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Influence of preparation conditions on properties of gold loaded on the supports containing group five elements
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Influence of preparation conditions on properties of gold loaded on the supports containing group five elements Izabela Sobczak*, Justyna Florek, Katarzyna Jagodzinska, Maria Ziolek A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland
Abstract Gold catalysts based on MCM-41 modified with vanadium and niobium, SBA-3 and group V metal oxides were prepared by several methods. The properties of the materials obtained were characterised by nitrogen adsorption, XRD, TEM, UV-Vis, and test reactions (AcoAc cyclisation and methanol oxidation). The best dispersion of gold was reached when it was introduced during the synthesis of MCM-41 with the use of H2SO4 as pH adjusting agent and Nb source besides Na silicate. Acid/base properties were determined by the preparation methods and the presence of group five elements. Keywords: AuNbVMCM-41; AuSBA-3; Au/V,Nb,Ta-oxides; acidity/basicity
1. Introduction Following the breakthrough research results of Hutchings and Haruta, there has been a dramatic increase in the interest in gold catalysis [1]. It has been demonstrated that the physicochemical and catalytic properties of gold catalysts depend mainly on the type of support and the preparation method. Both parameters influence the size of Au clusters [1]. Interaction between gold and the metals localized in the support plays also an important role and can determine the catalytic activity of the catalysts. The idea of this work is to use two groups of materials, silicate or metalosilicate (Me=Nb, V) hexagonally ordered mesoporous molecular sieves and transition metal oxides (V2O5, Nb2O5, Ta2O5) as supports for gold. The first group of samples exhibits very high surface area and the presence of isolated metal species, whereas bulk metal oxides are characterized by smaller surface areas and much higher concentration of metals on the surface. The main focus of this study is the influence of the methods of support syntheses and gold loading and their effect on the physicochemical properties of materials prepared.
2. Experimental 2.1. Preparation of the catalysts SBA-3 material was synthesized following the procedure reported originally by Stucky et al. [2]. NbMCM-41 samples were synthesized by hydrothermal method [3] and modified with gold according to [4,5]. Au/MCM-41 (with gold loading of 1 wt%) and Au/SBA-3 (3wt % of Au) were prepared by incipient wetness impregnation (IMP) of the support with HAuCl4 (Johnson Matthey). The alternative, direct synthesis of AuSBA-3, AuMCM-41, AuVMCM-41 and AuVNbMCM-41 (COP- co-precipitation of all components in one pot synthesis) was performed in the same manner as conventional MCM-41 [6] and SBA-3 [2]. The only difference was the admission of HAuCl4,
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vanadium(IV) oxide sulphate hydrate and ammonium niobate(V) oxalate as the sources of Au, V or Nb, respectively and the use of HCl or H2SO4 as pH adjustment agent. Commercial oxides (V2O5,Ta2O5 –Aldrich, Nb2O5 –Alfa Aesar) were modified by goldsol method [7] with tetrakis(hydroksymethyl)phosphonium chloride(THPC) as reducing agent and HAuCl4 as a source of gold (1 wt.% of Au) and, additionally, by depositionprecipitation (DP) method with urea [7]. The prepared materials were calcined at 623 K.
2.2. Characterization The XRD patterns were obtained on a D8 Advance diffractometer (Bruker) using CuKα radiation (λ=0.154 nm). The surface area and pore volume of the samples were measured by nitrogen adsorption at 77 K, using the conventional procedure on a Micromeritics 2010 apparatus. The UV–visible spectra were recorded on a Cary 300Scan (Varian) spectrometer in the range from 800-180 nm. For transmission electron microscopy (TEM) measurements powders were deposited on a grid with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV. The catalysts were tested in acetonylacetone (AcoAc) cyclisation at 623 K and methanol oxidation at 473 and 523 K as the probe reactions under conditions described in [4,5].
