(206b) Role of Water in Methane Oxidation Observed By Transient Drifts at Low Temperatures Using Bimetallic Aupd Catalysts

Methane is an abundant, inexpensive raw material, so it naturally becomes a desirable carbon feedstock for chemical processes. Unfortunately, current approaches are energy-intensive to methane's utilization despite the thermodynamic possibility of low-temperature activation. This awareness beckons for a catalyst that can activate methane at low temperatures. An additional complication is a propensity of overoxidation once one of the C-H bonds is broken. A stabilization agent might mitigate this over-oxidizing tendency.

AuPd nanoparticles have provided some exciting results in methane activation by the Hutchings group in partial oxidation of methane to methanol using AuPd catalysts.^1,2 It is of particular interest to understand and optimize this unique activity of AuPd that the individual components do not possess. In our work, we use in-situ­ vibrational spectroscopy via DRIFTS to observe surface species interactions and the apparent relationship to activity.

We observe changes in surface species as both a function of temperature and composition of the atmosphere both steady-state and transient operating modes. The spectroscopic studies performed on Au, Pd, and AuPd nanoparticles dispersed onto TiO₂ revealed surface hydroxides on the single component catalysts, while in contrast, hydroxide on AuPd is not present in a hydrated atmosphere. Comparing spectroscopic signatures to the steady-state kinetic data, we note that water's presence improves the rate of methane oxidation at low temperatures. At the same time, there is no significant change to activity for the other materials. DRIFTS with a transient composition of feed gas a few interesting data points. Following a purge of dry helium for Au and Pd catalysts, formate remained on the surface to a greater extent than the AuPd bimetallic metal. Observations of spectroscopic signatures and the relationship to activity are addressed, including hydroxide, adsorbed C-H, and carbonyls. In addition to AuPd, a stabilizing material, CuO_x was prepared with AuPd to observe the stabilizing agent's role in the change of material composition. Interestingly, we observed the positive consequence of adsorbed C-H and an improvement in selectivity in the materials with CuO_x.

To summarize, we analyze Au and Pd nanoparticles dispersed on TiO₂ for oxidation of methane by analyzing surface composition via adsorbed species and the relationship to the conversion and the important role of water in this reaction.

(1) Ab Rahim, M. H.; Armstrong, R. D.; Hammond, C.; Dimitratos, N.; Freakley, S. J.; Forde, M. M.; Morgan, D. J.; Lalev, G.; Jenkins, R. L.; Lopez-Sanchez, J. A.; Taylor, S. H.; Hutchings, G. J. Low Temperature Selective Oxidation of Methane to Methanol Using Titania Supported Gold Palladium Copper Catalysts. Catal. Sci. Technol. 2016, 6 (10), 3410–3418. https://doi.org/10.1039/c5cy01586c.

(2) Williams, C.; Carter, J. H.; Dummer, N. F.; Chow, Y. K.; Morgan, D. J.; Yacob, S.; Serna, P.; Willock, D. J.; Meyer, R. J.; Taylor, S. H.; Hutchings, G. J. Selective Oxidation of Methane to Methanol Using Supported AuPd Catalysts Prepared by Stabilizer-Free Sol-Immobilization. ACS Catal. 2018, 8 (3), 2567–2576. https://doi.org/10.1021/acscatal.7b04417.