Al2O3 Using Chemical Vapor

Dec 29, 2006 - Characterization of SiO2 Deposited by CVD on MoxOy/Al2O3 using NMR and TOF-SIMS. K. A. Boateng , J. M. Hill. Chemical Vapor Deposition ...
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Ind. Eng. Chem. Res. 2007, 46, 684-690

Controlled Pore Opening of Ni/Al2O3 Using Chemical Vapor Deposition in a Fluidized Bed Reactor Kenneth A. Boateng, Linjie Hu, and Josephine M. Hill* Department of Chemical and Petroleum Engineering, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta T2N 1N4, Canada

The modification of catalyst structures can be very important for achieving high activities, selectivities, and stabilities. Chemical vapor deposition (CVD) is one method that can be used for this purpose. In this work, SiO2 has been deposited on the external surface of Ni/Al2O3 catalysts using CVD in a fluidized bed and the deposition parameters (time and temperature) studied. The catalysts were characterized using N2 physisorption, H2 and CO chemisorption, NH3 temperature-programmed desorption, X-ray diffraction, and reactivity for n-octane hydrocracking. The pore openings of Ni/Al2O3 were narrowed such that H2 (2.9 Å) uptake remained essentially constant while N2 (3.6 Å) and CO (3.8 Å) were excluded. As well as changing the pore structure, the deposited silica increased the Brønsted acidity of the Ni/Al2O3 catalysts. The n-octane conversion achieved over the coated catalysts was a balance between the number of strong acid sites and the accessibility to these sites. Introduction Chemical vapor deposition (CVD) provides an efficient technique to precisely control the pore opening of porous catalysts. The technique has found wide industrial application for the modification of the selectivity of catalysts by controlling the pore-opening size. CVD for shape selectivity has predominantly been applied to zeolites because of their inherently microporous structures. For example, CVD modified zeolites have been used for increasing the selectivity in hydrocracking, alkylation, and isomerization reactions.1,2 The mechanism for the formation of SiO2 by CVD of tetramethylorthosilane (TMOS) has been studied by Katada and co-workers.3,4 They proposed that in the presence of water vapor and high temperature (>200 °C), TMOS vapor is adsorbed on the Al2O3 where it is hydrolyzed by water vapor on the Al2O3 surface to form a silanol (eq 1 and eq 2). The silanol is very reactive and reacts further with TMOS to form the siloxane SiO-Si bond (eq 3). In this work, water vapor is in excess such that adsorbed TMOS (silicon alkoxide) may react directly with silanol groups to produce methanol (eq 4) on the solid surface.3-5

dAl-OH + Si(OCH3)4 f dAl-O-Si(OCH3)3 + CH3OH (adsorption) (1) tSi-OCH3 + H2O f tSi-OH + CH3OH (hydrolysis) (2) tSi-OH + OH-Sit f tSi-O-Sit + H2O (condensation) (3) tSi-OCH3 + OH-Sit f tSi-O-Sit + CH3OH (condensation) (4) The hydrolysis and condensation processes that occur in the CVD of TMOS are similar to the sol-gel process used to form silica glass and ceramics.6-8 Whereas the formation of silica * To whom correspondence should be addressed. Tel.: (403) 2109488. Fax: (403) 284-4852. E-mail: [email protected].

glass by the sol-gel process requires a catalyst, CVD does not require any catalyst as the reaction is thermally activated. Solgel is usually performed in the liquid phase, but CVD occurs in the vapor phase and requires high temperature, typically above the bubble point of the silica precursor (i.e., TMOS) at the given reaction pressure to vaporize the silica precursor. By depositing silica on the external surface of zeolite A, Niwa et al. have demonstrated the separation of O2 and N2 molecules as a result of the difference in the kinetic diameters of O2 and N2.9 Oxygen, which has the smaller molecular size, was adsorbed in preference to N2. On catalysts with no size-exclusion modification, N2 is adsorbed in preference to O2. Hibino et al.2 and Kim et al.10 have shown that the pore opening of HZSM-5 can be modified by depositing a silicon alkoxide on the external surface to change the selectivity for toluene alkylation and disproportionation. The modified catalyst was selective toward the formation of para-isomers. On alumina supports, however, little work has been done to utilize CVD for shape-selective catalysts, even though alumina is used in many heterogeneous catalytic processes. Until recently, most of the work done on alumina supports was for the creation of acidity by coating with silica.1,2,5,10-13 For example, Katada et al. have prepared solid acid catalysts by depositing a monolayer of silica on γ-Al2O3 using TMOS as the silica precursor.5 The catalyst was active for the isomerization of 1-butene. Sato and co-workers have also demonstrated that the acidity created by the deposition of SiO2 on Al2O3 exhibited high catalytic activity.11,12 The catalyst was used for several chemical reactions including the cracking of cumene and n-hexane, m-xylene isomerization, and 2-butanol dehydration. The work presented herein demonstrates the use of CVD to control the pore openings of Ni/Al2O3 catalysts for the selective chemisorption of H2 (2.9 Å) and exclusion of larger molecules (N2, 3.6 Å; CO, 3.8 Å). A fluidized bed reactor was used with steam injection through an annulus of the reactor, and silicon alkoxide introduction through the bottom of the reactor was carried by an inert gas. The effect of deposition time on the subsequent adsorption of H2, CO, and N2 was investigated. The catalysts were characterized before and after modification with N2 physisorption, H2 and CO chemisorption, temperature-

