Non‐Thermal Plasma Activation of Gold‐Based Catalysts for Low‐Temperature Water–Gas Shift Catalysis

Abstract Non‐thermal plasma activation has been used to enable low‐temperature water‐gas shift over a Au/CeZrO4 catalyst. The activity obtained was comparable with that attained by heating the catalyst to 180 °C providing an opportunity for the hydrogen production to be obtained under conditions where the thermodynamic limitations are minimal. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), structural changes associated with the gold nanoparticles in the catalyst have been observed which are not found under thermal activation indicating a weakening of the Au−CO bond and a change in the mechanism of deactivation.

Such reversible exothermic reactions come with difficulties in terms of process development since thermodynamics and kinetics have antagonist temperature requirements.From at hermodynamic point of view,o nly low temperature operation allows access to high equilibrium conversion whilst from ak inetics point av iew,t he higher the reaction temperature the better.I nt he specific case of the WGS reaction (DH8 8 298 = À41.09 kJ mol À1 )the thermodynamic equilibrium at high temperature limits the CO conversion, to the point that WGS processes usually require multiple reactors in series with decreasing operation temperatures.Insome cases, this is not sufficient to achieve low enough output CO concentrations and afurther CO purification step is added. If lower temperature operations could be achieved, significant process intensification and operational cost saving would ensue.F igure 1i llustrates the issue associated with the relationship between catalyst activity and thermodynamic limitations.Ast he reaction temperature is raised to increase the activity,t he equilibrium limitation is approached and limits the achievable conversion.
Close coupling of plasmas with heterogeneous catalysts presents many advantages,i ncluding the possibility of opening up alternative reaction pathways from the plasmagenerated species [13] and structural changes of the active phase due to the plasma;however,the activation mechanism depends on many factors including the type of catalyst, reactants,r eaction conditions. [3,4,[18][19][20][21] Thep resent study demonstrates that areversible exothermic reaction of significant industrial importance can be performed at close to room temperature using heterogeneous catalysis and dielectricbarrier discharge (DBD) plasma activation;a nd thus overcoming the equilibrium limitations whilst maintaining high catalytic activity.
Light-off experiments with af ull WGS reaction mixture  [22] and Tibiletti et al. [23] Importantly,C Oc onversion was observed at low temperatures,f or example,1 5% conversion at 100 8 8C. Figure 2r eports ac omparison of the CO conversions obtained with an empty reactor, the pure CeZrO 4 support and the Au/CeZrO 4 catalyst under plasma conditions (7.5 kV and 22.5 kHz) using the full WGS feed. Thee mpty reactor and the CeZrO 4 support both gave very low conversions about 7%.Asignificant increase in CO conversion was observed when the gold catalyst was used, reaching 70 % under these conditions.Whilst no heat source was applied, the application of the plasma led to an increase in the reactor temperature.This rise was not related to the exothermicity of the reaction since,i na ll three cases,t he measured temperature of the system was approximately 115 8 8C, irrespective of the extent of conversion obtained. Theenhancement effect of the plasma-activated catalytic system over the thermal system is clear. At 115 8 8C, only 20 %COc onversion was found over the gold catalyst under thermal activation ( Figure 1) compared with 70 %conversion under plasma conditions.Inorder to explore the importance of the Joule heating during plasma activation, asimplified mixture,containing only CO and H 2 O was also tested. Under these conditions the CO conversion obtained under plasma activation was close to 90 %. To achieve as imilar performance thermally requires temperatures above 400 8 8C, as shown in Figure S1 in the Supporting Information. Therefore,t he effect of plasma on the CO conversion is not solely associated with aJoule heating effect.
Thev ariation of CO conversion with applied voltage is shown in Figure S2. As ignificant increase in the conversion was observed from < 40 %a t6 .0 kV to 70 %a t7 .5 kV.T he calculated specific energy input for an applied voltage of 7kV was 9.2 W. No change in the selectivity was observed for the different voltages tested or under thermal conditions.I na ll cases,the reaction was selective to CO 2 ,with mass balances of 97 % AE 3% (Table S1) which is consistent with the previous studies under thermal control. [22,23] Furthermore,t emperature-programmed oxidation of the used samples confirmed that little carbon deposition took place and that the carboncontaining species evolution could be attributed to carbonate decomposition after both thermal and plasma reactions [24] ( Figure S3). This dependence on applied voltage is in contrast with Sekine et al. which may reflect the application of aD C compared with AC electric field. [2] Despite the excellent activity at relatively low reaction temperatures,A u-based catalysts are often affected by significant deactivation with time on stream. [24][25][26][27][28][29] Herein, as imilar deactivation behaviour was observed for similar starting conversions of CO under thermal (178 8 8C) and plasma conditions ( Figure S4). This deactivation was previously attributed [27] to surface hydrolysis leading to al oss of the gold-support interaction and the rate of deactivation was observed to increase with increasing water concentration. A similar behavior was observed under plasma conditions ( Figure S5). Interestingly,w hilst the trends in the initial rate of deactivation are similar under both thermal and plasma activation, the effect on gold differs. [30] Longer-term deactivation experiments have also been performed over 35 htime on stream ( Figure S6). Theinitial rapid deactivation rate was over 2.5 %h À1 ( Figure S5);h owever after 10 hr eaction the rate slowed to 0.6 %h À1 and, thereafter,t he rate remained constant.
