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Effects of hydroxylation of a silica surface on the metal dispersion in supported platinum catalysts
141
Applied Catalysis, 77 (1991) 141-148 Elsevier Science Publishers B.V., Amsterdam
Effects of hydroxylation of a silica surface on the metal dispersion in supported platinum catalysts Masahiko Arai*, Shi-Ling Guo’ and YoshiyukiNishiyama Chemical Research Institute of Non-aqueous Solutions, TO/K&U University, Sendai 980 (Japan), tel. (+ 81-22) 2276266, fax. (+ 81-22) 2238956
Katahira,
(Received 13 March 1991, revised manuscript received 21 May 1991)
Abstract The influence of the surface hydroxyl groups of porous silica on the degree of platinum dispersion was studied using silica supports with the same surface area but with an increasing amount of these functional groups. The supports were prepared by immersing a calcined silica in water at 70°C for various periods of time. Platinum was deposited on the supports by wet impregnation with aqueous solutions of platinum tetraamine dichloride (PTD ) and chloroplatinic acid (CPA). For catalysts with PTD reduced at 4OO”C,the degree of platinum dispersion indicated a maximum for a support whose surface was moderately hydroxylated. Such an effect was not observed for the catalysta reduced at the lower temperature of 330°C and for those with CPA. The surface hydroxyl groups affected the reduction of PTD and certain types of surface hydroxyl groups may be responsible for the effects observed. Keywords: catalyst preparation (wet impregnation), hydroxylation, platinum dispersion, silica.
INTRODUCTION
The metal dispersion of supported metal catalysts is dependent on various factors and changes in metal dispersion are often attended by significant changesin the catalyticperformanceof the catalysts [l-3 1.One of the important factors is the surfacepropertiesof the support materialsused and several recentstudieshave demonstratedthat certainfunctionalgroupson the surface of silica and carbon supports may play an important role in dispersingmetals [4-7 1.In a previousstudy [ 41, the degreeof nickel dispersionon a porous silica gel was found to increasewhen it was merely calcined at 400-8OO”Cprior to metal loading in spite of using a wet impregnationtechniquewith an aqueous solution. The nickel dispersion was related to the change of the surface hydroxyl groups on the silica although its surface area and porosity were also changed by the calcination. This view was supported by an additional result that rehydroxylationof calcined silica decreasedthe degree of nickel disper‘Present address: Chemiatry Department, Zheng Zhou Industry College, People’s Republic of China.
01669934/91/$03.50
0 1991 Elaevier Science Publishere B.V. All righta reserved.
142
sion. Prado-Burgueteet al. [ 51 usedcarbon materialswith the samepore structure but with an increasingamount of oxygen-containingsurface groups prepared by mild oxidation;they noted the importanceof these surfacefunctional groupson the dispersionof supportedplatinum on carbon. The present study has been focused, to a greater degree, on the effect of surfacehydroxyl groups on the dispersion of metals on porous silica. We prepared supportswith increasingamounts of these surfacefunctional groupsby rehydroxylatinga calcined silica gel. Platinum was loaded on these supports by wet impregnationwith aqueoussolutionsof PTD and CPA, differingin that the platinumwas includedin either a cation or an anion part and the adsorption of the precursorby ion exchangemay or may not contributeto the metal loading. The effect of the surface hydroxyl groups was shown for PTD in a similarmannerto the nickel nitrateas mentionedabove but not for CPA. The possible actions of the surfacehydroxylgroupsin dispersingplatinumover the silica surfacewill be discussed. EXPERIMENTAL
The support materialused was a powdered (32-60 mesh), porous silica gel, Silbead-N (MizusawaIndustrialChemicals), which includesaluminain 2 wt.%. The silica gel was calcined in air at 700°C for five hours and was immersed in distilledwater at 70’ C for variousperiods of time in order to prepare silica supportswith different degreesof rehydroxylation.After immersionin water, the silica powder was vacuumdried at 110’ C and stored in a desiccatorbefore use. The differently rehydroxylated silica supports will be hereinafter expressed as e.g. SI (3) where SI denotes SiOz and the figure in parenthesesis the period of time of the rehydroxylationin hours. A wet impregnationmethod was used to prepare 0.5 wt.