrich) were performed following the same catalyst pre-treatment and procedures described above. All experiments were performed at least in duplicate, with the figures presented being representative of those observed for the catalyst under the specified reaction conditions. In Section 3.2, a catalyst ageing process is described for the Au/Fe2O3 catalyst. All the figures presented correspond to the aged catalyst, whereas replicated results obtained upon immediate receipt of the WGC catalyst are also described.
3. Results A series of propyne conversion plots as a function of pulse number for Au/TiO2, Au/Fe2O3, and their corresponding supports are displayed in Fig. 1. Fig. 1 is complicated due to the requirement to consider the performance of the Au/Fe2O3 catalyst after different activation treatments. That aspect of the work is considered in Section 3.2. At increasing pulse number, propyne conversion values displayed in Fig. 1 [(number of propyne molecules converted) (number of propyne molecules in the feed) 1 100] indicate a decrease in activity for the Au/ TiO2 catalyst (a), whereas the Au/Fe2O3 catalyst displays a progressive increase approaching a constant conversion of ca. 40% (c). The catalysts clearly display different activity patterns. The pulse-flow technique permits observation of the first steps in the evolution of the catalyst performance, and this has permitted evaluation of carbon retention, evolution of selectivity, activity and deactivation. In this manner, the pulse-flow technique is complementary to testing under continuous flow conditions.
Fig. 1. Conversion plots for the first pulses of (a) Au/TiO2 (pre-treatment 523 K, test 423 K); (b) TiO2 (pre-treatment 523 K, test 423 K); (c) Au/ Fe2O3 (pre-treatment 523 K, test 523 K); (d) Au/Fe2O3 (pre-treatment 663 K, test 523 K); and (e) Fe2O3 (pre-treatment 663 K, test 523 K).
3.1. Au/TiO2 Firstly, catalytic testing of fresh Au/TiO2 samples carried out at several temperatures evaluated the preferred reaction temperature for this catalyst. Propene and unreacted propyne are identified as the only compounds emanating from the reactor at any given reaction temperature. This signifies that the catalyst is 100% selective to propene [(number of product molecules of propene) (number of converted educt molecules) 1 100]. The catalytic results displayed in Fig. 2 indicate 423 K to be an appropriate temperature for catalyst evaluation. Previous work by Buchanan and Webb indicated that temperatures in excess of 473 K were required for butadiene hydrogenation over alumina and silica-supported gold catalysts [14]. However, Fig. 2 reveals favourable quantities of propene to be formed at 393 K, with increasing temperature leading to a reduction in product formation beyond 423 K. Fig. 1(a) indicates that at 423 K 25% of molecules are converted at pulse 1, which then declines to ca. 10% at pulse 8. This signifies a substantial degree of catalyst deactivation over the pulse sequence studied. Fig. 2 also indicates progressive deactivation at all temperatures, although this reduction on consecutive pulsing is greatest at 393 K. Fig. 3 displays the product composition for testing performed over the Au/TiO2 catalyst at 423 K as a function of pulse number. The reduction in propene yield and increasing propyne breakthrough is readily apparent: pulse 1 yields 4.88 mmol propene g 1 whereas pulse 4 yields only 3.32 mmol propene g 1 (a reduction of 32%, or 0.52 mmol propene g 1 pulse 1) and diminishes to 1.95 mmol propene g 1 in pulse 8 (a total reduction of 60%, or 0.41 mmol propene g 1 pulse 1). In addition, the extent of propyne retention is seen to decline on increased pulsing. Seventy-five percent of
Fig. 2. Number of moles of propene in the product stream per gram of catalyst for tests performed at 398, 423, 523 and 623 K. Fresh catalyst was used for each reaction temperature studied. A number of consecutive pulses indicate substantial deactivation for all temperatures.
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Fig. 3. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over the Au/TiO2 catalyst at 423 K: (a) propene detected; (b) unreacted propyne; and (c) C3 units retained by the catalyst.
the incident propyne molecules are retained in pulse 1, however, this is reduced to 40% by pulses 7 and 8. The reduction in propene production does not directly correlate with the carbon retention and so two processes are thought to be occurring at the catalyst surface. Firstly, propyne is being hydrogenated to propene with an associated deactivation channel, which is causing a progressive reduction in propene yields (0.52 mmol propene g 1 pulse 1 in a four-pulse cycle). Secondly, the catalyst is retaining large quantities of propyne, the extent of which decreases on increasing exposure. Given the amounts of retained hydrocarbon involved (cumulative value for the eighth pulse is 86.7 mmol propyne g 1), the majority of this propyne is thought to be associated with the support material. In fact, this build up of propyne on the support is associated with a slow desorption stage that is not quantifiable within the pulse-flow technique. This progressive release does not appear as a pulse of propyne, rather it is seen as a small increase in time in the baseline of the GC trace. Confirmation of this phenomenon was made by in-line mass spectrometry (Leda Mass LM22) that detected small levels of propyne release for periods of up to 2 h after pulsing the catalyst with hydrogen:propyne mixtures. This propyne retention process is thought to reflect a weak adsorption of propyne on the catalyst support surface. Crucially, no propane is observed, making this catalyst 100% selective to formation of the alkene. This is consistent with a previous report of propyne hydrogenation over a highly dispersed Au/ZrO2 catalyst [7,18], and with previous work on acetylene hydrogenation [16]. In order to evaluate if higher temperatures beyond 500 K favoured conversion, a series of experiments were performed at 623 K. The first pulse yielded 1.65 mmol propene g 1, which reduced to 0.55 mmol propene g 1 for the second pulse and 0.4 mmol propene g 1 for the 10th pulse. These results signify modest activity at elevated temperatures.
