Influence of Excess Sodium Ions On the Specific Surface Area Formation in a NiO-Al 2 O 3 Catalyst Prepared by Different Methods

The influence of sodium ions on the specific surface area of a NiO-Al2O3 catalyst in dependence of nickel loading (5, 10, and 20 wt% Ni), temperature of heat treatment (400, 700 and 1100C) and the method of sample preparation was investigated. Low temperature nitrogen adsorption (LTNA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied for sample characterization. Dramatic differences in the specific surface area were registered between non-rinsed and rinsed Al2O3 and NiO-Al2O3 samples. The lagged sodium ions promote sintering of non-rinsed catalyst samples.


Introduction
The NiO-Al 2 O 3 oxide system finds broad application in a number of important industrial catalytic processes as steam reforming, partial oxidation of methane, methanation of carbon monoxide and hydrogenation of unsaturated organics [1,2].These catalysts have perspective in future technologies as dry reforming of methane with carbon dioxide [3,4] and tri-reforming [5,6].Alumina supported nickel oxide as a nanocomposite material is also interesting for ceramic industry [7], as well as humidity sensors [8] and mine-safety sensors [9].
Besides active nickel and a support, industrial catalysts often contain promoters.Promoters can cause a significant influence on the metal dispersion and catalytic properties.The addition of small amounts of alkali metal compounds has been known to modify activity and selectivity of catalysts, as well as to improve their lifetime [1].Alkali and alkaline-earth metal ions as catalyst promoters could prevent sulfur poisoning and the choking rate.The promoter content in these catalysts is usually less than 2-3 mas %.Potassium and sodium promoters are introduced into the catalyst during precipitation of nickel and aluminum ions with their hydroxide solution.However, the amount of alkali ions may prevail the optimal promoter quantity in the catalyst and can cause serious problems of thermal stability of catalysts during their application.
In this study, the influence of excess sodium ions on specific surface area formation and sintering behavior of the NiO-Al 2 O 3 oxide catalyst was investigated.Two series of nickel-alumina catalyst were prepared by precipitation of aluminum ions with a sodium hydroxide solution.The first series of the catalyst samples were prepared by carefully washing off sodium ions from the catalyst precursor using distilled water.The second series of samples were prepared in the same way as the first one, however, sodium ions were not removed from precipitates of the catalyst precursor with adequate rinse of samples.The main goal of this study is to determine the influence of excess Na + ions on the physico-chemical and catalytic properties of the NiO-Al 2 O 3 catalyst.

Experimental 2.1 Sample preparation
In this work all samples were synthesized using E. Merck chemicals of analytical grade.The alumina support of the catalyst was prepared by successive precipitation of Al 3+ ions from the nearly saturated aluminum nitrate solution with 20 wt% sodium hydroxide solution at pH=9.5 and ambient temperature.The precipitate was left to stay for 24 hours and than it was filtered through a Buchner funnel in vacuum.About 100 times lager volume of distilled water than the starting volume of the precipitate was used for this operation.The obtained precipitate was dried at 105 o C during 6 hours and thereafter was ground in an agate mortar.The well ground sample was then calcined at 450 o C for 2 hours.
The synthesized γ-Al 2 O 3 like alumina sample was wet impregnated with a nickel nitrate solution at ambient temperature.The impregnated samples contained 5.1; 10.0 and 19.9 wt% of nickel as Ni 2+ ions.The obtained precipitate was dried at 105 o C during 6 hours and thereafter was ground in an agate mortar.
The mechanical powder mixed samples were prepared by mixing of the synthesized γ-Al 2 O 3 with a commercial NiO.The metal oxide powders were homogenized in a rotational mixer during one hour.NiO-Al 2 O 3 samples prepared by mechanical powder mixing contained 5.0; 10.0 and 20.0 wt% of nickel.
The co-precipitated catalysts were prepared by simultaneous precipitation of nickel and aluminum ions from their mixed nitrate solution with 20 wt% NaOH up to pH=9.5 at ambient temperature.The Ni/Al ratio in the initial nitrate solutions was varied to obtain different nickel contents.The precipitates were matured for 24 hours, subsequently vacuum filtered and washed until being free of nitrate anions.The obtained precipitates were air-dried at 105 o C for 6 hours.Thereafter the well-ground samples were calcined at 450 o C for 2 hours.The nickel content in the co-precipitated samples was 5.1, 9.93 and 19.96 wt %.
The exact nickel content in synthesized catalysts was determined gravimetrically as described elsewhere [10].
All the synthesized samples were later thermally treated at 400 o , 700 o and 1100 o C in static air atmosphere during 6 hours.

