Using Phosphonic Acid Monolayers to Control CO2 Adsorption and Hydrogenation on Pt/Al2O3

Phosphonic acid (PA) self-assembled monolayers (SAMs) were deposited onto Pt/Al2O3 catalysts to modify the support to enable control over CO2 adsorption and CO2 hydrogenation activity. Significant differences in catalytic activity toward CO2 hydrogenation (reverse water-gas shift, RWGS) were observed after coating Al2O3 with PAs, suggesting that the reaction was mediated by CO2 adsorption on the support. Amine-functionalized PAs were found to outperform their alkyl counterparts in terms of activity, however there was little effect of amine location in the SAM (i. e., spacing between the amine functional group and phosphonate attachment group). One amine-PA and one alkyl-PA, aminopropyl phosphonic acid (C(3)NH(2)PA) and methyl phosphonic acid (C(1)PA), respectively, were investigated in more detail. The C(3)NH(2)PA-modified catalyst was found to bind CO2 as a combination of carbamate and bicarbonate. Additionally, at 30 degrees C, both PAs were found to reduce CO2 adsorption uptake by approximately 50 % compared to unmodified 5 %Pt/Al2O3. CO2 adsorption enthalpy was measured for the catalysts and found to be strongly correlated with hydrogenation activity, with the trend in binding enthalpy and CO2 hydrogen rate trending as uncoated >C(3)NH(2)PA>C(1)PA. PA SAMs were found to have weaker effects on CO binding and CO selectivity, consistent with selective modification of the Al2O3 support by the PAs.