CO adsorption, NO adsorption and CO þ NO reaction on various Pd model catalysts have been studied using vibrational spectroscopy from ultrahigh vacuum (UHV) up to elevated pressures (B1 bar) and the kinetics of the reaction compared with the conventional high surface area Pd/g-Al 2 O 3 catalysts. The structure sensitivity of the CO þ NO reaction on different Pd surfaces is explained using Pd(111), Pd(100) single crystals and planar Pd/ SiO 2 /Mo(110), Pd/SiO 2 /Mo(112), Pd/Al 2 O 3 /Ta(110) supported model catalysts by emphasizing the particle size/ morphology effects and particle-support interactions. A reaction intermediate, isocyanate (-NCO), is detected via in situ vibrational spectroscopy at elevated pressures on Pd(111) single crystal surface and the significance of this reaction intermediate on the improvement of the catalytic NO x removal is discussed.

CO adsorption on a Pd(111) single-crystal surface was investigated using in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) within the pressure range 10 -6 -800 mbar. The coverage-dependent CO overlayer structures found on the Pd(111) surface are identical throughout this pressure regime, that is, no new surface species at elevated pressures or adsorbate-induced substrate reconstructions were observed. The transition from an adsorbate superstructure dominated by bridged-bound CO to an adsorbate overlayer having 3-fold/atop CO sites was followed by varying the adsorbate pressure over 9 orders of magnitude. The derived phase diagram indicates an apparent activation energy of 44.35 ( 1.63 kJ/mol for the bridged-to-3-fold-hollow/atop transition. A comparison between these data and recent sum frequency generation (SFG) results is made.

Formic acid (HCOOH) has a great potential as a safe and a convenient hydrogen carrier for fuel cell applications. However, efficient and CO-free hydrogen production through the decomposition of formic acid at low temperatures (<363 K) in the absence of additives constitutes a major challenge. Herein, we present a new heterogeneous catalyst system composed of bimetallic PdAg alloy and MnO x nanoparticles supported on amine-grafted silica facilitating the liberation of hydrogen at room temperature through the dehydrogenation of formic acid in the absence of any additives with remarkable activity (330 mol H 2 ·mol catalyst −1 ·h −1 ) and selectivity (>99%) at complete conversion (>99%). Moreover this new catalytic system enables facile catalyst recovery and very high stability against agglomeration, leaching, and CO poisoning. Through a comprehensive set of structural and functional characterization experiments, mechanistic origins of the unusually high catalytic activity, selectivity, and stability of this unique catalytic system are elucidated. Current heterogeneous catalytic architecture presents itself as an excellent contender for clean hydrogen production via room-temperature additive-free dehydrogenation of formic acid for on-board hydrogen fuel cell applications.

Interaction of NO2 with an ordered theta-Al2O3/NiAl(100) model catalyst surface was investigated using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The origin of the NO(x) uptake of the catalytic support (i.e., Al2O3) in a NO(x) storage catalyst is identified. Adsorbed NO2 is converted to strongly bound nitrites and nitrates that are stable on the model catalyst surface at temperatures as high as 300 and 650 K, respectively. The results show that alumina is not completely inert and may stabilize some form of NO(x) under certain catalytic conditions. The stability of the NO(x) formed by exposing the theta-Al2O3 model catalyst to NO2 adsorption increases in the order NO2 (physisorbed or N2O4) < NO2 (chemisorbed) < NO2- < NO3-.

The catalytic conversion of environmentally hazardous pollutants in automobile exhausts is one of the most thoroughly studied problems in heterogeneous catalysis. Three-way catalysts (TWC) are used to remove the major pollutants in the exhaust gases by the simultaneous reduction of NO x and the oxidation of CO and unburned hydrocarbons. Recently, Pd-based catalysts have been considered as an alternative to the more commonly used Pt/Rhbased catalysts. 1,2 In this study vibrational spectroscopy is used to examine the in situ adsorption of CO + NO to form CO 2 , N 2 O and N 2 on a Pd(111) model catalyst under high-pressure conditions. We employ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), [3][4][5] an adaption of the well-known IRAS technique. The surface sensitivity of PM-IRAS relies on the fact that IR absorption by an adsorbed species on a metal surface shows a strong dependence on the polarization of the incoming IR beam, while gas-phase species are isotropic with respect to the polarization. Figure 1 shows PM-IRA spectra of Pd(111) in the presence of a 240 mbar CO + NO mixture within the temperature range 300-600 K (CO:NO ) 3:2). The spectra were obtained by increasing the CO + NO gas mixture pressure at T surface ) 300 K until an equilibrium pressure of 240 mbar was established. The data were acquired in the presence of 240 mbar of ambient pressure at the given temperatures. The spectrum at 350 K shows a feature at 1922 cm -1 , which corresponds to CO residing on either two-fold or threefold Pd sites. [6][7][8][9][10] The feature at 1876 cm -1 can be assigned to CO located on the three-fold sites 7,[8][9][10] while the peak at 1745 cm -1 is assigned to NO bound to atop sites. 11 At 350 K, atop sites are predominantly occupied by NO whereas three-fold sites are occupied by CO and possibly NO (considering the broad band at approximately 1550 cm -1 ). Interestingly, at 600 K and under reaction conditions, the spectrum is dominated by a new band at 2255 cm -1 besides the CO-related feature at 1908 cm -1 . The feature at 2255 cm -1 is assigned to the asymmetric stretching mode of an isocyanate (-NCO) species. [12][13][14][15][16][17][18][19][20] Once produced at high temperatures (500-625 K), this species is stable within the 300-625 K temperature range as well as after reducing the chamber pressure to 1 × 10 -7 mbar at room temperature. In previous infrared studies on Pd/Al 2 O 3 16,17 a broad band near 2242 cm -1 has been observed and assigned to a substrate-bound isocyanate species. According to these previous studies, the formation of isocyanate takes place on metal sites followed by spillover to the substrate. Although a metal-bound isocyanate species was not reported for the Pd/Al 2 O 3 system, 16 isocyanate formation in the CO + NO reaction on Pd(111) was postulated in a recent theoretical study. 20 To explore the conditions under which the metal-bound isocyanate species is formed on Pd(111), experiments with different total pressures were conducted (see Figure 2). At 10 -4...