Temperature programme reduction ( TPR ) studies of cobalt phases in γ-alumina supported cobalt catalysts

Temperature programmed reduction (TPR) is one of the techniques for obtaining information about phases or bulk species in heterogeneous catalysts. Information from TPR analysis can give insights about phase-support interaction and extent of reduction of the phases at different temperatures. TPR technique is a common tool in the characterisation of cobalt based Fischer-Tropsch (FT) catalysts. However, interpretation of TPR profiles of γ-alumina supported cobalt FT catalysts had been characterised with different views on the nature of phases and reduction processes involved. In this report, we use reduction behaviour of unsupported Co3O4 to gain insight for more explicit analysis of TPR profiles of γ-alumina supported Co3O4 catalysts. The transition Co3O4 → CoO → Co in γ-alumina supported catalysts prepared with wet impregnation with aqueous cobalt nitrate and calcined at temperatures ≤ 350°C gave reduction peaks at 300 to 350°C. Reduction peaks at 500 to 600°C were due to Co-Al mixed oxide phases; most likely Co2AlO4 and probably routes formation of the mixed oxide were also discussed. Consideration of tendency of dissolution of γ-alumina during the impregnation of metal salt is instructive toward achieving higher reducibility of Co3O4 in the design of cobalt based Fischer-Tropsch catalysts.


INTRODUCTION
Increasing energy demand coupled with the awareness of declining petroleum reserves has motivated renewed research and commercial interest in Fischer-Tropsch synthesis (FTS).Different feedstock options (natural gas, coal and biomass) are being explored to produce liquid hydrocarbons as alternative to petroleum (Kagan et al., 2008;Schulz, 1999;Dry, 2002).Commercial FTS operations are based on iron and cobalt based catalysts.H 2 /CO ratio of syngas from natural gas do not required water gas shift step which matches the low water-gas shift (WGS) activity of cobalt based catalysts.Moreover, Co-based catalysts are selective towards paraffins and have higher activity for hydrocarbon formation than Febased catalysts; hence, cobalt catalysts are preferred for *Corresponding author.E-mail: sharafeldin99@yahoo.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License  (Jacobs et al., 2007).
Active sites in cobalt based catalysts are cobalt metal nanoparticles and several studies have shown positive correlations between cobalt dispersion and FTS activity and hydrocarbon selectivity (Iglesia, 1997;Girardon et al., 2007).Thus, maximizing cobalt dispersion on the support is a key objective in the design of Co-based catalysts (Khodakov, 2009).Generally, preparations of cobalt catalysts involve impregnation of high surface area support material with cobalt salt solution, followed by drying and calcination.During calcination, the impregnated salt is transformed into oxide phase(s).The active phase for the FT synthesis are generated in situ by reduction under hydrogen stream prior to passage of syngas feed (Zhang et al., 2002;Sirijaruphan et al., 2003).
Gamma (γ) alumina is often a prefer support for cobalt catalysts because of its ability to stabilize small size clusters of cobalt particles and high resistance to attrition especially in the continuously stirred tank reactor or slurry bubble column reactor.However, several reports have advanced that after activation, considerably large fraction of the impregnated cobalt precursor on γ-alumina are catalytically inactive for the hydrocarbon synthesis (Chu et al., 2007;Sirijaruphan et al., 2003).An important of objective in the design of cobalt-based Fischer-Tropsch synthesis is to maximise the proportion of impregnated cobalt precursor on γ-alumina that become reduced to the metallic phase.Temperature programmed reduction (TPR) technique is useful barometric tool for studying the reduction behaviour of catalysts.However, there are varied interpretations of H-TPR profiles of γ-alumina supported FTS cobalt catalysts.These interpretations was categorised and summarised by Jacobs et al., (2007) (Figure 1).Case 1 scenario is the most popular, with the view that the predominant phase of the cobalt is as

Temperature programmed reduction (TPR)
TPR profiles of the samples were recorded with ChemiSorb 2720 (M/s Micrometrics, USA) equipped with a TCD detector.The TPR profiles were obtained by reducing the catalyst samples by a gas mixture of 10% H2 in Ar with a flow rate of 20 ml/min while the temperature was increased from ambient to 800°C at a rate of 10°C/min.

