The catalytic properties of thin film Pd-rich GaPd2 in methanol steam reforming

• An isolated, unsupported Pd-rich GaPd2 intermetallic compound has been prepared.
• High thermochemical stability under methanol steam reforming conditions.
• No active redox chemistry of Ga species detected.
• Unsupported Pd-rich GaPd2 is inactive and unselective in methanol steam reforming.
• Bi-functional synergism between GaPd2 and Ga2O3 is present.

An unsupported Pd-rich GaPd2 sample in form of a thin film has been prepared by alternating layer deposition of Pd and Ga metal and was subsequently used as a structurally and chemically stable model system to clarify the catalytic properties of the unsupported intermetallic compound GaPd2 in methanol steam reforming (MSR). The sample revealed a slightly Pd-richer GaPd2 bulk composition of Ga28Pd72, as evidenced by EDX analysis, low-energy ion scattering, X-ray diffraction measurements, and depth profiling by in situ X-ray photoelectron spectroscopy. The latter additionally showed a high stability of GaPd2 both under methanol and oxidative methanol steam reforming conditions. No active redox chemistry of Ga species or other reaction-induced oxidative Ga surface segregation has been detected during catalytic MSR reaction. Corroborating these observations, corresponding catalytic experiments under methanol steam reforming conditions revealed only a, in comparison with elemental Pd, very small activity in methanol dehydrogenation (CO formation rate at maximum 0.019 mbar min−1; 0.08 site−1 s−1). Unsupported thin film Pd-rich GaPd2 with the given surface and bulk stoichiometry must therefore be considered a poor methanol steam reforming/dehydrogenation catalyst. In oxidative steam reforming experiments, only total oxidation without significant H2 formation has been observed.

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