Hexagonal boron nitride (hBN) is attracting a lot of attention in catalysis due to its high selectivity in oxidative dehydrogenation of alkanes. hBN-supported catalysts also offer an interesting alternative in the design of multifunctional catalysts that take advantage of both bare hBN sites and metal/hBN sites. In order to explore these capabilities, the partial oxidation of methane to syngas was studied on Pt/hBN where the nature of the support was modified by heat and steam treatment prior to loading Pt. Four different families of catalysts were synthesized: one using untreated hBN, two with air at 850-900 °C (one with humid and one with dry air), and one more heated in air at 650 °C. For each family a sonochemical wet impregnation method was compared with regular wet impregnation in water. When applicable, a post treatment to remove borate was applied and compared with the non-post treated sample. It was found that the highest activity and syngas selectivity (67% methane conversion and ~75% carbon based selectivity for CO and H
2 at 600 °C) were achieved on a material where some of the support had been converted to borate. Syngas yields were better on hBN than a reference support, Al
2O
3. The importance of borate appears to be related to the ability to enhance platinum dispersion. For example, the dispersion increased to 3.3% in the borated sample compared with 2.8 % in the untreated hBN. However, if the borated sample is purified to remove part of the B
2O
3, the catalytic activity is reduced even in catalysts with similar Pt dispersion. This is an indication of the role of B
2O
3 which could be changing the chemistry of Pt by the formation of a B
2O
3 layer between hBN and deposited Pt. A 1090 cm
-1 FTIR peak assigned to B-N-O vibration confirms the presence of this layer.
Unlike previous works, in our method we don’t add an external boron source to the hBN, instead, we treat hBN at medium-high temperature to produce a functionalized material with boric acid as a byproduct. The presence of boric acid is shown in the XRD pattern and its amount in the final material is a function of the treatment time, atmosphere (dry or water saturated air) and temperature. All these variables affect the catalytic activity of Pt/hBN. FTIR spectroscopy in the region 3200 – 3450 cm-1 is used to evaluate the OH groups. An estimate of the Pt attachment sites is made based on the intensity of the OH peak in the FTIR spectra. This material shows a high activity for partial oxidation of methane for syngas production. The activity of the final catalyst is a function of the impregnation method, the hBN treatment, and the B2O3 content. Adding boron oxide to hBN increase the dispersion determined with H2 chemisorption and the acid sites according to NH3-TPD.
These results indicate that hBN can be an effective support for the catalytic partial oxidation of methane, and suggest that specific pretreatment methods can lead to enhanced catalytic performance.