Reducibility
Reducibility of the supported catalysts was determined by O2titration which was designated by Ro and calculation of which was expressed by the Equation -1.The catalysts were first reduced at 350°C for 8 h with pure hydrogen and re-oxidised with 5% O2 in Helium gas at the same temperature using pulse chemisorption technique.The percentage of Ro was calculated by assuming complete re-oxidation of Co to Co3O4 according to the chemical equation: 3Co + 2O2 → Co3O4.
Reducibility determined by TPR technique is expressed as RH and calculated according to equation 2. Complete reduction of Co3O4 phase was assumed and the TPR profile was deconvoluted to corresponding peak areas.The deconvolution of the TPR signal was made with the ChemiSoft software package provided by M/s Micrometrics, USA.The peak temperature was fixed for the deconvolution of the TPR profiles.

Gravimetric measurements of thermal decomposition
It has been reported that thermal decomposition of cobalt nitrate to cobalt oxides (Co 2 O 3 and Co 3 O 4 ) in an inert atmosphere starts at 185°C (Ehrhardt et al., 2005).This informed the choice of lowest calcination temperature for unsupported cobalt catalysts for this study.Result of the gravimetric measurements of thermal decomposition of cobalt nitrate in static air is presented in Table 2. Due to possibility presence of residual nitrate due to incomplete decomposition of the nitrate salt we anticipated that the calculated value will be higher than the experimental mass loss. is the most probable oxide of phase of cobalt after calcination of the supported catalysts.This inference from our hypothesis of presence of residual nitrate agrees with many reports in which Co 3 O 4 phase is obtained after calcination of cobalt nitrate.We also obtained peaks assigned to residual nitrate in the H-TPR profile, and hydrogen consumption from those peaks are related to residual nitrate content after calcination of the cobalt salt (infra verde).However, report of Ehrhardt et al., (2005) indicated presence of Co 3 O 4 and Co 2 O 3 phases when cobalt nitrate is calcined in inert atmosphere.Concurrent transformation of the oxides to metallic cobalt takes place in a reducing atmosphere.The oxides are completely reduced to metallic state at temperature less than 400°C.The transformation from cobalt nitrate to metallic cobalt proceeds as follows: Where the coefficient 'm' decreases with the increase of temperature and duration of calcinations. (

Crystallite size
XRD of unsupported and γ-alumina supported catalysts also depict presence of Co 3 O 4 phase (Figures 2 and 3).This corroborates the inference from the gravimetric analysis.Crystallite size of the Co 3 O 4 of the catalysts calcined at different temperature is presented in Table 3.
In both unsupported and γ-alumina supported catalysts crystallites size of Co 3 O 4 increases with increase in calcination temperature except the unsupported sample calcined at 185°C which is an outlier.Reason for this outlier situation is not clear at the moment.At the same temperature Co 3 O 4 crystallite sizes are smaller in the supported than in unsupported catalysts.This is due to stabilization of Co 3 O 4 nanoparticles by the support and in each case the increasing crystallite size with increasing calcination temperature can be attributed to sintering which is promoted at higher temperatures.

Unsupported Co 3 O 4 samples
Figure 4 shows the H-TPR profiles of the unsupported  is proportional with the value of 'x'.They report that the dominant reduction peak temperatures decreased as the values of 'x' increases.The assigned transitions also agree with the report of Yuvaraj et al., (2003).These authors investigated direct reduction of transition metal nitrates with hydrogen.They advanced that reduction of transition metal nitrates is activated by their decomposition and nitrate salts decompose at temperature range between 177 to 270°C.In particular, the temperature ranges 242 ± 30° and 242 ± 20°C was reported for decomposition and reduction of cobalt nitrate, respectively.The peak temperatures assigned to reduction of residual nitrate decreases with increase in calcination temperature and are similar for catalysts C and D. Peak temperatures attributed to CoO → Co transition increases with the increase of calcination temperature and catalyst B and C also show similar temperature for CoO→ Co reduction.The trend of CoO → Co reduction peak temperature agrees with the report of Tang et al., (2008) in which it was shown that temperature of reduction of CoO x species increases with the increase of calcination temperature.Potoczna-Petru and Kȩpiński (2001) also demonstrated that initiation temperature of reduction of Co 3 O 4 increases with increase in calcination temperature.With the help selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) analyses the authors showed that crystallite Temperature °C

