Catalyst Properties Via the Selection of Dopant M = Gd , La , Mg . Part 2 . Catalytic Activity

To elucidate the role of support composition in autothermal reforming of ethanol (ATR of C2H5OH), a series of Ni catalysts (Ni content 2–15 wt.%) supported on different ceria-based oxides (Ce1-xGdxOy, Ce1-xLaxOy and Ce1-xMgxOy; x = 0.1–0.9) were prepared. The synthetized materials were tested in ATR of ethanol at 200–700 °C. It was established that supports themselves show catalytic activity in ATR of C2H5OH and provide 10–15% yield of H2 at 700 °C. Upon the increase of Ni content from 2 to 15 wt.% the temperature of 100% ethanol conversion decreases from 700 tо 300 °С, hydrogen yield increases from 25 to 60%, the inhibition of С2-С3 by-products formation, as well as the promotion of decomposition of acetaldehyde occur. The enhancement of catalyst performance in ATR of C2H5OH has been observed in the next series of supports: Ce1-xMgxOy < Ce1-xGdxOy < Ce1-xLaxOy and with a decrease of x to an optimal value that correlates with the improvement of Ni active component reducibility. At 600 °C on 10Ni/Ce0.8La0.2O1.9 catalyst the H2 yield of 50% was achieved at C2H5OH conversion of 100%. Stable and high performance of developed catalysts in ATR of C2H5OH indicates the promise of their use in the production of hydrogen. Article info Received: 15 March 2018 Received in revised form: 19 May 2018 Accepted: 24 July 2018


Introduction
At present, the global energy consumption is increasing [1, 2] while the conventional hydrocarbon resources are depleting [3,4].Sustainable development of society implies a decrease in dependence on fossil fuels [5,6].Ethanol is one of the most significant chemicals in large quantity derived from biomass [7,8].The use of this renewable feedstock for producing hydrogen provides a green way of production of clean fuel of future [9,10].
Autothermal reforming (ATR) of ethanol, combining the endothermic steam reforming (SR) reaction and the exothermic partial oxidation, is the most promising method of producing hydrogen, thanks to a favorable energy balance and a low rate of formation of carbon deposits on the catalyst [11][12][13].According to the proposed reaction scheme for ethanol reforming [12,13], the main reaction steps include ethanol dehydrogenation to acetaldehyde, its decomposition to methane and carbon monoxide, methane steam reforming and water gas shift reaction.However many side reactions might also occur, thus resulting in coke formation and less hydrogen yield.Possible side reactions involve ethanol dehydration to ethylene; its polymerization to coke; acetone formation via acetaldehyde condensation followed by decarboxylation; methane cracking and Boudouard reaction [12,13].So the task of the catalyst for ATR of ethanol is to maximize the yield of hydrogen and reduce the contribution of side reactions.The general requirements for catalysts include the following points [14][15][16]: catalyst should break C-C bond rather than promote the C-O bond activation; reform methane to generate hydrogen; activate water and oxygen to produce highly mobile oxygen species and to inhibit coke formation.
Both transition metals (Ni, Co, Cu) and noble metals (Rh, Pt, Pd, Ir, Ag) have been applied as the active component for ethanol reforming while different oxides (Al 2 O 3 , CeO 2 , La 2 O 3 , MgO, ZnO) are used as supports [17][18][19][20][21].The nature of the metal and the support greatly affect the final product distribution [22].According to the study of M/Al 2 O 3 catalysts in oxidative steam reforming of ethanol [20], Ru-, Rh-and Ir-containing samples efficiently rupture the C-C bond of ethanol, resulting in the best hydrogen yield, while in the case of Co, Ni, Pd and Pt metals the dehydration of ethanol preferentially occurs with higher selectivity of ethylene formation.Often bimetallic catalysts have advantages over monometallic ones.It was demonstrated [23] that co-presence of Ni and Co in the catalyst composition favors the high hydrogen yield due to the enhanced reducibility of the bimetallic sample.The interaction of Rh and Pt in the bimetallic RhPt sample improved catalyst activity and stability against coking during ATR of C 2 H 5 OH [19].It was shown that the quantity of carbon deposits formed in ATR reaction decreases in the following row of active metals: Ni >> Pt > Rh ~ RhPt [19].However, the high price of noble based catalysts restricts their wide use.
It was shown that modification of supports is an effective way to control the catalyst performance in reforming of fuels [24][25][26][27][28][29][30].This study is devoted to the elucidation of the role of support composition in autothermal reforming of ethanol over Ni/Ce 1-x Gd x O y , Ni/Ce 1-x La x O y and Ni/Ce 1-x Mg x O y catalysts.The regulation of support characteristics and, consequently, the catalyst properties is carried out through doping of ceria by Gd 3+ , La 3+ , Mg 2+ cations of different molar portion (x is equal to 0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1.0).To establish the structure-property-performance relationships, the results of catalyst tests in ATR of C 2 H 5 OH were correlated with physicochemical properties of catalysts.

