Bimetallic Catalyst System for Steam Reforming
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To improve catalytic performance of Ni supported catalysts for hydrogen production, a monolayer of a noble metal is introduced on the surface of Ni particles. A series of surface alloy M/Ni catalysts was prepared by redox reaction of the reduced Ni catalyst with various types of noble metals (M: Ag, Pt, Pd, Rh, Re, Ru) at different surface composition. Hydrotalcite-like (HT) material was used as support for the catalyst. The loading amount of Ni was fixed at 12 wt% for all catalysts (obtained atomic ratio Ni:Mg:Al = 0.73:5.27:2). The second metal M loading was varied 0-100% of surface Ni atoms on the Ni particles. The surface modifications of Ni particles by alloying with M are determined by means of XRD, hydrogen chemisorption, and XPS. It was found that M atoms are selectively located at the Ni surfaces, and form a stable surface alloy with Ni. H2 chemisorption measurements showed a dramatic change in the amount of absorbed hydrogen on the particles which is caused by the surface M-Ni alloy. Particularly, the Ni dispersion in bimetallic Ag/Ni catalyst exponentially decreased with increasing surface fraction of Ag. XPS results confirmed the existence of metallic M and Ni in the bimetallic catalysts, and reveal a shift in binding energy of both Ni metal and M metal. The suppression of Ni peak is observed when surface M fraction increased. The effects of surface Ni-Ag alloy on carbon formation during methane decomposition (MD) reaction and on catalytic activity of the catalyst in steam reforming of methane (SRM) are performed in this work. The selective deposition of Ag on surface Ni substantially decreases carbon formation during MD and the catalytic activity during SRM. The very small amount of 0,075wt% Ag deposited on the surface Ni (surface Ag fraction of 0.03) reduces the carbon formation in methane decomposition 10 times compared with the mono Ni catalyst. From the correlation of initial rate and surface Ag fraction, the active Ni(211) sites are predicted to be 0.055. Both experimental estimations and theoretical density functional theory (DFT) calculations suggested that Ag atoms deposit first on all Ni(211) sites. As a result, blockage of the steps on Ni nanoparticles dramatically decreased the initial methane decomposition rate and the TOF of methane steam reforming. After filling all the active Ni(211) sites, the deposition of Ag on the Ni surface was followed by the less active Ni(100) facet, and finally the lowest Ni(111) facet, which causes a relatively small decrease of the initial rate of methane decomposition and TOF of methane steam reforming with increasing Ag loading. The kinetic study of methane decomposition shows that surface Ni-Ag alloy leads to decrease in both the pre-exponential factor and surprisingly also of the activation energy of Ag/Ni catalysts with increasing Ag loading. But both the kinetic study and DFT calculation of methane steam reforming indicate that the apparent activation energy increases simultaneously when the surface Ag fraction increased. The effects of surface Ni alloying with various types of second metals (M: Ag, Pt, Pd, Rh, Re, Ru) at different surface compositions, on the catalytic behavior in MD reaction and SRM reaction, were also conducted. It was found that the surface M-Ni alloy influenced the activity of the catalyst for the two reactions, and that changes were a function of the surface M fraction. Among all bimetallic catalysts, the Ag/Ni-HT catalyst displays the lowest values for the methane decomposition rate (rd) and methane steam reforming rate (rr), and the both rates decrease with increasing Ag loading. The Pt/Ni-HT exhibits the highest rd and rr at a surface fraction (a) of 0.25. The Rh/Ni-HT presents the moderate rd at both a = 0.25 and a = 1, but the highest rr is observed at high Rh loading (a = 1). The M/Ni-HT catalysts show more resistance toward carbon formation than the monometallic Ni catalyst in the following order monometallic Ni << Pt0.25Ni(s)0.75 < Pd0.25Ni(s)0.75 ≤ Ru0.25Ni(s)0.75 ≤ Rh0.25Ni(s)0.75 ≤ Re0.25Ni(s)0.75 < Ag0.25Ni(s)0.75 at surface M fraction of 0.25 and monometallic Ni < Pt1Ni(s)0 < Rh1Ni(s)0 < Re1Ni(s)0 < Ag1Ni(s)0 at surface M fraction of 1. Interestingly, M1Ni(s)0 catalysts (M: Pt, Pd, or Rh) exhibits higher carbon formation than M0.25Ni(s)0.75 catalyst. The activation energies of methane decomposition over M/Ni-HT increase in the following order: Ag0.25Ni(s)0.75 < Rh0.25Ni(s)0.75 ≤ Ru0.25Ni(s)0.75 < Pt0.25Ni(s)0.75 < Pd0.25Ni(s)0.75 < Re0.25Ni(s)0.75 and Ag1Ni(s)0 < Rh1Ni(s)0 = Pt1Ni(s)0 < Re1Ni(s)0 < Pd1Ni(s)0. The effect of changes in the d-band center of Ni catalyst by alloying with M, on the catalytic activity over both reactions, is also addressed. If the d-band center shifts toward more negative, the activity of surface alloy for both reactions is increased. However when the reactions take place at high temperature, the structure of PtNi is rearranged; as a result, extremely higher activity than the trend is observed for PtNi in both reactions.