Influences of Interfaces and Promoters on the Water-Gas Shift Reaction Over Supported Noble Metal Catalysts

Yanran Cui, Purdue University

Abstract

The water-gas shift reaction (WGS) is an important reaction to produce high purity hydrogen for various industrial processes such as ammonia synthesis and hydrotreating of petrochemicals. Finding a catalyst with higher activity is desired to speed up the reaction at lower temperatures where the equilibrium conversion is higher. Supported noble metal catalysts have been identified as a class of active catalysts for this reaction. The interface sites between the metal and support play an important role in determining the overall activity and can be modified by properly tuning the metal-support interactions. On the other hand, WGS reaction can be used as a probe reaction to study the fundamental catalytic processes over various heterogeneous catalysts. The overall goal of this work is to develop a model based approach to catalyst design that we call Discovery Informatics, which involves building a database with sufficient chemical and information diversity to allow identification of active sites, modelling the kinetics and discovering descriptors of the kinetic parameters. The first part of this work focuses on Fe promotion effects on a rutile supported Au catalyst. By properly adding Fe to the Au/rutile catalysts, WGS rate per mole of Au at 120 °C could be promoted to a maximum of about 4 times. As Fe loading increased, significant changes in the WGS kinetics were observed, that is, a decrease in apparent reaction order with respect to CO (0.7 to -0.3) and an increase in apparent activation energy (53 kJ/mol to 98 kJ/mol). The changes in the WGS kinetics imply stronger binding of CO on the active sites. Operando FTIR experiments identified an increase in CO adsorbed on strongly backdonating Au sites as the Fe loading was increased. The results showed that Fe-doping can modify the CO adsorption properties of interface Au sites, which changes the WGS rates and the nature of the active sites. Besides CO adsorption, H2O dissociation is another important factor that influences the activity of WGS catalysts. The second part of this work focuses on studying the H2O dissociation by using an Au/MgO catalyst as a model system. In this work, MgO and Mg(OH)2 were adopted as supports and loaded with 2.5 wt% Au. WGS rates and kinetics were measured on these catalysts. Au/MgO showed higher WGS rates than Au/Mg(OH) 2 but a lower apparent order with respect to H2O. This implies a higher H2O/OH coverage over the Au/MgO compared with Au/Mg(OH) 2, which corresponds to a higher binding affinity for H2O/OH on Au/MgO. A kinetic isotope effect (KIE), which is the ratio between the WGS rate with H2/H2O and WGS rate with D2/D 2O, was measured for both catalysts and both showed the same KIE ratio of about 2.0±0.3. This similar KIE implies a similar reaction mechanism on both catalysts and that breaking of a hydrogen bond is involved in the rate-determining step. Density Functional Theory (DFT) calculations also revealed a decrease of about 0.7 eV in the energy barrier for H2O dissociation at the Au/MgO interface compared with pure MgO and pure Au. Further experimental studies on other supports such as TiO2, ZrO2, Al 2O3 etc. also shows that a lower apparent order with respect to H2O (about -0.3) results in a higher WGS rate. Thus the hydroxyl group participates in the rate-determining step and H2O order can be used as a potential descriptor for the activity. The last two studies focus on the active sites for WGS over supported Pt catalysts and the Na promotion effects on those catalysts. Multiple types of sites exist on supported Pt catalysts (single Pt atoms, Pt clusters and Pt nanoparticles) but it is still debated in the literature which kind of sites are more important for the WGS reaction. A Pt/TiO2 catalyst with only Pt nanoparticles on the support was prepared by organic solvothermal method. It showed similar activity and WGS kinetics to the normal Pt/TiO 2 catalyst, which implies that single Pt atoms or small clusters of Pt atoms are not the dominant active sites. Variations of the rate with support imply the importance of the Pt-support interface in controlling activity. Na has been reported to promote supported Pt catalysts. In order to study the reason for promotion, a series of Pt-Na catalysts supported on multi-walled carbon nanotubes (MWCNT) was prepared with different Pt:Na ratio. Na was observed to promote the TOR of Pt/MWCNT catalysts by a factor of more than 20. The addition of Na changed the kinetic parameters of Pt/MWCNT (increase in apparent activation energy, decrease in CO and CO2 orders) similarly to the modifications previously reported for Na-promoted Pt/Al2O 3, Pt/TiO2 and Pt/ZrO2 catalysts. The independence of response of the apparent kinetic parameters to Na on the underlying parent support for Pt suggests that Na leads to a support-type effect of its own. As confirmed by in situ ΔXANES experiments, Na enhanced the binding of CO with Pt. XAS data showed that Pt remained in reduced or metallic state under the WGS conditions. It is suggested that Na forms islands over the Pt particles and forms a new type of Pt-NaOx interface as the active site. A washing procedure could remove the Na from the MWCNT and re-distribute it over the surface of Pt. The washed catalysts showed much lower Na loadings but similar WGS TOR at 250°C compared to their as-prepared counterparts, which further supports the conclusion that the Pt-NaOx interfaces are the active sites.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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