Syngas Production via SteamâCO2 Dual Reforming of Methane over

Research Article pubs.acs.org/journal/ascecg

Syngas Production via Steam−CO2 Dual Reforming of Methane over LA-Ni/ZrO2 Catalyst Prepared by L‑Arginine Ligand-Assisted Strategy: Enhanced Activity and Stability Weizuo Li, Zhongkui Zhao,* Fanshu Ding, Xinwen Guo, and Guiru Wang State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering, School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116024, People’s Republic of China S Supporting Information *

ABSTRACT: A highly dispersed supported nickel catalyst (LA-Ni/ZrO2), synthesized by a facile L-arginine ligandassisted incipient wetness impregnation (LA-IWI) approach, demonstrates much superior catalytic activity and exceptional stability for steam−CO2 dual reforming of methane in comparison with the classical Ni/ZrO2 catalyst by the IWI method. The origin of the enhanced activity and stability of the developed LA-Ni/ZrO2 catalyst as well as the role of the Ni− {(L-Arg)} complex as the Ni precursor is revealed by employing diverse characterization techniques including Xray diffraction (XRD), N2 adsorption (BET), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (Raman), CO chemisorption, temperature-programmed hydrogenation (TPH), and thermogravimetric analysis (TGA). The superior catalytic activity of the developed LA-Ni/ZrO2 catalyst to the classical Ni/ZrO2 can be ascribed to the higher Ni dispersion, intensified Ni−support interaction, the enlarged oxygen vacancies, as well as the increased t-ZrO2 content and enhanced reducibility of NiO led by oxygen vacancies. More interestingly, although a larger amount of coke depositing on the spent LA-Ni/ZrO2 catalyst in comparison with that on the spent Ni/ZrO2 can be observed by TGA and TPH measurement, the developed LA-Ni/ZrO2 illustrates much higher catalytic stability to Ni/ZrO2, ascribed to the superior thermal sintering resistance of Ni nanoparticles and the different coke morphologies confirmed by TEM images led by intensified interaction of Ni and the ZrO2 support. The much superior catalytic activity and stability of the developed LA-Ni/ZrO2 catalyst endows it to be a promising candidate for syngas production with diverse H2/CO ratios via steam−CO2 dual reforming of methane. KEYWORDS: L-Arginine, Nickel, Robust catalyst, Oxygen vacancies, Dual reforming of methane, Syngas production

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thesis.4−6 The steam−CO2 dual reforming of methane has been considered as a sophisticated approach for potential industrial application in syngas production, in which the H2/ CO can be easily adjusted by controlling H2O/CO2 in the feed, and the coke deposition on the catalyst can be suppressed due to the introduction of steam.4,7−10 Ni-based catalysts with activity and selectivity comparable to those of noble metals, but with lower cost, have been extensively employed as promising catalysts for reforming of methane.11−16 However, the fatal drawback of the Ni-based catalyst is the rapid deactivation deriving from carbon deposition and the thermal sintering of the dispersed Ni species.17−19 Therefore, it is highly desirable for exploring stable and active Ni-based reforming catalysts. Pioneer studies have demonstrated that the size of the Ni particles has a crucial role in suppressing coke.17,20−22 It has been illustrated that

INTRODUCTION Methane is the predominant component of natural gas, coalbed gas, shale gas, and methane hydrate, which form a major part of the energy market today; therefore, studies on efficient transformations are highly desirable. The combustion of natural gas and other fossil fuels to meet domestic, industrial, and automotive energy demands causes emission of CO2. Methane and CO2 are the two most important greenhouse gases contributing to global warming. CO2 reforming of methane for syngas production with a H2/CO ratio around 1 contributes not only to mitigation of the global environmental problem but also to the supply of a valuable chemical feedstock for synthesizing valuable hydrocarbons through Fischer−Tropsch (F-T) synthesis and oxosynthesis.1−3 However, additional steps are required to adjust the H2/CO ratio to meet the specific requirement for downstream transformation of syngas. It is highly desirable for developing an efficient one-step route to produce syngas with variable H2/CO ratios according to the target processes like Fischer−Tropsch or methanol syn© 2015 American Chemical Society

Received: October 11, 2015 Published: October 22, 2015 3461

DOI: 10.1021/acssuschemeng.5b01277 ACS Sustainable Chem. Eng. 2015, 3, 3461−3476

Research Article

ACS Sustainable Chemistry & Engineering

preparing highly dispersed supported Ni for steam−CO2 dual reforming of methane. L-Arginine, as a low-cost compound, with a carboxylic acid group at one end and an amino group at the other, may be used to act as a complexing agent due to its zwitterionic characteristics, which may maintain compositional homogeneity in the precursor solution. We envision that the highly dispersed supported Ni catalyst would be fabricated by a ligand-assisted approach using L-arginine as ligands. Herein, we synthesized highly dispersed supported Ni catalysts on ZrO2 by a facile LA-IWI approach, which demonstrate much superior catalytic activity and stability for steam−CO2 dual reforming of methane to the classical Ni/ ZrO2 catalysts prepared by the IWI method. Various characterization techniques including XRD, BET, TEM, H2-TPR, Raman, CO chemisorption, TPH, TGA, and FT-IR were employed to explore the origin of enhanced activity and stability of the developed LA-Ni/ZrO2 catalyst for steam−CO2 dual reforming of methane. This work presents a new route for the synthesis of highly active and stable Ni-based catalysts for the steam−CO2 dual reforming of methane. The fundamental understanding on the origin of enhanced activity and stability of the LA-Ni/ZrO2 catalyst is also quite important for the rational design of a more active and stable candidate for the reaction.

carbon deposition only can occur when the metal cluster is greater than a critical size. Therefore, to prepare highly dispersed supported Ni catalysts with nanosized Ni thermal sintering-resistance may be a promising approach for improving the stability of Ni-based catalysts for reforming of methane.23−26 It was previously demonstrated that the preparation method and support type related strong metal support interaction (SMSI) can efficiently suppress the thermal sintering of Ni nanoparticles, and thus in turn improve stability of Ni-based catalysts for reforming of methane through maintaining a high metal surface area and inhibiting the coke deposition.27−30 Also, the thermal sintering can be suppressed as a result of the confinement effect of either the “well-defined” structures31−35 or the nanopore channels of mesoporous materials.24−26,36−38 The “well-defined” structures like perovskites, spinels, and solid solution cannot provide sufficient “accessible” active centers for the reactants; however, the pore channels in the mesoporous materials inevitably suffer from the pore collapse in a high temperature atmosphere for reforming of methane, and especially in the presence of steam for the steam−CO2 dual reforming. Moreover, the SMSI of a Ni-based catalyst can efficiently suppress the formation of shell-like carbon that mainly contributes to decreased catalytic activity owing to the access of the reacting gases to the active species being hindered by the carbon layers encapsulated on the Ni nanoparticles.39−41 Therefore, the development of a new and efficient approach for preparing highly dispersed supported Ni catalysts with SMSI is highly desirable. Furthermore, much research has previously demonstrated that support type significantly affects the catalytic performance of Ni-based catalysts for reforming of methane,42−45 and the interest in ZrO2 as a support for Ni-based reforming catalysts is ascribed to strengthening of metal support interaction between the Ni and ZrO2, and its unique acidic and basic properties as well as reducing and oxidizing abilities.46−48 Generally, ZrO2 exists in three different polymorphs at ambient pressures: monoclinic ZrO2 (m-ZrO2, room temperature−1175 °C), tetragonal ZrO2 (t-ZrO2, 1175−2370 °C), and cubic ZrO2 (cZrO2, 2370−2680 °C).49 In the reforming process, the tetragonal phase (t-phase) is more desirable compared to the monoclinic and cubic phases.50,51 However, in comparison with m-ZrO2, t-ZrO2 is thermodynamically metastable at low temperature and would tend to transform to m-ZrO2 upon thermal treatment.52 Therefore, the preparation of low temperature stable t-ZrO2 still remains a challenge. Recent reports show that the t-ZrO2 phase can be efficiently stabilized by the presence of oxygen vacancies resulted from the incorporation of heteroatom ions such as Cu, Ag, and Na into the ZrO2 lattice for the supported catalysts.53,54 Therefore, we envision that the enhanced incorporating of Ni into the ZrO2 matrix by strengthened Ni−ZrO2 interaction also can stabilize t-ZrO2, and therefore increase the catalytic performance of Ni-based catalysts for steam−CO2 dual reforming of methane. To develop a facile and efficient strategy to prepare highly dispersed supported metal catalysts is highly desirable for obtaining an excellent catalyst for steam−CO2 dual reforming of methane. It was previously demonstrated that the pyrolysis of molecularly defined metal complexes was an efficient method for synthesizing highly dispersed catalysts for nitrobenzene reduction and hydrolysis of ammonia borane for hydrogen generation.55,56 However, the precious ligands were required. Therefore, it is highly desirable to find a low-cost ligand for

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EXPERIMENTAL SECTION

Synthesis of ZrO2 and Ni-Based Catalysts. ZrO2 support was synthesized using a modified hydrothermal method.57 In a typical synthesis, 13.7 g of Zr(NO3)4·5H2O (Sinopharm Chemical Reagent Co., Ltd.) and urea were dissolved in deionized water. The solution was vigorously stirred for 1 h at the room temperature, and then transferred into a Teflon-lined stainless-steel autoclave and maintained at a desired temperature for a certain time, followed by cooling to room temperature. The resulting precipitate was collected, washed, dried overnight, milled, and subsequently calcined at 400 °C for 4 h. The obtained oxide precursor was ground, and the final ZrO2 support was prepared. The LA-NiO/ZrO2 catalyst was prepared by the LA-IWI method. In a typical synthesis, a certain L-arginine was dissolved in the aqueous solution and then the required amount of Ni(NO3)2·6H2O was added into the above solution, followed by stirring at room temperature to obtain the Ni−{(L-Arg)} complex aqueous solution. The LA-NiO/ ZrO2 catalyst was prepared by the facile IWI method using the Ni− {(L-Arg)} complex aqueous solution as impregnant with a subsequent calcination process at 550 °C for 6 h in muffle. Through changing the molar ratios of L-arginine to Ni and the mass ratios of Ni to ZrO2 support, a series of LA-NiO/ZrO2 catalysts with diverse LA/Ni ratios (1.0:1, 2.0:1, and 2.5:1) and Ni loadings (6, 8, 10, and 12%) were prepared. The real Ni loading was measured by ICP experiment. For comparison, the classical NiO/ZrO2 catalyst was also prepared by the IWI method by using nickel nitrate aqueous solution as impregnant. The as-synthesized LA-NiO/ZrO2 and NiO/ZrO2 catalysts were reduced at 650 °C for 2 h under the mixture of 20% H2 in N2 at a flow rate of 30 mL min−1 to obtain the reduced samples, which are denoted as LA-Ni/ZrO2 and Ni/ZrO2, respectively. Catalyst Characterization. The transmission electron microscopy (TEM) images were obtained by using a Tecnai F30 HRTEM instrument (FEI Corp.) at an acceleration voltage of 300 kV. The samples were dispersed into ethanol with ultrasonic treatment for 10 min, and a drop of the suspension was placed on a copper grid for TEM observation. The specific surface area of the samples was measured by using N2 adsorption desorption isotherms determined on a Beishide apparatus of model 3H-2000PS1 system at −196 °C. The specific surface areas were calculated by the Brunauer−Emmett−Teller (BET) method. The XRD patterns were collected from 10 to 80° at a step width of 0.02° using a Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a Cu Kα source (λ = 1.5406 Å). The average crystallite 3462