-% platinum/silica catalystsfrom aqueoussolutions of PTD and CPA. A known amount of silica powder was added to the precursor solution in a recovery flask and kept immersedovernightat room temperaturewithout agitation.The initialpH of the solution was 7.0 for PTD and 2.1 for CPA. The suspensionof silica powder in the solution was rotationallyagitatedat about 60 rpm for 10 min and then the solvent was evaporated under reduced pressure at around 50°C within one hour by rotational agitation using a rotary evaporator.To examine the possibility of adsorption of PTD, the residualamount of platinum in the impregnating solution was measuredby atomic absorption spectrophotometry.The silica supports impregnatedwith the precursor were vacuum dried at 110°C for four hoursand reducedby flowinghydrogenat 400 or 330’ C usinga heating schedule such that the supports were heated at 7 K min-’ up to the desired reductiontemperature,kept at this temperaturefor two hours, and cooled to room temperaturein about five hours. The surface area and porosity of the silica gels were measuredby nitrogen
143
adsorptionmethodsand their surfacehydroxylgroupswere measuredby Fouriertransforminfraredspectroscopy (FT-IR) with a diffusereflectancemethod for a mixture of silica gel (ca. 1 wt.-% ) and potassium bromide. The mixture was preparedby grindingknown amounts of silica and potassium bromide in a mortar in an air-conditionedroom and the FT-IR spectrumwas collected for the mixture in flowing dry nitrogen. The PI’-IR spectra obtained with this procedure gave the relativeamounts of surface hydroxyl groupsbeing in good agreementwith those obtained from thermogravimetryas described previously [41The degreeof platinumdispersionof the preparedcatalystswas determined by temperature-programmeddesorption (TPD) of hydrogen. They were exposed to flowing hydrogenat 30 ml min-’ for 15 min at 30” C and then heated in argon at 30 ml rain-’ by linearheatingat 30 K min-’ up to 450” C usingthe apparatusand procedures described previously [81. The dispersion of metal precursorswas examined by FT-IR with the above-mentioned diffuse reflectance method and X-ray photoelectron spectroscopy (XPS ). RESULTS
Dispersion of platinum
The influence of the rehydroxylationof silica gel on the degreeof platinum dispersion for the catalysts reduced at 400°C is shown in Fig. la. The rehydroxylationaffected the platinumdispersionin the case of PTD, for which the dispersionincreased,reacheda maximum,and then decreasedwith increasing extent of rehydroxylation.This change is in accordance with the changes in nickel dispersionwith dehydroxylationby calcination and rehydroxylationof the same silica supportas reportedpreviously [ 41. Fig. la showsthe maximum degree of platinum dispersion attained for SI (6)) which was more than thirtyfold and sevenfold greaterthan that compared to SI (0) and SI (15)) respec(b)
i“ .
1
l
o-
.
.
Time
(h)
Fig. 1. Influence of the hydroqdation of eilica by immersion in water at 70°C on (a) the degree of platinum dispersion mewned by TPD ( l : PTD, 0: CPA) and-(b) the total amount of surface hydroxyl groups relative to SI (0) measured by FT-IR.
144
tively. For the catalysts with CPA, in contrast, the rehydroxylation had little influence on the platinum dispersion. In the case of PTD, when reduction was performed at a lower temperature of 330’ C, the resultant platinum dispersion was not greatly influenced by the rehydroxylation; it was 35, 32, and 31% for SI(O), SI(6), and SI(15), respectively. Impregnation and dispersion of PTD
The effect of rehydroxylation of the platinum dispersion was observed for PTD as described above and as a result we chiefly focussed our attention on it and we examined the dispersion of PTD precursor before reduction. The adsorption of PTD on silica was previously reported by Benesi et al. [91 and they demonstrated that this was due to ion exchange between the platinum tetraamine ions and protons of the surface hydroxyl groups having sufficient Brensted acidity. To examine the adsorption of PTD on our silica gel, we measured the residual amount of platinum in the impregnating solution after immersing silica in it overnight. The amount of platinum in the solution decreased to less than one tenth of the initial value for the cases of SI (0)) SI (1)) SI (4.5)) SI (6.5)) and SI (10). These silica supports, namely, adsorbed more than 90% of the PTD initially present in the solution. PTD was therefore shown to be deposited on all of our silica gels by ion exchange irrespective of
1
4000
I
I
I
3200
2400
Wovenumber
EOO
860
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(cm’)
Fig.2.FT-IRspectraofPTD (a),SI(O) (b),andPTDaupportedonSI(O) (c),SI(3) (d),SI(8.5) (e), and SI(15) (f) before reduction. All the -plea were dried at 110°C. Arrows indicate absorption bands at 1414 cm-’ which appeared for supported PTD samples.