The fact that propene formation is observed at temperatures as low as 393 K is initially surprising (Fig. 2), as previous work has established that gold will not activate molecular hydrogen at temperatures below 473 K [14,39]. Zanella et al. have considered the role of hydrogen supply for Au/TiO2 catalysts [28] and indicate hydrogen dissociation to be the rate determining step for crotonaldehyde hydrogenation. Furthermore, they assigned low coordinated gold atoms as being responsible for the dissociative adsorption of hydrogen. The Au/TiO2 studied here is well dispersed and it is possible that edge atoms on gold particles can provide sufficient hydrogen supply at temperatures lower than that accessible on catalysts containing larger particles. Moreover, it is also possible that these sites are being compromised by increasing propyne exposure forming hydrocarbonaceous overlayers [40], that ultimately leads to reduced conversion. It is also possible that the presence of electron donor defects in the TiO2 structure [24] is modified by the presence of the
Fig. 4. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over TiO2 at 423 K: (a) unreacted propyne and (b) C3 units retained by the catalyst.
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Fig. 5. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst at 423 K: (a) propene; (b) propane; and (c) C3 units retained by the catalyst. No propyne was detected in the eluant stream.
hydrocarbonaceous overlayer, that additionally result in deactivation of the catalyst. Replicate blank experiments were performed to evaluate the role of the support in the observed surface chemistry. Fig. 4 shows a representative outcome for the titania support treated in exactly the same manner as the Au/TiO2 catalyst. It is evident that the support has the capacity to retain substantial quantities of propyne. Furthermore, no reaction is apparent, with only propyne detected in the product stream. For completeness, we note that one of a series of four experimental blank runs actually produced some propene, but this outcome could not be repeated in the other three runs. This irreproducibility with the support indicates some variability in performance, which deserves further investigation. For example, Buchanan and Webb [14] and Sermon et al. [41] established that a Boehmite/g-Al2O3 support contributed to the hydrogenation activity reported over alumina-supported gold catalysts. However, with reference
to Fig. 4, no such role is apparent with the Au/TiO2 catalyst studied here. 3.2. Au/Fe2O3 The Au/Fe2O3 catalyst revealed different reaction characteristics to those observed for the Au/TiO2 catalyst. Initially, the catalyst was tested at a range of increasing temperatures indicating that temperatures higher than that observed for Au/TiO2 were more favourable (523 K). Superior activity than obtained with the Au/TiO2 catalyst was observed while retaining a 100% selectivity towards propene at temperatures up to 623 K. Furthermore, similar carbon retention and deactivation features were observed. However, a series of repeat measurements performed four months later showed the catalyst to display a considerable ageing effect. The results for the aged catalyst are presented above in Fig. 5 at a reaction temperature of 423 K.
Fig. 6. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst at 523 K: (a) propene evolution; and (b) propyne retention. No propyne was detected in the eluant stream.
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Fig. 7. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst pre-treated at 663 K. Regions A, B and C are distinguished on the basis of reaction temperature (523, 423 and 523 K, respectively). Key: (a) propene; (b) propyne; and (c) C3 units retained by the catalyst. No propane was detected in the eluant stream in any case.
Firstly, exclusive selectivity to propene is lost, with propane and propene produced at changing amounts as the pulse number increases: the amount of propene produced increases while propane decreases. These results indicate a conditioning process to be taking place, where activity and selectivity towards propene steadily increases up to pulse 7. A second sample was then tested for propyne hydrogenation at 523 K. The results shown in Fig. 6 indicate that 100% selectivity is now achieved. Still, after ten pulses, no propyne is detected. However, when the reaction temperature was returned to 423 K (results not shown) propane was formed (47%) together with propene (53%). These observations indicate that the catalyst surface is sensitive to reaction conditions and that higher temperatures are required to stabilise the active sites to selectively hydrogenate propyne to propene.
Aiming to obtain a catalyst with invariable 100% selectivity to propene, a third sample was pre-treated using the previously described procedure but at an elevated temperature of 663 K, rather than the 523 K used for the runs presented in Figs. 5 and 6. Fig. 7 shows the evolution of the catalyst activity for propyne hydrogenation at 523 and 423 K. At a reaction temperature of 523 K (region A in Fig. 7) the catalyst displays similar activity to that observed when the catalyst was pre-treated at 523 K (Fig. 6). However, in this case no induction process is apparent, and the catalyst effectively displays the same activity from pulse 2 to pulse 14. In order to evaluate the stability of this surface, the reaction temperature was decreased to 423 K (region B in Fig. 7). At this reaction temperature increasing amounts of unreacted propyne are detected, activity is significantly
Fig. 8. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over Fe2O3 (pre-treated at 663 K) at a reaction temperature of 523 K. (a) propene detected; (b) unreacted propyne; and (c) C3 units retained by the catalyst.