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reaction can be calculated. The transformed CH4 in steam reforming reaction can be calculated by the total transformed CH4 minus the transformed CH4 in dry reforming reaction. That is to say, although the steam was not analyzed by GC, the section of FH2O,in − FH2O,out can be indirectly calculated by the combination of FCH4,in, FCO2,in, FCH4,out, and FCO2,out.

sizes were estimated on the basis of the Scherrer formula over the multiple characteristic diffraction peaks by the MDI Jade 5 software. The Ni dispersion and metal particle size of the samples were measured by CO titration at 35 °C using ChemBET Pulsar TPR/TPD equipment (Quantachrome, USA). The sample (∼100 mg) was reduced in situ with H2 at 650 °C for 1.5 h, then flushed at 400 °C with He for 50 min. After prereduction, the sample was cooled to 30 °C and CO chemisorption was carried out. The total metal dispersion of Ni metal was calculated from eq 1, where DM, VS, SF, SW, and MW are metal dispersion (%), volume of active gas chemisorbed (cm3 at STP), stoichiometry factor, sample weight (g), and molecular weight of active metal (g/mol), respectively:58

DNi(%) = 100 ×

⎛ Vs × SF ⎞ ⎜ ⎟ × MW ⎝ SW × 22414 ⎠

CH4 conversion:

FCH4,in − FCH4,out

xCH4(%) =

FCH4,in

× 100 (2)

CO2 conversion: (1)

xCO2(%) =

The H2-TPR experiments were performed in an in-house constructed instrument equipped with a TCD (thermal conductivity detector) to measure H2 consumption. 50 mg of catalyst was loaded in a quartz tube between two quartz wool plugs, and purged in Ar at 300 °C for 30 min and then was cooled down to ambient temperature in Ar. After that, it was reduced with a 10 vol % H2−Ar mixture (30 mL min−1) by heating up to 800 °C at a ramp rate of 10 °C min−1. The Raman spectra were collected on powdered samples by using a laser with an excitation wavelength of 532 nm at room temperature on a Thermo Scientific DXR Raman microscope. FT-IR spectra were collected in the wavenumber range of 4000−400 cm−1 on an EQUINOX-55 Fourier Transform Infrared Spectrometer (BRUKER). The TGA analysis was conducted to study the amount of coke deposited on the spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts using a PerkinElmer STA 6000 with a heating rate of 10 °C min−1 from 30 to 800 °C in an air stream. TPH analysis of spent catalyst was conducted following a previously reported method.58,59 The TPH technique was employed to characterize coke species deposit on the catalyst during the catalytic steam−CO2 dual reforming of methane. The spent catalyst was degassed in Ar at 150 °C for 1 h before it was subject to heat treatment in a mixed gas containing 10% H2 in Ar from room temperature to 900 °C with a ramp rate of 10 °C min−1. The effluent gas for TPH was analyzed with a thermal conductivity detector (TCD). Catalytic Performance Measurement. The catalytic performance measurement of the prepared catalysts was performed in a quartz tube fixed-bed continuous reactor (6 mm O.D.) at atmospheric pressure. Typically, 50 mg of catalyst with the 40−60 mesh particle size was loaded between two quartz wool plugs. The temperatures were measured by using K-type thermocouples and controlled by a PID controller. Before the reaction, the catalyst was reduced at 650 °C for 2 h under the mixture of 20% H2 in N2 at a flow rate of 30 mL min−1. The reaction feed contained the CH4, CO2, H2O, and N2 (N2 was used as internal standard gas, CH4:CO2:H2O:N2 = 1:0.8:0.4:0.2) at 40 mL min−1 of the total flow rate (gas hourly space velocity, GHSV, 48 000 mL g−1 h−1), which was controlled by the mass flow controllers. The reforming reaction was performed at 650−850 °C, and the N2 was used an internal standard to calculate the CH4 and CO2 conversions. The effluent gas was passed through a trap to condense the residual steam and then analyzed by using a gas chromatograph online with a molecular sieve column and a Porapaq Q column. The CH 4 conversion, CO 2 conversion, CO selectivity, H 2 selectivity, and H2/CO (eqs 2−6) were calculated on the basis of the following equations. Where, the FCH4,in, FCO2,in, and FH2O,in are the flow rate of each component (CH4, CO2, and H2O, respectively) in the feed; the FCH4,out, FCO2,out, FH2O,out, FCO,out, and FH2,out are the flow rate of each component (CH4, CO2, H2O, CH4, CO, and H2, respectively) in the effluent. In eq 5 for H2 selectivity, the section of FH2O,in − FH2O,out is denoted as the produced hydrogen by the steam reforming of methane, which was actually calculated on the basis of the transformed CH4 in steam reforming reaction. On the basis of the amount of transformed CO2, the transformed CH4 in dry reforming

FCO2,in − FCO2,out FCO2,in

× 100 (3)

CO selectivity:

SCO(%) =

FCO,out FCH4,in − FCH4,out + FCO2,in − FCO2,out

× 100 (4)

H2 selectivity:

SH2(%) =

FH2,out 2 × (FCH4,in − FCH4,out) + FH2O,in − FH2O,out (5)

× 100 H2/CO molar ratio:

H 2 /CO =

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FH2,out FCO,out

× 100 (6)

RESULTS AND DISCUSSION Preparations and Characterization of LA-Ni/ZrO2 Catalyst. The LA-Ni/ZrO2 and classical Ni/ZrO2 catalysts were prepared by the LA-IWI and IWI methods described in the Experimental Section. From Figure 1, the peak on the FTIR spectra of the Ni−{(L-Arg)} complex corresponding to the out-of-plane bending band of −NH2 shifts from 1682 to 1668 cm−1, and the peak corresponding to the stretching bending band of −CO of the Ni−{(L-Arg)} complex shifts from 1562 to 1617 cm−1 with respect to those on the FT-IR spectra of L-

Figure 1. FT-IR spectra of the as-synthesized Ni−{(L-Arg)} complex and the L-arginine. Inset is photo of L-arginine (LA), nickel nitrate (Ni), and Ni−{(L-Arg)} complex (LA-Ni) aqueous solutions. 3463


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Figure 2. TEM images of the as-synthesized (a) LA-NiO/ZrO2 and (b) NiO/ZrO2 catalysts. The insets are the corresponding particle size distribution histograms and the NiO crystal lattice characteristics of the selected particle.

arginine, suggesting the existence of interaction between Ni and the functional groups of the L-arginine for the Ni−{(L-Arg)} sample.60,61 Thus, the Ni−{(L-Arg)} complex was successfully synthesized for the following use in preparing the LA-Ni/ZrO2 catalyst, which can be further confirmed by the significant difference in color of the L-arginine, nickel nitrate, and Ni−{(LArg)} complex aqueous solutions. According to the above analysis and literature,61 the coordination patterns of the Ni− {(L-Arg)} complex and L-arginine can be speculated, displayed as the insets in Figure 1. By employing the Ni−{(L-Arg)} complex and the nickel nitrate aqueous solutions as impregnant, the LA-Ni/ZrO2 and Ni/ZrO2 catalysts were finally prepared after the calcination and the subsequent reduction processes. The metal particle morphology, particle size distribution, and NiO crystal lattice of the samples were investigated using TEM characterization. Figure 2 shows the TEM images of LA-NiO/ ZrO2 and NiO/ZrO2 samples. From Figure 2a, the LA-NiO/ ZrO2 catalyst shows a relatively regular particle without any appreciable agglomeration to be observed. In contrast, for the NiO/ZrO2 catalyst (Figure 2b), the edge of the particles becomes dim in comparison with the LA-NiO/ZrO2 catalyst, suggesting the aggregation of the NiO particles. From the particle size distribution histograms for LA-NiO/ZrO2 and NiO/ZrO2 shown as the insets in Figure 2, the mean NiO particle sizes of the former and the latter are 20.1 and 22.8 nm, respectively, which reveals that the use of the Ni−{(L-Arg)} complex as impregnant can efficiently enhance the dispersion of NiO on the ZrO2 support. The promoting effect in NiO particle dispersion may be resulted from the stereohindrance effect of the Ni−{(L-Arg)} complex. From the insets in Figure 2, it was observed that the lattice spacing values corresponding to the (200) plane of cubic phase NiO for LA-NiO/ZrO2 and NiO/ZrO2 samples are 2.13 and 2.04 Å, respectively. The enlarged crystallite cell can be ascribed to the incorporation of Zr4+ into the NiO crystalline phase possibly enhanced by inducing effect of LA in the Ni−{(L-Arg)} complex, which was further confirmed by the following XRD characteristics. Figure 3 presents the XRD patterns of the LA-NiO/ZrO2 and NiO/ZrO2 catalysts, and the NiO was also included for comparison. From Figure 3, the NiO phase is identified by comparison with the corresponding JCPDS file (JCPDS No. 65-2901). The tetragonal and monoclinic ZrO2 phases are identified by comparison with those reported in references 53−55. The broader diffraction peaks corresponding to the NiO phase on the XRD patterns of LA-NiO/ZrO2 in comparison with those of NiO/ZrO2 can be observed,