Catalyst preparation
The Ni/Ce 1-x Gd x O y , Ni/Ce 1-x La x O y and Ni/ Ce 1-x Mg x O y catalysts (Ni content -2-15 wt.%) were prepared by incipient wetness impregnation of appropriate supports (x is equal to 0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1.0) with aqueous solutions of nickel nitrate.After impregnation, the samples were dried at 90 °C and calcined in air for 4 h at 500 °C.The number before nickel in the catalyst name corresponds to the Ni content (wt.%).The physicochemical properties of Ce 1-x M x O y supports and Ni/Ce 1-x M x O y catalysts were given and discussed in Part I of this article.

Catalytic activity measurements
ATR of C 2 H 5 OH was measured in a flow setup with a quartz reactor (14 mm i.d.) at atmospheric pressure, temperature 200-700 °C, a flow rate of 230 mL/min and the molar ratio between reagents C 2 H 5 OH : H 2 O : O 2 : He = 1 : 3 : 0.5 : 1 according to the method described in [25].

Catalytic activity of Се 1-х M x O y supports in ATR of C 2 H 5 OH reaction
The effect of chemical composition of prepared materials on their catalytic performance in ATR of C 2 H 5 OH is shown in Fig. 1-7 and Tables 1-3.Regardless of support composition over studied supports the ethanol conversion (X C2H5OH ) and hydrogen yield (Y H2 ) rise with a growth of the temperature of process (Fig. 1).
The CeO 2 oxide and Се 1-х M x O y mixed oxides (x = 0.1-0.9)are characterized by higher ethanol conversion in the low-temperature region in comparison to X C2H5OH over Gd 2 O 3 , La 2 O 3 and MgO samples.However over all Се 1-х M x O y supports (x = 0-1) complete conversion of C 2 H 5 OH could be only obtained at 700 °C.Supports themselves provide 10-15% yield of H 2 at the reaction temperature of 600 °C and their performance has trend to improve in the following sequence: Support composition: Irrespectively of support composition, in addition to hydrogen, the formation of a wide range of carbon-containing products (C-products) was observed: acetaldehyde, acetone, ethylene, methane and carbon oxides.As a typical example, Fig. 2 shows the selectivity of C-products formation in the ATR of C 2 H 5 OH over Ce 0.8 La 0.2 O 1.9 support.
As a rule, with an increase of the reaction temperature the selectivity of C 2 H 4 O and C 3 H 6 O formation decreases, selectivity to CO and CH 4 increases, while those of the rest C-product have volcano-type dependence (Fig. 2).The C-product distribution is controlled by support composition (Table 1 1).Comparatively large selectivity of C 2 -C 3 product formation and relatively low selectivity of CH 4 formation (Fig. 2, Table 1) indicate that supports have a weak capability of breaking the C-C bond in ethanol [31].