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reduced and a deactivation process is accessed. However, the catalyst retains 100% selectivity to propene, in contrast to the outcome when an activation temperature of 523 K is used (Fig. 6 and text). It is apparent that this additional heat treatment has produced a surface of similar catalytic properties to that observed before the ageing process. In region C the reaction temperature was returned to 523 K, and the catalyst immediately recovered the same activity characteristics as seen in region A. These results indicate that the effects of ageing can be reversed, and is a further indication of a dynamic surface, where the thermal history can have a dramatic effect on catalytic performance. Such trends resemble characteristics of a strong metal-support interaction [42]. Following on from these observations, two samples of Au/TiO2 were pre-treated at 663 K and tested at 423 and 523 K, respectively. In both cases, the samples selectivity, activity and deactivation rate were the same as those pretreated at 523 K (Figs. 1–3), indicating the Au/TiO2 to be stable, and in contrast to the Au/Fe2O3, its hydrogenation performance to be relatively insensitive to the sample pretreatment in the range of temperatures studied. It is known that iron oxide can undergo several phase transformations, sometimes involving a topotactic transformation at low temperatures (e.g. formation of hematite at 473 K by thermal dehydration of goethite) [43]. The changes in the catalytic properties of Au/Fe2O3 due to ageing and thermal treatment might be correlated with the transformations of the iron oxide crystalline phases present in the surface layer. The equilibrium between one and other of these phases could then be responsible for a variable catalytic performance (metastable phase?), with different catalytic behaviour reflecting the interaction of gold and the iron oxide support. Mo¨ ssbauer spectroscopy has previously displayed differences in the relative amounts of iron oxide phases (ferrihydrite, haematite and goethite) in iron oxidesupported gold catalysts depending on the preparation method utilised, that could be correlated with differences in catalytic activity for CO oxidation [44]. As with Au/TiO2, the cumulative propyne retained by the Au/Fe2O3 is substantial. This quantity most likely exceeds the gold active site density (unknown), with the support material thought to be acting as a reservoir. The propyne retention profiles for the Au/TiO2 catalyst (Fig. 3) differs from that seen for the Au/Fe2O3 (Figs. 5 and 7). Given that the gold crystallites are of comparable particle size in the two catalysts, these differences in quantities of propene produced and propyne retention profiles indicate that the support is indeed perturbing the gold-based chemistry. To consider the effect of the support material in the observed trends for the Au/Fe2O3 catalyst, blank experiments were performed on a Fe2O3 sample after the same pretreatment carried out for the gold catalyst. These experiments produced essentially the same results independently of whether Fe2O3 was pre-treated at 523 K or at 663 K. A representative reaction profile for these experiments is
presented in Fig. 8, which shows the effect of pulsing the hydrogen/propyne mixture at 523 K. Minimal propene formation is observed (0.54 mmol propene g 1 for the first pulse), which slowly decreases to zero at increasing pulse numbers. This indicates the support activity to have a minimal direct role in the chemistry seen for the Au/Fe2O3 catalyst. A negative carbon mass balance indicates that the iron oxide surface is able to retain substantial amounts of propyne, although propyne is detected in increasing amounts with increasing pulse number. It is worthwhile comparing the optimum specific activities of both catalysts. The 1.5% Au/TiO2 (pre-treated at 523 K) produced 325 mmol propene gAu1 at 423 K (pulse 1, Fig. 3), whereas the 4.5% Au/Fe2O3 (pre-treated at 663 K) produced 182 mmol propene gAu1 at 523 K (pulse 10, Fig. 7). Thus, this analysis indicates the Au/TiO2 to have a higher specific activity than the Au/Fe2O3. However, in using specific activities, one has to be aware of the different preparative procedures used. Au/TiO2 and Au/Fe2O3 were prepared by deposition–precipitation and coprecipitation respectively, therefore, it is expected that the concentration of gold on the surface of the two supports will be different. In deposition–precipitation gold is deposited on the surface of the TiO2 support, whereas gold should be homogeneously distributed in both the surface and bulk of the iron oxide support [1]. Furthermore, the metal surface area is unknown. Despite these limitations, and given the fact that these catalysts have comparable particle sizes, the observations made on the different catalytic performances suggest different support interactions. As Figs. 1, 3 and 8 show the isolated supports to have zero (TiO2) or negligible (Fe2O3) hydrogenation activity, then this contribution is thought to occur at the Au/support interface.
4. Summary and conlusions Two supported gold catalysts have been investigated for the hydrogenation of propyne. The Au/TiO2 catalyst (mean particle size 3.7 1.50 nm) prepared by the deposition–precipitation method was 100% selective towards propene production but was additionally characterised by deactivation on increased pulsing of the reaction mixture and substantial propyne retention, presumably by the support material. The as received Au/Fe2O3 catalyst (mean particle size 3.7 0.93 nm) prepared by co-precipitation, initially exhibited broadly similar behaviour to the titaniasupported catalyst (100% propene selectivity, deactivation, propyne retention by the support material). However, the catalytic properties of the sample changed in time (4 months) where a significant reduction in propene selectivity and the absence of deactivation were observed. Higher reaction temperatures were necessary to recover
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the propene selectivity. However, pre-treatment at a higher temperature of 663 K ensured 100% selectivity at low and high reaction temperatures. The two gold catalysts exhibited optimum propene formation at two different reaction temperatures: 423 K for Au/TiO2, and 523 K for Au/Fe2O3. Differences in the two temperature profiles are thought to reflect differences in the metal-support interaction. This work underlines the ability of gold catalysts to selectively hydrogenate propyne. The catalyst support, pretreatment temperature, and reaction temperature must be carefully selected to provide a catalyst with suitable selectivity and to minimise deactivation. For the Au/ Fe2O3 catalyst, the ageing of the sample was also shown to be a major issue, for which further investigations are required. The ageing effects and the variable selectivity observed indicate an intricate catalytic system.
