Low Temperature Water Gas Shift: Evaluation of Pt/HfO₂ and Correlation between Reaction Mechanism and Periodic Trends in Tetravalent (Ti, Zr, Hf, Ce, Th) Metal Oxides

Abstract

It is well-known that catalysts containing small quantities of noble metals (e.g., Pt, Au) that are highly dispersed on Group IV and related tetravalent metal oxides (e.g., TiO2, ZrO2, CeO2, ThO2) display, once activated, high conversion for water-gas shift (WGS). The mechanism suggested for this by many authors is bimolecular in nature. In one view, adsorption of CO on the partially reducible oxide produces formates by reaction with active bridging OH groups, followed by water-assisted formate decomposition by dehydrogenation, with this second step being facilitated by metal particles at the metal–support interface. Considering this group of oxides, HfO2, as far as we are aware, had not yet been studied as a potential candidate for low temperature water-gas shift (LTWGS). Thus, in this contribution, catalysts composed of Pt supported on HfO2 were prepared and tested. Catalyst testing results suggest that this system is, like Pt/ZrO2 and Pt/CeO2, active for the WGS reaction, although additional studies are required to obtain an ordering of reactivity. Insight into the possible mechanism was gained through the use of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Formate bands were observed to form when defect-associated bridging OH groups, present on hafnia after partial reduction, were reacted with CO. Pt facilitated partial reduction of hafnia and thus the formation of the bridging OH groups on its surface. The formate bands readily disappeared in the presence of steam at 130 °C, converting to the second intermediate, carbonate species, which liberated CO2 and presumably H2 products. Moreover, in switching from CO adsorption alone to steady state LTWGS, the formate bands became reaction rate limitedand more so at higher temperature. The implication is that formate is turning over more rapidly at the latter condition, with its decomposition (probably via C–H bond breaking) being the most likely rate limiting step. These experiments suggest that the reaction probably follows a bimolecular associative pathway, with adsorbed formate being an intermediate, in the same manner as observed with the other Group IV and related tetravalent oxide based LTWGS catalysts. Moreover, an interesting relationship may exist between the C–H bond strength of adsorbed formate (i.e., observed during CO adsorption) and the metal ion radius in the oxide.

Promotion of the Water‐Gas‐Shift Reaction by Nickel Hydroxyl Species in Partially Reduced Nickel‐Containing Phyllosilicate Catalysts

“…The water‐gas‐shift (WGS) reaction has garnered much attention in producing hydrogen gas and removing carbon monoxide (CO) from the syngas, which is a mixture of CO and H 2 …”

“…To strike ab alance between the kinetics and thermodynamics, this reaction is industrially operated in two stages:o ne at lower temperatures ( % 200-250 8C) catalyzed by Cu-Zn catalysts;t he other at higher temperatures ( % 350-450 8C) catalyzed by Fe-Cr catalysts. [1][2][3][4] In the high-temperature WGS reaction, Ni is widely studied as ac andidate to replaceF e-Cr catalysts, whicha re known to be toxic owing to theC rc ontent. [5] Despite the high catalytic activity of Ni at high temperatures, it catalyzes the undesirable methanation side reaction, which decreasest he H 2 selectivity and hinders the commercialization of Ni-basedc atalysts.…”

Improved activity and stability of Pd@CeO2 core–shell catalysts hybridized with multi-walled carbon nanotubes in the water gas shift reaction

“…Therefore, while hightemperature WGSR (HT-WGSR) catalysts may take advantage of the hot gases generated by the reformer, a low-temperature process (LT-WGSR) is preferable from a thermodynamic point of view [3]. The heterogeneous catalysts most widely investigated are based on supported Fe, Pt, Au, and Cu metals [4][5][6].…”

Theoretical Study on Electronic Properties and Structural Evolution in Hf2On-/0 (n=1~6) Clusters

“…The trends of structural evolution and the behavior of sequential oxidation of the Hf 2 O n -(n＝1～6) clusters were observed. For the anionic species, starting from the C 2v triangular structure 由 Pt 和 HfO 2 组成的催化剂可以作为水煤气转化过程的 催化剂 [2] . 此外, HfO 2 可以作为催化剂或催化助剂光降 解废水中的有机污染物 [7] .…”