Heterogeneous reaction of formaldehyde on the surface of γ-Al2O3 particles

Formaldehyde is one of the most important carbonyl organic compounds. Heterogeneous reactions of formaldehyde on the surface of oxides of crustal elements could be an important sink for formaldehyde in the atmosphere. In this study, the kinetics of the heterogeneous reaction of formaldehyde on the surface of γ-Al2O3 was investigated in situ by diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) combined with ion chromatography (IC), X-ray diffraction (XRD), and field emission environmental scanning electron microscope (ESEM). The adsorbed species, formate, dioxymethylene, polyoxymethylene, and formaldehyde, on γ-Al2O3 particles, were identified by infrared spectroscopy. On the surface of γ-Al2O3 particles, formaldehyde is first oxidized to dioxymethylene, which is further oxidized to formate. The reaction order was determined as 0.74 ± 0.05, and the initial reactive uptake coefficients at room temperature were calculated with the geometric and Brunauer–Emmett–Teller specific surface areas as (3.6 ± 0.8) × 10−4 and (1.4 ± 0.31) × 10−8, respectively. The influences of oxygen concentration, humidity, and temperature on the reaction products and reactive uptake coefficients were studied. The results indicated the reaction is independent of the O2 concentration within the range used in this experiment, and humidity is negatively correlated with the uptake of formaldehyde by the particles. Temperature not only plays a role on the heterogeneous reaction products but also is positively correlated with the uptake rate of formaldehyde by γ-Al2O3 particles in the troposphere. The apparent activation energy of the reaction was determined.

► Main products of HCHO on γ-Al2O3 were identified qualitatively and quantitatively. ► The reaction order and the reactive uptake coefficients were determined. ► The reaction is independent of the O2 concentration. ► Humidity is negatively correlated with the uptake of HCHO by γ-Al2O3. ► Temperature is positively correlated with the uptake of HCHO by γ-Al2O3.