Acknowledgements The authors are grateful for financial support from the Auricat EU network (HPRN-CT-2002-00174). The World Gold Council is thanked for provision of the Au/TiO2 and Au/Fe2O3 catalysts.
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The use of titania- and iron oxide-supported gold catalysts for the hydrogenation of propyne Jose Antonio Lopez-Sanchez, David Lennon * Department of Chemistry, Joseph Black Building, The University of Glasgow, Glasgow G12 8QQ, Scotland, UK Received 25 October 2004; received in revised form 21 January 2005; accepted 24 January 2005 Available online 4 June 2005
Abstract Propyne hydrogenation over titania-supported and iron oxide-supported gold catalysts has been investigated under pulse-flow conditions. The two catalysts exhibit different catalytic profiles. The Au/TiO2 catalyst displayed complete selectivity to propene and progressive deactivation, whereas Au/Fe2O3 exhibited selectivities and deactivation patterns dependent on ageing, catalyst pre-treatment and reaction temperature. These results suggest significant differences in the active surfaces of these two catalysts, due to the interaction of the gold metal with the support. The activity of gold catalysts can be modified by the selection of the oxide support and pre-treatment to produce catalysts completely selective to the partially hydrogenated product. # 2005 Elsevier B.V. All rights reserved.
1. Introduction In the last few years, gold catalysis has generated a great amount of interest [1]. This has been the result of two main findings: gold is the most active catalyst for ethyne hydrochlorination [2–4], and it is also effective for carbon monoxide oxidation at low temperatures [5,6]. Because of these findings and their industrial importance, the potential of gold catalysts for CO oxidation and selective oxidation reactions is now the subject of intensive research. As a consequence, progress has been made in the understanding of the effect of the many variables that govern catalytic activity in oxidation reactions, such as gold particle morphology and catalyst preparation method [1,7]. However, relatively little work has been carried out in assessing the applicability of gold in hydrogenation reactions and the variables that define the catalyst activity [1]. Gold catalysts have been reported to hydrogenate a range of olefins, such as ethene [8], propene [9,10], pent-1-ene [11], cyclohexene [12,13], 1,3-butadiene [14,15], ethyne [16,17], propyne [18], but-2-yne [19], phenylacetylene [20] and aromatic compounds such as toluene and naphthalene * Corresponding author. Tel.: +44 141 330 4372; fax: +44 141 330 4888. E-mail address: [email protected] (D. Lennon). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.048
[21]. Moreover, early work on gold films examined the hydrogenation of acetone to iso-propanol [22], although more recently, supported gold catalysts were reported to be inactive for the same reaction [23]. Favourable results are reported in the hydrogenation of a,b-unsaturated aldehydes to the corresponding unsaturated alcohol: acrolein (2propenal) [24–26] and crotonaldehyde (2-butenal) [15,27,28], as well as the selective hydrogenation of unsaturated ketones in the liquid phase [29,30]. These results are encouraging and suggest that gold catalysis could have application in a number of selective hydrogenation reactions, normally the domain of metals such as palladium or platinum [31]. Inspection of the gold hydrogenation literature indicates that there still remains some uncertainty on the catalyst specifications that favour hydrogenation activity. For example, in contrast to the particle size dependency established for oxidation reactions [7], Okumara et al. report the hydrogenation of 1,3-butadiene to be almost insensitive to the particle size and nature of the support [15]. This contradicts an earlier study by Buchanan and Webb that established strong support effects for the same reaction [14]. Other discrepancies arise in the study of crotonaldehyde hydrogenation with gold catalysts. For example, Bailie et al. proposed that sites active for the selective formation of
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crotyl alcohol on Au/ZnO catalysts were associated with large gold particles [27], whereas Zanella et al. observed favourable crotyl alcohol selectivities of ca. 60–70% for gold particles having diameters below 4 nm for a series of Au/TiO2 catalysts [28]. However, it is acknowledged that the different support materials could be playing a substantial role in perturbing the surface chemistry of the crotonaldehyde conversion in these two cases [28]. Against this background, it was decided to investigate a relatively simple hydrogenation reaction over some wellcharacterised supported gold catalysts. Recognising the success and benefits of the EuroPt-1 exercise [32], the World Gold Council (WGC) has recently made available a number of supported gold catalysts to provide standardisation in an area that has a range of catalyst preparative techniques available [33]. Examples of recent investigations on WGC reference catalysts are studies by Corma and co-workers [34] and Dumestic and co-workers [35] in CO oxidation reactions, and by Galvagno and co-workers [30] on hydrogenation reactions. Galvagno et al. found the reference catalyst Au/Fe2O3 to exhibit lower surface area, activity and selectivity than similar Au/Fe2O3 prepared in their laboratories in the liquid phase hydrogenation of a,bunsaturated ketones to a,b-unsaturated alcohols. This communication examines the hydrogenation of propyne over Au/TiO2 and Au/Fe2O3 catalysts, using pulse-flow methods. The Au/TiO2 and Au/Fe2O3 catalysts were prepared by deposition–precipitation and coprecipitation, respectively, and have been reported elsewhere [36,37]. Catalysts prepared by co-precipitation and deposition–precipitation contrast with catalysts utilised in the early hydrogenation studies where an impregnation method was used [19]. The newer preparative procedures are thought to yield narrower particle size distributions than those attainable via wet impregnation [1]. The support is also reported to play an important role in hydrogenation reactions [14,15,24,29,30], although this role might be dependent on the reagent, being for some reactions practically negligible [15,30]. This role can be the one providing a source of hydrogen to the metal sites [14], or interacting with gold to produce more selective electron-rich gold particles [24].