3. Results and discussion 3.1. AuMCM-41 and AuSBA-3 The main difference in the preparation of AuMCM-41 and AuSBA-3 in one pot synthesis is the use of an alkaline medium (pH=11) in the first one and an acidic medium (pH=1) in the second synthesis. Moreover, different sources of silicon (sodium silicate and TEOS, respectively) are applied. Results of this study have proved a significant difference in the surface properties of both materials containing metallic gold particles on the surface. Acetonylacetone (AcoAc) cyclisation allows us to evaluate acidity and basicity of the surface on the basis of the selectivity to methylcyclopentanone (MCP) and dimethyl furan (DMF) [8]. MCP/DMF<<1 indicates the acidic character of the surface and MCP/DMF>>1 – basic properties of the surface. Highly basic character of AuMCM-41 is demonstrated by MCP/DMF ratio equal 104, whereas it is only 0.11 for AuSBA-3 (Table 1). The acidity of AuSBA-3 is confirmed by dehydration activity (dimethyl ether formation) in methanol oxidation (Table 2). Interestingly, SBA-3 impregnated with gold (Au/SBA-3) reveals oxidative properties like AuMCM-41 demonstrated by selectivity to formaldehyde and methyl formate. It means that acidity of the surface is not generated during the synthesis of silicate SBA-3 and impregnation with chloroauric acid but it results from gelation of TEOS together with HAuCl4 in the presence of Pluronic and HCl at pH=1. Table 1. Texture properties of the catalysts and selectivity ratio in AcoAc cyclisation at 623 K. Catalyst AuSBA-3 AuMCM-41(HCl) Au/NbMCM-41(IMP) AuNbMCM-41(HCl) AuVMCM-41(HCl) AuVMCM-41(H2 SO4 ) AuVNbMCM-41(HCl) AuVNbMCM-41(H2SO4)
Average pore vol. BJH, Surface area (ads.) cm3g-1 BET, (ads.) m2g-1 996 0.45 886 0.81 900 1.00 870 0.86 813 0.80 1055 1.34 851 1.08 1042 1.05
* MCM = methylcyclopentenon; DMF = dimethylfuran
MCP/DMF* (AcoAc cyclisation) 0.11 104 0.23 9.00 22.0 0.19 16.0 0.05
Influence of the preparation conditions on the properties of gold loaded
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Participation of gold source in the formation of acidic centers is clearly deduced from these results. Table 2. The results of methanol oxidation at 473 K. Catalyst
MeOH conv., % 25 8 38
AuSBA-3 Au/SBA-3 AuMCM-41
HCOH sel., % 56 2
HCOOCH3 sel., % 91
CH3OCH3 sel., % 99.9 -
CO2 sel., % 0.1 44 7
3.2. MeMCM-41(Me=Nb, V) materials containing gold – effect of preparation method and synthesis conditions MCM-41 material has been chosen for the further study of the interaction of gold with group V metals located in the structure of mesoporous material. Gold containing NbMCM-41 materials were prepared by two manners: impregnation (Au/NbMCM-41) and co-precipitation (AuNbMCM-41). The introduction of gold during the synthesis leads to the catalysts with more disordered structure and lower surface area and pore volume compared to the material obtained by impregnation (Table 1). The Au-metal crystallites are present on both Au-catalysts (XRD, UV-Vis – Fig. 1). However, the peaks assigned to the metallic gold in the XRD patterns (at 2Θ = 38.2° and 44.8°) are sharper for the impregnated material indicating larger Au agglomerates. The introduction of Au during the synthesis strongly enhances the dispersion of Au. AuVNbMCM-41 (HCl)
Intensity, a.u.
10
AuVNbMCM-41 (H2SO4)
0.5
Au
AuVNbMCM-41 (HCl) AuVMCM-41 (HCl) 35
40
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2Θ ,
50 o
55
60
F (R)
Au/NbMCM-41 (IMP) AuVNbMCM-41 (H2SO4)
0
AuVNbMCM-41 (H2SO4)
AuVMCM-41 (HCl)
AuVNbMCM-41 (HCl)
Au/MCM-41 (IMP) 300 400 500 600 700 800
Wavelenght, nm
Fig. 1. XRD patterns, TEM images and UV-Vis spectra of selected MCM-41 materials.