Supported Co 3 O 4 samples
We attempted interpretation of the reduction pattern of γalumina supported cobalt catalysts with the backgrounds from the TPR profiles of unsupported Co 3 O 4 .Figure 6 shows the TPR profiles of the alumina supported Co 3 O 4 samples.The TPR profiles can be deconvoluted into three peaks as presented in Table 5.The first peak (αpeak) is assigned to nitrate reduction.Compared to the unsupported catalysts, these peaks appeared at higher temperatures, which increased with increasing temperature of calcination of the supported catalysts.This may be due to interaction with the surface of the alumina support.Highest peak temperature in the unsupported Co 3 O 4 is 400°C, and invoking crystallites effect of highest lower peak temperature of Co 3 O 4 phase in the supported catalyst is expected to be below 400°C.However, the profiles of the supported Co 3 O 4 contain peaks at temperature above 400°C.Moreover, the temperature gaps between the two higher temperature peaks in supported Co 3 O 4 are much wider (191 to 436°C) than in the unsupported samples.Based on these observations we posit that the transitions in the third or γpeaks of the unsupported and γ-alumina supported cobalt catalysts are different.
Except sample E, the second peaks in the profiles of γalumina supported are assigned to Co 3 O 4 → Co transition.These peaks are at lower temperatures compared to similar transition in the unsupported counterparts.The lowered Co 3 O 4 → Co transition temperature here is attributable to crystallites size effect   Petru and Kȩpiński, 2001;Wang et al., 2004).The prominent peak in the uncalcined catalyst (E) consists of overlap of two peaks: nitrate and Co 3 O 4 → Co transition peaks.The third peaks (γpeaks) which range from 517°C (for calcined catalyst) to 654°C (for uncalcined catalyst) are designated to the reduction of cobaltaluminium mixed oxide.The following trends are observed from the profiles of the supported catalysts: (i) temperature of nitrate (αpeak) increases but peak area decreases with increasing calcination temperature; (ii) temperature and peak area of Co 3 O 4 → Co (βpeaks) transitions increasing with increasing calcination temperature; (iii) increasing visibility of the third (γ-peaks) with increasing calcination temperature.These third peaks are absent in the unsupported catalyst.Dispersion and reducibility of supported cobalt catalysts is usually estimated using H 2 chemisorption and O 2 titration technique.Depending on cobalt loading, the reducibility values for unpromoted alumina based catalysts commonly vary between to 45% (Jacobs et al., 2002;Borg et al., 2007).The temperature, 350°C, is Calcination temperature (°C) widely adopted as the standard reduction temperature of supported cobalt catalysts for these analyses.This adopted temperature presupposes formation of metallic cobalt phase should take place at ≤ 350°C.It corroborates with the reduction temperatures obtained for unsupported Co 3 O 4 , and in effect ruled out Case# 1 scenario (Figure 1).
Results of reducibility of the supported Co 3 O 4 catalysts are shown in Figure 7. Reducibility values obtained from H-TPR agrees well with values obtained from O 2 -titration method.It is observed that the reducibility determined from both the techniques has little variation of values with reference to the increase of calcination temperature.Since residual cobalt nitrate is readily transformed to cobalt during reduction in hydrogen at 350°C, the small difference may be attributed to possibility of metallic cobalt contribution from residual nitrate.The results indicated that calcination temperature has little or no influence on the reducibility of the Co/Al 2 O 3 catalysts; crystallite size of Co 3 O 4 phase and by extension dispersion of metallic cobalt nanoparticles in reduced catalysts decreases with increasing calcination temperature.
Figure 8 shows the summary of deconvolution analysis of the profiles in Figure 6.The area of the α-peaks decreases expectedly with increasing calcination temperature.It was expected that the areas of α peaks will decreased progressively from catalyst E to J.But F (calcined at 150°C) showed higher percentage hydrogen consumption than catalyst E (calcined at 110°C).
According to TGA profiles of cobalt nitrate by Ehrhardt et al., (2005), calcination temperatures of the two catalysts (E and F) are below decomposition of cobalt nitrate, but may contain different amount of hydrated water molecules.The hydrated water molecules contribute to mass of the catalyst and it is expected to be higher in catalyst E than in catalyst F. Thus catalyst F will contain higher amount of cobalt nitrate per unit mass than catalyst E. This may account for the observed higher percentage hydrogen consumption production of nitrate peak of catalyst F compared to catalyst E.