Sample nomenclature
The following unified I/M/C-W-T nomenclature of the catalyst samples was used in the paper: I/M/C-designated the impregnation/mechanical powder mixing/co-precipitation preparation method; W-nickel content in wt%, T-temperature of the heat treatment (for example: I-5-400 is a sample prepared by impregnation, containing around 5 wt% of Ni 2+ , thermally treated at 400 o C).The alumina support with no nickel induced is designated as A.
The samples where sodium ions were rinsed off with distilled water are designated with (r), for example I-5-400r; or A-700r (rinsed alumina support, with no nickel induced, thermally treated at 700 o C).

Sample characterization
The BET specific surface area of the samples was determined by low temperature N 2 adsorption (LTNA) from a helium gas mixture in a Micromeritics ASAP 2000 apparatus.
XRD analysis was used for investigation of the catalyst structure.The investigation was carried out in XRD equipment, Philips APD 1700 with Cu Kα in the angular range 2θ=3-65.
Catalyst morphology was studied by means of scanning electron microscopy (SEM), (JEOL JSM-6460LV).Before investigation the samples were sputter coated with gold in a vacuum apparatus (BAL-TEC-SCD005).The sodium content in the samples was determined with energy disperse spectroscopy (EDS) connected with the JEOL JSM-6460LV device.
The catalyst activity test for partial methane oxidation was carried out in a fix-bed stainless-steel flow reactor as described elsewhere [11,12].

Texture analysis
BET specific surface area values of alumina samples are listed in Tab.I. Specific surface areas of the non-rinsed alumina samples have very low values after thermal treatment at 400 o C, 700 o C and 1100 o C during 6 hours.The sample treated at 400 o C has a 39 times smaller specific surface area than the corresponding alumina sample rinsed with distilled water.The sample treated at 700 o C has an about 4 times higher specific surface area than the sample treated at 400 o C.However, the increase of specific surface area of the rinsed alumina samples is only 10 % compared to the sample treated at 400 o C. The specific surface area of the rinsed alumina samples thermally treated at 700 o C is only 11 times higher than the specific surface area of the corresponding non-rinsed alumina sample.Samples treated at 1100 o C have low specific surface areas and the specific surface area of the rinsed alumina samples is only 2 times higher than the specific surface area of non-rinsed alumina sample.BET specific surface area values of alumina samples loaded with nickel oxide are listed in Tab.II.The specific surface area of the catalyst samples is higher than the specific surface area of the corresponding unloaded alumina.Rinsed catalyst samples thermally treated at 400 o C and non-rinsed catalyst samples thermally treated at 700 o C have the highest specific surface area values, independently of the way of catalyst preparation and nickel loading.The specific surface areas of the rinsed catalyst samples treated at 700 o C have significantly lower values comparing to the samples thermally treated at 400 o C. For instance, the specific surface area of the rinsed catalyst samples prepared by co-precipitation treated at 400 o C has 2.1-2.8time higher specific surface area than the samples treated at 700 o C. The difference is smaller in the catalyst samples prepared by impregnation, 1.7-2.4times, and in mechanical powder mixed samples, about 1.5 times.These results show that the way of catalyst preparation significantly influences catalyst specific surface area formation.Samples treated at 1100 o C have a low specific surface area, which is caused by intensive sintering of the catalyst samples at such a high temperature, as well as by phase transformation of thermodynamically instable aluminas into a stable α-form.

