Identifying the Thermal Decomposition Mechanism of Guaiacol on Pt(111): An Integrated X-ray Photoelectron Spectroscopy and Density Functional Theory Study

Abstract

Using a concerted effort from both experiment and theory, we determine the thermal decomposition mechanism for guaiacol on Pt(111), a reaction of interest in the area of bio-oil upgrading. This work serves as a demonstration of the power of combining in situ temperature-programmed X-ray photoelectron spectroscopy (TPXPS) and density functional theory (DFT) to elucidate complex reaction mechanisms occurring on heterogeneous surfaces. At low temperature (230 K), guaiacol was found to chemisorb with the aromatic ring parallel to the Pt(111) surface with five distinct carbon species and three oxygen species. As the temperature was increased, TPXPS showed several significant changes to the surface species. The increase in the species associated with the decomposition of the functional groups of guaiacol is followed by their subsequent disappearance and an increase in the nonaromatic carbon signal. On the basis of an energetic analysis of the various mechanisms using DFT, along with the comparison of the experimentally and theoretically derived core-level binding energies, we determined that guaiacol’s decomposition mechanism occurs via the dehydrogenation of both the methyl and hydroxyl functional groups, followed by demethylation of the CH2 or CH group to form 1,2-benzoquinone. Further heating to above 375 K likely breaks the aromatic ring and results in the rapid formation and desorption of CO, accounting for the disappearance of the O 1s signal above 450 K. These results show that a knowledgeable application of TPXPS and DFT can result in the quantitative identification of surface species during complex reactions, providing insight useful for the design of future heterogeneous surfaces.

Atomistic-Level Description of GaN/Water Interface by a Combined Spectroscopic and First-Principles Computational Approach

“…13−15 This has made it possible to distinguish subtle differences and changes in the structure of solid surfaces. 16 Given the circumstances, Pham et al have discussed in detail the applicability of the combined approach using firstprinciples calculations and AP-XPS measurements to probe the surface chemistry at a complex solid/liquid interface and concluded that the it is a promising way to reveal the relation between surface chemistry and key electronic features at the interface. 17 The fact that the solid/liquid interface can be successfully modeled using atomistic simulation techniques encourages us to examine the predictive power of these methods.…”

“…In addition to the band alignment perspective, atomistic level description of the interface structure is essential for studying the reactions that take place at the interface; e.g., it is well-known that coadsorbates on the surface and the surface oxidation drastically change the kinetics of interface reactions. ,, In this regard, one may recall that XPS also gives the information about the geometric structure at the interface. Recently, it has become increasingly common to utilize first-principles calculations when interpreting XPS results. − This has made it possible to distinguish subtle differences and changes in the structure of solid surfaces . Given the circumstances, Pham et al have discussed in detail the applicability of the combined approach using first-principles calculations and AP-XPS measurements to probe the surface chemistry at a complex solid/liquid interface and concluded that the it is a promising way to reveal the relation between surface chemistry and key electronic features at the interface …”

Synergistic Application of XPS and DFT to Investigate Metal Oxide Surface Catalysis

“…This combined XPS and DFT-computed XPS approach is evolving as a new characterization technique to study reaction mechanisms, identify stable intermediates, understand the nature of active sites and evaluate the catalyst performance, but, the current application is limited to systems where U value does not apply (non-semiconductors). Some excellent examples of this approach include the identification and quantification of surface intermediates during Guaiacol thermal decomposition on Pt(111) in the recent work by Hensley et al, 43 the structural characterization of bimetallic Pd−Fe alloy catalyst for the conversion of furfural by Pinto et al, 44 the detection of vacancy and substitution defects in a nitrogencontaining graphene-like matrix by Artyushkova et al 45 and the insightful investigation into the electronic properties for selfassembled monolayer structures by Taucher et al 46 It is expected that such integrated methods could also be extended for studying heterogeneous catalysis over TMOs; however, the application of such combined approaches for TMO catalyzed reactions requires U value estimation for DFT+U so that predictions are accurate and reliable. Therefore, provided that the appropriate U value is known, theoretical (DFT computed XPS) methods could be extended to accurately interpret experimental XPS shifts and assign them to respective surface moieties to provide mechanistic insights into TMO catalyzed reactions, to guide the development of more active and selective TMO-based catalysts.…”

Rational design of heterogeneous catalysts for biomass conversion – Inputs from computational chemistry

“…benzoquinone) structure with a double, and thus strengthened, C=O bond (see resonance structures for phenoxy in Figure 5). [31,34,58] In the case of guaiacol, similar reactions happen under UHV conditions to yield 1,2-benzoquinone with now two C=O bonds. In this regard, the study by Hensley et al combining DFT and state-of-the-art XPS measurements is particularly instructive.…”

“…In this regard, the study by Hensley et al combining DFT and state-of-the-art XPS measurements is particularly instructive. Unlike phenol or anisole, the vicinity of the two oxygenated groups significantly activates the aromatic C-C bond on benzoquinone (activation barrier of only 40 kJ/mol [58] vs 110-160 kJ/mol for phenoxy [31]) leading to the full decomposition of the aromatic moiety on Pt(111). Under an atmosphere of hydrogen, the chemistry is however very different.…”