Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal–Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

Abstract

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH → CHOH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.

Thermodynamic Limitations of the Catalyst Design Space for Methanol Production from Methane

“…More importantly, this indicates that kinetic limitations are absolutely necessary for selectively converting CH 4 into alcohols rather than CO 2 . Experimental studies show that catalysts with well‐defined active sites can sufficiently lower the activity for complete oxidation and produce alcohols from methane in gas phase reactions at steady state …”

“…Although alcohol selectivity is generally improved in chemical looping processes, conversion and CH 3 OH yield on a per mole CH 4 basis are typically low and difficult to quantify because a large excess of CH 4 is often used to populate a fraction of the active sites, and CH 3 OH yields are typically reported relative to the amount of catalyst or the number of active sites. Semi‐continuous chemical looping processes can have long catalyst cycle times (>20 hours) and are less economically attractive than steady‐state reactions, but few reports exist for one‐pot selective oxidation of CH 4 to CH 3 OH using O 2 as a low‐cost oxidant . A strategy that has been explored for one‐pot synthesis of alcohols is to include steam in the feed stream to drive the equilibrium away from combustion (reaction 7) and toward alcohols ,,.…”

“…Semi‐continuous chemical looping processes can have long catalyst cycle times (>20 hours) and are less economically attractive than steady‐state reactions, but few reports exist for one‐pot selective oxidation of CH 4 to CH 3 OH using O 2 as a low‐cost oxidant . A strategy that has been explored for one‐pot synthesis of alcohols is to include steam in the feed stream to drive the equilibrium away from combustion (reaction 7) and toward alcohols ,,. The presence of H 2 O in continuous feed streams is also reported to provide kinetic inhibition for over‐oxidation by occupying active sites, while chemical looping systems have shown H 2 O to be effective for facilitating methanol desorption .…”

“…Methanol is released when the resulting surface species are exposed to steam, but a high‐temperature calcination step is needed to return these materials to the state in which they can activate methane. Recently, a number of studies showed that methane can be converted to methanol and in some cases ethanol in the presence of O 2 and steam . Even methanol formation from chemical looping of methane and steam in the absence of O 2 was reported …”

Palladium Dimer Supported on Mo2CO2(MXene) for Direct Methane to Methanol Conversion

“…[10] Among these catalysts, Pdbased catalysts have attracted considerable attention due to their high selectivity, though the Pd utilization in these supported metal nanoparticle catalysts is very low, and its commercializations are hampered due to the high cost. [12,[16][17][18][19][20][21][22][23] Downsizing catalyst nanoparticles to single atoms could be desirable to enhance utilization efficiency of noble metal atoms. [24] Recently, single-atom catalysts (SACs) have been attracted increasingly as their high activity for a wide variety of chemical reactions, [24][25][26][27][28] such as oxygen reduction reaction (ORR) [27] and hydrogen evolution reaction (HER).…”