Single-crystal adsorption calorimetry on well-defined surfaces: from single crystals to supported nanoparticles.

Single-crystal adsorption calorimetry (SCAC) measures the energetics of gas-surface interactions in a direct way and can be applied to a broad range of well-defined model surfaces. In this Personal Account we review some of the recent advances in understanding the interaction of gaseous molecules with single-crystal surfaces and well-defined supported metallic nanoparticles by this powerful technique. SCAC was applied on single-crystal surfaces to determine formation enthalpies of adsorbed molecular fragments typically formed during heterogeneously catalyzed reactions involving hydrocarbons.

Bond energies of molecular fragments to metal surfaces track their bond energies to H atoms.

The bond energy of molecular fragments to metal surfaces is of great fundamental importance, especially for understanding catalytic reactivity. Thus, the energies of adsorbed intermediates are routinely calculated to understand and even predict the activity of catalytic materials. By correlating our recent calorimetry measurements of the adiabatic bond dissociation enthalpies of three oxygen-bound molecular fragments [-OH, -OCH3, and -O(O)CH] to the Pt(111) surface, it is found that these RO-Pt(111) bond enthalpies vary linearly with the RO-H bond enthalpies in the corresponding gas-phase molecules (water, methanol, and formic acid), with a slope of 1.

Energetics of formic acid conversion to adsorbed formates on Pt(111) by transient calorimetry.

Carboxylates adsorbed on solid surfaces are important in many technological applications, ranging from heterogeneous catalysis and surface organo-functionalization to medical implants. We report here the first experimentally determined enthalpy of formation of any surface bound carboxylate on any surface, formate on Pt(111). This was accomplished by studying the dissociative adsorption of formic acid on oxygen-presaturated (O-sat) Pt(111) to make adsorbed monodentate and bidentate formates using single-crystal adsorption calorimetry.

Energetics of adsorbed CH3 on Pt(111) by calorimetry.

The enthalpy and sticking probability for the dissociative adsorption of methyl iodide were measured on Pt(111) at 320 K and at low coverages (up to 0.04 ML, where 1 ML is equal to one adsorbate molecule for every surface Pt atom) using single crystal adsorption calorimetry (SCAC). At this temperature and in this coverage range, methyl iodide produces adsorbed methyl (CH(3,ad)) plus an iodine adatom (I(ad)).

Energetics of adsorbed methanol and methoxy on Pt(111) by microcalorimetry.

The heat of adsorption and sticking probability of methanol were measured on clean Pt(111) at 100, 150, and 210 K and on oxygen-precovered Pt(111) at 150 K by single-crystal adsorption calorimetry (SCAC). On clean Pt(111) at 100 K, the heat of methanol adsorption was found to be 60.5 ± 0.

Insights into catalysis by gold nanoparticles and their support effects through surface science studies of model catalysts.

One important aid in understanding catalysis by gold nanoparticles would be to understand the strength with which they bond to different support materials and the strength with which they bond adsorbed intermediates, and how these strengths depend on nanoparticle size. We present here new measurements of adsorption energies by single crystal adsorption calorimetry, and new analyses of other recent measurements by this technique in our lab, which imply that: (1) small nanoparticles of metals like Au bind much more strongly to supports like titania and iron oxide which are generally observed to be effective in making Au nanoparticles active in catalysis than to supports like MgO which are considered less effective, (2) the thermodynamic stability of adsorbed intermediates for catalytic reactions can either increase strongly or decrease strongly with decreasing metal nanoparticle size below 8 nm, depending on the system, and (3) the reaction to insert O2 into the Au-H bond of adsorbed H on the Au(111) surface to make Au-OOH (O2,g + H(ad) --> OOH(ad)) is exothermic by -80 kJ mol(-1). This adsorbed hydroperoxy species is thought to be a key intermediate in selective oxidation reactions over Au nanoparticle catalysts, but its production by this reaction may also provide a route for O2 activation in less demanding reactions (like CO oxidation) as well.