Study On Mo₂C-based Catalysts For Low-Temperature Water Gas Shift Reaction

The catalytic properties of early transition metal carbides have been the subject ofmany investigations since it was reported that some carbides possess catalyticproperties that resemble those of Pt group metals. Water gas shift (WGS) reaction isan important industrial process for hydrogen production. Previously we reported thatthe high surface area Mo2C prepared from the temperature programmed reaction andthe Pt deposited onto Mo2C (Pt/Mo2C) were active for this reaction. In this thesis, thesupported Mo2C catalysts, the influences of the alkali metal additions and the stabilityof the Mo2C catalysts were investigated.Chapter2describes the synthesis and characterization of Mo2C/Al2O3and Pt-Mo2C/Al2O3catalysts and their evaluation for the WGS reaction. Mo2C/Al2O3catalystwas prepared via the temperature programmed reaction from AM/Al2O3precursorwhich was prepared by the incipient wetness impregnation of Al2O3and AM. The Pt-Mo2C/Al2O3catalysts were prepared from the Mo2C/Al2O3catalyst using a wetimpregnation method. The materials were characterized using techniques includingphysical adsorption, X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD),CO chemisorption, temperature programmed desorption and transmission electronmicroscopy and the activities were evaluated for the WGS reaction under differentialconditions. The presence of Mo2C enhanced the deposition of Pt from H2PtCl6solutions. The Pt-Mo2C/Al2O3catalysts were much more active than thecorresponding Pt/Al2O3catalyst. For example, the turnover frequency for the3.8wt%Pt-Mo2C/Al2O3catalyst at240°C (0.81s-1) was two orders of magnitude higher thanthat for the3.9wt%Pt/Al2O3catalyst (0.007s-1). Trends with increasing Pt loadingfor the Pt-Mo2C/Al2O3catalysts were similar to those previously observed forPt/Mo2C catalysts. The results were consistent with Pt being co-located with Mo2C, aconsequence of the strong affinity of the Pt precursor for Mo2C. Interaction of the Ptwith Mo2C would account for the significant differences between the catalytic andsurface chemical properties of the Pt-Mo2C/Al2O3and Pt/Al2O3catalysts.In chapter3, high surface area SiO2was used as the support for the preparation ofthe Mo2C/SiO2and Pt-Mo2C/SiO2catalysts. These materials were also characterizedby physical adsorption, XRD and STEM-XEDS and evaluated for WGS reaction to identify the catalyst structures and compositions. The Mo2C particles supported on the16.9and29.0wt.%Mo2C/SiO2were in the range of the1-2nm and the loadingamount of the Mo2C had little influence on the particle size distribution. Thenormalized activities of the16.9and29.0wt.%Mo2C/SiO2catalyst were higher thanthe unsupported Mo2C catalyst because of the higher dispersion.4.2wt.%Pt-16.3wt.%Mo2C/SiO2catalyst was also prepared from the wet-impregnation method and the Ptparticle distribution was in the range of5-10nm. Pt and Mo2C were also co-locatedand the interaction enhanced the reactivity of the Pt.In chapter4, the influences of the alkali metal addition were investigated. Thesodium and potassium were loaded on the high surface Mo2C via the wetimpregnation method. The composition and structure of the catalysts werecharacterized and the activities were compared. The addition of the sodium andpotassium decreased the surface areas of the Mo2C catalysts, CO uptakes and also thereaction rates. The reaction rates of Na/Mo2C catalysts normalized by the surfaceareas were similar to each other and also silimar to the Mo2C catalyst indicating thatthe presences of the alkali metals didnot change the chemical properties of the activesites on the Mo2C catalyst while blocked the pores thus decreased the activities of thecatalysts. Alkali metals were reported as the promoters for the enhanced activities inthe WGS reaction which could modify the properties of the supports, such as TiO2andAl2O3. However, the Mo2C was a good catalyst for the water activation compared toalkali metal species. Thus, the addition of the alkali metals decreased the activities ofMo2C and Pt/Mo2C catalysts.In chapter5, the stability of the Mo2C in the WGS reaction was investigated. Thehigh surface area Mo2C (~120m2/g) manifested a higher deactivation rate comparedto the commercial Cu/Zn/Al2O3catalyst. The post reaction characterization showedthat the bulk structure of Mo2C was maintained. Thus, the deactivation was due to thesurface properties change of the Mo2C. The reaction rates of the Mo2C before andafter each reactant or reactants treatments were compared. The results showed thatrate of the Mo2C catalyst after the water treatment at240°C was much lower than thatof before the treatment indicating the water may be responsible for the rate decrease.Carbon species were also formed on the post reaction catalyst surface while may notbe the dominating deactivation reason. The water activation on the Mo2C can beprocessed at240°C to produce H2and the oxygen species. The oxygen species wereaccumulated on the Mo2C surface which resulted in the catalyst deactivation. The regeneration methods for the Mo2C catalyst were also investigated and the partialactivity of Mo2C catalyst could be restored after the treatment in H2at240°C.