Following the reaction under plasma activation, transmission electron microscopy showed an increase in the average gold particle size from 0.7 to 1.8 nm ( Figure S7),

Angewandte Chemie
Communications 5580 www.angewandte.org suggesting sintering of gold may be ac ause of deactivation under these conditions.T his is in contrast with previously reported data for the thermally activated reaction which showed no significant change in particle size. [26,27] Furthermore,adecrease of the surface area from 73 m 2 g À1 (fresh catalyst) to 56 m 2 g À1 after plasma exposure has also been observed. However,itis important to note that the activity of the gold catalyst was still significant even after 500 min reaction time with 57 %COconversion observed for the Au/ CeZrO 4 catalyst under plasma conditions under full WGS reaction conditions.C haracterization of the catalyst before and after plasma exposure did not show changes in the XRD patterns ( Figure S8). Only CeO 2 and ZrO 2 features were present [31] with no Au peaks suggesting that gold particles were well dispersed on the surface of support and the size of gold particles was smaller than the detection limit of XRD, smaller than 5nm [25,31] which is in agreement with the TEM results. Figures S9 and S10 show the X-ray photoelectron spectral (XPS) lines of Au 4f,a nd Ce 3d regions for the Au/CeZrO 4 catalyst and as ummary of the XPS binding energy data is given in Table S2. Binding energies of 84.0 eV and 86.3 eV were observed for the Au 4f 7/2 corresponding to Au 0 and Au 3+ ; however, no changes were observed before and after the plasma reaction. Theo nly significant change observed was adecrease in the Ce 3+ /(Ce 3+ + Ce 4+ )surface ratio from 0.34 to 0.17 which indicates some surface oxidation of the support had occurred when the catalyst was exposed to plasma.
In order to further probe the gold active site under the plasma activation, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used. This used ap reviously reported DRIFTS reactor setup where the plasma was set to only penetrate as mall portion of the sample,t herefore,e nsuring differential conditions for the spectroscopic measurements. [32] Theinlet gas composition was set to the simplified mixture forward WGS feed (2 %C O, 7.5 %H 2 O), therefore,e nsuring that the CO 2 observed was entirely derived from the plasma activation. Thei nsitu DRIFTS spectra were recorded as af unction of time during three plasma on-off cycles over the Au/CeZrO 4 catalyst (see Figure S11). Thef ormation and disappearance of gas phase CO 2 (2300 and 2400 cm À1 )a ssociated with ad ecrease in the IR bands of both CO and H 2 Owhen plasma was switched on confirmed the activity of the catalyst for the WGS reaction under these conditions.Acomparison of the spectrum recorded at 150 8 8Cu nder thermal conditions,t hat is,n o plasma activation, and the spectrum obtained when the plasma was on showed the presence of formates in the region 3000-2500 cm À1 ,c arbonates and formates in the region 1700 and 800 cm À1 and carbonyl bands in the region 2200-2000 cm À1 . [27,33,34] Moreover,as ignificant reduction in the carbonate bands observed under plasma conditions may indicate that the CO 2 release was promoted. Thepresence of surface carbonates blocking the redox sites on the catalyst have been proposed as potential causes of deactivation. [28,29,[35][36][37] Importantly,the presence of the plasma reduces the carbonate features present which indicates that this is probably not the cause of the deactivation in the present system.
In order to characterize the state of the catalyst further, an examination of the carbonyl bands has been undertaken. These features have been used extensively to characterize the state of the surface gold sites. [23,38,39] Forexample,IRbands at 2110-2090 cm À1 have been assigned to CO on metallic Au particles (Au 0 ÀCO) [25,[38][39][40] whereas bands between 2120 and 2180 cm À1 are thought to be due to CO adsorbed on Au d+ .In addition, bands in the region of 2050-1950 cm À1 have been assigned to CO adsorbed on very small gold cluster/negatively charged Au. [25] Figure S12 compares the carbonyl region (2200-2000 cm À1 )u nder thermal conditions and also during plasma on-off cycles.F or each spectrum, the gas phase CO was subtracted from the original spectra recorded during the reaction. During thermal activation at 150 8 8C, the predominant feature was the band at 2095 cm À1 associated with the CO adsorbed on Au 0 ,s pecifically at the step sites or the perimeter of Au nanoparticles,c onsistent with previous studies. [23,27,34] As mall contribution of Au d+ ÀCO species at 2120 cm À1 was also observed. Exposure of the fresh catalyst to WGS feed under plasma activation (first cycle) led to the appearance of similar adsorbed features,w ith Au 0 ÀCO species more dominant than the Au d+ À CO species (Figure 3). Previous studies showed that under thermal conditions,m etallic Au is the most stable and an active species for the water-gas shift reaction. [23,26,27,41,42] No CO bands were observed when the pure support was examined under similar conditions.O ne xtinguishing the plasma over the catalyst, as ignificant decrease in the adsorbed CO was observed predominantly associated with the band at 2095 cm À1 ,t hat is,a ssociated with adsorption on the gold nanoparticles.Asmall decrease was also observed in the band at 2120 cm À1 attributed to adsorption on partially oxidized gold.
