Mechanistic and spectroscopic identification of initial reaction intermediates for prenal decomposition on a platinum model catalyst

Abstract

The prediction of a reaction mechanism and the identification of the corresponding chemical intermediates is a major challenge in surface science and heterogeneous catalysis, due to a complex network of elementary steps and surface species. Here we demonstrate how to overcome this difficulty by tracking the temperature dependent formation of the initial reaction intermediates and identifying the decomposition pathways in the case of prenal, an α,β-unsaturated aldehyde, on the Pt(111) model catalyst surface by combining vibrational spectroscopy, thermal reaction/desorption spectroscopy (TPRS) experiments and detailed theoretical analysis. TPRS characterization of this reaction up to 600 K shows a series of desorption states of H(2) (∼280 K, 410 K and 473 K) and CO (∼414 K), giving valuable insights into the sequence of elementary steps suggesting that the loss of hydrogen and the carbonyl functions are among the first elementary steps. HREELS experiments recorded after annealing to specific temperatures result in complex spectra, which can be assigned to several subsequently formed and transformed surface intermediates. Starting from stable prenal adsorption structures, complementary DFT calculations allow the determination of the most likely reaction pathway for the initial decomposition steps and the identification of the corresponding intermediates by comparison with HREELS. The decomposition occurs from the strongly bonded prenal adsorption structures via a dehydro-η(3)-triσ(CCC)-H1 intermediate to the highly stable η(1)-isobutylidyne species at high temperatures.

Guidelines to Achieving High Selectivity for the Hydrogenation of α,β-Unsaturated Aldehydes with Bimetallic and Dilute Alloy Catalysts: A Review

“…31,32,104,110,188,248 The experimentally observed selectivity trends were attributed to the adsorption geometry of the unsaturated aldehyde in key comparative studies of the three species. [31][32][33] Initial results from the semi-empirical extended Hückel theory, 33 followed by first-principles calculations, 31,32,160,161,165,174,175,203,248 have established the following. Acrolein adsorbed mainly via its C=C bond over a large range of coverage, promoting the formation of propanal.…”

“…Hydrogenation products were not observed for acrolein, 94,95,106,107,243,244 crotonaldehyde, 106,108,109,187,243,[245][246][247] and prenal. 110,248 Instead, decarbonylation dominated, yielding CO, H 2 , and propylene. Using DFT, the decomposition pathway was modeled for prenal, which proceeded by the formyl C-H bond scission, decarbonylation, and isomerization to the highly stable η 1 -isobutylidyne species.…”

From Static to Reacting Systems on Transition-Metal Surfaces

“…As already alluded to above, the reactivity of Pt surfaces toward selective hydrogenation of UALs could be favorably changed by the addition of a second metal M ¼ Sn, [245][246][247][248] Ni, [249][250][251] Co, 251 and Cu. 251 Such an oxophilic modifier as Sn is expected to interact mainly with the O atom and thus to improve the desired selectivity toward UOLs by facilitating activation of the C]O bond.…”

Partial Hydrogenation of Unsaturated Carbonyl Compounds: Toward Ligand-Directed Heterogeneous Catalysis

“…[42][43] The appearance of this new band evidences acrolein decomposition via decarbonylation resulting in carbonaceous depositsethylidyne speciesand a fragment involving the carbonyl group of acrolein previously described in the literature. [44][45][46][47][48] Since the propenol formation rate decreases over time and this process is correlated with the appearance of ethylidyne-like species, it can be speculated that decomposition products lead to the catalyst's deactivation by blocking the surface sites.…”