Catalytic activity of Ni/Се 1-х M x O y catalysts in ATR of C 2 H 5 OH reaction
The introduction of Ni in Се 1-х M x O y support changes the material performance in ATR of C 2 H 5 OH.As follows from Fig. 3a, at low Ni content the composition of products and its temperature dependence are still similar to those in the presence of support.Upon the increase of Ni content from 2 to 15 wt.% the inhibition of formation of ethylene and acetone occurs, as well as the promotion of decomposition of acetaldehyde (Fig. 3b, Fig. 4a).So at high Ni content (10-15 wt.%), irrespectively of the composition of Се 1-х M x O y support (x = 0.1-0.9), at 600 °C the amount of these compounds is below detected limits and C-products consist of CH 4 and carbon oxides only (Fig. 3b, Table 2).It is noted that in this case, the high yield of methane is already observed at low values of reaction temperature and it decreases with a growth of reaction temperature (Fig. 3b).Dependence of Y СН4 and Y CO vs. temperature shows the increase of contribution of methane steam reforming reaction with increasing of reaction temperature.The comparison of the obtained product yields over the 10Ni catalyst (Fig. 3b) with thermodynamic equilibrium yields (Fig. 3d) shows that these values are close to each other.On the contrary, at low Ni content the significant deviation is observed (Fig. 3a, Fig. 3d).It means that in this case the reaction studied is far from the equilibrium state and it is controlled by kinetic limitations.
The increase of Ni content also leads to a decrease in the temperature of complete ethanol conversion and an increase of hydrogen yield (Fig. 3, Fig. 4b).In particular, over Ni/Ce 0.8 La 0.2 O 1.9 catalyst upon the increase of Ni content from 2 to 15 wt.%X C2H5OH decreases from 700 to 300 °С (Fig. 3) while Y H2 at 600 °C grows from 15 to 60% (Fig. 4b).The performance of samples with 10 and 15 wt.%Ni is comparable and 10 wt.% Ni may be regarded as appropriate content for high catalyst performance in the studied reaction.In general, the optimal Ni content depends on support composition and reaction conditions.For example, the 30 wt.% Ni is selected as the optimal value for Ni/CeO 2 -ZrO 2 catalysts for steam reforming of C 2 H 5 OH [31].It can be seen (Fig. 4b) that the best hydrogen yield in ATR of C 2 H 5 OH is achieved in the presence of Ni catalysts on the basis of Се 0.8 La 0.2 O 1.9 support while the least activity was shown by the catalysts based on Се 0.8 Mg 0.2 O 1.8 support.As was shown in Part 1 of this article, these 10Ni/Ce 0.8 M 0.2 O y samples have a similar average size of NiO particles but different ability in reduction that consequently can affect the catalyst performance (Fig. 4b).
At high Ni content (10-15 wt.%) the features of the supports own activity in the reaction is mainly exhibited by catalysts on the individual oxides while the performance of catalysts on mixed oxides differs more strongly at T reaction < 450 °C (Table 2).In this case, dopant type and content have the impact on the catalyst performance mainly through the regulation of active component properties.The optimal value of the dopant content in support composition depends on the kind of dopant (Fig. 5).For example, with a decrease of a molar fraction of lanthanum in the support, the dependence (Y H2 vs. T reaction ) removes to lower temperatures, and the yield of hydrogen increases (Fig. 3c, Table 3).This correlates with a deterioration of nickel dispersion and improvement of reducibility of nickel cations (Part 1 of this article).It is in agreement with findings in earlier works [31][32][33].Different oxygen storage capacity of supports also may have an impact on catalyst performance [13,31].Figure 6 shows the dependence of H 2 yield in ATR of C 2 H 5 OH over 10Ni/Се 1-х La x O y catalysts on its structural and redox characteristics.The variation of La content in Се 1-х La x O y affects the mode of interaction between Ni supported species and Се 1-х La x O y support.In particular, the intensification of such interaction appears as change in phase composition and dispersion of Ni active component as well as its T reduction : at x = 0-0.2→ NiO (13-25 nm) → T = 470-500 °C, at x = 0.5-0.8→ NiO (< 8 nm) → T= 540-580 °C and at x = 0.9-1 → LaNiO 3 → T = 650 °C.It turns out that an increase in the degree of interaction between the components of the catalyst has both positive (improvement of the dispersion and anti-sintering ability) and negative (decrease of the ability to reduction and, accordingly, the concentration of Ni o active centers) effects [28,31,34].In this connection, a volcano-type dependence of the hydrogen yield on the degree of interaction between metal and support is observed.It is reported [35] that both the moderate metal-support interaction and the right ability to be reduced contribute to the high performance of Ni/Mg 0.75 Ti 0.25 O and Ni/Mg 0.5 Ti 0.5 O catalysts in tri-reforming of methane.So, the yield of H 2 can be maximized through optimization of support composition which will provide the necessary for this system mode of the interaction of the catalytically active species with the support matrix.
The developed catalyst shows the stable characteristics in the studied catalytic process (Fig. 7).It is noted that these experiments were conducted without pre-reduction of catalysts.The data of Fig. 7 indicates that catalysts are capable of self-activation.The samples are reduced under reaction mixture that provides the formation of Ni o active sites.The decrease of reducibility of Ni n+ cations in 10Ni/Ce 1-x La x O y in comparison to those in 10Ni/CeO 2 leads to the appearance of an induction period of the reaction.
According to [32, [36][37][38], Y H2 (mol of H 2 /mol of ethanol) in ATR of C 2 H 5 OH can be equal to 2.1-4.5.In the presence of our catalyst the value of 3.5 mol of H 2 /mol of ethanol is attained.At 600 °C on 10Ni/Ce 0.8 La 0.2 O 1.9 catalyst the H 2 yield of 50% was achieved at C 2 H 5 OH conversion of 100%.Stable and high performance of developed catalysts in ATR of C 2 H 5 OH indicates the promise of their use in the production of hydrogen.(M = Gd, La, Mg).Irrespectively of support composition, the increase of Ni content up to 10-15 wt.% provides an increase of H 2 yield and a decrease of C 2 -C 3 -products yield.At a low nickel content, support composition determines the selectivity of by-products formation, especially in the low-temperature region.The growth of H 2 yield in ATR of C 2 H 5 OH over Ni/Се 1-х M x O y catalysts is observed in the next series of supports: Ce 1-x Mg x O y < Ce 1-x Gd x O y < Ce 1-x La x O y or with a decrease of the molar fraction of a dopant up to an optimal value that correlates with the enhancement of the active component reducibility.