Figure 3. XRD patterns of the as-synthesized (a) LA-NiO/ZrO2, (b) NiO/ZrO2, (c) bare ZrO2 support, and (d) the bulk NiO.

suggesting the degree of NiO dispersion would be increased with replacement of nickel nitrate by the Ni−{(L-Arg)} complex. The average crystallite sizes of NiO for the two catalysts are estimated by the Scherrer equation on the basis of the (200) plane of NiO, demonstrating the decrease from 21.3 to 18.3 nm (Table 1) whereas the L-arginine-assisted IWI method is adopted. From Table 1, the slight increased surface area of LA-NiO/ZrO2 in comparison with NiO/ZrO2 presents possible evidence for higher NiO dispersion. The XRD results further confirm the enhancing effect of the Ni−{(L-Arg)} complex used as impregnant, which is consistent with the results obtained from TEM (Figure 2). The Ni metal dispersion and metal size of the reduced LA-NiO/ZrO2 and NiO/ZrO2 were further investigated through the following CO chemisorption and TEM measurement, and assuming the chemisorption stoichiometry of CO to Ni metal was 1.61 The Ni dispersity and the calculated Ni particle size based on its dispersity58 as well as the Ni particle size from TEM for the two samples are listed in Table 1. From Table 1, the much higher degree of Ni dispersion (9.7%) and much smaller Ni particle (10.4 nm) for LA-Ni/ZrO2 catalyst to those for Ni/ZrO2 (7.5% and 13.4 nm, respectively) can be clearly seen, further confirming the enhancing effect of the use of the Ni−{(LArg)} complex as impregnant. From Figure 3, the XRD peaks toward the (111), (200), and (220) planes of NiO for both LANiO/ZrO2 and NiO/ZrO2 shift to lower diffraction angles in comparison with those for bulk NiO, suggesting that the lattice expansion of the NiO crystalline phase occurs owing to the replacement of Ni2+ (0.78 Å) with the Zr4+ (0.84 Å). The 3464


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Table 1. Characteristics of the As-Synthesized LA-NiO/ZrO2 and NiO/ZrO2 Samples and the Bare ZrO2 Support samples LA-NiO/ ZrO2 NiO/ZrO2 Bare ZrO2

SBETa (m2 g−1)

PSTEMb NiO/Ni (nm)

PSTEMc Ni (nm)

CSd NiO (nm)

DNie (%)

PScsf Ni (nm)

H2 uptake α/β (mmol g−1)

DRg (%)

Xt‑ZrO2h (%)

52

20.1/11.6

15.1

18.2

9.7

10.4

0.77/0.98

76.6

7.8

49 141

22.8/13.7

25.6

21.3

7.5

13.4

0.39/1.19 0.06

69.5

2.9 1.9

a

BET specific surface area from N2 adsorption desorption. bAverage NiO particle size for as-synthesized LA-NiO/ZrO2 and NiO/ZrO2 catalysts and the average metallic Ni particle size for their reduced samples measured by TEM (Figures 2 and 14). cAverage metallic Ni particle size for the spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts (suffering from prereducing) measured by TEM (Figure 14). dDenoted as average crystallite size of NiO, calculated from the Scherrer equation according to the characteristic peaks toward the (111), (200), and (220) planes of NiO of the LA-NiO/ZrO2 and NiO/ZrO2 catalysts (Figure 3). eDenoted as metallic Ni dispersion of the reduced samples (LA-Ni/ZrO2 and Ni/ZrO2) from CO chemisorption, calculated from eq 1. fDenoted as Ni particle size of the reduced samples (LA-Ni/ZrO2 and Ni/ZrO2), calculated from the formula of PSNi (nm) = 101/DNi.58 gDenoted as the degree of reduction of the as-synthesized LA-NiO/ZrO2 and NiO/ZrO2 catalysts, calculated by the percentage of the actual total H2 uptake to the theoretical H2 uptake for the NiO being complete reduced. hDenoted as the percentage of t-ZrO2 to total ZrO2 support including both m-ZrO2 and t-ZrO2, calculated on the basis of references.53−55

insertion of Zr4+ into the NiO lattice shows the existence of interaction between NiO and the ZrO2 support. Furthermore, the enlarged shift of XRD peaks toward the (111), (200), and (220) planes of NiO for LA-NiO/ZrO2 in comparison with those for NiO/ZrO2 illustrates the intensified NiO−ZrO2 interaction. The strengthened expansion in NiO unit cell for LA-NiO/ZrO2 in comparison with that for NiO/ZrO2 is also confirmed by the increased d-spacing observed in TEM images. Moreover, a shift to a higher angle for XRD characteristic peaks toward the (201), (022), and (310) of the m-ZrO2 phase can be observed, demonstrating the shrinkage in the ZrO2 lattice cell for the two supported NiO catalysts in comparison with the bare ZrO2 support due to the larger ionic radius (0.84 Å) of Zr4+ being replaced by the Ni2+ (0.78 Å) with a slight smaller ionic radius. Furthermore, the more obvious shift for LA-NiO/ ZrO2 than NiO/ZrO2 can be observed, further showing the stronger NiO−ZrO2 interaction. In combination of the TEM, XRD, and CO chemisorption characterization results, it can be proposed that the use of the Ni−{(L-Arg)} complex as a Ni precursor can efficiently enhance the nickel dispersion and in turn intensify SMSI. The intensified SMSI on the developed LA-Ni/ZrO2 in comparison with Ni/ZrO2 both can inhibit Ni thermal sintering and can change the NiO electronic state confirmed by H2-TPR and Raman spectra. In addition, the insertion of Ni2+ ion into the ZrO2 crystal lattice results in the generation of oxygen vacancies in the ZrO2 support to compensate the difference between the oxidation states of Zr4+ and Ni2+ and thus maintain the electroneutrality of the network.62 More interestingly, the significantly intensified diffraction peak at 59.9° corresponding to the t-ZrO2 phase on the LA-Ni/ZrO2 in comparison with that on Ni/ZrO2 or on bare ZrO2 can be observed. On the basis of references 53−55 and 58, the t-ZrO2 contents in the ZrO2 support for LA-NiO/ ZrO2, NiO/ZrO2, and bare ZrO2 support are calculated and listed in Table 1. The presence of oxygen vacancies led by heteroatom doping can stabilize the thermodynamically unstable t-ZrO2.53−55 The use of the Ni−{(L-Arg)} complex as a Ni precursor strengthens the Ni−ZrO2 interaction, which leads to more Ni2+ inserting into the ZrO2 support on the LANi/ZrO2 in comparison with that on Ni/ZrO2.62 As is known, the Ni2+ would be changed to metallic Ni by either the reduction or catalytic reaction process (because suffering from prereduction before reaction and at reducing atmosphere for reaction process), then it is essential to ensure whether the stable t-ZrO2 phase can be maintained even if the Ni2+ suffered from reduction and catalytic reaction, although the LA

impregnation method can more efficiently stabilize the t-ZrO2 phase for the LA-NiO/ZrO2 catalyst in comparison with the traditional one. In Figure S1, the XRD analyses of the reduced and spent LA-NiO/ZrO2, as well as bare ZrO2, are presented. From the patterns, the obviously higher XRD peaks toward the (311) facet of ZrO2 on the two supported Ni catalysts than that on the bare ZrO2 can be observed, suggesting the stable t-ZrO2 by Ni2+ can be maintained even if the Ni2+ suffered from reduction and catalytic reaction. Furthermore, from the above, the LA impregnation method can lead to stronger metal− support interaction in comparison with a traditional one, and as a consequence, the LA impregnation method strengthens the heteroatom ions doping for stabilizing the thermodynamically unstable t-ZrO2. The stable t-ZrO2 by Ni2+ doping does not reversibly return to m-ZrO2, but maintains its current crystal phase under the reduction and reaction conditions. The smaller Ni crystal size and the stable t-ZrO2 phase may favor the steam−CO2 dual reforming of methane reaction. The H2-TPR analysis was also conducted to reveal the reducibility of the LA-Ni/ZrO2 and Ni/ZrO2 catalysts, as well as to study the interaction between NiO and ZrO2 support. The H2-TPR profiles are presented in Figure 4, and the quantitative analytical results are listed in Table 1. From the H2-TPR results, the ZrO2 support could hardly exhibit H2 uptake, and therefore the H2-TPR profiles of the two catalysts mainly correspond to the H2 uptake of Ni-containing species. From Figure 4, both samples exhibited similar profiles of hydrogen reduction, the

Figure 4. H2-TPR profiles of the as-synthesized LA-NiO/ZrO2 and NiO/ZrO2 catalysts. Inset is the magnified region. 3465


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ACS Sustainable Chemistry & Engineering presence of two main reduction peaks (α and β) of the NiO can be observed. The α-peak (300−420 °C) can be assigned to free NiO species on the ZrO2 surface and the NiO located on the support with weak interaction between NiO and ZrO2.64 The β-peak (420−600 °C) can be assigned to the reduction of NiO located on the support that has strong interaction with the ZrO2 support and the Ni2+ ions penetrating into the ZrO2 lattice.14,65 Compared with NiO/ZrO2 catalyst, the α-peak corresponding to LA-NiO/ZrO2 catalyst slightly migrated to relatively lower temperatures (from 383 to 377 °C) and the H2 consumption increased (Table 1), might be ascribed to higher mobility of surface oxygen of the ZrO2 support.66,67 The smaller the crystal is, the higher the surface energy is. Hence a larger amount of surface oxygen species can be reduced at a lower temperature.68,69 The improved mobilization should be resulted from the smaller NiO crystalline size of the LA-NiO/ ZrO2 catalyst. In addition, the β-peak for the LA-NiO/ZrO2 catalyst shifts to higher temperatures (from 469 to 476 °C) in comparison with that for NiO/ZrO2, suggesting the intensified Ni support interaction identified by TEM and XRD analysis. Moreover, the slight decrease in H2 consumption for β-peak on the LA-NiO/ZrO2 catalyst in comparison with that for NiO/ ZrO2 can be seen, which may attribute to the Ni2+ ions penetrating into the ZrO2 lattice. From Table 1, the developed LA-NiO/ZrO2 catalyst in this work by using the Ni−{(L-Arg)} complex as a Ni precursor had a higher degree of nickel oxides species reduction than that the classical NiO/ZrO2 prepared by nickel nitrate as a Ni precursor, which is favorable for higher activity for reforming of methane. Figure 5 shows the Raman spectra of LA-NiO/ZrO2 and NiO/ZrO2 samples. The peaks of LA-NiO/ZrO2 located at