2. Experimental 2.1. Catalyst preparation The 1.5% Au/TiO2 (Lot No. 02-5) and 4.5% Au/Fe2O3 (Lot No. 02-3) catalysts were provided by the World Gold Council as reference catalysts The titania-supported catalyst was prepared by the deposition precipitation method using P-25 (Degussa) powder as the TiO2 support, whereas the Fe2O3-supported catalyst was prepared by co-precipitation from an aqueous solution. The detailed synthesis of these samples has been described elsewhere [36,37]. Character-
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isation data provided by the WGC indicate the gold content to be 1.5 and 4.5% for the titania and iron oxide-supported catalysts, respectively, as indicated by inductively coupled plasma analysis. Transmission electron microscopy (TEM) indicated both catalysts to have well dispersed gold particles; the 1.5% Au/TiO2 catalysts comprises of particles of 3.7 nm average diameter (standard variation 1.5 nm) and 4.5% Au/Fe2O3 3.7 nm (standard variation 0.93 nm). Inspection of the catalyst samples after the catalytic testing described below by transmission electron microscopy indicated no major changes in particle size distributions. 2.2. Reaction testing A pulse-flow microcatalytic reactor system was used throughout this study. The sample of catalyst was supported on a glass sinter in the centre of the reactor (8 mm i.d. down flow), which was placed inside a furnace. Catalyst temperatures were measured by means of a chromel– alumel thermocouple placed alongside the catalyst bed. The standard reduction/activation stage follows a similar reducing pre-treatment to that utilised by Zanella et al. [28]. Approximately 0.220 g of the catalyst sample is reduced in a 25% H2 in He flow (67 ml min 1) while the temperature is raised from ambient temperature to the desired temperature (typically 523 K) at 5 K min 1, then held for 2 h. Experiments showed the performance of Au/ Fe2O3 catalyst to be sensitive to this pre-treatment stage, and so pre-treatment temperatures of 523 and 663 K were examined. The sample was then set to the reaction temperature under a flow of helium. Pulses of reactant gas (78 mmol g 1) of a 3:1 hydrogen/propyne mixture were injected into the helium carrier gas (50 ml min 1), by passing the carrier flow through a sample loop (volume = 5.01 cm3) in the glass line. The sample loop is situated immediately above the catalyst and was previously filled with the desired amount of reactant mixture following a procedure described elsewhere [38]. Thus, each pulse corresponds to an incident propyne pulse of 19.5 mmol propyne g 1. On elution from the catalyst bed the full pulse was analysed by on-line gas chromatography, using a glass column packed with Porapak Type QS 80–100 and a thermal conductivity detector. The amount of gas adsorbed/reacted was determined from the difference between calibration peak areas and the peak areas obtained following injections of pulses of comparable size onto the catalyst. When tests were performed at several temperatures, temperature ramps were typically of 5 K min 1. Helium (BOC, 99% purity) and hydrogen (BOC, 99% purity) were purified using in-line deoxygenating and drying traps. The propyne (BDH, 96% purity) was purified through vacuum distillation prior to use. Propane and propene (Messer Griesheim, 99.95% purity) were used for gas chromatography calibration. Blank experiments performed on the empty reactor at 423, 523 and 623 K displayed no activity. Blank experiments of the catalyst’s
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support materials, titania (TiO2 P-25, Degussa) and ferric oxide (Fe2O3, 99.999%, Aldrich) were performed following the same catalyst pre-treatment and procedures described above. All experiments were performed at least in duplicate, with the figures presented being representative of those observed for the catalyst under the specified reaction conditions. In Section 3.2, a catalyst ageing process is described for the Au/Fe2O3 catalyst. All the figures presented correspond to the aged catalyst, whereas replicated results obtained upon immediate receipt of the WGC catalyst are also described.
3. Results A series of propyne conversion plots as a function of pulse number for Au/TiO2, Au/Fe2O3, and their corresponding supports are displayed in Fig. 1. Fig. 1 is complicated due to the requirement to consider the performance of the Au/Fe2O3 catalyst after different activation treatments. That aspect of the work is considered in Section 3.2. At increasing pulse number, propyne conversion values displayed in Fig. 1 [(number of propyne molecules converted) (number of propyne molecules in the feed) 1 100] indicate a decrease in activity for the Au/ TiO2 catalyst (a), whereas the Au/Fe2O3 catalyst displays a progressive increase approaching a constant conversion of ca. 40% (c). The catalysts clearly display different activity patterns. The pulse-flow technique permits observation of the first steps in the evolution of the catalyst performance, and this has permitted evaluation of carbon retention, evolution of selectivity, activity and deactivation. In this manner, the pulse-flow technique is complementary to testing under continuous flow conditions.