Fig. 1 .
Fig. 1.Conversion of C 2 H 5 OH (open symbols) and yield of H 2 (bold symbols) in ATR of C 2 H 5 OH vs. reaction temperature over supports of different chemical composition.

Fig. 5 .
Fig. 5. Yield of H 2 in the ATR of C 2 H 5 OH at 600 °C over Ni/Ce 1-x Gd x O y , Ni/Ce 1-x La x Oy and Ni/Ce 1-x Mg x O y catalysts.

Fig. 6 .
Fig.6.Yield of H 2 in ATR of C 2 H 5 OH over 10Ni/Се 1-х La x O y catalysts versus its structural and redox characteristics.

Fig. 7 .
Fig. 7. Yield of H 2 in the ATR of C 2 H 5 OH over 10Ni/Се 1-х La x O y catalysts versus time on stream at 600 °C.
).In particular, at 600 °C Gd 2 O 3 and La 2 O 3 are noted for high selectivity of C 2 H 4 and C 3 H 6 O formation, respectively, while at 400 °C MgO is Fig. 2. Selectivity for C-products obtained in the ATR of C 2 H 5 OH over Ce 0.8 La 0.2 O 1.9 support.O which is subsequently decomposed to СO and CH 4 .Among studied samples, the lowest selectivity of ethylene formation is observed over MgO which correlates with its basic properties.According to [13], in this case, the rate of C 2 H 5 OH dehydrogenation to acetaldehyde is faster than C 2 H 5 OH dehydration to ethylene.The C-product distribution over Се 1-х M x O y samples lies close to those on CeO 2 at a low molar fraction of the dopant (Table

Table 3
Effect of La molar fraction on performance of 10Ni/Се 1-х La x O y catalysts in ATR of C 2 H 5 OH at 600 °C