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the extent of the rehydroxylation of their surfaces. Therefore PTD was believed to be very highly dispersed on the silica supports after the evaporation of water followed by vacuum drying. Fig. 2 shows the FT-IR spectra for supported and unsupported PTD and SI (0) after vacuum drying at 110” C. The rehydroxylated supports indicated similar spectra to SI (0). Unsupported PTD has a sharp band due to Pt-N at 511 cm-’ [ 10,111 and several sharp and broad bands due to the ligand of ammonia at around 900, 1400, 1600, and 3230 cm-’ [12,13]. These absorption bands are not seen but a weak absorption band appears similarly at 1414 cm-’ for ail of the supported PTD samples examined as can be seen from comparison with the spectrum of SI (0). It is not clear whether this absorption at 1414 cm- ’ is a shift of the band for the ammonia ligand due to the presence of the support or other causes. It was reported that the absorption due to the formation of SiO-metal appeared at wavenumbers of 1000 to 900 cm-’ [ 14-161. The XPS measurements for supported PTD indicated that the ratios of peak areas of Pt 4d to Si 2p were 0.8, 1.0,0.9, and 0.9 for SI(O), SI(4.5), SI(8.5), and SI (15)) respectively, being the same for these samples within experimental errors. This suggested that the degree of PTD dispersion was similar for these supports and it could not be responsible for the significant differences in the degree of platinum dispersion observed after reduction as shown in Fig. la. The binding energy of platinum could not be determined accurately mainly due to its small loading. Rehydroxylution of silica Fig. lb shows the increase in the amount of surface hydroxyl groups on the rehydroxylation as estimated by integrating absorbance from 3600 to 2800 cm- ’ of FT-IR spectra. Typical FT-IR spectra are given in Fig. 3. Before the rehydroxylation, the silica gel had a surface area of 410 m2 g-l, a total pore volume of 0.20 ml g-’ with an average pore diameter of 2.5 nm, and a total amount of
Wavenumber (cm-‘) Fig.3.E”I’-IRspectraofSI(0)
(a),SI(3)
(b),SI(6)
(c),andSI(12)
(d).
146
surface hydroxyl groups of 8.2 x 10m4mol g-’ corresponding to a density of 1.2 groups nm -‘. The surface area and pore volume were not influenced by the rehydroxylation [ 41. The amount of surface hydroxyl groups was increased by a factor of about 1.7 with the rehydroxylation for fifteen hours. The SI(15) had a density of 2.0 groups nme2 and so it was still not fully hydroxylated [ 171. The total amounts of the surface hydroxyl groups of all the supports used were larger than the amount of platinum loaded. For SI (0)) for example, the molar ratio of these functional groups to platinum atoms loaded was about 30. FTIR spectra indicated that the silica supports had a few types of surface hydroxyl groups [ 181 but it was not clear in Fig. 3 whether a certain type significantly varied or not upon rehydroxylation. DISCUSSION
The present results demonstrate that the dispersion of platinum is significantly influenced by the rehydroxylation of silica prior to metal loading in the case of PTD. The maximum degree of metal dispersion was observed for the silica that was moderately rehydroxylated (Fig. la and b ) . The PTD was chiefly deposited on the silica supports by ion exchange with the protons of the surface hydroxyl groups of Br0nsted acidity, and it was highly dispersed on all the supports used. It is therefore believed that the following reduction of PTD precursors was affected by the rehydroxylation of the support, which influenced the reducibility of the PTD and the sintering of species formed during the reduction; this sintering effect may be more important. This idea would also be supported by the fact that the effect of rehydroxylation appeared in the case of reduction at 400°C but not at the lower temperature of 330°C. The PTD could be reduced at a temperature as low as 300 ’ C and the complex of Pt ( NHs)2H2 was suggested to be