Fig. 1. Conversion plots for the first pulses of (a) Au/TiO2 (pre-treatment 523 K, test 423 K); (b) TiO2 (pre-treatment 523 K, test 423 K); (c) Au/ Fe2O3 (pre-treatment 523 K, test 523 K); (d) Au/Fe2O3 (pre-treatment 663 K, test 523 K); and (e) Fe2O3 (pre-treatment 663 K, test 523 K).
3.1. Au/TiO2 Firstly, catalytic testing of fresh Au/TiO2 samples carried out at several temperatures evaluated the preferred reaction temperature for this catalyst. Propene and unreacted propyne are identified as the only compounds emanating from the reactor at any given reaction temperature. This signifies that the catalyst is 100% selective to propene [(number of product molecules of propene) (number of converted educt molecules) 1 100]. The catalytic results displayed in Fig. 2 indicate 423 K to be an appropriate temperature for catalyst evaluation. Previous work by Buchanan and Webb indicated that temperatures in excess of 473 K were required for butadiene hydrogenation over alumina and silica-supported gold catalysts [14]. However, Fig. 2 reveals favourable quantities of propene to be formed at 393 K, with increasing temperature leading to a reduction in product formation beyond 423 K. Fig. 1(a) indicates that at 423 K 25% of molecules are converted at pulse 1, which then declines to ca. 10% at pulse 8. This signifies a substantial degree of catalyst deactivation over the pulse sequence studied. Fig. 2 also indicates progressive deactivation at all temperatures, although this reduction on consecutive pulsing is greatest at 393 K. Fig. 3 displays the product composition for testing performed over the Au/TiO2 catalyst at 423 K as a function of pulse number. The reduction in propene yield and increasing propyne breakthrough is readily apparent: pulse 1 yields 4.88 mmol propene g 1 whereas pulse 4 yields only 3.32 mmol propene g 1 (a reduction of 32%, or 0.52 mmol propene g 1 pulse 1) and diminishes to 1.95 mmol propene g 1 in pulse 8 (a total reduction of 60%, or 0.41 mmol propene g 1 pulse 1). In addition, the extent of propyne retention is seen to decline on increased pulsing. Seventy-five percent of
Fig. 2. Number of moles of propene in the product stream per gram of catalyst for tests performed at 398, 423, 523 and 623 K. Fresh catalyst was used for each reaction temperature studied. A number of consecutive pulses indicate substantial deactivation for all temperatures.
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Fig. 3. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over the Au/TiO2 catalyst at 423 K: (a) propene detected; (b) unreacted propyne; and (c) C3 units retained by the catalyst.
the incident propyne molecules are retained in pulse 1, however, this is reduced to 40% by pulses 7 and 8. The reduction in propene production does not directly correlate with the carbon retention and so two processes are thought to be occurring at the catalyst surface. Firstly, propyne is being hydrogenated to propene with an associated deactivation channel, which is causing a progressive reduction in propene yields (0.52 mmol propene g 1 pulse 1 in a four-pulse cycle). Secondly, the catalyst is retaining large quantities of propyne, the extent of which decreases on increasing exposure. Given the amounts of retained hydrocarbon involved (cumulative value for the eighth pulse is 86.7 mmol propyne g 1), the majority of this propyne is thought to be associated with the support material. In fact, this build up of propyne on the support is associated with a slow desorption stage that is not quantifiable within the pulse-flow technique. This progressive release does not appear as a pulse of propyne, rather it is seen as a small increase in time in the baseline of the GC trace. Confirmation of this phenomenon was made by in-line mass spectrometry (Leda Mass LM22) that detected small levels of propyne release for periods of up to 2 h after pulsing the catalyst with hydrogen:propyne mixtures. This propyne retention process is thought to reflect a weak adsorption of propyne on the catalyst support surface. Crucially, no propane is observed, making this catalyst 100% selective to formation of the alkene. This is consistent with a previous report of propyne hydrogenation over a highly dispersed Au/ZrO2 catalyst [7,18], and with previous work on acetylene hydrogenation [16]. In order to evaluate if higher temperatures beyond 500 K favoured conversion, a series of experiments were performed at 623 K. The first pulse yielded 1.65 mmol propene g 1, which reduced to 0.55 mmol propene g 1 for the second pulse and 0.4 mmol propene g 1 for the 10th pulse. These results signify modest activity at elevated temperatures.
The fact that propene formation is observed at temperatures as low as 393 K is initially surprising (Fig. 2), as previous work has established that gold will not activate molecular hydrogen at temperatures below 473 K [14,39]. Zanella et al. have considered the role of hydrogen supply for Au/TiO2 catalysts [28] and indicate hydrogen dissociation to be the rate determining step for crotonaldehyde hydrogenation. Furthermore, they assigned low coordinated gold atoms as being responsible for the dissociative adsorption of hydrogen. The Au/TiO2 studied here is well dispersed and it is possible that edge atoms on gold particles can provide sufficient hydrogen supply at temperatures lower than that accessible on catalysts containing larger particles. Moreover, it is also possible that these sites are being compromised by increasing propyne exposure forming hydrocarbonaceous overlayers [40], that ultimately leads to reduced conversion. It is also possible that the presence of electron donor defects in the TiO2 structure [24] is modified by the presence of the
Fig. 4. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over TiO2 at 423 K: (a) unreacted propyne and (b) C3 units retained by the catalyst.