10.1021/ie0607346 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006

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Figure 1. Chemical vapor deposition apparatus.

programmed desorption of NH3, X-ray diffraction, and inductively coupled plasma. In addition, the catalysts have been tested with a model reaction of n-octane hydrocracking to demonstrate the influence of the coating on the access to the active sites as well as on the acidity of the catalyst. The results indicate that CVD is an appropriate method for reducing the size of the pore openings in Ni/Al2O3 catalysts. The modified catalysts will be useful in a catalytic reaction in which H2 is competing with larger molecules for adsorption sites. Experimental Method Apparatus. The CVD apparatus used in this work is shown in Figure 1. The fluidized bed reactor consisted of a quartz tube mounted vertically inside an electric furnace. The reactor tube had an inside diameter of 1 cm and overall length of about 41 cm. One feature of the reactor is the attachment of an annular tube made of 1/8-in. stainless steel tubing for steam injection into the reaction zone inside the bed (Figure 1). A 1/32-in. thermocouple is also extended through the annulus to measure the temperature of the bed. Quartz frits with openings of 15-40 micrometers were used as the distributor plate. The reactor operated at atmospheric pressure. A piston pump (Alltech 426 HPLC pump) pumped water through an evaporator into the fluidized bed with flowing N2 (20 sccm) as the carrier. The TMOS was pumped by a syringe infusion pump (Cole Parmer), was evaporated, and was carried to the reactor by N2 flowing at 60 sccm. The flow of N2 was controlled by a mass flow controller (Type 1179A by MKS Instruments). FieldPoint and LabView (National Instruments) were used for data acquisition and readout. Preparation of Ni/Al2O3. A 25-g batch of Ni/Al2O3 was prepared by the wetness impregnation method. The γ-Al2O3 (60 mesh, Alfa Aesar) was impregnated with an aqueous solution of Ni(NO3)2‚6H2O. The mixture was dried at room temperature for about 16 h in a fume hood and then was transferred to a muffle furnace and was treated at 80 °C for 2 h followed by drying at 110 °C for 10 h. The impregnated Al2O3 was then calcined in the muffle furnace at 550 °C for 8 h. On the basis of temperature-programmed reduction, a reduction temperature of 550 °C was chosen. Thus, the catalysts were reduced in flowing H2 by heating to 550 °C at 10 °C/min and were held at 550 °C for 4 h. The resulting Ni/Al2O3 catalysts had Ni loadings of 17% (Galbraith Laboratories Inc.) and a surface area of 129