the species of high surface mobility [ 191. There was a maximum in the effect of rehydroxylation on the platinum dispersion for a moderately rehydroxylated support although all the supports examined had sufficient amounts of surface hydroxyl groups compared with the amount of platinum loaded. It was thus suggested that the observed effect was not related to all the surface hydroxyl groups but to those of a certain type. This type could be specified by the strength of the Brnrnstedacidity or other parameters. Such a specification could not be attained in the present work although we attempted a differential thermal analysis of silica gels as well as FT-IR. We will compare the present results for PTD with those for nickel reported previously [ 41. Similar effects were also found for nickel/silica catalysts prepared from nickel nitrate with the same procedures as used in the present work. However, the dispersion of the nickel precursors was dependent on the amount of surface hydroxyl groups of the support used and this is not the case for PTD as mentioned above. We previously invoked a dynamic wettability for the role
147
of these surface groups in the observed effect for nickel. At a later stage in water evaporation, droplets of the impregnating solution could be formed on the surface of the support, the droplets on the more hydrophobic surfaces would be more easily split or shrunk because it is difficult for the solvent, water, to wet such surfaces. This would result in the formation of smaller droplets on the more hydrophobic surfaces than on the less hydrophobic ones. Smaller crystallites of the nickel salt would then be deposited from the smaller droplets onto the more hydrophobic surfaces at the end of evaporation of the solvent rather than onto the less hydrophobic surfaces. This idea may be valid for catalyst preparations when the deposition of precursors takes place through a forced process by the evaporation of the solvent. In the case of nickel, the nickel nitrate was generally deposited in such a forced manner but it was also deposited by adsorption, like PTD, although the amount adsorbed was less than one tenth of the total amount impregnated [4]. This fact and the results for the PTD suggest an additional explanation to that of dynamic wettability for the results on nickel. One possibility is that a small amount of nickel precursor is adsorbed on the surface of the support upon its immersion in the impregnating solution and the remainder is forced to deposit preferentially on those pre-adsorbed precursors upon the evaporation of water. The contribution of adsorption may be dependent on the acid strength of the surface hydroxyl groups of the support and the basic strength of the metal precursors and Ni2+ in the present cases]. [PUNW:’ In contrast with the PTD and nickel nitrate, the dispersion of platinum was not greatly affected by the rehydroxylation of silica in the case of CPA. It is different from the PTD and the nickel salt in that the metal was included in its anion part and it was not adsorbed on the support judging by the fact that the colour of its dark yellow solution did not vary during the immersion of the support. The CPA was forced to deposit on the surface during the evaporation of water and this deposition was not dependent on the amount of surface hydroxyl groups probably due to the anionic character of the precursor. In conclusion, it should be again noted that the significance of surface hydroxyl groups in dispersing metal on silica was also demonstrated for PTD; however, the adsorption of PTD by ion exchange contributed much more to the impregnation of the precursors than was the case with nickel nitrate which was reported previously. In addition, the degree of metal dispersion was maximal for the support which was hydroxylated to a certain degree in the cases of PTD and nickel nitrate, and it is hence believed that a certain type of surface hydroxyl groups is responsible for the effects observed.
