Using phosphonic acid monolayers to control CO2 adsorption and hydrogenation onPt/Al2O3

Phosphonic acid (PA) self‐assembled monolayers (SAMs) were deposited onto Pt/Al 2 O 3 catalysts to modify the support to enable control over CO 2 adsorption and CO 2 hydrogenation activity. Significant differences in catalytic activity toward CO 2 hydrogenation (reverse water‐gas shift, RWGS) were observed after coating Al 2 O 3 with PAs, suggesting that the reaction was mediated by CO 2 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 CO 2 as a combination of carbamate and bicarbonate. Additionally, at 30 °C, both PAs were found to reduce CO 2 adsorption uptake by approximately 50 % compared to unmodified 5 %Pt/Al 2 O 3 . CO 2 adsorption enthalpy was measured for the catalysts and found to be strongly correlated with hydrogenation activity, with the trend in binding enthalpy and CO 2 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 Al 2 O 3 support by the PAs.