The areas of the β-and γ-peaks increase with increase in calcination temperature.The shape of the Co 3 O 4 reduction peak is similar for both the supported and unsupported catalysts, but the signature of two stage reduction of Co 3 O 4 particle to Co is not distinguished in the TPR profile of the supported catalysts.Due to smaller crystallite size of Co 3 O 4 phase in the supported compared to unsupported catalysts, the two stage reduction process, Co 3 O 4 → CoO→ Co, are combined in the β peaks.It is worth noting that there is a compromise between nitrate removal and Co 3 O 4 crystallite in the supported catalysts.Temperature 250°C appears to be a reasonable compromise between the two ends.The same has also been suggested in the literature as optimum calcination temperature towards achieving higher cobalt dispersion on γ-Al 2 O 3 (Borg et al., 2007;Belambe et al., 1997).
Cobalt phase in catalyst E is essentially in the nitrate form.As reported by Yuvaraj et al. (2003), concurrent decomposition and reduction of cobalt nitrate is expected at 242 ± 30°C, thus H-TPR profile of catalyst E ought not to show any peak at temperature above 350°C.However, the profiles of catalyst E showed unexpected hydrogen consumption over a broad temperature range with peak temperature at 650°C.This suggest that prior to calcination the impregnated cobalt nitrate had interacted with γ-alumina supported to form cobalt phases that are not reducible at typical reduction temperature of Co 3 O 4 phase.While the scope of the present does not cover identification of this cobalt phase, it is important to draw attention to the point that contrary to the widely held view that γ-alumina is inert in aqueous environment, it has been shown that γ-alumina is active in aqueous media.Carrier et al., (2007) demonstrated that hydroxide polymorphs of aluminium are thermodynamically more stable phases than γ-alumina in aqueous media.They also showed that γ-alumina transformed to hydroxide polymorphs in aqueous environment.The transformation takes place via surface hydration and dissolution.The dissolution can produce autonomous aluminium hydroxide phase from a supersaturated solution of dissolved aluminium ions.The dissolution process can also be aided by presence of H + , OH -or by metal ions (for example, Ni 2+ , Co 2+ ) (Trueba and Trasatti, 2005).Thus, in effect γ-alumina in contact with aqueous metal ions solution during catalyst preparation via impregnation method has two active phasessurface hydroxide and dissolved Al 3+ .The surface O-Hs accounts stabilize nano clusters or particles, leading to the widely recognised high dispersion of impregnated metal of metal oxide in γalumina support.The second implication of alteration of γ-alumina in aqueous media to catalyst preparation is that, during evaporation stage of impregnation process it leads to supersaturation of dissolved Al 3+ ions which precipitate as independent hydroxide phase or coprecipitate with metal ions being impregnated on the γalumina.Co-precipitation of Al 3+ ions with divalent metal ions like Co 2+ can form hydrotalcite-like mixed oxide phase (Ay et al., 2009).
Formation of hydrotalcites-like structures in coprecipitated of Co 2+ and Al 3+ ions had been reported by Khassin et al., (2001).They demonstrated that during calcination the hydrotalcites-like structures are transformed into aluminates through inverted spinel-like structure.
But at moderate temperatures the hydrotalcites-like structures transformed into Co oxide phase on a highly defective inverted spinel-like structure, in which Co 2+ enter the support structure and occupy both tetrahedral and octahedral positions.The authors explained that octahedron coordinated Co species are reduced at 580 to 620°C, reduction at 470 to 480°C can also yield Co° supported on inverted spinel-like structure, which contains Co 2+ in the octahedral coordination.Reduction at 600°C transforms the support to 'ideal' spinel, which contains no octahedron coordinated Co 2+ .According to Pe et al., (2001), these cobalt phases may yield metallic when reduced in hydrogen at ≥ 480°C.
Thus, we suggest that the cobalt phase responsible for the γ-peaks of catalyst F-J are attributed to hydrotalcite-like structures, dehydration and dehydroxydecarbonation of these structures appear to at 150 to 200 and 250 to 300°C in air, respectively leading to inverted spinel-like structures (Co 2 AlO 4 or CoAl 2 O 4 ).In line with the discussion on alteration of γ-alumina in aqueous media, the peak at 650°C may be assigned to surface Co-Al mixed oxide phase which is formed after impregnation as amorphous phase and crystallises in situ during the course of the TPR analysis.The crystallisation of the Co-Al mixed oxide phase on calcination can also be inferred by the observed progressive sharpening of γ-peaks with increasing calcination temperature.This inference agrees with similar conclusion by Ji et al., (2000) who proposed that Co 3 O 4 phase on Co/Al 2 O 3 prepared by impregnation method is interfaced with other cobalt surface phase.We proposed that the cobalt phase in the third peak (γ-peak) of the calcined alumina supported catalysts is likely to be Co 2 AlO 4 since the peak temperature is ~ 520°C which near to 480°C.But this Co 2 AlO 4 phase could not be identified by XRD analysis due to very similar lattice parameters of the phases namely Co 3 O 4 , Co 2 AlO 4 and CoAl 2 O 4 (Walsh et al., 2007).