148 REFERENCES 1 2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
M. Boudart and G. Djega-Mariadassou, Kinetics of Heterogenous Catalytic Reactions, Princeton University Press, Princeton, NJ, 1984. B. Imelik, C. Naccache, G. Coudurier, H. PraIiaud, P. Meriaudeau, G. GaIlezot, G.A. Martin and J.C. Vedrine (Editor), Metal-Support and Metal-Additive Effects in Catalysis, Stud. Surf. Sci. CataI., Vol. 11, Elsevier, Amsterdam, 1982. D.L. Trimm, Design of Industrial Catalysts, Elsevier, Amsterdam, 1980. S.-L. Guo, M. Arai and Y. Nishiyama, Appl. Catal., 65 (1990) 31. C. Prado-Burguete, A. Linares-Solano, F. Rodriguez-Reinoso, and C. Salinas-Martinez de Lecea, J. Catal., 115 (1989) 98. A.A. Chen, M.A. Vannice and J. Phillips, J. Phys. Chem., 91 (1987) 6257. M. Arai, Y. Ikushima and Y. Nishiyama, Bull. Chem. Sot. Jpn., 59 (1986) 347. Y. Ikushima, M. Arai, and Y. Nishiyama, Appl. Catal., 11 (1984) 305. H.A. Benesi, R.M. Curtis and H.P. Studer, J. Catal., 10 (1968) 328. D.B. Powell and N. Sheppard, J. Chem. Sot., (1956) 3108. K. Nakamoto, J. Fujita and H. Murata, J. Am. Chem. Sot., 80 (1958) 4817. G.F. Svatos, C. Curran and J.V. Qua&no, J. Am. Chem. Sot., 77 (1955) 6159. S. Mizushima and I. Nakagawa, Nippon Kagaku Zasshi, 80 ( 1959 ) 124. M. Nogami and Y. Moriya, Yogyo Kyokai Shi, 85 (1977) 59. D. Bradley, R. Mehrotra and D. Gaur, Metal AIkoxides, Academic Press, New York, 1978. H. Suzuki, S. Takasaki, F. Koga, A. Ueno, Y. Kotera, T. Sat0 and N. Todo, Chem. Lett., (1982) 127. L.T. Zhuravlev, Langmuir, 3 (1987) 316. H.-P. Boehm and H. KnGzinger, in J.R. Anderson and M. Boudart(Editor), Catalysis: Science and Technology, Springer-Verlag, New York, 1983, Vol. 4, p. 39. J.R. Anderson, Structure of Metallic Catalysts, Academic Press, New York, 1975.
Applied Catalysis, 77 (1991) 141-148 Elsevier Science Publishers B.V., Amsterdam
Effects of hydroxylation of a silica surface on the metal dispersion in supported platinum catalysts Masahiko Arai*, Shi-Ling Guo’ and YoshiyukiNishiyama Chemical Research Institute of Non-aqueous Solutions, TO/K&U University, Sendai 980 (Japan), tel. (+ 81-22) 2276266, fax. (+ 81-22) 2238956
Katahira,
(Received 13 March 1991, revised manuscript received 21 May 1991)
Abstract The influence of the surface hydroxyl groups of porous silica on the degree of platinum dispersion was studied using silica supports with the same surface area but with an increasing amount of these functional groups. The supports were prepared by immersing a calcined silica in water at 70°C for various periods of time. Platinum was deposited on the supports by wet impregnation with aqueous solutions of platinum tetraamine dichloride (PTD ) and chloroplatinic acid (CPA). For catalysts with PTD reduced at 4OO”C,the degree of platinum dispersion indicated a maximum for a support whose surface was moderately hydroxylated. Such an effect was not observed for the catalysta reduced at the lower temperature of 330°C and for those with CPA. The surface hydroxyl groups affected the reduction of PTD and certain types of surface hydroxyl groups may be responsible for the effects observed. Keywords: catalyst preparation (wet impregnation), hydroxylation, platinum dispersion, silica.
INTRODUCTION
The metal dispersion of supported metal catalysts is dependent on various factors and changes in metal dispersion are often attended by significant changesin the catalyticperformanceof the catalysts [l-3 1.One of the important factors is the surfacepropertiesof the support materialsused and several recentstudieshave demonstratedthat certainfunctionalgroupson the surface of silica and carbon supports may play an important role in dispersingmetals [4-7 1.In a previousstudy [ 41, the degreeof nickel dispersionon a porous silica gel was found to increasewhen it was merely calcined at 400-8OO”Cprior to metal loading in spite of using a wet impregnationtechniquewith an aqueous solution. The nickel dispersion was related to the change of the surface hydroxyl groups on the silica although its surface area and porosity were also changed by the calcination. This view was supported by an additional result that rehydroxylationof calcined silica decreasedthe degree of nickel disper‘Present address: Chemiatry Department, Zheng Zhou Industry College, People’s Republic of China.
01669934/91/$03.50
0 1991 Elaevier Science Publishere B.V. All righta reserved.
