Carbon dioxide is a greenhouse gas, and needs to be converted into one of the useful feedstocks, such as carbon monoxide and methanol.

Tuning the surface energetics, especially work function (ϕ) of the materials, is of a great deal of interest for a wide range of surface- and interface-based devices and applications. How the ϕ of a solid surface changes under the reaction conditions is of paramount interest to the chemists, particularly in the areas of surface-dependent phenomena such as, catalysis and electrochemistry. In the present study, by using the valence band and core-level photoelectron spectroscopy, surface electronic changes from Mo to MoO3 via MoO2 was studied under relevant near-ambient pressure (NAP) and high temperature conditions. A very significant change in ϕ from Mo to MoO3 was observed and it is well corroborated with the changes in gas-phase vibrational features of O2 in both near-ambient pressure ultraviolet photoelectron spectra (NAPUPS) as well in NAP X-ray photoelectron spectroscopy. Reversible changes in the electronic structure is observed when MoO3 was reduced in H2 to MoO2. On the basis of the extent of oxidation/reduction of MoO x , NAPUPS has shown, one or two additional peaks in the band gap at 0.6 and 1.6 eV below the Fermi level. Mo5+ features are identified in the VB and in the Mo 3d core levels with distinct features. Mo5+ features are also stable and essential to bridge MoO2 and MoO3 layers, and their co-existence. In addition, characteristic changes in Mo 4d and O 2p features observed from Mo to MoO3 and well corelated to the band gap of MoO3. Oxidation and reduction propagate from the surface to bulk; indeed, this has significant implications in surface-dependent phenomena. The present study demonstrates (a) the uniqueness of NAPUPS in identifying the subtle to large changes in the electronic structure on solid surfaces under common oxidation and reduction (in general, under reaction) conditions, and (b) relevance of NAPUPS to all surface-dependent phenomena, such as catalysis and electrochemistry.

Work function alteration due to any chemical/electronic changes can be measured precisely by NAPUPS and correlated to the activity changes.

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This study demonstrates a sustainable catalytic CO2 conversion to near 100% CO selectivity at ambient pressure on In2O3. Critically, high CO yield could be observed at the cost of undesired methanation, using a lower than stoichiometric amount of hydrogen in the feed; 1:1 and 1:0.67 CO2:H2 ratios exhibit 98–99.6% CO selectivity with 25–38% CO2 conversion between 773 and 873 K. CO2 and H2 conversion under steady-state conditions at 773–873 K suggests a 1:1 ratio of adsorbed reactants (with 1:0.67 CO2:H2 feed) on the catalyst surface, underscoring the presence of an ideal reactant composition for the reverse water-gas shift reaction, while H2-rich feed compositions show the H2-dominated surface. Surface electronic structure changes, under near-operating conditions, were explored with near ambient pressure photoelectron spectroscopy (NAPPES), and the interesting findings are as follows: (a) A shift in the valence band to lower binding energy, up to 0.6 eV, was observed because of electron filling at high temperatures. (b) An observation of heterogeneous nature of the catalyst surface under NAPPES measurement conditions is attributed to the generation of active oxygen vacancy (Ov) sites, which in turn changes the work function of In2O3. (c) The above changes are found to be reversible, when the reaction was stopped. Vibrational features of the reactant molecules were observed to be broadened in the active temperature window of the catalyst supporting the heterogeneous character of the catalyst surface because of dynamic Ov generation. By optimizing gas hourly space velocity, CO2:H2 ratio, and reaction temperature, exclusive CO selectivity is possible with a H2:CO2 ratio of ∼0.67, which will avoid the product separation stage altogether, while minimizing the expensive H2 in the reactant feed.

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