On consecutive cycling of the plasma, an overall decrease in the Au 0 ÀCO band was observed, whereas the intensity of the band at 2120 cm À1 increased with cycling,b oth during application of the plasma and in its absence.T his change in the nature of the adsorption of the CO is consistent with the change in the electron microscopy before and after the plasma treatment. Therein, particle growth of the gold was observed which would decrease the amount of interfacial gold atoms responsible for the band at 2095 cm À1 .P revious studies have shown that the size of this feature under thermal conditions can be related to the activity of the catalyst. [26,27,43] This may also be the case under plasma conditions which shows deactivation with time on stream;t he fact that after three cycles this band has decreased considerably,yet the catalyst is still active would suggest that this species is,i ndeed, being converted into al ess active form (Au d+ ). Under the plasma conditions,i ti sp ostulated that reactive oxygen species (ROS) [1,[44][45][46] formed through the activation of the water leads to oxidation of the gold. This reactive oxygen species, which may be OH, for example,g enerated in the gas phase plasma will also increase the hydrolysis of the gold-support interface and lead to the loss of the interfacial gold species, represented by the band at 2095 cm À1 .P reviously reported density functional theory (DFT) calculations have demonstrated that the decoration of the surface with hydroxy groups significantly weakens the metal-support interaction and ad ewetting of the gold nanoparticle.T his loss of the interfacial gold which is exacerbated by the sintering of the gold particles,l eads to ad eactivation of the catalyst. As neither sintering or oxidation of the surface is observed under thermal conditions this highlights the significantly higher reactivity of the water derived species which is activated by ionization in the gas phase rather than by the oxygen vacancies on the support. These changes are also consistent with the XPS results which show surface oxidation of the support on exposure to the plasma, that is,areduction in the surface concentration of oxygen vacancies.T he formation of oxygen vacancies are catalyzed by surface metal sites and, therefore,t he loss of surface gold would also reduce their surface concentration.
To obtain ab etter understanding of the mechanisms governing the high catalytic activity achieved under plasma conditions,DFT was used to calculate the free energy profile of the WGS over Au(100) and Au(111) for both redox and carboxyl pathways shown in Figures S15 and S16, respectively.The results strongly suggest that the water activation is the rate-determining step on gold surfaces,w hich is in agreement with previous results. [47] Water has been observed to be activated in the gas phase under plasma conditions to form, for instance,OHand H 2 O + species. [1,[44][45][46]48] Theformer is thought to be ak ey intermediate from the water dissociation on the surface of gold and the subsequent reaction to form COOH before releasing CO 2 .Prior formation of the OH before adsorption would reduce the effective barriers,u sing the method reported by Wang et al., [49] from 2.87 to 0.73 eV on Au(111) and from 2.44 to 0.99 eV on Au(100). Further-more,t he activation barriers for the dissociation of H 2 O x as af unction of the charge x over gold was calculated and was found to be negative for cationic water ( Figure S17), suggesting that H 2 O + is not stable and will dissociate into OH and H spontaneously.T herefore,t he plasma activated low temperature WGS is likely to be due to the facile activation of H 2 O reducing the overall activation barrier. It should be noted although the role of the support has not been taken into account in these calculations,a ctivation of water is also thought to be akey step in the WGS process at the interface between the gold and the support. Therefore,t he activation barriers are likely to be even lower in the presence of the oxide under plasma activation.
In conclusion, the reported results clearly demonstrate that it is possible to decouple the thermodynamics of the WGS process from the kinetics using ad ielectric-barrier discharge activated heterogeneous catalyst system that enables the reaction to occur at low temperature.I nsitu diffuse reflectance infrared spectroscopy coupled with plasma activation lead to the formation of various types of both gas phase and surface species including CO (ads), CO 2 (g and ads), formates,c arbonates,a nd water. Similar species were reported to be formed under various WGS reaction conditions over avariety of catalysts.The plasma could affect the rate-determining step on the gold surface by activating the water in the gas phase.M ost importantly,t he DRIFTS study demonstrated an impact of the DBD on the structural properties of the gold leading to as ignificant change in the adsorption properties of CO indicative of aweakening of the Au À CO bond.