Enhancement of Methanol Synthesis by Oxidative Fluorination of Cu/ZnO Catalysts─Insights from Surface Analyses

Abstract

A Cu/ZnO catalyst system for methanol synthesis promoted by oxidative fluorination was studied. Gaseous F2 reacts in a first step mainly with CuO to give CuF2 (XPS, thermodynamics). In the active system, the entire fluoride content transforms to ZnF2 and the catalyst system should be formulated as Cu/ZnO1–x /ZnF2 (XPS). Tested for methanol production using a (1 + x) H2/CO x syngas (x = 1, ..., 2, eight steps@40 bar, 473 and 513 K), the fluorinated systems have optimal performance at x = 2, that is, a 3H2/CO2 mixture, and inhibit CO2 promotion at low CO2 concentrations. The number of surface ZnO1–x oxygen defect sites and the number of active sites for methanol synthesis increased in the fluorinated systems after H2 reduction (refined chemisorption measurements, XPS, and BET analysis). Concomitantly, the number of active sites for the (reverse) water–gas shift reaction decreased. Both account for the increased methanol activity and selectivity of the fluorinated catalyst systems and imply negligible water inhibition for the fluorinated case.

Realistic Operando‐DRIFTS Studies on Cu/ZnO Catalysts for CO2 Hydrogenation to Methanol – Direct Observation of Mono‐ionized Defect Sites and Implications for Reaction Intermediates

“…According to this investigation, [53] oxidative fluorination of the pre-catalyst followed by H 2 -activation led to the active CZ_ XXX catalyst system described Figure 1. First, mainly CuF 2 and minor amounts of fluoride in a ZnO matrix (ZnO 1-x F 2x ) formed.…”

“…Inspired by the remarkable enhancement of the MeOH production rate from a 3H 2 /CO 2 feed gas after the oxidative fluorination of a binary Cu/ZnO 1-x (CZ) catalyst system [54] and by the ambiguous assignment [42] of surface intermediates in DRIFT spectroscopy of MeOH catalysis, we had studied the catalytic performance of a well-defined set of highly active CZ catalyst systems supported by specialized N 2 O chemisorption and XPS analyses. [53] Here we report on an extended DRIFTS investigation of the same batch of these CZ, CZ_200 and CZ_2000 catalyst systems (XXX after CZ_XXX indicates the total F 2 -pressure, to which the CZ sample was exposed: i. e., 200 mbar for CZ_200 (contains 1.2 w% F by ion chromatography IC) and 2000 mbar for CZ_2000 (2.7 w% F by IC). The essential characteristics of these catalyst systems with a typical Cu : Zn ratio of 2.1 : 1 are summarized in Table 1.…”

“…See Ref. [53], [c] Calculated according to Goldschmidt with r(O 2À ) = 0.132 nm (r(Zn 2 + ) = 0.083 nm), resulting in 5.47 • 10 À 20 m 2 /oxygen atom. See Ref.…”

“…Hence, this work reports on a full operando-DRIFTS investigation of pristine and to some extent also oxidatively fluorinated Cu/ZnO catalyst systems that have improved performance for CO 2 hydrogenation. [53] The catalytic performance of this catalyst system and surface analyses (XPS, chemisorption) were already studied. [53,54] The operando-DRIFT spectra of active Cu/ZnO 1-x catalysts were recorded under realistic reaction conditions (30 bar, 513 K).…”

“…[53] The catalytic performance of this catalyst system and surface analyses (XPS, chemisorption) were already studied. [53,54] The operando-DRIFT spectra of active Cu/ZnO 1-x catalysts were recorded under realistic reaction conditions (30 bar, 513 K). Simultaneously, DRIFT spectra, optical absorption and quantitative product composition (MeOH, H 2 O, CO by GC) were recorded with varying gas mixtures (H 2 , CO 2 , CO, N 2 ).…”