m2/g ((2 m2/g). The surface area of the purchased Al2O3 was 208 m2/g as measured by N2 physisorption using an Autosorb1C adsorption instrument (Quantachrome Instruments). Silica Deposition. The SiO2 deposition was performed using the CVD apparatus described above (Figure 1). One gram of Ni/Al2O3 was placed in the fluidized bed reactor and was fluidized with N2 (60 sccm). TMOS (1.75 mol %) was vaporized and injected into the bottom of the reactor while 14 mol % H2O was evaporated with N2 (20 sccm) as carrier gas and was injected into the annulus of the reactor. The purpose of steam injection was to suppress carbon formation and at the same time hydrolyze the TMOS. Preliminary experiments were done using the blank Al2O3 support to develop the CVD procedure. SiO2 deposition experiments were done with temperatures between 150 °C and 500 °C. After SiO2 deposition, the samples were calcined in flowing air at 500 °C for 2 h to remove any traces of carbon and organic matter remaining in the catalyst caused by side reactions. The Ni catalysts were coated for times of 0.5, 1, 1.5, 2, 2.5, or 3 h at 350 °C and were identified according to this deposition time. That is, Ni-0 refers to an uncoated catalyst while Ni-2.5 refers to a catalyst treated in the CVD apparatus for 2.5 h. Catalyst Characterization. The characterization techniques used were N2 physisorption, H2 and CO chemisorption, X-ray diffraction (XRD), and NH3 temperature-programmed desorption (TPD). The coated samples were analyzed for H2 chemisorption before and after coating. As such, the coated samples have been reduced twice while the uncoated samples have only been reduced once. N2 physisorption was performed using an Autosorb-1C adsorption apparatus (Quantachrome Instruments) to determine surface area, pore volume, and pore size distribution. The surface area was calculated using the Brunauer, Emmett, and Teller (BET) method, while the pore volume and pore size distribution were calculated by the Barret-JoynerHalenda (BJH) method using the desorption leg of the isotherm. The desorption leg of the isotherm is preferred for pore analysis because it is thermodynamically more stable than the adsorption leg because of the lower Gibb’s free-energy change.14 The error in the surface area measurements is 2% on the basis of repeat analysis of the samples. H2 and CO chemisorption were performed on a ChemBET 3000 (Quantachrome Instruments) to determine the H2 and CO

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uptakes before and after SiO2 coating of the Ni/Al2O3. All catalysts (with and without SiO2 deposition) were pretreated by reduction in flowing H2 at 550 °C for 4 h before the chemisorption measurements. To test whether CO can penetrate the SiO2 coating, each of the samples that were coated for 2 and 2.5 h was exposed to 60 mL/min of pure CO for 30 min at 40 °C. Following the CO exposure, each of the samples was purged with flowing N2 for 1 h to remove any physically adsorbed CO. Each of the samples was tested for H2 uptake following the exposure to CO. An uncoated Ni/Al2O3 catalyst was also tested for H2 uptake after CO exposure to obtain a baseline for comparison. Powder X-ray diffraction (XRD) spectra were recorded on a Rigaku Multiflex X-ray diffractometer using Cu/KR1 radiation (λ ) 1.54056 Å) at 40 kV tube voltage and 40 mA tube current with a scanning speed of 2°/min. The XRD patterns were referenced to the powder diffraction files (ICDD-FDP database) for identification. XRD was performed to monitor changes in the oxidation state of the Ni phase during the coating procedure. NH3 TPD was performed using the ChemBET 3000 instrument with 10% NH3 diluted in He, before and after SiO2 deposition, to determine the effect of the deposition on the acidity of the Al2O3 support. The NH3 was adsorbed at 40 °C and desorption was performed in the temperature range of 40550 °C with a heating rate of 10 °C/min. Silicon elemental analysis of the coated Ni/Al2O3 catalysts was performed using inductively coupled plasma-mass spectroscopy (ICP-MS, Galbraith Laboratories). The ICP-MS technique used to perform the Si elemental analysis has a relative error margin of (10%. Carbon elemental analysis was performed using a Perkin-Elmer 2400 CHN Analyzer to determine the carbon content before and after calcination of coated catalysts. n-Octane Hydrocracking Procedure. The hydrocracking of n-octane was carried out in a quartz fixed bed reactor with 100 mg of catalyst at 400 °C, a weight-hourly space velocity (WHSV) of 2.0 h-1, and H2/n-octane molar ratio of 20 under atmospheric pressure. Before reaction, the catalyst was reduced at 550 °C under flowing H2 for 4 h. The reaction products were analyzed online using a gas chromatograph (Agilent 6890 GC) with a 60-m long, 0.32-mm i.d. GS-GasPro PLOT column and a flame ionization detector (FID). A mass spectrometry (Cirrus by MKS Instruments) was also used to analyze the product stream. Results Development of CVD Procedure. The SiO2 deposition procedure was developed by coating plain commercial γ-Al2O3 with SiO2 for different deposition times and temperatures. Figure 2 illustrates the change in measured surface area with deposition temperatures 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, and 500 °C at a constant deposition time of 1 h. The surface area is calculated directly from the nitrogen uptake and, thus, representative of the accessibility of the pores to nitrogen. As expected, the surface area decreases as the deposition temperature increases. The measured surface areas in Figure 2 steadily decreased until a temperature of 400 °C at which point the surface area increased from 80 m2/g to 100 m2/g. With a further increase in temperature to 500 °C, the surface area decreased to 83 m2/g. This increase in nitrogen adsorption at 400 °C may be due to surface coverage by methoxy species and other decomposition products, which are removed from the surfaces and pores upon calcination. At deposition temperatures of 400 °C and above, significant carbon formation was visible on the Al2O3 after the

Figure 2. Effect of deposition temperature on measured surface area of Al2O3 for a deposition time of 1 h.