Raman peaks corresponding to both t- and m-ZrO2 on the LANiO/ZrO2 catalyst to NiO/ZrO2 sample imply the strengthened metal−support interaction by LA impregnation method in comparison with the traditional one. Catalytic Performance. Figure 6 shows the initial activity of the developed LA-Ni/ZrO2 catalyst by the LA-IWI approach, and the classical supported Ni catalyst on ZrO2 prepared by the IWI method is also included for comparison. The LA-Ni/ZrO2 catalyst demonstrates much superior catalytic activity in CH4 and CO2 to the Ni/ZrO2 catalyst (Figure 6a,b). It is also observed that the CO2 conversion was lower than the CH4 conversion for both catalysts, which can be ascribed to the steam reforming of methane or the possible presence of decomposition of methane at higher temperatures.38 Furthermore, From Figure S2, we can clearly see the decreased H2/CO ratio on LA-Ni/ZrO2 catalyst in comparison with that on Ni/ ZrO2. The possible reason is that the new LA-IWI method mainly enhances the activity in the CO2 reforming of methane. Moreover, the obtained H2/CO is lower than the calculated value, ascribed to the possible reverse water−gas shift reaction. From Figure 6c,d, the similar H2 selectivity for both of the catalysts with an increase in reaction temperature can be observed, but the CO selectivity increased with then increased reaction temperature, differing from the reported result.38 This shows that the increased reaction temperature leads to the obvious carbon gasification, but not methane decomposition or water−gas shift. The similar H2 selectivity and slight lower CO selectivity of LA-Ni/ZrO2 in comparison with Ni/ZrO2 catalyst can be observed, which might be owing to the more methane decomposition over the former, confirmed by TGA of the spent catalyst. However, from the following investigation on the catalytic stability, although the carbon amount on it is higher, the LA-Ni/ZrO2 catalyst exhibited much higher stability, and this could be owing to the coke morphologies.38,39 Anyway, the developed LA-Ni/ZrO2 catalyst exhibits significantly enhanced activity in methane reforming. Correlated the reaction results to the above characterization results, the improvement in catalytic activity for steam−CO2 dual reforming reaction of methane on the LA-Ni/ZrO2 catalyst in comparison with the classical Ni/ ZrO2 catalyst may be ascribed to the increased NiO dispersity, intensified Ni−support interaction, the enlarged oxygen vacancies, and the increased t-ZrO2 content and enhanced reducibility of NiO.1,7,13,25−27,50,51 Molar Ratio of LA/Ni. On the basis of the above work, the facile and efficient LA-IWI strategy for fabricating a high efficient Ni-based catalyst with high NiO dispersion and intensified SMSI for steam−CO2 dual reforming of methane has been established. Herein the LA role was further studied by changing the molar ratio of LA to Ni (nLA/Ni) in the Ni−{(LArg)} complex. Figure 7 depicts the effect of nLA/Ni on the steam−CO2 dual reforming of methane over the LA-Ni/ZrO2 catalysts with different nLA/Ni values. Results show that the catalytic activity of LA-Ni/ZrO2 catalysts is dependent on the nLA/Ni, and the appropriate nLA/Ni value is required. The catalyst prepared with 2.0 of nLA/Ni exhibits the highest catalytic activity with medium H2/CO (Figure S3). At 750 °C, the CH4 conversion on the catalyst with 2.5 of nLA/Ni is much higher than CO2 conversion, showing it has higher activity in steam reforming than in CO2 dry reforming or the enhanced methane decomposition. This, in turn leads to the highest H2/CO ratio among the three samples. The possible reverse water−gas shift (RWGS) over catalyst with 1.0 of nLA/Ni leads to the lowest H2/ CO ratio.63 At 800 °C, the three catalysts show the similar H2/

Figure 5. Raman spectra of the as-synthesized LA-NiO/ZrO2 and NiO/ZrO2 catalysts.

175, 328, 371, and 613 cm−1 are assigned to m-ZrO2, and the peak at 469 cm−1 is ascribed to the Eg mode of the t-ZrO2 phase.70 From Figure 5, the Eg mode frequency of Raman peak of NiO/ZrO2 (474.0 cm−1) is similar to bulk t-ZrO2 phase (473.7 cm−1). However, the Raman peak of LA-NiO/ZrO2 is found to be a down-shift of the Eg frequency (469.0 cm−1) in comparison with that for bulk t-ZrO2 phase can be ascribed to the presence of oxygen vacancies.70−72 The oxygen vacancies may lead to a decline of the ionic character-electron of the Zr4+ states in the Zr−O band and an increase in the s-electron density surrounding the Zr atom. The increased oxygen vacancies can improve the reducibility of NiO confirmed by H2-TPR analysis. Furthermore, from Figure 5, the stronger 3466


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Figure 6. (a) CH4 conversion, (b) CO2 conversion, (c) H2 selectivity, and (d) CO selectivity as a function of reaction temperature for steam−CO2 dual reforming of methane over the LA-Ni/ZrO2 and Ni/ZrO2 catalysts with 10% Ni loading and 2.0 of Arg/Ni ratio (error bars equal 95% confidence interval). Reaction conditions: mcat = 50 mg, CH4/CO2/H2O = 1:0.8:0.4, GHSV = 48 000 mL h−1 g−1, and performed at atmospheric pressure.

CO ratio; however, at 850 °C, the catalyst with 1.0 of nLA/Ni shows the highest H2/CO ratio among the three samples and the other two exhibit similar H2/CO ratios. By comparing the samples with 1.0 and 2.5 of nLA/Ni, the catalyst with 1.0 of nLA/Ni shows the much higher CO2 conversion but similar CH4 conversion, correlated to the H2/CO ratios on the two samples (higher H2/CO on catalyst with 1.0 of nLA/Ni), showing that the CH4 decomposition on the catalyst with 2.5 of nLA/Ni is enhanced, which decreases the steam reforming of methane. As a result, the lower H2/CO on the catalyst with 2.5 of nLA/Ni can be observed. Furthermore, by comparing the samples with 2.0 and 2.5 of nLA/Ni, the former shows the higher catalytic activity in both CH4 and CO2 with the similar H2/CO ratio (Figure S3). While carefully comparing the CH4 and CO2 conversions on the two catalysts, we can find that the increase in CO2 conversion on the catalyst with 2.0 of nLA/Ni in comparison of that on the other one is higher than the increase in CH4 conversion. The increase in CO2 conversion, either by CO2 reforming or by RWGS, leads to the decrease in H2/CO ratio, however, the similar H2/CO ratios can be seen on the two catalysts. It may be resulted from the enhancing effect of the catalyst with 2.0 of nLA/Ni on the steam reforming or decomposition of methane. From Figure 7c,d, the H2 selectivity increased with the increase in nLA/Ni, and this might be the occurrence of the carbon gasification, and methane decom-

position.38 The CO selectivity increased as nLA/Ni increased from 1 to 2 can be ascribed to the enhanced CO2 conversion, carbon gasification, and/or RWGS. The further increase in nLA/Ni from 2 to 2.5 does not lead to an increase in CO selectivity, but results in the decrease in CH4 and CO2 conversion. The 2 of nLA/Ni can be chosen. All in all, the LANi/ZrO2 catalyst with 2.0 of nLA/Ni demonstrates the best catalytic activity among the three samples at the different reaction temperatures. The change in the order of H2/CO ratios over the three catalysts at the different reaction temperatures may be ascribed to the complex competition process including the steam reforming, CO2 reforming, and the possible methane decomposition, RWGS, etc. The 2.0 of nLA/Ni is selected, and also the reaction temperature can be chosen on the basis of the required H2/CO ratio for the specific downstream applications. Then what is the reason for the reduced LA-NiO/ZrO2 with 2.0 of nLA/Ni exhibiting much higher catalytic activity than the other two? Herein the BET and XRD analysis was done to explore the origin of its higher activity. Figure 8 presents the XRD patterns of the LA-NiO/ZrO2 catalysts with diverse nLA/Ni values; the quantitative data and the BET results are listed in Table S1. From the XRD and BET results, the increase in nLA/Ni from 1.0 to 2.0, the visible decrease in average NiO crystallite size (from 22.5 to 18.2 nm) with the increase in t-ZrO2 content 3467


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Figure 7. (a) CH4 conversion, (b) CO2 conversion, (c) H2 selectivity, and (d) CO selectivity as a function of reaction temperature for steam−CO2 dual reforming of methane over the 10% LA-Ni/ZrO2 catalysts with diverse LA/Ni molar ratios (error bars equal 95% confidence interval). Reaction conditions: mcat = 50 mg, CH4/CO2/H2O = 1:0.8:0.4, GHSV = 48 000 mL h−1 g−1, and performed at atmospheric pressure.

between Ni and ZrO2 support, and in turn leads to the decrease in t-ZrO2 content. The higher Ni dispersity and t-ZrO2 content of the LA-NiO/ZrO2 catalyst with 2.0 of nLA/Ni in comparison with those of the other two samples allows it to be higher activity for steam−CO2 dual reforming of methane. Ni Loadings. The effect of Ni loading on the catalytic performance in steam−CO2 dual reforming over the LA-Ni/ ZrO2 catalysts with 2.0 of nLA/Ni and the diverse Ni loadings from 6 to 12 wt % was investigated (the real loadings by ICP are presented in Table S2), and the reaction results are presented in Figure 9. From Figure 9, we can clearly see that the LA-Ni/ZrO2 catalyst with the loading of 10 wt % has demonstrated the excellent catalytic performance in steam− CO2 dual reforming. As for the LA-Ni/ZrO2 catalysts with diverse Ni loadings, the order of catalytic activity in CH4 conversion is as follows: 10 > 8 > 6 > 12 wt %. From Figure 9b, the CO2 conversion on 8 wt % is higher than that on 10 wt %. From Figure S4, the LA-Ni/ZrO2 catalysts with diverse Ni loadings show similar H2/CO ratios (lower than the theoretical value, ascribed to the possible RWGS). Then which one is better, 8 or 10 wt %? The catalytic stability of the two catalysts was measured, and Figure S5 presents the reaction results. The 10%LA-Ni/ZrO2 catalyst shows higher catalytic stability than the one with 8 wt % Ni loading. Therefore, 10 wt % is chosen for steam−CO2 dual reforming over the developed LA-Ni/ ZrO2 catalysts. From Figure 9a, the H2 selectivity decreased