To obtain the catalysts with high gold dispersion, AuVMCM-41 and AuVNbMCM-41 materials were prepared by COP method with the use of HCl or H2SO4 as pH adjustment agent. As shown in Table 1, the use of HCl leads to the catalysts with lower surface area and pore volume when compared to those of AuVMCM-41 (H2SO4) and AuVNbMCM-41(H2SO4) materials. Moreover, MCM-41 prepared with HCl shows disordering of hexagonal structure. Considering the size and dispersion of gold, it was found on the basis of XRD patterns (Fig. 1) that bigger Au agglomerates are formed on the surface of Au(V,Nb)MCM-41 (HCl). TEM images (Fig. 1) confirm this conclusion. The average size of Au crystallites in AuVMCM-41(HCl) and AuVNbMCM-41(HCl) was 45-50 nm. The application of H2SO4 to adjust pH during the synthesis leads to much smaller gold particles (~20 nm). Moreover, Nb species located in MCM-41 samples plays the role of a structural promoter that decreases the agglomeration of gold. TEM images do not show the smaller Au particles located inside the channels. The use of HCl or H2SO4 during the synthesis of MCM-41 materials influences also the acid-base properties of the catalyst surface studied by AcoAc transformation (Table 1). AuVMCM-41(H2SO4) and AuVNbMCM-41(H2SO4) materials exhibit acidic properties
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(MCP/DMF << 1). The application of HCl during the synthesis significantly increases the basicity of the material (MCP/DMF >>1). There is no doubt that the presence of chlorine near Au species is responsible for a very high basicity of gold-MCM-41, as was indicated earlier for AuMCM-41 [9]. The introduction of V and mainly Nb into MCM-41 together with Au in one pot synthesis diminishes the basicity. The interaction between Au and group five elements in MCM-41 determines the surface properties also in methanol oxidation. Such interaction in AuVNbMCM-41 (H2SO4) results in the highest selectivity to formaldehyde because of the weaker chemisorption of HCHO, whereas bimetallic catalysts (AuVMCM-41(HCl) and AuNbMCM-41(HCl)) are the most active in CO2 formation because of their basicity [5].
3.3. Group V metals oxides - effect of preparation method
o
Au
Au/Nb2O5 (THPC)
Nb2O5 30
4. Conclusions
Au/Nb2O5 (DPU)
500
Intensity a.u.
Gold was introduced into group V metal oxides (V2O5, Nb2O5, Ta2O5) by two methods: via deposition-precipitation with urea and via gold-sol method with THPC as a reducing agent. In XRD patterns of all calcined materials the reflections characteristic of metallic gold are well visible on the catalysts prepared by DP method indicating bigger Au particles than that when gold sol method is used (Fig. 2 -example for Au/Nb2O5) as confirmed by TEM images. The average particle size in the Au/Nb2O5 prepared by DP method is about 20 nm, whereas in the sample prepared by gold-sol method it is of about 6 nm. THPC used during gold-sol method stabilizes the colloid gold solutions and that is why gold particle sizes are smaller and their dispersion is higher.
35
40
2Θ,o
45
Dispersion of gold is better when Au is introduced into Fig. 2. XRD patterns of Nb2O5 samples. ordered mesoporous material in one pot synthesis than in the case of using the impregnation method. Au crystallite size depends on the nature of acid used as pH adjusting agent in one pot synthesis (HCl promotes agglomeration of gold) and on the chemical composition of MCM-41 material (the presence of Nb decreases Au agglomeration because of strong Au-Nb interaction). Dispersion of Au on metal oxides is higher than on MeMCM-41 because of higher concentration of group V metals. Gold-sol method using for the modification of metal oxides gives rise to a higher gold dispersion than deposition-precipitation one. That is why the modification with Au using THPC as reducing agent is recommended. Acid/base properties of mesoporous gold – silica prepared in one pot strongly depends on pH of the synthesis medium. Acidity dominates for AuSBA-3, whereas basicity is characteristic of AuMCM-41. Oxidative properties of Au-MCM-41 catalysts are determined by the preparation methods and the presence of V and Nb.
Acknowledgements Polish Ministry of Science (Grant No. N N204 032536) is acknowledged for the partial financial support of this work. Acknowledge is made also to Johnson Matthey (UK) for supplying HAuCl4.
Influence of the preparation conditions on the properties of gold loaded
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References 1. 2. 3. 4. 5. 6. 7. 9.