Figure 3. Change in measured surface area of Ni/Al2O3 (b) and Al2O3 (O) with deposition time, at a deposition temperature of 350 °C.

deposition. That is, the white Al2O3 powder turned black in color. The carbon was removed by calcination in flowing air at 500 °C (the powder turned white or slightly beige in color). At deposition temperatures of 350 °C and below, there was no visible carbon formation. Because of increased carbon formation at higher temperatures, a deposition temperature of 350 °C was used for the silica deposition on the Ni/Al2O3 catalysts. Nitrogen Physisorption on Ni/Al2O3. The Ni/Al2O3 catalysts were reduced immediately before N2 physisorption measurements. Figure 3 shows the change in measured surface area as a function of SiO2 deposition time for an Al2O3 sample and a Ni/Al2O3 catalyst. As expected, the nitrogen uptake decreased with increasing deposition time. The rate of decrease in surface area is different for the Ni/Al2O3 than for the blank Al2O3 support. After 1.5 h of deposition, the Al2O3 surface area is ∼3 m2/g while that of Ni/Al2O3 is ∼35 m2/g, indicating that the deposition of SiO2 on the Al2O3 is more rapid than the deposition on Ni/Al2O3. The difference in behavior is ascribed to the stronger affinity of SiO2 for the alumina surface. In the case of Ni/Al2O3, some of the surface has likely been covered by Ni deposited on the exterior surface of the alumina particles. The pore volumes of Ni/Al2O3 as a function of deposition time at 350 °C are given in Table 1. Consistent with the surface area measurements, the pore volume decreased as the deposition time increased. After 2.5 h of deposition, the pore volume had decreased to essentially zero compared to a value of 0.193 cm3/g

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Figure 4. Change in pore-size distribution of Ni/Al2O3 with deposition time, at a deposition temperature of 350 °C.

Figure 6. Change in H2 (b) and CO (O) uptakes on Ni/Al2O3 with deposition time at a deposition temperature of 350 °C. Table 2. H2 Uptake on Ni/Al2O3 after Exposure of Catalysts to 100% CO Flow for 30 Min. Ni/Al2O3 Was Coated with SiO2 at 350 °C

Figure 5. Amount of SiO2 on Ni/Al2O3 as a function of deposition time at a deposition temperature of 350 °C. Table 1. Pore Volumes of Ni/Al2O3 Coated with SiO2 for Various Times at 350 °Ca sample ID

deposition time (h)

average pore volume (cm3/g)

Ni-0 Ni-1 Ni-1.5 Ni-2 Ni-2.5 Ni-3

0 1.0 1.5 2.0 2.5 3.0

0.193 0.107 0.022 0.005 0.0007 0.0005

a

Measurements were performed by N2 physisorption.

before deposition. Figure 4 shows that the pore size distribution changes as the amount of SiO2 deposition increases; that is, the pores decrease in diameter as the amount of SiO2 deposition increases. The reduction in pore size may be a result of SiO2 deposition within the pores since the original Ni/Al2O3 pores (38 Å modal pore diameter) are large enough for the silicon alkoxide molecule to penetrate the catalyst (TMOS has a kinetic diameter of 8.9 Å). Amount of Deposition and Carbon Formation. Figure 5 shows the amount of SiO2, as determined by ICP-MS, deposited on Ni/Al2O3 as a function of deposition time. After 1 h of deposition, the SiO2 fraction was 16%. The amount of silica deposited increased to 30% after 1.5 h and then the amount deposited remained constant up to 2.5 h of deposition. The surface may have been saturated after 1.5 h with physisorbed species that hindered the growth of Si-O-Si bonds.15 To verify if other species were deposited on the surface, the carbon content

sample ID

deposition time (h)