Figure 8. XRD patterns of the as-synthesized LA-NiO/ZrO2 catalysts with diverse LA/Ni molar ratios.

can be observed,53−55 although it decreases the surface area from 58.8 to 51.9 m2 g−1. The higher NiO dispersion and tZrO2 content for the catalyst with 2.0 of nLA/Ni makes it exhibit higher catalytic activity. However, the further increase in nLA/Ni from 2.0 to 2.5 leads to the further decrease in surface area from 51.9 to 43.4 m2 g−1. Moreover, the NiO dispersity on the catalyst with 2.5 of nLA/Ni is worse than that on the one with 2.0 of nLA/Ni (from 18.2 to 22.4 nm), which may weaken the SMSI 3468


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Figure 9. (a) CH4 conversion, (b) CO2 conversion, (c) H2 selectivity and (d) CO selectivity as a function of reaction temperature for steam−CO2 dual reforming of methane over the as-synthesized LA-Ni/ZrO2 catalysts with diverse metallic Ni loadings and 2.0 of Arg/Ni ratio (error bars equal 95% confidence interval). Reaction conditions: mcat = 50 mg, CH4/CO2/H2O = 1:0.8:0.4, GHSV = 48 000 mL h−1 g−1, and performed at atmospheric pressure.

with the increase in reaction temperature, this could be owing to the RWGS reaction. The increased CO selectivity along with the increased reaction temperature may be ascribed to the enhanced CO2 conversion, the carbon gasification, and RWGS reaction. The LA-Ni/ZrO2 catalyst with 10 wt % Ni loading shows medium H2 selectivity and similar CO selectivity in comparison with the other loadings at the reaction temperature above 750 °C, besides showing the highest CH4 conversion. Therefore, 10 wt % Ni loading is optimum. The XRD, BET, and H2-TPR techniques were employed to explore the reason why the catalytic activity of reduced LANiO/ZrO2 catalysts increases up to the maximum and then decrease as the Ni loading is continuously increased from 6 to 12 wt %. Figure 10 and 11 present the XRD patterns and H2TPR profiles of the LA-NiO/ZrO2 catalysts with diverse Ni loadings, and the quantitative analysis results of XRD, H2-TPR and BET measurements are listed in Table S2. From Figure 10, the diffraction peaks corresponding to the NiO phase become stronger and narrower with the increase in Ni loading from 6 to 12 wt %, implying the increase in average crystalline size. We estimate the average crystalline size of the supported NiO on the ZrO2 carrier with the Ni loading of 6, 8, 10, and 12 wt % by the Scherrer equation. The average NiO crystalline size for the above three catalysts is 14.5, 17.3, 18.2, and 22.1 nm, respectively (Table S2). This trend can be attributed to “nanoparticle size effect”.73 Correlated to the

Figure 10. XRD patterns of the as-synthesized LA-NiO/ZrO2 catalysts with diverse metallic Ni loadings: (a) 6%; (b) 10%; (c) 12%.

reaction results shown in Figure 9 to catalyst nature, the amount of active sites may increase with the increase in Ni loading, which is favorable for the steam−CO2 dual reforming reaction of methane. But too high of Ni loading may lead to agglomeration of the metal, resulting in poor metal dispersion. As a consequence, the number of accessible Ni atoms (per gram of catalyst) decreases as the Ni loading rises from 6 to 12 wt %, obtained by CO chemisorption experiments. Further3469


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catalysts in reforming reaction of methane is strongly dependent on the NiO dispersion and SMSI as well as the reducibility and the t-ZrO2 content affected by NiO dispersion, SMSI and oxygen vacancies. The appropriate molar ratio of LA to Ni in the Ni−{(L-Arg)} complex (2:1) and the Ni loading (10 wt %) is essential for achieving excellent catalytic performance in steam−CO2 dual reforming of methane. From above, we develop a highly active Ni-based catalyst for steam−CO2 dual reforming of methane by a facile and efficient LA-IWI strategy and elucidate the role of LA in the enhancing effect on catalytic activity. As follows, we investigate the effect of reaction conditions on the catalytic activity over the developed LA-Ni/ZrO2 catalyst. Molar Ratio of H2O to CO2 in the Feed. The introduction of steam into the feed can adjust the H2/CO ratio by tuning the H2O/CO2 ratio, besides suppressing the coke deactivation of the Ni-based catalyst. Herein we investigate the effect of molar ratio of H2O to CO2 in the feed, and the reaction results are presented in Figure S6. For the convenience in carrying steam, the increased N2 concentration in the feed was adopted (CH4:N2 = 1:2, but for the left in this work, the 1:0.2 of CH4/ N2 was used). From Figure S6, the initial CO2 conversion is about 30% lower than CH4 conversion. This is due to the fact that the feed (H2O+CO2)/CH4 ratio was fixed at 1.2, which is slightly higher than the stoichiometric feed ratio. Moreover, as shown in Figure S6b, the negative value conversion of CO2 at low CO2/H2O feed ratios indicates that the dry reforming of methane is suppressed by the water−gas shift reaction. In addition, from Figure S6c, the H2/CO molar ratios decrease with increasing temperature, especially at low CO2/H2O molar ratios in the feed stream. This result is explained by the fact that the effect of the water−gas shift reaction increases at low temperature, whereas the H2/CO ratio is affected by the highly endothermic steam and dry reforming of methane at elevated temperature because the water−gas shift reaction is thermodynamically unfavorable at elevated temperature.74 Moreover, the possible RWGS might be a reason for the change taking place with the diverse reaction temperature. Effect of Gas Hourly Space Velocities (GHSV). Furthermore, we investigate the effect of GHSV on the catalytic performance of the developed LA-Ni/ZrO2 catalyst by changing the GHSV from 24 000 to 72 000 mL g−1 h−1, and the results are shown in Figure S7. As shown in Figure S7a,b, the CH4 and CO2 conversions suffered a notable decline with an increase in the feed flow rate from 24 000 to 72 000 mL g−1 h−1. It may be attributed to that the decrease in the residence time on the surface of the catalyst and limited active centers for the increasing number of the reactants that caused the decline in the catalytic conversion.3,76 In addition, it also could be observed in Figure S7c that the general trend of the H2/CO ratios increased as the GHSV increased, this may be ascribed to that the residence time becomes shorter, CO2 reforming of methane could not complete under this reaction condition. This also suggested that the influence of the GHSV on CO2 reforming of CH4 was much more obvious than the one over the steam reforming of CH4. Stability of LA-Ni/ZrO2. Until now, coke deactivation is the key problem for Ni-based catalysts for reforming of methane, which hinders their use as a long-term catalyst in industrial applications. The highly dispersed Ni-based catalysts demonstrate high catalytic activity for reforming of methane and also have a crucial role in suppressing coke;20−24 however, controlling the size of Ni nanoparticles is not so straightforward

Figure 11. H2-TPR profiles of the as-synthesized LA-NiO/ZrO2 catalysts with diverse metallic Ni loadings. Inset: the magnified region for bare ZrO2.

more, the smaller NiO crystallite is, the stronger the interaction between NiO and ZrO2 is, and in turn leads to more oxygen vacancies. Therefore, the higher t-ZrO2 amount can be observed on the LA-NiO/ZrO2 in comparison with that on the other two samples shown in Table S2 because the oxygen vacancies can stabilize the unstable t-ZrO2 phase.53−55 The high t-ZrO2 content in the supported Ni catalysts on ZrO2 is favorable for high catalytic activity for reforming of methane.50,51 As a result, the continuous increase in Ni loading does not lead to the monotonous improvement in the CH4 and CO2 conversion, and the catalyst with 10 wt % Ni loading demonstrates the highest catalytic activity for steam−CO2 dual reforming reaction of methane. Moreover, from Table S2, we cannot see a visible change in surface area of the LA-NiO/ZrO2 catalysts with diverse Ni loadings. From Figure 11 and Table S2, the increase in Ni loading from 6 to 10 wt % leads to the obvious increase in H2 uptake from 0.97 to 1.75 mmol g−1 with similar reduction temperature for the α-peak assigned to the reduction of free NiO on the ZrO2 surface and the NiO that only has weak interaction with the support.64 The further increase in Ni loading from 10 to 12 wt % only results in a slight increase in H2 uptake toward the αpeak, but the reduction temperature quick increases from 376 to 386 °C. As the loading increases from 6 to 12 wt %, the reduction temperature toward to β-peak assigned to the reduction the NiO with SMSI shifts to a higher temperature from 470 to 490 °C.14,64 The H2 uptake for β-peak increases from 0.64 to 0.98 mmol g−1 with the Ni loading increasing from 6 to 10 wt %; however, the further increase in Ni loading from 10 to 12 wt % leads to a decrease in H2 uptake from 0.98 to 0.94 mmol g−1 for the β-peak. The LA-NiO/ZrO2 catalyst with 10 wt % of appropriate Ni loading has the highest total H2 uptake and reduction degree shown in Table S2, mainly ascribed to its higher NiO dispersion and more oxygen vacancies. Correlated to the reaction result, the increased H2 uptake toward the reduction of Ni species by high Ni loading favors the steam−CO2 dual reforming of methane. However, the too high of Ni loading leads to the larger crystal size and low dispersity of Ni (Table S2), which unfavors the reaction. Therefore, the appropriate Ni loading is essential. Correlating the XRD, BET, and H2-TPR characterization results to the reaction results for the steam−CO2 dual reforming over the reduced LA-NiO/ZrO2 catalysts, we can see the catalytic performance of the developed Ni-based 3470


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Figure 12. (a) CH4 conversion, (b) CO2 conversion, (c) H2 selectivity, and (d) CO selectivity as a function of time for stream for steam−CO2 dual reforming of methane over the developed LA-Ni/ZrO2 catalyst (error bars equal 95% confidence interval for conversion). Reaction conditions: mcat = 50 mg, CH4/CO2/H2O = 1:0.8:0.4, GHSV = 48 000 mL h−1 g−1, and performed at atmospheric pressure.