Tab.I BET specific surface area of non-rinsed
Tab. II BET specific surface area of the catalysts with 5, 10 and 20 wt% of nickel, prepared by impregnation, mechanical mixing, and co-precipitation.The samples were treated previously at 400, 700 and 1100 o C during 6 hours in a static air atmosphere, m 2  The ratio of specific surface areas of the rinsed and non-rinsed catalyst samples is listed in Tab.III.The highest ratio of catalyst specific surface area values was obtained with the samples treated at 400 o C.This ratio for the catalyst samples treated at 400 o C is 34-106.8,and for the catalyst samples treated at 700 o C is 7.1-30.0.
The ratio has moderate values in samples treated at 1100 o C, i.e. the high temperature of heat treatment has a leveling effect on the catalyst specific surface area, independent of the way of catalyst preparation, nickel loading and sodium ion content.The obtained results show that the lagging sodium ions in alumina gel have a decisive influence on the formation of the catalyst specific surface area.The removal of sodium ions from the catalyst precipitate is a difficult and subtle process.This process demands a lot of distilled water for precipitate rinsing and it is time consuming.The sodium ion content in the non-rinsed sample can reach even 20 wt.%, Fig. 1.The lagging sodium ions also influence significantly the pore structure of the samples.Some typical adsorption/desorption curves of the samples are given in Fig. 2. The shape of the adsorption isotherms of the rinsed samples belong to type IV referring to IUPAC classification, indicating a microporous/mesoporous type of material [13].As can be seen in Fig. 2 the nitrogen adsorption-desorption isotherm exhibits a distinctive hysteresis loop similar to H2 type according to the IUPAC (Brunauer-Deming-Deming-Teller) classification [13].Materials with a hysteresis loop type H2 have heterogeneous pores with various radii and shapes.H2 distribution of pores in appropriate conditions would lead to H1 distribution, which is characteristic of materials having tubular pores open at both ends.Contrary, the adsorption-desorption isotherms of non-rinsed samples exhibit a hysteresis loop which belongs to type H3.These materials have tapered slit pores with narrow necks, Fig. 2.  Tab.IV Total pore volume of the catalysts with 5, 10 and 20 wt% of nickel, prepared by impregnation, mechanical mixing, and co-precipitation.The samples were treated previously at 400, 700 and 1100 o C during 6 hours in a static air atmosphere, cm 3  The average particle size of the samples, listed in Tab.V, is calculated using the data for theoretical density of NiO (6.96 g/cm 3 ) and Al 2 O 3 (4.0g/cm 3 ), [14].The theoretical density of the catalyst samples was calculated taking account only the weight fraction of NiO and Al 2 O 3 , however, the weight fraction of different sodium oxide species in non-rinsed samples was not considered.

XRD-analysis and morphology
The XRD patterns of rinsed and non-rinsed catalyst samples substantially differ, i.e. rinsed catalyst samples show only a few diffraction signals, contrary to the XRD patterns of the non-rinsed samples.XRD patterns of rinsed catalyst samples are shown in Fig. 4. The unloaded alumina support is in γ-form (ASTM-10-425).Catalysts prepared by different methods with 10 wt.% of nickel and thermally treated at 700 o C have a similar crystal structure as the unloaded alumina support.However, in co-precipitated and impregnated catalyst samples the characteristic spinel appears, NiAl 2 O 4 (ASTM 10-339) and also slight nickel oxide lines.At this temperature of heat treatment and nickel oxide content, the formation of solid solutions between nickel spinel, nickel oxide and γ-Al 2 O 3 is very likely [15].In the XRD patterns of the samples prepared by mechanical powder mixing the characteristic NiO diffraction lines (ASTM 4-0835) are clearly present, and the nickel spinel diffraction lines are absent.The XRD diffraction patterns of non-rinsed catalyst samples are more complex than the rinsed ones, Fig. 5.The alumina support after heat treatment at 700 o C is in β-form (ASTM 10-414).Sodium β-alumina is represented as Na 1+x Al 11 O 17+x/2 , with x ranging from about 0.15 to 0.3 [16].This alumina, as a solid electrolyte, has a very high electrical conductivity, and today it has good perspective for commercial production in the field of ceramics for hightemperature batteries [16].In the XRD patterns of the non-rinsed catalyst support, besides sodium β-alumina, the diffraction lines of different sodium oxide compounds are also present, such as NaO 3 (ASTM 18-1235), Na 5 AlO 4 (ASTM 14-500) and Na 2 O defined according (ASTM 02-1285) and [17].Balshin's theory generally explains the increase of sintering by the migration of atoms [19].
The presence and migration of Na + ions significantly influences the sample morphology.These phenomena can be seen clearly in the SEM pictures of the rinsed and nonrinsed alumina samples.The rinsed alumina sample treated at 700 o C contains very well formatted uniform crystals compared to the corresponding non-rinsed alumina samples.In this sample the road-like crystals likely belong to one of the free sodium oxide forms, Fig. 6.The rinsed catalyst samples have a significant activity in partial methane oxidation reaction [11,12], while, the catalyst samples with excess of sodium ions did not show any activity, Tab.VI.A free NiO phase in rinsed catalyst samples has a potential for active site formation [11], but in the catalyst with excess of sodium ions the free NiO phase is not accessible, because of strong sintering caused by intensive migration of sodium ions in β-Al 2 O 3 bulk.The registered NiO crystallites (Fig. 5) very likely are encapsulated in β-Al 2 O 3 bulk and become unreachable for the reactants.
Tab. VI Average catalytic activity of the rinsed catalysts in partial methane oxidation, μmolCH 4 /g cat •s.The catalysts contain 5, 10 and 20 wt% of nickel, prepared by impregnation, mechanical mixing, and co-precipitation.The samples previously were treated at 400, 700 and 1100 o C during 6 hours in a static air atmosphere, m 2