Ni-0 Ni-2 Ni-2.5

0 2 2.5

H2 uptake (µL/g) before exposure after exposure 398 441 430

96 139 308

percent change -76 -68 -28

of a Ni/Al2O3 catalyst was determined by carbon, hydrogen, and nitrogen (CHN) analysis after coating the catalyst with SiO2 for 2 h at 400 °C. The Ni/Al2O3 catalyst was gray in color after the deposition, and the CHN analysis revealed a carbon content of 0.7%. This catalyst was then calcined and the carbon content was reduced to 0.2%. N2 physisorption was also performed on the same sample before and after calcination. The surface area of the sample before calcination was 50 m2/g compared to 4 m2/g after calcination. H2 and CO Chemisorption. Figure 6 shows H2 and CO uptakes on Ni/Al2O3 after SiO2 deposition for various times. The H2 uptake actually increased after 1 h of deposition from 398 µL/g to 493 µL/g. This increase is probably due to a further reduction of NiAl2O4 within the structure during the second reduction after the coating.16 After 2.5 h of deposition, the average H2 uptake was 430 µL/g. In contrast, the CO uptake decreased from 405 µL/g before coating to 5.8 µL/g, after 2.5 h of deposition, indicating that the deposited silica had reduced the pore openings and that the technique was successful. To further test the size-exclusion properties of the coated Ni/ Al2O3 catalysts, three catalysts were exposed to pure flowing CO for 30 min. The H2 uptakes, before and after this exposure, are listed in Table 2. The uncoated catalyst (Ni-0) was severely poisoned by exposure to CO with the H2 uptake decreasing from 398 µL/g before exposure to 96 µL/g after exposure (76% change). The second and third samples, coated for 2 and 2.5 h, respectively, were less affected by the exposure to CO with decreases in H2 uptakes of 68% and 28%, respectively. These results indicate that the pores reduced the accessibility for CO. X-ray Diffraction. The XRD spectra for Ni/Al2O3 at various stages in the deposition process are shown in Figure 7. After reduction (Figure 7a), the spectrum had peaks at 44.5, 51.8, and 76.4° 2θ corresponding to Ni and peaks at 37.3 and 67.3° 2θ corresponding to Al2O3 (matched to ICDD-FDP database). After deposition (Figure 7b), most of the Ni has been oxidized, as evidenced by peaks at 37.2, 43.3, and 62.9° 2θ corresponding to NiO. The peaks around 37° 2θ overlap; 37.2° 2θ is associated

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Figure 7. XRD spectra for Ni/Al2O3: (a) reduced, with no deposition, (b) 3-h deposition at 350 °C, before reduction, and (c) 3-h deposition at 350 °C, after calcination and reduction.

Figure 8. Temperature-programmed desorption (10 °C/min) of NH3 on Ni/Al2O3: (a) no deposition, (b) 1-h deposition, (c) 1.5-h deposition, (d) 2-h deposition, (e) 2.5-h deposition, and (f) 3-h deposition, all at a deposition temperature of 350 °C. Table 3. Ammonia Uptake on Ni/Al2O3 as a Function of SiO2 Deposition Time at 350 °C sample ID

deposition time (h)

total NH3 uptake (µmol/g)

Ni-0 Ni-0.5 Ni-1 Ni-1.5 Ni-2 Ni-2.5 Ni-3

0 0.5 1.0 1.5 2.0 2.5 3.0

461 448 246 224 143 113 4.1

with NiO, 37.0° 2θ is associated with NiAl2O4, and 37.3° 2θ is associated with Al2O3. After deposition and reduction (Figure 7c), the XRD spectra is very similar to the spectra of the originally reduced Ni/Al2O3 catalyst (Figure 7a), except that the Ni peak intensities have decreased. This decrease is due to the silica coating. NH3 Temperature-Programmed Desorption. Temperatureprogrammed desorption (TPD) of NH3 on the Ni/Al2O3 catalyst was done to monitor changes in the accessibility of the acid sites on the alumina support. The TPD spectra are shown in Figure 8 for six different samples with deposition times ranging from 0 to 3 h. The total ammonia uptake was determined by integrating the area under the TPD curves, and these areas are listed in Table 3. Two main peaks are observed in the TPD spectra around 160-180 °C and 400-440 °C. These peaks correspond to weak and strong acid sites, respectively, and are

Figure 9. Change of n-octane conversion as a function of SiO2 deposition time on Ni/Al2O3 during hydrocracking of n-octane at 400 °C and atmospheric pressure.