intensified Ni−support interaction, the enlarged oxygen vacancies, and the increased t-ZrO2 content and enhanced reducibility of NiO. The origin of the expectedly higher stability of the LA-Ni/ZrO2 catalyst for steam−CO2 dual reforming of methane in comparison of that of Ni/ZrO2 was explored by using the HRTEM, TGA, and TPH techniques. Figure 13 displays the TEM images of the fresh and spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts, and the results for the mean size of Ni particle are listed in Table 1. The thermal sintering of metal particle on the two samples after they suffer the severe reforming reaction conditions can be observed when we compare the TEM images between the fresh (Figure 13a,b and Table 1) and spent (Figure 13c,d and Table 1) catalysts. However, the developed LA-Ni/ZrO2 catalyst demonstrates a much superior sintering resistance to the classical supported Ni catalyst. The average metal particle of the spent LA-Ni/ZrO2 catalyst grows by 30% in comparison with the fresh one (from 11.6 to 15.1 nm), but approximately 87% on the classical Ni/ ZrO2 can be observed (from 13.7 to 25.6 nm). The developed LA-Ni/ZrO2 by LA-IWI approach demonstrates much higher Ni thermal sintering resistance in comparison with the classical Ni/ZrO2 by the IWI method, which might be ascribed to higher Ni dispersion and the intensified SMSI. The much higher thermal sintering resistance of the LA-Ni/ZrO2 catalyst in comparison with that of the Ni/ZrO2 catalyst may be one of the important reasons for its high stability shown in Figure 12. Furthermore, coke deposition is a crucial factor for deactivation

because the thermal sintering of the nanosized Ni particles easily takes place under the severe reaction conditions for reforming of methane, especially for the supported Ni catalysts owing to its low Tammann temperature.1 As a result, the catalytic activity for reforming of methane decreases as the time on stream extends. Herein we investigate the catalytic stability of the developed LA-Ni/ZrO2 catalyst and also explore the origin of enhanced stability of Ni-based catalyst by the LA-IWI method for steam−CO2 dual reforming of methane in comparison of classical Ni/ZrO2 catalyst by employing the TEM, TGA, and TPH techniques. Figure 12 demonstrates the CH4 conversion, CO2 conversion, and the H2/CO ratio as the function of time on stream over the developed LA-Ni/ZrO2 catalyst for steam−CO2 dual reforming of methane, and the classical Ni/ZrO2 catalyst is also included for comparison. From Figure 12, the developed LANi/ZrO2 catalyst exhibits much higher initial activity (e.g., around 94% and 95% for the conversions of CH4 and CO2, respectively) and stable catalytic behavior with similar H2/CO ratio shown in Figure S8 (lower than theoretical value because of the possible RWGS) in comparison with the classical Ni/ ZrO2 in the whole 24 h time on stream. From Figure 12c,d, the two catalysts exhibit similar selectivity for both H2 and CO, as well as no visible change can be observed along with the time on stream. As mentioned above, the excellent catalytic activities of the LA-Ni/ZrO2 catalyst to that of Ni/ZrO2 has been established to be ascribed to the increased Ni dispersity, 3471


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Figure 13. TEM images of fresh and spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts: (a) fresh LA-Ni/ZrO2, (b) fresh Ni/ZrO2, (c) spent LA-Ni/ZrO2, and (d) spent Ni/ZrO2, respectively. Insets are particle size distributions and magnified region.

of Ni-based catalysts in reforming of methane.18 From Figure 13c, a great deal of carbon tubes coiling on the spent LA-Ni/ ZrO2 catalyst can be observed; however, strangely, it looks like no coke on the spent Ni/ZrO2 catalyst can be clearly seen. Therefore, the HRTEM experiment on the spent Ni/ZrO2 catalyst was performed, and the inset in Figure 13d depicts the image. From the inset in Figure 13d, the shell-shaped coke encapsulated on Ni of the spent Ni/ZrO2 catalyst with about 8 nm graphitic carbon layers can be clearly seen. From reference 39, the intensified NiO−support interaction on the developed LA-NiO/ZrO2 catalyst confirmed by XRD, H2-TPR, and Raman analysis can efficiently inhibit the growth of shell-like carbon, but just carbon whiskers on the spent LA-NiO/ZrO2 catalyst can be observed (Figure 13c). However, owing to weaker interaction on the Ni/ZrO2 catalyst, a large amount of shell-like carbon is formed on the Ni particle. The presence of carbon whiskers does not decrease the accessibility of Ni to reactants but instead shell-like inhibiting the reactants from approaching the Ni active sites.39,43 Generally, the carbon whiskers grow based on two modes: one is metal was lifted up on the supported catalyst with weak interaction, and located at the tip of the whiskers; the other is that the carbon whiskers grow on the supported metal, and the metal does not leave the support. Because of the strong Ni−support interaction of the LA-NiO/ZrO2 catalyst, the carbon whiskers grow on the supported metal, but almost all Ni is not lifted up (just a few Ni particles are lifted up). From the TEM analysis, no visible shell carbon encapsulated on the Ni particle and high thermal sintering resistance of LA-the Ni/ZrO2 catalyst owing to the intensified SMSI led by the employment of the Ni−{(L-Arg)} complex as the Ni precursor endows it to be a highly stable robust reforming catalyst.

Furthermore, TGA analysis was employed to evaluate the rate of coke deposited on the two spent catalysts, and the TGA curves are presented in Figure 14. From Figure 14, the relative

Figure 14. TGA curves of the spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts.

higher coking rate of 0.17 mg gcat−1 h−1 for LA-Ni/ZrO2 compared to that of the Ni/ZrO2 catalyst (0.05 mg gcat−1 h−1) can be observed (the increased weight owing to the oxidation of the reduced metallic Ni has been taken into account), which is quite different from the previously reported result that highly dispersed Ni can suppress the coke formation.20−26 It is generally accepted that the existence of oxygen vacancies may reduce the coke through coke combustion reaction. However, in this work, the improved oxygen vacancies identified by the above on the highly dispersed LA-Ni/ZrO2 catalyst does not suppress coke 3472


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Correlated to the TEM, TGA, and TPH characterization results to the reaction performance as a function of time on stream, we can safely say that the catalytic stability of Ni-based catalysts is strongly dependent on the thermal sintering resistance and coke morphology significantly affected by the interaction between Ni and the support. The intensified Ni− ZrO2 interaction on the LA-Ni/ZrO2 catalyst by an L-arginine ligand-assisted incipient wetness impregnation method can significantly improve the Ni sintering resistance and also can change the morphology of coke on spent catalyst, attaching its higher catalytic activity led by the high Ni dispersity, enhanced NiO reducibility, enlarged oxygen vacancies, and the increased t-ZrO2 content, the developed LA-Ni/ZrO2 catalyst would be considered as a promising candidate for the production of syngas with tunable H2/CO ratios through steam−CO2 dual reforming of methane.

deposition. The reason may be that the formed tube-like carbon on the spent LA-Ni/ZrO2 catalyst does not inhibit the methane from approaching the Ni, and thus in turn the tubes can continuously grow. However, the shell-like carbon layers closely encapsulated on the Ni particles, and therefore further growth can be inhibited owing to the lost accessibility of Ni active sites. More interestingly, the higher coke amount on the spent LANi/ZrO2 catalyst in comparison with that on the spent Ni/ ZrO2 catalyst does not depress the catalytic activity, and thus in turn the developed LA-Ni/ZrO2 catalyst in this work by employing an L-arginine ligand-assisted incipient wetness impregnation method demonstrates much superior stability to the classical Ni/ZrO2 catalyst although a much higher coking rate on the former than that on the latter can be clearly seen. The results illustrate that the stability is strongly dependent on the morphology of coke on the spent catalyst.39 The shell-like carbon encapsulated on Ni particles can inhibit the coke further deposition and therefore can suppress coke. However, the shelllike carbon encapsulated on Ni particles, at the same time, can depress the accessibility of Ni to reactants, and thus in turn leads to the decrease in activity for reforming. Although there is a much higher coke forming rate on the LA-Ni/ZrO2 catalyst, higher catalytic stability can be achieved in comparison with that on the Ni/ZrO2 catalyst, which is ascribed to the maintained higher accessibility of Ni particles to reactants. Moreover, the maximum on the TGA curves can be observed, ascribed to the increase in weight of the spent samples resulted from the oxidation of Ni to NiO. The TPH experiments were conducted to elucidate further the nature of coke on the two spent catalysts. Generally, temperatures between 300 and 600 °C on the TPH profiles are assigned as α-carbon that can easily react with hydrogen. Carbon species above 600 °C is assigned as β-carbon, which is much less reactive toward hydrogen than the α-carbon. Carbon species located around 805 °C is assigned as γ-carbon that reacts with hydrogen difficultly.38,59,60 From Figure 15, there

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CONCLUSIONS Supported Ni catalysts on ZrO2 with high Ni dispersity were synthesized by an L-arginine ligand-assisted incipient wetness impregnation method. Evidenced by TEM, CO chemisorption, and XRD, the forming of a Ni−{(L-Arg)} complex used as a Ni precursor facilitates the Ni dispersion on support and strengthens the Ni−ZrO2 interaction. Such SMSI in turn gives rise to more oxygen vacancies owing to the replacement of Zr4+ in the ZrO2 lattice by Ni2+ on LA-NiO/ZrO2 catalyst in comparison with classical NiO/ZrO2, and therefore both Ni2+ cation and the thermodynamically unstable tetragonal ZrO2 phase can be stabilized as well as the reduction of supported NiO on ZrO2 and sintering-resistance can be improved. The improvement in Ni dispersity, reducibility, oxygen vacancies, and the increasing t-ZrO2 content can significantly accelerate the steam−CO2 dual reforming of methane. Furthermore, the high Ni sintering-resistance and the change in coke characteristics of the developed LA-Ni/ZrO2 catalyst by the LA-IWI method make it exhibit remarkably higher catalytic stability in comparison with the classical Ni/ZrO2 catalyst prepared by the IWI method, but not by suppressing coke deposition. This work presents a new route to the synthesis of highly efficient and stable Ni-based catalysts for producing syngas with tunable H2/CO ratios through steam−CO2 dual reforming of methane. The fundamental understanding on the origin of enhanced activity and stability of the LA-Ni/ZrO2 catalyst is also quite important for the rational design of a more active and stable candidate for the methane reforming reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01277.

Figure 15. TPH profiles of the spent LA-Ni/ZrO2 and Ni/ZrO2 catalysts.

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are quite different characteristics toward the TPH profiles on the spent LA-Ni/ZrO2 catalyst from that on the spent Ni/ZrO2 catalyst, suggesting the different types of coke depositing on the two catalysts confirmed by TEM analysis. Furthermore, the higher H2 uptake for the spent LA-Ni/ZrO2 than that for the spent Ni/ZrO2 catalyst can be clearly observed, which is consistent with the results obtained by TGA analysis.