142
sion. Prado-Burgueteet al. [ 51 usedcarbon materialswith the samepore structure but with an increasingamount of oxygen-containingsurface groups prepared by mild oxidation;they noted the importanceof these surfacefunctional groupson the dispersionof supportedplatinum on carbon. The present study has been focused, to a greater degree, on the effect of surfacehydroxyl groups on the dispersion of metals on porous silica. We prepared supportswith increasingamounts of these surfacefunctional groupsby rehydroxylatinga calcined silica gel. Platinum was loaded on these supports by wet impregnationwith aqueoussolutionsof PTD and CPA, differingin that the platinumwas includedin either a cation or an anion part and the adsorption of the precursorby ion exchangemay or may not contributeto the metal loading. The effect of the surface hydroxyl groups was shown for PTD in a similarmannerto the nickel nitrateas mentionedabove but not for CPA. The possible actions of the surfacehydroxylgroupsin dispersingplatinumover the silica surfacewill be discussed. EXPERIMENTAL
The support materialused was a powdered (32-60 mesh), porous silica gel, Silbead-N (MizusawaIndustrialChemicals), which includesaluminain 2 wt.%. The silica gel was calcined in air at 700°C for five hours and was immersed in distilledwater at 70’ C for variousperiods of time in order to prepare silica supportswith different degreesof rehydroxylation.After immersionin water, the silica powder was vacuumdried at 110’ C and stored in a desiccatorbefore use. The differently rehydroxylated silica supports will be hereinafter expressed as e.g. SI (3) where SI denotes SiOz and the figure in parenthesesis the period of time of the rehydroxylationin hours. A wet impregnationmethod was used to prepare 0.5 wt.-% platinum/silica catalystsfrom aqueoussolutions of PTD and CPA. A known amount of silica powder was added to the precursor solution in a recovery flask and kept immersedovernightat room temperaturewithout agitation.The initialpH of the solution was 7.0 for PTD and 2.1 for CPA. The suspensionof silica powder in the solution was rotationallyagitatedat about 60 rpm for 10 min and then the solvent was evaporated under reduced pressure at around 50°C within one hour by rotational agitation using a rotary evaporator.To examine the possibility of adsorption of PTD, the residualamount of platinum in the impregnating solution was measuredby atomic absorption spectrophotometry.The silica supports impregnatedwith the precursor were vacuum dried at 110°C for four hoursand reducedby flowinghydrogenat 400 or 330’ C usinga heating schedule such that the supports were heated at 7 K min-’ up to the desired reductiontemperature,kept at this temperaturefor two hours, and cooled to room temperaturein about five hours. The surface area and porosity of the silica gels were measuredby nitrogen
143
adsorptionmethodsand their surfacehydroxylgroupswere measuredby Fouriertransforminfraredspectroscopy (FT-IR) with a diffusereflectancemethod for a mixture of silica gel (ca. 1 wt.-% ) and potassium bromide. The mixture was preparedby grindingknown amounts of silica and potassium bromide in a mortar in an air-conditionedroom and the FT-IR spectrumwas collected for the mixture in flowing dry nitrogen. The PI’-IR spectra obtained with this procedure gave the relativeamounts of surface hydroxyl groupsbeing in good agreementwith those obtained from thermogravimetryas described previously [41The degreeof platinumdispersionof the preparedcatalystswas determined by temperature-programmeddesorption (TPD) of hydrogen. They were exposed to flowing hydrogenat 30 ml min-’ for 15 min at 30” C and then heated in argon at 30 ml rain-’ by linearheatingat 30 K min-’ up to 450” C usingthe apparatusand procedures described previously [81. The dispersion of metal precursorswas examined by FT-IR with the above-mentioned diffuse reflectance method and X-ray photoelectron spectroscopy (XPS ). RESULTS
Dispersion of platinum
The influence of the rehydroxylationof silica gel on the degreeof platinum dispersion for the catalysts reduced at 400°C is shown in Fig. la. The rehydroxylationaffected the platinumdispersionin the case of PTD, for which the dispersionincreased,reacheda maximum,and then decreasedwith increasing extent of rehydroxylation.This change is in accordance with the changes in nickel dispersionwith dehydroxylationby calcination and rehydroxylationof the same silica supportas reportedpreviously [ 41. Fig. la showsthe maximum degree of platinum dispersion attained for SI (6)) which was more than thirtyfold and sevenfold greaterthan that compared to SI (0) and SI (15)) respec(b)
i“ .
1
l
o-
.
.
Time
(h)
Fig. 1. Influence of the hydroqdation of eilica by immersion in water at 70°C on (a) the degree of platinum dispersion mewned by TPD ( l : PTD, 0: CPA) and-(b) the total amount of surface hydroxyl groups relative to SI (0) measured by FT-IR.