consistent with the literature.17 The ammonia uptake decreased with increasing deposition time, indicating that the acid sites were progressively blocked. SiO2 is significantly less acidic than Al2O3. After 0-3 h of deposition, the uptake decreased from 461 µmol/g to 4 µmol/g (Table 3). The NH3 uptake decreased because of coverage of the external sites on the particles as well as narrowing of the pores preventing NH3 from accessing internal sites. Catalytic Performance for Hydrocracking of n-Octane. Figure 9 shows the conversion of n-octane as a function of the SiO2 deposition time. The conversions shown in Figure 9 are taken after 20 min on stream. With no SiO2 deposition, n-octane conversion was 29%. The maximum conversion (67%) was obtained on the Ni/Al2O3 catalyst coated for 0.5 h. The conversion decreased to zero for catalysts coated for 1.5 h or longer. The product stream consisted of only one C4 species that is likely 1-butene. The stabilities of the catalysts varied. That is, the loss of activity over 3 h on stream was 47%, 23%, and 0% for the uncoated catalyst, the catalyst coated for 0.5 h, and the catalyst coated for 1 h, respectively. Discussion Deposition Process. As expected, the surface areas of the Al2O3 and Ni/Al2O3 samples decreased with increasing deposition temperature and time (Figures 2 and 3), consistent with the results reported by Sato et al.11 for a SiO2/Al2O3 system and by Fodor et al.18 for a SiO2/MCM-41 system. During the deposition, some additional carbonaceous material was deposited on the surface. The CHN analysis and N2 physisorption results before and after calcination are consistent with a porous layer of contamination being formed during the deposition. This layer was removed with a mild calcination, leaving behind a compact SiO2 structure.15 In an inert carrier gas, the thermal decomposition of silicon alkoxide tends to produce carbon and other undesired organic products.2,11,19 The addition of steam helps to reduce the formation of undesirable products and also helps to propagate the growth of Si-O-Si bonds so that the growth of SiO2 can be faster.15 The carbonaceous layer likely prevented additional silica from being deposited beyond a deposition time of 1.5 h (Figure 5). Additional silica can be deposited by calcining the catalysts between deposition runs. The silica layer narrowed the pore openings of the catalyst and prevented the penetration of the larger molecules (Figures

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Figure 10. Schematic of the structure of Ni/Al2O3 catalyst coated with SiO2.

3, 4, and 6). That is, CO (3.8 Å) and N2 (3.6 Å) were excluded from the pores, while H2 (2.9 Å) could still penetrate to the Ni sites. In catalyst reaction systems of noble metals using H2/CO mixture as feedstock (or H2 with CO as contaminant), the CO tends to decrease the reactivity of the noble metal by strongly adsorbing on the surface and by inhibiting further adsorption of H2.20,21 This technique may be a new way for separating H2 from CO in the reaction systems involving noble metals, thereby preserving the reactivity of noble metal catalyst against the poisoning effect of CO. The silica precursor, TMOS, has a molecular dimension of approximately 8.9 Å, which is significantly smaller than the modal pore diameter of the Al2O3 support (∼38 Å). Therefore, the TMOS likely penetrated some of the pores of the alumina creating a framework through which H2 could still penetrate. The presence of Ni in the Al2O3 structure influenced the deposition process (Figure 3). The silica will interact more strongly with the Al2O3 than the Ni.10 Thus, a complete shell of silica is not formed on the Ni/Al2O3 catalyst and the measured surface area remains at 17 m2/g after 1.5 h of deposition rather than being reduced to zero as for the Al2O3 sample. Addition of Ni to the Al2O3 resulted in a decrease in surface area from 208 m2/g to 129 m2/g, indicating that the Ni is situated at the pore mouths and is blocking access to some of the internal pores. The low Ni dispersion of 1% is likely a result of some Ni being inaccessible because of this pore-mouth blocking, as well as some Ni being situated on the external surface of the particles, or some Ni being associated with the alumina in a spinel phase.16 Interestingly, the N2 uptake reached a minimum after 1.5 h of deposition (Figure 2), corresponding to a maximum silica uptake at 1.5 h (Figure 5), while the CO uptake (Figure 6) and ammonia uptake (Table 3) continued to decrease up to 3 h of deposition. The activity of the catalyst (Figure 9) reached a minimum (at zero conversion) after 1.5 h of deposition. These results may indicate that the silica shell was not uniformly formed. That is, after 1.5 h of deposition, sufficient silica had been deposited to remove the activity of the catalyst but not to completely exclude all molecules larger than H2. Further deposition may have filled in some gaps in the coating so that ammonia and CO adsorption continued to decrease. The N2 physisorption and ICP measurements will be less sensitive to small changes in the coating than the selective chemisorption measurements. The thickness of the coating is still under investigation, but initial analysis indicated that the coating could not be detected by scanning electron microsopy (SEM). The fact that the Ni peaks are still visible in the XRD spectra (Figure 7) suggests that the coating is relatively thin (