G.C. Bond, C. Luis, D.T. Thompson, 2006, Catalysis by Gold, Imperial College Press Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, 1994, Chem. Mater., 6, 1176 M. Ziolek, I. Nowak, 1997, Zeolites,18, 377 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, Stud. Surf. Sci. Catal., 70, 1300 I. Sobczak, N. Kieronczyk, M. Trejda, M. Ziolek, 2008, Catal. Today 139, 188 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, 1992, Nature, 359, 710 S. Demirel-Gulen, M. Lucas, P. Claus, 2005, Catal. Today 102–103, 1668. R.M. Dessau, 1990, Zeolites, 10, 205 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, J. Catal., 245, 259
Influence of preparation conditions on properties of gold loaded on the supports containing group five elements Izabela Sobczak*, Justyna Florek, Katarzyna Jagodzinska, Maria Ziolek A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland
Abstract Gold catalysts based on MCM-41 modified with vanadium and niobium, SBA-3 and group V metal oxides were prepared by several methods. The properties of the materials obtained were characterised by nitrogen adsorption, XRD, TEM, UV-Vis, and test reactions (AcoAc cyclisation and methanol oxidation). The best dispersion of gold was reached when it was introduced during the synthesis of MCM-41 with the use of H2SO4 as pH adjusting agent and Nb source besides Na silicate. Acid/base properties were determined by the preparation methods and the presence of group five elements. Keywords: AuNbVMCM-41; AuSBA-3; Au/V,Nb,Ta-oxides; acidity/basicity
1. Introduction Following the breakthrough research results of Hutchings and Haruta, there has been a dramatic increase in the interest in gold catalysis [1]. It has been demonstrated that the physicochemical and catalytic properties of gold catalysts depend mainly on the type of support and the preparation method. Both parameters influence the size of Au clusters [1]. Interaction between gold and the metals localized in the support plays also an important role and can determine the catalytic activity of the catalysts. The idea of this work is to use two groups of materials, silicate or metalosilicate (Me=Nb, V) hexagonally ordered mesoporous molecular sieves and transition metal oxides (V2O5, Nb2O5, Ta2O5) as supports for gold. The first group of samples exhibits very high surface area and the presence of isolated metal species, whereas bulk metal oxides are characterized by smaller surface areas and much higher concentration of metals on the surface. The main focus of this study is the influence of the methods of support syntheses and gold loading and their effect on the physicochemical properties of materials prepared.
2. Experimental 2.1. Preparation of the catalysts SBA-3 material was synthesized following the procedure reported originally by Stucky et al. [2]. NbMCM-41 samples were synthesized by hydrothermal method [3] and modified with gold according to [4,5]. Au/MCM-41 (with gold loading of 1 wt%) and Au/SBA-3 (3wt % of Au) were prepared by incipient wetness impregnation (IMP) of the support with HAuCl4 (Johnson Matthey). The alternative, direct synthesis of AuSBA-3, AuMCM-41, AuVMCM-41 and AuVNbMCM-41 (COP- co-precipitation of all components in one pot synthesis) was performed in the same manner as conventional MCM-41 [6] and SBA-3 [2]. The only difference was the admission of HAuCl4,
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vanadium(IV) oxide sulphate hydrate and ammonium niobate(V) oxalate as the sources of Au, V or Nb, respectively and the use of HCl or H2SO4 as pH adjustment agent. Commercial oxides (V2O5,Ta2O5 –Aldrich, Nb2O5 –Alfa Aesar) were modified by goldsol method [7] with tetrakis(hydroksymethyl)phosphonium chloride(THPC) as reducing agent and HAuCl4 as a source of gold (1 wt.% of Au) and, additionally, by depositionprecipitation (DP) method with urea [7]. The prepared materials were calcined at 623 K.
2.2. Characterization The XRD patterns were obtained on a D8 Advance diffractometer (Bruker) using CuKα radiation (λ=0.154 nm). The surface area and pore volume of the samples were measured by nitrogen adsorption at 77 K, using the conventional procedure on a Micromeritics 2010 apparatus. The UV–visible spectra were recorded on a Cary 300Scan (Varian) spectrometer in the range from 800-180 nm. For transmission electron microscopy (TEM) measurements powders were deposited on a grid with a holey carbon film and transferred to JEOL 2000 electron microscope operating at 80 kV. The catalysts were tested in acetonylacetone (AcoAc) cyclisation at 623 K and methanol oxidation at 473 and 523 K as the probe reactions under conditions described in [4,5].