Extra XRD patterns, reaction results, and the catalyst nature quantitative analytic results (PDF).

AUTHOR INFORMATION

Corresponding Author

*Z. Zhao. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3473


Research Article


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(16) Kuhn, J. N.; Zhao, Z.; Felix, L. G.; Slimane, R. B.; Choi, C. W.; Ozkan, U. S. Olivine catalysts for methane-and tar-steam reforming. Appl. Catal., B 2008, 81 (1), 14−26. (17) Bradford, M.; Vannice, M. CO2 reforming of CH4. Catal. Rev.: Sci. Eng. 1999, 41 (1), 1−42. (18) Liu, C. J.; Ye, J.; Jiang, J.; Pan, Y. Progresses in the Preparation of Coke Resistant Ni-Based Catalyst for Steam and CO2 Reforming of Methane. ChemCatChem 2011, 3 (3), 529−541. (19) Gadalla, A. M.; Bower, B. The Role of Catalyst support on the activity of nckel for reforming methane with CO2. Chem. Eng. Sci. 1988, 43 (11), 3049−3062. (20) Zhu, X.; Huo, P.; Zhang, Y. P.; Cheng, D. G.; Liu, C. J. Structure and reactivity of plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane. Appl. Catal., B 2008, 81 (1), 132−140. (21) Chen, D.; Christensen, K. O.; Ochoa-Fernández, E.; Yu, Z.; Tøtdal, B.; Latorre, N.; Monzón, A.; Holmen, A. Synthesis of carbon nanofibers: effects of Ni crystal size during methane decomposition. J. Catal. 2005, 229 (1), 82−96. (22) Sun, N.; Wen, X.; Wang, F.; Wei, W.; Sun, Y. Effect of pore structure on Ni catalyst for CO2 reforming of CH4. Energy Environ. Sci. 2010, 3 (3), 366−369. (23) Xu, B. Q.; Wei, J. M.; Yu, Y. T.; Li, Y.; Li, J. L.; Zhu, Q. M. Size Limit of Support Particles in an Oxide-Supported Metal Catalyst: Nanocomposite Ni/ZrO2 for Utilization of Natural Gas. J. Phys. Chem. B 2003, 107 (22), 5203−5207. (24) Xu, L.; Song, H.; Chou, L. One-Pot Synthesis of Ordered Mesoporous NiO-CaO-Al2O3 Composite Oxides for Catalyzing CO2 Reforming of CH4. ACS Catal. 2012, 2 (7), 1331−1342. (25) Wang, N.; Shen, K.; Huang, L.; Yu, X.; Qian, W.; Chu, W. Facile Route for Synthesizing Ordered Mesoporous Ni-Ce-Al Oxide Materials and Their Catalytic Performance for Methane Dry Reforming to Hydrogen and Syngas. ACS Catal. 2013, 3 (7), 1638− 1651. (26) Zhang, S.; Muratsugu, S.; Ishiguro, N.; Tada, M. Ceria-Doped Ni/SBA-16 Catalysts for Dry Reforming of Methane. ACS Catal. 2013, 3 (8), 1855−1864. (27) Li, Z.; Mo, L.; Kathiraser, Y.; Kawi, S. Yolk-Satellite-Shell Structured Ni-Yolk@ Ni@ SiO2 Nanocomposite: Superb Catalyst toward Methane CO2 Reforming Reaction. ACS Catal. 2014, 4 (5), 1526−1536. (28) Li, S.; Gong, J. Strategies for improving the performance and stability of Ni-based catalysts for reforming reactions. Chem. Soc. Rev. 2014, 43 (21), 7245−7256. (29) Ewbank, J. L.; Kovarik, L.; Diallo, F. Z.; Sievers, C. Effect of metal-support interactions in Ni/Al2O3 catalysts with low metal loading for methane dry reforming. Appl. Catal., A 2015, 494, 57−67. (30) Wang, C.; Sun, N.; Zhao, N.; Wei, W.; Zhang, J.; Zhao, T.; Sun, Y.; Sun, C.; Liu, H.; Snape, C. E. The Properties of Individual Carbon Residuals and Their Influence on the Deactivation of Ni-CaO-ZrO2 Catalysts in CH4 Dry Reforming. ChemCatChem 2014, 6 (2), 640− 648. (31) Liu, B.; Au, C. Carbon deposition and catalyst stability over La2NiO4/γ-Al2O3 during CO2 reforming of methane to syngas. Appl. Catal., A 2003, 244 (1), 181−195. (32) Gallego, G. S.; Mondragón, F.; Barrault, J.; Tatibouët, J. M.; Batiot-Dupeyrat, C. CO2 reforming of CH4 over La-Ni based perovskite precursors. Appl. Catal., A 2006, 311, 164−171. (33) Xu, Z.; Zhen, M.; Bi, Y.; Zhen, K. Catalytic properties of Ni modified hexaaluminates LaNiyAl12−yO19−δ for CO2 reforming of methane to synthesis gas. Appl. Catal., A 2000, 198 (1), 267−273. (34) Luna, A. E. C.; Iriarte, M. E. Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst. Appl. Catal., A 2008, 343 (1), 10−15. (35) Hu, Y. H. Solid-solution catalysts for CO2 reforming of methane. Catal. Today 2009, 148 (3), 206−211. (36) Liu, D.; Quek, X.-Y.; Wah, H. H. A.; Zeng, G.; Li, Y.; Yang, Y. Carbon dioxide reforming of methane over nickel-grafted SBA-15 and MCM-41 catalysts. Catal. Today 2009, 148 (3), 243−250.

ACKNOWLEDGMENTS This work is financially supported by the Joint Fund of Coal, set up by National Natural Science Foundation of China and Shenhua Co., Ltd. (No. U1261104), and also sponsored by the Chinese Ministry of Education via the Program for New Century Excellent Talents in University (Grant no. NCET-120079), and the Natural Science Foundation of Liaoning Province (grant no. 2015020200), as well as by the Fundamental Research Funds for the Central Universities (DUT15LK41).

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REFERENCES

(1) Gonzalez Delacruz, V. M.; Pereniguez, R.; Ternero, F.; Holgado, J. P.; Caballero, A. Modifying the Size of Nickel Metallic Particles by H2/CO Treatment in Ni/ZrO2 Methane Dry Reforming Catalysts. ACS Catal. 2011, 1 (2), 82−88. (2) Chu, W.; Wang, L. N.; Chernavskii, P. A.; Khodakov, A. Y. GlowDischarge Plasma-Assisted Design of Cobalt Catalysts for Fischer− Tropsch Synthesis. Angew. Chem., Int. Ed. 2008, 47 (27), 5052−5055. (3) Wei, J. M.; Xu, B. Q.; Li, J. L.; Cheng, Z. X.; Zhu, Q. M. Highly active and stable Ni/ZrO2 catalyst for syngas production by CO2 reforming of Methane. Appl. Catal., A 2000, 196 (2), L167−L172. (4) Olah, G. A.; Goeppert, A.; Czaun, M.; Prakash, G. S. Bi-reforming of Methane from Any Source with Steam and Carbon Dioxide Exclusively to Metgas (CO-2H2) for Methanol and Hydrocarbon Synthesis. J. Am. Chem. Soc. 2013, 135 (2), 648−650. (5) Elmalik, E. E.; Tora, E.; El-Halwagi, M.; Elbashir, N. O. solvent selection for commercial supercritical Fischer−Tropsch synthesis process. Fuel Process. Technol. 2011, 92 (8), 1525−1530. (6) Song, C.; Pan, W. Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios. Catal. Today 2004, 98 (4), 463−484. (7) Ryi, S. K.; Lee, S. W.; Park, J. W.; Oh, D. K.; Park, J. S.; Kim, S. S. Combined steam and CO2 reforming of methane using catalytic nickel membrane for gas to liquid (GTL) process. Catal. Today 2014, 236, 49−56. (8) Gardner, T. H.; Spivey, J. J.; Kugler, E. L.; Pakhare, D. CH4-CO2 reforming over Ni-substituted barium hexaaluminate catalysts. Appl. Catal., A 2013, 455, 129−136. (9) Koo, K. Y.; Roh, H.-S.; Seo, Y. T.; Seo, D. J.; Yoon, W. L.; Park, S. B. Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process. Appl. Catal., A 2008, 340 (2), 183−190. (10) Danilova, M.; Fedorova, Z.; Zaikovskii, V.; Porsin, A.; Kirillov, V.; Krieger, T. Porous Nickel-based catalysts for combined steam and carbon dioxide reforming of methane. Appl. Catal., B 2014, 147, 858− 863. (11) Li, D.; Nakagawa, Y.; Tomishige, K. Methane reforming to synthesis gas over Ni catalysts modified with noble metals. Appl. Catal., A 2011, 408 (1), 1−24. (12) Hao, Z.; Zhu, Q.; Jiang, Z.; Hou, B.; Li, H. Characterization of aerogel Ni/Al2O3 catalysts and investigation on their stability for CH4CO2 reforming in a fluidized bed. Fuel Process. Technol. 2009, 90 (1), 113−121. (13) Ö zkara-Aydınoğlu, Ş.; Aksoylu, A. E. CO2 Reforming of methane over Pt-Ni/Al2O3 catalysts: effects of catalyst composition, and water and oxygen addition to the feed. Int. J. Hydrogen Energy 2011, 36 (4), 2950−2959. (14) Son, I. H.; Lee, S. J.; Soon, A.; Roh, H. S.; Lee, H. Steam treatment on Ni/γ-Al2O3 for enhanced carbon resistance in combined steam and carbon dioxide reforming of methane. Appl. Catal., B 2013, 134, 103−109. (15) Koo, K. Y.; Roh, H. S.; Jung, U. H.; Yoon, W. L. Combined H2O and CO2 reforming of CH4 over Ce-promoted Ni/Al2O3 catalyst for gas to liquid (GTL) process: enhancement of Ni-CeO2 interaction. Catal. Today 2012, 185 (1), 126−130. 3474