Conclusions
The influence of sodium ions on surface area formation of the NiO-Al 2 O 3 oxide system in dependence of nickel loading (5; 10; 20 wt% Ni), temperature of heat treatment (400; 700; 1100 o C) and the way of sample preparation were investigated.Dramatic differences in sample specific surface areas were registered between non-rinsed and distilled water rinsed Al 2 O 3 and NiO-Al 2 O 3 samples.The lagged sodium ions hinder nickel spinel, nickel oxide and γ-Al 2 O 3 solid solution formation.Their high mobility strongly accelerates sintering processes in samples even at low temperatures such as 700 o C.

Fig. 2
Fig. 2 Adsorption-desorption isotherms of rinsed and non-rinsed free alumina support treated at 400 o C during 6 hours

Fig. 3
Fig. 3 Adsorption-desorption isotherms of the co-precipitated non-rinsed catalyst with 5 wt% of nickel, treated at 400, 700 and 1100 o C during 6 hours

Fig. 6
Fig.6 SEM pictures of rinsed (left) and non-rinsed (right) free alumina samples, treated at 400 o C during 6 hours, Magnifications 20 000XThe rinsed catalyst samples have a significant activity in partial methane oxidation reaction[11,12], while, the catalyst samples with excess of sodium ions did not show any activity, Tab.VI.A free NiO phase in rinsed catalyst samples has a potential for active site formation[11], but in the catalyst with excess of sodium ions the free NiO phase is not accessible, because of strong sintering caused by intensive migration of sodium ions in β-Al 2 O 3 bulk.The registered NiO crystallites (Fig.5) very likely are encapsulated in β-Al 2 O 3 bulk and become unreachable for the reactants.
Al 2 O 3 and rinsed Al 2 O 3 (r), treated at different temperatures during 6 hours in a static air atmosphere, m 2 /g /g (r) rinsed with 100 times larger volume of distilled water than the volume of the precipitate Tab.III BET specific surface area ratio of the rinsed and non-rinsed catalyst samples.The catalyst samples contain 5, 10 and 20 wt% of nickel, prepared by impregnation, mechanical mixing, and co-precipitation.The samples were previously treated at 400, 700 and 1100 o C during 6 hours in a static air atmosphere.(r)rinsed with 100 times larger volume of distilled water than the volume of the precipitate Average particle size of the catalysts with 5, 10 and 20 wt% of nickel, prepared by impregnation, mechanical mixing, and co-precipitation.The samples were treated previously at 400, 700 and 1100 o C during 6 hours in a static air atmosphere, μm