3. Results and discussion 3.1. AuMCM-41 and AuSBA-3 The main difference in the preparation of AuMCM-41 and AuSBA-3 in one pot synthesis is the use of an alkaline medium (pH=11) in the first one and an acidic medium (pH=1) in the second synthesis. Moreover, different sources of silicon (sodium silicate and TEOS, respectively) are applied. Results of this study have proved a significant difference in the surface properties of both materials containing metallic gold particles on the surface. Acetonylacetone (AcoAc) cyclisation allows us to evaluate acidity and basicity of the surface on the basis of the selectivity to methylcyclopentanone (MCP) and dimethyl furan (DMF) [8]. MCP/DMF<<1 indicates the acidic character of the surface and MCP/DMF>>1 – basic properties of the surface. Highly basic character of AuMCM-41 is demonstrated by MCP/DMF ratio equal 104, whereas it is only 0.11 for AuSBA-3 (Table 1). The acidity of AuSBA-3 is confirmed by dehydration activity (dimethyl ether formation) in methanol oxidation (Table 2). Interestingly, SBA-3 impregnated with gold (Au/SBA-3) reveals oxidative properties like AuMCM-41 demonstrated by selectivity to formaldehyde and methyl formate. It means that acidity of the surface is not generated during the synthesis of silicate SBA-3 and impregnation with chloroauric acid but it results from gelation of TEOS together with HAuCl4 in the presence of Pluronic and HCl at pH=1. Table 1. Texture properties of the catalysts and selectivity ratio in AcoAc cyclisation at 623 K. Catalyst AuSBA-3 AuMCM-41(HCl) Au/NbMCM-41(IMP) AuNbMCM-41(HCl) AuVMCM-41(HCl) AuVMCM-41(H2 SO4 ) AuVNbMCM-41(HCl) AuVNbMCM-41(H2SO4)
Average pore vol. BJH, Surface area (ads.) cm3g-1 BET, (ads.) m2g-1 996 0.45 886 0.81 900 1.00 870 0.86 813 0.80 1055 1.34 851 1.08 1042 1.05
* MCM = methylcyclopentenon; DMF = dimethylfuran
MCP/DMF* (AcoAc cyclisation) 0.11 104 0.23 9.00 22.0 0.19 16.0 0.05
Influence of the preparation conditions on the properties of gold loaded
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Participation of gold source in the formation of acidic centers is clearly deduced from these results. Table 2. The results of methanol oxidation at 473 K. Catalyst
MeOH conv., % 25 8 38
AuSBA-3 Au/SBA-3 AuMCM-41
HCOH sel., % 56 2
HCOOCH3 sel., % 91
CH3OCH3 sel., % 99.9 -
CO2 sel., % 0.1 44 7
3.2. MeMCM-41(Me=Nb, V) materials containing gold – effect of preparation method and synthesis conditions MCM-41 material has been chosen for the further study of the interaction of gold with group V metals located in the structure of mesoporous material. Gold containing NbMCM-41 materials were prepared by two manners: impregnation (Au/NbMCM-41) and co-precipitation (AuNbMCM-41). The introduction of gold during the synthesis leads to the catalysts with more disordered structure and lower surface area and pore volume compared to the material obtained by impregnation (Table 1). The Au-metal crystallites are present on both Au-catalysts (XRD, UV-Vis – Fig. 1). However, the peaks assigned to the metallic gold in the XRD patterns (at 2Θ = 38.2° and 44.8°) are sharper for the impregnated material indicating larger Au agglomerates. The introduction of Au during the synthesis strongly enhances the dispersion of Au. AuVNbMCM-41 (HCl)
Intensity, a.u.
10
AuVNbMCM-41 (H2SO4)
0.5
Au
AuVNbMCM-41 (HCl) AuVMCM-41 (HCl) 35
40
45
2Θ ,
50 o
55
60
F (R)
Au/NbMCM-41 (IMP) AuVNbMCM-41 (H2SO4)
0
AuVNbMCM-41 (H2SO4)
AuVMCM-41 (HCl)
AuVNbMCM-41 (HCl)
Au/MCM-41 (IMP) 300 400 500 600 700 800
Wavelenght, nm
Fig. 1. XRD patterns, TEM images and UV-Vis spectra of selected MCM-41 materials.
