Differentiating the Reactivity of ZrO2-Bound Formates Formed on Cu/ZrO2 during CO2 Hydrogenation

Abstract

The surface species formed during the hydrogenation of CO2 with H2 over a ZrO2-supported Cu catalyst were investigated by operando diffuse reflectance FT-IR spectroscopy at 220 °C and 3 bar. The reactivity of two different formates located on zirconia could be unraveled. The data pointed to ZrO2 hydroxyl groups at 3755 cm−1 as the sites on which carbonates and then formates were hydrogenated to methoxy species. Formate hydrogenation appeared as the slowest step. The most reactive ZrO2-bound formates exhibited a rate constant of reaction about 65 times higher than that of the slower formate.

Cu‐Bound Formates are Main Reaction Intermediates during CO2 Hydrogenation to Methanol over Cu/ZrO2

“…The operando DRIFTS spectrum obtained at steadystate at 220 °C and 3 bar of 20 % CO 2 + 60 % H 2 in He is shown in Figure 1A. As described earlier, [21] IR bands of formates (2873, 1575 and 1380 cm À 1 ), carbonates (1468 cm À 1 ) and methoxides (2927, 2822, 1150 and 1049 cm À 1 ) were observed, located in most part on the zirconia support (Table 1). Most methoxides were likely spectators formed from the readsorption of methanol on ZrO 2 , which are removed by hydrolysis with water rather than by reductive elimination, as shown by Fisher and Bell.…”

“…[19,26] The carbonates observed were likely those located on ZrO 2 far away from the Cu interface and were thus only slowly reduced because of limited hydrogen spillover. [12] As discussed earlier, [21] the decay observed using region 3 can be associated with so-called "fast formates", presumably located near the Cu interface and decaying within 25 min and "slow formates", presumably located far away from Cu and decaying much slower.…”

“…The fast formate on ZrO 2 signal, after correction of the slow formate signal (see Ref. [21] for details), exhibited a single exponential decay over the 1 to 25 min period, indicating uniform reactivity (Figure 4B). This observation also stresses the relevance of the overall signal decomposition into these different contributions.…”

“…We report here such comparison obtained through quantitative operando diffuse reflectance FT-IR spectroscopy (DRIFTS) on our Cu/ZrO 2 . [21] Specific formate decomposition rates (in nmol s À 1 g À 1 ) are derived from formate concentrations and decomposition rate constants.…”

“…We recently reported that at least two types of formates were present on the ZrO 2 phase on a 6 wt.% Cu/ZrO 2 catalyst and that the reactivity of those greatly differed [21] . The fastest reacting formates, which exhibited a 65‐fold higher decomposition rate at 220 °C, was proposed to be located closer to the Cu‐ZrO 2 interface.…”