Conclusion
H-TPR profiles of unsupported cobalt catalysts calcined at different temperatures are used as reference for TPR profiles of the supported catalysts.The amount of residual nitrate decreases with increasing calcination temperature.Reduction Co 3 O 4 to Co occurs in two stages: Co 3 O 4 → CoO → Co at temperature ranges of 200 to 400°C and 220 to 330°C for unsupported and supported catalysts, respectively.The two reduction steps take place within same temperature region with single peak maximum in the supported catalysts.Calcination temperature does not have significant effect on reducibility of alumina supported Co 3 O 4 , but 250°C is considered a reasonable compromise between achieving high nitrate removal and Co 3 O 4 dispersion.The result of this study is not consistent with the interpretation of TPR profile of phase Co 3 O 4 on alumina support in terms of a two stage reduction process with peak temperatures at 200 to 400°C and 400 to 800°C, respectively.We suggest that consideration of the tendency of dissolution of alumina during the impregnation of metal salt will be beneficial toward achieving higher reducibility of Co 3 O 4 in the design of cobalt based Fischer-Tropsch catalysts.

Figure 1 .
Figure 1.Case scenarios in the interpretation of observed peaks in standard H-TPR patterns for Co/Al2O3 catalysts(Jacobs et al., 2007).
Co 3 O 4 and it undergo reduction to metallic cobalt in a two stage process: Co 3 O 4 → CoO → Co, at temperatures ranges 200 to 400°C and 400 to 800°C, respectively.Case 2 and Case 3 scenarios recognised Co 3 O 4 phase with a one-stage reduction to Co, at the temperature ~ 300 to 350°C.The advocates of the Case 2 and Case 3 viewpoints believed that γ-alumina supported FTS cobalt and γ are the reduction peaks assigned to nitrate, Co3O4 and cobalt-aluminate in the resolved TPR profiles of the supported catalysts.

Figure 5 .
Figure 5. Percentage residual nitrate and CoO → Co / Co3O4 → CoO peak area ratio in the unsupported Co3O4 samples calcined at different temperatures.

Figure 7 .
Figure 7.Comparison of Reducibility values from TPR profile estimates and O2 -titration analysis.

Figure 8 .
Figure 8. Summary of decomposition analysis of TPR profiles of the supported Co3O4.
3 cobalt phases.It is argued that reduction of these phases account for H-TPR peaks in the temperature range of 400 to 800°C.The three proposals recognised the possibility of cobalt-aluminate phase which is reduced above 800°C.While there is yet a consensus on reduction behaviour of alumina supported cobalt catalyst, the aforementioned highlighted perspectives in the interpretations of H-TPR profiles have influenced approaches to design of alumina supported cobalt based FT catalysts.This study is motivated by the need for correct interpretations of H-TPR profiles alumina supported cobalt catalysts that are intended for Fischer-Tropsch application.Hence, we present results of H-TPR studies of unsupported cobalt oxide as reference to gain insight about reduction behaviour of γ-alumina cobalt catalysts.X-ray diffraction (XRD) was also used to identify the Co-oxide phases and determine their crystallite sizes.Reducibility of the supported catalysts was determined by O 2 -titration.METHODOLOGYCatalyst preparationUnsupported Co3O4 samples were prepared by calcining cobalt nitrate hexahydrate [Co(NO3)2.6H2O,Merck, India] at 185,

Table 2 .
Gravimetric results of thermal decomposition of cobalt nitrate in static air at different temperatures.
Table 2 showed difference between percentage calculated and experimental mass loss of decomposition of Co(NO 3 ) 2 .6H 2 O decreases with increasing calcination temperature.Positive values are obtained for differences based on decomposition of Co(NO 3 ) 2 .6H 2 O to Co 3 O 4 at all temperatures.While values obtained based on decomposition of Co(NO 3 ) 2 .6H 2 O to Co 2 O 3 gave negative values at 250 and 350°C.Since negative different is impracticable, we inferred that Co 3 O 4

Table 3 .
Particles size of Co3O4 phase in unsupported and γ-alumina supported samples.

Table 5 .
Peak temperatures of the TPR profiles of the Co/Al2O3 samples.