Methane steady state with formaldehyde

Lowe and Schmidt (1983) have measured formaldehyde over the Atlantic Ocean as a function of latitude. Their results are shown in Fig. 4-6. In the region 33°S-40°N the data scatter around a mean mixing ratio of wn(HCHO) = 0.22 ppbv. Further northward they decline. The lifetime of formaldehyde due to photodecomposition by sunlight is about 5 h at midlatitudes, and 15 h when averaged over a full day (Warneck et al., 1978). In addition, formaldehyde reacts with OH. Where the oxidation of methane is the sole source, formaldehyde mixing ratios are expected to be in steady state with methane [dm(HCHO)/dr = 0] ... 

A conmercial catalyst frcm Harshaw was used, a 3 1 mixture of molybdenum trioxide and ferric molybdate, as well as the two separate phases. Kinetic experiments were done previously in a differential reactor with external recycle using these same catalysts as well as several other preparations of molybdenun trioxide, including supported samples. Hie steady state kinetic experiments were done in the temperature range 180-300 C, and besides formaldehyde, the following products were observed, dimethylether, dimethoxymethane, methyl formate, and carbon-monoxide. Usually very little carbon dioxide was obtained, and under certain conditions, hydrogen and methane can be produced. 

Comparison with Fig. 4-6 shows that the steady-state prediction is in very good agreement with the observational data. This justifies the assumption that methane is the dominant source of formaldehyde in the marine atmosphere. Figure 4-6 includes the results from a two-dimensional model calculation (Derwent, 1982). The values are somewhat higher than the measured ones, but they confirm the constancy with latitude in the equatorial region. The decline of m(HCHO) toward high latitudes, which is evident... 

The greater activity of Pd for methanol decomposition reaction was also found by using the steady state isotopic transient kinetic analysis (SSITKA) method over noble metal (Pt, Pd, Rh)/ceria catalysts. Their activity increased in the order Rh < Pt < Pd, while the by-products were (i) methane, carbon dioxide, water, methyl formate and formaldehyde in most cases and (ii) ethylene and propylene, formed only over Rh/Ce02, at 553 K. SSITKA measurements indicated that two parallel pools exist for the formation of CO (via formation and decomposition of formaldehyde and methyl formate). The difference in the activity order of noble metal/ceria catalysts seems to correlate with the surface coverage of active carbon containing species, which followed the same order. The latter implies that a part of these species is formed on the ceria surface or/and metal-ceria interface. ... 

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