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Fig. 5. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst at 423 K: (a) propene; (b) propane; and (c) C3 units retained by the catalyst. No propyne was detected in the eluant stream.
hydrocarbonaceous overlayer, that additionally result in deactivation of the catalyst. Replicate blank experiments were performed to evaluate the role of the support in the observed surface chemistry. Fig. 4 shows a representative outcome for the titania support treated in exactly the same manner as the Au/TiO2 catalyst. It is evident that the support has the capacity to retain substantial quantities of propyne. Furthermore, no reaction is apparent, with only propyne detected in the product stream. For completeness, we note that one of a series of four experimental blank runs actually produced some propene, but this outcome could not be repeated in the other three runs. This irreproducibility with the support indicates some variability in performance, which deserves further investigation. For example, Buchanan and Webb [14] and Sermon et al. [41] established that a Boehmite/g-Al2O3 support contributed to the hydrogenation activity reported over alumina-supported gold catalysts. However, with reference
to Fig. 4, no such role is apparent with the Au/TiO2 catalyst studied here. 3.2. Au/Fe2O3 The Au/Fe2O3 catalyst revealed different reaction characteristics to those observed for the Au/TiO2 catalyst. Initially, the catalyst was tested at a range of increasing temperatures indicating that temperatures higher than that observed for Au/TiO2 were more favourable (523 K). Superior activity than obtained with the Au/TiO2 catalyst was observed while retaining a 100% selectivity towards propene at temperatures up to 623 K. Furthermore, similar carbon retention and deactivation features were observed. However, a series of repeat measurements performed four months later showed the catalyst to display a considerable ageing effect. The results for the aged catalyst are presented above in Fig. 5 at a reaction temperature of 423 K.
Fig. 6. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst at 523 K: (a) propene evolution; and (b) propyne retention. No propyne was detected in the eluant stream.
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Fig. 7. Carbon mass balance for propyne hydrogenation over the Au/Fe2O3 catalyst pre-treated at 663 K. Regions A, B and C are distinguished on the basis of reaction temperature (523, 423 and 523 K, respectively). Key: (a) propene; (b) propyne; and (c) C3 units retained by the catalyst. No propane was detected in the eluant stream in any case.
Firstly, exclusive selectivity to propene is lost, with propane and propene produced at changing amounts as the pulse number increases: the amount of propene produced increases while propane decreases. These results indicate a conditioning process to be taking place, where activity and selectivity towards propene steadily increases up to pulse 7. A second sample was then tested for propyne hydrogenation at 523 K. The results shown in Fig. 6 indicate that 100% selectivity is now achieved. Still, after ten pulses, no propyne is detected. However, when the reaction temperature was returned to 423 K (results not shown) propane was formed (47%) together with propene (53%). These observations indicate that the catalyst surface is sensitive to reaction conditions and that higher temperatures are required to stabilise the active sites to selectively hydrogenate propyne to propene.
Aiming to obtain a catalyst with invariable 100% selectivity to propene, a third sample was pre-treated using the previously described procedure but at an elevated temperature of 663 K, rather than the 523 K used for the runs presented in Figs. 5 and 6. Fig. 7 shows the evolution of the catalyst activity for propyne hydrogenation at 523 and 423 K. At a reaction temperature of 523 K (region A in Fig. 7) the catalyst displays similar activity to that observed when the catalyst was pre-treated at 523 K (Fig. 6). However, in this case no induction process is apparent, and the catalyst effectively displays the same activity from pulse 2 to pulse 14. In order to evaluate the stability of this surface, the reaction temperature was decreased to 423 K (region B in Fig. 7). At this reaction temperature increasing amounts of unreacted propyne are detected, activity is significantly
Fig. 8. Carbon mass balance plot for successive pulsing of a 3:1 mixture of hydrogen and propyne over Fe2O3 (pre-treated at 663 K) at a reaction temperature of 523 K. (a) propene detected; (b) unreacted propyne; and (c) C3 units retained by the catalyst.