144
tively. For the catalysts with CPA, in contrast, the rehydroxylation had little influence on the platinum dispersion. In the case of PTD, when reduction was performed at a lower temperature of 330’ C, the resultant platinum dispersion was not greatly influenced by the rehydroxylation; it was 35, 32, and 31% for SI(O), SI(6), and SI(15), respectively. Impregnation and dispersion of PTD
The effect of rehydroxylation of the platinum dispersion was observed for PTD as described above and as a result we chiefly focussed our attention on it and we examined the dispersion of PTD precursor before reduction. The adsorption of PTD on silica was previously reported by Benesi et al. [91 and they demonstrated that this was due to ion exchange between the platinum tetraamine ions and protons of the surface hydroxyl groups having sufficient Brensted acidity. To examine the adsorption of PTD on our silica gel, we measured the residual amount of platinum in the impregnating solution after immersing silica in it overnight. The amount of platinum in the solution decreased to less than one tenth of the initial value for the cases of SI (0)) SI (1)) SI (4.5)) SI (6.5)) and SI (10). These silica supports, namely, adsorbed more than 90% of the PTD initially present in the solution. PTD was therefore shown to be deposited on all of our silica gels by ion exchange irrespective of
1
4000
I
I
I
3200
2400
Wovenumber
EOO
860
-
(cm’)
Fig.2.FT-IRspectraofPTD (a),SI(O) (b),andPTDaupportedonSI(O) (c),SI(3) (d),SI(8.5) (e), and SI(15) (f) before reduction. All the -plea were dried at 110°C. Arrows indicate absorption bands at 1414 cm-’ which appeared for supported PTD samples.
145
the extent of the rehydroxylation of their surfaces. Therefore PTD was believed to be very highly dispersed on the silica supports after the evaporation of water followed by vacuum drying. Fig. 2 shows the FT-IR spectra for supported and unsupported PTD and SI (0) after vacuum drying at 110” C. The rehydroxylated supports indicated similar spectra to SI (0). Unsupported PTD has a sharp band due to Pt-N at 511 cm-’ [ 10,111 and several sharp and broad bands due to the ligand of ammonia at around 900, 1400, 1600, and 3230 cm-’ [12,13]. These absorption bands are not seen but a weak absorption band appears similarly at 1414 cm-’ for ail of the supported PTD samples examined as can be seen from comparison with the spectrum of SI (0). It is not clear whether this absorption at 1414 cm- ’ is a shift of the band for the ammonia ligand due to the presence of the support or other causes. It was reported that the absorption due to the formation of SiO-metal appeared at wavenumbers of 1000 to 900 cm-’ [ 14-161. The XPS measurements for supported PTD indicated that the ratios of peak areas of Pt 4d to Si 2p were 0.8, 1.0,0.9, and 0.9 for SI(O), SI(4.5), SI(8.5), and SI (15)) respectively, being the same for these samples within experimental errors. This suggested that the degree of PTD dispersion was similar for these supports and it could not be responsible for the significant differences in the degree of platinum dispersion observed after reduction as shown in Fig. la. The binding energy of platinum could not be determined accurately mainly due to its small loading. Rehydroxylution of silica Fig. lb shows the increase in the amount of surface hydroxyl groups on the rehydroxylation as estimated by integrating absorbance from 3600 to 2800 cm- ’ of FT-IR spectra. Typical FT-IR spectra are given in Fig. 3. Before the rehydroxylation, the silica gel had a surface area of 410 m2 g-l, a total pore volume of 0.20 ml g-’ with an average pore diameter of 2.5 nm, and a total amount of
Wavenumber (cm-‘) Fig.3.E”I’-IRspectraofSI(0)
(a),SI(3)
(b),SI(6)
(c),andSI(12)
(d).
