My present research interests are in the area of heterogeneous catalysis with an emphasis on CH bond activation in oxidative dehydrogenation reactions, partial oxidation and total oxidation reactions (1, 2). In the past, I have modeled:

1) Early stages of epitaxial thin film growth (3)

2) Ion selectivity observed in selectivity filters present in biological membrane proteins (4)

3) Effects of elastic strain on adsorptive and catalytic properties (5)

4) Influence of oxide supported metal clusters on reactant diffusion and kinetics of product formation (6).

In all the above research efforts, I have used density functional calculations and force field calculations to support the models and mechanisms proposed. In the near future, I will be pursuing my interests in heterogeneous catalysis and will venture into the area of modeling electrochemical systems.

My post doctoral work with Prof. Horia Metiu at the University of California at Santa Barbara involves the use density functional calculations to investigate how the catalytic properties of oxide surfaces can be modulated by replacing some of the host cations with other metal dopants. Many oxidation reactions operate via the Mars and Van Krevelen mechanism in which the oxygen atom from the oxide surface is supplied to the reactant that is getting oxidized, leaving behind a surface oxygen vacancy. Hence, the energy needed to create an oxygen vacancy from the oxide surface can be used to calibrate the propensity of the surface towards the Mars and Van Krevelen mechanism (1).

We have performed calculations on doped ZnO, MgO, CaO, BaO and CdO to identify the trends in the energetics of surface oxygen vacancy formation. We show that the doped oxide can operate in two qualitatively different methods with respect to the manner in which the oxygen can be supplied to the reactant that is oxidized. In one scenario, the substituted dopant may be “activated” and this leads to the generation of activated adsorbed oxygen molecules and also to the generation of peroxo complexes. In the second scenario, the dopant activates the oxide surface by facilitating the creation of a surface oxygen vacancy. The reactive oxygen species generated in the two different scenarios also leads to differences in reactivity of the oxide in oxidation reactions. Based on these predictions, the different forms of active oxygen species have been characterized by spectroscopic probes and by using various reaction chemistries (7).

We also elucidate how the oxide surface can be doped to facilitate CH bond activation which is important in oxidative dehydrogenation reactions. When many reaction channels are possible in an undoped oxide surface, doping also makes it possible to increase the selectivity by facilitating a particular reaction channel while suppressing others. Our calculations also support the experiments performed on methanol chemistry on doped ZnO (7).

My doctoral work was with Prof. Feng Liu at the University of Utah, and involved research mainly in three areas namely, early stages of epitaxial thin film growth, using externally applied strain to modulate adsorption and catalysis on a metal surface and how reactant diffusion and product evolution kinetics are influenced by oxide supported metal clusters.

In the area of epitaxial growth, we proposed a new concept called “critical epi-nucleation” to distinguish nucleation on surfaces with and without reconstruction (3). On a reconstructed surface, the critical classical nucleus is stable against dissociation, but may not yet break the underlying surface reconstruction. Consequently, there must exist a “critical epi-nucleus” that is not only stable but also has established the epi-configuration by removing the reconstruction of the underlying substrate. We illustrated this concept by first-principle calculations of homonucleation on reconstructed Si(001) surface where the critical epi-nucleus consists of six adatoms.

In the area of modifying adsorptive and catalytic properties using elastic strain, we demonstrated a model for determining the adsorptive and catalytic properties of strained metal surfaces based on linear elastic theory. We employed first-principle calculations of CO adsorption on Au and K surfaces and CO dissociation on Ru surface (4). The model involved a single calculation of the adsorption-induced surface stress on the unstrained metal surface, which determines quantitatively how adsorption energy changes with external strain. The model is generally applicable to both transition and non-transition metal surfaces, as well as to different adsorption sites on the same surface. Extending the model to both the reactant and the transition state of surface reactions allows determination of the effect of strain on surface reactivity.

In the area of catalysis using oxide supported metal clusters, experiments on the CO oxidation reaction using seven-atom Au clusters deposited on TiO₂surface correlated CO₂ formation with oxygen associated with Au clusters. We performed first principle calculations using a seven-atom Au cluster supported on a reduced TiO₂ surface to explore potential candidates for the form of reactive oxygen (5). These calculations suggested a thermodynamically favorable path for O₂ diffusion along the surface Ti row, resulting in its dissociated state to be bound to Au cluster and TiO₂ surface. Subsequently, CO can approach along the same path and react with the dissociated O₂ to form CO₂. The origin of the slow kinetic evolution of products observed in experiments was also investigated and was attributed to the strong binding of CO₂ to the Au cluster and the surface simultaneously.

1) Raj Pala and Horia Metiu “Modification of the oxidative power of ZnO surface by substituting some surface Zn atoms with other metals” (In press, J. Phys. Chem. C)

2) Raj Pala and Horia Metiu (In preparation)

3) Raj Pala and Feng Liu, “Critical epi-nucleation on reconstructed surfaces and a model study of Si(001) homoepitaxy”, Phys. Rev. Lett. 95, 136106 (2005).

4) Raj Pala, B.Chanda, S.K. Gupta, M.K.Mathew and J.Chandrasekhar, “Modeling of ion permeation in calcium and sodium channel selectivity filter”, Proteins: structure, function and genetics, 38, 384 (2000).

5) Raj Pala and Feng Liu “Determining the adsorptive and catalytic properties of strained metal surface using adsorption-induced stress”, J. Chem. Phys., 120, 7720 (2004).

6) Raj Pala and Feng Liu “Nature of reactive O₂ and slow CO₂ evolution kinetics in CO oxidation by TiO₂supported Au cluster”, J. Chem. Phys., 125, 16, 144714(2006)

7) M. Sushchikh, R. Pala, J.N. Park, T. Wei, H. Metiu, E.W.McFarland (in preparation)