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reduced and a deactivation process is accessed. However, the catalyst retains 100% selectivity to propene, in contrast to the outcome when an activation temperature of 523 K is used (Fig. 6 and text). It is apparent that this additional heat treatment has produced a surface of similar catalytic properties to that observed before the ageing process. In region C the reaction temperature was returned to 523 K, and the catalyst immediately recovered the same activity characteristics as seen in region A. These results indicate that the effects of ageing can be reversed, and is a further indication of a dynamic surface, where the thermal history can have a dramatic effect on catalytic performance. Such trends resemble characteristics of a strong metal-support interaction [42]. Following on from these observations, two samples of Au/TiO2 were pre-treated at 663 K and tested at 423 and 523 K, respectively. In both cases, the samples selectivity, activity and deactivation rate were the same as those pretreated at 523 K (Figs. 1–3), indicating the Au/TiO2 to be stable, and in contrast to the Au/Fe2O3, its hydrogenation performance to be relatively insensitive to the sample pretreatment in the range of temperatures studied. It is known that iron oxide can undergo several phase transformations, sometimes involving a topotactic transformation at low temperatures (e.g. formation of hematite at 473 K by thermal dehydration of goethite) [43]. The changes in the catalytic properties of Au/Fe2O3 due to ageing and thermal treatment might be correlated with the transformations of the iron oxide crystalline phases present in the surface layer. The equilibrium between one and other of these phases could then be responsible for a variable catalytic performance (metastable phase?), with different catalytic behaviour reflecting the interaction of gold and the iron oxide support. Mo¨ ssbauer spectroscopy has previously displayed differences in the relative amounts of iron oxide phases (ferrihydrite, haematite and goethite) in iron oxidesupported gold catalysts depending on the preparation method utilised, that could be correlated with differences in catalytic activity for CO oxidation [44]. As with Au/TiO2, the cumulative propyne retained by the Au/Fe2O3 is substantial. This quantity most likely exceeds the gold active site density (unknown), with the support material thought to be acting as a reservoir. The propyne retention profiles for the Au/TiO2 catalyst (Fig. 3) differs from that seen for the Au/Fe2O3 (Figs. 5 and 7). Given that the gold crystallites are of comparable particle size in the two catalysts, these differences in quantities of propene produced and propyne retention profiles indicate that the support is indeed perturbing the gold-based chemistry. To consider the effect of the support material in the observed trends for the Au/Fe2O3 catalyst, blank experiments were performed on a Fe2O3 sample after the same pretreatment carried out for the gold catalyst. These experiments produced essentially the same results independently of whether Fe2O3 was pre-treated at 523 K or at 663 K. A representative reaction profile for these experiments is
presented in Fig. 8, which shows the effect of pulsing the hydrogen/propyne mixture at 523 K. Minimal propene formation is observed (0.54 mmol propene g 1 for the first pulse), which slowly decreases to zero at increasing pulse numbers. This indicates the support activity to have a minimal direct role in the chemistry seen for the Au/Fe2O3 catalyst. A negative carbon mass balance indicates that the iron oxide surface is able to retain substantial amounts of propyne, although propyne is detected in increasing amounts with increasing pulse number. It is worthwhile comparing the optimum specific activities of both catalysts. The 1.5% Au/TiO2 (pre-treated at 523 K) produced 325 mmol propene gAu1 at 423 K (pulse 1, Fig. 3), whereas the 4.5% Au/Fe2O3 (pre-treated at 663 K) produced 182 mmol propene gAu1 at 523 K (pulse 10, Fig. 7). Thus, this analysis indicates the Au/TiO2 to have a higher specific activity than the Au/Fe2O3. However, in using specific activities, one has to be aware of the different preparative procedures used. Au/TiO2 and Au/Fe2O3 were prepared by deposition–precipitation and coprecipitation respectively, therefore, it is expected that the concentration of gold on the surface of the two supports will be different. In deposition–precipitation gold is deposited on the surface of the TiO2 support, whereas gold should be homogeneously distributed in both the surface and bulk of the iron oxide support [1]. Furthermore, the metal surface area is unknown. Despite these limitations, and given the fact that these catalysts have comparable particle sizes, the observations made on the different catalytic performances suggest different support interactions. As Figs. 1, 3 and 8 show the isolated supports to have zero (TiO2) or negligible (Fe2O3) hydrogenation activity, then this contribution is thought to occur at the Au/support interface.
4. Summary and conlusions Two supported gold catalysts have been investigated for the hydrogenation of propyne. The Au/TiO2 catalyst (mean particle size 3.7 1.50 nm) prepared by the deposition–precipitation method was 100% selective towards propene production but was additionally characterised by deactivation on increased pulsing of the reaction mixture and substantial propyne retention, presumably by the support material. The as received Au/Fe2O3 catalyst (mean particle size 3.7 0.93 nm) prepared by co-precipitation, initially exhibited broadly similar behaviour to the titaniasupported catalyst (100% propene selectivity, deactivation, propyne retention by the support material). However, the catalytic properties of the sample changed in time (4 months) where a significant reduction in propene selectivity and the absence of deactivation were observed. Higher reaction temperatures were necessary to recover
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the propene selectivity. However, pre-treatment at a higher temperature of 663 K ensured 100% selectivity at low and high reaction temperatures. The two gold catalysts exhibited optimum propene formation at two different reaction temperatures: 423 K for Au/TiO2, and 523 K for Au/Fe2O3. Differences in the two temperature profiles are thought to reflect differences in the metal-support interaction. This work underlines the ability of gold catalysts to selectively hydrogenate propyne. The catalyst support, pretreatment temperature, and reaction temperature must be carefully selected to provide a catalyst with suitable selectivity and to minimise deactivation. For the Au/ Fe2O3 catalyst, the ageing of the sample was also shown to be a major issue, for which further investigations are required. The ageing effects and the variable selectivity observed indicate an intricate catalytic system.
Acknowledgements The authors are grateful for financial support from the Auricat EU network (HPRN-CT-2002-00174). The World Gold Council is thanked for provision of the Au/TiO2 and Au/Fe2O3 catalysts.
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