To obtain the catalysts with high gold dispersion, AuVMCM-41 and AuVNbMCM-41 materials were prepared by COP method with the use of HCl or H2SO4 as pH adjustment agent. As shown in Table 1, the use of HCl leads to the catalysts with lower surface area and pore volume when compared to those of AuVMCM-41 (H2SO4) and AuVNbMCM-41(H2SO4) materials. Moreover, MCM-41 prepared with HCl shows disordering of hexagonal structure. Considering the size and dispersion of gold, it was found on the basis of XRD patterns (Fig. 1) that bigger Au agglomerates are formed on the surface of Au(V,Nb)MCM-41 (HCl). TEM images (Fig. 1) confirm this conclusion. The average size of Au crystallites in AuVMCM-41(HCl) and AuVNbMCM-41(HCl) was 45-50 nm. The application of H2SO4 to adjust pH during the synthesis leads to much smaller gold particles (~20 nm). Moreover, Nb species located in MCM-41 samples plays the role of a structural promoter that decreases the agglomeration of gold. TEM images do not show the smaller Au particles located inside the channels. The use of HCl or H2SO4 during the synthesis of MCM-41 materials influences also the acid-base properties of the catalyst surface studied by AcoAc transformation (Table 1). AuVMCM-41(H2SO4) and AuVNbMCM-41(H2SO4) materials exhibit acidic properties
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(MCP/DMF << 1). The application of HCl during the synthesis significantly increases the basicity of the material (MCP/DMF >>1). There is no doubt that the presence of chlorine near Au species is responsible for a very high basicity of gold-MCM-41, as was indicated earlier for AuMCM-41 [9]. The introduction of V and mainly Nb into MCM-41 together with Au in one pot synthesis diminishes the basicity. The interaction between Au and group five elements in MCM-41 determines the surface properties also in methanol oxidation. Such interaction in AuVNbMCM-41 (H2SO4) results in the highest selectivity to formaldehyde because of the weaker chemisorption of HCHO, whereas bimetallic catalysts (AuVMCM-41(HCl) and AuNbMCM-41(HCl)) are the most active in CO2 formation because of their basicity [5].
3.3. Group V metals oxides - effect of preparation method
o
Au
Au/Nb2O5 (THPC)
Nb2O5 30
4. Conclusions
Au/Nb2O5 (DPU)
500
Intensity a.u.
Gold was introduced into group V metal oxides (V2O5, Nb2O5, Ta2O5) by two methods: via deposition-precipitation with urea and via gold-sol method with THPC as a reducing agent. In XRD patterns of all calcined materials the reflections characteristic of metallic gold are well visible on the catalysts prepared by DP method indicating bigger Au particles than that when gold sol method is used (Fig. 2 -example for Au/Nb2O5) as confirmed by TEM images. The average particle size in the Au/Nb2O5 prepared by DP method is about 20 nm, whereas in the sample prepared by gold-sol method it is of about 6 nm. THPC used during gold-sol method stabilizes the colloid gold solutions and that is why gold particle sizes are smaller and their dispersion is higher.
35
40
2Θ,o
45
Dispersion of gold is better when Au is introduced into Fig. 2. XRD patterns of Nb2O5 samples. ordered mesoporous material in one pot synthesis than in the case of using the impregnation method. Au crystallite size depends on the nature of acid used as pH adjusting agent in one pot synthesis (HCl promotes agglomeration of gold) and on the chemical composition of MCM-41 material (the presence of Nb decreases Au agglomeration because of strong Au-Nb interaction). Dispersion of Au on metal oxides is higher than on MeMCM-41 because of higher concentration of group V metals. Gold-sol method using for the modification of metal oxides gives rise to a higher gold dispersion than deposition-precipitation one. That is why the modification with Au using THPC as reducing agent is recommended. Acid/base properties of mesoporous gold – silica prepared in one pot strongly depends on pH of the synthesis medium. Acidity dominates for AuSBA-3, whereas basicity is characteristic of AuMCM-41. Oxidative properties of Au-MCM-41 catalysts are determined by the preparation methods and the presence of V and Nb.
Acknowledgements Polish Ministry of Science (Grant No. N N204 032536) is acknowledged for the partial financial support of this work. Acknowledge is made also to Johnson Matthey (UK) for supplying HAuCl4.
Influence of the preparation conditions on the properties of gold loaded
337
References 1. 2. 3. 4. 5. 6. 7. 9.
G.C. Bond, C. Luis, D.T. Thompson, 2006, Catalysis by Gold, Imperial College Press Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, 1994, Chem. Mater., 6, 1176 M. Ziolek, I. Nowak, 1997, Zeolites,18, 377 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, Stud. Surf. Sci. Catal., 70, 1300 I. Sobczak, N. Kieronczyk, M. Trejda, M. Ziolek, 2008, Catal. Today 139, 188 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, 1992, Nature, 359, 710 S. Demirel-Gulen, M. Lucas, P. Claus, 2005, Catal. Today 102–103, 1668. R.M. Dessau, 1990, Zeolites, 10, 205 I. Sobczak, A. Kusior, J. Grams, M. Ziolek, 2007, J. Catal., 245, 259