Research Article

ACS Sustainable Chemistry & Engineering (37) Liu, D.; Quek, X. Y.; Cheo, W. N. E.; Lau, R.; Borgna, A.; Yang, Y. MCM-41 supported nickel-based bimetallic catalysts with superior stability during carbon dioxide reforming of methane: Effect of strong metal-support interaction. J. Catal. 2009, 266 (2), 380−390. (38) Wang, N.; Shen, K.; Yu, X.; Qian, W.; Chu, W. Preparation and characterization of a plasma treated NiMgSBA-15 catalsyst for methane reforming with CO2 to produce syngas. Catal. Sci. Technol. 2013, 3, 2278−2287. (39) Huang, T.; Huang, W.; Huang, J.; Ji, P. Methane reforming reaction with carbon dioxide over SBA-15 supported Ni-Mo bimetallic catalysts. Fuel Process. Technol. 2011, 92 (10), 1868−1875. (40) Horváth, A.; Stefler, G.; Geszti, O.; Kienneman, A.; Pietraszek, A.; Guczi, L. Methane dry reforming with CO2 on CeZr-oxide supported Ni, NiRh and NiCo catalysts prepared by sol-gel technique: relationship between activity and coke formation. Catal. Today 2011, 169 (1), 102−111. (41) Snoeck, J.-W.; Froment, G.; Fowles, M. Filamentous carbon formation and gasification: thermodynamics, driving force, nucleation, and steady-state growth. J. Catal. 1997, 169 (1), 240−249. (42) Barroso-Quiroga, M. M.; Castro-Luna, A. E. Catalytic activity and effect of modifiers on Ni-based catalysts for the dry reforming of methane. Int. J. Hydrogen Energy 2010, 35 (11), 6052−6056. (43) Kambolis, A.; Matralis, H.; Trovarelli, A.; Papadopoulou, C. Ni/ CeO2-ZrO2 catalysts for the dry reforming of methane. Appl. Catal., A 2010, 377 (1), 16−26. (44) Chen, W.; Zhao, G.; Xue, Q.; Chen, L.; Lu, Y. High carbonresistance Ni/CeAlO3-Al2O3 catalyst for CH4/CO2 reforming. Appl. Catal., B 2013, 136, 260−268. (45) Bachiller Baeza, B.; Mateos Pedrero, C.; Soria, M.; GuerreroRuiz, A.; Rodemerck, U.; Rodríguez Ramos, I. Transient studies of low-temperature dry reforming of methane over Ni-CaO/ZrO2-La2O3. Appl. Catal., B 2013, 129, 450−459. (46) Güell, B. M.; da Silva, I. T.; Seshan, K.; Lefferts, L. Sustainable route to hydrogen-design of stable catalysts for the steam gasification of biomass related oxygenates. Appl. Catal., B 2009, 88 (1), 59−65. (47) Goscianska, J.; Ziolek, M.; Gibson, E.; Daturi, M. Novel mesoporous zirconia-based catalysts for WGS reaction. Appl. Catal., B 2010, 97 (1), 49−56. (48) Yasu-eda, T.; Kitamura, S.; Ikenaga, N.-O.; Miyake, T.; Suzuki, T. Selective oxidation of alcohols with molecular oxygen over Ru/ CaO-ZrO2 catalyst. J. Mol. Catal. A: Chem. 2010, 323 (1), 7−15. (49) Shukla, S.; Seal, S. Thermodynamic Tetragonal Phase Stability in Sol-Gel Derived Nanodomains of Pure Zirconia. J. Phys. Chem. B 2004, 108 (11), 3395−3399. (50) Bellido, J. D.; Assaf, E. M. Effect of the Y2O3-ZrO2 support composition on nickel catalyst evaluated in dry reforming of methane. Appl. Catal., A 2009, 352 (1), 179−187. (51) Rezaei, M.; Alavi, S.; Sahebdelfar, S.; Yan, Z.-F. Mesoporous nanocrystalline zirconia powders: a promising support for nickel catalyst in CH4 reforming with CO2. Mater. Lett. 2007, 61 (13), 2628− 2631. (52) Xie, S.; Iglesia, E.; Bell, A. T. Water-Assisted Tetragonal-toMonoclinic Phase Transformation of ZrO2 at Low Temperatures. Chem. Mater. 2000, 12 (8), 2442−2447. (53) Xie, H.; Lu, J.; Shekhar, M.; Elam, J. W.; Delgass, W. N.; Ribeiro, F. H.; Weitz, E.; Poeppelmeier, K. R. Synthesis of Na-Stabilized Nonporous t-ZrO2 Supports and Pt/t-ZrO2 Catalysts and Application to Water-Gas-Shift Reaction. ACS Catal. 2013, 3 (1), 61−73. (54) Grabowski, R.; Słoczyński, J.; Sliwa, M.; Mucha, D.; Socha, R.; Lachowska, M.; Skrzypek, J. Influence of Polymorphic ZrO2 Phases and the Silver Electronic State on the Activity of Ag/ZrO2 Catalysts in the Hydrogenation of CO2 to Methanol. ACS Catal. 2011, 1 (4), 266− 278. (55) Westerhaus, F. A.; Jagadeesh, R. V.; Wienhöfer, G.; Pohl, M. M.; Radnik, J.; Surkus, A. E.; Rabeah, J.; Junge, K.; Junge, H.; Nielsen, M.; Brückner; Beller, M. Heterogenized cobalt oxide catalysts for nitroarene reduction by pyrolysis of molecularly defined complexes. Nat. Chem. 2013, 5, 537−543.

(56) Li, P. Z.; Aijaz, A.; Xu, Q. Highly dispersed surfactant-free nickel nanoparticles and their remarkable catalytic activity in the hydrolysis of ammonia borane for hydrogen generation. Angew. Chem., Int. Ed. 2012, 51, 6753−6756. (57) Wang, S.; Yin, K.; Zhang, Y.; Liu, H. Glycerol Hydrogenolysis to Propylene Glycol and Ethylene Glycol on Zirconia Supported Noble Metal Catalysts. ACS Catal. 2013, 3 (9), 2112−2121. (58) Fan, M. S.; Abdullah, A. Z.; Bhatia, S. Utilization of greenhouse gases through carbon dioxide reforming of methane over Ni-Co/MgOZrO2: preparation, characterization and activity studies. Appl. Catal., B 2010, 100 (1), 365−377. (59) Becerra, A.; Dimitrijewits, M.; Arciprete, C.; Luna, A. C. Stable Ni/Al2O3 catalysts for methane dry reforming. Granular Matter 2001, 3 (1−2), 79−81. (60) Alagha, A.; Brown, D. A.; Elawad, M.; Müller-Bunz, H.; Nimir, H.; Yanovsky, A. I.; Nolan, K. B. The preparation and crystal structure of acetatobis (L-arginine) zinc (II) acetate trihydrate, the first reported X-ray structure of a zinc (II)−arginine complex. Inorg. Chim. Acta 2011, 377 (1), 185−187. (61) Zhang, Z.; Verykios, X. E. Carbon dioxide reforming of methane to synthesis gas over Ni/La2O3 catalysts. Appl. Catal., A 1996, 138 (1), 109−133. (62) Hegarty, M.; O’Connor, A.; Ross, J. Syngas production from natural gas using ZrO2-supported metals. Catal. Today 1998, 42 (3), 225−232. (63) Li, S.; Zhang, C.; Huang, Z.; Wu, G.; Gong, J. A Ni@ZrO2 nanocomposite for ethanol steam reforming: enhanced stability via strong metal-oxide interaction. Chem. Commun. 2013, 49 (39), 4226− 4228. (64) Huang, B.; Li, X.; Ji, S.; Lang, B.; Habimana, F.; Li, C. Effect of MgO promoter on Ni-based SBA-15 catalysts for combined steam and carbon dioxide reforming of methane. J. Nat. Gas Chem. 2008, 17 (3), 225−231. (65) Sun, G.; Hidajat, K.; Wu, X.; Kawi, S. A crucial role of surface oxygen mobility on nanocrystalline Y2O3 support for oxidative steam reforming of ethanol to hydrogen over Ni/Y2O3 catalysts. Appl. Catal., B 2008, 81 (3), 303−312. (66) Lin, S. S.; Chen, C. L.; Chang, D. J.; Chen, C. C. Catalytic wet air oxidation of phenol by various CeO2 catalysts. Water Res. 2002, 36 (12), 3009−3014. (67) Campbell, C. T.; Peden, C. H. Oxygen vacancies and catalysis on ceria surfaces. Science 2005, 309 (5735), 713−714. (68) Dow, W. P.; Wang, Y. P.; Huang, T. J. Yttria-stabilized zirconia supported copper oxide catalyst: I. Effect of oxygen vacancy of support on copper oxide reduction. J. Catal. 1996, 160 (2), 155−170. (69) Li, C.; Li, M. UV Raman spectroscopic study on the phase transformation of ZrO2, Y2O3-ZrO2 and SO42−/ZrO2. J. Raman Spectrosc. 2002, 33 (5), 301−308. (70) Merle, T.; Guinebretiere, R.; Mirgorodsky, A.; Quintard, P. Polarized Raman spectra of tetragonal pure ZrO2 measured on epitaxial films. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65 (14), 144302. (71) Fadda, G.; Zanzotto, G.; Colombo, L. First-principles study of the effect of pressure on the five zirconia polymorphs. II. Static dielectric properties and Raman spectra. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82 (6), 064106. (72) Bouvier, P.; Lucazeau, G. Raman spectra and vibrational analysis of nanometric tetragonal zirconia under high pressure. J. Phys. Chem. Solids 2000, 61 (4), 569−578. (73) Cong, Y.; Li, B.; Yue, S.; Fan, D.; Wang, X. J. Effect of Oxygen Vacancy on Phase Transition and Photoluminescence Properties of Nanocrystalline Zirconia Synthesized by the One-Pot Reaction. J. Phys. Chem. C 2009, 113 (31), 13974−13978. (74) Liu, B.; Goldbach, A.; Xu, H. Sour water-gas shift reaction over Pt/CeO2 catalysts. Catal. Today 2011, 171 (1), 304−311. (75) Xu, L.; Song, H.; Chou, L. Carbon dioxide reforming of methane over ordered mesoporous NiO-MgO-Al2O3 composite oxides. Appl. Catal., B 2011, 108, 177−190. 3475


Research Article

ACS Sustainable Chemistry & Engineering (76) Van Dillen, A. J.; Terörde, R. J.; Lensveld, D. J.; Geus, J. W.; de Jong, K. P. Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes. J. Catal. 2003, 216 (1), 257−264.

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