Importance of the oxygen bond strength for catalytic activity in soot oxidation

Abstract

The oxygen bond strength on a catalyst, as measured by the heat of oxygen chemisorption, is observed to be a very important parameter for the activity of the catalyst in soot oxidation. With both intimate contact between soot and catalyst (tight contact) and with the solids stirred loosely together (loose contact) the rate constants for a number of catalytic materials outline a volcano curve when plotted against their heats of oxygen chemisorption. However, the optima of the volcanoes correspond to different heats of chemisorption for the two contact situations. In both cases the activation energies for soot oxidation follow linear Brønsted-Evans-Polanyi relationships with the heat of oxygen chemisorption. Among the tested metal or metal oxide catalysts Co 3 O 4 and CeO 2 were nearest to the optimal bond strength in tight contact oxidation, while Cr 2 O 3 was nearest to the optimum in loose contact oxidation. The optimum of the volcano curve in loose contact is estimated to occur between the bond strengths of α-Fe 2 O 3 and α-Cr 2 O 3 . Guided by an interpolation principle Fe a Cr b O x binary oxides were tested, and the activity of these oxides was observed to pass through an optimum for an FeCr 2 O x binary oxide catalyst, which exhibited a rate constant at 550 °C that was 2.3 times higher than the one for pure α-Cr 2 O 3 and 29 times higher than the one for pure α-Fe 2 O 3 .

Visualizing the mobility of silver during catalytic soot oxidation

“…The silver catalyst generally shows a high activity for soot oxidation. The significant activity in loose contact, where the temperature of maximal oxidation rate is shifted downwards by 123 °C compared to pure soot, is especially noteworthy, since it is very challenging to achieve a significant activity in this situation [11,20]. The reason for the high activity in loose contact will be discussed further in section 3.4 below.…”

“…The catalytic activity of Ag in soot oxidation was measured using temperature programmed oxidation (TPO) in a flow reactor setup described elsewhere [20]. For the activity tests soot (~2 mg) and catalyst in a ratio of 1:5 (wt:wt) were stirred together with a spatula (loose contact) or crushed together for 6 minutes in an agate mortar (tight contact).…”

“…It is known [11,20] to be a great challenge to achieve a significant lowering of the oxidation temperature with loose contact between catalyst and soot, and the significant effect of metallic silver in loose contact with soot is thus a particularly striking feature in Figure 4Figure 4. Assuming an intermediate oxygen coverage the heat of oxygen chemisorption on metallic silver should be in the order of 130 kJ/mol [39][40][41], which should make silver well suited for oxygen activation [20,42].…”

“…Assuming an intermediate oxygen coverage the heat of oxygen chemisorption on metallic silver should be in the order of 130 kJ/mol [39][40][41], which should make silver well suited for oxygen activation [20,42].…”

“…In terms of what constitutes a good catalyst both the surface area [17][18][19] and the strength of the oxygen-catalyst bond are very important for the catalytic activity [20]. Silver is able to activate oxygen by dissociative adsorption already at low temperature [21,22], and in a number of cases silver-based catalysts have been reported to exhibit high activity for catalytic soot oxidation [18,[23][24][25][26][27][28][29][30][31][32][33].…”