(739d) Determination of Quantitative Kinetics and the Structural Mechanism for Ni-Co Dry Reforming Catalysts

Dry reforming Ni-Co bimetallics catalysts supported on MgAlOx were synthesized using coprecipitation and incipient wetness impregnation methods. The mechanism of metal particle growth during reduction of the monometallic Ni or Co and bimetallic Ni–Co catalysts was studied using pair distribution function (PDF). This study reveals quantitative kinetics for NiCo catalysts with various compositions and synthesis techniques.

Introduction

CO₂ reforming of CH₄ (CDRM), also known as dry reforming of methane, has attracted attention among the other CH₄ reforming reactions because it uses CO₂ and CH₄ which are greenhouse gases and major component from shale gas, and produces synthesis gas or syngas, which is desired for industry. Production of syngas with a H₂/CO ratio of 1 is suitable feed for Fischer-Tropsch (F-T) and methanol synthesis.¹

CH₄ + CO₂ → 2H₂+ 2CO;             CDRM  DH° = 247 kJ/mol

Ni-based catalysts mostly used for dry reforming reaction due to their availability and low cost compare to other metals. Ni-Co/AlMgO_x bimetallic catalysts with a Ni/Co ratio were prepared by the precipitation method, which is highly active and stable for dry reforming reaction.²The effects of various preparation methods, Ni/Co ratios on Ni-Co bimetallic catalysts, and Ni or Co monometallic catalysts using dry reforming reaction have been investigated. 

Materials and Methods

The bimetallic Ni-Co/MgZlOx catalysts with Ni/Co ratio 1:0, 1:1 and 0:1 were synthesized one-step precipitating and impregnation.² The reduction of the catalysts was conducted in H₂/He mixture at 750 ^oC. Scattering data suitable for PDF analysis were collected at beamline 11-ID-B at the Advanced Photon Source at Argonne National Laboratory. High energy X-rays (58 keV, λ=0.2127Å) were used in combination with a large amorphous silicon-based area detector to collect data to high values of momentum transfer.The two dimensional images were reduced to one-dimensional scattering intensity data within Fit2D.3 The contributions from the sample environment and background to the measured diffraction intensities were subtracted and corrections for multiple scattering, X-ray polarization, sample absorption, and Compton scattering were applied within PDFgetX2,³ to obtain G(r), the pair distribution function. The PDFs, G(r) = 4πr[ρ(r)−ρ_o] where ρ(r) and ρ_oare the instantaneous and average densities.

Results and Discussion

The in situ PDFs (Fig. 1) show atomic structural changes during the 3 h reduction at 750 ^oC for NiCo bimetallic catalysts with ratio of 0:1, 1:1 and 1:0 synthesized with two techniques coprecipitation and impregnation. The oscillation peaks around 2 Å and 3 Å are metal-oxygen (M-O) bonds contributed from both the MgAlOx support and the precursors of Ni and Co as labeled in Fig 1.1. Both of these two peaks went down during the reduction mostly from the reduction of NiOx and CoOx. Meanwhile a new peak at 2.5 Å kept growing which corresponds to the formation of metal-metal (M-M) bonds and illustrate the reduction extent. From Fig 1, we can tell NiOx is generally easier to be reduced than CoOx regardless of the preparation method. The coprecipitated  Co shows higher reducibility than impregnated Co (Fig 1.1 and Fig 1.4) while the coprecipitated Ni shows slightly lower reducibility than impregnated one (Fig. 1.3 and Fig 1.6).  This shows the moderation effect from the support for the coprecipitation method implying stronger interaction between the metal and the support lattice. More interestingly, the impregnated bimetallic NiCo catalysts exhibit less reduction than the single metal Ni, which indicates the Co component hinders the reduction. On the contrary the coprecipitated NiCo shows the highest reducibility possibly due to the synergetic effect from the support and bimetals.  By quantifying the disappearance of M-O peaks and formation of the M-M peak in the in situ PDFs,⁴the quantitative reduction kinetics is obtained and facilitates understanding on the reduction mechanism for the impregnated and coprecipitated NiCo catalysts. This analysis will be presented in the full presentation as the limited volume here.

Figure 1. The in situ PDFs for reduction of NiCo bimetallic catalysts with ratio of 0:1, 1:1 and 1:0 synthesized with two techniques coprecipetation and impregnation at 750 ^oC for 3 h.

Significance

The in situ PDF study provides the atomic structural kinetics and mechanism for NiCo bimetallic catalysts synthesis which will help understanding the correlation between structure and synthesis condition and future optimize the catalysts design.

References

1. M.S. Fan, A.Z. Abdullah, S. Bhatia, ChemCatChem, 1 (2009), p. 192.; J.R.H. Ross, A.N.J. van Keulen, M.E.S. Hegarty, K. Seshan, Catal. Today, 30 (1996), p. 193.; M.A. Vannice, Catal. Rev., 14 (1976), p. 153.
2. J. Zhang, H. Wang, A.K. Dalai, J. Catal. 249 (2007), p. 298.; J. Zhang, H. Wang, A.K. Dalai, Appl. Catal.A 339 (2008), p. 321.
3. X. Qiu, J. W. Thompson, S. J. L. Billinge, J. Appl. Cryst. 37, 678-678.
4. Zhao, H.; Nenoff, T. M.; Jennings, G.; Chupas, P. J.; Chapman, K. W., J. Phys. Chem. Letters 2011, 2 (21), 2742-2746.