Heterogeneous Catalysis Using UV and VUV Ionizations

Transition metal (M_n) and metal oxide (M_nO_x) clusters are important industrial heterogeneous catalysts. Neutral metal and metal oxide clusters composed of limited numbers of atoms in the gas phase which are fully accessible by both experiment and theory are excellent model systems to investigate the reaction mechanisms for the condensed phase catalytic processes

Experimental methods

Metal (M_n) and metal oxide (M_nO_x) clusters are generated by laser ablation with a focused 532 nm Nd:YAG laser onto a metal disk in the presence of pulsed helium carrier gas or an O₂/He mixture controlled by a R. M. Jordan supersonic nozzle. Metal and metal oxide clusters are formed in an adjustable length gas channel which is coupled directly to a tube/reactor. The reactant gases are injected into the reactor by a second pulsed valve (General Valve, Series 9). The reactions in the flow tube reactor are estimated to occur at near room temperature due to the large number of collisions between metal and/or metal oxide clusters and the bath and reactant gases. Ions are deflected from the molecular beam by an electric field and only neutrals enter the ion source. Ionization is through UV (193 nm, 6.4 eV), VUV (118nm, 10.5 eV) and x-ray (46.5 nm, 26.5 eV) laser radiation depending on different research system. Ions are accelerated and detected by a time of flight mass spectrometer, and signals are recorded and averaged by a digital storage oscilloscope.

Reaction of Nb_n and Ta_n clusters with C₆H₆ and C₇H₈

Product distribution in the reaction of Nb_n and Ta_n with 1.4 mTorr toluene in the pick up cell. Ionization is by a 193 nm laser with fluence of 770 mJ/cm²

Reaction of Nb_n clusters with CO + H₂

Product distribution for the reaction of V_n (a), Nb_n (b)­, and Ta_n (c) (n = 7-10) with 55 ppm, 92 ppm and 147 ppm CH₃OH in a flow tube reactor, respectively. Ionization is by a 193 nm laser at a fluence of 200 mJ/cm². ▲, ▼, ●, ■, ¿ and ★ indicate carbide impurities, oxide impurities, M_nCO, M_n(CO)₂, Nb_n(COH₄)₂/Nb_n(CH₃OH)₂ and Nb_nCOH₄/Nb_nCH₃OH, respectively. Note that except for Nb₈ and Nb₁₀, CH₃OH is dehydrogenated on all the metal clusters

A potential energy surface profile for the reaction Nb₈ + CO + 2H₂ → Nb₈ + CH₃OH. Energies are in eV and relative to the initial reactant energy Nb₈ + CO + 2H₂. Energy levels are calculated by BPW91/LANL2DZ/6-311+G(2d, p)

Reaction of V_mO_n clusters with SO₂

TOF mass spectra for reactions of V_mO_n with different concentrations of SO₂ seeded in He. The concentrations of SO₂ are 0%, 2%, 5%, and 10% from top to bottom traces.

TOF mass spectra for reaction of V_mO_n with SO₂ employing a 26.5 eV soft x-ray laser for ionization. A product SO₃ is observed when SO₂ is added (bottom trace), and it disappears with the V_mO_n cluster signal when the ablation laser is blocked (top trace)

Enthalpy (at 298.15 K) for V_xO_y + SO₂ -> V_xO_y+1 + SO and V_xO_y + SO₂ -> V_xO_y-1 + SO₃ reactions.

TOF mass spectra for reaction of neutral iron oxide clusters with carbon monoxide in flow tube reactor. CO concentrations are 0% (top trace), 1% (middle), and 5% (bottom) in helium carrier gas

DFT calculated reaction pathways of FeO₂ + CO → FeO + CO₂. Reaction intermediates (In), transition states (TSn), and their relative Gibbs free energies (ΔG in eV) at 298 K are given as In/ΔG or TSn/ΔG. Bond lengths in Å are also given for each geometry. Superscripts 3 and 5 are used in this figure to differentiate the reactants, products, intermediates, and transition states in triplet and quintet spin states, respectively

A possible catalytic cycle for CO oxidation by O₂ over iron oxide catalysts at the molecular level. A, B, and C mark different surface Fe atoms while D and d mark lattice Fe atoms that are underneath the surface.