146
surface hydroxyl groups of 8.2 x 10m4mol g-’ corresponding to a density of 1.2 groups nm -‘. The surface area and pore volume were not influenced by the rehydroxylation [ 41. The amount of surface hydroxyl groups was increased by a factor of about 1.7 with the rehydroxylation for fifteen hours. The SI(15) had a density of 2.0 groups nme2 and so it was still not fully hydroxylated [ 171. The total amounts of the surface hydroxyl groups of all the supports used were larger than the amount of platinum loaded. For SI (0)) for example, the molar ratio of these functional groups to platinum atoms loaded was about 30. FTIR spectra indicated that the silica supports had a few types of surface hydroxyl groups [ 181 but it was not clear in Fig. 3 whether a certain type significantly varied or not upon rehydroxylation. DISCUSSION
The present results demonstrate that the dispersion of platinum is significantly influenced by the rehydroxylation of silica prior to metal loading in the case of PTD. The maximum degree of metal dispersion was observed for the silica that was moderately rehydroxylated (Fig. la and b ) . The PTD was chiefly deposited on the silica supports by ion exchange with the protons of the surface hydroxyl groups of Br0nsted acidity, and it was highly dispersed on all the supports used. It is therefore believed that the following reduction of PTD precursors was affected by the rehydroxylation of the support, which influenced the reducibility of the PTD and the sintering of species formed during the reduction; this sintering effect may be more important. This idea would also be supported by the fact that the effect of rehydroxylation appeared in the case of reduction at 400°C but not at the lower temperature of 330°C. The PTD could be reduced at a temperature as low as 300 ’ C and the complex of Pt ( NHs)2H2 was suggested to be the species of high surface mobility [ 191. There was a maximum in the effect of rehydroxylation on the platinum dispersion for a moderately rehydroxylated support although all the supports examined had sufficient amounts of surface hydroxyl groups compared with the amount of platinum loaded. It was thus suggested that the observed effect was not related to all the surface hydroxyl groups but to those of a certain type. This type could be specified by the strength of the Brnrnstedacidity or other parameters. Such a specification could not be attained in the present work although we attempted a differential thermal analysis of silica gels as well as FT-IR. We will compare the present results for PTD with those for nickel reported previously [ 41. Similar effects were also found for nickel/silica catalysts prepared from nickel nitrate with the same procedures as used in the present work. However, the dispersion of the nickel precursors was dependent on the amount of surface hydroxyl groups of the support used and this is not the case for PTD as mentioned above. We previously invoked a dynamic wettability for the role
147
of these surface groups in the observed effect for nickel. At a later stage in water evaporation, droplets of the impregnating solution could be formed on the surface of the support, the droplets on the more hydrophobic surfaces would be more easily split or shrunk because it is difficult for the solvent, water, to wet such surfaces. This would result in the formation of smaller droplets on the more hydrophobic surfaces than on the less hydrophobic ones. Smaller crystallites of the nickel salt would then be deposited from the smaller droplets onto the more hydrophobic surfaces at the end of evaporation of the solvent rather than onto the less hydrophobic surfaces. This idea may be valid for catalyst preparations when the deposition of precursors takes place through a forced process by the evaporation of the solvent. In the case of nickel, the nickel nitrate was generally deposited in such a forced manner but it was also deposited by adsorption, like PTD, although the amount adsorbed was less than one tenth of the total amount impregnated [4]. This fact and the results for the PTD suggest an additional explanation to that of dynamic wettability for the results on nickel. One possibility is that a small amount of nickel precursor is adsorbed on the surface of the support upon its immersion in the impregnating solution and the remainder is forced to deposit preferentially on those pre-adsorbed precursors upon the evaporation of water. The contribution of adsorption may be dependent on the acid strength of the surface hydroxyl groups of the support and the basic strength of the metal precursors and Ni2+ in the present cases]. [PUNW:’ In contrast with the PTD and nickel nitrate, the dispersion of platinum was not greatly affected by the rehydroxylation of silica in the case of CPA. It is different from the PTD and the nickel salt in that the metal was included in its anion part and it was not adsorbed on the support judging by the fact that the colour of its dark yellow solution did not vary during the immersion of the support. The CPA was forced to deposit on the surface during the evaporation of water and this deposition was not dependent on the amount of surface hydroxyl groups probably due to the anionic character of the precursor. In conclusion, it should be again noted that the significance of surface hydroxyl groups in dispersing metal on silica was also demonstrated for PTD; however, the adsorption of PTD by ion exchange contributed much more to the impregnation of the precursors than was the case with nickel nitrate which was reported previously. In addition, the degree of metal dispersion was maximal for the support which was hydroxylated to a certain degree in the cases of PTD and nickel nitrate, and it is hence believed that a certain type of surface hydroxyl groups is responsible for the effects observed.
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